2D-ciystal

Next to the m-interactions, the arrangement of alkyl chains along the axis of symmetry of the graphite seems to be crucial. While over the years chemists have accumulated ample empirical evidence on the formation of the 3D structures in single crystals, the knowledge of 2D-ciystal formation, i.e of monolayers on surfaces, is still in its infancy. We have seen that, reducing the symmetry of hexaalkyl HBCs by going to a functionalized bromoderivative, such as... 

The kinetics by which UPD layers form are qualitatively the processes already discussed. There are the electron transfer kinetics from the metal substrate to the depositing ion and the surface diffusion of the adions formed to edge sites on terraces. Complications occur, however, for there is the adsorption of ions to take care of and that brings up questions of which isotherm to use (Section 6.8). Three kinds of UPD formations are shown in Fig. 7.146. Thus Fig. 7.146 (c) shows ID phase formation along a monatomic step in the terraces on the single ciystal Fig. 7.146 (b) shows 2D nucleation at a step, and Fig. 7.146 (a) shows 2D nucleation on an atomically flat plane. 

In the X-ray pattern for "HCOOH sol" Hf02 films fired at temperatures below 560 °C, only halo patterns representing the amorphous state were observed. At 560 °C [Fig. 12(a)], a small diffraction peak was observed at 2d = 28° in the halo pattern. At 570 °C, the diffraction peak at 26 = 28° became clearer and higher, indicating that partial crystallization from the amorphous state commenced at 560 °C. The observed diffraction peak was identified to correspond to monoclinic (111) (JCPDS card) and full ciystallization was attained at 700 °C. 

TPR-profiles are shown in Fig. 2 for samples with various copper contents. Note that all copper of calcined samples is quantitatively reduced to Cu below 523 K in 5% HjAr. Increasing the calcination temperature results in a shift of the Tu -values of the reduction profiles to lower temperatures (Fig. 2d,e). The small peak at about 740 K occurring with all samples, except the one calcined at 923 K, was accompanied by a measurable exothermicity and is attributed to the ciystallization of amorphous zirconia. Pure uncaldned zirconia support yielded also a small peak at 718 K, followed by two broad peaks at 793 and 913 K, while a calcined sample (723 K) did not show the ciystallization-peak at 718 K. Uncalcined samples resulted in TPR profiles which depended on the nature of the precursor salts used for preparation. While pure nitrate precursors showed a single reduction peak at 500 K (Fig. 2f), samples prepared in the presence of acetate-ions resulted in TPR profiles consisting of three peaks at about 500, 567 and 623 K (Fig. 2b,c). We conclude that the reduction peaks at 500 and 567 K are attributable to the reduction of CuO, whilst the peaks at 623 K are due to the reductive decomposition of acetate-modified zirconia. Calcination at 623 K led to the disappearence of the reduction peak at 623 K and to a shift of the peaks at 500 and 567 K to lower temperatures (Fig. 2d). 

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