Growth and characterization of silicene, germanene and hexagonal boron nitride

In this thesis the properties and synthesis of silicene and germanene were addressed. ‘Because of their position in the periodic table, we think of them as “graphene’s cousins”, Adil Acun says. ‘The discovery of graphene, inspired the exploration of other 2D-materials: silicon and germanium are elements with similar features as carbon.’  

Notable differences were found in the structural and electronic properties of silicene and germanene versus graphene. After that - following the same reasoning - hexagonal boron-nitride was proposed as a graphene-like material.

 The properties and synthesis of these materials were addressed in this thesis as well. ‘The two-dimensional (2D) materials of interest possess two sub-lattices and a honeycomb structure,’ Adil says. ‘These ingredients together with respective pz-orbitals overlapping in the two sub-lattices, account for fascinating properties in these so-called Dirac materials.’  

Hexagonal boron-nitride is an insulator that may act as a buffer layer of 2D-materials. The two sub-lattices are comprised of two different elements, namely boron and nitrogen. This is leading to different properties, of which the most important ones are the wide band gap (5.9 eV) and its chemical inertness.

‘By using highly specialized (in fact: unique) experimental equipment, we were able to follow the growth of h-BN on Ir(111) real-time,’ Adil says. ‘The large size of the h-BN islands we obtained, is promising for epitaxially grown graphene, silicene or germanene on top of h-BN. This encouragement is a major deliverable of this thesis for future research and promising applications.’

Among the experimental methods used in this PhD work, Adil mentions LEEM (Low Energy Electron Microscopy).

‘The University of Twente started with this experimental setup already in 2006, with the great vision of Professor Bene Poelsema,’ Adil says. ‘Now Mesa+ owns one of the, I suppose, ten LEEM setups present worldwide. We are the only one within nanotechnological research, being able to follow materials grow processes real-time. The importance of the top Nano-lab facilities and expertise in Twente, cannot be emphasized enough.’


The growth of h-BN on Ir(111) was studied also with LEED (using electron diffraction) and PEEM (using electron photoemission). The study of growth of silicene on Ag(111) was performed in-situ, using LEEM and µLEED. In studying the growth of germanene, STM (Scanning Tunneling Microscopy) was used. ‘During the first part of my PhD project, I visited the École Polytechnique Fédérale de Lausanne,’ Adil says. ‘During this exchange program I was privileged to learn about the state-of-the-art of this experimental domain.’

Adil also collaborated with the University of Tokyo, to learn about the theoretical framework on the 2D materials involved in this project, by dr. Motohiko Ezawa. In Radboud University Nijmegen, Adil collaborated with Spinoza laureate professor Misha Katsnelson, being an expert on 2D graphene.

‘During the PhD I gained more and more confidence,’ Adil says. ‘In Twente, bachelor and master students in Applied Physics gain a deeply rooted base of knowledge, allowing them to mature quickly into academic research, as well as developing the soft skills needed. In my case, I was happy to present my results in international conferences.’

Academic career

The work on germanene and silicene was published in papers such as Condensed Matter and Applied Physics Letters, leading to several thousands of downloads and top-rated amounts of scientific citations.

Adil likes to pursue an academic career. ‘At the moment I am busy writing a Rubicon proposal,’ he says. ‘Studying molecular spintronics - which is highly relevant for DNA molecules as well - at Berkeley University would be a major challenge. Also I worked as a post-doc within the Physics of Interfaces and Nanomaterials Group, in order to transfer knowledge to the PhD candidate continuing my research topics.’

Adil: ‘Another feature of academic work I like very much is education. To teach contributes to put one’s own work into perspective, being alert on developments and forcing oneself to explain your findings. Also the social cohesion aspects, speaks to me. As for my immigrant origin, I was given the opportunity to develop myself by education. So, the chances are there to be taken. I would like to contribute to that.’