Two-Dimensional Silver-Chalcogenolate-Based Cluster-Assembled Material: A p-type Semiconductor.

We have synthesized and characterized a new two-dimensional honeycomb architecture resembling a single-layer of atomically precise silver cluster-assembled material (CAM), [Ag12(StBu)6(CF3COO)6(4,4'-azopyridine)3] (Ag12-azo-bpy). The interlayer noncovalent van der Waals interactions within the single-crystals were successfully disrupted, leading to the creation of this unique structure. The optimized Ag12-azo-bpy CAM demonstrates a valence band that is localized on the Ag12 cluster node situated near the Fermi energy level. This localization induces electron injection from the linker to the cluster node, facilitating efficient charge transportation along the plane. Exploiting this single-layer structure as a distinctive platform for p-type channel material, it was employed in a field-effect transistor configuration. Remarkably, the transistor exhibits a high hole mobility of 1.215 cm2 V-1 s-1 and an impressive ON/OFF current ratio of ∼4500 at room-temperature.

Modulation of Singlet-Triplet Gap in Atomically Precise Silver Cluster-Assembled Material.

Silver cluster-based solids have garnered considerable attention owing to their tunable luminescence behavior. While surface modification has enabled the construction of stable silver clusters, controlling interactions among clusters at the molecular level has been challenging due to their tendency to aggregate. Judicious choice of stabilizing ligands becomes pivotal in crafting a desired assembly. However, detailed photophysical behavior as a function of their cluster packing remained unexplored. Here, we modulate the packing pattern of Ag12 clusters by varying the nitrogen-based ligand. CAM-1 formed through coordination of the tritopic linker molecule and NC-1 with monodentate pyridine ligand; established via non-covalent interactions. Both the assemblies show ligand-to-metal-metal charge transfer (LMMCT) based cluster-centered emission band(s). Temperature-dependent photoluminescence spectra exhibit blue shifts at higher temperatures, which is attributed to the extent of the thermal reverse population of the S1 state from the closely spaced T1 state. The difference in the energy gap (ΔEST ) dictated by their assemblies played a pivotal role in the way that Ag12 cluster assembly in CAM-1 manifests a wider ΔEST and thus requires higher temperatures for reverse intersystem crossing (RISC) than assembly of NC-1. Such assembly-defined photoluminescence properties underscore the potential toolkit to design new cluster- assemblies with tailored optoelectronic properties.

Unusually High-Spin Fe12C12- Metallo-Carbohedrene Clusters.

Ferromagnets constructed from nanometals of atomic precision are important for innovative advances in information storage, energy conversion, and spintronic microdevices. Considerable success has been achieved in designing molecular magnets, which, however, are challenging in preparation and may suffer from drawbacks on the incompatibility of high stability and strong ferromagnetism. Utilizing a state-of-the-art self-developed mass spectrometer and a homemade laser vaporization source, we have achieved a highly efficient preparation of pure iron clusters, and here, we report the finding of a strongly ferromagnetic metal-carbon cluster, Fe12C12-, simply by reacting the Fen- clusters with acetylene in proper conditions. The unique stability of this ferromagnetic Fe12C12- cluster is rooted in a plumb-bob structure pertaining to Jahn-Teller distortion. We classify Fe12C12- as a new member of metallo-carbohedrenes and elucidate its structural stability mechanism as well as its soft-landing deposition and magnetization measurements, providing promise for the exploration of potential applications.

Periodic Trends in the Infrared and Optical Absorption Spectra of Metal Chalcogenide Clusters.

We have investigated the optical absorption, infrared spectra, binding energies, and other cluster properties to investigate whether periodic trends can be observed in the electronic structure of transition metal chalcogenide clusters ligated with CO ligands. Our studies demonstrate the existence of several periodic trends in the properties of pure and mixed octahedral metal chalcogenide clusters, TM6Se8(CO)6 (TM = W-Pt). We find that octahedral metal chalcogenide clusters with 96, 100, and 114 valence electrons have larger excitation energies, consistent with these clusters having closed electronic shells. Periodic trends were observed in the infrared spectra, with the CO bond stretch having the highest energy at 100 and 114 valence electrons due to the closed electronic shell minimizing back-bonding with the CO molecule. A periodic trend in the antisymmetric TM-C stretch was also observed, with the vibrational energy increasing as the valence electron count increased. This is due to decrease in the TM-C bond length, resulting in a larger force constant. These results reveal that periodic trends seen earlier in simple or noble-metal clusters can be observed in symmetric transition metal chalcogenide clusters, showing that the superatom concept in metal chalcogenide clusters goes beyond electronic excitations, and can be seen in other observable properties.

