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Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2 thin films requires high-temperature reduction of Nb2O5 films using H2 or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2 films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2 phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation on c -sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2 films without post-reduction steps which will be assumed to be a cost-effective process for NbO2 based devices. [ABSTRACT FROM AUTHOR]

Silver (Ag) nanoparticles can be spontaneously oxidized and present in different oxidized surface phases. The impact of oxidation induced photo absorption property and related photocatalytic activity are still unclear in Ag-decorated semiconductor photocatalysts. Herein, Ag-decorated BiOCl with the metallic Ag0 to oxidized Ag+ were employed to investigate the effect of surface state of Ag on relative photocatalyst properties. A redshift of localized surface plasmon resonance was observed as the Ag0 oxidized to Ag+ and a reversible manipulation was realized in UV light-driven photocatalysis. It is found that the Ag0/BiOCl presents higher photocatalytic activity than Ag+/BiOCl, but this difference is gradually decreasing under UV light irradiation compared with visible light irradiation. A controlled experiment suggests that the reduction of Ag+ under UV light reduced the difference between Ag0/BiOCl and Ag+/BiOCl. The possible mechanism for electron transport and the conversion between Ag+ and Ag0 via the assistance of the photoelectric effect from BiOCl has been elucidated. This photocatalytic reaction assisted reversible tuning the surface state of Ag/BiOCl will open up the possibility of rationally designing Ag-decorated semiconductors for light harvesting. [ABSTRACT FROM AUTHOR]

In this article, we present a comprehensive review on recent research progress in design and fabrication of active tunable metamaterials and devices based on phase transition of VO2. Firstly, we introduce mechanisms of the metal-to-insulator phase transition (MIPT) in VO2 investigated by ultrafast THz spectroscopies. By analyzing the THz spectra, the evolutions of MIPT in VO2 induced by different external excitations are described. The superiorities of using VO2 as building blocks to construct highly tunable metamaterials are discussed. Subsequently, the recently demonstrated metamaterial devices based on VO2 are reviewed. These metamaterials devices are summarized and described in the categories of working frequency. In each working frequency range, representative metamaterials based on VO2 with different architectures and functionalities are reviewed and the contributions of the MIPT of VO2 are emphasized. Finally, we conclude the recent reports and provide a prospect on the strategies of developing future tunable metamaterials based on VO2. [ABSTRACT FROM AUTHOR]

The appearance of stripe phases is a characteristic signature of strongly correlated quantum materials, and its origin in phase-changing materials has only recently been recognized as the result of the delicate balance between atomic and mesoscopic materials properties. A vanadium dioxide (VO2) single crystal is one such strongly correlated material with stripe phases. Infrared nano-imaging on low-aspect-ratio, single-crystal VO2 microbeams decorated with resonant plasmonic nanoantennas reveals a novel herringbone pattern of coexisting metallic and insulating domains intercepted and altered by ferroelastic domains, unlike previous reports on high-aspect-ratio VO2 crystals where the coexisting metal/insulator domains appear as alternating stripe phases perpendicular to the growth axis. The metallic domains nucleate below the crystal surface and grow towards the surface with increasing temperature as suggested by the near-field plasmonic response of the gold nanorod antennas. [ABSTRACT FROM AUTHOR]

We built and measured the dynamical current versus time behavior of nanoscale niobium oxide crosspoint devices which exhibited threshold switching (current-controlled negative differential resistance). The switching speeds of 110 × 110 nm2 devices were found to be ΔtON = 700 ps and ΔtOFF = 2.3 ns while the switching energies were of the order of 100 fJ. We derived a new dynamical model based on the Joule heating rate of a thermally driven insulator-to-metal phase transition that accurately reproduced the experimental results, and employed the model to estimate the switching time and energy scaling behavior of such devices down to the 10 nm scale. These results indicate that threshold switches could be of practical interest in hybrid CMOS nanoelectronic circuits. [ABSTRACT FROM AUTHOR]