Si3N4 Tip Research Articles

Micromachined Si 3N 4-tip on Cantilever for parallel SFM and NSOM applications

Micromachining of a nanoscale tip for near field scanning optical microscopy (NSOM) and scanning force microscopy(SFM) has been described. The tapered optical metal-coated fiber is generally used to provide a sub-wavelength sized aperture on tip. Several micromachining methods have been performed in order to have a metal aperture with radius less than @l/2. Apertures provided with less than @l/2 and hollow tips would provide a suitable probe for both NSOM and SFM. A tip coated with thin metal film will meet these requirements. The Si tip have been initially fabricated using reacting ion etching (RIE). The SiO2 etch masks with [email protected] and [email protected] were patterned followed by Si etching. The etched Si post was at least 3 @mm tall and the radius of the tip was found to be 30nm and ~10nm depending on the fabrication methods. A Si3N4 thin film was deposited on the fabricated Si -tip using a low pressure chemical vapor deposition technique in order to provide a capability for atomic force microscope. A Cr metal film was deposited using thermal evaporator. The thick photoresist film was coated using two-stage methods in order to cover the tall [email protected] tip. The PR film was carefully etched to have a metal aperture size with less than @l/2.

Characterization of the interfacial properties of modified polypropylene

The interfacial properties of a rubber blended polypropylene were investigated by the combination of complementary techniques in order to characterize the effects of different surface treatments. The wettability was investigated by contact angle measurements with water, streaming potential measurements indicated the Bronsted acidity/basicity of the surfaces, and X-ray photoelectron spectroscopy (XPS) identified the chemical elements, which were incorporated by the various treatments. The topography and the roughness of the samples were quantitatively analyzed by scanning force microscopy (SFM). Direct force measurements showed the possibility of changing the sign of the surface charge by choosing suitable buffer solutions, leading to attraction and repulsion to the Si3N4 tip, respectively. In the case of technical-type polymer samples used in this study, direct force measurements are in exact agreement with the streaming potential measurements and offer a valuable tool for interface characterization.

Nano-Tribological and Wear Behavior of Boric Acid Solid Lubricant©

One uses atomic force microscopy to explore the surface morphology, frictional characteristics (i.e., friction mapping) and surface adhesive forces of boric acid-treated aluminum surfaces. In earlier studies, boric acid was shown to be an effective solid lubricant providing 0.02 to 0.1 friction coefficients to the sliding surfaces of metallic and ceramic materials. In this study, the authors mainly focused on the friction and wear behavior of cleaved single crystals of boric acid. Tests were performed on uncoated and boric acid-coated surfaces of aluminum substrates, as well as the bulk boric acid material. The nano-scale wear of the boric acid crystals manifested itself in the displacement of atomic planes which, in turn, resulted in the formation of the worn area grown by a spiral-growth mechanism. In addition, new crystals in the vicinity of the sliding contact areas were formed. These crystals formed by a spiral dislocation mechanism. The nanoscale friction measurements showed that the friction coefficient of a Si3N4 tip against the boric acid single crystal, was in the range of 0.07 to 0.13, depending on the sliding direction with respect to the crystallographic orientation. The friction coefficients of the boric acid-coated aluminum substrates varied between 0.11 and 0.19. While these values were consistent with earlier findings and further demonstrated the lubricity of boric acid, they were higher than the previous measurements (i.e., 0.011–0.1) obtained on a pin-on-disk machine using steel or ceramic pins. An explanation is provided for this phenomenon. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998

Adsorption of ions onto polymer surfaces and its influence on zeta potential and adhesion phenomena

The adhesion behavior that governs many technologically and biologically relevant polymer properties can be investigated by zeta potential measurements with varied electrolyte concentration or pH. In a previous work [1] it was found that the difference of the adsorption free energies of Cl- and K+ ions correlates with the adhesion force caused by van der Waals interactions, and that the decrease of adhesion strength by adsorption layers can be elucidated by zeta potential measurements. In order to confirm these interrelations, zeta potential measurements were combined with atomic force microscopy (AFM) measurements. Force–distance curves between poly(ether ether ketone) and fluorpolymers, respectively, and the Si3N4 tip of the AFM device in different electrolyte solutions were measured and analysed. The adsorption free energy of anions calculated from the Stern model correlates with their ability to prevent the adhesion between the polymer surface and the Si3N4 tip of the AFM device. These results demonstrate the influence of adsorption phenomena on the adhesion behavior of solids. The results obtained by AFM confirm the thesis that the electrical double layer of solid polymers in electrolyte solutions is governed by ion adsorption probably due to van der Waals interactions and that therefore van der Waals forces can be detected by zeta potential measurements.

