Chapter 1. Introduction

The investigation of morphology and electronic structure of well-ordered functional molecular nanostructures via self-assembly on metal surfaces has been extensively carried out for future applications of molecular electronics, nanoscale biological systems and solid state quantum computations. Yet, how self-assembly is influenced by the material of the substrate, coverage dependence and temperature is not fully understood. Moreover, the realization of molecular electronics requires understanding parameters influencing molecular electronic structure like adsorption structure, molecular orientation and lateral interactions. Indeed, several systems [1-3] shown molecules of different orientations adsorbed at the 1st layer. However, only few works have reported the influence of such differences in orientation on the electronic structure of molecules [4]. As for lateral interactions, Kroger et al. [5] reported that molecules of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) on Au(788) located inside different superstructures have different electronic structures. They suggest that the origin is due to differences in lateral interactions: number of neighboring molecules and differences in hydrogen bond lengths. However, the origin remains unclear and there are too few reports on the effect of lateral interactions on the electronic structure. Thus further research is required. Based on the situation as above, I investigated the influence of material of the substrate, coverage, and temperature on the self-assembly of molecules. I also investigated how molecular orientation, lateral interactions and molecule-surface interactions influences the electronic structure of a molecule with scanning tunneling microscopy (STM) and spectroscopy (STS) and core-level spectroscopies (CLS).

Chapter 2. Experimental Techniques

All STM and STS data were obtained at 4.7 K with a low-temperature ultra-high vacuum (UHV) STM. CLS experiments were carried out at SPring-8, BL17SU, Japan. The characteristics of the beamline and undulator have been reported elsewhere [6]. Surfaces of Au(111) and Cu(110) were cleaned by repeating cycles of Ar+ sputtering and annealing. The molecules were evaporated onto the clean Au(111) and Cu(110) surfaces at 300 K for sub-monolayers (20 K for multilayer) by resistively heating the molecules powder inside a crucible under UHV.

Chapter 3. Self-Assembly of Functionalized Aromatic Molecules on Metal Surfaces

3.1. Benzoic Acid on Au(111) [7]

On Au(111), benzoic acid molecules physisorb and form dimers via hydrogen-bonding between carboxyl groups. These dimers have three orientations reflecting the Au(111) substrate three-fold symmetry and are adsorbed in the hcp and fcc regions of the Au(111) reconstruction, but seldom at the corrugation lines (boundary region between hcp and fcc stacking regions). They assemble into dimer rows with a total of six equivalent orientations.

3.2. Benzoate on Cu(110) [8]

In contrast to the case of Au(111), benzoic acid (C6H5COOH) molecules dissociatively chemisorb in the form of benzoate (C6H5COO-) on Cu(110) at 300 K and their self-assembly on Cu(110) has been reported before [1, 9]. However, some contradictions appear in the proposed models of previous reports as Cu-complexes with several molecules adsorbed on the same adatoms can hardly stand-up at high coverage and thus hardly explain the STM images of the coverage dependence. In this study, I discuss these contradictions and propose new molecular models of the self-assembly.

3.3. Meta-Aminobenzoate on Cu(110) [10, 11]

I characterized the molecular structures of m-AB on Cu(110), consisting of molecular chains, 〓 and 〓 superstructures with STM (Figure 1a), and proposed models of the molecular arrangements with precise lattice matching (Figure 1b). Coverage dependence measured with STM (Figure 1a) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy (Figure 1c) revealed that the molecules adsorb in a flat-lying orientation at low coverage, while some molecules adsorb in an upright orientation at high coverage due to local crowding. These upright molecules appear brighter in the STM images (Figure 1a D) and their proportion is directly related to surface coverage. Additionally, thermal diffusion of the molecules induced a step-edge reconstruction, and a slow phase transition between the 〓and 〓 structures.

Chapter 4. Electronic Structure of Functionalized Aromatic Molecules on Metal Surfaces [12]

Investigation with coverage dependence of benzoate, m-AB and p-AB on Cu(110) with STM-STS and CLS reveals that the molecules orientation affect their own electronic structure by inducing a shift of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (Figure 2). Measured X-ray photoelectron spectroscopy (XPS) data at O1s, C1s and N1s reveals that coverage-dependent charge transfers occur between the molecule and the Cu(110) and are most probably at the origin of the energy shifts in HOMO and LUMO levels and core-levels.

