Publications 2016–2009

2016

201. Introduction: Nanoparticle Chemistry
D. V. Talapin and E. V. Shevchenko. Chem. Rev. 2016, 116, 10343.

Abstract
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.

Abstract
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.

Abstract
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. 2016, 116, 11220.

Abstract
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 2016, 353, 885.

Abstract
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.

Abstract
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. 2016, 138, 7464.

Abstract
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.

Abstract
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.

Abstract
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.

2015

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.

Abstract
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 2015, 9, 9475.

Abstract
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., 2015, 15, 6309. 

Abstract
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., 2015, 27, 5941.

Abstract
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.

188. Many-Body Effects in Nanocrystal Superlattices: Departure from Sphere Packing Explains Stability of Binary Phases
Michael A. Boles and Dmitri V. Talapin. J. Am. Chem. Soc. 2015, 137, 4494.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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 2015, 347, 425.
Technology Update: “Semiconductor solder seals nanojoints“

Abstract
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) 2015, 229, 85.

Abstract
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.

2014

181. Soft Epitaxy of Nanocrystal Superlattices
Sara Rupich, Fernando Castro, William T.M. Irvine, and Dmitri Talapin. Nat. Commun. 2014, 5, 5045.

Abstract
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 2014, 8, 9388.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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. 2014, 53, 9126.

Abstract
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.

Abstract
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 2014, 8, 7359.

Abstract
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 2014, 344, 1340.

Abstract
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. 2014, 26, 4038.

Abstract
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. 2014, 126, 7596.

Abstract
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.

Abstract
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. 2014, 14, 2772.

Abstract
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.

Abstract
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. 2014, 53, 1909.

Abstract
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 ∼180  nm window of the available bandwidth at ∼10  fs 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.

Abstract
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.

Abstract
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.

Abstract
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 2014, 8, 977.

Abstract
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. 2014, 5, 196.

Abstract
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.

2013

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.

Abstract
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.

Abstract
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.

Abstract
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. 2013, 82, e50731.

Abstract
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.

Abstract
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. 2013, 42, 2484.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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. 2013, 13, 65.

Abstract
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. 2013, 138, 014705.

Abstract
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.

2012

149. Colloidal InSb Nanocrystals
Wenyong Liu, Angela Y. Chang, Richard D. Schaller, and Dmitri V. Talapin. 2012, 134, 20258.

Abstract
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.

Abstract
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.

147. Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media
Sandrine Ithurria and Dmitri Talapin. J. Am. Chem. Soc. 2012, 134, 18585.

Abstract
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.

Abstract
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.

Abstract
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.

144. Soluble precursors for CuInSe2, CuIn1-xGaxSe2 and Cu2ZnSn(S,Se)4 based on colloidal nanocrystals and molecular metal chalcogenide surface ligands
Chengyang Jiang, Jong-Soo Lee, and Dmitri V. Talapin. J. Am. Chem. Soc. 2012, 134, 5010.

Abstract
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.

Abstract
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.

Abstract
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.

Abstract
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. 2012, 12, 5797.

Abstract
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 2012, 86, 075412.

Abstract
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 2012, 86, 045303.

Abstract
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.

Abstract
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.

2011

136. Structural Defects in Periodic and Quasicrystalline Binary Nanocrystal Superlattices
Maryna I. Bodnarchuk, Elena V. Shevchenko, and Dmitri V. Talapin. J. Am. Chem. Soc. 2011, 133, 20837.

Abstract

Binary nanocrystal superlattices (BNSLs) emerge as an important class of man-made materials where components and functionalities can be added, tuned, or combined in a predictable manner. These amazingly complex structures spontaneously self-assemble from colloidal solutions containing binary mixtures of functional (semiconducting, magnetic, plasmonic, etc.) nanocrystals. Further developments of the BNSL-based materials require a deep understanding and control over BNSL formation and structural perfection. Like any solid, BNSL can contain different kinds of structural defects. It is well-known that defects can have a tremendous effect on the material’s behavior. Defect engineering is used to modify and improve many of the mechanical, electrical, magnetic, and optical properties of conventional solids. In this work, we provide the first systematic analysis of structural defects in various BNSL structures. We used BNSLs as a platform for studying structural defects in both periodic (crystalline) and aperiodic (quasicrystalline) lattices, as well as for direct imaging of the interfaces between crystalline and quasicrystalline domains. Such direct observation of local imperfections in complex multicomponent lattices provides a unique insight into the fundamental aspects of crystal formation.

