Byung-Ryool Hyun

Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles

Injection of photoexcited electrons from colloidal PbS quantum dots into TiO(2) nanoparticles is investigated. The electron affinity and ionization potential of PbS quantum dots, inferred from cyclic voltammetry measurements, show strong size dependence due to quantum confinement. On the basis of the measured energy levels, photoexcited electrons should transfer efficiently from the quantum dots into TiO(2) only for quantum-dot diameter below approximately 4.3 nm. Continuous-wave fluorescence spectra and fluorescence transients of PbS quantum dots coupled to titanium dioxide nanoparticles are consistent with electron transfer for small quantum dots. The measured electron transfer time is surprisingly slow ( approximately 100 ns), and implications of this for future photovoltaics will be discussed. Initial results obtained from solar cells sensitized with PbS quantum dots are presented.

Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.

Electrogenerated Chemiluminescence from PbS Quantum Dots

We report the first observation of electrogenerated chemiluminescence (ECL) from PbS quantum dots (QDs). Different ECL intensities are observed for different ligands used to passivate the QDs, which indicates that ECL is sensitive to surface chemistry, with the potential to serve as a powerful probe of surface states and charge transfer dynamics in QDs. In particular, passivation of the QD surfaces with trioctylphosphine (TOP) increases ECL intensity by 3 orders of magnitude when compared to passivation with oleic acid alone. The observed overlap of the ECL and photoluminescence spectra suggests a significant reduction of deep surface trap states from the QDs passivated with TOP.

Control of Electron Transfer from Lead-Salt Nanocrystals to TiO2

The roles of solvent reorganization energy and electronic coupling strength on the transfer of photoexcited electrons from PbS nanocrystals to TiO(2) nanoparticles are investigated. We find that the electron transfer depends only weakly on the solvent, in contrast to the strong dependence in the nanocrystal-molecule system. This is ascribed to the larger size of the acceptor in this system, and is accounted for by Marcus theory. The electronic coupling of the PbS and TiO(2) is varied by changing the length, aliphatic and aromatic structure, and anchor groups of the linker molecules. Shorter linker molecules consistently lead to faster electron transfer. Surprisingly, linker molecules of the same length but distinct chemical structures yield similar electron transfer rates. In contrast, the electron transfer rate can vary dramatically with different anchor groups.

A Generic Method for Rational Scalable Synthesis of Monodisperse Metal Sulfide Nanocrystals

A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic nonpolar solutions by using (NH(4))(2)S as a sulfide precursor. (NH(4))(2)S is stabilized in an organic primary amine solution and exhibits high reactivity toward metal complexes. This novel technique exhibits wide applicability for organic phase metal sulfide NC synthesis: a large variety of monodisperse NCs have been synthesized, including Cu(2)S, CdS, SnS, ZnS, MnS, Ag(2)S, and Bi(2)S(3). The stoichiometric reactions between (NH(4))(2)S and metal salts afford high conversion yields, and large-scale production of monodisperse NCs (more than 30 g) can be synthesized in a single reaction. The high reactivity of (NH(4))(2)S enables low temperature (<100 °C) syntheses, and the air-stable materials (such as CdS NCs) can be produced in air. Moreover, this low-temperature technique can be used to produce small size NCs which are difficult to be synthesized by the conventional high temperature methods, such as sub-5 nm Ag(2)S and Bi(2)S(3) quantum dots.

Role of Solvent Dielectric Properties on Charge Transfer from PbS Nanocrystals to Molecules

Transfer of photoexcited charge from PbS nanocrystals to ligand molecules is investigated in different solvents. We find that the charge transfer rate increases dramatically with solvent dielectric constant. This trend is accounted for by a modified Marcus theory that incorporates only static dielectric effects. The choice of solvent allows significant control of the charge transfer process. As an important example, we find that PbS nanocrystals dispersed in water exhibit charge transfer rates 1000 times higher than the same nanocrystals in organic solvent. Rapid charge extraction will be important to efficient nanocrystal-based photovoltaic and photodetector devices.

