Four nanocrystal inks have been developed by Kagan’s group that make up the transistor, which are deposited on a flexible backing.
Four nanocrystal inks have been developed by Kagan’s group that make up the transistor, which are deposited on a flexible backing.
The inks’ specialized surface chemistry allowed them to stay in configuration without losing their electrical properties.
The inks’ specialized surface chemistry allowed them to stay in configuration without losing their electrical properties.

Making electronics mobile, wearable, and implantable demands cheap, flexible and large-area devices. Now an international team of researchers has devised a new approach to fabricating such devices using layers of nanocrystal ‘inks’ deposited onto flexible plastics [Choi et al., Science 352 (2016) 205].

Led by Cherie R. Kagan of the University of Pennsylvania and Ji-Hyuk Choi, now at Korea Institute of Geoscience and Mineral Resources, together with colleagues from Korea and Yonsei Universities, the team used solution processing to create all the elements of a field-effect transistor (FET) from colloidal nanoparticles for the first time.

‘‘These materials are colloids just like the ink in your inkjet printer,’’ explains Kagan, ‘‘but you can get all the characteristics that you want and expect from the analogous bulk materials, such as whether they’re conductors, semiconductors or insulators.’’

The nano-sized inorganic colloidal particles are grown in solution and stabilized with a surface layer of surfactants, which enables self-assemble into ordered layers. Using this approach, Kagan and her colleagues deposited layers of metallic Ag, semiconducting CdSe, and insulating Al2O3 nanocrystals onto polymer substrates using spin coating. Ultimately, the researchers suggest, it should be possible to construct the devices using nanocrystal colloidal inks and additive manufacturing techniques like 3D printing.

‘‘The trick to working with solution-based materials is making sure that, when you add the second layer, it doesn’t wash off the first, and so on,’’ says Kagan.

So after the deposition of each metallic, semiconducting and insulating nanocrystal colloidal layer through a mask, the layer is treated with polyelectrolytes to control surface charge and passivation. The result is structurally stable layers that enable high performance devices. The advantage of the approach is that synthetic methods can produce vast ‘libraries’ of colloidal nanocrystals with different properties.

The other advantage is that the assembly process can be carried out at lower temperatures than vacuum-based techniques so several transistors can be fabricated on the same plastic substrate at the same time.

‘‘Making transistors over larger areas and at lower temperatures have been goals for an emerging class of technologies including the Internet of things, large area flexible electronics and wearable devices,’’ says Kagan.

Although the devices cannot be printed yet, she admits, the stage is set for additive manufacturing in the future because all the constituents are solution-based materials.

‘‘The demonstrated materials and fabrication techniques may help open new avenues towards manufacturing of electronic devices and circuits,’’ comments Leszek A. Majewski of Manchester University. ‘‘The next natural step would be to demonstrate thin-film integrated circuits — which would require the development of p-type nanocrystal semiconductors. Once demonstrated, more serious uses of these materials (e.g. integrated circuits, sensor arrays, CMOS-like circuitry) could be envisaged.’’

This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.05.003