135. Colloidal self-assembly: Interlocked octapods
Sara M. Rupich and Dmitri V. Talapin. Nat. Mater. 2011, 10, 815.

Abstract

Suspensions of octapod-shaped nanocrystals are seen to spontaneously interlock into chains, which in turn aggregate side-by-side to form three-dimensional crystals. The observed hierarchical self-assembly can be explained by the octapod’s shape and the solvent-tunable van der Waals interactions.

134. Observation of Size-Dependent Thermalization in CdSe Nanocrystals Using Time-Resolved Photoluminescence Spectroscopy
Daniel C. Hannah, Nicholas J. Dunn, Sandrine Ithurria, Dmitri V. Talapin, Lin X. Chen, Matthew Pelton, George C. Schatz, and Richard D. Schaller. Phys. Rev. Lett. 2011, 107, 177403.

Abstract

We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ∼25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions.

133. Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH–, and NH2– as Surface Ligands
Angshuman Nag, Maksym V. Kovalenko, Jong-Soo Lee, Wenyong Liu, Boris Spokoyny, and Dmitri V. Talapin. J. Am. Chem. Soc. 2011, 133, 10612.

Abstract

All-inorganic colloidal nanocrystals were synthesized by replacing organic capping ligands on chemically synthesized nanocrystals with metal-free inorganic ions such as S2–, HS–, Se2–, HSe–, Te2–, HTe–, TeS32–, OH– and NH2–. These simple ligands adhered to the NC surface and provided colloidal stability in polar solvents. The versatility of such ligand exchange has been demonstrated for various semiconductor and metal nanocrystals of different size and shape. We showed that the key aspects of Pearson’s hard and soft acids and bases (HSAB) principle, originally developed for metal coordination compounds, can be applied to the bonding of molecular species to the nanocrystal surface. The use of small inorganic ligands instead of traditional ligands with long hydrocarbon tails facilitated the charge transport between individual nanocrystals and opened up interesting opportunities for device integration of colloidal nanostructures.

132. Themed issue: Chemical transformations of nanoparticles
Dmitri V. Talapin and Yadong Yin. J. Mater. Chem. 2011, 21, 11454.

Abstract

For a long time, chemists studied systems where composition and structure could be defined at the atomic level. Such determinism is natural in molecular compounds and in macroscopic bulk crystalline solids. As molecules became larger and larger, it was more and more difficult to keep this level of control. For the first time, chemists experienced this problem in polymers. Indeed, description of macromolecular substances required statistical approaches dealing with the average molecular weight and polydispersity. This transition from fully deterministic to statistical descriptions of chemical species was an important paradigm shift that required accepting new chemistry rules and inventing new characterization techniques. It definitely paid off. It is difficult to overestimate the importance of polymers for modern society…

131. Three-Dimensional Nanocrystal Superlattices Grown in Nanoliter Microfluidic Plugs
Maryna I. Bodnarchuk, Liang Li, Alice Fok, Sigrid Nachtergaele, Rustem F. Ismagilov, and Dmitri V. Talapin. J. Am. Chem. Soc. 2011, 133, 8956.

Abstract

We studied the self-assembly of inorganic nanocrystals (NCs) confined inside nanoliter droplets (plugs) into long-range ordered superlattices. We showed that a capillary microfluidic platform can be used for the optimization of growth conditions for NC superlattices and can provide insights into the kinetics of the NC assembly process. The utility of our approach was demonstrated by growing large (up to 200 μm) three-dimensional (3D) superlattices of various NCs, including Au, PbS, CdSe, and CoFe2O4. We also showed that it is possible to grow 3D binary nanoparticle superlattices in the microfluidic plugs.

130. Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays
Jong-Soo Lee, Maksym V. Kovalenko, Jing Huang, Dae Sung Chung, and Dmitri V. Talapin. Nature Nanotech. 2011, 6, 348.

Abstract
Flexible, thin-film electronic and optoelectronic devices typically involve a trade-off between performance and fabrication cost. For example, solution-based deposition allows semiconductors to be patterned onto large-area substrates to make solar cells and displays, but the electron mobility in solution-deposited semiconductor layers is much lower than in semiconductors grown at high temperatures from the gas phase. Here, we report band-like electron transport in arrays of colloidal cadmium selenide nanocrystals capped with the molecular metal chalcogenide complex In2Se42−, and measure electron mobilities as high as 16 cm2 V−1 s−1, which is about an order of magnitude higher than in the best solution-processed organic and nanocrystal devices so far. We also use CdSe/CdS core–shell nanoparticles with In2Se42− ligands to build photodetectors with normalized detectivity D* > 1 × 1013 Jones (I Jones = 1 cm Hz1/2 W−1), which is a record for II–VI nanocrystals. Our approach does not require high processing temperatures, and can be extended to different nanocrystals and inorganic surface ligands.

