F. Weigl

Chemically Induced Metal-to-Insulator Transition in Au55 Clusters

The cluster compound Au55(PPh3)12Cl6 has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.

Growth of thin, flat, epitaxial (111) oriented gold films on c-cut sapphire

AbstractAnewgrowthprocedurehasbeendevelopedtopreparethin(<50nm),flat(roughness 0.3–1nm rms ),epitaxialAufilmsontopofaninsulatingsubstratewithoutusingamagneticseedlayer.ThiscouldbeobtainedbydepositingfirstaNbseedlayer(61nm)atroomtemperatureontopofc-cutsapphirefollowedbyevaporatingAuatarateof0.05–0.1nm/sat300 C.Thisprocedureresultedin(111)orientedflatAufilmsevenforthicknesswellbelow50nm.Theflatnessofthesefilmswasconsistentlyconfirmedbyatomicforceandscanningtunnelingmicroscopy,theirexcellentepitaxialquality(rocking width 0.1 –0.3 ) byX-raydiffractometryand reflected high energyelectron diffraction. 2001ElsevierScienceB.V.Allrightsreserved. Keywords:Gold;Niobium;Aluminumoxide;Epitaxy;Growth;Metallicfilms;X-rayscattering,diffraction,andreflection;Reflectionhigh-energyelectrondiffraction(RHEED) 1. IntroductionThere is immense interest in flat epitaxial andthin, yet electrically conducting, metallic films,which then can be used for experiments likee.g. attenuated total reflection (ATR) [1], electro-depositionofnanoclusters[2,3]orscanningtunnel-ing microscopy(STM) on organic monolayers [4].In this context, Au films are especiallyattractivedue to their chemical stability, which is essential,if, after film preparation under ultrahigh vacuum(UHV),experimentsdemandfurtherexsitusteps.In our case, aiming at studying surface sensitiveelectrical transport properties of such thin films,the application of magnetic seedlayers had to beexcluded, since otherwise additional spin-depen-dent scattering mechanisms would contributeto the resistance. Furthermore, to make surfacescatteringthedominantprocess,competingdefectscatteringprocesseshavetobeminimizedleadingtothedemandofthin,flat,highqualityepitaxialAu films. As a consequence of these combinedqualityrequirements, the well known substratesusedtogrowepitaxialAufilms,i.e.mica[5–7]andMgO [8] turn out to be unsuitable. On mica, thepercolation thickness of epitaxial Au films neces-saryto observe electrical conductance appears tobe close to 30 nm. Thus, for epitaxial Au filmsbelow30nmelectricalconductancecannolongerbeguaranteed.ForMgO,ontheotherhand,Feor

The Self-organization of Metal Loaded Micelles - An Approach to Prepare Ordered Arrays of Metallic Nanoislands

The preparation and characterization of hexagonally ordered pure Au nanoparticles is described. Self-assembly of diblock copolymers in solution is the driving force which leads to micellar structures. Starting, for example, with Au-salt loaded inverse micelles, monomicellar arrays exhibiting a significant hexagonal order can be prepared with taylored intermicellar distances and structure heights on top of various substrates. In order to remove the polymer matrix and to finally obtain arrays of pure Au nanoparticles, the micelles are first exposed to an oxygen plasma followed by an annealing process. Special attention is given to the chemical state of the nanoparticles applying photoelectron spectroscopy to control all preparational steps.