Adsorption of Single Magnesium Phthalocyanine Molecules on V2O3

to interact with VdO vacancies, being the dominant structural defect in the vanadyl-terminated V2O3 surface. The interaction with such defects is, how...
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J. Phys. Chem. C 2008, 112, 10027–10031

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Adsorption of Single Magnesium Phthalocyanine Molecules on V2O3 Thin Films N. Nilius* and V. Simic-Milosevic Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: February 4, 2008; ReVised Manuscript ReceiVed: April 8, 2008

The adsorption characteristic of single magnesium phthalocyanine molecules on a V2O3 thin film grown on Au(111) has been investigated by scanning tunneling microscopy and spectroscopy. The molecules are found to interact with VdO vacancies, being the dominant structural defect in the vanadyl-terminated V2O3 surface. The interaction with such defects is, however, weak and leaves the molecular electronic system essentially unperturbed. Introduction A tremendous amount of scanning tunneling microscopy (STM) work has been accomplished in the past decade to understand the interaction of large organic molecules with metal and semiconductor surfaces.1–5 The experiments focused on a characterization of the binding configuration of the molecules, the degree of hybridization with substrate electronic states and the role of charge transfer upon adsorption. Also, self-organization effects into supramolecular networks have been a major interest, aiming for the fabrication of functional building blocks for molecular electronic devices.6–8 It soon turned out that even on weakly interacting noble metal surfaces, such as Ag(111), the coupling between molecular orbitals and electronic states of the support is substantial.9 For example, no vibrational progressions and optical excitations could be observed for molecules on metal supports, which relates to the dramatically reduced lifetime of excited molecular states in the presence of metal-induced decay channels.10 The interaction is, however, strongly reduced when depositing the organic molecules onto a thin insulating film that decouples the molecular species from the metal support.11–14 The reduced interplay in this case results from the absence of electronic states around the Fermi level of the insulator that are available for a hybridization with the molecular orbitals. Experiments on thin oxide and halide films therefore provided valuable insights into the intrinsic electronic properties of molecules and enabled their optical characterization at the single molecule level.15–17 In contrast to metal surfaces, insulators and especially oxides are characterized by a rather inhomogeneous potential landscape for adsorption due to the abundance of defects and structural imperfections.18 Those defects are known to dominate the interaction of small inorganic molecules (CO, CO2, H2O) and metal adatoms with the otherwise inert materials and often represent the only available adsorption sites on their surfaces.19 The desired decoupling of the adsorbate electronic system from the metal support is, in such a case, just replaced by a new interaction mechanism that involves hybridization of molecular orbitals with defect states in the oxide surface. While the importance of oxide defects is well established for the binding of small adsorbates, little is known on their interplay with large conjugated molecules. In principle, the binding mechanism should differ considerably in both cases, as adsorption of small molecules and adatoms is governed by the * Corresponding author: [email protected].

