Energy-filtered Electron Research Articles

A TEM Study of Structural Degradation in LiFePO4 Batteries after Hybrid Vehicle Use

Establishing a robust, efficient circular economy for batteries hinges on understanding how they decay over time under various conditions. This understanding is crucial to maximize material use before recycling or disposal. A key aspect of this endeavor is the study of lithium iron phosphate (LiFePO4), a pivotal battery technology whose structural integrity over time remains a subject of inquiry.1 Known aging mechanisms in LiFePO4 (LFP) batteries include electrode degradation, SEI growth, and electrolyte decomposition.1 These processes extend to the cathode, where degradation can manifest as Fe dissolution, Li inventory loss, Fe/Li anti-site defects, and LFP amorphization.1–3 However, these mechanisms are typically studied in lab-aged cells under controlled conditions, leaving a gap in our understanding of their behavior in real-life, commercialized applications.Our research aims to bridge this gap by characterizing the degradation mechanisms of LFP cathode material in various states of health (SOH) after use in a hybrid vehicle. We sourced our cathode samples from 26650 cells extracted from a BAE ESS-A123 hybrid bus battery pack. After selecting cells with drastically different SOHs based on previous analyses,4 we employed transmission electron microscopy (TEM) for detailed characterization.In this talk, we will share insights gained from high-resolution TEM characterization, alongside chemical analysis using energy-filtered TEM and electron energy loss spectroscopy (EELS). Our focus will be on changes in the olivine structure, including lattice parameter alterations, Li and Fe migration, and Fe oxidation. By combining cell-level SOH analyses with TEM characterization, we will highlight how electrochemical test results correlate with material degradation mechanisms, enhancing our understanding of battery health, longevity, and diagnostics. Wang, L. et al. Insights for understanding multiscale degradation of LiFePO4 cathodes. eScience 2, 125–137 (2022). Li, X. et al. First Atomic-Scale Insight into Degradation in Lithium Iron Phosphate Cathodes by Transmission Electron Microscopy. J. Phys. Chem. Lett. 11, 4608–4617 (2020). Xu, P. et al. Efficient Direct Recycling of Lithium-Ion Battery Cathodes by Targeted Healing. Joule 4, 2609–2626 (2020). Ramirez-Meyers, K., Rawn, B. & Whitacre, J. F. A statistical assessment of the state-of-health of LiFePO4 cells harvested from a hybrid-electric vehicle battery pack. J. Energy Storage 59, 106472 (2023).

Direct observation of the atomic density fluctuation originating from the first sharp diffraction peak in SiO2 glass

The intermediate-range order of covalently bonded glasses has been extensively studied in terms of their diffraction peaks observed at low scattering angles; these peaks are called the first sharp diffraction peaks (FSDPs). Although the atomic density fluctuations originating from the quasilattice planes are a critical scientific target, direct experimental observations of these fluctuations are still lacking. Here, we report the direct observation of the atomic density fluctuations in silica glass by energy-filtered angstrom-beam electron diffraction. The correspondence between the local electron diffraction patterns of FSDPs and the atomic configurations constructed based on the X-ray and neutron diffraction results revealed that the local atomic density fluctuations originated from the quasi-periodic alternating arrangements of the columnar chain-like atomic configurations and interstitial tubular voids, as in crystals. We also discovered longer-range fluctuations associated with the shoulder of the FSDP on the low-Q side. The hierarchical fluctuations inherent in materials could aid in the elucidation of their properties and performance.

Observation of turbulence-induced reduced electrostatic particle flux in the presence of QL whistlers in large laboratory plasma

The electrostatic particle flux is measured in the presence of obliquely propagating quasi-longitudinal (QL) whistler turbulence (ωci<ωLH≈ω<ωce) in the large-volume plasma device (LVPD). The QL whistler is observed with frequency band between 40 and 100 kHz, and the characteristic wave numbers k∥≪k⊥ are excited by the reflected energetic electrons via loss cone (localised mirror type magnetic geometry) formation in the presence of a transverse magnetic field [BEEF(x̂)] of electron energy filter and axial magnetic field of LVPD [Bo(ẑ)]. The effect of mirror strength on radial particle flux is explored by changing the BEEF as this variation changes the excited QL-whistler turbulence. We observed that the increase in QL-whistler turbulence level, the radial particle transport, i.e., the radial particle flux subsides, is accompanied by particle flux direction reversal from radially inward to outward.

