Transmission Electron Energy Loss Spectroscopy Research Articles

(Invited) Microscopic Insights into Interfacial Conductivity and Stability of Solid-State Electrolytes for Li-Metal Batteries

Solid-state electrolytes are intensively being investigated as a critical component in future generation Li-metal battery systems, including their uses as the primary electrolyte in all-solid-state batteries, and as the protection layer for Li-metal in Li-air, Li-S, or aqueous batteries. While several solid electrolyte materials were developed recently that demonstrated sufficient conductivity, unexpectedly high resistance and cycling instability from grain boundaries and interfaces are often observed. The practical applications of solid-state electrolytes cannot be realized without a thorough understanding of these phenomena. This talk will cover our recent efforts in elucidating the fundamental origin of (1) interfacial resistivity between solid electrolytes and Lithium metal, and (2) preferential dendrite growth along grain boundaries, by using in situ and atomic-resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Interfacial phenomena among different solid electrolytes, e.g. Li(7−3×)AlxLa3Zr2O12 (LLAZO), Li1+xTi2–xAlx(PO4)3 (LATP) and LIPON, will be compared and potential strategies to mitigate interfacial challenges will be proposed.

Unique Structural Characteristics of Catalytic Palladium/Gold Nanoparticles on Graphene.

Adding Au to Pd nanoparticles (NPs) can impart high catalytic activity with respect to hydrogenation of a wide range of substances. These materials are often synthesized by reducing metallic precursors; hence, sonochemical and solvothermal processes are commonly used to anchor these bimetals onto thin supports, including graphene. Although similar NPs have been studied reasonably well, a clear understanding of structural characteristics relative to their synthesis parameters is lacking, due to limitations in characterization techniques, which may prevent optimization of this very promising catalyst. In this report, a strategic approach has been used to identify this structural and material synthesis correlation, starting with controlled sample preparation and followed by detailed characterization. This includes advanced scanning transmission electron microscopy and electron energy loss spectroscopy; the latter using a state-of-the-art instrumentation to map the distribution of Pd and Au, and to identify chemical state of the Pd NPs, which has not been previously reported. Results show that catalytic bimetal NP clusters were made of small zero-valent Pd NPs aggregating to form a shell around an Au core. Not only can the described characterization approach be applied to similar material systems, but the results can guide the optimization of the synthesis procedures.

Direct observation of an electrically degenerate interface layer in a GaN/sapphire heterostructure.

An electrically degenerate layer deteriorates the optoelectric performance of a wide band gap semiconductor grown on an insulator substrate. This detrimental effect can be passively avoided by using a buffer layer to harbor various lattice defects. However, the longstanding scientific questions regarding the microscopic origin of the degenerate interface layer and the effect of local changes in the atomic structure and chemical environment at an interface on the functionality of a desired film have remained unanswered. Moreover, this is key information for the development of ultrathin optoelectronic devices. In this study, we discuss the direct observation of a degenerate interface phase at the GaN/sapphire interface on an atomic scale. By combining high-resolution transmission electron microscopy and electron energy loss spectroscopy, we detect the presence of an ultrathin (∼6.5 Å) α-Ga2O3-x layer near the GaN/sapphire interface, which is subjected to ∼4.5% biaxial compressive strain and contains many oxygen vacancies. Density functional theory calculations show that the presence of a defective α-Ga2O3-x thin layer in the GaN and sapphire heterostructure remarkably reduces the band offset between the α-Ga2O3-x conduction band and the GaN valence band, thereby exerting a significant influence on the conductivity enhancement of the interface. Our results provide an unprecedented integrated picture of the degenerate interface phenomenon on an atomic scale, which would evolve the fundamental understanding about a wide band gap semiconductor heterostructure system.

Back-End Integrable On-Chip MIM Decoupling Capacitors Featuring High Capacitance With Ultra-Low Leakage Current by Nitrogen-Incorporated HfZrOx

HfZrOx (HZO) crystallized in orthorhombic phase formed by plasma-enhanced atomic layer deposition (ALD) deposition and subsequent 400 °C annealing with TiN capping layer was employed as the dielectric for on-chip metal-insulator-metal (MIM) decoupling capacitors. With appropriate NH3 plasma treatment on HZO/TiN interface and bulk HZO to respectively, suppress interfacial scavenging effect and passivate defects pertinent to oxygen vacancies, enhanced k value of 28.9, high capacitance of 23.3 fF/μm 2 and low leakage of 0.16 fA/μm 2 at 1 V, 25 °C are achieved. With the plasma treatment, the improved interfacial and bulk quality that are confirmed by transmission electron microscopy and electron energy loss spectroscopy also lead to more desirable reliability of 10-year lifetime at 125 °C. The MIM capacitor technology exhibits performance that is notably advantageous over other high k dielectrics. More importantly, HfO2, ZrO2 and NH3 plasma treatment are all fab-friendly processes, with proper process integration, it is possible to further push the limit of their capabilities to realize decoupling capacitors eligible for sub-7 nm technology.

Local Structure of High Performance TiOx Electron‐Selective Contact Revealed by Electron Energy Loss Spectroscopy

AbstractRecent increases in the power conversion efficiency of crystalline silicon (c‐Si) heterojunction (SHJ) solar cells are due to implementation of carrier‐selective contacts (CSCs) with high passivation performance and low contact resistivity. These electrical properties of CSCs significantly depend on the nature of the CSCs/c‐Si interface at the atomistic scale, and precise control of the interface with the SiOx interlayer is the key to obtaining superior electrical properties. It is shown that a TiOx/c‐Si structure with an interlayer prepared by nitric acid at room temperature shows the best performance among points with five different interlayer formation methods. The underlying mechanisms are investigated by combining high‐resolution transmission electron microscopy and electron energy loss (EEL) spectroscopy. The EEL spectra reveal that the Si:O ratio of an as‐deposited SiOx interlayer is nonstoichiometric (x < 2), which could contribute to lower contact resistivity. Furthermore, the as‐deposited SiOx leads to formation of a Ti containing SiOx layer with few oxygen vacancies after forming gas annealing, resulting in the significant enhancement of passivation performance. These results show that the control of the interlayer with atomistic scale is of crucial importance to realize higher power conversion efficiency of SHJ solar cells.

Effect of electrolyte composition on rock salt surface degradation in NMC cathodes during high-voltage potentiostatic holds

Surface degradation of lithium-ion cathodes at high voltages represents a significant barrier to increasing the useable energy of conventional cathodes. Though much work has been focused on mitigating the detrimental effects of such phenomena, the complex and correlated mechanisms involved have made progress difficult. Herein, the effect of electrolyte identity on surface degradation is examined. LiNi0.5Mn0.3Co0.2O2 (NMC532)/Li4Ti5O12 (LTO) full cells were constructed using three different electrolytes: a baseline organic carbonate electrolyte, a baseline organic carbonate electrolyte with an oxidizable additive, and a fluorinated carbonate electrolyte. Each system was subjected to a 60 h potentiostatic hold at 3.05 V (4.6 V vs. Li+/Li) and subsequently deconstructed for characterization via transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Molecular simulations were carried out on each distinct component of the three electrolytes in contact with crystal surfaces of an NMC532 particle. Results indicate an affinity for the additive-containing electrolyte to extract atomic oxygen from the cathode surface, whereas the fluorinated electrolyte components have negligible interactions. Hydrogen abstraction from the baseline electrolyte is facilitated by the NMC532 surface. TEM analysis of cathode surfaces, after the potentiostatic holds, revealed that the NMC532 particles cycled with the additive-containing electrolyte underwent significant surface degradation, including changes to local crystal structure in a region penetrating 20–40 nm from the surface; consistent with oxygen and lithium loss from the cathode material. However, the NMC532 in contact with the fluorinated electrolyte showed a disordered layer of just a few nm in thickness, similar to the depth of crystal disorder observed in pristine NMC532 primary particles. EELS data of the oxygen K-edge also indicated transition metal (TM) reduction, consistent with oxygen loss and changes in crystal structure extending into the cathode surface for each of the electrolytes surveyed. These surface reactions, including oxygen loss, are shown to correlate well with elevated oxidation currents and surface reconstructions of cathode particles.

Accurate control of the covalent functionalization of single-walled carbon nanotubes for the electro-enzymatically controlled oxidation of biomolecules.

Single-walled carbon nanotubes (SWCNTs) were functionalized by ferrocene through ethyleneglycol chains of different lengths (FcETGn) and the functionalized SWCNTs (f-SWCNTs) were characterized by different complementary analytical techniques. In particular, high-resolution scanning electron transmission microscopy (HRSTEM) and electron energy loss spectroscopy (EELS) analyses support that the outer tubes of the carbon-nanotube bundles were covalently grafted with FcETGn groups. This result confirms that the electrocatalytic effect observed during the oxidation of the reduced form of nicotinamide adenine dinucleotide (NADH) co-factor by the f-SWCNTs is due to the presence of grafted ferrocene derivatives playing the role of a mediator. This work clearly proves that residual impurities present in our SWCNT sample (below 5 wt. %) play no role in the electrocatalytic oxidation of NADH. Moreover, molecular dynamic simulations confirm the essential role of the PEG linker in the efficiency of the bioelectrochemical device in water, due to the favorable interaction between the ETG units and water molecules that prevents π-stacking of the ferrocene unit on the surface of the CNTs. This system can be applied to biosensing, as exemplified for glucose detection. The well-controlled and well-characterized functionalization of essentially clean SWCNTs enabled us to establish the maximum level of impurity content, below which the f-SWCNT intrinsic electrochemical activity is not jeopardized.

Interface properties and built-in potential profile of a LaCrO3/SrTiO3 superlattice determined by standing-wave excited photoemission spectroscopy

LaCrO$_3$ (LCO) / SrTiO$_3$ (STO) heterojunctions are intriguing due to a polar discontinuity along (001), two distinct and controllable interface structures [(LaO)$^+$/(TiO$_2$)$^0$ and (SrO)$^0$/(CrO$_2$)$^-$], and interface-induced polarization. In this study, we have used soft- and hard x-ray standing-wave excited photoemission spectroscopy (SW-XPS) to generate a quantitative determination of the elemental depth profiles and interface properties, band alignments, and the depth distribution of the interface-induced built-in potentials in the two constituent oxides. We observe an alternating charged interface configuration: a positively charged (LaO)$^+$/(TiO$_2$)$^0$ intermediate layer at the LCO$_\textbf{top}$/STO$_\textbf{bottom}$ interface and a negatively charged (SrO)$^0$/(CrO$_2$)$^-$ intermediate layer at the STO$_\textbf{top}$/LCO$_\textbf{bottom}$ interface. Using core-level SW data, we have determined the depth distribution of species, including through the interfaces, and these results are in excellent agreement with scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS) mapping of local structure and composition. SW-XPS also enabled deconvolution of the LCO-contributed and STO- contributed matrix-element-weighted density of states (MEWDOSs) from the valence band (VB) spectra for the LCO/STO superlattice (SL). Monitoring the VB edges of the deconvoluted MEWDOS shifts with a change in probing profile, the alternating charge- induced built-in potentials are observed in both constituent oxides. Finally, using a two-step simulation approach involving first core-level binding energy shifts and then valence-band modeling, the built-in potential gradients across the SL are resolved in detail and represented by the depth distribution of VB edges.

Determination of the structure and properties of an edge dislocation in rutile TiO2

A global optimization procedure is used to predict the structure and electronic properties of the b = c[001] edge dislocation in rutile TiO2. Over 1000 different atomic configurations have been generated using both semi-empirical and density functional theory estimates of the energy of the system to identify the most stable structure. Both stoichiometric and oxygen deficient dislocation core structures are predicted to be stable depending on the oxygen chemical potential. The latter is associated with Ti3+ species in the dislocation core. The dislocation is predicted to act as a trap for electrons but not for holes suggesting they are not strong recombination centers. The predicted structures and properties are found to be consistent with experimental results obtained using scanning transmission electron microscopy and electron energy loss spectroscopy on samples produced using the bicrystal approach.

Local Structure and Point-Defect-Dependent Area-Selective Atomic Layer Deposition Approach for Facile Synthesis of p-Cu2O/n-ZnO Segmented Nanojunctions

Area-selective atomic layer deposition (AS-ALD) has attracted much attention in recent years due to the possibility of achieving accurate patterns in nanoscale features, which render this technique compatible with the continuous downscaling in nanoelectronic devices. The growth selectivity is achieved by starting from different materials and results (ideally) in localized growth of a single material. We propose here a new concept, more subtle and general, in which a property of the substrate is modulated to achieve localized growth of different materials. This concept is demonstrated by selective growth of high-quality metallic Cu and semiconducting Cu2O thin films, achieved by changing the type of majority point defects in the ZnO underneath film exposed to the reactive species using a patterned bilayer structure composed of highly conductive and highly resistive areas, as confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The selective growth of these materials in a patterned ZnO/Al-doped ZnO substrate allows the fabrication of p-Cu2O/n-ZnO nanojunctions showing a nonlinear rectifying behavior typical of a p-n junction, as confirmed by conductive atomic force microscopy (C-AFM). This process expands the spectra of materials that can be grown in a selective manner by ALD and opens up the possibility of fabricating different architectures, taking advantage of the area-selective deposition. This offers a variety of opportunities in the field of transparent electronics, catalysis, and photovoltaics.

Impact of boron diffusion at MgO grain boundaries on magneto-transport properties of MgO/CoFeB/W magnetic tunnel junctions

Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.

