Aberration-corrected High-resolution Transmission Electron Microscopy Research Articles

Ordered Porous Pd Octahedra Covered with Monolayer Ru Atoms.

Monolayer Ru atoms covered highly ordered porous Pd octahedra have been synthesized via the underpotential deposition and thermodynamic control. Shape evolution from concave nanocube to octahedron with six hollow cavities was observed. Using aberration-corrected high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, we provide quantitative evidence to prove that only a monolayer of Ru atoms was deposited on the surface of porous Pd octahedra. The as-prepared monolayer Ru atoms covered Pd nanostructures exhibited excellent catalytic property in terms of semihydrogenation of alkynes.

Strain-induced preferential dissolution at the dislocation emergences in MnS: an atomic scale study

The long-standing problem of dislocation-preferential dissolution in a crystal has been generally ascribed to the distortion energy stored in the vicinity of the dislocation core. However, due to lack of experimental means, the relationship between the local distortion state and the electrochemical behaviour of a single dislocation has not been established so far. via in situ ex-environment transmission electron microscopy (TEM), we demonstrate that the emergences of both edge and screw dislocations on MnS surfaces are the preferential sites for dissolution of the MnS inclusions within a stainless steel. In addition, we map the strain-induced variation of the standard electrode potential around the edge dislocation by a combination of the aberration-corrected high-resolution TEM and strain-analysis-based mechanochemistry theory. Significantly, our report provides a new approach to investigate the strain–corrosion correlation at an atomic scale.

Surface determination through atomically resolved secondary-electron imaging.

Unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we report a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 × 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO5 units. Dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.

Local octahedral rotations and octahedral connectivity in epitaxially strained LaNiO3/LaGaO3 superlattices

For ABO3 perovskites, octahedral rotations and distortions couple strongly to the functional properties. However, in short period perovskite superlattices, the characterization of the octahedral behavior remains challenging due to the local structural variations of the BO6 octahedra. By aberration-corrected high-resolution transmission electron microscopy, we investigated the local octahedral rotations in a [(4 unit cell (u.c.)//4 u.c.) × 8] LaNiO3/LaGaO3 superlattice grown on a (001) SrTiO3 substrate. The octahedral behavior varies along the growth direction even though the superlattice is coherently strained. Near the substrate, octahedral rotations about [100] and [010] axes in the superlattice are suppressed due to the octahedral connectivity—rotational magnitudes and patterns—between the NiO6 and TiO6 octahedra. Away from the substrate, the magnitudes of [100] and [010] rotations are enhanced as a response to substrate-induced tensile strain. Near the surface of the superlattice, the [100] and [010] rotational magnitudes of NiO6 and GaO6 relax to the bulk values of LaNiO3 and LaGaO3, respectively. Our results indicate that the response of octahedral rotations to epitaxial strain in superlattices is significantly different from that in thin films.

Investigation of disclinations in Marks decahedral Pd nanoparticles by aberration-corrected HRTEM

Temperature plays a vital role in the nucleation and growth of nanocrystals in the hydrothermal method, but the heating methods would cause some side reactions during the temperature rising period. Here, we modify the synthesis process of Marks decahedral Pd nanoparticles and do a detailed analysis of the internal structure of a Marks decahedral Pd Nanoparticle using aberration-corrected high resolution transmission electron microscopy (HRTEM) and the geometric phase analysis (GPA). The yield rate of Marks decahedral Pd nanoparticles increases to more than 60%. Our measurements confirm the existence of disclinations, which is consistent with the results given by the inhomogeneous strain model.

Bottom-up formation of robust gold carbide.

A new phenomenon of structural reorganization is discovered and characterized for a gold-carbon system by in-situ atomic-resolution imaging at temperatures up to 1300 K. Here, a graphene sheet serves in three ways, as a quasi transparent substrate for aberration-corrected high-resolution transmission electron microscopy, as an in-situ heater, and as carbon supplier. The sheet has been decorated with gold nanoislands beforehand. During electron irradiation at 80 kV and at elevated temperatures, the accumulation of gold atoms has been observed on defective graphene sites or edges as well as at the facets of gold nanocrystals. Both resulted in clustering, forming unusual crystalline structures. Their lattice parameters and surface termination differ significantly from standard gold nanocrystals. The experimental data, supported by electron energy loss spectroscopy and density-functional theory calculations, suggests that isolated gold and carbon atoms form – under conditions of heat and electron irradiation – a novel type of compound crystal, Au-C in zincblende structure. The novel material is metastable, but surprisingly robust, even under annealing condition.

