John P. Folkers

Manipulation of the Wettability of Surfaces on the 0.1 to 1 Micrometer Scale Through Micromachining and Molecular Self-Assembly

Micromachining allows the formation of micrometer-sized regions of bare gold on the surface of a gold film supporting a self-assembled monolayer (SAM) of alkanethiolate. A second SAM forms on the micromachined surfaces when the entire system—the remaining undisturbed gold-supported SAM and the micromachined features of bare gold—is exposed to a solution of dialkyl disulfide. By preparing an initial hydrophilic SAM from HS(CH2)15COOH, micromachining features into this SAM, and covering these features with a hydrophobic SAM formed from [CH3(CH2)11S]2, it is possible to construct micrometer-scale hydrophobic lines in a hydrophilic surface. These lines provide new structures with which to manipulate the shapes of liquid drops.

X-ray grazing incidence diffraction from alkylsiloxane monolayers on silicon wafers

X‐ray reflection (both specular and off‐specular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers (n‐C18H37SiO1.5) formed by self‐assembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 A. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid n‐alkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermal‐expansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational corr...

Heterogeneous catalysis on platinum and self-assembled monolayers on metal and metal oxide surfaces

This paper describes research in two areas: heterogeneous catalysis and molecular self-assembly. The work in heterogeneous catalysis used the platinum-catalyzed hydrogenation of diolefin(dialkyl)platinum(II) complexes to generate platinum surface alkyls of known structure. The structure and reactivity of these suiface alkyls were inferred from characteristic processes occurring on the surface. The incorporation of deuterium from isotopically labelled protic solvents (e.g. EtOD) was particularly useful mechanistically. The technique of self-assembly was used to form organic monolayers by coordination of a ligand to a metal or metal oxide surface. In addition to the chemisorption of alkyl thiols on gold (as alkyl thiolates, RS-Au+), a number of other systems were demonstrated to givs ordered organic surface species.

Acid-base interactions in wetting

The study of the ionization of carboxylic acid groups at the interface between organic solids and water demonstrates broad similarities to the ionizations of these groups in homogeneous aqueous solution, but with important systematic differences. Creation of a charged group from a neutral one by protonation or deprotonation (whether -NH3 + from -NH2 or -CO2- from -CO2H) at the interface between surface-functionalized polyethylene and water is more difficult than that in homogeneous aqueous solution. This difference is probably related to the low effective dielectric constant of the interface (e≃9) relative to water (e≃80). It is not known to what extent this difference in e (and in other properties of the interphase, considered as a thin solvent phase) is reflected in the stability of the organic ions relative to their neutral forms in the interphase and in solution, and to what extent in differences in the concentration of H+ and OH- in the interphase and in solution. Self-assembled monolayers (SAMs)-esp...

Self-assembled monolayers of alkanethiols on gold: the adsorption and wetting properties of monolayers derived from two components with alkane chains of different lengths

This paper describes the preparation and wetting properties of two-component self-assembled monolayers (SAMs) obtained by the competitive adsorption of one short-chain (HS(CH2)10Sh) and one long-chain (HS(CH2)21Lg) alkanethiol onto gold from dilute ethanolic solutions. The four possible combinations of the tail groups CH3 and CH2OH were investigated: Sh = CH3/Lg = CH2OH, Sh = CH2OH/Lg = CH3, Sh = CH2OH/Lg = CH2OH, and Sh = CH3/Lg = CH,. The compositions of these SAMs are not the same as the compositions of the solutions from which they were formed. Although the relationship between the composition of the SAM and the composition of the solution suggests that some phase separation may be occurring within the SAM, contact angles with water and hexadecane show that significant disorder still remains in the interfacial region.

MOLECULAR SELF-ASSEMBLY AND MICROMACHINING

We are developing molecular self-assembly as a strategy for the fabrication of nanostructures. We have used self-assembled monolayer films formed by the chemisorption of organic molecules onto gold substrates, in combination with micromachining, (i) to pattern gold surfaces with well-defined regions of SAMs with contrasting properties, (ii) to transfer these patterns to the underlying gold film using SAMs as nanometerthick chemical resists for wet etching, (iii) to construct microelectrodes by taking advantage of the dielectric barrier properties of SAMs, and (iv) to create contamination-resistant films on the surfaces of metals that are otherwise highly susceptible to contamination by adventitious molecular adsorbates and particles.