A measurement of the local electric field on field emitter crystals using T-F emission

A method is introduced to measure in situ the absolute value of the local field strength on the surface of a field emitter tip by using T?F emission. The method is based on the unified theory of electron emission (Christov) which is now experimentally well confirmed. The method is tested on several single crystal faces of tungsten tips. The absolute value of the local field can be determined within an error of about 5%. Relative field strengths at different points on one single crystal face can be measured with an error of 2%. Once the absolute value of the field strength is thus measured, the absolute value of the work function can be obtained additionally, but so far only with a fairly great error.     

Nucleation on substrates from the vapour phase

Recent developments in the theory of nucleation of vapour deposits on crystalline substrates are reviewed. To facilitate comparison, the theories are formulated in a common dimensionless notation, and an examination of the major underlying assumptions reveals the basic similarity of many of them. The capture and dissociation rates are expressed in terms of the cluster geometry and the pertinent energy parameters, and from these the ‘chemical’ rate equations are set up. The following types of approximate solutions are discussed: (a) long-time asymptotic solutions, from which the conditions for saturation in the cluster concentration may be deduced, (b) a generalization of the type of approximate solution used by Logan (1969), and (c) numerical solutions employing a minimum number of simplifying assumptions. Based on a simple model, agreement between the latter two seems reasonably good. For any given set of fixed parameters (energies, geometrical constants, and arrival rate from the vapour) several temperature ranges may be distinguished. The main division is between ‘initially complete condensation’ at low temperatures and ‘initially incomplete condensation’ at high temperatures. Within each of the latter cases there are further transitions (a) between different values of the ‘critical size’ i*, and (b) from negligible growth to rapid growth of the supercritical clusters. The influence of all of these factors on the final cluster concentration is described.

The distributions of the clusters in size and spacing are discussed briefly and qualitatively, as are the types of effects that can be induced by defects or other ‘special sites’ on the substrate.

Comparisons are made with some recent experimental studies. In many of these, defects in the substrate seem to play a dominant role, and no detailed comparison with theory seems possible. One notable exception is the nucleation of rare gas crystals on graphite substrates (Venables and Ball 1970), and here, for at least two of the three gases studied, excellent quantitative agreement is obtained.     

Simple-to-prepare multipoint field emitter

We investigate multitip field emitters prepared by electroerosion treatment of the surface of molybdenum samples. Their characteristics are determined for operation with a protecting activated fullerene coating. Our experiments indicate that such cathodes are promising for high-voltage electron devices operating in technical vacuum.

     

Effect of the emitter temperature and emitter atom ionization potential on field evaporation

The field evaporation of nickel, nichrome alloy, and tungsten carbide at different temperatures is studied with a time-of-flight atomic probe and a field emission microscope. The charge of evaporating ions does not depend on the emitter temperature: it decreases with decreasing evaporating field F _ev. If F _ev does not vary with temperature, so does the charge of the ions. In the case of multicomponent emitters with different ionization potentials of the components, the components evaporate at the same values of F _ev in the form of atoms and ionized clusters. The reason for such behavior is that the initial evaporation of the easily ionizable component decreases the binding energy of harder-to-ionize ones to the point where they can evaporate at the same field.     

Nucleation of crystals from their liquid phase

Liquids can be supercooled below their melting temperature T_m or pressurized above their melting pressure P_m. Many authors relate the maximum degree of supercooling—or overpressurization—to a value of the liquid–solid interfacial tension by using the standard theory of nucleation. The main goal of this review is to examine whether this relation is justified or not. We consider general arguments and two main examples: liquid helium which is simple and pure, consequently a model system, and liquid water which is complex but ubiquitous. To cite this article: S. Balibar, F. Caupin, C. R. Physique 7 (2006).     

Frequency converter based on a field electron emitter

An electron field emitter based on a carbon nanotube is considered as a frequency converter of the voltage applied to its electrodes. This property of the emitter relates to the nonlinear form of the current-voltage characteristic described by the classical Fowler-Nordheim expression. Calculations show that the number of higher harmonics in the spectrum of the emission current increases upon a decrease in the applied voltage and with increasing relative amplitude of the ac signal.     

