The superatom states of fullerenes and their hybridization into the nearly free electron bands of fullerites

Understanding the Electronic Structure and Chemical Bonding in the 2D Fullerene Monolayer

Recently synthesized two-dimensional (2D) monolayer quasi-hexagonal-phase fullerene (qHPC60) demonstrates excellent thermodynamic stability. Within this monolayer, each fullerene cluster is surrounded by six adjacent C60 cages along an equatorial plane and is connected by both C-C single bonds and [2 + 2] cycloaddition bonds that serve as bridges. In this study, we investigate the stability mechanism of the 2D qHPC60 monolayer by examining the electronic structure and chemical bonding through state-of-the-art theoretical methodologies. Density functional theory (DFT) studies reveal that 2D qHPC60 possesses a moderate direct electronic band gap of 1.46 eV, close to the experimental value (1.6 eV). It is found that the intermolecular bridge bonds play a crucial role in enhancing the charge flow and redistribution among C60 cages, leading to the formation of dual π-aromaticity within the C60 sphere and stabilizing the 2D framework structure. Furthermore, we identify a series of delocalized superatom molecular orbitals (SAMOs) within the 2D qHPC60 monolayer, exhibiting atomic orbital-like behavior and hybridization to form nearly free-electron (NFE) bands with σ/π bonding and σ*/π* antibonding properties. Our findings provide insights into the design and potential applications of NFE bands derived from SAMOs in 2D qHPC60 monolayers.

Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Superatom molecular orbital in C80

The Superatom Molecular Orbitals (SAMO) in fullerene derivatives are of great interests which gives a wide basement for many electronic applications. In this work, the Density Functional Theory reveals the SAMO states of endohedrally doped C80 derivatives with Li, Sc, Mn, Ti, Ca, Fe, and Co atoms in molecular and periodic structures. The choice and position of metal atoms in endohedrally doped C80 derivatives largely affects the orientation of SAMO energies and wavefunction distributions. Among various derivatives, the Co-substituted C80 constitutes the lowest SAMO energy. The charge transfer study infers the influence of metal atoms inside the cage on SAMO energies. At higher energies, pz-, 2s-, and pxy- SAMO bands have been overlapped with higher dispersion bands which depict the increased intermolecular interaction in delocalized bands causing a larger dispersion. These results give new insights for future studies on lowering SAMO energy nearly to the fermi level in higher fullerenes.

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.

Superatom Molecular Orbitals of Endohedral C82

Understanding superatom molecular orbital (SAMO) states in fullerene derivatives has been in the limelight ever since the first discovery of SAMOs owing to the fundamental interest in this topic as well as to the possible applications in molecular switches and other organic electronics. Nevertheless, very few reports have been published on SAMO states of larger fullerenes so far. Using density functional theory, we attempt to partially remedy this situation by presenting a study on SAMO states in C82 and its Ca and Sc endohedrally doped derivatives, comparing results with previous relevant findings for C60. We find that C82 possesses higher SAMO energies compared to C60, as associated with the symmetry of the molecule, and that endohedral doping leads to energetically favorable side positions of Ca and Sc inside the C82 cage. Among the two, Sc@C82 has more stable SAMO states compared to Ca@C82 as reflected by the shift in the density of states, while the charge states are found to be similar. In the case of the monolayer form, the pz- and 2s-SAMO orbitals overlap with the nearest neighbors, causing parabolic band dispersion with the formation of near free electron states and that the SAMO state energies move closer to the Fermi energy compared to the related molecules. These findings provide promising information about the distribution of SAMO states in C82 fullerene, which can be further relevant in studies of SAMO states of higher fullerenes and for coming applications of these systems.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Superatomic states under high pressure

The study of superatoms has attracted great interest since they apparently go beyond the traditional understanding of the periodic table of elements. In this work, we clearly show that superatoms can be extended from conventional structures to states under pressure condition. By studying the compression process of the CH₄@C₆₀ system formed via embedding methane molecules inside fullerene C₆₀, it is found that the system maintains superatomic properties in both static states, and even dynamic rotation situations influenced by quantum tunneling. Remarkably, the simulations reveal the emergence of new superatomic molecular orbitals by decreasing the confined space to approach the van der Waals boundary between CH₄ and C₆₀. Our current results not only establish a complete picture of superatoms from ambient condition to high pressure, but also offer a perspective for the discovery and exploration of new properties in superatom systems under extreme conditions.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C₆₀) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C₆₀ can be visualized by Ag_n NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Ag_n NCs on C₆₀ layers.

