Surface Chemistry.ppt 42191k

Surface Characterization by Spectroscopy and Microscopy

Dr. Nizam M. El-Ashgar

Importance of surface characterization 12345-

Heterogeneous Catalysis. Semiconductor thin film technology. Corrosion and adhesion mechanisms Activity of metal surfaces. Studies of the behavior and functions of biological membranes.

Introduction

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Definition of Solid Surface: • The boundary layer between a solid and a vacuum, a gas or a liquid. • A part of solid that differs in composition from the average composition of the bulk of the solid. Top layer of atoms with a non-uniform composition that varies from the bulk. The surface may be several of atomic layers deep. surface measurements dose not affect the measurement of average composition of bulk (tiny fraction of the total solid). Classical Methods: Provide useful information about the physical nature of surfaces but less about their chemical nature. Involve obtaining optical and electron microscopic images of surfaces and adsorption isotherms, surface areas, surface roughness, pore sizes and reflectivity. Spectroscopic Methods: (1950) Provide information about the chemical nature of surfaces.

Spectroscopic Surface Method The chemical composition of the solid surface often differs from the interior or bulk. Spectroscopic surface methods provide both qualitative and quantitative chemical information about the composition of a surface layer of a solid that is a few angstrom units to a few tens of angstrom units in thickness

General Technique in surface spectroscopy The solid sample is irradiated with a primary beam made up of photons, electrons or neutral molecules.  This impact on a solid surface and results in formation of a secondary beam also consisting of photons, electrons, molecules or ions form the solid surface.  The type of particles making up the primary beam is not necessarily the same as the particle of secondary beam.  The secondary beam which results from scattering, sputtering or emission is studied by a variety of spectroscopic methods. Most Effective Surface Methods: Those in which the primary, the secondary beam or both is made up of either electrons, ions or molecules and not photons to ensure the measurement be restricted to surface not the bulk. 

Examples: Maximum penetration depth of abeam of 1-keV electrons or ions is 25 Αo. Whereas the penetration depth of a photon beam of the same energy is about 104 Αo. Precautions must be taken in case of using two beams of photons (X-Ray, FL, Raman, IR reflection spectroscopy). Studying surfaces by these methods is possible but bulk inreferences must be avoided. Surface Spectroscopic Methods Classified according to the nature of the primary and secondary beams. Table:

Sampling Surfaces Three types of sampling methods are employed regardless of the type of spectroscopic surface method being used. First method: Involves focusing the primary beam on a single small area of the sample and observing the secondary beam. Second method: Mapping the surface in which a region of the surface is scanned by moving the primary beam across the surface in a raster pattern of measured increments (linear or two dimensional mapping). Third method: Depth profiling in which a beam of ions from an ion gun is used to etch a hole in the surface by sputtering. A finer primary beam is used to produce a secondary beam from the center of the 

Surface Contamination Contamination of surfaces (by adsorption of some atmospheric components: water oxygen and CO2) is a problem in analysis.  Even in vacuum these contaminations occurs in a relatively short time. For example: At P of 10-6 torr, a clean surface will become covered by a monolayer of gas molecules in just 3 s. At P of 10-8 torr coverage occurs in about 1 hr. At P of 10-10 torr, 10 hr is required. Provision must often be made to clean the sample surface, in the chamber used for the irradiating the sample. Cleaning Methods: Baking the sample at high temperature, sputtering the sample with a beam of inert gas ions from an electron gun, mechanical scraping or polishing the surface with an abrasive, ultrasonic washing with various solvents and bathing the sample in reducing P to remove oxides. Other Contaminations: The primary beam itself can alter the surface as a measurement progresses, damage occurs that depends on the momentum of the primary beam particles. 

Electron Spectroscopy XPS, AES and UPS methods are based on analysis of emitted electrons produced by various incident beams.  The signal from the analyte consists of a beam consists of abeam of electrons rather than photons.  Determination is of the power of the electrons beam as a function of E (h ν ) or frequency.  Studying needs high resolution spectral measurements of electrons .  Used to study surfaces. Types of Electron Spectroscopy: 1- Photoelectron Spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) : Sample surface irradiated with monochromatic X-radiation. 2- Auger electron spectroscopy (AES). Excitation occurs by a beam of electrons. 3- Ultraviolet Photoelectron Spectroscopy (UPS). A monochromatic beam of UV causes ejection of electrons from the analyte (not common). 

Electron spectroscopy is a powerful tool for the identification of all the elements in the periodic table except H and He.  The method permits determination of the oxidation state of an element and the type of species to which it is bonded.  The technique provides useful information about the electronic structure of molecules.  Applied to gases and solids and to solutions and liquids.  Restricted to surface layer of a few atomic layers thick (20 to 50 Ao) because of poor penetrating power of electrons.  Composition of layers is different from average composition of entire sample. Valuable Current Applications: Qualitative analysis of solid surfaces such as metal alloys, semiconductors, and heterogeneous catalysis. Limited applications of quantitative analysis. 

X-Ray Photoelectron Spectroscopy  

In XPS the kinetic E of emitted electrons is recorded. The spectrum thus consists of a plot of the number of emitted electrons or the power of the electron beam as a function of the E or ν or λ of the emitted electrons .

Principles of XPS: XPS provides information: 1- About the atomic composition of a sample. 2- about the structure and oxidation state of the compounds.

Physical Bases Based upon photon in/electron out process.  Photon E (Einstein ): E=hν where : h - Planck constant ( 6.62 x 10 -34 J s ). ν - frequency (Hz) of the radiation.  In XPS the photon is absorbed by an atom in a molecule or solid, leading to ionization and the emission of a core (inner shell) electron.  But in case of UPS the photon interacts with valence levels (ionization by lost of one of them) Process of Photoionization A + hν A+* + eWhere A can be an atom, a molecule or an ion and A+* is an electronically excited cation. E(A) + h ν = E(A+ ) + E(e-) Since the electron's energy is present solely as kinetic energy (KE) this can be rearranged to give the following expression for the KE of the photoelectron : 

KE = h ν – [ E(A+ ) - E(A) ]

The difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron KE = h ν - BE Which corrected to: KE = h ν - BE –W W is the work function of the spectrometer, which corrects for the electrostatic environment in which the electron is formed and measured. BE of an electron is characteristic of the atom and orbital from which the electron was emitted.

