Adsorption and self-assembly of hexa-tert-butyl-hexa-peri-hexabenzocoronene on the si(111)-$\sqrt{3}\times \sqrt{3}$-Ag surface

Highlights

• Adsorption and self-assembly of hexa-tert-butyl-peri-hexabenzocoronene on the Si(111)-$\sqrt{3}$x$\sqrt{3}$-Ag surface.

• The HB-HBC molecules are assembled into a $\sqrt{21}$x$\sqrt{21}$-R$\pm 10.9{}^{\circ }$ structure with respect to the Si(111)-1 $\times$ 1 surface.

• At submonolayer coverages, the tert-butyl groups of HB- HBC are adsorbed on both the Si and Ag trimers of the Si(111)-Ag surface, and only on the Si trimers at a full monolayer.

Abstract

Self-assembled structures of hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC) on the Si(111)-$\sqrt{3}\times \sqrt{3}$-Ag surface have been investigated by low-energy electron diffraction and low-temperature scanning tunneling microscopy (STM). The HB-HBC molecules are assembled into a $\sqrt{21}\times \sqrt{21}-$R$\pm 10.9^{\circ }$ structure with respect to the Si(111)-$1\times 1$ surface. Although the periodic structure is unchanged as a function of molecular coverage, we find that the orientations and adsorption sites of HB-HBC within the molecular islands are changed at near a full monolayer. At submonolayer coverages, the tert-butyl groups of HB-HBC are adsorbed on both the Si and Ag trimers of the Si(111)-Ag surface, and only on the Si trimers at a full monolayer. This result should be associated with a balance between substrate-molecule and inter-molecule interactions.

Introduction

Organic molecular thin-films have attracted much attention, due to the potential application in devices [1], such as organic light-emitting diodes and field-effect transistors. The performance of the devices is highly dependent on the interface structures between the film and substrate, which are strongly influenced by initial structural order of the adsorbed molecules during the thin films growth. Thus, the understanding and control of the molecular arrangement in the first organic layer on various substrate surface are essential for optimization of molecular electronics.

Hexa-peri-hexabenzocoronene (HBC) is one of the discotic polycyclic aromatic hydrocabons [2], [3], which can be regarded as the smallest graphene fragment consisting of 13 fused benzene rings, as shown in Fig. 1(a). The large aromatic core of HBC has been reported to show remarkable properties as the charge transport and electron reservoir materials for the molecular devices, which depend on the molecular arrangements [2]. Through the strong $\pi$ -$\pi$  intermolecular interactions, the formation of columnar self-assembled structures has been reported for the HBC derivatives [3], [4], [5]. On metal surfaces, such a columnar stacking of unsubstituted HBC has been reported as a result of the multi-layer growth [4], whereas a lacking of attractive intermolecular forces has been observed between flat-lying molecules at submonolayer coverages [6]. Nevertheless, by substituting peripheral bulky groups, a very dense packing of the molecules has been formed even at submonolayer coverages [6], [7].

For the molecular assembly of HBC and its derivatives, the Au(111), Cu(111), and highly oriented pyrolytic graphite (HOPG) surfaces have been used as a substrate due to the relatively weak molecule-surface interactions [4], [6], [7], [8], [9], [10], [11]. Although a more disordered structure of adsorbed molecules is expected on reactive semiconductor surfaces, the molecule-surface interactions should be weakened by the formation of the metal induced superstructures. In particular, the Si(111)-$\sqrt{3}\times \sqrt{3}$-Ag surface is known as a relatively inert substrate for molecular self-assembly [12], [13], [14], which is composed of topmost Si and Ag trimers, as shown in Fig. 1(b). The ground-state atomic arrangement of the surface has been described by an inequivalent triangle (IET) model [15], whereas a honeycomb-chain-trimer (HCT) model has been observed above around 150 K [16], [17]. Within the unit cell, two Ag trimers are equivalent in the HCT model [19]. In the IET model, the two Ag trimers become inequivalent through slight rotations of Ag triangles from the symmetric positions of the HCT model, resulting in the formation of large and small Ag trimers as shown in Fig. 1(b) [15], [16], [17]. The IET model has been further distinguished into IET(-) and IET(+) configurations with respect to the relative positions of the small and large Ag trimers within the unit cell.

In this paper, we report on adsorption and self-assembly of hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC) on the Si(111)-$\sqrt{3}\times \sqrt{3}$-Ag surface, which have been investigated by low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (STM). The HB-HBC molecules have been selected because the large aromatic core should be decoupled with the surface by the bulky tert-butyl groups after the adsorption. Our obtained self-assembled structures of HB-HBC have two symmetry-equivalent domains mirrored along the [11$\overline{2}$], which have been formed depending on the underlying IET(-) and IET(+) structures. We find that the orientations and adsorption sites of HB-HBC are different between at submonolayer and near a full monolayer, although the self-assembled structures are unchanged. The tert-butyl groups of HB-HBC are adsorbed on both the Ag and Si trimers at submonolayers, and only on the Si trimers at near a full monolayer. We also investigate distinct interactions of the tert-butyl groups with underlying large and small Ag trimers of the IET structure.