Electron emission and work function: past, present and future

The history of electron emission is reviewed from a standpoint of the work function that determines the electron emission capability and of applications in the fields of scientific instruments and displays. For years, in thermionic emission, a great deal of effort has been devoted to the search for low work function materials with high melting temperature, while reduction of the local change in time of the work function rather than the work function itself has been the main issue of field emission investigations. High brightness and long life are the central targets of emission material investigations for scientific instrument applications, while high current density and low power consumption are the guiding principles for display applications.In most of the present day industries, thermionic emission materials are exclusively used in such fields requiring high current and high reliability as cathode ray tubes, transmission and receiving tubes, x-ray sources and various electron beam machines. Field electron emission sources, however, since applied to high resolution electron microscopes in the 1970s have recently become dominant in research and development in the fields of scientific instruments as well as in the fields of various electron tubes and beam machines.The main issue in this report is to analyse the work function on the atomic scale and thereby to understand the fundamental physics behind the work function, the change in time of the local work function leading to field emission current fluctuation and the relationship between microscopic (on atomic scale) and macroscopic work functions.Our attempt is presented here, where the work function on the atomic scale is measured by utilizing a scanning tunnelling microscopy technique, and it is made clear how far the local work function extends its influence over neighbouring sites. As a result, a simple relationship is established between microscopic and macroscopic work functions.

Low work function thin film is important for various electron devices to realize high-efficiency electron emission capability. Lanthanum Hexaboride (LaB6) is well known as a low-work-function bulk-crystal material ( 2.7eV) [1] which has been extensively used for field emission devices. We have developed sputtered LaB6 thin films using the sintered LaB6 target, and the LaB6 thin film with work function of 2.8 eV was realized in the previous study [2]. In this report, we investigated the dependence of impurities such as nitrogen and oxygen in the LaB6 target on the work function of sputtered thin films, and found that the work function of sputtered films can be reduced by doping small amount of nitrogen in the target. Fig.1(a) shows a work function of the sputtered films as a function of nitrogen doping content in the target. In this case, oxygen content in the target was controlled at 0.22-0.30 wt%. LaB6 thin film was deposited using the rotation magnet sputtering equipment [2], where approximately 100-nm thick LaB6 films were deposited on Si substrate with thermally grown SiO2 at room temperature. Work function was obtained by the vacuum ultraviolet photoelectron spectroscopy (UPS) method. It was found that the work function is lowest for the nitrogen content of approximately 0.4 wt%, where very low work function of 2.4 eV was achieved. Fig.1(b) shows a work function of the sputtered films as a function of oxygen doping content in the target. In this case, nitrogen content in the target was controlled at approximately 0.2 wt%. It was found that oxygen content must be less than approximately 1wt% to keep work function low. To investigate the observed dependence of nitrogen and/or oxygen doping content upon work function of the sputtered film, we carried out X-ray diffraction (XRD) to measure crystallinity and crystal orientation. However, no significant difference was observed regardless of nitrogen content, where (100) orientation with almost the same peak height and width was obtained. Also, it was difficult to detect nitrogen content in the thin film at present. On the other hand, clear dependence of nitrogen content in the target was found in the Hall effect measurements for the sputtered film: Fig.2 shows mobility and density of carriers (electrons) of LaB6 films as a function of nitrogen doping content obtained by the Hall effect measurements. Conduction type was confirmed to be n-type conduction. It was found that both carrier mobility and carrier density are highest at the nitrogen content at approximately 0.4%, for which lowest work function was obtained. The results suggested that orderliness between atoms was improved at this condition, resulting in the realization of low work functions, although the mechanism is not well understood yet. The low work function sputtered LaB6 thin films obtained by nitrogen doped LaB6 target has a potential to realize various high performance electron devices exhibiting high efficiency electron emissions.[1] M. Lafferty, J. Appl. Phys. 22 (1951) 299.[2] T. Goto et al., J. Vac. Sci. Technol. A 27 (2009) 653. Figure 1

For continuous one‐year operation without deterioration of the surface ionization engine characteristic, the neutral efflux should be in the 0.1% range. For the neutral efflux reduction the Saha equation implies the use of high work function metals. Therefore, tungsten‐rhenium alloys and iridium filaments were investigated. For completeness, tungsten and rhenium filaments were also studied with regard to their thermionic work function and threshold temperature for cesium surface ionization. The data were measured in an all‐metal guard ring diode with directly heated filament under clean surface conditions. The electron work functions and the threshold temperatures of the tungsten‐rhenium alloys increase with increasing amount of rhenium in tungsten. For φ‐I≤0.5 eV the electron work function governs the surface ionization.One porous tungsten‐25% rhenium pellet and two porous tungsten substrates chemically coated with rhenium and iridium yielded work functions below those measured with the corresponding fil...

In this article, we demonstrate that the electron work function (EWF) as an intrinsic parameter can provide information or clues in a simple or straightforward way for material design, modification, and development. A higher work function of a material represents a more stable electronic state, which consequently generates a higher resistance to any attempt of changing the electronic state and other corresponding states, e.g., changes in structure or microstructure caused by mechanical and electrochemical actions. Using Cu-Ni alloy as an example, we demonstrate the correlation between the EWF and Young's modulus of the material as well as its hardness. The properties of a material can be modified using elements with appropriate work functions. This is also applicable for tailoring inter-phase boundaries or interfaces.

