GENERIC METHODOLOGIES
2.5 SPECTROSCOPY TECHNIQUES
Spectroscopy techniques are methods that use radiated energy to analyze properties or characteristics of mate- rials. It measures intensity as a function of wavelengths and produces spectrums for comparison purposes.
2.5.1 FOURIER TRANSFORM INFRARED SPECTROSCOPY
Fourier transform infrared (FTIR) spectroscopy is a type of absorbance analyzing technique. It meas- ures the amount of light at different wavelengths that is absorbed by a sample and express these meas- urements into a spectrum. Since different materials will absorb and transmit different range and levels of light, each will have its unique spectrum. So, by looking at the spectrums, this technique can be used to analyze characteristics such as film thickness, optical characteristics of nanoparticles, optical proper- ties of coating, particle size, and composition.
The technique was first proposed in the early years of the 20th century when Albert Abraham Michelson invented a configuration of beam splitter and mirrors that allows a beam of light to change wavelengths due to interferences (Nicolet, 2001). This component is known as the interferometer in any Fourier transform spectrometer and led to his design being named the Michelson interferometer. It is also the simplest configuration of any interferometer.
The first interferometers designed for this technique became available in the 1960s in England and were marketed by Research and Industrial Instruments Corporation (Griffiths, 1983). It was a slow- scanning instrument that used a paper-tape punch and a remote computer to digitize its results. A faster scanning spectrometer was produced by Block Engineering in the United States afterward. It could detect signals more easily (Griffiths, 1983). The end of the 1960s saw more developments in Fourier spectrometer technology including the Fourier transform algorithm (fast Fourier transform) by Cooley FIGURE 2.17
Typical sin2ψ plot for (A) Au and (b) Ni ultrathin (few nanometer) layers within Au/Ni multilayer.
From Thomas, O., Loubens, A., Gergaud, P., & Labat, S. (2006). X-ray scattering: a powerful probe of lattice strain in materials with small dimensions. Science Direct, 182–187.
37 2.5 SPECTROSCOPy TECHNIQUES
and Tukey in 1965, which enabled higher resolution spectrums to be produced at a faster pace. The algorithm was first implemented in an interferometer in 1969 by Digilab (Griffiths, 1983). Digilab later collaborated with Nicolet to produce a spectrometer that became an industry standard and contributed to the popularity of FTIR spectroscopy today (Griffiths, 1983). Many more improvements have been made since then in various other countries, most of them based on the Michelson interferometer.
This FTIR spectroscopy technique is similar to other absorption spectroscopy techniques, where a light of certain wavelength is passed through a sample. The amount of light transmitted or absorbed is then recorded, and another wavelength is selected to be recorded the same way. The main difference between this method and other absorption spectroscopy methods is its ability to process multiple wave- lengths simultaneously with the use of the Fourier transform algorithm. Fourier transform allows the machine to convert raw data of different wavelengths into a spectrum much faster and more efficiently.
There are several components to the FTIR spectrometer: the light source, the interferometer, the sample compartment, and the detector. The light source can contain any wavelength of light from ultra- violet to far-infrared. The FTIR spectrometer gives the most accurate and precise result in the infrared spectrum compared to other methods. Fig. 2.18 shows the general layout of each component. The light source will pass through the interferometer which will adjust the wave to the desired wavelength com- bination. Then, this light will go through the sample and hit the detector.
The interferometer is the key part of the instrument, using mirrors and a beam splitter in various configurations in order to convert the light source into a beam with different combination of frequen- cies. The beam splitter is usually a thin film made of plastic that is opaque to wavelengths longer than 2.5 µm (Griffiths, 1983).
The most well-known configuration as mentioned earlier is the Michelson interferometer.
FIGURE 2.18
The general layout of an FTIR spectrometer.
From How an FTIR spectrometer operates, ChemWiki, UCDavis.
38 CHAPTER 2 GENERIC METHODOLOGIES FOR CHARACTERIZATION
FIGURE 2.19
Schematic diagram of a Michelson interferometer.
From https://commons.wikimedia.org/wiki/File:Michelson_interferometer_with_labels.svg.
Fig. 2.19 shows the Michelson interferometer configuration. It has a single beam splitter in the middle that splits the light source into two equal parts. One beam travels to a fixed mirror and bounces back toward the center and the other beam bounces off a moving mirror. When the two beams join back again in the center, the difference in distance travelled will result in interference between the two waves. The overall effect will produce a light beam of different frequency depending on the distance of the moving mirror.
A more recent interferometer, the refractively scanned interferometer, was discussed in the article,
“Fourier Transform Infrared Spectroscopy,” by Peter R. Griffiths. The schematic diagram of this con- figuration is shown in Fig. 2.20.
For this interferometer, a moving wedge beam splitter and two fixed corner mirrors are used. By adjusting the scanning wedge in the center, the ratio between the light pass through to the first mirror and the light reflected to the second mirror will be changed. When the two beams joined back together in the middle, the difference in ratio will create interference resulting in a light of different wavelengths.
This configuration is said to exhibit more stability than the Michelson interferometer (Griffiths, 1983).
After exiting the interferometer, the beam is shone through a sample contained in the sample com- partment. Part of the radiation will be absorbed while the rest will reach the detector. The detector will then collect the absorbance data, also known as interferograms, and converts them using FT into an absorbance spectrum for ease of comparison with other results.
There are many advantages to using this technique to determine the characteristics of nanoparti- cles. Compared to other methods, the FTIR spectrometer has a multiplex or Fellgett advantage, which means it is able to analyze information faster because it can process radiation of different frequencies simultaneously. Another advantage is the high quality and precision of its data, made possible because
39 2.6 SURFACE ANALySIS AND DEPTH PROFILING
FIGURE 2.20
Schematic diagram of a refractively scanned interferometer.
From Peter R. Griffiths, James A. De Haseth, James D. Winefordner (Series Editor), Fourier Transform Infrared Spectrometry, 2nd Edition. Copyright © 2007, John Wiley and Sons
the FT calculations are done digitally (Griffiths, 1983). A third advantage is the use of interferences in the light, giving this technique a high signal-to-noise ratio which allows it to boost the amplitude of the targeted wavelengths. In addition, remote sensing, which enables the process to not have physical contact with the sample, ensures that the property of the sample remains unchanged.
The FTIR technique has gone through many developments in its instrumentation since the 1960s, mainly to improve the speed of the scanning process. It has provided a new way to test samples using infrared spectroscopy, making analyzing materials “virtually limitless” (Nicolet, 2001). However, there are some limitations to this technique, one of which is that FTIR spectroscopy focuses more on qualita- tive results as opposed to quantitative (Van de Voort, 2009). As an example, if the FTIR spectroscopy method was used to track relative changes in a substance, the results may be thrown off if the formula- tion of the substance changes or is covered by another substance.