Hydrophilic Silicon Surfaces Research Articles

Laser Produced Hydrophilic and Hydrophobic Silicon Surfaces

Two lasers were utilized for silicon processing using photoelectrochemical etching and laser texturing in order to produce nano/micro structures, respectively. Photoelectrochemical etching process utilizes a CW diode laser of 532 nm wavelength was used to support electrochemical etching for both n-type and p-type conductivity. While laser texturing process was employed using pulsed fiber laser of 1064 nm wavelength. Various characterization methods were devoted to examine silicon micro/nanostructures surfaces produced by lasers. These methods include AFM, SEM and Raman scattering to provide clear evidence about formation of micro/nanostructures abundant at silicon surfaces. Moreover, FTIR analysis for the laser produced silicon surfaces could emphasize whether the resultant silicon surface is hydrophilic or hydrophobic. Image analysis software adopted a side view micro image was used to measure the contact angle between the water droplet and silicon micro/nano-surfaces. It is found that the laser produced silicon nanostructure by photoelectrochemical etching creates a hydrophobic surface and even super hydrophobic with contact angle of 130 degrees for 50 nm average size. In addition, utilizing fiber laser of high repetition rate for laser texturing produces microstructures that are super hydrophilic with contact angle could reach 8 degrees for a surface dimension of 50 μm.

Tuning contact line dynamics on slippery silicone oil grafted surfaces for sessile droplet evaporation

Controlling the dynamics of droplet evaporation is critical to numerous fundamental and industrial applications. The three main modes of evaporation so far reported on smooth surfaces are the constant contact radius (CCR), constant contact angle (CCA), and mixed mode. Previously reported methods for controlling droplet evaporation include chemical or physical modifications of the surfaces via surface coating. These often require complex multiple stage processing, which eventually enables similar droplet-surface interactions. By leveraging the change in the physicochemical properties of the outermost surface by different silicone oil grafting fabrication parameters, the evaporation dynamics and the duration of the different evaporation modes can be controlled. After grafting one layer of oil, the intrinsic hydrophilic silicon surface (contact angle (CA) ≈ 60°) is transformed into a hydrophobic surface (CA ≈ 108°) with low contact angle hysteresis (CAH). The CAH can be tuned between 1° and 20° depending on the fabrication parameters such as oil viscosity, volume, deposition method as well as the number of layers, which in turn control the duration of the different evaporation modes. In addition, the occurrence and strength of stick–slip behaviour during evaporation can be additionally controlled by the silicone oil grafting procedure adopted. These findings provide guidelines for controlling the droplet-surface interactions by either minimizing or maximising contact line initial pinning, stick–slip and/or constant contact angle modes of evaporation. We conclude that the simple and scalable silicone oil grafted coatings reported here provide similar functionalities to slippery liquid infused porous surfaces (SLIPSs), quasi-liquid surfaces (QLS), and/or slippery omniphobic covalently attached liquid (SOCAL) surfaces, by empowering pinning-free surfaces, and have great potential for use in self-cleaning surfaces or uniform particle deposition.

Protein (BSA) adsorption on hydrophilic and hydrophobic surfaces

Ultrathin films of model protein bovine serum albumin (BSA) are deposited at room temperature on hydrophilic and hydrophobic silicon surfaces in absence and presence of divalent ions and their hydrophobic, structural and morphological properties are investigated and compared. The hydrophobic property of the BSA films is evaluated using the water contact angle (CA) study. After adsorption on hydrophilic silicon surface (CA ≈ 20°), the hydrophobicity of pure and Ca2+ ions interacted BSA film is found to increase as CA values are obtained as ≈ 60° and 66.6° respectively, whereas on adsorption to hydrophobic silicon surface (CA ≈ 82°), the hydrophobicity decreases as CA changes to ≈ 72° and 77° respectively. The increment in CA or hydrophobicity of BSA films on both surfaces could be due to higher adsorption of BSA molecules in presence of divalent ions and due to more exposure of their hydrophobic residues towards the outer surface. Atomic force microscopy (AFM) images show deposition of smooth BSA films on both surfaces maintaining the globular structure. The structural properties obtained from X-ray reflectivity (XRR) studies show that a monolayer of tilted BSA molecules (≈ 45 Å) with its innate globular morphology is deposited on hydrophilic surface in absence of any ions but in the presence of divalent Ca2+ ions, bilayer of BSA molecules (≈ 70 Å) with tilted side-on orientation is deposited. Although, similar tilted monolayer is found to deposit on hydrophobic surface in absence of ions but the tilting is more with respect to surface as thicker (≈ 55 Å) and denser BSA layer is deposited. In presence of Ca2+ ions, tilted bilayer of BSA molecules (≈ 90 Å) is deposited on hydrophobic surface. Thus, in absence as well as in presence of divalent ions, higher adsorption of BSA molecules is confirmed on hydrophobic surface compared to hydrophilic surface.

