## 3 Results and Discussion
### Optical Properties of N-CQDs  
The optical characteristics of N-CQDs were analyzed by Ultraviolet visible (UV-vis) absorption spectroscopy, photoluminescence (PL) excitation spectroscopy, and emission spectroscopy. The UV-Vis absorption spectrum of N-CQDs was examined, showing a prominent absorption peak between 300 and 350 nm, as illustrated in Figure 4a. This signifies that N-CQDs possess the ability to absorb ultraviolet energy. The excitation wavelength of the 1 mg/mL N-CQDs solution shown in Figure 4a is around 380 nm, but the peak emission wavelength is about 550 nm. The Stokes shift between the excitation and emission wavelengths is considerable, approximately 170 nm. This phenomenon indicates  
Figure 3: (**a**) The complete XPS spectrum of N-CQDs, high-resolution XPS spectra for (**b**) C1s, (**c**) N1s, and (**d**) O1s of N-CQDs. Figure 2: The FTIR spectrum of the N-CQDs. that there is a large energy loss between absorption and emission in the fluorescence process of CQDs. From the perspective of individual carbon point, the influence of molecular structure of carbon point is mainly manifested as energy dissipation in the process of vibration relaxation and radiation transition [34; 35]. N-CQDs often exhibit a range of oxygen-containing functional groups. The distinctive fluorescence band of CQDs originates from particular electron transitions between the anti-bonding and bonding molecular orbitals of functional groups, including \(\pi\)-\(\pi\), \(\sigma\)-\(\pi\)-n, and \(\pi\)-\(\sigma\)[36; 37]. Figures 2 and 3 demonstrate that the incorporation of a minimal quantity of nitrogen elements results in the creation of novel functional groups and chemical bonds (including C=O and C-N). The newly formed intermediate states efficiently reduce the HOMO-LUMO gap, while surface states diminish the band gap of N-doped carbon quantum dots (N-CQDs), leading to a substantial Stokes shift [38; 39]. The concentration of N-CQDs markedly affects the fluorescence (FL) intensity and emission wavelength of the solution. Figure 4b illustrates the effect of solutions with differing N-CQD concentrations on the fluorescence peak. With illumination at a consistent wavelength of 440 nm, a rise in N-CQD concentration led to a progressive redshift of  
Figure 4: (**a**) Absorption spectrum of N-CQDs and the excitation and emission spectra of a 1 mg/mL N-CQDs solution; (**b**) emission spectra of N-CQDs solutions at varying concentrations under 440 nm light excitation; (**c**) emission spectra of a 0.25 mg/mL N-CQDs solution excited at different wavelengths; (**d**) CIE 1931 chromaticity diagram for a 0.25 mg/mL N-CQDs solution (NL and UV denote N-CQDs solutions under natural light and ultraviolet light irradiation, respectively). the fluorescence wavelength and the initial enhancement in fluorescence intensity. The fluorescence intensity maximizes at a concentration of 0.25 mg/mL of N-CQDs. Nevertheless, as the concentration of N-CQDs increased more, the intensity began decreasing. At high concentrations, the fluorophores in the N-CQDs can absorb excitation light and emit light reciprocally, leading to the internal filter effect [24]. Thus, when the fluorescence intensity attains its maximum, the increase in concentration leads to the emission of fluorescence exclusively at longer wavelengths. This illustrates the phenomena wherein the increase in concentration causes a decrease in fluorescence intensity and a redshift in the emission wavelength. Figure 4c demonstrates the N-CQDs solution at a concentration of 0.25 mg/mL being excited at different wavelengths. With the shift in the excitation wavelength from 280 nm to 360 nm, the fluorescence peak progressively broadened and intensified accordingly. The peak's shape displayed considerable asymmetry, being broader on the left and slenderer on the right. At the excitation wavelength of 380 nm, the fluorescence peak was seen at around 500 nm, where its intensity was maximized.