Scanning Transmission X-ray Microscopy Research Articles

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.

Co-aromatization of methane with propane over Zn/HZSM-5: The methane reaction pathway and the effect of Zn distribution

The co-aromatization of methane with propane is investigated using Zn/HZSM-5 as the catalyst at 400 °C. The presence of methane has a pronounced effect on the product distribution in terms of improved formation of phenyl rings and substitution groups of aromatic species. Isotopic labeling and solid-state NMR spectroscopy studies reveal that the favored site for methane incorporation varies at different stages along the co-aromatization reaction, from the benzylic sites at the initial stage to the phenyl rings and then substitution groups as the reaction proceeds. SIMS spectroscopy of the involved reaction intermediates suggests the formation of C4 species over the Zn sites that are bonding to the oxygen atoms of the zeolite framework, which is supported by theoretical calculations. The interaction between Zn species and the zeolite framework is also confirmed by XAS and changes in coordination environments are identified through XANES and solid-state NMR spectroscopy. The redistribution of Zn during the reaction is observed on the surface of the zeolite support. Scanning transmission X-ray microscopy images show that Zn is rich in the inner pores compared with the external surface of the catalyst particles in the pristine catalysts. XANES and XPS spectra demonstrate that the Zn concentration increases on the external surface after the reaction, a conclusion that is further supported by the theoretical study. The Zn redistribution might be among the reasons for methane participation pathway alternation during the co-aromatization, since the Zn species located within the inner pores and on the external surface might prefer to catalyze the methane incorporation to the phenyl rings and substitution groups of the formed aromatics, respectively.

Three-dimensional focal stack imaging in scanning transmission X-ray microscopy with an improved reconstruction algorithm.

Focal stack (FS) is an effective technique for fast 3D imaging in high-resolution scanning transmission X-ray microscopy. Its crucial issue is to assign each object within the sample to the correct position along the optical axis according to a proper focus measure. There is probably information loss with previous algorithms for FS reconstruction because the old algorithms can only detect one focused object along each optical-axial pixel line (OAPL). In this study, we present an improved FS algorithm, which utilizes an elaborately calculated threshold for normalized local variances to extract multiple focused objects in each OAPL. Simulation and experimental results show its feasibility and high efficiency for 3D imaging of high contrast, sparse samples. It is expected that our advanced approach has potential applications in 3D X-ray microscopy for more complex samples.

Correlative Spectromicroscopy and Tomography for Biomedical Applications Involving Electron, Ion, and Soft X-ray Microscopies

Abstract

Emission and propagation of 1D and 2D spin waves with nanoscale wavelengths in anisotropic spin textures.

Spin waves offer intriguing perspectives for computing and signal processing, because their damping can be lower than the ohmic losses in conventional complementary metal-oxide-semiconductor (CMOS) circuits. Magnetic domain walls show considerable potential as magnonic waveguides for on-chip control of the spatial extent and propagation of spin waves. However, low-loss guidance of spin waves with nanoscale wavelengths and around angled tracks remains to be shown. Here, we demonstrate spin wave control using natural anisotropic features of magnetic order in an interlayer exchange-coupled ferromagnetic bilayer. We employ scanning transmission X-ray microscopy to image the generation of spin waves and their propagation across distances exceeding multiples of the wavelength. Spin waves propagate in extended planar geometries as well as along straight or curved one-dimensional domain walls. We observe wavelengths between 1 μm and 150 nm, with excitation frequencies ranging from 250 MHz to 3 GHz. Our results show routes towards the practical implementation of magnonic waveguides in the form of domain walls in future spin wave logic and computational circuits.

Dependence of substrate work function on the energy-level alignment at organic–organic heterojunction interface

The dependence of substrate work function (WF) on organic–organic heterojunction (OOH) interface was investigated using ultraviolet and X-ray photoelectron spectroscopy. We studied the interface of boron subphthalocyanine chloride (SubPc)/α-sexithiophene (6T) deposited on MoO3, SiO2, Cs2CO3. We observed that MoO3 and Cs2CO3 induce a p-doping and n-doping, respectively, due to the WF position, that can generate charge transfer at the OOH interface. However, the same effect was not observed after annealing the organic layers. Using scanning transmission X-ray microscope combining with near-edge X-ray absorption fine structure, we could observe that SubPc film became well-ordered after annealing the thin film. Thus, we suggested that the control of charge transfer arises from the reduction of the molecules misorientation on the film that induces a reduction in the density of gap states.

