Delocalized radiation damage in polymers

Seeing structural evolution of organic molecular nano-crystallites using 4D scanning confocal electron diffraction (4D-SCED)

Direct observation of organic molecular nanocrystals and their evolution using electron microscopy is extremely challenging, due to their radiation sensitivity and complex structure. Here, we introduce 4D-scanning confocal electron diffraction (4D-SCED), which enables direct in situ observation of bulk heterojunction (BHJ) thin films. 4D-SCED combines confocal electron optic setup with a pixelated detector to record focused spot-like diffraction patterns with high angular resolution, using an order of magnitude lower dose than previous methods. We apply it to study an active layer in organic solar cells, namely DRCN5T:PC₇₁BM BHJ thin films. Structural details of DRCN5T nano-crystallites oriented both in- and out-of-plane are imaged at ~5 nm resolution and dose budget of ~5 e⁻/Ų. We use in situ annealing to observe the growth of the donor crystals, evolution of the crystal orientation, and progressive enrichment of PC₇₁BM at interfaces. This highly dose-efficient method opens more possibilities for studying beam sensitive soft materials.

Studying organic molecular nanocrystals with electron microscopy has been challenging due to complex structures and radiation sensitivity. Here, the authors present 4D-scanning confocal electron diffraction, and demonstrate direct in situ observation of structural evolution of bulk heterojunction thin films.

Reducing electron beam damage through alternative STEM scanning strategies, Part I: Experimental findings

The highly energetic electrons in a transmission electron microscope (TEM) can alter or even completely destroy the structure of samples before sufficient information can be obtained. This is especially problematic in the case of zeolites, organic and biological materials. As this effect depends on both the electron beam and the sample and can involve multiple damage pathways, its study remained difficult and is plagued with irreproducibility issues, circumstantial evidence, rumors, and a general lack of solid data. Here we take on the experimental challenge to investigate the role of the STEM scan pattern on the damage behavior of a commercially available zeolite sample with the clear aim to make our observations as reproducible as possible. We make use of a freely programmable scan engine that gives full control over the tempospatial distribution of the electron probe on the sample and we use its flexibility to obtain multiple repeated experiments under identical conditions comparing the difference in beam damage between a conventional raster scan pattern and a newly proposed interleaved scan pattern that provides exactly the same dose and dose rate and visits exactly the same scan points. We observe a significant difference in beam damage for both patterns with up to 11 % reduction in damage (measured from mass loss). These observations demonstrate without doubt that electron dose, dose rate and acceleration voltage are not the only parameters affecting beam damage in (S)TEM experiments and invite the community to rethink beam damage as an unavoidable consequence of applied electron dose.

Chemical Fingerprinting of Polymers Using Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) is becoming an important tool in the characterization of polymeric materials. The sensitivity of EELS to changes in the chemical structure of polymeric materials dictates its applicability. In particular, it is important for compositional analysis to have reference spectra of pure components. Here, we report the spectra of the carbon K-edge of six polymers (polyethylene, polypropylene, polybutylene terephthalate, and polylactic acid) including copolymers (styrene acrylonitrile and acrylonitrile butadiene styrene), to be used as reference spectra for future EELS studies of polymers. We have successfully decomposed the carbon K-edge of each of the polymers and assigned the observed peaks to bonding transitions. The spectra have been acquired in standard experimental conditions, and electron beam damage has been taken into account during establishment of spectral-structural relationships. We found that the more commonly available low-energy resolution spectrometers are adequate to chemically fingerprint linear saturated hydrocarbons such as PE, PP, and PLA. We have thus moved a step closer toward creating an atlas of polymer EELS spectra, which can be subsequently used for chemical bond mapping of polymeric materials with nanoscale spatial resolution.

