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Scanning transmission electron microscopy (STEM) allows for imaging, diffraction, and spectroscopy of materials on length scales ranging from microns to atoms. By using a high-speed, direct electron detector, it is now possible to record a full two-dimensional (2D) image of the diffracted electron beam at each probe position, typically a 2D grid of probe positions. These 4D-STEM datasets are rich in information, including signatures of the local structure, orientation, deformation, electromagnetic fields, and other sample-dependent properties. However, extracting this information requires complex analysis pipelines that include data wrangling, calibration, analysis, and visualization, all while maintaining robustness against imaging distortions and artifacts. In this paper, we present py4DSTEM, an analysis toolkit for measuring material properties from 4D-STEM datasets, written in the Python language and released with an open-source license. We describe the algorithmic steps for dataset calibration and various 4D-STEM property measurements in detail and present results from several experimental datasets. We also implement a simple and universal file format appropriate for electron microscopy data in py4DSTEM, which uses the open-source HDF5 standard. We hope this tool will benefit the research community and help improve the standards for data and computational methods in electron microscopy, and we invite the community to contribute to this ongoing project.

Gadolinium titanate (Gd2Ti2O7, or GTO) and other lanthanide pyrochlores are interesting candidates for actinide waste disposal and fast ion conduction because the relevant material properties are intimately dependent on local cation structure. Therefore, a deep understanding of the kinetics associated with cation ordering and disordering is required if such material properties are to be tuned for specific device designs. To this end, single crystals of GTO were irradiated with 190 keV helium ions to a total fluence of 1 × 1017 ions/cm2, amorphizing the sample surface to a depth of ~1 μm and resulting in significant He bubble accumulation. FIB lamellae lifted out from the irradiated sections were examined during heat treatment in the (scanning) transmission electron microscope. Two distinct stages of the re-crystallization of the amorphized material were observed. The material near the end of the ions’ range transformed first and with the same orientation as the pristine material. This was due to the close proximity of the pristine material and the presence of small defect fluorite seeds, but the propagation of this growth front was frustrated by the large pores (formerly bubbles) in the He accumulation layer. This was followed by heterogeneous nucleation of new crystallites at random orientations at the top of the He accumulation layer, which is attributed to the high surface area associated with the many small He bubbles in that region. It is inferred that the kinetics of grain growth in this material are significantly faster than the kinetics of grain nucleation.

Previous studies of the reaction of hydrogen gas with uranium metal have been mainly limited to microscale or higher observations of uranium surfaces. We have characterized subsurface features of uranium hydride structures formed in a depleted uranium pellet aged for over 9 years under a controlled atmosphere with an initial H2 partial pressure of 0.667 kPa (5 torr) using electron energy-loss spectroscopy (EELS), giving information at lower length scales than previously reported. We demonstrate that multiple linear least-squares fitting of the O4,5 edge of the uranium EEL spectrum, with input spectra generated from the dataset being analyzed, can be used to map hydride and metal phases in aged uranium metal at the nanometer scale. While the interface between the bulk metal and the subsurface hydride structures is sharp and well defined, the interior of the reacted region is heterogeneous, containing a mix of polycrystalline UH3 and stranded, apparently unreacted, U metal.

Understanding the structure-property relationship for materials destined for irradiation extremes is a key step in developing materials with reliable, long-term performance. One crucial relationship is the ability of a material to withstand or accommodate amorphization, as this dictates its potential use as a nuclear waste form. Pyrochlores are one such class of materials for consideration as waste forms and there has been significant work examining how both the crystal structure and chemistry impacts amorphization resistance, leading to the important conclusion that the amorphization resistance of pyrochlores (A2B2O7) is very sensitive to the nature of the B cations. For example, pyrochlores with B Ti amorphize much more readily than B Zr compounds. However, there are still questions regarding how these types of materials respond to prolonged or high-dose irradiation conditions. In this work, Gd2Ti2O7 (GTO) and Gd2Zr2O7 (GZO) pyrochlores were implanted with 400 keV Kr++ ions at room temperature to calculated peak damages of 119 and 135 displacements per atom (dpa), respectively. As expected, GTO amorphized completely under irradiation. However, discrete bubbles of Kr coalesced within the amorphous matrix without micro-cracking or spallation. In contrast, GZO transforms to a disordered fluorite structure under irradiation with no indications of localized amorphization. But, the accumulation of Kr within the host material leads to sub-grain structures, extended defects, and the development of micro-cracks. Thus, while GTO readily amorphizes even under low dose irradiations, the resistance of the amorphous GTO matrix to micro cracking and gas release, even in the presence of large bubble formation, suggests an enhanced propensity to retain gaseous species. Consideration of long-term dose accumulation effects in nuclear waste forms would suggest reconsideration of amorphization processes in pyrochlores and related materials as a potential beneficial effect for immobilization and long-term storage of actinide materials.

SiO2-TiO2 nanofibers were electrospun from partially hydrolyzed tetraethyl orthosilicate, and titanium isopropoxide using sol-gel chemistry. SiO2-TiO2 sol phase diagrams were created summarizing the role of composition on solution homogeneity and electrospinnability. Inorganic nanofiber spinnability was studied without the addition of any organic polymer, oligomer, gelator, or binder. TiO2 concentration within SiO2-TiO2 fibers ranged from 25 to 100 mol%. SiO2, SiO2-TiO2, and TiO2 nanofiber structures were investigated using scanning electron microscopy and transmission electron microscopy. Inorganic fiber spinning was highly dependent on sol reaction temperature, time, and solution composition. At high TiO2 concentrations, twisted and ribbon-like nanofibers with dumbbell-shaped cross-sections were observed. This was attributed to jet branching and splitting during electrospinning. Electrospun fibers were amorphous at room temperature, but thermally converted into crystalline anatase, which underwent additional structural changes at higher temperatures into rutile. This anatase-rutile thermal phase transformation was highly dependent upon TiO2 concentration. Nanofiber composition, thermal stability, and crystalline structures were characterized by energy-dispersive X-ray spectroscopy; Fourier transform infrared spectroscopy, thermal gravimetric analysis, and wide-angle X-ray diffraction.

Heterogeneous redox reactions on clay mineral surfaces control mobility and bioavailability of redox-sensitive nutrients and contaminants. Iron (Fe) residing in clay mineral structures can either catalyze or directly participate in redox reactions; however, chemical controls over its reactivity are not fully understood. In our previous work we demonstrated that converting a minor portion of Fe(III) to Fe(II) (partial reduction) in the octahedral sheet of natural Fe-rich clay mineral nontronite (NAu-1) activates its surface, making it redox-active. In this study we produced and characterized synthetic ferric nontronite (SIP), highlighting structural and chemical similarities and differences between this synthetic nontronite and its natural counterpart NAu-1, and probed whether mineral surface is redox-active by reacting it with arsenic As(III) under oxic and anoxic conditions. We demonstrate that synthetic nontronite SIP undergoes the same activation as natural nontronite NAu-1 following the partial reduction treatment. Similar to NAu-1, SIP oxidized As(III) to As(V) under both oxic (catalytic pathway) and anoxic (direct oxidation) conditions. The similar reactivity trends observed for synthetic nontronite and its natural counterpart make SIP an appropriate analog for laboratory studies. The development of chemically pure analogs for ubiquitous soil minerals will allow for systematic research of the fundamental properties of these minerals.

Author(s): Janish, Matthew T; Schneider, Matthew M; Ophus, Colin; Ciston, Jim; Valdez, James A; McClellan, Kenneth J; Byler, Darrin D; Chen, Di; Wang, Yongqiang; Holesinger, Terry G; Uberuaga, Blas P

The lithiation of molybdenum disulfide (MoS 2 ) has been directly studied in situ in the TEM by observing specimens with the viewing direction parallel to the basal planes. The MoS 2 lamella was characterized by bright-field imaging during the lithiation, and both selected-area diffraction and high-resolution imaging before and after. An overall expansion of ∼5% along the c -direction was observed with concurrent local contraction. The contraction can be related to the expulsion of Mo as Li reduces it to form Li 2 S.

