Surface mobility, metallic films

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

The majority of deposits formed in this group have been on Au electrodes, as they are robust, easy to clean, have a well characterized electrochemical behavior, and reasonable quality films can be formed by a number of methodologies. However, Au is a soft metal, there is significant surface mobility for the atoms, which can lead to surface reconstructions, and alloying with depositing elements. In addition, Au it is not well lattice-matched to most of the compounds being formed by EC-ALE. 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

The microstructure and morphology of thick single-phase films have been extensively studied for a wide variety of metals, alloys, and refractory compounds. Structural models have been proposed (12,13). Three zones with different microstructure and surface morphology were described for thick (tens of micrometers) deposits of pure metal. At low temperature (< 0.3 Tm ), where Tm is the melting point (K) of the deposit metal, the surface mobility of the adatoms is reduced, and the deposit was reported to grow as tapered crystallites. The surface is not full density (Zone 1). At higher substrate temperature (0.3-0.45 Tm ), the surface mobility increases. The surface... 

Reduction of thin CU2O layers reproduces smooth terraces of the Cu surface, however, with multi-atomic steps in between. This result is further proof for the high surface mobility of Cu atoms on pure Cu(lll) terraces. Reduction of the thicker duplex film yields a rougher metal surface. During the rearrangement of several atomic layers smooth large terraces cannot be achieved. 

For ill-designed composite membranes, for example, formed by depositing palladium onto substrates which it does not wet, surface tension will force the thin film to contract and ball up if the palladium atoms acquire sufficient surface mobility. Pinholes may form as a prelude to complete de-wetting, or pinholes may remain from the initial fabrication if the palladium did not fully wet its substrate. Kinetics of de-wetting is accelerated at elevated temperature and in the presence of adsorbates such as CO, which increase surface mobility of Pd. If molten metals do not wet ceramics, they will be expelled from ceramic pores. During sintering of cermets, Pd and other metals will not adhere to the ceramic phase, if the metal and ceramic do not wet. 

Surface mobility can explain the experiments of Ditch-bum (32) on the deposition by sputtering of cadmium films on glass and metal surfaces. He found that when the cadmium particles were directed on to the surfaces through fine slits, no deposit could be observed if the width of the slit was below 5 X 10 mm., a phenomenon which he attributed to a loss by surface diffusion of particles which could not be sufficiently rapidly replaced by condensation. 

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