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Public defence Lijie Zhang

structural and electronic properties of one- and two- dimensional systems studied by scanning tunneling microcopy 

The research of low-dimensional electron systems has received quite some attention during the last two decades. This thesis deals with the synthesis and characterization of metal-induced nanowires on Ge(110) surface and germanene, a novel two-dimensional material, which can be considered as the germanium analogue of graphene. Experiments of Au- and Pt- induced nanowires on Ge(110) are presented in this thesis. Inspired by the Pt/Ge system we have exploited a novel way to synthesize germanene by using the formation of the eutectic phase. This eutectic phase is composed of 22% Pt and 78% Ge. Upon cooling the eutectic undergoes spinodal decomposition into a Ge2Pt and a pure Ge phase. Furthermore, the growth of germanene on a band gap material – MoS2 – has been presented in the final part of this thesis.

In chapter 1, a brief introduction of the low-dimensional electron systems has been given. The experimental details have been addressed in chapter 2.

In chapter 3, we have demonstrated the properties of the bare Ge(110) surface and Au-induced nanowires. After several cycles of cleaning the bare Ge(110) surface displayed several reconstructions, which are all composed of pentagons i.e. five-membered atom rings. In our case, there are three different reconstruction domains i.e. 16x2, c(8x10) and a disordered phase. Three well-defined electronic states are extracted from the normalized differential conductive spectrum. Two of the states, located near the Fermi level, respectively (0.4 eV and -0.3 eV), are related to the pentagons. The third state has been tentatively ascribed to the Ge-Ge back bond. Subsequently, we have studied the structural and electronic properties of Ge(110) after the deposition of a sub-monolayer amount of Au. Upon Au deposition and annealing nanowires are formed that are aligned along the ‘ridges’ of the unreconstructed Ge(110) surface (this direction is referred to as the [1-10] direction). Scanning tunneling spectroscopy measurements recorded on these nanowires reveal that the nanowires are semiconducting, suggesting that the nanowires are composite of Ge.

In chapter 4, we have studied the structural and electronic properties of Pt-induced nanowires on Ge(110) surfaces by scanning tunneling microscopy and low energy electron microscopy. The deposition of a sub-monolayer amount of Pt and subsequent annealing at 1100 (±30) K results into nanowires which are aligned along the densely packed [1-10] direction of the Ge(110) surface. With increasing Pt coverage the nanowires form densely packed arrays with separations of 1.1 ± 0.1 nm, 2.0 ± 0.1 nm, and 3.4 ± 0.1 nm. Ge pentagons reside in the troughs for nanowire separations of 3.4 nm, however, for smaller nanowire separations no pentagons are found. Spatially resolved scanning tunneling spectroscopy measurements reveal a filled electronic state at -0.35 eV. This electronic state is present in the troughs as well as on the nanowires. The -0.35 eV state has the strongest intensity on the pentagons, which is in line with the results of the bare Ge(110) surface as discussed in the previous chapter. For Pt depositions exceeding two monolayers, pentagon-free nanowire patches are found, that coexist with Pt/Ge clusters. Upon annealing at 1050 K these Pt/Ge clusters become liquid-like, indicating that we are dealing with eutectic Pt0.22Ge0.78 clusters. Low energy electron microscopy videos reveal the formation and spinodal decomposition of these eutectic Pt/Ge clusters.

In chapter 5, we suggest that the Pt/Ge clusters emerged in the previous chapter, are actually Ge2Pt crystallites. The outermost of the crystalline exhibit a honeycomb structure with a nearest-neighbor distance of 2.5 ± 0.1 Å. This honeycomb lattice is composed of two hexagonal sub-lattices that are displaced vertically by 0.2 Å with respect to each other. The interior of the germanene sheets displays a V-shaped density of states indicative of a 2D Dirac system. Based on our experimental observations we propose that the outermost layer of the Pt/Ge crystals is a germanene layer. However, the density of states at the minimum of the V-shaped curve does not completely vanish, which we ascribe to the underlying metallic Ge/Pt substrate. Two types of step edges are found on the germanene sheets: straight reconstructed zigzag step edges with a 4x periodicity and rough step edges. Both types of step edges have a parabolic shaped dI/dV curves.

In chapter 6, we have synthesized germanene on a band gap material: MoS2 and investigated the electronic properties of germanene. The germanene islands preferentially nucleate at pre-existing defects of the MoS2 surface. Germanene's lattice constant is about 20% larger than that of MoS2 and the angle between the two lattices is 0 degrees. The density of states of the germanene layer exhibits a well-defined V-shape around the Fermi level, which is one of the hallmarks of a two-dimensional Dirac system. These experimental observations are in very good agreement with state-of-the-art DFT calculations.