Characterization of monolayers and LB films of octadecyltrialkoxysilanes

Thin Solid Films 284-285 ( 1996) 80-84

Characterization of monolayers and LB films of octadecyltrialkoxysilanes D.M. Taylor a3*,S.K. Gupta a, P. Dynarowicz b a Institute of Molecular and Biomolecular Electronics, University of Wales, Dean Street, Bangor, Gwynedd LL57 I UT, UK b Department of General Chemistry, Jagiellonian University, ul Ingardena 3, 30-060 Krakow, Poland

Abstract The film-forming behaviour of octadecyltrimethoxysilane (TMS) and octadecyltriethoxysilane (TES) was investigated. At sufficiently low subphase temperatures (less than 25 “C for TMS and less than 15 “C for TRS), the surface pressure isotherms show a temperaturedependent transition from a liquid-expanded to a liquid-condensed phase. On further compression, a second transition is observed to an even more condensed phase in which it is believed that the alkoxy headgroups assume a staggered arrangement so as to minimize the area per molecule. Monolayers of TMS deposited from this phase are uniform and highly ordered. Fourier transform infrared (FAIR) studies carried out over a range of incident angles suggest a vertical alignment of the aliphatic chains in the monolayer. Surface potential measurements on deposited monolayers indicate the possibility of a long-term chemical reaction between the alkoxy headgroup and the underlying aluminium substrate. Keywords: Langmuir-Blodgett films (LB films); Monolayers; Octadecyltrialkoxysilanes; AFM

1. Introduction Alkoxysilanes are widely used for surface modification, e.g. to promote coupling between polymers and metal substrates [ 1 ] and for the covalent binding of proteins to inorganic supports [ 21. Prior to this investigation, surprisingly little work had been carried out on the Langmuir-Blodgett (LB) deposition of long-chain alkoxysilanes, presumably because of their perceived reactivity with water. In one of the few studies reported, Wolpers et al. [ 3-61 chose to investigate the more reactive trichlorosilanes which spontaneously polymerize on the aqueous subphase surface. In this paper, we report the monolayer behaviour and LB film-forming characteristics of octadecyltrimethoxysilane (TMS) and octadecyltriethoxysilane (TES). These were chosen because their reactivity with water is relatively low. Furthermore, these compounds may also be deposited by chemisorption, thus enabling a comparison between LB and chemisorption methods to be made in a later stage of this investigation.

2. Experimental

details

TMS was purchased from Aldrich Ltd. (Gillingham, UK) and TES from Lancaster Synthesis Ltd. (Morecambe, UK). * Corresponding author. 0040~6090/96/$15.00 0 1996 Elsevier Science S.A. AH rights reserved SSDIOO40-6090(95)08276-X

All materials were used as-received without further purification after differential scanning calorimetry (DSC) measurements had been performed using a UNIPAN type 660K calorimeter to confirm their purity. For this measurement, samples were placed in aluminium and platinum crucibles and heated at a rate of 5 “C mini from - 50 “C to 30 “C. Sharp melting transitions from crystal to isotropic liquid were found to occur at 17 f 0.5 “C for Th4S and 5 III0.5 “C for TES, indicative of high purity material. LB films were prepared by spreading an aliquot of the material, dissolved ( 1 g 1-l) in chloroform (HPLC), on the surface of ultrapure water (pH 5.7 + 0.1) held in a PTFE sliding barrier trough, located on an antivibration table in a class II semiconductor clean room. The pure water used for the subphase and for cleaning was obtained from a Millipore SuperQ system comprising reverse osmosis, activated carbon, nuclear grade deionizer cartridges and a 0.2 p_.mpointof-use filter. The subphase temperature was controlled to within f 1 “C by means of a heat exchanger fitted to the metal base plate under the trough. After spreading, the monolayers were left for at least 5 min for the solvent to evaporate, after which compression was initiated at a rate of about 0.1 nm’ molecule - ’ min - ‘. Surface pressures were measured within f 0.5 mN m- ’ with a Wilhelmy plate electrobalancearrangement. The surface potential of the floating monolayers was measured within + 10 mV using a vibrating plate voltmeter, designed and built in-house [7].

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Monolayers were deposited at surface pressures of 25 mN m-’ (TMS) and 14 mN m-’ (TES) at adipping speed of 5 mm min-’ onto glass microscope slides, both uncoated and metallized with 200 nm of aluminium or gold evaporated in a turbopumped vacuum unit. After preparation, samples were stored in silica-gel desiccators to minimize the risk of attack by atmospheric moisture. Scanning surface potential measurements [ 81 were performed by placing a sample on a motor-driven stage under the sensing probe of a Trek model 320B voltmeter. Surface plasmon resonance (SPR) studies were carried out using an apparatus designed and built in-house [ 91. Fourier transform infrared (FTIR) spectra were obtained for a range of incidence angles ( lo”-70”) using a Brooker model IFS1 13v spectrometer fitted with a vacuum stage into which samples were placed approximately 3 h before spectra were taken. Atomic force micrographs of TMS on gold were obtained using a Nanoscope III STM/AFM. The substrate used was cleaved mica onto which gold was evaporated in a turbopumped system while maintaining a substrate temperature of 300 “C.

3. Results Fig. 1(a) and Fig. 1(b) show pressure-area (T-A) and surface potential-area (A V-A) isotherms obtained at 20 “C for TMS and TES respectively. Fig. 1(a) shows that the pressure isotherms obtained for TMS are independent of the compression rate between 0.01 and 0.05 nm’ molecule-’ min-’ , except for a slight increase in the collapse pressure as the compression rate increases. At this temperature, the isotherms of TMS and TRS are very different. For TMS, a transition from a liquid-expanded to a liquid-condensed phase [ lo] occurs at about 0.4 nm* molecule - ’ . A second transition to a more condensed phase then occurs at about 0.25 nm* molecule - I. When the area per molecule is less than about

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0.2 nm*, collapse occurs at a pressure determined by the compression rate. For TES, the monolayer remains in the liquid-expanded phase down to approximately 0.4 mn* molecule- ’ when a sharp collapse occurs. The surface potential of the two compounds becomes measurable well before any significant increase in pressure is observed. Indeed, as the monolayers enter the liquidexpanded phase and the pressure begins to rise, A V reaches a plateau at approximately 240 mV. A rapid increase in A V then occurs on further compression. (The feature at approximately 1.4 nm* molecule- ’ in the A V-A isotherm in Fig. 1(b) is probably caused by an island of condensed TES floating under the probe.) It is seen in Fig. 2(a) and Fig. 2(b) that the apparently different behaviour of TMS and TElS at room temperature represents two extremes of a trend in which stable, highly condensed monolayers are obtained at low subphase temperatures, but expanded monolayers with low collapse pressures at higher subphase temperatures. At room temperature, the rate of area loss of compressed TMS monolayers is less than 0.6% min - ’ of the original area as long as the surface pressure is less than 24 mN m- ‘. TES monolayers are only stable below 14 mN m- ’ when the area loss is approximately 1% per min- ‘. Monolayer transfer is effected onto both bare and metallized glass slides with a transfer ratio close to unity on the first withdrawal from the subphase. It is difficult to assemble multilayer films of these compounds. The only success was achieved with TMS on aluminized glass substrates when X-type layers (deposition on entry only) were transferred with a reduced deposition ratio of 0.8 and with about 15% of the previously deposited layer being lost on the upstroke during the first few dipping cycles. Surface potential scans (Fig. 3(a) ) show that, on a macroscopic scale at least, a monolayer of TMS is deposited uniformly onto an aluminium substrate. Similar scans were obtained for TMS on gold. Soon after deposition, all mono-

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Fig. 1. Surfacepressu~~area ( +A) and surface potential-area (A V-A) isotherms obtained from TMS (a) and TE!3 (b) at 20 “C. The pressureisotherms for TMS were obtained over a range of compression rates from 0.01 to 0.05 nm* molecule-’ min- ’ (curvcsa-e). Allotherisothcrn~ wereobtainedatapproximateiy 0.02 nm2 molecule-’ min-‘.

