Vanadium dioxide (VO2) is a fascinating metal oxide material because of its remarkable ability to undergo a reversible metal-insulator transition (MIT) at temperatures close to room temperature, specifically at a critical temperature of 68 °C. This unique property allows VO2 to switch between conductive and insulating states. Recently, lithium-doped VO2 (Li-VO2), particularly the B-phase, has attracted significant attention as a promising electrode material for lithium-ion batteries. However, synthesising VO2 is challenging due to the multiple polymorphs that can coexist within VO2 thin films. In particular, when a transition metal like Li+ is doped beyond its solubility limit, typically around 1 at.%, in VO2 thin films, it can induce phase separation, segregation, lattice distortion, and defects in the host crystal structure. In this study, we investigated the structural, microstructural, and metal-insulator transition-temperature properties of Li+-doped VO2 thin films (Li+ ion concentration >1.0 at.%). A high-repetition-rate femtosecond pulsed laser deposition (fs-PLD) was utilised to grow Li2O-doped VO2 thin films. TEM analysis indicated that no phase-induced segregation or separation occurred at a Li + concentration <5 at.%, but segregation started to appear at Li+ concentration ≥6 at.%. XPS confirmed the successful incorporation of Li + ions within the VO2 matrix, while both XRD and Raman spectroscopy revealed the coexistence of VO2(M) and VO2(B) phases, as evidenced by the emergence of new peaks that correlated with VO2(B) as Li+ content increased. A temperature-dependent XRD pattern was employed to determine the transition temperature of the samples. Temperature-dependent resistivity measurements were performed to confirm the transition temperature obtained from XRD, indicating that the MIT decreases with increasing Li+ ion concentration. The decrease in the MIT from ∼64 °C (0 at.% Li) to 60 °C (6.87 at.% Li) as Li+ ion content increased is due to the presence of the VO2(B) phase, which is a clear indication of induced lattice distortion of VO2(M). Understanding the structural, microstructural, and MIT properties of the coexistence of the VO2(B) and VO2(M1) phases in Li-doped VO2 thin films prepared via fs-PLD would pave the way for developing innovative energy-storage devices.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)