Direct Observation of Two Dimensional Magic Clusters

Abstract

Two dimensional magic clusters have been directly observed on the p 3 3p 3 R30 ± reconstructed Ga͞Si(111) surface using scanning tunneling microscopy. The magic numbers are n͑n 1 1͒͞2, where n (2, 3, 4, or 5) is the number of atoms on each side of these triangular clusters with preferred orientation with respect to the substrate. The p 3 3p 3 R30 ± adatom lattice surrounding the magic clusters exhibits characteristic vacancies. A structural model is proposed to account for the cluster orientation and lattice vacancies as well as the extraordinary abundance and stability of the decamers ͑n 4͒. [S0031-9007(98)06520-X] PACS numbers: 68.35.Bs, 61.16.ChMagic clusters, i.e., clusters with enhanced stability at certain sizes, have been intensively studied over the last decade. For magic clusters in free space, many interesting results about their electronic and atomic shell structures are found in the literature [1][2][3][4][5][6]. In contrast, magic clusters supported by a substrate (henceforth referred to as 2D magic clusters) are rarely discussed. Only recently have researchers found indirect evidence of such clusters in a molecular beam scattering experiment [7] and issues related to the stability of clusters on various substrates caught researchers' attention [8,9]. The main difference between free and supported clusters lies in the presence of the supporting substrate for the latter. The geometric constraint and electronic effect exerted by the substrate may or may not destroy the stability of an approaching magic cluster. For example, according to a calculation [8], Na 8 (a magic cluster in free space) retains its intrinsic structure after landing on the insulating NaCl(100) surface, but it spontaneously collapses on the Na(110) surface. Therefore, it is of fundamental interest to search for direct evidence of 2D magic clusters and study the cluster-surroundings interaction that needs not to be considered for magic clusters in free space. The recent interests in 2D magic clusters are also related to emerging attempts to coat surfaces with size-selected clusters [10]. Since the formation of 2D magic clusters is a plausible approach to the creation of this type of novel material, it is important to experimentally demonstrate the existence of 2D magic clusters and understand the conditions and mechanism for their formation.

Experimental and Theoretical Investigation of Single Cu, Ag, and Au Atoms Adsorbed onSi(111)−(7×7)

“…The binding energy difference for Cu, Ag, and Au at the stable and metastable adsorption sites in both FHUC and UHUC is listed in Table I to compare with the experimental values. The somewhat stronger binding in the FHUC is consistent with the preferential occupation of Ag, Au, and Cu in the FHUC at thermal equilibrium as observed in our experiment and also in other adsorbate systems [5,20,21]. The above conclusion can be directly compared to a recent first principles study by Cho and Kaxiras [18,19] for K, Mg, Ga, Si, and Ge, using a (4 4) supercell rather than a full (7 7) cell.…”

Stepwise self-assembly of C60 mediated by atomic scale moiré magnifiers

“…Such a new approach to the substrate-assisted self-assembly of size-selected structures via 'moiré shell closure' is conceptually very different from the previous approach that uses surfaces with large superstructures of quasi-periodic inhomogeneous adsorption sites [2][3][4][5][6][7] as 'anchor sites' to assist the aggregation of particles and achieves size selection. The creation of an ensemble of monodispersed surfaceclusters of 37-mer is a remarkable achievement, especially, in light of fact that the largest monodispersed surface-clusters that has been reported to date contains merely 10 galliums 22 . This dramatic increase in the cluster size is primarily due to the use of C 60 as building blocks rather than atoms.…”

Dynamics of copper atoms on Si(111)-7×7 surfaces

“…The reactivity of nanoclusters with substrate surfaces affects the electronic characteristics of the interfaces. Recently, more works have addressed the formation and chemical characteristics of self-assembled nanoclusters on surfaces, perhaps because they dominate IC processes [1][2][3][4][5][6].…”