Fig. 3a shows practical Raman spectrum of stacking C
3N. The ethanol solution of C
3N (0.1 mg/mL) was spin-coated (6000 rpm, 40 s) on a Silicon substrate. A flake of C
3N sheet (lateral size of ~2 µm, thickness of 1.9-3.7 nm) was chosen to be detected by a LabRAM-HR spectrometer (Jobin Yvon Co., Ltd., French). Then the sample was treated by annealing at 200 ℃ (high purity nitrogen, 99.99%) for 2 h in the tube furnace, after which the same region of the C
3N sheet was detected. All the Raman collections were performed at room temperature using a 532 nm laser source. The laser power and spot size were 50 mW and 1 µm, respectively. A parallel Raman collection was operated with completely same preparation and characterization approach, which indicated the universality of our dealing process (Fig. S7 in Supporting information). The Raman peaks before annealing are strongly overlapped in the range of 1300-1600 cm
−1 (gray curve). This typical coherence of Raman shifts indicates the rich structural information of as-synthesized C
3N. The Raman peaks narrowed with enhanced Raman peak intensity on the order of 10
3 after annealing. The full width at half maximum (FWHM) of Raman peaks at 1244.4, 1342.8, 1375.6, 1410.0, 1489.2 and 1566.0 cm
−1 were decreased as 18.4, 16.8, 14.4, 19.2, 14.4, 16.8 and 17.6 cm
−1, respectively. It is assumed that the high density ordered arrays of C
3N after annealing induced a stronger surface plasmon resonance effect due to molecular assembly and enhanced surface plasmon effect [
42], which significantly improved the legibility of Raman signals because of the enhancement of in-plane crystallization (green curve) [
43-
45]. In addition, the area ratio of breath-like peaks decreased from 49.5% to 39.6%, indicating the reduction of defects in 2D lattice [
46-
48]. The result shows eight characteristic peaks at 1150.0, 1244.4, 1342.8, 1375.6, 1410.0, 1489.2 and 1566.0 cm
−1 in range of 1050-1600 cm
−1 (
Table 1). Notably, the peak located at 1150.0 and 1489.2 cm
−1 can be due to ZO and LO/TO modes of AA stacking type, respectively. These two peaks only have a few contributions in the range of 1050-1600 cm
−1, indicating relatively low content of AA′ stacking type C
3N, which corresponds to the theoretical calculation as mentioned in Table S2 [
19]. It can be seen that the characteristic Raman peaks of C
3N products are well in line with the theoretical predictions, indicating a successful synthesis of C
3N as well as a clear distinguishment of four typical stacking types.