Atomic scale microstructure and properties of Se-deficient two-dimensional MoSe2.

We study the atomic scale microstructure of nonstoichiometric two-dimensional (2D) transition metal dichalcogenide MoSe2-x by employing aberration-corrected high-resolution transmission electron microscopy. We show that a Se-deficit in single layers of MoSe2 grown by molecular beam epitaxy gives rise to a dense network of mirror-twin-boundaries (MTBs) decorating the 2D-grains. With the use of density functional theory calculations, we further demonstrate that MTBs are thermodynamically stable structures in Se-deficient sheets. These line defects host spatially localized states with energies close to the valence band minimum, thus giving rise to enhanced conductance along straight MTBs. However, electronic transport calculations show that the transmission of hole charge carriers across MTBs is strongly suppressed due to band bending effects. We further observe formation of MTBs during in situ removal of Se atoms by the electron beam of the microscope, thus confirming that MTBs appear due to Se-deficit, and not coalescence of individual grains during growth. At a very high local Se-deficit, the 2D sheet becomes unstable and transforms to a nanowire. Our results on Se-deficient MoSe2 suggest routes toward engineering the properties of 2D transition metal dichalcogenides by deviations from the stoichiometric composition.

Structural Heterogeneity of the Melt-spun (Fe, Co)-Si-B-P-Cu Alloy with Excellent Soft Magnetic Properties

A structural heterogeneity of a newly developed soft magnetic Fe81.2Co4Si0.5B9.5P4Cu0.8 melt-spun alloy has been studied by transmission electron microscopy (TEM) and electron diffraction. Hollow-cone dark-field TEM imaging revealed that the density of coherent scattering regions in the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy is as high as that in the Fe85.2Si2B8P4Cu0.8 hetero-amorphous alloy (NANOMET®). According to the aberration-corrected high-resolution TEM, crystalline atomic clusters, typically of ∼1nm in diameter, are densely distributed in an amorphous matrix of Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, while nanobeam electron diffraction detected weak Bragg reflections from Fe clusters with the body centered cubic structure. These results unambiguously reveal structural heterogeneity of the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, which is similar to that observed in the NANOMET®.

Implantation and atomic-scale investigation of self-interstitials in graphene.

Crystallographic defects play a key role in determining the properties of crystalline materials. The new class of two-dimensional materials, foremost graphene, have enabled atomically resolved studies of defects, such as vacancies,1-4 grain boundaries,(5-7) dislocations,(8,9) and foreign atom substitutions.(10-14) However, atomic resolution imaging of implanted self-interstitials has so far been reported neither in any three-dimensional nor in any two-dimensional material. Here, we deposit extra carbon into single-layer graphene at soft landing energies of ∼ 1 eV using a standard carbon coater. We identify all the self-interstitial dimer structures theoretically predicted earlier,(15-17) employing 80 kV aberration-corrected high-resolution transmission electron microscopy. We demonstrate accumulation of the interstitials into larger aggregates and dislocation dipoles, which we predict to have strong local curvature by atomistic modeling, and to be energetically favorable configurations as compared to isolated interstitial dimers. Our results contribute to the basic knowledge on crystallographic defects and lay out a pathway into engineering the properties of graphene by pushing the crystal into a state of metastable supersaturation.

Low temperature epitaxy of Ge-Sb-Te films on BaF2 (111) by pulsed laser deposition

Pulsed laser deposition was employed to deposit epitaxial Ge2Sb2Te5-layers on the (111) plane of BaF2 single crystal substrates. X-ray diffraction measurements show a process temperature window for epitaxial growth between 85 °C and 295 °C. No crystalline growth is observed for lower temperatures, whereas higher temperatures lead to strong desorption of the film constituents. The films are of hexagonal structure with lattice parameters consistent with existing models. X-ray pole figure measurements reveal that the films grow with one single out-of-plane crystal orientation, but rotational twin domains are present. The out-of-plane epitaxial relationship is determined to be Ge2Sb2Te5(0001) || BaF2(111), whereas the in-plane relationship is characterized by two directions, i.e., Ge2Sb2Te5 [-12-10] || BaF2[1-10] and Ge2Sb2Te5[1-210] || BaF2[1-10]. Aberration-corrected high-resolution scanning transmission electron microscopy was used to resolve the local atomic structure and confirm the hexagonal structure of the films.

Efficient hydrogen production from methane over iridium-doped ceria catalysts synthesized by solution combustion

This study reports on the investigations performed on an Ir-CeO2 (0.1wt% Ir) catalyst, prepared for the first time by one-step solution combustion synthesis (SCS) and submitted to a number of analyses: N2 volumetry, elemental analysis, X-ray diffraction, scanning electron microscopy, aberration-corrected high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed methods. The catalyst was studied in the steam reforming of methane (SRM) at 750°C, under water-deficient conditions (CH4/H2O ratio of 10) and in the presence of 220ppm H2S. The obtained catalytic performances were compared with those of an Ir/CeO2 catalyst prepared by incipient wetness impregnation (IWI). The influence of the support and the metal was investigated through testing of Ir/SiO2-Al2O3-IWI and Rh-CeO2-SCS catalysts, respectively. Using CeO2 as carrier, Ir-based catalysts are highly efficient in SRM and resistant to carbon and irreversible sulfur poisoning. The catalytic activity of Ir-CeO2-SCS is superior to that of Ir/CeO2-IWI. Unlike the Ir-CeO2 catalyst, Ir/SiO2-Al2O3 is completely deactivated in presence of sulfur, while Rh-CeO2-SCS does not recover its initial activity after exposure to an H2S-containing feed.

Evaluation of stacking faults and associated partial dislocations in AlSb/GaAs (001) interface by aberration-corrected high-resolution transmission electron microscopy

The stacking faults (SFs) in an AlSb/GaAs (001) interface were investigated using a 300 kV spherical aberration-corrected high-resolution transmission electron microscope (HRTEM). The structure and strain distribution of the single and intersecting (V-shaped) SFs associated with partial dislocations (PDs) were characterized by the [110] HRTEM images and geometric phase analysis, respectively. In the biaxial strain maps εxx and εyy, a SF can be divided into several sections under different strain states (positive or negative strain values). Furthermore, the strain state for the same section of a SF is in contrast to each other in εxx and εyy strain maps. The modification in the strain states was attributed to the variation in the local atomic displacements for the SF in the AlSb film on the GaAs substrate recorded in the lattice image. Finally, the single SF was found to be bounded by two 30° PDs. A pair of 30° PDs near the heteroepitaxial interface reacted to form a Lomer-Cottrell sessile dislocation located at the vertices of V-shaped SFs with opposite screw components. The roles of misfit dislocations, such as the PDs, in strain relaxation were also discussed.

Numerical correction of anti-symmetric aberrations in single HRTEM images of weakly scattering 2D-objects

Here, we present a numerical post-processing method for removing the effect of anti-symmetric residual aberrations in high-resolution transmission electron microscopy (HRTEM) images of weakly scattering 2D-objects. The method is based on applying the same aberrations with the opposite phase to the Fourier transform of the recorded image intensity and subsequently inverting the Fourier transform. We present the theoretical justification of the method, and its verification based on simulated images in the case of low-order anti-symmetric aberrations. Ultimately the method is applied to experimental hardware aberration-corrected HRTEM images of single-layer graphene and MoSe2 resulting in images with strongly reduced residual low-order aberrations, and consequently improved interpretability. Alternatively, this method can be used to estimate by trial and error the residual anti-symmetric aberrations in HRTEM images of weakly scattering objects.

Silicon carbide embedded in carbon nanofibres: structure and band gap determination.

Materials drastically alter their electronic properties when being reduced to the nanoscale due to quantum effects. Concerning semiconductors, the band gap is expected to broaden as a result of the quantum confinement. In this study we report on the successful synthesis of wide bandgap SiC nanowires (with great potential for applications) and the local determination of their band gap. Their value was found to be higher with respect to bulk SiC. The nanowires are grown as a heterostructure, i.e. encapsulated in carbon nanofibres via dc hot-filament Plasma-Enhanced Chemical Vapour Deposition on the Si/SiO2 substrate. The structure of the as-produced carbon nanofibres was characterized by means of aberration-corrected high-resolution transmission electron microscopy. Two different pure SiC polytypes, namely the 3C (cubic) and the 6H (hexagonal) as well as distorted structures are observed. The SiC nanowires have diameters in the range of 10-15 nm and lengths of several hundred nanometers. The formation of the SiC is a result of the substrate etching during the growth of the CNFs and a subsequent simultaneous diffusion of not only carbon, but also silicon through the catalyst particle.

