Self-assembly and horizontal orientation growth of VO2 nanowires

Facile Synthesis of Ag NP Films via Evaporation-Induced Self-Assembly and the BA-Sensing Properties

Herein, we design and prepare large-area silver nanoparticle (Ag NP) films based on evaporation-induced self-assembly, which offers the visual and real-time detection of chilled broiler meat freshness. The color change is based on the fact that an increase in the biogenic amine (BA) concentration causes a change in the absorption wavelength of Ag NPs caused by aggregation and etch of the Ag NPs, resulting in a yellow to brown color change, thus enabling a naked-eye readout of the BA exposure. The Ag NP films exhibit a rapid, sensitive, and linear response to BAs in a wide detection range of 2 µM to 100 µM. The Ag NP films are successfully applied as a quick-response, online, high-contrasting colorimetric sensor for visual detection of the freshness of chilled broiler meat.

Kinetics of Catalyst-Free and Position-Controlled Low-Pressure Chemical Vapor Deposition Growth of VO2 Nanowire Arrays on Nanoimprinted Si Substrates

In the present article, the position-controlled and catalytic-free synthesis of vanadium dioxide (VO₂) nanowires (NWs) grown by the chemical vapor deposition (CVD) on nanoimprinted silicon substrates in the form of nanopillar arrays was analyzed. The NW growth on silicon nanopillars with different cross-sectional areas was studied, and it has been shown that the NWs' height decreases with an increase in their cross-sectional area. The X-ray diffraction technique, scanning electron microscopy, and X-ray photoelectron spectroscopy showed the high quality of the grown VO₂ NWs. A qualitative description of the growth rate of vertical NWs based on the material balance equation is given. The dependence of the growth rate of vertical and horizontal NWs on the precursor concentration in the gas phase and on the growth time was investigated. It was found that the height of vertical VO₂ NWs along the [100] direction exhibited a linear dependence on time and increased with an increase in the precursor concentration. For horizontal VO₂ NWs, the height along the direction [011] varied little with the growth time and precursor concentration. These results suggest that the high-aspect ratio vertical VO₂ NWs formed due to different growth modes of their crystal faces forming the top of the growing VO₂ crystals and their lateral crystal faces related to the difference between the free energies of these crystal faces and implemented experimental conditions. The results obtained permit a better insight into the growth of high-aspect ratio VO₂ NWs and into the formation of large VO₂ NW arrays with a controlled composition and properties.

High quality VO2 thin films synthesized from V2O5 powder for sensitive near-infrared detection

Vapor transport method has been successfully used to synthesize high quality VO₂ thin films on SiO₂/Si substrate using V₂O₅ as a precursor in an inert-gas environment. The morphological and structural evolutions of the intermediate phases during the nucleation and growth processes were investigated by SEM and Raman spectroscopy, respectively. The results showed that the conversion of V₂O₅ powder to VO₂ thin films was dominated by a melting-evaporation-nucleation-growth mechanism. Further characterization results demonstrated that the high quality crystals of monoclinic VO₂ thin films exhibit a sharp resistance change up to 4 orders of magnitude. In addition, the VO₂ thin films exhibited good near-infrared response, high stability, and reproducibility under ambient conditions, which should be promising for sensitive near-infrared detection. Our work not only provided a simple and direct approach to synthesize high quality VO₂ thin films with distinct phase transition properties but also demonstrated the possible infrared sensing application in the future.

Recent Progress on Vanadium Dioxide Nanostructures and Devices: Fabrication, Properties, Applications and Perspectives

Vanadium dioxide (VO₂) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO₂ is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO₂, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO₂ and their corresponding structural features, the typical fabrication methods of VO₂ nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO₂. In addition, the challenges and opportunities for VO₂ in future studies and applications are also discussed.

Synthesis of Self-Assembled Randomly Oriented VO2 Nanowires on a Glass Substrate by a Spin Coating Method

Randomly oriented vanadium dioxide (VO₂) nanowires were produced on a glass substrate by spin coating from a cosolvent. SEM studies reveal that highly dense VO₂ nanowires were grown at an annealing temperature of 400 °C. X-ray diffraction (XRD) provides evidence of the high crystallinity of the VO₂ nanowires-embedded VO₂ thin films on the glass substrate at 400 °C. Characterization by high-resolution transmission electron microscopy (HR-TEM) confirmed the formation of VO₂ nanowires. The optical band gap of the nanowires-embedded VO₂ thin films was also calculated from the transmittance data to be 2.65-2.70 eV. The growth mechanism of the solution-processed semiconducting VO₂ nanowires was proposed based on both solvent selection and annealing temperature. Finally, the solar water splitting ability of the VO₂ nanowires-embedded VO₂ thin films was demonstrated in a photoelectrochemical cell (PEC).