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.

Time- and momentum-resolved image-potential states of 2H-MoS2 surface

Rydberg-like image potential states (IPSs) form special series surface states on metal and semiconducting surfaces. Here, using time-resolved and momentum-resolved multi-photon photoemission (mPPE), we measured the energy positions, band dispersion, and carrier lifetimes of IPSs at the 2H-MoS₂ surface. The energy minima of the IPSs (n = 1 and 2) were located at 0.77 and 0.21 eV below the vacuum level. In addition, the effective masses of these two IPSs are close to the rest mass of the free electron, clearly showing nearly-free-electron character. These properties suggest a good screening effect in the MoS₂ parallel to the surface. The multi-photon resonances between the valence band and IPS (n = 1) are observed, showing a k_‖-momentum-dependent behavior. Our time-resolved mPPE measurements show that the lifetime of photoexcited electrons in the IPS (n = 1) is about 33 fs.

Direct Visualization of Nearly Free Electron States Formed by Superatom Molecular Orbitals in a Li@C60 Monolayer

Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we directly determine the spatial and energetic distributions of superatom molecular orbitals (SAMOs) of an Li@C₆₀ monolayer adsorbed on a Cu(111) surface. Utilizing a weakly bonded [Li⁺@C₆₀] NTf₂^- (NTf₂^-: bis(trifluoromethanesulfonyl)imide) salt makes it possible to produce a Li@C₆₀ monolayer with high concentration of Li@C₆₀ molecules. Because of the very uniform adsorption geometry of Li@C₆₀ on Cu(111), the p_z-SAMO, populated above the upper hemisphere of the molecule, exhibits an isotropic and delocalized nature, with an energy that is significantly lower compared to that of C₆₀. The isotropic overlapping of p_z-SAMOs in the condensed monolayer of Li@C₆₀ results in a laterally homogeneous STM image contributing to the formation of a free-electron-like states. These findings make an important step toward further basic research and applicative utilization of Li@C₆₀ SAMOs.

Endocircular Li Carbon Rings

By employing accurate state-of-the-art many-electron quantum-chemistry methods, we establish that monocyclic carbon rings can accommodate Li guest atoms. The low-lying electronic states of these endocircular systems are analyzed and found to include both charge-separated states where the guest Li atom appears as a cation and the ring as an anion and encircled-electron states where Li and the ring are neutral. The electron binding energies of the encircled-electron states increase drastically at their highly symmetric equilibrium geometries with increasing size of the ring, and in Li@C24 , this state becomes the ground state. Li is very weakly bound vertical to the rings in the low-lying encircled-electron states, hinting to van-der-Waals binding. Applcations are mentioned.

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as zim = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined zim was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

The Emergence and Evolution of Borophene

Neighboring carbon and sandwiched between non-metals and metals in the periodic table of the elements, boron is one of the most chemically and physically versatile elements, and can be manipulated to form dimensionally low planar structures (borophene) with intriguing properties. Herein, the theoretical research and experimental developments in the synthesis of borophene, as well as its excellent properties and application in many fields, are reviewed. The decade-long effort toward understanding the size-dependent structures of boron clusters and the theory-directed synthesis of borophene, including bottom-up approaches based on different foundations, as well as up-down approaches with different exfoliation modes, and the key factors influencing the synthetic effects, are comprehensively summarized. Owing to its excellent chemical, electronic, mechanical, and thermal properties, borophene has shown great promise in supercapacitor, battery, hydrogen-storage, and biomedical applications. Furthermore, borophene nanoplatforms used in various biomedical applications, such as bioimaging, drug delivery, and photonic therapy, are highlighted. Finally, research progress, challenges, and perspectives for the future development of borophene in large-scale production and other prospective applications are discussed.

Electric control of nearly free electron states and ferromagnetism in the transition-metal dichalcogenides monolayers

Using the first-principles calculations, we explore the nearly free electron (NFE) states in the transition-metal dichalcogenidesMX₂(M= Mo, W;X= S, Se, Te) monolayers. It is found that both the external electric field and electron (not hole) injection can flexibly tune the energy levels of the NFE states, which can shift down to the Fermi level and result in novel transport properties. In addition, we find that the valley polarization can be induced by both electron and hole doping in MoTe₂monolayer due to the ferromagnetism induced by the charge injection, which, however, is not observed in other five kinds ofMX₂monolayers. We carefully check band structures of all theMX₂monolayers, and find that the exchange splitting in the top of the valence band and the bottom of conduction band plays the key role in the ferromagnetism. Our researches enrich the electronic, spintronic, and valleytronic properties ofMX₂monolayers.

