Metal deposition under ultrahigh vacuum

We will only briefly discuss the surface enrichment model proposed for the Ni-Cu alloy, since it has been reviewed several times elsewhere (38, 39). The phase diagram of this alloy has a miscibility gap. Figures 1 and 2 show the results of two experiments, which demonstrate composition differences between bulk and surface in Ni-Cu alloys (4a, 4b). The alloys are films deposited under ultrahigh vacuum. After sintering the binary systems all have nearly the same work function, despite the fact that the overall Cu Ni ratio in the copper-rich system is about four times that of the metal-rich system. 

In the present study we have used a thin, well-ordered atomically flat alumina film grown on a NiAl(llO) single crystal surface as a model support [27]. The atomic arrangement within line defects of this film have recently been investigated [28]. In addilion, Ihere exists a proposed structure of the film based on X-ray diffraction data which is controversially debated at the moment [29]. The structure and size of the oxide-supported metal particles were controlled utilizing nucleation and growth of vapor deposited metal atoms under ultrahigh vacuum conditions. 

Several striking examples demonstrating the atomically precise control exercised by the STM have been reported. A "quantum corral" of Fe atoms has been fabricated by placing 48 atoms in a circle on a flat Cu(lll) surface at 4K (Fig. 4) (94). Both STM (under ultrahigh vacuum) and atomic force microscopy (AFM, under ambient conditions) have been employed to fabricate nanoscale magnetic mounds of Fe, Co, Ni, and CoCr on metal and insulator substrates (95). The AFM has also been used to deposit organic material, such as octadecanethiol onto the surface of mica (96). New appHcations of this type of nanofabrication ate being reported at an ever-faster rate (97—99). 

In typical surface science experiments as presented previously, oxide-supported metal nanoparticles are deposited onto a clean oxide surface by physical vapor deposition. The precursor in this process is metal atoms in the gas phase, which impinge on the surface, diffuse until they eventually get trapped (either at surface defects or by dimer formation), and then act as nuclei for the growth of larger particles. These processes are well understood for ideal model systems under ultrahigh vacuum (UHV) conditions [56, 57]. In contrast, most realistic supported metal catalyst... 

When calcium is deposited by vapor deposition upon clean surfaces under ultrahigh vacuum conditions, calcium diffuses into the near surface region, donates electrons to the ir-system, and forms Ca " ions. The interfacial region between the Ca-metal contact and the polymer has an approximate scale in the range of 20 to 30 A [189]. 

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