129. Evaluation of Ordering in Single-Component and Binary Nanocrystal Superlattices by Analysis of Their Autocorrelation Functions
Stefan Pichler, Maryna I. Bodnarchuk, Maksym V. Kovalenko, Maksym Yarema, Gunther Springholz, Dmitri V. Talapin, and Wolfgang Heiss. ACS Nano 2011, 5, 1703.

Abstract
Self-assembly of colloidal nanocrystals and other nanosized building blocks has led to numerous large-scale and well-ordered superstructures. To quantify the superlattice quality we present a simple and efficient method, based on analysis of the autocorrelation function to determine characteristic order parameters for short-range and long-range ordering. This provides a feedback for further improvements of deposition techniques and self-assembly processes. To show the power of this method, it is applied to various two-dimensional ordered single component and binary nanocrystal assemblies. A quantitative comparison of the normalized long-range order parameter for various colloidal or epitaxially grown superlattice structures evidences that the long-range ordering in monodisperse colloidal superlattices by far supersedes that obtained at best by epitaxially grown quantum dots. Astonishingly, for selected binary nanocrystal superlattices the long-range ordering parameter reaches almost the same values as for single component superlattices. Besides the high sensitivity of the introduced quantification method to lattice imperfections our analysis also reveals any anisotropy in the ordering of the superlattices, which again can be quantified, for example, to identify the areas of highest quality within one specific sample.

2010

128. The Role of Particle Morphology in Interfacial Energy Transfer in CdSe/CdS Heterostructure Nanocrystals
Nicholas J. Borys, Manfred J. Walter, Jing Huang, Dmitri V. Talapin, and John M. Lupton. Science 2010, 330, 1371.

Abstract

Nanoscale semiconductor heterostructures such as tetrapods can be used to mimic light-harvesting processes. We used single-particle light-harvesting action spectroscopy to probe the impact of particle morphology on energy transfer and carrier relaxation across a heterojunction. The generic form of an action spectrum [in our experiments, photoluminescence excitation (PLE) under absorption in CdS and emission from CdSe in nanocrystal tetrapods, rods, and spheres] was controlled by the physical shape and resulting morphological variation in the quantum confinement parameters of the nanoparticle. A correlation between single-particle PLE and physical shape as determined by scanning electron microscopy was demonstrated. Such an analysis links local structural non-uniformities such as CdS bulbs forming around the CdSe core in CdSe/CdS nanorods to a lower probability of manifesting excitation energy–dependent emission spectra, which in turn is probably related to band alignment and electron delocalization at the heterojunction interface.

127. Multiexcitonic Dual Emission in CdSe/CdS Tetrapods and Nanorods
Andrey A. Lutich, Christian Mauser, Enrico Da Como, Jing Huang, Aleksandar Vaneski, Dmitri V. Talapin, Andrey L. Rogach, and Jochen Feldmann. Nano Lett. 2010, 10, 4646.

Abstract

CdSe/CdS semiconductor nanocrystal heterostructures are currently of high interest for the peculiar electronic structure offering unique optical properties. Here, we show that nanorods and tetrapods made of such material combination enable efficient multiexcitonic emission, when the volume of the nanoparticle is maximized. This condition is fulfilled by tetrapods with an arm length of 55 nm and results in a dual emission with comparable intensities from the CdS arms and CdSe core. The relative intensities of the dual emission, originating from exciton phase-space filling and reduced Auger recombination, can be effectively modulated by the photon fluence of the pump laser. The results, obtained under steady-state detection conditions, highlight the properties of tetrapods as multiexciton dual-color emitters.

126. Alkyl Chains of Surface Ligands Affect Polytypism of CdSe Nanocrystals and Play an Important Role in the Synthesis of Anisotropic Nanoheterostructures
Jing Huang, Maksym V. Kovalenko, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 15866.

