Table of Contents
Further Reading
Title Page
Copyright
Contents to Volume 1
Preface
List of Contributors
The Past, the Present, and the Future of Nanoscopy
The Past
The Present and the Future
Acknowledgments
References
Part I: Methods
Chapter 1: Transmission Electron Microscopy
1.1 Introduction
1.2 The Instrument
1.3 Imaging and Diffraction Modes
1.4 Dynamical Diffraction Theory
References
Chapter 2: Atomic Resolution Electron Microscopy
2.1 Introduction
2.2 Principles of Linear Image Formation
2.3 Imaging in the Electron Microscope
2.4 Experimental HREM
2.5 Quantitative HREM
2.6 Appendix 2.A: Interaction of the Electron with a Thin Object
2.7 Appendix 2.B: Multislice Method
2.8 Appendix 2.C: Quantum Mechanical Approach
References
Chapter 3: Ultrahigh-Resolution Transmission Electron Microscopy at Negative Spherical Aberration
3.1 Introduction
3.2 The Principles of Atomic-Resolution Imaging
3.3 Inversion of the Imaging Process
3.4 Case Study: SrTiO3
3.5 Practical Examples of Application of NCSI Imaging
References
Chapter 4: Z-Contrast Imaging
4.1 Recent Progress
4.2 Introduction to the Instrument
4.3 Imaging in the STEM
4.4 Future Outlook
Acknowledgments
References
Chapter 5: Electron Holography
5.1 General Idea
5.2 Image-Plane Off-Axis Holography Using the Electron Biprism
5.3 Properties of the Reconstructed Wave
5.4 Holographic Investigations
5.5 Special Techniques
5.6 Summary
Acknowledgments
References
Books
Articles
Basics
Method
Interpretation
Inverse Problem
Applications
Electric Potentials
Semiconductors
Li-Ion Batteries
Holographic Tomography
Magnetic
Atomic Resolution
Chapter 6: Lorentz Microscopy and Electron Holography of Magnetic Materials
6.1 Introduction
6.2 Lorentz Microscopy
6.3 Off-Axis Electron Holography
6.4 Discussion and Conclusions
Acknowledgments
References
Chapter 7: Electron Tomography
7.1 History and Background
7.2 Theory of Tomography
7.3 Electron Tomography, Missing Wedge, and Imaging Modes
7.4 STEM Tomography and Applications
7.5 Hollow-Cone DF Tomography
7.6 Diffraction Contrast Tomography
7.7 Electron Holographic Tomography
7.8 Inelastic Electron Tomography
7.9 Advanced Reconstruction Techniques
7.10 Quantification and Atomic Resolution Tomography
Acknowledgments
References
Chapter 8: Statistical Parameter Estimation Theory – A Tool for Quantitative Electron Microscopy
