Low frequency photothermal excitation of AFM microcantilevers

Abstract

Photothermal excitation at frequencies below the mechanical resonance of the atomic force microscopy (AFM) microcantilever can be utilized in force modulation microscopy, fast force displacement curve acquisition, and tip-based mass spectroscopy. To understand the microcantilever bending response in these modes, accurate models of the thermoelastic response of the AFM microcantilever are needed. We study the sub-resonance photothermal vibrational response of coated and uncoated AFM microcantilevers as a function of laser modulation frequency and spot location. The sub-resonance microcantilever response shows distinct thermoelastic regimes. Below the microcantilever's thermal roll-off frequency, the vibration amplitude is mostly constant. Past this frequency, the vibration amplitude decreases with increasing frequency. At modulation frequencies below the thermal roll-off frequency, the most efficient photothermal laser spot to excite harmonic motion is near the free end of both coated and uncoated microcantilevers. For the tested coated microcantilevers, the most efficient photothermal laser location migrates from near the free end of the microcantilever to near the fixed end as modulation frequency increases. For the tested uncoated microcantilever, the most efficient photothermal laser location remains unchanged at the tested frequencies. To predict the bending response of the coated microcantilever, a bilayer bending model is implemented. At low frequencies, this model underpredicts the bending response compared to experiments by up to 90%. This may be due to neglecting microcantilever bending contributed by a through-thickness temperature gradient. Our results illustrate different aspects of the frequency-dependent photothermal laser spot optimization that can guide users to maximizing microcantilever response to a given input power.

Nanoscale Rheology: Dynamic Mechanical Analysis over a Broad and Continuous Frequency Range Using Photothermal Actuation Atomic Force Microscopy

“…Therefore, despite the local temperature increase, the elimination of disrupting stimuli (magnetic fields) and spurious resonances render PT-AFM nano-DMA more versatile for nano-DMA measurements. We note that although the measurement sensitivity is expected to increase when k c approaches |k*| , , the correct application of PT-AFM nano-DMA requires that k c > | k *|. PT-AFM nano-DMA measurements are robust and synergize well with measurements collected via other AFM techniques such as AM–FM AFM.…”

“…To directly apply eqs and to PT-AFM nano-DMA, the optical lever calibration of both A R and A S must be identical. This calibration depends on the shape in which the cantilever vibrates, which generally changes with the photothermal laser spot position, drive frequency, and cantilever boundary conditions , (these factors are analyzed further in Supporting Information SI4). Without correcting for these shape changes, measurements at different ratios of | k *|: k c are not directly comparable.…”

Simultaneous resonance in multi-frequency atomic force microscopy

“…Almost four decades after its invention, atomic force microscopy (AFM) has become a standard tool to generate high-quality topographic images and produce high-resolution compositional mapping of the surface mechanical, chemical and electrical properties [1][2][3][4][5]. In recent years, multi-frequency AFM has been in the focus of attention of scanning probe microscopy researchers.…”