Solvent regulation synthesis of single-component white emission carbon quantum dots for white light-emitting diodes

2.1 Chemicals and materials

Ammonium citrate (AC, A.R.), N,N-dimethylformamide (DMF, 99.8%), N-(b-aminoethyl)-g-aminopropyl methldimethoxy silane (AEAPMS, 96%), petroleum ether (PE, A.R.), and cellulose acetate (CA) were purchased from Aladdin Biochemical Technology Co., Ltd (Shanghai, China). All chemical reagents were used as received without further purification. Deionized water (18.2 MΩ) was used for all experiments.

2.2 Synthesis of RCQDs and SCWE-CQDs

In a typical process for the synthesis of RCQDs, 5.2 g AC was dissolved in 50 mL of DMF with vigorous stirring to achieve a homogeneous solution. After that, the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 200°C for 6 h, resulting in a dark-red solution. Subsequently, the products were purified with PE three times to remove impurities and were dialyzed (1,000 Da of cut-off molecular weight) for 24 h to eliminate the unreacted precursor. Finally, a red small molecular sol was obtained. To prepare SCWE-CQDs, 1.0 mL RCQDs were added into 10 mL AEAPMS, followed by slight stirring at room temperature. Afterward, the mixture was diluted by DMF to produce the SCWE-CQDs. Before testing, the products were also purified with PE three times to remove impurities. The graphical abstract is shown in Figure 1.

Figure 1

Schematic illustration of the preparation of RCQDs and SCWE-CQDs using a facile hydrothermal method with ammonium citrate (AC), N,N-dimethylformamide (DMF), and their application in WLEDs.

2.3 Fabrication of SCWE-CQDs WLEDs devices

To evaluate the performance of SCWE-CQDs application in WLEDs, the CA with different contents was added into as-prepared SCWE-CQDs solution and then intensely stirred in a vacuum mixer for 6 minutes to degas bubbles. Afterward, the mixture was added dropwise to a 395 nm ultraviolet LED (UV-LED) chip and solidified at 130°C for 2 h under ambient conditions. The CCT, CRI, CIE color coordinates, and emission spectra of the WLEDs devices were characterized by using a high-accuracy integrating sphere system (Multi Spectrums T-950/930).

2.4 Characterization

TEM observation was performed on a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) with a 200 kV accelerating voltage. XRD patterns were recognized with an X-ray diffractometer (XRD, D8-Advance, Bruker, Germany) using a Cu-Kα radiation source. Absorption and fluorescence spectra were collected using an UV-visible spectrometer (Tu-1901, Purkinje, China) and a fluorescence spectrophotometer (RF-6000, Shimadzu, Japan) at room temperature, respectively. FTIR spectra were recorded from 4,000 to 400 cm⁻¹ on a Fourier Transform Infrared Spectrometer (Vertex 33, Bruker, Germany). XPS data were obtained with an X-ray photoelectron spectrometer (XPS, Kratos Axis Ultra DLD, Kratos, UK) under a mono Al-Kα excitation source. The PLQY of CQDs solutions were determined by utilizing the following equation [19].

where the Q is the PLQY, the subscript “st” denotes the reference quinine sulfate, “I” is the integrated fluorescence intensity, “A” is the UV-Vis absorbance at 365 nm, “n” is the different refractive index of the solvent, and the superscript “2” means the square of “n”. Quinine sulfate dissolved in 0.1 M H₂SO₄ (PLQY: 55%) was used as the standard.

3.1 Morphological and structural characterizations of RCQDs and SCWE-CQDs

The morphological and structural characterizations of as-prepared RCQDs and SCWE-CQDs were measured by using TEM and XRD to confirm the chemical structure of CQDs. Figure 2a and c demonstrate the TEM images of RCQDs and SCWE-CQDs, indicating that they are well-dispersive. Additionally, the high-resolution TEM (HR-TEM) image in Figure 2a inset reveals that RCQDs have a conspicuous crystalline characteristic with 0.21 nm lattice distance, corresponding to the (100) lattice planes of graphitic carbon, suggesting the successful synthesis of well-crystallized CQDs. Interestingly, a similar crystal structure was observed in SCWE-CQDs (Figure 2c). Beyond that, the size distribution histograms of RCQDs (Figure 2b) and SCWE-CQDs (Figure 2d) show relatively narrow size distribution, with average diameter of 3.47 and 2.65 nm, respectively. Compared with the SCWE-CQDs, the obtained RCQDs are easy to form “clusters” because of aggregation.

