Carbon-based materials such as fullerenes (C60), carbon nanotubes (CNTs), carbon nanodots (CNDs), activated carbons, and graphene have constituted an emerging field in research for decades. In this chapter, various carbon allotropes are explored and these are shown in Fig. 4.1 [1]. Carbon quantum dots (CQDs) also belong to carbon family and these were discovered during the purification of single-walled carbon nanotubes in 2004 [2], which have zero dimension and diameter of around 1–10 nm in size. CQDs have their own optical, electrical, and mechanical properties and exhibit characteristic strong florescence. CQDs possess many physical properties such as nano size, high thermal and electrical conductivity, long-term fluorescence stability, and good thermal stability. CQDs also possess good water solubility, photobleaching resistance, and excellent photo and chemical stability, excellent biocompatibility, low toxicity, and magnetism [3], [4], [5], [6]. Incorporation of appropriate functional groups and heteroatoms leads to desired tailored applicability. This chapter aims to present a comprehensive overview of physical properties of CQDs and their characterization. According to their nature of the cores, CQDs are classified into three types: graphene quantum dots (GQDs), polymer dots (PDs), and CNDs (Fig. 4.2) [7].

Carbon nanomaterials can be classified in several ways: dimensionality, that i…

The size-dependent physical properties such as absorbance, PL, magnetic behavior, melting and boiling point, particle size and morphology, density, tensile nature, conductivity, heat absorption, etc. of CQDs play a vital role in their different applications. These properties are mainly controlled by the synthesis procedure as both the carbon core and the surface passivation play critical role in controlling such parameters. CQDs are generally quasi-spherical in shape, and depending on the precursors and synthetic pathways utilized, they can be crystalline or amorphous carbon with a variety of adjustable surface groups [6]. TEM and SEM can be used to analyze the morphology, size distribution, and particle size of CQDs. If the particle size varies from 1 to 20 nm, SEM is used; however, if the measurement surpasses the resolution of SEM, TEM can provide greater resolving power [10]. CQDs uniform dispersion and spherical shape with its agglomeration are analyzed by TEM, and the lateral size distribution in particle size and lattice fringes may measure by HRTEM. The uniformity, 2D and 3D topographic structure can be observed by atomic force microscopy (AFM). The peak width decreases as the relative intensity of the XRD peak increases, indicating that CQDs carbonation and crystallinity are improving [25]. Thus, XRD technique provides a valuable information on the particle size, crystal structure, and purity of CQDs [21]. CQDs are primarily made up of carbon atoms, with oxygen, nitrogen, and ma…

4.3.1. Structural characterization

Structure analysis and characterization focuses on contribution of significant components such as crystal structure, structural parameters, and position of the functional groups as these have important effects on the characteristics and properties of the material. These are generally known to be structure–activity or structure–property. In other words, these are collectively termed as structural characterization techniques—namely, XRD, SEM, TEM, Raman, XPS, FTIR, AFM, UV–vis, etc.

4.3.2. Photophysical analysis

Excitation-dependent PL, also known as excitation-dependent fluorescence emission, is perhaps the most remarkable attribute of CQDs. Typical excitation-dependent luminescence spectra are shown, along with the accompanying colors. The multicolor features of CQDs are highlighted by their wide spectrum range and relatively high emission peak intensities. Indeed, one of the distinctive aspects of CQDs is that the emission color can be changed according to the excitation wavelength, which has implications in a variety of applications.