Lead-salt quantum-dot ionic liquids.

The electronic energies of lead–salt quantum dots (QDs) are determined primarily by quantum confinement due to their large exciton Bohr radii. The fundamental electronic structure of the QDs (PbS and PbSe) has been worked out by Kang et al., and now research with these materials is turning towards applications. For instance, lead–salt QDs have been used as active materials in photovoltaic devices due to their size-tunable infrared (IR) absorption. They are also efficient IR emitters and could be used in biomedical imaging and in electroluminescent devices. In order for QDs to realize their full potential, their stability (e.g., photostability) and compatibility with other materials must be improved. Accordingly, much effort is devoted to surface passivation and functionalization of QDs, with increasing attention being paid to the use of ionic liquids to passivate the QD surface. Using certain ionic liquid ligands, solid materials can be transferred to a new state that exhibits liquidlike behavior at room temperature. To date, metal nanoparticles and oxide nanoparticles have been functionalized using a polymer ionic liquid. Some semiconductor nanoparticles (e.g., CdSe) functionalized using small-molecule ionic ligands have been reported. In this work, we report the first lead–salt (PbS, PbSe, and PbTe) QD ionic liquid where polymer ionic liquid ligands are used as capping ligands for QDs. The resulting amphiphilic QD ionic liquids exhibit fluidlike behavior at room temperature, even in the absence of solvents. The ionic liquid capping ligands also dramatically improve the photostability of

Single molecule and single cell epigenomics

Dynamically regulated changes in chromatin states are vital for normal development and can produce disease when they go awry. Accordingly, much effort has been devoted to characterizing these states under normal and pathological conditions. Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is the most widely used method to characterize where in the genome transcription factors, modified histones, modified nucleotides and chromatin binding proteins are found; bisulfite sequencing (BS-seq) and its variants are commonly used to characterize the locations of DNA modifications. Though very powerful, these methods are not without limitations. Notably, they are best at characterizing one chromatin feature at a time, yet chromatin features arise and function in combination. Investigators commonly superimpose separate ChIP-seq or BS-seq datasets, and then infer where chromatin features are found together. While these inferences might be correct, they can be misleading when the chromatin source has distinct cell types, or when a given cell type exhibits any cell to cell variation in chromatin state. These ambiguities can be eliminated by robust methods that directly characterize the existence and genomic locations of combinations of chromatin features in very small inputs of cells or ideally, single cells. Here we review single molecule epigenomic methods under development to overcome these limitations, the technical challenges associated with single molecule methods and their potential application to single cells.

Heterojunction PbS nanocrystal solar cells with oxide charge-transport layers.

Oxides are commonly employed as electron-transport layers in optoelectronic devices based on semiconductor nanocrystals, but are relatively rare as hole-transport layers. We report studies of NiO hole-transport layers in PbS nanocrystal photovoltaic structures. Transient fluorescence experiments are used to verify the relevant energy levels for hole transfer. On the basis of these results, planar heterojunction devices with ZnO as the photoanode and NiO as the photocathode were fabricated and characterized. Solution-processed devices were used to systematically study the dependence on nanocrystal size and achieve conversion efficiency as high as 2.5%. Optical modeling indicates that optimum performance should be obtained with thinner oxide layers than can be produced reliably by solution casting. Room-temperature sputtering allows deposition of oxide layers as thin as 10 nm, which enables optimization of device performance with respect to the thickness of the charge-transport layers. The best devices achieve an open-circuit voltage of 0.72 V and efficiency of 5.3% while eliminating most organic material from the structure and being compatible with tandem structures.

PbS quantum dot photoluminescence quenching induced by an applied bias

We demonstrate that a DC voltage applied on a PbS quantum dot embedded organic light emitting diode can partially quench the quantum dot photoluminescence and propose a way to avoid this quenching.