Nucleation kinetics for gold deposited onto mica substrates

The nucleation behavior of gold deposited under ultra high vacuum conditions onto cleaved mica substrates has been measured over the temperature range 270–450 °C at impingement fluxes of 6 × 10¹³ and 8 × 10¹² atoms cm^?2sec^?1. A step-kinetic technique was used wherein separate substrate areas were exposed for successively longer times to the vapor beam from a thermal source. The deposits were examined in a high resolution electron microscope. For mica cleaved in situ in UHV, the variation in number density of nuclei with time of exposure to the atom beam revealed that nucleation occurred rapidly on preferred sites which were exhausted within the first few seconds of exposure. After the initial saturation, nucleation appeared to occur randomly over the rest of the surface. The number density of preferred sites increased with decreasing temperature indicating that a spectrum of preferred sites of varying degrees of potency were active. The random nucleation rates could by analyzed satisfactorily using the atomistic model for nucleation. A critical sized nucleus consisting of one atom is consistent with the data. The characteristic energy for nucleation (2ΔG_des ?ΔG_sd), was found to be 1.1 eV. Nucleation behavior on mica substrates cleaved in air did not indicate the presence of active preferred sites, but nucleation was more rapid.     

Surface reconstruction of a field electron emitter

The surface and emission images of a metal field’s electron cathode in the form of a tip are simulated. The surface structure is calculated in the thin-shell and broken-bond (local-environment) models for the perfect crystal lattice. The cathode shape and macroscopic electric field are represented by the sphere-on-cone model. The amplification of a local electric field is the adjustable parameter of the model. The method of determination of the emitter tip’s crystal faces based on the analysis of the surface atoms’ environment geometry is proposed. It is shown that it is enough to restrict the consideration of geometric environment by the fifth order of the nearest neighbors for the emitter radius of 100–1000 lattice parameters (31.6–316 nm for the tungsten). The crystallographic model of work function anisotropy in the broken-bond approach is used: the local work function’s value is set in accordance with Miller indices of the face containing this area. The model adequacy is corroborated by the comparison of current-voltage characteristics and emission images with the data of the natural experiment.     

Laser-initiated metal deposition on GaAs substrates

The local deposition of metal structures by thermal dissociation of trimethylaluminium, dimethylzinc and dimethylcadmium on GaAs surfaces heated by a cw krypton laser has been investigated. Piles of amorphous aluminium and zinc and crystalline structures of Cd have been deposited at temperatures between 200 and 1000°C. The smallest size of the deposits was ≈4 μm.     

Arc plasma synthesized LaB<Subscript>6</Subscript> nanocrystallite film on various substrates as a field emitter

Lanthanum hexaboride (LaB₆) nanocrystallites/nanowires have been successfully synthesized using gas phase condensation in arc plasma. Our results show that the LaB₆ nanowires have ~20 nm diameter and length of several micrometers. The as-synthesized LaB₆ nanocrystallites and nanowires are crystalline in nature. Field emission studies were carried out on as synthesized LaB₆ nano-powder deposited on W and Re tips and foils, and also on Si foil substrates under ultra high vacuum. The Fowler–Nordheim plots obtained from the current–voltage characteristics are found to be linear in accordance with the quantum mechanical tunneling phenomenon. High value of field enhancement factor (8177) is observed for LaB₆ on Re substrate, indicating that the electron emission is from the nanometric features of the LaB₆. Our results show that the arc plasma synthesized LaB₆ nanocrystalline film exhibits good emission stability as emitter without severe deviations from the initial set value and also a strong ability to withstand the ion bombardment, which is useful for low operation voltage vacuum micro/nano electronic devices.     