4D-STEM of Beam-Sensitive Materials

ConspectusScanning electron nanobeam diffraction, or 4D-STEM (four-dimensional scanning transmission electron microscopy), is a flexible and powerful approach to elucidate structure from "soft" materials that are challenging to image in the transmission electron microscope because their structure is easily damaged by the electron beam. In a 4D-STEM experiment, a converged electron beam is scanned across the sample, and a pixelated camera records a diffraction pattern at each scan position. This four-dimensional data set can be mined for various analyses, producing maps of local crystal orientation, structural distortions, crystallinity, or different structural classes. Holding the sample at cryogenic temperatures minimizes diffusion of radicals and the resulting damage and disorder caused by the electron beam. The total fluence of incident electrons can easily be controlled during 4D-STEM experiments by careful use of the beam blanker, steering of the localized electron dose, and by minimizing the fluence in the convergent beam thus minimizing beam damage. This technique can be applied to both organic and inorganic materials that are known to be beam-sensitive; they can be highly crystalline, semicrystalline, mixed phase, or amorphous.One common example is the case for many organic materials that have a π-π stacking of polymer chains or rings on the order of 3.4-4.2 Å separation. If these chains or rings are aligned in some regions, they will produce distinct diffraction spots (as would other crystalline spacings in this range), though they may be weak or diffuse for disordered or weakly scattering materials. We can reconstruct the orientation of the π-π stacking, the degree of π-π stacking in the sample, and the domain size of the aligned regions. This Account summarizes illumination conditions and experimental parameters for 4D-STEM experiments with the goal of producing images of structural features for materials that are beam-sensitive. We will discuss experimental parameters including sample cooling, probe size and shape, fluence, and cameras. 4D-STEM has been applied to a variety of materials, not only as an advanced technique for model systems, but as a technique for the beginning microscopist to answer materials science questions. It is noteworthy that the experimental data acquisition does not require an aberration-corrected TEM but can be produced on a variety of instruments with the right attention to experimental parameters.

Analytical Cryo-Scanning Electron Microscopy of Hydrated Polymers and Microgels

Despite the fact that scanning electron microscopes (SEM) coupled with energy-dispersive X-ray microanalysis (EDS) has been commercially available for more than a half-century, SEM/EDS continues to develop and open new opportunities to study the morphology of advanced materials. This is particularly true in applications to hydrated soft matter. Developments in field-emission electron sources that enable low-voltage imaging of uncoated polymers, silicon-drift detectors that enable high-efficiency collection of X-rays characteristic of light elements, and cryogenic methods to effectively cryo-fix hydrated samples have opened new opportunities to apply techniques relatively well established in hard-materials applications to challenging new problems involving synthetic polymers. We have applied cryo-SEM imaging and spatially resolved EDS to collect new information characterizing polyelectrolyte microgels. These are charged gel particles with dimensions in the range of 0.1-100 μm. Perhaps most notable is the fact that the high hydration levels-the samples are mostly water-allow robust calibration curves to be generated using frozen-hydrated buffers with known salt and/or hydrocarbon compositions. Such calibration curves enable quantitative composition measurements in the low-concentration extremes associated with high-swelling hydrogels. We use an experimentally derived carbon calibration curve to determine the microgel swell ratio, Q. The swell ratio, arguably, is the single most important gel characteristic because it is directly related to the mesh size of the networked polymer, which in turn determines many of the gel's mechanical and transport properties. While Q can be experimentally measured in macroscopic gels based on weight measurements in the dry and hydrated states, it is very difficult to measure in a microgel, and the fact that EDS in a cryo-SEM can determine Q from a single X-ray spectrum is significant. Furthermore, because of the electrostatic charge distributed along the polymer chains, the presence and concentration of counter-ions play a critical role in polyelectrolyte systems. While conceptually understood for decades, experimental measurements of counter-ion concentrations have been largely limited to a relatively small set of materials that involve macroscopic samples. By developing calibration curves from frozen-hydrated buffer of known ionic strength, we measure the concentration of Na counter-ions in microgels of poly(acrylic acid) (PAA) with a limit of detection of ∼0.014 M. Such measurements may help resolve some long-standing questions in polyelectrolyte science concerning counter-ion condensation. Even in the absence of a calibration curve, we show that spatially resolved X-ray spectroscopy can map the spatial distribution of a cationic oligopeptide complexed within a hydrated PAA microgel because of the nitrogen fingerprint that, albeit at very low concentration, is unique to the peptide. We look specifically at the case of a microgel with a so-called core-shell structure, where, again, the underlying polyelectrolyte science responsible for core-shell formation remains incompletely understood. These examples highlight how a modern cryo-SEM can be exploited to quantitatively characterize hydrated soft matter. The approach is almost certain to continue its development and impact as the base of experienced practitioners, the accessibility to well-configured microscopes, and the abundance of challenging problems involving hydrated soft matter all continue to grow.

