Precision Raman: Effect in the field

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  • Published: Dec 1, 2016
  • Author: David Bradley
  • Channels: Raman
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Changing electronics

New technology that could change the way in which electronics are printed facilitates high quality, direct spray deposition of semiconducting molecular crystals on to any surface. TIPS Pentacene structure drawn by David Bradley

New technology that could change the way in which electronics are printed facilitates high quality, direct spray deposition of semiconducting molecular crystals on to any surface. The precise nature of a single crystal field-effect transistor made this was has been ascertained using polarised Raman spectroscopy.

Sebastian Wood, Grigorios-Panagiotis Rigas, Alina Zoladek-Lemanczyk, James Blakesley, and Fernando Castro of the National Physical Laboratory, in Teddington, Grigorios-Panagiotis Rigas and Maxim Shkunov of the Advanced Technology Institute, University of Surrey, Guildford, UK, and Stamatis Georgakopoulos and Marta Mas-Torrent of the Institut de Ciencia de Materials de Barcelona in Cerdanyola, Spain, have found a way to convert organic semiconducting inks into isolated crystals through a scalable process, suitable for a wide range of materials. As a proof of principle the team experimented with the polyaromatic hydrocarbon pentacene, modified with two TIPS (triisopropylsilane groups) on the central benzene ring.

The approach could allow flexible, light-weight and low-cost circuitry for use in wearable devices, advanced photodetector arrays, chemical and biological sensors, robotic skin tensile sensors, X-ray medical detectors, light emitting transistors and diodes, and miniature lasers to be developed, according to the team.

Single, again

Inorganic semiconductors, such as silicon, have been the mainstay of the electronics age for more than 70 years. They are at the heart of almost every electronic device. However, inorganic crystals need to be grown typically from a melt at very high temperatures with special inert gas chambers in ultra-clean rooms with time-consuming and energy intensive processes. In contrast, organic semiconductors can be formed from solution at room temperature in ambient conditions. This could lead to large-scale production of far cheaper electronic devices with a range of applications as well as extending the repertoire of electronics into areas from which they have traditionally been excluded.

Printable electronics

The NPL and Surrey researchers and their colleagues have now developed for the first time a low-cost, scalable spray-printing process that allows them to make high-quality isolated organic single crystals. They suggest that the same technique is widely applicable to a vast range of molecular semiconductors, many of which are soluble and can be formulated as semiconducting inks for printing on to almost any substrate surface. Key to the success of the process is the use of so-called anti-solvent crystallization wherein the addition of a more soluble, second solute forces the compound of interest to precipitate from the solvent as crystals. This in combination with solution shearing allows the team to control the size, shape, and orientation of the crystals formed by fine tuning spray angle and distance to the substrate. The trick is to cover the surface with a non-solvent so that semiconductor molecules float on top and self-assemble into highly ordered crystals. The resulting crystals are of high quality as revealed by polarised optical and scanning electron microscopy, X-ray diffraction, polarised Raman spectroscopy. They also demonstrate their prowess in field-effect transistor tests.

"This method is a powerful, new approach for manufacturing organic semiconductor single crystals and controlling their shape and dimensions," explains Surrey's Shkunov. "We can make single crystals in a much simpler way [than silicon], entirely at room temperature with an artist's spray brush. With a new class of organic semiconductors based on carbon atoms, we can spray-coat organic inks onto anything, and get more or less the right size of crystals for our devices right away."

"We can also beat silicon by using light emitting molecules to make lasers, for example, something you can't do with traditional silicon," Shkunov adds. "This molecular crystals growth method opens amazing capabilities for printable organic electronics."

"We now have probably two objectives, one technical and one applied," NPL's Wood told SpectroscopyNOW. "Technically, we would like to develop our characterisation techniques to give higher spatial resolution. At the moment we can map our samples with micrometre resolution, but we are working on a 'tip-enhanced spectroscopy' technique that will give us ~20 nanometre resolution. In terms of applications, we have already demonstrated transistors, so the next step is to move towards printable (low-cost) biological and medical sensors. At present, we are looking for potential industrial collaborators who would like to work with us to develop these applications."

Related Links

Sci Rep 2016, 6, 33057: "Precise Characterisation of Molecular Orientation in a Single Crystal Field-Effect Transistor Using Polarised Raman Spectroscopy"

Nature Commun 2016, 7, 13531: "Spray printing of organic semiconducting single crystals” single crystals "

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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