Intentional polarity conversion of AlN epitaxial layers by oxygen

Nitride materials (AlN, GaN, InN and their alloys) are commonly used in optoelectronics, high-power and high-frequency electronics. Polarity is the essential characteristic of these materials: when grown along c-direction, the films may exhibit either N- or metal-polar surface, which strongly influences their physical properties. The possibility to manipulate the polarity during growth allows to establish unique polarity in nitride thin films and nanowires for existing applications but also opens up new opportunities for device applications, e.g., in non-linear optics. In this work, we show that the polarity of an AlN film can intentionally be inverted by applying an oxygen plasma. We anneal an initially mixed-polar AlN film, grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), with an oxygen plasma in a molecular beam epitaxy (MBE) chamber; then, back in MOVPE, we deposit a 200 nm thick AlN film on top of the oxygen-treated surface. Analysis by high-resolution probe-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) evidences a switch of the N-polar domains to metal polarity. The polarity inversion is mediated through the formation of a thin AlxOyNz layer on the surface of the initial mixed polar film, induced by the oxygen annealing.

Arsenate adsorption on iron-impregnated ordered mesoporous carbon: Fast kinetics and mass transfer evaluation

Removal of arsenic (As) from drinking water sources is much needed worldwide, and point-of-use (POU) treatment could be an effective solution for residential applications. Adopting existing removal technologies that are successful in large-scale treatment to POU units, however, is rather ineffective due to the short hydraulic residence time in POU systems. In this study, we investigated the use of iron-containing ordered mesoporous carbon (FeOMC) for rapid arsenate [As(V)] sorption in batch reactors. The OMC was prepared by an evaporation-induced self-assembly approach followed by impregnation of iron oxides, which was identified mainly as maghemite (γ-Fe2O3) by coupled high-resolution transmission electron microscopy and electron energy loss spectroscopy microanalysis. A rapid kinetics of As(V) adsorption was observed, showing that at an initial concentration of 52.1 μg As/L and solid loading of 2.0 g/L, 84.8% of As(V) was removed within 90 s at pH 6.1. The kinetics was satisfactorily modelled according to the mass transfer of As(V) in a single FeOMC grain; and the results suggested that the intraparticle diffusion along the mesoporous channels could be hindered by adsorption. In summary, this study has not only developed ordered mesoporous materials for rapid arsenic adsorption, but also gained insights into the factors controlling the sorption kinetics by evaluating the mass transfer inside the straight and uniform channels of surface-functionalized OMC.

Impact of carbon impurities on the initial leakage current of AlGaN/GaN high electron mobility transistors

We have systematically studied the origin of high gate-leakage currents in AlGaN/GaN high electron mobility transistors (HEMTs). Devices that initially had a low gate-leakage current (good devices) are compared with ones that had a high gate-leakage current (bad devices). The apparent zero-bias Schottky barrier height of bad devices (0.4 < ϕB0 < 0.62 eV) was found to be lower than that of the good devices (ϕB0 = 0.79 eV). From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analysis, we found that this difference is due to the presence of carbon impurities in the nickel layer in the gate region.

Complex momentum behavior of electronic excitations in β-CuPc

The electronic excitation spectrum of β-CuPc has been investigated using electron energy-loss spectroscopy in transmission. Our results demonstrate a rather strong momentum dependence of the lowest exciton features. Both main components show a negative dispersion, and the momentum dependence indicates that this negative dispersion is parallel to the molecular stacks in β-CuPc. In addition, the spectral shape also varies upon increasing momentum transfer indicating a particular momentum dependence of the inter-molecular interactions.

Quantitative Operando Visualization of Electrochemical Reactions and Li Ions in All-Solid-State Batteries by STEM-EELS with Hyperspectral Image Analyses

All-solid-state lithium-ion batteries (LIBs) are one of the promising candidates to overcome some issues of conventional LIBs with liquid electrolytes. However, high interfacial resistance of Li-ion transfer at the electrode/solid electrolyte limits their performance. Thus, it is important to clarify interfacial phenomena in a nanometer scale. Here, we present a new method to dynamically observe the Li-ion distribution and Co-ion electronic states in a LiCoO2 cathode of the all-solid-state LIB during charge and discharge reactions using operando scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). By applying a hyperspectral image analysis of non-negative matrix factorization (NMF) to the STEM-EELS, we succeeded in clearly observing the quantitative Li-ion distribution in the operando condition. We found from the operando observation with NMF that the Li ions did not uniformly extract/insert during the charge/discharge reactions, and the activity of the electrochemical reaction depended on the Li-ion concentration in a pristine state. An electrochemically inactive region was formed about 10-20 nm near the LiCoO2/Li2O-Al2O3-TiO2-P2O5-based solid electrolyte interfaces. The STEM-EELS, electron diffraction, and Raman spectroscopy experimentally showed that the inactive region was a mixture of LiCoO2 and Co3O4, leading to the higher interfacial resistance of the Li-ion transfer because Co3O4 does not have pathways of Li-ion diffusion in its crystal.