Forecasting imminent atrial fibrillation in long-term ECG recordings

Ultrathin films of model protein bovine serum albumin (BSA) are deposited at room temperature on hydrophilic and hydrophobic silicon surfaces in absence and presence of divalent ions and their hydrophobic, structural and morphological properties are investigated and compared. The hydrophobic property of the BSA films is evaluated using the water contact angle (CA) study. After adsorption on hydrophilic silicon surface (CA ≈ 20°), the hydrophobicity of pure and Ca2+ ions interacted BSA film is found to increase as CA values are obtained as ≈ 60° and 66.6° respectively, whereas on adsorption to hydrophobic silicon surface (CA ≈ 82°), the hydrophobicity decreases as CA changes to ≈ 72° and 77° respectively. The increment in CA or hydrophobicity of BSA films on both surfaces could be due to higher adsorption of BSA molecules in presence of divalent ions and due to more exposure of their hydrophobic residues towards the outer surface. Atomic force microscopy (AFM) images show deposition of smooth BSA films on both surfaces maintaining the globular structure. The structural properties obtained from X-ray reflectivity (XRR) studies show that a monolayer of tilted BSA molecules (≈ 45 Å) with its innate globular morphology is deposited on hydrophilic surface in absence of any ions but in the presence of divalent Ca2+ ions, bilayer of BSA molecules (≈ 70 Å) with tilted side-on orientation is deposited. Although, similar tilted monolayer is found to deposit on hydrophobic surface in absence of ions but the tilting is more with respect to surface as thicker (≈ 55 Å) and denser BSA layer is deposited. In presence of Ca2+ ions, tilted bilayer of BSA molecules (≈ 90 Å) is deposited on hydrophobic surface. Thus, in absence as well as in presence of divalent ions, higher adsorption of BSA molecules is confirmed on hydrophobic surface compared to hydrophilic surface.

Growth of monolayer and trilayer of gold nanoparticles attached octadecanethiol films on hydrophilic silicon surface using Langmuir–Blodgett method

Growth of monolayer and trilayer of gold nanoparticles attached octadecanethiol films on hydrophilic silicon surface using Langmuir–Blodgett method

Self-Organization of Di- and Triglycine Oligopeptides in Thin Films on the Hydrophilic and Hydrophobic Silicon Surface under Exposure to Organic Compounds Vapors

Self-Organization of Di- and Triglycine Oligopeptides in Thin Films on the Hydrophilic and Hydrophobic Silicon Surface under Exposure to Organic Compounds Vapors

Structure and morphology of adsorbed protein (BSA) layer on hydrophilic silicon surface in presence of mono-, di- and tri-valent ions

