By Josh Perry, Editor
Scientists from Nagoya University (Japan) have developed a new segregation method for creating monolayers of post-graphene 2-D materials, specifically germanium, which is easier than the standard process and creates more uniform layers.
A structural model of the germanium grown with the new process. (Nagoya University)
According to a report from the university, the common practice for growing germanium monolayers is molecular beam epitaxy, but it is problematic. It requires precise temperature control of the environment, has a long preparation period, and the layers can become contaminated with oxides from the substrate.
In the new process, researchers used an ultra-high vacuum to prevent oxidation. They covered a disk of germanium with a 60-nanometer film of silver atoms with a standard evaporation technique. When the sample was heated to 500°C, the germanium atoms dissolved into the silver and when the sample was cooled to room temperature the atoms formed a layer or germanium on the surface of the solution.
“The growing process is gentler and much more ordered than the evaporation technique,” the article explained, “and the germanene grows in a ‘carpet-like’ manner, meaning that it is able to grow over ridges formed by multiple silver layers underneath, so the germanene can extend over huge areas - the team's sample grew to around 10 millimeters square.”
Although the germanium layer has some surface protrusions, the researchers called it “good quality” and are confident that even higher quality is possible with more study.
The research was recently published in ACS Nano. The abstract read:
“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.”