Work function determination

N.D. Lang, and A.R. Williams, Theory of local-work-function determination by photoemission from rate-gas adsorbates, Physics Review B 25(4), 2940-2942 (1982). 

Many investigations with surfaces have been carried out in this and other laboratories using the ion-bombardment method of cleaning. These include (1) structure investigations of the surface plane on clean surfaces, (2) work-function determinations, (3) adsorption measurements, (4) catalysis, (5) surface recombination velocity, (6) surface conductivity, and (7) field effect. One of the significant finds indicates that the relative positions of the atoms in the clean 100 surface planes of germanium and silicon are not the same as those of similar planes in the bulk crystals, but that these relative positions are the same when a monolayer of oxygen is adsorbed on these surfaces (9). 

The work function changes determined by field emission, listed in Table III, are in good agreement with the values obtained at low fields. The differences between the two sets are of the same order of magnitude as differences in the work function determined by a single technique. The high fields necessary in field emission do not appear to seriously affect work function measurements. 

Even a clean, single crystal emitter surface is not made up of a single plane alone for such a surface the work function, determined from the Fowler-Nordheim plot, is an average quantity. The total emission from a point is the sum of currents from the microscopic regions of uniform characteristics, which will be denoted by the subscript i. In the high applied fields, each contributes independently, and... 

Work function determination by photoemission measurement Work function determination by thermoemission measurement Work function determination by field emission measurement Work function determination by field emission retarding potential measurement... 

Work function determination by diode thermoemission retarding potential measurement... 

To ascertain the possible presence of interface dipole effects in Figure 19.12, work function measurements of both ITO and PEDOTiPSS surfaces were carried out through a Kelvin Probe (KP) [119]. The work functions determined were 4.8 + 0.1 eV for ITO, and 5.2 + 0.1 eV for PEDOTiPSS. Note that the difference between the two determinations (0.4 eV) is independent of the reference used. A similar... 

If the energy of the iacident x-rays and the spectrometer work function are known, the measured kinetic energy can be used to determine the binding energy E from... 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

The metallic electrode materials are characterized by their Fermi levels. The position of the Fermi level relative to the eneigetic levels of the organic layer determines the potential barrier for charge carrier injection. The workfunction of most metal electrodes relative to vacuum are tabulated [103]. However, this nominal value will usually strongly differ from the effective workfunction in the device due to interactions of the metallic- with the organic material, which can be of physical or chemical nature [104-106]. Therefore, to calculate the potential barrier height at the interface, the effective work function of the metal and the effective ionization potential and electron affinity of the organic material at the interface have to be measured [55, 107],... 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

For a metal/solution interface, the pcz is as informative as the electron work function is for a metal/vacuum interface.6,15 It is a property of the nature of the metal and of its surface structure (see later discussion) it is sensitive to the presence of impurities. Its value can be used to check the cleanliness and perfection of a metal surface. Its position determines the potential ranges of ionic and nonionic adsorption, and the region where double-layer effects are possible in electrode kinetics.8,10,16... 

Experiments at present are concentrated on sd-metals and Pt-group metals. The sp-metals, on which theories of the double layer have been based, are somewhat disregarded. In some cases the most recent results date back more than 10 years. It would be welcome if double-layer studies could be repeated for some sp-metals, with samples prepared using actual surface procedures. For instance, in the case of Pb, the existing data manifest a discrepancy between the crystalline system and the crystal face sequence of other cases (e.g., Sn and Zn) the determination of EgaQ is still doubtful. For most of sp-metals, there are no recent data on the electron work function. 

Depending on the rate behaviour upon variation of the catalyst potential UWr and, equivalently work function , a catalytic reaction can exhibit two types of behaviour, electrophobic or electrophilic. These terms, introduced since the early days of electrochemical promotion, are synonymous to the terms electron donor and electron acceptor reaction introduced by Wolkenstein113 in the fifties. Electrochemical promotion permits direct determination of the electrophobicity or electrophilicity of a catalytic reaction by just varying UWr and thus 0. 

Significant observations regarding the origin of NEMCA have been also made using Ultra-violet Photoelectron Spectroscopy (UPS) with Pt and Ag electrodes deposited on YSZ. In this case the work function of the electrode can be determined from the cutoff energy of secondary electrons (Fig. 5.43).24,68 As shown in Fig. 5.8b the change in the work function of the gas-exposed Ag surface is very close to the imposed electrode potential change AUwr. 

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