clusters

Cluster Studies in the Duncan Lab

Metal-Carbide and Oxide Cages and Nanocrystals

Metal carbon clusters are produced using the same techniques originally used to produce C₆₀ ("Buckminsterfullerene"). These species are now recognized to form metal-carbon cages (so-called "met-cars" clusters - M₈C₁₂) or metal carbon "nanocrystals" (M₁₃C₁₄), depending on the metals employed and the growth conditions. We produce these clusters and study their chemistry via gas phase adsorption reactions and we study their decomposition via mass-selected photodissociation. Our laboratory was the first to document the competition between met-cars cage and nanocrystal production, the first to show that certain nanocrystals could dissociatively reconstruct to form met-cars cages, and the first to show that laser excitation of large nanocrystals leads to photo-induced crystal cleavage to produce smaller nanocrystals.

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Exciting work has been done in collaboration with the research group of Prof. Gerard Meijer at the University of Nijmegen, The Netherlands. In their lab, we have used the Free Electron Laser for Infrared eXperiments ("FELIX") together with a metal cluster experiment. (visit the FELIX lab) Using FELIX, we have obtained the first-ever infrared spectra of gas phase metal clusters for the met-cars and nanocrystal species. The experiments use resonance-enhanced multiphoton ionization with infrared radiation (IR-REMPI) in the 400-1700 cm^-1 region from the pulsed free electron laser at the F.O.M. Institute for Plasma Physics (Utrecht). Infrared spectra are obtained for Ti₈C₁₂, Ti₁₄C₁₃ and for many larger metal carbide nanocrystals. The nanocrystals have a strong resonance at 500 cm^-1(20 microns), and these spectra are essentially unchanged with the size of the cluster. In a new collaboration with astronomers, these TiC nanocrystal spectra have been identified as having the identical signature of the so-called "21 micron line" (which actually occurs at 20.1 microns) seen in the IR spectra of carbon rich stars as they burn out in the last stage of their growth. This identifies metal carbide nanocrystals in space for the first time and provides significant new insight into interstellar dust formation.

Closely related to the work on metal carbide clusters are studies of other metal compound systems (metal oxides, nitrides, etc.) which also form strongly bound clusters with novel geometries, and which also have potential astrophysical significance.

This research is sponsored by the Air Force Office of Scientific Research and by the U.S. Department of Energy.

Selected Publications:

A. M. Ketch, C. L. Anfuso, K. S. Molek and M. A. Duncan, "Photodissociation of small indium oxide cluster cations," Int. J. Mass Spectrom. 304, 29 (2011).

O. Kostko, S. R. Leone, M. A. Duncan, M. Ahmed, "Determination of ionization energies of small silicon clusters: Vacuum ultraviolet (VUV) photoionization experiments and ab initio calculations," J. Phys. Chem. A 114, 3176 (2010).

B. W. Ticknor, B. Bandyopadhyay and M. A. Duncan, “Photodissociation of noble metal-doped carbon clusters,” J. Phys. Chem. A 112, 12355 (2008).

K. S. Molek, C. Anfuso-Cleary and M. A. Duncan, “Photodissociation of iron oxide cluster cations,” J. Phys. Chem. A 112, 9238 (2008).

Z. D. Reed and M. A. Duncan, "Photodissociation of yttrium and lanthanum oxide cluster cations," J. Phys. Chem. A 112, 5354 (2008).

L. Belau, S. E. Wheeler, B. W. Ticknor, M. Ahmed, S. R. Leone, W. D. Allen, H. F. Schaefer, and M. A. Duncan, "Ionization Thresholds of Small Carbon Clusters: Tunable VUV Experiments and Theory," J. Am. Chem. Soc. 129, 10229 (2007).

K. S. Molek, Z. D. Reed, A. M. Ricks and M. A. Duncan, “Photodissociation of Chromium Oxide Cluster Cations,” J. Phys. Chem. A, 111, 8080 (2007).

J. B. Jaeger, T. D Jaeger and M. A. Duncan, "Photodissociation of Metal-Silicon Clusters: Encapsulated versus Surface-Bound Metal," J. Phys. Chem. A 110, 9310 (2006).

K. S. Molek, T.D. Jaeger and M. A. Duncan, "Photodissociation of Vanadium, Niobium and Tantalum Oxide Cluster Cations," J. Chem. Phys. 123, 144313 (2005).

B. W. Ticknor and M. A. Duncan, "Photodissociation of size-selected silicon carbide cluster cations," Chem. Phys. Lett. 405, 214 (2005).

G. von Helden, A. G. G. M. Tielens, D. van Heijnsbergen, M. A. Duncan, S. Hony, L. B. F. M. Waters and G. Meijer, "Titanium Carbide Nanocrystals in Circumstellar Environments," Science 288, 313 (2000).

Synthesis and Characterization of Novel Organometallic Clusters

We have produced C₆₀ in the gas phase with metal atoms bound to its external surface. Complexes with a variety of metals have been formed, some of which form metal monolayer films on the C60 surface, and others which form cluster-cluster (M_x-C₆₀) complexes. These species are produced by laser ablation of a metal sample rod coated with a fullerene film. The film is applied to the metal rod via a sublimation oven in a separate sample preparation chamber. In recent experiments, we have used similar methods to bind metal on the "molecular surfaces" of polyaromatic hydrocarbons (PAH's; coronene, phenanthrene, etc.), to prepare novel sandwich complexes (coronene-Fe-coronene) and to make metal complexes in the gas phase with other large polyatomic species (e.g., porphyrins). These various clusters and complexes are interrogated with mass selected laser photodissociation. Especially stable species detected in the gas phase become candidates for macroscopic isolation.

This research is sponsored by the Air Force Office of Scientific Research, by the National Science Foundation and by the U.S. Department of Energy.

Selected Publications:

A. C. Scott, J. W. Buchanan, N. D. Flynn, M. A. Duncan, “Photodissociation of Calcium-coronene and Calcium-pyrene Cluster Cations,” Int. J. Mass Spectrom. 269, 55 (2008).

A. C. Scott, J. W. Buchanan, N. D. Flynn, M. A. Duncan, “Photodissociation of Iron-Pyrene and Iron-Perylene Cation Complexes,” Int. J. Mass Spectrom. 266, 149 (2007).

A.C. Scott, N.R. Foster, G.A. Grieves and M.A. Duncan, "Photodissociation of lanthanide metal cation complexes with cyclooctatetraene," Int. J. Mass Spectrom. 263, 171 (2007).

E.D. Pillai, K.S. Molek and M.A. Duncan, "Growth and Photodissociation of U⁺(C₆H₆)_n (n=1-3) and UO_m⁺(C₆H₆) (m=1,2) Complexes," Chem. Phys. Lett. 405, 247 (2005).

T.D. Jaeger and M.A. Duncan, "Photodissociation of M⁺(benzene)_x Complexes (M=Ti, V, Ni) at 355 nm," Intl. J. Mass Spectrom. 241, 165 (2005).

T. Ayers, B.C. Westlake, D.V. Preda L.T. Scott and M.A. Duncan, "Laser Plasma Production of Metal-Corannulene Ion-Molecule Complexes," Organometallics 24, 4573 (2005).

T.M. Ayers, B.C. Westlake and M.A. Duncan, “Laser Plasma Production of Metal and Metal-Compound Complexes with PAH’s,” J. Phys. Chem. A 108, 9805 (2004).

Laser Desorption Mass Spectrometry

We have built a special version of a time-of-flight mass spectrometer for analysis of involatile materials. Recent studies show that fast pulsed laser excitation can lead to volatilization and ionization of molecules with virtually no intrinsic vapor pressure. We have used this technique to analyze novel polymer films of C₆₀ and metal colloidal "quantum dot" materials. Additional studies in our lab use Matrix Assisted Laser Desorption Ionization ("MALDI") to produce mass spectra of proteins, enzymes and polymers. We are collaborating to analyze particle sizes and extent of polymerization in materials produced in other laboratories, and we are investigating the mechanism of laser desorption/ionization in its various forms.

This research is sponsored by the University of Georgia Research Foundation and the Air Force Office of Scientific Research.

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This is the laser desorption mass spectrometer system. A "MiniLite" YAG laser is used to desorb samples.

Selected Publications:

A.M. Rao, P.C. Eklund, U.D. Venkateswaran, J. Tucker, M.A. Duncan, G. Bendele, P.W. Stephens, J.L. Hodeau, L. Marques, M. Nunez-Regueiro, I.O. Bashkin, E.G. Ponyatovsky and A.P. Morovsky, "Properties of C₆₀ polymerized under high pressure and temperature," Appl. Phys. A 64, 231 (1997).

A.M. Rao, M. Menon, K.A. Wang, P.C. Eklund, K.R. Subbaswamy, D.S. Cornett, M.A. Duncan and I.J. Amster, "Photoinduced Polymerization of Solid C₇₀ Films," Chem. Phys. Lett. 224, 106 (1994).

D.S. Cornett, M.A. Duncan and I.J. Amster, "Liquid Mixtures for Matrix Assisted Laser Desorption Mass Spectrometry of Proteins," Anal. Chem. 65, 2608 (1993).

D.S. Cornett, I.J. Amster, M.A. Duncan, A.M Rao, and P.C. Eklund, "Laser Desorption Mass Spectrometry of Photopolymerized C₆₀ Films," J. Phys. Chem. 97, 5036 (1993).

A.M. Rao, P. Zhou, K.A. Wang, G.T. Hager, J.M. Holden, Y. Wang, W.T. Lee, X.X. Bi, P.C. Eklund, D.S. Cornett, M.A. Duncan, and I.J. Amster, "Photo-induced Polymerization of Solid C₆₀ Films," Science 259, 955 (1993).

Synthesis of Macroscopic Amounts of Metal Clusters and Metal Nanoparticles

A new laser vaporization flow reactor (LVFR) has been constructed consisting of a laser ablation cluster source combined with a fast flowtube reactor for the production and isolation of ligand-coated metal clusters. The source includes high repetition rate laser vaporization with a 100 Hz KrF (248 nm) excimer laser, while cluster growth and passivation with ligands takes place in a flowtube with ligand addition via a nebulizer spray. Samples are isolated in a low temperature trap and solutions containing the clusters are analyzed with laser desorption time-of-flight mass spectrometry. Initial experiments with this apparatus have trapped Ti_x(ethylenediamine)_y complexes which apparently have linear metal units with octahedral ligand coordination. Other experiments have produced and isolated clusters of the form Ti_xO_y(THF)_z that apparently have linear metal oxide cores and larger (TiO₂)_x(THF)_y nanoparticle species, where x=10-14 and y=5-8. The isolation of these new cluster species suggest that the LVFR instrument has considerable potential for the production of new nanocluster materials.

This research is sponsored by the Air Force Office of Scientific Research.

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Selected Publications:

S. Ard, C. Dibble, S. T. Akin and M. A. Duncan, "Ligand-coated vanadium oxide nanoclusters: Capturing gas phase magic numbers in solution," J. Phys. Chem. C 115, 6438 (2011).

T.M. Ayers, J.L. Fye, Q. Li and M.A. Duncan, “Synthesis and Isolation of Titanium Metal Cluster Complexes and Ligand-coated Nanoparticles with a Laser Ablation Flowtube Reactor,” J. Clus. Sci. 14, 97 (2003).

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