Exploring the Emissive States of Heteroatom-Doped Graphene

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Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots Guancao Yang, Chuanli Wu, Xiaojun Luo, Xiaoyan Liu, Yuan Gao, Ping Wu, Chenxin Cai, and S. Scott Saavedra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01385 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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The Journal of Physical Chemistry

Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots

Guancao Yang,† Chuanli Wu,† Xiaojun Luo,† Xiaoyan Liu,† Yuan Gao,† Ping Wu,*† Chenxin Cai*† and S. Scott Saavedra*‡

† Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, P.R. China. ‡Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721-0041, United States.

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ABSTRACT: The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped-GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on a combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multi-emissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a high-energy transition originating from quantum confinement of the carbon core (carbon π‒π* transitions in sp2 domain), and an excitation-dependent PL emission band (band II) originates from a low-energy edge band transition, which is attributed to radiative recombination associated with both the n‒π* transition of N/O/B-containing groups and π–π* charge-transfer between the carbon core to the edge of the GQDs. Moreover, PL emission maxima (both band I and II) for NGQDs and BGQDs show a blueshift and redshift, respectively, relative to UGQDs because the doping altered the electronic structure and the distribution of molecular orbitals in the GQDs. These results clarify previous inconsistencies regarding the PL emission mechanism and electronic properties of GQDs and can thus provide a foundation for the application of doped GQDs in electronics, photonics, and biology.


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1. INTRODUCTION Graphene quantum dots (GQDs), including doped-GQDs, have recently emerged as fascinating fluorescent probes with distinct advantages, such as broadband optical absorption, good chemical stability, high stability against photobleaching, good biocompatibility, and low cytotoxicity etc. and have been utilized in various fields, such as bioimaging, optical sensing, and light emitting devices.1‒9 However, their photoluminescence (PL) behavior remains to be clarified. Some contradictory experimental observations have been reported. A typical example is the ongoing debate regarding shifts in the PL emission peak of GQDs induced by the dopant atom (such as nitrogen and boron atom), which is a widely used strategy for manipulation of the PL properties of GQDs. Qu et al.10 and Li et al.11 reported that nitrogen-doped GQDs (NGQDs) emitted blue luminescence, which differs from the green luminescence emitted by their N-free counterparts. Our results have also demonstrated that the PL maximum for NGQDs shows a blueshift compared with that of undoped GQDs (UGQDs).12 While Jin et al. found a redshift in the PL for NGQDs compared to UGQDs,13 Tetsuka et al. further observed that the PL for NGQDs shifted to even longer wavelengths as the amount of N doping content increased.14 In the case of boron-doped GQDs (BGQDs), it has been widely held that B-doping can lead to a redshift in the PL,3‒5,15,16 whereas 72-nm and 15-nm blueshifts in the PL of BGQDs were observed by Shan17 and Zhang et al.,18 respectively, compared to UGQDs. In addition to shifts in the PL emission, several intriguing aspects of GQDs also remain unknown. For example, their PL excitation (PLE) spectra, in most cases, do not resemble the absorption spectra even for the lower energy edge band (more than 330 nm), which is usually assigned to the n‒π* transition. However, the exact assignment for this low-energy excitation band also remains to be disclosed. Moreover, a uniform explanation that can address most observations for the PL characteristics of GQDs is yet to emerge. The excitation-dependent PL of GQDs is usually ascribed to the emission originating from band gap transitions in sp2 π-domains and the emission from surface defects.13,15 However, the PL of GQDs was also assigned to both the crystalline cores and organic fluorophores that are formed in the ACS Paragon Plus Environment

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synthesis of GQDs. For example, the fluorophore IPCA (5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α] pyridine-7-carboxylic acid) can be formed by citric acid (a common precursor of GQD synthesis) and ethylenediamine in a typical synthesis of NGQDs,19 and the fluorescent molecule TPA (5-oxo-2,3dihydro-5H-[1,3] thiazolo [3,2-α]-pyridine-3,7-dicarboxylic acid) can be generated by reaction of citric acid and L-cysteine in a typical synthesis of nitrogen-sulfur-co-doped GQDs.20 Therefore, fundamental questions regarding the origin of the PL in GQDs remain unanswered. These inconsistent observations complicate the interpretation of the emissive mechanism for GQDs and are largely due to the high heterogeneity of doped GQDs, which are typically synthesized by different methods and precursors, resulting in different sizes, different surface functional groups, and defects. Consequently, it is hard to correctly evaluate PL behavior using the available data from previous reports. Since the PL of GQDs is sensitive to the size, edge configuration, and surface functional groups and defects,21‒23 synthesis of homogeneous, heteroatom-doped GQDs using the same method in a similar medium is crucial for understanding the emissive states in doped GQDs and the effects of the heteroatoms on the emissive mechanism in GQDs. Here, we address this issue by employing the same method (a constant potential electrolysis method via oxidizing a graphite rod in aqueous solution) to synthesize three different types of GQDs: UGQDs, BGQDs, and NGQDs. B and N atoms were selected as electron-rich and electron-deficient dopant models, respectively, because of their comparable atomic size to the carbon atom, as well as the difference in electronegativity (electronegativity value of C, N, and B is 2.55, 3.04, and 2.04, respectively). The as-synthesized GQDs show similar sizes, chemical compositions (types and compositions of surface functional groups), and defects. The PL emission states of the GQDs and the effects of the mechanism of heteroatom doping on the PL characteristics of the GQDs were elucidated based on spectroscopic methods and theoretical calculations. 2. EXPERIMENTAL SECTION


