STM image showing the detailed structure of the conjugated polymer C14DPPF-F. The polymer backbones appear as bright rows and the alkyl sidechains are seen as darker rows perpendicular to the backbones. Image: University of Warwick.
STM image showing the detailed structure of the conjugated polymer C14DPPF-F. The polymer backbones appear as bright rows and the alkyl sidechains are seen as darker rows perpendicular to the backbones. Image: University of Warwick.

The first ever detailed pictures of the structure of conjugated polymers have been produced by a research team led by Giovanni Costantini at the University of Warwick in the UK.

The ability of conjugated polymers to conduct electricity makes them highly sought after, but until now they could also be described as extremely camera shy as there has been no easy means to determine their structure. The new technique developed by Costantini’s team not only allows researchers to determine this structure but to clearly see it with their own eyes.

Conjugated polymers are able to conduct electricity because they comprise a chain of conjugated molecules through which electrons can move freely due to their overlapping electron p-orbitals. Effectively, they are excellent molecular wires. Moreover, they are akin to semiconductor materials (they have energy gaps), so they can be used for electronic (plastic electronics) and photovoltaic (organic solar cells) applications.

Modern conjugated polymers are often co-polymers, made from an (ideally regular) sequence of different monomers. The order of these monomers is critical for the polymer’s opto-electronic properties, which can be severally damaged by errors in how the monomers actually link up in a chain to form the polymer (so called polymerization errors). However, detecting the nature and exact position of these errors has proved problematic using current analytical methods. Mass spectrometry does not provide a solution, as shorter polymer chains are more likely to be ionized and thus tend to be over-represented in the spectra.

Costantini and co-workers have proposed and implemented a completely novel approach to overcome this fundamental analytical problem. The underlying idea is extremely simple, yet at the same time transformative: deposit the polymers onto a surface and image them by high-resolution scanning tunneling microscopy (STM). This approach effectively realizes one of the visionary predictions of Richard Feynman in his famous 1959 speech There's Plenty of Room at the Bottom, where he said that in the future "it would be very easy to make an analysis of any complicated chemical substance; all one would have to do would be to look at it and see where the atoms are".

The atomic-scale resolution of STM is ideal for this aim but the problem remains that the chains of polymer molecules have first to be deposited intact in a vacuum onto atomically clean and flat surfaces. The usual method of doing this involves heating the molecular material until it sublimes, but for molecules as large as polymers this effectively melts the structure that should be studied.

The researchers have thus opted for a new method that sprays a cloud of the polymer through a series of tiny openings into a vacuum chamber, allowing a single unjumbled layer to be deposited onto a surface. This layer is fully representative of the original polymer sample. Conducting STM on these layers produced stunningly resolved pictures, clearly revealing sub-monomer details of the conjugated polymers.

The research team, which also included scientists from Imperial College London and the universities of Cambridge and Liverpool in the UK, reported its results in a paper in Science Advances. Their high-resolution STM images of the structure of conjugated polymers are so detailed that not only can they help with quality control and fine-tuning of polymer design, but they can even be used as something akin to an intellectual property (IP) passport photo for polymers. Such precise and clear images could help synthetic researchers to demonstrate exactly the design they wish to legally protect by dramatically improving the information available to support an application for IP protection.

In their paper, the researchers demonstrate the power of the new technique by examining the conjugated polymer poly(tetradecyl-diketopyrrolopyrrole-furan-co-furan) (C14DPPF-F). This is a conjugated polymer of the DPP-based family that is currently demonstrating some of the best performances in optoelectronic devices.

This material is most effective when its polymer chains form in an alternating sequence of one large ‘A’ monomer and a smaller ‘B’ monomer. However, flaws can occur during synthesis to break that ideal sequence, damaging the polymer’s appealing conducting and light-harvesting properties. Scientists had speculated that this mainly occurs when two of the larger ‘A’ monomers join directly together in an BAAB sequence.

When such flaws happen, gaps or voids form in the conjugated polymer's assembly. The University of Warwick-led research team was able to use their new visualization technique to very clearly show all of these gaps and then to zoom in further onto the polymer chains, precisely spotting each of the defective monomer sequences. On doing so, to their great surprise, they found not the expected BAAB flaws but ABBA defects.

“This new capability to image conjugated polymers with sub-monomeric spatial resolution, allow us, for the first time, to sequence a polymeric material by simply looking at it,” said Costantini, a physicist in the University of Warwick's Department of Chemistry. “Some of the first images we produced using this technique were so detailed that when the researchers who synthesized the polymers first saw them, their overjoyed impression reminded me of how new parents react to the first ultrasound scans of their babies.”

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.