Modification of Charge Transfer and Energy Level Alignment at

Jul 1, 2009 - For the unmodified TiOPc/TiO2 system, a strong charge transfer is observed from the first layer TiOPc into the substrate, which leads to...
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J. Phys. Chem. C 2009, 113, 13765–13771

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Modification of Charge Transfer and Energy Level Alignment at Organic/TiO2 Interfaces Shun Yu,*,† Sareh Ahmadi,† Pa˚l Palmgren,|,‡ Franz Hennies,‡ Marcelo Zuleta,§ and Mats Go¨thelid*,† Materials Physics, ICT, Royal Institute of Technology, Electrum 229, SE-164 40 Stockholm, Sweden, MAX-lab, Lund UniVersity, Box 118, SE-22100 Lund, Sweden, and Physical & Analytical Chemistry Department, Uppsala UniVersity, Box 259, SE-751 05 Uppsala, Sweden ReceiVed: March 28, 2009; ReVised Manuscript ReceiVed: May 26, 2009

Adsorption of titanyl phthalocyanine (TiOPc) on rutile TiO2(110) modified by a set of pyridine derivatives (2,2′-bipyridine, 4,4′-bipyridine, and 4-tert-butyl pyridine) has been investigated using synchrotron radiation based X-ray photoelectron spectroscopy (XPS). For the unmodified TiOPc/TiO2 system, a strong charge transfer is observed from the first layer TiOPc into the substrate, which leads to a molecular layer at the interface with a depleted highest occupied molecular orbital (HOMO). However, precovering the TiO2 surface with a saturated pyridine monolayer effectively reduce this process and leave the TiOPc in a less perturbed molecular state. Furthermore, the TiOPc HOMO and core levels are observed at different binding energies ranging by 0.3 eV on the three pyridine monolayers, which is ascribed to differences in surface potentials set up by the different pyridine/TiO2 systems. Introduction Interfaces between organic molecules and inorganic substrates are of both scientific and technological significance,1,2 with the appearance of electronic devices based on organic materials: organic light emitting diodes,3,4 transistors,5 organic solar cells,6 dye-sensitized solar cells,7 etc. Surface properties and interfacial interactions can severely change the electronic properties of both molecule and substrate,8 critically affecting the performance of the final devices. Therefore, much effort has been put into understanding and controlling these effects. Coadsorption or preadsorption has been used to modify or control these interactions, either by metal atoms9,10 or organic molecules11–16 directly bound to the substrate. Coadsorption can provide an efficient route for nanopatterning and constructing supramolecular structures9,11–14 by manipulation of the molecular selforganization. In addition, the preadsorbed layer could also work as protective layers on the reactive substrates, as shown by Sasahara et al.15 who used pivalic acid to passivate the TiO2 surface before exposing the substrate to air and subsequently immersing into a dye solution for the purpose of ultrahigh vacuum STM observation. Furthermore, co- or preadsorption induced modifications of the electronic structure and energy level alignement10,16–18 at interfaces have also been reported. In the dye sensitized solar cells (DSSCs), the addition of 4-tertbutyl pyridine (4TBP), which is integrated in the electrolyte and can coadsorb with dyes on the TiO2 substrate,19–21 causes a favorable shift of the band potential to higher cell voltage and hinders the electronic regeneration of the electrolyte from the oxide support leading to increased cell efficiency. Titanyl phthalocyanine (TiOPc) is a widely used dye within the organic electronic and photovoltaic industry. It has a suitable * To whom correspondence should be addressed. (S.Y.) Tel: +46(0)8 790 4162. Fax: +46(0)8 752 7850. E-mail: [email protected] (M.G.) Tel: +46(0)8 790 4154. Fax: +46(0)8 752 7850. E-mail: [email protected]. † Royal Institute of Technology. ‡ Lund University. § Uppsala University. | Department of Physics & Materials Science, Uppsala University, Box 530, SE-751 21 Uppsala, Sweden.

energy gap around 1.7 eV for solar cell applications.22 However, according to recent studies,23,24 a strong charge transfer occurs across the Pc-TiO2 interface resulting in an oxidized state of the first organic layer. Later, experiments showed that preadsorption of a monolayer of 4,4′-bipyridine (4,4′ Bipy) proved to successfully modify the rutile TiO2(110) surface, avoiding the oxidation of the first FePc layer on top and leaving FePc in an unperturbed molecular state.25 In the present work, we extend the previous study based on 4,4′ Bipy and additionally introduce two new pyridine molecules, 4TBP and 2,2′-bipyridine (2,2′ Bipy) coadsorbed with TiOPc. Both pyridines are involved in DSSC applications and are therefore technologically relevant. 4TBP is an important additive in DSSC as mentioned above, and 2,2′ Bipy derivatives are basic components of the ruthenium complex dyes, which are still preferentially used as the best sensitizers in DSSC fabrication. We use X-ray photoelectron spectroscopy (XPS) to explore the electronic structure of the interface of TiOPc and TiO2 with and without preadsorbed pyridines layer. By analyzing core level and valence band spectra, we investigate the interaction between different molecules and substrate, and compare the modification effects. A charge transfer from the first layer of TiOPc on TiO2(110) is revealed, consistent with the previous observation.23 The comparison shows that all three different saturated pyridine monolayers successfully protect TiOPc against the oxidation. Furthermore, the surface potentials on the three modified surfaces are different leading to different TiOPc HOMO positions at the interface. Experimental Methods XPS was performed at the surface end station of beamline I511 at MAX-lab, Lund, Sweden. This beamline gives a horizontally polarized photon beam in the soft X-ray spectral region from an undulator. A modified Zeiss SX-700 monochromator is used to select the desired photon energy. Spectra are recorded using a Scienta R4000 spectrometer in the analysis chamber, which can rotate around the axis of the photon beam. The preparation chamber, with a base pressure lower than 1 ×

10.1021/jp902814d CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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Figure 1. TiOPc/TiO2 core level spectra: (a) C1s spectra, hυ ) 347 eV and (b) N1s spectra, hυ ) 455 eV. The numerical fit of the C1s spectrum from TiOPc thick film (top, a) illustrates the contributions from benzene carbon (B), pyrole carbon (P), and their respective shakeup structures (Bsu and Psu).

10-10 mbar, contains a sputter gun, sample annealing, low energy electron diffraction (LEED) optics and evaporators. A load lock is connected to the preparation chamber for fast sample entry. The sample crystal of rutile TiO2(110) was purchased from Surface Preparation Laboratory, The Netherlands, and aligned to within 0.2° from the (110) plane. After introduction into the ultrahigh vacuum (UHV) chamber, thermal treatment at around 1000 K in UHV created oxygen vacancies in the sample and increased the conductivity, changing the sample color from transparent to blue. Before each deposition, the surface was cleaned by several rounds of Ar+ sputtering followed by annealing in UHV. LEED showed a clear 1 × 1 pattern. TiOPc (Sigma-Aldrich, 95% purity) was deposited on the surface by sublimation in the preparation chamber using a homemade Knudsen-type evaporation cell. The molecular powder was thoroughly outgassed before deposition, until no impurities (water) were detected in mass-spectra. Pyridine evaporators (for 2,2′ Bipy and 4,4′ Bipy deposition) and a leak valve (for 4TBP deposition) were mounted on the load lock. The base pressure in the load-lock was 6 × 10-10 mbar. The modifier molecules, 2,2′ Bipy (Sigma-Aldrich, 99%+ purity), 4,4′ Bipy (Fluka, 98% purity), and 4TBP (Sigma-Aldrich, 99% purity), were deposited without further purification at room temperature. Formation of the saturated single pyridine monolayer was confirmed from core level (Ti2p, O1s, C1s, and N1s) and valence band spectra. Saturation is reached when no spectral changes are observed after further exposure. The C1s intensity of the saturated monolayers is very stable and reproducible and is used to relate the molecular coverage of different preparations. 4,4′ Bipy forms a (1 × 1) overlayer where each molecule occupies one surface unit cell 0.3 nm by 0.6 nm. These molecules stand up with one nitrogen atom down and the other one pointing up.23 The size of a phthalocyanine molecule is around 1.2 nm by 1.2 nm. Here we define one monolayer of TiOPc as a layer with flat lying TiOPc packed close together. This leads to an estimated TiOPc C1s intensity around 50% of that of the saturated 4,4′ Bipy layer, based on the surface density ratios of C atoms. The thicknesses of TiOPc on top of pyridines were around 1 monolayer.

Photoelectron spectra were collected after each deposition and normalized to the background. C1s, N1s and valence band (VB) spectra were measured with photon energy 347 eV, 455 and 110 eV, respectively. The experimental resolution for core level spectra and valence spectra are 110 and 12 meV, correspondently. To avoid beam damage to the organic layers, the sample was moved (1.7 µm/s) during X-ray exposure. The energy scale in XPS was calibrated to the Fermi level, recorded from a tantalum foil in electrical contact with the substrate. Curve fitting of core level spectra is done by using the software XPSPeak41.26 Unless otherwise mentioned, all TiOPc C1s spectra were fitted with the full width at half-maximum (fwhm) 0.9 eV, Lorentzian 9% and Gaussian 91%. The relationship between different components is determined from the thick bulklike TiOPc film. Results and Discussion TiOPc on the Bare Substrate. C1s and N1s core level spectra from monolayer and thick TiOPc films on TiO2(110) are shown in Figure 1. The C1s spectrum from the thick film (top left) demonstrates the typical three-peak structure observed for TiOPc27 and other phthalocyanines.23–25,28–30 It can be deconvoluted into four components27,28 representing benzene carbon (B), pyrrole carbon (P), and their respective shakeup structures (Bsu and Psu). The intensity of B + Bsu and P + Psu is consistent with the molecular stoichiometry (i.e., 3:1, Figure 2). The shakeup structure is mainly due to excitations of HOMO electrons into the LUMO by the outgoing photoelectron. The shakeup thus appears shifted to higher binding energy by the HOMO-LUMO gap. Other excitations are also possible but with lower intensity.28 The N1s spectrum from the thick film (top right) shows a single peak at 399.2 eV with a weak shakeup structure at 401.0 eV. A C1s spectrum from monolayer coverage (bottom left) shows a broad peak with a rather flat summit, disproportional to the molecular stoichiometry. This line profile appears in a rather broad coverage range (0.7-1.4 ML) due to island growth. The N1s spectrum (bottom right) displays two clearly resolved peaks at 398.9 and 400.6 eV. The peak at lower binding energy

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Figure 2. Molecular structures of (a) 4,4′ Bipy, (b) 4 TBP, (c) 2,2′ Bipy, and (d) TiOPc.

later develops as the dominant peak of thick film. A similar peak splitting, for both C1s and N1s, was reported by Palmgren et al.23 for a thin FePc film on TiO2(110). It is attributed to an interfacial charge transfer from molecules into the surface. This interaction leads to a reduction of charge in the HOMO in the first FePc layer. At increasing coverage C1s spectra clearly comprised two different states with a 1.2 eV binding energy difference. N1s also showed two peaks separated by 1.2-1.3 eV as well.23 Here, in the case of TiOPc, the C1s splitting is 1.3 eV and the N1s splitting is 1.5 eV. Hence, in agreement with previous results, we suggest the existence of two different TiOPc: those at the interface which experience a charge transfer into the substrate and those which are not in direct contact with the surface and retain a molecular character. These two species coexist in the monolayer region, which is explained by island growth. This is not surprising considering that TiOPc in its “bulk” form adopts a double layer structure.31,32 Furthermore, the absence of a clear shakeup structure for the peak at higher binding energy is in line with a charge reduction in the HOMO. This is also supported by the disappearance of the HOMO feature in valence band spectra from 1 ML TiOPc (Figure 5b). Hence we conclude that the interface charge transfer appears to be common for H2Pc, FePc, and TiOPc. This charge transfer from the interface TiOPc layer may severely influence the injection of photoexcited electrons from the organic dye into TiO2 substrate during photovoltaic processes. The low binding energy peak, related to molecules not in direct contact with the surface, shifts to higher binding energy (from 398.9 to 399.2 eV) when increasing the layer thickness. This effect is commonly observed in molecular systems and is often assigned to a reduced core hole screening.23,33 Pyridine Modification. Based on the previous successful experiments that a reduction of the charge transfer from the phthalocyanine molecules was accomplished by modification of TiO2(110) using 4,4′ Bipy,25 the modification method is still done by predeposition of pyridines (4,4′ Bipy, 2,2′ Bipy, and 4TBP) that form saturated monolayers between the TiO2(110) surface and the TiOPc thin film. Figure 3 presents N1s spectra from gradually thicker TiOPc layers, up to monolayer thickness, deposited on the following pyridine monolayers: (a) 2,2′ Bipy, (b) 4,4′ Bipy, and (c) 4TBP. A comparison between the resulting monolayer films and TiOPc films (6 ML and a thick film) on unmodified TiO2 is illustrated in Figure 3d. The bottom spectra of panels a-c correspond to the saturated pyridine monolayers before the TiOPc deposition. It is observed that 4TBP and 4,4′ Bipy show very symmetric peaks around 400.0 eV, whereas 2,2′ Bipy shows a main peak at 400.7 eV with a shoulder at 399.9 eV. The nitrogen atoms in some

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13767 pyridine derivatives are believed to prefer binding to the metal ion of the transition metal oxide.34,35 Gra¨tzel et al.19 suggested that 4TBP adsorb on TiO2 through the nitrogen lone pair to the Ti(IV) ion. It has also been shown that 4,4′ Bipy binds to the surface with one nitrogen atom while the other points out of the surface.23 However, pyridine and some other pyridine derivatives are found to adsorb flat with the molecular plane parallel to the surface.36–38 2,2′ Bipy has a different distribution of nitrogen atoms compared to the other two molecules, and NEXAFS demonstrates that it adopts an almost lying down geometry on the surface.39 Hence, the asymmetric line profile of 2,2′ Bipy can be explained by a more complex film structure with multiple adsorption configuration. The higher binding energy can be interpreted as a reduced charge density at, or near, these nitrogen atoms. Upon deposition of TiOPc on the different pyridine monolayers, a peak appears around 399.0 eV. This is near the main peak from the thicker TiOPc layers. Curve fitting shows that the pyridine related N1s peaks (green lines in Figure 3a-c) shift to lower binding energy after TiOPc deposition. The curve fitting of spectra from the coadsorption systems were done by locking the line profiles (Lorentzian and Gaussian fwhm and the relative peak positions and intensities if multi peaks are needed) to the results from fitting the monolayer pyridine spectra. N1s peaks of 4,4′ Bipy moves by 0.25 eV, close to the reported value 0.3 eV for FePc/4,4′ Bipy.25 The only N atom of 4TBP which binds to the surface shifts to lower binding energy as well. This shift lies probably in that atop TiOPc molecules change the charge distribution within the pyridine layer, resulting in a slightly different electronic environment. N1s of 2,2′ Bipy shifts by 0.4 eV, which is more than the other pyridines after TiOPc deposition. This may be due to the π-π interaction between TiOPc and 2,2′ Bipy and also the fact that the N1s from 2,2′ Bipy had a higher binding energy before TiOPc adsorption. In Figure 3d, fitted N1s spectra from the coadsorption systems are compared with spectra from TiOPc multilayers with different thicknesses. On TiOPc/4TBP and TiOPc/4,4′ Bipy the TiOPc related N1s peak is aligned with the peak from the 6 ML TiOPc film, whereas on TiOPc/2,2′ Bipy the TiOPc N1s peak aligns with the thick TiOPc film. The N1s energy difference between the two cases is 0.3 eV. This indicates that the pyridine monolayers modify the TiO2 surface differently; presenting slightly different surface potentials, work functions, for the adsorbed TiOPc films. The work function change induced by an adsorbate depends on the charge transfer between adsorbate and surface and the resulting surface dipole changes. Clearly the 2,2′ Bipy is different in this respect, which can be traced back to the higher N1s core level binding energy position before TiOPc adsorption indicating a larger charge transfer from 2,2′ Bipy than the other two pyridines. The evolution of C1s spectra from the coadsorption systems is shown in Figure 4a-c, with a spectrum from each pyridine monolayer in the bottom and successive depositions of TiOPc on top in the following spectra. The preparations are the same as for the N1s spectra. The 4TBP monolayer displays one sharp peak at 285.9 eV. The 2,2′ Bipy peak is found at 286.4 eV and is much broader while the 4,4′ Bipy spectrum has a clear structure with two resolved peaks (at 286.5 and 285.8 eV), which was explained earlier as due to chemical shifts between C-N and C-C and also the influence of the upright geometry.25 The additional width in the 2,2′ Bipy spectrum is in line with the split N1s and a more complex adsorption geometry. As TiOPc is deposited a strong peak gradually develops on the low binding energy side together with a small peak on the

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Figure 3. N1s spectra from coadsorption, hυ)455 eV: (a) TiOPc/2,2′ Bipy, (b) TiOPc/4,4′ Bipy, (c) TiOPc/4TBP, and (d) comparison of coadsorption with 6 ML and thick TiOPc film. Bp and Pc in (c) represent the contribution from pyridines and phthalocyanine, respectively. The dashed lines in (d) mark the peak position in the thick TiOPc film and in the 6 ML TiOPc film as the reference in order to show the relative position of coadsorption peaks.

high binding energy side. These intensities appear where the benzene C1s peak and shakeup from the TiOPc are expected; see Figure 4d. The coadsorption spectra were numerically fitted; the fitting parameters for TiOPc are defined in the experimental section and the fitting parameters for the pyridines are determined from the respective monolayer. The pyridine related C1s peaks shift from their respective original positions to lower binding energy, and these shifts are identical to the shifts previously found in N1s. In order to verify the information on the C1s line shape representing the TiOPc layer we also created difference spectra by subtracting the pyridine signals from the total spectra (considering both intensity and position). They are plotted in Figure 4d together with fitted coadsorption spectra from Figure 4a-c. The difference spectra clearly display all the features in the spectrum from the thick film and the development follows the fitted curves excellently. This demonstrates that the experimental spectra from the coadsorption systems in all cases can be reproduced by addition of the spectrum from the pyridine film and the TiOPc film, and that the TiOPc film on top of the pyridine layers retains its unperturbed molecular electronic structure and that the strong interfacial charge transfer observed on the bare substrate is reduced.

Although the line profiles are very similar, their exact positions vary slightly; TiOPc/4TBP C1s is well aligned with the spectrum from the 6 ML film, whereas the TiOPc/4,4′ Bipy C1s is located at higher binding energy. For TiOPc/2,2′ Bipy the peak maximum is at a even higher position. Although the C1s relative positions are different from N1s upon referring to the thick TiOPc film, they follow a similar trend as N1s: (TiOPc/ 2,2′ Bipy) > (TiOPc/4,4′ Bipy) > (TiOPc/4TBP) with a difference around 0.3 eV between TiOPc/2,2′ Bipy and TiOPc/4TBP. This observation supports the idea that the pyridine layers set up different work functions for the TiOPc layer, but the differences clearly indicate that this is not the only factor; the local chemical interaction between the molecular layers plays different roles for N and C. This is not surprising since the interface chemistry is mediated through the nitrogen lone pair (4,4′ Bipy), the methyl groups (4TBP), and the π-system (2,2′ Bipy). Photoemission spectra from the valence band and the Fermi level region are shown in Figure 5. The Fermi level region from the clean TiO2 surface is dominated by a defect induced surface state at 0.9 eV. The valence band maximum (VBM) of the substrate was determined from the photoemission on-set (that is when the derivative in the spectrum + zero) and is found at

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Figure 4. C1s spectra of coadsorption, hυ ) 347 eV: (a) TiOPc/2,2′Bipy, (b) TiOPc/4,4′Bipy, (c) TiOPc/4TBP, and (d) difference spectra representative of the TiOPc contribution, made by subtracting the pyridine signals from the coadsorption spectra and compared to TiOPc/TiO2 system. The dashed line is used to show the relative position of coadsorption system referenced to the thick TiOPc film.

3.0 eV below the Fermi level, in agreement with previous literature.40,41 Adsorption of pyridines results in a slightly increased intensity between the gap state and the VBM, and also an apparent shift of the leading edge of the valence band toward to the Fermi level. Due to expected spectral contributions from molecular orbitals in the same energy region this shift cannot be directly assigned to a VBM shift. However, the bulk O1s core level shifts to lower binding energy (0.1-0.3 eV), with different shifts for the three pyridines as 4TBP > 4,4′ Bipy >2,2′ Bipy. As mentioned above, 4TBP has been shown to increase the open circuit voltage (Voc) in DSSCs, and the reason has been referred to this induced movement of the valence band.20,21 Upon deposition of TiOPc on the pyridine monolayers a new structure appears between the gap state and the valence band edge (between 1.0 and 2.0 eV), which we assign to the HOMO of TiOPc. Comparing all the valence band spectra from the coadsorption systems reveals different HOMO positions on different pyridines. In Figure 5b we summarize and compare spectra from the coadsorption systems with those from 1 ML, 6 ML and thick films of TiOPc on bare TiO2(110). At 1 ML TiOPc coverage no HOMO is observable, which explains the vanishing of the shakeup structure in the core level spectra. At

6 ML the HOMO structure emerges and it gradually shifts to higher binding energy as thickness increases within the molecular film. There is clearly a double structure (0.5-0.6 eV separation), which is not caused by spectral overlap with the gap state. HOMO structure splitting has previously been observed due to mixed adsorption geometries with differently oriented molecular dipole,42,43 but the observed splits were much smaller in that case. Recent unpublished results demonstrate that a HOMO split can be traced to intermolecular interactions within the TiOPc film through the titanyl moiety and the organic ring.44 Hence, it is necessary to point out that the solid markers in the figure identify the high binding energy component of the split HOMO to make the comparison easier. In the bottom panel of Figure 5b, HOMO structures after the surface state peak (0.9 eV) have a broad width in all cases as well, which may also be ascribed to the splitting HOMO structure.42–44 In Figure 6 we plot all of the binding energy positions of the main TiOPc core level peaks and HOMO positions of the coadsorption system with the thick TiOPc film as a reference. The tendency and extent of chemical shifts of core level and HOMO of TiOPc are quite systematic. The reason for these shifts may be either due to initial or final state effects; screening effect or charge transfer induced dipole formation,45 or more

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Figure 5. Valence band spectra from the coadsorption systems, hυ ) 110 eV (a) TiOPc/2,2′ Bipy (bottom), TiOPc/4,4′ Bipy (middle), and TiOPc/ 4TBP (top). (b) HOMO of coadsorbed TiOPc compared with that of different thicknesses TiOPc films on TiO2. The solid markers are indicating the position of the high binding energy HOMO component all spectra for easy comparison.

Figure 6. Core level and HOMO positions in the thin TiOPc film on top of the different pyridines relative to those of TiOPc thick film, data points for N1s and C1s (benzene C) are from Figures 3d and 4d. HOMO data points are the marker points in Figure 5b.

probably a combination of both. The final state screening is indirectly proportional to the distance from the substrate via e2/4r in the interface system and is strongly related with the screening environment.33 The core level and valence band can be expected to be screened differently depending on the local charge distribution.46 In the present case, around 1 ML TiOPc

has been deposited on the saturated pyridine monolayer. Both 4,4′ Bipy and 4TBP are standing on the surface, whereas the 2,2′ Bipy is lying down. So TiOPc is expected to be closer to the substrate when deposited on 2,2′ Bipy than on 4,4′ Bipy or 4TBP. Thus the TiOPc/2,2′ Bipy system should be more efficiently screened, which would result in lower binding energies. However, the opposite is observed here. Comparison of the TiOPc/4TBP and TiOPc/2,2′ Bipy shows that the binding energies of C1s, N1s and VB are about 0.3 eV higher for TiOPc/ 2,2′ Bipy. This type of shift is more likely due to a homogeneous static electric field existing between adsorbate and substrate, a typical characteristic of dipole formation.18,45,47 Thus, for the TiOPc/pyridine coadsorption systems, the interfacial dipole induced by different charge transfers is the main reason for the shifts in core levels and HOMO. On the other hand, molecular interactions can create different charge distributions within the molecules, which in turn, may induce different additional chemical shifts of core level and HOMO. Further detailed analysis of TiOPc/4,4′ Bipy illustrates that there are still small differences between the shifts of C1s, N1s and HOMO. These small differences could be ascribed to the different electronic environments caused by intermolecular interaction. Hence, the energy alignment of this “sandwich” structure can be tuned by preadsorption of pyridine modifiers that create different surface dipoles on binding to the surface and change the work function systematically, thereby putting the TiOPc molecular orbitals at different positions in the TiO2 band gap. Similar modification results have previously been reported for other modifier molecules on a metal surface.17 But, the details of how the molecular interactions influence the properties of this structure must rely on a more detailed theoretical analysis. Conclusions The modification effects of preadsorbed pyridine monolayers (4,4′ Bipy, 4TBP, and 2,2′ Bipy) on TiOPc on the rutile TiO2(110) surface have been systematically investigated. When

CT and Alignment at Organic/TiO2 Interfaces TiOPc is adsorbed directly on TiO2, the first layer of molecules experience a strong charge transfer from molecule to substrate, in agreement with the previous results of FePc on TiO2.23 Surface modifications with all three pyridines lead to a reduction of this effect leaving a layer of TiOPc that has an essentially unperturbed electronic structure. The charge transfer between the pyridine layers and the oxide substrate are different for the three systems, leading to different interface dipoles and corresponding work functions which cause different TiOPc HOMO and core level positions. Acknowledgment. We acknowledge Dr. C. Puglia for the great generousness to support our work. Great thanks go to the kind staff at Max-lab. The Swedish Energy Agency (STEM), the Swedish Research Council (VR), the Go¨ran Gustafsson and Carl Trygger Foundations are kindly acknowledged for financial support. Supporting Information Available: Nitrogen K-edge NEXAFS spectra of 2,2′-bipyridine saturated monolayer on top of rutile TiO2(110), full valence band spectrum of clean TiO2, and complete ref 41. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fahlman, M.; Crispin, A.; Crispin, X.; Henze, S. K. M.; de Jong, M. P.; Osikowicz, W.; Tengstedt, C.; Salaneck, W. R. J. Phys.: Condens. Matter. 2007, 19, 183202–183221. (2) Kahn, A.; Koch, N.; Gao, W. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2529–2548. (3) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539–541. (4) Braun, S.; Osikowicz, W.; Wang, Y.; Salaneck, W. R. Org. Electron. 2007, 8, 14–20. (5) Hamadani, B. H.; Ding, H.; Gao, Y.; Natelson, D. Phys ReV. B 2005, 72, 235302–235306. (6) Crispin, X. Sol. Energy Mater. Sol. Cells 2004, 83, 147–168. (7) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (8) Rosei, F.; Schunack, M.; Naitoh, Y.; Jiang, P.; Gourdon, A.; Laegsgaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F. Prog. Surf. Sci. 2003, 71, 95–146. (9) Nilson, K., Phthalocyanines on Surface, Monolayers, Films and Alkali Modified Structure, Ph.D. dissertation, Uppsala University, 2007. (10) Wang, Y.; Yamachika, R.; Wachowiak, A.; Grobis, M.; Crommie, M. F. Nat. Mater. 2008, 7, 194–200. (11) Zhang, H. L.; Chen, W.; Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 2720–2721. (12) Nath, K. G.; Ivasenko, O.; Macleod, J. M.; Miwa, J. A.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Phys. Chem. C 2007, 111, 16996–17007. (13) Langner, A.; Tai, S. L.; Lin, N.; Chandrasekar, R.; Ruben, M.; Kern, K. Angew. Chem., Int. Ed. 2008, 47, 8835–8838. (14) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031. (15) Sasahara, A.; Pang, C. L.; Onishi, H. J. Phys. Chem. B 2006, 110, 4751–4755. (16) Salzmann, I.; Duhm, S.; Heimel, G.; Oehzelt, M.; Kniprath, R.; Johnson, R. L.; Rabe, J. P.; Koch, N. J. Am. Chem. Soc. 2008, 130, 12870– 12871. (17) Franke, K. J.; Schulze, G.; Henningsen, N.; Ferna´ndez-Torrente, I.; Pascual, J. I.; Zarwell, S.; Ru¨ck-Braun, K.; Cobian, M.; Lorente, N. Phys. ReV. Lett. 2008, 100, 036807-036810.

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