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

Area of Science:

• Biomolecular electronics
• Solid-state physics
• Protein biophysics

Background:

• Efficient electron transport (ETp) across solid-state junctions up to 10 nm is a key challenge in biomolecular electronics.
• Existing charge transport mechanisms struggle to explain ETp in non-conjugated molecules like proteins, especially over larger distances.
• Coherent quantum tunneling is proposed for small protein junctions (2-3 nm) but is problematic for thicker films.

Purpose of the Study:

• To investigate the limits of electron transport (ETp) across bacteriorhodopsin (bR) protein films as a function of junction width.
• To explore ETp mechanisms in thick protein layers (9-60 nm) and their temperature dependence.

Main Methods:

• Fabrication of large-area (∼10^-3 cm²) gold-protein-silicon junctions using bacteriorhodopsin (bR) protein.
• Assembly of bR into single and multiple bilayers, creating junction widths from approximately 9 to 60 nm.
• Measurement of photoemission spectra and junction currents as a function of temperature and junction width.

Main Results:

• Junction currents exhibited an exponential decrease with increasing junction width, with low length-decay constants (0.05-0.5 nm^-1).
• Electron transport remained nearly temperature-independent, particularly below 160 K, even for the widest junctions.
• Observed behavior challenges hopping and coherent quantum tunneling as primary transport mechanisms over these distances.

Conclusions:

• The findings suggest that electron transport is limited by charge injection into the protein film contacts, rather than propagation through the protein.
• The electrostatics of the protein films further restrict carrier injection, creating a bottleneck for efficient ETp.
• The riddle of efficient charge propagation across dozens of nanometers of protein layers, surpassing injection efficiency, requires further investigation.