Figure 2. (A) Fabrication of GQDs via a soft-template method
based on Hexa-peri-hexabenzocoronene as the carbon source.
Reproduced with
permission.[49] Copyright 2011, American Chemical
Society. (B) Schematic diagram of GQDs synthesized by catalytic solution
chemistry using glucose. Reproduced with
permission.[52] Copyright 2019, American Chemical
Society. (C) Pyrolysis technique for producing GQDs using CA as a raw
material. Reproduced with permission.[51]Copyright 2012, Elsevier. (D) Solubilization strategy towards colloidal
GQDs using 2, 4, 6-triakyl phenyl groups. Reproduced with
permission.[53] Copyright 2010, American Chemical
Society. (E) Schematic energy level diagram for GQDs specimens and
quantum confinements introduced in the sp2 domain.
Reproduced with permission.[55] Copyright 2016,
American Chemical Society. (F) The synthetic route towards GQDs from
CX-72 carbon black. Reproduced with
permission.[56] Copyright 2012, Royal Society of
Chemistry. (G) Illustration of band gap narrowing by enlarging the
π-conjugated system via conjugating GQDs with poly-aromatic rings or by
introducing an intermediate n-orbital level in the band gap via
conjugating with electron-donating groups. Reproduced with permission.
Reproduced with permission.[57] Copyright 2018,
American Chemical Society.
As with GQDs, CQD can be synthesized by top-down and bottom-up routes
(Figure 1C). Methods for preparing the CQDs by top-bottom routes are
similar to those described for GQDs and does not involve the aggregation
of precursors. Tao et al. [59] obtained three
different CQDs from graphite, single-walled carbon nanotubes and
multi-walled carbon nanotubes, respectively, using acid treatments
(Figure 3A). Due to the mixed
H2SO4/HNO3 treatments,
the obtained CQDs possessed a high oxygen content (up to 55%). TEM and
AFM images shown that all CQDs samples consisted of spherical
nanoparticles. The fluorescence emission of the CQDs is attributed to
the poly-aromatic sp2 carbon nanostructures, as well
as the various functional groups on their surface. Hu et al.[60] synthesized CQDs by laser irradiation of
graphite flakes in polymer solution. By regulating the laser pulse
width, cavitation bubbles of various sizes and densities were formed,
leading to CQDs of different sizes (Figure 3B). After a laser pulse,
nuclei/clusters were first formed. As the cavitation bubbles shrunk, the
nuclei/clusters were forced to interact with aggregation creating CQDs.
For the bottom-up synthesis of CQDs, the most commonly used methods are
microwave methods, thermal decomposition, and hydrothermal methods.
Unlike GQDs, the precursors aggregate by dehydration to form tiny
spherical nuclei. The remaining reaction precursors then further
aggregate on the surface of the initially formed nuclei, eventually
forming CQDs. Tang et al. [61] prepared CQDs using
glucose as a precursor using a hydrothermal method (Figure 3C). TEM and
AFM showed the diameter and height of the CQDs to 3.4 ± 0.5 nm and 3.2
nm, respectively, confirming a near spherical structure.
The bottom-up formation process of CQDs typically involved the following
steps: Precursors are first dehydrated to form C=C-containing nuclei.
Additional precursor molecules then deposit on the surface of the nuclei
and generate new C=C bonds by dehydration. By this pathways, spherical
CQDs and formed which becomes larger as the reaction time increases
(assuming precursor remains available). Ma et al.[62] showed that intermediates produced during
ethylene diamine tetraacetic acid decarboxylation gradually fused
together to give graphite-like structures (CQDs) under the solid-state
reaction conditions (Figure 3D), with the same compounds being converted
into graphitic carbons at much higher pyrolysis temperatures.
The PL of CQDs is affected by quantum confinement effects and also size,
functional groups, and edge effects.[63] Surface
defects formed by surface oxidation can capture excitons and generate
fluorescence associated with surface-states. With the increase of
surface oxidation degree of CQDs, more surface defects were formed. This
results in a narrowing of energy levels and a redshift in PL emission.
Single sheets are often required for band gap PL emissions in
graphene-based materials in order to restrain interlayer quenching.
However, the single sheet requirement does not hold for defect-origin
fluorescence [64] in quasi-spherical CQDs. Wang et
al. demonstrated that CQDs tend to have green PL centers, attributed to
edge states (carbon atoms on the edge of carbon backbone and functional
groups that include C=O like carbonyl and carboxyl groups). They found
that competition between the emission centers (edge states) and the
traps control the optical properties of CQDs.[65]
2.3. CPDs