Work function considerations

An additional consideration in the selection of source and drain electrodes is the work function difference between the semiconductor and the energy levels in the semiconductor. Gold is a popular choice for PFETs, since its large work function provides reasonable access to the HOMO level of many materials. There are some indications that noble metals like Pt with a higher work function (and, in principle at least, a better energy match) may provide even better contact performance. It is not entirely clear, however, whether these 

Conduction band Fermi level Valence band 

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Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

As discussed already in Chapter 2 the work function, , of a solid surface is one of the most important parameters dictating its chemisorptive and catalytic properties. The work function, (eV/atom) of a surface is the minimum energy which an electron must have to escape from the surface when the surface is electrically neutral. More precisely is defined as the energy to bring an electron from the Fermi level, EF, of the solid at a distance of a few pm outside of the surface under consideration so that image charge interactions are negligible. 

Therefore it is much more useful to use the second way of splitting the energy difference between zero and p, which is common in the area of surface science, i.e. to consider -p as the sum of the work function vacuum level of the surface under consideration ... 

In order to attempt a more quantitative description one may start from the early theoretical considerations of Boudart9 who was first to tackle the problem of predicting the change in heats of adsorption with changing work function O. According to his early semiempirical electrostatic model when the work function of a surface changes by AO then the heat of adsorption, -AHad, of covalently bonded adsorbed species should change by ... 

Derivation of simple and unambiguous quantitative relations between the signal amplitude of a sensor, i.e., the value of the change of electric conductivity, work function, etc. and concentration of detected traces of admixture in the medium under study is also important for successful development of the sensor measuring technique. Theoretical considerations given in this book show that such relations exist in most simple form. The purpose of experiment consists in statistical substantiation that these dependencies rigorously hold at proper conditions. 

Excrements show that all the alkyl, hydroxyl, and amine radicals which we have studied considerably reduce the conductivity and increase the work function of oxide semiconductors like ZnO, Ti02, CdO, WO2, M0O3, etc. during chemisorbtion. It should be noted that the revealed effects are rather profound especially if we are dealing with the effect of chemisorbtion of active particles on conductivity of a thin (less than 1 pm) sintered polycrystal semiconductor films. Thus, conductivity of such films in the presence of free CH3-radicals with the concentration of even 10 cm and less may change from initial value by dozens or hundreds percent depending on experimental conditions. 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

The changes in the dipole potentials are typically small, of the order of a few tenths of a volt, while work functions are of the order of a few volts. If we keep the solvent, and hence 3>ref, fixed and vary the metal, the potential of zero charge will be roughly proportional to the work function of the metal. This is illustrated in Fig. 3.6. A more detailed consideration of the dipole potentials leads to a subdivision into separate correlations for sp, sd, and transition metals [3]. 

The work function of the rubbing surfaces and the electron affinity of additives are interconnected on the molecular level. This mechanism has been discussed in terms of tribopolymerization models as a general approach to boundary lubrication (Kajdas 1994, 2001). To evaluate the validity of the anion-radical mechanism, two metal systems were investigated, a hard steel ball on a softer steel plate and a hard ball on an aluminum plate. Both metal plates emit electrons under friction, but aluminum produced more exoelectrons than steel. With aluminum, the addition of 1% styrene to the hexadecane lubricating fluid reduced the wear volume of the plate by over 65%. This effect considerably predominates that of steel on steel. Friction initiates polymerization of styrene, and this polymer formation was proven. It was also found that lauryl methacrylate, diallyl phthalate, and vinyl acetate reduced wear in an aluminum pin-on-disc test by 60-80% (Kajdas 1994). 

In the study of work-function changes of metals caused by the presence of adsorbates, the most troublesome aspect is that of obtaining a sufficiently well-defined surface. There is considerable experimental evidence based on... 

Capacitor. The potential difference between two conductors A and B at the same temperature connected by a circuit containing no source of e.m.f. is given by the difference in work function (pa — a according to Equation (2). The application of a potential Fo = < s — a between the two conductors brings them to the same potential, but for a value F 5 Fo there will be a net charge on one of the conductors of (F — Vo)/C, where C is the capacitance. If F is adjusted so that F = Fo, no current flows when the capacitance is varied and F = 4>a — A This is the basis of the capacitor method, where the surface under consideration and a reference electrode form the plates of a capacitor. The reference electrode must remain inert at normal temperatures when exposed to gas or vapor during the measure-... 

Turning now to the applications considered in Secs. IX, X, and XI, we note that the activation energies of desorption and migration are conveniently measured by S.P. methods, while the results obtained for the adsorption of N2O on Pt show that there is considerable scope for the application of work-function measurements in investigating catalytic reactions at metal surfaces. 

It is now believed, however, that this intrinsic or a priori surface heterogeneity is not the most general cause of the effect under consideration and that much of the observed decrease in the heat of adsorption with increasing surface -coverage arises from effects associated with the adsorption process itself (Boudart, 85). Among these are repulsions between the adsorbed molecules and changes in the work function of the metal. The latter effect, which is due to the dipoles associated with the adsorbed species, is particularly important and has been termed induced heterogeneity (Boudart, 85) or the work function effect (de Boer, 69, US). 

As early as 1928, Roginskil and Schul tz (4) stressed the importance of electronic considerations, and Rideal and Wansbrough-Jones (5) related the work function of metals to the activation energy for their oxidation. Brewer, 1928 (6), Schmidt, 1933 (7), and Nyrop, 1935 (8) proposed that the surface must be capable of effecting ionization of the adsorbed species in some catalytic processes. Lennard-Jones (9) in his... 

The second column of Table II shows the numbers of the first (wi), second (ni), and third (nj) neighboring atoms for an atom situated inside (in) or located outside (ad) the plane under consideration. Planes having the same numbers of Wi and n2 for in-atoms show identical values of work function increases with packing density of the crystal planes, their specific surface energy a decreases [Straiiski and Suhrmann (48)] in a reverse linear relation to 4> (see Table II, column 3. and Fig. 12). 

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