We present experimental results on the use of quantitative contact resonance force microscopy (CR-FM) for mapping the planar location and depth of 50 nm silica nanoparticles buried beneath polystyrene films 30 nm to 165 nm thick. The presence of shallowly buried nanoparticles, with stiffness greater than the surrounding matrix, is shown to locally affect the surface contact stiffness of a material for all depths investigated. To achieve the necessary stiffness sensitivity, the CR-FM measurements are obtained utilizing the fifth contact eigenmode. Stiffness contrast is found to increase rapidly with initial increases in force, but plateau at higher loads. Over the explored depth range, stiffness contrast spans roughly one order of magnitude, suggesting good depth differentiation. Scatter in the stiffness contrast for single images reveals non-uniformities in the model samples, which are explained by particle size dispersity. Finite element analysis is used to simulate the dramatic effect particle size has on contact stiffness contrast. It is found that measurements at a range of forces can be used to deconvolve particle size effects from depth effects.
Pub Type: Journals
atomic force acoustic microscopy (AFAM), contact resonance force microscopy (CR-FM), subsurface, nanocomposite