Article pubs.acs.org/JPCC
Unraveling Mechanism on Reducing Thermal Hysteresis Width of VO2 by Ti Doping: A Joint Experimental and Theoretical Study Shi Chen,†,‡ Jianjun Liu,*,† Lihua Wang,§ Hongjie Luo,†,‡ and Yanfeng Gao*,†,‡ †
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China ‡ School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, China § Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 210204, China ABSTRACT: Tailoring effectively the hysteresis loop width of VO2 semiconductor-metal transition (SMT) is crucial to develop thermal sensor devices. The Ti-doping is known as the most effective method to reduce ΔTc and has been considered as the prototypical model in understanding the mechanism by which the ΔTc of VO2 MST could be manipulated. Here we present a joint experimental and first-principles computational study on nondoped and Ti-doped VO2 to clarify the mechanism of Ti-doping on narrowing the hysteresis loop width of VO2. On the basis of the analyses of differential scanning calorimetry (DSC), we found that phase transition temperatures in the cooling circle increase faster than those in the heating circle with increasing Ti concentrations, exhibiting a hysteresis width reduction at 2 °C per Ti at. %. First-principles calculations reveal that dopant Ti atoms break the octahedral symmetry of local structure in VO2 (R) phase. This distortion is propagated in anisotropy and exhibits an obvious nonlocal effect. In contrast, the Ti-doping-induced structural change in VO2 (M) phase is only constrained in Ti-involved chain along the a-axis. The calculated energy profiles for TixV1−xO2 phase transition shows that structural stability and activation energies increased with the increase of Ti concentration. The activation barriers in Ti-doped VO2 (R) phases are increased more remarkably than that in Ti-doped VO2 (M) phases, which is consistent with experimental observation of concentration-dependent reduction of thermal hysteresis width.
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ΔTc is doping some ions with an electron acceptor and using similar ionic radii.11,25 Many efforts have been made in doping Ti to reduce the ΔTc of VO2.11,22−24 Our previous studies showed that the ΔTc of Ti-doped VO2 film prepared by polymer-assisted deposition decreased from 38.2 to 3.5 °C, probably due to an increase in doping-induced defectnucleation site density.11 Considering interfacial stress and doping techniques, Nishikawa et al. fabricated TixV1−xO2 (x = 0.14) films on silica glass, in which thermal hysteresis disappeared.24 The Ti-doped epitaxial VO2 films showed that substitutive structural defects may promote the structural phase transition without superheating or supercooling (ΔTc
