D. Östling

Theory for Collective Resonances of the C60 Molecule

We discuss the collective resonances of the C60 molecule described by a spherical shell. Detailed results are given for the π and π + σ plasmons of C60, the polarisability and the dielectric function. We present results in good quantitative agreement with recent experiments. Some new features in the spectrum, like a monopole mode of oscillation, are predicted for a doped C60 molecule.

Collective resonances of carbon onions

Abstract Based on a simple equation of motion for the induced density in a spherical carbon particle we demonstrate the existence of a rich spectrum of collective resonances as a function of the number of shells for n = 1 ( C 60 ), 2,10 and 40. We also present results for the photoabsorption cross section and discuss the nature of the collective resonances by examining the eigenmodes of the systems.

Electronic properties of the C60 molecule doped with potassium

Molecular cluster calculations within the local density approximation have been performed in a study of the electronic structure of C60 doped with K, where the K atom has been located inside or outside the cage. The results support our earlier calculations for endohedral KC60 as well as those for endohedral and exohedral K2C60 clusters, i.e. the 4s valence electron of K is transferred to the t1u, LUMO, level of C60. Ionization energies evaluated in this work as well as in the earlier work, are found to be in good agreement with recently determined photoelectron spectra for KxC−60 clusters in gas phase.

Electronic structure and optical properties of bare and Li, Na, K and Ca coated C60 molecule

Abstract We study the electronic structure, geometric and low-energy optical properties of C 60 and metal-coated M 32 C 60 , where M = Li, Na, K or Ca. The one-electron Kohn–Sham orbitals are used together with the RPA matrix to calculate the screened oscillator-strength distribution. The deep-lying levels in M 32 C 60 have C 60 character while those close to and above the Fermi energy have the character of the metallic M 32 layer. Isolated alkali M 32 shells are characterized by a double-peak structure in the range 1–3 eV, well below the π plasmon of C 60 at about 6 eV. In M 32 C 60 the M 32 shell is coupled to C 60 , but the characteristic double-peak can still be identified.

Electronic structure and optical properties of bare and coated C60 molecules

Abstract Ground state electronic structures of C 60 and metal coated C 60 , modeled with the clusters Li 12 C 60 and Li 32 C 60 , are analyzed within the local density functional approximation. The low-energy part of the linear-response spectra of a bare C 60 , Li 12 C 60 and Li 32 C 60 is also studied. The Kohn-Sham molecular wavefunctions are used for the evaluation of dipole matrix elements and polarizabilities, and the effect of screening the oscillator strengths is found using a simplified RPA approach. For the Li-coated C 60 species the electronic level diagram shows correspondence to the C 60 level diagram but with new levels originating from Li close to the Fermi level. New structure in the oscillator strength distribution from the Li-coating is found at lower energy than the π plasmon of C 60 at about 6 eV.

Collective resonances of the molecule: effects of electron-density profile

Previously, we discussed a purely classical model for analysing surface plasmons of the molecule. The molecule was described as an abrupt spherical shell containing an effective number of or electrons. This simple model was found to be quite applicable for analysis of electron energy loss spectra (EELS), describing the main features of the experimentally observed collective resonances of electrons. In this study we have extended our earlier classical model using spherically averaged radial charge distributions of constructed using different numbers of valence electrons. These charge distributions have been obtained from a self-consistent local density calculation, based on a molecular-orbital linear combination of atomic orbitals (MO-LCAO) approach. Use of this type of smooth charge distribution introduces, together with causality, a natural broadening of the collective resonances.

Surface plasmons ofC60

We discuss the collective dipolar resonances of theC60 molecule described by a spherically symmetric shell. The shell is modeled by a step in the radial direction. We present results for π and π+σ plasmons in good quantitative agreement with recent experiments. New features, like a monopole mode of oscillation, are predicted for theC60 molecule.

Calculations of the electronic structure of doped Buckminsterfullerene

Molecular cluster calculations within the local density approximation have been performed in a study of the electronic structure of theC60 molecule — “Buckminsterfullerene” doped with different atoms asK, Ca, Sc andAl-Cl. Doping withK shows how the valence 4s electron is easily transferred to thet1u LUMO state of the bareC60 molecule. Use ofCa andSc as dopant element seems to give a somewhat stronger hybridization between the 4s, 3d wavefunctions with the carbon 2p levels of theC60 molecule. Doping with the 3p elements show the existence ofn- andp-type dopedC60, where a partly occupied level occurs in the band gap similar to the donor or acceptor levels in traditionally doped semiconductors.

Plasmons in C 60 , Carbon Onions and Carbon Tubes

After the discovery of the C 60 molecule a lot of attention has been given to its optical and collective properties. The latest development in fullerene related research was the synthesis of coaxial carbon sheets called carbon or nano tubes and also spherical concentric graphitic shells called carbon onions. With a model describing the collective dynamics of electrons we may gain insight in the physics of the collective resonances. Based on a simple equation of motion for the induced density in a carbon particle we demonstrate the existence of a rich spectrum of collective resonances for both carbon onions and carbon tubes.

Electronic Structure of Doped Buckminsterfullerene

Molecular cluster calculations within the local density approximation have been performed in a study of the electronic structure of the C 60 molecule - “Buckminsterfullerene” doped with K, B and N. Calculations for the KC 60 molecule, with the K atom located at the centre of the cage as well as at different positions inside or outside the cage, show how the valence 4s electron is transferred to the LUMO state of the bare C 60 molecule. Doping with a B or N atom located at the centre of the cage creates a molecule with a partly occupied level of 2p character in the HOMO and LUMO gap, similar to donor and acceptor levels in the band gap of traditionally doped semiconductors. Doping by substitution of one or two of the carbon atoms in the cage with X = B or N, as modelled with the C 59 X 1 or C 58 X 2 clusters, gives a different structure with a splitting of the HOMO and LUMO levels in the pure C 60 molecule and with the creation of acceptor and donor levels with the substitution of B and N, respectively.