Molecular teamwork is key to efficient organic semiconductors

(Nanowerk News) This result may lead to more efficient smart watches, solar cells and other organic electronics of the future.
Researchers at the Beckman Institute for Advanced Science and Technology at the University of Illinois Urbana-Champaign have discovered a way to trigger cooperative behavior often found in viruses in organic semiconductors. The energy- and time-saving phenomenon may help enhance the performance of smartwatches, solar cell and other organic electronics.
Their work appears in Nature Communications ("Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics"). The research team made use of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory.
This video shows two phase transitions in organic semiconductor material captured at APS beamline 8-ID-E as the material was slowly heated. The first transition occurs at 164 degrees Celsius, the second at 223 degrees Celsius. Understanding these transitions could lead to better materials for electronic devices.
“Our research brings semiconductors to life by unlocking the same dynamic qualities that natural organisms like viruses use to adapt and survive,” said Ying Diao, a researcher at the Beckman Institute and a co-author of the study.
The behavior the researchers focused on is called molecular cooperativity, which is often observed in nature but rarely seen in non living systems. It can be seen in the virus that causes E. coli infections, for example. This virus alights on an unassuming host cell and grips the surface with its tubular tail. Then, the proteins in the tail contract in unison, flattening its structure and reeling the virus’s body in for the critical strike. Thanks to the proteins’ teamwork, the tail can flex and flatten with ease.
Viruses may have mastered molecular cooperativity, but the same cannot be said of crystals: non living molecular structures classified by their symmetry. Though aesthetically pleasing, the molecules that comprise crystalline structures have diva-like dispositions and seldom work together. Instead, they test researchers’ patience by plodding through structural transitions one molecule at a time — a process famously demonstrated by diamonds growing from carbon, which demands blistering heat, intense pressure and thousands of years sequestered deep beneath the earth.
“Imagine taking down an elaborate domino display brick by brick. It’s exhausting and laborious, and once you’ve finished, you would most likely not have the energy to try it again,” said Daniel Davies, the study’s lead author and a researcher at the Beckman Institute at the time of the study.
By contrast, cooperative transitions occur when molecules shift their structure together, like a row of dominoes flowing seamlessly to the floor. The collaborative method is fast, energy-efficient and easily reversible — it’s why the bacteria responsible for E. coli infection can tirelessly contract its protein-packed tail with little energy lost.
For a long time, researchers have struggled to replicate this cooperative process in non living systems to reap its time- and energy-saving benefits. This problem was of particular interest to Diao and Davies, who wondered how molecular teamwork might impact the electronics sector.
“Molecular cooperativity helps living systems operate quickly and efficiently,” Davies said. “We thought, ‘If the molecules in electronic devices worked together, could those devices display those same benefits?’ ”
Diao and Davies study organic electronic devices, which rely on semiconductors made from molecules like hydrogen and carbon rather than inorganic ones like silicon, a ubiquitous ingredient in the laptops, desktops and smart devices saturating the market today.
“Since organic electronics are made from the same basic elements as living beings, like people, they unlock many new possibilities for applications,” said Diao, also of the University of Illinois Urbana-Champaign. “In the future, organic electronics might be able to attach to our brains to enhance cognition or be worn like a Band-Aid to convert our body heat into electricity.”
Diao studies the design of solar cells, wafer-thin window clings that soak up sunlight to convert into electricity. Organic semiconductors that can flex without breaking and contour to human skin would likewise be “an important part of the future of organic electronic devices,” Davies said.
An important step toward designing dynamic organic electronics like these is fashioning dynamic organic semiconductors. And for that to happen, the semiconductor molecules must cooperate.
Dominoes inspired the researchers’ approach to trigger molecular teamwork in a semiconductor crystal. They discovered that rearranging the clusters of hydrogen and carbon atoms spooling out from a molecule’s core — otherwise known as alkyl chains — causes the molecular core itself to tilt, triggering a crystal-wide chain of collapse the researchers refer to as an “avalanche.”
“Just like dominoes, the molecules don’t move from where they are fixed. Only their tilt changes,” Davies said.
But tilting a string of molecules is neither as easy nor as tactile as picking up a domino and rotating it 90 degrees. On a scale much smaller than a plastic game piece, the researchers gradually applied heat to the molecule’s alkyl chain; the increased temperature induced the domino-like effect.
Using heat to rearrange the molecules’ alkyl chains also caused the crystal itself to shrink — just like the virus’s tail prior to E. coli infection. In an electronic device, this property translates to an easy, temperature-induced on-off switch.
To confirm this discovery, Davies drove his material (a thin film on a silicon chip) on ice from Urbana to the APS. The high intensity beam at APS beamline 8-ID-E allowed him to track how the X-ray scattering changed as the sample was gradually warmed through the transition between two different polymorphs, or crystal structures.
Together with collaborating Argonne scientist Joseph Strzalka, a co-author on the paper, Davies recorded a movie of the changes in the X-ray scattering data, which showed the molecular rearrangement definitively. “It was a dramatic example of the power of time-resolved X-ray scattering to follow the evolution of a material’s structure,” said Strzalka.
The APS itself is also about to undergo a dramatic transition. A comprehensive upgrade will provide a more brilliant and more coherent X-ray beam. When the APS returns to operation in 2024, the grazing-incidence wide-angle X-ray scattering (GIWAXS) instrument will also be able to measure the dynamic properties of the material and show, for instance, how the diffusivity of the material changes approaching the avalanche.
“The APS Upgrade will open up new avenues for studying the interrelationship between materials’ properties and structure,” Strzalka said.
The applications of this discovery have yet to be fully realized; for now, the researchers are thrilled with the first step.
“The most exciting part was being able to observe how these molecules are changing and how their structure is evolving throughout these transitions,” Davies said.
Source: By Jenna Kurtzweil and Andre Salles, Argonne National Laboratories (Note: Content may be edited for style and length)
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