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D.M. Taylor et al. /Thin Solid Films 284-285 (19%) 80-84

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hours. On longer storage, the potential decreases steadily with time, eventually changing sign and gradually increasing in the reverse direction. After several days, potentials of - 120 mV are common. Interestingly, when freshly deposited films are scanned under vacuum (Fig. 3(b) ), a negative surface potential is observed which partially recovers on re-exposure to the atmosphere (Fig. 3(c) ) . This partially reversible negative shift occurs in both fresh and aged films and is different from the long-term negative shift described above. It is probably caused by the removal of water from the films. The intensity of the C-H stretching bands, clearly visible near 2900 cm- ’ in the FTIR spectra of deposited monolayers (to be reported elsewhere), is at a maximum at an incident angle of 10” or less, suggesting an almost vertical orientation of the aliphatic chains. Bands changing in intensity as a function of the incident angle are also seen in the spectral range 1050-1300 cm- ‘. For TMS, the intensities of the two methoxy bands at approximately 1100 cm-’ and 1200 cm-’ are greatest at an angle of incidence of 45”. For TES, the same bands are at a maximum at 70” and 20” respectively. The CH2 (scissors) band at 1450 cm-’ is at a maximum at 45” for TMS, but at 20” for TES. These differences suggest that the orientation of the molecules is different in the two cases. No evidence is obtained for polymerization even though the films are several days old. It should be noted, however, that the Si0-Si bands, if present, would have appeared in the range 1000-l 150 cm-’ [ 1l-131 and may have been obscured by the alkoxy bands. SPR micrographs of TMS on gold are considerably more uniform than those of TES, which often gives only weak contrast. Fitting the Fresnel equations [ 141 to the SPR reflectance spectra of TMS monolayers on gold yields a thickness of 1.8 nm and a relative permittivity of 2.3 for the monolayer. Fig. 4 shows an atomic force microscopy (AFM) image of a TMS monolayer deposited onto gold-covered mica. The

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layers exhibit a positive surface potential; a value in excess of 300 mV is obtained for a TMS monolayer immediately after deposition, but this decreases rapidly in the first few

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D.M. Taylor et al. /Thin Solid Films 286285

regular molecular arrangement is clearly visible as are some dislocations (bottom right). The lattice constants were measured to be 0.37 and 0.44 nm, giving the area per molecule as 0.16 nm*, only slightly smaller than the estimated value at the deposition pressure.

4. Discussion The isotherms of TMS and TES are essentially similar in that they both show the presence of a transition from a liquidexpanded to a liquid-condensed phase. In both cases, the area per molecule and the pressure at which the transition occurs are temperature sensitive, as expected. Generally, the isotherms of TES are more expanded than those of TMS which is not surprising in view of the larger headgroup and lower melting point of the former. At sufficiently low temperatures, both compounds exhibit a second transition which is almost independent of temperature, occurring at approximately 20 mN m-’ for TMS and approximately 17 mN m-’ for TES. We believe that this stage of condensation is caused by alternate alkoxy headgroups slipping vertically, thus allowing the molecular packing to be determined by the aliphatic chains: the area per molecule above this transition is less than 0.25 nm*. Once into this phase, collapse begins, gently at first, but accelerating as the surface pressure rises. Surprisingly, no discontinuity is observed in the behaviour of the pressure isotherms of TMS when the subphase temperature passes through the melting point of the compound (approximately 17 “C). The isotherms of both compounds are influenced very little by the time for which the monolayer is left floating on the subphase prior to compression, confirming the low reactivity with water. Interestingly, at the onset of the pressure rise in the liquidexpanded phase (approximately 0.8 nm* for TMS and approximately 1.2 nm* for TES) , the surface potential of both compounds is about 240 mV. According to the Helmholtz equation [ 71, this implies that the vertical component of the molecular moment in TES is at least 50% greater than for TMS. It is tempting to ascribe this difference to the headgroups. However, such a large difference in the moments of methoxy and ethoxy groups is not observed in esters of, for example, acetic acid [ 151. All our results to date suggest that the first monolayer of TMS LB-deposited onto aluminium or gold is highly uniform. On a macroscopic scale, this is confirmed by scanning surface potential measurements. Phase contrast and SPR microscopy confirm the microscopic uniformity of the monolayer, while AFM studies confirm the high degree of molecular order in the films. The FTIR spectra obtained for different incident angles are consistent with a near-vertical alignment of the aliphatic chains. The surface potential of an LB monolayer of TMS on aluminium is similar in magnitude to that of the floating monolayer, suggesting a similar molecular arrangement in

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the two cases. The long-term reduction and eventual reversal of A Vin the deposited monolayer probably reflect a chemical reaction between the alkoxy moiety and the aluminium substrate, leading to the formation of Si-C&Al bonds. Such a reaction would be accelerated by the presence of water entrapped in the film during deposition. Some confirmation for this view was obtained in subsidiary experiments which showed that TMS films chemisorbed onto aluminium do not exhibit the same negative swing in potential. A similar range of tests on monolayers of TES deposited onto gold and aluminium show these to be of lower quality, with those on gold being especially poor. Presumably, the larger headgroup of this compound militates against a high packing density.

5. Conclusions By the correct choice of subphase temperature, it is possible to form stable monolayers of both TMS and TES at the air-water interface. On compression, both compounds exhibit a transition from a liquid-expanded to a liquid-condensed phase. Further compression results in a second transition to a more condensed phase in which the area per molecule is determined by the hydrocarbon chains. Monolayers of TMS are deposited uniformly onto both aluminium and gold. Monolayers of TES of reasonable quality are obtained on aluminium, with the deposition onto gold being rather poor. The FTIR evidence suggests a vertical alignment of the aliphatic chains in high quality monolayers. Long-term changes in the surface potential of the deposited monolayers can be interpreted as a chemical reaction between the alkoxy group and the aluminium substrate. Further experiments are being carried out to confirm this hypothesis.

Acknowledgements The authors are pleased to acknowledge the financial support of the European Community Commission (Grant number ERBCIPECT 926013175) for this work. They are also grateful to Dr. M. Kalaji for obtaining the FTIR spectra, Dr. J. Phelps for the SPR measurements and Griel Ltd for permission to publish the AFM image.

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[61M. Wolpers, M. Stratman and H. Viefhaus, Fresenius J. Anal. Chem., 341 (1991) 337. [71 D.M. Taylor, O.N. Oliveira, Jr. and H. Morgan, J. Colloid Interface Sci., 139 (1990) 508. [81 SK. Gupta, D.M. Taylor, A.E. Underbill and C.E.A. Wainwright, Synth. Met., 58 (1993) 373. [91 H. Morgan and D.M. Taylor, Appl. Phys. Len., 64 (1994) 1330. [lOI A.W. Adamson, Physical Chemistry of Surfaces, Interscience, New York, 1960.

[ 11I E.G. Rochow. An Introduction to the Chemistry of Silicones, Wiley,

New York, 2nd edn., 1951.

[ 121 D. Braun, H. Chedron and W. Kern, Praktikum der Makromolekularen Organischen Chemie, Httthig, Heidelberg, 3rd e&t., 1979. [ 131 W. Nell, Chemie und Technologie der Silicone, Verlag Chemie, Weinheim, 2nd edn.. 1968. [ 141 J. Phelps and D.M. Taylor, .I. Phys. D: Appl. Phys., submitted for publication. [ 151 R.C. Weast (ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, PL, 68th edn., 1988.