Adsorption, film growth, and electronic structures of 2,7-dioctyl[1]benzothieno-[3,2-b][1]benzothiophene (C8-BTBT) on Cu (100)

Abstract

Using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM), and grazing X-ray diffraction measurement(GIXRD), we systematically investigate the correlations of interface energy level structure, film growth and the molecular orientation of 2, 7-dioctyl[1]benzothieno-[3, 2-b][1]benzothiophene (C8-BTBT) on Cu(100). We find that the adsorption of the first layer of C8-BTBT molecules on Cu(100) is a stable physical one, and there is no chemical shift of the S 2p peaks of XPS and the ratio of the output of C to that of S is the same as the stoichiometric value of the molecular C8-BTBT. The heights of the steps of the upper layers of C8-BTBT in the AFM images are ～ 30 , close to the length of the molecular long c-axis, indicating the standing-up configuration of the upper molecules. AFM image shows that the upper molecules tend to grow into islands while the bottom molecules tend to grow into layer, suggesting an Stranski-Krastanov growth mode of multilayer C8-BTBT on Cu(100). The GIXRD shows an out-of-plane period of 30.21 , which consistently proves the standing-up configuration of the outer molecule layer. There is an electric dipole of 0.41 eV at the very interface pointing from the substrate copper to C8-BTBT, which will reduce the barrier for electron transport and increase the barrier for hole transport from Cu to C8-BTBT. The vacuum level (Evac) starts to bend downward after 16 deposition, and with the increase of the thickness of the film, a total downward shift of 0.42 eV is observed. The downward shift is ascribed to the changing of molecular orientation from lying down before 16 to standing up after 16 , which establishes an outward-pointing layer of C-H bonds and accordingly forms a dipole layer depressing the surface barrier. The shape and leading edge of the hightest occupied molecular orbit (HOMO) also change with the increase of film thickness. These changes are due to the anisotropy of electron ionization from molecular orbit. The total downward shift of the HOMO is about 0.63 eV. The downward bending of 0.42 eV for Evac and 0.63 eV for HOMO with increasing film thickness lead to a slightly decreasing ionization potential (IP) about 0.1 eV before 32 and then an increasing IP about 0.31 eV, which finally results in a total increase of 0.21 eV for IP. The bending electronic structures facilitate electron transport from interface to surface and hole transport from surface to interface. Our Investigation provides valuable information for relevant device design.