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- Sprache: Englisch
In-Situ Transmission Electron Microscopy Experiments
Design and execute cutting-edge experiments with transmission electron microscopy using this essential guide
In-situ microscopy is a recently-discovered and rapidly-developing approach to transmission electron microscopy (TEM) that allows for the study of atomic and/or molecular changes and processes while they are in progress. Experimental specimens are subjected to stimuli that replicate near real-world conditions and
their effects are observed at a previously unprecedented scale. Though in-situ microscopy is becoming an increasingly important approach to TEM, there are no current texts combining an up-to-date overview of this cutting-edge set of techniques with the experience of in-situ TEM professionals.
In-Situ Transmission Electron Microscopy Experiments meets this need with a work that synthesizes the collective experience of myriad collaborators. It constitutes a comprehensive guide for planning and performing in-situ TEM measurements,
incorporating both fundamental principles and novel techniques. Its combination of technical detail and practical how-to advice makes it an indispensable introduction to this area of research.
In-Situ Transmission Electron Microscopy Experiments readers will also find:
- Coverage of the entire experimental process, from method selection to experiment design to measurement and data analysis
- Detailed treatment of multimodal and correlative microscopy, data processing and machine learning, and more
- Discussion of future challenges and opportunities facing this field of research
In-Situ Transmission Electron Microscopy Experiments is essential for graduate students, post-doctoral fellows, and early career researchers entering the field of in-situ TEM.
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Table of Contents
Cover
Title Page
Copyright
Preface
Acknowledgments
List of Abbreviations
About the Author
1 In‐Situ TEM
1.1 Introduction
1.2 General Scope of the Book
1.3 Why In‐Situ TEM
1.4 TEM: Overview
1.5 TEM/STEM‐Based Characterization Techniques
1.6 Other Techniques
1.7 Introduction to Different Stimuli Used for In‐Situ TEM
1.8 Potential Limitations and Cautions
1.9 Take‐Home Messages
References
2 Experiment Design Philosophy
2.1 General
2.2 Choice of Technique and the Microscope
2.3 TEM Holder Design and Selection
2.4 Specimen Design and Preparation
2.5 Guidelines for Experimental Setup
2.6 Practical Example of Designing In‐Situ TEM Experiment
2.7 Review
References
3 In‐Situ Heating
3.1 History
3.2 Currently Available Heating Holders
3.3 Experimental Considerations
3.4 Select Applications
3.5 Limitations and Possibilities
3.6 Chapter Summary
References
4 In‐Situ Cryo‐TEM
4.1 Historical Perspective
4.2 Specimen Holder Design and Function
4.3 Specimen Design and Preparation
4.4 Practical Aspects of Performing Cryogenic Cooling
4.5 Some Noteworthy Applications
4.6 Benefits and Limitations
4.7 Chapter Summary
References
5 Designing Liquid and Gas Cell Holders
5.1 Historical Perspective
5.2 Design Philosophy
5.3 Windows
5.4 Microfabricated Window Cell (Microchips)
5.5 Examples of Modified Window Holders
5.6 Take Home Message
References
6 In‐Situ Solid–Liquid Interactions
6.1 Historical Perspective
6.2 Holder Design and Selection
6.3 Specimen Design and Preparation
6.4 Data Acquisition
6.5 Practical Challenges
6.6 Select Examples of Applications
6.7 Limitations
6.8 Take‐Home Messages
References
7 In‐Situ Gas–Solid Interactions
7.1 Historical Perspective
7.2 Current Strategies
7.3 Gas Manifold Design and Construction
7.4 Practical Aspects of Performing Experiments in Gas Environment
7.5 Select Examples of Applications
7.6 Review of Benefits and Limitations
7.7 Take‐Home Messages
References
8 Multimodal and Correlative Microscopy
8.1 Multimodal TEM
8.2 Correlative Approaches
8.3 Take Home Messages
References
9 Data Processing and Machine Learning
9.1 History of Image Simulation and Processing
9.2 Current Status
9.3 Data Management
9.4 Data Processing and Machine Learning (ML)
9.5 Select Applications
9.6 Future Needs
9.7 Limitations
9.8 Take Home Messages
References
10 Future Vision
10.1 Historical Aspect
10.2 Current Status
10.3 Technical Challenges
10.4 Developing Relevant Strategies
10.5 Take Home Messages
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Choosing the TEM/STEM/Holder for specific applications.
Table 2.2 Relevant properties of grid and support materials.
a)
Chapter 5
Table 5.1 Some common considerations for the fabricating microchips with win...
Chapter 7
Table 7.1 Closed versus open cell configurations.
Chapter 8
Table 8.1 Modifications reported to focus light on the sample for in‐situ TE...
Chapter 10
Table 10.1 Possible combination of stimuli in controlled environment and tec...
List of Illustrations
Chapter 1
Figure 1.1 Number of publications reporting results obtained using in‐situ o...
Figure 1.2 Distinct signals generated due to elastic and inelastic interacti...
Figure 1.3 A schematic illustration of various components of (a) basic TEM a...
Figure 1.4 Simple ray diagrams showing the origin of spherical (a) and chrom...
Figure 1.5 Measured DQE as a function of spatial frequency for the DE‐20 (gr...
Figure 1.6 Selected area EDP with (a) continuous rings indicating amorphous ...
Figure 1.7 EDS from a CdTe solar cell lamella, collected in STEM mode, peaks...
Figure 1.8 (a) Zero‐loss, low‐loss, and the core‐loss regions of EEL spectra...
Figure 1.9 Schematic showing the process of choosing windows for collecting ...
Figure 1.10 Schematic of a “data cube” obtained by collecting EELS data from...
Figure 1.11 (a) The ultrafast electron microscope. Shown are the basic compo...
Figure 1.12 Schematic showing the tip of a nano‐indenter holder with magnifi...
Figure 1.13 Overview of the layout of the MIAMI‐2 system, which incorporates...
Figure 1.14 A schematic showing the entire system used for
I
–
V
measurements,...
Chapter 2
Figure 2.1 Block diagram describing the strategic elements of in‐situ TEM ex...
Figure 2.2 (a) Powder sample loaded, using dry or wet technique, on a holey ...
Figure 2.3 Mechanical preparation of epitaxially grown thin films on a subst...
Figure 2.4 (a) Schematic illustration of the H‐bar FIB technique. Material o...
Figure 2.5 (a) TEM image of amorphous carbon film supported on Cu grid after...
Figure 2.6 (a) TEM image of an Au nanoparticle covered by a contamination la...
Figure 2.7 Evolution of energy loss spectrum in the specimen chamber with ti...
Figure 2.8 (a) Au–Ga phase diagram showing existence of a solid phase in the...
Chapter 3
Figure 3.1 (a) A single wire heating element placed in the tip area of a sid...
