Tuning the Metal-Insulator Transition in TiO2/VO2Superlattices by Modifying the Layer Thickness or Inducing Defects

Vanadium dioxide (VO2) is a thermochromicmaterial that can be used in advanced applications such as smartenergy-saving windows and other smart optical/electronic devices.However, obtaining a comfortable metal-insulator transitiontemperature while improving solar utilization in VO2remains anunresolved question at both the fundamental and application levelsof research. Although studies on designing TiO2/VO2multilayersto address the above issues have been widely reported, the natureof the metal-insulator transition and how thickness and defectsaffect phase transition behaviors are still subjects of ongoingdebate. Herein, by varying the VO2or TiO2layer thicknesses orinducing defects such as oxygen vacancies and interstitial Ti/Vatoms, the metal-insulator behavior including the atomic andelectronic structures of TiO2/VO2superlattices was systematically investigated. Our results show that the V-V distances in (m+n)TiO2/VO2(001) superlattices exhibit discontinuous dimerization characteristic and the superlattices exhibit alternating metal-insulator transition characteristics as the layer thicknessmincreases from 0 to 10. When 0 <m=n< 10, the band gaps for (m+n)TiO2/VO2(001) superlattices exhibit a downward-opening parabola. However, when 0 <m< 10 andm+n= 10, the band gapsfluctuate around 0.4 eV. Additionally, defects such as oxygen vacancies or cationic interstitial Ti/V atoms have a great impact on themetal-insulator transition in (m+n)TiO2/VO2(001) superlattices. Oxygen vacancies are preferentially located in the VO2layer.When oxygen vacancies are present in the TiO2layer, they migrate across the interface into the VO2layers, indicating that there isconsiderable interdiffusion of V/Ti interstitial atoms across the interface. The interstitial V atoms diffuse more easily into the VO2layer than interstitial Ti atoms. The currentfindings may be useful in understanding the metal-insulator behavior of VO2/TiO2superlattices by varying the layer thickness or inducing defects, thereby providing a new approach for designing VO2-basedheterostructures for smart energy-saving windows or other smart optical/electronic devices.