New Analytical Methods in Nanotechnology-A Review

Somsubhra Ghosh*, Sowjanya Bomma, V. Laxmi Prasanna, P. Srivani, Dr. David Bhanji

Nalanda College of Pharmacy, Nalgonda, Andhra Pradesh-5008001, India.

*Corresponding Author E-mail: som_subhra_ghosh@yahoo.co.in

ABSTRACT

Nanotechnology may be able to create many new materials and devices with a vast range of applications, such as in medicine, electronics, biomaterials and energy production. On the other hand, nanotechnology raises many of the same issues as any new technology, including concerns about the nanotoxicity and environmental impact of nano materials, and their potential effects on global economics. Like electricity or computers before it, nanotech will offer greatly improved efficiency in almost every facet of life. But as a general-purpose technology, it will be dual-use, meaning it will have many commercial uses and it also will have many military uses making far more power -full weapons and tools of surveillance. Thus it represents not only wonderful benefits for Humanity, but also grave risks. A key understanding of nanotechnology is that it offers not just better products, but a vastly improved manufacturing process. A computer can make copies of data files—essentially as many copies as you want at little or no cost. It may be only a matter of time until the building of products becomes as cheap as the copying of files. That's the real meaning of nanotechnology and why it is sometimes seen as "the next industrial revolution." Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometers. Quantum mechanical effects are important at this quantum-realm scale. In this project a discussion about each method and its principle, applications, and limitations are done in details. These techniques are discussed in detailed further. Applications for the future: Efficient and energy-saving Cube-shaped nanostructures known as metal organic frameworks (MOFs) are the storage medium of the future.

KEYWORDS: Nanotechnology, Analysis, Contamination, characterization, composition

INTRODUCTION:

Nanotechnology is one of the most important emerging technologies worldwide. Through the controlled manufacture and structuring of materials, it allows the creation of completely new properties in product development. Nanotechnology is the engineering of functional systems at the molecular scale, the prefix “nano” (Greek for dwarf) describes a spatial dimension. Now a day’s growth in the micro and nano engineering industry has led to demand for analytical and characterization methods for these materials and system. Materials characterization at increasingly small dimensions is a critical part of many manufacturing industries, including semiconductors, optoelectronics, automotive and aerospace.

Nanomaterials: The nonmaterial’s field includes subfields which develop or study materials having unique properties arising from their nanoscale dimensions. Interface and colloid science has given rise to many materials which may be useful in nanotechnology, such as carbon nanotubes and other fuller -enes, and various nanoparticles and nano rods

.

Nanomaterials with fast ion transport are related also to nanoionics and nanoelectronics. Progress has been made in using these materials for medical applications. Nano scale materials are sometimes used in solar cells, which combats the cost of traditional silicon solar cells development of applications incorporating semiconductor nanoparticles to be used in the next generation of products, such as display technology, lighting, solar cells and biological imaging. Many analytical techniques available today can provide data on nano-engineered materials. Development of analytical tools specifically for nanotechnology is underway, yet use of a new measurement with a new technology may result in many questions or unexpected variables in contrast to trusted data with considerable historical context. Some analytical methods in nanotechnology are, TEM – Transmission Electron Microscopy, XPS-X-ray Photoelectron Spectroscopy, XRD- X-ray Diffraction, AES-Auger Electron.

A. Transmission electron microscopy:

Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de Broglie wavelength of electrons. TEM forms a major analysis method in a range of scientific fields, in both physical and biological sciences. TEMs find application in cancer research, virology, materials science as well as pollution, nanotechnology, and semiconductor research ⁽¹⁾.

Electrons: Theoretically, the maximum resolution, d, that one can obtain with a light microscope has been limited by the wavelength of the photons that are being used to probe the sample, λ and the numerical aperture of the system. Early twentieth century scientists theorized ways of getting around the limitations of the relatively large wavelength of visible light by using electrons. Like all matter, electrons have both wave and particle properties. The wavelength of electrons is found by equating the de Broglie equation to the kinetic energy of an electron. An additional correction must be made to account for relativistic effects, as in TEM an electron’s velocity approaches the speed of light, c.

