The Fresnel effect of a defocused biprism on the fringes in inelastic holography

We present energy filtered holography experiments on a thin foil of Al. By propagating the reduced density matrix of the probe electron through the microscope, we quantitatively predict the fringe contrast as a function of energy loss. Fringe contrast simulations include the effect of Fresnel fringes created at the edges of the defocused biprism, the effect of partial coherence in combination with inelastic scattering, and the effect of a finite energy distribution of the incoming beam.     

Inelastic electron holography as a variant of the Feynman thought experiment

Using a combination of electron holography and energy filtering, interference fringes produced after inelastic interaction of electrons with hydrogen molecules are examined. Surprisingly, the coherence of inelastic scattering increases when moving from the surface of a hydrogen-containing bubble to the vacuum. This phenomenon can be understood in terms of the Feynman two-slit thought experiment with a variable ambiguity of the which-way registration.     

Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, part II: inelastic scattering

The implementation of spherical aberration-corrected pre- and post-specimen lenses in the same instrument has facilitated the creation of sub-Angstrom electron probes and has made aberration-corrected scanning confocal electron microscopy (SCEM) possible. Further to the discussion of elastic SCEM imaging in our previous paper, we show that by performing a 3D raster scan through a crystalline sample using inelastic SCEM imaging it will be possible to determine the location of isolated impurity atoms embedded within a bulk matrix. In particular, the use of electron energy loss spectroscopy based on inner-shell ionization to uniquely identify these atoms is explored. Comparisons with scanning transmission electron microscopy (STEM) are made showing that SCEM will improve both the lateral and depth resolution relative to STEM. In particular, the expected poor resolution of STEM depth sectioning for extended objects is overcome in the SCEM geometry.     

Imaging using inelastically scattered electrons in CTEM and STEM geometry

It is shown that energy filtered transmission electron microscopy images are closely related to energy spectroscopic scanning transmission electron microscopy images. For the case of a single atom, we explore this similarity using both the coupled channels and density matrix approaches. We extend the result to the crystal case and find that the similarity persists, the limiting effects due to energy differences in the scattered electrons being small for typical specimen thicknesses in high-resolution transmission electron microscopy.     

Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L_2,3 edges was parametrized by measurement of the chemical shift of the L₃ edge and the ratio of integrated intensity under the L₃ and L₂ edges. These two parameterizations were correlated to Cr valence and also the dⁿ orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin dⁿ orbital configurations can be readily distinguished.     

Extending the analysis of EELS spectrum-imaging data, from elemental to bond mapping in complex nanostructures

Multiple least squares fitting has been employed for long time in elemental electron energy-loss spectroscopy (EELS) analysis, in particular in biology, but with the hypothesis of a rather stable shape for the used core-loss signals. In the present case, we explore its use for identifying the variations in the edges' fine structures in complex boron nitride samples and in particular for mapping the bonding types of boron in such samples. Details about this improved procedure applied to data acquired in the spectrum-imaging mode are reported here.     

Multivariate statistical analysis of electron energy-loss spectroscopy in anisotropic materials

Recently, an expression has been developed to take into account the complex dependence of the fine structure in core-level electron energy-loss spectroscopy (EELS) in anisotropic materials on specimen orientation and spectral collection conditions [Y. Sun, J. Yuan, Phys. Rev. B 71 (2005) 125109]. One application of this expression is the development of a phenomenological theory of magic-angle electron energy-loss spectroscopy (MAEELS), which can be used to extract the isotropically averaged spectral information for materials with arbitrary anisotropy. Here we use this expression to extract not only the isotropically averaged spectral information, but also the anisotropic spectral components, without the restriction of MAEELS. The application is based on a multivariate statistical analysis of core-level EELS for anisotropic materials. To demonstrate the applicability of this approach, we have conducted a study on a set of carbon K-edge spectra of multi-wall carbon nanotube (MWCNT) acquired with energy-loss spectroscopic profiling (ELSP) technique and successfully extracted both the averaged and dichroic spectral components of the wrapped graphite-like sheets. Our result shows that this can be a practical alternative to MAEELS for the study of electronic structure of anisotropic materials, in particular for those nanostructures made of layered materials.     

