Methods of electron microscopy

Electron microscopy is a collection of investigation methods. Microstructure of bodies (up to atom-molecule level), their local composition and electric and magnetic fields (microfields) located on the surfaces or in microvolume of bodies are investigated with a help of electron microscopes

Electron microscope is a device which makes possible to obtain high–magnification objects using electrons for their lighting. Electron microscope allows to see such small parts which are not allowed in light (optical) microscope. This microscope is widely used for substance structure researches

On the principle of operation and method of object investigation there are some types of electron microscopes: transparent, reflecting, emissive, scanning and shadow. The most common types of microscopes are transparent and scanning microscopes in possession of high resolution and universality

The first electron microscope Ernst Ruska, 1931 yr. Magnification 400х

Electron microscopes Karl Zeiss: а)– transparent; b)– scanning.

According to resolution there are three classes of microscopes: Class of microscopeSpatial resolution first0,2–1,5 nm (2–15 А) second2–3 nm (20–30 А) third5–15 nm (50–150 А)

The main types of electron microscopy:  Transparent electron microscopy (TEM)  Scanning electron microscopy (SEM)  Electron-probe microanalysis (EPMA)

Differences of optical, transparent and scanning electron microscopy picture sample objective lamp ОМ condenser projector screen picture sample objective Source of electrons condenser coils SE detector CRT TEМ SEМ screen scan projector

 Transparent electron microscopy (TEM) allows to solve an extensive range of mineralogical tasks. As the development progresses the method this range becomes wider. In TEM according to solved tasks the different methods (such as suspension, replica, ion etching, ultramicrotoming, decoration, plane grid direct-view and etc.) are used Transparent electron microscopy (TEM)

Transparent electron microscope (TEM) is similar with a light microscope. They are different in that electron beam is used for lighting of samples in the transparent electron microscope instead of light used in the light microscope. The common transparent electron microscope consists of an electron gun, a number of condenser lenses, an objective lens and a projection system. The projection system corresponds with ocular but it projects a picture on a screen. Heated tungsten or lanthanum cathode is a source of electrons

Characteristics  Resolution: nm (crystallographic lattice of gold)  Magnification: х50 - х  Accelerator: kV Possibilities  Wide visual field  High contrast  Special construction of lens for biological samples Hitachi H-7650 TEM Modern transparent electron microscope (TEM)

Cold field Incandescence (W) Lanthanum hexaboride (LaB 6 ) Schottky (thermofield) Temperature RT Spread E<2 эВ<1.5 эВ<0.8 эВ<0.2 эВ Brightness 5x10 5 A/cm 2 sr5x10 6 A/cm 2 sr5x10 8 A/cm 2 sr2x10 9 A/cm 2 s r Beam current100 нА 200 нА10 нА Service life<100 h1 year1-2 years>3 years Vacuum Field cathodes Types of cathodes

 Scanning electron microscope (SEM) is widely used in research laboratories.  According to its technological capabilities it combines both qualities of light microscope (LM) and transparent electron microscope (TEM), but it is more multifunctional Scanning electron microscopy (SEM)

Fields of geology Possibilities and characteristics of scanning electron microscope (SEM) Descriptive petrology Petrological description and classification of rocks Identification of mineralsDetermination of mineral composition Experimental petrology Investigation of phase relations and element balance of coexisting phases Geothermobarometry For assessment of temperature and pressure under which the rock was formed ZonalityStudying of zonality in mineral grains Modal analysis Volumetric relation of mineral fractions in rocks Localization of rare-earth phases Grains of rare-earth phases can be identified with a help of automatic searching procedure and X-ray signal.

 SEM is intended for scanning of sample surface with electron probe and it is also intended for detection (recognition) of broad-spectral beaming.  Secondary, returned and absorbed electrons are signals for imaging in SEM.  Mode of functioning of SEM is based on using of certain effects, which appear when the surface of objects are irradiating by thinly focused electron beam – probe. Different signals are generated in the result of electrons-sample (substance) interaction Томск, ТПУ, ИГНД, ГЭГХ 16

Out-lens S-3400 Resolution 3 n m Semi-in-lens S-4800 Resolution 1 n m In-lens S-5500 Resolution 0.4 n m SЕDSЕD Electron beam Virtual lens Sample Objective lens

Characteristics of SEM Hitachi S-3400N Resolution Resolution when the detector uses secondary electrons: 3,0 nm (accelerating voltage 30 kV, when working with high amount of vacuum) 10 nm ( accelerating voltage 3 kV, when working with high amount of vacuum) Resolution when the detector uses back-scattering electrons : 4,0 nm ( accelerating voltage 30 kV, when working with low amount of vacuum) Degree of magnification От 5 до ( Degree of magnification of optical microscope to ) Vacuum system Management of amount of vacuum formation: fully automatic Accessible amount of vacuum : 1,5 х pascal Range of tuned amount of vacuum : from 6 to 270 pascal (22 discrete levels)

Type of detectorCharacteristicsField of application SE (secondary electron detector) Supplying information about high resolution surface. High contrast of boundaries. Detector is sensitive to sample charge. Detector is unworkable with low amount of vacuum. Detector is advantageous for morphological investigations of the sample surface. BSE (back-scattering electron detector) Supplying composite information with high sensitivity. Detector is less sensitive to sample charge. Lower contrast of boundaries. Detector allows to work with low amount of vacuum. Detector is advantageous for investigations of conducting surface, which don’t require contrast demonstration of boundaries; it supplies composite information; influence of surface charges is reduced General characteristics of signals, depending on choice of the detector type.

Low Voltage BSE (BSE-H) Pure SE (SE) Pictures obtained in accordance with different types of detectors

X-ray spectrum microanalysis (XSMA) with electron probe (electron microprobe analysis EMPA) It is a chemical analysis method of a small part of solid sample where X-ray radiation is excited by focused electron beam.  The term “electron-probe microanalysis” or EPMA is used more often.  X-ray spectrum contains lines which characterize this element presence in sample, therefore qualitative analysis is carried out easier after the lines identification in accordance with wavelengths (or in accordance with photon energies)

Томск, ТПУ, ИГНД, ГЭГХ 23  The element composition determination of local part of investigated substance can be carried out with a help of electron probe microanalysis.  Electron probe microanalysis allows to detect presence in 0,1-2 мкм3 practically all elements of Periodic table.  Error of determination achieves 1 % (relative portion), and limit in certain cases can achieve 50 ppm (mass portion 0,005 %), though in certain cases it can achieve lower values. Spatial resolution (cross locality) is limited approximately 1 мкм because of electron probe scattering in sample.  With a help of this It can be carried out quantitative chemical analysis of sections and polished sections from alloys, minerals, slag, organic and inorganic compounds for all elements without initial sample destruction.  Absolute sensitivity of electron probe microanalysis is rather less than sensitivity of methods of emissive spectrum analysis and X-ray fluorescence analysis.

 Modern electron probe microanalyzers are compound vacuum devices, consisting of electron-optical system (electron gun and electron magnetic lenses), optical microscope and device using for scanning of element distribution on the object surface (X-ray spectrometer).  X-ray spectrometers detect appearing in the sample X-ray emission, and special addition attachments record automatically lines strength and all parameters of process Microanalyzers (Oxford instruments)

At present there are two types of X-ray spectrometers. Old, classical spectrometers with wave dispersion(WD or WS), using conditions of reflection from crystal, and spectrometers with energy dispersion, using conditions of reflection from Bragg-Vulf crystal, and spectrometers with energy dispersion (ED), made on the base of semiconductor detector

 Separately methods of WDS and EDS are not universal to solve all tasks of X-ray microanalysis.  WDS possesses good spectral resolution and sensitivity but it is a rather slow consecutive method.  EDS – high-speed method of simultaneous analysis, very flexible in application Томск, ТПУ, ИГНД, ГЭГХ 26

 WD spectrometers work in serial mode, i.e. intensity of one X-ray line is measured. WD spectrometer has better spectral resolution, but in ED spectrometer all spectrum comes up quicker and in comfortable type for interpretation characteristic lines.  SEM is usually completed by ED spectrometers though sometimes one (rarely two) WD spectrometer is placed on them.  Usually some WD spectrometers (as a rule, no less than three, to cover all range of analyzed elements) are placed on electron microprobe. They are often supplied with ED spectrometer

 Qualitative analysis in point  Quantitative analysis in point  Line profile of concentrations  Mapping of elemental composition