Higher-Harmonic Force Detection in Dynamic Force Microscopy

Abstract

In atomic force microscopy, a force-sensing cantilever probes a sample and thereby creates a topographic image of its surface. The simplest implementation uses the static deflection of the cantilever to probe the forces. More recently, dynamic operation modes have been introduced. The dynamic modes either work at a constant oscillation frequency and sense the amplitude variations caused by tip–sample forces (amplitude-modulation or tapping mode) or operate at a constant amplitude and varying frequency (frequency-modulation mode). Here, we report on new operational concepts that capture the higher harmonics in either amplitude-modulation or frequency-modulation mode. Higher-harmonic detection in AM force microscopy allows the measurement of time-resolved tip–sample forces that contain detailed information about the material characteristics of the sample, while higher-harmonic detection in small-amplitude frequency-modulation mode allows a significant improvement in spatial resolution, in particular when operating in vacuum at low temperatures.