Ferroelectric switching dynamics in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 thin films

In this work, the ferroelectric characteristics of 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) thin films grown on 0.7 wt. % Nb-doped (001)-SrTiO3 (Nb:STO) single-crystal have been investigated. High-resolution transmission electron microscopy and electron energy loss spectroscopy revealed a very sharp Nb:STO/BCZT interface, while selected area electron diffraction revealed the epitaxial growth of the BCZT layer on the Nb:STO substrate. The ferroelectric nature of the BCZT films have been investigated by piezoresponse force microscopy and hysteresis loops. The effect of electric field on polarization switching kinetics has been investigated and has been analyzed by the nucleation limited switching model with a Lorentzian distribution function. The local field variation was found to decrease with the increase in the electric field, and thus, the switching process becomes faster. The peak value of the polarization current and the logarithmic characteristic switching time exhibited an exponential dependence on the inverse of electric field. This model gave an excellent agreement with the experimental polarization reversal transients throughout the whole time range.

Intensive EELS study of epoxy composites reinforced by graphene‐based nanofillers

ABSTRACTA detailed analysis of composites with epoxy matrix and three types of graphene‐based nanofillers combining scanning transmission electron microscopy and electron energy loss spectroscopy was reported here. The composites were prepared by a three‐roll milling method and the nanofillers were well dispersed in the matrix, as confirmed by SEM and scanning transmission electron microscopy observation. Mass density profiles through the interface from the matrix to the nanofillers were obtained thanks to the plasmon energy values measured from the low‐loss spectra. The core‐loss spectra revealed the chemical bonding states of different phase regions and identified the different chemical composition of graphite nanoplatelets (GNPs) in GNPs/epoxy and GNP‐CNTs/epoxy composites. Energy dispersive spectroscopy mapping was also conducted to further confirm the components in different phase regions. The successful characterization of the chemical structure and the bonding states paves the way for a better understanding of the structure–property relationships in composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46748.

Coordinated Nanoscale Compositional and Oxidation State Measurements of Lunar Space‐Weathered Material

AbstractSpace weathering on airless bodies includes a number of processes, such as micrometeorite impacts and solar wind bombardment, which leads to a variety of microscale to nanoscale alteration features, including vapor deposited layers on grain and rock surfaces and creation of nanophase opaque inclusions. The nanophase inclusions cause reddening and darkening of the visible to near‐infrared spectra of space weathered material, features associated with increasing space exposure of many airless body regoliths. On the Moon, most nanophase inclusions are metallic iron (npFe0), but recent work using aberration‐corrected scanning transmission electron microscopy and electron energy loss spectroscopy has provided evidence of oxidized nanoparticles in space weathered lunar soil grains. We examined three different lunar soils in order to confirm the finding of oxidized nanophase inclusions and to provide detailed elemental and mineralogical information about the surrounding material. Our data show that substrate and rim composition are key factors in determining whether highly localized oxidation occurs; for example, nanophase inclusions in rims on low Fe substrates are more prone to oxidation. Detailed understanding of the phases and features present in these samples is necessary for the correct interpretation of remotely sensed data as well as extrapolation from processes on the lunar surface to those on other airless bodies.

Mechanism for the formation of nitrogen-filled voids after annealing of GaN on a sapphire substrate

This study investigated the formation of nitrogen-filled voids at the interface between a GaN layer grown on a sapphire substrate by metalorganic vapor phase epitaxy. These voids were formed in the sapphire substrate at the interface after annealing and previous research shows that they can induce an inversion domain in AlN, which affects the film quality and the device performance. We investigated the void formation using scanning electron microscopy, scanning transmission electron microscopy, energy-dispersive X-ray spectrometry, and electron energy loss spectroscopy. The voids are known to originate from the decomposition of sapphire in the presence of ammonia and hydrogen. Our analysis confirmed that the outgassing reaction between the decomposing sapphire and the low temperature GaN buffer layer resulted in the diffusion of aluminum and oxygen into the GaN buffer layer. During the annealing process, oxygen ions replaced nitrogen ions and created nitrogen-filled voids at the interface between the GaN buffer layer and the sapphire substrate. The presence of molecular nitrogen in the voids was confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy.