Superatom States Research Articles

The role of TM’s (M’s) d valence electrons in TM@X12 and M@X12 clusters

Using the density functional theory method, the icosahedral TM@X12 (M@X12) clusters (TM=Mn, Tc, Re; M=Zn, Cd, Hg; and X=Sn, Ge), which are composed of Sn12 (Ge12) shell covering a single TM (M) atom, have been systematically examined to explore the role of TM’s (M’s) d valence electrons playing in the clusters. The results show that the magnetism originate from the contribution of TM’s d valence electrons to TM@X12 clusters, where TM’s (M’s) d valence electrons are not included in the superatomic electronic states to TM@X12 (M@X12) clusters. Taking into account the structural stability (imaginary frequency, binding energy, embedding energy, and core-shell interaction) as well as the chemical stability (HOMO-LUMO gap) after, we proposed that TM@X12 and M@X12 clusters can be assigned as the protyle superatoms. Furthermore, the results suggest that M@C60 clusters can not be superatoms, because their negative embedding energies and the distance from the center atom (M) to C atom is larger than the sum of their Van Waals radii. Interestingly enough, we may obtain a simple judging method: for a magnetic superatom, the smaller the energy gap between the highest occupied magnetic state (HOMS) and Fermi level or HOMO (MOgap, or MFgap), the easier on the change of its spin magnetic moment.

Superatom State-Resolved Dynamics of the Au25(SC8H9)18(-) Cluster from Two-Dimensional Electronic Spectroscopy.

Superatom state-resolved dynamics of the Au25(SC8H9)18(-) monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au25(SC8H9)18(-) MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump-probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

Superatom Model for Ag–S Nanocluster with Delocalized Electrons

Several Ag–S nanoclusters where the cluster core comprises mixed metal (main component) and sulfur atoms show superatomic orbitals in the conduction band edge. However, there are no superatomic states, i.e., delocalized electrons, in the valence band, and the clusters in question can be labeled as “zerovalent”. We show here an example of an Ag–S cluster which fulfills the superatom model and has delocalized electrons: The recently synthesized and characterized [Ag62S12(StBu)32]2+ cluster has four delocalized valence electrons based on a simple counting rule, and we compare it to the zerovalent cluster [Ag62S13(StBu)32]4+. Our electronic structure analysis confirms the existence of superatomic states in the valence and conduction bands, but the locations of these states do not agree completely with the conventional prediction based on the spherical Jellium model. [Ag62S12(StBu)32]2+ displays the 1S2 electronic shell closure at the Fermi energy instead of the 1S21P2 configuration as suggested by its electro...

Influence of caged noble-gas atom on the superatomic and valence states of C60−

Using high-level ab initio coupled cluster methods, we have conducted a detailed study of two endofullerene anions, He@C−60 and Ne@C−60. The main focus was on elucidating the effect of the noble-gas atom on the energy and electronic structure of the stable anion states of the parent C−60. Our study revealed that the noble-gas atom has no influence on the valence-like states of C−60, but strongly affects the superatomic-like 2Ag state. The latter was found to undergo pronounced destabilisation in both the He@C−60 and Ne@C−60 species, losing more than 70% of its original electron binding energy. The observed destabilisation is due to perturbation of the excess electron density of the 2Ag state inside the C60 cage by the caged noble-gas atom. We show how the results of our analysis can be used to predict the behaviour of the 2Ag state in other closed-shell endohedral complexes. Some possible implications of the observed features of the 2Ag state are discussed.

Buckybowl superatom states: a unique route for electron transport?

A unique paradigm for intermolecular charge transport mediated by diffuse atomic-like orbital (SAMOs), typically present in conjugated hollow shaped molecules, is investigated for C20H10 molecular fragments by means of G0W0 theory. Inclusion of many body screening and polarization effects is seen to be important for accurate prediction of electronic properties involving these diffuse orbitals. Theoretical predictions are made for the series of bowl-shaped fullerene fragments, C20H10, C30H10, C40H10, C50H10. Interesting results are found for the LUMO-SAMO energy gap in C20H10, which is shown to be nearly an order of magnitude lower that that determined for C60. Given the ability to support bowl fragments on metal surfaces, these results suggest the concrete possibility for exploiting SAMO-mediated electron transport in supramolecular conducting layers.

