Article pubs.acs.org/JPCC
Self-Assembly of Cobalt-Phthalocyanine Molecules on Epitaxial Graphene on Ir(111) Sampsa K. Ham ̈ al̈ aï nen, Mariia Stepanova, Robert Drost, Peter Liljeroth, Jouko Lahtinen, and Jani Sainio* Department of Applied Physics, Aalto University School of Science, P.O. Box 11100, FI-00076, Finland ABSTRACT: We have studied the adsorption and selfassembly of cobalt phthalocyanine (CoPc) on epitaxial graphene grown on iridium (111) by scanning tunneling microscopy (STM), Auger electron spectroscopy, and lowenergy electron diffraction (LEED). CoPc deposited on graphene/Ir(111) at room-temperature self-assembles into large, well-ordered domains with a nearly square unit cell. On the basis of the observed LEED pattern and STM images, a detailed structure for the overlayer is proposed. Despite the corrugation of the moiré pattern of graphene on Ir(111), its hexagonal symmetry is not translated to the CoPc layer. This is in contrast to systems with stronger graphene−metal interaction that makes graphene on Ir(111) a convenient, clean, and welldefined model system for studying molecular doping of graphene.
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INTRODUCTION There has been a huge push to translate the exciting electronic properties of graphene into practical device applications.1−3 One of the remaining hurdles is the absence of a gap in the graphene band structure that results in poor on/off ratios for graphene transistors.4 Several approaches have been used to overcome this problem: most notably, quantum confinement in small graphene nanostructures (quantun dots or nanoribbons) can be used to open a gap at the Dirac point in the graphene band structure.5−10 In addition to the modification of the graphene band structure, methods for controllably doping graphene are required in device applications. Research into using substitutional atoms, such as nitrogen, to dope graphene is still in its beginning: for example, the adsorption geometry of the dopants needs to be elucidated.11 Second, as the device sizes shrink, it would be extremely desirable to have welldefined dopant configurations, for example, dopants organized in an array. It has even been suggested that periodic potential modulation on the graphene (induced, for example, by periodic strain or doping) could be used to tune the graphene band structure and not only shift the doping level.12−14 Phthalocyanines form a series of commercially available molecules that consist of a metal center and a surrounding organic macrocycle, where the nature of the central metal atom (Zn, Fe, Mn, Co, Cu, Ni, etc.) controls the spin state and the energetic position of the metal-centered molecular orbitals. The molecule size and molecule−substrate interactions can also be tuned by introducing additional side groups. These molecules typically adsorb on a surface in a flat configuration, with the nearest-neighbor distance being governed by their overall size (ca. 1.5 nm without additional side groups). This potential tunability makes phthalocyanines a versatile model template for studying molecule−substrate interactions. © 2012 American Chemical Society
Adsorption of phthalocyanines (Pc's) on graphite has been well-studied:15−18 they interact only weakly with the graphite substrate and commonly adopt a square lattice with a very similar lattice constant to a bulk molecular crystal. On the other hand, metal phthalocyanine (MPc) adsorption on strongly corrugated graphene on a Ru(0001) substrate (corrugation is due to the moiré pattern formed by the lattice mismatch between graphene and the metal substrate) results in a Kagome lattice where the molecules only reside on the fcc and hcp sites of the moiré unit cell.19 We have investigated adsorption of cobalt phthalocyanine (CoPc) molecules on epitaxial graphene on an Ir(111) substrate. Graphene on Ir(111) also displays a moiré pattern, but the corrugation is significantly smaller than that on the Ru(0001) and Rh(111) substrates.20−22 The moiré of graphene on Ir(111) has been used to create homogeneous arrays of small metal clusters that prefer to nucleate on the fcc sites of the moiré.23,24 It is an interesting question how this weaker moiré pattern will affect the adsorption geometry of MPc's compared to atomically flat substrates. Further motivation for studying this effect comes from the fact that the topographic corrugation of graphene devices on hexagonal boron nitride is very similar to the moiré corrugation on Ir(111). Graphene on h-BN represents the current state-of-the-art in graphene devices on solid supports in terms of the achievable extremely high electron mobilities and low charge density fluctuations.25 With its similar corrugation, epitaxial graphene on Ir(111) forms a convenient model system for investigating molecular selfassembly on this type of corrugated surface. Received: June 29, 2012 Revised: August 28, 2012 Published: August 28, 2012 20433
dx.doi.org/10.1021/jp306439h | J. Phys. Chem. C 2012, 116, 20433−20437
The Journal of Physical Chemistry C
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Article
