Visualizing the Subsurface of Soft Matter: Simultaneous Topographical Imaging, Depth Modulation, and Compositional Mapping with Triple Frequency Atomic Force Microscopy

Abstract

Characterization of subsurface morphology and mechanical properties with nanoscale resolution and depth control is of significant interest in soft matter fields like biology, polymer science, and even in future applications like nanomanufacturing, where buried structural and compositional features are important to the functionality of the system. However, controllably "feeling" the subsurface is a challenging task for which the available imaging tools are relatively limited. In this paper, we propose a trimodal atomic force microscopy (AFM) imaging scheme, whereby three eigenmodes of the microcantilever probe are used as separate control "knobs" to simultaneously measure the topography, modulate sample indentation by the tip during tip-sample impact, and map compositional contrast, respectively. We illustrate this multifrequency imaging approach through computational simulation and experiments conducted on ultrathin polymer films with embedded glass nanoparticles in ambient air. By actively increasing the tip-sample indentation using a higher eigenmode of the cantilever, we are able to gradually and controllably reveal glass nanoparticles which are buried tens of nanometers deep under the surface, while still being able to refocus on the surface.

Subsurface imaging of two-dimensional materials at the nanoscale

“…22 In particular, Ebeling et al have been able 3 to image glass nanoparticles (NPs) embedded in a soft Polydimethylsiloxane (PDMS) film. 23 UFM has specifically proven a valid tool to localize subsurface defects in stiff materials.…”

Optimization of phase contrast in bimodal amplitude modulation AFM

“…It has several configurations depending on the feedback schemes [16][17][18][19][20][21][22][23][24]. In the first bimodal AFM configuration (bimodal AM) [15,16], the feedback acts on the amplitude of the first mode by keeping it at a fixed value during imaging while the second mode operates in an open loop.…”

Enhancing the multiple harmonics by step-like cantilever

“…2 The higher harmonics are generated by the tip-sample interactions, which can be used to extract information on the material properties. [3][4][5][6][7][8][9][10] The enhanced resolution, 6,9 sensitivity 7 and better contrast 10,11 can be obtained in higher harmonics. For example, García et al 9 demonstrated that bimodal AFM enables fast, accurate and angstrom-scale Young's modulus mapping on a wide range of materials in air and liquid.…”

“…The trimodal AFM can be used as separate control "knobs" to simultaneously measure the topography, map compositional contrast and modulate sample indentation by the tip during tip-sample impact, respectively. 10 However, the higher harmonic signals are suppressed due to the rapid decay of frequency response curve of the uniform cantilever, 12 which may result in a small signal output that is even lower than the effective noise level. The vibration amplitudes of the higher harmonics are several orders of magnitude smaller than that of the fundamental component.…”

“…An application example is the trimodal AFM, which offers a way to locate and characterize the subsurface structures of the materials. 10 All intersection points are obtained as ξ o changing from 0.2 to 0.8, and the corresponding values of (ξ o , γ, χ) are listed in Table I. Sahin et al 5 demonstrated that the 24th harmonic is more sensitive to harder samples and the 8th harmonic is more sensitive to softer samples when monitoring the cantilever deflection at the harmonic corresponding to the third mode.…”