The New Ag-S Cluster [Ag50S13(StBu)20][CF3COO]4 with a Unique hcp Ag14 Kernel and Ag36 Keplerian-Shell-Based Structural Architecture and Its Photoresponsivity.

In metal nanoclusters (NCs), the kernel geometry and the nature of the surface protecting ligands are very crucial for their structural stability and properties. The synthesis and structural elucidation of Ag NCs is challenging because the zerovalent oxidation state of Ag is very reactive and prone to oxidization. Here, we report the NC [Ag50S13(StBu)20][CF3COO]4 with a hexagonal close-packed (hcp) cagelike Ag14 kernel. A truncated cubic shell and an octahedral shell encapsulate the hcp-layered kernel via an interstitial S2- anionic shell to form an Ag36 Keplerian outer shell of the NC. A theoretical study indicates the stability of this NC in its 4+ charge state and the charge distribution between the kernel and Keplerian shell. The unprecedented electronic structure facilitates its application toward sustainable photoresponse properties. The new insights into this novel Ag NC kernel and Keplerian shell structure may pave the way to understanding the unique structure and developing electronic structure-based applications.

High-Spin Superatom Stabilized by Dual Subshell Filling.

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment (S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt4]5[Fe6S8(CN)6]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

Magic Numbers in Octahedral Ligated Metal-Chalcogenide Superatoms.

The attainment of the superatomic state offers a unifying framework for the periodic classification of atomic clusters. Metallic clusters attain the superatomic state via the confined nearly free electron gas model that leads to groupings of quantum states marked by radial and angular momentum quantum numbers. We examine ligated octahedral metal-chalcogenide clusters where the nearly free electron gas model is invalid; however, the high symmetry can also lead to the bunching of electronic states. For octahedral TM6E8L6 clusters (TM = transition metal; E = chalcogen; L = ligand), the electronic shells are filled for valence electron counts of 96, 100, and 114 electrons. These magic electron counts are marked by large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, high ionization energies, and low electron affinity─all classic signatures of the superatomic state. We also find that clusters with electron counts differing from the magic counts show periodic patterns reminiscent of those observed in the periodic table of elements.

A Magnetic Superatomic Dimer with an Intense Internal Electric Dipole Moment.

The electronic and magnetic properties of the ligand-decorated Fe6S8 cluster and fused superatomic dimer are investigated using first-principles density functional theory. It is shown that the redox properties of the Fe6S8 cluster can be effectively controlled by altering the nature of the attached ligands. Donor ligands such as phosphines reduce the ionization energy of the Fe6S8 cluster, whereas the acceptor ligands such as CO increase the electron affinity. Such variation in the redox properties of the Fe6S8 cluster is the result of the ligand-induced shift in the cluster's electronic levels, so the occupation number remains mostly unaffected, leading to a marginal change in the spin magnetic moment of the cluster. A combination of two identical Fe6S8 clusters decorated by unbalanced ligands results in a superatomic dimer with a massive dipole moment and a large spin magnetic moment. Donor ligands on one side of the superatomic dimer with acceptor ligands on the other cause significant intercluster charge transfer. The resulting superatomic dimer offers an interesting motif for spintronics-related applications.

The superatomic state beyond conventional magic numbers: Ligated metal chalcogenide superatoms.

The field of cluster science is drawing increasing attention due to the strong size and composition-dependent properties of clusters and the exciting prospect of clusters serving as the building blocks for materials with tailored properties. However, identifying a unifying central paradigm that provides a framework for classifying and understanding the diverse behaviors is an outstanding challenge. One such central paradigm is the superatom concept that was developed for metallic and ligand-protected metallic clusters. The periodic electronic and geometric closed shells in clusters result in their properties being based on the stability they gain when they achieve closed shells. This stabilization results in the clusters having a well-defined valence, allowing them to be classified as superatoms-thus extending the Periodic Table to a third dimension. This Perspective focuses on extending the superatomic concept to ligated metal-chalcogen clusters that have recently been synthesized in solutions and form assemblies with counterions that have wide-ranging applications. Here, we illustrate that the periodic patterns emerge in the electronic structure of ligated metal-chalcogenide clusters. The stabilization gained by the closing of their electronic shells allows for the prediction of their redox properties. Further investigations reveal how the selection of ligands may control the redox properties of the superatoms. These ligated clusters may serve as chemical dopants for two-dimensional semiconductors to control their transport characteristics. Superatomic molecules of multiple metal-chalcogen superatoms allow for the formation of nano-p-n junctions ideal for directed transport and photon harvesting. This Perspective outlines future developments, including the synthesis of magnetic superatoms.