Pushing the limits of high-resolution polymer microscopy using antioxidants

High-resolution transmission electron microscopy (HRTEM) has been transformative to the field of polymer science, enabling the direct imaging of molecular structures. Although some materials have remarkable stability under electron beams, most HRTEM studies are limited by the electron dose the sample can handle. Beam damage of conjugated polymers is not yet fully understood, but it has been suggested that the diffusion of secondary reacting species may play a role. As such, we examine the effect of the addition of antioxidants to a series of solution-processable conjugated polymers as an approach to mitigating beam damage. Characterizing the effects of beam damage by calculating critical dose DC values from the decay of electron diffraction peaks shows that beam damage of conjugated polymers in the TEM can be minimized by using antioxidants at room temperature, even if the antioxidant does not alter or incorporate into polymer crystals. As a consequence, the addition of antioxidants pushes the resolution limit of polymer microscopy, enabling imaging of a 3.6 Å lattice spacing in poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3″'-di(2-octyldodecyl)-2,2';5',2″;5″,2″'-quaterthiophene-5,5″'-diyl)] (PffBT4T-2OD).

Nanoscale Chemical Heterogeneity in Aromatic Polyamide Membranes for Reverse Osmosis Applications

Reverse osmosis membranes are used within the oil and gas industry for seawater desalination on off-shore oilrigs. The membranes consist of three layers of material: a polyester backing layer, a polysulfone support and a polyamide (PA) thin film separating layer. It is generally thought that the PA layer controls ion selectivity within the membrane but little is understood about its structure or chemistry at the molecular scale. This active polyamide layer is synthesized by interfacial polymerization at an organic/aqueous interface between m-phenylenediamine and trimesoyl chloride, producing a highly cross-linked PA polymer. It has been speculated that the distribution of functional chemistry within this layer could play a role in solute filtration. The only technique potentially capable of probing the distribution of functional chemistry within the active PA layer with sufficient spatial and energy resolution is scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS). Its use is a challenge because organic materials suffer beam-induced damage at relatively modest electron doses. Here we show that it is possible to use the N K-edge to map the active layer of a PA film using monochromated EELS spectrum imaging. The active PA layer is 12 nm thick, which supports previous neutron reflectivity data. Clear changes in the fine structure of the C K-edge across the PA films are measured and we use machine learning to assign fine structure at this edge. Using this method, we map highly heterogeneous intensity variations in functional chemistry attributed to N-C═C bonds within the PA. Similarities are found with previous molecular dynamics simulations of PA showing regions with a higher density of amide bonding as a result of the aggregation process at similar length scales. The chemical pathways that can be deduced may offer a clearer understanding of the transport mechanisms through the membrane.

Chemical fingerprinting of polyvinyl acetate and polycarbonate using electron energy-loss spectroscopy

This work demonstrates that the high sensitivity of EELS can be used to identify the changes in the chemical structure of polymeric materials.

Mapping structure and morphology of amorphous organic thin films by 4D-STEM pair distribution function analysis

Imaging the phase distribution of amorphous or partially crystalline organic materials at the nanoscale and analyzing the local atomic structure of individual phases has been a long-time challenge. We propose a new approach for imaging the phase distribution and for analyzing the local structure of organic materials based on scanning transmission electron diffraction (4D-STEM) pair distribution function analysis (PDF). We show that electron diffraction based PDF analysis can be used to characterize the short- and medium-range order in aperiodically packed organic molecules. Moreover, we show that 4D-STEM-PDF does not only provide local structural information with a resolution of a few nanometers, but can also be used to image the phase distribution of organic composites. The distinct and thickness independent contrast of the phase image is generated by utilizing the structural difference between the different types of molecules and taking advantage of the dose efficiency due to use of the full scattering signal. Therefore, this approach is particularly interesting for imaging unstained organic or polymer composites without distinct valence states for electron energy loss spectroscopy. We explore the possibilities of this new approach using [6,6]-phenyl-C61- butyric acid methyl ester (PC61BM) and poly(3-hexylthiophene-2,5-diyl) (P3HT) as the archetypical and best-investigated semiconductor blend used in organic solar cells, compare our phase distribution with virtual dark-field analysis and validate our approach by electron energy loss spectroscopy.

Electron beam damage of perfluorosulfonic acid studied by soft X-ray spectromicroscopy

Scanning transmission X-ray microscopy (STXM) was used to study chemical changes to perfluorosulfonic acid (PFSA) spun cast thin films as a function of dose imparted by exposure of a 200 kV electron beam in a Transmission Electron Microscope (TEM). The relationship between electron beam fluence and absorbed dose was calibrated using a modified version of a protocol based on the positive to negative lithography transition in PMMA [Leontowich et al, J. Synchrotron Rad. 19 (2012) 976]. STXM was used to characterize and quantify the chemical changes caused by electron irradiation of PFSA under several different conditions. The critical dose for CF₂-CF₂ amorphization was used to explore the effects of the sample environment on electron beam damage. Use of a silicon nitride substrate was found to increase the CF₂-CF₂ amorphization critical dose by ∼x2 from that for free-standing PFSA films. Freestanding PFSA and PMMA films were damaged by 200 kV electrons at ∼100 K and then the damage was measured by STXM at 300 K (RT). The lithography cross-over dose for PMMA was found to be ∼2x higher when the PMMA thin film was electron irradiated at 120 K rather than at 300 K. The critical dose for CF₂-CF₂ amorphization in PFSA irradiated at 120 K followed by warming and delayed measurement by STXM at 300 K was found to be ∼2x larger than at 300 K. To place these results in the context of the use of electron microscopy to study PFSA ionomer in fuel cell systems, an exposure of 300 e^-/nm² at 300 K (which corresponds to an absorbed dose of ∼20 MGy) amorphizes ∼10% of the CF₂-CF₂ bonds in PFSA. At this dose level, the spatial resolution for TEM imaging of PFSA is limited to 3.5 nm by radiation damage, if one is using a direct electron detector with DQE = 1. This work recommends caution about 2D and 3D morphological information of PFSA materials based on TEM studies which use fluences higher than 300 e^-/nm².

Nanoscale Probing of Local Hydrogen Heterogeneity in Disordered Carbon Nitrides with Vibrational Electron Energy-Loss Spectroscopy

In graphitic carbon nitrides, (photo)catalytic functionality is underpinned by the effect that residual hydrogen content, manifesting in amine (N-H _x) defects, has on its optoelectronic properties. Therefore, a detailed understanding of the variation in the local structure of graphitic carbon nitrides is key for understanding structure-activity relationships. Here, we apply aloof-beam vibrational electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM) to locally detect variations in hydrogen content in two different layered carbon nitrides with nanometer resolution. Through low dose rate TEM, we obtain atomically resolved images from crystalline and disordered carbon nitrides. By employing an aloof-beam configuration in a monochromated STEM, radiation damage can be dramatically reduced, yielding vibrational spectra from carbon nitrides to be assessed on 10's of nanometer length scales. We find that in disordered graphitic carbon nitrides the relative amine content can vary locally up to 27%. Cyano (C≡N) defects originating from uncondensed precursor are also revealed by probing small volumes, which cannot be detected by infrared absorption or Raman scattering spectroscopies. The utility of this technique is realized for heterogeneous soft materials, such as disordered graphitic carbon nitrides, in which methods to probe catalytically active sites remain elusive.

