Study of Spin-Dependent Charge Transfer through Self-Assembled Monolayers of DNA on Metal Surfaces

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Deoxyribonucleic acid (DNA) molecules, the genetic code of life, have recently been shown to possess a surprising filtering capacity for the flow of electric charge. The charge is carried by electrons that are transmitted through each molecule. Electrons also possess an intrinsic property known as spin, and can exist in one of two (usually) equally probable states: spin up or spin down. Surprisingly, DNA molecules can filter transmitted electrons based on their spin, holding exciting implications for their use in emerging technological applications that require the generation and manipulation of spin-polarized electrical currents within the nascent field of spintronics. However, the mechanism responsible for the filtering effect remains elusive and controversial. In order to assess the practicality of DNA molecules as components in next generation spintronic devices, the parameters that have been theoretically predicted to influence electron spin-filtering capacity by DNA molecules assembled on metal surfaces must be tested and determined. The proposed research will leverage two decades worth of studies on manipulating molecular assemblies and recent advances to develop and to understand spin-polarized sources of electrons. This research will combine the fields of nanoscience, nanotechnology, spintronics, bioelectronics, and biomagnetics. Specific advances in terms of experimental tests and their results will be made widely available both through UCLA, national, and global collaborations. Deeper understanding of these effects will lead to important discoveries across disciplines; findings will be broadly disseminated via presentations, publications, and strong collaborative networks. Outreach activities will target young, underrepresented scientists and the public at large by engaging local high school teachers and students, as well as the global entertainment industry based in Southern California. Recent observations of spin-selective phenomena involving chiral molecules prevalent in biological systems demonstrate the unique and exciting potential to utilize diamagnetic molecules for organic spintronics. In particular, unprecedented spin-selective electron transmission has been observed through self-assembled monolayers of deoxyribonucleic acid (DNA) adsorbed on metal surfaces. While experimental evidence has established that assembled, oriented, chiral DNA molecules have the capacity to filter electron spins at room temperature, the mechanism responsible for this phenomenon remains controversial. One community suggests that the phenomenon may be intrinsic to the molecule itself as many theoretical models attribute the observed effects to unconventional Rashba-type molecular spin-orbit coupling. In contrast, leading theory groups argue that the substrates and interfaces play major roles in spin-orbit coupling. Careful experimental tests will deconvolute the relative contributions and will establish a means to optimize spin selection and filtering. Robust figures of merit will be established to characterize spin-dependent charge transport via electrochemical methods. Molecular properties and substrate materials will be tuned systematically to test the theoretically predicted contributions to this phenomenon. Specifically, the modification of molecular chemical and electronic structures including the inclusion of heavy metal species and substrate identity will directly probe spin orbit coupling within the molecule and interface. This work will simultaneously develop a foundational understanding of spin-selective electron-molecule interactions and critically evaluate the practicability of ultimately implementing DNA and/or other chiral molecular assemblies as sources of polarized electrons in next generation organic spin electronic devices such as spin filters, spin-transfer torque magnetoresistive random access memory devices and spin-polarized organic light emitting diodes.