Energy Filtering Research Articles

The Impact of Energy Filtering on Fluctuation Electron Microscopy

Abstract Fluctuation electron microscopy (FEM) analyzes intensity fluctuations within diffraction patterns in order to draw conclusions regarding the structure of amorphous materials by calculating the normalized variance V(k,R). Ideally, such experiments only evaluate elastically diffracted electrons. However, an undesired inelastic background intensity is always present and degrades the FEM data. Energy filtered FEM experiments were performed on amorphous germanium created by self-ion implantation. FEM data were acquired in a transmission electron microscope at 60 and 300 kV with different electron doses as well as varying energy filter slit widths at two sample thicknesses. Generally, the measurements reveal that energy filtering greatly improves FEM data at both beam energies and sample thicknesses by removing a certain amount of the inelastic background intensity in the diffraction patterns. The narrower the energy filter, the larger the normalized variance. This brings energy filtered FEM data closer to the normalized variance determined by simulations under idealized conditions. Furthermore, preliminary results indicate that the medium range order length scale extracted from the pair-persistence analysis used in FEM is strongly affected by energy filtering.

Improving data quality for three-dimensional electron diffraction by a post-column energy filter and a new crystal tracking method.

Three-dimensional electron diffraction (3D ED) has become an effective technique to determine the structures of submicrometre- (nanometre-)sized crystals. In this work, energy-filtered 3D ED was implemented using a post-column energy filter in both STEM mode and TEM mode [(S)TEM denoting (scanning) transmission electron microscope]. The setups for performing energy-filtered 3D ED on a Gatan imaging filter are described. The technique and protocol improve the accessibility of energy-filtered 3D ED post-column energy filters, which are available in many TEM laboratories. In addition, a crystal tracking method in STEM mode using high-angle annular dark-field imaging is proposed. This method enables the user to monitor the crystal position while collecting 3D ED data at the same time, allowing a larger tilt range without foregoing any diffraction frames or imposing extra electron dose. In order to compare the differences between energy-filtered and unfiltered 3D ED data sets, three well known crystallized inorganic samples have been studied in detail. For these samples, the final R 1 values improved by 10-30% for the energy-filtered data sets compared with the unfiltered data sets, and the structures became more chemically reasonable. Possible reasons for improvement are also discussed.

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe‐Sb2Te3 Superlattice‐Like Films

AbstractInterfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p‐type (MnTe)x(Sb2Te3)y superlattice‐like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle‐resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb2Te3 layer in the SLs, originating from the work function difference between MnTe and Sb2Te3. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum‐dots (MnTe)x(Sb2Te3)12 superlattice‐like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)0.1(Sb2Te3)12 obtains the highest room‐temperature PF of 2.50 mWm−1K−2, while the (MnTe)0.25(Sb2Te3)12 possesses the highest PF of 2.79 mWm−1K−2 at 381 K, remarkably superior to the values acquired in binary MnTe and Sb2Te3 films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.

Improving thermoelectric properties of Cu2O powder via interface modification

We report thermoelectric properties of Cu2O powder through interface modification. Seebeck coefficient (SC) of Cu2O–Au composite powder was measured to be 900–1500 μV/K in the temperature range of 300–420 K, and power factor was ∼1.95 μW/m⋅K2 at 420 K, in contrast to raw Cu2O powder (0.86 μW/m⋅K2). The improved Seebeck coefficient and power factor relate very likely to the band structure modification by Au atoms, energy filtering and remote doping. Optic measurement and band structure calculated using density functional theory confirmed the band gap of Cu2O–Au composite powder (1.78eV) is slightly narrower than the raw Cu2O powder (1.85eV) and higher electronic density of states (DOS) in the conductance band of Cu2O–Au composite powder. Powder form thermoelectric devices (P-TE) easily prepared by pressing in a die without sintering might offer an alternative paradigm for mass fabrication with low cost. The high Seebeck coefficient and power factor of P-TE could find applications as power generator, temperature/touch sensors, transient thermoelectric coolers, etc.