Electron beam damage of epoxy resin films studied by scanning transmission X-ray spectromicroscopy

Focused ion beam coupled with scanning electron microscopy (FIB-SEM) is a popular technique for advanced electron microscopy with applications such as, high-precision site-specific lamella sample preparation for transmission electron microscopy (TEM) and slice-and-view FIB 3-dimensional tomography. Damage caused by the electron imaging component of FIB-SEM may be compounded with damage from the ions during the ion milling process. There are known strategies for mitigating damage from ions and electrons (cryo-SEM, dose-control, voltage control), but the electron damage on common embedding resins for EM has not been explored in detail beyond their resistance to shape-change. The relationship between beam parameters and damage mechanisms remains unclear. Since we are relying on the physical, chemical and thermal stability of embedded samples during ion-beam milling, it is important to distinguish electron beam damage from ion beam damage. Scanning transmission X-ray microscopy (STXM) has been used for analyzing the electron beam radiation damage on polymer films by characterizing the chemical bonding changes. In this paper, we focus on the effect of beam voltage and electron dose on electron beam damage to epoxy resin thin films. Irradiated areas on polymer thin films were characterized by near edge X-ray absorption fine structure (NEXAFS) in STXM. We found that, even when using low current and voltage, the electron beam can still cause noticeable chemical changes within the polymer film. The degree of electron beam damage depends not only on the beam energy, but also on the amount of inelastic scattering occurring within the material, as determined by the sample thickness.

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.

Scanning transmission X-ray microscopy with efficient X-ray fluorescence detection (STXM-XRF) for biomedical applications in the soft and tender energy range.

Scanning transmission X-ray microscopy, especially in combination with X-ray fluorescence detection (STXM-XRF) in the soft X-ray energy range, is becoming an increasingly important tool for life sciences. Using X-ray fluorescence detection, the study of biochemical mechanisms becomes accessible. As biological matrices generally have a low fluorescence yield and thus a low fluorescence signal, high detector efficiency (e.g. large solid angle) is indispensable for avoiding long measurement times and radiation damage. Here, the new AnImaX STXM-XRF microscope equipped with a large solid angle of detection enabling fast scans and the first proof-of-principle measurements on biomedical samples are described. In addition, characterization measurements for future quantitative elemental imaging are presented.

Nanoscale detection of spin wave deflection angles in permalloy

Magnonics is a potential candidate for beyond CMOS and neuromorphic computing technologies with advanced phase encoded logic. However, nanoscale imaging of spin waves with full phase and magnetization amplitude information is a challenge. We show a generalized scanning transmission x-ray microscopy platform to get a complete understanding of spin waves, including the k-vector, phase, and absolute magnetization deflection angle. As an example, this is demonstrated using a 50 nm thin permalloy film where we find a maximum deflection angle of 1.5° and good agreement with the k-vector dispersion previously reported in the literature. With a spatial resolution approximately ten times better than any other methods for spin wave imaging, x-ray microscopy opens a vast range of possibilities for the observation of spin waves and various magnetic structures.

Application of Electron Microscopes in Nanotoxicity Assessment.

In this chapter, we highlight the applications of electron microscopes (EMs) in nanotoxicity assessment. EMs can provide detailed information about the size and morphology of nanomaterials (NMs), their localization in cells and tissues, the nano-bio interactions, as well as the ultrastructural changes induced by NMs exposure. Here, we share with the readers how we prepare the tissue sample, and the different types of EMs used among the nanotoxicologists. It is possible to deploy conventional EMs along or in combination with other analytical techniques, such as electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS or EDX), and TEM-assisted scanning transmission X-ray microscopy (STXM), toward further elemental and chemical characterization. Appropriate images are inserted to illustrate throughout this chapter.

Dynamic Imaging of the Delay- and Tilt-Free Motion of Néel Domain Walls in Perpendicularly Magnetized Superlattices.

