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Menno Bokdam (promotion date: 15 November 2013)

CHARGE TRANSFER AND REDISTRIBUTION AT

INTERFACES BETWEEN METALS AND 2D MATERIALS

Promotion date: November 15.

Promotor: Prof.dr. Paul J. Kelly

Assistant promotor: Dr. Geert Brocks

The discovery of graphene in 2004 has led to a new HYPE. The prospect of HighlY Planar Electronics is an appealing view for Nanotechnology. Graphene - a conducting, one atom thick carbon sheet with peculiar electronic characteristics - is one of

the primary building blocks for very thin planar electronic components. One can think of resistors, transistors, diodes and planar capacitors as multilayers consisting of graphene and other 2D materials.

Considering that these components would be very thin and that the in-plane xy-dimensions are much larger than the z-dimension, it is not strange to assume that the interfaces between the different materials in the multilayer are important factors. In this thesis the physical processes controlling these interfaces are studied.

The primary method that we used to study these systems is Density Functional Theory (DFT). The second method is to use phenomenological models that give a generalized picture of a complex system to interpret the results of the DFT calculations.

Three types of interfaces are investigated, metal|organic, metal|2D-insulator and 2D-insulator|graphene interfaces.

In experiment, constant work function levels (pinning levels) are measured after deposition of the same molecules on different metal surfaces. The transfer of electrons between the metal surface and the molecular layer affects the work function of the molecule-covered surface. A planar capacitor model is constructed and together with DFT calculations it is able to accurately predict pinning levels.

A graphene field effect device is studied: a graphene|hexagonal Boron-Nitride (h-BN) multilayer on a copper surface. The charge transfer between the graphene sheet and the copper surface through the ultra-thin h-BN film is studied.

A deeper look at the origins of the large potential steps occurring at weakly interacting metal|insulator interfaces is given. Also the interface effects at the interfaces between weakly bonded 2D materials are studied

Was your theoretical research application driven?

Yes, graphene is one of the most promising materials for nanotechnology. For its discovery Andre Geim and Konstantin Novoselov won the Nobel Prize in 2010. Promising applications of graphene are its use as a high frequency transistor and as a transparent electrode. Because the mobility of the charge carriers is very high, less heat is produced during operation, saving a lot of energy.

In order to make functional 2D graphene structures, metal|isolator|graphene multilayers will be needed. Using DFT calculations and phenomenological models we determined the behavior of the electrons at these planes and interfaces. The understanding of the physical processes at these interfaces is of prime importance for experimental graphene research. It makes prediction of graphene properties in devices possible. I hope my work will give them more insight and will inspire them to make these kinds of structures.

Did you publish on some of those?

For example an article on graphene field effect devices consisting of graphene, Boron-Nitride multilayers and a copper surface, is published in Nano Letters, showing tunneling phenomena. Recently, experimentalists in Switzerland actually found the phenomena as described here. The predicted behavior was clearly observed.

Also I had publications in: Physical Review B, Applied Physics Letters and Organic Electronics.

Are you a different kind of scientist now, compared to four years ago when you started your PhD project?

Now I am regularly questioning the relevance of the theoretical work. This I do by listening and discussing with experimentalists and colleagues on conferences, in meetings and during daily collaborations. We strive to be of added value in the front row of this promising field of research. This is obliged even more so as a large part of this research was funded by the European FP7 project: Monitor, in which different groups from other universities and knowledge institute collaborate.

What, in your opinion, is important for Mesa+ to stay successful in future?

During conferences I experienced the good reputation Mesa+ has internationally. I guess a research theme considering 2D materials involving graphene can be adopted by different groups within Mesa+, each bringing in their own expertise and research approaches. In my opinion, more collaboration between groups is possible on such an exciting research topic. A lot of chances are within reach here, that I am sure of!