Abraham Ulman

Surface potential studies of alkyl-thiol monolayers adsorbed on gold

Abstract The surface potential (contact potential difference) of monolayers of alkyl thiols adsorbed on gold have been measured using the Kelvin technique. The potential has been found to vary linearly with increasing chain length, n , for 6⩽ n ⩽22. The variation is discussed in terms of the changing nature of the permittivity of the hydrocarbon portion of the monolayer, a factor often ignored in studies self-assembled monolayers.

Absorption and second-harmonic generation of monomer and aggregate hemicyanine dye in Langmuir–Blodgett films

Absorption spectra and second-harmonic generation of Langmuir–Blodgett monolayers containing a hemicyanine dye with a large second-order hyperpolarizability are reported. The dye is found to be in the form of H aggregates in a pure-dye film and mostly monomeric in a 1:4 dye–arachidic acid film. This observation explains a report in the literature that mixed dye–arachidic acid films display greater second-harmonic generation efficiency than a pure-dye film. It may also be a factor in the reported failure of second-harmonic generation to increase quadratically with the number of layers in noncentrosymmetric multilayer films.

Wetting studies of molecularly engineered surfaces

Abstract The wettability studies of mixed monolayers containing Hydrophobie (CH3) and hydrophilic (OH) terminal groups are discussed. We describe and explain a concentration-driven transition in the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible surface phase transition, resulting in the formation of a prewetting water layer. The self-assembly method opens exciting new possibilities of engineering smooth surfaces. Chemical properties fine-tuned at the molecular level will lead to new levels of control of physicochemical properties of surfaces. Long-chain thiol derivatives can be used in model surfaces to attain control of surface roughness and chemical properties. Combinations of surface functionalities give a large variety of surfaces, thus allowing a systematic control of surface free energy, and chemical affinity.

Blocking temperatures of amorphous iron nanoparticles coated by various surfactants

Abstract Amorphous iron nanoparticles obtained by the sonication of Fe(CO) 5 in decalin were coated by various surfactants using the self-assembled monolayer (SAM) technique. They have been shown previously to behave as superparamagnetic materials. These measurements are now extended to include information regarding their blocking temperature ( T B ). The results show that for one functional group T B is only slightly dependent on the length of the alkyl chain. On the other hand, the nature of the functional group bonded to the iron particles is strongly affecting T B .

Use of functionalized WS2 nanotubes to produce new polystyrene/polymethylmethacrylate nanocomposites

Multiwall WS2 nanotubes of 40 ‐ 50 nm diameter were functionalized with n-octadecyl phosphonic acid by sonication in toluene and blended with mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) to form new nanocomposite (NC) materials. The surface and domain structures were studied by atomic force microscopy (AFM), scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM) for various levels of loading of nanotubes up to 20 wt%. Phase-separated domain size and surface roughness of the nanocomposite films were found to be dramatically reduced relative to the pure homopolymer blend and good dispersal of the nanotubes in the blend matrix was attained. q 2003 Published by Elsevier Science Ltd.

Self-assembled monolayers of rigid thiols.

The preparation, structure, properties and applications of self-assembled monolayers (SAMs) of rigid 4-mercapto-biphenyls are briefly reviewed. The rigid character of the biphenyl moiety results in a molecular dipole moment that affects both the adsorption kinetics on gold surfaces, as well as the equilibrium structure of mixed SAMs. Due to repulsive intermolecular interaction, the Langmuir isotherm model does not fit the adsorption kinetics of these biphenyl thiols, and a new Ising model was developed to fit the kinetics data. The equilibrium structures of SAMs and mixed SAMs depend on the polarity of the solution from which they were assembled. Infrared spectroscopy suggests that biphenyl moieties in SAMs on gold have small tilt angles with respect to the surfaces normal. Wetting studies shows that surfaces of these SAMs are stable for months, thus providing stable model surfaces that can be engineered at the molecular level. Such molecular engineering is important for nucleation and growth studies. The morphology of glycine crystals grown on SAM surfaces depends on the structure of the nucleating glycine layer, which, in turn, depends on the H-bonding of these molecules with the SAM surface. Finally, the adhesion of PDMS cross-linked networks to SAM surfaces depends on the concentration of interfacial H-bonding. This non-linear relationship suggests that the polymeric nature of the elastomer results in a collective H-bonding effect.

