Research Paper on Microscopy and Spectroscopy Technologies

Scanning Electron Microscopy (SEM)

The Scanning Electron Microscopy (SEM) is an analytical technique that acquires an image of a sample using a focused beam. SEM relies on a variety of signals generated at the surface of solid specimens by a focused beam of high-energy electrons. Such electron-sample interactions provide details about the specimen including chemical composition, texture, and the crystalline structure and orientation of constituent components of the sample (Goldstein, 2003). In most applications, a 2-dimensional image is generated that displays spatial variations in these properties from data over a selected area of the surface of the specimen (Clarke, 2002; Reimer, 1998). Conventional scanning electron microscopy techniques (magnification ranging 20X ~30,000X, a spatial resolution of 50 100 nm) can be used to scan areas ranging from approximately 1 cm to 5 microns in width in. The scanning electron microscopy is also capable of analyzing selected point locations on the specimen, which is particularly useful in determining qualitative or semi-quantitative chemical compositions (using Energy-Dispersive X-Ray Spectroscopy), crystalline structure, and crystal orientations (using Electron Backscatter Diffraction). The design and function of the scanning electron microscopy are very similar to the Electron probe micro-analyzer and considerable overlap in capabilities exists between the two devices.

Accelerated electrons in a scanning electron microscope carry significant amounts of kinetic energy dissipated as a variety of signals generated during the interaction between the electron and the specimen when the incident electrons are decelerated in the sample. Among the various signals include secondary electrons (that produce scanning electron microscopy images), backscattered electrons, diffracted backscattered electrons (Electron Backscatter Diffraction that is used to determine crystal structures and orientations of materials), photons, visible light, and heat. According to Reimer (1998) and Egerton (2005) secondary electrons and backscattered electrons are often used to generate images of samples. Whereas secondary electrons are most useful for generating the morphology and topography of specimens (Clarke, 2002), backscattered electrons are mostly used to produce contrasts in the composition in multiphase samples (Egerton, 2005). From his work on SEM and X-ray microanalysis, Goldstein (2003) observed that X-rays are generated by inelastic collisions of the incident electrons with electrons in discrete orbitals of atoms. The excited electrons yield X-rays that are of a fixed wavelength (related to the difference in energy levels of electrons in different orbitals) as they return to lower energy states. Therefore, each element that is excited by the electron beam produces characteristic X-rays. Scanning Electron Microscopy analysis is considered valuable because it is a non-destructive technique, meaning that the production of x-rays via electron interactions do not lead to volume loss of the specimen. Consequently, the same samples can be analyzed repeatedly without loss of volume.

The Scanning Electron Microscopy is critical to the characterization of solid materials. Most Scanning Electron Microscopes are comparatively easy to operate with minimal sample preparation required. In addition, data acquisition is rapid. However, one of the limitations of SEM is that specimens must be solid. Secondly, Energy-Dispersive X-Ray Spectroscopy detectors on Scanning Electron Microscopes cannot very light elements such as lithium.

Infrared (IR) Spectroscopy

Infrared spectroscopy is a widely used and extremely important analytical method in science. It is a sensitive tool used to identify specimens since each sample has a characteristic spectrum. A molecule consists of at least two atoms with distances between them dictated by the interaction of their respective outer electrons and the sum of all forces between the atoms. Atoms are excited and vibrate around their equilibrium state when a molecule takes up energy. Radiation is emitted during the transition of molecule vibration from the excited state to a lower state. The frequency of the incident light is equal to the energy difference between electron orbits (molecule vibrations) of the electrons involved. Rotations are excited when the incident energy is too low, i.e. far infrared region (below 200 cm-1).

According to Hooke's law, the frequency of the vibration is related to the mass and force. This means, that the energy or frequency of a harmonic oscillator depends on the force. However, due to the fact that a bond cannot compress or stretch indefinitely, the molecule behaves like an anharmonic oscillator. The energy levels are not equivalent to distances anymore like it would in harmonic oscillation as the distance between atoms increase the energy reaches a maximum.

Vibrational modes can be bending vibrations (in-plane and out-of-plane) or stretching (symmetric and asymmetric). As a rule, vibrations are only infrared active if changes in the molecular dipole moment occur during the vibration. Since no dipole moment is present in diatomic molecules with the same atoms, vibration cannot occur. In contrast, various types of atoms in molecules interact with incident radiation, and where a dipole moment is not present, the antisymmetric displacement of the centre of charge can induce it.

A non-linear molecule of n atoms has 3 N degrees of freedom, in which three of these are rotational, three are translational, and the remaining belong to normal vibrations modes. In a linear molecule, however, two degrees are rotational and three degrees are translational. Therefore, the number of fundamental vibrations in a linear molecule it is 3N-5 and a non-linear molecule is 3N-6. The CO2 and the water molecule are examples of linear molecule and non-linear molecules respectively.

In degenerated vibrations such as the CO2 bending vibrations, symmetrically equivalent vibrations have the same wavenumber. In coupled vibrations, the network of molecules in a structure can lead to bands with slightly different positions. Overtone bands, which have a lower intensity than the fundamental band have a slightly lower intensity than the fundamental vibrational frequency. Lastly, the combination vibrations which include the sum or difference of two vibrations help in identifying species of molecules in a sample.

The infrared radiation spectra are divided into the following regions:

Near-IR: 12500 - 4000 cm-1 combination vibrations and overtones

Mid-IR: 4000- 400 cm-1 H2O bending and stretching vibrations

Far-IR: 400 - 0 cm-1 MO4, MO6, lattice vibrations

Infrared spectroscopy has very wide applications; it has been used in speciation and determination of concentration, identification of minerals, information about atoms and atomic bonds, qualitative and quantitative assessment of structural incorporated molecules and defects in samples etc.

Raman Spectroscopy

Since Sir Chandrasekhara Venkata Raman predicted the Raman Effect in 1928, Raman spectroscopy has been applied in science to study the inelastic scattering of light. Unlike in infrared spectroscopy, there must be a change of the polarization potential for a molecule to exhibit a Raman Effect. The amount of change of the polarization potential determines the intensity of the scattered light. In Raman spectroscopy, photons are scattered through their interaction with vibrational and rotational molecular transitions (McMillian and Hofmeister, 1988).

All matter vibrates at temperatures higher than absolute zero. Upon absorption of a photon, a molecule can vibrate to an excited state. Infrared spectroscopy which relies on the frequency principle works when the energy is equal to the energy difference between the two vibrational states. In Raman spectroscopy, however, ultraviolet, visible or near-infrared light, which has a much higher energy than those energy differences is used as radiation source. Thus, instead of absorption, the incident radiation excites the system to a high-energy state. Scattering reactions occur immediately the molecule recovers immediately from this state with the elastically scattered light having the same energy as the incident radiation (Raleigh scattering). However, the system may gain energy during this process, which is lost and the system reaches a higher energy state during scattering (Stokes scattering). In the event of an already vibrating system is excited, it loses energy and the system reaches a lower energy level (Anti-Stokes scattering) (Beran and Libowitzky, 2004).

Polarizability is described as the measure of the ability for the electron cloud to deform in contrast to the atomic nuclei. The electrons are attracted to the positive charge and the nuclei to the negative charge when placed into an electric field the electrons. Molecular geometry determines the change of the polarizability. For instance, the polarizability gets smaller during the stretching during the symmetric stretching vibration of the CO2 molecule. The polarizability changes and the vibration are Raman-active (but IR-inactive) (Beran and Libowitzky, 2004; McMillian and Hofmeister, 1988). On the other hand, no change in the polarizability during the asymmetric stretching vibration, and the vibration is Raman-inactive (but IR-active). This analytic technique has much application including identification of minerals, identification of molecular structures, speciation, and concentration among a host of other applications. These applications are due to the non-destructive nature of Raman spectroscopy, small sample amounts required to study without sample preparation are necessary (Beran and Libowitzky, 2004; McMillian and Hofmeister, 1988).

X-Ray Fluorescence Microscopy

Like the Scanning Electron Microscope, X-ray fluorescence (XRF) spectrometer is a relatively non-destructive analytical technique for materials. An XRF spectrometer works on the principles of wavelength-dispersive spectroscopy that is similar to an electron probe micro-analyzer (Fitton, 1997). However, an X-ray fluorescence spectrometer cannot generally analyze small size samples typical of an electron probe micro-analyzer (2-5 microns). Therefore, X-ray fluorescence microscopy is usually applied to the analysis of larger fractions of samples. X-ray fluorescence spectrometers are widely used to analyze samples due to their relative ease of use and low cost of sample preparation.

The behaviour of atoms when they interact with radiation makes it possible to analyze these behaviours x-ray fluorescence. Materials can be ionized by exposure to high-energy, short wavelength radiation. Focusing a material with radiation of sufficient energy of the radiation will dislodge an inner electron an exercise that makes the atom become unstable leading to the replacement of the missing inner electron with an outer electron. As the outer electron falls into the inner orbital to replace the missing electron, energy is released in form of a photon. The emitted radiation represents the energy difference between the two orbitals involved. The resulting fluorescent X-rays are valuable in detecting the abundances of elements present in the sample.

The analysis of elements in materials by fluorescent X-rays is made possible by the atom X-radiation interaction behaviour. When a sample is illuminated by an incident beam of intense X-rays of an XRF spectrometer, some of the energy is scattered, and the rest is absorbed within the sample in a manner dependent upon the chemistry of the material. The sample is said to be excited when the primary (incident) X-ray beam illuminates it. When the illuminated sample is excited, it, in turn, produces X-rays whose spectrum of wavelengths is attributabl...