CAREER: Superelastic Organic Semiconductors (SOSs): A New Class of Molecular Crystals of Responsive Shape Memory

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This Faculty Early Career Development (CAREER) grant will promote fundamental understanding of the responsive shape memory effect in a new class of superelastic organic semiconductors (SOSs). The shape memory effect can be illustrated by the ability of a material to remember and recover a programmed shape via thermal and mechanical input. Superelasticity refers to the ability of a material to recover a large amount of mechanical deformation at a given temperature. Superelasticity in organic crystals - through interconvertible phase changes - is a recently discovered materials phenomenon. This discovery will likely breed a new research field on polymorphic engineering of molecular crystals, providing a way of overcoming the intrinsic fragility (brittleness) of organic crystals in deformable electronics. Polymorphism is the ability of a material to exist in more than one crystal structure (ordered molecular arrangement). This award explores the fundamental relationship between those structural states and the functionalities of superelasticity, ferroelasticity, and shape memory. Electronic devices with SOSs as active layers can respond to environmental stimuli without additional circuits and find a variety of applications such as remote sensing, memory devices, and programmable electronics. SOSs are also intriguing when it comes to their mechanically and thermally tunable electrical and optical properties. With the potential to create new forms of electronic and optical devices, it is vital to understand and rationalize the mechanics of superelastic organic semiconductors. This research project will both theoretically and experimentally analyze the deformability and shape memory in organic semiconductors under mechanical and thermal load and further understand the fundamental relationship between the mechanical, thermal, and optoelectronic properties of SOSs. The research will leverage the educational and outreach activities based on new curriculum development integrating data sciences, engineering education for K-12 students through an existing collaboration with Women in Engineering Program at Purdue, and engagement of underrepresented groups in engineering sciences.The specific goal of the research is to understand the mechanics and molecular mechanism of superelasticity, ferroelasticity, and shape memory effect in a new class of organic semiconductors using multi-scale theoretical modeling and experimentation approaches. The research will (i) understand the cooperative molecular mechanism underlying the deformability and shape memory effect in the solid-state molecular crystal, (ii) understand the thermodynamics, kinetics, and stress profiles along the trajectory of the martensitic transition under the thermal and mechanical load, (iii) understand the molecular kinetics and deformation twinning/detwinning in organic crystals responsible for the superelasticity, ferroelasticity, and shape memory effect, and (iv) establish the structure-property relationship by understanding the mechanical, electronic, optical, and thermal properties of SOSs for the use in optoelectronics. Overall, the research project is to address a grand challenge in the fundamental understanding of molecular structures of macroscopic and reversible deformation in response to external stimuli. The fundamental understanding of superelasticity/ferroelasticity in organic crystals will create new knowledge about the martensitic phase transition in solid-state molecules. Such knowledge can open new avenues for rapid, reversible modulation of electronic and optical properties by means of molecular design.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.