Elemental and nanostructural analysis of multicolor CQDs
The transmission electron microscope images demonstrate the spherical nature of the produced C-CQDs as illustrated in Fig. 2a,b with an interlayer spacing of 0.21 nm shown in the inset of Fig. 2a. Furthermore, the size distribution analysis of the C-CQDs shown in Fig. 2c reveals an average particle size of approximately 1.7 nm. Similarly, Fig. 2d,e shows that the G-CQDs also exhibit a semi-spherical shape with a 0.21 interlayer distance, as depicted in the inset of Fig. 2d. The size of the G-CQDs was found to be around 2.6 nm, as illustrated in Fig. 2f. Likewise, the Y-CQDs were observed to have a structure that is nearly identical to the C-CQDs and G-CQDs, as shown in Fig. 2g,h with dimensions of about 3.8 nm, as illustrated in Fig. 2i, and a d-spacing of 0.21 nm.
Energy dispersive X-ray spectroscopy equipped with TEM was utilized to analyze the chemical composition of C-CQDs, G-CQDs, and Y-CQDs. The results indicated that carbon is the predominant element in all three samples, accompanied by different proportions of boron and oxygen. Elemental ratios of C, O, and B in the synthesized C-CQDs, G-CQDs, and Y-CQDs are presented in Table 1.
Table 1
Chemical composition of the synthesized multicolor using energy dispersive X-ray detector equipped with TEM.
| C (%) | B (%) | B (%) |
C-CQDs | 80.85 | 17.43 | 1.72 |
G-CQDs | 78.01 | 16.6 | 5.39 |
Y-CQDs | 82.57 | 12.7 | 4.73 |
TEM analysis, along with X-ray diffraction spectroscopy, suggests that the C, G, and Y-CQDs produced are carbon-based structures with a semi-crystalline nature. Figure 3 exhibited a prominent broad XRD peak at 2θ = 22.3° for all the samples corresponding to an interlayer lattice spacing of 0.398 nm, this value exceeds the crystallographic plane distance of bulk graphite (typically 0.33nm) due to functional groups attached to the surface of multicolor CQDs28. Additionally, faint peaks at 2θ = 42.9° indicate another interplanar distance of 0.21nm, mostly attributed to specific planes; this demonstrates a certain level of crystallinity in the synthesized CQDs. Moreover, the observed XRD peak at 2θ = 13.6° for Y-CQDs is often caused by their deformation resulting from the addition of sulfuric acid into the reaction precursors.
Fourier transform infrared analysis was used to examine the surface functional groups and bond structure of the C, G, Y-CQDs that were synthesized. As seen from Fig. 4, it was observed that both C-CQDs and G-CQDs exhibited a broad peak at 3328 cm− 1 attributed to O-H stretching hydroxyl groups. Additionally, the recorded peaks at 2970 and 2879 cm− 1 are related to C-H stretching vibration. The absorption peak at 1656 cm− 1 was identified as vibrations of C = O/C = C29. Moreover, distinct absorption bands at 1328, 1270, 1090, and 1040 cm− 1 corresponded to B-O stretching vibration, B-O-H bending vibration, C-B stretching followed by B-O-H deformation vibration for both samples, respectively30,31. For the synthesized Y-CQDs there is a shift in the O-H absorption peak from 3328 to 3425 cm− 1. The recorded peaks also include those found previously with slight variations: 2943 and 1612 cm− 1 categorized as C-H and B-O vibrational stretching, respectively; finally, peaks at 1380, and 1041 cm− 1 signifying the B-O and stretching vibration B-O-H deformation vibration. These findings indicate that boronic acid groups had been successfully transferred from boric acid molecules onto the surface during particle formation.
Optical properties of multicolor CQDs
The UV − visible absorption spectra of the synthesized CQDs illustrated in Fig. 5a exhibited similar absorbance in the UV range (200–350 nm) and varying absorbance at longer wavelengths, with absorption edges observed at 462, 505, and 535 nm for C-CQDs, G-CQDs, and Y-CQDs, respectively. Intense peaks of absorption in the ultraviolet range indicate the existence of electronic transitions from π to π* orbitals, largely associated with the conjugated C = C/C = O domain29. Furthermore, the change in absorption boundaries of the synthesized CQDs due to n → π* transitions is a consequence of size increase from 1.7 nm to 3.8 nm, emphasizing the impact of quantum confinement on these CQD samples32. In addition to the UV–Vis spectra, Tauc-Plot curves presented in Fig. 5b yield information on the calculated optical bandgap for these CQDs samples, revealing values of 2.59 eV for C-CQDs, 2.34 eV for G-CQDs, and 2.26 eV for Y-CQDs.
Figure 6 illustrates the photoluminescence spectra of the synthesized CQDs with emission maxima at 482 nm, 527nm, and 560 nm for the synthesized C-CQDs, G-CQDs, and Y-CQDs, respectively. The emissions of the CQDs samples are most notably characterized by their narrow bandwidth emissions, with full-width at half-maximum measuring 28 nm for C-CQDs, 42 nm for G-CQDs, and 33 nm for Y-CQDs. The photoluminescence excitation of multicolor CQDs depicted in Fig. 6(a–c), which is perfectly match with the absorption spectra of these multicolor CQDs samples seen in the inset Fig. 5; along with the significantly small FWHM values suggest high color purity due to the direct exciton recombination, indicating their potential application in display technology and other photonic applications. Furthermore, both C-CQDs and G-CQDs show a small Stokes shift between photoluminescence excitation and emission of approximately 20 nm while Y-CQD exhibits only 13 nm shift, which are mostly attributed to band edge direct exciton recombination as well as weak electron–phonon coupling in the multicolor CQDs33,34. Moreover, Fig. 6 (d–f) illustrate a two-dimensional excitation emission map that confirms the intrinsic narrow bandwidth characteristics of the synthesized multicolor CQDs samples as well as demonstrate excitations-independent nature of the synthesized C-, G-, Y-CQDs. By using fluorescein as the fluorescence quantum yield (FLQY)reference solution, the FLQY of the examined C-, G-, Y-CQDs were found to be 65%, 54%, and 60%, respectively.
