Carbon Dots in Water and Mesoporous Matrix: Chasing the Origin of their Photoluminescence

The optical properties of microwave-synthesized carbon dots dispersed in water and in mesoporous ordered silica are reported. The fluorescence characteristics of bare carbon dots are compared to those of nitrogen-doped samples, nitrogen doping being exploited to increase the emission efficiency. Two emission bands in the blue-green region, at about 430 and 520 nm, are singled out by exciting the samples in the far UV (250 nm), in the near UV (350 nm), and in the visible (at 420 nm). The analysis of excitation/emission maps allows discussing the wavelength tunability of the CD emission features, singling out the red-shift trend as related to CDs polydispersity and distribution of green emitting centers. By dispersing the carbon dots in mesoporous silica, the relative contribution of the two emission bands is modified, promoting the green photoluminescence. The properties of the emission bands are studied as a function of excitation energy power and cumulative exposure time at fixed excitation energy. The presented results offer insights into the origin of the emitting centers, looking at the interaction with a suitable inorganic silica matrix and at the parameters affecting the photoluminescence of carbon dots, mainly with reference to implications on applicability in solid-state photonics.