Interfacial control

Some of the earliest approaches to interfacial modification for organic semiconductors included results from the Penn State group involving vapor-deposited silanes [32], a material of choice in traditional semiconductor processing, because of ease of deposition. 

Phosphonic acid based SAMs [7c], which bind preferentially to alumina surfaces, can be prepared similarly, but it was also found experimentally that spin-coating a solution of 0.1% (w/w) SAM from a solvent such as toluene, followed by baking for 10-30 min on a hot plate at 100-120 °C, then a good rinse in dean toluene, and blow dry produced superior results in terms of phosphonic acid-SAM quality compared with overnight soaking in the same solution. This may be because of 

Self-assembly of SAMs Spin coating of SAMs Spin coating of polymers 

Polymeric surface treatments are applied in similar fashion, namely spin-coating from a low-concentration solution of the polymer in organic solvent, e.g. 0.1% (w/w) polymer in toluene. Spin conditions can vary widely but, as reported by 3M [30a], 500 rpm for 20 s then 2000 rpm for 40 s provides 100 A thick dry polymer films on smooth dielectrics with good surface quality and contact-angle uniformity. Spun layers are typically air-dried for several hours, or dried at an elevated temperature of 110-130 °C for 30 min in an oven or 5 min on a hot plate. These materials are typically not rinsed after they are baked, because the homopolymer version would redissolve, removing almost all of the polymer. In a recent report from Fris-bie s group, in collaboration with 3M, device oxides were intentionally roughened 

The Infineon group reported a pure polymer dielectric which has also been shown to improve pentacene performance, both as an unmodified polymer, and with a silane treatment performed on it [7b]. In addition, Samsung SDI has recently reported a proprietary polymer dielectric which enables them to achieve high mobility in pentacene TFTs [12]. Table 2.2 emphasizes some of the reports of increasing pentacene mobility. 

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Molecular modeling techniques, augmented by careful measurements of the stmcture of the interfacial regions, hold promise for elucidating details of these three aspects of interfacial control of matrix polymerization. 

AH Goldberg, WI Higuchi. Mechanisms of interphase transport II Theoretical considerations and experimental evaluation of interfacially controlled transport in solubilized systems. J Pharm Sci 58 1341-1352, 1969. 

All this material about the influence of the rate of transport in a solution, and how if it is too slow it influences the overall current density observed, has again used an extreme situation—the opposite of the diffusion-free picture given in earlier sections. In this section a relation is deduced that connects the diffusion control to the interfacial control. It is... 

Usually one tries to emphasize the one (interfacial) control or the other (diffusion). Further Reading Seminal... 

P. Delahay, New Instrumental Methods in Electrochemistry, Interscience, New York (1952). Contains much seminal work showing the evolution away from the pure diffusion control and the introduction of activation overpotential. i.e.. interfacial control. 

Correspondingly, the electroanalytical chemist frequently refers to electrode processes having a finite rate as reversible when they have moved only a small degree away from equilibrium. This happens as a result of the competition between iL and ip, the diffusion-controlled limiting current and the interfacially controlled Faradaic current. 

In the initial steep ascent part of the current density then, one is just viewing a part of a normal i — r) curve under interfacial control, the beginning of the normal exponential (Tafelian) relation of current to overpotential. At such low times, 8f is small and hence iL is too large to have an observable effect. 

The summary given by this author rests on the work of several theorists who followed the work of Sevcik. Among the most outstanding of these is Paul Delahay who. with Strassner and others in 1951 1953 contributed much to the basic theory of linear sweep voltammetry with partial interfacial control. Students interested in programs for such simulations should contact Prof. David K. Gosser, Chemistry Department, City College of New York, NY, 10031. 

Asymptotic ripening Adatom emission, diffusion, capture diffusion or interfacial control Universal CSD d as f(t). Analytical compare CSD or d with experiment to distinguish between diffusion or interfacial good for long times, sintering. Supersaturation must not be too large valid only after large process times needd ... 

