- Email: [email protected]
Present developments in high-resolution transmission electron microscopy
489
Present developments in high-resolution transmission electron microscopy Frank Ernst* and Manfred Riihle? High-resolution
transmission
now approaches instrumental
electron microscopy
a resolution of 1 A,combining
developments
with innovative strategies
imaging and image processing. become
a truly ‘quantitative’
(HRTEM) recent
Moreover,
technique
HRTEM
that enables one to
reliability. At the same time, HRTEM
new applications: at reconstructed observation
has found
plan-view imaging of the atom configuration crystal surfaces; and atomistic in situ
of diffusion reactions and defect kinetics in solids.
Addresses Max-Ptanck-lnstitut fiir Metallforschung, SeestraEe 92, 70174 Stuttgart, Germany *e-mail: [email protected] fe-mail: [email protected] Current Opinion in Solid State & Materials 2:469-476
pattern
constitutes
the
of has
retrieve the atomistic structure of materials with high and well-known
distribution of their interference HRTEM image.
Science 1997,
Electronic identifier: 1359-0266-002-00469 0 Current Chemistry Ltd ISSN 1359-0286 Abbreviations HAADF high-angle annular dark-field HRTEM high-resolution transmission electron microscopy
Introduction According to the paradigm of materials science the properties and the behavior of ‘real’ materials depend uniquely on their microscopic structure. Often the technically relevant properties, such as hardness, electrical conductivity, or corrosion resistance do not so much depend on the underlying equilibrium structure but rather originate from defects disturbing the atom configuration. Therefore, in order to obtain not only a phenomenological but also a ‘physical’ understanding of real materials it is most important to assess their structure at an atomic resolution. Among the experimental techniques that achieve atomic resolution, high-resolution transmission electron microscopy (HRTEM) has the major advantage of imaging the interior of the object, not only the morphology of its surface. ‘Conventional’ HRTEM images are formed by the interference of coherent electron waves; one illuminates the object (a thin foil of the material under investigation) with a (nearly) planar electron wave. The object transmits this wave, interacts with it, and the resulting electron wave ve at the exit plane of the object carries information about the atom arrangement in the object. \ve corresponds to a set of ‘diffracted’ coherent plane waves. The electron optics transfers these waves to the image plane, and the intensity
However, aberrations of the electron optics and instrumental instabilities limit the fine detail in the image. Moreover, the electron optics re-distribute the information of each object point over an extended area of the image. This ‘point spread’ blurs the image and complicates image interpretation down to the ‘information resolution limit’, idest the finest details contained in the image. Several approaches compete with each other to overcome these problems. Development of improved electron optics, new techniques of imaging, and innovative methods of image processing aim to extract more information from HRTEM images. This paper summarizes major progress in these ‘advanced’ methods of HRTEM, as documented by publications from 1996 and 1997. In the same period, new techniques have extended the applicability of HRTEM to a larger field of materials research. By means of sophisticated image processing it has become possible to obtain plan-view images of the atom configuration at reconstructed crystal surfaces. Another exciting development concerns in situ HRTEM, which enables one to observe, at high-resolution, how defects in materials respond to heating, changes in chemical environment, electron beam irradiation, and mechanical stress.
Advances Iterative
in HRTEM techniques
digital
Image
matching
Three complications generally inhibit a straight-forward interpretation of conventional HRTEM images, in terms of a projected structure. First, most real objects are not thin enough to avoid dynamical electron diffraction, which complicates the relationship between the object structure and the electron wave function, we at the exit plane of the object. Second, lens aberrations and instrumental instabilities limit the resolution at which the instrument reproduces this electron wave function in the image plane. The present development of an electron optical spherical aberration corrector [l] will significantly reduce this problem in the future. Third, while both, the amplitude and the phase of the electron wave carry information about the object only the electron intensity (square of the amplitude) constitutes an observable quantity. The phase, in contrast, remains non-observable. One well-established method to correctly interpret HRTEM images relies on numerically simulating the images that hypothetical object structures would yield, under the experimental imaging conditions. The simulated image
470
Characterization
techniques
that yields the best match with the experimental image under consideration identifies the best model for the real structure. This method has been automated to perform a structure refinement by ‘iterative digital image matching’ (Figure 1) [2,3,4*,5,6”,7]. Mathematically, this means a description of the discrepancy between an experimental image and the simulated image of each possible structure, as a function on a highly-dimensional space of structures, and to search this space for the absolute minimum of that function. In such a many-parameter optimization it remains difficult to verify that a given solution constitutes the global minimum and not only a secondary one. Moreover, optimization on a high-dimensional space constitutes a major computing task. Major progress concerning these problems has been achieved by implementing the global optimization algorithm of simulated evolution [6”,7] for image matching. Compared to steepest descent optimization this algorithm locates the global optimum with more trustworthiness and higher efficiency. Therefore, simulated annealing enables a determination of electron optical imaging parameters, such as objective lens defocus, specimen thickness, beam tilt, and so on, along with the position of atom columns.
Fiaure
1
image
-7T
I !!
column positions with an average reliability between 0.01 ‘and 0.02 nm [3,4’,5]. As expected, the reliability increases with increasing projected electrostatic potential (scattering power) of the columns [S]. On the other hand, a simulated image never exactly reproduces an experimental HRTEhI image, even if the input data for the structure and the imaging parameters strictly corresponded to reality. Depending on the material under study, the contrast in the simulated images often largely exceeds the contrast in the corresponding experimental images and shows more fine detail. For structure refinements by iterative digital image matching such ‘contrast misfit’ does not introduce a severe problem because adjustment of column positions in the trial structure does not substantially alter the contrast of the corresponding simulated image (6”,7]. While the source of the contrast misfit remains unknown, one may account for the excess contrast damping in experimental images by a phenomenological, ‘non-physical’ modulation transfer function. When it comes to determining imaging parameters, however, knowledge of absolute contrast becomes more important. The results of systematic investigations (CB Boothroyd, personal communication; T Gemming, G Mobus, F Ernst and M Riihle, unpublished data) have already ruled out several plausible hypotheses about the origin of contrast misfit. Moreover, quantitative analysis of diffraction patterns obtained with a convergent electron beam (CBED) has revealed that the multislice method and the Bloch wave method employed for HRTEM image simulations deliver accurate diffraction intensities [8], provided that one chooses the thickness of the slices small enough and includes correct Debye-Waller factors [9]. This means that the problem of simulating HRTEM images concerns the phases of the diffracted beams rather than their amplitudes. Still,
the
major
source(s)
of the contrast
misfit
remain(s)
to be identified. According to another hypothesis, the contrast misfit arises from rapid stochastic lateral movements of the object during image recording. If this turns out to be true one might solve the problem of contrast misfit by time-resolved image recording, which means to build up the final HRTEM image by digital image processing from a series of subsequent images recorded with extremely short exposure times (T Gemming, personal communication).
Flow diagram of object structure determination by iterative digital matching (gray box) of experimental and simulated HRTEM images.
Matching of the imaging parameters reduces the ‘residual discrepancy’ between simulated and experimental images, and thus improves the reliability of the structure refinement [2,3,4*,5]. Depending on the imaging conditions, the reliability of atom positions can be significantly better than the point resolution of the microscope. At grain boundaries, for example, one can determine the atom
Depending on the material under investigation and the high voltage of the electron microscope, the transmission of high-energy electrons may damage the object during observation. Systematic beam damaging experiments on grain boundaries [4’,10*] and metal/oxide interfaces [lo’] have quantitatively revealed the effect of prolonged electron irradiation on the reliability of atom coordinates, determined by iterative digital image matching. Even when imaging these interfaces by high-voltage high-resolution TEM (HVHRTEM), in a microscope operating
Present developments
in high-resolution transmission electron microscopy Ernst and Riihle
voltage as high as 1250 kV, irradiation damaging did not severely decrease the reliability of structure determination-provided that observation times remained reasonably short (the order of 10 minutes). This result is important because it demonstrates that object damage induced by electron beam irradiation does not necessarily mean a handicap of HVHRTEM. at an accelerating
Figure 2
TI
object ‘Ye
hologram
-i
1997 Current Opinion m Solid State & titerials
Science
Off-axis holography in a transmission electron microscope. A biprism (charged quartz wire) causes the electron wave diffracted by the object to interfere with a reference wave that has passed through the hole of the TEM specimen. The hologram (interference pattern) serves to retrieve I& the electron wave function at the exit plane of the object.