Figure 3.2 Photographs of the tip section of a Gatan heating holder: (a) top...
Figure 3.3 Schematic illustration of the MEMS sample holder. The bottom sect...
Figure 3.4 (a) Backscattered electron image showing the pattern of holes in ...
Figure 3.5 (a–h) Images showing cooperative motion of large numbers of atoms...
Figure 3.6 Photographs showing the gradual disappearance of an extrinsic sta...
Figure 3.7 (a) Measured bulk plasmon peak shift of Al nanoparticles as a fun...
Figure 3.8 (a) Spiral‐shaped microheater uses a four‐point‐probe technique t...
Figure 3.9 Sequence of operations: (a–c) SE ion beam images showing the tran...
Figure 3.10 A sequence of weak‐beam TEM video images recorded during cooling...
Figure 3.11 TEM images recoded from a 270 nm thick Cu film during cooling, a...
Figure 3.12 Video‐captured TEM bright‐field images recorded during in‐situ h...
Figure 3.13 The growth of an initial‐stage precipitate. (a, b) Images extrac...
Figure 3.14 TEM images recorded at different temperatures during in‐situ hea...
Figure 3.15 In‐situ chemical evolution of Cu
5
FeS
4
nanoparticles during therm...
Figure 3.16 (a–h) With increasing temperature, the voids denoted by the shad...
Figure 3.17 (a–d) Time‐resolved BF TEM images recorded at 600 °C during the ...
Chapter 4
Figure 4.1 (a) Liquid N
2
holder with reservoir (orange) and (b) cryo transfe...
Figure 4.2 (a) Photograph of the HennyZ liquid‐nitrogen double‐tilt specimen...
Figure 4.3 Monolithic micro mixing–spraying device. (a) Device layout. (b) D...
Figure 4.4 Block diagram showing the various fields of application of cryo‐T...
Figure 4.5 (a) Cryo‐TEM image of vitrified solutions of P3HT 1 wt%, aged for...
Figure 4.6 (a) HRTEM cryo‐TEM image of ZIF‐8 particles taken along 〈111〉 dir...
Figure 4.7 Structure and elemental composition of dendrites and their interp...
Figure 4.8 Cryo‐TEM images of (a) freshly prepared suspension of ferrihydrit...
Figure 4.9 Atomic‐resolution HAADF‐STEM imaging of non‐commensurate‐to‐comme...
Figure 4.10 (a) TEM image of Hg
1.25
TaS
2
crystal at −170 °C prior to deinterc...
Figure 4.11 2D displacement vector maps of the (111)‐BTO film acquired from ...
Figure 4.12 BF‐TEM micrographs of a temperature cycle of BaTiO
3
(heating to ...
Figure 4.13 (a) The HAADF‐STEM image of the 1‐unit cell FeSe film on SrTiO
3
...
Figure 4.14 EELS spectrum of the O K edge at 300 K (27 °C), 85 K (−188 °C), ...
Figure 4.15 Mix of perfect and partial dislocations in the development of pl...
Chapter 5
Figure 5.1 Schematic image of a side‐entry TEM holder for a Titan microscope...
Figure 5.2 Theoretical maximum image resolution versus thickness of water. C...
Figure 5.3 (a) Cross‐sectional view depicting bulging of a square‐shaped sil...
Figure 5.4 (a) Membrane deflection as a function of pillar spacing for diffe...
Figure 5.5 (a) Top microchip with SiN
x
windows and (b) bottom microchip with...
Figure 5.6 (top) Fabrication steps for free‐standing SiN
x
windows: (a) SiN
x
...
Figure 5.7 Static cells: (a, b) Side and top views of static cell where the ...
Figure 5.8 Schematic drawings showing various methods for incorporating gas ...
Figure 5.9 (a) Schematic for introducing heating and/or biasing connections ...
Figure 5.10 (a–e) Schematic of various fabrication steps for monolithic chip...
Figure 5.11 (a) Schematic showing the location of a window cell and EDS dete...
Figure 5.12 Schematic of the two electrode configurations commercially avail...
Figure 5.13 (a) The schematic for a modified Nanofactory scanning tunneling ...
Chapter 6
Figure 6.1 Diagrammatic representation of the improved environmental cell sp...
Figure 6.2 Schematic of the process flow for graphene‐based LC fabrication. ...
Figure 6.3 Basic types of graphene liquid cells. For type A, a sample is imm...
Figure 6.4 (a) Nanofluidic cell design with a feedback loop to control the l...
Figure 6.5 (a) Schematic illustration of the unassembled LC device showing t...
Figure 6.6 The liquid cell. (a) Components of the cell. The viewing window i...
Figure 6.7 (a) Measured temperature using a TEM LC heater (orange) and in‐si...
Figure 6.8 Measurement of the variations in liquid thickness due to the bulg...
Figure 6.9 The facet development of a Pt nanocube viewed along the [011] axi...
Figure 6.10 Etching process of regular and corner defected cubes. Time seque...
Figure 6.11 Time series of in‐situ TEM images, extracted from a video, showi...
Figure 6.12 Growth of core–shell structure: Image series showing the Pd grow...
Figure 6.13 (a) Micelle–micelle fusion event captured by LCTEM. (a) Single f...
Figure 6.14 (a) Cyclic voltammograms of the first two scans from a glassy ca...
Figure 6.15 (a) Schematic drawing showing the experimental setup of the open...
Figure 6.16 Temporal evolution of a LiFePO
4
/FePO
4
cluster during one charge/...
Chapter 7
Figure 7.1 (a) Schematic of a differentially pumped physical cell placed bet...
Figure 7.2 A flow chart explaining the concept of enclosing sample in a gas ...
Figure 7.3 Illustration of the nanoreactor device. (a) Schematic cross secti...
Figure 7.4 (a) Design of a spiral heating element with gas injection system,...
Figure 7.5 (a) General principle of designing a differentially pumped (DP) T...
Figure 7.6 (a) Pumping speed required to maintain a particular cell pressure...
Figure 7.7 Design of gas manifold used at NIST‐ESTEM lab, with flow rates co...
Figure 7.8 Observed increase in resistance of the single SnO
2
nanowire devic...
Figure 7.9 (a) Intensity recorded on the CCD camera as a function of gas pre...
Figure 7.10 Au nanoparticle supported on CeO
2
(a) in vacuum, (b) in pure N
2
...
Figure 7.11 (a) A snapshot of a nanoparticle, extracted from a video during ...
Figure 7.12 Effect of increasing oxygen pressure on Si wire growth kinetics:...
Figure 7.13 In‐situ ETEM images and electron diffraction patterns (inset at ...
Figure 7.14 (a) High‐resolution image of Au{100}‐hex reconstructed surface u...
Chapter 8
Figure 8.1 Block diagram showing the length scale, spatial and energy resolu...
Figure 8.2 Schematic layout (a) of the PIES instrument. D1 and D2 indicate t...
