A series of 2-phenylpropane-1,3-dithiol derivatives with single (R1ArDT), double (R2ArDT), and triple (R3ArDT) octadecyloxy chains substituted at the 4-, 3,5-, and, 3,4,5-positions, respectively, on the aromatic ring were synthesized and used to form self-assembled monolayers (SAMs) on gold. Insight into the relationship between the surface chain and headgroup packing densities was investigated by varying the number of surface chains for the bidentate adsorbates in these monolayers. Characterization of the resulting SAMs using ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy, and contact angle goniometry revealed that the tailgroups become more comformationally ordered and more densely packed as the number of alkyl chains per adsorbate was increased. Conversely, the molecular packing density (i.e., number of molecules per unit area) decreased as the number of alkyl chains per adsorbate was increased. Of particular interest, the desorption profiles obtained in isooctane at 80 °C suggested that the bidentate adsorbate with the most densely packed alkyl chains, R3ArDT, was significantly more stable than the other SAMs, producing the following relative order for thermal stability for the dithiolate SAMs: R3ArDT > R2ArDT > R1ArDT.

Ternary two-dimensional (2D) transition metal dichalcogenide compounds exhibit a tunable electronic structure allowing for control of the interlayer and the intralayer atomic displacement to efficiently tune their physical and electronic properties. Using a diamond anvil cell, hydrostatic pressure was applied to Mo 0.5 W 0.5 S 2 up to 40 GPa in order to study the optical phonon vibrational modes. Analysis of the high-pressure Raman spectra shows that the two in-plane E 2g modes resembling that of pristine MoS 2 and WS 2 , as well as disorder-activated longitudinal acoustic phonon mode, are hardened and suppressed as pressure increases. The two A 1g modes of the ternary compound that resemble the A 1g modes of pristine MoS 2 and WS 2 , displayed similar Raman shifts to the pristine compounds as pressure increases. A Raman peak at 470 cm −1 that is close to A 1g peaks emerges at ∼8 GPa, which represents a disorder-activated pressure-induced out-of-plane Raman mode observed only in the ternary compound under high pressure. At pressures above ∼30 GPa, a Raman peak at approximately 340 cm −1 is observed, signifying additional disorder-activated vibration mode. Our results reveal the enhanced interactions in the structural and vibrational behavior of the MoS 2 and WS 2 domains in the Mo 0.5 W 0.5 S 2 compound under hydrostatic pressure. These results could have implications in understanding the electronic, optical, and structural properties of the new 2D ternary compound materials under extreme mechanical conditions.

Molecular interactions at the solid−liquid interface have a profound impact on the macroscale wetting properties of surfaces. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and is used to determine the orientation of molecules at the interface. This study utilized SFG spectroscopy to study the interactions between the polar liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF 3 (CH 2 ) 17 SH and CD 3 (CF 2 ) 6 (CH 2 ) 10 SH on gold; these SAMs exhibit strong oriented molecular dipoles at their chain termini. We used SFG spectroscopy to examine the C−N and C−H stretching regions, 2000−2300 and 2800−3050 cm −1 , respectively, to probe the solid−liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive-or negative-going peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the orientation (CH 3 up or down) of the acetonitrile molecules with respect to changes in the direction of the FSAM dipole. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided direct evidence for the dipole-influenced wettability behavior of these unique model surfaces.

The direction and magnitude of surface dipoles directly affect the interfacial properties and can be tuned through molecular design. This article examines the effect of a hydrocarbon−fluorocarbon, "HC−FC", dipole on the structural and interfacial properties of self-assembled monolayers (SAMs) as the dipole is buried into the film. A series of selectively fluorinated alkanethiols with a progressively extended alkyl chain atop six fluorocarbons and an alkyl spacer of 11 hydrocarbons, H(CH 2 ) n (CF 2 ) 6 (CH 2 ) 11 SH, where n = 1−7 (HnF6H11SH) were prepared and used to generate SAMs on evaporated gold, allowing for the systematic burying of the HC−FC dipole into the film. Structural analyses of the films revealed well-ordered films with slight disorder/loose packing in the top alkyl chains. In addition, odd−even effects were observed in the orientation and wettability of the SAMs corresponding to the number of carbon atoms in the top alkyl chain, leading to the conclusion that the fluorinated segment behaves as a surrogate surface. As for the effect of the dipole on the wetting behavior of the films, the effect appears to be minimized after three methylene units; however, the structural features of the monolayers were also found to influence the wettability of the films.

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