Where, h is Planck's constant, m₀ is the rest mass of an electron and E is the energy of the accelerated electron ⁽²⁾.

Figure No1: Shows the layout of optical components in a basic TEM.

Source formation: It may be a tungsten filament, or a lanthanum hexa boride (LaB6) source. LaB6 sources utilize small single crystals. By connecting this gun to a high voltage source (100-300 kV) the gun will given sufficient current, begin to emit electrons either by thermionic or field electron emission into the vacuum. This extraction is usually aided by the use of a Wehnelt cylinder. Once extracted, the upper lenses of the TEM allow for the formation of the electron probe to the desired size and location for later interaction with the sample. The interaction of electrons with a magnetic field will cause electrons to move according to the right hand rule, thus allowing for electromagnets to manipulate the electron beam.

Optics: The lenses of a TEM allow for beam convergence, with the angle of convergence as a variable parameter, giving the TEM the ability to change magnification simply by modifying the amount of current that flows through the coil, quadrupole or hexapole lenses. Typically a TEM consists of three stages of lensing. The stages are the condensor lenses, the objective lenses, and the projector lenses. The condensor lenses are responsible for primary beam formation, whilst the objective lenses focus the beam.

Display: Imaging systems in a TEM consist of a phosphor screen, which may be made of fine (10-100 μm) particulate zinc sulphide, for direct observation by the operator. Optionally, an image recording system such as film based or doped YAG screen coupled CCDs ⁽³⁾.

Components: A TEM is composed of several components, which include a vacuum system in which the electrons travel, an electron emission source for generation of the electron stream, a series of electromagnetic lenses, and electrostatic plates to allow the operator to guide and manipulate the beam as required and Imaging devices.

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.

Electron gun: 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 at some given angle, known as the gun divergence semi angle, α. By constructing the Wehnelt cylinder such that it has a higher negative charge than the filament itself, electrons that exit the filament in a diverging manner are, under proper operation, forced into a converging pattern the minimum size of which is the gun crossover diameter.The thermionic emission current density, J, can be related to the work function of the emitting material and is a Boltzmann distribution given below, where A is a constant, Φ is the work function and T is the temperature of the material.

This equation shows that in order to achieve sufficient current density it is necessary to heat the emitter, taking care not to cause damage by application of excessive heat, for this reason materials with either a high melting point, such as tungsten, or those with a low work function (LaB₆) are required for the gun filament ⁽⁴⁾

Electron lens:

Electron lenses are designed to act in a manner emulating that of an optical lens, by focusing parallel rays at some constant focal length. Lenses may operate electro statically or magnetically. The majority of electron lenses for TEM utilise electromagnetic coils to generate a convex lens.

Arrangement of the main components at the TEM:

A great advantage of the transmission electron microscope is in the capability to observe, by adjusting the electron lenses, both electron microscope images (information in real space) and diffraction patterns (information in reciprocal space) for the same region. By inserting a selected area aperture and using the parallel incident beam illumination, we get a diffraction pattern from a specific area as small as 100 nm in diameter. The contrast in these images is attributed to the change of the amplitude of either the transmitted beam or diffracted beam due to absorption and dynamic scattering in the specimens. Thus the image contrast is called the absorption-diffraction, or the amplitude contrast. Amplitude-contrast images are suitable to study mesoscopic microstructures, e.g., precipitates, lattice defects, interfaces, and domains ⁽⁵⁾.Three observation modes in electron microscope using an objective aperture. The center of the objective aperture is on the optical axis.

High Resolution Transmission Electron Microscopy:

The image results from the multiple beam interference (because of the differences in phase of the transmitted and diffracted beams) and is called the phase contrast image. For a very thin specimen and aberration-compensating condition of a microscope, the phase contrast corresponds closely to the projected potential of a structure. For a thicker specimen and less favorable conditions the phase contrast has to be compared with calculated images. Theory of dynamic scattering and phase contrast formation is now well developed for multislice and Bloch wave’s methods. HREM can be used to determine an approximate structural model, with further refinement of the model using much higher resolution powder x-ray or neutron diffraction. Other major advantages in using electron scattering for crystallographic studies is that the scattering cross section of matter for electrons is 103 to 104 larger than for x rays and neutrons, typical wavelengths are one hundredth of those for x rays and neutrons, and the electron beam can be focused to extremely fine probe sizes (1 nm). It means a great sensitivity to small deviations from an average structure caused by ordering, structural distortions, short-range ordering, or presence of defects. Such changes often contribute either very weak superstructure reflections, or diffuse intensity, both of which are very difficult to detect by x-ray or neutron diffraction. In addition, modern transmission electron microscopes provide a number of complementary capabilities known as analytical electron microscopy. ⁽⁶⁾

Study of Phase Transitions:

The essential feature of a structural phase transition is the change of symmetry. Most often it is a low temperature (room temperature) phase that is studied, which has a lower symmetry than the high temperature phase(s). Its space group often is a subgroup of that of the high temperature phase. Therefore, a single crystal (grain) of the high temperature phase at low temperature is subdivided into a number of symmetry-related domains, or structural variants. The variants are either rotational (twins) or translational (anti-phase domains, or APDs), and separated from each other by specific interfaces.

Strengths of TEM Analysis:

The ultimate elemental mapping resolution of any analytical technique. Sub 0.2 nm image resolution. Small area crystallographic information

Limitations:

There are a number of drawbacks to the TEM technique.

1. Many materials require extensive sample preparation to produce a thin sample.

2. Relatively time consuming process.

3. The structure of the sample may also be changed during the preparation process.

4. The field of view is relatively small, raising the possibility that the region analyzed may not be characteristic of the whole sample.

5. There is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials ⁽⁷⁾.

Applications of TEM:

1. The technique was employed in the diagnosis of viral infections such as hepatitis B and parvovirus B19.

2. Electron microscopy has led to the discovery of many new viruses, most notably the various viruses associated with gastroenteritis, for which it remained the principal diagnostic method until fairly recent times

3. Both in the speed of diagnosis and the potential for detecting, by a single test, any viral pathogen or even multiple pathogens present within a sample.

4. Electron microscopy has been used, however, to elucidate the structure and function of many bacterial features, such as flagellae, fimbriae and spores and in the study of bacteriophages.

5. The important advantage of TEM over other characterization techniques is that information can obtain and both from real and reciprocspace.

6. TEM techniques works with very high magnification only a small amount of material is required⁽⁸⁾

B. X-ray photoelectron spectroscopy:

XPS (X-ray Photoelectron Spectroscopy) is based on the principle that X-rays hitting atoms generate photoelectrons.

Figure No 2: Shows Photoelectron spectroscopy uses monochromatic sources of radiation

Figure No 3: Shows the photoionization levels

And it also called as Electron Spectroscopy for Chemical Analysis or ESCA. It is a typical example of a surface-sensitive technique. Only electrons that are generated in the top few atomic layers are detected. In this way quantitative information can be obtained about the elemental composition of the surface of all kinds of solid material (insulators, conductors, polymers). An important strength of XPS is that it provides both elemental and chemical information. ⁽⁹⁾

Basic principle:

Photoelectron spectroscopy is based upon a single photon in/electron out process and from many viewpoints this underlying process is a much simpler phenomenon than the Auger process.

The energy of a photon is given by the Einstein relation

E = h ν

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. By contrast, in UPS the photon interacts with valence levels of the molecule or solid, leading to ionisation by removal of one of these valence electrons.

The kinetic energy distribution of the emitted photoelectrons (i.e. the number of emitted photoelectrons as a function of their kinetic energy) can be measured using any appropriate electron energy analyser and a photoelectron spectrum can thus be recorded.

