Pressure-induced phase transitions and insulator-metal transitions in VO2 nanoparticles

We present pressure-induced phase transitions and metallization in VO2 nanoparticles characterized by synchrotron x-ray diffraction, Raman spectroscopy, and infrared reflectivity measurements. The M1-M1′-Mx phase transitions were found in VO2 nanoparticles upon compression. The results of IR reflectivity shown that pressure-induced metallization occurs in the M1′ phase with increasing pressure and the sample becomes fully metallic at the transition of M1′ to Mx. The metallic Mx phase transforms to metastable mixed phases displaying insulating properties upon decompression. We attribute the pressure-induced metallization of the M1′ phase to the strong electron correlations, while the metal-insulator transition (MIT) from the Mx to the mixed phases is found to be associated with the structural phase transitions. Both the electron-correlation-driven Mott transition and the structure-driven MIT can be achieved in VO2 by applying pressure. Compared with bulk samples, the VO2 nanoparticles exhibit larger bulk moduli and increased transition pressure due to their nanosize effects. High pressure provides an effective method to study the MIT in strongly correlated materials and paves the way for modifying electronic properties of VO2.