Caged-electron states and split-electron states in the endohedral alkali C60

The low-lying electronic states of neutral X@C60 (X = Li, Na, K, Rb) have been computed and analyzed by employing state-of-the-art high level many-electron methods. Apart from the common charge-separated states, well known to be present in endohedral fullerenes, one non-charge-separated state has been found in each of the investigated systems. In Li@C60 and Na@C60, the non-charge-separated state is a caged-electron state already discussed before for Li@C60. This indicates that the application of this low-lying state of Li@C60 discussed before is also applicable for Na@C60. In K@C60 and Rb@C60, the electronic radial distribution analysis shows that this hitherto unknown non-charge-separated state possesses a different nature from that of a caged-electron state.

Two-dimensional ultrathin van der Waals heterostructures of indium selenide and boron monophosphide for superfast nanoelectronics, excitonic solar cells, and digital data storage devices

Semiconducting indium selenide (InSe) monolayers have drawn a great deal of attention among all the chalcogenide two-dimensional materials on account of their high electron mobility; however, they suffer from low hole mobility. This inherent limitation of an InSe monolayer can be overcome by stacking it on top of a boron phosphide (BP) monolayer, where the complementary properties of BP can bring additional benefits. The electronic, optical, and external perturbation-dependent electronic properties of InSe/BP hetero-bilayers have been systematically investigated within density functional theory in anticipation of its cutting-edge applications. The InSe/BP heterostructure has been found to be an indirect semiconductor with an intrinsic type-II band alignment where the conduction band minimum (CBM) and valence band maximum (VBM) are contributed by the InSe and BP monolayers, respectively. Thus, the charge carrier mobility in the heterostructure, which is mainly derived from the BP monolayer, reaches as high as 12 × 10³ cm² V^-1 s^-1, which is very much desired in superfast nanoelectronics. The suitable bandgap accompanied by a very low conduction band offset between the donor and acceptor along with robust charge carrier mobility, and the mechanical and dynamical stability of the heterostructure attests its high potential for applications in solar energy harvesting and nanoelectronics. The solar to electrical power conversion efficiency (20.6%) predicted in this work surpasses the efficiencies reported for InSe based heterostructures, thereby demonstrating its superiority in solar energy harvesting. Moreover, the heterostructure transits from the semiconducting state (the OFF state) to the metallic state (the ON state) by the application of a small electric field (∼0.15 V Å^-1) which is brought about by the actual movement of the bands rather than via the nearly empty free electron gas (NFEG) feature. This thereby testifies to its potential for applications in digital data storage. Moreover, the heterostructure shows strong absorbance over a wide spectrum ranging from UV to the visible light of solar radiation, which will be of great utility in UV-visible light photodetectors.

Collective Effects of Band Offset and Wave Function Dimensionality on Impeding Electron Transfer from 2D to Organic Crystals

Excited-state electron transfer (ET) across molecules/transition metal dichalcogenide crystal (TMDC) interfaces is a critical process for the functioning of various organic/TMDC hybrid optoelectronic devices. Therefore, it is important to understand the fundamental factors that can facilitate or limit the ET rate. Here it is found that an undesirable combination of the interfacial band offset and the spatial dimensionality of the delocalized electron wave function can significantly slow down the ET process. Specifically, it is found that whereas the ET rate from TMDCs (MoS₂ and WSe₂) to fullerenes is relative insensitive to the band offset, the ET rate from TMDCs to perylene molecules can be reduced by an order of magnitude when the band offset is large. For the perylene crystal, the sensitivity of the ET rate on the band offset is explained by the 1D nature of the electronic wave function, which limits the availability of states with the appropriate energy to accept the electron.

