Atomic-Scale Understanding of Li Storage Processes in the Ti4C3 and Chemically Ordered Ti2Ta2C3 MXenes: A Theoretical and Experimental Assessment

Daniel Maldonado-Lopez ◽  
Jassiel R. Rodriguez ◽  
Vilas G. Pol ◽  
Ravuri Syamsai ◽  
Nirmala Grace Andrews ◽  
H. Hashimoto ◽  
Y. Sugimoto ◽  
Y. Takai ◽  
H. Endoh

As was demonstrated by the present authors that atomic structure of simple crystal can be photographed by the conventional 100 kV electron microscope adjusted at “aberration free focus (AFF)” condition. In order to operate the microscope at AFF condition effectively, highly stabilized electron beams with small energy spread and small beam divergence are necessary. In the present observation, a 120 kV electron microscope with LaB6 electron gun was used. The most of the images were taken with the direct electron optical magnification of 1.3 million times and then magnified photographically.1. Twist boundary of ZnSFig. 1 is the image of wurtzite single crystal with twist boundary grown on the surface of zinc crystal by the reaction of sulphur vapour of 1540 Torr at 500°C. Crystal surface is parallel to (00.1) plane and electron beam is incident along the axis normal to the crystal surface. In the twist boundary there is a dislocation net work between two perfect crystals with a certain rotation angle.

R. J. Wilson ◽  
D. D. Chambliss ◽  
S. Chiang ◽  
V. M. Hallmark

Scanning tunneling microscopy (STM) has been used for many atomic scale observations of metal and semiconductor surfaces. The fundamental principle of the microscope involves the tunneling of evanescent electrons through a 10Å gap between a sharp tip and a reasonably conductive sample at energies in the eV range. Lateral and vertical resolution are used to define the minimum detectable width and height of observed features. Theoretical analyses first discussed lateral resolution in idealized cases, and recent work includes more general considerations. In all cases it is concluded that lateral resolution in STM depends upon the spatial profile of electronic states of both the sample and tip at energies near the Fermi level. Vertical resolution is typically limited by mechanical and electronic noise.

Alexis T. Bell

Heterogeneous catalysts, used in industry for the production of fuels and chemicals, are microporous solids characterized by a high internal surface area. The catalyticly active sites may occur at the surface of the bulk solid or of small crystallites deposited on a porous support. An example of the former case would be a zeolite, and of the latter, a supported metal catalyst. Since the activity and selectivity of a catalyst are known to be a function of surface composition and structure, it is highly desirable to characterize catalyst surfaces with atomic scale resolution. Where the active phase is dispersed on a support, it is also important to know the dispersion of the deposited phase, as well as its structural and compositional uniformity, the latter characteristics being particularly important in the case of multicomponent catalysts. Knowledge of the pore size and shape is also important, since these can influence the transport of reactants and products through a catalyst and the dynamics of catalyst deactivation.

A. K. Datye ◽  
D. S. Kalakkad ◽  
L. F. Allard ◽  
E. Völkl

The active phase in heterogeneous catalysts consists of nanometer-sized metal or oxide particles dispersed within the tortuous pore structure of a high surface area matrix. Such catalysts are extensively used for controlling emissions from automobile exhausts or in industrial processes such as the refining of crude oil to produce gasoline. The morphology of these nano-particles is of great interest to catalytic chemists since it affects the activity and selectivity for a class of reactions known as structure-sensitive reactions. In this paper, we describe some of the challenges in the study of heterogeneous catalysts, and provide examples of how electron holography can help in extracting details of particle structure and morphology on an atomic scale.Conventional high-resolution TEM imaging methods permit the image intensity to be recorded, but the phase information in the complex image wave is lost. However, it is the phase information which is sensitive at the atomic scale to changes in specimen thickness and composition, and thus analysis of the phase image can yield important information on morphological details at the nanometer level.

David J. Smith

The era of atomic-resolution electron microscopy has finally arrived. In virtually all inorganic materials, including oxides, metals, semiconductors and ceramics, it is possible to image individual atomic columns in low-index zone-axis projections. A whole host of important materials’ problems involving defects and departures from nonstoichiometry on the atomic scale are waiting to be tackled by the new generation of intermediate voltage (300-400keV) electron microscopes. In this review, some existing problems and limitations associated with imaging inorganic materials are briefly discussed. The more immediate problems encountered with organic and biological materials are considered elsewhere.Microscope resolution. It is less than a decade since the state-of-the-art, commercially available TEM was a 200kV instrument with a spherical aberration coefficient of 1.2mm, and an interpretable resolution limit (ie. first zero crossover of the contrast transfer function) of 2.5A.