We report on the time-resolved investigation of current- and field-induced domain wall motion in the flow regime in perpendicularly magnetized microwires exhibiting antisymmetric exchange interaction by means of scanning transmission X-ray microscopy with a 200 ps time step. The sub-ns time step of the dynamical images allowed us to observe the absence of incubation times for the motion of the domain wall within an uncertainty of 200 ps, together with indications for a negligible inertia of the domain wall. Furthermore, we observed that, for short current and magnetic field pulses, the magnetic domain walls do not exhibit a tilting during their motion, providing a mechanism for the fast, tilt-free, current-induced motion of magnetic domain walls.

Development and prospect of biological effects and safety of nanomaterials

Nanomaterials have lots of unique physical and chemical properties, such as quantum size effect, surface effect and macroscopic quantum tunnel effect. The biological effects of nanomaterials might be different from those of bulk materials larger than microns even if their chemical compositions are same. Therefore, the biological effects and safety evaluation results obtained from the bulk materials might not be suitable for the nanomaterials. Since 2003, governments worldwide have initiated the researches on biological effects and safety issues of nanomaterials with increasing financial support. China is one of the earliest countries in the world to carry out researches on biological effects and safety issues of nanomaterials. And some research fields have entered the forefront of the world. The CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety established jointly by National Center for Nanoscience and Technology (NCNST) and Institute of High Energy Physics of Chinese Academy of Sciences (IHEP of CAS) was the first professional laboratory focusing on the biological effects and safety of nanomaterials in China. Scientists in this laboratory have done many in-depth studies of the relationship between the physical-chemical properties and biological responses of nanomaterials. They found several factors which affected the biological effects of carbon nanotubes significantly, such as nanoparticle-Crona, metal impurities, surface, size, shape and so on. They have also established multiple models for hazard recognition and risk assessment. Nanomaterials/nanoparticles (NMs/NPs) are exposed to human beings mostly through four routes, i.e., oral intake, skin contact, inhalation, and intravenous injection. Upon entering the body, NMs/NPs are quickly distributed into specific organs and then metabolized primarily by the liver. The final excretion of NMs/NPs usually occurs in the liver and kidney in the form of urine and feces. In general, the most important issues regarding the absorption, distribution, metabolism and excretion (ADME) processes of NMs/NPs include: (1) Where do and how much NMs/NPs get in (via absorption)? (2) Where do and how much NMs/NPs go (via distribution)? (3) How much, when do, and what form of NMs/NPs remain intact (via metabolism)? (4) Where do, how much, and what form of NMs/NPs stay in the system (via excretion)? At various stages of the ADME processes, the challenges for in vivo analysis of NMs/NPs may become largely different. Important analytical methods to resolve the analytical challenges for each absorption, distribution, metabolism and excretion (ADME) process are summarized. The state-of-the art techniques with high sensitivity and high resolution suitable for safety evaluation have been developed to ensure the sustainable development of nanotechnology, such as inductively coupled plasma mass spectrometry (ICPMS), inductively coupled plasma optical emission spectrometry (ICP-OES), nuclear analysis such as neutron activation analysis (NAA), laser ablation inductively coupled plasma mass spectrometry (LAICPMS), secondary ion mass spectrometry (SIMS), synchrotron radiation X-ray fluorescence spectroscopy (SRXRF), X-ray absorption spectroscopy (XAS), X-ray fluorescence spectroscopy (XRF), scanning transmission X-ray microscopy (STXM), transmission X-ray microscopy (TXM) and so on. In this review, we summarize these research progresses in recent years.

In Situ/Operando X-ray Spectroscopies for Advanced Investigation of Energy Materials.