Reversible spin storage in metal oxide-fullerene heterojunctions

We show that hybrid MnO_x/C₆₀ heterojunctions can be used to design a storage device for spin-polarized charge: a spin capacitor. Hybridization at the carbon-metal oxide interface leads to spin-polarized charge trapping after an applied voltage or photocurrent. Strong electronic structure changes, including a 1-eV energy shift and spin polarization in the C₆₀ lowest unoccupied molecular orbital, are then revealed by x-ray absorption spectroscopy, in agreement with density functional theory simulations. Muon spin spectroscopy measurements give further independent evidence of local spin ordering and magnetic moments optically/electronically stored at the heterojunctions. These spin-polarized states dissipate when shorting the electrodes. The spin storage decay time is controlled by magnetic ordering at the interface, leading to coherence times of seconds to hours even at room temperature.

One-dimensional nearly free electron states in borophene

Two-dimensional boron (borophene) features structural polymorphs and distinct in-plane anisotropy, opening opportunities to achieve tailored electronic properties by intermixing different phases. Here, using scanning tunneling spectroscopy combined with first-principles calculations, delocalized one-dimensional nearly free electron states (NFE) in the (2,3) or β₁₂ borophene sheet on the Ag(111) surface were observed. The NFE states emerge from a line defect in borophene, manifested as a structural unit of the (2,2) or χ₃ sheet, which creates an in-plane potential well that shifts the states toward the Fermi level. The NFE states are held near the 2D plane of borophene, rather than in the vacuum region as observed in other nanostructures. Furthermore, borophene can provide a rare prototype to further study novel NFE behaviors, which may have potential applications in transport or field emission nanodevices based on boron.

Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces. Here we report how a black phosphorous (BP) substrate concatenates a nearly-free-electron (NFE) like conduction band of a C60 monolayer. Scanning tunneling microscopy reveals the C60 lowest unoccupied molecular orbital (LUMO) band is strongly delocalized in two-dimensions, which is unprecedented for a molecular semiconductor. Experiment and theory show van der Waals forces between C60 and BP reduce the inter-C60 distance and cause mutual orientation, thereby optimizing the π-π wave function overlap and forming the NFE-like band. Electronic structure and carrier mobility calculations predict that the NFE band of C60 acquires an effective mass of 0.53-0.70 me (me is the mass of free electrons), and has carrier mobility of ~200 to 440 cm2V-1s-1. The substrate-mediated intermolecular van der Waals interactions provide a route to enhance charge delocalization in fullerenes and other organic semiconductors.

Li@C60 as a multi-state molecular switch

The field of molecular electronics aims at advancing the miniaturization of electronic devices, by exploiting single molecules to perform the function of individual components. A molecular switch is defined as a molecule that displays stability in two or more states (e.g. "on" and "off" involving conductance, conformation etc.) and upon application of a controlled external perturbation, electric or otherwise, undergoes a reversible change such that the molecule is altered. Previous work has shown multi-state molecular switches with up to four and six distinct states. Using low temperature scanning tunnelling microscopy and spectroscopy, we report on a multi-state single molecule switch using the endohedral fullerene Li@C₆₀ that displays 14 molecular states which can be statistically accessed. We suggest a switching mechanism that relies on resonant tunnelling via the superatom molecular orbitals (SAMOs) of the fullerene cage as a means of Li activation, thereby bypassing the typical vibronic excitation of the carbon cage that is known to cause molecular decomposition.

Angle-resolved photoelectron spectroscopy and scanning tunnelling spectroscopy studies of the endohedral fullerene Li@C60

Gas phase photoelectron spectroscopy (Rydberg Fingerprint Spectroscopy), TDDFT calculations and low temperature STM studies are combined to provide detailed information on the properties of the diffuse, low-lying Rydberg-like SAMO states of isolated Li@C60 endohedral fullerenes. The presence of the encapsulated Li is shown by the calculations to produce a significant distortion of the lowest-lying S- and P-SAMOs that is dependent on the position of the Li inside the fullerene cage. Under the high temperature conditions of the gas phase experiments, the Li is mobile and able to access different positions within the cage. This is accounted for in the comparison with theory that shows a very good agreement of the photoelectron angular distributions, allowing the symmetry of the observed SAMO states to be identified. When adsorbed on a metal substrate at low temperature, a strong interaction between the low-lying SAMOs and the metal substrate moves these states to energies much closer to the Fermi energy compared to the situation for empty C60 while the Li remains frozen in an off-centre position.

