Experimental High-resolution Transmission Electron Research Articles

Experimental and theoretical studies on cobalt ruthenium phosphate structure optimization towards supercapacitor application

Recent studies have given indications that nanoarchitectured binary transition metal phosphates can act as potential supercapacitor (SC) electrode materials due to their exceptional energy storage capacity and remarkable electrochemical stability. However, the highly amorphous nature or broad X-ray diffraction peaks of metal phosphates put a limit on the structural optimization of these materials. We demonstrate a combined experimental and theoretical study leading to an optimized cobalt-ruthenium phosphate (CRP) structure for high-performance SC application. The material structure was optimized by detailed density functional theory (DFT) calculations, including the Bader charge analysis, DOS, and PDOS, revealed the lattice and volume expansion, reduced band gap, and improved conductivity of the cobalt phosphate lattice by including ruthenium in the cobalt phosphate. These predictions are verified with experimental band gap and high-resolution transmission electron microscope plane image indexing. Further, the CRP was developed by potentiostatic deposition onto the carbon cloth substrate. The flexible CRP electrode exhibited a high specific capacitance of 1258 F g−1 (943.5C g−1) in the aqueous 2 M KOH electrolyte. The asymmetric SC device (CRP//activated carbon) has exhibited an excellent specific energy of ∼59 Wh kg−1 at a specific power of ∼751 W kg−1.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures

WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider $\mathrm{C}\ensuremath{\leftrightarrow}\mathrm{vacancy}$ and $\mathrm{W}\ensuremath{\leftrightarrow}\mathrm{Ti}$ substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Co materials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.

Atomic column heights detection in metallic nanoparticles using deep convolutional learning

The physico-chemical and mechanical properties of metallic (e.g., Au, Pt) nanoparticle complexes highly depend on the order and the distribution of their atomic structure. While transmission electron microscopy (TEM) provides the highest spatial resolution to an image of the nanoparticle’s atomic structure, it is time consuming and cumbersome to estimate the atomic column height from the two-dimensional projected TEM images. With continued progress of in-situ or operando TEM techniques in discovery of nanoscale science, it is paramount to develop artificial intelligence approaches that can be integrated with real-time TEM imaging. In this work, we present a modeling framework based upon deep learning approach (i.e., convolutional neural network – CNN) for the detection of the atomic column heights in the experimental high-resolution transmission electron microscopy (HRTEM) images of gold nanoparticles of different sizes. For this purpose, we propose a method for the generation of the training dataset based on the Wulff construction, in order to bring a physically realistic treatment to the network’s learning process. Moreover, we introduce a model based on the regression scheme, as a valid alternative to a classification approach reported in the prior literature. In addition to counting atoms in verity of columns and nanoparticles, the model also provides insights concerning the experimental conditions suitable for the appropriate identification of atomic column heights by the neural network. Thus, the developed modeling approach establishes a basis for accelerated or ‘on-the-fly’ analysis of nanoparticles as well as a framework for extending deep learning models to broad applications in nanoscience.

Measuring Dynamic Structural Changes of Nanoparticles at the Atomic Scale Using Scanning Transmission Electron Microscopy.

We propose a new method to measure atomic scale dynamics of nanoparticles from experimental high-resolution annular dark field scanning transmission electron microscopy images. By using the so-called hidden Markov model, which explicitly models the possibility of structural changes, the number of atoms in each atomic column can be quantified over time. This newly proposed method outperforms the current atom-counting procedure and enables the determination of the probabilities and cross sections for surface diffusion. This method is therefore of great importance for revealing and quantifying the atomic structure when it evolves over time via adatom dynamics, surface diffusion, beam effects, or during insitu experiments.

Large-scale experimental and theoretical study of graphene grain boundary structures

We have characterized the structure of 176 different single-layer graphene grain boundaries grown with chemical vapor deposition using $g1000$ experimental high-resolution transmission electron microscopy images using a semiautomated structure processing routine. We introduce an algorithm for generating grain boundary structures for a class of hexagonal two-dimensional materials and use this algorithm and molecular dynamics to simulate the structure of $g79\phantom{\rule{0.16em}{0ex}}000$ linear graphene grain boundaries covering 4122 unique orientations distributed over the entire parameter space. The dislocation content and structural properties are extracted from all experimental and simulated boundaries, and various trends are explored. We find excellent agreement between the simulated and experimentally observed grain boundaries. Our analysis demonstrates the power of a statistically significant number of measurements as opposed to a small number of observations in atomic science.

