Interfacial Electron Beam Lithography Converts an Insulating Organic Monolayer to a Patterned Single-Layer Conductor with Puzzling Charge Transport Performance.

The direct generation of conducting paths within an insulating surface represents a conceptually unexplored approach to single-layer electrical conduction that opens vistas for exciting research and applications fundamentally different from those based on specific layered materials. Herein we report surface channels with single-layer -COOH functionality patterned on insulating n-octadecyltrichlorosilane monolayers on silicon that exhibit unusual ionic-electronic conduction when equipped with ion-releasing silver electrodes. The strong dependence of charge transport in such channels on their lateral dimensions (nanosize, macro-size), the type (p, n) and resistivity (doping level) of the underlying silicon substrate, the nature of the insulating spacer layer between the conducting channel and the silicon surface, and the postpatterning chemical manipulation of channel's -COOH functionality allows designing channels with variable resistivities, ranging from that of a practical insulator to some unexpectedly low values. The unusually low resistivities displayed by channels with nanometric widths and micrometer-millimeter lengths are attributed primarily to enhanced electronic transport within ultrathin nanowire-like silver metal films formed along their conductive paths. Function-structure correlations derived from a comprehensive analysis of electrical, atomic force microscopy, and Fourier transform infrared spectral data suggest an unconventional mode of conduction in these channels, which has yet to be elucidated, apparently involving coupled ionic-electronic transport mediated and enhanced by interfacial electrical interactions with charge carriers located outside the conducting channel and separated from those carrying the measured current. These intriguing findings hint at effects akin to Coulomb pairing in the proposed mechanisms of excitonic superconductivity in interfacial nanosystems structurally related to the present metalized surface channels.

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.

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.

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.

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.

A bipolar electrochemical approach to constructive lithography: metal/monolayer patterns via consecutive site-defined oxidation and reduction.

Experimental evidence is presented, demonstrating the feasibility of a surface-patterning strategy that allows stepwise electrochemical generation and subsequent in situ metallization of patterns of carboxylic acid functions on the outer surfaces of highly ordered OTS monolayers assembled on silicon or on a flexible polymeric substrate. The patterning process can be implemented serially with scanning probes, which is shown to allow nanoscale patterning, or in a parallel stamping configuration here demonstrated on micrometric length scales with granular metal film stamps sandwiched between two monolayer-coated substrates. The metal film, consisting of silver deposited by evaporation through a patterned contact mask on the surface of one of the organic monolayers, functions as both a cathode in the printing of the monolayer patterns and an anodic source of metal in their subsequent metallization. An ultrathin water layer adsorbed on the metal grains by capillary condensation from a humid atmosphere plays the double role of electrolyte and a source of oxidizing species in the pattern printing process. It is shown that control over both the direction of pattern printing and metal transfer to one of the two monolayer surfaces can be accomplished by simple switching of the polarity of the applied voltage bias. Thus, the patterned metal film functions as a consumable "floating" stamp capable of two-way (forward-backward) electrochemical transfer of both information and matter between the contacting monolayer surfaces involved in the process. This rather unusual electrochemical behavior, resembling the electrochemical switching in nanoionic devices based on the transport of ions in solid ionic-electronic conductors, is derived from the nanoscale thickness of the water layer acting as an electrolyte and the bipolar (cathodic-anodic) nature of the water-coated metal grains in the metal film. The floating stamp concept introduced in this report paves the way to a series of unprecedented capabilities in surface patterning, which are particularly relevant to nanofabrication by chemical means and the engineering of a new class of molecular nanoionic systems.

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.

Contact electrochemical replication of hydrophilic-hydrophobic monolayer patterns.

Contact electrochemical replication (CER) is a novel pattern replication methodology advanced in this laboratory that offers the unprecedented capability of direct one-step reproduction of monolayer surface patterns consisting of hydrophilic domains surrounded by a hydrophobic monolayer background (hydrophilic @ hydrophobic monolayer patterns), regardless of how the initial "master" pattern was created. CER is based on the direct electrochemical transfer of information, through aqueous electrolyte bridges acting as an information transfer medium, between two organosilane monolayers self-assembled on smooth silicon wafer surfaces. Upon the application of an appropriate voltage bias between a patterned monolayer/silicon specimen playing the role of "stamp" and a monolayer/silicon specimen playing the role of "target", the hydrophilic features of the stamp are copied onto the hydrophobic surface of the target. It is shown that this electrochemical printing process may be implemented under a variety of experimental configurations conducive to the formation of nanometric electrolyte bridges between stamp and target; however, using plain liquid water for this purpose is, in general, not satisfactory because of the high surface tension, volatility, and incompressibility of water. High-fidelity replication of monolayer patterns with variable size of hydrophilic features was achieved by replacing water with a sponge-like hydrogel that is nonvolatile, compressible, and binds specifically to the hydrophilic features of such patterns. Since any copy resulting from the CER process can equally perform as stamp in a subsequent CER step, this methodology offers the rather unique option of multiple parallel reproduction of an initially fabricated master pattern.

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.

Wetting driven self-assembly as a new approach to template-guided fabrication of metal nanopatterns.

Wetting driven self-assembly (WDSA) of appropriate materials in their liquid state on organic monolayer nanopatterns consisting of wettable (lyophilic) surface features surrounded by a nonwettable (lyophobic) monolayer background is shown to provide the basis of a versatile new approach to template-guided fabrication of metal nanopatterns. Monolayer nanopatterns with planned distributions of lyophilic/lyophobic surface regions are conveniently generated by constructive nanolithography upon local electrochemical oxidation of the top -CH3 groups of a highly ordered OTS (n-octadecyltrichlorosilane) monolayer self-assembled on silicon to -COOH (Adv. Mater. 2000, 12, 725-731). Retraction of such a patterned monolayer from a liquid that does not wet its nonpolar -CH3 surface (lyophobic) results in selective, site-defined immobilization of nanosized volumes of the liquid on the locally generated polar -COOH groups (lyophilic). Examples are given of WDSA of organic materials that offer further options for post-assembly chemical processing, such as nonvolatile low-melting olefins, acids, or thiols, the former being in situ reacted to generate polar functions like -COOH or -SH. Loading surface patterns created in this manner with silver or gold ions followed by further chemical processing results in elemental metal nanoparticles generated within the ion-binding organic material, which thus functions as a guiding template for planned metal deposition at predefined surface sites. WDSA is particularly versatile, as any nonvolatile material with appropriate melting temperature and surface wetting characteristics or solubility in a liquid displaying such properties may in principle be utilized to fabricate potentially useful surface nanostructures.

Transient charge accumulation in a capacitive self-assembled monolayer.

Charge accumulation in an organosilane monolayer self-assembled on silicon is studied using electron-spectroscopy-based chemically resolved electrical measurements (CREM). By resolving the net electrical response of the organic layer, a significant capability of holding extra charge is indicated. Quantum size effects at a molecularly thin layer and the role of competing discharge mechanisms, including defect-assisted leakage currents, are discussed.