G. Behan

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.

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.

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.

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.

Experimental setup for energy-filtered scanning confocal electron microscopy (EFSCEM) in a double aberration-corrected transmission electron microscope

Scanning confocal electron microscopy (SCEM) is a new imaging mode in electron microscopy. Spherical aberration corrected electron microscope instruments fitted with two aberration correctors can be used in this mode which provides improved depth resolution and selectivity compared to optical sectioning in a conventional scanning transmission geometry. In this article, we consider a confocal optical configuration for SCEM using inelastically scattered electrons. We lay out the necessary steps for achieving this new operational mode in a double aberration-corrected instrument with uncorrected chromatic aberration and present preliminary experimental results in such mode.

Optical depth sectioning in the aberration-corrected scanning transmission and scanning confocal electron microscope

The use of spherical aberration correctors in the scanning transmission electron microscope (STEM) has the effect of reducing the depth of field of the microscope, making three-dimensional imaging of a specimen possible by optical sectioning. Depth resolution can be improved further by placing aberration correctors and lenses pre and post specimen to achieve an imaging mode known as scanning confocal electron microscopy (SCEM). We present the calculated incoherent point spread functions (PSF) and optical transfer functions (OTF) of a STEM and SCEM. The OTF for a STEM is shown to have a missing cone region which results in severe blurring along the optic axis, which can be especially severe for extended objects. We also present strategies for reconstruction of experimental data, such as three-dimensional deconvolution of the point spread function.

Optical depth sectioning of metallic nanoparticles in the aberration-corrected scanning transmission electron microscope

Aberration correctors have proved themselves an important addition to the electron microscope. The resulting increased numerical aperture of the objective lens has allowed sub-angstrom resolution [1]. The increased numerical aperture also reduces the depth of field [2], which is just a few nanometres for an aberration-corrected scanning transmission electron microscope (STEM). We can use this reduction to provide threedimensional information by optically sectioning our sample much like in confocal optical microscopy, or probe specific depths in our sample. While we do not expect this technique will be able to compete with tomography for spatial resolution, the advantage is that data acquisition is a matter of minutes as opposed to hours and specific depths within a sample can be probed without the need for a lengthy reconstruction.

Scanning confocal electron microscopy in a double aberration corrected transmission electron microscope

The development of spherical aberration correctors has allowed the numerical aperture of electron lenses to be increased by typically 2–4 times, allowing significant improvements in resolution. The depth of field of an image is inversely proportional to the square of the numerical aperture, and therefore the reduction in depth of field in aberration corrected microscopes has been dramatic. The depth of field of a spherical aberration corrected scanning transmission electron microscope (STEM) may only be a few nm, creating an opportunity to perform nanoscale optical sectioning, as has previously been demonstrated [1, 2]. The optical transfer function (OTF) for incoherent STEM optical sectioning (Figure 1a), however, shows a large missing cone region which severely restricts the depth resolution for extended objects.