Formation of aromatic siloxane self-assembled monolayers

Heterostuctures of 4-(chloromethyl)phenyltrichlorosilane and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine prepared on Si(111) using particle lithography: Nanoscale characterization of the main steps of nanopatterning

Nanostructures of 4-(chloromethyl)phenyltrichlorosilane (CMPS) were used as a foundation to attach and grow heterostructures of porphyrins and organosilanes. A protocol was developed with particle lithography using steps of immersion in organosilane solutions to selectively passivate the surface of Si(111) with octadecyltrichlorosilane (OTS). A methyl-terminated matrix was chosen to direct the growth of CMPS nanostructures to fill the uncovered sites of Si(111) to enable spatial confinement of the surface reaction. Silica spheres with a diameter of 500 nm were used as a surface mask to prepare nanoscopic holes within the OTS matrix film. Next, the samples were immersed in solutions of CMPS dissolved in toluene or bicyclohexane. Nanostructures of CMPS formed within the nanoholes, to furnish spatially selective sites for binding porphyrins. The samples were then characterized with AFM to evaluate the height and morphology of the CMPS nanostructures that had formed within the nanoholes of OTS. The samples were then refluxed in a porphyrin solution for selective binding to produce heterostructures. The attachment of porphyrins was evidenced by increases in the height and width of the CMPS nanopatterns. The measurements of size indicate that multiple layers of porphyrins were added. Through each step of the surface reaction the surrounding matrix of OTS showed minimal areas of nonspecific adsorption. The AFM studies provide insight into the mechanism of the self-polymerization of CMPS as a platform for constructing porphyrin heterostructures.

Compact test apparatus for evaluation of flow erosion of marine coatings

An apparatus designed and manufactured for evaluation of flow erosion of coatings or layers is presented in this paper. The setup was primarily designed for coatings intended to perform in dynamic marine environments but can be also used for evaluation using fresh water. The concept is based on an in-line flow test cell and modular design allowing good flexibility of varying testing parameters. The flow rate that can be achieved depends on the flow cell geometry and can reach 28 km/h (15 kn) with the presented setup. Temperature may be adjusted between 15 and 35 °C. Particle and metal ion filters are parts of this setup. The dimensions of the apparatus including all components do not exceed 2 m × 2 m × 2 m. The use of the apparatus is illustrated with the results of evaluation of self-polishing anti-fouling coatings and model, silicon wafer grafted layers.

Olefin epoxidation by a (salen)Mn(iii) catalyst covalently grafted on glass beads

A newly synthesized robust (salen)Mn(iii) monolayer on glass bead substrates provides an active catalyst for asymmetric epoxidation of 6-cyano-2,2-dimethylchromene, cis-β-ethylstyrene, 1,2-dihydronaphthalene and indene.

Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.

Sulfobetaine-based polymer brushes in marine environment: is there an effect of the polymerizable group on the antifouling performance?

Three different zwitterionic polymer brush coatings for marine biofouling control were prepared by surface-initiated atom transfer radical polymerization (ATRP) of sulfobetaine-based monomers including methacrylamide (SBMAm), vinylbenzene (SBVB) and vinylimidazolium (SBVI). None of these brush systems have been assessed regarding marine antifouling performance. Antifouling tests performed indicate that surfaces featuring these three brush systems substantially reduce the adhesion of the marine microalgae, Amphora coffeaeformis, and the settlement of cyprid larvae of the barnacle, Amphibalanus amphitrite, in a similar way, displaying comparable performance. Thus, it appears that the chemical structure of the polymerizable group has no substantial influence on marine antifouling performance.

Enhanced stability of low fouling zwitterionic polymer brushes in seawater with diblock architecture

The successful implementation of zwitterionic polymeric brushes as antifouling materials for marine applications is conditioned by the stability of the polymer chain and the brush-anchoring segment in seawater. Here we demonstrate that robust, antifouling, hydrophilic polysulfobetaine-based brushes with diblock architecture can be fabricated by atom-transfer radical polymerization (ATRP) using initiator-modified surfaces. Sequential living-type polymerization of hydrophobic styrene or methyl methacrylate and commercially available hydrophilic sulfobetaine methacrylamide (SBMAm) monomer is employed. Stability enhancement is accomplished by protecting the siloxane anchoring bond of brushes on the substrate, grafted from silicon oxide surfaces. The degradation of unprotected PSBMAm brushes is clearly evident after a 3 month immersion challenge in sterilized artificial seawater. Ellipsometry and atomic force microscopy (AFM) measurements are used to follow changes in coating thickness and surface morphology. Comparative stability results indicate that surface-tethered poly(methyl methacrylate) and polystyrene hydrophobic blocks substantially improve the stability of zwitterionic brushes in an artificial marine environment. In addition, differences between the hydration of zwitterionic brushes in fresh and salt water are discussed to provide a better understanding of hydration and degradation processes with the benefit of improved design of polyzwitterionic coatings.

