Growth of nanometer-size metallic particles on CaF2(111)

Abstract

Fe, Co, and Ag particles grown on various CaF2 substrates have been studied using ultrahigh vacuum scanning electron microscopy with nanometer resolution. Fe and Co show a very high nucleation density which is remarkably independent of deposition temperature in the range 20<T<300 °C, on both bulk CaF2(111), and on thin CaF2(111) films grown on Si(111). This feature is characteristic of nucleation at defect sites with a high trapping energy. An atomistic nucleation model has been extended to cover this case. The comparison with experiment requires adsorption, pair binding, and defect trapping energies all to be around 1 eV. The trapping sites occupy 1% of the surface, and are thought to be chemical (F-vacancy, oxide, or hydroxide) in nature. In contrast, the growth of Ag on the same substrates shows a more usual nucleation and growth pattern, though the growth of Ag on Fe islands shows interesting features which are discussed. A self-similar coalescence model is tested using the data obtained. The agreement is excellent for Ag, while Co and Fe show the expected deviations due to limited surface diffusion around the islands.

Microscopic view of epitaxial metal growth: nucleation and aggregation

“…There are two slopes separated by a plateau at a density of one island per superstructure unit cell. 43 For temperatures below this plateau, the ÐÐÐÐ 43 This Arrhenius behavior is also obtained for nucleation in the presence of trapping defects [58,261]. The authors of Refs.…”

Nucleation and growth of Ag islands on fivefold Al-Pd-Mn quasicrystal surfaces: Dependence of island density on temperature and flux

“…In some of those systems, the dependence of island density on temperature has been measured, 40,45 showing a plateau and then a decline. 40,45 For one system, Pd/ MgO, these data were initially interpreted in terms of a model like the one we have used, incorporating strong trapping of adatoms at defects and subsequent island nucleation ͑with i =1͒ below the threshold temperature.…”

“…In some of those systems, the dependence of island density on temperature has been measured, 40,45 showing a plateau and then a decline. 40,45 For one system, Pd/ MgO, these data were initially interpreted in terms of a model like the one we have used, incorporating strong trapping of adatoms at defects and subsequent island nucleation ͑with i =1͒ below the threshold temperature. 45 However, this model was challenged on the basis of density functional theory ͑DFT͒ calculations, which indicated that cluster diffusion, as well as monomer diffusion, should be operative, and that the surface dimer binding energy necessary to fit the data, 1.2 eV, was unreasonably high.…”

“…40 Figure 6͑c͒ shows a schematic of the model. The model parameters describing the system are as follows: the trap density ͑n t ͒; the trap energy ͑E t Ͼ 0͒ reflecting the additional binding at trap sites; the critical nucleus size ͑i͒ above which islands are stable ͑and for which we allow different values i t for islands at traps, and i f on trap-free terrace regions͒; the effective diffusion barrier ͑E d Ͼ 0͒, which describes diffusion between heterogeneous One of the traps is marked both on the STM image and on the PES.…”

Nucleation Kinetics on Inhomogeneous Substrates: Al/Au(111)

“…There are mean-field models for heterogeneous nucleation in the presence of attractive or repulsive point defects [15], however, for the repulsive line defects in the present system there is no theoretical treatment so far. Hence we adopted a kinetic Monte Carlo (KMC) model [16] to simulate diffusion and nucleation on the complex potential energy surface experienced by an Al adatom on the reconstructed Au (111) [17] indicate that the adatom potential is affected within a range of several atomic distances around the dislocations.…”