Nanowire manipulation yields boosted optical devices
A team of researchers from the Norwegian University of Science and Technology (NTNU) have established that making changes at the atomic level in nanowires with a wurtzite lead to the nanowires' enhanced effectiveness in LEDs and solar cells.
NTNU researchers Dheeraj Dasa and Helge Weman, in cooperation with IBM, tuned a gallium arsenide (GaAs) with a small strain. The tuning was facilitated by the special hexagonal crystal structure, referred to as wurtzite, which the researchers succeeded in growing in the MBE lab at NTNU. The results were published in Nature Communications.
The current breakthrough builds on another research that was conducted in 2010, where a team—with Helge Weman as a member—went public with its success in controlling a change in the crystal structure during nanowire growth. By altering the crystal structure in a substance, i.e. changing the positions of the atoms, the substance can gain entirely new properties. This team developed a way to alter the crystal structure in nanowires made of GaAs and other semiconductors.
In 2012, the researchers managed to make semiconductor nanowires grow on the super-material graphene. Graphene then was identified as the thinnest and strongest material ever made, with a potential to revolutionise solar cell and LED component development, as well as replace silicon as a component in electronic circuits.
The research group received a lot of international attention for the graphene method. Helge Weman soon formed the company CrayoNano AS, with fellows Bjrn-Ove Fimland and Dong-Chul Kim as co-founders. The company started working with a patented invention that grows semiconductor nanowires on graphene. The method is called molecular beam epitaxy (MBE), which generates a hybrid material that exhibits good electric and optical properties.
The combination of nanowires and graphene facilitates much broader and more flexible solar cells.
"Our goal [now] is to create solar cells that are more effective than when they are made with thin film technology," Weman emphasised.
In thin films like GaAs, the atoms are placed cubically in a fixed, predefined structure. When the researchers manipulate the atom structure inside the nanowire, they can grow both cubic and hexagonal crystal structures. The different structures have completely different properties, for example when it comes to optical properties.
Electron microscope picture of wurtzite GaA/AIGaAs core-shell nanowires. Source: Dr. Dheeraj Dasa and Prof. Helge Weman, NTNU
Since the graphene breakthrough, the research group has, among other things, studied the hexagonal crystal structure in the GaAs nanowires.
"In cooperation with IBM, we have now discovered that if we stretch these nanowires, they function quite well as light-emitting diodes. Also, if we press the nanowires, they work quite well as photodetectors. This is facilitated by the hexagonal crystal structure, called wurtzite. It makes it easier for us to change the structure to optimise the optical effect for different applications," explained Weman.
"It also gives us a much better understanding, allowing us to design the nanowires with a built-in compressive stress, for example to make them more effective in a solar cell. This can for instance be used to develop different pressure sensors, or to harvest electric energy when the nanowires are bent," he added.
Because of the new ability to manipulate the nanowires' crystal structure, it is possible to create highly effective solar cells that produce a higher electric power. Also, the fact that CrayoNano now can grow nanowires on super-light, strong and flexible graphene, allows production of very flexible and lightweight solar cells.
The CrayoNano group will now start growing gallium nitride nanowires for use in white light-emitting diodes.
One of the group's objectives is to create gallium nitride nanowires in a MBE machine to create light-emitting diodes with better optical properties and grow them on graphene to make them flexible, lightweight and strong.
- Paul Buckley
EE Times Europe
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