Terahertz electrodynamic properties of graphene doped with nitrogen plasma

In this study, we present a detailed examination of the influence of the defects induced by the nitrogen plasma on the electrodynamic properties of few-layer graphene using terahertz (THz) time-domain spectroscopy (TDS). Initially, few-layer graphene is obtained using the chemical vapor deposition technique. Then, it is repeatedly treated with sub-3 kV nitrogen plasma that results in the creation of multiple lattice defects and the insertion of nitrogen observed by means of Raman and x-ray photoelectron spectroscopy. According to obtained spectra, the graphene lattice transferred onto a quartz substrate withstands up to 600 s of plasma treatment. However, the number of defects increases with treatment time: even 10 s treatment of initial graphene considerably reflects in Raman spectra. At the same time, 600 s of plasma treatment leads to the insertion of up to ∼9 at. % nitrogen, predominantly in pyridinic and pyrrolic/pyrazolic forms. Notably, the ratio between pyridinic, pyrrolic/pyrazolic and graphitic forms of nitrogen insertion in graphene remains constant independently on the treatment time. The described structural changes lead to the increase in THz transmittance with treatment time, as observed using THz- TDS. According to the proposed theoretical explanation based on the Kubo formalism, such dependence of THz spectra on an extension of treatment time indicates the decrease in total conductivity of graphene corresponding to the sufficient increase in electron collision broadening and the decrease in chemical potential caused by plasma treatment. Therefore, nitrogen plasma treatment is proven as an effective, robust and scalable method for adjusting the conductivity and transport properties of graphene widening its potential applications in THz electronics and photonics.