Multiple-Valence Aluminum and the Electronic and Geometric Structure of Al nO m Clusters.

Electronic stability in aluminum clusters is typically associated with either closed electronic shells of delocalized electrons or a +3 oxidation state of aluminum. To investigate whether there are alternative routes toward electronic stability in aluminum oxide clusters, we used theoretical methods to examine the geometric and electronic structure of Al nO m (2 ≤ n ≤ 7; 1 ≤ m ≤ 10) clusters. Electronically stable clusters with large HOMO-LUMO (highest occupied molecular orbital and lowest unoccupied molecular orbital) gaps were identified and could be grouped into two categories. (1) Al2 nO3 n clusters with a +3 oxidation state on the aluminum and (2) planar clusters including Al4O4, Al5O3, Al6O5, and Al6O6. The structures of the planar clusters have external Al atoms bound to a single O atom. Their electronic stability is explained by the multiple-valence Al sites, with the internal Al atoms having an oxidation state of +3, whereas the external Al atoms have an oxidation state of +1.

Al Valence Controls the Coordination and Stability of Cationic Aluminum-Oxygen Clusters in Reactions of Aln+ with Oxygen.

The reactivity of cationic aluminum clusters with oxygen is studied via a customized time-of-flight mass spectrometer. Unlike the etching effect for anionic aluminum clusters exposed to oxygen, here, the cationic Aln+ clusters react and produce a range of small AlnOm+ clusters. Relatively large-mass abundances are found for Al3O4+, Al4O5+, and Al5O7+ at lower O2 reactivity, while at higher O2 concentration, oxygen addition leads to Al2O7+, Al3O6,8-10+, and Al4O7,9+, showing relatively high abundance, and Al5O7+ remains as a stable species dominating the Al5Om+ distribution. To understand these results, we have investigated the structures and stabilities of the AlnOm+ clusters. First-principles theoretical investigations reveal the structures, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, fragmentation energies, ionization energies, and Hirshfield charge of the AlnOm+ clusters (2 ≤ n ≤ 7; 0 ≤ m ≤ 10). Energetically, Al3O4+, Al4O5+, and Al5O7+ are calculated to be most stable with high fragmentation energies; however, they still allow for the chemisorption of additional O2 with large binding energies leading to clusters with higher O/Al ratios. The stability of the species is consistent with Al possessing three valence electrons, while O typically accepts two, leading to the expectation that Al3O4+, Al5O7+, and Al7O10+ are reasonably stable. In addition to this, Al3O+, Al5O3+, and Al7O5+ are found to exhibit large HOMO-LUMO gaps associated with the different oxidation states of Al. The oxygen-rich species such as Al2O7+, Al3O10+, and Al4O9+ all display superoxide structures providing further insights into the oxidation of aluminum clusters.

Superatoms: Electronic and Geometric Effects on Reactivity.

The relative role of electronic and geometric effects on the stability of clusters has been a contentious topic for quite some time, with the focus on electronic structure generally gaining the upper hand. In this Account, we hope to demonstrate that both electronic shell filling and geometric shell filling are necessary concepts for an intuitive understanding of the reactivity of metal clusters. This work will focus on the reactivity of aluminum based clusters, although these concepts may be applied to clusters of different metals and ligand protected clusters. First we highlight the importance of electronic shell closure in the stability of metallic clusters. Quantum confinement in small compact metal clusters results in the bunching of quantum states that are reminiscent of the electronic shells in atoms. Clusters with closed electronic shells and large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gaps have enhanced stability and reduced reactivity with O2 due to the need for the cluster to accommodate the spin of molecular oxygen during activation of the molecule. To intuitively understand the reactivity of clusters with protic species such as water and methanol, geometric effects are needed. Clusters with unsymmetrical structures and defects usually result in uneven charge distribution over the surface of the cluster, forming active sites. To reduce reactivity, these sites must be quenched. These concepts can also be applied to ligand protected clusters. Clusters with ligands that are balanced across the cluster are less reactive, while clusters with unbalanced ligands can result in induced active sites. Adatoms on the surface of a cluster that are bound to a ligand result in an activated adatom that reacts readily with protic species, offering a mechanism by which the defects will be etched off returning the cluster to a closed geometric shell. The goal of this Account is to argue that both geometric and electronic shell filling concepts serve as valuable organizational principles that explain a wide variety of phenomena in the reactivity of clusters. These concepts help to explain the fundamental interactions that allow for specific clusters to be described as superatoms. Superatoms are clusters that exhibit a well-defined valence. A superatom cluster's properties may be intuitively understood and predicted based on the energy gained when the cluster obtains its optimal electronic and geometric structure. This concept has been found to be a unifying principle among a wide variety of metal clusters ranging from free aluminum clusters to ligand protected noble metal clusters and even metal-chalcogenide ligand protected clusters. Thus, the importance of electronic and geometric shell closing concepts supports the superatom concept, because the properties of certain clusters with well-defined valence are controlled by the stability that is enhanced when they retain their closed electronic and geometric shells.

Metal Chalcogenide Clusters with Closed Electronic Shells and the Electronic Properties of Alkalis and Halogens.

Clusters with filled electronic shells and a large gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are generally energetically and chemically stable. Enabling clusters to become electron donors with low ionization energies or electron acceptors with high electron affinities usually requires changing the valence electron count. Here we demonstrate that a metal cluster may be transformed from an electron donor to an acceptor by exchanging ligands while the neutral form of the clusters has closed electronic shells. Our studies on Co6Te8(PEt3)m(CO)n (m + n = 6) clusters show that Co6Te8(PEt3)6 has a closed electronic shell and a low ionization energy of 4.74 eV, and the successive replacement of PEt3 by CO ligands ends with Co6Te8(CO)6 exhibiting halogen-like behavior. Both the low ionization energy Co6Te8(PEt3)6 and high electron affinity Co6Te8(CO)6 have closed electronic shells marked by high HOMO-LUMO gaps of 1.24 and 1.39 eV, respectively. Further, the clusters with an even number of ligands favor a symmetrical placement of ligands around the metal core.

CO ligands stabilize metal chalcogenide Co6Se8(CO)n clusters via demagnetization.

The role of carbon monoxide ligands on the magnetic moment of Co6Se8(CO)n clusters, n = 0-6 was investigated to better understand the interplay between the electronic structure of metal chalcogenide clusters and their ligands. We find that the addition of CO ligands to Co6Se8 results in the gradual demagnetization of the cluster. Generally, the addition of a CO ligand effectively adds two electrons to the cluster that occupy deeper states and further pushes up an antibonding orbital out of the valence manifold of cluster states. Through such processes, the fully ligated Co6Se8(CO)6 cluster attains a closed electronic shell with a large gap between occupied and unoccupied states. Each removal of a CO ligand from the cluster then stabilizes an antibonding state that adds unoccupied states to the valence manifold. Such a cluster with partially filled states may either deform as in a Jahn-Teller distortion to quench the spin, or maintain its core structure and be stabilized in a high spin state as in Hund's rules. As these clusters generally maintain their octahedral core, the high spin state prevails and the removal of the ligands results in an increase in spin multiplicity.

Complete Ag4M2(DMSA)4 (M = Ni, Pd, Pt, DMSA = Dimercaptosuccinic Acid) Cluster Series: Optical Properties, Stability, and Structural Characterization.

The cluster series Ag4M2(DMSA)4 (M = Ni, Pd, Pt) has been synthesized and the optical spectra and stability have been examined as a function of the metal, M. We have also obtained the structure of Ag4Ni2(DMSA)4 using X-ray crystallography, confirming the previously calculated structure. In the optical spectrum, there is a significant blue shift as the substituted metal M progresses down the periodic table. Theoretical calculations suggest that the blue shift is due to the lowering in energy of the d orbitals of the transition metal, M; however the expected metal-metal excitations are optically weak, and the spectra are dominated by metal-ligand excitations. The Ag4Pd2(DMSA)4 species has exceptionally high stability relative to the previously reported Ni and Pt analogues.

Evolution of the Spin Magnetic Moments and Atomic Valence of Vanadium in VCux+, VAgx+, and VAux+ Clusters (x = 3-14).

