Theoretical Study of Graphene Nanoflakes with Nitrogen and Vacancy Defects: Effects of Flake Sizes and Defect Types on Electronic Excitation Properties

Abstract

We have established finite-sized model molecules of hexagonal graphene nanoflakes which consisted of up to about 400 carbon atoms and certain defects and calculated their geometric structures, molecular orbitals, and valence electron excitation properties by DFT and TD-DFT approaches. Several types of defects such as single nitrogen atom dopants, vacancies, combinations of nitrogen atoms and vacancies, etc. have been investigated. The vertical excitation energies and the corresponding absorption spectra of all low-lying excited states of all these defects are found strongly depending on nanoflake sizes with few exceptions. The reason is revealed by examining molecular orbital configuration changes upon excitation, where in most cases the electron density does not localize in the defect center but disperses to the edges of the nanoflake instead. As a result, graphene nanoflakes with these defects do not present definite spectral features like a zero-phonon line in the nanodiamond. The energy gaps and absorption spectra could be tuned by varying nanoflake sizes and defect types, where the size determines the positions of the first major absorption peaks and the defect type affects spectral shapes of the following bands.