Bandgap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical absorption

Abstract

Thermochromic Mg-doped VO2 films were deposited by reactive direct current magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were inferred from Rutherford backscattering. Optical bandgaps were obtained from spectral transmittance and reflectance measurements—from both the film side and the back side of the samples—and ensuing determination of absorption coefficients. The bandgap of Mg-doped films was found to increase by 3.9 ± 0.5 eV per unit of atom ratio Mg/(Mg + V) for 0 < Mg/(Mg + V) < 0.21. The presence of ∼0.45 at. % Si enhanced the bandgap even more.

Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows

“…37 A similar approach has been used in earlier work. 8,9,38 Some of the samples had too large surface roughness to give satisfactory fits under the constraints imposed by this analysis and are not considered below. The optical constants for the remaining 22 samples were obtained from ,…”

“…Eight samples contained Si and were excluded from the analysis in the present work but were discussed elsewhere. 8 Film density ρ was evaluated from RBS data and…”

“…24 Magnesium doping was found to be able to increase T lum and to simultaneously decrease τ c by ~3 °C/at.% Mg; 1,25 the boost in optical performance was seen to be connected with band gap widening and concomitant lowering of the spectral absorptance in the luminous wavelength range. 8,10 These features could be reconciled with hybrid functional calculations. 10 Fluorine doping is another way to enhance T lum .…”

“…The overarching goal is to assess VO 2 -based materials for energy efficient fenestration, and the present paper is a sequel to earlier ones. [1][2][3][4][5][6][7][8][9][10] Vanadium dioxide is a well known thermochromic material. 11 It undergoes a first-order metal-to-insulator transition, accompanied by a crystal structure transformation from monoclinic (M 1 ) to tetragonal rutile, at a critical temperature τ c ≈ 68 °C.…”

Durability of thermochromic VO2 thin films under heating and humidity: Effect of Al oxide top coatings

“…The deposition rate was ~7 nm/minute. The deposition conditions were carefully monitored and kept since they are of crucial importance for reproducible formation of thermochromic VO 2 films, as discussed elsewhere [16,23,24].…”