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Abstract
To tune the electronic and optoelectronic properties of graphene quantum dots (GQDs), heteroatom doping (e.g., nitrogen (N), boron (B), and sulfur (S)) is an effective method. However, it is difficult to incorporate S into the carbon framework of GQDs because the atomic size of S is much larger than that of C atoms, compared to N and B. In this study, we report a simple and one-step method for the synthesis of sulfur-doped GQDs (S-GQDs) via the pulsed laser ablation in liquid (PLAL) process. The as-prepared S-GQDs exhibited enhanced fluorescence quantum yields (0.8% → 3.89%) with a huge improved absorption band in ultraviolet (UV) region (200 ∼ 400 nm) and excellent photo stability under the UV radiation at 360 nm. In addition, XPS results revealed that the PLAL process can effectively facilitate the incorporation of S into the carbon framework compared to those produced by the chemical exfoliation method (e.g., hydrothermal method). And also, the mechanisms related with the optical properties of S-GQDs was investigated by time-resolved photoluminescence (TRPL) spectroscopy. We believe that the PLAL process proposed in this study will serve as a simple and one-step route for designing S-GQDs and opens up to opportunities for their potential applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene quantum dots (GQDs) are a new type of carbon nanomaterial, a few nanometers in size [1], that exhibit a distinct energy band gap, which has been claimed to be due to edge effects [2] and quantum confinement [3]. Recently, GQDs have attracted much attention because of their tunable optical properties, excellent photo stability, and low toxicity which offer great potential for future applications in bio-imaging [4], and optoelectronic devices [5]. Although, GQDs are promising optical nanomaterials with significant advantages, their relatively poor optical properties (e.g., quantum yield (QY)) have been the major barrier to the practical applications.
In recent years, heteroatom doped GQDs (H-GQDs) has provided an attractive way to significantly improve their optical and electrical properties [6]. Consequently, the H-GQDs has been widely researched in potential applications such as bio-imaging, oxygen reduction reaction, photo catalyst, sensor, and solar cells [7–15]. In particular, utilizing a tunable ultraviolet absorption band by sulfur doping of GQDs are promising for increasing the light conversion efficiency and long term stability of Dye-sensitized solar cells (DSSCs) [16,17]. Several researches have demonstrated the successful doping of S into the GQDs by chemical exfoliation methods (e.g., hydrothermal, solvothermal) [8,16–18]. However, chemical exfoliation methods are usually performed under strong acidic or basic conditions, which require an extensive period of washing. In addition, the atomic size of S is much larger than that of C atoms, andf the length of the C–S bond (1.78 A) is greater than that of the C–C bond (1.53 A). And also, the difference in electronegativity between S atoms and C atoms shows to be too small to promote significant charge transfer in C–S composites [19]; thus, the chemical doping of S into the carbon framework would seem to be significantly difficult. Hence, S-GQDs have been rarely reported, and it is essential to develop a simple, high quantum yield, environmentally benign, and efficient doping methods for S-GQDs.
To overcome such issues, the pulsed laser ablation in liquid (PLAL) can be one of the alternative methods to the synthesis of S-GQDs. The PLAL process is simple and chemically clean, because it does not require steps involving strong acidic chemicals or long periods of washing [19–24]. Furthermore, it has been effectively used for various heteroatoms doping in GQDs due to the nature of the PLAL process such as high-temperature and high-pressure environments [19]. Previously, our groups proposed a PLAL process for preparing nitrogen doped GQDs (N-GQDs), which shows that the enhanced optical properties (e.g., PL intensity and absorption) and fluorescence quantum yield after N doping [23]. However, to our best knowledge, the synthesis of S-GQDs by the PLAL process has not been described in the literature.
In this paper, we report a facile and one-step approach to the synthesis of S-GQDs through the PLAL process. The morphology, optical properties, and chemical composition of S-GQDs were studied using various characterization techniques, including transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Furthermore, the mechanisms related with the optical properties are explained by time-resolved PL (TRPL).
