Atomic Resolution Z-contrast Imaging Research Articles

In-plane and vertical heterostructures from 1T′/2H transition-metal dichalcogenides

Abstract An avalanche of research has been carried out on two-dimensional (2D) transition metal dichalcogenides (TMDs) due to their potential applications in advanced electronics and flexible devices. To take full use of the emerging 2D TMDs materials, their in-plane/vertical heterostructures have been explored, enabling effective tuning of their physical and chemical properties. However, structural differences between the various phases impede the formation of functional heterostructures. Therefore, robust synthesis strategies for heterostructures with different phases have been explored in this study. A chemical vapor deposition process has been proposed in which the key parameters like reaction sources and deposition sites have been carefully adjusted, trying to achieve simultaneous synthesis of 1T′/2H in-plane and vertical heterostructures. Consequently, 2D in-plane RexMo1−xS2/MoS2 and vertical ReS2/MoS2 heterostructures have been produced in different regions at the same time. Atomic-resolution Z-contrast images reveal the detailed atomic structure of the 1T′/2H interfaces. The lateral interface is found to contain Mo atoms with only 5-fold coordination with S due to the phase mismatch. This work demonstrates a route to exploit heterostructures of different phases and opens the possibility to build more complicated 2D heterostructures using CVD.

Nature and Distribution of Cu and Pd Species in CuPd/TiO2-Na Bimetallic Catalysts for Glycerol Hydrodeoxygenation.

We demonstrated recently that CuPd/TiO2–Na bimetallic catalysts synthesized by sequential wet impregnation are active, selective, and stable for the hydrodeoxygenation (HDO) of glycerol into propylene glycol at low H2 pressure. The present study reports on the nature and distribution of Cu and Pd surface species in CuPd/TiO2–Na bimetallic catalysts using different scanning transmission electron microscopy techniques that supply cluster-specific alloying details. In particular, we used atomic-resolution Z-contrast imaging, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. We also include X-ray photoelectron spectroscopy results. Our analysis shows that the metallic nanoparticles adopt mainly five different structures according to how the Cu and Pd atoms coordinate among themselves: a homogeneous CuPd alloy structure (45–61%), a Cu shell/CuPd core (15–23%), a smaller number of particles formed by Cu on the surface and Pd in the nucleus (10–17%), and there are also nanoparticles formed only by Pd (4–7%) or by Cu (8–13%). We determined that there is a inhomogeneous distribution of Cu and Pd in the bimetallic nanoparticles, with Cu being predominant on the surface (between 76 and 90% of the total area analyzed for each particle). Most bimetallic nanoparticles have sizes below 6 nm, the Pd monometallic nanoparticles are in the 2–4 nm range, whereas the monometallic Cu nanoparticles are larger than 8 nm. Bimetallic nanoparticles with sizes smaller than 6–7 nm are fundamentally made up of Cu0–Pd0 and Cu1+–Pd0. The nanoparticles with sizes greater than 7 nm consist of Cu2+ and Cu2+–Pd2+. Our obtained results also help describe reports about the activation of HDO by Pd–Cu in the absence of H2, an effect apparently not observed with other bimetallic systems.

The effect of nano-twins on the thermoelectric properties of Ga2O3(ZnO)m (m = 9, 11, 13 and 15) homologous compounds

Ga2O3(ZnO)m (m = integer) homologous compounds are naturally occurring nanostructured materials. Their intrinsically low thermal conductivity makes them attractive for thermoelectric applications. High density Ga2O3(ZnO)m (m = 9, 11, 13, and 15) single phase ceramics were prepared by solid-state reaction. Nano-sized, twin-like V-shaped boundaries parallel to b-axis (apex angle ∼ 60°) were observed for all compositions. Atomic resolution Z-contrast imaging and EDS analysis for m = 15 showed segregation of Ga ions at the interface of V-shaped twin boundaries. Thermal and charge transport properties depend on the value of m. Compositions with m = 9 exhibited very low lattice thermal conductivity of 2 to 1.5 W/m.K at 300 K–900 K; compositions with m=15 showed improved power factor of 140 μW/m. K2 at 900 K leading to a thermoelectric figure of merit (ZT value) of 0.055. This study explores the structural variants and routes to improve the thermoelectric properties of these materials

A decagonal quasicrystal with rhombic and hexagonal tiles decorated with icosahedral structural units.

