Article pubs.acs.org/JPCC
Electronic and Optical Properties of Edge-Functionalized Graphene Quantum Dots and the Underlying Mechanism Yunhai Li, Huabing Shu, Xianghong Niu, and Jinlan Wang* Department of Physics, Southeast University, Nanjing, Jiangsu 211189, China S Supporting Information *
ABSTRACT: We systematically investigate the electronic structure and optical properties of edge-functionalized graphene quantum dots (GQDs) utilizing density functional and many-particle perturbation theories. A mechanism based on the competition and collaboration between frontier orbital hybridization and charge transfer is proposed. The frontier orbital hybridization of the GQD moiety and functional group reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while the charge transfer from the GQD moiety to the functional group enlarges it. Contrarily, frontier orbital hybridization and charge transfer collaborate to shift down the energy of the first bright exciton, the former through activation of low-lying dark excitons and the latter via increased exciton binding energy. Functional groups containing a carbon−oxygen double bond (CO), namely, aldehyde (−CHO), ketone (−COCH3), and carboxyl (−COOH), are more favorable for tailoring the electronic and optical properties of pristine GQD among all the functional groups investigated here. The amino group (−NH2), although frequently employed in experiments, has a much weaker influence on electronic structure since the large charge transfer cancels out the effect of frontier orbital hybridization. reaction,26 etc., refer to the preparation of GQDs through cutting graphene-derived raw materials. On the contrary, bottom-up methods, including stepwise solution chemistry,27 precursor pyrolysis,28,29 catalyzed fullerene cage-opening,30 etc., synthesize GQDs from polycyclic compounds with aromatic structures or other molecular precursors. Although advantageous in terms of raw-material abundance, product quantity, and operation simplicity, top-down methods rely on strong oxidizing agents to a large extent, which result in the introduction of oxygen-containing functional groups into GQDs including hydroxyl (−OH), epoxy (−O−), ether (−OCH3), carbonyl (−(CO)−), and carboxyl (−COOH) groups, as revealed by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS).5 These oxygencontaining functional groups can enhance the solubility, yet they may alter the electronic and optical properties of GQDs. Wang et al. have investigated the origin of green luminescence of GQDs and attributed it to the special edge states consisting of carbon atoms on the edge of the carbon backbone and functional groups with carbon−oxygen double bond CO.31 Li et al. have found that the removal of oxygen-containing groups through NaBH4 reduction enhances the quantum yield (QY) from 11.7% to 28.9% as well as a blue shift of the
I. INTRODUCTION Graphene-based nanomaterials have attracted tremendous research interest since the successful preparation of graphene in 2004.1 Graphene, graphene oxide, reduced graphene oxide, and graphene quantum dots (GQDs) have been intensively investigated for both fundamental science and practical applications.2−4 Among graphene-based materials, GQDs, single- or few-layer graphene fragments with the diameter generally below 10 nm,5 have drawn particular attention.5−8 GQDs are superior in terms of chemical inertness, tunable photoluminescence, low cytotoxicity, and long-term photobleaching resistance.9 Their applications in electronics and optoelectronics such as super capacitor,10 flash memory,11 photodetector,12 and phototransistor,13 in medicine and pharmacology including cellular imaging,14 drug delivery,15 cancer phototherapy,14 and wound disinfection,16 and in renewable energy resources like fuel cell electrocatalyst17 and photovoltaics18 have already been demonstrated by an enormous amount of research, highlighting the great application potential of GQDs. The successful synthesis of GQDs with regulated size and morphology is a prerequisite to facilitate the application of GQDs. Various synthetic approaches have been developed so far, which can be generally classified into two categories, namely, top-down and bottom-up methods.5−8 Top-down methods, including acidic oxidation,19,20 hydrothermal21,22 and solvothermal23 cutting, electrochemistry,24,25 photo-Fenton © 2015 American Chemical Society
