Langmuir 1998, 14, 3067-3070
3067
Monolayers Prepared from Phosphazene Functionalized Fluorocarbon Surfactants Yongchi Tian, Quock Y. Ng, and Janos H. Fendler* Department of Chemistry, Clarkson University, Potsdam, New York 13699-5814, and MAXTOR Corp., 2121 Miller Drive, Longmont, Colorado 80501 Received November 10, 1997 Surface pressure vs surface area measurements, surface plasmon spectroscopy, imaging by atomic force microscopy (AFM), and molecular modeling have established the formation of well-packed monolayers from hexakis(tetrahydroperfluorododecanoxyl)cyclotriphosphazene, IT, with the hydrophilic phosphazene ring lying on the water surface and the fluorinated aliphatic chains protruding into the air. This molecular orientation was preserved upon transferring the monolayers to solid substrates, where different load force behaviors have been observed for well-ordered, randomly packed, and spin-coated layers by the approaching AFM tip.
Introduction Bulky fluorocarbon surfactants self-associate into micelles and pack into monolayers in a different fashion from their hydrocarbon counterparts.1 In general, monolayers prepared from inherently rigid fluorocarbon surfactants are less tilted and more densely packed than monolayers prepared from hydrocarbon surfactants.2 Additionally, fluorocarbon surfactants are chemically inert, show a great degree of thermal and photolytic stability, and have a low surface tension and a high degree of water repellency. These properties have rendered fluorocarbon surfactants to be desirable as lubricants.3-5 For application in the microelectronic industry the lubricant should be ultrathin, ideally a monomolecular layer, should be strongly adsorbed to the substrate surface, and should have lateral fluidity. Evidence is provided in the present work for the formation of stable monolayers from hexakis(tetrahydroperfluorododecanoxyl)cyclotriphosphazene, IT, on water surfaces which, upon transfer, strongly adsorb onto substrates and thus become a potentially useful lubricant for technological applications. Experimental Section Hexakis(tetrahydroperfluorododecanoxyl)cyclotriphosphazene, IT, was dissolved, in perfluorohexane (PF5060, Aldrich) to give a stock solution of 1.7 × 10-3 M. A 10-20 µL aliquot of this stock IT solution was spread on water in a Lauda P model Langmuir trough. Surface pressure (Π) vs surface area (A) isotherms were obtained at a compression rate of 90-150 mm2/min. The monolayer was then transferred, at different stages of compression, onto freshly cleaved mica for AFM imaging or onto a gold-coated glass plate for surface plasmon measurements, respectively. Surface plasmon measurements were carried out by using a homemade system described previously.6 Briefly, one-side goldcoated glass slides (50 nm thick) were employed as the reflection element. The uncoated side was optically coupled to the base of * To whom correspondence should be addressed at Clarkson University. (1) Brace, N. O. J. Org. Chem. 1962, 27, 4491. (2) Naselli, C.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1989, 90, 3855. (3) Homola, A. M. Adv. Info. Storage Syst. 1991, 1, 279. (4) Mate, C. M.; Novotny, V. J. J. Chem. Phys. 1991, 94, 8420. (5) Mate, C. M. Surf. Coat. Technol. 1993, 62, 3730. (6) Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637641.
a right angle prism (n ) 1.52) by an index matching oil. A p-polarized, 632.8 nm beam from a HeNe laser (Hughes, 3235HPC, 20 mW) was directed to the middle of the prism. The prism was driven by a stepping motor with an angular resolution of 0.01° on a rotator (Oriel) capable of synchronously varying the angle of incidence, θ, and the direction of a large area silicon detector (Newport, 818-SL). Each angular scanned curve was fitted to the Fresnel equation by adjusting thicknesses and dielectric constants chosen for an appropriate multilayer model. AFM images were taken by a Topometrix Explorer scanning probe microscope. The AFM images were taken in the contact mode in air at room temperature, using a 2.6 µm × 2.6 µm scan head and a silicon nitride tip. When scanning, the tip was maintained at approximately constant height while the cantilever deflection was monitored by a top-minus-bottom mode detector. The PID parameters were set to 0.0002, 0.00001, and 0. This measurement mode, the so-called “force mode”, was found to be the best way to achieve molecular resolution. To rule out possible artifacts caused by electronic oscillation, mechanical vibration, and thermal drift, Fourier transforms of images scanned in opposite directions (forward and reverse) were compared. The initial variations between object positions from forward and reverse scans were found in the order of 0.5 nm in amplitude and 3-5° in angle. After 1-2 h stabilization, Fourier transforms of the images, obtained from forward and reverse scans, coincided within the error order of 0.05 nm and 1°. Molecularly resolved images were taken with high scan rates (100-200 Hz) as well as low scan rates (
