Hydrophilic Silicon Surfaces Research Articles

Water structuring and collagen adsorption at hydrophilic and hydrophobic silicon surfaces

The adsorption of a collagen fragment on both a hydrophobic, hydrogen-terminated and a hydrophilic, natively oxidised Si surface is investigated using all-atom molecular dynamics. While favourable direct protein-surface interactions via localised contact points characterise adhesion to the hydrophilic surface, evenly spread surface/molecule contacts and stabilisation of the helical structure occurs upon adsorption on the hydrophobic surface. In the latter case, we find that adhesion is accompanied by a mutual fit between the hydrophilic/hydrophobic pattern within the protein and the layered water structure at the solid/liquid interface, which may provide an additional driving force to the classic hydrophobic effect.

A comparative study of the growth of octadecyltrichlorosilane and 3-mercaptopropyltrimethoxysilane self-assembled monolayers on hydrophilic silicon surfaces

Self-assembled monolayers of two different organosilane precursors, methyl-terminated nonpolar n-octadecyltrichlorosilane (OTS, Cl 3Si(CH 2) 17CH 3) and thiol-terminated polar 3-mercaptopropyltrimethoxysilane (MPTMS, (CH 3O) 3SiCH 2CH 2CH 2SH), were prepared separately on hydrophilic silicon surfaces by immersion in millimolar solutions of the respective precursors in toluene at room temperature. Ex situ atomic force microscopy (AFM), lateral force microscopy (LFM) and X-ray photoelectron spectroscopy (XPS) were used to study the growth and the properties of OTS and MPTMS SAMs. For OTS SAMs, generally speaking, small islands surrounded large dendrite-shaped islands. But for MPTMS SAMs, sporadic small round islands appeared, but no dendrites. The impact of the solution age was more significant on the growth of OTS SAMs than MPTMS SAMs. At the same precursor concentration and solution age, the growth of OTS SAMs was much faster than MPTMS SAMs due to the greater hydrolysis ability of Si–Cl bonds in OTS as compared with that of Si–OCH 3 bonds in MPTMS. The difference in hydrolysis ability was confirmed by the absence of a Cl signal in the XP spectrum of OTS SAMs and the existence of a C 1s peak corresponding to unhydrolyzed Si–OCH 3 bonds in the XP spectrum of MPTMS SAMs. This trend together with the difference in alkyl chain length had a strong influence on the surface morphology and coverage of these two SAMs. According to the individual adsorption behavior of the components, the predictable kinetic difficulty of preparing OTS/MPTMS mixed SAMs by co-adsorption is pointed out. Furthermore, a potential reaction condition for stepwise adsorption is suggested.

Ice nucleation on hydrophilic silicon

We have used Fourier transform infrared spectroscopy to study thin water films on a hydrophilic silicon surface in the temperature range from 20 to -20 degrees C. Throughout that range, the spectra of the water adjacent to the silicon surface are consistent with that of bulk water near 25 degrees C. Thicker films (>1 microm) freeze at -11+/-1 degrees C. We reconcile the apparent paradox of a thin film of water which is quite liquidlike at a temperature where freezing of thicker films occurs by hypothesizing that the nucleation event in the thicker film is triggered by a critical ice embryo which forms at some small distance from the silicon surface, as opposed to in direct contact with it.

Comparison of Interfacial Forces During Post-Chemical-Mechanical-Polishing Cleaning

This research investigates the interfacial forces involved in tribological interactions while removing nanosized particles during post-chemical-mechanical polishing cleaning. Surface and interfacial forces are discussed to understand the particle adhesion and subsequent removal through physical and chemical interactions. Approaches include theoretical analysis combined with experimental study. The theoretical analysis was focused on the forces that exist between particles and a substrate. Surface interaction consideration includes applied pressure, frictional force, and hydrodynamic drag. The polishing experiments were carried out on silicon wafers with SiO2 slurry. Cleaning experiments were performed in de-ionized water using a polyvinyl acetal brush to remove particles from a hydrophilic-silicon surface. The fluid-drag force was found to affect the lubricating behavior of cleaning through changing material properties. Values of interfacial forces and their effects on cleaning were discussed along with a lubricating model system.

Molecular dynamics simulations of elasto-hydrodynamic lubrication and boundary lubrication for automotive tribology

Friction control of machine elements on a molecular level is a challenging subject in vehicle technology. We describe the molecular dynamics studies of friction in two significant lubrication regimes. As a case of elastohydrodynamic lubrication, we introduce the mechanism of momentum transfer related to the molecular structure of the hydrocarbon fluids, phase transition of the fluids under high pressure, and a submicron thickness simulation of the oil film using a tera-flops computer. For boundary lubrication, the dynamic behavior of water molecules on hydrophilic and hydrophobic silicon surfaces under a shear condition is studied. The dynamic structure of the hydrogen bond network on the hydrophilic surface is related to the low friction of the diamond-like carbon containing silicon (DLC-Si) coating.

Local modification of the silicon surface with protein molecules

Adsorption of bovine serum albumin, horse radish peroxidase, and green fluorescent protein on the hydrophilic silicon surface was studied. The possibility of preparing microstructured one-, two-, and three-component films on the solid surface by combining the methods of microcontact printing and self-arrangement of proteins from solution was demonstrated.

Microtribological properties of silicon and silicon coated with self-assembled monolayers: effect of applied load and sliding velocity

This paper investigates the influence of the applied load and sliding velocity on the microfrictional properties of native oxide-covered Si(100) and Si(100) coated with octadecyltrichlorosilane (OTS) and perfluorodecyltrichlorosilane (FDTS) self-assembled monolayers (SAMs) using a precision microtribometer. Microfriction was investigated as a function of the applied load and sliding velocity. As has been confirmed in earlier studies, in the microtribological regime, OTS and FDTS significantly reduce the friction force in comparison to the bare native oxide-covered (hydrophilic) silicon surface. The friction versus applied load curve of the substrate material as well as the SAMs-covered surfaces can be described by a model based on contact mechanics. For the native oxide surface, microfriction is reduced with increasing sliding speed. The friction force of the OTS- and FDTS-covered surfaces increases with load and is proportional to the natural logarithm of sliding speed. The increase with sliding velocity gets larger for higher normal loads. It can be shown that this increase is proportional to the contact area of the counter sample with the SAMs.

Load dependence and lifetime studies of self‐assembled monolayers

AbstractA comparative study of the microtribological properties of native oxide on Si (100), Si (100) coated with octadecyltrichlorosilane (OTS) and perfluorode‐cyltrichlorosilane (FDTS) self‐assembling monolayers (SAMs) is presented. The frictional properties between these samples and a bare silicon sphere were examined using a microtribometer. Microfriction was investigated as a function of the normal load and relative humidity. Also, the microfriction of OTS‐ and FDTS‐coated surfaces was studied as a function of sliding time and normal load to examine the lifetime of these monolayers. Confirming the results of earlier studies, in the microtribological regime OTS and FDTS significantly reduce the friction force in comparison to the bare, native oxide covered (hydrophilic) silicon surface. The friction vs. normal load curve of oxide‐covered surfaces as well as the SAMs can be described by contact mechanics. Lifetime measurements of the SAMs, examined as a function of the normal load and relative humidity, indicate that the OTS monolayers wear quickly in both dry and moist environments, while the lifetime of FDTS monolayers appears to increase in moist environments. Copyright © 2006 John Wiley & Sons, Ltd.

Adsorption Behaviour of Creatine Phosphokinase onto Silicon Wafers: Comparison between Ellipsometric and Atomic Force Microscopy Data

Protein adsorption plays a major role in a variety of important technological and biological processes [1-2] and the understanding of the fundamental factors that determine protein adsorption are imperative to the development of biocompatible materials and biotechnological devices [3-4] as for example biosensors [5]. The adsorption of proteins on surfaces is a complex process. Due to the large size and different shapes of these adsorbing particles, the interactions between the adsorbed proteins on the surface can be strongly influentiated by the fact that the particles may undergo conformational changes upon adsorption [6-7]. In a previous work the adsorption behaviour of creatine phosphokinase (CPK) onto hydrophilic (silicon wafers and amino-terminated surfaces) and hydrophobic (Polystyrene, PS, coated wafers) substrates was investigated by means of null-ellipsometry and contact angle measurements [8]. This previous ellipsometric study led to a model, where CPK adsorption takes place in four stages: (i) a diffusive one, where all the arriving biomolecules are immediately adsorbed; (ii) the arriving biomolecules might stick on the latter and afterward diffuse to the free sites on the substrate, followed by conformational changes [6-7], (iii) formation of a monolayer and (iv) continuous and irreversible adsorption. A multilayer system might be formed, as well as aggregation processes might play a role at this stage. In this work Atomic Force Microscopy (AFM) measurements under water were done in order to confirm this four steps model and to observe changes in the film topography and homogeneity along the adsorption process. The thickness of the adsorbed CPK biofilm obtained by ellipsometry was also compared with that obtained by the wet AFM method.

Adsorption of Mixed Anionic and Nonionic Surfactants at the Hydrophilic Silicon Surface

Specular neutron reflectivity has been used to determine the structure and composition of the mixed anionic and nonionic surfactants adsorbed at the hydrophilic silica solid−solution interface. Measurements show that the anionic surfactant sodium dodecyl sulfate (SDS) is adsorbed at the hydrophilic silica surface in the presence of the nonionic surfactant hexaethylene monododecyl ether but has no affinity for the surface in the absence of the nonionic surfactant. The variations of adsorbed amount, composition, and structure of the adsorbed layer are shown to reflect the different affinities of the two surfactants for the hydrophilic surface. At a solution concentration greater than the critical micellar concentration, the adsorbed amount decreases for solutions increasingly rich in SDS, and the surface composition is not consistent with the pseudo phase approximation.

Direct bonding of silicon wafers with the concurrent formation of diffusion layers

An original technique for Si-Si direct bonding combined with impurity diffusion in a single process is suggested. A dopant (aluminum) source is located at the interface. The high-temperature treatment of the polished wafers in an oxidizing atmosphere results in the diffusion of Al atoms and the formation of a p-n junction in n-silicon. The presence of aluminum is shown to improve the continuity of the interface. Results obtained are explained within a model whereby the initial contact between the hydrophilic silicon surfaces in a water solution of aluminum nitrate Al(NO3)3 serves to increase the bonding area of the wafers at room temperature due to the interaction of Al-OH groups with water molecules adsorbed on the surfaces of the wafers.

Adsorption of Mixed Cationic and Nonionic Surfactants at the Hydrophilic Silicon Surface from Aqueous Solution: The Effect of Solution Composition and Concentration

The adsorption of the mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide (C16TAB) and hexaethylene glycol monododecyl ether (C12EO6) at the hydrophilic silicon−solution i...

The modeling of excimer laser particle removal from hydrophilic silicon surfaces

We summarize experimental results on the successful removal of submicron-sized polystyrene latex, carboxylate-modified latex, SiO2, and Al2O3 particles from hydrophilic silicon surfaces by excimer laser, using both dry and steam cleaning; the cleaning and damage thresholds have also been determined for these particles. Adhesion and removal models for an ideal sphere particle, that include van der Waals forces, hydrogen bonding, and thermoelastic effects, theoretically explain the laser cleaning results. Two models of the removal force due to the explosive evaporation of liquid film have been calculated and compared. The effects of both asperities on the particle surface and particle aggregation have also been considered. The results of the calculations show that even those surface asperities which are small compared with the particle dimension can cause a large reduction in both adhesion and thermoelastic removal forces. The theoretical predictions are consistent with the experimental observations.

Quantification of adsorbed human serum albumin at solid interfaces: a comparison between radioimmunoassay (RIA) and simple null ellipsometry

Radioimmunoassay (RIA) and null ellipsometry are two common methods to quantify adsorbed proteins. However, the accuracy of null ellipsometry with a constant protein refractive index ( n=1.465, k=0) at λ=632.8 nm has this far not been explored. The present study compared the methods, and the degree of agreement between the simplified single wavelength null ellipsometry and RIA to quantify adsorbed proteins was explored on different surfaces. The quantification methods agreed well when Ångström smooth hydrophilic or hydrophobic silicon surfaces, and freshly radio-labelled proteins were used. Some discrepancies were noted when either rough surface or stored and aged labelled proteins were used. The differences decreased when the aged protein solution was equilibrated with freshly dissolved proteins at room temperature (RT) for a few hours prior to the surface incubations. Significant differences were also noted between the methods when albumin was adsorbed at it’s iso-electric point (pH 4.8).

The effects of hydrogen bonds on the adhesion of inorganic oxide particles on hydrophilic silicon surfaces

Hydrogen bonds have a dominant effect on the adhesion of inorganic oxide particles, such as SiO2 and Al2O3, to hydrophilic silicon surfaces. An analysis of adhesion forces due to hydrogen bonds between particle and substrate surfaces has been carried out, and is used to interpret the efficiencies of removing polystyrene latex, SiO2, and Al2O3 particles from a hydrophilic silicon surface by laser cleaning. Evidence of the dominant effect of hydrogen bonding was confirmed by using alcohol instead of water during particle deposition.

In-situ detection of butane gas at a hydrophobic silicon surface

It has been suggested that attractive forces at hydrophobic surfaces arise from the cavitation of nanobubbles in the interfacial water region eventually leading to film instability and rupture. These phenomena may be the basis for bubble attachment at a hydrophobic surface and the flotation of hydrophobic particles. Now, for the first time the accommodation of gas by interfacial water at a hydrophobic silicon surface has been demonstrated by in-situ FTIR/IRS measurements for the butane/silicon system Such gas accommodation does nor occur at a hydrophilic silicon surface. The origin and long-range nature of hydrophobic interactions are discussed based on these spectroscopic measurements which support the presence of gas nanobubbles/cavities at hydrophobic surfaces. Cavitation of these nanobubbles may explain the very strong long-range attractive forces which have been measured between macroscopic hydrophobic surfaces in some systems.

Interactions between hydrophobic and hydrophilic silicon surfaces using a tapered probe in near-field scanning optical microscopy.

We examined the tip-sample interaction of a Si(111) surface processed with HF solution. The interaction length of the probe with its hydrophobic surface is approximately 30, and the frequency responses show an interaction force of elastic type. By contrast, the damping curve on a hydrophilic surface, Si with thin oxide, operates on a longer length of 100. Here, the frequency responses indicate an interaction force of the viscous type.

Excimer Laser Induced Removal of Particles from Hydrophilic Silicon Surfaces

A pulsed KrF excimer laser was used to remove several types of submicron-sized particles from silicon surfaces. Polystyrene latex particles, 0.1 μm and larger, were removed from silicon surfaces by dry laser cleaning (no water layer condensed on the surface) but SiO2 particles could not be so removed. However, during steam laser cleaning, in which a thin film of water is deposited on the surface as both an energy transfer medium and an adhesion force reduction agent, these 0.1–0.2 μm SiO2 particles were almost entirely removed. Calculations of the various forces contributing to adhesion indicate that hydrogen bonds are the major contributor to the adhesion of inorganic particles to substrate surfaces. Photoacoustic detection, using piezoelectric transducers, monitored the surface vibrations induced by the laser pulses.

Study of aged Langmuir-Blodgett monolayers by AFM and ToF-SIMS mapping

Langmuir-Blodgett (LB) monolayer samples of stearic acid transferred on hydrophilic plasma-oxidized silicon surfaces in the presence of barium chloride have been studied by combined atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping. By ageing, these samples develop holes of increasing size, and after some weeks these boles are large enough to be mapped by ToF-SIMS (lateral resolution ∼200 nm) but still in the domain of AFM imaging. High mass resolution ToF-SIMS spectra from such surfaces display Ba, CI, and stearate-related peaks, as well as substrate-related peaks, Chemical imaging by ToF-SIMS shows that barium, chlorine and stearate maps are correlated with each other and complementary to the signals from the substrate. Height image and interaction force (adhesion and friction) maps have been obtained by AFM on this chemically well-characterized system and show that it is possible to discriminate the organic film regions from the uncovered substrate on the basis of different interaction forces. In addition, the measurements allow us to state that film and substrate have good homogeneity. Furthermore, the interaction force images can be used to follow the early stages of the evolution of these systems, which are not accessible with other chemical mapping techniques.

Competitive Adsorption of Metal Ions onto Hydrophilic Silicon Surfaces from Aqueous Solution

Si surfaces prepared using the IMEC‐clean were exposed to a dilute solution of ten metals, Ba, Ca, Co, Cr, Cu, Fe, K, Ni, Sr, and Zn, in dilute nitric acid solution. The metal deposition onto hydrophilic silicon surfaces was found to be dependent on the metal concentration, pH, and adsorption time, the power‐dependence of metal surface concentrations, as expressed in , ranged from 0.27 for to 0.73 for , with an average value of 0.54. The power dependence of metal surface concentrations, as , ranged from −0.07 for K to −0.56 for Sr, with an average value of −0.30. We interpret these results to be consequences of all cations, in solution, including , competing for a limited number of adsorption sites. Using the Langmuir assumptions for adsorption behavior, we derive mathematical relationships for such behavior, with the key result being is the surface concentration of the metal, the solution concentration of the metal cation, and is the equilibrium constant governing its attachment to the surface, for the ith species. σ is the areal density of adsorption sites on the surface. Summation proceeds over all cations, including , in solution. Consequences of this relationship include the observed fractional power dependencies of and the metal cations, and the reduced adsorption onto the surface of any given metal cation when in the presence of other cations, including . © 1999 The Electrochemical Society. All rights reserved.