Abstract

We show that the length of the alkyl chain of surface ligands can shift the equilibrium between the wurtzite and zinc blende polytypes of CdSe nanocrystals. In-situ wide-angle X-ray scattering measurements reveal that short-chain (e.g., propyl) phosphonic acids stabilize CdSe nanocrystals with the zinc blende phase whereas octadecylphosphonic acid stabilize nanocrystals with the wurtzite phase. We also demonstrate how this effect can be used to improve the shape selectivity in the synthesis of anisotropic CdSe/CdS and ZnSe/CdS nanoheterostructures.

125. Highly Monodisperse Bismuth Nanoparticles and Their Three-Dimensional Superlattices
Maskym Yarema,  Maksym V. Kovalenko, Guenter Hesser, Dmitri V. Talapin, and Wolfgang Heiss. J. Am. Chem. Soc. 2010, 132, 15158.

Abstract

A simple and reproducible synthesis of highly monodisperse and ligand-protected bismuth nanoparticles (Bi NPs) is reported. The size of the single-crystalline and spherically shaped NPs is controlled between 11 and 22 nm mainly by the reaction temperature. The high uniformity of the NPs allows their self-assembly into long-range-ordered two- and three-dimensional superstructures.

124. Nanocrystal Superlattices with Thermally Degradable Hybrid Inorganic-Organic Capping Ligands
Maksym V. Kovalenko, Maryna I. Bodnarchuk, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 15124.

Abstract

Colloidal metallic and semiconductor nanocrystals (NCs) functionalized with metal chalcogenide complexes (MCCs) have shown a promise for designing materials that combine high carrier mobility with the electronic structure of strongly quantum-confined solids. Here we report a simple and general methodology for switching the repulsive forces responsible for colloidal stabilization of MCC-capped NCs from long-range electrostatic to short-range steric through the formation of tight ionic pairs with cationic surfactants. This noncovalent surface modification remarkably improved the ability of MCC-capped NCs to self-assemble into long-range ordered superlattices. These NCs are highly soluble in nonpolar solvents and compatible with various technologically relevant organic molecules and polymers. The hybrid inorganic−organic coating can be thermally decomposed at significantly lower temperatures compared to those required for removal of conventional organic ligands.

123. Enhanced color conversion from colloidal CdSe/CdS dot/rods by vertical microcavities
H. Puehringer, J. Roither, Maksym V. Kovalenko, M. Eibelhuber, T. Schwarzl, Dmitri V. Talapin, and Wolfgang Heiss. Appl. Phys. Lett. 2010, 97, 111115.

Abstract

Colloidal CdSe/CdS dot/rods exhibit efficient photoluminescence from the spherical CdSedots at wavelengths well below the absorption edge of the rod material. This property makes dot/rods advantageous for color conversion applications, especially when they are embedded in optical microcavities to improve light extraction in forward direction. Here, surface emitting half-wavelength microcavities are demonstrated containing films of dot/rods as active material, exhibiting luminescence enhancement factors of up to 21 at the resonator wavelengths, whereas with conventional CdSe/ZnS core-shell nanocrystals only half of this value is obtained.

122. Probing the Surface of Transition-Metal Nanocrystals by Chemiluminesence
Galyna Krylova, Nada M. Dimitrijevic, Dmitri V. Talapin, Jeffrey R. Guest, Holger Borchert, Arun Lobo, Tijana Rajh, and Elena V. Shevchenko. J. Am. Chem. Soc. 2010, 132, 9102.

Abstract
We propose a simple chemiluminescence (CL) method for investigation of the surface of Co-based nanocrystals (NCs). Using a combination of CL and spin-trap electron paramagnetic resonance techniques, we systematically studied the generation of reactive oxygen species (ROS) at the surface of differently sized CoPt3 spherical NCs and CoPt3/Au nanodumbbells. We have shown that differently sized CoPt3 NCs can promote the formation of ROS and as a result can lead to the oxidation of luminol accompanied by the emission of the light. CL allows monitoring the stability of transition-metal-based NCs against oxidation and dissolution. We found by CL that cobalt ions slowly leach from the surface of CoPt3 NCs even under very mild conditions; however, the amount of the leached cobalt ions does not exceed the maximal concentration of cobalt at the NC surface indicating that only surface atoms can go into solution.

121. The Role of Order, Nanocrystal Size, and Capping Ligands in the Collective Mechanical Response of Three-Dimensional Nanocrystal Solids
Paul Podsiadlo, Galyna Krylova, Byeongdu Lee, Kevin Critchley, David J. Gosztola, Dmitri V. Talapin, Paul D. Ashby, and Elena V. Shevchenko, J. Am. Chem. Soc. 2010, 132, 8953.

