Imaging Molecular Orbitals of PTCDA on Graphene on Pt(111

Article pubs.acs.org/JPCC

Imaging Molecular Orbitals of PTCDA on Graphene on Pt(111): Electronic Structure by STM and First-Principles Calculations Antonio J. Martínez-Galera,*,†,# Nicoleta Nicoara,†,∇ José I. Martínez,‡,§,○ Yannick J. Dappe,⊥ José Ortega,‡,§ and José M. Gómez-Rodríguez†,§,∥ †

Departamento Física de la Materia Condensada, ‡Departamento Física Teórica de la Materia Condensada, §Condensed Matter Physics Center (IFIMAC), and ∥Instituto Nicolás Cabrera (INC), Universidad Autónoma de Madrid, E-28049, Madrid, Spain ⊥ Service de Physique de l’Etat Condensé, DSM/IRAMIS/SPEC, CEA Saclay URA CNRS 2464, F-91191 Gif-Sur-Yvette Cedex, France ABSTRACT: The adsorption and growth of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on graphene monolayers epitaxially grown on Pt(111) surfaces is studied by a combination of experimental scanning tunneling microscopy (STM) and spectroscopy (STS) measurements and first-principles density functional theory (DFT) calculations. For submonolayer coverage, until the completion of the first layer, PTCDA molecules form a well-ordered herringbone structure with molecules lying flat on the graphene surface weakly coupled to the Pt(111) substrate. High-resolution STM imaging at different sample biases has allowed the identification of intramolecular features that can be related to the original PTCDA frontier orbitals. Theoretical STM calculations, based on local-orbital DFT, have been carried out on the full PTCDA/graphene/Pt(111) system. The comparison of theoretical and experimental STM images has allowed us to ascribe the origin of intramolecular features to the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the free PTCDA molecules. Moreover, the experimental STS spectra display well-resolved peaks centered at −2.2 and +1.2 eV in excellent agreement with DFT calculations. This study reveals that the growth and electronic structure of PTCDA retain all of the essential electronic features of the molecular layer upon adsorption on this weakly coupled graphene on Pt(111) surface.

■

INTRODUCTION

advantage of the intrinsic properties of each material to design more efficient devices. An archetypal organic compound of special relevance because of its interesting properties and promising applications in the field of optoelectronics is the 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). For this reason, studies concerning the interaction between graphene and PTCDA are of great interest not only from a fundamental point of view but also for technological purposes. During the past decade, the exciting properties of PTCDA have stimulated a large number of studies by means of different experimental techniques related to its adsorption on different surfaces. More specifically, PTCDA monolayers have been studied on surfaces of noble metals such as Au(111),9 Ag(110), 10 Ag(111), 10,11 or Cu(111),12 on surfaces of semiconductors such as Si(111)13 or GaAs(001),14 as well as on highly oriented pyrolithic graphite (HOPG) substrates.15 Much more recently, the study of the growth of PTCDA has also been extended to graphene surfaces epitaxially grown on different substrates. The first of such studies was carried out at 4.7 K by STM/STS on bilayers of epitaxial graphene on SiC(0001) substrates.16 In this case, an arrangement of PTCDA

Graphene, because of its exceptional electronic structure, has become a material that holds great expectations for future technology.1 For this reason, since it was for the first time isolated from a graphite sample in 2004, large amounts of resources have been devoted to graphene research. As a straightforward consequence, the step from laboratory research to the design and fabrication of devices with applications in a huge range of technological areas could be getting closer. Regarding this issue, it is important to outline that advances in the design of graphene-based nanodevices are becoming a fact. Examples include the development of prototypes of field effect transistors,2 integrated circuits,3 or flexible touchscreens.4 Until now, in many of these advances, organic compounds have been employed together with graphene layers. However, when combining graphene with other materials, it is crucial to take into account that the properties of graphene could be affected by the interaction with its local environment. Also, as suggested in recent works, the combination of appropriate materials with graphene could allow tuning of its electronic structure.5−8 Nevertheless, sometimes it is more interesting to employ materials that interact weakly with graphene monolayers in such a way that properties of graphene as well as those of the materials placed in its local environment remain without significant changes. In these cases, it would be possible to take © 2014 American Chemical Society

Received: January 22, 2014 Revised: March 24, 2014 Published: March 28, 2014 12782

