Conformational disorder in alkylsiloxane monolayers at elevated temperatures Jan Weber, Thorsten Balgar, and Eckart Hasselbrink Citation: The Journal of Chemical Physics 139, 244902 (2013); doi: 10.1063/1.4846298 View online: http://dx.doi.org/10.1063/1.4846298 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/139/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer J. Chem. Phys. 134, 084701 (2011); 10.1063/1.3518457 Infrared spectroscopy of the organic monolayer sandwiched between a Hg electrode and a Si substrate Rev. Sci. Instrum. 81, 053103 (2010); 10.1063/1.3422256 Chain-length-dependent change in the structure of self-assembled monolayers of n-alkanethiols on Au(111) probed by broad-bandwidth sum frequency generation spectroscopy J. Chem. Phys. 118, 1904 (2003); 10.1063/1.1531098 Effects of mechanical compression on the vibrational spectrum of a self-assembled monolayer J. Appl. Phys. 90, 5070 (2001); 10.1063/1.1407850 The structure and spectroscopy of monolayers of water on MgO: An ab initio study J. Chem. Phys. 113, 3344 (2000); 10.1063/1.1287276
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THE JOURNAL OF CHEMICAL PHYSICS 139, 244902 (2013)
Conformational disorder in alkylsiloxane monolayers at elevated temperatures Jan Weber, Thorsten Balgar,a) and Eckart Hasselbrink Fakultät für Chemie and Center for Nanointegration (CENIDE), Universität Duisburg-Essen, D-45117 Essen, Germany
(Received 19 September 2013; accepted 26 November 2013; published online 23 December 2013) Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C–H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850–3000 cm−1 . We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4846298] I. INTRODUCTION
Silane-based self-assembled monolayers (SAMs) are intensively discussed in the context of heterogeneous catalysis, microelectronics or biosensing since they allow for chemical tailoring of various oxidic surfaces.1–6 Alkylsiloxane monolayers exhibit a high mechanical and thermal stability when, e.g., compared to alkylthiols on gold as they can be chemically grafted to various oxidic substrates.7, 8 Thus, they allow to prepare templates for applications where elevated processing temperatures are applied. Despite their high thermal stability, alkylsiloxane monolayers are laterally less ordered when compared to, e.g., alkylthiols on gold.6, 9 Therefore, alkylsiloxane monolayers reveal a higher tendency towards thermally induced gauche defects. This is particular important for applications in which not only the thermal stability of the monolayer but also a proper alignment of the headgroups at elevated temperatures is crucial, e.g., nanoelectronics or some kinds of biosensing. Otherwise, terminal functional groups may be buried within the monolayer to some extent. Octadecylsiloxane (ODS) monolayers have been successfully prepared on various oxidic substrates including metal oxides, minerals, oxidized silicon or glass.6, 8–10 The routinely used preparation method proceeds via wet chemical coating in a millimolar solution of trichlorosilane or trimethoxysilane precursors. Upon polymerization the anchor groups are crosslinked and build up a polysiloxane network which is subsequently covalently bound to the surface. Therefore, not every siloxane headgroup is covalently attached to a surface silanol group, resulting in a final monolayer with an area taken per ODS molecule of about 21 Å2 .9 It is supposed that the alkyl chains are tilted at an angle of 7◦ to 30◦ with respect to the surface normal.6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2013/139(24)/244902/8/$30.00
Several studies of ODS monolayers on glass including atomic force microscopy (AFM), ellipsometry and contact angle (CA) measurements, indicate a stability of the monolayer in air up to temperatures of ≈490 K.11–13 High resolution electron energy loss spectra (HREELS) indicate a thermal stability of the ODS monolayer on an oxidized silicon surface up to 740 K in vacuum.12, 14 X-ray photoelectron spectroscopy (XPS) studies in vacuum of siloxane layers with perfluorinated alkyl chains show thermal stability up to 620 K.7 Hence, the thermal stability of the ODS layer is drastically reduced under ambient conditions, i.e., in the presence of oxygen and water. Kim et al. investigated the thermal stability of ODS monolayers on oxidized silicon at ambient conditions in a combined AFM and XPS study which provided some information about ordering parameters like the tilt angle and chemical information about the oxidation process.13 These studies suggest chain degradation via cleavage of the C–C bonds in vacuum whereas under ambient conditions the dominant reaction pathway is decomposition proceeding via partial oxidation. However, only a few studies addressed reversible structural changes like the formation of gauche defects or a change of the molecular tilt angle at elevated temperatures, as either the techniques used are insensitive to these or the data have only been obtained after returning to room temperature. Grazing incidence X-ray diffraction (GIXD) measurements by Koga et al. provided information about lateral chain order but only in the small temperature range from room temperature to 328 K.15 Thus, whereas the degradation mechanism at temperatures above 500 K is sufficiently characterized, there is a lack of information regarding the crystallinity and the extent of chain defects at moderate temperatures from 300 to 500 K, which is the temperature range of importance for technical processing. Here, we present a sum frequency generation (SFG) spectroscopy study on the thermal stability of an ODS monolayer
