A New Approach to the IR Spectroscopic Study of Molecular Orientation and Packing in Adsorbed Monolayers. Orientation and Packing of Long-Chain Primary Amines and Alcohols on Quartz

Abstract

A new method is suggested for determining the molecular orientation in adsorbed films with uniaxial and biaxial anisotropy from two (s-and p-polarized) IRRAS spectra of the same sample, measured at the optimal angle of incidence. The method is simple, does not use the film thickness, and is internally stable with respect to the uncertainty in the input optical parameters of the anisotropic film. The advantages and limitations of the method are discussed. To validate the method, we determined the orientation of hexadecylamine and hexadecyl alcohol adsorbed on a quartz surface. It is shown that at a concentration above the concentration of 2D precipitation but below the concentration of 3D precipitation, hydrocarbon chains in the adsorbed amine monolayer are well-packed in a monoclinic (biaxial) subcell with a tilt angle of about 30°. Chaotically arranged crystallites of the amine molecules appear at the surface at a concentration higher than the concentration of 3D precipitation. Adsorbed monolayers of the alcohol turn out to have a hexagonal structure, in which the hydrocarbon tails are "flip-flop" positioned and tilted by 25-30°from the surface normal.

Electronic Current Transport through Molecular Monolayers: Comparison between Hg/Alkoxy and Alkyl Monolayer/Si(100) Junctions

“…1, left). [24][25][26] The presence of Si-O-R bonds for alkoxy monolayers can not be proven clearly by FTIR, because their characteristic bands, which are in the 1110-1000 cm À1 range for organo-Si compounds, [27] overlap with the Si-O-Si bands from O impurities in Czochralski Si substrates (Si(Cz)). Therefore, XPS was used to verify the chemical nature of the Si-molecule bond.…”

Interpretation of IR Spectra of Ultrathin Films

“…However, this approach demands a priori knowledge of the polarizability of the adsorbed molecule -a parameter that can be calculated assuming a certain microscopic model of the molecular packing only. The MO in the film can be determined by fitting the dichroic ratio (DR) of a selected band in ATR [517, 518,559,[608][609][610][611][612][613][614][615][616][617] or IRRAS [558,[618][619][620] spectra or the ratio of the band intensities in the metallic IRRAS and normalincidence transmission spectra [551,621], which will be referred to below as the "DR fitting" approach. In the spectroscopy of ultrathin films, the DR is defined as the ratio of the peak (or integrated) intensity of the absorption band in the s-polarized spectrum, A s , to that in the p-polarized spectrum, A p : DR = A s A p .…”

“…Chernyshova and Rao [558] suggested to characterize the MO and MP from IRRAS of ultrathin films on transparent substrates by fitting the DR calculated (3.60) where ϕ 1 is the angle of incidence, ξ 3 ≡ n 2 3 − sin 2 ϕ 1 is the generalized complex refractive index of the substrate, n 2 and k 2 are, respectively, the refractive and absorption indices of the film, and n 3 is the refractive index of the substrate. As seen from Eq.…”

“…in the error due to uncertainty in the monolayer optical parameters limits the optimal angle range from above by ∼75 • . Hence, the angle of 73 • optimal from the SNR viewpoint is also optimal for the IRRAS-DR fitting method [558]. Finally, it is important to know absolute accuracy in the IR spectroscopic measurements of MO.…”

“…Although the effect of the mutual arrangement of the molecules in a subcell on the CH stretching bands has been found to be negligible [507,527], the ratio of the band intensities can be used for determining the film symmetry [558] (see Section 3.11.3 for details).…”