Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Ultra-compact waveguide electroabsorption optical switches and photodetectors with micron- and sub-micron lengths and compatible with silicon (Si) waveguides are demonstrated using the insulator-metal phase transition of vanadium dioxide (VO(2)). A 1 μm long hybrid Si-VO(2) device is shown to achieve a high extinction ratio of 12 dB and a competitive insertion loss of 5 dB over a broad bandwidth of 100 nm near λ = 1550 nm. The device, operated as a photodetector, can measure optical powers less than 1 μW with a responsivity in excess of 10 A/W. With volumes that are about 100 to 1000 times smaller than today's active Si photonic components, the hybrid Si-VO(2) devices show the feasibility of integrating transition metal oxides on Si photonic platforms for nanoscale electro-optic elements.

A 1 μm-long hybrid Si-VO 2 electroabsorption switch with integrated electrical contacts is demonstrated. A record extinction ratio of ~12 dB/μm is achieved over a bandwidth of ~100 nm with a power dissipation of 2.55 mW.

The optical and electrical characteristics of the insulator-metal phase transition of vanadium dioxide (VO2) enable the realization of power-efficient, miniaturized hybrid optoelectronic devices. This work studies the current-controlled, two-step insulator-metal phase transition of VO2 in varying microwire geometries. Geometry-dependent scaling trends extracted from current-voltage measurements show that the first step induced by carrier injection is delocalized over the microwire, while the second, thermally-induced step is localized to a filament about 1 to 2 μm wide for 100 nm-thick sputtered VO2 films on SiO2. These effects are confirmed by direct infrared imaging, which also measures the change in optical absorption in the two steps. The difference between the threshold currents of the two steps increases as the microwires are narrowed. Micron- and sub-micron-wide VO2 structures can be used to separate the two phase transition steps in photonic and electronic devices.

Highly tunable optical transmission through one-dimensional gold gratings patterned on top of a film of the phase transition material, vanadium dioxide (VO2), is demonstrated. Dense electrical integration is enabled by grating features that also function as electrical contacts to the VO2. Extraordinary optical transmission is observed in the VO2 insulator phase, and the optical transmission is extinguished by up to about 6 dB in a 170 nm thick VO2 film. Measurements of gratings with varying duty cycles demonstrate the dependence of the optical transmission and tuning on the device geometry.

We present metallic gratings on a VO 2 film with electrical control of the transmission spectrum. The extraordinary optical transmission was tunable by up to a factor of 2.75X to achieve an extinction ratio of 26 dB/μm.

In this study, three different types of nanothermites, i.e., Al/CuO, MoO3, and Al/Bi2O3, were produced using the widely spread wet (i.e., isopropanol based) method. In addition to the above method, the three nanothermites were also produced using a Resodyn LabRAM mixer. A paraffin-coated spherical Al nanopowder (100 nm) was used as the fuel source, while the oxidizers were nanometric powders of CuO (40 nm), MoO3 (100 nm), and Bi2O3 (200 nm). The effect of nanothermite composition on the sensitivity for the tests of electrostatic discharge (ESD), impact, and friction was investigated. Scanning electron microscopy (SEM) was used to analyze the morphology and homogeneity of the nanothermites. Next, the nanothermites were thermally analyzed in terms of energy release, ignition temperature, and flame temperature using a thermogravimetric analysis differential and scanning calorimetry (TGA/DSC) technique. A low-power diode laser was used to evaluate the ability of different laser wavelengths (661, 532, and 445 nm) to produce the ignition energy needed for a specific thermite reaction. Low ignition delays (less than 15 ms) were obtained at approximately 300 mW laser power output for both Al/MoO3 and Al/Bi2O3 thermites. Finally, a forward-looking infrared camera was used to estimate the ignition and burning temperatures of the Al/MoO3 nanothermite.

By controlling the thermal transport of VO2 nano-gap junctions using device geometry, contact material, and applied voltage waveforms, the electronically induced insulator-metal phase transition is investigated in the adiabatic heating and transient carrier injection regimes. With a gradual ramping of an applied voltage on a microsecond time scale, the transition electric field threshold can be directly reduced by the Joule heating. With an abrupt applied voltage, the transition threshold is initiated by carriers injected within the first tens of nanoseconds, but the complete insulator-metal phase transition is limited by thermal redistribution times to hundreds of nanoseconds.

