241. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes
Vladislav Kamysbayev, Alexander S. Filatov, Huicheng Hu, Xue Rui, Francisco Lagunas, Di Wang, Robert F. Klie, and Dmitri V. Talapin. Science 2020eaba8311.

UChicago News: UChicago chemists invent way to customize compounds just a few atoms thick


Versatile chemical transformations of surface functional groups in 2D transition-metal carbides (MXenes) open up a new design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with O, NH, S, Cl, Se, Br, and Te surface terminations, as well as bare MXenes (no surface termination) was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn (n = 1, 2) MXenes terminated with Te2− ligands show a giant, (>18%) in-plane lattice expansion compared to the bulk TiC lattice. Nb2C MXenes exhibited surface-group-dependent superconductivity.

240. sasPDF: pair distribution function analysis of nanoparticle assemblies from small-angle scattering data
Chia-Hao Liu, Eric M. Janke, Ruipen Li, Pavol Juhás, Oleg Gang, Dmitri V. Talapin, and Simon J. L. Billinge. J. Appl. Cryst. 202053, 699.


sasPDF, a method for characterizing the structure of nanoparticle assemblies (NPAs), is presented. The method is an extension of the atomic pair distribution function (PDF) analysis to the small-angle scattering (SAS) regime. The PDFgetS3 software package for computing the PDF from SAS data is also presented. An application of the sasPDF method to characterize structures of representative NPA samples with different levels of structural order is then demonstrated. The sasPDF method quantitatively yields information such as structure, disorder and crystallite sizes of ordered NPA samples. The method was also used to successfully model the data from a disordered NPA sample. The sasPDF method offers the possibility of more quantitative characterizations of NPA structures for a wide class of samples.

239. Bright trion emission from semiconductor nanoplatelets
Lintao Peng, Matthew Otten, Abhijit Hazarika, Igor Coropceanu, Moritz Cygorek, Gary P. Wiederrecht, Pawel Hawrylak, Dmitri V. Talapin, and Xuedan Ma. Phys. Rev. Materials 2020, 4, 056006.


The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. Here, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

238. Heat-driven acoustic phonons in lamellar nanoplatelet assemblies
Benjamin T. Diroll, Vladislav Kamysbayev, Igor Coropceanu, Dmitri V. Talapin, and Richard D. Schaller. Nanoscale 2020, 12, 9661.


Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C–H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3–8 μeV and 0.5–1 ns oscillation period) depending on the thickness of the nanoplatelets—but not their lateral areas—and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.

237. Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy
Burak Guzelturk, James K. Utterback, Igor Coropceanu, Vladislav Kamysbayev, Eric M. Janke, Marc Zajac, Nuri Yazdani, Benjamin L. Cotts, Suji Park, Aditya Sood, Ming-Fu Lin, Alexander H. Reid, Michael E. Kozina, Xiaozhe Shen, Stephen P. Weathersby, Vanessa Wood, Alberto Salleo, Xijie Wang, Dmitri V. Talapin, Naomi S. Ginsberg, and Aaron M. Lindenberg. ACS Nano 2020, 14, 4792.


Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron–phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron–phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.


The morphology of nanocrystals serves as a powerful handle to modulate their functional properties. For semiconducting nanostructures, the shape is no less important than the size and composition, in terms of determining the electronic structure. For example, in the case of nanoplatelets (NPLs), their two-dimensional (2D) electronic structure and atomic precision along the axis of quantum confinement makes them well-suited as pure color emitters and optical gain media. In this study, we describe synthetic efforts to develop ZnSe NPLs emitting in the ultraviolet part of the spectrum. We focus on two populations of NPLs, the first having a sharp absorption onset at 345 nm and a previously unreported species with an absorption onset at 380 nm. Interestingly, we observe that the nanoplatelets are one step in a quantized reaction pathway that starts with (zero-dimensional (0D)) magic-sized clusters, then proceeds through the formation of (one-dimensional (1D)) nanowires toward the (2D) “345 nm” species of NPLs, which finally interconvert into the “380 nm” NPL species. We seek to rationalize this evolution of the morphology, in terms of a general free-energy landscape, which, under reaction control, allows for the isolation of well-defined structures, while thermodynamic control leads to the formation of three-dimensional (3D) nanocrystals.

235. Quantum dot solids showing state-resolved band-like transport
X. Lan, M. Chen, M. H. Hudson, V. Kamysbayev, Y. Wang, P. Guyot-Sionnest, and D. V. Talapin. Nature Mater. 2020, 19, 323.

Improving charge mobility in quantum dot (QD) films is important for the performance of photodetectors, solar cells and light-emitting diodes. However, these applications also require preservation of well defined QD electronic states and optical transitions. Here, we present HgTe QD films that show high mobility for charges transported through discrete QD states. A hybrid surface passivation process efficiently eliminates surface states, provides tunable air-stable n and p doping and enables hysteresis-free filling of QD states evidenced by strong conductance modulation. QD films dried at room temperature without any post-treatments exhibit mobility up to μ ~ 8 cm2 V−1 s−1 at a low carrier density of less than one electron per QD, band-like behaviour down to 77 K, and similar drift and Hall mobilities at all temperatures. This unprecedented set of electronic properties raises important questions about the delocalization and hopping mechanisms for transport in QD solids, and introduces opportunities for improving QD technologies.

234. Hot-Carrier Relaxation in CdSe/CdS Core/Shell Nanoplatelets
M. Pelton, Y, Wang, I. Fedin, D. V. Talapin, S. K. O’Leary. J. Phys. Chem. C 2020, 124, 1020.


We present time-resolved photoluminescence (PL) spectroscopy of a series of colloidal CdSe/CdS core/shell nanoplatelets with different core and shell thicknesses. Exciton numbers are determined from the integrated PL intensities, and carrier temperatures are determined from the high-energy exponential tail of the PL spectra. For times between 10 and 1000 ps, the measured carrier relaxation dynamics are well described by a simple model of Auger reheating: biexcitonic Auger recombination continually increases the average energy of the carriers (while decreasing their number), and this reheating sets a bottleneck to cooling through electron–phonon coupling. For times between 1 and 10 ps, the relaxation dynamics are consistent with electron–phonon coupling, where the bottleneck is now the decay of the longitudinal optical phonon population. Comparison of relaxation dynamics to recombination dynamics reveals changes in the carrier–phonon coupling for shell thicknesses greater than 4 monolayers.

233. Titanium Nitride Modified Photoluminescence from Single Semiconductor Nanoplatelets
L. Peng, X. Wang, I. Coropceanu, A. B. Martinson, H. Wang, D. V. Talapin, and X. Ma. Adv. Funct. Mater. 2020, 30, 1904179.


Titanium nitride (TiN) is an alternative plasmonic material that has the potential for visible and near‐infrared optical applications due to its distinct properties. Here, coupling effects between TiN nanohole array films and nearby excitonic emitters, semiconductor nanoplatelets (NPLs), are investigated using single particle spectroscopy. At the emission wavelength of the NPLs, the local field enhancement close to the surface of the TiN nanohole array films induces an increase in the radiative decay rates of the emitters by a factor of up to 2. This effect diminishes quickly as the distance between the TiN nanohole array films and emitters increases. At short wavelengths where the NPLs are excited, the TiN nanohole array films exhibit lossy dielectric characteristics. Local field modification at these wavelengths leads to a reduced local density of electromagnetic states, and hence the photoluminescence intensity of the emitters. This study shows the potential of TiN as an alternative plasmonic material for optoelectronic and photonic applications, especially in the long wavelength ranges.


232. Direct Wavelength-Selective Optical and Electron-Beam Lithography of Functional Inorganic Nanomaterials
Y. Wang, J.-A. Pan, H. Wu, and D. V. Talapin. ACS Nano 2019, 13, 13917.


Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is a photoresist-free method for high-resolution patterning of inorganic nanocrystals (NCs) that has been demonstrated using deep UV (DUV, 254 nm) photons. Here, we expand the versatility of DOLFIN by designing a series of photochemically active NC surface ligands for direct patterning using various photon energies including DUV, near-UV (i-line, 365 nm), blue (h-line, 405 nm), and visible (450 nm) light. We show that the exposure dose for DOLFIN can be ∼30 mJ/cm2, which is small compared to most commercial photopolymer resists. Patterned nanomaterials can serve as highly robust optical diffraction gratings. We also introduce a general approach for resist-free direct electron-beam lithography of functional inorganic nanomaterials (DELFIN) which enables all-inorganic NC patterns with feature size down to 30 nm, while preserving the optical and electronic properties of patterned NCs. The designed ligand chemistries and patterning techniques offer a versatile platform for nano- and micron-scale additive manufacturing, complementing the existing toolbox for device fabrication.

231. Colloidal Gelation in Liquid Metals Enables Functional Nanocomposites of 2D Metal Carbides (MXenes) and Lightweight Metals
V. Kamysbayev, N. M. James, A. S. Filatov, V. Srivastava, B. Anasori, H. M. Jaeger, Y. Gogotsi, and D. V. Talapin. ACS Nano 2019, 13, 12415.


Nanomaterials dispersed in different media, such as liquids or polymers, generate a variety of functional composites with synergistic properties. In this work, we discuss liquid metals as the nanomaterials’ dispersion media. For example, 2D transition-metal carbides and nitrides (MXenes) can be efficiently dispersed in liquid Ga and lightweight alloys of Al, Mg, and Li. We show that the Lifshitz theory predicts strong van der Waals attraction between nanoscale objects interacting through liquid metals. However, a uniform distribution of MXenes in liquid metals can be achieved through colloidal gelation, where particles form self-supporting networks stable against macroscopic phase segregation. This network acts as a reinforcement boosting mechanical properties of the resulting metal–matrix composite. By choosing Mg–Li alloy as an example of ultralightweight metal matrix and Ti3C2Tx MXene as a nanoscale reinforcement, we apply a liquid metal gelation technique to fabricate functional nanocomposites with an up to 57% increase in the specific yield strength without compromising the matrix alloy’s plasticity. MXenes largely retain their phase and 2D morphology after processing in liquid Mg–Li alloy at 700 °C. The 2D morphology enables formation of a strong semicoherent interface between MXene and metal matrix, manifested by biaxial strain of the MXene lattice inside the metal matrix. This work expands applications for MXenes and shows the potential for developing MXene-reinforced metal matrix composites for structural alloys and other emerging applications with metal–MXene interfaces, such as batteries and supercapacitors.

230. Polarized near-infrared intersubband absorptions in CdSe colloidal quantum wells
B. T. Diroll, M. Chen, I. Coropceanu, K. R. Williams, D. V. Talapin, P. Guyot-Sionnest, and R. D. Schaller. Nat. Commun. 201910, 4511.


Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30–200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900–1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.

229. High Carrier Mobility in HgTe Quantum Dot Solids Improves Mid-IR Photodetectors
M. Chen, X. Lan, X. Tang, Y. Wang, M. H. Hudson, D. V. Talapin, and P. Guyot-Sionnest. ACS Photonics 2019, 6, 2358.


Improved mid-infrared photoconductors based on colloidal HgTe quantum dots are realized using a hybrid ligand exchange and polar phase transfer. The doping can also be controlled n and p by adjusting the HgCl2 concentration in the ligand exchange process. We compare the photoconductive properties with the prior “solid-state ligand exchange” using ethanedithiol, and we find that the new process affords ~ 100-fold increase of the electron and hole mobility, ~100-fold increase in responsivity and ~10-fold increase in detectivity. These photodetector improvements are primarily attributed to the increase in mobility (μ) because the optical properties are mostly unchanged. We show that the specific detectivity (D*) of a photoconductive device is expected to scale as √μ. The application potential is further verified by long-term device stability.

228. Colloidal Atomic Layer Deposition with Stationary Reactant Phases Enables Precise Synthesis of “Digital” II-VI Nano-heterostructures with Exquisite Control of Confinement and Strain
A. Hazarika, I. Fedin, L. Hong, J. Guo, V. Srivastava, W. Cho, I. Coropceanu, J. C. Portner, B. T. Diroll, J. P. Philbin, E. Rabani, R. F. Klie, and D. V. Talapin. J. Am. Chem. Soc. 2019141, 13487.


In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics, which detrimentally affect practically every property of functional nanomaterials. We discuss a novel synthetic strategy, colloidal Atomic Layer Deposition (c-ALD) with stationary reactant phases, which largely circumvent the limitations of traditional colloidal syntheses of nano-heterostructures with atomic precision. This approach allows for significant reduction of inhomogeneity in nanomaterials in complex nanostructures without compromising their structural perfection and enables the synthesis of epitaxial nano-heterostructures of unprecedented complexity. The improved synthetic control ultimately enables bandgap and strain engineering in colloidal nanomaterials with close-to-atomic accuracy. To demonstrate the power of new c-ALD method, we synthesize a library of complex II-VI semiconductor nanoplatelet heterostructures. By combining spectroscopic and computational studies, we elucidate the subtle interplay between quantum confinement and strain effects on the optical properties of semiconductor nanostructures.

