Elsevier

Organic Electronics

Volume 25, October 2015, Pages 170-177
Organic Electronics

Sexithiophene ultrathin films on passivated Si(0 0 1) surfaces: Growth and electronic structure

https://doi.org/10.1016/j.orgel.2015.05.047Get rights and content

Highlights

  • Core levels of α-6T thin films were investigate by high-resolution photoelectron spectroscopy.

  • The origin of the in-plane orientation is partly ascribed to the interaction with the Si–OH species.

  • The origin of the in-plane orientation is partly ascribed to the geometrical effect.

  • The origin of the molecular orientation/packing structure was examined.

Abstract

We investigated the electronic states of α-sexithiophene (α-6T) on three passivated Si(0 0 1) surfaces, oxidized Si(0 0 1), water-adsorbed Si(0 0 1), and ethylene-adsorbed Si(0 0 1), by means of high-resolution photoelectron spectroscopy using synchrotron radiation. The main purpose of the present study was to identify the origin of the marked difference in molecular orientation on these substrates. We found that the interaction of α-6T molecules with Si–OH species should be the dominant reason for the in-plane orientation. On the other hand, the predominance of the in-plane orientation on ethylene-adsorbed Si(0 0 1) could not be simply explained by the chemical interaction but was ascribed to the geometrical effect of the surface corrugation. We investigated changes in the core levels upon α-6T deposition on passivated Si(0 0 1) surfaces and interpreted the results based on the charge transfer scheme.

Introduction

Molecular organic semiconductor materials such as conjugated oligothiophenes have been extensively studied because of their potential application in electronic and optoelectronic devices [1]. Here, we focused on α-sexithiophene (α-6T), which contains six thiophene rings linked together at the para positions. It is known that α-6T is a p-type semiconductor with high carrier mobility. It is important to study the electronic properties of organic layers grown on silicon surfaces because of the wide range of applications of the resulting structures. One of the major approaches for growing organic layers on silicon surfaces is to exploit covalent bonding between the adsorbed molecules and the Si dimers on Si(0 0 1) to build functionalized units [2]. The other approach is to use a passivated surface in order to prevent chemical bonding, and instead promote a self-assembly process leading to agglomerated island formation [3]. It has been reported that α-6T molecules dissociate into 1T to 5T fragments on a clean Si(0 0 1) surface [4]. We have chosen the latter approach in this study in order to investigate the intrinsic electronic property of α-6T. On passivated Si(0 0 1) surfaces, α-6T molecules tend to be aligned perpendicular to the surface [3]. This trend is considered to be valid in many cases when the surface is less reactive [5]. Therefore, it is a challenging task to promote the in-plane orientation of α-6T molecules on passivated Si(0 0 1) surfaces.

In our work, we first determined the molecular orientation of α-6T on three passivated Si(0 0 1) surfaces: oxidized Si(0 0 1), water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1) by means of optical measurements [6]. We found that α-6T molecules tend to be aligned perpendicular to the surface for oxidized Si(0 0 1), while they tend to be aligned parallel to the surface for ethylene-adsorbed Si(0 0 1). It is noted that the in-plane orientation of α-6T molecules can be promoted using artificial periodic grooves with a depth of 10 nm [7], [8]. Here, we propose a possible use of nano-scale grooves with a depth less than 0.5 nm on the ethylene-adsorbed Si(0 0 1) to promote the in-plane orientation of α-6T molecules. For water-adsorbed Si(0 0 1), both types of molecules coexist. Ultraviolet photoelectron spectroscopy (UPS) study showed that the work function decreases upon the deposition of α-6T in the case of oxidized Si(0 0 1), while it is almost unchanged for ethylene-adsorbed Si(0 0 1) at the thickness lower than 0.5 nm [9]. The amount of the change in the work function is intermediate for water-adsorbed Si(0 0 1). Therefore, the molecular orientation might be directly associated with the work function change. In order to clarify the origin of these differences, we investigated the core levels (Si 2p, O 1s, C 1s, S 2p) using high-resolution photoelectron spectroscopy.

Section snippets

Experimental

Present experiments were carried out in ultrahigh vacuum (UHV) chambers, one of which contained an electron spectrometer for photoelectron spectroscopy measurements, and the other contained a low-energy electron diffraction (LEED) apparatus for surface preparation, at Saga University beamline BL13 in the Saga Light Source [10]. A clean Si(0 0 1) surface (n-type, 4–8 Ωcm) was prepared by flashing the sample at 1400 K under UHV, with a base pressure below 2 × 10−8 Pa. Formation of the Si(0 0 1)-(2 × 1)

Molecular orientation

In this work, we consider three types of molecular orientation for α-6T, as shown in Fig. 1. In our previous work, we addressed the difference between the upright-standing orientation and the flat-lying orientation as the in-plane orientation, because our optical techniques can detect the transition moment of the long molecular axis [6]. To be more precise, the Buddha-lying orientation can also be the in-plane orientation, where the α-6T molecule is within the plane that is perpendicular to the

Summary

We investigated the core levels in α-6T thin films grown on three passivated Si(0 0 1) surfaces, oxidized Si(0 0 1), water-adsorbed Si(0 0 1), and ethylene-adsorbed Si(0 0 1), using high-resolution photoelectron spectroscopy with synchrotron radiation. We found that the C 1s states may be sensitive to the molecular orientation, while the S 2p states may be related to the crystallinity of the films. The origin of the work function such as the molecular orientation and the interaction of α-6T molecules

Acknowledgements

The photoemission experiments were performed at Saga University Beamline (SAGA-LS/BL13) with proposals of H25-101 V, H25-213 V, and H26-108 V under the support by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work was partly supported by a Grant-in-Aid for Scientific Research (B) (21360020, 25286016).

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