Oxygen Vacancies Control Transition of Resistive Switching Mode in Single-Crystal TiO₂ Memory Device

Epitaxial TiO₂ thin films were grown by radio-frequency magnetron sputtering on conductive Nb-SrTiO₃ substrates. X-ray photoelectron spectroscopy reveals that the oxygen vacancies inside the TiO₂ films can be dramatically reduced by postannealing treatment under an oxygen atmosphere. The decreasing concentration of oxygen vacancies modifies the resistive switching (RS) mechanism from a valence change mode to a electrochemical metallization mode, resulting in a high switching ratio (≥10⁵), a small electronic leakage current in the high-resistance (≥10⁹ Ω) state, and a highly controlled quantized conductance (QC) in the low-resistance state. These results allow for understanding the relationship between different RS mechanisms as well as the QC for multilevel data storage application

Effect of Surface Oxidation on the Interaction of 1-Methylaminopyrene with Gold Nanoparticles

The effect of the surface chemistry of gold nanoparticles (GNPs) on the GNP–amine (−NH₂) interaction was investigated via conjugating an amine probe1-methylaminopyrene (MAP) chromophorewith three Au colloidal samples of the same particle size yet different surface chemistry. The surface of laser-irradiated and ligand-exchanged-irradiated GNPs is covered with acetonedicarboxylic ligands (due to laser-introduced citrate oxidization) and citrate ligands, respectively, and both surfaces contain oxidized Au species which are essentially lacking for the citrate-capped GNPs prepared by the pure chemical approach. Both laser-irradiated samples show inferior adsorption capacity of MAP as compared with the purely chemically prepared GNPs. Detailed investigations indicate that MAP molecules mainly complex directly with Au atoms via forming Au-NH₂R bonds, and the oxidization of the GNP surface strongly influences the ratio of this direct bonding to the indirect bonding originating from the electrostatic interaction between protonated amine (−NH₃⁺) and negatively charged surface ligands. The impact of the oxidized GNP surface associated with the laser treatment is further confirmed by aging experiment on GNP–MAP conjugation systems, which straightforwardly verifies that the surface oxidation leads to the decrease in the MAP adsorption on GNPs

High-Efficiency Broadband C₃N₄ Photocatalysts: Synergistic Effects from Upconversion and Plasmons

A plasmon and upconversion enhanced broadband photocatalyst based on Au nanoparticle (NP) and NaYF₄:Yb³⁺, Er³⁺, Tm³⁺ (NYF) microsphere loaded graphitic C₃N₄ (g-C₃N₄) nanosheets (Au-NYF/g-C₃N₄) was subtly designed and synthesized. The simple one-step synthesis of NYF in the presence of g-C₃N₄, which has not been reported in the literature either, leads to both high NYF yield and high coupling efficiency between NYF and g-C₃N₄. The Au-NYF/g-C₃N₄ structure exhibits high stability, wide photoresponse from the ultraviolet (UV), to visible and near-infrared regions, and prominently enhanced photocatalytic activities compared with the plain g-C₃N₄ sample in the degradation of methyl orange (MO). In particular, with the optimization of Au loading, the rate constant normalized with the catalysts mass of the best-performing catalyst 1 wt % Au-NYF/g-C₃N₄ (0.032 h^–1 mg^–1) far surpasses that of NYF/g-C₃N₄ and g-C₃N₄ (0.009 h^–1 mg^–1) by 3.6 times under λ > 420 nm light irradiation. The high performance of the Au-NYF/g-C₃N₄ nanocomposite under different light irradiations was ascribed to the distinctively promoted charge separation and suppressed recombination, and the efficient transfer of charge carriers and energy among these components. The promoted charge separation and transfer were further confirmed by photoelectrochemical measurements. The 1 wt % Au-NYF/g-C₃N₄ exhibits enhanced photocurrent density (∼6.36 μA cm^–2) by a factor of ∼5.5 with respect to that of NYF/g-C₃N₄ sample (∼1.15 μA cm^–2). Different mechanisms of the photodegradation under separate UV, visible, and NIR illuminations are unveiled and discussed in detail. Under simulated solar light illumination, the involved reactive species were identified by performing trapping experiments. This work highlights the great potential of developing highly efficient g-C₃N₄-based broadband photocatalysts for full solar spectrum utilization by integrating plasmonic nanostructures and upconverting materials

Fabrication of Buckling Free Ultrathin Silicon Membranes by Direct Bonding with Thermal Difference

