Nanoscale chemical analysis of beam‐sensitive polymeric materials by cryogenic electron microscopy

Abstract

Nanoscale chemical analysis of functional polymer systems by electron microscopy, to gain access into degradation processes during the materials life cycle, is still a formidable challenge due to their beam sensitivity. Here a systematic study on the different stages of degradation in a P3HT‐PCBM organic photovoltaic (OPV) model system is presented. To this end pristine samples, samples with (reversibly) physisorbed oxygen and water and samples with (irreversibly) chemisorbed oxygen and water are imaged utilizing the full capabilities of cryogenic STEM‐EELS. It is found that oxygen and water are largely physisorbed in this system leading to significant effects on the band structure, especially for PCBM. Quantification proves that degradation concomitantly decreases the amount of CC bonds and increases the amount of COC bonds in the sample. Finally, it is shown that with a pulsed electron beam utilizing a microwave cavity, beam damage can be significantly reduced which likely extends the possibilities for such studies in future.

High-Resolution Imaging of Unstained Polymer Materials

“…HAADF-STEM and ADF-STEM are thus ideally suited for studying thin films of metal and semiconductors. There are, however, a few examples where these techniques have been used to study polymer thin films. ,− The presence of light elements in these systems complicates the interpretation of scattering signals.…”

“No-dose” imaging

“…In this contribution I will be discussing how appropriate averaging schemes can be utilized to access the structure of organic photovoltaics [3,4] and the dynamics of polymer vesicle formation [7] from noisy cryo-TEM and liquid-phase TEM data. These approaches can also be employed to obtain compositional information of beam-sensitive materials at high resolution from cryo-STEM-EELS [4,8]. Furthermore, for liquid-phase STEM experiments it will be shown how quantifying the reaction/assembly dynamics of amorphous cobalt carbonate (Figure 1C) in dependence of the applied electron flux permits extrapolation to "no-dose" imaging conditions, i.e., zero-electron flux [9].…”

“…Along the same lines, it will be shown how dynamic control over the liquid layer thickness in a liquid cell can be achieved to mediate diffusion limitations for most part of the synthesis while at the same time enabling a rapid reduction in thickness whenever needed to overcome dose or resolution limitations [10]. Finally, the benefits of precise control over electron exposure in time via ultrafast beam blanking (Figure 1D) are introduced as a means to extend the critical damage threshold for beam sensitive materials [8]. (SNR) of carbon nanotube (CNT) plotted as a function of electron dose, inner collection angle of ADF detector for various polymer section thicknesses 100 nm (red), 200 nm (green), 500 nm (blue), 1 µm (gold), 2 µm (magenta), and 5 µm (yellow).…”

“…bright CNT in dark background) [5]; (C) growth rate of cobalt carbonate nanoparticles measured from liquid-phase STEM data in dependence of distance from imaging area where electrons generate CO32-ions by a reaction cascade [9]; (D) fading of P3HT diffraction ring intensity in dependence of accumulated electron dose for continuous (blue) and a pulsed electron beam at 75 MHz (orange). The critical dose threshold at which the intensity has decreased to 1/e is indicated by the gray doted horizontal line [8].…”