Mapping Polymer Molecular Order in the SEM with Secondary Electron Hyperspectral Imaging

R. C. Masters, N. Stehling, K. J. Abrams, V. Kumar, M. Azzolini, N. M. Pugno, M. Dapor, A. Huber, P. Schäfer, D. G. Lidzey and C. Rodenburg

Adv. Sci. 6, 1801752 (2019)
Understanding nanoscale molecular order within organic electronic materials is a crucial factor in building better organic electronic devices. At present, techniques capable of imaging molecular order within a polymer are limited in resolution, accuracy, and accessibility. In this work, presented are secondary electron (SE) spectroscopy and secondary electron hyperspectral imaging, which make an exciting alternative approach to probing molecular ordering in poly(3‐hexylthiophene) (P3HT) with scanning electron microscope‐enabled resolution. It is demonstrated that the crystalline content of a P3HT film is reflected by its SE energy spectrum, both empirically and through correlation with nano‐Fourier‐transform infrared spectroscopy, an innovative technique for exploring nanoscale chemistry. The origin of SE spectral features is investigated using both experimental and modeling approaches, and it is found that the different electronic properties of amorphous and crystalline P3HT result in SE emission with different energy distributions. This effect is exploited by acquiring hyperspectral SE images of different P3HT films to explore localized molecular orientation. Machine learning techniques are used to accurately identify and map the crystalline content of the film, demonstrating the power of an exciting characterization technique.