Out-of-plane structure and surface morphology of the adsorbed thin films of globular protein bovine serum albumin (BSA) are investigated using X-ray reflectivity (XRR) and atomic force microscopy (AFM). BSA adsorption on hydrophilic silicon surface is done by dip coating method in absence and presence of mono- (Na + ), di- (Ca 2+ ) and tri- (La 3+ and Y 3+ ) valent ions in BSA solution for different salt concentrations. Unlike mono- and di-valent ions, BSA shows re-entrant phase transition for tri-valent ions like La 3+ and Y 3+ with their concentration variation. XRR study shows that the thickness of the adsorbed BSA layer slightly increases with the increase of mono- and di-valent ions concentration, while for tri-valent ions, considerable amount of thickness is obtained when the film is deposited from the condensed phase under re-entrant condensation and becomes relatively less with salt concentrations lower or higher than that particular salt concentration, however, thickness is again increased for higher salt concentration. Surface morphologies of the films are also modified with the variation of the salt type and their concentration. Thus, structural and morphological changes of the adsorbed protein layer on hydrophilic surface and the probable reason for such modifications are explored in the presence of different valent ions with their concentration variation. • BSA adsorption on hydrophilic surface is done in absence/presence of ions. • In presence of mono- and di-valent ions, bilayer-like structure is formed. • For tri-valent ions, concentration dependent structures are formed. • At turbid phase, tilted molecules are shifted vertically to form a trilayer-like structure. • Surface morphologies of adsorbed BSA films depend on salt type and concentration.

Interactions between Asphaltenes and a Model Demulsifier in Bulk and at an Interface Studied by Small-Angle Neutron Scattering (SANS) and Neutron Reflectometry

This article describes neutron reflectometry to probe the structure of the asphaltene layer adsorbed onto a hydrophilic silicon surface and the interactions between asphaltenes and a model demulsif...

Exploring Hidden Local Ordering in Microemulsions with a Weak Directive Second Order Parameter

So far, the near-surface ordering of microemulsions was focused on lamellar ordering while the bulk microemulsion was bicontinuous. In a series of different non-ionic surfactants the near-surface ordering of microemulsions at a hydrophilic silicon surface was studied using grazing incidence small angle neutron scattering. For the surfactant C8E3, most likely a gyroid structure was found at the solid–liquid interface, while the more efficient surfactants find lamellar ordering up to lamellar capillary condensation. The ranges for near-surface ordering are deeper than the bulk correlation lengths. These findings point towards theories that use directional order parameters that would lead to deeper near-surface ordering than simple theories with a single scalar order parameter would predict. Rheology experiments display high viscosities at very low shear rates and, therefore, support the existence of a directional order parameter.

Studying Hemoglobin and a Bare Metal-Porphyrin Complex Immobilized on Functionalized Silicon Surfaces Using Synchrotron X-ray Reflectivity.

We evaluate here, using synchrotron X-ray reflectivity, hemoglobin adsorption characteristics on silicon substrates with varying chemical functionalities. Hemoglobin at isoelectronic point and at negative charge is immobilized on functionalized hydrophilic (hydroxyl, carboxylic, amine) and hydrophobic (alkylated) silicon surfaces for the study. As a control, the bare cofactor hemin (containing only the metal and porphyrin with no amino acid residues) is also studied under similar conditions. Ordered layers (grown using the Langmuir-Blodgett technique) are observed to be less affected by the surface chemistry compared to the multilayers formed by physical absorption. Surface chemistry and charge of the proteins are critical in controlling the protein adsorption characteristics on silicon, such as thickness (correlated to molecule size) and roughness. In this study, this is very well realized by varying both the hydrophobicity and hydrophilicity of the substrate. The fundamental studies discussed here provide us with a set of important guidelines as to how electrode surface functionalization can control molecular conformation/orientation, especially protein adsorption on the substrate. This in turn is expected to have a significant impact on the protein electrochemical function and response of biomolecular devices.

