Haotian Shi

Enhanced Photocatalytic Reduction of CO2 to CO through TiO2 Passivation of InP in Ionic Liquids

A robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18×) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn-junction. Photoelectrochemical reactions were carried out in a nonaqueous solution consisting of ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4), dissolved in acetonitrile, which enables CO2 reduction with a Faradaic efficiency of 99% at an underpotential of +0.78 V. While the photocatalytic yield increases with the addition of the TiO2 layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause an increase in the electron-hole pair recombination rate. NMR spectra show that the [EMIM](+) ions in solution form an intermediate complex with CO2(-), thus lowering the energy barrier of this reaction.

Layer Control of WSe2 via 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, N-layer WSe2 is reduced to N-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 WOx 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.

Cross-plane Thermoelectric and Thermionic Transport across Au/ h -BN/Graphene Heterostructures

The thermoelectric voltage generated at an atomically abrupt interface has not been studied exclusively because of the lack of established measurement tools and techniques. Atomically thin 2D materials provide an excellent platform for studying the thermoelectric transport at these interfaces. Here, we report a novel technique and device structure to probe the thermoelectric transport across Au/h-BN/graphene heterostructures. An indium tin oxide (ITO) transparent electrical heater is patterned on top of this heterostructure, enabling Raman spectroscopy and thermometry to be obtained from the graphene top electrode in situ under device operating conditions. Here, an AC voltage V(ω) is applied to the ITO heater and the thermoelectric voltage across the Au/h-BN/graphene heterostructure is measured at 2ω using a lock-in amplifier. We report the Seebeck coefficient for our thermoelectric structure to be −215 μV/K. The Au/graphene/h-BN heterostructures enable us to explore thermoelectric and thermal transport on nanometer length scales in a regime of extremely short length scales. The thermoelectric voltage generated at the graphene/h-BN interface is due to thermionic emission rather than bulk diffusive transport. As such, this should be thought of as an interfacial Seebeck coefficient rather than a Seebeck coefficient of the constituent materials.

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 atomically sharp lateral heterojunction 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.

Plasmon Resonant Amplification of a Hot Electron-Driven Photodiode

We report plasmon resonant excitation of hot electrons in a photodetector based on a metal/oxide/metal (Au/Al2O3/graphene) heterostructure. In this device, hot electrons, excited optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer, producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent produced in this device is measured using 633-nm-wavelength light as a function of incident angle. We observe the maximum photocurrent at ±10° from normal incidence under irra-diation with light polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating, and a constant (angle-independent) photocurrent under irradiation with light polarized perpendicular to the incident plane (s-polarization) and parallel to the grating. These data show an amplification factor of 4.6× under resonant conditions. At the same angle (±10°), we also observe sharp dips in the photoreflectance corresponding to waveve-ctor matching between the incident light and the plasmon mode in the grating. In addition, finite-difference time-domain simulations predict sharp dips in the photoreflectance at ±10°, and the electric field intensity profiles show clear excitation of a plasmon resonant mode when illuminated with p-polarized light at this angle.

Plasmon resonant amplification of hot electron-driven photocatalysis

We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate. At these same angles, we also observe sharp dips in the photoreflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain simulations also predict sharp dips in the photoreflectance at ±9°, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate...

Enhanced thermoelectric efficiency in topological insulator Bi2Te3 nanoplates via atomic layer deposition-based surface passivation

We report in-plane thermoelectric measurements of Bi2Te3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around −34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al2O3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to −169 μV/K. Here, we believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi2Te3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (∼10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomaterials in general. An increase in the sample resistance is also observed after the ALD process, which is consistent with the decrease in doping.We report in-plane thermoelectric measurements of Bi2Te3 nanoplates, a typical topological insulator with Dirac-like metallic surface states, grown by chemical vapor deposition. The as-grown flakes exposed to ambient conditions exhibit relatively small thermopowers around −34 μV/K due to unintentional surface doping (e.g., gas adsorption and surface oxidation). After removal of the unintentional surface doping and surface passivation by deposition of 30 nm of Al2O3 using atomic layer deposition (ALD), the Seebeck coefficient of these flakes increases by a factor of 5× to −169 μV/K. Here, we believe that the ALD-based surface passivation can prevent the degradation of the thermoelectric properties caused by gas adsorption and surface oxidation processes, thus, reducing the unintentional doping in the Bi2Te3 and increasing the Seebeck coefficient. The high surface-to-volume ratio of these thin (∼10 nm thick) nanoplates make them especially sensitive to surface doping, which is a common problem among nanomat...

Monitoring Local Electric Fields at Electrode Surfaces Using Surface Enhanced Raman Scattering-Based Stark-Shift Spectroscopy during Hydrogen Evolution Reactions

We report the use of surface-enhanced Raman scattering (SERS) to measure the vibrational Stark shifts of surface-bound thiolated-benzonitrile molecules bound to an electrode surface during hydrogen evolution reactions (HERs). Here, the electrode surface consists of Au nanoislands deposited both with and without an underlying layer of monolayer graphene on a glass substrate. The Stark shifts observed in the nitrile (C-N) stretch frequency (around 2225 cm-1) are used to report the local electric field strength at the electrode surface under electrochemical working conditions. Under positive (i.e., oxidative) applied potentials [vs normal hydrogen electrode (NHE)], we observe blue shifts of up to 7.6 cm-1, which correspond to local electric fields of 22 mV/cm. Under negative applied potentials (vs NHE), the C-N stretch frequency is red-shifted by only about 1 cm-1. This corresponds to a regime in which the electrochemical current increases exponentially in the hydrogen evolution process. Under these finite electrochemical currents, we estimate the voltage drop across the solution ( V = IR). Correcting for this voltage drop results in a highly linear electric field versus applied electrochemical voltage relation. Here, the onset potential for the HER lies around 0.2 V versus NHE and the point of zero charge (PZC) occurs at 0.04 V versus NHE, based on the capacitance-voltage ( C- V) profile. The solution field is obtained by comparing the C-N stretch frequency in solution with that obtained in air. By evaluating the local electric field strength at the PZC and the onset potential, we can separate the solution field from the reaction field (i.e., electrode field), respectively. At the onset of HER, the solution field is -0.8 mV/cm and the electrode field is -1.2 mV/cm. At higher ion concentrations, we observe similar electric field strengths and more linear E-field versus applied potential behavior because of the relatively low resistance of the solution, which results in negligible voltage drops ( V = IR).