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2.1. Synthesis of UGQDs, NGQDs, and BGQDs. UGQDs were synthesized by a constant potential electrolysis method in a two-compartment two-electrode cell with a sample volume of ~20 mL (as illustrated in Figure S1; the instruments used in this work are shown in detail in Supporting Information). A high purity graphite rod (99.9%, 3 mm in diameter, Shanghai Carbon Co. Ltd.) was used as an anode and placed into an aqueous 0.1 M phosphate buffer solution (PBS, pH ∼7); the length of the graphite rod below the solution level was approximately 10 cm. A Pt sheet (1.5 cm × 1.5 cm) was used as a cathode. The voltage between the anode and cathode was maintained at 3 V, which is high enough to oxidize C–C bonds and to further drive electrolyte ions into the graphene layers.4 After 2 hours of electrolysis, UGQDs were collected by filtering the resulting solution using a 0.22 µm microporous nylon membrane to remove precipitated graphite oxide and graphite particles. The paleyellow solution obtained was then dialyzed in deionized water in a dialysis bag (retained molecular weight of 3500 Da) for 48 hours to remove the PBS electrolyte; the deionized water was changed every 12 hours. Note that the P atom cannot be doped into the carbon networks using the present experimental conditions, as indicated by the XPS measurements in Supporting Information. The BGQDs were prepared by similar procedures to those used for the synthesis of UGQDs except for the addition of 0.1 M borax (99.5%, Sigma-Aldrich; the solution pH was adjusted to ∼7) to the electrolyte. The detailed procedures have been previously reported in our previous studies.3‒5 The NGQDs were also synthesized by similar procedures, with the inclusion of 0.1 M TMAH (tetramethylammonium hydroxide) instead of 0.1 M borax in the electrolyte (the solution pH was adjusted to ∼7). Of note, for synthesis of NGQDs, the N source needs to be selected carefully. We have ever tried to synthesize the NGQDs using several different types of N-containing compounds such as ammonium chloride, ammonium perchlorate, urea, ethylenediaminetetraacetic acid disodium salt (EDTA), and TMAH etc. as N source. However, our results indicate that the NGQDs can only be synthesized in the presence of TMAH. Ammonium chloride, urea, and EDTA etc. cannot be used as N source and electrolyte to synthesize NGQDs under our conditions.


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2.2. Spectroscopic Measurements. UV-Vis spectra were measured by a Cary 5000 UV-vis-NIR spectrometer (Varian). Fluorescence and photoluminescence excitation (PLE) spectra were collected using a FluoroSENS fluorescence spectrophotometer (Gilden Photonics) equipped with a xenon lamp excitation source. Time-resolved fluorescence measurements were carried out using a time-correlated single photon counting (TCSPC) system. The samples were excited at 360 nm (Nanoled). A repetition rate of 1 mHz was used. The fluorescence decays were collected at an emissive wavelength of 460 nm by a Fluoro-Maxs®-4 Spectrofluorimeter (FM-4P-TCSPC, with standard configuration, Horiba Jobin Yvon) using an RR928P-2 detector. The lifetime data were obtained by application of deconvolution techniques using IBH DAS6 analysis software. 2.3. Calculation Details. We constructed a model containing 13 hexagonal rings with delocalized p electrons (C43H16O4) for UGQD, a model containing 42 carbon atoms and a pyridinic N (C42NH16O4) for NGQDs, and a model containing 42 carbon atoms and a sp2-hybridized boron atom (C42BH15O4) for BGQDs. For simplification, each model contains one ‒OH, one ‒COOH, and one C‒O‒C group as the oxygen-containing group. The carbon atoms at the edge of the graphene were terminated with hydrogen atoms. The reasons we chose these models for calculations are that (i) these models contain the same types of the oxygen-containing surface functional groups (such as ‒OH, ‒COOH, and C‒O‒C groups) and C‒B, or C‒N moieties as in the synthesized GQDs (refer to XPS measurements presented in Supporting Information), and (ii) more importantly, the bond length of C‒C in these models (1.421 Å) is identical to the theoretical value (1.422 Å, the C–C distance in graphite),24 which is crucial for constructing the model of GQDs in theoretical calculations. The calculation was performed based on density functional theory (DFT) using Gaussian 03 (revision B.03).25 Geometrical optimizations were carried out using a DFT method with Becke’s hybrid three-parameter nonlocal exchange functional combined with the Lee-Yang-Parr gradient-corrected correlation functional (B3LYP). The 6-31G (d, p) basis set was used for all elements. We optimized the geometrical structure and calculated the charge densities of each atom in the optimized structure for UGQD, NGQD, and BGQD using Mulliken charge density analysis. The spatial distributions for α and β electrons in the HOMO (highest occupied ACS Paragon Plus Environment