227. Uniaxial transition dipole moments in semiconductor quantum rings caused by broken rotational symmetry
N. F. Hartmann, M. Otten, I. Fedin, D. V. Talapin, M. Cygorek, P. Hawrylak, M. Korkusinski, S. Gray, A. Hartschuh, and X. Ma. Nat. Commun. 2019, 10, 3253. 


Semiconductor quantum rings are topological structures that support fascinating phenomena such as the Aharonov–Bohm effect and persistent current, which are of high relevance in the research of quantum information devices. The annular shape of quantum rings distinguishes them from other low-dimensional materials, and enables topologically induced properties such as geometry-dependent spin manipulation and emission. While optical transition dipole moments (TDMs) in zero to two-dimensional optical emitters have been well investigated, those in quantum rings remain obscure despite their utmost relevance to the quantum photonic applications of quantum rings. Here, we study the dimensionality and orientation of TDMs in CdSe quantum rings. In contrast to those in other two-dimensional optical emitters, we find that TDMs in CdSe quantum rings show a peculiar in-plane linear distribution. Our theoretical modeling reveals that this uniaxial TDM originates from broken rotational symmetry in the quantum ring geometries.



Self-assembly of two sizes of nearly spherical colloidal nanocrystals (NCs) capped with hydrocarbon surface ligands has been shown to produce more than 20 distinct phases of binary nanocrystal superlattices (BNSLs). Such structural diversity, in striking contrast to binary systems of micron-sized colloidal beads, cannot be rationalized by models assuming entropy-driven crystallization of simple spheres. In this work, we show that the PbS ligand binding equilibrium controls the relative stability of two closely related BNSL structures featuring alternating layers of PbS and Au NCs. At an intermediate size ratio, as-prepared PbS NCs assemble with Au NCs into CuAu BNSLs featuring orientational coherence of PbS NCs across the lattice. Measurement of interparticle separations within CuAu and modeling of the structure reveal that PbS inorganic cores are nearly in contact through (100) NC surfaces in the square tiling of the CuAu basal plane. On the other hand, AlB2BNSLs with PbS NCs packed in random orientations were found to be the dominant self-assembly product when the same binary NC solution was evaporated in the presence of added oleic acid (OAH). Solution nuclear magnetic resonance titration experiments confirmed that added OAH binds to PbS NCs, implicating ligand surface coverage as an important factor influencing the relative stability of CuAu and AlB2 BNSLs at the experimental size ratio. From these results, we conclude that as-prepared PbS NCs feature sparsely covered (100) surfaces and thus effectively flat patches along NC x-, y-, and z-directions. Such anisotropic PbS–PbS interactions can be efficiently screened by restoring effectively spherical NC shape via addition of OAH to the binary assembly solution. Our findings underscore the important contribution of NC surfaces to superlattice phase stability and offer a strategy for targeted BNSL assembly.

225. Nanocrystals in Molten Salts and Ionic Liquids: Experimental Observation of Ionic Correlations Extending beyond the Debye Length
V. Kamysbayev, V. Srivastava, N. B. Ludwig, O. J. Borkiewicz, H. Zhang, J. Ilavsky, B. Lee, K. W. Chapman, S. Vaikuntanathan, D. V. Talapin. ACS Nano 201913, 5760.


The nature of the interface between the solute and the solvent in a colloidal solution has attracted attention for a long time. For example, the surface of colloidal nanocrystals (NCs) is specially designed to impart colloidal stability in a variety of polar and nonpolar solvents. This work focuses on a special type of colloids where the solvent is a molten inorganic salt or organic ionic liquid. The stability of such colloids is hard to rationalize because solvents with high density of mobile charges efficiently screen the electrostatic double-layer repulsion, and purely ionic molten salts represent an extreme case where the Debye length is only ∼1 Å. We present a detailed investigation of NC dispersions in molten salts and ionic liquids using small-angle X-ray scattering (SAXS), atomic pair distribution function (PDF) analysis and molecular dynamics (MD) simulations. Our SAXS analysis confirms that a wide variety of NCs (Pt, CdSe/CdS, InP, InAs, ZrO2) can be uniformly dispersed in molten salts like AlCl3/NaCl/KCl (AlCl3/AlCl4) and NaSCN/KSCN and in ionic liquids like 1-butyl-3-methylimidazolium halides (BMIM+X, where X = Cl, Br, I). By using a combination of PDF analysis and molecular modeling, we demonstrate that the NC surface induces a solvent restructuring with electrostatic correlations extending an order of magnitude beyond the Debye screening length. These strong oscillatory ion–ion correlations, which are not accounted by the traditional mechanisms of steric and electrostatic stabilization of colloids, offer additional insight into solvent–solute interactions and enable apparently “impossible” colloidal stabilization in highly ionized media.

224. Systematic Mapping of Binary Nanocrystal Superlattices: The Role of Topology in Phase Selection
I. Coropceanu, M. A. Boles, D. V. Talapin. J. Am. Chem. Soc. 2019141, 5728.


The self-assembly of two sizes of spherical nanocrystals has revealed a surprisingly diverse library of structures. To date, at least 15 distinct binary nanocrystal superlattice (BNSL) structures have been identified. The stability of these binary phases cannot be fully explained using the traditional conceptual framework treating the assembly process as entropy-driven crystallization of rigid spherical particles. Such deviation from hard sphere behavior may be explained by the soft and deformable layer of ligands that envelops the nanocrystals, which contributes significantly to the overall size and shape of assembling particles. In this work, we describe a set of experiments designed to elucidate the role of the ligand corona in shaping the thermodynamics and kinetics of BNSL assembly. Using hydrocarbon-capped Au and PbS nanocrystals as a model binary system, we systematically tuned the core radius (R) and ligand chain length (L) of particles and subsequently assembled them into binary superlattices. The resulting database of binary structures enabled a detailed analysis of the role of effective nanocrystal size ratio, as well as softness expressed as L/R, in directing the assembly of binary structures. This catalog of superlattices allowed us to not only study the frequency of different phases but to also systematically measure the geometric parameters of the BNSLs. This analysis allowed us to evaluate new theoretical models treating the cocrystallization of deformable spheres and to formulate new hypotheses about the factors affecting the nucleation and growth of the binary superlattices. Among other insights, our results suggest that the relative abundance of the binary phases observed may be explained not only by considerations of thermodynamic stability, but also by a postulated preordering of the binary fluid into local structures with icosahedral or polytetrahedral symmetry prior to nucleation.


223. Describing screening in dense ionic liquids with a charge-frustrated Ising model
N. B. Ludwig, K. Dasbiswas, D. V. Talapin, S. Vaikuntanathan.  J. Chem. Phys. 2018149, 164505.

Charge correlations in dense ionic fluids give rise to novel effects such as long-range screening and colloidal stabilization which are not predicted by the classic Debye–Hückel theory. We show that a Coulomb or charge-frustrated Ising model, which accounts for both long-range Coulomb and short-range molecular interactions, simply describes some of these ionic correlations. In particular, we obtain, at a mean field level and in simulations, a non-monotonic dependence of the screening length on the temperature. Using a combination of simulations and mean field theories, we study how the correlations in the various regimes are affected by the strength of the short ranged interactions.

222. Origin of Broad Emission Spectra in InP Quantum Dots: Contributions from Structural and Electronic Disorder
E. M. Janke, N. E. Williams, C. She, D. Zherebetskyy, M. Hudson, L. Wang, D. J. Gosztola, R. D. Schaller, B. Lee, C. Sun, G. S. Engel, D. V. Talapin. J. Am. Chem. Soc. 2018140, 15791.


The ensemble emission spectra of colloidal InP quantum dots are broader than achievable spectra of cadmium- and lead-based quantum dots, despite similar single-particle line widths and significant efforts invested in the improvement of synthetic protocols. We seek to explain the origin of persistently broad ensemble emission spectra of colloidal InP quantum dots by investigating the nature of the electronic states responsible for luminescence. We identify a correlation between red-shifted emission spectra and anomalous broadening of the excitation spectra of luminescent InP colloids, suggesting a trap-associated emission pathway in highly emissive core–shell quantum dots. Time-resolved pump–probe experiments find that electrons are largely untrapped on photoluminescence relevant time scales pointing to emission from recombination of localized holes with free electrons. Two-dimensional electronic spectroscopy on InP quantum dots reveals multiple emissive states and increased electron–phonon coupling associated with hole localization. These localized hole states near the valence band edge are hypothesized to arise from incomplete surface passivation and structural disorder associated with lattice defects. We confirm the presence and effect of lattice disorder by X-ray absorption spectroscopy and Raman scattering measurements. Participation of localized electronic states that are associated with various classes of lattice defects gives rise to phonon-coupled defect related emission. These findings explain the origins of the persistently broad emission spectra of colloidal InP quantum dots and suggest future strategies to narrow ensemble emission lines comparable to what is observed for cadmium-based materials.

221. Direct Synthesis of Six-Monolayer (1.9 nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 nm
W. Cho, S. Kim, I. Coropceanu, V. Srivastava, B. T. Diroll, A. Hazarika, I. Fedin, R. D. Schaller, G. Galli, D. V. Talapin. Chem. Mater201830, 6957.


Quasi-two-dimensional semiconductor nanoplatelets (NPLs) have garnered widespread interest because of their uniquely narrow emission spectra and favorable characteristics for optical gain and lasing. Strongly quantum confined along thickness, NPLs exhibit the electronic structure of quantum wells determined by their thickness, which can be controlled with atomic precision. Among all semiconductor NPLs, CdSe NPLs are probably the most studied.

220. Semiconductor Nanoplatelet Excimers
B. T. Diroll, W. Cho, I. Coropceanu, S. Harvey, A. Brumberg, N. Holtgrewe, S. A. Crooker, M. R. Wasielewski, V. B. Prakapenka, D. V. Talapin, R. D. Schaller. Nano Lett. 201818, 6948.


Excimers, a portmanteau of “excited dimer”, are transient species that are formed from the electronic interaction of a fluorophore in the excited state with a neighbor in the ground state, which have found extensive use as laser gain media. Although common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of cofacial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

219. Colloidal Chemistry in Molten Salts: Synthesis of Luminescent In1–xGaxP and In1–xGaxAs Quantum Dots
V. Srivastava, V. Kamysbayev, L. Hong, E. Dunietz, R. F. Klie, D. V. Talapin. J. Am. Chem. Soc. 2018140, 12144.


Control of composition, stoichiometry, and defects in colloidal quantum dots (QDs) of III–V semiconductors has proven to be difficult due to their covalent character. Whereas the synthesis of colloidal indium pnictides such as InP, InAs, and InSb has made significant progress, gallium-containing colloidal III–V QDs still remain largely elusive. Gallium pnictides represent an important class of semiconductors due to their excellent optoelectronic properties in the bulk; however, the difficulty with the synthesis of gallium-containing colloidal III–V QDs has largely prohibited their exploration as solution-processed semiconductors. Here we introduce molten inorganic salts as high-temperature solvents for the synthesis and manipulation of III–V QDs. We demonstrate cation exchange reactions on presynthesized InP and InAs QDs to form In1–xGaxP and In1–xGaxAs QDs at temperatures above 380 °C. This approach produces novel ternary alloy QDs with controllable compositions that show size- and composition-dependent absorption and emission features. Emission quantum yields of up to ∼50% can be obtained for In1–xGaxP/ZnS core–shell QDs. A comparison of the optical properties of InP/ZnS core–shells with In1–xGaxP/ZnS core–shells reveals that Ga incorporation leads to significant improvement in the optical properties of III–V/II–VI core–shell emitters which is of great importance for quantum dot-based lighting and display applications. This work also demonstrates the potential of molten inorganic salts as versatile solvents for the synthesis and processing of colloidal nanomaterials at temperatures inaccessible for traditional solvents.

218. Conduction Band Fine Structure in Colloidal HgTe Quantum Dots
M. H. Hudson, M. Chen, V. Kamysbayev, E. M. Janke, X. Lan, G. Allan, C. Delerue, B. Lee, P. Guyot-Sionnest, D. V. Talapin.  ACS Nano 201812, 9397.


HgTe colloidal quantum dots (QDs) are of interest because quantum confinement of semimetallic bulk HgTe allows one to synthetically control the bandgap throughout the infrared. Here, we synthesize highly monodisperse HgTe QDs and tune their doping both chemically and electrochemically. The monodispersity of the QDs was evaluated using small-angle X-ray scattering (SAXS) and suggests a diameter distribution of ∼10% across multiple batches of different sizes. Electron-doped HgTe QDs display an intraband absorbance and bleaching of the first two excitonic features. We see splitting of the intraband peaks corresponding to electronic transitions from the occupied 1Se state to a series of nondegenerate 1Pe states. Spectroelectrochemical studies reveal that the degree of splitting and relative intensity of the intraband features remain constant across doping levels up to two electrons per QD. Theoretical modeling suggests that the splitting of the 1Pe level arises from spin–orbit coupling and reduced QD symmetry. The fine structure of the intraband transitions is observed in the ensemble studies due to the size uniformity of the as-synthesized QDs and strong spin–orbit coupling inherent to HgTe.

217. Anisotropic photoluminescence from isotropic optical transition dipoles in semiconductor nanoplatelets
X. Ma, B. T. Diroll, W. Cho, I. Fedin, R. D. Schaller, D. V. Talapin, and G. P. Wiederrecht. Nano Lett. 201818, 4647.


