Aberration-Corrected STEM Imaging Research Articles

Regional consistency between cation displacement and oxygen octahedral tilt in nanodomain of BNKT-SBT relaxor ferroelectrics revealed by atomic-resolution AC-STEM

Since being discovered, relaxor ferroelectrics have attracted continued interest due to their unusual properties. Relaxor behavior is attributed to nanodomains or PNRs. Therefore, nano-scale even atomic-scale structural analysis is crucial to understanding the relaxor properties. However, the atomic-scale nanodomain structure has not been fully understood. For Bi0.5Na0.5TiO3, cation displacement and oxygen octahedral tilt are two common structural distortions. However, the relationship between them has not been proved experimentally. Here, the cation displacement and octahedral tilt were both investigated based on aberration-corrected STEM images. Cation displacement vectors illustrate a vortex structure. Octahedral tilt is regionally consistent with cation displacement that both of them increase gradually from the domain wall to the interior of the nanodomain. The formation of nanodomains may originate from their combined effect, thereby producing relaxor properties. Moreover, oxygen octahedral displacement stemmed from the complex coordinating environments was observed. These findings provide further insights into the nanodomain structure.

Electrical spectroscopy of defect states and their hybridization in monolayer MoS2

Defects in solids are unavoidable and can create complex electronic states that can significantly influence the electrical and optical properties of semiconductors. With the rapid progress in the integration of 2D semiconductors in practical devices, it is imperative to understand and characterize the influence of defects in this class of materials. Here, we examine the electrical response of defect filling and emission using deep level transient spectroscopy (DLTS) and reveal defect states and their hybridization in a monolayer MOCVD-grown material deposited on CMOS-compatible substrates. Supported by aberration-corrected STEM imaging and theoretical calculations, we find that neighboring sulfur vacancy pairs introduce additional shallow trap states via hybridization of individual vacancy levels. Even though such vacancy pairs only represent ~10% of the total defect concentration, they can have a substantial influence on the off currents and switching slopes of field-effect transistors based on 2D semiconductors. Our technique, which can quantify the energy states of different defects and their interactions, allows rapid and nondestructive electrical characterization of defect states important for the defect engineering of 2D semiconductors.

Robust 2 nm-sized gold nanoclusters on Co3O4 for CO oxidation.

In this study, gold nanoparticles were dispersed on Co3O4 nanoplates, forming a specific Au-Co3O4 interface. Upon calcination at 300 °C in air, aberration-corrected STEM images evidenced that the gold nanoclusters (NCs) on Co3O4{111} were maintained at ca. 2.2 nm, which is similar to the size of the parent Au colloidal particles, demonstrating the stronger metal-support interaction (SMSI) on Co3O4{111}. Au/Co3O4{111} showed good catalytic activity (a full CO conversion achieved at 80 °C) and durability (over 10 hours) in CO oxidation, which was mainly due to the promotion by the surface oxygen vacancies and intrinsic defects of Co3O4{111} for activating O2 and by Au0, Auδ+, and Au+ species on the surface of gold NCs for CO activation, as evidenced by Raman and Fourier-transform infrared (FT-IR) spectroscopy analysis. Au/Co3O4 catalyzed CO oxidation obeyed the Langmuir-Hinshelwood mechanism at low temperatures.

Aberration-corrected STEM imaging of 2D materials: Artifacts and practical applications of threefold astigmatism.

High-resolution scanning transmission electron microscopy (HR-STEM) with spherical aberration correction enables researchers to peer into two-dimensional (2D) materials and correlate the material properties with those of single atoms. The maximum intensity of corrected electron beam is confined in the area having sub-angstrom size. Meanwhile, the residual threefold astigmatism of the electron probe implies a triangular shape distribution of the intensity, whereas its tails overlap and thus interact with several atomic species simultaneously. The result is the resonant modulation of contrast that interferes the determination of phase transition of 2D materials. Here, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast and its unintended impact on violating the power-law dependence of contrast on coordination modes between transition metal and chalcogenide atoms. The finding illuminates the correlation between atomic contrast, spatially inequivalent chalcogenide orientation, and residual threefold astigmatism on determining the atomic structure of emerging 2D materials.

