Charge transfer mediated photoluminescence enhancement in carbon dots embedded in TiO2 nanotube matrix

Carbon quantum dots (CQDs) have drawn a lot of attention in recent years for imaging applications due to their luminescence properties in the visible range. There has been a strong thrust in achieving CQDs with enhanced luminescence efficiency. We report nearly three times increase in the total photoluminescence emission of CQD films by depositing them on TiO2 nanotube (NT) substrates, instead of glass. The net increase in PL arose from higher CQD core emission and the appearance of a strong surface state emission. High-resolution X-ray photoelectron spectroscopy (XPS) was employed to analyze the composition of the CQD surfaces, which confirmed more carboxyl groups on the surface of CQD in the CQD:NT composite. The band alignment of the composite, determined from XPS measurements, favoured carrier transfer from TiO2 NT to CQD. The TiO2 NT, having better surface coverage and wide absorption spectrum, had high absorption cross-section. The efficient transfer of the carriers from NT to CQD is found to be the dominant phenomenon leading to a significant increase in the PL emission intensity.