Figure 2

TEM images of (a) RCQDs and (c) SCWE-CQDs, insets are HRTEM images demonstrating the well-dispersive CQDs. The size distribution histograms of (b) RCQDs and (d) SCWE-CQDs show an average diameter of 3.47 and 2.65 nm, respectively.

Furthermore, the crystal structures of RCQD and SCWE-CQD were measured by XRD, indicating the typical graphite structure feature of CQDs, with significant diffraction peaks, as shown in Figure 3. The XRD diffraction patterns of RCQDs and SCWE-CQDs show broad diffraction peaks at 21.3° and 21.1°, respectively, indexing as (100) crystal face, which indicated these CQDs are arranged in a disordered order [12]. Additionally, SCWE-CQDs have another sharp diffraction peak at 27.7° due to the stacking of the conjugated aromatic system, corresponding to the (002) crystal face. Based on the above results, it is revealed that the XRD diffraction pattern of SCWE-CQDs is consistent with good crystallization characteristics and the amorphous nature of graphitic carbon [27]. Therefore, these SCWE-CQDs both have crystalline cores and amorphous surfaces simultaneously.

Figure 3

XRD diffraction patterns of RCQDs and SCWE-CQDs.

3.2 DMF dilution regulating the optical properties of CQDs

The SCWE-CQDs were prepared by adding a certain amount of DMF into the RCQDs solution, so it is necessary to optimize DMF dilution ratios to obtain SCWE-CQDs with desired optical properties. We studied five typical CQDs, which were synthesized by adding different DMF dilution ratios (0, 5, 50, 100, 500 times) to the RCQDs original solution. The optical properties of these products are characterized by UV-visible spectroscopy and photoluminescence spectroscopy. From the result of Figure 4a, these CQDs have different absorption characteristics from UV to visible regions. Without adding DMF, the RCQDs (0 times) have a distinct absorption band at 621 nm. When the dilution ratios are five times, the spectra have two intense absorption bands centered at 536 and 593 nm, respectively. As the dilution ratios increased to 50 and 100 times, the number of absorption peaks changed from two to three, and the first characteristic absorption peak gradually blues shifted. Continuing to increase dilution ratios, four characteristic absorption peaks were observed and the first characteristic absorption peak blue-shifted to the UV region (318 nm). The absorption band less than 350 nm can be attributed to the π–π* interlayer transition of oxygen-functionalized graphitic core or n–π* transitions caused by the CQD “edge band” transition [28]. In addition, the absorption band at 350–800 nm of these products can be assigned to the π–π* aromatic fluorophore structure and the n–π* transitions of the C═O bond [29]. The “edge band” transition refers to a transition between the nonbonding orbital of edge groups to π* orbital of the core in CQDs at the boundary of the sp² and sp³ hybridized carbon, especially in some blue-emitting CQDs, which is similar to many previously reported cases [28,30,31]. The broad absorption band at 486 nm (100 times diluted sample) or longer wavelength can be considered as the “surface band” of CQDs, which probably occur because of the low energy transitions of functional groups attached to the edge of the carbon core [32].

Figure 4

(a) UV-Vis absorption spectra of CQDs synthesized by adding 5 times (red solid line, 5×), 50 time (blue solid line, 50×), 100 times (green solid line, 100×), and 500 times (purple solid line, 500×) DMF dilution ratios into RCQDs solution, and the original RCQDs was denoted as “0” (black solid line), respectively. The PL emission spectra of RCQDs diluted by DMF in (b) 0 times, (c) 5 times, (d) 50 times, (e) 100 times, and (f) 500 times at different excitation wavelengths (360–500 nm), respectively. Insets are photographs of the solutions under 395 nm UV light.