2. Experimental details
2.1 Preparation of pristine GQDs and S-GQDs
Graphite flakes were purchased from HQ Graphene (The Netherlands), and 3-mercaptopropionic (MPA) and high-purity ethanol (>99.99%) were purchased from Sigma Aldrich. S-GQDs were synthesized by the facile and simple PLAL method using graphite flakes in high-purity ethanol with MPA. Generally, 500 mg of graphite flakes were dispersed in 200 ml of high- purity ethanol and MPA with different concentrations (0, 5, 10, and 15 mol%). Pulsed laser ablation was injected on the graphite solution for 30 min at room temperature and in air using a Q-Switch Nd:YAG laser system. The graphite solution was injected by a horizontal pulsed laser beam (355 nm wavelength and third harmonic) at a repetition rate of 10 Hz. The pulse laser width was 10 nm/sec, and the ablation power was 1 J. After completion of the PLAL treatment, the S-GQDs suspension was dried overnight at 80 °C in vacuum condition.
2.2 Characterization of PGQDs and S-GQDs
High resolution transmission electron microscopy (HR-TEM) images of the pristine GQD (PGQDs) and S-GQDs samples were captured with a JEM-2100F transmission electron microscope equipped with a field-emission gun (200 kV; Jeol, USA). XPS spectra were recorded for both samples using a VG ESCALAB 220i system (Thermo Scientific, USA). XPS survey and high-resolution scans were performed at pass energies of 100 and 20 eV, respectively, and an X-ray beam size of approximately 100 µm. PGQDs and S-GQDs samples for XPS measurement were prepared on a silicon substrate by spin-coating method, with the rotation speed adjusted to 2000rpm. The samples were dried in vacuum oven at 80 °C for overnight prior to measurement. Room-temperature PL spectra were collected using a PL spectrophotometer (Horiba, Fluoromax-plus with 150 W xenon arc lamp) in the wavelength range of 300–800 nm. The PL emission spectra were measured at the excitation wavelengths of 400, 450, and 500 nm. A time-correlated single photon counting (TCSPC) spectrometer (Horiba Jobin Yvon) was used to measure the nanosecond-level lifetimes. The above experimental sections follow our work [23].
2.3 QY measurements
Quinine sulfate in water (QY = 0.58) and fluorescein in ethanol (QY = 0.79) were chosen as the standards for measuring the QYs of excited PGQDs and S-GQDs. The QYs of pristine and S-GQDs were calculated according to:
3. Results and discussion
S-GQDs were fabricated by PLAL process using graphite as a carbon precursor and MPA as a S precursor with various concentrations (0, 5, 10, and 15 mol%) in high-purity ethanol, as illustrated in Fig. 1. The synthesis of S-GQDs involves the following steps. First, the decompositions of S molecules (i.e., derived from the MPA) and carbon clusters (i.e., derived from the graphite) were simultaneously performed by a pulsed laser. These phenomena are correlated to the thermal heating, melting and evaporation by PLAL process [19]. The pulse laser injected on the surface of the graphite flakes, which can lead to high-temperature and high-pressure plume forms around the graphite flakes. These rapid environmental changes make it possible to decompose the carbon clusters and sulfur molecules. Second, the aggregation of each precursor (i.e., C and S) occurred in the synthesis area, resulting in forms sulfur doped graphene nanosheet (S-graphene nanosheet). Finally, the S-graphene sheets are further ablated by the injected pulsed laser to form several nanosized S-GQDs with homogeneous size and high crystallinity (Fig. 2). The above mechanism for the synthesis of GQDs by PLAL process is presented in detail in our previous study [23,24].
Fig. 1. Representation schematic for the possible mechanisms of the transformation of graphite to S-GQDs by the PLAL process.
Fig. 2. TEM images of S-GQDs: (a) S5-GQDs, (c) S10-GQDs, and (e) S15-GQDs. HR-TEM images of S-GQDs: (b) S5-GQDs, (d) S10-GQDs, and (f) S15-GQDs. Insets are the FFT pattern (left), which shows the high-quality crystalline hexagonal patterns of the GQDs. Right side insets show the lattice distance of the S-GQDs.
Figure 2 shows HR-TEM images of S-GQDs with 5 mol% S (S5-GQDs, Figs. 2(a) and (b)), S-GQDs with 10 mol% S (S10-GQDs, Figs. 2(c) and(d)), and S-GQDs with 15 mol% S (S15-GQDs, Figs. 2(e) and (f)). The average diameter of 6 nm ± 0.5 nm are obtained for the above GQDs by averaging more than 60 GQDs measurements. The comparison of HR-TEM images and size distribution of S-GQDs indicates the negligible changes in their size and shape due to incorporation of S into the carbon framework. In addition, the fast Fourier transformation patterns of the three types of S-GQDs (the left insets of Figs. 2(b),(d), and (f)) show high crystallinity in the pristine graphene structure with an interlayer spacing of 0.24 nm, as confirmed by the HR-TEM images (the right insets of Figs. 2(b),(d), and (f)). No crystalline features that correspond to another carbon precursor, such as the [002] plane, were observed for any of the GQDs by HR-TEM, and is similar to that previously reported [24]. In addition, the atomic force microscopy (AFM) analysis results are shown in Fig. 3. The statistical interpretation is that more than 90% of the PGQDs and three types of S-GQDs exhibit a thickness below 1.8 nm, which reveals that the prepared GQDs contain 3-4 graphene layers [25,26].