An active transverse energy filter to differentiate low energy particles with large pitch angles in a strong magnetic field

We present the idea and proof of principle measurements for an angular-selective active filter for charged particles. The motivation for the setup arises from the need to distinguish background electrons from signal electrons in a spectrometer of MAC-E filter type. While a large fraction of the background electrons exhibit predominantly small angles relative to the magnetic guiding field (corresponding to a low amount of kinetic energy in the motion component transverse to the field lines, in the following referred to as transverse energy) and pass the filter mostly unhindered, signal electrons from an isotropically emitting source interact with the active filter and are detected. The concept is demonstrated using a microchannel plate (MCP) as an active filter element. When correctly aligned with the magnetic field, electrons with a small transverse energy pass the channels of the MCP without interaction while electrons with large transverse energies hit the channel walls and trigger an avalanche of secondary electrons that is subsequently detected. Due to several drawbacks of MCPs for an actual transverse energy filter, an alternative detection technique using microstructured Si-PIN diodes is proposed.

Fast preparation of AgxPb20SbTe20 (x = 0.5–3) component with high thermoelectric performance

PbTe-based quaternary AgPbmSbTem+2 (LAST) has been a kind of promising thermoelectric materials due to its relatively low lattice thermal conductivity and good electrical performance. However, traditional melting or mechanical alloying methods are time-consuming, which is not suitable for applications. To solve this problem, a fast method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) has been adopted in this work. Nanostructured n-type AgxPb20SbTe20 (x = 0.5–3) materials with high thermoelectric performance have been successfully prepared. On one hand, power factors of all samples reached over 1.6 mW·m−1·K−2 at high temperature, because Ag assembles grain boundaries and forms Ag2Te precipitates in the SHS process, enhancing the thermoelectric properties by modulation doping, energy filtering and dynamic doping. On the other hand, the Ag2Te-PbTe coherent interfaces bring multiple phonon scattering resources to reduce the thermal conductivity. As a result, a high ZT value of ∼1.32 at 773 K is achieved in Ag1.25Pb20SbTe20. This work provides a facile, rapid and energy-saving way to prepare PbTe-based materials with good thermoelectric performance.

Mapping structural heterogeneity at the nanoscale with scanning nano-structure electron microscopy (SNEM)

Here we explore the use of scanning electron diffraction (also known as 4D-STEM) coupled with electron atomic pair distribution function analysis (ePDF) to understand the local order (structure and chemistry) as a function of position in a complex multicomponent system, a hot rolled, Ni-encapsulated, Zr65Cu17.5Ni10Al7.5 bulk metallic glass (BMG), with a spatial resolution of 3 nm. We show that it is possible to gain insight into the chemistry and chemical clustering/ordering tendency in different regions of the sample, including in the vicinity of nano-scale crystallites that are identified from virtual dark field images and in heavily deformed regions at the edge of the BMG. In addition to simpler analysis, unsupervised machine learning was used to extract partial PDFs from the material, modeled as a quasi-binary alloy, and map them in space. These maps allowed key insights not only into the local average composition, as validated by EELS, but also a unique insight into chemical short-range ordering tendencies in different regions of the sample during formation. The experiments are straightforward and rapid and, unlike spectroscopic measurements, don’t require energy filters on the instrument. We spatially map different quantities of interest (QoI’s), defined as scalars that can be computed directly from positions and widths of ePDF peaks or parameters refined from fits to the patterns. We developed a flexible and rapid data reduction and analysis software framework that allows experimenters to rapidly explore images of the sample on the basis of different QoI’s. The power and flexibility of this approach are explored and described in detail. Because of the fact that we are getting spatially resolved images of the nanoscale structure obtained from ePDFs we call this approach scanning nano-structure electron microscopy (SNEM), and we believe that it will be powerful and useful extension of current 4D-STEM methods.

High-resolution cryo-EM performance comparison of two latest-generation cryo electron microscopes on the human ribosome

Recent technological advances in cryo electron microscopy (cryo-EM) have led to new opportunities in the structural biology field. Here we benchmark the performance of two 300 kV latest-generation cryo electron microscopes, Titan Krios G4 from Thermofisher Scientific and CRYO ARM 300 from Jeol, with regards to achieving high resolution single particle reconstructions on a real case sample. We compare potentially limiting factors such as drift rates, astigmatism & coma aberrations and performance during image processing and show that both microscopes, while comprising rather different technical setups & parameter settings and equipped with different types of energy filters & cameras, achieve a resolution of around 2 Å on the human ribosome, a non-symmetric object which constitutes a key drug target. Astigmatism correction, CTF refinement and correction of higher order aberrations through refinement in separate optics groups helped to account for astigmatism/coma caused by beam tilting during multi-spot and multi-hole acquisition in neighbouring holes without stage movement. The obtained maps resolve Mg2+ ions, water molecules, inhibitors and side-chains including chemical modifications. The fact that both instruments can resolve such detailed features will greatly facilitate understanding molecular mechanisms of various targets and helps in cryo-EM structure based drug design. The methods and analysis tools used here will be useful also to characterize existing instruments and optimize data acquisition settings and are applicable broadly to other drug targets in structural biology.

