Electron affinity, relation work function

What happens with the outer orbitals of an atom when it approaches a metal surface Discuss the role of the atom s ionization potential and electron affinity in relation to the work function of the metal for the strength of the eventual chemisorption bond. 

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

The degree of ionicity in the bond between a metal atom and a polymer, or molecule, is related to the ionization potential and electron affinities of the substituents. The metals we have studied are of interest as electron injecting contacts in electronic devices. These metals must have a low ionization potential (or work function), of the same order as the electron affinity of the polymer, in order for the charge transfer process to occur. If the ionization potential of the metal is lower than the polymery-electron affinity, spontaneous charge transfer occurs which is the signature of an ionic bond. Thus, the character of the charge distribution in the metal-polymer complexes we are studying is related to the situation in the electronic device. 

Here 4> is the work function of the metal and x is the electron affinity of the insulator. Table 3.3 shows that N0 is very sensitive to the values of 0 - x - In the organic polymer devices the injecting contact is made as nearly ohmic as possible and — x is small. In computation of the I-V relations 0 — X is assumed to be zero. In this case the value of No is very large and can be taken as infinity [37,38],... 

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

Let us recall in this connection that the thermodynamic work function is equal to w = x P Ec. Here x is the electron affinity of the semiconductor, and the position of F relative to the conduction-band bottom, Ec, in the semiconductor bulk is given by the following relations ... 

However, experimental ]V curves often deviate from the ideal /scl- In these cases, the measured current /inj is injection limited caused by a nonohmic contact or poor surface morphology. When the MO interface is nonohmic, carrier injection can be described by the Richardson-Schottky model of thermionic emission the carriers are injected into organic solid only when they acquire sufficient thermal energy to overcome the Schottky barrier ((()), which is related to the organic ionization potential (/p), the electron affinity (AJ, the metal work function (O, ), and the vacuum level shift (A) [34,35]. Thus, the carrier injection efficiency (rj) can be calculated by the following equation ... 

The purpose of this work is to start from the basic equations of density functional theory to describe the changes in the energy associated with the transition from one ground-state to another, in terms of different sets of variables. In this process one will find the natural definitions of the hardness and softness kernels, the local hardness, the local softness, the global hardness and the global softness [23]. Then, we will proceed to establish their relation with ionization potentials and electron affinities, in order to confirm their behavior as a measure of chemical hardness or softness [14, 24]. Finally, this theoretical framework will be used to analyze the maximum hardness and the HSAB principles. 

A more useful measure of the ionization energy in semiconductors is related to the depth of the conduction band edge below This energy difference is defined as the electron affinity X=E — E given the alternate symbol by some authors. The electron affinity unlike the work function is a constant of the material, independent of impurity content, as shown in Fig. S.2b. 

Consistent with Relations 3.8(a-b) and 3.9(a-b), these relations apply only over limited ranges of work-function, ionization potential, or electron affinity. 

Experimental measurements generally determine the work function or other related doping-dependent quantities. Careful conversion to the doping-independent value is therefore needed to determine the electron affinity. The measurements are difficult and many factors can affect the outcome. Therefore, electron affinity values have often been the source of significant debate in the experimental literature. Real device... 

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