Transport Properties of Methyl-Terminated Germanane Microcrystallites.

Germanane is a two-dimensional material consisting of stacks of atomically thin germanium sheets. It's easy and low-cost synthesis holds promise for the development of atomic-scale devices. However, to become an electronic-grade material, high-quality layered crystals with good chemical purity and stability are needed. To this end, we studied the electrical transport of annealed methyl-terminated germanane microcrystallites in both high vacuum and ultrahigh vacuum. Scanning electron microscopy of crystallites revealed two types of behavior which arise from the difference in the crystallite chemistry. While some crystallites are hydrated and oxidized, preventing the formation of good electrical contact, the four-point resistance of oxygen-free crystallites was measured with multiple tips scanning tunneling microscopy, yielding a bulk transport with resistivity smaller than 1 Ω·cm. When normalized by the crystallite thickness, the resistance compares well with the resistance of hydrogen-passivated germanane flakes found in the literature. Along with the high purity of the crystallites, a thermal stability of the resistance at 280 °C makes methyl-terminated germanane suitable for complementary metal oxide semiconductor back-end-of-line processes.

Account of the diversity of tunneling spectra at the germanene/Al(1 1 1) interface.

Despite the wealth of tunneling spectroscopic studies performed on silicene and germanene, the observation of a well-defined Dirac cone in these materials remains elusive. Here, we study germanene grown on Al(1 1 1) at submonolayer coverages with low temperature scanning tunneling spectroscopy. We show that the tunnelling spectra of the Al(1 1 1) surface and the germanene nanosheets are identical. They exhibit a clear metallic behaviour at the beginning of the experiments, that highlights the strong electronic coupling between the adlayer and the substrate. Over the course of the experiments, the spectra deviate from this initial behaviour, although consecutive spectra measured on the Al(1 1 1) surface and germanene nanosheets are still similar. This spectral diversity is explained by modifications of the tip apex, that arise from the erratic manipulation of the germanium adlayer. The origin of the characteristic features such as a wide band gap, coherence-like peaks or zero-bias anomalies are tentatively discussed in light of the physical properties of Ge and AlGe alloy clusters, that are likely to adsorb at the tip apex.

Resolving the Controversial Existence of Silicene and Germanene Nanosheets Grown on Graphite.

The highly oriented pyrolytic graphite (HOPG) surface, consisting of a dangling bond-free lattice, is regarded as a potential substrate for van der Waals heteroepitaxy of two-dimensional layered materials. In this work, the growth of silicon and germanium on HOPG is investigated with scanning tunneling microscopy by using typical synthesis conditions for silicene and germanene on metal surfaces. At low coverages, the deposition of Si and Ge gives rise to tiny and sparse clusters that are surrounded by a honeycomb superstructure. From the detailed analysis of the superstructure, its comparison with the one encountered on the bare and clean HOPG surface, and simulations of the electron density, we conclude that the superstructure is caused by charge density modulations in the HOPG surface. At high coverages, we find the formation of clusters, assembled in filamentary patterns, which indicates a Volmer-Weber growth mode instead of a layer-by-layer growth mode. This coverage-dependent study sets the stage for revisiting recent results alleging the synthesis of silicene and germanene on the HOPG surface.