Effect of Electron Beam Irradiation on Polymers

Electron energy loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM) is widely used for chemical state analysis of variety of chemical compounds. High beam sensitivity of substances like polymers hinders the possibility of exploring in-depth analysis provided through the high spatially resolved EELS spectroscopy. In this study, the electron beam irradiation damage on polymers were analyzed with varying dose of electron beams. The stability of the polymers under electron beam exposure depends on the chemical structure on the polymers. In this study the polymers with and without phenyl groups namely Polycarbonate, Polyethylene terephthalate, Polystyrene, Styrene Maleic Anhydride and Polymethylmethacrylate are selected for the comparative degradation study. Effect of varying the electron dose on the stability of polymers were monitored by recording the low-loss EELS spectrum in π to π* transition and (π+σ) to (π+σ)* transition region.

Quantitative Analysis of Electron Beam Damage in Organic Thin Films

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.

Electron energy loss spectroscopy for polymers: a review

Electron energy loss spectroscopy (EELS) allows imaging as well as extraction of spatially resolved chemical information and this review presents how EELS can be ap plied to polymeric systems.

Damage-free vibrational spectroscopy of biological materials in the electron microscope

Vibrational spectroscopy in the electron microscope would be transformative in the study of biological samples, provided that radiation damage could be prevented. However, electron beams typically create high-energy excitations that severely accelerate sample degradation. Here this major difficulty is overcome using an ‘aloof' electron beam, positioned tens of nanometres away from the sample: high-energy excitations are suppressed, while vibrational modes of energies <1 eV can be ‘safely' investigated. To demonstrate the potential of aloof spectroscopy, we record electron energy loss spectra from biogenic guanine crystals in their native state, resolving their characteristic C–H, N–H and C=O vibrational signatures with no observable radiation damage. The technique opens up the possibility of non-damaging compositional analyses of organic functional groups, including non-crystalline biological materials, at a spatial resolution of ∼10 nm, simultaneously combined with imaging in the electron microscope.

Use of electron microscopy to determine morphology, or find where functionally significant biomolecules are located with high spatial resolution is of great interest. Here, Rez, Cohen et al. use aloof electron beam vibrational spectroscopy to probe different bonds in biological samples with no significant radiation damage.

Control of radiation damage in the TEM

The problem of electron-beam damage in the transmission electron microscope is reviewed, with an emphasis on radiolysis processes in soft materials and organic specimens. Factors that determine the dose-limited resolution are identified for three different operational modes: bright-field scattering-contrast, phase-contrast and dark-field microscopy. Methods of reducing radiation damage are discussed, including low-dose techniques, cooling or encapsulating the specimen, and the choice of imaging mode, incident-beam diameter and incident-electron energy. Further experiments are suggested as a means of obtaining a better understanding and control of electron-beam damage.

Mechanisms of radiation damage in beam-sensitive specimens, for TEM accelerating voltages between 10 and 300 kV

Ionization damage (radiolysis) and knock-on displacement are compared in terms of scattering cross section and stopping power, for thin organic specimens exposed to the electrons in a TEM. Based on stopping power, which includes secondary processes, radiolysis is found to be predominant for all incident energies (10-300 keV), even in materials containing hydrogen. For conducting inorganic specimens, knock-on displacement is the only damage mechanism but an electron dose exceeding 1000 C cm(-2) is usually required. Ways of experimentally determining the damage mechanism (with a view to minimizing damage) are discussed.