Pushing the limits of electrostatic adsorption: charge enhanced dry impregnation of SBA-15

Abstract Charge enhanced dry impregnation (CEDI) is a method to synthesize supported metal nanoparticles which combines the simplicity of incipient wetness impregnation with the small particle size obtained from electrostatic adsorption of metal precursors onto the oxide support. We have explored the utility of CEDI by applying it to a difficult to impregnate support – a largely one dimensional porous SBA-15 silica. Monometallic (Pt, Pd, Co, Ni, and Cu) catalysts at multiple metal loadings (1–20 wt%) as well as their bimetallic pairs were supported on mesoporous SBA-15 and characterized by high sensitivity powder XRD and in select formulations, with aberration-corrected z-contrast STEM imaging. CEDI applied to SBA-15 without washing is more effective than dry impregnation (DI) of SBA-15. Compared to amorphous silica, CEDI of SBA-15 without washing gives larger particles at lower metal loadings, while at higher metal loadings, the pores of the SBA-15 help render the particle size smaller than amorphous silica. CEDI without washing of SBA-15 with nitrate salts gives somewhat smaller particles than CEDI with chloride salts. CEDI with washing to remove residual counterions of the metal salt typically gives nanoparticles smaller than 2 nm. The smallest particles are always obtained when the counterion is removed by washing, or when a hydroxide salt is employed; ultrasmall particles are obtained without washing in this case.

Catalytic decomposition of N2O on supported Rh catalysts

Abstract Numerous Rh catalysts were evaluated for N2O decomposition for automotive applications. Some Rh-containing spinel materials exhibit excellent fresh activities in the absence of H2O but become inactive after hydrothermal aging or when tested in a wet feed. Rh catalysts supported on zeolites can be very active in a dry feed even after aging but are extremely sensitive to H2O. Rh/CeO2 is an exceptional catalyst for this reaction in the presence of both H2O and O2. Hydrothermal aging (750 °C/20 h) significantly increases its activity. A similar activity enhancement was found by calcining the support before Rh impregnation. XPS results show a surface enrichment of Rh species on the aged Rh/CeO2 catalyst relative to the fresh catalyst. Aberration corrected STEM images reveal that Rh is buried in the bulk on the fresh catalyst and pulled out onto the surface of the support after thermal treatments. All catalysts are inhibited by H2O with the zeolite-based Rh catalysts being the worst. The aged Rh/CeO2 catalyst is less sensitive to H2O relative to others. DRIFTS data show that H2O sensitivity is related to catalyst hydrophilicity; a high coverage of OH groups on a catalyst reduces its N2O decomposition activity. H2-TPR results show that a Rh/CeO2 catalyst can be readily reduced at

Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO3 Thin Films.

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO3 (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO3)0.29(SrTa1/2Al1/2O3)0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c1 domains and [001̅] for c2 domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

Aberration measurement of the probe-forming system of an electron microscope using two-dimensional materials

The geometric and chromatic aberration coefficients of the probe-forming system in an aberration corrected transmission electron microscope have been measured using a Ronchigram recorded from monolayer graphene. The geometric deformations within individual local angular sub-regions of the Ronchigram were analysed using an auto-correlation function and the aberration coefficients for the probe forming lens were calculated. This approach only requires the acquisition of a single Ronchigram allowing rapid measurement of the aberration coefficients. Moreover, the measurement precision for defocus and two-fold astigmatism is improved over that which can be achieved from analysis of Ronchigrams recorded from amorphous films. This technique can also be applied to aberration corrected STEM imaging of any hexagonal two-dimensional material.

Aberration-corrected STEM Imaging and EELS Mapping of BaTiO3/SrTiO3 Interfacial Defects

Aberration-corrected STEM Imaging and EELS Mapping of BaTiO₃/SrTiO₃ Interfacial Defects

Integrative structural and advanced imaging characterization of manganese oxide nanotubes doped with cobaltite

