cluster research group, university of hawaii at manoa, honolulu
Micrometer- and nanometer-sized powders have many applications in industry. Their structural, chemical, and electrical properties are related and need to be determined in order to make use of these powders. In collaboration with research groups at Asea Brown Bovery (ABB) corporation and the Argonne National Laboratory we studied powders of metals as well as combustion related materials, using Repeated-Contact Electrification, STM, STS, inelastic electron tunneling, AFM, and XPS.
The adsorption sites for single adatoms and adclusters on HOPG have been studied using STM. Also, cluster structures were determined with the STM at atomic resolution. Self-assembled films of gold particles emit light due to ineleastic electron tunneling. Electrons injected from the STM tip excite extended and localized surface plasmons which decay accompanied by photon emission. The emitted light spectrum gives the plasmon resonances.
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These are STM images of charge density waves on TaSe2 single crystals, showing three different lock-in positions relative to the substrate atoms.
Charge-density waves (CDW) together with periodic lattice distortions (PLD) occur in low-dimensional solids due to the so-called Peierls distortion. They can be observed by STM due to the dependence of the tunneling current on the local density of states on a surface. Taking into account our earlier observations of different lock-in pattern for charge-density waves on 1T-TaSe2 we studied the effects of small metal clusters and islands on graphite substrates. Charge density modulations on the support are induced by the adsorbed clusters, and by feedback, they affect the clusters itself by locking them into high charge density locations.
Carbon nanotubes were first generated by gas discharge on the graphite electrodes in 1991 by a research group in Japan. Subsequently many researchers thought that the electric field between the electrodes and the applied He buffer gas would be required for the tubular growth and their formation was theoretically described in this way.
In 1994 we could produce carbon nanotubes without any electric field lines and without a buffer gas present. We generated the nanotubes in ultrahigh vacuum at 2x10-10 torr by quenching very hot (3500°) carbon vapor on a cold single-crystal substrate. This made the original growth model obsolete. Using scanning tunneling microscopy we obtained atomic resolution on the nanotubes and could directly prove the graphitic network structure. In addition, we could determine the chiralities of the two outer atomic layers of multiwall tubes. We have produced nanotubes and nanocones of carbon by vapor deposition in UHV. They were analyzed with atomic resolution by scanning tunneling microscopy. The helicities of the surface and subsurface atomic networks were determined.
Carbon nanocones were grown in UHV in the same way as the nanotubes. For pentagon-hexagon networks, five types of possible cone structures were identified. Only the one with the smallest opening angle (of 19.2 degrees) did form in vapor phase growth.
We surprisingly found, that LiOH, NaOH and KOH crystals, when exposed to a CO2 laser beam, emit water clusters containing one alkali atom each. Time-of-flight mass spectra for a wide size range of clusters were recorded. Clusters with 20 water molecules show enhanced stability. They have cage structure of the pentagonal dodecahedron with the alkali atoms being centered in the cage.
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laser power dependence plotNatural coal contains sulfur in different amounts depending on which mine it is taken from. While burning the coal the sulfur is released and has to be collected in huge filters. This leads to high operating costs of coal combustion plants. In addition, since the filters are not 100% effective, part of the sulfur is released and adds to hazardous air pollution. Also, in some countries the filters have been removed even though there are international regulations about installation and operation of the filters. Asea Brown Boveri (ABB) company is one of the largest builders of coal power plants worldwide. In the research lab in Baden, Switzerland, we built an apparatus to fundamentally study a process for removing the sulfur from the coal before it is burned. Since the coal is pulverized before it is put into the burners, the sulfur containing powder particles could be separated from pure coal particles by contact electrification followed by electrostatic separation. We studied the charge transfer when powder particles come into contact with metal surfaces and showed in a number of experiments that electrostatic separation of sulfur containing coal particles indeed can be done.
Surface Properties of Coal and Pyrite Powders; B. A. Kwetkus, and K. Sattler; Appl. Surf. Sci. 68, 139 (1993)