8.1 Introduction
8.2 Methodology
8.3 Electron Microscopy Applications
8.4 Conclusions
Acknowledgments
References
Chapter 9: Dynamic Transmission Electron Microscopy
9.1 Introduction
9.2 Time-Resolved Studies Using Electrons
9.3 Building a DTEM
9.4 Applications of DTEM
9.5 Future Developments for DTEM
9.6 Conclusions
Acknowledgments
References
Chapter 10: Transmission Electron Microscopy as Nanolab
10.1 TEM and Measuring the Electrical Properties
10.2 TEM with MEMS-Based Heaters
10.3 TEM with Gas Nanoreactors
10.4 TEM with Liquid Nanoreactors
10.5 TEM and Measuring Optical Properties
10.6 Sample Preparation for Nanolab Experiments
References
Chapter 11: Atomic-Resolution Environmental Transmission Electron Microscopy
11.1 Introduction
11.2 Atomic-Resolution ETEM
11.3 Development of Atomic-Resolution ETEM
11.4 Experimental Procedures
11.5 Applications with Examples
11.6 Nanoparticles and Catalytic Materials
11.7 Oxides
11.8 In situ Atomic Scale Twinning Transformations in Metal Carbides
11.9 Dynamic Electron Energy Loss Spectroscopy
11.10 Technological Benefits of Atomic-Resolution ETEM
11.11 Other Advances
11.12 Reactions in the Liquid Phase
11.13 In situ Studies with Aberration Correction
11.14 Examples and Discussion
11.15 Applications to Biofuels
11.16 Conclusions
Acknowledgments
References
Chapter 12: Speckles in Images and Diffraction Patterns
12.1 Introduction
12.2 What Is Speckle?
12.3 What Causes Speckle?
12.4 Diffuse Scattering
12.5 From Bragg Reflections to Speckle
12.6 Coherence
12.7 Fluctuation Electron Microscopy
12.8 Variance versus Mean
12.9 Speckle Statistics
12.10 Possible Future Directions for Electron Speckle Analysis
References
Chapter 13: Coherent Electron Diffractive Imaging
13.1 Introduction
13.2 Coherent Nanoarea Electron Diffraction
13.3 The Noncrystallographic Phase Problem
13.4 Coherent Diffractive Imaging of Finite Objects
13.5 Phasing Experimental Diffraction Pattern
13.6 Conclusions
Acknowledgments
References
Chapter 14: Sample Preparation Techniques for Transmission Electron Microscopy
14.1 Introduction
14.2 Indirect Preparation Methods
14.3 Direct Preparation Methods
14.4 Summary
Acknowledgments
References
Chapter 15: Scanning Probe Microscopy – History, Background, and State of the Art
15.1 Introduction
15.2 Detecting Evanescent Waves by Near-Field Microscopy: Scanning Tunneling Microscopy
15.3 Interaction of Tip–Sample Electrons Detected by Scanning Near-Field Optical Microscopy and Atomic Force Microscopy
15.4 Methods for the Detection of Electric/Electronic Sample Properties
15.5 Methods for the Detection of Electromechanical and Thermoelastic Quantities
15.6 Advanced SFM/SEM Microscopy
Acknowledgments
References
Chapter 16: Scanning Probe Microscopy – Forces and Currents in the Nanoscale World
16.1 Introduction
16.2 Scanning Probe Microscopy – the Science of Localized Probes
16.3 Scanning Tunneling Microscopy and Related Techniques
16.4 Force-Based SPM Measurements
16.5 Voltage Modulation SPMs
16.6 Current Measurements in SPM
16.7 Emergent SPM Methods
16.8 Manipulation of Matter by SPM
16.9 Perspectives
Acknowledgments
References
Chapter 17: Scanning Beam Methods
17.1 Scanning Microscopy
17.2 Conclusions
References
Chapter 18: Fundamentals of the Focused Ion Beam System
18.1 Focused Ion Beam Principles
18.2 FIB Techniques
Acknowledgments
References
Further Reading
Contents to Volume 2
Chapter 19: Low-Energy Electron Microscopy
19.1 Introduction
19.2 Theoretical Foundations
19.3 Instrumentation
19.4 Areas of Application
19.5 Discussion