Terminal silanization of perfluoropolyether, polydimethylsiloxane, their block polymer and the self-assembled films on plasma-treated silicon surfaces

In this paper, terminal silanization of perfluoropolyether, polydimethylsiloxane and their block polymer was studied and N-(Triethoxysilylpropyl) urethano-perfluoropolyether (silanized-PFPE), N-(triethoxysilylpropyl) urethano-polydimethylsiloxane (silanized-PDMS), N-(Triethoxysilylpropyl) urethano-polydimethylsiloxane -block-perfluoropolyether (silanized-PSPF) were obtained, respectively. The self-assembled films of terminal silanized polymers were prepared on plasma-treated silicon surfaces via the liquid phase deposition method (LPD), and the surface structures and properties were investigated by contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM). It was discovered that the hydrophilic plasma-treated silicon surfaces became hydrophobic and possessed lower surface-free energies after treating by all three silanized polymers. Among them, the self-assembled film of silanized-PFPE provided the lowest surface-free energy of 12.77 mN/m with the water contact angle being 118.2°. Self-assembled films with different deposition concentrations were prepared. Results showed that the higher the treating concentration, the higher water contact angle (WCA) obtained. The terminal silanized polymers were coated on the silicon substrates uniformly. This was identified by XPS and EDS measurements of chemical compositions on the surface of the films. AFM was used to analyze the surface morphologies of silanized PFPE-deposited films, which was found to be rough at all the different concentrations of treatment solution. A concentration of 5% could provide the completely covered film with the roughest surface and thus develop the surface with excellent hydrophobicity.

A Systematic Study of Plasma Activation of Silicon Surfaces for Self Assembly

We study the plasma activation systematically in an attempt to simplify and optimize the formation of hydrophilic silicon (Si) surface critical for self-assembly of nanostructures that typically uses piranha solution, a high molarity cocktail of sulfuric acid and hydrogen peroxide at elevated temperatures. In the proposed safer and simpler approach, O2 plasma is used under optimized process conditions in a capacitively coupled parallel-plate chamber to induce strong hydrophilic behavior on silicon surfaces associated with the formation of suboxide groups. Surface activation is validated and studied via contact angle measurements as well as XPS spectra and consequently optimized using a novel atomic force spectroscopy approach, which can streamline characterization. It is found that plasma power around 100 W and exposure duration of ∼65 s are the most effective parameters to enhance surface activation for the reactive ion etcher system used. Other optimum plasma process conditions for pressure and flow-rate are also reported along with temporal development of activation, which peaks within 1 h and wears off in 24 h scale in air. The applicability of the plasma approach to nanoassembly process was demonstrated using simple drop coating and spinning of polystyrene (d < 500 nm, 2.5-4.5% w/v) and inkjet printing on polydimethylsiloxane.

Capillary-induced crack healing between surfaces of nanoscale roughness.

Capillary forces are important in nature (granular materials, insect locomotion) and in technology (disk drives, adhesion). Although well studied in equilibrium state, the dynamics of capillary formation merit further investigation. Here, we show that microcantilever crack healing experiments are a viable experimental technique for investigating the influence of capillary nucleation on crack healing between rough surfaces. The average crack healing velocity, v̅, between clean hydrophilic polycrystalline silicon surfaces of nanoscale roughness is measured. A plot of v̅ versus energy release rate, G, reveals log-linear behavior, while the slope |d[log(v̅)]/dG| decreases with increasing relative humidity. A simplified interface model that accounts for the nucleation time of water bridges by an activated process is developed to gain insight into the crack healing trends. This methodology enables us to gain insight into capillary bridge dynamics, with a goal of attaining a predictive capability for this important microelectromechanical systems (MEMS) reliability failure mechanism.

Solution-processable graphene oxide as an insulator layer for metal–insulator–semiconductor silicon solar cells

Development of a high quality but low cost insulating layer is important for its application in photovoltaic cells. In this work, solution processable graphene oxide (GO) thin films were first utilized as an insulating layer to construct MIS silicon solar cells. The efficient water-soluble GO nano-sheets with controlled thickness produced a good contact with the hydrophilic silicon surfaces. The average open circuit voltage (VOC) of the GO incorporated MIS silicon solar cell was increased by 0.2 V, while their power conversion efficiency (PCE) was demonstrated as 88% higher than that of the corresponding Schottky solar cells. The improvement of the device performance in the GO-based MIS silicon solar cell is attributed to the increased built-in potential as well as the reduced interface defect, resulting in reduced carrier recombination. The ability to establish a low-cost and solution-processable insulating layer will open the door for wide application in photovoltaic and other optoelectronic devices.