EXPERIMENTAL SECTION
The whole sample preparation and characterization was performed in an ultra-high-vacuum (UHV) chamber with a base pressure below 1 × 10−10 mbar. The system is equipped with a RHK UHV750 variable-temperature scanning tunneling microscope (STM), PerkinElmer 15-255G double pass cylindrical mirror analyzer (DPCMA) for Auger electron spectroscopy (AES), and Omicron SpectraLEED low-energy electron diffraction (LEED) optics. The STM can be cooled by a continuous flow liquid helium cryostat. All STM images of the CoPc layers were taken at a sample temperature of 50 K using cut PtIr tips. Before imaging, the tips were annealed in the load lock at 150 °C for around 10 h. The sample was also cooled with liquid nitrogen down to 120 K for LEED. The Ir(111) crystal was cleaned by repeated cycles of 2 kV Ar+ sputtering and annealing to 1500 K. Monolayer graphene (MG) was grown on the clean Ir(111) from ethylene (C2H4) by the combination of temperature-programmed growth (TPG) and chemical vapor deposition (CVD), as described in ref 26. The combination of the two methods yields graphene that is better aligned with the underlying Ir(111) than that produced by a single CVD step. The simulation of LEED patterns was done using the free software LEEDpat3,27 and the STM images were analyzed using the open source SPM software Gwyddion.28 The CoPc was acquired from Sigma Aldrich (307696, βform, dye content = 97%). A home-built thermal evaporator was used to deposit the CoPc on the MG/Ir(111) at room temperature. A deposition rate of 0.3 monolayers/min calibrated by AES was used in all the experiments. A monolayer thickness of 3.4 Å was used in converting the AES signal to a number of CoPc layers.29,30 There was a good match between coverages estimated by STM imaging and AES using this conversion factor. Before the deposition of CoPc, the quality of the MG/Ir(111) was checked by STM and LEED.
Figure 1. (a) 150 × 150 nm2 STM image (100 pA/0.16 V) of graphene on the Ir(111) surface before CoPc deposition. (inset) Atomically resolved STM image of the moiré (1 nA/0.1 V). The hexagonal pattern is the moiré caused by the lattice mismatch between graphene and Ir. (b) LEED pattern from clean graphene on Ir(111). The satellite spots around the first-order Ir spots arise from the moiré pattern. (c) 50 × 50 nm2 STM image (4 pA/0.66 V) of the surface after the deposition of a monolayer of CoPc. (d) LEED pattern after CoPc deposition. The hexagonal pattern in the center is the first-order spots of the moiré pattern.
from the STM images (Figures 1c and 2a). Several different rotational domains were observed in STM, as one would expect for a square lattice on a hexagonally ordered substrate. The rotational domains can be clearly seen in the LEED pattern in Figures 1d and 2b, which also reveals that the molecular lattice is rotated with respect to the substrate lattice. From interpretation of the LEED pattern, a rotation of 9.0 ± 0.5° with respect to the graphene lattice can be estimated. The molecular lattice is found to be nearly square, with an angle of 88.6 ± 0.5°. It is worth noting that the pattern of the higherorder LEED spots is extremely sensitive to shifting of the lattice vectors of the CoPc structure. From LEED images showing the first-order moiré spots together with the CoPc structure (Figure 1d), the lattice constant for the molecules can be estimated to be about 0.55 moiré repeat distances (amoiré = 25.3 Å31), consistent with the STM observations. By assuming at least a degree of epitaxy between the CoPc and graphene, the resulting best model for the CoPc overlayer 1 structure (Figure 2c). The simulated LEED is a −54 6.5 pattern from this structure matches well with the observed one, as can be seen in Figure 2d. The unit vector lengths derived from this model are (31)1/2 × ag and (129)1/2/2 × ag, where ag ≈ 2.45 Å is the graphene unit vector length.31 The obtained lattice vector lengths from the model are thus 13.6 and 13.9 Å The molecules themselves are rotated by 30° ± 5° with respect to the CoPc lattice, as can be seen from low-bias STM images (Figure 2a). The symmetry of the moiré pattern is not translated in the CoPc layer; instead, the observed molecular lattice is similar to
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RESULTS AND DISCUSSION STM images of the bare graphene layer on Ir(111) show the well-known moiré pattern, which is formed due to the slight mismatch of lattice constants of graphene and iridium (Figure 1a). The moiré is also visible in LEED as satellite spots around the Ir diffraction spots (Figure 1b). CoPc was deposited on the graphene at temperatures of 275−300 K without any annealing step. Annealing of the CoPc layers after deposition as well as deposition on a preheated sample (>50 °C) was found to result in a poorer LEED pattern or no pattern at all. After the CoPc deposition, large well-ordered islands of CoPc can be observed in STM images (Figure 1c). The diffraction pattern from the CoPc layer is also observed in LEED with low beam energies (