formation of covalent bonds, while conjugated molecules mainly interact via polarization interactions involving their delocalized π-electronic system. In most STM experiments dealing with the adsorption of organic molecules to insulator surfaces, the role of defects remained unclear and the perturbation of the molecular electronic system due to coupling with defect states was not studied explicitly.11,12,14,15 Local investigations of the interplay between organic molecules and oxide defects are therefore highly needed in order to judge whether oxide films are suitable spacers to decouple organic molecules from metal supports and to preserve their intrinsic electronic properties. In this work, we have investigated the adsorption of individual magnesium phthalocyanine (MgPc) molecules on [0001]-oriented V2O3 films. This particular oxide is chosen because it has a relatively simple surface structure and exposes a small number of well-defined defects. In addition, V2O3 is a prototype material for a covalently bound oxide with a small band gap of 500-700 meV that opens below a Mott-like metal-insulator transition at around 150 K. Due to the small gap size, even thick films of V2O3 are accessible to STM experiments. In contrast to wide band gap insulators, V2O3 has a sufficiently high electron density to enable substantial polarization interactions with organic molecules, thus stabilizing the molecular species onto the oxide support. Experimental Section The experiments are performed with a custom-built, ultrahigh vacuum STM operated at 5 K.20 The V2O3 film is prepared by depositing 5 ML of vanadium in 2 × 10-7 mbar O2 onto a sputtered/annealed Au(111) surface at room temperature.21–23 Subsequent annealing of the sample to 700 K leads to the formation of large, hexagonally shaped vanadia islands of 20-50 nm diameter and 3-5 nm height.24 The crystalline nature of the film is verified by the hexagonal superstructure visible in LEED and the well-ordered atomic arrays resolved in the STM. The MgPc molecules (Sigma Aldrich) are thermally evaporated from an alumina crucible and closed onto the sample held at 300 K. The coverage is adjusted to approximately 2 × 1012 molecules per cm2. The electronic properties of the molecules and the bare oxide support are determined by differential conductance spectroscopy performed with lock-in technique (Vmod ) 10 mV, 900-1100 Hz) and open feedback loop. Following the Tersoff-Hamann approach, the dI/dV signal provides a measure of the local density of states (LDOS) of the sample surface.

10.1021/jp801039x CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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Figure 1. (A) STM topographic image of a V2O3 island on Au(111) after exposure to MgPc molecules (22 × 22 nm2, -1.8 V, 0.1 nA). (B) Structure model of the V2O3 surface. (C) dI/dV spectrum of a V2O3 island showing the correlation band gap (set point 1.0 V).

Results and Discussion Figure 1A shows an STM topographic image of a V2O3 island carrying a dozen MgPc molecules. The pristine oxide film exposes a hexagonal array of protrusions with 4.95 Å lattice constant. On the basis of earlier LEED and STM measurements, these maxima are attributed to single vanadyl (VdO) groups that protrude by ∼2.3 Å from the topmost oxygen layer of a bulk-truncated (0001) V2O3 surface (Figure 1B).21,23,25 At UHV conditions and moderate temperatures, the surface termination with one VdO group per unit cell (1 × 1 vanadyl structure) has the highest thermodynamic stability of all possible V2O3 terminations, as revealed by recent DFT calculations.26 The dark spots in the bright array of VdO groups represent vanadyl vacancies, where a single VdO group is missing at the surface (also termed VdO defect). Such vacancies are the dominant defect type in the vanadia surface and are usually associated with the chemical activity and the adsorption characteristic of the oxide film.27 The electronic structure of vanadia, as deduced from dI/dV spectroscopy, is determined by the correlation band gap that opens up below the Mott transition at 150 K.24,28,29 The gap size depends sensitively on the local oxide stoichiometry. For oxide compositions close to the ideal 2:3 ratio between V and O atoms, the gap size amounts to 600-700 meV (Figure 1C). Vanadyl vacancies have little influence on the bulklike correlation band gap and cause only a slight reduction of the surface LDOS at the conduction band edge.24,26 Individual MgPc molecules are imaged with submolecular resolution on top of the vanadia islands (Figure 1A). Surprisingly, the distinct 4-fold symmetry of the gas-phase molecule is perturbed in almost every case with one, rarely two, of the four legs (corresponding to a MgPc indole group) appearing with reduced topographic contrast. As no deviations from the gas-phase symmetry are observed for MgPc deposited onto bare Au(111), fragmentation of the molecules during the evaporation process is excluded. Furthermore, damaged molecules are clearly identified on the oxide film by the absence of their characteristic cloverleaf shape (see circled species in Figure 1A). The asymmetric appearance of single MgPc molecules in the STM seems therefore to be associated with their adsorption characteristic on the V2O3 surface. Details of the MgPc binding geometry are deduced from atomically resolved images as the one shown in Figure 2A. The image exhibits an unperturbed molecule with a 2-fold symmetry (bottom) in addition to a distorted one (top). In both cases, the main molecular axis aligns with one of the three directions of the hexagonal VdO lattice. As expected from the superposition of the 2-fold molecular and the 3-fold substrate symmetry,