High-resolution
electron holography
Another approach of HRTEM aims to retrieve the complex-valued electron wave function, ye at the exit plane of the object, directly from experimental images, without simulating electron diffraction in model structures. ve carries amplitude and phase information. Empirically, the corresponding phase image often provides more intelligible information on the object structure than the amplitude image or the intensity distribution of a conventional HRTEM image, which generally intermixes amplitude and phase information and suffers from point spread. The major interest in retrieving ye, however, arises from the perspective of using ve to directly reconstruct the structure of the object. Off-axis electron holography by means of a MGllenstedt biprism (Figure 2) allows direct high-resolution retrieval of we from a single image or ‘hologram’ [ 11,121. This method offers many advantages. On the other hand, it requires precise knowledge of microscope aberrations and consideration of artefacts [ 131. Moreover, the need for a reference wave passing through the specimen hole limits the field of view to regions no more than some 1Onm away from the hole. The preparation of TEM specimens is difficult to control at this length scale, and in ‘randomly’ prepared specimens it can be hard to find the crystal defect of interest within the
471
small region suitable for imaging. A recent study [ 14**] of a large angle grain boundary in gold demonstrates, however, that high-resolution off-axis electron holography can solve not only crystal structures but also the atomistic structure of crystal defects. Besides off-axis holography, which provides a ‘direct’ way of retrieving the phase and amplitude of ye, several other methods proceed along an ‘indirect’ way. Among the latter, the ‘focus variation method’ (Figure 3) probably constitutes the most advanced method. This method aims to a retrieve ye from intensity distributions recorded at the image plane of the microscope [15-171. Such reconstruction, which includes correction of lens aberrations, has primary importance for the interpretation of conventional high-resolution images obtained under highly coherent illumination, predominantly in microscopes equipped with field-emission guns. As input data the focus variation method requires a series of images recorded at different objective lens foci. The images may (and normally will) include non-linear contributions. From this data one retrieves the exit-plane wave function ye by the ‘paraboloid method’ [18”], which separates linear from non-linear image transfer, or by the more general ‘maximum likelihood method’ [15]. When tested on simulated images, for which one knows the reasonable wave function ve, both methods have proven to possess sufficient stability against noise and large contributions of non-linear imaging, even for images of non-periodic objects (defects) [17]. Real HRTEM images, however, often exhibit less contrast and less fine detail than simulated images (‘contrast misfit’). Moreover, while recording a focus series the object may drift, tilt, or become contaminated, and the prolonged electron irradiation may introduce damage. Nevertheless, application of the paraboloid method to HRTEM images of oxides with complex structures has demonstrated that the method works under ‘real’ conditions, too. The applicability of focus variation methods to real non-periodic objects, such as interfaces, remains to be shown. Besides their benefits, all present methods of retrieving ve share the problem of depending on accurate data for the aberration parameters (spherical aberration constant) of the electron microscope. Therefore, the success of these procedures depends on further development of experimental methods to determine these parameters with sufficient accuracy. Object reconstruction
Given the electron wave, we at the exit plane of the object, one may try to ‘reconstruct’ the structure of the object. This means to determine the spatial distribution of the electrostatic potential, at least in projection along the viewing direction. This inversion problem constitutes a difficult task, except for ‘weak phase’ (very thin) objects, where the square of we directly indicates the projected electrostatic potential. Most real TEM
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Characterization
techniques
Fiaure 3
Figure 4
M
Yl
object We
object (atom columns)
electron optics
I
channeling images:
4
‘focus
series’
1s eigenfunction
C 1997 Current Opinmn I” Soled Stats 8 Materials Scmnca
Scheme of the ‘focus variation method’ for retrieving the electron wave function ve at the exit plane of the object from a series of HRTEM images recorded at different focus settings f,, f2 ... fN of the objective lens.
specimens, however, do not qualify as weak phase objects because one cannot prepare them thin enough. Normally, therefore, the image formation process involves dynamical electron diffraction, which is difficult to invert. For objects consisting of largely spaced atom columns along the viewing direction, however, a new method [19] simplifies the interpretation of the exit-plane wave function in terms of atom column positions in the object. The new method rests on a particularly simple view of dynamical electron diffraction in crystals [20**,21”]. In this view the high-energy electrons, that propagate through the object, occupy highly localized channeling eigenfunctions of the atom columns parallel to the optic axis (Figure 4). Provided that each atom column contributes only one eigenstate, which should be true for elements with atomic numbers 2 up to about 50, the electron wave function at the exit plane of the object correlates in a simple fashion with the projected potential of the atom columns. This simple relationship even holds for crystal defects-such as interfaces in edge-on orientation or dislocations in end-on orientation-as long as they do not disrupt the atom columns along the viewing direction. Comparisons with real-space image simulations of individual columns have confirmed the channeling theory [21**]. This theory also allows one to eliminate residual aberrations from the reconstructed object-plane wave function [21**,22]. Application of the channeling theory to retrieve the complex structure of several different oxides from experimental HRTEM images has shown that this method delivers ‘true’ results [19]. On the other hand, viewing electron scattering as channeling of electrons along atom columns clearly reveals the intrinsic limits of object structure retrieval from the
We B 1997 Current Opinion in Solid State 8 Mater&
Science
J
According to the ‘channeling theory: the electrons that propagate through the object occupy highly localized 1s eigenfunctions (shaded regions) of the atom columns parallel to the optic axis.
wave function ve. The average mass of the columns (not the particular kind and vertical distribution of the atoms within the columns) determines the corresponding eigenfunctions. Thus, while object reconstruction may provide accurate information on column positions, it will be difficult to derive the chemical composition of the columns or the vertical distribution of the atoms from ve [19]. Thus, object reconstruction will most likely deliver a set of possible structures rather than a unique solution, and prior knowledge about the object (composition, thickness, and so on) may be necessary to select the best solution out of this set-similar to the prior knowledge required in iterative digital image matching. The channeling theory requires an object consisting of well-separated atom columns. Another new approach [23] to object reconstruction does not depend on this assumption. This new approach employs a simulatedannealing algorithm to find a potential-distribution for which simulation of dynamical electron diffraction yields the same exit-plane wave function ye as obtained for the real object. However, in addition to the above limitations of object reconstruction, the latter method only works for periodic structures. Summarizing, significant progress has been made concerning the difficult problem of reconstructing object structures from given exit plane wave functions ye. However, these methods suffer from intrinsic limitations. Moreover, object reconstruction by inverting the information in the exit-plane electron wave ye will become ‘quantitative’ only when it becomes possible to quantify error limits for ve and to deduce from these the reliability of the atom (column) coordinates in the reconstructed structure.
