246. Direct Optical Lithography of Colloidal Metal Oxide Nanomaterials for Diffractive Optical Elements with 2π Phase Control
Jia-Ahn Pan, Zichao Rong, Yuanyuan Wang, Himchan Cho, Igor Coropceanu, Haoqi Wu, and Dmitri V. Talapin. J. Am. Chem. Soc. 2021, 143, 5, 2372–2383.


Spatially patterned dielectric materials are ubiquitous in electronic, photonic, and optoelectronic devices. These patterns are typically made by subtractive or additive approaches utilizing vapor-phase reagents. On the other hand, recent advances in solution-phase synthesis of oxide nanomaterials have unlocked a materials library with greater compositional, microstructural, and interfacial tunability. However, methods to pattern and integrate these nanomaterials in real-world devices are less established. In this work, we directly optically pattern oxide nanoparticles (NPs) by mixing them with photosensitive diazo-2-naphthol-4-sulfonic acid and irradiating with widely available 405 nm light. We demonstrate the direct optical lithography of ZrO2, TiO2, HfO2, and ITO NPs and investigate the chemical and physical changes responsible for this photoinduced decrease in solubility. For example, micron-thick layers of amorphous ZrO2 NPs were patterned with micron resolution and shown to allow 2pi phase control of visible light. We also show multilayer patterning and use it to fabricate features with different thicknesses and distinct structural colors. Upon annealing at 400 C, the deposited structures have excellent optical transparency across a wide wavelength range (0.3 – 10 um), a high refractive index (n = 1.84 at 633 nm) and are optically smooth. We then fabricate diffractive optical elements, such as binary phase diffraction gratings, that show efficient diffractive behavior and good thermal stability. Different oxide NPs can also be mixed prior to patterning, providing a high level of material tunability. This work demonstrates a general patterning approach that harnesses the processability and diversity of colloidal oxide nanomaterials for use in photonic applications.

245. Stoichiometry of the Core Determines the Electronic Structure of Core–Shell III–V/II–VI Nanoparticles
Mariami Rusishvili, Stefan Wippermann, Dmitri V. Talapin, and Giulia Galli. Chem. Mater. 202032, 9798-9804.

Recently, III–V quantum dots (QDs) emerged as an environmentally friendly alternative to CdSe; however, they exhibit broader emission spectra and inferior photoluminescence quantum yield. Here, we report a computational study of the optoelectronic properties of InxPz and InxGayPz QDs interfaced with zinc chalcogenide shells. Using density functional theory, we show that fine-tuning the composition of the core is critical to achieving narrow emission lines. We show that core–shell nanoparticles, where the core has the same diameter but different stoichiometries, may absorb and emit at different wavelengths, leading to broad absorption and emission spectra. The value of the fundamental gap of the core–shell particles depends on the ratio between the number of group III and P atoms in the core and is maximized for the 1:1 composition. We also show that the interplay between quantum confinement and strain determines the difference in the electronic properties of III–V QDs interfaced with ZnS or ZnSe shells.

244. Functional materials and devices by self-assembly
Dmitri V. Talapin, Michael Engel, and Paul V. Braun. MRS Bull. 2020, 45, 799.

Precise patterning of quantum dot (QD) layers is an important prerequisite for fabricating QD light‐emitting diode (QLED) displays and other optoelectronic devices. However, conventional patterning methods cannot simultaneously meet the stringent requirements of resolution, throughput, and uniformity of the pattern profile while maintaining a high photoluminescence quantum yield (PLQY) of the patterned QD layers. Here, a specially designed nanocrystal ink is introduced, “photopatternable emissive nanocrystals” (PENs), which satisfies these requirements. Photoacid generators in the PEN inks allow photoresist‐free, high‐resolution optical patterning of QDs through photochemical reactions and in situ ligand exchange in QD films. Various fluorescence and electroluminescence patterns with a feature size down to ≈1.5 µm are demonstrated using red, green, and blue PEN inks. The patterned QD films maintain ≈75% of original PLQY and the electroluminescence characteristics of the patterned QLEDs are comparable to thopse of non‐patterned control devices. The patterning mechanism is elucidated by in‐depth investigation of the photochemical transformations of the photoacid generators and changes in the optical properties of the QDs at each patterning step. This advanced patterning method provides a new way for additive manufacturing of integrated optoelectronic devices using colloidal QDs.

243. Direct Optical Patterning of Quantum Dot Light‐Emitting Diodes via In Situ Ligand Exchange
Himchan Cho, Jia‐Ahn Pan, Haoqi Wu, Xinzheng Lan, Igor Coropceanu, Yuanyuan Wang, Wooje Cho, Ethan A. Hill, John S. Anderson, and Dmitri V. Talapin. Adv. Mater. 2020, 32, 2003805.

Precise patterning of quantum dot (QD) layers is an important prerequisite for fabricating QD light‐emitting diode (QLED) displays and other optoelectronic devices. However, conventional patterning methods cannot simultaneously meet the stringent requirements of resolution, throughput, and uniformity of the pattern profile while maintaining a high photoluminescence quantum yield (PLQY) of the patterned QD layers. Here, a specially designed nanocrystal ink is introduced, “photopatternable emissive nanocrystals” (PENs), which satisfies these requirements. Photoacid generators in the PEN inks allow photoresist‐free, high‐resolution optical patterning of QDs through photochemical reactions and in situ ligand exchange in QD films. Various fluorescence and electroluminescence patterns with a feature size down to ≈1.5 µm are demonstrated using red, green, and blue PEN inks. The patterned QD films maintain ≈75% of original PLQY and the electroluminescence characteristics of the patterned QLEDs are comparable to thopse of non‐patterned control devices. The patterning mechanism is elucidated by in‐depth investigation of the photochemical transformations of the photoacid generators and changes in the optical properties of the QDs at each patterning step. This advanced patterning method provides a new way for additive manufacturing of integrated optoelectronic devices using colloidal QDs.

242. Area and thickness dependence of Auger recombination in nanoplatelets
 John P. Philbin, Alexandra Brumberg, Benjamin T. Diroll, Wooje Cho, Dmitri V. Talapin, Richard D. Schaller, and Eran Rabani. J. Chem. Phys. 2020, 153, 054104.

The ability to control both the thickness and the lateral dimensions of colloidal nanoplatelets offers a test-bed for area and thickness dependent properties in 2D materials. An important example is Auger recombination, which is typically the dominant process by which multiexcitons decay in nanoplatelets. Herein, we uncover fundamental properties of biexciton decay in nanoplatelets by comparing the Auger recombination lifetimes based on interacting and noninteracting formalisms with measurements based on transient absorption spectroscopy. Specifically, we report that electron–hole correlations in the initial biexcitonic state must be included in order to obtain Auger recombination lifetimes in agreement with experimental measurements and that Auger recombination lifetimes depend nearly linearly on the lateral area and somewhat more strongly on the thickness of the nanoplatelet. We also connect these scalings to those of the area and thickness dependencies of single exciton radiative recombination lifetimes, exciton coherence areas, and exciton Bohr radii in these quasi-2D materials.

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 2020369, 979.

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.