International Journal of Physics and Research (IJPR) ISSN 2250-0030 Vol. 3, Issue 2, Jun 2013, 1-6 © TJPRC Pvt. Ltd.
ENERGETICS OF SMALL CLUSTERS OF ALKALI METALS (Li, Na, AND K) ADSORBED ON GRAPHENE GAGANDEEP KAUR1,2, SHUCHI GUPTA1,3 & KEYA DHARAMVIR1 1
Department of Physics and Centre of Advanced Studies in Physics, Panjab University, Chandigarh, India 2 3
Chandigarh Engineering College, Landran, Mohali, Punjab, India
University Institute of Engineering and Technology, Panjab University, Chandigarh, India
ABSTRACT Graphene has unique properties arising from its honeycomb-lattice structure that could maximize the interaction of adsorbate on the layer. Many experimental methods have been devised to form stable metallic clusters of different sizes. Metal clusters, especially those of alkali-metal atoms Li, Na, and K, have played an important role in the development of cluster physics as a branch of modern physics and chemistry. Moreover metal-graphene contacts would play a crucial role in graphene based electronics. In this paper, we use a method involving interatomic model potentials to investigate the structure and binding energies of clusters of alkali metal Mn (M=Li, Na, K; n=1, 3), adsorbed on graphene. For M-M interaction, we use Gupta potential while Lennard-Jones potential was used for M-C interactions (C = carbon). We find that for each alkali cluster in the minimum energy configuration, adsorbation at the hollow site on grapheme is favored. Stability of the Mn - graphene system increases with the increase in size of the cluster. All trimers adopted a geometry close to an equilateral triangle with significant changes in bond lengths.
KEYWORDS: Graphene, Metal Clusters, Interatomic Potentials INTRODUCTION In recent years, graphene, the two-dimensional building block of graphite, has been the focus of attention as a substrate for new nano structure devices using its two-dimensional property. Fabrication of nanoclusters on graphene is of great interest in studies of applications in electronics, sensors, biodevices, catalysis, energy storage, etc [1, 2]. Metals adsorbed on graphene can form different types of structures and change graphene’s electronic behavior, giving rise to interesting physical properties. It is therefore essential to have a good understanding of metal–carbon (M-C) bond strength as well as cohesive energies of the system as these play significant roles in the cluster formation process. Small clusters composed of alkali-metal atoms have been the subject of numerous theoretical studies in recent years due to their structural and spectroscopic properties [3]. The enhancement of electrical conductivity in alkali atoms doped single-wall carbon nanotubes has been studied both experimentally and theoretically [4-6]. Spin-valve device structures operated by side gates were proposed utilizing the site dependence of alkali metal absorption on graphene nano ribbons [7]. It has been proposed that alkali-metal-plated graphite could have practical applications as a substrate in studying normal and superfluid He films [8, 9]. Adsorbed alkali atoms have been shown to act as chemical dopants on the CNTs and have been used to fabricate field effect transistors [10]. Potassium has also been used to chemically dope graphene, allowing graphene quasiparticle dynamics [11] and minimum conductivity [12] to be studied. Furthermore, the electronic structure of graphene bilayers [13] can be tuned by K atoms.
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Gagandeep Kaur, Shuchi Gupta & Keya Dharamvir
Motivated by the interesting results of adsorption of small metal clusters on graphene, we carry out a systematic analysis of small sized alkali metal clusters Mn (M= Li, Na, K; n=1,3) adsorbed on graphene using interatomic potentials. In this work the focus will be entirely on the adsorption site, the structure of small metal clusters adsorbed on grapheme, and relative stabilities of these various configurations.
THEORETICAL METHODS Graphene consists of C atoms arranged in a two-dimensional honeycomb lattice with a hexagonal primitive unit cell containing two atoms. The Mn - graphene system is modeled using Mn adatoms in a 20* 30 hexagonal graphene supercell. We take up clusters one by one. This setup corresponds to a coverage of Mn (n=1-3) adatoms per 2480 C atoms. In our notation, we let the y direction be perpendicular to the graphene plane, x and z axes lying in the plane. We consider the binding of the alkali atom on three sites: the hollow H site at the center of a hexagon, the bridge B site at the midpoint of a carbon-carbon bond, and the top T site directly above a carbon atom (Figure 1).
Figure 1: Three Adsorption Sites on the Graphene Sheet We use the atom-atom interaction approach, with Lennard – Jones (LJ) potential [14] for M-C interactions and Gupta Potential (GP) [15] for M-M interactions, keeping the graphene sheet rigid. The LJ parameters which have been used in our calculations are taken from reference [14]. The parameters for Gupta potential are taken from [15, 16]. We first find the minimum energy configuration for an Mn cluster. By placing the minimized M1-3 cluster a certain distance above the graphene sheet at various positions and in various orientations, the total energy of the cluster - graphene system is obtained by summing over M – C pairs interacting via LJ potential.
RESULTS AND DISCUSSIONS Binding Energy of a Single Atom on Graphene For the single adatom - graphene system, the adatom is moved along the y direction for each adsorption site (T, B, H). By placing the M atom above any of these three sites and then varying the distance between M atom and the sheet, total energy is obtained by summing over the potential energy (PE) of interaction between all the atoms on the graphene sheet and M, using LJ potential. Of the three adsorption sites considered above, the site with the minimum binding energy is referred to as the favored site. Binding energies for the three sites considered are summarized in Table 1. Alkali metal adatoms prefer to bind more strongly to the H site. For the isolated atom-graphene system, our conclusions for Li and Na are in agreement with Chan et al. [17] while for K adatom, he predicted B- site as favored site. For single atom adsorption, the hollow sites were found to be more favorable than carbon bridge or top of carbon atoms for all alkali metals as predicted by Caragiu et al. [18]. The adsorption energies and adsorption sites on graphene sheet for alkali metals calculated by K. Nakada et al. [19] using the DFT formulation also shows the H-site as the most preferred site for adsorption, though values of binding energies were
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Energetics of Small Clusters of Alkali Metals (Li, Na, and K) Adsorbed on Graphene
somewhat different from our calculations. So our results corroborate their findings on the whole. Table1: Interaction between Metal Adatom and Graphene Sheet. EB is Binding Energy and d is the Perpendicular Distance between the Metal Atom and the Graphene Sheet. In Each Case, LJ Potential Has Been Used ATOM
STABLE SITE
Li
H-site
Na
H-site
K
H-site
SITES T B H T B H T B H
E(eV) -0.315 -0.336 -0.337 -0.317 -0.318 -0.328 -0.362 -0.363 -0.369
d(A0) 3.14 3.04 3.04 3.44 3.43 3.38 3.84 3.83 3.81
Binding Energy of Isolated Clusters Using Gupta potential, we find binding energy and bond lengths for optimized structures of isolated Mn clusters which are summarized in Table 2. Our results are in good agreement with those available in literature. Our results for Li dimer matches well with experimentally derived equilibrium bond length of 2.673 Å for the lithium dimer [20]. For the Li trimer, the initial configurations considered are equilateral, isosceles as well as scalene triangles. However, after minimization, an isosceles triangle as predicted in ref. [21] is found to be the stable configuration. For the Na dimer, the calculated bond length is 3.31Å which is comparable to the quoted value of 3.21 Å [22]. However the optimized geometry obtained by us for this trimer is an isosceles triangle with bond lengths slightly different from the above value. Bond length of K dimer predicted by our calculations 4.23 Å matches with 4.21 Å and 4.22 Å as obtained by [23] and [24]. For potassium trimer, results for binding energy per atom -0.331 eV are same (-.3306 eV) as calculated by [25]. Table 2: Binding Energy and Bond Length of Isolated Optimized Mn Clusters M
Li
Na
K
2
Binding Energy/Atom (eV) -0.321
3
-0.437
2.59, 2.47, 2.59
2
-0.288
3.31
3
-0.383
3.44, 3.74, 3.44
2
-0.245
4.23
3
-0.331
4.36, 4.32, 4.36
n
Bond Length (Å) 2.67
Binding Energies of Minimized Dimers and Trimers on Graphene Sheet Calculations were performed for a variety of initial orientations (parallel and perpendicular) for the M2 dimers placed above the graphene sheet. All calculational details were the same as in the case of the single atom. For each dimer, initial configuration is not necessarily the same as that obtained after the system has relaxed. The preferred orientation of each dimer was found to be parallel to graphene sheet with the two atoms of the dimer occupying adjacent H-sites on the graphene as shown in figure 2. Our results for bond lengths of dimers of all three alkali metals compare well with ref. [26] where the values obtained were 2.95Å (2.68), 3.07Å (3.05), 4.65Å (4.04) respectively for Li2, Na2, K2 adsorbed on graphene. The value in parentheses denotes the gas phase (or free dimer) value.
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Gagandeep Kaur, Shuchi Gupta & Keya Dharamvir
Figure 2: Parallel Orientations of Dimer on Graphene Sheet after Minimization (Top View) For all trimers, we take the minimized isosceles triangle structures obtained by using Gupta potential as the initial structure. After minimization on the graphene sheet, however, M3 adopts a geometry close to an equilateral triangle with significant changes in bond lengths. For the Na3 trimer, optimized geometry on graphene is an equilateral triangle with bond length 3.47 Å which is comparable to the DFT work of Rytkönen et al. [27] who find a value of 3.35 Å.
Figure 3: Parallel Orientations of Trimer on Graphene Sheet after Minimization (Top View) Figure 4 shows the binding energies (BE) of clusters M1-3 on graphene. The minimized energy configurations of Mn have been tabulated in Table 3.
Figure 4: Binding Energy of M1-3 on a Graphene Sheet
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Energetics of Small Clusters of Alkali Metals (Li, Na, and K) Adsorbed on Graphene
Table 3: Minimized Energy Structures for Mn – Grapheme System Metal (M) Li Na K
n
EB (eV/atom)
dC-M (Å)
1 2 3 1 2 3 1 2 3
-0.337 -0.6576 -0.7738 -0.328 -0.613 -0.7113 -0.369 -0.6134 -0.6995
3.04 3.04 3.04 3.38 3.4 3.4 3.81 3.81 3.81
Initial Bond Lengths d M-M ( Å) 2.67 2.59,2.47,2.59 3.31 3.44,3.74,3.44 4.23 4.36,4.32,4.36
Initial Structure Isosceles Isosceles Isosceles
Final Bond Length d M-M ( Å ) 2.56 2.69 3.28 3.47 4.16 4.32
Final Structure Equilateral Equilateral Equilateral
CONCLUSIONS The structure and binding energies of small clusters of alkali metals on graphene has been studied using interatomic potential model. Of the three possible symmetric sites of adsorption, the H-site was found to be energetically favourable. For dimers and trimers, parallel orientations were preferred over perpendicular, on the graphene sheet. In order of stability of a single metal atom and their dimers on the grapheme sheet, we find Li > K > Na, indicating weak binding of Na and Na2 with graphene sheet. However, Na trimer was found to be more stable than K trimer on graphene sheet. Our results are in good agreement with available literature. Since for large clusters (of hundreds or thousands of atoms) ab initio electronic structure calculations are still, at present, computationally expensive, there has been much interest in developing atomic potentials for the simulation of such clusters.
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