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molecular orbital) and LUMO (lowest unoccupied molecular orbital) for UGQD, NGQD, and BGQD were also evaluated based on the calculations.

Figure 1. (A1‒A3) TEM, (B1‒B3) HRTEM, (C1‒C3) fast Fourier transform image, and (D1‒D3) lineprofile analysis of the diffraction fringes for UGQDs (1), NGQDs (2), and BGQDs (3). The insets in A1, A2, and A3 show the size distribution of the synthesized UGQDs, NGQDs, and BGQDs, respectively, as obtained by dynamic light scatter analysis.

3. RESULTS AND DISCUSSION To unravel the PL emission states of the doped-GQDs, we first synthesized UGQDs, NGQDs, and BGQDs using a constant potential electrolysis method. Due to use of the same experimental parameters, including applied voltage, electrolysis time, carbon precursor, and pH environment, the three different types of GQDs show a similar size of ~(4.3 ± 0.5) nm (A1‒3, Figure 1), as revealed from transmission electron microscopy (TEM). The lattice spacing of UGQDs, NGQDs, and BGQDs was ~0.22, 0.21, and 0.24 nm, respectively, as revealed by high-resolution TEM (HRTEM, B1‒3), fast Fourier transform imaging (C1‒3), and line-profile analysis (D1‒3). The lattice spacing observed is similar to that reported ACS Paragon Plus Environment

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previously (0.21 nm).26 Moreover, the as-synthesized UGQDs, NGQDs, and BGQDs retain a similarity for the chemical compositions (types and compositions of surface functional groups) and defects, as characterized by Raman, FTIR, and XPS (see the detailed characterizations provided in Supporting Information and Figure S2‒S4). These characteristics are helpful for unraveling the emissive states in doped GQDs and for elucidating the effect of the doping heteroatom on the PL.

Figure 2. (A) The absorption spectra of UGQDs (a), NGQDs (b), and BGQDs (c). (A‒C) The absorption spectra of UGQDs (a), NGQDs (b), and BGQDs (c) in the core (B), edge (C), and surface state transition region (D).

We then studied the absorption spectra of the synthesized GQDs, as shown in Figure 2, because the absorption spectra can directly reflect the changes of the transition energy levels in the GQDs upon the heteroatom doping, and thus understanding these changes of the transition energy levels is of great help in elucidating the PL emission states of the GQDs. Moreover, the detailed study on the effects of the heteroatom doping on the absorption spectra of GQDs is presently unavailable. The absorption spectra exhibit features expected for core, edge, and surface state transitions.27,28 The absorption band maximum for UGQDs, NGQDs, and BGQDs is observed at ~230 (curve a, Figure 2A and 2B), 228 (curve b), and 232 nm (curve c), respectively. This band can be assigned to the π‒π* transition for C=C within the sp2 ACS Paragon Plus Environment

