Assembly of iron phthalocyanine and pentacene molecules on a graphene monolayer grown on Ru(0001)

H. G. Zhang, J. T. Sun, T. Low, L. Z. Zhang, Y. Pan, Q. Liu, J. H. Mao, H. T. Zhou, H. M. Guo, S. X. Du, F. Guinea, and H.-J. Gao

Phys. Rev. B 84, 245436 – Published 19 December 2011

Abstract

Anisotropic triangular graphene monolayers grown on a Ru(0001) surface represent unique two-dimensional templates for creating ordered, large-scale assembly of functional molecules. Using scanning tunneling microscopy and spectroscopy, we demonstrate the selective adsorption and the formation of ordered molecular arrays of iron phthalocyanine and pentacene molecules of different structural symmetries on the graphene/Ru(0001) templates. With in-depth investigations of the molecular adsorption and assembly processes, we reveal the existence of site-specific, lateral electric dipoles (or lateral electric fields) in the epitaxial graphene monolayers and the capability of the dipoles in directing and driving the molecular adsorption and assembly. We show that the lateral dipoles originate from the inhomogeneous distribution of charge due to the epitaxial constraint of graphene on a Ru(0001) surface. The adsorption mechanism is rather general and applicable to similar molecular systems on graphene monolayers formed on other transition metal surfaces.

Received 16 November 2011

DOI:https://doi.org/10.1103/PhysRevB.84.245436

Authors & Affiliations

H. G. Zhang¹, J. T. Sun¹, T. Low², L. Z. Zhang¹, Y. Pan¹, Q. Liu¹, J. H. Mao¹, H. T. Zhou¹, H. M. Guo¹, S. X. Du¹, F. Guinea³, and H.-J. Gao^1,*

¹Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
²IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA
³Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Sor Juana Inés de la Cruz 3, 28049, Madrid, Spain

^*hjgao@iphy.ac.cn

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Issue

Vol. 84, Iss. 24 — 15 December 2011

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Images

Figure 1
STM images of FePc molecules adsorbed on a hexagonal moiré pattern of g/Ru(0001) at varying coverage. (a) High-resolution image ( $U$ $=$ 0.1 V, $I$ $=$ 0.5 nA) of three distinct regions, top, fcc, and hcp, marked by triangles and dashed and solid hexagons, respectively. (b) STM image (9 nm × 9 nm, $U$ $=$ −1.5 V, $I$ $=$ 0.05 nA) revealing that molecules first adsorb at the fcc regions. (c), (d) Sequences of STM images ( $U$ $=$ −2.0 V, $I$ $=$ 0.05 nA) of FePc molecules with increasing coverage.Reuse & Permissions
Figure 2
Local work function within one graphene moiré unit cell. (a) Energy shift between STS peaks of the same order as a function of the order. The peaks of the hcp region are used as the reference energies. Dashed lines mark the constant energy shifts at higher-order peaks. (b) Plots of current $I$ vs tip-sample separation $z$ at a constant voltage 1.0 V (initial current $I$ $=$ 0.03 nA) on three distinct regions of the g/Ru(0001) moiré unit cell. Inset shows the resulted values for local barrier height (LBH, Φ $_{a}$ ) by fitting to Eq. (1). The $d I$ / $d V$ spectra with the feedback loop on were acquired by using a lock-in technique with a 10 mV (rms) sinusoidal voltage modulation at 800 Hz. Current-distance ( $I$ - $z$ ) spectroscopy measurements were performed with the feedback loop off.Reuse & Permissions
Figure 3
DFT calculation of local work functions of the g/Ru(0001) surface. (a) Two-dimensional map of local work functions for the fully relaxed structure of g/Ru(0001) before molecular adsorption. (b) Line profile of work functions along $A$ to $A^{'}$ [marked in (a)] before (solid black line) and after (dotted blue line) molecular adsorption.Reuse & Permissions
Figure 4
DFT calculation of lateral dipole moments on the g/Ru(0001) surface. (a) Averaged electron density difference along the top-hcp and top-fcc directions, indicated by the red dashed and black solid arrows in Fig. 3a. Two points, B $_{1}$ and B $_{2}$ , represent the boundary between the top-hcp site and top-fcc site, respectively, and are also marked in Fig. 3a. (b) Perspective view of the inhomo-geneous lateral dipole moments created by the g/Ru(0001) superstructure. Color arrows highlight different orientation and magnitudes.Reuse & Permissions
Figure 5
(a) Pentacene molecular structure. Blue dashed arrow is the molecular axis. (b) STM image (15.5 nm × 15.5 nm, $U$ $=$ −2 V, $I$ $=$ 0.09 nA) of the molecules adsorbed on the g/Ru(0001) surface. White triangles indicate the top regions. At low coverage the molecules adsorb preferably at the fcc region, which is consistent with what the adsorption model suggested.Reuse & Permissions
Figure 6
(a) Model height profile for the graphene superlattice on Ru(0001). (b) Pseudomagnetic field induced by strains. (c) Scalar potential induced by strains.Reuse & Permissions

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