**3. Fabrication strategies of indium oxide based nanomaterials**

## **3.1 Indium oxide based thin films**

Indium oxide based nanomaterials including nanostructured thin films and bulk nanomaterials are found to be a highly exploration area in the nano domain [34–45]. For deposition of nanostructured IO based thin films, several techniques including sol-gel, sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), ink-jet printing, spray pyrolysis, atomic layer deposition (ALD) are available [34–45]. These are discussed in the next sub-sections.

#### *3.1.1 Sol-gel coating technique*

Sol-gel coating technique such as dip coating, spin coating or sometimes drain coating (especially for large size and heavy weight substrate) is also a very much useful for development/fabrication of various functional nanostructured thin films that are suitable for the use especially in the field of microelectronics and optoelectronics [4, 34–38]. The coating technique can fulfill to obtain desired physical and optical properties of the nanomaterials [34–38]. In this respect, sol-gel coating technique as a facile fabrication strategy has already been established as cost effective one for deposition of nanostructured IO based thin films [34–38]. Among other thin film fabrication techniques such as sputtering, CVD, ALD, PVD, spray pyrolysis and ink-jet printing, sol-gel technique is become a convenient one where high chemical and environmental stabilities of the nanomaterials can be obtained [34–38]. Sol-gel coating techniques can be applicable to deposit a huge numbers of highperformance nanostructured IO based thin films including ITO, IZO, IZGO, ZIO, IAO, Sb-doped IO, Cd-ITO, Cr-ITO for various applications [34–45]. A schematic diagram as shown in **Figure 2** where IO based thin films by sol-gel coating technique is described. In addition, some sol-gel based nanostructured IO thin films with their area of applications are highlighted in **Table 2**.

#### **Figure 2.**

*Schematic presentation for the fabrication of IO based thin films by sol-gel coating technique.*

## *3.1.2 Spray pyrolysis*

In spray pyrolysis technique for IO based thin-film deposition includes spraying of suitable metal salt solution onto a heated substrate [39]. In this technique, the main steps are precursor solution atomization, transportation of resultant aerosol and decomposition of precursor onto a substrate [39]. Another important factor of this technique is to select an appropriate type of atomizer for desired application. Important parameters that have to be controlled are atomization, solute concentration, temperature gradient and carrier gas [39]. Adopting this technique, the fabrication of different IO based nanostructured thin films have been reported [2, 39]. Some of the potential nanostructured thin films and their applications are given in **Table 2**.

#### *3.1.3 Ink-Jet printing*

Ink-Jet printing technique (IJP) can mainly be divided into nozzle based digital inkjet printing and non-digital screen, offset, flexography and gravure printing [40]. In this technique with appropriate ink solvents, a widespread range of flexible thin-film devices can be fabricated such as transistors, light-emitting devices, sensors and energy harvesting and storage devices [40]. Although, gravure printing results high-throughput with high resolution and noble pattern fidelity but due to the characteristic contact nature of gravure printing along with the use of highviscosity ink with binders, the contamination/residue related issues and degradation of printing materials may occur [40]. Few reports with respect to IO based nanostructured thin films are listed in **Table 2**.

#### *3.1.4 Sputtering physical vapor deposition*

A commonly used method for IO based thin film deposition is PVD in which the coating generates onto a substrate through atom by atom [41]. The PVD involves the atomization or vaporization of material from a solid source called target [41]. In this technique, the substrate majorly influences the properties of thin film. It is worthy to note that the deposition method must be performed under vacuum, plasma, gaseous or electrolytic environment. In this technique, the stresses generated into a thin film during cooling process or melting of substrate (mostly for an organic polymer)

can limit the deposition process [41, 45]. Transparent conducting ITO thin films over silicon wafer can be fabricated by PVD techniques such as magnetron sputtering, vacuum evaporation, ion-plating [41, 45] towards gas-sensors, SERS, electrochromism and self-cleaning applications [41, 45].
