50 research outputs found
Photoelectrochemistry of Semiconductor Nanowire Arrays
This project supported research on the growth and photoelectrochemical characterization of semiconductor nanowire arrays, and on the development of catalytic materials for visible light water splitting to produce hydrogen and oxygen. Silicon nanowires were grown in the pores of anodic aluminum oxide films by the vapor-liquid-solid technique and were characterized electrochemically. Because adventitious doping from the membrane led to high dark currents, silicon nanowire arrays were then grown on silicon substrates. The dependence of the dark current and photovoltage on preparation techniques, wire diameter, and defect density was studied for both p-silicon and p-indium phosphide nanowire arrays. The open circuit photovoltage of liquid junction cells increased with increasing wire diameter, reaching 350 mV for micron-diameter silicon wires. Liquid junction and radial p-n junction solar cells were fabricated from silicon nano- and microwire arrays and tested. Iridium oxide cluster catalysts stabilized by bidentate malonate and succinate ligands were also made and studied for the water oxidation reaction. Highlights of this project included the first papers on silicon and indium phosphide nanowire solar cells, and a new procedure for making ligand-stabilized water oxidation catalysts that can be covalently linked to molecular photosensitizers or electrode surfaces
Non-Volatile Control of Valley Polarized Emission in 2D WSe2-AlScN Heterostructures
Achieving robust and electrically controlled valley polarization in monolayer
transition metal dichalcogenides (ML-TMDs) is a frontier challenge for
realistic valleytronic applications. Theoretical investigations show that
integration of 2D materials with ferroelectrics is a promising strategy;
however, its experimental demonstration has remained elusive. Here, we
fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and a
AlScN ferroelectric dielectric, and experimentally demonstrate efficient tuning
as well as non-volatile control of valley polarization. We measured a large
array of transistors and obtained a maximum valley polarization of ~27% at 80 K
with stable retention up to 5400 secs. The enhancement in the valley
polarization was ascribed to the efficient exciton-to-trion (X-T) conversion
and its coupling with an out-of-plane electric field, viz. the quantum-confined
Stark effect. This changes the valley depolarization pathway from strong
exchange interactions to slow spin-flip intervalley scattering. Our research
demonstrates a promising approach for achieving non-volatile control over
valley polarization and suggests new design principles for practical
valleytronic devices.Comment: Manuscript (22 pages and 5 figures), supporting informatio
Light-driven C-H bond activation mediated by 2D transition metal dichalcogenides
C-H bond activation enables the facile synthesis of new chemicals. While C-H
activation in short-chain alkanes has been widely investigated, it remains
largely unexplored for long-chain organic molecules. Here, we report
light-driven C-H activation in complex organic materials mediated by 2D
transition metal dichalcogenides (TMDCs) and the resultant solid-state
synthesis of luminescent carbon dots in a spatially-resolved fashion. We
unravel the efficient H adsorption and a lowered energy barrier of C-C coupling
mediated by 2D TMDCs to promote C-H activation. Our results shed light on 2D
materials for C-H activation in organic compounds for applications in organic
chemistry, environmental remediation, and photonic materials
Exciton Confinement in Two-Dimensional, In-Plane, Quantum Heterostructures
Two-dimensional (2D) semiconductors are promising candidates for
optoelectronic application and quantum information processes due to their
inherent out-of-plane 2D confinement. In addition, they offer the possibility
of achieving low-dimensional in-plane exciton confinement, similar to
zero-dimensional quantum dots, with intriguing optical and electronic
properties via strain or composition engineering. However, realizing such
laterally confined 2D monolayers and systematically controlling size-dependent
optical properties remain significant challenges. Here, we report the
observation of lateral confinement of excitons in epitaxially grown in-plane
MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2
monolayer film via a sequential epitaxial growth process. Various optical
spectroscopy techniques reveal the size-dependent exciton confinement in the
MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low
temperature as compared to continuous monolayer MoSe2. Finally, single-photon
emission was also observed from the smallest dots at 1.6 K. Our study opens the
door to compositionally engineered, tunable, in-plane quantum light sources in
2D semiconductors.Comment: Main Manuscript: 29 pages, 4 figures Supplementary Information: 14
pages, 12 figure
In situ epitaxial MgB2 thin films for superconducting electronics
A thin film technology compatible with multilayer device fabrication is
critical for exploring the potential of the 39-K superconductor magnesium
diboride for superconducting electronics. Using a Hybrid Physical-Chemical
Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure
necessary to keep the MgB phase thermodynamically stable can be achieved
for the {\it in situ} growth of MgB thin films. The films grow epitaxially
on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like of
39 K, a (4.2K) of A/cm in zero field, and a
of 29.2 T in parallel magnetic field. The surface is smooth with a
root-mean-square roughness of 2.5 nm for MgB films on SiC. This deposition
method opens tremendous opportunities for superconducting electronics using
MgB