Characterization of Hydrothermally Synthesized Carbon Quantum Dots from Ligusticum Wallichii by Transmission Electron Microscopy

The structure of carbon quantum dots (CQDs) synthesized by one-pot hydrothermal method using Ligusticum wallichii as carbon source was observed and analyzed by the transmission electron microscopy (TEM). Aiming at the problem of weak contrast of CQDs in TEM bright field imaging, the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) was carried out to rapidly locate the CQDs in the copper grid. The corresponding TEM images showed that the CQDs were irregular shape with a size of about 3 nm. By the high-resolution transmission electron microscopy (HRTEM) and the corresponding fast Fourier transform (FFT), the lattice fringe of 0.21 nm, diffraction with six-fold symmetry, as well as the graphitic layer spacing of 0.34 nm were observed. The indexing method of characteristic indices of crystalline CQDs was also discussed. The HRTEM analysis indicated that the CQDs were crystalline hexagonal phase graphene-like structure.