Scandium: An efficient dopant to modulate the optical spectrum of vanadium dioxide (VO 2 )
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Scandium: An efficient dopant to modulate the optical spectrum of vanadium dioxide ( VO2) Devanshi Bhardwaj1 · A. M. Umarji1 Received: 30 March 2020 / Accepted: 12 August 2020 © Springer Nature Switzerland AG 2020
Abstract Transparency of VO2 thin films in the visible region is an important aspect of study for its use in smart window applications. In this regard, we experimentally validate the theoretical prediction regarding the influence of scandium (Sc) doping on modulating the optical spectrum of VO2. Sc-doped nano-crystalline VO2, bulk and thin films were synthesised using a two-step process involving a rapid non-equilibrium solution combustion process and ultrasonic nebulised spray pyrolysis of aqueous combustion mixture respectively; followed by a reduction step. The presence of Sc was determined by X-ray photoelectron spectroscopy where a peak observed at 530.06 eV is attributed to Sc–O interaction. Raman spectra showed a shift in 611 c m−1 peak position to a lower wavenumber due to tensile strain arising from Sc doping. Sc-doped VO2 thin films showed semiconductor to metal transition of four orders of magnitude change in resistance. Sc-doping increased the band gap from 1.76 eV for undoped to 2.02 eV with 2.0 at. % of doping thereby making the films more transparent in the visible region of the optical spectrum. This shift in the band gap of VO2 was consistent with the theoretical reports. Thus, Sc is a potential dopant for modulating the optical spectrum of VO2 for its application in smart windows. Keywords Vanadium dioxide · Doping · Smart windows · Optical spectrum · Scandium
1 Introduction Vanadium dioxide ( VO2) is a transition metal oxide which finds use in various applications like thermochromic devices [1], micro-bolometer [2], IR-photodetector [3], metamaterial [4, 5], smart windows [6, 7], thermal switching [8], batteries [9, 10], memristic devices [11], THz modulation devices [12] etc. It is been extensively studied due to its ability to undergo a first order phase transition accompanied by semiconductor-to-metal transition (SMT) at 68 °C (TSMT ). During the phase transition, a change in crystal structure from monoclinic (M1) phase (P21/c) to tetragonal (R) phase (P42/mnm) is observed which leads to change in the V–V bond length [13]. During the SMT transition, VO2 shows a change in electrical resistivity, high resistance
(T TSMT ) as well as in the optical response with a slight hysteresis which is attributed to the stress involved due to structural change and the change in latent heat [14]. VO2 acts as IR transparent at T TSMT [15]. Due to this property, it is been for used as a coating material on windows called as smart windows [6, 16]. Various methods like sol–gel [17], pulsed laser deposition [18–20], chemical vapour deposition [21], sputtering [22, 23] etc. have been employed for fabrication of doped as well as undoped VO2 thin films. There are few limitations to use VO2 for smart window applications: (a) High T SMT , which has been overcome by adding dopants in va
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