Nilius and Simic-Milosevic rotation of (60° leads to a replication of the molecular configuration with respect to the V2O3 lattice. This rotational invariance is indeed found in the STM images, revealing all three possible molecular orientations with equal probability. In addition, a rotation of an individual molecule by (60° can be induced by exploiting the enhanced tip-sample interaction at low scan bias, as demonstrated in the subsequently taken images in Figure 2A,C. The adsorption site of the symmetric molecule is determined by extrapolating an ideal VdO lattice onto the MgPc position and overlying it with a model structure of the planar molecule, thereby neglecting possible deformations (Figure 2B). In the resulting configuration, the central Mg atom and two molecular legs of the MgPc are located on top of a vanadyl group, while the other two legs occupy bridge positions between two VdO groups. The legs in on top position always exhibit reduced topographic contrast, independently of the applied bias. Asymmetric molecules show a similar adsorption geometry with the MgPc center located on top of a VdO group. The distorted leg corresponds to one in a VdO top position in the majority of cases (Figure 2D). Only in rare examples where two MgPc legs show reduced contrast does one leg occupy a VdO top and the other one a bridge site (Figure 2B). Further insight into the adsorption environment, especially the atomic structure below the MgPc, is obtained by desorbing selected molecules via scanning at low sample bias and reduced tip-sample distances. The underlying desorption mechanism is related to the strong Pauli repulsion between tip electronic states and the molecular orbitals. As the adhesion of the molecule to the V2O3 surface is substantial, such manipulation experiments are often accompanied with geometrical changes of the tip apex and a loss of spatial resolution. Desorption of a symmetric molecule exposes a defect-free VdO lattice in the interaction region (Figure 2C,D). Two protrusions remain visible on the surface after the MgPc removal, most likely fragments of the desorbed molecule (Figure 2D). Their position on top of a VdO group would be compatible with H atoms, as stable V-OH groups are known to form easily on the vanadia surface.30 Whether those atoms have been involved in anchoring the molecule to the support or result from the desorption process itself is not known. The VdO lattice below an asymmetric MgPc always reveals a vacancy site, i.e., a missing vanadyl group (Figure 3A,BfC,D). The defect site hereby matches the former position of the distorted MgPc leg(s), indicating that the observed asymmetry is introduced by the influence of the VdO vacancy on the molecular configuration. The electronic properties of regular and defect-bound MgPc on V2O3 are investigated by dI/dV spectroscopy. Spectra of symmetric molecules exhibit two pronounced maxima at approximately -1.5 and +1.7 V that might contain an unresolved fine structure (Figure 4, red curve).11 The dI/dV peaks mark the position of the HOMO and the LUMO level. Conductance mapping reveals the shape of the underlying molecular orbitals. The HOMO has a 4-fold symmetry with a bright central region and eight clearly distinguishable lobes (Figure 4, top). Several molecular orbitals contribute to the unoccupied LDOS. At +1.6 V, a bright ring with dark central region appears in the dI/dV maps. With increasing sample bias, the central part gains more and more intensity, until a bright and dark lobe pair with 2-fold symmetry emerges in the conductance maps at 2.0 V. The bright lobes correspond to the two legs that protrude in the topographic images of the molecules (Figure 2A).

Adsorption of MgPc on a V2O3 Thin Film

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Figure 2. (A) STM topographic image of a V2O3 patch with a regular and a defect-bound MgPc molecule (10 × 10 nm2, -1.6 V, 0.1 nA). (B) As in part A, but with the vanadyl lattice and a MgPc structure model overlaid. (C) As in part A, but after a tip-induced rotation of both molecules by 60°. (D) As in part C, but after controlled desorption of the lower molecule by scanning at -0.5 V sample bias.

Surprisingly, MgPc molecules attached to a V2O3 defect are almost indistinguishable from their regular-bound counterparts from an electronic point of view. They have rather similar HOMO and LUMO energies (Figure 4, blue curve) and exhibit essentially the same orbital shapes as the symmetric molecules. Only the distorted leg is characterized by less pronounced peaks in the dI/dV spectra and smaller intensity in the conductance maps. The similar conductance signature of molecules bound to regular and defect sites demonstrates that the influence of the VdO vacancy does not produce new electronic states and leaves the molecular electronic system essentially intact. The HOMO-LUMO gap of MgPc molecules on the vanadia surface has been determined to 2.8 ( 0.2 eV by averaging over more than 50 molecules in different binding configurations (Figure 4, inset). Information on the binding mechanism of MgPc to regular and defect sites of the V2O3 surface can be deduced from analyzing the specific adsorption geometry of the molecules. In all cases, the molecular center is attached to a VdO group, suggesting that the Mg interaction with the vanadyl oxygen plays a dominant role in the adsorption process. This conclusion is further supported by the bright appearance of the molecular center in conductance maps and clearly rules out a convex distortion of the molecule. The Mg-O interaction is ascribed

to an electrostatic attraction between the positively charged Mg ion and the negatively charged vanadyl oxygen.31 Also the formation of a true covalent bond between the two species cannot be ruled out. The molecular frame, on the other hand, interacts with the vanadium 3d states in the oxide via its delocalized π-electronic system.32 As the V atoms in the VdO groups are in the oxidation state +5 (V 3d0 configuration), polarization interactions are efficient only with the regular V atoms (V 3d2 configuration) located in the bulklike V2O3 layers. The VdO groups therefore mainly act as inert spacers that decrease the overlap between the molecular π-system and the oxide LDOS. In regular-bound molecules, the four legs experience a pairwise different interplay with the oxide surface, depending on their position with respect to the vanadyl groups. The two legs in the VdO bridge positions show a large topographic height and an enhanced dI/dV intensity, suggesting a strong overlap of their indole groups with the oxide states due to a reduced distance from the surface. The legs in the VdO top positions, on the other hand, are bent away from the surface due to Pauli repulsion with the filled 2p⊥ orbitals of the vanadyl oxygen.26 As a result, the overlap between the π-states of those indole groups and the oxide LDOS is smaller and tunneling into the legs is less efficient. The unequal interaction of both leg pairs with the VdO groups

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Nilius and Simic-Milosevic

Figure 3. (A) STM image of a V2O3 patch with two defect-bound MgPc molecules (9.5 × 9.5 nm2, -1.6 V, 0.1 nA). (B) As in part A, but with the vanadyl lattice and a MgPc structure model overlaid. (C, D) As in part A, but after controlled desorption of the upper molecule by scanning at -0.4 V sample bias.

reduces the 4-fold symmetry of gas-phase MgPc to the 2-fold symmetry observed on the oxide surface and might also alter the planar geometry of the molecular frame.33 The asymmetric binding configuration is related to the absence of one VdO group below the molecule. The leg located above the vacancy experiences a smaller steric repulsion and can slightly approach the oxide surface. The π-electronic system of the respective indole group is now able to overlap more efficiently with the V 3d states below the VdO termination. As the two orthogonal legs remain in VdO bridge sites and only the leg vis-a`-vis the defect position still occupies an unfavorable VdO top site, the total binding energy of the molecule slightly increases with respect to a regular-bound species. The preferred adsorption of MgPc close to vacancy sites in the V2O3 film is therefore traced back to the enhanced coupling of the π-states in the leg above the VdO defect to the oxide LDOS. Apart from this, no additional impact of the defect is revealed on the molecular electronic system. The molecular leg above the VdO vacancy neither shows a new spectral feature nor enhanced dI/dV intensity in conductance spectra or images. Furthermore, the reactive center of the MgPc is never found to bind directly to a VdO vacancy. This negligible “direct” influence of the defect is ascribed to two effects: (i) According

to DFT calculations, the absence of a single VdO group does not introduce a large perturbation of the oxide electronic structure.26 (ii) The defect state associated with the missing VdO group is spatially confined to the topmost bulklike V2O3 layer and therefore well separated from the MgPc binding plane via the remaining VdO groups. This relatively large distance between the VdO defect state and the MgPc molecular orbitals inhibits effective hybridization. In contrast, single Au atoms are able to approach the VdO vacancy site due to their smaller size and efficiently couple to the defect state formed upon VdO removal, as revealed by changes in their dI/dV signature.35 The surprising insensitivity of the MgPc electronic system against perturbation by the oxide defect is therefore connected to the rigidity of the molecular frame, which stabilizes the molecule above the remaining VdO groups and prevents substantial overlap with the defect state located below the VdO spacer layer. Additional conclusions on the interaction between MgPc orbitals and the oxide LDOS can be drawn from the dI/dV measurements. The nearly symmetric position of the HOMO and LUMO with respect to the Fermi level (as fixed by the Au support) indicates the absence of substantial charge transfer into or out of the molecular orbitals. Also, the symmetry of the two frontier orbitals is compatible with the simulated orbital shapes

Adsorption of MgPc on a V2O3 Thin Film

J. Phys. Chem. C, Vol. 112, No. 27, 2008 10031 Acknowledgment. The work was supported by the DFG priority program “Quantum transport at the molecular scale” (SPP 1243). References and Notes

Figure 4. dI/dV spectra of a regular (red) and a defect-bound MgPc molecule (blue) in addition to the V2O3 reference (black) (set point +2.0 V). The shape of the underlying molecular orbitals is deduced from dI/dV mapping and shown for a regular molecule in the upper part of the figure. The inset depicts the energy position of the HOMO and LUMO for more than 50 different molecules on the V2O3 surface.

of gas phase phthalocyanines.36 Only, the HOMO-LUMO gap of MgPc on the oxide surface is enlarged by 30% with respect to the gas-phase value (2.85 eV versus 2.16 eV).33 This difference is, however, not related to an alteration of the molecular electronic structure upon adsorption, but reflects the dielectric response of the oxide film in the tip-sample junction. Being an insulator with finite dielectric constant (εr) at low temperature, V2O3 is subject to band bending effects in the tip electric field.37,38 As a result, only a part of the sample bias drops in the tip-molecule junction, whereas the rest declines in the oxide film. The ratio between the voltage-drop in the oxide and the total bias amounts to η ) d/(zεr), where d is the oxide thickness and z the tip-molecule distance. Due to this bias division in the tunneling gap, the molecular orbitals shift from their flat-band position (Eo) to EV ) Eo/(1 - η) in the biased junction. The dielectric function of V2O3 depends strongly on temperature and oxide doping level and is only approximately known for the given experimental conditions. Using an εr of 24,39 a mean film thickness of 30 Å, and a tip-molecule distance of 5 Å, the increase in the HOMO-LUMO gap is estimated with 33% with respect to the free molecule, in reasonable agreement with the experimental result. In conclusion, the adsorption behavior of MgPc molecules on vanadyl-terminated V2O3 islands is found to be modified by the presence of VdO vacancies in the oxide surface. In the preferential binding configuration, a single leg of the MgPc aligns with a vacancy site, thereby reducing the steric repulsion with the vanadyl groups and enhancing the binding energy of the molecule. The VdO vacancy itself has only little influence on the MgPc electronic system, most likely because the associated defect state is located too far below the MgPc to allow substantial overlap with the molecular orbitals. The experiments demonstrate that defects in vanadyl-terminated V2O3 help to anchor large conjugated molecules on the surface without perturbing their intrinsic electronic properties.

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