Periodic Arrays of Cu-Phthalocyanine Chains on Au(110) - The

Jun 27, 2008 - The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 × 2)-Au(110) surface has been studied. The overlayer deposition h...
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10794

J. Phys. Chem. C 2008, 112, 10794–10802

Periodic Arrays of Cu-Phthalocyanine Chains on Au(110) Luca Floreano,* Albano Cossaro, Roberto Gotter, Alberto Verdini, Gregor Bavdek,† Fabrizio Evangelista,‡ Alessandro Ruocco,‡ Alberto Morgante,§ and Dean Cvetko† CNR-INFM Laboratorio Nazionale TASC, BasoVizza SS-14, Km 163.5, I-34012 Trieste, Italy ReceiVed: NoVember 23, 2007; ReVised Manuscript ReceiVed: May 6, 2008

The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 × 2)-Au(110) surface has been studied. The overlayer deposition has been monitored in real time by helium atom scattering (HAS) and low energy electron diffraction (LEED). Throughout the monolayer regime the Cu-Pc molecules are systematically observed to line-up edge-to-edge along the [11j0] direction of the Au substrate, yielding a commensurate 5-fold periodicity (14.4 Å). Cu-Pc chains deconstruct the 2-fold Au missing row order in the early stage of deposition. A set of higher order periodicities (5-, 7-, and 3-fold) are progressively observed along [001] with increasing CuPc deposition, the 3-fold phase appearing at the monolayer saturation coverage. The corresponding molecular orientation has been studied by variable polarization absorption spectroscopy (XAS), whereas the Au substrate structure has been determined by out-of-plane surface X-ray diffraction. The (5 × 5) phase is found to be rather corrugated, and it exhibits a high degree of long-range order yielding the most prominent diffraction pattern. In the (5 × 5) phase, the Cu-Pc chains are found to lift the underneath missing row reconstruction, being separated by residual Au rows. Similarly, in the more compressed 3-fold monolayer phase, the Cu-Pc molecules were formerly found to lie within a shallow (1 × 3) Au reconstruction [Cossaro, A.; et al. J. Phys. Chem. B 2004, 108, 14671]. From comparison of the different deposition stages, as measured in real time by HAS, we can draw a comprehensive picture of the system evolution. In fact, the observed periodicities at different coverage are always formed by an array of Cu-Pc chains in shallow troughs that are equally spaced by a number of uncovered Au rows, as dictated by the Cu-Pc coverage. The growth of Cu-Pc arrays in the submonolayer range is thus driven by an interchain repulsion mechanism. 1. Introduction Most of planar π-conjugated molecules like polycyclic aromatic hydrocarbons (PAH) 1–5 and metal-phthalocyanine (M-Pc)6–11 adsorb flat on metal surfaces in order to maximize the overlap between the delocalized molecular orbitals and the surface charge density of the substrate. Such a configuration is highly desirable to improve the charge transfer at the electrodes of organic based electronic devices.12 On the other hand, the first molecular layer often looses its semiconducting properties due to a charge transfer from the metal to the lowest unoccupied molecular orbitals (LUMO). Still, the first flat organic layer can be a good candidate to drive the planar growth of next layers in both homogeneous13,14 and heterogeneous 15,16 organic films. In fact, the optimal π overlap associated with the planar stacking of flat organic molecules yields the best charge transport properties.17–19 This general behavior is also expected for the Cu-phthalocyanine (Cu-Pc) molecule, an aromatic dye formed by four pyrrolic and four benzene macrocycles arranged around a central Cu atom in a planar geometry, that is widely employed in organic-based electron devices. In fact, the partial filling of the LUMO in the first layer has been reported for deposition on Al(100), also corresponding to a flat adhesion of the Cu-Pc molecules.20 On the other hand, X-ray photoemission, XPS, * Corresponding author. Fax: +39-040-226767. E-mail: floreano@ tasc.infm.it. † Also at: Department of Physics, University of Ljubljana, Ljubljana, Slovenia. ‡ Also at: Department of Physics and CNISM, University of Roma Tre, Roma, Italy. § Also at: Department of Physics, University of Trieste, Trieste, Italy.

measurements of the C1s core level in a Cu-Pc monolayer on Au already display the characteristic fingerprint of the bulk phase LUMO, irrespective of the molecular and crystal orientations.21,22 This result is somehow unexpected since the Cu-Pc interaction with the Au surfaces is not negligible. In fact, molecular azimuthal reorientation has been reported for Cu-Pc deposited on stepped Au(111),23 whereas a large substrate reconstruction is observed in the monolayer phase on Au(110).24 In the latter case, Cu-Pc induces a 1 × 3 substrate reconstruction where molecules lie along the Au troughs.22 In addition, molecules line up into long correlated chains along the [11j0] direction displaying a commensurate 5-fold periodicity (14.4 Å), that well matches the Cu-Pc edge size (13.8 Å). Cu-Pc deposition on Au(110) thus offers the opportunity of forming well ordered templates of an organic semiconductor already in the submonolayer range. In addition, the different periodicities of these anisotropic geometries can be potentially exploited for driving the coupling with different organic molecules. However, in the submonolayer regime, the ordering and growth of the Cu-Pc overlayer is less well understood; in particular, issues like the site of Cu-Pc adsorption, the degree of molecular diffusion and ordering, and the spatial correlation of molecular orientation are still open. In this paper, we explore the Cu-Pc molecular adsorption and ordering throughout the submonolayer coverage range. Adsorption of Cu-Pc is monitored by helium atom scattering (HAS) and low energy electron diffraction (LEED) in real time. Whereas HAS exclusively probes the outermost electron density, i.e., the order of the molecular overlayer, LEED is also sensitive to the substrate Au order. It is shown how the different sensitivity of the two probes may yield apparent discrepancies

10.1021/jp711140e CCC: $40.75  2008 American Chemical Society Published on Web 06/27/2008

Cu-Phthalocyanine Chains on Au(110)

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in the measurement of the surface symmetry during deposition. The structure of the substrate beneath the Cu-Pc chains is determined by means of out of plane grazing incidence X-ray diffraction, GIXRD, which is almost unaffected by the low Z and few atoms of the molecular overlayer. The molecule to surface adsorption geometry is determined by studying the polarization dependence of near-edge X-ray absorption fine spectroscopy, NEXAFS. In particular, new NEXAFS measurements, taken at the ionization thresholds of C, N, and Cu, partially correct our previous determination of the molecular tilt angle and shed new light on the modification of the LUMOs induced by the substrate interaction. 2. Experimental Setup The measurements have been taken at the Aloisa beamline and its branchline HASPES (Elettra synchrotron in Trieste, Italy). We modified our former HAS apparatus26 to be attached at the Aloisa branchline. The incoming X-ray beam path is coaxial to the outgoing He beam one; moreover, the series of collimators placed between the sample and the quadrupole mass spectrometer (for He selection and detection) also serves to align the photon beam on the same scattering center of the He beam. We equipped the HAS apparatus with a homemade hemispherical electron spectrometer (150 mm mean radius), which lenses are designed to match the photon beam spot size and shape at the Aloisa branchline (about 0.5 × 1 mm transverse size at the sample). The electron spectrometer is equipped with a multichannel detection system that enables the parallel acquisition of 48 energy channels. The scattering geometry allows us to perform simultaneously HAS and XPS. In addition, the HAS chamber is equipped with an electron gun that, thanks to the relatively small angular acceptance of the spectrometer (∼1.5°), can be used to acquire low energy electron diffraction patterns by rotating the sample in the same way as He diffraction. X-ray absorption spectra, XAS, can be taken in partial electron yield by means of a channeltron detector. Complementary valence band spectroscopy can be measured off-line thanks to a He lamp. A characteristic of the HAS setup is the possibility to perform all of the measurements in real time during surface preparation, being it sputtering, gas dosing, or evaporation. Cu-Pc was evaporated from a homemade boron nitride crucible hosted inside a three-slot cryopanel. The sample is mounted on a variable temperature (120-1200 K) manipulator (CTPO with Omniax translator, by VG) with six degrees of freedom (precision of 0.01° for the three rotations). The sample holder (a modified POD assembly) hosts two tungsten filaments for radiative heating and two thermocouples. The POD is fully interchangeable with the manipulator of the Aloisa experimental chamber that is a homemade realization (made by CINEL, Italy) with improved precision for the rotations (of the order of 0.001°). At the Aloisa experimental chamber both photoelectron spectroscopy and X-ray diffraction measurements can be performed thanks to the wide photon energy range available (from 130 to 1500 eV and from 3 to 8 keV).31 The branchline exploits the same monochromator, but its optical configuration limits the maximum photon energy to ∼1200 eV. Both electron spectrometers (homemade) and energy resolved Si photodiodes (Eurisys, France) are mounted inside the Aloisa experimental chamber on rotating frames. XAS spectra are collected by means of a channeltron detector provided with a retarding grid electrode to filter out low energy secondary electrons. The manipulator is mounted coaxial to the photon

Figure 1. Left: the chemical structure and molecular size of Cu-Pc is shown. Right: a sketch of the experimental geometry adopted for the variable polarization XAS measurements. The sample is kept at a constant grazing angle R and azimuthal orientation φ (with the [11j0] direction aligned in the scattering plane). The orientation θ of the surface with respect to the vector of the linearly polarized photon beam is selected by rotating the sample around the photon beam axis.

beam, that impinges on the surface at grazing incidence (grazing angle R variable from 0 to 20°). The grazing angle is determined by measuring the specularly reflected photon beam either by collimated photodiodes placed behind the electron spectrometer lenses or by a TV camera coupled to a phosphor screen placed downstream the manipulator. The rotation around the beam axis allows to change the orientation θ of the surface with respect to the linearly polarized X-ray electric field, while keeping the surface at a constant grazing angle, as depicted in Figure 1. XAS spectra measured at the C and N K -edges have been calibrated with a precision of 0.01 eV by simultaneous acquisition of the 1s f π/ gas phase transitions of CO and N2 at hν ) 287.40 and 401.10 eV, respectively (see ref 31 for details about the operation of the window-less ionization chamber). XPS, XAS and GIXRD measurements can be thus performed in situ at Aloisa, whereas a reflection high energy electron diffraction, RHEED, system is employed in the preparation chamber to check the surface preparation during evaporation. Two Au(110) samples of high crystal quality (miscut lower than 0.1°) have been used and routinely exchanged between the two experimental chambers. The missing row (1 × 2)Au(110) surface (unit cell size of 2.884 × 8.156 Å 2) was prepared by real time monitoring of the diffraction pattern (either by HAS or by RHEED) during sputtering and annealing. The sample is ion bombarded (Ar+) first at room temperature and 3 keV. The ion energy is progressively reduced to 0.5 keV during annealing, as the critical temperature of ∼700 K is approached, where the (1 × 2)f(1 × 1) order-disorder phase transition takes place.27 Sample heating and sputtering are only stopped when the half-integer diffraction peaks fully disappear (some 50 K above the critical temperature). The Cu-Pc evaporation cells have been operated at typical temperatures of 600-620 K, corresponding to deposition rates in the range of 0.1-1 Å/min. A quartz microbalance was employed in the Aloisa experimental chamber to adjust the deposition rate (assuming a Cu-Pc density F ∼ 1.6 g/cm3). 3. Results 3.1. Phase Symmetry Evolution. Because of the large Au atom mobility, well ordered submonolayer phases of Cu-Pc, involving large scale rearrangements of the substrate structure,

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Figure 2. Consecutive one-dimensional diffraction scans taken during deposition by LEED (upper panel, color) and HAS (lower panel, color) along the [001] direction. LEED scans have been taken with a 200 eV kinetic energy, whereas HAS measurements have been performed at a beam energy of 19 meV, corresponding to a wavevector kHe ) 6.12 Å -1. The diffraction intensities are given on a logarithmic scale. The deposition time is on the vertical y axis. The substrate temperature was held at 300 K and the Cu-Pc deposition rate was kept in the 0.05 ML/scan range in order to avoid major surface evolution within a single scan. The Cu-Pc deposition starts from the second scan on. The initial (1 × 2)Au(110) diffraction pattern displays a characteristic half-integer peak at 0.77 Å -1 due to the 2-fold missing row reconstruction. Well ordered terraces extend over 250 Å, as judged from the widths of the integer and half-integer diffraction peaks. The spacings corresponding to the side-peaks of the HAS (0,0) and (0,-1/2) peaks are reported in the graphic at the bottom-left. The spacing of the (0,-1/2) side-peaks is always twice that of the (0,0) side-peaks. The side-peaks drift until they become the (0,-1/5) and (0,-2/5) peaks of the (5 × 5) phase.

can be grown already at room temperature. First of all, we notice that diffraction along the [11j0] direction yields a 5-fold periodicity (14.4 Å) in any of the different phases encountered during the first layer deposition. This is consistent with previous reports24 and the good matching of this periodicity with the CuPc edge size (13.8 Å) points to the alignment of molecules into edge-to-edge chains extending along [11j0]. The diffraction patterns are much richer and more complex along the [001] direction, that is known to have a low stability against higher order reconstructions.32 A sequence of one-dimensional angular scans taken along the [001] direction during Cu-Pc evaporation at TS ) 300 K are shown in Figure 2 for both electron and He scattering, upper and lower panel, respectively. In both cases the initial stages of deposition display the same evolution of the diffraction pattern, with a gradual splitting of the half-integer peak and additional peaks arising from the (0,0) specular reflection. These side-peaks gradually shift away from the original ones, eventually yielding an intermediate phase with a neat 5-fold periodicity, i.e., a (5 × 5) phase, as clearly detected by both HAS and LEED. The observed half-integer peak decomposition into a central narrow spike with two symmetric side-peaks points to the

Floreano et al. formation of domains of opposite missing row order. This process may occur via the formation of monatomic steps across the [001] direction, since the atom displacement associated with a step on an fcc(110) surface has both a vertical and a lateral component. In particular, domains separated by two monatomic steps have opposite missing row order.33 The appearance of sidepeaks at small parallel momentum transfer ∆q aside the (0,0) specular reflection is indicative of the proliferation of steps, indeed.34 These steps are up-down correlated, since the separation of the half-integer side-peaks from the reciprocal lattice vector G1×2 (yielding the spacing between domains of same missing row order) is half the separation of the specular side-peaks from the specular reflection (yielding the spacing between equivalent steps); see bottom-left panel of Figure 2. This kind of correlated step/domain wall proliferation induced by molecular adsorption seems to be a common feature for the Au(110) surface when the deposited molecules line up head-totail along the [11j0] direction. In fact, it has been recently reported for the early deposition stage of pentacene36 and studied in larger detail for the growth of sexithiophene (6T),37 where the 6T chain yields a 4-fold periodicity along [001] at the saturation of the first monolayer.38 In the latter case, the evolution of the diffraction sidepeaks is accompanied by the simultaneous appearance of a 4-fold symmetry pattern originated by the periodicity within the growing 6T islands (due to chain bunching).37 In the present case, no additional Bragg’s peaks, corresponding to a possible intraisland periodicity, have been observed during the evolution of the sidepeaks toward the 5-fold phase. In LEED, a 3-fold periodicity is established after the 5-fold one that does not evolve any longer but simply slowly fades away with further deposition. This behavior is characteristic of a Stranski-Krastanov regime of growth, where 3D islands are formed on top of a wetting layer. The islands grow without a lateral correlation, even if a preferential orientation of the CuPc molecular plane parallel to the surface can be preserved for many layers on both Au(110)39 and Au(001).10 The appearance of the (5 × 3) LEED pattern can be associated with the completion of the first monolayer (ML). In HAS, the (5 × 5) phase is followed by the appearance of a very weak 7-fold periodicity, that soon disappears together with the integer order diffraction peaks. This disappearance is to be associated with a disordered growth (HAS is a few orders of magnitude more sensitive than LEED to defects), consistent with the StranskiKrastanov growth model. To better understand this apparent discrepancy, we studied in more detail the 7-fold (5 × 7) phase as a function of the temperature. In fact, the 7-fold phase demonstrated to be affected by a strong Debye-Waller attenuation (dynamical vibrations) and the peak detection by HAS is enhanced by cooling the sample. In addition, Cu-Pc deposition at slightly higher substrate temperature is found to improve the ordering of the organic overlayer. The optimal 7-fold phase is obtained by deposition at TS ) 350 K, where, on the other hand, the Debye-Waller attenuation heavily inhibits its detection by HAS. By cooling to 140 K the (5 × 7) phase grown at 350 K, the dynamical disorder is strongly reduced and the peaks can be detected, as shown in Figure 3. In this case, LEED patterns taken in situ on the same phase also display the presence of the 7-fold peaks. On the contrary, the (5 × 3) symmetry is never detected by HAS. To explain this behavior, one must consider the different probing depth of the techniques employed in the present study, as depicted in Figure 4, where HAS and grazing incidence XRD (GIXRD) represent the two extreme cases. HAS exclusively probes the outermost surface charge density, its turning point

