Non-contact AFM imaging in water using electrically driven cantilever vibration

Area of Science:

• Surface Science
• Nanotechnology
• Analytical Chemistry

Background:

• Atomic Force Microscopy (AFM) is a powerful tool for nanoscale imaging.
• Existing AFM techniques often require operation in air or vacuum, limiting their use in aqueous solutions.
• Characterizing electrical properties alongside topography in liquid environments presents significant challenges.

Purpose of the Study:

• To present a novel AFM imaging mode capable of simultaneous topography and electrical property measurement in aqueous solutions.
• To adapt and extend the Scanning Probe Force Microscopy (SPFM) technique for operation in water.
• To address the theoretical and practical complexities of performing AFM in polarizable media like water.

Main Methods:

• Utilizing a non-contact AFM mode where tip-sample interactions in an aqueous medium induce cantilever vibrations.
• Employing two distinct vibration modes: one at the applied voltage frequency (ω) and another at twice this frequency (2ω).
• Implementing feedback control on the 2ω vibration amplitude for topography imaging and analyzing the 1ω vibration for electrical properties.

Main Results:

• Demonstrated simultaneous imaging of surface topography and local electrical properties (charge/potential) in aqueous solutions.
• Achieved topography imaging with high sensitivity to tip-sample separation (1-10 nm) using the 2ω vibration.
• Successfully recorded amplitude and phase of 1ω vibration for detailed surface electrical analysis.

Conclusions:

• The developed AFM mode effectively overcomes the challenges of imaging in aqueous environments.
• This technique significantly expands the applicability of SPFM-based methods for nanoscale surface analysis in liquids.
• Provides a valuable new tool for studying interfacial phenomena in biologically and chemically relevant aqueous systems.