Clusters/Nanoparticles

Small carbon clusters, including those terminated by heteroatoms, have been of theoretical and experimental interest for many years and we have explored both aspects in our labs. Computational studies provide critical information for the evolution of the field. We have completed surveys of the geometries and electronic structures of CnS and CnS2 (and the anions and cations), as well CnN and CnN2, using density functional theory. We have (via mass spectrometry) examined some of these clusters experimentally. The cluster abundance depends not on the cluster stability, but on a convoluted function of the stability and the ionization potential. We have also produced the CnS2 clusters (of 8-9 carbon atoms) in bulk using an apparatus similar to that employed for fullerenes. These clusters have applications as nanocantilevers in sensors.

Laser ablation of ZnS or mixtures of elemental zinc and sulfur was used to produce clusters of zinc and sulfur. Mass spectrometric detection was used to identify the products. Regardless of the composition of the ablation sample, the mass spectra were dominated by mixed cluster ions. In particular, the mass peaks corresponding to Zn13S13 and Zn34S34 cations were present in an abundance much greater than neighbor ions, indicating that the geometry of these particular clusters confers a special stability. The mass spectra of the analogous ZnO clusters did not exhibit magic numbers. We find that the most stable structures are predicted to be cage molecules consisting of six- and four-membered ZnS rings. 'agic numbers' were observed for cluster spectra of CdS and ZnSe. We have focused our current work on understanding the physical basis for these observations. Recently, we have modified our fullerene production apparatus to produce larger quantities of ZnS nanoparticles and ZnS nanotubes (directly without templating). These materials are the focus of additional research.

Silver clusters have received particular attention because they have interesting catalytic properties and silver film coatings are widely used. We are interested in such clusters both as free molecules and as absorbed species on surfaces. Our Ag3 studies include the examination of various properties, including Jahn-Teller effects. We have obtained the full BO surface of neutral Ag3, as well as electronic properties of trimer and its anion and cation. Calculations are also completed for the related Cu and Au cases. We have also examined the related CuRg van der Waals molecules and Cu3Rg molecules and have successfully predicted the observed blue shifts upon complexation with the rare gas atom. Much of this computational research will now be directed towards exploring these small clusters on surfaces.

We are interested in the structure and formation mechanisms of endohedral fullerenes. Fullerenes provide a nanometer-sized hollow space within the carbon cage.  Considering that the size of the cages may vary from C60 to C240, this hollow space spans a wide range of cavity sizes. The cavities may be filled by metal atoms, by small clusters of metals/semiconductors, by non-metal atoms or by molecular fragments/radicals. Our work has focused on understanding the relative stability of series of related fullerenes, for example when the endohedral atoms are the Group IV atoms, N, P and As. We are also exploring the transition states involved in the production of the endohedral cages via several possible mechanisms, including windowing and insertion.