Density-functional theory study of structures, stabilities, and electronic properties of the Cu2-doped silicon clusters: Comparison with pure silicon clusters

doi:10.1016/j.physb.2012.07.038

Physica B: Condensed Matter

Volume 407, Issue 21, 1 November 2012, Pages 4379-4386

https://doi.org/10.1016/j.physb.2012.07.038 Get rights and content

Abstract

Equilibrium geometric structures, stabilities, and electronic properties of Si_nCu₂ (n=1–8) clusters and pure silicon Si_n (n=3–10) clusters are investigated systematically by exchange-correlation density functional (B3LYP). The optimized geometries show that the most stable isomers have 3D structure for n=2, 4–8, and Cu-substituted Si_n+2 clusters is dominating growth pattern for the Si_nCu₂ clusters. The calculated averaged binding energies, fragmentation energies, second-order difference of energies, and the HOMO–LUMO gaps show that Si₂Cu₂ and Si₅Cu₂ clusters have enhanced relative stabilities and chemical stability than their neighboring clusters. Electronic properties of Si_nCu₂ (n=1–8) clusters are studied by calculating the natural population analysis and electrostatic potential, where the results show that the two copper atoms always possess positive charge and positive potential surround them. In addition, the VIP, VEA and the chemical hardness (η) are also analyzed and compared.

Introduction

In recent years, silicon clusters have been studied extensively by both theoretical and experimental techniques because they may be employed as the building blocks for developing new silicon-based nanomaterials with tunable properties. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] Previous investigations indicate that the pure silicon is unfavorable for forming large-sized clusters and bulk solids; however, the encapsulation of transition metal in the silicon cluster contributes to enhance stability of pure silicon clusters and simultaneously exhibit many magic behaviors. [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31] After the transition metal (TM) is doped into the silicon frame, the TM mixed silicon clusters tend to form closed shell electronic structures and contribute to generate novel transition metal encapsulated clusters with novel properties.

Copper is a common trace impurity in the manufacturing of silicon devices, such as integrated circuits and solar cells. Since Cu plays an important role in altering the electronic properties of Si-based semiconductors, the behavior of Cu in Si has been studied extensively [32], [33]. In 1987, Beck first studied copper silicides in a molecular beam coupled with a mass spectrometer. The dominant clusters detected in this study contained a single copper atom and between 6 and 16 silicon atoms [34], [35]. Plass et al. [36] reported on pseudopotential investigations of the molecules Cu₂Si₂, Cu₂Sn₂, Cu₄Si₄ and Cu₄Sn₄. For these clusters, the relative stabilities of several possible structures were compared and their bonding situation was discussed. Scherer et al. [37] studied Cu_xSi_y clusters by time-of-flight mass spectrometry, and they observed Cu_xSi_y species containing up to three Cu atoms and as many as nine silicon atoms. The geometry and electronic properties of CuSi_n (n=4, 6, 8, 10 and 12) clusters have been reported in several recent publications [38], [39], [40], [41], [42]. These studies used locally optimized structures based on those that can be constructed by the adsorption of a Cu atom into a Si_n atomic cluster or by substituting a Si atom by a Cu atom in Si_n+1 cluster. CuSi_n clusters, for up to n=9–15, have been observed by Hossain et al. [43] and they reported the structures and relative stabilities in the homologous Cu at Si_n (n=9–15) cluster series.

When two copper atoms are doped into the silicon clusters, whether their structure and properties differ from those of the single-doped and pure silicon cluster? In order to reveal the unusual properties of the small size Cu₂ doped silicon clusters, we reported a systematical density functional theory investigation on them, and the pure silicon clusters were also studied by using identical methods and basis sets for comparison. The main objective of this research is to provide a detailed investigation of the geometrical structure, relative stabilities, and electronic properties of the small size Si_nCu₂ (n=1–8) clusters; To explore the growth-pattern behaviors, which can provide significant help for such kind of cluster assemble materials; the last but not least is to understand the influence of copper atom on pure silicon structure and offer more detailed information for further investigations in the future.

Section snippets

Computational details

Geometrical structure optimization and frequency analysis of Cu₂ doped silicon clusters at the size range n=1–8 have been performed by the density functional theory (DFT) using the GAUSSIAN 03 program [44] with the B3LYP exchange-correlation potential [45], [46]. In present calculations, full electron calculation for the Cu atom is rather time-consuming, so it is better to introduce effective core potential including relativistic effects (RECP) for it to describe the inner-core electrons. Then,

Geometrical structures

A number of optimized low-lying isomers for Si_nCu₂ (n=1–8) clusters are shown in Fig. 1, and their corresponding relative energies, symmetries, and electronic state are summarized in Table 2. According to the energies from low to high, these isomers are designated by na, nb, nc, and nd, where n represents the number of Si atoms in the Si_nCu₂ clusters. Meanwhile, in order to examine the effects of dopant Cu atom on silicon clusters, geometry optimizations of Si_n (n=3–10) clusters have been

Conclusions

The systematic investigation on the geometrical structures, growth-pattern behaviors, relative stabilities, electronic properties, VIPs, VEAs, and hardnesses of the Si_nCu₂ (n=1–8) clusters have been performed theoretically at the B3LYP/GENECP level. All calculated results can be summarized as follows:

(i)
The lowest energy structures of Si_nCu₂ (n=1–8) clusters favor the planar structures for n=1, 3 and three-dimensional (3D) structures for n=2, 4–8. The two Cu atoms is found to occupy exclusively

Acknowledgment

The authors are grateful to the National Natural Science Foundation of China (Nos. 10974138 and 11104190) and the Doctoral Education Fund of Education Ministry of China (Nos. 20100181110086 and 20111223070653)

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Density-functional theory study of structures, stabilities, and electronic properties of the Cu2-doped silicon clusters: Comparison with pure silicon clusters

Abstract

Introduction

Section snippets

Computational details

Geometrical structures

Conclusions

Acknowledgment

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J. Mol. Struct. (Theochem)

Chem. Phys. Lett.

J. Mol. Struct. (Theochem)

J. Mol. Struct. (Theochem)

Chem. Phys. Lett.

J. Am. Chem. Soc.

J. Chem. Phys.

J. Phys. Chem. A

Phys. Rev. Lett.

J. Chem. Phys.

J. Chem. Phys.

J. Phys. Chem. B

Phys. Rev. Lett.

Phys. Rev. B

Phys. Rev. B

J. Chem. Phys.

Phys. Rev. B

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. Lett

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. B

Phys. Rev. B

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Density-functional theory study of structures, stabilities, and electronic properties of the Cu₂-doped silicon clusters: Comparison with pure silicon clusters