Atomic-scale Characterization Research Articles

Atomic-scale characterization of plasma-induced damage in plasma-enhanced atomic layer deposition

Plasma-enhanced atomic layer deposition (PE-ALD) has many advantages for the deposition of thin films. However, an appropriate control of the plasma frequency in the PE-ALD process is required to reduce the plasma-induced damage of the thin films during deposition. In this study, we comparatively studied the effects of conventional 13.56MHz, radio frequency (RF) and 60MHz, very high frequency (VHF) plasma reactants in the PE-ALD during the deposition of Al2O3. The plasma damage and the degree of strain of the substrate are investigated by transmission electron microscopy at the atomic scale. In addition, a correlation between the atomic structure and plasma damage at RF and VHF is suggested. Compared to the RF PE-ALD, Al2O3 thin films deposited with VHF PE-ALD show a clearly effective reduction of the plasma-induced damage. Moreover, the Al2O3 thin films are grown into a tetrahedral structure near the surface and are then further grown into an octahedral structure, indicating the presence of an increased number of ions and radicals during the plasma-enhanced process. It is evident that the VHF PE-ALD is a more important deposition process for reducing plasma-induced damage to thin films than its RF counterpart.

Atom probe study on microstructure change in severely deformed pearlitic steels: application to rail surfaces and drawn wires

Pearlitic steel is used as the material for high tensile steel wires, rails and wheels due to its high work hardening and wear resistance. These properties arise from a layered structure comprising deformable lamellar ferrite and hard lamellar cementite. This paper reviews the microstructural change in heavily drawn pearlitic steels wires and worn surfaces of pearlitic rails using atom probe tomography analysis. The cementite decomposition mechanism was elucidated for heavily drawn pearlitic steel wires. For pearlitic rail steels, atomic scale characterization of worn surfaces and of the white etching layer (WEL) were performed, and a mechanism for the formation of the WEL was proposed. The differences and similarities in microstructure and in the state of the cementite in these severely deformed pearlitic steels are discussed.

Atomic-scale characterization of thermoelectric oxides using high spatial and energy resolution STEM-EELS

Atomic-scale characterization of thermoelectric oxides using high spatial and energy resolution STEM-EELS

Atomic-Scale Characterization of Electrode Materials in Lithium/Sodium-ion Batteries by STEM

Atomic-Scale Characterization of Electrode Materials in Lithium/Sodium-ion Batteries by STEM

Towards the atomic-scale characterization of isolated iron sites confined in a nitrogen-doped graphene matrix

Atomic scale characterization of the surface structure of powder catalysts is essential to the identification of active sites, but remains a major challenge in catalysis research. We described here a procedure that combines atomic force microscopy (AFM), operated in air, and scanning tunneling microscopy (STM), operated in UHV, to obtain the atomic structure and local electronic properties of powder catalysts. The atomically dispersed Fe-N-C catalyst was used as an example, which was synthesized by low temperature ball milling methods. We discussed the effect of solvents in the dispersion of powder catalysts on a planar support, which is key to the subsequent atomic characterization. From the morphology, atomic structure and local electronic properties of the Fe-N-C catalyst, our combined measurements also provide an insight for the effect of ball milling in the preparation of atomically dispersed metal catalysts.

Deformation stimulated precipitation of a single-phase CoCrFeMnNi high entropy alloy

We report transmission electron microscope characterization of phase decomposition in a single-phase CoCrFeMnNi high entropy alloy (HEA) subjected to cold rolling and subsequent annealing at intermediate temperatures. It is found that plastic deformation plays a key role in phase destabilization of the HEA during intermediate annealing. The formation of Cr-rich precipitates in recrystallized grains and recovered samples suggests that dislocations generated by cold rolling is one of the critical factor leading to the destabilization of the stable HEA. Atomic-scale characterization of the Cr-rich precipitates reveals a complex crystallographic structure which is comprised of one-dimensional pentagons with selective atomic occupation. This study indicates that the phase stability of HEAs is very fragile at intermediate temperatures at which the multicomponent solid solutions stabilized by high configurational entropy may break down by local chemical disturbance at dislocations and grain boundaries.

Persistent Electrochemical Performance in Epitaxial VO2(B).

