Doh C. Lee

Colloidal Spherical Quantum Wells with Near-Unity Photoluminescence Quantum Yield and Suppressed Blinking

Thick inorganic shells endow colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell heterostructure NCs in applications has been limited due to the low photoluminescence quantum yield (PL QY ≤ 60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits ∼100% PL QY for SQW NCs with a thick CdS shell (≥5 nm). High PL QY of thick-shell SQW NCs is preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state lightings and luminescent solar concentrators.

Multifunctional Dendrimer Ligands for High-Efficiency, Solution-Processed Quantum Dot Light-Emitting Diodes

We present multifunctional dendrimer ligands that serve as the charge injection controlling layer as well as the adhesive layer at the interfaces between quantum dots (QDs) and the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs). Specifically, we use primary amine-functionalized dendrimer ligands (e.g., a series of poly(amidoamine) dendrimers (PADs, also referred to PAMAM)) that bind to the surface of QDs by replacing the native ligands (oleic acids) and also to the surface of ZnO ETL. PAD ligands control the electron injection rate from ZnO ETL into QDs by altering the electronic energy levels of the surface of ZnO ETL and thereby improve the charge balance within QDs in devices, leading to the enhancement of the device efficiency. As an ultimate achievement, the device efficiency (peak external quantum efficiency) improves by a factor of 3 by replacing the native ligands (3.86%) with PAD ligands (11.36%). In addition, multibranched dendrimer ligands keep the QD emissive layer intact during subsequent solution processing, enabling us to accomplish solution-processed QLEDs. The approach and results in the present study emphasize the importance of controlling the ligands of QDs to enhance QLED performance and also offer simple yet effective chemical mean toward all-solution-processed QLEDs.

Quantum Dot/Siloxane Composite Film Exceptionally Stable against Oxidation under Heat and Moisture

We report on the fabrication of a siloxane-encapsulated quantum dot (QD) film (QD-silox film), which exhibits stable emission intensity for over 1 month even at elevated temperature and humidity. QD-silox films are solidified via free radical addition reaction between oligosiloxane resin and ligand molecules on QDs. We prepare the QD-oligosiloxane resin by sol-gel condensation reaction of silane precursors with QDs blended in the precursor solution, forgoing ligand-exchange of QDs. The resulting QD-oligosiloxane resin remains optically clear after 40 days of storage, in contrast to other QD-containing resins which turn turbid and ultimately form sediments. QDs also disperse uniformly in the QD-silox film, whose photoluminescence (PL) quantum yield (QY) remains nearly unaltered under harsh conditions; for example, 85 °C/5% relative humidity (RH), 85 °C/85% RH, strongly acidic, and strongly basic environments for 40 days. The QD-silox film appears to remain equally emissive even after being immersed into boiling water (100 °C). Interestingly, the PL QY of the QD-silox film noticeably increases when the film is exposed to a moist environment, which opens a new, facile avenue to curing dimmed QD-containing films. Given its excellent stability, we envision that the QD-silox film is best suited in display applications, particularly as a PL-type down-conversion layer.

Slow colloidal growth of PbSe nanocrystals for facile morphology and size control

We report colloidal growth of PbSe nanosheets and finely size-tuned PbSe nanocrystals (NCs) via simple control of reaction parameters. The approach involves slow injection of precursors with excess amounts of oleic acid. Retarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, causes attachment of PbSe NCs through the (110) planes, which are more reactive than the (100) facet, into a two-dimensional geometry. In contrast, such attachment processes can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs dispersed in chloroform. For instance, the slow injection of Pb and Se precursors into a reaction solution containing Cd chalcogenide NCs results in the growth of spherical PbSe NCs, as the Cd chalcogenide NCs hinder the PbSe nuclei from merging via (110) planes. Compared to conventional rapid-injection methods, PbSe NCs grow slowly, which enables fine control of NC size. Ab initio calculations suggest that Cd precursors strongly bound on the surface of PbSe NCs may impede nanosheet formation and may slow PbSe NC growth.

