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Organic semiconductors can shift shapes, electrical properties

By Doug Main March 14, 2018
Organic semiconductors
Schematic showing the various color changes that the semiconducting material 1-benzothiophene (ditBu-BTBT) can undergo when heated, revealing a shape-shifting ability uncommon for semiconducting materials. (a) In situ polarized optical microscopy showing the change from blue to yellow; the arrows show the direction of the phase change. In Type I, the top and bottom scale bars are 100 µm and 25 µm, respectively; in Type II the scale bar is 25 µm. (b) Schematic showing the cooperative molecular planes; top and bottom scale bars are 100 µm and 25 µm, respectively. Credit: Nature Communications

Metals such as shape-memory alloys have long been known to reversibly shift orientation, but an analogous phenomenon has now been discovered in organic semiconductors. The discovery happened accidentally. Researchers at the University of Illinois were studying polymorphisms, or different molecular arrangements in crystals of an organic semiconductor. These large crystals “looked like a starry night,” says Ying Diao, a professor of chemical and biomolecular engineering, who encouraged her students to use these to see if they could observe interesting phase transitions. Upon annealing one of the crystals, heating it up and slowly cooling, something shocking happened: a sudden wave of color swept through the material.

“When we first saw this... we almost fell out of our chairs,” says Diao, lead author of a study describing the findings published in Nature Communications.

The group had found that when a crystal of 1-benzothiophene (ditBu-BTBT) passed a certain temperature, its color changed from blue to brown, or blue to yellow. These two shifts take place depending on which way the crystals are oriented. The researchers also note a similar phenomenon in another organic semiconductor called TIPS-pentacene, which also includes a color change, a change in curvature, and an elongation of about 10 percent.

Along with these shifts, the electrical properties of the semiconductors changed, reversibly altering the charge carrier mobility by a factor of three in the case of ditBu-BTBT.

Further investigation showed that this shift was caused by a cooperative transition of atoms within the crystal. The semiconductor crystals consist of a solid core, made of fused benzyl rings, which allow electrons to dissociate. Connected to this core are side chains that help to stabilize the materials. The team showed that the phase change was driven by the rotation of these side chains, which when thermally excited lock into each other like “gears in a wheel,” Diao explains. “The topology is corrugated, like little molecular gears,” Diao says. “When one gear rotates, it brings along all the other gears as well.”

One of the exciting aspects of the new discovery is that such organic semiconductors can be “printed the way you print newspapers,” allowing engineers to lay down these circuits on a flexible and stretchable substrate. Combined with this newfound ability to change shape and electrical function, the finding opens the door to many new applications for biomedical devices and the like, Diao says.

While the color change was especially notable in crystals of ditBu-BTBT, the charge carrier mobility only changed by 50%. However, in the TIPS-pentacene material, the color change was less noticeable but the charge carrier mobility changed by a factor of four.

John Anthony, a researcher at the University of Kentucky who was not involved in the work, says this is one of the first times cooperativity has been applied to semiconducting materials, although the phenomenon is well known in other molecules like proteins and lipids. Derivatives of TIPS-pentacene, which his group developed, may “undergo much more substantial solid-state rearrangements based on precisely the type of molecular rotations described here,” he says.

The paper should help spur others to look for such shape-shifting behavior in semiconducting materials, he says. Although he does not see imminent commercial applications coming from this work, it could pave the way for the discovery of new materials that dramatically improve in performance once they are transformed.

The cooperative interactions the team observed explain why this material does not fall apart upon heating. “Absent that sort of long-range cooperativity, crystals like this will tend to shatter rather than smoothly expand or contract,” he says. While “this cooperative behavior is already well known in shape-memory and shape-changing organic materials, this work shows that such capabilities may be possible in materials that are also semi-conductive rather than insulating.”

Diao says that this idea of cooperativity could be applied to molecular machines, amplifying small molecular changes all the way up to the macroscopic level.

This type of cooperative shape-shifting can be seen in nature. For example, bacteriophages use an analogous method to change their conformation and make their way into host cells. Finding out how the same basic process can take place in organic semiconductors is exciting, she says. It could also help unite research findings from the world of semiconductors and metal alloys, two fields that “almost never talk to each other,” she says.

Read the article in Nature Communications.