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Development of Integrated Dry−Wet Synthesis Method for Metal Encapsulating Silicon Cage Superatoms of M@Si16 (M = Ti and Ta) Hironori Tsunoyama,†,§ Hiroki Akatsuka,† Masahiro Shibuta,§,‡ Takeshi Iwasa,†,§,⊥ Yoshiyuki Mizuhata,∥ Norihiro Tokitoh,∥ and Atsushi Nakajima*,†,§,‡ †

Department of Chemistry, Faculty of Science and Technology, and ‡Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan § Nakajima Designer Nanocluster Assembly Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, 3-2-1 Sakado, Kawasaki 213-0012, Japan ∥ Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: Nanoclusters (NCs) of several to hundreds of atoms in size are prospective functional units for future nanomaterials originating in their unique, size-specific properties. To explore the field of NC-based materials science, the development of large-scale, size-exclusive synthesis methods is in high demand, as one can see from the successful evolution of fullerene science. We have developed a large-scale synthesis method for main group-based NC compounds by scaling up the clean dry-process with a high-power impulse magnetron sputtering. The 100 mg scale synthesis of binary NCs of M@ Si16 (M = Ti and Ta) stabilized by poly(ethylene glycol) dimethyl ether enables us to characterize their structures by an array of methods, for example, mass spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and 29Si nuclear magnetic resonance. Spectroscopic evidence indicates that the M@Si16 NCs are the metal-encapsulating tetrahedral silicon-cage structure satisfying the 68 electrons, closed-electronic-shell superatom.

1. INTRODUCTION Nanoclusters (NCs) comprised of several to hundreds of atoms exhibit remarkable physical and chemical properties that can be designed not only by changing the number of atoms (cluster size) but also by alloying composition elements.1 Various important concepts have been elaborated, such as the superatom model for metals and metalloids,2−8 based on significant findings of the size specificity of magnetic,9 catalytic,10−12 thermodynamic,13 optical,14 and structural properties,15,16 which are remarkably different from their bulk counterparts. These investigations for the gas-phase NCs have resulted from extensive developments of dry NC generation methods,12,17−22 high-sensitive spectrometry,21,23 and immobilization methods onto surfaces.10−12,20,24−26 Although these findings show that NCs play a role in nanomaterials science, the limited yield (∼fmol/s) in conventional gas-phase NC synthesis, for example, laser vaporization method, prohibits the application of these unique NCs to “real world” functional materials. As the discovery of C60 fullerene27,28 opened a new field of carbon science29,30 including carbon nanotubes,31 it is ultimately important to develop a new strategy for high-yield synthesis of NCs under exclusive size control, facilitating the development of NC-based materials science. © XXXX American Chemical Society

Among the numerous NCs, composite NCs consisting of multiple components have attracted considerable attention, because the compositions provide a new parameter of the combinations of the constituent atoms for design of the electronic properties that can be used together with the cluster size parameter. In particular, metal encapsulating silicon cage M@Si16 NCs have attracted significant interest due to their unique cage structure and superatomic character, enabling electronically fine optimization by a central atom substitution with keeping the structural symmetry. Two types of M@Sin NCs with different compositions have been discovered in transition-metal−silicon binary systems, M@Si16 (M = groups 3−5)5−7,32−39 and MSi12 (M = group 6).40 For the latter, a central metal atom was proposed to be sandwiched by two hexagonal Si6 rings. For the former, two isomers were proposed:6,7,38,39 a 28-face polyhedron with a 16-coordinated central atom (Frank−Kasper (FK) phase) and a 10-face fullerene-like symmetric cage. The structures of M@Si16 NCs are different from those of well-known transition metal silicides, Received: June 30, 2017 Revised: August 23, 2017 Published: August 28, 2017 A

