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Avijit Kumar (promotion date: 26 September 2013)

The Art of Catching and Probing Single Molecules

Promotion date: September 26.

Promotor: Prof.dr. Harold Zandvliet

The thesis presents several studies performed in the field of “molecular electronics” using scanning tunneling microscopy (STM) and spectroscopy. We have investigated electronic (transport) properties of a single octanethiol, a single CuPc molecule and molecular assemblies of β–cyclodextrin (β-CD) molecules immobilized on Pt/Ge(001), Au/Ge(001), and Au(111) substrates, respectively.

A single octanethiol molecule is trapped between an STM tip and Pt/Ge(001) substrate, and the transport properties of this junction are investigated. The junction acts as a molecular switch which can be opened and closed by controlling the gap between the electrodes and voltage applied across them. The threshold field for the attachment/detachment was found to be 4-6 x 109 Vm-1.

We have also studied CuPc molecules adsorbed in a “molecular bridge” configuration on Au-induced nanowires on a Ge(001) substrate, where the core, Cu2+ ion, of the CuPc molecule was decoupled from its environment. The charging of the core takes place at sample biases larger than 3.5 V as revealed from the STM images. Further, our STM measurements on a monolayer of β–CD molecules self-assembled on the Au(111) surface indicate a very rich dynamical behavior of single β–CD molecules.

This dynamics is induced by electrons that tunnel inelastically and they are not observed in macroscopic molecular junctions. This indicates that great care must be taken when interpreting transport measurements data obtained using macroscopic molecular junctions.

Above studies corroborate that molecular electronics is very interesting from the fundamental and application point of view as we witness phenomena such as switching, charging and dynamics for various molecular junctions.

What made your approach special?

Using the combination of scanning tunneling microscopy together with spectroscopy, allowed us to simultaneously look, probe and change individual molecules and their properties. This new way of research was fundamental but can have practical implications, as this way of contacting with molecules is quite new. Quantum-mechanical behavior plays an important role on this level. We were able to make a molecular switch using an octanethiol molecule which is driven by electric field.

Did you manage to have some important articles published?

Publications appeared in Small, Journal of Physical Chemistry C, Applied Surface Science, Journal of Chemical Physics and Langmuir.

How did you develop personally, as a researcher and scientist?

I am more objective, meaning I developed a good, healthy habit of questioning things if necessary. By doing so I am more confident about the knowledge base I work from. As an experimentalist I now have much more experience in performing new experiments in an ultra-vacuum environment. Troubleshooting problems with the equipment is a tricky task. This is a tricky job as the equipment is very fragile inside though it looks robust from the outside. I learned to work with the equipment in a careful and independent way, making good use of the technical support and professionalism of my colleagues. Last but not least, I am more open to new ideas and perspectives of research. Here also, the discussions and collaboration with colleagues is very important. Theory and experimental work rely upon each other more and more these days.

What are your future plans?

I prefer to stay in academics. I like the freedom and life style and working environment more than in industry. Here I worked before. The freedom is less and consumer driven research is very different from the topics and research culture that exists for example at Mesa+.

Can you tell something more about the way you experienced this Mesa+ culture?

During my internship in France, I experienced how big and well known an organization Mesa + really is, everybody knowing this institute and the way people are working on research there. The great thing I believe is, that as a PhD you are learning to do all the research steps yourself. The equipment and support necessary, is available. This saves lots of time. I experienced this in my efforts to independently perform my sample preparation. When doing so, you are aware if ambiguity or flaws in the sample could be present. This positively contributes to the way I performed the experimental measurements, with greater awareness.