Optical imaging beyond the diffraction limit by SNEM: effects of AFM tip modifications with thiol monolayers on...

Area of Science:

• Nanoscale optical imaging
• Surface science
• Materials science

Background:

• Scanning near-field optical microscopy (SNOM) techniques like a-SNOM and SNEM are powerful tools for nanoscale imaging.
• Performance degradation occurs due to atmospheric contaminant adhesion on metal-coated tips, limiting imaging consistency.
• Atomic force microscopy (AFM) probes are commonly used as tips in these techniques.

Purpose of the Study:

• To investigate the effectiveness of self-assembled monolayers (SAMs) in protecting gold-coated AFM probe tips from surface contamination.
• To extend the period of consistent and reproducible results for SNEM imaging.
• To analyze the impact of SAM thickness on optical signal intensity and image contrast.

Main Methods:

• Fabrication of gold-coated AFM probe tips.
• Formation of ethanethiol (EtSH) SAMs on probe tips.
• Comparison of SNEM imaging with bare and SAM-protected tips before and after ambient air exposure.
• Analysis of optical signal intensity decay over time.
• Modification of tips with varying alkanethiol chain lengths and comparison with point dipole model calculations.

Main Results:

• SAM protection with EtSH reduced optical signal intensity loss from 66% (bare tip) to 14% (SAM-protected tip) after five days of air exposure.
• Longer alkanethiols (e.g., 1-dodecanethiol) suppressed field enhancement but could be locally removed under load, restoring contrast.
• Experimental results were consistent with point dipole model calculations.

Conclusions:

• Self-assembled monolayers of ethanethiol effectively protect gold-coated AFM tips from atmospheric contaminants, significantly improving SNEM imaging stability.
• SAMs extend the operational lifetime and reproducibility of nanoscale optical imaging.
• Tip modification strategies, including SAM removal under load, offer potential for dynamic control of imaging parameters.