Structures of a Long-Chain n-Alkane, n-C₄₄H₉₀, on a Au(111) Surface:  An Infrared Reflection Absorption Spectroscopic Study

Abstract

Vacuum-deposited films of a long-chain n-alkane, n-C44H90, tetratetracontane (TTC), on a Au(111) surface were investigated by infrared reflection absorption spectroscopy. We found four different structures for TTC on the Au surface. One is a “flat-on” structure formed by molecules in an all-trans conformation with the molecular plane composed of carbon atoms parallel to the surface. This structure was found in the first monolayer physisorbed on the surface. The second structure includes the gauche conformation, which shows a change in the orientation of the methylene groups with an increase in the thickness of TTC. The third structure is the crystalline state formed by all-trans molecules, which was found in the third and thicker layers with the molecular long axis parallel to the surface. These three structures were found for films deposited and measured at room temperature. With an increase in temperature after deposition at room temperature, the first and second structures were easily converted to another structure. This fourth structure consists of TTC molecules in the vicinity of the surface with their molecular axes parallel to the surface and their molecular planes tilted from the surface. The molecules in this structure interact with the Au(111) surface through some of their methylene groups, while the molecules in the flat-on structure contact the surface through all of their methylene groups. The appearance of these two structures is controlled by the balance of two kinds of interaction: methylene group−Au surface interaction and the interaction among TTC molecules. In the second layer, the formation of TTC molecules including a gauche conformation is ascribed to the geometrical incompatibility between flat-on TTC and crystalline TTC.

Self‐Assembly of Normal Alkanes on the Au (111) Surfaces

“…Figure 6c shows another example, in which the zigzag methylene groups for nC 22 H 46 all show up by adjusting the molecules perpendicular to the scanning direction. Therefore, we conclude that alkanes adopt a flat-on structure when adsorbed on gold surfaces; this is in agreement with the infrared spectroscopy results, [4] and the LEED results. [2,3] Marchenko and Cousty emphasized the importance of the geometric mismatch between the periodicity of gold (interatomic distance 0.288 nm neglecting the reconstruction of gold), and the two neighboring CH 2 groups (0.251 nm).…”

“…A flat-on structure was found to be formed by molecules in an all-trans conformation with the carbon skeleton parallel to the surface for the first monolayer physisorbed on the surface. [4] Early LEED studies of light alkanes (C 3 -C 8 ) on Pt(111) and Ag(111) surfaces advocate the same flat-on structure. High-resolution STM images may not show every methylene group of an alkane molecule.…”

Concentration-dependent switching of the mode of phase separation in ternary self-assembled monolayers of 2-mercaptoethane sulfonic acid, 2-aminoethanethiol and 1-dodecanethiol on Au(1 1 1)

“…The m as (CH 2 ) at this wavenumber is an indication that the alkyl group is lying flat on the surface [41]. With increasing t imrs , this peak shifted to 2927 cm À1 .…”

Structural and kinetic properties of laterally stabilized, oligo(ethylene glycol)-containing alkylthiolates on gold: A modular approach

“…43 Interestingly, a strong peak that normally does not appear in spectra of alkanethiolate SAMs is found at 2907 cm −1 . In previous studies by Yamamoto et al, this peak was assigned to "softening" of methylene CH vibrations upon direct interaction with a metal surface 32,44,45 A comparison of the relative peak intensities in the CH region, Fig. 4͑a͒, with the above cited spectroscopic investigations of long chain alkane films on gold suggests that the amount of the material adsorbed during the first minutes corresponds to a fraction of a monolayer of EG 6 C 16 molecules with disordered alkyl chains in a flat-on arrangement.…”