Issues related to energy and the environment have now become of central and crucial importance in our societies. Low-carbon green energy will have a critical role in a necessary third industrial revolution. To reduce global greenhouse gas emissions in response to globalization and increasingly stringent carbon emission policies, large scale green energy production technologies must be established worldwide. A new age of human demand for green energy is thus coming and scientists are focused on finding new functional efficient and low-cost materials to generate clean and sustainable energy. Improving the energy conversion, generation, and storage efficiency of energy materials has always been a daunting challenge. For many important energy material systems, such as nanostructured catalysts, artificial photosynthetic systems, smart energy saving materials, and energy storage devices, monitoring the atomic and electronic structures close to the interfacial region in a real working environment is of paramount importance. Designing a better-performing material without comprehending its fundamental properties such as chemical states, atomic and electronic structures and how they are altered close to the interfacial regions during the physical and chemical reactions involved in their applications is very challenging. Understanding, controlling and tuning the interfaces in energy conversion and storage materials requires in situ/operando characterization tools, of which synchrotron X-ray spectroscopies, which have several unique features, are very suitable ones. X-ray absorption spectroscopy can be used to elucidate the local unoccupied electronic structure in the conduction band, and X-ray emission spectroscopy can be used to characterize the occupied electronic structure in the valence band. The derived resonant inelastic X-ray scattering reveals inter- and/or intra-electric transitions (i.e. d-d, f-f excitation and charge-transfer excitation) that reflect intrinsic chemical and physical properties. Scanning transmission X-ray microscopy is a chemical mapping technique with elemental sensitivity and spatial selectivity, which can therefore yield information about chemical composition in various spatial regions. This unique characteristic makes the method effective for investigating interfacial phenomena (such as electron transport, interface formation/deformation, defects, doping etc.). In situ/operando approaches have made the probing and understanding of changes in the atomic and electronic structures of energy materials in an operational environment feasible. This article presents a perspective of the pioneering developments as well as the recent achievements in in situ/operando synchrotron X-ray spectroscopies for the advanced investigation of energy materials. Four major energy material systems are identified: energy storage, energy generation, energy conversion, and energy saving material systems. Selected representative investigations of each systems are showcased and discussed demonstrating that in situ/operando synchrotron X-ray spectroscopy is truly essential for unraveling better fundamental knowledge for mechanism understanding and efficiency optimization of existing and emerging energy material systems.

Sub-micron level investigation reveals the inaccessibility of stabilized carbon in soil microaggregates

Direct evidence-based approaches are vital to evaluating newly proposed theories on the persistence of soil organic carbon and establishing the contributions of abiotic and biotic controls. Our primary goal was to directly identify the mechanisms of organic carbon stabilization in native-state, free soil microaggregates without disrupting the aggregate microstructure using scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS). The influence of soil management practices on microaggregate associated-carbon was also assessed. Free, stable soil microaggregates were collected from a tropical agro-ecosystem in Cruz Alta, Brazil. The long-term experimental plots (>25 years) comparing two tillage systems: no-till and till with a complex crop rotation. Based on simultaneously collected multi-elemental associations and speciation, STXM-NEXAFS successfully provided submicron level information on organo-mineral associations. Simple organic carbon sources were found preserved within microaggregates; some still possessing original morphology, suggesting that their stabilization was not entirely governed by the substrate chemistry. Bulk analysis showed higher and younger organic carbon in microaggregates from no-till systems than tilled systems. These results provide direct submicron level evidence that the surrounding environment is involved in stabilizing organic carbon, thus favoring newly proposed concepts on the persistence of soil organic carbon.

Comparative study of catalytic activities among transition metal-doped IrO2 nanoparticles

Catalytic activities of transition metal-doped IrO2 nanoparticles (TM-IrO2 NPs; TM = Cr, Mn, Fe, Co, or Ni) are compared for various oxidation reactions such as electrochemical oxygen evolution reaction (OER), gas-phase photo-oxidation of thiol function group, and CO oxidative conversion. Here, we discovered a series of TM-IrO2 catalysts have a common activity trend for these oxidation reactions, and their activities are closely related with modified electronic states of IrO2, strongly affected by the types of the transition metal across the periodic table. For all oxidation reactions, Cr- and Mn-IrO2 achieved the highest oxidation catalytic activity, and sequentially decreased activities were obtained with Fe, Co, and Ni doped IrO2. For instance, the highest OER activity was achieved by Cr or Mn doping exhibiting the smallest overpotential η = 275~230 mV at 10 mA/cm2, while Ni-IrO2 showed rather larger overpotential (η = 347 mV) even compared with non-doped IrO2 (η = 314 mV). Scanning transmission X-ray microscopy and high-resolution photoemission spectra of TM-IrO2 indicated dopant metals modified the Ir-O interaction and thus increasing oxygen vacancy defects in IrO2. Strongly positive correlation was observed between the catalytic activities and vacancy states. The amount of defect related signals was observed the most for Cr- or Mn-IrO2, less so for Fe- or Co-IrO2, and unnoted for Ni-IrO2 compared with bare IrO2. Based on these catalytic activities and surface spectroscopic analysis results, vacancy defects induced by doping in TM-IrO2 NPs are proposed to contribute to enhance the oxidation activities.

