**3. Indium selenide series**

The indium selenide series exhibits similar characteristics to the indium sulfide series, such as multiple crystallographic phases and polymorphic materials which have unique properties already found use in various applications. The chemistry, synthesis and application of the indium selenide series is already disseminated in comprehensive literature reviews available elsewhere [39, 40].

Among recent developments in the synthesis of indium selenide, is a novel reaction protocol which has been designed to growing ultrathin films of stoichiometric indium selenide (InSe) by precipitation of the thermally evaporated InSe crystal on a chemically neutral oil [41]. In another study, the thermal evaporation technique was used albeit to synthesize InSe nanowires on silicon and quartz silica substrates through an edge-epitaxial growth mechanism [42], this work presents a solution on challenges associated with growing nanowires on these substrates as a result of the lattice mismatch. This provides easy access to investigate the efficiency of nanowires on fabricated electronic and optoelectronic devices. The epitaxial growth approach has also been employed in the fabrication of few-layer β-In2Se3 thin films on c-plane sapphire and silicon substrates through the metalorganic chemical vapor deposition method [43], the synthetic protocols have potential scale up capabilities while retaining good quality uniform film. Obtaining defect-free nanomaterials from bulk counterparts through exfoliation mediated processes still remains an economically ideal route, however, the most common issue is low yields. Recent efforts towards this direction is the development of ultrafast electrochemicalassisted delamination of bulk In2Se3 through intercalation by tetrahexylammonium ions in a typical setup provided in **Figure 6(d)** [44], the authors demonstrated that the results are reproducible and the obtained yields of up to 83% flakes which have large micron-scale lateral sizes suitable for fabricating various nanodevices.

Applications of binary indium selenide nanomaterials are provided in **Table 1**. As observed, the choice of synthetic method is crucial since it produces nanomaterials suitable for specific applications. Recent interests are towards synthesizing good quality nanosheets and thin films, attributed to the development of novel next-generation devices for use in various fields. It is apparent that the sought-after features of binary indium selenide nanomaterials are optical properties-related, hence exploitation predominantly observed in optoelectronic applications.

#### **Figure 6.**

*(a) Top and (b) side view of the layered crystal structure In2Se3; (c) the chemical structure of the tetrahexylammonium-based intercalant; (d) experimental setup; (e) images showing the beginning and completion of the experiment; (f) dispersion of delaminated In2Se3 nanosheets in dimethylformamide. Reprinted with permission from Ref. [44]. Copyright 2020 WILEY-VCH.*


**Table 1.**

*Recent advances in the application of binary indium selenide nanomaterials.*

*Indium Chalcogenide Nanomaterials in the Forefront of Recent Technological Advancements DOI: http://dx.doi.org/10.5772/intechopen.94558*

### **3.1 Indium selenide-based ternary and quaternary nanomaterials**

Multinary indium selenide-based nanomaterials, with the exception of the binary system, are rarely subjects of research interest compared to the sulfide counterparts, most probably due to synthetic challenges associated with limited economic precursors. Hence, recent technologies outlined in section 3 above rely mostly on pre-synthesized (at extreme reaction conditions) and commercial indium sulfide bulk material. Regardless of this, recent efforts on multinary indium selenide-based nanomaterials have been reported.

Silver indium sulfide nanocrystals of the AgIn5Se8 phase have been synthesized through an eco-friendly electrochemical method using L-glutathione as a stabilizing agent [56]. The photoluminescence spectra of the nanocrystals showed an increase in quantum yields with an increase in silver-to-indium ratio used during synthesis. Furthermore, the nanocrystals displayed good photothermal responses which are ideal for hyperthermia applications. Layered manganese indium sulfide nanosheets of the MnIn2Se4 phase prepared by mechanical exfoliation, have recently been demonstrated as a potential candidate for use in magnetic and optoelectronic devices due to their interesting magnetic and transport properties [57]. Computational studies using first-principle calculations have predicted properties of the layered indium selenide bromide (InSeBr) which have significantly been ignored [58]. The comprehensive Raman scattering measurements have predicted that InSeBr would be a good potential candidate for use in optoelectronic properties. Research interests on quaternary indium selenide-based nanomaterials have primarily focused on copper indium gallium selenide [Cu(In,Ga)Se2] materials which are heavily invested in the fabrication of next-generation semiconductor solar cells; a recent, comprehensive review on the science, synthesis and application of Cu(In,Ga)Se2 nanomaterials is available elsewhere [59].

## **4. Indium telluride series**

Indium telluride and derived nanomaterials are rarely common, due to a limited application scope. The most common application of indium telluride nanomaterials is in thermoelectrics. There has been attempts in gas sensing applications showing unsatisfactory sensitivity, attributed to the low electrical resistance of the nanomaterial [60]. Other applications have been mentioned elsewhere with references therein [61]. In a recent report, the authors devised a method of preparing In2Te3 thin films composed of nanowire structures from bulk InTe using a chemical vapor deposition technique through a gold-catalyzed vapor-liquid–solid growth mechanism [62]. It was however observed that the low electrical resistivity and thermal conductivity cannot be improved by simply changing the morphology of the particles. A separate study has reported that these properties can be effectively improved by doping In2Te3 with aluminum and antimony [63]. The stoichiometric InTe phase is also used in thermoelectric applications; recent studies also identify that the thermoelectric performance is improved by doping with antimony [64].

#### **4.1 Indium telluride-based ternary and quaternary nanomaterials**

Ternary analogues of indium tellurides also find use in thermoelectric applications, such as copper indium telluride (CuInTe2) and silver indium sulfide (AgInTe2). The thermoelectric properties of the former have recently been reported to be enhanced by doping with manganese [65], while for the latter, adjusting

only the silver concentration *x* (in Ag1-*x*InTe2) was sufficient [66]. The interesting properties of another ternary material potassium indium telluride (KInTe2), for the first time, have been recently predicted and investigated through theoretical first-principle calculations [67]; preliminary studies suggest the material is a semiconductor with an indirect energy band gap.