Si3N4-on-SOI and VO2-on-SOI integrated photonic devices and circuits, including an ultra-broadband and efficient grating coupler, adiabatic polarization rotator splitters, and wavelength-scale modulators, are described. Hybrid integration enables functionalities not attainable in each material separately.

We measured the current-voltage and infrared optical transmission characteristics of thin film VO2 wires over the metal-insulator phase transition. The phase transition had electrical or thermal characteristics depending on the wire geometry and applied current.

Voltage-controlled switching in lateral VO2 nano-gap junctions with different gap lengths and thermal properties was investigated. The effect of Joule heating on the phase transition was found to be strongly influenced by the device geometry, the contact material, and the current. Our results indicate that the VO2 phase transition was likely initiated electronically, which was sometimes followed by a secondary thermally induced transition.

The conversion of solar energy into hydrogen via water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. Therefore, development of visible light-responsive and efficient photocatalyst material has been of immense interest, but with limited success. Here, we show that overall water splitting under visible-light irradiation can be achieved using a single photocatalyst material. Multiband InGaN/GaN nanowire heterostructures, decorated with rhodium (Rh)/chromium-oxide (Cr2O3) core-shell nanoparticles can lead to stable hydrogen production from pure (pH ∼ 7.0) water splitting under ultraviolet, blue and green-light irradiation (up to ∼560 nm), the longest wavelength ever reported. At ∼440-450 nm wavelengths, the internal quantum efficiency is estimated to be ∼13%, the highest value reported in the visible spectrum. The turnover number under visible light well exceeds 73 in 12 h. Detailed analysis further confirms the stable photocatalytic activity of the nanowire heterostructures. This work establishes the use of metal-nitrides as viable photocatalyst for solar-powered artificial photosynthesis for the production of hydrogen and other solar fuels.

The insulator-metal phase transition of a correlated-electron material, vanadium dioxide, is used to demonstrate electrically controlled, compact, broadband, and low voltage plasmonic switches. The devices are micron-scale in length and operate near a wavelength of 1550 nm. The switching bandwidths exceed 100 nm and 400 mV is sufficient to attain extinction ratios in excess of 20 dB. The results illustrate the promise of using phase transition materials for efficient and ultra-compact plasmonic switches and modulators.

We report on the achievement of wafer-level photocatalytic overall water splitting on GaN nanowires grown by molecular beam epitaxy with the incorporation of Rh/Cr(2)O(3) core-shell nanostructures acting as cocatalysts, through which H(2) evolution is promoted by the noble metal core (Rh) while the water forming back reaction over Rh is effectively prevented by the Cr(2)O(3) shell O(2) diffusion barrier. The decomposition of pure water into H(2) and O(2) by GaN nanowires is confirmed to be a highly stable photocatalytic process, with the turnover number per unit time well exceeding the value of any previously reported GaN powder samples.

This paper describes the design and modeling of a smart uncooled infrared detector with wavelength selectivity in the long-wavelength infrared (LWIR) band. The objective is to enhance the probability of detecting and identifying objects in a scene. This design takes advantage of the smart properties of vanadium dioxide (VO2): it can switch reversibly from an IR-transparent to an IR-opaque thin film when properly triggered. This optical behavior is exploited here as a smart mirror that can modify the depth of the resonant cavity between the suspended thermistor material and a patterned mirror on the substrate, thereby altering wavelength sensitivity. The thermistor material used in the simulation is vanadium oxide (VO2). The simulation results show that, when VO2 is used in the metallic phase, it reflects IR radiation back to the suspended VOX and enhances IR absorption in the 9.4-10.8-mu m band. When the film is switched to the semiconductor phase, it admits most IR radiation, which is then reflected back to the suspended VOX by a patterned gold thin film under an SiO2 spacer layer. The spacer layer is used to increase the resonant cavity depth underneath the microbolometer pixel. Thus, the peak absorption value is shifted to 8-9.4 mu m, creating the second spectral band. The detector is designed with a relatively low thermal conductance of 1.71 x 10(-7) W/K to maximize responsivity (R-v) to values as high as 1.27 x 10(5) W/K and detectivity (D*) to as high as 1.62 x 10(9) cm Hz(1/2)/W, both at 60 Hz. The corresponding thermal time constant is equal to 2.45 ms. Hence, these detectors could be used for 60-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 14 and 16 mK for the 8-9.4- and 9.4-10.8-mu m bands, respectively. The calculated absorption coefficients in the two spectral bands were 59% and 65%, respectively.