Shell thickness determination for PTFE-PS core-shell nanoparticles using scanning transmission X-ray microscopy (STXM)

A scanning transmission X-ray microscopy (STXM)-based methodology is introduced for determining the dimensions (shell thickness, core and total diameter) of core-shell nanoparticles, which exhibit a strong X-ray absorption contrast and a well-defined interface between core and shell material. A low radiation dosage during data acquisition and, therefore, less X-ray beam-induced damage of the sample is achieved by recording STXM images only at 2 predetermined energies of maximum absorption contrast, instead of recording a stack of images across the whole absorption edge. A model core-shell nanoparticle, polytetrafluoroethylene (PTFE) cores with polystyrene (PS) shell, is used for demonstration. Near-edge X-ray absorption fine structure spectroscopy confirms the significant difference in X-ray absorption behavior between PTFE and PS. Additionally, because of the insolubility of styrene in PTFE a well-defined interface between particle core and shell is expected. To validate the STXM results, both the naked PTFE cores as well as the complete core-shell nanoparticles are examined by scanning electron microscopy (SEM). The introduced STXM-based methodology yields particle dimensions in agreement with the SEM results and provides additional information such as the position of the particle core, which cannot be extracted from a SEM micrograph.