Present developments in high-resolution trensmission electron microscopy Ernst and Riihle
Z-contrast Imaging with a STEM
While conventional HRTEM and off-axis electron holography rely on image formation by interference of coherent electron waves, high-resolution imaging in a nanoprobe scanning transmission electron microscope (STEM) exploits the benefits of ‘incoherent imaging’ [24”,25**]. Figure 5 depicts the basic idea: while a focused electron probe scans the object, a high-angle annular dark-field (HAADF) detector collects the electrons that the object scatters into directions making large angles with the axis of the incident probe. The entity of the signals recorded at each scan point yields an image of the object. Provided that the annular detector has a suitable design (outer and inner radius) [26”] it suppresses intensity modulations resulting from interference and yields largely incoherent images. The latter term implies a linear transfer of intensities, not amplitudes, from the exit plane of the object to the image plane. The resulting images appear to allow a straight-forward interpretation in terms of projected atom columns and their average atomic number 2 [24”,25**]. Examples that demonstrate the power of such ‘Z-contrast’ imaging include the atomistic structure of tilt grain boundaries in strontium titanate [24”,25”,27*,28] and semiconductor heterointerfaces [24**,25”,29-311. In spite of this success, some severe problems of Z-contrast imaging still need to be overcome. Instrumental instabilities, either electronic or from mechanical vibrations, introduce severe distortions in HAADF images. This restricts reliable determination of atom column positions to small regions, no larger than a few atom spacings in diameter. This problem could be severe, for example, when trying to determine the relative translation of two crystallites forming a grain boundary. Moreover, even within regions of perfect crystal, HAADF images sometimes show contrast variations that one would not expect for ideal Z-contrast. Another drawback of HAADF imaging lies in the high electron dose required to record an image. Similar to HVHRTEM, therefore, one has to consider potential damaging of the object during observation. In contrast to the apparent simplicity of HAADF images and their simple interpretation in terms of Z-contrast, the theoretical interpretation of HAADF imaging has been controversial. Two recent papers [26**,32**] have advanced the theoretical understanding. According to these studies, the contrast mainly arises from ‘thermal diffuse’ scattering, which means large-angle multiphonon, incoherent scattering. Different from earlier suggestions, this kind of electron scattering does not require prior dynamical elastic diffraction. According to one theory [32”], elastic scattering to the first order Laue zone does not play a central role, either. Most importantly, both studies [26”,32”] suggest that the simple interpretation of HAADF images in terms of column scattering power (Z-contrast) may not strictly hold in practical situations. A plain Z-contrast only occurs with ‘thin’ objects [26**].
473
Figure 5
electron probe
(D 1997 Current Opu%an in Solid State 6. Materials Sciem
High-resolution imaging in a nanoprobe scanning transmission electron microscope (STEM) by recording electrons scattered under large angles against the optic axis with a high-angle annular dark-field (HAADF) detector.
Real objects, however, are mostly ‘thick’ and can produce a more complicated HAADF contrast. In particular, depending on the inner angle of the annular detector, the contribution of elastic scattering may lead to contrast reversal [26”]. Moreover, HAADF image simulations indicate that under some circumstances the contrast depends strongly on the focus of the objective lens [32**], and that no simple relation exists between image intensity and object thickness [26”]. Thus, it appears that quantitative interpretation of HAADF images, for instance in terms of column occupancy, generally does require image simulations. Plan-view HRTEM imaging of surface structures
Recent work [33”,34] has also established HRTEM as a powerful tool for plan-view imaging of the two-dimensional arrangement of atoms at crystal surfaces. By means of sophisticated image processing, including noise reduction using a Wiener filter [35], it is possible to extract the contribution of surface atoms from conventional HRTEM images and to de-convolute the overlap of the TEM specimen’s top and bottom surface to obtain images revealing the two-dimensional surface structure. Because the surface atoms alone constitute an ideally thin object, one may interpret such images directly in terms of a projected structure. As a major advantage over scanning tunneling microscopy (STM), HRTEM can probe the surface reconstruction into some depth, too. A HRTEM
474
Characterization
techniques
study of the 7 x 7 reconstruction of the {ill} silicon surface demonstrates that such in-depth probing may significantly improve the physical understanding of surface reconstruction phenomena [33”].
accuracy. Moreover, improved resolution and reliability of structure retrieval increase the demand for high-quality TEM specimens. This means that the techniques of TEM specimen preparation will require increasing attention in the future.
In situ HRTEM
Observation of processes rather than static structures at the atomic scale allows one to tackle a wide range of interesting problems in materials science. Major progress in understanding materials behavior has been made recently by means of in situ HRTEM, which is rapidly expanding, owing to increasing stability of high-resolution microscopes. In the period under survey in this article, in situ HRTEM has delivered a wealth of new insights about the formation and stability of complex carbon structures. Pioneering work in this field includes direct observation of fullerene molecule formation under an environment simulating an arc-discharge chamber [36], formation of single fullerene cages [37] or carbon shell clusters (‘onions’) [38] under electron irradiation, and generation of diamond under the internal pressure of carbon onions [39**]. Other in situ HRTEM studies have shed light on the microscopic motion of grain boundaries [40,41*,42] and the interaction between grain boundaries and dislocations [43*]. The latter work concerns grain boundaries that play a key role for the primary recrystallization of face-centered cubic metals. The in situ observations suggest the atomistic mechanism by which growing recrystallized grains accommodate dislocations from the deformed matrix. Furthermore, in situ observations have provided insight into the solid-liquid phase transitions of metals [44] and the reconstruction mechanism of the Si{lll), (OOl), (Zll), and {311) surfaces [45]. Work on dislocation motion in silicon by kink formation and migration [46*] impressively demonstrates that in situ HRTEhl can also provide ‘quantitative’ data, such as dislocation velocity and kink formation energy.
Conclusions High-resolution transmission electron microscopy (HRTEM) constitutes one of the most powerful experimental techniques available to investigate materials at the atomic level. In addition to instrumental developments that aim to reduce the spherical aberration of the objective lens, several new methods have been developed to improve the resolution limit and the reliability of structure retrieval from HRTEhI images. These methods include iterative digital matching of simulated and experimental images, several variants of high-resolution electron holography, object reconstruction, and incoherent high-resolution imaging in a dedicated scanning transmission electron microscope, employing a high-angle annular dark-field detector. For all these methods, the development of efficient image processing methods and optimization strategies has become a central issue. Still, major problems remain be solved, like the contrast misfit between simulated and experimental images, or the experimental determination of electron optical imaging parameters with sufficiently high
Nevertheless, the resolution of HRTEM imaging presently approaches 18. The reliability of determining the position of atom columns at crystal defects has even reached 0.01 A. This development marks an important transition: while formerly HRTEM yielded only qualitative results, involving visual inspection and personal judgement of an individual observer, HRTEM has now developed into a truly ‘quantitative’ technique that delivers ‘hard numbers’ about the atomistic structure of defects in materials. Plan-view imaging of surface structures and in situ experiments constitute exciting new domains of HRTEhI characterization of materials. The exploration of these new domains has hardly begun.
References
and recommended
reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
. ..
of special interest of outstanding interest
1.
Haider M, Braunshausen G, Schwan E: Correction of the spherical aberration of a 200 kV TEM by means of a hexapolecorrector. Oprik 1995, 99:167-l 79.
2.
Hofmann D, Ernst F: Quantitative HRTEM of the incoherent twin boundary in Cu. Ultramicroscopy 1994, 53:205-221.
3.
Ernst F, Hofmann D, Nadarzinski K, Stemmer S, Streiffer SK: Quantitative high-resolution electron microscopy of interfaces. In Intergranular and Interphase Boundaries in Materials, vol 1. Edited by Ferro AC, Conde EP, Fortes EA (Materials Science Forum 207-209). ZiJrich: Trans Tech Publications; 1996:23-34.
4. .
Nadarzinski K, Ernst F: The atomistic structure of a 213, (111) grain boundary in NiAI, studied by quantitative high-resolution transmission electron microscopy. Phil Mag A 1996, 74:641664. Method of iterative digital matching of simulated and experimental HRTEM images, applied to determine the atomistic structure of a grain boundary in NiAI, and quantitative analysis of electron beam damage on the reliability of atom column positions. 5.