dx.doi.org/10.1021/jp500768y | J. Phys. Chem. C 2014, 118, 12782−12788

The Journal of Physical Chemistry C

Article

in a brick-wall type structure, where molecules would be adsorbed with their perylene nucleus tilted with respect to the surface, was reported. Furthermore, by means of STS measurements, these authors also suggested the presence of a slight charge transfer from the graphene layer to the PTCDA monolayer.16 Thereafter, Wang and Hersam17 observed by STM that PTCDA molecules adsorbed on graphene/ SiC(0001) surfaces at room temperature are arranged in a herringbone-type lattice analogous to that of molecules in the (102) plane of the crystal structure of the two phases of bulk PTCDA.17 Here, it is important to note that similar structures have also been observed in multilayer islands14 and on the monolayer of PTCDA adsorbed on Au(111),9 HOPG,15 Cu(111),12 and Ag(111)10 substrates. Moreover, the analysis performed by Wang and Hersam through STS measurements suggested the absence of a significant charge transfer between the molecules of PTCDA and the graphene surface.17 Afterward, a combination of studies conducted at 77 K with STM and photoemission spectroscopy by Huang et al.18 agreed with the molecular ordering of PTCDA monolayers adsorbed on graphene/SiC(0001) described by Wang and Hersam. That study also determined that, only for PTCDA coverages higher than 1 ML, a very small transfer of electrons from the graphene layer to PTCDA molecules takes place. The same herringbonetype structure was also observed by Alaboson et al., who employed PTCDA monolayers adsorbed on graphene/ SiC(0001) surfaces for the growth of insulating materials such as HfO2 and Al2O3.19 It is particularly interesting to note that until now most experimental studies related to PTCDA adlayers adsorbed on graphene have been carried out on graphene surfaces epitaxially grown on SiC(0001) substrates. The epitaxial growth on different metal surfaces also gives rise to graphene monolayers with high structural perfection, and in some cases they present an electronic structure similar to that of free-standing graphene.20−23 However, the properties of PTCDA monolayers adsorbed on graphene surfaces epitaxially grown on metal substrates have been much less studied. In particular, to the best of our knowledge, there are only two studies,24,25 both conducted at room temperature by STM, on surfaces of graphene/Ru(0001), which is a strongly interacting graphene− metal system where the electronic structure is very different from that of ideal graphene.26,27 In both cases, the STM images displayed a herringbone-type structure of PTCDA molecules distorted by a periodic modulation due to the highly corrugated moiré pattern present on the underlying graphene/Ru(0001) surface. Here, we present a combined experimental and theoretical study performed, for the first time, for PTCDA monolayers adsorbed on a weakly interacting graphene−metal system where graphene properties are closer to that of ideal graphene. In particular, this study was performed by STM/STS measurements at room temperature and theoretical STM calculations for PTCDA monolayers adsorbed on graphene grown on Pt(111) surfaces. Our results have allowed for gaining a better understanding of the molecule−graphene interaction in this remarkable system.

and with a LEED/AUGER optics for sample characterization, as well as with devices for in situ sample preparation. Pt(111) surfaces were prepared by repeated cycles of Ar+ bombardment at 1 kV followed by annealing at 600 °C at a partial pressure of 10−6 Torr of O2. After each annealing step, the sample was flashed to 1000 °C in the same partial pressure of O2. The growth of graphene monolayers on Pt(111) surfaces was performed by UHV-CVD. More specifically, graphene monolayers were obtained by exposing the Pt(111) surface at 1000 °C to a partial pressure of 3 × 10−7 Torr of ethylene (C2H4) during 60 s. The quality of graphene/Pt(111) samples was initially checked by LEED and then at the atomic scale by STM. Afterward, PTCDA (from Sigma-Aldrich) was deposited on the graphene/Pt(111) sample at room temperature from a home-built Ta crucible. An accurate calibration of the deposition rate was obtained through STM measurements. The tips employed in our experiments were made out of electrochemically etched tungsten, and they were prepared in situ, in UHV, by means of annealing at high temperatures followed by field emission. Data acquisition and analysis were carried out by using the WS×M software from Nanotec Electrónica S.L.30 Finally, it should be mentioned that all of the STM measurements were performed at room temperature in the constant current mode and that the bias voltage was applied to the sample while the tip was grounded. Theoretical Details. Theoretical calculations, based on local-orbital DFT,31 were performed to analyze the electronic structure and STM images for PTCDA on Graphene/Pt(111). In a first step, we determine the PTCDA/graphene/Pt(111) interface geometry and, in particular, the graphene/Pt(111) and PTCDA/graphene equilibrium distances, combining localorbital DFT and intermolecular perturbation theory to describe the “weak chemical” and van der Waals (vdW) interactions.32 Within this approach, each interacting subsystem comprising the PTCDA/graphene/Pt(111) interface is calculated separately by using local-orbital DFT;31 then, we add the interaction between subsystems using perturbation theory.31 This formalism has already provided excellent results in the study of a wide range of graphitic materials,33 physisorbed molecules on surfaces,34−37 or encapsulated molecules in carbon nanotubes.38,39 Using this approach, the structural model for the PTCDA/ graphene/Pt(111) interface geometry is obtained as follows: (i) the distance between graphene and Pt(111) is obtained from a calculation in which the interface is represented by a slab with (√3 × √3)-periodicity composed by 7 Pt-layers (21 Pt atoms in the unit-cell), interacting with one graphene sheet (8 C atoms in the unit-cell); (ii) regarding the PTCDA adsorption distance on graphene, we have used a periodic graphene sheet (72 C atoms in the 2D-unit-cell) in interaction with a PTCDA molecule; and (iii) we constructed the PTCDA herringbone adlayer on graphene supported on the Pt(111) surface on the basis of the experimentally detected PTCDA herringbone geometry; this is justified by the weak interactions between the different subsystems, which do not alter significantly the electronic structure of each subsystem.34,40−43 From these results, and to assemble a unit cell feasible for a theoretical analysis, the PTCDA herringbone structure on graphene/ Pt(111) was considered as a Pt(111) 4 layers slab, with a graphene sheet located at 3.2 Å on the Pt(111) surface (which is in good agreement with the experimental distance of 3.30 Å),44 and the PTCDA herringbone adlayer located at 2.9 Å on the graphene sheet (similar to other DFT calculations of

