Field-assisted transport

Several issues remain to be resolved in studies of electron transport at nanocrystalline semiconductor-electrolyte interfaces. Among these are those related to the variation of excess carrier density with distance (and consequent variations in D and T across the film), the importance of field-assisted transport phenomena, the role of temporary localization of electrodes in traps, the influence of solution species on transient profiles and the like. However, rapid progress is being made in these directions as exemplified by several published studies [338, 349, 352, 353-355]. 

The penetration of chlorine atoms into the passive films is suggested by close examination of the relaxed structures in Figures 7.10 and 7.11. It seems to occur independently of the O-enriched or O-deficient nature of the films and of the implemented defect site. However, this aspect was not addressed by the authors in their study and thus cannot be further discussed. Detailed studies relevant for testing the penetration-induced voiding mechanism of passivity breakdown would require implementing O vacancies as point defects not only at the surface but also in the bulk of the passive films of appropriate crystalline structure. Implementation of field-assisted transport in the passive film and at its interfaces would also be required. 

As electroneutrality is demanding, it is generally assumed that the field-assisted transport of charged species is more rapid than the transport of neutral species and, consequently, solvation equilibria can only be established slowly. Therefore, the equilibria associated with electronic, ionic and solvation processes may be established on quite different time scales, but at long enough time scales thermodynamics will prevail and processes will attain a state of global equilibrium. However, the relative rates of all the processes involved in the charge compensation are still an open question. 

However, this does not preclude the possibility that in a portion of the oxide at least (the outer layer), the OH transport mechanism is operative, with the release of protons at the interface between the two oxide layers. Hence, in such a case, some field-assisted proton transfer is likely to take place through the outer layer while chemical dissolution should be operative at the outer O/S interface. 

Narayanan et al. conducted a theoretical analysis on the cell parameters that determined this maximum shuttling current. By assuming that the mass transport of the redox couple [R]/[0] is mainly realized by means of diffusion—which is reasonable because the low concentration of [R]/[0] at the additive level makes the field-assisted migration negligible—they applied the finite linear diffusion... 

It has been established from conductivity measurements that thermally activated and field-assisted hole hopping is responsible for the charge transport in solid polysilanes [48,49]. The mobility of the hole is as high as 10 m /V sec, while the mobility of the electron is a few orders of magnitude lower. In this section, we will show the reason why only the hole is mobile in polysilanes and how we can construct electron-conductive polysilanes. 

We suggest that this approach might be exploited at two levels. Firstly, field assistance of ion transfers (migration) will lead to their being more rapid than neutral species transfers. Secondly, size effects (for ions or neutral species) will lead to a diversity of transport rates. These effects are likely to be more pronounced in the confined geometry of polymer films than for the same species in solution. The extent to which transfer of a given species dominates the net transfer process (on a given time scale) will depend on its availability, i.e. solution concentration. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

In organic cells, however, the steps involved in the generation of photo-current are (1) light absorption, (2) exciton creation, (3) exciton diffusion, (4) exciton dissociation in the bulk or at the surface, (5) field-assisted carrier separation, (6) carrier transport, and (7) carrier delivery to external circuit. Assuming that only the excitons which reach the junction interface produce free carriers, if the blocking contact is illuminated [65],... 

Dufva M (2005) Fabrication of high quality microarrays. Biomol Eng 22 173-184 Wellman AD, Sepaniak MJ (2006) Magnetically-assisted transport evanescent field fluor-oimmunoassay. Anal Chem 78 4450-4456... 

Wellman AD, Sepaniak MJ (2007) Multiplexed, waveguide approach to magnetically assisted transport evanescent field fluoroassays. Anal Chem 79 6622-6628 Loete F, Vuillemin B, Oltra R et al (2006) Application of total internal reflection fluorescence microscopy for studying pH changes in an occluded electrochemical cell development of a waveguide sensor. Electrochem Commun 8 1016—1020... 

The first time-of-flight measurements of PVK were by Regensburger (1968). At 4.0 x K)5 V/cm, a mobility of 10-6 cm2/Vs was reported. The mobility was field dependent and thermally activated with a high-field activation energy of 0.7 eV. No evidence for deep trapping was observed. The field dependence was attributed to the ionization of shallow traps or field-assisted hopping. The study of Regensburger, and the earlier work of Vannikov (1967), were the first to clearly show that electronic transport occurs in polymers. Kiyszewski et al. (1968) and Szymanski and Labes (1969) later reported a... 

The above observations have been interpreted within the framework of two distinct models, one involving trapping/detrapping of the photogenerated electrons [345, 346] and the other based on electron diffusion (or field-assisted diffusion) not attenuated by electron localization [347, 348]. The millisecond transit times also mean that the transit times are very long compared with equilibration of majority carriers in a bulk semiconductor or electron-hole pair separation within the depletion layer of a flat electrode. The slow transport is rationalized by a weak driving force and by invoking percolation effects [338]. 

The inward motion of anions is assumed to be the dominant ionic transport across the oxide. The ionic movement is field-assisted drifting and is the rate-limiting process. The diffusion of ions at room temperature is considered to be too slow to account for the oxide growth rates. The current density is written as... 

Superlattice structures yield efficient charge transport normal to the layers, because the charge carriers can move through the minibands the narrower the barrier, the wider the miniband and the higher the carrier mobility. Transport in MQWs with thick barriers requires thermionic emission of carriers over the barriers, or if electric fields are applied, field-assisted tunnelling through the barriers (Parsons et al, 1990). 

In the present paper, a sandwich type model for the transport characteristics of the S-N-S junction is developed. The role of the Andreev reflection at the S-N interface is taken into account. We analyze the photon-assisted transport process due to both intersubband transitions (when the radiation field is in that transverse polarization) and to intrasubband transition (when the ac field is in the longitudinal polarization). 

This mechanism is consistent with the hypothesis that in the second stage dissolution kinetics is dependent on diffusion within the concentration boundary layer. It is conceivable that in the first stage field assisted dissolution may be the controlling step. In this stage formation of Ti(OH)4 or of hydroxy-cations, e.g. Ti(OH)3, has different effects on titanium transport. While Ti(OH)4 does not react with organic molecules, Ti(OH)3 can form organometallic complexes which may be transported systemically. 

---