Electron Energy Loss Spectroscopy Mapping Research Articles

Hydroxide Modified Synthesis of Atomically‐Doped Photoluminescent WS2 Monolayers

AbstractAtomically doping in thin film poses a significant challenge due to the potential for dopant precipitation caused by self‐purification effects. This challenge is particularly pronounced in the case of doping monolayers of photoluminescent transition metal dichalcogenides, where high energy barriers are also required to break the in‐plane bonds between the transition metal (TM) and chalcogen atoms. To address this issue, this study introduces hydroxide ions to adsorb onto the surface of WS2 monolayers. This results in a significant reduction in the formation energy of the TM─S bonds, enabling them to substitute for W sites and overcoming the self‐purification effect of WS2 monolayers. The in‐plane doping of TMs including Cr, Mn, Fe, Co, and Ni atoms is confirmed through precise atomic‐scale chemical imaging using scanning transmission electron microscopy with electron energy loss spectroscopy mapping. Photoluminescence measurements reveal that the band structure of WS2 monolayers can be systematically modulated by different doping metals, owing to their distinct atomic sizes. In addition, the atomically‐doped WS2 monolayers exhibit room‐temperature ferromagnetism, which has never been seen in pristine WS2 monolayers. The modulation of the band structure and the emergence of magnetism in TMs‐doped WS2 monolayers hold significant promise for optoelectronic and magnetoelectric applications.

Spectral Tuning of Plasmonic Activity in 3D Nanostructures via High‐Precision Nano‐Printing

AbstractPlasmonic nanoparticles reveal unique optical properties and are increasingly incorporated into commercial products and technologies, ranging from photovoltaics to biological and chemical sensors. Shifting and tuning their plasmonic response according to the targeted application strongly depends on the ability to control the geometry in every detail and has not been reliably demonstrated for complex 3D nano‐architectures yet. Following that motivation, it herein presents how Focused Electron Beam Induced Deposition (FEBID), a highly flexible additive 3D direct‐write technology with spatial nano‐scale precision, is used for the controlled and tunable fabrication of plasmonically active 3D nanostructures that exhibit highly concentrated, well defined and predictable local plasmonic resonances. As model systems, planar Au nanowires and 3D nano‐tips of various geometries are prepared via FEBID and plasmonically characterized via scanning transmission electron microscopy based electron energy loss spectroscopy (STEM‐EELS) mapping measurements. The findings are complemented with corresponding plasmon simulations, revealing very good agreement with experimental findings. This way, on‐demand spectral tuning of the plasmonic response becomes accessible via upfront modeling and design of suitable 3D nanostructures, to achieve customized plasmonic responses, therefore paving the way for yet unrealized plasmonic applications in 3D space.

Atomic Layer Deposition of the Geometry Separated Lewis and Brønsted Acid Sites for Cascade Glucose Conversion.

Solid acid catalysts with bi-acidity are promising as workhouse catalysts in biorefining to produce high-quality chemicals and fuels. Herein, we report a new strategy to develop bi-acidic cascade catalysts by separating both acid sites in geometry via the atomic layer deposition (ALD) of Lewis acidic alumina on Brønsted acidic supports. Visualized by transmission electron microscopy and electron energy loss spectroscopy mapping, the ALD-deposited alumina forms a conformal alumina domain with a thickness of around 3 nm on the outermost surface of mesoporous silica-alumina. Solid state nuclear magnetic resonance investigation shows that the dominant Lewis acid sites distribute on the outermost surface, whereas intrinsic Brønsted acid sites locate inside the nanopores within the silica-rich substrate. In comparison to other bi-acidic solid catalyst counterparts, the special geometric distance of Lewis and Brønsted acid sites minimized the synergetic effect, leading to a cascade reaction environment. For cascade glucose conversion, the designed ALD catalyst showed a highly enhanced catalytic performance.