Discovering high-performance energy storage materials is indispensable for renewable energy, electric vehicle performance, and mobile computing. Owing to the open atomic framework and good room temperature conductivity, bronze-phase vanadium dioxide [VO2(B)] has been regarded as a highly promising electrode material for Li ion batteries. However, previous attempts were unsuccessful to show the desired cycling performance and capacity without chemical modification. Here, we show with epitaxial VO2(B) films that one can accomplish the theoretical limit for capacity with persistent charging-discharging cyclability owing to the high structural stability and unique open pathways for Li ion conduction. Atomic-scale characterization by scanning transmission electron microscopy and density functional theory calculations also reveal that the unique open pathways in VO2(B) provide the most stable sites for Li adsorption and diffusion. Thus, this work ultimately demonstrates that VO2(B) is a highly promising energy storage material and has no intrinsic hindrance in achieving superior cyclability with a very high power and capacity in a Li-ion conductor.

Atomic scale characterization of mismatched graphene layers

In the bourgeoning field of two dimensional layered materials and their atomically thin counterparts, it has been established that the electronic coupling between the layers of the material plays a key role in determining its properties [1,2]. We are just beginning to understand how each material is unique in that respect while working our way up to building new materials with functionalities enabled by interlayer interactions. In this review, we will focus on a system that despite its apparent simplicity possesses a wealth of intriguing physics: layers of graphene with various degree of coupling. The situations discussed here are graphene layers vertically twisted with respect with each other, weakly decoupled electronically and laterally twisted forming grain boundaries. We emphasize experiments that atomically resolve the electronic properties.

Dielectric properties and atomic-scale microstructural characterizations of cubic-pyrochlored ceramics in the system of Bi2O3-MgO-Nb2O5

Single-phase pyrochlore-type ceramic samples of Bi1.5MgNb1.5O7 (BMN) in the system of Bi2O3-MgO-Nb2O5 were synthesized via solid-state reactions, and their microstructures, dielectric properties, and leakage current characteristics were also investigated. X-ray diffraction patterns demonstrate the synthesized BMN samples have cubic phase pyrochlore-type structure, which is also confirmed by selected area electron diffraction (SAED) patterns. The highest density of the BMN samples sintered at different temperatures was 6.511 g/cm3, which was about 98.0% of the theoretical one. More than six Raman vibrational peaks observed in the BMN samples also confirmed the cubic phase pyrochlore-type structure of the synthesized BMN samples. TEM images reveal that the BMN samples have clean grain boundaries and no voids are observed at triple-grain boundary. Both SAED patterns and HRTEM images confirm the single crystalline nature of the BMN ceramic grains, and the superlattice structures (or modulated pyrochlore structures) along the [001] and 〈112〉 directions in the BMN samples. Such long-range ordered pyrochlore structures (superlattice structures) could enhance the polarizations of the electric dipoles formed along the superstructure direction, resulting in high dielectric constants of the BMN samples. The dielectric constants of the BMN samples were measured to be 180–195, which exhibit almost frequency independent characteristic in the frequency range of 102–106 Hz. The dielectric losses of the BMN samples are much stable and smaller than 0.002 below 105 Hz, which is ascribed to the existence of clean grain boundaries since they are benefit to reducing the extrinsic dielectric loss. The leakage current values of the BMN ceramics are in the range of 0.13–0.75 μA/cm2 (measured @ electric field E = 13.3 MV/m). A flat frequency independence of the relative dielectric constant, low dielectric loss, and small leakage current characters, make the BMN dielectrics attractive in the fields of high volume efficient multilayered ceramic capacitors with the capacitance weakly dependent on the frequency.

Three-dimensional atomic models from a single projection using Z-contrast imaging: verification by electron tomography and opportunities.

In order to fully exploit structure-property relations of nanomaterials, three-dimensional (3D) characterization at the atomic scale is often required. In recent years, the resolution of electron tomography has reached the atomic scale. However, such tomography typically requires several projection images demanding substantial electron dose. A newly developed alternative circumvents this by counting the number of atoms across a single projection. These atom counts can be used to create an initial atomic model with which an energy minimization can be applied to obtain a relaxed 3D reconstruction of the nanoparticle. Here, we compare, at the atomic scale, this single projection reconstruction approach with tomography and find an excellent agreement. This new approach allows for the characterization of beam-sensitive materials or where the acquisition of a tilt series is impossible. As an example, the utility is illustrated by the 3D atomic scale characterization of a nanodumbbell on an in situ heating holder of limited tilt range.

