Quantitative High-resolution Electron Microscopy Research Articles

Electron image contrast analysis of mosaicity in rutile nanocrystals using direct electron detection.

Direct electron detection provides high detective quantum efficiency, significantly improved point spread function and fast read-out which have revolutionized the field of cryogenic electron microscopy. However, these benefits for high-resolution electron microscopy (HREM) are much less exploited, especially for in situ study where major impacts on crystallographic structural studies could be made. By using direct detection in electron counting mode, rutile nanocrystals have been imaged at high temperature inside an environmental transmission electron microscope. The improvements in image contrast are quantified by comparison with a charge-coupled device (CCD) camera and by image matching with simulations using an automated approach based on template matching. Together, these approaches enable a direct measurement of 3D shape and mosaicity (∼1°) of a vacuum-reduced TiO2 nanocrystal about 50 nm in size. Thus, this work demonstrates the possibility of quantitative HREM image analysis based on direct electron detection.

Experimental measurements and theoretical calculations of the atomic structure of materials with subangstrom resolution and picometer precision

Lattice defects are unavoidable structural units in materials and play an important role in determining material properties. Compared with the periodic structure of crystals, the atomic configurations of the lattice defects are determined by the coordinates of a large number of atoms, making it difficult to experimentally investigate them. In computational materials science, multiparameter optimization is also a difficult problem and experimental verification is usually required to determine the possibility of obtaining the structure and properties predicted by calculations. Using our recent studies on oxide surfaces as examples, we introduce the method of integrated aberration-corrected electron microscopy and the first-principles calculations to analyze the atomic structure of lattice defects. The atomic configurations of defects were measured using quantitative high-resolution electron microscopy at subangstrom resolution and picometer precision, and then the electronic structure and dynamic behavior of materials can be studied at the atomic scale using the first-principles calculations. The two methods complement each other and can be combined to increase the understanding of the atomic structure of materials in both the time and space dimensions, which will benefit materials design at the atomic scale.

More than meets the Eye—Myelinated axons crowd the subthalamic nucleus

High frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a successful treatment for patients with advanced Parkinson's disease (PD). Although its exact mechanism of action is unknown, it is currently believed that the beneficial effects of the stimulation are mediated either by alleviating pathological basal ganglia output patterns of activity or by activation of the axons of passage that arise from the cerebral cortex and other sources. In this study, we show that the anatomical composition of the primate STN provides a substrate through which DBS may elicit widespread changes in brain activity via stimulation of fibers of passage. Using quantitative high-resolution electron microscopy, we found that the primate STN is traversed by numerous myelinated axons, which occupy as much as 45% of its sensorimotor territory and 36% of its associative region. In comparison, myelinated axons occupy only 27% of the surface areas of the sensorimotor and associative regions of the internal segment of the globus pallidus (GPi), another target for therapeutic DBS in PD. We also noted that myelinated axons in the STN, on average, have a larger diameter than those in GPi, which may render them more susceptible to electrical stimulation. Because axons are more excitable than other neuronal elements, our findings support the hypothesis that STN DBS, even when carried out entirely within the confines of the nucleus, mediates some of its effects by activating myelinated axons of passage.

Quantitative high resolution electron microscopy image matching applied to the strontium titanate Σ3(112) grain boundary

Quantitative image matching has been employed in order to compare experimentally observed grain boundary structure to predicted model structures for the strontium titanate (SrTiO3) Σ3(112) grain boundary. Initially, quantitative image matching of image regions containing bulk crystal were used to determine the specimen and microscope parameters, including specimen thickness and tilt, and image defocus. Using the fitted values for thickness and defocus, images have been simulated from two proposed model structures of the SrTiO3 Σ3(112) grain boundary and quantitatively matched to experimental images.

Strain measurements in s-Si/SiGe nanostructures by quantitative high-resolution electron microscopy

AbstractWe have studied strained Si layers grown on Si1-xGex virtual substrate (VS) by high-resolution transmission electron microscopy (HRTEM). Aberration-corrected HRTEM coupled with geometric phase analysis (GPA) provides precise measurements of strain. Different parameters are investigated: the VS composition (x=20, 30, 40 and 50%) and the s-Si layer thickness. Finite element method simulations confirm our measurements. Measurements and simulations lead to the conclusion that the strain state of the deposited layer is independent of the layer thickness. We apply the technique to measuring strains in the channel region of a p-MOSFET and show that the technique is a promising metrological tool for nanoelectronic devices.

Quantitative high resolution transmission electron microscopy of nanostructured semiconductors

Peak-finding procedures and the geometric phase method of quantitative high resolution electron microscopy (qHRTEM) were applied to determine the local strain and the chemical composition of nanostructured semiconductor materials. The growth of the structures investigated was induced by minimization of strain energy. The analysis of strain distribution is necessary for the understanding of the self-organized formation of nanostructures. The possibilities and limitations of the methods are discussed in detail by analysing HRTEM images of (Si,Ge) islands and of a double layer of stacked quantum dots of (In,Ga)As and Ga(Sb,As).

Analysis of Stresses and Strains around Dislocations at Grain Boundaries by Quantitative High-Resolution Electron Microscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Energy-filtered electron-diffracted beam holography

A method of energy-filtered electron holography is described where any two electron-diffracted beams can be interfered using an electron biprism. A Gatan image filter is used to select the energy loss of the electrons produced in the holograms. Gallium arsenide is used as the TEM specimen. This method of microscopy confirms that fringes extending beyond a limiting aperture were due to inelastically scattered electrons and specifically electrons scattered from the bulk plasmon. The degree of coherence of the zero-loss and energy-loss electrons were high and measured to be ∼0.3, which was maintained even for the high energy-loss electrons up to 100 eV. Future systematic studies using this method should help understand the Stobbs factor and contribute to the development of quantitative high-resolution electron microscopy.

Nanoscale Measurement of Stress and Strain by Quantitative High-Resolution Electron Microscopy

The geometric phase technique (GPA) for measuring the distortion of crystalline lattices from high-resolution electron microscopy (HRTEM) images will be described. The method is based on the calculation of the “local” Fourier components of the HRTEM image by filtering in Fourier space. The method will be illustrated with a study of an edge dislocation in silicon where displacements have been measured to an accuracy of 3 pm at nanometre resolution as compared with anisotropic elastic theory calculations. The different components of the strain tensor will be mapped out in the vicinity of the dislocation core and compared with theory. The accuracy is of the order of 0.5% for strain and 0.1° for rigid-body rotations. Using bulk elastic constants for silicon, the stress field is determined to 0.5 GPa at nanometre spatial resolution. Accuracy and the spatial resolution of the technique will be discussed.

The Effect of Substrates / Ligands on Metal Nanocatalysts Investigated By Quantitative Z-Contrast Imaging and High Resolution Electron Microscopy

AbstractOur direct density function-based simulations of Ru-, Pt- and mixed Ru-Pt clusters on carbon-based supports reveal that substrates can mediate the PtRu5 particles [1]. Oblate structure of PtRu5 on C has been found [2]. Nevertheless, the cluster-substrate interface interactions are still unknown. In this work, we present the applications of combinations of quantitative z-contrast imaging and high resolution electron microscopy in investigating the effect of different substrates and ligand shells on metal particles. Specifically, we developed a relatively new and powerful method to determine numbers of atoms in a nanoparticle as well as three-dimensional structures of particles including size and shape of particles on the substrates by very high angle (~96mrad) annular dark-field (HAADF) imaging [2-4] techniques. Recently, we successfully synthesize icosahedra Au13 clusters with mixed ligands and cuboctahedral Au13 cores with thiol ligands, which have been shown by TEM to be of sub-nanometer size (0.84nm) and highly monodisperse narrow distribution. X-ray absorption and UV-visible spectra indicate many differences between icosahedra and cuboctahedral Au13 cores. Particles with different ligands show different emissions and higher quantum efficiency has been found in Au11 (PPH3) SC12)2C12. We plan to deposit those ligands-protected gold clusters onto different substrates, such as, TiO2 and graphite, etc. Aforementioned analysis procedure will be performed for those particles on the substrates and results will be correlated with that of our simulations and activity properties. This approach will lead to an understanding of the cluster-substrates relationship for consideration in real applications.

Atomic structure of the interface between silicon (111) and amorphous germanium

The structure of the transition region between crystalline Si(111) and amorphous germanium has been studied by means of quantitative high-resolution electron microscopy. Using iterative image matching techniques the two-dimensional distribution function of germanium atoms in the transition region has been determined from focal image series of such interfaces. The distribution function reveals lateral ordering close to the crystalline substrate in addition to a pronounced layering usually observed for solid-liquid interfaces. It further shows that the transition region is elastically strained due to the volume misfit between crystalline silicon and amorphous germanium. The width of the transition region is $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ corresponding to about four (111) layers of crystalline silicon or germanium. Finally, the width of the bond-angle distribution for the first layer of germanium atoms on the silicon substrate is determined as 8.9\ifmmode^\circ\else\textdegree\fi{} which is close to the corresponding value of 9.7\ifmmode^\circ\else\textdegree\fi{} for bulk amorphous germanium.

The effect of the translational symmetry of crystalline silicon on the structure of amorphous germanium in the interfacial region

The structure of an amorphous Ge layer near an interface with a Si(111) crystal was studied by quantitative high-resolution electron microscopy. It was found that the translational symmetry of a Si crystal leads to the crystal-like order in the positions of Ge atoms in the interfacial region, the width of which is about 1.4 nm. In this region, the average orientation of interatomic bonds tilted with respect to the interface compensates for the difference in the bond lengths in crystalline Si and amorphous Ge and is responsible for the tetragonal distortion of the most likely atomic positions.

Impact of misfit dislocations on the polarization instability of epitaxial nanostructured ferroelectric perovskites.

Defects exist in almost all materials and defect engineering at the atomic level is part of modern semiconductor technology. Defects and their long-range strain fields can have a negative impact on the host materials. In materials with confined dimensions, the influence of defects can be even more pronounced due to the enhanced relative volume of the 'defective' regions. Here we report the dislocation-induced polarization instability of (001)-oriented Pb(Zr(0.52)Ti(0.48))O(3) (PZT) nanoislands, with an average height of approximately 9 nm, grown on compressive perovskite substrates. Using quantitative high-resolution electron microscopy, we visualize the strain fields of edge-type misfit dislocations, extending predominantly into a PZT region with a height of approximately 4 nm and width of approximately 8 nm. The lattice within this region deviates from the regular crystal structure. Piezoresponse force microscopy indicates that such PZT nanoislands do not show ferroelectricity. Our results suggest that misfit engineering is indispensable for obtaining nanostructured ferroelectrics with stable polarization.

Atomic-scale analysis of interfaces in an all-oxide magnetic tunnel junction

We use High Resolution Electron Microscopy (HRTEM) together with Electron Energy Loss Spectroscopy (EELS) to analyze the crystallography and the chemical configuration of interfaces in a state- of-the-art La2/3Sr1/3MnO3/SrTiO3/La2/3Sr1/3MnO3 tunnel junction. EELS indicates that manganese ions keep their bulk valency up to the last atomic plane in contact with the insulator. Tunnel magnetoresistance however decreases with temperature faster than magnetisation in these samples. Quantitative HRTEM reveals some local departures from chemical abruptness at the interfaces, which could play a role in this decrease.

Characterization of strain fields in Si1-xGex island structures by means of quantitative high-resolution electron microscopy

Characterization of strain fields in Si_1-xGe_x island structures by means of quantitative high-resolution electron microscopy

Recent developments in transmission electron microscopy imaging

A review of recent developments in imaging with a transmission electron microscope is proposed here. A first part is devoted to applications in the “imaging” mode: interest of field emission, “Z-contrast”, quantitative high resolution electron microscopy and reconstruction techniques. A second part will deal with the “diffraction” mode: convergent beam diffraction and “electron cristallography”.

Nanometre-scale defects in shear bands in a metallic glass

Abstract We have compared the structure of shear bands with that of undeformed regions of a metallic glass. Using quantitative high-resolution electron microscopy, we observed void-like defects, approximately 1 nm in diameter and at a concentration of one in 100nm3, which are generated as a result of plastic deformation in the shear bands. These defects may result from the coalescence of excess free volume upon cessation of the flow. By comparing the free energy of the shear band containing uniformly distributed free volume with that of the relaxed shear band with voids present, we show that the coalescence is thermo-dynamically possible.

Phonon Scattering in Quantitative High-Resolution Electron Microscopy – effects, problems and approaches

Phonon Scattering in Quantitative High-Resolution Electron Microscopy – effects, problems and approaches

Quantitative High Resolution Electron Microscopy

With the resolution becoming sufficient to reveal individual atoms, HREM is now entering the stage where it can compete with X-ray methods to quantitatively determine atomic structures of materials without much prior knowledge, but with the advantage of being applicable to aperiodic objects such as crystal defects. In our view the future electron microscope will be characterised by a large versatility in experimental settings under computer control such as the illumination conditions (TEM-STEM), CBED, detecting conditions (diffraction, image, ptychography) and many other tunable parameters such as focus (g), voltage, spherical aberration (C s ), beam tilt, etc. Since modern detectors can detect single electrons, also the counting statistics is known. The only limiting factor in the experiment will be the total number of electrons that interact with the object during the experiment due to the limitations in the exposure time or in the object damage. However, instrumental potentialities will never be exploited fully if not guided by an experimental strategy. Here intuitive guidelines can be very deceptive. For instance an image made with the best electron microscope (C s = 0) at the best focus (g = 0) from the best object (phase object) would show no contrast at all. Hence, questions such as what is the best C s , focus, object thickness, etc. can only be answered properly if done using a method of experiment design.

Current work at the Stuttgart UHV diffusion bonding facility

Abstract The Stuttgart ultra-high-vacuum (UHV) diffusion bonding facility serves to produce high-quality solid solid model interfaces. An overview is givef on the more recent work comprising bicrystals of metal–metal, ceramics–ceramics, and metal–ceramics combinations. Orientation errors below 0.1° have been obtained routinely with sapphire–sapphire bicrystals. The bi- and tricrystals produced serve as samples for mechanical testing and as specimens for both analytical and quantitative high-resolution electron microscopy. The close control of the interface chemistry allows for segregation studies and studies of the influence dopants have on the interface properties. UHV diffusaon-bonded low-angle grain boundaries present arrays of edge or screw dislocations of just one type and thus facilitate investigations of these dislocations and their core structures.