Hydrophobic Si Research Articles

Molecular Dynamics Simulation of Bubble Formation in Confined Water in Si/SiO2 Nanochannels

We used molecular dynamics simulations to study bubble formation in confined water in Si and SiO2 nanochannels. Our simulations showed that surface wettability plays a major role in determining the mechanism type of bubble formation in confined geometry of nanofeatures. Hydrophobic Si surfaces and hydrophilic SiO2 surfaces led to heterogenous and homogenous bubble formation, respectively. Such observations can help in understanding cavitation effects during etching/cleaning processes of features at nano-scale dimensions.

Gravimetric analysis of water vapor adsorption kinetics on hydrophilic and hydrophobic Si surfaces using microcantilevers

Understanding the nature of the adsorption of water molecules on a Si surface is of great interest in basic sciences as well as in broad range of industrial processes. Traditional techniques such as Fourier transform infrared spectroscopy cannot capture the adsorption phenomena on a larger surface area of Si. Herein, we report the adsorption kinetics of water molecules on hydrophilic and hydrophobic Si surfaces by means of a precision gravimetric method using microcantilevers (MC). Si MCs with hydrophilic and hydrophobic surfaces were synthesized by chemical route, and their resonance frequency with increasing Relative Humidity (RH) was measured. Shift in resonance frequency of MCs due to adsorption of water molecules was used to estimate the added mass on their surface. In hydrophilic MCs, added mass Vs RH data revealed three distinctive slopes, indicating variation in the molecular configuration of water molecules with increasing RH as predicted by spectroscopic techniques. These results are independently validated by measuring the adhesion force on the MC surface using force-distance curves. In contrast to this, in hydrophobic MCs, added mass due to adsorption was very low and found to increase marginally at higher RH values. An increase in MC surface roughness leading to the clustering of water molecules was shown to be the reason for these observations.

Interfacial interactions in urease/stearic acid and urease/DOPC binary mixed systems at the air-water and the air-solid interfaces

In the present study, the protein-lipid interactions between water-soluble metalloenzyme urease (Urs) and insoluble stearic acid (SA) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were investigated at the air–water interface using the Langmuir-Blodgett (LB) technique. Surface pressure (π) - molecular area (A) isotherms were measured for the pure SA and pure DOPC monolayers in the presence of urease in the water subphase. From the (π – A) isotherms of the binary system, it was found that urease strongly interacts with SA and DOPC. The compressibility of the SA monolayer was reduced with the incorporation of urease; a similar trend was also found with DOPC. Moreover, an association of urease with lipid monolayers was found to be surface pressure (π) dependent. The presence of a little hump in the mixed isotherms of Urs-SA at low surface pressure supports this observation. However, the miscibility studies and the calculations of excess Gibb’s free energy indicate an attractive interaction between urease and SA molecules. The considerable morphological variations observed by atomic force microscopy (AFM) in urease-SA deposited films onto hydrophilic and hydrophobic Si(100) surfaces suggest that urease was strongly bound with the surrounding lipid SA on the hydrophilic surfaces but the urease molecules were rigorously denatured on the hydrophobic surfaces. The AFM analysis of Urs-DOPC mixed LB films at room temperature (RT) and elevated temperature suggests substantial denaturation of urease in a hydrophobic environment leading us to speculate the exposure of tryptophan moieties of the urease molecules to the hydrophobic air phase.

Hydrophobic Si nanopillar array coated with few-layer MoS2 films for surface-enhanced Raman scattering

In this study, we covered Si nanopillar (NP) array with few-layer MoS2 films to convert their wettability characteristics from hydrophilic to hydrophobic for applications as a surface-enhanced Raman scattering (SERS) substrate. The Si NP array was fabricated using a semiconductor process. We then sulfurized and transferred MoO3 films coated onto the Si NP array to MoS2 films. The surface morphology and cross-sectional profile of the MoS2-coated Si NP array structure was examined using scanning electron microscopy and transmission electron microscopy. The SERS results indicate that the substrate exhibits a favorable enhancement factor of 1.76 × 103 and a detection limit of approximately 10−5M for Rhodamine 6G (R6G) utilized as the test molecule, attributed to the charge transfer (CT) mechanism at the interface between MoS2 and R6G. Contact angle measurements showed that the MoS2-coated Si NP array possesses a hydrophobic surface. Our results suggest that an MoS2-coated Si NP array with CT and hydrophobicity characteristics is extremely promising SERS substrates for SERS applications.