Abstract
Chemically synthesized PbS, CdSe, and CoPt3 nanocrystals (NCs) were self-assembled into highly periodic supercrystals. Using the combination of small-angle X-ray scattering, X-ray photoelectron spectroscopy, infrared spectroscopy, thermogravimetric analysis, and nanoindentation, we correlated the mechanical properties of the supercrystals with the NC size, capping ligands, and degree of ordering. We found that such structures have elastic moduli and hardnesses in the range of ∼0.2−6 GPa and 10−450 MPa, respectively, which are analogous to strong polymers. The high degree of ordering characteristic to supercrystals was found to lead to more than 2-fold increase in hardnesses and elastic moduli due to tighter packing of the NCs, and smaller interparticle distance. The nature of surface ligands also significantly affects the mechanical properties of NCs solids. The experiments with series of 4.7, 7.1, and 13 nm PbS NCs revealed a direct relationship between the core size and hardness/modulus, analogous to the nanoparticle-filled polymer composites. This observation suggests that the matrices of organic ligands have properties similar to polymers. The effective moduli of the ligand matrices were calculated to be in the range of ∼0.1−0.7 GPa.

120. Energetic and Entropic Contributions to Self-Assembly of Binary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor
Maryna I. Bodnarchuk, Maksym V. Koyalenko, Wolfgang Heiss, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 11967.

Abstract
We studied the effect of temperature on self-assembly of monodisperse colloidal nanocrystals into single-component and binary superlattices. Temperature, which serves as a weighting factor for the internal energy (U) and entropy (S) contributions to the Helmholtz free energy F = U − TS, allows tailoring relative weights of the interparticle interactions and free-volume entropy during the formation of nanocrystal superlattices. Temperature also provides a convenient tool for directing self-assembly of nanocrystals toward desired superlattice structures. We found that temperature strongly affects the structures of binary superlattices self-assembled from the mixtures of CdSe + PbS nanocrystals and PbSe + Pd nanocrystals. In the former case, small Hamaker constants for CdSe and PbS nanocrystals led to a relatively simple phase diagram, including only high-density NaZn13–, AlB2–, and NaCl-type binary superlattices. In contrast, binary superlattices self-assembled at different temperatures from PbSe and Pd nanocrystals showed a number of low-density complex phases stabilized by strong local van der Waals interactions between Pd nanocrystals. The structural diversity of nanoparticle superlattices is shown to be a result of the cooperative effect of the entropy-driven crystallization and the interparticle interactions. Both ΔU and TΔS terms associated with the superlattice formation should be of the same order of magnitude, with |ΔU| < |TΔS| for the assembly of CdSe and PbS nanocrystals and |ΔU| > |TΔS| for the PbSe and Pd nanocrystals.

119. Spatio-temporal dynamics of coupled electrons and holes in nanosize CdSe-CdS semiconductor tetrapods
C. Mauser, E. Da Como, J. Baldauf, A. L. Rogach, J. Huang, D. V. Talapin, and J. Feldmann. Phys. Rev. B 2010, 82, 081306.

Abstract
We report on coupled electron-hole transfer, Coulomb drag, in CdSe/CdS semiconductor nanotetrapods. We demonstrate that photoexcited holes can either be transferred to the CdSe core or become trapped in one of the CdS arms. By combining time-resolved pump-probe and photoluminescence measurements we investigate how the Coulomb potential drags the electron to the hole localization site. As supported by effective-mass calculations taking into account Coulomb effects we conclude that the hole dynamics determines the fate of the electron in a coupled dynamics.

118. Expanding the Chemical Versatility of Colloidal Nanocrystals Capped with Molecular Metal Chalcogenide Ligands
Maksym V. Kovalenko, Maryna I. Bodnarchuk, Jana Zaumseil, Jong-Soo Lee, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 10085.

