Jeffrey Christenson


Infrared spectra were collected on an FT-IR spectrometer equipped with the Harrick DiaMaxATR™ single-reflection high-throughput diamond ATR accessory or ConcentratIR2 multiple-reflection diamond ATR (see Figure 1). The system was purged to remove water vapor and CO2. Spectra were collected at 8 cm

 resolution, and signal averaged over 32 scans. The spectra were referenced to the clean ATR crystal.

Two diamond ATRs: Single reflection DiaMaxATR (left) and ConcentratIR2 (right).

The sample used here was a High Five Nitrile Disposable Glove (lightly powdered, textured, P/N N842). For the single-reflection ATR measurement, a portion of the glove was pressed against the ATR crystal using the maximum force supplied by the build-in pressure applicator. For the multiple-reflection ATR measurements, the sample was gently pressed against the ATR crystal with compressible foam to fill the trough, and to apply force as uniformly as possible.

Figure 2 shows the two spectra. The upper spectrum measured with the multiple-reflection ATR shows only noise in the 2300–1900 cm-1 region, due to the high absorbance of diamond in that region. The lower spectrum, however, clearly shows the C≡N stretch at 2238 cm-1. Another weaker band is also apparent in the single-reflection ATR spectrum at 2162 cm-1, possibly indicating the presence of an alkyne group.

 Diamond ATR spectra of a nitrile glove measured using a multiple reflection ATR (blue) and a single reflection ATR (red).

The Harrick DiaMaxATR high-throughput, single-reflection diamond ATR is effective at measuring functional groups that are frequently obscured by the strong diamond lattice bands in the 2300–1900 cm-1 region. This makes it more suitable for analysis of the function groups in that region, including nitriles, isocyanates, isothiocyanates, diimides, azides, and ketenes.


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Articles

Diamond ATR has become one of the most commonly used FT-IR spectroscopy methods. However, the strong diamond lattice bands in the 2300–1900 cm-1 region make it difficult to measure the functional groups from nitriles, isocyanates, isothiocyanates, diimides, azides, and ketenes that would normally appear in that region. This applications note compares the sensitivity of a single-reflection ATR to multiple-reflection ATR for the nitrile functional group infrared transition.


Diamond ATR-FTIR Study of Nitriles

Many polymers and thin films on semiconductors are designed with specific molecular orientation to take advantage of mechanical, chemical, or physical properties. Some plastics, for example, exhibit different mechanical strengths depending on the orientation. Molecular orientation can be extracted from polarized ATR measurements.


This application note demonstrates determining the correct orientation of the polarizer for these measurements and illustrates the importance of polarized measurements in specific examples.


For all measurements, a Harrick Wire Grid Polarizer (KRS-5 substrate) was used. The polarizer was oriented in the same position for both background and sample measurements. All spectra were collected using an FT-IR spectrometer at an 8 cm-1 resolution, averaging over 32 scans.
Harrick’s Horizon, Seagull, and VariGATR accessories were used here. Such accessories redirect the infrared to and from the sample via mirrors, changing the direction of the electric field, which makes it tricky to ascertain the polarizer position for s- or p-polarization. For ATR, the polarizer orientation is straightforward to determine. The effective thickness for p-polarization is nearly twice that for s-polarization at a 45° incident angle (1), which is directly reflected in band intensity.


Three samples were investigated. An unoriented liquid, water, was used to demonstrate the method used to determine polarizer orientation with the Horizontal multiple reflection ATR. Then, two oriented samples were examined: Tunicin was measured with the Seagull at a 45° incident angle and an oriented thin film on silicon was examined with the VariGATR at a 65° grazing incidence.



Figure 1 shows the ATR spectra of water, measured with the two perpendicular orientations of the polarizer. Clearly, the 0 setting gives bands that are roughly twice as strong as those for the 90 setting, indicating that the former gives p-polarized radiation.

 ATR spectra of water recorded with the polarizer set to 90 (red) and 0 (blue).


Figures 2 and 3 show spectroscopic differences resulting from polarized measurements. In Figure 2, the O-H and C-H stretching regions change with polarization, suggesting different orientations of functional groups on the surface. Figure 3 shows differences at 1500 cm

. These bands are not visible with s-polarization but are with p-polarization, which probes bonds perpendicular to the interface. This shows the preferential orientation of functional groups on the surface.

2: ATR spectrum of tunicin measured with s-polarization (red) and p-polarization (blue).

ATR spectrum of a 16 Å thick photoresist ultrathin film on a silicon wafer, measured with unpolarized (black), s-polarized (red), and p-polarized (blue) incident radiation.

Combining a polarizer with ATR measurements provides valuable information about the sample. It also determines that the polarization delivered is straightforward. Polarization can enhance band intensities and provide information on oriented functional groups.


N.J. Harrick, Internal Reflection Spectroscopy (Wiley, New York, 1967).


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Polarization in FT-IR ATR

Grazing angle Ge ATR spectroscopy is extremely sensitive to monolayers and thin films on high refractive index substrates such as Si. As with any ATR method, the sensitivity increases as the critical angle is approached, as demonstrated herein.

Monolayers and other thin coatings on Si and other high refractive index substrates are of key importance in  materials science, bioscience, synthetic organic, and semiconductor research. These films are often close to the FTIR detection limit so methods that improve sensitivity are vital. Traditionally, these samples were examined by transmission or grazing angle specular reflectance spectroscopy. Grazing angle Ge-ATR has  recently gained wider use because of its higher sensitivity. Grazing angle Ge-ATR measurements have typically been recorded at 65°, an angle sufficiently above the critical angle for the Si-Ge interface, 58.7°.   

This note explores the increased sensitivity gained by optimizing the incident angle and also discusses the potential spectral anomalies if the nominal critical angle of a given sample is exceeded. 

The grazing angle Ge-ATR measurements were carried out in a commercial FTIR spectrometer, using Harrick's VariGATR™ accessory at selected incident angles from 60° to 65° (see Figure 1). Spectra were collected at an 8 cm

  resolution and signal averaged over 32 scans. The samples investigated were thin films on Si substrates. Each sample was pressed against the ATR crystal using the same force using the VariGATR's built-in pressure applicator and force sensor. 

Figure 1: The VariGATRâ¢ shown with its Force Sensor.			

Figures 2a and 2b show spectra recorded from two different samples at several incident angles. Both spectra have higher band intensities at the lowest angle measured. However, the spectrum of sample B shows significant dispersion effects at 60° which may complicate interpretation. These include a sloping baseline and a visible step between the high and low wavenumber edges of the bands. These effects are not apparent in the spectrum of sample A. 

Figure 2: (a): ATR spectrum of sample A recorded at an incident angle of 65Â° (red), 63Â° (blue), and 60Â° (black) with an applied force of 722 N. (b): ATR spectrum of sample B recorded at an incident angle of 65Â° (blue) and 60Â° (black) with an applied force of 700 N.			

In conclusion, it is clear that selecting the incident angle to approach the critical angle does increase sensitivity. However, for ease of interpretation, the incident angle should be optimized for greatest sensitivity without significant dispersion effects. 

141 Tompkins Ave., 2nd floor, Pleasantville NY 10570

Grazing angle Ge ATR spectroscopy is extremely sensitive to monolayers and thin films on high refractive index substrates such as Si.

Jeffrey Christenson

Articles

Diamond ATR-FTIR Study of Nitriles

Polarization in FT-IR ATR

Optimized Grazing Angle Ge-ATR of Monolayers and Thin Films on Si