Hydrophilic Surface Modification of Amorphous Hydrogenated Carbon Nanocomposite Films via Atmospheric Oxygen Plasma Treatment

Herein we investigated hydrophilic surface modification of SiOx containing amorphous hydrogenated carbon nanocomposite films (DLC:SiOx) via the use of atmospheric oxygen plasma treatment. The modified films exhibited effective hydrophilic properties with complete surface wetting. More detailed water droplet contact angle (CA) measurements revealed that oxygen plasma treated DLC:SiOx films maintained good wetting properties with CA of up to 28 ± 1° after 20 days of aging in ambient air at room temperature. This treatment process also increased surface root mean square roughness from 0.27 nm to 1.26 nm. Analysis of the surface chemical states suggested that the hydrophilic behavior of DLC:SiOx treated with oxygen plasma is attributed to surface enrichment with C-O-C, SiO2, and Si-Si chemical bonds as well as significant removal of hydrophobic Si-CHx functional groups. The latter functional groups are prone to restoration and are mainly responsible for the increase in CA with aging. Possible applications of the modified DLC:SiOx nanocomposite films could include biocompatible coatings for biomedical applications, antifogging coatings for optical components, and protective coatings to prevent against corrosion and wear.

A new single-step technique to fabricate transparent hydrophobic surfaces utilizable in perovskite solar cells

We report a simple low-cost novel method for coating an ultrathin hydrophobic film usable for different applications. A hydrophobic layer is fabricated on a TiO2 substrate by hydrolysis of silicone rubber evaporation. The hydrophobic Si (CH3)3 groups bonded to the substrate by the reaction of hydrophilic SiOH groups with the hydroxyl groups on the TiO2 surface. In addition, the silicone film was introduced in a carbon-based perovskite solar cell as a tunneling passivation layer between the electron transport layer and the active layer to reduce interfacial and extraction losses. An improved perovskite layer with better crystallinity, larger grain size, and higher stability was achieved. The optimal energy conversion efficiency was 17.42% with FF, VOC, and JSC values of 73.76%, 1.1 V, and 21.39 mA cm−2, respectively.

Fabrication of free standing graphene oxide membranes for efficient adsorptive removal of cationic dyes

Free standing graphene oxide membranes (FSGOMs) were prepared by controlled infra-red irradiation of aqueous GO dispersions on hydrophobic Si substrate and evaluated for their adsorption ability towards methylene blue (MB) class cationic dyes. The intactness and morphological characteristics of FSGOMs are determined by precursor GO dispersion medium and GO mass accumulated in membrane. The MB adsorption ability of FSGOMs was examined at different membrane weights, MB concentrations, ambience of interaction, initial pH, heat treatment conditions and these results were analyzed in light of corresponding morphological and vibrational features. Disintegration of FSGOMs in aqueous medium liberate plentiful GO layers enriched with unsaturated chemical bonds, which are prone to react with tertiary amine nitrogen, sulphur entities of MB molecules through electrostatic/π-π interactions and assist in MB adsorption. FSGOMs displayed superior adsorption efficiency towards MB as compared to rhodamine B, rhodamine 6G, malachite green dyes, which is attributed to relatively smaller size, planarity and alignment of MB molecules with GO layers and their affinity towards diverse GO functionalities. The demonstrated solid state FSGOM based procedure facilitate to provide platform for substantial adsorption of variety of cationic dyes, limits generation of secondary liquid waste and has significant potential in waste water purification applications.

