Pyramidal Tip Research Articles

The exponent 3/2 at pyramidal nanoindentations

The analysis of published loading curves reveals the exponent 3/2 to the depth for nanoindentations with sharp pyramidal or conical tips. This has geometric reasons, as it occurs independent on the bonding states and indentation mechanisms. Nevertheless, most mathematical deductions and finite element simulations of nanomechanical parameters in the literature continue using the experimentally not supported Hertzian exponent 2. Therefore, numerous published loading curves of various authors are plotted using the experimental exponent 3/2 to present unbiased proof for its generality with metals, oxides, semiconductors, biomaterials, polymers, and organics. Linearity is independent of equipment and valid for load controlled, or depth controlled, or continuous stiffness, or AFM force measurements. The linearity with exponent 3/2 often extends from the nano- into the microindentation ranges. The tip rounding and taper influence of the "geometrical similar" indenters are discussed. When kinks occur in such linear plots through the origin, these indicate change of the materials' mechanical properties under pressure by phase transition. These events are discussed for nanoindentations with respect to the known hydrostatic transformation pressures that are, of course, always higher than the necessary indentation mean pressure. Numerous Raman, as well as X-ray and electron diffraction results from the literature support the phase transitions that are now easily detected. Nanoporous materials first fill the pores upon indentation. Published loading curves exhibit more information than hitherto assumed.

GaAs/AlAs/InGaP heterostructure: a versatile material basis for cantilever designs

We report on the design, fabrication and initial mechanical testing of cantilevers with tips based on a GaAs/In0.485Ga0.515P/AlAs heterostructure grown by metal organic chemical vapor deposition. They were produced using a dedicated technological process based on (1) the formation of integrated tips through an AlAs-assisted surface sacrificial wet-etching process and (2) the GaAs cantilever release fully protected between two InGaP etch-stop layers. 2 µm thick InGaP/GaAs/InGaP cantilevers had integrated pyramidal tips with the sides at ∼45° to (1 0 0). Metallic elements were processed close to the tip apexes using non-standard optical lithography. The cantilever release was accomplished using photolithography, Ar ion milling of InGaP and wet chemical etching of GaAs via resist layers deposited by a draping technique. A tip–cantilever prototype with length, width and thickness of 150, 35 and 2 µm, respectively, exhibited a resonance frequency of 66.2 kHz, which correlated well with a theoretical value of 57 kHz for a GaAs cantilever of identical dimensions.

Surface roughness of intraocular lenses with different dioptric powers assessed by atomic force microscopy

To analyze the optic surface roughness and morphology of 2 types of hydrophobic acrylic intraocular lenses (IOLs) with various dioptric powers using atomic force microscopy (AFM). Technical University of Cluj-Napoca, Faculty of Mechanics, Cluj-Napoca, Romania. Atomic force microscopy was used to characterize the topography of 2 types of hydrophobic acrylic IOLs from a single manufacturer (SN60AT and SA30AL) with dioptric powers ranging from 10.0 diopters (D) to 30.0 D. The AFM analysis was performed in contact mode using a V-shaped silicon nitride cantilever with a pyramidal tip curvature of 15 nm and a nominal spring constant of 0.2 N/m. Detailed surface characterization of the IOL optic was obtained using 6 quantitative parameters provided by the AFM software. Five of 6 roughness parameters indicated statistically significant differences (P<.05) between IOLs with different dioptric powers, with the 10.0 D IOL in both models providing the smoothest optic surface. Between models with the same dioptric power, the SN60AT model had lower values of each surface roughness parameter than the SA30AL model. Atomic force microscopy was an accurate tool for assessing the surface properties of IOL optics. Manufacturing processes were responsible for introducing detectable differences in the topography of IOL biomaterials with identical copolymer constituents but different dioptric powers. Nanometric analysis may assist IOL manufacturers in developing IOLs with optimal surface characteristics.

Observation of Si Pattern Sidewall Using Inclination Atomic Force Microscope for Evaluation of Line Edge Roughness

Inclination atomic force microscope (AFM) imaging has been studied on the possibility to observe a pattern sidewall in contact mode or digital probing (step-in) mode for a line edge roughness (LER) or line width roughness (LWR). Analysis of the AFM tip bending and slipping indicates that it is serious problem to measure and control very fine patterns within an error of less than 1 nm in contact of the tip on the steep slop of the pattern, and it is very important directly to observe the sidewall at inclination angle. In experiments using pyramidal tip and steep Si pattern with about 90 degrees slop, it has demonstrated that the inclination angle is 35-40 degrees for faithful observation of the sidewall. We have observed the etched strip lines on the sidewall with a width of about 100 nm and a depth of about 6.4 nm. We have demonstrated that the inclination AFM is very useful for evaluation of the LER or LWR.

Design and fabrication of a pyramid wavefront sensor

A new pyramid wavefront sensor (PWFS), which utilizes a reflective pyramid mirror instead of a refractive pyramid prism at the focus of a telescope, is presented. As a key optical component in this PWFS, the pyramid mirror requires accurate microfabrication for excellent quality of the tip, the turned edges, and the surfaces. The moving mask lithography process is proposed for its economic, simple, and precise control to make the cross-sectional shape of the structure. The completed pyramid mirror has a square base of 1-mm length and four side facets inclined to the base at 3.7 deg. The sizes of the pyramid tip and turned edges are both about 6 µm, which show excellent aspects of sharpening-tip and knife-edges. The root mean square of four facets is approximately 70 nm, and the maximum profile deviation is 0.2 µm.

Single-crystal diamond probes for atomic-force microscopy

The results of investigations aimed at the development and testing of diamond probes for scanning atomic-force microscopy are presented. Plasmochemical deposition of diamond polycrystalline films and selective thermal oxidation were used to manufacture diamond probes. The obtained single-crystal diamond pyramidal tips of micron size had a radius of curvature of 2–20 nm at the top. The diamond tips were attached to a cantilever with an epoxy adhesive and then tested as probes in scanning atomic-force microscopy. Tests have shown that the manufactured diamond probes have appreciable advantages over conventional probes.

Noncontact microrheology at acoustic frequencies using frequency-modulated atomic force microscopy

We report an atomic force microscopy (AFM) method for assessing elastic and viscous properties of soft samples at acoustic frequencies under non-contact conditions. The method can be used to measure material properties via frequency modulation and is based on hydrodynamics theory of thin gaps we developed here. A cantilever with an attached microsphere is forced to oscillate tens of nanometers above a sample. The elastic modulus and viscosity of the sample are estimated by measuring the frequency-dependence of the phase lag between the oscillating microsphere and the driving piezo at various heights above the sample. This method features an effective area of pyramidal tips used in contact AFM but with only piconewton applied forces. Using this method, we analyzed polyacrylamide gels of different stiffness and assessed graded mechanical properties of guinea pig tectorial membrane. The technique enables the study of microrheology of biological tissues that produce or detect sound.

Edge Chipping of Rock: An Experimental Study

This paper investigates experimentally the edge chipping of a rock to assess the cutting performance of a conical and a pyramidal tip. It was found that the conical tip generates many radiated cracks and results in a larger amount of fine rock grains due to crashing. The size of chips produced by the pyramidal tip is bigger. It was concluded that the critical chipping energy has approximately a linear relation with h9/4 of which h is the depth of cut of the tip.

Micro- and Nanomechanical Analysis of Articular Cartilage by Indentation-Type Atomic Force Microscopy: Validation with a Gel-Microfiber Composite

As documented previously, articular cartilage exhibits a scale-dependent dynamic stiffness when probed by indentation-type atomic force microscopy (IT-AFM). In this study, a micrometer-size spherical tip revealed an unimodal stiffness distribution (which we refer to as microstiffness), whereas probing articular cartilage with a nanometer-size pyramidal tip resulted in a bimodal nanostiffness distribution. We concluded that indentation of the cartilage's soft proteoglycan (PG) gel gave rise to the lower nanostiffness peak, whereas deformation of its collagen fibrils yielded the higher nanostiffness peak. To test our hypothesis, we produced a gel-microfiber composite consisting of a chondroitin sulfate-containing agarose gel and a fibrillar poly(ethylene glycol)-terephthalate/poly(butylene)-terephthalate block copolymer. In striking analogy to articular cartilage, the microstiffness distribution of the synthetic composite was unimodal, whereas its nanostiffness exhibited a bimodal distribution. Also, similar to the case with cartilage, addition of the negatively charged chondroitin sulfate rendered the gel-microfiber composite's water content responsive to salt. When the ionic strength of the surrounding buffer solution increased from 0.15 to 2 M NaCl, the cartilage's microstiffness increased by 21%, whereas that of the synthetic biomaterial went up by 31%. When the nanostiffness was measured after the ionic strength was raised by the same amount, the cartilage's lower peak increased by 28%, whereas that of the synthetic biomaterial went up by 34%. Of interest, the higher peak values remained unchanged for both materials. Taken together, these results demonstrate that the nanoscale lower peak is a measure of the soft PG gel, and the nanoscale higher peak measures collagen fibril stiffness. In contrast, the micrometer-scale measurements fail to resolve separate stiffness values for the PG and collagen fibril moieties. Therefore, we propose to use nanostiffness as a new biomarker to analyze structure-function relationships in normal, diseased, and engineered cartilage.

Fabrication of cantilever arrays with nano-aperture hollow tips for parallel microplasma etching

For the application of parallel microplasma etching, cantilever arrays with nano-aperture hollow pyramid tips have been successfully fabricated. The SiO 2 cantilever arrays and hollow tips are formed by thermal oxidation on a Si (1 0 0) wafer with pyramid cavities. Due to the stress-dependent nonuniform oxidation, the oxide thickness is about 400 nm at the tip apexes which is much thinner than the 1.2 μm thick sidewalls. Based on these nonuniform oxide hollow tips, nano-apertures of 50–200 nm in diameter are obtained at the apexes after the following 1:10 water-diluted HF isotropic etching. The base widths of the hollow tips are designed to be 50 and 100 μm with the final sidewall thickness of only 600 nm. Consequently, the width–thickness ratio (hollow pyramid tip base width/sidewall thickness) is up to 150:1. Some improvements are made in the fabrication process and these fragile tips are obtained with a product yield of more than 90%. Then, cantilever arrays with hollow tips are released consistently and the bending behavior is discussed. In addition, preliminary experiments and simulations of microplasma generation and extraction confirm the application feasibility of this structure in parallel microplasma etching.

ZnO Pyramidal Arrays: Novel Functionality in Antireflection

We introduce a new type of ZnO array structure that can effectively suppress the reflection of light at a range of wavelength from ultraviolet through visible part to the near-infrared region, with reflectivity <2.5%, resulting in its black color. This black ZnO is composed of a random pyramidal array growing directly on Zn foil through a simple hydrothermal method. We have investigated the causes of antireflection by removing the pyramid tips via scratching and ion etching on the ZnO surface. All results demonstrate that the antireflection nature of the black ZnO originates from the pyramidal array structure, which causes a gradient change of refractive index between air and substrate.

High stability and electronic structures of noble-metal covered W(111) atom perfect pyramidal tips

Noble-metal covered W 111 single-atom pyramidal tips SATs are artificially created, highly stable atom perfect nanostructures with broad scientific and technological interests. Using first-principles calculations, the binding energy of the topmost atom is found to be nearly identical to the cohesive energy of the metal despite its greatly reduced coordination. This explains the extraordinary high stability of the SATs. In addition, the tip states below Ef that localize not only in the real space but also in the energy domain is a genetic property of the reduced dimension of the apex of the pyramidal tip structure. The calculated spectral features may also provide a foundation for future scanning tunneling spectroscopy measurements as well as an elemental analysis of surface atoms using SATs in scanning tunneling spectroscopy.

Mechanical performance of metallic glasses during nanoscratch tests

Nanoscratch experiments on four bulk metallic glasses using a pyramidal diamond tip (Berkovich) indenter have been carried out under different loads and scratching velocities. There exists a general correlation between the hardness and wear resistance, i.e., a lower hardness produces a lower wear resistance. Scratch velocity appears to have little effect on the scratch depths of the four alloys studied. However, faster scratch velocity leads to finer shear bands. Compared with ribbon sample with the same chemical composition, Ti-based bulk glassy sample that experienced slower cooling rate during solidification, and thus contained less free volume, exhibits better wear resistance.

Enhanced Density Control of Al:ZnO Nanowires via One-by-One Coupling of Nanowires and Pyramids

Al doped ZnO nanowire arrays with controlled growth densities were fabricated on silicon without using catalysts via sputtering followed by thermal chemical vapor deposition (CVD). Scanning electron microscopy and high-resolution transmission electron microscopy results show that the Al:ZnO single-crystalline nanowires synthesized by CVD prefer growing epitaxially on the tips of the ZnO pyramids pre-synthesized by sputtering with the c-axis perpendicular to the substrate. Consequently, the densities of the as-grown Al:ZnO nanowires were controllable by changing the particle densities of the pre-grown ZnO seed layers. The Al concentration of the Al:ZnO nanowires were measured to be around 2.63 at.% by electron energy loss spectrum. Field-emission measurements show the turn-on fields of the Al:ZnO nanowire arrays with controllable area densities are tunable. Room-temperature cathodoluminescence spectra of the Al:ZnO nanowires show relatively strong and sharp ultraviolet emissions centered at 383 nm and broad green emissions at around 497 nm. This work provides a simple method to control the field emission and luminescence densities of Al doped ZnO nanowire arrays, which also shows good potential for developing nano-pixel optical devices.

Single crystal diamond tips for scanning probe microscopy

Single crystal diamond tips with perfect pyramidal geometry were obtained by a combination of chemical vapor deposition and selective oxidation of polycrystalline films. The parameters of the deposition process were chosen to provide growth of a textured film consisting of micrometer sized diamond crystallites embedded into nanodiamond ballas-like material. The heating of the film in an air environment was used for selective oxidation of the nanodiamond component. The films obtained contain free standing pyramidal single crystal diamond tips oriented by their apexes to the substrate surface. The tips were used for the fabrication of atomic force microscopy probes and their evaluation in comparison to common silicon probes.

Fabrication of Nanometer-Scale Si Field Emitters Using Self-Assembled Ge Nanomasks

Large-area, nanometer-scale Si field emitters have been fabricated by selective chemical etching of self-assembled Ge islands on Si. Taking advantage of the relatively low etching rate, uniform Ge islands act as virtual nanomasks for the underlying Si substrate. During selective chemical etching, Ge nanomasks shrink into small Ge-core islands, which determine the apex sharpness of the resulting Si pyramidal tips. The results demonstrate that Si pyramidal tips exhibited improved antireflective and electron field emission characteristics compared to as-grown Ge islands. The high field enhancement factor can be attributed to high tip density, nanoscale apex, and well-controlled spacing between Si pyramidal tips. This work offers a low cost alternative for designing and fabricating high efficiency Si-based field emitters or nanodevices.

Fabrication and characterization of coaxial scanning near-field optical microscopy cantilever sensors

A process scheme for the fabrication of scanning near-field optical and force microscopy cantilever sensors with coaxial tips is presented. A hollow fourfold pyramidal tip is used, coated from both sides by 100 nm aluminium. Focused Ion Beam milling is performed to structure the tips. First measurements on a model system demonstrate the performance of these sensors.

Wave propagation in pyramidal tip-like structures with cubic material properties

The three-dimensional wave propagation in cubic materials like silicon AFM tips is investigated numerically. The used VS-FDTD (Velocity Stress Finite-Difference Time-Domain) code is explained starting with the fundamental equations for the wave propagation in cubic materials. The discretisation, the boundary conditions, and the stability conditions are considered. As a validation of the numerical code, first the simulated wave velocities are compared with analytically calculated results for different polarizations and propagating directions. Second the code is validated by monitoring the internal energy of the simulated structure. Third, the results of the numerical code for long wavelengths agree well with a simplified analytical model for wave propagation in horn geometries. With the presented code pyramidal focusing tips are investigated regarding their focusing and detection properties and regarding the influence of the crystallographic orientation. It turns out that the focusing effect for the primary waves is strong in tips with lateral dimensions at the sharp end which are smaller than the dominant wave length. In contrast, the detection properties of a tip are better if the lateral dimensions at the sharp end of the tip are larger than the dominant wave length.

Structure and Field‐Emission Properties of Sub‐Micrometer‐Sized Tungsten‐Whisker Arrays Fabricated by Vapor Deposition

Sub-micrometer-sized tungsten-whisker arrays are synthesized by a catalyst- and template-free vapor-deposition process. The resultant whiskers have a hexagonal cross-section, sharp pyramidal tip, and large aspect ratio. The excellent field-emission properties of the whisker arrays, which can be tuned varying the diameters of the synthesized whiskers, are demonstrated.

Reactivity indexes for different geometries of palladium leads

Electronic transport through metallic break junctions or molecules is clearly dependent not only on the electronic structure of the central nanodevice connecting the leads, but also the shape and crystalline orientation of the contacts which can define the possible conduction channels. In this work we examine different geometries of contacts of palladium characterizing them through global and local reactivity indexes as electrophilicity, chemical hardness and Fukui functions. In molecules, these indicators are essentially defined by the energies of the frontier molecular orbitals and in solids they are related with the local and partial density of states. We use for this purpose an ab-initio based code (FIREBALL), applied to plane contacts with (001) fcc faces and also pyramidal tips grown following a (001) and (111) packaging. The results allow us to have an insight about the chemical features of this type of nanojunctions.