Atomic-resolution Transmission Electron Microscopy Research Articles

Line-defect mediated formation of hole and Mo clusters in monolayer molybdenum disulfide

The production of hole and Mo cluster by electron beam irradiation in molybdenum disulfide (MoS2), which consists of S-Mo-S layers, is monitored over time using atomic resolution transmission electron microscopy. S vacancies are firstly formed due to knocking off of S atoms and then line defects are induced due to accumulation of S vacancies in MoS2 sheet instead of forming a hole. The line defects tend to be merged at a point and a hole is formed subsequently at the point. Mo atoms tend to be clustered discretely as a nano sheet along the edge of the hole due to difference in displacement threshold energy between Mo and S atoms under electron irradiation. After Mo clusters are nearly separated from MoS2 sheet, the clusters are transformed into body-centered cubic nanocrystal of Mo during prolonged electron beam irradiation. The line defect mediated formation of hole and Mo cluster only occurs within a single grain of monolayer MoS2 sheet.

Observation of antisite domain boundaries in Cu2ZnSnS4 by atomic-resolution transmission electron microscopy.

Atomic resolution transmission electron microscopy has been used to examine antisite defects in Cu2ZnSnS4 (CZTS) kesterite crystals grown by a hot injection method. High angle annular dark field (HAADF) imaging at sub-0.1 nm resolution, and lower magnification dark field imaging using reflections sensitive to cation ordering, are used to reveal antisite domain boundaries (ADBs). These boundaries, typically 5-20 nm apart, and extending distances of 100 nm or more into the crystals, lie on a variety of planes and have displacements of the type ½[110] or ¼[201], which translate Sn, Cu and Zn cations into antisite positions. It is shown that some ADBs describe a change in the local stoichiometry by removing planes of S and either Cu or Zn atoms, implying that these boundaries can be electrically charged. The observations also showed a marked increase in cation disorder in regions within 1-2 nm of the grain surfaces suggesting that growth of the ordered crystal takes place at the interface with a disordered shell. It is estimated that the ADBs contribute on average ∼0.1 antisite defect pairs per unit cell. Although this is up to an order of magnitude less than the highest antisite defect densities reported, the presence of high densities of ADBs that may be charged suggests these defects may have a significant influence on the efficiency of CZTS solar cells.

Electron Microscopy of Probability Currents at Atomic Resolution.

Atomic resolution transmission electron microscopy records the spatially resolved scattered electron density to infer positions, density, and species of atoms. These data are indispensable for studying the relation between structure and properties in solids. Here, we show how this signal can be augmented by the lateral probability current of the scattered electrons in the object plane at similar resolutions and fields of view. The currents are reconstructed from a series of three atomic resolution TEM images recorded under a slight difference of perpendicular line foci. The technique does not rely on the coherence of the electron beam and can be used to reveal electric, magnetic, and strain fields with incoherent electron beams as well as correlations in inelastic transitions, such as electron magnetic chiral dichroism.

Covalently Binding Atomically Designed Au9 Clusters to Chemically Modified Graphene.

Atomic-resolution transmission electron microscopy was used to identify individual Au9 clusters on a sulfur-functionalized graphene surface. The clusters were preformed in solution and covalently attached to the surface without any dispersion or aggregation. Comparison of the experimental images with simulations allowed the rotational motion, without lateral displacement, of individual clusters to be discerned, thereby demonstrating a robust covalent attachment of intact clusters to the graphene surface.

Direct and Scalable Deposition of Atomically Thin Low-Noise MoS2 Membranes on Apertures.

Molybdenum disulfide (MoS2) flakes can grow beyond the edge of an underlying substrate into a planar freestanding crystal. When the substrate edge is in the form of an aperture, reagent-limited nucleation followed by edge growth facilitate direct and selective growth of freestanding MoS2 membranes. We have found conditions under which MoS2 grows preferentially across micrometer-scale prefabricated solid-state apertures in silicon nitride membranes, resulting in sealed membranes that are one to a few atomic layers thick. We have investigated the structure and purity of our membranes by a combination of atomic-resolution transmission electron microscopy, elemental analysis, Raman spectroscopy, photoluminescence spectroscopy, and low-noise ion-current recordings through nanopores fabricated in such membranes. Finally, we demonstrate the utility of fabricated ultrathin nanopores in such membranes for single-stranded DNA translocation detection.

Geometrically constrained self-assembly and crystal packing of flattened and aligned carbon nanotubes

While the mechanical properties of highly aligned carbon nanotube (CNT) thin films and their nanocomposites have been widely studied, the load transfer mechanisms and failure modes of aligned CNT composites have not been sufficiently explored and understood. In this research, super-aligned CNT thin films with a measured alignment fraction of up to 0.93 are fabricated by mechanical stretching. High concentration (50–60wt% CNT) CNT reinforced bismaleimide (CNT/BMI) nanocomposites are fabricated from the aligned network to study mechanical properties and microstructures. Atomic resolution transmission electron microscopy (TEM) analysis reveal unusual CNT crystal packing and permit the observation of interesting structural features of the CNTs and their assemblages, including collapse, flattened packing, preferred stacking, folding and twisting phenomena, as well as CNT pullouts from bundles and the resin matrix. The large surface-to-surface contact areas between aligned and flattened nanotubes, driven by van der Waals interactions, give rise to a high density packing of the flattened CNTs in the nanocomposite, resembling a graphitic material. Molecular dynamics (MD) simulations are performed to model the packing structure and understand the dependence of density on the relative content of flattened nanotube and void space. The modeling results support the conclusions drawn from the experimental observations.

Boundary Observation and Contrast Tuning of Ni/YSZ Anode by TEM and FIB-SEM

Ni/yttria stabilized zirconia (Ni/YSZ) anode was investigated in the aspect of microstructure. Atomic resolution transmission electron microscopy and high resolution focused ion beam-scanning electron microscopy (FIB-SEM) were used to study the atomic structure of boundaries and phase contrast, respectively. Images of Ni/YSZ interface and Ni/YSZ/pore triple-phase boundary were obtained. High-Index contact surfaces were found. Distinct phase contrast among Ni, YSZ and resin was observed during the slice and view process in FIB-SEM.

Antimony-doped graphene nanoplatelets.

Heteroatom doping into the graphitic frameworks have been intensively studied for the development of metal-free electrocatalysts. However, the choice of heteroatoms is limited to non-metallic elements and heteroatom-doped graphitic materials do not satisfy commercial demands in terms of cost and stability. Here we realize doping semimetal antimony (Sb) at the edges of graphene nanoplatelets (GnPs) via a simple mechanochemical reaction between pristine graphite and solid Sb. The covalent bonding of the metalloid Sb with the graphitic carbon is visualized using atomic-resolution transmission electron microscopy. The Sb-doped GnPs display zero loss of electrocatalytic activity for oxygen reduction reaction even after 100,000 cycles. Density functional theory calculations indicate that the multiple oxidation states (Sb3+ and Sb5+) of Sb are responsible for the unusual electrochemical stability. Sb-doped GnPs may provide new insights and practical methods for designing stable carbon-based electrocatalysts.

Dissociation of the 1/3〈[formula omitted]〉 dislocation and formation of the anion stacking fault on the basal plane in α-Al2O3

Dislocation structures in the (0001)/〈11¯00〉 2° tilt grain boundary of alumina (α-Al2O3) were investigated by atomic-resolution transmission electron microscopy (TEM). We found that the grain boundary basically consists of a periodic array of 1/3[1¯101], 1/3[01¯11], and 1/3[10 1¯ 1] dislocations, and these dislocations were dissociated into two partial dislocations with a stacking fault in between on the (0001) plane. Atomic-resolution scanning TEM analysis revealed that the stacking fault is present on the anion sublattice, which has not been observed in alumina so far. The presence of the stable (0001) anion stacking faults was theoretically supported using a first-principles calculation. Combining experimental and theoretical results, we demonstrate the dissociation reactions of the 1/3〈1¯101〉 dislocation and the associated stacking fault.

Influence of size, shape and core-shell interface on surface plasmon resonance in Ag and Ag@MgO nanoparticle films deposited on Si/SiO x.

Ag and Ag@MgO core–shell nanoparticles (NPs) with a diameter of d = 3–10 nm were obtained by physical synthesis methods and deposited on Si with its native ultrathin oxide layer SiOx (Si/SiOx). Scanning electron microscopy and transmission electron microscopy (TEM) images of bare Ag NPs revealed the presence of small NP aggregates caused by diffusion on the surface and agglomeration. Atomic resolution TEM gave evidence of the presence of crystalline multidomains in the NPs, which were due to aggregation and multitwinning occurring during NP growth in the nanocluster source. Co-deposition of Ag NPs and Mg atoms in an oxygen atmosphere gave rise to formation of a MgO shell matrix surrounding the Ag NPs. The behaviour of the surface plasmon resonance (SPR) excitation in surface differential reflectivity (SDR) spectra with p-polarised light was investigated for bare Ag and Ag@MgO NPs. It was shown that the presence of MgO around the Ag NPs caused a red shift of the plasmon excitation, and served to preserve its existence after prolonged (five months) exposure to air, realizing the possibility of technological applications in plasmonic devices. The Ag NP and Ag@MgO NP film features in the SDR spectra could be reproduced by classical electrodynamics simulations by treating the NP-containing layer as an effective Maxwell Garnett medium. The simulations gave results in agreement with the experiments when accounting for the experimentally observed aggregation.

Understanding and Controlling Nucleation and Growth of TiO2 Deposited on Multiwalled Carbon Nanotubes by Atomic Layer Deposition

Controlled deposition of thin conformal oxide films on carbon nanotubes (CNTs) by atomic layer deposition (ALD) for applications in solar energy and photocatalysis is still challenging, as the early stages of nucleation and subsequent growth are not yet well understood. In this work, we employed ALD to grow TiO2 on multiwalled carbon nanotubes (MW-CNTs). The effects of deposition temperature (120–240 °C), number of ALD cycles (20–750), and surface pretreatment of the MW-CNTs with oxygen plasma on the morphology and crystallinity of the TiO2 were systematically studied using transmission electron microscopy (TEM). By tuning the deposition conditions, controllable nucleation and growth of TiO2 on CNTs was achieved. In particular, high-quality crystalline anatase conforming to the CNTs was obtained with an ALD growth temperature as low as 200 °C. Direct observation using aberration-corrected atomic-resolution TEM imaging at 120 keV revealed an island structure of crystalline TiOx at the very early stage of nucleation, followed by coalesced growth of crystalline anatase at this temperature. The study also paves the way to understand the interface between the two materials on an atomic level.

The Effect of Crystallographic Imperfections on the Photoluminescence of ZnO Thin Films

ABSTRACTZnO thin films were synthesized by radio-frequency (RF) magnetron sputtering of high purity ZnO solid targets on sapphire substrates. Depositions were carried out at selected temperatures between 293 K and 1173 K, and post-deposition annealing was performed at 1173 K for 3 min. in an O2 atmosphere. Samples for electron microscopy investigations were prepared by lift-out technique in a multi-beam FIB/SEM instrument. The ZnO thin films show generally uniform thickness (about 1µm), determined by transmission electron microscopy (TEM) imaging. Irrespective of the deposition temperature, the ZnO thin films are polycrystalline, with individual grains exhibiting columnar morphology with the long axis oriented perpendicular to the ZnO/sapphire interface. The grain size varies with the deposition temperature, and a direct correlation between grain size and photoluminescence has been observed. Analyses performed using low-temperature photoluminescence spectroscopy measurements at 12 K revealed luminescence peaks at 3.361, 3.317, 3.218 and 3.115 eV. The intensity of the luminescence peak at 3.317 eV decreased with increasing deposition temperature. The films deposited at lower temperatures also exhibited a higher density of stacking faults as observed from the atomic resolution TEM. The crystallographic imperfections/photoluminescence relationship is not clear. The purpose of this study is to quantify the observed crystallographic imperfections and understand their effect on the photoluminescence of undoped ZnO thin films deposited on sapphire substrates.

Atomic-scale dynamics of triangular hole growth in monolayer hexagonal boron nitride under electron irradiation

The production of holes by electron beam irradiation in hexagonal boron nitride (hBN), which has a lattice similar to that of graphene, is monitored over time using atomic resolution transmission electron microscopy. The holes appear to be initiated by the formation of a vacancy of boron and grow in a manner that retains an overall triangular shape. The hole growth process involves the formation of single chains of B and N atoms and is accompanied by the ejection of atoms and bundles of atoms along the hole edges, as well as atom migration. These observations are compared to density functional theory calculations and molecular dynamics simulations.

Heterogeneous nucleation and shape transformation of multicomponent metallic nanostructures

To be able to control the functions of engineered multicomponent nanomaterials, a detailed understanding of heterogeneous nucleation at the nanoscale is essential. Here, by using in situ synchrotron X-ray scattering, we show that in the heterogeneous nucleation and growth of Au on Pt or Pt-alloy seeds the heteroepitaxial growth of the Au shell exerts high stress (∼2 GPa) on the seed by forming a core/shell structure in the early stage of the reaction. The development of lattice strain and subsequent strain relaxation, which we show using atomic-resolution transmission electron microscopy to occur through the slip of {111} layers, induces morphological changes from a core/shell to a dumbbell structure, and governs the nucleation and growth kinetics. We also propose a thermodynamic model for the nucleation and growth of dumbbell metallic heteronanostructures.

