Quantitative Analysis of Electron Beam Damage in Organic Thin Films

Abstract

In transmission electron microscopy (TEM) the interaction of an electron beam with polymers such as P3HT:PCBM photovoltaic nanocomposites results in electron beam damage, which is the most important factor limiting acquisition of structural or chemical data at high spatial resolution. Beam effects can vary depending on parameters such as electron dose rate, temperature during imaging, and the presence of water and oxygen in the sample. Furthermore, beam damage will occur at different length scales. To assess beam damage at the angstrom scale, we followed the intensity of P3HT and PCBM diffraction rings as a function of accumulated electron dose by acquiring dose series and varying the electron dose rate, sample preparation, and the temperature during acquisition. From this, we calculated a critical dose for diffraction experiments. In imaging mode, thin film deformation was assessed using the normalized cross-correlation coefficient, while mass loss was determined via changes in average intensity and standard deviation, also varying electron dose rate, sample preparation, and temperature during acquisition. The understanding of beam damage and the determination of critical electron doses provides a framework for future experiments to maximize the information content during the acquisition of images and diffraction patterns with (cryogenic) transmission electron microscopy.

Imaging Beam‐Sensitive Materials by Electron Microscopy

“…49 As another example, the morphological evolution of beam‐sensitive macromolecules can be directly monitored with subsecond time resolution using cryo‐EM. The macromolecular self‐assembly, nucleation, and growth processes can be clearly observed without considerable structural damage, which have been reported and summarized by Patterson et al32 Leijten et al quantitatively evaluate the beam damage effects on crystalline organic polymers and observe a significantly enhanced irradiation durability under cryogenic conditions, which increases the critical dose up to ≈7 times higher 79…”

“…For example, dehydration and chemical doping of the zeolites have been observed to remarkably stabilize their structures against electron beam irradiation 217,218. The dehydration and dioxygen treatment of organic thin films have also been proved to remarkably extend their critical doses under cryogenic conditions 79…”

“…On the contrary, an “inverse” dose‐rate effect refers to decreased radiation sensitivity upon increased dose rate, which mainly originates from slower beam damage events such as diffusion‐limited mass loss, precipitation and segregation 57. The underlying dose‐rate dependent damage mechanisms, such as heating, charging (or damage that fits the “damage by the induced electric field, DIEF” model56) and diffusion allow the recovery of structures against beam damage,75,79 possibly through heat and charge dissipations,56 as well as back‐diffusion processes 69,80,81. Notably, under certain circumstances, the beam damages are affected by multiple mechanisms while the dominant mechanism as well as the overall dose‐rate effect may vary depending on the illumination conditions and material properties 21,56.…”

“…b) Fading of relative diffraction intensity versus the accumulated dose for P3HT prepared by conventional method (CF), at room temperature (RT), direct spin‐coating (DS) and at 80 K (Cryo) at a dose rate of 10 e Å −2 s −1 , respectively. Reproduced with permission 79. Copyright 2017, American Chemical Society.…”

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

“…P3HT films are well known to be susceptible to electron beam damage, with loss of molecular ordering being one of the damage effects occurring at lowest dose . In Figure , we consider how the measured P3HT SE spectrum is affected by the electron dose and dose rate.…”