Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 +/- 1 degrees from the surface normal with a 43 +/- 5 degrees twist, a highly oleophobic and hydrophobic ambient surface, and direct S-GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to approximately 100 degrees C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au[111] surfaces. A two-step solution assembly process is observed: rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au[111], is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au[111].

Self-assembled monolayers (SAMs) formed from 4,4‘-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2) n SH (TPn, n = 1−6) on polycrystalline (111) gold and silver substrates have been characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS, near-edge X-ray absorption fine structure spectroscopy, ellipsometry, and water contact angle measurements. The packing density of the SAM constituents and the orientation of the terphenyl moieties exhibited a pronounced odd−even variation with the number (n) of methylene groups in the aliphatic (CH2) n linker, which was opposite on silver as compared to gold. A higher packing density and a corresponding smaller inclination of the terphenyl moieties was observed for odd numbers of the methylene units in TPn on Au, and for even numbers of these units in TPn on Ag. The observed odd−even effects are explained by the strong dependence of the bending potentials in the metal−S−C bond on the deviation of the metal−S−C bond angle from an optimal value of ≈104° (Au) and ≈180° (Ag), respectively. The optimal metal−S−C angles are determined by the hybridization of the sulfur in the metal−thiolate bond, which is proposed to have sp3 hybridization on Au and an sp hybridization on Ag, respectively.

The C, N, and O K-edge near-edge X-ray absorption fine structure spectra of the 22 most common proteinogenic alpha-amino acids in the zwitterionic form collected from solvent-free polycrystalline powder films in the partial electron yield mode are reported. Spectral features common to all amino acids, as well as distinctive fingerprints of specific subgroups of these compounds, are presented and discussed.