Current fluctuations from various crystal faces of a clean tungsten field emitter

The temperature, frequency and current dependence of current fluctuations have been measured for the (310), (112) and (100) planes of a tungsten field emitter. The rms percent fluctuation of the current at 900 K was 8.7, 1.5 and 0.13% and the threshold temperature for current fluctuation was 300, 650 and 1000 K for the (310), (112) and (100) planes respectively. Based on the results reported here it is concluded that the current fluctuations arise from the generation of mobile atoms on net plane terraces.     

Measurement of the adsorption energy on single crystal faces by field emission (hydrogen on tungsten)

As a preliminary study has shown¹), the isosteric energy of adsorption on single crystal faces can be measured, if the coverage is kept constant on controlling the field electron currents. This method has been improved (a) by the introduction of the probe hole technique, (b) by special cleaning procedures, and (c) by a more precise measurement of gas pressure and temperature. The method is used to obtain the adsorption energy q of hydrogen on the tungsten faces (100), (111), (112), (013), (122), (123) and (114). Initial q-values vary between 33 kcal/mole on (013) and 40 kcal/mole on (112). q decreases with coverage down to a few kcal/mole. The results indicate a relation between the adsorption states and the structure of each face.     

Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms     

Anisotropy of secondary ion emission from a Ni4Mo single crystal

The azimuthal and polar angle distribution of Ni⁺ and Mo⁺ ions emitted from an ordered Ni⁴ Mo single crystal irradiated with 10-keV Ar⁺ ions was studied. Different azimuthal distributions for Ni⁺ and Mo⁺ ions emitted from the (001) Ni₄Mo face were detected; emission maxima were observed in 〈 011 〉 and 〈 001 〉 directions for Ni⁺ and Mo⁺ ions, respectively. It was shown that polar distributions of nickel’s secondary ions vary with its energy. The observed systematic features were explained by correlated collisions in the upper layers of a Ni₄Mo single crystal.     

Validity of the Fowler-Nordheim parameters measured for silicon adsorption on single crystal faces of tungsten

The adsorption of silicon on the (112), (111), (100), (110), (113) and (116) planes of tungsten field emitters has been studied. As with Ge and CdS adsorption, anomalous behaviour of the Fowler-Nordheim parameters A and φ was observed together with non-linearity in the Fowler-Nordheim plots. This non-linearity is not explicable in terms of the theories proposed for bulk semiconductors. The present results support the suggestion that field-induced Si migration from the W substrate is responsible for anomalies in the F-N parameters.     

Electron emission induced by keV protons from tungsten surface at different temperatures

The electron emission yield is measured from the tungsten surface bombarded by the protons in an energy range of 50 keV-250 keV at different temperatures. In our experimental results, the total electron emission yield, which contains mainly the kinetic electron emission yield, has a very similar change trend to the electronic stopping power. At the same time, it is found that the ratio of total electron emission yield to electronic stopping power becomes smaller as the incident ion energy increases. The experimental result is explained by the ionization competition mechanism between electrons in different shells of the target atom. The explanation is verified by the opposite trends to the incident energy between the ionization cross section of M and outer shells.     

Highly efficient light emission at lambda = 1.5 microm by a three-dimensional tungsten photonic crystal

For what is believed to be the first time, a three-dimensional tungsten photonic crystal is demonstrated to emit light effectively at wavelength lambda = 1.5 microm. At a bias of V = 7 V, the thermal emission exhibits a full width at half-maximum of delta lambda = 0.85 microm. Within this narrow band, the emitted optical power is 4.5 W and the electrical-to-optical conversion efficiency is approximately 22% per emitting surface. This unique emission is made possible by a large, absolute bandgap in the infrared lambda and flat photonic dispersion near the band edges and in a narrow absorption band.     

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.     

Modification of the tungsten carbide field emitter surface to localize the electron and ion emission

Using field emission microscopy, the shape modification of the tungsten carbide emitter simultaneously exposed to high electric fields and high temperatures is studied. It is shown that in this case the emitter shape changes observed on the emitter surface are the same as those observed in the pure metal emitters. The possibility to grow a single nanoprotrusion on the emitter surface which can emit charged particles with stability similar to that for the carbon material emitters is demonstrated. The values of the emission current, current density, emission angle, and reduced brightness are comparable to those for the carbon nanotube emitters, and the advantage of this single nanoprotrusion is its complete reproducibility and capability to emit not only electrons but also ions.     

Electron yield per ion charge-state correction for an ion collector with unsuppressed secondary electron emission

The electron yield per ion charge-state γ/q was measured for emission of electrons from clean polycrystalline gold induced due to impact of Ta ^q+ (11≤q≤41) ions with kinetic energy per chargeE _i/q from 15 keV/q to 150 keV/q. The dependence of γ on angle of incidence was analyzed with use of relation γ(ϑ)=γ₀ cos^−f ϑ. The fitting of experimental data gives a range of γ₀/q from 1 to 1.75 for Ta¹³⁺ and from 1.5 to 1.73 for Ta³⁹⁺. The dependence of γ₀/q onq andE _i is discussed with respect to measurement of ion currents emitted from laser-produced plasmas with an ion collector with unsuppressed secondary electron emission. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, and by grant A1010819 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

Adsorption of nitrogen on single crystal faces of iridium,studied by field emission microscopy

The adsorption of nitrogen on iridium was studied with a field emission microscope equipped with a probe-hole assembly to enable emission experiments on individual emitter regions. The adsorption of nitrogen is markedly face-specific. Temperature programmed desorption reveals three binding states: γ₁ on the (100) face with a maximum heat of adsorption of 7–8 kcal/mole, γ₂ on the regions around (110) with a maximum heat of adsorption of 10–11 kcal/mole and γ₃ on the roughest tip regions (210), (320), (531) and (731) with a maximum heat of adsorption of 13–14 kcal/mole. Nitrogen adsorbed in the γ₁ and γ₃ states causes a decrease, but in the γ₂ state a small increase, in the work function. These results are discussed in relation with data on nickel, palladium, platinum and rhodium. While nitrogen is only weakly adsorbed on all these metals there is a marked difference in the nature of the pertinent adsorption complex.     

Electron scattering in a crystal lattice

For some types of crystals it is permissible to assume that within a spherical region surrounding each ion the potential is spherically symmetric and that in the innerspace between these spherical regions the potential is constant. Schrödinger's equation is solved for such a potential distribution in which the spherical potential wells are not all alike. Both propagating and non-propagating solutions are considered. An asymptotic evaluation of the wave scattered by an impurity ion is carried out. From this, a calculation of residual resistance in alkali metal alloys is obtained.     

Correlation of field emission flicker noise and work function for tungsten single crystal planes with adsorbed potassium

The flcker noise power of the (112), (110), (100), (111) and (122) planes as well of (110) vicinals of a tungsten field emitter has been measured as a function of potassium coverage. Distinct minima of the noise power for coverages that correspond exactly to the work function minima could be found. It seems that this coincidence might be explained — apart from big lateral interactions of the adparticles — by the stronger adsorbate-substrate interaction in a coherent adiayer than in an incoherent one and/or by an adlayer transition to the “metallic” state resulting in compensation of the ionic adsorbate charge by a common electron gas.