E. C. Cosgriff

Three-dimensional imaging by optical sectioning in the aberration-corrected scanning transmission electron microscope

The depth resolution for optical sectioning in the scanning transmission electron microscope is measured using the results of optical sectioning experiments of laterally extended objects. We show that the depth resolution depends on the numerical aperture of the objective lens as expected. We also find, however, that the depth resolution depends on the lateral extent of the object that is being imaged owing to a missing cone of information in the transfer function. We find that deconvolution methods generally have limited usefulness in this case, but that three-dimensional information can still be obtained with the aid of prior information for specific samples such as those consisting of supported nanoparticles. We go on to review how a confocal geometry may improve the depth resolution for extended objects. Finally, we present a review of recent work exploring the effect of dynamical diffraction in zone-axis-aligned crystals on the optical sectioning process.

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.

Three-dimensional imaging in double aberration-corrected scanning confocal electron microscopy, Part I:

A transmission electron microscope fitted with both pre-specimen and post-specimen spherical aberration correctors enables the possibility of aberration-corrected scanning confocal electron microscopy. Imaging modes available in this configuration can make use of either elastically or inelastically scattered electrons. In this paper we consider image contrast for elastically scattered electrons. It is shown that there is no linear phase contrast in the confocal condition, leading to very low contrast for a single atom. Multislice simulations of a thicker crystalline sample show that sample vertical location and thickness can be accurately determined. However, buried impurity layers do not give strong, nor readily interpretable contrast. The accompanying paper examines the detection of inelastically scattered electrons in the confocal geometry.

Imaging Modes for Scanning Confocal Electron Microscopy in a Double Aberration-Corrected Transmission Electron Microscope

Aberration correction leads to reduced focal depth of field in the electron microscope. This reduced depth of field can be exploited to probe specific depths within a sample, a process known as optical sectioning. An electron microscope fitted with aberration correctors for both the pre- and postspecimen optics can be used in a confocal mode that provides improved depth resolution and selectivity over optical sectioning in the scanning transmission electron microscope (STEM). In this article we survey the coherent and incoherent imaging modes that are likely to be used in scanning confocal electron microscopy (SCEM) and provide simple expressions to describe the images that result. Calculations compare the depth response of SCEM to optical sectioning in the STEM. The depth resolution in a crystalline matrix is also explored by performing a Bloch wave calculation for the SCEM geometry in which the pre- and postspecimen optics are defocused away from their confocal conditions.

Bright-field scanning confocal electron microscopy using a double aberration-corrected transmission electron microscope

Scanning confocal electron microscopy (SCEM) offers a mechanism for three-dimensional imaging of materials, which makes use of the reduced depth of field in an aberration-corrected transmission electron microscope. The simplest configuration of SCEM is the bright-field mode. In this paper we present experimental data and simulations showing the form of bright-field SCEM images. We show that the depth dependence of the three-dimensional image can be explained in terms of two-dimensional images formed in the detector plane. For a crystalline sample, this so-called probe image is shown to be similar to a conventional diffraction pattern. Experimental results and simulations show how the diffracted probes in this image are elongated in thicker crystals and the use of this elongation to estimate sample thickness is explored.

ADF STEM imaging of screw dislocations viewed end-on

This paper presents annular dark-field scanning transmission electron microscope image simulations of a screw dislocation viewed end-on in a thin single crystal of Mo, taking into account surface relaxation (the Eshelby twist). The image contrast can be understood in terms of the effects of the displacements normal to the dislocation arising from the Eshelby twist on the channelling behaviour and interband scattering of the incident beam. With the beam focussed at the entrance surface, the image peak positions reflect the positions of the atoms at the entrance surface. For atomic columns at distances from the core less than the foil thickness, the image peak positions are predicted to lie between the perfect crystal and actual surface atom positions. The predicted intensity distribution of the image is qualitatively similar to that of a published experimental image of a screw dislocation in GaN [Phys. Rev. Lett. 91 (2003) p. 165501]. An assessment is made of the possibility of imaging core displacements by focussing near the foil centre, where surface relaxation effects should be minimised.

Image Contrast in Aberration-Corrected Scanning Confocal Electron Microscopy

Abstract The larger objective lens numerical aperture allowed by spherical aberration correction in electron optics leads to a reduced depth of focus, which typically becomes less than the thickness of the sample. Although this may complicate image interpretation, it leads to an opportunity to measure three‐dimensional information though the technique of optical sectioning. This chapter presents a theoretical analysis of transfer functions and image contrast for aberration‐corrected scanning confocal electron microscopy (SCEM). A comparison is made to optical sectioning using conventional scanning transmission electron microscopy (STEM). It is shown that for bright‐field SCEM there is little contrast, and a missing cone in the transfer function that is also seen for STEM. Energy‐filtered SCEM is seen to have strong transfer and no missing cone in the transfer function.

Selection rules for Bloch wave scattering for HREM imaging of imperfect crystals along symmetry axes

A Bloch wave analysis is used to investigate high-resolution electron microscope (HREM) imaging of crystals containing atomic displacements due to strain. In the absence of interband scattering, the shifts of peaks and troughs in the image will correspond to the displacements of the atoms in the exit surface. Interband scattering will shift the image peaks away from the actual atom positions and modify the apparent magnitude of the displacement identified by the observed image peak positions. By considering the case of seven-beam imaging of a cubic crystal aligned along a ⟨111⟩ axis, it is shown that the symmetry of the Bloch waves leads to selection rules for the interband scattering, similar to those seen for dipole electron excitations in atoms. It is also shown that, to first order, no intraband scattering can occur.

Theoretical interpretation of electron energy-loss spectroscopic images

We discuss the theory of electron energy‐loss spectroscopic images in scanning transmission electron microscopy. Three case studies are presented which have as common themes issues of inelastic scattering, coherence and image interpretation. The first is a state‐by‐state inelastic transitions analysis of a spectroscopic image which does not admit direct visual interpretation. The second compares theory and experiment for two‐dimensional mapping. The third considers imaging in three dimensions via depth sectioning.

Three-dimensional imaging using aberration-corrected scanning transmission and confocal electron microscopy

A reduction in the focal depth of field as a result of the installation of aberration correctors in scanning transmission electron microscopy, allows three-dimensional information to be retrieved by optical depth sectioning. A three-dimensional representation of the specimen is achieved by recording a series of images over a range of focal values. Optical depth sectioning in zone-axis crystals is explored computationally using a Bloch wave analysis to explain the form of the electron intensity in the crystal as a function of depth. We find that the intensity maximum deviates from that of the expected defocus value due to pre-focusing by the atomic column and also due to channelling pendellosung. The possibility of performing bright-field imaging in a double corrected two lens system in a confocal arrangement is also investigated computationally. The method offers some advantages over depth sectioning using conventional transmission electron microscopy.