We employed an in situ electrochemical cell in the transmission electron microscope (TEM) together with ex situ time-of-flight, secondary-ion mass spectrometry (TOF-SIMS) depth profiling, and FIB-helium ion scanning microscope (HIM) imaging to detail the structural and compositional changes associated with Na/Na(+) charging/discharging of 50 and 100 nm thin films of Sb. TOF-SIMS on a partially sodiated 100 nm Sb film gives a Na signal that progressively decreases toward the current collector, indicating that sodiation does not proceed uniformly. This heterogeneity will lead to local volumetric expansion gradients that would in turn serve as a major source of intrinsic stress in the microstructure. In situ TEM shows time-dependent buckling and localized separation of the sodiated films from their TiN-Ge nanowire support, which is a mechanism of stress-relaxation. Localized horizontal fracture does not occur directly at the interface, but rather at a short distance away within the bulk of the Sb. HIM images of FIB cross sections taken from sodiated half-cells, electrically disconnected, and aged at room temperature, demonstrate nonuniform film swelling and the onset of analogous through-bulk separation. TOF-SIMS highlights time-dependent segregation of Na within the structure, both to the film-current collector interface and to the film surface where a solid electrolyte interphase (SEI) exists, agreeing with the electrochemical impedance results that show time-dependent increase of the films' charge transfer resistance. We propose that Na segregation serves as a secondary source of stress relief, which occurs over somewhat longer time scales.

Materials such as tin (Sn) and silicon that alloy with lithium (Li) have attracted renewed interest as anode materials in Li-ion batteries. Although their superior capacity to graphite and other intercalation materials has been known for decades, their mechanical instability due to extreme volume changes during cycling has traditionally limited their commercial viability. This limitation is changing as processes emerge that produce nanostructured electrodes. The nanostructures can accommodate the repeated expansion and contraction as Li is inserted and removed without failing mechanically. Recently, one such nano-manufacturing process, which is capable of depositing coatings of Sn “nanoneedles” at low temperature with no template and at industrial scales, has been described. The present work is concerned with observations of the lithiation and delithiation behavior of these Sn nanoneedles during in situ experiments in the transmission electron microscope, along with a brief review of how in situ TEM experiments have been used to study the lithiation of Li-alloying materials. Individual needles are successfully lithiated and delithiated in solid-state half-cells against a Li-metal counter-electrode. The microstructural evolution of the needles is discussed, including the transformation of one needle from single-crystal Sn to polycrystalline Sn–Li and back to single-crystal Sn.

The metal tantalum has many varied uses including in microelectronics (especially in capacitors) as thin films, in medical applications as an implant material or for surgical instruments, in X-ray lithography for masks, and in high-temperature structural applications. Ta is particularly useful because it is relatively ductile, refractory in nature, and does not readily react with corrosive materials. The body-centered cubic (bcc) crystal structure of pure Ta, also known as the α-phase, is the most commonly observed, but Ta is also known to exist in two other allotropes, one tetragonal and the other (much less-well-known) face-centered cubic (fcc). The tetragonal form (β-Ta) has been produced by various deposition techniques and often occurs mixed with the α-phase; the fcc phase has only previously been reported in thin films deposited by thermal evaporation. There have been other reports of ‘bcc metals’ such as V and Fe existing with an fcc crystal structure when the metal is deposited as a thin film. In the present study, fcc Ta with a = 0.43 nm has been observed using transmission electron microscopy in bulk samples of Ta that have been subjected to quasi-static tensile deformation that was so large as to cause fracture of the material. The fcc phase has a relatively small grain size but appears to be stable at room temperature. It is also shown that relatively large grains (10–20 nm in diameter) of Ta can also exist with an hcp structure with a = 0.304 nm and c = 0.494 nm.

Thin tantalum films have been studied during in situ heating in a transmission electron microscope. Diffraction patterns from the as-deposited films were typical of amorphous materials. Crystalline grains were observed to form when the specimen was annealed in situ at 450 °C. Particular attention was addressed to the formation and growth of grains with the face-centered cubic (fcc) crystal structure. These observations are discussed in relation to prior work on the formation of fcc Ta by deformation and during thin film deposition.

Understanding how Lithium is driven into layer materials such as graphite and the transition metal dichalcogenides (TMDs) is critical in optimizing performance in lithium-ion batteries (LIBs). Understanding how these processes change if Li is replaced by Na will be important if Na is ever to replace Li. The large interlayer spacing in, e.g., MoS2 (∼0.65nm), can accommodate species such as alkali metal ions (Li+/Na+/K+) during the intercalation reaction. Intercalation is reported to change the electronic structure of the host molecule, resulting in variations in their electrical and optical properties. However the solid-state reactions between Li and MoS2 have not been examined in atomistic detail. It is universally accepted that Li+ ions can be inserted into vdW gap but just how this reaction proceeds is still unclear. Plan-view imaging has been extensively used, but really it is essential to visualize the process with the electron beam being parallel to the basal planes of the layer material. Lattice-fringe images have actually been considered for several systems by microtoming specimens or simply using curved regions of the specimen; until the present work the orientation of the specimen was less than ideally uncontrolled. In the present study, TEM specimens are made using FIB, and oriented to allow direct observation of the basal planes. This study of reactions uses both a Tecnai F30 and a Cs/image-corrected Titan equipped with a direct electron detector camera, K2. This camera has two major advantages: the electron dose can be minimized and quick changes during reactions are recorded; a second ChemiSTEM Titan is equipped with both EELS and XEDS capabilities. As the reaction between MoS2 and Li proceeds, white-line defects have been observed using HRTEM. Lower-magnification images show that the defects are not equally spaced and do not correspond to ‘stage’ development. In fact, these defects can cross several basal planes in the MoS2 (either forwards or backwards), this breaking covalent bonds within the layers but maintain essentially the same width after the step; they are not completely constrained to the vdW gap. Density functional theory (DFT) simulations are carried out to investigate the structural accommodation of the layered material during insertion and exertion of the intercalating species (energy barriers, volumetric expansion, and phase transformations). The structural stability of the 2H and 1T phases of MoS2 during lithiation suggests that a phase transformation of the 2H phase of MoS2 to the 1T phase may occur when MoS2 is reacted with Li; the computational study allows different dosages of Lithium ion to be assessed with the aim of testing these the validity of these models using in-situ characterization of the solid-state reactions between Li and MoS2 in the transmission electron microscope (TEM). The role of atomic scale bonding on the energetics and mechanisms of strain relaxation (and phase transformation) due to Li/Na/K intercalation in MoS2, MoSe2, WS2 and WSe2 at the atomic scales will be presented. This work is funded by NSF GOALI grant No. 1820565. MTJ is now at Los Alamos National Laboratory (LANL), WBM is now at Sandia National Laboratories (SNL). The TEM was all carried out at CINT, an Office of Science User Facility operated for the U.S. DOE, or in SNL. SNL is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s NNSA under contract DE-NA-0003525. The views expressed in the abstract do not necessarily represent the views of the U.S. DOE or the U.S. Government.

One-dimensional (1D) nanostructures, often referred to as nanowires, have attracted considerable attention due to their unique mechanical, chemical, and electrical properties. Although numerous novel technological applications are being proposed for these structures, many of the processes used to synthesize these materials involve a vapor phase and require high temperatures and long growth times. Potentially faster methods requiring templates, such as anodized aluminum oxide, involve multiple fabrication steps, which would add significantly to the cost of the final material and may preclude their widespread use. In the present study, it is shown that template-free electrodeposition from an alkaline solution can produce arrays of Sn nanoneedles directly onto Cu foil substrates. This electrodeposition process occurs at 55 °C; it is proposed that the nanoneedles grow via a catalyst-mediated mechanism. In such a process, the growth is controlled at the substrate/nanostructure interface rather than resulting from random plating-induced defects such as dendrites or aging defects such as tin whiskers. There are multiple potential applications for 1D Sn nanostructures—these include anodes in lithium-ion and magnesium-ion batteries and as thermal interface materials. To test this potential, type 2032 lithium-ion battery button cells were fabricated using the electrodeposited Sn. These cells showed initial capacities as high as 850 mAh/g and cycling stability for over 200 cycles.

Lead halide perovskites are increasingly considered for applications beyond photovoltaics, for example, light emission and detection, where an ability to pattern and prototype microscale geometries can facilitate the incorporation of this class of materials into devices. Here we demonstrate laser direct write of lead halide perovskites, a remarkably simple procedure that takes advantage of the inverse dependence between perovskite solubility and temperature by using a laser to induce localized heating of an absorbing substrate. We demonstrate arbitrary pattern formation of crystalline CH3NH3PbBr3 on a range of substrates and fabricate and characterize a microscale photodetector using this approach. This direct write methodology provides a path forward for the prototyping and production of perovskite-based devices.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.