Silicon Cages Research Articles

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 superatomswas 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.

Isolation of R6Si6 Dianion: A Bridged Tricyclic Isomer of Dianionic Hexasilabenzene.

A new strategy for the highly selective synthesis of tricyclo[2,2,0,02,5]hexasilanes R6Si6X2 (R = 2,4,6-Me3C6H2; X = H, Cl) and a bridged tricyclic R6Si6 dianion starting from the tetrachlorotrisilane RCl2SiSi(H)RSiCl2R (1) was described. Reduction of 1 with lithium naphthalene afforded tricyclohexasilane R6Si6H2 (2), which was halogenated to give the dichloride R6Si6Cl2 (3). Reduction of 3 with four equivalents of potassium graphite in the presence of 18-crown-6 afforded the first R6Si6 dianion (5) paired with [K(18-crown-6)]+2 counterions. The dianion 5 could act as a two-electron reductant toward transition metal halides and a nucleophile toward chlorosilanes. These reactions allowed the efficient and selective access to three types of silicon cages. The structures of the representative cages were confirmed by X-ray diffraction studies. Density functional theory calculations on 5 indicate that the negative charges are localized mainly on the anionic silicon atoms.

Development of Integrated Dry–Wet Synthesis Method for Metal Encapsulating Silicon Cage Superatoms of M@Si16 (M = Ti and Ta)

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 ...

Synthesis and thermal behaviour of phenyl-substituted POSSs linked by aliphatic and aromatic bridges

Five novel bridged heptaphenyl polyhedral oligomeric silsesquioxanes (POSSs), in which two identical silicon cages R7(SiO1.5)8 (with R = Phenyl) are linked to various aliphatic [(CH2) n with n = 2, 6 and 10] and aromatic (Ar–Ar and Ar–O–Ar, where Ar = p-C6H4) bridges, were synthesized. The obtained compounds were characterized by elemental analysis and 1 H NMR spectroscopy, and the results were in very good agreement with those of expected products. The synthesized heptaphenyl POSSs were thermally degraded, in dynamic heating conditions (25–700 °C), in both flowing nitrogen and static air atmosphere. The temperatures at 5% mass loss (T 5%) and residues at 700 °C were thus determined to evaluate their resistance to thermal degradation, but no substantial difference was found between the values in the two studied environments. The obtained T 5% values were much higher than those of the corresponding isobutyl and cyclopentyl POSSs we investigated in the past. Also, the T 5% values of the phenyl POSSs with aliphatic bridges decreased quite linearly on increasing the number of bridge carbon atoms (n C). This behaviour was interpreted and attributed to the presence of the external corona formed by voluminous phenyl groups. The residue at 700 °C, which was largely higher than those of the corresponding isobutyl and cyclopentyl POSSs, was a further confirmation of the better thermal stability of the compounds here studied.

Structure Dependent Magnetic Coupling in Cobalt-Doped Silicon Clusters

The structure of cobalt-doped silicon clusters, SinCo+ (n = 5–8) and SinCo2+ (n = 8–12), is investigated in a combined infrared multiple photon dissociation spectroscopy and density functional theory study. The singly doped clusters have exohedral structures in which the Co atom substitutes an atom of bare Sin+1+ clusters. In the doubly doped SinCo2+ clusters, the second Co atom is adsorbed to the singly doped counterparts and, for n ≥ 9, one of the Co atoms is encapsulated by a silicon cage. Computational analysis of the electronic and magnetic properties of the identified isomers indicates a distance dependent magnetic coupling between the Co atoms in the SinCo2+ clusters.

Ferrocenyl-substituted Bicyclo[1.1.1]pentasilanes: Evaluation of Significant Interactions between Ferrocenyl Units and Rigid Silicon Cages

Ferrocenyl-substituted bicyclo[1.1.1]pentasilanes 1, 2, and 3 were synthesized by the reactions of bromo-substituted bicyclo[1.1.1]pentasilanes with lithioferrocene. Significant interactions betwee...