Synthesis and Properties of Tetraphenylporphyrin Molecules Containing Heteroatoms Other than Nitrogen. 5. High Resolution Nuclear Magnetic Resonance Studies of Inner and Outer Aromaticity

Tetraphenylporphyrine, in denen zwei der NH-Gruppen durch Heter atome ersetzt sind, wie z.B. in (I), weisen Wechselwirkungen mit dem Kern des Porphyrins auf, die durch Protonierung oder Komplexierung unterbunden werden.

MIXED ALKANETHIOL MONOLAYERS ON GOLD SURFACES : WETTING AND STABILITY STUDIES

Abstract Studies of wetting and stability of mixed monolayers containing hydrophobie and hydrophilic components are discussed. We are reporting the observation of an apparent concentration-driven transition in the cosine of the contact angles of liquids on mixed monolayers. It is suggested that this phenomenon is due to a possible (true or rounded) surface phase transition, resulting in the formation of a prewetting water layer. This formation is triggered by variations in the quenched distribution of random surface fields. The variation of the surface free-energy, both polar and dispersive parts, has been determined as a function of surface OH-concentration. The surface free-energy of the 100% OH surface is close to that found for water, as might be expected for a surface coated with several monolayers of water. Zisman plots obtained for several of the surfaces using polar and nonpolar liquids give γ c values which follow the observed dispersive contribution to the total surface free energy, and thus do not present a good approximation to the surface free energy (i.e., γ c sv ). Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH 3 groups at their air-monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (%OH). The effect of different storage environments and temperature on the changing contact angles are discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize over time, depending on the chain length, by surface reorganization and the adsorption of contaminants.

SURFACE-PLASMON ENHANCED RAMAN-SPECTROSCOPY WITH HS(CH2)21OH ON DIFFERENT METALS

A method is presented that expands plasmon surface polariton field enhanced Raman spectroscopy (PSPR) to arbitrary metal substrates. Using this technique the ratio of the Raman tensor elements αzz/αxx for the alkane chain in HS(CH2)21OH on silver is determined. Based on this knowledge we also investigate PSPR spectra of HS(CH2)21OH adsorbed on copper and gold in order to determine its tilt angle with respect to the metal surface.

Electrochemical Sensing of Membrane Potential and Enzyme Function Using Gallium Arsenide Electrodes Functionalized with Supported Membranes

We deposit phospholipid monolayers on highly doped p-GaAs electrodes that are precoated with methyl-mercaptobiphenyl monolayers and operate such a biofunctional electrolyte-insulator-semiconductor (EIS) setup as an analogue of a metal-oxide-semiconductor setup. Electrochemical impedance spectra measured over a wide frequency range demonstrate that the presence of a lipid monolayer remarkably slows down the diffusion of ions so that the membrane-functionalized GaAs can be subjected to electrochemical investigations for more than 3 days with no sign of degradation. The biofunctional EIS setup enables us to translate changes in the surface charge density Q and bias potentials Ubias into the change in the interface capacitance Cp. Since Cp is governed by the capacitance of semiconductor space charge region CSC, the linear relationships obtained for 1/Cp2 vs Q and 1/Cp2 vs Ubias suggests that Cp can be used to detect the surface charges with a high sensitivity (1 charge per 18 nm2). Furthermore, the kinetics of phospholipids degradation by phospholipase A2 can also be monitored by a significant decrease in diffusion coefficients through the membrane by a factor of 104. Thus, the operation of GaAs membrane composites established here allows for electrochemical sensing of surface potential and barrier capability of biological membranes in a quantitative manner.