On potential core-shell morphologies of δ-hydride precipitates in zircaloy-2: A microstructural characterization approach by electron diffraction and energy-loss spectroscopy

The crystal structure of the interfacial areas of a δ-FCC water-quenched nano-hydride was characterized by electron energy-loss spectroscopy (EELS) and electron diffraction. EELS revealed ribbons with plasmon energy (PE) values of 17.4 ± 0.01 eV and 18.3 ± 0.01 eV (nominally characteristic of the ζ- and γ-hydride phases, respectively) in the interfacial area between the δ-core and α-Zr matrix. Electron diffraction patterns (DPs) obtained from the <21¯1¯0> axes of the interface contained reflections that could be indexed as {0001} reflections of the ζ-phase. Such ζ-type reflections, however, disappeared after tilting the interface away from the <21¯1¯0>-axes; implying that they originated from sources other than a hypothetical ζ-phase. Moreover, electron DPs obtained from multiple zone axes of the interface, did not show characteristic {110}/{112} superlattice reflections of the γ-phase. These results ruled out the existence of ζ- and γ-phases in the interface (down to the spatial resolution of the utilized techniques) despite the measured plasmon energy values.Subsequently, dielectric theory was utilized to clarify the origin of interfacial ribbons with PE values characteristic of the ζ- and γ-phases. Dielectric functions of the α-Zr and δ-core were extracted from the energy-loss spectra of the two phases, to simulate the energy-loss functions across the interface. Simulations suggested that the observed interfacial ribbons in EELS maps (with PE values of 17.4 ± 0.01 eV and 18.3 ± 0.01 eV) could have stemmed from the delocalized nature of plasmon vibration and the effect of interface on shifting the plasmon vibration frequency, but not necessarily from the existence of the ζ- and γ-phases.

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.

Behavior and properties of helium cavities in ductile-phase-toughened tungsten irradiated with He+ ions at an elevated temperature

Ductile-phase toughened tungsten (DPT W) composites have demonstrated a great potential for plasma-facing components in fusion power systems. These materials can retain the outstanding thermomechanical properties of W while significantly enhancing the fracture toughness. The DPT W composite in this study consists of W particles (88 wt.%) embedded in a ductile NiFeW (12 wt.%) solution matrix. Irradiation of the composite was performed with 90 keV He+ ions to 1.0 × 1017 He+/cm2 at 973 K. The He cavities formed in both W and NiFeW phases are examined using convergent-beam scanning transmission electron microscopy (CB-STEM) at an atomic-level resolution. Full-range depth profiles of the He cavity diameters and number densities are analyzed. The results show that the average cavity diameter in NiFeW is ∼2.7 times that in W, while the number density is an order of magnitude lower. A quantitative analysis of the He density and pressure in the cavities is also achieved using STEM electron energy loss spectroscopy (STEM-EELS) mapping. The He atoms in the cavities in NiFeW are found to be under-pressurized, while those in similar-sized cavities in W are not detectable. The He energy-loss shift exhibits a linear relationship with the He density in cavities in NiFeW, with the slope comparable to that for martensitic steel.

Heterogeneity in Cation Exchange Ag+ Doping of CdSe Nanocrystals.

Cation exchange is becoming extensively used for nanocrystal (NC) doping in order to produce NCs with unique optical and electronic properties. However, despite its ever-increasing use, the relationships between the cation exchange process, its doped NC products, and the resulting NC photophysics are not well characterized. For example, similar doping procedures on NCs with the same chemical compositions have resulted in quite different photophysics. Through a detailed single molecule investigation of a postsynthesis Ag+ doping of CdSe NCs, a number of species were identified within a single doped NC sample, suggesting the differences in the optical properties of the various synthesis methods are due to the varied contributions of each species. Electrostatic force microscopy (EFM), electron energy loss spectroscopy (EELS) mapping, and single molecule photoluminescence (PL) studies were used to identify four possible species resulting from the Ag+-CdSe cation exchange doping process. The heterogeneity of these samples shows the difficulty in controlling a postsynthesis cation exchange method to produce homogeneous samples needed for use in any potential application. Additionally, the heterogeneity in the doped samples demonstrates that significant care must be taken in describing the ensemble or average characteristics of the sample.