Robust Magnetic Moments on Impurities in Metallic Clusters: Localized Magnetic States in Superatoms

Introducing magnetic impurities into clusters of simple metals can create localized states for higher angular momentum quantum numbers (l = 2 or 3) that can breed magnetism analogous to that in virtual bound states in metallic hosts, offering a new recipe for magnetic superatoms. In this work, we demonstrate that MnCa(n) clusters containing 6-15 Ca atoms show a spin magnetic moment of 5.0 μB irrespective of the cluster size. Theoretical analysis reveals that the Mn d states hybridize only partially with superatomic states and introduce extra majority and minority d states, largely localized at the Mn site, with a large gap. Successive addition of Ca atoms introduces superatomic states of varying angular momentum that are embedded in this gap, allowing control over the stability of the motifs without altering the moment. Assemblies of such clusters can offer novel electronic features due to the formation of localized magnetic "quasibound states" in a confined nearly free electron gas.

Superatom representation of high-TC superconductivity

A “super-atom” conceptual interface between chemistry and physics is proposed in order to assist in the search for higher TC superconductors. The plaquettes generating the checkerboard superstructure in the cuprates, the C60 molecules in K3C60, and the Mo6S82- clusters in Chevrel phase materials offer such candidate super-atoms. Thus, in the present study high-TC superconductivity HTSC is articulated as the entanglement of two disjoint electronic manifolds in the vicinity of a common Fermi energy. The resulting HTSC ground state couples near-degenerate protected local super-atom states to virtual magnons in an antiferromagnetic AFM embedding. The composite Cooper pairs emerge as the interaction particles for virtual magnons mediated “self-coherent entanglement” of super-atom states. A Hückel type resonating valence bond RVB formalism is employed in order to illustrate the real-space Cooper pairs as well as their delocalization and Bose Einstein condensation BEC on a ring of super-atoms. The chemical potential μBEC for Cooper pairs joining the condensate is formulated in terms of the super-exchange interaction, and consequently the TC in terms of the Neél temperature. A rationale for the robustness of the HTSC ground state is proposed: achieving local maximum “electron correlation entropy” at the expense of non-local phase rigidity.

The Electronic Properties of Superatom States of Hollow Molecules

Electronic and optical properties of molecules and molecular solids are traditionally considered from the perspective of the frontier orbitals and their intermolecular interactions. How molecules condense into crystalline solids, however, is mainly attributed to the long-range polarization interaction. In this Account, we show that long-range polarization also introduces a distinctive set of diffuse molecular electronic states, which in quantum structures or solids can combine into nearly-free-electron (NFE) bands. These NFE properties, which are usually associated with good metals, are vividly evident in sp(2) hybridized carbon materials, specifically graphene and its derivatives. The polarization interaction is primarily manifested in the screening of an external charge at a solid/vacuum interface. It is responsible for the universal image potential and the associated unoccupied image potential (IP) states, which are observed even at the He liquid/vacuum interface. The molecular electronic properties that we describe are derived from the IP states of graphene, which float above and below the molecular plane and undergo free motion parallel to it. Rolling or wrapping a graphene sheet into a nanotube or a fullerene transforms the IP states into diffuse atom-like orbitals that are bound primarily to hollow molecular cores, rather than the component atoms. Therefore, we named them the superatom molecular orbitals (SAMOs). Like the excitonic states of semiconductor nanostructures or the plasmonic resonances of metallic nanoparticles, SAMOs of fullerene molecules, separated by their van der Waals distance, can combine to form diatomic molecule-like orbitals of C(60) dimers. For larger aggregates, they form NFE bands of superatomic quantum structures and solids. The overlap of the diffuse SAMO wavefunctions in van der Waals solids provides a different paradigm for band formation than the valence or conduction bands formed by interaction of the more tightly bound, directional highest occupied molecular orbitals (HOMOs) or the lowest unoccupied molecular orbitals (LUMOs). Therefore, SAMO wavefunctions provide insights into the design of molecular materials with potentially superior properties for electronics. Physicists and chemists have thought of fullerenes as atom-like building blocks of electronic materials, and superatom properties have been attributed to other elemental gas-phase clusters based on their size-dependent electronic structure and reactivity. Only in the case of fullerenes, however, do the superatom properties survive as delocalized electronic bands even in the condensed phase. We emphasize, however, that the superatom states and their bands are usually unoccupied and therefore do not contribute to intermolecular bonding. Instead, their significance lies in the electronic properties they confer when electrons are introduced, such as when they are excited optically or probed by the atomically sharp tip of a scanning tunneling microscope. We describe the IP states of graphene as the primary manifestation of the universal polarization response of a molecular sheet and how these states in turn define the NFE properties of materials derived from graphene, such as graphite, fullerenes, and nanotubes. Through low-temperature scanning tunneling microscopy (LT-STM), time-resolved two-photon photoemission spectroscopy (TR-2PP), and density functional theory (DFT), we describe the real and reciprocal space electronic properties of SAMOs for single C(60) molecules and their self-assembled 1D and 2D quantum structures on single-crystal metal surfaces.

