Improving Photocatalytic Hydrogen Generation via Polycitric Acid-based Carbon Nanodots.

Bottom-up syntheses of carbon nanodots (CND) using solvothermal treatment of citric acid are known to afford nanometer-sized, amorphous polycitric acid-based materials. The addition of suitable co-reactants in the form of in-situ synthesized N-hetero-π-conjugated chromophores facilitates hereby the overall functionalization. Our incentive was to design a CND model that features phenazine (P-CND) - a well-known N-hetero-π-conjugated chromophore - to investigate the influence of the CND matrix on its redox chemistry as well as photochemistry.

Transparent TiO nanotubes supporting silver sulfide for photoelectrochemical water splitting.

Differences between photoelectrochemical and electrochemical activity were thoroughly investigated for the oxygen evolution reaction mediated by AgS deposited on two types of ordered titania substrates. Titanium dioxide nanotubes were fabricated by anodization of magnetron sputtered Ti films on ITO-coated glass substrates or directly from Ti foil. Further, AgS deposition on the nanotubes was carried out using successive ionic layer adsorption and reaction, known as SILAR, with 5, 25, and 45 cycles performed.

Monolithic Two-Terminal Tandem Solar Cells Using SbS and Solution-Processed PbS Quantum Dots Achieving an Open-Circuit Potential beyond 1.1 V.

Multijunction solar cells have the prospect of a greater theoretical efficiency limit than single-junction solar cells by minimizing the transmissive and thermalization losses a single absorber material has. In solar cell applications, SbS is considered an attractive absorber due to its elemental abundance, stability, and high absorption coefficient in the visible range of the solar spectrum, yet with a band gap of 1.7 eV, it is transmissive for near-IR and IR photons.

Understanding the Visible Absorption of Electron Accepting and Donating CNDs.

Carbon nanodots (CNDs) synthesized from citric acid and formyl derivatives, that is, formamide, urea, or N-methylformamide, stand out through their broad-range visible-light absorbance and extraordinary photostability. Despite their potential, their use has thus far been limited to imaging research. This work has now investigated the link between CNDs' photochemical properties and their chemical structure.

Probing Excitons in Ultrathin PbS Nanoplatelets with Enhanced Near-Infrared Emission.

Colloidal PbS nanoplatelets (NPLs) are highly interesting materials for near-infrared optoelectronic applications. We use ultrafast transient optical absorption spectroscopy to study the characteristics and dynamics of photoexcited excitons in ultrathin PbS NPLs with a cubic crystal structure. NPLs are synthesized at near room temperature from lead oleate and thiourea precursors; they show an optical absorption onset at 680 nm (1.

Engineering the Band Alignment in QD Heterojunction Films via Ligand Exchange.

Colloidal quantum dots (QDs) allow great flexibility in the design of optoelectronic devices, thanks to their size-dependent optical and electronic properties and the possibility to fabricate thin films with solution-based processing. In particular, in QD-based heterojunctions, the band gap of both components can be controlled by varying the size of the QDs. However, control over the band alignment between the two materials is required to tune the dynamics of carrier transfer across a heterostructure.

Controlling Superstructure-Property Relationships via Critical Casimir Assembly of Quantum Dots.

The assembly of colloidal quantum dots (QDs) into dense superstructures holds great promise for the development of novel optoelectronic devices. Several assembly techniques have been explored; however, achieving direct and precise control over the interparticle potential that controls the assembly has proven to be challenging. Here, we exploit the application of critical Casimir forces to drive the growth of QDs into superstructures.

Highly Photoconductive InP Quantum Dots Films and Solar Cells.

InP and InZnP colloidal quantum dots (QDs) are promising materials for application in light-emitting devices, transistors, photovoltaics, and photocatalytic cells. In addition to possessing an appropriate bandgap, high absorption coefficient, and high bulk carrier mobilities, the intrinsic toxicity of InP and InZnP is much lower than for competing QDs that contain Cd or Pb-providing a potentially safer commercial product. However, compared to other colloidal QDs, InP QDs remain sparsely used in devices and their electronic transport properties are largely unexplored.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts.

Enhanced Multiple Exciton Generation in PbS|CdS Janus-like Heterostructured Nanocrystals.

Generating multiple excitons by a single high-energy photon is a promising third-generation solar energy conversion strategy. We demonstrate that multiple exciton generation (MEG) in PbS|CdS Janus-like heteronanostructures is enhanced over that of single-component and core/shell nanocrystal architectures, with an onset close to two times the PbS band gap. We attribute the enhanced MEG to the asymmetric nature of the heteronanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate.

Hot-electron transfer in quantum-dot heterojunction films.

Thermalization losses limit the photon-to-power conversion of solar cells at the high-energy side of the solar spectrum, as electrons quickly lose their energy relaxing to the band edge. Hot-electron transfer could reduce these losses. Here, we demonstrate fast and efficient hot-electron transfer between lead selenide and cadmium selenide quantum dots assembled in a quantum-dot heterojunction solid.

