Letter pubs.acs.org/JPCL
Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence Te-Fu Yeh,† Wei-Lun Huang,‡ Chung-Jen Chung,§ I-Ting Chiang,‡ Liang-Che Chen,† Hsin-Yu Chang,∥ Wu-Chou Su,‡ Ching Cheng,⊥ Shean-Jen Chen,§,∥ and Hsisheng Teng*,†,§ †
Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan ‡ Department of Internal Medicine, College of Medicine and Hospital, National Cheng Kung University, Tainan 70101, Taiwan § Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan ∥ Department of Engineering Science, National Cheng Kung University, Tainan 70101, Taiwan ⊥ Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan S Supporting Information *
ABSTRACT: Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp2 domains, which leads to a change in the π*−π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology.
fluorescence.3,8,9 The oxidized sp3 sites can be regarded as fluorophores, and they contribute to long-wavelength fluorescence.14,15 Exploring the size-dependent properties of GO is essential for elucidating the quantum-confinement fundamentals underlying the optical design of GO. Numerous methods have been developed for synthesizing graphene oxide dots (GODs), which exhibit PL emissions of various wavelengths.4,5,16−20 To obtain size-dependent PL, we should consider the three major origins of luminescence emissions in GODs: (I) sp2 domains,3,5,20−22 (II) zigzag edge sites,4,5 and (III) surface defect sites.20−23 For investigating quantum confinements in the sp2 domains (origin I), previous studies have regulated the sp2-domain size by removing the oxygen groups from GODs; however, these GODs did not show sp2-domain-dependent PL emissions,3 and this observation is attributable to uneven oxidation in the sp2 domains. Peng et al. reported luminescence in three colors in the GODs of three sizes. The size-dependent luminescence was attributed to emissions from the zigzag edge structure (origin II) that shows a controllable energy difference between the σ and π states in a carbene-like triplet ground state.5 However, the PL emissions were dependent on the excitation wavelength,6,7
T
he zero-mass-electron feature for graphene indicates that finite-sized graphene can exhibit a quantum-confined energy gap.1 The quantum-confined energy gaps of graphene quantum dots carved from graphene were determined using a single-electron transistor with quantum point contacts.2 Because graphene is an environmentally sensitive material, its intrinsic electronic properties can be determined only under ultraclean and strictly regulated conditions, which restricts its practical applications. Graphene oxide (GO)3 and graphene quantum dots bearing oxygen functionalities4 provide an opportunity to explore the electronic structures of graphene under ambient conditions. Despite considerable efforts, studying the optical properties of GO to elucidate the quantum size effect has been difficult because the presence of defect states results in excitation-wavelength-dependent photoluminescence (PL) emissions,5−7 and the inhomogeneous distribution of oxygenated sites prevents the optical properties from being strongly dependent on the size.8,9 The synthesis of homogeneously oxygenated graphene quantum dots is the key to deciphering the electronic structure of GO with quantum confinement. GO contains sp2 domains and sp3-hybridized carbon atoms covalently bound to oxygen functional groups.10−13 The CO bonds disrupt the extended sp2 conjugated network and isolate the sp2 domains, where the π electrons are confined; consequently, the band gap becomes finite, giving rise to © XXXX American Chemical Society
Received: April 7, 2016 Accepted: May 18, 2016
2087
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
Letter
The Journal of Physical Chemistry Letters indicating that surface defect states might be involved in the PL process, and the size effect was not strong. The surface defect sites on GODs (origin III), such as oxygen functionalities,20−23 can provide intermediate states (i.e., the nonbonding states (nstates) in oxygen atoms) within the π−π* energy gap, which is reflected by red shifts in both light absorption (n → π* transition) and PL emissions. Quantum confinement in GODs is difficult to detect because the reported synthesis processes generally cause inhomogeneous oxygen distribution and vacancy defects.24−26 Previous studies reported quantum confinement in graphene and carbon dots synthesized using amidative cutting27 and ultrafiltration,28 respectively. However, these studies neglected the influence of heterogeneities in oxygen functionalities on PL emissions. In the present study, we develop a method for synthesizing high-crystallinity, high-uniformity GODs emitting excitation-wavelength-independent PL and devise a strategy for particle sieving to elucidate how quantum confinement affect PL emissions from GODs. We demonstrate that the PL is associated with the π* → n transition of electrons. The observed quantum confinement is ascribed to the change in the size of the sp2 domains, which leads to a change in the π−π* energy gap; the n-state levels remain unaffected by the change in size. The results make a meaningful contribution to existing fundamental understanding