Recent observations of spin-dependent and enantioselective interactions between electrons and chiral biomolecules (e.g., DNA, -helical peptides, and proteins) at room temperature have inspired studies to elucidate the roles of spin and chirality in biology and in charge transfer at metal-molecule interfaces. Electrons of a certain spin orientation are transmitted through chiral molecules more easily in one direction vs the other, a phenomenon described as the chiral-induced spin selectivity effect.
However, identifying the preferred spin-velocity relationship for electrons confined to move along helical potentials has proven to be difficult, with conflicting experimental results regarding preferred polarization orientation. Thus, to elucidate the preferred spin polarization direction in DNA-mediated charge transport, I applied our group’s expertise in molecular self-assembly, large-scale molecular patterning, and data processing and analysis from information-rich images, to investigate the effect via fluorescence microscopy.
Fluorescent perylenediimide derivatives were precisely incorporated within hydrophobic pockets in double-stranded DNA helices. The DNA/dye complexes were subsequently patterned on ferromagnetic substrates that could be magnetized parallel or antiparallel to the nominally vertically aligned DNA strands. There are two relaxation pathways following photoexcitation. The dye molecules either fluoresce, or when well-coupled to the DNA, display competitive quenching due to charge transfer to the underlying ferromagnetic surface. Because charge injection into ferromagnetic materials is spin dependent, a dependence of the fluorescence intensity on substrate magnetization direction is indicative of spin filtering; lower fluorescence intensities in this system correspond to higher degrees of charge quenching and transfer from the dye to the substrate. My results suggest that electron helicity, or spin projection along the helical axis of DNA, is preferentially aligned parallel to its velocity direction within this charge transport regime.
Yet, while I and others have demonstrated that chiral molecules can polarize transmitted electrons, unifying mechanisms that account for the magnitude of spin polarization, and that can predict the strength of the relativistic effects due to helix-induced spin-orbit coupling, remain elusive. Development of accurate models has been impeded, in part, by the lack of quantitative, experimental analyses on the relative energy barriers to spin-dependent scattering of electrons within chiral electrostatic fields with precise orientation control.
To tackle this challenge, I developed experiments to test spin selectivity in a second charge transport regime: photoelectron transmission through adsorbed chiral molecule assemblies. Ultraviolet photoelectron spectroscopy was used to measure the ionization energy and work function of these systems, and therefore the spin-selective energy barriers to photoemission from chiral molecule films. I hypothesize that photoelectrons emitted by ionization of chiral molecular films using unpolarized ultraviolet radiation leave behind spin-polarized holes. Underlying ferromagnetic substrates provide a source of replenishing spin-polarized electrons, thus, effective ionization energies depend on substrate magnetization orientation. I measured significant differences in the ionization energies and work function values of ferromagnetic substrates coated with chiral films of ca. 100 and 80 meV, respectively, that depended on substrate magnetization orientation, relative saturation of the substrate magnetization, and molecular handedness.
Having shown that the chiral-induced spin selectivity effect is subtle in the context of charge-transport through self-assembled monolayers of chiral molecules, I internalized the necessity of repeated measurements, unbiased statistical analysis of large data sets, and careful design of control experiments. Continuing these practices, my measurements have enabled the unprecedented determination of the relative spin-dependent energy barriers to transmission through chiral molecules, which will be critical in the development and evolution of theoretical models necessary for foundational understanding of this phenomenon.
Moving forward, elucidating the mechanistic contributions to spin filtering from the adsorbed chiral species, underlying ferromagnetic materials, and metal-molecule interfaces will enable us to critically assess the practicality of chiral organic materials for spintronics applications. Devices that utilize stable organic layers may facilitate the design, development, and implementation of next-generation electronic device architectures that exploit spin injection and detection at metal/semiconductor, chiral-molecule interfaces for information storage, memory technology, sensors, optics, and energy-efficient electronics.