Materials Science and Technology of Polymers at the University of Twente

Surface dynamics of organic layers explored by scanning probe microscopy techniques

Hairong Wu thesis cover: Surface dynamics of organic layers explored by scanning probe microscopy techniques

Organic thin layers, especially self-assembled monolayers (SAMs), on well-defined solid surfaces have attracted tremendous attention owing to their interesting physical and chemical behavior as well as potential applications. The aim of this PhD thesis was to study the structural properties, electronic properties and dynamical process of SAMs. The objective was achieved by using scanning probe microscopy techniques, like time-resolved scanning tunneling microscopy (TR STM) and electrochemical atomic force microscopy (EC-AFM), with high spatial resolution and time resolution. Employing a current-voltage (I-V) converter with a large bandwidth, current-time spectroscopy, while the feedback loop disabled, can be recorded (hereafter referred as I-t traces). In this way, a high temporal resolution for STM can be achieved. This makes STM a versatile tool for the characterization of the structure, electronic properties and dynamics of SAMs. Recently developed peak force tapping AFM enables one to measure soft biological samples with high spatial resolution and minimal damage for the sample. With the combination of peak force tapping mode in EC-AFM, morphological responsiveness of redox-active metalloprotein azurin molecular layers were investigated.

The motivation and the introduction of the topics are presented in Chapter 1.

In Chapter 2, a literature overview of SAMs and their dynamics as studied by TR STM were described. Firstly, fundamental aspects, potential applications, growth mechanisms and structure of SAMs on Au(111) were introduced. Secondly, the basics and spectroscopic methods of STM were briefly introduced. More importantly, developments of the temporal resolution of STM and its application in studying the dynamics of SAMs were surveyed. Based on TR STM measurements, energetics and dynamics of decanethiol SAMs on Au(111) surfaces were ascribed in Chapter 3. It was revealed that the massive dynamics of the decanethiol SAMs were due to diffusion of decanethiol-Au complexes, rather than the diffusion of individual decanethiolate molecules. In addition, the boundary free energies between the disordered and order phases were studied using a statistical analysis of the thermally induced meandering of the domain boundaries. On the basis of these results, it is possible to accurately predict the two-dimensional phase diagram of the decanethiolate/Au(111) system.

Chapter 4 focused on the ordering and dynamics (including phase transition and conformational changes) of monothiol oligo(phenylene ethynylene) (termed as OPE hereafter) SAMs and dithiol OPE SAMs. It was revealed that the monothiol OPE molecules at the edges of the vacancy lines exhibited dynamic behavior and frequently jump back and forth between neighboring stripes. I-t traces recorded on dithiol OPE SAM on Au(111) suggested that the molecules continuously switch back and forth between two nearly degenerate configurations.

In Chapter 5, current-voltage (I-V) spectroscopy and current-distance (I-Z) spectroscopy were applied to study the transport properties of a single octanethiol molecule in the temperature range from 77 K to room temperature. The conductance of octanethiol is temperature independent, demonstrating that either quantum mechanical tunneling or ballistic transport is the main transport mechanism.

In Chapter 6, stimuli responsiveness (via changing the electrochemical potential) of the redox-active metalloprotein Cu-azurin on Au(111) surface was investigated by in-situ EC-AFM. It was revealed that the height of the Cu-azurin can be modulated by the surface potential.

Finally, in the Outlook, directions for future research were provided. For instance, TR STM could be employed to study dynamics of mixed SAMs, which is under debate to date. Another future direction, related to the dynamics of organic layers on surfaces, would be to improve the time resolution of AFM. The bulk part of this Thesis describes the dynamic of SAMs from a true molecular perspective by using TR STM. Deep insights discovered in the dynamics of the SAMs, down to the single molecular level, were obtained. In addition, monitoring molecular dynamics of responsive systems with TR STM, EC-AFM was also attempted with success to investigate the responsiveness, for instance the redox responsiveness of redox active molecules like azurin. We documented that by single molecular imaging, electrochemical potential stimulated apparent height changes can be resolved. Our work in this thesis further enhances knowledge of dynamics of SAMs and other molecular systems, which will find applications in molecular devices.