K/graphite: Uniform energy shifts of graphite valence states

Ultra-thin graphite overlayers prepared by heating SiC crystals can give highly resolved valence band photoemission spectra. This is exploited to monitor K deposition induced energy shifts of graphite valence band states. States near the corners of the Brillouin zone, responsible for the semimetal character of graphite, are observed to show shifts that are nearly equal to the shifts of the uppermost filled sigma state and of an empty state 7.6 eV above EF. The results give credence to the rigid band shift model often used to discuss the electronic structure and charge transfer for graphite ad- and absorption systems. (c) 2006 Elsevier B.V. All rights reserved

Narrow UV photoemission lines from graphite

Stimulated by the recent observation by LEED that well ordered graphite overlayers may be prepared by heating SiC crystals [1] we have used LEED, STM and UPS (hv = 20-140 eV) to study such layers and compared the photoemission spectra to those we obtain from a natural single crystal. Two results are of particular interest. One is that the overlayers give spectra having a quality well on par with those from the natural crystal. This means that the overlayers can be attractive as an alternative to natural crystals, which are inconveniently small for many experiments. The other result is that there are graphite states, which give quite narrow emission lines. One is due to the upper σ state at Γ and another to states near the Fermi level, which for graphite means states at the zone corners. Both lines should be useful for ador absorption studies as an alternative to the C 1s core line and the σ line may provide testing ground for the current modeling of excitation lifetimes in graphite

Narrow photoemission lines from graphite valence states

Valence-band photoelectron angular distributions, measured in the photon energy range 20-150 eV, are obtained from crystalline graphite overlayers prepared by heating SiC(0001) and from a graphite natural single crystal. The dispersion of the valence bands for the overlayers agrees well with that of the single crystal. The valence electrons have binding energies, which agree with LDA calculations if the calculated binding energies are multiplied by a factor of 1.13. The upper sigma state at Gamma and states near the Fermi level at the zone corners give quite narrow emission lines. Since the widths are on par with that of the C 1s level the lines are of interest as an alternative to the core line when graphite is used as substrate for adsorption or absorption studies

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the pi states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage I compounds C8K or CgRb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 x 2) ordered alkali metal layer adjacent to the uppermost carbon layer. (c) Elsevier B.V. All rights reserved