Field ion microscopy of W and PtTi at about 1000°C

Bright new images including the image of atoms have been observed when a positive high voltage (～ 15 kV) was applied to a specimen heated to ～ 1000°C in a field ion microscope. A channel plate multiplier was especially effective to this effect. Since those images were observed even in a high vacuum of 10^?7 Torr, where imaging gas hardly exists, and the brightness of the images does not depend on the residual gas pressure, they can not be the usual images caused by gas ions emitted from the tip of the specimen. A tentative interpretation is that they are caused by metal gas ions which are vaporized from the heating loop of tungsten and ionized at the tip of the specimen.     

Use of ion processing to improve the quality of fullerene-coated field emitters

The efficiency of tip field emitters covered by activated fullerene coatings is studied in a wide range of emission currents and residual gas pressures. Main mechanisms behind the influence of the gaseous medium and ion bombardment on the emitter efficiency are determined. The feasibility of improving the homogeneity of the fullerene coating by potassium ion bombardment is demonstrated. From data on the emitter performance in a technical vacuum, a previously unknown effect is discovered: the structure of activated fullerene coatings is reproduced under intense ion bombardment. It is found that intense bombardment by residual gas ions increases a limiting current extracted from fullerene-coated tip field emitters.     

Operation of activated-fullerene-coated field emitters in technical vacuum

The operation of tungsten tip field emitters coated by activated fullerenes in technical vacuum is studied. Ways of emitter training to provide the self-reproducibility of the coatings and their long service life under intense ion bombardment in wide ranges of pressures, from 2 × 10^?8 to (5–7) × 10^?7 Torr, and emission currents, from (1.5–2.0) to (10–20) μA, are developed.     

同心圆环型场发射阵列阴极的粒子模拟

 模拟了同心圆环型场发射阵列阴极的特性。采用时域有限差分法计算了电场，利用Fowler-Nordheim方程计算锥尖处的发射电流，利用Lorentz方程计算了电子的运动轨迹。得到了微阴极电子的运动轨迹、相空间图以及电子束的发散度、亮度等，结果表明该结构能形成良好的聚焦电子束。     

A model for streamer propagation

A model is presented to describe the propagation of positive corona streamers in the low field region of a non-uniform field gap in atmospheric air. It has been assumed that the growth is a property solely of the streamer tip, uninfluenced by the channel conductivity. Calculations from the model indicate that the criterion for propagation of a streamer in zero external field is that the number of ions in the tip be 10⁸ and the radius about 3×10^?3 cm. It is proposed that the streamer ceases to propagate as a result of the loss of energy of the tip due to the formation of ion pairs in the channel. The results of previous experimental observations of streamers are compared with calculations derived from the model, and a prediction from the model of the lifetime of streamers after voltage removal is discussed.     

Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

Field ion microscopy of metal-metal contacts

Tungsten-platinum and platinum-platinum mechanical contacts were investigated using field ion microscopy. A clean field evaporated field ion microscope tip served as one contact member, and a fiat outgassed platinum foil was used as the other. The experiments were conducted in-situ using an ultra-high-vacuum field ion microscope pumped to 2 × 10^?10 Torr before being backfilled to 5 × 10^?4 Torr of helium or neon for imaging. A refined contact device was employed having an ultimate applied load resolution of 5 micromg. In contrast to previous results, gross tip deformation could be avoided, while the contact area could clearly be seen against the untouched parts of the tip. It was found in most cases that the damage to the tip at the contact interface extended only 5–15 Å deep, and that this damage depth was independent of both the magnitude of the applied load and the size of the contact area. The functional variation of the contact area with the applied load was found to be in basic agreement with the Hertz theory of elastic deformation. Fundamental limitations on the minimum observable size of the contact area due to impact are discussed.     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

Vordurchschlagsphänomene der Feldemission im Nanosekundenbereich

Variation of field emitters on Mo- and Cu-cathodes were investigated by field emission microscopy after application of nanosecond voltage pulses. The emitters delivered emission currents with densities of more than 10¹³ Am^?2 without breakdown. Heating by these currents caused desorption and melting of emitter tips. From the desorption events resonance tunneling was deduced with typical UHV-conditions. Melting caused blunting of the emitters. Hence sharp micro-points exploded only by sufficiently fast heating (with time constants ≦ 1 ns) or by secondary effects due to contaminations. By introduction of air in the pressure region of 10^?4 - 10^?3 Pa new emitting sites appear with time constants of about 10^?8 s, probably by formation of oxides.     

The influence of argon ion bombardment on the electrical and optical properties of clean silicon surfaces

The effect of low energy noble gas ion bombardment on the electrical and optical properties of Si(211) surfaces has been investigated by surface conductivity and field effect measurements, ellipsometry and AES. With this combination of techniques, information is obtained concerning the electrical properties, the chemical composition and the damage of the surface layer. Upon ion bombardment in the energy range of 500–2000 eV, ellipsometry shows the formation of a damaged surface layer with optical properties close to those of an evaporated amorphous silicon film. In order to measure the conductivity changes as sensitive as possible, nearly intrinsic silicon crystals were used. For the clean, 5200 Ω cm Si(211) surface, bombarded only with a mass-analyzed argon ion beam, a small increase in conductivity is found to occur after a small ion dose (saturation after 5 × 10¹⁴ ions cm^?2 while after 5 × 10¹³ ions cm^?2 already half of the increase has occurred). The effect was found to be independent of ion energy between 500 and 2000 eV. As the field effect signal did not change after this treatment, it is concluded that the surface state density in the neighbourhood of the Fermi level shows a slight decrease.     

Evaluations of carbon nanotube field emitters for electron microscopy

Brightness of carbon nanotube (CNT) emitters was already reported elsewhere. However, brightness of electron emitter is affected by a virtual source size of the emitter, which strongly depends on electron optical configuration around the emitter. In this work, I-V characteristics and brightness of a CNT emitter are measured under a practical field emission electron gun (e-gun) configuration to investigate availability of CNT for electron microscopy. As a result, it is obtained that an emission area of MWNT is smaller than its tip surface area, and the emission area corresponds to a five-membered-ring with 2nd nearest six-membered-rings on the MWNT cap surface. Reduced brightness of MWNT is measured as at least 2.6×10⁹ A/m² sr V. It is concluded that even a thick MWNT has enough brightness under a practical e-gun electrode configuration and suitable for electron microscopy.     

Effect of dipole structures on field emission of wide-gap semiconductor emitters

It is shown that dipole structures placed in a thin (less than 1 nm) near-surface layer of a high-resistivity field emitter produce small domains on the emitting surface in which the electric field may exceed 10⁸ V/cm. In these domains, the emitter surface potential is positive, providing effective electron transport from inside the emitter to the emission boundary. Optimal dipole orientations ensuring maximal electric fields at the surface are found. When the surface density of dipoles localized in the near-surface layer is on the order of 10⁶ cm⁻², one can expect an emitter-averaged emission current density of higher than 1 A/cm². The dipole structures in the near-surface layer may persist owing to incorporated impurity molecules having a dipole moment or result from a random combination of positively charged ionized impurities and electrons captured by deep traps. Trap charging/discharging asymmetry accounts for the hysteresis of the emission I–V characteristics.     

Auger electron spectroscopy (AES) studies of argon ion induced desorption of carbon from tantalum

AES studies of argon ion induced desorption of carbon from tantalum were performed. The carbon adlayer was allowed to adsorb from a well characterized residual gas atmosphere, that was unvarying within 20%. The argon ions impact on the surface at an angle of 60° from the surface normal with energies between 0.2 to 1.0 keV. The total desorption cross section values measured under these conditions are 0.07–1.1 × 10^?15 cm².     

Field evaporation experiments with a magnetic sector atom-probe FIM

A magnetic sector atom-probe FIM has been successfully operated for dc field evaporation of tip materials such as Rh, W, Ir, Mo and Ti. A limited number of evaporated metal ions were clearly identified forming a line spectrum. Field evaporation of Rh in the presence of ³He and ⁴He gases showed that the formation of the helium compound (RhHe)²⁺ is quite sensitive to He gas pressure; no helium compound were observed below 5 × 10^?7 Torr and all ions detected as helium compound above 5 × 10^?5 Torr at 78 K.     

Enhancement in electrochemical properties of ionic liquid-based nanocomposite polymer electrolytes by 100 MeV Si9+ swift heavy ion irradiation

Swift heavy ion (SHI) irradiation has been used as a tool to enhance the electrochemical properties of ionic liquid-based nanocomposite polymer electrolytes dispersed with dedoped polyaniline (PAni) nanorods; 100 MeV Si⁹⁺ ions with four different fluences of 5?×?10¹⁰, 1?×?10¹¹, 5?×?10¹¹, and 1?×?10¹² ions cm^?2 have been used as SHI. XRD results depict that with increasing ion fluence, crystallinity decreases due to chain scission up to fluence of 5?×?10¹¹ ions cm^?2, and at higher fluence, crystallinity increases due to cross-linking of polymer chains. Ionic conductivity, electrochemical stability, and dielectric properties are enhanced with increasing ion fluence attaining maximum value at the fluence of 5?×?10¹¹ ions cm^?2 and subsequently decrease. Optimum ionic conductivity of 1.5?×?10^?2 S cm^?1 and electrochemical stability up to 6.3 V have been obtained at the fluence of 5?×?10¹¹ ions cm^?2. Ac conductivity studies show that ion conduction takes place through hopping of ions from one coordination site to the other. On SHI irradiation, amorphicity of the polymer matrix increases resulting in increased segmental motion which facilitates ion hopping leading to an increase in ionic conductivity. Thermogravimetric analysis (TGA) measurements show that SHI-irradiated nanocomposite polymer electrolytes are thermally stable up to 240–260 °C.     

Energy spectrum of field ionization at a single atomic site

Energy deficit spectra of field ions coming from above a single atomic site are measured by using an atom-probe FIM modified with a Möllenstedt energy analyzer. This device offers a resolution of 5 × 10^?5 and is inherently more efficient and less noisy than a retarder. The energy spectra made up of 10 to 100 ions/sec are displayed on the screen of an assembly of two microchannel plates and are photographically recorded within a few seconds. Jason peaks for H₂⁺ and Ne⁺ are confirmed, and are also found for He⁺. High-order multiple peaks appear when ions are taken from the flat, closely packed net planes of W and Ir field ion emitters. The results are in quantitative agreement with a resonance model similar to one by Alferieff and Duke and by Jason. Noble gas ions are also observed from the forbidden zone near the surface, and interpreted as apex-adsorbed atoms ionized by or after excitation by the electron shower coming from farther-out field ionization of other gas atoms. Energy from excited metastable apex-adsorbed atoms may account for artifact vacancies observed particularly when field evaporation is performed in neon.     

Higher order averages of primary recoil distributions

A molecular beam detector is described which ionizes by electron bombardment. The ions are then separated in a quadrupole mass filter and detected by a multiplier. The special advantages of the mass filter allow a high overall transmission so that every 1000th molecule of the beam is measured as an ion. The ion current due to the residual gas is reduced by a factor of 10^?6 for all masses >45. These results were achieved without separately pumping or baking out the ionisation chamber. Since magnets are not used the detector is comparatively light and small in size. The smallest detectable beam was found to have a current density of 1,8·10^?6 molecules/sec mm² corresponding to 4·10³ molecules/cm³, at a vacuum pressure of 8·10^?7 torr in the apparatus, and using a time constant of 1,25 sec.     

Effect of vacuum level on field emission from nanographite films

The effect of vacuum level on field emission from nanographite films obtained by plasma-chemical deposition is studied. The stable emission of electrons from the nanographite is observed at a threshold field of 1?C2 V/??m, a current density of 0.1 mA/cm², and a residual gas pressure in the measuring chamber of less than 10^?5 Torr. At a higher pressure, the emission properties of the films gradually degrade with time. Repeat evacuation of the chamber to 10^?5 Torr restores the emission properties. Such behavior of the nanographite emitters is explained by adsorption/desorption processes (reversible degradation of the emission) and the destruction of the film under the action of residual gas ion bombardment (irreversible changes).     

Neon gas imaging of gold in the field ion microscope

An improved field ion microscope (FIM) technique has been developed for the neon gas imaging of gold specimens. The technique produces images which are stable at best image voltage at a tip temperature (T_T) of 30 K or less. The first stage of the technique consisted of the development of an end form at 55 K in the presence of a partial pressure of air (～ 2 × 10^?8 Torr gauge pressure) and neon gas (～ 3 × 10^?5 Torr gauge pressure) followed by further field evaporation at 28 K. The second stage involved neon gas imaging of the previously developed end form in a baked FIM in a background pressure of (0.5 to 3) × 10^?9 Torr. The FIM images obtained in conjunction with the field ionization characteristic curves showed that there is a working range (in the sense defined by Southon and Brandon). A detailed study was made of artifact vacancies detected in the {203}, {321}, {315}, {421}, {671} and {731} planes, and it was found that at 28 K their concentration was < 2.5 × 10^?3 at.fr. Approximately 191,000 atomic sites were examined for artifact vacancies. The artifact vacancy concentrations measured in the present study were a factor of 13 to 60 lower than those measured earlier by Schmid and Balluffi who employed a background pressure of ～ 5 × 10^?8 Torr in their FIM. Hence, the artifact vacancy concentrations detected in gold are dependent upon the background partial pressure employed in the FIM. This latter result plus the result that the images are only stable in ultra-high vacuum (UHV) conditions indicates the need for UHV conditions for the successful imaging of gold surfaces.     

Gas Phase Emitter Effect of Thulium within Ceramic Metal Halide Lamps in Dependence on Frequency

The gas phase emitter effect within ceramic metal halide (CMH) lamps reduces the effective work function of the electrode material and, therewith, the electrode temperature. An investigation of the gas phase emitter effect of thulium (Tm) within CMH lamps seeded with Tm iodide (TmI3) is carried out. For this purpose, phase resolved images of the arc attachment and measurements of the electrode temperature, Tm atom and ion densities are performed in dependence on operating frequency by pyrometry and optical emission spectroscopy. Additionally, the influence of a sodium iodide (NaI) admixture is studied. The emitter effect is generated by means of a monolayer of Tm atoms on the electrode surface generated by a Tm ion current within the cathodic phase. It overlaps onto the anodic phase at higher frequencies of some hundreds of hertz. The reason is the finite life time of the monolayer, which is determined by the adsorption energy of Tm on the tungsten surface. Due to the low electric field strength in front of the anode and the mass inertia, the emitter ions and atoms remain in front of the anode. They retard the decay of the monolayer and with it the increase of the work function. Moreover, a comparison of a lamp seeded with TmI3 and sodium iodide (NaI) with a lamp seeded only with TmI3 illustrates a slight reduction of the electrode tip temperature caused by a higher Tm saturation vapour pressure and a higher Tm amount within the lamp filling. The influence of Na appears to be quite low. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)