## 3 Results and Discussion
### Formation Mechanism of N-CQDs  
The process of generating fluorescent carbon quantum dots can be divided into two stages in this study. Since cellulose and hemicellulose have lower thermal stability than lignin, most of the cellulose and hemicellulose and a small amount of lignin in poplar wood are first carbonized under high pressure and high temperature conditions, thus obtaining abundant bulk carbon materials. Then, NH\({}_{3}\)-H\({}_{2}\)O served as the nitrogen source, while H\({}_{2}\)O\({}_{2}\) and ultraviolet light facilitated the treatment of the solid biological carbon. During the experiment, H\({}_{2}\)O\({}_{2}\) and NH\({}_{3}\)\(\cdot\)H\({}_{2}\)O are mixed together according to the volume ratio of 2:1, and H\({}_{2}\)O\({}_{2}\) will accelerate the decomposition under alkaline conditions and gradually react violently with NH\({}_{3}\)\(\cdot\)H\({}_{2}\)O to heat up. This promotes H\({}_{2}\)O\({}_{2}\) to absorb the high energy of ultraviolet light under the light of UV-LED lamps (385-395 nm, 30 W), which causes the electrons inside its molecules to be excited to a high energy state, resulting in the break of the peroxygen bond (O-O bond) (NH\({}_{3}\)\(\cdot\)H\({}_{2}\)O provides an alkaline environment, hydrogen peroxide plays the oxidation role, and diluted with water to balance the reaction strength) [42; 43]. During this reaction, H\({}_{2}\)O\({}_{2}\) decomposes to form intermediates including generate hydroxyl (HO-) and hydroperoxyl (HOO\(\bullet\)) radicals, then to form oxygen and water [44; 45]. The HO- radical exhibits strong oxidative propertiesthat can effectively break down larger carbon sources into smaller fragments [46; 47]. The FTIR and XPS data indicate that the HO- radical simultaneously cleaves both sp2 and sp3 hybridized carbons, resulting in the breakdown of certain C-C and C=C bonds into oxygen-containing functional groups. Furthermore, the HO- radical further disassembles the already formed small carbon particles, transforming them into even smaller CQDs, which leads to their contraction into diminutive CQDs. However, the self-assembly of these tiny particles into larger structures is facilitated by ammonia. The CQDs size can be controlled below 10 nm and concentrated in the range of 4-5 nm, which is the result of the simultaneous action of NH\({}_{3}\cdot\)H\({}_{2}\)O and H\({}_{2}\)O\({}_{2}\). The synthesized N-CQDs exhibit a significant presence of oxygen-containing functional groups and numerous defects, which facilitate electron transitions between bonding and antibonding energy levels. The characterization of FTIR and XPS showed the formation of -COOH and -NH\({}_{2}\) groups on the surface of CQDs, where pyrrole nitrogen (-C-N-C) often existed in conjugated form in the graphite structure of CQDs during doping [33; 48; 49; 50]. The content of nitrogen in lignin of poplar wood was 0.20 to 0.44%, which further confirmed the presence of nitrogen doping on the surface of CQDs [51; 52; 53; 54]. This characteristic enables N-CQDs to emit green fluorescence signals under specific excitation wavelengths. Under the combined effects of ultraviolet light and ammonia, both components in the solution promote the decomposition of H\({}_{2}\)O\({}_{2}\), leading to a rapid and substantial production of CQDs. In the process of preparing CQDs, poplar wood with the same weight was selected as far as possible to carry out experiments under the same conditions. The serial number in Table 1 represents the sequence of experiments. After many experiments, the mass yield of CQDs obtained with 12 g poplar wood block was basically stable at about 42 wt%. Table 2 describes a comparative analysis of the mass yield of CQDs obtained in this study with the mass yield of CQDs recorded in the existing literature. It is evident that among the various methods for synthesizing CQDs, the hydrothermal method usually exhibits a superior quality yield compared to other techniques, and the sources of precursor materials are also notably more diverse.