■

EXPERIMENTAL AND THEORETICAL METHODS Experimental Details. The experiments were performed in an ultrahigh vacuum (UHV) system with a base pressure below 1 × 10−10 Torr equipped with a home-built variabletemperature scanning tunneling microscope (VT-STM)28,29 12783



Article

PTCDA on free-standing graphene).42 The lattice parameters for the periodically repeated rectangular (β = 90°) unit cell (with two PTCDA molecules) are a = 19.67 Å and b = 13.36 Å (see below), where the two PTCDA molecules of each unit cell form an angle of γ = 76°. A perpendicular vacuum distance of 100 Å has been considered, to avoid any possible interaction between perpendicular neighboring cells. In these calculations, a basis set of sp3d5 numerical atomic orbitals (NAOs)37 is used for Pt and C, sp3 for O, and s for H, with cutoff radii (in au): s = 4.6, p = 5.8, and d = 4.2 (Pt); s = 4.0, p = 4.5, and d = 5.4 (C); s = 3.4 and p = 3.8 (O); and s = 4.1 (H). Core−electrons are taken into account by means of norm-conserving scalarrelativistic pseudopotentials.45 The 2D Brillouin-zone has been discretized by using a set of 32 k-points, enough to get converged results. To simulate theoretically the STM images for the PTCDA/ graphene/Pt(111) system, we combine our local-orbital DFT calculations with a Keldysh−Green function formalism.31,46,47 Within this theoretical framework, the tip (a standard pyramidal clean W-tip in our case) and the sample, the PTCDA/graphene/Pt(111), are calculated by using localorbital DFT,31 separately providing the Hamiltonian in the local-orbital basis for the tip and sample independently. The Hamiltonian hopping matrix elements between tip and sample are then obtained by using a dimer approximation: a dimer formed by one W atom (corresponding to the tip) and another one (H, C, O, or Pt coming from the sample) is calculated for different atom−atom distances and for all of the nonzero interactions, using the Keldysh−Green formalism to propagate the tunnel current between both subsystems.46,47 Some recent examples of the application of this approach for organic molecules on surfaces can be found in refs 35,48. An important issue for obtaining accurate and realistic STM images is that simply using the LDA Kohn−Sham energy levels provides a PTCDA transport gap that is too small.34,48 However, the effective charging energy of the molecule, U, can be used to effectively correct the Kohn−Sham energy gap, EKS, and calculate the transport gap as Et = EKS + U. In a previous work, we have used this approach to determine the value of the HOMO−LUMO transport gap for PTCDA on Au(111), obtaining a value of 2.9 eV.34 Within the same approach, we have calculated the value of the HOMO−LUMO transport gap for PTCDA on pristine graphene, for the PTCDA/graphene/Pt(111) system, obtaining a value of 3.5 eV, in good agreement with the experimental value (see below). Notice that the screening due to the substrate should be smaller for PTCDA on the graphene/Pt(111) surface as compared to PTCDA on the metal Au(111) surface, in excellent agreement with the slightly larger transport gap obtained for the PTCDA/ graphene/Pt(111) system.

Figure 1. (a) STM image acquired over an area of a graphene/Pt(111) sample where a single moiré pattern is observed. The unit cell associated with this moiré pattern is outlined in yellow. Tunneling parameters: Vs = −1.02 V, It = 0.11 nA, size 47 × 47 nm2. (b and c) STM images obtained on areas of graphene/Pt(111) surfaces exhibiting other moiré patterns. To facilitate their interpretation, a schematic drawing of the superstructure associated with the moiré pattern is provided at the right of each image. Tunneling parameters: (b) Vs = +0.27 V, It = 7.1 nA, size 4 × 4 nm2; (c) +Vs = +0.1 V, It = 5 nA, size 7 × 7 nm2.

image shown in Figure 1b is the most frequently found, and it can be explained by a rotation of 19.1° of the graphene layer with respect to the Pt(111) substrate. The resulting superstructure has a 3 × 3 periodicity with respect to the graphene lattice, and it can be denoted as G/Pt(111)-(√7 × √7)R19.1° with respect to the atomic periodicity of the Pt(111) surface. The moiré pattern observed in the STM image shown in Figure 1c takes place when the graphene layer is rotated with respect to the Pt(111) substrate by a small angle of 1.7°. This moiré pattern can be described by a G/Pt(111)-(2√13 × 2√13)-R13.9° supercell with respect to the Pt(111) lattice and also by a [G/Pt(111)-(√67 × √67)-R12.2°]G superstructure with respect to the graphene periodicity. The existence of a large number of different moiré structures, in agreement with previous findings,49−53 is a consequence of the low interaction of graphene with Pt(111) surfaces. PTCDA is an organic molecule with a flat geometry and rectangular shape comprising a perylene nucleus and two carboxylic anhydride groups (OC−O−CO), which are located at the ends of the molecule. Figure 2a shows a schematic drawing of this molecule. An STM image acquired after the adsorption of a submonolayer coverage of PTCDA on a graphene/Pt(111) surface at room temperature is shown in

