Low work function cathode development Summary report
Low work function cathode configurations and nickel encapsulated powder for withstanding intensive ion bombardment
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Dependence of the mechanical behavior of alloys on their electron work function—An alternative parameter for materials design
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.
36 MATERIALS SCIENCE↗
Decreasing Work Function of Carbon Nanotubes Hydrogenated in Hydrogen Plasma
Experimental data showing a decrease in the work function of carbon nanotube (CNT) bundles and arrays hydrogenated in hydrogen-containing plasma are presented. This plasma treatment leads to hydrogen chemisorption on CNTs, which results in a work-function decrease from 4.8 to 3.3 eV. The experimental data confirm quantum-mechanical calculations for single-walled CNTs of variable chirality. Calculations indicate that a decrease in the CNT work function depends on both the properties of CNT and the degree of its passivation by hydrogen.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Work function lowering of LaB 6 by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodes
We report a lowering of work function for lanthanum hexaboride (LaB 6 ) by monolayer hexagonal boron nitride (hBN) coating. Photoemission electron microcopy (PEEM) and thermionic emission electron microscopy (TEEM) both revealed that the hBN coated region of a LaB 6 (100) single crystal has a lower work function compared to the bare (i.e., non-coated) and graphene coated regions. A broad and uniform brighter image of the hBN coated region in PEEM was quantitatively supported by a 0.4 eV decrease in the work function in photoelectron spectra compared to the bare region. TEEM results were consistent in that the hBN coated region exhibited thermionic emission at 905 °C, whereas the bare and graphene coated regions did not. A larger decrease in the work function for hBN coated LaB 6 (100) compared to graphene coated LaB 6 (100) was qualitatively supported by our density functional theory calculations. Adding an oxide layer in the calculations improved consistency between the calculation and experimental results. We followed up our calculations with synchrotron-radiation x-ray photoelectron spectroscopy and confirmed the presence of an oxide layer on our LaB 6 .
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Coverage dependent work function of graphene on a Cu(111) substrate with intercalated alkali metals
Using first-principles calculations, it is shown that the work function of graphene on copper can be adjusted by varying the concentration of intercalated alkali metals. Using density functional theory, we calculate the modulation of work function when Li, Na, or K are intercalated between graphene and a Cu(111) surface. Furthermore, the physical origins of the change in work function are explained in terms of phenomenological models accounting for the formation and depolarization of interfacial dipoles and the shift in the Fermi-level induced via charge transfer.
36 MATERIALS SCIENCE↗
Low work function silicon collector for thermionic converters
To improve the efficiency of present thermionic converters, single crystal silicon was investigated as a low work function collector material. The experiments were conducted in a test vehicle which resembled an actual thermionic converter. Work function as low as 1.0eV was obtained with an n-type silicon. The stabilities of the activated surfaces at elevated temperatures were tested by raising the collector temperature up to 829 K. By increasing the Cs arrival rate, it was possible to restore the originally activated low work function of the surface at elevated surface temperatures. These results, plotted in the form of Rasor-Warner curve, show a behavior similar to that of metal electrode except that the minimum work function was much lower with silicon than with metals.
Chang, K. H.↗
Work function and surface stability of tungsten-based thermionic electron emission cathodes
Materials that exhibit a low work function and therefore easily emit electrons into vacuum form the basis of electronic devices used in applications ranging from satellite communications to thermionic energy conversion. W–Ba–O is the canonical materials system that functions as the thermionic electron emitter commercially used in a range of high-power electron devices. However, the work functions, surface stability, and kinetic characteristics of a polycrystalline W emitter surface are still not well understood or characterized. In this study, we examined the work function and surface stability of the eight lowest index surfaces of the W–Ba–O system using density functional theory methods. We found that under the typical thermionic cathode operating conditions of high temperature and low oxygen partial pressure, the most stable surface adsorbates are Ba–O species with compositions in the range of Ba 0.125 O–Ba 0.25 O per surface W atom, with O passivating all dangling W bonds and Ba creating work function-lowering surface dipoles. Wulff construction analysis reveals that the presence of O and Ba significantly alters the surface energetics and changes the proportions of surface facets present under equilibrium conditions. In conclusion, analysis of previously published data on W sintering kinetics suggests that fine W particles in the size range of 100-500 nm may be at or near equilibrium during cathode synthesis and thus may exhibit surface orientation fractions well described by the calculated Wulff construction.
36 MATERIALS SCIENCE↗
Surface work function of gadolinium
Gadolinium surface work function from contact potential difference against platinum
Katzberg, S. J.↗
Cesiated work functions of 0001 rhenium
Cesiated work functions of directed monocrystalline Re thermionic emitter from saturated electron emission currents over temperature range
Lieb, D.↗
A mechanism of Cu work function reduction in CsBr/Cu photocathodes
Thin films of CsBr deposited on Cu(100) have been proposed as next-generation photocathode materials for applications in particle accelerators and free-electron lasers. However, the mechanisms underlying an improved photocathode performance remain poorly understood. We present density Functional Theory (DFT) calculations of the work function reduction following the application of CsBr thin film coatings to Cu photocathodes. The effects of structure and van der Waals forces are examined. Calculations suggest that CsBr films can reduce the work function by around 1.5 eV, which would explain the exponential increase in quantum efficiency (QE) of coated vs. uncoated photocathodes. In conclusion, a model explaining experimentally observed laser activation of photocathode is provided whereby the photo-induced creation of di-vacancies at the surface, and their subsequent diffusion throughout the lattice and segregation at the interface leads to a further increase in QE after a period of laser irradiation.
