Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.

We have used low energy electron microscopy to directly visualize the formation and stability of silicene layers on a Ag(111) substrate. Theoretical calculations call into question the stability of this graphene-like analog of silicon. We find that silicene layers are intrinsically unstable against the formation of an “sp3-like” hybridized, bulk-like silicon structure. The irreversible formation of this bulk-like structure is triggered by thermal Si adatoms that are created by the silicene layer itself. To add injury to insult, this same instability prevents the formation of a fully closed silicene layer or a thicker bilayer, rendering the future large-scale fabrication of silicene layers on Ag substrates unlikely.

The electronic and energetic properties of the elementary building block, i.e. a five-membered atom ring (pentagon), of the Ge(110) surface was studied by scanning tunneling microscopy and spectroscopy at room temperature. The Ge(110) surface is composed of three types of domains: two ordered domains ((16x2) and c(8x10)) and a disordered domain. The elementary building block of all three domains is a pentagon. Scanning tunneling spectra recorded on the (16x2), c(8x10) and disordered domains are very similar and reveal three well-defined electronic states. Two electronic states are located 1.1 eV and 0.3 eV below the Fermi level respectively, whereas the third electronic state is located 0.4 eV above the Fermi level. The electronic states at -0.3 eV and 0.4 eV can be ascribed to the pentagons, whilst we tentatively assigned the electronic state at -1.1 eV to a Ge-Ge back bond or trough state. In addition, we have analyzed the straight [1-12] oriented step edges. From the kink density and kink-kink distance distributions we extracted the nearest neighbor interaction energy between the pentagons, which exhibit a strong preference to occur in twins, as well as the strain relaxation energy along the pentagon-twin chains.

The temperature dependence of the density of states of germanene, synthesized on Ge/Pt crystals, has been investigated with scanning tunneling spectroscopy. After correction for thermal broadening, a virtually perfect V-shaped density of states, which is a hallmark of a two-dimensional Dirac system, has been found. In an attempt to directly measure the energy dispersion relation via quasiparticle interference we have recorded spatial maps of the differential conductivity near the edges and defects of germanene. Unfortunately, we did not find any sign of Friedel oscillations. The absence of these Friedel oscillations hints to the occurrence of Klein tunneling.

The ionic component of the strong bond in hexagonal boron nitride (hBN) has been grossly disregarded in literature. Precisely this quantity is demonstrated to govern the shape of monolayer hBN islands grown at high temperatures. HBN zigzag edges are charged and energetically less favorable than the neutral armchair edges, in contrast to those of the purely covalent graphene. Nucleation of hBN islands occurs exclusively on either the inner or the outer corners of substrate steps. Taking into account the charge at edges of hBN islands offers a powerful framework to understand the nucleation of the islands and their orientation with respect the founding steps, as well as various equilibrium shapes, including prominently a rightangled trapezoid. BN dimers are identified as basic building blocks for hBN. A surprisingly strong interaction between hBN and the pre-existing steps on the moderately reactive Ir(1 1 1) substrate is uncovered. Localized charges are probably relevant for all 2D-materials lacking inversion symmetry.

Variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are performed on heptathioether β-cyclodextrin (β-CD) self-assembled monolayers (SAMs) on Au. The β-CD molecules exhibit very rich dynamical behavior, which is not apparent in ensemble-averaged studies. The dynamics are reflected in the tunneling current-time traces, which are recorded with the STM feedback loop disabled. The dynamics are temperature independent, but increase with increasing tunneling current and sample bias, thus indicating that the conformational changes of the β-CD molecules are induced by electrons that tunnel inelastically. Even for sample biases as low as 10 mV, well-defined levels are observed in the tunneling current-time traces. These jumps are attributed to the excitations of the molecular vibration of the macrocyclic β-CD molecule. The results are of great importance for a proper understanding of transport measurements in SAMs.

No abstract

No abstract