Isolated C60 Molecules Research Articles

Interactions between a water molecule and C60 in the endohedral fullerene H2O@C60.

A water molecule encapsulated inside a C60 fullerene cage behaves almost like an asymmetric top rotor, as would be expected of an isolated water molecule. However, inelastic neutron scattering (INS) experiments show evidence of interactions between the water molecule and its environment [Goh et al., Phys. Chem. Chem. Phys., 2014, 16, 21330]. In particular, a resolved splitting of the 101 rotational level into a singlet and a doublet indicates that the water molecule experiences an environment of lower symmetry than the icosahedral symmetry of a C60 cage. Recent calculations have shown that the splitting can be explained in terms of electrostatic quadrupolar interactions between the water molecule and the electron clouds of nearest-neighbour C60 molecules, which results in an effective environment of S6 symmetry [Felker et al., Phys. Chem. Chem. Phys., 2017, 19, 31274 and Bačić et al., Faraday Discussions, 2018, 212, 547-567]. We use symmetry arguments to obtain a simple algebraic expression, expressed in terms of a linear combination of products of translational and rotational basis functions, that describes the effect on a water molecule of any potential of S6 symmetry. We show that we can reproduce the results of the electrostatic interaction model up to ≈12 meV in terms of two unknown parameters only. The resulting potential is in a form that can readily be used in future calculations, without needing to use density functional theory (DFT) for example. Adjusting parameters in our potential would help identify whether other symmetry-lowering interactions are also present if experimental results that resolve splittings in higher-energy rotational levels are obtained in the future. As another application of our model, we show that the results of DFT calculations of the variation in energy as a water molecule moves inside the cage of an isolated C60 molecule, where the water molecule experiences an environment of icosahedral symmetry, can also be reproduced using our model.

Encapsulation of Isolated C60 Molecules in a Cyclodextrin-based Metal-Organic Framework

Encapsulation of Isolated C60 Molecules in a Cyclodextrin-based Metal-Organic Framework

Electromagnetic energy within an isolated C60 molecule

By using the equation of motion for the dynamic response of the σ and π electrons in the carbon nanostructure, the classical Poynting theorem of conservation of electromagnetic energy is extended so as to be applicable to derive an exact expression for the time-averaged electromagnetic energy within an isolated C60 molecule, which is irradiated by a plane and time-harmonic electromagnetic wave. Numerical results show that the stored energy exhibits resonant enhancement near resonances of extinction cross section of the system.

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing.

The understanding and control of molecule-metal interfaces is critical to the performance of molecular electronics and photovoltaics devices. We present a study of the interface between C60 and W, which is a carbide-forming transition metal. The complex solid-state reaction at the interface can be exploited to adjust the electronic properties of the molecule layer. Scanning tunneling microscopy/spectroscopy measurements demonstrate the progression of this reaction from wide band gap (>2.5 eV) to metallic molecular surface during annealing from 300 to 800 K. Differential conduction maps with 104 scanning tunneling spectra are used to quantify the transition in the density of states and the reduction of the band gap during annealing with nanometer spatial resolution. The electronic transition is spatially homogeneous, and the surface band gap can therefore be adjusted by a targeted annealing step. The modified molecules, which we call nanospheres, are quite resistant to ripening and coalescence, unlike any other metallic nanoparticle of the same size. Densely packed C60 and isolated C60 molecules show the same transition in electronic structure, which confirms that the transformation is controlled by the reaction at the C60-W interface. Density functional theory calculations are used to develop possible reaction pathways in agreement with experimentally observed electronic structure modulation. Control of the band gap by the choice of annealing temperature is a unique route to tailoring molecular-layer electronic properties.

Electronic Structure of Crystalline Buckyballs: fcc-C60

The electronic properties of pristine fcc-C60 are calculated by utilizing a variety of density functional theory (DFT) approaches including the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), PBE-GGA+DFT-D3(vdW), Engel and Vosko GGA (EV-GGA), GGA plus Hubbard U parameter (GGA+U), hybrids Becke–Perdew–Wang hybrid functional (B3PW91), Becke–Lee–Yang–Parr hybrid functional (B3LYP), the PBE exchange-correlation functional (PBE0), and Tran and Blaha regular and non-regular modified Becke and Johnson (TB-mBJ) potential within a DFT frame work using augmented plane waves plus local orbital method. The comparison of the calculated results with the experimental values shows that the non-regular TB-mBJ method reproduces a correct experimental direct band gap of 2.12 eV at X symmetry for this compound. The effectiveness of this theoretical approach in the reproduction of the experimental band gap is due to the proper treatment of the electrons in the interstitial region of the crystal. Our results show that the C60 clusters are weakly interacting with each other in the fcc crystal. This study also reveals that the five-fold degeneracies of the isolated C60 molecule due to its icosahedral symmetry are completely lifted at an X symmetry point by the crystal field.

Two-dimensional van der Waals C60 molecular crystal.

Two-dimensional (2D) atomic crystals, such as graphene and transition metal dichalcogenides et al. have drawn extraordinary attention recently. For these 2D materials, atoms within their monolayer are covalently bonded. An interesting question arises: Can molecules form a 2D monolayer crystal via van der Waals interactions? Here, we first study the structural stability of a free-standing infinite C60 molecular monolayer using molecular dynamic simulations, and find that the monolayer is stable up to 600 K. We further study the mechanical properties of the monolayer, and find that the elastic modulus, ultimate tensile stress and failure strain are 55–100 GPa, 90–155 MPa, and 1.5–2.3%, respectively, depending on the stretching orientation. The monolayer fails due to shearing and cavitation under uniaxial tensile loading. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the monolayer are found to be delocalized and as a result, the band gap is reduced to only 60% of the isolated C60 molecule. Interestingly, this band gap can be tuned up to ±30% using strain engineering. Owing to its thermal stability, low density, strain-tunable semi-conducting characteristics and large bending flexibility, this van der Waals molecular monolayer crystal presents aplenty opportunities for developing novel applications in nanoelectronics.

Multipole plasmon excitations of C60 dimers.

We study the multipole plasmon mode frequencies of a pair of C60 molecules by means of the linearized hydrodynamic theory for electronic excitations on the each C60 surface. We apply the two-center spherical coordinate system for mathematical convenience and find an explicit form of the surface plasmon energies. Numerical result shows when approaching the two C60 molecules, the coupling between the bare plasmon modes leads to the appearance of additional modes having energies that are different from those of the isolated C60 molecules.

Extinction properties of an isolated C60 molecule

We study the extinction spectra of an isolated C60 molecule, within the framework of the vector wave function method. Electronic excitations on the C60 molecule surface are modeled by an infinitesimally thin spherical layer of the σ and π electrons, which is described by means of the two-dimensional two-fluid model. Numerical results show that a strong interaction between the fluids gives rise to the splitting of the extinction spectra into two peaks in a quantitative agreement with the π and σ+π plasmon energies.

Plasmon excitations in C60 by fast charged particle beams

For an isolated C60 molecule, we study plasmon excitations that are induced by an external, fast moving electron, by using a two-dimensional, spherical, two-fluid hydrodynamic model for the dynamic response of the σ and π electrons in the carbon nanostructure. Second quantization of the linearized hydrodynamic model allows us to discuss how effective is multiple excitation of various plasmon modes. Mean numbers of the excited plasmon modes, differential cross sections, and the total energy loss of the incident electron are calculated by both a quantized model with zero damping and by a semi-classical model with phenomenological damping. Our calculated differential cross sections are compared with experiment.

(e,2e) experiments on C60

Electron impact experiments combined with the coincidence detection of the charged particles in the final state represent one of the richest sources of information on the dynamics of molecular processes and the structure of the targets, where the targets can extend from rare gases to molecules of increasing complexity. In this work the electron impact ionization of isolated C60 molecules in the gas phase has been studied via (e,2e).

Scanning tunneling microscopy/spectroscopy on self-assembly of a glycine/Cu(111) nanocavity array

The step-by-step analysis of a hierarchical self-assembly revealed the incorporation of nanocavity blocks in a metastable orientation to stabilize the organized array. The confinement of 2D electrons by a quantum corral was verified. Furthermore, manipulation of an isolated C(60) molecule was realized using nanocavities of ∼1.3 nm diameter as a template.

“Soft” Metallic Contact to Isolated C60 Molecules

C60 adsorbed on a monolayer of hexaazatriphenylene-hexanitrile (HATCN) on Ag(111) is investigated by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy. UPS and quantum-mechanical modeling show that HATCN chemisorbed on Ag(111) displays metallic character. This metallic molecular layer decouples C60 electronically from the Ag substrate and simultaneously acts both as template for the stable adsorption of isolated C60 molecules at room temperature and as "soft" metallic contact for subsequently deposited molecules.