Received: June 21, 2015 Revised: September 16, 2015 Published: October 15, 2015 24950
DOI: 10.1021/acs.jpcc.5b05935 J. Phys. Chem. C 2015, 119, 24950−24957
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The Journal of Physical Chemistry C photoluminescence (PL) emission peak.32 Furthermore, functional groups such as amine have been introduced into GQDs for the purpose of improving solubility and QY and tuning emission wavelength.33−35 Jin et al. have shown that alkyl amine groups can lead to the red-shift of the PL spectra and the increase of PL peak intensity.33 The emission wavelength of amino group functionalized GQDs can be further tuned from 420 to 535 nm by increasing the content of amino groups, as demonstrated by Tetsuka et al.34 Moreover, the redox potential can be tuned via functionalization.36 Fluorine37 and chlorine38 functionalized GQDs have also been successfully prepared. In addition to experiments, theoretical calculations have also been utilized to investigate functionalized GQDs.39−43 Cocchi et al. have clarified the influence of functionalization-induced structural distortions on the electronic and optical properties of GQDs41 and predicted charge-transfer excitations in GQD nanojections.43 Mandal et al. have demonstrated that porphyrin-functionalized GQDs can be good candidates for applications in solar cells.42 Chen et al.40 have systemically investigated the influence of size, edge configuration, shape, functionalization, heteroatom doping, and defects on the photoluminescence of GQDs, while the impact of solvent effects has been demonstrated by Guo et al.44 Both Chen and Guo’s studies report the reduction in energy gap and redshift in optical spectra induced by oxygen-containing −OH and −COOH groups. However, an investigation on the underlying mechanism of how the functional group affects pristine GQD and a detailed comparison on the effects of common functional groups encountered in experiments are still desirable, as the existing studies have not given a detailed analysis, which are the main topics of this research. In this work the electronic and optical properties of GQDs with edge functionalization with −NH2, −OH, −F, −CHO, −COCH3, and −COOH groups are systematically investigated by employing a combination approach of density functional theory (DFT),45,46 the GW method,47−49 and the Bethe− Salpeter equation (BSE).50−52 Electronic structure and optical properties are determined by the competition and collaboration between frontier orbital hybridization of the GQD moiety and functional group and charge transfer. Functional groups containing a carbon−oxygen double bond (CO) are more favorable for tailoring the HOMO−LUMO energy gap and excitation energy of the first bright exciton. The amino (−NH2) group could also tune the optical properties of GQD, but its influence on energy gap is much weaker due to the cancellation of the frontier orbital hybridization effect by large charge transfer.
Figure 1. (a)−(h) Top and side views of relaxed structures of edgefunctionalized GQDs. Carbon, hydrogen, nitrogen, oxygen, and fluorine atoms are colored in gray, white, blue, red, and green, respectively. For clarity, carbon and hydrogen atoms in functional groups are highlighted in black and cyan. The label below each graph indicates the functional group with which the GQD is functionalized. Pristine GQD is regarded as functionalized with a hydrogen atom as aforementioned.
GQDs are denoted as GQD-R throughout the remaining part of this work, where R indicates the functional group. The electronic and optical properties of edge-functionalized GQDs are investigated via a three-step procedure.49,52 In the first step, all GQDs are fully relaxed, and Kohn−Sham energies, wave functions, and exchange-correlation matrix elements are extracted. Quasi-particle energies are then evaluated in the second step via the perturbative solution to the Dyson equation47−49 ⎡ ℏ2 2 ⎤ QP QP QP ∇ + Vion + VH + Σ(Enk )⎥ψnkQP = Enk ψnk ⎢− ⎣ 2me ⎦
(1)
where me is the mass of electron; Vion is the electrostatic potential contributed by ions; VH is the Hartree potential; 47,48 Σ(EQP nk ) is the self-energy operator in GW approximation; QP QP and Enk and ψnk are the quasi-particle energy and the quasiparticle wave function, respectively. Both G (Green’s function) and W (screened Coulomb interaction) are built from Kohn− Sham energies and wave functions and not iterated during the calculation; i.e., single-shot G0W0 calculation is performed. Finally the optical excitation energies and exciton wave functions are determined through BSE50−52 S QP (EckQP − Evk )A vck +
