Tungsten Field Emitter Tip Research Articles

Field ionization of helium in a supersonic beam: Kinetic energy of neutral atoms and probability of their field ionization

High detection efficiency combined with spatial resolution on a nm-scale makes the field ionization process a promising candidate for spatially resolved neutral particles detection. The effective cross-sectional area σ eff can serve as a measure for the effectiveness of such a field ion detector. In the present contribution, we combine quantum-mechanical calculations of the field-modified electron density distribution near the tungsten tip surface and of the resulting local field distributions, performed using the functional integration method, with a classical treatment of the atom trajectories approaching the tip in order to calculate the σ eff values for ionization of free He atoms over an apex of a tungsten field emitter tip. The calculated values are compared with experimental data for supersonic He atomic beams at two different temperatures 95 and 298 K.

Field electron emission from CdTe deposited tungsten tip: light-induced effects

Cadmium telluride (CdTe) film has been deposited on tungsten field emitter tip by metal-organic chemical vapor deposition method. The deposited film on tungsten tip was characterized by scanning electron microscopy and energy dispersive X-ray analysis. In an all-glass field emission microscopy experiment, current–voltage characteristics were recorded after annealing the tip at ∼950 K for 30 s. Field emission current stability was recorded for 1, 3 and 5 μA current levels. While recording the stability, the field emitter was also exposed to the light (incandescent lamp, power ∼500 W) from a distance of ∼23 cm for 5 min. This resulted into enhancement in the current levels up to 40%. After switching off the lamp, current decreased to its originally set value with a finite decay time. The results are discussed in the light of existing literature and properties of CdTe and other solar energy materials.

Measurement of field emission current variations caused by an amplitude modulated laser

A laser diode ( λ=658 nm, 30 mW) was focused on a tungsten field emitter tip in vacuum. The time dependent emitted current was measured while the laser diode was TTL-amplitude modulated at frequencies from 10 Hz to 50 kHz. It was found that below 100 Hz, the rise and fall times of the field emitted current ranged from 2 to 3 ms, but beginning at 300 Hz, the rise and fall times become shorter as the frequency is increased. The incremental change in the total emitted current is greater at 5 kHz than at any of the other frequencies. Since the only parameter changed was the modulation frequency, we conclude that these effects are not consistent with the assumptions from photofield studies that (1) the increase in the total current is mainly thermal and therefore slow, (2) the current increase is small, and (3) the effect is negligible at the red end of the spectrum. From the carried out experiments, we conclude that the current increase is not caused by tip heating, but rather, they show the significance of the circuit parameters of the field emitter tube itself.

Interaction of Hydrogen with Vanadium Layers Preadsorbed on Tungsten Field Emitter Tip

Interaction of hydrogen with vanadium layers preadsorbed on a thermally cleaned tungsten field emitter was studied at room temperature and 78 K through measurements of the total work function changes. An increase in the work function followed by its slight decrease at higher exposure can be understood taking into account the possibility of negatively (β ) and positively (β+) polarized adspecies formation on thin vanadium layer. This process leads to vanadium hydride formation. The work function results suggest that hydrogen diffusion into the vanadium layer is meaningful at room temperature. Thermal desorption of hydrogen adsorbate carried out within the temperature range 409-461 K from thin va^iadium layer (θv = 40) provided a value of 127 ± 6 kJ/mol for the activation energy for desorption.

Decapitation of tungsten field emitter tips during sputter sharpening

The sharpening of tungsten tips by sputtering with neon ions shows a surprising phenomenon when the tip radius is smaller than about 20 nm: the gas ions produce a neck in the tip which becomes thinner until it breaks due to the electrostatically induced stress. This decapitation phenomenon limits the attainable tip sharpness to a radius of about 4 nm. The process can repeat in a regular, oscillating manner and is probably at the origin of the unreliability often mentioned in connection with this sharpening method. The effect can be used to determine experimentally the theoretical shear stress of refractory metals.

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.

Observation of doubly charged Ne 2 molecular ions by field desorption

Using an isotopic mixture of 20Ne/ 22Ne = 1 : 1 as an imaging gas for a tungsten field emitter tip, simultaneous emission of two Ne field ions as well as a small Ne 2+ 2 yield was observed and characterized. The onset of the Ne dimer detection corresponds to an increase in the Ne surface coverage, which suggests that a second adsorption layer may exist. For increasing field strengths above 4 V Å −1 decreasing activation energies of desorption for the surface dimer are obtained as a function of field strength. This gives experimental support for the theoretical calculations of activation energies of field desorption for He, Ne, and Ar adatoms from a tungsten surface.

Design and performance of a novel field electron/field ion microscope combination based on image processing

The construction of a novel field emission/field ion microscope (FEM/FIM) combination based on image processing techniques is described. The field electron/field ion image formed on a fluorescent screen is converted into a computer image. From these images more detailed information can be obtained about processes taking place upon adsorption of gases or during reactions. The most reactive areas on a tip can be more easily determined and colouring of the different areas makes the differences in work function of these tip areas more pronounced. The working of this new microscope is demonstrated using the reaction between oxygen and a tungsten field emitter tip. The results obtained with this experimental set-up are compared with literature data on this well-studied system for the sake of calibration. It is shown that the reproducibility of work function measurements is about 0.03 eV.

Ion production from LiF-coated field emitter tips

Ion emission has been obtained from a LiF-coated tungsten field-emitter tip. Ion formation is thought to be caused by the high electric field experienced by the LiF. At the time of emission the electric field at the surface of the LiF is calculated to be on the order of 100 MV/cm. Inside the LiF the field is on the order of 10 MV/cm. These fields exceed the value needed to produce bulk dielectric breakdown in LiF. The surface field is of sufficient magnitude to produce ion emission by field evaporation from the crystal surface. Even prior to dielectric breakdown, precursor processes can lead to ion formation. Electric-field-stress fragmentation of the LiF layer is thought to occur, followed by ionization of the fragments.

Characteristics of a helium field ion gun

A helium field ion gun for a focused ion beam system was designed and constructed. The main feature of the ion gun structure is the use of a gas jet nozzle with a tungsten field emitter tip near the nozzle end. Since the helium gas jet is collimated towards the emitter tip, the local pressure in the vicinity of the tip is kept 110 times higher than the value for the location of the ion gauge which is about 20 cm away from the helium jet ion gun. Using this ion gun, current density of 1 μA/sr was obtained although the helium gas flow rate ejected from the nozzle is as small as 1.6×10−3 Torr l/s. This result indicates high efficiency of the helium gas supply to the emitter tip surface.

Laser-stimulated field desorption of molecular ions from a tungsten emitter

We report here on a experimental observation of photon-stimulated field emission of molecular anthracene ions from the surface of a layer adsorbed on a tungsten field-emitter tip. When the tip is irradiated with laser pulses 249, 308, and 400 nm in wavelength falling within the absorption bands of anthracene, the stimulated ion signal is proportional to the pulse fluence. The efficiency of the process decreases with the increasing laser wavelength. Photon stimulation is believed to be due to the resonance excitation of the anthracene molecules, followed by the field ionization of the excited molecules.

Crystalline co-adsorption layers of indium and antimony on tungsten field emitter tips

Results of preliminary observation of the growth process of indium and antimony co-adsorption layers on tungsten field emitter tips are presented. The deposition of indium and antimony was carried out from two separate vapour sources in ultra high vacuum. Crystalline structure was observed in a number of cases, mostly on the tip surface prepared by field evaporation. The presence of thermally induced disorder in the atomic arrangement of the substrate surface was found to be favourable for nucleation. Surface diffusion of the deposited material is assumed to play an essential role in the early stages of growth of nuclei. Field emission patterns of crystalline layers which were grown on the field evaporated surface of tungsten indicate the crystal structure obtained to be that of indium antimonide.

Condensation of tungsten vapours on its own surface

AbstractThe condensation of tungsten vapours onto tungsten field emitter tips is studied. It is shown that at temperatures, at which the desorption flux can be neglected, the nuclei appear at a definite adatom concentration. Its value is found to be about 1013 atoms/cm2, which corresponds to 1% from the monolayer. This value confirms our previous theoretical considerations, i. e. nucleation from a dilute adlayer takes place only if the desorption energy is lower than the vaporisation heat of the condensate. With condensation on its own surface this condition is always satisfied.

Field emission and field ion microscope studies of the epitaxial growth of nickel on tungsten

Single crystals of nickel have been grown on tungsten field emitter tips by vapour deposition in UHV, using substrate temperatures in the range 425–700° K, and nickel fluxes of the order of 0.1–10 atom layers per minute. The orientation relationship is {111} Ni ∥ {110} W, with 〈211〉 Ni ∥ 〈110〉 W. The directional misfit is 3.5%. The crystals grow from initial nuclei formed on the {110} step ledges at multilayer coverages. To permit direct FIM study of the crystals, some specimens were grown on nearly perfect, field evaporated tungsten surfaces, held at 500°K. Examination at 78°K, using hydrogen and helium as the imaging gases, gave information regarding the structure, form and degree of perfection of the crystals. An interfacial reaction was observed, which produced considerable disordering at the tungsten surface. The mechanism of this reaction apparently involved the substitutional replacement of tungsten atoms by nickel, in the interface lattice. The implications of these observations with respect to the theory of epitaxial growth are discussed.

NUCLEATION OF CRYSTALS IN MULTILAYER METALLIC ADSORBATES

The adpopulations of zinc, cadmium, nickel, and gold which are critical for appreciable nucleation rate of crystallites on clean tungsten field emitter tips have been measured. These critical adconcentrations range from 1.1 monolayers for cadmium to 3.7 monolayers for nickel and are independent of temperature from 75°K to above room temperature. This phenomenon can be described by none of the existing theories of heterogeneous nucleation.

Nucleation of Zinc Crystals in Multilayer Adsorption

AbstractThe adpopulation of zinc which is critical for an appreciable nucleation rate of crystallites on clean tungsten field emitter tips has been measured. This critical adconcentration is 2.7 monolayers for zinc, independent of temperature from 75 °K to above room temperature. This phenomena can be described by none of the existing theories of heterogeneous nucleation because all of these theories presuppose that adsorption prior to nucleation is less than a monolayer. For concentrations greater than a monolayer, the exact supersaturation at the substrate becomes indeterminate.