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  1. 1. 1 Surface Characterization Techniques Dr Amber Solangi
  2. 2. 2 • Topics: – Introduction – Auger Electron Spectroscopy – Electron Spectroscopy for Chemical Analysis OR X-ray Photoelectron Spectroscopy (XPS) – Electron Microscopy (TEM and SEM)– Atomic Force Microscopy (AFM) – Fourier-Transform Infrared Spectroscopy (FTIR)
  3. 3. 3 How do we Define the Surface The surface behaviour of materials is crucial to our lives. • one considers a car body shell, a biological cell, tissue or implant, a catalyst, a solid state electronic device or a moving component in an engine, it is the surface which interfaces with its environment. The surface reactivity will determine how well the material behaves in its intended function.
  4. 4. 4 How Many Atoms in a Surface? The top layer of surface atoms are those that are the immediate interface with the other phases (gas, liquid or solid) impinging on it, this could be regarded as the surface.
  5. 5. 5 Continue…………………... • The most inclusive way to define a surface is to state that a surface of interface exists in any case where there is an abrupt change in the system properties with distance, with many degrees of abruptness.
  6. 6. 6 • In a very real sense therefore, the surface could said to be the top 2–10 atomic or molecular layers (say, 0.5–3 nm). • beyond 100 nm it is more appropriate to begin to describe such a layer in terms of its bulk solid state properties. Continue…………………...
  7. 7. 7 The surface can be considered in terms of three regimes: • the top surface monolayer, • the first ten or so layers and • the surface film, no greater than 100 nm Continue…………………...
  8. 8. 8 Surface of a Solid • The surface of a solid in contact with a liquid or gaseous phase usually has very different chemical composition and physical properties from the interior of the solid. • Characterization of these surface properties is often important in many fields, including heterogeneous catalysis, semiconductor thin-film technology, corrosion and adhesion mechanisms, activity of metal surfaces, and studies of the behavior and functions of biological membranes. • To understand fully the surface of a solid material, we need techniques that not only distinguish the surface from the bulk of the solid, but also ones that distinguish the properties of these three regimes.
  9. 9. 9 Surface Methods • The chemical composition of a surface of a solid is often different from the interior of the solid. • One should not focus solely on this interior bulk composition because the chemical composition of the surface layer of a solid is sometimes much more important.
  10. 10. 10 Surface Measurements • Classical methods • They involve obtaining optical and electron microscopic images, as well as measurements of adsorption isotherms, surface areas, surface roughness, pore sizes and reflectivity. • useful information about the physical nature of surfaces but less about their chemical nature.
  11. 11. 11 Measurements (cont.) • Spectroscopic methods • provided information about the chemical nature of surfaces, as well as determine their concentration • began in the 1950s
  12. 12. 12 Measurements (cont.) • Microscopic methods • imaging surfaces and determining their morphology, or physical features
  13. 13. 13 The figure below illustrates the general principle by which a spectroscopic examination of surface is performed.
  14. 14. 14 Electron spectroscopy • In electron spectroscopy, the spectroscopic measurement consists of the determination of the power of the electron beam as a function of the energy (or frequency hv) of the electrons.
  15. 15. 15 • The most common type is based upon the irradiation of the sample surface with monochromatic X-radiation. • This is called X-ray photoelectron spectroscopy (XPS). • This method is also known as Electron spectroscopy for chemical analysis (ESCA).
  16. 16. 16 • X-Ray Photoelectron Spectroscopy (XPS), not only provided information about the atomic composition of the sample, but also information about the structure and oxidation state of the compounds being examined.
  17. 17. 17 • The second type of electron spectroscopy is called Auger electron spectroscopy (AES). • Auger spectra are most commonly excited by a beam of electrons, although X-rays are also used.
  18. 18. 18 • The third type of electron spectroscopy is ultraviolet photoelectron spectroscopy (UPS). • In this method, a monochromatic beam of ultraviolet radiation causes the ejection of electrons form the analyte. • This method is not as common as the first two methods.
  19. 19. 19 • Electron spectroscopy can be used for the identification of all of the elements in the periodic table except for helium and hydrogen. • The method also permits the determination of the oxidation state of an element and the type of species to which it is bonded. • This technique also provides useful information about the electronic structure of molecules.
  20. 20. 20 • Secondary-ion mass spectrometry (SIMS) is the most highly developed mass spectrometric surface methods, with several manufacturers offering instruments for this technique. • SIMS is useful from determining both atomic and the molecular composition of solid surfaces.
  21. 21. 21 Secondary Mass Ion Spectroscopy (SIMS) In SIMS, ion bombardment sputters off surface ions (secondary ions) that are then counted. Spectra are compared to a database to determine species, quantity, orientation information. SIMS components: sample and ion gun, mass analyzer (filter), and processor/computer SIMS can damage surface, but gives quantitative data on composition as a function of depth in a sample
  22. 22. 22 • In secondary-ion mass analyzers that serve for general surface analysis and for depth profiling, the primary ion beam diameter ranges from 0.3 to 0.5mm. • Double-focusing, single-focusing, time-of- flight and quadrapole spectrometers are used for mass determination.
  23. 23. 23 • Ion microprobe analyzers are more sophisticated (and thus more expensive) instruments that are based upon a focused beam of primary ions that has a diameter of 1 to 2 m. This beam can be moved across a surface for about 300 m in both x and y directions.
  24. 24. 24 Microscopic methods • In many fields of chemistry, material science, geology and biology, a detailed knowledge of the physical nature of the surface of solids is of great importance. • The classical method of obtaining this information was optical microscopy. • The resolution of optical microscopy is limited by diffraction effects to about the wavelength of light.
  25. 25. 25 Current surface information at considerably higher resolution is obtained by following techniques:  Scanning electron microscopy (SEM)  Transmission Electron microscopy (TEM)  Scanning tunneling microscopy (STM)  Atomic force microscopy (AFM)
  26. 26. 26 Transmission Electron Microscoy –Scanning Electron Microscopy (TEM, SEM) TEM in comparison to light microscopy. Above right: sperm cells in light microscopy, below right: sperm cells in a TEM
  27. 27. 27 SEM Electrons can penetrate deeply into a sample, giving averaged chemical information with depth . SEM has great depth of focus. Left: osteoblast cells cultured on a titanium mesh. Right: schematic of an SEM
  28. 28. 28 SEM Experiment Trochodiscus longispinus in OM and SEM. Note improved depth of field and resolving capability of the SEM experiment.
  29. 29. 29 Scanning Probe Microscopes • Scanning probe microscopes (SPMs) are capable of resolving details or surfaces down to the atomic level. Unlike optical and electron microscopes, scanning probe microscopes reveal details not only on the lateral x and y axis of a sample but also the z axis.
  30. 30. 30 Scanning Probe Microscopy (SPM) Atomic Force Microscopy (AFM) AFM surface topography of poly (D,L- lactic acid)-poly(ethylene glycol)- monomethyl ether diblock copolymer Right: AFM instrumentation. Stylus is placed on sample surface. Laser tracks movement of stylus, and cantilever deflection is monitored. Stage is moved up and down to maintain contact between tip and sample. Above right: the greater the tip radius, the lower the spatial resolution.
  31. 31. 31 Information Required To understand the properties and reactivity of a surface, the following information is required: • physical topography, • chemical composition, • chemical structure, • atomic structure, the electronic state • a detailed description of bonding of molecules at the surface.
  32. 32. 32 Surface analysis techniques and the information they can provide Radiation IN Radiation DETECTED Photon Electron Photon Photon Electron Electron Ion Ion Neutron Neutron Surface Information SEM Physical topography STM Chemical composition ESCA – XPS AES SIMS ISS Chemical structure ESCA – XPS EXAFS IR & SFG EELS SIMS INS Atomic structure EXAFS LEED RHEED ISS Adsorbate bonding EXAFS IR EELS SIMS INS
  33. 33. 33 • ESCA/XPS – Electron analysis for chemical analysis/X-ray photoelectron spectroscopy. X-ray photons of precisely defined energy bombard the surface, electrons are emitted from the orbitals of the component atoms, electron kinetic energies are measured and their electron binding energies can be determined enabling the component atoms to be determined. • AES – Auger electron spectroscopy. Basically very similar to the above except that a keV electron beam may be used to bombard the surface. • SIMS – Secondary ion mass spectrometry. There are two forms, i.e. dynamic and molecular SIMS. In both a beam of high energy (keV) primary ions bombard the surface while secondary atomic and cluster ions are emitted and analysed with a mass spectrometer. • ISS – Ion scattering spectrometry. An ion beam bombards the surface and is scattered from the atoms in the surface. The scattering angles and energies are measured and used to compute the composition and surface structure of the sample target.
  34. 34. 34 • IR – Infrared (spectroscopy). Various variants on the classical methods – irradiate with infrared photons which excite vibrational frequencies in the surface layers; photon energy losses are detected to generate spectra. • EELS – Electron energy loss spectroscopy. Low energy (few eV) electrons bombard the surface and excite vibrations – the resultant energy loss is detected and related to the vibrations excited. • INS – Inelastic neutron scattering. Bombard a surface with neutrons – energy loss occurs due to the excitation of vibrations. It is most efficient in bonds containing hydrogen. • SFG – Sum frequency generation. Two photons irradiate and interact with an interface (solid/gas or solid liquid) such that a single photon merges resulting in electronic or vibrational information about the interface region. • LEED – Low energy electron diffraction. A beam of low energy (tens of eV) electrons bombard a surface; the electrons are diffracted by the surface structure enabling the structure to be deduced.
  35. 35. 35 • RHEED – Reflection high energy electron diffraction. A high energy beam (keV) of electrons is directed at a surface at glancing incidence. The angles of electron scattering can be related to the surface atomic structure. • EXAFS – Extended X-ray absorption fine structure. The fine structure of the absorption spectrum resulting from X-ray irradiation of the sample is analysed to obtain information on local chemical and electronic structure. • STM – Scanning tunnelling microscopy. A sharp tip is scanned over a conducting surface at a very small distance above the surface. The electron current flowing between the surface and the tip is monitored; physical and electron density maps of the surface can be generated with high spatial resolution. • AFM – Atomic force microscopy (not included in table). Similar to STM but applicable to non-conducting surfaces. The forces developed between the surface and the tip are monitored. A topographical map of the surface is generated.
  36. 36. 36 Overview of Characterization Methods Penetration depths can go deeply below the surface.
  37. 37. 37 X-ray Techniques
  38. 38. Basic Principle of Electron microscopes and AFM Muhammad Ashraf
  39. 39. What are Electron Microscopes? Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. This examination can yield the following information: Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties (hardness, reflectivity...etc.) Morphology The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity...etc.)
  40. 40. Composition The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties (melting point, reactivity, hardness...etc.) Crystallographic Information How the atoms are arranged in the object; direct relation between these arrangements and materials properties (conductivity, electrical properties, strength...etc.)  Electron microscopes were developed due to the limitations of Light Microscopes which are limited by the physics of light.  In the early 1930's this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organic cells (nucleus, mitochondria...etc.).  This required 10,000x plus magnification which was not possible using current optical microscopes.
  41. 41. How do Electron Microscopes Work?  Electron Microscopes(EMs) function exactly as their optical counterparts except that they use a focused beam of electrons instead of light to "image" the specimen and gain information as to its structure and composition.  The basic steps involved in all EMs:  A stream of electrons is formed (by the Electron Source) and accelerated toward the specimen using a positive electrical potential  This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam.  This beam is focused onto the sample using a magnetic lens  Interactions occur inside the irradiated sample, affecting the electron beam  These interactions and effects are detected and transformed into an image  The above steps are carried out in all EMs regardless of type.
  42. 42. Comparison of OM,TEM and SEM
  43. 43. Types of Electron microscopes TEM: transmission electron microscope SEM: scanning electron microscope
  44. 44. Introduction of TEM The transmission electron microscope (TEM) was the first type of Electron Microscope to be developed and is patterned exactly on the light transmission microscope except that a focused beam of electrons is used instead of light to "see through" the specimen. It was developed by Max Knoll and Ernst Ruska in Germany in 1931.
  45. 45. • In the TEM, the focused, monochromatic electron beam interacts with and is transmitted through the sample, focused into an image and projected onto a phosphor coated screen which emits visible light. The brighter areas of the image represent areas where more electrons have passed through the sample. The darker areas represent areas where fewer electrons have passed through as a result of higher specimen density. A TEM can magnify up to about 500,000x Basic Principle of TEM
  46. 46. TEM
  47. 47. TEM Images Thin section of budding yeast cell Thin section of E. coli bacteria
  48. 48. The first scanning electron microscope (SEM) debuted in 1938 ( Von Ardenne) with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample. Introduction of SEM
  49. 49. • In the SEM, a set of scan coils moves the electron beam across the specimen in a 2 dimensional grid fashion. When the electron beam scans across the specimens, different interactions take place. These interactions are decoded with various detectors situated in the chamber above the specimen. Some electrons from the surface material are knocked out of their orbitals by the electron beam, and are called SECONDARY ELECTRONS. These electrons are detected by the secondary electron detector. Different interactions give images based on topography, elemental composition or density of the sample. A SEM can magnify up to about 100,000x. Basic Principle of SEM
  50. 50. SEM
  51. 51. SEM Images Budding yeast cell, original magnification 32 000X E. coli bacteria, original magnification 30 000X
  52. 52. Advantages of Using SEM over OM The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time and produces an image that is a good representation of the three-dimensional sample. The combination of higher magnification, larger depth of field, greater resolution, compositional and crystallographic information makes the SEM one of the most heavily used instruments in academic/national lab research areas and industry.
