Understanding the structural and chemical changes in vertical graphene nanowalls upon plasma nitrogen ion implantation

Abstract

Shallow plasma ion implantation is a versatile method for nitrogen incorporation in vertical graphene nanowalls (VGNs). However, the defects introduced by the process and the preference of nitrogen to occupy various locations in the 2D layered structure make the characterization complex. We have simplified the analysis of 2 kV nitrogen plasma ion implanted VGNs by correlating the binding energy of N_1s electrons with the chemical state of nitrogen as lone-pair localized (N1), lone-pair de-localized (N2) and quaternary nitrogen (N3). This new approach helps to understand the electronic nature of implanted VGNs, based on the occupancy of structural locations by nitrogen. The C_1s photoelectron spectra and G-peak intensity normalized comparison of the entire Raman spectra revealed large scale sp²C to sp³C conversion and generation of defects upon implantation. The increase in relative stiffness of implanted VGNs, as observed in atomic force acoustic microscopic studies, was correlated with the formation of graphitic CN_x (N2), crosslinking of layers by nitrogen (N3) and interlayer sp³ carbon.