IR-visible sum-frequency generation (SFG) spectroscopy has been used in a total internal reflection geometry to study the molecular structure of polystyrene (PS) at PS/sapphire and PS/air interfaces, simultaneously. The symmetric vibrational modes of the phenyl rings dominate the SFG spectra at the PS/air interface as compared to the antisymmetric vibrational modes at the PS/sapphire interface. This indicates approximately parallel orientation of the phenyl rings at the PS/air interface while nearly perpendicular orientation at the PS/sapphire interface, with respect to the surface normal.
We present an experimental demonstration of the effect of solvent structure on the interfacial reactivity of the silica/water interface using in situ vibrational Sum-frequency Generation (vSFG) spectroscopy. The response of the molecular arrangement of the interfacial solvent to the presence of cations is pH dependent with the highest sensitivity at neutral pH, relevant to geochemical and biological environments. The pH-dependent changes in vSFG spectra are in excellent correlation with the enhancement of quartz dissolution in salt water, which was hypothesized by Dove et al. to be due to changes of the interfacial solvent structure at the silica surface. vSFG provides mechanistic insights into silica dissolution and sheds light on the role of ions in altering interfacial solvent ordering, which has implications in fields ranging from protein-water interactions to oil recovery.
IR−visible sum frequency generation (SFG) spectroscopy was used to study surface modification of polystyrene by its exposure to a UV light source or plasma. It was found that the polystyrene surface underwent dramatic changes after exposure to these treatments, as evidenced by marked changes in the surface SFG spectra. Before the treatments, the surface spectrum showed a pronounced peak at 3068 cm-1 which is characteristic of the symmetric stretch of the aromatic C−H of polystyrene. This peak decreased markedly, and other vibrational bands associated with the CH2 and CH3 groups appeared after the treatments. The observed spectral changes provided direct evidence of surface reactions involving the aromatic ring. In addition, our data showed that the degrees of oxidation of the polystyrene surface were different with the two processes. The oxidation to a higher oxidation state, resulting in the formation of carbonyl/carboxyl species, was observed with plasma treatment but not with UV irradiation. This difference was also reflected in contact angle measurements. Before the treatments, the contact angle was 95 ± 4°. It decreased to 45 ± 4° and to less than 10° with UV irradiation and plasma treatment, respectively. The different pathways for the two treatments are discussed. In addition, the kinetics of photooxidation of the polystyrene surface was measured in situ, yielding a half-life of 15 min, which is much shorter than that of the bulk.
Infrared-visible sum-frequency generation (SFG) is used to spectroscopically probe the interfacial structure of a self-assembled monolayer of alkyl thiol on a Au(111) surface. The SFG spectra of the CH3 group measured as a function of azimuthal angle indicates that sulfur atoms cannot be situated at sites of a single type, e.g. , hollow or bridge, but must be in a mixed arrangement. SFG is also used to study the reconstruction of the gold substrate by alkyl thiol.PACS numbers: 68.45.v, 42.65.k, 61.16.d Nearly 10 years ago Nuzzo and Allara [1] discovered that when gold is immersed in a dilute organosulfur solution an organic monolayer is spontaneously chemisorbed on the surface of the gold, producing a self-assembled monolayer (SAM). Such coatings are potentially useful for corrosion inhibition, wear protection, and organic sensors. Perhaps the best characterized SAM system is CH3(CH2)"~SH (C,), adsorbed on a Au surface. The C, /Au system, specifically its surface and chain orientation, has been widely studied. He [2] and x-ray [3] scattering studies have shown that the C H3 outer surface has a rectangular unit cell which includes four tilted hydrocarbon chains. Infrared spectroscopy [4], in agreement with Raman scattering [5] and ellipsometry [6], as well as an earlier sum-frequency generation (SFG) study [7], suggests that the chains are densely packed and the number of the gauche defects is insignificant. Despite all the available experimental results, the location of sulfur chemisorption sites is known only by inference. The conventional belief has been that the S atoms reside at hollow sites of the gold substrate, generating a simple (~3 X~3)R30 structure. The location of these sites is essential for a detailed evaluation of Au-S, Au-Au, and S-S bonds at the interface. These in turn affect the monolayer orientation and stability.Single frequency second-order nonlinear optical measurement has proven to be a unique technique for studying interface structural symmetry [8]. In this work infraredvisible SFG was used to probe the structural symmetry of the C~s/Au (111) interface. The present measurements in which the frequency dependence of the signal was used to differentiate the substrate and adlayer structures demonstrate the sensitivity of SFG spectroscopy to the structural symmetry of an interface. Our experiments reveal an unexpected threefold rotational symmetry at the sulfur adsorbate sites of the C~s/Au(111) system. This result precludes the sulfur atoms being located solely at single-type hollow sites of the gold substrate. Such a configuration would give rise to an isotropic SFG signal. Recently, we learned of an elegant x-ray experiment that independently produced results leading to the same conclusion [9]. The experimenters suggest a disulfide configuration with sulfur in the hollow and bridge sites.This arrangement would produce SFG results consistent with ours.In addition to adsorbate site geometry, the effect of the thiol chemisorption on the surface stress of the gold substrate was also exami...
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