Probing the Mysterious Behavior of Tungsten as a Dopant Inside Pristine Cobalt‐Free Nickel‐Rich Cathode Materials

AbstractNickel‐rich cathode materials with small amounts of tungsten (W) dopants have attracted extensive attention in recent years. However, the chemical state, crystalline form, compound chemistry, and location of W in these layered cathodes are still not well‐understood. In this study, these missing structural properties are determined through a combination of macro‐, to atomic‐sensitive characterization techniques and density functional theory (DFT). W‐doped LiNiO2 (LNO) particles, prepared with mechanofusion and coprecipitation methods, are used to probe changes in the structure and location of W‐species. The results indicate that W is mainly distributed on the surfaces and inside grain boundaries of the secondary particles, regardless of the doping method. Electron energy loss spectroscopy (EELS) mapping confirms the simultaneous presence of W, O, with and without Ni in the grain boundaries as well as W‐ and O‐rich regions on the very surface. The W‐rich areas inside the grain boundaries are found to be in two forms, crystalline and amorphous. This paper suggests the presence of kinetically stabilized‐Li4+xNi1‐xWO6 (x = 0, 0.1) with the possibility of LixWyOz phases in LNO which are consistent with the electron microscopy, X‐ray absorption and diffraction data. The multiple roles of W in this complex microstructure are discussed considering the W distribution.

Mussel-inspired polydopamine functionalized with ionic liquid as a novel, eco-efficient adsorbent for the selective removal of anionic pollutants from aqueous solutions

In this work, novel mussel-inspired polydopamine nanoparticles functionalized with ionic liquid (PDA-IL) were synthesized via one step oxidative polymerization and Schiff-base addition reaction using dopamine and ionic liquid (3-aminopropylammonium acetate). The synthesized polydopamine-ionic liquid (PDA-IL) was utilized as an efficient adsorbent in the removal of anionic dyes from aqueous solutions, using Alizarin Red S (ARS) as a model pollutant. Multiple analytical tools, including scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analyzer (TGA), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and Zeta potential were applied in the characterizations of PDA-IL. A parametric study was conducted to examine the effects of pH, contact time, initial concentration, adsorbent dose, and temperature on PDA-IL adsorption performance. Characterization results confirmed the formation of nano-sized PDA-IL particles with polycrystalline structure. Additionally, the porous structure, surface charge, surface moieties and thermal stability were progressively tuned along with the introduction of the custom-synthesized IL within the matrix of PDA. Results showed a significant maximum ARS adsorption capacity of PDA-IL, up to 234.2 mg g−1 at 25 °C. Adsorption kinetics data revealed a pseudo-second-order kinetics, governed by chemisorption. Equilibrium data for ARS removal were in agreement with Freundlich isotherm, indicating multilayer adsorption on the heterogeneous PDA-IL surface. Furthermore, regeneration studies demonstrated an efficient re-use of PDA-IL adsorbent in at least 4 adsorption/desorption cycles. A selectivity study was also conducted, which showed that PDA-IL selectivity towards anionic dyes can be tuned via pH control. Finally, an adsorption mechanism was proposed, based on FTIR and electron energy loss spectroscopy (EELS) mapping results, among others, for an understanding of the underlying adsorption phenomena on PDA-IL. Overall, the developed PDA-IL adsorbent had a great removal capacity of anionic ARS dye, fast adsorption kinetics, excellent selectivity towards anionic dyes and high regeneration efficiency.

Piezoresistive Behavior of Polypyrrole:Carboxymethyl Cellulose Composites.

The unique properties of conducting polymers make them ideally suited for applications in organic electronics, photovoltaics, and energy storage systems. Depending on the specific application, they can outperform metal-based electronics by cost, mechanical flexibility, molecular design opportunities, and environmental impact. Many composites of conducting polymers with polyanions can be processed in water. However, the facile processing of such composites comes at a cost of reduced conductivity. In this manuscript, electronic conductivity dependence on composition for a composite of polypyrrole (PPy) with carboxymethyl cellulose (CMC) has been studied. Secondary ion mass spectrometry and electron energy loss spectroscopy mapping indicate the formation of a nanostructure forming PPy-rich nanospheres with a CMC-rich surface coverage. This structure requires inter-particle electron conduction to occur via quantum tunneling. Variations in the tunneling distance are dependent on the applied pressure, giving rise to a pressure-dependent electronic conductivity and thus piezoresistance. This behavior opens new applications of conducting polymer composites in pressure-sensitive electronic devices, providing metal-free alternatives to quantum tunneling composites.

