**1. Introduction**

Over the years, there has been an increasing demand on state-of-art solutions to solve real world problems such as the energy crises and efficient early-detection protocols in biomedical services. The current systems in place suffer from a range of issues, e.g. an increase in the depletion rate of fossil fuel and petroleum reserves as precursors in the electrical power generation plants [1]. Although, in the context of electricity generation, there exists alternatives such as nuclear power, unwavering challenges such as toxicity of nuclear waste still persist [2]. Another good example is the use of conventional dyes for the detection of tumors (typically having issues with stability and sensitivity) and drug delivery systems, which both generally lack selectivity i.e. in crucial need of smart, guide-assisted delivery to an affected target area [3]. As a response to these issues, among many that exist, scientists and engineers have presented a range of nanotechnology-based solutions through successes in the development and pioneering work on functional nanomaterials and related devices. There are, however, reservations in trusting these technologies in the general public domains, attributed to insufficient knowledge and/or lack of educational strategies [4]. Thus, progress in introducing these systems for general use still remains a challenge, with few successes such as QLED televisions [5] already available to the general public consumers for everyday use.

The core fundamental principle to grasp on nanotechnology and nanomaterials is that when the particle size dimensions of a bulk material decrease to the nanometer scale, improved and/or novel properties emerge. Thus, properties of a material can be tuned to desired standards best suited for specific applications, by simply manipulating particle size and shape. Indium chalcogenide nanomaterials are among many functional materials which boast rich literature in the aforementioned context, hence, their technological importance continues to be showcased in widespread applications to date.

The surge in the interest of indium chalcogenide nanomaterials has mainly been fueled by their recognition as alternative candidates against giants in the field of photovoltaics and sustainable energy solutions, such as cadmium chalcogenides which are known for their toxicity issues albeit achieving high performance and efficiency in metal chalcogenide-based semiconductor solar cells and other optoelectronic applications [6]. There are other less-to-non-toxic candidates which have been identified, such as antimony [7] and tin [8] chalcogenides, among others. However, indium chalcogenides contain a broad spectrum of crystallographic phases/species which exhibit unique properties attributed to different atomic compositions and crystal lattice orientation (polymorphism), contrary to antimony and tin chalcogenides. An example of this can be seen in the indium sulfide series, where InS, In3S4, In6S7 and In2S3 (α-In2S3, *β*-In2S3 and *γ*-In2S3 [9]) phases have been obtained experimentally [10]. This, in addition to manipulating particle size and shape, as well as employing other enhancement techniques such as doping and composite fabrications, present endless opportunities to harness tailor-made properties.

The most common and easy route to tune the properties of nanomaterials is by tweaking reaction parameters during synthesis. Therefore, the choice of synthetic methods best suited for specific precursors is of crucial importance [11]. Over the years, there has been an intensive research invested on precursor design necessary to produce high quality nanomaterials [12]. Hence, metalorganic compounds gained unprecedented attention as molecular precursors compatible with a range of fabrication protocols. These molecular precursors have made it possible to access various classes of nanomaterials, although the overall nanomaterial fabrication protocols were initially a hit-or-miss process. As a result of this approach, useful data has been obtained which has formed an integral part of theoretical models used to predict novel nanomaterials and their corresponding properties. As much as molecular precursors have demonstrated their preference and superiority over conventional salt-based precursors in the context of nanomaterial fabrication, the latter is however currently ideal for the development of devices which are sensitive to impurities, among other factors. Hence, recent technological advances (from late 2019 to date of this book chapter) of indium chalcogenide nanomaterials presented in the next sections are predominantly obtained through conventional salt-based precursor routes. Interesting literature on molecular precursors for indium chalcogenide nanomaterials is available elsewhere [13, 14].
