Electron Nanodiffraction Patterns Research Articles

Correlation symmetry analysis of electron nanodiffraction from amorphous materials

Angular symmetry in diffraction reflects rotational symmetry in the sample. We introduce the angular symmetry coefficient as a method to extract local symmetry information from electron nanodiffraction patterns of amorphous materials. Symmetry coefficients are the average of the angular autocorrelation function at the characteristic angles of a particular rotational symmetry. The symmetry coefficients avoid non-structural features arising from Fourier transformation and Friedel symmetry breakdown that affect the angular power spectrum approach to determining angular symmetries in amorphous nanodiffraction. Both methods require thin samples to avoid overlapping diffraction from clusters of atoms separated in the thickness of the sample, but symmetry coefficients are more forgiving. Electron nanodiffraction experiments on a Pd-based metallic glass sample demonstrate both potentially misleading information in angular power spectrum and the utility of symmetry coefficients.

(Invited) Temperature-Dependent Thermal Diffuse Scattering for Scanning Transmission Electron Microscope Thermometry

Scanning transmission electron microscopy (STEM) thermometry techniques open new opportunities for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on measuring thermally-induced strains must contend with small thermal expansion coefficients (<10 parts per million (ppm)/K) for some materials, as well as potentially non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements, and Debye-Waller theory predicts that many materials should display temperature coefficients >1000 ppm/K at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, because the Debye-Waller effect on the Bragg peak intensity is typically overwhelmed by the effects of thermal tilts and thermal drift. Here, we demonstrate quantitative TDS measurements using STEM by measuring the diffuse background intensity (rather than the Bragg peak intensity) in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS in the diffuse background between the Bragg spots; previous TEM work (with the beam in flood mode) showed that this diffuse signal is relatively insensitive to thermal tilts and drift. Using this diffuse signal, we measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K, and compare our results with the predictions of Debye-Waller theory. The measurements display typical temperature uncertainties of 8 K and temperature sensitivities of 51 K Hz-1/2. This TDS-based STEM thermometry demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures. Figure 1

Electron microscopy used in measuring temperature-dependent thermal diffusion scattering

Measuring the diffuse scattering in energy-filtered scanning electron nanodiffraction patterns provides way forward for nanoscale temperature readings.

Measuring temperature-dependent thermal diffuse scattering using scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) thermometry techniques offer the potential for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on thermally induced strains must contend with small thermal expansion coefficients [&amp;lt;10 parts per million (ppm)/K] for some materials of interest, as well as non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements with larger temperature coefficients (&amp;gt;1000 ppm/K) for almost all materials at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, due to experimental difficulties in quantifying the relatively small thermal signals. Here, we demonstrate quantitative TDS measurements using STEM by measuring diffuse scattering in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS between the Bragg spots. We measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K and compare this result with the predictions of Debye-Waller theory. This TDS-based STEM thermometry technique demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures.

Statistical Analyses of Electron Nanodiffraction Patterns to Examine Order and Structural Variability in Amorphous Materials

Statistical Analyses of Electron Nanodiffraction Patterns to Examine Order and Structural Variability in Amorphous Materials

Determination of the Projected Atomic Potential by Deconvolution of the Auto-Correlation Function of TEM Electron Nano-Diffraction Patterns

We present a novel method to determine the projected atomic potential of a specimen directly from transmission electron microscopy coherent electron nano-diffraction patterns, overcoming common limitations encountered so far due to the dynamical nature of electron-matter interaction. The projected potential is obtained by deconvolution of the inverse Fourier transform of experimental diffraction patterns rescaled in intensity by using theoretical values of the kinematical atomic scattering factors. This novelty enables the compensation of dynamical effects typical of transmission electron microscopy (TEM) experiments on standard specimens with thicknesses up to a few tens of nm. The projected atomic potentials so obtained are averaged on sample regions illuminated by nano-sized electron probes and are in good quantitative agreement with theoretical expectations. Contrary to lens-based microscopy, here the spatial resolution in the retrieved projected atomic potential profiles is related to the finer lattice spacing measured in the electron diffraction pattern. The method has been successfully applied to experimental nano-diffraction data of crystalline centrosymmetric and non-centrosymmetric specimens achieving a resolution of 65 pm.

Interpretation of angular symmetries in electron nanodiffraction patterns from thin amorphous specimens.

The interpretation of angular symmetries in electron nanodiffraction patterns from thin amorphous specimens is examined. It is found that in general there are odd symmetries in experimental electron nanodiffraction patterns. Using simulation, it is demonstrated that this effect can be attributed to dynamical scattering, rather than other divergences from the ideal experimental conditions such as probe-forming lens aberrations and camera noise. The departure of opposing diffracted intensities from Friedel's law in the phase grating formalism is calculated using a general structure factor for disordered materials. On the basis of this, a simple correction procedure is suggested to recover the kinematical angular symmetries, and thus readily interpretable information that reflects the symmetries of the original projected object. This correction is numerically tested using both the phase object and multislice calculations, and is demonstrated to fully recover all the kinematical diffracted symmetries from a simulated atomic model of a metallic glass.

Finestructures study of the diamond/titanium interface by transmission electron microscopy

It is well known that a TiC layer can be formed and should act as a buffer layer in diamond films deposited on Ti alloy. Through our cross-sectional investigation in HRTEM, a thin layer (20–30 nm) was first identified between the outermost diamond film and the inner reactive TiC layer adjacent to the substrate. This layer consists of numerous crystalline nanoparticles with grain sizes of 5–20 nm. Through electron nanodiffraction patterns combined with EDS and EELS analysis, these nanoparticles can be identified as a TiC1−xOx phase with a similar structure to cubic TiC. Besides, C atoms and O atoms in TiC1−xOx randomly occupy the vacancies of C in TiC. The thickness of this TiC1−xOx layer does not change significantly with increasing deposition time, and the diamond phase directly nucleates and grows on it.

Systematic mapping of icosahedral short-range order in a melt-spun Zr36Cu64 metallic glass.

By analyzing the angular correlations in scanning electron nanodiffraction patterns from a melt-spun Zr(36)Cu(64) glass, the dominant local order was identified as icosahedral clusters. Mapping the extent of this icosahedral short-range order demonstrates that the medium-range order in this material is consistent with a face-sharing or interpenetrating configuration. These conclusions support results from atomistic modeling and a structural basis for the glass formability of this system.

Structures of Carbon Nanocrystals

Single-crystal electron nanodiffraction patterns have been obtained from nanocrystals of various phases formed within carbonaceous balls produced by chemical vapor deposition (CVD). For the face-centered cubic phase, n-diamond, with cell dimension a = 0.36 nm, the relative intensities of the (200) reflection (forbidden for the diamond structure) and the (111) reflection suggest that the structure may include hydrogen atoms, and theoretical analyses support this possibility. There is evidence for a body-centered cubic phase with a = 0.31 nm, and the incorporation of hydrogen in this structure is suggested. Small amounts of hexagonal diamond, i-carbon, and other cubic phases are also present.

A Quantitative Nanodiffraction System for Ultrahigh Vacuum Scanning Transmission Electron Microscopy

Of all the long-lived particles available as probes of condensed matter, and of all the signals available on a modern electron microscope, electron nanodiffraction patterns provide the strongest signal from the smallest volume. The technique is therefore perfectly suited to nanostructural investigations in inorganic chemistry and materials science. The Vacuum Generators HB501S, an ultrahigh vacuum (UHV) variant of the HB501 scanning transmission electron microscope (STEM), with side-entry double-tilt stage, specimen preparation and analysis chamber, three postspecimen lenses, and cold field-emission tip with integral magnetic gun lens, has therefore been modified to optimize nanodiffraction and quantitative convergent beam electron diffraction (QCBED) performance. A one-micrometer grain-size phosphor screen lying on a fiber-optic faceplate atop the instrument is fiber-optically coupled to a 2048 x 2048 charge-coupled device (CCD), 16-bit camera. This arrangement promises to provide much greater sensitivity, larger dynamic range, and a better modulation transfer function (MTF) than conventional single crystal scintillator (YAG) CCD systems, with noticeable absence of cross talk between pixels. The design of the nanodiffraction detector system is discussed, the gain of the detector is measured, the spherical aberration constant of the objective lens is measured by the Ronchigram method, and preliminary results from the modified instrument are shown.

SURFACE BONDING STATES OF NANO-CRYSTALLINE DIAMOND BALLS

The rough surface of nano-crystalline diamond spheres induces surface electronic states which appear as a broadened pre-peak over approx. 15 eV at the C K-edge energy threshold for carbon in the parallel electron energy loss spectrum (PEELS). This appears to be at least partially due to 1s-π* transitions, although typically the latter occupy a range of only 4 eV for the sp2 edge of highly-oriented pyrollytic graphite (HOPG). No π* electrons appear in the conduction band inside the diamond particles, where all electrons are sp3 hybridized. PEELS data were also obtained from a chemical vapour deposited diamond film (CVDF) and gem-quality diamond for comparison with the spectra of nano-diamonds. The density of sp2 and sp3 states on the surface of diamond nano-crystals is calculated for simple structural models of the diamond balls, including some conjecture about surface structures. The results are used to interpret the sp2/sp3 ratios measured from the PEELS spectra recorded as scans across the particles. Surface roughness at the atomic scale was also examined using high-resolution transmission electron microscopy (HRTEM) and electron nano-diffraction patterns were used to confirm the crystal structures.

SIZE AND SURFACE STRUCTURE OF DIAMOND NANO-CRYSTALS

High-resolution transmission electron microscopy and electron nano-diffraction patterns provide useful information concerning the size distribution and surface condition of nano-crystalline diamond powder produced by shock detonation. Analysis of X-ray powder diffractometer data, using the Debye equation to predict the powder profile, starting with a set of spherical diamond ball models, allows the mean size to be obtained for bulk quantities of the same powders. The electron powder diffraction patterns for some diamond spheres, and also spherical shell, models of the surface structures were calculated using the Debye equation, and compared with experiment. It is suggested that the surface structure may become accessible to diffraction methods, if a single nano-crystal is irradiated and both the single crystal pattern plus the diffuse scattering are recorded, as a function of the orientation of that single crystal.

