Epitaxial crystallization crystals

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

Epitaxial crystallization was accomplished by the immersion of a preheated substrate (the substrates for epitaxial crystallization were single crystals of alkali halides from Harshaw Chemical Co.) into the monomer solution followed by its in-situ cleavage to expose two fresh (100) surfaces. 

Epitaxial thin films, 24 742 Epitaxy, 22 152, 185. See also Epitaxial growth Heteroepitaxy in FET fabrication, 22 163-164 in HBT fabrication, 22 166, 167 in RTD fabrication, 22 170 silicon purification via, 22 496 197 vitreous silica in, 22 442 Epitaxy crystallization, ion-beam-induced, 14 447-448... 

Ion-beam-induced epitaxy crystallization (IBIEC), 14 447-448 Ion-beam lithography... 

Orent d cylinder formation and epitaxial crystal azation... 

Epitaxially grown thin films of vapor-deposited organic compounds and defect structures in these films have been studied by electron diffraction and high-resolution imaging with TEM [23]. Here, epitaxial crystallization of polymers is briefly described. 

In this system, the fomiation of CdS is strictly limited to the surface substrates so that no influence to the film growtn can be considered to result from the solution phase reaction. Deposition of nanosized epitaxial dots on Au(l 11) has been successfully achieved by using this strategy.40,41) However, long-range epitaxial crystal growth seems to be difficult, probably due to the very low solubility of CdS in DMSO... 

Analysis of Helical Hand Selection in Epitaxially Crystallized Films. 23... 

Epitaxial Crystallization of Isotactic Poly(l-Butene), Form I. 26... 

Epitaxial Crystallization of Syndiotactic Polypropylene, Isochiral Form II. 28... 

Epitaxial crystallization of polymers has been investigated for a wide variety of substrates minerals (alkali halides, talc, mica, and so on), low molecular weight organic materials (condensed and linear aromatics, benzoic acid and many of its substituted variants and their salts or hemiacids, other organic molecules of different types), and other crystalline polymers. 

Epitaxial crystallization utilizes some form of lattice (dimensional) and structural (surface topographies) matching of the deposit (usually the polymer) and the substrate crystal. This matching has been demonstrated in a number of ways in the field of polymer crystallization [18] and only the general rules are (briefly) recalled here. 

For polymers that can exist in different crystal structures based on different chain conformations, epitaxial crystallization can induce these various crystal structures (it can impose different chain conformations). The most... 

Epitaxial crystallization of helical polymers may involve three different features of the polymer chain or lattice. These are (a) the interchain distance (as for stretched out polymers), (b) the chain axis repeat distance, and (c) the interstrand distance - the distance between the exterior paths of two successive turns of the helix. The two former periodicities are normal and parallel to the chain axis direction, and are therefore not usually sensitive to the chirality of the helix (unless the substrate topography is asymmetric and favors a given helical hand). However, the interstrand distance is oblique to the helix axis (it is normal to the orientation of the outer chain path) and therefore has different, symmetric orientations relative to the helix axis for left-handed and right-handed helices (Fig. 2). In other words, epitaxies that involve the interstrand distances are discriminative with respect to helix chirality. This discrimination becomes visible if the crystal structure is based on whole layers of isochiral helices. Such a situation does indeed exist for isotactic poly(l-butene), Form I, that will be considered soon. 

The second and third illustrations of epitaxial crystallization deal with syndiotactic polypropylene (sPP). Syndiotactic polymers are by design susceptible to forming either right-handed or left-handed helices, and are therefore suitable materials in the present context of helical hand selection. 

Both Form I (considered later) and Form II can be obtained by epitaxial crystallization. At atmospheric pressure, the unstable, chiral Form II can be forced to crystallize by using an appropriate substrate, namely 2-quinoxalinol, as assessed by the electron diffraction pattern (Fig. 5a) (no AFM images are available) [32], The contact plane is found to be (110)F... 

Fig.6 (a) AFM image of the be contact face of sPP in its antichiral Form I, epitaxially crystallized on p-terphenyl. Note the strict alternation of right- and left-handed helices (helix axis at one o clock). Left-handed helices are better resolved than right-handed helices. Reproduced from [33] with permission (b) Fourier-filtered image of the upper left area of part (a) showing more clearly the right-handed helices that alternate with the left-handed ones. Reproduced from [33] with permission (c) The surface structure of p-terphenyl used as a substrate for the epitaxial crystallization of syndiotactic polypropylene (parts... 

The first layer epitaxially crystallized on a substrate of a different nature has many or all of the features of a normal layer produced in bulk crystallization. AFM imaging of the first layer provides the most direct approach that can be imagined to visualize the outcome of the crystallization pro-... 

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