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Thursday 14 September 2017, 14:30, Prof.dr. G. Berkhoff-zaal

PhD defence Adil Acun

The growth and characterization of silicene, germanene and hexagonal boron nitride

In this thesis various 2D-materials have been described. The two-dimensional materials of interest possess two sub-lattices and a honeycomb structure. These ingredients together with respective pz -orbitals overlapping in the two sub-lattices account for fascinating properties that are ascribed to Dirac materials. Graphene, silicene and germanene are examples of Dirac materials. Another example of a material with a comparable structure is hexagonal boron nitride. However, in this case, the two sub-lattices are comprised of two different elements, namely boron and nitrogen. Henceforth leading to different properties of which the most important ones are the wide band gap (5.9 eV) and its chemical inertness.

 

In Chapter 1 the instruments, the materials and the methods were introduced. LEEM (Low Energy Electron Microscopy) was used in order to visualize the growth of silicene and hexagonal boron-nitride real-time. Fur- thermore, the structural properties of both materials was investigated by using low energy electron diffraction. Measurements on the electronic properties of germanene were conducted by low temperature scanning tunneling microscopy accompanied with scanning tunneling spectroscopy. 

An extensive introduction to the electronic structure of 2D-materials was provided in Chapter 2. The nearest- neighbor tight-binding method was applied on pristine graphene. We showed that around the K and K’-points graphene is a semimetal of which the dispersion relation features linear properties. The band structure in the vicinity of the K and K’-points has been given the name Dirac cone. Next, we exploited the Dirac cone by using the long wavelength limit and found indeed a linear dispersion relation.  By including the theory of special relativity in the Hamiltonian  of the Schr¨odinger equation derived from the Dirac equation. Consequently, it was found that the electrons behave as massless fermions and have a Fermi velocity equal to the speed of light. The tight-binding method was applied on both zigzag and armchair graphene edges with various widths. The type of edge termination and the width significantly changed the electronic properties of the graphene edges.

Chapter 3 described the growth of silicene on Ag(111).   The study was performed in-situ with LEEM and➭LEED. Silicene islands nucleate and grow during the deposition of silicon. Before the silicene layer is fully closed a phase transition occurs from silicene to ”sp3-like” hybridized silicon upon continuing the deposition. In another experiment the phase transition was induced by increasing the substrate temperature. Both experiments clearly show that silicene is unstable. The instability of silicene on Ag(111) is problematic, since the Ag(111) was ought to be ideal due to the relatively small lattice mismatch between silicene and Ag(111). Furthermore, no indications for multilayer silicene formation were found in this study. 

Germanene’s debut is addressed in Chapter 4. A review and a STM-study are provided in this Chapter. The electronic structure of pristine germanene and bilayer germanene is discussed. An overview of experimental work considering the growth of germanene is mentioned. The fascinating properties of germanene arising from the large spin-orbit coupling is put in a perspective that could lead to phenomenal applications in future. In order to attain this possibility the pristine properties of germanene need to be achieved.  H-BN is proposed as a possible substrate retrieving the properties of freestanding germanene. 

The growth of h-BN on Ir(111) was studied with LEEM, PEEM and LEED as described in Chapter 5. Due to the two different elemental sub-lattices two distinct rotational domains of h-BN islands were observed that have clearly different shapes, e.g. trapezoidal and triangular.  The two distinct types of islands have small variations in their respective lattice parameters. Additionally, our PEEM study shows that the  yield  of  photo-emitted electrons is less for the triangular island. This indicates a lower interaction between the triangular islands and the Ir substrate, compared to the trapezoidal h-BN islands. The growth of h-BN on Ir(111) is limited by the sticking of the borazine molecules at the Ir(111) surface. H-BN was thermally disintegrated and upon cooling down new phases occurred that have a (6 × 2) superstructure. A tentative model for this elongated phase, probably borophene, was proposed. 

In summary, various 2D-materials ranging from graphene, silicene, germanene, hexagonal boron-nitride and borophene have been addressed in this thesis. The large size of the h-BN islands 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 applications.