Abstract
Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.
Highlights
Thickness were characterized by monitoring the evolution of sharp spots of the lowenergy electron diffraction (LEED) pattern in figure 1(c), i.e. the typical commensurate 4 × 4 superstructure of the first monolayer [7]
It is a well-known fact that DFT calculations employing the local density approximation (LDA) or the generalized gradient approximation (GGA) for the exchange-correlation functional severely underestimate band gaps of semi-conductors and yield too small π-band widths of many systems including graphene—and graphene-like molecules—which can be corrected by self-energy calculations within the so-called GW-approximation [24]
For both samples we identify several peaks that we attribute to molecular signals
Summary
Thickness were characterized by monitoring the evolution of sharp spots of the lowenergy electron diffraction (LEED) pattern in figure 1(c), i.e. the typical commensurate 4 × 4 superstructure of the first monolayer [7]. The k-dependence of the PES intensity clearly makes the Ag sp-bands visible, which disperse between the Fermi edge and the 4d states at about 1 Å−1 and which are forming a nearly constant background in the angle-integrated EDCs. As in other organic monolayer systems [26,27,28], the Shockley state of Ag(111) [29] does not appear in the photoemission data, because it has most likely shifted above the Fermi level. The electronic states of coronene and HBC can be described in a quantum well approach.
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