II. MODEL AND METHODOLOGY The molecular structures of edge-functionalized GQDs are depicted in Figure 1. A hexagonal zigzag-edged GQD consisting of 54 carbon atoms with edges saturated with hydrogen atoms is regarded as the prototype for functionalization. The armchair edged GQDs are not included as it is easy to distort upon functionalization.41 Eight different functional groups, namely, amino (−NH2), oxygen-containing hydroxyl (−OH), aldehyde (−CHO), ketone (−COCH3), and carboxyl (−COOH), fluorine (−F), hydrogen atom (−H), and methyl (−CH3) are taken into account. The inclusion of −NH2, −OH, halogen (−F), and carbonyl (−(CO)−) containing −CHO, −COCH3, and −COOH is inspired by experiments,5,34,37,38 while −CH3 is included for comparison. Pristine GQD is regarded as functionalized with hydrogen atom. For simplicity,
∑
S ⟨vck|K eh|v′c′k′⟩A vS′ c ′ k ′ = ΩSA vck
v′c′k′
(2)
EckQP
QP Evk
where and are the quasi-particle energies for conduction and valence bands; ASvck is the component of the exciton wave function; Keh is the electron−hole interaction kernel; and ΩS is the excitation energy, respectively. The imaginary part of the macroscopic dielectric function, which determines the absorption spectrum, is evaluated from the excitation energies and exciton wave functions through52 ε2(ω) =
16πe 2 ω2
∑ |λ ⃗ ·⟨0|v ⃗|S⟩|2 δ(ω − ΩS) S
(3)
where λ⃗ is the polarization vector of the incident light; v⃗ is the velocity operator; and ⟨0|v|⃗ S⟩ is the transition matrix element. 24951
DOI: 10.1021/acs.jpcc.5b05935 J. Phys. Chem. C 2015, 119, 24950−24957
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The Journal of Physical Chemistry C The validity of the DFT+GW+BSE procedure for GQDs has been clarified in previous work.53 The HOMO−LUMO energy gap of coronene, a smaller GQD with similar geometry of GQDs investigated in this work, is predicted to be 4.43 eV, while the experimental result is 4.86 eV. This error of 0.43 eV is common for organic molecules as demonstrated by the benchmark of Blase et al.54 Also, the excitation energies of the lowest two excited states are predicted to be 1.90 and 1.97 eV, in good agreement with theoretical results of 1.88 and 1.98 eV from time-dependent density functional calculations. This ensures the validity of the current three-step procedure. The DFT and GW+BSE calculations are carried out with the Quantum-ESPRESSO55 and BerkeleyGW56 packages, respectively. Wave functions and charge density are expanded within plane-wave basis sets with kinetic energy cutoffs of 60 and 240 Ry, respectively. Electron−nucleus interaction is treated utilizing norm-conserving pseudopotentials. The exchangecorrelation of electrons is approximated by the Perdew and Zunger functional57,58 whose validity for GQDs has been carefully justified.53 To avoid the spurious interactions between periodic images, the supercell technique is employed with vacuum layers thicker than 10 nm along all three dimensions. All structures are relaxed without any constraint until the total energy is converged within 10−4 eV and the forces on every atom are smaller than 0.01 eV/Å. In GW calculations the dielectric matrix and self-energy operator are constructed using more than 1400 orbitals, which ensure the convergence of the band gap within 0.05 eV. The frequency dependence of the selfenergy operator is treated by the general plasmon-pole model.49 To get converged absorption spectra in the range of 0−8 eV, 24 occupied and 60 unoccupied orbitals are used to build the electron−hole interaction kernel. The Coulomb interaction is truncated at the edge of the Wigner−Seitz cell in both GW and BSE calculations to boost the convergence on the thickness of the vacuum layer.59 Only the gamma point is taken into account in all the calculations.
Figure 2. Energy diagrams of edge-functionalized GQDs. Blue and red lines refer to results from DFT and GW calculations, while solid and dashed lines denote occupied and unoccupied energy levels, respectively. For every GQD the energy levels corresponding to HOMO and LUMO are given. Both the DFT and GW energies have been corrected according to the vacuum level.
