Sharp Tungsten Tip Research Articles

Scanning thermal imaging by near-field fluorescence spectroscopy

A scanning thermal microscope that uses a fluorescent particle as a temperature probe hasbeen developed. The particle, made of a rare-earth ion-doped fluoride glass, is glued at theextremity of a sharp tungsten tip and scanned on the surface of an electronic device. Thetemperature of the device is determined by measuring the fluorescence spectrum of theparticle at every point on the surface and by comparing the intensity variations of twoemission lines. As an example, we will show some images obtained on a nickel stripe1 µm wide, heated by an electrical current. A good agreement is observed with a simulation ofthe temperature field on the device.

Experimental Evidence for Ice Formation at Room Temperature

The behavior of water under extreme confinement and, in particular, the lubrication properties under such conditions are subjects of long-standing controversy. Using a dedicated, high-resolution friction force microscope, scanning a sharp tungsten tip over a graphite surface, we demonstrate that water nucleating between the tip and the surface due to capillary condensation rapidly transforms into crystalline ice at room temperature. At ultralow scan speeds and modest relative humidities, we observe that the tip exhibits stick-slip motion with a period of 0.38+/-0.03 nm, very different from the graphite lattice. We interpret this as the consequence of the repeated sequence of shear-induced fracture and healing of the crystalline condensate. This phenomenon causes a significant increase of the friction force and introduces relaxation time scales of seconds for the rearrangements after shearing.

Transmission-type microfocus x-ray tube using carbon nanotube field emitters

A microfocus x-ray tube that can generate x rays with the focal spot size less than 5μm has been demonstrated using carbon nanotube (CNT) field emitters. A CNT cathode on a sharp tungsten tip, a magnetic solenoid lens, and a transmission-type x-ray target were adopted for the microfocus x-ray tube. The design characteristics and the operation performance of the microfocus x-ray tube are presented. Due to the small focal spot size, clear x-ray radiographic images of 6μm bars and x-ray images with the magnification factor of higher than 230 were obtained.

Recrystallization of tungsten wire for fabrication of sharp and stable nanoprobe and field-emitter tips

Atomically sharp tungsten tips made from single crystal tungsten wire are superior to those made from cold-drawn polycrystalline wire but are rarely used due to their high price. We have devised a method of obtaining highly crystalline tungsten wire by recrystallizing cold-drawn wire. The effect of various heat treatments on the wire microstructure was observed using scanning electron microscopy and x-ray diffraction. A dramatic difference in the shapes of tips etched from cold-drawn and recrystallized wires was observed using transmission electron microscopy. The described annealing process is an inexpensive alternative to using single crystal wires.

Ultrafast Electron Pulses from a Tungsten Tip Triggered by Low-Power Femtosecond Laser Pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.

Efficient electrochemical etching method to fabricate sharp metallic tips for scanning probe microscopes

A new technique based on electrochemical etching for the fabrication of sharp metallic tips for scanning probe microscopes is introduced. In the proposed method, a small Teflon mass is attached to the end of an immersed tungsten wire using an aluminum tape, which leads to a significant enhancement of yield rate of sharp tungsten tips with an apex size below 100nm to over 60%. The functionality of the tungsten tips fabricated by the proposed method is verified by measuring the topography of a standard sample using a shear-force scanning probe microscope.

Indentation mechanics of Cu–Be quantified by an in situ transmission electron microscopy mechanical probe

In situ transmission electron microscopy was used to study, in real time, the sub-surface deformation taking place in Cu–Be alloy during nanoindentation. A twinned region of the material was indented with a sharp tungsten tip in a specially developed transmission electron microscopy (TEM) holder. A flexible hinge-based force sensor was used to measure the force on the indenter, and the force–displacement curve for the tip was obtained by tracking the tip in the sequential images of a TEM video of the indentation process. Step-like structures ∼50 nm in size resulting from the tip surface roughness were observed to generate clusters of dislocations in the sample when they come in contact with the softer Cu–Be. With this setup, the forces and the mean pressure associated with such an individual deformation event in a nanostructured TEM sample were measured.

Nanoscale organic electroluminescence from tunnel junctions

Nanoscale organic electroluminescence was induced by positioning a sharp tungsten tip on the surface of a free-base porphyrin $({\mathrm{H}}_{2}\mathrm{TBPP})$ monolayer on the top of PtTBP porphyrin (PtTBPP) multilayers on a Cu(100) substrate in an ultrahigh vacuum scanning tunneling microscope (STM) system. The well-defined molecular fluorescence spectra are perfectly matched with the conventional photoluminescence spectrum from bulk ${\mathrm{H}}_{2}\mathrm{TBPP}$ molecules. The nanoscale PtTBPP multilayers do not fluoresce; rather, they act as spacers to enhance the decoupling of the electronic state of the ${\mathrm{H}}_{2}\mathrm{TBPP}$ monolayer from the Cu surface. The electronic property of molecules and the energy-level alignment of molecules with respect to the Fermi levels of electrodes are probably quite critical for observing STM-induced molecular fluorescence from molecular layers with a similar thickness. The molecule in proximity to the tip apex of a scanning tunneling microscope is locally excited by the hot electron injection mechanism, followed by radiative decay via Franck-Condon transitions.

Stable field emission from W tips in poor vacuum conditions

We report a stable field emission (FE) current from sharp tungsten (W) tips in relatively poor vacuum (up to 10−2 mbar) conditions. We use small tip–anode spacing to keep the extraction voltage low. A simple current regulator circuit, with a bandwidth of ∼1.6 kHz, was designed, which controls the voltage applied according to the emission current measured. Without a current regulator circuit, an uncleaned W tip in unbaked 6×10−7 mbar system pressure cannot emit stable FE current and short-term fluctuations at a few nA current were found to be more than 300%. The current regulator circuit improves the FE current stability dramatically. It was observed that at current level of ∼3.5 nA and regulation voltage of ∼120 V the short-term fluctuations in the current were ∼5% at 6×10−7 mbar unbaked system pressure. Subsequently, the system pressure was increased in steps up to 10−3 mbar of argon gas and it was observed that the current regulator circuit worked at almost the same efficiency. Around 10−2 mbar of Ar gas pressure larger short-term fluctuations started appearing and around 10−1 mbar of Ar gas pressure the current regulator circuit failed to regulate the current. In another experiment, at 10−3 mbar of Ar gas pressure, at ∼1.5 and 25 nA current levels long-term FE current stability was recorded. Our results show that the FE current stability in our experiments is much better than that reported in the literature with feedback on the tip–anode distance. This study may find applications in focused electron beam systems as well as in electron impact ion sources.

A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors

We present an electrochemical etching scheme for producing sharp tungsten tips for use in scanning probe microscopes. The motivation behind the development of this particular method comes from the need to have an etched probe attached to a quartz tuning fork. Comparisons with existing etching methods are made. This rather simple scheme incorporates the key advantages of previously established techniques to give reproducible and controlled etching cycles.

Preparation of cantilevered W tips for atomic force microscopy and apertureless near-field scanning optical microscopy

Tip characteristics play an important role in the resolution and sensitivity of scanning probe microscopy. Extensive efforts have been devoted to tip fabrication. Most of the research is focused on scanning tunneling microscopy applications, which require sharp and short tips. Long tips that can be bent into cantilevered tips have great potential in atomic force microscopy/apertureless near-field scanning optical microscopy applications. However, the fabrication of such tips has been rarely reported. The present work is carried out with the aim of optimizing the conditions suitable for fabricating long and sharp tungsten tips. Besides topography, optical, and spectroscopic information, electrical and magnetic measurements can also be carried out with such tips obtained with the recipe reported in this article. The long tips also make it possible to measure deep grooves/trenches.

A simple fabrication method for tunnel junction emitters

The tunnel junction emitter is a novel vacuum electron source, based on ballistic transmission of electrons through ultra-thin foils. This source can operate at low voltages and is expected to have a high brightness and low energy spread. In this paper, we will present a simple method to fabricate tunnel junction emitters in the form of a solid state device. These devices consist of a sharp tungsten tip with a tungsten oxide layer casted in an epoxy resin. The apex of the tip is in contact with a 20 nm thick gold foil, which is deposited on the resin surface. Such a device is similar to a conventional metal-oxide–metal cathode. The difference is that for this device, the lateral size of the tunnel contact is very small. Results are presented on the tunnel current and the emission current as a function of the bias voltage between tip and foil. The threshold voltage above which electrons are emitted is in agreement with the work function of the gold foil.

Atomic Force Microscopy and X‐ray Photoelectron Spectroscopy Investigation of the Onset of Reactions on Alkali Silicate Glass Surfaces

Atomic force microscopy was used to measure forces acting on a sharp tungsten tip as it was brought into contact with silica and 30 mol% binary alkali silicate glasses. Experiments were performed in controlled atmospheres and under vacuum. Attractive forces and liquid‐layer thicknesses were found to vary markedly between the glasses, and heterogeneity was observed on the binary alkali silicates analyzed in vacuo. Air or wet carbon dioxide exposure resulted in the penetration of the tip into a soft surface layer on the alkali silicates. In addition, liquid layer formation on the alkali silicates was found to be promoted by exposure to water vapor in the order lithium < sodium < potassium. X‐ray photoelectron spectroscopy indicated that reaction between the potassium silicate surface and water vapor occurred on exposure to only 10−4 torr (1 torr = 1.33 × 102 Pa) water. Surface segregation and leaching of potassium occurred under the same conditions.

Atomic-force-microscope study of polymer lubricants on silicon surfaces.

Atomic-force-microscope measurements of the forces on a sharp tungsten tip sliding across silicon substrates coated with perfluoropolyether polymers provide insight into how lubricants function at the molecular level. In particular, hydrodynamic lubrication is observed for tip-substrate separation distances as small as a few polymer chain diameters and a relatively small change in the composition of the polymer, namely, the addition of alcohol end groups, greatly increases the load required to compress and squeeze out the polymers from between the sliding surfaces

Interaction forces of a sharp tungsten tip with molecular films on silicon surfaces.

An atomic-force microscope was used to measure the surface forces between a sharp tungsten tip and several different types of molecularly thin organic films on a silicon substrate. The forces are dramatically influenced by the molecular films covering the surfaces. The films investigated fall into three categories: liquid for unbound films of perfluoropolyether, intermediate for bonded films of perfluoropolyether, and soft solid for multilayers of cadmium arachidate. Molecular-level origins of these forces are discussed.

Fabrication of atomically sharp tungsten tips

Atomically sharp tungsten tips (<20 Å) have been grown on a silicon substrate by chemical vapor reaction of WF6 with atomically sharp preformed silicon tips. Transmission electron microscope studies show that a thickness of about 50 Å of tungsten is optimum for complete coverage of silicon while retaining atomic sharpness at the tip. The tungsten is nearly amorphous and is porous. These tips have potential applications in vacuum microelectronics as electron emitters and can also be used as electronic and magnetic microsensors and probes.

A simple new technique for preparing STM tips

A new improved DC etching method for fabricating sharp tungsten tips for scanning tunnelling microscopy and field ion microscopy is discussed. Instead of the conventional current method, differential control of the etching current is used. it reliably produces tungsten tips with radius about 40 nm and aspect ratio near unity for wire diameters up to 0.66 mm.

Shaping of tungsten tips for scanning tunneling microscope

We present a new method for producing extremely sharp tungsten tips needed for Scanning Tunneling Microscopy studies. A lateral resolution less than 1 Å has been achieved in air on cleaved graphite samples.

A combined scanning tunnelling and field ion microscope

SUMMARYThe construction of a combined scanning tunnelling and field ion microscope allowing the in situ preparation and analysis of the tunnelling tip is described. The apparatus is based on the Besocke type STM and includes a fast, automated sample approach, a sample transfer manipulator and a tip‐cooling mechanism for the FIM operation mode. The design is simple and needs a mechanical feedthrough only for sample transfer. A method has been developed to produce sharp tungsten tips with small clusters on the apex plane. Test‐run results of FIM and STM operation modes are discussed.

The effect of oxygen on the adhesion of clean metal surfaces

Abstract The force of separation and the electrical resistance between a sharp tungsten tip and a clean or oxidized [ncirc]ickel single-crystal surface have been measured as ā funčtion of joining load. The adhesion diminishes with oxidation above monolayer coverage. The adhesion with clean surfaces is ductile, and changes to elastic, brittle behaviour with thick oxide layers, accompanied by an increase in hardness.