Metal‐to‐Insulator Transition of Hydrothermally Prepared VO₂ Crystal During Repeated Thermal Cycles

Abstract

One of the most well-known transition phenomena in solids is insulator-metal transition (IMT) of VO 2 , 1 which is related to the first-order transition between insulating monoclinic (M-VO 2 -low temperature phase) and metallic rutile (R-VO 2 -high temperature phase) VO 2 phase around 340 K. The electrical conductivity of VO 2 is suddenly changed by several orders of magnitude at the transition.1 The optical properties and other various physical properties of VO 2 are abruptly changed near the IMT. Even though the IMT of VO 2 was investigated for last several decades, the origin of the transition is still unclear. 10-14Strongly correlated electron theory (Mott-Hubbard) and electron-lattice interaction theory (Peierls) are equally accepted as explanation model for the transition.10 Especially, the transition of VO 2 accompanied with the structural transformation makes hard to unveil the origin of the transition.The IMT temperature (T H~3 40 K) can be founded by heating of the sample from below T H . The metal-to-insulator transition (MIT) temperature (T C~3 34 K), the reverse direction transition of the IMT, can be founded by cooling of the sample from above T C . Generally, there is hysteresis gap between T H and T C . 8,11,12 VO 2 bulk crystals show sharp and clear transitions in the resistance measurement with narrowed the hysteresis gap, as small as~5 K. 1,11,12,15,16 However, a thin film VO 2 , which consists of submicro-sized crystals of VO 2 , shows unclear and continuous transition at the both transitions. The T H of VO 2 film related with the thickness of the film-thinner film has higher T H , but still below T H of bulk. The increased aspect (length/width) ratio of the composed nanocrystals from 0.3 to 1.25 results the T H increases 18 K. 8,17 But, the transition temperatures of each VO 2 film keep at the nearly same points during repeated thermal cycles. 17There have been several reports for the preparation and characterization of VO 2 crystals, 11,12,14-16 thin film, 17 nanowire, 18 and so on. 19,20 In general, the observation of clear MIT and IMT of VO 2 in the resistance measurement was reported in the well-grown single crystalline samples. [11][12][13][14][15][16]18 And the most of hydrothermally prepared crystalline samples 9,19,20 and thin films showed unclear transition in the resistivity measurement as mentioned. 8,17 However, recently we reported that the hydrothermally prepared high quality VO 2 crystals, which have clear MIT and IMT in the electrical conductivity measurement. 21 In this investigation, the origin of the shift of the transition temperature, sharpness, and clearness of the first-order transition of hydrothermally prepared VO 2 crystals, were discussed.Figure 1(a) shows the XRD patterns of the product of the hydrothermal reaction, which prepared under various reaction temperatures (436, 463, 483, and 503 K). Even though there were reports 9,22 that claimed similar processes 9,22 produce VO 2 , all the patterns of the samples could be indexed as VO 2 ÁH 2 O (JCPDS No. 50-1797). Th...

The Optical Behavior of VO2 Film Modulated by the Morphology and Preferred Growing Axis

“…The mixture was stirred for more than 5 h at room temperature until the color of the solution turned to deep blue. The color change is an indication of the formation of VOCl 2 in the solution mixture . The solution was filtered with a filter paper.…”

“…Since the early report of Morin, the characteristic physicochemical properties of VO 2 near the phase transition have been extensively investigated . On heating through phase transition temperature ( T H = ~340 K), VO 2 undergoes a first‐order insulator to metal transition (IMT) and on cooling through the phase transition temperature ( T C = ~334 K), VO 2 undergoes a first‐order metal to insulator transition (MIT) . The gap between T H and T C depends on the physicochemical (particle size, impurities level, crystallinity, etc.)…”

“…The gap between T H and T C depends on the physicochemical (particle size, impurities level, crystallinity, etc.) state of the VO 2 phase . The phase transition includes a structural change between a metallic high temperature (high‐T) phase (R‐VO 2 : tetragonal rutile) and an insulating low‐temperature (low‐T) phase (M‐VO 2 : monoclinic) .…”

“…state of the VO 2 phase . The phase transition includes a structural change between a metallic high temperature (high‐T) phase (R‐VO 2 : tetragonal rutile) and an insulating low‐temperature (low‐T) phase (M‐VO 2 : monoclinic) . Furthermore, the phase transition could be initiated not only by a temperature, but also by an electric field, physical stress, electromagnetic wave, etc.…”

“…The phase transition includes a structural change between a metallic high temperature (high‐T) phase (R‐VO 2 : tetragonal rutile) and an insulating low‐temperature (low‐T) phase (M‐VO 2 : monoclinic) . Furthermore, the phase transition could be initiated not only by a temperature, but also by an electric field, physical stress, electromagnetic wave, etc. These transition properties of VO 2 make the material a good candidate for several practical applications—field effect transistors, sensor, optical switches, and smart windows …”