Charge-Transfer-Driven Nonplanar Adsorption of F4TCNQ Molecules on Epitaxial Graphene Avijit Kumar,† Kaustuv Banerjee,† Marc Dvorak,‡ Fabian Schulz,†,§ Ari Harju,‡,⊥ Patrick Rinke,‡ and Peter Liljeroth*,† †
Department of Applied Physics and ‡COMP/Department of Applied Physics, Aalto University School of Science, 00076 Aalto, Finland S Supporting Information *
ABSTRACT: π-conjugated organic molecules tend to adsorb in a planar configuration on graphene irrespective of their charge state. In contrast, here we demonstrate charging-induced strong structural relaxation of tetrafluorotetracyanoquinodimethane (F4TCNQ) on epitaxial graphene on Ir(111) (G/Ir(111)). The work function modulation over the graphene moiré unit cell causes siteselective charging of F4TCNQ. Upon charging, the molecule anchors to the face-centered cubic sites of the G/Ir(111) moiré through one or two cyano groups. The reaction is reversible and can be triggered on a single molecule by moving it between different adsorption sites. We introduce a model taking into account the trade-off between tilt-induced charging and reduced van der Waals interactions, which provides a general framework for understanding charging-induced structural relaxation on weakly interacting substrates. In addition, we argue that the partial sp3 rehybridization of the underlying graphene and the possible bonding mechanism between the cyano groups and the graphene substrate are also relevant for the complete understanding of the experiments. These results provide insight into molecular charging on graphene, and they are directly relevant for potential device applications where the use of molecules has been suggested for doping and band structure engineering. KEYWORDS: STM, F4TCNQ, self-assembly, Kondo effect, epitaxial graphene, Ir(111), DFT
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Here, we present adsorption of F4TCNQ molecules on epitaxial graphene on an Ir(111) (G/Ir(111)) substrate, where molecular charging is accompanied by a strong structural relaxation. The molecules are charged on certain sites of the G/ Ir(111) moiré due to a slight work function modulation over the moiré unit cell and the close vicinity of the lowest unoccupied molecular orbital (LUMO) of F4TCNQ to the substrate Fermi level. Upon charging, the molecule anchors to the face-centered cubic (fcc) sites of the G/Ir(111) moiré through one or two cyano groups, and scanning tunneling microscopy (STM) experiments clearly show signatures of bonding between the cyano groups and the graphene substrate. The reaction is reversible, similarly to the reported Diels− Alder-type reaction between isoindole moiety of phthalocyanine and the underlying graphene,20 and can be triggered on a single molecule by moving it between different adsorption sites. We confirm the charging experimentally by the observation of the Kondo resonance on the nonplanar molecules and unravel the factors responsible for the observed behavior by using two
hemical functionalization of graphene has attracted broad interest as a powerful route to modify its chemical and electronic properties.1−3 Covalent functionalization, that is, attaching molecules covalently to the graphene backbone, can be used to modify the graphene surface chemistry but is usually detrimental to its electronic properties. This stems from the strong in-plane bonding in graphene, making it chemically inert and requiring highly reactive species to induce the required sp2 to sp3 rehybridization of the carbon atoms.1,4−6 On the other hand, physisorbed self-assembled molecular layers leave the electronic structure of graphene intact and can be used, for example, to control the concentration of charge carriers in graphene.3,7,8 The self-assembled systems typically consist of relatively large π-conjugated molecules adsorbing in a planar geometry driven by the van der Waals (vdW) interactions between the molecules and graphene.3,9−15 For molecules with high electron affinity such as tetracyanoquinodimethane (TCNQ) and tetrafluorotetracyanoquinodimethane (F4TCNQ), adsorption can be accompanied by charge transfer.10−13,16 However, this does not normally result in dramatic changes in the adsorption configuration, in contrast to several examples on metallic substrates.17−19 © 2017 American Chemical Society
Received: March 6, 2017 Accepted: May 3, 2017 Published: May 3, 2017 4960
DOI: 10.1021/acsnano.7b01599 ACS Nano 2017, 11, 4960−4968
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Figure 1. F4TCNQ adsorption on graphene on Ir(111). (a) Overview STM topography image of F4TCNQ molecules adsorbed on G/Ir(111) surface. Most of the molecules are in the two-legged configuration; one example of a flat geometry is highlighted by a black circle. The moiré unit cell is indicated by the black parallelogram. The black dots indicate the corners (fcc sites) of the moiré pattern. Imaging parameters: 0.85 V; 3.4 pA. (b) Illustration of the G/Ir(111) moiré unit cell depicting different registries between the carbon atoms and the underlying Ir substrate. (c) STM topography image showing the two-legged and flat adsorption geometries with respect to the G/Ir(111) moiré. The fcc and hcp regions have been depicted by filled and open circles. Overlaid molecular structures (to scale) have been added to help to visualize the adsorption sites. One end of the two-legged molecule is anchored to the fcc site, and the flat molecule is adsorbed on the top site.