Surface yttrium-doping induced by element segregation to suppress oxygen release in Li-rich layered oxide cathodes

Doping electrochemically inert elements in Li-rich layered oxide cathodes usually stabilizes the structure to improve electrochemical performance at the expense of available capacity. Here, we use an element segregation principle to realize a uniform surface doping without capacity sacrifice. On the basis of Hume-Rothery rule, element yttrium is chosen as a candidate dopant to spontaneously segregate at particle surface due to mismatched ionic size. Combined with X-ray photoelectron spectroscopy and electron energy loss spectroscopy mapping, yttrium is demonstrated uniformly distributed on particle surface. More importantly, a significant alleviation of oxygen release after surface doping is detected by operando differential electrochemical mass spectrometry. As a result, the modified sample exhibits improved reversibility of oxygen redox with 82.1% coulombic efficiency and excellent cycle performances with 84.15% capacity retention after 140 cycles. Postmortem analysis by transmission electron microscopy, Raman spectroscopy and X-ray diffraction reveal that the modified sample maintains the layered structure without a significant structure transformation after long cycles. This work provides an effective strategy with a series of elements to meet the industrial application.

Electron energy-loss spectroscopy investigation of multipolar behavior in Al nanostructure as a function of surface coating with PVP and oxidation

The inherent, self-limiting oxidation characteristics of Aluminum (Al) nanospheres and nanocubes of similar dimensions (100 nm edge length) are illustrated utilizing the MATLAB based boundary element method (MNPBEM17) optical and electron energy loss spectroscopy (EELS) simulations. Such model systems conveniently establish not only the distinct dipolar and multi-polar plasmonic resonance behaviors with reference to their respective passivated ligand PVP as well as their self-limiting surface conformal/non-conformal oxide layers, demonstrating the vital fact that Al nanostructures could be a suitable substitution of conventional noble metals, especially for UV-plasmonic applications. In the case of Al nanocubes, both oxidation processes were found to be strongly site-selective (edges, corners and faces), whereas a differential variation is observed in Al nanospheres from the center to the surface, effectively corroborated through the simultaneous correlation of optical spectra with the spatially and energetically resolved EELS maps. Moreover, the robust contribution from dark plasmon resonances was also confirmed in the EELS data along with the induced surface charge distribution calculations, further substantiating their strong dependence on the surrounding dielectric environment concerning their oxide passivation as well. Thus, our present results demonstrate a better understanding of the spatial coherence of the plasmonic behavior with reference to the passivated ligand and (non)-conformal oxidation processes of Al nanostructures, in general, thereby providing a fundamental understanding of the plasmon physics necessary to improve the design and optimization of new nanostructures for nanophotonic, nanoantenna and sensing applications.

Improvement of Fermi-Level Pinning and Contact Resistivity in Ti/Ge Contact Using Carbon Implantation.

Effects of carbon implantation (C-imp) on the contact characteristics of Ti/Ge contact were investigated. The C-imp into Ti/Ge system was developed to reduce severe Fermi-level pinning (FLP) and to improve the thermal stability of Ti/Ge contact. The current density (J)-voltage (V) characteristics showed that the rectifying behavior of Ti/Ge contact into an Ohmic-like behavior with C-imp. The lowering of Schottky barrier height (SBH) indicated that the C-imp could mitigate FLP. In addition, it allows a lower specific contact resistivity (ρc) at the rapid thermal annealing (RTA) temperatures in a range of 450–600 °C. A secondary ion mass spectrometry (SIMS) showed that C-imp facilitates the dopant segregation at the interface. In addition, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping showed that after RTA at 600 °C, C-imp enhances the diffusion of Ge atoms into Ti layer at the interface of Ti/Ge. Thus, carbon implantation into Ge substrate can effectively reduce FLP and improve contact characteristics.

Effects of Carbon Incorporation on Electrical Characteristics and Thermal Stability of Ti/TiO2/n-Ge MIS Contact

The effects of carbon incorporation on the thermal stability of the interfacial TiO₂ layer and the electrical characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were investigated. The improved thermal stability and contact characteristics of Ti/TiO₂/<i>n</i>-Ge contacts were characterized in terms of Schottky barrier height (SBH) and specific contact resistivity (&#x03C1;_<i>c</i>) using the Schottky diode and circular transmission line model (CTLM). The values of SBH and &#x03C1;_<i>c</i> increased after the rapid thermal annealing (RTA) above 550 &#x00B0;C. The current density&#x2013;bias voltage (<i>J&#x2013;V</i>) curves of the Schottky diode showed a change of contact characteristics from Ohmic-like behavior to rectifying. This thermal instability was mainly caused by the decomposition of the interfacial TiO₂ layer after high-temperature annealing. The structural degradation was confirmed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. When carbon ions were incorporated into the interfacial TiO₂ layer, the SBH and &#x03C1;_<i>c</i> values showed relatively stable characteristics as the RTA temperature increased up to 600 &#x00B0;C. The EELS mapping showed that the diffusion of oxygen from the interfacial TiO₂ layer was effectively suppressed thanks to the incorporation of carbon. Thus, the carbon incorporation can improve the thermal stability of the interfacial TiO₂ layer and the metal&#x2013;insulator&#x2013;semiconductor contact characteristics for Ge-based device applications.

Unlocking the origin of compositional fluctuations in InGaN light emitting diodes

The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in $In_{x}Ga_{1-x}N$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in $In_{x}Ga_{1-x}N$ QWs at different Indium concentration and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim$ 18%) content $In_{x}Ga_{1-x}N$ QWs, there exists a striking deviation from the model in higher In content ($\geq$ 24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In-content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role on the material performance.

Synthesis, characterization, and single-crystal growth of a high-entropy rare-earth pyrochlore oxide

The synthesis and single-crystal growth of a rare-earth high-entropy oxide ${({\mathrm{Yb}}_{0.2}{\mathrm{Tb}}_{0.2}{\mathrm{Gd}}_{0.2}{\mathrm{Dy}}_{0.2}{\mathrm{Er}}_{0.2})}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ in the cubic pyrochlore phase $Fd\overline{3}m$ are achieved by solid-state reaction and optical floating-zone growth techniques. X-ray diffraction and a structure refinement analysis confirm a single-phase pyrochlore structure and lattice parameter of $a=10.1152(3)\phantom{\rule{0.16em}{0ex}}\AA{}$. Additional characterization techniques, including scanning transmission electron microscopy and nanoscale electron-energy loss spectroscopy mapping, support a single-phase pyrochlore structure with a homogeneous mixture down to \ensuremath{\sim}2.3 nm. Magnetization measurements on a single crystal reveal antiferromagnetic correlations with a spin-glass ground state. These results show that it is possible to grow large single crystals of a high entropy pyrochlore oxide, expanding the avenues for future high-entropy oxide research.

Investigation of Barrier Property of Amorphous Co–Ti Layer as Single Barrier/Liner in Local Co Interconnects

The scaling of interconnect dimensions is becoming increasingly difficult due to the fast rise of line/via resistance. To alleviate this problem, Co has a high potential as an alternative material to Cu or W in metallization schemes. In this article, Co is used as local interconnects, and Co–Ti is proposed as a single barrier/liner to replace the conventional thick TiN/Ti bilayer. The samples with and without Co–Ti were fabricated to investigate the barrier property of Co–Ti. Meanwhile, the reaction mechanism between Co/Co–Ti and Si was comprehensively studied. In order to further evaluate the barrier property of Co–Ti, the samples were annealed at 500 °C for different durations, including 1, 30, and 60 min. Interfacial quality, elemental distribution, and Co spikes were analyzed by cross-sectional and plane-view transmission electron microscopy (TEM) in combination with corresponding electron energy loss spectroscopy (EELS) mapping. Furthermore, refined transmission line model (RTLM) structures were employed to extract the specific contact resistivity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho _{c}$ </tex-math></inline-formula> ) of as-fabricated devices with different Co:Ti atomic ratios. Achieved results demonstrate that as a single barrier/liner, Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.65</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</sub> is highly effective in preventing Co diffusion into Si substrate, thus realizing a uniform Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.65</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.35</sub> /n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si interface, spike-free contacts, and ultralow <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rho _{c}$ </tex-math></inline-formula> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.3 \times 10^{-8} \Omega {-}cm^{2}$ </tex-math></inline-formula> .

