Modern-day transistors are based on semiconducting-type materials, usually silicon. In the quest for cheaper, less power-hungry microelectronics devices, such as those in laptops, tablets and smartphones, researchers are looking into alternatives to these materials.

A team led by Christian Klinke has now made transistors from metal nanoparticles. The small size of the particles means that they no longer show metallic characteristics under current flow but instead have an energy bandgap (akin to that in semiconductors) that arises from the Coulomb repulsion between electrons in the material. This effect is known as the Coulomb blockade, and it exists even at room temperature in the materials employed in the new devices (in this case cobalt-platinum nanoparticles).

Transport governed by Coulomb blockade

“By applying a voltage, we can shift the energy of this gap, which means that the current in the devices can be switched on and off as desired,” explains Klinke. “The transport mechanism in these devices is based on percolative tunnelling and hopping of electrons governed by Coulomb blockade instead of classical band transport as in silicon.

“This leads to a new type of transistor with novel characteristics,” he tells nanotechweb.org. “Instead of having only two states (ON and OFF, as in classical semiconductor transistors based on silicon), our devices show sinusoidal transport characteristics, meaning that several input voltages lead to the same output current.”

Scalable techniques

The researchers synthesized their cobalt-platinum metallic nanoparticles using a colloidal chemistry approach that can be well controlled and which is scalable and fabricated the transistors using electron-beam lithography. They deposited the nanoparticles as stripes on the device using the Langmuir–Blodgett method, which allows for high-quality monolayer films that can also be scaled up to the industrial level.

“Our approach makes use of standard lithography techniques for the design of the components and their integration into electrical circuits,” says Klinke. “This makes them inexpensive and industry-compatible.”

The devices have ON/OFF ratios of more than 90%.

The team, reporting its work in Science Advances DOI: 10.1126/sciadv.1603191, says that it will now further develop the device design using even smaller nanoparticles. “We will also be looking at how nanoparticle shape affects the final electronic properties of the transistors.”