Conductance and Surface-Enhanced Raman Scattering of Single Molecules Utilizing Dimers of Nanoparticles.

摘要

In the past few years, the field of molecular electronics and in particular, the development of new techniques for contacting and measuring single molecules, has emerged, providing new insights into this field. The relatively small size of a molecule, typically of the order of 1 nm, requires innovative approaches in order to develop functioning single-molecule devices. The experimental methods currently used for single-molecule measurements provide little control over the number of molecules bridging the gap or the local electronic properties of the metal-molecule contact. In this thesis, a new approach for contacting a single molecule is presented that provides better control of these parameters. Our method is based on synthesizing a dimer structure consisting of two gold colloids connected by a thiol group to either side of the molecule. This structure is then positioned between two electrodes by electrostatic trapping and, thus, the conductance of the molecule can be measured.;The fundamental questions addressed by the field of molecular electronics are as follows: "what is the conductivity of a junction containing an individual molecule and how is it affected by the molecule's specific structure?" We were able to shed some light on these questions by studying the electrical conduction through three short organic molecules that differ by their degree of conjugation. We will consider here a fully conjugated molecule, 4,4'-biphenyldithiol (BPD), Bis-(4-mercaptophenyl)-ether (BPE), in which the conjugation is broken at the center by an oxygen atom, and 1,4-benzenedimethanethiol (BDMT), where the conjugation is broken near the contacts by a methylene group. We found that the presence of localizing groups such as the oxygen in the BPE and the methylene groups in the BDMT suppresses the electrical conduction dramatically, relative to the conjugated molecule, BPD. A unique feature of the BPD molecule is the appearance of reproducible, pronounced peaks in its conductance at certain voltage values. The position of peaks in the spectrum was affected by the electrostatic environment, resulting in random gating.;In view of the above developments, my thesis focuses on surface-enhanced Raman scattering (SERS) measurement of single molecules. Single-molecule spectroscopy is an emerging field that provides detailed information on molecular response, which is unavailable in measurements performed on an assembly of molecules. The obvious problem, however, in implementing most spectroscopic techniques, such as Raman scattering, is the very weak signal obtained from a single molecule. Interestingly, the Raman signal from a molecule has been shown to increase dramatically when the molecule is adsorbed to metal particles of certain types having sub-wavelength dimensions [1, 2]. This enhancement technique, known as surface-enhanced Raman scattering, can increase the Raman signal by as much as 14--15 orders of magnitude, which has been shown to be sufficient for performing single-molecule spectroscopy successfully.;Dimer structures are not only attractive for conductance measurements on single-molecule devices; they could also serve as an efficient antenna system that greatly enhances the electromagnetic field at the center of the dimer, where the molecule resides. Dimers provide a basic experimental model for studying the fundamentals of the SERS enhancement, which are not well understood. Dimers have the advantage of possessing a small gap (on the order of a nanometer) that is beyond the limit of today's sophisticated lithography techniques. By utilizing the dimer structures that contain a Rhodamine 123 molecule, we were able to resolve some fundamental questions regarding the SERS enhancement mechanism. The issue of how the nanoparticles' surface plasmon properties affects the SERS enhancement was addressed both experimentally and by calculations. Moreover, it was predicted by our calculations that when the dimers consist of large nanoparticles, a non-uniform enhancement of the different molecular modes of Rhodamine 123 should occur. This was also observed experimentally where specific peaks of the SERS spectrum were more pronounced than others.;I will begin this thesis with an introduction to the field of molecular electronics; I will review the use of different molecular clips and I will briefly describe some theoretical approaches and the experimental methods used for single-molecule measurements. I follow this by introducing the field of surface-enhanced Raman spectroscopy of single molecules. More specifically, I will describe the mechanism underlying the Raman enhancement and its relation to surface plasmons of nanoparticles. The introduction ends with a detailed description of the dimer approach and its potential use for performing transport and SERS measurements of single molecules. The results are composed of two parts: the first is related to transport through single molecules, including characterization of dimers and their contacts to the electrodes as well as transport measurements through single molecules. The second part relates to the SERS originating from dimers embedding a single molecule. Next, I will discuss the distribution of the SERS intensity as a result of the different orientations of the dimers with respect to the laser polarization. In the final chapter of this work, I will discuss the dependence of the SERS intensity on the size of nanoparticles, and explain the results using calculations of the near-field in the dimer's junction.