Figure 4b–f show the PL emission spectra of the samples changed under different excitation wavelengths (360–500 nm) excited when the original solution of RCQDs was diluted with DMF by 0, 5, 50, 100, and 500 times, respectively. Figure 4b shows the PL emission spectrum of RCQDs centered at about 675 nm without adding DMF, which appears to be excitation-independent, and the intensity increases with the excitation wavelength range from 360 to 500 nm. When the dilution ratios are five times, the PL emission spectra have two sharp peaks at about 540 and 655 nm, and the peak intensity at 655 nm is significantly higher than that at 540 nm (Figure 4c). Generally, the multi-emissions of CQDs are related to the quantum size effect, surface defect states, and molecular states. In addition, the samples also exhibit excitation-independent behavior. This excitation-independent behavior possibly arises from the different particle sizes and the distribution of molecular states [33,34]. Besides, the TEM images demonstrate that the size distribution of the CQDs ranges from 1.0 to 5.0 nm, which is not very uniform. Therefore, all TEM and FL spectral characteristics indicate that the luminescence of CQDs is related to the particle size, suggesting that there are some particles with different sizes in the solution. As the dilution ratios increase, the PL emission peak position and peak intensity of the sample change obviously. When the dilution ratios are 50 times, the PL emission spectra show three characteristic peaks at 539, 580, and 667 nm, respectively. In addition, the peak intensity of the green region is different from that of the red area, so the color of the solution shows a composite color. When the dilution ratios increase to 100 times, the position of the peaks shifted to 437, 536, and 660 nm, respectively, and the corresponding peak intensities have significant differences. Accordingly, the color of the solution turns to pure white, namely SCWE-CQDs. However, when the dilution ratios further increase, the color of the solution gradually changes to blue, and the PL emission spectrum of CQD shows a double peak and then becomes a single peak as the excitation wavelength is in the range of 360 to 500 nm. These results indicate that the white light of SCWE-CQDs is composed of a mixture of colors. The relative PLQY of the SCWE-CQDs is 53% under 365 nm excitation, demonstrating a higher than most previous reported, as presented in Table 1.

3.3 Determining the relationship between DMF and CQDs

To study the blue shift characteristics caused by DMF induced, FTIR and XPS characterizations were further performed to distinguish the chemical bonding and chemical composition of RCQDs and SCWE-CQDs. As shown in Figure 5a and b, both RCQDs and SCWE-CQDs have a lot of hydrophilic groups such as O–H/N–H (3,450–3,150 cm⁻¹) and C–O (1,260 cm⁻¹) on their surfaces, which make them possess good solubility in water [38]. Furthermore, the stretching vibration band of the C–H (2,935 cm⁻¹), C═N (1,660 cm⁻¹), and C–N (1,398 and 1,460 cm⁻¹) was observed, revealing the formation of polyaromatic structures, which is consistent with absorption and PL emission results [39]. Comparing the FTIR spectra of these samples, we found the enhancement of the typical stretching vibration intensity of O–H/N–H (3,450–3,150 cm⁻¹) and C–H (2,935 cm⁻¹) bonds from the RCQDs to SCWE-CQDs, indicating the SCWE-CQDs with higher hydrophilicity and polarity than RCQDs. Similar characteristics are found in carbon nanotubes [40]. In addition, strong peaks at approximately 1,020–1,085 cm⁻¹ and 796 cm⁻¹ were observed in SCWE-CQDs, which can be ascribed to Si–CH₂ and Si–O–Si bonds stretching vibrations, respectively [26]. The Si–O–Si bonds can avoid the fluorescence quenching of the CQDs due to their chain network structure, thereby improving the stability of the SCWE-CQDs.

Figure 5

FTIR spectra of (a) RCQDs and (b) SCWE-CQDs.

Furthermore, the XPS characterizations were carried out to investigate the chemical composition of CQDs, as demonstrated in Figures 6 and 7. Analogously, five peaks at 100.1, 150.5, 284.8, 399.8, and 531.9 eV in the full spectra of XPS are observed in Figure 6a, corresponding to Si2p, Si2s, C1s, N1s, and O1s of RCQDs and SCWE-CQDs, respectively, which indicate the components of CQDs remain unchanged under DMF diluted. Apart from that, the relative contents of the atoms for these samples were compared as shown in Figure 6b. The XPS intensity at 399.8 eV gradually increases from the RCQDs to SCWE-CQDs, implying a corresponding increase in the number of amide groups in the CQDs, which reconfirms the FTIR analysis results. In addition, the high-resolution narrow scan spectra of atoms were recorded, as shown in Figure 7. The C1s band of two typical CQDs can be divided into three peaks, relevant to C–C/C═C (284.7 eV), C–O/C–N (285.7 eV), and C═O (288.2 eV), evidencing that there are sp² and sp³ carbons in these CQDs [41,42]. Accordingly, the N1s band of CQDs can be divided into three peaks, corresponding to C═N (398.9 eV), N–H (399.9 eV), and C–N (401.0 eV). Additionally, the O1s band of CQDs can be decomposed to two peaks of C–O (530.7 eV) and C═O (532.1 eV). All the above results indicate that these CQDs possess abundant functional groups, allowing them highly dissolvable in water and organic solvents, which is meaningful for further applications.

