Article pubs.acs.org/JPCC
Photoelectron and Absorption Spectroscopy Studies of Metal-Free Phthalocyanine on Au(111): Experiment and Theory Masumeh-Nina Shariati,† Johann Lüder,† Ieva Bidermane,†,‡ Sareh Ahmadi,§ Emmanuelle Göthelid,† Pål Palmgren,† Biplab Sanyal,† Olle Eriksson,† Maria Novella Piancastelli,† Barbara Brena,† and Carla Puglia*,† †
Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden Institut des Nanosciences de Paris, Université Pierre et Marie Curie, 4 place Jussieu, boîte courrier 840, 75252 Paris Cedex 05, France § Materials Physics, Royal Institute of Technology, Electrum 229, SE 164 40 Kista, Sweden ‡
ABSTRACT: The adsorption of monolayers and multilayers of metal-free phthalocyanine molecules on the Au(111) (√3 × 22) reconstructed surface has been investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Our results for the monolayer show that the molecules are arranged tightly onto the surface with their molecular plane parallel to it. In addition, the X-ray absorption spectra of the monolayer have been modeled by density functional theory, which could enlighten new aspect of the interaction between molecules and substrate. The XAS results evidence that also in the multilayer the molecules keep the orientation with the molecular plane parallel to the surface. These results are discussed in the framework of molecule− molecule/molecule−adsorbate interactions.
1. INTRODUCTION Organic molecules in ordered adlayers have a technological potential for the development of molecular film-based devices. As is well known, the electronic response of a device depends on the electronic structure of the film, which in turn is strictly linked to the geometrical arrangement of the molecules in the film. The study of the geometrical and electronic structure of these films is therefore a requirement to improve and control the properties of such devices. For this reason, the study of the adsorption process of organic molecules on different substrates and under different adsorption conditions is needed to gain control on the morphology and then on the functionality of the film. The deposition parameters (as deposition rate and substrate temperature), the intermolecular interaction, as well as the interaction between the molecules and the substrate regulate the adsorption process, as already shown in previous studies and summarized in the work by Kera et al.,1 dealing with the adsorption of H2Pc’s on Ag(111) at different temperatures and on Ni3Al(111) at room temperature (RT). Phthalocyanines (Pc’s) are very good candidates for many different technological applications for the reason that because of their high thermal and chemical stability they can grow in very ordered overlayers.2−4 They can be used in a variety of applications such as p-type organic semiconductors in solar cells,5,6 gas sensors,7 cancer therapy,8 organic light-emitting devices,9,10 and magnetic switches.11 A phthalocyanine is a planar molecule formed by four isoindole groups consisting of a pyrrole group coupled to a benzene ring. These isoindole © 2013 American Chemical Society
groups are connected via nitrogen bridging. In metal-free phthalocyanines (H2Pc’s), hydrogen atoms are bonded to two of the nitrogen atoms in the molecular center, thus resulting in a molecule with a two-fold symmetry (Figure 1), whereas metal phtalocyanines (MPc’s) exhibit four-fold symmetry. Our previous scanning tunneling microscopy (STM) study of monolayers of H2Pc on the Au(111) surface has shown that also the metal-free phthalocyanines are adsorbed with the molecular plane parallel to the substrate.12 Investigations of FePc, CuPc, and CoPc on Au(111) have revealed that the molecules at monolayer coverage are aligned with their molecular plane parallel to the surface and form a highly ordered adlayer with a square unit cell. In the multilayer film (from the second and third layers to higher thicknesses), these MPc molecules are instead found standing on the surface, that is, with the molecular plane tilted with respect to the surface normal.3,13−15 Multilayer adsorption where the phthalocyanines still have the molecular plane parallel to the metallic surface has been reported in an X-ray photoelectron spectroscopy (XPS) and Xray absorption spectroscopy (XAS) study of FePc on Au(111)16 and in another work on metal-free Pc on Ag(111) at RT.1 Recently by XPS and STM, a flat orientation of Received: August 1, 2012 Revised: March 10, 2013 Published: March 18, 2013 7018
dx.doi.org/10.1021/jp307626n | J. Phys. Chem. C 2013, 117, 7018−7025
The Journal of Physical Chemistry C
Article
