Structure-property relationship of 2D material modifications

The recent years have seen an explosive growth in the variety of new low-dimensional materials with fascinating properties and a diverse application potential. For this proposal, two-dimensional materials are at the focus of interest, in particular graphene but also other two-dimensional materials such as molybdenum disulfide, niobium diselenide, hexagonal boron nitride, mono-layer bismuth strontium calcium copper oxide (BSCCO), and related systems. In the ideal case, many of these materials are expected to have very unique electronic, mechanical, chemical and thermal properties. However, modifications such as doping, amorphization, functionalization, or combinations (heterostructures) of 2-D materials will be needed to tailor and fine-tune their properties, defince active areas, structure or join them into electronic circuits, or in some cases may simply be inevitable as a result of synthesis and processing. Moreover, new 2-D structures that have no 3-D counterpart might be derived by a modification of existing 2-D materials. This project aims to make significant advances in understanding the structure-property relationships and control of material modifications at the atomic level. For this purpose, we will join several atomic-resolution local probes which in their combination will allow us to determine structural, electronic, optical and mechanical properties at the level of individual defects, functional groups, dopants or adsorbates. We will explore the use of atom-sized electron beams for local modifications e.g. to create vacancies, incorporate dopants, or to introduce structural defects in pre- defined patterns. On the same structures we will employ electron energy loss spectroscopy, the analysis of charge redistribution from direct images, as well as scanning tunneling microscopy and spectroscopy in a novel double- probe configuration in order to assess their electronic, optical or mechanical properties at the same, atomic level, resolution. Overall, this project will bring significant advances in the understanding of these materials, in the control over properties by structural modifications, and new routes to their application.

In this project we investigated modifications of 2D materials on the atomic level. Individual atoms were removed from the crystal lattice, individual impurity atoms were introduced, or the arrangement of bonds was changed without changing the number of atoms. The direct observation of such structures on the atomic level became possible only recently and was enabled by new developments in electron microscopy. In particular, it has also become possible to also observe the dynamics of atomic configurations on the atomic level. For example, the difffusion of a vacancy in graphene with two missing atoms could be followed over many steps (Nat. Commun., 5, 3991 (2014), http://dx.doi.org/10.1038/ncomms4991), an understanding for the displacement of silicon atoms in the graphene lattice was obtained (Phys. Rev. Lett., 113, 115501 (2014), http://dx.doi.org/10.1103/PhysRevLett.113.115501), and even the formation of two-dimensional silicon carbide was captured on video (Scientific reports 7, 4399 (2017) http://dx.doi.org/10.1038/s41598-017-04683-9). However, the thus formed 2D silicon carbide samples are rather small, consisting of only about 10 atoms. Furthermore, hybrid systems made from multiple low dimensional materials were studied: A single layer of carbon (graphene) together with a single layer of boron nitride for example was observed to form a wavy structure instead of remaining flat and strictly two-dimensional (Nano Letters 17, 1409 (2017) http://dx.doi.org/10.1021/acs.nanolett.6b04360). In a sandwich structure from graphene and fullernes, the diffusion of individual fullerenes could be observed, and even their rotation could be detected however this rotation was blocked when the fullerenes merged into larger objects due to extended electron irradiation (Science Advances 3, e1700176 (2017) http://dx.doi.org/10.1126/sciadv.1700176). The graphene sandwich provides a nanoscale reaction chamber that allows the observation of molecular dynamics in the transmission electron microscope. Therefore it is expected that this work also opens many new avenues for studying the structure and dynamics of other molecules similarly encapsulated in the 2D space between graphene sheets.