To fabricate ordered molecular multi-layers over a wide area through π–π stacking interaction, the molecule frameworks lying-down parallel to the substrate are preferable specially for the light-emitting and photovoltaic molecular devices. The lying-down configuration in the layers is mostly dominated by the chemical properties at the interface between the substrate and the first layer molecules. For example, as a substrate, a Si(001)-2×1 surface has a large number of Si dangling bonds per unit area. The molecules with π orbitals deposited on the Si(001)-2×1 can be bonded with the dangling bonds, which are slant to the surface, possibly leading to their lying-down configuration. In this study, we examine the adsorption of 4,4”diamino-pterphenyl (DAT) vapor-deposited on the Si(001)-2×1 by a scanning tunneling microscope (STM) to pursue the stable lying-down configuration. An ultrahigh vacuum (UHV) chamber system equipped with a molecule evaporator and STM was used. At low coverages of DAT, a number of elongated protrusions were observed, frequently tilted at ~17° to the Si dimer rows of the Si(001)- 2×1. The protrusions corresponded to individual DAT molecules. The stability at 17° was ascribed to three chemical bonds to the Si surface; each amino group at both ends of a DAT covalently bonds to one Si atom of a Si dimer, and in addition the center benzene ring of the DAT forms a bond, resulted in a butterfly-like lying down structure (Fig. 1) .
References 1) A. Hassan, T. Nishimura, A. Sasahara, H. Murata, and M. Tomitori: Surf. Sci. 630 (2014) 96.
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