Many important light-matter coupling and energy-transfer processes depend critically on the dimensionality and orientation of optical transition dipoles in emitters. We investigate individual quasi-two-dimensional nanoplatelets (NPLs) using higher-order laser scanning microscopy and find that absorption dipoles in NPLs are isotropic in three dimensions at the excitation wavelength. Correlated polarization studies of the NPLs reveal that their emission polarization is strongly dependent on the aspect ratio of the lateral dimensions. Our simulations reveal that this emission anisotropy can be readily explained by the electric field renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium, and we conclude that emission dipoles in NPLs are isotropic in the plane of the NPLs. Our study presents an approach for disentangling the effects of dipole degeneracy and electric field renormalization on emission anisotropy and can be adapted for studying the intrinsic optical transition dipoles of various nanostructures.

216. Surface chemistry and buried interfaces in all-inorganic nanocrystalline solids
E. Scalise, V. Srivastava, E. M. Janke, D. Talapin, G. Galli, and S. Wippermann. Nature Nanotech. 201833, 841.


Semiconducting nanomaterials synthesized using wet chemical techniques play an important role in emerging optoelectronic and photonic technologies. Controlling the surface chemistry of the nano building blocks and their interfaces with ligands is one of the outstanding challenges for the rational design of these systems. We present an integrated theoretical and experimental approach to characterize, at the atomistic level, buried interfaces in solids of InAs nanoparticles capped with Sn2S64– ligands. These prototypical nanocomposites are known for their promising transport properties and unusual negative photoconductivity. We found that inorganic ligands dissociate on InAs to form a surface passivation layer. A nanocomposite with unique electronic and transport properties is formed, that exhibits type II heterojunctions favourable for exciton dissociation. We identified how the matrix density, sulfur content and specific defects may be designed to attain desirable electronic and transport properties, and we explain the origin of the measured negative photoconductivity of the nanocrystalline solids.

215. Monodisperse InAs Quantum Dots from Aminoarsine Precursors: Understanding the Role of Reducing Agent 
V. Srivastava, E. Dunietz, V. Kamysbayev, J. S. Anderson, and D. V. Talapin. Chem. Mater. 201830, 3623.


Materials that absorb and emit in the short-wavelength infrared (SWIR) region of the electromagnetic spectrum are important for applications in telecommunication, night vision, photovoltaics and in vivo biological imaging. However, further technological development of these applications requires high quality, inexpensive and solution-processed SWIR emitters. For instance, in vivo biological imaging requires the probes to be nontoxic, bright and narrow-band emitters. Materials with similar qualities are also desirable for large area partially transparent photovoltaic concentrators matched to Si photovoltaic cells. Solution processed semiconductor quantum dots (QDs) have naturally emerged as attractive candidates for such applications.

214. Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets
C. E. Rowland, I. Fedin, B. T. Diroll, Y. Liu, D. V. Talapin, and R. D. Schaller. J. Phys. Chem. Lett. 2018, 9, 286.


Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300 to 700 K using static and transient spectroscopies as well as in situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission line width with temperature elevation up to ∼500 K, losing a factor of ∼8 to 10 in PL intensity at 400 K relative to ambient. Temperature elevation above ∼500 K yields thickness-dependent, irreversible degradation in optical properties. Electron microscopy relates stability of the core-only NP morphology up to 555 and 600 K for the four and five monolayer NPs, respectively, followed by sintering and evaporation at still higher temperatures. Reversible PL loss, based on differences in decay dynamics between time-resolved photoluminescence and transient absorption, results primarily from hole trapping in both NPs and sandwich NPs.


213. Nonmonotonic Dependence of Auger Recombination Rate on Shell Thickness for CdSe/CdS Core/Shell Nanoplatelets
M. Pelton, J. J. Andrews, I. Fedin, D. V. Talapin, H. Lengd S. K. O’Leary. Nano Lett. 2017, 17, 6900.


Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2–4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.

212. Size-Dependent Biexciton Quantum Yields and Carrier Dynamics of Quasi-Two-Dimensional Core/Shell Nanoplatelets
X. Ma, B. T. Diroll, W. Cho, I. Fedin, R. D. Schaller, D. V. Talapin, S. K. Gray, G. P. Wiederrecht, and D. J. Gosztola. ACS Nano 201711, 9119.


Quasi-two-dimensional nanoplatelets (NPLs) possess fundamentally different excitonic properties from zero-dimensional quantum dots. We study lateral size-dependent photon emission statistics and carrier dynamics of individual NPLs using second-order photon correlation (g(2)(τ)) spectroscopy and photoluminescence (PL) intensity-dependent lifetime analysis. Room-temperature radiative lifetimes of NPLs can be derived from maximum PL intensity periods in PL time traces. It first decreases with NPL lateral size and then stays constant, deviating from the electric dipole approximation. Analysis of the PL time traces further reveals that the single exciton quantum yield in NPLs decreases with NPL lateral size and increases with protecting shell thickness, indicating the importance of surface passivation on NPL emission quality. Second-order photon correlation (g(2)(τ)) studies of single NPLs show that the biexciton quantum yield is strongly dependent on the lateral size and single exciton quantum yield of the NPLs. In large NPLs with unity single exciton quantum yield, the corresponding biexciton quantum yield can reach unity. These findings reveal that by careful growth control and core–shell material engineering, NPLs can be of great potential for light amplification and integrated quantum photonic applications.

211. Direct optical lithography of functional inorganic nanomaterials
Y. Wang, I. Fedin, H. Zhang, and D. V. Talapin. Science 2017357, 385.

Perspective: M. Striccoli. Photolithography based on nanocrystals. Science 2017357, 353.


Photolithography is an important manufacturing process that relies on using photoresists, typically polymer formulations, that change solubility when illuminated with ultraviolet light. Here, we introduce a general chemical approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials. The patterned materials can be metals, semiconductors, oxides, magnetic, or rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The conductivity, carrier mobility, dielectric, and luminescence properties of optically patterned layers are on par with the properties of state-of-the-art solution-processed materials. The ability to directly pattern all-inorganic layers by using a light exposure dose comparable with that of organic photoresists provides an alternate route for thin-film device manufacturing.

210. A room temperature continuous-wave nanolaser using colloidal quantum wells
Z. Yang, M. Pelton, I. Fedin, D. V. Talapin, and E. Waks.  Nat. Commun. 20178, 143.


Colloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 μW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.

209. Soluble Lead and Bismuth Chalcogenidometallates: Versatile Solders for Thermoelectric Materials
H. Zhang, J. S. Son, D. S. Dolzhnikov, A. S. Filatov, A. Hazarika, Y. Wang, M. H. Hudson, C.-J. Sun, S. Chattopadhyay, and D. V. Talapin. Chem. Mater. 2017, 29, 6396.


Here we report the syntheses of largely unexplored lead and bismuth chalcogenidometallates in the solution phase. Using N2H4 as the solvent, new compounds such as K6Pb3Te6·7N2H4 were obtained. These soluble molecular compounds underwent cation exchange processes using resin chemistry, replacing Na+ or K+ by decomposable N2H5+ or tetraethylammonium cations. They also transformed into stoichiometric lead and bismuth chalcogenide nanomaterials with the addition of metal salts. Such a versatile chemistry led to a variety of composition-matched solders to join lead and bismuth chalcogenides and tune their charge transport properties at the grain boundaries. Solution-processed thin films composed of Bi0.5Sb1.5Te3 microparticles soldered by (N2H5)6Bi0.5Sb1.5Te6 exhibited thermoelectric power factors (∼28 μW/cm K2) comparable to those in vacuum-deposited Bi0.5Sb1.5Te3 films. The soldering effect can also be integrated with attractive fabrication techniques for thermoelectric modules, such as screen printing, suggesting the potential of these solders in the rational design of printable and moldable thermoelectrics.

208. Orbitals, Occupation Numbers, and Band Structure of Short One-Dimensional Cadmium Telluride Polymers
A. J. S. Valentine, D. V. Talapin, and D. A. Mazziotti. J. Phys. Chem. A 2017, 121, 3142.


Recent work found that soldering CdTe quantum dots together with a molecular CdTe polymer yielded field-effect transistors with much greater electron mobility than quantum dots alone. We present a computational study of the CdTe polymer using the active-space variational two-electron reduced density matrix (2-RDM) method. While analogous complete active-space self-consistent field (CASSCF) methods scale exponentially with the number of active orbitals, the active-space variational 2-RDM method exhibits polynomial scaling. A CASSCF calculation using the (48o,64e) active space studied in this paper requires 1024 determinants and is therefore intractable, while the variational 2-RDM method in the same active space requires only 2.1 × 107 variables. Natural orbitals, natural-orbital occupations, charge gaps, and Mulliken charges are reported as a function of polymer length. The polymer, we find, is strongly correlated, despite possessing a simple sp3hybridized bonding scheme. Calculations reveal the formation of a nearly saturated valence band as the polymer grows and a charge gap that decreases sharply with polymer length.

207. Stable colloids in molten inorganic salts
H. Zhang, K. Dasbiswas, N. B. Ludwig, G. Han, B. Lee, S. Vaikuntanathan, and D. V. Talapin. Nature 2017, 542, 328.


A colloidal solution is a homogeneous dispersion of particles or droplets of one phase (solute) in a second, typically liquid, phase (solvent). Colloids are ubiquitous in biological, chemical and technological processes, homogenizing highly dissimilar constituents. To stabilize a colloidal system against coalescence and aggregation, the surface of each solute particle is engineered to impose repulsive forces strong enough to overpower van der Waals attraction and keep the particles separated from each other. Electrostatic stabilization of charged solutes works well in solvents with high dielectric constants, such as water (dielectric constant of 80). In contrast, colloidal stabilization in solvents with low polarity, such as hexane (dielectric constant of about 2), can be achieved by decorating the surface of each particle of the solute with molecules (surfactants) containing flexible, brush-like chains. Here we report a class of colloidal systems in which solute particles (including metals, semiconductors and magnetic materials) form stable colloids in various molten inorganic salts. The stability of such colloids cannot be explained by traditional electrostatic and steric mechanisms. Screening of many solute–solvent combinations shows that colloidal stability can be traced to the strength of chemical bonding at the solute–solvent interface. Theoretical analysis and molecular dynamics modelling suggest that a layer of surface-bound solvent ions produces long-ranged charge-density oscillations in the molten salt around solute particles, preventing their aggregation. Colloids composed of inorganic particles in inorganic melts offer opportunities for introducing colloidal techniques to solid-state science and engineering applications.

206. Understanding and curing structural defects in colloidal GaAs nanocrystals
V. Srivastava, W. Liu, E. M. Janke, V. Kamysbayev, A. S. Filatov, C. Sun, B. Lee, Tijana Rajh, R. D. Schaller, and D. V. Talapin. Nano Lett. 2017, 17, 2094.


GaAs is one of the most important semiconductors. However, colloidal GaAs nanocrystals remain largely unexplored because of the difficulties with their synthesis. Traditional synthetic routes either fail to produce pure GaAs phase or result in materials whose optical properties are very different from the behavior expected for quantum dots of direct-gap semiconductors. In this work, we demonstrate a variety of synthetic routes toward crystalline GaAs NCs. By using a combination of Raman, EXAFS, transient absorption, and EPR spectroscopies, we conclude that unusual optical properties of colloidal GaAs NCs can be related to the presence of Ga vacancies and lattice disorder. These defects do not manifest themselves in TEM images and powder X-ray diffraction patterns but are responsible for the lack of absorption features even in apparently crystalline GaAs nanoparticles. We introduce a novel molten salt based annealing approach to alleviate these structural defects and show the emergence of size-dependent excitonic transitions in colloidal GaAs quantum dots.


Amplified spontaneous emission (ASE) and lasing from solution-processed materials are demonstrated in the challenging violet-to-blue (430–490 nm) spectral region for colloidal nanoplatelets of CdS and newly synthesized core/shell CdS/ZnS nanoplatelets. Despite modest band-edge photoluminescence quantum yields of 2% or less for single excitons, which we show results from hole trapping, the samples exhibit low ASE thresholds. Furthermore, four-monolayer CdS samples show ASE at shorter wavelengths than any reported film of colloidal quantum-confined material. This work underlines that low quantum yields for single excitons do not necessarily lead to a poor gain medium. The low ASE thresholds originate from negligible dispersion in thickness, large absorption cross sections of 2.8 × 10–14 cm–2, and rather slow (150 to 300 ps) biexciton recombination. We show that under higher-fluence excitation, ASE can kinetically outcompete hole trapping. Using nanoplatelets as the gain medium, lasing is observed in a linear optical cavity. This work confirms the fundamental advantages of colloidal quantum well structures as gain media, even in the absence of high photoluminescence efficiency.

204. New forms of CdSe: molecular wires, gels, and ordered mesoporous assemblies
M. H. Hudson, D. S. Dolzhnikov, A. S. Filatov, E. M. Janke, J. Jang, B. Lee, C. Sun, and D. V. Talapin. J. Am. Chem. Soc. 2017139, 3368.