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domain (e.g., core state). Compared with UGQDs, NGQDs show a 2-nm blueshift (from 230 to 228 nm). This blueshift is caused by electron enrichment in the GQDs due to the doping of the electron-rich N atom.29 A 2-nm redshift is observed in the spectrum for BGQDs (from 230 to 232 nm), which is due to electron deficiency resulting from doping of the electron-deficient B atom. Moreover, the intensity of this band is clearly weaker for both NGQDs and BGQDs compared to the UGQDs (Figure 2B). This reduction in intensity implies a decrease in the number of delocalized π‒π* transitions in the core as a result of the low content of carbon sp2-domains in the NGQDs and BGQDs, which agrees well with (i) the calculated sp2-domain size for UGQDs (4.70 nm), NGQDs (4.47 nm), and BGQDs (4.42 nm) as determined from the Raman results (see the detailed calculation in Supporting Information), and (ii) the content of C=C moiety for the NGQDs and BGQDs (41.87 and 39.79%, respectively, Table S1) estimated from the XPS analysis, which is lower than that in UGQDs (48.34%). This decrease in the C=C moiety content in the NGQDs and BGQDs is caused by doping-induced breakage of the C=C bond. More importantly, the intensity of this band for NGQDs is almost the same as that for BGQDs (curve b and c, Figure 2B), because of the similar sp2-domain size in NGQDs and BGQDs (4.47 and 4.42 nm, respectively), further supporting the assignment of this absorption band to the π‒π* transition in the sp2 domain. In addition to the well-pronounced core transition band at ~230 nm, an absorption band is also observed to appear at ~300 nm (Figure 2C). We assign it to the edge transition. Compared to UGQDs (~300 nm, curve a, Figure 2C), the band for the NGQDs shows a ~8-nm blueshift to ~292 nm (curve b), while that for the BGQDs shows a ~10-nm redshift to ~310 nm (curve c). This edge transition has been previously assigned to the n‒π* transition of the electrons from the nonbonding orbital of the surface functional group to the π* orbital of the sp2-carbon domain.30‒33 Our analysis of the PLE spectra (see discussion presented below) suggests that both the n‒π* transition of the surface groups and the π‒π* charge-transfer transition of the edge of the sp2-carbon domain contribute to this transition. For example, in UGQDs, the n‒π* transition includes excitation of nonbonding electrons of the O atoms in C=O or/and C‒O bonds (nO2p‒π*) into the π* orbital of the C core, and the π‒π* charge-transfer transition ACS Paragon Plus Environment

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originates from charge transfer from the inner to outer part of the sp2-hybridized carbon core. In the NGQDs and BGQDs, in addition to the nO2p‒π* transition, the n‒π* transition also includes the nN2p‒π* and nB2p‒π* transitions. This edge transition is the main PL emissive center of GQDs. An additional weak, low-energy absorption band is observed in the absorption spectra at ~405 (curve a, Figure 2D), 370 (curve b), and 435 nm (curve c) for UGQDs, NGQDs, and BGQDs, respectively. This band shows a plateau (or a shoulder) within the tail of the edge state band. Similarly, the direction of the shift for this band for NGQDs and BGQDs relative to UGQDs is the same as that observed for the edge state, e.g., it shows a blueshift (~35 nm) and redshift (30 nm) for NGQDs and BGQDs, respectively. This band is rarely discussed in the literature. We assign this weak absorption band to the surface state of the GQDs, which is most likely induced by the surface functional groups attached at the edge of the carbon core. These surface groups have been previously assumed to create PL emissive states within the edge.27 To explore the PL emission mechanism of the doped GQDs and to connect the optical absorption with the PL emission for the core, edge, and surface bands discussed above, PLE spectra were also needed to be measured and analyzed; PLE can provide information on the number of emitted photons at each wavelength, rather than the light absorption as revealed by absorption spectra. PLE has been previously used as primary measure to characterize PL emissive centers.34,35 The PLE spectra of GQDs show three or four characteristic bands, which can be ascribed to core, edge, and one or two surface bands (Figure 3). These features are similar to those observed in the absorption spectra. We assign the band at ~270 nm to the core band, originating from the core state within the sp2-carbon domain. This excitation band shows a significant redshift with respect to the corresponding absorption band measured in the absorption spectrum (~230 nm, Figure 2). Despite the strong absorption, this state contributes less to the PL emission of GQDs. However, PLE spectra show a strong excitation band with a maximum centered at ~340 nm (Figure 3), which can be assigned to the edge band. This result implies that the edge band is the dominating emission center in GQDs. As


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discussed in the absorption spectra, both π–π* charge-transfer transitions in the sp2-carbon domain and the n–π* transition from N, B or O containing groups contribute this edge band.

Figure 3. Deconvoluted PLE spectra of NGQDs (A), UGQDs (B), and BGQDs (C) measured at the emission wavelength of 460 nm. The curve-fitting analysis was performed as a linear combination of the components identified in the spectrum. These components were approximated by Gaussian functions whose peak positions, widths, and heights were adjusted iteratively in the curve-fitting procedure. To confirm this result and to distinguish between transitions that mainly contribute to this edge band, we recorded the PLE spectra of GQDs in polar protic solvents such as water (dipole moment µ = 1.85 D), methanol (µ = 1.70 D), and ethanol (µ = 1.69 D) and in polar aprotic solvents such as acetonitrile (µ = 3.93 D), dimethylformamide (µ = 3.82 D), and dimethyl sulfoxide (µ = 3.96 D). This band for NGQDs shows a gradual blueshift of ~5 nm in polar protic solvent with increasing solvent polarity (Figure 4A). In contrast, a redshift of ~14 nm is observed in polar aprotic solvent as the solvent polarity increases. Similar shifts are observed in UGQDs and BGQDs (Figure 4B, C). Kumbhakar and coworkers reported ACS Paragon Plus Environment

