Characterization of self assembly layers of octadecanephosphonic acid by polarisation modulation FT-IRRA spectroscopy mapping

doi:10.1016/j.molstruc.2003.08.031

Journal of Molecular Structure

Volumes 661–662, 16 December 2003, Pages 429-435

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Abstract

Self assembly layers were studied by a polarization modulation FT-spectroscopy mapping technique. The optical lay out is based on polarization modulation FT infrared reflection absorption spectroscopy (PM-FT-IRRAS). Here we report for the first time on a PM-FT-IRRAS mapping instrument. Octadecanephosphonic acid adsorbed on a patterned aluminum/gold surface was investigated. The nature of chemical bonding at particular surface areas was evaluated by principal component analysis. The most prominent features of the PM-FT-IRRA spectra are the P–O and PO stretching vibrations. It is shown that octadecanephosphonic acid is adsorbed both on Al₂O₃ and on Au. Moreover, PM-FT-IRRAS maps reveal areas of non-equivalent structural features. Lateral dimensions of these areas are in the micrometer range. Such non-equivalencies may control the inhibition potential of SAMs on ignoble metals, hence become crucial to the quality of products as biosensors or microelectronic components.

Introduction

The application area of molecules capable of forming self assembly monolayers (SAMs) increased significantly due to achievements in chemical tailoring of reactive endgroups of SAM-forming molecules. SAMs strongly bound to metal surfaces are widely used in microelectronic devices, in sensors, or in general in coating metal interfaces [1], [2], [3], [4], [5]. Lack of uniformity of SAMs, e.g. variation in layer thickness or molecular ordering, are major limiting factors for application of SAMs in miniaturized systems.

The thickness of SAMs is in the nanometre range. Conventional microscopic methods capable of providing information about such thin layers are scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) [6], [7], [8]. Unfortunately, these methods are not appropriate for detecting and investigating disorder in SAM domains with lateral dimensions as large as the micrometer scale. Moreover, the information obtained by STM or AFM is mainly based on topology, less on details in molecular structure.

Raman and Infrared spectroscopy are able to provide information about molecular ordering. The application of Raman spectroscopy for monitoring SAM uniformity is restricted by its sensitivity. A particular method to obtain information about molecular ordering in SAMs is Infrared Reflection Absorption Spectroscopy (IRRAS). Since the early 1980s, a large number of layered molecular systems has been studied by FT-IRRAS [9], [10], [11], [12], [13]. Disordered molecules including atmospheric gases and non-adsorbed molecules near the surface cancel out in the ratio of p and s polarized spectra, only contributions by ordered species remain. Initially, detection of the very weak surface signals required very long accumulation times and, consequently, experimental drifts and fluctuations during recording of s and p polarized spectra became limiting factors. The problem could be overcome by introducing the photoelastic modulation (PEM) technique to FTIR spectroscopy, which permitted fast modulation of the polarisation state and to extract the two intensity signals for p- and s-polarization (I_p−I_s) and (I_p+I_s) recorded under identical experimental conditions [10]. This kind of investigations is referred to as polarization modulation FT-IRRAS (PM-FT-IRRAS).

During formation of SAMs on metal surface, the molecules’ long axes are expected to align perpendicular (or nearly perpendicular) to the metal surface. Vibrational modes with transition dipole moments perpendicular to the surface will be excited when p polarized light enters the sample (the electrical vector of p polarized light is parallel to the plane of incidence, that of s polarized light perpendicular to the plane of incidence). Vibrational modes with transition dipole moments parallel to the surface can be excited only by s polarized light. Statistically disordered molecules absorb to the same extend in either p and s polarized light, whereas, well-ordered molecules exhibit distinct differences in their p and s polarized spectra. For this reason, differences between p and s polarized spectra are calculated and normalized to the total measured intensity as (I_p−I_s)/(I_p+I_s), i.e. only spectral information about vibrations of ordered molecular species adsorbed on metal surface are displayed in the PM-FT-IRRA spectra.