Nearly Free Electron Superatom States of Carbon and Boron Nitride Nanotubes

By first-principles theory we study the nearly free electron (NFE) states of carbon and boron nitride nanotubes. In addition to the well-known π bands, we found a series of one-dimensional (1D) NFE bands with on-axis spatial distributions, which resemble atomic orbitals projected onto a plane. These bands are 1D counterparts of the recently discovered superatom orbitals of 0D fullerenes. In addition to the previously reported lowest energy NFE state with the angular quantum number l = 0 corresponding to s atomic orbital character, we find higher energy NFE bands with l > 0 corresponding to the p, d, etc., orbitals. We show that these atom-like states of nanotubes originate from the many-body screening, which is responsible for the image potential of the parent two-dimensional (2D) graphene or BN sheets. With a model potential that combines the short-range exchange-correlation and the long-range Coulomb interactions, we reproduce the energies and radial wave function profiles of the NFE states from the density functional theory calculations. When the nanotube radius exceeds the radial extent on NFE states, the NFE state energies converge to those of image potential states of the parent 2D molecular sheets. To explore possible applications in molecular electronics that take advantage of the NFE properties of nanotube building blocks, we investigate the modification of NFE states by transverse electric fields, alkali metal encapsulation, and lateral and concentric nanotube dimerization.

The Superatom States of Fullerenes and Their Hybridization into the Nearly Free Electron Bands of Fullerites

Motivated by the discovery of the superatom states of C60 molecules, we investigate the factors that influence their energy and wave function hybridization into nearly free electron bands in molecular solids. As the n = 3 solutions of the radial Schrodinger equation of the central attractive potential consisting of the short-range C atom core and the long-range collective screening potentials, respectively, located on the icosahedral C60 molecule shell and within its hollow core, superatom states are distinguished by their atom-like orbitals corresponding to different orbital angular momentum states (l = 0, 1, 2,...). Because they are less tightly bound than the pi orbitals, that is, the n = 2 states, which are often exploited in the intermolecular electron transport in aromatic organic molecule semiconductors, superatom orbitals hybridize more extensively among aggregated molecules to form bands with nearly free electron dispersion. The prospect of exploiting the strong intermolecular coupling to achieve metal-like conduction in applications such as molecular electronics may be attained by lowering the energy of superatom states from 3.5 eV for single chemisorbed C60 molecules to below the Fermi level; therefore, we study how the superatom state energies depend on factors such as their aggregation into 1D-3D solids, cage size, and exo- and endohedral doping by metal atoms. We find, indeed, that if the ionization potential of endohedral atom, such as copper, is sufficiently large, superatom states can form the conduction band in the middle of the gap between the HOMO and LUMO of the parent C60 molecule. Through a plane-wave density functional theory study, we provide insights for a new paradigm for intermolecular electronic interaction beyond the conventional one among the sp(n) hybridized orbitals of the organic molecular solids that could lead to design of novel molecular materials and quantum structures with extraordinary optical and electronic properties.

Atomlike, Hollow-Core–Bound Molecular Orbitals of C 60

The atomic electron orbitals that underlie molecular bonding originate from the central Coulomb potential of the atomic core. We used scanning tunneling microscopy and density functional theory to explore the relation between the nearly spherical shape and unoccupied electronic structure of buckminsterfullerene (C60) molecules adsorbed on copper surfaces. Besides the known pi* antibonding molecular orbitals of the carbon-atom framework, above 3.5 electron volts we found atomlike orbitals bound to the core of the hollow C60 cage. These "superatom" states hybridize like the s and p orbitals of hydrogen and alkali atoms into diatomic molecule-like dimers and free-electron bands of one-dimensional wires and two-dimensional quantum wells in C60 aggregates. We attribute the superatom states to the central potential binding an electron to its screening charge, a property expected for hollow-shell molecules derived from layered materials.