NITROGEN-INDUCED RECONSTRUCTION ON RH(110) - EFFECT OF OXYGEN ON THE GROWTH AND ORDERING OF RH-N CHAINS

The formation of (3 x 1) and (2 x 1)-N phases on a Rh(110) surface has been studied by STM and LEED. When the Rh(110) surface is exposed to NH3 at 400-450 K strings running in the [001] direction appear in the STM images which are due to nucleation and growth of -Rh-N- chains. The weak corrugation of approximately 0.2 angstrom along the strings with a periodicity equal to the [001] lattice constant and the large corrugation of approximately 0.7 angstrom and spacing greater-than-or-equal-to two [110BAR] lattice constants across the strings are indicative of a reconstruction involving absence of [001] rows. The nucleation of -Rh-N- chains occurs preferentially at step edges. With increasing coverage most of the strings grow on the lower terrace in an added-row mode remaining attached to the terrace boundaries. The periodicity in the [110BAR] direction changes from random to tripled and doubled lattice constant with (3 x 1) and (2 x 1) islands preferentially formed in the vicinity of the steps. The (3 x 1) and (2 x 1) islands coexist with areas on the terraces with a (n x 1) chain periodicity with n > 3, which accounts for the streaking in LEED patterns even at saturation. The presence of small amounts of oxygen was found to change the growth mode by increasing the nucleation and growth rate on the terraces, promoting the completion of the (3 x 1) and (2 x l)-N phases. The mechanism of the N-induced reconstruction and the effect of oxygen are discussed considering the thermodynamic and kinetic factors driving the restructuring process.