Physicochemical Properties of a Bi-aromatic Heterocyclic-Azo/BSA Hybrid System at the Air-Water Interface.

The interaction of a heterocyclic azo compound with itself and with bovine serum albumin (BSA) is realized by probing the structural modifications in Langmuir (L) monolayers and Langmuir–Blodgett (LB) films. It was found from the pressure–area/molecule isotherms that the elastic, thermodynamic, and hysteretic properties of the pure azo L monolayer were strongly altered due to the variation of temperature and pH of subphase water. In addition to that, the modification of such properties of the azo L monolayer due to mixing with BSA was also studied. The incorporation of BSA within the azo molecular assembly reduced the elasticity of that assembly. Such reduction of in-plane elasticity of the pure azo monolayer can also be achieved by reducing the temperature and pH of subphase water without adding BSA. A reduction in area per molecule of the azo assembly at the air–water interface associated with the conformational change from horizontal to vertical orientation facilitating π–π interaction was observed with increase in temperature and pH of the subphase. Such parameters also affected the interactions between azo and BSA molecules within the azo/BSA binary system. The structures of pure azo and binary films can be determined after they are transferred to hydrophilic and hydrophobic Si surfaces using the LB technique. Their out-of-plane and in-plane structures, as extracted from two complementary surface sensitive techniques, X-ray reflectivity and atomic force microscopy, were found to be strongly dependent on mixing with BSA, subphase pH, temperature, and substrate nature.

A novel one-step synthesis of bright luminescent silicon nanocrystals capped with hydrophobic surface

Visible photoluminescent silicon nanocrystals (Si NCs) capped with hydrophobic surface are synthesized via a novel and one-step bottom-up strategy by utilizing trialkoxysilane as silicon source. Ascorbate sodium as reducing agent facilitates the hydrothermal-assisted hydrolysis and condensation of trialkoxysilane molecule to generate a SiSi rich core and promotes the formation of Si NCs. The synthesized hydrophobic Si NCs give an average size of 3.4 nm and exhibit visible photoluminescence with a maximum emission wavelength around 450 nm accompanying a high absolute quantum yield of 17% which could be attributed to the enhanced radiative recombination by present efficient hydrophobic surface capping on Si NCs.

Tailoring Nanonets‐Engineered Superflexible Nanofibrous Aerogels with Hierarchical Cage‐Like Architecture Enables Renewable Antimicrobial Air Filtration

AbstractPurification of pathogenic air has become an essential part of infection prevention and control. Most present air filters can hardly achieve excellent air filtration performance and the effective inactivation of the airborne pathogens at the same time. Herein, a bottom‐up approach is reported upon to construct cage‐like structured superflexible nanofibrous aerogels (CSAs) with renewable antimicrobial properties by combining electrospun silica nanofibers, bacterial cellulose nanofibers, and the hydrophobic SiOSi elastic binder. The following efficient grafting of N‐halamine compounds endows the CSAs with biocidal function. The resultant aerogels exhibit intriguing features of high porosity, hydrophobicity, superelasticity, foldability, renewable chlorination ability (>5400 ppm), high filtration performance toward PM0.3 (>99.97%, 189 Pa), and excellent antibacterial and antiviral activity (6 logs reduction within 5 min contact), enabling the aerogels to intercept and inactivate the pathogenic contaminants in air. The successful synthesis of CSAs provides a new possibility to design high‐performance air filtration materials for public health protection.

Heterogeneous assembly of water from the vapor phase-Physical experiments and simulations with binding trifunctional organosilanes at the vapor/solid interface.