Interaction of Porphine and Its Metal Complexes with C60 Fullerene: A DFT Study

We performed DFT calculations (BLYP general-gradient approximation in conjunction with a double numerical basis set) for the interaction of free porphine ligand and a number of its metal complexes with C60 molecule to analyze how the nature of a central metal ion influences the geometry and electronic characteristics (electrostatic potential and spin density distribution and highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) structure). We found that the presence of a central metal ion is crucial for a strong interaction. The energy of interaction between H2P and C60 is -0.3 kcal mol(-1) only, whereas the formation energies for the metal complexes vary from -27.3 kcal mol(-1) for MnClP.C60 to -45.8 kcal mol(-1) for MnP.C60. As a rule, the formation energy correlates with the separations between porphinate and fullerene molecules; the Mn and Fe complexes exhibit the closest approach of ca. 2.2 A between the metal ion and carbon atoms of C60. In most porphine-C60 complexes studied, the two closest contacts of central metal ion or H are those with carbon atoms of the (6,6) bond; VOP.C60 is the only exception, where the closest V...C contacts involve the (5,6) bond. The macrocycle geometry changes, and the magnitude of the effect depends on the central atom, being especially dramatic for Mn, MnCl, and Fe complexes. The shape of LUMOs in most complexes with C60 is not affected notably as compared to the LUMO of the isolated C60 molecule. In the case of Fe, the HOMO extends from the central atom to two opposite pyrrol rings. At the same time, the HOMO-LUMO gap energy decreases drastically in most cases, by ca. 20-30 kcal mol(-1). For electrostatic potential distribution, we systematically observed that the negative lobe contacting C60 shrinks, whereas the opposite one becomes notably bigger. In the case of paramagnetic complexes of VO, Mn, FeCl, Co, and Cu, spin density distribution was analyzed as well.

Mechanical Interactions in All-Carbon Peapods

Structures and energetics of several types of all-carbon peapods made by C60s encapsulated inside single-wall carbon nanotubes are calculated. The interactions between the tubes and the molecules determine a minimal diameter for exothermic encapsulation of 11.74 A and a radial deformation of the C60 cage that is a linear function of the tube diameter. Many relative orientations of the “peas” have similar energies and can be thermally equilibrated, although preferential ways of interaction between fullerenes do exist. In particular, for a triplet of embedded fullerenes, the lowest energy arrangement is characterized by a facing describable as pentagon-hexagon-pentagon. The activation energy for the translation of isolated C60 molecules inside the tubes is very low and an isolated C60 can diffuse freely inside the pod: The major factor controlling the dynamics of molecular transport in the peapods is therefore the C60−C60 interaction.

Superconductivity in hole-dopedC60from electronic correlations

We derive a model for the highest occupied molecular orbital band of a C60 crystal which includes on-site electron-electron interactions. The form of the interactions are based on the icosahedral symmetry of the C60 molecule together with a perturbative treatment of an isolated C60 molecule. Using this model we do a mean-field calculation in two dimensions on the [100] surface of the crystal. Due to the multi-band nature we find that electron-electron interactions can have a profound effect on the density of states as a function of doping. The doping dependence of the transition temperature can then be qualitatively different from that expected from simple BCS theory based on the density of states from band structure calculations.

Direct measurement of the electric polarizability of isolated C60 molecules

We present the first direct measurement of the electric polarizability of isolated C60 molecules by molecular beam deflection technique. We have obtained a value of 76.5±8.0 Å3 which is consistent with most of the recent calculations and slightly lower than the value of the polarizability of C60 measured in fullerite crystals.

Supramolecular assembly of individual C 60 molecules on a monolayer of 4,4 ′ -dimethylbianthrone molecules

molecules adsorbed on a well-ordered monolayer of 4,4’-dimethylbianthrone (DMB), first grown on Cu(111). The surface binding energies and diffusion barriers of C60 molecules are substantially modified when the molecules bond to DMB instead of to Cu. The tip of the STM can be used to reposition single, isolated C60 molecules across the surface without disrupting the DMB film underneath.

Resonant Tunneling Through Discrete Electronic Levels of a C60Molecule in the Presence of Charging Effects

We have conducted tunneling spectroscopy studies for isolated C60 molecules in the double barrier tunnel junction configuration. The tunneling current-voltage (I-V) and dI/dV vs. V spectra of these molecules exhibit rich structures resulting from both resonant tunneling through the discrete levels and single electron charging effects. In particular, we observe degeneracy lifting within the molecular orbitals, probably due to the Jahn-Teller effect and local electric fields. Theoretical fits, performed using the orthodox model for single-electron tunneling modified to account for the discrete level spectrum of C60, agree well with our data.

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C60) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C60 molecule and that of a C60 crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C60, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals.

Single electron tunneling and level spectroscopy of isolated C60 molecules

The interplay between single electron tunneling effects and the discrete molecular levels of C60 molecules is studied using scanning tunneling microscopy. Isolated C60 molecules were deposited onto a gold substrate, covered by a thin insulating layer. The tunneling current–voltage characteristics of isolated molecules, both at room temperature and at 4.2 K, exhibit rich structures, resulting from the interplay between charging effects and the electronic spectrum. In particular, we observe degeneracy lifting within the C60 molecular orbitals, probably due to the Jahn–Teller effect and local electric fields.