The structure of a decagonal quasicrystal in the Zn58Mg40Y2 (at.%) alloy was studied using electron diffraction and atomic resolution Z-contrast imaging techniques. This stable Frank-Kasper Zn-Mg-Y decagonal quasicrystal has an atomic structure which can be modeled with a rhombic/hexagonal tiling decorated with icosahedral units at each vertex. No perfect decagonal clusters were observed in the Zn-Mg-Y decagonal quasicrystal, which differs from the Zn-Mg-Dy decagonal crystal with the same space group P10/mmm. Y atoms occupy the center of 'dented decagon' motifs consisting of three fat rhombic and two flattened hexagonal tiles. About 75% of fat rhombic tiles are arranged in groups of five forming star motifs, while the others connect with each other in a 'zigzag' configuration. This decagonal quasicrystal has a composition of Zn68.3Mg29.1Y2.6 (at.%) with a valence electron concentration (e/a) of about 2.03, which is in accord with the Hume-Rothery criterion for the formation of the Zn-based quasicrystal phase (e/a = 2.0-2.15).

Ab initio determination of atomic structure of Zn-Zr precipitates in a Mg-Nd-Zn-Zr alloy.

The atomic structure of nanometre-sized Zn-Zr precipitates in a Mg alloy is determined by combining tilt series of micro-beam electron diffraction with atomic resolution Z-contrast imaging. The stoichiometry of the Zn-Zr precipitates is Zn2Zr3 with a primitive tetragonal structure (space group P42/mnm, a = b = 0.761 nm, c = 0.682 nm). There are 20 atoms in the unit cell of tetragonal Zn2Zr3, comprising 12 Zr atoms at the 4d, 4f, 4g positions and eight Zn atoms at the 8j positions.

Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3: Experimental and Computational Evidence.

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1-xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.

Direct observation of incommensurate structure in Mo3Si.

Z-contrast imaging, electron diffraction, atom-probe tomography (APT) and density functional theory calculations were used to study the crystal structure of the Mo3Si phase which was previously reported to have an A15 crystal structure. The results showed that Mo3Si has an incommensurate crystal structure with a non-cubic unit cell. The small off-stoichiometry in composition of the sample which was revealed by APT and atomic resolution Z-contrast imaging suggested that site substitution caused the development of split atomic positions, disorder and vacancies.

(Invited) Transistor Strain Measurement Using Electron Beam Techniques

Strain and the related stress in nanoelectronic devices play a critical role in semiconductor technologies. In general, strain arises from the lattice mismatch between dissimilar materials and thus strain is critical for the design and growth of epitaxial heterostructures in micro- and nano-electronics. In transistors, strained silicon channels provide a major boost to the performance of metal-oxide-semiconductor field-effect transistors (MOSFETs), e.g., the use of epitaxial strain of Si and Ge alloys in PMOS [1]. In the near term, the transistor technology is pushing towards the sub 10 nm technology node. At this length scale, resolving strain in the devices requires a resolution of sub nanometer with not only high resolution, also high accuracy and reliability. The use of novel architectures and alternative materials in the transistor technology also demands metrology capable of handling these structural and material changes. High resolution strain measurement can be performed using high energy electrons either in reciprocal space using convergent beam electron diffraction (CBED), scanning electron nanodiffraction (SEND), nanobeam electron diffraction (NBED) [2], or scanning precession electron diffraction (SPED) or in real space through direct imaging using high resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) [3] or dark-field electron holography (DFEH)[4]. The resolution and strain measurement sensitivities range from Å to several nm and 10-3 to 10-4respectively. Recent developments in aberration corrected STEM especially have improved the spatial resolution to sub- Å and the precision of measuring atomic column positions to picometers [3]. This talk will review high resolution and high sensitivity electron beam techniques for strain characterization in nanodevices and device materials. Examples include planar and a finfet transistors and III-V heterostructures. For finfet transistors, we demonstrate that NBED provides an effective approach for strain mapping (See figure below) and some of challenges will also be highlighted. In III-V heterostructures, we will describe atomic resolution Z-contrast imaging based methods for high resolution interfacial strain analysis and its applications for the investigation of interfacial intermixing and atomic layer-by-layer strain profiles in InAs/GaSb and InAs/InAsSb superlattices, targeted for middle- and long-wavelength IR detection.

Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures.

The aberration-corrected scanning transmission electron microscope (STEM) has emerged as a key tool for atomic resolution characterization of materials, allowing the use of imaging modes such as Z-contrast and spectroscopic mapping. The STEM has not been regarded as optimal for the phase-contrast imaging necessary for efficient imaging of light materials. Here, recent developments in fast electron detectors and data processing capability is shown to enable electron ptychography, to extend the capability of the STEM by allowing quantitative phase images to be formed simultaneously with incoherent signals. We demonstrate this capability as a practical tool for imaging complex structures containing light and heavy elements, and use it to solve the structure of a beam-sensitive carbon nanostructure. The contrast of the phase image contrast is maximized through the post-acquisition correction of lens aberrations. The compensation of defocus aberrations is also used for the measurement of three-dimensional sample information through post-acquisition optical sectioning.

Facet-dependent disorder in pristine high-voltage lithium-manganese-rich cathode material.

Defects and surface reconstructions are thought to be crucial for the long-term stability of high-voltage lithium-manganese-rich cathodes. Unfortunately, many of these defects arise only after electrochemical cycling which occurs under harsh conditions, making it difficult to fully comprehend the role they play in degrading material performance. Recently, it has been observed that defects are present even in the pristine material. This study, therefore, focuses on examining the nature of the disorder observed in pristine Li1.2Ni0.175Mn0.525Co0.1O2 (LNMCO) particles. Using atomic-resolution Z-contrast imaging and electron energy loss spectroscopy measurements, we show that there is indeed a significant amount of antisite defects present in this material, with transition metals substituting on Li metal sites. Furthermore, we find a strong segregation tendency of these types of defects toward open facets (surfaces perpendicular to the layered arrangement of atoms) rather than closed facets (surfaces parallel to the layered arrangement of atoms). First-principles calculations identify antisite defect pairs of Ni swapping with Li ions as the predominant defect in the material. Furthermore, energetically favorable swapping of Ni on the Mn sites was observed to lead to Mn depletion at open facets. Relatively, low Ni migration barriers also support the notion that Ni is the predominant cause of disorder. These insights suggest that certain facets of the LNMCO particles may be more useful for inhibiting surface reconstruction and improving the stability of these materials through careful consideration of the exposed surface.

Vacancy-mediated ω-assisted α-phase formation mechanism in titanium–molybdenum alloy

This study investigates the mechanism of the ω-to-α phase transformation in a titanium–molybdenum alloy. Using aberration-corrected high annular angle dark-field scanning transmission electron microscopy and density functional theory (DFT), the authors demonstrate that the ω-to-α phase transformation is mediated by vacancy ordering. Atomic resolution Z-contrast images display high- and low-contrast regions in a single ω precipitate, and it was found that the low Z-contrast regions are composed of an unknown structure, which differs significantly from the ω-phase crystal structure. DFT calculations show that the vacancies could be stabilized by Mo and should preferably be assembled with ordering. The relaxed ω-phase structure with ordered defects explains the intricate atomic images in the low Z-contrast regions of the ω phase. It was found that this distorted ω phase with defects is similar to the hexagonal α-phase structure with vacancies. It was confirmed that these vacancies could easily be occupied by Ti atoms and could form a perfect α phase.

Roughness of the SiC/SiO2 vicinal interface and atomic structure of the transition layers

The SiC/SiO2 interface is generally considered to be the cause for the reduced electron mobility of SiC power devices. Previous studies have shown a correlation between the mobility and the transition layer width at the SiC/SiO2 interface. The authors investigated this interface with atomic resolution Z-contrast imaging and electron energy-loss spectroscopy, and discovered that this transition region was due to the roughness of the vicinal interface. The roughness of a vicinal interface consisted of atomic steps and facets deviating from the ideal off-axis cut plane. The authors conclude that this roughness is limiting the mobility in the channels of SiC MOSFETs.