Comparison of intensity and absolute contrast of simulated and experimental high-resolution transmission electron microscopy images for different multislice simulation methods

The Stobbs factor problem, a major difference of absolute contrast between experimental and simulated high resolution transmission electron microscopy images has been reported frequently. In this respect, some modifications to the multislice simulation techniques were proposed to improve the correspondence to the experiment. The influence of different suggestions on the simulated contrast is investigated by numerical simulations. The agreement between experiment and simulations is then checked by comparison with high-resolution micrographs of crystalline gold. The experimental data is therefore compared to simulated intensities computed for the thickness and orientation of the specimen measured by refinements of diffraction patterns. The agreement of both intensity and contrast is investigated and the remaining contrast discrepancy is determined. The results show a good agreement for a small objective aperture, while for a larger aperture a difference of contrast by a factor of 1.2 can still be observed. Without any aperture, the deviation between experiment and simulations is largest.

From Sphere to Multipod: Thermally Induced Transitions of CdSe Nanocrystals Studied by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations are used to show that a spherical zinc blende (ZB) nanocrystal (NC) can transform into a tetrapod or an octapod as a result of heating, by a local zincblende-to-wurtzite phase transformation taking place in the NC. The partial sphere-to-tetrapod or sphere-to-octapod transition occurs within simulation times of 30 ns and depends on both temperature and NC size. Surprisingly, the wurtzite (WZ) subdomains are not formed through a slip mechanism but are mediated by the formation of highly mobile Cd vacancies on the ZB{111} Cd atomic planes. The total potential energy of a tetrapod is found to be lower than that of a ZB sphere at the same numbers of atoms. The simulation results are in good agreement with experimental high-resolution transmission electron microscopy (HR-TEM) data obtained on heated colloidal CdSe NCs.

Dopant Segregation Analysis on Sb:SnO2 Nanocrystals

The development of reliable nanostructured devices is intrinsically dependent on the description and manipulation of materials' properties at the atomic scale. Consequently, several technological advances are dependent on improvements in the characterization techniques and in the models used to describe the properties of nanosized materials as a function of the synthesis parameters. The evaluation of doping element distributions in nanocrystals is directly linked to fundamental aspects that define the properties of the material, such as surface-energy distribution, nanoparticle shape, and crystal growth mechanism. However, this is still one of the most challenging tasks in the characterization of materials because of the required spatial resolution and other various restrictions from quantitative characterization techniques, such as sample degradation and signal-to-noise ratio. This paper addresses the dopant segregation characterization for two antimony-doped tin oxide (Sb:SnO(2)) systems, with different Sb doping levels, by the combined use of experimental and simulated high-resolution transmission electron microscopy (HRTEM) images and surface-energy ab initio calculations. The applied methodology provided three-dimensional models with geometrical and compositional information that were demonstrated to be self-consistent and correspond to the systems' mean properties. The results evidence that the dopant distribution configuration is dependent on the system composition and that dopant atom redistribution may be an active mechanism for the overall surface-energy minimization.

Unveiling the Chemical and Morphological Features of Sb−SnO2 Nanocrystals by the Combined Use of High-Resolution Transmission Electron Microscopy and ab Initio Surface Energy Calculations

Modeling of nanocrystals supported by advanced morphological and chemical characterization is a unique tool for the development of reliable nanostructured devices, which depends on the ability to synthesize and characterize materials on the atomic scale. Among the most significant challenges in nanostructural characterization is the evaluation of crystal growth mechanisms and their dependence on the shape of nanoparticles and the distribution of doping elements. This paper presents a new strategy to characterize nanocrystals, applied here to antimony-doped tin oxide (Sb-SnO(2)) (ATO) by the combined use of experimental and simulated high-resolution transmission electron microscopy (HRTEM) images and surface energy ab initio calculations. The results show that the Wulff construction can not only describe the shape of nanocrystals as a function of surface energy distribution but also retrieve quantitative information on dopant distribution by the dimensional analysis of nanoparticle shapes. In addition, a novel three-dimensional evaluation of an oriented attachment growth mechanism is provided in the proposed methodology. This procedure is a useful approach for faceted nanocrystal shape modeling and indirect quantitative evaluation of dopant spatial distribution, which are difficult to evaluate by other techniques.

Antimony Selenide Crystals Encapsulated within Single Walled Carbon Nanotubes‐A DFT Study

The structure and binding energies of antimony selenide crystals encapsulated within single‐walled carbon nanotubes are studied using density functional theory. Calculations were performed on the simulated Sb2Se3 structure encapsulated within single walled nanotube to investigate the perturbations on the Sb2Se3 crystal and tube structure and electronic structure and to estimate the binding energy. The calculated structures are in good agreement with the experimental high resolution transmission electron microscopy images of the Sb2Se3@SWNT. The calculated binding energy shows that larger diameter tube could accommodate the Sb2Se3 crystals exothermically. Minimal charge transfer is observed between nanotube and the Sb2Se3 crystals.

Image matching between experimental and simulated high‐resolution electron micrographs of sapphire on the orientation

The effects of imaging parameters have been studied on their roles of the severe mismatches between experimental and simulated high-resolution transmission electron micrographs of sapphire along the direction. Image simulation and convergent-beam electron diffraction techniques have been performed on misalignments of the electron beam and the crystal specimen. Based on this study, we have introduced an approach to achieve reliable simulation for experimental images of sapphire on the projection by the use of iterative digital image matching.

A transmission electron microscopy study on the crystal structure and atomic arrangement of Al–Nd thin films deposited on glass substrates

The crystal structure and atomic arrangement of Al–2 at.% Nd thin films were investigated by a transmission electron microscopy (TEM) study. As a result, we have revealed that Nd atoms exist in Al matrix as solid solutions in the state of as-deposited and annealed at low temperature, while the intermetallic compounds with the composition of Al 4Nd are precipitated as the annealing temperature increased above 300 °C. The precipitated Al 4Nd has a tetragonal structure with nine cyclic layers, and the atomic arrangement was identified by the fast Fourier transform (FFT) and Fourier filtered images as well as the experimental high-resolution transmission electron microscopy (HRTEM) images. Consequently, Nd atoms, forms of solid solutions or precipitates, effectively suppress the grain growth of Al.

Mercury telluride crystals encapsulated within single walled carbon nanotubes: A density functional study

AbstractThe structure and binding energies of mercury telluride crystals encapsulated within single walled carbon nanotubes (SWNTs) have been studied using density functional theory. The energies of three different pseudo one‐dimensional crystals of HgTe with 4:4, 3:3, and 2:2 coordination are compared. The initial structure for the 4:4 crystal was a 2 × 2 cubic motif derived from rock salt bulk structure, the 3:3 crystal corresponds to a novel structure found when HgTe was intercalated within SWNTs, and the 2:2 crystal is a chain motif derived from cinnabar (HgS) bulk structure. The isolated 3:3 crystal was found to be the most thermodynamically stable of the three structures. Calculations were performed on the 3:3 crystal inserted into three different SWNTs, (15, 0), (9, 9), and (17, 0), in order to investigate the perturbations on the molecular and electronic structure of the crystal and the SWNT, and the energy of formation of the HgTe@SWNT composites. The calculated structures are in good agreement with the experimental high resolution transmission electron microscopy images of the HgTe@SWNT composite. The calculated binding energies and density of states show that the interaction between nanotubes and the HgTe crystals is noncovalent. Since the energy difference of the “free” 4:4 and 3:3 structures is small and of the order of magnitude of the binding energies with the nanotubes, we carried out calculations on 4:4 HgTe structure inserted in to two different SWNTs, (15, 0) and (17, 0). The calculated binding energies show that, when the 4:4 structure is inserted into the smallest tube, the resultant composite has an energy comparable to the 3:3 structure, suggesting that this polymporph may also be found experimentally. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

REW– exit-wave reconstruction and alignments for focus-variation high-resolution transmission electron microscopy images

A Windows-based computer program has been developed for exit-wave reconstruction and experimental high-resolution transmission electron microscopy image alignment. While the exit-wave reconstruction is performed mainly using the maximum-likelihood method, image alignments may be carried out using several algorithms, including the time-consuming but robust genetic algorithm.