DOI: 10.1021/acs.jpcc.7b06449 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C such as MSi and MSi2, and silicon clathrate compounds of MxSi46 and MxSi136 (M = alkali and alkaline earth metals), in which M@Si20, M@Si24, and M@Si28 polyhedrons were interconnected.41 Furthermore, the structure could not be classified directly as containing multiple Si−Si bonds found in organosilicon complexes.42−45 In addition to symmetric geometry, both of the isomers for M@Si16 have 68 electron shell closing of superatomic orbitals (SAOs), which are delocalized over the NC.7,38 As a result, a superatom family of halogen-like (M@Si16−), rare-gas-like (M@Si16(0)), and alkali-like (M@Si16+) NCs could be formed for group 3, 4, and 5 elements, respectively.5 Replacement of the central metal atom in the polyhedral cage can potentially tune their properties and provide a series of superatoms while keeping their geometrical symmetry, which was first demonstrated in aluminum-based superatoms in the gas phase,46 and have been extensively studied in various binary superatoms of M@ Al12,47−51 M@E16 (E = Si, Ge, Sn, and Pb),5,7,38,52−54 M@E12 (E = Sn and Pb),55−58 and [email protected]−62 Although M@Si16 superatoms show stability superior to other sizes based on their electronic structure and the reactivity in the gas-phase molecular beam or on the surface,63−65 potential high reactivity of Si atoms may cause the difficulty in their chemical synthesis. On the basis of these points, we targeted superatomic M@Si16 NCs for the development of a large-scale synthetic method mainly based on the clean dry-physical process. We have developed a milligram-scale synthetic method for M@Si16 (M = Ti and Ta) superatomic NCs by integrating a dry physical method into wet chemical processes. The method is constructed by a physical approach for NC generation (magnetron sputtering and a flow cell reactor) and a chemical strategy for accumulation and isolation (crystallization) of superatomic NCs in a liquid dispersant. M@Si16 NCs stabilized with dispersants were successfully synthesized and isolated in milligram-scale, and their structures were identified for the first time as a metal-encapsulating tetrahedral silicon-cage (METS) by 29Si NMR and Raman spectroscopy combined with quantum chemical calculations based on the density functional theory (DFT).

Figure 1. Schematic of the NC synthesis apparatus based on HiPIMS and DiLET.

pressure in the cell to be 5−20 Pa; typical flow rates for Ar and He were 90−140 and 0−140 sccm, respectively. Generated NC ions and neutrals were introduced to a vacuum chamber (pressure 16). All of the purification procedures were carried out in the Ar filled glovebox. In the first isolation step that uses a mixed solvent of hexane (n-C6) and tetrahydrofuran (THF), n-C6:THF = 4.5:1, a polar fraction of M@Sin:PEG-DME NCs (#A) was separated as a precipitate from less polar M@Sin:PEG-DME and Sin:PEGDME NCs (#A′), and excess dispersant. In the second isolation step, slightly more polar M@Sin:PEG-DME NCs (#B) were separated from fraction #A using a slightly more polar mixed

2. METHODS 2.1. Gas-Phase Synthesis of M@Si16 NCs. Binary NCs of M@Sin (M = Ti and Ta) were generated by the intensive NC source66,67 based on a high-power impulse magnetron sputtering (HiPIMS) powered by a modulated-pulse power supply (Zpulser LLC, AXIA-3X) combined with a gas flow reactor (Figure 1). Magnetron sputtering (MSP) source (Angstrom Science Inc., ONYX-MAGII) with a disk target (o.d. = 50.8 mm, alloy of Ti−Si or Ta−Si with 3 at% of metal) was placed inside a NC growth cell (i.d. = 110 mm, length (L) = 460 mm, equipped with a liquid nitrogen jacket). Working gas of argon (Ar; 99.9995% pure, [O2] < 0.1 ppm) was introduced from multiple pin-holes on an anode (grounded) electrode. Atomic neutrals and ions were generated by HiPIMS, in which 1.0 ms width modulated pulse was utilized. Typical peak powers of discharge (Pp) and repetition rates of sputtering (Trep) were 0.5−3.0 kW and 120−200 Hz, respectively. Generated atomic species were condensed into NCs by the assistance of helium (He) buffer gas (99.9998% pure, [O2] < 0.1 ppm) cooled by liquid nitrogen, which was introduced upstream of the MSP source. Typical travel time of NCs in the growth cell is 50−100 ms. Flow rates of Ar and He gases were controlled by individual mass flow controllers to maintain B


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The Journal of Physical Chemistry C solvent (n-C6:THF = 2.5:1) in which a precipitate of #B was separable from larger, less polar M@Sin:PEG-DME NCs in the supernatant. In the final step, the soluble fraction (#C) of M@ Si16:PEG-DME in absolute THF was separated from insoluble, the most polar substances. A typical yield of fraction #C was 50−150 mg from the NC dispersion prepared for 20 h of the sputtering, in which 60 wt % of organic substances was included based on an elemental analysis. Fractional crystallization is one of the crucial steps in a successful isolation scheme, although the isolated fraction #C for M = Ti and Ta could not be completely purified despite extensive efforts; Sin NCs were probably contaminated based on elemental analyses (Si/Ti = 27/1 and Si/Ta = 22/1 based on inductively coupled-plasma atomic emission spectroscopy). 2.3. Preparation of SERS Substrate. Substrates for surface-enhanced Raman scattering (SERS) were fabricated by deposition of silver (Ag) NCs with average molecular weight of ca. 5000 amu onto strontium titanate (100) surface (Ag/ SrTiO3). Negative ions of Ag NCs generated by the HiPIMS NC source were deposited with kinetic energy less than 30 eV/ NC after coarse mass filtering by radio frequency electric field on the Q-MS filter. More than 80 monolayers of Ag NCs were deposited. Atomic force microscope images clearly indicate that a SERS suitable, tens-nanometer scale convexoconcave structure was formed for Ag/SrTiO3 (Figure S1). M@Si16 (M = Ti and Ta) NC cations were further deposited on the Ag/SrTiO3 without exposure of the substrate to air prior to the second deposition. Size selected M@Si16 NC (M = Ti and Ta) cations were soft-landed on the substrate with kinetic energy of less than 20 eV/NC. M@Si16 NCs with coverage of 5−10 ML were deposited (M@Si16/Ag/SrTiO3). 2.4. Characterization of M@Si 16:PEG-DME. Laser Desorption Ionization Mass Spectrometry (LDI-MS). LDI mass spectra were recorded with a reflectron time-of-flight mass spectrometer (ABSciex, Voyager DE-STR) equipped with a handmade sample transfer system under vacuum. The specimen was prepared by dropcasting a THF solution of isolated M@Si16:PEG-DME onto a gold plated stainless steel plate in the glovebox. The sample plate was transferred into a handmade transferring vessel, which was connected to the glovebox and evacuated. The plate was then transferred to the LDI-MS apparatus under vacuum. A 314 nm laser was used for desorption/ionization. Laser fluence was minimized to suppress fragmentation after desorption/ionization. Typical acceleration voltage was 7.5 kV. X-ray Photoemission Spectroscopy (XPS). XPS measurements were performed under ultrahigh vacuum conditions (600 rpm), and the center of stirring was offset from the center of the molecular beam to mix with the dispersant sufficiently. Isolation of M@Si16 NCs was conducted by fractional crystallization procedures. Because of the slightly polar character of M@Si16:PEG-DME NCs as compared to free PEG-DME, excess dispersant and less polar Sin:PEG-DME NCs were removed from the crude dispersion. Successful removal of excess PEG-DME was confirmed from 1H NMR and 13C NMR (Figure 3); sharp peaks corresponding to free PEG-DME disappeared in the NMR spectra of M@Si16:PEG-DME. To improve the purity of M@Si16:PEG-DME, the fraction was further fractionated by a slightly more polar mix solvent of THF and n-C6. Despite extensive efforts at crystallization, for example, variation of mixing ratio of THF/n-C6, use of other combinations of solvents, and temperature, elemental analyses of isolated #C fraction of M@Si16:PEG-DME (Ti 2.1 wt %, Si 32.2 wt % and Ta 9.2 wt %, Si 31.6 wt %) suggest simultaneous isolation of Si NCs in the samples even after the isolation. However, the ratio of Si/M (M = Ti and Ta) was greatly improved from the initial value of crude products (Si/M = 33); the values of Si/Ti and Si/Ta were determined to be 27/1 and 22/1 based on ICP-AES, respectively. The contaminant in fraction #C of Ta@Si16:PEG-DME is probably Sin−, which acts as a counteranion for [Ta@Si16]+. The increase of M@Si16 NCs in fraction #C was confirmed by LDI mass spectra as shown in Figure 4 and Figure S2. Only a series of M@Sin− (n = 14−16) ions were observed in the mass spectra (Figure S2). Because these fragmentations were reasonably explained by the stability of M@Si16− ions formed by the LDI process, the mass spectra provided clear fingerprints of parent M@Si16 NCs; the total valence electrons (NV) for [M@Si16]− (69e for M = Ti, 70e for M = Ta) exceed the shell closing number (68e), resulting in occupation of a high lying next subshell that tends to induce fragmentation to satisfy NV to be no more than 68. The absence of these ions in LDI-MS of fractions #A, #B, and the insoluble substances in THF indicates that the concentration of M@ Si16:PEG-DME increased in fraction #C throughout the crystallization steps. 3.3. Charge State of M@Si16:PEG-DME NCs. The charge states of isolated M@Si16:PEG-DME were examined by XPS. For fraction #C of Ti@Si16:PEG-DME, the main peak corresponding to Si 2p3/2,1/2 is almost the same as metallic Si in the zerovalent state (Figure 5a). The main peak for Ti 2p5/2 is consistently in agreement with metallic Ti, not with oxidized Ti(III) or Ti(IV) (Figure 5b), although the shoulder for these oxidation states appears as a minor feature. For the #C fraction of Ta@Si16:PEG-DME, the peaks for Si 2p3/2,1/2 (Figure 5c) and Ta 4f7/2 (Figure 5d) clearly indicate that the isolated Ta@

Figure 2. Representative mass spectra of M@Sin NC ions in the molecular beam generated by the MSP NC source. (a) Ti@Sin NC ions generated by HiPIMS. Sputtering conditions: peak power (Pp) = 3.9 kW, repetition rate (Trep) = 180 Hz, Ar flow = 140 sccm, and He flow = 140 sccm. (b) Ti@Sin NC ions generated by direct-current MSP. Sputtering conditions: discharge power = 0.35 kW, Ar flow = 76 sccm, and He flow = 140 sccm. (c) Ta@Sin NC cations generated by HiPIMS. Sputtering conditions: Pp = 1.1 kW, Trep = 140 Hz, Ar flow = 100 sccm, and He flow = 100 sccm.

factors affect the NC size in the same way, target NCs can be yielded at higher power under a low Ar pressure or lower power under a high Ar pressure. The selectivity of stable M@Si16 NC ions was found to increase at higher power under a lower Ar pressure, which is likely due to the fragmentation through evaporative cooling of hot, metastable, larger NCs. Even at the optimized condition of sputtering, selectivity of Ti@Si16 NC ions cannot be increased as high as 0.6% and 3.8% for NC anions and cations, respectively, by the direct-current MSP (Figure 2b) due to the formation of NCs larger than 600 amu. In contrast, production selectivity of Ti@Si16 NC ions in the HiPIMS method reaches 3.7% and 6.8% for NC anions and cations (Figure 2a), respectively, despite the lower stability of Ti@Si16+ (67e) and Ti@Si16− (69e) with open electronic shells. The enhancement of selectivity in HiPIMS is probably due to a higher plasma temperature and resulting increase of the internal temperature of NCs to induce evaporative cooling of the metastable NCs of MSin, M2Sin (n > 16). Therefore, more stable, target NCs made up of Ti@Si16 neutrals (68e) are formed more preferentially. Indeed, the shell closure ions of Ta@Si16+ (68e) are formed with more than 10% yield under optimum conditions (Figure 2c). Regardless of the high D


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The Journal of Physical Chemistry C

Figure 4. LDI-MS spectra of the fraction #C for (a) Ti@Si16:PEGDME and (b) Ta@Si16:PEG-DME in the negative ion mode. Red bars represent isotope distribution of M@Sin. Mass spectra in wider mass range are shown in Figure S2.

