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Stacking Principle and Magic Sizes of Transition Metal Nanoclusters Based on Generalized Wulff Construction S

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Stacking Principle and Magic Sizes of Transition Metal Nanoclusters Based on Generalized Wulff Construction S University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Xiaoshan Xu Papers Research Papers in Physics and Astronomy 2013 Stacking Principle and Magic Sizes of Transition Metal Nanoclusters Based on Generalized Wulff Construction S. F. Li Zhengzhou University X. J. Zhao Zhengzhou University X. S. Xu University of Nebraska-Lincoln, [email protected] Y. F. Gao University of Tennessee, Knoxville, [email protected] Zhenyu Zhang University of Science and Technology of China, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicsxu Part of the Atomic, Molecular and Optical Physics Commons, Condensed Matter Physics Commons, and the Engineering Physics Commons Li, S. F.; Zhao, X. J.; Xu, X. S.; Gao, Y. F.; and Zhang, Zhenyu, "Stacking Principle and Magic Sizes of Transition Metal Nanoclusters Based on Generalized Wulff onC struction" (2013). Xiaoshan Xu Papers. 27. https://digitalcommons.unl.edu/physicsxu/27 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Xiaoshan Xu Papers by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. week ending PRL 111, 115501 (2013) PHYSICAL REVIEW LETTERS 13 SEPTEMBER 2013 Stacking Principle and Magic Sizes of Transition Metal Nanoclusters Based on Generalized Wulff Construction S. F. Li,1,2,3,4 X. J. Zhao,1 X. S. Xu,4,5 Y.F. Gao,2,5,* and Zhenyu Zhang4,† 1School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China 2Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 3Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 4ICQD, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China 5Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, Tennessee 37831, USA (Received 16 February 2013; published 9 September 2013) Nanoclusters with extra stability at certain cluster sizes are known as magic clusters with exotic properties. The classic Wulff construction principle, which stipulates that the preferred structure of a cluster should minimize its total surface energy, is often invoked in determining the cluster magicity, resulting in close-shelled Mackay icosahedronal clusters with odd-numbered magic sizes of 13, 55, 147, etc. Here we use transition metal clusters around size 55 as prototypical examples to demonstrate that, in the nanometer regime, the classic Wulff construction principle needs to be generalized to primarily emphasize the edge atom effect instead of the surface energy. Specifically, our detailed calculations show that nanoclusters with much shorter total edge lengths but substantially enlarged total surface areas are energetically much more stable. As a consequence, a large majority of the nanoclusters within the 3d-, 4d-, and 5d-transition metal series are found to be fcc or hcp crystal fragments with much lower edge energies, and the widely perceived magic size of 55 is shifted to its nearby even numbers. DOI: 10.1103/PhysRevLett.111.115501 PACS numbers: 61.46.Bc, 31.15.AÀ, 36.40.Cg, 36.40.Mr The electronic, magnetic, catalytic, optical, and mechani- theoretical and/or experimental approaches, especially cal properties of nanoclusters are often drastically different concerning transition metal (TM) clusters with strongly from their bulk counterparts due to their distinct geometric directional d valence electrons. structures and quantum confinement effects and may lead to In this Letter, we employ first-principles total energy unique applications [1–4]. One class of nanoclusters, com- calculations within density functional theory to demon- monly known as ‘‘magic’’ clusters [5–8], has gained par- strate the need to generalize the classic Wulff construction ticular attention because of their extra stability. Such clusters principle by including the vital contribution of the edge are more abundant in typical fabrication processes (for atoms in the global energy minimization of the nanoclus- example, in a cluster beam [7]), and may also exhibit highly ters. Unlike the alkali or simple metal clusters, whose desirable functionalities either individually or as building valence electrons are dominantly itinerant s electrons, the blocks in cluster assembled materials [2,9–11]. For a given directional d orbitals within a TM cluster are more local- nanocluster of a fixed size and specific elemental composi- ized. Such localized d orbitals will be manifested in the tion, it is of fundamental interest and practical importance form of unsaturated dangling bonds on the edge atoms, to identify the dominant factors determining its preferred an aspect energetically undesirable. Furthermore, many of geometric structure and magicity, upon which the physical the surface atoms of a polyhedral nanocluster are actually and chemical properties of the cluster can be reliably located on the cluster edges, and such edge atoms can predicted and accurately tuned. contribute substantially in determining the structure and Two mechanisms have been frequently invoked in the properties of the nanocluster. The present study, reveal- explaining the magicity of a given cluster: atomic shell ing the dominance of the edge atom effect instead of the closure [5] and electronic shell closure [6,8], or both [7]. surface energy in structural optimization of the TM nano- The former is inherently rooted in the classic Wulff clusters, is particularly significant considering the crucial construction principle [12], which stipulates that the pre- functional roles played by the edge atoms in nanoscale ferred structure of a given cluster should minimize its catalysis and many other related physical and chemical total surface energy, resulting in high symmetry close- phenomena [19–23]. shelled Mackay icosahedronal (Ih) clusters [13] with Our calculations were carried out using the density odd-numbered magic sizes of 13, 55, 147, etc. [14–18]. functional theory [24] within the spin-polarized general- Nevertheless, for such clusters of nanometer sizes, the ized gradient approximation [25] as implemented in the validity of the classic Wulff construction principle has so VASP code [26]. The interaction of the valence electrons far not been rigorously examined using state-of-the-art with the ionic core was described with the projector 0031-9007=13=111(11)=115501(5) 115501-1 Ó 2013 American Physical Society week ending PRL 111, 115501 (2013) PHYSICAL REVIEW LETTERS 13 SEPTEMBER 2013 augmented wave method [27]. The atomic positions were optimized with the energy convergence of 0.001 eV. In obtaining the ground state configurations of the TMN clusters, we have considered many initial candidate configurations manually constructed or computationally generated via high temperature first-principles molecular dynamic simulations. We have also carried out selective checks on the most stable optimized structures using the particle swarm optimization (CALYPSO) code [28,29], as well as high temperature molecular dynamics simulations and vibrational frequency analysis, confirming that these structures are also dynamically stable. In our study, we first choose Ru nanoclusters of sizes around 55 atoms as prototypical examples. The preferred bulk structure of Ru is hcp, but it becomes a fcc-like crystal fragment for the Ru nanoclusters in the studied size range. I Furthermore, we reveal the nonmagic nature of the h-Ru55 cluster and identify the nearby even-numbered cluster of FIG. 1 (color online). Geometric structures and relative ener- Ru56 to be magic. We then extend our investigation to the 3 nd n ¼ gies of the 4 representative low energy configurations of Ru . series of TM clusters around -TM55 ( 3, 4, and 5), 55 The relative energies in (a)-(c) are measured from that of the Ih including fcc, hcp, and bcc elements. Most strikingly, we E ¼ Eð ÞEð ðI ÞÞ find that all these clusters prefer the fcc- or hcp-crystal- structure in (d), given by Á TM55 TM55 h . fragment (FCCCF or HCPCF) structures, except for the earliest and the latest TM elements in the periodic table. we first rule out the possibility that these low symmetry These structures have lower symmetries compared with the FCCCF and HCPCF Ru55 structures are stabilized via the Ih configuration, but each with significantly reduced edge usual electronic or geometric closed-shell mechanism. length, thereby compensating the energy increase associ- The former can be ruled out by the observation that there ated with the larger surface area. Consequently, the widely is no significant energy gap between the highest occupied assumed Ih magic clusters of size 55 actually possess much and lowest unoccupied molecular orbitals for either the less relative stability than their nearby even-numbered FCCCF or HCPCF structure, the latter by the fact that these clusters within the new configurations. The exceptions structures deviate severely from perfectly closed-shell of the earliest and latest TM55 clusters can be attributed configurations. Exclusion of other possible mechanisms, to the negligible numbers of d-type dangling bonds on the such as strong relativistic effects [31–34] that stabilize the edge atoms. s orbitals and destabilize the d orbitals, and enhanced Figure 1 displays 4 representative low energy Ru55 can- s-d hybridization [33], is presented in the Supplemental didate structures optimized from various initial configura- Material S3 [30]. tions. The 2 low symmetry structures in Figs. 1(a) and 1(b) To assess the relative importance of the edge atoms, are found to be much lower in energy than the high we can qualitatively separate the total energy of a given symmetry Ih structure shown in Fig. 1(d), by 3.120 and polyhedral cluster of size N into 3 terms: 2.691 eV, respectively. Each of the 2 more stable FCCCF E ¼ E þ E þ E ; structures contains 4 layers of atoms stacked in the A-B-C-A Tot Bulk Surf Edge (1) sequence, distributed as Að13Þ-Bð15Þ-Cð14Þ-Að13Þ,and E Að12Þ-Bð16Þ-Cð16Þ-Að11Þ in Figs.
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