LOW-COVERAGE SULFUR INDUCED RECONSTRUCTION OF NI(111)

STM, LEED and AES data are reported for S segregated on Ni(lll) betwee n 675 and 1175K. Two chemisorbed phases corresponding to different cov erages (root 39 for Theta(s)= 0.22 ML and (5 root 3 x 2)rect for Theta (s)= 0.40 ML) have been observed which are identical to phases produce d by adsorption and post-annealing of H2S. STM data obtained for the ( 5 root 3 x 2)rect-S phase con the missing row reconstruction of the fi rst Ni plane in this phase. For the root 39-S phase, STM and LEED data give evidence of reconstruction of the topmost Ni plane to generate 4 -fold coordinated S adsorption sites. Two models are proposed, one wit h a (100) arrangement of the first Ni plane to give an overstructure s imilar to that of the Ni(100)-(2 x 2)-S phase, the other with a clockw ise-anticlockwise reconstruction of the (100) arrangement of the first Ni plane similar to that reported for C and N adsorption on Ni(100). STM data provide direct evidence of a preferential formation of the ro ot 39-S phase at substrate monoatomic steps, on the upper terraces. Nu cleation takes place preferentially at step edges where the lower coor dination of Ni atoms allows an easier reconstruction of the upper terr ace to produce the topmost plane of the root 39 phase. The growth can Proceed then on both the upper and the lower terraces. (C) 1997 Elsevi er Science B.V.