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Phase change materials can enable temperature self-regulation due to their drastic changes in optical properties accompanying the phase transition. Significant reduction of the optical absorption after the transition is the key ingredient for an enhanced regulating performance. However, the absorptivity of unpatterned vanadium dioxide (VO2) thin films can hardly be reduced after phase transition at visual-to-infrared band. In this work, we combine the direct binary search (DBS) and particle swarm optimization (PSO) algorithms for an optimized design of temperature self-regulating nano-photonic devices on VO2 thin films. For a given incident wavelength, a pixelated structure is firstly inverse-designed by the DBS algorithm which maximizes the absorption contrast before and after the transition. To overcome fabrication challenges as pixel size is at deep sub-wavelength scale, the pixelated structure can then be replaced by geometric shapes which are more tractable in manufacturing processes. The geometrical parameters are optimized by the PSO algorithm where our optimized device brings the absorptivity down to 33% after the transition. These results provide an effective way for the inverse design of optimized nano-photonic structures based on phase change materials. [ABSTRACT FROM AUTHOR]

Zinc blende ZnSe longitudinal twinning nanowires and a sandwich structure with the wurtzite ZnSe inserting into the zinc blende ZnSe longitudinal twinning nanowires are fabricated via a simple thermal evaporation method. The high-resolution transmission electron microscope images of the two types of nanowires match well with simulated atomic models of them. The growth of them might be caused by the crystal plane slip during the phase transformation process between the wurtzite and the zinc blende ZnSe nanowire. The vibrating and luminescence properties of the as-grown longitudinal twinning nanowire are investigated by room-temperature Raman and low-temperature (10 K) photoluminescence spectroscopy, respectively. The electrical transport properties of the two types of longitudinal twinning ZnSe nanowires and the monocrystal ZnSe nanowires were compared using in situ measurement in transmission electron microscope. [ABSTRACT FROM AUTHOR]

Graphical abstract Micro-nano scale current mapping for annealed 0 min, 1 min, 2 min and 3 min of VO 2 samples, and its corresponded threshold switching voltage. Highlights • Annealed process reduced threshold switching voltage of VO 2 thin film. • Due to the resistance decreased, VO 2 threshold switching voltage decreased. • CAFM mapping shown joule-heating effects on electrical-induced VO 2 phase transition. Abstract It was research hotspot that how to lower the threshold switching voltage (V th), especially for the perpendicular structure of electrical-induced VO 2 insulator-metal phase transition (MIT). This structure can reduce the V th to the orders of 10−1 V. In micro-nano scale, combined with results of resistance change versus temperature, X-ray photoelectron spectroscopy measurements and conductive atomic force microscope, we had confirmed that V3+ was an important factor for electrical-induced VO 2 thin MIT. On the one hand, joule heating produced by conductive V3+ region induced the surrounding VO 2 region phase transition. On the other hand, with the annealing time increased from 0 min to 2 min at 470 °C in H 2 /N 2 mixture gases, V3+ contents increased, thin film surface conductive areas increased, thin film resistance decreased, and finally resulted in threshold switching voltage decreased. Our experimental results were consistent with theory analysis. However, when the annealing time was too long (≥3 min), VO 2 thin film threshold switching behavior disappeared, and current was linearly increased with voltage. [ABSTRACT FROM AUTHOR]

It has been proved challenging to fabricate the single crystal orientation of VO 2 thin film by a simple method. Based on chemical reaction thermodynamic and crystallization analysis theory, combined with our experimental results, we find out that when stoichiometric number of metallic V in the chemical equation is the same, the ratio of metallic V thin film surface average roughness R a to thin film average particle diameter d decreases with the decreasing sputtering Argon pressure. Meanwhile, the oxidation reaction equilibrium constant K also decreases, which will lead to the increases of oxidation time, thereby the crystal orientation of the VO 2 thin film will also become more uniform. By sputtering oxidation coupling method, metallic V thin film is deposited on c-sapphire substrate at 1 × 10 −1 Pa, and then oxidized in the air with the maximum oxidation time of 65s, high oriented (020) VO 2 thin film has been fabricated successfully, which exhibits ∼4.6 orders sheet resistance change across the metal-insulator transition. [ABSTRACT FROM AUTHOR]

Abstract: The nanocrystalline VO2 thin films, which surface has a flake grain structure, are achieved by DC sputtering deposition at different sputtering powers. It is found that the hysteresis loop of metal-insulator phase transition (MIT) is almost superposition, and the hysteresis width is only 0.4°C for the surface flake grain structure that obtained at 132W DC sputtering power. Moreover, it is shown that the phase transition is very steep, and the film displays 3–4 orders of the change of sheet resistance at MIT. The characterizations of SEM, AFM and four-point probe methods show that the hysteresis width, the orders of the change of sheet resistance and the phase transition become narrower, higher and steeper at MIT, respectively when the surface shapes of the nanocrystalline VO2 thin films change from nanoparticle structures to flake structures with the DC sputtering powers increased from 66W to 132W and the surface flake grain sizes reduced gradually to minimum at 132W. Meanwhile, the surface roughness also changes into minimum. However, with the powers further increased from 132W to 176W, the surface flake grain sizes become bigger, and then the surface roughness changes poor. At 176W, the surface flake structures begin to turn into nanoparticle structures. The hysteresis width, the orders of the change of sheet resistance and the phase transition become wider, lower and poorly steeper at MIT, separately. The results reveal that the nanocrystalline shapes and the surface roughness can affect the hysteresis width and the sheet resistance steepness in MIT. Our analysis shows that the mechanism of the narrowed hysteresis width mainly depends on the strain imbalance of the nanocrystalline VO2 thin film of the flake structures at MIT. [Copyright &y& Elsevier]