Nanocharacterisation: Volume 37 (Nanoscience & Nanotechnology Series) - Hardcover

Hardcover

ISBN 10: 184973805X ISBN 13: 9781849738057

Publisher: Royal Society of Chemistry, 2015

Synopsis

This new edition has been fully revised and updated to reflect the recent developments in instrumental characterisation methods for nanostructured materials.

About this title

About the Author

A I Kirkland is Professor of Materials at Oxford University and the author of over 170 refereed papers. He was awarded "best materials paper" of 2005 by the Microscopy Society of America. Since 2000 he has also been involved in the characterisation of CCD cameras for TEM. His most recent work involves the development of approaches to complex phase extension and diffractive imaging to further improve resolution.

From the Back Cover

Nanocharacterisation provides an overview of the main characterisation techniques that are currently used to study nanostructured materials. Following on from the success of the first edition, this new edition has been fully revised and updated to reflect the recent developments in instrumental characterisation methods. With contributions from internationally recognised experts, each chapter focuses on a different technique to characterise nanomaterials providing experimental procedures and applications. State of the art characterisation methods covered include Transmission Electron Microscopy, Scanning Transmission Electron Microscopy, Scanning Probe Microscopy, Electron Energy Loss Spectroscopy and Energy Dispersive X-ray Analysis, 3D Characterisation, Scanning Electron and Ion Microscopy and In situ Microscopy. Essentially a handbook to all working in the field this indispensable resource will appeal to academics, professionals and anyone working fields related to the research and development of nanocharacterisation and nanotechnology.

Excerpt. © Reprinted by permission. All rights reserved.

Nanocharacterisation

By Angus I. Kirkland, Sarah J. Haigh

The Royal Society of Chemistry

Copyright © 2015 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-805-7

Contents

Chapter 1 Characterization of Nanomaterials Using Transmission Electron Microscopy David J. Smith, 1, 
Chapter 2 Scanning Transmission Electron Microscopy A. R. Lupini, S. N. Rashkeev, M. Varela, A. Y. Borisevich, M. P. Oxley, K. van Benthem, Y. Peng, N. de Jonge, G. M. Veith, T. J. Pennycook, W. Zhou, R. Ishikawa, M. F. Chisholm, S. T. Pantelides and S. J. Pennycook, 30, 
Chapter 3 Scanning Tunnelling Microscopy of Surfaces and Nanostructures Martin R. Castell, 80, 
Chapter 4 Electron Energy-loss Spectroscopy and Energy-dispersive X-ray Analysis M. B. Ward, N. Hondow, A. P. Brown and R. Brydson, 108, 
Chapter 5 Electron Holography of Nanostructured Materials Rafal E Dunin-Borkowski, Takeshi Kasama and Richard J Harrison, 158, 
Chapter 6 Electron Tomography Matthew Weyland and Paul A. Midgley, 211, 
Chapter 7 Scanning Electron and Ion Microscopy of Nanostructures Natasha Erdman and David C. Bell, 300, 
Subject Index, 351, 

CHAPTER 1

Characterization of Nanomaterials Using Transmission Electron Microscopy

DAVID J. SMITH

Department of Physics, Arizona State University, Tempe, AZ 85287, USA Email: david.smith@asu.edu

1.1 Introduction

The transmission electron microscope (TEM) has evolved over many years into a highly sophisticated instrument that has found widespread application across many scientific disciplines. Because the TEM has an unparalleled ability to provide structural and chemical information over a range of length scales down to the level of atomic dimensions, it has developed into an indispensable tool for scientists who are interested in understanding the properties of nanostructured materials and manipulating their behavior.

The resolution of the optical microscope is restricted by the wavelength of visible light, which thus precludes atomic-scale imaging. In contrast, an energetic or fast-moving electron has a wavelength of much less than 1 Å (where 1 Å = 10-10 m), so that an enormous improvement in resolution can be achieved, at least in principle, by using an electron beam for imaging. A suitable combination of (magnetic) electron lenses is required, both for focusing the electron beam onto the object and also for providing an enlarged image. Maximum magnifications of the conventional, fixed-beam TEM are typically close to or exceeding one million times, so that key structural features of nanoscale objects are easily visualized on the final viewing screen or recording medium. Moreover, recent scanning TEMs can provide much larger magnifications, up to 50 million times or more, making feature visibility even easier.

Image formation in the TEM is more complicated in practice than is the case for the optical microscope. Strong magnetic fields are needed for focusing the electron beam, and these cause electrons to take a spiral trajectory through the lens field. In addition, a major restriction on ultimate microscope performance results from unavoidable aberrations of the electron lenses. Primarily due to the need for a compromise between small-angle diffraction effects and wide-angle spherical aberration limits, the resolution d can be roughly expressed by an equation of the form

d = A CS¼ λ¾ (1.1)

where CS is the spherical aberration coefficient of the objective lens, λ is the electron wavelength, and A is a constant with a value ranging from 0.43 to 0.7 depending on the type of imaging (coherent, incoherent, or phase contrast). Values of d typically range from about 3.0 Å down to 1.0 Å as electron energies are increased from 100 to 1250 keV. Modern-day TEMs operating at 200 or 300 keV have resolution limits well below 2.0 Å, which is comparable to the spacing between atoms. Individual columns of atoms can then be resolved in crystalline materials, which must first however be oriented so that the incident electron beam is aligned along some major crystallographic zone axis of the sample. In some special cases, such as along the edges of catalyst particles or in single-layered, two-dimensional sheets, isolated single atoms can even be imaged.

The power of the electron microscope is illustrated by the simple example in Figure 1.1, which shows the boundary region between two Al crystals, both of which are oriented so that the electron beam is parallel with a [001]-type zone axis. Each black spot in the image marks the position of a column of Al metal atoms viewed in an end-on geometry. It is obviously straightforward to visualize the periodic array of misfit dislocations (arrowed) which accommodate the angular misfit of 6º between the two crystals, and further analysis would enable the detailed atomic structure around the dislocation core to be determined.

This chapter begins by providing a brief introduction to the TEM and some of the key aspects of high-resolution imaging. The recent emergence of aberration-corrected instruments is also briefly mentioned. Applications to nanostructured materials are then described in greater detail, and some emerging trends and problems are discussed. For further information about microscope operation and more details about applications to a broader range of materials, the interested reader is referred to the review articles and monographs listed at the end of the chapter.

1.2 Imaging

1.2.1 Transmission Electron Microscopy: Standard Operating Mode

In the standard TEM operating mode, which is commonly referred to as amplitude or diffraction-contrast imaging, only a small fraction of those electrons which have passed through the sample are used to form the highly magnified final image. Most of the scattered (or diffracted) electrons are prevented from reaching the image plane by positioning a small objective aperture located in the back focal plane of the objective lens. This aperture thus serves to determine the image contrast. For crystalline samples, the electron diffraction pattern (EDP) is used to ensure that the orientation of the specimen relative to the direction of the incident electron beam will satisfy a strongly diffracting condition. Many common structural defects have a highly characteristic appearance under such diffraction-contrast conditions. The spacings and angles between crystal lattice planes can also be determined if the EDP is first calibrated using a known material. In addition, the availability of a crystalline substrate or support can provide a convenient method for sample orientation during observation. By using the substrate EDP for reference purposes, internal interfaces can be aligned perpendicular to the electron beam direction so that any changes in the microstructure of thin films and multilayers can then be determined as a function of film thickness. As an example, Figure 1.2 shows a multilayered magnetic tunneling transistor (MTT) deposited directly on the native oxide of a Si substrate. The individual layers of the MTT can be clearly recognized, and their thickness uniformity is easily confirmed. Finally, it should be appreciated that preparing such complex samples for examination with the TEM can represent a serious challenge to the electron microscopist. Because of considerable differences in thinning rates, it will often be difficult to prepare samples that are electron-transparent across the entire region of interest simultaneously. Descriptions of standard approaches developed for preparing electron-transparent specimens can be found elsewhere.

