Browsing by Author "Guvenc, Z. B."

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Citation - WoS: 15
Citation - Scopus: 15
A density functional study of small Li-B and Li-B-H clusters
(Pergamon-elsevier Science Ltd, 2009) Yildirim, E. K.; Guvenc, Z. B.; 1155
In this work we present the results of a detailed theoretical research for the small LinBm clusters and their hydrogen storage properties by means of density functional theory. All calculations were performed by using Gaussian03 program. For the optimization of the clusters 6-311++G(d,p) basis set was employed. We have chosen global minimum of B-6 cluster as the starting point and replaced the boron atoms one at a time, in a stepwise manner, with Li atoms. For these new structures we have searched for the stable configurations, and calculated their energies, HOMO-LUMO energy gaps, Raman and infrared data, average boron, and lithium charge distribution, and vibrational frequencies. Our results show that as the number of Li atoms increases, stability of the clusters decreases and they become more reactive. In addition to that, there are significant charge transfers from boron atoms to lithium atoms on average. The hydrogen storage capabilities of the most stable isomers of LinBm and B-6 clusters have also been investigated by the same methods, and the results are compared. The Li3B3 has the highest hydrogen storage capacity among the clusters investigated in this study. Furthermore, formation of hydrogen molecules is observed in the vicinity of the clusters, some of which are attached to the Li atoms. In addition, as the number of hydrogen atoms increases, the boron atoms are separated from the other boron atoms, and they form satellite BHx (x = 3,4) clusters around the center. These are attached to the system by a bridging bond of a hydrogen or a Li atom. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Citation - WoS: 18
Citation - Scopus: 18
Differences in melting behaviours of disordered and symmetric clusters: Au(N)(N=54-56)
(Iop Publishing Ltd, 2006) Yildirim, E. K.; Guvenc, Z. B.; 1155
We have investigated the melting behaviours of disordered and symmetric gold clusters (Au-N, N = 54 - 56) by means of molecular dynamics simulations. We have found that there is no single isolated lowest energy structure for this size of Au clusters. Instead there are many nearly degenerate disordered low lying structures. The melting behaviours of these disordered structures showed that the melting occurs as a gradual process in which initially, behaviours of the surface and the inner atoms are quite different from each other, and they do not mix until the beginning of the melting. On the other hand, the symmetric forms of the Au-N ( N = 54 - 56) present different melting behaviours from those of the corresponding disordered structures. Their melting occurs suddenly, i.e. over a very short temperature interval. During the heating of these symmetric forms no phase changes occur until the melting temperature at which melting occurs as a collective motion of all the atoms in the cluster. On the other hand, the phase changes in the disordered structures take place as a result of both collective motions of all the atoms in the cluster, and as local displacements of the atoms.
Citation - WoS: 8
Citation - Scopus: 9
Effects of hydrogen hosting on cage structures of boron clusters: density functional study of B(m)H(n) (m=5-10 and n <= m) complexes
(Iop Publishing Ltd, 2008) Boyukata, M.; Özdoğan, Cem; Ozdogan, C.; Guvenc, Z. B.; 120207; Ortak Dersler Bölümü
The structural stability of hydrogen bonded boron microclusters has been studied by using the density functional theory. Effects of the increasing number of hydrogen atoms on the cage geometries of B-5-B-10 clusters, and the distortion of the cage configurations of the boranes are assessed. The possible stable structures of BmHn(m = 5-10 and n <= m) boron hydrides, their binding energies, HOMO-LUMO energy gaps and the total atomic charges of the B-m in the complexes are determined. For the series of B5Hn, B7Hn, and B9Hn major structural changes are observed.
Citation - WoS: 2
Citation - Scopus: 2
Lyapunov exponent as an indicator of phase transition in melting Pd-13 clusters
(Royal Swedish Acad Sciences, 2006) Kurt, E.; Boyukata, M.; Guvenc, Z. B.; 13410; 120207
In this study, the chaotic behaviour of Pd13 clusters is investigated in a wide total energy range. To identify interactions between atoms, the embedded atom potential is used in the computer simulation. Various analyses including power spectra, maximal Lyapunov exponents (lambda) and rms bond-length fluctuations (delta) are carried out to identify the characteristics of the system. As a result of these analyses, it is observed that Lyapunov spectra can clearly indicate the co-existence region for Pd-13 clusters and exponents have fixed values above a certain total energy value of the system. In addition, while the fast Fourier transforms give the existence of diffusive motion above the transitional region, the correlation dimension values exhibit a smooth character relevant to the total energies of the system.
Citation - WoS: 15
Citation - Scopus: 15
Molecular dynamics simulation of melting behaviour of small gold clusters: Au-N (N=12-14)
(Iop Publishing Ltd, 2007) Yildirim, E. K.; Atis, M.; Guvenc, Z. B.; 1155
We have investigated the melting behaviour of AuN (N = 12 - 14) clusters by means of molecular dynamics simulation on the basis of the Voter - Chen version of the embedded-atom method. The melting behaviour of the clusters is described in terms of short-time average temperatures and atomic coordination numbers of the clusters. Results have shown that during the melting process, the phase changes occur as a collective and simultaneous motion of all the atoms in a very short-time interval. Furthermore the Au-14 cluster presents a two-stage melting behaviour which is different from those of the Au-12 and Au-13 clusters. The isomer sampling probabilities are obtained from the thermal quenching of the molten clusters, and their energy-spectrum widths are investigated. The results of the isomer forming probabilities showed that the global minimum structures of these clusters are not always the most probable ones to be formed in the experiments.
Citation - WoS: 153
The unusually stable B-100 fullerene, structural transitions in boron nanostructures, and a comparative study of alpha- and gamma-boron and sheets
(Amer Chemical Soc, 2010) Ozdogan, C.; Özdoğan, Cem; Mukhopadhyay, S.; Hayami, W.; Guvenc, Z. B.; Pandey, R.; Boustani, I.; Ortak Dersler Bölümü
Solid alpha-B-12 rhombohedral and gamma-B-28 orthorhombic boron as well as boron nanostructures in the form of spheres, sheets, and multirings beside a ring consisting of icosahedral B-12 units were investigated using ab initio quantum chemical and density functional methods. The structure of the 131(x) fullerene exhibits unusual stability among all noninteracting free-standing Clusters, and is more stable than the 13120 cluster fragment of the gamma-B-28 solid, recently predicted and observed by Oganov et al. (Nature 2009, 457, 863). In addition, we compared the stability of the multirings and reported the structural transition from double-ring to triple-ring systems. This structural transition Occurs between B-52 and B-54 Clusters. We confirm that the noninteracting free-standing triangular buckled-sheet is more stable than the gamma-sheet, assembled in this work, and than the a-sheet, proposed by Tang and Ismail-Beigi (Phys. Rev. Lett. 2007, 99, 115501). In contrast, however, when these sheets are considered as infinite periodic systems, then the a-sheet remains the most stable one.