Without Contact Resistance, Proteins in Thin‐Film Solid‐State Junctions Can Be Efficient Electronic Conducting Materials

The solid-state protein junctions have shown efficient electron transport over a few tens of nanometer lengthscale. This work demonstrates, how the contact resistance ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mover><mml:msub><mml:mi>R</mml:mi> <mml:mi>C</mml:mi></mml:msub> <mml:mo>̂</mml:mo></mml:mover> <mml:annotation>$\widehat {{R_{\mathrm{C}}}}$</mml:annotation></mml:semantics> </mml:math> ) of a solid-state protein junctions, treated as a contact-limited process, which can be extracted quantitatively from the measured junction resistance (R_P) by using the extrapolated zero-length resistance and series resistance (R_S). Alternating current (impedance spectroscopy) and direct current measurements are used to examine charge transport in junctions of human serum albumin (HSA) and bacteriorhodopsin (bR) films with varying thicknesses. Three contact configurations, Si-Au, Au-eutectic gallium indium (EGaIn), and, in a micropore device (MpD), Au-Pd, are compared. While Si-Au and Au-EGaIn junctions exhibit substantial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mover><mml:msub><mml:mi>R</mml:mi> <mml:mi>C</mml:mi></mml:msub> <mml:mo>̂</mml:mo></mml:mover> <mml:annotation>$\widehat {{R_{\mathrm{C}}}}$</mml:annotation></mml:semantics> </mml:math> that are ascribed to interfacial oxides and electrostatic protein-electrode interactions, MpD effectively eliminates <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mover><mml:msub><mml:mi>R</mml:mi> <mml:mi>C</mml:mi></mml:msub> <mml:mo>̂</mml:mo></mml:mover> <mml:annotation>$\widehat {{R_{\mathrm{C}}}}$</mml:annotation></mml:semantics> </mml:math> , enabling measuring the intrinsic electron transport across HSA and bR films. The exponential length-dependence of R_P shows a transport decay constant (β) that varies with interfacial conditions, underscoring the role of contact engineering. By minimizing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mover><mml:msub><mml:mi>R</mml:mi> <mml:mi>C</mml:mi></mml:msub> <mml:mo>̂</mml:mo></mml:mover> <mml:annotation>$\widehat {{R_{\mathrm{C}}}}$</mml:annotation></mml:semantics> </mml:math> , exceptionally low β values (≈0.7-1.1 nm^-1) are found, proving that, indeed, proteins can have outstanding charge transport efficiencies.