Comparing Adsorption of an Electron-Rich Triphenylene Derivative: Metallic vs Graphitic Surfaces

Crucial to the performance of devices based on organic molecules is an understanding of how the substrate–molecule interface influences both structural and electronic properties of the molecular layers. Within this context we studied the self-assembly of an alkoxy-triphenylene derived electron donor (HAT) in the monolayer regime on graphene/Ni(111). The molecules assembled into a close-packed hexagonal network commensurate with the graphene layer. Despite the commensurate structure, the HAT molecules only had a weak, physisorptive interaction with the substrate as pointed out by the photoelectron spectroscopy data. We discuss these findings in view of our recent reports for HAT adsorbed on Ag(111) and graphene/Ir(111). For all three substrates HAT adopts a similar close-packed hexagonal structure commensurate with the substrate while being physisorbed. The ionization potential is equal for all three substrates, supporting weak molecule–substrate interactions. These findings are remarkable, as commensurate overlayers usually only form at strongly interacting interfaces. We discuss potential reasons for this particular behavior of HAT which clearly sets itself apart from most studied molecule–substrate systems. In particular, these are the relatively weak but flexible intermolecular interactions, the molecular symmetry matching that of the substrate, and the comparatively weak but directional molecule–substrate interactions.


STM
Table S1: XPS binding energies (eV), FWHM (eV) and relative area (%) for the C 1s and O 1s core levels for graphene/Ni(111), 0.9 ML HAT and 8.4 ML HAT for the data shown in Figure 2    Graphene/Cu(111) 1.787 C 60 4 Gas phase 0.796 TCNQ 5 Graphene 1.43 0.51 F 4 -TCNQ 5 Graphene 1.26 0.1 Table S2: Adsorption energy; intermolecular interaction energy and molecule-substrate interaction energy for molecules on graphene discussed in the manuscript and for HAT on Ag(111).Calculations are for a single layer of molecules on graphene (no support simulated), unless otherwise noted.Adsorption energy is the total energy gained by adsorbing a full layer of molecules on the substrate, per molecule; molecule-molecule interaction energy is the amount of this that is due to moleculemolecule interactions.

Figure S1 :
Figure S1: Overview STM image of HAT on graphene/Ni(111).a) STM image (same image as Figure 1a in the manuscript, 100x100 nm 2 , 1.56 V, 20 pA).The inset shows the fast Fourier transformation (FFT).b) The same image as a), with an overlay marking the two mirror domains of HAT in green and blue.The corresponding spots in the FFT (see inset) are marked in the same colour.

Figure S4 :
Figure S4: LEED pattern of HAT on graphene on Ni(111).a) Experimental LEED pattern obtained from 1.1 monolayer of HAT on graphene on Ni(111) at 18 eV.b) The same pattern as in a), with the spots from two HAT mirror domains marked with orange and blue circles.Spots that occur in both domains and overlap are marked with an orange-blue circle.c) Simulated pattern.The grey spot in the centre is the (00) spot, the orange and blue spots are due to the two mirror domains of HAT.Overlapping spots are marked by a half blue, half orange circle.The diamonds indicate the unit cell of the reciprocal lattice of each the domain.

Figure S5 :
Figure S5: LEED pattern of HAT on graphene on Ni(111).Experimental LEED pattern obtained from 1.1 monolayer of HAT on graphene on Ni(111) at 75 eV.b) Simulated LEED pattern.The grey spot in the centre indicates the (00) spot, the black spots at the edges indicate the 1 st order of Gr/Ni(111) spots.The orange and blue spots are due to the two mirror domains of HAT.c) A close-up of the substrate (10) spot marked in a) and b) by a black rectangle, and the ring of HAT spots surrounding it.The spots are marked by rings.The rings are colour-coded as in b) except for the substrate spot which is marked in white.

Figure S7 :
Figure S7: O 1s core level spectra for Gr/Ni(111) (blue), monolayer HAT (orange) and multilayer HAT (green).The spectra are fitted with two peaks: one for the molecular oxygen (red) and one for signal from the crystal mask (grey).The position of the molecular peak is indicated by a dashed red line.

Figure S9 :
Figure S9: Secondary electron cut-off for HAT on Ag(111), measured with He I (21.2 eV) light at a sample bias of -5 V. Clean Ag(111) (blue, work function 4.5 eV) and one monolayer of HAT on Ag(111) (orange, work function 3.7). .