Optical Setup

For the study of the photochromic molecules presented in Chapter 8, a multi-mode optical fiber is installed in the cryostat via a fiber vacuum feed-through (see Fig. 4.3). The installed fiber is shown in Fig. 4.4(a). Because the optical absorption spectra of the molecules differ from each other and depend on the isomeric state of the species, a palette of different wavelengths has to be used. Therefore, a programmable high power (maximum 40 mW/cm²) LED source containing 365, 442, 550 and 630 nm is connected. The light spot on the sample has a diameter of about 1 mm. The LED spectroscopy is shown in Fig. 4.4(b).

The in-situ optical switching measurement under light irradiation was examined using Au-octanedithiol-Au molecular junctions for the first step in order to know the stability and to examine other effects by lights. As presented in Fig. 4.5, there is no strong effect of visible light, whereas there is a significant change under UV light. The UV light may heat the junctions and degrade the molecules. Increase of conductance under UV light was not observed in non-photoactive molecules. In photochromic molecular junctions (e.g. Au-4Py-Au), the reversible conductance switching was observed for a single period under UV and visible-light irradiation as shown in Fig. 4.6. Under UV light, the conductance increased about two times, and the conductance decreased back with visible light. However, this effect was not repeatedly observed, and cannot be discussed completely in the framework of this thesis.

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Figure 4.3. Schematic diagram of the electronic and optic set-up used for the in-situ optical switching measurements. Custom assembled multi-mode optical fiber is installed inside of cryostat through vacuum feed-through (VFT).

Figure 4.4. (a) A MCBJ system and optical fiber installation in a custom designed cryostat.

White box indicates the position where the single molecule sample is mounted. The junctions head for downward and the light is irradiated from the bottom. (b) The light spectra of source (LED) measured with minimum power of light. These four light sources are selectively exposed depending on the absorption properties of switching molecules.

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Figure 4.5. Au-Octanedithiol-Au junctions were exposed under (a) 630 nm and (b) 365 nm wavelength at 4.2 K while measuring conductance at a fixed contact distance. D and O indicate the dark state without light and the light on of each light, respectively. The light power at the end of optical fiber was 0.05 mW/cm². Under visible light irradiation (630 nm), the conductance does not change significantly, but the noise enhances. Under UV light irradiation (365 nm), the conductance decreases due to perhaps the change of configurations on metallic atoms.

Figure 4.6. The in-situ conductance switching of a Au-4Py-Au molecular junction at 230 K.

The conductance increased and decreased under UV and visible light, respectively. This effect was not observed repeatedly because the junctions were degraded under UV light while decreasing the conductance. This switching effect was not observed below 200 K. We assume that there is a temperature gating effect for the switching mechanism. In order to clarify this, the reversible switching measurements as a function of temperature is required.

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Chapter 5

Conductance and Vibrational States of Single-Molecule Junctions controlled by Mechanical

Stretching and Material Variation

This work has been published as Y. Kim, H. Song, F. Strigl, H.-F. Pernau, T. Lee, and E.

Scheer, Phys. Rev. Lett. 2011, 106, 196804. We reproduce here a slightly adapted manuscript.

The aim of this set of experiments is to reveal the role of the contacting electrode metal and the molecular conformation onto the transport properties of single molecule junctions. For that purpose, we chose the well-studied model molecule hexanedithiol (HDT), because it is known to provide a rather well-defined conductance with robust chemical bonds to gold and a prominent conformational change known from earlier experiments and theoretical studies.

The contact configuration is changed by stretching the MCBJ electrodes. The influence of the metal is studied by comparing junction of the same molecule, but one time contacted with gold electrodes, one time with platinum electrodes. This chapter presents inelastic electron tunneling spectroscopy (IETS) measurements carried out on HDT single molecule contacts at low temperature. Under stretching the alkanedithiol molecular junctions, both the molecular conformation and the contact geometry can be changed, and these effects were expected to influence the linear conductance by more than one order of magnitude. As explained in Chapter 2, the changes of the molecular conformation, the contact geometry and the molecule bonding influence the molecular vibrational modes and the metal-phonon modes are thus reflected in the IETS signals. By combining IETS with mechanical control and electrode material variation, we show that the metal electrodes influence the transport properties in a double manner, by virtue of their electrical and their mechanical properties. The electrical properties determine the bonding strength and the linear conductance, the mechanical properties are important for the molecular conformations and thus indirectly also for the transport properties. The mechanical strain of different electrode materials can be imposed onto the molecule, opening a new route for controlling the charge transport through individual molecules.

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5.1 Introduction

Extensive studies on charge transport through single molecules have been performed for the implementation of molecular-scale devices, as well as with the objective of understanding how the molecules are able to carry charges in such devices as discussed in Chapter 2 [12, 13, 26, 30, 97, 98]. The conductance of the same single molecule contacted with a mechanically controllable break-junction (MCBJ) or with a scanning tunneling microscope (STM) is reported to have various values because the contact geometry and the molecular conformation may vary as shown in Chapter 2. To understand precisely such dependences, more sophisticated experimental studies are required, in spite of the complexity of those measurements. In such single-molecule devices, both the contact geometry and the material of electrodes (e.g., gold (Au) or platinum (Pt)) can significantly influence the charge transport through the single molecule, although it is specifically anchored by functional end groups (e.g., thiol (-SH)) [95, 99-101]. Alkanedithiol is one of the most appropriate candidates to study these properties, owing to its simple and flexible structure with the -bonding (see also Fig. 2.20 in Chapter 2). Imbedded into a junction it can adopt the usual trans conformation as well as a defect conformation. A well-known defect is given by the so-called gauche conformation, which is predicted to give rise to alterations of the charge tunneling [27, 40, 73, 102]. To understand such manifold behavior of a single-molecule junction, inelastic electron tunneling spectroscopy (IETS) has been introduced as a powerful tool, which is very sensitive and applicable for detecting vibrational excitation in solid-state molecular devices as introduced in Chapter 2 [25, 26, 30, 34, 98]. Here IETS measurements for 1,6-hexanedithiol [SH-(CH₂)₆-SH, denoted as HDT] molecules when stretching both Au and Pt MCBJs used as adjustable electrodes at low-temperature are discussed. The signature of the different molecular conformations and contact geometry are demonstrated by the appearance of particular IETS signals as well as by changes in the conductance.