Kienzle 0, Exner M, Ernst F: Analysis of interface structures by quantitative high-resolution transmission electron microscopy. In Atomic Resolution Microscopy of Surfaces and Interfaces. Edited by Hamers R, Smith DJ (Mater Res Sot Symp Proc 466). Pittsburgh: Materials Research Society; 1997, in press.
6. ..
Mdbus G: Retrieval of crystal defect structures from HREM images by simulated evolution. I. Basic technique. Ultramicroscopy 1996, 65:205-216. This paper introduces a powerful implementation of structure refinement by iterative digital matching of simulated and experimental images, and discusses the limitation of this method. 7.
Mobus G, Dehm G: Retrieval of crystal defect structures from HREM images by simulated evolution. II. Experimental image evaluation. Ultramicroscopy 1996, 65:217-226.
0.
Cheng YF, Nuechter W, Mayer J, Weickenmeier A, Gjnnes J: Loworder structure-factor amblitude and Sian determination of an unknown structure AlmFe by quantitative convergent-beam electron diffraction. Acta Cryst A 1996, 52:923-936.
Present developments in high-resolution transmission electron microscopy Ernst and Riihle
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Jiang Hua C, Op de Beeck M, Van Dyck D: Can the multislice method be used to calculate HOLZ reflections in high-energy electron diffraction and imaging? Microsc Microanal Microstruct 1996, 7:27-47.
10. .
Dehm G, Nadarzinski K, Ernst F, Riihle M: Quantification of irradiation damage generated during HRTEM with 1250 keV electrons. Ultramicroscopy 1996, 63:49-55. Quantitative analysis of electron beam damage on the reliability of atom column positions as obtained by iterative digital matching of simulated and experimental images. 11.
Lichte H: Electron holography. I. Can electron holography 0.1 nm resolution? Ultramicroscopy 1992, 47:223-230.
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Pennycook SJ, Browning ND, McGibbon MM, McGibbon Al, Chisholm MF, Jesson DE: Determination of interface structure and bonding by Z - contrast STEM. Diffus Defect Data Part B 1996. 47:561-571. Principles of HAADF imaging and its combination with analytical TEM at high spatial resolution. 26.
Hartel P, Rose H, Dinges C: Conditions and reasons for incoherent imaging in STEM. Ultramicroscopy 1996, 63:93-l 14. Eeoretical fundament of HAADF imaging in a dedicated STEM. 27.
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in electron holography.
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Uftramicroscopy
Orchowski A, Lichte H: High-resolution electron holography of non-periodic structures at the example of a X-13 grain boundary in gold. Ultramicroscopy 1996, 64:199-209. This article reports on a first application of high-resolution electron holography to solve the atomistic structure of a grain boundary.
26.
McGibbon MM, Browning ND, McGibbon Al, Pennycook SJ: The atomic structure of asymmetric [OOll tilt boundaries in SrTiO3. Phil Mag A 1996, 73:625-641.
29.
McGibbon AJ, Pennycook SJ: Direct atomic resolution imaging of dislocation core structures in a 300 kV STEM. Microsc Semiconduc Mater 1995. 1995:79-82.
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McGibbon AJ, Pennycook SJ, Angelo JE, Mills MJ: Direct sub-lattice imaging of interface dislocation structures in CdTe/GaAs@OlL In Evolution of Thin Film and Surface Structure and Morphology. Edited by Demczyk BG et a/. (Mater Res Sot Symp Proc 355). Pittsburgh: Materials Research Society; 1995:625-630. McGibbon AJ, Pennycook SJ, Angelo JE: Direct observation of dislocation core structures in CdTe/GaAs(OOl). Science 1995, 269:519-521.
15.
Coene WMJ, Thust A, Op de Beeck M, Van Dyck D: Maximumlikelihood method for focus-variation image reconstruction in high resolution transmission electron microscopy. Ultramicroscopy 1996, 64:109-l 35.
16.
Thust A, Overwijk MHF, Coena WMJ: Numerical correction of lens aberrations in phase-retrieval HRTEM. Ultramicroscopy 1996, 64:249-264.
31.
Thust A, Coene WMJ: Focal-series reconstruction in HRTEM: simulation studies on non-periodic obiects. Ultramicroscopy 1996, 64:21 l-230.
32.
16. ..
Op de Beeck M, Van Dyck D, Coene W: Wave function reconstruction in HRTEM: the parabola method. Ultramicroscopy 1996. 64:167-i 63. Retrieval of ihe electron wave function at the exit plane of the object by evaluating a series of HRTEM images recorded at different objective lens focus settings. 19.
Op de Beeck M, Van Dyck D: Direct structure reconstruction HRTEM. Ultramicroscopy 1996,64:153-l 65.
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1z
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Van Dyck D, Lichte H, van der Mast KD: Sub-Ongstrsm structure characterisation: the Brite-Euram route towards one Ongstrsm. Ultramicroscopy 1996, 64:1-l 5. Overview about competing approaches to increaae the resolution limit of high-resolution transmission electron microscopy by retrieving the electron wave function at the exit plane of the object and to reconstruct the object structure from this wave function. 21. Van Dyck D, Op de Beeck M: A simple intuitive theory for .. electron diffraction. Ultramicroscopy 1996, 64:99-l 07. This paper presents a new concept to describe dynamical electron d#fraction in thin crystals. This concept allows an intuitive understanding of the correlation between structure of the object and the electron wave function that describes the electron diffraction at the exit plane of the object under coherent illumination. 22.
Tang D, Zandbergen HW,.Jansen J, Op de Beeck M, Van Dyck D: Fine-tuning of the focal residue in exit-wave reconstruction. Ultramicroscopy 1996. 64:265-276.
23.
Lentzen M, Urban K: Reconstruction of the projected crystal potential from a periodic high-resolution electron microscopy exit plane wave function. Ultramicroscopy 1996, 62:249-264.
24. ..
Pennycook SJ, Browning ND, McGibbon MM, McGibbon Al, Jesson DE, Chisholm MF: Direct determination of interface structure and bonding with the scannina transmission electron microscope. Philos T&s Royal Sot Lond Series A 1996, 354:2619-2634.
Amali A, Rez P: Theory of lattice resolution in high-angle annular dark-field images. Microsc Microanal 1997, 3:28-46. rtheory of high-angle electron scattering and the formation mechanism of high-resolution images obtained in scanning transmission electron microscopes with a high-angle annular dark-field detector. 33. ..
Bengu E, Plaas R, Marks LD, lchihashi T, Ajayan PM, lijima S: Imaging the dimers in Sit1 11)-(7x 7). Phys Rev Lett 1996, 77:4226-4226. Pioneering work demonstrating the power of image processing by solving the two-dimensional atomistic structure of reconstructed crystal surfaces from plan-view HRTEM images. 34.
Marks LD, Plass R: Atomic structure of Sit111 )-(5x2)-Au from high resolution electron microscopy and heavy-atom holography. Phys Rev Lett 1995, 76:2172-2175.
35.
Marks LD: Wiener-filter enhancement Ultramicroscopy 1996, 62:1-2.
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Burden AP, Hutchison JL: Real-time observation of fullerene generation in a modified electron microscope. J Cryst Growth 1996, 156:1-2.
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Zwanger MS, Banhart F, Seeger A: Formation and decay of spherical concentric-shell carbon clusters. J Cryst Growth 1996, 163~445-454.
of noisy HREM images.
39. Banhart F: Carbon onions as nanoscopic pressure cells for .. diamond formation. Nature 1996, 362:433-435. _ . fn situ observation ot diamond tormatlon In carbon onions under irradiation with high-energy electrons. 40.
Wunderlich W: Theoretical considerations about arain boundary migration in fee metals. In lntergranularkd interphase Boundaries in Materials, vol 1. Edited by Ferro AC, Conde EP, Fortes EA (Materials Science Forum 207-209). Zirich: Trans Tech Publications; 1996:141-l 44.
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techniques
41. .
Campbell GH, Chan DK, Medlin DL, Angelo JE, Carter CB: Dynamic observation of the FCC to 9R shear transformation in a copper S=3 incoherent twin boundary. Scr Mater 1996, 351037-642. In situ HRTEM observation of a structural transition in a Cu grain boundary.
42.
43.
Kamino T, Yaguchi T, Ukiana M, Yasutomi Y, Saka H: Application of HREM in situ heating experiments to the study of dynamical behaviour of grain boundaries at very high temperatures. In lntergranular and herphase Boundaries in Materials, vol2. Edited by Ferro AC, Conde EP, Fortes EA (Materials Science Forum 207-209). &rich: Trans Tech Publications; 1996:469-492.
Medlin DL: Climb and glide of a/3<11 I> dislocations in an aluminium Sigma =3 boundary. Phil Mag A 1997, 75:733-747. ;n situ HRTEM observations of moving grain boundary dislocations and implications for the accommodation of crystal dislocations by moving grain boundaries.
44.
Xiao S, Johnson E, Hinderberger S, Hohansen A, Bourdelle K, Dahmen U: /n-situ high-resolution electron microscopy observations of nanometre-sized Pb inclusions in Al near their melting point. I Microsc 1995, 160:61-69.
45.
Kamino T, Yaguchi T, Tomita M, Saka H: /n-situ high-resolution electron microscopy study on a surface reconstruction of Au-deposited Si at very high temperatures. Phil Mag A 1997, 75:105-l 14.
46. .
Kolar HR, Spence dislocation kinks 4034. This paper demonstrates tative data on dislocation
JCH, Alexander H: Observation of moving and unpinning. Phys Rev Lett 1996, 77:4031the application of in situ HRTEM to obtain quantimotion.
Present developments in high-resolution transmission electron microscopy Frank Ernst* and Manfred Riihle? High-resolution
transmission
now approaches instrumental
electron microscopy
a resolution of 1 A,combining
developments
with innovative strategies
imaging and image processing. become
a truly ‘quantitative’
(HRTEM) recent
Moreover,
technique
HRTEM
that enables one to
reliability. At the same time, HRTEM
new applications: at reconstructed observation
has found
plan-view imaging of the atom configuration crystal surfaces; and atomistic in situ
of diffusion reactions and defect kinetics in solids.
Addresses Max-Ptanck-lnstitut fiir Metallforschung, SeestraEe 92, 70174 Stuttgart, Germany *e-mail: [email protected] fe-mail: [email protected] Current Opinion in Solid State & Materials 2:469-476
pattern
constitutes
the
of has
retrieve the atomistic structure of materials with high and well-known
distribution of their interference HRTEM image.
Science 1997,
Electronic identifier: 1359-0266-002-00469 0 Current Chemistry Ltd ISSN 1359-0286 Abbreviations HAADF high-angle annular dark-field HRTEM high-resolution transmission electron microscopy
Introduction According to the paradigm of materials science the properties and the behavior of ‘real’ materials depend uniquely on their microscopic structure. Often the technically relevant properties, such as hardness, electrical conductivity, or corrosion resistance do not so much depend on the underlying equilibrium structure but rather originate from defects disturbing the atom configuration. Therefore, in order to obtain not only a phenomenological but also a ‘physical’ understanding of real materials it is most important to assess their structure at an atomic resolution. Among the experimental techniques that achieve atomic resolution, high-resolution transmission electron microscopy (HRTEM) has the major advantage of imaging the interior of the object, not only the morphology of its surface. ‘Conventional’ HRTEM images are formed by the interference of coherent electron waves; one illuminates the object (a thin foil of the material under investigation) with a (nearly) planar electron wave. The object transmits this wave, interacts with it, and the resulting electron wave ve at the exit plane of the object carries information about the atom arrangement in the object. \ve corresponds to a set of ‘diffracted’ coherent plane waves. The electron optics transfers these waves to the image plane, and the intensity
However, aberrations of the electron optics and instrumental instabilities limit the fine detail in the image. Moreover, the electron optics re-distribute the information of each object point over an extended area of the image. This ‘point spread’ blurs the image and complicates image interpretation down to the ‘information resolution limit’, idest the finest details contained in the image. Several approaches compete with each other to overcome these problems. Development of improved electron optics, new techniques of imaging, and innovative methods of image processing aim to extract more information from HRTEM images. This paper summarizes major progress in these ‘advanced’ methods of HRTEM, as documented by publications from 1996 and 1997. In the same period, new techniques have extended the applicability of HRTEM to a larger field of materials research. By means of sophisticated image processing it has become possible to obtain plan-view images of the atom configuration at reconstructed crystal surfaces. Another exciting development concerns in situ HRTEM, which enables one to observe, at high-resolution, how defects in materials respond to heating, changes in chemical environment, electron beam irradiation, and mechanical stress.
Advances Iterative
in HRTEM techniques
digital
Image
matching
Three complications generally inhibit a straight-forward interpretation of conventional HRTEM images, in terms of a projected structure. First, most real objects are not thin enough to avoid dynamical electron diffraction, which complicates the relationship between the object structure and the electron wave function, we at the exit plane of the object. Second, lens aberrations and instrumental instabilities limit the resolution at which the instrument reproduces this electron wave function in the image plane. The present development of an electron optical spherical aberration corrector [l] will significantly reduce this problem in the future. Third, while both, the amplitude and the phase of the electron wave carry information about the object only the electron intensity (square of the amplitude) constitutes an observable quantity. The phase, in contrast, remains non-observable. One well-established method to correctly interpret HRTEM images relies on numerically simulating the images that hypothetical object structures would yield, under the experimental imaging conditions. The simulated image
470
Characterization
techniques
that yields the best match with the experimental image under consideration identifies the best model for the real structure. This method has been automated to perform a structure refinement by ‘iterative digital image matching’ (Figure 1) [2,3,4*,5,6”,7]. Mathematically, this means a description of the discrepancy between an experimental image and the simulated image of each possible structure, as a function on a highly-dimensional space of structures, and to search this space for the absolute minimum of that function. In such a many-parameter optimization it remains difficult to verify that a given solution constitutes the global minimum and not only a secondary one. Moreover, optimization on a high-dimensional space constitutes a major computing task. Major progress concerning these problems has been achieved by implementing the global optimization algorithm of simulated evolution [6”,7] for image matching. Compared to steepest descent optimization this algorithm locates the global optimum with more trustworthiness and higher efficiency. Therefore, simulated annealing enables a determination of electron optical imaging parameters, such as objective lens defocus, specimen thickness, beam tilt, and so on, along with the position of atom columns.
Fiaure
1
image
-7T
I !!
column positions with an average reliability between 0.01 ‘and 0.02 nm [3,4’,5]. As expected, the reliability increases with increasing projected electrostatic potential (scattering power) of the columns [S]. On the other hand, a simulated image never exactly reproduces an experimental HRTEhI image, even if the input data for the structure and the imaging parameters strictly corresponded to reality. Depending on the material under study, the contrast in the simulated images often largely exceeds the contrast in the corresponding experimental images and shows more fine detail. For structure refinements by iterative digital image matching such ‘contrast misfit’ does not introduce a severe problem because adjustment of column positions in the trial structure does not substantially alter the contrast of the corresponding simulated image (6”,7]. While the source of the contrast misfit remains unknown, one may account for the excess contrast damping in experimental images by a phenomenological, ‘non-physical’ modulation transfer function. When it comes to determining imaging parameters, however, knowledge of absolute contrast becomes more important. The results of systematic investigations (CB Boothroyd, personal communication; T Gemming, G Mobus, F Ernst and M Riihle, unpublished data) have already ruled out several plausible hypotheses about the origin of contrast misfit. Moreover, quantitative analysis of diffraction patterns obtained with a convergent electron beam (CBED) has revealed that the multislice method and the Bloch wave method employed for HRTEM image simulations deliver accurate diffraction intensities [8], provided that one chooses the thickness of the slices small enough and includes correct Debye-Waller factors [9]. This means that the problem of simulating HRTEM images concerns the phases of the diffracted beams rather than their amplitudes. Still,
the
major
source(s)
of the contrast
misfit
remain(s)
to be identified. According to another hypothesis, the contrast misfit arises from rapid stochastic lateral movements of the object during image recording. If this turns out to be true one might solve the problem of contrast misfit by time-resolved image recording, which means to build up the final HRTEM image by digital image processing from a series of subsequent images recorded with extremely short exposure times (T Gemming, personal communication).
Flow diagram of object structure determination by iterative digital matching (gray box) of experimental and simulated HRTEM images.
Matching of the imaging parameters reduces the ‘residual discrepancy’ between simulated and experimental images, and thus improves the reliability of the structure refinement [2,3,4*,5]. Depending on the imaging conditions, the reliability of atom positions can be significantly better than the point resolution of the microscope. At grain boundaries, for example, one can determine the atom
Depending on the material under investigation and the high voltage of the electron microscope, the transmission of high-energy electrons may damage the object during observation. Systematic beam damaging experiments on grain boundaries [4’,10*] and metal/oxide interfaces [lo’] have quantitatively revealed the effect of prolonged electron irradiation on the reliability of atom coordinates, determined by iterative digital image matching. Even when imaging these interfaces by high-voltage high-resolution TEM (HVHRTEM), in a microscope operating
Present developments
in high-resolution transmission electron microscopy Ernst and Riihle
voltage as high as 1250 kV, irradiation damaging did not severely decrease the reliability of structure determination-provided that observation times remained reasonably short (the order of 10 minutes). This result is important because it demonstrates that object damage induced by electron beam irradiation does not necessarily mean a handicap of HVHRTEM. at an accelerating
Figure 2
TI
object ‘Ye
hologram
-i
1997 Current Opinion m Solid State & titerials
Science
Off-axis holography in a transmission electron microscope. A biprism (charged quartz wire) causes the electron wave diffracted by the object to interfere with a reference wave that has passed through the hole of the TEM specimen. The hologram (interference pattern) serves to retrieve I& the electron wave function at the exit plane of the object.
High-resolution
electron holography
Another approach of HRTEM aims to retrieve the complex-valued electron wave function, ye at the exit plane of the object, directly from experimental images, without simulating electron diffraction in model structures. ve carries amplitude and phase information. Empirically, the corresponding phase image often provides more intelligible information on the object structure than the amplitude image or the intensity distribution of a conventional HRTEM image, which generally intermixes amplitude and phase information and suffers from point spread. The major interest in retrieving ye, however, arises from the perspective of using ve to directly reconstruct the structure of the object. Off-axis electron holography by means of a MGllenstedt biprism (Figure 2) allows direct high-resolution retrieval of we from a single image or ‘hologram’ [ 11,121. This method offers many advantages. On the other hand, it requires precise knowledge of microscope aberrations and consideration of artefacts [ 131. Moreover, the need for a reference wave passing through the specimen hole limits the field of view to regions no more than some 1Onm away from the hole. The preparation of TEM specimens is difficult to control at this length scale, and in ‘randomly’ prepared specimens it can be hard to find the crystal defect of interest within the
471
small region suitable for imaging. A recent study [ 14**] of a large angle grain boundary in gold demonstrates, however, that high-resolution off-axis electron holography can solve not only crystal structures but also the atomistic structure of crystal defects. Besides off-axis holography, which provides a ‘direct’ way of retrieving the phase and amplitude of ye, several other methods proceed along an ‘indirect’ way. Among the latter, the ‘focus variation method’ (Figure 3) probably constitutes the most advanced method. This method aims to a retrieve ye from intensity distributions recorded at the image plane of the microscope [15-171. Such reconstruction, which includes correction of lens aberrations, has primary importance for the interpretation of conventional high-resolution images obtained under highly coherent illumination, predominantly in microscopes equipped with field-emission guns. As input data the focus variation method requires a series of images recorded at different objective lens foci. The images may (and normally will) include non-linear contributions. From this data one retrieves the exit-plane wave function ye by the ‘paraboloid method’ [18”], which separates linear from non-linear image transfer, or by the more general ‘maximum likelihood method’ [15]. When tested on simulated images, for which one knows the reasonable wave function ve, both methods have proven to possess sufficient stability against noise and large contributions of non-linear imaging, even for images of non-periodic objects (defects) [17]. Real HRTEM images, however, often exhibit less contrast and less fine detail than simulated images (‘contrast misfit’). Moreover, while recording a focus series the object may drift, tilt, or become contaminated, and the prolonged electron irradiation may introduce damage. Nevertheless, application of the paraboloid method to HRTEM images of oxides with complex structures has demonstrated that the method works under ‘real’ conditions, too. The applicability of focus variation methods to real non-periodic objects, such as interfaces, remains to be shown. Besides their benefits, all present methods of retrieving ve share the problem of depending on accurate data for the aberration parameters (spherical aberration constant) of the electron microscope. Therefore, the success of these procedures depends on further development of experimental methods to determine these parameters with sufficient accuracy. Object reconstruction
Given the electron wave, we at the exit plane of the object, one may try to ‘reconstruct’ the structure of the object. This means to determine the spatial distribution of the electrostatic potential, at least in projection along the viewing direction. This inversion problem constitutes a difficult task, except for ‘weak phase’ (very thin) objects, where the square of we directly indicates the projected electrostatic potential. Most real TEM
472
Characterization
techniques
Fiaure 3
Figure 4
M
Yl
object We
object (atom columns)
electron optics
I
channeling images:
4
‘focus
series’
1s eigenfunction
C 1997 Current Opinmn I” Soled Stats 8 Materials Scmnca
Scheme of the ‘focus variation method’ for retrieving the electron wave function ve at the exit plane of the object from a series of HRTEM images recorded at different focus settings f,, f2 ... fN of the objective lens.
specimens, however, do not qualify as weak phase objects because one cannot prepare them thin enough. Normally, therefore, the image formation process involves dynamical electron diffraction, which is difficult to invert. For objects consisting of largely spaced atom columns along the viewing direction, however, a new method [19] simplifies the interpretation of the exit-plane wave function in terms of atom column positions in the object. The new method rests on a particularly simple view of dynamical electron diffraction in crystals [20**,21”]. In this view the high-energy electrons, that propagate through the object, occupy highly localized channeling eigenfunctions of the atom columns parallel to the optic axis (Figure 4). Provided that each atom column contributes only one eigenstate, which should be true for elements with atomic numbers 2 up to about 50, the electron wave function at the exit plane of the object correlates in a simple fashion with the projected potential of the atom columns. This simple relationship even holds for crystal defects-such as interfaces in edge-on orientation or dislocations in end-on orientation-as long as they do not disrupt the atom columns along the viewing direction. Comparisons with real-space image simulations of individual columns have confirmed the channeling theory [21**]. This theory also allows one to eliminate residual aberrations from the reconstructed object-plane wave function [21**,22]. Application of the channeling theory to retrieve the complex structure of several different oxides from experimental HRTEM images has shown that this method delivers ‘true’ results [19]. On the other hand, viewing electron scattering as channeling of electrons along atom columns clearly reveals the intrinsic limits of object structure retrieval from the
We B 1997 Current Opinion in Solid State 8 Mater&
Science
J
According to the ‘channeling theory: the electrons that propagate through the object occupy highly localized 1s eigenfunctions (shaded regions) of the atom columns parallel to the optic axis.
wave function ve. The average mass of the columns (not the particular kind and vertical distribution of the atoms within the columns) determines the corresponding eigenfunctions. Thus, while object reconstruction may provide accurate information on column positions, it will be difficult to derive the chemical composition of the columns or the vertical distribution of the atoms from ve [19]. Thus, object reconstruction will most likely deliver a set of possible structures rather than a unique solution, and prior knowledge about the object (composition, thickness, and so on) may be necessary to select the best solution out of this set-similar to the prior knowledge required in iterative digital image matching. The channeling theory requires an object consisting of well-separated atom columns. Another new approach [23] to object reconstruction does not depend on this assumption. This new approach employs a simulatedannealing algorithm to find a potential-distribution for which simulation of dynamical electron diffraction yields the same exit-plane wave function ye as obtained for the real object. However, in addition to the above limitations of object reconstruction, the latter method only works for periodic structures. Summarizing, significant progress has been made concerning the difficult problem of reconstructing object structures from given exit plane wave functions ye. However, these methods suffer from intrinsic limitations. Moreover, object reconstruction by inverting the information in the exit-plane electron wave ye will become ‘quantitative’ only when it becomes possible to quantify error limits for ve and to deduce from these the reliability of the atom (column) coordinates in the reconstructed structure.
