**2.2 Semiconductors: introduction and utility**

Semiconductors are well known class of compounds, whose properties are considered to be between metals and insulators. The fundamental expression of this idea is the band gap energy, generated due to the presence of distinct valence band, VB (HOMO) and conduction band, CB (LUMO). The overlapping of both VB and CB is very much absent in semiconductors; instead they are differing by 'specific amount' of energy. However, there is a provision for the electrons in the VB to jump into CB by absorbing that particular 'specific amount' of energy. The required threshold energy to overcome the VB-CB difference is termed as 'Band Gap Energy'. Since the QDs are the diminutive form of the bulk semiconductors, hence they also follow the band gap theory. In QDs, the transition of electrons from the HOMO to LUMO, primarily, depends on the size of the individual particles. Smaller the size of the particle, higher energy is required for an electron to jump to conduction band and vice versa. Therefore, smaller particles show more blue shift from the bulk in both absorption and luminescence spectra than the bigger particles (**Figure 1**). Interestingly, in a solution, the particle size of QDs of a particular material is never homogeneous, i.e., the size of each dot cannot be exactly the same. Therefore, when absorption spectrum of QDs in a solution is recorded, due to the merging of peaks for each and every particle (responsible for individual electronic transition of individual particle), the absorption maximum is always broad, sometime edge only [21]. That is why, these QDs can cover a wide range of light spectrum to excite its electrons as per the requirement of energy. The broader excitation spectra of the QDs also help them in the illumination of multicolored QDs through the excitation of single light. The high stoke shift finds the QDs resistant toward the auto fluorescence, hence their sensitivity is enhanced drastically. These metamaterials are synthesized in laboratories according to their requirement in a specific application. A very interesting and important factor behind the utility of these nano materials is their tunability in size and shape. Different sized particles of different materials within the same

**Figure 1.** *Effect of size of QDs on electronic properties.*


**Table 1.**

*Band gap and excitonic wavelength of group IIB-VIA semiconductors revealing the capacity of accessing the visible light spectrum, thereby exhibiting their potential applications in optical and electronic devices.*

application are essential for providing improved efficacy of the device, e.g., quantum dot sensitized solar cells. Researchers around the world have reported various shapes of nanoparticles, viz., spherical, cubic, hexagonal, triangular, wire, ball, necklace, etc. They deliver different properties and can be utilized for different applications.

Among various semiconductors, the QDs of group IIB-VIA [zinc sulfide (ZnS), cadmium sulfide (CdS), zinc selenide (ZnSe), cadmium selenide (CdSe), zinc telluride (ZnTe) and cadmium telluride (CdTe)] are potential candidates in various applications, primarily in optoelectronics due to their wide and direct band gap (**Table 1**).

Chemistry of these semiconductors suggests that the formation of variable strong bonding between these molecules is responsible for their differences in band gap and absorption maxima. The covalency of the molecules from Sulfur to Tellurium is different in these hybrid molecules, so as their electronic transition properties are varied and hence the band gap.
