Evolution of the Electronic Structure at the Interface ... - ACS Publications

8654

J. Phys. Chem. C 2008, 112, 8654–8661

Evolution of the Electronic Structure at the Interface between a Thin Film of Halogenated Phthalocyanine and the Ag(111) Surface L. Giovanelli,*,†,‡ P. Amsalem,†,‡ J. M. Themlin,†,‡ Y. Ksari,†,‡ M. Abel,†,‡ L. Nony,†,‡ M. Koudia,†,‡ F. Bondino,§ E. Magnano,§ M. Mossoyan-Deneux,†,‡ and L. Porte†,‡ Aix-Marseille UniVersite´, IM2NP, Centre Scientifique de Saint-Je´roˆme, AVenue Escadrille Normandie-Niemen Case 142, F-13397 Marseille CEDEX 20, France, Centre National de la Recherche Scientifique, (CNRS), IM2NP (UMR 6242), Marseille-Toulon, France, and TASC National Laboratory, INFM-CNR, SS 14, km 163.5, I-34012 Trieste, Italy ReceiVed: January 7, 2008; ReVised Manuscript ReceiVed: February 20, 2008

The electronic structure of a thin film of chlorine-substituted Zn-phthalocyanine (ZnPcCl8) deposited on Ag(111) was studied by valence band photoemission and near-edge X-ray absorption fine structure spectroscopy. At the first stages of molecular adsorption the interface formation is accompanied by the promotion of intense interface states. A charge transfer from the substrate to lowest unoccupied molecular orbital (LUMO) of the molecules of the first layer appears as a new density of states close to the Fermi level in photoemission. The central role of the LUMO is corroborated by the reduction of the LUMO-derived transition observed in the low-coverage X-ray absorption spectrum taken at the N 1s edge. However, the lack of a sizable density of states at EF as well as the photon energy dependence of the interface state peak intensity suggest that more complicated mechanisms of intra- or extramolecular charge redistribution may also play a role. The Ag 4dderived valence band region also appears strongly modified upon molecular adsorption. In the paper it is shown that these modifications are merely due to the vanishing of the substrate surface states and the emerging of a bulk density of states at the interface. It is suggested that such behavior should be common for other organic monolayers adsorbed on noble metal surfaces. Finally, the study of the work function as a function of coverage is presented. Its behavior is interpreted in terms of coexistence of different structural phases and compared to a local-probe work function study on the same system. I. Introduction The study of the interface formation between thin films of organic molecules and well-ordered inorganic surfaces has stimulated an increasing scientific interest over the past decade. It is now well-established that the performance improvements of future electronic devices based on organic molecules will strongly depend on the advances made in the fundamental understanding of the physical phenomena taking place at these interfaces and during the organic thin film growth. These include, for instance, charge injection mechanisms between metal electrodes and organic layers, conducting properties, carrier recombination, etc.1–3 The relevance of conjugated organic molecules as new materials for (opto-) electronics, gas-sensing, and energy conversion comes from their wide range of physical properties and the possibility of tuning them during chemical synthesis, by acting, for instance, on their functionalization or by doping.4 More recently, the possibility of designing original lowdimensional systems based on self-assembled functionalized organic molecules has further stimulated the surface science community in getting a deeper understanding of the intermolecular and molecule-substrate interaction.5–7 When the molecules are deposited on the metal surface the interaction mechanism and the resulting physical properties of the hybrid system depend on a number of factors, such as the metal/ * Corresponding author e-mail: [email protected]. † Aix-Marseille Universite ´. ‡ Centre National de la Recherche Scientifique. § TASC National Laboratory.

molecule energy level alignment, the formation of molecular resonances and their interaction with the metal continuum, the promotion of new covalent bonds often accompanied by charge transfer from or to the metal, and the consequent charge redistribution across the molecular unit.8–10 In the scientifically relevant case of π-conjugated molecules such as C60, phthalocyanine (Pc), oligocenes, and perilene or naphthalene-derivatives such as PTCDA or NTCDA (to cite the more studied ones), the molecules of the first layer generally adsorb on the noble metal surface through a rearrangement of their π-electron system.3,11–13 The result is an ordered superstructure in which the commensurability to the substrate lattice is often accompanied by molecular distortion.14 Finally, dynamical aspects such as interface electron-vibron coupling have been shown to be decisive to determine the molecular adsorption site.15 The very rich picture of molecular self-assembly has stimulated the study of model systems with a slowly increasing degree of complexity. Following that purpose, a detailed study of zincphthalocyanine (Zn-Pc) derivatives deposited on Ag(111) has recently been undertaken.7,16–18 By substituting eight outermost hydrogens with halogen atoms (Cl and F), the two-dimensional self-assembling properties reveal a rich behavior through the appearance of various 2D phases. Most interestingly, when Cl was used as a peripheral substituent, the two-dimensional molecular structure shows a structural evolution. Ordered domains are observed from the earliest stages of molecular adsorption together with a disordered “gas” phase. As a function of time, the ordered domains change to two more compact phases, at the end increasing the packing density by 35%. With

10.1021/jp800116j CCC: $40.75  2008 American Chemical Society Published on Web 05/21/2008

Halogenated Phthalocyanine and the Ag(111) Surface

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8655 runs are presented and discussed in Section III. In the last part of the paper we summarize the main results. II. Experiment

Figure 1. The schematic representation of the ZnPcCl8 molecule.

the help of density functional theory (DFT) calculations, such evolution was rationalized in terms of a sequential intermolecular hydrogen-bond formation between the Cl and the hydrogens of neighboring molecules. By following the structural evolution in real-time for several hours it appears that the transformation between the phases happens through the disordered phase that is always present on the surface. The appearance of regularly spaced fault lines in the most compact phase suggests a competition mechanism between molecule/substrate and intermolecular interactions, resulting in the relaxation of the molecular film structure needed to minimize the energy. In this work we report on the evolution of the electronic structure of a thin film of ZnPcCl8 (Figure 1) grown on Ag(111). We compare the occupied and unoccupied electronic states measured when the surface coverage was less than one monolayer (sub-ML) to those of a thin film of increasing thickness. Their evolution allows to identify some interface-related features and to follow their modification in reaching a thickness at which the probed molecules are not interacting with the substrate anymore. The study was performed in two runs. In the first run the valence band (VB) and the surface work function (WF) were studied by UPS from the earliest stages of adsorption up to a thick molecular layer by gradually increasing the molecular layer thickness. In the second run we used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to measure the unoccupied states of a sub-ML, and we compare them with spectra characteristic of a thick film. Our results show that the molecular adsorption is accompanied by the formation of some intense interface states: a charge transfer from the substrate fills in a previously unoccupied molecular orbital with a possible hybridization with the substrate states. Accordingly, NEXAFS measurements showed a reduction (filling) of the lowest unoccupied molecular orbital (LUMO) and a redistribution of higher unoccupied states. The UPS spectrum of the 4d region of the Ag(111) valence band shows important changes after the molecular adsorption due to the vanishing of the substrate surface states. The changes of the surface WF as a function of coverage and the comparison with local WF measurements suggest that the coexistence of different phases is important in this system. All these observations helped in getting further insight into the mechanism of interface formation between a prototypical metal and a planar conjugated multifunctional organic molecule. The paper is organized as follows; in Section II we give the details of the experiments; the results of the two experimental

Two distinct runs were performed with two separate experimental apparatus. High-resolution VB photoelectron spectroscopy (UPS) as well as low-resolution core level spectroscopy (XPS) for thickness calibration were done in a combined STMphotoemission setup. Near-edge X-ray absorption fine structure (NEXAFS) was performed at the BACH photoemission endstation at the ELETTRA synchrotron radiation facility in Trieste.19 Both experiments were performed under ultrahigh vacuum (p ≈ 3.10-10 mbar), and particular care was taken to reproduce the experimental conditions as closely as possible. The Ag(111) surface was prepared by repeated cycles of Ar+ sputtering and annealing at 730 K. Its cleanliness and crystalline arrangement have been checked by UPS, XPS and low-energy electron diffraction (LEED). ZnPcCl8 molecules were gradually vapor deposited at a rate of about 0.03-0.05 nm/min at a pressure never rising above 10-9 mbar. The UPS and XPS spectra were recorded in normal emission geometry with a 125 mm radius hemispherical analyzer. All binding energy (BE) values are referred to the Fermi level (EF) of a Ta foil in contact with the sample. The VB spectra were performed by using a He discharge lamp (hν ) 21.2 eV or hν ) 40.8 eV), recorded with an acceptance angle of 8° and an energy resolution of about 100 meV limited by the thermal broadening. All VB spectra were recorded with the sample biased at -5 V with respect to the analyzer in order to detect the low kinetic energy cutoff from which the WF value was inferred. The contributions from the He I and He II satellite lines have been subtracted. The average film thicknesses were estimated from the attenuation of the Ag 3d core level signal obtained by using the Mg KR line of an X-ray source (hν ) 1253.6 eV) and a mean free path for the photoelectrons of 2.2 nm.20,21 In discussing the data we will often refer to the film thickness in terms of monolayers. This is obtained by assuming that in the present case the molecules adsorb with their molecular plane parallel to the substrate surface and taking a value of 0.4 nm for the interplanar distance.22 More reliable estimation is inferred from the details of the photoemission spectra. The NEXAFS spectra at the N K-edge of nitrogen were recorded in partial yield with the electron analyzer tuned on kinetic energies around 375 eV to collect the N KLL Auger electrons. The X-rays were polarized in the horizontal plane, and the light propagation vector was in grazing incidence (15° with respect to the sample surface). In this configuration the electric field vector was almost perpendicular to the surface normal. To effectively account for the (nonuniform) incident photon flux, the raw spectra were divided by the average current on the last mirror of the beamline and by the spectrum of the clean Ag(111) substrate.23 III. Results and Discussion Valence Band. Figures 2 and 3 show the evolution of the VB photoemission spectra of a thin film of ZnPcCl8 adsorbed on Ag(111) for nominal film thicknesses increasing from 0.3 to 2.7 nm. The same set of spectra is displayed in both figures for different energy regions to ease the analysis and discussion. The bottom-most spectrum represents the energy distribution curve (EDC) of the clean Ag(111). The intense density of states between BE of 4-8 eV is mainly due to the Ag 4d bands with some sp contribution. The almost flat density of states between the Fermi level and about 4 eV binding energy is primarily due

8656 J. Phys. Chem. C, Vol. 112, No. 23, 2008

Figure 2. He I, wide-range valence band UPS spectra of ZnPcCl8/ Ag(111) as a function of molecular film thickness. Some interface states around the Ag 4d band are indicated with vertical lines.

Figure 3. He I, valence band UPS spectra of ZnPcCl8/Ag(111) as a function of molecular film thickness. Fermi level region; thick markers evidence the HOMO of the thick film molecules and of the interface molecules.

to electrons having sp character but also with some 4d contribution.24 Looking at Figure 2 we observe that at the earliest stages of molecular adsorption (0.3 nm) the highest feature of the 4d silver band (BE∼4.8 eV) appears reduced to about half of its intensity (note that the two bottom-most spectra are

Giovanelli et al. normalized in intensity). Furthermore, an increased spectral weight appears at higher BEs together with some new narrow features (indicated by thin vertical lines) that increase in intensity for a thickness of 0.6 nm. The clearest of these features is at the low BE side of the 4d band at 4.2 eV, and three other develop at higher BEs: 5.6, 6.2, and 6.9 eV. Because their intensity relative to the remainder of the VB decreases for thickness higher than 0.6 nm (dashed lines), it would be natural to assign these features to electronic states localized at the interface between the molecular film and the metal surface. However, taking into account the photon energy dependence of these features will lead us to a different conclusion about their nature (see Photon Energy Dependence of Interface Features subsection). For higher thickness, broader molecular features grow up in this energy region, gradually reaching those of the thick film. At the highest coverage the substrate and the interface-related features are no longer visible (see also the EDC at EF in Figure 3); because the intermolecular interactions are expected to be weak (essentially van der Waals interactions), the spectrum is representative of isolated molecules.25 The molecule-derived less bound states, namely HOMO, HOMO-1, and HOMO-2 of the isolated molecule are observed between 0 and 4 eV BE. As the film thickness is increased they grow in intensity over the substrate-related background. Apart from a slight shift (see below), their line-shapes do not seem to be affected by the interaction with the substrate. Figure 3 shows the region close to EF. The VB in this region is modified after the first deposition (the spectrum of the clean substrate is superimposed for comparison). There is an overall enhancement of the EDC, and a new density of states grows up as a strong spectral feature centered at 0.4 eV. On the basis of the evolution at higher coverage, it is clear that this feature is distinct from the electronic states originating from the HOMO orbital of the isolated molecule, which form a spread feature between 1.1 and 1.6 eV. When the thickness is raised beyond 0.6 nm, the intensity of the new feature close to EF diminishes and eventually disappears. In accordance with previous argument, this behavior is the clear sign of an interface character. To explain this feature, we favor the hypothesis of a charge transfer from the metal substrate to the first molecular layer with the consequent creation of occupied interface states at the expense of unoccupied molecular states. Such a charge transfer can, in fact, be explained in terms of the chemical potential difference between the bare substrate (µ ) -WF) and the isolated molecule (µ ) -(IP + EA)/2).26 For the clean Ag(111) we measured µAg ) -4.55, whereas for the isolated molecule DFT calculations report µmol ) -4.8 eV.27 When the molecules are brought in contact with the surface the systems react to equalize their chemical potentials. This is performed through a charge transfer from the system with the higher chemical potential to the one with the lower,28 that is, from the surface to the molecules. In other words, the observed charge transfer from Ag(111) to ZnPcCl8 is energetically favorable at the interface, and the alignment of the Fermi energies induces a sizable interface dipole. The relevance of such a charge transfer will be corroborated later on through the reduction of the LUMO-derived transition in the low-coverage NEXAFS spectrum. This charge transfer may equally well be described in the framework of the recently introduced “induced density of interface states” model.29 Within this approach, the sign and amount of charge transfer at a metal-organics interface are determined by the relative positions of the bare metal work function φM and the charge neutrality level (CNL) of the organic

Halogenated Phthalocyanine and the Ag(111) Surface semiconductor, referenced to a common vacuum level. The position of the charge neutrality level of the organics is not known a priori and is not readily accessible experimentally. Nevertheless, because the terminating chlorine atoms of ZnPcCl8 tend to increase the electronegativity of the molecule, it is reasonable to infer that both the ionization energy and the electron affinity have rather high values with respect to the rather low value of the Ag work function. The CNL, which is located between HOMO and LUMO, is therefore expected to lie rather deeply. Interestingly, from Figure 3, no increased DOS is observed at the EF, testifying that the system remains insulating whatever the coverage. This is also the case of thin films of M-Pc doped with alkali metals, where, despite a charge transfer of up to four electrons per molecule, no metallic states could be detected by photoemission.30,31 Electronic correlation effects resulting from the large values of the (effective) Coulomb hole-hole interaction with respect to the very small valence bandwidth values (i.e., U/W . 1) may explain such a behavior.32 Other scenarios are also plausible, such as, for example, the redistribution of the transferred charge in other molecular states (different from the LUMO), possibly involving Ag states. A charge-transfer from the substrate was previously observed for another halogenated M-Pc, namely, CuPcF4 on Ag.21 We note, however, that in the present case the intensity of the newHOMO is quite high if compared to the new-HOMO-1. To appreciate the amount of charge transferred from the 0.3 nm spectrum is not straightforward. In a first approximation the charge transferred should be proportional to the spectral weight of the new feature close to EF. By assuming that the new-HOMO and the new-HOMO-1 give a similar spectral response one could extract the occupation of the new-HOMO from the ratio between the two π-derived states.33 Nevertheless, because the newHOMO-1 of the first layer is sensibly broadened by the interaction with the substrate, this procedure is somewhat difficult. As we will see later in the paper, the task is further complicated by the presence of final-state photoemission effects. Photon Energy Dependence of Interface Features. As pointed out at the beginning of this section, the formation of the interface promotes several features in the region of the Ag 4d states. A similar behavior is not uncommon in metal/organic interfaces. The presence of new electronic states at the interface between an organic single layer and a noble metal substrate were recently observed on a number of similar systems.34–37 In the case of smaller molecules like aromatic-thiol on Cu(100), with the help of ab initio calculations,38 bonding and antibonding states were identified in the photoemission spectrum.39,40 Nevertheless, the origin of the interface states observed with UPS is not always easy to unveil. To point out the presence of substrate-related features we performed UPS measurements of the clean Ag(111) and about 2 ML (0.7 nm) of ZnPcCl8/Ag(111) with He I (21.2 eV) and He II (40.8 eV) radiation. The results are displayed in Figure 4. The spectra are presented in couples according to the excitation energy employed. We will first address the results of the wide range-spectrum (Figure 4a), concentrating on the changes occurring in the 4-7 eV BE range. Because of band dispersion in k⊥ and because of different surface sensitivity, changing the photon energy from He I to He II strongly changes the EDC of the clean Ag(111). When the molecules are deposited on the surface, new structures at 4.2 and 6.9 eV develop, and their intensity appears strongly reinforced when He II radiation is employed. The effect is less striking on the structures around 5.6 and 6.2 eV, whose intensity increases only slightly when the photon energy is increased.

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8657

Figure 4. Valence band UPS spectra of the clean Ag(111) (thick lines) and ∼2 ML ZnPcCl8/Ag(111) (thin lines) taken with He I (top) and He II (bottom) radiation. (a) wide range spectra; (b) Fermi level region.

Figure 5. Valence band spectra of clean Ag(111) (symbols) taken with X-rays (courtesy of G. Panaccione23) and ∼2 ML ZnPcCl8/Ag(111) (thin line) taken with He II radiation.

What is the origin of these structures? In Figure 5 we present a comparison between the spectrum of 2 ML ZnPcCl8/Ag(111) taken with He II radiation and the spectrum of a clean Ag(111) sample taken with a Mg K-R X-ray source (1486 eV), which essentially reveals the bulk DOS of the silver crystal.24 The two spectra have been normalized to the intensity of the peak at 4.2 eV. Apart from the region below 4 eV, where the moleculerelated features superpose on the flat Ag sp band, and considering a different background due to higher lying molecular states and secondary electrons, the resemblance between the two spectra is striking. Particularly, the energy position, line-shape, and the relative intensities of the 4d band region are very similar. This proves that the surface DOS of Ag(111) is strongly quenched by the presence of the molecular layer and that the observed interface states detected right after molecular adsorption are, in fact, Ag bulk states.

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Giovanelli et al.

TABLE I: Photo-ionization Cross Sections in Mbarns of the Valence Band Atomic Orbitals of the ZnPcCl8/Ag(111) System atomic orbital

σ (He I)

σ (HeII)

C 2p C 2s N 2p Cl 3p Ag 4d Ag 5s

6 1.2 10 14 17 0.030

2 1.2 4 0.5 37 0.035

We now examine the photon energy dependence of the first interface state at 0.4 eV that we interpreted as the filling of the LUMO. Figure 4b displays the energy region close to the EF taken with different photon energies. Again, the intensity of the spectra was normalized to facilitate their comparison. Surprisingly, when He II radiation is employed, the new spectral intensity close to EF is quenched, whereas the one at about 1.2 eV BE is still present. We tried to interpret such effect in terms of a reduction of the atomic photoionization (PI) cross-section for the less bound states but it soon became evident that such thesis is not viable. In Table I we report the calculated values for the PI cross-sections of the atomic orbitals participating in the molecule and substrate VB. In fact, the decreasing of intensity of the feature at 0.4 eV in going from He I to He II suggests that the Ag 4d levels (for whom the PI cross-section increases) have a minor contribution to this state. On the other hand, the atomic composition of the HOMO and LUMO of the isolated molecule is quite similar, with the molecular orbitals well-delocalized over the macrocycle but with very low contribution from the Cl atoms. Consequently, if the newHOMO intensity were quenched because of a PI cross-section reduction, then that should happen for the new-HOMO-1 as well. However, this is clearly not the case. Other possible effects that can modify the intensity of the photoemission spectra are related to interference effects of the photoelectrons. In the case of fullerenes single layers,41,42 multilayers,43 and even isolated molecules,44–46 the HOMO/HOMO-1 intensity ratio oscillates considerably over a large photon energy range. The origin of the observed phenomenon was shown to be the interference of photoelectrons originating from different carbon atoms, whereas its amplitude and photon energy dependency are ascribed to the spherical shape and large size of the molecule. Very recently, however, the combination of gas-phase photoemission and firstprinciple calculations revealed a similar effect on smaller molecules of reduced symmetry such as bis(cyclopentadienyl)magnesium,47 concluding that the observed oscillations are in fact a widespread phenomenon potentially concerning other organic molecules. It is therefore possible that the strong variation of the HOMO/HOMO-1 ratio observed for the ZnPcCl8/Ag(111) interface may be due to a photoelectron interference effect. Yet another explanation is related to a possible intra- or extra-molecular charge rearrangement following molecular adsorption, evoked above in order to explain the lack of a sizable DOS around EF for the first adsorbed layer. Under this hypothesis, the shallower and prominent interface states could derive from other specific molecular states, such as, for example, the normally unoccupied Zn 4p. A more accurate investigation of the HOMO/HOMO-1 cross-section ratio as a function of the photon energy may be helpful in determining the atomic and orbital origin of the first interface state. Unoccupied States. Figure 6 displays the X-ray absorption spectra of a multilayer (top) and a sub-ML of ZnPcCl8/Ag(111). The NEXAFS spectra are closely related to the density of

Figure 6. N-K edge NEXAFS spectra of a thick layer of ZnPcCl8 (top) and a submonolayer of ZnPcCl8/Ag(111) (bottom).

unoccupied states of the system, under the additional influence of a core hole. More precisely, because it concerns transitions from core levels, the technique is chemically sensitive and probes the empty states around a given atomic species. This picture is complicated by the presence of multiple nonequivalent atomic sites having different participation to different unoccupied orbitals.48 In the case of N K-edge of M-Pc molecules having a 4-fold symmetry axis (D4h or C4V), one cannot separate aza and pyrrole atoms, and the final result is a mixture of the two transitions. For the multilayer spectrum, the first intense transition is to the LUMO of the isolated molecule. It occurs at about 398.1 eV, well below the N 1s BE (dashed line), testifying an important core-exciton energy. At slightly higher photon energy, an asymmetric structure likely includes two separate transitions to higher occupied molecular orbitals. Kera et al. have recently shown that the final-state relaxation energies are very important in these systems, and consequently, the interpretation of the NEXAFS spectra is better accomplished with the help of a robust theoretical support.48 Nevertheless, as for other Pcs30,48,49 the first two features have π character, are oriented perpendicularly to the molecular plane, and accordingly, since the molecules are known to adsorb flat on the surface, they are expected to be sensitive to the interface formation. It is interesting to note that the spectrum of the multilayer is virtually identical to the one of ZnPc.30 This indicates that the electronic structure close to the molecular center (where the N atoms reside) is unperturbed by the presence of the Cl atoms at the periphery. It is also important to stress out that the molecular orientation in the thick film does not affect the intensity ratio between the first two features in the NEXAFS spectrum. The spectrum of the sub-ML appears noticeably different from the multilayer. One easily recognizes all the main features, but there is a general redistribution of their intensities and relative energy positions. Most remarkably, the intensity ratio between the first two features has considerably changed, with the first one being reduced with respect to the second. The net disappearance of a significant part of the unoccupied states, especially around the LUMO, corroborates the charge-transfer scenario. The different trend of the two spectra in the pre-edge region is due to the fact that the two spectra are superposed to a different background. The energy of the absorption edge has also shifted to 397.9 eV, and the excitonic effect is still important. This aspect confirms the insulating character of the first molecular layer that was inferred from the absence of molecule-derived intensity at EF in the UPS spectra. In fact, for a metallic system, the electronic configuration at the edge of the N K-absorption (one electron at the EF) is the same as that of N 1s XPS final state (with one electron at the EF coming from the ground); contrary

Halogenated Phthalocyanine and the Ag(111) Surface

Figure 7. The low energy cutoff region of ZnPcCl8/Ag(111) as a function of molecular film thickness. The inset displays the work function, LUMO, and HOMO energy positions as a function of film thickness.

to what is presently observed, the absorption edge should be coincident with the N 1s BE.50,51 Also, the next feature at higher photon energy has changed; it is sensibly closer to the absorption edge and the asymmetry is now inverted. Although a direct interpretation is not trivial, such behavior indicates that also higher-lying molecular orbitals are affected by the moleculesurface interaction. This favors the hypothesis of a significant intramolecular charge rearrangement for the first layer molecules in close contact with silver. Work Function. The evolution of the work function (WF) at the metal/organic interface has been intensively studied in recent years.52–61 As a function of coverage, one measures the transition from the WF of the bare metal to the one of the organic thin layer. Such transition includes the formation of an interface dipole whose magnitude depends (i) on the modification of the metal surface electrostatic potential and (ii) on the possible formation of interface bonds. Altogether, the WF change at the monolayer regime will be the result of the charge redistribution at the metal surface and on the molecular unit. In Figure 7 the low-energy cutoff of the UPS spectra are displayed. The surface WF, that is the energy difference between the EF and the surface vacuum level,62 can be obtained directly from the low-energy cutoff energy by subtracting the (constant) UV-photon energy. For the Ag(111) surface we found WF ) 4.55 +/- 0.05 eV, a value that, although somewhat lower, is consistent to previously reported values63 and is due to the electric dipole created by the spill-out of electrons at the sample surface.64 Successive molecular depositions result in an initial increase (0.3 and 0.6 nm), a momentary decrease (0.7 and 0.8 nm), and a gradual increase of WF up to the highest thickness presently investigated. In the case of ZnPcCl8/Ag(111) studied here, the increasing of the WF observed at the earliest stages of molecular adsorption can be rationalized in terms of the charge transfer from the substrate to the molecules observed in UPS and NEXAFS. Such a charge transfer would result in an interface dipole with

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8659 negative charge spilled out into the molecules (dipolar moment toward the metal). The explanation of the transient decrease of WF for intermediate coverage is more involved. Density functional theory (DFT) calculations have recently shown that the WF changes at the organic monolayer coverage are extremely sensitive to the details of the charge rearrangement going on at the molecule-metal contact and within the molecule itself.52–61 For example, in the case of C60/Cu(111), where a metal-to-molecule charge transfer was measured by UPS,65 DFT calculations have shown that the overall dipole moment is pointing outward from the surface, explaining the observed decrease of WF.62 A similar experimental result was observed for CuPcF4/Ag(111),21 but at the moment a theoretical investigation is missing. In the present case, we also attribute the WF decrease to the appearance of a molecular dipole moment opposite to the initial interface dipole. The delayed appearance of this molecular dipole moment, which points toward vacuum, may be linked to the deformation of the planar Pc molecules associated with the formation of an ordered monolayer at the surface. Indeed, we have shown that upon vapor deposition of Pc molecules on Ag(111) at room temperature that the formation of an ordered monolayer proceeds via a two-dimensional precursor phase (the so-called gas phase), where the Pc molecules are adsorbed but remain highly mobile on the surface.8,17 For larger coverage toward the full 1 ML (or under mild annealing), the molecular deposit spontaneously evolves toward ordered phases of increasing compactness. We postulate that, in contrast to this gaseous phase where the Pc molecules remain flat, they significantly change their equilibrium geometry upon adsorption at fixed sites (ordered monolayers), where they develop a dipole moment pointing toward vacuum. Preliminary results of ongoing DFT calculations in our group66 do show that the Pc molecules are notably distorted once adsorbed in an ordered 2D phase. They show that the energetically preferred adsorption configuration places the central Zn atom in close contact with surface Ag atoms, the former being significantly taken out of the molecular plane and the adsorbed molecule then adopting a concave shape. These calculations also confirm the negative charge transfer toward the molecule. They also show that most of the excess charge is located on the Zn atom. Considering the curved equilibrium molecular shape, it is clear that this charge configuration results in a net static molecular dipole oriented toward vacuum. This molecular dipole intrinsic to the ordered first monolayer reduces the interface dipole and gives rise to the observed dip in the work function evolution at and just above 1 ML coverage (0.7 nm). As a matter of fact, a recent work has reported local contact potential difference (CPD) on the same system using Kelvin probe force microscopy.67 The resolution around 30 nm is sufficient to resolve coexisting ordered and disordered 2D domains at the surface. CPD maps reveal that ordered domains have a significantly lower work function than the disordered (gaseous) phase, a fact that largely corroborates our model. We notice a coincidental agreement between the WF value at 1 ML coverage with the molecular chemical potential as derived using DFT (both values around 4.8 eV). In view of the involved evolution of the interface WF with respect to molecular coverage, this agreement seems purely fortuitous. Finally, we observe that the gradual increase of WF observed beyond 0.85 nm goes in parallel with the shift of the HOMO toward lower BE testifying that at those coverages the molecular levels are aligned to the vacuum level and that at high coverage the WF is determined by the molecule’s IP. A possible change

