Rectification in Molecular Tunneling Junctions Based on Alkanethiolates with Bipyridine-Metal Complexes

Area of Science:

• Materials Science
• Molecular Electronics
• Chemistry

Background:

• Molecular tunneling junctions are crucial for nanoscale electronic devices.
• Alkanethiolate self-assembled monolayers (SAMs) on gold surfaces enable controlled molecular architectures.
• Bipyridine-metal complexes offer tunable electronic properties for molecular junctions.

Purpose of the Study:

• To investigate the mechanism of current rectification in molecular tunneling junctions.
• To understand the role of metal ions (Cobalt vs. Copper) in dictating junction behavior.
• To correlate electronic structure with observed rectification properties.

Main Methods:

• Fabrication of molecular tunneling junctions using alkanethiolates with bipyridine head groups complexed with CoCl2 or CuCl2.
• Electrical characterization of the Au-SAM-EGaIn junctions under varying bias.
• Analysis of molecular orbital accessibility (HOMO/LUMO) in relation to rectification.

Main Results:

• Au-S(CH2)11BIPY-CoCl2 junctions exhibited significant current rectification (r=82.0 at ±1.0 V).
• Au-S(CH2)11BIPY-CuCl2 junctions showed no rectification (r=1.0).
• Cobalt junctions had an accessible LUMO at positive bias and inaccessible HOMO/LUMO at negative bias, while copper junctions had accessible HOMO and LUMO at both biases.

Conclusions:

• Rectification in Co-based junctions is attributed to the selective accessibility of the LUMO orbital at positive bias.
• Lack of rectification in Cu-based junctions results from accessible HOMO and LUMO orbitals at both positive and negative biases.
• Differences in charge transport mechanisms (Fowler-Nordheim vs. direct tunneling) at negative bias explain the varying rectification ratios.