of quantum-confined photonics of graphene. The preparation of the graphene oxide quantum dots (GOQDs) used in this study is described in the Supporting Information. In brief, we first prepared GODs from mild oxidation of GO sheets that were derived from graphite oxidation and then sieved the GODs through poly(ether sulfone) membranes to obtain GOQDs. Here, we report the direct observation of the quantum size effect in GOQDs obtained by sieving high-quality GODs by using centrifugation tubes filled with the polyethersulfone membranes of different pore sizes. Figure S1 presents high-resolution transmission electron microscopy (HRTEM) images of GOQD specimens and Fourier transform (FFT) patterns of two GOQD specimens. All the particles have high crystallinity and the graphene {11̅00} planes with a d-spacing of 0.213 nm. However, aggregation of some particles was inevitable in TEM analysis (Figure S1i) because the GOQDs were not protected by capping reagents. We used Raman spectroscopic analysis to determine the mean sp2-domain size of the GOQD specimens according to the intensity ratio between the D and G bands in the Raman spectra (Figure S2).29−32 The sizes estimated by Raman spectroscopy were consistent with those approximately determined using HRTEM, although the former values were slightly smaller because the oxygenated regions were ignored in the Raman estimation. The GOQD specimens are denoted using the acronym QD followed by their mean sp2domain sizes (in angstrom), as determined by Raman analysis. For example, QD26 presented in Figure S1 has a mean size of 2.6 nm. Figure 1a shows the GOQD dispersions (in water at 400 mg L−1) under daylight lamp irradiation. The color of the dispersions varied slightly with the GOQD size. Under ultraviolet light irradiation at 365 nm, the GOQD specimens exhibited multiple PL colors, varying from orange-red for QD79 to blue for QD10 (Figure 1b). The size dependence of the PL color resulted from quantum confinements in the sp2 domain, which is elucidated in the following discussion.
Figure 1. Photographs of aqueous dispersions of GOQDs of different sizes. (a) GOQD dispersions under daylight lamp irradiation. (b) GOQD dispersions under UV lamp irradiation (365 nm center wavelength).
Considering the probable influence of the thickness of the GOQD discs on PL emissions,33 we analyzed the topography of GOQDs on mica substrates by using atomic force microscopy (AFM). The AFM images (Figure S3) show that the topographic heights of the GOQDs range from 1 to 2 nm and are monodisperse at approximately 1.5 nm, indicating that the GOQDs are principally 1−2 layers. We used X-ray photoelectron spectroscopy (XPS) to analyze the chemical composition of each GOQD specimen (Figure S4). The fullrange XPS spectra revealed that the O/C atomic ratios of the GOQDs with sizes exceeding 2 nm were approximately 0.4−0.5 (Table S1), indicating the similar degree of surface oxidation of the GOQDs. For the GOQDs with sizes smaller than 2 nm, the O/C ratios were slightly higher, suggesting that the number of oxygen groups at the periphery of the GOQDs began influencing the overall oxygen content. The composition of oxygen functionalities was obtained by deconvoluting the C 1s spectra (Figure S5). For sizes larger than 2 nm (Table S1), the GOQDs showed similar CO contents, indicating that the GOQDs had similar concentrations of basal-plane oxygen functionalities (mainly the epoxy group). The concentration of periphery functionalities (CO and OCO) slightly increased with a decrease in the particle size because of the increasing contribution of edge sites to the overall oxygen functionalities. For the size further decreased to less than 2 nm, the CO content increased at the expense of the CO/OCO loss, indicating that the formation of hydroxyl groups at the periphery was favorable for small GOQDs. Theoretical calculations will reveal that OH is stable functionalities at the periphery of small-size graphene. The above XPS analysis reflects the uniform concentrations of the epoxy CO bonding on the basal plane for all the GOQDs, which indicates the homogeneous distribution of oxidized sp3 sites on the basal plane. Fourier transform infrared (FTIR) spectroscopic measurements of the GOQDs (Figure S6) also reveal similar epoxy intensities for all the GOQDs. Figure 2a presents optical absorption spectra. The progressive shift of the absorption band edge to short wavelengths with a decrease in the size of the GOQDs is apparent. The maximal absorbance of each GOQD spectrum can be ascribed to the π → π* transition in the sp2 domains,3 which dominate the GOQD structures. The absorption edge may correspond to the n → π* transitions of nonbonding electrons of oxygen atoms involved in CO or CO bonds.23 We used ultraviolet photoelectron spectroscopy (UPS) to determine the highest occupied orbital level (i.e., the n-state level) of the GOQD 2088
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
Letter
The Journal of Physical Chemistry Letters
Figure 2. Optical spectroscopic analysis of GOQD suspensions. (a) Normalized optical absorption spectra of the GOQDs. (b) Normalized PL spectra of GOQDs excited by 350 nm monochromatic radiation grated from a xenon lamp.