1.2.2 High-Resolution Electron Microscopy

In the technique of high-resolution electron microscopy (HREM), a much larger objective aperture (or sometimes none at all) is used. The directly transmitted beam can then interfere with one or more diffracted beams, and the contrast across the image will depend on the relative phases of the various beams. This imaging mode is thus often referred to as phase-contrast imaging. When the microscope imaging conditions are properly adjusted (lens defocus, image astigmatism, incident-beam alignment) then it is possible to simply interpret phase-contrast images in terms of the projected crystal potential provided that the specimen thickness is not too great (less than 10 nm preferred). Indeed, individual atomic columns or even single atoms can be separately resolved in many crystalline inorganic materials using the latest generations of HREMs. High electron doses, typically ~500–2000 electrons per square Ångstrom, are required to record such images, which means that specimens intended for high-resolution studies must be relatively resistant to electron irradiation effects. It is impossible to examine most organic materials and polymers directly under such intense imaging conditions. By using a specimen heating holder, and by adding a TV-rate image-pickup system to the base of the electron microscope lens column, dynamic events such as chemical reactions can be followed in real time without significant loss of spatial resolution.

Over the past 40 years, HREM has been used to characterize a wide range of inorganic materials. Important applications include determining the microstructure of crystalline defects, interfaces and grain boundaries, investigating nanocrystalline features in amorphous films, and studying small particles in heterogeneous catalysts. The characterization of magnetic thin films and multilayers, for example, continues to be very important, since layer continuity and defect microstructure are crucial to the viability of recording media. High-resolution images are able to provide specific details that are usually unavailable using other techniques. As an illustration, Figure 1.3(a) and (b) compare two high-resolution electron micrographs that reveal the amorphous or polycrystalline nature of the barrier layers in simple magnetic tunnel junctions grown by dc reactive sputtering. The layer sequences in the images are: (a) Co (50 nm)/HfO2 (10 nm)/Fe (50 nm), and (b) Co (50 nm)/CoO (10 nm)/Fe (50 nm). Further high-resolution images of nanomaterials are presented in later sections.

1.2.3 Basis of High-resolution Imaging

Image formation in the electron microscope occurs in two stages. Electrons of the incident beam interact with the specimen, undergoing both elastic and inelastic scattering. The electron wavefunction emerging from the exit surface of the specimen passes through the objective lens and further magnifying lenses are used to form the final image. Electrons that are elastically scattered mainly contribute to the high-resolution bright-field image. Note that the inelastically scattered electrons can provide valuable information about sample composition via the technique of electron-energy-loss spectroscopy (EELS), while electrons scattered to very large angles can be used for Z-contrast annular-dark-field (ADF) imaging in the scanning transmission electron microscope (STEM). These possibilities are described and developed further in later chapters.

Unlike X-ray or neutron scattering, electron scattering is strongly dynamical, meaning that the kinematical scattering approximation will be inadequate for understanding image formation except for the very thinnest of samples. Multiple electron scattering with large phase changes is far more typical, so that knowledge about the relative heights and locations of different atoms in the specimen becomes important for quantitative interpretation of image features. Indeed, image simulations are considered as essential for extracting detailed information about atomic arrangements at dislocations and interfaces. Several approaches to image simulation have been developed over the years, with the most widespread, commonly known as the multislice method, being based on an n-beam dynamical theory of electron scattering. In this approach, atoms in the specimen are considered as being located on narrowly separated planes (or slices), normal to the beam direction. The electron wavefunction is then propagated slice-by-slice through the sample to eventually form the exit-surface wavefunction. This iterative process lends itself to convenient computer algorithms that enable rapid computations to be carried out, and these simulations are especially useful during the refinement of unknown defect structures. Further information about different but equivalent theories of electron scattering can be found in the monograph by Cowley.

The electron wavefunction at the exit surface of the specimen must still be transferred to the final viewing screen or recording medium. This process is determined primarily by the properties of the objective lens. The effect of this lens can be conveniently understood by reference to what is termed the phase-contrast transfer function (TF), as originally described by Hanszen. The basic form of the TF is both specimen- and microscope-independent, so that a single set of universal curves can be used to describe the transfer characteristics of all objective lenses. Electron microscopes with different objective lenses, or operating at different electron energies, are easily compared by using suitable scaling factors. Figure 1.4 shows TFs for the optimum defocus of the objective lens of a typical 400 kV HREM. The two curves correspond to (a) coherent and (b) partially coherent incident electron illumination.

