Role of V-V dimerization in the insulator-metal transition and optical transmittance of pure and doped ${\mathrm{VO}}_2$ thin films

An insulator to metal (IMT) transition $(T_t\sim 341\mathrm{K})$ in the ${\mathrm{VO}}_2$ accompanies a 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 ${\mathrm{VO}}_2$ thin film. Our results show that the W doping significantly lowers IMT temperature ($\sim 292\mathrm{K}$ to $\sim 247\mathrm{K}$ for 1.3% W to 3.7% W) by stabilizing the metallic rutile, $R$, phase while Tb doping does not alter the IMT temperature much and retains the insulating monoclinic, $\mathit{M}\mathit{1}$, 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 $3d_{\parallel }$ states.

Figure 1

Room-temperature XRD patterns of (a) pure and W-doped ${\mathrm{VO}}_2$ thin films, (b) pure and Tb-doped ${\mathrm{VO}}_2$ thin films. Room temperature Raman spectra of (c) pure and W-doped ${\mathrm{VO}}_2$ thin films and (d) pure and Tb-doped ${\mathrm{VO}}_2$ thin films.

Figure 2

Temperature-dependent resistivity of (a) pure and W-doped ${\mathrm{VO}}_2$ thin films (b) pure and Tb-doped ${\mathrm{VO}}_2$ thin films.

Figure 3

O $1s$ and V $2p$ XPS of (a) the pure ${\mathrm{VO}}_2$ (b) 1.3% W, (c) 2.9% W, (d) 1.3% Tb, (e) 3.3% Tb, and (f) 4.6% Tb-doped ${\mathrm{VO}}_2$ thin films

Figure 4

(a)–(c) W $4f$ XPS of the 1.3%, 2.9%, and 3.7% W-doped ${\mathrm{VO}}_2$ thin films. (d)–(f) Tb $3d$ XPS of the 1.3%, 3.3%, and 4.6% Tb-doped ${\mathrm{VO}}_2$ thin films. (g) Variation of the $V^{5+}$ and $V^{3+}$ contents for the Tb- and W-doping concentrations, along with the doping dependent $V^{4+}$ content variation in the ${\mathrm{VO}}_2$ thin film.

Figure 5

(a)–(g) Temperature-dependent Raman spectra of the pure and the W-/Tb-doped ${\mathrm{VO}}_2$ thin films collected in the cooling cycle. Temperature-dependent frequency shift of the ${\omega }_0$ Raman mode positions obtained from Lorentz fitting in (h) W-doped (i) pure, and Tb-doped ${\mathrm{VO}}_2$ thin films.

Figure 6

(a)–(f) Temperature-dependent variation of the insulating phase fraction (IPF) obtained from optical transmittance measurements and monoclinic phase fraction (MPF) obtained through the ${\omega }_0$ Raman mode in the pure and the W-/Tb-doped ${\mathrm{VO}}_2$ thin films. Data shown in these graphs are from the cooling cycles.

Figure 7

(a)–(g) Temperature-dependent optical transmittance spectra of the pure and W-/Tb-doped ${\mathrm{VO}}_2$ thin films. (h) Temperature-dependent IR transmittance at $\sim 2500\mathrm{nm}$ of the pure and W-/Tb-doped ${\mathrm{VO}}_2$ thin films in the cooling cycles.

Figure 8

(a) Tauc plot [(${\alpha h\nu )}^2$ vs $h\nu$] of the pure and W-doped ${\mathrm{VO}}_2$ thin films at room temperature. Insets: (i) Tauc plot in the IR range of photon energies and (ii) variation in the $T_{\text{lum}}$ and the ${\lambda }_K$ with the W-doping percentage. (b) Tauc plot of the pure and Tb-doped ${\mathrm{VO}}_2$ thin films at room temperature. Insets: (i) Tauc plot in the IR range of photon energies and (ii) variation in the $T_{\text{lum}}$ and the ${\lambda }_K$ with the Tb-doping percentage.

Figure 9

Room temperature (a) V ${\mathrm{L}}_{2,3}$ (b) O K-edge XAS measurements of pure and W-/Tb-doped ${\mathrm{VO}}_2$ thin films. Normalized O $K$-edge XAS of (c) pure and Tb-doped, (d) pure and W-doped ${\mathrm{VO}}_2$ thin films. The V $K$-pre-edge XAS (e) of 1.3% Tb-doped and (f) of 1.3% W-doped ${\mathrm{VO}}_2$ thin films at 300 K and 373 K.

Figure 10

(a)–(d) Room temperature (300 K) Raman-integrated intensity of the ${\omega }_{v_1}$ and ${\omega }_{v_2}$ vibrational modes in pure and Tb-doped ${\mathrm{VO}}_2$ thin films. (e)–(g) Raman integrated intensity of the ${\omega }_{v_1}$ and ${\omega }_{v_2}$ vibrational modes in the 1.3%, 2.9%, and 3.7% W-doped ${\mathrm{VO}}_2$ thin films at 200 K, 150 K, and 130 K, respectively. (h) Raman intensity variation of the V-O and V-W modes with the increase in the W% concentration.

Figure 11

Variation in the spectral position of the ${\pi }^{*}$ states, IR band gap $\left({\mathrm{E}}_{g2}\right)$, and the IR switching temperature ($T_{oc}$) with the W% and the Tb% concentration in the ${\mathrm{VO}}_2$ thin films.