Scanning Transmission X-ray Microscopy Measurements Research Articles

Elemental changes in heart and coronaries after breast cancer radiotherapy assessed by synchrotron radiation soft X-ray spectromicroscopy

Radiotherapy (RT) plays a pivotal role in the treatment of breast cancer (BC) and various thoracic malignancies. Radiation induced heart disease (RIHD) is one such long term toxicity which can offset the improvement in cancer specific mortality. Long term normal tissue toxicity is becoming a bigger concern, as early diagnosis and the improvement in the treatment of these cancers has led to patients surviving longer. Our research group on Physics applied to biomedical sciences has been investigating the side effects of BC treatment (RT and chemotherapy) for more than ten years. The cardiac regeneration has been studied to better understand the damage that occurs following radiation procedures in the heart tissue after many thoracic cancer treatments. One possible complication is coronary artery disease induced by irradiation after radiotherapy in thoracic area. Studies on the structures of cardiac tissue and the distribution of low atomic weight element can help to understand mechanisms associated with damage to healthy tissue, as these are of fundamental importance to metabolism in biological systems. The present study aimed to elucidate how radiotherapy in the thoracic area causes damage in the coronary artery, and to verify the potential use of losartan in reducing, or even preventing, the side effects of irradiation in this artery. To assess elemental and morphological differences in aortic and coronary samples, the Low Energy X-Ray Fluorescence (LEXRF) technique using Synchrotron Radiation was employed. SR- LEXRF and scanning transmission X-ray microscopy measurements were carried out at the beamline TwinMic at Elettra Sincrotrone Triste, Italy.

Quantifying signal quality in scanning transmission X-ray microscopy

While the general effects of experimental conditions such as photon flux and sample thickness on the quality of data acquired by scanning transmission X-ray microscopy (STXM) are widely known at a basic level, the specific details are rarely discussed. This leaves the community open to forming misconceptions that can lead to poor decisions in the design and execution of STXM measurements. A formal treatment of the uncertainty and distortions of transmission signals (due to dark counts, higher-order photons and poor spatial or spectral resolution) is presented here to provide a rational basis for the pursuit of maximizing data quality in STXM experiments. While we find an optimum sample optical density of 2.2 in ideal conditions, the distortions considered tend to have a stronger effect for thicker samples and so ∼1 optical density at the analytical energy is recommended, or perhaps even thinner if significant distortion effects are expected (e.g. lots of higher-order light is present in the instrument). (Note that X-ray absorption calculations based on simple elemental composition do not include near-edge resonances and so cannot accurately represent the spectral resonances typically employed for contrast in STXM.) Further, we present a method for objectively assessing the merits of higher-order suppression in terms of its impact on the quality of transmission measurements that should be useful for the design of synchrotron beamlines.

In-Situ Soft X-Ray Spectromicroscopy Characterization of Electrochemical Processes

Soft X-ray Scanning Transmission X-ray microscopy (STXM) is a synchrotron-based technique which can provide both spectroscopic characterization (near edge X-ray absorption fine structure, NEXAFS) and chemically selective imaging with high spatial resolution (~30 nm). Recently, we have developed in situ flow electrochemical devices [1,2] which allow control of the electrochemical environment while conducting STXM measurements, thus providing a platform for in-situ studies of electrochemical oxidation and reduction processes. This presentation reports results of in situ flow electrochemical STXM studies on three different systems to demonstrate the present capabilities. First, the ability to rapidly exchange the electrolyte is demonstrated by a STXM Fe 2p and in situ electrochemical study of the ferro/ferricyanide solution redox system. Second Cu and Ag-doped Cu catalysts for CO2 electrochemical reduction (CO2R) were successfully prepared using in situ electrodeposition. Third, the electrolyte was changed from CuSO4 to NaHCO3 (as substrate for CO2R) and the cell was operated under electrochemical CO2 reduction conditions, while monitoring the changes to the Cu deposited layer at various potentials, including –0.5 V where CO2 reduction is expected [3]. The figure shows a cyclic voltammogram (CV) and color-coded Cu oxidation state maps which were derived from Cu 2p stacks measured under chronoamperometric conditions at the indicated potentials. These results demonstrate that in situ flow electrochemical STXM measurements can be performed in our device under varying electrochemical reaction conditions, enabling visualization of the morphology changes from selective energy imaging, and quantitative tracking of electrochemical transformations from spectromicroscopy. This system will be used for in situ studies of CO2 electrochemical reduction catalysis with the goal of obtaining mechanistic insights to guide the development of catalysts with improved efficiency and selectivity. In addition, the system will be used to study a variety of material science, chemistry and environmental science related questions associated with oxidation or reduction processes, such as mechanisms of extra-cellular electron transport in marine sediment microbial biofilms [4].This research is supported by NSERC (Canada). STXM measurements were performed at the ambient STXM facility at the Canadian Light Source, which is funded by the Canadian Foundation for Innovation.[1] V. Prabu et al., Rev. Sci. Inst. 89 (2018) 063702.[2] P. Ingino, et al., in preparation[3] L. Wang, D.C. Higgins, et al., Proc. Nat. Acad. Sci. (2020) 01821683. DOI: 10.1073/pnas.1821683117[4] M. Obst, et al., Microsc. Microanal. 24 (S-2) (2018) 502-504. Figure 1

