On the possibility to identify N dimers in N-doped graphene by means of X-Ray Photoelectron Spectroscopy

Abstract

Graphene is a material with unique properties not seen yet in other compounds. In fact, this is the reason behind everyone talking about it. In the last years, the possible applications of this compound have been studied extensively due to its unexpected and exotic properties. Nowadays, the number of applications of graphene based devices and systems is ever growing. The present work focuses in one of these possible applications. Graphene has the capacity to behave like a metal, this is because some special features of its electronic structure; namely the absence of a band gap. This provides a great capability to act as an electrical conductor. In fact, graphene conductivity can overcome that of metallic compounds such as copper, one of the best electrical conductors. This property is worthy in itself but also implies some drawbacks in possible applications in electronics. Doping, as in the case of classical semiconductors based in Si and largely used in transistors and other electronic devices, offers the possibility to engineer the band gap of graphene as well. In fact, doping with nitrogen has been successfully achieved and the resulting system behaves effectively as an n-doped semiconductor. The problem of doping is, however, the lack of control in the synthesis and, depending on the method used, the nitrogen atoms introduced in the graphene network can occupy different sites resulting in different advantageous or undesired properties. The first step towards a control of the doping at an atomic level requires identifying the sites where the N atoms are located. In addition, N atoms in the graphene network may cluster in dimmers which have been suggested to introduce a structural instability. The main aim of the present work is to investigate whether the presence of N dimmers in the graphene networks can be identified through X-ray photoelectron spectroscopy (XPS), one of the most broadly used techniques in materials science. XPS provides the binding energy (BE) of core electrons that depends essentially on the ionized atom but also and its surroundings. Following, previous work on the theoretical study of the core level binding energy (CLBE) of isolated N atoms at distinct sites of the graphene network, we analyze different models for N dimers on graphene and inspect the corresponding N(1s) CLBE to provide information on whether these structures can be resolved by XPS. This is accomplished by the help of theoretical calculations based on Density Functional Theory (DFT), which allowed us to accurately determine the N(1s) CLBE and to, hence, to provide information regarding possible spectroscopic differentiation of the different possible atomic arrangements. The results are conclusive, the existence of near neighbour (n,n) dimers lead CLBEs different enough so as to unambiguously identify them. On the other hand, next near neighbour (n,n,n) dimers cannot be detected using XPS because the (n,n,n) dimer signal is too close to the isolated nitrogen signal