Publikationsserver der RWTH Aachen University
Electron transport studies - an electrochemical scanning tunneling microscopy approach
Abstract
dc:descriptionOur group works in the field of electrochemical surface science – a combination of approaches, characteristic for electrochemistry and physical surface science. The work presented here describes the application of scanning tunneling microscopy (STM) and electrochemical techniques to investigate the electron transfer (ET) in various systems. The experimental results are accomplished by the theoretical description of the studied processes, well known as well as derived in the frame of existing collaborations. An introduction, a description of principles and procedures of experiments are presented in chapters 1-3. Chapter 4 describes a basic electrochemical scanning tunneling microscopy (EC STM) experiment. Au(111)-(1x1) substrates were employed in all studies. The substrate was modified by ionic adlayers, formed by the coadsorption of Cu2+ with SO42- or Cl-. Steady state STM images were recorded and used to evaluate the structure of the respective adlayers. Transitions between different adlayers were followed in time resolved STM experiments. Current-distance scanning tunneling spectroscopy (STS) was used to probe the tunneling response of bare Au(111)-(1x1), Cu2+/SO42- and Cu2+/Cl- adlayers. The I-z curves exhibit characteristic responses of the bare and|or adsorbate-modified adlayer. Chapters 5-7 describe measurements of the conductance of single atomic and molecular junctions employing the STM “stretching” techniques in three systems. It consists of bringing an STM tip in contact with a metal surface or adlayer and monitoring the current through the junctions upon retraction of the tip. Stable quantum point contacts (QPCs) or single molecular junctions (SMJs) result in constant current plateaus in the I-z curves. They are than analyzed statistically to extract the conductance of single junctions. Chapter 5 describes the stretching experiments with Au QPCs. The obtained results are in good agreement with existing experimental and theoretical data. However, they also demonstrate new properties of Au QPCs, such as an electronic shell effect and a correlation between electrical and mechanical properties of Au QPCs. Chapter 6 describes stretching experiments in an archetypal molecular system. SMJs are formed with alkanedithiols (SnS) chemically bound to two Au electrodes via thiol (SH) linkers. Detailed experiments are carried out with 1,8-octanedithiol (S8S). Electrochemical properties and the structure of the self-assembled monolayers (SAMs) formed by S8S are in agreement with other results. Two types of SAMs – a densely packed adlayer of vertically oriented molecules or a layer of flat lying molecules – may be formed upon variation of the assembly conditions. The S8S SAMs were used as substrates in the stretching experiments. They demonstrated, that the S8S conductance is unaffected by the variation of the environment, but the probability of SMJ formation is enhanced by the presence of molecules in solution. The latter condition was used to explore the chain length dependence of the SnS conductance. The careful investigation demonstrated three types of SMJ formed for each SnS molecule. Complementary ab initio simulation (F. Evers, A. Bagrets, Forschungszentrum Karlsruhe) demonstrated that the variability of the SMJ conductance is caused by the interplay of the Au-SH contact and to an intrinsic conformational degree of freedom of the SnS molecules. Chapter 7 presents conductance studies with SMJs formed by Au and 4,4'-bipyridine (44BP) – a molecule with two pyridyl rings. The latter is an intrinsically conjugated, highly conductive anchoring group. It binds via the formation of a donoracceptor bond between a lone electron pair of the N atom and an empty s-orbital of Au. Four different types of SMJs, each having a potential-independent conductance, were found. The overall range of conductance values covers more than 2 orders in magnitude. Complementary ab initio simulations of the conductance in Au-44BP-Au SMJs demonstrated a much stronger effect of contact geometry and molecular conformation (i.e., tilt angle between two pyridyl rings), compared to the case of SnS. The latter is closely related to the nature of the donor-acceptor coupling between the pyridyl ring and the Au leads. Finally, the results of experiments on the ET enhancement employing the redoxactive molecules (N-hexyl-N'-(6-thiohexyl)-4,4'-bipyridinium) were analyzed (chapter 8). The current-voltage electrochemical scanning tunneling spectroscopy (EC STS) experiments carried out were reported elsewhere (Zhihai Li, Ph.D. thesis, RWTH Aachen, 2007). The theoretical model of A. Kuznetsov and J. Ulstrup adopted by me, was employed to describe the observed phenomena. A very good agreement between experimental data and predictions of the model was found. The applicability and limitation of the model and of the corresponding experiments were discussed.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Pobelov, Ilja Vladimirovic
- Contributors dc:contributor
-
- Simon, Ulrich
Subjects
dc:subject × 17- info:eu-repo/classification/ddc/540
- Rastertunnelmikroskopie
- Elektrochemie
- Molekularelektronik
- Festkörperoberfläche
- Monoschicht
- Freies Molekül
- Chemie
- elektrochemische Rastertunnelmikroskopie
- Ladungstransport
- atomare Systeme
- molekulare Systeme
- scanning tunneling microscopy
- electrochemistry
- molecular electronics
- charge transport
- single molecules
Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng