Scanning Transmission Electron Microscopy Energy Research Articles

Spalling induced van der Waals lift-off and transfer of 4-in. GaN epitaxial films

Epitaxial lift-off (ELO) of high-quality GaN layers allows for integration with a variety of materials enabling improved performance, reduced costs, and development of new electronics. Of the ELO technologies, two-dimensional (2D) material-based lift-off offers great promise but is still in the early stages of development and has yet to demonstrate the scale and yield of other ELO technologies. Here, we demonstrate the potential of this process's scalability, speed, and yield through epitaxial growth and lift-off of 4-in. GaN films using a 2D boron nitride (BN) van der Waals (vdW) buffer layer. Since the BN layer acts as the growth template and the mechanical release layer, both the quality and adhesion of the GaN layer are correlated with the BN morphology and uniformity. Detailed spectroscopic mapping demonstrates excellent BN uniformity, which translates into growth of high-quality GaN as shown in mapping of the x-ray rock curves (XRCs), atomic force microscopy, and photoluminescence. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal abrupt chemically distinct interfaces between the sapphire, BN, and AlN/GaN layers essential for efficient lift-off. Combined with the BN/GaN vdW heterostructure, Ni spalling is used to efficiently lift-off and transfer a full 4-in. GaN layer. Post transfer characterization of a 1.9 μm thick GaN layer transferred to a SiO2/Si wafer shows a very minimal change in the XRC and photoluminescence. Strain measurements before and after transfer show that the process fully relaxes residual strain formed in the GaN during high-temperature growth. This work highlights the potential for industry scalability of an exciting 2D material-based lift-off technology, which can facilitate higher power and more efficient radio frequency devices.

Hollow Psychography: Toward Simultaneous 4D Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy.

With the recent development of high-acquisition-speed pixelated detectors, 4D scanning transmission electron microscopy (4D-STEM) is becoming routinely available in high-resolution electron microscopy. 4D-STEM acts as a "universal" method that provides local information on materials that is challenging to extract from bulk techniques. It extends conventional STEM imaging to include super-resolution techniques and to provide quantitative phase-based information, such as differential phase contrast, psychography, or Bloch wave phase retrieval. However, an important missing factor is the chemical and bonding information provided by electron energy loss spectroscopy (EELS). 4D-STEM and EELS cannot currently be acquired simultaneously due to the overlapping geometry of the detectors. Here, the feasibility of modifying the detector geometry to overcome this challenge for bulk specimens is demonstrated, and the use of a partial or defective detector for ptycholgaphic structural imaging is explored. Results show that structural information beyond the diffraction-limit and chemical information from the material can be extracted together, resulting in simultaneous multi-modal measurements, adding the additional dimensions of spectral information to 4D datasets.

Molecular Layer Deposition of Organic–Inorganic Hafnium Oxynitride Hybrid Films for Electrochemical Applications

The molecular layer deposition (MLD) method can be used to deposit hybrid organic–inorganic films with precisely defined composition, flexible properties, and conformality on different substrates. In this study, hafnium-based organic–inorganic hybrid polymer films were studied as potential coatings for silicon nanoparticles (SiNPs) used in composite lithium-ion battery (LIB) anodes, an application which requires the film to be both flexible and stable under electrochemical conditions. Hf-hybrid films were successfully deposited by MLD using sequential exposure of the homoleptic tetrakis(dimethylamido) hafnium complex and ethanolamine as the reactants. The self-limiting surface reactions lead to a constant growth per cycle (GPC) of ∼2.0 Å/cycle at 120 °C. Temperature-dependent growth was observed, with the GPC decreasing from ∼2.5 to ∼1.1 Å/per cycle as the temperature was increased from 65 to 145 °C. Scanning transmission electron microscopy and electron energy loss spectroscopy mapping confirm that a thin, dense, and conformal Hf-based MLD layer is deposited on the SiNPs. The presence of expected C–N, C–O, and −CH2 moieties in the MLD films was confirmed by Fourier transform infrared spectroscopy. Hafnium nitride and hafnium oxide bonds within the hybrid thin films were identified by X-ray photoelectron spectroscopy. Characterization results indicated that the deposited hafnium-based organic–inorganic hybrid films contain both metal oxynitride bonds and organic bonds, including C–C, C–O, and C–N. This Hf-based MLD thin film was tested on LIB SiNP composite anodes as an artificial solid–electrolyte interphase, with results showing that the capacity retention increased by about 35% after 110 cycles in a LIB application.

Boosting the Catalytic Performance of AuAg Alloyed Nanoparticles Grafted on MoS2 Nanoflowers through NIR-Induced Light-to-Thermal Energy Conversion.

MoS2 nanoflowers (NFs) obtained through a hydrothermal approach were used as the substrate for the deposition of tiny spherical bimetallic AuAg or monometallic Au nanoparticles (NPs), leading to novel photothermal-assisted catalysts with different hybrid nanostructures and showing improved catalytic performance under NIR laser irradiation. The catalytic reduction of pollutant 4-nitrophenol (4-NF) to the valuable product 4-aminophenol (4-AF) was evaluated. The hydrothermal synthesis of MoS2 NFs provides a material with a broad absorption in the Vis-NIR region of the electromagnetic spectrum. The in situ grafting of alloyed AuAg and Au NPs of very small size (2.0-2.5 nm) was possible through the decomposition of organometallic complexes [Au2Ag2(C6F5)4(OEt2)2]n and [Au(C6F5)(tht)] (tht = tetrahydrothiophene) using triisopropilsilane as reducing agent, leading to nanohybrids 1-4. The new nanohybrid materials display photothermal properties arising from NIR light absorption of the MoS2 NFs component. The AuAg-MoS2 nanohybrid 2 showed excellent photothermal-assisted catalytic activity for the reduction of 4-NF, which is better than that of the monometallic Au-MoS2 nanohybrid 4. The obtained nanohybrids were characterised by transmission electron microscopy (TEM), High Angle Annular Dark Field-Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (HAADF-STEM-EDS), X-ray photoelectron spectroscopy and UV-Vis-NIR spectroscopy.

Constructing N-Coordinated Co and Cu Single-Atomic-Pair Sites toward Boosted CO2 Photoreduction

The visible-light-driven photocatalytic reduction reaction of carbon dioxide (CO2) (CO2RR) to value-added fuels presents a feasible approach to curb anthropogenic CO2 emissions and mitigate the increasing energy crisis. However, developing photocatalysts with excellent performance still remains a great challenge in this field. Herein Co,Cu,N-codoped carbon nanoparticles (Co1Cu1/NC) were fabricated through the pyrolysis of zeolitic imidazolate framework (ZIF-8) with Cu(NO3)2 adsorbed inside the cavities and CoTBPP decorated over the surface of ZIF. Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy measurements disclose the dual-metal single-atomic nature of Co1Cu1/NC consisting of atomically dispersed Co–Cu pair sites on a nitrogen-doped carbon support. Extended X-ray absorption fine-structure analysis reveals the tetra-N-coordinated nature of each metal in Co1Cu1/NC (N2–Co–N2–Cu–N2). For the purpose of comparative study, Co,N- and Cu,N-codoped carbon nanoparticles (Co1/NC and Cu1/NC) also with single atomic site nature have been fabricated following the same route. The as-prepared Co1Cu1/NC exhibits highly effective photocatalytic CO2-to-CO reduction with a considerably high CO-generating yield of 22.46 mmol g–1 and a CO selectivity of 83.4% after 2 h of visible-light irradiation. Experimental characterizations and in particular theoretical calculations disclose the close association of the remarkable CO2RR catalytic activity of Co1Cu1/NC with the synergetic effect of the Co–Cu atomic-pair sites, which facilitate the conversion of CO2 to CO via lowering the energy barrier for the formation of the *COOH intermediate. This work paves a new avenue for the rational design and construction of atomic-pair photocatalysts with boosted performance.

Advanced transmission electron microscopy investigation of defect formation in movpe-growth of gap on silicon using arsenic initial coverage

Integration of GaP layers on silicon substrates using AsH3 pre-exposure followed by a PH3-based GaP epitaxial growth allows the development of very promising processes for the photovoltaic industry, although many of the growth routines using this approach suffer from reproducibility issues when transferred to a new epitaxial system, leading to poor quality layers. This fact reveals a lack of knowledge on the mechanisms behind the formation of the most common planar defects (stacking faults and microtwins) and their dynamics for GaP/Si Metal Organic Vapor Phase Epitaxy using AsH3 and PH3. Therefore, in this work, a set of GaP/Si samples with a similarly high defect density grown between 700 °C and 725 °C, are analyzed by means of high-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. The results presented show contaminant-free Si surfaces for temperatures above 725 °C, ruling out the hypothesis of contaminant as the origin of these planar defects. Regarding the interface Si/GaP, the GaP growth starts, in all the samples, with GaSi bonds. Additionally, no traces of As are found, which reinforces the hypothesis of an effectively displacement of As on Si surface by Ga atoms at high temperature. Finally, it is observed complex chemical structures in the origin of the microtwins and the cause of the origin of these defects seems to be a localized gallium depletion at the GaP/Si interface.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes.

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO2 nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Iron Based Catalysts for Nitrogen Reduction Reaction

Ammonia (NH3) is a fundamental feedstock for the global population not only for its wide use as fertilizer and chemical, but also for its potential as energy storage medium. Its synthesis at the industrial scale is based on the well-known Haber Bosch process, operating at high temperature (400-500°C) and high pressure (150-300atm), thus making it highly pollutant. Considering the global climate emergency, it is mandatory to develop a green ammonia synthesis that relies on milder conditions and may adopt renewable energy sources. The electrochemical synthesis, of NH3 from N2 and H2O at ambient conditions and using renewable energy driven electricity could be a promising approach. An efficient electrochemical ammonia synthesis however, is currently still lacking. The main reasons are the absence of adequate catalysts capable of dissociating the N2 triple bond and the competition with the hydrogen evolution reaction (HER) at the cathode, since the required potential is close to that of nitrogen reduction reaction (NRR), required for the ammonia formation.There are some reaction models in the literature but it is commonly accepted that the dissociative adsorption of N2 is the rate limiting step and extensive research has been done on nitrogen interaction with metal surfaces. Ru, Co, Bi, Au, Fe, Mo, etc based materials have been extensively tested for the green ammonia synthesis, but results are still conflicting also because a complete evaluation of environmental contaminations is still lacking.In this work we adopted as NRR catalyst Fe based nanoparticles on carbon cloth substrate via a simple and fast electroless deposition technique. We prepared a FeCl3 solutions with different concentrations in the range 1-10mM and deposited a drop on (3x1.5) cm2 samples of carbon cloth, on a hot plate, at 80°C. After evaporation of the water, the as deposited substrates were dipped in NaBH4 solution while stirring.The catalyst morphology has been studied by Scanning Electron Microscopy (SEM), as a function of the concentration. Very small particles, with a size ranging from 10 to 70nm, were obtained with the most diluted solution. The cluster size increases with increasing FeCl3 concentration in the solution, giving rise to strong coalescence effects and to the formation of a thicker and continuous layer in the case of the most concentrated solution. Scanning Transmission electron Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) have been adopted to determine the composition. EELS spectra showed that the iron L-edge is that typical of Fe3O4 materials.The electrochemical ability to reduce nitrogen, with the formation of ammonia, was evaluated in a standard two compartment cell, with a phosphate buffered solution (PBS 0.1M) adopting a Zirfon membrane as a gas separator. Ar or N2 gas (both with a purity of 99.9999%) flowed in the cathode chamber after going through an acidic trap and a water trap, to be further purified. A rigorous protocol has been adopted to evaluate the ammonia production, including a two steps measurement of the environmental ammonia at the open circuit potential. The ammonia was measured by spectrophotometric analysis using the indophenol blue method. The electrochemical production of ammonia was obtained by chronoamperometry under constant voltage.For the iron-based catalysts, a very efficient activation procedure has been developed, based on cyclic voltammetry under N2 flow. The activation process allows an improvement up to 10 times in the ammonia generation rate. Moreover, a strong correlation has been found between the particle size of the catalyst and its activity for NRR, with the best results achieved with the sample covered with nanoparticles, exhibiting also the highest electrochemical active area. Iron based nanoparticles showed excellent activity for NRR, with a faradaic efficiency of 15% at -0.35 V vs RHE and a maximum ammonia production rate of 85µg mg-1 cat h-1. Acknowledgements:This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No101006941.

Insight into the cation migration and surface structural evolution of spinel LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is considered to be a promising cathode material for electric vehicles (EV) and hybrid electric vehicles (HEVs). Nevertheless, the structural transformation under a deep discharge voltage is still ambiguity. Herein, we report detailed studies of structural change of LNMO cathode under a wide voltage range of 5.0–1.5 V cycling by using synchrotron-based X-ray absorption spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The fundamental findings demonstrate that LNMO would undergo a complex phase transformation from cubic to tetragonal during deep discharge to a voltage lower than 3 V. It gives rise to a further reduction of Mn4+ to Mn3+ and this serious structural distortion leads to the formation of Li2Mn2O4-type tetragonal phase with space group of I41/amd, which is also confirmed by XANES simulations and density functional theory (DFT) calculations. Meanwhile, migration of Ni ions toward the surface from bulk lattice, leading to the rock-salt NiO phase formation with a space group of Fm-3 m. Finally, these observed evolutions on the surface are associated to the fast capacity fading for LNMO under such a wide voltage range. This work provides insight into understanding the capacity degradation mechanism of spinel cathodes and would inspire us to further design high-performance cathode materials for advanced lithium-ion batteries.

Atomic-Scale Observations of Oxygen Release Degradation in Sulfide-Based All-Solid-State Batteries with Layered Oxide Cathodes.

All-solid-state batteries exhibit considerable potential for applications in electric vehicles. Understanding and controlling the oxygen release from the layered cathode active material are essential in achieving long-term operation of all-solid-state batteries because the oxygen release degrades the cathode material and the solid electrolyte, triggering capacity degradation. In this study, we verified the specific interface where the oxygen release is accelerated by atomic-scale scanning transmission electron microscopy and electron energy loss spectroscopy analyses. Oxygen release is suppressed at the interface where the LiNbO3 coating layer is sufficiently formed. Decomposition products on the solid electrolyte and the antisite defect layers on the cathode surface are formed by oxygen release at the interface where the Li2S-P2S5 solid electrolyte and Li(Ni1/3Mn1/3Ni1/3)O2 cathode are in direct contact. These irreversible passivation layers lead to capacity degradation. In addition, we found that exfoliation of the LiNbO3 coating from the cathode not only physically breaks the Li conduction path but also results in oxygen release and the deterioration of the cathode. These atomic-scale insights can further advance the development of all-solid-state batteries by suppressing oxygen release.

AuAg Nanoparticles Grafted on TiO2@N-Doped Porous Carbon: Improved Depletion of Ciprofloxacin under Visible Light through Plasmonic Photocatalysis.

TiO2 nanoparticles (NPs) were modified to obtain photocatalysts with different composition sophistication and displaying improved visible light activity. All of them were evaluated in the photodegradation of ciprofloxacin. The band gap of TiO2 NPs was successfully tailored by the formation of an N-doped porous carbon (NPC)-TiO2 nanohybrid through the pyrolysis of melamine at 600 °C, leading to a slight red-shift of the absorption band edge for nanohybrid NPC-TiO2 1. In addition, the in-situ formation and grafting of plasmonic AuAg NPs at the surface of NPC sheets and in close contact with TiO2 NPs leads to AuAg-NPC-TiO2 nanohybrids 2–3. These nanohybrids showed superior photocatalytic performance for the degradation of ciprofloxacin under visible light irradiation, compared to pristine P25 TiO2 NPs or to AuAg-PVP-TiO2 nanohybrid 4 in which polyvinylpyrrolidone stabilized AuAg NPs were directly grafted to TiO2 NPs. The materials were characterized by transmission electron microscope (TEM), High Angle Annular Dark Field—Scanning Transmission Electron Microscopy—Energy Dispersive X-ray Spectroscopy HAADF-STEM-EDS, X-ray photoelectron spectroscopy and solid UV-vis spectroscopy. Moreover, the active species involved in the photodegradation of ciprofloxacin using AuAg-NCS-TiO2 nanohybrids were evaluated by trapping experiments to propose a mechanism for the degradation.

Enhanced Energetic Performance of Aluminum Nanoparticles by Plasma Deposition of Perfluorinated Nanofilms.

The performance of Al as nanoenergetic material in solid fuel propulsion or additive in liquid fuels is limited by the presence of the native oxide layer at the surface, which represents a significant weight fraction, does not contribute to heat release during oxidation, and acts as a diffusion barrier to Al oxidation. We develop an efficient technique in which the oxide layer is effectively turned into an energetic component via a reaction with fluorine that is coated in the form of a fluorocarbon nanofilm on the Al surface by plasma-enhanced chemical vapor deposition. Perfluorodecalin vapors are introduced in a low-pressure plasma reactor to produce nanofilms on the surface of Al nanoparticles, whose thickness is controlled with nanolevel precision as demonstrated by high-resolution transmission electron microscopy images. Coated particles show superior heat release, with a maximum enhancement of 50% at a thickness of 10 nm. This significant improvement is attributed to the chemical interaction between Al2O3 and F to form AlF3, which removes the oxide barrier via an exothermic reaction and contributes to the amount of heat released during thermal oxidation. The chemistry and mechanism of the enhancement effect of the plasma nanofilms are explained with the help of X-ray photoelectron spectroscopy, X-ray diffraction, high-angle annular dark-field scanning transmission electron microscopy-energy dispersive spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.

BiO 2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO 2 Reduction to Formate

BiO _2-x Nanosheets with Surface Electron Localizations for Efficient Electrocatalytic CO ₂ Reduction to Formate

Identification of Active Sites in HCHO Oxidation over TiO2-Supported Pt Catalysts

The cooperative catalysis between noble metals and oxygen vacancies in reducible oxides plays an important role in many heterogeneous catalytic reactions, but definite identification of active sites remains challenging. Herein, we report an experimental identification of active sites in HCHO oxidation over TiO2-supported Pt catalysts by means of microscopic and spectroscopic techniques. Pt/TiO2–x catalysts with tunable oxygen vacancy concentrations were designed and fabricated based on hydrogen spillover. A combination of aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy allowed us to directly observe the spatial distribution of oxygen vacancies and probe its correlation with the catalytic activity. In situ diffuse reflectance infrared Fourier transform spectroscopy in combination with the online gas chromatography technique was employed to investigate the functionalities of Pt and oxygen vacancies in HCHO oxidation. Our study revealed that HCHO oxidation occurred at the Pt/TiO2–x interface via a cooperative mechanism: oxygen vacancies served as active sites for chemically absorbing HCHO molecules and the adjacent Pt site dissociatively activated O2.

Feasibility to Improve the Stability of Lithium-Rich Layered Oxides by Surface Doping.

Li-rich layer-structured oxides are considered promising cathode materials for their specific capacities above 250 mAh·g-1. However, the drawbacks such as poor rate performance, fast capacity fading, and the continuous transition metal (TM) migration into the Li layer hinder their commercial applications. To address these issues, surface doping of Ti and Zr was conducted to the Li- and Mn-rich layered oxide (LMR), Li1.2Mn0.54Ni0.13Co0.13O2. The drop of the average discharge potentials of the Ti- and Zr-doped LMR was reduced by 593 and 346 mV in 100 cycles, respectively. With aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we clarified that Ti4+ and Zr4+ ions are located near the surface of the material, anchor the surface oxygen, and stabilize the LMR structure. The difference in the strengths of the Ti-O and Zr-O bonds and the doping-resultant electronic structures were determined with density functional theory (DFT) calculations and soft X-ray absorption spectroscopy (SXAS), responsible for the electrochemical performance of surface-doped materials. These findings verify our modification strategies to enhance the cycling performances of the promising LMR cathode materials.

Imaging Se diffusion across the FeSe/ SrTiO3 interface

Monolayer FeSe on SrTiO$_3$ superconducts with reported $T_\mathrm{c}$ as high as 100 K, but the dramatic interfacial $T_\mathrm{c}$ enhancement remains poorly understood. Oxygen vacancies in SrTiO$_3$ are known to enhance the interfacial electron doping, electron-phonon coupling, and superconducting gap, but the detailed mechanism is unclear. Here we apply scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to FeSe/SrTiO$_3$ to image the diffusion of selenium into SrTiO$_3$ to an unexpected depth of several unit cells, consistent with the simultaneously observed depth profile of oxygen vacancies. Our density functional theory (DFT) calculations support the crucial role of oxygen vacancies in facilitating the thermally driven Se diffusion. In contrast to excess Se in the FeSe monolayer or FeSe/SrTiO$_3$ interface that is typically removed during post-growth annealing, the diffused Se remains in the top few unit cells of the SrTiO$_3$ bulk after the extended post-growth annealing that is necessary to achieve superconductivity. Thus, the unexpected Se in SrTiO$_3$ may contribute to the interfacial electron doping and electron-phonon coupling that enhance $T_\mathrm{c}$, suggesting another important role for oxygen vacancies as facilitators of Se diffusion.