The process of photoionization can be considered in several ways : one way is to look at the overall process as follows:

A + hν → A+ + e-

Conservation of energy then requires that :

E (A) + hν = 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 = hν - (E (A+ ) - E (A) )

The final term in brackets, representing the difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron - this then leads to the following commonly quoted equation :

KE = hν - BE

An alternative approach is to consider a one-electron model along the lines of the following pictorial representation ; this model of the process has the benefit of simplicity but it can be rather misleading.

The BE is now taken to be a direct measure of the energy required to just remove the electron concerned from its initial level to the vacuum level and the KE of the photoelectron is again given by :

KE = hν - BE

NOTE - the binding energies (BE) of energy levels in solids are conventionally measured with respect to the Fermi-level of the solid, rather than the vacuum level. ⁽¹⁰⁾

The basic requirements for a photoemission experiment (XPS or UPS) are:

1. A source of fixed-energy radiation (an x-ray source for XPS or, typically, a He discharge lamp for UPS)

2. An electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy)

3. A high vacuum environment (to enable the emitted photoelectrons to be analysed without interference from gas phase collisions)

Such a system is illustrated schematically below:

Figure No 4: Shows the electron energy analyser

There are many different designs of electron energy analyser but the preferred option for photoemission experiments is a concentric hemispherical analyser (CHA). ⁽¹¹⁾

Applications:

· Elemental composition of the surface (top 1–10 nm usually)

· Empirical formula of pure materials

· Elements that contaminate a surface

· Chemical or electronic state of each element in the surface

· Uniformity of elemental composition across the top surface (or line profiling or mapping) and uniformity of elemental composition as a function of ion beam etching (or depth profiling)

· Analysis of thin film contamination: characterization of thin layers (< 10 nm) determination of composition and effective layer thickness of multilayer systems (high-k oxide layers on si, self assembled mono layers on a gold substrate, mono layers of biological materials such as proteins, antibodies and DNA

· Evaluation of adhesion failures.

· Measurement of elemental composition of insulating materials (e.g., polymers, glasses)

· Identification of the chemical state of surface films (e.g., metal or oxide)

· Quantitative elemental depth profiling of insulators ^{(12, 13)}

·

C. X-RAY DIFFRACTION:

X-ray diffraction (XRD) is a versatile, non destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials⁽¹⁴⁾.

Generation of X-rays:

When electrons strike a metal anode with sufficient energy, X-rays are produced. This process is typically accomplished using a sealed x-ray tube, which consists of a metal target (often copper metal) and a tungsten metal filament, which can be heated by passing a current through it (typically 10-15 mA), resulting in the “boiling off” of electrons from the hot tungsten metal surface. These “hot” electrons are accelerated from the tungsten filament (negative bias) to the metal target (positive bias) by an applied voltage (typically 15-30 kilovolts). The collision between these energetic electrons and electrons in the target atoms results in electron from target atoms being excited out of their core-level orbitals, placing the atom in a short-lived excited state. The atom returns to its ground state by having electrons from lower binding energy levels (i.e. levels further from the nucleus) make transitions to the empty core levels. The difference in energy between these lower and higher binding energy levels is radiated in the form of X-rays. This process results in the production of characteristic X-rays. Thus X-rays provide a convenient means of determining the elements present in a sample. A lower energy process that involves the interaction of electrons with the nucleus of an atom in the target metal produces a continuum of lower intensity X-radiation over a broad energy range known as Bremstrahllung. As the voltage on an X-ray tube is increased, the characteristic line spectra of the target element are superimposed upon the continuous spectrum (at right). As the spacing between atoms of crystals is on the same order as X-ray wavelengths (1-3 Å), crystals can diffract the radiation when the diffracted beams are in-phase. The Bragg equation is given as nλ = 2dsinθ. For a given wavelength (λ), diffraction can only occur at a certain angle (θ) for a given d-spacing ⁽¹⁵⁾

Diffraction:

Single Crystal (Laue) Diffraction –

A beam of X-rays of all wavelengths is directed at a single crystal, which sits stationary in front of a photographic plate. A series of diffraction spots surround the central point of the beam, corresponding to diffraction from a given series of atomic planes (at right).

Powder Diffraction –

A powder is used to ensure completely random crystal orientation to get diffraction from all possible planes. The diffraction pattern can be recorded on a flat photographic film or on a CRT (cathode ray tube). When the incident beam satisfies the Bragg condition, a set of planes forms a cone of diffracted radiation at an angle θ to the sample. Since the cone of X-rays intersects the flat photographic filmstrip in two arcs equally spaced from the direct X-ray beam, two curved lines will be recorded on the photographic film. The distance of the lines from the center can be used to determine the angle θ, which can then be used to determine the interplanar d spacing. X-ray powder diffractometers record all reflections using a scintillation detector (in counts per second of X-rays). The pattern of diffracted X-rays is unique for a particular structure type and can be used as a “fingerprint” to identify the structure type. Different minerals have different structure types, thus X-ray diffraction is an ideal tool for identifying different minerals.

Figure No 5: Shows at left; powder film of (a) spodumene (LiAlSi₂O₆), (b) aragonite (CaCO₃), (c) feldspar (KAlSi₃O₈) and (d) alpha-quartz (SiO₂).

Figure No 6: Shows at right, a diffraction pattern from a mixture of magnetite and hematite using a XRD powder diffract meter.

Ice:

A crystal lattice is a regular three-dimensional distribution (cubic, rhombic, etc.) of atoms in space. These are arranged so that they form a series of parallel planes separated from one another by a distance d, which varies according to the nature of the material. For any crystal, planes exist in a number of different orientations - each with its own specific d-spacing.

Constructive interference:

When a monochromatic X-ray beam with wavelength lambda is projected onto a crystalline material at an angle theta, diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by a complete number n of wavelengths.

Bragg's Law:

By varying the angle theta, the Bragg's Law conditions are satisfied by different d-spacings in polycrystalline materials. Plotting the angular positions and intensities of the resultant diffracted peaks of radiation produces a pattern, which is characteristic of the sample. Where a mixture of different phases is present, the resultant diffractogram is formed by addition of the individual patterns. Based on the principle of X-ray diffraction, a wealth of structural, physical and chemical information about the material investigated can be obtained. Diffracted waves from different atoms can interfere with each other and the resultant intensity distribution is strongly modulated by this interaction. If the atoms are arranged in a periodic fashion, as in crystals, the diffracted waves will consist of sharp interference maxima (peaks) with the same symmetry as in the distribution of atoms. Measuring the diffraction pattern therefore allows us to deduce the distribution of atoms in a material. The peaks in an x-ray diffraction pattern are directly related to the atomic distances. Let us consider an incident x-ray beam interacting with the atoms arranged in a periodic manner as shown in 2 dimensions in the following illustrations. The atoms, represented as green spheres in the graph, can be viewed as forming different sets of planes in the crystal (colored lines in graph on left). For a given set of lattice planes with an inter-plane distance of d, the condition for a diffraction (peak) to occur can be simply written as

2dsin(θ) =n (λ

Figure No 7: Shows the lattice planes and Bragg’s law basic diagram

In the equation, theta is the wavelength of the x-ray, theta the scattering angle, and n an integer representing the order of the diffraction peak. The Bragg's Law is one of most important laws used for interpreting x-ray diffraction data.

It is important to point out that although we have used atoms as scattering points in this example, Bragg's Law applies to scattering centers consisting of any periodic distribution of electron density. In other words, the law holds true if the atoms are replaced by molecules or collections of molecules, such as colloids, polymers, proteins and virus particles ^(15,16).

Why XRD?

· Measure the average spacings between layers or rows of atoms

· Determine the orientation of a single crystal or grain

· Find the crystal structure of an unknown material.

· Measure the size, shape and internal stress of small crystalline regions

X-Ray Analytical Instrumentation:

Basic XRD measurements made on thin film samples include:

· Rocking curve measurements made by doing a λ scan at a fixed 2θ angle, the width of which is inversely proportionally to the dislocation density in the film and is therefore used as a gauge of the quality of the film.

· Super lattice measurements in multilayered heteroepitaxial structures, which manifest as satellite peaks surrounding the main diffraction peak from the film. Film thickness and quality can be deduced from the data.

· Glancing incidence x-ray reflectivity measurements, which can determine the thickness, roughness, and density of the film. This technique does not require crystalline film and works even with amorphous materials.

· Texture measurements--will be discussed separately ^{(17, 18, 19).}

Applications of XRD

· To identify crystalline phases and orientation

· To determine structural properties: Lattice parameters (10-4Å), strain, grain size, expitaxy, phase composition, preferred orientation (Laue) order-disorder transformation, thermal expansion

· To measure thickness of thin films and multi-layers*

· To determine atomic arrangement

· Detection limits: ~3% in a two phase mixture; can be ~0.1% with synchrotron radiation Spatial resolution: normally none

· Identification/quantification of crystalline phase

· Measurement of average crystallite size, strain, or micro-strain effects in bulk and thin-film samples

· Determination of lattice parameters to quantify alloy content

· Powder XRD can be used to fingerprint minerals without any prior knowledge of crystal structure or symmetry ^(20,21)

Limitations:

XRD can only work with crystalline materials, hence glasses and partially crystalline materials cannot be identified using this method. Phases that comprise less than about 3-5 wt.% (depending on crystal symmetry) of a sample will not be detected using a bench-top XRD Mixtures of phases with low symmetry will be difficult to differentiate due to the larger number of diffraction peaks.

D. Auger Electron Spectroscopy :

Auger electron spectroscopy (AES) has now emerged as one of the most widely used analytical techniques for obtaining the chemical composition of solid surfaces. The basic advantages of this technique are its 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. When an electron is ejected from an inner shell of an atom the resultant vacancy can be filled by either a radiative (X-ray) or nonradiative (Auger) process. In AES the atomic core levels are ionized by the incident electron beam and the resulting Auger electrons are detected with an electron spectrometer. These electrons form small peaks in the total energy distribution fu

Figure No 8: Shows the electron beam sample interaction

Working:

The sample is irradiated with electrons from an electron gun. The emitted secondary electrons are analyzed for energy by an electron spectrometer. The experiment is carried out in a UHV environment because the AES technique is surface sensitive due to the limited mean free path of electrons in the kinetic energy range of 20-2500 eV

The essential components of an AES spectrometer are

• UHV environment

• Electron gun

• Electron energy analyzer

• Electron detector

• Data recording, processing, and output system

UHV Environment:

The surface analysis necessitates the use of a UHV environment (4) because the equivalent of one monolayer of gas impinges on a surface every second in a vacuum of 10–6 torr. A monolayer is adsorbed on the surface of the specimen in about 1 second at 10–6 torr. Contamination of the specimen surface is critical for highly reactive surface materials, where the sticking coefficient for most residual gases is very high (near unity). The sticking coefficient for surfaces that are passivated through exposure to air is very low. The vacuum requirements are much less stringent for such sample generally the background pressure is reduced to the low 10–10-torr range in order to minimize the influence of residual gases in surface analysis measurements.

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 range of beam currents normally used in AES is between 10–9 and 5 ´ 10–6 A. The lower current gives high spatial resolution whereas the higher current may be used to give speed and high sensitivity where spatial resolution is of little concern. In certain samples the high current used may induce surface damage to the specimen and should be avoided

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 spectrometers today because of the relative ease of stray magnetic field cancellation.