Cu-Phthalocyanine Chains on Au(110)

Figure 3. Comparison between LEED and HAS diffraction pattern taken along [001] after deposition of one Cu-Pc monolayer at TS ) 355 K and measured at TS ) 140 K. As a guide to the eye, a grid with 7-fold periodicity is superimposed to the graph. Surface diffraction peaks of the seventh order are clearly observed in the HAS pattern. The corresponding peak width yields a mean domain size of 150 Å. The 7-fold peaks can be clearly detected in the LEED pattern too, but, in this case, faint peaks with 3-fold symmetry are also reported. As explained in the text, these peaks arise from the substrate reconstruction beneath disordered Cu-Pc domains that cannot be detected by HAS.

Figure 4. Different probing depths of GIXRD, LEED, and HAS schematically represented in the three panels, from left to right, respectively. At an energy of 7 keV and a grazing angle of 0.5°, the attenuation length of photons in a Au bulk is 20 Å, whereas it is 5 µ m for graphitic carbon.28 The shaded area in the left panel roughly indicates the Au atoms that yield a scattering contribution to GIXRD data. The mean free path of electrons is almost independent of the material in the 50-2000 eV range, from which an universal curve of escape depth is drawn.29 At the kinetic energy of 200 eV presently used, the escape depth is in the range of 6-7 Å for LEED. For RHEED at 15 keV, a mean free path of ∼10 nm is reduced to 5 Å at a grazing angle of 3°. In this case, both the organic overlayer and the first substrate layers are probed (central panel), but the high Z Au atoms have a scattering cross section larger than low Z atoms of the organic layer. Thermal energy He atoms (20-60 meV) are completely nonpenetrating and they only probe the outermost surface charge density. The turning point of He atoms is situated at ∼3 Å above the plane of the atomic nuclei, as shown by the corrugated thick line in the right panel.30

being located about 3 Å above the plane of the atomic nuclei. As a consequence, HAS is not sensitive to the substrate rearrangement beneath the organic overlayer. On the contrary, GIXRD (below the critical angle) has a relatively large probing depth of a few nanometers. In addition, the cross section for X-ray scattering is proportional to the square of the atomic number. In the present case, where large rearrangements of the substrate Au atoms occur, GIXRD is weakly affected by the organic overlayer (monolayer) made by low Z atoms and is

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10797 effectively probing the substrate ordering. Midway, we find the electron diffraction techniques, such as RHEED and LEED, with a small probing depth of 5-10 Å. The cross section for electron scattering also depends on the electronic structure of the target atoms. Electrons effectively probe both the organic overlayer and the atomic structure of a few Au layers beneath. The 7-fold phase can be regarded as the highest coverage single layer phase displaying long-range order. A slightly larger amount of molecules leads to the formation of a laterally compressed phase with a large density of defects (either static or dynamical) that inhibits the detection of any HAS diffraction pattern, still being a monolayer phase.22 At the same time, the compressed/disordered phase yields a 3-fold reconstruction of the substrate that can still be detected by techniques with larger penetration depth, which sensitivity to defects is also much lower than that of HAS. Hereafter we will indicate this monolayer deposition stage as the “(5 × 3)” phase. 3.2. Molecular Orientation. To get a clear picture of the molecular orientation, we measured NEXAFS spectra at different coverage to be compared with our former study of the “(5 × 3)” phase. Giving the edge-to-edge alignment of molecules along the [11j0] direction for any surface phase, we restrict our study to the molecular tilt around an axis parallel to the [11j0] substrate direction. In our former study, the molecular orientation was extracted by the absorption spectra measured at the Cu L3 X-ray absorption edge as a function of the surface orientation θ with respect to the linearly polarized electric field of the photon beam. In particular, the analysis was performed on the most prominent line at 932 eV (the so-called white line), which corresponds to a transition into an empty state with the dipole moment lying in the molecular plane (σ-plane symmetry).44,45 LUMO resonances are thus expected to be enhanced when the polarization of the electric field is parallel to the molecular plane, and to vanish when the polarization is perpendicular to the molecular plane. In Figure 5, the NEXAFS spectra taken at the Cu L3 edge are shown for the (5 × 5), “(5 × 3)” and 3 ML films. A strong dichroism is always present, where the white line is always maximum in s polarization and minimum in p polarization. In particular, the white line almost vanishes in the (5 × 5) phase, whereas the residual relative intensity increases in the “(5 × 3)” and 3 ML films, suggesting an increase of the molecular tilt angle. For the present scattering geometry (photon beam along [11j0], molecules tilted around [11j0]), σ-plane LUMO symmetry and 2-fold substrate symmetry, the NEXAFS signal is proportional to44

1 - sin2 γ cos2 θ - cos2 γ sin2 θ cos2 R

(1)

where γ represents the molecular tilt angle between the molecular plane and the substrate surface. At small grazing angle R, the ratio Ip/Is between the NEXAFS intensity in p (θ ) 90°) and s polarization (θ ) 0°) is simply proportional to tan2 γ and the corresponding molecular tilt angles can be readily evaluated, as reported in Table 1. We remark that the intensity ratio of the (5 × 5) phase is at the detection limit of our measurements (see the noise level in the upper panel of Figure 5), as a consequence the tilt angle is affected by a large relative uncertainty (at least 50%). Notwithstanding the tilt differences are within the angular uncertainty, we see a trend to the increase of the tilt angle with the increase of the coverage. In addition, we notice that the tilt γ∼14° for the “(5 × 3)” phase is smaller than the previously reported value of 32°.22 We attribute this discrepancy to a number of concurrent reasons: mainly to the former use of a poor quality Au(110)

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Floreano et al.

Figure 5. NEXAFS spectra measured by partial electron yield at the Cu L3 edge (constant bias of -850 V applied to the channeltron grid). The sample was kept at a fixed grazing angle R ) 3° with the [11j0] direction along the photon beam. The white line was extracted after interpolation of the background intensity. The spectra have been normalized to the background before the onset of the white line transition. In absence of a reference signal for the absolute photon energy, we have conventionally calibrated the white line to 931.7 eV, after ref.44 Data points are indicated by open markers for the s (θ ) 0°) and p polarization (θ ) 90°), squares and circles, respectively. Upper panel: (5 × 5) phase. Center panel: “(5 × 3)” phase that corresponds to the saturation of the first monolayer (ML). Lower panel: 3 ML film.

TABLE 1: Tilt angle of the Molecular Plane around the [11j0] Direction, As Calculated from the Dichroism of the NEXAFS Transitions at the Cu L3 Edge phase/coverage

Ip/Is

γ (°)

(5 × 5) “(5 × 3)” 3 ML

3.4% 6.1% 7.6%

10.5 14 15.5

crystal for the first NEXAFS measurements (homemade spark cutting and surface polishing with a final miscut of ∼1°), then to the extremely low signal to background ratio of the first study, because of total electron yield measurements, instead of partial electron yield, that was implemented later. In general, because of the overall low intensity of the white line, its analysis resulted to be very sensitive to the background subtraction and normalization procedure. Thus, to get a consistent picture of the molecular orientation we also studied the K -edge of N and C. A set of XAS spectra at the N K edge is shown in Figure 6, as taken for films with different Cu-Pc coverage, where a strong linear dichroism is always clearly evident. N K edge transitions with π-plane symmetry (in the 398-404 eV range), which dipole moment is perpendicular to the molecular plane, are strongly enhanced when the electric field orientation is close to the surface normal (θ ) 90°). The opposite occurs for the transitions with σ-plane symmetry in the 405-415 eV range. We can conclude that the molecules within the Cu-Pc chains display a preferential orientation and they lie almost flat on the

Figure 6. NEXAFS spectra measured by partial electron yield at the N K shell (constant bias of -370 V applied to the channeltron grid). The sample was kept at a fixed grazing angle R ) 6° with the [11j0] direction along the photon beam. The photon energy scale has been calibrated to the absolute value of the second vibrational state of N2 in the gas phase at 401.10 eV.31 The spectra have been normalized to the background before the onset of the LUMO resonances. Data points are indicated by open markers for the s and p polarization (squares and circles, respectively). The best fittings are superimposed as full thick line. The spectra have been taken on four films of different coverage: (a) the (5 × 5) phase, (b) the “(5 × 3)”, (c) 1.5 ML, and (d) ∼5 ML. The thick vertical bars put in evidence the energy shift between the main resonance in p polarization and the residual spectral feature in s polarization.

surface. It is worth noticing that the fine structure of the N K shell resonance states, as taken in p polarization, is almost identical to that reported for thick films (50 nm) of Cu-Pc on the same surface,39 as well as for films of Zn-Pc40 and H2-Pc.41 However, a close inspection of the NEXAFS taken in s polarization on the (5 × 5) phase reveals the fingerprint of a distortion of the π symmetry LUMO resonances. In fact, the faint peak detected in the π region at θ ) 0° is not the residual of the most prominent π transition (main line at ∼398.3 eV) but is located at a higher photon energy (398.7 eV). This spectral feature might be associated either with a rehybridization of the LUMO states localized on some N atoms or with a distortion of some N bonds in the Cu-Pc atomic backbone. As a consequence, an analysis of the NEXAFS intensity ratio Ip/Is cannot be rigorously applied to the π transitions for an estimate of the molecular tilt angle. As the Cu-Pc coverage increases, the residual intensity in the 400-405 eV π region also increases.

Cu-Phthalocyanine Chains on Au(110)

J. Phys. Chem. C, Vol. 112, No. 29, 2008 10799 TABLE 2: Tilt Angle of the Molecular Plane around the [11j0] Direction, As Calculated from the Dichroism of the NEXAFS Transitions at the C K Edge phase/coverage (5 × 5) “(5 × 3)” 1.5 ML 5 ML

γ (°)

Is/Ip 2.3% 3.7% 6.6% 11.8%

8.5 11 15 19

The first hypothesis might be in contrast with XPS measurements of the C 1s core level and its shakeup satellites (but a more detailed XPS investigation, possibly at K edge photon energy, would be required). For the second hypothesis, one should consider the very small relative intensity of this feature (a few percents) as compared with former observations on benzene (20-30%).42 As a consequence, either the C-H bonds present a very small distortion or only a few of them (out of 16) are affected by a sizable out-ofplane bending. Contrary to the case of N K edge, the intensity of the π symmetry resonances can be easily discriminated from the background (and from the step-like transition at 287.4 eV) and an evaluation of the molecular tilt angle can be attempted. For the given scattering geometry (grazing incidence), 2-fold surface symmetry and π-plane transition symmetry, the NEXAFS intensity is proportional to43

cos2 γ sin2 θ + sin2 γ cos2 θ tan2

Figure 7. NEXAFS spectra measured by partial electron yield at the C K shell (constant bias of -230 V applied to the channeltron grid). The sample was kept at a fixed grazing angle R ) 6° with the [11j0] direction along the photon beam. The photon energy scale has been calibrated to the absolute value of the first vibrational state of CO in the gas phase at 287.40 eV.31 Due to the carbon contamination of the monochromator affecting the photon flux, we have normalized the spectra from Cu-Pc films to spectra taken on the clean Au(110) substrate in the same scattering and detection conditions. The spectra have been taken on the same films as the previous figure.

