Fig. 2

Physicochemical properties of GQDs. a TEM image (scale bar = 5 nm). b XPS survey spectra (insert: HR-XPS spectra of C1s). c Fluorescence emission spectrum at the excitation wavelength of different wavelengths ranging from 320 to 350 nm. d Raman spectra of GQDs with different defect densities (insert: Id/Ig ratio of GQDs with different defect densities). e UPS spectra of GQDs. f Band structure of GQDs. Conduction band(CV) = Valence band(VB) + \({\mathrm{E}}_{\mathrm{g}}^{\mathrm{opt}}\) [28]