A trace amount of interfacial water is required to initiate hydrosilation reactions of trifunctional organosilanes to form surface assemblies. In recent studies, we have learned that water also has a critical role in directing molecular placement on surfaces because water can react with silicon to provide oxygenated sites for surface binding. Consequently, the wettability nature of substrates influences the placement and density of organosilane films formed by vapor-phase reactions. Nanopatterning protocols were designed using vapor-phase organosilanes and colloidal lithography to compare the wettability differences of hydrophilic mica(0001) compared to relatively hydrophobic Si(100) as a strategy for tracking the location of water on surfaces. The competition between hydrophobic and hydrophilic domains for the adsorption and coalescence of water condensed from vapor can be mapped indirectly by mapping the organosilanes, which bind to water at the solid interface, using atomic force microscopy. Trifunctional octadecyltrichlorosilane (OTS) was used as a marker molecule to map out the areas of the surface where water was deposited. The effect of systematic changes in film thickness and surface coverage of OTS was evaluated at the vapor/solid interface by adding an incremental amount of water to sealed reaction vessels to wet the surface and assessing the outcome after reaction with vapor-phase trichlorosilane. Reactive molecular dynamics simulations of the silicon-water vapor interface combined with electronic structure calculations of oxygenated silicon clusters with methyltrichlorosilane provided insight of the mechanism for surface binding, toward understanding the nature of the interface and wettability factors, which influence the association and placement of silane molecules on surfaces.

Water droplet bouncing on a non-superhydrophobic Si nanospring array.

Self-cleaning surfaces often make use of superhydrophobic coatings that repel water. Here, we report a hydrophobic Si nanospring surface that effectively suppresses wetting by repelling water droplets. The dynamic response of Si nanospring arrays fabricated by glancing-angle deposition is investigated. These hydrophobic arrays of vertically standing nanosprings (about 250 nm high and 60 nm apart) allow the droplets to rebound within a few milliseconds after contact. Amazingly, the morphology of the nanostructures influences the impact dynamics. The rebound time and coefficient of restitution are higher for Si nanosprings than for vertical Si columns. By considering the droplet/nanospring surface as a coupled-spring system, we argue that the restoring force of the nanosprings may be responsible for the water-droplet rebound. The bouncing phenomena studied here are essential in the design of self-cleaning surfaces and are also of fundamental importance for the study of wetting behavior on nanostructures.

Silane Functionalized Polyvinyl-Alcohol Formaldehyde Sponges on Fast Oil Absorption

A series of silane functionalized polyvinyl-alcohol formaldehyde sponges (PVF-DTMS-x) containing hydrophobic Si–O–Si and dodecyl(−(CH2)12−) groups was synthesized. The PVF-DTMS-x sponges were obtai...

Structure and Elastic Properties of an Unsymmetrical Bi-Heterocyclic Azo Compound in the Langmuir Monolayer and Langmuir-Blodgett Film.

We study the structure and elastic properties of the bi-heterocyclic azo compound at the air–water interface through surface pressure (π)–area (A) isotherm recording followed by transferring them on hydrophilic and hydrophobic Si surfaces by the Langmuir–Blodgett (LB) deposition method. A substantial change in the area/molecule is observed as a function of subphase pH and temperature. Such parameters strongly influence intramolecular interactions within azo molecules and the interactions between azo molecules and water that manifested in higher surface activity at low temperature and high pH, which in turn modifies the elasticity of azo assembly at the air–water interface. A large pH-dependent hysteresis with negative change in entropy, indicating molecular rearrangements, is observed. Molecular assembly formed at the air–water interface is then transferred onto hydrophilic and hydrophobic Si surfaces at two different surface pressures (5 and 30 mN/m) by the LB technique. The structural analysis performed by X-ray reflectivity and atomic force microscopy techniques suggests that the LB films exhibit an abrupt layered structure on hydrophilic Si, whereas an overall rough film is formed on hydrophobic Si. The coverage and compactness of individual layers are found to increase with the deposition pressure (5 to 30 mN/m).

