Article pubs.acs.org/JPCC
Electronic Ground-State and Orbital Ordering of Iron Phthalocyanine on H/Si(111) Unraveled by Spatially Resolved Tunneling Spectroscopy M. Gruyters,*,† T. Pingel,† T. G. Gopakumar,† N. Néel,§ Ch. Schütt,‡ F. Köhler,‡ R. Herges,‡ and R. Berndt† †
Institut für Experimentelle und Angewandte Physik and ‡Institut für Organische Chemie, Christian-Albrechts-Universität zu Kiel, D-24098 Kiel, Germany § Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany ABSTRACT: The electronic properties of FePc molecules adsorbed on hydrogen-passivated Si(111) surfaces are investigated by scanning tunneling microscopy at low temperatures. Spatially resolved spectroscopy reveals a significant variation of the electronic states at different positions above the molecule. The highest occupied ligandand metal-based orbitals of FePc are determined by pronounced peaks in the tunneling spectra and voltagedependent changes in the microscopic images. Comparison with density functional theory calculations indicates that the electronic ground state is an 3A1g state.
■
under debate.10,11 It turns out that the choice of functional is crucial.9−12 A detailed understanding of the fundamental electronic structure of FePc would be desirable, for example, to explain and optimize the electronic and optoelectronic properties of MPc-based materials. Here we report on the electronic properties of adsorbed FePc molecules. Whereas MPcs chemisorbed to metal surfaces have been widely investigated, we use a weakly binding substrate, namely, hydrogen-passivated Si(111). A combination of microscopic and spectroscopic characterization enables a detailed description of the molecular orbitals and their energy ordering.
INTRODUCTION Metal phthalocyanines (MPcs) possess special physical and chemical properties that make them promising candidates as organic building blocks for electronic and optoelectronic devices such as transistors1 and solar cells,2 respectively. Moreover, MPcs provide model systems for the understanding of the properties of metal atoms in complexes. A few of the MPcs such as iron phthalocyanine (FePc) show magnetic and even ferromagnetic properties under certain conditions.3 There appears to exist an agreement now that the ground state of FePc corresponds to a triplet rather than a singlet spin state.4 The electronic configuration of the ground state, however, remains unclear despite numerous experimental and theoretical studies throughout the last decades.3−12 In MPcs containing transition metals, the d shell is only partially occupied by electrons, leading to versatile electronic and optical properties. The electronic configuration is usually described by the highest occupied molecular d orbitals of the metal atom and the electronic ground-state symmetry. As early as 1974, a (dxy)2 (dz2)2 (dxz,yz)2 configuration and correspondingly an 3A2g state has been suggested on the basis of magnetic circular dichroism spectroscopy for FePc molecules in solution.6 Recent studies of X-ray magnetic circular dichroism of thin FePc films7 and FePc powder8 have indicated an 3Eg state. Liao and Schreiner have found in a theoretical study that an 3A2g state has the lowest energy and also that an 3Eg state is only slightly higher in energy.9 Recent theoretical studies have shown that other electronic ground states such as 3B2g and 3A1g may be energetically more favorable, keeping the subject still © 2012 American Chemical Society
■
EXPERIMENTAL SECTION Experiments were performed in an ultrahigh vacuum apparatus providing a base pressure of ∼6 × 10−11 mbar. FePc molecules were deposited by an electron beam evaporator onto the surface of a hydrogen-terminated Si(111) substrate13 held at a temperature of 12 K. A home-built STM was operated at the same temperature. All STM images were recorded in constant current mode with the sample voltage V applied between the Si substrate (n-type, P-doped, ∼10 Ωcm resistivity) and the tunneling tip. For spectroscopy of differential conductance, the feedback loop of the tip was disabled, and the derivative of the current I with respect to V (dI/dV) was measured by a lock-in Received: June 14, 2012 Revised: September 12, 2012 Published: September 24, 2012 20882
dx.doi.org/10.1021/jp3058433 | J. Phys. Chem. C 2012, 116, 20882−20886
The Journal of Physical Chemistry C
Article
amplifier. Electrochemically etched W tips were cleaned in situ by Ar ion sputtering and annealing.
■
RESULTS AND DISCUSSION STM images of wet chemically prepared hydrogen-passivated Si(111) substrates reveal long-range ordered, ideally hydrogenterminated surfaces.13 Two characteristic features are a triangular shape of terrace edges (Figure 1a) and a hexagonal
Figure 2. Constant current STM images (I = 20 pA) of FePc adsorbed on H/Si(111) for different sample voltages of −2.0, −2.1, −2.25, and −2.6 V.
