Ever since the initial synthesis of graphene kicked off the search, people have been aware of the potential advantages of finding new 2-D materials. Graphene is a layer of carbon atoms, bound together in a simple hexagonal structure. Carbon was already well-known for forming a fascinating array of chemical bonds; allotropes and fullerenes including C-60 – ‘Bucky-balls’ – having already been synthesised. But it was soon realised that 2-D allotropes of other elements could be formed, like silicene and stanene for silicon and tin respectively. The different properties of these atoms resulted in differing properties for their 2-D analogues.
Germanium was first discovered in 1886: although originally it didn’t see many applications industrially, as it was considered to be a poor electrical conductor, by the 1940s and 1950s, its useful optical and electronic properties as a semiconductor were recognised. Early transistors were often made of germanium – and many still are, although once it became easier to synthesise silicon with the appropriate levels of purity, the allure of germanium fell.
In 2014, a decade after graphene was first isolated at the University of Manchester, two different research teams were able to create germanene. This material is essentially the cousin of graphene; it consists of a single layer of germanium atoms in the characteristic hexagonal structure associated with 2-D materials. Graphene had originally been studied on metal plates in the 1960s before it was rediscovered, isolated, and characterised in 2004; this germanene was created by a European team through molecular beam epitaxy onto a gold substrate, while a Chinese team used platinum. In this process, individual atoms are deposited onto the substrate at very low pressures and high temperatures.
In many ways it has similar properties to graphene. Both are characterised by their high electron mobility – a measure of how much the electrons respond to applied electrical fields. Electron mobility is one of the key quantities required for a good semiconductor material; graphene has a carrier mobility in excess of 15000 cm2/Vs, which is around ten times that seen in silicon. Although germanene has not yet been refined to the same extent, theoretical calculations suggest that the intrinsic mobility – that which can be obtained by a ‘perfect’ material where mobility is limited only by phonons, or scattering from vibrations in the crystal lattice – is even higher for germanene than graphene.spin-orbit
Most of the differences between germanene and graphene arise due to the difference in the hexagonal structure. While graphene’s hexagonal crystal structure is flat, germanene’s crystal structure is buckled; its lattice consists of two vertically separated sub-lattices, unlike the graphene lattice which is confined to a plane. There are important electronic differences in the material properties that result from this. In a paper published in Nano Letters in 2012, before germanene had been synthesised, it was calculated that a bandgap could be opened in germanene if a vertical electric field was applied – meaning that it can be ideal for use as a field effect transistor; a crucial component in much of modern electronics. In addition to this, the fact that germanium has been used alongside silicon in semiconductors before suggests that it may be easier to integrate germanene into existing semiconductor circuits and applications.
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Another crucial difference arises due to the atomic properties of germanium. It exhibits very high spin-orbit coupling, which means there are distinct energy levels based on the spin of the electron. The spin-orbit gap in germanene is 24meV (milli-electron-volts) compared to less than 0.05meV for graphene. This means that germanene is a 2D material that may have real uses in the burgeoning field of quantum computing. One of the key components required to build a quantum computer is to obtain a quantum state, which can be used to encode information in the form of a qubit. The spin on the electron is a classic and well-documented quantum state, and it can potentially be measured and manipulated. This is a new form of electronics – spintronics – and germanene may well be a useful material in these efforts. The spin-orbit gap also makes it of interest to quantum condensed matter physicists: it can be used as an experimental material to observe the quantum-spin Hall effect at accessible temperatures.
Quantum-spin Hall states are not just of interest to theoretical and experimental physicists – although theoretical breakthroughs in this area did win the Nobel Prize recently. They are linked to a new phase of matter, topological insulators. You can describe a topological insulator as a material that is “an insulator on the inside, while conducting on the outside”. The bulk material doesn’t conduct electricity, but the outside is superconducting! Synthesising such materials could lead new kinds of electronics: it has been suggested that nanoscale wires could be made out of topological insulators. This could result in extraordinarily fast and small circuits, allowing us to keep Moore’s Law going and increasing both the processing speed and density of computing power that we can obtain in our circuits. As well as this, the 2D topological insulator class has important spintronic properties. Graphene was first predicted to have a topologically-insulating phase, but because the spin-orbit coupling of carbon is so low, it is not expected to be possible to observe it any time soon – and it may not be possible to have such states at room temperature.
External strain of Germanene can cause its bandgap to change; this owes to the double-lattice structure which is in contrast to that of graphene. Given that you can tune the bandgap of germanene through this strain, it might find applications as a solar panel material, or in LEDs; this could also be useful for the nanoelectronic applications we mentioned earlier.
It seems likely that graphene and germanene will ultimately both find a very different set of uses. It’s possible to produce graphene much more easily; rather than slowly and expensively through molecular beam epitaxy, you can produce graphene by all kinds of methods, including simple exfoliation of a material like graphite. Also, carbon is a more abundant material than germanium. Yet, at the same time, germanene shares many of the properties that make graphene such an exciting material, and adds a few more that are of great interest for potential applications in spintronics, quantum computing, and in semiconductor devices. It seems likely that germanene will fill an important niche in one of these fields. And, given the growing trend to synthesise new properties in 2D materials by sandwiching together different 2D layers in van der Waals heterostructures, the two may end up being combined into something with an even greater range of applications.