Fabrication of high quantum yield quantum dot/polymer films by enhancing dispersion of quantum dots using silica particles

We have fabricated quantum dot (QD)/polymer films of high quantum yield by coating silica particles with quantum dots. When particles were dispersed in tetrahydrofuran, free QD suspension exhibited higher quantum yield than QD-coated silica particles. Scattering is a most likely reason for the drop in quantum yield for the QD-coated silica particles, as supported by results of silica particles with varying morphologies: for example, QD-coated hollow silica particles showed higher quantum yield than filled silica particles, as the hollowness gave rise to reduced scattering. In the QD/polymer films, however, QD-coated filled/hollow silica particles showed significant enhancement in quantum yield (i.e., up to 2.4 times higher than that of free QDs). Confocal microscopy revealed that the enhanced quantum yield likely results from improved dispersion of QD-coated silica particles. In addition, the quantum yield of QD-coated hollow silica particles in films was lower than that of filled particles because of lower structural stability. Introducing silica (either filled or hollow) particles prevents spectral redshift of emission peak when prepared in the form of film, as opposed to QD-only sample. Our findings point to the possibility that QD-coated filled/hollow silica particles exhibit superior stability, quantum efficiency, and color accuracy, which render them potentially useful for the next-generation light-emitting devices and photovoltaics.

A centrifuge-based stepwise chemical loading disc for the production of multiplex anisotropic metallic nanoparticles

We have proposed a novel rotary microdevice in which multiplex anisotropic metallic nanoparticles (NPs) can be synthesized under diverse conditions in a high-throughput manner. In this study, by tuning the concentration of ascorbic acid (AA) as a control solution, the shape evolution from hexagon to tripod of gold nanoparticles (Au NPs) was achieved.

Tetrapod CdSe-sensitized macroporous inverse opal electrodes for photo-electrochemical applications

Quantum dots (QDs) possess promising characteristics that are important to light harvesting, but their mesoscale size limits their application in the direct sensitization of TiO2 porous films for photo-electrochemical cells (PECs). Here, inverse opal (IO) TiO2 structures were sensitized by tetrapod-CdSe (tp-CdSe) QDs, which were used in visible-light PECs. Because of the interconnected macropores in the IO structure, tp-CdSe penetrated the entire film and deposited on its surface. In contrast, infiltration was limited to the surface of the conventional mesoporous TiO2 film. The amount of tp-CdSe deposited was dependent on the immersion time of the film in the tp-CdSe dispersion. Optimum deposition of tp-CdSe was observed at the highest photocurrent density. Light harvesting, thus the photocurrent, increased with increasing amounts of deposit, but there was a corresponding decrease in electron lifetime. The maximum photocurrent density per Cd mass was 0.474 mA cm−2, which is greater than previous results from experiments using QD-sensitized PECs. We thus believe that a combination of tetrapod QDs and the TiO2 IO film may provide a new platform for PEC electrodes.

Expanding depletion region via doping: Zn-doped Cu2O buffer layer in Cu2O photocathodes for photoelectrochemical water splitting

We report photoelectrochemical hydrogen evolution reaction using a Cu2O-based photocathode with a layer doped with Zn ions. The doping results in the shift of the onset flat-band potential of the photocathode, likely a consequence of maximized band-bending in the Cu2O/Zn : Cu2O heterojunction. Systematic electrochemical analysis reveals that expansion of depletion region is responsible for the enhanced photoelectrochemical performance, e.g., the increase of photocurrent and reduced internal resistance.

Bi2O3 as a Promoter for Cu/TiO2 Photocatalysts for the Selective Conversion of Carbon Dioxide into Methane

A significantly enhanced gas‐phase photocatalytic conversion of carbon dioxide into methane on Cu/TiO2 nanoparticles upon introducing Bi2O3 as a promoter in the vicinity of Cu was observed. The maximum rate of CH4 generation of 11.90 μmol g−1 h−1 recorded in the case of Cu‐Bi2O3/TiO2 is approximately one order of magnitude higher than that obtained with Cu/TiO2 nanoparticles. The enhanced performance was attributed to facilitated migration of CO* from Cu to the Bi2O3 surface.

Continuous Purification of Colloidal Quantum Dots in Large-Scale Using Porous Electrodes in Flow Channel

Colloidal quantum dots (QDs) afford huge potential in numerous applications owing to their excellent optical and electronic properties. After the synthesis of QDs, separating QDs from unreacted impurities in large scale is one of the biggest issues to achieve scalable and high performance optoelectronic applications. Thus far, however, continuous purification method, which is essential for mass production, has rarely been reported. In this study, we developed a new continuous purification process that is suitable to the mass production of high-quality QDs. As-synthesized QDs are driven by electrophoresis in a flow channel and captured by porous electrodes and finally separated from the unreacted impurities. Nuclear magnetic resonance and ultraviolet/visible/near-infrared absorption spectroscopic data clearly showed that the impurities were efficiently removed from QDs with the purification yield, defined as the ratio of the mass of purified QDs to that of QDs in the crude solution, up to 87%. Also, we could successfully predict the purification yield depending on purification conditions with a simple theoretical model. The proposed large-scale purification process could be an important cornerstone for the mass production and industrial use of high-quality QDs.