P-Type Chemical Doping-Induced High Bipolar Electrical Conductivities in a Thermoelectric Donor–Acceptor Copolymer

A new statistical method for determining the key factors of the process of field electron emission (FEE) from nanostructures based on a comparison of the microscopic measurements of the work function of electrons with the results of a macroscopic characterization of the FEE is presented. Using the example of nanocrystalline carbon films, the possibilities of this statistical method for a quantitative estimation of the influence of the local field enhancement and the work function reduction on the FEE threshold decrease are demonstrated.

Along with the development of cold field emissi on technology, more and more microscopes equipped with cold field emission gun (C-FEG ) are applied in scien tific research. Thanks to the lower working temperature, the service time of emitter has been ex tended. In the meantime, this technology allows to emit electrons from smaller emission radius compared to thermionic emission and Schottky emission [1]. Based on this property, higher curre nt density, higher brightness, be tter special coherence and lower energy distribution of the emission can be expected, wh ich also means that a better electron beam can be obtained [2]. For the further improvement of cold field emission, new emitter materials have been explored. As a rolled-up graphe ne sheet, carbon nanotubes (CNTs) have the similar property, for instance, low work function, high mechanic performan ce, high melting point, high carrier mobility [3-4]. In addition, CNTs have small apex radius curvature for its quasi-one -dimensional character at nanoscale. Therefore, CNTs are capable of working under the operating condition of cold field emission [5]. Furthermore, carbon nanocones are considered as candi date as well [6]. Significant improvements of such kind of carbon nanocones are demonstrated by repl acing the regular tungste n tip in transmission electron microscope [7]. For a higher performance of the cold field emission application, the electronic band structure of carbon nano-object can be modulat ed through the introduction of heteroatoms and a lower work function can be expected [8,9]. Our work is devoted to th e doping of these carbon nanostructures by nitrogen and/or boron, and to the evaluation of these doped carbon nano-objects by a characterization for the cold field emission application.

Effect of dielectric coating on the electron work function and the surface stress of a metal

Is work function a surface or a bulk property?

A self-consistent theory of phenomena occurring under passage of superhigh current densities through the surface-plasma interface is proposed. The electric field penetration into a conductor, processes near the plasma-surface interface, neutralization of spatial emission electron charge by ions have been considered. It has been found out that a gap in the cathode drop inside the conductor occurs at high emission current density; this phenomenon causes, in this turn, distortion of the Fermi surface and a reduction in the work function. As a result, the conditions for superhigh electron emission density are realized. Thus the current passage through the conductor-plasma interface acquires a self-organizing nature. An estimate of the critical current, beginning from which the electron emission selforganization emerges, is given.

Work function measurements and their relation to modern surface analysis techniques

This paper reports on characterization techniques for electron emission from potassium-intercalated boron nitride modified graphitic petals. Carbon-based materials offer potentially good performance in electron emission applications owing to high thermal stability and a wide range of nanostructures that increase emission current via field enhancement. Furthermore, potassium adsorption and intercalation of carbon-based nanoscale emitters decreases work functions from approximately 4.6 eV to as low as 2.0 eV. In this study, boron nitride modifications of graphitic petals were performed. Hexagonal boron nitride is a planar structure akin to graphene and has demonstrated useful chemical and electrical properties when embedded in graphitic layers. Photoemission induced by simulated solar excitation was employed to characterize the emitter electron energy distributions, and changes in the electron emission characteristics with respect to temperature identified annealing temperature limits. After several heating cycles, a single stable emission peak with work function of 2.8 eV was present for the intercalated graphitic petal sample up to 1000 K. Up to 600 K, the potassium-intercalated boron nitride modified sample exhibited improved retention of potassium in the form of multiple emission peaks (1.8 eV, 2.5 eV, and 3.3 eV) resulting in a large net electron emission relative to the unmodified graphitic sample. However, upon further heating to 1000 K, the unmodified graphitic petal sample demonstrated better stability and higher emission current than the boron nitride modified sample. Both samples deintercalated above 1000 K.

The electronic charge densities and work functions at sharp metallic facet edges are determined from ab initio calculations, combined with macroscopic averaging techniques. In particular, we examine how two different work functions coexist at close range near edges between inequivalent facets. The surface ionic relaxation at facet edges is shown to influence appreciably the local electrostatic potential in the vacuum. Various edges between Al(100) and Al(111) facets are studied, as well as between Na(110) facets. We also develop a model of electronic surface dipoles, which accounts for the surface charge transfer between inequivalent facets, and which allows us to predict the influence of the shape and size of a macroscopic crystal on its work functions.

Electron affinity of GaN(0001) surface doped with Al, Mg

In this work, density functional theory (DFT) computations with van der Waals (vdW) corrections were performed to investigate the dihydrogen bondings and their effects on the electronic and work functions of graphane/silicane bilayer and fully hydrogenated hexagonal boron nitride (fhBN)/silicane bilayer. The type of dihydrogen bonding (C-H···H-Si or N-H···H-Si) greatly affected the stability, electricity and photology of the bilayers, leading to significant band gap and work function modifications of the nanosystems. Interestingly, the fhBN/silicane bilayer combined by N-H···H-Si bilayers has an energy gap (∼0.533 eV) much lower than those of individual building blocks fHBN and silicane monolayer, and the work function (3.11 eV) of the fhBN/silicane bilayer combined by B-H···H-Si bilayers is much lower than that of graphene and other traditional metals. Changing the direction and strength of external electric field can effectively tune the band gap and work function of the bilayers, correspondingly cause a semiconductor-metal transition in the band gap and the work function showed a widely tunable range. These results offer new opportunities for developing electronic and opto-electronic devices based on graphane/silicane bilayer and fhBN/silicane bilayer.