Pyramidal Tip Research Articles

Numerical study of the hydrodynamic drag force in atomic force microscopy measurements undertaken in fluids

When atomic force microscopy (AFM) is employed for in vivo study of immersed biological samples, the fluid medium presents additional complexities, not least of which is the hydrodynamic drag force due to viscous friction of the cantilever with the liquid. This force should be considered when interpreting experimental results and any calculated material properties. In this paper, a numerical model is presented to study the influence of the drag force on experimental data obtained from AFM measurements using computational fluid dynamics (CFD) simulation. The model provides quantification of the drag force in AFM measurements of soft specimens in fluids.The numerical predictions were compared with experimental data obtained using AFM with a V-shaped cantilever fitted with a pyramidal tip. Tip velocities ranging from 1.05 to 105μm/s were employed in water, polyethylene glycol and glycerol with the platform approaching from a distance of 6000nm. The model was also compared with an existing analytical model. Good agreement was observed between numerical results, experiments and analytical predictions. Accurate predictions were obtained without the need for extrapolation of experimental data. In addition, the model can be employed over the range of tip geometries and velocities typically utilized in AFM measurements.

Fabrication of Mo pyramidal-shape single atom tips covered by a noble metal

• Provide another option for a material to produce single atom tips. • Selecting an oxide that is easier to remove from the surface of the metal is important in the manufacture of single-atom tips. • The single atom tips will have a self-assembled formation after annealing the Mo tips covered by noble metal. • There are two types of pyramidal structures formed on Mo (1 1 1) facets. The effect of annealing temperature on the faceting phenomena has been studied for pure molybdenum (Mo) and Mo tips covered with palladium (Pd), platinum (Pt), rhodium (Rh), or iridium (Ir) by field ion microscopy (FIM). For these Mo samples, three {2 1 1} facets were found to expand to {1 1 1} facets and form pyramidal structures after annealing at the temperature of 1100–1300 K. The pyramidal single atom tips (SATs) were formed on Pd-, Pt- and Rh-covered Mo tips. Two types of pyramidal structure, stacked by either 1, 3, 10 or 1, 6, 15 atoms for the top three layers were found. However, no SATs were found on pure Mo and Ir-covered tips. This indicates that noble metal adsorption, which can increase the difference of surface-free-energy anisotropy, indeed benefits the formation of SATs on Mo systems. Additionally, an SAT cannot be obtained for the Ir-covered Mo systems, because Ir is easily alloyed with Mo.

Individual Template-Stripped Conductive Gold Pyramids for Tip-Enhanced Dielectrophoresis.

Gradient fields of optical, magnetic, or electrical origin are widely used for the manipulation of micro- and nanoscale objects. Among various device geometries to generate gradient forces, sharp metallic tips are one of the most effective. Surface roughness and asperities present on traditionally produced tips reduce trapping efficiencies and limit plasmonic applications. Template-stripped, noble metal surfaces and structures have sub-nm roughness and can overcome these limits. We have developed a process using a mix of conductive and dielectric epoxies to mount template-stripped gold pyramids on tungsten wires that can be integrated with a movable stage. When coupled with a transparent indium tin oxide (ITO) electrode, the conductive pyramidal tip functions as a movable three-dimensional dielectrophoretic trap which can be used to manipulate submicrometer-scale particles. We experimentally demonstrate the electrically conductive functionality of the pyramidal tip by dielectrophoretic manipulation of fluorescent beads and concentration of single-walled carbon nanotubes, detected with fluorescent microscopy and Raman spectroscopy.

Patterning of titanium oxide surfaces using inductively coupled plasma for gas sensing

Abstract Titanium oxide thin films were deposited at room temperature by reactive magnetron sputtering in a mixture of oxygen and argon on oxidized silicon substrates. The optimal etching characteristics of TiO2 films by an inductively coupled plasma system were investigated. The maximum etch rate of TiO2 was 104 nm/min at fixed 200 W of ICP power and the highest investigated value of RF chuck power of 150 W. Patterning of TiO2 tip arrays by electron beam lithography and dry etching was conducted. Experimental results showed that the exposure dose optimization was a significant parameter for controlling the tip size and its shape. Pyramidal TiO2 tip arrays were successfully fabricated by dry etching in CF4/Ar plasma through a Au nanoparticle mask. The TiO2 tip arrays can be expected to have an important application in gas microsensors.