Planar Growth, Integration, and Applications of Semiconducting Nanowires

Silicon and other inorganic semiconductor nanowires (NWs) have been extensively investigated in the last two decades for constructing high-performance nanoelectronics, sensors, and optoelectronics. For many of these applications, these tiny building blocks have to be integrated into the existing planar electronic platform, where precise location, orientation, and layout controls are indispensable. In the advent of More-than-Moore's era, there are also emerging demands for a programmable growth engineering of the geometry, composition, and line-shape of NWs on planar or out-of-plane 3D sidewall surfaces. Here, the critical technologies established for synthesis, transferring, and assembly of NWs upon planar surface are examined; then, the recent progress of in-plane growth of horizontal NWs directly upon crystalline or patterned substrates, constrained by using nanochannels, an epitaxial interface, or amorphous thin film precursors is discussed. Finally, the unique capabilities of planar growth of NWs in achieving precise guided growth control, programmable geometry, composition, and line-shape engineering are reviewed, followed by their latest device applications in building high-performance field-effect transistors, photodetectors, stretchable electronics, and 3D stacked-channel integration.

Hydrothermal epitaxy growth of self-organized vanadium dioxide 3D structures with metal–insulator transition and THz transmission switch properties

The hydrothermal method is an effective approach for the synthesis of VO₂ films with unique crystallites morphology and sharp electrical and optical switch properties.

The Effect of Substrate Biasing during DC Magnetron Sputtering on the Quality of VO2 Thin Films and Their Insulator-Metal Transition Behavior

In this work, VO₂ thin films were deposited on Si wafers (onto (100) surface) by DC magnetron sputtering under different cathode bias voltages. The effects of substrate biasing on the structural and optical properties were investigated. The results show that the metal–insulator transition (MIT) temperature of VO₂ thin films can be increased up to 14 K by applying a cathode bias voltage, compared to deposition conditions without any bias. The decrease in the transition efficiency and increase in the transition temperature are attributed to the enlarged grain size, increased defects, and the residual stress in the VO₂ thin films induced by biasing. The optical transmittance measurements for different thickness films indicate an attenuation coefficient of 3.1 × 10⁷ m⁻¹ at 2000 nm or an extinction coefficient of 4.9 in the metal phase. The optical transmittance vs wavelength characteristics point to an indirect bandgap of 0.6 ± 0.5 eV and significant scattering in the bulk and/or at the interface.

In-Plane Nanowires with Arbitrary Shapes on Amorphous Substrates by Artificial Epitaxy

The challenge of nanowire assembly is still one of the major obstacles toward their efficient integration into functional systems. One strategy to overcome this obstacle is the guided growth approach, in which the growth of in-plane nanowires is guided by epitaxial and graphoepitaxial relations with the substrate to yield dense arrays of aligned nanowires. This method relies on crystalline substrates which are generally expensive and incompatible with silicon-based technologies. In this work, we expand the guided growth approach into noncrystalline substrates and demonstrate the guided growth of horizontal nanowires along straight and arbitrarily shaped amorphous nanolithographic open guides on silicon wafers. Nanoimprint lithography is used as a high-throughput method for the fabrication of the high-resolution guiding features. We first grow five different semiconductor materials (GaN, ZnSe, CdS, ZnTe, and ZnO) along straight ridges and trenches, demonstrating the generality of this method. Through crystallographic analysis we find that despite the absence of any epitaxial relations with the substrate, the nanowires grow as single crystals in preferred crystallographic orientations. To further expand the guided growth approach beyond straight nanowires, GaN and ZnSe were grown also along curved and kinked configurations to form different shapes, including sinusoidal and zigzag-shaped nanowires. Photoluminescence and cathodoluminescence were used as noninvasive tools to characterize the sine wave-shaped nanowires. We discuss the similarities and differences between in-plane nanowires grown by epitaxy/graphoepitaxy and artificial epitaxy in terms of generality, morphology, crystallinity, and optical properties.

Elaborately programmed nanowires fabricated using a tapered push-pull nozzle system

Elaborately programmed silver nanowire arrays can be prepared using a tapered push-pull nozzle system (TPPNS), which is used to directly write micro-nano wires on a substrate via a two-reagent reaction in the diffusion mixing region. The wires could be precisely positioned on the substrate and their width could be freely controlled from the micro to the nano scale, indicating an advance in the methodologies of controlling and fabricating nanowires. The as-prepared silver three-electrode device can serve as a three-electrode sensor.

Surface-Guided Core-Shell ZnSe@ZnTe Nanowires as Radial p-n Heterojunctions with Photovoltaic Behavior

The organization of nanowires on surfaces remains a major obstacle toward their large-scale integration into functional devices. Surface-material interactions have been used, with different materials and substrates, to guide horizontal nanowires during their growth into well-organized assemblies, but the only guided nanowire heterostructures reported so far are axial and not radial. Here, we demonstrate the guided growth of horizontal core-shell nanowires, specifically of ZnSe@ZnTe, with control over their crystal phase and crystallographic orientations. We exploit the directional control of the guided growth for the parallel production of multiple radial p-n heterojunctions and probe their optoelectronic properties. The devices exhibit a rectifying behavior with photovoltaic characteristics upon illumination. Guided nanowire heterostructures enable the bottom-up assembly of complex semiconductor structures with controlled electronic and optoelectronic properties.

