Understanding and controlling the depth sensitivity of scanning probe based infrared imaging and nanospectroscopy for buried polymeric structures

Abstract

Atomic force microscopy paired with infrared spectroscopy (AFM-IR) is a robust technique for investigating complex polymer blends and composites’ nanoscale surface topography and chemical composition. In this work, we measured bilayer polymer films to study the effect of laser power, laser pulse frequency, and laser pulse width on the depth sensitivity of the technique. Unique bilayer polystyrene (PS) and polylactic acid (PLA) samples with various film thicknesses and blend ratios were prepared. The depth sensitivity characterized by the amplitude ratio of the resonance bands of PLA and PS was monitored as the thickness of the top barrier layer was incrementally increased from tens of nanometers to hundreds of nanometers. In addition, incrementally increasing the incident laser power resulted in greater depth sensitivity due to the enhanced thermal oscillations generated in the buried layer. In contrast, incrementally increasing the laser frequency increased the surface sensitivity, as indicated by a reduced PLA/PS AFM-IR signal ratio. Finally, the dependence of the depth sensitivity on the laser pulse width was observed. Consequently, by precisely controlling the laser energy, pulse frequency, and pulse width, one can finely control the depth sensitivity of the AFM-IR tool from 10 nm to 100 nm. Our work provides the unique capability to study buried polymeric structures without the need for tomography or destructive etching.