Electronic Structure of Titanylphthalocyanine Layers on Ag(111)

We have investigated the electronic structures of axially oxo functionalized titanylphthalocyanine (TiOPc) on Ag(111) by X-ray and ultraviolet photoelectron spectroscopies, two-photon photoemission, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. Furthermore, we use complementary data of TiOPc on graphite and planar copper phthalocyanine (CuPc) on Ag(111) for a comparative analysis. Both molecules adsorb on Ag(111) in a parallel orientation to the surface, for TiOPc with an oxygen-up configuration. The interaction of nitrogen and carbon atoms with the substrate is similar for both molecules, while the bonding of the titanium atom to Ag(111) in the monolayer is found to be slightly more pronounced than in the CuPc case. Ultraviolet photoemission spectroscopy reveals an occupation of the lowest unoccupied molecular orbital (LUMO) level in monolayer thick TiOPc on Ag(111) related to the interaction of the molecules and the silver substrate. This molecule–metal interaction also causes an...


Introduction
The electronic properties of organic semiconductor interfaces on adequate substrates is of major importance for the development of devices based on molecular electronics. This is the case of organic photovoltaic cell (OPVCs) or organic light emitting diodes (OLEDs). The determination of the binding energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states of organic layers with respect to the Fermi level of the metal substrate is crucial, since these values define the energy barriers for hole and electron injection from the metal to the organic layer. Both, transparent insulating or semiconducting substrates but also metals have to be addressed here. 1 The latter case presents the metal electrode contact of the organic semiconductor in the device. While the growth of these structures is often well known, 2,3 the electronic structure has been less investigated. 4 Here we analyze the electronic properties of titanylphthalocyanine (titanyl-Pc or TiOPc), molecules that are well known as an organic p-type semiconductor and photoconductor, 5,6 that have been widely used in color laser printer toners but also have ability in further optoelectronic applications. [7][8][9] TiOPc is an oxometal phthalocyanine that has an axial TiO group with the oxygen atom protruding from the Pc molecular plane. As a result the molecule contains a strong dipole moment P calculated to have values between 1.43 and 3.73 D, 10,11 which is absent in planar Pc molecules, e.g., copper phthalocyanine (CuPc).
Several studies found that TiOPc on graphite (Highly Oriented Pyrolytic Graphite -HOPG) grows with its backbone parallel to the surface plane and with the oxygen atom pointing away from the substrate. 12-14 The same has been observed for other non-planar Pc molecules like VOPc 15,16 or AlCl-Pc. 17,18 However, some studies devoted to TiOPc on Ag(111) suggested a tilted orientation, 19,20 that could not be confirmed by more recent investigations. 21,22 These latest studies also suggest that the oxygen atom is pointing to the vacuum in the monolayer. Molecules in the second layer grow on top of the first one with the oxygen atom facing towards the surface in order to minimize the dipole-dipole interaction between layers 23 (see Figure 1 for a sketch). The different layer morphologies reveal characteristic molecular arrangements on the surface of Ag(111) ranging from the submonolayer till the completion of the monolayer (ML) and the bilayer formation. Prior to the completion of a full TiOPc ML the molecules display a commensurate phase (c-phase) that develops into a point-on-line (POL) phase when the thickness is close to a full ML.
The POL phase is characterized by a loss of the fixed 2D registry between molecules and the Ag(111) surface, while the unit cell vectors shift along the directions of the Ag atom rows. 24 In this phase the molecules arrange in order to optimize their positions and azimuthal alignment within the layer. At full saturation (1ML) the short-range intermolecular repulsion becomes dominant, which causes an azimuthal reorientation in order to minimize repulsive overlap of neighboring species. 21 The 1 ML POL phase of TiOPc has a molecular area occupation of 199Å 2 , which corresponds to one molecule over nearly 28 Ag atoms. 21 This molecular density is slightly lower compared to CuPc 25 that can compress more and occupy 192Å 2 . In both cases the molecules lie flat on the substrate and the small difference in the occupation is related to slight differences in both POL phases. In the case of TiOPc SPA-LEED analysis suggests a partially loss of registry with the Ag(111) surface, while in CuPc the loss of registry is even larger allowing a higher compression. The unit cell in both POL phases presents a squashed square with nearly identical base vectors and angles close to 90 • . 21,22,25 Therefore, on the hexagonal surface of the substrate, the molecules grow in three rotational domains. The formation of the second layer takes place when the ML is completely compressed and no further molecules can be incorporated in the first layer. 23 All stages in that growth process display characteristic scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) patterns that allow the identification of each phase depending on coverage. In this work we have used these characterization techniques for thickness calibration purposes prior to the electronic structure characterization of the layers.
Here we present a comprehensive analysis of the electronic structure of TiOPc grown on Ag(111) by means of X-ray, ultraviolet and two-photon photoemission spectroscopy as well as X-ray absorption and partially compare these results to the same molecule on a graphite (HOPG) surface, where the molecule-substrate interaction is reduced, and to flat CuPc molecules on Ag(111). We observe that the first TiOPc layer is weakly chemisorbed to Ag(111) confirming the oxygen-up orientation. On the other hand, already the second layer displays the same electronic properties as thick TiOPc films. The position of the characteristic HOMO level of the first monolayer adsorbed on Ag(111) still persists at the same energy due to the charge transfer between the molecular layer and the substrate independent of the additional layers evaporated on top of it. Additionally, a gradual energy shift of the HOMO level during the completion of the second layer is observed, which seems to be directly linked with the progressive change observed in the work function. At the same time the LUMO level that was found to be occupied in the monolayer range is vanishing with increasing thickness. From X-ray photoemission spectroscopy (XPS) data, we find that the strongest interaction of the molecules is by the nitrogen and carbon atoms. The axial Ti atom interaction with the substrate is small but not negligible. X-ray absorption spectra reveal a flat-lying configuration of the TiOPc molecules on Ag(111) excluding a tilt angle phase arrangement, a result that confirms recent investigations. 21,22 This spectroscopy technique also confirms the partial occupation of the LUMO level in the monolayer case. X-ray magnetic circular dichroism experiments of the ML film reveal the absence of paramagnetism on the Ti atom. Furthermore, two-photon photoemission spectroscopy indicates the transformation of the Ag(111) Shockley state into an unoccupied interface state after absorption of TiOPc which shifts up in energy when going from a TiOPc mono-to a bilayer.

Methods
The photoemission (PE) experiments were performed at the Philipps-University of Marburg (Germany) while the X-ray absorption experiments were taken at the BOREAS beamline of ALBA synchrotron in Barcelona (Spain). Both experimental systems were equipped with Low Energy Electron Diffraction (LEED) to check for the ordering of the molecular layers.
The Ag(111) crystal was prepared by repeated cycles of sputtering (700 V) and annealing (500 • C) until the Ag(111) surface state was well observed (Marburg) and the Scanning Tunneling Microscope indicated large flat terraces (Barcelona).
TiOPc was synthesized by the reaction of 1,2-dicyanobenzene and tetrabutoxy titanium modifying the established workup procedure. 26 Purification was accomplished by extensive continuous hot-extraction of by-products by dichloroethane and acetonitrile (80 • C, 2 h each).
The remaining microcrystalline product was degassed at 400 • C under vacuum conditions (10 −6 mbar) for 4 h and analyzed by Electron Ionization Mass Spectroscopy (EI-MS), combustion analysis and Attenuated total reflection Infrared Spectroscopy (ATR-IR). The TiOPc molecules were then further purified in UHV by holding the evaporator temperature approx. 50 K below the deposition temperature for several weeks. This procedure ensured highly pure TiOPc molecules during deposition. Then TiOPc and CuPc (Aldrich 99%) were deposited by a very small deposition rate (1-2 ML/hour) with the sample hold at 100 • C in order to facilitate ordering of the layers. The base pressure of the system was 3×10 −11 mbar and rised to 6×10 −11 mbar during deposition. The layer thickness was obtained by a combined analysis of the channelplate based Low Energy Electron Diffraction (OCI Vacuum Microeng. Inc.), evaporation time, and X-ray photoemission spectroscopy analysis of the attenuation of the substrate Ag 3d core level emission in Marburg and by Scanning Tunneling Microscopy in Barcelona. XPS and UPS were obtained by Specs XR50 Al K α emission and with a VG Microtech UVL-HI UPS/1 Helium lamp, respectively. The features due to the He I satellites were removed by numerical methods whose parameters were obtained for thick molecular films observing the Ag low density of states region between 4 eV binding energy and the Fermi level. The work function and the two-photon photoemission data were measured with low energy photons produced independently from visible and ultraviolet photons delivered by an optical parametric oscillator (OPO) setup. X-ray absorption (XAS) data were taken as a total yield signal normalized to a gold mesh absorption inserted in the beamline. The light incidence angle was 20 • with respect to the surface for carbon and nitrogen 1s absorption using horizontal and vertical polarized light. The measurement temperature for the normal XAS experiment was 300 K. For the Ti 2p measurement the sample was cooled to 3 K and rotated to normal incidence. In the last case for the X-ray magnetic circular dichroism experiment left and right circular polarized light was used and the magnetic field was applied perpendicular to the sample and along the incident photon propagation direction.
STM images were taken both at Philipps-University of Marburg and at ALBA synchrotron of Barcelona using Omicron VT and Specs Aarhus instruments, respectively.   Additionally, the STM image shows a large area with bilayer formation together with a patch of the compressed ML. For comparison TiOPc layers were grown on a graphite (HOPG) surface, which is well known for a very weak interaction with the molecular layers. The HOPG substrate consists of micron-size crystallites having different rotational orientations but a common perpendicular axis. This arrangement give rise to ring like LEED patterns of the pure substrate. One, therefore, also expects ring-like diffraction structures of TiOPc molecules grown on this substrate. The ring diameters are very similar to the LEED spots of the same molecules on Ag(111), taking into account the rotational domains of the HOPG substrate and reveal only minor differences between 1 and 2 ML coverage.  is rather small, therefore, molecule-molecule interactions dominate in this system. 13 On a more reactive metal surface like TiOPc/Ag(111), the molecule-metal interaction is stronger, which is reflected in shifts of the molecular core levels in the mono-and submonolayer range.

Structural analysis
Therefore, we will first consider the weak interacting molecule adsorption on HOPG. In the TiOPc/HOPG system one observes a rather broad peak at E B = 399 eV that changes its intensity according to the thickness but does not present a core level shift. The broad peak is produced by the sum of two contributions of non-equivalent nitrogen atoms within the Pc ligand with the lower/higher binding energy peaks corresponding to the four nitrogen atoms in the iminic/pyrrolic position, respectively. The individual peaks cannot be resolved due to the linewidth of the peaks and to the resolution of the experimental system. In FePc the peak separation was theoretically calculated to be 0.3 eV. 28 In order to observe both contributions, a peak fit analysis has been carried out here by using two individual Doniach Sunjic peaks 27    shift to lower binding energies is observed with respect to the thick layer case. For ML and sub-ML coverage, a further core level shift is observed, that amounts to 0.3 eV with respect to the thick layer. In the case of CuPc the multi-to monolayer shift is identical to TiOPc films. The C 1s core level analysis of the molecules on HOPG is complicated due to the overlapping of the substrate and molecular carbon atom emissions and was not carried out. like Fe or Co in FePc or CoPc, core level shifts of more than 1 eV have been found. 32,33 In the last cases similar to iron or cobalt tetraphenylporphyrines (FeTPP, CoTPP) an Fe-or Co-Ag bond is formed that can be well described by the surface trans effect. 34 In CoPc, as a result of such a strong interaction on a surface, the Co atom loses the gas-phase magnetic  moment. 35 We can compare the small core level shift for Ti in TiOPc/Ag(111) also to the V 2p core level shift in vanadyl-Pc (VOPc) on Ag(111) that was found to be 0.2 eV between 1 and 10 ML thickness. 36 On the contrary, VPc arguably the planar counterpart to VOPc, presents a core level shift of 0.9 eV between the multilayer and monolayer film. 37 The latter result signifies a much stronger interaction of the V atoms with Ag(111) than in the case of VOPc. In the VOPc case as well as for TiOPc the strong V=O and Ti=O bond and a consequent small V-Ag and Ti-Ag interaction are considered responsible for the observed minor shifts.

Lowest unoccupied molecular levels of TiOPc/Ag(111)
The inner nitrogen-containing chromophore of the phthalocyanine ligand as well as carbon atoms will largely determine the frontier orbitals 21 and, thereby, the optical absorption of observe that the first peak has a shoulder on the low energy side, which is indicated by an arrow and by the dotted line below the peak. The peak separation from the leading peak is approx. 0.8 eV. For F 16 CuPc and CuPc on Ag(111), such a doublet was explained by the lifting of the LUMO degeneracy. 30 Upon charge transfer from the substrate, the electronic energy can be minimized by splitting the two-fold degenerate e g LUMO into a partially filled F-LUMO and an empty LUMO levels. We ascribe our observation for TiOPc/Ag(111) to the same effect.
The O 1s absorption spectrum consists of a doublet peak at 529 and 530 eV that is attributed to the transition from the O 1s orbital to Ti=O π * and σ * orbitals, respectively. 36 Both peaks reveal an angular dependence, and confirm that the Ti=O bond is mainly oriented perpendicular to the surface. The main feature in the O K-edge XAS near 529 eV is designated as a transition from the O 1s core level to molecular LUMO+1 orbitals. The LUMO state in TiOPc/Ag(111) mainly consists of C and N 2p unoccupied orbitals, 21 and there is no direct interaction between the O ion and the Pc ligand, similarly as in VOPc. 36 Therefore, transitions to the LUMO levels are suppressed at the O K-edge. The O 1s XLD curve resembles very well that observed for 1ML VOPc/Ag(111), which was assigned to molecules with oxygen-up orientation. In a hypothetical oxygen-down configuration like in VOPc/Si(111), the π * peak would disappear due to the diminishing of the transition metaloxygen bond caused by surface atom-oxygen bond. 36 Figure 4(c) shows the XAS at the Ti 2p adsorption edge for circularly polarized light at normal incidence geometry and with a 6 T magnetic field applied perpendicular to the surface. The shape of the adsorption spectra was calculated in an early work of de Groot. 40 The Ti 2p 3/2 adsorption edge of multilayer TiOPc reveals a multi-peak structure of 4 peaks that come from the lifting of degenerated e g and t 2g levels, while the Ti 2p 1/2 edge has less structural features due to lifetime broadenings. 12,41 The Ti 2p 3/2 emission of the TiOPc ML shown here also consists of 4 peaks and is very similar to the more recent TiOPc publication 41 from which the multiplet scheme used in the figure was taken. Similarly as observed in the case of oxygen, for Ti one expects transitions into the LUMO+1 level rather than the LUMO due to the very weak Ti contributions to the latter. 21 As detailed in the inset in Figure 4(c), we observe a very faint pre-peak emission 0.7 eV below the leading peak marked by an arrow in the inset that coincide well with the calculated LUMO -LUMO+1 distance. 21 We assign this small feature to a weak transition into the LUMO level.
The Ti 4+ ion in TiOPc has a d 0 electronic configuration that would lead to a diamagnetic behavior. 42,43 In order to probe the magnetic properties, in the lower part of Figure 4(c) we show the X-ray magnetic circular dichroism signal taken at 3K and 6 T applied field. The same measurement on the Cu or V 2p adsorption edge of CuPc 44 or VOPc 36 results in XMCD spectra that amount to approx. 20% of the total XAS peak height. Here in TiOPc the relative height of the XMCD signal is lower than 3%. Therefore, we interpret this behavior as the absence of paramagnetism of the Ti atom as predicted earlier. In contrast, the V 4+ ion in VOPc has a d 1 electronic configuration. The presence of an unpaired 3d  CuPc 25,30 or SnPc 57 on Ag(111). For the peak-fitting process shown in Figure 6(b) we furthermore had to include a peak labeled as Kondo-peak at the Fermi energy position for a correct fitting. This last contribution has been observed previously on CuPc/Ag(111) 25 and derives from the many-body Kondo resonance that can arise from the interaction of localized spins, in this case that of the unpaired F-LUMO electron, with the surrounding electron sea of the metallic substrate. 25,58 In TiOPc such a peak was not necessary to include. From the fit we observe that the F-LUMO level measured in CuPc/Ag(111) coincide with previous results 25,58 and is found at 0.16 eV binding energy. In TiOPc/Ag(111) the peak shifts to slightly higher binding energy at 0.26 eV. Due to the large linewidth of the LUMO emission peak (500 meV), the error bar is in the same range as the peak shift. Nevertheless, also X-ray standing wave experiments revealed a smaller TiOPc molecular backbone-Ag distance than in CuPc/Ag(111). This means that the -¿ interaction between TiOPc and Ag(111) is stronger than that between CuPc and Ag(111) and explains the higher binding energy in TiOPc.

TiOPc-Ag interface state
Time-and angle-resolved two-photon photoemission experiments on PTCDA/Ag(111) found an unoccupied interface state and concluded from the dispersion and the rather short inelastic lifetime of that state, slightly above E F , that it must originate from the Shockley state of the bare Ag(111) substrate, which is upshifted from below the metallic Fermi level by as much as 0.7 eV due to the interaction with the molecular layer. 59,60 This interpretation was subsequently confirmed by density-functional theory (DFT) calculations. [61][62][63][64][65][66] In the meantime, additional molecules, e.g., NTCDA, 63  This points to an electronic corrugation of the interface state caused by the phthalocyanine molecules. The interface state is sensitive to the first angstroms near the metal-organic interface. 83 Consequently, a decrease in molecule-metal distance, which is indicated by the increasing state energy, could enhance the corrugation of the interface state, leading to a higher effective mass. The polar titanyl group of TiOPc might act as an additional distancedependent source of corrugation and, indeed, even if the state energies are taken into account the effective masses measured for TiOPc seems to be slightly higher than those observed for H 2 Pc or FePc. Nevertheless, the experimental uncertainties limit a closer comparison.

Conclusions
The electronic structure of non-planar TiOPc molecular layers on Ag (111)

Supporting Information Available
The Supporting Information contains information on the determination of the value of the dipole moment of 1 ML TiOPc. This material is available free of charge via the Internet at http://pubs.acs.org/.
Spanish Government for financial support through PTA2014-09788-I fellowships. ICN2 is funded by the CERCA Programme / Generalitat de Catalunya. We additionally acknowledge BOREAS beamline staff, especially Hari Babu, and ALBA for provision of synchrotron radiation and for synchrotron access funding.