This work investigates the structure and properties of soluble chalcogenidocadmates, a molecular form of cadmium chalcogenides with unprecedented one-dimensional bonding motifs. The single crystal X-ray structure reveals that sodium selenocadmate consists of infinite one-dimensional wires of (Cd2Se3)n2n charge balanced by Na+ and stabilized by coordinating solvent molecules. Exchanging the sodium cation with tetraethylammonium or didodecyldimethylammonium expands the versatility of selenocadmate by improving its solubility in a variety of polar and nonpolar solvents without changing the anion structure and properties. The introduction of a micelle-forming cationic surfactant allows for the templating of selenocadmate, or the analogous telluride species, to create ordered organic–inorganic hybrid CdSe or CdTe mesostructures. Finally, the interaction of selenocadmate “wires” with Cd2+ ions creates an unprecedented gel-like form of stoichiometric CdSe. We also demonstrate that these low-dimensional CdSe species show characteristic semiconductor behavior, and can be used in photodetectors and field-effect transistors.

203. Tandem Solar Cells from Solution-Processed CdTe and PbS Quantum Dots Using a ZnTe–ZnO Tunnel Junction
R. W. Crisp, G. F. Pach, J. M. Kurley, R. M. France, M. O. Reese, S. U. Nanayakkara, B. A. MacLeod, D. V. Talapin, M. C. Beard, and J. M. Luther. Nano Lett. 2017, 17, 1020.


We developed a monolithic CdTe–PbS tandem solar cell architecture in which both the CdTe and PbS absorber layers are solution-processed from nanocrystal inks. Due to their tunable nature, PbS quantum dots (QDs), with a controllable band gap between 0.4 and ∼1.6 eV, are a promising candidate for a bottom absorber layer in tandem photovoltaics. In the detailed balance limit, the ideal configuration of a CdTe (Eg  = 1.5 eV)–PbS tandem structure assumes infinite thickness of the absorber layers and requires the PbS band gap to be 0.75 eV to theoretically achieve a power conversion efficiency (PCE) of 45%. However, modeling shows that by allowing the thickness of the CdTe layer to vary, a tandem with efficiency over 40% is achievable using bottom cell band gaps ranging from 0.68 and 1.16 eV. In a first step toward developing this technology, we explore CdTe–PbS tandem devices by developing a ZnTe–ZnO tunnel junction, which appropriately combines the two subcells in series. We examine the basic characteristics of the solar cells as a function of layer thickness and bottom-cell band gap and demonstrate open-circuit voltages in excess of 1.1 V with matched short circuit current density of 10 mA/cm2 in prototype devices.

202. Transparent Ohmic Contacts for Solution-Processed, Ultrathin CdTe Solar Cells
J. M. Kurley, M. G. Panthani, R. W. Crisp, S. U. Nanayakkara, G. F. Pach, M. O. Reese, M. H. Hudson, D. S. Dolzhnikov, V. Tanygin, J. M. Luther, and D. V. Talapin. ACS Energy Lett20172, 270.


Recently, solution-processing became a viable route for depositing CdTe for use in photovoltaics. Ultrathin (∼500 nm) solar cells have been made using colloidal CdTe nanocrystals with efficiencies exceeding 12% power conversion efficiency (PCE) demonstrated by using very simple device stacks. Further progress requires an effective method for extracting charge carriers generated during light harvesting. Here, we explored solution-based methods for creating transparent Ohmic contacts to the solution-deposited CdTe absorber layer and demonstrated molecular and nanocrystal approaches to Ohmic hole-extracting contacts at the ITO/CdTe interface. We used scanning Kelvin probe microscopy to further show how the above approaches improved carrier collection by reducing the potential drop under reverse bias across the ITO/CdTe interface. Other methods, such as spin-coating CdTe/A2CdTe2 (A = Na, K, Cs, N2H5), can be used in conjunction with current/light soaking to improve PCE further.


201. Introduction: Nanoparticle Chemistry
D. V. Talapin and E. V. Shevchenko. Chem. Rev. 2016116, 10343.


Nanoparticle chemistry is a relatively young branch of chemical research. Even 30 years ago, these words would have sounded puzzling to many scientists despite the fact that nanoparticles, primarily in the form of dust and smoke, have always existed in nature. Nanoparticles were utilized in construction materials, pigments, and stained glass well before their nature and properties were uncovered and understood. For more than a century, transition metal nanoparticles were widely used as heterogeneous catalysts and generated impressive revenues for petrochemical companies. Despite these all-pervading examples, nanoparticle chemistry did not evolve into a rigorous academic field until the end of the 20th century, when the availability of electron microscopy and other modern characterization techniques equipped researchers with tools suitable for analyzing nanometer sized objects.

200. Facile, Economic and Size-Tunable Synthesis of Metal Arsenide Nanocrystals
V. Srivastava, E. M. Janke, B. T. Diroll, R. D. Schaller, and D. V. Talapin. Chem. Mater. 2016, 28, 6797.


Synthesis of colloidal nanocrystals (NC) of important arsenide nanomaterials (e.g., InAs, Cd3As2) has been limited by the lack of convenient arsenic precursors. Here we address this constraint by identifying a convenient and commercially available As precursor, tris-dimethylaminoarsine (As(NMe2)3), which can be used to prepare high quality InAs NCs with controlled size distributions. Our approach employs a reaction between InCl3 and As(NMe2)3 using diisobutylaluminum hydride (DIBAL-H) to convert As(NMe2)3 in situ into reactive intermediates AsHx(NMe2)3–x, where x = 1,2,3. NC size can be varied by changing DIBAL-H concentration and growth temperature, with colloidal solutions of InAs showing size dependent absorption and emission features tunable across wavelengths of 750 to 1450 nm. We also show that this approach works well for the colloidal synthesis of Cd3As2 NCs. By circumventing the preparation of notoriously unstable and dangerous arsenic precursors (e.g., AsH3 and As(SiMe3)3), this work improves the synthetic accessibility of arsenide-based NCs and, by extension, the potential of such NCs for use in infrared (IR) applications such as communications, fluorescent labeling and photon detection.

199. Surface-Area Dependent Electron Transfer Between Isoenergetic 2D Quantum Wells and a Molecular Acceptor
B. Diroll, I. Fedin, P. Darancet, D. V. Talapin, R. Schaller. J. Am. Chem. Soc. 2016, 138, 11109.


We report measurements of electron transfer rates for four isoenergetic donor–acceptor pairs comprising a molecular electron acceptor, methylviologen (MV), and morphology-controlled colloidal semiconductor nanoparticles of CdSe. The four nanoparticles include a spherical quantum dot (QD) and three differing lateral areas of 4-monolayer-thick nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the measurements, performed via ultrafast photoluminescence, relate the dependence of charge transfer rate on the spatial extent of the initial electron–hole pair wave function explicitly, which we show for the first time to be related to surface area in this regime that is intermediate between homogeneous and heterogeneous charge transfer as well as 2D to 0D electron transfer. The observed nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

198. Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials
M. Boles, M. Engel, and D. V. Talapin.  Chem. Rev. 2016116, 11220.


Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

197. Building Devices from Colloidal Quantum Dots (solicited review)
C. R. Kagan, E. Lifshitz, E. H. Sargent, and D. V. Talapin. Science 2016353, 885.


The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantum dots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials.

196. Colloidal CdSe Quantum Rings
I. Fedin and D. V. Talapin. J. Am. Chem. Soc. 2016, 138, 9771.


Semiconductor quantum rings are of great fundamental interest because their non-trivial topology creates novel physical properties. At the same time, toroidal topology is difficult to achieve for colloidal nanocrystals and epitaxially grown semiconductor nanostructures. In this work, we introduce the synthesis of luminescent colloidal CdSe nanorings and nanostructures with double and triple toroidal topology. The nanorings form during controlled etching and rearrangement of two-dimensional nanoplatelets. We discuss a possible mechanism of the transformation of nanoplatelets into nanorings and potential utility of colloidal nanorings for magneto-optical (e.g., Aharonov–Bohm effect) and other applications.

195.  Solution-Processed, Ultrathin Solar Cells from CdCl3-capped CdTe Nanocrystals: The Multiple Roles of CdCl3 Ligands 
H. Zhang, J. M. Kurley, J. C. Russell, J. Jang, D. V. Talapin. J. Am. Chem. Soc. 2016138, 7464.


Solution-processed CdTe solar cells using CdTe nanocrystal (NC) ink may offer an economically viable route for large-scale manufacturing. Here we design a new CdCl3-capped CdTe NC ink by taking advantage of novel surface chemistry. In this ink, CdCl3 ligands act as surface ligands, sintering promoters, and dopants. Our solution chemistry allows obtaining very thin continuous layers of high-quality CdTe which is challenging for traditional vapor transport methods. Using benign solvents, in air, and without additional CdCl2 treatment, we obtain a well-sintered CdTe absorber layer from the new ink and demonstrate thin-film solar cells with power conversion efficiency over 10%, a record efficiency for sub-400 nm thick CdTe absorber layer.

194. Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology
P. Cunningham, J. B. Souza Jr, I. Fedin, C. She, B. Lee, and D. V. Talapin. ACS Nano 2016, 10, 5769.


Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor–polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

193. The surface science of nanocrystals
M. A. Boles, D. Ling, T. Hyeon, D. V. Talapin. Nat. Mater. 2016, 15, 141.


All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. Surface ligands — molecules that bind to the surface — are an essential component of nanomaterial synthesis, processing and application. Understanding the structure and properties of nanoscale interfaces requires an intricate mix of concepts and techniques borrowed from surface science and coordination chemistry. Our Review elaborates these connections and discusses the bonding, electronic structure and chemical transformations at nanomaterial surfaces. We specifically focus on the role of surface ligands in tuning and rationally designing properties of functional nanomaterials. Given their importance for biomedical (imaging, diagnostics and therapeutics) and optoelectronic (light-emitting devices, transistors, solar cells) applications, we end with an assessment of application-targeted surface engineering.


192. Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te2– Ligands Lead To Charge Carrier Diffusion Lengths Over 2 μm
R. W. Crisp, R. Callahan, O. G. Reid, D. S. Dolzhnikov, D. V. Talapin, G. Rumbles, J. M. Luther, and N. Kopidakis. J. Phys. Chem. Lett. 2015, 6, 4815.


We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm2/(V s)). Our TRMC findings show that Te2– capped CdTe NCs show a marked improvement in carrier mobility (11 cm2/(V s)), indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films.

191. Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets
Chunxing She, Igor Fedin, Dmitriy S. Dolzhnikov, Peter D. Dahlberg, Gregory S. Engel, Richard D. Schaller, and Dmitri V. Talapin. ACS Nano 20159, 9475.


There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm2, lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

190. Solution-Processed Transistors Using Colloidal Nanocrystals with Composition-Matched Molecular “Solders”: Approaching Single Crystal Mobility
Jaeyoung Jang, Dmitriy S. Dolzhnikov, Wenyong Liu, Sooji Nam, Moonsub Shim, and Dmitri V. Talapin. Nano Lett.201515, 6309. 


Crystalline silicon-based complementary metal-oxide–semiconductor transistors have become a dominant platform for today’s electronics. For such devices, expensive and complicated vacuum processes are used in the preparation of active layers. This increases cost and restricts the scope of applications. Here, we demonstrate high-performance solution-processed CdSe nanocrystal (NC) field-effect transistors (FETs) that exhibit very high carrier mobilities (over 400 cm2/(V s)). This is comparable to the carrier mobilities of crystalline silicon-based transistors. Furthermore, our NC FETs exhibit high operational stability and MHz switching speeds. These NC FETs are prepared by spin coating colloidal solutions of CdSe NCs capped with molecular solders [Cd2Se3]2– onto various oxide gate dielectrics followed by thermal annealing. We show that the nature of gate dielectrics plays an important role in soldered CdSe NC FETs. The capacitance of dielectrics and the NC electronic structure near gate dielectric affect the distribution of localized traps and trap filling, determining carrier mobility and operational stability of the NC FETs. We expand the application of the NC soldering process to core–shell NCs consisting of a III–V InAs core and a CdSe shell with composition-matched [Cd2Se3]2– molecular solders. Soldering CdSe shells forms nanoheterostructured material that combines high electron mobility and near-IR photoresponse.

189. Development and Structure/Property Relationship of New Electron Accepting Polymers Based on Thieno[2′,3′:4,5]pyrido[2,3-g]thieno[3,2-c]quinoline-4,10-dione for All-Polymer Solar Cells
In Hwan Jung, Donglin Zhao, Jaeyoung Jang, Wei Chen, Erik S. Landry, Luyao Lu, Dmitri V. Talapin, and Luping Yu. Chem. Mater.201527, 5941.