Mapping of electrical double-layer force between tip and sample surfaces in water with pulsed-force-mode atomic force microscopy

Pulsed-force-mode atomic force microscopy (PFM-AFM) using a cantilever with a Si3N4 tip was applied to map charge distribution on a sample surface in water. In order to confirm the applicability of the present PFM, we prepared a patterned sample by vapor deposition of Al on a quartz plate covered with silica beads, followed by oxidation of Al with O2 and removal of the beads with ultrasonication. The two different areas of Al2O3 and SiO2 had different isoelectric points and bore positive and negative charge, respectively, at pH 8.6. The lateral resolution of the present method was found to be ca. 30 nm.

New approaches to atomic force microscope lithography on silicon

We have investigated new approaches to the formation of conducting nanowires on crystalline silicon surfaces using atomic force microscope (AFM) lithography. To increase processing speed and reduce wear of the AFM tip, large-scale structures are formed with a direct laser write setup, while the AFM is used to add the finer nanostructures. Both methods are based on selective oxidation of hydrogen-passivated silicon and subsequent etching to define conducting regions on the surface. This combined technique has previously been implemented on amorphous Si on oxide. To extend the technique to form crystalline silicon nanowires, we have used an arsenic implanted crystalline silicon layer on p-type Si, where the nanostructures are isolated from the substrate electrically due to p-n junction formation. Improvements in the reliability of the AFM lithography technique were achieved by using all-metal tips, which do not wear out as rapidly as metal-coated Si3N4 tips.

Modification of Silicon Nitride Tips with Trichlorosilane Self-Assembled Monolayers (SAMs) for Chemical Force Microscopy

Chemical modification of Si3N4 atomic force microscopy tips was performed by a mild pretreatment using ozone and alkaline/acid solutions and subsequent formation of self-assembled monolayers (SAMs) with two kinds of ω-alkenyltrichlorosilanes (CH2CH(CH2)nSiCl3, n = 4, 9; HTS and UTS, respectively). Hydroboration of their vinyl groups gave hydroxyl-terminated SAMs. The extent of SAM formation was assessed by determining the adhesion between Si3N4 tips on one hand and bare mica or mica substrates covered with UTS SAMs on the other hand. All adhesion forces were determined with the samples immersed in water to eliminate effects of capillary forces. Contact angles on analogously treated planar Si3N4 substrates were measured for comparison. Proper conditions for SAM formation on Si3N4 tips were found to be similar to those for formation of SAMs on Si3N4 substrates, as shown by the good temporal correlation between the adhesion forces observed between UTS-modified tips and mica and the contact angles of water on planar Si3N4 substrates modified under identical conditions. Due to hydrophobic interactions, strong adhesion between tips modified with HTS or UTS SAMs on one hand and UTS SAMs on mica on the other hand was observed. The adhesion of tips modified with HTS to hydrophobic surfaces decreased only slightly upon hydroboration of the vinyl groups. Furthermore, hydroxyl-terminated tips derived from HTS adhered more strongly to vinyl-terminated monolayers on mica than to hydroxyl-terminated monolayers. This suggests substantial disorder in the HTS-type SAMs formed on Si3N4 tips, resulting in strong hydrophobic interactions between exposed alkyl chains on the tips and the hydrophobic surfaces. This seems to conceal the specific interactions between the terminal functional groups. Evidence for such disorder is also given by FTIR spectra and by the larger friction forces obtained for HTS monolayers on Si3N4 than for UTS monolayers on Si3N4. On the other hand, conversion of the terminal vinyl groups of the UTS SAMs on the Si3N4 tips to hydroxyl groups led to a large decrease in the adhesion to hydrophobic UTS monolayers and a small increase in the adhesion to hydroxyl-terminated monolayers derived from UTS. This shows that formation of SAMs of trichlorosilanes with long alkyl chains on Si3N4 tips and subsequent functional group conversion is a promising method for the preparation of modified tips for chemical force microscopy.

AFM and STM Characterization of TiO2—Ultrafiltration Membranes

Atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) were used to investigate the surface of TiO2 ultrafiltration membranes with pore sizes of 10 nm. It is widely accepted that conventional AFM with Si3N4 or Si tips is not able to resolve pores in supported ceramic membranes with a diameter of <40 nm. In this study it should be tested whether it is possible to lower this border for further characterization of nanofiltration membranes. Here we present different ways of imaging and discuss alternative sample preparation techniques.© 1997 John Wiley & Sons, Ltd.

AFM and STM Characterization of TiO2?Ultrafiltration Membranes

Atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) were used to investigate the surface of TiO2 ultrafiltration membranes with pore sizes of 10 nm. It is widely accepted that conventional AFM with Si3N4 or Si tips is not able to resolve pores in supported ceramic membranes with a diameter of <40 nm. In this study it should be tested whether it is possible to lower this border for further characterization of nanofiltration membranes. Here we present different ways of imaging and discuss alternative sample preparation techniques.© 1997 John Wiley & Sons, Ltd.

The influence of pH on friction between oxide surfacesin electrolytes, studied with lateral force microscopy:application as a nanochemical imaging technique

We have investigated the lateral force (frictional) signalbetween the Si3N4 tip of an AFM andSiO2 and AlOx surfaces in a 1 mMNaCl solution, as a function of pH. It was found that thefrictional force depends strongly on both the pH and theisoelectric points (IEP) of the materials under investigation.A simple linear model describing the dependence of the lateralforce on the total normal force has been used to accountqualitatively for the observed lateral force signal from the single-asperity contact between tip and surface on a nanometerscale. The observed pH-dependence of the lateral force signalon different oxide surfaces has been applied to reverse thechemical contrast on an appropriate sample composed of twodifferent oxides, thus demonstrating the potential of this method for ``chemical imaging''.

Effects of electric potentials on surface forces in electrolyte solutions

Atomic force microscopy was employed to analyze the electric double layer on an Au film electrode deposited on mica. An Au-covered or a bare Si3N4 tip was used as a probe. An electrolyte solution was aqueous NaOH of pH 10.9. The electrode and the tip potentials were controlled electrochemically by a potentiostat. Repulsive forces were always observed regardless of the polarity of the potential, i.e., the positive and the negative side of the point of zero charge. The result indicates that the strongly adsorbed OH− anions overcompensated the positive charge at the Au surface. The potential of the outer Helmholtz plane at an electrode potential was estimated from the force–distance curves.

Friction forces on hydrogen passivated (110) silicon and silicon dioxide studied by scanning force microscopy

Scanning force microscopy (SFM) was used to study the surface topography and the low load, small contact area frictional behavior of hydrogen passivated (H-terminated) silicon and silicon dioxide in a dry nitrogen atmosphere. The unpassivated samples showed no significant contrast in frictional sliding of a SFM tip between the thermally grown silicon dioxide and the native silicon dioxide that typically covers exposed silicon surfaces. The H-termination of the sample surface leads to an increase in friction by a factor of 2 on silicon, whereas the friction of the silicon dioxide is not influenced. Coefficients of friction were determined by using the two-dimensional histogram technique. On SiO2 and H-terminated Si surfaces, using standard Si3N4 tips, coefficients of friction were determined to be 0.3±0.1 and 0.6±0.1, respectively.

Chain Length Dependence of the Frictional Properties of Alkylsilane Molecules Self-Assembled on Mica Studied by Atomic Force Microscopy

We show that the frictional properties of alkylsilane monolayers self-assembled on mica in contact with Si3N4 tips depend strongly on the length of the alkyl chains. Friction is particularly high with short chains of less than eight carbons. We attribute this to the large number of dissipative modes in the less ordered short chains. Longer chains, stabilized by van der Waals attractions form more compact and rigid layers and act as much better lubricants. This lubricating action is lost at a certain threshold load, where wear of the molecular layer occurs, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end groups is not sufficient to determine the frictional properties of monolayer films. The increased number of energy dissipation modes facilitated by the presence of molecular disorder (e.g., rotations about a C−C axis), in fact dominates the frictional behavior of monolayers with short chains.

Imaging and nano‐dissection of tobacco mosaic virus by atomic force microscopy

SummaryTobacco mosaic virus (TMV) has been deposited on freshly cleaved mica substrates. The topography was investigated by contact, non‐contact and lateral‐force microscopy under ambient conditions in air. The results were in accord with known dimensions of TMV (i.e. 18 nm in diameter and 300 nm in length). However, convolution of tip shape with TMV morphology resulted in an apparent width of 80–140 nm in the lateral plane, a factor of 4–7 greater than the known diameter. Other artefacts ‐ broadening and double images ‐ were observed and ascribed to tip anomalies.High force loadings and slow repetitive scanning resulted in controlled removal of parts of the TMV structure. Accordingly, it was possible to reveal and image the central core channel of the TMV. The precision and resolution of dissection induced by AFM is currently limited by the shape of the tip, having a 40‐nm radius of curvature for standard Si3N4 tips. It is estimated that sharper tips, with a radius of curvature of less than 10 nm, should be able to resolve, non‐destructively, the protein subunits in the non‐contact mode, and selectively remove single subunits in the contact mode.

Single-asperity friction in friction force microscopy: The composite-tip model

Friction measurements on muscovite mica and glass have been performed with a friction force microscope using Si3N4 tips. The environment was changed from ambient conditions to N2- or Ar-gas conditions. In ambient conditions, the friction-versus-load curves showed a nonlinear behavior closely following the Hertzian contact mechanics of a single-asperity contact: friction force ∼load2/3. For the same tip under gaseous conditions the friction force increased linearly with the load, indicating multi-asperity contact. A new model is proposed to explain this change in the nature of the friction behavior. In this composite-tip model, the tip is formed by the actual Si3N4 tip and ‘‘solidlike’’ contaminants present in enclosed cavities between tip and sample surface.

Measurements of Electrostatic Double-Layer Forces Due to Charged Functional Groups on Langmuir-Blodgett Films with an Atomic Force Microscope

Using an atomic force microscope, we measured the forces between a Si3N4 tip and monolayers with different polar functional groups ( –NH2, –COOH, –CONH2, and –OH) prepared by the Langmuir-Blodgett method while varying the pH value of aqueous solutions. The obtained force vs distance curves were related to the surface charges of the tip and the dissociation of the functional groups from the pH dependency, and the charged state of functional groups could thus be discerned. In addition, the electrostatic origin of the force has been confirmed assuming constant potentials on both surfaces.

Fluctuation in Two-Dimensional Stick-Slip Phenomenon Observed with Two-Dimensional Frictional Force Microscope

We used an atomic force microscope combined with a lateral force microscope (AFM/LFM) as a two-dimensional frictional force microscope (2D-FFM) to investigate the two-dimensional behavior of the atomic-scale friction between the cleaved surface of MoS2 and the Si3N4 tip apex of the microcantilever based on the two-dimensional stick-slip model. As a result, for the scan direction along the row of the stick-points, we found the unstable state where the tip apex shows fluctuation between two adjacent rows of stick-points. On the other hand, near the row of the stick-points, we also found the stable state where the tip apex takes a straight walk on a row of the stick-points without fluctuation.

The two-dimensional stick-slip phenomenon with atomic resolution

The authors investigated the atomic-scale stick-slip motion between muscovite mica and the Si3N4 tip apex of a cantilever using an atomic-force microscope combined with a lateral-force microscope (AFM/LFM). As a result, a square-wave behaviour of the cantilever motion was found across the scan direction, in addition to the well known sawtooth behavior along the scan direction. It was also found that the square-wave behavior was synchronized with the sawtooth behavior. To explain these phenomena, the authors proposed the two-dimensional stick-slip model, where the stick-points have the same periodicity as the lattice structure of the mica surface, so that the tip shows a zigzag walk along the scan. This new model seems to explain not only the one-dimensional line-scan profile, but also the contrast mechanism of the two-dimensional image.

Interaction Force Between Thin Film Disk Media and Elastic Solids Investigated by Atomic Force Microscope

Atomic force microscopy is used to investigate the interaction force between the sharp tips of various elastic solids and four different samples. The samples are: thin film disk media coated with functional liquid lubricant having diol end groups, unlubricated disk media, a single-crystal silicon wafer, and Au evaporated onto single-crystal silicon. Relationships between the interaction and static friction force of disk media and a taper flat type head slider are examined. The interaction force between a disk medium coated with a functional liquid lubricant greater than 11.0 nm thick and tungsten tips with radii of 5 μm-100 μm is caused by the functional liquid lubricant meniscus, as pointed out by McFarlane and Tabor. However, at a thickness of several nanometers, the interaction force has a lower value than that for lubricant thicknesses above 11.0 nm. The interaction force has a minimum value of 0.4 μN at the functional liquid lubricant thickness of 2.0 nm. Mean interaction forces of the tungsten, Al2O3 − TiC and Si3N4 tips on a disk medium coated with a 2.0-nm-thick functional liquid lubricant are less than 0.1 times those for an unlubricated disk medium. Interaction forces of the SiC tip show very low values, even when the disk medium is unlubricated. Static friction force between a thin-film disk medium and a head or sphere is dependent on the interaction force between the medium and a tip that is made of the same material as the head or sphere. The use of an atomic force microscope (AFM), may allow the surface structure to be more thoroughly analyzed.