Formation and diffusion of S-decorated clusters on Cu(111) Page: 4 of 20
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For Physical Review Letters, 3/13/00
This result shows that S can be a "skyhook," weakening the bonds of Cu ada-
toms to the rest of the substrate and promoting their transport. An effect of this kind
has been proposed for H on metals (M), through formation of HM dimers,3 but how
divalent S might act as a "skyhook" has not been known till now.
Because close-packing means better coordinated first-layer atoms, self-adsorp-
tion costs more energy on closer-packed surfaces. Cu/Cu(111) obeys this rule. The
present Density Functional Theory4 (DFT) calculations, based on the Generalized
Gradient Approximations (GGA), say that 0.79 eV is needed to form a Cu adatom
on Cu(111). The same logic that predicts this large Eform(ad-Cu) suggests that the
self-diffusion barrier on Cu(111), Ediff(ad-Cu), should be small. The present find-
ing, Edfgi(ad-Cu)= 57 meV, again agrees.
These results constrain ideas of how S promotes Cu-transport at 300K. E.g,
since little can be gained by lowering a barrier close to 2kBT (=51.7 meV), S must
act by increasing the concentration of diffusing adspecies. However, S does not act
by reducing the barrier to dissociating Cu atoms from island edges onto terraces.
On geometric grounds, this barrier must be close to Eform(ad-Cu) + Ediff(ad-Cu) =
0.85 eV (expt.6 = 0.78 0.04 eV), i.e., again only -57 meV larger than the mini-
mum needed to produce an ad-Cu on a terrace.
S impurities must therefore promote island-decay by forming tightly bound, and
correspondingly plentiful CunSm ad-species that diffuse easily.7 This, however,
raises a general issue: What complex with a divalent impurity can significantly
enhance metal adatom transport? For S/Cu(111), I show that ad-Cu3S3 is a good
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FEIBELMAN,PETER J. Formation and diffusion of S-decorated clusters on Cu(111), article, March 21, 2000; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc703777/m1/4/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.