Electron Detector:

Having passed through the analyzer, the secondary electrons of a particular energy are spatially separated from electrons of different energies. Various cross-sectional view of the CHA with input lens. The outer sphere has a negative voltage and the inner sphere has a positive voltage. The dashed lines indicate the trajectories followed by the emitted electrons. The central dashed line represents an equipotential surface. The entrance and the exit slits lie on a diameter and areas detectors are used to detect these electrons. Centered at the mean radius from the center of curvature

Data Recording, Processing, and Output System:

The Auger electrons appear as peaks on a smooth background of secondary electrons. If the specimen surface is clean, the main peaks would be readily visible and identified. However, smaller peaks and those caused by trace elements present on the surface may be difficult to discern from the background. Because the background is usually sloping, even increasing the gain of the electron detection system and applying a zero offset is often not a great advantage. Therefore, the Auger spectra are usually recorded in a differential form. In the differential mode it is easy to increase the system gain to reveal detailed structure not directly visible in the undifferentiated spectrum.

What It Does?

The high surface sensitivity of AES is due to the limited mean free path of electrons in the kinetic energy range 20 to 3000 eV. Auger electrons, which lose energy through plasma losses, core excitations, or interband transitions, are removed from the observed Auger peaks and contribute to the nearly uniform background on which the Auger peaks are superimposed. Because phonon losses are small compared with the natural width of Auger peaks, they do not affect the Auger escape depth. Hence the Auger yield is not dependent on the sample temperature.

Because the Auger transition probability and Auger electron escape depth are independent of the incident electron beam energy, Ep, the dependence of the Auger peak amplitude on Ep is governed completely by the ionization cross-section of the initial core level. Ionization occurs primarily by the incident electrons during their initial passage through the escape depth region.

Figure No 9: Shows the ionization process

The Auger Process and Auger Spectroscopy

Auger process is illustrated using the K, L1 and L2,3 levels. These could be the inner core levels of an atom in either a molecular or solid-state environment.

Ionization:

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 ionise all levels of the lighter elements, and higher core levels of the heavier elements.

In the diagram above, ionisation is shown to occur by removal of a K-shell electron, but in practice such a crude method of ionisation will lead to ions with holes in a variety of inner shell levels.

In some studies, the initial ionisation process is instead carried out using soft x-rays ( hν = 1000 - 2000 eV ). In this case, the acronym XAES is sometimes used. As we shall see, however, this change in the method of ionisation has no significant effect on the final Auger spectrum.

Electron transitions and the Auger effect

The Auger effect is an electronic process at the heart of AES resulting from the inter- and intrastate transitions of electrons in an excited atom. When an atom is probed by an external mechanism, such as a photon or a beam of electrons with energies in the range of 2 keV to 50 keV, a core state electron can be removed leaving behind a hole. As this is an unstable state, the core hole can be filled by an outer shell electron, whereby the electron moving to the lower energy level loses an amount of energy equal to the difference in orbital energies. 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:

E_kin = E_{Core State} − E_B − E_C'

Where E_{Core State}, E_B, E_C' are respectively the core level, first outer shell, and second outer shell electron energies, measured from the vacuum level. The apostrophe (tic) denotes a slight modification to the binding energy of the outer shell electrons due to the ionized nature of the atom; often however, this energy modification is ignored in order to ease calculations. Since orbital energies are unique to an atom of a specific element, analysis of the ejected electrons can yield information about the chemical composition of a surface. Figure 1 illustrates two schematic views of the Auger process.

Figure No 10: Shows the auger process.

Two views of the Auger process. (a) Illustrates sequentially the steps involved in Auger deexcitation. An incident electron creates a core hole in the 1s level. An electron from the 2s level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted. The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b) Illustrates the same process using spectroscopic notation, KL1L2,3.

AES - Instrumentation

.

Figure No 11: Show the cylindrical mirror analyzer

AES experimental setup using a cylindrical mirror analyzer (CMA). An electron beam is focused onto a specimen and emitted electrons are deflected around the electron gun and pass through an aperture towards the back of the CMA. These electrons are then directed into an electron multiplier for analysis. Varying voltage at the sweep supply allows derivative mode plotting of the Auger data. An optional ion gun can be integrated for depth profiling experiments.