This is an indication that the Cu-Pc molecules (or part of them) are tilting their molecular plane off the sample surface. However, the residual intensity detected for the “(5 × 3)” phase still displays an overall energy distribution different from that observed in the p -polarization. A quantitative analysis of the NEXAFS dichroism, i.e., Is/Ip ratio, thus cannot yield a reliable tilt determination much better than that drawn from the Cu L3 edge analysis, not even for the larger thickness films, where the unaltered Cu-Pc LUMO resonances are hardly discriminated from the hybridized features. A clearer hint of the evolution of the molecular orientation can be drawn from the comparison with the C K edge spectra, as shown in Figure 7. The NEXAFS measurements have been taken on the same films of Figure 6 and with the same scattering conditions. Very large π symmetry resonances are observed in p polarization that vanish in s polarization for the (5 × 5) phase. The very small residual intensity detected at 285 eV can be attributed to the 95% degree of polarization of the X-ray beam, together with the small indetermination of the absolute θ angle due to the azimuthal precession (1-2°) of the sample mounting assembly. On the contrary, the main residual intensity observed below the ionization threshold in the 287-291 eV range in s polarization appears as a step-like electronic transition from the core level into a continuum of states. This transition at ∼287 eV can be attributed to the close vicinity of the carbon atoms to the metallic substrate43 or to an out-of-plane bending of the C-H bonds in the Cu-Pc molecule.42

(2)

The ratio Is/Ip is thus proportional to γ, and the corresponding molecular tilt angles are reported in Table 2. We observe the same trend as found for the Cu L3 edge. Even by assuming a practically flat adsorption in the (5 × 5) phase, we detect a clear increase of the tilt angle for the “(5 × 3)” phase and higher coverage films. In conclusion, the close vanishing of all of the nonhybridized features at both the C and N K edges, as well as at the Cu L3 edge in the opposite polarization, suggests a practically flat molecular adsorption in the (5 × 5) phase, where the substrate charge exchange does not significantly alter the planar structure of the molecule. As the coverage increases, all the π components start to be detected in s polarization, indicating a gradual increase of the tilt angle. The resulting angle value might be the average tilt angle of all of the molecules in the phase or the average value between two coexisting populations of flat and more largely tilted molecules. The second model seems more suitable since the hybridization features observed in the (5 × 5) phase (both at the N and C K edges) are preserved also at higher coverage. The presence of differently oriented molecules in the “(5 × 3)” phase is consistent with the large density of point defects indicated by HAS measurements. Because of the Stranski-Krastanov growth mode, the tilt angle at higher coverage is also determined by the contribution from coexisting populations with different orientations. The tilt angle of molecules within clusters is expected to be larger than that of molecules in the first layer, that are anchored to the substrate. 3.3. Substrate Reconstruction. We studied the substrate reconstruction beneath the (5 × 5) phase by means of grazing incidence surface X-ray diffraction. In the present case, the measurement of the diffracted intensity as a function of the vertical momentum transfer demonstrated to be only sensitive to the atomic rearrangement of the topmost Au layers, because of (i) the much larger atomic number of Au with respect to C and N and (ii) the occurrence of a substrate reconstruction involving large mass displacement. The uniaxial anisotropy of the organic overlayer is also reflected in the substrate recon-

10800 J. Phys. Chem. C, Vol. 112, No. 29, 2008

Floreano et al. to a shallow 5-fold reconstruction with one added row. A slightly larger R factor is obtained for a surface made of the combination of an (1 × 2) missing row subunit and an (1 × 3) shallow one. The second model is appealing for its similarity with the reconstruction of the “(5 × 3)” saturation phase, that is the same of the (1 × 3) subunit.22 However, the practically flat orientation of the molecules is in favor of the shallow (1 × 5) reconstruction, which might host the molecule without requiring large out-of-plane distortion of the atomic backbone. Both structural models involve sizable rearrangement of atom positions down to the third layer. In particular, the second and third Au layers show an asymmetric lateral distortion (along [001]). This observation let us envisage an asymmetrical adsorption geometry of Cu-Pc molecules with one edge anchored to an Au added row. In fact, the width of the 5-fold shallow trough, although much larger than the molecular side, does not allow a good matching of the molecular carbon-nitrogen frame with the substrate lattice; that is, a symmetric accommodation of Cu-Pc within the shallow trough is also inhibited by the structural mismatch.3 The driving mechanism thus appears to be the preferential interaction of the molecular edges with the low coordinated Au atoms in the added rows and steps running along the [11j0] direction, as reported on the Au(111) surface.23 Within this frame, the weak residual intensity, detected at the C K edge in spolarization, might be associated with a more pronounced bending or rehybridization of the C-H bonds adjacent to the Au added row. 4. Discussion