Several electron accepting polymers having weak accepting–strong accepting (WA-SA) and strong accepting–strong accepting (SA-SA) monomer alternation were synthesized for studies of structure/property relationship in all-polymer solar cells. Two kinds of cyclic amide monomers, 4,10-bis(2-butyloctyl)-thieno[2′,3′:5,6]pyrido[3,4-g]thieno-[3,2-c]isoquinoline-5,11-dione (TPTI) and 5,11-bis(2-butyloctyl)-thieno[2′,3′:4,5]pyrido[2,3-g]thieno[3,2-c]quinoline-4,10-dione (TPTQ), were synthesized as weak accepting monomers (WA). Difluorinated TPTQ (FTPTQ) and well-known perylene diimide (PDI) monomers were synthesized as strong electron accepting monomers (SA). By using 1-chloronaphthalene (CN) as a cosolvent, the morphology of the polymer blended films can be finely tuned to achieve better ordering toward face-on mode and favorable phase separation between electron donor and acceptor, resulting in significant enhancement of short circuit current (Jsc) and fill factor (FF). The fluorination in the TPTQ unit reduced the dipole moment of the D–A complex and gave a negative effect on a polymer system. PFP showed worse electron accepting property with lower electron mobility than PQP. It is reasoned that the internal polarization plays an important role in the design of electron accepting polymers. As a result, PQP having TPTQ monomer exhibited the best photovoltaic performance with power conversion efficiency (PCE) of 3.52% (Voc = 0.71 V, Jsc = 8.57 mA/cm2, FF = 0.58) at a weight ratio of PTB7-Th:PQP = 1:1, under AM 1.5G.


This work analyzes the role of hydrocarbon ligands in the self-assembly of nanocrystal (NC) superlattices. Typical NCs, composed of an inorganic core of radius R and a layer of capping ligands with length L, can be described as soft spheres with softness parameter L/R. Using particle tracking measurements of transmission electron microscopy images, we find that close-packed NCs, like their hard-sphere counterparts, fill space at approximately 74% density independent of softness. We uncover deformability of the ligand capping layer that leads to variable effective NC size in response to the coordination environment. This effect plays an important role in the packing of particles in binary nanocrystal superlattices (BNSLs). Measurements on BNSLs composed of NCs of varying softness in several coordination geometries indicate that NCs deform to produce dense BNSLs that would otherwise be low-density arrangements if the particles remained spherical. Consequently, rationalizing the mixing of two NC species during BNSL self-assembly need not employ complex energetic interactions. We summarize our analysis in a set of packing rules. These findings contribute to a general understanding of entropic effects during crystallization of deformable objects (e.g., nanoparticles, micelles, globular proteins) that can adapt their shape to the local coordination environment.

187. Auger-Limited Carrier Recombination and Relaxation in CdSe Colloidal Quantum Wells
Erfan Baghani, Stephen K. O’Leary, Igor Fedin, Dmitri V. Talapin, and Matthew Pelton. J. Phys. Chem. Lett. 2015, 6, 1032.


Using time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron–hole recombination rate depends only on the number of electron–hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron–hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells.

186. Prospects of Nanoscience with Nanocrystals
M. V. Kovalenko, A. Cabot, L. Manna, Z. Hens, D. V. Talapin, C. Kagan, V. Klimov, A. Rogach, P. Reiss, D. Milliron, P. Guyot-Sionnest, G. Konstantatos, W. Parak, T. Hyeon, B. Korgel, C. Murray, and W. Heiss. ACS Nano 2015, 9, 1012.


Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

185. Picosecond Energy Transfer in Binary CdSe Nano-Platelet Solids: Outpacing Auger Recombination
C. E. Rowland, I. Fedin, H. Zhang, A. O. Govorov, S. K. Gray, D. V. Talapin, and R. D. Schaller. Nature Mater. 2015, 14, 484.


Fluorescence resonance energy transfer (FRET) enables photosynthetic light harvesting, wavelength downconversion in light-emitting diodes (LEDs), and optical biosensing schemes. The rate and efficiency of this donor to acceptor transfer of excitation between chromophores dictates the utility of FRET and can unlock new device operation motifs including quantum-funnel solar cells, non-contact chromophore pumping from a proximal LED, and markedly reduced gain thresholds. However, the fastest reported FRET time constants involving spherical quantum dots (0.12–1 ns) do not outpace biexciton Auger recombination (0.01–0.1 ns), which impedes multiexciton-driven applications including electrically pumped lasers and carrier-multiplication-enhanced photovoltaics. Few-monolayer-thick semiconductor nanoplatelets (NPLs) with tens-of-nanometre lateral dimensions exhibit intense optical transitions and hundreds-of-picosecond Auger recombination, but heretofore lack FRET characterizations. We examine binary CdSe NPL solids and show that interplate FRET (∼6–23 ps, presumably for co-facial arrangements) can occur 15–50 times faster than Auger recombination and demonstrate multiexcitonic FRET, making such materials ideal candidates for advanced technologies.

184. Size-dependent Energy Levels of InSb Quantum Dots Measured by Scanning Tunneling Spectroscopy 
T. Wang, R. Vaxenburg, W. Liu, S. M. Rupich, E. Lifshitz, A. L. Efros, D. V. Talapin, and S. J. Sibener. ACS Nano 2015, 9, 725.


The electronic structure of single InSb quantum dots (QDs) with diameters between 3 and 7 nm was investigated using atomic force microscopy (AFM) and scanning tunneling spectroscopy (STS). In this size regime, InSb QDs show strong quantum confinement effects which lead to discrete energy levels on both valence and conduction band states. Decrease of the QD size increases the measured band gap and the spacing between energy levels. Multiplets of equally spaced resonance peaks are observed in the tunneling spectra. There, multiplets originate from degeneracy lifting induced by QD charging. The tunneling spectra of InSb QDs are qualitatively different from those observed in the STS of other III–V materials, for example, InAs QDs, with similar band gap energy. Theoretical calculations suggest the electron tunneling occurs through the states connected with L-valley of InSb QDs rather than through states of the Γ-valley. This observation calls for better understanding of the role of indirect valleys in strongly quantum-confined III–V nanomaterials.

183. Composition-matched molecular “solders” for semiconductors
D. S. Dolzhnikov, H. Zhang, J. Jang, J. S. Son, M. G. Panthani, S. Chattopadhyay, T. Shibata, and D. V. Talapin. Science 2015347, 425.
Technology Update: Semiconductor solder seals nanojoints


We propose a general strategy to synthesize largely unexplored soluble chalcogenidometallates of cadmium, lead, and bismuth. These compounds can be used as “solders” for semiconductors widely used in photovoltaics and thermoelectrics. The addition of solder helped to bond crystal surfaces and link nano- or mesoscale particles together. For example, CdSe nanocrystals with Na2Cd2Se3 solder was used as a soluble precursor for CdSe films with electron mobilities exceeding 300 square centimeters per volt-second. CdTe, PbTe, and Bi2Te3 powders were molded into various shapes in the presence of a small additive of composition-matched chalcogenidometallate or chalcogel, thus opening new design spaces for semiconductor technologies.

182. Inorganic Surface Ligands for Colloidal Nanomaterials
A. Nag, H. Zhang, E. Janke, and D. V. Talapin.  Z. Phys. Chem. (Special issue Horst Weller 60th birthday) 2015229, 85.


Since the discovery of metal chalcogenide complexes (MCCs) as capping ligands for colloidal nanocrystals (NCs) in 2009, the chemistry of inorganic ligands for NCs has provided a new paradigm for surface design of nanomaterials. Various inorganic anions including MCCs, metal-free chalcogenides, oxoanions/oxometallates, and halides/pseudohalides/halometallates have been employed to replace the original long-chain organic ligands on NCs. This ligand exchange can also be achieved through a two-step route using ligands stripping agents like HBF4. This review outlines recent advances in inorganically-capped colloidal NCs and details the ligand exchange process for NCs using MCCs and metal-free chalcogenides. The binding affinities of ligands to NC surface have been rationalized in terms of Pearson’s hard and soft acids and bases (HSAB) principle. We also demonstrate that inorganic ligands broaden the functionality of NCs by tailoring their electro-optical properties or generating new inorganic phases through chemical reactions between nanomaterials and their surface ligands. Especially promising are the electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganically-capped colloidal NCs, which substantially outperform their organically-capped couterparts.


181. Soft Epitaxy of Nanocrystal Superlattices
Sara Rupich, Fernando Castro, William T.M. Irvine, and Dmitri Talapin. Nat. Commun. 20145, 5045.


Epitaxial heterostructures with precise registry between crystal layers play a key role in electronics and optoelectronics. In a close analogy, performance of nanocrystal (NC) based devices depends on the perfection of interfaces formed between NC layers. Here we systematically study the epitaxial growth of NC layers for the first time to enable the fabrication of coherent NC layers. NC epitaxy reveals an exceptional strain tolerance. It follows a universal island size scaling behaviour and shows a strain-driven transition from layer-by-layer to Stranski–Krastanov growth with non-trivial island height statistics. Kinetic bottlenecks play an important role in NC epitaxy, especially in the transition from sub-monolayer to multilayer coverage and the epitaxy of NCs with anisotropic shape. These findings provide a foundation for the rational design of epitaxial structures in a fundamentally and practically important size regime between atomic and microscopic systems.

180. Surface Functionalization of Semiconductor and Oxide Nanocrystals with Small Inorganic Oxoanions (PO43–, MoO42–) and Polyoxometalate Ligands
Jing Huang, Wenyong Liu, Dmitriy S. Dolzhnikov, Loredana Protesescu, Maksym V. Kovalenko, Bonil Koo, Soma Chattopadhyay, Elena V. Shevchenko, and Dmitri V. Talapin. ACS Nano 20148, 9388.


In this work, we study the functionalization of the nanocrystal (NC) surface with inorganic oxo ligands, which bring a new set of functionalities to all-inorganic colloidal nanomaterials. We show that simple inorganic oxoanions, such as PO43– and MoO42–, exhibit strong binding affinity to the surface of various II–VI and III–V semiconductor and metal oxide NCs. ζ-Potential titration offered a useful tool to differentiate the binding affinities of inorganic ligands toward different NCs. Direct comparison of the binding affinity of oxo and chalcogenidometallate ligands revealed that the former ligands form a stronger bond with oxide NCs (e.g., Fe2O3, ZnO, and TiO2), while the latter prefer binding to metal chalcogenide NCs (e.g., CdSe). The binding between NCs and oxo ligands strengthens when moving from small oxoanions to polyoxometallates (POMs). We also show that small oxo ligands and POMs make it possible to tailor NC properties. For example, we observed improved stability upon Li+ -ion intercalation into the films of Fe2O3 hollow NCs when capped with MoO42– ligands. We also observed lower overpotential and enhanced exchange current density for water oxidation using Fe2O3 NCs capped with [P2Mo18O62]6– ligands and even more so for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] with POM as the capping ligand.

179. Nanocrystal Grain Growth and Device Architectures for High-Efficiency CdTe Ink-Based Photovoltaics
Ryan W. Crisp, Matthew G. Panthani, William L. Rance, Joel N. Duenow, Philip A. Parilla, Rebecca Callahan, Matthew S. Dabney, Joseph J. Berry, Dmitri V. Talapin, and Joseph M. Luther. ACS Nano 2014, 8, 9063.


We study the use of cadmium telluride (CdTe) nanocrystal colloids as a solution-processable “ink” for large-grain CdTe absorber layers in solar cells. The resulting grain structure and solar cell performance depend on the initial nanocrystal size, shape, and crystal structure. We find that inks of predominantly wurtzite tetrapod-shaped nanocrystals with arms ∼5.6 nm in diameter exhibit better device performance compared to inks composed of smaller tetrapods, irregular faceted nanocrystals, or spherical zincblende nanocrystals despite the fact that the final sintered film has a zincblende crystal structure. Five different working device architectures were investigated. The indium tin oxide (ITO)/CdTe/zinc oxide structure leads to our best performing device architecture (with efficiency >11%) compared to others including two structures with a cadmium sulfide (CdS) n-type layer typically used in high efficiency sublimation-grown CdTe solar cells. Moreover, devices without CdS have improved response at short wavelengths.

178. Carrier Dynamics in Highly Quantum-Confined, Colloidal Indium Antimonide Nanocrystals
Angela Y. Chang, Wenyong Liu, Dmitri V. Talapin, and Richard D. Schaller. ACS Nano 2014, 8, 8513.


Nanometer-sized particles of indium antimonide (InSb) offer opportunities in areas such as solar energy conversion and single photon sources. Here, we measure electron–hole pair dynamics, spectra, and absorption cross sections of strongly quantum-confined colloidal InSb nanocrystal quantum dots using femtosecond transient absorption. For all samples, we observe a bleach feature that develops on ultrafast time scales, which notably moves to lower energy during the first several picoseconds following excitation. We associate this unusual red shift, which becomes larger for larger particles and more distinct at lower sample temperatures, with hot exciton cooling through states that we suggest arise from energetically proximal conduction band levels. From controlled optical excitation intensities, we determine biexciton lifetimes, which range from 2 to 20 ps for the studied 3–6 nm diameter particle sizes.

177. Probing the Surface of Colloidal Nanomaterials with Potentiometry in Situ
Igor Fedin and Dmitri V. Talapin. J. Am. Chem. Soc. 2014, 136, 11228.


