UV absorption in metal decorated boron nitride flakes: a theoretical analysis of excited states

The excited states of single metal atom (X = Co, Al and Cu) doped boron nitride flake (MBNF) B₁₅N₁₄H₁₄-X and pristine boron nitride (B₁₅N₁₅H₁₄) are studied by time-dependent density functional theory. The immediate effect of metal doping is a red shift of the onset of absorption from about 220 nm for pristine BNF to above 300 nm for all metal-doped variants with the biggest effect for MBNF-Co, which shows appreciable intensity even above 400 nm. These energy shifts are analysed by detailed wavefunction analysis protocols using visualisation methods, such as the natural transition orbital analysis and electron-hole correlation plots, as well as quantitative analysis of the exciton size and electron-hole populations. The analysis shows that the Co and Cu atoms provide strong contributions to the relevant states whereas the aluminium atom is only involved to a lesser extent.