Fig. 3. AFM images and insets are height distribution of (a) PGQDs, (b) S5-GQDs, (c) S-10GQDs, and (d) S15-GQDs.
The optical properties of prepared GQDs were investigated by PL (Photoluminescence), UV-vis (UV-visible) spectroscopy and PLE (PL excitation). Figure 4(a) shows the PL properties of the PGQDs (pristine GQDs) and three-types of S-GQDs. S-GQDs exhibit much stronger PL emission after S doping than PGQDs, with QYs calculated at 0.8%, 2.07%, 3.89%, and 3.11% for the PGQDs, S5-GQDs, S10-GQDs, and S15-GQDs, respectively. The PL spectra also showed that the peak intensity and QY gradually increased with S doping concentration up to a concentration of 10 mol%. The relatively strong electron affinity of the S atoms (strong withdrawing groups) in the GQDs could contribute to the optical properties (e.g., PL, QY) enhancement [27,28]. Thus, the successful doping of S results in more efficient PL radiative emission and PL QY by the high density electrons in the carbon frameworks. However, the peak intensity and QY decreased as the S doping concentration exceeded 10 mol%. This result indicate that excessive sulfur would block the passivated defects in the GQDs structure [18]. Additionally, PGQDs and the three types of S-GQDs observed the excitation-dependent PL spectra, as seen in Fig. 4(b), which shows PL properties similar to previous reports [29,30]. However, the PL excitation (PLE) spectra of the S-GQDs appear to be clearly different from those of the PGQDs. The PLE spectra of S-GQDs show two clear peaks at 260 and 360 nm, which implies that S-doping leads to an enhanced electron density in the intrinsic state of GQDs [9,23].
Fig. 4. Optical properties of PGQDs, S5-GQDs, S10-GQDs, and S15-GQDs. (a) Photoluminescence (PL) properties under 360 nm excitation, (b) PL excitation, (c) Uv-vis spectra, and (d) Dependence of PL emission spectra on UV excitation time for S10-GQDs
The UV-vis spectra generally show absorption bands at 210, 260, and ∼360 nm, presented in Fig. 4(c). The absorption bands at 210 and 260 nm are assigned to the π→π* transition of C = C of the sp2 C domain, whereas the broad peak at ∼360 nm in the absorption band spectra is attributed to the n→π* transition of the surface states. Interestingly, the absorption bands strongly increase after S doping, which is also correlated with the increased intrinsic density in the carbon framework [15]. The enhancement of the absorbance is attributed the alternation of the electronic structure of S-GQDs by the S doping, which restore intrinsic state (sp2 hybridization) and donates delocalized electrons to the carbon framework, thus giving more efficient absorbance in S-GQDs [18,31]. In addition, the photo-stability was tested under the continuous UV lamp illumination with a power of 250 W for different times. As shown in Fig. 4(d), there were negligible changes in PL intensities for the S10-GQDs during 180 min. Above strong absorption band in UV region and excellent photo stability could be useful for surface coating on DSSC devices such as UV absorbing layer [16,32].
