Orbital electronic occupation effect on metal–insulator transition in Ti _x V_1−x O₂

Abstract

A series of Ti V O (0% ⩽ x ⩽ 4.48%) thin films on c-plane sapphire substrates have been fabricated by co-sputtering oxidation solutions, and the metal-insulator transition temperature (T ) of Ti V O films rises monotonically at the rate of 1.64 K/at.% Ti. The x-ray diffraction measurement results show that, after Ti ion doping, the rutile structure expands along the c axis while shrinking along the a and b axis simultaneously. It makes the V-O bond length shorter, which is believed to upshift the π orbitals. The rising of π orbitals in Ti-doped VO has been illustrated by ultraviolet-infrared spectroscopy and first-principles calculation. With the Ti ion doping concentration increasing, the energy levels of π orbitals are elevated and the electronic occupation of π orbitals decreases, which weakens the shielding for the strong electron-electron correlations in the d orbital and result in the T rising. The research reveals that the T of VO can be effected by the electronic occupancy of π orbitals in a rutile state, which is helpful for developing VO-based thermal devices.

Impact of anharmonic effects on two phases of VO2 thin-film phase transition materials

“…Vanadium dioxide (VO 2 ) has attracted a lot of attention due to its significant change in optical and electrical properties [1][2][3][4] during phase transition near room temperature (∼340 K) [1][2][3][4][5][6][7]. Below 340 K, VO 2 is in the monoclinic phase [1][2][3][4][5][6][7][8] (VO 2 (M)) with the space group symmetry P21/c [1,7,8]. In this case it behaves as an insulator with very high resistance * Authors to whom any correspondence should be addressed.…”

“…value and the thermal conductivity appears at a relatively low value (nearly 3.6 Wm −1 K −1 ) [9]. However, it is shown that when the temperature increases to near 340 K, VO 2 film undergoes a metal-insulator transition [1][2][3][4][5][6][7]; from the monoclinic phase to the rutile phase (VO 2 (R)). As a consequence, above 340 K the film has metal-phase behaviors [1][2][3][4][5][6][7], with low resistance and high thermal conductivity (k ∼ 6.0 Wm −1 K −1 ) [9].…”

“…However, it is shown that when the temperature increases to near 340 K, VO 2 film undergoes a metal-insulator transition [1][2][3][4][5][6][7]; from the monoclinic phase to the rutile phase (VO 2 (R)). As a consequence, above 340 K the film has metal-phase behaviors [1][2][3][4][5][6][7], with low resistance and high thermal conductivity (k ∼ 6.0 Wm −1 K −1 ) [9]. During the significantly fast phase transition (in the femtosecond order) the metastable VO 2 (B) and VO 2 (A) phases [10,11] can hardly keep stable.…”

A Monoclinic V1-x-yTixRuyO2 Thin Film with Enhanced Thermal-Sensitive Performance

“…But the SMT temperature saturates at 80-85°C as the doping level reaches above about 8at% [37,50]. The previous literature demonstrated the SMT amplitude of Ti- doped VO 2 thin films obviously decreases with Tidoping level, owing to outstanding increase of the resistivity for the metal state [48]. This could originate from stronger Ti-O bonds than V-O ones.…”

“…This indicates the existence of monoclinic VO 2 in VTO from RT to 106°C. It has reported that Ti-doping increases the SMT temperature of VO 2 for a low doping level [48,49]. But the SMT temperature saturates at 80-85°C as the doping level reaches above about 8at% [37,50].…”