Showing papers by "Thalappil Pradeep published in 1999"

Self-assembled monolayers of small aromatic disulfide and diselenide molecules on polycrystalline gold films: a comparative study of the geometrical constraint using temperature-dependent surface-enhanced raman spectroscopy, X-ray photoelectron spectroscopy, and electrochemistry

Abstract: A detailed investigation of the self-assembled monolayers of diphenyl disulfide (DDS), diphenyl diselenide (DDSe), and naphthalene disulfide (NDS) on polycrystalline gold films using surface-enhanced Raman spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and electrochemistry is presented. Whereas DDS dissociatively chemisorbs on Au, in both DDSe and NDS, the Se-Se and S-S bonds, respectively, are preserved upon adsorption. All of the molecules adsorb with the molecular plane perpendicular to the surface. Temperature-dependent SERS studies suggest that the DDS monolayer is by far the most stable one and is stable up to a temperature of 423 K. Both DDSe and NDS desorb without breaking the diselenide and disulfide bonds. None of the monolayers show any structural change upon heating. XPS investigations show the presence of beam-induced damage upon X-ray exposure to DDS and NDS monolayers, and the damage is greater in the latter. Electrochemical investigations support the SERS and XPS data. Number of pinholes and defects are much less in the DDS monolayer than in NDS and DDSe. The impedance parameters such as double-layer capacitance, charge-transfer resistance, and diffusion coefficients measured at different frequencies support the above conclusion. It is suggested that the geometric constraint imposed by the rigid naphthalene ring inhibits the cleavage of the S-S bond, and consequently, the adsorption sites for sulfurs are not strongly bonded. For DDSe, it appears that the Se-Se distance is such that appropriate binding sites are available, thus leading to a more ordered monolayer. For DDS, the facile cleavage of the S-S bond leads to strong binding of the adsorbate molecules at the preferred surface sites, resulting in a rather well-ordered self-assembled structure.

Covalent chemical modification of self‐ assembled fluorocarbon monolayers by low‐ energy CH2Br2+· ions: a combined ion/surface scattering and X‐ray photoelectron spectroscopic investigation

Abstract: Specific covalent chemical modification at the outermost atomic layers of fluorinated self-assembled monolayers (F-SAMs) on gold is achieved by bombardment with low-energy polyatomic ions ( 2 Br 2 +· (m/z 172), mass and energy selected using a hybrid ion/surface scattering mass spectrometer and scattered from the F-SAM surface, CF 3 (CF 2 ) 7 (CH 2 ) 2 -S-Au, undergoes ion/surface reactions evident from the nature of the scattered ions, CH 2 F + (m/z 33), CHBrF + (m/z 111), and CF 2 Br + (m/z 129). The chemical transformation of the reactive F-SAM surface was independently monitored by in situ chemical sputtering with the projectile Xe +· . Representative species sputtered from the modified surface include CF 2 Br + , an indicator of terminal CF 3 to CF 2 Br conversion. X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of organic bromine at the surface; Br ( 3 P 3/2 ) and Br ( 3 P ½ ) peaks were present at binding energies of 182 and 190 eV, respectively. XPS analysis also revealed increased surface modification at higher collision energies in these reactive ion bombardment experiments, as exemplified by the increased hydrocarbon/fluorocarbon peak ratio in the C(1s) region and incorporation of oxygen in the surface seen in the observation of an O(1s) peak.

Adsorbate Geometry Distinction in Arenethiols by Ion/Surface Reactive Collisions

Abstract: Reactive scattering of low-energy ions from surfaces gives scattered product ions in which new bonds are formed with the adsorbate with a sensitivity to adsorbate geometry. Reactions of Cr•+ and C5...

Reactive ion scattering from surfaces bearing isomeric chlorinated adsorbates

Abstract: The value of low-energy ionic collisions for selective surface analysis is shown by the fact that reactive scattering allows differentiation of isomeric chemisorbates. Reactions of Cr•+ and Cr-containing cations at chlorobenzyl mercaptan (CBM) monolayers on Au surfaces show different reaction products, depending on the position of chlorine substitution in the phenyl ring. The chlorine atom abstraction product, CrCl+, is observed at 45-eV collision in 4-CBM and is completely absent in 2-CBM at the same energy. The sensitivity of reactive ion scattering to the isomeric adsorbate is further demonstrated by the fact that the peak corresponding to Cr•+ addition and Cl loss via Cl−C bond cleavage, C7H6SCr+, is large in the monolayer formed from 4-CBM, but weak in the 2-CBM monolayer, suggesting that the chlorine is below the first layer of atoms in the latter case. The inverse intensity relationship applies for the dehydrohalogenation product C7H5Cr+ where the Cl atom at the ortho position in 2-CBM facilitates ...

C–F and C–C bond activation by transition metals in low energy atomic ion/surface collisions

Abstract: The transition metal ions, Cr +· , Mo +· , W +· and Re + , abstract one or more fluorine atoms or C m F n groups (m = 1,2; n = 1-5) in collisions with fluorocarbon self-assembled monolayers (F-SAMs). The number of atoms abstracted increases with collision energy, and with W +· and Re + it is possible to maximize a specific scattered product ion by selecting the appropriate collision energy. The collision energy dependence suggests that dissociation of the products of multiple abstractions is not an important source of any of the observed ion/surface reaction products. The ions W +· and Re + activate and insert into C-C as well as C-F bonds. In Re + collisions, products of C-C bond activation are of comparable intensity to the C-F activation products. The reactivity of the ions towards fluorine abstraction is observed to be Cr +· +· +· + . The data are interpreted in terms of reaction at the surface and are rationalized by considering three factors (i) the electronic structures of the ions, (ii) the thermochemistry of fluorine abstraction, and (iii) the degree of orbital overlap of the metal ion and the F-SAM substrate.

Self assembled monolayers

Abstract: In recent years, a number of systems have been shown to spontaneously assemble on appropriate solid surfaces to form films with thicknesses of molecular dimensions. The special techniques needed to prepare, characterise and study such monolayers are described. The potential applications of these systems as catalysts, sensors, electrooptic and other materials are summarized.

Spectroscopic Investigations of M(CO)5-C60 (M = W, MO) Complexes: Precursors for Metal Fullerides

Abstract: Photochemical reactions of M(CO)6 (M = W, Mo) with C60 in solution yield η:2-complexes of M(CO)5 with C60. The complexes have been characterised by IR, UV/VIS, NMR and DSC. They do not show any orientational ordering down to 12 K and all the infrared bandwidths remain the same down to this temperature. The complexes can be decomposed thermally or photochemically yielding metal fullerides, which show characteristic reduction in peak width in the variable temperature IR spectra due to orientational ordering. Transitions are manifested in calorimetric studies also. Metal → C60 charge-transfer is observed in IR and XPS. A high temperature IR study of the C60-W(CO)5 complex reveals sequential elimination of the carbonyls yielding MC60. The study shows that carbonyl complexes can be used as precursors to make transition metal fullerides.