Electronic and Optical Studies of Coexisting 5- and 6-Atom Rings in Tetrahedral a -C
- PDF / 641,919 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 139 Views
Electronic and Optical Studies of Coexisting 5- and 6-Atom Rings in Tetrahedral a-C R.M. Valladares, A.G. Calles, M.A. Mc Nelis, Ariel A. Valladares§* Depto. de Física, Facultad de Ciencias-UNAM, Apartado Postal 70-542, México, D.F., 04510, MEXICO. Apartado Postal 70-464, México, D.F., 04510, MEXICO. *Instituto de Investigaciones en Materiales-UNAM, Apartado Postal 70-360, México, D.F., 04510, MEXICO. ABSTRACT We simulate both the amorphous cluster a-C57H52 that contains 5-atom planar rings and 6-atom boat-type rings and the crystalline cluster c-C59H60 that contains only chair-type 6-atom rings, as in the tetrahedral crystal (diamond structure). We carry out ab initio calculations using the DMol code and report the total energy, the electronic (density of states) structure and optical properties of the two types of clusters, with and without relaxation of the structures, in order to see the effect of the types of atom rings found in the amorphous structures compared to those found in the crystal structure. INTRODUCTION Recently, renewed interest in the atom topology of group IV semiconductors has appeared, in particular for silicon and carbon in themselves and also for the contrasting influences of 5-atom and other size rings and bonding in both materials ([1] and references contained therein). The silicon fourfold-coordinated crystalline clathrate structures Si(34) (fcc) and Si(46) (sc) have become relevant for their potential to generate wide band gap silicon semiconductors which makes them useful from the technological viewpoint [2]. The clathrate structures are formed with a higher proportion of 5-atom rings than that of 6-atom rings. It should be borne in mind that it is not possible to construct a crystalline structure with only 5atom rings [3]. Carbon is a more versatile element and its chemistry is richer than that of silicon, nevertheless finding crystalline clathrate structures experimentally has been an unsuccesful endeavour and the rigidity of the carbon-carbon bonds has been invoked as the responsible factor in these unfruitful attempts. Amorphous carbon, however, has a variety of bondings, atom topologies and coordination numbers. In general both sp2 and sp3 hybridizations coexist leading to 3d- and 2d-like structures and the presence of 5-atom and 7-atom rings, in addition to 6-atom boat-type rings, is not uncommon. It follows then, that in the amorphous counterpart it may be possible to create clathrate-like structures since the non-crystallinity allows the formation of structures that contain 5- and 6-atom rings. The influence of 5-atom ring topology on amorphous semiconductors is still not well understood since attempts to characterize its structure have led to experimental and theoretical results that are largely sample dependent [4]. There are indications that different topologies affect the properties and electronic structure of silicon and carbon, sometimes in opposite ways [5]. Adams et al. [6] have found that the theoretical clathrate structures of carbon manifest the §
Contacting auth
Data Loading...