Determination of peak ordering in the CrCoNi medium-entropy alloy via nanoindentation

Local chemical ordering (LCO) in the CrCoNi medium-entropy alloy was investigated by transmission electron microscopy (TEM) after different annealing treatments and their corresponding mechanical properties by bulk tensile tests and nanoindentation. A cold-rolled alloy was annealed at 1000°C for 0.5 h followed by ice water quenching and then aged at a number of different temperatures (600°C, 700°C, 800°C, 900°C, and 1000°C) under vacuum for 240 h to generate different degrees of chemical ordering. A splat-quenched sample rapidly cooled from the liquid phase was also examined. While bulk mechanical properties did not vary among samples with equivalent grain sizes, nanoindentation tests revealed notable differences. As indicated by the load at first pop-in using a Berkovich tip or the indentation yield strength via continuous stiffness measurements using a 10 μm spherical tip, the nanoindentation tests revealed that the stress for onset of plasticity during indentation varied with heat treatment and peaked in the 900°C aged sample. Energy-filtered TEM characterization indicated the presence of ordering in all specimens, with a higher degree of LCO in the aged samples relative to the splat-quenched and 1000°C-quenched samples. The evolution of LCO during aging was determined to occur on the time scale similar to those of bulk diffusion. The difference in nanoindentation strength was attributed to the difference in dislocation nucleation barriers imposed by different degrees of LCO.

Enhanced Thermoelectric Performance of ZnO-Based Thin Films via Interface Engineering

Zinc oxide (ZnO) is a potential thermoelectric material with good chemical and thermal stability as well as an excellent Seebeck coefficient. However, the extremely low carrier concentration brings poor electrical transport properties. Although Gallium (Ga) doping could increase the carrier concentration of ZnO film, its thermoelectric performance is still limited due to the deteriorated Seebeck coefficient and enhanced thermal conductivity. Interface engineering is an effective strategy to decouple electron-phonon interaction for thermoelectric materials. Thus, in this work, GZO (Ga-doped ZnO)/NAZO (Ni, Al co-doped ZnO) multilayer films were designed to further improve the thermoelectric properties of GZO films. It was found that GZO/NAZO multilayer films possessed better electrical conductivity, which was attributed to the increased carrier concentration and Hall mobility. Meanwhile, benefiting from the energy filtering that occurred at GZO/NAZO interfaces, the density of states effective mass increased, resulting in comparable Seebeck coefficient values. Ultimately, an enhanced power factor value of 313 μW m−1 K−2 was achieved in the GZO/NAZO multilayer film, which is almost 46% larger than that of GZO film. This work provides a paradigm to optimize the thermoelectric performance of oxide films and other thermoelectric systems by multilayer structure design with coherent interfaces.

Reactive spark plasma sintering of SiC-TiC-diamond composites

SiC-TiC-diamond composites were fabricated via spark plasma sintering at hold temperatures of 1600 °C, 1625 °C, and 1650 °C from a powder blend containing by weight 30 % Si, 30 % Ti, and either 40 % uncoated or TiC-coated diamond. Due to the sintering conditions, sp3-diamond decomposed into thermodynamically favorable sp2‑carbon, reacting with Si and Ti to form SiC and TiC at T ≥ 1600 °C. As the hold temperature increased, the composite density reduced due to excess graphite formation for the uncoated diamond composites. However, at too low of a temperature the porosity fraction increased in the SiC-TiC matrix for the TiC-coated diamond composites. Overall, the results revealed that composites with ~97 % theoretical density and minimal (up to 5 vol%) graphite can be achieved by sintering Si-Ti-uncoated diamond at 1600 °C and Si-Ti-TiC-coated diamond at 1625 °C. X-ray diffraction, electron and x-ray microscopies with imaging analyses were carried out to quantify the volume fractions of each composite phase and used to explain the composite densification behavior. Conventional and energy filtered transmission electron microscopy, in addition to electron energy loss spectroscopy, were conducted to understand the nature of diamond graphitization with respect to the nucleation and growth of SiC, TiC, and nanoscale graphite.

