Conventional silicon electronics rely on stiff, crystalline silicon, where a rigid and orderly lattice helps charges move quickly through devices. Organic semiconductors instead consist of carbon-based molecules that assemble into soft, bendable solids, making them attractive for rollable displays and lightweight circuitry but potentially changing how charges travel. Lead author Deepak Venkateshvaran from Cambridge's Cavendish Laboratory noted that silicon's stiffness contributes to its performance and that flexible electronics have been developed for decades without a clear picture of what flexibility looks like at the level of individual molecules or whether it affects conductivity.
To tackle this problem, the researchers turned to atomic force microscopy, using a nanoscale needle only about ten nanometres wide to gently press on the surface of thin organic semiconductor films. By measuring how strongly the surface resisted this deformation, they could infer local stiffness in regions only a few molecules across. Venkateshvaran likened the technique to feeling the ground with a stick, where a firm surface pushes back and a soft one gives way, except here the probing occurs on a nanometre scale comparable to a small cluster of molecules. Carefully controlling the applied force allowed the team to map variations in stiffness across the films with unprecedented resolution.
The group focused on a widely used organic semiconductor called DNTT, a small-molecule material often employed in flexible transistors. They compared unsubstituted DNTT with several closely related derivatives in which the same rigid molecular core carries different chemical side chains. These side chains act as molecular padding, spacing the rigid cores apart when the molecules pack together and altering both the packing structure and the mechanical response of the resulting solid. By systematically varying the length and flexibility of these side chains, the team could isolate how such modifications change out-of-plane stiffness.
Their AFM measurements revealed that materials with longer and more flexible side chains were softer when pressed perpendicular to the surface, while unsubstituted DNTT remained the stiffest of the set. Versions of DNTT bearing long chains showed a significant reduction in measured stiffness compared with the bare core. Researchers in the field had long assumed that introducing flexible side chains would soften an organic semiconductor, but Venkateshvaran stressed that this effect had not previously been quantified directly at the molecular level, in part because the differences are subtle and require extremely careful measurement conditions to detect reliably.
To validate the experimental observations, the team compared their data with computer simulations that calculated mechanical properties from molecular models of the materials. These calculations independently predicted the same trend: introducing flexible side chains reduces stiffness along the direction probed by the AFM tip. Venkateshvaran described the result using a brick-wall analogy, in which traditional views emphasised the mortar, representing the weak intermolecular forces that hold molecules together, while this work highlights that the bricks themselves, the individual molecules, also make a distinct contribution to the wall's overall rigidity. By separating the contribution from individual molecular units from the collective interactions between them, the study demonstrates an experimental route to disentangle these effects for the first time.
This ability to decompose mechanical behaviour into molecular and intermolecular components opens a path toward molecular-level design of mechanical properties in organic electronics. If chemists can systematically tune the stiffness of individual molecules and the way they pack, they may be able to engineer materials with targeted combinations of flexibility and electronic performance. The current study does not yet prove that stiffness directly controls charge transport in organic semiconductors, but it establishes the experimental tools needed to investigate that relationship carefully. It also shows that the nanoscale expression of flexibility can be mapped and quantified rather than treated as a vague macroscopic property.
In the longer term, understanding how softness and stiffness trade off in molecular semiconductors could inform the design of faster and more efficient flexible devices, from displays to wearable sensors. Venkateshvaran suggested there may be a glass ceiling on how well flexible molecular materials can conduct electricity, imposed by their mechanical nature. By clarifying the connection between nanoscale stiffness and charge transport, researchers might identify strategies to push beyond that ceiling, for example through new molecular architectures or packing motifs that preserve mechanical compliance while maintaining robust pathways for charge motion. The work drew support from the Royal Society, the Wiener-Anspach Foundation and the European Union, and highlights the role of precise nanoscale measurements in guiding the next generation of flexible electronics.
Research Report:Measuring the molecular origins of stiffness in organic semiconductors
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