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Graphitic C3N4 quantum dots for next-generation QLED displays
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Graphitic C3N4 quantum dots for next-generation QLED displays Liangrui He 1,†, Mi Fei 1,†, Jie Chen 1, Yunfei Tian 1, Yang Jiang 1, Yang Huang 2, Kai Xu 2, Juntao Hu 2,⇑, Zhi Zhao 3, Qiuhong Zhang 4, Haiyong Ni 4, Lei Chen 1,5,⇑ 1
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China 3 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China 4 Guangdong Province Key Laboratory of Rare Earth Development and Application, Guangdong Research Institute of Rare Metals, Guangdong Academy of Sciences, Guangzhou 510651, China 5 Intelligent manufacturing institute of Hefei University of Technology, Hefei 230051, China 2
Quantum dot light-emitting diode (QLED) displays are considered a next-generation technology, but previously reported quantum dots (QDs) consisting of heavy metals are toxic and harmful. This work examined earth-abundant, metal-free, graphitic C3N4 (g-C3N4) with exceptional optical and electronic properties, excellent chemical and thermal stability, an appropriate band gap, and non-toxicity for QLED applications. The dependence of the luminescence performance on the reaction atmosphere and temperature; the transformation of the crystal and electronic structures during the reaction, including crystal defects and surface functional groups; and the luminescence mechanisms of g-C3N4 were uncovered. The highest quantum yield of 49.8% was achieved by the sample possessing the highest graphitic-to-triazine carbon ratio synthesized at 500 °C under N2 atmosphere. The disappearance of the charge-transfer band, crystal defects (traps), and non-radiative transition (due to fast relaxation) from the absorption spectra demonstrates the enhanced quantum efficiency of the g-C3N4 QDs over that of the bulk powders. A QLED prototype device employing g-C3N4 QDs as the blue-emitting layer was demonstrated. Introduction With the development of artificial intelligence in terms of virtual and augmented reality, the demand for smart wearable and flexible displays has significantly increased and will continue to do so [1–6]. Distinguished from the solid-state light-emitting diodes (LED) used as backlights in liquid crystal displays, organic LEDs can be fabricated on flexible substrates [7]. Accordingly, flexible displays are currently dominated by organic light-emitting
⇑ Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China (L. Chen). E-mail addresses: Hu, J. ([email protected]), Chen, L. ([email protected]). † L.H. and M.F. contributed equally as co-first authors.
diodes (OLEDs). Nevertheless, the lifespan of the organic materials is still a great technical problem for OLEDs. Notably, the degradation of blue emitters occurs more rapidly than that of red and green materials, leading to color distortion [7], which is much more noticeable than a decrease in the overall brightness [8]. Quantum dot displays based on electro-emissive quantum dot LEDs (QLEDs) are considered the next-generation technology succeeding OLED displays [2–6]. QLED displays can easily realize large-area, full-color and flexible displays by patterning differentsized colloidal quantum dots (QDs) on a pixelated substrate similarly to active-matrix OLED (AMOLED) displays and can improve the color range of a device owing to its size-dependent
1369-7021/Ó 2018 The Authors. Published by Elsevier Ltd. https://doi.org/10.1016/j.mattod.2018.06.008This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
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optical properties [2–6]. Nevertheless, the QDs reported for QLEDs are mainly core–shell Gd(Se,S)/Zn(S,Se) [2–6]. The heavy metal Cd and unstable S are toxic and harmful to the environment, especially with respect to Cd, which is prohibited by the RoHS directive of the European Commission for use in electrical and electronic equipment. As an alternative, earth-abundant, metal-free, graphene-like C3N4 shows great potential as a blue emitter for QLED applications. Since the discovery of graphene, efforts have been made to exploit ultrathin two-dimension (2D) nanomaterials because of their enhanced photoresponsivity with respect to bulk materials [9]. Carbon nitrides exist in several allotropes, but graphitic phase C3N4 (g-C3N4), which has a graphene-like 2D structure, is the most stable in the ambient atmosphere [10–14]. C3N4 is the oldest synthetic polymer and can be traced back to melon, with a general formula (C3N3H)n, reported by Liebig in 1834 [15]. In 1989, Liu and Cohen theoretically predicted that b-C3N4 would be interesting as an ultra-hard material [16]. The lone pairs of the sp2 nitrogen atoms and the sp3-conjugated polymeric network (p) endow g-C3N4 with exotic features, such as facile synthesis, good chemical and thermal stability, non-toxicity, excellent biocompatibility, a medium band gap (2.7 eV), and excellent photo (electro)chemical activities [10–12]. The unique properties of gC3N4 have resulted in a wide range of studies aimed at investigating its luminescence [17–19], photocatalytic [10–12,21–22], fluorescence sensing [22–24], bio-imaging [9,24], bio-/cancer therapy [24–26], and solar energy [12,27] applications. Numerous routes have been developed to synthesize bulk and nanostructured g-C3N4, for example, by the polycondensation of nitrogen-rich precursors (such as urea [18,28], cyanamide [10], dicyanamide [24,29], and melamine [9,17,28,30], etc.) to form bulk powders followed by thermal/chemical etching or ultrasonic exfoliation in polar solvents to prepare nanosheets/nano wires/nanotubes/QDs [12]or by the one-step electrolysis of melamine in NaOH solution to produce g-C3N4 nanosheets [31]. However, defects are unavoidable because many intermediates (such as biuret, cyanuric acid, ammeline and ammelide) are involved in the synthesis of g-C3N4 [28–29]. In addition, precursors easily sublime during the thermal condensation process [31], which will change the functional groups on the surface of the particles and in turn affect the electronic structure of surface. Therefore, the photoelectric efficacy of g-C3N4 strongly depends on the synthetic routes and reaction conditions [28]. Some defects can serve as recombination centers to emit light, and other defects can facilitate charge separation to supply energy. Identifying the defects is the first step toward controlling and improving the performance of g-C3N4. For this, two types of defects were identified by Wu using surface photovoltage (SPV) spectroscopy [29]. Intrinsically, the luminescence performance is determined by the crystal and electronic structures and, more specifically, by the distribution of electrons in the excited states. Focusing on these problems, this work explores g-C3N4 for QLED display applications.
Materials and methods Bulk powders were synthesized by the thermal condensation of melamine at 450, 500, 550, 600, and 650 °C for 2 h under air
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and N2 atmospheres. Then, the QDs were prepared by ultrasonic exfoliation of the g-C3N4 powder in water or ethanol for approximately 16 h at room temperature [9]. After standing for 24 h, the suspension was centrifuged at approximately 8000 rpm to obtain a transparent colorless solution. The phases were examined by X-ray diffraction (XRD) analysis using an X’Pert PRO MPD diffractometer. The profile of the particles was characterized using an SU8020 scanning electron microscope (SEM). Highresolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses were performed using a JEM-2100 F equipped with a 2100F field-emission gun. The excitation and emission spectra were collected using a Hitachi F-4600 spectrophotometer. The absorption/reflection spectra were recorded on a Shimadzu UV-3600 ultraviolet–visible (UV–VIS) spectrophotometer. The Fourier transform infrared spectra (FTIR) were recorded using a Nicolet 67 Fourier infrared spectrometer. The quantum yield (QY) of the g-C3N4 QDs and bulk powders were determined by a steady-state lifetime fluorescence spectrometer (FluoRoLOG-3-TAU) and an integrating sphere fluorescence spectrometer (Orient KOJI QY-2000). QLED devices were fabricated by individually spin-coating each layer of organic compounds and g-C3N4 solution onto ITO glass.
Results and discussion To achieve maximal luminescence, the synthesis of bulk g-C3N4 powders was optimized at various temperatures. As shown in Fig. 1a and b, in the XRD patterns of the samples synthesized in air and N2 atmospheres, respectively, the characteristic diffraction peaks of g-C3N4 at 2h values of approximately 27.3° and 13.1°, corresponding to the (0 0 2) and (1 0 0) planes, were observed [20,30]. However, the residual raw material melamine and intermediates, such as melem and its derivatives of dimelem [32–35], were observed at 450 °C. Nevertheless, most of the impurity phases were confirmed to be melem (Supplementary Fig. 1) [32–35]. As the temperature increased to 500 °C, the diffraction peaks of melem disappear in Fig. 1a but are still present in Fig. 1b. This difference shows the significant effect of the reaction atmosphere on the products. As far as the diffraction peak at 27.3° is concerned, the shift in the diffraction peak position toward higher angles as the temperature increased from 450 to 650 °C suggests an intensified degree of thermal condensation with increasing temperature. Moreover, the enhanced diffraction intensity and the narrowing FWHM (full-width at halfmaximum) with increasing temperature suggests improved crystallinity and an increase in particle size, which is confirmed by the SEM images shown in Supplementary Fig. 1c, d. The bulk g-C3N4 particles have an irregular flake-like morphology, as shown in the SEM image in Fig. 1c. In addition to the agglomerates, some bars were observed (inset shown in Fig. 1c), which may be formed by the thermal-induced rolling of the 2D sheets of g-C3N4 at high temperatures to minimize the total surface free energy [36–38]. These rod-like particles were rarely observed in the powders condensed in air atmosphere (Supplementary Fig. 1c) but they were noticeable in the powders condensed in N2 atmosphere (Fig. 1c), which shows the important effect of reaction atmosphere on the surface morphology of the particles. Moreover, crumpling and wrinkling can be observed
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FIGURE 1
XRD patterns and SEM and TEM images of the g-C3N4 samples. (a, b) XRD patterns of the carbon nitride compounds synthesized via thermal condensation of melamine at different temperatures under air and N2 atmospheres, respectively; (c, d) SEM and TEM images of bulk g-C3N4 particles synthesized at 500 °C under N2 atmosphere, where the inset in (c) presents a close-up view of the rod-like particles; (e) HRTEM images of the QDs obtained by the ultrasonic exfoliation of bulk g-C3N4 in H2O; and (f) lattice fringe images of the QDs, where the inset is the SAED pattern.
on the surface of the bulk particles in Fig. 1d, indicating the 2D sheet structure of g-C3N4. After ultrasonic exfoliation, QDs with a diameter of approximately 2–3 nm were obtained, as shown in Fig. 1e. The lattice fringe spacing of approximately 0.21 nm in Fig. 1f agrees well with the (1 0 0) plane of g-C3N4 [30]. The inset of the SAED pattern, corresponding to the (0 02 ) plane of g-C3N4 with an interplanar distance of approximately 0.336 nm [30], indicates that the compound synthesized at 500 °C under N2 atmosphere has a crystalline structure. The emission spectra of the bulk powders synthesized under air and N2 atmospheres are presented in Fig. 2a, b, respectively. In ambient air, the strongest luminescence occurred at 450 °C,
and an inverse dependence of luminescence intensity on increasing temperature was found in Fig. 2a; however, the maximum luminescence appeared at 500 °C in Fig. 2b for the samples condensed under N2 atmosphere, and the intensity decreased as the temperature increased from 500 to 650 °C. Moreover, a redshift in the emission wavelength as the temperature increased from 450 to 650 °C was determined from the normalized emission spectra in Fig. 1a, b in Ref. [39], which was a common characteristic for samples synthesized under both air and N2 atmospheres. Compared to the strongest luminescence, the intensity of the sample synthesized at 500 °C under N2 atmosphere was approximately 2.5 times that of the sample synthesized at 450 °C under
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FIGURE 2
Emission and excitation spectra of the bulk g-C3N4 samples. (a, b) Emission spectrum of bulk g-C3N4 synthesized under air and N2 atmospheres, respectively, excited at 365 nm; (c, d) emission spectra of bulk g-C3N4 synthesized at 500 °C under air and N2 atmospheres, respectively, excited with various wavelengths; and (e, f) excitation spectra of bulk g-C3N4 synthesized under air and N2 atmospheres, respectively, by monitoring at the strongest emission.
air atmosphere. Fig. 2c, d present the emission spectra of bulk powders condensed at 500 °C under air and N2 atmospheres, respectively, excited at various wavelengths. The position of the emission peak remained nearly unchanged, but the intensity monotonously increased as the excitation wavelength increased from 305 to 325, 345, 365 and 385 nm. In addition, the FWHM of the emission spectra in Fig. 2b was narrower than that in Fig. 2a and was determined by the extent of the electron cloud around the atomic nucleus [40]. In short, the sharp bright green–blue luminescence of bulk g-C3N4 was obtained (inset in Fig. 2d). The temperature-dependent features of the g-C3N4 luminescence in Fig. 2a–d could be interpreted from the excitation spectra in Fig. 2e, f. The variation in the excitation intensity with temperature in Fig. 2e, f is consistent with the emission displayed in Fig. 2a, b; the emission intensity in Fig. 2c, d is also consistent with the excitation intensity varying with wavelength in Fig. 2e, f. Intrinsically, the emission and excitation spectra are determined by the electron transitions between the ground and excited states and depend on the distribution of electrons in the excited states. The asymmetrical configuration of the spectra displayed in Fig. 2a–f suggests that the emission and excitation spectra are comprised of more than one peak. With the excitation intensity normalized, four bands with peaks at approximately 339, 375, 399, and 431 nm can be distinguished in Fig. 1c, d in Ref. [39]. Since the states of C3N4 consist of an sp3 CAN d-bond, an sp2 C-N p-bond, and the lone pairs of bridged N atoms within the p-band, the bands with peaks at 339, 375, and 399 nm in the order of decreasing energy were attributed
to the electron transitions from the lone pair (LP) to the antibonding d⁄-orbital (LP-d⁄), from the LP to the anti-bonding p⁄orbital (LP-p⁄), and from the conjugated p-orbital to the antibonding p⁄-orbital (p–p⁄), respectively [17–18]. The band with a peak at approximately 431 nm appeared as the temperature increased from 450 to 500 and further decreased at 550 °C before disappearing when the temperature reached 600 and 650 °C, as shown in Fig. 2e, f and Fig. 1c, d in Ref. [39]. This trend likely originates from shallow traps (confirmed by the thermoluminescence below). In addition, the convex curve at 339 nm appears in the excitation spectra at 600 and 650 °C in Fig. 1c, d in Ref. [39], indicating electrons were easily excited from the LPs to the high level d⁄ state due to exorbitant condensation with the shrinkage of the C-N bonds in g-C3N4 at high temperatures. After being exfoliated into QDs, the sample synthesized at 500 °C under N2 atmosphere still exhibited the strongest luminescence (Fig. 3a, b). The highest QY of the g-C3N4 QDs was 49.8% (Supplementary Scheme 1), while the QY of the bulk powder synthesized at 500 °C under N2 atmosphere was approximately 22.9%. As the temperature increased from 450 to 650 °C, the change in the luminescence intensity and the redshift of the emission wavelength in Fig. 3 were consistent with those in Fig. 2. Nevertheless, the comparison of Fig. 3a– d with Fig. 2a–d reveals that the emission peak height of the QDs was shorter than that of the bulk powders and that the shift of the emission peak wavelength with varying temperature shown in Fig. 2a and b in Ref. [39] was smaller than that shown in Fig. 1a and b in Ref. [39], indicating the effect of size on the luminescence.
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FIGURE 3
Emission and excitation spectra of g-C3N4 QDs. (a, b) Emission spectra of transparent liquids containing g-C3N3 QDs achieved by exfoliating the above bulk powders synthesized at various temperatures under air and N2 atmospheres, respectively, and excited at 325 nm; (c, d) emission spectra of transparent liquids that contain the g-C3N3 QDs achieved by exfoliating the above bulk powders synthesized at 500 °C under air and N2 atmospheres, respectively, and excited at various wavelengths; and (e, f) excitation spectra of g-C3N4 QDs obtained by monitoring the emission at 470 nm in (a, b).
Fig. 3c, d present the emission spectra of the QDs obtained from the above bulk sample synthesized at 500 °C excited at different wavelengths. In addition to the asymmetrical main band peak at approximately 445 nm, one minor shoulder at approximately 405 nm was observed in Fig. 3d. The intensity of the minor band decreased as the excitation wavelength increased from 285 to 325 nm and then disappeared as the wavelength increased further, exhibiting the characteristics of selective excitation. The highest-energy emission at approximately 405 nm must come from the d⁄-LP transition due to the matched excitation of the d⁄-level with the 285–325 nm wavelength. The highest-energy emission of the d⁄-LP transition, which peaked at 405 nm, was clearly identified from the sample synthesized at 450 °C in N2 in Fig. 3b, and the d⁄-LP emission was also observed in Fig. 3a for the sample synthesized at 450 °C in air but had reduced intensity. Combined with the above discussion about the excited states of g-C3N4, the asymmetrical main band comprises the p⁄-LP and p⁄-p transitions. Thus, the emission spectra could be fitted with multiple Gaussian functions, as shown in Supplementary Fig. 2. Supplementary Table 1 summarizes the fitted emission peaks and the relative intensity of the d⁄LP, p⁄-LP and p⁄-p transitions. The d⁄-LP transition, which did not appear in the spectra of the bulk powders in Fig. 2, was present in the emission spectra of the QD in Fig. 3, potentially because the change in the crystal structure during the exfoliation process hindered the efficient relaxation of electrons from the d⁄
state to the p⁄ state. However, the d⁄-LP emission decreased as the temperature increased from 450 to 500 °C and then disappeared as the temperature increased to above 550 °C in Fig. 3a, b, suggesting that the conjugated p-orbital is not well formed at 450 °C (because the temperature is too low), especially in N2 atmosphere. This result in turn causes the inefficient relaxation of electrons to the p⁄ state. Some electrons radiate from the d⁄ state to the LP directly by emitting light. When the temperature was below 500 °C, two obvious excitation bands were observed in Fig. 3e, f, with one peak at approximately 323–336 nm and another at approximately 365–375 nm; however, a third peak at approximately 389 nm was observed at 600 and 650 °C. Combined with the above discussions about the C3N4 states, it is believed that the third peak at 389 nm originated from the lowest energy level of the p–p⁄ transition. Accordingly, the bands at approximately 323–336 and 365–375 nm are attributed to the LP-d⁄ and LP-p⁄ transitions, respectively. Comparatively, the excitation of shallow traps in Fig. 3e, f was not as evident as that in Fig. 2e, f, and the same result was seen when comparing Fig. 2c, d in Ref. [39] with Fig. 1c, d in Ref. [39]. Moreover, the comparison of Fig. 3e with 3f shows that, at the same temperature, more electrons were excited to the d⁄ state than to the p⁄ state for the sample condensed in air, whereas more electrons were excited to the p⁄ state than to the d⁄ state for the sample condensed in N2. The same conclusion holds true for bulk powers in Fig. 2e, f. The variation in the configuration of the
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emission and excitation spectra suggests a change in the electronic structures caused by changes in the reaction temperature, atmosphere and size. The absorption spectra of the bulk powders and QD were compared to uncover the mechanism of luminescence. In addition to the LP-d⁄, LP-p⁄ and p–p⁄ transitions, a typical band peak at 266 nm was observed in Fig. 4a, b for the bulk powders synthesized at 550–650 °C under both air and N2 atmospheres, which was likely caused by the charge effect from the transition of electrons from the valence band to the conduction band to form a photocurrent (denoted CTB) since g-C3N4 is an n-type semiconductor with a band gap of approximately 2.7 eV [17–19]. However, the CTB was not readily observed at 450 °C but at higher temperatures. The exorbitant condensation at 550–650 °C easily results in a photocurrent, which can explain the decrease in luminescence with an increase in the reaction temperature, as seen in Fig. 2. Moreover, the absorption within 440–600 nm improved significantly as the temperature increased from 450 to 650 °C. We think that this absorption is caused by defects related to the change in color with changing temperature [29]. However, this band overlaps with the emission spectrum very well (Fig. 3a, b in Ref. [39]). In addition to defects, the fast relaxation of electrons from the excited states to the ground state without the Stokes shift likely gives rise to the absorption in the 440–600 nm range. As shown in Fig. 4c, d, however, the absorption spectra of the QDs consist of the main LP-d⁄ transition and minor LP-p⁄ and p–p⁄ transitions. Nevertheless, the LP-p⁄ and p–p⁄ absorptions of the QDs in Fig. 4c, d are far weaker than those for bulk powders in Fig. 4a, b. After being exfoliated into QDs, the absorption bands of CTB and the non-radiative transition
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within 440–600 nm vanished, which could explain the enhanced photoresponsivity of the QDs compared to that of the bulk powders. The thermoluminescence (TL), which is considered to be the best tool for identifying defects [40], was used to investigate gC3N4. As shown in Fig. 4e, f, one band peak at approximately 75 °C and another band peak at approximately 250 °C were observed in the TL spectra of bulk g-C3N4. As determined using the symmetry (geometrical) factor [41], the band peak at 75 °C exhibited approximately second-order kinetics, while the band peak at 250 °C exhibited approximately first-order kinetics (Supplementary Table 2). Two types of defects (0.38 eV and 0.97 eV vs NHE) were identified using surface photovoltage (SPV) spectroscopy, as developed by Wu et al., who thought the defects were related to the amino/imino groups [29]. The deep-trap peak at 250 °C, whose depth was approximately 2.68 eV, as evaluated by Chen’s equation [41], was close to the band gap of g-C3N4 [17–20]. For samples synthesized in ambient air, the TL peak at 250 °C was also observed in the sample synthesized at 450 °C (Fig. 4e). For samples synthesized under N2 atmosphere, however, the peak at approximately 250 °C was not observed in a second test after being heated to 400 °C in air ambient (Supplementary Fig. 3). These results suggest that the peak at 250 °C may be related to residual H or adsorption on the surface of the particles, and the peak faded away due to oxidation at high temperatures. The depth of the shallow traps evaluated with Chen’s equation [41] was approximately 0.5–0.8 eV (Supplementary Table 2), which changed slightly with changes to the reaction temperature and atmosphere. The shallow traps may be related to hydroxylamine or hydroxyaldehyde groups, as
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Absorption and thermoluminescence spectra. (a, b) Absorption spectra of bulk g-C3N4 powders synthesized under air and N2 atmospheres, respectively; (c, d) absorption spectra of g-C3N4 QDs obtained by exfoliating the above bulk powders synthesized at various temperatures under air and N2 atmospheres, respectively; and (e, f) thermoluminescence of g-C3N4 bulk powders synthesized under air and N2 atmospheres, respectively. 6 Please cite this article in press as: L. He et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.008
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analyzed by FTIR below. The shallow traps play a key role in the design of the n-type semiconductor g-C3N4, in which the balance between concentration and depth is crucial to the photoelectric performance of g-C3N4, including the photoluminescence and photocatalysis performance [10–14,17–19]. Thus, the highest concentration of shallow traps observed in the sample synthesized at 500 °C in N2 suggested that the shallow traps exhibited a positive effect on the luminescence, which can explain the occurrence of the strongest luminescence in Fig. 2b. The compositions and chemical states of the bulk samples were investigated by XPS analysis. The C 1s and N 1s spectra of the bulk samples condensed at 500 °C are shown in Fig. 5a–d, and the others are provided in Fig. 4 in Ref. [39]. Two peaks were observed in the C 1s spectra in Fig. 5a, b, with one main peak at 287.9 eV for the sp2-bonded carbon in the NAC@N group and one at 284.7 eV for the sp2-bonded carbon in C–C [17–19,28– 38]. The former peak represents the sp2 C in triazine, whereas the latter corresponds to the graphitic sp2 C [17–19,28–38]. Three peaks were de-convoluted from the N 1s spectra in Fig. 5c, d: the minor peak at 400.9 eV for the amino functional group of HANAC (N1), the middle peak at 399.6 eV for the sp3 threecoordinated nitrogen from the NA(C)3 group (N2), and the main peak at 398.5 3 V for the sp2 hybridized nitrogen from the C@NAC group in triazine (N3) [17–19,28–38]. The analysis of atomic O discloses that the contamination from O decreases with increasing temperature, and the relative concentration of O in samples condensed under N2 atmosphere was lower than that condensed in ambient air (Fig. 4d and Table 1 in Ref. [39]). During the aromatization process, the N sp2/sp3 ratio increased
as result of the transformation from CANHAC to C@NAC [18]. As seen from Fig. 4b in Ref. [39], the highest intensity peak for graphic carbon was observed in the bulk sample condensed at 500 °C under N2 atmosphere, and the second highest intensity peak was observed in the bulk sample condensed at 450 °C in ambient air; these peaks are consistent with the strongest and the second strongest luminescence values in Fig. 2. Therefore, the type of C is related to the luminescence efficiency. From the FTIR spectra shown in Fig. 5e, f, the sharp absorption of the breathing mode of the triazine ring of g-C3N4, which shifted from 795 to 805 cm1 as the temperature increased from 450 to 650 °C, was observed [17–19,28–38]; typical stretching vibrations of the CAN bond at 1232, 1310 and 1396 cm1 and the C@N bond at 1534 and 1620 cm1 within the broad band region of 1000–1700 cm1, corresponded to the skeletal stretching modes of the aromatic rings [17–19,28–38]. The absorption within 2900–3500 cm1 was related to amino groups [17– 19,28–38]. A closer inspection of the FTIR spectra reveals subtle effects of the reaction temperature and atmosphere on the structures. In addition to NAH absorption at 3070 cm1, the intensified absorption at 3260 cm1 suggests that increasingly more NAOH bonds were generated as the temperature increased from 450 to 650 °C due to the formation of hydroxylamine (NH2OH) [42]. The absorption peak at 881 cm1, whose intensity also increased as the temperature increased from 450 to 650 °C, was caused by the formation of hemiacetal bonds between the aldehyde groups and the neighboring hydroxyl groups, i.e., C@O or O@CAOH [43]. The enhanced absorption of the aromatic rings as the temperature increased from 450 to 650 °C, as
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FIGURE 5
XPS and FTIR spectra of the g-C3N4 samples. (a, b) C 1s spectra of bulk samples synthesized at 500 °C in air and N2, respectively; (c, d) N 1s spectra of bulk samples synthesized at 500 °C in air and N2, respectively; and (e, f) FTIR spectra of bulk samples synthesized at various temperatures in air and N2, respectively. 7 Please cite this article in press as: L. He et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.008
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identified from the typical vibration of C@N at 1620 cm1, suggests that the degree of polycondensation increased, which was in accordance with the above XRD results. In addition, the formation of C@O, O@CAOH and NAOH suggests the breakage of heptazine units, which occurs at the surface of bulk particles. Fortunately, the damage was suppressed by the protection offered by the N2 atmosphere, as revealed by the comparison of Fig. 5f with 5e. To demonstrate its application potential, a QLED device was prepared by employing g-C3N4 QDs as an emitting layer with the spin-coating approach based on the device structure shown in Fig. 6a. The LiF/Al electrode was deposited by thermal evaporation. The matched energy levels of the various thin-film layers are shown in Fig. 6b. Fig. 6c is the image of the device in the light-off and light-on modes, with an applied voltage of 21 V. Two bands, one at 427 nm and one at 513 nm, were observed in the electroluminescence spectra shown in Fig. 6d, which differed from the electroluminescence peak at approximately 435 nm reported by Chen [44]. Fig. 6e suggests that the luminescence increases nearly linearly with the applied voltage. Concluded from the above discussion of the energy levels in the g-C3N4 QDs, the peaks at 427 and 513 nm should be emitted by the combination of the holes injected at the LP state in valence band with the electrons injected at the unoccupied d⁄ and p⁄ orbitals, respectively, in conduction band. Thus, the mechanism of electroluminescence can be described by the scheme in Fig. 6f.
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In addition to QLEDs, the g-C3N4 QDs are applicable for flexible transparent displays, such as smart windows for advertising [45], as demonstrated in Supplementary Fig. 4 by spincoating on flexible polyvinylchloride substrate, since the solution of g-C3N4 QDs in water or alcohol is nearly colorless and transparent. Compared with organic blue emitters, which have been practically used in OLEDs, such as the host material 4,40 -bis(9-carbazo lyl)-biphenyl (CBP) and its derivatives [46], the blue fluorescent material with biphenyl groups, the common blue phosphorescent material bis((4,6-difluorophenyl)-pyridine) iridium(III) picolinate (FIrpic) [47], and blue donor–acceptor small molecules [48], the g-C3N4 QDs exhibit the advantages of being nontoxic, earth-abundant, chemically stable, long lived, and fulfill the requirements of low-cost manufacturing at ambient conditions by spin-coating. With respect to the emerging halide perovskite nanocrystals [49], the g-C3N4 QDs have competitive superiority in terms of cost-effectiveness and the promising scaled-up fabrication with zero pollution release. Toward practical applications, however, the performance of g-C3N4 QDs in QLED devices, including emission color chromaticity and color saturation (determined by emission peak wavelength and band width), luminance, and energy conversion efficiency of electroluminescence still need to be optimized discreetly to meet with the high-quality requirement of displays on high-definition, highluminance and high-accuracy pictures.
FIGURE 6
QLED devices prepared using g-C3N4 QDs. (a) Schematic illustration of the device structure; (b) energy levels of various thin-film layers in the device; c the asprepared QLED device in natural light and a photograph of the light-emitting device with an applied voltage of 21 V; (d) emission spectra of the as-prepared QLED device; (e) the brightness and current density as function of the applied voltage; and f the mechanism of electroluminescence.
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Conclusions In summary, the strongest photoluminescence from g-C3N4 was obtained by the thermal condensation of melamine at 500 °C under N2 atmosphere, and the highest QY of approximately 49.8% was achieved for g-C3N4 QDs. The luminescence intensity of samples synthesized under N2 atmosphere is far higher than that of samples prepared in ambient air, and the emission wavelength of the QDs is shorter than that of the bulk powders. The increase in temperature intensifies the degree of polycondensation but causes the formation of hydroxylamine (NH2OH) and hemiacetal (O@CAOH) functional groups on the particle surface. Analysis of the XRD patterns demonstrates that minor residual melem can help modify g-C3N4. Damage to the functional groups on the particle surface was effectively inhibited by the N2 atmosphere, as revealed by the FTIR analysis. The variation in the configuration of the excitation spectra shows that more electrons are excited to the p* state in the samples synthesized under N2 atmosphere than in those synthesized in ambient air, indicating that more p-conjugated bonds are formed at 500 °C under N2 atmosphere. Moreover, the highest ratio of graphitic carbon to triazine carbon was observed in the sample synthesized at 500 °C under N2 atmosphere. The shallow traps present in the n-type semiconductor were identified by thermoluminescence, and the balance between the trap depth and concentration was crucial to the luminescence characteristics. After exfoliation to form QDs, the charge-transfer band and the non-radiative transition disappeared from the absorption spectra, which indicated the enhanced quantum efficiency of the QDs. The N2 atmosphere not merely alleviates surface oxidation and modifies surface morphology but more importantly exerts on chemical bonding in crystallization and alter electronic structure. The potential of the g-C3N4 QDs for next-generation QLED displays was demonstrated by fabricating a QLED prototype device.
Conflicts of interest None. The authors solemnly declare no competing financial interests.
Acknowledgments This work was supported by the National High-Tech R&D Program (863 program) (2013AA03A114), the National Natural Science Foundation (U1332133), the Project of Science and Technology of Guangdong Province (2017B090901070) and Anhui Province (1301022062), the Science and Technology Project of Guangzhou (201604016005), and the special fund for research and development of the Hefei Institute (IMICZ2015112), China.
Author contributions L.C., Y.J., and J.H. conceived the experiments and analyzed the data; L.H., M.F., J.C. and Y.T. completed the synthesis and characterization of the bulk and QDs samples; Y.H., K.X., and J.H. designed and fabricated the OLED devices; L.H. and M.F. grew
and prepared flexible transparent thin films; Z.Z, Q.Z. and H.N. measured the QYs. L.C., L.H., and M.F wrote the paper. All authors commented on the manuscript. Each author has made an indispensable contribution to this work.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.mattod.2018.06. 008. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
[40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
N. Guan et al., ACS Photon. 3 (2016) 597–603. T.H. Kim et al., Nat. Photon. 5 (2011) 176–182. J.S. Park et al., Nano Lett. 16 (2016) 6946–6953. X. Dai et al., Adv. Mater. 29 (2017). 1607022-22. J. Han et al., J. SID 24 (2016) 545–551. N. Oh et al., Science 355 (2017) 616–619. S.G. Ihn et al., Adv. Sci. 4 (2017) 1600502. Wikipedia, the free encyclopedia (EB/OL), OLED. 2017. at http://en.wikipedia. org/wiki/OLED. X. Zhang et al., J. Am. Chem. Soc. 135 (2013) 18–21. X. Wang et al., Nat. Mater. 8 (2009) 76–80. J. Liu et al., Science 347 (2015) 970–974. J. Liu, H. Wang, M. Antonietti, Chem. Soc. Rev. 45 (2016) 2308–2326. H.L. Lee et al., Appl. Mater. Today 8 (2017) 150–162. V. Devthade, D. Kulhari, S.S. Umare, Mater. Today: Proc. 5 (2018) 9203–9210. J. Liebig, Analyse der Harnsäure. Ann Pharm. 10 (1834) 10. A.Y. Liu, M.L. Cohen, Science 245 (1989) 841–842. Y. Zhang et al., Sci. Rep. 3 (2013) 1943. S. Bayan et al., J. Phys. Chem. C 121 (2017) 19383–19391. M. Rong et al., J. Mater. Chem. C 3 (2015) 10916–10924. S. Guo et al., Nat. Commun. 8 (2017) 1828–1829. Y. Dai et al., Nat. Commun. 9 (2018) 60–67. M. Xiong et al., Biosens. Bioelectron. 89 (2017) 23. G. Shiravand, A. Badiei, G.M. Ziarani, Sensors Actuators B 242 (2017) 244–252. Y. Dong et al., Small 12 (2016) 5376–5393. D.W. Zheng et al., Nat. Commun. 9 (2018). 1680-11. J.N. Tiwari et al., ACS Nano 11 (2017) 742–751. J. Xu et al., J. Power Sources 274 (2015) 77–84. Y. Zheng, Z. Zhang, C. Li, J. Photoch. Photobio. A 332 (2017) 32–44. P. Wu et al., J. Mater. Chem. A 2 (2014) 20338–20344. D. Das, S.L. Shinde, K.K. Nanda, ACS Mater. Interf. 8 (2016) 2181–2186. Q. Lu et al., Chem. Commun. 51 (2015) 12251–12253. I. Papailias et al., Appl. Surf. Sci. 358 (2015) 278–286. T. Tyborski et al., J. Phys. Condens. Matter. 24 (2012) 162201. X.Q. Wei et al., RSC Adv. 5 (2015) 26675. L. Stagi et al., Diam. Relat. Mater. 68 (2016) 84–92. L. Zhang et al., Mater. Today Energy 5 (2017) 91–98. S. Guo et al., Angew. Chem. Int. Ed. 55 (2016) 1830–1834. Q. Fan et al., Appl. Surf. Sci. 391 (2017) 360–368. L. He, M. Fei, J. Chen, Y. Tian, Y. Jiang, Y. Huang, K. Xu, J. Hu, Z. Zhao, Q. Zhang, H. Ni, L. Chen, Dataset of emission and excitation spectra, UV-VIS absorption spectra, and XPS spectra of graphitic C3N4, Data in Brief, submitted. L. Chen et al., Sci. Rep. 5 (2015) 11514. R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Princeton University Press, Oxford, 1981. W. Zheng, R.I. Kaiser, J. Phys. Chem. A 114 (2010) 5251–5255. P. Calvini et al., Vib. Spectrosc. 40 (2006) 177–183. Y. Li et al., Appl. Catal. B Environ. 229 (2018) 218–226. L. Chen et al., Nanoscale 5 (2013) 225–230. P. Strohriegl et al., Proc. SPIE 8829 (2013) 882906–882911. Y. Im et al., Adv. Funct. Mater. 27 (2017) 1603007–1603024. Y. Li et al., Mater. Today 20 (2017) 413–423. S. Adjokatse, H.H. Fang, M.A. Loi, Mater. Today 20 (2017) 258–266.
