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Register a new userRole of the V-V dimerization in insulator-metal transition and optical transmittance of pure and doped VO2 thin films
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S. S. Majid
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Insulator to metal (IMT) transition (T$_t$ $sim$ 341 K) in the VO2 accompanies transition from an infrared (IR) transparent to IR opaque phase. Tailoring of the IMT and associated IR switching behavior can offer potential thermochromic applications. Here we report on effects of the W and the Tb doping on the IMT and associated structural, electronic structure and optical properties of the VO2 thin film. Our results show that the W doping significantly lowers IMT temperature ($sim$ 292 K to $sim$ 247 K for 1.3% W to 3.7% W) by stabilizing the metallic rutile, $it{R}$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $it{M1}$, phase at room temperature. It is observed that the W doping albeit significantly reduces the IR switching temperature but is detrimental to the solar modulation ability, contrary to the Tb doping effects where higher IR switching temperature and solar modulation ability is observed. The IMT behavior, electrical conductivity and IR switching behavior in the W and the Tb doped thin films are found to be directly associated with the spectral changes in the V 3$it{d_{|}}$ states.
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In this paper we used Raman spectroscopy to investigate the optical properties of vanadium dioxide (VO2) thin films during the thermally induced insulating to metallic phase transition. We observed a significant difference in transition temperature in similar VO2 films grown on quartz and sapphire substrates: the film grown on quartz displayed the phase transition at a lower temperature (Tc=50C) compared a film grown on sapphire (Tc=68C). We also investigated differences in the detected Raman signal for different wavelengths and polarizations of the excitation laser. We found that for either substrate, a longer wavelength (in our case 785 nm) yielded the clearest VO2 Raman spectra, with no polarization dependence.
The vanadates VO$_2$ and V$_2$O$_3$ are prototypical examples of strongly correlated materials that exhibit a metal-insulator transition. While the phase transitions in these materials have been studied extensively, there is a limited understanding of how the properties of these materials are affected by the presence of defects and doping. In this study we investigate the impact of native point defects in the form of Frenkel defects on the structural, magnetic and electronic properties of VO$_2$ and V$_2$O$_3$, using first-principles calculations. In VO$_2$ the vanadium Frenkel pairs lead to a non-trivial insulating state. The unpaired vanadium interstitial bonds to a single dimer, which leads to a trimer that has one singlet state and one localized single-electron $S=1/2$ state. The unpaired broken dimer created by the vanadium vacancy also has a localized $S=1/2$ state. Thus, the insulating state is created by the singlet dimers, the trimer and the two localized $S=1/2$ states. Oxygen Frenkel pairs, on the other hand, lead to a metallic state in VO$_2$, but are expected to be present in much lower concentrations. In contrast, the Frenkel defects in V$_2$O$_3$ do not directly suppress the insulating character of the material. However, the disorder created by defects in V$_2$O$_3$ alters the local magnetic moments and in turn reduces the energy cost of a transition between the insulating and conducting phases of the material. We also find self-trapped small polarons in V$_2$O$_3$, which has implications for transport properties in the insulating phase.
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