Understanding the atomic-scale contrast in Kelvin probe force microscopy.

Area of Science:

• Surface Science
• Computational Materials Science
• Scanning Probe Microscopy

Background:

• Kelvin probe force microscopy (KPFM) is a powerful technique for mapping surface potential at the nanoscale.
• Understanding the origin of atomic-scale contrast in KPFM is crucial for accurate interpretation of experimental data.
• Previous studies have reported simultaneous atomic-scale topographical and contact potential difference (CPD) contrasts on ionic crystal surfaces.

Purpose of the Study:

• To numerically analyze the origin of atomic-scale contrast in Kelvin probe force microscopy.
• To investigate the relationship between KPFM contrast and the surface properties of ionic crystals.
• To elucidate the imaging mechanism responsible for atomic-scale CPD variations.

Main Methods:

• Combined atomistic simulations of tip-sample interaction force fields with a noncontact atomic force microscope simulator.
• Included a Kelvin module to mimic experimental KPFM conditions.
• Simulated the (001) surface of a bulk alkali halide crystal.

Main Results:

• The local contact potential difference (CPD) observed in simulations reflects the periodicity of the ionic crystal.
• The magnitude of the Madelung surface potential was not accurately reproduced by the CPD.
• The imaging mechanism was identified as induced polarization of ions at the tip-surface interface due to modulated bias voltage.

Conclusions:

• The atomic-scale contrast in KPFM on alkali halide surfaces originates from bias-induced polarization of surface ions.
• KPFM can reveal the atomic periodicity of ionic crystals, but not their bulk Madelung potential.
• The simulation results align well with existing theoretical predictions and experimental observations.