Local Specimen Thickness Research Articles

Site-specific angular dependent determination of inelastic mean free path of 300 keV electrons in GaN nanorods.

Inelastic mean free path (IMFP) of the electron is a very important parameter for quantitative analysis of several electron spectroscopies and transport properties. In spite of being a fundamental material property, its experimental determination is not trivial due to complexity of the various electron scattering processes in matter. In this report, we demonstrate a procedure to determine the IMFP of 300 keV electrons in GaN, using the log-ratio technique where the local specimen thickness needs to be accurately known. The GaN nanorod morphology of the sample used here allows the accurate measurement of thickness by 'thickness map' under EFTEM measurements which enable the site specific determination of IMFP. IMFP for different collection semi angles have also been measured to validate the angular dependence. Our experimental results estimates the IMFP of GaN for 300 keV electrons to be 143 ± 11 nm at no-aperture condition and exhibit a strong inverse angular dependence at smaller collection semi angles (β < 20 mrad) and a near angular independence at larger collection semi angles (β > 30 mrad). We discuss these results in the light of three different theoretical models prevalent in the literature.

Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM

Scanning transmission electron microscopy (STEM) at low energies (≤ 30 keV) in a scanning electron microscope is well suited to distinguish weakly scattering materials with similar materials properties and analyze their microstructure. The capabilities of the technique are illustrated in this work to resolve material domains in PTB7:PC71BM bulk-heterojunctions, which are commonly implemented for light-harvesting in organic solar cells. Bright-field (BF-) and high-angle annular dark-field (HAADF-) STEM contrast of pure PTB7 and PC71BM was first systematically analyzed using a wedge-shaped sample with well-known thickness profile. Monte-Carlo simulations are essential for the assignment of material contrast for materials with only slightly different scattering properties. Different scattering cross-sections were tested in Monte-Carlo simulations with screened Rutherford scattering cross-sections yielding best agreement with the experimental data. The STEM intensity also depends on the local specimen thickness, which can be dealt with by correlative STEM and scanning electron microscopy (SEM) imaging of the same specimen region yielding additional topography information. Correlative STEM/SEM was applied to determine the size of donor (PTB7) and acceptor (PC71BM) domains in PTB7:PC71BM absorber layers that were deposited from solution with different contents of the processing additive 1,8-diiodooctane (DIO).

Quantitative HAADF STEM of SiGe in presence of amorphous surface layers from FIB preparation

The chemical composition of four Si1−xGex layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of Si1−xGex in STEM.

Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe2

The phase and amplitude of the electron wavefunction that has passed through ultra-thin flakes of WSe2 is measured from high-resolution off-axis electron holograms. Both the experimental measurements and corresponding computer simulations are used to show that, as a result of dynamical diffraction, the spatially averaged phase does not increase linearly with specimen thickness close to an [001] zone axis orientation even when the specimen has a thickness of only a few layers. It is then not possible to infer the local specimen thickness of the WSe2 from either the phase or the amplitude alone. Instead, we show that the combined analysis of phase and amplitude from experimental measurements and simulations allows an accurate determination of the local specimen thickness. The relationship between phase and projected potential is shown to be approximately linear for extremely thin specimens that are tilted by several degrees in certain directions from the [001] zone axis. A knowledge of the specimen thickness then allows the electrostatic potential to be determined from the measured phase. By using this combined approach, we determine a value for the mean inner potential of WSe2 of 18.9±0.8V, which is 12% lower than the value calculated from neutral atom scattering factors.

Making sense of nanocrystal lattice fringes

The orientation dependence of thin-crystal lattice fringes can be gracefully quantified using fringe-visibility maps, a direct-space analog of Kikuchi maps [Nishikawa and Kikuchi, Nature (London) 121, 1019 (1928)]. As in navigation of reciprocal space with the aid of Kikuchi lines, fringe-visibility maps facilitate acquisition of crystallographic information from lattice images. In particular, these maps can help researchers to determine the three-dimensional lattice of individual nanocrystals, to “fringe-fingerprint” collections of randomly oriented particles, and to measure local specimen thickness with only a modest tilt. Since the number of fringes in an image increases with maximum spatial-frequency squared, these strategies (with help from more precise goniometers) will be more useful as aberration correction moves resolutions into the subangstrom range.

Cross-fringe Versus Single-fringe Probabilities

High-resolution TEM is well-suited to characterizing nanocrystals, where lattice fringes serve as a source of structural information [1, 2]. For example, 3D lattice parameters can be measured from 2D lattice fringe images taken at as few as two different specimen orientations [3-6]. Recent work has shown that lattice fringe-visibility maps, a thin-specimen extension of bend-contour and channeling-pattern maps, can assist crystallographic study in direct space much as do Kikuchi maps in reciprocal space [7]. For example with lattice resolution and a sufficiently precise multi-axis goniometer, a nanocrystal can be tilted while the condition for visualizing a set of lattice fringes is maintained so as to acquire new lattice fringe normals (co-vectors) and thus continually refine a basis triplet containing information on both the nanocrystal's lattice and its orientation. Local specimen thickness measurements are another promising possibility.

Development of Scanning-Type Synchrotron Radiation Mössbauer Microscope Using Focused X-Ray

A scanning-type synchrotron radiation Mössbauer microscope was designed and installed at the nuclear resonance beamline BL11XU of the SPring-8 facility. The focused Mössbauer probe beam (<φ20 µm) is obtained by the combination of a high-resolution monochromator, a multilayer X-ray focusing mirror and a pinhole slit. Using this Mössbauer microscope, we have performed, for the first time, the synchrotron radiation Mössbauer microscopic measurements of nuclear resonant forward scattering from an iron borate single crystal as a function of the sample position. It was found that the observed Mössbauer time spectra depend on the direction of the local hyperfine magnetic field and the local specimen thickness.

Compositional analysis based on electron holography and a chemically sensitive reflection

A method for compositional analysis of low-dimensional heterostructures is presented. The suggested procedure is based on electron holography and the exploitation of the chemically sensitive (0 0 2) reflection. We apply an off-axis imaging condition with the (0 0 2) beam strongly excited and centered on the optic axis. The first side band of the hologram is centered using an “empty” reference hologram obtained for a hole of the specimen. From the centered side band we use the phase of the central (0 0 0) and the amplitude of the (0 0 2) reflections to evaluate the local composition and the local specimen thickness in an iterative and self-consistent way. Delocalization effects that lead to a shift of the spatial information of (0 0 0) and (0 0 2) reflections are taken into account. The application of the procedure is demonstrated with an AlAs/ GaAs(0 0 1) superlattice with a period of 5 nm. The concentration profiles obtained are discussed in relation to segregation. The measured segregation efficiency is R=0.51±0.02.

Interfacial Segregation and Concentration Profiles by Energy-Filtered Transmission Electron Microscopy: Issues and Guidelines

Abstract Over the several years that imaging energy filters have been available commercially, numerous and wide-ranging applications have demonstrated elemental mapping at resolutions approaching 1 nm. From reviewing applications of energy-filtered transmission electron microscopy (EFTEM) to interfacial segregation and concentration profiles in metals and ceramics, some guidelines and issues may be identified. The work has been performed with a Gatan imaging filter (GIF) interfaced to a Philips CM30 operated at 300 kV with a LaB6 cathode. For quantitative elemental mapping by EFTEM a number of interrelated parameters [field of view, resolution, collection angle (β), slit width (Δ), incident beam divergence (α), illumination uniformity, probe current (i), drift rate, exposure time (T), and local specimen thickness (t)] have always to be properly selected and controlled. For example, the collection angle should be large enough for a strong signal, but not so large that the signal/background (S/B) and chromatic-aberration-limited spatial resolution (δ) are both degraded.

Direct imaging of spatially varying potential and charge across internal interfaces in solids

The importance of spatially varying potential (and thus charge) across lattice discontinuities in solids has been recognized in many technologically important systems, especially those containing electrically active interfaces, e.g. electroceramics. The presence of spatially varying potential across electroceramic interfaces has been indirectly deduced and analyzed using predominantly bulk measurement techniques like I-V, C-V curves and impedance spectroscopy. Direct imaging of spatially varying electrostatic potential profile and determination of the sign, magnitude and spatial distribution of the associated interface- and space- charge (and therefore defect density) in electroceramics have remained elusive.We have utilized the technique of transmission high energy electron holography to directly image and quantify the electrostatic potential across grain boundaries (GBs) in SrTiO3, a functional electroceramic. The phase of the exit wave function at the GB region can be altered by : (i) variation in local mean inner potential (related to the change in density of atoms at the GB), (ii) differential diffraction conditions across the interface, (iii) change in the local specimen thickness and, (iv) presence of local electrostatic (electrical charge) and magnetic potential.