Physical and electronic structure in carbon nanotubes

Electron nanodiffraction patterns and high-resolution images from bundles of single-walled carbon nanotubes (SWNTs) have been obtained to determine the helicity and the packing of tubes. It is found that there is no predominating helicity, only a weak tendency of being of `armchair' structure; the inter-tube spacing is close to 0.34 nm and several packing orientations may coexist in a single bundle. Electron energy loss spectra from the bundles of SWNTs and multi-walled carbon nanotubes are used to study electronic structure. The π-bonding is proposed as inter-tube bonding in the bundles. The plasmon energy Ep depends on the diameter and inter-tube (or inter-walled) spacing.

Helicity and packing of single-walled carbon nanotubes studied by electron nanodiffraction

Electron nanodiffraction patterns from raft-like bundles of single-walled carbon nanotubes have been obtained to analyze the helicity distribution and the packing of the tubules. In most cases, the helical angles were found to be quite evenly distributed, ranging from the zigzag structure of 0° helicity to the armchair structure of 30° helicity. Examples of preferred helicities were encountered occasionally, such as bundles of tubules of almost single-helicity, corresponding to the armchair structure. The lattice constant of the hexagonally packed bundles varied and two example values were determined to be 1.64 nm and 1.71 nm, respectively.

Coherent electron nanodiffraction from clean silver nano particles in a UHV STEM

Small metal particles have peculiar chemical and physical properties as compared to bulk materials. They are especially important in catalysis since metal particles are common constituents of supported catalysts. The structural characterization of small particles is of primary importance for the understanding of structure-catalytic activity relationships. The shape and size of metal particles larger than approximately 5 nm in diameter can be determined by several imaging techniques. It is difficult, however, to deduce the shape of smaller metal particles. Coherent electron nanodiffraction (CEND) patterns from nano particles contain information about the particle size, shape, structure and defects etc. As part of an on-going program of STEM characterization of supported catalysts we report some preliminary results of CEND study of Ag nano particles, deposited in situ in a UHV STEM instrument, and compare the experimental results with full dynamical simulations in order to extract information about the shape of Ag nano particles.

Ordered Structure in GalnP/AIGalnP Quantum Wells and p-Doped Multiquantum Well AIGalnp Laser Diodes

ABSTRACTZinc doping is performed on the GalnP/AIGalnP multiquantum well (MQW) structure with the aim of dissolving the ordered atomic arrangement which results in higher quantum levels and therefore shorter lasing wavelengths. It is shown that the photoluminescence (PL) peak wavelength gradually shortens with doping and decreases by 20 nm when the hole concentration reaches 1×1018 cm−3, while the PL relative intensity becomes half that of an undoped MQW layer. Therefore, a moderate level of zinc doping of around 4∼5×1017 cm−3 is desirable to shorten the PL wavelength without decreasing the crystal quality. Transmission electron nano-diffraction patterns confirm that the ordered structure in the MQW layers disappears as the hole concentration increases. On the basis of this data, uniformly p-doped and modulation p-doped MQW laser diodes are fabricated and their characteristics are compared with the undoped MQW lasers. CW operation is achieved at wavelengths of 631 to 633 nm, which is 10 nm shorter than the 643 nm in an undoped MQW laser. Comparatively low threshold currents of 73 and 88 mA are attained for uniformly p-doped and modulation p-doped MQW lasers, respectively. However, they are about 20∼30 mA higher than those of the undoped MQW lasers. This results from the large overflow of electrons from the active layer, and the fact that the differential gain becomes smaller in the 630-nm band.

Coherent electron nanodiffraction from small metal particles

Small electron probes of 10-15 Å in diameter can be easily formed at specimens in scanning transmission electron microscopes (STEM) using field emission guns (FEG). Nanodiffraction patterns from individual small metal particles (&lt; 50 Å) obtained with such a probe contain non-overlapping CBED disks. As a first order approximation, these patterns can be interpreted similarly to those for parallel beam illumination and then bulk crystallographic information is obtained. However there is more information contained in the nanodiffraction patterns. In the case of small metal particles, spot splitting occurs when the small electron beam is close to a particle edge, which results from the coherence of the probe. Spot splitting is a manifestation of a crystallographic surface plane of a particle being parallel to the beam since the splitting is along the direction normal to the particle edge. This phenomenon is one of the characteristic features of coherent electron nanodiffraction patterns of small metal particles. It will be shown that it is essentially a particle size effect!

Determination of atomic positions using electron nanodiffraction patterns from overlapping regions: Si[110

A procedure for determining atomic positions is described that utilizes electron nanodiffraction patterns from overlapping regions. The technique is applied using experimental data collected for Si[110] with a beam 4 Å in diameter. Autocorrelation functions are calculated for each beam position and correlated with theoretical functions. The positions of the Si atoms are established with an accuracy of ±0.2 Å, and images with 1 Å resolution are computed for individual beam positions. It is also demonstrated how the technique may be used to determine, with similar resolution, the structure of noncrystalline regions adjacent to crystalline regions.