Formation of sulfur clusters on Re(0001) surfaces observed with the scanning tunneling microscope

We have studied the various structures formed by sulfur on Re(0001) single-crystal surfaces using a scanning tunneling microscope. At coverages of 0.25 monolayer and below, open lattices formed by single adatoms are formed, mostly with a (2\ifmmode\times\else\texttimes\fi{}2) structure. Between 0.25 and 0.5 monolayer, phases consisting of three-, four-, or six-adatom clusters are observed. The sulfur trimers are formed first randomly in the (2\ifmmode\times\else\texttimes\fi{}2) regions and then ordered as their coverage increases to saturation at 0.45 monolayer. In all cases sulfur atoms are adsorbed in threefold hollow sites. The formation of sulfur clusters above 0.25 monolayer is interpreted as evidence of substrate-mediated many-body forces.