Carbon Dots: From Intense Absorption in Visible Range to Excitation-Independent and Excitation-Dependent Photoluminescence

Carbon dots (CDs) were hydrothermally synthesized using citric acid as carbon source and ethylenediamine as additive. The as-obtained CDs were then subjected to ultraviolet (UV) irradiation, resulting in ultraviolet-irradiated CDs (UVCDs). The morphology and size of CDs were characterized by transmission electron microscopy. Elemental analysis (EA) was employed to explore the composition of CDs. The surface functional groups of CDs were investigated with the help of Fourier transformation infrared spectroscopy. Fluorescence spectrometry and ultraviolet-visible (UV-vis) spectrophotometry were used to characterize the optical properties of CDs and UVCDs. The results show that the size of CDs was around 10 nm. C, H, O, and N constituted CDs with the empirical elemental composition CH1.26O0.52N0.22. Apart from excitation-independent photoluminescence (PL), CDs exhibited two UV-vis absorption peaks at 340 and 451 nm. They also revealed weak near infrared (NIF) emission with excitation at 340 and 360 nm. In addition to excitation-dependent PL, UVCDs showed one UV-vis absorption peak at 340 nm and extension of UV-vis absorption band to visible range, and they also displayed weak NIF emission at 340 and 360 nm excitation. Optical behaviors originated from surface defects and band bending. The as-prepared CDs could be used to harvest solar energy.