One-Nanometer Thin Monolayers Remove the Deleterious Effect of Substrate Defects in Molecular Tunnel Junctions

Area of Science:

• Materials Science
• Surface Chemistry
• Molecular Electronics

Background:

• Defects in self-assembled monolayers (SAMs) significantly impact the accuracy of predicted versus measured tunneling currents in molecular junctions.
• Understanding and mitigating these defects is crucial for advancing molecular electronics and reliable device fabrication.

Purpose of the Study:

• To investigate the counterintuitive finding that shorter, less-ordered SAMs yield higher quality tunneling barriers on defective substrates compared to taller, ordered SAMs.
• To elucidate the self-repair mechanisms of liquid-like SAMs on surface defects using molecular dynamics simulations.

Main Methods:

• Fabrication and characterization of self-assembled monolayer (SAM) based junctions with varying molecular lengths and order.
• Utilizing molecular dynamics simulations to model SAM behavior on defective substrates.
• Measuring and comparing tunneling currents across different SAM configurations.

Main Results:

• Shorter, less-ordered (liquid-like) SAMs form superior tunneling barriers on defective substrates.
• Taller, crystalline-like SAMs exhibit poorer barrier quality on the same defective substrates.
• Molecular dynamics revealed that short-chain molecules can reorient to fill and smooth surface defects more effectively than long-chain molecules.

Conclusions:

• Flexible, shorter SAMs possess a self-repairing capability that enhances tunneling barrier quality on imperfect surfaces.
• The findings suggest using flexible molecules as a strategy to overcome the detrimental effects of substrate defects in SAM-based devices.