Room-temperature direct bonding of graphene films by means of vacuum ultraviolet (VUV) / vapor-assisted method

A room temperature direct bonding between graphene thin films was realized by using vacuum ultraviolet (VUV) / vapor-assisted surface modification method at atmospheric pressure. In this paper, we examined the bonding between graphene mono layers and multi layers. X-ray photoelectron spectroscopy (XPS) showed that, in the ambient air, the graphene surfaces were easy to be oxidized, which resulted in the formation of CO₂ (C-O 286.6eV) and organic hydrocarbons. Additionally, chlorine from the graphene production process was found on the outmost surface. For such oxidized and adsorbed surfaces, the vacuum ultraviolet (VUV) treatment was found highly effective to modify the chemical binding condition. The VUV irradiation eliminated the oxide and contaminations, and changed the binding condition of carbon. For the monolayer graphene surface, the oxidized site was almost completely removed and C-C bond, which has the binding energy of 284.4 eV, became dominant. For the multilayer surface, although C-O bonds remained, the atomic weight percentage of carbon increased. Raman spectroscopy study results showed that the intensity of defect, which was indicated by D-band, decreased both on the monolayer and multilayer. Moreover, the intensity of 2D band, which was attributable to graphene formation, became stronger; the VUV irradiation turned out effective to modify the crystalline structure to create graphene outmost surface layer. After the VUV irradiation, the surfaces were exposed to the water vapor to make the surface hydrophilic, because molecular water can adsorbe on the hydrophilic carbon surfaces, create hydrogen bond between on the moment of contact, and make good adhesion between the surfaces. XPS observations indicated that the bridging layers including molecular layers, which were assigned as CHO (286.2eV) and/or COOH (288.2eV), were created by water exposure. Raman observations confirmed that the intensity ratio of D band and G band (I_D/I _G) decreased and the ratio of 2D band and G band (I _2D/I_G) increased apparently on the monolayer and multilayer, respectively. This inferred that the defect decreased and the disordered graphene layer was reduced on the outmost surface, and the quantitative quality of graphene was improved due to the water exposure. Similarly, the peak shifts showed that, both on the monolayer and multilayer, the crosslink between carbon and hydrogen became closer. Without using chemical process, the simple VUV irradiation combined with the water exposure turned out highly effective to make tight bond between graphene surfaces without degrading the quantitative quality.