Single crystals disorder

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

LEED is the most powerfiil, most widely used, and most developed technique for the investigation of periodic surface structures. It is a standard tool in the surface analysis of single-crystal surfaces. It is used very commonly as a method to check surface order. The evolution of the technique is toward greater use to investigate surface disorder. Progress in atomic-structure determination is focused on improving calculations for complex molecular surface structures. 

Usually, the collection of LEED 7-Vcurves requires single crystal surfaces with long-range order in the upper-most layers. Structural information can, however, also be obtained in a similar way for certain disordered surfaces, when the energy dependence of the diffusely scattered intensity is analyzed (diffuse LEED [2.264, 2.265]). 

Figure 6 Theoretical and experimental results for an ordered and a disordered system on the left theoretical data for a CusPt single crystal and on the right experimental findings for a CusPt alloy overlayer on a platinum substrate. The spectra for the ordered species are traced with continuous lines, whereas those for the disordered species are given in dotted lines. The spectra were normalized to equal peak height.

We Finally note that the MTR model is a priori more appropriate to disordered materials. It is not expected to give good results with single crystal OFET, especially when the mobility becomes temperature-independent (see Section 14.6.1.2). However, it has recently been invoked in the case of poly thiophene [112], the mobility of which is also thermally activated. 

Small needle-shaped single crystals were examined by transmission electron microscopy (TEM) and electron diffraction (ED) (see Fig. 16-17). The results show that the crystals are elongated along the b-axis, which is the direction of weak intermolecular n-n interactions, and have a well-developed (ab) top surface. It corresponds to the surface of aliphatic tails (direction of weak intermolecular interactions). There are indications of displacement of successive ( / )-laycrs along the fl-axis, in line with the other signs of disorder in the aliphatic layer. 

As an example. Fig. 8.1 shows a result from Bardi and co-workers obtained on a bimetallic AusPdClOO) single-crystal alloy [Kuntze et al., 1999]. The LEED pattern indicates a sharp (1 x 1) unit cell that corresponds to the bulk-tmncated stmcture of the substitutionaUy disordered Au3Pd alloy. Additionally, the authors determined the composition of the first outermost layer to be pure Au. These findings revealed that the (100) oriented surface of Au over AusPd alloy is not reconstmcted, which is unique, since pure Au, Pt, and Ir (100) crystals are all known to be reconstmcted in similar ways [Van Hove et al., 1981 Ritz et al., 1997]. In this case, the presence... 

The structural picture that was envisaged to represent the temperature-dependent fluctuations of the EFG tensor [15] is based on the X-ray structure of MbOa that exhibits a geometric disorder of Fe02 with two different positions of the terminal O-atom [28]. Within this stmcture, the projection of the 0-0 bond on the heme plane is rotated by about 40° in position 2 compared to 1 (Fig. 9.10). Conventional Mossbauer studies of single crystals of Mb02 have shown that the principal component of the FFG tensor lies in the heme plane and is oriented along the projection of the 0-0 bond onto this plane [29]. If the terminal O-atom is located in position 2, the EFG should be of the same magnitude as in position 1, but its orientation is different. The EFG fluctuates between positions 1 and 2 with a rate that depends on temperature. 

VII. Controlled Crystallographic Disorder in [Tp1 ]MX Complexes Bond Length Artifacts as Determined by Single Crystal X-Ray Diffraction... 

In order to study the effect of disorder between two atoms, single crystals composed of pairs of the complexes [TpBut]ZnCl, [TpBut]ZnI, and [TpBut]ZnMe, over a range of compositions, were studied. In each case, only a single atom was observed at the disordered site, and the Zn-X bond length that was measured corresponded to a composite for the... 

Atomic thermal parameters derived from single-crystal X-ray diffraction, which increase with increasing disorder and defects in the crystal [1]... 

Among crystalline solids, typical second-order transitions are associated with abrupt intermolecular conformational, rotational, and vibrational changes and/or with abrupt changes in crystalline disorder and/or defects [7], These changes in crystalline solids are sometimes difficult to assign without the use of appropriate spectroscopic techniques such as solid-state NMR or a diffraction procedure such as single-crystal X-ray diffraction. 

In the study of the M dependence of I and V of extended chain single crystals (FCSCs), samples were isothermally crystallized from the melt into the hexagonal (= disordered mobile ) phase at high pressure (P = 0.4 GPa). The range of AT was 3.3-9.4 K. 

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