Browsing by Author "Akpati, Hilary Chukwuma"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    A theoretical study of cluster reactivity and the effects of electric field on adatom-surface bond
    (1996) Akpati, Hilary Chukwuma; Nordlander, Peter J.
    We have used the density functional ab initio method to study the reactivity of small clusters (ammonia reacting with GaAs clusters), and the effects of electric field on adatom-surface interactions (H and Al adsorbed on Si(111) surface). In the case of NH$\sb3$ reactivity at various sites of a Ga$\rm\sb5As\sb5$ cluster, a strong correlation is found between the adsorbate-cluster binding energy and charge transfer. Ionizing the bare cluster to a cationic charge state enables multiple chemisorption of ammonia, which is much less favored on a neutral cluster and can be prohibited on a negatively charged cluster. In the case H and Al adsorbed on a Si(111) surface, the influence of an external electric field, such as that present in a scanning tunnelling microscope (STM), on the chemisorption bond is investigated. The changes in charge distributions, vibrational frequencies and adsorbate desorption barriers are calculated as a function of the strength and direction of the electric field. We find that the characteristics of the chemisorption bond can, to a large extent, be controlled through the externally applied field.
  • Thumbnail Image
    Item
    A theoretical study of the effects of an external electric field on adsorbate-surface systems
    (1997) Akpati, Hilary Chukwuma; Nordlander, Peter J.
    We have used the density functional ab initio method to conduct investigations on the effects of an applied electric field on the chemisorption bonds of adsorbate-surface systems, and on the reactivity of a gas phase semiconductor cluster. In STM current-induced excitation of adsorbates, lateral energy transfer among adsorbates tend to delocalize the excitation, and reduce resolution. We show that the strength of chemical bonds can be increased or decreased depending on the strength and direction of the applied electric field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from the site can be inhibited, and thereby lead to adsorbate excitation localization. The details of the field-induced shifts of an adsorbate-surface bonding features are shown to depend on their dipolar polarization. In the case of the reactivity of GaAs clusters with ammonia, recent experiments indicate that NH$\sb3$ adsorption rate depends strongly on cluster size, Ga/As composition ratio, and cluster charge state. We characterize the reactivity of NH$\sb3$ at various sites of a $\rm Ga\sb5As\sb5$ cluster in terms of the adsorbate binding energy and charge transfer, showing a strong correlation between the two. The dependence of the cluster reactivity on its charge state is deduced.