Analysis of neuronal iron deposits in Parkinson's disease brain tissue by synchrotron x-ray spectromicroscopy

BackgroundNeuromelanin-pigmented neurons, which are highly susceptible to neurodegeneration in the Parkinson’s disease substantia nigra, harbour elevated iron levels in the diseased state. Whilst it is widely believed that neuronal iron is stored in an inert, ferric form, perturbations to normal metal homeostasis could potentially generate more reactive forms of iron capable of stimulating toxicity and cell death. However, non-disruptive analysis of brain metals is inherently challenging, since use of stains or chemical fixatives, for example, can significantly influence metal ion distributions and/or concentrations in tissues. AimsThe aim of this study was to apply synchrotron soft x-ray spectromicroscopy to the characterisation of iron deposits and their local environment within neuromelanin-containing neurons of Parkinson’s disease substantia nigra. MethodsSoft x-ray spectromicroscopy was applied in the form of Scanning Transmission X-ray Microscopy (STXM) to analyse resin-embedded tissue, without requirement for chemically disruptive processing or staining. Measurements were performed at the oxygen and iron K-edges in order to characterise both organic and inorganic components of anatomical tissue using a single label-free method. ResultsSTXM revealed evidence for mixed oxidation states of neuronal iron deposits associated with neuromelanin clusters in Parkinson’s disease substantia nigra. The excellent sensitivity, specificity and spatial resolution of these STXM measurements showed that the iron oxidation state varies across sub-micron length scales. ConclusionsThe label-free STXM approach is highly suited to characterising the distributions of both inorganic and organic components of anatomical tissue, and provides a proof-of-concept for investigating trace metal speciation within Parkinson’s disease neuromelanin-containing neurons.

Formation of Néel-type skyrmions in an antidot lattice with perpendicular magnetic anisotropy

Magnetic skyrmions are particle-like chiral spin textures found in a magnetic film with out-of-planeanisotropy and are considered to be potential candidates as information carriers in next generationdata storage devices. Despite intense research into the nature of skyrmions and their dynamic prop-erties, there are several key challenges that still need to be addressed. In particular, the outstandingissues are the reproducible generation, stabilization and confinement of skyrmions at room tempera-ture. Here, we present a method for the capture of nanometer sized magnetic skyrmions in an arrayof magnetic topological defects in the form of an antidot lattice. With micromagnetic simulations,we elucidate the skyrmion formation in the antidot lattice and show that the capture is dependenton the antidot lattice parameters. This behavior is confirmed with scanning transmission x-ray mi-croscopy measurements. This demonstration that a magnetic antidot lattice can be implemented asa host to capture skyrmions provides a new platform for experimental investigations of skyrmionsand skyrmion based devices.

Polarization dependent X-ray absorption near-edge spectra of boron nitride nanotubes

We report polarization dependent, scanning transmission X-ray microscopy measurements of individual boron nitride nanotubes (BNNTs) and analyze them by means of first-principles calculations. X-ray absorption near-edge spectra of hexagonal boron-nitride (hBN) are calculated as a function of light polarization and layer stacking and best agreement with recent experimental data is obtained for the commonly assumed AA' stacking. Our experimental BNNT spectra show a pronounced polarization dependence which is qualitatively well reproduced in the calculations. We show that the BNNT has a mixed stacking sequence which may explain some of the differences seen between the hBN and BNNT line shapes.

