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  4. Near-Temperature-Independent Electron Transport Well beyond Expected Quantum Tunneling Range via Bacteriorhodopsin Multilayers
 
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2023
Journal Article
Title

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

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.
Author(s)
Bera, Sudipta
Weizmann Institute of Science
Fereiro, Jerry A.
Weizmann Institute of Science
Saxena, Shailendra K.
Chryssikos, Domenikos  
Fraunhofer-Einrichtung für Mikrosysteme und Festkörper-Technologien EMFT  
Majhi, Koushik
Bendikov, Tatyana
Sepunaru, Lior
Ehre, David
Tornow, Marc  
Fraunhofer-Einrichtung für Mikrosysteme und Festkörper-Technologien EMFT  
Pecht, Israel
Vilan, Ayelet
Sheves, Mordechai
Cahen, David
Journal
Journal of the American Chemical Society  
Open Access
DOI
10.1021/jacs.3c09120
Additional full text version
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