Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Synthesis and Characterization of Metal-Encapsulating Si16 Cage Superatoms Hironori Tsunoyama,† Masahiro Shibuta,‡ Masato Nakaya,§ Toyoaki Eguchi,∥ and Atsushi Nakajima*,†,‡ †
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡ Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan § Department of Energy Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ∥ Department of Physics, Graduate School of Science, Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
CONSPECTUS: Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si16, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)2(1P)6(1D)10(1F)14(2S)2(1G)18(2P)6(2D)10 for M = group 4 elements in neutral ground state. The most important issue for M@Si16 BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si16 cage; the replacement of M yields a series of M@Si16 BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si16 have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si16 BCSs (M = Ti and Ta). A series of M@Si16 BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, sizeselected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a softlanding apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C60 fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric “metal-encapsulating tetrahedral silicon-cage” (METS) structure with Frank−Kasper geometry.
1. INTRODUCTION Nanoclusters comprised of several to hundreds of atoms have been a central issue of nanomaterials sciences originating from their unique nature: local chemical bonding significantly deviating from their bulk analogues1 and novel properties arising from size-specific electronic structures.2−6 Once satisfying specific stability in terms of highly symmetry, cage © XXXX American Chemical Society
geometry, and electronic shell closure, nanoclusters are promising smallest functional units for future nanomaterials.7 Deviations in electronic structures of inorganic nanoclusters from atoms and the bulk originate from interactions among Received: February 28, 2018
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DOI: 10.1021/acs.accounts.8b00085 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research constituent atoms in finite space. An interesting physical concept of “superatoms (SAs)” has been introduced, where valence electrons delocalized over nanoclusters construct new atomic-like orbitals (superatomic orbital; SAO) in nanoclusters.7−10 Therefore, various nanocluster compounds can be generated by changing not only the nanocluster size but also their elements. Alloying of nanoclusters additionally opens designer properties of functional units by the modulation of their electronic properties along with their geometric factors, for example, homogeneous solid solution and heterogeneous phase segregation. According to geometric and electronic constraints in the stability of alloy nanoclusters, highly symmetric cage structures have frequently been found in binary systems not only in nanocluster compounds, for example, X@Al1211−18 and M@Sin,19−23 (Figure 1) but also
2. PHYSICAL AND CHEMICAL PROPERTIES OF ISOLATED METAL-ENCAPSULATING SILICON-CAGE SUPERATOMS, M@SI16 Systematic investigations on silicon-based inorganic BCSs is based on the development of the dual laser vaporization (DLV) nanocluster source (Figure 2),25,36 where the ratio of the two
Figure 2. A schematic DLV setup for the modified “face-to-face” nanocluster source combined with a flow-tube reactor.25,36 Reproduced with permission from ref 36. Copyright 2013 Chemical Society of Japan.
Figure 1. BCSs of M@Si16 and X@Al12.
components can be changed through independent control of fluence and timing for the two lasers along with the helium (He) buffer gas. In 2005, we have found a periodic family of M@Si16 BCSs based on mass spectrometry (Figure 3) and
in intermetallic compounds.24 Binary caged nanoclusters usually satisfy concurrent stabilities of geometry and electronic structures, which synergistically give chemical robustness to these nanoclusters. A particularly important feature in the binary superatoms is the opening of a systematic, periodic chemistry of “binary cage superatoms (BCS)” by the replacing of the central metal atom, which tune the electronic counts of the BCS, one by one under the same skeleton structure (geometric symmetry). Chemical and physical properties of BCSs have been extensively studied for X@Al12 (X = groups 13 to 15 atoms)11−14 experimentally and theoretically. The X@Al12 BCSs have been studied in gas phase by mass spectrometry, anion photoelectron spectroscopy (PES), and chemical reactivity toward small molecules. The charge-specific stability of X@Al12 has been demonstrated, where the negative (1−), neutral (0 valent), and positive (1+) nanoclusters are stable for X = B, Si, and P, respectively. Based on stable charge states, the formation of ionic SA complexes, for example, [Al@Al12−Na] and [B@Al12−Cs], have been proven experimentally,17,25,26 which can be extended to nanocluster assembled materials.7−10 For the elemental nanoclusters of silicon, charge-dependent stability and complex formation have also been investigated in the gas-phase by doping electron donors (Na) and acceptors (F)27−30 with photoionization spectroscopy and anion PES combined with theoretical calculations. In 1987, Dr. Beck found highly size-specific stability in transition metal−silicon binary systems, where the strong signals of M@Si15 and M@Si16 were found for M = group 6.19 The following molecular beam experiments by Sanekata et al.31,32 and Hiura et al.33 along with theoretical investigations by Kumar and Kawazoe,20 Nagase,34 Torres and Balbás23,35 have suggested the metal-encapsulating cage structure of silicon as featured in this Account.
Figure 3. Mass spectra showing size-selective formation of (A) TiSi16 neutrals, (B) ScSi16 anions, and (C) VSi16 cations. Reproduced with permission from ref 21. Copyright 2005 American Chemical Society.
anion PES (Figure 4),21 where halogen-like (MIII@Si16−), rare gas-like (MIV@Si16(0)), and alkali-like (MV@Si16+) SAs have been demonstrated by the doping of group 3, 4, and 5 atoms, respectively. Strong signals for Sc@Si16−, Ti@Si16(0), and Ta@ Si16+ in the mass spectra clearly indicate their electronic shell closure for the same number of valence electrons (68e), where 64 and 4 electrons come from the 16 Si atoms and the central atom including charged states. The electronic closed-shell Sc@ Si16− exhibits relatively larger adiabatic electron detachment energy (3.41 eV) compared to its neighbors, while smaller electron detachment energies for Ti@Si16− and Ta@Si16− show the existence of excess electrons on the next subshell. The high lying excess electron in Ti@Si16− is also probed by the B
DOI: 10.1021/acs.accounts.8b00085 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
packing structure at their size. More recently, spectroscopic characterizations in the gas-phase have been performed by Bowen, Duncan, Lau, von Issendorf, and Lievens.40−43 The DLV nanocluster source enables the elucidation of the electronic structures and chemical properties of BCSs. However, the flux of BCS ions (several pA = 6 × 109 NCs/ s) by the DLV method is not high enough to extend BCSs to their immobilization on surfaces, toward their applications in future materials sciences. In principle, more than 1013 nanoclusters per cm2 of a substrate are required for 1monolayer (ML) coverage. Since size separation by mass spectrometer is desired for fine nanocluster sciences, high flux of nanocluster ions must be generated in their sources.
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TiSi16−
3. FABRICATION OF SURFACE IMMOBILIZED AND LIGAND-STABILIZED SUPERATOMS
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Figure 4. PES of ScSi16 (A, B), (C, D), and VSi16 (F, G) at 266 nm (A, C, F) and at 213 nm (B, D, G). Comparison of the PES of TiSi16− with that of TiSi16F− (E) enables us to assign the HOMO− LUMO gap. Reproduced with permission from ref 21. Copyright 2005 American Chemical Society.
3.1. Intensive Nanocluster Source, Nanojima
In order to increase the ion flux, we have developed a nanocluster ion source based on high-power impulse magnetron sputtering (HiPIMS)44 method, where ionized fractions reach as high as 80%45 while maintaining similar sputtering rates to direct-current magnetron sputtering. Figure 6 illustrates the soft-landing apparatus for size-selected nanocluster ions with the advanced nanocluster source, nanojima. Atomic neutrals and ions were generated by sputtering target disks with argon (Ar) ions using a modulated-pulse power supply. A typical modulated pulse is comprised of first a gradual increase of the cathode voltage (0.5 ms) followed by a constant highpower period (peak power = 1−15 kW) of 0.5 ms.46 The generated atomic species are condensed into nanoclusters with the assistance of the buffer He gas cooled with liquid nitrogen. Elementary reaction processes for nanocluster growth are summarized in Figure 7 and discussed later. Generated neutrals and ions of nanoclusters are filtered by polarity with an ion
formation of ionic complexes with an electron acceptor, fluorine (F),37 and a donor, cesium (Cs),38 and the large electron binding energy for the [Ti@Si16−F]− complex. Along with the tunable properties of these BCSs by replacing the central metal, the group 4 BCS was extended to germanium (Ge),39 which gives slightly larger-sized BCS in a similar manner to period numbers in the periodic table. Geometric stability is probed by chemical reactions with water molecules, which are more strongly bound to metal atoms than to the surrounding Si atoms. In the mass spectra (Figure 5), the disappearance of water adducts with increasing Si atoms unveils the smallest Si-cage fulfilling metalencapsulation, where 12 to 14 Si atoms are required. Chemical inertness at M@Si16 clearly indicates their geometric close-
Figure 5. Relative reactivity of anionic, neutral, and cationic MnSim−/0/+ toward H2O vapor; for (a−c) group 3, (d−f) group 4, and (g−i) group 5 central atoms. Open circles and solid squares indicate reactivity for n = 1 and 2, respectively. Vertical arrows show the threshold size where the relative reactivity is lost.37 Reproduced with permission from ref 37. Copyright 2007 American Chemical Society. C
DOI: 10.1021/acs.accounts.8b00085 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 6. Fabrication method for surface immobilized BCSs based on a HiPIMS method (nanojima) and soft-landing of nanocluster ions. Reproduced with permission from ref 46. Copyright 2013 The Chemistry Society of Japan. Reproduced with permission from ref 51. Copyright 2013 American Chemical Society.
concentration of atomic species in contrast to negligible concentration of nanoclusters. Excess energy originating from binding is removed by thermalization with the He buffer gas. After the density of atoms decreases, aggregation (coalescence) of nanoclusters becomes dominant for further growth (2-a, 2-b, and 2-c). Therefore, the nanocluster density in the flow dramatically decreases downstream at the cell, terminating the nanocluster growth. Since the number of charged species is proportional to the number of initial atomic ions, HiPIMS enhances nanocluster ion formation. However, when the density of charged species is high enough to interact with each other, a charge annihilation process causes the yielding of neutral nanoclusters (3), which decreases the number of nanocluster ions. This process is indeed dominant under high power sputtering (>several kW). On the other hand, under high power sputtering, insufficient cooling with buffer He induces fragmentation processes required for populating stable magic-number nanoclusters (4), typically above 1 kW. Although nanocluster formations are always exothermic regardless of the size, size-dependent thermodynamic stability leads to various sized nanoclusters through reaction kinetics.46 Particularly, geometrically and electronically stable species are increasingly populated under high power, since the fragmentation rate is sensitive to size-dependent thermodynamic stability of nanoclusters. Figure 8 summarizes representative mass spectra of transition metal−silicon BCSs of Ta@Si16+ and Ti@Si16+ (Figure 8a). Owing to the high stability of closed-shell Ta@Si16+, its ion current reaches 3.5 nA (2 × 1010 NCs/s) after mass selection. Although Ti@Si16+ does not satisfy electronic shell closure, its
Figure 7. Summary of elementary reaction steps in nanocluster formation.52
deflector and by mass-number (cluster-size) with a quadrupole mass-filter. The resolution of the mass-filter was adjusted to increase the transmission of ions under appropriate resolutions for resolving each number of atoms: typical resolution is in the range of m/Δm = 20−200. Single-size nanocluster ions are deposited on a solid substrate with a typical kinetic energy of 5−30 eV (
