image: After cooling, germanium atoms float out of solution with the silver film: atoms that float upwards first settle near the corners of the hexagons of silver atoms on the surface. Then when enough germanium atoms are present on the surface, they form a sheet of germanene. view more
Credit: Junji Yuhara
Summary
In contrast to graphene (carbon), which is the best-known 2D material, flat, pure sheets of silicon (silicene), tin (stanene) and germanium (germanene) – "post-graphene" materials – are expected to easily exhibit properties of a topological insulator (specifically, Quantum Spin Hall insulator). This class of materials are electrically insulating in their interiors but have highly conductive surfaces and edges. A single-layered topological insulator is likely to be an ideal wiring material for nanoelectronics. Moreover, due to their "buckled" structure (meaning from side-on they appear zig-zagged, as if two separate hexagonal honeycomb lattices were bonded together), the "post-graphene" materials have a tunable band gap, so they could be the semiconductors of the future.
Up till now, production of germanene and the other post-graphene materials has been fraught with difficulties due to the complexity of the conventional process, which uses evaporation. In the conventional technique, atoms of the post-graphene material are evaporated onto a suitable substrate, which requires highly precise control of numerous parameters including evaporator temperature, evaporation time, sample temperature during and after deposition, and so on. Even then, for a uniform, single layer to be deposited is largely a matter of luck.
Now, a group led by Nagoya University's Junji Yuhara has solved the problem by using annealing and a novel approach for getting the germanium atoms to grow as a monolayer, called a "segregation method". The experiments were performed by Yuhara and his undergraduate student, Hiroki Shimazu. First, in an ultra-high vacuum – used to prevent oxidation of the surface – they covered a relatively thick disk of germanium with a 60 nanometre film of silver atoms using the conventional evaporation technique. They then simply heated the sample to 500 °C. It turns out that germanium atoms dissolve into silver at this temperature, much like sugar is better able to dissolve into hot water. Then they cooled the sample to room temperature and the germanium atoms come out of solution, forming a layer of germanene on the top surface.
The growing process is gentler and much more ordered than the evaporation technique, 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 Yuhara team's sample grew to around 10 millimetres square. The production of germanene with high crystalline quality is expected to be scalable: Junji Yuhara believes that one germanium substrate can be used to grow one million flat germanene sheets the size of a 10 cm diameter disk. This could indeed herald the advent of a new generation of electronics.
In Brief
Next generation electronics require a tenfold decrease in size and increase in energy efficiency. Pure monolayer materials theoretically predicted to be topological insulators are currently a promising candidate for achieving these goals. Initially, graphene, the first and best-known 2D material, had shown promise, and it still might prove to be useful. However, in the past five years, the so-called "post-graphene" materials – flat, pure sheets of silicon (silicene), tin (stanene) and germanium (germanene) – have appeared increasingly attractive for future electronics applications. The reason is two-fold. First, the presence of a strong spin-orbit interaction makes these materials likely to be topological insulators (specifically, Quantum Spin Hall insulator). In graphene this property is difficult to observe. These materials are electrically insulating in their interiors but have highly conductive surfaces and edges. A single-layered topological insulator is likely to be an ideal wiring material for nanoelectronics. Second, their "buckled" structure (meaning from side-on they appear zig-zagged, as if two separate hexagonal honeycomb lattices were bonded together) alters their electronic properties so the "band gap" – the energy difference between the valence and conduction bands – can be easily tuned, so the materials could be the semiconductors of the future. The recent explosive increase in research into post-graphene materials, as well as graphene.
While graphene is easy to produce (you can do it with a pencil "lead" at home), making the post-graphene materials has proved to be very difficult. The standard technique of Molecular Beam Epitaxy, whereby, say, germanium atoms from a source are heated and evaporated directly onto a clean crystal substrate, causing a thin film to be deposited, is fraught with difficulty. First, the wrong substrate harms the formation of the ultrathin layer. Second, the process requires a long preparation sequence and control of numerous experimental parameters. For example, the target substrate temperature has to be kept low to prevent the silicon, germanium or tin atoms from evaporating away from the surface or dissolving into the target substrate. The ultrathin layer easily become multilayered, uneven and contaminated with oxides or other substances. For a uniform, single layer to be deposited is largely a matter of luck.
Now, a group led by Nagoya University's Junji Yuhara has solved the problem by using annealing and a novel approach for getting the germanium atoms to grow as a monolayer, called a "segregation method". The experiments were performed by Yuhara and his undergraduate student, Hiroki Shimazu. First, in an ultra-high vacuum – used to prevent oxidation of the surface – they covered a relatively thick disk of germanium with a 60 nanometre film of silver atoms using the conventional evaporation technique. They then simply heated the sample to 500 °C. It turns out that germanium atoms dissolve into silver at this temperature, much like sugar is better able to dissolve into hot water. Then they cooled the sample to room temperature and the germanium atoms come out of solution. Some of the germanium atoms return to the germanium substrate while others float upwards and form a layer of germanene on the top surface.
After cooling, germanium atoms float out of solution with the silver film: atoms that float upwards first settle near the corners of the hexagons of silver atoms on the surface. Then when enough germanium atoms are present on the surface, they form a sheet of germanene.
The growing process is gentler and much more ordered than the evaporation technique, 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 Yuhara team's sample grew to around 10 millimetres square.
A scanning tunneling electron microscope (STM) image of the germanene layer is shown in Figure 3. The layer is not quite pure. Interestingly, regular arrangements of atoms – probably germanium with a dangling bond – appear on the germanene: besides hexagonal groups arranged in a diamond shape, pairs of these atoms are also arranged in a hexagon, with each pair rotated by 60 degrees relative to a pair on an adjacent corner, perfectly matching the silver Ag(111) crystalline periodicity over a long range – reminiscent of the hexatic phase in systems of two-dimensional hard disks. One could speculate that since no long-range interaction is known to exist in the germanene layer, the phenomenon could be due to jostling of neighbouring germanium atoms in thermal motion transmitting a torque over a long distance, similar to the 2D hard-disk systems in the hexatic phase.
While not free of the surface "protrusions", the germanene layer is of good quality. Its carpet growth ability is good reason to believe that production of germanene with high crystalline quality is scalable: indeed, Junji Yuhara believes that one germanium substrate with a thickness of 0.5 nm can be used to grow one million flat germanene sheets the size of a 10 cm diameter disk, if a technique can be found to transfer them off the substrate. This could indeed herald the advent of a new generation of electronics.
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The article, "Germanene Epitaxial Growth by Segregation through Ag(111) Thin Films on Ge(111)" was published in ACS NANO, published by American Chemical society at
DOI: 10.1021/acsnano.8b07006
Junji Yuhara1, Hiroki Shimazu1, Kouichi Ito1, Akio Ohta1, Masaaki Araidai1,2,3, Masashi Kurosawa1,2, Masashi Nakatake4, Guy Le Lay5
1Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan
3Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8601, Japan
4Aichi Synchrotron Radiation Center, Knowledge Hub Aichi, Seto, Aichi 489-0965, Japan
5Aix-Marseille Université, CNRS, PIIM UMR 7345, 13397 Marseille Cedex, France
Journal
ACS Nano