Figure 3. (a) 1H NMR and (b) 13C NMR spectra for the fraction #C of Ti@Si16:PEG-DME (blue) and PEG-DME (black) in THF-d8. (c) 1 H NMR and (d) 13C NMR spectra for that of Ta@Si16:PEG-DME (blue) and PEG-DME (black) in acetonitrile-d3.

Figure 5. XPS spectra of isolated #C fraction of M@Si16:PEG-DME for M = Ti (a and b) and M = Ta (c and d). Peak positions for reference compounds of metallic silicon (1), metallic titanium (2), and Ta@Si16 on a C60 monolayer (3)65 were indicated as black bars. In the spectrum for Ta 4f, the background was subtracted with cubic spline function, and the broad component of O 2s from PEG is shown as the black dotted curve along with the fitted result for Ta 4f (red dotted curves).

Si16:PEG-DME NCs are not oxidized throughout the chemical isolation multistep processes. Confidently, the binding energy of Ta 4f7/2 (22.6 eV) is almost the same as that of the [Ta@ Si16+−C60−] complex (22.5 eV) formed on the graphite surface.65 This agreement strongly suggests that all of the Ta atoms in isolated Ta@Si16:PEG-DME NCs are equivalent and very close to that of [Ta@Si16]+. For Si 2p, weak peaks corresponding to oxidized Si species (Si∼2+) were observed at higher electron binding energy (Figure 5a and c), which are probably caused by interaction with PEG-DME molecules. The quantitative agreement of the main peak positions indicates that the isolated samples are not oxidized throughout the purification process and the closed-electronic shells Ti@Si16 and Ta@Si16+ were isolated. The latter is also confirmed from the diamagnetic character of M@Si16:PEG-DME NCs as shown in Figure S3. In the temperature dependence of magnetic

susceptibility, paramagnetic and ferromagnetic behavior could not be identified. 3.4. Structures of M@Si16 NCs Based on DFT. To examine the spectroscopic data of M@Si16:PEG-DME NCs, the structures of free M@Si16 NCs were calculated by DFT. The optimized structures of Ti@Si16 and Ta@Si16+ are summarized in Figure 6. All of the geometries qualitatively match with previous calculations.6,7,38,39 The relative energy for these isomers depends on the exchange correlation functional; the fullerene-like D4d isomer (f-D4d) is more stable using the hybrid B3LYP functional, whereas the Frank−Kasper (FK) and the distorted FK (dist-FK) isomers become stable with the pure PBEPBE functional. Similar deviations of the relative energies among the isomers have been reported in the literature.6,7,39 In addition, the FK isomer becomes stable with improving the basis set from double-ζ (def-SV(P)) to triple-ζ (def-TZVP). On the basis of comparison with experimental spectroscopic E


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Figure 6. Optimized structures for M@Si16 NCs based on DFT (PBEPBE/def-TZVP). Values in parentheses indicate the relative energy from the most stable isomer in electronvolts. Figure 7. Raman spectra excited at 532 nm for isolated M@Si16:PEGDME and size-selected M@Si16 on the SERS substrate (M@Si16/Ag/ SrTiO3). (a) M = Ti and (b) M = Ta. Stick bars represent the selected Raman active modes calculated by DFT (Figure S6) for isomers of FK, dist-FK, and f-D4d.

data shown below, the PBEPBE/def-TZVP level of calculations gives reasonable results as compared to the others. For the raregas like Ti@Si16 NC, the FK isomer is more stable by 1 eV than f-D4d, whereas the energy becomes closer for the alkali-like Ta@Si 16 + , which is consistent with earlier theoretical predictions.6,39 According to the symmetry of each isomer, 16 Si atoms can be categorized into several sites as summarized in Figure S4. In the FK isomer, there are two Si sites: four Si atoms with tetrahedral (Td) symmetry and 12 Si atoms in triangular faces with Td symmetry. For the dist-FK isomer, one of the triangular faces in the FK isomer is rotated by 30°. As a result, three sites of Si atoms exist as the equatorial hexagon (six Si atoms), the bottom vertex (single Si atom), and residual nine Si atoms. For the f-D4d isomer of Ta@Si16+, there are again two sites of Si: eight Si atoms at two rectangular faces and eight Si atoms at equatorial positions. As a result of structure deformation of the f-D4d isomer for Ti@Si16, these equivalent sites split into totally eight. Chemical shifts in 29Si NMR (shown below) exhibit these symmetrically equivalent sites. Averaged distances between the central metal atom and surrounding Si atoms for FK isomers (2.78 Å for Ti@Si16, 2.81 Å for Ta@Si16+) are 4.5% and 3.1% shorter than those for f-D4d isomers (2.91 Å for Ti@Si16, 2.90 Å for Ta@Si16+). In contrast, bond lengths between the nearest neighbor Si atoms are longer in the FK isomer (2.48 Å for Ti@Si16, 2.50 Å for Ta@Si16+) than in the f-D4d isomers (2.32 Å for Ti@Si16, 2.33 Å for Ta@ Si16+); the latter is comparable with the fullerene-like Cl@Si20 compound (2.35 Å).45 The difference in bond lengths affects the vibrational frequencies discussed below. These bond lengths are almost the same between the FK and dist-FK isomers of Ta@Si16+. All of the geometrical parameters are summarized in Table S1. 3.5. Raman Spectroscopy of M@Si16 and M@Si16:PEGDME NCs. To characterize the structure of isolated M@ Si16:PEG-DME, Raman spectra are compared to those of the gas-phase synthesized, size-selected M@Si16 NCs and DFT calculations. Black curves in Figure 7 represent Raman spectra from the size-selected M@Si16 NCs in the M@Si16/Ag/ SrTiO3(100) SERS samples. Two strong broad peaks along with weak peaks at 300 and 620 cm−1 were commonly observed in the spectra; the strong peaks were red-shifted from Ti@Si16 (470 and 140 cm−1) to Ta@Si16 (450 and 125 cm−1). The same spectral features were observed in the isolated M@

Si16:PEG-DME NCs (473 and 145 cm−1 for M = Ti, 450 and 120 cm−1 for M = Ta) along with slight enhancement for the peak of 300 cm−1. The result combined with previous observations showing negligible deformation of the NC structures after soft-landing onto the substrate64 clearly indicates that the dominant species in the isolated M@ Si16:PEG-DME samples is M@Si16 NCs. Although the intensity could not be reproduced by the present DFT calculations (Figure S5) or by earlier DFT results,88 the red-shift for the stronger two peaks is reproduced by the FK isomer as summarized in Figure S6; the Raman active modes at 464 and 146 cm−1 for M = Ti and 448 and 131 cm−1 for M = Ta were found in this isomer. The deviation in intensity probably originates from omission of ligated PEG-DME molecules and NC−NC interactions in the soft-landed NCs in DFT calculations. Because the mode corresponding to 470 and 450 cm−1 could not be found for the dist-FK isomer, we can safely neglect this isomer from the structure candidates. Although the Raman active mode at 460 cm−1 was found for the f-D4d isomer of both M@Si16 NCs, the frequency is almost the same between M = Ti (463 cm−1) and M = Ta (464 cm−1). In addition, the corresponding modes for the peaks at 140 and 125 cm−1 could not be found in this isomer. These trends concerning the vibrational frequency were qualitatively equivalent in different levels of DFT calculations. The result strongly suggests that the structures of isolated M@Si16:PEGDME and soft-landed M@Si16 NCs are the metal-encapsulating tetrahedral silicon-cage (METS) with the 16 coordination Frank−Kasper structure. Because the vibration mode above 500 cm−1 could not be found in the FK isomer, the peak at 620 cm−1 probably originates from overtone of the fundamental mode. 3.6. NMR Spectroscopy of M@Si16:PEG-DME NCs. The 100-mg scale synthesis enabled us to characterize the M@Si16 compounds with NMR spectroscopy despite the lower sensitivity of 29Si nuclei. In the 1H NMR and 13C NMR spectra (Figure 3), broad peaks were observed with chemical F


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dynamics simulations (see the Supporting Information). In addition, (iv) mean displacement of the central metal atom, (v) vibrational expansion of the Si cage, and (vi) ligation of PEGDME molecules cause the shift in CS originating from modulation of electron distribution. On the basis of the potential energy curves (PECs) along displacement of the central metal atom (Figure S10), the central atom easily moves even at 300 K. The displacement of the central atom causes the distinguished variation of CS as summarized in Figure S11. On the basis of statistical analysis assuming the Boltzmann distribution in the PEC, the averaged 29Si CS shifted by +25−44 ppm in both cases. Breathing-like vibration of the Si cage causes more deviations on the CS as summarized in Figures S12 and S13. Expansion of the Si cage causes the positive shift in 29Si CS values. On the basis of the Boltzmann distribution assuming the pseudo harmonic oscillator, the averaged 29Si CS changes by +100 and +80 ppm for Ti@Si16 and Ta@Si16+, respectively. In contrast, variation of CS values for the f-D4d isomer with the displacement of the central metal atom is almost negligible (Figures S14−S17). Although expansion of the Si cage causes the positive shift of CS values in the f-D4d isomer, the shift is less than the FK isomer and raises more deviation from the experimental CS values. For the factor (v), ligations of PEG-DME probably cause the change of electron distribution to give the shift of CS. Preliminary calculations for CH3OCH3 adducts to the M@Si16 NC indicate the variation of CS by the ligation (−20 to +20 ppm), although the polarizable continuum model calculation gives no obvious CS (ΔCS = −5 to +5 ppm) for both isomers. For quantitative comparison, more precise calculations for the ligated M@Si16 NCs are required. On the basis of the above results, we conclude that the FK isomer qualitatively reproduces the experimental 29Si CS, which is consistent with the Raman spectroscopy. Larger deviation for Ta@Si16+ NC is likely caused by stronger ligation of the PEG-DME and/or complexation with counteranions to positively charged NCs. The structure of the FK M@Si16 NCs is remarkably different from those in silicon clathrates41 and organosilicon complex of Cl@Si20,45 in which the sp3 character of Si is dominant and dangling bonds are terminated by the interconnection of Si cages or terminal chlorine and Cl3Si groups. The contrasting structure clearly unveils the nature of Si−Si bonds in M@Si16 NCs. As summarized in Figure S18, molecular orbitals for valence electrons in the FK isomer of Ti@Si16 can be clearly categorized SAOs; the Ti@Si16 NCs has 68 valence electrons with closed electronic shell consisting of (1s)2(1p)6(1d)10(1f)14(2s)2(2p)6(1g)18(2d)10. The delocalized nature of SAOs helps to stabilize the symmetric Si cage with high coordination number (CN; CN = 6 for Si, CN = 16 for Ti) rather than smaller CN for the fullerene like f-D4d and Cl@ Si2045 compounds. 3.7. Stability of Ta@Si16:PEG-DME NC. Thermal stability of Ta@Si16:PEG-DME was examined on the basis of XPS. As reported for Ta@Si16+ NCs deposited on C60 fullerene,65 Ta@ Si16+ exhibits high thermal stability and high oxidation tolerance, which originate from the closed electronic shell of this superatom. Figure 9 shows the XPS spectra of TaSi16:PEGDME NCs after heat treatment at 673 K for 30 min. In the Si 2p region, the width and position of the metallic Si peak are almost the same as the dominant Si species of TaSi16:PEGDME (Figure 5c), although a broad peak corresponding to oxidized Si (blue dotted curve in Figure 9a) appears. For the Ta 4f region, Ta 4f7/2 and 4f5/2 (red curves in Figure 9b) can be

shifts (CS) similar to those of PEG-DME. Broadening of the peaks clearly indicates that PEG-DME ligates to the M@Si16, and the resulting increase in molecular weight causes slow diffusion of the organic moiety, making a more solid-like molecular environment. Small shifts to lower magnetic fields are consistent with the diamagnetic features of the NCs. In the 29 Si NMR spectra (Figure 8), two broad peaks were observed

