Figure 3. (A) Synthesis of CQDs from various carbon sources. Reproduced with permission. [59] Copyright 2012, Wiley-VCH. (B) Schematic of the mechanism of size control of CQDs obtained upon laser ablation in poly(ethylene glycol) liquid. Reproduced with permission.[60] Copyright 2011, Springer. (C) Preparation of GQDs by a microwave-assisted hydrothermal (MAH) method. Reproduced with permission.[61] Copyright 2012, American Chemical Society. (D) Schematic illustration of the reaction process for preparation of GQDs from EDTA (E-GQDs) and their graphene-like structures. Reproduced with permission.[62] Copyright 2015, Royal Society of Chemistry.
CPDs was first proposed by Yang et al [42, 66] in 2018, and since that time have attracted extensive research interest and attention. CPDs are transitional materials between polymer dots and fully carbonized materials.[67] Since their synthesis involves both polymerization and carbonization processes, CPDs offer a rich platform towards luminescent carbon-based nanomaterials.[68] CPDs has a unique polymer/carbon hybrid structure and special PL mechanisms.[69] Not surprising, the properties of CPDs are quite distinct from GQDs, CQDs and polymer dots. The polymeric component of CPDs confers three advantages: (1) abundant functional groups and short polymer chains resulting from incomplete carbonization, (2) polydispersity in CPDs structure, (3) and highly crosslinked network structure generated by the process of dehydration and carbonization.[70] Crosslink-enhanced emission (CEE) is a unique PL property of CPDs, which can be further divided into covalent-bond CEE and noncovalent-bond CEE (supramolecular-interaction CEE, ionic-bonding CEE and confined-domain CEE).[71]
CPDs synthesized by the carbonization of PDs by bottom-up methods usually have no clear boundary between the core and shell (Figure 1D). The carbon core of CPDs can exhibit intrinsic state or subdomain state emission.[72] Typically the core comprises a paracrystalline carbon core structure composed of tiny carbon clusters surrounded by polymer frames,[73] or instated partially dehydrated and carbonized crosslinked chains.[70] Tiny carbon clusters as the subdomain in the carbon core can adopt conjugated π-structures or diamond-like structure, with the conjugated π-structure being planar or curved (e.g. fullerene-like fragments).[74] Further, CPDs can also be synthesized by the post-synthetic decoration of CQDs with polymers or organic molecules. The CPDs synthesized by post-synthetic-decoration possess a well-defined boundary between the core (CQDs) and the polymer shell.
The precursors used to synthesize CPDs are extensive, with many organic molecules forming CPDs through hydrothermal cross-linking polymerization.[42] Since most of the precursors are asymmetric, intermolecular aggregations during dehydration lead to the formation of long polymer chains with a certain degree of crosslinking.[75] In such cases, the aggregation of the precursors is highly disordered. As the hydrothermal temperature rises, condensation, crosslinking and polymerization reactions occur in the chain segments of the preformed polymer chains, generating numerous random coils.[74, 76] Due to the shortened spatial distance, crosslinking further proceeds in the interior of polymer clusters and the structures get more compact and stable over time, resulting in CPDs with a small degree of carbonization. In the latter stages of the reaction, the polymeric fraction is reduced with a concomitant increase in the carbonization degree, leading to microcrystalline carbon regions in the interior of CPDs.[77, 78] Yang et al. [79]studied the structural evolution of CPDs (Figure 4A). CPDs with different emitting wavelengths were synthesized from CA-like precursors and ammonia, with the formation of CPDs involving three different stages. Taking CA as a representative example, CA and ammonia first form a six-membered ring molecules through amidation reactions. The π-conjugated domain of the ring systems were further extended through intermolecular amination, deacidification, and dehydration. Next, the intermediate products were further dehydrated to form a polymer. Finally, the polymers were further dehydrated and carbonized into CPDs. They also found that the carbon chain length of CA-like starting molecules controlled the cyclization mode, resulting in hexatomic, pentatomic, unstable four/three-membered ring systems or cyclization failure. CA-like starting molecules that formed hexatomic rings in the initial stages resulted in CPDs with the largest emission redshift. Shamsipur et al. [74] exhibited the existence of three different emission centers in CDs prepared through pyrolysis of CA and ethylenediamine (Figure 4B). These emission centers included molecular states, conjugate domain states, and carbon-core states. During dehydration processes at low temperatures, cyclization and polymerization of the raw materials generated highly fluorescent polymeric-like structures with a strong PL. The PL originated from molecular fluorophores incorporated in polymer structures. Further dehydration resulted in the growth of more π-conjugated structures (including polycyclic aromatic hydrocarbons, fullerene fragments, and even more complex aromatic structures) within the polymeric structures. Further carbonization generated nano-sized CPDs with carbon-cores, but the CPDs still contained fluorophore molecules associated with different aromatic domains. Wang et al. [80] explored the aggregation process of six different CPDs prepared with conjugated precursor o-phenylenediamine (Figure 4C). After being protonated by acid, aggregation was reported to take place in two ways: lateral growth (formation of long linear polymer chains) and longitudinal (formation of wide conjugated planar fragments). According to the calculated formation energies, the lateral growth needed much higher energies than longitudinal growth. The results revealed that o-phenylenediamine tends to aggregate into planar structures, then self-assemble into spherical CPDs with the polymer stacking inside the core being different from the surface polymer shell. The tangled polymer core more rapidly underwent dehydration and carbonization. The latter is the key to obtain highly photoluminescent CPDs from polymers without intrinsic fluorescence. Xia et al. [81] reported the nucleation process of CPDs was similar to the soap-free emulsion polymerization (Figure 4D). Firstly, the aggregation leads to the formation of polymer clusters. After dehydration and crosslinking, the hydrophobicity of the polymer clusters increases, leading to the internal hydrophobic structure and external hydrophilic structure, with the microphase separation aiding the formation of the carbon core.
A clear understanding of the CDs structure is essential to understanding the formation process of CDs and structure-property relationships. Owing to their small size and presence of heteroatoms, accurate structural information about CDs is difficult to obtain. This makes the main force driving their construction also uncertain. Recently, many researchers have started from the perspective of a single crystal, then tried to extend this model to capture the precise structure of CDs. Yang et al.[82] reported the successful synthesis of a new kind of crystalline luminescent organic nanodot by kinetically trapped self-assembly, which was then applied for a simplified π-packing model to simulate the structure of CDs (Figure 4E). The precise aggregation and structure induced PL of the nanodots allowed structural-property relationships between the nanodots and single crystal CDs to be established. This study shows that crystalline organic nanodots with precise structures provide a solid platform for exploring the structure of CDs.