Mobility of electrons

Usually tills is not tire case and Table 5.2 shows values for Eg, jig and ptp for a number of semiconductors having the diamond structure. It will generally be observed from this table that the mobilities of electrons are greater than those of positive holes making these materials n-type semiconductors. 

We have seen that the reasons for the mobility of electrons in metals are that they are readily removed from the atom (the ionization energy is... 

Figure 13. Voltage relaxation method for the determination of the diffusion coefficients (mobilities) of electrons and holes in solid electrolytes. The various possibilities for calculating the diffusion coefficients and from the behavior over short (t L2 /De ) and long (/ L2 /Dc ll ) times are indicated cc h is the concentration of the electrons and holes respectively, q is the elementary charge, k is the Boltzmann constant and T is the absolute temperature.

Referring to results obtained in study [16], we can assume that the conductivity of crystals in sintered ZnO film increases due to increase in number of conductivity electrons in the surface layer. Taking the sample mobility of electrons fd as 10 cm -s -V the temperature dependence being T (the data borrowed from [21]) one estimate the value A[e] from the following expression Acr = A[e, where Acr is the con-... 

M 1) and the ratios of mobilities of electrons and ions. Theoretical analysis by Mozumder (1971) produced a higher value, 1.0 x 1012 M-1s 1. Later experiments of Beck and Thomas (1972) gave ks - (2.2-3.0) x 1012 M Is 1, which is consistent with a recent mobility model (Mozumder, 1995 see Sect. 10.3.3). 

Conjugated polyenes exhibit large linear and nonlinear optical properties due to the mobility of electrons in extended TT-orbital systems. Hence, this is another reason for the growing interest shown in these molecules in recent years2,79-89. 

Generally speaking the mobilities of electrons and positive holes decrease and the band gaps increase as the bonding in the semiconductors becomes more... 

Although MEH-PPV 13 (at the time of discovery) was one of the most efficient soluble polymers for PLEDs application, its performance is not high enough for commercialization as LEP. One of the reasons is unbalanced hole-electron mobility in MEH-PPV (the mobility of holes is 100 times faster than the mobility of electrons) [133]. Copolymerization with other conjugated monomers, to some extent, can improve the electron-transporting properties and increase the EL performance. 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

The negative ions will also exist in the gas phase. However, the contribution of the negative ions may be negligibly small, because the mobility of electron is much larger than that of the negative ions. 

At low electric fields v is proportional to E and a mobility /u=er/m can be defined. The mobility of electrons and holes in bulk silicon is shown in the figure on the inner front cover of this book. 

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

Obviously, the donor activity of the nucleophile, that is, the sulfide ion is enhanced as the negative charge is dispersed along the polysulfide ion produced from the sulfide on the addition of elemental sulfur. This increases the mobility of electrons and facilitates electron transfer. That is why this reaction can be initiated in such a simple way as the addition of elemental sulfur. 

A representative example for the information extracted from a TRMC experiment is the work of Prins et al. [141] on the electron and hole dynamics on isolated chains of solution-processable poly(thienylenevinylene) (PTV) derivatives in dilute solution. The mobility of both electrons and holes as well as the kinetics of their bimolecular recombination have been monitored by a 34-GHz microwave field. It was found that at room temperature both electrons and holes have high intrachain mobilities of fi = 0.23 0.04 cm A s and = 0.38 0.02 cm / V s V The electrons become trapped at defects or impurities within 4 ps while no trapping was observed for holes. The essential results are (1) that the trap-free mobilities of electrons and holes are comparable and (2) that the intra-chain hole mobility in PTV is about three orders of magnitude larger than the macroscopic hole mobility measured in PTV devices [142]. This proves that the mobilities inferred from ToF and FET experiments are limited by inter-chain hopping, in addition to possible trapping events. It also confirms the notion that there is no reason why electron and hole mobilities should be principally different. The fact... 

In nonpolar liquids, bimolecular electron attachment rate constants, k , are much larger than those for conventional reactions of ions or radicals. This is, in part, related to the high mobility of electrons in these liquids but various other factors, like Vq, the kinetic energy of the electron, and dipole moment of the solute, are important as well. These and other factors are examined below the dependence of on the energy gap, AGr, in representative liquids is also shown and discussed. 

It is natural to conclude that the high rate constants for electron attachment reactions in nonpolar liquids are associated with the high mobility of electrons. Early studies [96,104,105] of attachment to biphenyl and SFg emphasized the dependence of on mobility. This relationship is apparent if the expression for the rate constant for a diffusion-controlled reaction ... 

The drift mobility of electrons in nonpolar liquids ranges from high values such as that for liquid xenon of 2000 cm /Vs to low values like that for tetradecane of 0.02 cm /Vs. It has often been suggested that the mobility is high for symmetrical molecules and low for straight chain molecules like -alkanes. Inspection of Table 2 shows that liquids with symmetrical molecules are indeed at the top of the list. However, other less symmetrical molecules like A-trimethylsilylmethane and 2,2,4,4-tetramethylpentane also show high drift mobility. A more important factor may be the existence of many methyl groups in the molecule. In any case, for liquids for which 10 cm /Vs, the electron is considered to be quasi-free. This is supported by the Hall mobility studies, as discussed below. 

It is of interest to note, at this point, the observation of Miller (38) to the effect that the mobility of electrons in the large grains of sintered... 

The diffusion coefficients of the ion are usually estimated from their mobilities (or conductances), which can be measured independently. Diffusion coefficients (or mobilities) vary over very large ranges for the solvated electron in different solvents (neopentane, D 2 x 10 4 m2 s 1 and n 7 x 10 3 m2 V-1 s 1 hexane, D 2 x 10 7 m2 s 1 and ju 8x 10 6 m2 V-1 s 1) [320]. There is considerable evidence that the mobility of electrons is not constant, but on the contrary, the mobility depends on the applied electric field, increasing approximately proportionately with electric field at high fields in solvents where n is small and decreasing with electric field in solvents where n is large. If the solvated electron mobility depends on the electric field, then the diffusion coefficient may also depend on the electric field. The implications of these complications are discussed in Sect. 2.2 and in Chap. 8, Sect. 2.7. 

The mobility of electrons in a metal accounts for the metals significant ability to conduct electricity and heat. Also, metals are opaque and shiny because the free electrons easily vibrate to the oscillations of any light felling on them, reflecting most of it. Furthermore, the metal ions are not rigidly held to fixed positions, as ions are in an ionic crystal. Rather, because the metal ions are held together by a fluid of electrons, these ions can move into various orientations relative to one another, which is what happens when a metal is pounded, pulled, or molded into a different shape. 

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