In order to identify areas of molecular disorder within a technical SAM sample, a microscopic mapping stage has to be included in the PM-FT-IRRAS optical layout. We developed such a microscopic PM-FT-IRRAS mapping system. The device is designed to allow both PM-FT-IRRAS mapping and surface plasmon resonance (SPR) imaging from the same SAM area. The SPR image provides information on slight changes in bulk properties (e.g. refractive index or effective layer thickness), whereas, the IRRAS map is based on molecular properties (e.g. conformation or non-bonding interactions). Here we report on the PM-FT-IRRAS part.

Currently the preferred model system for SAMs is thiols on gold. The Au–S bond is rather strong and stable. Problems arise for less noble metal substrates without gold cover. It is well known that SAMs on aluminum are less stable and less homogeneous. In particular on aluminium, SAMs of sufficient quality are needed as promotors for a well-controlled formation of high-quality coatings. Such chemically modified surfaces are conditioned for reacting selectively with organic molecules or for inhibiting any electrochemical reaction at the metal surface. Octadecanephosphonic acid is a promising compound for the formation of SAMs on aluminum. PM-FT-IRRAS is capable of detecting structural disorders across areas of technical interest.

Section snippets

Experimental setup

The PM-FT-IRRAS mapping optical set up (Fig. 1) was developed as an accessory to a Bruker FTIR spectrometer IFS 88 (Bruker Optik GmbH, Ettlingen, Germany). Infrared light emerging from the parallel beam port of the spectrometer is at first focused by spherical gold plated mirrors, subsequently it is linearly polarized by a wire grid polarizer and modulated by a ZnSe crystal (PEM-90, HINDS Instr., Hilsboro, OR, USA). The pinhole shields marginal rays and forms a point source for illumination of

Sample preparation

Octadecanephosphonic acid (CH₃(CH₂)₁₇PO₃H₂) was synthesised as described elsewhere [14]. Quartz glass windows were rinsed with ethanol, treated in a plasma cleaner and subsequently heated in a vacuum chamber to a temperature of approximately 200 °C. At first a 3 nm chromium layer was vapour deposited, subsequently a 50 nm gold film deposited. Finally, a mask consisting of a copper mesh with hexagons of a diameter of 80–120 μm was placed on the gold layer and a aluminum pattern (5 nm layer

Results and discussion

The spatial resolution of the PM-FT-IRRA maps is primarily limited by the spot size of the focused infrared beam on the sample. Diffraction effects, stray light and an imperfect optical adjustment may detract from achieving the theoretical spatial resolution. The angle of incidence of the light beam on the sample surface plays an important role as well. A bright field image of an octadecanephosphonic acid layer on the aluminum pattern on gold is shown in Fig. 2. The bright field image was

Conclusion

This study shows that PM-FT-IRRAS mapping combined with multivariate data evaluation provides a unique tool to characterize the structural order/disorder during processes like formation of SAMs. Lateral resolution down to 20 μm can be achieved even for non-crystalline samples of extremely low abundance. In case of octadecanephosphonic acid, the results indicate oriented attachment both on aluminum oxide as well as on gold. The presence of physisorbed as well as bidentate and tridentate

Acknowledgements

The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the ‘Sonderforschungsbereich Reaktive Polymere’ (SFB 287). The authors also thank W. Hartmann and F. Koschine of Bruker Saxonia GmbH (Leipzig, Germany) for their assistance during the development of the PM-FT-IRRAS mapping system.

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Characterization of self assembly layers of octadecanephosphonic acid by polarisation modulation FT-IRRA spectroscopy mapping

Abstract

Introduction

Section snippets

Experimental setup

Sample preparation

Results and discussion

Conclusion

Acknowledgements

J. Catal.

Vib. Spectrosc.

Prog. Org. Coat.

Phys. Chem. B

Langmuir

Chem. Mater.

Macromolecules

Langmuir

Langmuir