Using scanning tunneling microscopy and spectroscopy we have investigated the growth of a jmonolayer Ag film on Si(l 11)7x7 at substrate temperatures of 90 and 130°C. An Ag-induced subunit has been identified on top of three dangling bonds of the second atomic layer, which may be considered as the critical nucleus of Ag condensation. At 130°C Ag mostly condenses in the form of triangularshaped flat islands of equal size, which are preferentially located on faulted halves of the 7x7 unit cells and are explained by a close-packed structure of Ag atoms. On the Ag islands the local density of states near the Fermi level is reduced compared to the uncovered 7x7 unit cells. PACS numbers: 61.16.Di, 68.35.Bs In contrast to numerous investigations of Si(lll)V3 x VlR30 o -Ag l not much work has been devoted to the initial stage of Ag condensation on a low-temperature Si(lll) substrate. Low means in this context that the substrate temperature is not yet sufficient for the V3 x VJ phase transition, which is observed for temperatures above 500 °C and an Ag coverage (0) of at least j monolayer (ML). 2 Previous studies have already established the general nature of the growth process of Ag on Si(l 11)7x7. Room-temperature deposition leads to a layer-by-layer growth mode up to 0 of a few ML according to the nearly exponential dependency of the Si L2,yW and Ag M4W Auger intensities. 3 For 0 = 10 ML, the angle-resolved photoemission results could be explained by the growth of an ordered overlayer structure in the form of Ag(l 11) domains. 3 Additional information on the general growth process of this system may be inferred from electron microscopic work. 4 In the present Letter we report on a scanning-tunneling-microscopy (STM) study of the initial stages of Ag film growth on Si(l 11)7x7. The experiments have been performed at substrate temperatures (T SU b) of 90 and 130°C. As the first step of Ag condensation we have observed ringlike structures on the inner adatoms of both halves of the 7 x 7 unit cell, which show a triangular distortion and indicate bonding of the Ag atoms to the dangling bonds of the second atomic layer. For r su b = 130°C we find that the Ag deposit forms triangular and essentially two-dimensional islands with a closepacked arrangement of Ag atoms. These islands, as well as the subunits, preferentially occupy halves of the 7x7 unit cells where according to the dimer-adatomstacking-fault (DAS) model 5 a stacking fault is present. On the uncovered parts of the surface we observe the usual 7x7 reconstruction of clean Si(lll). The Ag islands still reflect details of the 7 x 7 reconstruction, which means that the 7x7 structure is preserved on the entire surface. The shape and the size of the Ag islands indicate a very weak or even repulsive interaction of Ag atoms with those substrate sites where according to the DAS structure the dimers of the second atomic Si layer are located. On the Ag islands we observe a reduction of the local density of states both above and below the Fermi level EY compared with the ad...

By means of X-ray investigations the structure of the SE and S,-phases of the title compounds has been studied. The lattice parameters of the orthorhombic S , structure are given. The thickness of the smectic layers is nearly the same in the SE and S, phases and agrees well with the length of the molecules in their most stretched form. That suggests a similar conformation of the alkyl chains in both phases. A model of the packing in the SA phase is discussed which IS based on a dense packing with antiparallel orientation of molecules with a non-symmetrical shape.Mit Hilfe von rontgenographischen Verfahren wurde die Struktur der SE und S , Phasen der genannten Verbindungen untersucht. Die Gitterparameter der orthorhombischen Struktur in der SE Phase werden angegeben. Die Dicke der smektischen Schichten in der SE und S, Phase ist nahezu gleich und stimmt gut mit der Lange der Molekule in ihrer gestreckten Form iiberein. Dieses Ergebnis legt eine ahnliche Konformation der Alkylketten in beiden Phasen nahe. Es wird ein Packungsmodell fur die SA Phase diskutiert, das auf einer dichten Packung antiparallel orientierter Molekiile von nichtsymmetrischer Form basiert.

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