Figure 6

(a) XPS full survey of RCQDs and SCWE-CQDs, (b) XPS elemental analysis results of the RCQDs and SCWE-CQDs.

Figure 7

High-resolution narrow scan spectra of (a) C1s, (b) N1s, and (c) O1s for the RCQDs, and (d) C1s, (e) N1s, and (f) O1s for the SCWE-CQDs.

3.4 DMF solvation effect on the fluorescent mechanism

Until now, the mechanism for the PL emission of CQDs can be concluded as follows: (i) the quantum confinement effect or the conjugated π-domains of carbon core, namely intrinsic states; (ii) the surface functional groups concatenated with the carbon skeleton, known as surface states; (iii) the fluorescent molecular group connected on the surface or interior of CQDs, called molecular state [43,44,45]. Usually, the PL emission of CQDs deriving from intrinsic states or surface states is insensitive to most solvents (e.g., DMF, water). On the contrary, the PL emission originating from molecular states is very sensitive to the solvent environment [46]. According to the characterization of absorption, PL emission, and PLQY of RCQDs and SCWE-CQDs (Figure 4), the optical properties of CQDs depend strongly on the DMF amount, suggesting that the emission of CQDs possibly originates from the molecular states. Based on molecule fluorescence principles, the spectrum shift induced by the solvation effect can be further divided into two classes: general solvation effect and specific solvation effect [46]. The general solvation effect is caused by solvent polarity, while the specific solvation effect is influenced by a combination of many complicated factors, such as hydrogen bonds (HBs), internal charge transfer, and conformational changes [29]. In this article, the spectrum shift is regulated by DMF dilution ratios, thus the specific solvation effect is responsible for the PL spectrum shift. In addition, both the adsorption and PL spectra changes demonstrate that the HB is formed between CQDs and DMF via amino or imino groups donating or accepting protons [46]. Lin’s group and Song’s group confirmed that the specific PL properties of CQDs are caused by the strong HB effect formed in polar aprotic solvents [28,46]. Moreover, the FTIR and XPS results (Figures 5–7) confirmed that the nitrogen atoms were attached to the surface of CQDs in the form of graphitic nitrogen and pyrrolic nitrogen. Therefore, with more DMF added, the exposed nitrogen atoms attached to CQDs surface build a powerful HB with DMF via nitrogen protonation. Meanwhile, the electron transmissions can be realized between the hydrogen atom and nitrogen atom by HB donor-to-acceptor bridges. With the HB effect reinforced, the bandgap between the lowest excited singlet state and the ground state increases, resulting in RCQDs blue-shifted and thereby SCWE-CQDs obtained. Additionally, from the results of Figures 2–4, the excitation-independent behavior and the multi-peak characteristics of PL emission of CQDs suggest that the particle sizes affect spectrum shift. However, unlike traditional semiconductor quantum dots, a large CQDs spectral shift (>100 nm) cannot be obtained by simply varying the particle size [47]; in most cases, the large spectral shift is related to the surface electron transmissions rather than the size [48]. In general, the combined action of the HB effect and the size effect leads to the blue shift of RCQDs, but the HB effect is more dominant than the particle size in causing large spectral shifts.

