Reliable and Low-Power Multilevel Resistive Switching in TiO₂ Nanorod Arrays Structured with a TiO_<i>x</i> Seed Layer

The electrical performance of TiO₂ nanorod array (NRA)-based resistive switching memory devices is examined in this paper. The formation of a seed layer on the fluorine-doped tin oxide (FTO) glass substrate after treatment in TiCl₄ solution, before the growth of TiO₂ NRAs on the FTO substrate via a hydrothermal process, is shown to significantly improve the resistive switching performance of the resulting TiO₂ NRA-based device. As fabricated, the Al/TiO₂ NRA/TiO_<i>x</i> layer/FTO device displayed electroforming-free bipolar resistive switching behavior while maintaining a stable ON/OFF ratio for more than 500 direct sweeping cycles over a retention period of 3 × 10⁴ s. Meanwhile, the programming current as low as ∼10^–8 A and 10^–10 A for low resistance state and high resistance state respectively makes the fabricated devices suitable for low-power memristor applications. The TiO_<i>x</i> precursor seed layer not only promotes the uniform and preferred growth of TiO₂ nanorods on the FTO substrate but also functions as an additional source layer of trap centers due to its oxygen-deficient composition. Our data suggest that the primary conduction mechanism in these devices arises from trap-mediated space-charge-limited current (SCLC). Multilevel memory performance in this new device is achieved by varying the SET voltage. The origin of this effect is also discussed

Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition

A close-proximity atmospheric pressure chemical vapor deposition (AP-CVD) reactor is developed for synthesizing high quality multicomponent metal oxides for electronics. This combines the advantages of a mechanically controllable substrate-manifold spacing and vertical gas flows. As a result, our AP-CVD reactor can rapidly grow uniform crystalline films on a variety of substrate types at low temperatures without requiring plasma enhancements or low pressures. To demonstrate this, we take the zinc magnesium oxide (Zn_1–<i>x</i>Mg_<i>x</i>O) system as an example. By introducing the precursor gases vertically and uniformly to the substrate across the gas manifold, we show that films can be produced with only 3% variation in thickness over a 375 mm² deposition area. These thicknesses are significantly more uniform than for films from previous AP-CVD reactors. Our films are also compact, pinhole-free, and have a thickness that is linearly controllable by the number of oscillations of the substrate beneath the gas manifold. Using photoluminescence and X-ray diffraction measurements, we show that for Mg contents below 46 at. %, single phase Zn_1–<i>x</i>Mg_<i>x</i>O was produced. To further optimize the growth conditions, we developed a model relating the composition of a ternary oxide with the bubbling rates through the metal precursors. We fitted this model to the X-ray photoelectron spectroscopy measured compositions with an error of Δ<i>x</i> = 0.0005. This model showed that the incorporation of Mg into ZnO can be maximized by using the maximum bubbling rate through the Mg precursor for each bubbling rate ratio. When applied to poly­(3-hexylthiophene-2,5-diyl) hybrid solar cells, our films yielded an open-circuit voltage increase of over 100% by controlling the Mg content. Such films were deposited in short times (under 2 min over 4 cm²)

PEGylated MoSe₂ Nanomaterials with Limited Oxidation via Femtosecond Laser Ablation for Photothermal Therapy

MoSe2 nanomaterials are promising photothermal agents for noninvasive cancer treatment. Their surfaces usually need to be functionalized with biocompatible polymers to improve their biocompatibility and colloidal stability and to reduce their cytotoxicity. Herein PEGylated MoSe2 nanomaterials are produced by femtosecond laser ablation of MoSe2 powder in aqueous poly(ethylene glycol) (PEG) solutions. Quantum dots are produced by laser ablation for 30 min with a power of 1.5 W, while larger spherical nanoparticles are produced by laser ablation for 10 min with various powers. PEG molecules attach to the nanomaterials through both physical absorption and Mo–O chemical bonds. A higher concentration of PEG in the solution results in more PEG being attached, and increasing the laser ablation power leads to more PEG molecules being attached through chemical bonds. Notably, the attachment of PEG to the nanomaterials through Mo–O bonds can efficiently suppress the oxidation of the MoSe2 nanomaterials to MoO3 nanoparticles. Both the MoSe2 quantum dots and spherical nanoparticles demonstrate high photothermal conversion efficiencies (PTCEs), and the PTCEs of the quantum dots are overall higher than those of the nanoparticles, making them a promising candidate agent for photothermal cancer therapy

Highly Sensitive Self-Actuated Zinc Oxide Resonant Microcantilever Humidity Sensor

A resonant microcantilever sensor is fabricated from a zinc oxide (ZnO) thin film, which serves as both the structural and sensing layers. An open-air spatial atomic layer deposition technique is used to deposit the ZnO layer to achieve a ∼200 nm thickness, an order of magnitude lower than the thicknesses of conventional microcantilever sensors. The reduction in the number of layers, in the cantilever dimensions, and its overall lower mass lead to an ultrahigh sensitivity, demonstrated by detection of low humidity levels. A maximum sensitivity of 23649 ppm/% RH at 5.8% RH is observed, which is several orders of magnitude larger than those reported for other resonant humidity sensors. Furthermore, the ZnO cantilever sensor is self-actuated in air, an advantageous detection mode that enables simpler and lower-power-consumption sensors

Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide

Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway

Polymer Crystallization as a Tool To Pattern Hybrid Nanostructures: Growth of 12 nm ZnO Arrays in Poly(3-hexylthiophene)

Well-ordered hybrid materials with a 10 nm length scale are highly desired. We make use of the natural length scale (typically 10–15 nm) of the alternating crystalline and amorphous layers that are generally found in semicrystalline polymers to direct the growth of a semiconducting metal oxide. This approach is exemplified with the growth of ZnO within a carboxylic acid end-functionalized poly­(3-hexylthiophene) (P3HT–COOH). The metal-oxide precursor vapors diffuse into the amorphous parts of the semicrystalline polymer so that sheets of ZnO up to 0.5 μm in size can be grown. This P3HT–ZnO nanostructure further functions as a donor–acceptor photovoltaic system, with length scales appropriate for charge photogeneration