Novel caged clusters of silicon: Fullerenes, Frank-Kasper polyhedron and cubic
TL;DR: A review of metal encapsulated caged clusters of Si and Ge obtained from computer experiments based on an ab initio pseudopotential method can be found in this article, where it is shown that one M atom changes drastically the properties of Si/Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed.
Abstract: We review recent findings of metal (M) encapsulated caged clusters of Si and Ge obtained from computer experiments based on an
ab initio pseudopotential method. It is shown that one M atom changes drastically the properties of Si and Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed. In particular M@Si16 silicon fullerene has been obtained for M= Zr and Hf, while a Frank-Kasper polyhedron has been obtained for M@X16, X = Si and Ge. These clusters show high stability and large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps which are likely to make these species strongly abundant. A regular icosahedral M@X12 cluster has also been obtained for X = Ge and Sn by doping a divalent M atom. Interactions between clusters are rather weak. This is attractive for developing self-assembled cluster materials.
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TL;DR: General principles for designing stable highly symmetrical clusters are proposed, which takes advantage of both the extra stability of cage aromaticity and the good geometrical balance between the outer cage and the endohedral atom.
Abstract: General principles for designing stable highly symmetrical clusters are proposed. This approach takes advantage of both the extra stability of cage aromaticity and the good geometrical balance between the outer cage and the endohedral atom. The applicability of these design principles was confirmed by gas-phase experimental observations on group 14 element cages with endohedral Al's and also is illustrated by many literature examples of diverse systems.
103 citations
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TL;DR: In this article, the most stable structures and magic clusters have been determined for silicon clusters doped with a Cr atom starting from many initial configurations, and the results of Raman activities and infrared intensities are presented for selected clusters.
Abstract: Silicon clusters doped with a Cr atom have been studied using ab initio plane-wave ultrasoft pseudopotential and Gaussian methods. The most stable structures and magic clusters have been determined for ${\mathrm{Si}}_{n\mathrm{Cr}}$ $(n=8--17)$ starting from many initial configurations. Our results show that for $n=8--11$ the number of Si atoms is not enough to surround a Cr atom fully and therefore the structures of these clusters are basket type in which the Cr atom has a bare part. A cage structure is formed for $n=12$. $\mathrm{Cr}@{\mathrm{Si}}_{12}$ and $\mathrm{Cr}@{\mathrm{Si}}_{15}$ show magic behavior. Among the charged clusters, anion of $\mathrm{Cr}@{\mathrm{Si}}_{12}$ and cation of $\mathrm{Cr}@{\mathrm{Si}}_{13}$ have high stability. The ionization potentials and electron affinities have been calculated. The dynamical stability of clusters is studied from vibrational calculations. The results of Raman activities and infrared intensities are presented for selected clusters. These can be used to identify the structures from experiments. The bonding nature in $\mathrm{Cr}@{\mathrm{Si}}_{n}$ clusters is found to change depending on the structure even when the cluster size is the same.
79 citations
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TL;DR: In this paper, an ab initio study on the stability of hydrogenated empty cages with different guest atoms was performed and it was shown that doping can be used to manipulate the HOMO-LUMO gap with the possibility of varying optical properties as well as to prepare species with large magnetic moments.
Abstract: We report results of an ab initio study on the stability of hydrogenated empty cages ${X}_{n}{\mathrm{H}}_{n}$ with $X=\mathrm{Si}$, Ge, and Sn, and $n=8$, 10, 12, 14, 16, 18, 20, 24, and 28. All these cages have large highest-occupied\char21{}lowest-unoccupied molecular orbital (HOMO-LUMO) gaps. The HOMO-LUMO gap for Ge cages is found to be even larger than the values for Si cages, though in bulk Ge has a smaller band gap than Si. Cages with $n=16$ and 20 are found to be particularly stable in the form of fullerene structures. The bonding in the dodecahedral ${X}_{20}{\mathrm{H}}_{20}$ cage is very close to $s{p}^{3}$ type and it leads to the highest stability of this cage with perfect icosahedral symmetry. Endohedral doping of the empty cages such as ${\mathrm{Si}}_{n}{\mathrm{H}}_{n}$ $(n=10\char21{}28)$, with different guest atoms shows that doping can be used to manipulate the HOMO-LUMO gap with the possibility of varying their optical properties as well as to prepare species with large magnetic moments. Depending upon the guest atom, the character of the HOMO and the LUMO states and their origins either from the cage or the guest atom changes. This could lead to their applications in sensors. In contrast to the metal-encapsulated silicon-caged clusters, the embedding energy of the guest atom in the hydrogenated silicon fullerenes is small in most cases due to the weak interactions with the cage and therefore these slaved guest atoms can keep their atomic properties to a large extent. We find that atoms with closed electronic shell configurations such as Ca, Ba,\dots{} generally occupy the center of the cage. However, Be and other open electronic shell atoms tend to drift towards the wall of the cage. Doping of halogens such as iodine and alkalis such as Na can be used to produce, respectively hole and electron doping while transition-metal atoms such as V, Cr, Mn, and Fe are shown to produce atomiclike magnetic moments in many cases. In most of these cases the HOMO-LUMO gap becomes small because the guest atom orbital(s) are only partially occupied. However, for Ni and Zn the HOMO-LUMO gap is large as the hybridized $d$ orbitals become fully occupied. An interesting finding is that the endohedral doping can lead to a higher-energy undoped cage isomer to become the lowest-energy doped isomer. Implications of this result for endohedral fullerenes of carbon are also discussed.
48 citations
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TL;DR: In this article, the Raman and infrared spectra have been calculated and they could help in the experimental identification of the structures, which are predicted to exhibit luminescence in the visible range and could find applications in biological systems, optoelectronics, and as tagging material.
Abstract: 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.
42 citations
References
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TL;DR: In this article, the authors report geometries calculated for medium-sized silicon clusters using an unbiased global search with a genetic algorithm, which are in excellent agreement with the values that they measure experimentally.
Abstract: Silicon is the most important semiconducting material in the microelectronics industry. If current miniaturization trends continue, minimum device features will soon approach the size of atomic clusters. In this size regime, the structure and properties of materials often differ dramatically from those of the bulk. An enormous effort has been devoted to determining the structures of free silicon clusters1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22. Although progress has been made for Sin with n < 8, theoretical predictions for larger clusters are contradictory2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22 and none enjoy any compelling experimental support. Here we report geometries calculated for medium-sized silicon clusters using an unbiased global search with a genetic algorithm. Ion mobilities23 determined for these geometries by trajectory calculations are in excellent agreement with the values that we measure experimentally. The cluster geometries that we obtain do not correspond to fragments of the bulk. For n = 12–18 they are built on a structural motif consisting of a stack of Si9 tricapped trigonal prisms. For n ⩾ 19, our calculations predict that near-spherical cage structures become the most stable. The transition to these more spherical geometries occurs in the measured mobilities for slightly larger clusters than in the calculations, possibly because of entropic effects.
539 citations
"Novel caged clusters of silicon: Fu..." refers background in this paper
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TL;DR: Mass analyses reveal that many types of transition metal ions M(+) react with silane to form dehydrogenated MSi( +)(n) cluster ions as an end product, indicating that the metal atom is endohedral and stabilizes the Si polyhedral cage.
Abstract: We report the formation of a series of metal-containing hydrogenated silicon clusters using an ion trap. Mass analyses reveal that many types of transition metal ions ${M}^{+}$ ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Hf}$, Ta, W, Re, Ir, etc.) react with silane $({\mathrm{SiH}}_{4})$ to form dehydrogenated $M{\mathrm{Si}}_{n}^{\phantom{\rule{0ex}{0ex}}+}$ cluster ions ( $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}14$, 13, 12, 11, 9, respectively) as an end product, indicating that the metal atom is endohedral and stabilizes the Si polyhedral cage. This finding is confirmed by our ab initio calculation that ${\mathrm{WSi}}_{12}$ is a W-encapsulating ${\mathrm{Si}}_{12}$ cage cluster, and is very stable owing to both the electronic and the geometrical shell closures.
389 citations
"Novel caged clusters of silicon: Fu..." refers background in this paper
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TL;DR: Metal-encapsulated caged clusters of silicon from ab initio pseudopotential plane wave calculations using generalized gradient approximation for the exchange-correlation energy are reported, making them attractive for cluster-assembled materials.
Abstract: We report metal-encapsulated caged clusters of silicon from ab initio pseudopotential plane wave calculations using generalized gradient approximation for the exchange-correlation energy. Depending upon the size of the metal ( $M$) atom, silicon forms fullerenelike $M@{\mathrm{Si}}_{16}$, $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Hf}$, Zr, and cubic $M@{\mathrm{Si}}_{14}$, $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Fe}$, Ru, Os, caged clusters. The embedding energy of the $M$ atom is $\ensuremath{\approx}12\mathrm{eV}$ due to strong $M\ensuremath{-}\mathrm{Si}$ interactions that make the cage compact. Bonding in these clusters is predominantly covalent and the highest-occupied--lowest-unoccupied molecular orbital gap is $\ensuremath{\approx}1.5\mathrm{eV}$. However, an exceptionally large gap (2.35 eV) is obtained for $\mathrm{Ti}@{\mathrm{Si}}_{16}$ Frank-Kasper polyhedron. Interaction between these clusters is weak, making them attractive for cluster-assembled materials.
314 citations
"Novel caged clusters of silicon: Fu..." refers background in this paper
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IBM1
TL;DR: Using the Car-Parrinello method, the authors have obtained unforeseen structures for the low-lying isomers of Si45 and other midsized Si clusters, providing for the first time a consistent interpretation of the available exptl.
Abstract: Using the Car-Parrinello method, the authors have obtained unforeseen structures for the low-lying isomers of Si45 and other midsized Si clusters. They are formed by two shells of atoms, the outer one (cage) being fullerenelike and the inner one (core) consisting of a few atoms satg. dangling bonds. These novel structures provide for the first time a consistent interpretation of the available exptl. data, including the reactivity trends and the structural transition at a size of .apprx.25 atoms. [on SciFinder (R)]
226 citations
"Novel caged clusters of silicon: Fu..." refers background in this paper
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TL;DR: In this paper, a reaction between a metal atom and silicon in a supersonic jet to form metal atom-silicon clusters was observed, and the dominant product peaks observed in the mass spectra obtained for all three group VIB metals correspond to identical but remarkable cluster stoichiometries.
Abstract: We report observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom–silicon clusters. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and F2 laser photoionization time‐of‐flight mass spectrometry. Three group‐VIB transition metals and copper have been investigated. The dominant product cluster peaks observed in the mass spectra obtained for all three group VIB metals corresponds to identical but remarkable cluster stoichiometries. The dominant product peaks have formulas given by MSin where n=16. Copper results are different than the other three metals, indicating the importance of the metal valence electronic structure to the chemistry. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interfa...
218 citations
"Novel caged clusters of silicon: Fu..." refers result in this paper
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