  53. 53. Differences between SEM and TEM TEM SEM Electron beam passes through thin sample. Electron beam scans over surface of sample. Specially prepared thin samples or particulate material are supported on TEM grids. Sample can be any thickness and is mounted on an aluminum stub. Specimen stage halfway down column. Specimen stage in the chamber at the bottom of the column. Image shown on fluorescent screen. Image shown on TV monitor. Image is a two dimensional projection of the sample. Image is of the surface of the sample.
  54. 54. Resolution of microscopes Microscope Resolution Magnification Optical ± 200 nm ± 1000X TEM ± 0.2 nm ± 500 000X SEM ± 2 nm ± 200 000X The RESOLUTION or RESOLVING POWER of a microscope is the instrument's ability to separate two objects that are close together.
  55. 55. Atomic Force Microscopy Introduction In all SPM techniques a tip interacts with the sample surface through a physical phenomenon. Measuring a “local” physical quantity related with the interaction, allows constructing an image of the studied surface. All the data are transferred to a PC, where, with the use of the appropriate software, an image of the surface is created. The scanning tunneling microscope (STM) is the ancestor of all scanning probe microscopes. It was invented in 1982 by Gerd Binning and Heinrich Rohrer at IBM Zurich. Five years later they were awarded the Nobel Prize in Physics for their invention. The atomic force microscope (AFM) was also invented by Binning et al. in 1986.
  56. 56. Introduction continue the STM measures the tunneling current (conducting surface), the AFM measures the forces acting between a fine tip and a sample. The tip is attached to the free end of a cantilever and is brought very close to a surface. Attractive or repulsive forces resulting from interactions between the tip and the surface will cause a positive or negative bending of the cantilever. The bending is detected by means of a laser beam, which is reflected from the back side of the cantilever. Following figure shows the basic concept of STM and AFM.
  57. 57. Basic Principle of AFM AFM provides a 3D profile of the surface on a nanoscale, by measuring forces between a sharp probe (<10 nm) and surface at very short distance (0.2-10 nm probe-sample separation). The probe is supported on a flexible cantilever. The AFM tip “gently” touches the surface and records the small force between the probe and the surface.
  58. 58. Figure shows Spring depiction of cantilever b) SEM image of triangular SPM cantilever with probe (tip).
  59. 59. Applications of SEM, AFM and TEM Ashfaque Ali Bhatti.
  60. 60. Introduction Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. Topography (surface features of an object) Morphology (shape and size of the particles making up the object) Composition (elements and compounds that the object is composed of and the relative amounts of them) Crystallographic information (how the atoms are arranged in the object).
  61. 61. Electron microscope is a valuable tool has developed scientific theory and it contributed greatly to biology, medicine and material sciences. This wide spread use of electron microscopes is based on the fact that they permit the observation and characterization of materials on a nanometer (nm) to micrometer (μm) scale.
  62. 62. SEM • Penetrate into the sample within a small depth, so that it is suitable for surface topology, for every kind of samples (metals, ceramics, glass, dust, hair, teeth, bones, minerals, wood, paper, plastics, polymers, etc) • The SEM permits the observation of materials in macro and submicron ranges. • The instrument is capable of generating three-dimensional images for analysis of topographic features . • chemical composition of the sample’s surface When used in conjunction with EDS i.e. an elemental analysis of the material or contaminants that may be present.
  63. 63. SEM SEM was used to examine the contamination at a higher magnification to determine Surface contamination, contamination was lying on the surface of the ceramic Embedded in the surface (contamination from the pressing operation) or within the surface (contamination in the glass powder) dark specks of sintered glass ceramic Contamination inside the holder ceramic
  64. 64. EDX EDX use to quantify the particle of contamination and compared to the composition of the holder ceramic EDX spectra of particle (red) and the normal ceramic (black outline)
  65. 65. Forensic Applications of Scanning Electron Microscopy • One of the most well know applications of SEM in forensics is the automated detection and classification of gun shot residue (GSR). • Automated SEM has also been used for the classification of minerals in soil and the detection of very small pieces of bone in fire debris
  66. 66. Find out the cause of death The fiber-end has a flat top with a lip and it can be clearly seen that there is a tool mark in the end surface. post explosion residue from flash powder Fibers investigate fiber fracture and damage Fiber-end of a parachute cord that probably has been cut with a knife Post explosion residues Improvised explosives may be based on pyrotechnic mixtures and these can suspect to prove they had been in the vicinity of the scene of crime
  67. 67. Ballistics • The examination of microtraces of foreign material embedded in or adhered to bullets provides critical information in the trajectory reconstruction of spent bullets. Surface of a bullet that hit MDF after hitting Greenboard (gypsum). MDF (black) is found on top of gypsum (grey).
  68. 68. Biology Sometimes SEM can be used to detect small bloodstains in order to reconstruct the trajectory of bullets. Bloodstain with some tiny fragments of bone near damage in a doorpost saying that the victim was hit first Another area in which SEM is used is the identification of animal hairs. SEM is very useful to visualize the characteristic scale patterns on hairs. Scale pattern on hair of a civet bloodstain with bone fragments
  69. 69. Atomic Force Microscope • Form of microscopy in which a sharp tip is scanned over the surface of a sample, while sensing the interaction force between the tip and the sample. • Atomic force microscopy is currently applied to various environments (air, liquid, vacuum) • Materials of types such as metal semiconductors, soft biological samples, conductive and non-conductive materials. • With this technique size measurements or even manipulations of nano-objects may be performed
  70. 70. Some possible applications of AFM are: - Substrate roughness analysis. - Step formation in thin film epitaxial deposition. - Pin-holes formation or other defects in oxides growth. - Grain size analysis. - Phase mode is very sensitive to variations in material properties, including surface stiffness, elasticity and adhesion. - Obtaining information of what is happening under indentation at very small loads.
  71. 71. Biological Applications of AFM One of the advantages of AFM is that it can image the non-conducting surfaces. Immediately extended to the biological systems, such as analyzing the crystals of amino acids and organic monolayers. Biosciences DNA and RNA analysis; Protein nucleic acid complexes; Chromosomes, Cellular membranes; Proteins and peptides; molecular crystals; Polymers and biomaterials; Ligand-receptor binding. Cell Biology Unique capabilities of AFM's to study the dynamic behavior of living and fixed cells such as red and white blood cells, bacteria, platelets, cardiac myocytes, living renal epithelial cells.
  72. 72. Microbiology The AFM has been used to viewing and analyzing the ultra structure of microbial cell surface studies. Function-related conformational changes in single proteins, Surface ultra structure of living cells, Cell surface dynamics, and Morphology of biofilms. The physical properties and biomolecular interactions such as Stiffness of cell walls, Local surface charge and h y d r o p h o b i c i t y, E l a s t i c i t y a n d conformational properties of single molecules, AFM image of Saccharomyces cerevisiae Yeast cell immobilized in a porous membrane. Surface of Phanerochaete chrysosporium Fungal Spores.
  73. 73. Nucleic acid Research • One area of significant progress is the imaging of nucleic acids. The ability to generate nanometer-resolved images of unmodified nucleic acids has broad biological applications. Chromosome mapping, transcription, translation and small molecule-DNA interactions such as intercalating mutagens.
  74. 74. AFM to Forensic Science AFM is an emerging powerful tool in forensic science At the present moment in forensic investigation AFM is used: • To analyse documents visualising ink deposis and differentiating them according to their origin • To analyse fibres Examination of the morphological changes in textile fibres exposed to different environmental stresses • To distinguish between the chemical domains on the fingerprints.
  75. 75. AFM examinationof ink depositon a paper
  76. 76. AFM examinationof ink depositon a paper
  77. 77. AFM in Polymer Materials • AFM is extremely useful for studying the local surface molecular composition and mechanical properties of a broad range of polymer materials, including block copolymers, bulk polymers, thin-film polymers, polymer composites, and polymer blends. AFM topographic images of polymer isotactic polypropylene with a scan size of 1.5 μm x 1.5 μm Styrene with a scan size of 1μm x 1μm
  78. 78. TEM A Transmission Electron Microscope is ideal for a number of different fields such as life sciences, nanotechnology, medical, biological and material research, forensic analysis, gemology and metallurgy as well as industry and education. TEMs provide topographical, morphological, compositional and crystalline information. This information is useful in the study of crystals and metals, but also has industrial applications. TEMs can be used in semiconductor analysis and production and the manufacturing of computer and silicon chips. The TEM allows imaging of the internal structure of a wide range of samples from biological, medical, and materials sciences, up to magnifications of 600,000x. Samples must be extremely thin < 60nm thick.
  79. 79. • Material study - Composite and nano-structured materials study - Defects from the manufacturing process in semi-conductors - Ceramic systems - Plastic Deformations - The study of layers and structures - Phase transformations - Nanometric systems - Ordered alloy structures - The study of cellular structures
  80. 80. Characterization of nanowire Characterization of composition and structure is crucial to understanding the properties and performance of nanomaterials. Cu−Ni core/shell nanowires
  81. 81. Mineralogical Applications • Mineral Identification • Morphological • Kaolinite: Hexagonal • Attapulgite: Needle
  82. 82. Application of transmission electron microscopy to the clinical study Clinical study of viral and bacterial infections Preparation of a herpesvirus from a skin lesion negatively stained with PTA. The virion is surrounded by a limiting lipid bi- layer. Thin section of a cultured cell containing replicating adenoviruses. Note crystalline arrays of virus assembling within the cell nucleus. Adenoviruses are about 80 nm in diameter.
  83. 83. Muhammad Ali Ph.D Research Fellow
  84. 84. Microscopy Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye (objects that are not within the resolution range of the normal eye).
  85. 85. There are three well-known branches of microscopy, optical, electron, and scanning probe microscopy. • Optical and electron microscopy involves the diffraction, reflection, or refraction of radiation incident upon the subject of study, and the subsequent collection of this scattered radiation in order to build up an image. • Scanning probe microscopy involves the interaction of a scanning probe with the surface or object of interest.
  86. 86. Electron Microscopy • Developed in the 1930s that use electron beams instead of light. • Because of the much lower wavelength of the electron beam than of light, resolution is far higher. Most commonly used: • Scanning electron microscopy (SEM) • Transmission electron microscopy (TEM)
  87. 87. General schematic diagram for the electron microscope
  88. 88. • It is a type of electron microscope capable of producing high-resolution images of a sample surface. • Due to the manner in which the image is created, SEM images have a characteristic 3D appearance and are useful for judging the surface structure of the sample. • It is not high enough to image individual atoms, as is possible in the TEM … as it is 1- 20 nm Scanning Electron Microscopy (SEM)
  89. 89. SEM
  90. 90. Components • Vacuum system: • To increase the mean free path of the electron-gas interaction, a standard TEM is evacuated to low pressures, typically on the order of 10−4 Pa. Specimen stage: The specimen holders are adapted to hold a standard size of grid upon which the sample is placed or a standard size of self- supporting specimen. Standard TEM grid sizes is a 3.05 mm diameter ring, with a thickness and mesh size ranging from a few to 100 μm. The sample is placed onto the inner meshed area having diameter of approximately 2.5 mm. Usual grid materials are copper, molybdenum, gold or platinum. This grid is placed into the sample holder which is paired with the specimen stage.
  91. 91. Electron lenses are designed to act in a manner emulating that of an optical lens, by focusing parallel rays at some constant focal length. The majority of electron lenses for TEM utilise electromagnetic coils to generate a convex lens. Electron lenses are manufactured from iron, iron-cobalt or nickel cobalt alloys,such as permalloy. These are selected for their magnetic properties, such as magnetic saturation, hysteresis and permeability.
  92. 92. Why do we need a lens? Because all electron sources generally produce a diverging beam of electrons. This beam must be "focussed" onto the specimen, to increase the intensity and thus to making the probe "smaller". Lenses in an TEM/STEM utilize either or combinations of Magnetic and Electrostatic Fields to direct the beams as desired.
  93. 93. The electron gun is formed from several components: the filament, a biasing circuit, a Wehnelt cap, and an extraction anode. By connecting the filament to the negative component power supply, electrons can be "pumped" from the electron gun to the anode plate, and TEM column, thus completing the circuit. The gun is designed to create a beam of electrons exiting from the assembly
  94. 94. TEM consists of an emission source, which may be a tungsten filament, or a lanthanum hexaboride (LaB6) source. For tungsten, this will be of the form of either a hairpin-style filament, or a small spike- shaped filament. LaB6 sources utilize small single crystals.
  95. 95. Secondary Electron Detector The electrons are detected by an Everhart-Thornley detector,18 which is a type of scintillator-photomultiplier system. The secondary electrons are first collected by attracting them towards an electrically biased grid at about +400 V, and then further accelerated towards a phosphor or scintillator positively biased to about +2,000 V.
  96. 96. Backscatter electron detector Dedicated backscattered electron detectors are positioned above the sample in a "doughnut" type arrangement, concentric with the electron beam, maximising the solid angle of collection. BSE detectors are usually either of scintillator or of semiconductor types. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality.
  97. 97. Transmission Electron Microscopy (TEM) • A beam of electrons is transmitted through a sample, then an image is formed, magnified and directed to appear either on a fluorescent screen or layer of photographic film or to be detected by a sensor (e.g. charge-coupled device, CCD camera. • It involves a high voltage electron beam emitted by a cathode, usually a tungsten filament and focused by electrostatic and electromagnetic lenses. • Electron beam that has been transmitted through a specimen that carries information about the inner structure of the specimen which reaches the imaging system of the microscope. Then magnified by a series of electromagnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or CCD camera.
  98. 98. Transmission Electron Microscopy (TEM) Human red blood cells Neurons CNS Neuron growing on astroglia
  99. 99. (GUN: Electrons are emitted from a tungsten filament a thin wire) (Electrons are accelerated with an electric field to sample)
  100. 100. Invented by Binnig, Quate, and Gerber at Stanford University in 1986. Atomic Force Microscopy (AFM) measures the interaction force between the tip and surface. The tip may be dragged across the surface, or may vibrate as it moves. The interaction force will depend on the nature of the sample, the probe tip and the distance between them. The AFM makes use of a sharp tip attached to a cantilever, acting as a spring. Unlike STM, where the tunneling current is a measure of the interaction, the force between tip and sample is detected via its mechanical influence on the cantilever deflection or resonance. The AFM can be used to study insulating, semiconducting and conducting samples. Atomic Force Microscopy (AFM)
  101. 101. Fig. Schematic set-up of an AFM using the beam deflection method. The piezo tube scans and approaches the sample to the tip attached to a cantilever. The force acting on the highly flexible cantilever is transduced into an electronic signal via a beam deflected onto a four quadrant photo-detector.
  102. 102. Auger electron spectroscopy Instrumentation Zahid Hussain Shar
  103. 103. Introduction  Identification of elements on surfaces of materials  Quantitative determination of elements on surfaces  Depth profiling by inert gas sputtering Phenomena such as adsorption, desorption, and surface segregation from the bulk  Determination of chemical states of elements  In situ analysis to determine the chemical reactivity at a surface  Auger electron elemental map of the system
  104. 104. Particle-Surface Interactions Auger Electron Spectroscopy Ions Electrons Photons Vacuum Ions Electrons Photons
  105. 105. AES Spectrometer
  106. 106. AES Spectrometer The essential components of an AES spectrometer are UHV environment Electron gun Electron energy analyzer Electron detector Data recording, processing, and output system
  107. 107. UHV Environment • Until 1960 the advancements in surface analysis techniques were inhibited by two difficulties:  constructing an apparatus suitable for operation in a UHV environment  production and measurement of UHV the glass enclosures were replaced by standardized, stainless steel hardware. • The UHV environment could be easily achieved by pumping a stainless steel chamber with a suitable combination of ion, cryo, turbo molecular, or oil diffusion pumps. • The surface analysis necessitates the use of a UHV environment because in order to minimize the influence of residual gases in surface
  108. 108. Electron Gun • The nature of the electron gun used for AES analysis depends on a number of factors: – The speed of analysis (requires a high beam current) – The desired spatial resolution (sets an upper limit on the beam current) – Beam-induced changes to the sample surface (sets an upper limit to current density) • The electron gun optical system has two critical components: the electron source and the focusing forming lens The commonly used electron sources are
  109. 109. Electron sources • A tungsten cathode source consists of a wire filament, which is bent in the form of a hairpin. The filament operates at ~2700 K by resistive heating. The tungsten cathodes are widely used, because they are both reliable and inexpensive. • A lanthanum hexaboride (LaB6) cathode provides higher current densities because LaB6 has a lower work function than tungsten. • A field emission electron source consists of a very sharp tungsten point at which the electric field can be >107 V/cm. Hence, electrons tunnel directly through the barrier and leave the emitter. A field emission gun provides the brightest beam
  110. 110. Electron Energy Analyzer • The function of an electron energy analyzer is to disperse the secondary emitted electrons from the sample according to their energies. An analyzer may be either magnetic or electrostatic. Because electrons are influenced by stray magnetic fields (including the earth’s magnetic field), it is essential to cancel these fields within the enclosed volume of the analyzer. The stray magnetic field cancellation is accomplished by using Mu metal shielding. Electrostatic analyzers are used in all commercial spectrometer.
  111. 111. Types of Electron Energy Analyzer • The Cylindrical Mirror Analyzer (CMA) • Concentric Hemispherical Analyzer (CHA)
  112. 112. The Cylindrical Mirror Analyzer (CMA) • The CMA consists of two coaxial cylinders with a negative potential (V ) applied to the outer cylinder (with radius r2) and ground potential applied to the inner cylinder (with radius r1).
  113. 113. The Cylindrical Mirror Analyzer (CMA)
  114. 114. Electron Detector • Having passed through the analyzer, the secondary electrons of a particular energy are spatially separated from electrons of different energies. • Various detectors are used to detect these electrons.
  115. 115. Electron multiplier • An electron multiplier consists of a series of electrodes called dynodes. Each is connected along a resistor string . The dynode potentials differ in equal steps along the chain. When a particle (electron, ion, high energy neutral, or high energy photon) strikes the first dynode, it produces secondary electrons. The secondary electrons are then accelerated to the next dynode. A cascade of secondary electrons ensues. The cascade is collected at the anode. The resulting current is then electronically amplified and measured
  116. 116. Electron Multiplier
  117. 117. Microchannel Plate Electron Multiplier Arrays • The detector comprises an array of small channel electron multipliers
  118. 118. Microchannel Plate Electron Multiplier Arrays
  119. 119. Ion guns • An AES system is commonly equipped with an argon ion beam. The Ar+ ion beam is used to sputter the sample surface. • The ion gun is employed for: • Cleaning the sample surface, and • Depth profiling
  120. 120. Ion guns
  121. 121. Data Recording, Processing, and Output System • At present, the data in commercial instruments are acquired digitally and can be presented in either analog or digital mode. The majority of AES instruments are controlled by computer. Major functions of the computer control system are to acquire and store data efficiently.
  122. 122. Faheem Shah PhD Scholar NCEAC
  123. 123. XPS technique is based on Einstein’s idea about the photoelectric effect, developed around 1905 The concept of photons was used to describe the ejection of electrons from a surface when photons were impinged upon it XPS is a technique used to investigate elemental composition of surfaces. X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) XPS was developed in the mid-1960’’s by Siegbahn in Sweden.
  124. 124. • XPS is also known as ESCA (Electron Spectroscopy for Chemical Analysis). • The technique is widely used because it is very simple to use and the data is easily analyzed. • XPS works by irradiating atoms of a surface of any solid material with X-Ray photons, causing the ejection of electrons.
  125. 125. The XPS is controlled by using a computer system. The computer system will control the X-Ray type and prepare the instrument for analysis.
  126. 126. • The instrument uses different pump systems to reach the goal of an Ultra High Vacuum (UHV) environment. • The Ultra High Vacuum environment will prevent contamination of the surface and aid an accurate analysis of
  127. 127. X-Ray Source Ion Source SIMS Analyzer Sample introduction Chamber
  128. 128. X-Ray source Ion source Axial Electron Gun Detector CMA sample SIMS Analyzer Sample introduction Chamber Sample Holder Ion Pump Roughing Pump Slits
  129. 129. • The sample will be introduced through a chamber that is in contact with the outside environment • It will be closed and pumped to low vacuum. • After the first chamber is at low vacuum the sample will be introduced into the second chamber in which a UHV environment exists. First Chamber Second Chamber UHV
  130. 130. • Contamination of surface XPS is a surface sensitive technique. Contaminates will produce an XPS signal and lead to incorrect analysis of the surface of composition. • The pressure of the vacuum system is < 10-9 Torr • Removing contamination To remove the contamination the sample surface is bombarded with argon ions (Ar+ = 3KeV). Heat and oxygen can be used to remove hydrocarbons • The XPS technique could cause damage to the surface, but it is negligible.
  131. 131. X-Rays The X-Ray source produces photons with certain energies: MgK photon with an energy of 1253.6 eV AlK photon with an energy of 1486.6 eV Normally, the sample will be radiated with photons of a single energy (MgK or AlK). This is known as a monoenergetic X-Ray beam.
  132. 132. Electrons from filament are accelerated to ~30 kV, then directed towards the anode which is coated with the target element of interest (Mg or Al here). The power dissipated by the electron beam is ~300-400 Watts, which requires cooling of the anode targets with flowing water. This is a very serious design problem, since the anodes are at 30 kV, and normal water is conductive. In some sources, both targets are Mg or Al to allow both sources to operate simultaneously, with double the output power.
  133. 133. • Irradiate the sample surface, hitting the core electrons (e-) of the atoms. • The X-Rays penetrate the sample to a depth on the order of a micrometer. • Useful e- signal is obtained only from a depth of around 10 to 100 X-Rays
  134. 134. The Atom and the X-Ray Core electrons Valence electrons X-Ray Free electron proton neutron electron electron vacancy The core electrons respond very well to the X-Ray energy
  135. 135. Atoms layers e- top layer e- lower layer with collisions e- lower layer but no collisions X-Rays Outer surface Inner surface
  136. 136. • The X-Rays will penetrate to the core e- of the atoms in the sample. • Some e-s are going to be released without any problem giving the Kinetic Energies (KE) characteristic of their elements. • Other e-s will come from inner layers and collide with other e-s of upper layers – These e- will be lower in lower energy. – They will contribute to the noise signal of the spectrum.
  137. 137. X-Ray Electron without collision Electron with collision The noise signal comes from the electrons that collide with other electrons of different layers. The collisions cause a decrease in energy of the electron and it no longer will contribute to the characteristic energy of the element.
  138. 138. For best spectral purity, a monochromator selects only the central intense X-ray emission, but this reduces flux by at least an order of magnitude
  139. 139. Kinetic energy and hence binding energy is measured using a hemispherical analyzer Electrons are injected at S along a tangent of the spherical sections. Electrons that match the pass energy are refocused back to the exit slit F, where they are detected using a channeltron or electron multiplier.
  140. 140. Detectors: How can we detect the electrons once they have passed through the analyzer?
  141. 141. Each dynode at progressively more +ve potential to accelerate electrons giving more and more electrons
  142. 142. The front of the channeltron ideally should be at a high positive potential to attract the electrons. The rear of the device must be at an even more positive potential to accelerate the electron cascade down to the detector. Since most detection systems presume a source signal near ground potential, some level decoupling must be used.
  143. 143. Electron Spectroscopy / XPS for Chemical Analysis By Tahira Qureshi
  144. 144. Context • X-RAY INTERACTIONS WITH MATTER • Introduction • ATOMIC STRUCTURE • XRF – A PHYSICAL DESCRIPTION • XPS Principles • Photoelectric Effect • Photoelectron vs Other Spectroscopies • Summary
  145. 145. X-RAY INTERACTIONS WITH MATTER When X-rays encounter matter, they can be: Absorbed or transmitted through the sample (Medical X-Rays – used to see inside materials) Diffracted or scattered from an ordered crystal (X-Ray Diffraction – used to study crystal structure) Cause the generation of X-rays of different “colours” (X-Ray Fluorescence – used to determine elemental composition)
  146. 146. • X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces. Introduction
  147. 147. An atom consists of a nucleus (protons and neutrons) and electrons Z is used to represent the atomic number of an element (the number of protons and electrons) Electrons spin in shells at specific distances from the nucleus Electrons take on discrete (quantized) energy levels (cannot occupy levels between shells Inner shell electrons are bound more tightly and are harder to remove from the atom ATOMIC STRUCTURE
  148. 148. K shell - 2 electrons L shell - 8 electrons M shell - 18 electrons N shell - 32 electrons Shells have specific names (i.e., K, L, M) and only hold a certain number of electrons The shells are labelled from the nucleus outward ELECTRON SHELLS X-rays typically affect only inner shell (K, L) electrons
  149. 149. Step 1: When an X-ray photon of sufficient energy strikes an atom, it dislodges an electron from one of its inner shells (K in this case) Step 2a: The atom fills the vacant K shell with an electron from the L shell; as the electron drops to the lower energy state, excess energy is released as a K X-ray Step 2b: The atom fills the vacant K shell with an electron from the M shell; as the electron drops to the lower energy state, excess energy is released as a K X-ray XRF – A PHYSICAL DESCRIPTION Step 1: Step 2a: Step 2b:
  150. 150. XPS Principles • If we consider a single atom with just one x-ray photon on the way, the total energy is hv+Ei, where hv is the photon energy and Ei the energy of the atom in its initial state. • Following the absorption of the photon and the emission of the photoelectron, the total energy is now KE+Ef, where KE is the electron kinetic energy and Ef the final state energy of the atom (now an ion). Because total energy is conserved hv+Ei = KE+Ef Or hv-KE = Ef-Ei = BE where we call the difference between the photon energy (which we know) and the electron energy (which we measure), the binding energy of the orbital from which the electron was expelled. The binding energy is determined by the difference between the total energies of the initial-state atom and the final-state ion. • It is roughly equal to the Hartree-Fock energy of the electron orbital and so peaks in the photoelectron spectrum can be identified with specific atoms and hence, a surface compositional analysis performed.
  151. 151. Ionization occurs when matter interacts with light of sufficient energy (Heinrich Hertz, 1886) Ehn = electron kinetic energy + electron binding energy hn e- e- e- Photoelectric Effect Photoelectron spectroscopy uses this phenomenon to learn about the electronic structure of matter
  152. 152. Photoelectrons • When light strikes an atom an electron may be ejected if the energy of the light is high enough. The energy in the light is determined by its wavelength or frequency (short wavelength = high energy and high frequency = high energy) X-rays have high energy. When X-rays strike a solid electrons are always ejected from the near-surface region of the solid.
  153. 153.  XPS spectral lines are identified by the shell from which the electron was ejected (1s, 2s, 2p, etc.).  The ejected photoelectron has kinetic energy: KE=hv-BE-  Following this process, the atom will release energy by the emission of an Auger Electron. Conduction Band Valence Band L2,L3 L1 K Fermi Level Free Electron Level Incident X-ray Ejected Photoelectron 1s 2s 2p The Photoelectric Process
  154. 154. Kai Seigbahn: Development of X-ray Photoelectron Spectroscopy Nobel Prize in Physics 1981 (His father, Manne Siegbahn, won the Nobel Prize in Physics in 1924 for the development of X-ray spectroscopy) C. Nordling E. Sokolowski and K. Siegbahn, Phys. Rev. 1957, 105, 1676.
  155. 155. Measurements with XPS  If we measure the energy of the ejected photoelectrons we can calculate its Binding Energy which is the energy required to remove the electron from its atom. From the binding energy we can learn some important facts about the sample under investigation: • The elements from which it is made • The relative quantity of each element • The chemical state of the elements present • Modern XPS instruments can also produce images or maps showing the distribution of the elements or their chemical states over the surface. A good instrument would have a spatial resolution of a few microns.
  156. 156. Electron Spectroscopy for Chemical Analysis Exciting Radiation Outcoming Radiation UV (~ 20 eV)  UPS electrons from occupied valence states X-Ray (~ 10 keV)  XPS electrons from (occupied) core states Ultraviolet photoelectron spectroscopy (UPS) refers to the measurement of kinetic energy spectra of photoelectrons emitted by ultraviolet photons, to determine molecular energy levels in the valence region.
  157. 157. Photoelectron vs Other Spectroscopies Others • Photon must be in resonance with transition energy • Measure absorbance or transmittance of photons • Scan photon energies Photoelectron • Photon just needs enough energy to eject electron • Measure kinetic energy of ejected electrons • Monochromatic photon source
  158. 158. Summary ESCA provides unique information about chemical composition And chemical state of a surface useful for biomaterials Advantages  surface sensitive (top few monolayers)  wide range of solids  relatively non-destructive Disadvantages  expensive, slow, poor spatial resolution, requires high vacuum
  159. 159. By Huma Ishaque X-ray Photoelectron Spectroscopy (XPS) Applications
  160. 160. Applications • X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures, • The elemental composition, • Empirical formula, • Chemical state and electronic state of the elements that exist within a material.
  161. 161. • Identifying stains and discolorations • Characterizing cleaning processes • Analyzing the composition of powders and debris • Determining contaminant sources • Examining polymer functionality before and after processing to identify and quantify surface changes
  162. 162. • Measuring lube thickness on hard disks • Obtaining depth profiles of thin film stacks (both conducting and non-conducting) for matrix level constituents • Assessing the differences in oxide thickness between samples
  163. 163. • It is a surface analysis technique with a sampling volume that extends from the surface to a depth of approximately 50-70 Angstroms
  164. 164. Where do Binding Energy Shifts Come From? -or How Can We Identify Elements and Compounds? Electron-electron repulsion Electron-nucleus attraction Electron Nucleus Binding Energy Pure Element Electron- Nucleus Separation Fermi Level Look for changes here by observing electron binding energies
  165. 165. KE versus BE E E E KE can be plotted depending on BE Each peak represents the amount of e-s at a certain energy that is characteristic of some element. 1000 eV 0 eV BE increase from right to left KE increase from left to right Binding energy # of electrons (eV)
  166. 166. Interpreting XPS Spectrum: Background • The X-Ray will hit the e-s in the bulk (inner e- layers) of the sample • e- will collide with other e- from top layers, decreasing its energy to contribute to the noise, at lower kinetic energy than the peak . • The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis. Binding energy # of electrons N1 N2 N3 N4 Ntot= N1 + N2 + N3 + N4 N = noise
  167. 167. XPS Spectrum • The XPS peaks are sharp. • In a XPS graph it is possible to see Auger electron peaks. • The Auger peaks are usually wider peaks in a XPS spectrum.
  168. 168. Identification of XPS Peaks • The plot has characteristic peaks for each element found in the surface of the sample. • There are tables with the KE and BE already assigned to each element. • After the spectrum is plotted you can look for the designated value of the peak energy from the graph and find the element present on the surface.
  169. 169. Elemental Shifts
  170. 170. Carbon-Oxygen Bond Valence Level C 2p Core Level C 1s Carbon Nucleus Oxygen Atom C 1s Binding Energy Electron-oxygen atom attraction (Oxygen Electro- negativity) Electron-nucleus attraction (Loss of Electronic Screening) Shift to higher binding energy Chemical Shifts- Electronegativity Effects
  171. 171. Chemical Shifts- Electronegativity Effects Functional Group Binding Energy (eV) hydrocarbon C-H, C-C 285.0 amine C-N 286.0 alcohol, ether C-O-H, C-O-C 286.5 Cl bound to C C-Cl 286.5 F bound to C C-F 287.8 carbonyl C=O 288.0
  172. 172. Electronic Effects Spin-Orbit Coupling 284 280 276 288 290 Binding Energy (eV) C 1s Orbital=s l=0 s=+/-1/2 ls=1/2
  173. 173. Electronic Effects Spin-Orbit Coupling 965 955 945 935 925 19.8 BindingEnergy (eV) Cu 2p 2p1/2 2p3/2 Peak Area 1 : 2 Orbital=p ls=1/2,3/2 l=1 s=+/-1/2
  174. 174. Electronic Effects Spin-Orbit Coupling 370 374 378 366 362 6.0 BindingEnergy (eV) Peak Area 2 : 3 Ag3d 3d3/2 3d5/2 Orbital=d ls=3/2,5/2 l=2 s=+/-1/2
  175. 175. Electronic Effects Spin-OrbitCoupling 3.65 87 91 83 79 Binding Energy (eV) Peak Area 3 : 4 Au4f 4f5/2 4f7/2 Orbital=f l=3 s=+/-1/2 ls=5/2,7/2
  176. 176. Electronic Effects- Spin-Orbit Coupling Ti Metal Ti Oxide
  177. 177. Relative Sensitivities of the Elements 0 2 4 6 8 10 12 Elemental Symbol Relative Sensitivity Li Be B C N O F Ne Na M Al Si P S Cl Ar K Ca Sc Ti V Cr M Fe Co Ni Cu Zn G G As Se Br Kr Rb Sr Y Zr Nb M Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Ce Pr Nd P S Eu G Tb Dy Ho Er T Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi 1s 2p 3d 4d 4f
  178. 178. XPS Analysis of Pigment from Mummy Artwork 150 145 140 135 130 Binding Energy (eV) PbO2 Pb3O4 500 400 300 200 100 0 Binding Energy (eV) O Pb Pb Pb N Ca C Na Cl XPS analysis showed that the pigment used on the mummy wrapping was Pb3O4 rather than Fe2O3 Egyptian Mummy 2nd Century AD World Heritage Museum University of Illinois
  179. 179. Analysis of Carbon Fiber- Polymer Composite Material by XPS Woven carbon fiber composite XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties. -C-C- -C-O -C=O -300 -295 -290 -285 -280 Binding energy (eV) N(E)/E
  180. 180. Analysis of Materials for Solar Energy Collection by XPS Depth Profiling- The amorphous-SiC/SnO2 Interface The profile indicates a reduction of the SnO2 occurred at the interface during deposition. Such a reduction would effect the collector’s efficiency. Photo-voltaic Collector Conductive Oxide- SnO2 p-type a-SiC a-Si Solar Energy SnO2 Sn Depth 500 496 492 488 484 480 Binding Energy, eV Data courtesy A. Nurrudin and J. Abelson, University of Illinois
  181. 181. Relevant Industries for XPS Analysis • Aerospace • Automotive • Biomedical/biotechnology • Compound Semiconductor • Data Storage • Defense • Electronics
  182. 182. • Industrial Products • Pharmaceutical • Photonics • Polymer • Semiconductor • Solar Photovoltaics • Telecommunications
  183. 183. Strengths of XPS Analysis • Chemical state identification on surfaces • Identification of all elements except for H and He • Quantitative analysis, including chemical state differences between samples • Applicable for a wide variety of materials, including insulating samples (paper, plastics, and glass) • Depth profiling with matrix-level concentrations • Oxide thickness measurements
  184. 184. Limitations of XPS Analysis • Detection limits typically ~ 0.1 at% • Smallest analytical area ~10 µm • Limited specific organic information • Sample compatibility with UHV environmen
  185. 185. Asif Bhatti
  186. 186. Contents Definition Discovery Basic Principle Advantages and Disadvantges
  187. 187. Definition A common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science It is a surface specific technique utilising the emission of low energy electrons in the Auger process and is one of the most commonly employed surface analytical techniques for determining the composition of the surface layers of a sample.
  188. 188. Discovery Discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Though the discovery was made by Meitner and initially reported in the journal Zeitschrift für Physik in 1922. Working with X rays and using a Wilson cloud chamber. Tracks corresponding to ejected electrons were observed along a beam of X rays
  189. 189. Basic Principle Auger spectroscopy can be considered as involving three basic steps : (1) Atomic ionization (by removal of a core electron) (2) Electron emission (the Auger process) (3) Analysis of the emitted Auger electrons
  190. 190. Auger effect Based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events The incident electron with sufficient primary energy 2 keV to 50 keV, Ep, ionizes the core level, such as a K level. • The vacancy thus produced is immediately filled by another electron from L1. • The energy (EK – EL1) released from this transition can be transferred to another electron, as in the L2 level. This electron is ejected from the atom as an Auger electron
  191. 191. The transition energy can be coupled to a second outer shell electron which will be emitted from the atom if the transferred energy is greater than the orbital binding energy An emitted electron will have a kinetic energy of: Ekin = ECore State − EB − EC' where ECore State, EB, EC' are respectively the core level, first outer shell, and second outer shell electron energies, measured from the vacuum level
  192. 192. 201
  193. 193. • This excitation process is denoted as a KL1L2 Auger transition. • It is obvious that at least two energy states and three electrons must take part in an Auger process. Therefore, H and He atoms cannot give rise to Auger electrons. • Several transitions (KL1L1, KL1L2, LM1M2, etc.) can occur with various transition probabilities. • The Auger electron energies are characteristic of the target material and independent of the incident beam energy
  194. 194. • Isolated Li atoms having a single electron in the outermost level cannot give rise to Auger electrons. • However, in a solid the valence electrons are shared and the Auger transitions of the type KVV occur involving the valence electrons of the solid. • In general, the kinetic energy of Auger electrons originating from an ABC transition can be estimated from the empirical relation
  195. 195. • Until the early 1950s Auger transitions were considered nuisance effects by spectroscopists, not containing much relevant material information, but studied so as to explain anomalies in x-ray spectroscopy data. • Since 1953 however, AES has become a practical and straightforward characterization technique for probing chemical and compositional surface environments and has found applications in metallurgy, gas-phase chemistry, and throughout the microelectronics industry
  196. 196. Advantages • High sensitivity for chemical analysis in the 5- to 20-Å region near the surface. • A rapid data acquisition speed. • Its ability to detect all elements above helium, and its capability of high-spatial resolution. • The high-spatial resolution is achieved because the specimen is excited by an electron beam that can be focused into a fine probe.
  197. 197. Disadvantages Analyzes conducting and semiconducting samples.  Special procedures are required for nonconducting samples.  Only solid specimens can be analyzed.  Samples that decompose under electron beam irradiation cannot be studied. Quantification is not easy.