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STM imaging, spectroscopy and manipulation of a self-assembled PTCDI monolayer on epitaxial graphene
Yang, H.,Mayne, A. J.,Comtet, G.,Dujardin, G.,Kuk, Y.,Sonnet, Ph.,Stauffer, L.,Nagarajan, S.,Gourdon, A. The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.14
<P>Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and manipulation studies were performed on an ordered self-assembled monolayer (SAM) of <I>N</I>,<I>N</I>′-bis(1-hexylheptyl)perylene-3,4:9,10-bis(dicarboximide) molecules on epitaxial graphene on hexagonal silicon carbide – SiC(0001). Four novel aspects of the molecular SAM on graphene are presented. Molecules adsorb in both armchair and zig-zag configurations, giving rise to six orientations of the molecular layer with respect to the underlying substrate. The interaction between the molecules and the graphene surface shifts the LUMO towards the Fermi level, inducing a charge transfer and the opening of a band gap in the graphene, with the LUMO inside. This decouples the LUMO from the surface rendering it invisible in the d<I>I</I>/d<I>V</I> spectroscopy. The HOMO only becomes visible at short tip-surface distances, as its energy lies within the band gap of the SiC substrate. Finally, the observed molecular defects are very particular, being composed exclusively of molecular dimers. These molecular dimers have a stronger interaction with the graphene than other molecules.</P> <P>Graphic Abstract</P><P>Scanning tunneling microscopy and spectroscopy studies of a self-assembled hexyl heptyl PTCDI monolayer on epitaxial graphene reveal molecules adsorbed in two configurations. Charge transfer induces band gap opening in graphene and decouples the LUMO. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cp42591f'> </P>
Yang, H.,Boudrioua, O.,Mayne, A. J.,Comtet, G.,Dujardin, G.,Kuk, Y.,Sonnet, Ph.,Stauffer, L.,Nagarajan, S.,Gourdon, A. The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.5
<P>Controlling the intrinsic optical and electronic properties of a single molecule adsorbed on a surface requires electronic decoupling of some molecular orbitals from the surface states. Scanning tunneling microscopy experiments and density functional theory calculations are used to study a perylene molecule derivative (DHH-PTCDI), adsorbed on the clean 3 × 3 reconstructed wide band gap silicon carbide surface (SiC(0001)-3 × 3). We find that the LUMO of the adsorbed molecule is invisible in <I>I</I>(<I>V</I>) spectra due to the absence of any surface or bulk states and that the HOMO has a very low saturation current in <I>I</I>(<I>z</I>) spectra. These results present a paradox that the molecular orbitals are electronically isolated from the surface of the wide band gap semiconductor even though strong chemical bonds are formed.</P> <P>Graphic Abstract</P><P>Organic molecule adsorption on the SiC surface forms strong chemical bonds but certain molecular orbitals are electronically decoupled from the substrate. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp23104b'> </P>