Figure 4. (A) Schematic diagram of CPDs formation process. Reproduced with permission.[79] Copyright 2022, Elsevier. (B) Schematic representation of the formation of CPDs from citric acid and ethylenediamine through a pyrolysis process. Reproduced with permission.[74] Copyright 2018, American Chemical Society. (C) Formation process and structural analysis of CPDs. Reproduced with permission.[80] Copyright 2022, Springer. (D) Schematic diagram of the nucleation and reaction process leading to CPDs. Reproduced with permission.[81]Copyright 2018, Wiley-VCH. (E) Schematic diagram of the crystallization mechanism of hexagonal crystals driven by kinetically trapped self-assembly. Reproduced with permission. [82]Copyright 2022, Wiley-VCH.
3. Aggregation in different types of CDs
The physical and chemical properties of nanoparticles are closely related to their morphology. A wide range of inorganic nanoparticles can be synthesized with well-defined morphologies, which typically involves regulating the crystal growth conditions. Morphology-engineering creates new properties, opening the door to new applications.[83-85] However, the morphology of CDs is often difficult to achieve owing to the ways they are synthesized (i.e. by rapid top-down or bottom-up strategies), hindering the further development of CDs.
As mentioned above, GQDs are typically anisotropic composed of single or multi-layer sheets, whereas CQDs and CPDs are spherical or quasi-spherical. A few studies have reported the synthesis of GQDs and CQDs with novel morphologies. However, the morphology of CPDs is especially difficult to change. In GQDs and CQDs, the aggregation of precursors occurs horizontally or vertically, while in CPDs, the aggregation of precursors is disorderly and random. Therefore, the aggregation in the synthesis of GQDs and CQDs is easier to control. Yuan et al. [86] used phloroglucinol as precursors to synthesize triangular CQDs of different sizes (Figure 5A). The raw material phloroglucinol possesses a unique molecular structure with three highly reactive hydrogen atoms at the three meta positions activated by three electron-donating hydroxyl groups, which was crucial for the synthesis of the triangular CQDs. Subedi et al.[87] reported the synthesis of anisometric CQDs using rigid-rod-shaped precursors (Figure 5B). Meng et al.[88] successfully obtained carbon quantum rings exploiting terephthalaldehyde and p-phenyldiacetonitrile as precursors (Figure 5C). The two precursor molecules undergo specific aggregation to form ribbons of different lengths. Cyano groups at the edges promoted curvature of conjugated aromatic carbon ribbons, forming carbon quantum rings with different diameters. To date, there has only been only one report demonstrating morphology-engineering of CPDs. Xiong et al.[89] synthesized carbon nanorods by a reverse micellar method (Figure 5D). First, aminopropylisobutyl polyhedral oligomeric silsesquioxane and citric acid self-assembled into complex structures (CPDs) in a nonpolar solvent, with the citric acid fraction in the assemblies then aggregating and carbonizing to form carbon nanorods.
In addition, the aggregation caused by the interaction between CDs leads to assemblies with different morphologies. Li et al.[90] first explored the mechanisms involved in the aggregation of multiple GQDs (Figure 5E). First, protonation/deprotonation of GQDs at different pH values caused GQDs to self-assemble into GQDs-water-GQDs units. Additionally, GQDs can assemble into large plates in the presence of Ca2+, which then convert into three-dimensional structures via π−π stacking. Ba et al. [91] prepared spherical arrays of CDs for solid state luminescence.
4. Special luminescence phenomenon of CDs caused by aggregation
In the previous section, we discussed the aggregation of different types of CDs in detail. When aggregation occurs between particles, their interactions can lead to special luminescence phenomena. In this section, we discuss these special luminescence phenomena resulting from CDs aggregation.