Monolayer metal coverage of fullerenes: the optical response

Abstract

A first-principles investigation of the photoabsorption cross section of the specially stable cluster Li C is carried out, including the icosahedral symmetry of the cluster and treating the ions via ab initio pseudopotentials. The role of the coating metal is assessed by computing the same quantity for the fullerene ''seed''. It is found that the main absorption features are determined by the carbon molecule, both at low and high excitation energies, in spite of a reduction of 60% for the ionization threshold of Li C as compared to C . Nonetheless the lithium coating is responsible for small yet clearly observable effects throughout the spectrum, in particular for a more structured and broad strength distribution at excitation energies below the ionization threshold of C , and a double peak structure in the Mie resonance at 20 eV.PACS: 36.40. #d; 31.50. #w; 33.20.Kf Inspired by electrochemical evidence of a stable Li C phase [1], the cluster Li C has been predicted to be particularly stable, due to the favourable combination of a closed-shell electronic structure, realized in a highlysymmetric icosahedral geometry [2]. This structural property is a somewhat surprising result, because with other alkali elements saturation occurs at x"6 for the intercalated compounds M V C . Prompted by these results, T. P. Martin and his group co-evaporated C and different alkali metals in a gas aggregation cell, registering the mass spectrum for several types of C clusters coated with metal atoms [3]. The lithium coating experiments confirmed the special stability of the Li C cluster, which develops also a well defined peak in the mass spectra of singly and doubly ionized lithium-coated C clusters. Theoretically, a good deal is known about the structural properties of Li C [2], as well as other lithiumcoated fullerenes [4]. This is not the case concerning their optical properties, which are expected to display a sizedependent behaviour. In fact, by increasing the number of alkali coating atoms, it is likely that the cluster undergoes a non-metal to metal transition, in which the C cage becomes the seed for a metallic nanostructure. Qualitatively, and taking as a reference the optical spectrum of the isolated C cage, one may expect that alkali coverage will induce important effects in the low-energy region (0(E(6 eV) of the absorption spectrum, where the collective charge density oscillations of alkali clusters take place, whereas at higher excitation energies (E 20 eV) the spectrum will be characterized by a (strongly screened) bump which originates in the so-called Mie plasmon [5] of C . Several physical phenomena may lead to such effects. On one hand, the layers of alkali-coating atoms will induce, in general, new geometrical equilibrium shapes for these clusters, as compared to the icosahedral shape of the C fullerene. The deviations from the icosahedral shape correspond to symmetry breakings which may induce important modifications in the energy sequence and degeneracies of the one-electron levels, thereby affecting the optic...

Optical response of cesium coated C60

“…But measurements of C 60 Cs 20 + and C 60 Cs 500 + in a wavelength-range between 2 and 2.8 eV do not show any additional structures. Recently, several theoretical approaches to the optical response of metal coated C 60 have been presented [13][14][15][16]. Rubio et al treat the problem of up to 93 sodium atoms coating C 60 by means of a two-shell jellium- Fig.…”