Structural effects recorded for AFM tips interacting with individual nanoparticles and their clusters deposited on substrates

Mica and alumina were coated with nanoparticles using aqueous suspensions while managing attractive substrate-particle electrostatic forces. Using nanoparticle-coated substrates, structural forces were measured for 10 nm silica particles deposited on the alumina substrate and 5-80 nm alumina particles on mica using an atomic force microscopy technique. For nanoparticles forming clusters, oscillation of structural forces was recorded with a periodicity that is close to the size of nanoparticles used. Positioning the AFM tip over the single particles allowed, on the other hand, the study of probe-nanoparticle colloidal forces.     

AFM colloidal forces measured between microscopic probes and flat substrates in nanoparticle suspensions

Colloidal forces between atomic force microscopy probes of 0.12 and 0.58 N/m spring constant and flat substrates in nanoparticle suspensions were measured. Silicon nitride tips and glass spheres with a diameter of 5 and 15 mum were used as the probes whereas mica and silicon wafer were used as substrates. Aqueous suspensions were made of 5-80 nm alumina and 10 nm silica particles. Oscillatory force profiles were obtained using atomic force microscope. This finding indicates that the nanoparticles remain to be stratified in the intervening liquid films between the probe and substrate during the force measurements. Such structural effects were manifested for systems featuring attractive and weak repulsive interactions of nanoparticles with the probe and substrate. Oscillation of the structural forces shows a periodicity close to the size of nanoparticles in the suspension. When the nanoparticles are oppositely charged to the probes, they tend to coat the probes and hinder probe-substrate contact.     

Attachment of nanoparticles to the AFM tips for direct measurements of interaction between a single nanoparticle and surfaces

Here we report a universal method of attachment/functionalization of tips for atomic force microscope (AFM) with nanoparticles. The particles of interest are glued to the AFM tip with epoxy. While the gluing of micron size particles with epoxy has been known, attachment of nanoparticles was a problem. The suggested method can be used for attachment of virtually any solid nanoparticles. Approximately every other tip prepared with this method has a single nanoparticle terminated apex. We demonstrate the force measurements between a single approximately 50 nm ceria nanoparticle and flat silica surface in aqueous media of different acidity (pH 4-9). Comparing forces measured with larger ceria particles ( approximately 500 nm), we show that the interaction with nanoparticles is qualitatively different from the interaction with larger particles.     

Colloidal interactions between asphaltene surfaces in aqueous solutions

Asphaltene at oil/water interfaces plays a dominant role in the recovery of crude oil. In this study, asphaltene monolayer films were deposited on hydrophobic silicon wafers and silica spheres from oil-water interfaces using a Langmuir interfacial trough. The morphology of the deposited asphaltene films was characterized with an atomic force microscope (AFM). The colloidal forces between the prepared asphaltene films in aqueous solutions were measured with AFM to shed light on the stabilization of water or oil droplets coated with asphaltene films. Factors such as solution pH, KCl concentration, calcium addition, and temperature all showed a strong impact on colloidal forces between the prepared asphaltene films. The findings provided a better understanding of asphaltene interfacial films at an oil/water interface in stabilizing bitumen-in-water and water-in-bitumen emulsions.     

Study of surface forces dependence on pH by atomic force microscopy

We used an atomic force microscope to investigate silicon nitride tip interactions with various materials (copper, nickel, silicon carbide) as a function of pH. The electrolyte used was 10(-3) M NaCl and the interactions observed through force versus distance curves (attraction or repulsion) depended on the pH value. Interaction forces calculation was derived from force versus distance curve data and the results are discussed in terms of electrostatic interactions using Zeta potential theory.     

AFM study of forces between silica, silicon nitride and polyurethane pads

Interaction of silica and silicon nitride with polyurethane surfaces is rather poorly studied despite being of great interest for modern semiconductor industry, e.g., for chemical-mechanical planarization (CMP) processes. Here we show the results from the application of the atomic force microscopy (AFM) technique to study the forces between silica or silicon nitride (AFM tips) and polyurethane surfaces in aqueous solutions of different acidity. The polyurethane surface potentials are derived from the measured AFM data. The obtained potentials are in rather good agreement with measurements of zeta-potentials using the streaming-potentials method. Another important parameter, adhesion, is also measured. While the surface potentials of silica are well known, there are ambiguous results on the potentials of silicon nitride that is naturally oxidized. Deriving the surface potential of the naturally oxidized silicon nitride from our measurements, we show that it is not oxidized to silica despite some earlier published expectations.     

Synthesis of dendron-protected porphyrin wires and preparation of a one-dimensional assembly of gold nanoparticles chemically linked to the pi-conjugated wires

A one-dimensional assembly of gold nanoparticles chemically bonded to pi-conjugated porphyrin polymers was prepared on a chemically modified glass surface and on an undoped naturally oxidized silicon surface by the following methods: pi-conjugated porphyrin polymers were prepared by oxidative coupling of 5,15-diethynyl-10,20-bis-((4-dendron)phenyl) porphyrin (6), and its homologues (larger than 40-mer) were collected by analytical gel permeation chromatography (GPC). The porphyrin polymers (>40-mer) were deposited using the Langmuir-Blodgett (LB) method on substrate surfaces, which were then soaked in a solution of gold nanoparticles (2.7 +/- 0.8 nm) protected with t-dodecanethiol and 4-pyridineethanethiol. The topographical images of the surface observed by tapping mode atomic force microscopy (AFM) showed that the polymers could be dispersed on both substrates, with a height of 2.8 +/- 0.5 nm on the modified glass and 3.1 +/- 0.5 nm on silicon. The height clearly increased after soaking in the gold nanoparticle solution, to 5.3 +/- 0.5 nm on glass and 5.4 +/- 0.7 nm on silicon. The differences in height (2.5 nm on glass and 2.3 nm on silicon) corresponded to the diameter of the gold nanoparticles bonded to the porphyrin polymers. The distance between gold nanoparticles observed in scanning electron microscopic images was ca. 5 nm, indicating that they were bonded at every four or five porphyrin units.     

Nanoparticle‐Imprinted Polymers for Size‐Selective Recognition of Nanoparticles

Citrate‐stabilized gold nanoparticles 15 nm and 33 nm in diameter were transferred concomitantly with a monolayer of positively charged polyaniline by Langmuir–Blodgett transfer at pH 5 onto a conducting indium‐doped tin oxide (ITO) support. Films consisting of one to three layers of polyaniline with thicknesses of 1–3 nm were prepared and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy. After electro‐oxidation of the Au nanoparticles in 0.1 M KCl, cavities were left behind in the film that could be analyzed by SEM. These cavities were able to recapture analyte nanoparticles from a solution of pH 10 and showed size‐exclusion properties. The amount of nanoparticles taken up by the cavities was conveniently analyzed by measuring the charge associated with the electro‐oxidation of these particles in 0.1 M KCl after the film had been rinsed with water. The size‐exclusion properties improved with the number of Langmuir–Blodgett layers transferred.     

硅表面复合自组装膜的制备及其摩擦性能

采用自组装技术在单晶硅表面制备了3-氨基丙基三乙氧基硅烷(APTES)-SiO2-APTES复合膜,并对其表面的组成、结构及摩擦性能进行了表征.结果表明:复合膜表面对水的接触角约为63°,且表面平整、致密,其平均粗糙度(Ra)约为0.963nm.通过原子力显微镜(AFM)和透射电子显微镜(TEM)观察到夹层中SiO2颗粒的粒径约为20-50nm,较均匀地分布在第一层APTES膜的表面.与APTES自组装单层膜(SAMs)相比,APTES-SiO2-APTES复合膜由于纳米SiO2颗粒的引入而表现出更低的摩擦系数和更长的耐磨寿命.     

Effects of degassing and ionic strength on AFM force measurements in octadecyltrimethylammonium chloride solutions

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.     

Quantifying adhesion bond parameters to distinguish interactions of hydrophilic and hydrophobic blocks of polystyrene-poly-2-vinylpyridine with a silicon nitride surface

An analysis of the loading rate dependence of the forces required to rupture an AFM tip from a block copolymer surface has provided insight into the structure of the surface-macromolecule contact, differentiation of the block contacting the tip, a measure of the polymer-surface binding energy, and the rigidity of the contact. Polystyrene-poly-2-vinylpyridine block copolymers were studied adsorbing to silicon nitride. Polystyrene makes stiff van der Waals contact with the silicon nitride surface in aqueous solution, while the bond of poly-2-vinylpyridine to the surface is more flexible and may involve a bridging water.     

Conversion of a metastable superhydrophobic surface to an ultraphobic surface

Superhydrophobic surfaces in Wenzel and metastable wetting state were prepared and the conversion of such surfaces to ultraphobic surfaces was reported by the application of a fine-scale roughness. Silicon nitride substrates with hexagonally arranged pillars were prepared by micromachining. The two-scale roughness was achieved by coating these substrates with 60 nm silica nanoparticles. The surface was made hydrophobic by silanization with octadecytrichlorosilane (OTS). Wettability studies of the silicon nitride flat surface, silicon nitride pillars, and the surfaces with two-scale roughness showed that a two-scale roughness can effectively improve the hydrophobicity of surfaces with a higher apparent contact angle and reduced contact angle hysteresis when the original rough surface was in a metastable or Wenzel state. This study shows the pathway of converting a metastable hydrophobic surface to an ultraphobic surface by the introduction of a fine-scale roughness, which adds to the literature a new aspect of fine-scale roughness effect.     

AFM interaction study of alpha-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions

Ionic strength dependence of interaction and friction forces between hydrophilic alpha-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force-distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10(-3) M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the alpha-alumina surfaces at pH 11.     

Interaction forces between salivary proteins and Streptococcus mutans with and without antigen I/II

The antigen I/II family of surface proteins is expressed by oral streptococci, including Streptococcus mutans, and mediates specific binding to, among others, salivary films. The aim of this study was to investigate the interaction forces between salivary proteins and S. mutans with (LT11) and without (IB03987) antigen I/II through atomic force microscopy (AFM) and to relate these interaction forces with the adhesion of the strains to saliva-coated glass in a parallel plate flow chamber. Upon approach of the bacteria toward a saliva-coated AFM tip, both strains experienced a similar repulsive force that was significantly smaller at pH 6.8 (median 3.0 and 3.1 nN for LT11 and IB03987, respectively) than at pH 5.8 (median 4.6 and 4.7 nN). The decay length of these repulsive forces was between 19 and 37 nm. Upon retraction at pH 6.8, the combined specific and nonspecific adhesion forces were significantly stronger for the parent strain LT11 (median -0.4 nN) than for the mutant strain IB03987 (median 0.0 nN), whereas at pH 5.8 the median of the adhesion forces measured was 0.0 nN for both strains. Moreover, at pH 6.8, the parent strain LT11 adhered in significantly higher numbers (9.6 x 106 cm-2) to a salivary coating than the mutant strain IB03987 (2.5 x 106 cm-2). Similar to the difference in adhesion forces between both strains at pH 5.8, the difference in adhesion between both strains also disappeared at pH 5.8, which suggests the involvement of attractive electrostatic forces in the interaction between antigen I/II and salivary coatings. In summary, this study shows that antigen I/II at the surface of S. mutans LT11 is responsible for its increased adhesion to salivary coatings under flow through an additional attractive electrostatic force.     

Isoelectric point of fluorite by direct force measurements using atomic force microscopy

Interaction forces between a fluorite (CaF2) surface and colloidal silica were measured by atomic force microscopy (AFM) in 1 x 10(-3) M NaNO3 at different pH values. Forces between the silica colloid and fluorite flat were measured at a range of pH values above the isoelectric point (IEP) of silica so that the forces were mainly controlled by the fluorite surface charge. In this way, the IEP of the fluorite surface was deduced from AFM force curves at pH approximately 9.2. Experimental force versus separation distance curves were in good agreement with theoretical predictions based on long-range electrostatic interactions, allowing the potential of the fluorite surface to be estimated from the experimental force curves. AFM-deduced surface potentials were generally lower than the published zeta potentials obtained from electrokinetic methods for powdered samples. Differences in methodology, orientation of the fluorite, surface carbonation, and equilibration time all could have contributed to this difference.     

Surfactant-resisted assembly of Fe-containing nanoparticles for site-specific growth of SWNTs on Si surface

This paper describes a facile approach to the site-specific growth of single-walled carbon nanotubes (SWNTs) on silicon surfaces by chemical vapor deposition (CVD). The approach is based on the use of a surfactant as a resist to define patterns of silicon oxide nanodomains onto which nanoparticles of iron hydroxide (Fe(OH)3), 1-5 nm diameter, could be deposited. In base growth mode, the SWNTs can grow from the oxide nanodomains. By controlling the location of oxide nanodomains, site-specific growth could be obtained. The iron hydroxide nanoparticles were prepared by hydrolysis of ferric chloride (FeCl3). Patterned hydroxylated silicon oxide nanodomains were created by scanning probe oxidation (SPO) of silicon substrates modified with aminopropyltrimethoxysilane (APTMS, H2N(CH2)3Si(OCH3)3). Due to electrostatic interaction, Fe(OH)3 nanoparticles can be selectively deposited on hydroxyl groups present on silicon oxide nanodomains. To inhibit the assembly of the nanoparticles on a APTMS-coated silicon surface, sodium dodecyl sulfate (SDS) was introduced, which restricted deposition to the hydroxylated nanodomains. A model mechanism for the selective deposition mechanism has been proposed. It was possible to convert the patterned Fe(OH)3 nanoparticles to iron oxide, which served as a catalyst for the site-specific growth of SWNTs. Raman spectroscopy and AFM were used to characterize the nanotubes on the Si substrate. This will offer the possibility for future integration with conventional microelectronics as well as the development of novel devices.     

Growth and fire resistance of colloidal silica-polyelectrolyte thin film assemblies

Thin films of colloidal silica were deposited on cotton fibers via layer-by-layer (LbL) assembly in an effort to reduce the flammability of cotton fabric. Negatively charged silica nanoparticles of two different sizes (8 and 27 nm) were paired with either positively charged silica (12 nm) or cationic polyethylenimine (PEI). PEI/silica films were thicker due to better (more uniform) deposition of silica particles that contributed to more than 90% of the film weight. Each coating was evaluated at 10 and 20 bilayers (BL). All coated fabrics retained their weave structure after being exposed to a vertical flame test, while uncoated cotton was completely destroyed. Micro combustion calorimetry confirmed that coated fabrics exhibited a reduced peak heat release rate, by as much as 20% relative to the uncoated control. The 10 BL PEI-8 nm silica recipe was the most effective because the coating is relatively thick and uniform relative to the other systems. Soaking cotton in basic water (pH 10) prior to deposition resulted in better assembly adhesion and flame-retardant behavior. These results demonstrate that LbL assembly is a useful technique for imparting flame retardant properties through conformal coating of complex substrates like cotton fabric.     

Hydration forces between silica surfaces: experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.     

Effect of pH and hydration on the normal and lateral interaction forces between alumina surfaces

Interaction forces between alumina surfaces were measured using an AFM-colloid probe method at different pHs. For an alpha-alumina-sapphire system at acidic pH, the force curve exhibited a well-defined repulsive barrier and an attractive minimum. At basic pH, the interactive force was repulsive at all separations with no primary minimum. Lateral force measurements under the same conditions showed that frictional forces were nearly an order of magnitude smaller at basic pH than those observed at acidic pH. This behavior was attributed to the hydration of the alumina surface. Normal and lateral force measurements with the strongly hydrated rho-alumina surfaces supported these findings.     

苯胺低聚物-二氧化硅复合纳米粒子作为高密度信息存储材料的初步探讨

近年来,在有机半导体中,聚苯胺低聚物已成为最为广泛的关注热点之一[1]．这不仅是因为它们与母体聚苯胺相比,有着良好的溶解性和确定的化学结构以及较低的杂质含量[2]．而且,通过改变共轭链长度或进行不同程度的化学修饰,如改变分子链上末端基团等,来调节它们的电子特性[3,4],使其能带值能够调制到所要求的范围内,以满足电子器件的诸多要求［’j．苯胺低聚物的这些优点及其3种相对稳定的氧化态形式使其作为高密度信息存储材料有巨大的潜力．将其以纳米粒子形式进行二维有序组装,使每一个粒子成为一个信息纪录点是我们目前所面临的…