Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7√7 × 7√7)R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate “(1.35 × 1.35)”R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and low buckling within 0.01 nm. As this 0.39 nm value is in good agreement with the theoretical lattice constant of free-standing germanene, conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure.

of a band gap in its electronic structure is difficult, these novel 2D materials are expected to possess sizeable band gaps, making them typically suitable for electronic applications. Furthermore, theory predicts that they are robust topological insulators, even up to about room temperature (RT) and above for germanene and stanene.In experiment, these artificial 2D materials were firstly realized in situ by molecular beam epitaxy (MBE) for silicene in 2012, [1,2] germanene in 2014, [3] stanene in 2015, [4] and finally plumbene in 2019. [5] One of the next targets is the growth of in-plane heterostructures because abrupt lateral interfaces promise controlled heterojunction functionalities. Lateral heterostructures of graphene and hexagonal boron nitride (h-BN) were grown on Cu(110) in 2014. [6] First-principles calculations on the electronic properties of graphene quantum dots embedded in monolayer h-BN sheets typically indicate that the h-BN band gap shrinks upon increasing the diameter of the graphene dots. [7] Besides lateral graphene/h-BN heterostructures, fabrication of several in-plane heterostructures of transition metal dichalcogenides has also been reported. [8] Very recently, vertical heterostructures have been prepared by boron intercalation underneath Group 14 elemental post-graphene materials receive much attention because of their outstanding properties, typically, as robust 2D topological insulators. Their heterostructures are a main target in view of disruptive applications. Here, the realization of striking in-plane lateral heterostructures between germanene and stanene are shown, which are sustainable 2D Ge-and Snbased graphene analogs, but with a strong intrinsic spin-orbit coupling. A unique combination of atomic segregation epitaxy (ASE) and molecular beam epitaxy (MBE) for the in situ continuous fabrication of nearly atomically precise lateral multijunction heterostructures, respectively, consisting of atom-thin germanene and stanene on a Ag(111) thin film is used. Scanning tunneling microscopy (STM) observations at atomic scale and low-energy electron diffraction testify that germanene and stanene sheets without intermixing are prepared simultaneously on the same terraces at wide scale: tin and germanium atoms neither exchange their sites nor adsorb on the germanene and stanene sheets. The atomic structure of the boundary between germanene and stanene is derived from STM images, while scanning tunneling spectroscopy reveals key electronic features at the heterojunction. This innovative synergetic approach of ASE and MBE growths offers great flexibility for the realization of unprecedented lateral 2D heterostructures.

No abstract

No abstract