Experimental Details The basic requirements:  a source of fixed-energy radiation (an xray source)  an electron energy analyzer (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy) 

a high vacuum environment (to enable the emitted photoelectrons to be analyzed without interference from gas phase collisions)

Instrumentation Expensive: 300,000 – 900,000.  Components: 1- Source 2- sample holder 3- Analyzer (same function as monochromator) . 4- detector 5- signal processor and read out. Electron spectrometers generally require elaborate vacuum systems to reduce P in all the components (10-5 – 10-8) torr. 1- Sources Simplest X-ray sources for XPS are X-ray tubes equipped with Mg or Al targets and suitable filters.  Mg Kα radiation : hν = 1253.6 eV  Al K α radiation : h ν = 1486.6 eV The emitted photoelectrons will therefore have kinetic energies in the range of ca. 0 - 1250 eV or 0 - 1480 eV The Kα lines for these two elements have considerably narrower band width (0.8 – 0.9 eV) than that of higher atomic number targets. Narrow bands lead to enhanced resolutions. Relatively sophisticated XPS employ a crystal monochromator to provide an X-ray beam having a bandwidth of about 0.3 eV. Monochromators improve S/N ratios. 

X-ray Photoelectron Spectrometer

2- Sample Holder: Solid samples: are mounted in a fixed position as close to photon or electron source and entrance slit of the spectrometer as possible. Sample compartment must be evacuated to a pressure of 10-5 torr or smaller to avoid attenuation of the electron beam. Much better vacuums 10-9 to 10-10 are required to avoid contamination of the sample surface by substances such as O2 or H2O that react with or are adsorbed on surface. Gas Samples: Are leaked into sample area through a slit of such a size as to provide P of 10-2 torr. Higher P lead to excessive attenuation of the electron beam due to inelastic collisions. If the sample P is two low weakened signal are obtained.

3- Analyzers: Most electron spectrometers are of hemispherical type in which the electron beam is deflected by an electrostatic magnetic field so electron travel in a curved path. Radius of curvature depends upon KE of the electron and the magnitude of the field. By varying the field electrons of various KE can be focused on the detector. P maintained at 10-5 or lower. 4- Transducers: Modern electron spectrometers are based upon solid state, channel electron multipliers. Consist of tubes of glass that have been doped with lead or vandium. When potential of several KV is applied across these materials a cascade or pulse of 106 – 108 electrons is produced for each incident electron. These pulses are then counted electronically. Two dimensional multichannel electron transducers are offered All resolution elements are stored simultaneously in a computer for subsequent display.

Applications of XPS XPS provides: - Qualitative and quantitative information about the elemental composition of matter, particularly solid surface. - Provides useful structural information. Qualitative Analysis: A low resolution, wide-scan XPS (survey spectrum) (next page). Used to determine the elemental composition of samples.

XPS Spectra 

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1- Characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. 2- The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study. 3- The intensity of the peaks is related to the concentration of the element within the sampled region. (quantitative analysis of the surface composition).

- With Mg or Al Kα source, all elements except H and He emit core electrons having characteristic BE. Survey spectrum has KE range of 250-1500 eV, which correspond to BE of about 0-1250 eV. Every element in the P.T. has one or more E levels that will result in the appearance of peaks in this region. The peaks are well resolved and lead to unambiguous identification provided the element if the element conc. >0.1%. Sometimes peak overlap is encountered such as O1s/Sb3d or Al2s, 2p/Cu3s, 3p. Overlapping problems can be resolved by investigating other spectral regions for additional peaks. Peaks from Auger electrons are found in XPS which are readily identified by comparing spectra produced by two X-ray sources (Mg and Al, Kα ) Auger peaks remain unchanged while photoelectric peaks are displaced on the KE scale.

Chemical shifts and oxidation states

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One of the peaks of the survey spectrum is examined under conditions of high E resolution. The position of the maximum is found to depend to a small degree upon the chemical environment of the atom responsible for the peak. So depends on: variations in the number of valence electrons and the type of bonds they form so influence the BE of the core electrons. BE increases as the oxidation state becomes more positive. This chemical shift can be explained by assuming that the attraction of the nucleus for a core electron is diminished by the presence of outer electrons. When one of these electrons removed the effective nuclear charge sensed for the core electron is increased so BE increases. Important application: Identification of the oxidation states of the elements for different kinds of inorganic compounds.

Chemical shifts and structure

The shift in binding E can be rationalized by taking into the effect of the various functional groups on the effective nuclear charge experienced by the 1s core electrons. As the electron nativity of the attached functional group increases it will withdraw electron density from carbon so the effective nuclear charge increases and the BE increases. XPS provides also the relative number of each type of atoms present in a compound. Example: The nitrogen 1s spectrum for sodium azide (Na+N3-) is made up of two peaks having relative areas in the ratio of 2:1 corresponding to the two end nitrogens and the center nitrogen respectively. Notes: The phtoelectrons in XPS are incapable of ing through more than 10-50 A0 of a solid. Thus the most important applications of electron spectroscopy are for surfaces. Examples of uses: Identification of active sites and poisons on catalytic surfaces. Determination of surface contaminants on semiconductors. Analysis of the composition of human skin Study of oxide surface layers on metal alloys. -

The XPS method has substantial potential in the elucidation of chemical structure. Its ability to distinguish among oxidation states of an element. Quantitative Applications: For determination of the elemental composition of various inorganic and organic materials. Both peak intensities and peak areas have been used as the analytical parameter as a function with concentration. Assumption that the surface composition and bulk are the same is Not always correct.

Chemical Shifts The exact binding energy of an electron depends not only upon the level from which photoemission is occurring, but also upon : 1) The formal oxidation state of the atom. 2) The local chemical and physical environment. Changes in either (1) or (2) give rise to small shifts in the peak positions in the spectrum - so-called chemical shifts Atoms of a higher positive oxidation state exhibit a higher binding energy due to the extra columbic interaction between the photo-emitted electron and the ion core.

Where do Binding Energy Shifts Come From? Or element or compound identification.

Elemental Shifts

Electronic Effect Spin-Orbit Splitting or Spin-Orbit Coupling  Some electronic levels (most obviously 3p and 3d ) give rise to a closely spaced doublet, Which appear when spectra expanded.  Permitted J values = L ± S  Coupling between L: The Angular Q.N., S: Unpaired Spin  The lowest energy final state is the one with maximum J (more than half full)  The relative intensities of the two peaks reflects the degeneracies of the final states (gJ = 2J + 1),  2D 5/2: gJ = 2x{5/2}+1 = 6 (lower B.E)  2D 3/2: gJ = 2x{3/2}+1 = 4 (higher B.E)  These two values determines the probability of transition to such a state during photoionization

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The inner core electronic configuration of the initial state of the Pd is: (1s)2(2s)2(2p)6(3s)2(3p)6(3d)10 .... with all sub-shells completely full. The removal of an electron from the 3d sub-shell by photo-ionization leads to a (3d)9 configuration for the final state - since the d-orbitals (l = 2) have non-zero orbital angular momentum, there will be coupling between the unpaired spin and orbital angular momenta. Spin-orbit coupling is generally treated using one of two models which correspond to the two limiting ways in which the coupling can occur these being the LS (or Russell-Saunders) coupling approximation and the j-j coupling approximation. If we consider the final ionized state of Pd within the Russell-Saunders coupling approximation, the (3d)9 configuration gives rise to two states (ignoring any coupling with valence levels) which differ slightly in energy and in their degeneracy ... 2D 5/2 g J = 2x{5/2}+1 = 6 2D 3/2 g J = 2x{3/2}+1 = 4 These two states arise from the coupling of the L=2 and S=1/2 vectors to give permitted J values of 3/2 and 5/2. The lowest energy final state is the one with maximum J (since the shell is more than half full), i.e. J = 5/2, hence this gives rise to the "lower binding energy" peak. The relative intensities of the two peaks reflects the degeneracies of the final states (g J = 2J+1), which in turn determines the probability of transition to such a state during photoionization.

s-Orbital

p-Orbital

d-Orbital

f-Orbital

Chemical Shifts-Electronegativity Effects

Chemical Shifts- Electronegativity Effects

C1s envelope has been resolved into five components of polystyrene surface exposed to an oxygen plasma.

Applications of X-ray Photoelectron Spectroscopy (XPS)

XPS Analysis of Pigment from Mummy Artwork

Analysis Carbon Fiber- Polymer Composite Material by XPS

Analysis of Materials for Solar Energy Collection by XPS Depth ProfilingThe amorphous SiC/SnO /SnO2 Interface

Polysiloxane Immobilized Ligand System O

O O

CH2

C O C2H5 + H2N(CH2)2NH2

N

Si

CH2

O

C O C2 H 5

Reflux/ Toluene - C2H5OH

O O O O O

Si

CH2

C

NH

NH2

CH2

C NH

NH2

N

O

XPS Results System

Element

Precursor

Core-line

 

%Composition

Product

Core-line

 

%Composition

C

Si

O

N

C1s

Si2p

O1s

N1s

285

102

532

399.5

38.8

17

41.4

2.85

C1s

Si2p

O1s

N1s

285

102

532

399.5

42.4

19.3

28.7

9.6

Conclusion 

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XPS is a powerful technique for characterizing solid surfaces. All types of inorganic solids can be analyzed. Elemental (except H, He) and chemical analyses within a depth of 10 nm. Quantitative technique. Extremely useful for surface treatment of materials.

Auger Electron Spectroscopy AES -

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Auger Electron Spectroscopy (Auger spectroscopy or AES) was developed in the late 1960's. Deriving its name from the effect first observed by Pierre Auger, a French Physicist, in the mid-1920's. It is a surface specific technique utilizing the emission of low energy electrons in the Auger process It is one of the most commonly employed surface analytical techniques for determining the composition of the surface layers of a sample.

Auger Electron Spectroscopy Is based upon a two-step process: First Step: (Ionization) Involves formation of an electronically excited ion A+* by exposing the analyte to a beam of electrons or sometimes X-rays. With X-rays: A + hν → A+* + eWith an electron beam: A + e-i → A+* + e’-i + e-A Where: e-i represents an incident electron from the source. e’-i represents the same electron after it has interacted with A and has thus lost some of its energy. e-A represents an electron that is ejected from one of the inner orbitals of A.

Energy levels in an isolated, multi-electron atom

Alternative nomenclature on the left that is used in Auger spectroscopy.

Electronic Structure - Solid State 

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In the solid state the core levels of atoms are little perturbed and essentially remain as discrete, localized (i.e. atomic-like) levels. The valence orbitals, however, overlap significantly with those of neighboring atoms generating bands of spatially-delocalized energy levels. The energy level diagram for the solid is therefore closely resemblant of that of the corresponding isolated atom, except for the levels closest to the vacuum level. The diagram below shows the electronic structure of Na metal:

The Auger Process

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The Auger process is initiated by creation of a core hole - this is typically carried out by exposing the sample to a beam of high energy electrons (typically having a primary energy in the range 2 10 keV). Such electrons have sufficient energy to ionize all levels of the lighter elements, and higher core levels of the heavier elements. Ionization is shown to occur by removal of a K-shell electron, but in practice such a crude method of ionization will lead to ions with holes in a variety of inner shell levels. In some studies, the initial ionization process is instead carried out using soft x-rays ( hν = 1000 - 2000 eV ). In this case, the acronym XAES is sometimes used. However, this change in the method of ionization has no significant effect on the final Auger spectrum.

Relaxation & Auger Emission Relaxation of the excited ion A+* can occur in either two ways. 2- X-ray Fluorescence: A+* → A+ + hνf hνf : Fluorescence photon The second process recognized as X-ray fluorescence. The energy of the Fl radiation hνf is independent of the excitation E. So polychromatic radiation can be used. 2- Auger emission: A+* → A++ + e-A e-A : Auger electron The E given up in relaxation results in the ejection of an electron (auger electron) with KE, Ek The energy of the auger electron is independent of the E of the photon or electron that originally created the vacancy in E level Eb. So mono-energetic source is not required for excitation. So it can be distinguished from XPS by this point.

KE of the auger electron is the difference between the E released in relaxation of the excited ion (Eb- E’b) and the energy required to remove the second electron from its orbit (E’b). Ek = (Eb – E’b) –E’b = Eb – 2E’b Auger emissions are described in of the type of orbitals involved in the production of the electron. Example: KLL Auger transition involves an initial removal of a K electron followed by a transition of an L electron to the K orbital with the simultaneous ejection of a second L electron. Other common transitions: LMM and MNN XPS Spectra: Consist of a few characteristic peaks lying in the region of 20 t0 1000 eV.

Auger emission

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In previous example, one electron falls from a higher level to fill an initial core hole in the K-shell. The energy liberated in this process is simultaneously transferred to a second electron. A fraction of this energy is required to overcome the binding energy of this second electron. The remainder is retained by this emitted Auger electron as kinetic energy. In the Auger process illustrated, the final state is a doubly-ionized atom with core holes in the L1 and L2,3 shells. We can make a rough estimate of the KE of the Auger electron from the binding energies of the various levels involved. In this particular example, KE = ( EK - EL1 ) - EL23 Note : the KE of the Auger electron is independent of the mechanism of initial core hole formation. The expression for the energy can also be re-written in the form : KE = EK - ( EL1 + EL23 )

It should be clear from this expression that the latter two energy could be interchanged without any effect i.e. it is actually impossible to say which electron fills the initial core hole and which is ejected as an Auger electron ; they are indistinguishable. An Auger transition is therefore characterized primarily by : the location of the initial hole  the location of the final two holes  although the existence of different electronic states () of the final doubly-ionized atom may lead to fine structure in high resolution spectra.  When describing the transition, the initial hole location is given first, followed by the locations of the final two holes in order of decreasing binding energy.  i.e. the transition illustrated is a KL1L2,3 transition .  If we just consider these three electronic levels there are clearly several possible Auger transitions : specifically,  K L1 L1, K L1 L2,3 , K L2,3 L2,3 

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In general, since the initial ionization is non-selective and the initial hole may therefore be in various shells, there will be many possible Auger transitions for a given element - some weak, some strong in intensity. AUGER SPECTROSCOPY is based upon the measurement of the kinetic energies of the emitted electrons. Each element in a sample being studied will give rise to a characteristic spectrum of peaks at various kinetic energies.

This is an Auger spectrum of Pd metal - generated using a 2.5 keV electron beam to produce the initial core vacancies and hence to stimulate the Auger emission process. The main peaks for palladium occur between 220 & 340 eV. The peaks are situated on a high background which arises from the vast number of so-called secondary electrons generated by a multitude of inelastic scattering processes.

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Auger spectra are also often shown in a differentiated form : the reasons for this are partly historical, partly because it is possible to actually measure spectra directly in this form and by doing so get a better sensitivity for detection. The plot below shows the same spectrum in such a differentiated form.

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Like XPS Auger spectra consist of a few characteristic peaks lying in the region of 20-1000 eV. The derivative of the counting rate as a function of the KE of the electron dN(E)/dE serves as the ordinate. Derivative spectra are standard for Auger spectroscopy in order to enhance the small peaks and repress the effect of the large but slowly changing, scattered electron background radiation. Well separated peaks obtained (useful for qualitative identification).

Comparison between XPS and AES - Auger electron emission and XPS are competitive processes and their relative rates depend upon atomic number of the element involved. Elements of high Z favor XPS (not sensitive for Z < 10). Elements of low Z favor AE. Both AES and XPS are complementary, the two techniques involved in a single instrument. Auger is of particular strength because of: 1- Its sensitivity to atoms of low Z. 2- Its minimal matrix effects. 3- Its high spatial resolution (detailed exam. Of solid surfaces). This arises because the primary beam is electrons ( more tightly focused on surfaces) than X-rays. Poor information for oxidation state obtained in case of AES. Difficulties in quant. analysis in case of AES.

Instrumentation of AES 

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Is similar to that for XPS except that the source is usually an electron gun rather than an X-ray tube. This source consists of a heated tungsten filament (0.1 mm d) and bent into V-shape tip. The cathodic filament is maintained at V of 1-50 kV with respect to the anode contained in the gun. Wehnelt cylinder surround the filament (-ve). The Electric Field causes electrons to converge on a tiny spot called crossover (of diameter d0) Cathodes contracted from LaB6 rods also used in electron guns (expensive source and requires better vacuum system to prevent oxide formation the decrease the efficiency). Third type: field emission W or C cathode of very sharp tip (100 nm or less). Held at high potential with intense E.F. at the tip (> 107 v/cm). Provides a beam of electrons of crossover diameter of only 10 nm (in case of W = 50 µm and LaB6 = 10 µm). Disadv: Fragility and requires better vacuum.

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Electron guns produce a beam of electrons with e of 1-10 KeV that focused on the surface of a sample. AES is of very high spatial-resolution scanning of solid surfaces. Normally electron beams with diameter ranging from 500 to 5 µm are used for this purpose. Guns producing beams of ≈ µm are called auger microprobes used to detect and determine the elemental composition of inhomogeneties

Applications of AES Qualitative Analysis of Solid Surfaces: - Auger spectra obtained by bombardment a small area of a surface ( diam 5-500 µm) with a beam of electrons from a gun). A derivative electron spectrum is obtained with an analyzer. Advantages: The low energy auger electrons (20-1000 eV) are able to penetrate only a few atomic layers (3-20 A) of solid. Thus electrons penetrate greater dept below sample surface but only these auger electrons from the first four or five atomic layers escape to reach the analyzer. (AES reflects the surface comp. of solids).

Depth Profiling of Surfaces: - That involves determination of the elemental composition of a surface as it is being etched away (sputtered) by a beam of argon ions. Either XPS or Auger spectroscopy can be used for elemental detection but AES is the more common. The microprobe (of d 5 µm) and etching beams are operated simultaneously. Intensity of one or more of auger peaks recorded as a function of time, a depth profile of elemental composition is obtained. Important for: Corrosion chemistry, catalyst behavior and properties of semiconductor junctions.

Line Scanning 

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Are used to characterize the surface composition of solids as a function of distance along a straight line of 100 µm or more. Auger microprobe is used that produces a beam that can be moved across a surface in a reproducible way.

Secondary ion Mass Spectrometry (SIMS) SIMS is the most highly developed of the mass spectrometric surface methods.  Proven useful for determining both atomic and molecular composition of solid surfaces. Two types: 1- Secondary-ion mass analyzers. 2- Microprobe analyzers. Both are based upon bombarding the surface of the sample with a beam of 5-20 keV ions such as Ar+, Cs+, N2+, or O+2. 

The ion beam is formed in an ion gun in which the gaseous atoms or molecules are ionized by an electron impact source. The positive ions are then accelerated by applying a high dc potential. The impact of these primary ion causes the surface layer of atoms of the sample to be stripped (sputtered off) largely as neutral atoms. A small fraction however forms positive or negative secondary ions that are drawn into a spectrometer for mass analysis.

Secondary mass analyzers: Serves for general surface analysis and depth profiling. The primary ion beam diameter ranges from 0.3-5 mm. Double-focusing, single focusing, time-of-flight and quadruple spectrometers are used for mass determination. These spectrometers yield qualitative and quantitative information about all the isotopes (hydrogen through uranium) present on surface. Sensitivities of 10-15 g or better are typical. Concentration profiles can be obtained with depth resolution of 50 to 100 Ao.

Ion Microprobe Analyzers 

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Are more sophisticated and more expensive instruments that are based upon a focused beam of primary ions that has a diameter of 1 to 2 µm. This beam can be moved across a surface for about 300 µm in both the x and y directions. A microscope is provided to permit visual adjustment of the beam position. Mass analysis is performed with a double-focusing spectrometer. The ion microprobe version of SIMS permits detailed studies of solid surfaces.

