As our electronic devices shrink in size, the physical limitations are starting to interfere with the expected increase in transistor density on silicon microchips, in line with Moore’s law, which predicts a doubling approximately every two years. Molecular electronics – the concept of using individual molecules as fundamental components for electronic devices – presents a promising approach to continue the trend of miniaturizing small electronic devices. These devices demand stringent control over electrical current flow, but the changing nature of these single molecules complicates device efficiency and reproducibility.
Researchers from the University of Illinois Urbana-Champaign have introduced an innovative approach to manage molecular conductance by employing molecules with rigid structures, specifically ladder-type molecules that are shape-persistent. They also showcased a simple “one-pot” method to synthesize these types of molecules, which has been applied to create a butterfly-shaped molecule, demonstrating the versatility of their method for regulating molecular conductance.
This groundbreaking study is led by Charles Schroeder, James Economy Professor of Materials Science and Engineering and Professor of Chemical and Biomolecular Engineering, along with postdoc Xiaolin Liu and graduate student Hao Yang, and their findings were published in the journal Nature Chemistry.
“When it comes to molecular electronics, we must consider how the flexibility and movement of the molecules influence their functional properties,” Schroeder remarks. “It turns out that this flexibility plays a crucial role in the electronic characteristics of these molecules. To tackle this issue and maintain steady conductivity regardless of the molecule’s shape, we aimed to develop molecules with rigid backbones.”
A significant hurdle in molecular electronics is the fact that many organic molecules display flexibility and can adopt various shapes – known as molecular conformations – each potentially leading to differing electrical conductance. Liu points out, “For molecules with several shapes, the conductance can vary widely, even by a factor of 1000. We decided to focus on ladder-type molecules because they maintain stable, rigid shapes which allow us to achieve consistent and reliable molecular junction conductance.”
Ladder-type molecules consist of an unbroken series of chemical rings with shared atoms that help “lock” the molecule into a specific shape. This structure enhances shape persistence and limits how much the molecule can rotate, which reduces conductance variability.
Consistent conductance is vital, especially with the long-term aim of using molecular electronics in practical devices. This requires billions of components with identical electronic characteristics. “Inconsistent conductance has been a barrier to the successful commercial implementation of molecular electronic devices. It’s challenging to produce the vast number of identical components needed and control the molecular conductance at the junctions,” Yang elaborates. “If we can master this, it will significantly accelerate the miniaturization of electronic devices.”
The team devised a unique one-pot synthesis method for these shape-persistent molecules, producing a variety of chemically diverse and charged ladder molecules. Traditional synthesis often relies on expensive starting materials and involves two-component reactions, which limits the resulting diversity. In contrast, this one-pot modular synthesis uses simpler and widely available starting materials. “This allows us to explore multiple combinations of starting materials and create a rich variety of molecules suited for molecular electronics,” Liu states.
Moreover, Liu and Yang applied their insights from ladder-type molecules to create and analyze the electronic properties of a butterfly-like molecule. These molecules feature two wing-like structures composed of chemical rings, and similar to ladder molecules, they have a fixed backbone and limited rotation. This approach will facilitate the development of additional functional materials, ultimately leading to more reliable and efficient electronic devices.
Charles Schroeder also collaborates with the departments of chemistry and bioengineering, as well as the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at Illinois.
Xiaolin Liu is connected to the department of chemistry and the Beckman Institute for Advanced Science and Technology at Illinois.
Hao Yang is part of the materials science and engineering department and the Beckman Institute for Advanced Science and Technology at Illinois.
Other contributors to this research include Jeffrey S. Moore (departments of chemistry and materials science and engineering, the Beckman Institute for Advanced Science and Technology, Illinois), JoaquÃn RodrÃguez-López (department of chemistry, the Beckman Institute for Advanced Science and Technology, Illinois), Qian Chen (departments of chemistry and materials science and engineering, the Beckman Institute for Advanced Science and Technology, Illinois), Adolfo I. B. Romo (department of chemistry, the Beckman Institute for Advanced Science and Technology, Illinois), Oliver Lin (department of chemistry, Illinois), Toby J. Woods (department of chemistry, Illinois), Rajarshi Samajdar (department of chemical and biomolecular engineering, the Beckman Institute for Advanced Science and Technology, Illinois), Hassan Harb (Materials Science Division, Argonne National Laboratory) and Rajeev S. Assary (Materials Science Division, Argonne National Laboratory).
This research received support from the U.S. Department of Energy Office of Science.
