Preparation and characterisation of two-dimensional ferroelectrics

Many two-dimensional materials are theoretically predicted to have remarkable properties, such as robust ferroelectricity for group-IV monochalcogenides. Ferroelectric materials exhibit a spontaneous polarisation in the absence of any external electrical field. Group-IV monochalcogenides consist of a metal (M = Ge, Sn) and a chalcogen (X = S, Se, Te) atom in equal parts. However, their investigation in terms of experimental preparation and characterisation is still scarce, even though such studies are the basis for all work and allow further insight into the growth process.
In this work, ultra-high vacuum chambers equipped with evaporators that allow molecular beam epitaxy were used to achieve highly controllable and clean environments. In addition, powerful tools like scanning tunneling microscopy (STM) and low electron energy diffraction (LEED) are used for crystallographic investigation with a focus on structure and shape determination of group-IV monochalcogenides. In particular, SnSe, GeSe, and SnS were investigated in this thesis.
Monolayer SnSe forms fractal-dendritic islands at room temperature on a graphene on Ir(111) surface. The shape is systematically studied during an annealing sequence. A change in shape could be observed and the equilibrium shape is reached at 570 K. This shape can be described as a rhombus consisting of only ⟨110⟩ edges. Due to coverage and island density not changing during this annealing sequence, it can be proven that no ripening process takes place, contrary to most growth processes of other two-dimensional materials. Desorption starts at around 600 K, which is indicated by a reduction in coverage and island density, until all islands vanish. A closer study of these edges, in terms of atomically resolved STM images and first-principle studies reveal an edge relaxation, which minimises the formation energy of such edges. Further investigations on the electronic structure show that this material is a semiconductor with a bandgap of 2.16 eV and the relaxation reduces the charge effect at the edges.
The formation of bilayer structures is highly present in this material and needs to be actively suppressed for monolayer investigations. This behaviour is also shown in a coverage study. However, no standard growth model can describe this tendency for bilayer preference. The study of bilayer SnSe reveals that multiple stacking orientations are possible, even when the islands grow together without clear boundaries. Electronic investigations show that an edge effect is present, which can not be explained by ferro- or antiferroelectric properties.
GeSe forms different amorphous structures on graphene substrates on multiple crystals under various conditions. This indicates that crystallisation is difficult. However, the crystallisation on Au(111) results in the out-of-plane ferroelectric β-polymorph. The growth can be described as self-limited since no second layer formation could be observed. Measurements reveal that the semiconductor, with a bandgap of 1.13 eV, expresses a hexagonal lattice with a complex superstructure.
The crystallisation of SnS can be achieved on various substrates, like graphene on Ir(111) and Au(111) presented here, as well as others in literature. First measurements aimed to experimentally determine the critical temperature with LEED measurements at elevated temperatures reveal significantly lower values than theoretically predicted.
This thesis demonstrates growth conditions of multiple group-IV monochalcogenide materials and focusses mainly on preparation and crystallographic properties. Despite the novelty and complexity of these materials, the presented results unveil promising properties and encourage for further in-depth investigations.