Layer Control of WSe₂ <i>via</i> Selective Surface Layer Oxidation

We report Raman and photoluminescence spectra of mono- and few-layer WSe₂ and MoSe₂ taken before and after exposure to a remote oxygen plasma. For bilayer and trilayer WSe₂, we observe an increase in the photoluminescence intensity and a blue shift of the photoluminescence peak positions after oxygen plasma treatment. The photoluminescence spectra of trilayer WSe₂ exhibit features of a bilayer after oxygen plasma treatment. Bilayer WSe₂ exhibits features of a monolayer, and the photoluminescence of monolayer WSe₂ is completely absent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. The mechanism of the changes observed in the photoluminescence spectra of WSe₂ is due to the selective oxidation of the topmost layer. As a result, <i>N</i>-layer WSe₂ is reduced to <i>N</i>–1 layers. Raman spectra and AFM images taken from the WSe₂ flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygen radicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, we confirmed that the oxidation will only affect the topmost layer of the WSe₂ flakes. X-ray photoelectron spectroscopy shows that the surface layer WO_<i>x</i> of the sample can be removed by a quick dip in KOH solution. Therefore, this technique provides a promising way of controlling the thickness of WSe₂ layer by layer

Microscopic Study of Atomic Layer Deposition of TiO₂ on GaAs and Its Photocatalytic Application

We report a microscopic study of <i>p</i>-GaAs/TiO₂ heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H₂ evolution and CO₂ reduction of these heterostructures shows a very strong dependence on the thickness of the TiO₂ over the range of 0–15 nm. Thinner films (1–10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl₄ atomic layer deposition (ALD) precursor, which is corrosive. Ti³⁺ defect states (i.e., O vacancies) in the TiO₂ film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO₂ molecules bind stably to these Ti³⁺ states. Thicker TiO₂ films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO₂ films are conducting

Field-Dependent Orientation and Free Energy of D₂O at an Electrode Surface Observed via SFG Spectroscopy

Polarization-selected vibrational sum frequency generation (SFG) spectroscopy of D2O is used to obtain the orientation of the free OD bond at a monolayer graphene electrode. We modulate the interfacial field by varying the applied electrochemical potential, and we measure the resulting change in the orientation. A hyperpolarizability model is used for the orientational analysis, which assumes a quadratic free energy orienting potential in the absence of the field, whose minimum and curvature determine the average tilt angle and the Gaussian width of the orientational distribution. The average free OD tilt angle changes in an approximately linear fashion with the applied field, from 46° from normal at −0.9 V vs Ag/AgCl (E = −0.02 V/Å) to 32° at −3.9 V vs Ag/AgCl (E = −0.17 V/Å). Using this approach, we map the free energy profile for the molecular orientation of interfacial water by measuring the reversible response to an external perturbation, i.e., a torque applied by an electric field acting on the molecule’s permanent dipole moment. This allows us to extract the curvature of the free energy orienting potential of interfacial water, which is (4.0 ± 0.8) × 10–20 J/rad2 (or 0.25 ± 0.05 eV/rad2 )

Direct <i>In Situ</i> Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO₂‑Protected GaP Photocathodes

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system

Confined Liquid-Phase Growth of Crystalline Compound Semiconductors on Any Substrate

The growth of crystalline compound semiconductors on amorphous and non-epitaxial substrates is a fundamental challenge for state-of-the-art thin-film epitaxial growth techniques. Direct growth of materials on technologically relevant amorphous surfaces, such as nitrides or oxides results in nanocrystalline thin films or nanowire-type structures, preventing growth and integration of high-performance devices and circuits on these surfaces. Here, we show crystalline compound semiconductors grown directly on technologically relevant amorphous and non-epitaxial substrates in geometries compatible with standard microfabrication technology. Furthermore, by removing the traditional epitaxial constraint, we demonstrate an <i>atomically sharp lateral heterojunction</i> between indium phosphide and tin phosphide, two materials with vastly different crystal structures, a structure that cannot be grown with standard vapor-phase growth approaches. Critically, this approach enables the growth and manufacturing of crystalline materials without requiring a nearly lattice-matched substrate, potentially impacting a wide range of fields, including electronics, photonics, and energy devices