■

RESULTS AND DISCUSSION An STM image acquired over a region of a high-quality graphene monolayer epitaxially grown on a Pt(111) substrate is observed in Figure 1a. In this image, the periodicity of a single moiré pattern associated with the presence of one rotational domain on the graphene/Pt(111) interface is visible. However, as expected for this weakly interacting graphene−metal system, several moiré patterns originated by different orientations of the graphene layer with respect to the Pt(111) surface have been observed in the present work. As examples, two moiré patterns frequently found during our experiments are shown in Figure 1b,c. In particular, the moiré pattern observed in the STM 12784



Article

PTCDA molecules adsorbed on graphene/Pt(111) surfaces to the predominant role of these molecule−molecule interactions over molecule−substrate ones. As above-mentioned, this molecular structure has also been observed in PTCDA multilayers14 as well as in monolayers of PTCDA adsorbed on Au(111),9 HOPG,15 Ag(111),10 Cu(111),12 and graphene/ SiC(0001) surfaces,17−19 among others. A similar arrangement has been observed for PTCDA monolayers adsorbed on graphene/Ru(0001),24,25 but in this case, the herringbone structure is distorted by the highly corrugated moiré pattern present on the graphene/Ru(0001) interface. This high corrugation of the moiré pattern is a consequence of the stronger graphene−metal coupling on graphene/Ru(0001) as compared to that on graphene/Pt(111). In all of those cases, the authors reported that the molecular arrangement is mainly induced by molecule−molecule interactions. High-resolution STM images such as those shown in Figure 3 reproducibly exhibit bias voltage-dependent submolecular

Figure 2. (a) Schematic drawing of the PTCDA molecule. (b) STM image where an island of PTCDA molecules adsorbed on a graphene/ Pt(111) surface is observed. Tunneling parameters: Vs = −2.1 V, It = 0.12 nA, size 35 × 35 nm2. (c) High-resolution STM image exhibiting the molecular ordering of PTCDA molecules in a herringbone-type structure. The vectors of the rectangular unit cell are indicated in yellow. Tunneling parameters: Vs = +1.2 V, It = 0.52 nA, size 4.2 × 4.2 nm2. (d) Schematics of the herringbone structure.

Figure 3. 4.5 × 4.5 nm2 STM images of PTCDA on graphene/ Pt(111) displaying submolecular resolution. (a) Tunneling parameters: Vs = −2.3 V, It = 0.23 nA. (b) Tunneling parameters: Vs = +1.24 V, It = 0.52 nA. The insets show a comparison of the zoom-in molecules with the DFT calculated HOMO (left) and LUMO (right) of the free molecule (calculation extracted from ref 9).

Figure 2b. In this image, an island composed of a single layer of molecules is observed over the graphene surface. Interestingly, it is possible to appreciate that the edges of the island present a noisy appearance caused by molecular diffusion in the edge and/or displacements of molecules induced by the influence of the STM tip during the scanning. Similar perturbations in the edge of the islands have also been observed for PTCDA adsorbed on graphene/SiC(0001) surfaces, and they were attributed to a weak molecule−substrate interaction. In contrast, as it can also be appreciated in Figure 2b, inside the island, PTCDA molecules exhibit a long-range molecular ordering according to a herringbone-type structure similar to that in the (102) plane of the crystal structure of PTCDA. For higher coverages, this herringbone structure extends over the surface, even over defects present on the graphene/Pt(111) interface, giving rise to a molecular layer of high perfection. This molecular ordering can be visualized in more detail in the STM image shown in Figure 2c, where the unit cell is indicated in yellow by the vectors b1 and b2. To facilitate the interpretation of this structure, a schematic representation of the unit cell is shown in Figure 2d. Our experimentally obtained values for the magnitude of b1 and b2 are 1.8 ± 0.2 and 1.2 ± 0.1 nm, respectively. Interestingly, in the STM image displayed in Figure 2c, it is possible to distinguish specific intramolecular features whose origin will be discussed later. Returning to the analysis of the molecular arrangement, it should be noticed that the herringbone structure in the (102) plane of bulk PTCDA is mainly due to quadrupolar interactions and C−H···O−C hydrogen bonds between the two molecules in the unit cell. Therefore, it is reasonable to attribute the molecular ordering of

features. In particular, molecules observed in the STM image displayed in Figure 3a, which has been acquired at a bias voltage of −2.3 V, exhibit intramolecular features consisting of eight lobes symmetrically distributed with respect to the major axis with four of them on each side. Likewise, in Figure 3b we can observe an STM image acquired at a voltage of +1.24 V where molecules present a different intramolecular structure. More specifically, this structure consists of 10 lobes symmetrically distributed with respect to the major axis in a way that each side presents five of them arranged in two rows parallel to the axis, consisting of three and two lobes, respectively, the row composed of two lobes being closer to the edge of the molecule. Theoretical calculations have been performed for the PTCDA monolayer adsorbed on graphene/Pt(111) to obtain an interpretation of the intramolecular features observed in STM images such as those displayed in Figure 3a and b. Figure 4 shows two simulated STM images obtained at two different voltage biases and at constant-current scanning conditions, moving the W-tip perpendicularly to the sample in each scanning stage to search a preselected fixed value of the tunnel current. The theoretical scanning parameters used here were It = 0.2 nA, and Vs = −2.3 and +1.4 V. At these voltages, the main contribution to the LDOS observed in the simulated STM images shown in Figure 4 is due to the HOMO (a) and to the LUMO (b) molecular orbitals of PTCDA adsorbed on graphene/Pt(111). Notice the excellent agreement between the experimental images shown in Figures 3 and the theoretical images in Figure 4. This comparison indicates that the 12785



Article

Figure 5. (a) Experimental STS spectra on one monolayer PTCDA on graphene/Pt(111); (b) Local Density of States (LDOS) around the transport gap on PTCDA in the DFT calculations for the PTCDA/ graphene/Pt(111) system. Two peaks are clearly resolved both in the experimental and in the theoretical spectra whose origin can be ascribed to the HOMO and LUMO of the PTCDA molecule.

