Cage Clusters Research Articles

Generation of argon-ion mixed silicon plasmas forming argon encapsulated silicon clusters

An inductively coupled argon (Ar) plasma is superimposed on a silicon (Si) plasma generated by an electron beam gun in order to realize the formation of gas-atom encapsulated Si cage clusters. The Si clusters, which are formed and deposited on a substrate, are analyzed by laser-desorption time-of-flight mass spectrometry and are found to have the mass spectra of not only pure Si cluster (Sin; n=1–17) but also Si cluster doped with Ar atom (ArSin; n=10–20) in the case that the large amount of Ar ions is generated in addition to the Si plasma. Together with the analysis of x-ray photoelectron spectroscopy, it is revealed that the Ar atom is included in the Si cluster, forming the structure of endohedral Ar@Sin complexes. Furthermore, the mass spectrum of Ar@Sin indicates the existence of the magic numbered cluster size n=15, 16 similar to the metal encapsulated Si clusters.

Magnetic property of potassium clusters in pressure-doped zeolite A

The pressure-doping technique was developed for further loading of guest potassium metal into porous crystals of zeolite beyond the upper limit at ambient pressure. A doping pressure dependence of the magnetic property of potassium clusters in cages of zeolite A is measured up to 320 MPa. The Curie constant monotonically decreases with increase in the doping pressure and shows very small values above 250 MPa. The magnetic susceptibility at 2 K, however, gradually increases and has a peak at 200 MPa. Observed decrease in the Curie constant is explained by the increase in the loading density of potassium. Simultaneously, the increase in the magnetic susceptibility at 2 K is explained by the superparamagnetism of ferromagnetic domains. A possible mechanism of change in the magnetic properties is discussed.

A Facile Route to the Non‐IPR Fullerene Sc3N@C68: Synthesis, Spectroscopic Characterization, and Density Functional Theory Computations (IPR=Isolated Pentagon Rule)

Owing to the unique feature of the non-IPR D3 (isomer 6140) C68 cage (IPR=isolated pentagon rule), Sc3N@C68 has been attracting great interest in the fullerene community. Herein we report the first high-yield synthesis of Sc3N@C68 by the "reactive gas atmosphere" method and its facile isolation by single-step HPLC to a high purity (>or=99 %). Thus, Sc3N@C68 is isolated in sufficient quantities for its further spectroscopic characterization, while the high purity of the sample ensures the reliability of the spectroscopic data obtained. In particular, the electronic and vibrational structures of Sc3N@C68 were studied in detail experimentally and by theoretical computations. The assignment of the observed absorption bands to particular electronic transitions is given in detail on the basis of time-dependent DFT computations. Vibrational spectroscopy of Sc3N@C68 reveals good agreement between the measured spectra and the theoretically calculated spectra. A detailed assignment of the vibrational modes, including the Sc3N cluster modes, cage modes, and vibrations of the adjacent pentagons are discussed. This study reveals that the effect of Sc3N encapsulation in the cage is much more complicated than just a formal transfer of six electrons. Consequently the electronic and vibrational spectra of the carbon cage in Sc3N@C68 cannot be adequately understood on the basis of a C68 (6-) cage alone.

Pb122-: Plumbaspherene

Although Si or Ge is not known to form empty cage clusters such as the fullerenes, we recently found a unique 12-atom icosahedral tin cluster, Sn12 2- (stannaspherene). Here we report photoelectron spectroscopy and theoretical evidence that Pb12 2- is also a highly stable icosahedral cage cluster and bonded by four delocalized radial pi bonds and nine delocalized on-sphere sigma bonds from the 6p orbitals of the Pb atoms. Following Sn12 2-, we coin a name, plumbaspherene, for the highly stable and nearly spherical Pb12 2- cluster, which is expected to be stable in solution and the solid state. Plumbaspherene has a diameter of approximately 6.3 A with an empty interior volume large enough to host most transition metal atoms, affording a new class of endohedral clusters.

Discovery of a Family of Isopolyoxotungstates [H4W19O62]6− Encapsulating a {WO6} Moiety within a {W18} Dawson‐like Cluster Cage

Caged up: A family of isopolyoxotungstate clusters of the form [H4W19O62]6− with a cluster cage (blue) identical to that of the Dawson-type heteropolyacids (red) has been discovered. These comprise a single trigonal-prismatic or octahedral {WO6}6− moiety in place of the two heteroanions found in the Dawson structure.