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similar phenomena in fluorescent carbon nanodots.36 The blueshift suggests a contribution from the n‒ π* transition to the edge band (as H bonding with solvent stabilizes the nonbonding electron pair in the ground state relative to that in the excited antibonding π state), while the redshift suggests a contribution from the π‒π* charge-transfer transition to this band (as the delocalized excited state is expected to show greater energy stabilization with increased polarity).37,38 Thus, we can conclude that both the π‒π* charge-transfer transition and n‒π* transition contribute to the edge band, rather than the general perception of an exclusive n‒π* transition contribution for this edge band.30‒33 Moreover, the observed broad absorption band at ~300 nm, as shown in Figure 2C, which is a characteristic of both π‒π* and n‒ π* transitions contributing to the edge band, can provide additional evidence for this assignment. This assignment is also in agreement with theoretical studies for carbon dots by Sudolská et al. based on DFT simulation,39 in which an experimentally obtained broad absorption band was determined to originate from both π‒π* charge-transfer and n‒π* transitions.

Figure 4. PLE spectra of NGQDs (A), UGQDs (B), and BGQDs (C) measured in a polar protic solvent including ethanol (a), methanol (b), and water (c), and in a polar aprotic solvent including dimethyl sulfoxide (a′), dimethylformamide (b′), and acetonitrile (c′). ACS Paragon Plus Environment

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Unlike the similar PLE shapes observed for core and edge state bands, the surface band in PLE for the NGQDs and BGQDs shows a significant difference in shape and position compared to that for UGQDs. The surface state band in the PLE of UGQDs shows only one peak (Figure 3B), extending at half-maximum from ~370 to 407 nm with the maximum located at ~390 nm. This band corresponds to the nO2p‒π* transition. In addition to the peak, the surface state in both NGQDs (Figure 3A) and BGQDs (Figure 3C) shows a new, low-energy transition peak at ~420 and 439 nm, respectively, which corresponds to the nN2p‒π* and nB2p‒π* transitions, respectively. To confirm this assignment, we studied the effects of the amount of doped B and N on the intensities of this low-energy transition peak in BGQDs, and NGQDs, respectively. The results depicted in Figure S5 indicate the intensities of the peak decrease with the reduction of amount of doped B in BGQDs. Similar results are also observed in the NGQDs (Figure S6). The intensities of this new peak also decrease with the reduction of amount of doped N in NGQDs. These results demonstrate that this new, low-energy transition peak is directly related to the doped B and N in BGQDs and NGQDs, respectively, not to other components, and thus provide direct evidence to the assignment of the new peak to nB2p‒π* and nN2p‒π* transitions. These new bands are caused by the sub-band gaps within the edge, which create additional energy levels and enhance light emission from the edge band.40 The dopant atoms can also induce shifts in the PLE spectra. From Figure 3, it can be observed that the core, edge, and nO2p‒π* bands for the surface state of NGQDs all show a slight blueshift, but they all show a slight redshift for BGQDs, which is consistent with the findings from the absorption spectra. Accordingly, the PL emissions from both core and edge bands will shift towards the blue direction for NGQDs and towards the red direction for BGQDs (see discussion below). After revealing the transition states responsible for the absorption and PLE spectra of UGQDs, NGQDs, and BGQDs, we next study their PL emission characteristics and the effects of the dopant atoms on PL emission. Figure 5 shows the excitation-dependent PL behavior for UGQDs, NGQDs, and BGQDs (A1, B1, and C1), implying the presence of multi-emissive centers (or deactivation pathways), ACS Paragon Plus Environment

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which is a common feature of fluorescent carbon materials.35,41 The existence of multi-emissive centers can also be verified by time-resolved PL measurements utilizing a time-correlated single photon counting (TCSPC) system; this is a useful tool for identifying each individual emissive component and enabling extraction of the associated fluorescent lifetime (τ) and weight of each lifetime component.42,43 The measurements were conducted using a 360 nm laser excitation source. All PL decay curves, which were recorded at 460 nm, can be fitted with a bi-exponential function (Figure S7) and comprise a slow component (τ1), ranging from 10.3–11.1 ns, and fast component (τ2), ranging from 1.6–2.2 ns, which suggests that there are possibly two emissive centers contributing to the PL of the synthesized GQDs.