Laser-Microprobe Mass Spectrometry

For solid surface studying.  Ionization and volatilization are accomplished with pulsed neodymium-YAG laser, which after frequency quadrupling, produces a 0.5 µm spot of 266nm radiation.  The power density of the radiation within this spot is 1010 to 1011 W/cm2.  Attenuation of the power of the beam to 1% by means of a 25-step optical filter.  A second lower power He-Ne laser (λ = 633 nm) is collinear with the ionization beam (illumination beam) so area chosen visually. Advantages:  Sensitivity is high (< 10-20).  Applicable to both inorganic and organic (including biological) samples.  Resolution is about 1 µm.  Produces data at rapid rate. Applications: Determination of Na/K conc. ratios in frog nerve fiber. Determination of calcium distribution in retinas. Classification of asbestos and coal mine dusts. Determination of Fluorine distributions in dental hard tissue. Analysis of amino acids. Study of polymer surfaces. 

Electron microprobe Electron microprobe analysis (EMPA) is a non-destructive method for determining the chemical composition of tiny amounts of solid materials.  X-ray emission is stimulated on the surface of the sample by a narrow focused beam of electrons.  The resulting X-ray emission is detected and analyzed with either a wavelength or an energy dispersive spectrometer. Instrumentation The instrument employs three integrated beams of radiation (electron, light and Xray) and < 10 an system is required and λ or E-dispersive X-ray spectrometer.  An electron source, commonly a W-filament cathode referred to as a "gun."  A series of electromagnetic lenses located in the column of the instrument, used to condense and focus the electron beam (0.1- 1 µm OD) emanating from the source; this comprises the electron optics and operates in an analogous way to light optics.  A sample chamber, with movable sample stage (X-Y-Z), that is under a vacuum to prevent gas and vapor molecules from interfering with the electron beam on its way to the sample; a light microscope allows for direct optical observation of the sample.  X-ray produced by the electron beam are collimated. 

A variety of detectors arranged around the sample chamber that are used to collect x-rays and electrons emitted from the sample.

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Electrons are produced by heating a filament similar to the filament in a light bulb (A). These electrons are then formed into a beam by accelerating them down a column at very high voltages, typically 15 to 20 thousand volts. The electrons through lenses that condense the beam (B), remove aberrations (C) and focus the beam (D). When the electrons arrive at the sample (E) the beam is focused into a spot much smaller than 0.001 millimeter in diameter. Upon entering the sample, the electrons interact with the atoms in the sample in what is called the interaction volume, causing X-rays to be produced. Each element produces X-rays with characteristic energies. These X-rays can then be counted by reflecting them through a crystal (F) and sending them on to a detector (G). By counting the X-rays generated by each element in the sample and comparing that number to the number of X-ray generated by a standard of known composition, it is possible to determine the chemical composition of a spot one one-thousandth of a millimeter in diameter with great accuracy.

Applications Provides a wealth information about the physical and chemical nature of surfaces (Quantitative and Qualitative analysis).  Phase studies in metallurgy and ceramics.  Investigation of grain boundaries in alloys.  Measurement of diffusion rates of impurities in semiconductors.  Determination of occluded species in crystals.  Study of the active sites of heterogeneous catalysts.

Scanning Electron Microscopy Great importance of studying of physical nature of solid surfaces in many fields (chemistry, material science, geology and biology). Classical methods: Is the optical microscopy, but of limitted resolution because of diffraction effects to about the wavelength of light. Higher resolution techniques (Three types) 1- Scanning electron microscopy (SEM). 2- Scanning tunneling microscopy (STM). 3- Atomic force microscopy (AFM). Basics: The surface of the solid is swipt in a raster pattern with finely focused beam of electrons or with a suitable probe. The raster is a scanning pattern similar to cathode-ray tube in TV: 

Raster (Scanning pattern) 1- Swept across a surface in a straight line (X-direction). 2- Returned to its starting position. 3- Shifted downward (y direction) by a standard increment. 4- Repeating the process until a desired area of the surface has been scanned. 5- During scanning process a signal is received above the surface (Z direction) and stored in a computer system which converted to an image.

Why Study Surfaces? 

Surface – the interface between two of matter’s common phases: • • • • • •

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Solid-gas (we will primarily focus on this) Solid-liquid Solid-solid Liquid-gas Liquid-liquid The majority of present studies are applied to this type of system, and the techniques available are extremely powerful

The properties of surfaces often control chemical reactions

Microscopy 

Why is microscopy useful? the analytical chemist? • • • •

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What can it tell

Sample topography Structural stress/strain Electromagnetic properties Chemical composition

Plus - a range of spectroscopic techniques, from IR to X-ray wavelengths/energies, have been combined with microscopy to create some of the most powerful analytical tools available…

Imaging Resolution and Magnification Some typical values for microscopic methods: Magnificati Method

Resolution

Human Eye

0.1-0.2 mm

on (x) -

Optical Microscopy Electron Microscopy Probe Microscopy

0.1-0.2 um

~1200

30-50 Å

10-75,000

<1 Å

> 500,000

Optical Microscopy - History 

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An ancient technique – the lens has been around for thousands of years. Chinese tapestries dating from 1000 B.C. depict eyeglasses. In 1000 A.D., an Arabian mathematician (Al Hasan) made the first theoretical study of the lens. Copernicus (1542 A.D.) made the first definitive use of a telescope. As glass polishing skills developed, microscopes became possible. John and Zaccharias Jannsen (Holland) made the first commercial and first compound microscopes. Then came lens grinding, Galileo, the biologists, and many great discoveries….

Modern Optical Microscopy in Chemistry 

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As optical microscopy developed, the compound microscope was applied to the study of chemical crystals. The polarizing microscope (1880): can see boundaries between materials with different refractive indices, while also detecting isotropic and anisotropic materials.

http://www.microscopyu.com/articles/polarized/polarizedintro.html

Optical Microscope Design 

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Microscope design has not changed much in 300 years • But the lenses are more perfect – free of aberrations Objective lenses are characterized NA (numerical apertures) • The numerical aperture of a microscope objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance • Large NA = finer detail = better light gathering

Diagram from Wikipedia (public domain)

http://www.microscopyu.com/articles/polarized/polarizedintro.html

Electron Microscopy (EM) Scanning electron microscopy (SEM) – an electron beam is scanned in a raster pattern with a beam of energetic electrons and “reflected” effects are monitored. Several types of signals are produced from a surface in this process including: 1- Backscattered electrons. 2- Secondary electrons. (Serve the basis of scanning electron microscopy) 3- Auger electrons. 4- X-ray fluorescence photons (used in electron microprobe analysis). 5- Other photons of various energies. All of these signals have been used for surface studies 

Ice crystals optical

SEM

Electron Microscopy: Resolution 

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Why can an electron microscope resolve things that are impossible to discern with optical microscopy? Example – calculate the wavelength of electrons accelerated by a 10 kV potential: 1 2

mv 2 = eV

2eV v= m

λ=

h m h λ= = m 2eV 2meV 6.63 ×10 −34 J s

Note: Resolution is limited by lens aberrations!

2(9.11×10-31 kg )(1.60 ×10-19 C)(10 4 V)

λ = 1.23 ×10 −11 m = 0.0123 nm = 0.123 x 10-3 µm. So EM can see >10000x more detail than visible light!

Electron Microscopy: Resolution 

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What about relativistic corrections? The electrons in an EM can in some cases be moving pretty close to the speed of light. Example – what is the wavelength for a 100 kV potential?

Using the relativistically corrected form of the previous equation:

λ=

λ=

h m h = m 2eV 2meV (1 + 2eV ) mc 2 6.63 ×10 −34 J s

2(9.11×10 kg )(1.60 ×10 C)(10 V)(1 + -31

-19

4

(1.60×10 -19 C)(10 4 V ) 2 ( 9.11×10 -31 kg) ( 3×108 m / s ) 2

λ = 3.7 ×10 −3 nm

At high potentials, EM can see atomic dimensions

)

Electron Microscopy: Sample-Beam Interactions 

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Sample-beam interactions control how both SEM and TEM (i.e. STEM) operate: • Formation of images • Spectroscopic/diffractometric analysis There are lots (actually eight) types of sample-beam interactions (which can be confusing and hard to !) It helps to classify these 8 types into two classes of sample-beam interactions: • bulk specimen interactions (bounce off sample – “reflected”) • thin specimen interactions (travel through sample-“transmitted”)

SEM: Sample-Beam Interactions Backscattered Electrons (~30 keV) 

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Caused by an incident electron colliding with an atom in the specimen which is almost normal to the incident electron’s path. The electron is then scattered "backward" 180 degrees. Backscattered electron intensity varies directly with the specimen's atomic number. This differing production rates causes higher atomic number elements to appear “brighter” than lower atomic number elements. This creates contrast in the image of the specimen based on different average atomic numbers. Backscattered electrons can come from a wide area around the beam impact point (see pg. 552 of Skoog) – this also limits the resolution of a SEM (along with abberations in the EM lenses)

SEM: Sample-Beam Interactions+ Secondary Electrons (~5 eV) 

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Caused by an incident electron ing "near" an atom in the specimen, close enough to impart some of its energy to a lower energy electron (usually in the Kshell). This causes a slight energy loss, a change in the path of the incident electron and ionization of the electron in the specimen atom. The ionized electron then leaves the atom with a very small kinetic energy (~5 eV). One incident electron can produce several secondary electrons. Production of secondary electrons is closely linked to sample topography. Their low energy (~5 eV) means that only electrons very near to the surface (<10 nm) are detected. They also don’t suffer from the backscattered electron lateral resolution problem depicted in Fig. 21-16 of Skoog. Changes in topography in the sample that are larger than this sampling depth can change the yield of secondary electrons via indirect effects (called collection efficiencies).

SEM: Sample-Beam Interactions Auger Electrons (10 eV – 2 keV) 

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Caused by relaxation of an ionized atom after a secondary electron is produced. The lower (usually K-shell) electron that was emitted from the atom during the secondary electron process has left a vacancy. A higher energy electron from the same atom can drop to a lower energy, filling the vacancy. This leaves extra energy in the atom which can be corrected by emitting a weakly-bound outer electron; an Auger electron. Auger electrons have a characteristic energy, which is unique and depends on the emitting element. Auger electrons have relatively low energy and are only emitted from the bulk specimen from a depth of several angstroms.

SEM: Sample-Beam Interactions X-ray Emission 

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Caused by relaxation of an ionized atom after a secondary electron is produced. Since a lower (usually K-shell) electron was emitted from the atom during the secondary electron process an inner (lower energy) shell now has a vacancy. A higher energy electron can "fall" into the lower energy shell, filling the vacancy. As the electron "falls" it emits energy in the form of X-rays to balance the total energy of the atom. X-rays emitted from the atom will have a characteristic energy which is unique to the element from which it originated. X-ray (elemental) mapping of sample surfaces is a common applications and a very powerful analytical approach.

SEM: Sample-Beam Interactions – X-rays

SEM: Sample-Beam Interactions Cathodoluminescence (CL)  Caused by electron hole pairs, which are created by the electron beam in certain kinds of materials. When the pairs recombine, cathodoluminescence (CL) can result. CL is the emission of UV-Visible-IR light by the recombination effect. CL is usually very weak and covers a wide range of wavelengths, and requires high beam currents, lowering resolution and challenging detector systems!  CL signals typically result from small impurities in an otherwise homogeneous material, or lattice defects in a crystal.  CL can be used effectively for some analytical problems. Some “random” examples: • Differentiation of anatase and rutile • Studying ferroelectric domains in sodium niobate • Location of subsurface crazing in ceramics • Forensic analysis of glasses

TEM: Sample-Beam Interactions (Thin Sample)

Unscattered Electrons 

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Incident electrons which are transmitted through the thin specimen without any interaction occurring inside the specimen. Used to image - the transmission of unscattered electrons is inversely proportional to the specimen thickness. Areas of the specimen that are thicker will have fewer transmitted unscattered electrons and so will appear darker, conversely the thinner areas will have more transmitted and thus will appear lighter.

TEM: Sample-Beam Interactions (Thin Sample) Elastically-Scattered Electrons  Incident electrons that are scattered (deflected from their original path) by atoms in the specimen in an elastic fashion (without loss of energy). These scattered electrons are then transmitted through the remaining portions of the specimen.  Electrons follow Bragg's Law and are diffracted. All incident electrons have the same energy (and wavelength) and enter the specimen normal to its surface. So all incident electrons that are scattered by the same atomic spacing will be scattered by the same angle. These "similar angle" scattered electrons can be collated using magnetic lenses to form a pattern of spots; each spot corresponding to a specific atomic spacing, This pattern can then yield information about the orientation, atomic arrangements and phases present in the area being examined.

TEM: Sample-Beam Interactions (Thin Sample) Inelastically-Scattered Electrons Incident electrons that interact with sample atoms inelastically (losing energy during the interaction). These scattered electrons are then transmitted through the rest of the sample. Inelastically scattered electrons have two uses: 1. Electron Energy Loss Spectroscopy (EELS): The amount of inelastic loss of energy by the incident electrons can be used to study the sample. These energy losses are unique to the bonding state of each element and can be used to extract both compositional and bonding (i.e. oxidation state) information on the sample region being examined. 2. Kakuchi bands: Bands of alternating light and dark lines caused by inelastic scattering, which are related to interatomic spacing in the sample. These bands can be either measured (their width is inversely proportional to atomic spacing) or used to help study the elasticity-scattered electron pattern

Electron Microscopy: Basic Design

Basic layout of an electron microscope: Electron gun (1-30 keV)

Computer

Magnetic lenses and scanning coils

Detectors electrons photons

Sample electrons Detectors

Electron Source (Gun) Thermionic electron gun – how it works: 

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Positive electrical potential applied to the anode The filament (cathode) is heated until a stream of electrons is produced The electrons are then accelerated by the positive potential down the column (can be up to 30 kV) A negative electrical potential (~500 V) is applied to the Wehnelt cap Electrons are forced toward the column axis by the Wehnelt cap Electrons collect in the space between the filament tip and Wehnelt cap (a space charge or “pool”) Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in the Wehnelt cap These electrons then move down the column towards the EM lens and scanning systems

The results: - Electrons are emitted from a nearly perfect point source (the space charge) - The electrons all have similar energies (monchromatic) - The electrons will travel parallel to the column axis

Electron Optics: Focusing and Scanning 

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Apertures – usually made of platinum foil, with circular holes of 2 to 100 µm. The magnetic condenser and objective lens serve to reduce the image to final spot size on sample (5-200 µm). The condenser lens system (one or more lenses) to accelerate electrons to reach the objective lens. The objective lens is responsible for the size of electron beam impinging on surface of sample. The individual lens is cylindrically symmetrical and of 10-15 cm in height.

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Magnetic lenses: Circular electro-magnets capable of projecting a precise circular magnetic field in a specified region. The field acts like an optical lens, having the same attributes (focal length, angle of divergence...etc.) and errors (spherical aberration, chromatic aberration....etc.). They are used to focus and steer electrons in an EM (SEM and STEM). Goal – a focused, monochromatic (I.e. same energy/wavelength) electron beam!

Scanning with SEM 

Accomplished by the two pairs of electromagnetic coils within the objective lens.

One pair deflects the beam in the X direction across the sample.  The other pair deflects it in the y direction.  By applying an electrical signal to one pair of the scan coils and varying the electrical signal (x coils) as a function of time, the electron beam is moved in a straight line across the sample and then returned to its original position.  After completion of the line scan, the other set of coils (y coil in this case) is used to deflect the beam slightly and the deflection of the beam using the x coils is repeated. By rapidly moving the beam the entire sample surface can irradiated with the electron beam. The signals to the scan coils can be either analog or digital (reproducible movement and location of the electron beam) The output signal from the sample can be encoded and stored in the digital form along with digital representations of the x and y positions of the beam. The signals are also used to drive the horizontal and vertical scans of a cathode-ray tube (CRT). 

The image of the sample Produced by using the output of a detector to control the intensity of the spot on CRT. This method of scanning produces a map of the sample in which there is one-to-one correlation between the signal produced at a particular location on the sample surface and corresponding point on the CRT display. Magnification (M) achievable in the SEM image is given by M = W/w Where W is the width of the CRT display and w is the width of the signal line scan across the sample. W is constant so magnification is achieved by decreasing w. So focusing the electron beam to infinity small point could provide infinite magnification. But variety of factors limit M to a range of 10X to 100,000X. 

Sample and sample Holder Sample chambers are designed for rapid changing of samples.  Large capacity vacuum pumps are used to hasten the switch from ambient P to 10-4 torr or less.  Sample holder is capable of holding samples many cm on an edge.  It can be moved in the x, y and z directions and can rotated about each axis. Samples that conduct electricity are easiest to study, because the unimpeded flow of electrons to ground minimizes artifacts associated with buildup of charge. It also conduct heat which minimizes the likelihood of their thermal degradation. (biological and most minerals samples do not conduct. A variety of techniques have been developed for obtaining SEM images of non conducting samples. Most common approaches involve coating the surface of the sample with thin metallic film produced by sputtering or by vacuum evaporation 

Transducers scintillation devices: most common type: The detector consists of a doped glass or plastic target that emits a cascade of visible photons when struck by an electron. The photons are conducted by a pipe to a photomultiplier tube that is housed outside the high-vacuum region of the instrument. Gains 105-106. Semiconductor transducers: (flat wafers of semiconductor material) are also used in electron microscopy. When high E electron strikes the detector, electron-hole pairs are produced that result in increased conductivity. Current gains are 103-104 The device is small enough so that it can be placed immediately adjacent to the sample (leads to high collection efficiency). Easy to use and are less expensive than scintillation transducers. X-rays produced: Usually detected and measured with E-dispersive systems.

Applications -

-

Provides morphologic and topographic information about the surfaces of solids that usually necessary in understanding the behavior of surfaces. First step in the study of the surface properties of a solid.

Electron Microscopy: Overall Design TEM design is similar – however, nowadays, TEM systems usually include a “cryo-stage” for keeping samples extremely cold during analysis

Transmission Electron Microscopy: Applications 

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Morphology • The size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters. Crystallographic Information • The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter • We will discuss this topic further during the crystallography lecture Compositional Information • The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter

Scanning Probe Microscopy (SPM)  

SPM, also known as profilimetry Are capable of resolving details of surfaces down to the atomic level.

Types: 1- The scanning tunnelling microscope: (primary use is measuring surface topography of samples). Unlike Optical and electron micropropes SPM reveal details not on the lateral x and y of a sample but also on the z axis which is perpendicular to the surface. Resolution: SPMs is ≈ 20 a in the x and y direction bu for ideal samples best resolution is 1 A. R in z direction is better than 1 A. SEMs is about 50 A.

The scanning tunneling microscope Capable of resolving features on an atomic scale on the surface of conducting solid surface. Main disadvantages: The requirement that the surface being examined must conduct electricity. Principles: The surface of the sample is scanned in a raster pattern by a very fine metallic tip. The tip is maintained at a constant distance d above the surface throughout the scan. The up and down motion of the tip then reflects the topology of the surface. In order to maintain the tip at constant distance from the sample surface, a tunneling current between the tip and the sample is monitored and held at a constant level. The TC is generated by the voltage V that is applied between the tip and sample.

A TC that es through a medium that contains no electrons: Examples of such media: Vacuum, nonpolar liquid, or even an aqueous electrolyte solution. Mechanism of current flow through an insulating medium: Rationalized by quantum mechanics but will not be dealt here. Tunneling currents become significant when conductors are within a few nanometers of one another and when one of conductors is the form of a sharp tip. Scanning Tunneling current It is given approximately by: 

It = Ve-cd Where V is the bias voltage between the conductors, C is a constant that is characteristic of conductor composition D is the capacity between the lowest atom on the tip and the highest atom on the sample. In the tunneling microscope current is held constant by a mechanism tht moves the tip up and down so d remains constant. Controlled by a piezoelectric transducer.