Hybridization of an unoccupied molecular orbital with an image potential state at a lead phthalocyanine/graphite interface

The interaction of a molecular orbital with a surface state is important to understand the spatial distribution of the wave function at the molecule/substrate interface. In this study, we focus on hybridization of an unoccupied state of lead phthalocyanine (PbPc) with the image potential state (IPS) on a graphite surface. The hybridization modifies the energy-momentum dispersions of the IPS on PbPc films as observed by angle-resolved two-photon photoemission. On the PbPc 1 monolayer film, the IPS band forms a band gap and back-folding appears at the first Brillouin zone boundary due to the periodic potential by the adsorbate lattice. The modification of the dispersion is accompanied by the intensity enhancement of the IPS. We attributed the origin of the modified dispersion and intensity enhancement to a hybridization of the IPS with a molecule-derived unoccupied level. From the photon energy-dependent measurement on multilayer films, we have found the diffuse unoccupied molecular level in the vicinity of the IPS. The tail part of the IPS wave function in the substrate is enhanced by the hybridization with the unoccupied state, and thus strengthens the transition from the occupied substrate band to the hybridized IPS.

Bound electronic states of the smallest fullerene C20- anion

We report on high-level coupled-cluster calculations for the anion states of the smallest fullerene C20. Using the state-of-the-art EA-EOM-CCSD method we revealed that the C20- anion has five bound electronic states at the C20 neutral ground-state D3d equilibrium configuration. These are two pairs of 2Eu and 2A2u states and one 2A1g state. The binding energies vary from 2.05 eV for the most bound 2Eu state to <1 meV for the 2A1g state. An analysis in terms of radial and angular density distribution of the excess electron revealed that the two 2Eu/2A2u pairs are valence-like states while 2A1g corresponds to a super-atomic-like (SAMO) state. The valence states of the first 2Eu/2A2u pair were found to be of p-type whereas those of the second pair are of hybrid sp-type. We have also applied a simple model to understand the binding of the excess electron in C20-. The model confirms the existence of only one SAMO state of the s-type for C20-. At the same time, it overestimates the number of the bound valence states of C20-.

Superatom Molecular Orbital as an Interfacial Charge Separation State

Hot electron cooling by energy loss to heat through electron-phonon (e-ph) interaction is an important mechanism that can limit the efficiency of solar energy conversion. To avoid such energy loss, sufficient charge separation needs to be realized by extracting hot carriers from the photoconverter before they cool, which requires fast interfacial charge transfer and slow internal hot carrier relaxation. Using ab initio time-dependent nonadiabatic molecular dynamics and taking C₆₀/MoS₂ as a prototype system, we show that the superatom molecular orbitals (SAMOs) of fullerenes, which are bound by the central potential of the whole molecule induced by the charge screening, are ideal media for charge separation. The diffuse character of SAMOs results in extremely weak e-ph interaction and therefore acts as a "phonon bottleneck" for hot electron cooling. Furthermore, it also leads to significant hybridization with other atoms at the interface that induces fast charge transfer. The interfacial charge-transfer rate at the C₆₀/MoS₂ interface is found to be 2 orders of magnitude faster than the hot electron cooling from s-SAMO in C₆₀. This conclusion is generally applicable for different carbon nanostructures that have SAMOs. The proposed SAMO-induced charge separation provides unique and essential insights into the material design and function for solar energy conversion.

Image potential states from the van der Waals density functional

The image potential state is one of the fundamental surface electronic states and has a great relevance to many surface phenomena, but its accurate description is a great challenge for the semilocal density functional. Here, we use the nonlocal van der Waals density functional to describe the image potential states of graphene, graphite, and carbon nanotubes. We found that although it does not yield the correct image potential outside the surface, the van der Waals density functional improves the description of image potential states because of the nonlocal correlation potential. Our study demonstrates the usefulness of the van der Waals density functional to study the surface electronic properties.

Cluster assemblies as superatomic solids: a first principles study of bonding & electronic structure

The synthesis of cluster based materials poses an exciting challenge for experimental chemistry. The main advantage of these materials compared to conventional bulk compounds is the simple tunability of the chemical and physical characteristics of individual clusters. As a consequence, cluster assemblies can theoretically be used for the creation of designer materials exhibiting specifically desired properties. Since superatoms reveal a large intrinsic thermodynamic stability and often very interesting tunable electronic characteristics, they seem to be an excellent choice as building blocks for the bulk. Here, we present a detailed first principles analysis of carefully chosen superatomic cluster binary and bulk assemblies, in order to determine which forces control the attractive interaction in superatomic solids, and how the individual cluster properties affect these assemblies. This study uses the highly tunable and stable Au₁₃(RS(AuSR)₂)₆ cluster with a variety of dopants as a model system, while the principles are likely transferable to other ligand protected systems with a straightforward superatomic electron count, such as aluminum or sodium clusters. Three different superatomic materials based on doped gold clusters, boranes and C₆₀s are constructed and evaluated. Beyond the verification that superatoms can be used to create materials that reveal emergent atom-based solid like properties, various factors influencing superatomic materials, such as the EA, IP and relative sizes of the clusters, have been identified and critically evaluated.

On-Surface Route for Producing Planar Nanographenes with Azulene Moieties

Large aromatic carbon nanostructures are cornerstone materials due to their increasingly active role in functional devices, but their synthesis in solution encounters size and shape limitations. New on-surface strategies facilitate the synthesis of large and insoluble planar systems with atomic-scale precision. While dehydrogenation is usually the chemical zipping reaction building up large aromatic carbon structures, mostly benzenoid structures are being produced. Here, we report on a new cyclodehydrogenation reaction transforming a sterically stressed precursor with conjoined cove regions into a planar carbon platform by incorporating azulene moieties in their interior. Submolecular resolution STM is used to characterize this exotic large polycyclic aromatic compound on Au(111) yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction. The resulting polycyclic aromatic carbon structure shows a [18]annulene core hosting peculiar pore states confined at the carbon cavity.

Direct visualization of diffuse unoccupied molecular orbitals at a rubrene/graphite interface

Using a combination of spectroscopic and microscopic imaging techniques, localized and delocalized unoccupied states are visualized at the molecular level.

Structural Transformation of Biochar Black Carbon by C60 Superstructure: Environmental Implications

Pyrogenic carbon is widespread in soil due to wildfires, soot deposition, and intentional amendment of pyrolyzed waste biomass (biochar). Interactions between engineered carbon nanoparticles and natural pyrogenic carbon (char) are unknown. This study first employed transmission electron microscopy (TEM) and X-ray diffraction (XRD) to interpret the superstructure composing aqueous fullerene C₆₀ nanoparticles prepared by prolonged stirring of commercial fullerite in water (nC₆₀-stir). The nC₆₀-stir was a superstructure composed of face-centered cubic (fcc) close-packing of near-spherical C₆₀ superatoms. The nC₆₀-stir superstructure (≈100 nm) reproducibly disintegrated pecan shell biochar pellets (2 mm) made at 700 °C into a stable and homogeneous aqueous colloidal (<100 nm) suspension. The amorphous carbon structure of biochar was preserved after the disintegration, which only occurred above the weight ratio of 30,000 biochar to nC₆₀-stir. Favorable hydrophobic surface interactions between nC₆₀-stir and 700 °C biochar likely disrupted van der Waals forces holding together the amorphous carbon units of biochar and C₆₀ packing in the nC₆₀ superstructure.

How many bound valence states does the C₆₀⁻ anion have?

We report on unprecedentedly large coupled cluster calculations for the C60(-) anion, and on a heuristic model uncovering the valence states of C60(-) that allow the resolution of the headlined question. Our results convincingly demonstrate that C60(-) possesses as many as four bound valence states: (2)T1u, (2)T1g, (2)T2u and (2)Hg. Our findings reconcile previous controversies regarding the existence of the bound (2)T2u and (2)Hg states. For all bound states of C60(-) we present an analysis of the radial and angular distributions of the excess electron, which reveals some unique properties of the valence states. Some interesting features of the introduced model are analyzed and discussed.

Evidence of enhanced photocurrent response in corannulene films

Experimental optical absorption and photoconductivity spectra of thin films with GW–BSE theoretical predictions provide evidence for diffuse super atomic molecular orbitals (SAMOs) in corannulene, C₂₀H₁₀.

Low-lying, Rydberg states of polycyclic aromatic hydrocarbons (PAHs) and cyclic alkanes

TD-DFT calculations of low-lying, Rydberg states of a series of polycyclic hydrocarbons and cyclic alkanes are presented.

