13 Nov 2017

Characterize thin films using FTIR polarization modulation | Spectroscopy Solutions

Thin films of organic and oxide materials are the building blocks of an extensive variety of nanomaterials and devices in optoelectronics, sensors and battery technology. FTIR allows the collection of spectrums of films around 2 nanometres thick and Specac's Infrared Wire Grid Polarizers help to improve FTIR sensitivity and reduce background noise.

There is currently a massive move towards research involving the growth of thin films based on organic (polymers or small molecules), metallic and oxide thin films. Thin film materials of a few nanometres or even a single molecules worth of thickness can demonstrate exciting electronic properties allowing the development of novel technology. These properties are typically related to the peculiar arrangement of the polymer backbones, and ultimately to the optical transition dipoles within the nanostructure of the thin film. 

FTIR Wire Grid Polarizers are perfect for polymer analysis.

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One of the largest growth areas for organic thin films is optoelectronics, which encompasses inorganic semiconductors, such as silicon or III-V compounds, and also ferroelectric oxides and inexpensive ‘plastic’ organic semiconductors.

Typical applications in this important area include low energy lighting, new types of lasers, charge-coupled imaging devices, phototransistors, photodiodes and solar cells (energy harvesting), photoresistors and sensors. 

Determining the properties of thin films 

The deposited thin films of different materials have to be characterised and analysed to determine their suitability for purpose and the extent of their electronic and chemical properties. Surface vibrational spectroscopy is one of the most powerful tools for procuring direct data on the structure of adsorbed molecules on solid surfaces as well as their overt bonding character. 

Infra-red reflection absorption FTIR spectroscopy (IRRAS) can be operated in relatively high-pressure environments, and has become a highly valuable tool for undertaking real surface studies. The sensitivity of IRRAS has been improved considerably using polarization modulation (PM) methods. This means PM-IRRAS FTIR is now a must-have for the high-sensitivity detection of adsorbates on metal surfaces in absorbing gaseous media. 

This technique permits the observation of weakly surface adsorbed species and metastable reaction intermediates, which persist only in the presence of gaseous molecules at room and elevated temperatures. This allows not only the determination of the chemical characteristics of the film but also provides an indication of the formation reactions.

PM-IRRAS FTIR is an excellent method for the analysis of nanometre scale thin films. It is possible to acquire FTIR spectrums of films of around 2 nm thick, and at the same time deduce molecular orientation.

How polarization modulation enhances FTIR 

The technique of polarization modulation‐infrared reflection‐adsorption spectroscopy (PM‐IRRAS) can be used to obtain the reflectance spectra of thin polymer films and spontaneously organised monolayers adsorbed onto metals such as gold, silver, and chromium. 

Specac Wire Grid FTIR Polarizers are perfect for assisting in the FTIR analysis of polymers.

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Differential PM-FTIR reflectance spectra are attained by photo-elastic modulation of the FTIR beam polarization and real-time sampling methodology to generate the average and differential FTIR interferograms from measurements of the Infrared signal during each modulation cycle. The use of polarization-independent optics after the metal surface ensures that a true surface Infrared differential reflectance spectrum is obtained. This technique has advantage of high surface sensitivity, and of the surface selection rule, where some infrared absorption bands are absent from the spectra of molecules adsorbed on metal particles. 

In addition, polarization modulation eliminates background signals from environmental factors such as water vapor and CO2. There is no need for a protective gas atmosphere or the optical section of the FTIR spectrometer to be in a vacuum.

Longer measurement times are also possible, because changes in the light source intensities or water surface height changes do not disturb the final spectrum.

Why must the light be polarized?

IRRAS is a technique dependent upon the optical constants of the thin film and substrate, the angle of incidence, as well as the polarization of the incident IR radiation. A photo‐elastic modulator generates alternating linear states of polarized light, the so‐called p and s components of radiation where p refers to parallel polarized radiation and s to perpendicular polarized radiation. 

A typical setup consists of two wire grid linear polarizers (for example polarizing filters from Specac) forming a crossed analyser setup, the optical sample introducing the change in the polarization of light, and a PEM further modulating the polarization state. The final detected intensities at the fundamental and second harmonic of the PEM operating frequency depend on the ellipticity and rotation introduced by the sample. 

The phase shift of the perpendicular component, s, exhibits no significant dependence upon the variation of the angle of incidence because the phase shift of the perpendicular component, s, is nearly 180° for all the angles of incidence, the net amplitude of the IR radiation parallel to the substrate surface is zero. In contrast, the phase shift of the parallel component, p, strongly depends upon the angle of incidence. 

The p‐ polarized component goes through a maximum at 88° leading to a net combined amplitude that is almost twice that of the incident radiation. This feature provides the enhanced differential reflectance spectrum of the adsorbed surface species i.e., the thin film.

Filters for polarization modulation 

PM-IRRAS FT-IR experiments require polarization filters where the transmittance of the filter remains constant irrespective of the polarization state of infrared radiation and the optical throughput is high. Specac polarizing filters permit a high resolving power, which allows the discrimination of vibrational bands in close proximity, especially at short wavelengths.

Specac IR grid polarizers are ideal for the FTIR analysis of thin films, because they provide superior resolution, a wide wavelength range (20 µm to 10mm), a high degree of polarization: > 99.9%, high transmission efficiency and no beam deviation or dispersion. 

Read our article on the part Specac's Infrared Polarizers have to play in polymer FTIR analysis.

Read how our FTIR Polarizers were used for nuclear fusion research.

References

  1. T. Buffeteau, B. Desbat, and J. M. Turlet, "Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-thin Films: Experimental Procedure and Quantitative Analysis," Appl. Spectrosc. 45, 380-389 (1991)
  2. Barbara J. Barner, Michael J. Green, et al., Polarization Modulation Fourier Transform Infrared Reflectance Measurements of Thin Films and Monolayers at Metal Surfaces Utilizing Real-Time Sampling Electronics, Anal. Chem. 1991, 63, 55-60 55
  3. Aritada Hatta, Toshimasa Wadayama, et al., A Polarization Modulation Infrared Reflection Applied to Study of Thin Films on Metal and Semiconductor Surfaces, Analytical Sciences December 1985, Vol. 1, 403

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