Figure 8. Analysis of eight nonequivalent GIXRD rod scans measured for the (5 × 5) Cu-Pc/Au(110) phase. L denotes the perpendicular momentum transfer in units of 2π/a3. The notation used for indicating the X-ray reflections are referred to the (1 × 5) unit cell depicted in the two lower panels. The vector lengths are a1 ) a3 ) 2.88 Å and a2 ) 4.08 Å. The photon energy was set to 7000 eV, with a grazing angle of 0.5°. Experimental data (open circles) are reported together with the best fit simulations (full and dotted lines) corresponding to the two real space models depicted in the bottom panels.

struction, which atomic displacement along the [11j0] direction is negligible (as expected by the very weak intensity of the corresponding diffraction peaks also in the more surface sensitive HAS and LEED patterns). In our former study of the saturation coverage phase, the substrate was considered to be reconstructed into a (1 × 3) unit cell.22 Similarly here, a one-dimensional approach can be followed to the analysis of the GIXRD data, and a (1 × 5) substrate reconstruction was considered in our fitting model. Figure 8 shows the integrated diffraction peak intensities as a function of vertical momentum transfer (rod scans) and the corresponding best-fit curves obtained with the numerical ROD program.46 Eight nonequivalent rod scans (∼500 reflections) along the [001] direction have been taken, both on bulk and reconstruction peaks. Due to the large instability of the (1 × 2)-Au(110) surface against higher order reconstructions,32 we considered several combinations of missing row units that were relaxed to fit the experimental data. Upon relaxation, two models yield reliability factors, R, much lower than the other ones. The best fit corresponds

The proliferation of up-down correlated steps across the [001] direction strongly supports the formation of Cu-Pc chains extending along [11j0] since the early stage of growth. The coupling of molecular chains into Cu-Pc islands can be excluded since no additional intraisland periodicity is observed along the [001] direction during the side-peak evolution toward the 5-fold symmetry phase. Rather the chains preserve the edge-to-edge molecular alignment throughout the deposition of the first layer, their spacing only being affected. As a consequence, the up-down correlated steps can be regarded as Au added rows decorating one side of the Cu-Pc chains. Since the density of monatomic steps, as measured by the satellite peak positions, increases with the increasing Cu-Pc coverage, we may argue that, across [001], there is an effective repulsive interaction among the Au steps associated with Cu-Pc chains. In the present case, an interchain repulsive mechanism must involve a considerable mass displacement, i.e., being stronger than a CDW, like proposed for the formation of pentacene arrays on Cu(110).47 In fact, GIXRD measurements clearly show that the different phase symmetries correspond to large reconstructions of the substrate. On the other hand, the observed substrate reconstruction is not as dramatic as reported for Cu-Pc submonolayers deposited on Ag(110), where the step-molecule interaction is strong enough to induce extended faceting over the surface with a density of steps varying according to the coverage.49 According to available calculations, the reconstructions we found for the Au(110) substrate are not the less expensive ones. Highly corrugated, missing row type reconstructions of third or higher order are favored50 that expose (111) microfacets. In fact, the surface energy difference between the (1 × 2) surface and the unreconstructed (1 × 1) was calculated to be triple the energy difference between the (1 × 2) and the (111) surface.48 Recent molecular dynamics calculations, within the model described in ref 51, indicate that the shallow (1 × 3) reconstruction requires an additional energy cost of 5.6 meV/

Cu-Phthalocyanine Chains on Au(110)

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Figure 9. Sideview of the model for four consecutive phases of Cu-Pc on Au(110). The (5 × 5) and “(5 × 3)” are drawn from the present study and ref 22. The low coverage (5 × 7) phase (3/7 ML) has been drawn on the basis of the observed domain wall proliferation leading to the formation of the (5 × 5) phase. The high coverage (5 × 7) phase (6/7 ML) has been drawn from the assumption that Cu-Pc molecules have the tendency to lie flat on the substrate.

atom with respect to the (1 × 2) missing row,52 i.e., ∼85 meV/ molecule. Given the larger surface energy of the (1 × 5) added row reconstruction, together with the larger unit cell, the energy cost per molecule would be at least double in the (5 × 5) phase, thus yielding a lower limit to the Cu-Pc adhesion energy. By comparison of the geometry of the “(5 × 3)” saturation phase with the (5 × 5) one, a model can be drawn of the surface morphology evolution during the monolayer deposition. As previously reported, at saturation the surface is reconstructed into shallow (1 × 3) troughs, where Cu-Pc molecules are hosted, tilted off the surface by at least 10-15°. The absence of a HAS diffraction pattern, yet with the persistence in LEED patterns of a 5-fold periodicity along the [11j0] direction,24 indicates that the edge-toedge chain alignment still dominates the surface morphology, even if with a large density of point defects (either static or dynamical). The surface evolution from the early step proliferation up to the saturation coverage can be thus depicted by a simple onedimensional picture, where several arrays of molecular chains are progressively formed, as shown in Figure 9. By conventionally assigning the coverage of 1 ML to the “(5 × 3)” phase, the intermediate coverage phases easily follow. In particular, the (5 × 5) phase with one flat chain within the shallow trough corresponds to a coverage of 3/5 ML. In between, a well ordered (5 × 7) phase can be grown, by properly tuning the substrate temperature during deposition (as shown in Figures 2 and 3), which would correspond to a 6/7 ML coverage with two Cu-Pc molecules in the unit cell. One

