High-frequency multimodal atomic force microscopy

Abstract

Multifrequency atomic force microscopy imaging has been recently demonstrated as a powerful technique for quickly obtaining information about the mechanical properties of a sample. Combining this development with recent gains in imaging speed through small cantilevers holds the promise of a convenient, high-speed method for obtaining nanoscale topography as well as mechanical properties. Nevertheless, instrument bandwidth limitations on cantilever excitation and readout have restricted the ability of multifrequency techniques to fully benefit from small cantilevers. We present an approach for cantilever excitation and deflection readout with a bandwidth of 20 MHz, enabling multifrequency techniques extended beyond 2 MHz for obtaining materials contrast in liquid and air, as well as soft imaging of delicate biological samples.

Keywords: atomic force microscopy; multifrequency imaging; nanomechanical characterization; photothermal excitation; small cantilevers.

Figure 1

a) Schematic of the optical drive and detection setup. The drive laser focus can be positioned relative to the readout laser focus through an adjustable kinematic mount. b) Photograph of the assembled readout head. The head can be mounted directly onto Bruker MultiMode scanners. c) Schematic of the constant current driver circuit for the photothermal drive laser. d) Simplified functional schematic of the high-bandwidth readout electronics. Transistor-based current arithmetic greatly improves bandwidth and reduces noise. Only the sum and vertical channels are shown for clarity; the horizontal deflection is also calculated. (CM = current mirror, −CM = current subtractor). e) Photograph of the readout electronics circuitry. One circuit board provides power conditioning and the drive laser control, the second board calculates the readout arithmetic.

Figure 2

a) Measured spectra of the major optical components in the readout design. b) Measurement of the beam waist of the readout and drive laser. The 1/e² waist of the readout and drive laser are 2.6 μm and 5.9 μm, respectively.

Figure 3

Cantilever drive and deflection readout characterization. a) In contrast to piezo excitation (top curves), photothermal excitation (lower curves) cleanly and consistently drives the first two resonances for more than 100 min. b) The photothermal tunes show resonances up to 19.5 MHz, demonstrating the wide bandwidth with clean phase responses for selected modes. By offsetting the drive laser laterally on a triangular cantilever (Bruker FastScan C), torsional resonances can be excited (red curve). Visible are the first three flexural modes (f₀, f₁ and f₂), the first two torsional modes (t₁ and t₂), and a complex higher resonant mode (hm). c) Thermal noise peak of the first flexural mode of a FastScan A cantilever, with a baseline noise floor of 45 fm/. d) Thermal noise peak of the second flexural mode of a FastScan A at 6.6 MHz.

Figure 4

Bimodal AFM imaging of a PS/PMMA polymer blend with small, high-frequency cantilevers in both air (panels a–c) and water (panels d–f). Panels a and c show topography based on amplitude modulation of the fundamental resonance. Panels b and e show the resonance frequency shift of the first higher resonant mode, and panels c and f show the drive amplitude needed to keep the first higher resonant mode at constant amplitude, related to the energy dissipation in the tip–sample interaction.

Figure 5

a) Schematic of the drive amplitude modulation feedback compared with standard amplitude modulation imaging. Instead of using the oscillation amplitude as feedback variable like in conventional amplitude modulation mode, the oscillation amplitude is kept constant and the drive amplitude required to keep it constant is used as feedback variable. The drive is then enforced to a setpoint above the free drive, resulting in a stable topography feedback. b) High-resolution DAM imaging in liquid of soft F-actin fibres on (3-aminopropyl)triethoxysilane coated glass. Both the sub- and superstructure of the protein are visible.