Wavelength-tunable visible to near-infrared photoluminescence of carbon dots: The role of quantum confinement and surface states

Carbon quantum dots (C-QDs) with different size distributions and surface characteristics can exhibit good emission properties in the visible and near-infrared (NIR) regions, which can be applicable in optoelectronic devices as well as biomedical applications. Optical properties of colloidal C-QDs in distilled water at different concentrations produced using a method of alkali-assisted surfactant-free oxidation of cellulose acetate is presented. The structural and optical properties of colloidal C-QDs at different concentrations were investigated, with the aim of clarifying the main mechanisms of photoluminescence emissions. We observed a wide range of tunable visible to NIR emissions with good stability from the C-QD colloids at different applied excitation wavelengths. The colloids show dual emissions with maxima at ∼420 and 775 nm (blue and NIR emissions) when excited at the wavelength range near the energy gaps of the C-QDs. Moreover, by increasing the excitation wavelength, tunable visible emissions at the spectral range of 475 to 550 nm are observed. A detailed analysis of the results shows that the blue and NIR luminescence of colloidal C-QDs originate from the oxide-related surface effects whereas quantum confinement is the responsible mechanism for tunable visible emissions of the C-QD colloid.