After differentiating and imitating, the XPS spectra of the Al, N, O, and C elements of the dark grey AlN powder sample are shown in Figure 6. In Figure 6a, three peaks of binding energies of 75.0, 74.0, and 73.1 eV were disclosed, which are consistent with the standard binding energies of the Al-O, Al-N, and Al-C bonds in Table 4, respectively. The N_1S peak in Figure 6b was divided into two peaks of binding energies of 396.7 and 400.0 eV, respectively, which were equal to the standard binding energies of the N-Al bond and N-C bonds in Table 4. The O_1S peak of a binding energy of 531.5 eV refers to the O-Al bond (Figure 6c). The C_1S peak in Figure 6d is trimodal, which corresponds to three single peaks of binding energies of 285.0, 284.5, and 288.6 eV, respectively, which are close to the standard binding energies of the C-C, C-Al and C=O bonds, respectively. In summary, in the XPS spectra of the AlN powder sample, the N-Al bond is associated with the N element in AlN, and the N-C bond corresponds to the C substance naturally adsorbed by AlN [28]. More importantly, the existence of the mesophase is confirmed indirectly by the O-Al, C-Al, and C=O bonds between two of the O, Al and C elements. In addition, most of the carbon in the sample exists in the form of a C-C bond, suggesting the elemental C exists in the dark grey AlN powder sample. The elemental C cannot be detected by XRD in Figure 3, due to its extremely low concentration and crystallinity.