In-Situ Electron Microscopy at High Resolution

The following sections are included:

• CONTENTS

This chapter gives an introduction to in-situ electron microscopy. The historical background, the achievements, and modern techniques of in-situ electron microscopy are briefly reviewed, and the limitations of the technique as well as the prospects for future developments are discussed.

Transmission electron microscopy (TEM) is one of the preferred techniques to obtain nano scale information of morphology, structure and chemistry of nanomaterials. Such in depth characterization of reactants and products is generally enough to deduce possible reaction mechanism. However, direct observation of the dynamic processes is needed to understand the fundamental properties of the reaction. Recent developments in the instrument and holder designs have made it possible to obtain atomic level structural and spatial resolution during gas-solid, liquid-solid and liquid-liquid interactions with a time resolution approaching 1/60th of a second using environmental transmission or scanning transmission electron microscopes (ETEM and ESTEM). Such combination of time, temperature, pressure, structural and chemical resolution has made ESTEM a versatile instrument that may be considered as a nano-scale laboratory for in situ synthesis and characterization. This chapter starts with a brief description of various approaches for making in situ TEM observations in gaseous and/or liquid environments including the design of ETEM or ESTEM. Special considerations for designing ETEM or ESTEM experiments and data collection are also provided. Its applications to obtain nano-scale information of the morphological, structural and chemical changes occurring during synthesis and/or functioning of nanomaterials are described. Such information is crucial in assisting us to understand and improve the synthesis and functioning of nanoscale processes.

The achievements of in-situ HREM observation carried out over the past decade by the author's group are reviewed. The subjects include solid-solid reaction (formation of SiC via solid-state reaction between Si and C, formation of void in SiC during sintering, vibration of a grain boundary and an interface), melting of metals with small dimensions, solid-liquid interfaces, wetting of non-metallic substrates with liquid metals and solid-gas reaction (oxidation of Si and catalyst). Analytical methods and holographic methods were also applied.

In the field of transmission electron microscopy there are still fundamental and practical reasons which hamper a straightforward correlation between microscopic structural information with the properties of materials. In this chapter it is argued that one should focus more on the generic features of defects, using a mesoscopic approach including various length scale transitions, and in particular on in-situ rather than on postmortem observations of solely atomic structures. This viewpoint has been exemplified with in-situ TEM indentation and in-situ straining studies at elevated temperatures of Al and Al-alloys with grain sizes ranging between submicrometers to submillimeters. It is concluded that in-situ Transmission Electron Microscopy has provided new insights into the interaction between dislocations and grain boundaries on various length scales, in which specifically the effect of Mg in Al-Mg alloys on these interaction mechanisms has been clarified.

Understanding the details of atomic motion at interphase boundaries is critical to understanding the mechanisms and behavior of phase transformations in materials. This article presents results obtained from in-situ hot-stage high-resolution transmission electron microscope (HRTEM) experiments that were used to determine the atomic structure, dynamics and mechanisms of transformation at order-disorder, precipitation and massive transformation interfaces in metal alloys at the atomic level. The similarities and differences among these various types of transformations are emphasized, as are the strengths and limitations of in-situ HRTEM for obtaining such information.

This chapter describes various kinds of in-situ TEM experiments performed on filled conventional carbon and non-carbon inorganic nanotubes. The results are summarized for nanotubes filled with liquid and solid metals, ceramics and inorganic compounds. The prospects of functional applications of analyzed nanotubes are particularly highlighted.

Ion implantation causes the formation of nano-phases as well as radiation damage. "In-situ" observation in a TEM is a unique technique to clarify such phenomena. One example is Xe nanocrystals embedded in a metal matrix. HRTEM observations revealed the atomic structures and the motion of atoms in a Xe nanocrystal. Electron beam-induced deposition is another technique to fabricate nano-structures. Nanostructures having desired shape and size can be obtained. Metal atoms are deposited using focused electron beam irradiation under the presence of a small amount of precursor gas molecules on the substrate. The details of these ion and electron beam effects are reviewed.

Electron irradiation of specimens is generally inevitable in electron microscopy. The energetic beam in the electron microscope can displace atoms in the specimens, and this may lead to visible structure transformations. Although radiation damage is often an unwelcome artefact, some classes of nanomaterials show the formation of novel phases or morphologies when subjected to electron irradiation. The advantage of in-situ electron microscopy is that structural transformations can be induced and imaged with the same electron beam and studied in real time with atomic resolution. This chapter summarizes the physical principles of irradiation phenomena with emphasis on effects in nanoparticles. In an experimental section the techniques of in-situ electron microscopy and electron irradiation at high specimen temperatures are treated. Examples of electron irradiation phenomena in graphitic nanoparticles such as carbon nanotubes or fullerene-like structures are given. Graphitic materials have a unique ability to reconstruct after atom displacements and thus show a great variety of structural transformations under the electron beam.

The defect formation and relaxation behavior in carbon materials under high-energy beam irradiation is of great scientific and technological importance. This chapter describes recent progress of in-situ HR-TEM imaging of carbon nanostructures for the direct observation of individual atomic defects induced by electron beam irradiation. Various types of atomic defects, such as mono-vacancies and interstitials, have been successfully identified. The temperature dependence of the formation rate of inter-layer defects has suggested a finite recombination energy barrier for the annihilation of the interstitial-vacancy pair. Moreover, the migration behavior of metal atoms through atomic defects in carbon nano-peapods has been demonstrated by a collaboration of a HR-TEM imaging and a DFT theory.

The following sections are included:

• INDEX