Colloidal nanomaterials represent an important branch of modern chemistry. However, we have very little understanding of molecular processes that occur at the nanocrystal (NC) surface during synthesis and post-synthetic modifications. Here we show that potentiometry can be used to study the surface of colloidal NCs under realistic reaction conditions. Potentiometric titrations of CdSe and InP nanostructures provide information on the active surface area, the affinity of ligands to the NC surface, and the surface reaction kinetics. These studies can be carried out at different temperatures in polar and nonpolar media for NCs of different sizes and shapes. In situ potentiometry can provide real-time feedback during synthesis of core–shell nanostructures.

176. Thermoelectric Tin Selenide: The Beauty of Simplicity
Hao Zhang and Dmitri V. Talapin. Angew. Chem. Int. Ed. 201453, 9126.


A thermoelectric figure of merit that is unprecedentedly high has been observed in SnSe crystals. This finding suggests that bulk materials with a layered structure and highly anharmonic lattice vibrations can feature an intrinsically low thermal conductivity and high thermoelectric efficiency. Together with other recent findings in this field, it urges us to revisit basic design principles for thermoelectric materials.

175.  Colloidal Quantum Rods and Wells for Lighting and Lasing Applications
C. She, I. Fedin, M. A. Boles, D. S. Dolzhnikov, R. D. Schaller, M. Pelton, and D. V. Talapin.  SID Digest 2014, 45, 134.


Colloidal semiconductor nanocrystals, also known as “quantum dots” (QDs), represent an example of a disruptive technology for display and lighting applications. Their high luminescence efficiency and tunable, narrow emission are nearly ideal for achieving saturated colors and enriching the display or TV color gamut. Our contribution will discuss the next generation of inorganic nanostructures with electronic structure optimized for achieving emission characteristics beyond traditional near‐spherical QDs. For example, nano‐heterostructures with spherical CdSe QDs epitaxially integrated into CdS quantum rods combine high luminescence efficiency with giant extinction coefficients, large Stokes shifts, and linearly polarized emission. Such a set of characteristics can be ideal for LCD backlighting. The other class of emitters includes colloidal quantum wells (QWs) whose ensemble luminescence is significantly narrower than emission spectra of the best QD samples. Moreover, we show that colloidal QWs produce amplified spontaneous emission (ASE) with pump‐fluence thresholds as low as 6 μJ/cm2 and gain as high as 600 cm−1, on par with the best values for any solution‐processed material.

174. Colloidal Nanocrystals with Inorganic Halide, Pseudohalide, and Halometallate Ligands
Hao Zhang, Jaeyoung Jang, Wenyong Liu, and Dmitri V. Talapin. ACS Nano 20148, 7359.


We investigate simple halides and pseudohalides as an important class of inorganic ligands for nanocrystals (NCs) in solution phase ligand exchange. These short, robust, and easy to model ligands bind to the NC surface and provide electrostatic stabilization of NC dispersions in N-methylformamide. The replacement of organic ligands on NCs with compact halide and pseudohalide ligands greatly facilitates electronic communication between NCs. For example, a high electron mobility of μ ≈ 12 cm2 V–1 s–1 has been observed in thin films made of I-capped CdSe NCs. We also studied charge transport properties of thin films based on the pseudohalide N3-capped InAs NCs, suggesting the possibility of obtaining “all III–V” NC solids. In addition, we extend the surface chemistry of halometallates (e.g., CH3NH3PbI3), which can stabilize colloidal solutions of lead chalcogenide NCs. These halide, pseudohalide, and halometallate ligands enrich the current family of inorganic ligands and can open up more opportunities for applications of NCs in the fields of electronics, optoelectronics, and thermoelectrics.

173. Connecting the Dots (Perspective)
Michael A. Boles and Dmitri V. Talapin. Science 2014344, 1340.


Nanocrystals—nanometer-sized inorganic crystals containing hundreds to thousands of atoms—offer exciting opportunities in areas as varied as photonics, catalysis, biotechnology, and medicine. For example, nanocrystals enrich the display color palette in tablet computers and advanced TVs.

172. Role of Precursor Reactivity in Crystallization of Solution-Processed Semiconductors: The Case of Cu2ZnSnS4
Chengyang Jiang, Wenyong Liu, and Dmitri V. Talapin.  Chem. Mater. 201426, 4038.


We study the formation of Cu2ZnSnS4 (CZTS) films from various liquid-phase precursors. Our experimental data point to the significant role that reactivities of precursor components play in the quality of the final material. Although reactive molecular precursors favor formation of CZTS under milder conditions, the formation of large crystalline domains requires using less reactive nanostructured precursors. We explain this effect using kinetics of nucleation and growth. We have also demonstrated a strategy to effectively enhance grain growth of CZTS using solid-state phase transition as the driving force for nanocrystal sintering. We hope this contribution will provide a useful guide toward the rational design of liquid-phase precursors for inorganic semiconductors for electronic and optoelectronic applications.

171. All-Inorganic Nanocrystals as a Glue for BiSbTe Grains: Design of Interfaces in Mesostructured Thermoelectric Materials
Jae Sung Son, Hao Zhang, Jaeyoung Jang, Bed Poudel, Al Waring, Luke Nally, and Dmitri V. Talapin.  Angew. Chem. Int. Ed. 2014126, 7596.


Nano‐ and mesostructuring is widely used in thermoelectric (TE) materials. It introduces numerous interfaces and grain boundaries that scatter phonons and decrease thermal conductivity. A new approach has been developed for the rational design of the interfaces in TE materials by using all‐inorganic nanocrystals (NCs) that serve as a “glue” for mesoscopic grains. For example, circa 10 nm Bi NCs capped with (N2H5)4Sb2Te7 chalcogenidometallate ligands can be used as an additive to BiSbTe particles. During heat treatment, NCs fill up the voids between particles and act as a “glue”, joining grains in hot‐pressed pellets or solution‐processed films. The chemical design of NC glue allowed the selective enhancement or decrease of the majority‐carrier concentration near the grain boundaries, and thus resulted in doped or de‐doped interfaces in granular TE material. Chemically engineered interfaces can be used as to optimize power factor and thermal conductivity.

170. Synthesis and Search for Design Principles of New Electron Accepting Polymers for All-Polymer Solar Cells
In Hwan Jung, Wai-Yip Lo, Jaeyoung Jang, Wei Chen, Donglin Zhao, Erik S. Landry, Luyao Lu, Dmitri V. Talapin, and Luping Yu. Chem. Mater. 2014, 26, 3450.


New electron withdrawing monomers, thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) and fluorenedicyclopentathiophene dimalononitrile (CN), have been developed and used to form 12 alternating polymers having different monomer combinations: (a) weak donating monomer–strong accepting monomer, (b) weak accepting monomer–strong accepting monomer, (c) weak accepting monomer–weak accepting monomer, and (d) strong donating monomer–strong accepting monomer. It was found that lowest unoccupied molecular orbital (LUMO) energy levels of polymers are significantly determined by stronger electron accepting monomers and highest occupied molecular orbital (HOMO) energy levels by the weak electron accepting monomers. In addition, fluorescent quantum yields of the TPTI-based polymers in chloroform solution are significantly decreased as the LUMO energy levels of the TPTI series of polymers become deeper. The quantum yield was found to be closely related with the photovoltaic properties, which reflects the effect of internal polarization on the photovoltaic properties. Only the electron accepting polymers showing SCLC mobility higher than 10–4 cm2/(V s) exhibited photovoltaic performance in blend films with a donor polymer, and the PTB7:PNPDI (1:1.8 w/w) device exhibited the highest power conversion efficiency of 1.03% (Voc = 0.69 V, Jsc = −4.13 mA/cm2, FF = 0.36) under AM 1.5G condition, 100 W/cm2. We provide a large set of systematic structure–property relationships, which gives new perspectives for the design of electron accepting materials.

169. Low-Threshold Stimulated Emission Using Colloidal Quantum Wells
Chunxing She, Igor Fedin, Dmitriy S. Dolzhnikov, Arnaud Demortière, Richard D. Schaller, Matthew Pelton, and Dmitri V. Talapin. Nano Lett. 201414, 2772.


The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/cm2 and gain as high as 600 cm–1, both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.

168. Self-Assembly of Tetrahedral CdSe Nanocrystals: Effective “Patchiness” via Anisotropic Steric Interaction
Michael A. Boles and Dmitri V. Talapin. J. Am. Chem. Soc. 2014, 136, 5868.


Controlling the spontaneous organization of nanoscale objects remains a fundamental challenge of materials design. Here we present the first characterization of self-assembled superlattices (SLs) comprised of tetrahedral nanocrystal (NCs). We observe self-assembly of CdSe nanotetrahedra into an open structure (estimated space-filling fraction φ ≈ 0.59) which has not been anticipated by many recent theoretical studies and simulations of tetrahedron packings. This finding highlights a gap in the understanding of the hierarchy of energy scales acting on colloidal NCs during self-assembly. We propose a strong dependence of ligand–ligand interaction potential on NC surface curvature. This effect favors spatial proximity of vertices in the dense colloidal crystal and may be considered an emergent “patchiness” acting through chemically identical ligand molecules.

167. Dispersion-free continuum two-dimensional electronic spectrometer
Haibin Zheng, Justin R. Caram, Peter D. Dahlberg, Brian S. Rolczynski, Subha Viswanathan, Dmitriy S. Dolzhnikov, Amir Khadivi, Dmitri V. Talapin, and Gregory S. Engel. Appl. Opt. 201453, 1909.


Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an 180nm window of the available bandwidth at 10fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll 𝑎.

166. Exploring size and state dynamics in CdSe quantum dots using two-dimensional electronic spectroscopy
Justin R. Caram, Haibin Zheng, Peter D. Dahlberg, Brian S. Rolczynski, Graham B. Griffin, Dmitriy S. Dolzhnikov, Dmitri V. Talapin, and Gregory S. Engel. J. Chem. Phys. 2014, 140, 084701.

Development of optoelectronic technologies based on quantum dots depends on measuring, optimizing, and ultimately predicting charge carrier dynamics in the nanocrystal. In such systems, size inhomogeneity and the photoexcited population distribution among various excitonic states have distinct effects on electron and hole relaxation, which are difficult to distinguish spectroscopically. Two-dimensional electronic spectroscopy can help to untangle these effects by resolving excitation energy and subsequent nonlinear response in a single experiment. Using a filament-generated continuum as a pump and probe source, we collect two-dimensional spectra with sufficient spectral bandwidth to follow dynamics upon excitation of the lowest three optical transitions in a polydisperse ensemble of colloidal CdSe quantum dots. We first compare to prior transient absorption studies to confirm excitation-state-dependent dynamics such as increased surface-trapping upon excitation of hot electrons. Second, we demonstrate fast band-edge electron-hole pair solvation by ligand and phonon modes, as the ensemble relaxes to the photoluminescent state on a sub-picosecond time-scale. Third, we find that static disorder due to size polydispersity dominates the nonlinear response upon excitation into the hot electron manifold; this broadening mechanism stands in contrast to that of the band-edge exciton. Finally, we demonstrate excitation-energy dependent hot-carrier relaxation rates, and we describe how two-dimensional electronic spectroscopy can complement other transient nonlinear techniques.

165. Temperature-Dependent Hall and Field-Effect Mobility in Strongly Coupled All-Inorganic Nanocrystal Arrays
Jaeyoung Jang, Wenyong Liu, Jae Sung Son, and Dmitri V. Talapin. Nano Lett. 2014, 14, 653.

We report on the temperature-dependent Hall effect characteristics of nanocrystal (NC) arrays prepared from colloidal InAs NCs capped with metal chalcogenide complex (MCC) ligands (In2Se42– and Cu7S4). Our study demonstrates that Hall effect measurements are a powerful way of exploring the fundamental properties of NC solids. We found that solution-cast 5.3 nm InAs NC films capped with copper sulfide MCC ligands exhibited high Hall mobility values over 16 cm2/(V s). We also showed that the nature of MCC ligands can control doping in NC solids. The comparative study of the temperature-dependent Hall and field-effect mobility values provides valuable insights concerning the charge transport mechanism and points to the transition from a weak to a strong coupling regime in all-inorganic InAs NC solids.

164. High Efficiency Solution Processed Sintered CdTe Nanocrystal Solar Cells: The Role of Interfaces
Matthew G. Panthani, J. Matthew Kurley, Ryan W. Crisp, Travis C. Dietz, Taha Ezzyat, Joseph M. Luther, and Dmitri V. Talapin. Nano Lett. 2014, 14, 670.

Solution processing of photovoltaic semiconducting layers offers the potential for drastic cost reduction through improved materials utilization and high device throughput. One compelling solution-based processing strategy utilizes semiconductor layers produced by sintering nanocrystals into large-grain semiconductors at relatively low temperatures. Using n-ZnO/p-CdTe as a model system, we fabricate sintered CdTe nanocrystal solar cells processed at 350 °C with power conversion efficiencies (PCE) as high as 12.3%. JSC of over 25 mA cm–2 are achieved, which are comparable or higher than those achieved using traditional, close-space sublimated CdTe. We find that the VOC can be substantially increased by applying forward bias for short periods of time. Capacitance measurements as well as intensity- and temperature-dependent analysis indicate that the increased VOC is likely due to relaxation of an energetic barrier at the ITO/CdTe interface.