Figure 4. First-principles simulated constant current STM images of PTCDA on graphene/Pt(111) corresponding to bias voltages Vs = −2.3 V (a) and Vs = +1.4 V (b). The relaxed atomic structure is shown in (c).

images in Figure 4 correspond to the PTCDA HOMO and LUMO states. Notice the excellent agreement between Figure 5a and b: we stress here that to obtain accurate theoretical STM images, it is important to properly correct the transport gap provided by the DFT calculations. For organic materials, it is well-known that the Kohn−Sham energy levels yield a transport gap that is too small: this is related to the molecule self-interaction energy.54,55 When a molecule is adsorbed on a surface, the dynamical polarization of the system to added electrons or holes in the molecule also has to be taken into account to obtain the transport gap of the molecule on the surface.56 In our work, both effects are introduced by means of the charging energy of the molecule on the surface, U. Using this approach (see Theoretical Details), we obtain a transport gap for PTCDA on graphene of 3.5 eV, in good agreement with the experimental transport gap of 3.4 eV; see Figure 5a and b. Once the transport gap for PTCDA on graphene has been effectively corrected using this value for the calculation of the electronic structure of the interface, the self-consistent electronic structure of the PTCDA/graphene/Pt(111) interface shows a slight mismatch of 0.25 eV between the experimental and theoretical values for the Fermi energy position within the PTCDA transport gap. In our theoretical STM calculations, we have shifted the PTCDA energy levels by 0.25 eV so that the theoretical Fermi level coincides with the experimental one. In this way, the bias voltage used for the theoretical STM images is directly related to the bias voltage used for the experimental STM images. In addition, it is interesting to point out that our differential conductance measurements are also similar to those reported by Wang et al. on the monolayer of PTCDA adsorbed at room temperature on graphene/SiC(0001) surfaces.17 Similarly, the results obtained by Huang et al. on PTCDA

intramolecular features observed in the STM images displayed in Figure 3a and b are originated by the HOMO and LUMO, respectively. Interestingly, there is also an excellent agreement between the intramolecular structure observed in our STM images and the HOMO and LUMO of PTCDA calculated for the free molecule in the gas phase9 (shown as insets in Figure 3). It indicates, again, a weak electronic coupling between the substrate and the molecule. Similar intramolecular features have also been observed in STM images acquired over PTCDA monolayers adsorbed on other substrates such as Au(111)9 and graphene/Ru(0001),25 although over this interface the whole molecular lattice presents a distortion due to the highly corrugated moiré pattern. In both cases, the authors also attributed the observation of that intramolecular resolution to a weak molecule−substrate interaction. Additional information about the electronic structure of PTCDA monolayers adsorbed at room temperature on graphene/Pt(111) surfaces can be obtained from the analysis of differential conductance curves. In Figure 5a is shown a representative plot of the experimental differential conductance G = dI/dV, as a function of the sample voltage, obtained by numerical differentiation of an I−V curve acquired by STS. This plot exhibits a LDOS with a relatively large gap around the Fermi Level and two peaks centered at −2.2 and +1.2 V. Similar dI/dV plots have been reproducibly obtained from I−V curves acquired with several tips on different regions of PTCDA monolayers adsorbed over graphene/Pt(111) surfaces at room temperature. Figure 5b shows the Local Density of States around the Fermi level in the DFT calculation for the theoretical STM images of Figure 4, showing that the STM 12786



■

monolayers adsorbed on graphene/SiC(0001) surfaces at 77 K are also consistent with the curve in Figure 5a.18 However, in that case, a broader band gap was observed, which was attributed to a possible influence of the temperature on the adsorption of molecules.18 Finally, curves comparable to that shown in Figure 5a were also obtained on PTCDA monolayers adsorbed at room temperature on Au(111) surfaces.9 In all of these cases, the peaks were found to be centered at bias voltages close to those of the peaks present in the curve shown in Figure 5a, and they were also associated with the HOMO and LUMO of PTCDA molecules physisorbed on those surfaces.