(Invited) Atomic Resolution, Coordinated Electron Microscopy Characterization of 2D Layers Synthesized By Confinement Heteroepitaxy (CHet) Technique

Since the discovery of graphene in 2004 [1] there has been a flurry of activity in investigating two dimensional (2D) materials, because of their superlative properties: spin-orbit coupling, massless Dirac fermions, mechanical strength, thermal and chemical stability [2]. Remarkably, the in-plane hexagonal symmetry of graphene is largely responsible for its behavior. However, the zero-band gap of graphene presents hurdles to its use as a semiconductor. This has spurred interest in synthesis other van der Waals 2D structures, such as h-BN [3] and more recently 2D Gallium Nitride (2D GaN) [4].The synthesis of 2D Nitrides was first reported in 2016 by applying Confinement Heteroepitaxy (CHet) process to form a 2D Gallium Nitride (GaN) layer with 1-2 atom thickness. In this technique, a silicon carbide (SiC) substrate is graphitized and deliberately damaged using oxygen plasma to provide a two-step intercalation path. First a metal (like Gallium, or other group V metal) through evaporation is intercalated between the graphene and SiC substrate. Then, to complete the half reaction, nitrogen is introduced by ammonia annealing at 700 °C to form a 1-layer thick III-nitride.In this study, the growth method is extended from 2D GaN to variety of other 2D metals (Indium), Nitrides (InN) and alloys (InGa alloys). We employ a coordinated approach to understand the surface in both plan and cross-section view – linking through a variety of electron microscopy techniques, including auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Bright-field, Dark-field, aberration-corrected STEM-HAADF imaging, and atomic-scale electron energy loss spectroscopy (EELS) mapping using a Gatan K2 SI direct electron detector and other techniques mapping were all applied to characterize the SiC/2D-layer/graphene interface. These electron microscopy techniques allow a precise, local measurement of the atomic structure of these new materials, including such growth parameters as the effect of graphene on the 2D layer stability, the effect of the SiC step edges on the layers structure, and the effect of the plasma type and source used in damaging the graphene on the 2D layer chemistry. Plasmon mapping also describes the unique bonding in the materials and their electronic structure. Ultimately, the measurements presented here characterize a whole new class of nanomaterials and provide insight on the fundamentals of the intercalation growth process. 1. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.2. Soldano, C., A. Mahmood, and E. Dujardin, Production, properties and potential of graphene. Carbon, 2010. 48(8): p. 2127-2150.3. Watanabe, K., T. Taniguchi, and H. Kanda, Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal. Nature Materials, 2004. 3(6): p. 404-4094. Al Balushi, Z.Y., et al., Two-dimensional gallium nitride realized via graphene encapsulation. Nature Materials, 2016. 15(11): p. 1166-1171. Figure 1

Magneto-dielectric effect in relaxor superparaelectric Tb2CoMnO6 film

We report magneto-dielectric properties of partially B-site ordered monoclinic Tb2CoMnO6 double perovskite thin film epitaxially grown by metalorganic aerosol deposition technique. Transmission electron microscopy and electron energy loss spectroscopy mapping shows the presence and distribution of both Co2+ and Co3+ ions in the film, evidencing a partial B-site disorder, which was further confirmed by the observation of reduced saturation magnetization at low temperatures. The ferromagnetic Curie temperature, TC=110 K, is slightly higher as compared to the bulk value (100 K) probably due to an in plane tensile strain. Remarkably, a short range ordering of spins at T*~190 K>>TC was established and assigned to the B-site disorder in the film. Two different dielectric relaxation peaks have been observed; they merge at the same temperature T* of short range spin correlations. Moreover, an unexpected high temperature dipolar relaxor-glass-like transition at T~T* was observed, at which a coupling to short range magnetic correlations results in a 4% magneto-dielectric coupling.

Beyond surface redox and oxygen mobility at pd-polar ceria (100) interface: Underlying principle for strong metal-support interactions in green catalysis

When ceria is used as a support for many redox catalysis involved in green catalysis, it is well-known that the overlying noble metal can gain access to a significant quantity of oxygen atoms with high mobility and fast reduction and oxidation properties under mild conditions. However, it is as yet unclear what the underlying principle and the nature of the ceria surface involved are. By using two tailored morphologies of ceria nanocrystals, namely cubes and rods, it is demonstrated from Scanning Transmission Electron Microscopy with Electron Energy Loss Spectroscopy (STEM-EELS) mapping and Pulse Isotopic Exchange (PIE) that ceria nanocubes terminated with a polar surface (100) can give access to more than the top most layer of surface oxygen atoms. Also, they give higher oxygen mobility than ceria nanorods with a non-polar facet of (110). A new insight for the possible additional role of polar ceria surface plays in the oxygen mobility is obtained from Density Functional Theory (DFT) calculations which suggest that the (100) surface sites that has more than half-filled O on same plane can drive oxygen atoms to oxidise adsorbate(s) on Pd due to the strong electrostatic repulsion.