Hydrophilic Silicon Substrates Research Articles

Multiscale Carbon Structures Fabricated by Direct Micropatterning of Electrospun Mats of SU-8 Photoresist Nanofibers

A novel method for the direct fabrication of arrays of micropatterned polymeric and carbon nanofiber structures on any substrate is developed. First SU-8, an epoxy-based negative photoresist, is electrospun under optimized conditions to produce a layer of polymeric nanofibers. Next, this nanofibrous mat is micropatterned using photolithography, and finally, pyrolysis produces ordered arrays of microdomains containing carbon nanofibers. The nanotextured surfaces of carbon nanofibers are shown to be very hydrophobic (water contact angle approximately 130 degrees). Micropatterning thus generates a substantial wettability contrast of nanofiber domains with intervening micropatches of very hydrophilic carbon (approximately 20 degrees) or silicon substrates.

Polyelectrolyte–surfactant complexes on solid surface

Polyelectrolyte–surfactant complexes have been deposited on hydrophilic silicon substrates by using a horizontal deposition technique. DNA and carboxymethyl cellulose (carboxyMC) were used as short and long polyelectrolyte and dodecyltrimethylammonium bromide (DTAB) was used as water-soluble surfactant. Varying the surfactant concentration, the structural and morphological information have been obtained for these polyelectrolyte–surfactant complexes. Morphology and out-of-plane structures have been obtained by atomic force microscopy and X-ray reflectivity studies. Electron density profiles obtained from the reflectivity study show that DNA–DTAB complexes form lamellar like multilayered structure but carboxyMC–DTAB complexes form coil-like structure. At lower DTAB concentration, these DNA–DTAB and carboxyMC–DTAB complexes form Gibbs layer, whereas at higher surfactant concentration, DTAB molecules themselves form lamellar like multilayered structure that coexists with the structure formed by the complexes.

Multiple Transmission−Reflection Infrared (MTR-IR) Spectroscopy of Arachidic Acid LB Films on Hydrophilic and Hydrophobic Silicon Surfaces

Our recently developed multiple transmission−reflection infrared (MTR-IR) spectroscopy method was applied to measure arachidic acid Langmuir−Blodgett films on hydrophilic and hydrophobic silicon substrates. High-quality MTR-IR spectra at both s and p polarizations were obtained. The MTR-IR absorption intensity at different incident angles was found to be linearly correlated with the transmission times (N) for arachidic acid monolayers on a hydrophilic silicon substrate. Significant differences in polarization-dependent intensities were observed for some specific vibration modes, for example, CH3 and CH2 stretching, carboxylic acid stretching, and CH2 bending, on different types of monolayer and multilayer films. By means of polarization-dependent spectra, the molecular orientation was numerically calculated, and the molecular structures such as the crystalline packing of hydrocarbon chains and the trans and cis configurations of carboxylic acid species were deduced. Obvious and repeatable intensity differ...

Phospholipid Langmuir Film as Template for in Situ Silica Nanoparticle Formation at the Air/Water Interface

This study describes for the first time the in situ formation of small-sized silica nanoparticles after the polycondensation of tetraethylorthosilicate (TEOS) at the air/water interface. Mixtures of TEOS and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) are prepared with different molar ratios (TEOS/DPPC 15:1, 50:1, 500:1, and 5000:1) and dissolved in chloroform. Spreading TEOS/DPPC 15:1 and 50:1 on the water surface of a Langmuir trough leads to an initial increase of the surface pressure ( approximately 26-29 mN/m) after allowing chloroform to evaporate. Within the reaction time of 21 h, only a slight decrease of the surface pressure by approximately 2 mN/m occurs. Films of silica/DPPC mixtures are transferred from the air/water interface to silicon wafers by dip-coating. The morphologies of the silica nanoparticles and agglomerates together with DPPC are observed using tapping mode atomic force microscopy (AFM). An ordered array of silica nanoparticles can be observed after 21 h reaction of the precursor solution of TEOS/DPPC 500:1 and transfer onto hydrophilic silicon substrate. A mixture with a molar ratio of TEOS/DPPC of 5000:1 is also placed onto the water surface. DPPC forms a uniform film observed by AFM after film transfer. Silica rich domains can be observed with mesoporous morphology.

Organosilane self-assembled multilayer formation based on activation of methyl-terminated surface with reactive oxygen species generated by vacuum ultra-violet excitation of atmospheric oxygen molecules

A xenon excimer lamp which irradiates vacuum ultra-violet (VUV) light at 172 nm in wavelength was applied to the photochemical surface conversion of n-octadecyltrimethoxysilane self-assembled monolayer (ODS-SAM) in the presence of atmospheric oxygen and subsequent multilayer fabrication. The terminal functional groups of ODS-SAM, –CH 3 groups, were converted into polar functional groups, like –COOH, by the reaction with atomic oxygen species generated photochemically through VUV excitation of atmospheric oxygen molecules. The structure of the resulting organosilane multilayer with different numbers of superimposed monolayers (from 1 to 11), prepared on a smooth and hydrophilic silicon substrate by the layer-by-layer (LbL) approach, was examined in terms of molecular organization as well as the intra- or interlayer binding modes in such novel films. Ellipsometry and grazing angle X-ray reflectivity measurements revealed that multilayer films of up to 11 discrete monolayers were successfully obtained, indicating that the self-assembly is a viable technique for the construction of relatively thick (16 nm and above) multilayer films.

Collapse of preformed cobalt stearate film on water surface

Preformed cobalt stearate (CoSt) molecules form a film on the water surface, which with barrier compression shows multilayers of different heights that are evidenced from the structures of the films deposited on hydrophilic silicon (0 0 1) substrates by using a horizontal deposition technique at different positions of the surface pressure ( π)–specific molecular area ( A) isotherm. In-plane morphology and out-of-plane structures are obtained from the atomic force microscopy (AFM) and X-ray reflectivity studies. Electron density profiles (EDPs), extracted from the reflectivity data, show that the monolayer coverage is maximum when π is far before the collapse point ( π c) but with barrier compression domains of multilayers start to form even before π c. After π c, two different bilayer repeat distances have been observed from the two different series of the Bragg peaks implying the formation of domains by both the tilted and untilted CoSt molecules. Far after π c, reflectivity decreases rapidly and morphology of the deposited films changes totally. Structures before and after π c of the CoSt film have also been obtained by changing the pH of the subphase water. From all the structural information it is clear that the preformed CoSt film collapses in a different way in comparison with the collapse of the standard cobalt stearate monolayer where cobalt stearate molecules were formed at the air–water interface. Reasons for obtaining different structures on the water surface with barrier compression have been proposed.

Hierarchical Organization of Poly(ethylene oxide)-block-poly(isobutylene) and Hydrophobically Modified Fe2O3 Nanoparticles at the Air/Water Interface and on Solid Supports

Langmuir monolayers and Langmuir-Blodgett (LB) film morphologies of block copolymers and hydrophobically modified iron oxide nanoparticles were studied by surface pressure-mean molecular area (pi-mmA) measurements and by tapping mode atomic force microscopy (AFM). The amphiphilic diblock copolymers consisted of a hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic poly(isobutylene) (PIB) block. The pi-mmA isotherm of PEO(97)-b-PIB(37) (the subscripts refer to the respective degrees of polymerization) at the air/water interface had an extended plateau reflecting the extension of PEO chains into the water subphase at a surface pressure of 10 mN.m(-1), which is absent for the more hydrophobic PEO(19)-b-PIB(130). Iron oxide (Fe(2)O(3)) nanoparticles capped with oleic acid ligands as the shell were dispersed in the amphiphilic block copolymers at the air/water interface to prevent macroscopic aggregation of the particles. When the nanoparticles were mixed with PEO(97)-b-PIB(37), using a particle to polymer chain ratio of 1:100, macroscopic aggregation of the nanoparticles was not observed, and the pi-mmA isotherm was dominated by PEO(97)-b-PIB(37). Monolayers of block copolymers were transferred at different surface pressures from the air/water interface to hydrophilic silicon substrates using the Langmuir-Blodgett technique. The AFM images of PEO(97)-b-PIB(37) LB films depicted not only the typical finger-like morphology of the crystallized PEO blocks but also PIB blocks arranged in vertical columns growing perpendicular to the substrate surface. The columns are characteristic for PEO(19)-b-PIB(130) LB films after transfer at high surface pressures and can be assigned to a mesomorphic PIB phase with ordered chains. Finally, it was observed that small clusters of a few Fe(2)O(3) nanoparticles occupy the top of PIB phases after compression and transfer of the block copolymer nanoparticle mixtures to solid supports.

Micromachined Hydrogel Stamper for Soft Printing of Biomolecules with Adjustable Feature Dimensions

We report on the development of a hydrogel stamper with built-in reservoirs for soft-printing of biomolecules with adjustable feature dimensions. The stamper consists of potassium hydroxide (KOH)-etched silicon cavities onto which biomolecule-soaked low cross-linked density hydrogel with large pores and low mechanical strength is loaded. Application of weight to the top surface allows for a controllable protrusion of hydrogel from the opposite nozzles. Such protrusion combined with a suitable spacer between the stamper and a substrate provides a means for printing features with dimensions depending on the applied weight. Utilizing the above method, we successfully stamped bovine serum albumin conjugated with fluorescein isothiocyanate (BSA-FITC) model proteins on hydrophilic silicon substrates with a feature dimension ratio of 20:1 using a single stamper.

Langmuir monolayer and Langmuir–Blodgett films of amphiphilic triblock copolymers with water-soluble middle block

Langmuir monolayers and Langmuir–Blodgett (LB) film morphology of amphiphilic triblock copolymers are studied using surface pressure-area measurements and atomic force microscopy (AFM), respectively. The triblock copolymers are composed of long water-soluble poly(ethylene oxide) (PEO) chains as middle block with very short poly(perfluorohexylethyl methacrylate) (PFMA) end blocks. The surface pressure-area isotherms show phase transitions in the brush regime. This phase transition is due to a rearrangement of PFMA block at the air–water interface. It becomes more significant with increasing PFMA content in the copolymer. LB films transferred at low surface pressures from the air–water interface to hydrophilic silicon substrates show surface micelles in the size range of 50–100 nm. A typical crystalline morphology of the corresponding PEO homopolymer is observed in LB films of copolymers with very short PFMA blocks, transferred in the brush region at high surface pressure. This crystallization is hindered with increasing PFMA content in the copolymer.

Growth of bimolecular films of three-tailed amphiphiles

The three-tailed amphiphile ferric stearate molecule, which forms a bimolecular layer on water surface with molecules in the lower and upper layers in different conformations, has been studied to understand transfer and growth of bimolecular films on the surface of hydrophilic silicon substrates. This bimolecular film forms a two-dimensional lattice on water with a slightly distorted hexagonal lattice where both the in-plane and out-of-plane domain sizes are small. The film also showed larger microscopic rigidity compared to its macroscopic mechanical response. This asymmetric bimolecular layer was found to be preserved when the film is transferred on the substrates at different values of surface pressures ranging from 1 mN/m to near-collapse (55 mN/m). Both the upper and lower layers become denser and interfaces between these layers become sharper with increase in deposition pressure but the growths have different natures. The lower layer of transferred film is dense from 1 mN/m and, except for a steplike increase between 20 and 30 mN/m, changes slowly in density. The density of the upper molecular layer grows continuously with surface pressure.

Langmuir–Blodgett and Langmuir–Schaefer films of poly(5-amino-1-naphthol) conjugated polymer

Langmuir-type films of poly(5-amino-1-naphthol) conductive polymer were formed at the air/water interface. The surface pressure versus molecular area isotherms of the polymeric layers showed a high tendency to aggregate and non-monomolecular behavior on the water surface. Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) procedures have been successfully used to transfer the Langmuir films of this polymer onto hydrophilic silicon substrates. Atomic force microscopy images showed that the films obtained by the LS method have much better quality than those obtained by the LB one. In particular, we have obtained uniform and smooth LS films covering practically all the substrate, while the LB films showed rather aggregated material, only partially covering the substrate.