Scanning Tunnelling Microscopy (STM) Rastering

Piezo actuators Z DC bias

X

control electronics

computer

Y

display tunnel current amp

Constant current imaging: A loop adjusts the separation between tip and sample to maintain a constant current. The voltages applied to the piezo are translated into an image. Image represents a convolution of topography and electronic structure

1/8 in

Besocke-beetle style STM head Slide courtesy of B. Mantooth and the Weiss Group at Penn State

Scanning Tunnelling Microscopy 

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Tunnelling current is caused by quantum mechanical phenomena (confinement of an electron to a “box” with finite walls) The tunnelling current It is given by:

I t = Ve − Cd   

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Where: V is the bias voltage C is a constant based on the conducting materials d is the spacing between the atom at the tip and the sample atom

Tips are prepared by cutting and electrochemical etching – atomic scale can be achieved because the tunnelling current falls off exponentially with increasing gap.

R. J. Hamers, “Scanned Probe Microscopies in Chemistry,” J. Phys. Chem., 1996, 100, 13103-13120.

The atomic Force Microscope  

Invented in 1986. The Atomic Force Microscope was developed to overcome a basic drawback with STM - that it can only image conducting or semiconducting surfaces. The AFM, however, has the advantage of imaging almost any type of surface, including polymers, ceramics, composites, glass, and biological samples.

Permits resolution of individual atoms on both conducting and insulating surfaces. Procedure: A flexible force-sensing cantilever stylus is scanned in rastyer pattern over the surface of the sample. The force acting between the cantilever and the sample surface causes minute deflections of the cantilever which are detected by optical means. As in STM the motion of the tip or sometimes the sample is achieved with a piezoelectric tube. During a scan the force on the tip is held constant by the up and down motion of the tip, which provides the topographic information. Advantages: It is applicable to nonconducting samples. 

An atomically sharp tip is scanned over a surface with mechanisms that enable the piezo-electric scanners to maintain the tip at a constant force (to obtain height information), or height (to obtain force information) above the sample surface.  The AFM works by scanning a fine ceramic or semiconductor tip over a surface much the same way as a phonograph needle scans a record (for those of you that what a record player was!).  The tip is positioned at the end of a cantilever beam shaped.  As the tip is repelled by or attracted to the surface, the cantilever beam deflects.  A diode laser is focused onto the back of a reflective cantilever.  As the tip scans the surface of the sample, moving up and down with the contour of the surface, the laser beam is deflected off the attached cantilever into a dual element photodiode.  The photodetector measures the difference in light intensities between the upper and lower photodetectors, and then converts to voltage.  from the photodiode difference signal, through software control from the computer, enables the tip to maintain either a constant force or constant height above the sample. In the constant force mode the piezoelectric transducer monitors real time height deviation.  A plot of the laser deflection versus tip position on the sample surface provides the resolution of the hills and valleys that constitute the topography of the surface.  The AFM can work with the tip touching the sample ( mode), or the tip can tap across the surface (tapping mode) much like the cane of a blind person.  The output from the photodiode then controls the force applied to tip so that it remains constant.  The optical control is similar to tunneling-current control system in the STM.

Measuring forces Because the atomic force microscope relies on the forces between the tip and sample, knowing these forces is important for proper imaging. The force is not measured directly, but calculated by measuring the deflection of the lever, and knowing the stiffness of the cantilever. Hook’s law gives F = -kz, where F is the force, k is the stiffness of the lever, and z is the distance the lever is bent.

AFM Modes of operation

The tip and cantilever 

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Performance of AFM depends on the physical charcteristics of the cantilever and tip. Early: cantilevers were cut from metal foil and tips were made from crushed diamond particles. The tip were glued manually to cantilevers. Currently: this crude method replaced by semiconductor production methods in which integral cantilever/tip assemblies are produced by etching single chips of silicon, silicon oxides or silicon nitride. Small and delicate cantilever and tip are suggested. Typically the cantilevers are a few micrometers in width and about one micrometer in thickness. The pyramid-or cone-shaped tips are a few micrometers in height and width at the base.

• • • • • •

Invented in 1986 Cantilever Tip Surface Laser Multi-segment photodetector

Figure 4. Three common types of AFM tip. (a) normal tip  (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall). 

Tapping mode operation Disadvantages of the mode scanning: The tip is in constant (mode) with the surfcae of the sample and the downward force of the tip may not be low enough to avoided damage to sample surface and distortion of image as consequence (specially in soft materials biological samples, polymers. or semi hard: silicon wafers). Overcome by: Tapping mode operation: a process in which the tip s the surface for only a brief time periodically and then removed from the surface. The cantilever is oscilated at frequency of a few hundred kilohertz. The oscillation is driven by a constant driving force and the amplitude is monitored continuously. The cantilever is positioned so that the tip touches the surface only at the bottom of each oscillation cycle. (variety materials images that difficult to image by constant- mode).

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Mode • High resolution • Damage to sample • Can measure frictional forces

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Non- Mode • Lower resolution • No damage to sample

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Tapping Mode • Better resolution • Minimal damage to sample

Applications Seeing surface structures with high resolution. Examples: Semiconductor field. Silicon surfaces and their defects. Magnetic domains on magnetic materials. Biotechnology (DNA, chromatin, protein/enzyme interactions, membrane viruses). Advantages: Permits imaging of biological samples under water without distortion 

DNA image

2.5 x 2.5 nm simultaneous topographic and friction image of  highly oriented pyrolytic graphic (HOPG). The bumps  represent the topographic atomic corrugation, while the  coloring reflects the lateral forces on the tip. The scan  direction was right to left

Comparison of AFM and other imaging techniques

1. AFM versus STM: It's interesting to compare AFM and its precursor -- Scanning Tunneling Microscope. In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is much more complex when characteristics such as tip shape and force are considered. STM is generally applicable only to conducting samples while AFM is applied to both conductors and insulators. In of versatility, needless to say, the AFM wins. Furthermore, the AFM offers the advantage that the writing voltage and tip-to-substrate spacing can be controlled independently, whereas with STM the two parameters are integrally linked. 2. AFM versus SEM: Compared with Scanning Electron Microscope, AFM provides extraordinary topographic contrast direct height measurements and unobscured views of surface features (no coating is necessary). 3. AFM versus TEM: Compared with Transmission Electron Microscopes, three dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available from cross-sectioned samples. 4. AFM versus Optical Microscope: Compared with Optical Interferometric Microscope (optical profiles), the AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials.