Interface space charge

Surface and other interface space charge regions have been... 

While in the bulk of materials local electroneutrality has to be fulfilled, at interfaces space charge zones occur which provide additional degrees of freedom defined by the contact chemistry. So it is the individual preference of a given charge carrier for a certain phase or the boundary core itself that is to the fore. The tradeoff of this additional flexibility is the occurrence of an electrical field which confines the effects to the immediate vicinity of the interface. 

Figure Bl.28.9. Energetic sitiration for an n-type semiconductor (a) before and (b) after contact with an electrolyte solution. The electrochemical potentials of the two systems reach equilibrium by electron exchange at the interface. Transfer of electrons from the semiconductor to the electrolyte leads to a positive space charge layer, W. is the potential drop in the space-charge layer.

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

The logaritlrmic law is also observed when the oxide him is an electrical insulator such as AI2O3. The transport of elecuons tlrrough the oxide is mainly due to a space charge which develops between tire metal-oxide interface and the oxide-gas interface. The incorporation of oxygen in the surface of tire oxide requhes the addition of electrons, and if this occurs by a charging process... 

Consider now the system Cu/CujO in oxygen gas at a pressure p (X signifies the oxide/oxygen interface in Fig. 1.75). Ignoring space charges, x the equilibrium concentration of cation vacancies or positive holes at the CujO/Oj interface, is given by... 

The layer is so thick that the space-charge regions at the two interfaces are unimportant and the oxide can be regarded as electrically neutral. 

Blom et al. [85] stated that the l/V characteristics in LEDs based on ITO/di-alkoxy-PPVs/Ca are determined by the bulk conductivity and not by the charge carrier injection, which is attributed to the low barrier heights at the interface ITO/PPV and PPV/Ca. They observed that the current flow in so called hole-only devices [80], where the work function of electrodes are close to the valence band of the polymer, with 1TO and Au as the electrodes, depends quadratically on the voltage in a logl/logV plot and can be described with following equation, which is characteristic for a space-charge-limitcd current (SCL) flow (s. Fig. 9-26) ... 

Although the observations for PPV photodiodes of different groups are quite similar, there are still discussions on the nature of the polymer-metal contacts and especially on the formation of space charge layers on the Al interface. According to Nguyen et al. [70, 711 band bending in melal/PPV interfaces is either caused by surface states or by chemical reactions between the polymer and the metal and... 

Dispersing a dielectric substance such as A1203 in Lil [34] enhances the ionic conductivity of Lil about two orders of magnitude. The smaller the particle size of the dielectrics, the larger is the effect. This phenomenon is explained on the basis that the space-charge layer consists of or Li, generated at the interface between the ionic conductor (Lil) and the dielectric material (A1203) [35],... 

Another technique consists of MC measurements during potential modulation. In this case the MC change is measured synchronously with the potential change at an electrode/electrolyte interface and recorded. To a first approximation this information is equivalent to a first derivative of the just-explained MC-potential curve. However, the signals obtained will depend on the frequency of modulation, since it will influence the charge carrier profiles in the space charge layer of the semiconductor. 

Figure 28. Semiconductor interfaces with increasing electric fields in the space charge layer (from top to bottom) compared with tubes of different diameters through which an equivalent amount of water is pressed per unit time (equivalent to limiting current).

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

The lattice gas has been used as a model for a variety of physical and chemical systems. Its application to simple mixtures is routinely treated in textbooks on statistical mechanics, so it is natural to use it as a starting point for the modeling of liquid-liquid interfaces. In the simplest case the system contains two kinds of solvent particles that occupy positions on a lattice, and with an appropriate choice of the interaction parameters it separates into two phases. This simple version is mainly of didactical value [1], since molecular dynamics allows the study of much more realistic models of the interface between two pure liquids [2,3]. However, even with the fastest computers available today, molecular dynamics is limited to comparatively small ensembles, too small to contain more than a few ions, so that the space-charge regions cannot be included. In contrast, Monte Carlo simulations for the lattice gas can be performed with 10 to 10 particles, so that modeling of the space charge poses no problem. In addition, analytical methods such as the quasichemical approximation allow the treatment of infinite ensembles. 

The simplest model for the ionic distribution at liquid-liquid interfaces is the Verwey-Niessen model [10], which consists of two Gouy-Chapman space-charge layers back to... 

As an example Fig. 6 shows the distribution of the ions for a potential difference of A(j) = 0(00) — 0(—00) = kT/cq between the two bulk phases. In these calculations the dielectric constant was taken as e = 80 for both phases, and the bulk concentrations of all ions were assumed to be equal. This simplifies the calculations, and the Debye length Lj), which is the same for both solutions, can be used to scale the v axis. The most important feature of these distributions is the overlap of the space-charge regions at the interface, which is clearly visible in the figure. 

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