Figure 8.3 Schematic showing the external laser input and signal collection ...
Figure 8.4 (a) Layout of in‐situ Raman assembly in the sample chamber of the...
Figure 8.5 (a) The schematic design of the specimen holder. (b, c) In‐situ t...
Figure 8.6 (a) Schematic diagram of an in‐situ TEM observation system for ph...
Figure 8.7 (a) Schematic cross‐sectional view of the lens‐based specimen hol...
Figure 8.8 (a) Schematic illustration of a specimen holder equipped with a p...
Figure 8.9 (a) Optical holder schematics: four laser diodes with 405, 488, 6...
Figure 8.10 (a) Morphological evolution of Cu
2
O cubes and re‐deposited NPs c...
Figure 8.11 (a) Diagram of the in‐situ STXM technique. Soft X‐ray light is f...
Figure 8.12 (a) Illustration of the in‐situ setup for 2D and 3D ptychography...
Figure 8.13 (a) Schematic of the microcell: the catalyst is confined between...
Figure 8.14 Top: Scanning transmission electron microscopy images and electr...
Figure 8.15 Summary chart showing the TEM image of synthesized particles and...
Chapter 9
Figure 9.1 Separation of the host and Hg sublattices by Fourier image proces...
Figure 9.2 Schematic description of the Crystal Ball Plus program to unequiv...
Figure 9.3 High‐performance computation microscopy workflow. Life cycle of n...
Figure 9.4 The electron microscopy framework. The framework for translating ...
Figure 9.5 Neural network data architecture and workflow for crystal space g...
Figure 9.6 Schematic of the workflow, from images to evaluating material pro...
Figure 9.7 Tracking complex defect transformations on the surface. (a–d) STE...
Figure 9.8 CNN predictions of the atomic column heights in the HRTEM experim...
Figure 9.9 Atomic manipulation workflow. Image is acquired and passed to tra...
Chapter 10
Figure 10.1 Working principle of the in‐situ quantitative structure‐property...
Figure 10.2 Block diagram of an apparatus for phase‐locked strobe
transmissi
...
Figure 10.3 Elements to be incorporated in a multimodal TEM/STEM/DTEM instru...
Figure 10.4 Proposed schematic of the integrated transmission electron micro...
Figure 10.5 Schematic of an adjustable pole piece gap, (a) small, (b) medium...
Figure 10.6 Modified specimen chamber with multiple ports to incorporate oth...
Figure 10.7 (a) Schematic showing the possible overlapping between the combi...
Figure 10.8 Overview of the reinforcement learning process for diffraction a...
Figure 10.9 Workflow for the training and discovery phase for automated expe...
Guide
Cover Page
Title Page
Copyright
Preface
Acknowledgments
List of Abbreviations
About the Author
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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In-Situ Transmission Electron Microscopy Experiments
Design and Practice
Renu Sharma
Author
Dr. Renu Sharma
Arizona State University
Tempe
AZ 85287
Cover Image: Courtesy of Renu Sharma
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing‐in‐Publication Data
A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2023 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐34798‐8
ePDF ISBN: 978‐3‐527‐83483‐9
ePub ISBN: 978‐3‐527‐83484‐6
oBook ISBN: 978‐3‐527‐83482‐2
Preface
The motivation to write this book originates from a book titled Dynamic Experiments in Electron Microscope by Butler and Hale published in 1981 that was very helpful at the initial stages of my journey in the field of in‐situ TEM. Although this book included most of the stimuli we are currently using, remarkable advances have been made since its publication. Moreover, while working with collaborators with different background and/or training students/postdoc, I also realized the complexity of the multifaceted challenges we face before (planning), during and after (data analysis) in‐situ experiments. We often learn from our mistakes, but they are not reported such that we can all learn from them collectively. Here I present a collective learning experience that I have gained through discussions and input from my peers who are acknowledged in a later section.
In‐situ TEM has become an integral part of TEM to understand the reaction mechanisms during synthesis and functioning of materials at nanoscale. Experiments are performed in a TEM/STEM instrument using diverse stimuli in vacuum, liquid, or gaseous environments to mimic real‐world conditions as much as possible. It is also used to make physical property measurements under specific stimulus and in controlled environment (liquid or gas). For example, measuring mechanical properties, such as strength and/or plasticity in H2 environment as a function of temperature; or measuring electrical properties in an electrochemical cell (liquid environment) as a function of temperature; or measuring magnetic properties or behavior as a function of temperature, etc. The title of the book was chosen to make a distinction that this book covers the former, i.e. performing in‐situ experiments, although the property measurements are briefly described in Chapter 1.
There are also two aspects of formal training that is required to perform successful in‐situ TEM experiments and obtain unambiguous results. First and foremost, is the training required for understanding the functioning and data collection techniques on a TEM platform. Second requirement for in‐situ or operando data collection is a deep understanding of physics and/or chemistry of material under observation. This need arises from the fact that during in‐situ data collection, we are using TEM column as an experimental platform as well as nanoscale characterizing technique.
The book does NOT cover fundamental training required for successful operation of TEM‐based instruments, but highlights the practical aspects of designing in‐situ TEM experiments. The intended audience includes graduate students, postdoctoral fellow, and/or scientists, who are excited about entering the field of in‐situ TEM and are trained in exploiting TEM‐based techniques to their fullest extent. A brief synopsis and reference books about the fundamental aspect of TEM and related techniques are included. Also, references to a large volume of review articles and edited books, which include a wide span of fundamentals and applications of in‐situ and/or operando TEM, are provided in the relevant chapters/sections. This book will make readers aware of the technique developments, possible pitfalls, and emphasizes on the “know how” for planning successful experiments. We include:
Foundational chapters:
Chapters 1
and
2
discuss the need for making in‐situ TEM observations, review the TEM fundamentals, briefly describe TEM‐based techniques (with appropriate references) and when to use them; introduction to various stimuli – their importance, limitation; best approaches to perform in‐situ experiments; provide answers to some practical questions such as which microscope to use, what is suitable holder, should I use imaging (high resolution/low magnification), diffraction (SAED, CBED, or NED), STEM, or ATEM, overview of specimen preparation, experimental planning with respect to choice of specimen grids, holders, temperature, etc., and take-home messages.
In‐situ technique
Chapters 3
to
7
:
Chapter 3
– heating and
Chapter 4
– cooling, describe specific modifications to a TEM holder needed for temperature control, how to set up an in‐situ experiment, applications, pros, and cons.
Chapter 5
is dedicated to the microchip fabrication and window holder design as it is an integral and/or essential part of in‐situ experiments in liquid (
Chapter 6
) or gas (
Chapter 7
) environment.
Chapters 6
and
7
include the introduction, history, design consideration, applications, and limitations for experiments in liquids and gasses.
Multimodal and correlative microscopy:
Chapter 8
covers the multimodal experiments performed on modified TEM using various techniques and stimuli and correlative experiments that combine TEM with other characterization platforms, such as X‐rays and/or photons under same or similar experimental conditions.
Data processing and machine learning:
Chapter 9
covers the need and methods to meet the big data challenge, includes the history of TEM data analysis such as image simulations and processing; introduction to machine learning and its application to TEM data processing; application of ML to automatic alignments and control of in‐situ TEM experiments, a step toward running autonomous experiments.
Future vision:
Chapter 10
reviews the outcome of few major workshops aimed at defining challenges for in‐situ TEM experiments; possible solutions, dream instruments, steps for increasing temporal resolution; integration of other techniques (combinatorial techniques); application of ML to run autonomous experiments.
This book does NOT include detailed insights for making property measurements but includes available technology and reference for them. It is hoped that the book will help in planning and performing successful in‐situ experiments, develop new analytical methods, motivate to think outside the box for new technological advancements.
DISCLAIMER:The book was written after retiring from NIST and the contents of this book are result of the research conducted in my personal time and the ideas are not supported by NIST in any form. – Renu Sharma
Renu SharmaNovember 4, 2022
Acknowledgments
It takes a village
– Hillary Rodham Clinton
First and foremost, I would like to acknowledge the contributions of Dr. Khalid Hattar (Sandia National Lab), starting from his encouragement, most of the interesting quotes (used throughout the book), ideas about the structure of the book and chapters, and multiple discussions during writing period of the book.
As one of my goals was to share the failures, lessons learnt, and remedies that are often not included in our publications, I needed to reach out to my peers to discuss their experiences. I am extremely thankful to the enthusiastic response I received from (alphabetic order) See Wee Chee, Peter Crozier, Katherine Jungjohann, Sergei Kalinin, Penghan Lu, Andrew Minor, Kristian Møhave, Joe Patterson, Søren B. Simonsen, Robert Sinclair, Eric Stach, Seiji Takeda, Michael Zachman, Jian‐Ming (JM) Zuo. Their contributions are included in forms of a quote or note in different chapters. I understand that there are only so many hours in the day and some of you were not able to reach back to me; however, I know that you wanted to, and I am thankful to have you as my friends and peers.
I would like to acknowledge those who have reviewed various chapters and their comments; David Smith (Chapter 1); Robert Sinclair (Chapter 2); Vijayan Sriram (Chapter 3); Katherine Jungjohann (Chapter 4); Eric Satch (Chapter 5); Joe Patterson (Chapter 6); Peter Crozier (Chapter 7); See Wee Chee (Chapter 8), Joshua Taillon (Chapter 9); Raymond Unocic (Chapter 10). Moreover, I am also thankful for the stimulating discussions with Rafal Dunin‐Borkowski, Joerg Jinschek, Marija Gajdardziska‐Josifovska, June Lau, James LeBeau, Molly (Martha) McCartney Goetz Veser, Judith Yang, and Wei‐Chang David Yang.
Last but not the least, I sincerely appreciate the ever‐present moral support from my family, friends, collaborators, and peers.
Renu SharmaNovember 4, 2022
List of Abbreviations
List of key terminology and abbreviations used in the book
1D
one dimensional
2D
two dimensional
3D
three dimensional
4D
four dimensional
ACN
artificial convolutional network
ACOM
automated crystal orientation mapping
AC‐TEM
aberration corrected transmission electron microscope or microscopy
ADF
annular dark field
AI
artificial intelligence
ATEM
analytical transmission electron microscope or microscopy
BF
bright field
BOE
buffered oxide etch
CAD
computer‐aided design
C
c
chromatic aberration constant
CBED
convergent beam electron diffraction
CCD
charge‐coupled detector
CL
cathodoluminescence
CNN
convolutional neural network
CNT
carbon nanotube
C
s
spherical aberration constant
CVD
chemical vapor deposition
DE
direct electron
DF
dark field
DFT
density functional theory
DM
digital micrograph
DoE
Department of Energy
DP
diffraction pattern
DSC
differential scanning calorimetry
EBID
electron‐beam‐induced deposition
E‐cell
environmental cell
EDP
electron diffraction pattern
EDS
energy‐dispersive X‐ray spectroscopy
EELS
electron energy loss spectroscopy
ELNES
energy‐loss near‐edge structure
ESTEM
environmental scanning transmission electron microscope or microscopy
ETEM
environmental transmission electron microscope or microscopy
EXAFS
extended X‐ray absorption fine structure
FEA
finite element analysis
FEG
field‐emission gun
FFT
fast Fourier transform
FIB
focused ion beam
GB
grain boundary
GC‐MS
gas chromatograph–mass spectrometer
GIF
Gatan imaging filter
GPU
graphic processing unit
GSS
graphene sandwich superstructure
GVS
graphene veil superstructure
HAADF
high angle annular dark field (aka Z‐contrast imaging)
HRTEM
high‐resolution transmission electron microscope or microscopy
HVEM
high‐voltage electron microscope
IBID
ion‐beam‐induced deposition
IBIL
ion‐beam‐induced luminescence
IPA
isopropanol alcohol
LPCVD
low‐pressure chemical vapor deposition
LSPR
localized surface plasmon resonance
Maglev‐TMP
magnetically levitated turbomolecular pump
MEMS
micro‐electromechanical system
ML
machine learning
MOCVD
metal organic chemical vapor deposition
MWNT
multiwalled nanotubes
MWCNT
multiwalled carbon nanotube
NBED
nano beam electron diffraction
NDA
non‐disclosure agreement
NED
nano electron diffraction
PED
precession electron diffraction
PL
photoluminescence
RGA
residual gas analyzer
RIE
reactive‐ion etching
RT
room temperature
SAED or SAEDP
selected area electron diffraction or selected area electron diffraction pattern
SEM
scanning electron microscope or scanning electron microscope/microscopy
SIMS
secondary ion mass spectrometer or spectrometry
SLG
single‐layer graphene
SNR
signal‐to‐noise resolution
STEM
scanning transmission electron microscope or microscopy
STM
scanning tunneling microscope or microscopy
STO
strontium titanate
STXM
scanning transmission X‐ray microscope/microscopy
SWNT
single‐walled nanotube
SWCNT
single‐walled carbon nanotube
TEM
transmission electron microscope or microscopy
TGA
thermogravimetric analysis
TMP
turbomolecular pump
UHV
ultra‐high vacuum
UV
ultra‐violet
VLS
vapor–liquid–solid
VSS
vapor–solid–solid
XAS
X‐ray absorption spectroscopy
XPS
X‐ray photoelectron spectroscopy
XRD
X‐ray diffraction
XTM
X‐ray transmission electron microscope
About the Author
Renu Sharma is a retired project leader from the Nanoscale Imaging Group in the Physical Measurement Laboratory and is currently working as an emeritus associate with the Materials Science and Engineering group, both at the National Institute of Standards and Technology (). She received a BS and BEd in Physics and Chemistry from Panjab University, India, and an MS and PhD degrees in Solid State Chemistry from the University of Stockholm, Sweden, where she had a Swedish Institute Fellowship. Renu joined the CNST/NIST in 2009, coming from Arizona State University (), where she began as a faculty research associate in the Department of Chemistry and Biochemistry and the Center for Solid State Science and most recently served as a senior research scientist in the LeRoy Eyring Center for Solid State Science and as an affiliated faculty member in the School of Materials and Department of Chemical Engineering. She has been a pioneer in the development of environmental scanning transmission electron microscopy (), combining atomic‐scale dynamic imaging with chemical analysis to probe gas‐solid reactions. She has applied this powerful technique to characterize the atomic‐scale mechanisms underlying the synthesis and reactivity of nanoparticles (including catalysts), nanotubes, nanowires, inorganic solids, ceramics, semiconductors, and superconductor materials. She is a fellow of the Microscopy Society of America, has received a Bronze Medal of Service from the Department of Commerce for developing new measurement techniques, a Deutscher Akademischer Austauschdienst () Faculty Research Fellowship, is a past President of the Arizona Imaging and Microanalysis Society, member of Editorial Board of Nanomaterials, and has given over 100 invited presentations, edited 1 book, published 3 book chapters, and over 200 research articles. At the NIST, she has established an advanced E(S)TEM measurement capability that combines Raman and cathodoluminescence spectroscopies with electron diffraction, electron spectroscopy, and high‐resolution imaging for nanoscience research (ETEM Lab). She has advised several graduate students and postdoctoral researchers and also has Emeritus position at Arizona State University since 2010.
1In‐Situ TEM
1.1 Introduction
Transmission electron microscope (TEM) and related techniques (scanning transmission electron microscope [STEM], tomography, holography, Lorentz microscopy, etc.) are preferred methods to understand the atomic‐scale structure and chemistry of materials, especially for nanomaterials. The size of grains, grain boundary structure, density of defects, dislocations, etc. control the materials properties making such information critical for their synthesis and applications. Beautiful atomic resolution images, electron diffraction patterns, and chemical maps provide unprecedented information about the structure and chemistry of the defects and grain boundaries in the material under investigation. Commonly employed procedure is to characterize the starting material (prenatal) and end products (postmortem) to deduce the pathway for chemical reactions or phase transformations occurring when the material is subjected certain stimuli, such as temperature, pressure, and/or mechanical stress. Main motivation behind this exercise is to be able to generate synthesis–structure–property relationships by identifying structure and chemistry of materials formed under different synthesis conditions and measuring their properties.
In‐situ TEM observations provide a direct visualization of structural and chemical changes under synthesis or operational conditions of nanomaterials when they are subjected to relevant external stimuli. Note that (i) TEM requires thin samples such that most of electrons are transmitted through after interacting with the sample, (ii) we need high vacuum to avoid electron scattering by the gas molecules, (iii) electrons can be treated as particles and/or waves, (iv) image formation optics is quite similar to light microscopes. The restrictions imposed by first two points require us to find methods to make thin electron transparent samples and modify the TEM column or sample holder, to accommodate required experimental conditions. We will address these requirements in this book as described below.
Seeing is believing but feeling is the truth.
– Thomas Fuller
1.2 General Scope of the Book
During past couple of decades, materials word is continuously shrinking in size where the size of semiconductor chips or batteries has dropped down to nanometers. As a result, fabrication methods have adapted from synthesizing building blocks for future assembly to combining synthesis and assembly process into one step, putting stringent control on the fabrication process.
Therefore, TEM‐based techniques have become a method of choice to understand and predict the desired synthesis or fabrication route for materials with desired properties. TEM community has recognized this need and responded accordingly. There has been an explosion in the breadth of combinatorial in‐situ TEM techniques that are now readily available due to advanced microscope controls, the development of microfabricated TEM sample holders, and automated data handling. Moreover, monochromated electron source and aberration‐corrected lenses have made it possible to use medium‐voltage microscope (200–400 keV TEM) with a pole‐piece gap of 5 mm to 7 mm, needed for inserting TEM holders equipped with heating, cooling, biasing, mechanical testing, liquid and gas containment, etc. for atomic‐scale structural and chemical characterization. These advancements and rapidly changing demand have attracted many more scientists to participate in the field, with or without formal training for employing TEM platform for performing experiments. As a result, a symposium or session (sometimes more than two) on the in‐situ TEM characterization is included in most of the major conferences in chemistry and materials field (American Chemical Society (ACS), American vacuum Society (AVS), Materials Research Society (MR S), American Institute of Chemical Engineers (AIChE)), beyond microscopy and microanalysis (M&M), and the number of publications has increased exponentially (Figure 1.1).
Figure 1.1 Number of publications reporting results obtained using in‐situ or operando techniques in the last 40 years. Note the exponential growth since 2010. Source: Data from Web of Science.
1.3 Why In‐Situ TEM
Going from pretty pictures to touching and feeling
– Murray Gibson
Let us first understand the meaning and difference between two commonly used terms: “in‐situ” and “operando.” First term “in‐situ TEM” that we will commonly use throughout this book is also valid for in‐situ STEM, in‐situ analytical transmission electron microscope (ATEM) (EDS or EELS), etc. “In‐situ” originates from Latin that means “in place” or “in position” and is used in many contexts. For us, it means TEM characterization of materials subjected to external stimuli at a specific “position or place” under synthesis or functioning conditions. Examples of external stimuli include, but are not limited to, temperature, gas or liquid environment, electrical biasing, magnetic or mechanical force, etc. to the material under observation using TEM‐based platform. The term “operando” also originates from Latin and means “working,” for example, we refer to the term in context of measuring the reactants, the product, and/or functionalities under working (functioning) conditions. In simple terms, while “in‐situ” observations provide information about the changes under specific environmental conditions, the “operando” implies measuring the consequence of these conditions. In catalyst community, operando is strictly used to measure the kinetics as function of reaction variables, such as composition of reactants, products, nature (including loading, size) of catalyst/support, temperature, and pressure. Although in‐situ TEM experiments have unveiled several catalytic mechanisms at atomic scale, most of the time, we do not operate under “working” reactor condition.
These are not strict definitions but are generally used by the materials, physical, and chemical scientists. In‐situ TEM observations are more frequently used to follow reactivity of the material; however, operando measurements are relevant for catalysis, battery operations, and nanomaterial synthesis. Also, we generally use the term “in‐situ” or “operando” when pursuing the changes with time, that may be termed as dynamic changes. In‐situ characterization and measurements have following advantages:
Same area or same nanoparticle is observed before, after, and during the reaction process such that all steps, including intermediate steps (if any), are identified.
A careful design of the experiments leads to the observation and understanding of the relationship between morphological, structural, and chemical changes concurrently.
Both the thermodynamic and the kinetic data of the reaction process may be obtained at nanometer or sub‐nanometer scale.
In‐situ observations result in considerable time saving as the synthesis, property measurement, and characterization can be performed simultaneously.
In‐situ
correlative light and electron microscopy
(
CLEM
) is used to combine nanoscale and microscale characterization of the same sample subjected to same experimental conditions.
However, in‐situ TEM experiments have their limitations and will be discussed later (Section 1.7).
1.4 TEM: Overview
This section is a brief reminder of some of the simple facts about the functioning of the TEM/STEM instruments. References to the books and articles are provided at the end of the chapter and should be consulted for detailed understanding of the electron optics, image formation, and chemical analysis.
1.4.1 Historical Perspective
After the advent of using coaxial magnetic coils to focus electron beam to a point, Ernst Ruska and Max Knoll, in 1931, built and demonstrated the first TEM, capable of magnifying objects to approximately 400 times, demonstrating the principles of electron microscopy [1]. The resolution limit of electron microscopes increased rapidly and, in 1939, and Siemens introduced first commercial instrument based on Ruska's design. Further development in the resolving power was slow, but a number of research groups worked on developing their own instruments leading to the formation of a number of commercial companies that took over the market from Siemens. Since then, there has been a steady development in improving the image as well as energy resolution. Moreover, several other applications that take advantage of the electron interaction with the sample such as diffraction and chemical analysis (energy‐dispersive X‐rays [EDS], electron energy‐loss spectroscopy [EELS], cathodoluminescence [CL], etc.) have also been developed. To continue to take advantage of the TEM platform for in‐situ observations, it is imperative for us to understand the principles of electron scattering and image formation in a TEM, basic design of TEM/STEM/ATEM instruments, and appreciate the developments over last 90 years that resulted in commercially available modern microscopes.
1.4.2 Electron–Sample Interactions
Electron–sample interactions can be divided into two main categories, elastic and inelastic (Figure 1.2). The collision of incident electrons with the nucleus is scattered by larger angles that results in the elastic scattering, whereas inelastic electrons scattering results from the collision of incident electron with the electrons cloud around the nucleus, including inner shell electrons, and is limited to small angles or small energy losses (below 50 eV). The elastic scattering contributes to the phase change in the transmitted electrons and is important for image formation as it contributes to the image contrast. In simple terms, elastic scattering allows us to acquire diffraction and images from the sample.
Apart from elastically and inelastically scattered electrons, the electron–sample interactions result in generating a host of signals, such as electron–hole pairs, back‐scattered electrons, secondary electrons, Auger electrons, characteristic X‐rays, and visible light (Figure 1.2). While secondary and back‐scattered electrons are generally not used during TEM characterization, they are important for scanning electron microscope (SEM). We also ignore Auger electrons, but the light generated as a result of electron–hole recombination, i.e. CL, is often used to understand the semiconducting properties in materials. EELS and characteristic X‐ray signal (EDS) are routinely used to obtain chemical composition of the sample.
Figure 1.2 Distinct signals generated due to elastic and inelastic interactions of electrons with the sample. Most of the TEMs are equipped to collect electron diffraction, image, EELS and EDS data.
1.4.3 Overview of Modern TEM
Let us now revisit the ways TEMS/STEM instruments enable us to collect some or all the signals generated as a result of the electron/sample interaction shown in Figure 1.2. Image formation in TEM is based on the same optical principles as for a light microscope. The difference is that in a TEM, the electrons are used as a source of light and magnetic lenses are used for image formation and magnification. TEM can stand for transmission electron “microscope” or “microscopy,” where one is an instrument (noun) and the other is a technique (adverb). For simplicity's sake, we will use the same acronym for both. We assume that the placement of the acronym in a sentence should be obvious enough to distinguish between “microscope” and “microscopy.” Also, we use TEM for the basic system and specialized instruments or techniques, such as STEM or ATEM, can be considered as extension of the basic system. Although we will see that this simple explanation is not entirely correct as their design and functioning could have noticeable differences, i.e. a dedicated STEM has different optics than a TEM and an ATEM may have larger objective pole‐piece gap. Here we will concentrate on TEM/STEM instruments as they are most effective for in‐situ experiments. Figure 1.3a shows a simple ray diagram and location of various components of a basic TEM, and Figure 1.3b shows the detailed features of a modern double‐corrected (Cs) TEM. A detailed description and functioning of each component are described in Sections 1.4.3.1–1.4.4.
1.4.3.1 Electron Source or Electron Gun
Electron gun can be considered as an “illumination source” for samples under observation and is an important component. High‐energy electrons can be extracted from a thermoionic source such as a tungsten filament, or LaB6 crystal, or field emission from a pointed tungsten or tungsten tip coated by ZrO2. Electrons are emitted from thermoionic sources by heating the source to a very high temperature. Electron density, brightness, coherence, and energy resolution of the illumination system are controlled by the electron source. Whereas thermoionic sources are easy to make and operate, they have low electron density, brightness, energy resolution and are incoherent. A field‐emission source/gun (FEG) provides high brightness, energy resolution, and coherence, where the electrons are extracted from the source by applying an electric field either at room temperature (cold FEG) or at high temperature (Schottky). Simple but detailed information about the sources and their properties can be found in Chapter 5 of Ref. [2]. Moreover, small probes, on the order of sub‐nanometer, can be formed using a FEG, which determines the spatial resolution for STEM imaging and atomic‐scale chemical mapping [3].
Figure 1.3 A schematic illustration of various components of (a) basic TEM and (b) double‐corrected modern TEM/STEM with EDS, EELS, and STEM detectors.
The type of electron gun is decided while constructing the microscope and cannot be changed afterward as the gun chamber, extraction and focusing systems are designed for specific source and are not easily interchangeable.
1.4.3.2 Lenses
Lenses are needed to focus the electrons on the sample and on the image screen after passing through the sample. In 1927, Busch successfully focused the electrons using electromagnets that motivated Ruska to design the first microscope in 1931 [1]. The same principle is used even today to construct lenses, where the magnetic field is generated by passing current through coils wound around a soft magnetic core. Since the magnetic field strength within the lens is function of current flowing through the electrostatic coils, both the focal length and the focal point can be controlled by varying the current. Following are the lenses used in a TEM:
Condenser lens:
This lens is used to focus the beam of electrons, emitted from the gun to a point on the sample such as to form a point source. Modern TEMs have a set of condenser lenses that can be used to control the probe size and location of crossover along the optical axis.
Objective lens:
Objective or image forming lens is located around the sample and focuses the transmitted electron beams on the image plane. The magnetic assembly generally consists of two parts, commonly known as pole pieces, placed above (top) and below (bottom) the sample, and is the most critical component that produces an axially symmetric magnetic field. The sample is located between these two pole pieces, collectively called objective lens, and defines the image resolution of the microscope. For optimum performance, the sample should be located at a specific position, called “eucentric height” between the two pole pieces. The gap between the pole pieces determines:
The achievable tilt angle of the holder tip, which is important, (i) to align the sample along one of the crystalline zone axes parallel to the incident beam, (ii) to collect highest X‐ray signal for chemical analyses, and (iii) to obtain 3D images using tomography.
The space to insert
objective aperture
() in the back focal plane of the sample or a CL detector.
The location of the sample within the gap for optimum resolution.
Note that objective lens is probe forming lens in STEM mode for a TEM/STEM instrument, thus also determines the image resolution in STEM mode.
Projector or magnifying lens:
Keep in mind that image resolution is determined by the objective lens and cannot be improved by employing other lenses; however, they can be used for image magnification that helps us see subtle changes in the image contrast. A set of electromagnetic lenses, known as projector lenses, provide this magnification. We can collect diffraction patterns or images by switching between image plane and back focal plane.
1.4.3.3 Lens Aberrations
Electromagnetic lenses, used to focus electrons, suffer from similar aberrations as the optical lenses, albeit they arise from different reasons. Spherical aberrations arise due to difference in the angle subtended by the electrons arriving at the lens from the electron source (for condenser lens) or from the sample (for objective lens). Electrons leaving a point object at large angle are scattered too strongly by the lens and brought to focus before the Gaussian plane giving rise to disc of confusion (Figure 1.4a). The diameter of the Gaussian image formed by point thus is given by
Figure 1.4 Simple ray diagrams showing the origin of spherical (a) and chromatic (b) aberrations.
where Cs is spherical aberration constant. The third‐order dependence on θ implies that the beams scattered at high angles are most affected by aberrations. On modern instruments, the Cs ranges about 0.4 and 2.5 mm and can be corrected by using appropriate optics as will be explained under Section 1.4.3.4.
On the other hand, electrons with varying energy, ΔV, will also be focused on different points due to the small variation in the wavelength, which is the source of chromatic aberration (Figure 1.4b). It is important to note that modern high‐voltage tanks, used to generate electrons, are very precise but not perfect. Following are the three important sources of fluctuation in the wavelength/voltage of the electrons:
The energy spread of the electron leaving the filament.
High voltage instabilities (typical Δ
V
o
/
V
o
is 2 × 10
−6
/min;
V
o
is the operating voltage of the microscope).
Varying energy losses in the specimen.
Currently, both aberrations can be corrected using a set of lenses that generate negative Cs or Cc such as the positive values of imaging lenses are canceled out. A monochromator is also employed to mitigate the energy spread of the electrons leaving the filament. As expected, achievable image and energy resolution of such microscopes is at sub‐nanometer and below 1 eV, respectively.
Astigmatism Just as for optical lenses, a deviation from perfect circularity is commonly present for electromagnetic lenses and could be due to imperfections in the iron core, machining error, asymmetrical windings, dirty apertures, etc. As a result, a stretching in the image is observed as a point object is focused. This problem can be easily recognized by changing the focus as the stretching direction will change going from under focus to overfocus. Most microscopes are equipped with individual stigmator coils for all lenses to compensate for this distortion. Modern microscopes with aberration correctors also provide a precise correction of astigmatism for condenser and/or objective lenses.
The lenses in a TEM have similar aberrations as optical lenses, but most of them can be corrected.
1.4.3.4 Aberration Correctors
Both the image and spectrum resolution are affected by the lens aberrations in a TEM [4–6]. Apart from Cs and Cc, described above, the energy of all electrons hitting the sample may not be the same, i.e. electron beam is not monochromatic. The high‐voltage tank of most microscopes is designed to keep the energy fluctuation to a minimum value, and some sources are equipped with a monochromator. There are a number of ways, such as using electrostatic energy filter [7], or using a combination of electrostatic and magnetic quadruples [8], to filter out electrons with higher energy spread but often results in loss of intensity of the source. The energy resolution of a monochromated source may vary from 8 to 80 meV. We also find that the image resolution is also improved by using a monochromated source.
A TEM with Cs and Cc corrector for objective lens is also readily available [9, 10]. Both the cost and height of the TEM column increase with incorporation of correctors (Figure 1.3b).
1.4.4 Data Acquisition Systems
Electron–sample interactions result in generating various imaging and spectroscopy data that need to be collected for further analysis and documentation. Modern microscopes use digital cameras to record images, videos, diffraction patterns, EDS and EELS data. A microscope can be equipped with different digital detectors for recording TEM, STEM, annular dark‐field (ADF), or bright‐field (BF) images, or spectroscopy data such as EELS, EDS, or CL. We will look at them in detail as we work out the experimental plans.
1.4.4.1 Types of Detectors
Recording medium for image, DP, and spectroscopy data has different requirements. Whereas spectroscopy data have always been acquired using digital detectors, photographic film or image plates were used for recording images and DP due to their high dynamic range (pixel resolution) in the past. We should keep in mind that although recording medium to acquire DP and images may be the same, the acquisition process and time is different due to different intensity range present in the signals. The films have recently been replaced by electronic media such as phosphor/photomultiplier (PMT) or fiber‐optic charge‐coupled device (CCD). Here the phosphor is used as a scintillator to convert electrons into photons that are then detected by a PMT or a CCD. PMT could be preferred for in‐situ measurements due to their faster acquisition time, but their performance, as measured by their detective quantum efficiency (DQE), reduces at higher operating voltages (>120 keV) making CCD a better option. With continued rapid improvement in technology, a high DQE ≈ 0.7 and faster acquisition rate can be achieved for medium‐voltage microscopes (200–300 keV) [11] by using direct electron detectors (DEDs) equipped with monolithic active pixel sensors. Furthermore, digital data acquisition enables data processing, including drift correction and adding a few frames to improve signal‐to‐noise ratio to obtain atomic resolution images at high time resolution [12]. The digital images are also compatible with machine learning approaches that improve our ability for unambiguous data analysis (Chapter 9). Figure 1.5 shows a comparison between three commercially available DED cameras and photographic film [13].
Figure 1.5 Measured DQE as a function of spatial frequency for the DE‐20 (green), Falcon II (red), and K2 Summit (blue). The corresponding DQE of photographic film is shown in black. Source: McMullan et al. [13]/Elsevier/CC BY‐3.0.
Apart from the DQE, time resolution, often termed as temporal resolution, is an important factor for in‐situ measurements. Ideally, we need both high spatial and temporal resolution to decipher metastable steps of a chemical or physical process under investigation. Currently, ms time resolution is possible using DED cameras or another technology based on “pump and probe” idea (see Section 1.6.3). CMOS camera with scintillator [14] and pixel array detector [15] are a few other types of detectors available, and technology is improving at a fast pace, and cameras with both high spatial and temporal resolution are available for in‐situ measurements.
1.5 TEM/STEM‐Based Characterization Techniques
As we mentioned earlier, we assume that the readers of this book have thorough knowledge and adequate experience to operate TEM/STEM instruments. This section is a rapid review of TEM‐based techniques that are available for in‐situ measurements.
1.5.1 Diffraction
Electron diffraction is formed in the back focal plane of the objective lens and provides following structural information:
Crystallinity:
Diffuse rings in a DP indicate that the sample is amorphous (
Figure 1.6a
), spots on a ring pattern indicate polycrystalline nature (
Figure 1.6b
), and a 2D spot pattern is indicative of a single crystal (
Figure 1.6d
).
Structure of crystalline sample:
A single crystal sample can be aligned along one of its crystallographic zones to obtain structural information using
selected area electron diffraction
(
SAD or SAED
),
convergent beam electron diffraction
(
CBED
) (
Figure 1.6e
),
nanobeam electron diffraction
(
NBED or NED
), or
nano area electron diffraction
(
NAED
) (see below for details).
Defects:
A streaking or faint lines along the spot patterns (
Figure 1.6c
) indicate the presence of defects in the structure, which can then be further investigated using imaging.
Long‐range order:
Presence of faint spots, or satellite spot, along with stronger spots, indicates the presence of long‐range order along one of the crystallographic directions or perpendicular to viewing direction, respectively (
Figure 1.6f
).
Strain:
presence of faint lines along 2D diffraction spots is indicative of the presence of strain in a single crystalline sample. A number of methods to analyze and measure strain from electron diffraction patterns, especially from CBED or NBED, have been developed [
17
–
19
].
We often use a parallel electron beam and place an aperture, i.e. selected area aperture, to form diffraction pattern from a desired area (SAED or SAD). Such DP provides us information about the crystallinity and orientation of the sample. Some of the structural information, such as d‐spacing and angle between planes in projection, is also obtained from these patterns. We can select the diffracted beams, using an objective aperture (OA) we want to use to form an image. For example, outer most diffraction spots arise from multiple scattering events, so we can improve image contrast by discarding them. We can also select one or two diffraction spots and discard central beam to form dark‐field TEM image.
A CBED, which is in form of discs instead spots (Figure 1.6e), provides detailed structural information, such as crystal class and symmetry. Methods have been developed to use a CBED pattern to determine crystal structure by removing contribution of inelastically scattered electrons [20]. Nowadays, we can converge electron beam to nanometer size to obtain diffraction pattern from nanometer‐size area, NBED or NED [21]. Interestingly, modern microscopes also allow us to obtain electron diffraction by using parallel electron beam of nanometer size to select an area of interest and are known as NAEDs [21]. Nano‐diffraction techniques allow as to meet the challenges of obtaining structural information from the building block of the nanoworld (Figure 1.6f). For example, chirality of individual single‐walled nanotubes can be determined using NAED [22].
Figure 1.6 Selected area EDP with (a) continuous rings indicating amorphous nature of the sample, (b) rings with diffraction spots from polycrystalline sample, (c) streaks in ordered 2D pattern indicate presence of defects, (d) from single crystal Si oriented in [111] zone axis, (e) EBED from single crystal, (f) NED from a bicrystal α‐Fe2O3, which can be indexed as two overlaying α‐Fe2O3 platelets oriented along the 〈0001〉 zone axis, with a coincidence‐site‐lattice boundary with a twist angle of 21.79. Source: (d, e) Zhu et al. [16]/from American Chemical Society.
1.5.2 TEM Imaging Modes
The dual nature of electrons, as particles and waves, is used in understanding the image formation process. While the electrons scattered by the atoms or crystalline lattice propagate as transmitted electrons, the wave function makes the image formation similar to an optical microscope and fundamental mechanism also has some similarities. For example, the resolution can be defined by the Rayleigh criteria; separation between two‐point objects should be more than 0.61λ, where λ is the wavelength of incident light. It implies that the image resolution can be improved by decreasing the wavelength, i.e. by increasing energy of the electrons. However, unlike light, electron scattering is both elastic and inelastic, and the Rayleigh's criteria do not hold as the resolution is impeded due to multiple scattering experienced by electrons while traveling through the sample and the magnetic lenses.
The wavelength of the electron in such a microscope is only 1/20 of an angstrom. So it should be possible to see the individual atoms. What good would it be to see individual atoms distinctly?
– Richard Feynman
In the high‐resolution TEM, a parallel incident electron beam interacts strongly with the sample, forming multiple diffracted beams (as explained above) that are brought together by the objective lens such as they can interfere to create an image, and the structural information can be obtained from the exit electron wave function that has been attenuated by the multiple scattering events. We should keep in mind that the exit‐wave carry the information about both the phase shifts and amplitude modification due to electron scattering by the sample. The phase information cannot be directly visualized as it is influenced by the lens aberrations and other imperfections; therefore, it is the amplitude of the electron wave that is recorded on the image plane.
However, we need the phase information of the exit waves to obtain structural information, which can be achieved by tuning the focus of the objective lens such that the phase of the wave is converted into amplitudes on the image plane. The combined effect of aberrations, objective aperture function, drift, and other instabilities on the image contrast can be mathematically treated as phase‐contrast transfer function (CTF). Scherzer has shown that there is a specific value of defocus, depending on the properties of the microscope, where low spatial frequencies are transformed into image intensities with similar phase. This value, known as Scherzer defocus, also determines the resolution limit of the microscope. However, we should keep in mind that the information beyond Scherzer resolution can be obtained by using a highly coherent and monochromated electron beam generated by a FEG along with aberration correctors. Moreover, the phase difference can also be measured from interference patterns such as formed by using holography, where separate phase and amplitudes image are collected (Section 1.6.2).
As mentioned above, the location of the sample within the objective pole piece is critical for obtaining the best electron optical performance of the TEM. With appropriate choice of the microscope, imaging conditions, and modifications, TEM can be used to obtain morphological, structural, and chemical information. Some of them are described below as they are the ones that we utilize to follow dynamic changes during in‐situ observations.
Low‐magnification TEM:
is used to obtain morphological information such as size and shape of nanoparticles, particle size distribution, dislocations, measuring Burger's vectors.
High‐resolution imaging:
atomic‐scale information, including, but not limited to, from defects, grain boundaries, nanoparticles, etc.
Dark‐field imaging:
objective aperture can be used to select the diffraction spots to form image that shows the corresponding region as bright, similar to STEM‐ADF images.
Tomography:
3‐D images reconstructed from a series of images recorded at incremental positive and negative tilts (±70°). Full rotation of sample is possible by using needle‐shaped sample geometry and a special TEM holder.