8660 J. Phys. Chem. C, Vol. 112, No. 23, 2008 in the molecular orientation at high coverage is expected to have only a minor impact on the WF change for this unpolar molecule. IV. Conclusions In this paper the electronic properties of a thin film of ZnPcCl8 on Ag(111) were investigated. The UPS data showed a new density of states close to the EF, whereas the NEXAFS spectra displayed a reduced intensity of the LUMO resonance. These evidence allowed us to conclude that the molecules of the first layer adsorb through a charge-transfer mechanism with electrons flowing from the substrate to the LUMO of the pristine molecules. Nevertheless, the lack of a sizable density of states (DOS) at EF as well as the photon energy dependence of the interface states peak intensity suggest that more complicated mechanisms of intra- or extramolecular charge redistribution may be important to understand the full picture. After molecular adsorption the Ag(111) 4d valence band was sensibly modified, and several interface states were detected. We argue that these features are not molecule-derived hybrid states but rather originate from Ag bulk states, which appear after the quenching of the surface states upon molecular adsorption. We suggest that such enhancement of metal bulk states after molecular adsorption may also show up for other similar systems. Finally, the surface WF was studied as a function of coverage, and the effect of the coexistence of several structural phases was addressed. Acknowledgment. We would like to acknowledge D. Catalin and A. Bliek for their precious technical support. We would also like to thank G. Panaccione for providing the XPS spectrum of the clean Ag(111). Finally, Ch. Loppacher and P. Milde are kindly acknowledged for fruitful discussion. References and Notes (1) Cahen, D. Kahn, A., Umbach, E. Mater. Today, 2005, July/August, 32. (2) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (3) Zahn, D. R. T.; Gavrila, G. N.; Salvan, G. Chem. ReV. 2007, 107, 1161. (4) Craciun, M. F.; Rogge, S.; Morpurgo, Alberto F. J. Am. Chem. Soc. 2005, 127, 12210. (5) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (6) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (7) Abel, M.; Oison, V.; Koudia, M.; Maurel, C.; Katan, C.; Porte, L. ChemPhysChem 2006, 82, 7. (8) Zhu, X.-Y. Surf. Sci. Rep. 2004, 54, 1. (9) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605. (10) Eremtchenko, M.; Schaefer, J. A.; Tautz, F. S. Nature 2003, 425, 602. (11) Tautz, F. S.; Eremtchenko, M.; Schaefer, J. A.; Sokolowski, M.; Shklover, V.; Umbach, E. Phys. ReV B 2002, 65, 125405. (12) Koch, N.; Duhm, S.; Rabe, J. P.; Rentenberger, S.; Johnson, R. L.; Klankermayer, J.; Schreiber, F. Appl. Phys. Lett. 2005, 87, 101905. (13) Maxwell, A. J.; Bru¨hwiler, P. A.; Arvanitis, D.; Hasselstro¨m, J.; Johansson, M. K.-J.; Mårtensson, N. Phys. ReV. B 1998, 57, 7312. (14) Hauschild, A.; Karki, K.; Cowie, B. C. C.; Rohlfing, M.; Tautz, F. S.; Sokolowski1, M. Phys. ReV. Lett. 2005, 94, 036106. (15) Eremtchenko, M.; Bauer, D.; Schaefer, J. A.; Tautz, F. S. New J. Phys. 2004, 6, 4. (16) Koudia, M.; Abel, M.; Maurel, C.; Bliek, A.; Catalin, D.; Mossoyan, M.; Mossoyan, J.-C.; Porte, L. J. Phys. Chem. B 2006, 10058, 110. (17) Amsalem, P.; Giovanelli, L.; Themlin, J. M.; Koudia, M.; Abel, M.; Oison, V.; Ksari, Y.; Mossoyan, M.; Porte, L. Surf. Sci. 2007, 601, 4185. (18) Oison, V.; Abel, M.; Koudia, M.; Porte, L. Phys. ReV. B 2007, 75, 035428. (19) Zangrando, M.; Zacchigna, M.; Finazzi, M.; Cocco, D.; Rochow, R.; Parmigiani, F. ReV. Sci. Instrum. 2004, 75, 31.

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