Electrochemically controlled rectification in symmetric single-molecule junctions

Area of Science:

• Single-molecule electronics
• Molecular electrochemistry
• Nanoscale device fabrication

Background:

• Single-molecule electrochemical science has evolved to include molecular electronics functions like rectification.
• Achieving rectification typically requires complex asymmetric molecular structures or electrode geometries.
• Developing simpler methods for single-molecule rectification is crucial for advancing molecular electronics.

Purpose of the Study:

• To propose and validate an experimental and theoretical strategy for in situ (in operando) rectification in symmetric molecular structures.
• To demonstrate the ability to tune rectification polarity and amplitude by controlling energy alignment and electrolyte concentration.
• To offer a pathway for constructing controllable single-molecule rectifying devices without asymmetric molecular designs.

Main Methods:

• Utilized electrochemical scanning tunneling microscopy (EC-STM) with a bipotentiostat for independent control of tip and substrate electrode potentials.
• Designed molecules capable of electronic conduction via lowest unoccupied molecular orbital (LUMO) or highest occupied molecular orbital (HOMO).
• Created asymmetric energy alignment between the STM tip, molecule, and substrate to induce rectification.

Main Results:

• Observed single-molecule rectification in symmetric molecules within a ±0.5 V voltage range due to asymmetric energy alignment.
• Successfully tuned rectification polarity and amplitude by varying dominant charge transport orbital (LUMO/HOMO) and electrolyte concentration.
• Extended existing theory to accurately predict and rationalize the observed in situ rectification phenomena, showing excellent agreement with experiments.

Conclusions:

• Demonstrated a novel method for achieving and tuning single-molecule rectification using symmetric molecules and controlled energy alignment.
• This approach bypasses the need for challenging asymmetric molecular synthesis, simplifying the fabrication of molecular electronic devices.
• The findings provide a foundation for developing controllable, electrolyte-tuned single-molecule rectifying devices.