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.

Nonlocal measurement of quasiparticle charge and energy relaxation in proximitized semiconductor nanowires using quantum dots

The lowest-energy excitations of superconductors do not carry an electric charge, as their wave function is equally electron-like and hole-like. This fundamental property is not easy to study in electrical measurements that rely on the charge to generate an observable signal. The ability of a quantum dot to act as a charge filter enables us to solve this problem and measure the quasiparticle charge in superconducting-semiconducting hybrid nanowire heterostructures. We report measurements on a three-terminal circuit, in which an injection lead excites a non-equilibrium quasiparticle distribution in the hybrid system, and the electron or hole component of the resulting quasiparticles is detected using a quantum dot as a tunable charge and energy filter. The results verify the chargeless nature of the quasiparticles at the gap edge and reveal the complete relaxation of injected charge and energy in a proximitized nanowire, resolving open questions in previous three-terminal experiments.

Design Considerations for 2-D Dirac-Source FETs—Part I: Basic Operation and Device Parameters

Dirac-source field-effect transistors (DS-FETs) have been proposed as promising candidates for low-power switching devices by leveraging the Dirac cone of graphene as a low-pass energy filter. In particular, using 2-D materials as the channel in a DS-FET is of interest for ultimate scaling purposes. In this article, we investigate the design considerations for 2-D DS-FETs using ballistic simulations based on the Landauer formalism. We study the impact of several key device parameters on the device performance, such as graphene doping, Schottky barrier heights, and effective mass of the 2-D channel. In Part II of this article, we study the impact of nonidealities on the performance of DS-FETs and benchmark the performance of DS-FETs for different channel materials.

Hybrid Graphene-Supported Aluminum Plasmonics.

Controlled fabrication of devices for plasmonics on suspended graphene enables obtaining tunable localized surface plasmon resonances (LSPRs), reducing the red-shift of LSPRs, and creating hybrid 3D–2D systems promising for adjustable dipole–dipole coupling and plasmon-mediated catalysis. Here, we apply a low-cost fabrication methodology to produce patterned aluminum nanostructures (bowties and tetramers) on graphene monolayers via electron-beam lithography and trap platinum (Pt) nanoclusters (NCs) within their hotspots by thermal annealing. We reveal the LSPRs of aluminum plasmonics on graphene using electron energy-loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) in a monochromated scanning transmission electron microscope (STEM). The LSPRs of these nanostructures are measured to be between visible and ultraviolet regions of the spectrum and are confirmed by electromagnetic simulations. The antibonding dipole and bonding dipole modes of both structures are tuned by controlling their gap size. The tetramers enable the simultaneous excitation of both antibonding and bonding dipole modes at the poles of nanoprisms, while bowties allow us to excite these modes separately either at the poles or within the hotspot. We further show that the hybrid nanocavity–NC systems are in the intermediate coupling regime providing an enhanced plasmon absorption in the Pt NCs via the energy transfer from the antibonding dipole mode to the Pt NCs. The dipole LSPR of Pt NCs also couples to the bonding-type breathing mode in bowties. Our findings suggest that these hybrid nanocavity–graphene systems are of high application potential for plasmon-mediated catalysis, surface-enhanced fluorescence, and quantum technologies.

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.

Surface Functionalization of Surfactant-Free Particles: A Strategy to Tailor the Properties of Nanocomposites for Enhanced Thermoelectric Performance.

The broad implementation of thermoelectricity requires high-performance and low-cost materials. One possibility is employing surfactant-free solution synthesis to produce nanopowders. We propose the strategy of functionalizing "naked" particles' surface by inorganic molecules to control the nanostructure and, consequently, thermoelectric performance. In particular, we use bismuth thiolates to functionalize surfactant-free SnTe particles' surfaces. Upon thermal processing, bismuth thiolates decomposition renders SnTe-Bi2S3 nanocomposites with synergistic functions: 1) carrier concentration optimization by Bi doping; 2) Seebeck coefficient enhancement and bipolar effect suppression by energy filtering; and 3) lattice thermal conductivity reduction by small grain domains, grain boundaries and nanostructuration. Overall, the SnTe-Bi2S3 nanocomposites exhibit peak z T up to 1.3 at 873 K and an average z T of ≈0.6 at 300-873 K, which is among the highest reported for solution-processed SnTe.

