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.