3.5 Performance of WLEDs based on SCWE-CQDs

Regarding high-quality WLEDs’ application in displays, optimizing relevant parameters is of great significance for obtaining high CRI and pure white light. During the fabrication of WLEDs devices, we found that the contents of cellulose and SCWE-CQDs have a significant effect on WLEDs’ performance. Therefore, we then focused on these two parameters. Initially, the contents of cellulose ranging from 0.05 to 0.3 g are investigated, keeping the content of SCWE-CQDs solution (1.0 mL) unchanged. Table 2 exhibits the performance of the obtained WLEDs, mainly containing the CIE chromaticity coordinates, luminous efficiency, CCT, and CRI. As shown in Table 2, by changing cellulose contents, the CIE color coordinates (0.330–0.363, 0.326–0.369) and CRI of 82–89 acquired in the white light region. In addition, with the increase of cellulose contents, the CIE coordinates are gradually approached pure white light (0.33, 0.33), and CCT also gradually changes from warm light color to cold light color and maintains a high CRI. Furthermore, the relevant electroluminescence (EL) spectra of the as-fabricated WLEDs with different CCTs are demonstrated in Figure 8a–e. With the increase of cellulose contents, the CCT of the WLEDs changes from 4,113 to 5,628 K (Figure 8a). At the same time, the corresponding CRI value at different CCTs did not drop significantly, and the maximum value reached 89, which is potential for micro-display or indicator applications. Besides, it is worth noting that during the transition from liquid to the solid phase, the PLQY of SCWE-CQDs is significantly reduced due to the aggregation, leading to a decrease in the luminous efficiency of WLEDs. These push us to explore new strategies to alleviate the degradation of fluorescence efficiencies in the liquid-solid phase transition process, such as finite-difference time-domain method and molecular dynamics simulation [49].

Figure 8

(a) The CIE coordinates of WLEDs, whose structure is composed of 0.05, 0.1, 0.2, and 0.3 g cellulose and UV LED (395 nm) chip. The electroluminescence (EL) patterns of the WLEDs prepared with (b) 0.05 g, (c) 0.1 g, (d) 0.2 g, and (e) 0.3 g of the cellulose, respectively, insets are the working images of devices.

Moreover, the effect of the SCWE-CQDs contents on the performance of WLEDs was studied, keeping other conditions the same, as presented in Table 3 and Figure 9. With the SCWE-CQDs contents increased, CIE chromaticity coordinates values, luminous efficiency, and CRI increased, while the CCT values decreased. Importantly, we obtained a white light emission with the CIE coordinates of (0.328, 0.336) approaching pure white light (0.33, 0.33), and corresponding CRI and CCT are 84 and 5,710 K, respectively. Figure 9a shows the CIE coordinates and CCT of WLEDs devices; the CIE coordinates of all devices are near the Planck Black Body radiation curve, which indicates that the WLEDs have a high CRI value, corresponding to Table 3. In addition, Figure 9b–e demonstrate the EL diagram of the normalized drive current of various devices at 20 mA, where the insets are the working images of WELDs devices. Similarly, with the increase of SCWE-CQDs contents, the ratio of long-wavelength in EL spectrum increased, and thereby the CRI increased. These results demonstrate that the SCWE-CQDs are promising to improve the color design of WLEDs for the green light source.

Figure 9

(a) The CIE coordinates of WLEDs, whose structure is composed of 0.2, 0.5, 0.8, and 1.1 mL SCWE-CQDs and UV-LED (395 nm) chip. The electroluminescence (EL) pattern of the WLEDs device prepared by diluting (b) 0.2 mL, (c) 0.5 mL, (d) 0.8 mL, and (e) 1.1 mL of the SCWE-CQDs, insets are the working images of devices.

Stability is an important index to value the reliability of WLEDs devices [50]. We use the CIE coordinates pattern and EL spectrum to evaluate the stability of SCWE-CQDs-based WLEDs devices under different working current drives and working time intervals. As shown in Figure 10a and c, although the CIE coordinates of WLEDs devices change greatly within the working current of 20–350 mA, the spectral shape of EL remains unchanged, indicating that WLEDs have excellent current stability. In addition, the time stability of the device within 0–50 h was investigated (Figure 10b and d), and it was found that the CIE coordinates and EL spectrum of these devices did not fluctuate much, indicating that the whole WLEDs device presented good time stability. At the same time, with the increase of working current or the extension of working time, the intensity of the two peaks of the EL spectrum in the green and red regions decreases slightly as SCWE-CQDs are easily affected by the temperature, which leads to the decrease of fluorescence efficiency.

Figure 10

(a) and (c) Variation of color coordinates and EL normalized intensity of WLEDs devices driven by different currents (20–350 mA), (b) and (d) Variation of color coordinates and EL normalized intensity of WLEDs devices under different working times (0–50 h).