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Graphitic C3N4 quantum dots for next-generation QLED displays Liangrui He 1,†, Mi Fei 1,†, Jie Chen 1, Yunfei Tian 1, Yang Jiang 1, Yang Huang 2, Kai Xu 2, Juntao Hu 2,⇑, Zhi Zhao 3, Qiuhong Zhang 4, Haiyong Ni 4, Lei Chen 1,5,⇑ 1
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China 3 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China 4 Guangdong Province Key Laboratory of Rare Earth Development and Application, Guangdong Research Institute of Rare Metals, Guangdong Academy of Sciences, Guangzhou 510651, China 5 Intelligent manufacturing institute of Hefei University of Technology, Hefei 230051, China 2
Quantum dot light-emitting diode (QLED) displays are considered a next-generation technology, but previously reported quantum dots (QDs) consisting of heavy metals are toxic and harmful. This work examined earth-abundant, metal-free, graphitic C3N4 (g-C3N4) with exceptional optical and electronic properties, excellent chemical and thermal stability, an appropriate band gap, and non-toxicity for QLED applications. The dependence of the luminescence performance on the reaction atmosphere and temperature; the transformation of the crystal and electronic structures during the reaction, including crystal defects and surface functional groups; and the luminescence mechanisms of g-C3N4 were uncovered. The highest quantum yield of 49.8% was achieved by the sample possessing the highest graphitic-to-triazine carbon ratio synthesized at 500 °C under N2 atmosphere. The disappearance of the charge-transfer band, crystal defects (traps), and non-radiative transition (due to fast relaxation) from the absorption spectra demonstrates the enhanced quantum efficiency of the g-C3N4 QDs over that of the bulk powders. A QLED prototype device employing g-C3N4 QDs as the blue-emitting layer was demonstrated. Introduction With the development of artificial intelligence in terms of virtual and augmented reality, the demand for smart wearable and flexible displays has significantly increased and will continue to do so [1–6]. Distinguished from the solid-state light-emitting diodes (LED) used as backlights in liquid crystal displays, organic LEDs can be fabricated on flexible substrates [7]. Accordingly, flexible displays are currently dominated by organic light-emitting
⇑ Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China (L. Chen). E-mail addresses: Hu, J. ([email protected]), Chen, L. ([email protected]). † L.H. and M.F. contributed equally as co-first authors.
diodes (OLEDs). Nevertheless, the lifespan of the organic materials is still a great technical problem for OLEDs. Notably, the degradation of blue emitters occurs more rapidly than that of red and green materials, leading to color distortion [7], which is much more noticeable than a decrease in the overall brightness [8]. Quantum dot displays based on electro-emissive quantum dot LEDs (QLEDs) are considered the next-generation technology succeeding OLED displays [2–6]. QLED displays can easily realize large-area, full-color and flexible displays by patterning differentsized colloidal quantum dots (QDs) on a pixelated substrate similarly to active-matrix OLED (AMOLED) displays and can improve the color range of a device owing to its size-dependent
1369-7021/Ó 2018 The Authors. Published by Elsevier Ltd. https://doi.org/10.1016/j.mattod.2018.06.008This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
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optical properties [2–6]. Nevertheless, the QDs reported for QLEDs are mainly core–shell Gd(Se,S)/Zn(S,Se) [2–6]. The heavy metal Cd and unstable S are toxic and harmful to the environment, especially with respect to Cd, which is prohibited by the RoHS directive of the European Commission for use in electrical and electronic equipment. As an alternative, earth-abundant, metal-free, graphene-like C3N4 shows great potential as a blue emitter for QLED applications. Since the discovery of graphene, efforts have been made to exploit ultrathin two-dimension (2D) nanomaterials because of their enhanced photoresponsivity with respect to bulk materials [9]. Carbon nitrides exist in several allotropes, but graphitic phase C3N4 (g-C3N4), which has a graphene-like 2D structure, is the most stable in the ambient atmosphere [10–14]. C3N4 is the oldest synthetic polymer and can be traced back to melon, with a general formula (C3N3H)n, reported by Liebig in 1834 [15]. In 1989, Liu and Cohen theoretically predicted that b-C3N4 would be interesting as an ultra-hard material [16]. The lone pairs of the sp2 nitrogen atoms and the sp3-conjugated polymeric network (p) endow g-C3N4 with exotic features, such as facile synthesis, good chemical and thermal stability, non-toxicity, excellent biocompatibility, a medium band gap (2.7 eV), and excellent photo (electro)chemical activities [10–12]. The unique properties of gC3N4 have resulted in a wide range of studies aimed at investigating its luminescence [17–19], photocatalytic [10–12,21–22], fluorescence sensing [22–24], bio-imaging [9,24], bio-/cancer therapy [24–26], and solar energy [12,27] applications. Numerous routes have been developed to synthesize bulk and nanostructured g-C3N4, for example, by the polycondensation of nitrogen-rich precursors (such as urea [18,28], cyanamide [10], dicyanamide [24,29], and melamine [9,17,28,30], etc.) to form bulk powders followed by thermal/chemical etching or ultrasonic exfoliation in polar solvents to prepare nanosheets/nano wires/nanotubes/QDs [12]or by the one-step electrolysis of melamine in NaOH solution to produce g-C3N4 nanosheets [31]. However, defects are unavoidable because many intermediates (such as biuret, cyanuric acid, ammeline and ammelide) are involved in the synthesis of g-C3N4 [28–29]. In addition, precursors easily sublime during the thermal condensation process [31], which will change the functional groups on the surface of the particles and in turn affect the electronic structure of surface. Therefore, the photoelectric efficacy of g-C3N4 strongly depends on the synthetic routes and reaction conditions [28]. Some defects can serve as recombination centers to emit light, and other defects can facilitate charge separation to supply energy. Identifying the defects is the first step toward controlling and improving the performance of g-C3N4. For this, two types of defects were identified by Wu using surface photovoltage (SPV) spectroscopy [29]. Intrinsically, the luminescence performance is determined by the crystal and electronic structures and, more specifically, by the distribution of electrons in the excited states. Focusing on these problems, this work explores g-C3N4 for QLED display applications.
Materials and methods Bulk powders were synthesized by the thermal condensation of melamine at 450, 500, 550, 600, and 650 °C for 2 h under air
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and N2 atmospheres. Then, the QDs were prepared by ultrasonic exfoliation of the g-C3N4 powder in water or ethanol for approximately 16 h at room temperature [9]. After standing for 24 h, the suspension was centrifuged at approximately 8000 rpm to obtain a transparent colorless solution. The phases were examined by X-ray diffraction (XRD) analysis using an X’Pert PRO MPD diffractometer. The profile of the particles was characterized using an SU8020 scanning electron microscope (SEM). Highresolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses were performed using a JEM-2100 F equipped with a 2100F field-emission gun. The excitation and emission spectra were collected using a Hitachi F-4600 spectrophotometer. The absorption/reflection spectra were recorded on a Shimadzu UV-3600 ultraviolet–visible (UV–VIS) spectrophotometer. The Fourier transform infrared spectra (FTIR) were recorded using a Nicolet 67 Fourier infrared spectrometer. The quantum yield (QY) of the g-C3N4 QDs and bulk powders were determined by a steady-state lifetime fluorescence spectrometer (FluoRoLOG-3-TAU) and an integrating sphere fluorescence spectrometer (Orient KOJI QY-2000). QLED devices were fabricated by individually spin-coating each layer of organic compounds and g-C3N4 solution onto ITO glass.
Results and discussion To achieve maximal luminescence, the synthesis of bulk g-C3N4 powders was optimized at various temperatures. As shown in Fig. 1a and b, in the XRD patterns of the samples synthesized in air and N2 atmospheres, respectively, the characteristic diffraction peaks of g-C3N4 at 2h values of approximately 27.3° and 13.1°, corresponding to the (0 0 2) and (1 0 0) planes, were observed [20,30]. However, the residual raw material melamine and intermediates, such as melem and its derivatives of dimelem [32–35], were observed at 450 °C. Nevertheless, most of the impurity phases were confirmed to be melem (Supplementary Fig. 1) [32–35]. As the temperature increased to 500 °C, the diffraction peaks of melem disappear in Fig. 1a but are still present in Fig. 1b. This difference shows the significant effect of the reaction atmosphere on the products. As far as the diffraction peak at 27.3° is concerned, the shift in the diffraction peak position toward higher angles as the temperature increased from 450 to 650 °C suggests an intensified degree of thermal condensation with increasing temperature. Moreover, the enhanced diffraction intensity and the narrowing FWHM (full-width at halfmaximum) with increasing temperature suggests improved crystallinity and an increase in particle size, which is confirmed by the SEM images shown in Supplementary Fig. 1c, d. The bulk g-C3N4 particles have an irregular flake-like morphology, as shown in the SEM image in Fig. 1c. In addition to the agglomerates, some bars were observed (inset shown in Fig. 1c), which may be formed by the thermal-induced rolling of the 2D sheets of g-C3N4 at high temperatures to minimize the total surface free energy [36–38]. These rod-like particles were rarely observed in the powders condensed in air atmosphere (Supplementary Fig. 1c) but they were noticeable in the powders condensed in N2 atmosphere (Fig. 1c), which shows the important effect of reaction atmosphere on the surface morphology of the particles. Moreover, crumpling and wrinkling can be observed
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FIGURE 1
XRD patterns and SEM and TEM images of the g-C3N4 samples. (a, b) XRD patterns of the carbon nitride compounds synthesized via thermal condensation of melamine at different temperatures under air and N2 atmospheres, respectively; (c, d) SEM and TEM images of bulk g-C3N4 particles synthesized at 500 °C under N2 atmosphere, where the inset in (c) presents a close-up view of the rod-like particles; (e) HRTEM images of the QDs obtained by the ultrasonic exfoliation of bulk g-C3N4 in H2O; and (f) lattice fringe images of the QDs, where the inset is the SAED pattern.
on the surface of the bulk particles in Fig. 1d, indicating the 2D sheet structure of g-C3N4. After ultrasonic exfoliation, QDs with a diameter of approximately 2–3 nm were obtained, as shown in Fig. 1e. The lattice fringe spacing of approximately 0.21 nm in Fig. 1f agrees well with the (1 0 0) plane of g-C3N4 [30]. The inset of the SAED pattern, corresponding to the (0 02 ) plane of g-C3N4 with an interplanar distance of approximately 0.336 nm [30], indicates that the compound synthesized at 500 °C under N2 atmosphere has a crystalline structure. The emission spectra of the bulk powders synthesized under air and N2 atmospheres are presented in Fig. 2a, b, respectively. In ambient air, the strongest luminescence occurred at 450 °C,
and an inverse dependence of luminescence intensity on increasing temperature was found in Fig. 2a; however, the maximum luminescence appeared at 500 °C in Fig. 2b for the samples condensed under N2 atmosphere, and the intensity decreased as the temperature increased from 500 to 650 °C. Moreover, a redshift in the emission wavelength as the temperature increased from 450 to 650 °C was determined from the normalized emission spectra in Fig. 1a, b in Ref. [39], which was a common characteristic for samples synthesized under both air and N2 atmospheres. Compared to the strongest luminescence, the intensity of the sample synthesized at 500 °C under N2 atmosphere was approximately 2.5 times that of the sample synthesized at 450 °C under
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FIGURE 2
Emission and excitation spectra of the bulk g-C3N4 samples. (a, b) Emission spectrum of bulk g-C3N4 synthesized under air and N2 atmospheres, respectively, excited at 365 nm; (c, d) emission spectra of bulk g-C3N4 synthesized at 500 °C under air and N2 atmospheres, respectively, excited with various wavelengths; and (e, f) excitation spectra of bulk g-C3N4 synthesized under air and N2 atmospheres, respectively, by monitoring at the strongest emission.
air atmosphere. Fig. 2c, d present the emission spectra of bulk powders condensed at 500 °C under air and N2 atmospheres, respectively, excited at various wavelengths. The position of the emission peak remained nearly unchanged, but the intensity monotonously increased as the excitation wavelength increased from 305 to 325, 345, 365 and 385 nm. In addition, the FWHM of the emission spectra in Fig. 2b was narrower than that in Fig. 2a and was determined by the extent of the electron cloud around the atomic nucleus [40]. In short, the sharp bright green–blue luminescence of bulk g-C3N4 was obtained (inset in Fig. 2d). The temperature-dependent features of the g-C3N4 luminescence in Fig. 2a–d could be interpreted from the excitation spectra in Fig. 2e, f. The variation in the excitation intensity with temperature in Fig. 2e, f is consistent with the emission displayed in Fig. 2a, b; the emission intensity in Fig. 2c, d is also consistent with the excitation intensity varying with wavelength in Fig. 2e, f. Intrinsically, the emission and excitation spectra are determined by the electron transitions between the ground and excited states and depend on the distribution of electrons in the excited states. The asymmetrical configuration of the spectra displayed in Fig. 2a–f suggests that the emission and excitation spectra are comprised of more than one peak. With the excitation intensity normalized, four bands with peaks at approximately 339, 375, 399, and 431 nm can be distinguished in Fig. 1c, d in Ref. [39]. Since the states of C3N4 consist of an sp3 CAN d-bond, an sp2 C-N p-bond, and the lone pairs of bridged N atoms within the p-band, the bands with peaks at 339, 375, and 399 nm in the order of decreasing energy were attributed
to the electron transitions from the lone pair (LP) to the antibonding d⁄-orbital (LP-d⁄), from the LP to the anti-bonding p⁄orbital (LP-p⁄), and from the conjugated p-orbital to the antibonding p⁄-orbital (p–p⁄), respectively [17–18]. The band with a peak at approximately 431 nm appeared as the temperature increased from 450 to 500 and further decreased at 550 °C before disappearing when the temperature reached 600 and 650 °C, as shown in Fig. 2e, f and Fig. 1c, d in Ref. [39]. This trend likely originates from shallow traps (confirmed by the thermoluminescence below). In addition, the convex curve at 339 nm appears in the excitation spectra at 600 and 650 °C in Fig. 1c, d in Ref. [39], indicating electrons were easily excited from the LPs to the high level d⁄ state due to exorbitant condensation with the shrinkage of the C-N bonds in g-C3N4 at high temperatures. After being exfoliated into QDs, the sample synthesized at 500 °C under N2 atmosphere still exhibited the strongest luminescence (Fig. 3a, b). The highest QY of the g-C3N4 QDs was 49.8% (Supplementary Scheme 1), while the QY of the bulk powder synthesized at 500 °C under N2 atmosphere was approximately 22.9%. As the temperature increased from 450 to 650 °C, the change in the luminescence intensity and the redshift of the emission wavelength in Fig. 3 were consistent with those in Fig. 2. Nevertheless, the comparison of Fig. 3a– d with Fig. 2a–d reveals that the emission peak height of the QDs was shorter than that of the bulk powders and that the shift of the emission peak wavelength with varying temperature shown in Fig. 2a and b in Ref. [39] was smaller than that shown in Fig. 1a and b in Ref. [39], indicating the effect of size on the luminescence.
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FIGURE 3
Emission and excitation spectra of g-C3N4 QDs. (a, b) Emission spectra of transparent liquids containing g-C3N3 QDs achieved by exfoliating the above bulk powders synthesized at various temperatures under air and N2 atmospheres, respectively, and excited at 325 nm; (c, d) emission spectra of transparent liquids that contain the g-C3N3 QDs achieved by exfoliating the above bulk powders synthesized at 500 °C under air and N2 atmospheres, respectively, and excited at various wavelengths; and (e, f) excitation spectra of g-C3N4 QDs obtained by monitoring the emission at 470 nm in (a, b).