J.K. Weiss ◽  
M. Gajdardziska-Josifovska ◽  
M. R. McCartney ◽  
David J. Smith

Interfacial structure is a controlling parameter in the behavior of many materials. Electron microscopy methods are widely used for characterizing such features as interface abruptness and chemical segregation at interfaces. The problem for high resolution microscopy is to establish optimum imaging conditions for extracting this information. We have found that off-axis electron holography can provide useful information for the study of interfaces that is not easily obtained by other techniques.Electron holography permits the recovery of both the amplitude and the phase of the image wave. Recent studies have applied the information obtained from electron holograms to characterizing magnetic and electric fields in materials and also to atomic-scale resolution enhancement. The phase of an electron wave passing through a specimen is shifted by an amount which is proportional to the product of the specimen thickness and the projected electrostatic potential (ignoring magnetic fields and diffraction effects). If atomic-scale variations are ignored, the potential in the specimen is described by the mean inner potential, a bulk property sensitive to both composition and structure. For the study of interfaces, the specimen thickness is assumed to be approximately constant across the interface, so that the phase of the image wave will give a picture of mean inner potential across the interface.

K. L. Merkle

The atomic structures of internal interfaces have recently received considerable attention, not only because of their importance in determining many materials properties, but also because the atomic structure of many interfaces has become accessible to direct atomic-scale observation by modem HREM instruments. In this communication, several interface structures are examined by HREM in terms of their structural periodicities along the interface.It is well known that heterophase boundaries are generally formed by two low-index planes. Often, as is the case in many fcc metal/metal and metal/metal-oxide systems, low energy boundaries form in the cube-on-cube orientation on (111). Since the lattice parameter ratio between the two materials generally is not a rational number, such boundaries are incommensurate. Therefore, even though periodic arrays of misfit dislocations have been observed by TEM techniques for numerous heterophase systems, such interfaces are quasiperiodic on an atomic scale. Interfaces with misfit dislocations are semicoherent, where atomically well-matched regions alternate with regions of misfit. When the misfit is large, misfit localization is often difficult to detect, and direct determination of the atomic structure of the interface from HREM alone, may not be possible.

D. J. Wallis ◽  
N. D. Browning

In electron energy loss spectroscopy (EELS), the near-edge region of a core-loss edge contains information on high-order atomic correlations. These correlations give details of the 3-D atomic structure which can be elucidated using multiple-scattering (MS) theory. MS calculations use real space clusters making them ideal for use in low-symmetry systems such as defects and interfaces. When coupled with the atomic spatial resolution capabilities of the scanning transmission electron microscope (STEM), there therefore exists the ability to obtain 3-D structural information from individual atomic scale structures. For ceramic materials where the structure-property relationships are dominated by defects and interfaces, this methodology can provide unique information on key issues such as like-ion repulsion and the presence of vacancies, impurities and structural distortion.An example of the use of MS-theory is shown in fig 1, where an experimental oxygen K-edge from SrTiO3 is compared to full MS-calculations for successive shells (a shell consists of neighboring atoms, so that 1 shell includes only nearest neighbors, 2 shells includes first and second-nearest neighbors, and so on).

J. L. Lee ◽  
C. A. Weiss ◽  
R. A. Buhrman ◽  
J. Silcox

BaF2 thin films are being investigated as candidates for use in YBa2Cu3O7-x (YBCO) / BaF2 thin film multilayer systems, given the favorable dielectric properties of BaF2. In this study, the microstructural and chemical compatibility of BaF2 thin films with YBCO thin films is examined using transmission electron microscopy and microanalysis. The specimen was prepared by using laser ablation to first deposit an approximately 2500 Å thick (0 0 1) YBCO thin film onto a (0 0 1) MgO substrate. An approximately 7500 Å thick (0 0 1) BaF2 thin film was subsequendy thermally evaporated onto the YBCO film.Images from a VG HB501A UHV scanning transmission electron microscope (STEM) operating at 100 kV show that the thickness of the BaF2 film is rather uniform, with the BaF2/YBCO interface being quite flat. Relatively few intrinsic defects, such as hillocks and depressions, were evident in the BaF2 film. Moreover, the hillocks and depressions appear to be faceted along {111} planes, suggesting that the surface is smooth and well-ordered on an atomic scale and that an island growth mechanism is involved in the evolution of the BaF2 film.

N. D. Browning ◽  
M. M. McGibbon ◽  
M. F. Chisholm ◽  
S. J. Pennycook

The recent development of the Z-contrast imaging technique for the VG HB501 UX dedicated STEM, has added a high-resolution imaging facility to a microscope used mainly for microanalysis. This imaging technique not only provides a high-resolution reference image, but as it can be performed simultaneously with electron energy loss spectroscopy (EELS), can be used to position the electron probe at the atomic scale. The spatial resolution of both the image and the energy loss spectrum can be identical, and in principle limited only by the 2.2 Å probe size of the microscope. There now exists, therefore, the possibility to perform chemical analysis of materials on the scale of single atomic columns or planes.In order to achieve atomic resolution energy loss spectroscopy, the range over which a fast electron can cause a particular excitation event, must be less than the interatomic spacing. This range is described classically by the impact parameter, b, which ranges from ~10 Å for the low loss region of the spectrum to <1Å for the core losses.

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