ADSORBATE-INDUCED STRUCTURAL-CHANGES OF RH FIELD EMITTER TIPS

Rh field emitter tips undergo considerable structural changes in the p resence of adsorbed CO layers. This results from extensive field-free reaction studies under low pressure conditions (10(-4)-10(-3) Pa CO) a t variable temperatures (200-450 K) and subsequent imaging by field io n microscopy (FIM). The onset temperature for strong morphological cha nges is at 360 K. Independent of the CO reaction pressure the almost h emispherical shape of a Rh tip is transformed into a shape which resem bles a polyhedron. Planes with high step density undergo faceting. A ( 1 x 2) missing-row reconstruction has been observed to occur on the {1 13} plane at 450 K. The mechanism underlying these (thermally activate d) changes likely involves the formation (and decomposition) of mobile subcarbonyls, Rh(CO)x (x = 1-3). Depending on the surface temperature , CO dissociation into carbon and oxygen atoms may occur and compete w ith subcarbonyl formation. There are indications that small amounts of surface carbon can inhibit the CO-induced transformation towards poly hedral morphologies. For example, it has been observed that carbon con taining {012} planes remain stable during interaction with CO at 390 K and appear enlarged at the expense of neighbouring planes.