139, 244902-1
© 2013 AIP Publishing LLC
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Weber, Balgar, and Hasselbrink
on a glass substrate in just that temperature range. Briefly, in SFG spectroscopy an infrared and a visible upconversion laser pulse are temporally and spatially overlapped at an interface. In non-centrosymmetric media the two waves mix in a second-order nonlinear optical process to give a signal wave with the sum frequency. The SFG signal is strongly enhanced if at least one of the incoming waves is resonant with a vibrational transition of a molecule at the interface. SFG spectroscopy is a powerful tool for studying molecules at interfaces16–19 and has been widely used to study the growth and structure of several types of monolayers including ODS on various substrates.20–23 SFG spectroscopy provides detailed information about the molecular configuration such as orientational and conformational disorder (gauche defects). In the case of an ODS monolayer at a surface one can also extract information about the molecular tilt angle when applying different polarization combinations of the light waves. We report in this paper that the ODS monolayers develop conformational disorder, namely, a significant increase in gauche defects in a temperature range far below the temperature of degradation. This finding calls into question the commonly assumed all-trans chain configuration. Our results are relevant with respect to thermal processing or photothermal patterning techniques since ODS monolayers are a favourable candidate for applications requiring elevated temperatures. II. EXPERIMENTAL SETUP A. Sample preparation
A well established procedure was used for the coating of the glass substrates with ODS.24 Briefly, the samples (commercial microscope slides, 0.5 mm thickness, 1 × 1 cm2 , Plano) were sonicated in ethanol for 10 min and subsequently immersed into a hot mixture (3:1) of concentrated H2 SO4 (96 wt. %) and H2 O2 (30 wt. %) for 30 min. This strongly oxidizing solution removes organic contaminations from the sample surface. Furthermore, it creates a hydrophilic surface with a high density of hydroxyl groups necessarily required for a proper binding of the ODS monolayer. Prior to coating, the cleaned samples were rinsed with deionized water several times and blown dry with a stream of argon. The subsequent ODS coating was carried out in a glove box under dry argon atmosphere to prevent the silane molecules from hydrolyzation. The samples were immersed into the coating solution (1 mM octadecyltrichlorosilane (OTS, 98%, ABCR chemicals, Germany), in toluene p.a. grade, used as provided, water content
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.204.37.217 On: Wed, 03 Dec 2014 18:59:35
THE JOURNAL OF CHEMICAL PHYSICS 139, 244902 (2013)
Conformational disorder in alkylsiloxane monolayers at elevated temperatures Jan Weber, Thorsten Balgar,a) and Eckart Hasselbrink Fakultät für Chemie and Center for Nanointegration (CENIDE), Universität Duisburg-Essen, D-45117 Essen, Germany
(Received 19 September 2013; accepted 26 November 2013; published online 23 December 2013) Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C–H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850–3000 cm−1 . We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4846298] I. INTRODUCTION
Silane-based self-assembled monolayers (SAMs) are intensively discussed in the context of heterogeneous catalysis, microelectronics or biosensing since they allow for chemical tailoring of various oxidic surfaces.1–6 Alkylsiloxane monolayers exhibit a high mechanical and thermal stability when, e.g., compared to alkylthiols on gold as they can be chemically grafted to various oxidic substrates.7, 8 Thus, they allow to prepare templates for applications where elevated processing temperatures are applied. Despite their high thermal stability, alkylsiloxane monolayers are laterally less ordered when compared to, e.g., alkylthiols on gold.6, 9 Therefore, alkylsiloxane monolayers reveal a higher tendency towards thermally induced gauche defects. This is particular important for applications in which not only the thermal stability of the monolayer but also a proper alignment of the headgroups at elevated temperatures is crucial, e.g., nanoelectronics or some kinds of biosensing. Otherwise, terminal functional groups may be buried within the monolayer to some extent. Octadecylsiloxane (ODS) monolayers have been successfully prepared on various oxidic substrates including metal oxides, minerals, oxidized silicon or glass.6, 8–10 The routinely used preparation method proceeds via wet chemical coating in a millimolar solution of trichlorosilane or trimethoxysilane precursors. Upon polymerization the anchor groups are crosslinked and build up a polysiloxane network which is subsequently covalently bound to the surface. Therefore, not every siloxane headgroup is covalently attached to a surface silanol group, resulting in a final monolayer with an area taken per ODS molecule of about 21 Å2 .9 It is supposed that the alkyl chains are tilted at an angle of 7◦ to 30◦ with respect to the surface normal.6 a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2013/139(24)/244902/8/$30.00
