CO₂ Reduction to Methanol on TiO₂‑Passivated GaP Photocatalysts

In the past, the electrochemical instability of III–V semiconductors has severely limited their applicability in photocatlaysis. As a result, a vast majority of the research on photocatalysis has been done on TiO₂, which is chemically robust over a wide range of pH. However, TiO₂ has a wide band gap (3.2 eV) and can only absorb ∼4% of the solar spectrum, and thus, it will never provide efficient solar energy conversion/storage on its own. Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated GaP photocathodes under 532 nm wavelength illumination. The TiO₂ layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO₂ deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO₂ passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause nonradiative carrier recombination. In addition to passivation effects, the TiO₂ deposited by ALD is n-type due to oxygen vacancies and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that assists in the separation of photogenerated electron–hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5 V. No enhancement is observed for TiO₂ thicknesses above 10 nm, due to the insulating nature of the TiO₂, which eventually outweighs the benefits of passivation

Zener Tunneling and Photocurrent Generation in Quasi-Metallic Carbon Nanotube pn-Devices

We investigate the electronic and optoelectronic properties of quasi-metallic nanotube pn-devices, which have smaller band gaps than most known bulk semiconductors. These carbon nanotube-based devices deviate from conventional bulk semiconductor device behavior due to their low-dimensional nature. We observe rectifying behavior based on Zener tunneling of ballistic carriers instead of ideal diode behavior, as limited by the diffusive transport of carriers. We observe substantial photocurrents at room temperature, suggesting that these quasi-metallic pn-devices may have a broader impact in optoelectronic devices. A new technique based on photocurrent spectroscopy is presented to identify the unique chirality of nanotubes in a functional device. This chirality information is crucial in obtaining a theoretical understanding of the underlying device physics that depends sensitively on nanotube chirality, as is the case for quasi-metallic nanotube devices. A detailed model is developed to fit the observed <i>I–V</i> characteristics, which enables us to verify the band gap from these measurements as well as the dimensions of the insulating tunneling barrier region

Thermoacoustic Transduction in Individual Suspended Carbon Nanotubes

We report an experimental measurement of the acoustic signal emitted from an individual suspended carbon nanotube (CNT) approximate 2 μm in length, 1 nm in diameter, and 10^–21 kg in mass. This system represents the smallest thermoacoustic system studied to date. By applying an AC voltage of 1.4 V at 8 kHz to the suspended CNT, we are able to detect the acoustic signal using a commercial microphone. The acoustic power detected is found to span a range from 0.1 to 2.4 attoWatts or 0.2 to 1 μPa of sound pressure. This corresponds to thermoacoustic efficiencies ranging from 0.007 to 0.6 Pa/W for the seven devices that were measured in this study. Here, the small lateral dimensions of these devices cause large heat losses due to thermal conduction, which result in the relatively small observed thermoacoustic efficiencies

Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions

Transition metal dichalcogenides (TMDCs), such as MoS₂ and WSe₂, are free of dangling bonds and therefore make more “ideal” Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS₂ and WSe₂ and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur <i>p</i>_<i>z</i> orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS₂ and WSe₂ (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model

Near-Field Surface Waves in Few-Layer MoS₂

Recently emerged layered transition metal dichalcogenides have attracted great interest due to their intriguing fundamental physical properties and potential applications in optoelectronics. Using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical modeling, we study propagating surface waves in the visible spectral range that are excited at sharp edges of layered transition metal dichalcogenides (TMDC) such as molybdenum disulfide and tungsten diselenide. These surface waves form fringes in s-SNOM measurements. By measuring how the fringes change when the sample is rotated with respect to the incident beam, we obtain evidence that exfoliated MoS₂ on a silicon substrate supports two types of Zenneck surface waves that are predicted to exist in materials with large real and imaginary parts of the permittivity. In addition to conventional Zenneck surface waves guided along one interface, we introduce another Zenneck-type mode that exists in multilayer structures with large dissipation. We have compared MoS₂ interference fringes with those formed on a layered insulator such as hexagonal boron nitride where the small permittivity supports only leaky modes. The interpretation of our experimental data is supported by theoretical analysis. Our results could pave the way to the investigation of surface waves on TMDCs and other van der Waals materials and their novel photonics applications

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

Twin-Free GaAs Nanosheets by Selective Area Growth: Implications for Defect-Free Nanostructures

Highly perfect, twin-free GaAs nanosheets grown on (111)B surfaces by selective area growth (SAG) are demonstrated. In contrast to GaAs nanowires grown by (SAG) in which rotational twins and stacking faults are almost universally observed, twin formation is either suppressed or eliminated within properly oriented nanosheets are grown under a range of growth conditions. A morphology transition in the nanosheets due to twinning results in surface energy reduction, which may also explain the high twin-defect density that occurs within some III–V semiconductor nanostructures, such as GaAs nanowires. Calculations suggest that the surface energy is significantly reduced by the formation of {111}-plane bounded tetrahedra after the morphology transition of nanowire structures. By contrast, owing to the formation of two vertical {11̅0} planes which comprise the majority of the total surface energy of nanosheet structures, the energy reduction effect due to the morphology transition is not as dramatic as that for nanowire structures. Furthermore, the surface energy reduction effect is mitigated in longer nanosheets which, in turn, suppresses twinning

Observation of Asymmetric Nanoscale Optical Cavity in GaAs Nanosheets

GaAs nanosheets with no twin defects, stacking faults, or dislocations are excellent candidates for optoelectrical applications. Their outstanding optical behavior and twin free structure make them superior to traditionally studied GaAs nanowires. While many research groups have reported optically resonant cavities (i.e., Fabry–Perot) in 1D nanowires, here, we report an optical cavity resonance in GaAs nanosheets consisting of complex 2D asymmetric modes, which are fundamentally different from one-dimensional cavities. These resonant modes are detected experimentally using photoluminescence (PL) spectroscopy, which exhibits a series of peaks or “fringes” superimposed on the bulk GaAs photoluminescence spectrum. Finite-difference time-domain (FDTD) simulations confirm these experimental findings and provide a detailed picture of these complex resonant modes. Here, the complex modes of this cavity are formed by the three nonparallel edges of the GaAs nanosheets. Due to the asymmetrical nature of the nanosheets, the mode profiles are largely unintuitive. We also find that by changing the substrate from Si/SiO₂ to Au, we enhance the resonance fringes as well as the overall optical emission by 5× at room temperature. Our FDTD simulation results confirm that this enhancement is caused by the local field enhancement of the Au substrate and indicate that the thickness of the nanosheets plays an important role in the formation and enhancement of fringes

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 )

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