(Figure 2b−d). At V = −2.6 V, the center is the dominating feature of the image. The images in Figure 2 approximately reflect superpositions of contours of molecular orbitals at energies below EF. Calculated contour plots of molecular orbitals of FePc in the gas phase are presented in Figure 3. Details of the calculations
Figure 1. Constant current STM images: (a) H/Si(111) with different terraces, V = +2.0 V, I = 100 pA; (b) atomically resolved H/Si(111), V = −2.2 V, I = 50 pA; and (c) FePc adsorbed on H/Si(111) V = −2.65 V, I = 30 pA. The color bars on the right cover height ranges of approximately 0.2 and 7.9 Å in panels b and c, respectively.
surface structure with bulk-like lateral atomic distances of 3.8 Å (Figure 1b). The bright round spots in Figure 1b correspond to electronic contributions from H atoms saturating the bonds of the first layer Si atoms underneath.14 It should be stressed that H/Si(111) surfaces are extremely inert against chemical reactions.15 They are stable in air against oxidation for several hours. For all images, the color scale is chosen such that the H/ Si(111) substrate appears mainly in blue. The FePc molecule is mainly represented by colors ranging from red over yellow to white (Figure 1c). At V = −2.0 V, FePc shows a cross-like shape with a maximum in relative height at the center (Figure 2a). Below a sample voltage of −2.0 V, FePc molecules adsorbed on H/ Si(111) show a flower-like shape (Figure 2b−d). In constant current STM images, this shape is found only for MPc adsorbed on semiconducting or insulating substrates.16,17 A cross-like shape is usually found for MPc molecules adsorbed on metallic surfaces.18−22 The flower-like shape is indicative of weak molecule−substrate binding. For FePc adsorption on H/ Si(111), there are two strong arguments for weak binding. First, molecular adsorption does not take place on the substrate held at room temperature. Second, there is no relation between the planar molecular orientation on the surface and the substrate geometry (Figure 1c). FePc is most likely physisorbed on H/ Si(111), one reason being the steric repulsion between hydrogen atoms in the benzene rings of the molecule and hydrogen atoms in the terminating layer of the substrate. With decreasing sample voltage, the circular center of the adsorbate image increases in diameter and relative height
Figure 3. Pseudo 3-D presentation of contour plots of the highest occupied molecular orbitals of FePc (a1u, a1g, eg, b2g) calculated using the B3LYP functional and a 6-31+G* basis set.
are given further below. For d-metal phthalocyanines, molecular orbitals are usually classified according to the irreducible representations of their D4h point group. Ligand-based orbitals correspond to a1u and eg. Metal-based orbitals correspond to a1g and b2g (i.e., dz2 and dxy). A comparison of the adsorbate images with the calculated contour plots shows that the flower-like shape in Figure 2b,c is best represented by an a1u orbital. This assignment is further illustrated by Figure 4, where the schematic molecular structure of FePc is superimposed onto 20883
dx.doi.org/10.1021/jp3058433 | J. Phys. Chem. C 2012, 116, 20882−20886
The Journal of Physical Chemistry C
Article
from the center across the ligand (pos. 3−5), a strong peak develops at −2.2 eV (L1), which is highest in intensity at the outer edge of the benzene ring. Additionally, smaller peaks are found at energies of about −2.8, −3.35, and −3.9 eV on ligand positions. The most prominent information from the dI/dV spectra are the positions and energy dependencies of the ligand- and metalrelated peaks, L1 and M1, respectively. A comparison of the tunneling spectra with the voltage-dependent images leads to the conclusion that peak L1 is related to the occurrence of the flower-like shape of the molecule (Figure 2b,c), whereas peak M1 is related to the central maximum of the molecule (Figure 2d). Concerning the orbital ordering, it follows that peak L1 at an energy of −2.2 eV can be best attributed to the ligand-based a1u orbital, whereas peak M1 at an energy of −2.65 eV is due to the metal-based a1g orbital. No peak is found at and above a voltage of −2.0 V. The cross-like shape in Figure 2a, therefore, cannot be attributed to any peak in the dI/dV spectra. It can be explained by an in-gap image of the molecule, which results from superposition of different orbitals even below and above the highest occupied and lowest unoccupied orbitals (HOMO and LUMO), respectively. For naphthalocyanine (NPc) molecules, a similar in-gap image of cross-like shape has been found for weak adsorption on ultrathin insulating NaCl films.23 STM images of molecules within the HOMO−LUMO gap have also been reported for other adsorbed molecules such as pentacene.24 According