Figure 5. (A1‒C1) PL spectra of NGQDs (A1), UGQDs (B1), and BGQDs (C1) under different excitation wavelengths (300‒500 nm). (A2‒C2) PL excitation-emission mappings corresponding to A1‒ C1 panels. (A3‒C3) PL emission of NGQDs (A3), UGQDs (B3), and BGQDs (C3) under excitation of 360 nm.

The PL emission of UGQDs increases with the excitation wavelength in the range of 300 to 340 nm, with a maximum PL peak observed at ~465 nm under 340 nm excitation; the PL emission then ACS Paragon Plus Environment

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decreases with a redshift in the emission peak observed for further increase in excitation wavelength (B1, Figure 5). Two PL emission bands corresponding to the different emissive centers can be clearly identified. One emission band is located at ~430 nm (band I) with the emission peak remaining almost invariant under excitation ranging from 300 to 380 nm; another emission band is located at 465‒580 nm (band II) with the emission peak moving to the longer direction with increasing excitation wavelength. We assign the high-energy transition (band I) to core band, resulted from the π‒π* transition of C=C in the carbon core (sp2 domain),44 and the low-energy transition (band II) to the edge band, originated from the surface group-related transitions. Our assignments for the PL emission of GQDs can be confirmed by measuring the size-, and oxygenamount-, and medium-pH-dependent PL emissions. We synthesized UGQDs with two different sizes, ~4.3 and 5.5 nm, and recorded the PL emission. As shown in Figure S8, we find that band I shows a redshift (10 nm) with increasing size of the UGQDs from ~4.3 and 5.5 nm; band II, however, remains almost invariant as the size of the UGQDs is increased. These results imply that band I in Figure 5 is caused by the carbon-core-related transitions. In addition, we also reduced the amount of the oxygencontaining groups at the surface of UGQDs via reduction by NaBH4 and then compared the oxygenamount-dependent PL emission. After reduction by NaBH4, the atomic ratio of O/C for UGQDs decreased to ~0.55 from 0.64 (Figure S9). The PL emission data shown in Figure S10 indicate that band I remains almost unchanged, implying that band I is not affected by the alteration of the amount of oxygen-containing groups. However, band II showed a clear suppression in intensity at ~465 nm, suggesting sensitivity to changes in the surface groups. Furthermore, we also studied the effects of solution pH on the PL characteristics of UGQDs to confirm our assignments. As presented in Figure S11, we can observe pH-dependent PL spectra. The position of band I almost remains invariant when the solution changes from neutral to basic (pH increases from 7 to 12); however, the maximum PL position of band II displays a ~13-nm redshift in basic solution (pH 12) in comparing with that in neutral solution (pH 7). This is because at pH 12, the surface carboxylic acid group (‒COOH) in the UGQDs is deprotonated (‒COO‒). The negative charge can weaken the electron-withdrawing abilities ACS Paragon Plus Environment

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of the carboxylic acid to UGQDs, causing the redshift of the surface group-related PL emission. Therefore, this PL band can be assigned to the transitions related to those surface groups attached to the edge of the carbon core (edge band). Similar emission features were also observed in NGQDs (A1, Figure 5) and BGQDs (C1, Figure 5): both exhibit an excitation-independent emission band I (~427 nm for NGQDs, and ~432 nm for BGQDs) under excitation ranging from 300 to 360 nm and an excitation-dependent emission band II under longer excitation wavelengths. Because the as-synthesized doped GQDs contain similar types of surface Ocontaining functional groups (as revealed by XPS and FTIR), similar surface defects (as revealed by Raman), and almost the same sized QDs (from TEM), the gradual blue and redshift phenomenon observed for the PL emission is unlikely to be caused by a change in the surface functional groups, defects, or sizes. Therefore, it is highly likely that the shift in the PL maximum is caused by the doping of N and B atoms into the carbon sp2 configurations (see DFT calculation presented below). Another PL emission feature that should be noted is that the strongest PL emission was observed at a wavelength longer than 455 nm (A2, B2, and C2). This should originate from band II (edge band transition), implying that the edge band transition is the main emission center in PL for UGQDs, NGQDs, and BGQDs, i.e., the PL emission is attributed to radiative recombination associated with both the n‒π* transition of N/O/B-containing groups and π–π* charge-transfer between the carbon core to the edge of the GQDs. This conclusion agrees well with the PLE spectra shown in Figure 3; however, such a result differs from that obtained in previous reports,30‒33 which assign this edge band transition (band II) only to the n‒π* transition. To explore the effect of the doped heteroatoms on the PL emission of GQDs, PL spectra were measured under the same excitation wavelength. Here, we selected 360 nm as the excitation source to record the PL spectra (A3, B3, and C3). Compared to UGQDs (B3, the PL emission maxima of band I is at ~430 nm), band I of NGQDs shows a slight blueshift (band I at ~427 nm, A3), while that of BGQDs shows a slight redshift (band I at ~432 nm, C3). These different shifts in the core band can be explained by considering the different electronegativity of the doped atoms (B and N) relative to the C atoms in ACS Paragon Plus Environment

