Carbon Contacts to Proteins Enable Robust, Biocompatible Electronic Junctions with Near-Activation-less Conduction Down to 10 K

The integration of functional proteins into solid-state electronic devices remains a central challenge in molecular bioelectronics due to the fragile nature of protein structures and their complex charge-transport behavior. Here, we present a robust crosswire evaporated top-contact device based on bacteriorhodopsin (bR) single-bilayers (SBL), configured as Au/Cys/bR(SBL)/eC/Au (simplified as Au/bR/eC). The evaporated carbon (eC) top electrode forms a conformal, non-invasive contact that suppresses filament formation and ensures electrical integrity across the crosswire intersecting area (≈200 µm²). Structural and spectroscopic analyses confirm that the solid-state bR films retain the native absorption spectrum and exhibit functional photocycle activity after electrode deposition, indicating that their native conformation is not significantly altered. Remarkably, electron transport (ETp) through the ≈9 nm bR-SBL junctions is temperature-independent within 300 K–10 K, excluding thermally activated hopping, while the length is incompatible with coherent tunneling. Under green illumination, the junctions exhibit a reversible, photo-induced current enhancement (J_green/J_dark ≈ 2), ascribed to light-driven conformational changes rather than direct photoexcitation. The Au/bR/eC architecture thus establishes a thermally non-activated, conformationally mediated transport mechanism via a stable, cryo-compatible solid-state protein junction. This work provides a scalable platform for integrating light-responsive biomolecules into future bio-optoelectronic and neuromorphic devices.