specimens and found that their n-state levels were almost identical, irrespective of the GOQD size (Figure S7). Figure 2b shows the normalized PL spectra of the GOQD specimens excited at a wavelength of 350 nm. The specimens exhibited single PL peaks at 580 nm (QD79), 560 nm (QD61), 540 nm (QD54), 520 nm (QD26), 490 nm (QD16), and 460 nm (QD10). The PL emissions encompass the orange-red to blue region of the visible spectrum. Comparison of the absorption and PL spectra revealed that the wavelength of the PL peak was close to that of the absorption edge for each GOQD specimen (although the absorption edge could not be precisely positioned in the spectra). Therefore, we assigned the single-peak PL emissions to radiative π* → n transitions.9 The energy values or wavelengths of the radiative transitions are governed by the particle size (i.e., by the degree of quantum confinement). All the GOQD dispersions showed excitationwavelength-independent PL emissions (Figure S8), indicating a scarcity of trap states between the π* and n-state energy levels. The trap-free characteristics enable the precise identification of the discrete energy levels of the orbitals or states in the electronic structures of the GOQDs. Figure 3 presents normalized PL excitation (PLE) spectra, which reveal the excitation wavelengths for local maximum PL emissions at the peak wavelengths depicted in the PL spectra (Figure 2b). The deconvolution of the PLE spectra revealed that each GOQD specimen exhibited three absorptive transition peaks. The long-wavelength peak is ascribed to the n → π* transition because this peak has an energy value slightly larger than that of PL emissions. The intermediate-wavelength peak has an energy value close to that of the absorbance maximum and is assigned to the π → π* transition. The shortwavelength peak with high energy is believed to result from the n → σ* transition. Such peaks have been reported for GO, in the sp3 domains associated with hydroxyls, amines, and
Figure 3. PLE spectra of the aqueous GOQD dispersions. (a) QD79, (b) QD61, (c) QD54, (d) QD26, (e) QD16, and (f) QD10. The PLE spectra were monitored at the wavelengths for the maximal PL emissions from the GOQDs shown in Figure 2b. Each PLE spectrum can be deconvoluted into three peaks that correspond to electron transitions: n → σ* (blue), π → π* (green), and n → π* (red).
ethers.34,35 As examples, the PL quantum yields of QD10 and QD54 were measured to be 6% and 16%, respectively (see the Supporting Information). The moderate quantum yields exhibited by the GOQDs can be explained by the fact that the present work does not cap the GOQDs using any reagents or specific functionalities. Figure 4 presents a schematic of the energy levels associated with the absorption and PL of the GOQDs. We located the nstate level (i.e., the highest occupied orbital level) of each GOQD specimen on the basis of the UPS analysis. The n-state levels of the specimens were located at similar energetic positions. The energy values required for the n → π*, π → π*, and n → σ* transitions in each GOQD specimen (Figure 3) were used to determine the levels of the π, π*, and σ* states on the basis of the identified n-state level. The excitationwavelength-independent PL emissions were ascribed to the π* → n transition. The PLE spectra depict an energy difference between the absorptive n → π* and radiative π* → n transitions. This energy difference cannot be attributed to the relaxation of electrons to shallow trap states below the π* level because the presence of trap states generally results in excitation-wavelength-dependent emissions. Instead, the energy level diagram in Figure 4 shows that the transition of electrons 2089
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
Letter
The Journal of Physical Chemistry Letters
Figure 4. Schematic energy level diagram for GOQD specimens. A schematic of the energy levels associated with size-dependent PL emissions from the GOQDs. The energy levels are determined on the basis of UPS (see Figure S7) and PLE (see Figure 3) analysis. The level of the n-states is not disturbed by a change in the particle size, whereas the levels of the π, π*, and σ* orbitals vary considerably with the size. The π* → n recombination, which involves phonon scattering and an electron transition, is responsible for the excitation-wavelengthindependent PL in the GOQD specimens. The relaxation of electrons from the σ* orbital to the π* orbital is essential for PL emissions induced by short-wavelength excitation. The PL color varies from orange-red (for QD79) to blue (for QD10).
Figure 5. Quantum confinements introduced in the sp2 domain. (a) Variation of the π−π* energy gap with the particle size for GOQDs (determined from measurements, Figure 4) and for oxidized graphene quantum dots (O-GQDs, see the structural schematics) as determined from calculations (see Figures S9−S11). (b) Schematic of the mechanism underlying the decrease in the size of the continuous sp2 phase, which surrounds the isolated sp3 sites or islands, with a decrease in the diameter of the GOQDs. The orange areas represent oxygenated sp3 sites or islands, which are homogeneously distributed in the basal plane of the GOQDs.
from the π* orbital to the n-state orbital for radiative recombination involves energy loss through phonon scattering for achieving momentum alignment, although we cannot rule out the possibility of other relaxation mechanisms causing the energy difference. Because the PL emissions were excitationwavelength-independent, electrons excited to the σ* orbital relaxed to the π* orbital for radiative π* → n recombination. Figure 5a shows the particle size dependence of the π−π* energy gap. Clearly, the quantum confinements in the sp2 domain led to the widening of the π−π* energy gap with a decrease in the particle size. For GOQDs smaller than 2 nm, the gap energy exhibits a sharp increase with a decrease in the size. The sharp increase can be ascribed to the increased confinement influence of the peripheral oxygen functionalities on the enclosed sp2 domains. Chemical analysis by using XPS revealed higher oxygen content in the GOQDs smaller than 2 nm. Figure 5b shows a conceptual schematic of quantum confinements introduced in GOQDs for elucidating the variation of the energy level with particle size. The basal plane of the GOQDs comprises an sp2-domain frame decorated with oxidized sp3 sites or islands. The sp2 region is a continuous phase, and in small GOQDs, confinements are introduced in the sp2 domain to regulate the π−π* energy gap. A decrease in the diameter of the disc-like GOQDs leads to a decrease in the size of the connected sp2 domain, thereby increasing the π−π* energy gap. Oxidized sp3 sites introduce n-state levels within the energy gap, and these levels become the highest occupied orbitals in the GOQDs. The energy levels of the n-states are fixed and unrelated to the GOQD size, as shown in Figure 4. For GOQDs smaller than 2 nm, the peripheral oxygen groups occupy an appreciable fraction of the GOQD framework to exert a strong influence on the electronic structure. To support the interpretation presented in Figure 5, we performed density functional theory (DFT)-based first-
principles calculations by using the Vienna ab initio simulation package code (Figures S9−S11).36,37 The calculations were performed to estimate the π−π* energy gap for graphene quantum dots and oxidized graphene quantum dots (GQDs and O-GQDs, respectively), in which the graphene layers are bonded with hydrogen atoms and hydroxyl groups, respectively, at the periphery (see Figure S9). The calculations also revealed that small graphene sheets bonded with edge oxygen atoms are not stable, which explains why the CO bonding was replaced by the OH for the small GOQDs. The OH or H groups represent the extremes for attachment at graphene periphery because OH strongly attracts electrons from graphene, whereas H donates electrons, and the results for attachments with carboxyl and carbonyl groups should be within those for OH or H groups shown in Figure S9. The shape of graphene, in addition to the periphery functionality type, influences the electronic structure of GQDs;38 however, we only used a round shape for calculation because the GOQDs are disc-like. Figure 5a shows the variation of the π−π* energy gap with the O-GQD particle size according to the DFT calculations. Using O-GQDs to simulate the energy gap of the GOQDs is more realistic relative to the use of GQDs. The measured and calculated data show similar trends for the variation of the π−π* energy gap with the particle size (Figure 5a); the calculated gap values are smaller than the values obtained from optical measurements by 1.5−2.0 eV. The gap values calculated 2090
