Abstract
Graphene produced by chemical vapor deposition (CVD) is now widely used. It has a polycrystalline structure, in which carbon atoms form 5- and 7-membered rings at the domain boundaries [1]. The relationship between such domain boundaries and transport properties is still poorly understood. Doping effects and conduction properties induced by thin films of organic molecules are of great importance in the device application.
In the present work, we focused on oligothiophene (6T) and phtyalocyanine (Pc) molecules because of their excellent electronic properties and other functionalities. It has been reported that thin films of 6T on CVD-graphene in field effect transistors (FETs) slightly improve their performances [2]. However, detailed knowledge of how the molecular adsorption structure and electronic states near the interface affect the performance has not been clarified. Here, we investigated the interaction between graphene and 6T or Pc thin films by atomic force microscopy (AFM), micro-Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy (XPS), and synchrotron radiation photoemission spectroscopy (KEK BL13B, SPring-8 BL23SU).We also fabricated a FET structure and measured transport properties.
Fig. 1. XPS spectra of 6T molecularly adsorbed CVD graphene. Fig. 1 shows the XPS spectra, which clearly show the S2s core level of the 6T molecule. Thickness dependence of the peak intensity is apparent. On the other hand, spectral decomposition of the C1s core level between the graphene substrate and the molecule is rather difficult. Hence, we applied photoelectron spectroscopy with synchrotron radiation to obtain the high-resolution core levels as well as the valence states. Annealing the 6T/Pc films has led significant structural changes, corresponding to the slight changes in the electronic states. We further analyzed the transport properties of a monolayer graphene layer using the FET structure. For example, we found the change in the On/Off ratio by the adsorption of the 6T thin films. Here, we also investigated Pc thin films grown on graphene, and compare the interfacial structures, electronic states and transport properties with those of 6T thin films.
[1] S. M. Fus et al., Prog. Surf. Sci. 92 (2017) 176.
[2] T.-J. Ha et al., Appl. Phys. Lett. 101 (2012) 033309.
