The Electronic Properties of Superatom States of Hollow Molecules

Abstract

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...

Ultrafast Multiphoton Thermionic Photoemission from Graphite

“…Because the IPS is a universal property of molecular sheets [56], the ILB is derived from IPS by stacking them into a 3D solid [57]; screening is inefficient, and similar hot electron dynamics could occur in other quasi-2D materials with low densities of states at the Fermi level. Moreover, thermionic emission can occur in other materials and is likely to be the dominant process for photoemission from plasmonic nanoparticles where hot electron generation and light localization are particularly efficient [91][92][93].…”

“…The ILB derives its 3D character from its physical origin in hybridization of IPSs of stacked graphene sheets, which give it characteristic probability density maxima in the interlayer space. Surprisingly, the ILB has never been reported in 2PP or optical spectroscopy of graphite; however, its existence is firmly established by theory as well as inverse photoemission and x-ray absorption spectroscopies [52][53][54][55][56][57].…”

Dimerization of Radical-Anions: Nitride Clusterfullerenes versus Empty Fullerenes

“…Interestingly, in contrast with empty fullerenes, 33 In the previous studies of Sc 3 N@C 80 (CF 3 ) 2 , we have found that the most stable structure has para-C 6 (CF 3 ) 2 hexagon on one pole (both CF 3 groups are bonded to PHHJs) and the Sc 3 N cluster located in such a way that one Sc atom is facing the cage hexagon on the opposite pole, whereas two other Sc atoms are coordinated close to the para-position of CF 3 -bearing carbon atoms. 35,36 Considering the idealized C 2 symmetry of this conformer (in the actual optimized structure C 2 symmetry is slightly distorted), we have studied 39 isomers of Sc 3 …”

Super-atom molecular orbital excited states of fullerenes

“…This clearly distinguishes the SAMO states from the higher lying Rydberg states. The unique nature of the potential was discussed in analogy with image potential states of graphene in the context of the STM experiments carried out by the Petek group [7]. For states with higher principal quantum numbers, the electron density is almost entirely outside the carbon cage and the electron-binding energies and properties more closely resemble those of conventional Rydberg series.…”