Optical & Electronic Properties of Germanene

Table of Contents

Geranium
Germanene Origins
Germanene Properties
Uses in Quantum Research

Geranium

Germanium was first discovered in 1886, although the developer of the periodic table, Dmitri Mendeleev, had predicted that it should exist a few years before. For quite some time, it was considered to be a weak conductor. However, in the 1940s and 1950s, its useful optical and electronic properties as a semiconductor were recognized.

Many early transistors were made of Germanium. However, it was displaced as the material of choice after it became easier to manufacture silicon with the appropriate purity. On the other hand, a sizeable fraction of transistors and semiconductor devices are still made of germanium today.

Germanene Origins

Germanene was only synthesized in 2014. In many ways, this new material is a cousin to graphene. Just as graphene consists of a single layer of carbon atoms, germanene contains a single layer of germanium atoms. It’s constructed in a similar way to graphene and silicene as 2D materials - it was first synthesized by molecular beam epitaxy, depositing atoms onto a substrate at very low pressures and high temperatures.

When the European team of scientists first involved with the synthesis of germanene published their paper, they were excited by the theoretical calculations that suggested germanene could have unusual and useful optical and electronic properties.

We have provided compelling evidence of the birth of nearly flat germanene - a novel, synthetic germanium allotrope which does not exist in nature. It is a new cousin of graphene.

'The synthesis of germanene is just the very beginning of a long quest. Indeed, success in the synthesis was not easy to achieve and quite demanding. A considerable amount of work is now needed to further characterize the electronic properties of the material.

Professor Le Lay, Lead Author

Germanene Properties

Like other 2-D materials, it forms a honeycomb structure, but the properties of germanene are different in many ways. Unlike graphene, it has a ‘buckled,’ rather than flat structure. In a paper published in Nano Letters in 2012, before germanene had been synthesized, 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).

Calculations suggest germanene may be a robust two-dimensional topological insulator. It is thought to retain these properties up to room temperature. A topological insulator can be identified as a material that is “an insulator on the outside while conducting on the inside.” Synthesising such materials could lead new kinds of electronics, and their properties may also be useful for “spintronics” – the use of the electron’s spin, rather than just its charge, for quantum information processing.

Germanene is also an optically active material with interesting features in the visible spectrum, suggesting that there may be optoelectronic applications. External strains of Germanene can cause its bandgap to change, an unusual property that may well allow it to be easily tuned in nanoelectronics.

Germanene has a very high thermoelectric figure of merit – up to 2.5 in certain nanostructures, with the highest ever observed being 2.6 in tin selenide. There is currently a hunt for an efficient thermoelectric device, which could be used in a thermoelectric generator, converting waste heat into electricity. This is typically the kind of device that is used to recycle some of the wasted energy from braking in cars, but materials with good thermoelectric properties can have all kinds of industrial applications. It seems likely that 2D materials may form part of this search.

Uses in Quantum Research

The extensive spin-orbit coupling in Germanene – hundreds of times larger than that in graphene – means that it can be used as an excellent experimental system for observing the quantum spin Hall effect at room temperatures.

These 2D topological insulators are exciting new phases of quantum matter that still need further research. Some think that exotic new forms or states of matter could arise in these materials – such as Majorana fermions, hypothetical particles that are their own antiparticle.

These states of matter can have applications from quantum information processing and quantum computing, all the way to understanding the nature of superconductivity. It’s such a significant field in theoretical physics at the moment that the 2016 Nobel Prize was awarded for theoretical calculations and explanations of exotic condensed matter phenomenon through topological phase theory.

Perhaps ultimately the most exciting – and unforeseeable – applications for 2D materials like germanene and graphene will come when we learn about ways to stack them together. Quoted in a Nature literature review in 2015, Andras Kis of EPFL in Switzerland set forth his view:

“Instead of trying to pick one and say this is the best, maybe the best thing to do is to combine them in such a way that all their different advantages are properly utilized,”  

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