Using GATR-FTIR for the Analysis of Polymer Brush Formation on Silicon Wafers

Polymer brush systems are surface immobilized thin films composed of ordered assemblies of polymeric chains, which are absorbed onto a surface or interface at one or more tethering points or terminally grafted.

They are gaining interest over the past few years owing to their potential application in a myriad of fields, including microelectronics, biomedical devices, thermo-responsive adhesives, nano-patterning, tailoring of surface properties, and controlled gene/drug delivery.

GATR-FTIR Spectroscopy

When polymer brushes are formed from flat silica surfaces, multiple reactions at the surface are often required so that the silicon dioxide layer can be efficiently functionalized with active sites for the subsequent polymerization from these sites.

Compared to other FTIR techniques, grazing angle attenuated total reflectance (GATR)-Fourier transform infrared (FTIR) spectroscopy enables direct acquisition of spectra from silica wafer substrates without deteriorating the surface functionality or devoid of an ideal substrate like a silicon ATR crystal (Figure 1).

Figure 1. GATR Ge-ATR accessory

This article discusses the applicability of GATR-FTIR to differentiate surface modifications of silica wafers with polymer brushes and thin films. Optical contact between the ATR crystal and the sample is required for this technique for better sensitivity.

Experimental Procedure

The surface contaminates from the Czochraclaki-grown <111> Si wafers were removed by cleaning with a 30:70 v/v hydrogen peroxide and concentrated sulfuric acid solution. This was followed by depositing an 11-carbon tertiary bromo-isobutyrate initiator deposited in an anhydrous toluene solution onto the cleaned Si wafers (Figure 2a).

The next step was the sequential cleaning of the substrates with toluene, methanol, and methylene chloride. GATR- FTIR spectra were acquired at 8cm^-1 resolution with 256 sample scans and 64 background scans.

The terminal bromine was then converted to a dithioester endgroup (Figure 2b) by subjecting samples with previously deposited bromo-initiator subjected to a modified atom transfer addition (ATA) reaction. Then wafer cleaning and GATR-FTIR spectra collection were carried out, as before.

The final step was the formation of homopolymer and diblock copolymer brushes of poly(styrene) (PSty) and PSty-bpoly( methyl acrylate) PMA from the dithioester moieties (Figure 2c and 2d, respectively). This was followed by sample cleaning and subsequent acquisition of GATR-FTIR spectra.

Figure 2. Reaction sequence for formation of a diblock copolymer brush on flat Si wafers

Experimental Results and Discussion

Peaks can be observed at the GATR- FTIR spectrum of the immobilized bromo-initiator, (Figure 3a) at roughly 2850 and 2930 cm^{- 1}, which are designated to the CH₂ stretching and the C-H stretching vibrations, respectively, and at roughly 1740 cm^-1, which is designated to the carbonyl stretching vibration of the ester group.

After the deposition and characterization of the bromo silane initiator, the terminal bromine was converted to a dithioester end group (Figure 2b) by subjecting it to a modified ATA reaction.

Figure 3. GATR-FTIR of surface immobilized (a) 11-C tertiary initiator and (b) dithioester surface structure

Figure 3b presents the GATR-FTIR spectrum of the sample subsequent to reaction with a dithioester containing compound, showing few discernable variations to that of the immobilized bromo-silane initiator spectrum.

This is attributed to the relatively weak intensity of aromatic C-H and C-C stretches, in particular in the presence of only one aromatic ring per immobilized molecule, and to the fact that the C=S stretching vibration can be observed in the finger print region.

This spectral region is subjected to large deviation in the GATR-FTIR of silicon wafers owing to the strong absorbance of the native silicon dioxide and lattice bands.

The synthesis of a PSty homopolymer brush was carried out to evaluate the efficiency of the immobilized dithioester surface towards surface initiated polymerizations (Figure 2c). The presence of PSty was confirmed from the GATR-FTIR spectra for the PSty homopolymer brush (Figure 4) owing to the expected appearance of C=C aromatic doublets at 1420-1480cm^-1 and aromatic C-H stretching around 3100cm^-1.

Figure 4. GATR-FTIR of PSty homopolymer brush

A PSty-b-PMA diblock copolymer brush was created using a homopolymer brush of PSty (Figure 2d). The inclusion of MA in the generation of the PSty-6-PMA diblock copolymer brush (Figure 5) was confirmed by the GATR-FTIR spectra owing to the appearance of an increment in the CH₂ stretch at roughly 2920cm^-1 and a carbonyl stretch at 1720 cm^-1.

Figure 5. GATR-FTIR of PSty-b-PMA diblock copolymer brush

Besides GATR-FTIR spectroscopy, ellipsometry, goniometry, and x-ray photoelectron spectroscopy were also used to characterize each Si wafer system. The thickness of the wafers before and after GATR-FTIR spectra acquisition was determined using ellipsometry, considering that some degree of force needs to be applied for better contact between the treated wafer and the ATR crystal. A sample wafer was placed at each modification step for spectral acquisition but not for thickness measurements due to changes in sample thicknesses.

Conclusions

The results clearly demonstrate the applicability of the GATR Ge-FTIR accessory in spectral elucidation of films, whose characteristic stretches appear in the spectral range of 1300-3300cm^-1. Nevertheless, other peaks in the fingerprint region are masked by the native silicon oxide stretches of the Si wafer mask.

The use of floatzone Si wafers could avoid this problem. Although GATR-FTIR technique is basically nondestructive, it requires high contact force, which affects the thickness of polymer brushes.

About Harrick Scientific Products, Inc.

Since its beginnings in 1969, Harrick Scientific has advanced the frontiers of optical spectroscopy through its innovations to transmission, internal reflection, external reflection, diffuse reflection, and emission spectroscopy. The president and founder of the corporation, Dr. N. J. Harrick, pioneered internal reflection spectroscopy and became the principal developer of this technique.

Harrick Scientific offers a large selection of standard and custom-built accessories for IR and UV-VIS spectrometers. Many of these attachments were originally forerunners in their field and their contemporary versions are considered industry standards. Harrick Scientific continues to introduce innovative new products. In addition to these state-of-the-art accessories, Harrick Scientific supplies a complete line of optical elements, including windows, ATR plates, prisms, and hemispheres.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

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