Surface state modulation of red emitting carbon dots for white light-emitting diodes†

Kang Yuan ^a, Xinghua Zhang *^a, Ruohan Qin ^a, Xuefeng Ji ^a, Yahui Cheng ^b, Lanlan Li ^a, Xiaojing Yang ^a, Zunming Lu ^a and Hui Liu *^c
aSchool of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China. E-mail: zhangxinghua@hebut.edu.cn
^bDepartment of Electronics, Nankai University, Tianjin 300350, China
^cSchool of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. E-mail: hui_liu@tju.edu.cn

First published on 18th October 2018

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1 Introduction

As a new type of fluorescent carbon material, carbon dots (CDs) have attracted broad scientific interest due to their unique properties.¹ Compared with conventional semiconductor quantum dots,² CDs are superior to dye-based fluorescence probes for imaging and sensing due to the advantages of excellent biocompatibility, low toxicity, tunable photoluminescence (PL) and less photobleaching.^3–6 They have been applied in various fields, such as light-emitting diodes (LEDs),^7,8 water splitting,^9,10 and photocatalysis.^11,12 Since the discovery of CDs,¹³ different luminescence mechanisms of CDs have been proposed. Although there is no uniform explanation, the luminescence mechanism of CDs is generally related to the quantum size effect,^14,15 recombination of electron–hole pairs,¹⁶ or surface states.¹⁷ The quantum size effect is not easy to use compared with the semiconductor quantum dots, since it is more difficult to control the size of CDs. Fortunately, the two latter luminescence mechanisms provide a facile method to modulate the fluorescence of CDs through controlling the surface states with different functional groups.¹⁸ Different functional groups can form various surface states during the formation of CDs. Traditionally, most researchers introduced a variety of functional groups in the preparation of CDs, which is not conducive to studying the impact of specific functional groups on the surface state. In addition, the surface state is a vague concept for the luminescence mechanism. It is significant to further elaborate the meanings of surface states and deeply investigate the effects of surface states formed by specific functional groups on luminescence properties.

Amino groups are very useful in modulating the luminescence properties of CDs because nitrogen not only can be used as a doping element, but can also form a variety of surface states on CDs, such as pyridinic N, pyrrolic N, amino N and so on, which greatly affects the luminescence properties of CDs.^19,20 Various materials and methods have been used to synthesize N-doped CDs, and the luminescence properties of CDs are influenced by the amounts, ratios, or speciation of doped nitrogen. Dong et al.¹⁶ proposed that nitrogen doping generated a new surface state, which would increase the radiative recombination rate of electrons as a recombination center. Xu et al.²¹ compared several kinds of N-doped CDs and found that nitrogen existed in the –NO₃ form which contributed little to the radiative recombination and only nitrogen bonding to carbon could really enhance the emission of CDs. Qian et al.^22,23 prepared a series of CDs and the results suggested that amino and pyrrolic N could promote radiative recombination by generating more protonation processes or aromatic structures, and these were responsible for the improvement in the luminescence properties. Sarker et al.²⁴ revealed that the presence of graphitic N could cause red-shifting of CDs due to the electron-doping effect. A large number of studies indicate that the effects of nitrogen species on photoluminescence properties are significant for improving the performance of CDs. However, the influence of nitrogen on the fluorescence properties has not been fully understood, and the chemical speciation and functions of doped nitrogen have only received limited attention. Therefore, it is necessary to clarify the effects of the speciation of nitrogen, especially the effects of their speciation on the surface of CDs on the luminescence properties, which is beneficial for understanding the luminescence mechanism and the meaning of surface states.

In this work, we used a one-pot hydrothermal method to synthesize N-doped fluorescent CDs with tunable surface states, employing p-phenylenediamine and typical raw materials with various types of amino group to achieve controllable emission of CDs. It was found that nitrogen related speciation is related to red, green and blue emitting states. Red emitting states are attributed to the fluorophore formed by the deformation of p-phenylenediamine on CDs. Pyridinic N is responsible for the green emission state and pyrrolic N can enhance the blue emission. Furthermore, the pro-CDs (p-phenylenediamine and L-proline as carbon/nitrogen sources) with multiple emission states are suitable for fabricating white LED devices. The pro-CDs based LED device emits bright white light with CIE coordinates of (0.31, 0.32) and a high CRI of 85, and it also has excellent stability.

2 Experimental

2.1 Materials

p-Phenylenediamine (PDDA) and ethylenediamine (EDA) were obtained from Damao Chemical Reagent Factory (Tianjin, China). L-Proline was purchased from Tianjin Guangfu Fine Chemical Research Institute. Polyethyleneimine (PEI) was bought from Aladdin Company.