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the sp2 domain.45 Generally, the atom with higher electronegativity has a stronger ability to bind with the n electrons, and thus, higher energy is required to excite the electron transition. The electronegativity of C, N, and B is 2.55, 3.04, and 2.04, respectively. As a result, the transition energy increases in the order of BGQDs, UGQDs, and NGQDs. Note that some previous reports observed that the NGQDs exhibited a redshift of the core band (band I),13,14 which may be explained by considering both the GQD size and the surface functional groups. It is well accepted that increasing the size of the GQDs can result in narrowing the band gap,13 and thus causing a redshift of the core band, as depicted in Figure S8. Moreover, the shift direction of the core band for the PL emission of NGQDs is also significantly related to the form of the N atoms in NGQDs, which depends on the synthesis conditions. For example, a pyridinic N in NGQDs forms two σ-bonds and one π-bond with neighboring C atoms and contributes a lone pair electron to the occupied π orbitals (HOMO) across the sp2 orbitals, causing the HOMO energy level to decrease, as depicted in Figure 6A below; this widens the band gap, leading to the blueshift of the core band. In contrast, when the N atom is doped in carbon core in the form of graphitic N, one excess electron is injected into the unoccupied π* orbitals (LUMO) of the carbon sp2 lattice. As a result, the energy level of LUMO is lowered because of the increase of the electron density, causing the band gap to narrow, and thus leading to the redshift of the core band. Sarkar et al. carried out a computational study on the absorption spectra of N-doped carbon dots and concluded that graphitic N causes a redshift.46 The experimental results of Holá et al. demonstrated a redshift of PL emission of NGQDs induced by graphitic N.47 In the synthesis of the NGQDs, amine groups can also be generated at surface of GQDs, as observed in previous works.14 The unpaired electrons in these amine groups can contribute electron density to the GQDs based on their electron-withdrawing and electron-accepting behaviors. Increasing the electron density can narrow the band gap of GQDs,48 resulting in the redshift of the core band. In our synthesized NGQDs, the N atoms exist in the forms of pyridinic and pyrrolic, as revealed by XPS (Figure S4), and there are no amine group. Pyridinic N causes the increase of the band gap; thus the maximum of PL emission of NGQDs displays a blueshift as depicted in Figure 5 (A3, B3, and C3). In contrast, a pyrrolic N atom forms three ACS Paragon Plus Environment

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σ bonds with two C atoms and one H atom. There are no extra electrons to participate in π-conjugated system, e.g. no charge doping effect occurs. Our DFT calculation indicate that the energy levels of HOMO and LUMO in pyrrolic N-doping GQDs are similar to those in UGQDs (Figure S12), and thus negligible effects on the band gap are predicted. In contrast to the slight shift observed for the core band, band II of NGQDs and BGQDs shows a large shift relative to that of UGQDs, but the shift direction is identical to that of band I, i.e., band II of NGQDs shows a blueshift (~10 nm), and that of BGQDs shows a redshift (~70 nm). These results suggest a primary effect of doping on PL emission for band II (edge band transition) rather than band I (core band transition). To gain further insight into the effects of introducing dopant N and B atoms in GQDs on the PL emission, we calculated the electronic character and molecular orbital distribution in GQD, as the electronic structure and orbital hybridization can give a clear picture for the mechanism underlying blue- or redshifted emission caused by doping.

Figure 6. (A) DFT simulated and (B) experimentally measured HOMO and LUMO energy levels for NGQDs, UGQDs, and BGQDs. The HOMO and LUMO of the GQDs shown in panel B were determined from a combination of absorption spectrum data and electrochemical measurements (see detailed calculations shown in Supporting Information and the cyclic voltammograms for NGQDs, UGQDs, and BGQDs shown in Figure S14). ACS Paragon Plus Environment

