Field-induced barrier

Pfister (1977) measured hole mobilities of TPA doped PC. Figure 51 shows the temperature dependencies for different concentrations. The field was 7.0 x 105 v/cm. The concentration is expressed as the weight ratio X of TPA to PC. The mobilities were thermally activated with activation energies that increase with decreasing TPA concentration. The concentration dependence was described by the lattice gas model with a wavefunetion decay constant of 1.3 A. Figure 52 shows the field dependencies at different temperatures for X - 0.40. The solid lines were derived from the Scher-Montroll theoiy (1975) using the listed parameters. Pfister concluded that the theoiy provides a self-consistent interpretation of all experimental observations if field-induced barrier lowering and temperature-dependent dispersion are formally introduced into the expression for the transit time. 

Field emission potential diagram. Large electric fields induce barrier narrowing, which increases the number of electrons tunnelling from the Fermi level in the metallic, electron-rich, surface into the vacuum. Variations in the density of occupied states, N E), and current density, J[E), as a function of electron energy. Surface contamination and charging effects (Schottky rounding) can be seen to drastically alter the potential profile. 

Under reverse bias, the FN tunnelling mechanism did not apply. On the other hand, the current-density was found to follow Richardson-Schottky thermionic emission model, where tunnelling through the barrier is ignored and field-induced barrier lowering is taken into consideration. The current density at a temperature T is given by [13] ... 

Under high applied electric fields, electrons can surmount a potential barrier even at very low temperatures. This process is based on field-induced tunneling of the charge carriers across potential barrier. The probability for the tunneling depends strongly on the height and the width of the potential barrier. 

The field-induced tunneling across a potential barrier can be described using the Fowler-Nordheim equation [79] ... 

Figure 9-25. Field-induced tunneling relative t]uanlunt efficiencies of organic LEDs calculated for various carrier mobilities and barrier heights (I KT , ,=,=0.2 2...//,= If) /( =I0"8,...

The small and weakly time-dependent CPG that persisLs at longer delays can be explained by the slower diffusion of excitons approaching the localization edge [15]. An alternative and intriguing explanation is, however, field-induced on-chain dissociation, a process that does not depend on the local environment but on the nature of the intrachain state. The one-dimensional Wannier exciton model describes the excited state [44]. Dissociation occurs because the electric field reduces the Coulomb barrier, thus enhancing the escape probability. This picture is interesting, but so far we do not have any clear proof of its validity. 

When the electric field-induced tunneling currents are analyzed in a log WE1) versus ME plot, a straight line should be obtained. The height of the potential barrier can be derived from the slope of this straight line using Eq. (9.12). 

Therefore, the following method was suggested and realized (the scheme is shown in Fig. 17). A 1.5 M solution of KCl or NaCl (the effect of preventing BR solubility of these salts is practically the same) was used as a subphase. A platinum electrode was placed in the subphase. A flat metal electrode, with an area of about 70% of the open barriered area, was placed about 1.5-2 mm above the subphase surface. A positive potential of +50 -60 V was applied to this electrode with respect to the platinum one. Then BR solution was injected with a syringe into the water subphase in dark conditions. The system was left in the same conditions for electric field-induced self-assembly of the membrane fragments for 1 hour. After this, the monolayer was compressed to 25 mN/m surface pressure and transferred onto the substrate (porous membrane). The residual salt was washed with water. The water was removed with a nitrogen jet. 

Obviously, the model is crude and does not take into account many of the factors operating in a real molecular stack. Lack of symmetry with respect to the polar axis and the fact that dipoles may not necessarily be situated in one plane represent additional complications. The angle a could also be field dependent which is ignored in the model. The model also requires that interactions between molecules in adjacent stacks be very weak in order for fields of 10 to 20KV/cm to overcome barriers for field induced reorientation. The cores are then presumably composed of a more or less ordered assembly of stacks with a structure similar to smectic liquid crystals. 

A field effect is also seen in the photocurrent decay time at high light intensity, interpreted in terms of bimolecular carrier recombination. Finally we discuss the rise time of a photocurrent following a rectangular light pulse. The latter results pertain to the time domain where transport is barrier controlled and bear out the importance of photo-induced barrier crossing processes. 

Figure 12. Particle transfer in a double-well potential. The initial wavepacket is localized in the potential well at positive distances (panel (a)). The LCT field induces a vibrational motion and drives the packet over the barrier, as can be taken from the coordinate expectation value displayed in panel (b). A phase jump is introduced in the field (panel (c)) at the time the barrier is passed, and afterwards a trapping in the second potential well takes place.

As an initial wavefunction we employ a Gaussian that is located in the potential well at negative distances, as indicated in Fig. 43. The interaction of the LCT field induces an average motion over the reaction barrier where afterwards a cooling takes place. This is demonstrated in Fig. 44 which contains the coordinate expectation value as a function of time (upper panel). Also shown is the target-state population, which we define as... 

It is instructive to compare the ratio an/aj 1.5 for ionic processes with a much higher ratio ajj,/a 10 — 10 for the conductivity due to dielectric losses. In the first case, the orientational nematic potential Wn does not influence the translational motion of ions and cj /a L. In the secmid case, the losses eJJ come in from the field induced alignment of the lOTigitudinal molecular axes against the potential barrier and dramatically depend on Wn whereas losses e" caused by molecular rotation about the longitudinal molecular axes are independent of Therefore, ratios e"n/ "j. (and are very large. 

Within this construct, the case of defect pinning is illustrated in Fig. 36b. The activation barrier around the local minimum is much larger than thermal energy, so the defect cannot escape. An important point is that the effects of defect interactions become stronger with increasing defect density, not only because of the increased number of defect interactions but also because of the reduced size of the average wall defect (and thus reduced electric field-induced driving force). 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

---