Accuracy and precision in model based EELS quantification

We present results on model based quantification of electron energy loss spectra (EELS), focusing on the factors that influence accuracy and precision in determining chemical concentrations. Several sources of systematical errors are investigated. The spectrometer entrance aperture determines the collection angle, and the effects of its position with respect to the transmitted beam are investigated, taking into account the diffraction by the crystal structure. The effect of the orientation of the sample is tested experimentally and theoretically on SrTiO3, and finally, a simulated experiment on c-BN at different thicknesses confirms the superior results of the model based method with respect to the conventional method. A test on a set of experimental reference compounds is presented, showing that remarkably good accuracy can be obtained. Recommendations are given to achieve high accuracy and precision in practice.     

Sharing of secondary electrons by in-lens and out-lens detector in low-voltage scanning electron microscope equipped with immersion lens

To understand secondary electron (SE) image formation with in-lens and out-lens detector in low-voltage scanning electron microscopy (LV-SEM), we have evaluated SE signals of an in-lens and an out-lens detector in LV-SEM. From the energy distribution spectra of SEs with various boosting voltages of the immersion lens system, we revealed that the electrostatic field of the immersion lens mainly collects electrons with energy lower than 40 eV, acting as a low-pass filter. This effect is also observed as a contrast change in LV-SEM images taken by in-lens and out-lens detectors.     

Optimizing EELS acquisition

A method for spectral acquisition, called binned gain averaging, will be described and tested. Systematic or correlated noise is efficiently suppressed with this method by averaging the gain over a series of CCD pixels. As a result, improved signal-to-noise ratios are obtained that allow the detection of very weak signals. At the same time, the spectral energy resolution is not degraded--even for long acquisition periods. It will be demonstrated that with this method, it is possible to significantly enhance the acquisition speed and quality of electron energy-loss (EEL) spectra and EELS maps. Examples will be given of double ionic scattering (i.e. the detection of the second boron K-edge) and the mapping of gold surface plasmons in the near-infrared and visible energy range.     

Atomic force microscopy characterization of the chemical contrast of nanoscale patterns fabricated by electron beam lithography on polyethylene glycol oxide thin films

The present paper shows that atomic force microscopy (AFM) imaging of friction force and phase lag in ambient air can be used to characterize the chemical contrast induced by electron beam (EB) irradiation on polyethylene glycol oxide (PEO) surface. Time-of-flight secondary emission mass spectroscopy measurements showed that the EB irradiation generates chemical contrast on PEO surface by decreasing the ether bond density. The AFM measurements showed smaller phase lag and lower friction and adhesive forces on the EB irradiated PEO surface, as compared to the non-irradiated PEO surface. While the chemical contrast in friction force had a linear dependence on the EB irradiation dose, the dependence of the chemical contrast in the phase lag was strongly non-linear. As the friction and adhesive forces depended on the AFM probe hydrophilicity and air humidity, the contrast in friction and adhesive forces is ascribed to different capillary condensation of ambient water vapour at the AFM tip contact with the EB irradiated and non-irradiated PEO surfaces, respectively.     

Model-based quantification of EELS spectra: treating the effect of correlated noise

Correlated noise is generally present in experimentally recorded electron energy loss spectra due to a non-ideal electron detector. In this contribution we describe a method to experimentally measure the noise properties of the detector as well as the consequences it has for model-based quantification using maximum likelihood. The effect of the correlated noise on the maximum likelihood fitting results can be shown to be negligible for the estimated (co)variance of the parameters while an experimentally obtained scaling factor is required to correct the likelihood ratio test for the reduction of noise power with frequency. Both effects are derived theoretically under a set of approximations and tested for a range of signal-to-noise values using numerical experiments. Finally, an experimental example shows that the correction for correlated noise is essential and should always be included in the fitting procedure.     

光阑尺寸对干涉条纹对比度的影响

本文讨论用He-Ne激光器作光源时,光阑尺寸对干涉条纹可见度的影响。通过实验我们找到了最佳光阑的变化范围。     

Exchange of angular momentum in EMCD experiments

In energy loss magnetic chiral dichroism (EMCD) experiments a chiral electronic transition is induced that obeys the dipole selection rule for the magnetic quantum number Δm=±1$\mathrm{\Delta }m=\pm 1$ or Δ_Lz=±?$\mathrm{\Delta }{L}_z=\pm ?$. The incident plane electron wave is inelastically scattered and is detected in the diffraction plane, i.e. again in a plane wave state. Naïve reasoning suggests that the angular momentum _Lz$L_z$ of the probe electron has not changed in the interaction since plane waves have 〈_Lz〉=0$〈L_z〉=0$. This leads to the seeming contradiction that angular momentum is not conserved in the interaction. A closer inspection shows that the density matrix of the probe has indeed 〈_Lz〉=±?$〈L_z〉=\pm ?$ after a chiral interaction. However, 〈_Lz〉$〈L_z〉$ is not conserved when the probe electron propagates further to the exit surface of the specimen because the rigid lattice breaks rotational symmetry. Thus, the angular momentum of the photo electron that is created in a chiral electronic transition stems from both the probing electron and the crystal lattice.     

