Structure of photoluminescence spectra of oxygen-doped graphitic carbon nitride

Abstract

The relationships governing variation of the photoluminescence of graphitic carbon nitride synthesized by heat treatment of melamine in a closed air medium containing oxygen in the temperature range of 10–300 K were investigated. It was shown that the concentration of oxygen in the obtained material 4–5 at.% increases with increase of temperature and decreases with increase in the duration of the synthesis process. By measurements at reduced temperatures right down to 10 K it was possible to resolve bands due to radiative recombination processes in the photoluminescence spectra of the graphitic carbon nitride. It was found that increase of the synthesis temperature from 500 to 600oC and also increase of the duration at the given temperature from 30 to 240 min shift the maximum in the photoluminescence spectrum from 2.74 eV into the region of lower energies to 2.71–2.67 eV. This is due to the bigger role of the molecular system formed by the π bonds of carbon and nitrogen atoms with sp2 hybridization and characterized by a smaller forbidden band width in the emission of light. Transitions due to recombination through oxygen-induced levels in the forbidden band of the semiconductor lead to the appearance of a "tail" in the photoluminescence spectra in the region of low energies (2.40–2.33 eV). Increase of the carbon nitride synthesis temperature to 600oC leads to a change in the structure of the energy bands and to increase of the energy of the radiative transitions as a result of increase in the degree of doping with oxygen atoms and thermal stratifi cation.