Ultra-sharp Tungsten Research Articles

Molecular dynamics of monocrystal copper nano-scratched with tungsten tip

The utilization of ultra-sharp tungsten tips for nano-scratching has been employed in the fabrication of copper micro/nanostructures. Understanding the material removal mechanisms and the evolution of subsurface defects during this process is crucial for optimizing manufacturing processes. Nevertheless, the mechanisms of material removal and subsurface defect evolution of ultra-sharp tungsten tips nano-scratching monocrystal copper remain elusive. This study utilizes molecular dynamics simulation to investigate the material removal mechanism, subsurface defect, and dislocation evolution of monocrystal copper nano-scratched with tungsten tip. Furthermore, the formation mechanism of stress-induced stacking faults is revealed through spatially distributed hydrostatic pressure analysis. Finally, the effect of scratching parameters on subsurface damage and workpiece machinability is explored. The results indicate that plastic deformation mechanisms of nano-scratching monocrystal copper include amorphization, dislocation slip, and atomic phase transition. During the nano-scratching process, V-shaped dislocation loops and prismatic dislocation loops are formed in the shearing-plowing region, and atomic clusters and stacked fault tetrahedrons are generated in the subsurface region. Scratching along the [100] crystal orientation, coupled with relatively increased scratching distance, velocity, and appropriate depth facilitates the attainment of shallow subsurface deformed layer and excellent machinability.

Diamond-like Carbon Patterning by the Submerged Discharge Plasma Technique via Soft Solution Processing.

Submerged plasma-assisted discharge direct patterning of diamond-like carbon (DLC) onto the silicon substrate in ambient conditions has succeeded as a new and novel soft solution process. In this environmentally benign technique, a copious amount of pure ethanol (ca. 4 mL) was locally activated with a maximum of ca. 0.23 mkWh by an as-electrochemically synthesized ultrasharp tungsten tip. With the assisted submerged plasma, the decomposed ethanol molecules are anodically patterned directly onto the silicon substrate in ambient conditions. The physical nature of DLC patterns was accessed by profilometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy analysis. Furthermore, Fourier-transform infrared, Raman, and X-ray photoelectron spectra were analyzed for chemical compositions and structures, such as surface functionalization, carbon-carbon bonding, and sp2-sp3 ratio, respectively. From a Berkovich-configured nanoindentation analysis, Young's modulus and hardness have shown increasing trend with increasing sp3-sp2 ratio in DLC patterns of 68.5 and 2.8 GPa, respectively. From the electrochemical cyclovoltammetry analysis, a maximum areal specific capacitance of 205.5 μF/cm2 has been achieved at a scan rate of 5 mV/s. The one-step, green, and environmentally sustainable approach of rapid formation of DLC patterns is thus a promising technique for various carbon-based electrode fabrication processes.

Fabricating ultra-sharp tungsten STM tips with high yield: double-electrolyte etching method and machine learning

The double-electrolyte etching method is a simple and effective way to fabricate ultra-sharp scanning tunnel microscopy (STM) tungsten tips. However, it is still challenging for this method to get ultra-sharp tungsten tips with high yield. In this work, significant enhancing of the yield is presented through optimized etching parameters as follows: temperature of 26 °C, applied voltage 7.5 V, electrolyte concentration 4 mol L−1 and length below the liquid lamellae of 2 mm. Under these conditions, the smallest tip radius is around 8 nm and the yield (radius < 10 nm) is 63.5%. These tips are capable of producing high-quality atomic resolution STM images, as demonstrated by testing on Si (111) and highly oriented pyrolytic graphite (HOPG) samples at room temperature. Furthermore, in order to find the relationship between the tip features and experimental etching parameters, an artificial neural network (ANN) model is built by machine learning. Garson’s algorithm is used to analyze the relative importance of each experimental parameter on tip features. The tip features can be estimated by this model with a correlation factor R over 0.85 indicating great predictive performance. Importance analysis indicates that the length of the tungsten wire below the liquid lamellae is the most important parameter to obtain high-quality STM tungsten tips in the double-electrolyte etching method. This result provides a clear direction for rapidly selecting optimized tip fabrication parameters in the future.

Electrochemical Etching of Tungsten for Fabrication of Sub-10-nm Tips with a Long Taper and a Large Shank

From the perspective of maximizing the practicability of tungsten nano-tips, a sharp tip, long taper, and large shank are usually expected. However, simultaneously satisfying the requirements for tip radius, taper length, and shank diameter is theoretically impossible with the conventional drop-off tip fabrication method, which is based on lamellae electrochemical etching. In this study, a two-step etching method is proposed to fabricate a sub-10-nm tungsten tip directly from a 1-mm rod. First, a floating electrolyte-based drop-off process is carried out to fabricate a tungsten needle with a long length taper of 10 mm. Then, an inversed lamellae drop-off process is conducted to realize fine etching of the needlepoint. As a result, an ultra-sharp tungsten nano-tip with a radius of 5.5 nm and taper length of 10 mm is successfully fabricated from a 1-mm tungsten rod.

Understanding the Image Contrast of Material Boundaries in IR Nanoscopy Reaching 5 nm Spatial Resolution

Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale resolved Infrared (IR) and Terahertz (THz) imaging, and thus has manifold applications ranging from materials to biosciences. However, a quantitatively accurate understanding of image contrast formation at materials boundaries, and thus spatial resolution is a surprisingly unexplored terrain. Here we introduce the write/read head of a commercial hard disk drive (HDD) as a most suitable test sample for fundamental studies, given its well20 defined sharp material boundaries perpendicular to its ultra-smooth surface. We obtainunprecedented and unexpected insights into the s-SNOM image formation process, free of topography-induced artifacts that often mask and artificially modify the pure near-field optical contrast. Across metal-dielectric boundaries, we observe non-point-symmetric line profiles for both IR and THz illumination, which are fully corroborated by numericalsimulations. We explain our findings by a sample-dependent confinement and screening of the near fields at the tip apex, which will be of crucial importance for an accurate understanding and proper interpretation of high-resolution s-SNOM images of nanocomposite materials. We also demonstrate that with ultra-sharp tungsten tips the apparent width (and thus resolution) of sharp material boundaries can be reduced to about 5 nm.

In situ fabrication and optoelectronic analysis of axial CdS/p-Si nanowire heterojunctions in a high-resolution transmission electron microscope

A high-precision technique was utilized to construct and characterize axial nanowire heterojunctions inside a high-resolution transmission electron microscope (HRTEM). By an in-tandem technique using an ultra-sharp tungsten probe as the nanomanipulator and an optical fiber as the optical waveguide the nanoscale CdS/p-Si axial nanowire junctions were fabricated, and in situ photocurrents from them were successfully measured. Compared to a single constituting nanowire, the CdS/p-Si axial nanowire junctions possess a photocurrent saturation effect, which protects them from damage under high voltages. Furthermore, a set of experiments reveals the clear relationship between the saturation photocurrent values and the incident light intensities. The applied technique is expected to be valuable for bottom-up nanodevice fabrications, and the regarded photocurrent saturation feature may solve the Joule heating-induced failure problem in nanowire optoelectronic devices caused by a fluctuating bias.

Tungstate sharpening: A versatile method for extending the profile of ultra sharp tungsten probes

The benefits of a new electrochemical etching method for the controlled sharpening of sub-micron tungsten probes are demonstrated. The proposed technique only utilizes the insulating effect of the WO₄(2-) by-product which offers more practical ways of controlling the process parameters. The electrosharpening method was fully automated through the analysis of the process current, bulk coulometry, shadowgraphs, and time lapse microscopy. Tip radii smaller than 15 nm were maintained over a wide range of controlled lengths up to 4.5 mm with conic angles of less than 1°.

Ultrasharp tungsten tips—characterization and nondestructive cleaning

Abstract We study the treatment of ultrasharp tungsten tips used for applications in nanoscience and introduce a fast and simple method for estimation of the tip radius using a single measurement of the autoemission current. The method is based on a detailed investigation of the influence of an arrangement of electrodes on the electric field layout in close proximity of the tip apex. The electric field was calculated using Monte Carlo Floating Random Walk algorithm. The most frequently used cleaning procedures (heating the whole tip to high temperature, electron bombardment and self-sputtering) were investigated by electrical measurements and microscopy techniques (SEM, TEM) and the results of the particular methods are compared. We report on the effectiveness and limiting conditions of the cleaning methods with respect to the damage they cause to the tip apex.

Electrostatic Potential Fluctuations on Oxide-Passivated Si(111) Surfaces Measured Using Integrated Scanning Probe Microscopy

Variations of the electrostatic potential were investigated for oxide-passivated n-Si(111) surfaces with atomically flat terraces by measuring the force acting on an ultra-sharp tungsten probe that was attached to the quartz resonator of an atomic force microscope. When the probe-sample gap maintained a constant tunneling current, an enhancement of electrostatic force with a lateral extent of ∼5 nm was observed around underlying donor atoms and charged defects. Additional variations of the surface potential and the probe-sample capacitance across the surface steps were associated with excess electric charge at the step edge. [DOI: 10.1380/ejssnt.2011.117]

Influence of Particle Size and Polymer−Filler Coupling on Viscoelastic Glass Transition of Particle-Reinforced Polymers

The viscoelastic glass-to-rubber softening transition is analyzed for various cross-linked polymers reinforced with filler particles. We find that the loss modulus peak corresponding to the segmental relaxation process (glass transition) is not significantly affected by the particle surface area in carbon black-filled polybutadiene or by silane chemical coupling of poly(styrene-co-butadiene) to silica. Large differences in shape and magnitude of the peak in the loss tangent (tan δ) vs temperature are noted for these materials; however, this is due to variations in the storage modulus at small strains in the rubbery state, which is influenced by the nature of the jammed filler network. The use of a simple relaxation model demonstrates this feature of the viscoelastic glass transition in filled rubber. It is not necessary to invoke concepts involving a mobility-restricted polymer layer near the filler surfaces to explain the viscoelastic results. Atomic force microscopy conducted with an ultrasharp tungsten tip indicates that there may be some stiffening of the elastomer in the proximity of filler particles, but this does not translate into an appreciable effect on the segmental dynamics in these materials.

Microdrilling for fabricating micrometer-scale holes in soft matter

Probing the response of soft materials at small scales requires examining fundamental behaviors that are often distinct from large-scale interactions. In the development of micrometer- and nanometer-sized holes in soft materials, understanding failure modes becomes essential. We observe fracture behavior in a soft material through a novel method, which leads to the fabrication of small-scale holes in polydimethylsiloxane. We utilize an ultra-sharp tungsten needle to drill this soft elastomeric polymer; this results in controlled hole size and exhibits fracture characteristics observed in brittle materials at larger length scales. We also examine the macroscopic characteristics known to contribute to brittleness and hardness for this material’s response with respect to curing times. This understanding will contribute to many applications including the development of porous materials and DNA sequencing efforts.

Improved emission stability of carburized HfC〈100〉 and ultrasharp tungsten field emitters

We have evaluated the cold-field-emission characteristics of HfC〈100〉 and ultrasharp tungsten emitters. We found that proper acetylene treatment improved both the angular current confinement and the emission stability of thermally cleaned HfC〈100〉 tips. Stable emission exceeding 10 μA/sr for over 1 h and angular confinement to a 3° semicone angle have been observed. The improvements are probably related to the modified work function and surface chemical composition induced by the acetylene treatment at the tip apex. Carburization of W〈100〉 and W〈111〉 tips also significantly improved the emission current stability. This study indicates the usefulness of surface processing in the development of cold-field emitters.

Nanostructuring of alkanethiols with ultrasharp field emitters

We have investigated the electron beam exposure of self-assembled monolayers (SAM) of alkanethiols on gold. SAM resist systems are of growing interest for proximal probe lithography due to their high chemical selectivity though being as thin as a monolayer. For this purpose a custom built proximal probe lithography device was used. The electrons emanate from an ultrasharp tungsten tip, providing extraordinary field emission properties—small emission angle and low emission energy. In contrast to common scanning tunneling microscope (STM) lithography our system can be operated not only in tunneling mode, but also in true field emission mode, i.e., at much larger tip-to-sample distances (typically 10–100 nm). Therefore the latter mode allows much higher writing speeds, as the tip does not have to follow the sample topography as accurately as in the STM mode. The first results obtained by exposure in the field emission mode are presented.

Probe hole field electron/field ion microscopy and energy spectroscopy of ultrasharp [111]-oriented tungsten tips

Tungsten field emitter tips are frequently being used in electron, scanning tunneling and point projection microscopes. To characterize the electronic properties as well as the electron and ion emission behavior for ultrasharp tungsten needles, we have carried out energy-resolved measurements. Hemispherical mirror and retarding potential analyses of field-emitted electrons and ions have been combined with FEM/FIM examinations of [111]-oriented tungsten emitter prepared in different ways, for example by neon ion sputtering and by a thermo-field treatment, creating a single atomically sharp microprotrusion. In the electron emission mode the free electron gas approximation gives satisfactory fits to the total energy distributions measured to date. Argon ion energy distributions show a field-dependent high energy secondary structure possibly indicating a field-gradient-enhanced migration of molecules, such as water, to the microprotrusion.

Application of Electrosurgery in Scalp Reduction

An ultrasharp tungsten electrode was used in several hundred scalp reduction procedures from 1988-1992 at Peterson Medical Institute, Santa Monica, California. This unique instrument has proved to be a useful adjunct in facilitating hair restoration surgery.

Brightness measurements of nanometer-sized field-emission-electron sources

The brightness of nanometer-sized field-emission-electron sources have been measured experimentally. Ultrasharp tungsten (111) single-crystal tips were fabricated in situ using Ne sputtering and field evaporation, and monitored using field ion microscopy. The average brightness of single-atom-terminated nanotips was found to be 3.3×108 A cm−2 sr−1 at 470 V, or 7.7×1010 A cm−2 sr−1 when extrapolated to 100 kV. These results show an improvement of about two orders of magnitude in source brightness over existing cold field-emission-electron sources, and produce a beam with greater particle flux per unit energy than those obtainable using current synchrotron/wiggler/undulator devices.

Nanotips by reverse electrochemical etching

A simple, two-stage procedure is shown to produce slender and ultrasharp tungsten tips of nanometer and subnanometer apex dimensions (nanotips). Tip sharpening is achieved by electrochemical etching through bubble dynamics induced by ac voltage in a novel configuration in which the wire end is oriented upward. Tip shape is characterized by high-resolution transmission electron microscopy.