Interaction of nanometer-sized gold nanocrystals with rutile (110) surface steps revealed at atomic resolution

We show that progress in atomic resolution Z-contrast imaging now enables a detailed understanding of nanocrystal (NC) interaction with surface steps. The interaction is measured based on the shape, orientation, strain and interfacial energy of Au NCs on surface steps of rutile (TiO2) (110) surfaces with a small miscut. Au NCs with the approximate Au(111)[−110] || TiO2(110)[001] epitaxial relationship are selected for study. Results show that the presence of surface steps induces a small rotation in the NCs in an amount less than the surface miscut angle. Atomic displacements measured from atomic resolution images show significant strain near the surface steps inside Au NCs. The shape of Au NCs on low miscut surfaces, however, is approximately similar. From this, an analysis of the average NC shape is performed using the modified Wulff–Kaishew theorem. Compared to Au NCs on flat surfaces, the measurements show a large height/width ratio, lowered interfacial energy and increased triple line energy for NCs observed on surface steps.

Lattice and strain analysis of atomic resolution Z-contrast images based on template matching

A real space approach is developed based on template matching for quantitative lattice analysis using atomic resolution Z-contrast images. The method, called TeMA, uses the template of an atomic column, or a group of atomic columns, to transform the image into a lattice of correlation peaks. This is helped by using a local intensity adjusted correlation and by the design of templates. Lattice analysis is performed on the correlation peaks. A reference lattice is used to correct for scan noise and scan distortions in the recorded images. Using these methods, we demonstrate that a precision of few picometers is achievable in lattice measurement using aberration corrected Z-contrast images. For application, we apply the methods to strain analysis of a molecular beam epitaxy (MBE) grown LaMnO₃ and SrMnO₃ superlattice. The results show alternating epitaxial strain inside the superlattice and its variations across interfaces at the spatial resolution of a single perovskite unit cell. Our methods are general, model free and provide high spatial resolution for lattice analysis.

Measuring phase transitions in La1-xSrxCoO3 using in-situ atomic-resolution Z-contrast imaging and EELS

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Antisite defects in La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3

Complex oxide thin films and superlattices with the perovskite ABO3 structure have been found to possess multifunctional properties. Here, we present our discovery of antisite defects, La ions in Fe(Mn) sites (denoted as LaB), in a La0.7Sr0.3MnO3/La0.7Sr0.3FeO3 superlattice. The antisite defect was directly characterized by atomic resolution Z-contrast imaging and the composition and electronic structure were analyzed by electron energy loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Density functional theory was used to calculate the formation energy, showing that the formation of the detected antisite defects is a consequence of the slightly reducing conditions during sample growth.

General schema for [0 0 1] tilt grain boundaries in dense packing cubic crystals

Atomic resolution Z-contrast images from a series of CeO2 [001] tilt grain boundaries at coincident site lattice (CSL) or near-CSL misorientations can all be explained within a structural unit model. These structural units (which cover all boundaries from 0° to 90°) show striking similarities to comparable CSL boundaries observed in cubic crystal structures that are also derived from dense packing (face centered cubic metal; rocksalt, perovskite, etc.). A general model for the structure of grain boundaries in such similarly structured materials systems has been developed that is based on the crystallography of the parent structures. Changes away from these predicted grain boundary symmetries can be interpreted as showing the frustration of symmetry caused by the incorporation of point defects (vacancies and impurities). This general model for grain boundary structures can, in principle, provide a means to infer the structure–property relationships in broad classes of materials.

Measuring spin-state and ferromagnetic transitions in La1-xSrxCoO3 using in-situ atomic-resolution Z-contrast imaging and EELS in

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Seeing the atoms more clearly: STEM imaging from the Crewe era to today

This review covers the development of scanning transmission electron microscopy from the innovations of Albert Crewe to the two-dimensional spectrum imaging in the era of aberration correction. It traces the key events along the path, the first atomic resolution Z-contrast imaging of individual atoms, the realization of incoherent imaging in crystals and the role of dynamical diffraction, simultaneous, atomic resolution electron energy loss spectroscopy, and finally the tremendous impact of the successful correction of lens aberrations, not just in terms of resolution but also in single atom sensitivity.

Polytypes of long-period stacking structures synchronized with chemical order in a dilute Mg–Zn–Y alloy

A series of structural polytypes formed in an Mg–1 at.%Zn–2 at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hexagonal-close-packed Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB′C′A, where B′ and C′ layers are commonly enriched by Zn/Y atoms.