Received: July 25, 2007

In Final Form: October 25, 2007

Abstract:

We have investigated the UV photooxidation of a homologous series of methyl-terminated self-assembled alkanethiolate monolayers adsorbed on GaAs(001) using time-of-flight secondary ion mass spectrometry. We observe that for undecanethiol (UDT) and tetradecanethiol (TDT), the reaction kinetics are well-described by pseudo-first-order rate constants. For hexadecanethiol (HDT), octadecanethiol (ODT), and eicosanethiol (ECT), there is a change in the apparent kinetics from first-order to second-order. In agreement with previous studies of the photooxidation of self-assembled monolayers (SAMs) adsorbed on metals, for short-chain alkanethiols (n 15) adsorbed on GaAs(001), the rate determining step is the penetration of oxygen to the substrate/S interface. However, for longer-chain alkanethiols, the photooxidation rate is not solely determined by the penetration of the active oxygen species to the interface. The kinetic analyses indicate that the reaction proceeds via the simultaneous photooxidation of the alkanethiols to alkylsulfonates and alkylsulfates as well as the decomposition of the alkanethiols. In the case of UDT, TDT, and HDT, once formed, the alkylsulfonates can also oxidize to form alkylsulfates or decompose. Also, unlike the case of SAMs adsorbed on metals, the GaAs substrate photooxidizes. The rates of photooxidation of SAMs adsorbed on GaAs(001) are higher than those for SAMs adsorbed on Au. This can be related to differences in the structures of methyl-terminated SAMs adsorbed on GaAs(001) and Au.

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