Quantum Yield Enhancement in Graphene Quantum Dots via Esterification with Benzyl Alcohol

The quantum yield of graphene quantum dots was enhanced by restriction of the rotation and vibration of surface functional groups on the edges of the graphene quantum dots via esterification with benzyl alcohol; this enhancement is crucial for the widespread application of graphene quantum dots in light-harvesting devices and optoelectronics. The obtained graphene quantum dots with highly graphene-stacked structures are understood to participate in π–π interactions with adjacent aromatic rings of the benzylic ester on the edges of the graphene quantum dots, thus impeding the nonradiative recombination process in graphene quantum dots. Furthermore, the crude graphene quantum dots were in a gel-like solid form and showed white luminescence under blue light illumination. Our results show the potential for improving the photophysical properties of nanomaterials, such as the quantum yield and band-gap energy for emission, by controlling the functional groups on the surface of graphene quantum dots through an organic modification approach.

www.nature.com/scientificreports www.nature.com/scientificreports/ Fig. S1), whereas that of GQD-Bn remaining in water (GQD-Bn (W)) was 4.8 ± 1.4 nm (see Supplementary  Fig. S1). The size distribution of GQD-Bn (W) showed another peak at an average diameter of 20.1 ± 6.0 nm for a large round thin layer, which was counted at the same rate in HRTEM observations. A crossed lattice pattern with a lattice parameter of 0.327 ± 0.005 nm, corresponding to the (002) interlayer spacing and similar to the value of bulk graphite 9 , was observed in the HRTEM image of GQD-Bn (O), indicating the high crystallinity of the obtained GQDs. In addition, 12% of the observed individual GQD-Bn (O) (32 GQDs counted) also showed a lattice pattern with an interlayer spacing of 0.245 ± 0.010 nm, corresponding to the (1120) lattice fringe of graphene 19,20 in addition to the (002) lattice patterns (Fig. 1c). The lattice patterns observed in GQD-Bn (O) indicate that GQD-Bn (O) has a highly graphene-stacked structure. In the case of GQD-Bn (W), a lattice parameter of 0.256 ± 0.023 nm was observed for the small particle, whereas no patterns appeared for the large round thin layer (Fig. 1b,d). The esterification of GQDs with benzyl alcohol seems to result in formation of a highly stacked structure of large, round GQD layers that are soluble in toluene. GQDs with a small particle structure and a large thin  layer were observed in water, which suggests that hydrophilic functional groups such as hydroxyl and carboxyl groups remain in the GQD-Bn (W) due to insufficient esterification within their structures. When 1-hexanol or 1-decanol-aliphatic ester-was used for the esterification instead of benzyl alcohol, the obtained GQDs (GQD-Hex and GQD-Dec, respectively) had a small particle shape with a diameter of approximately 5 nm, which is the same as that of GQD-Bn (W). Note that the size distributions of GQD-Hex and GQD-Dec do not contain the large GQDs, which are understood to remain in water when the esterified GQDs were extracted into toluene due to insufficient functionalization to be soluble in an organic solvent.
As shown in Fig. 3a, the esterification of GQD-Bn (O) was confirmed by Fourier transform infrared (FTIR) spectra from the peak shift of the C=O stretching mode observed at 1698 cm −1 for GQD-Bn (O), which is different from the C=O stretching mode observed for water-soluble GQDs, as previously reported (~1720 cm −1 ) 9,16,21 . In the case of GQD-Hex, peaks related to the vibration modes of the ester group and the saturated C-H moieties were observed at 1740 cm −1 and 2920 cm −1 , respectively, in the FTIR spectra and are attributed to the resultant aliphatic ester with 1-hexanol. These FTIR spectra confirm that the esterification of GQDs was successfully completed via the chemical reaction performed in this work. The high-resolution X-ray photoelectron spectroscopy (XPS) spectra shown in Fig. 3b,c confirm the modification of the functional groups in GQDs via esterification with benzyl alcohol because of the absence of a C-O peak (285 eV) and a C=O/O-C=O peak (287 eV) 16 . Note that GQD-Bn (O) on a fluorine-doped tin oxide substrate for the XPS measurement showed film like form, which seems to affect the detection of XPS signal related to ester groups on the edges of the GQDs. The Raman spectra of GQD-Bn could not be obtained at a wavelength of 780 nm due to a strong photoluminescence background of the GQD-Bn itself.
Optical characterization. Figure 4a shows the excitation and emission contour maps of fluorescence intensity for the GQD-Bn (O) sample. The emission maximum was 416 nm for GQD-Bn (O) and was independent of the excitation wavelength, indicating that the photoluminescence (PL) of GQD-Bn (O) was attributed to intrinsic state emission (electron-hole recombination, quantum size effect/zig-zag sites) 11 . The PL excitation spectrum of GQD-Bn (O) was well correlated with the absorption spectrum, with the characteristic feature observed in the range of 350-400 nm ( Fig. 4b) as arising from a transition from the highest occupied molecular orbital (π-orbital) to the lowest unoccupied molecular orbital 3,9 . As summarized in Table 1, all esterified GQDs showed blue fluorescence ascribed to intrinsic state emission. When aliphatic alcohols were used for esterification instead of benzyl alcohol, the emission wavelength was blueshifted to 400-406 nm with a decrease in the size of GQDs, which demonstrates that the obtained GQDs behaved as quantum dots (see Supplementary Fig. S3).
The absolute QY of GQD-Bn (O) in toluene was 0.25 at a wavelength of 400 nm, and the relative QY of GQD-Bn (O) was 3-5 times higher than those of GQD-Hex and GQD-Dec (see Table 1). In addition, as seen by comparing the relative QY of GQD-Bn (O) with those of non-functionalized GQDs, the QY of GQD-Bn (O) was approximately 100 times higher than those of non-functionalized GQDs. The highly stacked structure in GQD-Bn (O) leads to π-π interactions with adjacent aromatic rings of the benzylic ester on the edges of the GQDs, decreasing the flexibility arising from vibration and rotation of the surface functional groups and thus impeding the nonradiative recombination process and increasing the QY of GQDs. Although the doped GQDs still have higher QYs than GQDs with rigid functional groups 22 , our approach to utilize π-π interactions to impede the nonradiative recombination process in GQDs efficiently improves the QY of GQDs. Note that the same QY enhancement was observed when graphite was used as the starting material instead of carbon nanotubes 23 (see Table 1 and Supplementary Fig. S4). Figure 5 shows the time-resolved PL decay profiles of the esterified GQDs, and the corresponding lifetimes, obtained by fitting simulations using exponential decay functions, are also summarized in Table 1. The obtained PL decay consisted of two components 24 , and the PL decay time of the fast decay component was τ 1 = ~2 ns, regardless of the type of ester groups, whereas that of the slow decay component (τ 2 = 5-8 ns), and the intensity ratio of the slow decay component to the fast decay component (C 2 /C 1 ) depended on the type of ester group. The C 2 /C 1 values of GQDs formed with benzylic ester were higher than those of GQDs with an aliphatic ester, corresponding to the Φ/Φ Bn(O) value in Table 1. The two relaxation paths are understood to exist in the esterified GQDs; both a high probability and a long decay time following the slow relaxation path are understood to increase the QYs of GQDs with the aromatic ester. GQD-Bn (W) with the highest C 2 /C 1 , however, showed slightly lower QYs than GQD-Bn (O), suggesting that several factors, such as the polarity of the solvent, hydrogen bonding and nonradiative recombination due to the remaining carboxyl and epoxy groups, affect both of the relaxation path of excited carriers and the QYs in water.