Work-hardening of metal crystals

Some recent problems relating to work-hardening of metal crystals, such as the contribution of dislocation stress fields to the flow stress, the mechanism of the second stage, or linear, hardening, and the transition to the strongly temperature dependent third stage are discussed. It is shown that linear hardening can be explained by recourse to the strain invariance of the geometrical pattern of the dislocation network. The thermodynamic origin of this invariance is considered. Experimental data on the temperature and strain-rate dependence of the stress at which transition to the third stage occurs are shown to accord best with the view that at temperatures at which diffusion effects are negligible the third stage begins with the onset of appreciable dynamic recovery, involving the formation of point defects, probably through non-conservative movement of elementary intersection jogs.     

Superheating of metal crystals

We report new observations on samples which can exhibit superheating under uniform thermal conditions. The free surface of single crystal Ag spheres was coated with continuous layer of Au to inhibit surface initiated melting of Ag at its melting point. Superheating of up to 25 K was observed for time periods of about one minute. Hydrostatic stresses could be excluded as the reason for the superheating. The results disagree with the lattice stability model and may open the way to the study of crystals in the superheated state.     

Magnetic properties of 3d transition metal monolayers on metal substrates

We report results of systematic calculations for magnetic properties of 3d transition metal monolayers on Pd(001) and Ag(001). We find large similarities to interactions of magnetic 3d impurities in the bulk. Therefore the overlayer results are supplemented with results for 3d dimers in Cu, Ag, and Pd. Differences between the two classes of systems are utilized to reveal the interaction within the overlayers and between overlayers and substrates. In virtually all cases we find both ferromagnetic and antiferromagnetic solutions, showing large magnetic moments and similar densities of states. From the trend of the calculations we conclude that V, Cr, and Mn overlayers favor the antiferromagnetic c(2×2) structure, while Ti, Fe, Co, and Ni prefer the ferromagnetic one.     

Li-modified oscillations in CO oxidation on a Pt field emitter

The first observations of Li-modified isothermal kinetic oscillations during CO oxidation on a Pt field emitter tip are reported. The oscillations are realized on the surface of a [111]-oriented Pt tip in the presence of a Li submonolayer coverage of about 2.5 × 10¹⁴cm⁻² and are compared to the corresponding Li-free oscillations occurring on the same tip. In both cases the oscillatory process is visualized using the field electron emission (FEM) mode of the lithium field desorption microscope (Li-FDM). The origin of the FEM imaging contrast of coadsorbed LiCO and LiO layers on Pt is clarified. The influence of the coadsorbed Li submonolayer on the oscillating behavior of CO oxidation on Pt is discussed.     

Effect of ionization of condensing flux and transverse electric field on nucleation of metal films on ionic crystals

Experimental data on the effect of ions in the flux of condensing metal on the orientation of island metal films on alkali halide crystals are given. The effect of ionization of the condensing flux and the transverse electric field is explained on the basis of a previously proposed model of nucleation and oriented growth on point defects on the crystalline substrate.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 35–39, November, 1979.     

The effect of the crystal structure of the field emitter on field ion energy distributions

The field ionization probability of an atom as a function of distance from the field emitter is discussed in terms of the atomic arrangement and the electron scattering properties of the ion cores of the emitter in the immediate neighborhood of the atom to be ionized, and the electron transmission properties of the potential barrier between the emitter and that atom. This approach to field ionization calculations is somewhat similar to field ionization calculations based on low energy electron diffraction (LEED) procedure in that it takes into account electron scattering from the first few atomic layers of the emitter. It differs from LEED type calculations, because it considers the highly localized nature of the ionization near a surface atom. This localization makes the ionization probability relatively insensitive to the two-dimensional periodicity of the emitter surface. A one-dimensional calculation, in which only the potential barrier and three ion core scatterers in line with the field are considered, shows secondary structure in the predicted field ion energy distributions near the critical energy deficit, as well as the well known, primary field induced resonance peaks. The surface orientation dependence of these distributions arises naturally from this model because the secondary structure depends strongly upon the crystal parameter along a line parallel to the field. This one-dimensional calculation can be no more than an approximation to a complete calculation. It is interesting, however, that such a simple physical model, in which scattering from the image potential and only two or three ion cores is considered, rather than scattering from a complete crystal, can give prodicted field ion onergy distributions which are similar to those experimentally observed.