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

Assembling Magnetic Nanoparticles on Nanomechanical Resonators for Torque Magnetometry

We report a highly compliant process for patterning nanoparticle arrays on micro- and nanomechanical devices. The distinctive step involves the single layer self-assembled nanoparticles on top of released nanomechanical devices. We demonstrate the process by fabricating sizable arrays of nanomechanical devices on silicon-on-insulator substrates, acting as nanomechanical torque magnetometers. Later, the nanoparticles were self-assembled in geometrical shapes on top of the devices by a unique combination of top-down and bottom-up methods. The self-assembled array of nanoparticles successfully showed a magnetic torque signal by magnetic actuation of the magnetometer. This patterning process can be generalized for any shape and for a wide range of nanoparticles on the nanomechanical resonators.

Surface Coupling of Octaethylporphyrin with Silicon Tetrachloride

The surface assembly of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) using silicon tetrachloride as a coupling agent was investigated using atomic force microscopy (AFM). Nanopatterned films of Si-OEP were prepared by protocols of colloidal lithography to evaluate the morphology, thickness, and molecular orientation for samples prepared on Si(111). The natural self-stacking of porphyrins can pose a challenge for molecular patterning. When making films on surfaces, porphyrins will self-associate to form co-planar configurations of random stacks of molecules. There is a tendency for the flat molecules to orient spontaneously in a side-on arrangement that is mediated by physisorption to the substrate as well as by π-π interactions between macrocycles to form a layered arrangement of packed molecules, analogous to a stack of coins. When silicon tetrachloride is introduced to the reaction vessel, the coupling between the surface and porphyrins is mediated through covalent Si-O bonding. For these studies, surface structures of Si-OEP were formed that are connected with a Si-O-Si motif to a silicon atom coordinated to the center of the porphyrin macrocycles. Protocols of colloidal lithography were used as a tool to prepare surface structures and films of Si-OEP to facilitate surface characterizations. Conceptually, by arranging the macrocycles of porphyrins with defined orientation, local AFM surface measurements can be enabled to help address mechanistic questions about how molecules self-assemble and bind to substrates.

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.

Low-Voltage Voltammetric Electrowetting of Graphite Surfaces by Ion Intercalation/Deintercalation

We demonstrate low-voltage electrowetting at the surface of freshly cleaved highly oriented pyrolytic graphite (HOPG). Using cyclic voltammetry (CV), electrowetting of a droplet of a sodium perchlorate solution is observed at moderately positive potentials on high-quality (low step edge coverage) HOPG, leading to significant changes in the contact angle and relative contact diameter that are comparable to the results of the widely studied electrowetting on dielectric (EWOD) system, but over a much lower voltage range. The electrowetting behavior is found to be reasonably fast, reversible, and repeatable for at least 20 cyclic scans (maximum tested). In contrast to classical electrowetting, e.g., EWOD, the electrowetting of the droplet on HOPG occurs with the intercalation/deintercalation of anions between the graphene layers of graphite, driven by the applied potential, observed in the CV response, and detected by X-ray photoelectron spectroscopy. The electrowetting behavior is strongly influenced by those factors that affect the extent of the intercalation/deintercalation of ions on graphite, such as potential range scan rate, potential polarity, quality of the HOPG substrate (step edge density and step height), and type of anion in the solution. In addition to perchlorate, sulfate salts also promote electrowetting, but some other salts do not. Our findings suggest a new mechanism for electrowetting based on ion intercalation, and the results are important to fundamental electrochemistry as well as to diversifying the means by which electrowetting can be controlled and applied.

Environmental Applications of Interfacial Materials with Special Wettability

Interfacial materials with special wettability have become a burgeoning research area in materials science in the past decade. The unique surface properties of materials and interfaces generated by biomimetic approaches can be leveraged to develop effective solutions to challenging environmental problems. This critical review presents the concept, mechanisms, and fabrication techniques of interfacial materials with special wettability, and assesses the environmental applications of these materials for oil-water separation, membrane-based water purification and desalination, biofouling control, high performance vapor condensation, and atmospheric water collection. We also highlight the most promising properties of interfacial materials with special wettability that enable innovative environmental applications and discuss the practical challenges for large-scale implementation of these novel materials.

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.

Layer-by-layer assembly of a metallomesogen by dip-pen nanolithography

Palladium alkanethiolates are introduced here as a novel liquid ink for dip-pen nanolithography (DPN). These structures exhibit the unusual characteristic of layer-by-layer assembly, allowing one to deposit a desired number of metal ions on a surface, which can subsequently be reduced via thermolysis to form active catalytic structures. Such structures have been used to generate contiguous metallic or conducting polymer nanoscale architectures by electroless deposition.

Direct patterning of engineered ionic gold nanoparticles via nanoimprint lithography

Gold nanoparticles are engineered for direct imprinting of stable structures. This imprinting strategy provides access to new device architectures, as demonstrated through the fabrication of a prototype photoswitchable device.

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.

Octadecyltrichlorosilane (OTS)-coated ionic liquid drops: Micro-reactors for homogenous catalytic reactions at designated interfaces

An ionic liquid (IL), 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) can assemble on prefabricated carboxylic acid-terminated chemical patterns on octadecyltrichlorosilane (OTS) film. The chemical pattern controls the position, shape and size of the IL on the surface. After the IL assembly - by incubating IL drops assembled on sample surface in an OTS silane vapor - an OTS layer was coated on the IL drop surface which encapsulated the IL drop. The OTS-coated capsule can exist stably under aqueous solution. The OTS coating protected the IL drops from being instantaneously dissolved by other solutions. We found that a homogenous catalyst (FeCl(3)) dissolved in [Bmim]Cl can be assembled together on the chemical patterns and subsequently encapsulated together with [Bmim]Cl by OTS coating. The pinhole defects within the vapor-coated silane layer provide space for the catalyst inside the capsule and reactants outside the capsule to meet and react. When the OTS-coated capsule containing a FeCl(3)/IL mixture was soaked under H(2)O(2) solution, the Fe(3+) ions catalyzed the decomposition reaction of hydrogen peroxide at the vapor-coated OTS-water interface. Since the shape and position of the interface is defined by the underneath chemical pattern, our findings show that the OTS-coated IL drops assembled on chemical patterns can be used as novel micro-reactors. This allows homogenous catalytic reactions to occur at the designated interfaces.

Nanopatterning of ferritin molecules and the controlled size reduction of their magnetic cores

A nanopatterning method to deposit ferritin proteins with nanoscale accuracy over large areas is reported. Selective deposition is driven by the electrostatic interactions existing between the proteins and nanoscale features. Upon deposition, the protein shell can be removed by heating the patterns in an oxygen atmosphere. This leaves exposed the iron oxide core, which can be further reduced in size by plasma-etching methods. In this way, the initial ferritin molecules, which have a nominal size of 12 nm, are reduced to 2 nm nanoparticles. Magnetic force measurements confirm the magnetic activity of the as-deposited and etched nanoparticles.

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.

Conductive AFM patterning on oligo(ethylene glycol)-terminated alkyl monolayers on silicon substrates: proposed mechanism and fabrication of avidin patterns

Micro- and nanopatterns of biomolecules on inert, ultrathin platforms on nonoxidized silicon are ideal interfaces between silicon-based microelectronics and biological systems. We report here the local oxidation nanolithography with conductive atomic force microscopy (cAFM) on highly protein-resistant, oligo(ethylene glycol) (OEG)-terminated alkyl monolayers on nonoxidized silicon substrates. We propose a mechanism for this process, suggesting that it is possible to oxidize only the top ethylene glycol units to generate carboxylic acid and aldehyde groups on the film surface. We show that avidin molecules can be attached selectively to the oxidized pattern and the density can be varied by altering the bias voltage during cAFM patterning. Biotinylated molecules and nanoparticles are selectively immobilized on the resultant avidin patterns. Since one of the most established methods for immobilization of biomolecules is based on avidin-biotin binding and a wide variety of biotinylated biomolecules are available, this approach represents a versatile means for prototyping any nanostructures presenting these biomolecules on silicon substrates.

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.

Supramolecular functionalization of electron-beam generated nanostructures

Electron-beam lithography was used to pattern poly(styrene-co-(methyldiaminotriazine) styrene) (PS-Triaz). These polymer nanopatterns were utilized as molecular scaffolds for assembling complementary thymine-functionalized CdSe-ZnS quantum dots (Thy-QDs) via three-point hydrogen-bonding molecular recognition. This interaction was very specific, with N-methyl thymine-functionalized QDs (MeThy-QDs) not depositing on the surfaces. The "lock and key" specificity of the assembly is mirrored in the disassembly process, where complete removal of the QD was observed using a competing thymine guest.

Tunable Fabry-Perot interferometer from ferroelectric polymer based on surface energy modification

Surface energy modification was utilized in the fabrication of hollow transmission Fabry-Perot interferometer (FPI) for the first time. Polydimethylsiloxane (PDMS) was used to modify the surface energy of substrate for the self-assembly of poly(vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)] 70/30 mol% copolymer film on given areas, which is simple and low destructive for the photoelectric device. A strain of 7.12% under a field of 22.3 MV/m was observed from the copolymer film, which led to the FPI with a tunable range of 54 nm at wavelength of 604 nm.

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.

Controllable assembly of diverse rare-earth nanocrystals via the Langmuir-Blodgett technique and the underlying size- and symmetry-dependent assembly kinetics

The Langmuir-Blodgett (LB) technique provides a facile and robust method for the formation of large-area films of various nanoparticles (NPs), including 24.9 nm NaYF(4):Yb,Er nanospheres, 12.0 nm LiYF(4) nanopolyhedra, 14.1 x 1.8 nm triagonal-shaped LaF(3), 12.6 nm square CaF(2), 9.5 x 2.0 nm hexagonal EuF(3), and so forth. The assembly patterns of the deposited films were studied in accordance with the pi-A isotherms. Combined with the TEM observations, several representative stages of assembly process can be distinguished. The scrutiny of the self-assembly process by means of their pi-A isotherms elucidates that the concentration, size, and symmetry of nanoparticles play crucial roles in this process. The concept of "effective concentration", which is defined as the amount of nanoblocks in the "gas phase" rather than the actual number of nanoparticles at the air-water interface, was first proposed as a control parameter to elucidate the possible assembly kinetics. The similarly shaped 12.0 nm LiYF(4) and the 24.9 nm NaYF(4):Yb,Er were selected as the size-dependent examples. The smaller nanoparticles show a strong tendency of congregation to lower the surface energy. Three representative samples, namely, 24.9 nm NaYF(4):Yb,Er nanospheres (O(h)), 14.1 x 1.8 nm oblate triagonal LaF(3) nanosheets (D(3h)), and 41.3 nm x 24.6 nm NaYF(4) rods (D(6h)), were selected as the shape-dependent samples, which showed that the assembly patterns were contributed by the stability arising from the geometry of the nanoparticles, the tendency of aggregation of nanoparticles, and the probable rotation energy during the compression. More importantly, guided by the above assembly kinetics, for the 9.5 x 2.0 nm hexagonal EuF(3), we can effectively acquire the desirable assembly pattern.

Controlled assembly of gold nanoparticles and graphene oxide sheets on dip pen nanolithography-generated templates

The ability to organize nanomaterials, e.g., Au nanoparticles (NPs) and graphene oxide (GO) sheets, into ordered structures with high accuracy and resolution on a substrate is crucially important for fundamental studies and applications. In this letter, we developed a simple and efficient method to generate positively charged 11-amino-1-undecanethiol (AUT) templates on Au substrates, which were successfully used for controlled assembly of negatively charged Au NPs or GO sheets from aqueous solution. The templates were obtained by passivation of the exposed Au area with AUT after 16-mercaptohexadecanoic acid (MHA) patterns were generated by dip-pen nanolithography (DPN) on Au. The electrostatic interaction ensures that the Au NPs and GO sheets only adsorb on the designed AUT areas. Importantly, by using this method, the number of Au NPs adsorbed on patterned areas can be controlled, and a single Au NP array was successfully achieved.

