**2. Thin film growth of different VO2 polymorphs**

## **2.1 VO2 (M1) and VO2 (R) phase thin films**

Monoclinic VO2 (M1) (a = 5.74 Å, b = 4.52 Å, c = 5.38 Å, β = 122.6°) with space group P21/c is the most widely studied inorganic thermochromic material which is an insulator at room temperature. It shows a first-order MIT at 68°C with a concomitant structural transition into rutile tetragonal VO2 (R) (a = b = 4.55 Å, c = 2.86 Å) having space group P42/mnm [61]. Because of MIT and the associated huge changes in the structural, electronic and optical properties, VO2 (M1) and VO2 (R) are attractive for applications in smart windows, switching devices, memory materials and so on [16, 17].

**Figure 2** shows the structural arrangement of four different phases of VO2 [64]. In the VO2 (R) phase, the vanadium atoms are equally spaced along the rutile c axis (*c*R), while in the VO2 (M1) phase, simultaneous dimerization and tilting in equivalent chains occur, leading to a zigzag pattern.

Highly oriented VO2 (M1) thin films on R-cut sapphire substrate were prepared by Borek et al. using PLD [41]. They ablated metallic vanadium target by a KrF pulsed excimer laser in an ultrahigh vacuum deposition chamber with Ar and O2 (10:1) atmosphere of 100–200 mTorr, and a substrate temperature (*T*s) ∼ 500°C followed by 1 hour post deposition annealing in the same environment. Since then PLD was employed by number of groups to grow good quality VO2 (M1) thin films by varying the deposition parameters and post deposition treatment [44–46, 65]. Several other techniques such as sputtering, CVD, etc. were also employed to grow polycrystalline and epitaxial VO2 (M1) thin films on various substrates of different orientation [34, 42, 43, 66–69]. To date, most VO2 (M1) films have been grown on substrates such as sapphire (c-type, m-type, r-type and a-type), TiO2, perovskite oxides, Si and Quartz. **Figure 3(a)** shows the grazing incidence X-ray diffraction (GIXRD) data of polycrystalline VO2 (M1) thin film by Kumar et al. which was

#### **Figure 2.**

*The schematic structures for (a) rutile (R), (b) monoclinic M1, and (c) M2 phases of VO2. Red and blue balls denote vanadium and oxygen atoms, respectively. (d) The arrangement of vanadium chains in the four phases without oxygen atoms (a-d reprinted from Ref. [64]).*

grown on quartz substrate by sputtering VO2 at room temperature and post deposition annealing at 500°C [69]. **Figure 3(b)**–**d** depict the X-ray diffraction (XRD) patterns of VO2 (M1) thin film grown on TiO2 and Al2O3 substrates of different orientation [46, 70].

VO2 (R) is the high temperature phase of VO2 (M1). So, VO2 (M1) thin films generally transforms to VO2 (R) phase when heated above the MIT temperature. Apart from this, thin films showing VO2 (R) phase at room temperature can also be stabilized by strain, hydrogenation, oxygen vacancies and doping etc. [71–76]. Fan et al. reported the growth of ultrathin VO2 (R) phase thin film on TiO2 (002) substrate [71]. Y. Zhao et al. showed that hydrogenation can also lead to growth of VO2 (R) phase thin film [72]. Very recently, Liang et al. described that increase in concentration of W dopant in V1−xWxO2/Si thin films favors the growth of VO2 (R) phase [73]. **Figure 4** shows the XRD patterns of VO2 (R) phase thin films grown by different groups.