The atomic structures, bonding characteristics, spin magnetic moments, and stability of VCux+, VAgx+, and VAux+ (x = 3-14) clusters were examined using density functional theory. Our studies indicate that the effective valence of vanadium is size-dependent and that at small sizes some of the valence electrons of vanadium are localized on vanadium, while at larger sizes the 3d orbitals of the vanadium participate in metallic bonding eventually quenching the spin magnetic moment. The electronic stability of the clusters may be understood through a split-shell model that partitions the valence electrons in either a delocalized shell or localized on the vanadium atom. A molecular orbital analysis reveals that in planar clusters the delocalization of the 3d orbital of vanadium is enhanced when surrounded by gold due to enhanced 6s-5d hybridization. Once the clusters become three-dimensional, this hybridization is reduced, and copper most readily delocalizes the vanadium's valence electrons. By understanding these unique features, greater insight is offered into the role of a host material's electronic structure in determining the bonding characteristics and stability of localized spin magnetic moments in quantum confined systems.

Symmetry and magnetism in Ni9Te6 clusters ligated by CO or phosphine ligands.

The removal of a single ligand from the magnetic Ni9Te6(L)8 (L = P(CH3)3, CO) clusters is found to quench the magnetic moment. The reduction in magnetic moment is caused by a geometric deformation of the Ni9Te6 core that breaks the octahedral symmetry of the cluster. This effect is observed in both the CO and phosphine based ligands. The octahedral symmetry bare cluster is also found to have a large magnetic moment. These results highlight the dilemma faced by magnetic ligand protected clusters whose symmetry has been broken: whether to break the spin symmetry as in Hund's rules or to break the spatial symmetry as in the Jahn-Teller effect. The spatial symmetry breaking is found to be an oblate distortion that forms additional Ni-Te bonds resulting in the enhanced stability of the cluster.

Ionic versus metallic bonding in AlnNam and AlnMgm (m ≤ 3, n + m ≤ 15) clusters.

First principles electronic structure studies on the ground state geometries, stability, and the electronic structure of AlnNam and AlnMgm (m ≤ 3, n + m ≤ 15) clusters have been carried out to examine the nature of bonding between Na or Mg and Al. Identifying whether the bonding is ionic or metallic in bulk materials is typically straightforward; however, in small clusters where quantum confinement is important, the nature of bonding may become unclear. We have performed a critical analysis of the bonding in these bimetallic clusters using charge analysis, electrical dipole moments, hybridization of the atomic orbitals, the Laplacian of the charge density at the bond critical points, and the change in the bonding energy between neutral and anionic forms of the cluster. For NanAlm clusters, we find that the Na binding is primarily ionic, while the bonding in AlnMgm is primarily metallic. We find that the Mulliken population of the 3p orbital of Na and Mg can provide a rapid assessment of the nature of bonding. We also find that the Hirshfeld charge and dipole moments are effective indicators, when placed in context. We found that the Laplacian of the charge density at the bond critical points can be misleading in identifying whether the bonding is ionic or metallic in small clusters.

Transforming Ni9Te6 from Electron Donor to Acceptor via Ligand Exchange.

The ability to donate or accept charge is a fundamental property of a chemical species. This property is typically rooted in the valence electron count and may be determined from the ionization potential and electron affinity. First-principles theoretical studies have been carried out to show that a cluster may be transformed from a donor to an acceptor by changing only the ligand. Our studies on a chalcogenide Ni9Te6 cluster show that the ionization potential and electron affinity undergo substantial changes as the attachment of phosphine PH3 decreases the ionization potential to be less than that of sodium, whereas the attachment of PCl3 or CO increases the adiabatic electron affinity to be greater than iodine. The ligands change the electronic properties by creating a coulomb well that can shift the electronic spectrum. Studies on Co9Te6(CO)8 clusters show agreement with experiment and demonstrate that the ideas developed here are applicable to a wider group of clusters.

Geometry controls the stability of FeSi14.

First-principles theoretical studies have been carried out to investigate the stability of Sin cages impregnated with a Fe atom. It is shown that FeSi9, FeSi11, and FeSi14 clusters exhibit enhanced local stability as seen through an increase in Si binding energy, Fe embedding energy, the gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), and the Ionization Potential (IP). The conventional picture for the stability of such species combines an assumption of electron precise bonding with the 18-electron rule; however, we find this to be inadequate to explain the enhanced stability in FeSi11 and FeSi14 because the d-band is filled for all FeSin clusters for n≥ 9. FeSi14 is shown to be the most stable due to a compact and highly symmetric Si14 cage with octahedral symmetry that allows better mixing between Fe 3d- and Si 3p-electronic states.