MESA+ University of Twente
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Tjeerd Bollmann (promotion date: 23 September 2011)

Escape from Flatland: strain and quantum size effect
driven growth of metallic nanostructures

Promotion date: 23. September 2011

Promotor: Prof. dr. ir. Bene Poelsema & prof. dr Joost Frenken. (Universiteit Leiden)

Assistant promotor: dr. Raoul van Gastel

In this thesis, we show the influence of and subtle balance between Quantum Size Effects (QSE) and strain stabilizing interactions on the growth as well as on structural and electronic properties of nanostructures.

We present a Low Energy Electron Microscopy (LEEM) and Low Energy Electron Diffraction (µLEED) study in combination with Tensor LEED calculations illustrating the relevance of the QSE for quantization of island heights and ultimate film structures for Pb and Bi on Ni(111).

We show distinct QSE driven preferred heights (3,5,7 and 9) for the growth of Pb/Ni(111), apparent from height transitions. For the allotrope Bi the ultimate surface structure is determined by the QSE as well. Alloying results in intercallated BiNi9 nanowires, that lead to meandering steps during dealloying.

Large flat top Pb islands of 40 layers in height decay within 1-10 ms at about 526 K to form compact 3D structures. Their decay is induced by a thermal instability in the bi-domain wetting layer, leading to an energetic balance that is tipped in favor of the surface free energy.

As an example of strain dominated growth, we studied the morphology and structure of Cu on W(100), showing strong 3D growth of hut shaped Cu crystallites with steep facets. The boundary between the Cu crystallite and the pseudomorphic adlayer can be explained by the complete cancellation of shear stress.

Was there a special moment during the thesis project that you recall?

Using Low Energy Electron Microscopy is quite special. Worldwide not very many researchers use this kind of equipment. Although the resolution is not on an atomic level, the big advantage of video-rate imaging opens new research areas.

Using this tool, I was very surprised observing the extraordinary behaviour of Pb, forming compact 3D structures at about 526 K. The unanticipated behaviour surprised us. By verifying many times in different experiments I managed to filter out all doubts.

The search for an exact explanation of this phenomenon lasted very long though. Nine months before the end of the thesis project, we succeeded in linking the observed phenomena to the models and calculations we developed. This led to a nice publication in Physical Review Letters.

It was one of the successes of this project, sponsored by FOM. Also the QSE driven surface structures for Bi on Ni(111) were published in PRL.

I think my research is inspiring for follow-up research.

How did you develop personally, as a researcher and scientist during the PhD project?

I think it is a good thing to sometimes be left on your own for a while as a researcher. In doing so I learned to work much more independently, not acting as a passive researcher anymore. Also in everyday life I apply this attitude, paying attention in a critic manner in dealing with complex processes. For me as a person, I experience this as a big achievement.

What are your future plans?

Right now I am working as a post-doc researcher at the University of Osnabrück. Here non-contact Atomic Force Microscopy is at the centre of my work. In this way I enlarge my skills as an academic researcher even more. I like to stay in university research, as I am very interested in fundamental science, having the freedom to continue further and deeper when interesting phenomena appear, not being restrained by fixed targets coming from outside.

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

I really liked the Mesa+ days. It is a great way of enjoying the bond the researchers in different fields have in common. One can communicate in an interdisciplinary way easily and getting to know the various instrumentation used, sometimes bringing new ideas for your own research as well.