Transistors created from metal nanoparticles

Researchers from the University of Hamburg have developed transistors based on metal nanoparticles, in a production method based on a completely different principle to current techniques. Their results have been published in the journal Science Advances.

Transistors are nowadays based on semiconductor-type materials, usually silicon. But as the demands for computer chips in laptops, tablets and smartphones continue to rise, new possibilities are being sought out to fabricate transistors in an inexpensive, energy-saving and flexible manner.

Led by Dr Christian Klinke, the University of Hamburg researchers produced transistors using metal nanoparticles which are so small that they no longer show their metallic character under current flow but do exhibit an energy gap caused by the Coulomb repulsion of the electrons among one another. Via a controlling voltage, this gap can be shifted energetically and the current can thus be switched on and off as desired.

In contrast to previous approaches, the nanoparticles are not deposited as individual structures — a process that renders the production very complex and the properties of the corresponding components unreliable. Instead, they are deposited as thin films with a height of only one layer of nanoparticles. Employing this method, the electrical characteristics of the devices become adjustable and almost identical.

The Coulomb transistors are said to have three main advantages that make them suitable for commercial applications. First, the synthesis of metal nanoparticles by colloidal chemistry is very controllable and scalable. Second, it provides very small nanocrystals that can be stored in solvents and are easy to process. Third, the Langmuir-Blodgett deposition method provides high-quality monolayered films and can also be implemented on an industrial scale.

This approach therefore enables the use of standard lithography methods for the design of the components and the integration into electrical circuits, which renders the devices inexpensive, flexible and industry-compatible. The resulting transistors show a switching behaviour of more than 90% and function up to room temperature, indicating that inexpensive transistors and computer chips with lower power consumption could be possible in the future.

“Of course there is still a lot of research to be done, but our work shows that there are alternatives to traditional transistor concepts that can be used in the future in various fields of application,” said Dr Klinke. “The devices developed in our group can not only be used as transistors, but they are also very interesting as chemical sensors because the interstices between the nanoparticles, which act as so-called tunnel barriers, react highly sensitive to chemical deposits.”

Pictured: the new transistor concept. Thin films made of metal nanoparticles (in grey) are electrically contacted using gold electrodes. The current flowing through the films (in blue) is adjustable by the voltage of a local electrode that is below the film. For the characterisation, needle-shaped probes are applied to the components, which provide the corresponding voltages and measure the current. Image credit: UHH/Klinke.