prominent down-shift of the energy levels, due to the π electrons in CO that can extend the π-electron system in the GQD moiety.41 More detailed analysis reveals that for GQDs with CO the energy shifts of LUMO are larger than those of HOMO, while for the ones without CO the situation is the opposite. These distinct behaviors of energy shifts can be elucidated with frontier orbital hybridization defined as the sum of projection of edge-functionalized GQD frontier orbitals onto the atomic orbitals of the functional group. As illustrated in Figure 3(a), the hybridization of HOMO is much larger than that of LUMO for GQDs without CO, while for GQDs with CO it is the LUMO that is much more hybridized. The uneven energy shifts of HOMO and LUMO result in reduction of the energy gap for both of the groups of GQDs, although the mechanism is distinct. GQD−NH2 has the lowest HOMO− LUMO energy gap among the GQDs without CO, while for GQDs with CO it is GQD−CHO as shown in Figure 3(b) and listed in Table 1. The reduction of energy gap is consistent with frontier orbital hybridization where a larger hybridization indicates a larger reduction. Energy gap reduction has been observed in edge-functionalized GQDs in earlier studies39−43, and 35−55% of the reduction has been considered to arise from functionalization-induced distortions.41 Nevertheless, as the distortions are negligible in this work the energy gap reduction is thus induced by frontier orbital hybridization mainly. The inclusion of many-body effects via the GW method results in a significant increase in the HOMO−LUMO energy gap, with quasi-particle corrections larger than 2.7 eV. Such large quasi-particle corrections stem from the enhanced electron−electron interaction due to the low dimensionality of GQDs.53 Apart from the quasi-particle corrections, it is notable that the energy gap ordering is different from the DFT case. At DFT level, the energy gap for GQD−NH2 is 1.81 eV, close to 1.74 eV of GQD−CHO and 1.75 eV of GQD− COCH3. The energy gap of GQD−CHO is slightly larger than that of GQD−COCH3. However, the quasi-particle energy gap of GQD−NH2 is much larger (4.64 eV) than that of GQD− CHO (4.48 eV) and GQD−COCH3 (4.47 eV). Also, the energy gap of GQD−CHO is larger than that of GQD− COCH3. This abnormality is caused by the charge transfer
III. RESULTS AND DISCUSSION The relaxed structures of edge-functionalized GQDs are shown in Figure 1. Different from the results obtained by Cocchi et al.,41 structural distortions here are negligible, with the carbon backbone of the GQD moiety remaining its planar geometry and planar functional groups parallel to the GQD plane. Displacements induced by edge functionalization are in the range of 0.01−0.04 Å. The reason for such small distortions is that only one functional group is attached to zigzag-edged GQDs. Such small distortions should exclude the structural influences on the electronic and optical properties of edgefunctionalized GQDs. The energy levels of frontier orbitals of edge-functionalized GQDs are depicted in Figure 2. Depending on the presence of the carbon−oxygen double bound CO, the edge-functionalized GQDs can be divided into two groups, one consisting of GQD−CH3, GQD−NH2, GQD−OH, and GQD−F and the other including GQD−CHO, GQD−COCH3, and GQD− COOH. For the GQDs with CO, both the DFT HOMO and LUMO energies shift toward lower energies with respect to pristine GQD. The situation for GQDs without CO is more complicated, where GQD−CH3, GQD−NH2, and GQD−OH shift toward higher energies, while GQD−F shifts toward lower energies. The differences in energy level alignment for −CH3/ −COCH3 and −OH/−COOH functionalized GQDs suggest that the carbon−oxygen double bound CO will introduce a 24952
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Figure 3. (a) Frontier orbital hybridization of HOMO and LUMO; (b),(c) HOMO−LUMO energy gaps of edge-functionalized GQDs at DFT and GW levels, respectively; (d) charge transferred from the GQD moiety to function group.
−NH2 group (1.01) is much larger than that of −CHO (0.12) and −COCH3 (0.09). Consequently, the quasi-particle correction of GQD−NH2 is larger than GQD−CHO and GQD−COCH3, which leads to a larger HOMO−LUMO energy gap eventually. It is similar for the case of GQD−CHO and GQD−COCH3 where the −CHO group withdraws more electrons from GQD. The above dicussions clearly reveal the competetion between frontier orbital hybridization and charge transfer in determining the HOMO−LUMO energy gap as illustrated in Figure 4(a). Frontier orbital hybridization reduces the gap via uneven shifts of HOMO and LUMO energies, while charge transfer enlarges the gap through reducing the electronic screening. The most obvious example is GQD−NH2, for which the large quasiparticle correction arising from charge transfer almost cancels out the energy gap reduction induced by frontier orbital hybridization. Therefore, the −NH2 group is not as efficient as the CO-containing functional groups in reducing the energy gap, although it is commonly used in experiments to tune the photoluminescence (PL) of GQDs. Alternatively, changing the content of oxygen-containing groups via the modification of oxidation degree of GQDs would be a more effective way. The optical absorption spectra of edge-functionalized GQDs calculated utilizing random-phase approximation (RPA) and BSE in conjugation with the GW method are shown in Figure 5. Electron−hole interaction introduces great changes into the profiles of absorption spectra. The first absorption peaks are significantly red-shifted and suppressed. The absorption spectra of GQD−CH3, GQD−OH, and GQD−F are very similar to that of pristine GQD, with the first peak located around 3.10 eV. However, absorption spectra of GQD−NH2, GQD−CHO, GQD−COCH3, and GQD−COOH are much different. The peak at 3.10 eV is split into two peaks, one shifted to lower energies (2.70 eV) and the other shifted to higher energies
Table 1. HOMO−LUMO Energy Gaps at DFT and GW Theory Levels, Quasi-Particle Corrections to Energy Gap, Binding Energies of the First Bright Exciton, and Transferred Bader Charges from the GQD Moiety to the Functional Groupa GQD
Eg-DFT
Eg-GW
ΔEg
Eb
charge
−H −CH3 −NH2 −OH −F −CHO −COCH3 −COOH
1.96 1.94 1.81 1.90 1.94 1.74 1.75 1.83
4.68 4.65 4.64 4.66 4.68 4.48 4.47 4.54
2.72 2.71 2.83 2.76 2.75 2.74 2.72 2.71
1.58 1.55 1.84 1.55 1.25 1.78 1.76 1.74
−0.04 −0.05 1.01 0.93 0.80 0.12 0.09 0.16
a
Energies are in eV, and charges are in elementary charge. A minus sign in the charge indicates that the functional group donates electrons to the GQD moiety. For pristine (−H), −CH3, −OH and −F functionalized GQDs the first bright exciton refers to the 9th exciton, while for other GQDs it refers to the 3rd exciton.
between the GQD moiety and function group. As demonstrated in Figure 3(d) and Table 1, all the function groups withdraw electrons from the GQD except −H and −CH3. Previous researches have revealed that electronic screening is closely related to the density of carriers and will be enhanced if the carrier density increases.60,61 The reduction of electrons, on the other hand, will reduce screening and enhances electron− electron interaction and eventually results in larger quasiparticle correction. This is confirmed by the fact that most of the electron-withdrawing groups functionalized GQDs have larger quasi-particle corrections, while the electron-donating −CH3 functionalized GQD has lower quasi-particle corrections than pristine GQD. The number of electrons transferred to the 24953
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1.58 eV (pristine GQD), 1.55 eV (GQD−CH3), 1.57 eV (GQD−F), and 1.55 eV (GQD−OH). For GQD−NH2, GQD−CHO, GQD−COCH3, and GQD−COOH the indices of first bright excitons are 3 and 4, which are mainly composed of transitions among HOMO−2, HOMO−1, HOMO, LUMO, LUMO+1, and LUMO+2. It is clear that for these GQDs more frontier orbitals are involved in the optical transitions than the other GQDs, consistent with the extent of frontier orbital hybridization demonstrated in Figure 3(a) and Table 1. The binding energies for the third exciton are 1.84 eV (GQD− NH2), 1.78 eV (GQD−CHO), 1.76 eV (GQD−COCH3), and 1.74 eV (GQD−COOH), respectively, much larger than that of pristine GQD and GQD−CH3, GQD−F, and GQD−OH. It seems abnormal that even though −F and −OH functional groups withdraw more electrons from the GQD moiety than −CHO, −COCH3, and −COOH groups the corresponding exciton binding energies are still lower. This is conflicting with the common knowledge that decreased density of carriers would enhance electron−hole interaction and lead to larger exciton binding energy. This abnormality is attributed to the collaboration between frontier orbital hybridization and charge transfer, as elucidated below. Indeed, charge transfer will result in large exciton binding energies. Moreover, frontier orbital hybridization will activate the low-lying dark excitons, further shifting the first bright exciton toward lower energies as illustrated in Figure 4(b). This is directly reflected in the indices of the first bright exciton for GQD−F/GQD−OH and GQD−CHO/GQD−COCH 3 / GQD−COOH, respectively. For GQD−F/GQD−OH the charge transfer is very large, but the frontier orbital hybridization is much smaller compared to GQD−CHO/GQD− COCH3/GQD−COOH, as shown in Figure 3(a). This results in the higher indices of the first bright excitons. On the contrary, although GQD−CHO/GQD−COCH 3 /GQD− COOH have smaller charge transfer, their frontier orbital hybridization is very large, which compensates the effect of charge transfer and eventually results in large exciton binding energies. A more intuitive picture of the activation of dark excitons via frontier orbital hybridization is demonstrated in the
Figure 4. Schematics of (a) competetion and (b) collaboration between frontier orbital hybridization and charge transfer in determining HOMO−LUMO energy gap and energy of the first bright exciton, respectively. The x-axis of figure (b) is energy. Label ΔI in figure (b) indicates the increase of oscillator strength due to frontier orbital hybridization.