Manganese oxide nanotubes (MnO2) were efficiently produced through a hydrothermal method, using SiO2 powder as nucleation points, then doped with cobaltite (Co3O4) nanoparticles uniformly deposited along the surface of the MnO2 nanotubes. An integrative approach using advanced analytical electron microscopy techniques (UHR FE-SEM, HR-TEM, and BF/HAADF-STEM, coupled with EDX) in combination with spectroscopy allowed the determination of the structural characteristics of this composite nanomaterial. Advanced imaging clearly revealed the tubular structure of the MnO2 nanotubes (diameter of 30–80 nm and length of 3–5 μm) and the arrangement of the discrete Co3O4 deposits (10–40 nm). Remarkably, high-resolution and spherical aberration-corrected STEM imaging allowed for the determination of the crystalline arrangement of the nanomaterials, particularly at the interface between MnO2 and Co3O4 particles with high spatial sub-Angstrom resolution, revealing the distribution and high structural consistency of the novel composite materials produced. Furthermore, X-ray diffraction and Raman spectroscopy confirmed that MnO2 corresponded to the crystallographic phase cryptomelane (K2-xMn8O16), while the dopant cobalt nanoparticles adopted a cobaltite (Co3O4) phase. We demonstrated the catalytic properties of the composite MnO2–Co3O4 nanotubes as an electrocatalyst material for oxygen evolution, where it showed superior behaviour, with a significantly higher catalytic activity (6.8 times) than pure MnO2 in the OER region.

Enhancement of the Hydrogen Evolution Reaction from Ni-MoS2 Hybrid Nanoclusters.

This report focuses on a novel strategy for the preparation of transition metal–MoS2 hybrid nanoclusters based on a one-step, dual-target magnetron sputtering, and gas condensation process demonstrated for Ni-MoS2. Aberration-corrected STEM images coupled with EDX analysis confirms the presence of Ni and MoS2 in the hybrid nanoclusters (average diameter = 5.0 nm, Mo:S ratio = 1:1.8 ± 0.1). The Ni-MoS2 nanoclusters display a 100 mV shift in the hydrogen evolution reaction (HER) onset potential and an almost 3-fold increase in exchange current density compared with the undoped MoS2 nanoclusters, the latter effect in agreement with reported DFT calculations. This activity is only reached after air exposure of the Ni-MoS2 hybrid nanoclusters, suggested by XPS measurements to originate from a Ni dopant atoms oxidation state conversion from metallic to 2+ characteristic of the NiO species active to the HER. Anodic stripping voltammetry (ASV) experiments on the Ni-MoS2 hybrid nanoclusters confirm the presence of Ni-doped edge sites and reveal distinctive electrochemical features associated with both doped Mo-edge and doped S-edge sites which correlate with both their thermodynamic stability and relative abundance.

Aberration Corrected STEM Imaging of Domain Walls in Congruent LiNbO 3

Aberration Corrected STEM Imaging of Domain Walls in Congruent LiNbO 3

Maximising Information from Aberration-Corrected STEM images: Applications to Plasmonic, Semiconductor and Battery Materials

Maximising Information from Aberration-Corrected STEM images: Applications to Plasmonic, Semiconductor and Battery Materials

The rational synthesis of Pt-Pd bimetallic catalysts by electrostatic adsorption

To improve the often poor metal dispersion and poor metal1–metal2 contact of bimetallic catalysts prepared by dry impregnation (incipient wetness), we demonstrate several ways to employ strong electrostatic adsorption using Pt and Pd over silica, alumina, carbon, and oxidized carbon. Metal uptake versus pH was monitored for single metal solutions and mixtures of metal precursors. Catalysts were characterized by powder XRD, aberration-corrected STEM imaging, and EDXS line scans and mapping of individual nanoparticles.Electrostatic adsorption of mixed metal precursors (co-SEA) results in well dispersed, homogeneously alloyed nanoparticles, while sequential SEA gives well dispersed nanoparticles with core–shell morphologies. Shell metal loading can be increased by additional cycles of SEA, which gives some flexibility at catalyst composition in exchange for more synthesis steps. Dry impregnation yields larger, agglomerated, inhomogeneously alloyed particles.

Ge1-ySny (y = 0.01-0.10) alloys on Ge-buffered Si: Synthesis, microstructure, and optical properties

