Water-mediated height artifacts in dynamic atomic force microscopy

Area of Science:

• Surface science
• Nanotechnology
• Materials characterization

Background:

• Amplitude modulation atomic force microscopy (AM-AFM) is widely used for nanoscale surface characterization under ambient conditions.
• Water films on surfaces and tips, common in ambient AM-AFM, cause apparent height variations due to capillary forces and water menisci.
• Differential surface affinities (hydrophilic/hydrophobic) exacerbate height measurement errors.

Purpose of the Study:

• To systematically investigate the impact of water layers and heterogeneous surface affinities on height measurements in AM-AFM.
• To explore the phenomenon of apparent height variations and contrast inversion under different operating conditions.
• To provide a theoretical and experimental basis for re-evaluating critical height measurements in published AM-AFM studies.

Main Methods:

• Experimental study using self-assembled monolayers of stearic acid (hydrophobic) on mica (hydrophilic).
• Theoretical simulation using a simplified point mass on a spring model to mimic AFM tip dynamics.
• Systematic variation of AM-AFM operation parameters (free oscillation amplitude, setpoint).

Main Results:

• Apparent heights measured by AM-AFM can vary significantly in magnitude and even sign (contrast inversion) depending on operation parameters.
• True surface heights cannot be accurately determined in the presence of water layers on surfaces with non-uniform water affinity, even if water menisci are not formed.
• The study demonstrates a critical limitation of AM-AFM for accurate height determination in ambient conditions with heterogeneous surfaces.

Conclusions:

• Published AM-AFM studies relying on accurate height measurements, especially on heterogeneous surfaces under ambient conditions, should be critically reviewed.
• The influence of water layers and surface properties on tip-sample interactions needs careful consideration for reliable nanoscale characterization.
• Further research into advanced AFM techniques or controlled environments may be necessary to overcome these height measurement limitations.