19.6 Concluding Remarks
References
Chapter 20: Spin-Polarized Low-Energy Electron Microscopy
20.1 Introduction
20.2 Theoretical Foundations
20.3 Instrumentation
20.4 Areas of Application
20.5 Discussion
20.6 Concluding Remarks
References
Chapter 21: Imaging Secondary Ion Mass Spectroscopy
21.1 Fundamentals
21.2 SIMS Techniques
21.3 Biological SIMS
21.4 Conclusions
References
Chapter 22: Soft X-Ray Imaging and Spectromicroscopy
22.1 Introduction
22.2 Experimental Techniques
22.3 Data Analysis Methods
22.4 Selected Applications
22.5 Future Outlook and Summary
Acknowledgments
References
Chapter 23: Atom Probe Tomography: Principle and Applications
23.1 Introduction
23.2 Basic Principles
23.3 Field Ion Microscopy
23.4 Atom Probe Tomography
23.5 Conclusion
References
Chapter 24: Signal and Noise Maximum Likelihood Estimation in MRI
24.1 Probability Density Functions in MRI
24.2 Signal Amplitude Estimation
24.3 Noise Variance Estimation
24.4 Conclusions
References
Chapter 25: 3-D Surface Reconstruction from StereoScanning Electron Microscopy Images
25.1 Introduction
25.2 Matching Stereo Images
25.3 Conclusions
Acknowledgments
References
Further Reading
Part II: Applications
Chapter 26: Nanoparticles
26.1 Introduction
26.2 Imaging Nanoparticles
26.3 Electron Tomography of Nanoparticles
26.4 Nanoanalytical Characterization of Nanoparticles
26.5 In situ TEM Characterization of Nanoparticles
References
Chapter 27: Nanowires and Nanotubes
27.1 Introduction
27.2 Structures of Nanowires and Nanotubes
27.3 Defects in Nanowires
27.4 In situ Observation of the Growth Process of Nanowires and Nanotubes
27.5 In situ Mechanical Properties of Nanotubes and Nanowires
27.6 In situ Electric Transport Property of Carbon Nanotubes
27.7 In situ TEM Investigation of Electrochemical Properties of Nanowires
27.8 Summary
References
Chapter 28: Carbon Nanoforms
28.1 Imaging Carbon Nanoforms Using Conventional Electron Microscopy
28.2 Analysis of Carbon Nanoforms Using Aberration-Corrected Electron Microscopes
28.3 Ultrafast Electron Microscopy
28.4 Scanning Tunneling Microscopy (STM)
28.5 Scanning Photocurrent Microscopy (SPCM)
28.6 X-Ray Electrostatic Force Microscopy (X-EFM)
28.7 Atomic Force Microscopy
28.8 Scanning Near-Field Optical Microscope
28.9 Tip-Enhanced Raman and Confocal Microscopy
28.10 Tip-Enhanced Photoluminescence Microscopy
28.11 Fluorescence Quenching Microscopy
28.12 Fluorescence Microscopy
28.13 Single-Shot Extreme Ultraviolet Laser Imaging
28.14 Nanoscale Soft X-Ray Imaging
28.15 Scanning Photoelectron Microscopy
Acknowledgments
References
Chapter 29: Metals and Alloys
29.1 Formation of Nanoscale Deformation Twins by Shockley Partial Dislocation Passage
29.2 Minimal Strain at Austenite–Martensite Interface in Ti-Ni-Pd
29.3 Atomic Structure of Ni4Ti3 Precipitates in Ni-Ti
29.4 Ni-Ti Matrix Deformation and Concentration Gradients in the Vicinity of Ni4Ti3 Precipitates
29.5 Elastic Constant Measurements of Ni4Ti3 Precipitates
29.6 New APB-Like Defect in Ti-Pd Martensite Determined by HRSTEM
29.7 Strain Effects in Metallic Nanobeams
29.8 Adiabatic Shear Bands in Ti6Al4V
29.9 Electron Tomography
29.10 The Ultimate Resolution
Acknowledgments
References
Chapter 30:In situ Transmission Electron Microscopy on Metals
30.1 Introduction
30.2 In situ TEM Experiments
30.3 Grain Boundary Dislocation Dynamics Metals
30.4 In situ TEM Tensile Experiments
30.5 In situ TEM Compression Experiments
30.6 Conclusions
Acknowledgments
References
Chapter 31: Semiconductors and Semiconducting Devices
31.1 Introduction