Study of the Structural and Electronic Properties of Neutral and Charged Niobium-Doped Silicon Clusters: Niobium Encapsulated in Silicon Cages

We performed systematic structure searches for low energy structures of neutral and singly charged niobium-doped silicon clusters NbSinQ (n = 2–20; Q = 0, ± 1) by means of the CALYPSO structure searching method. A large population of low energy clusters is collected from the unbiased structure search. Subsequent geometry optimizations using density-functional theory with the B3LYP exchange-correlation functional are carried out to determine structural patterns and relative stabilities of various low energy candidates for Nb-doped silicon clusters. Based on the calculated binding energies along with measured photoelectron spectroscopy data, we are able to confirm that our lowest energy structures are the true minima. It is shown that the localized position of the Nb impurity atom in NbSin0/±1 clusters gradually moves from the convex capping position, to surface-substituted, to the concave, and in the end to the encapsulated state as the number of Si atoms increases from 2 to 20. The lowest energy isomer of both neutral and anionic NbSi12 cluster is very stable in a high-symmetry endohedral D6h structure in which the Nb atom is placed at the center of a regular hexagonal prism of Si atoms. This makes it an attractive building block for cluster-assembled materials.

Synthesis and Thermal Behaviour of Novel Aliphatic/Aromatic Hepta-Cyclopentyl Bridged Polyhedral Oligomeric Silsesquioxanes (POSSs)/Polystyrene (PS) Nanocomposites

The resistance to the thermal degradation of some polystyrene (PS) based nanocomposites, containing as filler novel aliphatic and aromatic hepta-cyclopentyl bridged polyhedral oligomeric silsesquioxanes (POSSs), was evaluated in both inert and oxidative atmospheres. The fillers were formed by two identical silicon cages R7(SiO1.5)8 (R = cyclopentyl) linked to several aliphatic [–(CH2)2–, –(CH2)6– and –(CH2)10–] and aromatic (Ar, Ar–Ar, Ar–O–Ar and Ar–S–Ar) bridges, where Ar = p-C6H4. Nanocomposites were prepared by in situ polymerization of styrene in the presence of 5 % w/w of appropriate POSS. The actual filler content in the products obtained, checked by 1 H NMR spectroscopy, resulted in all cases slightly higher than in starting mixtures. The glass transition temperature (T g) was also determined by Differential Scanning Calorimetry. The degradation of nanocomposites was carried out into a thermobalance and the temperatures at 5 % mass loss (T 5 %) were determined to evaluate the resistance to the thermal degradation which resulted, for the compounds here studied, higher not only than PS, but also than those of the nanocomposites filled with the corresponding hepta-isobutyl POSSs. The results were compared and discussed and suggested that the increments of resistance to thermal degradation in respect to neat PS (T 5 % of nanocomposite − T 5 % of PS) obey to an additivity group law.

Synthesis and thermal characterization of new dumbbell-shaped cyclopentyl-substituted POSSs linked by aliphatic and aromatic bridges

Seven aliphatic and aromatic dumbbell-shaped bridged hepta-cyclopentyl polyhedral oligomeric silsesquioxanes (POSSs), in which two identical silicon cages R7(SiO1.5)8 (with R = cyclopentyl) are linked to various aliphatic [(CH2)n with n = 2, 6 and 10] and aromatic (Ar, Ar–Ar, Ar–O–Ar and Ar–S–Ar, where Ar = p-C6H4) bridges, were prepared through a literature method opportunely modified by us to obtain higher yields. The obtained products were checked by elemental analysis and 1 H NMR spectra, and the results were in very good agreement with those of expected products. The synthesized hepta-cyclopentyl POSSs were degraded, in dynamic heating conditions, in both flowing nitrogen and static air atmosphere. The determined temperatures at 5 % mass loss (T 5%) and residues at 700 °C evidenced a much better resistance to thermal degradation than the corresponding isobutyl POSSs we investigated in the past. For aromatic POSSs no substantial difference was found between the T 5% values in inert environment and those in oxidative atmosphere. By contrast the T 5% values of aliphatic POSSs in air were slightly lower than the corresponding ones in nitrogen and decreased, in both environments, on increasing the aliphatic chain length, then showing an opposite trend in respect of the corresponding isobutyl derivatives. This behaviour was interpreted as due to the presence in the POSSs molecules of bulky cyclopentyl groups in the place of smaller isobutyls.

