Faster, more efficient, thinner, more energy-saving! These are some of the watchwords used in the development of better mobiles, solar cells and LED lighting. The common denominator for improving transistors in electronics or lighting diodes are ultra-small semi-conductors known as nanowires. Using a new “supermicroscope”, Reine Wallenberg, professor of solid state physics, is studying how nanowires grow and how their properties can be controlled.
“If we understand what happens, we can control the construction process when nanowires are formed. Our new ETEM microscope (Environmental transmission electron microscope) enables us to directly observe when and how a nanowire is built up, atom by atom”, says Reine Wallenberg.
Just over 20 years ago, researchers at Lund University succeeded in showing that it was possible to build wires with completely unique material combinations and properties on the nano level. The nanowires currently included in the NanoLund strategic research area’s development of solar cells and light diodes are the result of long and arduous experimentation with materials and production processes.
The new microscope enables researchers to see what happens, as it happens. Nanowires are formed when gases with the desired building blocks flow over metal particles, usually made of gold. They function as a platform for construction. The researchers determine the exact gas mixture required to create a certain type of nanowire, as well as the right temperature and gas pressure, through trial and error. Today, the search for new combinations of materials and new properties can be speeded up significantly, thanks to the ability to observe the construction process in detail and in real time.
We are interested in the precise moment of genesis.
“Previously, finding the correct parameters was a huge task. Now, all experiments take place in a chamber inside the microscope as we observe and film the process. In one day, we can conduct ten different experiments, which could previously take up to 10 months to carry out”, explains Reine Wallenberg, continuing:
“We have a video that illustrates the advantages of being able to observe the entire process. At the beginning of the video, you see the gold drops before the nanowires have started to form. If you then look at the end of the video, when the experiment is complete, it looks as though no real nanowires have been formed. But in fact, a lot has happened, which is revealed in the rest of the film. When we reduced the temperature during the course of the experiment, the nanowires started to grow, but when we increased the temperature once more, the balance was disrupted and the nanowires were ‘eaten up’ again!”
To enable the design of new types of structures for new areas of application, the researchers need to understand exactly what happens at the atomic level as the nanowires’ structure is built up. Another question the researchers are investigating is what determines when the wires start to grow in a certain direction, i.e. what determines when the atoms penetrate the gold particle and the crystal starts to develop into a thin nanowire.
“We are interested in the precise moment of genesis. In the process of aerotaxis, when you have floating particles in a flow of gas, you could think there is no reason why the atoms would combine to form a wire, specifically – but they do! So we want to understand how they grow – how the atoms connect to one another. We have been able to build nanowires for close to twenty years but we still don’t know exactly how it happens.”
Once the researchers have solved the question of what premises are required to build a certain type of nanowire, others take over and scale up the process to develop fast and cheap production. Ultimately, these tiny nanowires studied by Reine Wallenberg and his colleagues can be used to improve conditions for reducing the energy consumption in our computers, to purify water from bacteria using UV light in a process run by solar cells, or to develop flexible electrodes for use in brain surgery, that help us to improve treatments for Parkinson’s disease and epilepsy, for example.
The ETEM supermicroscope (Environmental transmission electron microscope) is located at nCHREM (the national Centre for High-Resolution Electron Microscopy) at Kemicentrum in Lund.