Near-Temperature-Independent Electron Transport Well beyond Expected Quantum Tunneling Range via Bacteriorhodopsin Multilayers

Sudipta Bera, Jerry A. Fereiro, Shailendra K. Saxena, Domenikos Chryssikos, Koushik Majhi, Tatyana Bendikov, Lior Sepunaru, David Ehre, Marc Tornow, Israel Pecht, Ayelet Vilan, Mordechai Sheves, David Cahen

Research output: Contribution to journalArticlepeer-review

Abstract

A key conundrum of biomolecular electronics is efficient electron transport (ETp) through solid-state junctions up to 10 nm, often without temperature activation. Such behavior challenges known charge transport mechanisms, especially via nonconjugated molecules such as proteins. Single-step, coherent quantum-mechanical tunneling proposed for ETp across small protein, 2-3 nm wide junctions, but it is problematic for larger proteins. Here we exploit the ability of bacteriorhodopsin (bR), a well-studied, 4-5 nm long membrane protein, to assemble into well-defined single and multiple bilayers, from ∼9 to 60 nm thick, to investigate ETp limits as a function of junction width. To ensure sufficient signal/noise, we use large area (∼10-3 cm2) Au-protein-Si junctions. Photoemission spectra indicate a wide energy separation between electrode Fermi and the nearest protein-energy levels, as expected for a polymer of mostly saturated components. Junction currents decreased exponentially with increasing junction width, with uniquely low length-decay constants (0.05-0.5 nm-1). Remarkably, even for the widest junctions, currents are nearly temperature-independent, completely so below 160 K. While, among other things, the lack of temperature-dependence excludes, hopping as a plausible mechanism, coherent quantum-mechanical tunneling over 60 nm is physically implausible. The results may be understood if ETp is limited by injection into one of the contacts, followed by more efficient charge propagation across the protein. Still, the electrostatics of the protein films further limit the number of charge carriers injected into the protein film. How electron transport across dozens of nanometers of protein layers is more efficient than injection defines a riddle, requiring further study.

Original languageEnglish
JournalJournal of the American Chemical Society
Early online date6 Nov 2023
DOIs
StateE-pub ahead of print - 6 Nov 2023
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2023 The Authors. Published by American Chemical Society.

Funding

D.Ca., M.S., and S.B. thank the Israel Science Foundation (ISF) for initial funding. D.Ca., M.S., M.T., and D.Ch. thank the DFG for partial funding (Middle East Collaboration, Grant TO266/10-1). M.S. thanks the Kimmelman Center for Biomolecular Structure and Assembly for partial support. We thank Antoine Kahn for UPS data analysis, Roman Korobko and Omer Yaffe for making impedance measurement facilities available to us and Kavita Garg for some early work with double bR bilayer junctions. M.S. holds the Katzir-Makineni Chair in Chemistry. We thank the reviewers for constructive criticism.

FundersFunder number
Deutsche ForschungsgemeinschaftTO266/10-1
Israel Science Foundation

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