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
Letter
The Journal of Physical Chemistry Letters using the Kohn−Sham method are known to be underestimated.39−41 The use of the GW approximation39−41 for quasiparticle calculations, which are nevertheless not employed in the present study due to the limit in computing resources, would increase the calculated gap values in Figure 5a by approximately 1.5−2 eV, considerably reducing the difference between the absolute values obtained from calculations and the measurements. The agreement, at least in the trend, between the measured and calculated gap values lends strong support to the quantum-confinement model presented in Figure 5b. Figure S12 presents the time-resolved photoluminescence (TRPL) decay profiles of the GOQD specimens excited by a femtosecond pulse laser at 730 nm. The two-photon PL intensities of the individual GOQD specimens were measured at the characteristic peak wavelengths presented in Figure 2b. Figure S12 shows that the average lifetime of the GOQDs decreases with increasing particle size, from 1.1 ns for QD10 to 0.75 ns for QD79. A previous study reported that large graphitic domains facilitated delocalization of excited electrons to result in frequent energy loss at defect states.17 In the GOQDs of the present study, larger QDs (such as QD79) imposed less restriction on the delocalization of the excited electrons in the π* orbital, which resulted in higher probability of nonradiative recombination at defect sites. By contrast, small QDs (such as QD10) confined excited electrons in discrete orbitals, therefore to extend the lifetime of the excitons. The TRPL results further illustrate the significant influence of quantum confinements on the performance of the GOQDs. In summary, we have reported the facile synthesis of highcrystallinity GOQDs that show excitation-wavelength-independent PL emissions because of the introduction of sizedependent quantum confinement. PL emissions, whose color changed from orange-red to blue upon reducing the GOQD size, originated from the π* → n transition and the energy loss associated with phonon scattering. The quantum confinement resulted from a change in the particle size, and it led to a change in the sp2-domain size and thus in the π*−π gap energy. By contrast, the oxygen n-state orbitals located in the π*−π gap showed similar energy levels for all the GOQDs, irrespective of the particle size. The prominent confinement effects of the GOQDs can be explained by the strong conjugation of electrons in the sp2 domains in the basal plane, which allowed the introduction of confinement in the sp2 domain through the regulation of the particle size. The oxygenated sp3 sites or islands in the basal plane are isolated by the continuous sp2 phase, and the peripheral oxygen functionalities strongly influence the electronic structure in GOQDs smaller than 2 nm. This work provides important insights suggesting that rationally designed GOQDs can be applied in electronic, photonics, and biology applications. The metal-free feature makes GOQDs a promising medium for biological cell imaging and proliferation tracking. Additionally, the multiple-color emissions suggest the applicability of the GOQDs to cell labeling.
■
■
of GOQDs, XPS and FTIR analyses of GOQDs, UPS analysis for the valence band edges of GOQDs, excitation wavelength-independent PL feature of GOQDs, theoretical energy gap calculations for GOQDs. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: 886-6-2344496. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology, Taiwan (104-2221-E-006-231-MY3, 104-2221-E006-234-MY3, 104-3113-E-006-005, and 104-3113-E-006-011CC2), and by the Ministry of Education, Taiwan, The Aim for the Top University Project to the National Cheng Kung University.
■
REFERENCES
(1) Li, L. S.; Yan, X. Colloidal Graphene Quantum Dots. J. Phys. Chem. Lett. 2010, 1, 2572−2576. (2) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320, 356−358. (3) Eda, G.; Lin, Y. Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. A.; Chen, I. S.; Chen, C. W.; Chhowalla, M. Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505−509. (4) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots. Adv. Mater. 2010, 22, 734−738. (5) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L.; Song, L.; Alemany, L. B.; Zhan, X.; Gao, G.; Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J. J.; Ajayan, P. M. Graphene Quantum Dots Derived from Carbon Fibers. Nano Lett. 2012, 12, 844−849. (6) Tang, L.; Ji, R.; Cao, X.; Lin, J.; Jiang, H.; Li, X.; Teng, K. S.; Luk, C. M.; Zeng, S.; Hao, J.; Lau, S. P. Deep Ultraviolet Photoluminescence of Water-Soluble Self-Passivated Graphene Quantum Dots. ACS Nano 2012, 6, 5102−5110. (7) Sun, Y. P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang, H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S. Y. QuantumSized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756−7757. (8) Chien, C. T.; Li, S. S.; Lai, W. J.; Yeh, Y. C.; Chen, H. A.; Chen, I. S.; Chen, L. C.; Chen, K. H.; Nemoto, T.; Isoda, S.; Chen, M.; Fujita, T.; Eda, G.; Yamaguchi, H.; Chhowalla, M.; Chen, C. W. Tunable Photoluminescence from Graphene Oxide. Angew. Chem., Int. Ed. 2012, 51, 6662−6666. (9) Wang, L.; Wang, H. Y.; Wang, Y.; Zhu, S. J.; Zhang, Y. L.; Zhang, J. H.; Chen, Q. D.; Han, W.; Xu, H. L.; Yang, B.; Sun, H. B. Direct Observation of Quantum-Confi ned Graphene-Like States and Novel Hybrid States in Graphene Oxide by Transient Spectroscopy. Adv. Mater. 2013, 25, 6539−6545. (10) Putri, L. K.; Ong, W. J.; Chang, W. S.; Chai, S. P. Heteroatom Doped Graphene in Photocatalysis: A Review. Appl. Surf. Sci. 2015, 358, 2−14. (11) Kamat, P. V. Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242−251. (12) Xiang, Q.; Yu, J. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2013, 4, 753−759. (13) Li, Q.; Li, X.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J. CdS/ Graphene Nanocomposite Photocatalysts. Adv. Energy Mater. 2015, 5, 1500010.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00752. Methods, high-resolution TEM images of GOQDs, Raman spectroscopy for the estimation of the sp2domain size of GOQDs, AFM images and height profiles 2091
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
Letter
The Journal of Physical Chemistry Letters (14) Gokus, T.; Nair, R. R.; Bonetti, A.; Böhmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. Making Graphene Luminescent by Oxygen Plasma Treatment. ACS Nano 2009, 3, 3963−3968. (15) Galande, C.; Mohite, A. D.; Naumov, A. V.; Gao, W.; Ci, L.; Ajayan, A.; Gao, H.; Srivastava, A.; Weisman, R. B.; Ajayan, P. M. Quasi-Molecular Fluorescence from Graphene Oxide. Sci. Rep. 2011, 1, 85. (16) Lee, J.; Kim, K.; Park, W. I.; Lim, B. H.; Park, J. H.; Kim, T. H.; Bong, S.; Kim, C.-H.; Chae, G. S.; Jun, M.; Hwang, Y.; Jung, Y. S.; Jeon, S. Uniform Graphene Quantum Dots Patterned from SelfAssembled Silica Nanodots. Nano Lett. 2012, 12, 6078−6083. (17) Song, S. H.; Jang, M. H.; Chung, J.; Jin, S. H.; Kim, B. H.; Hur, S. H.; Yoo, S.; Cho, Y. H.; Jeon, S. Highly Efficient Light-Emitting Diode of Graphene Quantum Dots Fabricated from Graphite Intercalation Compounds. Adv. Opt. Mater. 2014, 2, 1016−1023. (18) Song, S. H.; Jang, M. H.; Jeong, J. M.; Yoon, H.; Cho, Y. H.; Jeong, W. I.; Kim, B. H.; Jeon, S. Primary Hepatocyte Imaging by Multiphoton Luminescent Graphene Quantum Dots. Chem. Commun. 2015, 51, 8041−8043. (19) Wang, L.; Zhu, S. J.; Wang, H. Y.; Wang, Y. F.; Hao, Y. W.; Zhang, J. H.; Chen, Q. D.; Zhang, Y. L.; Han, W.; Yang, B.; Sun, H. B. Unraveling Bright Molecule-Like State and Dark Intrinsic State in Green-Fluorescence Graphene Quantum Dots via Ultrafast Spectroscopy. Adv. Opt. Mater. 