No abstract

Here we describe a new and simple method for preparing alkyl monolayers on silicon, which consists of mechanically scribing oxide-coated silicon while it is wet with 1-alkenes or 1-alkynes (neat or in inert solvents) under ambient conditions. X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, wetting data, and stability tests suggest covalent bonding of unsaturated species to exposed silicon surfaces. Enclosures (hydrophobic corrals) made by scribing silicon that is wet with unsaturated hydrophobic species hold droplets of water and liquids with substantially lower surface tensions. Wetting tests suggest that 1-alkynes make better hydrophobic corrals than 1-alkenes, and theoretical results suggest it should be more difficult for alkyl chains of chemisorbed 1-alkenes to pack than those of 1-alkynes. Underivatized interior regions of hydrophobic corrals are functionalized with polyelectrolyte multilayers. Theoretical energies for water and methanol droplets (gravitational and surface) in hydrophobic corrals are calculated, and a model of failure of liquid droplets in hydrophobic corrals is presented.

Silicon is arguably the most important material in modern technology and there has been much recent interest in chemically modifying its surface. 1,2 Linford and co-workers 3 recently published a new method of simultaneously preparing alkyl monolayers on silicon and patterning silicon by scribing it with a diamond-tipped rod while it is wet with 1-alkenes or 1-alkynes. They proposed that scribing creates highly active Si species that condense with unsaturated molecules. Here, we report that monolayers on Si can also be produced and Si surfaces concomitantly patterned by scribing Si that is wet with 1-chloro-, 1-bromo-, and 1-iodoalkanes. 4 As before, 3 this process takes place under ambient conditions, without the need to degas reagents. A dry Si surface with its thin (10-20 Å) native oxide layer is simply wet with an alkyl halide and the surface is scribed. We propose that surface species on scribed silicon, which may include SidSi (double) bonds and Si dangling bonds (Si • ), as are present on Si(100)-(2 × 1) and Si(111)-(7 × 7), respectively, 2 react with alkyl halides to produce Si-X (X is Cl, Br, or I) and Si-alkyl species. This process is shown below for Si • : homolytic scission of a C-X bond is followed by condensation of Si • with an alkyl radical.While • CH 2 (CH 2 ) n-1 H could diffuse away from the surface, it is likely that it will return to it by a random walk (a cage effect would also increase the likelihood of reaction with the surface). Bond strength tabulations support this mechanismsthe CH 3 -X 5 and C-X 6 bonds are weaker than the Si-X bond.Step (2) is clearly energetically favorable. 5 Bronikowski and Hamers 7 similarly suggested the following mechanism to explain the 2-fold excess of -Cl over -CH 3 on Si(001) dosed with gaseous CH 3 Cl:A number of reports on unpassivated silicon 2 and on monolayer formation on silicon 1 and gold serve as important precedents to this work. Abbott, Folkers, and Whitesides 8 removed regions of thiol monolayers on gold by micromaching techniques, for example, with a scalpel or a carbon fiber, and subsequently formed a second thiol monolayer in the exposed regions. Xu and Liu 9 used an AFM tip to scrape away areas of a thiol monolayer on gold while it was immersed in a solution of a different thiol. They showed that the thiol in solution adsorbs in the exposed regions. Linford 10 has shown that functionalized particles can be produced in a single step by grinding silicon in the presence of reactive compounds. CH 3 I, 11 CH 3 Cl, 7,12 and CH 3 CH 2 -Br 13,14 undergo dissociative adsorption onto Si(100)-(2 × 1) under ultrahigh vacuum to form Si-X and Si-CH 3 (or Si-CH 2 CH 3 ) species. Sailor and co-workers 15 derivatized both planar and porous silicon by electrochemical reduction of haloalkanes. Monolayers on planar and/or porous Si have been prepared by reacting H-terminated Si with diacylperoxides, 16 alkenes, Dorff, M. J.; Berges, D. A.; Linford, M. R. Langmuir 2001, 19, 5889-5900.(4) Preliminary results from our laboratory also indicate that monolayers are ...

No abstract