2.2 Synthesis of CDs

The CDs (p-CDs (carbon source is PDDA), EDA-CDs (PDDA and EDA), PEI-CDs (PDDA and PEI) and pro-CDs (PDDA and L-proline)) with different surface states were synthesized using a hydrothermal method. For the synthesis of EDA-CDs, 0.25 mmol PDDA and 0.5 mmol EDA were added to 60 mL deionized water. Next, the reaction solution was transferred to a 15 mL Teflon lined stainless steel autoclave and it was kept at 180 °C for 12 h. After that, the autoclave was cooled down to room temperature naturally, then an equal volume of ethanol was added to the prepared solution. PEI-CDs, p-CDs and pro-CDs were synthesized in the same way.

2.3 Fabrication of the white LED

Since the pro-CDs have blue, green and red emissions under 365 nm excitation, we selected a UV LED chip as a light source to excite the pro-CDs and realize white light emission. To fabricate the white LEDs based on pro-CDs as a single light converter, the specific steps are as follows. First, 9 mL pro-CDs solution (1.41 g L⁻¹) was dried to obtain solid powders. Next, the pro-CDs powders were dissolved in 9 mL absolute ethanol and mixed with 9 mL epoxy in a water bath at 60 °C for 4 h accompanied by stirring. During this time, ethanol will generally be removed. After that, 3 mL curing agent was added into the above solution and stirred for more than 10 minutes at room temperature. Then, the above uniform mixture was added to a self-made mold with an ultraviolet LED chip (365 nm), and cured at room temperature for 12 h. At last, the pro-CDs based white LED was obtained.

2.4 Measurements

Morphologies of the CDs were characterized using a transmission electron microscope (TEM, JEOL 2100). X-ray photoelectron spectra (XPS) of CDs were obtained with a PHI1600EXCA photoelectron spectrometer. A spectrometer (Bruker, WQF-410) was used to obtain Fourier transform infrared (FTIR) spectra of CDs. The ultraviolet visible (UV-Vis) absorption spectra were carried out on a spectrophotometer (Hitachi, U-3900H). The excitation and emission spectra of CDs were collected using a steady and transient state spectrophotometer (Horiba, FL-3-22). In addition, the emission spectra of the pro-CDs based white LED under different currents were also collected with the steady and transient state spectrophotometer, and the LED tester was used to provide the power. The other parameters such as color temperature (CCT), color coordinates and color rendering index (CRI) were obtained with a spectra irradiance illuminometer (Konica Minolta, CL-500A). The luminous efficiency of the pro-CDs based white LED was measured using a spectroradiometer (SpectraScan PR-650).

3 Results and discussion

3.1 Surface state modulation of CDs

To investigate the effects of amino groups on surface emission states, different raw materials with amino groups were used to prepare carbon dots (Fig. 1). PDDA was used to synthesize the CDs with multiple emission states, and the addition of EDA, PEI or L-proline was selected to modulate the surface states of CDs during the synthesis process. As shown in Fig. 2, the four samples are well-dispersed nanoparticles with similar sizes. According to the TEM images, the p-CDs (Fig. 2a), EDA-CDs (Fig. 2b), and PEI-CDs (Fig. 2c) have an average diameter of 6.0 nm, and the average particle size of pro-CDs (Fig. 2d) is about 7.5 nm. Therefore, the difference in luminescence properties for the four samples should not originate from the quantum size effect.

3.2 Spectral properties of CDs

Fig. 3 shows the emission spectra of p-CDs, EDA-CDs, PEI-CDs, and pro-CDs samples under different excitation wavelengths. It can be seen that p-CDs (Fig. 3a) have three emission peaks centered at 460 nm, 520 nm and 605 nm under 365 nm excitation, and the red emission exhibits excitation-independent features. When EDA with two amino groups was introduced to the p-CDs, the EDA-CDs (Fig. 3b) display only one emission peak centered at 508 nm under different excitation wavelengths and the emission peak intensity increases with the increase in EDA amounts (as shown in Fig. S1a, ESI†). It is observed that the EDA-CDs show excitation properties before 460 nm excitation, which is different from the p-CDs with excitation independence. The introduction of EDA greatly changes the emission states of CDs, which may be attributed to the deformation of the amino groups. PEI with a large number of imino groups was also used to modulate the surface emission states of p-CDs. From Fig. 3c, one can see that PEI-CDs have similar photoluminescence properties as EDA-CDs except for an extra emission peak at 460 nm. In addition, the emission intensity of PEI-CDs also increases with the increase in PEI amounts (shown in Fig. S1b, ESI†). Comparing the differences between EDA and PEI, the blue emission state is probably due to the presence of the imine group. To further prove the assumption, L-proline with imino groups was chosen to modulate the blue emission states. As shown in Fig. 3d, the intensity of the blue emission peak at 460 nm is comparable to the intensity of the red emission peak under 365 nm excitation. Compared with the red emission, the blue emission intensity is increased and reaches the maximum at a molar ratio of PDDA to L-proline of 1:1.25 with increasing L-proline (shown in Fig. S1c, ESI†), demonstrating the effects of imino groups on blue emission states. Excitation spectra of these CDs are shown in Fig. S2 (ESI†), and are consistent with the change of emission spectra.