Abstract
We developed different strategies toward the synthesis of colloidal nanocrystals stabilized with molecular metal chalcogenide complexes (MCCs). Negatively charged MCCs, such as SnS44−, Sn2S64−, SnTe44−, AsS33−, MoS42−, can quantitatively replace the organic ligands at the nanocrystal surface and stabilize nanocrystal solutions in different polar media. We showed that all-inorganic nanocrystals composed of metals, semiconductors, or magnetic materials and capped with various MCC ligands can be synthesized using convenient and inexpensive chemicals and environmentally benign solvents such as water, formamide, or dimethylsulfoxide. The development of mild synthetic routes was found to be crucial for the design of highly luminescent all-inorganic nanocrystals, such as CdSe/ZnS and PbS capped with Sn2S64− MCCs, respectively. We also prepared conductive and luminescent layer-by-layer assemblies from inorganically capped colloidal nanocrystals and polyelectrolytes. In close-packed films of 5-nm Au nanocrystals stabilized with Na2Sn2S6 we observed very high electrical conductivities (>1000 S cm−1).

117. “Magnet-in-the-Semiconductor” FePt−PbS and FePt−PbSe Nanostructures: Magnetic Properties, Charge Transport, and Magnetoresistance
Jong-Soo Lee, Maryna I. Bodnarchuk, Elena V. Shevchenko, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 6382.

Abstract
We report a synthesis of colloidal nanostructures combining a magnetic material (FePt) with a narrow-gap semiconductor (PbS and PbSe) in form of core−shells or nanodumbbells and explore their optical, magnetic, electrical, and magnetotransport properties. The arrays of “magnet-in-the-semiconductor” nanostructures show semiconductor-type transport properties with magnetoresistance typical for magnetic tunnel junctions, thus combining the advantages of both functional components. We observed gate-controlled charge transport through the arrays of FePt−PbS and FePt−PbSe core−shell nanostructures with an electron mobility of 0.01 cm2/(V s) and 0.08 cm2/(V s), respectively, combined with ferro- and superparamagnetic behavior and large tunneling magnetoresistance. This work shows that multicomponent colloidal nanostructures can be used as the building blocks for design of multifunctional materials for electronics and optoelectronics.

116. Semiconductor Nanocrystals Functionalized with Antimony Telluride Zintl Ions for Nanostructrued Thermoelectrics
Maksym V. Kovalenko, Boris Spokoyny, Jong-Soo Lee, Marcus Scheele, Andrew Weber, Susanthri Perera, Daniel Landry, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 6686.

Abstract
The energy efficiency of heat engines could be improved by the partial recovery of waste heat using thermoelectric (TE) generators. We show the possibility of designing nanostructured TE materials using colloidal inorganic nanocrystals functionalized with molecular antimony telluride complexes belonging to the family of Zintl ions. The unique advantage of using Zintl ions as the nanocrystal surface ligands is the possibility to convert them into crystalline metal chalcogenides, thus linking individual nanobuilding blocks into a macroscopic assembly of electronically coupled functional modules. This approach allows preserving the benefits of nanostructuring and quantum confinement while enabling facile charge transport through the interparticle boundaries. A developed methodology was applied for solution-based fabrication of nanostructured n- and p-type Bi2−xSbxTe3 alloys with tunable composition and PbTe−Sb2Te3 nanocomposites with controlled grain size. Characterization of the TE properties of these materials showed that their Seebeck coefficients, electrical and thermal conductivities, and ZT values compared favorably with those of previously reported solution-processed TE materials.

115. Increased Color-Conversion Efficiency in Hybrid Light-Emitting Diodes utilizing Non-Radiative Energy Transfer
Soontorn Chanyawadee, Pavlos G. Lagoudakis, Richard T. Harley, Martin D. B. Charlton, Dmitri V. Talapin, Hong Wen Huang, and Chung-Hsiang Lin. Adv. Mater. 2010, 22, 602.

Abstract
An efficient hybrid color‐conversion light‐emitting device consisting of colloidal nanocrystal quantum dots (NQDs) and a surface‐patterned GaN‐based LED is demonstrated (see figure). Excitation in a surface‐patterned LED is efficiently transferred to NQD emitters via non‐radiative energy transfer. A twofold enhancement of the NQD emission is achieved.

114. Size-Dependent Multiple Twinning in Nanocrystal Superlattices
Sara M. Rupich, Elena V. Shevchenko, Maryna I. Bodnarchuk, Byeongdu Lee, and Dmitri V. Talapin. J. Am. Chem. Soc. 2010, 132, 289.

