Formation of metal-encapsulating Si cage clusters

Anion Photoelectron Spectroscopy and Quantum Chemistry Calculations of Gas-Phase TaSi17̅ and TaSi18̅ Clusters: Structural Determination, Bonding Characteristics, and Multiplicity of Structural Forms

This study explores the structures and chemical bonding properties of TaSi17̅ and TaSi18̅ clusters by employing anion photoelectron spectroscopy and theoretical computations. Utilizing CALYPSO and ABCluster programs for initial structure prediction, B3LYP hybrid functional for optimization, and CCSD(T)/def2-TZVPPD level for energy calculations, the research identifies the most stable isomers of these clusters. Key findings include the identification of two coexisting low-energy isomers for TaSi17̅, exhibiting Ta-endohedral fullerene-like cage structures, and the lowest-energy structures of TaSi17̅ and TaSi18̅ anions can be considered as derived from the TaSi16̅ superatom cluster. The study enhances the understanding of group 14 element chemistry and guides the design of novel inorganic metallic compounds, potentially impacting materials science.

Exploring the stability and aromaticity of rare earth doped tin cluster MSn16- (M = Sc, Y, La)

Rare earth elements have high chemical reactivity, and doping them into semiconductor clusters can induce novel physicochemical properties. The study of the physicochemical mechanisms of interactions between rare earth and tin atoms will enhance our understanding of rare earth functional materials from a microscopic perspective. Hence, the structure, electronic characteristics, stability, and aromaticity of endohedral cages MSn16- (M = Sc, Y, La) have been investigated using a combination of the hybrid PBE0 functional, stochastic kicking, and artificial bee colony global search technology. By comparing the simulated results with experimental photoelectron spectra, it is determined that the most stable structure of these clusters is the Frank-Kasper polyhedron. The doping of atoms has a minimal influence on density of states of the pure tin system, except for causing a widening of the energy gap. Various methods such as ab initio molecular dynamics simulations, the spherical jellium model, adaptive natural density partitioning, localized orbital locator, and electron density difference are employed to analyze the stability of these clusters. The aromaticity of the clusters is examined using iso-chemical shielding surfaces and the gauge-including magnetically induced currents. This study demonstrates that the stability and aromaticity of a tin cage can be systematically adjusted through doping.

Theoretical prediction of superatom WSi12-based catalysts for CO oxidation by N2O

Catalytic conversion of N2O and CO into nonharmful gases is of great significance to reduce their adverse impact on the environment. The potential of the WSi12 superatom to serve as a new cluster catalyst for CO oxidation by N2O is examined for the first time. It is found that WSi12 prefers to adsorb the N2O molecule rather than the CO molecule, and the charge transfer from WSi12 to N2O results in the full activation of N2O into a physically absorbed N2 molecule and an activated oxygen atom that is attached to an edge of the hexagonal prism structure of WSi12. After the release of N2, the remaining oxygen atom can oxidize one CO molecule via overcoming a rate-limiting barrier of 28.19 kcal mol-1. By replacing the central W atom with Cr and Mo, the resulting MSi12 (M = Cr and Mo) superatoms exhibit catalytic performance for CO oxidation comparable to the parent WSi12. In particular, the catalytic ability of WSi12 for CO oxidation is well maintained when it is extended into tube-like WnSi6(n+1) (n = 2, 4, and 6) clusters with energy barriers of 25.63-29.50 kcal mol-1. Moreover, all these studied MSi12 (M = Cr, Mo, and W) and WnSi6(n+1) (n = 2, 4, and 6) species have high structural stability and can absorb sunlight to drive the catalytic process. This study not only opens a new door for the atomically precise design of new silicon-based nanoscale catalysts for various chemical reactions but also provides useful atomic-scale insights into the size effect of such catalysts in heterogeneous catalysis.

A Computational Exploration of Exohedrally Transition Metal Doped Si94- Superatom Based Magnetic MSi9M' Clusters (M, M' = Sc(II) to Cu(II))

Nanosized clusters are drawing immense attention of the scientific community due to their size and composition dependent tunability of physical and chemical properties. Silicon nanoclusters are especially important because of their abundance and ample utility in the domains of electronics and semiconductor industry. Zintl phases of Si offer an excellent opportunity in the domain of nanocluster research owing to their superior stability and multifarious possibilities of tunability of electronic properties through doping with other elements. Doping silicon clusters with transition elements is a prevalent strategy to induce magnetic properties in such clusters. Although doping silicon clusters with single transition metal atoms can induce significant magnetism in nanoclusters, the dominant covalent interaction between silicon and the transition metal causes the magnetic moment to quench. The rational strategy of inducing a sustainable magnetic moment can be to introduce ferromagnetic interaction between two sites carrying nonvanishing magnetic moments. In the present work, such a possibility is explored in terms of the stability of the clusters and corresponding magnetic exchange coupling in them. The Si94-superatomic cluster is doped with two transition metal atoms exohedrally and the neutral clusters designed thereby are investigated computationally if they reduce or reinforce the high stability of the superatom and substantiate the possibility of obtaining nanosized magnetic units as building blocks of tunable materials for various applications.

Structural Determination and Bonding Characteristics of the Gas-Phase Ta2Si2̅ Anion and Its Neutral: Anion Photoelectron Spectroscopy and Theoretical Studies

The structures and bonding characteristics of Ta2Si2̅/0 clusters are investigated using anion photoelectron spectroscopy and quantum chemical calculations. The vertical detachment energy of the Ta2Si2̅ anion is measured to be 2.00 ± 0.08 eV using the 266 nm photon. It is found that the Ta2Si2̅ anion has three low-energy isomers with a C2v symmetric Ta-Ta dibridged structural framework, all of which contribute to the experimental photoelectron spectrum, while the Ta2Si2 neutral also has a C2v symmetric Ta-Ta dibridged structural framework. The charge-transfer from Ta atoms to Si atoms is discovered using atomic dipole moment corrected Hirshfeld analysis for the Ta2Si2̅ anion and Ta2Si2 neutral. Chemical bonding investigations show that both the Ta2Si2̅ anion and Ta2Si2 neutral have a strong covalent Ta-Ta bond, as well as σ and π double bonding patterns. Furthermore, the Ta atoms are linked together by a single 2c-2e Ta2 σ bond, whereas the Si atoms are linked together with the Ta atoms via four 2c-2e TaSi σ bonds, two 3c-2e TaSi2 σ bonds, one 4c-2e Ta2Si2 σ bond, and one 4c-2e Ta2Si2 π bond.

Making Sense of the Growth Behavior of Ultra-High Magnetic Gd2-Doped Silicon Clusters

The growth behavior, stability, electronic and magnetic properties of the Gd₂Si_n^- (n = 3-12) clusters are reported, which are investigated using density functional theory calculations combined with the Saunders 'Kick' and the Artificial Bee Colony algorithm. The lowest-lying structures of Gd₂Si_n^- (n = 3-12) are all exohedral structures with two Gd atoms face-capping the Si_n frameworks. Results show that the pentagonal bipyramid (PB) shape is the basic framework for the nascent growth process of the present clusters, and forming the PB structure begins with n = 5. The Gd₂Si₅^- is the potential magic cluster due to significantly higher average binding energies and second order difference energies, which can also be further verified by localized orbital locator and adaptive natural density partitioning methods. Moreover, the localized f-electron can be observed by natural atomic orbital analysis, implying that these electrons are not affected by the pure silicon atoms and scarcely participate in bonding. Hence, the implantation of these elements into a silicon substrate could present a potential alternative strategy for designing and synthesizing rare earth magnetic silicon-based materials.

Electron Detachments of NbSin-/0 (n = 1-3) Clusters from Density Matrix Renormalization Group-CASPT2 Calculations

The electronic states of NbSi_n^-/0/+ (n = 1-3) clusters have been explored using the state-of-the-art DMRG-CASPT2 method with relatively large active spaces. The leading configurations, bond distances, vibrational frequencies, and relative energies of the low-lying states were identified. Electron detachment energies of the anionic cluster and ionization energies of the neutral clusters were reported at the DMRG-CASPT2 level. The ground states of the NbSi_n^-/0/+ (n = 1-3) clusters were predicted as the ³Δ, ⁴Π, and ⁵Π states of the linear NbSi^-/0/+, the ³A₂, ⁴B₁, and ³B₁ states of cyclic NbSi₂^-/0/+, and the ¹A', ²A', and ³A″ states of tetrahedral NbSi₃^-/0/+ isomers. The first feature in the photoelectron spectrum of NbSi^- was attributed to the transitions from the anionic ³Δ ground state to the neutral ⁴Π, ⁴Δ, and ⁴Φ states, whereas the second feature was assigned to the transitions to the neutral ²Δ, ²Σ⁺, and ²Φ states. The first band in the photoelectron spectrum of NbSi₃^- was ascribed to the transition from the anionic ¹A' ground state to the neutral 1²A' and 1²A″ states; the second band was attributed to the transitions to 2²A', 2²A″, and 3²A' states; and the third band was assigned to the transition to 3²A' states.

Structural evolution and electronic properties of neutral and anionic TiASil (A = Sc, Ti; l ≤ 12): relatively stable TiASi4 as a structural unit

Molecular orbitals (MO) and the HOMO–LUMO energy gaps (HLgs) of neutral TiASi4 clusters (A = Sc, Ti).

Structural and Electronic Properties of LaSin-/0 (n = 2-6) Clusters: Anion Photoelectron Spectroscopy and Density Functional Calculations

The structures and electronic properties of LaSi_n^- (n = 2-6) anions and their neutral counterparts were investigated by anion photoelectron spectroscopy and theoretical calculations. The vertical detachment energies of the most stable structures of LaSi_n^- (n = 2-6) were measured to be 1.28, 1.58, 2.30, 2.05, and 2.91 eV, respectively. The lowest-energy isomer of LaSi₂^- is an isosceles triangle with a C_2v symmetry. For LaSi_3-6^- clusters, the most stable isomers are polyhedrons with La atom face-capping the Si_n frameworks. The lowest-energy structures of neutral LaSi_2,4,5 clusters are similar to their anionic counterparts. The most stable isomer of neutral LaSi₃ is a planar structure with C_2v symmetry, which is different from the triangular pyramid structure of LaSi₃^- anion. The lowest-energy isomer of LaSi₆^- is a C_5v symmetric pentagonal bipyramid structure, while for neutral LaSi₆ cluster, the C_5v structure is not the most stable one. The natural population analysis showed that there is electron transfer from La atoms to Si atoms in LaSi_n^-/0 (n = 2-6). The ZZ tensor component in isochemical shielding surfaces and the anisotropy of the induced current density analyses indicate that the most stable isomer of LaSi₆^- has aromaticity.

Quantum chemistry calculations of the growth patterns, simulated photoelectron spectra, and electronic properties of LaASil (A = Sc, Y, La; l ≤ 10) compounds and their anions

The growth patterns, simulated photoelectron spectra, and electronic properties of LaASi_l (A = Sc, Y, and La; l ≤ 10) compounds and their anions were studied via quantum chemistry calculations using the Perdew-Burke-Ernzerhof (PBE) method and unprejudiced structural searching software ABCluster. The results revealed that the growth patterns of the most stable structures of neutral and anionic LaASi_l showed an adsorptive mode. The lowest-energy structures (LESs) of the LaASi_l (l ≤ 7) clusters were similar, except for those of anionic LaYSi₄^- and LaYSi₅^- and neutral LaScSi₇. Additionally, we investigated and calculated the photoelectron spectra, vertical detachment energies, adiabatic electron affinities, relative stability, charge transfer, magnetic moment, and chemical bond analysis of the LaASi_l ground-state structures. The La₂Si_l clusters exhibited higher stability than the LaYSi_l and LaScSi_l systems owing to their higher dissociation energies (DEs). The DEs of the LESs in the LaASi₃ molecule are higher than those of other clusters. Thus, the LaASi₃ cluster shows potential as a building framework for Si-based cluster materials with good stability. The natural population analysis data and chemical bond analysis results showed that the spd hybridization of the orbitals of the metal atoms in the LaASi_l system occurred. Except for the LaScSi₉ and LaScSi₁₀ clusters, the neutral LaASi_l compounds transform into the corresponding anions when an extra electron is accepted by the Si clusters.

Structural Evolution and Electronic Properties of TaSin-/0 (n = 2-15) Clusters: Size-Selected Anion Photoelectron Spectroscopy and Theoretical Calculations

The structural evolution and electronic properties of TaSi_n^-/0 (n = 2-15) clusters are explored using anion photoelectron spectroscopy accompanied by quantum chemical calculations. The Ta atom in TaSi_n^-/0 is inclined to interact with more Si atoms and has high coordination numbers. The theoretical calculations show that TaSi₂^-/0 have trianglur structures and TaSi₃^-/0 adopt pyramid structures, while the geometries of TaSi_n^-/0 (n = 4-7) are all exohedral structures dominated by bipyramid-based configurations with the Ta atom face-capping the Si_n motifs. TaSi₈^-/0 and TaSi_9-10^- have boat-shaped geometries, whereas TaSi_9-10 neutrals adopt bipyramid-based geometries instead of boat-shaped ones. TaSi₁₁^- and TaSi₁₂ are confirmed as the critical size of transiting from exohedral to endohedral structures for anionic and neutral clusters, respectively. TaSi_12-15^-/0 have pentagonal or hexagonal prism-based geometries. Natural population analysis shows that the electron transfers from Si_n skeletons to Ta atom. The second-order energy differences (Δ₂E) and incremental binding energy (ΔE^I) values exhibit strong odd-even alternations, suggesting that the TaSi_n-odd^-/0 clusters are more stable than the adjacent TaSi_n-even^-/0 ones, except that TaSi_12^-/0 are more stable than TaSi_11^-/0 and TaSi_13^-/0.

Au3Si4- and Au4Si4: Electronically Equivalent but Different Polarity Superatoms

A series of Au_xSi₄^- cluster anions (x = 1-4) were generated most abundantly by laser ablation of a Au₄Si alloy target. Photoelectron spectroscopy and density functional theory (DFT) calculation of Au_xSi₄^- (x = 1-4) revealed that Au₃Si₄^- can be viewed as an electronically closed superatom and is composed of a Si₄ unit whose three adjacent edges of a single facet are bridged by three Au atoms. Such phase-segregated structure is facilitated by aurophilic interaction between the three Au atoms and results in a large permanent dipole moment (4.43 D). DFT calculations on an electronically equivalent superatom Au₄Si₄ predicted a new structure in which the uncoordinated Si atom of Au₃Si₄^- is bonded by Au⁺. This Au₄Si₄ is much more stable than a cubic structure previously reported and has a large HOMO-LUMO gap (1.68 eV) and a small permanent dipole moment (0.41 D).

Quantum chemical models for the absorption of endohedral clusters on Si(111)-(7 × 7): a subtle balance between W–Si and Si–Si bonding

The absorption of endohedral clusters on Si(111)-7 × 7 generates a new bond between W and a surface silicon adatom.

Synthesis and Characterization of Metal-Encapsulating Si16 Cage Superatoms

Nanoclusters, aggregates of several to hundreds of atoms, have been one of the central issues of nanomaterials sciences owing to their unique structures and properties, which could be found neither in nanoparticles with several nanometer diameters nor in organometallic complexes. Along with the chemical nature of each element, properties of nanoclusters change dramatically with size parameters, making nanoclusters strong potential candidates for future tailor-made materials; these nanoclusters are expected to have attractive properties such as redox activity, catalysis, and magnetism. Alloying of nanoclusters additionally gives designer functionality by fine control of their electronic structures in addition to size parameters. Among binary nanoclusters, binary cage superatoms (BCSs) composed of transition metal (M) encapsulating silicon cages, M@Si₁₆, have unique cage structures of 16 silicon atoms, which have not been found in elemental silicon nanoclusters, organosilicon compounds, and silicon based clathrates. The unique composition of these BCSs originates from the simultaneous satisfaction of geometric and electronic shell-closings in terms of cage geometry and valence electron filling, where a total of 68 valence electrons occupy the superatomic orbitals of (1S)²(1P)⁶(1D)¹⁰(1F)¹⁴(2S)²(1G)¹⁸(2P)⁶(2D)¹⁰ for M = group 4 elements in neutral ground state. The most important issue for M@Si₁₆ BCSs is fine-tuning of their characters by replacement of the central metal atoms, M, based on one-by-one adjustment of valence electron counts in the same structure framework of Si₁₆ cage; the replacement of M yields a series of M@Si₁₆ BCSs, based on their superatomic characteristics. So far, despite these unique features probed in the gas-phase molecular beam and predicted by quantum chemical calculations, M@Si₁₆ have not yet been isolated. In this Account, we have focused on recent advances in synthesis and characterizations of M@Si₁₆ BCSs (M = Ti and Ta). A series of M@Si₁₆ BCSs (M = groups 3 to 5) was found in gas-phase molecular beam experiments by photoelectron spectroscopy and mass spectrometry: formation of halogen-, rare-gas-, and alkali-like superatoms was identified through one-by-one tuning of number of total valence electrons. Toward future functional materials in the solid state, we have developed an intensive, size-selected nanocluster source based on high-power impulse magnetron sputtering coupled with a mass spectrometer and a soft-landing apparatus. With scanning probe microscopy and photoelectron spectroscopy, the structure of surface-immobilized BCSs has been elucidated; BCSs can be dispersed in an isolated form using C₆₀ fullerene decoration of the substrate. The intensive nanocluster source also enables the synthesis of BCSs in the 100-mg scale by coupling with a direct liquid-embedded trapping method into organic dispersants, enabling their structure characterization as a highly symmetric "metal-encapsulating tetrahedral silicon-cage" (METS) structure with Frank-Kasper geometry.

Ab initio study of medium sized boron-doped silicon clusters SinBm, n = 11-13, m = 1-3

The ground state and energetically low structures of neutral SinBm clusters, of medium size with n = 11-13, m = 1-3, are identified, presented and rationalized. Structures of the nanoclusters are predicted using density functional theory (DFT) and employing the HSE06 range-separated hybrid exchange-correlation functional. For these systems the functional is shown to offer systematic performance when benchmarked against high accuracy coupled-cluster CCSD(T) and compared to well known functionals used in the literature. Discrepancies for small size systems present in the literature are addressed and resolved. The structural evolution patterns of the clusters are discussed and common structural features (substructures) are identified. Cluster geometries are extensively searched via a particle swarm optimization algorithm alongside more traditional methodologies. In addition to the binding energies (that include zero-point energy corrections) of the structures, the optical gaps and UV/visible absorption spectra are reported, employing the CAM-B3LYP functional that was benchmarked against the high level EOM-CCSD level of theory. The computed infrared spectra are provided and discussed in length with respect to structural details. Their effectiveness as charge transfer units is examined. Optical gaps range between 1.4-2.5 eV, and are adjustable through the boron and silicon content of the clusters, which, along with the increased structural stability, offers promise for applications in optoelectronics.

The structural landscape in 14-vertex clusters of silicon, M@Si14: when two bonding paradigms collide

The structural chemistry of the title clusters has been the source of controversy in the computational literature because the identity of the most stable structure appears to be pathologically dependent on the chosen theoretical model. The candidate structures include a D_3h-symmetric 'fullerene-like' isomer with 3-connected vertices (A), an 'arachno' architecture (B) and an octahedral isomer with high vertex connectivities typical of 'closo' electron-deficient clusters (C). The key to understanding these apparently very different structures is the fact that they make use of the limited electron density available from the endohedral metal in very different ways. Early in the transition series the favoured structure is the one that maximises transfer of electron density from the electropositive metal to the cage whereas for later metals it is the one that minimises repulsions with the increasingly core-like d electrons. The varying role of the d electrons across the transition series leads directly to strong functional dependency, and hence to the controversy in the literature.

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

Stepwise oxidative reaction of a Ta-encapsulating Si₁₆ caged nanocluster superatom upon exposure to nitric oxide is investigated by monitoring N 1s core level signals.

Structure, Stability, and Electronic and Magnetic Properties of VGen (n = 1-19) Clusters

We systematically study the equilibrium geometries and electronic and magnetic properties of Ge_n+1 and VGe_n (n = 1-19) clusters using the density functional theory approach within the generalized gradient approximation. Endohedral structures in which the vanadium atom is encapsulated inside a Ge_n cage are predicted to be favored for n ≥ 10. The dopant V atom in the Ge_n clusters has not an immediate effect on the stability of small germanium clusters (n < 6), but it largely contributes to strengthen the stability for n ≥ 7. Our study enhances the large stability of the VGe₁₄ cluster, which presents an O_h symmetry cagelike geometry and a peculiar electronic structure in which the valence electrons of V and Ge atoms are delocalized and exhibit a shell structure associated with the quasi-spherical geometry. Consequently, this cluster is proposed to be a good candidate to be used as the building blocks for developing new materials. The cluster size dependence of the stability, the vertical ionization potentials, and electron affinities of Ge_n+1 and VGe_n are presented. Magnetic properties and the partial density of states of the most stable VGe_n clusters are also discussed.

Au20Si12: A hollow Catalan pentakis dodecahedron

A stable hollow Au₂₀Si₁₂ cage with I_h symmetry has been predicted using first-principles density functional theory. The stability of the cage-like Au₂₀Si₁₂ structure is verified by vibrational frequency analysis and molecular dynamics simulations. A relatively large highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1.057 eV is found. Electronic structure analysis shows that clearly p-d hybridizations between Si atoms and Au atoms are of great importance for the stability of Au₂₀Si₁₂ cage. The cage-like Au₂₀Si₁₂ structure may have potential applications in semiconductor industry and microelectronics.

B40 cluster stability, reactivity, and its planar structural precursor

We report a comprehensive first-principles study of the structural and chemical properties of the recently discovered B₄₀ cage. It is found to be highly reactive and can exothermically dimerize, regardless of the orientation, by overcoming a small energy barrier ≃0.06 eV. The energy gap of the system varies widely with the aggregation of the increasing number of B₄₀ cages, from 3.14 eV in a single B₄₀, to 1.54 eV in the dimer, to 1.25 eV in the trimer. We also explore a recipe for protecting the B₄₀ cage by sheathing it within a carbon shell and identify carbon nanotubes with a radius of ∼6 Å as optimal hosts for an isolated cage. It is demonstrated that B₄₀ can be unfolded into a planar 'molecule' that tessellates the plane. The corresponding 2D boron sheet constitutes a structural precursor foldable into this unique boron cage structure of current interest.