Quantum size effect across semiconductor-to-metal phase transition in vanadium dioxide thin films

The phase transition temperature of vanadium dioxide (VO2) thin films shifts to the vicinity of room temperature by reducing the dimensions from microscale to nanoscale without the use of any dopant. This quantum size effect elucidated by studying the structural, optical and electrical properties of nanothin and microthin films of VO2 across their semiconductor-metal phase transitions has been demonstrated in this reported work. The films fabricated on glass substrates via the inorganic sol-gel route were characterised by Rutherford backscattering spectrometry (RBS), X-ray diffractometery, scanning electron microscopy, ultraviolet-visible spectroscopy and the four-probe method. The RBS results estimate a layer thickness of 15 and 291 nm for nanothin and microthin films, respectively. The films were polycrystalline in their nature of [011] orientation with regular and irregular crystallites of a cuboidal surface morphology. Estimation of crystallite size revealed that nanothin films attain poor crystallinity as compared with microthin films. The blue shift was observed in nanothin films from microthin films as maximum absorption occurred at wavelengths of 360 and 443 nm, respectively. Optical bandgaps of 2.3 and 1.87 eV were found for nanothin and microthin films, respectively. Depletion in metal-to-semiconductor and semiconductor-to-metal transition temperatures were experienced from higher temperature regimes of 341 and 335 K for microthin films to lower temperature regimes of 319 and 305 K for nanothin films, respectively.