of the frequency shift dependence on the tip−sample distance (Supporting Information). This yields an estimate of the tilt angle of 13 ± 3°. In addition to the two-legged and flat molecules, we also see “three-legged” molecules where three cyano groups of the molecules are visible, especially at higher coverages (Supporting Information). These observations differ from the flat geometries observed for F4TCNQ on graphene on Ru(0001) surfaces12,13,21,22 and graphene on boron nitride.10,11 The parent molecule, TCNQ, also exhibits flat adsorption geometries on G/Ir(111)16 and G/Ru(0001).12 Along with the asymmetric adsorption geometry, F4TCNQ molecules prefer selected adsorption sites on the moiré pattern of the G/Ir(111) surface, consistent with earlier reports.16 A careful observation reveals that the ends of the two-legged molecules (where the cyano groups are not visible) follow a hexagonal symmetry with a periodicity similar to that of the G/ Ir(111) moiré, as illustrated in the top-left region of Figure 1a. A schematic of the G/Ir(111) moiré pattern is shown in Figure 1b, in which the different adsorption registries have been marked. In the top site, the center of the carbon ring of graphene is directly above an Ir atom, corresponding to the region of weakest graphene−metal interaction and largest adsorption height from the Ir(111) substrate. Conversely, regions corresponding to alternate carbon atoms of the ring being positioned over the surface metal atoms are denoted as (i) fcc site where carbon atoms not on top of metal atoms are on the fcc hollow site and (ii) hexagonal close-packed (hcp) site where carbon atoms not on top of metal atoms are on the hcp hollow site. As the transition from one registry to the other is smooth, each region has been depicted by a circle. By overlaying scaled molecular structures on the STM images, it is apparent that the two-legged molecules are anchored to the fcc
different modeling approaches derived from our density functional theory (DFT) calculations. The former takes into account the trade-off between tilt-induced charging and reduced vdW interactions and can be used as a general framework for understanding charging-induced structural relaxation on weakly interacting substrates. The latter includes partial sp3 rehybridization of the underlying graphene and points to the possibility of a bonding mechanism between the cyano groups and the graphene substrate. These results provide understanding on charging-induced structural relaxation of molecules on weakly interacting substrates. They are directly relevant for potential device applications where the use of molecules has been suggested for doping and band structure engineering of graphene.7
RESULTS AND DISCUSSION An STM overview image of F4TCNQ molecules adsorbed on epitaxial graphene on Ir(111) is shown in Figure 1a along with the chemical structure of the molecule in the inset. Two adsorption geometries are evident from the topography image: (i) two cyano groups show up as bright protrusions while the other two are not visible (referred to as two-legged molecules), and (ii) all four cyano groups are bright (referred to as flat molecules). A molecule with flat adsorption geometry has been encircled in Figure 1a. At this low coverage (
