Clamping Instability and van der Waals Forces in Carbon Nanotube Mechanical Resonators

We investigate the role of weak clamping forces, typically assumed to be infinite, in carbon nanotube mechanical resonators. Due to these forces, we observe a hysteretic clamping and unclamping of the nanotube device that results in a discrete drop in the mechanical resonance frequency on the order of 5–20 MHz, when the temperature is cycled between 340 and 375 K. This instability in the resonant frequency results from the nanotube unpinning from the electrode/trench sidewall where it is bound weakly by van der Waals forces. Interestingly, this unpinning does not affect the <i>Q</i>-factor of the resonance, since the clamping is still governed by van der Waals forces above and below the unpinning. For a 1 μm device, the drop observed in resonance frequency corresponds to a change in nanotube length of approximately 50–65 nm. On the basis of these findings, we introduce a new model, which includes a finite tension around zero gate voltage due to van der Waals forces and shows better agreement with the experimental data than the perfect clamping model. From the gate dependence of the mechanical resonance frequency, we extract the van der Waals clamping force to be 1.8 pN. The mechanical resonance frequency exhibits a striking temperature dependence below 200 K attributed to a temperature-dependent slack arising from the competition between the van der Waals force and the thermal fluctuations in the suspended nanotube

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

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

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