Intercrystalline Gaps Research Articles

On the porosity of coldly condensed sers active Ag films

Abstract In this paper we report on the geometric structure of Ag films, deposited under UHV conditions and annealed at temperatures ( T an ) ranging from 58 to 430 K, as deduced from UPS, AES, TDS and work function measurements of adsorbed xenon. The macroscopic work function of the bare films increases continuously from 4.25 eV ( T an = 60 K) to 4.72 eV ( T an = 330 K). Evidence is provided that coldly deposited Ag films are highly porous and that the pores persist up to T an = 170 K, but are irreversibly healed between 170 and 250 K. The minimum thickness of the evaporated Ag films needed to develop these pores is found to be 50 A. The width of the pores, which are most likely intercrystalline gaps, is estimated to be 5–15 A. Besides the“macroscopic” pores the films contain atomic scale defects, which, in contrast to the pores, are healed continu- ously with increasing T an . Films annealed at 330 K are composed of (111) grains with still a few percent of defect sites. The implications of these structural features on the adsorption properties of pyridine as well as on the interpretation of SERS results from such Ag films are dealt with in part II of this work.

Structure and properties of evaporated metal films

Metal films particularly of nickel and tungsten, deposited on glass have been studied by electron microscopy in transmission and by replication, and by the measurement of gas adsorption and electrical resistance. From the transmission electron micrographs crystal widths have been obtained and the presence of gaps between the crystals established for nickel films examined as deposited at 273 °K. On sintering, the first stage is the removal of intercrystalline gaps while in later stages surface asperities are removed and crystal growth occurs. The effect of sintering on gas adsorption and electrical resistance has been studied. From the extinction contours on individual nickel crystals in the transmission electron micrographs the detailed crystal shape has been deduced. Films deposited when the substrate was at an elevated temperature showed an area for gas adsorption considerably below that for films which had been sintered at that temperature subsequent to deposition at 273 °K.