163. Thermal Stability of Colloidal InP Nanocrystals: Small Inorganic Ligands Boost High-Temperature Photoluminescence
Clare E. Rowland, Wenyong Liu, Daniel C. Hannah, Maria K. Y. Chan, Dmitri V. Talapin, and Richard D. Schaller. ACS Nano 20148, 977.

We examine the stability of excitons in quantum-confined InP nanocrystals as a function of temperature elevation up to 800 K. Through the use of static and time-resolved spectroscopy, we find that small inorganic capping ligands substantially improve the temperature dependent photoluminescence quantum yield relative to native organic ligands and perform similarly to a wide band gap inorganic shell. For this composition, we identify the primary exciton loss mechanism as electron trapping through a combination of transient absorption and transient photoluminescence measurements. Density functional theory indicates little impact of studied inorganic ligands on InP core states, suggesting that reduced thermal degradation relative to organic ligands yields improved stability; this is further supported by a lack of size dependence in photoluminescence quenching, pointing to the dominance of surface processes, and by relative thermal stabilities of the surface passivating media. Thus, small inorganic ligands, which benefit device applications due to improved carrier access, also improve the electronic integrity of the material during elevated temperature operation and subsequent to high temperature material processing.

162. Persistent Interexcitonic Quantum Coherence in CdSe Quantum Dots
Justin R. Caram, Haibin Zheng, Peter D. Dahlberg, Brian S. Rolczynski, Graham B. Griffin, Andrew F. Fidler, Dmitriy S. Dolzhnikov, Dmitri V. Talapin, and Gregory S. Engel. J. Phys. Chem. Lett. 20145, 196.

The creation and manipulation of quantum superpositions is a fundamental goal for the development of materials with novel optoelectronic properties. In this Letter, we report persistent (∼80 fs lifetime) quantum coherence between the 1S and 1P excitonic states in zinc-blende colloidal CdSe quantum dots at room temperature, measured using two-dimensional electronic spectroscopy. We demonstrate that this quantum coherence manifests as an intradot phenomenon, the frequency of which depends on the size of the dot excited within the ensemble of QDs. We model the lifetime of the coherence and demonstrate that correlated interexcitonic fluctuations preserve the relative phase between excitonic states. These observations suggest an avenue for engineering long-lived interexcitonic quantum coherence in colloidal quantum dots.


161. Bi1–xSbx Alloy Nanocrystals: Colloidal Synthesis, Charge Transport, and Thermoelectric Properties 
Hao Zhang, Jae Sung Son, Jaeyoung Jang, Jong-Soo Lee, Wee-Liat Ong, Jonathan A. Malen, and Dmitri V. Talapin. ACS Nano 2013, 7, 10296.


Nanostructured Bi1–xSbx alloys constitute a convenient system to study charge transport in a nanostructured narrow-gap semiconductor with promising thermoelectric properties. In this work, we developed the colloidal synthesis of monodisperse sub-10 nm Bi1–xSbx alloy nanocrystals (NCs) with controllable size and compositions. The surface chemistry of Bi1–xSbx NCs was tailored with inorganic ligands to improve the interparticle charge transport as well as to control the carrier concentration. Temperature-dependent (10–300 K) electrical measurements were performed on the Bi1–xSbx NC based pellets to investigate the effect of surface chemistry and grain size (∼10–40 nm) on their charge transport properties. The Hall effect measurements revealed that the temperature dependence of carrier mobility and concentration strongly depended on the grain size and the surface chemistry, which was different from the reported bulk behavior. At low temperatures, electron mobility in nanostructured Bi1–xSbx was directly proportional to the average grain size, while the concentration of free carriers was inversely proportional to the grain size. We propose a model explaining such behavior. Preliminary measurements of thermoelectric properties showed a ZT value comparable to those of bulk Bi1–xSbx alloys at 300 K, suggesting a potential of Bi1–xSbx NCs for low-temperature thermoelectric applications.

160. Quantum dot light-emitting devices 
Dmitri V. Talapin and Jonathan Steckel. MRS Bull. 2013, 38, 685.


Colloidal semiconductor nanocrystals, also known as “quantum dots” (QDs), represent an example of a disruptive technology for display and lighting applications. The QDs’ high luminescence efficiency and precisely tunable, narrow emission are nearly ideal for achieving saturated colors and enriching the display or TV color gamut. Quantum dot light-emitting diodes (QLEDs) can provide saturated emission colors and allow inexpensive solution-based device fabrication on almost any substrate. The first incorporation of QDs into the consumer market is using them as optical down-converters. Blue light from an efficient high energy light source (e.g., GaN blue LED) is absorbed and reemitted at any desired lower energy wavelength. Alternatively, electric current can be used for direct excitation of QDs. QLEDs are an exciting technical challenge and commercial opportunity for display and solid-state lighting applications. Recent developments in the field show that efficiency and brightness of QLEDs can match those of organic LEDs.

159. Magnet-in-the-Semiconductor Nanomaterials: High Electron Mobility in All-Inorganic Arrays of FePt/CdSe and FePt/CdS Core-Shell Heterostructures
Jae Sung Son, Jong-Soo Lee, Elena V. Shevchenko, and Dmitri V. Talapin. J. Phys. Chem. Lett. 2013, 4, 1918.


We report a colloidal synthesis and electrical and magnetotransport properties of multifunctional “magnet-in-the-semiconductor” nanostructures composed of FePt core and CdSe or CdS shell. Thin films of all-inorganic FePt/CdSe and FePt/CdS core–shell nanostructures capped with In2Se42– molecular chalcogenide (MCC) ligands exhibited n-type charge transport with high field-effect electron mobility of 3.4 and 0.02 cm2/V·s, respectively. These nanostructures also showed a negative magnetoresistance characteristic for spin-dependent tunneling. We discuss the mechanism of charge transport and gating in the arrays of metal/semiconductor core–shell nanostructures.

158. Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals
Karthish Manthiram, Brandon J. Beberwyck, Dmitri V. Talapin, and A. Paul Alivisatos. J. Vis. Exp. 201382, e50731.


We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

157. Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays 
Wee-Liat Ong, Sara M. Rupich, Dmitri V. Talapin, Alan J. H. McGaughey, and Jonathan A. Malen. Nat. Mater. 2013, 12, 410.


Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, optoelectronic and energy-related applications. Hard/soft interfaces in these nanocrystal arrays (NCAs) create a complex and uncharted vibrational landscape for thermal energy transport that will influence their technological feasibility. Here, we present thermal conductivity measurements of NCAs (CdSe, PbS, PbSe, PbTe, Fe3O4 and Au) and reveal that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces, and the volume fractions of nanocrystal cores and surface ligands. NCA thermal conductivities are controllable within the range 0.1–0.3 W m−1 K−1, and only weakly depend on the thermal conductivity of the inorganic core material. This range is 1,000 times lower than the thermal conductivity of silicon, presenting challenges for heat dissipation in NCA-based electronics and photonics. It is, however, 10 times smaller than that of Bi2Te3, which is advantageous for NCA-based thermoelectric materials.

156. The chemistry of functional nanomaterials
Yadong Yin and Dmitri Talapin. Chem. Soc. Rev. 201342, 2484.


Nanostructured materials are the core components of nanotechnology, providing basic building blocks for fabricating complex devices with desired functions. Thanks to their inherent quantum size and shape effects, nanomaterials have many important applications in electronics, optoelectronics, information processing, catalysis, biomedical science, environmental science, energy conversion and storage, advanced defense technologies, and many other fields. Chemistry plays a central role in the development of novel nanostructured materials. This themed issue on the chemistry of functional nanomaterials gives a broad overview of the synthesis, structural manipulation, characterization, surface modification, self-assembly, processing and integration of nanoscale materials into functional devices.

155. Spin-dependent electronic processes and long-lived spin coherence of deep-level trap sites in CdS nanocrystals
K. J. van Schooten, J. Huang, D. V. Talapin, C. Boehme, and J. M. Lupton. Phys. Rev. B. 2013, 87, 125412.

Carrier trapping in colloidal nanocrystals represents a major energy loss mechanism for excitonic states crucial to devices. Surprisingly little is known about the influence of the spin degree of freedom on the nature of these intrinsic trap centers or the types of coupling that these states experience. Here, a pulsed microwave optically detected magnetic resonance study is presented that aims to probe the interaction pathways existing between shallow band-edge trap states and the deep-level emissive chemical defect states responsible for the broad, low-energy emission common to CdS nanocrystals. Due to long spin coherence times (T2) of these states, Rabi flopping detected in the luminescence under magnetic resonance provides access to information regarding the modes of coupling of shallow-trapped electron-hole pairs, both of isolated species and of those in proximity to the emissive defect. Corresponding optically detected spin-echo experiments expose an extraordinarily long intrinsic spin coherence time (T2 ≈ 1.6 μs) for colloidal nanocrystals, and an electron spin-echo envelope modulation indicative of local spin interactions. This effect provides opportunities for gaining the detailed chemical and structural information needed in order to eliminate energy loss mechanisms during the synthetic process.

154. Indirect Exciton Formation due to Inhibited Carrier Thermalization in Single CdSe/CdS Nanocrystals
Eyal Shafran, Nicholas J. Borys, Jing Huang, Dmitri V. Talapin, and John M. Lupton. J. Phys. Chem. Lett. 2013, 4, 691.

Temperature-dependent single-particle spectroscopy is used to study interfacial energy transfer in model light-harvesting CdSe/CdS core–shell tetrapod nanocrystals. Using alternating excitation energies, we identify two thermalized exciton states in single nanoparticles that are attributed to a strain-induced interfacial barrier. At cryogenic temperatures, emission from both states exemplifies the effects of intraparticle disorder and enables their simultaneous characterization, revealing that the two states are distinct in regards to emission polarization, spectral diffusion, and blinking.

153. III-IV Nanocrystals Capped with Molecular Metal Chalcogenide Ligands: High Electron Mobility and Ambipolar Photoresponse
Wenyong Liu, Jong-Soo Lee, and Dmitri V. Talapin. J. Am. Chem. Soc. 2013, 135, 1349.

In this work, we synthesized InP and InAs nanocrystals (NCs) capped with different inorganic ligands, including various molecular metal chalcogenide complexes (MCCs) and chalcogenide ions. We found that MCCs and chalcogenide ions can quantitatively displace organic ligands from the surface of III–V NCs and serve as the inorganic capping groups for III–V NC surfaces. These inorganic ligands stabilize colloidal solutions of InP and InAs NCs in polar solvents and greatly facilitate charge transport between individual NCs. Charge transport studies revealed high electron mobility in the films of MCC-capped InP and InAs NCs. For example, we found that bridging InAs NCs with Cu7S4 MCC ligands can lead to very high electron mobility exceeding 15 cm2/(V s). In addition, we observed unprecedented ambipolar (positive/negative) photoresponse of MCC-capped InAs NC solids that changed sign depending on the ligand chemistry, illumination wavelength, and doping of the NC solid. For example, the sign of photoconductance of InAs NCs capped with Cu7S4 or Sn2S64– ions converted from positive at 0.80 and 0.95 eV to negative at 1.27 and 1.91 eV. We propose an explanation of this unusually complex photoconductivity of InAs NC solids.

152. Light-Induced Charged and Trap States in Colloidal Nanocrystals Detected by Variable Pulse Rate Photoluminescence Spectroscopy
Michele Saba, Mauro Aresti, Francesco Quochi, Marco Marceddu, Maria Antonietta Loi, Jing Huang, Dmitri V. Talapin, Andrea Mura, and Giovanni Bongiovanni. ACS Nano 2013, 7, 229.

Intensity instabilities are a common trademark of the photoluminescence of nanoemitters. This general behavior is commonly attributed to random fluctuations of free charges and activation of charge traps reducing the emission yield intermittently. However, the actual physical origin of this phenomenon is actively debated. Here we devise an experiment, variable pulse rate photoluminescence, to control the accumulation of charges and the activation of charge traps. The dynamics of these states is studied, with pulse repetition frequencies from the single-pulse to the megahertz regime, by monitoring photoluminescence spectrograms with picosecond temporal resolution. We find that both photocharging and charge trapping contribute to photoluminescence quenching, and both processes can be reversibly induced by light. Our spectroscopic technique demonstrates that charge accumulation and trap formation are strongly sensitive to the environment, showing different dynamics when nanocrystals are dispersed in solution or deposited as a film.

151. Spin-Dependent Exciton Quenching and Spin Coherence in CdSe/CdS Nanocrystals
Kipp J. van Schooten, Jing Huang, William J. Baker, Dmitri V. Talapin, Christoph Boehme, and John M. Lupton. Nano Lett. 201313, 65.

Large surface-to-volume ratios of semiconductor nanocrystals cause susceptibility to charge trapping, which can modify luminescence yields and induce single-particle blinking. Optical spectroscopies cannot differentiate between bulk and surface traps in contrast to spin-resonance techniques, which in principle avail chemical information on such trap sites. Magnetic resonance detection via spin-controlled photoluminescence enables the direct observation of interactions between emissive excitons and trapped charges. This approach allows the discrimination of three radical species located in two functionally different trap states in CdSe/CdS nanocrystals, underlying the fluorescence quenching and thus blinking mechanisms: a spin-dependent Auger process in charged particles; and a charge-separated state pair process, which leaves the particle neutral. The paramagnetic trap centers offer control of the energy transfer yield from the wide-gap CdS to the narrow-gap CdSe, that is, light harvesting within the heterostructure. Coherent spin motion within the trap states of the CdS arms of nanocrystal tetrapods is reflected by spatially remote luminescence from CdSe cores with surprisingly long coherence times of >300 ns at 3.5 K, illustrating coherent control of light harvesting.