(Keynote) Developments in Visualizing Electrochemistry in Action with Liquid Cell Electron Microscopy

In situ transmission electron microscopy (TEM) can provide unparalleled structural and compositional information with high spatial and temporal resolution and is a standard tool for understanding and quantifying electrochemical nucleation and growth processes. Both solid state reactions and processes involving liquids can be examined. However, liquid phase reactions require particular care since the liquid needs to be confined in a thin layer and protected from the vacuum in the microscope. This problem is solved in the technique of electrochemical liquid cell TEM by confining the liquid between microfabricated electron transparent membranes on which the electrodes are patterned. Electrochemical liquid cell TEM has enabled quantification of nucleation and growth phenomena, dendrite formation, additive effects on morphology and SEI formation. Recent experimental developments have broadened the application space, but many more opportunities remain to be explored. Here we discuss how the existing strategies for electrochemical liquid cell TEM experiments can be adapted to allow quantitative data acquisition under an expanded range of electrochemical conditions. We first discuss the control of temperature, an important parameter in electrochemical systems such as batteries and fuel cells. Increasing the temperature while simultaneously applying a bias is achieved by customizing liquid cell chips to incorporate an electrically isolated heater strip beneath the electrodes. We find, as expected, that elevated temperature increases the rates of deposition and etching reactions and we discuss potential applications in electrocatalysis and codeposition. We then address the limitations of image resolution in liquid cell experiments. For nucleation and growth processes that take place on the surface of the electrode, the electrode materials such as Au or Pt often used in microfabricated liquid cells contribute strongly to electron scattering and thereby reduce the signal to noise ratio in the images. We show how few-layer graphene can be added to liquid cell chips to produce electrodes with minimal scattering, resulting in nucleation and growth movies with improved discrimination between nucleus shapes. We next consider the exciting consequences of instrumental improvements in microscopy - aberration correction, energy filtering and efficient electron detectors - in improving the resolution and lowering the dose required for imaging. Electron beam effects are unavoidable, however, and we discuss how the chemical changes caused by the beam can be modeled and perhaps mitigated for different electrolyte compositions and as a function of temperature. More speculative developments in liquid cell technique may allow us to measure stress, electrolyte composition and even the electrochemical double layer. Although a lot of work remains to be done, I hope to convince you that electrochemical TEM of liquids can address grand challenge questions in nucleation and growth through its unique view of electrochemical processes.

First-Principles Study of Half Metallic Ferromagnetic and Optical Properties of Nb Doped Cubic ZnS using TB-mBJ Approximation

Structural, electronic and optical properties of niobium doped ZnS are studied by using full-potential linearized augmented plane wave plus local orbital (FPLAPW+lo) method within the density functional theory (DFT). Computed results of Nb doped ZnS are compared with that of the pristine zinc blende. Tran-Blaha approach of modified Becke and Johnson local spin density approximation (TB-mBJ) is used to study electronic and optical properties. Estimated result shows that Nb reduces the bandgap of ZnS due to hybridization of Nb-4d orbital with S-3p orbital near the Fermi level. Niobium dopant provides half metallic nature to ZnS with 100% spin polarization. Maximum photo-response is noticed in the ultraviolet range for Zn1-xNbxS (x = 25, 12.5, 6.25 %). Highest peaks are shifted toward the lower energy range for higher dopant percentage. All these suggest that Nb doped ZnS solid solutions are suitable candidate for both energy filter of UV spectrum and spintronic device. Dhaka Univ. J. Sci. 69(3): 194-201, 2022 (June)

Electronic structure of partially fluorinated graphene: The impact of adsorption patterns and dynamic stability

The impact of fluorination on the stability and electronic structure of graphene is investigated by employing density functional theory calculations. It is observed that both the cell symmetry and dynamical stability (phonon dispersion spectra) are necessary for the identification of energetically optimum configurations. In general, an increase in band gap is observed with fluorination, which is in good agreement with the experimental studies. These results underscore the importance of employing key stability (energy, dynamical and thermal) filters for correct assessment of electronic properties of partially fluorinated graphene and may help to explain the inconsistencies of previous theoretical and experimental findings.