Surface sensitivity in AES arises from the fact that emitted electrons usually have energies ranging from 50 eV to 3 keV and at these values, electrons have a short mean free path in a solid. The escape depth of electrons is therefore localized to within a few nanometers of the target surface, giving AES an extreme sensitivity to surface species. Because of the low energy of Auger electrons, most AES setups are run under ultra high vacuum (UHV) conditions. Such measures prevent electron scattering off of residual gas Atoms as well as the formation of a thin "gas (adsorbate) layer" on the surface of the specimen which degrades analytical performance. A typical AES setup is shown schematically in figure 11. In this configuration, focused electrons are incident on a sample and emitted electrons are deflected into a cylindrical mirror analyzer (CMA). In the detection unit, Auger electrons are multiplied and the signal sent to data processing electronics. Collected Auger electrons are plotted as a function of energy against the broad secondary electron background spectrum.

The instrumentation of AES can be divided into three parts: an electron gun, a cylindrical mirror analyzer, and an electron detector.

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. A lanthanum hexaboride (LaB₆) cathode provides higher current densities because LaB₆ has a lower work function than tungsten. At 2000 K, the current density can be as high as ~100 A/cm². 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.

Electron energy analyzers

Normally, AES employs a cylindrical mirror analyzer (CMA) as an electron energy analyzer. In both XPS and AES, the heart of these techniques is the measurement of an electron energy spectrum. An electron energy analyzer (strictly speaking, an electron velocity analyzer) is often employed to measure the kinetic energy of electrons. It is also called a spectrometer. The basic function of an electron spectrometer is to separate out only those electrons in a desired narrow band of energies from all other electrons with a wide range of energies entering the spectrometer. Basically, an electron spectrometer is capable of directing and focusing the electrons ejecting from the sample surface under X-ray excitation

Electron detectors

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 strikes the first dynode, it produces secondary electrons. The secondary electrons are then accelerated to the next dynode

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 energy of the Ar+ ions ranges from 0.1 to 5 keV. The ion gun is employed for:

(1) Cleaning the sample surface, and

(2) Depth profiling

Advantages of AES

• Spatial resolution is high.

• Analysis is relatively rapid.

• Surface or subsurface analysis can be performed.

• It is sensitive to light elements (except H and He).

• It provides reliable semiquantitative analysis.

• Chemical information is available in some cases.

Disadvantages

• Insulators are difficult to study due to surface charging.

• Surface may be damaged by the incident electron beam.

• Precise quantitative analysis may require extensive work.

• Sensitivity is modest (0.1 to 1 atom %).

• Depth profiling by ion sputtering or sectioning is destructive.

Limitations

• 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.

• The Auger spatial resolution common to most commercial instruments is of the order of 0.2 μm or less and is a function of analysis time.

• The sampling depth is about three monolayers.

Common Applications

• Qualitative analysis through fingerprinting spectral analysis

• Identification of different chemical states of elements

• Determination of atomic concentration of elements

• Depth profiling

• Adsorption and chemisorption of gases on metal surfaces

• Interface analysis of materials deposited in situ on surfaces

Sensitivity and Detection Limits

The sensitivity is of the order of 0.3%.Complementary or Related Techniques.

CONCLUSION:

Each of the short cases studies detailed above and also the discussions of the various analytical techniques have been used to demonstrate some of the capabilities of currently available analytical methods. Further techniques not detailed here can also contribute significantly to nanomaterials charaterisation. Those include raman spectroscopy, particularly with respect to carbon nanotubes and AFM(Atomic Force Microscopy), for dimensional and morphology studies. These established techniques can successfully be applied routinely to new materials. Further development and specialization of these techniques is also underway, as is the development and establishment of new analytical methods, to meet the needs of both nano materials community and needs of all high technology industries. A further broad, elaborate study can be done in future to get more details in this field.

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Received on 05.01.2013 Accepted on 10.02.2013

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