SEM-EDX Characterization Techniques PDF

SCANNING ELECTRON MICROSCOPY (SEM) AND ENERGY DISPERSIVE X-ray ANALYSIS (EDX)  red blood cell diatom Simple DNA ~5 m (SEM) 30 m molecules...

SCANNING ELECTRON MICROSCOPY (SEM) AND ENERGY DISPERSIVE X-ray ANALYSIS (EDX)  red blood cell diatom Simple DNA ~5 m (SEM) 30 m molecules proteins 5x109 V/m, the current density is of 105 A/cm2 and the brightness is hundreds times higher with respect to a thermoinic source.  V(x) φeff EF Tunneling V =qEx x  Cold Field Emitters (CFE) The emission derives from a few nanometers area and it is indipendent from the source temperature. Even if the total current is relatively small (1-10 μA), the brightness is particulary high (108 A/cm2sr @ 20 kV ) thanks to the very small beam dimensions. The beam is focused and accelerated by two anodes. The bias voltage between the first anode and the cathode (3-5 kV) determines the electric field to extract the electrons. The bias voltage between the second anode and the cathode (from some hundreds of Volts to 30 kV) accelerates the electrons. Cold Field Emitters (CFE) The proper operation of a CFE requires a very clean enviroment, for this reason a vacuum level of 10-8-10-9 Pa is needed. Before working, the emitter is heated for few seconds at very high temperature (2500K). This procedure damages slowly the emitter tip. Nevertheless, even if cleaning procedure is performed every day, the tip damage occurs at very long time. Advantages: the very small dimensions of the beam (3 nm) requires a very small focusing process (by the electromagnetic lenses) to reach the proper dimension of the spot (1 nm). low energetic spread. since the beam source is changed rarely, the system (source-lenses) remains aligned and clean for long time, assuring reproducibility and stability in the measurements. Cold Field Emitters (CFE) The Schottky Field Emitter – single crystal of tungsten – single crystal of tungsten – operate at room temperature coated with zirconium – very bright oxide (ZrO2) – long lasting – heated to 1800 K – require very low P (< 10-10 Torr) – ZrO2 lowers the work function – require frequent flashing – larger virtual source size (sudden heating) – small energy spread – poor current stability – high current density Thermal Field Emitter – good current stability – like a cold field emitter, but – does not easily contaminate; heated to 1800 K no flashing – does not contaminate easily, no – long life flashing – larger energy spread than CFE  PARAMETERS OF AN ELECTRON GUN Brightness, b (A/cm2 steradiant) is defined as the beam current for unit area and solid angle. The brightness is constant and it is the same in all the column points. For this reason, the brightness on the sample is almost the same of the brightness close to the source. The defects and the aberrations of the lenses decrease the effective value of the brightness.  Electromagnetic lenses act mainly like optical lenses They consist of current flow conductors with iron core The magnetic field depends on the current and the number of windings The magnetic field sets the optical refraction power of the lens, which is variable In SEM: lenses are solely used to minimize the beam diameter The focal length of the lens can be adjusted changing the amount of current running through the coils.  Electron beam scan: X-direction scanning coil Two sets of coils are used Holizontal line scan for scanning the electron Blanking beam across the specimen surface in a raster pattern similar to y-direction that on a TV screen. scanning coil This effectively samples the specimen surface point by point over the Objective scanned area. lens specimen Astigmatism can derive by: inhomogeneities in the pole-pieces; asymmetry in the lens windings, dirty apertures All these factors can produce an astigmatic image depending to an electrons divergence resulting in splitted focus lines and ultimately in a stretched image. This effect can be corrected by using a stigmator, a device which applies an additional weak magnetic field to make the lens symmetric to the electron beam and to to balance the inhomogeneities.  The combined effect of elastic and inelastic scattering is to limit the penetration of the beam into the solid. The resulting region over which the incident electrons interact with the solid, depositing energy and radiation forms is known as the interaction volume. Electron energy decreases with depth and the effects that can be produced are directly connected to absorption and scattering events in the sample. Secondary electrons, for instance, are produced throughout the interaction volume, but have very low energies and can only escape from a thin layer near the sample's surface. Secondary electrons are ejected from the sample with an energy lower than 50eV. The main part of this signal ( 90 %) is The emission coefficient for secondary characterized by energies electrons is defined as: around 10eV. They derived from inelastic interaction between = Nse/Nbeam the primary electron beam and the sample valence electrons. The fraction of ejected secondary electrons is independent from the Z se atomic number of sample atoms. E0 We define the emission coefficient for back scattered electrons as: η = Nbacksct/Nbeam η depends on the Z atomic number of analyzed material. BSE images show characteristics of atomic number contrast, i.e., high average Z appear brighter than those of low average Z. Z coefficient calculated by Henrich (1966) Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties. Morphology The shape and size of the particles making up the object; direct relation between these structures and materials properties. Composition The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties.  (Coupled with EDX) Crystallographic Information How the atoms are arranged in the object; direct relation between these arrangements and material properties. ENERGY DISPERSIVE X-ray SPECTROSCOPY (EDXS) when a charged particle (des-)accelarates or changes of direction, it emits an electromagnetic wave. – This is widely used to produce synchrotron radiation  It is useful to consider the appearance of the K, L, and M families in EDX spectra as a function of position in the energy range 0.7-10 keV. The K family consists of two recognizable line, Kα and Kβ. The ratio of the intensities of the Kα peak to the Kβ peak is approximately 10:1 when these peaks are resolved. This is one of the important criteria for the identification of an element. The penetration or, more precisely, the interaction volume depends on the acceleration voltage (energy of electron) and on the atomic number of the specimen.  EDX MAP OF A METALLIC ALLOY Quantitative results 60 50 Weight% 40 30 20 10 0 C O Cu Ag Os Ir Au A piece of stone analized at an acceleration voltage of 20 kV  EDX LINE SCAN ON ZnO/CuO NANORORDS 20 nm 100 nm (d) Intensity (a. u.) (a) Zn Cu 100 nm 0 100 200 300 400 nm 20 nm 100 nm (e) Intensity (a. u.) (b) Zn 100 nm Cu 0 100 200 300 400 nm 20 nm 100 nm (f) Intensity (a. u.) (c) Zn 100 nm Cu 0 100 200 300 400 nm Q. Simon et al J. Mater. Chem., 2012, 22, 11739