Selective antimony reduction initiating the nucleation and growth of InSb quantum dots.

Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties, but widespread experimentation with InSb QDs is lacking due to the difficulty in synthesizing this material. The key experimental challenge in fabricating InSb QDs is preparing a suitable Sb-precursor in the correct oxidation state that reacts with the In-precursor in a controllable manner. Here, we review and discuss the synthetic strategies for making colloidal InSb QDs and present a new reaction scheme yielding small (∼1 nm diameter) InSb QDs.

Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals.

Carrier multiplication is a process in which one absorbed photon excites two or more electrons. This is of great promise to increase the efficiency of photovoltaic devices. Until now, the factors that determine the onset energy of carrier multiplication have not been convincingly explained.

Repairing Nanoparticle Surface Defects.

Solar devices based on semiconductor nanoparticles require the use of conductive ligands; however, replacing the native, insulating ligands with conductive metal chalcogenide complexes introduces structural defects within the crystalline nanostructure that act as traps for charge carriers. We utilized atomically thin semiconductor nanoplatelets as a convenient platform for studying, both microscopically and spectroscopically, the development of defects during ligand exchange with the conductive ligands Na SnS and (NH ) Sn S . These defects can be repaired via mild chemical or thermal routes, through the addition of L-type ligands or wet annealing, respectively.

Tandem Solar Cells from Solution-Processed CdTe and PbS Quantum Dots Using a ZnTe-ZnO Tunnel Junction.

We developed a monolithic CdTe-PbS tandem solar cell architecture in which both the CdTe and PbS absorber layers are solution-processed from nanocrystal inks. Due to their tunable nature, PbS quantum dots (QDs), with a controllable band gap between 0.4 and ∼1.

Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te(2-) Ligands Lead To Charge Carrier Diffusion Lengths Over 2 μm.

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 cm(2)/(V s)).

Comparison of Recombination Dynamics in CH3NH3PbBr3 and CH3NH3PbI3 Perovskite Films: Influence of Exciton Binding Energy.

Understanding carrier recombination in semiconductors is a critical component when developing practical applications. Here we measure and compare the monomolecular, bimolecular, and trimolecular (Auger) recombination rate constants of CH3NH3PbBr3 and CH3NH3PbI3. The monomolecular and bimolecular recombination rate constants for both samples are limited by trap-assisted recombination.

Synthetic Conditions for High-Accuracy Size Control of PbS Quantum Dots.

Decreasing the variability in quantum dot (QD) syntheses is desirable for better uniformity of samples for use in QD-based studies and applications. Here we report a highly reproducible linear relationship between the concentration of ligand (in this case oleic acid, OA) and the lowest energy exciton peak position (nm) of the resulting PbS QDs for various hot-injection temperatures. Thus, for a given injection temperature, the size of the PbS QD product is purely controlled by the amount of OA.

Preparation of Cd/Pb Chalcogenide Heterostructured Janus Particles via Controllable Cation Exchange.

We developed a strategy for producing quasi-spherical nanocrystals of anisotropic heterostructures of Cd/Pb chalcogenides. The nanostructures are fabricated via a controlled cation exchange reaction where the Cd(2+) cation is exchanged for the Pb(2+) cation. The cation exchange reaction is thermally activated and can be controlled by adjusting the reaction temperature or time.

Metal halide solid-state surface treatment for high efficiency PbS and PbSe QD solar cells.

We developed a layer-by-layer method of preparing PbE (E = S or Se) quantum dot (QD) solar cells using metal halide (PbI2, PbCl2, CdI2, or CdCl2) salts dissolved in dimethylformamide to displace oleate surface ligands and form conductive QD solids. The resulting QD solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. We find that the PbI2 treatment is the most successful in removing alkyl surface ligands and also replaces most surface bound Cl(-) with I(-).

Nanocrystal grain growth and device architectures for high-efficiency CdTe ink-based photovoltaics.

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.

High efficiency solution processed sintered CdTe nanocrystal solar cells: the role of interfaces.

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.

Coherent exciton delocalization in strongly coupled quantum dot arrays.

Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron-hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter.

Role of dopants in long-range charge carrier transport for p-type and n-type graphene transparent conducting thin films.

Monolayer to few-layer graphene thin films have several attractive properties such as high transparency, exceptional electronic transport, mechanical durability, and environmental stability, which are required in transparent conducting electrodes (TCs). The successful incorporation of graphene TCs into demanding applications such as thin film photovoltaics requires a detailed understanding of the factors controlling long-range charge transport. In this study, we use spectroscopic and electrical transport measurements to provide a self-consistent understanding of the macroscopic (centimeter, many-grain scale) transport properties of chemically doped p-type and n-type graphene TCs.