|
Quick Links Related Websites |
October 23, 2002 Formation of Alkylsilane-Based Monolayers on GoldT.M. Owens1, K.T. Nicholson1, M.M. Banaszak Holl1, and S. Süzer2 A team of chemists from the University of Michigan in Ann Arbor have made a surprising discovery: At room temperature, the chemical reaction of compounds with silicon-hydrogen bonds with gold surfaces yields a new class of alkylsilane(CH3(CH2)nSi)-based monolayers. The monolayers are reminiscent of the self-assembled monolayers (SAMs) formed by alkanethiols (CH3(CH2)nSH). However, they exhibit a different chemical behavior that may prove advantageous in microelectronics and micro-contact printing. The alkylsilane monolayers have been characterized using soft X-ray photoelectron spectroscopy (SXPS) and reflection absorption infrared spectroscopy (RAIRS).
We synthesized monolayers of hexylsilane (C6H13SiH3), octylsilane (C8H17SiH3), and octadecylsilane (C18H37SiH3) by exposing gold surfaces to one of the alkylsilanes (figure 1) in an ultrahigh vacuum (UHV) chamber. The gold-alkylsilane samples have been characterized in UHV by reflection absorption infrared spectroscopy (RAIRS) at the University of Michigan in Ann Arbor, and soft x-ray photoelectron spectroscopy (SXPS) at beam line U8B at the National Synchrotron Light Source at Brookhaven National Laboratory.
The silicon 2p core levels were measured to have a binding energy of -99.8 electronvolts (eV), as shown in figure 2A, suggesting that silicon is bound to the gold surface and that no silicon-hydrogen bonds remain. We measured a full width at half-maximum of 0.4 eV, which is smaller than that observed for bulk, crystalline silicon using the same beamline and monochromator, thus indicating that silicon atoms are in a chemically homogenous environment. The valence band spectra of the alkylsilane-based monolayers (figure 2) bear a striking resemblance to valence band data of frozen alkanes (CnH2n+2) of the same length, indicating that the hexyl, octyl, and octadecyl chains have remained intact.
We have not observed a silicon-hydrogen stretch in the RAIRS
spectra for alkylsilane monolayers on gold (figure 3). The
silicon-hydrogen stretch at 2150 cm-1 is the most intense feature
observed for liquid alkylsilane (figure 3D). The absence of this
feature in the monolayers suggests that no silicon-hydrogen bonds
remain after chemical adsorption. The observed carbon-hydrogen
stretching modes between 2850 and 3000 cm-1, are consistent with the
alkyl (CnH2n+1) chains being angled away from the surface.
The combination of SXPS and RAIRS data suggests that when the alkylsilane monolayers bind to the gold surface, all three silicon-hydrogen bonds react with the gold surface and silicon forms bonds to three surface gold atoms, which is unlike single gold-sulfur bonds in structures made of thiol bound to gold. Also, the alkanethiol and alkylsilane layers exhibit different oxidation behaviors. Oxidation of sulfur atoms causes alkanethiol monolayers to fall apart, while oxidation of the alkylsilane layers knits the silicon atoms together by forming a siloxane network and maintaining the integrity of the alkyl chains and the surface monolayer. The unique properties of the alkylsilane layers may lend them to applications requiring durable layers and may provide an interesting route for the formation of patterned structures. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
The physical properties of metal surfaces can be modified through
the adsorption of small molecules. For example, hydrocarbons
modified with carboxylic acid (RCOOH) or thiol (RSH) functional
groups have been particularly well studied. In recent years,
scientists have shown that the adsorption of alkanethiols
(CH3(CH2)nSH) on gold surfaces may have many applications, including
molecular electronics and micro-contact printing. But these
applications have met some limitations due to surface defects
introduced by the adsorption process and to degradation caused by
atmospheric ozone. We have explored the use of alkylsilanes
(CH3(CH2)nSiH3) as a replacement for the thiol groups. 