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The calculation indicated that the heteroatom doping can alter the whole electronic structure as well as the molecular orbital distribution in GQDs. Both the HOMO and LUMO of GQDs show p-π and p-π* orbital characteristics. Before doping, carbon atoms near the H-terminated end show a negative charge due to hydrogen bonding; however, the negative charge is very weak (Figure S13A). Carbon atoms, a few hexagons away from the H-terminated end, become almost neutral with delocalized electrons distributed at each carbon atom within the aromatic ring. The HOMO and LUMO are delocalized, and the π electron cloud density is uniformly distributed at each carbon atom (Figure 6A). In the NGQDs, the N atom is relatively strongly negative, with a charge of –0.544 a.u. negative charge (Figure S13B). This is a consequence of the higher electronegativity (3.04) and strong electron-accepting ability of the N atom compared to carbon (2.55). Most of the compensating positive charge is distributed on the adjacent carbon atoms in the sp2-carbon domain; therefore, the charges of the carbon atoms (marked with green) and adjacent N atom in NGQDs show significant enhancement relative to UGQDs. The pyridinic N contributes one electron to the π system across the sp2 orbitals as a delocalized electron. The electron is excited into the LUMO with the π electron of the sp2 C atoms and resides in the plane perpendicular to the conjugated π-system (Figure 6A). As a result, the orbitals show a more even hybridization than that found in UGQDs, and the energy level of HOMO decreases. In the BGDQs, the alteration in charge distribution induced by B-doping, as shown in Figure S13C, is less significant than that induced by N-doping since the electronegativity of the B atom (2.04) is less than that of carbon atoms. Moreover, the B atom does not provide the delocalized electron to the π system across the C atoms, but the empty orbital, and thus causing the decrease of the electron density in HOMO and a lift in HOMO level. A resultant narrowing of band gap occurs, and a localized distribution of HOMO and LUMO orbitals relative to UGQDs (Figure 6A). The doping-induced alteration of molecular orbital hybridization in NGQDs and BGQDs will affect the PL emission energies. It is well-accepted that an even hybridization of molecular orbitals leads to a wider energy gap between the HOMO and LUMO


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(π‒π*); in contrast, an uneven distribution can result in a narrower energy gap.49 Therefore, N-doping induces a broad band gap in NGQDs, while B-doping leads to a narrowing of the band gap in BGQDs. The calculated energy gap (Eg) between the HOMO and LUMO is 3.18, 3.27, and 2.88 eV for UGQDs, NGQDs, and BGQDs (Figure 6A), which is close to the energy gap estimated from the absorption spectra (Eg of UGQDs, NGQDs, and BGQDs is estimated to be 3.06, 3.35 and 2.85 eV (Figure 6B), respectively). The HOMO and LUMO energy levels for the GQDs shown in Figure 6B were determined by combining the absorption spectrum data and electrochemical measurements (see detailed calculation shown in Supporting Information; cyclic voltammograms for NGQDs, UGQDs, and BGQDs are shown in Figure S14). It is found that the values for Eg increase in the following order: BGQDs, UGQDs, and NGQDs. Thus, we can explain the blueshift and redshift of the PL emission for the edge transition (band II) of NGQDs and BGQDs, respectively, relative to UGQDs, as shown in Figure 5. 4. CONCLUSIONS In summary, we have synthesized UGQDs, NGQDs, and BGQDs with similar sizes, chemical compositions, and defects and elucidated their PL emissive states and the effects of the dopant heteroatom on PL emission. The GQDs showed multi-emissive centers for the PL emission mechanism, with an excitation-independent PL emission band (band I), resulting from a high-energy transition caused by quantum confinement of the carbon core (carbon π‒π* transitions in sp2 domain), and an excitation-dependent PL emission band (band II), originating from a low-energy edge band transition, which is attributed to radiative recombination associated with both the n‒π* transition of N/O/Bcontaining groups and π–π* charge-transfer between the carbon core to the edge of the GQDs. Moreover, due to alteration of the whole electronic structure and the molecular orbital distribution, NGQDs and BGQDs show a blueshift and redshift, respectively, relative to UGQDs for the PL emission maxima (both band I and II). Our results have clarified previous inconsistencies regarding the PL emission


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mechanism and electronic properties of GQDs and can thus be used to provide a foundation for the application of doped GQDs in electronics, photonics, and biology. ASSOCIATED CONTENT Supporting Information Instruments, procedures of electrochemical measurements, and detailed characterizations of GQDs, calculations of the level of HOMO, LUMO, and band gap of GQDs, Tables, and Figures. This information is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (P. Wu); [email protected] (C. Cai); [email protected] (S. S. Saavedra). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by NSFC (21335004, 21405083, and 21675088) and Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1) Shen, J.; Zhu, Y.; Yang, X.; Li, C. Graphene Quantum Dots: Emergent Nanolights for Bioimaging, Sensors, Catalysis and Photovoltaic Devices. Chem. Commun. 2012, 48, 3686‒3699.


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Exploring the Emissive States of Heteroatom-Doped Graphene

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