Roadblocks in Cation Diffusion Pathways: Implications of Phase Boundaries for Li-Ion Diffusivity in an Intercalation Cathode Material.

Increasing intercalation of Li-ions brings about distortive structural transformations in several canonical intercalation hosts. Such phase transformations require the energy dissipative creation and motion of dislocations at the interface between the parent lattice and the nucleated Li-rich phase. Phase inhomogeneities within particles and across electrodes give rise to pronounced stress gradients, which can result in capacity fading. How such transformations alter Li-ion diffusivities remains much less explored. In this article, we use layered V2O5 as an intercalation host and examine the structural origins of the evolution of Li-ion diffusivities with phase progression upon electrochemical lithiation. Galvanostatic intermittent titration measurements show a greater than 4 orders of magnitude alteration of Li-ion diffusivity in V2O5 as a function of the extent of lithiation. Pronounced dips in Li-ion diffusivities are correlated with the presence of phase mixtures as determined by Raman spectroscopy and X-ray diffraction, whereas monophasic regimes correspond to the highest Li-ion diffusivity values measured within this range. First-principles density functional theory calculations confirm that the variations in Li-ion diffusivity do not stem from intrinsic differences in diffusion pathways across the different lithiated V2O5 phases, which despite differences in the local coordination environments of Li-ions show comparable migration barriers. Scanning transmission X-ray microscopy measurements indicate the stabilization of distinct domains reflecting the phase coexistence of multiple lithiated phases within individual actively intercalating particles. The results thus provide fundamental insight into the considerable ion transport penalties incurred as a result of phase boundaries formed within actively intercalating particles. The combination of electrochemical studies with ensemble structural characterization and single-particle X-ray imaging of phase boundaries demonstrates the profound impact of interfacial phenomena on macroscopic electrode properties.

Observation of the Interface between Resin and Carbon Fiber by Scanning Transmission X-ray Microscopy

The sub-micron spatial distribution of chemical states of carbon fiber (CF) and the interface between CF and resin must affect physical properties of carbon-fiber-reinforced plastic (CFRP). In order to evaluate scanning transmission X-ray microscopy (STXM) techniques for application to the chemical-state analysis of CFRP, we performed STXM measurements near C K-edge energies. The results of the spectral deconvolution analysis suggest the presence of another phase at the interface of CF and resin, which may be a coating layer. In addition, the preferred orientation of the stuck of graphene sheets to the fiber axis direction of CF was observed by using linear polarized X-ray beams.

Point focusing with flat and wedged crossed multilayer Laue lenses.

Point focusing measurements using pairs of directly bonded crossed multilayer Laue lenses (MLLs) are reported. Several flat and wedged MLLs have been fabricated out of a single deposition and assembled to realise point focusing devices. The wedged lenses have been manufactured by adding a stress layer onto flat lenses. Subsequent bending of the structure changes the relative orientation of the layer interfaces towards the stress-wedged geometry. The characterization at ESRF beamline ID13 at a photon energy of 10.5 keV demonstrated a nearly diffraction-limited focusing to a clean spot of 43 nm × 44 nm without significant side lobes with two wedged crossed MLLs using an illuminated aperture of approximately 17 µm × 17 µm to eliminate aberrations originating from layer placement errors in the full 52.7 µm × 52.7 µm aperture. These MLLs have an average individual diffraction efficiency of 44.5%. Scanning transmission X-ray microscopy measurements with convenient working distances were performed to demonstrate that the lenses are suitable for user experiments. Also discussed are the diffraction and focusing properties of crossed flat lenses made from the same deposition, which have been used as a reference. Here a focal spot size of 28 nm × 33 nm was achieved and significant side lobes were noticed at an illuminated aperture of approximately 23 µm × 23 µm.