Surface energy engineering for LiTaO3 and α-quartz SiO2 for low temperature (<220 °C) wafer bonding

Wafer bonding can be substituted for heteroepitaxy when manufacturing specific heterojunction-based devices. Devices manufactured using wafer bonding include multijunction solar cells, integrated sensors, heterogeneously integrated photonic devices on Si (such as high-performance laser diodes), Mach-Zehnder modulators, photodetectors, optical filters, and surface acoustic wave devices. In these devices, creating heterointerfaces between different semiconductors with heavily mismatched lattice constants and/or significant thermal expansion mismatch presents significant challenges for heteroepitaxial growth. High costs and poor yields in heavily mismatched heteroepitaxy can be addressed by wafer bonding in these optoelectronic devices and sensors, including the LiTaO3/Si and LiTaO3/SiO2 heterostructures. In the present work, heterostructure formation between piezoelectric LiTaO3 (100) and Si (100) and α-quartz SiO2 (100) is investigated via wafer bonding. Direct bonding is selected instead of heteroepitaxy due to a significant thermal expansion mismatch between LiTaO3 and Si-based materials. The coefficient of thermal expansion (CTE) of LiTaO3 is 18.3 × 10−6/K. This is 1 order of magnitude larger than the CTE for Si, 2.6–2.77 × 10−6/K and 25–30 times larger than the CTE for fused SiO2 and quartz (which ranges 0.54–0.76 × 10−6/K). Thus, even at 200 °C, a 4 in. LiTaO3/Si bonded pair would delaminate with LiTaO3 expanding 300 μm in length while Si would expand only by 40 μm. Therefore, direct wafer bonding of LiTaO3/Si and LiTaO3/SiO2 is investigated with low temperature (T < 500 K) Nano-Bonding™, which uses surface energy engineering (SEE). SEE is guided by fast, high statistics surface energy measurements using three liquid contact angle analysis, the van Oss/van Oss–Chaudhury–Good theory, and a new, fast Drop Reflection Operative Program analysis algorithm. Bonding hydrophobic LiTaO3 to hydrophilic Si or SiO2 is found to be more effective than hydrophilic LiTaO3 to hydrophobic Si or SiO2 temperatures for processing LiTaO3 are limited by thermal decomposition LiTaO3 into Ta2O5 at T ≥ 180 °C due to Li out-diffusion as much as by LiTaO3 fractures due to thermal mismatch.

Pd/SiO2 Inorganic–Organic Composite Membrane Calcined Under N2 Atmosphere: Thermal Stability and H2/CO2 Separation

A novel Pd/SiO₂ inorganic-organic composite material was developed for the selective separation of H₂ from a mixture of H₂ and CO₂. Its thermal stability and microstructure calcined under N₂ atmosphere were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and N₂ sorption-desorption measurements. Pd element in Pd/SiO₂ gel material exists in PdCl₂ form, calcination at 350 °C can result in the complete transformation of Pd2+ to metallic Pd0. With the increase of calcination temperature, the hydrophobic Si-CH₃ bands decreased in intensity. The residue of Pd/SiO₂ material calcined at 800 °C was mainly composed of Si-O-Si, metallic Pd0, CSi₄ and some elemental C0. The mean pore size, BET specific surface area and total pore volume of the as-prepared Pd/SiO₂ material calcined at 350 °C was about 2.26 nm, 417.35 m² g-1 and 0.288 m³ g-1, respectively. The mean H₂ and CO₂ permeances of the corresponding Pd/SiO₂ membrane were 9.90×10-6 and 9.10×10-7 mol m-2 Pa-1 s-1, respectively, when operating at 200 °C and a pressure difference of 0.3 MPa. After the steam exposure at 200 °C for 168 h, the H₂ permeance decreased by 3.23% while the H₂/CO₂ permselectivity increased by 2.50%.

Quantum-Chemical Modeling of Vinpocetine Desorption from Silicon and Silicon-Dioxide Particle Surfaces

Activation energies for desorption of vinpocetine from silicon (Si) and silicon-dioxide (SiO2) surfaces were analyzed and compared. The molecular mechanism of vinpocetine desorption was found to differ depending on the adsorbent and ionization state, which in turn depended on the pH. Nonpolar interactions between the adsorbent and vinpocetine played a decisive role during desorption from a hydrophobic Si surface without substituents. Quantum-chemical estimates of the activation energy led to the conclusion that vinpocetine in aqueous solution at pH 6.8 and 7 was bound more strongly to SiO2 than to Si. The activation energies for vinpocetine desorption from Si and SiO2 surfaces at pH 2 were statistically indistinguishable and statistically significantly less than the activation energies at pH 6.8 and 7.

POSS Hybrid Robust Biomass IPN Hydrogels with Temperature Responsiveness.

In order to improve the performance of traditional sodium alginate (SA) hydrogels cross-linked by Ca2+ ions to meet greater application demand, a strategy was designed to structure novel SA-based gels (named OP-PN gels) to achieve both stimulus responsiveness and improved mechanical strength. In this strategy, the SA chains are co-cross-linked by CaCl2 and cationic octa-ammonium polyhedral oligomeric silsesquioxane (Oa-POSS) particles as the first network, and an organically cross-linked poly(N-isopropyl acrylamide) (PNIPA) network is introduced into the gels as the second network. Several main results are obtained from the synthesis and characterization of the gels. For OP-PN gels, their properties depend on the content of both uniformly dispersed Oa-POSS and PNIPA network directly. The increased Oa-POSS and PNIPA network content significantly improves both the strength and resilience of gels. Relatively, the increased Oa-POSS is greatly beneficial to the modulus of gels, and the increased PNIPA network is more favorable to advancing the tensile deformation of gels. The gels with hydrophilic PNIPA network exhibit better swelling ability and remarkable temperature responsiveness, and their volume phase transition temperature can be adjusted by altering the content of Oa-POSS. The deswelling rate of gels increases gradually with the increase of POSS content due to the hydrophobic Si–O skeleton of POSS. Moreover, the enhanced drug loading and sustained release ability of the target drug bovine serum albumin displays great potential for this hybrid gel in the biomedical field.

Thermal decomposition mechanism and kinetics of Pd/SiO2 nanocomposites in air atmosphere

SiO2 and Pd/SiO2 nanocomposites were prepared using sol–gel method and characterized by X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy and different heating rate thermogravimetric–differential thermogravimetric analysis. The apparent activation energies (Ea) were calculated using the Kissinger’s and Ozawa’s methods. The most probable kinetic mechanism functions were obtained by master plot method. The entropy (ΔS≠), enthalpy (ΔH≠) and activation free energy (ΔG≠) were calculated using three methods. The results demonstrated that the thermal decomposition processes of both materials are the three-step reactions. After thermal condition in air atmosphere, the introduced hydrophobic Si–CH3 group in Pd/SiO2 gel materials was completely decomposed into inorganic composition, and the Pd element was transformed into PdO and metallic Pd. The Ea values obtained by Kissinger’s method are close to those by Ozawa’s method. The Pd-doping can increase the thermal decomposition activation energies and improve the thermal stability of SiO2 material. The decomposition mechanisms for the three stages are the assumed random nucleation and its subsequent growth, irrespective of the Pd-doping. Compared with SiO2 material, the thermal decomposition of Pd/SiO2 requires more energy due to the increasing ΔH≠. The ΔH≠ values calculated by method II are nearly equal to the Ea values from the Kissinger’s and Ozawa’s methods, which indicated that the thermodynamic parameters obtained by method II are the most accurate.

Water Adsorption on Hydrophilic and Hydrophobic Surfaces of Silicon

The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si–OH surface, the average thickness of the strongly hydrogen-bonded water layer increases up to ∼2 molecular layers as relative humidity (RH) increases, beyond which the weakly hydrogen-bonded structure is dominant. On the hydrophobic Si–H surface, the adsorbed water layer consists predominantly of the weakly hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements provide critical insights needed for better understanding of various physical processes affected by water adsorption in ambient conditions.