Orientation of One-Dimensional Silicon Polymer Films Studied by X-Ray Absorption Spectroscopy

Md Abdul Mannan,Iwao Shimoyama,Tetsuhiro Sekiguchi,Yuji Baba,Masamitsu Nagano,Hideyuki Noguchi,Norie Hirao

doi:10.1155/2012/528256

Abstract

Molecular orientations for thin films of one-dimensional silicon polymers grown by vacuum evaporation have been assigned by near-edge X-ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The polymer investigated was polydimethylsilane (PDMS) which is the simplest stable silicon polymer, and one of the candidate materials for one-dimensional molecular wire. For PDMS films deposited on highly oriented pyrolytic graphite (HOPG), four resonance peaks have been identified in the SiK-edge NEXAFS spectra. Among these peaks, the intensities of the two peaks lower-energy at 1842.0 eV and 1843.2 eV were found to be strongly polarization dependent. The peaks are assigned to the resonance excitations from the Si 1s toσ∗ pyzandσ∗pxorbitals localized at the Si–C and Si–Si bonds, respectively. Quantitative evaluation of the polarization dependence of the NEXAFS spectra revealed that the molecules are self-assembled on HOPG surface, and the backbones of the PDMS are oriented nearly parallel to the surface. The observed orientation is opposite to the previously observed results for PDMS on the other surfaces such as oxide (indium tin oxide) and metal (polycrystalline copper). The flat-lying feature of PDMS observed only on HOPG surface is attributed to the interaction between CH bonds in PDMS andπorbitals in HOPG surface.

Highlights

Polysilanes composed of silicon-silicon backbone have excellent properties in comparison with carbon-based polymers [1]
For PDMS films deposited on highly oriented pyrolytic graphite (HOPG), four resonance peaks have been identified in the Si K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra
Quantitative evaluation of the polarization dependence of the NEXAFS spectra revealed that the molecules are self-assembled on HOPG surface, and the backbones of the PDMS are oriented nearly parallel to the surface

Summary

Introduction

Polysilanes composed of silicon-silicon backbone have excellent properties in comparison with carbon-based polymers [1]. Polysilanes are considered to be an ideal onedimensional (1D) molecular wire with high electric conductivity since the σ electrons in polysilanes are delocalized comparable to those of π electrons in the conjugated carbonbased polymers. It has been predicted that the polysilanes possess electronic structure similar to direct band-gap semiconductor [2]. One of the features of PDMS is the high electric conductivity along the 1D chain due to the smaller HOMOLUMO band gaps [3]. It should be noted that the electric and optical properties in thin films of 1D polymer depend on the configuration of the molecular skeleton. For ultrathin films around monolayer or thinner than monolayer, electric conductivity would become anisotropic depending on the molecular orientation. It is much important to precisely determine the molecular orientation of polymers on solid surfaces

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Results

Conclusion