## 3 Results and Discussion
The presence of only two vibration modes at 222 cm\({}^{-1}\) (\(\omega_{3}\)) and 355 cm\({}^{-1}\) (\(\omega_{4}\)) confirms the formation of Ti\({}_{2}\)N MXene sheets after etching the Al layer from the Ti\({}_{2}\)AlN MAX phase observed in the Raman spectrum shown in Figure 3c. Comparative analysis of the Raman spectra of the Ti\({}_{2}\)AlN MAX phase and the as-synthesized Ti\({}_{2}\)N MXene sheets shows that the Ti-Al vibration mode (\(\omega_{1}\)) is completely absent in the case of the Ti\({}_{2}\)N MXene sheets, indicating the complete removal of the Al layer and formation of the Ti\({}_{2}\)N MXene structure. Simultaneously, a clear red shift in the peak positions of \(\omega_{3}\) and \(\omega_{4}\) was observed for the Ti\({}_{2}\)N MXene sheets compared to its parent MAX phase, clearly showing better etching of the Al layer along with exfoliation [10]. The exfoliated Ti\({}_{2}\)N MXene sheets were further decomposed into 0D Ti\({}_{2}\)N MQDs using a simple hydrothermal method similar to the synthesis of QDs of other 2D materials, such as graphene and TMDs [30,59,60]. The effective synthesis of Ti\({}_{2}\)N MQDs is based on two crucial steps: first, the Al layer is etched from the 3D Ti\({}_{2}\)AlN MAX phase to obtain 2D Ti\({}_{2}\)N MXene, and second, the MXene nanosheets are broken down into ultra-small Ti\({}_{2}\)N MQDs, which determines the quality of the MQDs [26]. Since MXene is metastable it may yield its oxidation products in alkaline medium at high temperature reaction system. However, we emphasize that the hydrothermal reaction process at 100 \({}^{\circ}\)C using NH\({}_{4}\)OH solution was adopted for the synthesis of MQDs where surface-terminated -NH groups from the NH\({}_{4}\)OH solution maintain the crystal structure of pristine MXene, preventing the formation of TiO\({}_{2}\) or TiO\({}_{2}\) quantum dots [35]. The morphology and size of the Ti\({}_{2}\)N MQDs were analyzed by HR-TEM and are presented in Figure 4a,b, respectively. The low-and high-magnification HR-TEM images in Figure 4a,b reveal the formation of uniform and ultra-small Ti\({}_{2}\)N MQDs, respectively. The fringes observed in the single MQD in the HR-TEM image presented in Figure 4b are highly parallel with separation distance of 0.26 nm that corresponds to (1 0 1) lattice space of the Ti\({}_{2}\)N [26] and reveal good crystallization of the Ti\({}_{2}\)N MQDs. The fast Fourier transform (FFT) pattern of the MQDs is shown in the inset of Figure 4b. The average lateral diameter of the Ti\({}_{2}\)N MQDs was determined to be 3.2 \(\pm\) 0.2 nm from the bell-shaped size distribution curve, as shown in Figure 4c, which matches the size of the single MQD shown in Figure 4b. The element distribution was measured by XPS and the content of nitrogen in the MQDs was approximately 17.6%, which is comparable with the previously reported value of Ti\({}_{2}\)N MXene [10]. Figure 3: (**a**) XRD pattern, (**b**) FE-SEM image, (**c**) Raman spectrum of the layered Ti\({}_{2}\)N MXene sheets. To study and understand the optical properties of the as-synthesized Ti\({}_{2}\)N MQDs, absorption, PLE, and PL spectra were acquired from aqueous Ti\({}_{2}\)N MQDs, and the results are presented in Figure 5. As-synthesized Ti\({}_{2}\)N MQDs exhibited the PL emission with the peak wavelength around at 420 nm upon the excitation wavelength between 390 nm to 230 nm (Figure 5c). The demonstration of PL behavior indicated the successful fabrication of light emitting Ti\({}_{2}\)N MQDs. The light emitting properties were observable due to quantum confinement effect which caused the bandgap expansion of semi-metal MXene that were previously observed from the fabrication of other MQDs and 2D material-derived QDs [26; 33; 35; 46; 60; 61]. PL spectrum is the plot of PL intensity at a fixed detection wavelength vs. the excitation wavelength. Because PLE intensity corresponds to the probability of optical transition, PLE spectra resemble the absorption spectra but with higher signal-to-noise ratio [62].