Quantification of the thickness of TEM samples by low-energy scanning transmission electron microscopy

Precise knowledge of the local sample thickness is often required for quantitative scanning (transmission) electron microscopy (STEM). The local sample thickness can be determined by the comparison of measured intensities from high-angle annular dark-field (HAADF)-STEM at low energies (<30 keV) with Monte-Carlo simulations. However, a suitable choice of the scattering cross-section (CS) used in the simulations is necessary to gain reliable thickness results. In this work, simulations using different CS, including the Screened Rutherford CS and different Mott CSs, were performed. The results were then compared with measurements on samples with known thickness and composition, for which an SEM equipped with a STEM detector was used. In most cases, the Screened Rutherford CS describes the experiment better than other CSs

Quantitative analysis of backscattered‐electron contrast in scanning electron microscopy

Backscattered-electron scanning electron microscopy (BSE-SEM) imaging is a valuable technique for materials characterisation because it provides information about the homogeneity of the material in the analysed specimen and is therefore an important technique in modern electron microscopy. However, the information contained in BSE-SEM images is up to now rarely quantitatively evaluated. The main challenge of quantitative BSE-SEM imaging is to relate the measured BSE intensity to the backscattering coefficient η and the (average) atomic number Z to derive chemical information from the BSE-SEM image. We propose a quantitative BSE-SEM method, which is based on the comparison of Monte–Carlo (MC) simulated and measured BSE intensities acquired from wedge-shaped electron-transparent specimens with known thickness profile. The new method also includes measures to improve and validate the agreement of the MC simulations with experimental data. Two different challenging samples (ZnS/Zn(O$_x$S$_{1–x}$)/ZnO/Si-multilayer and PTB7/PC$_{71}$BM-multilayer systems) are quantitatively analysed, which demonstrates the validity of the proposed method and emphasises the importance of realistic MC simulations for quantitative BSE-SEM analysis. Moreover, MC simulations can be used to optimise the imaging parameters (electron energy, detection-angle range) in advance to avoid tedious experimental trial and error optimisation. Under optimised imaging conditions pre-determined by MC simulations, the BSE-SEM technique is capable of distinguishing materials with small composition differences

Beam broadening measured in transmission mode at low electron energies in a scanning electron microscope

The broadening of the electron beam in the sample has to be considered when performing scanning transmission electron microscopy (STEM) at low primary electron energies. This work presents direct measurements of the beam broadening in a range of materials. The experimental results are compared with the theoretical model by Gauvin and Rudinsky that uses the concept of anomalous diffusion to obtain an analytical equation for the beam broadening

Electron-beam broadening in electron microscopy by solving the electron transport equation

Scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) are prominent techniques for the structural characterization of materials. STEM in particular provides high spatial resolution down to the sub-ångström range. The spatial resolution in STEM and SEM is ultimately limited by the electron-beam diameter provided by the microscope\u27s electron optical system. However, the resolution is frequently degraded by the interaction between electron and matter leading to beam broadening, which depends on the thickness of the analyzed sample. Numerous models are available to calculate beam broadening. However, most of them neglect the energy loss of the electrons and large-angle scattering. These restrictions severely limit the applicability of the approaches for large sample thicknesses in STEM and SEM. In this work, we address beam broadening in a more general way. We numerically solve the electron transport equation without any simplifications, and take into account energy loss along the electron path. For this purpose, we developed the software package CeTE (Computation of electron Transport Equation). We determine beam broadening, energy deposition, and the interaction volume of the scattered electrons in homogeneous matter. The calculated spatial and angular distributions of electrons are not limited to forward scattering and small sample thicknesses. We focus on low electron energies of 30 keV and below, where beam broadening is particularly pronounced. These electron energies are typical for SEM and STEM in scanning electron microscopes

Fast Determination of the Thickness of Electron-Transparent Specimens using Quantitative STEM-in-SEM and Monte-Carlo Simulations

Accurate values for the thickness of electron-transparent specimens in electron microscopy are of general interest, e.g. as a parameter for quantitative simulations and calculations in the field of transmission electron microscopy (TEM). Several thickness-determination techniques exist, e.g. based on plasmon losses in electron energy loss spectra, convergent-beam electron diffraction, or exploitation of thickness contours in images acquired under two-beam diffraction conditions. However, the accuracy, precision, and time consumption differs significantly and often yields thickness values only for a small sample region. We will show in this work that scanning transmission electron microscopy (STEM) in a scanning electron microscope (STEM-in-SEM) is well suited for thickness determination with rather satisfying accuracy (error within a few percent). This technique has been further elaborated by us after previous initial work. We will give an in-depth instruction and discussion of the technique so that users can avoid pitfalls

Room-temperature liquid-phase synthesis of aluminium nanoparticles

Aluminium nanoparticles, Al(0), are obtained via liquid-phase synthesis at 25 °C. Accordingly, AlBr₃ is reduced by lithium naphthalenide ([LiNaph]) in toluene in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA). The Al(0) nanoparticles are small (5.6 ± 1.5 nm) and highly crystalline. A light yellow colour and absorption at 250–350 nm are related to the plasmon-resonance absorption. Due to TMEDA functionalization, the Al(0) nanoparticles are colloidally and chemically stable, but show high reactivity after TMEDA removal

Ionic-liquid-based synthesis of GaN nanoparticles

GaN nanoparticles, 3–8 nm in diameter, are prepared by a microwave-assisted reaction of GaCl3 and KNH2 in ionic liquids. Instantaneously after the liquid-phase synthesis, the β-GaN nanoparticles are single-crystalline. The band gap is blue-shifted by 0.6 eV in comparison to bulk-GaN indicating quantum confinement effects. The GaN nanoparticles show intense green emission with a quantum yield of 55 ± 3%

Versatile application of a modern scanning electron microscope for materials characterization

Scanning electron microscopy (SEM) is an indispensable characterization technique for materials science. More recently, scanning electron microscopes can be equipped with scanning transmission electron microscopy (STEM) detectors, which considerably extend their capabilities. It is demonstrated in this work that the correlative application of SEM and STEM imaging techniques provides comprehensive sample information on nanomaterials. This is highlighted by the use of a modern scanning electron microscope, which is equipped with in-lens and in-column detectors, a double-tilt holder for electron transparent specimens and a CCD camera for the acquisition of on-axis diffraction patterns. Using multi-walled carbon nanotubes and Pt/Al$_{2}$O$_{3}$ powder samples we will show that a complete characterization can be achieved by combining STEM (mass-thickness and diffraction) contrast and SEM (topography and materials) contrast. This is not possible in a standard transmission electron microscope where topography information cannot be routinely obtained. We also exploit the large tilt angle range of the specimen holder to perform 180 degrees STEM tomography on multi-walled carbon nanotubes, which avoids the missing wedge artifacts

Electron Microscopic Investigation of Post-Annealed Superconducting GdBa2_2Cu3_3O7−δ_{7-δ} Thin Films on MgO

GdBa$_2$Cu$_3$O$_{7-δ}$ (GdBCO) is a promising high-temperature superconductor with potential application as coated conductor for power applications (e.g. transformers). However, further improvements in cost-effectiveness and superconducting properties are desired. The latter are influenced by O content, film texture, and film quality, where non-superconducting defects can effectively pin magnetic flux lines if they have favorable shape, size, and density. The O content in GdBCO can be set in-situ after film deposition or afterward by annealing in a different furnace (ex-situ). The correlation of oxygenation processes and defect formation/healing in GdBCO is not fully understood. In this work, we apply scanning transmission electron microscopy (STEM) to investigate the effect of different ex-situ oxygenation routes on the GdBCO microstructure