Abstract
We report a size-dependent change in the morphology of superlattices self-assembled from monodisperse colloidal PbS nanocrystals. Superlattices of large (>7 nm) PbS nanocrystals showed a strong tendency to form multiply twinned face-centered cubic superlattices with decahedral and icosahedral symmetry, exhibiting crystallographically forbidden five-fold symmetry elements. On the other hand, superlattices of small (<4 nm) PbS nanocrystals exhibited no twinning. To explain such a dramatic difference in the twinning probability, we showed that twinning energy in a nanocrystal superlattice is strongly size-dependent. In addition, the interparticle potentials acting during the self-assembly process are “softer” in the case of larger PbS nanocrystals, thus favoring the formation of multiply twinned superlattices. Our work introduces a new class of materials exhibiting multiple twinning, while offering flexibility in designing interparticle potentials.

113. Prospects of Nanocrystal Solids as Electronic and Optoelectronic Materials
Dmitri V. Talapin, Jong-Soo Lee, Maksym V. Kovalenko, and Elena V. Shevchenko. Chem. Rev. 2010, 110, 389. (Invited review, cover highlight)

Abstract
Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2−3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? 

2009

112. Comparison of Structural Behavior of Nanocrystals in Randomly Packed Films and Long-Range Ordered Superlattices by Time-Resolved Small Angle X-ray Scattering
Byeongdu Lee, Paul Podsiadlo, Sara Rupich, Dmitri V. Talapin, Tijana Rajh, and Elena V. Shevchenko. J. Am. Chem. Soc. 2009, 131, 16386.

Abstract

We evaluated the difference between randomly packed NCs (disordered films), periodic films, and three-dimensional crystals in terms of their lattice structure and interparticle spacing using time-resolved small-angle X-ray scattering (SAXS) technique. The work was performed on nanocrystal solids formed by 7 nm PbS nanocrystals capped with oleic acid. We have found that interparticle spacing in faceted three-dimensional crystals is ∼25% smaller as compared with three-dimensional films formed by solvent evaporation. We showed that interparticle spacing in faceted three-dimensional crystals is significantly smaller than the length of a fully extended molecule of oleic acid, and hence, full interdigitation of molecules from neighboring particle is doubtful. Also we demonstrated that postpreparative mild thermal treatment allows further manipulation of interparticle spacing.

111. Energetic disorder limits energy transfer in semiconductor nanocrystal–DNA–dye conjugates
Klaus Becker, Andrey L. Rogach, Jochen Feldmann, Dmitri V. Talapin, and John M. Lupton. App. Phys. Lett. 2009, 95, 143101.

Abstract

We demonstrate the influence of spectral linewidths of individual donor-acceptor couples on energy transfer efficiency in semiconductor nanocrystal–DNA–organic dye conjugates. Temperature-dependent single molecule and ensemble spectroscopy data are analyzedusing the Förster theory within the macroscopic and microscopic approaches. The results obtained evidence on the importance of the spectral overlap between emission of a single donor and absorption of a single acceptor in its close vicinity, which determines the microscopic resonance and transfer efficiency between individual neighbors. This realization poses important implications on the applicability of ensemble spectral overlap for the analysis of distance dependencies of nanoscopic objects.

110. Quasicrystalline order in self-assembled binary nanoparticle superlattices
Dmitri V. Talapin, Elena V. Shevchenko, Maryna I. Bodnarchuk, Xingchen Ye, Jun Chen, and Christopher B. Murray. Nature 2009, 461, 964.

Abstract

The discovery of quasicrystals in 1984 changed our view of ordered solids as periodic structures and introduced new long-range-ordered phases lacking any translational symmetry. Quasicrystals permit symmetry operations forbidden in classical crystallography, for example five-, eight-, ten- and 12-fold rotations, yet have sharp diffraction peaks. Intermetallic compounds have been observed to form both metastable and energetically stabilized quasicrystals; quasicrystalline order has also been reported for the tantalum telluride phase with an approximate Ta1.6Te composition. Later, quasicrystals were discovered in soft matter, namely supramolecular structures of organic dendrimers and tri-block copolymers, and micrometre-sized colloidal spheres have been arranged into quasicrystalline arrays by using intense laser beams that create quasi-periodic optical standing-wave patterns. Here we show that colloidal inorganic nanoparticles can self-assemble into binary aperiodic superlattices. We observe formation of assemblies with dodecagonal quasicrystalline order in different binary nanoparticle systems: 13.4-nm Fe2O3 and 5-nm Au nanocrystals, 12.6-nm Fe3O4 and 4.7-nm Au nanocrystals, and 9-nm PbS and 3-nm Pd nanocrystals. Such compositional flexibility indicates that the formation of quasicrystalline nanoparticle assemblies does not require a unique combination of interparticle interactions, but is a general sphere-packing phenomenon governed by the entropy and simple interparticle potentials. We also find that dodecagonal quasicrystalline superlattices can form low-defect interfaces with ordinary crystalline binary superlattices, using fragments of (33.42) Archimedean tiling as the ‘wetting layer’ between the periodic and aperiodic phases.

109. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands
Maksym V. Kovalenko, Marcus Scheele, and Dmitri V. Talapin. Science 2009, 324, 1417.

Abstract

Similar to the way that atoms bond to form molecules and crystalline structures, colloidal nanocrystals can be combined together to form larger assemblies. The properties of these structures are determined by the properties of individual nanocrystals and by their interactions. The insulating nature of organic ligands typically used in nanocrystal synthesis results in very poor interparticle coupling. We found that various molecular metal chalcogenide complexes can serve as convenient ligands for colloidal nanocrystals and nanowires. These ligands can be converted into semiconducting phases upon gentle heat treatment, generating inorganic nanocrystal solids. The utility of the inorganic ligands is demonstrated for model systems, including highly conductive arrays of gold nanocrystals capped with Sn2S64– ions and field-effect transistors on cadmium selenide nanocrystals.

 

108. Exciton–Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals
Michele Saba, Stefan Minniberger, Francesco Quochi, Juergen Roither, Marco Marceddu, Agnieszka Gocalinska, Maksym V. Kovalenko, Dmitri V. Talapin, Wolfgang Heiss, Andrea Mura, and Giovanni Bongiovanni. Adv. Mater. 2009, 21, 4942.

Abstract

Exciton–exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton–exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution‐processable lasers, with projected pump threshold densities below 1 kW cm−2 for continuous wave lasing.

107. Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanoscrystals
S. Wu, G. Han, D. Milliron, S. Aloni, V. Altoe, D. Talapin, B. Cohen, and P. J. Schuck. Proc. Nat. Acad. Sci. 2009, 106, 10917.

Abstract

The development of probes for single-molecule imaging has dramatically facilitated the study of individual molecules in cells and other complex environments. Single-molecule probes ideally exhibit good brightness, uninterrupted emission, resistance to photobleaching, and minimal spectral overlap with cellular autofluorescence. However, most single-molecule probes are imperfect in several of these aspects, and none have been shown to possess all of these characteristics. Here we show that individual lanthanide-doped upconverting nanoparticles (UCNPs)—specifically, hexagonal phase NaYF4 (β-NaYF4) nanocrystals with multiple Yb3+ and Er3+ dopants—emit bright anti-Stokes visible upconverted luminescence with exceptional photostability when excited by a 980-nm continuous wave laser. Individual UCNPs exhibit no on/off emission behavior, or “blinking,” down to the millisecond timescale, and no loss of intensity following an hour of continuous excitation. Amphiphilic polymer coatings permit the transfer of hydrophobic UCNPs into water, resulting in individual water-soluble nanoparticles with undiminished photophysical characteristics. These UCNPs are endocytosed by cells and show strong upconverted luminescence, with no measurable anti-Stokes background autofluorescence, suggesting that UCNPs are ideally suited for single-molecule imaging experiments.

106. Wavefunction Mapping of Immobilized InP Semiconductor Nanocrystals
G. Maruccio, C. Meyer, T. Matsui, D. V. Talapin, S. G. Hickey, H. Weller, and R. Wiesendanger. Small 2009, 5, 808.

Abstract
On the map: The electronic structure of InP nanocrystals (NCs) is studied and the symmetry of the squared wavefunctions (WFs) corresponding to the confined states in the spectral density is mapped directly (see picture). The WF maps show s‐ and p‐type character and charging energies that decrease when higher orbitals are occupied. Coupling of the NCs with the environment influences the WF energies and symmetries.

105. Photocurrent enhancement in hybrid nanocrystal quantum dot / p-i-n photovoltaic devices
S. Chanyawadee, R. T. Harley, M. Henini, D. V. Talapin, and P. G. Lagoudakis. Phys. Rev. Lett. 2009, 102, 077402.

Abstract
We fabricate a hybrid nanocrystal quantum-dot patterned p−i−n structure that utilizes nonradiative energy transfer from highly absorbing colloidal nanocrystal quantum dots to a patterned semiconductor slab to demonstrate a sixfold increase of the photocurrent conversion efficiency compared to the bare p−i−n semiconductor device.
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