36 MATERIALS SCIENCE↗
Low work function in the 122-family of iron-based superconductors
Herein we determine the work functions of the iron arsenic compounds A Fe 2 As 2 ( A = Ca , Ba , Cs ) using photoemission spectroscopy to be 2.7 eV for CaFe 2 As 2 , 1.8 eV for BaFe 2 As 2 , and 1.3 eV for CsFe 2 As 2 . The work functions of these 122 iron-based superconductors track those of the elementary metal A but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an A -layer. The low work function and good photoemission cross section of BaFe 2 As 2 and CsFe 2 As 2 enable photoemission even from a common white LED light.
36 MATERIALS SCIENCE↗
Work function reduction in lanthanum hexaboride hollow cathodes operated in gas discharges
Thermal characterization of lanthanum hexaboride (LaB6) hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass ow rates of 5 to 25 sccm in cathodes with three di erent ori ce diameters. Thus, the accepted value of the work function for polycrystalline LaB6, 2.66 eV, does not describe well the emission characteristics of LaB6 hollow cathodes operating with internal gas discharges at steady state. The measured temperatures and a model of the hollow cathode emitter and xenon discharge were used to estimate the value of the work function in these experiments, yielding a value ranging from 2.1 - 2.44 eV. Measurements of the work function as a function of depth on a hollow cathode emitter using X-ray photoelectron spectroscopy and ion beam milling indicate that the work function in a thin layer near the emitting surface is lower than in the bulk material. We postulate that a lanthanum-rich emitting surface with a lower work function develops as a result of lanthanum recycling in the internal plasma.
Lopez Ortega, Alejandro↗
Mechanisms for carbon adsorption on Au(110)-(2 × 1): A work function analysis
In this paper, the variation of the work function upon carbon adsorption on the reconstructed Au(110) surface is measured experimentally and compared to density functional calculations. The adsorption dynamics is simulated with ab-initio molecular dynamics techniques. Finally, the contribution of various energetically available adsorption sites on the deposition process is analyzed, and the work function behavior with carbon coverage is explained by the resultant electron charge density distributions.
74 ATOMIC AND MOLECULAR PHYSICS↗
Self-consistent many-electron theory of electron work functions and surface potential characteristics for selected metals
Electron work functions, surface potentials, and electron number density distributions and electric fields in the surface region of 26 metals were calculated from first principles within the free electron model. Calculation proceeded from an expression of the total energy as a functional of the electron number density, including exchange and correlation energies, as well as a first inhomogeneity term. The self-consistent solution was obtained via a variational procedure. Surface barriers were due principally to many-body effects; dipole barriers were small only for some alkali metals, becoming quite large for the transition metals. Surface energies were inadequately described by this model, which neglects atomistic effects. Reasonable results were obtained for electron work functions and surface potential characteristics, maximum electron densities varying by a factor of over 60.
Smith, J. R.↗
Systems and methods for producing low work function electrodes
According to an exemplary embodiment of the invention, systems and methods are provided for producing low work function electrodes. According to an exemplary embodiment, a method is provided for reducing a work function of an electrode. The method includes applying, to at least a portion of the electrode, a solution comprising a Lewis basic oligomer or polymer; and based at least in part on applying the solution, forming an ultra-thin layer on a surface of the electrode, wherein the ultra-thin layer reduces the work function associated with the electrode by greater than 0.5 eV. According to another exemplary embodiment of the invention, a device is provided. The device includes a semiconductor; at least one electrode disposed adjacent to the semiconductor and configured to transport electrons in or out of the semiconductor.
36 MATERIALS SCIENCE↗
Anisotropy of the Surface Energy and Work Function of IIB Metals
On the basis of electron-statistical method of calculation of the surface energy of metals, a technique for estimating the electron work function of hexagonal and rhombohedral metallic crystals has been developed. This technique relates surface energy to work function and can thus be applied to estimate the surface energy of crystal faces from experimental data for a work function. Computations have been made for cadmium, zinc, and mercury macrocrystals. The temperature and orientation dependences of both quantities have been constructed. Our results are in good agreement with literature experimental data.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Theoretical studies on the relationship between the thermionic work functions of refractory-metal intermetallic compounds and their electronic and crystal structures Final summary report
The Group VIA-Re binary systems are characterized by the presence of several intermediate phases. The u phase is present in all of the systems, but the a-Mn phases occur in only the Mo-Re and W-Re systems. The occurrence of these intermediate phases appears to be governed largely by electronic factors and is predictable on the basis of electron/atom ratio. The thermionic work functions of the intermediate phase materials were experimentally determined. function and the screening of the outermost electrons in the Group VIA metals was found. A correlation between the work As might be expected for purely metallic substances, the work functions were relatively high, in the range 4. 5 to 5. 0 eV, and in each case were bracketed by the work functions of the pure constituents. A discontinuous change in the average work function with composition is proposed, based on a thermodynamical treatment of the electron vapor.
REFRACTORY METAL↗
Control of work function of graphene by plasma assisted nitrogen doping
Nitrogen doping is expected to provide several intriguing properties to graphene. Nitrogen plasma treatment to defect-free and defective highly oriented pyrolytic graphite (HOPG) samples causes doping of nitrogen atom into the graphene layer. Nitrogen atoms are initially doped at a graphitic site (inside the graphene) for the defect-free HOPG, while doping to a pyridinic or a pyrrolic site (edge of the graphene) is dominant for the defective HOPG. The work function of graphene correlates strongly with the site and amount of doped nitrogen. Nitrogen atoms doped at a graphitic site lower the work function, while nitrogen atoms at a pyridinic or a pyrrolic site increase the work function. Control of plasma treatment time and the amount of initial defect could change the work function of graphite from 4.3 eV to 5.4 eV, which would open a way to tailor the nature of graphene for various industrial applications.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