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (2) Lin, Y. M.; Dimitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H. Y.; Grill, A.; Avouris, P. 100-GHz Transistors from WaferScale Epitaxial Graphene. Science 2010, 327, 662−662. (3) Lin, Y. M.; Valdes-Garcia, A.; Han, S. J.; Farmer, D. B.; Meric, I.; Sun, Y. N.; Wu, Y. Q.; Dimitrakopoulos, C.; Grill, A.; Avouris, P.; et al. Wafer-Scale Graphene Integrated Circuit. Science 2011, 332, 1294− 1297. (4) Bae, S.; Kim, H.; Lee, Y.; Xu, X. F.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; et al. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (5) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; et al. Control of Graphene’s Properties by Reversible Hydrogenation: Evidence for Graphane. Science 2009, 323, 610−613. (6) Leenaerts, O.; Partoens, B.; Peeters, F. M. Adsorption of H2O, NH3, CO, NO2, and NO on Graphene: A First-Principles Study. Phys. Rev. B 2008, 77, 125416. (7) Sun, J. T.; Lu, Y. H.; Chen, W.; Feng, Y. P.; Wee, A. T. S. Linear Tuning of Charge Carriers in Graphene by Organic Molecules and Charge-Transfer Complexes. Phys. Rev. B 2010, 81, 155403. (8) Coletti, C.; Riedl, C.; Lee, D. S.; Krauss, B.; Patthey, L.; von Klitzing, K.; Smet, J. H.; Starke, U. Charge Neutrality and Band-Gap Tuning of Epitaxial Graphene on SiC by Molecular Doping. Phys. Rev. B 2010, 81, 235401. (9) Nicoara, N.; Roman, E.; Gomez-Rodriguez, J. M.; Martín-Gago, J. A.; Mendez, J. Scanning Tunneling and Photoemission Spectroscopies at the PTCDA/Au(111) Interface. Org. Electron. 2006, 7, 287−294. (10) Glockler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bohringer, M.; Berndt, R.; Schneider, W. D. Highly Ordered Structures and Submolecular Scanning Tunnelling Microscopy Contrast of PTCDA and DM-PBDCI Monolayers on Ag(111) and Ag(110). Surf. Sci. 1998, 405, 1−20. (11) Temirov, R.; Soubatch, S.; Luican, A.; Tautz, F. S. FreeElectron-Like Dispersion in an Organic Monolayer Film on a Metal Substrate. Nature 2006, 444, 350−353. (12) Wagner, T.; Bannani, A.; Bobisch, C.; Karacuban, H.; Moeller, R. The Initial Growth of PTCDA on Cu(111) Studied by STM. J. Phys.: Condens. Matter 2007, 19, 056009. (13) Nicoara, N.; Paz, O.; Mendez, J.; Baro, A. M.; Soler, J. M.; Gomez-Rodriguez, J. M. Adsorption and Electronic Properties of PTCDA Molecules on Si(111)-(7 × 7): Scanning Tunneling Microscopy and First-Principles Calculations. Phys. Rev. B 2010, 82, 075402. (14) Nicoara, N.; Custance, O.; Granados, D.; Garcia, J. M.; GomezRodriguez, J. M.; Baro, A. M.; Mendez, J. Scanning Tunnelling Microscopy and Spectroscopy on Organic PTCDA Films Deposited on Sulfur Passivated GaAs(001). J. Phys.: Condens. Matter 2003, 15, S2619−S2629. (15) Kendrick, C.; Kahn, A.; Forrest, S. R. STM Study of the Organic Semiconductor PTCDA on Highly-Oriented Pyrolytic. Appl. Surf. Sci. 1996, 104, 586−594. (16) Lauffer, P.; Emtsev, K. V.; Graupner, R.; Seyller, T.; Ley, L. Molecular and Electronic Structure of PTCDA on Bilayer Graphene on SiC(0001) Studied with Scanning Tunneling Microscopy. Phys. Status Solidi B 2008, 245, 2064−2067. (17) Wang, Q. H.; Hersam, M. C. Room-Temperature MolecularResolution Characterization of Self-Assembled Organic Monolayers on Epitaxial Graphene. Nat. Chem. 2009, 1, 206−211. (18) Huang, H.; Chen, S.; Gao, X. Y.; Chen, W.; Wee, A. T. S. Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene. ACS Nano 2009, 3, 3431−3436.

■

CONCLUSIONS To summarize, we have studied the adsorption of PTCDA on graphene/Pt(111) surfaces by means of a combination of STM/STS measurements at room temperature and firstprinciples DFT calculations. For submonolayer coverages of PTCDA, our STM images reveal the formation of islands where molecules are arranged in a herringbone-type structure. With increasing coverage, the formation of a molecular monolayer exhibiting long-range ordering in a herringbone-type structure takes place. This molecular ordering presents high structural perfection. By means of STM measurements, we have reproducibly observed bias voltage-dependent submolecular features. A comparison with our DFT calculations shows that these intramolecular features correspond to the HOMO and LUMO molecular orbitals of PTCDA molecules adsorbed on graphene/Pt(111), revealing, thus, the weak electronic molecule−graphene coupling. Finally, this work shows the way in which a combination of STM/STS measurements with DFT calculations has allowed one to extract valuable information regarding the interaction of an archetypal organic molecule with graphene monolayers weakly coupled to their local environment. It is important to note that the graphene/Pt(111) interface is probably the one with an electronic structure closer to that of pure graphene in which the adsorption of PTCDA has been studied.