Atomic scale characterization of white etching area and its adjacent matrix in a martensitic 100Cr6 bearing steel

Atom probe tomography was employed to characterize the microstructure and C distribution in the white etching area (WEA) of a martensitic 100Cr6 bearing steel subjected to rolling contact fatigue. Different from its surrounding matrix where a plate-like martensitic structure prevails, the WEA exhibits equiaxed grains with a uniform grain size of about 10nm. Significant C grain boundary enrichment (>7.5at.%) and an overall higher C concentration than the nominal value are observed in the WEA. These results suggest that the formation of WEA results from severe local plastic deformation that causes dissolution of carbides and the redistribution of C.

Molten-salt synthesis of BaTiO3 powders and their atomic-scale structural characterization

In this work, BaTiO3 powders were synthesized by molten-salt synthesis method using BaCO3, TiO2, and the eutectic salts (NaCl-KCl) as the raw materials. A systematic study indicating the effects of calcination temperature as well as the molar ratio of NaCl to KCl in the mixed salt with respect to BaCO3 -TiO2 on the microstructures of BaTiO3 powders is reported. The results showed that all the powders crystallized in a tetragonal perovskite structure, as confirmed by X-ray diffraction pattern and Raman spectra. With increasing the calcination temperature, the morphology of the as-prepared BaTiO3 powders developed from rectangular/spherical aggregates to circular or cubic shapes and the average particle size was also increased from 320 nm to 900 nm. With increasing the molar ratio of BaCO3:TiO2: NaCl: KCl, the average particle size was slightly decreased from 440 nm to 360 nm, and the morphology of the as-synthesized BaTiO3 powders transformed from rectangular and cubic shapes to sphere shapes. The chemical compositions of the as-synthesized BaTiO3 powders obtained from energy dispersive X-ray analysis are in agreement with the nominal ratios. The present results provide a generalized methodology to large-scale synthesize other perovskite transitional metal oxide powders with controllable sizes and shapes.

Site-Specific Preparation of Intact Solid-Liquid Interfaces by Label-Free In Situ Localization and Cryo-Focused Ion Beam Lift-Out.

Scanning transmission electron microscopy (STEM) allows atomic scale characterization of solid-solid interfaces, but has seen limited applications to solid-liquid interfaces due to the volatility of liquids in the microscope vacuum. Although cryo-electron microscopy is routinely used to characterize hydrated samples stabilized by rapid freezing, sample thinning is required to access the internal interfaces of thicker specimens. Here, we adapt cryo-focused ion beam (FIB) "lift-out," a technique recently developed for biological specimens, to prepare intact internal solid-liquid interfaces for high-resolution structural and chemical analysis by cryo-STEM. To guide the milling process we introduce a label-free in situ method of localizing subsurface structures in suitable materials by energy dispersive X-ray spectroscopy (EDX). Monte Carlo simulations are performed to evaluate the depth-probing capability of the technique, and show good qualitative agreement with experiment. We also detail procedures to produce homogeneously thin lamellae, which enable nanoscale structural, elemental, and chemical analysis of intact solid-liquid interfaces by analytical cryo-STEM. This work demonstrates the potential of cryo-FIB lift-out and cryo-STEM for understanding physical and chemical processes at solid-liquid interfaces.

Facet-Selective Epitaxial Growth of δ-Bi2O3 on ZnO Nanowires

Integration of ZnO nanowires with Bi2O3 coating layers can modify their physicochemical properties and efficiently improve the performance of the resulting nanostructured composites for specific applications. The interfacial structures often control the properties of such nanocomposite systems. We report a novel approach to achieving selective epitaxial growth of δ-Bi2O3 layers onto the {11–20} nanoscale facets of ZnO nanowires. By detailed atomic scale characterization of the surfaces and interfaces of the Bi2O3/ZnO nanocomposites we proposed that growth of the δ-Bi2O3 species on ZnO {11–20} surfaces follows the Stranski–Krastanov mechanism with an epitaxial relationship of ZnO [10–10] (11–20)∥δ-Bi2O3 [001] (100). An atomic model is proposed to explain the epitaxial relationship, the interfacial atomic structure, and the facet-selective growth.

