Enhanced stability of magic clusters: A case study of icosahedral Al12X, X=B, Al, Ga, C, Si, Ge, Ti, As

Alkaline Earth Metal Superatom of W@Si16: Characterization of Group 6 Metal Encapsulating Si16 Cage on Organic Substrates

Transition metal atom (M)-encapsulating silicon cage nanoclusters (M@Si16) exhibit a superatomic nature, depending on the central M atom owing to the number of valence electrons and charge state on organic substrates. Since M@Si16 superatom featuring group 4 and 5 transition metal atoms exhibit rare-gas-like and alkali-like characteristics, respectively, group 6 transition metal atoms are expected to show alkaline earth-like behavior. In this study, M@Si16, comprising a central atom from group 6 (MVI = Cr, Mo, and W) were deposited on C60 substrates, and their electronic and chemical stabilities were investigated in terms of their charge state and chemical reactivity against oxygen exposures. In comparison to alkali-like Ta@Si16, the extent of charge transfer to the C60 substrate is approximately doubled, while the oxidative reactivity is subdued for MVI@Si16 on C60, especially for W@Si16. The results show that a divalent state of MVI@Si162+ appears on the C60 substrate, which is consistently calculated to be a symmetrical cage structure of W@Si162+ in C3v, revealing insights into the "periodic law" of M@Si16 superatoms pertaining to the characteristics of alkaline earth metals.

Oxidative Activation of Small Aluminum Nanoclusters with Boron Atom Substitution prior to Completing the Endohedral B@Al12- Superatom

Elemental substitution and doping validate the optimization of chemical and physical properties of functional materials, and the composition ratio of the substituting atoms generally determines their properties by changing their geometric and electronic structures. For atomically precise nanoclusters (NCs) consisting of countable atom aggregates, the composition can be controlled accurately to provide an ideal model to study the heteroatom substitution effects. Since aluminum (Al) and boron (B) both belong to group 13 in the periodic table, the effect of B atom substitution on Aln NCs can be investigated while maintaining the total number of valence electrons in AlnBm NCs. In this study, oxidative reactivities of small Al NCs with B atom substitution are studied for AlnBm NCs (m = 1, n = 6-14 and m = 2, n = 11) supported on organic surfaces by using X-ray photoelectron spectroscopy and oxygen molecule (O2) exposure measurements. Before completing the endohedral B@Al12- superatomic NC, one B atom substitution in Al NCs (AlnB) enhances oxidative reactivities 3-20 times compared to those of Aln+1, particularly for n ≤ 11. When one Al atom of Al12B is further substituted by a B atom to form Al11B2, the reactivity drastically increases (6.6 × 102 times), showing that the B atom substitution makes the NC chemically active or inactive geometrically depending on the exohedral or endohedral site for the B atom in the Al NC. In addition, density functional theory calculations show that the electronegative B atom contributes to forming a locally positive Al site to facilitate O2 adsorption except in Al12B, in which the B atom is geometrically shielded by the surface of the Al12 cage in B@Al12.

Reconsidering the Structures of C2 Al4 - and C2 Al5. Chemistry 2023;29:e202301338. [PMID: 37498677 DOI: 10.1002/chem.202301338] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The study of C2 Al4 -/0 and C2 Al5 -/0 was conducted using anion photoelectron spectroscopy and quantum chemical computations. The present findings reveal that C2 Al4 - has a boat-like structure, with a single C2 unit surrounded by four aluminum atoms. In contrast, the neutral C2 Al4 species adopts a D2h planar structure with two planar tetracoordinate carbon (ptC) units, consistent with previous reports. Furthermore, the global minimum isomer of C2 Al5 - adopts a D3h symmetry, where the C2 unit interacts with five aluminum atoms. It was also found that a lower symmetry structure of C2 Al5 - , where all five aluminum atoms are located on the same side of the C2 unit, albeit slightly higher in energy compared to the D3h structure. These computations show that the D3h structure of C2 Al5 - is highly stable, exhibiting a large HOMO-LUMO gap.

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations

Carbon-doped aluminum cluster anions, Al_nC^- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of Al_nC^- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al₆C^- is a carbon endohedral triangular prism. The Al₇C^- anion is a magic cluster with high stability. The structures of Al_7-9C^- can be viewed as the additional aluminum atoms attached around the triangular prism Al₆C^-. Two isomers of Al₁₀C^- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al₉C^- with the carbon atom located in a pentagonal bipyramid. The Al₁₁C^- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al_12-14C^- can be viewed as evolving from Al₁₁C^- by adding aluminum atoms to interact with the carbon atom. In Al₁₅C^-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al₁₃ unit can be identified as a part of the geometric structure of Al₁₅C^-.

Al13− and B@Al12− superatoms on a molecularly decorated substrate

Aluminum nanoclusters (Al_n NCs), particularly Al₁₃⁻ (n = 13), exhibit superatomic behavior with interplay between electron shell closure and geometrical packing in an anionic state. To fabricate superatom (SA) assemblies, substrates decorated with organic molecules can facilitate the optimization of cluster–surface interactions, because the molecularly local interactions for SAs govern the electronic properties via molecular complexation. In this study, Al_n NCs are soft-landed on organic substrates pre-deposited with n-type fullerene (C₆₀) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C₆₆H₆₆), and the electronic states of Al_n are characterized by X-ray photoelectron spectroscopy and chemical oxidative measurements. On the C₆₀ substrate, Al_n is fixed to be cationic but highly oxidative; however, on the HB-HBC substrate, they are stably fixed as anionic Al_n⁻ without any oxidations. The results reveal that the careful selection of organic molecules controls the design of assembled materials containing both Al₁₃⁻ and boron-doped B@Al₁₂⁻ SAs through optimizing the cluster–surface interactions.

Anionic aluminium clusters are promising candidates for the fabrication of superatom-assembled nanomaterials. Here, the authors report enhanced stability for Al₁₃⁻ and boron-doped B@Al₁₂⁻ on a molecularly decorated p-type organic substrate.

Exploring water adsorption and reactivity in a series of doped aluminum cluster anions

We present a systematic density functional study of central- and surface-doped aluminum cluster anions Al₁₂X^- (X = Mg, B, Ga, Si, P, Sc-Zn), their interactions and reactivity with water. Adsorption of water molecules on central-doped clusters is governed by the cluster electron affinity. Doping introduces a dramatic change in the cluster electronic structure by virtue of different ordering and occupation of super-atomic shells, which leads to the creation of complementary active sites controlling the reactivity with water. Surface doping creates unequal charge distribution on the cluster surface, resulting in the adsorption and reactivity of surface-doped clusters being dominated by electrostatic effects. These results demonstrate the strong influence of the doping position on the nature of the interaction and reactivity of the cluster, and contribute to a better understanding of doping effects.

Account of the diversity of tunneling spectra at the germanene/Al(1 1 1) interface

Despite the wealth of tunneling spectroscopic studies performed on silicene and germanene, the observation of a well-defined Dirac cone in these materials remains elusive. Here, we study germanene grown on Al(1 1 1) at submonolayer coverages with low temperature scanning tunneling spectroscopy. We show that the tunnelling spectra of the Al(1 1 1) surface and the germanene nanosheets are identical. They exhibit a clear metallic behaviour at the beginning of the experiments, that highlights the strong electronic coupling between the adlayer and the substrate. Over the course of the experiments, the spectra deviate from this initial behaviour, although consecutive spectra measured on the Al(1 1 1) surface and germanene nanosheets are still similar. This spectral diversity is explained by modifications of the tip apex, that arise from the erratic manipulation of the germanium adlayer. The origin of the characteristic features such as a wide band gap, coherence-like peaks or zero-bias anomalies are tentatively discussed in light of the physical properties of Ge and AlGe alloy clusters, that are likely to adsorb at the tip apex.

A designer 32-electron superatomic CBe8H12 cluster: core–shell geometry, octacoordinate carbon, and cubic aromaticity

A 32-electron CBe₈H₁₂ cluster is designed with cubic octacoordinate carbon. It features core–shell geometry, two-fold superatomic bonding, and cubic aromaticity.

Face-Sharing Homo- and Hetero-Bitetrahexahedral Superatomic Molecules M1M2@Li20 (M1/M2 = Ti and W)

Superatoms, being a class of clusters with chemical behavior similar to atoms, can be used as building blocks for constructing novel functional materials. Here, superatomic molecules M₁M₂@Li₂₀ (M₁/M₂ = Ti and W) are built with our recently discovered tetrahexahedral superatoms Ti@Li₁₄ and W@Li₁₄. A low-energy face-sharing bi-tetrahexahedral structure with a high symmetry of D_{6 h} is identified by global minimum structure search and frequency calculations. The number of shared Li atoms amounts to 6, which is rarely seen before. Molecular orbital and chemical bonding analyses reveal that although isolated superatoms Ti@Li₁₄ and W@Li₁₄ are nonmagnetic, Ti₂@Li₂₀ is an open-shell superatomic molecule with a magnetic moment of 2 μ_B, whereas for W₂@Li₂₀, the electronic shell remains closed. In both Ti₂@Li₂₀ and W₂@Li₂₀, a quadruple superbonding between superatoms is found. Interestingly, an assembly of two hetero-tetrahexahedral superatoms Ti@Li₁₄ and W@Li₁₄ also gives a face-sharing bi-tetrahexahedral structure but with a notable dipole moment. This study provides a basic understanding for the superatomic bonding of Ti@Li₁₄ and W@Li₁₄, which may aid their application in developing multi-superatom molecules or even bulk crystals.

Account of chemical bonding and enhanced reactivity of vanadium-doped rhodium clusters toward C–H activation: a DFT investigation

The chemical bonding and enhanced reactivity of vanadium-doped rhodium clusters toward C–H activation were investigated using DFT.

The stability of binary Al12X nanoclusters (X = Sc and Ti): superatom or Wade's polyhedron

Binary nanoclusters (NCs) exhibit strong potential as building blocks for tailor-made scientific materials based on the precise tuning of their electron countings and spin states along with the synergistic effects that originate from the constituent elements. Herein, we studied the electronic and geometric structures of transition metal (TM) doped aluminum (Al) Al₁₂X NCs (X  =  Sc and Ti), which are binary systems that extend from representative superatom [Formula: see text] anions. On the basis of the photoelectron spectroscopy (PES) and density functional theory (DFT) calculations, Al₁₂X anion and neutral structures are characterized as vertex-replaced icosahedron. The highly stable exohedral Al₁₂X icosahedron is described based on an electron counting rule derived from the coupling of Wade-Mingos' rule and the jellium model.

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.

Geometric and electronic properties of Si-atom doped Al clusters: robustness of binary superatoms against charging

Chemically stabilized binary superatoms are formed with Si-atom doping into Al superatoms.

Boron substitution in aluminum cluster anions: magic clusters and reactivity with oxygen

We have studied the size-selective reactivity of AlnBm(-) clusters m = 1,2 with O2 to investigate the effect of congener substitution in energetic aluminum clusters. Mixed-metal clusters offer an additional strategy for tuning the electronic and geometric structure of clusters and by substituting an atom with a congener; we may investigate the effect of structural changes in clusters with similar electronic structures. Using a fast-flow tube mass spectrometer, we formed aluminum boride cluster anions and exposed them to molecular oxygen. We found multiple stable species with Al12B(-) and Al11B2(-) being highly resistant to reactivity with oxygen. These clusters behave in a similar manner as Al13(-), which has previously been found to be stable in oxygen because of its icosahedral geometry and its filled electronic shell. Al13(-) and Al12B(-) have icosahedral structures, while Al11B2(-) forms a distorted icosahedron. All three of these clusters have filled electronic shells, and Al12B(-) has a larger HOMO-LUMO gap due to its compact geometry. Other cluster sizes are investigated, and the structures of the AlnB(-) series are found to have endohedrally doped B atoms, as do many of the AlnB2(-) clusters. The primary etching products are found to be a loss of two Al2O molecules, with boron likely to remain in the cluster.

Structural and electronic properties of uranium-encapsulated Au₁₄ cage

The structural properties of the uranium-encapsulated nano-cage U@Au₁₄ are predicted using density functional theory. The presence of the uranium atom makes the Au₁₄ structure more stable than the empty Au₁₄-cage, with a triplet ground electronic state for U@Au₁₄. Analysis of the electronic structure shows that the two frontier single-occupied molecular orbital electrons of U@Au₁₄ mainly originate from the 5f shell of the U atom after charge transfer. Meanwhile, the bonding orbitals and charge population indicate that the designed U@Au₁₄ nano-cage structure is stabilized by ionocovalent interactions. The current findings provide theoretical basis for future syntheses and further study of actinide doped gold nanoclusters, which might subsequently facilitate applications of such structure in radio-labeling, nanodrug carrier and other biomedical applications.

Superatomic orbitals in sixteen-coordinate M@Li16 bonded by metallic bonds

Based on density-functional calculation and genetic algorithm structure search, we propose a series of 16-coordinate core-shell clusters: M@Li(16)(M = Ca, Sr, Ba, Ti, Zr, Hf). A tetrahedral (T(d)) structure with an outer shell of 16 lithium atoms and one enclosed heavy atom is found to be the global minimum in the structural exploration of BaLi(16) based on genetic algorithm. This structure also has lower energy compared to the other isomers we employed in all the MLi(16) clusters. In this structure, the atoms are bonded together by metallic bonds with alkali (IA) and alkaline-earth (IIA) metal atoms. Their corresponding first electronic shells are closed with significant energy gaps because their total numbers of valence electrons fulfil the 18-electron rule. Such a combination could be extended to 20-electron systems by enclosing IVB elements. With simple valence electrons and highly symmetric structures, superatomic molecular orbitals are identified in all of the T(d) clusters.

Cage structure formation of singly doped aluminum cluster cations Al(n)TM+ (TM = Ti, V, Cr)

Structural information on free transition metal doped aluminum clusters, Al(n)TM(+) (TM = Ti, V, Cr), was obtained by studying their ability for argon physisorption. Systematic size (n = 5-35) and temperature (T = 145-300 K) dependent investigations reveal that bare Al(n)(+) clusters are inert toward argon, while Al(n)TM(+) clusters attach one argon atom up to a critical cluster size. This size is interpreted as the geometrical transition from surface-located dopant atoms to endohedrally doped aluminum clusters with the transition metal atom residing in an aluminum cage. The critical size, n(crit), is found to be surprisingly large, namely n(crit) = 16 and n(crit) = 19-21 for TM = V, Cr, and TM = Ti, respectively. Experimental cluster-argon bond dissociation energies have been derived as function of cluster size from equilibrium mass spectra and are in the 0.1-0.3 eV range.