2013, 1, 264−271. (20) Hao, Y.; Gan, Z.; Xu, J.; Wu, X.; Chu, P. K. Poly(ethylene glycol)/Carbon Quantum Dots Composite Solid Films Exhibiting Intense and Tunable Blue-Red Emission. Appl. Surf. Sci. 2014, 311, 490−497. (21) Zhu, S.; Zhang, J.; Tang, S.; Qiao, C.; Wang, L.; Wang, H.; Liu, X.; Li, B.; Li, Y.; Yu, W.; Wang, X.; Sun, H.; Yang, B. Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up-Conversion Bioimaging Applications. Adv. Funct. Mater. 2012, 22, 4732−4740. (22) Dong, Y.; Son, D. H. Strongly Nonlinear Dependence of Energy Transfer Rate on sp2 Carbon Content in Reduce Graphene OxideQuantum Dot Hybrid Structures. J. Phys. Chem. Lett. 2015, 6, 44−47. (23) Liu, F.; Jang, M. H.; Ha, H. D.; Kim, J. H.; Cho, Y. H.; Seo, T. S. Facile Synthetic Method for Pristine Graphene Quantum Dots and Graphene Oxide Quantum Dots: Origin of Blue and Green Luminescence. Adv. Mater. 2013, 25, 3657−3662. (24) Gan, Z.; Xiong, S.; Wu, X.; Xu, T.; Zhu, X.; Gan, X.; Guo, J.; Shen, J.; Sun, L.; Chu, P. K. Mechanism of Photoluminescence from Chemically Derived Graphene Oxide: Role of Chemical Reduction. Adv. Opt. Mater. 2013, 1, 926−932. (25) Wang, L.; Zhu, S. J.; Wang, H. Y.; Qu, S. N.; Zhang, Y. L.; Zhang, J. H.; Chen, Q. D.; Xu, H. L.; Han, W.; Yang, B.; Sun, H. B. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8, 2541−2547. (26) Park, H.; Noh, S. H.; Lee, J. H.; Lee, W. J.; Jaung, J. Y.; Lee, S. G.; Han, T. H. Latge Scale Synthesis and Light Emitting Fibers of Tailor-Made Graphene Quantum Dots. Sci. Rep. 2015, 5, 14163. (27) Kwon, W.; Kim, Y. H.; Lee, C. L.; Lee, M.; Choi, H. C.; Lee, T. W.; Rhee, S. W. Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite. Nano Lett. 2014, 14, 1306−1311. (28) Bao, L.; Liu, C.; Zhang, Z. L.; Pang, D. W. PhotoluminescenceTunable Carbon Nanodots: Surface-State Energy-Gap Tuning. Adv. Mater. 2015, 27, 1663−1667. (29) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Studying Disorder in Graphite-Based Systems by Raman Spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276−1291. (30) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotechnol. 2013, 8, 235−246. (31) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095−14107.
(32) Lucchese, M. M.; Stavale, F.; Ferreira, E. H. M.; Vilani, C.; Moutinho, M. V. O.; Capaz, R. B.; Achete, C. A.; Jorio, A. Quantifying Ion-Induced Defects and Raman Relaxation Length in Graphene. Carbon 2010, 48, 1592−1597. (33) Dong, Y.; Chen, C.; Zheng, X.; Gao, L.; Cui, Z.; Yang, H.; Guo, C.; Chi, Y.; Li, C. M. One-Step and High Yield Simultaneous Preparation of Single- and Multi-Layer Graphene Quantum Dots from CX-72 Carbon Black. J. Mater. Chem. 2012, 22, 8764−8766. (34) Li, M.; Cushing, S. K.; Zhou, X.; Guo, S.; Wu, N. Fingerprinting Photoluminescence of Functional Groups in Graphene Oxide. J. Mater. Chem. 2012, 22, 23374−23379. (35) Teng, C. Y.; Yeh, T. F.; Lin, K. I.; Chen, S. J.; Yoshimura, M.; Teng, H. Synthesis of Graphene Oxide Dots for ExcitationWavelength Independent Photoluminescence at High Quantum Yields. J. Mater. Chem. C 2015, 3, 4553−4562. (36) Kresse, G.; Furthmüller. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (37) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Methods. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (38) Zhu, S.; Wang, L.; Li, B.; Song, Y.; Zhao, X.; Zhang, G.; Zhang, S.; Lu, S.; Zhang, J.; Wang, H.; Sun, H.; Yang, B. Investigation of Photoluminescence Mechanism of Graphene Quantum Dots and Evaluation of Their Assembly into Polymer Dots. Carbon 2014, 77, 462−472. (39) Shishkin, M.; Kresse, G. Self-Consistent GW Calculations for Semiconductors and Insulators. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 235102. (40) Fuchs, F.; Furthmüller, J.; Bechstedt, F.; Shishkin, M.; Kresse, G. Quasiparticle Band Structure Based on a Generalized Kohn-Sham Scheme. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 115109. (41) Shishkin, M.; Marsman, M.; Kresse, G. Accurate Quasiparticle Spectra from Self-Consistent GW Calculations with Vertex Corrections. Phys. Rev. Lett. 2007, 99, 246403.
2092
DOI: 10.1021/acs.jpclett.6b00752 J. Phys. Chem. Lett. 2016, 7, 2087−2092
