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Advancements in Molecular Electronics: Controlling Conductance with Rigid Molecules

As the miniaturization of electronic devices continues, the traditional scaling of silicon-based microchips is encountering limits due to size constraints. This phenomenon challenges the long-standing prediction known as Moore’s Law, which posits a doubling of transistor density approximately every two years. An emerging frontier in electronics, molecular electronics, utilizes individual molecules as fundamental components, offering a promising avenue for further miniaturization. However, the efficiency of these devices relies heavily on controlling the flow of electrical current, a task complicated by the inherent motion of single-molecule components that can affect device performance and reproducibility.

Recent research from the University of Illinois Urbana-Champaign presents an innovative approach to managing molecular conductance by leveraging molecules with rigid backbones, particularly those characterized as shape-persistent structures, like ladder-type molecules. This research also introduces an effective “one-pot” synthesis method for creating these complex molecules. The team applied their findings to develop a butterfly-like molecule, reinforcing the versatility of their approach to controlling molecular conductance.

Leading the research is Charles Schroeder, a distinguished professor in the fields of Materials Science and Engineering as well as Chemical and Biomolecular Engineering. His team, which includes postdoctoral researcher Xiaolin Liu and graduate student Hao Yang, shared their findings in the peer-reviewed journal Nature Chemistry.

Schroeder explains, “In molecular electronics, one must account for the flexibility and motion of the molecules and their impact on functional properties. These factors significantly influence the electronic characteristics of molecules. To address this, we focused on designing molecules with rigid backbones to ensure consistent conductivity despite any conformational changes.”

One prevailing challenge in molecular electronics is the flexibility of many organic molecules, leading to various conformations that significantly impact their electrical conductance. Liu elaborates, “For molecules with numerous conformations, the conductance can vary drastically—sometimes up to 1000-fold. By choosing ladder-type molecules, which maintain stable rigid conformations, we can achieve reliable and robust molecular junction conductance.”

Ladder-type molecules consist of continuous chains of chemical rings with shared atoms linking them, thereby constraining their motion and maintaining a consistent structure. This rigidity is essential in stabilizing conductance and reducing variability.

Ensuring stable conductance is crucial, especially considering the goal of integrating molecular electronics into functional devices. The necessity for billions of identical components with uniform electronic properties presents a formidable barrier to the commercialization of these technologies. Yang notes, “The inconsistency in conductance has hindered the successful scaling of molecular electronic devices. Precision in this area could facilitate manufacturing simpler, smaller electronic devices.”

The research team’s synthesis strategy involved a unique one-pot ladderization technique that generates a variety of chemically distinct, charged ladder molecules without relying on expensive materials or multi-step processes. “By utilizing this multicomponent modular synthesis, we have access to simpler, commercially available starting materials, enabling us to produce a wide array of suitable molecules for molecular electronics,” Liu explains.

Furthermore, the researchers extended their insights from ladder-type molecules to create and analyze the electronic properties of butterfly-like structures. These molecules possess two chemically ringed “wings” and maintain a locked backbone, akin to ladder molecules. This development is expected to lay the groundwork for designing additional functional materials, leading to more efficient and dependable electronic devices.

Schroeder is also affiliated with the departments of chemistry and bioengineering, along with the Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at the University of Illinois.

Liu and Yang are linked with the department of chemistry and the Beckman Institute for Advanced Science and Technology. Other notable contributors to the study include Jeffrey S. Moore, Joaquín Rodríguez-López, Qian Chen, Adolfo I. B. Romo, Oliver Lin, Toby J. Woods, Rajarshi Samajdar, Hassan Harb, and Rajeev S. Assary.

This research project received funding from the U.S. Department of Energy Office of Science, underscoring the significance of advancing molecular electronics in the quest for next-generation miniature devices.

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