Observation of surface enhanced IR absorption coefficient in alkanethiol based self-assembled monolayers on GaAs(001)

Alkanethiol self-assembled monolayers (SAMs) of various methylene group chain lengths [HS-(CH(2))(n)-CH(3)] (n=9,11,13,15,17) were fabricated on the GaAs (001) surface followed by characterization using Fourier transform infrared spectroscopy. Modal analysis of the CH(2) stretching mode region (2800-3000 cm(-1)) showed that linear scaling of the n-dependent factors accurately reproduced the spectral data, supporting a chain-length consistent physical model upon which a measurement of the absorption coefficient was based. Evaluated from the linearity of the absorbance data, a peak coefficient of 3.5 x 104 cm(-1) was obtained and a domain for ordered self-assembly was assigned for values n>9. Compared with measurements of the absorption coefficient made in the liquid phase, the SAM phase coefficient was determined to be about six times greater. This enhancement effect is discussed in terms of contributions relating to the locally ordered environment and is largely attributed to the chemical properties of the interface. We believe this to be the first demonstration of IR spectral enhancement of a molecular species chemisorbed on the semiconductor surface. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3122052]