Electronic functionalisation of graphene via external doping and dosing

There exist many reports on functionalisation of graphene on a non-spatially resolved scale; this report concentrates on reviewing atomic-scale interactions of functionalising agents, i.e. on the electronic behaviour of single atoms, which are introduced as adatoms or lattice site impurities for the purpose of external doping and dosing to achieve bandgap engineering and electrical contacting of graphene; it also reviews the associated defects. Emphasis is put on visualisation of such interactions by advanced imaging in conjunction with localised spectroscopy techniques. Whereas the existing literature describing the development of such techniques in the application to graphene warrants a review in its own right, here the authors focus on observations, with modelling support, of the interaction phenomena themselves and not on the evaluation of measurements by such techniques. Atomic resolution transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS) in imaging and scanning mode, as well as scanning tunnelling microscopy (STM) are the most frequently applied techniques in aid of revealing topography and defect assisted interactions of graphene with foreign atoms and molecules. Electron-probe based investigations additionally lead to electron beam assisted interactions of foreign species with graphene. The graphene–metal interaction observed in a transmission electron microscope is a prevalent example of how reactions occurring between metals and graphene can be emphasised and thereby assessed: metal-mediated etching of graphene has proven to be a common phenomenon after metal dosing, e.g. to fabricate electrical contacts. The review reports furthermore on investigations revealing atomic position, bonding and dynamics of non-metal p- and n-dopants as well as on revealing the functionalisation of graphene via molecular self-assembly, intercalation and nano-sculpting. Literature till the end of 2013/begin of 2014 is reviewed.

Materials analysis: Good vibrations.

A newly constructed electron-energy monochromator for an atomic-resolution transmission electron microscope has resolved spectroscopic signatures of chemical-bond vibrations that are spatially highly localized. See Letter p.209 Spectroscopies sensitive to the vibrational behaviour of materials and chemical compounds — infrared and Raman spectroscopy for instance — are widely used to give insights into chemical and physical properties. These vibrational excitations can in principle also be detected by electron energy loss spectroscopy (EELS); but the effect is relatively weak and the energy resolution needed to extract such signals has not hitherto been available in electron microscopy. Here Ondrej Krivanek and colleagues demonstrate that recent advances in electron microscopy now mean that vibrational spectroscopy can be undertaken at high spatial resolution in the scanning transmission electron microscope. The authors present examples of applications in inorganic and organic materials, including the direct detection of hydrogen, a capability that could be of great use in the analysis of systems as diverse as hydrogen storage materials and biological tissues.

Surface-step-induced oscillatory oxide growth.

We report in situ atomic-resolution transmission electron microscopy observations of the oxidation of stepped Cu surfaces. We find that the presence of surface steps both inhibits oxide film growth and leads to the oxide decomposition, thereby resulting in oscillatory oxide film growth. Using atomistic simulations, we show that the oscillatory oxide film growth is induced by oxygen adsorption on the lower terrace along the step edge, which destabilizes the oxide film formed on the upper terrace.

Optimization of In2Se3/Si(111) Heteroepitaxy To Enable Bi2Se3/In2Se3 Bilayer Growth

Systematic optimization of molecular beam epitaxy growth parameters enabled high quality heteroepitaxy of In2Se3 on Si(111) surfaces. Surfaces of the best epilayers were characterized by atomically flat terraces that extended laterally for several hundred nanometers. These terraces were separated by single quintuple layer high steps. These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3. The quality of the In2Se3/Bi2Se3 interface was confirmed using atomic resolution transmission electron microscopy.

Elemental mapping in achromatic atomic-resolution energy-filtered transmission electron microscopy.

We present atomic-resolution energy-filtered transmission electron microscopy (EFTEM) images obtained with the chromatic-aberration-corrected FEI Titan PICO at the Ernst-Ruska Centre, Jülich, Germany. We find qualitative agreement between experiment and simulation for the background-subtracted EFTEM images of the Ti-L2,3 and O-K edges for a specimen of SrTiO3 oriented down the [110] zone axis. The simulations utilize the transition potential formulation for inelastic scattering, which permits a detailed investigation of contributions to the EFTEM image. We find that energy-filtered images of the Ti-L2,3 and O-K edges are lattice images and that the background-subtracted core-loss maps may not be directly interpretable as elemental maps. Simulations show that this is a result of preservation of elastic contrast, whereby the qualitative details of the image are determined primarily by elastic, coherent scattering. We show that this effect places a constraint on the range of specimen thicknesses which could theoretically yield directly useful elemental maps. In general, interpretation of EFTEM images is ideally accompanied by detailed simulations.

Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth.

Here, we show a novel solid–solid–vapor (SSV) growth mechanism whereby epitaxial growth of heterogeneous semiconductor nanowires takes place by evaporation-induced cation exchange. During heating of PbSe-CdSe nanodumbbells inside a transmission electron microscope (TEM), we observed that PbSe nanocrystals grew epitaxially at the expense of CdSe nanodomains driven by evaporation of Cd. Analysis of atomic-resolution TEM observations and detailed atomistic simulations reveals that the growth process is mediated by vacancies.