Field variations, measured mobility

The effect of field variations on measured mobility are considerable and discussion of this in terms of double layer relaxations provides the major thrust of Stotz paper. When the field is applied to a particle and its double layer, the particle moves in one direction relative to the double layer and there is, therefore, an induced asymmetry. The relaxation time required to restore the original symmetry can be defined as... 

This result is useful in understanding the variation of the field dependence of the TOF measured mobility from sample to sample, following the carrier density gradients (Fc dn/cbc). For example, the role of the diffusion carrier stream would explain the field dependence of jx in single crystals whenever their near-surface layer is strongly populated... 

Nuclear magnetic resonance is sensitive to molecular mobility and local magnetic fields. Motion is modified in liquids contained in confined geometry. Local magnetic field variation results from susceptibility effects at interfaces. Both phenomena are observed for liquids contained in porous solids. This paper critically examines these effects and their use in characterisation of porous materials. The principles are illustrated with porous silicas and preliminary results are given of diffusion measurements on n-butane in silica as a function of temperature and pore geometry. 

FET mobility measurements thus constitute a sound method for investigating changes in the mobility of an organic semiconductor due to morphology variations. On the basis of the FET characteristics of MDMO-PPV films spin-cast from different solvents, we will discuss the influence of interchain polymer aggregates on the hole field-effect mobility and further consequences for the short-circuiting of solar cells. 

Good descriptions of practical experimental techniques in conventional electrophoresis can be found in references 29, 30, and 32. For the most part, these techniques are applied to suspensions and emulsions, rather than foams. In bubble microelectrophoresis, the dispersed bubbles are viewed under a microscope, and their electrophoretic velocity is measured taking the horizontal component of motion, because bubbles rapidly float upwards in the electrophoresis cells (33, 34). A variation on this technique is the spinning cylinder method, in which a bubble is held in a cylindrical cell that is spinning about its long axis. An electric field is applied, and the electrophoretic mobility is determined (2, 35). Other elec-trokinetic techniques, such as the measurement of sedimentation potential (36) have been used as well. 

The secondary electron distribution is not yet thermal. In order to measure the time taken to attain a thermal distribution, a technique sensitive to the energy of the secondary electrons is required. Warman [13,14] and coworkers have elegantly developed and utilised the a.c microwave absorption method. This technique relies on the variation of the mobility of electrons with energy and/or number density to produce a change in the absorption of energy from an applied microwave field. 

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