Rivka Maoz

On the formation and structure of self-assembling monolayers. I. A comparative atr-wettability study of Langmuir—Blodgett and adsorbed films on flat substrates and glass microbeads

Abstract Organized oleophobic monolayers of several long chain compounds and steroid derivatives produced on flat solid substrates by spontaneous adsorption from organic solutions are compared with Langmuir—Blodgett (LB) monolayers transferred on identical substrates from the water-air interface. Quantitative infrared ATR and polarized ATR spectroscopy, and wettability measurements are used to correlate the various films and to determine their molecular density and orientation, mode of film-to-surface binding, and other structural characteristics. Formation of oleophobic adsorbed monolayers on a model powder substrate—smooth glass microbeads—is also investigated. It is concluded that, irrespective of the mode of film-to-surface binding (ionic, covalent, or hydrogen bonding), and the nature of the substrate (Ge, Si, ZnSe, glass slides, glass microbeads), saturation of the adsorption leads in all studied systems to the formation of tightly packed and highly oriented monolayers, structurally equivalent to LB monolayers of same or similar compounds deposited on the bare surfaces of the respective substrates. These findings are interpreted in terms of a cooperative surface process leading to aggregation of molecules into a characteristic “monolayer phase.” Significant structural differences may develop in LB built-up films thicker than one monolayer. A mechanism for the formation of covalently bonded silane monolayers is proposed.

Hydrogen-bonded multilayers of self-assembling silanes: structure elucidation by combined Fourier transform infra-red spectroscopy and X-ray scattering techniques

Abstract Hydrogen-bonded multilayer stacks of laterally interconnected long-chain silanes are a new class of synthetic self-assembling thin film organizates endowed with a somewhat unusual combination of structural and dynamic characteristics. In this paper we present experimental results obtained from a combined Fourier transform infra-red (FTIR) spectroscopic and X-ray scattering study, on the basis of which it is possible to derive a rather detailed picture of some of the main features of the microstructure of these novel multilayer films and their monolayer precursors. The films are shown to be composed of discrete monolayers, coupled to each other in a flexible, non-epitaxial manner, via interlayer multiple hydrogen bonds. The hydrocarbon tails assume a perpendicular average orientation on the layer planes and form a ‘rotator phase’-like hexagonal lattice with a lateral packing density of ca. 21 A 2 per molecule and a positional coherence length of ca.70 A. Extensive lateral coupling of the silane head groups appears to be responsible for the high structural robustness and defect self-healing capability of these films, while the interlayer hydrogen bonding accounts for the facile post-assembly intercalation of various polar guest species into their vertically expandable interlayer polar regions. As the SiO bond is too short to permit extensive intralayer polymerization under the steric constraints imposed by the compact packing of the perpendicularly oriented hydrocarbon tails, the observed high interconnectivity of the silane head groups is rationalized in terms of a dynamic equilibrium model involving continuous redistribution of the SiO bonds within a two-dimensional network of oligomeric siloxane and silanol species. This model of dynamic equilibriation of siloxane linkages can also help to explain other intriguing properties of such silane monolayers.

Postassembly Chemical Modification of a Highly Ordered Organosilane Multilayer: New Insights into the Structure, Bonding, and Dynamics of Self-Assembling Silane Monolayers

Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic acid function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectroscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3D molecular arrangement of the initial alcohol-silane multilayer stack is well preserved upon virtually quantitative conversion of the alcohol to carboxylic acid and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.

Penetration controlled reactions in organized monolayer assemblies III. Organic permanganate interaction with self-assembling monolayers of long chain surfactants☆

Surface reactions involving the penetration of reagents from an external fluid phase into the inner core of a solid-supported monolayer assembly may be used to probe some important structural features of the assembly, such as the spatial arrangement, packing density and degree of mobility of its molecular constituents, as well as its overall structural stability under the action of a given reagent. Information derived from the study of such reactions is essential in the evaluation of a number of potential applications of monolayer assemblies depending on their performance as molecular diffusion barriers. We have investigated several model systems involving the oxidation of ethylenic double bond functions by KMnO4. In order to distinguish between ion rejection caused by the hydrophobic effect and real structural impenetrability resulting from the high packing density of the material in an ordered film assembly, both aqueous and organic permanganate solutions were tested. A first report of this study, dealing with the interaction of some mono and multilayer assemblies (Langmuir-Blodgett and self-assembled) with permanganate in aqueous solution, is now in preparation and will be published elsewhere. In the present paper we wish to present results obtained for a number of self-assembled monolayer films treated with crown-ether-solubilized permanganate in benzene. The penetrability of the oxidant molecules into the inner core of the monolayer structure was followed by monitoring the extent of oxidation of unsaturated monolayer constituents as a function of their density of packing in the film, their mode of binding to the surface (ionic, covalent), and the nature of the substrate (ZnSe, Si). Similar monolayers made of saturated compounds were used to check the stability of each type of film under the conditions of the oxidation reaction. The structural transformations suffered by the films upon their exposure to the solution of the oxidant as well as the identity of the reaction products were determined using Fourier-transform IR-attenuated total reflection spectroscopy and wettability measurements. Covalently bonded silane monolayers were found to exhibit high stability and good barrier efficiency in both aqueous and organic environments. Slow oxidation of unsaturated silane monolayers may occur via a mechanism of lateral propagation starting from edges and defect sites in the film.

Patterned Organosilane Monolayers as Lyophobic−Lyophilic Guiding Templates in Surface Self-Assembly: Monolayer Self-Assembly versus Wetting-Driven Self-Assembly†

Monolayer self-assembly (MSA) was discovered owing to the spectacular liquid repellency (lyophobicity) characteristic of typical self-assembling monolayers of long tail amphiphiles, which facilitates a straightforward visualization of the MSA process without the need of any sophisticated analytical equipment. It is this remarkable property that allows precise control of the self-assembly of discrete, well-defined monolayers, and it was the alternation of lyophobicity and lyophilicity (liquid affinity) in a system of monolayer-forming bifunctional organosilanes that allowed the extension of the principle of MSA to the layer-by-layer self-assembly of planed multilayers. On this basis, the possibility of generating at will patterned monolayer surfaces with lyophobic and lyophilic regions paves the way to the engineering of molecular templates for site-defined deposition of materials on a surface via either precise MSA or wetting-driven self-assembly (WDSA), namely, the selective retention of a liquid repelled by the lyophobic regions of the pattern on its lyophilic sites. Highly ordered organosilane monolayer and thicker layer-by-layer assembled structures are shown to be ideally suited for this purpose. Examples are given of novel WDSA and MSA processes, such as guided deposition by WDSA on lyophobic-lyophilic monolayer and bilayer template patterns at elevated temperatures, from melts and solutions that solidify upon cooling to the ambient temperature, and the possible extension of constructive nanolithography to thicker layer-by-layer assembled films, which paves the way to three-dimensional (3D) template patterns made of readily available monofunctional n-alkyl silanes only. It is further shown how WDSA may contribute to MSA on nanoscale template features as well as how combined MSA and WDSA modes of surface assembly may lead to composite surface architectures exhibiting rather surprising new properties. Finally, a critical evaluation is offered of the scope, advantages, and limitations of MSA and WDSA in the bottom-up fabrication of surface structures on variable length scales from nano to macro.

Single-layer ionic conduction on carboxyl-terminated silane monolayers patterned by constructive lithography

Ionic transport plays a central role in key technologies relevant to energy, and information processing and storage, as well as in the implementation of biological functions in living organisms. Here, we introduce a supramolecular strategy based on the non-destructive chemical patterning of a highly ordered self-assembled monolayer that allows the reproducible fabrication of ion-conducting surface patterns (ion-conducting channels) with top -COOH functional groups precisely definable over the full range of length scales from nanometre to centimetre. The transport of a single layer of selected metal ions and the electrochemical processes related to their motion may thus be confined to predefined surface paths. As a generic solid ionic conductor that can accommodate different mobile ions in the absence of any added electrolyte, these ion-conducting channels exhibit bias-induced competitive transport of different ionic species. This approach offers unprecedented opportunities for the realization of designed ion-conducting systems with nanoscale control, beyond the inherent limitations posed by available ionic materials.

Parallel- and serial-contact electrochemical metallization of monolayer nanopatterns: A versatile synthetic tool en route to bottom-up assembly of electric nanocircuits

Summary Contact electrochemical transfer of silver from a metal-film stamp (parallel process) or a metal-coated scanning probe (serial process) is demonstrated to allow site-selective metallization of monolayer template patterns of any desired shape and size created by constructive nanolithography. The precise nanoscale control of metal delivery to predefined surface sites, achieved as a result of the selective affinity of the monolayer template for electrochemically generated metal ions, provides a versatile synthetic tool en route to the bottom-up assembly of electric nanocircuits. These findings offer direct experimental support to the view that, in electrochemical metal deposition, charge is carried across the electrode–solution interface by ion migration to the electrode rather than by electron transfer to hydrated ions in solution.

Self-Assembling Monolayers: A Study of Their Formation, Composition and Structure

The construction of organized molecular systems involving the participation of a large number of individual molecules (artificial supermolecular organizates) is becoming an important new aim of modern chemistry. Novel synthetic concepts will have to be developed for this purpose, most likely through the use of processes of molecular self-organization occurring at appropriate interfaces. Learning how to plan and efficiently control such processes is, undoubtedly, a first necessary step in this development and a major challenge chemistry faces today.

Site-Targeted Interfacial Solid-Phase Chemistry: Surface Functionalization of Organic Monolayers via Chemical Transformations Locally Induced at the Boundary between Two Solids

Effective control of chemistry at interfaces is of fundamental importance for the advancement of methods of surface functionalization and patterning that are at the basis of many scientific and technological applications. A conceptually new type of interfacial chemical transformations has been discovered, confined to the contact surface between two solid materials, which may be induced by exposure to X-rays, electrons or UV light, or by the application of electrical bias. One of the reacting solids is a removable thin film coating that acts as a reagent/catalyst in the chemical modification of the solid surface on which it is applied. Given the diversity of thin film coatings that may be used as solid reagents/catalysts and the lateral confinement options provided by the use of irradiation masks, conductive AFM probes or stamps, and electron beams in such solid-phase reactions, this approach is suitable for precise targeting of different desired chemical modifications to predefined surface sites spanning the macro- to nanoscale.

Interfacial Electron Beam Lithography: Chemical Monolayer Nanopatterning via Electron-Beam-Induced Interfacial Solid-Phase Oxidation

Chemical nanopatterning-the deliberate nanoscale modification of the chemical nature of a solid surface-is conveniently realized using organic monolayer coatings to impart well-defined chemical functionalities to selected surface regions of the coated solid. Most monolayer patterning methods, however, exploit destructive processes that introduce topographic as well as other undesired structural and chemical transformations along with the desired surface chemical modification. In particular in electron beam lithography (EBL), organic monolayers have been used mainly as ultrathin resists capable of improving the resolution of patterning via local deposition or removal of material. On the basis of the recent discovery of a class of radiation-induced interfacial chemical transformations confined to the contact surface between two solids, we have advanced a direct, nondestructive EBL approach to chemical nanopatterning-interfacial electron beam lithography (IEBL)-demonstrated here by the e-beam-induced local oxidation of the -CH3 surface moieties of a highly ordered self-assembled n-alkylsilane monolayer to -COOH while fully preserving the monolayer structural integrity and molecular organization. In this conceptually different EBL process, the traditional resist is replaced by a thin film coating that acts as a site-activated reagent/catalyst in the chemical modification of the coated surface, here the top surface of the to-be-patterned monolayer. Structural and chemical transformations induced in the thin film coating and the underlying monolayer upon exposure to the electron beam were elucidated using a semiquantitative surface characterization methodology that combines multimode AFM imaging with postpatterning surface chemical modifications and quantitative micro-FTIR measurements. IEBL offers attractive opportunities in chemical nanopatterning, for example, by enabling the application of the advanced EBL technology to the straightforward nanoscale functionalization of the simplest commonly used organosilane monolayers.