Observing How Fischer-Tropsch Synthesis Catalysts Work at the Nanoscale via Operando STXM

In a recent Angewandte Chemie paper, Weckhuysen and coworkers report an elegantly designed operando scanning transmission X-ray microscopy study on the industrially important Co/TiO2 catalyst for Fischer-Tropsch synthesis. They reveal the structure-performance relationship between the Co valence state and the chemical nature of the organic films deposited on the catalyst grains.

Probing the Location and Speciation of Elements in Zeolites with Correlated Atom Probe Tomography and Scanning Transmission X-Ray Microscopy.

Characterizing materials at nanoscale resolution to provide new insights into structure property performance relationships continues to be a challenging research target due to the inherently low signal from small sample volumes, and is even more difficult for nonconductive materials, such as zeolites. Herein, we present the characterization of a single Cu‐exchanged zeolite crystal, namely Cu‐SSZ‐13, used for NOX reduction in automotive emissions, that was subject to a simulated 135,000‐mile aging. By correlating Atom Probe Tomography (APT), a single atom microscopy method, and Scanning Transmission X‐ray Microscopy (STXM), which produces high spatial resolution X‐ray Absorption Near Edge Spectroscopy (XANES) maps, we show that a spatially non‐uniform proportion of the Al was removed from the zeolite framework. The techniques reveal that this degradation is heterogeneous at length scales from micrometers to tens of nanometers, providing complementary insight into the long‐term deactivation of this catalyst system.

Aqueous alteration of porous microchondrules in Semarkona: Implications for hydration, oxidation and elemental exchange processes

Porous, igneous glassy microchondrules are relatively common in Semarkona and other unequilibrated ordinary chondrites (UOC), occurring embedded within the fine-grained matrices and the chondrule rims. We have investigated the effects of aqueous alteration on two porous microchondrules and associated amorphous silicate materials in the fine-grained matrix of the Semarkona (LL3.00) ordinary chondrite. The iron valency was measured by synchrotron-based scanning transmission X-ray microscopy (STXM) and detailed compositional and mineralogical data were obtained by transmission electron microscopy (TEM). Our STXM data show that the iron in both microchondrules and the amorphous matrix materials is highly oxidized, with elevated ferric iron contents (up to Fe3+/∑Fe ratio 81%). The oxidation process appears to be the result of the interaction of aqueous fluids produced by melting of water ice that accreted into the LL chondrite parent body. The lack of secondary phases such as phyllosilicates or FeO-rich olivines in the microchondrules and the surrounding fine-grained matrix indicates that aqueous alteration was extremely limited. The distribution of major and minor elements indicates limited elemental exchange between the porous microchondrules and the adjacent fine-grained matrix, suggesting that the chemical composition of the two porous microchondrules was not extensively modified during the aqueous alteration. Therefore, these results strongly support the recent conclusions that porous and nonporous microchondrules have different precursor compositions, since their bulk compositions are different and have not been extensively modified by secondary processes.

Four-Dimensional Imaging of ZnO-Coated Alumina Aerogels by Scanning Transmission X-ray Microscopy and Ptychographic Tomography

Functional porous materials are of increasing importance in energy conversion and catalysis. Improved analysis is needed to guide optimization. Four-dimensional (4D) imaging [chemical mapping in three-dimensional (3D) by spectro-tomography] of an Al2O3 aerogel coated with ZnO by atomic layer deposition was performed using soft X-ray ptychography at the Zn L-edge. A two-dimensional spatial resolution of 14(2) nm was achieved. Visualizations of the 3D chemical structure are provided. The degree of ZnO coverage of the surface of the Al2O3 aerogel framework in two different samples was estimated and found to be both thicker and less homogeneous than expected. Other analyses of the 4D results, including the degree of contact between Al2O3 and ZnO, were extracted from the reconstructed 3D data. This pioneering soft X-ray spectro-ptycho-tomography study will anchor further studies of functional porous materials.