Deposition Of Metal Clusters Research Articles

Laser-assisted cluster deposition from the liquid phase

The experimental results on laser-assisted localized deposition of metal clusters are presented. A simple model is proposed which predicts the existence of an activation time for the deposition. The experimental results are qualitatively consistent with the model.

The structure of deposited metal clusters generated by laser evaporation

Metal clusters have been produced using a laser evaporation source. A Nd-YAG laser beam focused onto a solid silver rod was used to evaporate the material, which was then cooled to form clusters with the help of a pulsed high pressure He beam. TOF mass spectra of these clusters reveal a strong occurrence of small and medium sized clusters (n<100). Clusters were also deposited onto grid supported thin layers of carbon-films which were investigated by transmission electron microscopy. Very high resolution pictures of these grids were used to analyze the size distribution and the structure of the deposited clusters. The diffraction pattern caused by crystalline structure of the clusters reveals 3-and 5-fold symmetries as well asfcc bulk structure. This can be explained in terms of icosahedron and cuboctahedron type clusters deposited on the surface of the carbon layer. There is strong evidence that part of these cluster geometries had already been formed before the depostion process. The non-linear dependence of the cluster size and the cluster density on the generating conditions is discussed. Therefore the samples were observed in HREM in the stable DEEKO 100 microscope of the Fritz-Haber-Institut operating at 100 KV with the spherical aberrationcS=0.5 mm. The quality of the pictures was improved by using the conditions of minimum phase contrast hollow cone illumination. This procedure led to a minimum of phase contrast artefacts. Among the well-crystallized particles were a great amount of five- and three-fold symmetries, icosahedra and cuboctahedra respectively. The largest clusters with five- and three-fold symmetries have been found with diameters of 7 nm; the smallest particles displaying the same undistorted symmetries were of about 2 mm. Even smaller ones with strong distortions could be observed although their classification is difficult. The quality of the images was improved by applying Fourier filtering techniques.

Schottky-limit barrier heights for CO-coated metal clusters on GaAs(110)

This letter discusses band bending induced by the deposition of metal clusters coated with onto GaAs(110). The layer of CO between the metallic clusters and the undisrupted semiconductor simulates a metal-insulator-semiconductor junction. The observed barrier height shows Schottky-limit-like dependence on the work function of the metal.

Abrupt interfaces with novel structural and electronic properties: Metal-cluster deposition and metal-semiconductor junctions.

Abrupt interfaces with no observed substrate disruption are produced by a novel method of metal-semiconductor junction formation. This method involves the condensation of a thin Xe buffer layer on cleaved surfaces to isolate the semiconductor from impinging metal atoms. This Xe buffer layer provides a surface upon which the metal atoms diffuse, nucleate, and grow into metallic clusters. These clusters are then brought into contact with the substrate when the Xe is thermally desorbed. The result is an abrupt, nondisrupted, nearly ideal interface. Photoemission studies of Al, Ag, Au, Ga, Ti, and Co clusters grown on n- and p-type GaAs(110) show unique Fermi-level positions \ensuremath{\sim}0.3 and 1.0 eV below the conduction-band minimum, respectively, that are nearly metal and coverage independent. We find no evidence that metal-induced gap states or conventional defect levels are important in determining the Fermi-level position in the gap, but photoemission results indicate surface unrelaxation around the clusters. This unrelaxation results in the reappearance of states in the gap. High-resolution electron-microscopy results for Au(clusters)/GaAs(110) show intimate contact with no intermixing at the interface, with sintering of Au clusters to form an interconnected network of metal islands at high coverages. Comparisons of these results with those for interfaces formed by atom deposition at 60 and 300 K emphasize the novel properties of the cluster interface.

The Properties of Deposited Metal Clusters generated by Laser Evaporation

Metal clusters have been produced using a laser evaporation source. A Nd-YAG laser beam focused onto a solid silver rod was used to evaporate the material, which was then cooled to form clusters with the help of a pulsed high pressure He beam. TOF mass spectra of these clusters reveal a strong occurrence of small and medium sized clusters (n less than 100). Clusters were also deposited onto grid supported thin layers of carbon-films which were investigated by transmission electron microscopy. Very high resolution pictures of these grids were used to analyze the size distribution and the structure of the deposited clusters. The diffraction pattern caused by crystalline structure of the clusters reveal 3- and 5-fold symmetries besides the pattern reflecting the fcc bulk structure. This can be explained in terms of icosahedron and cuboctahedron type clusters deposited on the surface of the carbon layer. There is strong evidence that part of these cluster geometries had already been formed before the deposition process. The non-linear dependence of the cluster size and the cluster density on the generating conditions is discussed.

Energies and symmetries in interface formation: In/GaP(110) and Ga/InP(110)

We have used high-resolution synchrotron radiation photoemission to study adatom–substrate interactions and growth morphologies for In/GaP(110) and Ga/InP(110). Room-temperature experiments reveal extensive adatom clustering, but also substrate disruption and cation segregation for both interfaces, with greater amounts for Ga/InP(110). Ga deposition in InP(110) at 60 K also results in substrate disruption, but with kinetic trapping of the released In atoms close to the interface and a greater tendency toward layer-by-layer growth. In constrast, the deposition of preformed metallic Ga clusters shows no evidence for substrate disruption. We conclude that atom condensation and coalescence are responsible for disruption, with different activation barriers being present for cluster deposition and atom deposition because of the details of surface and interface bonding. For Ga deposition on n-type InP(110) at 60 K, the appearance of states at the Fermi level is correlated to changes in band bending. Metal cluster deposition leads to pinning positions that can be related to surface unrelaxation around the perimeter of the clusters; we find no evidence for metal induced gap states or defect levels.