Depth-sensitive subsurface imaging of polymer nanocomposites using second harmonic Kelvin probe force microscopy

We study the depth sensitivity and spatial resolution of subsurface imaging of polymer nanocomposites using second harmonic mapping in Kelvin Probe Force Microscopy (KPFM). This method allows the visualization of the clustering and percolation of buried Single Walled Carbon Nanotubes (SWCNTs) via capacitance gradient (∂C/∂z) maps. We develop a multilayered sample where thin layers of neat Polyimide (PI) (∼80 nm per layer) are sequentially spin-coated on well-dispersed SWCNT/Polyimide (PI) nanocomposite films. The multilayer nanocomposite system allows the acquisition of ∂C/∂z images of three-dimensional percolating networks of SWCNTs at different depths in the same region of the sample. We detect CNTs at a depth of ∼430 nm, and notice that the spatial resolution progressively deteriorates with increasing depth of the buried CNTs. Computational trends of ∂C/∂z vs CNT depth correlate the sensitivity and depth resolution with field penetration and spreading, and enable a possible approach to three-dimensional subsurface structure reconstruction. The results open the door to nondestructive, three-dimensional tomography and nanometrology techniques for nanocomposite applications.