New developments in the characterization of dislocation loops from LACBED patterns

The characterization of the Burgers vector of dislocations from large‐angle convergent‐beam electron diffraction (LACBED) patterns is now a well‐established method. The method has already been applied to relatively large and isolated dislocation loops in semiconductors. Nevertheless, some severe experimental difficulties are encountered with small dislocation loops. By using a 2 µm selected‐area aperture and a carbon contamination point to mark the loop of interest, we were able to characterize both the plane and the Burgers vector of dislocation loops of a few tens of nanometres in size present in Al‐Cu‐Mg alloys.     

Optimisation of a scanning atom probe with improved mass resolution using post deceleration

In this paper, we present calculations and experimental results obtained using post deceleration of ions in a scanning atom probe (SAP) geometry to improve the mass resolution. Various electrode geometries, tip to electrode distances in the range 50-170 microm and three different pulse shapes have been evaluated. Experimental mass resolutions of 750 FWHM and 200 FWTM have been achieved reproducibly for the 184W3+ peak without the use of a reflectron lens. 3D finite element electrostatics software has been used to simulate the ion trajectories through the instrument and thus to calculate the variations in velocities for the different electrode configurations. The observed trends are found to agree well with experimental results.     

Depth sectioning in scanning transmission electron microscopy based on core-loss spectroscopy

Recent and ongoing improvements in aberration correction have opened up the possibility of depth sectioning samples using the scanning transmission electron microscope in a fashion similar to the confocal scanning optical microscope. We explore questions of principle relating to image interpretability in the depth sectioning of samples using electron energy loss spectroscopy. We show that provided electron microscope probes are sufficiently fine and detector collection semi-angles are sufficiently large we can expect to locate dopant atoms inside a crystal. Furthermore, unlike high angle annular dark field imaging, electron energy loss spectroscopy can resolve dopants of smaller atomic mass than the supporting crystalline matrix.     

Charge compensation by in-situ heating for insulating ceramics in scanning electron microscope

With a steady temperature increase under high vacuum (HV) in an environmental scanning electronic microscope, we observed charge-free characterization and fine secondary electron (SE) images in focus for insulating ceramics (alumina (Al₂O₃), aluminum nitride (AlN), pure magnesium silicate (Mg₂SiO₄)). The sample current I_sc increased from −8.18×10⁻¹³ to 2.76×10⁻⁷ A for Al₂O₃ and −9.28×10⁻¹² to 2.77×10⁻⁶ A for AlN with the temperature increased from 298 to 633 K. The surface conductance σ increased from 5.6×10⁻¹³ to 5.0×10⁻¹¹/Ω for Al₂O₃ and 1.1×10⁻¹² to 1.0×10⁻⁷/Ω for AlN with the temperature increased from 363 to 593 K. The SE image contrast obtained via heating approach in high vacuum with an Everhart–Thornley SE-detector was better than that via conventional approach of electron–ion neutralization in low vacuum (LV) with a gaseous SE-detector. The differences of compensation temperatures for charge effects indicate dielectric and thermal properties, and band structures of insulators. The charge compensation mechanisms of heating approach mainly relate to accelerated release of trapped electrons on insulating surface and to increase of electron emission yield by heating.     

Reconstruction technique for off-axis electron holography using coarse fringes

A reconstruction technique for off-axis electron holography not requiring Fourier transformation is presented. Background intensity and amplitude modulation recorded in a hologram are normalized using an envelope function, and a cosine-function image corresponding to interference fringes is retrieved from the hologram. A reconstructed phase image is then calculated from the retrieved cosine image. After phase unwrapping, the phases due to carrier frequency and Fresnel diffraction from the biprism are removed using a reference hologram, and the corrected phase image is obtained. One advantage of this method is that the spatial resolution does not rely on the interference fringe spacing. Another advantage is that the phase image has no artifacts due to windowing of the sideband, which occurs in the usual Fourier-transformation method. Details of the calculation process and demonstrations of the method using a latex sphere particle and self-assembled Co nanoparticles are described.