An innovative method to fabricate large area (up to several squared millimeters) ultrathin (100 nm) monocrystalline silicon (Si) membranes is described. This process is based on the direct bonding of a silicon-on-insulator wafer with a preperforated silicon wafer. The stress generated by the thermal difference applied during the bonding process is exploited to produce buckling free silicon nanomembranes of large areas. The thermal differences required to achieve these membranes (≥1 mm²) are estimated by analytical calculations. An experimental study of the stress achievable by direct bonding through two specific surface preparations (hydrophobic or hydrophilic) is reported. Buckling free silicon nanomembranes secured on a 2 × 2 cm² frame with lateral dimensions up to 5 × 5 mm² are successfully fabricated using the optimized direct bonding process. The stress estimated by theoretical analysis is confirmed by Raman measurements, while the flatness of the nanomembranes is demonstrated by optical interferometry. The successful fabrications of high resolution (50 nm half pitch) tungsten gratings on the silicon nanomembranes and of focused ion beam milling nanostructures show the promising potential of the Si membranes for X-ray optics and for the emerging nanosensor market

Highly Sensitive Switchable Heterojunction Photodiode Based on Epitaxial Bi₂FeCrO₆ Multiferroic Thin Films

Perovskite multiferroic oxides are promising materials for the realization of sensitive and switchable photodiodes because of their favorable band gap (<3.0 eV), high absorption coefficient, and tunable internal ferroelectric (FE) polarization. A high-speed switchable photodiode based on multiferroic Bi₂FeCrO₆ (BFCO)/SrRuO₃ (SRO)-layered heterojunction was fabricated by pulsed laser deposition. The heterojunction photodiode exhibits a large ideality factor (<i>n</i> = ∼5.0) and a response time as fast as 68 ms, thanks to the effective charge carrier transport and collection at the BFCO/SRO interface. The diode can switch direction when the electric polarization is reversed by an external voltage pulse. The time-resolved photoluminescence decay of the device measured at ∼500 nm demonstrates an ultrafast charge transfer (lifetime = ∼6.4 ns) in BFCO/SRO heteroepitaxial structures. The estimated responsivity value at 500 nm and zero bias is 0.38 mA W⁻¹, which is so far the highest reported for any FE thin film photodiode. Our work highlights the huge potential for using multiferroic oxides to fabricate highly sensitive and switchable photodiodes

Enhanced Long-term and Thermal Stability of Polymer Solar Cells in Air at High Humidity with the Formation of Unusual Quantum Dot Networks

Due to the practical applications of polymer solar cells (PSCs), their stability recently has received increasing attention. Herein, a new strategy was developed to largely enhance the long-term and thermal stability of PSCs in air with a relatively high humidity of 50–60% without any encapsulation. In this strategy, semiconductor PbS/CdS core/shell quantum dots (QDs) were incorporated into the photoactive blend of poly­(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM). By replacing the initial ligands of oleic acid with halide ligands on the surface of PbS/CdS QDs via solution-phase ligand exchange, we were able to form unusual, continuous QD networks in the film of P3HT:PCBM, which effectively stabilized the photoactive layer. Air-processed PSCs based on the stabilized P3HT:PCBM film showed excellent long-term stability under high humidity, providing over 3% of power conversion efficiency (PCE) simultaneously. Around 91% of pristine PCE was retained after 30 days storage in high-humidity air without encapsulation. This constitutes a remarkable improvement compared to ∼53% retained PCE for the QD-free devices, which can be ascribed to the efficient suppression of both PCBM aggregation and oxidation of the thiophene ring in P3HT, thanks to the formation of robust QD networks. Furthermore, the presence of QD networks was able to enhance the stability of the P3HT:PCBM film against thermal stress/oxidation under high-humidity environment (50–60%) as well. The device kept 60% of pristine PCE after thermal treatment for 12 h at 85 °C in air, which is more than twice higher than that for the QD-free device. To the best of our knowledge, the work represents the first unambiguous demonstration of the formation of QD networks in the photoactive layer and of their important contribution to the stability of PSCs. This strategy is highly promising for other fullerene-based PSCs and opens a new avenue toward achieving PSCs with high PCE and excellent stability

Optical Resonance Engineering for Infrared Colloidal Quantum Dot Photovoltaics

We report optically enhanced infrared-harvesting colloidal quantum dot solar cells based on integrated Fabry–Perot cavities. By integrating the active layer of the photovoltaic device between two reflective interfaces, we tune its sensitivity in the spectral region at 1100–1350 nm. The top and bottom electrodes also serve as mirrors, converting the device into an optical resonator. The front conductive mirror consists of a dielectric stack of SiN_<i>x</i> and SiO₂ with a terminal layer of ITO and ZnO in which current can flow, while the back mirror consists of a highly reflective gold layer. Adjusting the reflectivity and central wavelength of the front mirror as well as the thickness of the active layer allowed increases in absorption by a total of 56% in the infrared, leading to a record external quantum efficiency of 60% at 1300 nm. This work opens new avenues toward low-cost, high-efficiency rear-junction photovoltaic harvesters that add to the overall performance of silicon solar cells