Individual Titanate Nanoribbons Studied by 3D-Resolved Polarization Dependent X-ray Absorption Spectra Measured with Scanning Transmission X-ray Microscopy

Polarization dependent X-ray absorption spectroscopy (XAS) is a powerful probe of the anisotropic electronic structure of bulk single crystals, but its application to nanostructured samples is challenging. Here we describe a method for obtaining linearly polarized XAS spectra along three orthogonal axes of an individual nano-object using scanning transmission X-ray microscopy (STXM). The technique is applied to a single sodium titanate nanoribbon [(Na,H)Ti NR]. Significant linear dichroism is observed at both the Ti 2p and O 1s edges. The experimental results are compared with first-principles calculations; good agreement is achieved. The spectral changes among the three axes are attributed to the anisotropic Ti–O bonding of the various Ti and O sites in the monoclinic crystal structure of the nanoribbon. The methodology for 3D dichroic STXM measurements developed in this study is a powerful way to investigate the anisotropic geometric and electronic structure of nanomaterials.

Probing the origin of photocurrent in nanoparticulate organic photovoltaics

Varying the donor–acceptor ratio is a common technique in optimising organic photovoltaic (OPV) device performance. Here we fabricate poly(3-hexylthiophene) (P3HT): phenyl C61 butyric acid methyl ester (PCBM) nanoparticle OPVs with varied donor–acceptor ratios from 1:0.5 to 1:2. Device performance increases with PCBM loading from 1:0.5 to 1:1, then surprisingly from 1:1 to 1:2 the performance plateaus, unlike reported trends in bulk heterojunction (BHJ) OPVs where device performance drops significantly as the donor:acceptor ratio increases beyond 1:1. Scanning transmission X-ray microscopy (STXM) measurements reveal core–shell nanoparticles for all donor:acceptor ratios with a systematic increase in the PCBM nanoparticle core volume observed as the PCBM loading is increased. This increases the functional PCBM domain size available for exciton harvesting, contrary to the result observed in BHJ OPV devices where increasing the PCBM loading does not lead to an increase in functional PCBM domains. In addition, STXM measurements reveal that the core–shell nanoparticles have core and shell compositions that change with PCBM loading. In particular, we observe that the PCBM component in the nanoparticle shell phase increases from a concentration that is below the percolation limit to one that is close to the optimal weight fraction for charge transport. This increase in the functional PCBM volume is reflected in an increase in PCBM photocurrent calculated from external quantum efficiency (EQE) measurements.

Phase evolution in single-crystalline LiFePO₄ followed by in situ scanning X-ray microscopy of a micrometre-sized battery.

LiFePO₄ is one of the most frequently studied positive electrode materials for lithium-ion batteries during the last years. Nevertheless, there is still an extensive debate on the mechanism of phase transformation. On the one hand this is due to the small energetic differences involved and hence the great sensitivity with respect to parameters such as size and morphology. On the other hand this is due to the lack of in situ observations with appreciable space and time resolution. Here we present scanning transmission X-ray microscopy measurements following in situ the phase boundary propagation within a LiFePO₄ single crystal along the (010) orientation during electrochemical lithiation/delithiation. We follow, on a battery-relevant timescale, the evolution of a two-phase-front on a micrometre scale with a lateral resolution of 30 nm and with minutes of time resolution. The growth pattern is found to be dominated by elastic effects rather than being transport-controlled.

An environmental sample chamber for reliable scanning transmission x-ray microscopy measurements under water vapor

We have designed, fabricated, and tested a compact gas-phase reactor for performing in situ soft x-ray scanning transmission x-ray microscopy (STXM) measurements. The reactor mounts directly to the existing sample holder used in the majority of STXM instruments around the world and installs with minimal instrument reconfiguration. The reactor accommodates many gas atmospheres, but was designed specifically to address the needs of measurements under water vapor. An on-board sensor measures the relative humidity and temperature inside the reactor, minimizing uncertainties associated with measuring these quantities outside the instrument. The reactor reduces x-ray absorption from the process gas by over 85% compared to analogous experiments with the entire STXM instrument filled with process gas. Reduced absorption by the process gas allows data collection at full instrumental resolution, minimizes radiation dose to the sample, and results in much more stable imaging conditions. The reactor is in use at the STXM instruments at beamlines 11.0.2 and 5.3.2.2 at the Advanced Light Source.