Novel hydride chemistries are employed to deposit light-emitting Ge1-ySny alloys with y ≤ 0.1 by Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) on Ge-buffered Si wafers. The properties of the resultant materials are systematically compared with similar alloys grown directly on Si wafers. The fundamental difference between the two systems is a fivefold (and higher) decrease in lattice mismatch between film and virtual substrate, allowing direct integration of bulk-like crystals with planar surfaces and relatively low dislocation densities. For y ≤ 0.06, the CVD precursors used were digermane Ge2H6 and deuterated stannane SnD4. For y ≥ 0.06, the Ge precursor was changed to trigermane Ge3H8, whose higher reactivity enabled the fabrication of supersaturated samples with the target film parameters. In all cases, the Ge wafers were produced using tetragermane Ge4H10 as the Ge source. The photoluminescence intensity from Ge1−ySny/Ge films is expected to increase relative to Ge1−ySny/Si due to the less defected interface with the virtual substrate. However, while Ge1−ySny/Si films are largely relaxed, a significant amount of compressive strain may be present in the Ge1−ySny/Ge case. This compressive strain can reduce the emission intensity by increasing the separation between the direct and indirect edges. In this context, it is shown here that the proposed CVD approach to Ge1−ySny/Ge makes it possible to approach film thicknesses of about 1 μm, for which the strain is mostly relaxed and the photoluminescence intensity increases by one order of magnitude relative to Ge1−ySny/Si films. The observed strain relaxation is shown to be consistent with predictions from strain-relaxation models first developed for the Si1−xGex/Si system. The defect structure and atomic distributions in the films are studied in detail using advanced electron-microscopy techniques, including aberration corrected STEM imaging and EELS mapping of the average diamond–cubic lattice.

Study of Au/Pd and Au/Co Bimetallic Nanoparticles Using Aberration Corrected STEM

The synergetic combination of two metals in bimetallic (BM) nanoparticle (NP) significantly changes the optical, catalytic, electronic and magnetic properties. The possible reason for synergetic effects is the heterometallic bond between two metals, dilution of active site, the combination of two kinds of metals and their fine structures. The enhanced catalytic properties of BM nanoparticles are widely used in several industrial reactions. For practical applications and better understanding of this system, there should be well-controlled synthesis of shape and size selected structures, powerful structural characterization tools including advanced in situ atomic-level analytical techniques, modern modeling tools with supercomputers, and in-depth understanding of structure-property relationship. In this paper, we are presenting the use of Z-contrast imaging in aberration-corrected HAADF STEM images in the study of atomic ordering in nanoalloys and core/shell and also investigating the growth mechanism in Au-Pd and the interface region between Au and Pd.

Thickness Variations and Absence of Lateral Compositional Fluctuations in Aberration-Corrected STEM Images of InGaN LED Active Regions at Low Dose

Aberration-corrected scanning transmission electron microscopy images of the In(0.15)Ga(0.85)N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer's upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.

Inside Cover: Revealing the Atomic Structure of Intermetallic GaPd2 Nanocatalysts by using Aberration‐Corrected Scanning Transmission Electron Microscopy (ChemCatChem 9/2013)

Up close and personal with nanocatalysts The cover picture shows the generation of a false-color image of GaPd2. In their Full Paper on p. 2599 ff., R. Leary et al. describe the atomic structure of GaPd2 nanocatalysts by using aberration-corrected STEM (AC-STEM) and annular dark-field imaging. AC-STEM provided direct insight into the distinct structure and chemical ordering of the intermetallic phase GaPd2 in nanosized particles. It also revealed deviations from the ideal “bulk” structure. The study highlights the importance of considering nanoparticle morphologies and nanocrystalline defects.

Pt/g--Al2O3 Reduction and Cluster Evolution Characterized by Aberration-Corrected STEM Imaging and EXAFS

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

Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process

Nanocrystalline LiFePO 4 powders synthesized by a microwave-assisted solvothermal (MW-ST) process have been structurally characterized with a combination of high resolution powder neutron diffraction, synchrotron X-ray diffraction, and aberration-corrected HAADF STEM imaging. A significant level of defects has been found in the samples prepared at 255 and 275 °C. These temperatures are significantly higher than what has previously been suggested to be the maximum temperature for defect formation in LiFePO 4 , so the presence of defects is likely related to the rapid MW-ST synthesis involving a short reaction time (∼5 min). A defect model has been tentatively proposed, though it has been shown that powder diffraction data alone cannot conclusively determine the precise defect distribution in LiFePO 4 samples. The model is consistent with other literature reports on nanopowders synthesized at low temperatures, in which the unit cell volume is significantly reduced relative to defect-free, micron-sized LiFePO 4 powders. Temperature-dependent antisite defect formation has been observed in nanocrystalline LiFePO 4 powders synthesized by a microwave solvothermal process, using high resolution diffraction and STEM imaging. • LiFePO 4 nanopowders synthesized by a microwave-assisted solvothermal process. • Defects directly observed by aberration-corrected HAADF STEM imaging. • Antisite defects present from synthesis at 255 and 275 °C. • Defects present from higher temperature synthesis than previously reported. • Powder diffraction data have been analyzed in detail for various defect models.