Directed surface assembly of 4-(chloromethyl)phenyltrichlorosilane: self-polymerization within spatially confined sites of Si(111) viewed by atomic force microscopy

The self-polymerization of 4-chloromethylphenyltrichlorosilane (CMPS) was studied within spatially confined nanoholes on Si(111) using atomic force microscopy (AFM). Surface platforms of nanoholes were fabricated within a film of octadecyltrichlorosilane using immersion particle lithography. A heating step was developed to temporarily solder the silica mesospheres to the surface, to enable sustained immersion of mesoparticle masks in solvent solutions for the particle lithography protocol. Substrates with a film of mesospheres were heated briefly to anneal the particles to the surface, followed by a rinsing step with sonication to remove the silica beads to generate nanopores within an octadecyltrichlorosilane (OTS) film. Nanopatterned surface templates were immersed in CMPS solutions and removed at different time points to monitor the successive growth of nanostructures over time. Analysis of AFM images after progressive exposure of the nanoholes to solutions of CMPS provided quantitative information and details of the surface self-assembly reaction. Pillar nanostructures of CMPS with different heights and diameters were produced exclusively within the exposed areas of the substrates. Throughout the reaction, the surrounding matrix of OTS-passivated substrate did not evidence growth of CMPS; the surface assembly of CMPS was strictly confined within the nanopores. The diameter of the CMPS nanostructures grew to match the initial sizes of the confined areas of Si(111) but did not spread out beyond the edges of the OTS nanocontainers. However, the vertical growth of columns was affected by the initial size of the sites of uncovered substrate, evidencing a direct correspondence; larger sites produced taller structures, and correspondingly the growth of shorter structures was observed within smaller nanoholes. The heights of CMPS nanostructures indicate that multilayers were formed, with taller columns generated after longer immersion times. These experiments offer intriguing possibilities for using particle lithography as a general approach for nanoscale studies of molecular self-assembly.

Functionalization of SnO₂ crystals with a covalently-assembled porphyrin monolayer

The functionalization of micro- and nano-sized metal-oxide powders offers many advantages because of their large surface areas and, therefore, the large number of functional molecules that can be grafted onto the grain surfaces. Porphyrin molecules on large band-gap semiconducting metal oxides represent key materials for many different optical and electronic applications. Herein, we have proposed a general two-step procedure for the functionalization of metal-oxide crystals with dye-sensitizers. In particular, we functionalized SnO₂ nanoparticles with a monolayer of the bifunctional trichloro[4-(chloromethyl)phenyl]silane. Then, a monolayer of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyne was covalently bound to the silanized SnO₂ grains. IR, UV/Vis, and luminescence measurements were used for optical characterization. The measured footprint of the grafted porphyrin molecules indicated total surface coverage of the grains. The surface electronic characterization was performed by using X-ray photoelectron spectroscopy. Emission measurements revealed two strong bands at 664.1 and 721.0 nm that were attributed to the porphyrin monolayer assembled on the surface of the SnO₂ crystals.

Novel temperature-responsive polymer brushes with carbohydrate residues facilitate selective adhesion and collection of hepatocytes

Temperature-responsive glycopolymer brushes were designed to investigate the effects of grafting architectures of the copolymers on the selective adhesion and collection of hypatocytes. Homo, random and block sequences of N-isopropylacrylamide and 2-lactobionamidoethyl methacrylate were grafted on glass substrates via surface-initiated atom transfer radical polymerization. The galactose/lactose-specific lectin RCA120 and HepG2 cells were used to test for specific recognition of the polymer brushes containing galactose residues over the lower critical solution temperatures (LCSTs). RCA120 showed a specific binding to the brush surfaces at 37 °C. These brush surfaces also facilitated the adhesion of HepG2 cells at 37 °C under nonserum conditions, whereas no adhesion was observed for NIH-3T3 fibroblasts. When the temperature was decreased to 25 °C, almost all the HepG2 cells detached from the block copolymer brush, whereas the random copolymer brush did not release the cells. The difference in releasing kinetics of cells from the surfaces with different grafting architectures can be explained by the correlated effects of significant changes in LCST, mobility, hydrophilicity and mechanical properties of the grafted polymer chains. These findings are important for designing 'on-off' cell capture/release substrates for various biomedical applications such as selective cell separation.

Fine wettability control created by a photochemical combination method for inkjet printing on self-assembled monolayers

Wettability tuning for organic solvents is demonstrated with the "combination method", a reversal of the conventional "cleavage method". Several advantages are inherent to this method: for example, the syntheses are simple, various surface-active groups can be used, and the reaction proceeds with a low-energy light source. The image shows the result after UV irradiation through a patterning mask.

Halogen-bonding mediated stepwise assembly of gold nanoparticles onto planar surfaces

In this study halogen bonding (XB) is used as the driving force for the noncovalent assembly of gold nanoparticles (AuNPs) on silicon and quartz substrates functionalized with organic monolayers. The AuNPs are functionalized with XB-donor ligands, whereas the monolayers have pyridine groups as XB-acceptors. The surface-confined systems are formed by iteratively exposing the monolayers to solutions of organic cross-linkers having 2-4 pyridine groups and functionalized AuNPs. UV-vis spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) reveal how the structure of the resulting surface-bound assemblies are controlled by (i) the properties of the monolayers, (ii) the molecular structure and the number of XB binding sites of the organic cross-linker, and (iii) the number of functionalized AuNP and cross-linker deposition steps. Moreover, these structures exhibit surface-enhanced Raman scattering and significant changes are observed in the morphology of some of the surface-bound assemblies upon aging.

Single and binary self-assembled monolayers of phenyl- and pentafluorophenyl-based silane species, and their phase separation with octadecyltrichlorosilane

In this paper, we first present the study of the formation of phenyltrichlorosilane film and self-assembled monolayers of phenylalkyltrichlorosilane (PATCl), pentafluoro-phenylalkyltrichlorosilane (PFATCl), and a mixture of the two, on silicon covered by its native oxide. These monolayers are shown to grow in two steps with characteristic time constants. The first step is characterized by a similar time constant of growth for all the studied trichlorosilane molecules and attributed to chemisorption. The second step corresponds to the arrangement between molecules, accelerated by the presence of the short alkyl chain (3-4 carbon atoms), and by mixing phenyl and pentafluoro-phenyl terminal moieties, which is accounted for by hydrogen bonding CH···FC and/or attractive quadrupolar interactions within a face-to-face phenyl/pentafluoro-phenyl alternating stack arrangement. Such results should allow improvement of intermolecular stacking within conjugated molecular domains, which is particularly important for molecular electronic devices. In the second part, we studied how PATCl, PFATCl, and their mixture phase separate with octadecyltrichlorosilane (OTS) molecules in various ratios. The way to improve phase separation was studied modifying aromatic ring to ring as well as aromatic-aliphatic interactions. OTS island size and coverage are shown to be smaller with the aromatic phase that involves stronger ring to ring interactions, i.e., attractive interactions between the phenyl species by mixing phenyl and pentafluoro-phenyl rings. The best phase separation is obtained with PFATCl as the aromatic molecule. If nanoislands of aromatic molecules could not be observed in these experiments, we show that they are attainable by mixing OTS and aromatic small organotriethoxysilanes whose grafting kinetics is slower. These results pave the way to the control improvement of the composition and nanostructuration of SAMs, essential for their further use within molecular devices.

Selective electroless metallization of patterned polymeric films for lithography applications

The fabrication of electrical interconnects to provide power for and communication with computers as their component complementary metal oxide semiconductor (CMOS) devices continue to shrink in size presents significant materials and processing compatibility challenges. We describe here our efforts to address these challenges using top-surface imaging and hybrid photoresist/self-assembled monolayer patterning approaches, in conjunction with selective electroless metal deposition, to develop processes capable of fabricating appropriate submicron and nanoscale metal features useful as electrical interconnects, as well as plasma-etch-resistant masks and metal diffusion barriers. Our efforts focus on the development of cost-effective methods compatible with a manufacturing environment that satisfy materials and process constraints associated with CMOS device production. We demonstrate the fabrication of approximately 50-nm-width features in metal with high fidelity and sufficient control of edge acuity to satisfy current industry design rules using our processes and discuss the challenges and opportunities for fabrication of analogous sub-10-nm metal features.