The partially degraded hydrophilic silane pattern and its application in studying the structures of long chain alkane films

We developed a protocol to fabricate hydrophilic patterns over an octadecyltrichlorosilane (OTS) film surface with an atomic force microscope (AFM). Through a local probe oxidation under a 100% humidity environment, the OTS was converted into a hydrophilic, carboxylic acid-terminated surface (OTSpd). The OTSpd pattern grew with the voltage dwell time applied on the conducting AFM probe. Eighty nanometer to submillimeter sized OTSpd patterns could be fabricated with a single scanning probe. The OTSpd patterns were used to study the spreading of long chain alkanes. Hexatriacontane (C36H74) was dip-coated on an OTSpd pattern. Subsequently, an additional hydrophilic OTSpd region was fabricated surrounding the coated C36H74. The alkane spread over this newly created region when heated above its melting point. After cooling to room temperature, the shape and structures of the solidified alkane patterns were characterized. On the methyl-terminated, low-energy surface, the alkane molecules stood directly on the surface. In contrast, on the hydrophilic, high-energy surface, the alkane formed seaweed-shaped patterns after spreading. On the OTSpd surface, the alkane molecules initially adsorbed on the hydrophilic surface with their alkyl chains parallel to the surface. Additional alkane molecules stood vertically or tilted on top of the parallel layer, forming the seaweed-shaped layer. The seaweed patterns were previously thought to consist of only vertically standing alkane molecules. We found that three additional tilted phases existed in the seaweed-shaped structures.

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.

Sequential electrochemical oxidation and site-selective growth of nanoparticles onto AFM probes

In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH 4 and further development (if desired) to give a metal nanoparticle-modified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).

Control of wettability of molecularly thin liquid films by nanostructures

The patterning of liquid thin films on solid surfaces is very important in various fields of science and engineering related to surfaces and interfaces. A method of nanometer-scale patterning of a molecularly thin liquid film on a silicon substrate using the lyophobicity of the oxide nanostructures has recently been reported (Fukuzawa, K.; Deguchi, T.; Kawamura, J.; Mitsuya, Y.; Muramatsu, T.; Zhang, H. Appl. Phys. Lett. 2005, 87, 203108). However, the origin of the lyophobicity of the nanostructure with a height of around 1 nm, which was fabricated by probe oxidation, has not yet been clarified. In the present study, the change in thickness of the liquid film on mesa-shaped nanostructures and the wettability for the various combinations of the thickness of the liquid films and the height of ridge-shaped nanostructures were investigated. These revealed that lyophobicity is caused by a lowering of the intermolecular interaction between the liquid and silicon surfaces by the nanostructure and enables the patterning of a liquid film along it. The tendency of the wettability for a given liquid film and nanostructure size can be predicted by estimating the contributions of the intermolecular interaction and capillary pressure. In this method, the height of the nanostructure can control the wettability. These results can provide a novel method of nanoscale patterning of liquid thin films, which will be very useful in creating new functional surfaces.

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.

Self-assembly of patterned monolayers with nanometer features: molecular selection based on dipole interactions and chain length

Patterned cocrystal monolayers self-assemble on HOPG in contact with solutions containing complementary pairs of 1,5-chain-substituted anthracene derivatives. Monolayer unit cells containing three or four molecules and spanning 9-11 nm are generated. The monolayers consist of alternating aromatic and aliphatic columns. The designs and dimensions of the cocrystal patterns (unit cells) are determined by (i) the preferred packing alignment of identical length side chains, (ii) the selectivity of each side chain for neighboring chains, (iii) the identities of the two side chains on each anthracene, and (iv) the 2D-chirality of 1,5-substituted anthracenes. The aliphatic columns form by interdigitation of identical length side chains arrayed in an antiparallel alignment, with the nth heavy atom of one side chain in registration with the (omega+2-n)th heavy atom of two adjacent chains ((omega <--> 2) packing). Adjacent side chains are attached, alternately, to anthracenes in one of the two flanking aromatic columns. The preference for (omega <--> 2) packing optimizes side-chain van der Waals interactions. The composition and fidelity of patterning in the cocrystal monolayers requires an additional source of "molecular recognition" in addition to side-chain length. Dipolar interactions, both attractive and repulsive, between ether groups in neighboring, (omega <--> 2) packed side chains, constitute a second recognition element needed for cocrystal self-assembly.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.