One-Step Hydrothermal Synthesis of W-Doped VO2 (M) Nanorods with a Tunable Phase-Transition Temperature for Infrared Smart Windows

Vanadium dioxide (VO₂), with reversible metal-semiconductor transition near room temperature, is a compelling candidate for thermochromic windows. Nanocomposite coatings derived from VO₂ nanoparticles are particularly superior to VO₂ films due to their advantages in large-scale preparation, flexible shaping, and regulation of optical properties. In this work, we developed a novel method for one-step hydrothermal synthesis of W-doped VO₂ (M) nanorods and studied their application in large-scale infrared smart windows. On introducing tartaric acid as a new reductant, VO₂ underwent a two-stage phase evolution from the pure phase comprising VO₂ (A) nanobelts to VO₂ (M) nanorods, instead of the conventional three-stage B-A-M phase evolution during hydrothermal synthesis. This transition is very favorable for the large-scale hydrothermal synthesis of VO₂ (M). The phase-transition temperature of VO₂ (M) nanoparticles can be regulated systematically by W doping, with a reduction efficiency of about 24.52 °C/atom % W. Moreover, VO₂ (M) composite films were fabricated using a convenient roller coating method, which exhibited significant midinfrared transmission switching up to 31%, with a phase-transition temperature of about 37.3 °C. This work demonstrates the significant progress in the one-step hydrothermal synthesis of VO₂ (M) nanorods and provides significant insights into their applications in infrared smart windows.

Three Dimensional Sculpturing of Vertical Nanowire Arrays by Conventional Photolithography

Ordered nanoarchitectures have attracted an intense research interest recently because of their promising device applications. They are always fabricated by self-assembling building blocks such as nanowires, nanodots. This kind of bottom up approaches is limited in poor control over height, lateral resolution, aspect ratio, and patterning. Here, we break these limits and realize 3D sculpturing of vertical ZnO nanowire arrays (NAs) based on the conventional photolithography approach. These are achieved by immersing nanowire NAs in thick photoresist (PR) layers, which enable the cutting and patterning of ZnO NAs as well as the tailoring of NAs. Our strategy of 3D sculpturing of NAs promisingly paves the way for designing novel NAs-based nanoarchitectures.

Nanostructural adsorption of vanadium oxide on functionalized graphene: a DFT study

Electrostatic Potential (EP) contours of rutile and M1-monoclinic vanadium oxide nanostructures adsorbed on hollow sites of pure graphene showing respectively horizontal and vertical growth tendencies.

Electrochemical gating-induced reversible and drastic resistance switching in VO2 nanowires

Reversible and drastic modulation of the transport properties in vanadium dioxide (VO₂) nanowires by electric field-induced hydrogenation at room temperature was demonstrated using the nanogaps separated by humid air in field-effect transistors with planer-type gates (PG-FET). These PG-FETs allowed us to investigate behavior of revealed hydrogen intercalation and diffusion aspects with time and spatial evolutions in nanowires. These results show that air nanogaps can operate as an electrochemical reaction field, even in a gaseous atmosphere, and offer new directions to explore emerging functions for electronic and energy devices in oxides.

A steep-slope transistor based on abrupt electronic phase transition

Collective interactions in functional materials can enable novel macroscopic properties like insulator-to-metal transitions. While implementing such materials into field-effect-transistor technology can potentially augment current state-of-the-art devices by providing unique routes to overcome their conventional limits, attempts to harness the insulator-to-metal transition for high-performance transistors have experienced little success. Here, we demonstrate a pathway for harnessing the abrupt resistivity transformation across the insulator-to-metal transition in vanadium dioxide (VO₂), to design a hybrid-phase-transition field-effect transistor that exhibits gate controlled steep (‘sub-kT/q') and reversible switching at room temperature. The transistor design, wherein VO₂ is implemented in series with the field-effect transistor's source rather than into the channel, exploits negative differential resistance induced across the VO₂ to create an internal amplifier that facilitates enhanced performance over a conventional field-effect transistor. Our approach enables low-voltage complementary n-type and p-type transistor operation as demonstrated here, and is applicable to other insulator-to-metal transition materials, offering tantalizing possibilities for energy-efficient logic and memory applications.

The intrinsic properties of conventional semiconductors limits the speed and efficiency of field-effect transistors. Here, the authors take advantage of the insulator-to-metal transition in vanadium dioxide to create a transistor with reversible and steep-slope switching at room temperature.

Guided Growth of Horizontal ZnSe Nanowires and their Integration into High-Performance Blue-UV Photodetectors

Perfectly aligned horizontal ZnSe nano-wires are obtained by guided growth, and easily integrated into high-performance blue-UV photodetectors. Their crystal phase and crystallographic orientation are controlled by the epitaxial relations with six different sapphire planes. Guided growth paves the way for the large-scale integration of nanowires into optoelectronic devices.

Star-shaped VO2 (M) nanoparticle films with high thermochromic performance

Star-shaped VO₂ (M) nanoparticles demonstrate adjustable phase transition temperature and excellent reversible infrared modulations in response to external voltage.