plausible geometry is the coupling into pairs of flat Cu-Pc chains separated by Au rows (see the central panel of Figure 9). This model is consistent with the tendency of Cu-Pc to lie flat on a shallow unit cell, and the higher substrate temperature required for the (5 × 7) stabilization easily overcomes a possible additional energy cost associated with the large substrate reconstruction (with a maximum energy difference of 19 meV/ atom between the missing row (1 × 2) surface and the unreconstructed (1 × 1) one52). As the coverage increases, the lateral interaction between adjacent chains, would induce an increase of the tilt angle of Cu-Pc molecules, thus forming the (1 × 3) subunits of the substrate that dominate in the compressed phase at the saturation of the monolayer. This morphology evolution, driven by a mechanism of chain repulsion, let us envisage the possibility to produce further arrays of Cu-Pc chains with larger spacings at low coverage, by finely tuning the substrate temperature during or after deposition. A representative low coverage (5 × 7) phase, as can be stabilized before the formation of the (5 × 5) phase (see HAS diffraction scans in Figure 2), might be built up by the merging of one (1 × 2) missing row subunit with one (1 × 5) subunit (that is a domain wall for the missing row lattice) hosting a Cu-Pc chain, as shown at the top of Figure 9. 5. Conclusions Several ordered deposition stages may be produced in the submonolayer range that involve large rearrangements of the

10802 J. Phys. Chem. C, Vol. 112, No. 29, 2008 Au(110) substrate. Common to all submonolayer phases appears a quasi unidimensional character of the overlayer ordering in which Cu-Pc molecules line up in extended chains along the Au missing row trough. This is clearly witnessed by the 5-fold periodicity along the [11j0], observed by both HAS and LEED since the early stage of deposition. A 3-fold reconstructed Au substrate is evidenced by LEED and GIXRD at the saturation of the first monolayer along the [001] direction. The formation of this “(5 × 3)” phase is accompanied by a diffuse HAS scattering from the molecular overlayer due to a large density of defects. Substantial ordering of the molecular overlayer takes place at 350 K or higher, where HAS diffraction puts in evidence a 7-fold periodicity (28.5 Å, i.e., twice the Cu-Pc edge size) along [001], ΓY, close to monolayer coverage. At a lower coverage, a (5 × 5) phase can be stabilized already at room temperature. Cu-Pc molecules are found to relax the missing row geometry inducing a shallow substrate reconstruction, as previously found for the “(5 × 3)” monolayer phase.22 These shallow reconstructions are consistent with the adsorption geometry of the Cu-Pc. Molecules are found to lie flat on the surface for the (5 × 5) phase, whereas they are tilted off the surface around the [11j0] in the laterally compressed “(5 × 3)” monolayer phase, where they coexist with a large number of point defects (mainly flat molecules). Acknowledgment. This work is dedicated to Giacinto Scoles and Fernando Tommasini in the year of their retirement. Although their names are indisputably linked to the progress of molecular beams and scattering techniques, they always pursued interdisciplinarity in surface science. The latest project of Tommasini, the Aloisa beamline, is the utmost accomplishment to the aim of integrating multiple investigation techniques. L.F. is also indebted to Riccardo Ferrando for useful discussions and updated calculations of the Au(110) energetics. This project has been cofinanced by the University of Trieste and MIUR (PRIN 2006020543). References and Notes (1) Glo¨ckler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bo¨hringer, M.; Berndt, R.; Schneider, W.-D. Surf. Sci. 1998, 405, 1. (2) So¨hnchen, S.; Ha¨nel, K.; Witte, G.; Wo¨ll, C. Chem. Mater. 2005, 17, 5297. (3) Witte, G.; Ha¨nel, K.; Busse, C.; Birkner, A.; Wo¨ll, Ch. Chem. Mater. 2007, 19, 4228. (4) Henze, S. K. M.; Bauer, O.; Lee, T.-L.; Sokolowski, M.; Tautz, F. S. Surf. Sci. 2007, 601, 1566. (5) Gao, L.; Deng, Z. T.; Ji, W.; Lin, X.; Cheng, Z. H.; He, X. B.; Shi, D. X.; Gao, H.-J. Phys. ReV. B 2006, 73, 075424. (6) Rocco, M. L. M.; Frank, K.-H.; Yannoulis, P.; Koch, E.-E. J. Chem. Phys. 1990, 90, 6859. (7) Fritz, T.; Hara, M.; Knoll, W.; Sasabe, H. Mol. Cryst. Liq. Cryst. 1994, 253, 269. (8) Lackinger, M.; Hietschold, M. Surf. Sci. 2002, 520, L619. (9) Park, K. T.; Miller, A.; Klier, K.; Opila, R. L.; Rowe, J. E. Surf. Sci. 2003, 529, L285. (10) Ellis, T. S.; Park, K. T.; Hulbert, S. L.; Ulrich, M. D.; Rowe, J. E. J. Appl. Phys. 2004, 95, 982. (11) Stadler, C.; Hansen, S.; Pollinger, F.; Kumpf, C.; Umbach, E.; Lee, T.-L.; Zegenhagen, J. Phys. ReV. B 2006, 74, 035404. (12) Dholakia, G. R.; Meyyappan, M.; Facchetti, A.; Marks, T. J. Nano Lett. 2006, 6, 2447. (13) Fenter, P.; Burrows, P. E.; Eisenberger, P.; Forrest, S. R. J. Cryst. Growth 1995, 152, 65. Fenter, P.; Schreiber, F.; Zhou, L.; Eisenberger, P.; Forrest, S. R. Phys. ReV. B 1997, 56, 3046. (14) Krause, B.; Du¨rr, A. C.; Ritley, K.; Schreiber, F.; Dosch, H.; Smilgies, D. Phys. ReV. B 2002, 66, 235404. (15) Yim, S.; Heutz, S.; Jones, T. S. Phys. ReV. B 2003, 67, 165308. (16) Chen, W.; Huang, H.; Chen, S.; Gao, X. Y.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 5036.

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