Physics-agnostic inverse design using transfer matrices

Inverse design is an application of machine learning to device design, giving the computer maximal latitude in generating novel structures, learning from their performance, and optimizing them to suit the designer’s needs. Gradient-based optimizers, augmented by the adjoint method to efficiently compute the gradient, are particularly attractive for this approach and have proven highly successful with finite-element and finite-difference physics simulators. Here, we extend adjoint optimization to the transfer matrix method, an accurate and efficient simulator for a wide variety of quasi-1D physical phenomena. We leverage this versatility to develop a physics-agnostic inverse design framework and apply it to three distinct problems, each presenting a substantial challenge for conventional design methods: optics, designing a multivariate optical element for compressive sensing; acoustics, designing a high-performance anti-sonar submarine coating; and quantum mechanics, designing a tunable double-bandpass electron energy filter.

Femtosecond-resolved imaging of a single-particle phase transition in energy-filtered ultrafast electron microscopy

Using an energy filter in transmission electron microscopy has enabled elemental mapping at the atomic scale and improved the precision of structural determination by gating inelastic and elastic imaging electrons, respectively. Here, we use an energy filter in ultrafast electron microscopy to enhance the temporal resolution toward the domain of atomic motion. Visualizing transient structures with femtosecond temporal precision was achieved by selecting imaging electrons in a narrow energy distribution from dense chirped photoelectron packets with broad longitudinal momentum distributions and thus typically exhibiting picosecond durations. In this study, the heterogeneous ultrafast phase transitions of vanadium dioxide (VO2) nanoparticles, a representative strongly correlated system, were filmed and attributed to the emergence of a transient, low-symmetry metallic phase caused by different local strains. Our approach enables electron microscopy to access the time scale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale.

Enhancement of thermoelectric power factor via electron energy filtering in Cu doped MoS2 on carbon fabric for wearable thermoelectric generator applications

The design and construction of state-of-the-art wearable thermoelectric materials are important for the development of self-powered wearable thermoelectric generators (WTEGs). Molybdenum disulfide (MoS2) has been reported as a noteworthy thermoelectric (TE) material because of its large intrinsic bandgap and high carrier mobility. In this work, Cu-doped two-dimensional layered MoS2 nanosheets were grown on carbon fabric (CF) via a hydrothermal method. The electrical conductivity, Seebeck coefficient, and power factor for the Cu-doped MoS2 were found to increase with increasing temperature. The maximum Seebeck coefficient was obtained for a MoS2 sample doped with 4 at% of Cu (CM4) was ∼10 μV/K at 303 K and ∼13 μV/K at 373 K. The enhancement in the Seebeck coefficient was attributed to an energy-filtering effect caused by the interfacial barrier between MoS2 and Cu. In addition, a thermoelectric device was designed with four pairs of TE materials, where CM4 (4 at%) was used as a p-type material and Cu wire was used as an n-type material. These p- and n-type materials were connected electrically in series and thermally in parallel to generate a voltage of 190.7 μV at a temperature gradient of 8 K.

Effects of Er atoms on graphitization process and structural defects for epitaxial graphene

Thermal decomposition of SiC at high temperature usually brings about excessively fast Si sublimation and a very rough surface. In order to fabricate high-quality homogeneous epitaxial graphene on a SiC(0001) substrate, highly reactive erbium atoms are employed in this work. Scanning tunneling microscopy and Raman spectroscopy have been utilized to investigate the modulations of Er atoms on graphitization evolution and structural defects for graphene after annealing durations. Experimental results show that Er atoms pre-deposited on clean substrates can definitely enhance the surface graphitization of SiC and make graphene grow in a controllable way. The existence of Er layer is believed to break Si–C bonds at low temperature and to decrease the Si sublimate rate. It is also demonstrated that Er atoms can modify the type of structural defects in graphene, and the areal density of flower defects increases to 1.22 × 1012 cm−2, quadrupling that in pristine graphene. This work puts forward a fabrication method for epitaxial graphene with flower defects in high density and will enlighten some future applications of graphene in nanoelectronics, electron energy filtering, and chemical catalysis.

Wider transmission forbidden gaps and tamm state in a complex optoelectronic superlattice based on monolayer MoS2

We have designed a complex optoelectronic superlattice based on monolayer MoS2 consisting of two singly periodic superlattices and investigated its electronic transport properties. It is found that the complex superlattice possesses wider transmission forbidden gaps compared with the two superlattices as its components, and we give a theoretic proof to support this phenomenon. In addition, a Tamm state can be found within the original forbidden gaps, and its tunneling transmission decays greatly with the width of potential regions. These excellent properties imply the complex optoelectronic superlattice proposed here can be applied to electron energy filters and efficient optoelectronic devices.

Origin of spurious intensity in vacuum near sample edge in bright field TEM images

Bright-field transmission electron microscope (BFTEM) images exhibit spurious image intensity in the vacuum near the sample edge. The spurious intensity gradually decreases with increasing distance from the sample edge. By taking into account angular and energy loss distribution of the scattered electrons and lens aberrations, the origin of the spurious intensity of BFTEM images can be explained. The spurious intensity extent and magnitude can be significantly reduced by using either electron energy filtering or a small collection semiangle.

Applications of a novel electron energy filter combined with a hybrid-pixel direct electron detector for the analysis of functional oxides by STEM/EELS and energy-filtered imaging

The performance and suitability of a new electron energy filter in combination with a hybrid pixel, direct electron detector for analytical (scanning) transmission electron microscopy are demonstrated using four examples. The STEM-EELS capabilities of the CEOS Energy Filtering and Imaging Device (CEFID) were tested with focus on weak signals and high spatio-temporal resolution. A multiferroic, multilayer structure of REMnO3 (RE = Yb, Er, Tb, Y), grown on yttria-stabilized zirconia (YSZ), is used to exemplify that this new instrumental setup produces valuable electron energy-loss spectroscopy (EELS) data at high energy losses even when using short acquisition times, providing detailed chemical information about the interfaces in this complex multilayer sample. Another functional oxide, namely a ferromagnetic La2NiMnO6 thin film grown on SrTiO3, demonstrates that atomically resolved spectrum images can be recorded, using short dwell times and moderate beam currents in order to warrant the integrity of the sample. In a third example, inhomogeneously Er-doped YSZ shows by EELS spectrum imaging that elements at low concentrations can be detected semi-quantitatively, uncovering the expected layered Er distribution but revealing substantial interdiffusion. In a final example, we simply demonstrate that the hybrid pixel detector in combination with the energy filter can also be used for energy-filtered imaging and thus for elemental mapping complementary to EELS in scanning transmission mode.

Nanoporous Vesicular Membranes of Amphiphilic Polymers Containing Trans / Cis Isomers

Nanoporous Vesicular Membranes of Amphiphilic Polymers Containing <i>Trans</i> / <i>Cis</i> Isomers

Design of a novel correlative reflection electron microscope for in-situ real-time chemical analysis* *Project supported by the Shanghai Tech University and the National Natural Science Foundation of China (Grant No. 11774039).

A novel instrument that integrates reflection high energy electron diffraction (RHEED), electron energy loss spectroscopy (EELS), and imaging is designed and simulated. Since it can correlate the structural, elemental, and spatial information of the same surface region via the simultaneously acquired patterns of RHEED, EELS, and energy-filtered electron microscopy, it is named correlative reflection electron microscopy (c-REM). Our simulation demonstrates that the spatial resolution of this c-REM is lower than 50 nm, which meets the requirements for in-situ monitoring the structural and chemical evolution of surface in advanced material.

Single-particle cryo-EM at atomic resolution.

The three-dimensional positions of atoms in protein molecules define their structure and their roles in biological processes. The more precisely atomic coordinates are determined, the more chemical information can be derived and the more mechanistic insights into protein function may be inferred. Electron cryo-microscopy (cryo-EM) single-particle analysis has yielded protein structures with increasing levels of detail in recent years1,2. However, it has proved difficult to obtain cryo-EM reconstructions with sufficient resolution to visualize individual atoms in proteins. Here we use a new electron source, energy filter and camera to obtain a 1.7Å resolution cryo-EM reconstruction for a human membrane protein, the β3 GABAA receptor homopentamer3. Such maps allow a detailed understanding of small-molecule coordination, visualization of solvent molecules and alternative conformations for multiple amino acids, and unambiguous building of ordered acidic side chains and glycans. Applied to mouse apoferritin, our strategy led to a 1.22Å resolution reconstruction that offers a genuine atomic-resolution view of a protein molecule using single-particle cryo-EM. Moreover, the scattering potential from many hydrogen atoms can be visualized in difference maps, allowing a direct analysis of hydrogen-bonding networks. Our technological advances, combined with further approaches to accelerate data acquisition and improve sample quality, provide a route towards routine application of cryo-EM in high-throughput screening of small molecule modulators and structure-based drug discovery.

On space charge effects in laboratory-based photoemission electron microscopy using compact gas discharge extreme ultraviolet sources

The analysis of electronic and structural properties of surfaces has been greatly advanced by photoemission electron microscopy and spectroscopy techniques. To further improve lateral and energy resolution of the instruments, it is necessary to optimize parameters of the radiation sources employed for photoemission studies (e.g. photon flux, pulse duration, spot size etc). We studied space charge effects observed in an energy-filtering photoemission electron microscope operated with a compact laboratory-scale gas-discharge extreme ultraviolet light source. In this system, we found limits of spatial- and energy-resolution controlled by the source radiation parameters. The pulse repetition rate can be varied in the kHz range and the duration of the EUV emission was measured to be several tens of nanoseconds long, and thereby very different from the standard synchrotron sources typically used for similar experiments. The spatial resolution could be improved by a factor of 5, but only on the expense of the photon density per pulse, which had to be decreased by a factor of 17 in order to reduce the image blur due to space charge effects. Furthermore, we found broadening of the x-ray photoelectron spectroscopy peaks for high photon fluxes. We have also performed a n-body Monte Carlo simulation to evaluate the difference between core-level photoelectrons and secondary electrons with respect to space charge.

Graphene-diamond junction photoemission microscopy and electronic interactions

Polycrystalline graphene was transferred onto differently terminated epitaxial layers of boron-doped diamond deposited onto single crystal substrates. Chemical and electronic characterisation was performed using energy-filtered photoemission electron microscopy and angle-resolved photoemission spectroscopy. Electronic interaction between the diamond and graphene was observed, where doping of the graphene on the hydrogen and oxygen terminated diamond was n-doping of 250 meV and 0 meV respectively. We found that the wide window of achievable graphene doping is effectively determined by the diamond surface dipole, easily tuneable with a varying surface functionalisation. A Schottky junction using the graphene-diamond structure was clearly observed and shown to reduce downward band bending of the hydrogen terminated diamond, producing a Schottky barrier height of 330 meV.

Theoretical Bounds on Electron Energy Filtering in Disordered Nanomaterials.