31.2 Nanoscopic Applications on Silicon-Based Semiconductor Devices
31.3 Conclusions
Acknowledgments
References
Chapter 32: Complex Oxide Materials
32.1 Introduction
32.2 Aberration-Corrected Spectrum Imaging in the STEM
32.3 Imaging of Oxygen Lattice Distortions in Perovskites and Oxide Thin Films and Interfaces
32.4 Atomic-Resolution Effects in the Fine Structure–Further Insights into Oxide Interface Properties
32.5 Applications of Ionic Conductors: Studies of Colossal Ionic Conductivity in Oxide Superlattices
32.6 Applications of Cobaltites: Spin-State Mapping with Atomic Resolution
32.7 Summary
Acknowledgments
References
Chapter 33: Application of Transmission Electron Microscopy in the Research of Inorganic Photovoltaic Materials
33.1 Introduction
33.2 Experimental
33.3 Atomic Structure and Electronic Properties of c-Si/a-Si:H Heterointerfaces
33.4 Interfaces and Defects in CdTe Solar Cells
33.5 Influences of Oxygen on Interdiffusion at CdS/CdTe Heterojunctions
33.6 Microstructure Evolution of Cu(In,Ga)Se2 Films from Cu Rich to In Rich
33.7 Microstructure of Surface Layers in Cu(In,Ga)Se2 Thin Films
33.8 Chemical Fluctuation-Induced Nanodomains in Cu(In,Ga)Se2 Films
33.9 Conclusions and Future Directions
Acknowledgment
References
Chapter 34: Polymers
34.1 Foreword
34.2 A Brief Introduction on Printable Solar Cells
34.3 Morphology Requirements of Photoactive Layers in PSCs
34.4 Our Characterization Toolbox
34.5 How It All Started: First Morphology Studies
34.6 Contrast Creation in Purely Carbon-Based BHJ Photoactive Layers
34.7 Nanoscale Volume Information: Electron Tomography of PSCs
34.8 One Example of Electron Tomographic Investigation: P3HT/PCBM
34.9 Quantification of Volume Data
34.10 Outlook and Concluding Remarks
Acknowledgment
References
Chapter 35: Ferroic and Multiferroic Materials
35.1 Multiferroicity
35.2 Ferroic Domain Patterns and Their Microscopical Observation
35.3 The Internal Structure of Domain Walls
35.4 Domain Structures Related to Amorphization
35.5 Dynamical Properties of Domain Boundaries
35.6 Conclusion
References
Chapter 36: Three-Dimensional Imaging of Biomaterials with Electron Tomography
36.1 Introduction
36.2 Biological Tomographic Techniques
36.3 Examples of Electron Tomography Biomaterials
36.4 Outlook
References
Chapter 37: Small Organic Molecules and Higher Homologs
37.1 Introduction
37.2 Optical Microscopy
37.3 Scanning Electron Microscopy–SEM
37.4 Atomic Force and Scanning Tunneling Microscopy (AFM and STM)
37.5 Transmission Electron Microscopy (TEM)
37.6 Summary
References
Index
Further Reading
Ohser, J. Schladitz, K.
3D Images of Materials Structures
Processing and Analysis
2009
Hardcover
ISBN: 978-3-527-31203-0
Codd, S. L., Seymour, J. D. (eds.)
Magnetic Resonance Microscopy
Spatially Resolved NMR Techniques and Applications
2009
Hardcover
ISBN: 978-3-527-32008-0
Maev, R. G.
Acoustic Microscopy
Fundamentals and Applications
2008
Hardcover
ISBN: 978-3-527-40744-6
Fukumura, H., Irie, M., Iwasawa, Y., Masuhara, H., Uosaki, K. (eds.)
Molecular Nano Dynamics
Vol. I: Spectroscopic Methods and Nanostructures/Vol. II: Active Surfaces, Single Crystals and Single Biocells
2009
Hardcover
ISBN: 978-3-527-32017-2
Roters, F., Eisenlohr, P. Bieler, T. R., Raabe, D.
Crystal Plasticity Finite Element Methods
in Materials Science and Engineering
2010
Hardcover
ISBN: 978-3-527-32447-7
Guo, J. (ed.)
X-Rays in Nanoscience
Spectroscopy, Spectromicroscopy, and Scattering Techniques
2010
Hardcover
ISBN: 978-3-527-32288-6
Tsukruk, V., Singamaneni, S.
