Access to techniques to produce and characterize free clusters built up from two to thousands of atoms has during the last decades generated several interesting discoveries and established cluster science as a research field of its own [1]. In addition to the discoveries of clusters, which show periodicities in the form of magic numbers also very unique clusters have been found as the fullerenes and nanotubes [2]. In addition to generating a lot of scientific progress, these new discoveries within the field of cluster science, and in particularly species such as nanotubes, have opened up the doors to different areas of science such as mesoscopic physics and modern material science. The general trend is from small to large systems, bottom-up approach, contrary to the general trend of modern mesoscopic physics or microelectronics, where the movement is from large to small, top-down approach. It is especially interesting how the area of fullerene research was initiated to solve problems in astrophysics.
Early experiments in cluster science were focused on to find clusters with unique reactivities or catalytic properties. We have in Göteborg used a pulsed laser vaporization source to generate a cluster beam, which passes reaction cells, containing low pressure of reactive gas. By measuring in the beam with a time-of-flight mass spectrometry the relative intensity of pure clusters and clusters with one or more molecules adsorbed as a function of the reaction pressure, reaction probabilities and catalytic activity can be studied. [3]
Another interesting use of clusters has been in the growth of single-wall carbon nanotubes, SWNT. We have in our research studied the growth of SWNT, which are produced by the catalytic chemical vapour deposition technique, CCVD, in the temperature range of 800-1400 K. In the production, high temperature catalysis is used in which, carbon containing feedstock as carbon monoxide, methane, alcohol etc will react on the surfaces of 1-2 nm transition metal particles and produce carbon atoms, which will diffuse on the metal cluster surface or dissolve in the clusters [4]. These MD studies have shown that the value of the carbon-metal adhesion strength is crucial for the growth of open SWNT. The calculations also show that to maintain an open end of the SWNT it is necessary that the SWNT adhesion strength to the metal cluster is comparable to the cap formation energy of the SWNT. The simulations have more recently [5] been extended to the use of Density Functional Theory to calculate the adhesion between the metal cluster and a SWNT. It was found that Fe, Co and Ni, commonly used to catalyze SWNT have larger adhesion strengths to SWNT than Cu, Pd and Au and are therefore likely to be more efficient for supporting growth. The calculations are now extended for tube segments of (n+m) = 9-13 tubes of armchair/armchair-like and zigzag/zigzag-like tubes to determine the most stable ones within a series.
References:
1.A. Rosén, A Periodic Table in Three Dimensions: A Sightseeing Tour in the Nanometer World, Adv. Quantum Chem. 30, 235 (1998).
2.A. Rosén, A Sightseeing Tour in the world of clusters-serendity and scientific progress,
Journal of Molecular Graphics and Modelling .19, 236 (2001)
3 M. Andersson and A. Rosén, J. Chem. Phys. 117, 7051 (2002)
4. F. Ding, K. Bolton and A. Rosén, J. Phys.Chem B108, 17369-17377 (2004).
5. F. Ding, P. Larsson, J. A. Larsson, R. Ahuja, H. Duan, A. Rosén and K. Bolton,
Nano Lett 8, 463 (2008)