Theoretical Study On Metal Superatoms And Bonding Characteristics Between Superatoms

Some specific clusters,which can mimic chemical behaviors of a single atom or a group elements in the periodic table of chemical elements,are termed as superatoms.The grasp of regular patterns of superatoms has brought new opportunities for the development of cluster science.The superatomic model successfully revealed electronic nature of spherical clusters,but this model still has some limitations.Hence,the super valence bond model?SVB?is recently proposed and developed.In the SVB model,a prolate cluster is divided into two spherical blocks sharing nucleus and valence pairs at the border to achieve electronic closed shell.It has achieved great success in explaining the electronic structures of clusters from a new perspective.We attempted to extend the SVB model to spherical aromatic clusters and nano-brass systems.In addition,the superatom-assembled materials have retained the unique properties of superatom units,which have attracted much attention in recent years.However,there is no proper theory to guide superatoms assembly.The assembly of the superatoms is basically equivalent to superatoms stacking.Based on the SVB model,we also demonstrated the feasibility of assembling new superatomic molecules and crystals from superatomic clusters.In summary,we studied characteristics of metal superatoms,superatomic molecules and superatomic crystals,and focuse on the bonding patterns between superatoms,which provides a new paradigm for the extension of the SVB model to other systems.1.Electronic shell of the tubular Au26 cluster:a cage-cage superatomic molecule based on spherical aromaticityGold clusters,which display a variety of unusual geometric structures due to the strong relativistic effects,have attracted much attention.Among them,Au₂₆ has a high-symmetry tubular structure(D_6d)with a large HOMO-LUMO energy gap,but its electronic stability still remains unclear.In this paper,the electronic nature of Au₂₆cluster is investigated using density functional theory method.Depending on the super valence bond model,the tubular Au₂₆ with 26 valence electrons could be viewed as a superatomic molecule composed of two open cages based on spherical aromaticity,and its molecule-like electronic shell closure is achieved via super triple bond??,2??between two cages.From this new cage-cage superatomic structural model,series of similar tubular clusters are predicted from the Au₂₆ skeleton.The capped two Au atoms are replaced by Cu,Ag and In respectively,to form tubular D_6d Au₂₄Cu₂ and Au₂₄Ag₂?26e?,and Au₂₄In₂?30e?clusters,where the super triple bonds also exist.Moreover,tubular D_5d Au₂₀In₂?26e?is obtained by replacing hexatomic Au₆ rings in bulk of the Au₂₄In₂ with pentagonal Au₅ rings.Chemical bonding analysis reveals that there is a super quintuple bond??,2?,2??between two open(Au₁₀In)cages,in accordance with the 26e Li₂₀Mg₃ superatomic molecule composed of two icosahedral superatoms.Our study proposes the new cage-cage structural model of superatomic molecule based on spherical aromaticity,which extends the range of super valence bonding pattern and gives inferences for further study of superatomic clusters.2.Bonding characteristics of the open shell Au16 superatom in new superatomic molecules and superatomic crystalCluster-assembled materials have attracted much interest because they exploit the uniqueness of clusters and situate them among potentially functional materials.The formation of super bonding between superatoms?SVB model?is a quite recent and fertile field,which has been greatly successful on understanding the stability and interactions of superatomic clusters.Hence the SVB model provides a new direction to assemble superatomic materials from superatom clusters.In this work,based on the structural characters of the pyramidal Au₂₀,superatomic molecules Au₁₉-Au₁₉ and Au₁₆-(Au₁₉)₄ are built and confirmed to be local minimums on the potential energy surface by density functional theory?DFT?calculations.It is found that the superatomic characters of Au₂₀ are retained in these two clusters,where the open-shell Au₁₆superatom can mimic the feature of sp³ hybrid C atom in bonding framework.Inspired by bonding characters of the assembled molecules,we use the diamond lattice as a template in which all carbon atoms are substituted by the Au₁₆ superatom to construct a stable Au₁₆-fcc crystal.Our DFT computations demonstrate that the structure has rather good dynamic and thermal stabilities.Dramatically,it is a direct semiconductor with a band gap of⁰.25 eV at the PBE level,suggesting that free electrons are localized.Chemical bonding analysis reveals that the electronic structure of the Au₁₆-fcc crystal follows the SVB model.Our work shows that the bonding patterns between superatoms is consistent in both superatomic molecules and superatomic crystal,and gives a better understanding of assembly behaviors of an all-metal semiconductor from a new superatomic perspective.3.Structural evolution and superatomic properties of Cu_xZn_y?x+y=3-13?Brass?Cu-Zn?is one of the typical Hume-Rothery alloys,which is extensively used in material science and industry.Earlier studies revealed that the structures of Cu-Zn alloys correlate with the average number of valence electrons or the electron-per-atom?e/a?ratio,but structural evolution rule still remains unclear.In this work,the Cu-Zn nanoalloy clusters?elementary cells of bulk brass?are studied to reveal the evolutional rule of their structures with sizes.Systematic unbiased global search is performed for structural prediction of Cu_xZn_y nanoalloys in a size range?x+y=3-13?by using genetic algorithm with density functional theory.The global minimum and low-lying isomers of the series of nanoalloy clusters are obtained,and the structural phase diagrams are plotted depending on the relative energy.The results show that geometric structures of Cu-Zn nanoalloys are also determined by the total number of valence electrons?n*?,just as in bulk brass.Cu-Zn nanoalloys with same n*have similar geometric motifs.When n*=6,the clusters adopt planar motif,which have?-aromaticity following the?4n+2?rule.When n*=8,18 and 20,the clusters keep spherical motifs,which satisfy the magic numbers of Jellium model and could be viewed as stable superatoms.In most of cases for n*=10,12 and 14,the clusters adopt prolate motifs,which have similar electronic structures to N₂,O₂,and F₂ molecules,respectively,based on the super valence bond model.Moreover,in some cases for n*=10 and 12,the clusters can be seen as an 8e-superatom combined with one?8e+2e?or two?8e+2e+2e?separate Zn atoms.4.Structures and superatomic characters of Ni atom doped Au clustersDue to the relativistic effect,Au clusters exhibits many unusual geometries.Among them,the small-size gold clusters display remarkable planar?2D?structures?<13 atoms?.The way in which heteroatoms are introduced can greatly change the geometries and electronic properties of Au clusters.Therefore,we located structures of NiAu₇^-?NiAu₈ and NiAu₉⁺clusters using the genetic algorithm program combined with the density functional theory.The results show that the three Ni atoms doped binary clusters are 3D spherical structures.Molecular orbitals and density of states analysis indicate that NiAu₇^-?NiAu₈ and NiAu₉⁺clusters have completely filled superatomic shells?1S²1P⁶?,which satisfy the jellium model and can be regarded as stable superatoms.The electronic structure of Ni atom in the clusters is d¹⁰.