Multiatom migration/ coalescence/ emission Crystallites of all sizes move, contacting crystallites or adatoms coalesce, crystallites and adatoms emitted diffusion or interfacial control. Reduce to KR or NMC models show effects of loading, solubility, emission, and diffusion good f( sintering or redispersion. Numerical computation elaborate multiatom emission very slow. 

The understanding of the interfacial control of crack tip plasticity for systems where the dissipation is no longer confined to a localized cavitational zone but is diffuse or away from the interface. 

Although the emphasis here will, by necessity, be placed on more recent data, several key reviews of transport in nanocrystalline ionic materials have been presented, the details of which will be outlined first. An international workshop on interfacially controlled functional materials was conducted in 2000, the proceedings of which were published in the journal Solid State Ionics (Volume 131), focusing on the topic of atomic transport. In this issue, Maier [29] considered point defect thermodynamics and particle size, and Tuller [239] critically reviewed the available transport data for three oxides, namely cubic zirconia, ceria, and titania. Subsequently, in 2003, Heitjans and Indris [210] reviewed the diffusion and ionic conductivity data in nanoionics, and included some useful tabulations of data. A review of nanocrystalline ceria and zirconia electrolytes was recently published [240], as have extensive reviews of the mechanical behavior (hardness and plasticity) of both metals and ceramics [13, 234]. 

FIGURE 9.7 Variation of radius with time for a drop initially containing 80 mol% -decane and 20 mol% squalane following injection into a 2.5 wt% solution of CijEg at 23°C. The solid and dashed curves represent theoretical predictions based on interfacially controlled and diffusion-controlled mass transfer, respectively. (From Pena, A.A. and Miller, C.A., J. Colloid Interface ScL, 244, 154, 2001. With permission.)... 

Here a is the drop radius, k a specific solubilization rate determined experimentally, c, the concentration of surfactant in micelles, and 0g and 9 the ratios of concentrations of the soluble species in the bulk and at the interface to the equilibrium solubilization capacity at c,. This equation for interfacially controlled transport is the counterpart to the well-known von Smoluchowski equation for diffusion-controlled transport ... 

The analysis was extended to predict mean drop size evolution for mixed emulsions consisting initially of some drops of pure decane and some of pure squalane. Its predictions based on interfacially controlled transport were in better agreement with the experimental results of Binks et al. than were those of the authors model, which was based on diffusion-controlled transport. 

Fig. 2. Solute distribution and transport phenomena at the interface of a growing crystal (a) Instability of the crystal-liquid interface and formation of a nonplanar pattern (schematically), (b) Faceted growth. It is assumed that the solute concentration in the liquid far from the interface (Cq) is constant due to forced and natural convection (stirring) whereas a thin solute diffusion layer (S) is quiet and possesses a solute distribution profile depending on the crystallization process type (a) interfacial control or a surface reaction (interfacial kinetics), Ce < C RJ C a difference between Cj and Cj is responsible for the driving force to buUd up the crystal surface (c) diffusion control Cj < Cj, providing a driving force for bulk diffusion in the liquid (b) mixed control.

While acid corrosion in glass fibers is diffusion-controiied and therefore /f-kinetics are expected, the process in aqueous and aikaiine soiutions is considered much more complicated because of the many influencing factors. The reaction kinetics depends on the (local) pH value. It is conventional opinion that, with the switch from a diffusion-controlled corrosion mechanism to an interfacial-controlled mechanism, a rapid shift from ft-to t-kinetics takes place, and the process follows linear t-kinetics except for short exposure times and low temperatures. However, in the literature, dependencies on t are also found, with values for a varying between 0.5 and 1 [819]. The chemical stability of glass fibers under alkaline attack is also significantly influenced by insoluble corrosion or reaction products on the fiber surface. 

K. Yoshida, Y. Aoyagi, H. Akimoto, T. Yano, M. Kotani, T. Ogasawara, Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties. Compos. Sci. Technol. 72 (2012) 1665-1670. 

K. Yoshida, Application of electrophoretic deposition for interfacial control of high-performance... 

Application of Electrophoretic Deposition for Interfacial Control of High-Performance SiC Fiber-Reinforced SiC Matrix (SiCf/SiC) Composites... 

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