Structural and Electronic Properties of BaO/MgO(001)-type Interface Studied via Aberration-corrected Transmission Electron Microscopy and First-principles Calculations

The properties of BaO/MgO-type interface in the BaZrO3/MgO(001) heterostructure are studied by aberration-corrected high-resolution transmission electron microscopy combined with first-principles calculations. Experimental evidence demonstrates that cation displacement and vacancies occur at the interface. Our first-principle calculations show that cation displacement results from the electrostatic potential effect at the interface, and cation vacancies could lower the interfacial work of separation and enhance the interfacial stability of BaO/MgO-type interface. The results highlight that the effect of interfacial defects should be taken into account in understanding the film growth kinetics and properties in oxide heteroepitaxy.

Dry-cleaning of graphene

Studies of the structural and electronic properties of graphene in its pristine state are hindered by hydrocarbon contamination on the surfaces. Also, in many applications, contamination reduces the performance of graphene. Contamination is introduced during sample preparation and is adsorbed also directly from air. Here, we report on the development of a simple dry-cleaning method for producing large atomically clean areas in free-standing graphene. The cleanness of graphene is proven using aberration-corrected high-resolution transmission electron microscopy and electron spectroscopy.

Near-Surface Strain in Icosahedra of Binary Metallic Alloys: Segregational versus Intrinsic Effects

A systematic structural analysis of FePt, CuAu, and Au icosahedral nanoparticles is presented. The uncovered particles are prepared by inert gas condensation and thermally equilibrated through in-flight optical annealing. Aberration-corrected high-resolution transmission electron microscopy reveals that the crystal lattice is significantly expanded near the particle surface. These experimental findings are corroborated by molecular statics simulations that show that this near-surface strain originates from both intrinsic strain due to the icosahedral structure and a partial segregation of the larger of the two alloy constituents to the particle surface.

Laser-deposited Cu/α–Al2O3 nanocomposite: experiment and modeling

A Nd:YAG laser operating at a wavelength of 266 or 355 nm is used to deposit a thin layer of copper on the (0 0 0 1)α-Al2O3 surface. The formation process is precisely controlled by identification of time distribution of two characteristics: energy and flux density of particles incident on the substrate. For this purpose, the Cu-plasma expansion is described by means of an analytical hydrodynamic model whose self-similar solutions are fitted to the experimental plasma images and time-of-flight spectra. The obtained nanocomposite is examined by the aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) method. The results reveal that copper crystals assume one main orientation relative to the substrate (1 1 1)[2 −1 −1]Cu\(\Vert\) (0 0 0 1)[−1 −1 2 0]α–Al2O3 and the formed interface has a specific microstructure. To reconstruct the phase boundary region, molecular dynamic (MD) and static (MS) simulations are carried out. The results show that strong bonding between copper and sapphire induces structural changes in the (1 1 1) Cu layer nearest the substrate and leads to formation of the system of partially dissociated dislocations in the next layer. In consequence, the Cu/α–Al2O3 interface becomes the semicoherent system. The lattice matching regions of the individual Cu layers are significantly lowered, which results in strong deformations along the closed packed planes. The reconstructed interface is used for Cs-HRTEM image simulation. A good accordance with the experimental results indicates that the MD model correctly maps the microstructure at the phase boundary of the synthesized nanocomposite.

Electron dose dependence of signal-to-noise ratio, atom contrast and resolution in transmission electron microscope images

In order to achieve the highest resolution in aberration-corrected (AC) high-resolution transmission electron microscopy (HRTEM) images, high electron doses are required which only a few samples can withstand. In this paper we perform dose-dependent AC-HRTEM image calculations, and study the dependence of the signal-to-noise ratio, atom contrast and resolution on electron dose and sampling. We introduce dose-dependent contrast, which can be used to evaluate the visibility of objects under different dose conditions. Based on our calculations, we determine optimum samplings for high and low electron dose imaging conditions.

Atomic surface diffusion on Pt nanoparticles quantified by high-resolution transmission electron microscopy

Aberration-corrected high-resolution transmission electron microscopy allows for the delocalization-free observation of atomic motions on metallic surfaces and thus enables measurements of the diffusion of single atoms on the surfaces of nanoscopic objects such as nanoparticles. Using this recently introduced method, the diffusion coefficient for surface self-diffusion of Pt nanoparticles is determined through the fluctuating occupation of the particle's atomic columns. This diffusion coefficient is determined to lie in the range D∼(10−17…10−16)cm2/s.