Present developments in high-resolution trensmission electron microscopy Ernst and Riihle
Z-contrast Imaging with a STEM
While conventional HRTEM and off-axis electron holography rely on image formation by interference of coherent electron waves, high-resolution imaging in a nanoprobe scanning transmission electron microscope (STEM) exploits the benefits of ‘incoherent imaging’ [24”,25**]. Figure 5 depicts the basic idea: while a focused electron probe scans the object, a high-angle annular dark-field (HAADF) detector collects the electrons that the object scatters into directions making large angles with the axis of the incident probe. The entity of the signals recorded at each scan point yields an image of the object. Provided that the annular detector has a suitable design (outer and inner radius) [26”] it suppresses intensity modulations resulting from interference and yields largely incoherent images. The latter term implies a linear transfer of intensities, not amplitudes, from the exit plane of the object to the image plane. The resulting images appear to allow a straight-forward interpretation in terms of projected atom columns and their average atomic number 2 [24”,25**]. Examples that demonstrate the power of such ‘Z-contrast’ imaging include the atomistic structure of tilt grain boundaries in strontium titanate [24”,25”,27*,28] and semiconductor heterointerfaces [24**,25”,29-311. In spite of this success, some severe problems of Z-contrast imaging still need to be overcome. Instrumental instabilities, either electronic or from mechanical vibrations, introduce severe distortions in HAADF images. This restricts reliable determination of atom column positions to small regions, no larger than a few atom spacings in diameter. This problem could be severe, for example, when trying to determine the relative translation of two crystallites forming a grain boundary. Moreover, even within regions of perfect crystal, HAADF images sometimes show contrast variations that one would not expect for ideal Z-contrast. Another drawback of HAADF imaging lies in the high electron dose required to record an image. Similar to HVHRTEM, therefore, one has to consider potential damaging of the object during observation. In contrast to the apparent simplicity of HAADF images and their simple interpretation in terms of Z-contrast, the theoretical interpretation of HAADF imaging has been controversial. Two recent papers [26**,32**] have advanced the theoretical understanding. According to these studies, the contrast mainly arises from ‘thermal diffuse’ scattering, which means large-angle multiphonon, incoherent scattering. Different from earlier suggestions, this kind of electron scattering does not require prior dynamical elastic diffraction. According to one theory [32”], elastic scattering to the first order Laue zone does not play a central role, either. Most importantly, both studies [26”,32”] suggest that the simple interpretation of HAADF images in terms of column scattering power (Z-contrast) may not strictly hold in practical situations. A plain Z-contrast only occurs with ‘thin’ objects [26**].
473
Figure 5
electron probe
(D 1997 Current Opu%an in Solid State 6. Materials Sciem
High-resolution imaging in a nanoprobe scanning transmission electron microscope (STEM) by recording electrons scattered under large angles against the optic axis with a high-angle annular dark-field (HAADF) detector.
Real objects, however, are mostly ‘thick’ and can produce a more complicated HAADF contrast. In particular, depending on the inner angle of the annular detector, the contribution of elastic scattering may lead to contrast reversal [26”]. Moreover, HAADF image simulations indicate that under some circumstances the contrast depends strongly on the focus of the objective lens [32**], and that no simple relation exists between image intensity and object thickness [26”]. Thus, it appears that quantitative interpretation of HAADF images, for instance in terms of column occupancy, generally does require image simulations. Plan-view HRTEM imaging of surface structures
Recent work [33”,34] has also established HRTEM as a powerful tool for plan-view imaging of the two-dimensional arrangement of atoms at crystal surfaces. By means of sophisticated image processing, including noise reduction using a Wiener filter [35], it is possible to extract the contribution of surface atoms from conventional HRTEM images and to de-convolute the overlap of the TEM specimen’s top and bottom surface to obtain images revealing the two-dimensional surface structure. Because the surface atoms alone constitute an ideally thin object, one may interpret such images directly in terms of a projected structure. As a major advantage over scanning tunneling microscopy (STM), HRTEM can probe the surface reconstruction into some depth, too. A HRTEM
474
Characterization
techniques
study of the 7 x 7 reconstruction of the {ill} silicon surface demonstrates that such in-depth probing may significantly improve the physical understanding of surface reconstruction phenomena [33”].
accuracy. Moreover, improved resolution and reliability of structure retrieval increase the demand for high-quality TEM specimens. This means that the techniques of TEM specimen preparation will require increasing attention in the future.
In situ HRTEM
Observation of processes rather than static structures at the atomic scale allows one to tackle a wide range of interesting problems in materials science. Major progress in understanding materials behavior has been made recently by means of in situ HRTEM, which is rapidly expanding, owing to increasing stability of high-resolution microscopes. In the period under survey in this article, in situ HRTEM has delivered a wealth of new insights about the formation and stability of complex carbon structures. Pioneering work in this field includes direct observation of fullerene molecule formation under an environment simulating an arc-discharge chamber [36], formation of single fullerene cages [37] or carbon shell clusters (‘onions’) [38] under electron irradiation, and generation of diamond under the internal pressure of carbon onions [39**]. Other in situ HRTEM studies have shed light on the microscopic motion of grain boundaries [40,41*,42] and the interaction between grain boundaries and dislocations [43*]. The latter work concerns grain boundaries that play a key role for the primary recrystallization of face-centered cubic metals. The in situ observations suggest the atomistic mechanism by which growing recrystallized grains accommodate dislocations from the deformed matrix. Furthermore, in situ observations have provided insight into the solid-liquid phase transitions of metals [44] and the reconstruction mechanism of the Si{lll), (OOl), (Zll), and {311) surfaces [45]. Work on dislocation motion in silicon by kink formation and migration [46*] impressively demonstrates that in situ HRTEhl can also provide ‘quantitative’ data, such as dislocation velocity and kink formation energy.
Conclusions High-resolution transmission electron microscopy (HRTEM) constitutes one of the most powerful experimental techniques available to investigate materials at the atomic level. In addition to instrumental developments that aim to reduce the spherical aberration of the objective lens, several new methods have been developed to improve the resolution limit and the reliability of structure retrieval from HRTEhI images. These methods include iterative digital matching of simulated and experimental images, several variants of high-resolution electron holography, object reconstruction, and incoherent high-resolution imaging in a dedicated scanning transmission electron microscope, employing a high-angle annular dark-field detector. For all these methods, the development of efficient image processing methods and optimization strategies has become a central issue. Still, major problems remain be solved, like the contrast misfit between simulated and experimental images, or the experimental determination of electron optical imaging parameters with sufficiently high
Nevertheless, the resolution of HRTEM imaging presently approaches 18. The reliability of determining the position of atom columns at crystal defects has even reached 0.01 A. This development marks an important transition: while formerly HRTEM yielded only qualitative results, involving visual inspection and personal judgement of an individual observer, HRTEM has now developed into a truly ‘quantitative’ technique that delivers ‘hard numbers’ about the atomistic structure of defects in materials. Plan-view imaging of surface structures and in situ experiments constitute exciting new domains of HRTEhI characterization of materials. The exploration of these new domains has hardly begun.
