Frequency-modulated atomic force microscopy operation by imaging at the frequency shift minimum: the dip-df mode.

Area of Science:

• Surface Science
• Microscopy Techniques
• Nanotechnology

Background:

• Frequency Modulated Non-Contact Atomic Force Microscopy (FM-NC-AFM) typically uses cantilever frequency shift (Δf) for topography feedback.
• Standard PID controllers struggle with stable operation around the Δf(z) minimum due to slope sign changes.
• Imaging in liquid environments is challenging due to moderate interaction peaks and cantilever frequency drift.

Purpose of the Study:

• To develop a novel feedback approach for stable FM-NC-AFM operation, particularly in liquid environments.
• To enable direct access to the Δf(z) minimum and imaging in the attractive tip-sample interaction regime.
• To overcome limitations of conventional feedback methods and improve operational stability.

Main Methods:

• Implementing a feedback loop that utilizes the derivative of Δf with respect to tip-sample separation (z).
• Regulating the slope of Δf(z) to zero, rather than a fixed setpoint.
• Acquiring isosurfaces of the Δf minimum on a calcite CaCO3(101̅4) surface in liquid.

Main Results:

• The new method demonstrates significantly increased operation stability.
• The approach is immune to cantilever frequency (f0) drift, a common issue in liquid.
• Successful imaging in the attractive tip-sample interaction regime was achieved.

Conclusions:

• Regulating the derivative of Δf offers a robust solution for challenging AFM imaging conditions.
• This technique enhances stability and accessibility to attractive interaction regimes in liquid NC-AFM.
• The method paves the way for more reliable nanoscale imaging in complex environments.