Research

, Volume: 16( 4) DOI: 10.21767/0972-768X.1000292

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Fundamentals and Applications of Scanning and Transmission Electron Microscopes

*Correspondence:

Zea H , Department of Chemical and Environmental Engineering, Faculty of Engineering, National University of Colombia, Bogota, Colombia, Tel: 571-3165000; E-mail: hrzear@unal.edu.co

Abstract

Electron microscopes are equipment that use an accelerated electron beams as probes to generated images with magnifications and resolution not possible to obtain with optical microscopes (due to fact that electron wavelength can be 100,000 times shorter than visible light photons). Electron microscopes operating in the conventional high vacuum mode require conductive imaging specimens; therefore, non-conductive materials need the deposition of a conductive layer (Au-Pd alloys, carbon and osmium, among others). Low voltage mode of modern microscopes makes possible to observe non-conductive uncoated specimens. Transmission electron microscopes require thin samples (below 100 nm), placed onto appropriate sample holders. Electron microscopes are state of the art equipment that requires high operation and maintenance standards, therefore having a clear understanding of the operation fundamentals, equipment capabilities, suitable sample preparations and appropriate results interpretation is of critical importance to use the technique in the most suitable fashion.

Keywords

Electron microscopy; Morphological characterization; Chemical characterization; Crystallographic characterization

Introduction

When a beam of charged particles hits the constituent atoms of a sample, they interact with the sample atoms and yield several effects that are generally a function of the nature of charged particle, its energy and the type of atoms of which the sample is constituted. Most electron microscopy characterization equipment uses an electron beam to probe the sample, although there are equipment’s that can use positively charged particles.

Secondary electrons occur when an electron from the beam passes very close to the sample atom nucleus, providing enough energy to one or more of the inner electrons to overcome the energy that holds them attached to the nucleus and to migrate out of the sample. These electrons are very low in energy (generally less than 5eV), so they must be very close to the surface to escape; they provide valuable topographic information of the sample and are used mainly in scanning electron microscopy. Backscattered electrons are produced when an beam electron directly interacts with the nucleus of an atom of the sample, being repelled in the opposite direction outside the sample, the signal intensity of the backscattered electrons is strongly related to the atomic number (Z) of the atoms that constitute the sample, the signal generated in this type of interactions allows to generate spatial distribution images of composition of the sample, especially in scanning electron microscopy. When a secondary electron is ejected from the atom, another electron at an outside energy level may jump inward to fill the energy vacuum generated by the ejected secondary electron.

The excess energy caused by this displacement is balanced by the emission of X-rays, which have energy values that are characteristic of each chemical element of the sample, so they are used to obtain information about the chemical composition in techniques such as spectroscopy (EDS, Energy Dispersive X-ray Spectroscopy). Transmitted or non-dispersed electrons are those that pass through the sample without interacting with the sample atoms; the number of electrons transmitted is generally inversely proportional to the thickness and density of the sample; such differentiation in intensity by thickness and density produce lighter or brighter areas in the image formed from the transmitted electrons [1-3]. Elastically dispersed electrons are those that are deviated from their original path by the atoms of the sample without loss of energy and which are then transmitted (not dispersed) through the sample. In crystalline materials, these electrons are deflected at a given angle that is dependent on the wavelength of the incident beam and the distance between the atomic planes of the sample, providing electron diffraction images that give important information of the spatial distribution of the atoms in the observed sample (Bragg's Law). The interference of these electrons with the transmitted ones dramatically increases the contrast and is essential to obtain high resolution images (HRTEM). Inelastic dispersed electrons are those that are diverted from their original trajectory by the atoms of the sample with a loss of energy and are then transmitted or dispersed through the sample. Electrons that are elastically dispersed a second time form the so-called Kikuchi lines, of great importance in the study of crystalline structures [1-4].

Methodology

Electron microscope techniques

Scanning electron microscope: The Scanning Electron Microscope (SEM) is a characterization instrument that forms images of the surface of the sample observed by the interaction of a beam of high energy electrons in a sweeping sequence on the surface of the material. The electron beam penetration in the sample depends on many variables, the most important are the electron beam acceleration and the nature of the material to be characterized [5-9].

Beam electrons interact in different ways with the sample atoms, each of these interactions generates characteristic signals and contain information of the topography of the surface, composition, and other properties of the sample [10-13]. Another of the interaction signals of the electron beam with the sample corresponds to the secondary electrons, which are collected in a type of detector called Backscattered Secondary Electrons (BSE), which converts them into an amplified voltage signal, this amplified signal is transmitted to an image generating device Which in many cases is a charge-couple camera (CCD) on which a variable light intensity point is generated, depending on the intensity of the amplified signal that produces it. The image of the sample surface is formed from the combination of multiple points of varying intensity on the screen of the CCD [14].

SEM can produce detailed images of the surface of the sample with a resolution generally ranging from 10 to 50 nm. Due to the way the beam of electrons is generated and directed on the sample, SEM images are characterized by having a reduced depth of field producing images with three-dimensional information useful for understanding the structure of the surface of the sample [14-16]. FIG. 1 presents scanning electron microscopy photographs of ZnO hexagonal crystals synthesized from aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine [17].