Rotational dynamics of Li+ ions encapsulated in C60 cages at low temperatures

Li⁺ ions encapsulated in fullerene C₆₀ cages (Li⁺@C₆₀) are expected to be suitable as molecular switches that respond to local electric fields. In this study, the rotational dynamics of Li⁺ ions in C₆₀ cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li⁺@C₆₀](PF₆^-), the Li⁺ ion rotates in the carbon cage even at 150 K. The rotational mode gradually changes into a librational mode below 120 K, which is associated with the localization of Li⁺ ions due to the electrostatic interactions with its screening image charge on the C₆₀ cage as well as with the neighboring Li⁺@C₆₀ and PF₆^- ions. A simple rotational/librational energy scheme for the Li⁺ ions successfully explains the spectroscopic results, and the potential of Li⁺@C₆₀ as a molecular switch is discussed based on the energy scheme.

Transition from SAMO to Rydberg State Ionization in C60 in Femtosecond Laser Fields

The transition between two distinct ionization mechanisms in femtosecond laser fields at 785 nm is observed for C₆₀ molecules. The transition occurs in the investigated intensity range from 3 to 20 TW/cm² and is visualized in electron kinetic energy spectra below the one-photon energy (1.5 eV) obtained via velocity map imaging. Assignment of several observed broad spectral peaks to ionization from superatom molecular orbitals (SAMOs) and Rydberg states is based on time-dependent density functional theory simulations. We find that ionization from SAMOs dominates the spectra for intensities below 5 TW/cm². As the intensity increases, Rydberg state ionization exceeds the prominence of SAMOs. Using short laser pulses (20 fs) allowed uncovering of distinct six-lobe photoelectron angular distributions with kinetic energies just above the threshold (below 0.2 eV), which we interpret as over-the-barrier ionization of shallow f-Rydberg states in C₆₀.

Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C₆₀ fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp²-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C₆₀ film: after hot excitons are nascently formed in C₆₀ via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C₆₀ film into the HOPG is evaluated to be 1.31 × 10¹² s⁻¹, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C₆₀. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C₆₀ layers is 3.3 ± 1.2 × 10¹³ s⁻¹.

Super-atom molecular orbital excited states of fullerenes

Super-atom molecular orbitals are orbitals that form diffuse hydrogenic excited electronic states of fullerenes with their electron density centred at the centre of the hollow carbon cage and a significant electron density inside the cage. This is a consequence of the high symmetry and hollow structure of the molecules and distinguishes them from typical low-lying molecular Rydberg states. This review summarizes the current experimental and theoretical studies related to these exotic excited electronic states with emphasis on femtosecond photoelectron spectroscopy experiments on gas-phase fullerenes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Electrochemical reduction of cationic Li+@C60 to neutral Li+@C60˙-: isolation and characterisation of endohedral [60]fulleride

Li@C₆₀ was synthesised by electrochemical reduction of ionic Li⁺@C₆₀ salt. This is the first report of isolation and unambiguous characterisation of endohedral metallo[60]fullerene.

Lithium-encapsulated [60]fullerene Li@C₆₀, namely, lithium-ion-encapsulated [60]fullerene radical anion Li⁺@C₆₀˙^–, was synthesised by electrochemical reduction of lithium-ion-encapsulated [60]fullerene trifluoromethanesulfonylimide salt [Li⁺@C₆₀](TFSI^–). The product was fully characterised by UV-vis-NIR absorption and ESR spectroscopy as well as single-crystal X-ray analysis for the co-crystal with nickel octaethylporphyrin. In solution Li@C₆₀ exists as a monomer form dominantly, while in the crystal state it forms a dimer (Li@C₆₀–Li@C₆₀) through coupling of the C₆₀ radical anion cage. These structural features were supported by DFT calculations at the M06-2X/6-31G(d) level of theory.

Single molecules as whispering galleries for electrons

Whispering gallery modes, well-known for acoustic and optical waves, have been shown recently for electrons in molecules on surfaces. The existence of such waves opens new possibilities for nanoelectronic devices. Here we propose a simple analytical textbook model which allows the main characteristic features of such electronic waves to be understood. The model is illustrated by two- and three-dimensional experimental situations.

