Low-Resistance Molecular Wires Propagate Spin-Polarized Currents.

Ruobing Wang,Suryakant Mishra,George Bullard,Amit Kumar Mondal,Ron Naaman,Francesco Tassinari,Michael J Therien,Chih-Hung Ko

doi:10.1021/jacs.9b06142

Abstract

Spin based properties, applications, and devices are typically related to inorganic ferromagnetic materials. The development of organic materials for spintronic applications has long been encumbered by its reliance on ferromagnetic electrodes for polarized spin injection. The discovery of the chirality-induced spin selectivity (CISS) effect, in which chiral organic molecules serve as spin filters, defines a marked departure from this paradigm because it exploits soft materials, operates at ambient temperature, and eliminates the need for a magnetic electrode. To date, the CISS effect has been explored exclusively in molecular insulators. Here we combine chiral molecules, which serve as spin filters, with molecular wires that despite not being chiral, function to preserve spin polarization. Self-assembled monolayers (SAMs) of right-handed helical (l-proline)8 (Pro8) and corresponding peptides, N-terminal conjugated to (porphinato)zinc or meso-to-meso ethyne-bridged (porphinato)zinc structures (Pro8PZnn), were interrogated via magnetic conducting atomic force microscopy (mC-AFM), spin-dependent electrochemistry, and spin Hall devices that measure the spin polarizability that accompanies the charge polarization. These data show that chiral molecules are not required to transmit spin-polarized currents made possible by the CISS mechanism. Measured Hall voltages for Pro8PZn1-3 substantially exceed that determined for the Pro8 control and increase dramatically as the conjugation length of the achiral PZnn component increases; mC-AFM data underscore that measured spin selectivities increase with an increasing Pro8PZn1-3 N-terminal conjugation. Because of these effects, spin-dependent electrochemical data demonstrate that spin-polarized currents, which trace their genesis to the chiral Pro8 moiety, propagate with no apparent dephasing over the augmented Pro8PZnn length scales, showing that spin currents may be transmitted over molecular distances that greatly exceed the length of the chiral moiety that makes possible the CISS effect.

Highlights

Molecular electronics promises both new opportunities for device miniaturization and possibilities for revolutionary device concepts
PZn2 and PZn3 belong to a larger class of meso-to-meso ethyne-bridgedzinc(II) structures (PZnn oligomers) that manifest exceptional electro-optical properties that include substantial polarizabilities and low-lying singlet states that are aligned exclusively along the long molecular axis.[6−10] ESR data acquired for charge-doped PZnn demonstrate globally delocalized hole and electron polaron states through substantial oligomer lengths[4,5] and corresponding spin relaxation times 3 orders of magnitude greater than that established for classic organic semiconductor Alq3.22 Scanning tunneling microscopy break-junction measurements show that metal-(thiol-PZnn-thiol)-metal junctions express quasi-ohmic resistances and afford one of the lowest β values (β = 0.034 Å−1; i.e., the phenomenological resistance decay parameter across the barrier) yet determined for thiol-terminated single molecules.[23]
The combination of spin Hall device, spin-dependent electrochemical, and magnetic conducting atomic force microscopy data, acquired for these Pro[8] and Pro8PZnn systems, demonstrates that once a spin current has been generated via tunneling through a chiral oligoproline unit, it persists through the appended achiral PZnn structure, which, in contrast to vParlou8e,30(βsu=pp0o.r0ts34exÅte−n1s;ivie.ec.,htahrgeepdheeloncoamlizeantoiolong4i,c5aalnrdesaisltoawncβe decay parameter across the barrier).[23]

Summary

■ INTRODUCTION

Molecular electronics promises both new opportunities for device miniaturization and possibilities for revolutionary device concepts. In the electron’s rest frame, the current transmitted through the chiral molecule generates a magnetic field proportional to the velocity of the moving electron and the electric field operating on the electron. This effective magnetic field acts on the electron’s magnetic moment, stabilizing one spin state and destabilizing the other. The CISS effect has been demonstrated in selfassembled monolayers (SAMs) composed of a wide range of chiral structures that include low-molecular-weight molecules,[16] DNA,[13,14,17,18] oligopeptides,[19] and proteins,[20] spinpolarized currents realized by this mechanism have far been transmitted exclusively through chiral molecular insulators. A description of the synthesis and characterization of all new compounds, detailed reaction schemes, device design, fabrication, and characterization, methods used to prepare and characterize selfassembled monolayers, and instrumentation utilized in this study may be found in the Supporting Information

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES