Photoemission spectroscopy of clean and potassium-intercalated carbon onions

Abstract

Hollow onionlike carbon ͑OLC͒, generated by annealing nanodiamond at 2140 K, has been studied by core-level and valence-band photoemission spectroscopy. Upon intercalation with potassium, core and valence states of the OLC show an almost rigid shift to higher binding energies, and the density of states at the Fermi level (E F ) is observed to increase. An asymmetric broadening of the C1s line from the OLC as intercalation proceeds indicates an increase in electron-hole pair excitations. Both core and valence-band spectra are consistent with charge transfer from the intercalated potassium to the OLC, and support the conclusion that the electronic structure of the carbon onions bears strong similarity to that of graphite, although differences do exist. In consequence the conclusion can be drawn that these species behave as graphite ''nanocrystals'' rather than as large fullerene molecules.

Photoemission study of onionlike carbons produced by annealing nanodiamonds

“…37 A photoemission study of potassium intercalated polygonal hollow onions produced at 2140 K showed that these carbon onions behave as small graphite crystals and hence display bulklike rather than molecularlike behavior. 38 The x-ray emission study of OLC produced in the temperature range of 1600-1900 K revealed that their C K ␣ spectra exhibit a significant increase of the high-energy maximum and are markedly different from the spectrum of particles formed at 2140 K, which is very similar to the C K ␣ spectrum of polycrystalline graphite. 32 However, due to dipole selection rules, the x-ray emission spectra yield information on the partial density of states.…”

“…The first peak, at 2.8± 0.1 eV, is related to bonding in graphite. 32,38,62 Progressive graphitization of the ND in samples annealed at 1420 and 1600 K, and the eventual vanishing of sp 3 carbon from the C1s photoemission spectra ͑Fig. 4͒ in the sample annealed at 1800 K results in increasing intensity and sharpness of the peak at 2.8± 0.1 eV in the valence band spectra.…”

“…This process is accompanied by an increasing intensity in the shoulder at 4.2± 0.1 eV in the valence band data, which indicates growth of the population of mixed -states. 32,38 The second peak at 7.5± 0.1 eV is related to bonding in graphite. 32,38 It should be noted that in our previous work ͑Ref.…”

“…32,38 The second peak at 7.5± 0.1 eV is related to bonding in graphite. 32,38 It should be noted that in our previous work ͑Ref. 38͒ for the sample annealed at 2140 K we found positions for and peaks of 2.92 eV and 7.84 eV respectively.…”

In situ formation of onion-like carbon from the evaporation of ultra-dispersed nanodiamonds

“…A typical AFM topographic image is shown in When produced directly by annealing ultra-dispersed nanodiamond, OLC samples show a clear π plasmon loss in the C1s core-level photoelectron spectra 6 [11,18], which is notably absent in the spectra shown in Fig. 3.…”