Organized Monolayers of Carbosilane Dendrimers with Mesogenic Terminal Groups

The structural ordering of mesogenic-terminated carbosilane dendrimers within surface monolayers was studied. We have shown that despite their symmetrical spherical shape layering with average spacing close to 6 nm is observed for dendrimers with butoxyphenylbenzoate terminal groups deposited on hydrophilic silicon substrates. The molecular layered packing is formed by flattened dendritic molecules with terminal groups stacking into bilayers. This layered ordering is controlled by strong interactions between the polar mesogenic groups and the hydrophilic surface and by microphase separation of the dendritic core and terminal groups. The dilution of the outer mesogenic shell with a nonmesogenic component partially disturbs the layered packing within the monolayer. Complete disruption of the regular lamellar ordering is observed if butoxyphenylbenzoate terminal groups are replaced with shorter and more polar cyanbiphenyl groups. For all dendrimers studied here the replacement of the hydrophilic support with hydrophobic one prevents the formation of dense surface layers due to antagonistic interactions between polar terminal groups and a methyl-terminated surface. However, the presence of the nonmesogenic hydrophobic terminal groups stimulates the formation of two-dimensional circular molecular structures which are characteristic of columnar LC dendrimer phases.

Preparation of stacked organosiloxane bilayers on hydrophilic and hydrophobic silicon substrates by spin coating

A simple one-step procedure for the preparation of stacked organosiloxane bilayers is demonstrated. Millimolar solutions of long-chain alkyl- and alkenyltrichlorosilanes, respectively, have been spin coated onto silicon samples with distinct hydrophilicity. After coating, the samples are characterized using optical microscopy, atomic force microscopy (AFM) and contact angle measurements. Generally, the procedure yields thin films consisting of highly aligned bilayers. The wettability of the substrates by the silane solution, however, strongly affects both the macroscopic and the microscopic homogeneity of the coatings across the surface. Cross-linking via Si–O–Si bonds appears to set in after some weeks and stabilize the films. For several days, however, the bilayers show a remarkable flexibility, i.e. using the AFM tip the self-assembling entities can be easily shifted between the distinct levels maintaining their vertical alignment.

Unusual Expansion and Contraction in Ultrathin Glassy Polycarbonate Films

The thermal expansion behavior of thin polycarbonate (PC) films supported on hydrophilic and hydrophobic silicon substrates is studied with specular X-ray reflectivity. These measurements suggest a suppression of the apparent glass transition temperature (Tg) with decreasing film thickness h on hydrophilic silicon oxide substrates, while no evidence for an h-dependent Tg shift is observed on hydrophobic passivated silicon substrates. More interestingly, we find that when h is comparable to the bulk radius of gyration, the expansion becomes nonmonotonic with temperature T in a reversible manner. Cooling one of these ultrathin PC films well below the apparent Tg gives rise to an apparently negative coefficient of thermal expansion (CTE). This unusual behavior stems from the fact that nominally Rg-thick glassy films exhibit an appreciable isothermal thickening over extended periods of time, with the propensity for this thickening increasing with the depth of cooling into the glassy state; normal thermal contraction upon cooling becomes convoluted with the sub-Tg thickening phenomenon. This is counterintuitive since deep in the glassy state the molecular mobility is anticipated to be small. The magnitude and rate of the isothermal thickening depend on the ratio of the macromolecular size to the film thickness, suggesting that chain distortion by the film boundaries is associated with this effect. The surface energy of the supporting substrate also influences these unusual behaviors.

Functionalization of carbon nanotubes with amphiphilic molecules and their Langmuir–Blodgett films

Crown ether-modified full-length multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) have been prepared by a direct heating method. Subsequently, the Langmuir–Blodgett (LB) method was used to prepare a monolayer of modified carbon nanotubes on an air–water interface. The corresponding Langmuir–Blodgett (LB) films appeared to be very stable and could be transferred onto a hydrophilic silicon substrate easily. Scanning electron microscopy (SEM) images revealed the morphology of the films, in which locally aligned structures could be observed.

Hydroxyapatite layers deposited from aqueous solutions on hydrophilic silicon substrate

Devices of micro-electro-mechanical systems or biochips for diagnosis to be implanted into human body require a biocompatible and protective coating to avoid inflammatory effects. We have developed a method to deposit a bioactive hydroxyapatite (HA) layer onto pretreated hydrophilic Si(1 1 1) from supersaturated solutions containing Ca 2+ and PO 4 3− ions with an initial pH value of 7.40±0.05 at 298 K. The deposition starts with an initial uniform deposit at a low rate approximately 25 nm/h. Growth then takes place predominantly along the direction normal to the substrate through in-plane nucleation of multiple embryos at a high rate approximately 325 nm/h. The deposition rate increases with increasing time duration, attaining a maximum at a thickness approximately 8 μm, and then decreases. X-ray photoelectron spectroscopy (XPS) indicates typical chemical states of HA phase. X-ray diffraction pattern of a 4-day deposit shows the direct formation of crystalline HA corroborating with the XPS results. XPS results indicate the possible incorporation of carbonate.

Substrate Effect on Topographical, Elastic, and Frictional Properties of Hydrogels

Cross-linked poly(N,N‘-dimethyl acrylamide) hydrogels were prepared on hydrophilic silicon substrates or hydrophobic polystyrene substrates and their topographical elastic properties were investigated by atomic force microscopy (AFM). The gels prepared on the hydrophobic substrate have a much rougher surface morphology with a lower surface elastic modulus than those prepared on the hydrophilic substrate. The topographical and mechanical properties of the gel have little influence on the surface friction, and the lower surface friction observed for the gel prepared on the hydrophobic substrate was attributed to the presence of dangling free polymer chains formed in the course of the polymerization.

X-ray, Micro-Raman, and Infrared Spectroscopy Structural Characterization of Self-Assembled Multilayer Silane Films with Variable Numbers of Stacked Layers

The structure of a series of alcohol-terminated bifunctional long-tail organosilane films with varying numbers of superimposed monolayers (between 1 and 11), prepared on smooth, hydrophilic silicon substrates by the layer-by-layer self-assembly approach, has been investigated with the purpose of elucidating details of the molecular organization and the intra- and interlayer modes of binding in such films. To this end, experimental results obtained by synchrotron X-ray scattering, micro-Raman, and Fourier transform infrared (FTIR) spectroscopic techniques have been combined and compared. A comprehensive analysis of all data demonstrates that the studied multilayer films consist of stacks of uncorrelated discrete monolayers, the inner molecular order of which is preserved with the growing total number of superimposed layers. Similar to self-assembled films of long-tail silanes with terminal −COOH groups,1 the intermolecular binding in the present films is characterized by partial intra- and interlayer covalent bond formation. The molecular hydrocarbon tails are perpendicularly oriented on the layer planes, forming a densely packed rotator phase like hexagonal lattice with a molecular surface area of ∼20 Å2 and a lateral correlation length of the order of 16 molecular diameters.

Photovoltaic characteristics of BR/p-silicon heterostructures using surface photovoltage spectroscopy

Purple membrane (PM) monolayers were deposited on hydrophilic or hydrophobic silicon substrates through the Langmuir–Blodgett (LB) technique. The photovoltaic features and interfacial charge separation of p-Si/PM/indium tin oxide (ITO) heterostructure were studied by surface photovoltage spectroscopy (SPS). The different photovoltaic response values were obtained due to the nonrandom orientation of PM in the LB films on the hydrophilic versus hydrophobic silicon substrates. The photovoltaic response value versus external potential of the p-Si/cytoplasmic-extracellular/ITO heterostructure showed an obviously rectifying behavior. Compared with the p-Si/ITO heterostructure, the response value of SPS increased more rapidly in the case of the positive external bias.

Formation of Mesoporous Silica Nanotubes

[29] M. A. van Dijk, R. van den Berg, Macromolecules 1995, 28, 6773.[30] From the morphology of the polymer blend thin film, it was alreadypointed out that there is no clear affinity for one of the two hydropho-bic phases to preferentially wet the hydrophilic silicon substrate.[31] V. Z.-H.Chan,E. L. Thomas, R. G. H.Lammertink, M. A. Hempenius,G. J. Vancso, H. Wang,R. J. Composto, unpublished.[32] Auger electron spectroscopy (AES) data obtained for oxygen etchedPFS homopolymer are consistent with the XPS results. Depth profiling(argon ion sputtering) indicated an approximately 10 nm thin layerrich of iron, silicon, and oxide at the surface of the oxygen etchedpolymer film.[33] A. B. Fischer, J. B. Kinney, R. H. Staley, M. S. Wrighton, J. Am.Chem. Soc. 1979, 101, 6501.