The four kinds of CDs have similar sizes and distinct photoluminescence properties, which indicates that the changes of luminescence mainly originated from the surface groups instead of the quantum size effect. Fig. S3 (ESI†) shows that p-CDs exhibit various emission spectra in different polar solvents under 365 nm excitation. The emission peaks of p-CDs are located at 522, 534, 556 and 600 nm in toluene, dichloromethane, acetone and ethanol, respectively. The polar dependence of the emission for p-CDs suggests that the emission of prepared CDs is mainly related to the surface states.²⁵ In order to further confirm the luminescence origin of CDs, the effects of pH values on luminescence of the four CDs were investigated. As shown in Fig. S4 and S5 (ESI†), the emission peak intensity of CDs gradually decreased with increasing pH value or decreasing pH value from 7, which further proved that the fluorescence of prepared CDs is mainly related to the surface structure of CDs.²⁶ The detailed effects of pH values on prepared CDs will be discussed below. Taking into account the fact that the raw material contains only amino groups, the multiple emission states of CDs are mainly related to the amino groups and their deformation.

The UV-Vis absorption spectra of the four kinds of CDs display several absorption peaks/bands (Fig. S6, ESI†). p-CDs show characteristic absorption bands at 225–282 nm and 340–502 nm, which are assigned to the π → π* transition of the aromatic CC bonds, and the n → π* transition of the aromatic sp² system containing CO and CN bonds, respectively.²⁷ This indicates that the different energy states resulted from functional surface states. EDA-CDs and PEI-CDs exhibit similar absorption peaks centered at 241 nm, 304 nm, and 404 nm, except for the peak above 404 nm of PEI-CDs with a long absorption tail and the absence of an absorption band at 500 nm. Compared with p-CDs, pro-CDs show the same absorption peaks at 225–282 nm and 405 nm, but pro-CDs exhibit obvious absorption peaks at 504 nm instead of the broad peak of p-CDs.

These absorption changes disclose that surface states have been changed through adding different amino-containing precursors, which significantly affects the photoluminescence performance of CDs.

3.2 Chemical bonding states of CDs

To analyze the elemental composition and chemical bonding of prepared CDs, FTIR and XPS spectra were investigated. As shown in Fig. S7 (ESI†), all CDs display characteristic IR peaks centered at 831 (C–H stretching), 1043–1350 (C–O stretching), 1403 (C–N stretching), 1514 (CC stretching), 1642 (CO/CN stretching) and 3450 (–NH₂ stretching) cm⁻¹.^20,28 The FTIR results suggest that the amounts of chemical bonds result in the changes of emission since the four samples consist of the same types of chemical bonds.

According to the XPS spectra (Fig. 4), all samples contain C, N and O elements. As shown in Table S1 (ESI†), oxygen amounts are increased from p-CDs to pro-CDs, but the emission spectra of the samples do not have a red shift with the increasing degree of surface oxidation, which is different from the luminescence mechanisms where the band gap is decreased with an increasing number of oxygen atoms in the structure.²⁹ We have demonstrated that the emission of the CDs originates from the surface states, so the changes of luminescence spectra are mainly attributed to the N related surface states. Fine scan XPS spectra of C 1s and N 1s for the CDs are given in Fig. 4(e–h) and (i–l), respectively. The C 1s spectra of CDs can be well-fitted into four curves centered at 284.3, 285.1, 286.4 and 288.1 eV, which corresponded to C–C/CC, C–N, C–O and CN/CO bonds, respectively. The N 1s spectra can be fitted well with four curves centered at 398.4, 399.2, 400.1 and 401.1 eV, corresponding to pyridinic, amino, pyrrolic and graphitic N,^25,30,31 respectively.