Chemical Control of Surfaces: From Fundamental Understanding to Practical Application

In the early days of the microelectronics industry, it became clear that even trace contaminants could have detrimental impact on the electronic properties of fabricated devices. This realization led to the development of the so-called RCA clean for silicon surfaces [], which uses sequential baths in basic and acidic hydrogen peroxide solutions, now known as SCA-1 and SCA-2, to oxidize organic materials, remove particulates, and bind metallic impurities. The detailed characterization of this process as well as its simplicity and economic viability soon led to its widespread industrial adoption. Although the RCA clean includes an optional etch in dilute HF between the two cleaning solutions to remove the native oxide layer, the overall process results in an extremely clean but electronically defectiveoxide-terminatedand thus extremely hydrophilic silicon surface, which we now know is quite rough on an atomic scale [].

Adsorption Behavior of Hydrophobin and Hydrophobin/Surfactant Mixtures at the Solid–Solution Interface

The adsorption of surface-active protein hydrophobin, HFBII, and HFBII/surfactant mixtures at the solid-solution interface has been studied by neutron reflectivity, NR. At the hydrophilic silicon surface, HFBII adsorbs reversibly in the form of a bilayer at the interface. HFBII adsorption dominates the coadsorption of HFBII with cationic and anionic surfactants hexadecyltrimethyl ammonium bromide, CTAB, and sodium dodecyl sulfate, SDS, at concentrations below the critical micellar concentration, cmc, of conventional cosurfactants. For surfactant concentrations above the cmc, HFBII/surfactant solution complex formation dominates and there is little HFBII adsorption. Above the cmc, CTAB replaces HFBII at the interface, but for SDS, there is no affinity for the anionic silicon surface hence there is no resultant adsorption. HFBII adsorbs onto a hydrophobic surface (established by an octadecyl trimethyl silane, OTS, layer on silicon) irreversibly as a monolayer, similar to what is observed at the air-water interface but with a different orientation at the interface. Below the cmc, SDS and CTAB have little impact upon the adsorbed layer of HFBII. For concentrations above the cmc, conventional surfactants (CTAB and SDS) displace most of the HFBII at the interface. For nonionic surfactant C(12)E(6), the pattern of adsorption is slightly different, and although some coadsorption at the interface takes place, C(12)E(6) has little impact on the HFBII adsorption.

Grafting of monoglyceride molecules for the design of hydrophilic and stable porous silicon surfaces

Hydrophilic chemically stable porous silicon surfaces are generated by surface functionalisation with polar head terminated lipid biomolecules of the monoglyceride type. Two approaches to anchor the monoglyceride moiety to porous silicon surfaces are presented.

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...

Dynamic Wetting of Polyisoprene Melts: Influence of the End Group

The spontaneous spreading of drops of polyisoprene melt terminated with methyl (PI-CH(3)), hydroxyl (PI-OH), and carboxyl groups (PI-COOH) on hydrophilic silicon surfaces has been studied experimentally. Despite the fact that all three polymers have a similar surface tension (0.032 N/m) at the polymer-air interface, the equilibrium contact angles were 29 degrees, <5 degrees, and 20 degrees, respectively. Spreading of PI-OH and PI-COOH is slowed down as compared to PI-CH(3), most probably due to the strong interfacial binding of the hydroxyl or carboxyl end group to the surface. We interpret and discuss the dynamic wetting experiments using the hydrodynamic and molecular kinetic theory of wetting.

SIM-08 DYNAMIC STRUCTURE OF BOUNDARY LUBRICATING WATER FILM ON HYDROPHILIC SURFACES USING MOLECULAR DYNAMICS SIMULATION(Simulations of Micro/Nano Scale Phenomena III,Technical Program of Oral Presentations)

Molecular dynamics simulations are used to study the dynamic behavior of confined water molecules on hydrophilic and hydrophobic silicon surfaces under a shear condition. The dynamically stable structure of the hydrogen bond network on the hydrophilic surface is found which is related to the low friction of the DLC-Si coating.