Local variation in Bi crystal sites of epitaxial GaAsBi studied by photoelectron spectroscopy and first-principles calculations

Epitaxial Bi-containing III–V crystals (III-V1-xBix) have attracted increasing interest due to their potential in infrared applications. Atomic-scale characterization and engineering of bulk-like III-V1-xBix properties (e.g., Bi incorporation and defect formation) are challenging but relevant to develop applications. Toward that target, we report here that the traditional surface-science measurement of photoelectron spectroscopy (PES) is a potential, non-destructive method to be combined in the studies of bulk-like properties, when surface effects are properly removed. We have investigated epitaxial GaAs1-xBix films, capped by epitaxial AlAs layers, with high-resolution photoelectron spectroscopy. The Bi5d core-level spectra of GaAs1-xBix together with ab-initio calculations give direct evidence of variation of Bi bonding environment in the lattice sites. The result agrees with photoluminescence (PL) measurement which shows that the studied GaAs1-xBix films include local areas with higher Bi content, which contribute to PL but do not readily appear in x-ray diffraction (XRD). The measured and calculated Bi core-level shifts show also that Ga vacancies and Bi clusters are dominant defects.

Atomic-scale characterization of subsurface damage and structural changes of single-crystal silicon carbide subjected to electrical discharge machining

Single-crystal silicon carbide (SiC) is an important semiconductor material used in power electronics. Due to its high hardness and brittleness, SiC is very difficult to machine using mechanical methods. Electrical discharge machining (EDM) has recently garnered extensive research interest as a potential machining method for SiC. However, this technique induces severe subsurface damage on the workpiece. To date, mechanisms leading to EDM-induced subsurface damage in SiC have not been clarified. This study aims to investigate the atomic-scale subsurface damage in SiC induced by EDM using Raman spectroscopy and transmission electron microscopy (TEM). In cross-sectional TEM observations, three regions of subsurface damage were identified, namely, the re-solidified layers, heat-affected zones, and microcracks. It was found that SiC decomposed into silicon and carbon in the re-solidified layers, and the degree of decomposition depended on the discharge energy and workpiece polarity. The re-solidified layer was a mixture of crystalline/amorphous silicon, crystalline/amorphous carbon, and nano-crystalline SiC. The presence of an extremely thin graphite layer was observed in the re-solidified layer. The heat-affected zone remained crystalline but showed a different crystal structure distinct from that of the bulk.

Substrate-Induced Nanoscale Undulations of Borophene on Silver.

Two-dimensional (2D) materials tend to be mechanically flexible yet planar, especially when adhered on metal substrates. Here, we show by first-principles calculations that periodic nanoscale one-dimensional undulations can be preferred in borophenes on concertedly reconstructed Ag(111). This "wavy" configuration is more stable than its planar form on flat Ag(111) due to anisotropic high bending flexibility of borophene that is also well described by a continuum model. Atomic-scale ultrahigh vacuum scanning tunneling microscopy characterization of borophene grown on Ag(111) reveals such undulations, which agree with theory in terms of topography, wavelength, Moiré pattern, and prevalence of vacancy defects. Although the lattice is coherent within a borophene island, the undulations nucleated from different sides of the island form a distinctive domain boundary when they are laterally misaligned. This structural model suggests that the transfer of undulated borophene onto an elastomeric substrate would allow for high levels of stretchability and compressibility with potential applications to emerging stretchable and foldable devices.

Connection Between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Single Crystals on Ni-Cr and Ni-Cr-Mo Alloys

Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective-oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electrochemistry are rarely connected with atomic scale ex situ techniques such as scanning tunneling (STM), atomic force (AFM) and transmission electron (TEM) microscopies to yield crucial information on structure, molecular and electronic properties of the oxides [5]. The main goal of this work is to integrate single crystal electrochemical measurements with nanoscale characterization to advance fundamental understanding and eventually predict the roles of Cr and Mo on surface stability in corrosive environments. Such a connection between global electrochemistry and atomic studies is infrequently reported [6].Polycrystalline alloys contain many grain orientations that may affect passivation behavior [7]. This work focuses on examining the oxide growth kinetics on single crystals. It is expected that grain surface facet evolution and oxide thickness will vary according to crystallographic orientation [8, 9]. Model alloys Ni-22Cr and Ni-22Cr-6Mo, wt%, were arc-melted, cold-rolled to achieve ~12% deformation, and then heat treated to allow for re-crystallization and growth of sufficiently large grains to enable electrochemical measurements. A ~4 mm2 area of a sample was first scanned with Electron Backscattered Diffraction (EBSD) to obtain a “catalog” of grain orientations, and then each grain within the EBSD map was probed regarding its electrochemical behavior. The kinetics of the passive state of single grains were determined potentiostatically at +0.2 VSCE in solution, where a previously formed on air oxide was partially reduced at -1.3 VSCE. Electrochemical impedance spectroscopy (EIS) was then collected after various stages of oxide growth and the data was fit assuming a constant phase element to enable calculation of thickness [10]. Oxide charge formation, current density, and thickness for each alloy and grain orientation were studied using Mott-Cabrera and Point Defect models. Ex situ Raman and XPS conducted on polycrystalline samples aided in determining molecular identity and the efficiency of oxide growth versus dissolution. AFM topography images and profiles across single grain surfaces at intermediate stages of potentiostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed on various grains (Figure 1a). Furthermore, the structural and electrochemical properties of oxides grown potentiostatically on specific crystallographic orientations were correlated with the TEM analysis performed on the oxides grown chemically under equivalent conditions using potassium persulfate or hydrogen peroxide oxidants. It was found the EIS, TEM and XPS oxide thicknesses were in agreement with each other. The broader relevancy of this presentation, besides connecting electrochemistry and nano- to mesoscale characterization, is its contribution to concepts beneficial towards development of predictive correlations associated with the passivity of Ni alloys during aqueous exposure. Acknowledgements This project was funded by U.S. ONR (SP0028970-PROJ0007990) and MURI Program under the direction of Dr David Shifler (N00014-14-1-0675).

Physisorbed-precursor-assisted atomic layer deposition of reliable ultrathin dielectric films on inert graphene surfaces for low-power electronics

Among the most fundamental challenges encountered in the successful incorporation of graphene in silicon-based electronics is the conformal growth of ultrathin dielectric films, especially those with thicknesses lower than 5 nm, on chemically inert graphene surfaces. Here, we present physisorbed-precursor-assisted atomic layer deposition (pALD) as an extremely robust method for fabricating such films. Using atomic-scale characterisation, it is confirmed that conformal and intact ultrathin Al2O3 films can be synthesised on graphene by pALD. The mechanism underlying the pALD process is identified through first-principles calculations based on density functional theory. Further, this novel deposition technique is used to fabricate two types of wafer-scale devices. It is found that the incorporation of a 5 nm-thick pALD Al2O3 gate dielectric film improves the performance of metal-oxide-graphene field-effect transistors to a greater extent than does the incorporation of a conventional ALD Al2O3 film. We also employ a 5 nm-thick pALD HfO2 film as a highly scalable dielectric layer with a capacitance equivalent oxide thickness of 1 nm in graphene-based tunnelling field-effect transistors fabricated on a glass wafer and achieve a subthreshold swing of 30 mV/dec. This significant improvement in switching allows for the low-voltage operation of an inverter within 0.5 V of both the drain and the gate voltages, thus paving the way for low-power electronics.

Localized electronic states at grain boundaries on the surface of graphene and graphite

Recent advances in large-scale synthesis of graphene and other 2D materials have underscored the importance of local defects such as dislocations and grain boundaries (GBs), and especially their tendency to alter the electronic properties of the material. Understanding how the polycrystalline morphology affects the electronic properties is crucial for the development of applications such as flexible electronics, energy harvesting devices or sensors. We here report on atomic scale characterization of several GBs and on the structural-dependence of the localized electronic states in their vicinity. Using low temperature scanning tunneling microscopy  and spectroscopy, together with tight binding and ab initio numerical simulations we explore GBs on the surface of graphite and elucidate the interconnection between the local density of states and their atomic structure. We show that the electronic fingerprints of these GBs consist of pronounced resonances which, depending on the relative orientation of the adjacent crystallites, appear either on the electron side of the spectrum or as an electron-hole symmetric doublet close to the charge neutrality point. These two types of spectral features will impact very differently the transport properties allowing, in the asymmetric case to introduce transport anisotropy which could be utilized to design novel growth and fabrication strategies to control device performance.