Thermochromic Vanadium Dioxide Nanostructures for Smart Windows and Radiative Cooling.

The pursuit of energy-saving materials and technologies has garnered significant attention for their pivotal role in mitigating both energy consumption and carbon emissions. In particular, thermochromic windows in buildings offer energy-saving potential by adjusting the transmittance of solar irradiation in response to temperature changes. Radiative cooling (RC), radiating thermal heat from an object surface to the cold outer space, also offers a potential way for cooling without energy consumption.

Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications.

Vanadium dioxide (VO) is one of the strongly correlated materials exhibiting a reversible insulator-metal phase transition accompanied by a structural transition from a low-temperature monoclinic phase to high-temperature rutile phase near room temperature. Due to the dramatic change in electrical resistance and optical transmittance of VO, it has attracted considerable attention towards the electronic and optical device applications, such as switching devices, memory devices, memristors, smart windows, sensors, actuators, etc. The present review provides an overview of several methods for the synthesis of nanostructured VO, such as solution-based chemical approaches (sol-gel process and hydrothermal synthesis) and gas or vapor phase synthesis techniques (pulsed laser deposition, sputtering method, and chemical vapor deposition).

Modulation of Switching Characteristics in a Single VO Nanobeam with Interfacial Strain via the Interconnection of Multiple Nanoscale Channels.

We demonstrate the modulation of electrical switching properties through the interconnection of multiple nanoscale channels (∼600 nm) in a single VO nanobeam with a coexisting metal-insulator (M-I) domain configuration during phase transition. The Raman scattering characteristics of the synthesized VO nanobeams provide evidence that substrate-induced interfacial strain can be inhomogeneously distributed along the length of the nanobeam. Interestingly, the nanoscale VO devices with the same channel length and width exhibit distinct differences in hysteric current-voltage characteristics, which are explained by theoretical calculations of resistance change combined with Joule heating simulations of the nanoscale VO channels.

Plasmonic Core-Shell-Satellites with Abundant Electromagnetic Hotspots for Highly Sensitive and Reproducible SERS Detection.

In this work, we develop a Ag@AlO@Ag plasmonic core-shell-satellite (PCSS) to achieve highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS) detection of probe molecules. To fabricate PCSS nanostructures, we employ a simple hierarchical dewetting process of Ag films coupled with an atomic layer deposition (ALD) method for the AlO shell. Compared to bare Ag nanoparticles, several advantages of fabricating PCSS nanostructures are discovered, including high surface roughness, high density of nanogaps between Ag core and Ag satellites, and nanogaps between adjacent Ag satellites.

Enhanced Ultraviolet Photoresponse Characteristics of Indium Gallium Zinc Oxide Photo-Thin-Film Transistors Enabled by Surface Functionalization of Biomaterials for Real-Time Ultraviolet Monitoring.

Indium gallium zinc oxide (IGZO) is one of the most promising materials for diverse optoelectronic applications based on thin-film transistors (TFTs) including ultraviolet (UV) photodetectors. In particular, the monitoring of UV-A (320-400 nm) exposure is very useful for healthcare applications because it can be used to prevent various human skin and eye-related diseases. However, the relatively weak optical absorption in the UV-A range and the persistent photoconductivity (PPC) arising from the oxygen vacancy-related states of IGZO thin films limit efficient UV monitoring.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film.

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO) in monolayer molybdenum disulfide (MoS) when interfacing with a VO film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS directly synthesized on a SiO/Si substrate by chemical vapor deposition was first transferred onto a VO/c-AlO substrate in which the VO film was prepared by a sputtering method. We compared the MoS interfaced with the VO film with the as-synthesized MoS by using Raman spectroscopy.

Automated Assembly of Wafer-Scale 2D TMD Heterostructures of Arbitrary Layer Orientation and Stacking Sequence Using Water Dissoluble Salt Substrates.

We report a novel strategy to assemble wafer-scale two-dimensional (2D) transition metal dichalcogenide (TMD) layers of well-defined components and orientations. We directly grew a variety of 2D TMD layers on "water-dissoluble" single-crystalline salt wafers and precisely delaminated them inside water in a chemically benign manner. This manufacturing strategy enables the automated integration of vertically aligned 2D TMD layers as well as 2D/2D heterolayers of arbitrary stacking orders on exotic substrates insensitive to their kind and shape.

Effects of Applied Voltages on the Charge Transport Properties in a ZnO Nanowire Field Effect Transistor.

We investigate the effect of applied gate and drain voltages on the charge transport properties in a zinc oxide (ZnO) nanowire field effect transistor (FET) through temperature- and voltage-dependent measurements. Since the FET based on nanowires is one of the fundamental building blocks in potential nanoelectronic applications, it is important to understand the transport properties relevant to the variation in electrically applied parameters for devices based on nanowires with a large surface-to-volume ratio. In this work, the threshold voltage shift due to a drain-induced barrier-lowering (DIBL) effect was observed using a Y-function method.

Self-protective GaInN-based light-emitting diodes with VO nanowires.

We presented a new functional GaInN-based light-emitting diode (LED) that is capable of protecting itself from unwanted thermal damage (a so-called self-protective LED). This functionality was achieved by incorporating VO2 nanowires on the LED chip. VO2 nanowires, as metal-insulator transition materials, show a phase transition from insulating to metallic at a characteristic transition temperature.

Dose-dependent effect of proton irradiation on electrical properties of WSe ambipolar field effect transistors.

The irradiation effect of high energy proton beams on tungsten diselenide (WSe) ambipolar field-effect transistors was investigated. We measured the electrical characteristics of the fabricated WSe FETs before and after the 10 MeV proton beam irradiation with different doses of 10, 10, 10, and 10 cm. For low dose conditions (10, 10, and 10 cm), the threshold voltages shifted to the negative gate voltage direction, and the current in the hole and electron accumulation regimes decreased and increased, respectively.

Protein Biophotosensitizer-Based IGZO Photo-thin Film Transistors for Monitoring Harmful Ultraviolet Light.

The development of health monitoring devices to prevent skin cancers or various diseases arising from exposure to harmful light has attracted increasing scientific interest and has led to the exploration of hybrid inorganic-biological systems through the incorporation of biomolecules. Here, ultraviolet (UV) photodetectors based on transistors incorporating green fluorescent protein (GFP) molecules on multilayer-stacked indium-gallium-zinc-oxide (IGZO) thin films are studied, where the top layer of the IGZO films has different surface properties. Light-sensitive GFP can play a role as a biophotosensitizer due to light-induced electron transfer during photoexcitation.

Studies on mass production and highly solar light photocatalytic properties of gray hydrogenated-TiO sphere photocatalysts.

In this paper, it is first reported that gray hydrogenated TiO sphere photocatalysts (H-TiO) with high reactivity to solar light are mass produced within a few minutes using an underwater discharge plasma modified sol-gel method at room temperature and atmospheric pressure. This plasma modified system is an easy one-step in-situ synthetic process and the crystallinity, hydrogenation, and spherical structure of H-TiO are achieved by the synergy effect between the continuous reaction of highly energetic atomic and molecular species generated from the underwater plasma and surface tension of water. The resultant H-TiO demonstrated high anatase/rutile bicrystallinity and extended optical absorption spectrum from the ultraviolet (UV) to visible range.

Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe-WSe van der Waals Heterojunction-Based Field-Effect Transistors.

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe-WSe films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metal-semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe-WSe vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current.

Thermodynamically Stable Synthesis of Large-Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n-n Heterojunction Devices.

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition.

Metastable state-induced consecutive step-like negative differential resistance behaviors in single crystalline VO nanobeams.

We demonstrate the current-dependent consecutive appearance of two different negative differential resistance (NDR) transitions in a single crystalline VO nanobeam epitaxially grown on a c-cut sapphire substrate. It is revealed that the first NDR occurs at an approximately constant current level as a result of the carrier injection-induced transition, independent of a thermally induced phase transition. In contrast, it is observed that the second NDR exhibits a temperature-dependent behavior and current values triggering the metal-insulator transition (MIT) are strongly mediated by Joule heating effects in a phase coexisting temperature range.

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS p-n heterojunction diodes.

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS and pentacene. The pentacene/MoS p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices.

The influence of interfacial tensile strain on the charge transport characteristics of MoS-based vertical heterojunction devices.

We demonstrate the charge transport characteristics of MoS-based vertical heterojunction devices through the formation of interfacial strain. Atomically thin MoS bilayers were directly synthesized on a p-type Si substrate by using chemical vapor deposition to introduce an interfacial tensile strain in the vertical heterojunction diode structure, which was confirmed by Raman, X-ray and ultraviolet photoelectron spectroscopy techniques. The electrical and optoelectronic properties of the heterojunction devices with the as-grown MoS (A-MoS) on p-Si were compared with those of transferred MoS (T-MoS)/p-Si devices.

Robust and stretchable indium gallium zinc oxide-based electronic textiles formed by cilia-assisted transfer printing.

Electronic textile (e-textile) allows for high-end wearable electronic devices that provide easy access for carrying, handling and using. However, the related technology does not seem to be mature because the woven fabric hampers not only the device fabrication process directly on the complex surface but also the transfer printing of ultrathin planar electronic devices. Here we report an indirect method that enables conformal wrapping of surface with arbitrary yet complex shapes.

Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine.

Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface.

Electrical and Optical Characterization of MoS2 with Sulfur Vacancy Passivation by Treatment with Alkanethiol Molecules.

We investigated the physical properties of molybdenum disulfide (MoS2) atomic crystals with a sulfur vacancy passivation after treatment with alkanethiol molecules including their electrical, Raman, and photoluminescence (PL) characteristics. MoS2, one of the transition metal dichalcogenide materials, is a promising two-dimensional semiconductor material with good physical properties. It is known that sulfur vacancies exist in MoS2, resulting in the n-type behavior of MoS2.

Substrate-mediated strain effect on the role of thermal heating and electric field on metal-insulator transition in vanadium dioxide nanobeams.

Single-crystalline vanadium dioxide (VO2) nanostructures have recently attracted great attention because of their single domain metal-insulator transition (MIT) nature that differs from a bulk sample. The VO2 nanostructures can also provide new opportunities to explore, understand, and ultimately engineer MIT properties for applications of novel functional devices. Importantly, the MIT properties of the VO2 nanostructures are significantly affected by stoichiometry, doping, size effect, defects, and in particular, strain.

Defect-mediated modulation of optical properties in vertically aligned ZnO nanowires via substrate-assisted Ga incorporation.

We report the defect-mediated modulation of optical properties in vertically aligned ZnO nanowires via a substrate-assisted Ga incorporation method. We find that Ga atoms were incorporated into a ZnO lattice via the diffusion of liquid Ga droplets from a GaAs substrate in which as-grown ZnO nanowires were placed face down on the GaAs substrate and annealed at 650 °C. Based on structural and compositional characterization, it was confirmed that the substrate-assisted incorporation of Ga can induce a high defect density in vertically aligned ZnO nanowires grown on a Si substrate.

Hydrogen plasma-mediated modification of the electrical transport properties of ZnO nanowire field effect transistors.

We investigated the effects of hydrogen plasma treatment on the electrical transport properties of ZnO nanowire field effect transistors (FETs) with a back gate configuration. After hydrogen plasma treatment of the FET devices, the effective carrier density and mobility of the nanowire FETs increased with a threshold voltage shift toward a negative gate bias direction. This can be attributed to the desorption of oxygen molecules adsorbed on the surface of the nanowire channel, to passivation and to doping effects due to the incorporation of energetic hydrogen ions generated in plasma.

Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity.

ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement.

In situ probing of doping- and stress-mediated phase transitions in a single-crystalline VO₂ nanobeam by spatially resolved Raman spectroscopy.

We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO₂ nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO₂ nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO₂ nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under air.