Metal-insulator Phase Transition In VO₂Nanoparticles

A number of transition-metal oxides exhibit phase transitions caused by heat. Among these, VO2is one of the most extensively studied materials because its phase transition occurs close to room temperature (Tc≈340K).Above Tc, VO2has a tetragonal rutile structure while its low-temperature phase is monoclinic, and its transition is accompanied by a103～105increase in resistivity,and a sharp changes in transmissivity and reflectivity in near-IR.Using the Ultra High Vacuum Cluster Beam System (UHV-CBS) with thermal annealing process, VOx thin films with thermal-induced phase transition were prepared. The optical character and the change of lattice vibration mode are studied in this thesis, analyse the phenomenon that the extinction peak near460nm and the plasmon resonance peak red-shifted and expand as temperature rise. Temperature-dependence Raman spectrum exhibit the structure transition during VO2metal-insulator transition. And the effect of laser irradiation when Raman spectrum was carried has been discussed.Our great attention has been focused on VO2nanoparticle films’optical character, especially on the change of their extinctivity dependence on temperature. It is found that the metal VO2nanoparticles exhibited an extinction peak near460nm (FWHM=50nm) and another one at near-IR (FWHM>1000nm) above Tc. Different from bulk and films, extinction peak near460nm exists in insulator VO2nanoparticles. And its change dependance on temperature is caused by Mie resonance. However, the extinction band at near-IR should be due to VO2metallic nanoparticle’s surface plasmon resonance absorption.The extinction peak position undergoes red shift along with the temperature rising, and the extinction peak becomes wider. The extinction peak broadening and red shift caused by higher temperature is mainly originated from the size effect of the plasmon resonance. VO2nanoparticles phase transition temperature and resonance wavelength have a different dependence on size. Larger VO2nanoparticles have higher phase transformation temperature and longer resonant peak.Under a higher temperature, more VO2namoparticles of larger size change into metal state, which leads to the extinction peak red shift. And more VO2nanoparticles have endured semiconductor-metal transition, they can absorb light of wider optical frequencies, so the extinction peak broadening, forming higher half peak width. There exists thermal hysteresis of VO2transmittance during MIT, and the width of its thermal hysteresis loop is about15℃.In this thesis, we have used Raman spectra to study the structure transition in VO2nanoparticles. From the data of the peak intensity in both heating and cooling process, we discoverethat the MIT is a reversible process and the VO2film is modeled with coexistence of metallic and insulating domains during metal-insulator transition. Also we found a hysteretic phenomenon across the structural transition. Because of laser irradiation, the temperature of VO2nanoparticles optical transition and structure transition are different.