150. Two-dimensional electronic spectroscopy of CdSe nanoparticles at very low pulse power
Graham B. Griffin, Sandrine Ithurria, Dmitriy S. Dolzhnikov, Alexander Linkin, Dmitri V. Talapin, and Gregory S. Engel. J. Chem. Phys. 2013138, 014705.

Nanoparticles have been proposed as a promising material for creating devices that harvest, transport, and manipulate energy and electrons. Ultrafast charge carrier dynamics represent a critical design aspect and are dependent on both size and shape of the nanoparticle.Spectroscopic investigation of the electronic structure and dynamics of these systems is complicated by sample inhomogeneity, which broadens peaks and leads to ambiguity in interpretation of both spectra and dynamics. Here, we use two-dimensional electronic spectroscopy to remove inhomogeneous broadening and to clarify interpretation of measured dynamics. We specifically investigate the effect of nanoparticle shape on the electronic structure and ultrafast electronic dynamics in the band-edge exciton states of CdSe quantum dots, nanorods, and nanoplatelets. Particle size was chosen to enable straightforward comparisons of the effects of particle shape on the spectra and dynamics without retuning the laser source. The spectra were measured with low pulse powers (generally <1 nJ/pulse), using short pulses (∼12 fs) to minimize interference from solventcontributions to the spectra, ambiguities in the dynamics due to pulse-overlap effects, and contributions to the dynamics from multi-exciton effects. The lowest two exciton states are clearly resolved in spectra of quantum dots but unresolved for nanorods and nanoplates, in agreement with previous spectroscopic and theoretical results. In all nanoparticles, ultrafast dynamics measurements show strong evidence of electronic relaxation into the lowest energy exciton state within ∼30 fs, a timescale not observable in previous dynamics measurements of similar systems. These dynamics are unambiguously assigned to hole relaxation, as the higher lying electronic excited states are not energetically accessible in these experiments. Clear evidence of coherent superpositions of the lowest two excitonstates were not seen in any of the particles studied, in contrast to recent results from work on quantum dots.


149. Colloidal InSb Nanocrystals
Wenyong Liu, Angela Y. Chang, Richard D. Schaller, and Dmitri V. Talapin. s 2012134, 20258.


We report the colloidal synthesis of monodisperse nanocrystals (NCs) of InSb, which is an important member of III–V semiconductor family. Colloidal InSb NC quantum dots showed well-resolved excitonic transitions in the near-infrared spectral range, with the optical band gaps tunable from ∼1.03 eV (1200 nm) to ∼0.71 eV (1750 nm) corresponding to 3.3 and 6.5 nm InSb NCs, respectively. We observed size-tunable band edge photoluminescence that could be significantly enhanced by growing InSb/CdSe or InSb/CdS core–shell nanostructures. Films of InSb NCs capped with S2– ions showed ambipolar charge transport.

148. Carrier Cooling in Colloidal Quantum Wells
Matthew Pelton, Sandrine Ithurria, Richard D. Schaller, Dmitriy S. Dolzhnikov, and Dmitri V. Talapin. Nano Lett. 2012, 12, 6158.


It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators.


Atomic layer deposition (ALD) is widely used for gas-phase deposition of high-quality dielectric, semiconducting, or metallic films on various substrates. In this contribution we propose the concept of colloidal ALD (c-ALD) for synthesis of colloidal nanostructures. During the c-ALD process, either nanoparticles or molecular precursors are sequentially transferred between polar and nonpolar phases to prevent accumulation of unreacted precursors and byproducts in the reaction mixture. We show that binding of inorganic ligands (e.g., S2–) to the nanocrystal surface can be used as a half-reaction in c-ALD process. The utility of this approach has been demonstrated by growing CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of colloidal nanoheterostructures with mixed confinement regimes for electrons and holes. In these materials holes are confined to a thin (∼1.8 nm) two-dimensional CdSe quantum well, while the electron confinement can be gradually relaxed in all three dimensions by growing epitaxial CdS layers on both sides of the quantum well. The relaxation of the electron confinement energy caused a shift of the emission band from 510 to 665 nm with unusually small inhomogeneous broadening of the emission spectra.

146. Effect of Metal Ions on Photoluminescence, Charge Transport, Magnetic and Catalytic Properties of All-Inorganic Colloidal Nanocrystals and Nanocrystal Solids
Angshuman Nag, Dae Sung Chung, Dmitriy S. Dolzhnikov, Nada M. Dimitrijevic, Soma Chattopadhyay, Tomohiro Shibata, and Dmitri V. Talapin. J. Am. Chem. Soc. 2012, 134, 13604.


Colloidal semiconductor nanocrystals (NCs) provide convenient “building blocks” for solution-processed solar cells, light-emitting devices, photocatalytic systems, etc. The use of inorganic ligands for colloidal NCs dramatically improved inter-NC charge transport, enabling fast progress in NC-based devices. Typical inorganic ligands (e.g., Sn2S64–, S2–) are represented by negatively charged ions that bind covalently to electrophilic metal surface sites. The binding of inorganic charged species to the NC surface provides electrostatic stabilization of NC colloids in polar solvents without introducing insulating barriers between NCs. In this work we show that cationic species needed for electrostatic balance of NC surface charges can also be employed for engineering almost every property of all-inorganic NCs and NC solids, including photoluminescence efficiency, electron mobility, doping, magnetic susceptibility, and electrocatalytic performance. We used a suite of experimental techniques to elucidate the impact of various metal ions on the characteristics of all-inorganic NCs and developed strategies for engineering and optimizing NC-based materials.

145. Low Voltage, Hysteresis Free, and High Mobility Transistors from All-Inorganic Colloidal Nanocrystals
Dae Sung Chung, Jong-Soo Lee, Jing Huang, Angshuman Nag, Sandrine Ithurria, and Dmitri V. Talapin. Nano Lett. 2012, 12, 1813.


High-mobility solution-processed all-inorganic solid state nanocrystal (NC) transistors with low operation voltage and near-zero hysteresis are demonstrated using high-capacitance ZrOx and hydroxyl-free Cytop gate dielectric materials. The use of inorganic capping ligands (In2Se42– and S2–) allowed us to achieve high electron mobility in the arrays of solution-processed CdSe nanocrystals. We also studied the hysteresis behavior and switching speed of NC-based field effect devices. Collectively, these analyses helped to understand the charge transport and trapping mechanisms in all-inorganic NCs arrays. Finally, we have examined the rapid thermal annealing as an approach toward high-performance solution-processed NCs-based devices and demonstrated transistor operation with mobility above 30 cm2/(V s) without compromising low operation voltage and hysteresis.


We report a new platform for design of soluble precursors for CuInSe2 (CIS), Cu(In1–xGax)Se2(CIGS), and Cu2ZnSn(S,Se)4 (CZTS) phases for thin-film potovoltaics. To form these complex phases, we used colloidal nanocrystals (NCs) with metal chalcogenide complexes (MCCs) as surface ligands. The MCC ligands both provided colloidal stability and represented essential components of target phase. To obtain soluble precursors for CuInSe2, we used Cu2–xSe NCs capped with In2Se42– MCC surface ligands or CuInSe2 NCs capped with {In2Cu2Se4S3}3– MCCs. A mixture of Cu2–xSe and ZnS NCs, both capped with Sn2S64– or Sn2Se64– ligands was used for solution deposition of CZTS films. Upon thermal annealing, the inorganic ligands reacted with NC cores forming well-crystallized pure ternary and quaternary phases. Solution-processed CIS and CZTS films featured large grain size and high phase purity, confirming the prospects of this approach for practical applications.

143. Inorganically Functionalized PbS–CdS Colloidal Nanocrystals: Integration into Amorphous Chalcogenide Glass and Luminescent Properties
Maksym V. Kovalenko, Richard D. Schaller, Dorota Jarzab, Maria A. Loi, and Dmitri V. Talapin. J. Am. Chem. Soc. 2012, 134, 2457.

Inorganic semiconductor nanocrystals (NCs) with bright, stable, and wavelength-tunable luminescence are very promising emitters for various photonic and optoelectronic applications. Recently developed strategies for inorganic surface capping of colloidal NCs using metal chalcogenide complexes have opened new perspectives for their applications. Here we report an all-inorganic surface functionalization of highly luminescent IR-emitting PbS–CdS NCs and studies of their luminescence properties. We show that inorganic capping allows simple low-temperature encapsulation of inorganic NCs into a solution-cast IR-transparent amorphous As2S3 matrix. The resulting all-inorganic thin films feature stable IR luminescence in the telecommunication wavelength region. The high optical dielectric constant of As2S3 also helps reduce the dielectric screening of the radiating field inside the quantum dot, enabling fast radiative recombination in PbS–CdS NCs.

142. Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals
Ilka Kriegel, Chengyang Jiang, Jessica Rodriguez-Fernández, Richard D. Schaller, Dmitri V. Talapin, Enrico da Como, and Jochen Feldmann. J. Am. Chem. Soc. 2012, 134, 1583.

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ∼1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu2–xS and Cu2–xSe, x = 0) into their nonstoichiometric counterparts (Cu2–xS and Cu2–xSe, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu2–xTe, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump–probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.

141. Nanocrystal solids: A modular approach to materials design
Dmitri V. Talapin. MRS Bull. 2012, 37, 63.

Colloidal nanocrystals can combine the benefits of inorganic semiconductors with size-tunable electronic structure and inexpensive solution-based device fabrication. Single- and multicomponent nanocrystal assemblies, also known as superlattices, provide a powerful general platform for designing two- and three-dimensional solids with tailored electronic, magnetic, and optical properties. Such assemblies built of “designer atoms” can be considered as a novel type of condensed matter, whose behavior depends both on the properties of individual building blocks and on the interactions between them. Efficient charge transport is crucial for applications of nanocrystal-based materials in various electronic and optoelectronic devices. For a long time, nanocrystals were considered poor electronic conductors. To facilitate charge transport, we developed novel surface chemistry using all-inorganic ligands, namely metal chalcogenide complexes that transformed colloidal nanomaterials into a very competitive class of solution-processed semiconductors for electronic, thermoelectric, and photovoltaic applications.

140. Particle-Level Engineering of Thermal Conductivity in Matrix-Embedded Semiconductor Nanocrystals
Daniel C. Hannah, Sandrine Ithurria, Galyna Krylova, Dmitri V. Talapin, George C. Schatz, and Richard D. Schaller. Nano Lett. 201212, 5797.

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a “bottom-up” means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.

139. Measurement of electronic splitting in PbS quantum dots by two-dimensional nonlinear spectroscopy
Elad Harel, Sara M. Rupich, Richard D. Schaller, Dmitri V. Talapin, and Gregory S. Engel. Phys. Rev. B 201286, 075412.

Quantum dots exhibit rich and complex electronic structure that makes them ideal for studying the basic physics of semiconductors in the intermediate regime between bulk materials and single atoms. The remarkable nonlinear optical properties of these nanostructures make them strong candidates for photonics applications. Here, we experimentally probe changes in the fine structure on ultrafast timescales of a colloidal solution of PbS quantum dots through their nonlinear optical response despite extensive inhomogeneous spectral broadening. Using continuum excitation and detection, we observe electronic coupling between nearly degenerate exciton states split by intervalley scattering at low exciton occupancy and a sub-100 fs frequency shift presumably due to phonon-assisted transitions. At high excitation intensities, we observe multi-exciton effects and sharp absorbance bands indicative of exciton-exciton coupling. Our experiments directly probe the nonlinear optical response of nearly degenerate quantum confined nanostructures with femtosecond temporal resolution despite extensive line broadening caused by the finite size distribution found in colloidal solutions.

138. Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states
Su Liu, Nicholas J. Borys, Jing Huang, Dmitri V. Talapin, and John M. Lupton. Phys. Rev. B 201286, 045303.

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95% can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.

137. Charged excitons, Auger recombination and optical gain in CdSe/CdS nanocrystals
Marco Marceddu, Michele, Saba, Francesco Quochi, Adriano Lai, Jing Huang, Dmitri V. Talapin, Andrea Mura, and Giovanni Bongiovanni. Nanotechnology 2012, 23, 015201.

CdSe/CdS colloidal nanocrystals are members of a novel class of light-emitting nanoparticles with remarkable optical properties such as suppressed fluorescence blinking and enhanced emission from multiexciton states. These properties have been linked to the suppression of non-radiative Auger recombination. In this work we employ ultrafast spectroscopy techniques to identify optical signatures of neutral and charged excitonic and multiexcitonic states. We show that Auger recombination of biexcitons is not suppressed, while we observe optical gain and amplified spontaneous emission from multiexciton states and from long-lived charged-exciton states.