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.