References
and recommended
reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
. ..
of special interest of outstanding interest
1.
Haider M, Braunshausen G, Schwan E: Correction of the spherical aberration of a 200 kV TEM by means of a hexapolecorrector. Oprik 1995, 99:167-l 79.
2.
Hofmann D, Ernst F: Quantitative HRTEM of the incoherent twin boundary in Cu. Ultramicroscopy 1994, 53:205-221.
3.
Ernst F, Hofmann D, Nadarzinski K, Stemmer S, Streiffer SK: Quantitative high-resolution electron microscopy of interfaces. In Intergranular and Interphase Boundaries in Materials, vol 1. Edited by Ferro AC, Conde EP, Fortes EA (Materials Science Forum 207-209). ZiJrich: Trans Tech Publications; 1996:23-34.
4. .
Nadarzinski K, Ernst F: The atomistic structure of a 213, (111) grain boundary in NiAI, studied by quantitative high-resolution transmission electron microscopy. Phil Mag A 1996, 74:641664. Method of iterative digital matching of simulated and experimental HRTEM images, applied to determine the atomistic structure of a grain boundary in NiAI, and quantitative analysis of electron beam damage on the reliability of atom column positions. 5.
Kienzle 0, Exner M, Ernst F: Analysis of interface structures by quantitative high-resolution transmission electron microscopy. In Atomic Resolution Microscopy of Surfaces and Interfaces. Edited by Hamers R, Smith DJ (Mater Res Sot Symp Proc 466). Pittsburgh: Materials Research Society; 1997, in press.
6. ..
Mdbus G: Retrieval of crystal defect structures from HREM images by simulated evolution. I. Basic technique. Ultramicroscopy 1996, 65:205-216. This paper introduces a powerful implementation of structure refinement by iterative digital matching of simulated and experimental images, and discusses the limitation of this method. 7.
Mobus G, Dehm G: Retrieval of crystal defect structures from HREM images by simulated evolution. II. Experimental image evaluation. Ultramicroscopy 1996, 65:217-226.
0.
Cheng YF, Nuechter W, Mayer J, Weickenmeier A, Gjnnes J: Loworder structure-factor amblitude and Sian determination of an unknown structure AlmFe by quantitative convergent-beam electron diffraction. Acta Cryst A 1996, 52:923-936.
Present developments in high-resolution transmission electron microscopy Ernst and Riihle
9.
Jiang Hua C, Op de Beeck M, Van Dyck D: Can the multislice method be used to calculate HOLZ reflections in high-energy electron diffraction and imaging? Microsc Microanal Microstruct 1996, 7:27-47.
10. .
Dehm G, Nadarzinski K, Ernst F, Riihle M: Quantification of irradiation damage generated during HRTEM with 1250 keV electrons. Ultramicroscopy 1996, 63:49-55. Quantitative analysis of electron beam damage on the reliability of atom column positions as obtained by iterative digital matching of simulated and experimental images. 11.
Lichte H: Electron holography. I. Can electron holography 0.1 nm resolution? Ultramicroscopy 1992, 47:223-230.
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Lichte H, Vdlkl E: Electron holography. II. First steps of high resolution electron holography into materials science. Ultramicroscopy 1992, 47~231-240.
Principles of high-resolution imaging in a nanoprobe STEM by means of a HAADF detector, and the impact of this technique on analytical TEM. 25. ..
Pennycook SJ, Browning ND, McGibbon MM, McGibbon Al, Chisholm MF, Jesson DE: Determination of interface structure and bonding by Z - contrast STEM. Diffus Defect Data Part B 1996. 47:561-571. Principles of HAADF imaging and its combination with analytical TEM at high spatial resolution. 26.
Hartel P, Rose H, Dinges C: Conditions and reasons for incoherent imaging in STEM. Ultramicroscopy 1996, 63:93-l 14. Eeoretical fundament of HAADF imaging in a dedicated STEM. 27.
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Lichte H: Artefacts 1996, 64~67-77.
in electron holography.
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Uftramicroscopy
Orchowski A, Lichte H: High-resolution electron holography of non-periodic structures at the example of a X-13 grain boundary in gold. Ultramicroscopy 1996, 64:199-209. This article reports on a first application of high-resolution electron holography to solve the atomistic structure of a grain boundary.
26.
McGibbon MM, Browning ND, McGibbon Al, Pennycook SJ: The atomic structure of asymmetric [OOll tilt boundaries in SrTiO3. Phil Mag A 1996, 73:625-641.
29.
McGibbon AJ, Pennycook SJ: Direct atomic resolution imaging of dislocation core structures in a 300 kV STEM. Microsc Semiconduc Mater 1995. 1995:79-82.
30.
McGibbon AJ, Pennycook SJ, Angelo JE, Mills MJ: Direct sub-lattice imaging of interface dislocation structures in CdTe/GaAs@OlL In Evolution of Thin Film and Surface Structure and Morphology. Edited by Demczyk BG et a/. (Mater Res Sot Symp Proc 355). Pittsburgh: Materials Research Society; 1995:625-630. McGibbon AJ, Pennycook SJ, Angelo JE: Direct observation of dislocation core structures in CdTe/GaAs(OOl). Science 1995, 269:519-521.
15.
Coene WMJ, Thust A, Op de Beeck M, Van Dyck D: Maximumlikelihood method for focus-variation image reconstruction in high resolution transmission electron microscopy. Ultramicroscopy 1996, 64:109-l 35.
16.
Thust A, Overwijk MHF, Coena WMJ: Numerical correction of lens aberrations in phase-retrieval HRTEM. Ultramicroscopy 1996, 64:249-264.
31.
Thust A, Coene WMJ: Focal-series reconstruction in HRTEM: simulation studies on non-periodic obiects. Ultramicroscopy 1996, 64:21 l-230.
32.
16. ..
Op de Beeck M, Van Dyck D, Coene W: Wave function reconstruction in HRTEM: the parabola method. Ultramicroscopy 1996. 64:167-i 63. Retrieval of ihe electron wave function at the exit plane of the object by evaluating a series of HRTEM images recorded at different objective lens focus settings. 19.
Op de Beeck M, Van Dyck D: Direct structure reconstruction HRTEM. Ultramicroscopy 1996,64:153-l 65.
Brownina ND. Pennvcook SJ: Direct exDerimental determination at internal in&aces. J Phys D 1996,
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Application of HAADF imaging to solve interface structures.
14. ..
1z
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in
20. ..
Van Dyck D, Lichte H, van der Mast KD: Sub-Ongstrsm structure characterisation: the Brite-Euram route towards one Ongstrsm. Ultramicroscopy 1996, 64:1-l 5. Overview about competing approaches to increaae the resolution limit of high-resolution transmission electron microscopy by retrieving the electron wave function at the exit plane of the object and to reconstruct the object structure from this wave function. 21. Van Dyck D, Op de Beeck M: A simple intuitive theory for .. electron diffraction. Ultramicroscopy 1996, 64:99-l 07. This paper presents a new concept to describe dynamical electron d#fraction in thin crystals. This concept allows an intuitive understanding of the correlation between structure of the object and the electron wave function that describes the electron diffraction at the exit plane of the object under coherent illumination. 22.
Tang D, Zandbergen HW,.Jansen J, Op de Beeck M, Van Dyck D: Fine-tuning of the focal residue in exit-wave reconstruction. Ultramicroscopy 1996. 64:265-276.
23.
Lentzen M, Urban K: Reconstruction of the projected crystal potential from a periodic high-resolution electron microscopy exit plane wave function. Ultramicroscopy 1996, 62:249-264.
24. ..
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