■

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 221 470 3599. E-mail: [email protected]. Present Addresses #

IPhysikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany. ∇ International Iberian Nanotechnology Laboratory, 4715-330 Braga, Portugal. ○ Departamento Superficies y Recubrimientos, Instituto de Ciencia de Materiales de Madrid-CSIC, E-28049 Madrid, Spain. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the CAM, under contract nos. CPI/ 0256/2007 and S2009/MAT-1467, and from the Spanish MICIIN under Grant nos. MAT2010-14902, CSD2010-00024, and FIS2010-16046, is gratefully acknowledged. J.I.M. acknowledges funding from Spanish MICIIN and CSIC through “Juan de la Cierva” and “JaeDoc” Programs. 12787



Article

(19) Alaboson, J. M. P.; Wang, Q. H.; Emery, J. D.; Lipson, A. L.; Bedzyk, M. J.; Elam, J. W.; Pellin, M. J.; Hersam, M. C. Seeding Atomic Layer Deposition of High-k Dielectrics on Epitaxial Graphene with Organic Self-Assembled Monolayers. ACS Nano 2011, 5, 5223− 5232. (20) Coraux, J.; N’Diaye, A. T.; Busse, C.; Michely, T. Structural Coherency of Graphene on Ir(111). Nano Lett. 2008, 8, 565−570. (21) Wintterlin, J.; Bocquet, M. L. Graphene on Metal Surfaces. Surf. Sci. 2009, 603, 1841−1852. (22) Batzill, M. The Surface Science of Graphene: Metal Interfaces, CVD Synthesis, Nanoribbons, Chemical Modifications, and Defects. Surf. Sci. Rep. 2012, 67, 83−115. (23) Martínez-Galera, A. J.; Brihuega, I.; Gomez-Rodriguez, J. M. Ethylene Irradiation: A New Route to Grow Graphene on Low Reactivity Metals. Nano Lett. 2011, 11, 3576−3580. (24) Roos, M.; Uhl, B.; Kuenzel, D.; Hoster, H. E.; Gross, A.; Behm, R. J. Intermolecular vs Molecule-Substrate Interactions: A Combined STM and Theoretical Study of Supramolecular Phases on Graphene/ Ru(0001). Beilstein J. Nanotechnol. 2011, 2, 365−373. (25) Zhou, H. T.; Mao, J. H.; Li, G.; Wang, Y. L.; Feng, X. L.; Du, S. X.; Muellen, K.; Gao, H. J. Direct Imaging of Intrinsic Molecular Orbitals Using Two-Dimensional, Epitaxially-Grown, Nanostructured Graphene for Study of Single Molecule and Interactions. Appl. Phys. Lett. 2011, 99, 153101. (26) Marchini, S.; Gunther, S.; Wintterlin, J. Scanning Tunneling Microscopy of Graphene on Ru(0001). Phys. Rev. B 2007, 76, 075429. (27) Preobrajenski, A. B.; Ng, M. L.; Vinogradov, A. S.; Martensson, N. Controlling Graphene Corrugation on Lattice-Mismatched Substrates. Phys. Rev. B 2008, 78, 073401. (28) Custance, O.; Brochard, S.; Brihuega, I.; Artacho, E.; Soler, J. M.; Baro, A. M.; Gomez-Rodriguez, J. M. Single Adatom Adsorption and Diffusion on Si(111)-(7 × 7) Surfaces: Scanning Tunneling Microscopy and First-Principles Calculations. Phys. Rev. B 2003, 67, 235410. (29) Martínez-Galera, A. J.; Gomez-Rodriguez, J. M. Nucleation and Growth of the Prototype Azabenzene 1,3,5-Triazine on Graphite Surfaces at Low Temperatures. J. Phys. Chem. C 2011, 115, 11089− 11094. (30) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool For Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (31) Lewis, J. P.; Jelinek, P.; Ortega, J.; Demkov, A. A.; Trabada, D. G.; Haycock, B.; Wang, H.; Adams, G.; Tomfohr, J. K.; Abad, E.; et al. Advances and Applications in the FIREBALL Ab Initio Tight-Binding Molecular-Dynamics Formalism. Phys. Status Solidi B 2011, 248, 1989−2007. (32) Dappe, Y. J.; Basanta, M. A.; Flores, F.; Ortega, J. Weak Chemical Interaction and van der Waals Forces Between Graphene Layers: A Combined Density Functional and Intermolecular Perturbation Theory Approach. Phys. Rev. B 2006, 74, 205434. (33) Savini, G.; Dappe, Y. J.; Oberg, S.; Charlier, J. C.; Katsnelson, M. I.; Fasolino, A. Bending Modes, Elastic Constants and Mechanical Stability of Graphitic Systems. Carbon 2011, 49, 62−69. (34) Martínez, J. I.; Abad, E.; Flores, F.; Ortega, J.; Brocks, G. Barrier Height Formation for the PTCDA/Au(111) Interface. Chem. Phys. 2011, 390, 14−19. (35) Martínez, J. I.; Abad, E.; Gonzalez, C.; Ortega, J.; Flores, F. Theoretical Characterization of the TTF/Au (111) Interface: STM Imaging, Band Alignment and Charging Energy. Org. Electron. 2012, 13, 399−408. (36) Abad, E.; Dappe, Y. J.; Martínez, J. I.; Flores, F.; Ortega, J. C6H6/Au(111): Interface Dipoles, Band Alignment, Charging Energy, and van der Waals Interaction. J. Chem. Phys. 2011, 134, 044701. (37) Basanta, M. A.; Dappe, Y. J.; Jelínek, P.; Ortega, J. Optimized Atomic-Like Orbitals for First-Principles Tight-Binding Molecular Dynamics. Comput. Mater. Sci. 2007, 39, 759−766.