Table 1 gives nitrogen speciation percentages of four kinds of CDs. Here p-CDs can serve as a reference sample with three emission states for different surface structures including pyridinic, amino, pyrrolic and graphitic N. When the EDA was introduced to form EDA-CDs, the disappearance of amino N and decrease of pyrrolic N are related to the change of red and blue emission states, and the sample has only one green emission state with drastic increasing of pyridine N, which reveals that pyridine N should be responsible for the green emission. The PEI-CDs also prove that pyridine N contributes to the green emission owing to the highest content of pyridine N in the PEI-CDs. Moreover, the PEI-CDs show a blue emission peak in addition to the green emission accompanied with the increase of pyrrolic N compared with EDA-CDs, disclosing that pyrrolic N is related to the blue emission state. Specifically, graphitic N is the dominant nitrogen configuration in pro-CDs while the photoluminescence of pro-CDs is similar to that of p-CDs except for the enhancement of blue emission due to the further increase of pyrrolic N, which suggests that graphitic N plays an important role in enhancing the PL in CDs instead of leading to significantly red-shifted light absorption.³² Based on the above discussion, it can be concluded that luminescence of prepared CDs mainly originated from the surface states, and tunable nitrogen speciation is the essential cause for the change of luminescence properties.

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3.3 Schematic structures and energy levels of CDs

Combining all of the factors, the possible structures of the four types of CDs are given in Fig. 5, which reveals apparent differences of N doping. As shown in Fig. 5a, when forming p-CDs, the surface amino groups of PDDA will produce different groups, eventually forming multiple emission states on the surface of CDs. Some of the N atoms will form pyridinic N and pyrrolic N on the surface. In addition, PDDA or its deformation will be connected to the surface of the carbon core to form a luminescent center, which may be the origin of the red light emission. As a part of the molecular state, amino groups reflect changes of this molecular state because the red emission state is the most sensitive to pH values and the intensity of red emission is related to the number of amino groups. The other part of the benzene ring becomes a broken chain connecting to the CDs in a hydrothermal reaction.

From the XPS of EDA-CDs, the emission state of the EDA-CDs becomes a single green emission. Meanwhile, the amounts of pyridinic N are obviously increased and the amino groups almost disappear. This indicates that the introduction of EDA can transform the original heterocyclic N to pyridinic N, and these changes are closely related to green emission (Fig. 5b). Similar to EDA-CDs, PEI-CDs (Fig. 5c) have an increase in pyridine N due to its amino groups on the long-chain branch. At the same time, it is easier for imino structures to form pyrrolic N, and blue emission has also been enhanced. As we can see, additional amino groups result in an increase of pyridine N with strong green emission and the imino groups are beneficial to form pyrrolic N with enhanced blue emission. Moreover, L-proline is used to introduce only imino groups (Fig. 5d), and the carboxyl groups reacted with basic amino groups to make it connect to the surface of CDs, which increases the amounts of pyrrolic N, leading to the enhancement of blue emission. In addition, the contents of graphitic N are increased with the introduction of L-proline, which also enhances the luminescence intensity of CDs.

The results of acid–base treatment can further confirm the rationality of the structure of prepared CDs (Fig. S4 and S5, ESI†). Under alkaline conditions, the protonation degree of N species decreases with increased pH values from 7 to 14, and the possible proton transfer from the protonated nitrogen to the conjugated carbon structure can enhance the fluorescence of the CDs, thus decreasing the emission intensity of p-CDs as the pH value increases.²³ Under acidic conditions, the influence of protonation is not dominant, and the destruction of acids for N species leads to the decrease of the luminescent center. Various N species have different responses to acid, which results in the decrease of fluorescence intensity in varying degrees. The red emission state is first destroyed because amino groups in the molecular state are liable to react with the acid. Pyridinic N is more basic than pyrrolic N since that lone-pair of electrons of the pyridinic nitrogen atom does not participate in the conjugated system of the pyridine ring. Therefore, the green emission is completely destroyed before the blue emission at low pH values.

A possible band structure was proposed to explain the photoluminescence process of the CDs with multiple emission states, as shown in Fig. 6. It can be seen clearly that there are N related surface emission states for CDs, which correspond to different electron transition processes. Electrons are excited from HOMO levels to LUMO levels when the excitation energy is higher than 3.40 eV (λ = 365 nm). Then the N related speciation as a rearrangement of the irradiation center results in blue, green and red emission from CDs. Specifically, the irradiation center formed by pyrrolic N emits blue light (λ = 460 nm) as the excitation energy is ∼3.35 eV (λ = 370 nm). Green light (λ = 508 nm) is emitted from the irradiation center formed by pyridinic N. Red light emission (λ = 605 nm) may be formed by the deformation of PDDA, which can be excited by 2.46 eV (λ = 504 nm).

