Improving phase transition temperature of VO₂ via Ge doping: a combined experimental and theoretical study

Enhancing the semiconductor–metal phase transition temperature (T_SMT) of VO₂ is of great consequence for further exploring the potential applications of VO₂ at elevated temperatures. In this study, Ge⁴⁺-doped VO₂ (Ge_xV_1−xO₂) samples were prepared by the hydrothermal and annealing approach. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC) and resistivity–temperature (R-T) analyses were used to investigate the influence of Ge doping on the lattice structures and phase transition properties of Ge_xV_1–xO₂ samples. We found that the lattice parameter of Ge_xV_1−xO₂ decreased with the Ge concentration increasing from 2 at% to 18 at%, which was further supported by density functional theory (DFT)-based first-principle simulations. T_SMT firstly increased from 64.5 to 73.0 °C at 8 at% Ge and then decreased to 71.5 °C at higher Ge concentration. Furthermore, DFT analysis revealed that the impact of lattice distortion induced by Ge doping rather than the changes in electronic structure is more pronounced on modulating T_SMT of Ge_xV_1−xO₂. The present work has pointed out the direction that the T_SMT could be enhanced and illustrated the physical reason behind the regulation of T_SMT in ions-doped VO₂ systems.

The d (logρ)/dT vs T curves are plotted for Ge_xV_1−xO₂ (0≤x≤0.18) samples (a) un-doped VO₂ ; (b) 2%; (c) 8%; (d) 18%, the transition temperatures upon heating, T_h, and cooling, T_c. The difference between T_h and T_c gives the hysteresis width, ΔT_t, while the FWHM determines the sharpness of the semiconductor-to-metal transition