to the dI/dV spectra, the ligand-based a1u and the metal-based a1g orbitals are the HOMO and HOMO−1, respectively. The ordering and especially the energy separation of the a1u and a1g orbitals determined here provide valuable information to determine the electronic ground-state configuration of FePc. To do so, density functional theory (DFT) calculations as implemented in Gaussian 0925 were performed using the B3LYP functional and a 6-31+G* basis set. Previous works26,27 have shown that metal complexes of similar size and structure such as Ni porphyrins are accurately described by the B3LYP functional28−30 and a 6-31+G* basis set. Geometries, the order of electronic states, and their energy difference are in very good agreement with experiments. Previous DFT studies on FePc using the B3LYP functional10,11 have also demonstrated good agreement with experiments such as light absorption and photoelectron spectroscopy. In our calculations, a free FePc molecule in the gas phase was assumed, which is reasonable considering the weak molecule− substrate binding. Furthermore, a D4h symmetry and a triplet spin state were assumed. For these conditions, the energetically most favorable ground state is found to be 3A1g, which is ∼0.4 eV lower in energy than the 3B2g state. Moreover, it turns out that for the 3A1g state the ordering and the energy separation of the a1u and a1g orbitals obtained by DFT are in good correspondence with our experimental results. For comparison, the energies of occupied orbitals calculated for the three most relevant electronic configurations of FePc (3A1g, 3A2g, 3B2g) are summarized in Table 1. In general, the energy difference between spin-down (↓) and spin-up (↑) orbitals for metalbased orbitals such as a1g is significant, whereas it is negligible for ligand-based orbitals such as a1u. The orbital ordering and the energies are similar to results from recent DFT calculations on FePc.10,11 Several occupied orbitals are listed in Table 1, but the main interest is on a1u and a1g. For all three states (3A1g, 3A2g, 3B2g), the ligand-based a1u orbital corresponds to the HOMO, as
Figure 4. Schematic molecular structure of FePc superimposed onto the constant current STM image of FePc adsorbed on H/Si(111) for a sample voltages of −2.1 V.
the adsorbate image. The maximum in relative height and diameter of the central protrusion found in Figure 2d is assigned to contributions from the metal-based a1g orbital. To elucidate further the molecular orbitals of FePc and their energy ordering, spectra of dI/dV have been recorded at different positions of the surface (Figure 5), which provide
Figure 5. Spectra of dI/dV as a function of sample voltage at different positions of the H/Si(111) substrate and a FePc molecule, as indicated in the STM image at the top. Spectra are shifted along the ordinate axis while keeping the same scaling.
information on the energy-resolved local density of states (LDOS) of the surface. On the H/Si(111) surface (pos. 1), dI/ dV continuously increases with decreasing sample voltage, reflecting the increase in LDOS with increasing energy distance from the valence band edge. The continuously varying dI/dV signal of the substrate allows for a straightforward identification of peaks related to the adsorbed molecule. In the center of the molecule (pos. 2), two pronounced peaks occur corresponding to energies of −2.65 (M1) and −3.8 eV (M2). Going away 20884
dx.doi.org/10.1021/jp3058433 | J. Phys. Chem. C 2012, 116, 20882−20886
The Journal of Physical Chemistry C
Article
ligand-based eg orbital in this energy range, 1.52 eV below the ligand-based a1u orbital. One may therefore speculate that this series of peaks has its origin in vibrational excitations of the molecule,32 especially because they are equidistant with an energy separation of ∼0.55 eV. At variance with this assumption, however, the highest vibrational energy of FePc has been calculated to amount to only 0.38 eV.33 In this context, it should be mentioned that for semiconductors, the relation between the applied sample voltage V and the energy of the tunneling electrons in units of electronvolts may not be straightforward, as assumed above. In fact, the potential difference between the tip and the sample surface may differ from the applied sample voltage.34 A part of the voltage drops within the interior of the sample due to tip-induced band bending. The amount depends on parameters of the semiconductor and the tip such as doping concentration, electron effective mass, electron affinity, band gap width, tip work function, and tip−sample separation. For the parameters used in our experiments, band bending effects are calculated by the method proposed by Feenstra.34 They lead to a correction of the energy of the tunneling electrons of
