To date germanene has only been synthesized on metallic substrates. A metallic substrate is usually detrimental for the two-dimensional Dirac nature of germanene because the important electronic states near the Fermi level of germanene can hybridize with the electronic states of the metallic substrate. Here we report the successful synthesis of germanene on molybdenum disulfide (MoS2), a band gap material. Pre-existing defects in the MoS2 surface act as preferential nucleation sites for the germanene islands. The lattice constant of the germanene layer (3.8 ± 0.2Å) is about 20% larger than the lattice constant of the MoS2 substrate (3.16Å). Scanning tunneling spectroscopy measurements and density functional theory calculations reveal that there are, besides the linearly dispersing bands at the K points, two parabolic bands that cross the Fermi level at the Γ point. The discovery that graphene, a single layer of sp 2 hybridized carbon atoms arranged in a honeycomb registry, is stable has resulted in numerous intriguing and exciting scientific breakthroughs [1,2]. The electrons in graphene behave as relativistic massless fermions that are described by the Dirac equation, i.e. the relativistic variant of the Schrödinger equation. One might anticipate that elements with a similar electronic configuration, such as silicon (Si), germanium (Ge) and tin (Sn), also have a "graphene-like" allotrope. Unfortunately, silicene (the silicon analogue of graphene), germanene (the germanium analogue of graphene) and stanene (the tin analogue of graphene) have not been found in nature and therefore these two-dimensional (2D) materials have to be synthesized. Theoretical calculations have revealed that the honeycomb lattices of the "graphene-like" allotropes of silicon and germanium are not fully planar, but slightly buckled [3,4]. The honeycomb lattices of these 2D materials consist of two triangular sub-lattices that are slightly displaced with respect to each other in a direction normal to the honeycomb lattice. Despite this buckling the 2D Dirac nature of the electrons is predicted to be preserved [3,4]. Another salient difference with graphene is that silicene and germanene have a substantially larger spin-orbit gap than graphene (<0.05 meV). Silicene's spin-orbit gap is predicted to be 1.55 meV, whereas the predicted spin-orbit gap of germanene is even 23.9 meV. This is very interesting because graphene and also silicene and germanene are in principle 2D topological insulators and thus ideal candidates to exhibit the quantum spin Hall effect. The interior of a 2D topological insulator exhibits a spin-orbit gap, whereas topologically protected helical edge modes exist at the edges of the material [5,6]. The two topologically protected spin-polarized edge modes have opposite propagation directions and therefore the charge conductance vanishes, whereas the spin conductance has a non-zero value.In the past few years various groups have successfully synthesized silicene [7][8][9] and germanene [10-13] on a variety of substrates. T...

The structural and electronic properties of germanene coated Ge2Pt clusters have been determined by scanning tunneling microscopy and spectroscopy at room temperature. The interior of the germanene sheet exhibits a buckled honeycomb structure with a lattice constant of 4.3 Å and a buckling of 0.2 Å. The zigzag edges of germanene are reconstructed and display a 4 periodicity. The differential conductivity of the interior of the germanene sheet has a V-shape, which is reminiscent of the density of states of a two-dimensional Dirac system. The minimum of the differential conductivity is located close to the Fermi level and has a non-zero value, which we ascribe to the metallic character of the underlying Ge2Pt substrate. Near the reconstructed germanene zigzag edges the shape of the differential conductivity changes from a V-shape to a more parabolic-like shape, revealing that the reconstructed germanene zigzag edges do not exhibit a pronounced metallic edge state. 2In the past decade a new class of materials has been developed, which is not threedimensional (3D), but two-dimensional (2D) in nature. Graphene is by far the most famous example of this new class of 2D materials [1,2]. Graphene consists of a single layer of sp 2 hybridized carbon atoms that are arranged in a planar honeycomb registry. Graphene is a very appealing material because of its unique physical properties [1,2]. The charge carriers in graphene behave as relativistic massless particles that are described by the Dirac equation, i.e. the relativistic variant of the Schrödinger equation. In the vicinity of the Dirac point the dispersion relation is linear, i.e., where F v is the Fermi velocity,  the reduced Planck constant and k the wave vector. Graphene is a semimetal and the density of states scales linearly with energy. One of the interesting properties of finite graphene sheets is the existence of electronic states that are localized at the edges of graphene. Theory predicts that a zigzag terminated graphene edge is metallic, whereas an armchair terminated graphene edge is semiconducting [3][4][5]. Scanning tunneling microscopy and spectroscopy studies of zigzag and armchair monatomic step edges of graphite have indeed confirmed these theoretical predictions [6][7][8].Since the rise of graphene there has been a growing interest in other two-dimensional materials that exhibit 'graphene'-like properties. The most obvious alternatives for graphene are the group IV elements, i.e. silicon, germanium and tin. Unfortunately, these graphene analogues of silicon (silicene), germanium (germanene) and tin (stanene) do not occur in nature and therefore these materials have to be synthesized. Germanene is one of the youngest members of the graphene family and has not been studied extensively. In contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled. Theoretical calculations have shown that despite this buckling the 2D Dirac properties of germanene are preserved [9] [13]. In particular the work of Dávila et al. [12] provides a...

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.

As a two-dimensional semiconductor with many physical properties, including, notably, layer-controlled electronic bandgap and coupled spin-valley degree of freedom, monolayer MoSe2 is a strong candidate material for next-generation opto- and valley-electronic devices. However, due to substrate effects such as lattice mismatch and dielectric screening, preserving the monolayer’s intrinsic properties remains challenging. This issue is generally significant for metallic substrates whose active surfaces are commonly utilized to achieve direct chemical or physical vapor growth of the monolayer films. Here, we demonstrate high-temperature-annealed Au foil with well-defined (100) facets as a weakly interacting substrate for atmospheric pressure chemical vapor deposition of highly crystalline monolayer MoSe2. Low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a honeycomb structure of MoSe2 with a quasi-particle bandgap of 1.96 eV, a value comparable with other weakly interacting systems such as MoSe2/graphite. Density functional theory calculations indicate that the Au(100) surface exhibits the preferred energetics to electronically decouple from MoSe2, compared with the (110) and (111) crystal planes. This weak coupling is critical for the easy transfer of monolayers to another host substrate. Our study demonstrates a practical means to produce high-quality monolayers of transition-metal dichalcogenides, viable for both fundamental and application studies.

We report an investigation of the electronic inhomogeneities in a single germanene layer grown on a molybdenum disulfide (MoS 2) substrate. Using scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of the Dirac point of germanene. The Dirac point maps reveal the presence of charge puddles in the germanene sheet. The Dirac point varies from À30 meV to þ15 meV, corresponding to a charge density in the puddles in the range of 2.6 Â 10 À3 electrons to 6.6 Â 10 À4 holes per nm 2. The radius of these puddles is about 10-20 nm, resulting in a total charge of the order of one charge carrier per puddle. The defect concentration in the top layer of the MoS 2 substrate is very comparable to the density of charge puddles, suggesting that the charge puddles are caused by the charged defects in the top layer of the MoS 2 substrate.