Moreover, exciton recombination dynamics by using time resolved PL spectra of our synthesized multicolor CQDs samples. The 439 nm picosecond pulsed laser was used to pump the CQDs samples and observe fluorescence lifetime decay at 482 nm, 527 nm, and 560 nm for C-, G-, and Y-CQDs respectively. Figure 7 display a single exponential fluorescence lifetime decay of almost 4.39 ns for C-CQDs solution indicates stable excitons with mainly radiative decay and minimal nonradiative contribution. Unlike C-CQDs, the fluorescence decay profiles of G-CQDs and Y-CQDs showed different features. As seen from Fig. 7, the decay profile of G-CQDs exhibited biexponential decays; where the fast decay was found to be 0.83 ns, and slow decay of 3.25 ns. Similarly, the fluorescence decay of Y-CQDs exhibited biexponential decay; the fast decay was found to be 0.52 ns and 3.80 ns. The differences in the fluorescence decay profiles could be caused by the presence of defect states introduced by the DMF synthetic solvent for both G-CQDS and Y-CQDs. The fluorescence lifetime of both G-CQDs and Y-CQDs have been fitted according to Eq. (1)
\(\:I\left(t\right)={I}_{0}+{B}_{1}{e}^{(-\frac{t}{{\tau\:}_{1}})}+{B}_{2}{e}^{(-\frac{t}{{\tau\:}_{2}})}\) 1
Where \(\:I\left(t\right)\) is the fluorescence intensity as a function of time, \(\:{I}_{0}\) is the initial fluorescence intensity, \(\:t\) normalized to the intensity at \(\:t=0\), \(\:{\tau\:}_{1}\)and \(\:{\tau\:}_{2}\) are the fluorescence decay for the first and second decay components, \(\:{B}_{1}\)and \(\:{B}_{2}\) are the fractional intensity of the first and second decay components.
The intensity average lifetime depicted in Table 2 has been calculated for both G-CQDs and Y-CQDs according to Eq. (2)35.
\(\:{⟨\tau\:⟩}_{int}=\:\frac{\sum\:_{i=1}^{2}{B}_{i}{{\tau\:}_{i}}^{2}}{\sum\:_{i=1}^{2}{B}_{i}{\tau\:}_{i}}=\frac{{B}_{1}{{\tau\:}_{1}}^{2}+{B}_{2}{{\tau\:}_{2}}^{2}}{{B}_{1}{\tau\:}_{1}+{B}_{2}{\tau\:}_{2}}\) 2
Table 2
Fluorescence decay profiles for G-CQDs and Y-CQDs.
| \(\:{\tau\:}_{1}\:\left(sec\right)\) | \(\:{\tau\:}_{2}\left(sec\right)\) | \(\:{B}_{1}\) | \(\:{B}_{2}\) | \(\:{⟨\tau\:⟩}_{int}\) |
G-CQDs | \(\:8.26\times\:{10}^{-10}\) | \(\:3.249\times\:{10}^{-9}\) | \(\:15\) | \(\:85\) | \(\:3.145\times\:{10}^{-9}\) |
Y-CQDs | \(\:5.22\times\:{10}^{-10}\) | \(\:3.80\times\:{10}^{-9}\) | \(\:11\) | \(\:89\) | \(\:3.745\times\:{10}^{-9}\) |
Characterization of white light-emitting diode based on multicolor CQDs
The high fluorescence quantum yield of the synthesized multicolor C-, G-, and Y-CQDs allowed them to be combined with blue LED chips to create WLEDs. Initially, the C-, G-, and Y-CQDs were dispersed in ethanol and mixed in various proportions to measure the resulting emission using a Spectrofluorophotometer. This process yielded a white emitting solution of multicolor CQDs. Subsequently, PVP was added to the mixed solution to produce a 15 wt.% multicolor CQDs solution in PVP. A photo-driven white light-emitting diode based on fluorescent multicolor CQDs was then successfully assembled by applying a thick layer of the prepared PVP mixed solution onto the surface of a flat blue LED. Figure 8a shows an electroluminescence spectrum of blue LED chip with an emission peak at 450 nm, while Fig. 8b demonstrates that after encapsulating the blue LED with a fluorescent mixture of multicolor CQDs, it generated a white electroluminescence spectrum. The luminous intensity increased as the induced current increased, without changing the emission spectrum shape, as indicated by Fig. 8c. Additionally, the prepared WLED depicted in the inset of Fig. 2b showed a color rendering index CRI of 79% and correlated color temperature CCT of 5723 K and color coordinates (0.33, 0.30) when applied current is at 30 mA which corresponds to neutral white light suitable for indoor lighting. Furthermore, Fig. 8d showcases a shift from neutral white to cool white illumination as the applied current is increased.