adjustment of the current used to heat the glass tube. The sample has been kept at room temperature during the deposition of the molecules. The monolayer has been prepared by desorbing the multilayer by annealing to 670 K. The experiments have been performed at beamline I511 at MAX-lab, the national synchrotron radiation laboratory in Lund, Sweden. This is an undulator-based beamline.18 The surface end station of the beamline consisted (at the time of the measurements we are presenting) of two chambers, an analysis chamber, and a preparation chamber, with base pressures of 8 × 10−11 and 5 × 10−10 Torr, respectively. The analysis chamber was equipped with a Scienta R4000 hemispherical electron analyzer, which could be rotated around the photon beam axis. The preparation chamber was equipped with sputtering and annealing facilities, LEED, mass spectrometer, and evaporator. Because of the construction of the end station, the incoming light was always at grazing incidence (7° tilt angle) with respect to the sample surface. The XPS spectra of the C1s and N1s core lines have been measured at photon energies of 400 and 530 eV with an overall resolution of the spectra of 102 and 125 meV, respectively. The binding energy (BE) scales of C1s and N1s photoemission (PE) spectra have been calibrated by measuring the Fermi edge, the Au 4f core line BE (at 84 eV), or both with the same photon energy used for the core-level photoemission spectrum. Considering the photon energies used in the experiment and the intensity attenuation of the Au4f signal for multilayer in comparison with the signal for clean surface, we can estimate a multilayer thickness of about 10 ± 2 layers. The XAS spectra have been measured in Auger electron yield, where the energy window has been defined by the kinetic energy of the Auger transitions of the element of interest. The XAS spectra have been detected at two different geometries, namely, with the electric field E vector of the light parallel (E∥) and perpendicular (E⊥) to the surface. The photon energy has been calibrated by measuring the difference between the kinetic energies of a PE line measured with first- and second-order light. The sample has been rastered during PE and XA measurements to avoid beam damage of the molecular film. All experiments have been performed at room temperature (RT). The presented data fits have been performed with Voigt functions (a convolution between Lorentzian and Gaussian functions).
Figure 1. Hexagonal gold cluster consisting of 37 gold atoms of the Au(111) substrate top layer plus the relaxed adsorbed H2Pc. Yellow spheres: gold atoms; brown spheres: carbon atoms; light-blue spheres: nitrogen atoms; white spheres: hydrogen atoms.
multilayer of metal free-Pc tetrasulfonic acid on Au(111) has been characterized.17 The present work is an experimental and theoretical study of adsorption of monolayers and multilayers of metal-free phthalocyanine (H 2 Pc) on the Au(111) (√3 × 22) reconstructed surface at RT using XPS and XAS. Information about occupied and unoccupied electronic states as well as geometric orientation of the molecules on the surface are obtained. Our experimental results indicate that the H2Pc molecules in multilayer films are still aligned parallel to the Au(111) substrate as for the monolayer coverage. The experimental XAS results are modeled by density functional theory (DFT), which gives deeper insights into the interaction between the adsorbed molecules and the gold surface.
3. THEORETICAL METHODS In addition to the measurements, DFT calculations have been performed. We have determined the adsorption structure of a metal-free phthalocyanine on the Au(111) surface by the Vienna ab initio simulation package (VASP).19−22 The Au(111) surface was modeled with a lattice parameter (a0) of 4.08 Å. It consisted of 270 atoms in three layers, where the atoms of the lowest layer were kept fixed. Because of the system size, it was sufficient to sample the Brillouin zone with only the gamma point and set the kinetic energy cutoff to 500 eV. We have used the projector-augmented wave (PAW) method23,24 in a plane-wave basis. Generalized gradient approximation (GGA)25,26 within the PBE25 description was applied for the exchange-correlation interaction. van der Waals interaction was taken into account through the DFT-D2 method of Grimme,27,28 applied in previous studies of Pc molecules adsorbed on surfaces.29 The size of the unit cell was
2. EXPERIMENTAL METHODS The single-crystal Au(111) substrate has been purchased from Surface Preparation Laboratory (SPL). The (√3 × 22) reconstruction of the Au(111) surface has been prepared by repeated cycles of argon sputtering and annealing to 720 K. The H2Pc molecules have been purchased from Aldrich (98% dye content). The molecules have been deposited in situ onto the sample in an evaporation chamber under UHV conditions using a home-built evaporator positioned a few centimeters away from the substrate. The evaporator consisted of a quartz glass tube with a diameter of ∼5 mm with a heating tungsten wire twisted around. The evaporator and the molecules have been outgassed carefully before deposition. The evaporation temperature has been controlled by careful 7019
dx.doi.org/10.1021/jp307626n | J. Phys. Chem. C 2013, 117, 7018−7025
The Journal of Physical Chemistry C
Article
large enough to exclude intermolecular interactions, and a vacuum layer of 13 Å was included. The ionic relaxation has been performed until the Hellmann−Feynman forces were lower than 0.02 eV/Å. The inclusion of the dispersion interactions in the geometry optimization leads to several adsorption sites with optimized energies differing by
