Design and discovery of chiral semiconductors with tunable optical and electrochemical responses

About This Grant

Non-technical summary Chirality is the phenomenon where two objects, called enantiomers, can exist as non-superimposable mirror images of each other, such as one's left and right hands. Chirality is ubiquitous in organic molecules which are essential to life, including amino acids and carbohydrates. These are selectively found in nature as only one of the two mirror image forms. Some inorganic crystals such as quartz also possess chiral structures. While inorganic crystals that are both chiral and light-absorbing semiconductors are rare, they could enable new types of devices such as photodetectors. Photodetectors can be used in enhanced biomedical imaging devices and can be used to selectively produce chiral pharmaceutical agents. However, a significant challenge in realizing these devices is that the chiral semiconductor must be prepared with high excess of one of its two mirror image forms, known as enantiomeric excess. With support from the Solid State and Materials Chemistry Program in NSF's Division of Materials Research, Professors Bryce Sadtler and Rohan Mishra at Washington University in St. Louis combine experiments and theory to discover new types of chiral semiconductors and synthesize them with enantiomeric excess. By identifying new chiral semiconductors and synthesizing them with enantiomeric excess, Profs. Sadtler and Mishra tune their properties for applications of national interest including chiral electrodes for the electrochemical synthesis of chiral pharmaceutical compounds. Additionally, Profs Sadtler and Mishra are organizing a symposium at a national conference on the synthesis, characterization, theory, and applications of chiral materials. Technical summary Chiral semiconductors that absorb light to generate mobile charge carriers offer unique light-matter interactions and electron-transport properties including polarization-dependent photocurrents, chiral-induced spin selectivity for electron transport, and ability to discriminate between enantiomers of chiral molecules during photoinduced redox reactions. While several novel classes of chiral semiconductors have recently been developed, the current pool of inorganic compounds with chiral structures is still small, and the methods to produce these materials with enantiomeric excess are largely empirical and compound specific. The development of general strategies to identify and prepare new chiral semiconductors with control over enantioselectivity remains a fundamental scientific challenge. With support from the Solid State and Materials Chemistry Program in the NSF's Division of Materials Research, Prof. Sadtler is testing the hypothesis that the introduction of chiral ligands during solution-phase synthesis, such as chemical bath deposition and colloidal nanocrystal synthesis, can provide a general route to template the growth of chiral metal oxide and chalcogenide semiconductors with enantiomeric excess. Prof. Sadtler is first applying this method to known chiral semiconductors, including the metastable chiral phases of tin sulfide and tin selenide. Simultaneously, Prof. Mishra is employing calculations using density functional theory and group theoretical methods to screen for hidden metastable phases of metal oxides and chalcogenides that possess chiral crystal structures. Through iteration between theory and experiment, Profs. Sadtler and Mishra develop new insights into how chemical strain via alloying of the ions that comprise the compound semiconductor and epitaxial strain with the growth substrate can stabilize these metastable chiral phases. They perform structural, optical, and electrochemical characterization of the chiral semiconductors to elucidate how the crystal structure, the degree of enantiomeric excess of crystallites within the film, and the nanoscale morphology control the optical response and enantioselectivity for electrochemical transformations of chiral molecules. A general method for identifying and synthesizing chiral semiconductors with enantiomeric excess provides a broader palette to tailor the optical and electrochemical properties of these materials for applications in sensing, photonics, and enantioselective catalysis. 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.