Room-Temperature Ordered Spin Structures in Cluster-Assembled Single V@Si12 Sheets

Since most of the existing pristine two-dimensional (2D) materials are either intrinsically nonmagnetic or magnetic with small magnetic moment per unit cell and weak strength of magnetic coupling, introducing transition metal atoms in various nanosheets has been widely used for achieving a desired 2D magnetic material. However, the problem of surface clustering for the doped transition metal atoms is still challenging. Here we demonstrate via first-principles calculations that the recently experimentally characterized endohedral silicon cage V@Si12 clusters can construct two types of single cluster sheets exhibiting hexagonal porous or honeycomb-like framework with regularly and separately distributed V atoms. For the ground state of these two sheets, the preferred magnetic coupling is found to be ferromagnetic due to a free-electron-mediated mechanism. By using external strain, the magnetic moments and strength of magnetic coupling for these two sheets can be deliberately tuned, which would be propitious to their advanced applications. More attractively, our first-principles molecular dynamics simulations show that both the structure and strength of ferromagnetic coupling for the pristine porous sheet are stable enough to survive at room temperature. The insights obtained in this work highlight a new avenue to achieve 2D silicon-based spintronics nanomaterials.

English

We need good data to practise good medicine. However, if a system becomes dominated by electronic health records (EHRs), computerized decision-making programs, and excessive guidelines and protocols, physicians can become pawns in “the silicon cage.” Arnold Eiser, professor of medicine and associate dean at Drexel University College of Medicine, is concerned about the erosion of both the patient-physician relationship and professionalism in the corporate world of American medicine. This postmodern world is characterized by what he calls “the three big C’s” of American medicine: consumerism, computerization, and corporatization. He notes that it is difficult to gain a broad perspective of the changes in health care when you are living through it. To provide an overview, he employs a wide-angle lens that includes postmodern philosophers, contemporary commentators, bioethicists, policy makers, and experiences from other countries (although this final aspect is relatively thin). Eiser begins by recounting the changes since 1970, when the health care system became more corporate. This business model increasingly viewed health care as marketable services and commodities, which was aided by challenges to the tradition of physician power and paternalism, coupled with social trends that favoured individualism, autonomy, and entitlements.

Dumbbell-shaped polyhedral oligomeric silsesquioxanes/polystyrene nanocomposites: The influence of the bridge rigidity on the resistance to thermal degradation

A comparative study concerning the resistance to the thermal degradation of polystyrene-based nanocomposites loaded with five novel aromatic dumbbell-shaped polyhedral oligomeric silsesquioxanes was carried out in dynamic heating conditions. The fillers were formed by two identical silicon cages R7(SiO1.5)8 (R = isobutyl) linked to several aromatic bridges (Ar, Ar–Ar, Ar–O–Ar, Ar–S–Ar and Ar–SO2–Ar) where Ar = p-C6H4. Nanocomposites were prepared by in situ polymerization of styrene in the presence of 5% of appropriate polyhedral oligomeric silsesquioxanes. The actual filler content in the products obtained, which was checked by 1H NMR spectroscopy, resulted in all cases slightly higher than that in starting mixtures. Glass transition temperature ( Tg) was determined by differential scanning calorimetry. Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis were carried out in both flowing nitrogen and static air atmosphere, and temperatures at 5% mass loss ( T5%) were determined to investigate the resistance to the thermal degradation. The results obtained were compared with each other and discussed. The resistance to the thermal degradation showed modest increments in respect to neat polystyrene, differently from analogous nanocomposites containing as fillers aliphatic bridged polyhedral oligomeric silsesquioxanes, as supported by scanning electron microscopy measurements which evidenced polyhedral oligomeric silsesquioxanes auto-aggregation phenomena. This behaviour was interpreted as due to the rigidity of polyhedral oligomeric silsesquioxane's aromatic bridges, which leads to low miscibility between filler and polymeric matrix.

Structural and Electrochemical Performance of Ternary Type-I Clathrate As Anode Materials for Lithium-Ion Batteries

Due to its high theoretical specific capacity, silicon has been the subject of intense interest and research as a high energy density anode for lithium-ion batteries. In the last several years, nanostructured silicon has been shown to be very effective for relieving lithiation induced strain and avoiding pulverization, which can lead to capacities > 3000 mAh/g, a 10X improvement over the commercially used graphite anode. However, despite this impressive proof-of-principle, nanostructured silicon can still suffer from degradation over long cycling times due to several reasons such as collapse of the nanostructure, increased porosity, structural instability, and insufficient surface passivation. The work to be presented here will introduce our investigations into the lithiation of silicon clathrates, (fullerene-like materials where silicon atoms are arranged in cage-like structures) that have been shown, through first principles calculations, to be able to allow lithium insertion without a large volume change or pulverization [1].In our group, we have investigated silicon clathrate materials, which have cage-like structures, as an anode material of Li-ion battery. Our studies on type II clathrate based on Na24Si136 show that lithium insertion is energetically feasible inside silicon cages that are fully occupied by other guest atoms. However, large amounts of lithium insertion cause a transformation between from the clathrate structure to an amorphous silicide and eventually Li15Si4 [2].This presentation discusses the structure and electrochemical performance of ternary Ba-X-Si type-I clathrate (where X = Cu, Al). Polycrystalline silicon clathrate materials are synthesized using thermal annealing and arc-melting methods. Electrochemical and structural analysis show these cage-like silicon structures have the ability to allow lithium insertion and removal without a large volume change or pulverization. Lithiation/delithiation of silicon clathrate anodes will be presented comparing to that of diamond cubic and amorphous silicon. Ex-situ X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic resonance are also employed to understand the structural changes upon lithiation and delithiation.

Structure assignment, electronic properties, and magnetism quenching of endohedrally doped neutral silicon clusters, Si(n)Co (n = 10-12).

The structures of neutral cobalt-doped silicon clusters have been assigned by a combined experimental and theoretical study. Size-selective infrared spectra of neutral Si(n)Co (n = 10-12) clusters are measured using a tunable IR-UV two-color ionization scheme. The experimental infrared spectra are compared with calculated spectra of low-energy structures predicted at the B3P86 level of theory. It is shown that the Si(n)Co (n = 10-12) clusters have endohedral caged structures, where the silicon frameworks prefer double-layered structures encapsulating the Co atom. Electronic structure analysis indicates that the clusters are stabilized by an ionic interaction between the Co dopant atom and the silicon cage due to the charge transfer from the silicon valence sp orbitals to the cobalt 3d orbitals. Strong hybridization between the Co dopant atom and the silicon host quenches the local magnetic moment on the encapsulated Co atom.

Optical Properties and Quasiparticle Band Gaps of Transition-Metal Atoms Encapsulated by Silicon Cages

Semiconductors assembled upon nanotemplates consisting of metal-encapsulating Si cage clusters (M@Sin) have been proposed as prospective materials for nanodevices. To make an accurate and systematic prediction of the optical properties of such M@Sin clusters, which represent a new type of metal–silicon hybrid material for components in nanoelectronics, we have performed first-principles calculations of the electronic properties and quasiparticle band gaps for a variety of M@Si12 (M = Ti, Cr, Zr, Mo, Ru, Pd, Hf, and Os) and M@Si16 (M = Ti, Zr, and Hf) clusters. At first stage, the electronic structure calculations have been performed within plane-wave density functional theory in order to predict equilibrium geometries, polarizabilities, and optical absorption spectra of these endohedral cagelike clusters. The quasiparticle calculations were performed within the GW approximation, which predict that all of these systems are semiconductors exhibiting large band gaps. The present results have demonstrated that the independent-particle absorption spectra of M@Sin, calculated within the local density or generalized gradient approximations to density functional theory, are dramatically influenced by many-body effects. On average, the quasiparticle band gaps were significantly increased, in comparison with the independent-particle gaps, giving values in the 2.45–5.64 eV range. Consequently, the inclusion of many-body effects in the electron–electron interaction, and going beyond the mean-field approximation of independent particles, might be essential to realistically describe the optical spectra of isolated M@Sin clusters, as well as their cluster-assembled materials.