Beam pen lithography based on arrayed polydimethylsiloxane (PDMS) micro-pyramids spin-coated with carbon black photo-resist

This paper presents a new method for preparing a polydimethylsiloxane (PDMS) mold which can be used in beam pen lithography for patterning a photo-resist (PR) layer in a maskless and direct-write manner. The PDMS mold contains an array of micro-pyramids on its surface and is spin-coated with a layer of carbon black PR which is an opaque material. Because of the arrayed pyramidal surface profile, the spin-coated carbon black PR layer is either thinner at the pyramid tips or does not cover the tips at all, which allows ultraviolet (UV) light to pass through the PDMS mold and forms an array of UV beams. The aperture size of the UV beams can be controlled at a sub-micrometer scale and hence can be used for micro/nano-patterning. Applying this carbon black-PR-coated PDMS mold in beam pen lithography along with a metal lift-off process, various metal dot patterns with a dot-size around 400 to 500 nm are successfully obtained. Both experimental results and theoretical analysis are given along with possible improvements and applications in the future.

Structural, Electrical, and Optical Properties of Faceted In$_{2}$ O$_{3}$ Nanostructures

We reported the study of the growth mechanism of the In2O3 nanorods on the silicon oxide substrate and its electrical and optical properties. Both of the vapor-liquid-solid and vapor-solid (VS) mechanism occurred concurrently during the growth of In2O3 nanorods. The catalyst on the top of the nanorod is evident for the VLS mechanism. Meanwhile, the diameter of the nanorod mismatched the size of the catalyst and transmission electron microscopy study revealed that the sidewall of the nanorod was rough at the atomic scale during the growth, which is evident for the VS mechanism. The In2O3 nanorods became faceted eventually with a pyramidal tip, composed of {1 0 0} and {1 1 1} facets with lower total surface energy at the growth temperature. The electrical measurement presents the conductivity and the estimated carrier concentration, while the photoluminescence exhibits the optical characteristics. This study provides the insight for the understanding of the growth mechanism and physical properties of the In2O3 nanostructures.

Atomic force microscopy determination of Young׳s modulus of bovine extra-ocular tendon fiber bundles

Extra-ocular tendons (EOTs) transmit the oculorotary force of the muscles to the eyeball to generate dynamic eye movements and align the eyes, yet the mechanical properties of the EOTs remain undefined. The EOTs are known to be composed of parallel bundles of small fibers whose mechanical properties must be determined in order to characterize the overall behavior of EOTs. The current study aimed to investigate the transverse Young׳s modulus of EOT fiber bundles using atomic force microscopy (AFM).Fresh bovine EOT fiber bundle specimens were maintained under temperature and humidity control, and indented 100nm by the inverted pyramid tip of an AFM (Veeco Digital Instruments, NY). Ten indentations were conducted for each of 3 different locations of 10 different specimens from each of 6 EOTs, comprising a total of 1800 indentations. Young׳s modulus for each EOT was determined using a Hertzian contact model.Young׳s moduli for fiber bundles from all six EOTs were determined. Mean Young׳s moduli for fiber bundles were similar for the six anatomical EOTs: lateral rectus 60.12±2.69 (±SD)MPa, inferior rectus 59.69±5.34MPa, medial rectus 56.92±1.91MPa, superior rectus 59.66±2.64MPa, inferior oblique 57.7±1.36MPa, and superior oblique 59.15±2.03. Variation in Young׳s moduli among the six EOTs was not significant (P>0.25).The Young׳s modulus of bovine EOT fibers is highly uniform among the six extraocular muscles, suggesting that each EOT is assembled from fiber bundles representing the same biomechanical elements. This uniformity will simplify overall modeling.

Controlling morphologies and growth mechanism of hexagonal prisms with planar and pyramid tips of ZnO microflowers by microwave radiation

Flower-like ZnO microstructures of hexagonal prisms with planar and hexagonal pyramid tips were successfully synthesized by a microwave radiation method. The precursor solutions were synthesized from zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT) at pH 9 and 13. The as-synthesized products were pure hexagonal wurtzite ZnO microstructured flowers of hexagonal prisms with planar tips at pH 9 and of hexagonal prisms with hexagonal pyramid tips at pH 13. Sharp and strong optical phonon Raman-active mode of E2H was detected at 438cm−1. The excitonic characters of the absorption peaks were also detected at 397nm for the first product and 384nm for the second one, and the decolorization efficiency of RhB by Hg-lamp irradiation for 140min was 77% and 87%, respectively.

Increased Cytoskeletal Stiffness of Schlemm's Canal Endothelial Cells in Glaucoma

The elevated intraocular pressure characteristic of glaucoma has been associated with a decreased pore density in the cells of the inner wall of Schlemm's canal (SC). SC cells form these pores in response to a transcellular pressure drop, facilitating the flow of aqueous humor across this endothelium. We hypothesize that impaired pore formation in glaucoma is due to an elevated stiffness of SC cells.Atomic force microscopy (AFM) measurements of elastic modulus were performed using pyramidal or spherical (4.5 or 10 µm) tips on SC cells isolated from 6 healthy and 5 glaucomatous human eyes. Using finite element modeling and AFM experiments with latrunculin-A (an F-actin depolymerizing agent), we previously showed that sharp pyramidal tips characterize the cortex stiffness while larger, spherical tips characterize the stiffness of the subcortical cytoskeleton. The geometry of an SC cell was reconstructed based on electron microscopy images and used to model cell deformation under pressure (3-6 mmHg).When probed with spherical tips, the modulus of glaucomatous cells (1.36±0.14 kPa) was significantly higher (p<0.02) than that of healthy cells (0.89±0.10 kPa). No significant difference was detected between healthy and glaucomatous cells using sharp tips (7.40±1.33 vs. 7.99±0.97 kPa). The higher modulus measured on glaucomatous cells using spherical tips suggest that the altered stiffness is likely in the subcortical cytoskeleton and not in the cell cortex. Preliminary studies using finite element modeling predict a 34-40% decrease in cell deformation solely due to the increased cell stiffness measured on glaucomatous cells. This is consistent with our hypothesis that cell deformation is likely a precursor to the pressure-driven pore formation process and that increased cell stiffness may inhibit this process.Acknowledgements: BrightFocus Foundation, NIH EY019696, NIH T32 EY007128.

Selective growth of metal particles on ZnO nanopyramids via a one-pot synthesis

Hybrid nanostructures of metal (Cu, Au, Ag)-ZnO nanopyramids were synthesized. These hybrid nanostructures possess two distinct morphologies where the metal can be selectively attached to either the base or the tip of the ZnO pyramids. This is the first time that such morphologies are reported for Cu-ZnO and Ag-ZnO hybrid nanostructures.

Solution-processed high-haze ZnO pyramidal textures directly grown on a TCO substrate and the light-trapping effect in Cu2O solar cells

We have prepared pyramidally textured ZnO (PT-ZnO) films with a high light-scattering effect using a simple low temperature (75 °C) solution process that involved no annealing and vacuum processes. The PT-ZnO films, which were composed of dense ZnO columnar grains with a (101)-faceted pyramidal tip, were formed directly on a TCO substrate by electrodeposition. The electrodeposition was performed within 10 min from Zn(NO3)2 aqueous solutions that contained NaCl; during deposition, a two-step electrolytic current was applied to separately control the nucleation density and grain size of ZnO grains. Light-scattering effects from the pyramid structure were examined by measuring haze values in the visible region and comparing the results with those from planar and nanorod structures. The PT-ZnO films showed a haze value of ∼52%, which is 3.7 and 2.2 times higher than those of planar and nanorod ZnO films, respectively. On the basis of total transmittance and haze, the optimal thickness of the pyramidal textures was determined to be 0.7–1.2 μm. Theoretical simulation of light scattering on the PT-ZnO was performed using a simple geometric model which indicated that the refraction angle, path length and intensity of transmitted light vary significantly depending on the outer refractive index of ZnO. The light-trapping effect derived from the PT-ZnO was demonstrated by fabricating ZnO/Cu2O thin-film solar cells. The Cu2O solar cell using the PT-ZnO showed efficient light absorption, achieving a 1.3 times higher short circuit current density than that using the planar ZnO.

The odontoid process: various configuration types in MR examinations

Several developmental anomalies have been described with respect to the odontoid process. The purpose of the study was to describe the MR appearance of the odontoid process and to calculate the prevalence of its morphological variants. We retrospectively reviewed 133 patients (85 males and 48 females, age range of 19-81years, mean 39.5), examined within a 7-year period. Patients were divided into two age groups: ≤30 and >30-year-old. The morphology of the odontoid process was classified into type I: pyramidal tip and type II: ovoid or convex tip. The incidence of os odontoideum and os terminale and the presence of longitudinal cleft were also recorded. Fischer's exact test was used for statistical analysis (p<0.05). Type I was identified in 54 cases (40.6%) and type II in 79 cases (59.4%). Os odontoideum was recorded in one case (0.7%), os terminale in 24 cases (18%) and the presence of a longitudinal cleft (partial or complete) in 86 cases (64.7%). Type I showed a higher prevalence in the ≤30 age group (p<0.05). No differences were shown between sexes. The configuration types of odontoid process are reported herein. The clinical relevance of this morphological variation needs to be elucidated with further studies.

Theoretical analysis and simulations of micro-dosing locomotive robot with drug-release mechanism

A micro-dosing system model of overall size 5×5×4.2 mm3 for drug delivery is proposed and presented. The drug delivery system mainly consists of a micro-wheel and a micro-drug release mechanism. The motion of the micro-wheel is controlled by changing the gravity center of a running disk, which is placed within the hollow micro-wheel and attracted by the actuated micro-solenoids fabricated on the inner wall of micro-wheel in shift. In addition, the micro-wheel is controlled to roll forwards/backwards to the designated location by two sliding mode control strategies: one for long-distance motion (to transport the drug to the vicinity of the spots under disease) and the other for short-distance motion (to decelerate down and stop at the exact drug-release location). On the other hand, the micro-drug release mechanism is composed by a cantilever beam and a chamber filled up by medicine. The pyramid tip of the cantilever beam deflected by the applied electrostatic force is designed to penetrate the micro-film which seals the chamber so that the medicine can be released at the specified spot. The so called “pyramid tip” is, in fact, to replace the conventional medical needle. Its profile is like a pyramid to comply with the MEMS (Micro Electro Mechanical System) fabrication process. This “pyramid” is constructed at the free end of a cantilever beam and hence named as a “pyramid tip”.

Atomic force microscopy to investigate spatial patterns of response to interleukin-1beta in engineered cartilage tissue elasticity

Atomic force microscopy (AFM) has been proposed as a tool to evaluate the structural and mechanical properties of cartilage tissue. Here, we aimed at assessing whether AFM can be employed to quantify spatially resolved elastic response of tissue engineered cartilage (TEC) to short exposure to IL-1β, thus mimicking the initially inflammatory implantation site.TEC generated by 14 days of pellet-culture of expanded human chondrocytes was left untreated (ctr) or exposed to IL-1β for 3 days. TEC pellets were then cut in halves that were glued on a Petri dish. Profiles of elasticity were obtained by sampling with a nanometer sized, pyramidal indenting tip, with 200µm step resolution, the freshly exposed surfaces along selected directions. Replicate TECs were analyzed biochemically and histologically.GAG contents and elasticity of pellets decreased (1.4- and 2.6-fold, respectively, p<0.05) following IL-1β stimulation. Tissue quality was evaluated by scoring histological pictures taken at 200μm intervals, using the Bern-score grading system. At each distance, scores of ctr TEC were higher than those IL-1β treated, with the largest differences between the two groups observed in the central regions. Consistent with the histological results, elasticity of IL-1β-treated TEC was lower than in ctr pellets (up to 3.4-fold at 200μm from the center). IL-1β treated but not ctr TEC was intensely stained for MMP-13 and DIPEN (cryptic fragment of aggrecan) especially in the central regions.The findings indicate the potential of AFM to investigate structure/function relationships in TEC and to perform tests aimed at predicting the functionality of TEC upon implantation.

Effect of cantilever deformation and tip-sample contact area on AFM nanoscratching

The present work investigates the effect of cantilever deformation and tip-sample contact area on the performance of atomic force microscopy (AFM)-based scratching tests. Nanoscratching tests are carried out using a pyramidal diamond tip on aluminum alloy surfaces. Three typical AFM cantilever deformation states are illustrated, and the actual normal loads applied on the surface of the sample are obtained using a new method. A theoretical model for AFM-based nanoscratching using a three-sided pyramidal tip is also established to calculate the effect of the tip-sample contact area for different scratching directions. The corresponding theoretical normal loads can be obtained with this model. Experimental and theoretical results are compared for the normal loads at an expected machined depth, with different scratching directions. The contact length between the chip and the rake face of the tip is found to be the key factor leading to an increase in the tip-sample contact area. This results in an actual normal load larger than the theoretical normal load at an expected depth, especially in the face-forward scratching direction. Similar variation in the normal load is observed in the edge- and sideface-forward scratching directions. The scratching direction has little effect on the depth of the scratched grooves. Comparing the actual normal load with the set normal load along different scratching directions reveals that the cantilever deformation in different scratching directions has little effect on the scratching process.

Template-Stripped Asymmetric Metallic Pyramids for Tunable Plasmonic Nanofocusing

We demonstrate a novel scheme for plasmonic nanofocusing with internally illuminated asymmetric metallic pyramidal tips using linearly polarized light. A wafer-scale array of sharp metallic pyramids is fabricated via template stripping with films of different thicknesses on opposing pyramid facets. This structural asymmetry is achieved through a one-step angled metal deposition that does not require any additional lithography processing and when internally illuminated enables the generation of plasmons using a Kretschmann-like coupling method on only one side of the pyramids. Plasmons traveling toward the tip on one side will converge at the apex, forming a nanoscale "hotspot." The asymmetry is necessary for these focusing effects since symmetric pyramids display destructive plasmon interference at the tip. Computer simulations confirm that internal illumination with linearly polarized light at normal incidence on these asymmetric pyramids will focus optical energy into nanoscale volumes. Far-field optical experiments demonstrate large field enhancements as well as angle-dependent spectral tuning of the reradiated light. Because of the low background light levels, wafer-scale fabrication, and a straightforward excitation scheme, these asymmetric pyramidal tips will find applications in near-field optical microscopy and array-based optical trapping.

Oxidation Sharpening, Template Stripping, and Passivation of Ultra‐Sharp Metallic Pyramids and Wedges

Ultra-sharp metallic pyramids and wedges with tunable tip angles and 5-nm tip radii are replicated from oxidation-sharpened silicon templates with high throughput (80 million pyramids per wafer). Atomic layer deposition of Al2 O3 shells can protect these sharp pyramidal tips for subsequent usage in near-field imaging.

Fabrication and Characterization of Polymer Hollow Microneedle Array Using UV Lithography Into Micromolds

Drug delivery through micromachined needles is an attractive alternative to intramuscular and subdermal injection by hypodermic needles, due to the potential for reduced pain caused by the micro-sized needles. In this paper, a polymer-based fabrication process using UV lithography into micromolds is developed, allowing the fabrication of microneedle (MN) shafts, tips, lumens, and substrate baseplate using lithography. Using UV lithography into micromolds allows complex three-dimensional structures to be defined, since both mask patterns and mold topography are available to define the structures. A hollow MN array and baseplate, in which the needle lumens extend through the thickness of the baseplate, are demonstrated. Fabricated SU-8 MNs are 825 μm in height and 400 μm in width, with a pyramidal tip; the needle lumen, 120 μm in diameter, intersects with one of the faces of the pyramidal tip. Mechanical characterization of the fabricated MNs shows that the fracture force of a single needle against a rigid surface is 12.0 N. The insertion force of a single needle into porcine skin is empirically determined to be 2.4 N. The fracture force of the needle against porcine skin is observed to be in excess of 90 N.

Light Extraction Enhancement of Vertical LED by Growing ZnO Nano-Rods on Tips of Pyramids

Creating a pyramidal structure on an n-GaN surface is considered the most effective approach for maximizing light extraction of n-side-up GaN-based light-emitting diodes (LEDs). This letter shows that the light extraction efficiency of a pyramidal n-GaN surface can be further enhanced by growing ZnO nano-rods (NRs) specifically on the tips of the pyramids on the n-GaN surface. Using tip-only ZnO NRs, the light-output power of an n-side-up LED with a pyramidal n-GaN surface can be further enhanced by 49.6% at 250 mA. This improved light extraction efficiency is due to the multiple facets on the ZnO NRs.

Determination of local and global elastic moduli of valve interstitial cells cultured on soft substrates

The elasticity of the extracellular matrix profoundly affects biological responses of cells, but also their mechanical properties. Single cell mechanical properties are often measured by atomic force microscopy (AFM), but technical guidelines for AFM measurement of cells grown on soft substrates are not well established. In this study, the local and global elastic moduli of aortic valve interstitial cells (VICs) cultured on soft polyacrylamide substrates (3–144kPa) were characterized via AFM force mapping using pyramidal and spherical tips, respectively. Local and global VIC modulus values both increased with substrate stiffness (p<0.05), with the average local cell modulus being consistently two to three times greater than the global modulus (p<0.05). For local measurements, a minimum of four measurements was required to observe the trend of increasing cell modulus with substrate stiffness, but there was no advantage to testing additional spots. The Hertz model was relatively accurate in estimating the global cell elastic modulus (<12% error, based on validated finite element analyses), despite the cells being of finite thickness and grown on deformable substrates, neither of which are accounted for in the Hertz model. The results of this study provide practical guidelines for efficient AFM-based measurement of the mechanical properties of cells on gels. They also provide new physiologically-relevant data on VIC mechanical properties and their correlation with substrate stiffness-dependent cytoskeletal changes, with relevance to heart valve mechanobiology and disease.