Paper TF-ThM10
Synthesis of Graphene via Surface Segregation and Reaction

Thursday, December 11, 2014, 11:00 am, Room Makai

Single-layer, bi-layer and few-layer thick nanosheets of graphene have been attracting significant amount of attention due to their excellent physical, chemical and mechanical properties. The first isolation of few-layer graphene (FLG) was demonstrated in 2004. However, single-layer graphene (SLG) was first synthesized 40 year ago by surface segregation, and was identified by Blakely et al. in 1974 using surface sensitive techniques. In 1980s, we initiated the growth and characterization of FLG and h-BN nanosheets using surface segregation and surface reaction. For the graphene growth, there are three important steps; surface segregation of doped atoms, surface reaction to form a monolayer phase, and subsequent 3-D growth (surface precipitation). Such surface phase transition was demonstrated on C-doped Ni(111) by in situ X-ray photoelectron spectroscopy (XPS) at elevated temperatures, and the growth mode was clarified by inelastic background analysis. Among the three, the surface segregation plays the most important role for the SLG synthesis. The surface segregation approach has been applied to Pt(111) and Pd(111) substrates, where weak coupling is expected, and controllable growth has been demonstrated successfully. As one of the derivatives, we recently proposed a promising method for producing SLG covering an entire substrate at low temperature using a Ni film deposited on an HOPG substrate. By heating the Ni/HOPG in high vacuum, carbon atoms forming graphene are diffused from the HOPG substrate through the Ni template. In this paper, we will put more focus on the effect of competitive surface-site occupation between carbon and other surface-active impurities on the SLG growth. It is known that sulfur is a typical impurity of metals and the most surface-active element. Even with a high purity single crystal, the surface sites are finally occupied by sulfur at the elevated temperatures by surface segregation. In the case of Ni(110) surface, it is confirmed by scanning Auger microscopy (SAM) and scanning tunneling microscopy (STM) that the available surface sites is nearly occupied by sulfur with a centered 2x2 arrangement. When the Ni(110) is doped with carbon, surface segregation of carbon and following graphene growth shall be strongly affected or restricted by the surface active elements such as sulfur. In this situation, we discovered a strongly directional growth of SLG, exhibiting rectangular-like shapes and nearly straight step edges. The detailed characterization at the nanoscale and interesting growth mechanism shall be discussed based on high resolution microscopes like UHV-STM and scanning helium ion microscopy (SHIM).