Crystalline size effects

Nitta, Y., Kawabe, M., Kojita, H., Imanaka, T. (1986) Crystalline size effect in enantioselective hydrogenation on modified Ni-silica catalysts, Chem. Express, 1,631-634. 

Markovic N, Gasteiger H, Ross PN (1997) Kinetics of oxygen reduction on Pt (hkl) electrodes implications of the crystalline size effect with supported Pt electrocatalysts. J Electrochem Soc 144 1591-1597... 

Band gap engineetring confined hetetrostruciutres. When the thickness of a crystalline film is comparable with the de Broglie wavelength, the conduction and valence bands will break into subbands and as the thickness increases, the Fermi energy of the electrons oscillates. This leads to the so-called quantum size effects, which had been precociously predicted in Russia by Lifshitz and Kosevich (1953). A piece of semiconductor which is very small in one, two or three dimensions - a confined structure - is called a quantum well, quantum wire or quantum dot, respectively, and much fundamental physics research has been devoted to these in the last two decades. However, the world of MSE only became involved when several quantum wells were combined into what is now termed a heterostructure. 

Another shortcoming of the present model is that the MD cell size in the y-axis direction, the direction parallel to the crystalline chain axis, was rather small. There must be serious size effects along this direction, even if we adopt the periodic condition in this direction. Here again we need a much larger MD cell in order to reproduce the lamella thickness vs. temperature relation in crystallization from the melt. 

The performance of semiconducting inorganic materials as a photocatalyst depends much on the physical properties of the particles such as crystallinity, size and morphology. The effect can be even more pronounced when using two or more materials together, since an intimate interaction between particles is more important in the multicomponent system (So et al., 2004). 

Polymers such as polystyrene, poly(vinyl chloride), and poly(methyl methacrylate) show very poor crystallization tendencies. Loss of structural simplicity (compared to polyethylene) results in a marked decrease in the tendency toward crystallization. Fluorocarbon polymers such as poly(vinyl fluoride), poly(vinylidene fluoride), and polytetrafluoroethylene are exceptions. These polymers show considerable crystallinity since the small size of fluorine does not preclude packing into a crystal lattice. Crystallization is also aided by the high secondary attractive forces. High secondary attractive forces coupled with symmetry account for the presence of significant crystallinity in poly(vinylidene chloride). Symmetry alone without significant polarity, as in polyisobutylene, is insufficient for the development of crystallinity. (The effect of stereoregularity of polymer structure on crystallinity is postponed to Sec. 8-2a.)... 

Sears and Schulman (51) measured surface pressures and potentials vs. molecular area (20 to 110 sq. A. per molecule) for the alkali metal stearates over 0.5N solutions of LiOH, NaOH, and KOH. Like Adam and Miller, they detected a specific cation effect on the 7r-A and AV-A isotherms of the ionized monolayer at high pH the ir-A curves were expanded in the order of the crystalline sizes of the alkali metal cations K > Na > Li. The sequence is the reverse observed for the long-chain... 

Crystallinity and disorder are important structural parameters for understanding relationships between structure and physical properties. Flaws and distortions are the main features that limit the ultimate properties of textile fibers. Some of these crazes, cracks and voids are revealed under the electron microscope, either on the surface or in cross sections stained with heavy metals (J, 2). However, these staining techniques (that reveal the main morphological features) make it much more difficult to determine the degree of distortion of the crystalline fraction. Theoretically, line profile studies permit separation of effects due to crystalline size from those due to structural distortions. However, the lack of peaks in semicrystalline fiber x-ray patterns hinders that approach. 

Because X-ray powder diffraction deals with solid samples, the analytical variables are different from those associated with the analysis of liquid or solution samples. Principle among these are particle size effects, uniform sample surface, crystallinity and X-ray absorption. Although particle size and a non-uniform sample surface are serious problems, their... 

Table V shows the X-ray parameters obtained from the cokes by two different calcining methods. X-ray diffractometry of interlayer spacing (doo2) and apparent crystalline size (Lc) indicated no particular difference between cokes processed under the two methods. Furthermore, no specific differences were found on these samples after graphitization, These facts show that the new calcining method would not lead to adverse effects on the development and rearrangement of coke crystallines.

Crystalline size and morphology, through their effects on stress in the glass or nucleation sites for hydrated phases. 

Furthermore, we have shown that oxidation in air can also be used to control the average crystal size in ND powders with subnanometer accuracy. Three different characterization techniques were used for measuring the crystal size because such analysis is very complex for nanocrystals. While HRTEM is able to visualize ND crystals, the calculated size distributions are statistically not reliable and the average size is often overestimated. Agglomeration and difficulties in sample preparation do not allow accurate estimates on average crystal size values. XRD, which directly probes the crystalline structure of a material, is more reliable in terms of statistics and average values, but lattice distortion and strain can interfere with size effects in XRD pattern and lead to an incorrect interpretation of the results. 

A. G. Cullis and L. T. Canham, Visible light emission due to quantum size effects in highly porous crystalline silicon. Nature 3S3, 353, 1991. 

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