Scanning Probe Interrogation of Soft Matter
2012
Hardcover
ISBN: 978-3-527-32743-0
The Editors
Prof. Gustaaf Van Tendeloo
Univ. of Antwerp (RUCA)
EMAT
Groenenborgerlaan 171
2020 Antwerpe
Belgium
Prof. Dirk Van Dyck
Univ. of Antwerp (RUCA)
EMAT
Groenenborgerlaan 171
2020 Antwerp
Belgium
Prof. Dr. Stephen J. Pennycook
Oak Ridge National Lab.
Condensed Matter Science Div.
Oak Ridge, TN 37831-6030
USA
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Contents to Volume 1
Preface
Since the edition of the previous “Handbook of Microscopy” in 1997 the world of microscopy has gone through a significant transition.
In electron microscopy the introduction of aberration correctors has pushed the resolution down to the sub-Angstrom regime, detectors are able to detect single electrons, spectrometers are able to record spectra from single atoms. Moreover the object space is increased which allows to integrate these techniques in the same instrument under full computer support without compromising on the performance. Thus apart from the increased resolution from microscopy to nanoscopy, even towards picoscopy, the EM is gradually transforming from an imaging device into a true nanoscale laboratory that delivers reliable quantitative data on the nanoscale close to the physical and technical limits. In parallel, scanning probe methods have undergone a similar evolution towards increased functionality, flexibility and integration.
As a consequence the whole field of microscopy is gradually shifting from the instrument to the application, from describing to measuring and to understanding the structure/property relations, from nanoscopy to nanology.
But these instruments will need a different generation of nanoscopists who need not only to master the increased flexibility and multifunctionality of the instruments, but to choose and combine the experimental possibilities to fit the material problem to be investigated.
It is the purpose of this new edition of the “Handbook of Nanoscopy” to provide an ideal reference base of knowledge for the future user.
Volume 1 elaborates on the basic principles underlying the different nanoscopical methods with a critical analysis of the merits, drawbacks and future prospects. Volume 2 focuses on a broad category of materials from the viewpoint of how the different nanoscopical measurements can contribute to solving materials structures and problems.
The handbook is written in a very readable style at a level of a general audience. Whenever relevant for deepening the knowledge, proper references are given.
Gustaaf van Tendeloo, Dirk van Dyck, and Stephen J. Pennycook
List of Contributors
ois VurpillotThe Past, the Present, and the Future of Nanoscopy
Gustaaf Van Tendeloo and Dirk Van Dyck
Science stems from the curiosity of humankind. We all want to look behind the curtain and discover new things that have not been exposed yet. We explore the world, like a child, with our five senses. For the present book, we mainly, although not exclusively, focus on the eyesight. With our eyes, we explore the world around us at different scales: from the stars and the cosmos down to the sub-millimeter scale. However, details below 0.1 mm are hardly visible to the naked eye. In order to improve our eyesight, we use a magnifying glass, but for further magnifications, we have to use combined lenses, which form an instrument that we know as a microscope.
The first multilens microscopes were built in the seventeenth century by Jan Swammerdam and Robert Hooke, but it was the Dutchman Antoni van Leeuwenhoek who captured the interest of scientists for the new technique [1]. He was able to produce such strong lenses that the magnification could go up to several hundred times. This evidently created a whole new inside in the “invisible” world of micron-scale details and organisms such as bacteria and cells. van Leeuwenhoek was not a scientist by education, but he got large recognition in the scientific world because of his amazing results. Nevertheless, he kept the secret of producing such strong lenses for himself until he died. Once the interest of the scientists was captured, the optical microscope evolved in the eighteenth and nineteenth centuries through the introduction of more lenses to minimize aberrations.
The resolution of “classical optical microscopy” kept improving, but at the end of the nineteenth century, Ernst Karl Abbe stated that the resolution of an optical microscope is actually limited by the wavelength of light; that is, of the order of about half a micrometer.