(38) Dappe, Y. J.; Ortega, J.; Flores, F. Intermolecular Interaction in Density Functional Theory: Application to Carbon Nanotubes and Fullerenes. Phys. Rev. B 2009, 79, 165409. (39) Dappe, Y. J.; Martínez, J. I. Effect of van der Waals Forces on the Stacking of Coronenes Encapsulated in a Single-Wall Carbon Nanotube and Many-Body Excitation Spectrum. Carbon 2013, 54, 113−123. (40) Otero, G.; Gonzalez, C.; Pinardi, A. L.; Merino, P.; Gardonio, S.; Lizzit, S.; Blanco-Rey, M.; Van de Ruit, K.; Flipse, C. F. J.; Mendez, J.; et al. Ordered Vacancy Network Induced by the Growth of Epitaxial Graphene on Pt(111). Phys. Rev. Lett. 2010, 105, 216102. (41) Vanin, M.; Mortensen, J. J.; Kelkkanen, A. K.; Garcia-Lastra, J. M.; Thygesen, K. S.; Jacobsen, K. W. Graphene on Metals: A van der Waals Density Functional Study. Phys. Rev. B 2010, 81, 081408. (42) Tian, X. Q.; Xu, J. B.; Wang, X. M. Self-Assembly of PTCDA Ultrathin Films on Graphene: Structural Phase Transition and Charge Transfer Saturation. J. Phys. Chem. C 2010, 114, 20917−20924. (43) Emery, J. D.; Wang, Q. H.; Zarrouati, M.; Fenter, P.; Hersam, M. C.; Bedzyk, M. J. Structural Analysis of PTCDA Monolayers on Epitaxial Graphene with Ultra-High Vacuum Scanning Tunneling Microscopy and High-Resolution X-Ray Reflectivity. Surf. Sci. 2011, 605, 1685−1693. (44) Sutter, P.; Sadowski, J. T.; Sutter, E. Graphene on Pt(111): Growth and Substrate Interaction. Phys. Rev. B 2009, 80, 245411. (45) Fuchs, M.; Scheffler, M. Ab Initio Pseudopotentials for Electronic Structure Calculations of Poly-Atomic Systems Using Density-Functional Theory. Comput. Phys. Commun. 1999, 119, 67− 98. (46) Blanco, J. M.; Gonzalez, C.; Jelinek, P.; Ortega, J.; Flores, F.; Perez, R. First-Principles Simulations of STM Images: From Tunneling to the Contact Regime. Phys. Rev. B 2004, 70, 085405. (47) Blanco, J. M.; Flores, F.; Perez, R. STM-Theory: Image Potential, Chemistry and Surface Relaxation. Prog. Surf. Sci. 2006, 81, 403−443. (48) Martínez, J. I.; Abad, E.; Gonzalez, C.; Flores, F.; Ortega, J. Improvement of Scanning Tunneling Microscopy Resolution with HSensitized Tips. Phys. Rev. Lett. 2012, 108, 246102. (49) Land, T. A.; Michely, T.; Behm, R. J.; Hemminger, J. C.; Comsa, G. STM Investigation of Single Layer Graphite Structures Produced on Pt(111) by Hydrocarbon Decomposition. Surf. Sci. 1992, 264, 261−270. (50) Enachescu, M.; Schleef, D.; Ogletree, D. F.; Salmeron, M. Integration of Point-Contact Microscopy and Atomic-Force Microscopy: Application to Characterization of Graphite/Pt(111). Phys. Rev. B 1999, 60, 16913−16919. (51) Martínez-Galera, A. J.; Gomez-Rodriguez, J. M. Surface Diffusion of Simple Organic Molecules on Graphene on Pt(111). J. Phys. Chem. C 2011, 115, 23036−23042. (52) Ugeda, M. M.; Fernandez-Torre, D.; Brihuega, I.; Pou, P.; Martínez-Galera, A. J.; Perez, R.; Gomez-Rodriguez, J. M. Point Defects on Graphene on Metals. Phys. Rev. Lett. 2011, 107, 116803. (53) Merino, P.; Svec, M.; Pinardi, A. L.; Otero, G.; Martín-Gago, J. A. Strain-Driven Moire Superstructures of Epitaxial Graphene on Transition Metal Surfaces. ACS Nano 2011, 5, 5627−5634. (54) Flores, F.; Ortega, J.; Vazquez, H. Modelling Energy Level Alignment at Organic Interfaces and Density Functional Theory. Phys. Chem. Chem. Phys. 2009, 11, 8658−8675. (55) Flores, F.; Abad, E.; Martínez, J. I.; Pieczyrak, B.; Ortega, J. On the Organic Energy Gap Problem. J. Phys.: Condens. Matter 2013, 25, 094007. (56) Abad, E.; González, C.; Ortega, J.; Flores, F. Charging Energy, Self-Interaction Correction and Transport Energy Gap for a Nanogap Organic Molecular Junction. Org. Electron. 2010, 11, 332−337.

12788


Imaging Molecular Orbitals of PTCDA on Graphene on Pt(111

Recommend Documents