Electron energy filters have recently been proposed as a method of reducing the effects of thermal broadening in device and sensing applications, enabling substantial improvements in their room temperature performance. Nanostructured materials can act as electron energy filters by funneling thermally broadened electrons through discrete energy levels. In this study, we develop a theoretical model of the electron filtering properties of nanostructured materials that explicitly includes the effects of thermal broadening and size heterogeneity on the heterogeneity of nanostructure energy levels. We find that under certain conditions quantum dot solids can perform as effective electronic energy filters. We identify a material-specific length scale parameter, Lcrit, that specifies the maximum mean quantum dot size that can yield effective energy filtering. Moreover, we show that energy filtering materials composed of quantum dots with size near Lcrit are maximally robust to heterogeneity in quantum dot size, tolerating variations ∼10% of the mean size. The length scale Lcrit can be estimated directly from the widely tabulated density of states effective mass and shows that semiconductors with light conduction band electrons, such as III-V type materials InSb and GaAs, are the most forgiving for energy filtering applications. Taken together, these results provide a practical set of quantitative design principles for semiconductor electron filters.

Specific features of the miniature ionisation BSE multi-detector unit for 3D imaging in environmental conditions

The author has been striving to develop SEM equipment that could extend the capabilities of standard instruments to allow advanced environmental techniques, including 3D imaging. Now its improved version is presented in a form of the miniature ionisation multi-detector unit that can be accommodated inside a bore of the objective lens and does not limit the minimal Working Distance (WD). The miniature detector unit seems well suited to VP/ESEM of any type to be used instead a standard Pressure Limiting Aperture and even to the classic SEM when combined with the vacuum-detector head and an intermediate vacuum system. An acceptable pressure of the environmental gas (preferably water vapour of ca 10 hPa) and the capability of 3D imaging make this solution particularly suitable in applications for highly hydrated samples of smooth surfaces. The unit can also deliver an abundance of signals which may be a source of better contrasts and additional information when digitally processed. Its present investigations are focused on the surface shape fidelity obtainable with the system working in gaseous environment of air or water and tested on metal and glass wires or hydrated organic samples. The detector unit may be reckoned an underpass electron energy filter which facilitates imaging details under sample surface, the feature demanded strongly in biomedical applications.

(Invited) Temperature-Dependent Thermal Diffuse Scattering for Scanning Transmission Electron Microscope Thermometry

Scanning transmission electron microscopy (STEM) thermometry techniques open new opportunities for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on measuring thermally-induced strains must contend with small thermal expansion coefficients (<10 parts per million (ppm)/K) for some materials, as well as potentially non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements, and Debye-Waller theory predicts that many materials should display temperature coefficients >1000 ppm/K at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, because the Debye-Waller effect on the Bragg peak intensity is typically overwhelmed by the effects of thermal tilts and thermal drift. Here, we demonstrate quantitative TDS measurements using STEM by measuring the diffuse background intensity (rather than the Bragg peak intensity) in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS in the diffuse background between the Bragg spots; previous TEM work (with the beam in flood mode) showed that this diffuse signal is relatively insensitive to thermal tilts and drift. Using this diffuse signal, we measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K, and compare our results with the predictions of Debye-Waller theory. The measurements display typical temperature uncertainties of 8 K and temperature sensitivities of 51 K Hz-1/2. This TDS-based STEM thermometry demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures. Figure 1

Electron microscopy used in measuring temperature-dependent thermal diffusion scattering

Measuring the diffuse scattering in energy-filtered scanning electron nanodiffraction patterns provides way forward for nanoscale temperature readings.

Measuring temperature-dependent thermal diffuse scattering using scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) thermometry techniques offer the potential for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on thermally induced strains must contend with small thermal expansion coefficients [&amp;lt;10 parts per million (ppm)/K] for some materials of interest, as well as non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements with larger temperature coefficients (&amp;gt;1000 ppm/K) for almost all materials at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, due to experimental difficulties in quantifying the relatively small thermal signals. Here, we demonstrate quantitative TDS measurements using STEM by measuring diffuse scattering in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS between the Bragg spots. We measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K and compare this result with the predictions of Debye-Waller theory. This TDS-based STEM thermometry technique demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures.