Synthesis and Characterization of Metal-Encapsulating Si16 Cage Superatoms.

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, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing 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.

Nitric oxide oxidation of a Ta encapsulating Si cage nanocluster superatom (Ta@Si16) deposited on an organic substrate; a Si cage collapse indicator.

The chemical reaction kinetics of an alkali-like superatom comprising a tantalum encapsulating Si16 cage nanocluster (Ta@Si16) deposited on an n-type organic substrate composed of overlayered C60 fullerene upon exposure to nitric oxide (NO) as a reactive gas are investigated. Core level X-ray photoelectron spectroscopy reveals that Ta@Si16 oxidation with NO proceeds stepwise from the outer Si16 cage to the central Ta atom; during the initial stage, NO is dissociatively chemisorbed by the cage surface of Ta@Si16 without penetrating the cage, while under extreme reaction conditions, the collapse of the Si16 cage leads to NO oxidation of the central Ta atom. In particular, molecular NO adsorption is associated with Ta oxidation only after the collapse of the Si16 cage of Ta@Si16. The reaction kinetics of M@Si16 with NO in the earlier stages of oxidation are discussed in conjunction with density functional theory calculations. Due to the superatomic nature of the shell closure with valence electrons coupled with metal encapsulation, surface oxidation of the caged Si in Ta@Si16 takes place gently compared to that of a naked Si surface, with molecularly physisorbed NO functioning as an indicator of Si cage collapse.

Geometric and electronic properties of Si-atom doped Al clusters: robustness of binary superatoms against charging.

The geometric and electronic properties of silicon-atom-doped aluminum clusters, AlnSim (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of AlnSim show that the stability of AlnSim is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al10Si and Al12Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al10 and Si@Al12. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al11Si2. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al11Si2 is characterized to be, Si@Al11Si as an alkaline-like binary superatom. For Al21Si2, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.

Chemical characterization of an alkali-like superatom consisting of a Ta-encapsulating Si16 cage.

Chemical characterization was performed for an alkali-like superatom consisting of a Ta-encapsulating Si16 cage, Ta@Si16, deposited on a graphite substrate using X-ray photoelectron spectroscopy (XPS) to element-specifically clarify the local electronic structure of the cage atoms. The XPS spectra derived from Ta 4f and Si 2p core levels have been well modeled with a single chemical component, revealing the formation of a symmetric Si cage around the Ta atom in the deposited nanoclusters. On chemical treatments by heating or oxygen exposure, it is found that the deposited Ta@Si16 is thermally stable up to 700 K and is also exceptionally less reactive toward oxygen compared to other Ta-Si nanoclusters, although some heat degradation and oxidation accompany the treatments. These results show the promising possibility of applying Ta@Si16 as a building block to fabricate cluster-assembled materials consisting of naked nanoclusters.

Development of ultrafine multichannel microfluidic mixer for synthesis of bimetallic nanoclusters: catalytic application of highly monodisperse AuPd nanoclusters stabilized by poly(N-vinylpyrrolidone).

On account of their novel properties, bimetallic nanoparticles and nanoclusters (NCs) are strong potential candidates for optical, magnetic, and catalytic functional materials. These properties depend on the chemical composition and size (number of constituent atoms) of the NCs. Control of size, structure, and composition is particularly important for fabricating highly functional materials based on bimetallic NCs. Size- and structure-controlled synthesis of two-element alloys can reveal their intrinsic electronic synergistic effects. However, because synergistic enhancement of activity is strongly affected by composition as well as by size and structure, controlled synthesis is a challenging task, particularly in catalytic applications. To investigate catalytic synergistic effects, we have synthesized highly monodisperse, sub-2 nm, solid-solution AuPd NCs stabilized with poly(N-vinylpyrrolidone) (AuPd:PVP) using a newly developed ultrafine microfluidic mixing device with 15 µm wide multiple lamination channels. The synergistic enhancement for catalytic aerobic oxidation of benzyl alcohol exhibited a volcano-shaped trend, with a maximum at 20-65 at. % Pd. From X-ray photoelectron spectroscopic measurements, we confirmed that the enhanced activity originates from the enhanced electron density at the Au sites, donated by Pd sites.

Advanced nanocluster ion source based on high-power impulse magnetron sputtering and time-resolved measurements of nanocluster formation.

We developed a new nanocluster (NC) ion source based on the high-power impulse magnetron sputtering (HiPIMS) technique coupled with a gas flow cell reactor. Silver NC anions (Ag(n)(-)) with a maximum intensity of 5.5 nA (Ag11(-)) are generated with the size ranging from the atomic anion to the 70-mer, which is well-controlled by simply adjusting the peak power and repetition rate of the HiPIMS. By time-resolved density profiles of Ag(n)(-), we find that the ion beam generated by HiPIMS is characterized by individual 100 ms duration "bunches" below a repetition rate of 10 Hz, which is well-thermalized with a group velocity of 5 m/s. The high intensity of the NCs is attributable to the high ionization fraction by this HiPIMS ion source, while the underlying mechanism of the flexible size tuning of the ion source is understood by time-resolved mass spectrometry coupled with the sequential growth mechanism; the increment of the density of the target species in the bunches with the peak power and the overlapping of the bunches with the repetition rate cause the formation of large NCs.

Platonic hexahedron composed of six organic faces with an inscribed Au cluster.

The structures of nanomaterials determine their individual properties and the suprastructures they can form. Introducing anisotropic shapes and/or interaction sites to isotropic nanoparticles has been proposed to extend the functionality and possible suprastructure motifs. Because of symmetric anisotropy, Platonic solids with regular polygon faces are one of the most promising nanoscale structures. Introduction of Platonic solid anisotropy to isotropic nanomaterials would expand the functionality and range of possible suprastructure motifs. Here, we demonstrate a novel strategy to obtain nano-Platonic solids through the face coordination of square porphyrins on an inscribed Au sphere with adequate size. The face coordination of the multidentate porphyrin derivatives, with four acetylthio groups facing the same direction, on the Au cluster encased the Au cluster in a Platonic hexahedron with six porphyrin faces. Transmission electron microscopy, mass spectrometry, elemental analysis, and scanning tunnelling microscopy were used to confirm the formation of the nano-Platonic hexahedron.

A new binding motif of sterically demanding thiolates on a gold cluster.

A gold cluster, Au(41)(S-Eind)(12), was synthesized by ligating the bulky arenethiol 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacene-4-thiol (Eind-SH) to preformed Au clusters. Extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and the fragmentation pattern in the mass spectrometry analysis indicated that formation of gold-thiolate oligomers at the interface was suppressed, in sharp contrast to conventional thiolate-protected Au clusters.

Organogold clusters protected by phenylacetylene.

A new class of monolayer-protected Au clusters with Au-C covalent bonds (organogold clusters) was synthesized by ligating phenylacetylene (PhC≡CH) to PVP-stabilized Au clusters. Matrix-assisted laser desorption ionization mass spectrometry revealed for the first time a series of stable compositions of the organogold (Au:C(2)Ph) clusters.

Synthesis and characterization of Au102(p-MBA)44 nanoparticles.

The synthesis of Au(102)(p-MBA)(44) nanoparticles on a preparative scale in high yield is described. Various analytical methods are shown to give results consistent with the composition and known structure of the particles, showing the preparation is essentially homogeneous, and attesting to the validity of the methods as well. Derivatization of the particles with proteins and DNA is demonstrated, and conditions are described for imaging individual particles by cryo-EM at low electron dose, close to focus, conditions optimal for recording high-resolution details.

Chirality and electronic structure of the thiolate-protected Au38 nanocluster.

Structural, electronic, and optical properties of the thiolate-protected Au(38)(SR)(24) cluster are studied by density-functional theory computations (R = CH(3) and R = C(6)H(13)) and by powder X-ray crystallography (R = C(12)H(25)). A low-energy structure which can be written as Au(23)@(Au(SR)(2))(3)(Au(2)(SR)(3))(6) having a bi-icosahedral core and a chiral arrangement of the protecting gold-thiolate Au(x)(SR)(y) units yields an excellent match between the computed (for R = C(6)H(13)) and measured (for R = C(12)H(25)) powder X-ray diffraction function. We interpret in detail the electronic structure of the Au(23) core by using a particle-in-a-cylinder model. Although the alkane thiolate ligands are achiral, the chiral structure of the ligand layer yields strong circular dichroism (CD) in the excitations below 2.2 eV for Au(38)(SCH(3))(24). Our calculated CD spectrum is in quantitative agreement with the previously measured low-energy CD signal of glutathione-protected Au(38)(SG)(24). Our study demonstrates a new mechanism for the strong chiral response of thiolate-protected gold clusters with achiral metal cores and ligands.

Magic numbers of gold clusters stabilized by PVP.

We applied matrix-assisted laser desorption/ionization (MALDI) for mass analysis of Au clusters stabilized by poly(vinyl-pyrrolidone) (PVP) and discovered a series of magic numbers for Au cluster size: 35 +/- 1, 43 +/- 1, 58 +/- 1, 70 +/- 3, 107 +/- 4, 130 +/- 1, and 150 +/- 2. Magic numbers smaller than approximately 70 agree with those of free Au clusters and can be explained qualitatively by the electronic shell model. In contrast, magic numbers larger than approximately 100 are obviously different from those of the free clusters. We suggest that the deviation from the electronic shell model is due to perturbation of the electronic and/or geometric structures caused by interaction with PVP.

Effect of electronic structures of Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) on aerobic oxidation catalysis.

Au clusters smaller than 1.5 nm and stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) showed higher activity for aerobic oxidation of alcohol than those of larger size or stabilized by poly(allylamine) (PAA). X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO, and X-ray absorption near edge structure measurements revealed that the catalytically active Au clusters are negatively charged by electron donation from PVP, and the catalytic activity is enhanced with increasing electron density on the Au core. Based on similar observations of Au cluster anions in the gas phase, we propose that electron transfer from the anionic Au cores of Au:PVP into the LUMO (pi*) of O(2) generates superoxo- or peroxo-like species, which plays a key role in the oxidation of alcohol. On the basis of these results, a simple principle is presented for the synthesis of Au oxidation catalysts stabilized by organic molecules.

Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution.

Single-size platinum Pt6 subnanoclusters exhibit superior mass-specific and surface-specific activities for the oxygen reduction reaction. The enhanced activity is attributed to polarized electron distributions based on rigorous structure characterization by X-ray absorption fine structure spectroscopy and density functional theory.

Ubiquitous 8 and 29 kDa gold:alkanethiolate cluster compounds: mass-spectrometric determination of molecular formulas and structural implications.

The molecular formulas and charge state distributions of thus-far known ubiquitous alkanethiolate-protected gold clusters with core-masses of 8 and 29 kDa were assessed using electrospray ionization mass spectrometry. The 8 and 29 kDa clusters were determined to be composed of single species, [Au38(SCn)24]z and [Au144(SCn)59]z, respectively, with charge states of z >/= 0. Possible geometric structures for Au38(SCn)24 and Au144(SCn)59 are discussed, based on the structures of relevant systems that have been recently determined experimentally and theoretically: [Au25(SR)18]- and Au102(SR)44, in which the Au cores are protected by monomers [-SR-Au-SR-] and/or dimers [-SR-Au-SR-Au-SR-]. Their preferential formation and chemical robustness are proposed as being associated with high stability due to geometric factors, while the Au-thiolate interface takes on common motifs regardless of the underlying Au core.

The stability of binary Al12X nanoclusters (X = Sc and Ti): superatom or Wade's polyhedron.

Binary nanoclusters (NCs) exhibit strong potential as building blocks for tailor-made scientific materials based on the precise tuning of their electron countings and spin states along with the synergistic effects that originate from the constituent elements. Herein, we studied the electronic and geometric structures of transition metal (TM) doped aluminum (Al) Al12X NCs (X = Sc and Ti), which are binary systems that extend from representative superatom [Formula: see text] anions. On the basis of the photoelectron spectroscopy (PES) and density functional theory (DFT) calculations, Al12X anion and neutral structures are characterized as vertex-replaced icosahedron. The highly stable exohedral Al12X icosahedron is described based on an electron counting rule derived from the coupling of Wade-Mingos' rule and the jellium model.

A designer ligand field for blue-green luminescence of organoeuropium(ii) sandwich complexes with cyclononatetraenyl ligands.

A novel η9-coordinated double-decker sandwich complex of divalent europium is synthesized. The complex exhibits blue-green photoluminescence at 516 nm, which is significantly blue-shifted from those of other organoeuropium(ii) sandwich complexes (∼630 nm). The blue-shift was quantitatively explained by the weakened electrostatic field of the expanded 10-π ring.

Formation of a superatom monolayer using gas-phase-synthesized Ta@Si16 nanocluster ions.

The controlled assembly of superatomic nanocluster ions synthesized in the gas phase is a key technology for constructing a novel series of functional nanomaterials. However, it is generally difficult to immobilize them onto a conductive surface while maintaining their original properties owing to undesirable modifications of their geometry and charge state. In this study, it has been shown that this difficulty can be overcome by controlling the donor-acceptor interaction between nanoclusters and surfaces. Cations of Ta-atom-encapsulated Si(16) cage nanoclusters (Ta@Si(16)) behaving as rare-gas-like superatoms are synthesized in the gas phase and deposited on conductive surfaces terminated with acceptor-like C(60) and donor-like α-sexithiophene (6 T) molecules. Scanning tunneling microscopy and spectroscopy have demonstrated that Ta@Si(16) cations can be densely immobilized onto C(60)-terminated surfaces while retaining their cage shape and positive charge, which is realized by creating binary charge transfer complexes (Ta@Si(16)(+)-C(60)(-)) on the surfaces. The Ta@Si(16) nanoclusters exhibit excellent thermal stability on C(60-)terminated surfaces similar to those in the gas phase, whereas the nanoclusters destabilize at room temperature on 6 T-terminated surfaces owing to the loss of electronic closure via a change in the charge state.

Nanoparticle imaging. Electron microscopy of gold nanoparticles at atomic resolution.

Structure determination of gold nanoparticles (AuNPs) is necessary for understanding their physical and chemical properties, but only one AuNP larger than 1 nanometer in diameter [a 102-gold atom NP (Au102NP)] has been solved to atomic resolution. Whereas the Au102NP structure was determined by x-ray crystallography, other large AuNPs have proved refractory to this approach. Here, we report the structure determination of a Au68NP at atomic resolution by aberration-corrected transmission electron microscopy, performed with the use of a minimal electron dose, an approach that should prove applicable to metal NPs in general. The structure of the Au68NP was supported by small-angle x-ray scattering and by comparison of observed infrared absorption spectra with calculations by density functional theory.