All-polymer solar cells utilizing low band gap polymers as donor and acceptor

All-polymer solar cells based on blends of the low band gap polymers poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]} (PTB7) and poly{[N,N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) are demonstrated. The use of the donor polymer PTB7 instead of poly(3-hexylthiophene) results in a higher open-circuit voltage and an overall spectral response better matched to the solar spectrum. A power conversion efficiency of 1.1% is reported with a peak external quantum efficiency of 18% at a wavelength of 680 nm. The microstructure of PTB7:P(NDI2OD-T2) blends is also investigated using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS), near-edge X-ray fine-structure (NEXAFS) spectroscopy, atomic force microscopy (AFM), and scanning transmission X-ray microscopy (STXM). GIWAXS measurements show that PTB7:P(NDI2OD-T2) blends contain P(NDI2OD-T2) crystallites with a (100) thickness of 9.5 nm dispersed in an amorphous PTB7 matrix. STXM measurements indicate a lack of mesoscale phase separation, with AFM and NEXAFS measurements revealing a P(NDI2OD-T2)-rich top surface with fibrillar morphology. These results indicate that the pairing of low band gap polymers as both donor and acceptor polymers in all-polymer solar cells may be an effective strategy for realizing high-efficiency all-polymer solar cells. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Sub‐Micrometer Charge Modulation Microscopy of a High Mobility Polymeric n‐Channel Field‐Effect Transistor

Electro-optical mapping of the charge density with sub-micrometer resolution can be obtained in a high mobility, top-gate n-channel polymer field-effect transistor by charge modulation microscopy. Local features on the 1 μm scale are unveiled and, using scanning transmission X-ray microscopy measurements, are attributed to structural variations within the polymeric film.

Effect of polymers on the nanostructure and on the carbonation of calcium silicate hydrates: a scanning transmission X-ray microscopy study

This study investigated the effects of organic polymers (polyethylene glycol and hexadecyltrimethylammonium) on structures of calcium silicate hydrates (C–S–H) which is the major product of Portland cement hydration. Increased surface areas and expansion of layers were observed for all organic polymer modified C–S–H. The results from attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopic measurements also suggest lowered water contents in the layered structures for the C–S–H samples that are modified by organic polymers. Scanning transmission X-ray microscopy (STXM) results further supports this observation. We also observed difference in the extent of C–S–H carbonation due to the presence of organic polymers. No calcite formed in the presence of HDTMA whereas formation of calcite was observed with C–S–H sample modified with PEG. We suggest that the difference in the carbonation reaction is possibly due to the ease of penetration and diffusion of the CO2. This observation suggests that CO2 reaction strongly depends on the presence of organic polymers and the types of organic polymers incorporated within the C–S–H structure. This is the first comprehensive study using STXM to quantitatively characterize the level of heterogeneity in cementitious materials at high spatial and spectral resolutions. The results from BET, XRD, ATR–FTIR, and STXM measurements are consistent and suggest that C–S–H layer structures are significantly modified due to the presence of organic polymers, and that the chemical composition and structural differences among the organic polymers determine the extent of the changes in the C–S–H nanostructures as well as the extent of carbonation reaction.

Carrier mobility in pentacene as a function of grain size and orientation derived from scanning transmission X-ray microscopy

Pentacene field-effect transistors were prepared on silicon nitride membranes for scanning transmission X-ray microscopy (STXM) investigations. The membranes were modified by different self-assembled monolayers (SAMs). Pentacene was deposited atop the SAM-treated membrane and the in-plane orientation of the grains were subsequently investigated by polarization dependent STXM measurements. The grain sizes were determined and compared to those obtained from atomic force microscopy (AFM) measurements. Statistical analysis of the grain orientation was correlated with the charge carrier mobility of the films, in which we observed an increase in the mobility with increasing grain size and decreasing surface roughness of the SAM.

Insulating behavior of magnetic spots in proton-bombarded graphite

Kelvin probe force microscopy measurements on micrometer small magnetic spots produced by proton bombardment on bulk graphite reveal a charge transfer from the center of the spot to an external ring with potential variation on the order of 50 mV. The total charge in the spot is neutral. The results can be well understood in terms of practically unscreened potentials, an insulating property, although the nonbombarded, surrounding graphite region exhibits good conductance. Scanning transmission x-ray microscopy measurements on magnetic spots prepared on graphitic films reveal similar charge distribution. The insulating behavior is fundamental to understand the magnetism in graphite.