Several studies of ODS monolayers on glass including atomic force microscopy (AFM), ellipsometry and contact angle (CA) measurements, indicate a stability of the monolayer in air up to temperatures of ≈490 K.11–13 High resolution electron energy loss spectra (HREELS) indicate a thermal stability of the ODS monolayer on an oxidized silicon surface up to 740 K in vacuum.12, 14 X-ray photoelectron spectroscopy (XPS) studies in vacuum of siloxane layers with perfluorinated alkyl chains show thermal stability up to 620 K.7 Hence, the thermal stability of the ODS layer is drastically reduced under ambient conditions, i.e., in the presence of oxygen and water. Kim et al. investigated the thermal stability of ODS monolayers on oxidized silicon at ambient conditions in a combined AFM and XPS study which provided some information about ordering parameters like the tilt angle and chemical information about the oxidation process.13 These studies suggest chain degradation via cleavage of the C–C bonds in vacuum whereas under ambient conditions the dominant reaction pathway is decomposition proceeding via partial oxidation. However, only a few studies addressed reversible structural changes like the formation of gauche defects or a change of the molecular tilt angle at elevated temperatures, as either the techniques used are insensitive to these or the data have only been obtained after returning to room temperature. Grazing incidence X-ray diffraction (GIXD) measurements by Koga et al. provided information about lateral chain order but only in the small temperature range from room temperature to 328 K.15 Thus, whereas the degradation mechanism at temperatures above 500 K is sufficiently characterized, there is a lack of information regarding the crystallinity and the extent of chain defects at moderate temperatures from 300 to 500 K, which is the temperature range of importance for technical processing. Here, we present a sum frequency generation (SFG) spectroscopy study on the thermal stability of an ODS monolayer
139, 244902-1
© 2013 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 132.204.37.217 On: Wed, 03 Dec 2014 18:59:35
244902-2
Weber, Balgar, and Hasselbrink
on a glass substrate in just that temperature range. Briefly, in SFG spectroscopy an infrared and a visible upconversion laser pulse are temporally and spatially overlapped at an interface. In non-centrosymmetric media the two waves mix in a second-order nonlinear optical process to give a signal wave with the sum frequency. The SFG signal is strongly enhanced if at least one of the incoming waves is resonant with a vibrational transition of a molecule at the interface. SFG spectroscopy is a powerful tool for studying molecules at interfaces16–19 and has been widely used to study the growth and structure of several types of monolayers including ODS on various substrates.20–23 SFG spectroscopy provides detailed information about the molecular configuration such as orientational and conformational disorder (gauche defects). In the case of an ODS monolayer at a surface one can also extract information about the molecular tilt angle when applying different polarization combinations of the light waves. We report in this paper that the ODS monolayers develop conformational disorder, namely, a significant increase in gauche defects in a temperature range far below the temperature of degradation. This finding calls into question the commonly assumed all-trans chain configuration. Our results are relevant with respect to thermal processing or photothermal patterning techniques since ODS monolayers are a favourable candidate for applications requiring elevated temperatures. II. EXPERIMENTAL SETUP A. Sample preparation
A well established procedure was used for the coating of the glass substrates with ODS.24 Briefly, the samples (commercial microscope slides, 0.5 mm thickness, 1 × 1 cm2 , Plano) were sonicated in ethanol for 10 min and subsequently immersed into a hot mixture (3:1) of concentrated H2 SO4 (96 wt. %) and H2 O2 (30 wt. %) for 30 min. This strongly oxidizing solution removes organic contaminations from the sample surface. Furthermore, it creates a hydrophilic surface with a high density of hydroxyl groups necessarily required for a proper binding of the ODS monolayer. Prior to coating, the cleaned samples were rinsed with deionized water several times and blown dry with a stream of argon. The subsequent ODS coating was carried out in a glove box under dry argon atmosphere to prevent the silane molecules from hydrolyzation. The samples were immersed into the coating solution (1 mM octadecyltrichlorosilane (OTS, 98%, ABCR chemicals, Germany), in toluene p.a. grade, used as provided, water content