Chapter 5. Tunneling Electron Induced Chemical Reactions of 4-Aminobenzenethiolate on Cu(110) [13]

This study reveals that both 4-aminobenzenethiol (4-H2NC6H4SH; 4-ABT) and 4,4'-Diamino- Diphenyl Disulfide (4,4'-H2NC6H4SSC6H4NH2; 4,4'DDD) adsorbs on Cu(110) at 298 K in the form of 4-aminobenzenethiolate (4-H2NC6H4S-; 4-ABTA) via S-H and S-S bonds dissociation, respectively. Independently of the precursor (4-ABT or 4, 4' DDD) two kinds of protrusions with different apparent heights appear on the surface, corresponding to two different molecular orientations. I found that it is possible to selectively induce change of orientation (1.7 eV), amine group dissociation (2.1 eV) or S group dissociation (2.5 eV) of the chemisorbed 4-ABTA molecule by controlling the energy of tunnel electrons injected into the molecules, as given in the parentheses.

Chapter 6. Summary and Future Directions

The influence of the substrate, coverage and temperature on molecular self-assembly has been investigated. Interestingly, m-AB molecules form a large variety of angles with the surface. This is quite different from the long belief that this kind of molecules has only two orientations (namely, fully up-right and fully flat-lying). Moreover, the molecule-surface interactions are coverage-dependent and are responsible for charge transfers resulting in energy shifts of core-levels, HOMO and LUMO levels. I also shown that it is possible to selectively induced changes in orientation and chemical groups dissociation of 4-ABTA.

Figure 1: (a) STM images of the chain (A), 〓(B) and〓structure (C) and brighter molecules adsorbed inside 〓(D). (b) Model of molecular arrangement for the〓structure. (c) O1s NEXAFS spectra of m-AB with coverage dependence.

Figure 2: STS spectra of benzoate, para-aminobenzoate and meta-aminobenzoate on Cu(110).

The present thesis presents the self-assembly and electronic structure of aromatic molecules on metal surfaces. In order to gain insight into the effect of lateral interactions and molecular orientation on the self-assembly and electronic structure of adsorbates, the study was performed by combining complementary techniques: scanning tunneling microscopy (STM) and spectroscopy (STS) at 4.7 K, near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The thesis consists of 6 chapters. Chapter 1 presents the background and purpose of the present research. Chapter 2 describes the experimental techniques. Chapter 3 focuses on the self-assembly of functionalized aromatic molecules, while Chapter 4 present the electronic structure of these molecules. Chapter 5 presents the tunneling electron induced chemical reactions of 4-aminobenzenetiolate (4-NH2-C6H4S-)o n Cu(110).

Chapter 1 introduces the background and purpose of the present research. Lateral interactions and molecular orientation of adsorbates on metal electrodes critically impacts their self-assembly and electronic structure and could have important applications for functional materials and organic molecular devices.

Chapter 3 describes the self-assembly of functionalized aromatic molecules on metal surfaces: benzoic acid (C6H5COOH) on Au(111), benzoate (C6H5COO-) on Cu(110) and meta-aminobenzoate (3-H2NC6H5COO-) on Cu(110). Benzoic acid molecules physisorb on Au(111) and form dimers via hydrogen bonding while they chemisorb after deprotonation in the form of benzoate molecule on Cu(110) due to the higher reactivity of the substrate. Molecular models were proposed for the self-assembly of benzoate on Cu(110) to explain the complex coverage-dependence of the self-assembly. The self-assembly of meta-aminobenzoate was characterized with STM and revealed a variety of superstructures and molecule-surface angles.

Chapter 4 presents the effect of lateral interactions and molecular orientation on the electronic structure of benzoate, para-aminobenzoate (4-H2NC6H5COO-a) nd meta-aminobenzoate on Cu(110). The study reveals a molecular orientation-dependant electronic structure, which consists of an upward energy shift of the LUMO level accompanied with downward energy shifts of the O1 s and C1s core-levels and an upward energy shift of the N1 s core-level of up-right molecules as compared to flat-lying molecules. The origin of the energy shifts turned out to be orientation-dependant charge transfers between the molecules and the metal electrode.

Chapter 5 shows that it is possible to induce a change of molecular orientation of selectively dissociate a chemical group from 4-aminobenzenethiolate molecules on Cu(110) by tuning the energy of tunnel electrons injected from an STM tip into the molecules.

Chapter 3 contains contributions from Prof Maki Kawai, Dr. Yousoo Kim and Dr. Sylvain Clair. Additionally, Chapter 3 includes courtesy images from Mr; Masafumi Hon and Dr. Satoshi Katano. Chapter 4 contains contributions from Prof Maki Kawai, Dr. Yousoo Kim, Dr. Yasutaka Takata, Dr, Jun Miyawaki, Prof Shik Shin and Mr. Shinji Doi. Chapter 5 contains contributions from Prof Maki Kawai, Dr. Yousoo Kim and Dr. Lee Hyun-Sook. The main contribution of the full thesis is the work of Caroline Rabot.