Solid-state GQD-Bn.
The crude GQD-Bn in the brown gel-like solid form showed white luminescence under UV illumination at a wavelength of 410 nm. The contour map of the fluorescence intensity for as-synthesized solid GQD-Bn is shown in Fig. 6, and a new broad emission peak appeared at approximately 600 nm. The new PL peak seems to arise from redshifted excimer-like emission 25,26 due to the form of assembled GQDs in the gel; the nonuniform structure and ester moiety of GQD-Bn would cause the heterogeneity of interactions in neighbouring GQD-Bn, resulting in broadening of the emission feature. The time-resolved PL decay in the solid state (see Supplementary Fig. S5) was not well-fitted by using a double exponential function, indicating that GQD-Bn in the solid state includes GQDs with various PL decay times and/or complicated relaxation paths. www.nature.com/scientificreports www.nature.com/scientificreports/ atmosphere was cooled in a 3-necked flask in an ice bath. The obtained GQD solution was gradually added to the flask, followed by refluxing in N 2 for 3 h at 120 °C. The obtained esterified GQDs with benzyl alcohol were in a gel form and partially dissolved in water. After neutralization of the esterified GQDs in water with NaHCO 3 , the organic-soluble GQDs were extracted into toluene. The GQDs in toluene were then dehydrated by the addition of MgSO 4 , following the removal of the resulting salt by filtration.

Synthesis of esterified GQDs.
Structural and optical characterization of GQDs. HRTEM images were obtained on an electron microscope at 120 kV (Tecnai G2 F20 S-TWIN, FEI, Thermo Fisher Scientific, USA). Infrared spectra were  www.nature.com/scientificreports www.nature.com/scientificreports/ obtained on an FT-IR spectrophotometer (FT/IR-4100, JASCO, Japan). X-ray photoelectron spectroscopy was carried out on an AXIS Nova (KRATOS ANALYTICAL, SHIMADZU Corp., Japan). Optical absorption spectra of the obtained samples were collected on a UV-vis-NIR spectrophotometer (UV-3600, SHIMADZU corp., Japan). All fluorescence spectra including excitation and emission contour maps were obtained by a )·(D i /D st ), where i is the sample, st is the reference, A is the absorbance at the excitation wavelength at which the PL spectra are measured, F is the peak area, n is the refractive index, and D is the dilution rate. GQD-Bn (O) in toluene was used as the reference for the calculation. A i and F i are calculated by using the absorbance at λ ex, max and the peak area of the PL spectrum excited at λ ex, max , respectively, of each sample. [d] The intensity ratio of the slow decay component (C 2 ) to the fast decay component (C 1 ). [e] τ avg calculated by the function τ avg = Σ(C·τ 2 )/Σ(C·τ) = (C 1 ·τ 1 2 + C 2 ·τ 2 2 )/ (C 1 ·τ 1 + C 2 ·τ 2 ). [f] Graphite was used as the starting material for synthesizing GQD-Bn (O).

conclusion
In conclusion, the QY of GQDs was successfully enhanced by restricting the rotation and vibration of surface functional groups on the edges of GQDs via esterification with benzyl alcohol. The obtained GQDs had a highly graphene-stacked structure with high crystallinity. GQD-Bn in toluene had an absolute QY value of up to 25%, whereas GQDs with aliphatic ester groups had QYs that were 1/3-1/5 lower. The type of ester groups on GQDs affected the relaxation path for the emission of GQDs, and the slower relaxation component of PL decay preferentially occurred in GQD-Bn. The as-synthesized GQD-Bn in a gel-like form showed white luminescence with two emission regions, 400-550 nm and 550-700 nm, which is expected to have promising applications in light-harvesting devices and optoelectronics.