Energy Loss Spectroscopy Research Articles

Confining Ultrafine MoO2 in a Carbon Matrix Enables Hybrid Li Ion and Li Metal Storage.

Poor cycle and rate performance caused by volume effects and sluggish kinetics is the main bottleneck for most lithium-ion battery (LIB) anode materials run on the conversion reaction. Although nanostructure engineering has shown to be an effective method to reduce the undesirable volume effects, cycling instability usually remains in nanostructured electrodes owning to particle aggregation in discharge and loss of active materials in charge. Here, to make these kinds of materials practical, we have developed a structure of ultrafine MoO2 nanoparticles (<3 nm) confined by a conductive carbon nanosheet matrix (MoO2/C). Instead of running on the conversion mechanism, the Li storage in the MoO2/C composite is through a two-step mechanism in discharge: intercalation followed by the formation of metallic Li, acting as a hybrid host for both Li ion intercalation and metallic Li plating. The Li-storage mechanism has been revealed by in situ X-ray diffraction analysis and in situ scanning transmission electron microscopy with corresponding electron energy loss spectrum analysis, which explains the natural origin of such high capacity along with good cyclability. This unique MoO2/C structure exhibits an excellent discharge capacity (810 mAh g-1 at 200 mA g-1) and cyclability (75% capacity retention over 1000 cycles). The carbon sheet plays a vital role in both a conductive network and a structure supporter with a robust confining effect that keeps the size of MoO2 uniformly under 3 nm even after high-temperature calcination. Our finding provides insights for the design of next-generation LIB anode materials with high capacity and longevity.

Size‐Mediated Recurring Spinel Sub‐nanodomains in Li‐ and Mn‐Rich Layered Cathode Materials

Li- and Mn-rich layered oxides are among the most promising cathode materials for Li-ion batteries with high theoretical energy density. Its practical application is, however, hampered by the capacity and voltage fade after long cycling. Herein, a finite difference method for near-edge structure (FDMNES) code was combined with in situ X-ray absorption spectroscopy (XAS) and transmission electron microscopy/electron energy loss spectroscopy (TEM/EELS) to investigate the evolution of transition metals (TMs) in fresh and heavily cycled electrodes. Theoretical modeling reveals a recurring partially reversible LiMn2 O4 -like sub-nanodomain formation/dissolution process during each charge/discharge, which accumulates gradually and accounts for the Mn phase transition. From the modeling of spectra and maps of the valence state over large regions of the cathodes, it was found that the phase change is size-dependent. After prolonged cycling, the TMs displayed different levels of inactivity.

Size‐Mediated Recurring Spinel Sub‐nanodomains in Li‐ and Mn‐Rich Layered Cathode Materials

AbstractLi‐ and Mn‐rich layered oxides are among the most promising cathode materials for Li‐ion batteries with high theoretical energy density. Its practical application is, however, hampered by the capacity and voltage fade after long cycling. Herein, a finite difference method for near‐edge structure (FDMNES) code was combined with in situ X‐ray absorption spectroscopy (XAS) and transmission electron microscopy/electron energy loss spectroscopy (TEM/EELS) to investigate the evolution of transition metals (TMs) in fresh and heavily cycled electrodes. Theoretical modeling reveals a recurring partially reversible LiMn2O4‐like sub‐nanodomain formation/dissolution process during each charge/discharge, which accumulates gradually and accounts for the Mn phase transition. From the modeling of spectra and maps of the valence state over large regions of the cathodes, it was found that the phase change is size‐dependent. After prolonged cycling, the TMs displayed different levels of inactivity.

Using [formula omitted]erenkov radiation for measuring the refractive index in thick samples by interferometric cathodoluminescence

Cathodoluminescence (CL) has evolved into a standard analytical technique in (scanning) transmission electron microscopy. CL utilizes light excited due to the interactions between the electron-beam and the sample. In the present study we focus on Cˇerenkov radiation. We make use of the fact that the electron transparent specimen acts as a Fabry-Pérot interferometer for coherently emitted radiation. From the wavelength dependent interference pattern of thickness dependent measurements we calculate the refractive index of the studied material. We describe the limits of this approach and compare it with the determination of the refractive index by using valence electron energy loss spectrometry (VEELS).

