Spin-Dependent Charge Transport through Chiral Assemblies

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Professor Paul S. Weiss of the University of California, Los Angeles is supported by the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program of the Division of Chemistry to study charge transport through organic chiral (e.g., helical) molecules and assemblies. The chiral molecules and molecular assemblies are placed on magnetic surfaces and nanostructures. A number of versatile spectroscopic and microscopic measurements and imaging techniques are used to probe the ultrafast charge transfer movements through molecules and assemblies that are attached to surfaces. The research results may advance the general fields of spintronics and magnetism. Better understanding of these organic assemblies paves the way for their use as complements or replacements for current spintronic devices, such as computer logic and memory with low power and high density. In the course of conducting the project, students are trained in a multidisciplinary environment, including international collaboration. In addition, Professor Weiss gives popular talks through different venues, such as National Public Radio (NPR) and courses for high school students and teachers. The project aims to elucidate the molecular, substrate, and interfacial properties that contribute to spin-selective charge transport in chiral molecular assemblies adsorbed on metal and semiconducting surfaces. The ultimate goal is to understand the processes involved and to assess the future practicality of implementing chiral organic assemblies as spin filters within device architectures. First, the mechanistic role of molecular spin-orbit coupling on electron spin filtering by DNA monolayers are determined. Heavy metal ions are incorporated within DNA molecules to manipulate and to enhance helix-induced spin-orbit-coupling. Monolayers of DNA are formed on ferromagnetic substrates and on the surfaces of GaN Hall effect devices, and the effects of mercuration are probed using spin-dependent electrochemistry, photoelectron spectroscopy, and Hall voltage measurements. Second, the coupling between ferromagnetic substrates and quantum dots tethered via chiral ligands is investigated. The dependence of photoluminescence lifetime of the chiral molecule-functionalized quantum dots are measured as a function of handedness and substrate magnetization using fluorescence lifetime imaging of patterned arrays of chiral quantum dots. The chiral signature, composition, and geometry of the quantum dots, as well as the lengths of tethering molecules (and thus the electronic coupling to the substrate) are tested. Finally, ultrafast charge transfer measurements between carboranethiolate enantiomers and magnetized substrates using core-hole clock spectroscopy are conducted.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.