It is important to appreciate that the TF has an oscillatory nature, as visible in Figure 1.4(a), which means that electrons scattered to different angles will undergo reversals in phase. These phase oscillations will thus cause artefactual details in the final image that are liable to be misinterpreted. The TF is also focus-dependent, meaning that further phase changes occur when the focus is changed, and these will also affect the appearance of the image. Thus, much of the detail visible in the recorded micrograph could be uninterpretable unless the lens defocus is accurately known. Also note that the incident electron beam is ideally a coherent, monochromatic plane wave, whereas some loss of coherence results from focal spread (temporal coherence) and finite beam divergence (spatial coherence). These effects of partial coherence are conveniently represented by envelope functions which cause dampening of the TF at larger scattering angles. Specimen information scattered to higher spatial frequencies, equivalent to improved image resolution, is therefore lost. These incoherent effects are illustrated by the curve labeled (b) in Figure 1.4, where it should also be noted that the positions of the TF zeroes are not affected by the envelopes. Finally, note that additional specimen information may become available through the use of the highly coherent field-emission electron gun (FEG), as reflected by the flood of results emerging since the development of the aberration-corrected electron microscope.

1.2.4 Resolution Limits

The resolution of any imaging system is closely coupled to the illumination wavelength. Thus, resolution limits on the picometer scale might reasonably be expected for high-energy electrons. As mentioned earlier, the compromise between diffraction and spherical aberration gives what is an approximate estimate of the image resolution. In practice, high-resolution imaging is considerably more complicated, and there are several alternative definitions that are applicable depending on the sample and the microscope operating conditions. These resolution limits are most easily understood by considering the TF of the objective lens.

The interpretable image resolution, which is sometimes referred to as the structural or point resolution, is defined only at the optimum or Scherzer defocus, where the TF has the largest possible band of spatial frequencies without any phase reversal. The corresponding first-zero crossover, as indicated by the arrow in Figure 1.4, gives the interpretable resolution. CS values increase slightly at higher electron energies. However, because of the reduction in λ, improvements in theoretical resolution limits are obtained. Typical interpretable resolutions are in the range of 2.5 Å down to 1.2 Å for corresponding accelerating voltages of 200 kV up to 1000 kV. The size and cost of higher-voltage electron microscopes, as well as the increasing likelihood of electron irradiation damage for higher-energy electrons, are further practical factors that need to be taken into account. Intermediate-voltage HREMs, operating in the range of 200–400 keV, became widespread because of these considerations.

The envelope functions define the instrumental resolution or information limit of the HREM. A value of roughly 15% (i.e., exp(-2)) is usually taken as the resolution cutoff since this level is commonly regarded as the minimum acceptable for image processing requirements. This resolution limit can extend well beyond the interpretable resolution for 200 or 300 keV HREMs equipped with an FEG electron source, as illustrated in Figure 1.5. Very fine image detail is thus often present but it is not easily related to specimen features because of the TF oscillations mentioned earlier. An objective aperture of suitable diameter can be used to prevent beams with inverted phase from contributing to the image. Alternatively, the phase modulations caused by the TF can be removed by a posteriori image processing when the defocus and CS values are well enough known. Improved resolution of image features can then be achieved, as demonstrated by the pioneering studies of Coene et al. who were able to resolve columns of oxygen atoms in a high-temperature superconductor for the first time using an approach based on focal-series reconstruction. The information limit is the resolution criterion that is applicable for aberration-corrected instruments since TF oscillations are no longer an issue in such cases.

(Continues...)

Excerpted from Nanocharacterisation by Angus I. Kirkland, Sarah J. Haigh. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2015
• Language: English
• ISBN 10: 184973805X
• ISBN 13: 9781849738057
• Binding: Hardcover
• Edition number: 2
• Number of pages: 358
• Editor: Angus I Kirkland