Stabilization of fullerene-like boron cages by transition metal encapsulation

The stabilization of fullerene-like boron (B) cages in the free-standing form has been long sought after and a challenging problem. Studies that have been carried out for more than a decade have confirmed that the planar or quasi-planar polymorphs are energetically favored ground states over a wide range of small and medium-sized B clusters. Recently, the breakthroughs represented by Nat. Chem., 2014, 6, 727 established that the transition from planar/quasi-planar to cage-like Bn clusters occurs around n = ∼38-40, paving the way for understanding the intriguing chemistry of B-fullerene. We herein demonstrate that the transition demarcation, n, can be significantly reduced with the help of transition metal encapsulation. We explore via extensive first-principles swarm-intelligence based structure searches the free energy landscapes of B24 clusters doped by a series of transition metals and find that the low-lying energy regime is generally dominated by cage-like isomers. This is in sharp contrast to that of bare B24 clusters, where the quasi-planar and rather irregular polyhedrons are prevalent. Most strikingly, a highly symmetric B cage with D3h symmetry is discovered in the case of Mo or W encapsulation. The endohedral D3h cages exhibit robust thermodynamic, dynamic and chemical stabilities, which can be rationalized in terms of their unique electronic structure of an 18-electron closed-shell configuration. Our results indicate that transition metal encapsulation is a feasible route for stabilizing medium-sized B cages, offering a useful roadmap for the discovery of more B fullerene analogues as building blocks of nanomaterials.

All for one and one for all: accommodating an extra electron in C60

Much like the neutral C60 fullerene, the C60(-) anion possesses certain unique properties which have attracted a great deal of research. One of these special properties, only recently fully uncovered, is that the C60(-) anion supports a substantial number of electronically stable excited states in contrast to other molecular anions with comparable electron affinity. In this work, we clarify how the C60(-) anion can support so many stable states by analyzing the radial and angular distributions of the excess electron bound to the anion. The analysis is based on ab initio calculations which are by far the most accurate on the C60(-) anion to date. Surprisingly, the radial distributions are highly similar for states of very different binding energies and the analysis stresses the importance of angular correlation in binding the excess electron. We further analyze the effect of the single excess electron on the electrons of the underlying neutral molecule. We demonstrate how this substantially modifies the actual distribution of the excess charge by shifting the underlying electron density. Implications of these findings are discussed.

Buckybowl superatom states: a unique route for electron transport?

C₂₀H₁₀ assembled on a Cu(111) surface: 1D conducting layers through SAMOs.

Structure-property relationships of curved aromatic materials from first principles

CONSPECTUS: Considerable effort in the past decade has been extended toward achieving computationally affordable theoretical methods for accurate prediction of the structure and properties of materials. Theoretical predictions of solids began decades ago, but only recently have solid-state quantum techniques become sufficiently reliable to be routinely chosen for investigation of solids as quantum chemistry techniques are for isolated molecules. Of great interest are ab initio predictive theories for solids that can provide atomic scale insights into properties of bulk materials, interfaces, and nanostructures. Adaption of the quantum chemical framework is challenging in that no single theory exists that provides prediction of all observables for every material type. However, through a combination of interdisciplinary efforts, a richly textured and substantive portfolio of methods is developing, which promise quantitative predictions of materials and device properties as well as associated performance analysis. Particularly relevant for device applications are organic semiconductors (OSC), with electrical conductivity between that of insulators and that of metals. Semiconducting small molecules, such as aromatic hydrocarbons, tend to have high polarizabilities, small band-gaps, and delocalized π electrons that support mobile charge carriers. Most importantly, the special nature of optical excitations in the form of a bound electron-hole pairs (excitons) holds significant promise for use in devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and molecular nanojunctions. Added morphological features, such as curvature in aromatic hydrocarbon structure, can further confine the electronic states in one or more directions leading to additional physical phenomena in materials. Such structures offer exploration of a wealth of phenomenology as a function of their environment, particularly due to the ability to tune their electronic character through functionalization. This Account offers discussion of current state-of-the-art electronic structure approaches for prediction of structural, electronic, optical, and transport properties of materials, with illustration of these capabilities from a series of investigations involving curved aromatic materials. The class of curved aromatic materials offers the ability to investigate methodology across a wide range of materials complexity, including (a) molecules, (b) molecular crystals, (c) molecular adsorbates on metal surfaces, and (d) molecular nanojunctions. A reliable pallet of theoretical tools for such a wide array relies on expertise spanning multiple fields. Working together with experimental experts, advancements in the fundamental understanding of structural and dynamical properties are enabling focused design of functional materials. Most importantly, these studies provide an opportunity to compare experimental and theoretical capabilities and open the way for continual improvement of these capabilities.