Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

184 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSAlthough most of the work has been done on polycrystalline porous films, morerecent research has shown ways of producing single crystal filmsSZ which are welldefined and exhibit diffraction patterns characteristic of particular faces. CarefuIchoice of substrate, substrate temperature and evaporation rate are required. Ingeneral single crystal films are most easily formed on substrates which are themselveswell defined crystals, e.g. silver on mica53, and germanium on calciumfluorideS4.(d) Cleavage and crushing of crystalsThe generation of a clean surface in vacuo by crushing large crystals is a relativelysimple technique, provided that the material fractures easily. Impurity layers arenot removed from the original surface, but this can be neglected by comparisonwith the new surface exposed. Clean surfaces of germanium have been producedin this manners6, but the technique has not been widely applied. The avoidanceof high temperature heating reduces the fikelihood that impurity will be sufficientlymobile to diffuse to the surface.Crushing of a crystal produces a great many crystal planes. A single crystalplane can more readily be produced by cleavage, although the totaI clean surfaceproduced is normally very small, of the order of a few square millimeters. Cleavageto obtain a specific crystal plane requires the use of a single crystal whoseorientation is accurately known. Most materials will onIy cleave readily aIongcertain crystal planes, thus limiting the number of different faces which can bestudied. The cleavage can be obtained by forcing a wedge into a premachined slotin the cry~tal~~.~~or by applying a bending forceS8. Surfaces produced in this wayare not always smooth and often contain a large number of steps58. Examples ofmaterials which have been cleaved to generate clean surfaces are given in Table 2.(e) Ion bombardmentThe method of ion bombardment to remove surface layers was introduced byTABLE 2GENERATION OF CLEAN SURFACES BY CLEAVAGEMaterial Faces produced ReferencesInSbGaAsBiTeGeSiNaCI, KCIMicaFeMi30Pyrolytic graphite5958, 595959, 60585761626364