Study of Two-Dimensional Materials with Honeycomb Geometry
Honeycomb structures have already fascinated mankind since ancient times. They were observed in various natural occurring phenomena, from the structure of the beehive of the honeybee that granted the structure its name, to the inner structure of butterfly wings, bones, and insect eyes. The honeycomb structure found many applications in engineering and design. Halfway last century it was predicted that atomic honeycomb structures have special electronic properties. With the rise of graphene in the last decade, many of these properties have been shown, and many new questions have risen. This has also sparked an interest in artificial honeycomb structures with length scales in the nanometer regime. This thesis describes the quest to structurally resolve honeycomb systems, both on the atomic and on the nanometer scale. The first part of this thesis deals with graphene: a honeycomb lattice of carbon atoms. The graphene is grown epitaxially on an iridium (111) surface by means of chemical vapor deposition. The structure of this epitaxial graphene is studied using atomic force microscopy (AFM), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Various contrast patterns observed with AFM could be explained in terms of tip reactivity and tip-sample distance. The lattice mismatch between graphene and iridium (111) gives rise to a moire pattern and physical buckling of the graphene. This physical buckling is quantified using LEED, and local variations therein are studied by AFM. The buckling of the graphene is then used as a model system to study the influence of non-homogeneous backgrounds on the image mechanism of AFM using CO molecule modified tips. The work on graphene is concluded with a chapter on the electronic properties of nanometer-sized graphene islands. Using STM and scanning tunneling spectroscopy (STS) it was shown that the charge carriers in the graphene islands behave as relativistic particles confined in a 2-D box. The second half of the thesis deals with honeycomb structures prepared from semiconductor nanocrystals. First, it describes a binary superlattice with a novel crystal structure, prepared by self-assembly of PbSe and CdSe nanocrystals. Strictly speaking the structure is not a honeycomb, but it consists of stacked layers of PbSe nanocrystals with a kagome structure, i.e., hexagons connected by triangles, sandwiched by layers of CdSe nanocrystals. The full crystal structure is resolved by electron tomography, showing the value of this technique for nanocrystal research. Secondly, this part of the thesis describes the formation of atomically coherent honeycomb structures formed by oriented attachment of PbSe nanocrystals. The atomic and nanoscale crystal structure is resolved by a combination of high-angular annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron tomography, electron diffraction (ED), STM, and small-angle X-ray scattering (SAXS). From these techniques it was found that the honeycomb structure is buckled – the two sublattices have a different height – and exhibits octahedral symmetry. Then cation exchange was used to transform the honeycomb structures into CdSe, with a zinc blende atomic lattice. Finally, the thesis is concluded with an outlook into future research directions and possible applications of the presented research.