Guest Atom Displacements in Silicon Cluster Na<SUB>2</SUB>@Si<SUB>50</SUB>H<SUB>44</SUB>

We present the displacements of the guest sodium atoms in double caged Na 2 @Si 50 H 44 cluster. The cluster consists of a piece of clathrate II consisting of two adjacent Si 28 cages filled with guest sodium atoms, being hydrogenated in order to terminate the dangling bonds. Although in a single Si 28 H 28 cage the endohedral sodium atom locates at the center, the sodium atoms in each of the double cage cluster displace about 0.06 nm away from each center of the cages to form a dimer between the endohedral sodium atoms. The displacements are attributed to the formation of covalent bond between the endohedral sodium atoms and the ionic bonding between the sodium atoms and the cage silicon atoms.

Thorium Encapsulated Caged Clusters of Germanium: Th@Gen, n = 16,18, and 20.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Novel Cage Clusters of MoS2 in the Gas Phase

Laser evaporation of MoS(2) nanoflakes gives negatively charged magic number clusters of compositions Mo(13)S(25) and Mo(13)S(28), which are shown to have closed-cage structures. The clusters are stable and do not show fragmentation in the post-source decay analysis even at the highest laser powers. Computations suggest that Mo(13)S(25) has a central cavity with a diameter of 4.5 A. The nanosheets of MoS(2) could curl upon laser irradiation, explaining the cluster formation.

Recent theoretical progress on electronic and structural properties of clusters: Permanent electric dipoles, magnetism, novel caged structures, and their assemblies

We present a brief account of the recent progress in the theoretical understanding of the electronic and structural properties of clusters of metals and semiconductors from ab initio calculations. The origin of the recently observed permanent electric dipoles in Nb clusters, the occurrence of magnetism in clusters of non-magnetic elements such as Pd, Rh, and Ru, as well as the findings of the metal encapsulated clusters of Si, Ge, Sn, and Pb are discussed. Empty caged clusters of Si, Ge, and Sn have also been shown to be stable with H capping. These can be functionalized by exohedral doping for different applications while endohedral doping of the cages can be used to tailor highest occupied-lowest unoccupied molecular orbital gaps as well as the magnetic properties. Assemblies of such clusters could lead to novel nanostructures and new phases of these materials.

Selective Formation of MSi16 (M: Sc, Ti, and V).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stability and properties of the cluster assembled solid phases ofX8C12 and YX7C12

We have investigated several of the most stable cluster assembled phases ofX8C12 and YX7C12(X, Y = B, N, Si) using pseudopotential plane-wave density functional theory. We havefound that many of these were stable, conductive and retained the integrity ofthe cluster cage, along with some of its bonding characteristics. A large rangeof metastable molecular solids can be produced in this way. Some of these newcovalent conductors and one semiconductor were considered good candidates for highTc superconductivity and may constitute an improvement over the recently discoveredfcc-C22 superconducting molecular solid.

Selective Formation of MSi16(M = Sc, Ti, and V)

Metal-encapsulated silicon cage clusters are a new class of clusters and are opening up new avenues for silicon-based nanoscale materials. We present experimental evidence for a highly stable cluster corresponding to M@Si16 (M = Sc, Ti, and V). Mass spectrometry and anion photoelectron spectroscopy show that the cluster features an electronically closed TiSi16 neutral core which undergoes a change in the number of valence electrons involving (i) substitution of neighboring metals with Sc and V, or (ii) addition of a halogen atom to the TiSi16 anion, and that VSi16F is predicted to form an ionically bound superatom complex.

A cage cluster with nine vertices composed of three molybdenum, four sulfur and two carbon atoms

A novel cage cluster with nine-vertices [Mo 3(μ 3-S)(μ 3-SCH(CO 2CH 3)CHS 2)(μ-OAc) 2(dtp) 3] ( 1) is obtained by the reaction of [Mo 3(μ 3-S)(μ-S) 3(μ-OAc)(dtp) 3(CH 3CN)] ( 2) with methylpropiolate (MP) in acetonitrile containing acetic acid. X-ray crystallography revealed that the open space of the incomplete cubane-type Mo 3S 4 core was covered by a methyl-propiolate ligand through three C–S bonds. Cluster 1 has been characterized by 13C and 1H NMR and electronic spectroscopy.

Predictions of novel nanostructures of silicon by metal encapsulation

Recent studies using ab initio total energy calculations have shown exciting possibilities of developing novel metal encapsulated caged clusters of silicon with fullerene-like, Frank–Kasper and other polyhedral structures. In contrast to carbon for which empty cage fullerene structures are stable with 20 or more atoms, 10–16 atom silicon cage structures are stabilized by a guest metal atom. These nanoclusters are predicted to exhibit luminescence in the visible range and could find applications in biological systems, optoelectronics, and as tagging material. The Raman and infrared spectra have been calculated and they could help in the experimental identification of the structures. Interaction of these clusters with metal as well as oxygen or hydrogen atoms show that the fullerene structure is stable. Also the interaction between clusters themselves is weak and the ionization potentials, large. These properties make them attractive for cluster assembled materials such as nanowires, nanotubes, and other 2 and 3D structures. Studies on hydrogen interaction have led to the predictions of empty center hydrogenated silicon fullerenes Si n H n with large HOMO–LUMO gaps. These could further be doped endohedrally or exohedrally to produce novel silicon fullerenes with a variety of properties opening new ways of using silicon for diverse applications.

Kinetics of gel formation in dilute dispersions with strong attractive particle interactions

We report a combined experimental and modeling study to investigate the kinetics of the transformation of a colloidal dispersion with strong attractive particle interactions into a gel. In the model, a two step mechanism is proposed, where an initial aggregation step is followed by an interconnection step leading to a particle network spanning the entire system volume. Using the Smoluchowski aggregation equation for the first step, we introduce a criterion to determine when the aggregation process is arrested and clusters become caged. In the second step the caged clusters diffuse over short distances and interconnect to form the particle network. The reliability of the gelation times computed by the model has been tested by comparison with a suitable set of experimental data. In addition, we show that the concept of mathematical gelation within the Smoluchowski aggregation equation is not appropriate for the description of the entire gel formation process in this system.

Smallest Magic Caged Clusters of Si, Ge, Sn, and Pb by Encapsulation of Transition Metal Atom

Ten atom clusters of X = Si, Ge, Sn, and Pb are known to be magic, suggesting that addition of an atom to such clusters is generally unfavorable. However, here we report using an ab initio ultrasoft pseudopotential method that these clusters can be further stabilized by doping of a Ni or Pt atom, leading to some of the smallest metal encapsulated clusters of these elements. For Si and Ge, doping of Ni is optimal while for Sn and Pb, Pt is the best. Our results agree with the recent observations of strong abundances and magic nature of X10Co- clusters of these elements. These findings could lead to the development of novel cluster-based nanomaterials for optoelectronic and other nanoscale applications.

Photodissociation of hydrogen iodide on the surface of large argon clusters: the orientation of the librational wave function and the scattering from the cluster cage.

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size n = 139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2 + 1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin-orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin-orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI-Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.

Carbon dimer in silicon cage: A class of highly stable silicon carbide clusters

A class of silicon carbide cage clusters with two carbon atoms inside the silicon cage and with high stabilities are presented. The theoretical formalism used is Hartree-Fock theory followed by second-order many-body perturbation theory to account for correlation effects, and geometry optimizations at the second-order perturbation theory level are performed without any symmetry constraints. The smallest ``cage'' is found to be a silicon cube with the carbon dimer inside the cube. Based on the simultaneous criteria of high binding energy, high vertical ionization potential, high [highest occupied--lowest unoccupied molecular orbital (HOMO-LUMO)] gap, and a low vertical electron affinity, ${\mathrm{Si}}_{14}{\mathrm{C}}_{2},$ with a close fullerenelike structure, is predicted to be a particularly stable cluster both at the all-electron and at the pseudopotential level of calculations. The $\mathrm{C}---\mathrm{C}$ bond lengths and the HOMO-LUMO gaps of the clusters are both found to oscillate with cluster size.

Kinetics of gel formation in dilute dispersions with strong attractive particle interactions

We report a combined experimental and modeling study to investigate the kinetics of the transformation of a colloidal dispersion with strong attractive particle interactions into a gel. In the model, a two step mechanism is proposed, where an initial aggregation step is followed by an interconnection step leading to a particle network spanning the entire system volume. Using the Smoluchowski aggregation equation for the first step, we introduce a criterion to determine when the aggregation process is arrested and clusters become caged. In the second step the caged clusters diffuse over short distances and interconnect to form the particle network. The reliability of the gelation times computed by the model has been tested by comparison with a suitable set of experimental data. In addition, we show that the concept of mathematical gelation within the Smoluchowski aggregation equation is not appropriate for the description of the entire gel formation process in this system.

Theoretical studies of CO 2(H 2O) 20,24,28 clusters: stabilization of cages in hydrates by CO 2 guest molecules

Theoretical studies on dodecahedral (20-mer), tetrakaidecahedral (24-mer) and hexakaidecahedral (28-mer) water clusters containing CO 2 guest molecules are carried out by optimizing geometry at the Hartree–Fock (HF) level with 6-31G* basis set followed by single point energy calculations with 6-311++G** basis set and applying the Becke-3-parameter density functional theory (DFT) and Lee-Yang-Parr correlation functional (B3LYP). While the filled tetrakaidecahedral and hexakaidecahedral cage clusters are stabilized by 7.79 and 3.42 kcal/mol, respectively (relative to unfilled cage and separated CO 2 molecule, SEC), the filled dodecahedral cage shows no such stabilization. The largest SEC value for tetrakaidecahedral cluster, resulting from two H-bonds between the guest and the host, explains the dominance of CO 2-filled tetrakaidecahedral structures in hydrates.