Photothermal excitation setup for a modified commercial atomic force microscope.

Area of Science:

• Nanoscience and nanotechnology
• Surface science
• Analytical chemistry

Background:

• Frequency modulation atomic force microscopy (FM-AFM) in liquids can be hindered by spurious peaks in the resonance spectrum.
• These peaks originate from acoustic modes in the liquid and setup, caused by indirect excitation via a piezoelectric transducer.
• Direct cantilever excitation methods are needed to improve FM-AFM performance in liquid environments.

Purpose of the Study:

• To present a simple design for implementing photothermal excitation in a modified Bruker Multimode scan head.
• To demonstrate the effectiveness of photothermal excitation in overcoming acoustic interference for high-resolution liquid AFM.
• To maintain the low noise levels of the original AFM setup.

Main Methods:

• Modification of a Bruker Multimode scan head to incorporate photothermal excitation.
• Comparison of resonance spectra obtained using piezoelectric (indirect) and photothermal (direct) excitation.
• Acquisition of high-resolution AFM images in aqueous solution.

Main Results:

• Photothermal excitation successfully eliminated unwanted acoustic peaks observed with piezoelectric excitation.
• The modified system maintained a low deflection noise density of approximately 15 fm/√[Hz].
• High-resolution images of bare calcite and Alizarin Red S adsorbed on calcite were obtained with atomic resolution of the substrate.

Conclusions:

• The developed photothermal excitation method is a simple and effective upgrade for FM-AFM in liquids.
• This approach enables artifact-free, high-resolution imaging of surfaces and molecular adsorbates.
• The technique preserves the low-noise performance crucial for atomic-resolution studies in liquid environments.