Spontaneous Formation and Stability of GaP Cage Structures: A Theoretical Prediction of a New Fullerene
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Spontaneous Formation and Stability of GaP Cage Structures: A Theoretical Prediction of a New Fullerene Francesco Buda1 , Valentina Tozzini2 and Annalisa Fasolino2 1 Department of Theoretical Chemistry, Vrije Universiteit, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands 2 Istituto Nazionale per la Fisica della Materia and Scuola Normale Superiore, Piazza dei Cavalieri, 7 I-56126 Pisa, Italy 2 Research Institute for Materials, Institute of Theoretical Physics, University of Nijmegen, Toernooiveld, NL-6525ED Nijmegen, The Netherlands
ABSTRACT We report the spontaneous formation of a GaP fullerene cage in ab-initio Molecular Dynamics simulations starting from a bulk fragment. A systematic study of the geometric and electronic properties of neutral and ionized III-V (GaP, GaAs, AlAs, AlP) clusters suggests the stability of hetero-fullerenes formed by compounds with zincblend bulk structure. Our prediction is supported by several indicators: these clusters show closed electronic shells and relatively large energy gaps; the ratio between the cohesive energy per atom in the cluster and in the bulk is very close to the value found for carbon fullerenes of the same size; the clusters are thermally stable up to a temperature range of 1500-2000 K and they do not dissociate when ionized.
INTRODUCTION The discovery of carbon fullerenes and nanotubes has opened a completely new eld at the borderline between chemistry and physics leading to many new phenomena and applications. Most e orts to identify fullerenes based on other elements have focused on BN which is the most similar to carbon and exists in nature in the hexagonal(graphite-like) structure[1-8]. However, the observed (nested-)cages and wires[1,2] do not present the characteristic pentagonal rings of carbon fullerenes. Here we explore the possibility of fullerene cages based on typical semiconductors of the III-V family, like GaAs, GaP, or AlAs, which do not possess a graphite-like bulk structure. We have recently shown[9] by means of ab-initio Car-Parrinello Molecular Dynamics[10], that a small GaP bulk fragment spontaneously organizes in a cage formed by a di erent number of atoms of the two elements arranged as in carbon fullerenes. Here we extend the analysis of the geometric and electronic structure of cages with the stoichiometry IIIx-Vx4 for various III-V clusters up to a total of 52 atoms.
COMPUTATIONAL DETAILS Our results are obtained by means of the Car-Parrinello approach[10] using a Density Functional in the Generalized Gradient Approximation proposed by Becke and Perdew[11,12]. This approximation reproduces the experimental cohesive energy
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of typical bulk semiconductors within 5 valence to conduction band excitation energies[13]. We use nonlocal norm-conserving rst-principles pseudopotentials[14] and expand the single particle wavefunctions on a plane wave basis set with a cut-o of 12 Rydberg. We use a periodically repeated cubic simulation box of 24 A side, a size found large enough to describe isolated clusters. The electronic opt
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