Poly crystals from thin films

The substituted five-ring OPVs have been processed into poly crystal line thin films by vacuum deposition onto a substrate from the vapor phase. Optical absorption and photolumincscence of the films are significantly different from dilute solution spectra, which indicates that intermolecular interactions play an important role in the solid-state spectra. The molecular orientation and crystal domain size can be increased by thermal annealing of the films. This control of the microstruc-ture is essential for the use of such films in photonic devices. 

Poly(TCDU) in thin film exists in the acetylenic phase 2, as opposed to the butatriene phase 1 found in bulk crystals polymerized from macroscopic monomer crystals (7). Polymerization to completion in thin film, thereby removing the constraint of the monomer lattice, could account for the acetylenic phase. 

Reticulate composites comprising needles of TTT-TCNQ (tetrathiotetra-cene-tetracyaiioquinodimethane) in poly(bisphenol-A-carbonate) were first described in the early 1980s. The composites were prepared by casting thin films from a solution of the components in o-dichloromethane. The composite contained a network of fine interconnected needle crystals of the CT salt and had a percolation threshold of 2-3 mass% of the CT salt, as shown in Fig. 8.22. 

From a materials perspective, the field of semiconductor electrochemistry and photoelectroehemistry has evolved from the use of semiconduetor single crystals to polycrystalline thin films and, more recently, to nanocrystalline films. The latter have been variously termed membranes, nanoporous or nanophase films, meso-porous films, nanostructured films, etc. they are all distinguished from their poly-... 

Polycrystallization of micrometer-order-thick a-Si films by FLA exhibits curious phenomena different from the case of crystallization of thin (<1 pm) a-Si films, and consequently forms poly-Si films with characteristic microstructures. Figure 11.9 shows surface images of the Si films after FLA under various lamp irradiances [35]. Only the edges are crystallized in the case of the lowest lamp irradiance, and the crystallized area expands towards the center of the... 

The liquid crystallinity of poly(3-alkylthiophene) has been pointed out [121]. Enhancement of crystallinity by molecular alignment in a liquid crystal phase is prominent in poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b] thiophene) exhibiting a lamellar mesophase. When a thin film of the polymer fabricated by the spin-coating method is annealed at 100 °C, domain size remarkably extends from several tens of nanometers to a several micrometers, maintaining molecular alignment within domains because of the thermal motion of the polymer chains. The hole mobility of the thin film transistors based on this polymer reached 0.7 cm2 V-1 s 1 [122]. 

The most common sample forms used for the measurement of electrical properties are pressed pellets, thin films, and single crystals. Electrical measurements on pellets are often difficult to interpret because of the presence of polycry staUinity, grain boundaries, large surface area, and unknown amounts of adsorbed gases. Films are useful in some instances, but in many cases also cause uncertainties because of poly cry staUinity. Electrical properties are most easUy and reUably interpreted with single crystals free from unintentional impurities and solvent occlusions. 

In order to visualize how dendritic crystals grow and how a square-shape envelope is formed from such dendritic structures, we have superposed two images from the same crystal taken at an interval of 95 sec (see Fig. 11.8). As we want to focus on single crystals only, we have chosen a rather thin film of about 40 nm in order to avoid growth front nucleation (GFN) [13,14j. GFN depends on the number of molecules present. Thus, in such thin films the transition from poly-crystals to single crystals can occur at lower temperatures than in the 108 nm thick 111 ins discussed above. 

Thakur and Meyler [22] have reported the growth of thin-film single crystals of poly(diacetylenes) by a shear process that produces aligned areas of about 1 cm-from melts and about 50 mm- from solution. These thin-film crystals were reported to be four orders of magnitude larger than those previously prepared by Isoda [23]. 

Fig. 16. (a). Phase contrast optical micrograph of a single crystal of iPKBES formed in thin film at 130°C (b). Transmission electron micrograph of poly(L-hydroxyproline) grown from a 0.1% solution in a 1/9 (v/v) mixture of formic acid and ethanol at room temperature (120). 

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