(3.15 eV). Detailed analysis reveals that the first peak of pristine GQD is contributed by the 9th and 10th excited states which mainly involve transitions between HOMO−1, HOMO, LUMO, and LUMO+1. It is the same for GQD−CH3 and GQD−F, while for GQD−OH the excited state indices become 9 and 11. The exciton binding energies for the ninth exciton are
Figure 5. (a)−(h) Absorption spectra of edge-functionalized GQDs at RPA and BSE theory levels. Plotted quantity is the imaginary part of the dielectric function. The unit of dielectric function is arbitrary. All spectra have been broadened with Gaussian-type broadening of 0.05 eV. 24954
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out the effect of frontier orbital hybridization. Furthermore, both frontier orbital hybridization and charge transfer shift the energy of the first bright exciton toward lower energies, the former through activation of dark excitons and the latter through large exciton binding energies. This work also reveals that functional groups containing a carbon−oxygen double bond (CO) have advantages over the other groups in tuning the electronic and optical properties of GQDs due to small charge transfer and large frontier orbital hybridization. The −NH2 functional group is also effective in tuning the optical properties but not as effective for electronic properties for its large charge transfer. These insightful understandings can be used for better experimental designs for efficient engineering of electronic and optical property of GQDs.
spatial distributions of the third exciton in Figure 6(a)−(f). It is clear that for the third exciton of GQD−CHO/GQD−
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05935. Details of transitions according to the lowest dark state (Table S1) and the first bright state (Table S2) (PDF)
Figure 6. (a)−(f) Spatial distributions of the 3rd exciton for pristine and −OH, −F, −CHO, −COCH3, and −COOH functionalized GQDs. White and blue colors denote low and high field values, respectively. The hole is fixed at the center of GQD as indicated by the solid circle. Integration over the z-axis has been performed.
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AUTHOR INFORMATION
Corresponding Author
*Prof. Jinlan Wang (
[email protected]). Phone number: +8625-52090600-8210.
COCH3/GQD−COOH the electron mainly locates at the functional group, while for GQD−F and GQD−OH the electron locates at the GQD moiety, similar to the case of pristine GQD. To illustrate the effects of functional groups on optical excitations of GQDs, the composition of the lowest dark excited state and the first bright state is listed in Tables S1 and S2. For all the GQDs, the lowest dark state is mainly contributed by two transitions between HOMO−1/HOMO and LUMO/LUMO+1 that have similar contribution. It is notable that the transitions for pristine and −CH3 functionalized GQDs are different from that for other GQDs, reflecting that the −CH3 group has little influence on the optical properties of pristine GQD. For the first bright state it is more complicated. The bright states of GQD−CHO, GQD− COCH3, and GQD−COOH are composed by transitions HOMO−1 → LUMO+2 and HOMO → LUMO+1, while more transitions contribute to the bright states for other functionalized GQDs. It is obvious from Table S2 that the transitions in GQD−NH2, GQD−CHO, GQD−COCH3, and GQD−COOH involve more frontier orbitals than other GQDs except GQD−OH, consistent with the extent of frontier orbital hybridization in Figure 3(a) as aforementioned, indicating that more orbitals have been activated.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the NSFC (21525311, 21173040, 21373045) and NSF of Jiangsu (BK20130016) and SRFDP (20130092110029) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin.
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REFERENCES
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IV. CONCLUSION In summary, the electronic and optical properties of edgefunctionalized GQDs have been investigated with density functional theory, the GW method, and the Bethe−Salpeter equation. A mechanism based on competition and collaboration between frontier orbital hybridization and charge transfer is proposed. Frontier orbital hybridization lowers the HOMO− LUMO energy gap via uneven shifts of HOMO/LUMO, while charge transfer enlarges the energy gap through the modification of electronic screening. The most obvious example is the −NH2 functionalized GQD, where the large quasi-particle corrections due to charge transfer almost cancel 24955
DOI: 10.1021/acs.jpcc.5b05935 J. Phys. Chem. C 2015, 119, 24950−24957
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