Figure 8. 29Si NMR spectra of M@Si16:PEG-DME dispersed in THFd8 for (a) M = Ti (300 K) and for (b) M = Ta (318 K). Stick bars represent CSs calculated by ZORA-DFT for isomers of FK, dist-FK, and f-D4d at the PBE0/TZ2P level. The CSs averaged over the sites are shown with faint colors.

after subtracting background signals from the glass tubing at −100 ppm. Prominently broadened signals were observed at −80 and from −30 to −80 ppm for M = Ti and Ta, respectively, while very broad signals at +30 ppm were observed for both M@Si16 and another fraction of #A′. No other peaks were found in the CS ranging from −400 to +400 ppm (Figure S7). Similar CS values of the major peaks between Ti@ Si16:PEG-DME and Ta@Si16:PEG-DME NCs indicate similar electron distribution of both M@Si16, which is consistent with XPS results. The 29Si CS values calculated with the ZORA-DFT method (Figure 8, sticks) for the FK and f-D4d isomers are clearly distinguishable; the f-D4d isomer shows positive CS in contrast to negative CS for the FK-type isomers. It is also noteworthy that the 29Si CS values sensitively depend on the density functional as summarized in Figures S8 and S9. Because core electrons affect the CS more, we have chosen the ZORA-DFT method for the comparison. In contrast to clearly distinct CS values in ZORA-DFT calculations, only broad features were observed in the experiments. Under the present experimental conditions, the broad features in the spectra intrinsically originate from the merging of multiple peaks for different Si atom sites by factors (ii) and (iii), along with broadening by factor (i); (i) nuclear magnetic coupling of 29Si with a magnetic quadrupole of the central metal atom, (ii) fluctuation of the position of the central metal, and (iii) rearrangements of the polyhedral Si scaffold on the NMR time scale, induced thermally or through ligation of PEG-DME. Indeed, the polyhedral Si scaffold fluctuates at 298 K based on molecularG


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Movie S2: MD simulation of the FK isomer of Ti@Si16+ at 298 K (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoshiyuki Mizuhata: 0000-0001-5301-0024 Atsushi Nakajima: 0000-0003-2650-5608 Present Address ⊥

Department of Chemistry, Faculty of Science, Hokkaido University, 060-0810 Sapporo, Japan.

Notes

The authors declare the following competing financial interest(s): H.T., H.A., and A.N. are inventors on JAPAN patent JP 5493139, submitted by JST agency and Ayabo Corp., which covers Nanocluster generator.

Figure 9. XPS spectra in the vicinity of (a) Si 2p and (b) Ta 4f for Ta@Si16:PEG-DME after heat treatment at 673 K for 30 min under vacuum (90% of Ta in Figures 5c and 5d remain unchanged through the heat treatment. In addition, the final atomic ratio of Si(0)/Ta = 17 is very close to that for Ta@Si16, which suggests the counteranion made by Si was oxidized through the heat treatment. The result suggests the high stability of the Ta@Si16:PEG-DME NCs.

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4. CONCLUSIONS We have developed the dry−wet combination synthesis method suitable for highly reactive, main-group, and early transition metal NCs based on HiPIMS and DiLET techniques. Superatomic NCs of M@Si16 (M = Ti and Ta) have been synthesized using the present method with stabilization by PEG-DME. Raman and NMR spectroscopies indicate that the most probable structure of M@Si16 stabilized by PEG-DME is the metal encapsulating tetrahedral Si cage with Frank−Kasper structure, which is achieved for the first time by the present large-scale synthesis. The NCs exhibit high thermal stability originating from closed electronic shell, 68 electrons in SAOs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06449. Full citation of ref 70, AFM images of SERS substrate, LDI-MS, magnetic susceptibility, 29Si NMR of M@ Si16:PEG-DME, and details of DFT calculations (Raman spectra, 29Si chemical shift, potential energy curves for M@Si16, variation of 29Si chemical shift by vibrations, molecular orbital analysis, and optimized Cartesian coordinates) (PDF) Movie S1: MD simulation of the FK isomer of Ti@Si16 at 298 K (AVI) H


Article

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