Fig. 3c, d present the emission spectra of the QDs obtained from the above bulk sample synthesized at 500 °C excited at different wavelengths. In addition to the asymmetrical main band peak at approximately 445 nm, one minor shoulder at approximately 405 nm was observed in Fig. 3d. The intensity of the minor band decreased as the excitation wavelength increased from 285 to 325 nm and then disappeared as the wavelength increased further, exhibiting the characteristics of selective excitation. The highest-energy emission at approximately 405 nm must come from the d⁄-LP transition due to the matched excitation of the d⁄-level with the 285–325 nm wavelength. The highest-energy emission of the d⁄-LP transition, which peaked at 405 nm, was clearly identified from the sample synthesized at 450 °C in N2 in Fig. 3b, and the d⁄-LP emission was also observed in Fig. 3a for the sample synthesized at 450 °C in air but had reduced intensity. Combined with the above discussion about the excited states of g-C3N4, the asymmetrical main band comprises the p⁄-LP and p⁄-p transitions. Thus, the emission spectra could be fitted with multiple Gaussian functions, as shown in Supplementary Fig. 2. Supplementary Table 1 summarizes the fitted emission peaks and the relative intensity of the d⁄LP, p⁄-LP and p⁄-p transitions. The d⁄-LP transition, which did not appear in the spectra of the bulk powders in Fig. 2, was present in the emission spectra of the QD in Fig. 3, potentially because the change in the crystal structure during the exfoliation process hindered the efficient relaxation of electrons from the d⁄
state to the p⁄ state. However, the d⁄-LP emission decreased as the temperature increased from 450 to 500 °C and then disappeared as the temperature increased to above 550 °C in Fig. 3a, b, suggesting that the conjugated p-orbital is not well formed at 450 °C (because the temperature is too low), especially in N2 atmosphere. This result in turn causes the inefficient relaxation of electrons to the p⁄ state. Some electrons radiate from the d⁄ state to the LP directly by emitting light. When the temperature was below 500 °C, two obvious excitation bands were observed in Fig. 3e, f, with one peak at approximately 323–336 nm and another at approximately 365–375 nm; however, a third peak at approximately 389 nm was observed at 600 and 650 °C. Combined with the above discussions about the C3N4 states, it is believed that the third peak at 389 nm originated from the lowest energy level of the p–p⁄ transition. Accordingly, the bands at approximately 323–336 and 365–375 nm are attributed to the LP-d⁄ and LP-p⁄ transitions, respectively. Comparatively, the excitation of shallow traps in Fig. 3e, f was not as evident as that in Fig. 2e, f, and the same result was seen when comparing Fig. 2c, d in Ref. [39] with Fig. 1c, d in Ref. [39]. Moreover, the comparison of Fig. 3e with 3f shows that, at the same temperature, more electrons were excited to the d⁄ state than to the p⁄ state for the sample condensed in air, whereas more electrons were excited to the p⁄ state than to the d⁄ state for the sample condensed in N2. The same conclusion holds true for bulk powers in Fig. 2e, f. The variation in the configuration of the
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emission and excitation spectra suggests a change in the electronic structures caused by changes in the reaction temperature, atmosphere and size. The absorption spectra of the bulk powders and QD were compared to uncover the mechanism of luminescence. In addition to the LP-d⁄, LP-p⁄ and p–p⁄ transitions, a typical band peak at 266 nm was observed in Fig. 4a, b for the bulk powders synthesized at 550–650 °C under both air and N2 atmospheres, which was likely caused by the charge effect from the transition of electrons from the valence band to the conduction band to form a photocurrent (denoted CTB) since g-C3N4 is an n-type semiconductor with a band gap of approximately 2.7 eV [17–19]. However, the CTB was not readily observed at 450 °C but at higher temperatures. The exorbitant condensation at 550–650 °C easily results in a photocurrent, which can explain the decrease in luminescence with an increase in the reaction temperature, as seen in Fig. 2. Moreover, the absorption within 440–600 nm improved significantly as the temperature increased from 450 to 650 °C. We think that this absorption is caused by defects related to the change in color with changing temperature [29]. However, this band overlaps with the emission spectrum very well (Fig. 3a, b in Ref. [39]). In addition to defects, the fast relaxation of electrons from the excited states to the ground state without the Stokes shift likely gives rise to the absorption in the 440–600 nm range. As shown in Fig. 4c, d, however, the absorption spectra of the QDs consist of the main LP-d⁄ transition and minor LP-p⁄ and p–p⁄ transitions. Nevertheless, the LP-p⁄ and p–p⁄ absorptions of the QDs in Fig. 4c, d are far weaker than those for bulk powders in Fig. 4a, b. After being exfoliated into QDs, the absorption bands of CTB and the non-radiative transition
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within 440–600 nm vanished, which could explain the enhanced photoresponsivity of the QDs compared to that of the bulk powders. The thermoluminescence (TL), which is considered to be the best tool for identifying defects [40], was used to investigate gC3N4. As shown in Fig. 4e, f, one band peak at approximately 75 °C and another band peak at approximately 250 °C were observed in the TL spectra of bulk g-C3N4. As determined using the symmetry (geometrical) factor [41], the band peak at 75 °C exhibited approximately second-order kinetics, while the band peak at 250 °C exhibited approximately first-order kinetics (Supplementary Table 2). Two types of defects (0.38 eV and 0.97 eV vs NHE) were identified using surface photovoltage (SPV) spectroscopy, as developed by Wu et al., who thought the defects were related to the amino/imino groups [29]. The deep-trap peak at 250 °C, whose depth was approximately 2.68 eV, as evaluated by Chen’s equation [41], was close to the band gap of g-C3N4 [17–20]. For samples synthesized in ambient air, the TL peak at 250 °C was also observed in the sample synthesized at 450 °C (Fig. 4e). For samples synthesized under N2 atmosphere, however, the peak at approximately 250 °C was not observed in a second test after being heated to 400 °C in air ambient (Supplementary Fig. 3). These results suggest that the peak at 250 °C may be related to residual H or adsorption on the surface of the particles, and the peak faded away due to oxidation at high temperatures. The depth of the shallow traps evaluated with Chen’s equation [41] was approximately 0.5–0.8 eV (Supplementary Table 2), which changed slightly with changes to the reaction temperature and atmosphere. The shallow traps may be related to hydroxylamine or hydroxyaldehyde groups, as
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Absorption and thermoluminescence spectra. (a, b) Absorption spectra of bulk g-C3N4 powders synthesized under air and N2 atmospheres, respectively; (c, d) absorption spectra of g-C3N4 QDs obtained by exfoliating the above bulk powders synthesized at various temperatures under air and N2 atmospheres, respectively; and (e, f) thermoluminescence of g-C3N4 bulk powders synthesized under air and N2 atmospheres, respectively. 6 Please cite this article in press as: L. He et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.008
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analyzed by FTIR below. The shallow traps play a key role in the design of the n-type semiconductor g-C3N4, in which the balance between concentration and depth is crucial to the photoelectric performance of g-C3N4, including the photoluminescence and photocatalysis performance [10–14,17–19]. Thus, the highest concentration of shallow traps observed in the sample synthesized at 500 °C in N2 suggested that the shallow traps exhibited a positive effect on the luminescence, which can explain the occurrence of the strongest luminescence in Fig. 2b. The compositions and chemical states of the bulk samples were investigated by XPS analysis. The C 1s and N 1s spectra of the bulk samples condensed at 500 °C are shown in Fig. 5a–d, and the others are provided in Fig. 4 in Ref. [39]. Two peaks were observed in the C 1s spectra in Fig. 5a, b, with one main peak at 287.9 eV for the sp2-bonded carbon in the NAC@N group and one at 284.7 eV for the sp2-bonded carbon in C–C [17–19,28– 38]. The former peak represents the sp2 C in triazine, whereas the latter corresponds to the graphitic sp2 C [17–19,28–38]. Three peaks were de-convoluted from the N 1s spectra in Fig. 5c, d: the minor peak at 400.9 eV for the amino functional group of HANAC (N1), the middle peak at 399.6 eV for the sp3 threecoordinated nitrogen from the NA(C)3 group (N2), and the main peak at 398.5 3 V for the sp2 hybridized nitrogen from the C@NAC group in triazine (N3) [17–19,28–38]. The analysis of atomic O discloses that the contamination from O decreases with increasing temperature, and the relative concentration of O in samples condensed under N2 atmosphere was lower than that condensed in ambient air (Fig. 4d and Table 1 in Ref. [39]). During the aromatization process, the N sp2/sp3 ratio increased
as result of the transformation from CANHAC to C@NAC [18]. As seen from Fig. 4b in Ref. [39], the highest intensity peak for graphic carbon was observed in the bulk sample condensed at 500 °C under N2 atmosphere, and the second highest intensity peak was observed in the bulk sample condensed at 450 °C in ambient air; these peaks are consistent with the strongest and the second strongest luminescence values in Fig. 2. Therefore, the type of C is related to the luminescence efficiency. From the FTIR spectra shown in Fig. 5e, f, the sharp absorption of the breathing mode of the triazine ring of g-C3N4, which shifted from 795 to 805 cm1 as the temperature increased from 450 to 650 °C, was observed [17–19,28–38]; typical stretching vibrations of the CAN bond at 1232, 1310 and 1396 cm1 and the C@N bond at 1534 and 1620 cm1 within the broad band region of 1000–1700 cm1, corresponded to the skeletal stretching modes of the aromatic rings [17–19,28–38]. The absorption within 2900–3500 cm1 was related to amino groups [17– 19,28–38]. A closer inspection of the FTIR spectra reveals subtle effects of the reaction temperature and atmosphere on the structures. In addition to NAH absorption at 3070 cm1, the intensified absorption at 3260 cm1 suggests that increasingly more NAOH bonds were generated as the temperature increased from 450 to 650 °C due to the formation of hydroxylamine (NH2OH) [42]. The absorption peak at 881 cm1, whose intensity also increased as the temperature increased from 450 to 650 °C, was caused by the formation of hemiacetal bonds between the aldehyde groups and the neighboring hydroxyl groups, i.e., C@O or O@CAOH [43]. The enhanced absorption of the aromatic rings as the temperature increased from 450 to 650 °C, as
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XPS and FTIR spectra of the g-C3N4 samples. (a, b) C 1s spectra of bulk samples synthesized at 500 °C in air and N2, respectively; (c, d) N 1s spectra of bulk samples synthesized at 500 °C in air and N2, respectively; and (e, f) FTIR spectra of bulk samples synthesized at various temperatures in air and N2, respectively. 7 Please cite this article in press as: L. He et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.06.008
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identified from the typical vibration of C@N at 1620 cm1, suggests that the degree of polycondensation increased, which was in accordance with the above XRD results. In addition, the formation of C@O, O@CAOH and NAOH suggests the breakage of heptazine units, which occurs at the surface of bulk particles. Fortunately, the damage was suppressed by the protection offered by the N2 atmosphere, as revealed by the comparison of Fig. 5f with 5e. To demonstrate its application potential, a QLED device was prepared by employing g-C3N4 QDs as an emitting layer with the spin-coating approach based on the device structure shown in Fig. 6a. The LiF/Al electrode was deposited by thermal evaporation. The matched energy levels of the various thin-film layers are shown in Fig. 6b. Fig. 6c is the image of the device in the light-off and light-on modes, with an applied voltage of 21 V. Two bands, one at 427 nm and one at 513 nm, were observed in the electroluminescence spectra shown in Fig. 6d, which differed from the electroluminescence peak at approximately 435 nm reported by Chen [44]. Fig. 6e suggests that the luminescence increases nearly linearly with the applied voltage. Concluded from the above discussion of the energy levels in the g-C3N4 QDs, the peaks at 427 and 513 nm should be emitted by the combination of the holes injected at the LP state in valence band with the electrons injected at the unoccupied d⁄ and p⁄ orbitals, respectively, in conduction band. Thus, the mechanism of electroluminescence can be described by the scheme in Fig. 6f.
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In addition to QLEDs, the g-C3N4 QDs are applicable for flexible transparent displays, such as smart windows for advertising [45], as demonstrated in Supplementary Fig. 4 by spincoating on flexible polyvinylchloride substrate, since the solution of g-C3N4 QDs in water or alcohol is nearly colorless and transparent. Compared with organic blue emitters, which have been practically used in OLEDs, such as the host material 4,40 -bis(9-carbazo lyl)-biphenyl (CBP) and its derivatives [46], the blue fluorescent material with biphenyl groups, the common blue phosphorescent material bis((4,6-difluorophenyl)-pyridine) iridium(III) picolinate (FIrpic) [47], and blue donor–acceptor small molecules [48], the g-C3N4 QDs exhibit the advantages of being nontoxic, earth-abundant, chemically stable, long lived, and fulfill the requirements of low-cost manufacturing at ambient conditions by spin-coating. With respect to the emerging halide perovskite nanocrystals [49], the g-C3N4 QDs have competitive superiority in terms of cost-effectiveness and the promising scaled-up fabrication with zero pollution release. Toward practical applications, however, the performance of g-C3N4 QDs in QLED devices, including emission color chromaticity and color saturation (determined by emission peak wavelength and band width), luminance, and energy conversion efficiency of electroluminescence still need to be optimized discreetly to meet with the high-quality requirement of displays on high-definition, highluminance and high-accuracy pictures.
FIGURE 6
QLED devices prepared using g-C3N4 QDs. (a) Schematic illustration of the device structure; (b) energy levels of various thin-film layers in the device; c the asprepared QLED device in natural light and a photograph of the light-emitting device with an applied voltage of 21 V; (d) emission spectra of the as-prepared QLED device; (e) the brightness and current density as function of the applied voltage; and f the mechanism of electroluminescence.
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Conclusions In summary, the strongest photoluminescence from g-C3N4 was obtained by the thermal condensation of melamine at 500 °C under N2 atmosphere, and the highest QY of approximately 49.8% was achieved for g-C3N4 QDs. The luminescence intensity of samples synthesized under N2 atmosphere is far higher than that of samples prepared in ambient air, and the emission wavelength of the QDs is shorter than that of the bulk powders. The increase in temperature intensifies the degree of polycondensation but causes the formation of hydroxylamine (NH2OH) and hemiacetal (O@CAOH) functional groups on the particle surface. Analysis of the XRD patterns demonstrates that minor residual melem can help modify g-C3N4. Damage to the functional groups on the particle surface was effectively inhibited by the N2 atmosphere, as revealed by the FTIR analysis. The variation in the configuration of the excitation spectra shows that more electrons are excited to the p* state in the samples synthesized under N2 atmosphere than in those synthesized in ambient air, indicating that more p-conjugated bonds are formed at 500 °C under N2 atmosphere. Moreover, the highest ratio of graphitic carbon to triazine carbon was observed in the sample synthesized at 500 °C under N2 atmosphere. The shallow traps present in the n-type semiconductor were identified by thermoluminescence, and the balance between the trap depth and concentration was crucial to the luminescence characteristics. After exfoliation to form QDs, the charge-transfer band and the non-radiative transition disappeared from the absorption spectra, which indicated the enhanced quantum efficiency of the QDs. The N2 atmosphere not merely alleviates surface oxidation and modifies surface morphology but more importantly exerts on chemical bonding in crystallization and alter electronic structure. The potential of the g-C3N4 QDs for next-generation QLED displays was demonstrated by fabricating a QLED prototype device.
Conflicts of interest None. The authors solemnly declare no competing financial interests.
Acknowledgments This work was supported by the National High-Tech R&D Program (863 program) (2013AA03A114), the National Natural Science Foundation (U1332133), the Project of Science and Technology of Guangdong Province (2017B090901070) and Anhui Province (1301022062), the Science and Technology Project of Guangzhou (201604016005), and the special fund for research and development of the Hefei Institute (IMICZ2015112), China.
Author contributions L.C., Y.J., and J.H. conceived the experiments and analyzed the data; L.H., M.F., J.C. and Y.T. completed the synthesis and characterization of the bulk and QDs samples; Y.H., K.X., and J.H. designed and fabricated the OLED devices; L.H. and M.F. grew
and prepared flexible transparent thin films; Z.Z, Q.Z. and H.N. measured the QYs. L.C., L.H., and M.F wrote the paper. All authors commented on the manuscript. Each author has made an indispensable contribution to this work.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.mattod.2018.06. 008. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
[40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
N. Guan et al., ACS Photon. 3 (2016) 597–603. T.H. Kim et al., Nat. Photon. 5 (2011) 176–182. J.S. Park et al., Nano Lett. 16 (2016) 6946–6953. X. Dai et al., Adv. Mater. 29 (2017). 1607022-22. J. Han et al., J. SID 24 (2016) 545–551. N. Oh et al., Science 355 (2017) 616–619. S.G. Ihn et al., Adv. Sci. 4 (2017) 1600502. Wikipedia, the free encyclopedia (EB/OL), OLED. 2017. at http://en.wikipedia. org/wiki/OLED. X. Zhang et al., J. Am. Chem. Soc. 135 (2013) 18–21. X. Wang et al., Nat. Mater. 8 (2009) 76–80. J. Liu et al., Science 347 (2015) 970–974. J. Liu, H. Wang, M. Antonietti, Chem. Soc. Rev. 45 (2016) 2308–2326. H.L. Lee et al., Appl. Mater. Today 8 (2017) 150–162. V. Devthade, D. Kulhari, S.S. Umare, Mater. Today: Proc. 5 (2018) 9203–9210. J. Liebig, Analyse der Harnsäure. Ann Pharm. 10 (1834) 10. A.Y. Liu, M.L. Cohen, Science 245 (1989) 841–842. Y. Zhang et al., Sci. Rep. 3 (2013) 1943. S. Bayan et al., J. Phys. Chem. C 121 (2017) 19383–19391. M. Rong et al., J. Mater. Chem. C 3 (2015) 10916–10924. S. Guo et al., Nat. Commun. 8 (2017) 1828–1829. Y. Dai et al., Nat. Commun. 9 (2018) 60–67. M. Xiong et al., Biosens. Bioelectron. 89 (2017) 23. G. Shiravand, A. Badiei, G.M. Ziarani, Sensors Actuators B 242 (2017) 244–252. Y. Dong et al., Small 12 (2016) 5376–5393. D.W. Zheng et al., Nat. Commun. 9 (2018). 1680-11. J.N. Tiwari et al., ACS Nano 11 (2017) 742–751. J. Xu et al., J. Power Sources 274 (2015) 77–84. Y. Zheng, Z. Zhang, C. Li, J. Photoch. Photobio. A 332 (2017) 32–44. P. Wu et al., J. Mater. Chem. A 2 (2014) 20338–20344. D. Das, S.L. Shinde, K.K. Nanda, ACS Mater. Interf. 8 (2016) 2181–2186. Q. Lu et al., Chem. Commun. 51 (2015) 12251–12253. I. Papailias et al., Appl. Surf. Sci. 358 (2015) 278–286. T. Tyborski et al., J. Phys. Condens. Matter. 24 (2012) 162201. X.Q. Wei et al., RSC Adv. 5 (2015) 26675. L. Stagi et al., Diam. Relat. Mater. 68 (2016) 84–92. L. Zhang et al., Mater. Today Energy 5 (2017) 91–98. S. Guo et al., Angew. Chem. Int. Ed. 55 (2016) 1830–1834. Q. Fan et al., Appl. Surf. Sci. 391 (2017) 360–368. L. He, M. Fei, J. Chen, Y. Tian, Y. Jiang, Y. Huang, K. Xu, J. Hu, Z. Zhao, Q. Zhang, H. Ni, L. Chen, Dataset of emission and excitation spectra, UV-VIS absorption spectra, and XPS spectra of graphitic C3N4, Data in Brief, submitted. L. Chen et al., Sci. Rep. 5 (2015) 11514. R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Princeton University Press, Oxford, 1981. W. Zheng, R.I. Kaiser, J. Phys. Chem. A 114 (2010) 5251–5255. P. Calvini et al., Vib. Spectrosc. 40 (2006) 177–183. Y. Li et al., Appl. Catal. B Environ. 229 (2018) 218–226. L. Chen et al., Nanoscale 5 (2013) 225–230. P. Strohriegl et al., Proc. SPIE 8829 (2013) 882906–882911. Y. Im et al., Adv. Funct. Mater. 27 (2017) 1603007–1603024. Y. Li et al., Mater. Today 20 (2017) 413–423. S. Adjokatse, H.H. Fang, M.A. Loi, Mater. Today 20 (2017) 258–266.
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