Ab initio DFT study of the properties of the Ge_n+1 and GaGe_n nanomaterials

C. SIOUANI^a,b, S. MAHTOUT^a et F. RABILLOUD^c

^aLaboratoire de Physique Théorique, Faculté des Sciences Exactes, Université de Bejaia, 06000 Bejaia, Algérie.

^bDépartement des Sciences de la matií¨re, Faculté des Sciences, Université d'Alger 1, 02 rue Didouche Mourad - Alger Centre 16000, Algérie.

^cInstitut Lumií¨re Matií¨re, UMR5306 Université Lyon1-CNRS, Université de Lyon, 69622 Villeurbanne Cedex, France.

Email : siouanichaouki@gmail.com

Abstract: This work is an ab initio theoretical investigation of the physicochemical properties of pure germanium nanomaterials (clusters) (Ge_n+1) and germanium doped with gallium (GaGe_n) in the range of 1 to 19 atoms through the density functional theory (DFT) with the generalized gradient (GGA) approximation, implemented in the SIESTA program. We show the relationship between the electronic and chemical behavior of these small systems with their structural properties. For each size, several initial structures have been relaxed to determine the most stable one. The structural relaxation and the search of the ground states energies of the Ge_n+1 and GaGe_n clusters are performed using the conjugate gradient method with the polarized spin option. Our results show the considerable influence of the sizes of the systems on the structure and the electronic properties of the GaGe_n clusters. We have also shown the influence of doping with a gallium atom on the stability and other physicochemical properties of pure germanium clusters and the relationship between binding energies and the size of new clusters. The values "‹"‹of the HOMO-LUMO gaps decrease with the increasing of their sizes. For each GaGe_n clusters, the HOMO-LUMO spin-up gaps are higher than the HOMO-LUMO spin-down gaps. This characteristic provides these clusters with properties that can have many potential applications, especially in the field of spin electronics applications in the coming years. The small clusters with high HOMO-LUMO gaps are higher stability and low reactivity than those with large sizes having very small gaps. The very small values of HOMO-LUMO gaps indicates that the gallium atom enhance the metallic character and the chemical activity during the crystal growth. In order to give more information on the electronic and chemical behavior of these systems, the ionization potentials, electron affinities, chemical hardness, total and partial densities of states are calculated and analyzed. The vertical electron affinity (VEA) and vertical ionization potential (VIP) of GaGe_n clusters are used to determine the energies needed to add or remove an electron from a cluster without any structural relaxation. The results obtained show that the values "‹"‹of the AEVs show an increasing tendency with the increase of the cluster sizes. This indicates that the clusters GaGe_n of great size will capture electrons more readily compared to those of small sizes. However, the values "‹"‹of the VIP increase with increasing of size. This implies that the clusters of great sizes require more energy to lose an electron compared to the clusters of small sizes indicating theirs high stabilities.

Key words: nanomaterials, ab initio, DFT, SIESTA, germanium-gallium, physicochemical properties.