Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so...

Area of Science:

• Molecular electronics
• Condensed matter physics
• Nanotechnology

Background:

• High current rectification ratios (RRs) in molecular electronics are often predicted by the potentiometer rule, relating RR to the localization of molecular orbitals.
• Alkane monothiol (CnT) molecular junctions were expected to exhibit high RRs due to their HOMO localized on the thiol group.

Purpose of the Study:

• To investigate the discrepancy between predicted and measured rectification ratios in CnT molecular junctions.
• To understand the role of the Stark effect and contact quality in molecular rectification.

Main Methods:

• Fabrication and characterization of CnT molecular junctions using conducting probe atomic force microscopy (CP-AFM).
• Measurement of current-voltage (I-V) characteristics for various molecular lengths and metallic contacts (Ag, Au, Pt).
• Computational analysis using ab initio OVGF (outer valence Green's function) quantum chemistry calculations.

Main Results:

• Measured RRs (∼1.5) were significantly lower than predicted by the potentiometer rule.
• The linear shift in HOMO position with applied bias (γ) was smaller than expected and had an opposite sign to predictions.
• Calculations confirmed a Stark shift (γm) with the correct sign but a magnitude much larger than experimentally observed γ, indicating significant bias screening by contacts.

Conclusions:

• The Stark effect plays a role in molecular rectification, but its impact is diminished by non-ideal molecule-electrode contacts that screen the applied bias.
• Optimizing contacts could dramatically increase rectification ratios in CnT junctions, potentially reaching RR ≈ 500.
• The findings highlight the general importance of considering Stark shifts and contact quality for advancing molecular electronics.