Metallated Schiff-Base Macromolecules as Alternative Metalloprotein Electron Transfer Intermediates

In the construction of biosensors, enzymes function as mediators converting biological signals generated by specific biological processes, into electrochemical signals. The ideology of bio-sensor design is retention of electron transfer activity of the enzyme utilizing superior interfacial architecture. In this work a Schiff-base macromolecule has been synthesized by reflux of 2, 3-diaminonaphthalene and pyrrole-2-carboxaldehyde starting materials. The Schiff-base ligand was subsequently complexed with FeCl2?4H2O under reflux, to produce the Fe-Schiff-base complex. The Schiff-base ligand and Fe-Schiff-base complex were characterized using nuclear magnetic resonance (NMR) spectroscopy, Ultra Violet/Visible (UV/Vis) spectroscopy, Fourier transfer infrared resonance (FTIR) and electron energy loss spectroscopy (EELS) to confirm the structure of the synthesis products. NMR spectroscopy confirmed the imide linkage of Schiff-base formation as two symmetrical peaks at 8.1 and 7.7 ppm respectively. Comparison of starting materials and product spectra by UV/Vis spectroscopy confirmed the disappearance of the diaminonaphthalene peak at 250 nm as evidence of complete conversion to product. FTIR spectroscopy of the Schiff-base ligand confirmed the formation of the imine bond at 1595 cm-1. EELS spectra comparing FeCl2?4H2O and the Fe-Schiff-base complex, showed good agreement in the energy loss profiles associated with changes to the electronic arrangement of Fe d-orbitals. EDS clearly identified a spectral band for Fe (7 - 8 eV) in the Fe-Schiff-base complex. Electrochemical evaluation of the Fe-Schiff-base complex was compared to the electrochemical signature of denatured cytochrome-C using cyclic voltammetry and square wave voltammetry. The Fe2+/Fe3+ quasi-reversible behavior for iron in the metallated complex was observed at -0.430 V vs. Ag/AgCl, which is consistent with reference values for iron in macromolecular structures.

Comparison of the interface and border traps of nanolaminate and bilayer structures of Al2O3 and HfO2 on In0.53Ga0.47As

This study is a comparison of interface and border traps of nanolaminate and bilayer structures of Al2O3 and HfO2 with those of single layers on an In0.53Ga0.47As substrate. The nanolaminate structure shows better trap minimalization in both trap cases along with lower hysteresis and leakage current density and higher breakdown field than the bilayer structures. X-ray photoelectron spectroscopy (XPS) was used to quantify the microstructural properties, and a combination of XPS and reflected electron energy loss spectrum analysis was used to calculate the bandgap of deposited films. Al incorporation into HfO2 increases the bandgap and band offsets.

Direct Quantification of Cu Vacancies and Spatial Localization of Surface Plasmon Resonances in Copper Phosphide Nanocrystals

Copper chalcogenides and pnictogenides often behave as heavily doped p-type semiconductors, because of the presence of a high density of Cu vacancies, with corresponding hole carriers in the valence band. If the free-carrier concentration is high enough, localized surface plasmon resonances can be sustained in nanocrystals of these materials, with frequencies that are typically observed in the infrared region of the spectrum (<1 eV), differently from the typical resonances featured in the visible range by metallic nanoparticles. Here, we demonstrate that Cu vacancies in hexagonal Cu3–xP nanocrystals can be directly quantified by scanning transmission electron microscopy (STEM) analysis. We also report, for the first time, the spatial localization of the plasmon resonances in individual Cu3–xP nanocrystals by means of STEM energy loss spectroscopy (EELS), which is an achievement that, to date, had been possible only on nanoparticles of noble metals. Two plasmon modes can be seen from STEM-EELS, which are in agreement with the resonances calculated from the vacancy concentration obtained from the STEM analysis.

Orientation-Dependent Intercalation Channels for Lithium and Sodium in Black Phosphorus.