3.4 Performance of a pro-CDs based white LED

We have synthesized different CDs with multiple emission states and modulated the emission peaks and intensities by changing the N related groups. From Fig. 3d, it can be seen that the pro-CDs have blue, green, and red emission peaks centered at 460 nm, 500 nm and 605 nm under 365 nm with CIE coordinates of (0.34, 0.33), which makes them suitable for fabricating white LED devices. Unfortunately, pro-CDs possess aggregation-induced luminescence quenching in the solid state. To solve this problem, people embed fluorescent dopants in solid matrices such as BaSO₄,³³ silica xerogel,³⁴ organically modified silicate gel glasses^35,36 and PMMA^37,38 to realize solid-state fluorescence of CDs. The matrices actually act as media like water. Here epoxy resin is chosen as the matrix because it not only acts as media like water, but also is suitable for LED encapsulation. As shown in Fig. S8 (ESI†), the fluorescence properties of pro-CDs dispersed in epoxy resin are similar to those in solution except for 23 nm blue shift of red emission and 17 nm blue shift of blue emission which are induced by polarity changes.²⁵ pro-CDs in epoxy resin also emit white light under 365 nm excitation, which indicates that pro-CDs in epoxy resin can be a good single light converter for white light-emitting diodes. Therefore, we employed pro-CDs as a single light converter for white LEDs by dispersing them into epoxy resin on an UV LED chip with 365 nm emission. Fig. 7a displays the emission spectra of the fabricated white LED, and blue shifts of emission spectra occur due to the polarity changes of epoxy resin for pro-CDs.⁴⁵ The white LED has CIE coordinates of (0.31, 0.32) which are very close to balanced white-light emission (0.33, 0.33), and the CRI is 85 which is also close to white light (Fig. 7b). The corresponding color temperature (CCT) of the white LED is 5837 K, which is superior to commercial white LEDs for daily lighting. Compared with other CDs-based white LEDs, the pro-CDs-based white LED is prepared with only one kind of CDs as a converter. The luminous efficiency of the white LED is 8.8 lm W⁻¹. In addition, the CIE chromaticity coordinates of (0.31, 0.32) are superior to those (0.27, 0.32) of LED devices reaching the white region.⁴⁴ A comparison of details is shown in Table 2, and it can be seen that the prepared pro-CDs based white LED also has good performance. Moreover, the white LED device also exhibits excellent stability within 12000 s (inset of Fig. 7a). The luminescence performance of the white LED suggests that pro-CDs can be a good candidate for white LED applications.

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4 Conclusions

In summary, fluorescent CDs with tunable surface states were successfully prepared via a hydrothermal method using PDDA and different raw materials with amino groups as carbon/nitrogen sources. The experimental results disclose that the fluorescence of CDs mainly originates from the surface states of nitrogen related speciation. The red emission state is attributed to the fluorophore formed by the deformation of PDDA on the surface of CDs. Pyridinic N is responsible for the green emission state and pyrrolic N can enhance the blue emission. These interesting phenomena reveal that the essence of the surface state mechanism is the change in electron transition processes caused by different chemical speciations on CDs and effectively controlling them can modulate the luminescence performance of CDs. Furthermore, the pro-CDs with multiple emission states can serve as a single light converter for fabricating white LED devices. The pro-CDs based white LED device has white CIE coordinates of (0.31, 0.32) and a high CRI of 85 with 8.8 lm W⁻¹, and it also exhibits excellent optical stability at long working time intervals.

Conflicts of interest

We declare that there are no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51771068, 21603052, 51671079, 51771067, 51571123, 51671108), the Natural Science Foundation of Hebei Province (No. E2018202082), Financial Support for Scientific and Technological Activities of Returnees from Abroad (CL201606), and the Innovation Fund for Excellent Youth of Hebei University of Technology (No. 2015005).

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Footnote

† Electronic supplementary information (ESI) available: Emission and excitation spectra of p-CDs, EDA-CDs, PEI-CDs, and pro-CDs. Photoluminescence spectra of the p-CDs in different solvents excited by 365 nm. Emission spectra of the prepared CDs by changing pH values (1–7) with addition of HCl. Emission spectra of the prepared CDs by changing pH values (7–14) with addition of NaOH. The UV-Vis absorption and FTIR spectra of p-CDs, EDA-CDs, PEI-CDs and pro-CDs, respectively. The atomic percentages of four kinds of CDs. See DOI: 10.1039/c8tc04468f

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