296
P. Veerakumar et al.
and the LUMO state of amine group are almost identical, though the energy level of
the GQD HOMO state becomes gradually higher than the amine LUMO state as a
result of increasing number of NH 2 groups which leads to the difference in the band
gap of GQD [143].
Similar to GQDs, the fluorescence of C-dots is originated from the diverse factors
such as quantum confinement effect, carbon core effect, surface states, defect/trap
sites, and heteroatom/element doping [132–136]. Even so, it was verified that the
multi-emission of C-dots was predominated by the carbon core and surface states.
The shorter wavelength emissions are mainly due to the recombination of electronhole pairs in the carbon core, whereas the longer wavelength emissions originate
from the surface states [132]. The carbon core in C-dots is mainly composed
of sp 2 carbon clusters in the amorphous state. To understand the fluorescing
mechanism originated from carbon core, Zhu et al. have analyzed the carbon core
structure where the Gaussian 09 package (B3LYP/6-31G(d)) was used to optimize
the electronic structures and calculate the energies of frontier molecular orbitals
[144]. They used two different model compounds of fused aromatic rings (FARs)
and cyclo-1,4-naphthylenes with different C-dots microstructure corresponding to
graphitized carbon core (class I) and disordered carbon core (class II), respectively.
The energy gaps between the HOMO and LUMO for the class I were calculated to
be 2.82 and 1.63 eV for FAR-19 (1.2 nm) and FAR-61 (2.2 nm), respectively. But
the inverse trend was calculated for the class II to be 2.80 and 3.46 eV for CN-8
(1.5 nm) and CN-16 (2.7 nm), respectively. From these calculation data regarding
the size-dependency of C-dots fluorescence, it can be concluded that for the C-dots
with a graphitized core, the smaller the size of the core, the higher the PL energy,
while an inverse trend is observed for C-dots with an amorphous core.
Apart from the carbon core, the fluorescence was controlled by the surface states
caused by the surface passivation with small organic molecules. The functional
groups possess different energy levels leading to the various emissive states on the
surface. Upon excitation of functionalized C-dots at certain excitation wavelength,
the domination of surface emissive states generally generates fluorescence at
the longer wavelength as compared to the fluorescence emitted by carbon core.
Dhenadhayalan et al. have studied the effect of surface functional groups (-COOH
and –NH 2 ) on the fluorescence of C-dots using time-resolved fluorescence spectroscopy techniques [132]. The surface functional groups are capable of regulating
the multi-fluorescence behavior based on the electron-withdrawing and -donating
properties of functional groups. The electron transfer takes place from the carbon
core to surface domain by the presence of −COOH on the surface and vice
versa for the case of −NH 2 present on the surface. Hola et al. have reported
the red-shifted emission for the carboxylic functionalized C-dots [145]. Further,
they have analyzed coronene derivatives as a model system to understand the
influence of functional groups using TDDFT computation. The maximum emission
of functionalized coronene (tricarboxylcoronene, ester of tricarboxylcoronene) was
calculated to red shift with respect to that of coronene, as a result of an extension of
π-conjugated electrons system. Moreover, upon the addition of carboxyl groups,
the electrostatic potential surfaces show a shift of charge toward the edge. This
P. Veerakumar et al.
and the LUMO state of amine group are almost identical, though the energy level of
the GQD HOMO state becomes gradually higher than the amine LUMO state as a
result of increasing number of NH 2 groups which leads to the difference in the band
gap of GQD [143].
Similar to GQDs, the fluorescence of C-dots is originated from the diverse factors
such as quantum confinement effect, carbon core effect, surface states, defect/trap
sites, and heteroatom/element doping [132–136]. Even so, it was verified that the
multi-emission of C-dots was predominated by the carbon core and surface states.
The shorter wavelength emissions are mainly due to the recombination of electronhole pairs in the carbon core, whereas the longer wavelength emissions originate
from the surface states [132]. The carbon core in C-dots is mainly composed
of sp 2 carbon clusters in the amorphous state. To understand the fluorescing
mechanism originated from carbon core, Zhu et al. have analyzed the carbon core
structure where the Gaussian 09 package (B3LYP/6-31G(d)) was used to optimize
the electronic structures and calculate the energies of frontier molecular orbitals
[144]. They used two different model compounds of fused aromatic rings (FARs)
and cyclo-1,4-naphthylenes with different C-dots microstructure corresponding to
graphitized carbon core (class I) and disordered carbon core (class II), respectively.
The energy gaps between the HOMO and LUMO for the class I were calculated to
be 2.82 and 1.63 eV for FAR-19 (1.2 nm) and FAR-61 (2.2 nm), respectively. But
the inverse trend was calculated for the class II to be 2.80 and 3.46 eV for CN-8
(1.5 nm) and CN-16 (2.7 nm), respectively. From these calculation data regarding
the size-dependency of C-dots fluorescence, it can be concluded that for the C-dots
with a graphitized core, the smaller the size of the core, the higher the PL energy,
while an inverse trend is observed for C-dots with an amorphous core.
Apart from the carbon core, the fluorescence was controlled by the surface states
caused by the surface passivation with small organic molecules. The functional
groups possess different energy levels leading to the various emissive states on the
surface. Upon excitation of functionalized C-dots at certain excitation wavelength,
the domination of surface emissive states generally generates fluorescence at
the longer wavelength as compared to the fluorescence emitted by carbon core.
Dhenadhayalan et al. have studied the effect of surface functional groups (-COOH
and –NH 2 ) on the fluorescence of C-dots using time-resolved fluorescence spectroscopy techniques [132]. The surface functional groups are capable of regulating
the multi-fluorescence behavior based on the electron-withdrawing and -donating
properties of functional groups. The electron transfer takes place from the carbon
core to surface domain by the presence of −COOH on the surface and vice
versa for the case of −NH 2 present on the surface. Hola et al. have reported
the red-shifted emission for the carboxylic functionalized C-dots [145]. Further,
they have analyzed coronene derivatives as a model system to understand the
influence of functional groups using TDDFT computation. The maximum emission
of functionalized coronene (tricarboxylcoronene, ester of tricarboxylcoronene) was
calculated to red shift with respect to that of coronene, as a result of an extension of
π-conjugated electrons system. Moreover, upon the addition of carboxyl groups,
the electrostatic potential surfaces show a shift of charge toward the edge. This