Dangling “tails

Lyklema and Vliet8 determined the equilibrium thickness to of free liquid films stabilized by poly(vinyl alcohol) (PVA) adsorbed at the air-water interface. They estimated to at different applied hydrostatic pressures by measuring the intensities of light reflected from the surface of the film to that of the silvery film. The to values obtained increased with rising hydrostatic pressure and were extrapolated to zero pressure to obtain to for a free film. The extrapolated to should correspond to twice the thickness of the adsorbed PVA layer, but it far exceeded twice the latter determined by ellipsometry. The great difference was interpreted in terms of the presence of long dangling tails which are probably not to be seen by ellipsometry. 

FIGURE 17.17 (a) Figure depicting stabilization role of surfactant. Long dangling tails of nonionic... 

The statistical thermodynamics of block copolymer adsorption was considered elsewhere.Many theories attempt to characterize adsorption by smface density, block segment distribution profile, and the thickness of adsorbed layer. As a rule, an adsorbed diblock copolymer has one block adsorbed on the surface in a rather flat conformation, whereas the other block, having a lower surface activity, forms dangling tails. Because of their freely dangling blocks, adsorbed diblock copolymers are often interpenetrated. The adsorption of block copolymers leads to the segregation of blocks in the adsorption layer. It was found that both kinetic and equilibrium features of the block copolymer adsorption are intimately related to the phase behavior of the block copolymer solution. In particular, a very strong increase in the adsorbed amount is observed when the system approaches the phase boundary. As a consequence, a partial phase separation phenomenon may proceed in the surface zone. 

Equilibrium adsorption of polymers is only one of the methods used to create a change in the polymer concentration close to a surface. For an adsorbed polymer, it is interesting to look at the detailed conformation of a single polymer chain at the substrate. One distinguishes sections of the polymer which are bound to the surface, so-called trains, sections which form loops, and the end sections of the polymer chain, which can form dangling tails. This is schematically depicted in Fig. 3a. Two other methods to produce polymer layers at surfaces are commonly used for polymers which do not spontaneously adsorb on a given surface. 

A fiuther rather subtle result of these improved mean-field theories is the oc-eurrenee of a depletion hole, i.e., a region at a eertain separation from the adsorbing surfaee where the monomer concentration is smaller than the bulk eoneentration [42], This depletion hole results from an interplay between the depletion of free polymers from the adsorbed layer and the slowly deeaying density profile resulting from dangling tails. It occurs at a distance from the surfaee eomparable with the radius of gyration of a free polymer, but also shows some dependence on the bulk polymer eoneentration. These and other effeets, related to the occurrence of loops and tails in the adsorbed layer, have been recently reviewed [43]. 

Global AMI.5 sun illumination of intensity 100 mW/cm ). The DOS (or defect) is found to be low with a dangling bond (DB) density, as measured by electron spin resonance (esr) of - 10 cm . The inherent disorder possessed by these materials manifests itself as band tails which emanate from the conduction and valence bands and are characterized by exponential tails with an energy of 25 and 45 meV, respectively the broader tail from the valence band provides for dispersive transport (shallow defect controlled) for holes with alow drift mobiUty of 10 cm /(s-V), whereas electrons exhibit nondispersive transport behavior with a higher mobiUty of - 1 cm /(s-V). Hence the material exhibits poor minority (hole) carrier transport with a diffusion length <0.5 //m, which puts a design limitation on electronic devices such as solar cells. 

FIG. 1. Schematic density of states distribution. Bands of (mobile) extended states exist due to short-range order. Long-range disorder causes tails of localized states, whereas dangling bonds show up around midgap. The dashed curves represent the equivalent states in a crystal. 

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

Recent studies of doped a-Si H have found that the background density of localized states, that is, the electrically active dopants and dangling bond defects, are metastable (Ast and Brodsky, 1979 Street et al., 1986, 1987a Muller et al., 1986). After annealing above 150°C in the dark, the dark conductivity at room temperature of n- and p-type doped a-Si H decreases by nearly a factor of two over a time scale of several weeks for n-type and several hours for p-type a-Si H. As shown in Fig. 9 (Street et al., 1987a), the relaxation rate of the occupied band tail density nBT is a sensitive function of temperature, so that the time to reach... 

As shown in Fig. 2- 3, localized electron levels arise (A and C in the figure) near the band edges at relatively high state densities tailing into the band gap these are called diffuse band tail states. Further, localized electron levels may occur due to dangling bonds and impurities (B in the figure) in the band gap, which are called gap states. 

For block copolymers in which one block acts as anchor and the other block dangles into bulk solution, the conformation of the adsorbed polymer was readily determined due to the characteristic molecular structure. It was shown that the conformation is either tail-train or tail-train-tail, depending on whether the block copolymer is AB or ABA, and that the tails are more elongated than the end-to-end distance of the block chain in bulk solution. 

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