XPS was employed to investigate the chemical composition and structural analysis of the three types of S-GQDs. Figure 5(a) shows the XPS full-scan survey of the PGQDs and three types of S-GQDs (5 mol%, 10mol%, and 15mol%). Three peaks located at ∼167 eV, ∼284 eV, and ∼532 eV correspond to S2p, C1s, and O1s, respectively [33]. The chemical compositions of C and S are revealed by C1s (Fig. 5(b)) and S2p (Fig. 5(c)) spectra (S-10GQDs). As shown in Fig. 5(b), the C1s XPS spectrum can be deconvoluted into four peaks, 284.2 eV (sp2 carbon), 285.8 eV (hydroxyl), 286.3 eV (C–S), and ∼288.7 eV (carboxyl) [18]. The sp2 component is the most intense peak among the deconvoluted peaks, indicating a graphitic matrix structure. The sp2 spectra of S-10GQDs consist of a peak centered at 163 eV, which can be deconvoluted into two separate components at 163.3 and 164.4 eV. The former peak at 163.3 eV corresponds to the 2p1/2 and 2p3/2 sites of the -C-S-C- covalent bond present in the S-GQDs structure (thiophene-S). The latter peak at 164.4 eV is attributed to the -C-SOx- bond. These two significant peaks assigned to the doping of the S atom takes place in both the carbon framework as well as across the edge of the GQDs. Moreover, the different dopant concentrations presented different S concentrations in the carbon framework (Fig. 6). It is clearly shown that when the dopant concentration is increased from 5 to 15 mol%, the S concentration in the carbon structure gradually increased (2.35%→6.97%→8.06%). In addition, it was found that the S10-GQDs exhibit a higher individual area for C-S-C bonding (65.3%) compared to S5-GQDs (52.6%) and S15-GQDs (61.6%). This signifies that when S-GQDs are synthesized through the PLAL process, the doping of S atom prefers to exist in the carbon frameworks than other synthesis method [8,15,18]. Thus, the above XPS results indicate that the PLAL process might be more effective method for S incorporation into the carbon framework, which can lead to enhancements of electronic density and improving optical properties.
Fig. 5. Structural analysis of S-GQDs. (a) XPS full-survey spectra of PGQDs, S5-GQDs, S10-GQDs, and S15-GQDs. XPS C1s spectra of (b) S10-GQDs and S2p spectra of the (c) S10-GQDs. (d) Dependence of the S doping concentration and quantum yield of the S-GQDs on the MPA concentration.
To explore the recombination mechanisms of S-GQDs, we conducted TRPL analysis (Figs. 7(a)–(d)). Table 1 depicts the values obtained by TCSPC. The fluorescence decay curve is fitted with a triexponential function (Eq. (1)), in which fluorescence decay curves arise through three different relaxation pathways.
Fig. 7. TCSPC decay curves of the (a) PGQDs, (b) S5-GQDs, (c) S10-GQDs, and (d) S15-GQDs. The solid line (red line) is fitted by a tri-exponential function.
Table 1. Excitation emission values, χ2 values, excitation lifetimes, and their corresponding amplitudes for PGQDs, S5-GQDs, S10-GQDs, and S15-GQDs.
The proportion of the short-lifetime GQDs (τ1) from the intrinsic states in the carbon framework gradually increased with S doping (5 mol%→10 mol%). However, the short-lifetime proportion gradually decreased with an increase in S doping concentration to 15 mol%. In addition, the proportions of the long-lifetime GQDs (τ2 and τ3) from the surface states gradually decreased as the S doping concentration increased up to 10 mol%. However, the long-lifetime proportion was shown to increase with an increase in S doping concentration to 15 mol%. Generally, the surface-state emission shows a longer recombination lifetime (τ2 and τ3) than that of the intrinsic-state emission (short recombination lifetime; τ1), which implies that S doping significantly enhances the optical properties owing to the increased GQDs proportion in the intrinsic state. In addition, the increase in the fraction of the short lifetimes explained by effective electron-hole recombination among the carbon framework, resulting in enhanced PL QY [28]. Excessive S doping leads to a reduction in this proportion, which is in agreement with the above optical property analysis (Fig. 4).
4. Conclusion
In conclusion, we successfully prepared S-GQDs with optimum concentrations in a simple and one-step route via the PLAL process. The as-prepared S-GQDs exhibited significantly enhanced PL intensity and QY (0.8% →3.89%) after S doping with 10 mol%. However, when the S concentration exceeded 10 mol% in the carbon structure, the PL intensity gradually decreased. According to the chemical composition and structural analysis, the two types of S bonding (-C-S-C-, -C-SOx-) were formed in the S-GQDs. It was also found that the enhancement of optical properties by the S doping is mainly attributed to -C-S-C- bonding. To explore the recombination mechanism of S-GQDs was explained by TRPL spectroscopy. Excessive S doping leading to reduced PL intensity was explained by the decreased fraction of the short-lifetime S-GQDs in the intrinsic state. We expect that our PLAL process is a simple and one-step route to achieve controllable optical properties of S-doped GQDs, allowing for further enhancement of optoelectronic and solar cell applications.
Funding
Basic Science Research Program; Ministry of Education (2019R1I1A3A0106266212); National Research Foundation of Korea; Korea Institute of Industrial Technology (KITECH JA-20-0004).
Disclosures
The authors declare no conflicts of interest.
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