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In-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...