Understanding high-temperature cycling-induced crack evolution and associated atomic-scale structure in a Ni-rich LiNi0.8Co0.1Mn0.1O2 layered cathode material

X-ray nano-computed tomography (nano-CT) and deep learning combined with Cs-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy were employed to investigate the atomic to microscopic structural evolution of LiNi0.8Co0.1Mn0.1O2 (NCM) upon cycling at 55 °C. Two types of intergranular cracks were clearly distinguished by nano-CT for cycled cathode particles; denoted open and closed cracks depending on whether or not the cracks reach the surface of the NCM secondary particles. The volume of high-temperature cycling-induced cracks quantified by deep learning increased drastically, particularly for the open cracks, and this phenomenon was accompanied by rapid degradation of capacity retention. Further precise STEM analysis of the crack regions revealed that migration of transition metal (TM) ions to the Li layer forms a rocksalt-like structure, and the associated reduction of TM ions, e.g., Ni3+ to Ni2+, predominately occurred in the open crack regions in the presence of penetrated electrolyte, even for regions extending to the center of the secondary particle. In contrast, in the closed crack regions, no significant atomic-scale structure distortion and limited reduction of TM ions was observed. Thus, our results provide fundamental guidance for improving the electrochemical performance of Ni-rich layered cathode materials for cycling at elevated temperatures.

Atomic-resolved structural and electric field analysis of the passivation interface of MIS-HEMTs

The passivation interfaces of GaN-based MIS-HEMTs with Si3N4 and ZrO2/Si3N4 bilayers were investigated through atomic resolution scanning transmission electron microscope–energy dispersive spectroscopy–differential phase contrast microscopy methods. It is found that the Si3N4/GaN interface exhibits atomic disorder fluctuation, and the GaN surface is discontinuous at the depth of 1–2 atomic layers. An oxide layer of ∼2 nm is formed at the ZrO2/GaN interface, and the GaN surface is atomically flat. Furthermore, the local minimum of the potential is located at the Si3N4/GaN interface, while it is distributed in the GaN side at the ZrO2/GaN interface. The electric field or potential distribution is affected by the crystal orientation of the polycrystalline ZrO2 layer. Finally, the difference in passivation mechanism is discussed.

Epitaxial Growth of CaMnO3–y Films on LaAlO3(112¯0) by Pulsed Direct Current Reactive Magnetron Sputtering

CaMnO3 is a perovskite with attractive magnetic and thermoelectric properties. CaMnO3 films are usually grown by pulsed laser deposition or radio frequency magnetron sputtering from ceramic targets. Herein, epitaxial growth of CaMnO3–y (002) films on a ‐oriented LaAlO3 substrate using pulsed direct current reactive magnetron sputtering is demonstrated, which is more suitable for industrial scale depositions. The CaMnO3–y shows growth with a small in‐plane tilt of <≈0.2° toward the (200) plane of CaMnO3–y and the () with respect to the LaAlO3 substrate. X‐ray photoelectron spectroscopy of the electronic core levels shows an oxygen deficiency described by CaMnO2.58 that yields a lower Seebeck coefficient and a higher electrical resistivity when compared to stoichiometric CaMnO3. The LaAlO3 substrate promotes tensile‐strained growth of single crystals. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal antiphase boundaries composed of Ca on Mn sites along <101> and <002>, forming stacking faults.

Probing the Thermal Behavior and Stability of Metal‐Fe3O4 Heterodimer Nanoparticles Utilizing In Situ Pulsed Laser Heating TEM

In situ laser heating with transmission electron microscopy (ILH‐TEM) is utilized as a neoteric method to probe the thermal behavior and stability of metal‐ (Pt, Au‐) metal oxide (Fe3O4) heterodimer (HD) nanoparticles. Two different supporting membrane types are used for the laser heating: pure‐carbon and silicon‐nitride, requiring different amounts of laser power to induce morphological changes. Diffraction pattern analysis and scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) are utilized to analyze reduction of the iron oxide under laser heating, which has differing effects depending on the support film heating mode and the heterodimer particle. A unique wetting behavior of Pt and Au onto Fe3O4 is observed in both heterodimers of Au and Pt. However, this wetting behavior is found to be partially reversible only in the case of the Au‐Fe3O4 HD system, whereas for the Pt‐Fe3O4 HD system, this wetting process was found to be irreversible under the applied experimental conditions. This observation of a novel wetting behavior has future implications for understanding the performance of oxide supported metal nanoparticles in high temperature applications, such as catalysis.