Black phosphorus (BP) with unique 2D structure enables the intercalation of foreign elements or molecules, which makes BP directly relevant to high-capacity rechargeable batteries and also opens a promising strategy for tunable electronic transport and superconductivity. However, the underlying intercalation mechanism is not fully understood. Here, a comparative investigation on the electrochemically driven intercalation of lithium and sodium using in situ transmission electron microscopy is presented. Despite the same preferable intercalation channels along [100] (zigzag) direction, distinct anisotropic intercalation behaviors are observed, i.e., Li ions activate lateral intercalation along [010] (armchair) direction to form an overall uniform propagation, whereas Na diffusion is limited in the zigzag channels to cause the columnar intercalation. First-principles calculations indicate that the diffusion of both Li and Na ions along the zigzag direction is energetically favorable, while Li/Na diffusion long the armchair direction encounters an increased energy barrier, but that of Na is significantly larger and insurmountable, which accounts for the orientation-dependent intercalation channels. The evolution of chemical states during phase transformations (from Lix P/Nax P to Li3 P/Na3 P) is identified by analytical electron diffraction and energy-loss spectroscopy. The findings elucidate atomistic Li/Na intercalation mechanisms in BP and show potential implications for other similar 2D materials.

Spherical tarball particles form through rapid chemical and physical changes of organic matter in biomass-burning smoke

Biomass burning (BB) emits enormous amounts of aerosol particles and gases into the atmosphere and thereby significantly influences regional air quality and global climate. A dominant particle type from BB is spherical organic aerosol particles commonly referred to as tarballs. Currently, tarballs can only be identified, using microscopy, from their uniquely spherical shapes following impaction onto a grid. Despite their abundance and potential significance for climate, many unanswered questions related to their formation, emission inventory, removal processes, and optical properties still remain. Here, we report analysis that supports tarball formation in which primary organic particles undergo chemical and physical processing within ∼3 h of emission. Transmission electron microscopy analysis reveals that the number fractions of tarballs and the ratios of N and O relative to K, the latter a conserved tracer, increase with particle age and that the more-spherical particles on the substrates had higher ratios of N and O relative to K. Scanning transmission X-ray spectrometry and electron energy loss spectrometry analyses show that these chemical changes are accompanied by the formation of organic compounds that contain nitrogen and carboxylic acid. The results imply that the chemical changes increase the particle sphericity on the substrates, which correlates with particle surface tension and viscosity, and contribute to tarball formation during aging in BB smoke. These findings will enable models to better partition tarball contributions to BB radiative forcing and, in so doing, better help constrain radiative forcing models of BB events.

Extending ζ-factor microanalysis to boron-rich ceramics: Quantification of bulk stoichiometry and grain boundary composition

Accurate quantification of light elements which produce only soft X-ray lines via X-ray energy dispersive spectrometry (XEDS) has been traditionally difficult due to poor X-ray emission and detector efficiencies at low energies and significant X-ray absorption effects. The ζ-factor microanalysis method enables one to correct for these shortcomings; however, ζ-factor microanalysis has not yet been thoroughly applied to inorganic materials which are entirely or mostly composed of light elements such as boron carbide, boron nitride, or boron suboxide. This work successfully extended ζ-factor microanalysis to boron-rich ceramics and accurately determined stoichiometries of multiple boron carbides and measured grain boundary compositions of a boron carbide mixed with additives consisting of rare-earth ions. Various strategies were employed to experimentally determine a full range of ζ-factors and measurements were validated using materials of known composition including silicon hexaboride and silicon carbide. Overall, this work has shown that XEDS is a viable technique for light element quantification in (scanning) transmission electron microscopy, in terms of both the accuracy and precision, which is comparable or superior to the complementary electron energy loss spectrometry.

Direct observation of MgO formation at cathode electrolyte interface of a spinel MgCo2O4 cathode upon electrochemical Mg removal and insertion

Identifying a Mg2+ insertion mechanism into Mg cathode is one crucial challenge of designing Mg batteries that own comparable specific energy to the lithium ion systems. Spinel Mg cathodes are attractive candidates mainly attributed to its high intercalation voltage and theoretical capacity, however, electrochemically removing Mg from its solid structure with identified capacity dedicated to actual reversible Mg intercalation introduce complications. Work presented here applies Scanning/Transmission Electron Microscopy with Energy Loss Spectroscopy and diffraction method to investigate the cathode electrolyte interface for a spinel MgCo2O4 material electrochemically cycled in the presence of a non-aqueous, low water content Mg electrolyte. Findings at an atomic scale suggest a complete de-magnesiation is occurring for thin MgCo2O4 particles upon electrochemical charging, but a vast majority of MgCo2O4 shows formation of an amorphous MgO layer with a thickness of ∼10 nm at the cathode electrolyte interface. The amorphous MgO layer is directly jointed with the crystalline MgCo2O4 at the outer edge of the cathode. Formation of MgO at the cathode electrolyte interface is believed to be the detrimental factor preventing further electrochemical cell cycling of MgCo2O4. A possible reaction pathway of intercalation-initiated surface conversion is proposed to explain reversible Mg insertion with MgCo2O4.

Structural analysis of the initial lithiation of NiO thin film electrodes.

Observations of the initial lithiation of NiO electrodes demonstrate how to seed conversion reactions using interfaces in a thin film Ni/NiO bilayer architecture. Operando X-ray reflectivity (XRR) reveals that structural changes in a NiO film begin at potentials near the theoretical reduction potential (1.8-2.0 V) with detectable lithiation of both the buried Ni/NiO interface and the outer NiO surface that occur prior to the reaction of the NiO film. This initial conversion reaction is most pronounced in ultrathin NiO films (∼20 Å) with only small changes to the NiO film surface for thicker films (∼67 Å). The limited reactivity of thicker NiO films probed using operando grazing incidence small-angle X-ray scattering (GISAXS) shows the growth of nanoparticles at the electrode/electrolyte interface during initial lithium ion insertion, with a 16-20 Å average radius. Ex situ X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS) confirm our conclusions about the morphological changes accompanying initial stage of lithiation in these conversion reaction electrodes. The present study reveals the interconnected challenges of solid-solid transitions, overpotentials, interfacial nucleation and kinetics, and transition metal dissolution in conversion-type electrodes that are critical for their use as electrodes in lithium-ion batteries.

Phase transitions and critical phenomena of tiny grains carbon films synthesized in microwave‐based vapor deposition system

Different peak trends of tiny grains carbon film have been observed under the investigations of the Raman spectroscopy and energy loss spectroscopy. Carbon films known in nanocrystalline and ultrananocrystalline diamond films are synthesized by employing microwave‐based vapor deposition system. Carbon atoms exhibit several state behaviors depending on the incurred positions of their electrons. Different morphology of tiny grains under different chamber pressure is related to different rate of arriving typical energies at/near substrate surface. Those tiny grains of carbon film, which evolved in graphitic state atoms are converted to structure of smooth elements where elongation of atoms of one‐dimensional arrays is as per exerting surface format forces along opposite poles from their centers. Such tiny grains in the film are the cause of v1 peak under the investigation of the Raman spectrum because of the enhanced propagation of input laser signals through channelized inter‐state electron gaps of elongated graphitic state atoms. Those tiny grains of carbon film, which evolved in fullerene state are the cause of v2 peak. The tiny grains related to v1 peak possess a low intensity as compared with the ones which comprised atoms having state behaviors known in their exceptional hardness. Tiny grains representing v1 peak in the Raman spectrum are also the cause of field emission characteristic of a carbon film. Different peak recordings were made for the Raman at defined positions indicating a different state of carbon atoms for a different phase of deposited tiny grains, which is in line to their energy loss spectroscopy.

Microstructure-property relationship in a low carbon Nb-B bearing ultra-high strength steel by direct-quenching and tempering

This work describes here the synergistic effect of niobium micro-alloying in combination with boron addition on the development of a 900 MPa grade ultra-high strength low carbon cost-effective bainitic steel processed by direct-quenching and induction tempering. A mixed microstructure consisting of acicular ferrite and lath bainite associated with high density of high angle grain boundaries was obtained by controlled rolling and direct-quenching. Nano-sized precipitation behavior during controlled rolling and induction tempering was studied by transmission electron microscopy (TEM). A number of nano-sized precipitates were observed in the matrix after controlled rolling and direct-quenching. These precipitates were identified by electron energy loss spectrometry (EELS) in scanning TEM (STEM) to be TiN-Nb(C,N) or TiN-NbC composite precipitates, and were associated with mean austenite grain size of ~ 34 ± 6 µm in steel before finishing rolling. A finish rolling reduction of 67% below non-austenite recrystallization temperature pancaked the austenite grains to 10–15 µm in thickness. Nano-sized NbC formed during induction tempering at 670 °C had an average diameter of ~ 4.3 nm and 9.5 nm for tempering duration of 5 min and 30 min, respectively. It is noted that while nano-sized NbC precipitates smaller than 5 nm provide significant precipitation hardening effect to increase the mechanical strength, acicular ferrite in the mixed microstructure helps in retaining high elongation of low carbon direct-quenched and tempered steel. Ultra-high yield strength of 944 MPa with high ductility (uniform elongation of 6.3% and total elongation of 20%) was obtained after induction tempering at 670 °C for 5 min.

Investigation of Degradation Pathway in High Ni-Content Cathode Materials at Primary and Secondary Particle Level By Multi-Scale Characterization

There is an increasing interest in studying the high Ni-content layered oxide materials for Li-ion batteries, especially for their application in electrical vehicles, since they are promising candidates for next-generation energy storage materials. Compared to traditional layered oxide materials such as LiCoO2 (LCO) or LiNi1/3Mn1/3Co1/3 O2 (NMC333), high Ni-content materials can provide higher specific capacity and energy density due to the two electron transfer per Ni by the Ni2+/Ni4+ redox couple. However, these materials are still suffering from the problems of capacity fading. It is quite important to understand the relationship between performance deterioration and structural degradation. The in-depth understanding of such relation can provide valuable guidance for future material design. The results obtained from our newly developed multi-scale characterization techniques will be reported, including the state-of-the-art aberration-corrected scanning transmission microscopy (STEM) imaging, STEM-electron energy loss spectroscopy (EELS), X-ray imaging, X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). These results show that such multi-scale combined characterization tools can help us to study the formation of internal nano-pore structure at primary particle level and understand its impact on the formation of micro-cracks at secondary particle level. This structure degradation is highly correlated to the irreversible cation migration and intermixing at atomic level during extensive electrochemical cycling. Also, we found that the surface plays a vital role in the function of these materials. By combining the synchrotron based X-ray techniques with electron microscopy and spectroscopy, this presentation will report the degradation pathway of high Ni-content cathode materials for Li-ion batteries.

Mechanical and electrical characterization of CVD-grown graphene transferred on chalcogenide Ge2Sb2Te5 layers

In the present work, we investigate some mechanical and electrical properties of a Chemical Vapor Deposition (CVD)-grown graphene layer transferred on 50 nm-thick Ge2Sb2Te5 chalcogenide and discuss the possible application of graphene as a scaled contact replacing metal lines in phase-change memory devices. At first, the graphene-chalcogenide interface was extensively investigated. The expected chemical composition was confirmed by means of Electron Energy Loss Spectrometry (EELS), and the absence of bond distortions in the chalcogenide layer after graphene transfer was proved through Raman spectroscopy. The latter evidenced also the presence of defects, further confirmed by Transmission Electron Microscopy (TEM) investigations. The quality of the contact stack was evaluated by means of the adhesion among layers and by the sheet resistance of the layers themselves. Scratch tests and numerical simulations revealed a stress distribution compatible with a failure of the interface where the graphene layer features a 2 %-void. Next, the contact resistance proved an effective good Ohmic quality with a high graphene sheet resistance, about 1400 Ohm/square, which suggests that a better optimized transfer process should be applied to reduce it more than one order of magnitude.

Growth, thermal desorption and low dose ion bombardment damage of C60 films deposited on Cu(111)

Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and reflection electron energy loss spectrometry (REELS) were used to characterize the growth and thermal stability of C60 films deposited on Cu(111). By means of LEED we found that while C60 grows in an ordered fashion up to the first monolayer (ML) at room temperature (RT), it turns amorphous beyond that point. On the other hand, when the substrate temperature is kept at 450 K, films up to two ML with crystalline structure are obtained. For substrate temperatures beyond 570 K thick films (more than 1 ML) do not grow at all. By using AES, we found that a thick C60 film starts to desorb at a temperature around 470 K but the first ML remains stable up to temperatures as high as 900 K. A ML with a better crystalline order is obtained after desorption than that growth with the substrate at RT or higher temperatures. When the substrate is heated at 970 K, the first ML is not fully removed but the C60 molecular structure is altered or molecules break up into smaller pieces. The ion induced damage on C60 on Cu(111) films was studied for typical ions, incoming energies and irradiation doses used in low energy ion scattering (LEIS) experiments. The D-value of C(KLL) Auger spectra, the π-plasmon of REELS and the evolution of the LEIS spectra, were monitored to characterize the damage caused to the film. We found that, at low doses (∼1014 ions cm−2), damage is only detectable for massive ions like Ar, but not for H and He in the 2–8 keV range.

Directed Self-Assembly and Pattern Transfer of Five Nanometer Block Copolymer Lamellae.

The directed self-assembly (DSA) and pattern transfer of poly(5-vinyl-1,3-benzodioxole-block-pentamethyldisilylstyrene) (PVBD-b-PDSS) is reported. Lamellae-forming PVBD-b-PDSS can form well resolved 5 nm (half-pitch) features in thin films with high etch selectivity. Reactive ion etching was used to selectively remove the PVBD block, and fingerprint patterns were subsequently transferred into an underlying chromium hard mask and carbon layer. DSA of the block copolymer (BCP) features resulted from orienting PVBD-b-PDSS on guidelines patterned by nanoimprint lithography. A density multiplication factor of 4× was achieved through a hybrid chemo-/grapho-epitaxy process. Cross-sectional scanning tunneling electron microscopy/electron energy loss spectroscopy (STEM/EELS) was used to analyze the BCP profile in the DSA samples. Wetting layers of parallel orientation were observed to form unless the bottom and top surface were neutralized with a surface treatment and top coat, respectively.

Microstructure and thermal stability of rf‐plasma‐nitridated α‐(AlGa)2O3 grown by mist CVD

A new lattice‐matching substrate material for the growth of AlGaN is proposed. This substrate is prepared by the rf‐radical beam‐based nitridation of α‐(AlGa)2O3, which is grown by mist chemical vapor deposition (CVD). We consider the effect of the nitridation process using reflection high‐energy electron diffraction (RHEED), transmission electron microscopy (TEM), scanning transmission electron microscopy‐electron energy loss spectroscopy (STEM‐EELS) and X‐ray photoelectron spectroscopy (XPS). Surface of α‐(AlGa)2O3 changed from corundum structure to wurtzite structure of AlGaN by nitridation using rf‐radical beam was observed. In addition, we discuss the thermal stability of α‐(AlGa)2O3. X‐ray diffraction ω–2θ profile to characterize the phase transition was performed.

Development and Characterization of P-doped Ba-122 Superconducting Tapes

Among the recently discovered Fe-based superconducting compounds, the Ba-122 phase has proved to be the more attracting for the development of powder in tube (PIT) processed conductors. In fact, after some years of development, critical current densities ( Jc ) of about 105 A/cm2 at 4.2 K and magnetic fields up to 10 T have been obtained in (Ba0.6K0.4)Fe2As 2, PIT wires and tapes. To develop a safe upscaling method, the synthesis of the powders is a crucial point. In order to avoid the use of highly reactive K we have developed BaFe2(P1-xAsx)2 PIT tapes. This compound has proved to be very stable and in form of crystals and thin films exhibits excellent critical current: We succeeded in manufacturing PIT tapes with this phase with Jc values of about 103 A/cm2 at 4.2 K in self-field. Detailed microstructural and chemical investigations of the samples have been performed by high-resolution TEM, scanning TEM, and electron energy loss spectrometry analysis. A deformation network inside the grains that can induce strong pinning has been observed. However, chemical inhomogeneities at the grain boundaries are also present, which limit the transport current.