Graphitic Nitrogen Doping in Carbon Dots Causes Red-Shifted Absorption

The knowledge gap on how different types of nitrogen centers affect the optical properties of N-doped carbon dots (CDs) hinders the rational design and synthesis of these nanostructures. We present a systematic theoretical study of 1 nm small CD models containing nitrogen and oxygen functional groups designed to explore the effects of various nitrogen centers on the absorption characteristics of CDs. Graphitic nitrogen is shown to have an electron-doping effect that alters the systems' electronic energy levels and causes pronounced red-shift of their absorption spectra. Other kinds of nitrogens including pyridinic, pyrrolic, and amino centers had no appreciable effects on the CDs' absorption properties.