'Research into topologically insulating materials can revolutionize’

| Hans Wolkers

Topological insulators are promising materials for application in electronics because they efficiently conduct electricity, reducing energy consumption. Use in computer chips is also possible, and there are promising uses in quantum computers. Their application in consumer products is very challenging though, since they only function at very low temperatures.

Pantelis Bampoulis, from the Physics of Interfaces and Nanomaterials group, has now developed a new topological material, made from so-called germanium, which overcomes this challenge.

‘Research on topological insulating substances opens a completely new field in physics and may revolutionize electronics’, Bampoulis says. ‘These special materials may substantially reduce energy consumption, and they also have a lot of potential in quantum computers.’ He places a holder, containing a small, flat piece of the material, germanium, inside an impressively looking machine.

His colleague, scientist Carolien Castenmiller, closely involved in the research, connects a tube to this so-called Scanning Tunneling Microscope (STM), supplying it with liquid nitrogen, to cool it. ‘Using the STM, we can study the properties of the sample’, she explains. ‘We can measure and visualize the structure of the surface, which defines the properties of the material. Even individual atoms are visible.’

After placing the sample and cooling down the system, a voltage is applied and a small metal tip scans the sample, to measure the electrical current at different locations. The result of the machine’s measurements is a spectacularly looking figure on a curved computer screen, red in the middle and bright on the edges. ‘This figure shows that our sample has the right characteristics to function as a topological insulator, Castenmiller explains. She points to the figure: ‘The red color in the middle indicates less electrical conductivity, while the bright edges show more conductivity. This ‘edge conductivity’ is a typical feature of topological materials.’ 

Lossless electricity flow

Common insulators, like wood, glass, or rubber don’t conduct electricity. Topological insulators are very different and have special properties: they insulate in the middle, while they conduct electricity along the edges, where they allow an undisturbed and lossless electricity flow. ‘Normal conductors, like a copper wire, contain impurities, resulting in scattering of electrons, increasing the resistance’, Bampoulis explains. ‘This generates heat and results in energy loss.’

But in topological insulators, electricity flow is unaffected by impurities and consequently, there are no energy losses.’ This makes topological insulators efficient electricity conductors. However, they are not applied in consumer products yet, because they are difficult to make and can only operate at very low temperatures.

Science on these special insulators is still in the very fundamental phase. ‘Although use in consumer products is still a long way off, studying these topological materials opens the door to completely new aspects of physics’, Bampoulis says enthusiastically. ‘We are still identifying and looking for the properties of suitable materials. Well-known examples of two-dimensional topological insulators are graphene, tungsten-ditelluride, and mercury telluride.’ Since these materials only behave ‘topologically’ at low temperatures, Bampoulis and his team are trying to identify new topological materials that also function at higher temperatures. 

Sophisticated method

Already during his MSc and PhD in Twente, Bampoulis experimented with germanium, which is a semiconductor. But a single layer of germanium atoms, crammed together in a chicken-wire (honeycomb) structure, exhibits properties very similar to graphene, a known topological insulator. This thin layer of germanium atoms is called germanene and it is a potentially excellent candidate to be used as a topological insulator.

Bampoulis and his colleagues managed to fabricate germanene in the lab using a sophisticated method. They heated germanium together with platinum to 850 degrees Celsius. The two materials mix and form tiny liquid droplets. ‘When cooling down these droplets, a very thin layer of germanium, just a few atoms thick, settles on top of the germanium-platinum alloy’, Bampoulis explains. ‘Using an STM, we could confirm that this layer is indeed germanene, and we have our potential topological insulator that we can investigate further.’

Electrical properties

To test whether germanene may act as a topological insulator, the scientist compared its structure and behavior under different conditions. For example, the first step often involves the evaluation of the material’s atomic structure using an STM. ‘We operate this machine at -196 degrees Celsius, to reduce all atomic movement ‘, Bampoulis says. ‘In our germanene research, we were first seeking to understand the atomic structure of the germanene layer. We found out that the atoms were arranged in a honeycomb structure and that this honeycomb didn’t form a flat surface, like in graphene. Instead, they were arranged in a more buckled way: every second atom was a little bit higher and stuck a tiny bit out of the main plane.’

Using the STM, Bampoulis and his team (including Carolien Castenmiller, Dennis Klaassen and Harold Zandvliet from the University of Twente) measured the electronic properties of the material: at which locations did the material conduct electricity and where did it insulate? They discovered that germanene functions as an insulator in its interior, and was conductive at the edges. This edge conductivity was not affected by any material defects, indicating a lossless electricity flow. This makes germanene a promising topological insulator. Manipulation by applying an electric field or increasing or decreasing temperature also gave information about how the material responded to change.

Unexpected feature

Although graphene is a very versatile material, it can only perform as a topological insulator near -273 Celsius, making practical applications impossible. For germanene that proved to be different. ‘Based on its atomic honeycomb structure, we hypothesized that germanene could be a suitable topological material at room temperature, so we decided to test this experimentally’, Bampoulis says. ‘We already confirmed the topological insulation and the edge conductivity of germanene at -196 degrees Celsius. Our preliminary data also indicate that this material exhibits these properties at room temperature.’

Bampoulis and his team discovered another useful and unexpected feature of germanene, which was related to its buckled honeycomb structure: by applying an electric field, they could alter the material’s topological properties. This resulted in the ability to switch the topological characteristics on and off: below a critical electric field strength, germanene is a  topological insulator that does not dissipate energy. When the electric field is increased beyond the critical value, germanene loses its topological characteristics and acts like an ordinary insulator, where no electrical current can pass through.

Fundamental level

Together with the successful operation at room temperature, the consequences of this finding are huge. With the ‘on’ and ‘off’ switch activated by applying an electric field, germanene functions similarly to a high-tech transistor. This sets the framework for the eventual replacement of traditional transistors in consumer products, where topological insulators guarantee a more energy-efficient operation. Bampoulis: ‘One can imagine that with billions of such topological switches in modern electronics, like our phones, energy savings could be massive.’  

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