At the end of the nineteenth century and in the beginning of twentieth century, a number of things changed. Thomson discovered the electron, Einstein introduced relativity theory, and quantum mechanics got solid foundations, thanks to Planck, Bohr, de Broglie, Heisenberg, Schrodinger, and Dirac. Particularly, the description of the particle – wave duality for accelerated electrons was important for the discovery of the “electron microscope” in 1931 by Ernst Ruska. Based on the fact that electrons could be deflected by an electrostatic or an electromagnetic field, Max Knoll and Ernst Ruska demonstrated a two-stage magnification of a simple object with a magnification of only 17 times. The first experiments with a real electron microscope were really disappointing from a scientific point of view. Although already in 1933 Ruska had no problem proving a resolution of 50 nm, considerably better than the resolution of an optical microscope, virtually all materials were burnt to a cinder under the electron beam and the interest of scientists faded away.
In that same period, Marton and others found a way to avoid severe burning of the samples, and the interest of industry to build a commercial instrument revived. In 1939, Siemens and Halske promoted the first commercial electron microscope, but it was only after the second World War that the real success of electron microscopy started and that biologists, chemists, physicists, materials scientists, and engineers got interested in the developments and applications [[2, 3]]. Technology of electron microscopy progressed very fast, and resolution was one of the key issues. Different European, American, and Japanese companies were involved in the race for the better resolution. In the middle of the 1950s, Menter [4] impressed the scientific community by showing lattice resolution and lattice imperfections with a resolution of about 1 nm. The first atomic resolution images in transmission electron microscopy TEM of heavy atoms such as thorium or gold appeared in the beginning of the 1970s [[5–7]]. This triggered a whole new research on defect studies at an atomic or nearly atomic scale, with S. Iijima being one of the pioneers in the field of solid state chemistry with his high-resolution studies of Nb2O5-based materials [8]. Independently, atomic resolution images of heavy atoms on a carbon support were obtained by Crewe and coworkers in Chicago using a high-resolution scanning transmission electron microscope (STEM) [9].
During the 1980s and the 1990s, the resolution steadily improved, and by the turn of the century, the instrumental resolution of most commercial instruments approached 0.1 nm. At this value, the limit seemed to be reached since spherical aberration and chromatic aberration (and often also the sample) limited further progress. However, the introduction of spherical-aberration-corrected lenses [10] opened a new world of subangstrom resolution and improved signal-to-noise ratio. A state-of-the-art description is given by Urban [11].
A remarkable fact is that in the 1960s and the 1970s, researchers as well as commercial companies pushed the accelerating voltage of microscopes, in order to benefit from the decreased wavelength of the electrons for voltages of 1 MeV or higher. This increased voltage had the extra effect that the penetration depth increased and that much thicker samples could be analyzed. Famous high-voltage centers were Toulouse, Cambridge, Osaka, Argonne, Berkeley, etc. However, nowadays, with the interest in nanostructured materials, nanotubes, and single-sheet materials such as graphene where the radiation damage at higher voltages becomes important, the tendency is more toward lower voltages: 40 keV and even lower, maintaining a subangstrom resolution.
Although TEM and STEM are now mainly used to study atomic details, the first technique that allowed atomic resolution was field emission microscopy or field ion microscopy (FIM). Cooper and Müller [12] showed already in 1958 clean atomic patterns of an iron needle. For a long time, the delicate sample preparation and the limited sample region that could be imaged hampered the applications, but recently, three-dimensional imaging using the atom probe technique and improved sample preparation by focused ion beam (FIB) have revived the technique.
Surface imaging for a long time has been the field of optical microscopy and later scanning electron microscopy (SEM), but atomic resolution has never been obtained. For the first technique, the wavelength is the limiting factor, and for SEM, the probe size and the signal-to-noise ratio were the limiting factors. However, with the introduction of scanning near-field optical microscopy (SNOM), the resolution limit has been pushed forward. By placing a detector very close to the sample, the resolution is limited by the size of the detector aperture rather than by the wavelength of the illuminating light. In this way, a lateral resolution of 20 nm has been demonstrated [13].
New developments in SEM and scanning ion microscopy (SIM) have revived the “atomic resolution dream,” and recently, Zhu showed atomic resolution using secondary electrons in an electron microscope [14].
However, in the 1980s, scanning tunneling microscopy (STM) and later atomic force microscopy (AFM) was invented and fine tuned by Binnig et al. [15] in order to produce atomically sharp images of (metallic) surfaces. It is worth noting that in 1986, more than 50 years after his invention of the electron microscope, Ernst Ruska obtained the Nobel Prize together with the inventors of the STM, Binnig and Roher.
The reason why technological advancements increase almost exponentially with time, as demonstrated by the well-known curve of Moore, is because they can profit directly from current-state technology. This is also the case with the improvement in the performance of microscopes. But every process of technological improvement will ultimately bounce against the physical limits. And in nanoscopy, we are now in a stage where we see these limits at the horizon. However, this does not mean the end of the road. In the future, nanoscopy will continuously evolve to nanology, from observing and describing to understanding. And understanding structure–property relations means interaction between experimentalists and theorists, which is based on the language of numbers such as atom positions or other structural parameters. In the future, images will thus become merely intermediate dataplanes from which these numbers can be extracted quantitatively. Thus, more important than the performance specifications of the instrument will be the precision (“error bar”) on the fitted parameters. For instance, as shown in Ref. [16], the precision to which the position of an atom can be determined is not only proportional to the “resolution” of the instrument but also inversely related to the square root of the number of imaging particles that interact with the atom. Thus, if one wants to design a better microscope or a better experimental setting, one has to take account of both factors.
With the development of aberration correctors [17], the resolution of the electron microscope is not limited anymore by the quality of the lenses but by the “width” of the atom itself, which is determined by the electrostatic potential and the thermal motion of the atom. Furthermore, when the electron collides with an atom at very close distance, it will inevitably transfer energy to the motion of the atom, which influences its position. This puts an ultimate limit to the resolution as the closest distance that nature allows to approach an atom without altering its position. This limit is of the order of 0.2 Å [18]. If that limit is reached, the images contain all the information that can be obtained with electrons, which makes HREM superior to diffraction since the experimental data are in the same real space where one wants to use this information.
In HAADF–STEM imaging [19], where the main part of the signal is caused by phonon scattering, which is very localized at the atom core, the ultimate resolution is mainly limited by the size of the probe, which is also in the subangstrom regime. An advantage of ADF STEM is that images can be interpreted more easily in terms of the atomic number of the atoms, but at the disadvantage of a weaker signal.
Imaging can also be combined with spectroscopy both in TEM (EFTEM) and in STEM (EELS), and spectral image maps can be obtained with atomic resolution [[19, 20]].
Most atomic resolution work is still done in two dimensions in which one can only observe the projected structure of the object. But the ultimate goal is to achieve 3D electron tomography with atomic resolution [21]. This dream is not yet achieved, mainly because of demanding flexibility and stability requirements of the object holders. Miniaturized MEMS holders can hopefully overcome these difficulties. There also remain theoretical difficulties in combining multiple scattering with tomographic schemes.
Another important technological achievement is that advanced detectors are able to detect single electrons so that the signal-to-noise ratio is ultimately determined by the unavoidable quantum statistics, which again is a function of the number of interacting particles. The main limitations on the number of imaging particles are the electron source, the recording time, and the stability of the object. The present electron sources are still far from the physical limits imposed by phase space so that one can still expect improvement [22]. The recording time is mainly limited by the mechanical stability of the holders. Also, there is a big improvement that can be expected from miniaturized piezo holders as has been used in SPM.
But a very important limitation can be the radiation damage in the specimen caused by the incident electrons. Knock-on damage in which atoms are displaced can be avoided by using lower electron energy. And both TEM and STEM instruments are moving toward lower accelerating voltages below 50 keV, and with the use of advanced chromatic aberration correctors [17], it is possible to keep the resolution at the atomic level. But ionization damage that occurs mainly in organic and biological objects cannot be avoided and only reduced by cooling (cryoprotection). In that case, the resolution is not anymore limited by the instrument but by the object and can only be improved by averaging over many identical objects such as in single-particle cryo-EM.
An alternative to electron microscopy that gets a lot of attention is the use of femtosecond X-ray pulses generated by a free electron laser (XFEL) [30]. If the pulse length is shorter than the time needed to destroy the structure, one can obtain a diffraction pattern from a single undamaged particle. Methods are being developed with success to bring the particles successively in the beam at a very high rate, to align the diffraction patterns on the fly, and to average them [21]. Although the original hope is to use the technique on single particles, the best demonstration up till now has been on small microcrystals of a virus.
And finally, it is worth mentioning that for many years, specimen preparation methods for electron microscopy kept on operating on the macroscale, which was anachronistic in comparison with the resolution performance and requirements of the EM. But during the past decade, we have seen an immensely growing impact of FIB preparation methods [24].
An important new development is ultrafast microscopy in which stroboscopic images can be made at nanosecond time intervals, which opens new ways to study the dynamics of various processes [23].
But the most exciting new development, stimulated by the Nobel Prize winner Ahmad Zewail is the so-called 4D microscopy in which femtosecond electron pulses are generated. At this level of time resolution, it becomes possible to use the same femtosecond laser pulse to radiate the object and to emit the electron pulse so that the imaging is quantum mechanically linked to the local process [25].
Another challenge that is far from being reached is to perform atomic resolution electron microscopy in realistic conditions (in situ EM) and to combine it with other physical measurements (nanolaboratory). Recently, there has been a major improvement in the development in the atomic resolution environmental transmission electron microscope (ETEM) under controlled environments to investigate gas–solid reactions in situ in which the dynamic nanostructure of the solid in working environments of gas and temperatures can be monitored in real time, under environmental pressures up to 1 bar and at high temperatures [26].
Aberration correctors make it possible not only to increase the working place inside the EM but also to visualize the images at atomic resolution without processing so as to observe dynamical processes in real-time atomic resolution and exactly at the position of the structural discontinuities (surfaces, defects, …) where they occur. The disposal of such instrument will give a boost to the field; SPM holders have already been installed in EM's since the 1980s. But nowadays, they are used to investigate surfaces and as extremely fine probes for local mechanical and electrical measurements, which transforms the specimen chamber in the electron microscope into a nanolaboratory [[27, 28]]. Recently, in the “atomscope” project, one even inserts an atom probe in a TEM so as to combine structural and chemical investigations on the same specimen and at the nanoscale [29].
It is unlikely that LEEM will be able to reach the level of the atomic dimensions because the fundamental limits are determined by wavelength and numerical aperture. With decreasing wavelength, (increasing energy) the backscattering decreases rapidly, up to 2 orders of magnitude, which requires long acquisition times associated with practical difficulties (specimen and electronics stability). Increasing the numerical aperture would require correction of higher order aberrations, both of the objective lens and of the beam separator, an unlikely endeavor. The second limit is the pressure limit. In situ microscopy at relatively high pressure as in TEM is not possible because of the high field between objective lens and specimen. Nevertheless, surface processes at high pressures will be done in high-pressure cells attached to the specimen chamber, allowing before/after-process studies. Time-resolved LEEM and SPLEEM with pulsed beams is certainly a possibility, but probably not at time resolutions of practical interest. Magnetic imaging with SPLEEM is certainly still far from its limits. The new high-brightness, high-polarization spin-polarized electron gun should extend this field considerably.
There is a trend to combine LEEM with synchrotron radiation photoemission electron microscopy in order to complement the structural information with chemical, electronic, and magnetic information. LEEM does not have one of the driving forces of TEM biology because most of these specimens are not crystalline and suitable for UHV. LEEM cannot study powder samples, which are important, for example, in catalysis and in environmental studies. Nevertheless, further growth of LEEM and techniques associated with it (SPLEEM, PEEM) can be expected due to the improvements of resolution, brightness, and ease of operation, which is expected to broaden the field of their applications considerably. When comparing LEEM with TEM, it should not be forgotten that LEEM is not used to study externally prepared small specimens but is basically a technique in which the emphasis is not on maximum lateral resolution but analytical versatility. This will also determine its future.