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STM studies of charge transfer and transport through metal-molecule complexes on ultrathin insulating films

Choi, Taeyoung
http://rave.ohiolink.edu/etdc/view?acc_num=osu1299525515

Abstract Details

2011, Doctor of Philosophy, Ohio State University, Physics.
Traditional electronic and memory devices face a fundamental limit as their size shrinks to an order of one nanometer. An understanding of the electronic and magnetic properties of single atoms and molecules is needed for their incorporation into passive or active components of such devices. Ultrathin insulating films comprising only a few atomic layers are useful materials for building nanostructures from atoms and molecules and for controlling their electronic and magnetic coupling at the nanoscale. Scanning Tunneling Microscopy (STM) is used to study the geometric and electronic structure of an ultrathin insulating film, Cu2N, grown on a Cu(100) surface. We find that the Cu2N lattice is incommensurate with the Cu(100) lattice, suggesting that the resulting strain contributes to the self-assembly of Cu2N into square shaped, ~25nm2 islands. Atomically resolved STM images enable us to register Cu and N atoms within the islands and reveal a contrast reversal with voltage that is similar to that for the GaAs(110) surface. Scanning Tunneling Spectroscopy (STS) indicates that the Cu2N islands suppress the local density of state (LDOS) of Cu(100) within a range from +2eV to -4eV and thus act as an insulator. We study single magnetic atoms (Fe and Co) and clusters on the Cu2N islands grown on Cu(100). Our tunneling spectra show step-like features which are due to the opening of inelastic tunneling channels. The inelastic conductance originates from excitations among spin states (spin-IETS). We observe the evolution of magnetic moments as a function of the number of atoms in clusters or chains. Transition metal ion (M=V, Mn, Co, and Fe)-tetracyanoethylene (TCNE, C6N4) compounds (e.g. V(TCNE)2) are well known organic semiconductors exhibiting ferromagnetism above room temperature. As a bottom-up approach, we studied single TCNE molecules on Cu(111). The single molecules display five distinct configurations which are reversibly switchable by voltage pulses from the STM tip. One of states shows a Kondo resonance in tunneling spectroscopy. Our density functional theory calculation suggests that molecular deformation is responsible for a rearrangement of molecular orbitals, such that unpaired spin in an orbital activates the Kondo resonance. We also study single TCNE molecules on Cu2N and successfully demonstrate the formation of single atom-molecule complexes (Fe-, Co-, and Cu-tetracyanoethylene) on Cu2N via atomically controlled manipulation. We find that the magnetic moments of metal atoms in these complexes change due to bonding with TCNE. The variation in spin-IETS could be useful for quantifying the degree of charge transfer and understanding spin-spin interactions in organic magnets. In order to realize charge/spin transport through a single molecule, we studied azobenzene and alkanedithiol molecules on Cu2N. Molecules bridging Co atoms on Cu2N and the Cu(100) substrate act as an additional pathway for current flow. This pathway is evidenced as a change in the Co Kondo resonance lineshape from a positive Lorentzian, to a more derivative-like Fano resonance.
Jay Gupta, PhD (Advisor)
Arthur Epstein, PhD (Committee Member)
Louis DiMauro, PhD (Committee Member)
David Stroud, PhD (Committee Member)
Lafont Jean-Francois, PhD (Committee Member)
274 p.
Physics
Scanning tunneling microscopy; Spectroscopy; Ultrathin insulating layer; Cu2N; Spin IETS; Charge transport; Charge transfer; Kondo; Single molecule; Switch; Spin excitation;

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Citations

  • Choi, T. (2011). STM studies of charge transfer and transport through metal-molecule complexes on ultrathin insulating films [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1299525515

    APA Style (7th edition)

  • Choi, Taeyoung. STM studies of charge transfer and transport through metal-molecule complexes on ultrathin insulating films. 2011. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1299525515.

    MLA Style (8th edition)

  • Choi, Taeyoung. "STM studies of charge transfer and transport through metal-molecule complexes on ultrathin insulating films." Doctoral dissertation, Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1299525515

    Chicago Manual of Style (17th edition)

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This open access ETD is published by The Ohio State University and OhioLINK.