Coupled redox transformations of catechol and cerium at the surface of a cerium(III) phosphate mineral

Highly insoluble Ce-bearing phosphate minerals form by weathering of apatite [Ca5(PO4)3.(OH,F,Cl)], and are important phosphorous repositories in soils. Although these phases can be dissolved via biologically-mediated pathways, the dissolution mechanisms are poorly understood. In this paper we report spectroscopic evidence to support coupling of redox transformations of organic carbon and cerium during the reaction of rhabdophane (CePO4·H2O) and catechol, a ubiquitous biogenic compound, at pH 5. Results show that the oxic–anoxic conditions influence the mineral dissolution behavior. Under anoxic conditions, the release of P and Ce occurs stoichiometrically. In contrast, under oxic conditions, the mineral dissolution behavior is incongruent, with dissolving Ce3+ ions oxidizing to CeO2. Reaction product analysis shows the formation of CO2, polymeric C, and oxalate and malate. The presence of more complex forms of organic carbon was also confirmed. Near edge X-ray absorption fine structure spectroscopy measurements at Ce-M4,5 and C-K absorption edges on reacted CePO4·H2O samples in the absence or presence of catechol and dissolved oxygen confirm that (1) the mineral surface converts to the oxide during this reaction, while full oxidation is limited to the near-surface region only; (2) the Ce valence remains unchanged when the reaction between CePO4·H2O and O2 but in the absence of catechol. Carbon K-edge spectra acquired from rhabdophane reacted with catechol under oxic conditions show spectral features before and after reaction that are considerably different from catechol, indicating the formation of more complex organic molecules. Decreases in intensity of characteristic catechol peaks are accompanied by the appearance of new π∗ resonances due to carbon in carboxyl (ca. 288.5eV) and carbonyl (ca. 289.3eV) groups, and the development of broad structure in the σ∗ region characteristic of aliphatic carbon. Evolution of the C K-edge spectra is consistent with aromatic-ring cleavage and polymerization. These results further substantiate that the presence of catechol, O2 (aq) causes both the oxidation of structural Ce3+ and the transformation of catechol to more complex organic molecules. Scanning Transmission X-Ray Microscopy measurements at the C K and Ce M4,5 edges indicate three dominant organic species, varying in complexity and association with the inorganic phase. Untransformed catechol is loosely associated with CeO2, whereas more complex organic molecules that exhibit lower aromaticity and stronger CO π∗ resonances of carboxyl-C and carbonyl-C groups are only found in association with the grains. These results further serve as basis to postulate that, in the presence of O2, CeO2 can mediate the oxidative polymerization of catechol to form higher molecular weight polymers. The present work provides evidence for a pathway of biologically-induced, non-enzymatic oxidation of cerium and formation of small CeO2 particles at room temperature. These findings may have implications for carbon cycling in natural and cerium-contaminated soils and aqueous environments.

NephilaclavipesSpider Dragline Silk Microstructure Studied by Scanning Transmission X-ray Microscopy

Nephila clavipes dragline silk microstructure has been investigated by scanning transmission X-ray microscopy (STXM), a technique that allows quantitative mapping of the level of orientation of the peptide groups at high spatial resolution (<50 nm). Maps of the orientation parameter P2 have been derived for spider silk for the first time. Dragline silk presents a very fine microstructure in which small, highly oriented domains (average area of 1800 nm2, thus clearly bigger than individual beta-sheet crystallites) are dispersed in a dominant, moderately oriented matrix with several small unoriented domains. Our results also highlight the orientation of the noncrystalline fraction in silk, which has been underestimated in numerous structural models. No evidence of either a regular lamellar structure or any periodicity along the fiber was observed at this spatial resolution. The surface of fresh spider silk sections consists of a approximately 30-120 nm thick layer of highly oriented protein chains, which was found to vary with the reeling speed, where web building (0.5 cm/s) and lifeline (10 cm/s) spinning speeds were investigated. While the average level of orientation of the protein chains is unaffected by the spinning speed, STXM measurements clearly highlight microstructure differences. The slowpull fiber contains a larger fraction of highly oriented domains, while the protein chains are more homogeneously oriented in the fastpull fiber. In comparison, cocoon silk from the silkworm Bombyx mori presents a narrower orientation distribution. The strength-extensibility combination found in spider dragline silk is associated with its broad orientation distribution of highly interdigitated and unoriented domains.