Thermal behaviour of a series of novel aliphatic bridged polyhedral oligomeric silsesquioxanes (POSSs)/polystyrene (PS) nanocomposites: The influence of the bridge length on the resistance to thermal degradation

Abstract The thermal degradation of Polystyrene based nanocomposites, containing as fillers novel dumbbell-shaped bridged POSSs in which two identical silicon cages R 7 (SiO 1.5 ) 8 (R = isobutyl), are linked to aliphatic bridges (CH 2 ) n of different lengths ( n = 2, 4, 6, 8 and 10), was studied. Various nanocomposites were prepared by in situ polymerization of styrene in the presence of 5% of appropriate POSS. The obtained compounds were characterized by 1 H NMR spectroscopy, to verify their actual filler content, which was slightly higher than that in starting mixtures, and by Differential Scanning Calorimetry (DSC) to determine glass transition temperature ( T g ). Thermogravimetric (TG) and Differential Thermogravimetric (DTG) analysis, in both flowing nitrogen and static air atmosphere, were carried out and temperatures at 5% mass loss ( T 5% ) were determined to investigate the resistance to the thermal degradation. The results obtained for various samples were compared with each other and with those of virgin Polystyrene, and were interpreted and discussed.

Synthesis, characterization and thermal stability of new dumbbell-shaped isobutyl-substituted POSSs linked by aromatic bridges

Five new dumbbell-shaped polyhedral oligomeric silsesquioxanes (POSSs), in which two identical silicon cages R7(SiO1.5)8 (with R = isobutyl), linked to various aromatic bridges (Ar, Ar–Ar, Ar–O–Ar, Ar–S–Ar and Ar–SO2–Ar, where Ar = p-C6H4) were prepared through a literature method opportunely modified by us to make easier preparation and increase yield, which was higher than 70 % in all cases. The obtained products were the expected ones, as supported by the results of elemental analysis and 1H NMR spectra. Their resistance to the thermal degradation in both flowing nitrogen and static air atmosphere was checked by degrading samples at 10 °C min−1 and determining temperatures at 5 % mass loss (T5%) and residues at 700 °C. The T5% values in air were lower than the corresponding ones in nitrogen, but the trend among the various POSSs investigated was the same in both used atmospheres, with the most high value for the compound having the Ar–O–Ar aromatic bridge. The residues at 700 °C in air of the compounds having not hetero-atoms (O or S) in the aromatic bridge were higher than those in nitrogen, whilst no substantial difference was observed for the other ones.

Smallest fullerene-like silicon cage stabilized by a V2 unit

We conducted a combined anion photoelectron spectroscopy and density functional theory study on V2Si20 cluster. Our results show that the V2Si20 cluster has an elongated dodecahedron cage structure with a V2 unit encapsulated inside the cage. It is the smallest fullerene-like silicon cage and can be used as building block to make cluster-assembled materials, such as pearl-chain style nanowires.

Synthesis and thermal properties of new dumbbell-shaped isobutyl-substituted POSSs linked by aliphatic bridges

Five new dumbbell-shaped polyhedral oligomeric silsesquioxanes (POSSs), in which two identical silicon cages are linked to various length aliphatic bridges, were prepared by corner capping reaction between hepta isobutyltricycloheptasiloxane trisilanol (HIBT) and suitable bis(triethoxysilyl) derivatives. The products obtained were characterized by elemental analysis and 1H NMR spectroscopy, and the results were in very good agreement with the expected ones. Degradations were carried out in flowing nitrogen and in static air atmosphere, and temperatures at 5 % mass loss (T 5 %) and residues at 700 °C were determined to investigate the resistance to the thermal degradation. The T 5 % values were lower in oxidative atmosphere than in inert environment, and increased linearly as a function of organic bridge length in either used atmosphere. The residues at 700 °C were higher in static air than in flowing nitrogen. The results obtained for various dumbbell-shaped POSSs were discussed and compared with each other. A comparison with the results previously obtained with the corresponding un-bridged phenyl, hepta isobutyl-POSSs showed a higher resistance to the thermal degradation of bridged POSSs.

High Magnetic Moments in Manganese‐Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese-doped silicon clusters cations, Si(n)Mn(+) with n=6-10, 12-14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si(n)Mn(+) (n=6-10) clusters are found to be substitutive derivatives of the bare Si(n+1)(+) cations, while the endohedral Si(n)Mn(+) (n=12-14 and 16) clusters adopt fullerene-like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si(n)Mn(+) (n=6-10) clusters have local magnetic moments of 4 μ(B) or 6 μ(B) and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition-metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.