Quantitative comparison of image contrast and pattern between experimental and simulated high-resolution transmission electron micrographs

Aiming to determine the contrast mismatch factor i.e. the Stobbs factor between the experimental and simulated high-resolution transmission electron micrographs, we have systematically compared the experimental images and simulations of a cleaved silicon sample for a series of focal settings and specimen thicknesses. For zero-loss energy filtered images, a mismatch factor of about 1.5–2.3 is measured for the image contrast, where the mismatch factor is focal dependent and higher mismatch appears around the focus value of 10 nm. Attention is also given to the effects of the sample vibration and drift to the image contrast and pattern of the high-resolution micrographs.

Direct observation of an extended complex stacking fault in the γ′ phase of a Ni-base superalloy

A creep deformation mechanism in a polycrystalline nickel-base superalloy (Rene 88 DT) has been examined using crystallographic analysis and comparison between simulated and experimental high-resolution transmission electron microscopy (HRTEM) images. Confirmation of the magnitude of the Burgers vector of the partial dislocations involved in the creep deformation was critical in conclusively determining its nature. A new approach to determining a complex stacking fault was developed using superlattice image contrast from an experimental HRTEM image.

Atomic Structure and Relaxation Behavior at AlN(0001)/Al2O3(0001)Interface

The atomic structure of AlN(0001)/Al2O3(0001) interface, which is a model of a large lattice-mismatched film/substrate interface, was investigated by using static lattice calculations. Detailed analysis of the calculated structure revealed that the atomic relaxation behavior strongly depends on local atomic configurations of the interface. Interface distance became larger when Al atoms of AlN side and O atoms of Al2O3 side at the interface are in on-top configurations, while it became smaller when they are not in on-top configurations. The calculated structure qualitatively reproduced the experimental high-resolution transmission electron microscopy (HRTEM) image, which was obtained in our previous study. Such varieties of the atomic relaxation can be concluded to play one of the important roles for the formation of stable hetero-interface structures.

Formation of porous silicon: Electron microscope investigation

Porous silicon structure was investigated by scanning electron microscope images of secondary electrons as well as by transmission electron microscope images of diffraction and phase contrasts. Different structure types in each one of the dissolution zones are found: crystalline structure of primary silicon, changed crystalline structure as a result of a partial dissolution process, and amorphous structure in the porous zone. A hypothesis about the type of the changed structure in the intermediate dissolution zone is put forward and an atomic model of this structure is presented. A theory of the mechanism of the dissolution process based on structure peculiarities, observed in the images, was formulated: the dissolution proceeds along the most favorable atomic planes of types (110) and (111). The dissolution front advances mainly in the direction [010]—it is zigzag shaped and parallel to the dissolution surface. Images of phase and diffraction contrasts have been simulated on the basis of models accounting for structure changes. A good correspondence between experimental and simulated high-resolution transmission electron microscopy images has been found.

Cyclic silicate active site and stereochemical match for apatite nucleation on pseudowollastonite bioceramic–bone interfaces

Hydroxyapatite (Ca 5(PO 4) 3(OH)) forms on pseudowollastonite (psW) ( α-CaSiO3) in vitro in simulated body fluid, human parotid saliva and cell-culture medium, and in vivo in implanted rat tibias. We used crystallographic constraints with ab initio molecular orbital calculations to identify the active site and reaction mechanism for heterogeneous nucleation of the earliest calcium phosphate oligomer/phase. The active site is the planar, cyclic, silicate trimer (Si 3O 9) on the (0 0 1) face of psW. The trimer has three silanol groups (>SiOH) arranged at 60° from each other, providing a stereochemical match for O atoms bonded to Ca 2+ on the (0 0 1) face of hydroxyapatite. Calcium phosphate nucleation is modeled in steps as hydrolysis of surface Ca–O bonds with leaching of Ca 2+ into solution, protonation of the surface Si–O groups to form silanols, calcium sorption as an inner-sphere surface complex and, attachment of HPO 4 2−. Our model explains the experimental solution and high resolution transmission electron microscopy data for epitaxial hydroxyapatite growth on psW in vitro and in vivo. We propose that the cyclic silicate trimer is the universal active site for heterogeneous, stereochemically promoted nucleation on silicate-based bioactive ceramics. A critical active site-density and a point of zero charge of the bioceramic less than physiological pH are required for bioactivity.