Abstract
Thermochromic M-phase vanadium dioxide VO2(M) films with different morphologies have been grown directly on smooth fused quartz substrates using low deposition rate pulsed laser deposition without posttreatment. When the substrate temperature was increased in the range 450°C–750°C, better (011) texturization of VO2(M) films was observed along with an enhancement of their crystallinity. Morphology evolved from small-grained and densely packed VO2(M) grains at 450°C to less packed micro/nanowires at 750°C. Mechanisms behind the crystallinity/morphology evolution were discussed and correlated with the effect of the temperature on the diffusion of the adatoms as well as on the V5+ valence states content in VO2(M) films. Resistivity measurements as a function of temperature revealed that the insulator-to-metal transition features of VO2(M) films (i.e., transition temperature (TIMT), resistivity variation (ΔR), hysteresis width (ΔH), and transition sharpness (ΔT)) are strongly dependent on the processing temperature. In terms of optical properties, it was found that the open (i.e., porous) structure of the films achieved at high temperature induced an improvement of their luminous transmittance. Simultaneously, the enhancement of the films crystallinity with the temperature resulted in better IR modulation ability. The present contribution provides a one-step process to control the morphology of VO2(M) films grown on smooth quartz substrates for applications as switches, memory devices, and smart windows.
Highlights
Introduction ermochromicM-phase Vanadium dioxide VO2(M) undergoes an insulator-to-metal transition (IMT) that takes place around a temperature of transition temperature (TIMT) ≈ 340K
TIMT, VO2(M) displays a tetragonal phase with metallic characteristics. e IMT is reversible and takes place at ultrafast timescales and is characterized by a dramatic change in its resistivity as well as in its infrared optical properties from being highly transmissive to being highly reflective, while the optical properties in the visible range remain almost unchanged across TIMT [1,2,3]. is makes VO2(M) very promising for ultrafast electronic switching devices, memristors, and smart windows applications, especially since the critical temperature can be decreased to room temperature by donor-level doping [2]
Smooth fused quartz, used as the substrate, was kept at 7 cm away from the target and the substrate temperature was varied for the different experiments. e laser was pulsed at a frequency of 2 Hz. e choice of this value is based on preliminary tests on the influence of the laser pulsing frequency on the morphology of the grown films toward the synthesis of VO2(M) micro/nanowires
Summary
M-phase Vanadium dioxide VO2(M) undergoes an insulator-to-metal transition (IMT) that takes place around a temperature of TIMT ≈ 340K. TIMT, VO2(M) has a monoclinic phase characterized by a high resistivity (insulator). TIMT, VO2(M) displays a tetragonal phase with metallic characteristics. E IMT is reversible and takes place at ultrafast timescales and is characterized by a dramatic change in its resistivity as well as in its infrared optical properties from being highly transmissive to being highly reflective, while the optical properties in the visible range remain almost unchanged across TIMT [1,2,3]. Full exploitation of the IMT in VO2(M) requires a thorough control of its IMT features, such as TIMT, hysteresis width ΔH, and modulation capability of its electrical and/or optical properties depending on the targeted application. It is worth mentioning that the IMTcharacteristics in VO2(M) films depend on their crystallinity and grain morphology, in addition to the impurity/dopants content [4, 5]
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