Tuning Electron Transport in Metal Films and Graphene with Organic Monolayers
Promotion date: December 4.
Promotor: Prof. dr. ir. Wilfred van der Wiel
Introducing organic molecules into electronic devices has attracted significant research interest due to its promise in both technological development and in fundamental research. Organic molecules are used for many purposes such as uniform and patterned doping of silicon with donor atoms, and the modification of the electronic properties of graphene. However, they were not used for magnetic doping of metals before.
In this thesis transition metal complexes are used as magnetic doping agents. This led to a novel molecular fabrication method for inserting isolated localized magnetic moments in a thin gold film with tunable density and with high reproducibility. This molecular spin doping technique can be used for further understanding impurity/electron, impurity/impurity interactions.
The method also enabled tuning the Kondo effect by capping the monolayers with a thin layer of gold. The Au-capping by e-beam evaporation increased the observed Kondo effect slightly, whereas capping by magnetron sputtering resulted in much significant enhancement. These results imply that it is possible to change the strength of impurity/host electron interactions, by tuning the deposition parameters of capping Au-layers.
The molecular complexes with the same metal containing group as the ones used for Au-doping, were also used to modify electronic properties of graphene. The monolayers were inserted between at the graphene/substrate interface. It was observed that the monolayers induced p-type doping of the graphene and resulted in a hysteresis effect. It was shown that the hysteresis effect in the molecular-layer/graphene system can be used to make memory devices.
Was your research fundamental in nature or also application oriented?
The initial parts of my thesis research was aimed at fundamental research. The interaction of the spin of conduction electrons with the spin of an isolated impurity was investigated. To investigate such interactions, we developed a new method to fabricate a system for doping of a metal with isolated magnetic impurities.
Our method consisted of inserting paramagnetic organic molecules to act as spin dopants into a thin layer of gold, such that we built an organic/inorganic hybrid system. It is interesting to study the hybrid electronic organic/inorganic systems due to their promise for downsizing electronic devices. Organic molecules are easy to process, cheap, and can be chemically tailored to gain various functionalities. They are employable on nanoscale, as they are small and flexible.
Our method not only enabled controlled and tunable doping, it also enabled us to change the strength of impurity/host electron interactions.
In the final part, besides fundamental research, application related aspects were involved as well. The fundamental aspects were aimed at investigating how the electrical properties of graphene are affected by a monolayer inserted at the graphene/substrate interface. The characterization of the electronic properties revealed that memory devices can be built from the monolayer/graphene systems, which was more application oriented.
Did you collaborate with colleagues from other groups?
I collaborated closely with many colleagues from my group, with other groups in the university and from abroad. From the university, we collaborated with the Molecular Nanofabrication Group. They had great contribution to this work with their advanced expertise on molecular synthesis and self-assembly processes This thesis would not be realized without their efforts.
Our collaboration with the Medical Cell Biophysics Group gave important insight in the effect of Au-deposition on the molecules. In addition, we collaborated with the University of Exeter. Our colleagues there grew the graphene, transferred and patterned it. From this work we were able to characterize the monolayer/graphene structures and built graphene memory devices from there.
What are your future plans?
Currently I am working as a post-doc researcher within the NanoElectronics Group. This project is supported by Toyota. The aim of the project is to fabricate multiple graphene layers to be used for energy storage applications. The first step is to develop a procedure to produce the graphene itself. We will use two different approaches for this: chemical vapor deposition and atomic layer deposition.
I choose to further develop my skills in a post-doc position, and after this perhaps in some other academic projects. It is my wish to eventually return to Turkey to be closer to my family, and to bring back my expertise by contributing to academic research in my home country.
After these four years of thesis research, my knowledge based on thin film growth, patterning and device fabrication techniques are much broader now. I found the chance to gain experience in working with organic compounds in terms of building self-assembled monolayers and characterizing them. I developed skills to perform structural, chemical, electrical and magnetic characterization and perform data analysis. Especially the low temperature electronic measurements techniques I got to learn, are of great value for my future job, I believe. I am sure to be able to use these skills in my research work in Turkey one day.
By collaborating with various colleagues within Mesa+ and from abroad, I am now used to work with a multidisciplinary attitude within research projects. Also I learned to extract the right information from literature much more efficiently now when compared to the starting of my PhD project. Also my writing and presentation skills have improved.
In what journals were you able to publish your results?
One article was published in Nature Nanotechnology, and another one in Nanotechnology. One article - which was not directly related to this thesis - was also published in Nanotechnology. Another article about the results of the final chapter is under preparation.
Did you feel part of the Mesa+ research community during your PhD project?
Yes, I definitely did. I had collaborations with other groups of Mesa+ and worked with very experienced and friendly people. I extensively used the facilities in the cleanroom and laboratories of the Molecular Nanofabrication Group. When problems arose people were always willing to help, and they came up with creative solutions. Discussions with colleagues had fruitful results.
I liked the weekly team meetings and our group meetings. In the meetings we discussed about draw-backs and exchanged ideas to overcome problems.
For Mesa+ it is important, I believe, to keep up the strategy of combining scientific research and industry applications arising from there. This is both good for come up with products from there and contribute to society, as well as for getting more appealing results out of scientific research.