Au(111) Interface: Coupling

Article pubs.acs.org/JPCC

Bis(triisopropylsilylethynyl)pentacene/Au(111) Interface: Coupling, Molecular Orientation, and Thermal Stability Andrea Gnoli,† Hande Ustunel,‡ Daniele Toffoli,§ Liyang Yu,∥ Daniele Catone,⊥ Stefano Turchini,⊥ Silvano Lizzit,# Natalie Stingelin,% and Rosanna Larciprete*,† †

CNR-ISC Istituto dei Sistemi Complessi Via Fosso del Cavaliere 100, 00133 Roma, Italy Department of Physics and §Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey ∥ King Abdullah University of Science and Technology, 3326-WS06, Al Kindi Building 4600 KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia ⊥ CNR-ISM Istituto di Struttura della Materia Via Fosso del Cavaliere 100, 00133 Roma, Italy # Elettra-Sincrotrone Trieste S.C.p.A., AREA Science Park, S.S. 14 km 163.5, 34149 Trieste, Italy % Department of Materials, Imperial College London, SW7 2AZ London, United Kingdom ‡

S Supporting Information *

ABSTRACT: The assembly and the orientation of functionalized pentacene at the interface with inorganics strongly influence both the electric contact and the charge transport in organic electronic devices. In this study electronic spectroscopies and theoretical modeling are combined to investigate the properties of the bis(triisopropylsilylethynyl)pentacene (TIPS-Pc)/Au(111) interface as a function of the molecular coverage to compare the molecular state in the gas phase and in the adsorbed phase and to determine the thermal stability of TIPS-Pc in contact with gold. Our results show that in the free molecule only the acene atoms directly bonded to the ligands are affected by the functionalization. Adsorption on Au(111) leads to a weak coupling which causes only modest binding energy shifts in the TIPS-Pc and substrate core level spectra. In the first monolayer the acene plane form an angle of 33 ± 2° with the Au(111) surface at variance with the vertical geometry reported for thicker solution-processed or evaporated films, whereas the presence of configurational disorder was observed in the multilayer. The thermal annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption at ∼470 K, which leaves the backbone of the decomposed molecule flat-lying on the metal surface as in the case of the unmodified pentacene. The weak interaction with the metal substrate causes the molecular dissociation to occur 60 K below the thermal decomposition taking place in thick drop-cast films.

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INTRODUCTION The electronic properties of organic semiconductors critically depend on the ability to organize themselves at the molecular level since charge transport occurs mainly through carrier hopping between adjacent molecules. High mobilities are obtained when the molecules assemble with favorable high degree of electronic overlap. In this context, the addition of side groups to acenes and in particular to pentacene (Pc) has revealed to be a successful strategy to modify the molecular crystal structure and enhance intermolecular orbital overlap.1−3 Triisopropylsilylethynyl (TIPS) substituents, which are bulky and nearly spherical insulating functional groups, partially isolate the acene core from the interactions with nearby molecules in the condensed phase.2 TIPS functionalization not only makes possible the solubility in common organic solvents but also features good transport properties arising from the “brick-wall” face-to-face arrangement of the molecular planes, which increases the orbital overlapping with respect to the edge-to-face “herringbone” stacking of Pc.4 Despite these © 2014 American Chemical Society

qualities, the use of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pc) in organic devices has been restricted by the difficulty in controlling the crystal size and orientation in films deposited by solution processes due to the strong role that uncontrollable factors such as solvent evaporation, stochastic nucleation, and fluid flow instabilities play on the molecular ordering,5,6 affecting the device performances.7 Recently, however, it has been demonstrated that these difficulties can be overcome by engineering the way to deliver and spread the solution onto the substrate. With the appropriate protocols room temperature (RT) carrier mobilities as high as 4.6 cm2/ (V s)8 and even 11 cm2/(V s)9 have been obtained.8−10 Such unprecedented results, besides reviving the technological appealing of TIPS-Pc, stimulate fundamental studies on the mechanisms that, starting from the molecule−substrate interReceived: May 5, 2014 Revised: September 5, 2014 Published: September 7, 2014 22522

dx.doi.org/10.1021/jp504401v | J. Phys. Chem. C 2014, 118, 22522−22532

The Journal of Physical Chemistry C

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(purity grade >99%) were purchased from Sigma-Aldrich. The experiments concerning the adsorption of TIPS-Pc on Au(111) were carried out in the ultrahigh-vacuum chamber (base pressure = 1 × 10−10 mbar) of the SUPERESCA beamline. The Au(111) crystal was Ar+ sputtered (600 V) and annealed at 820 K. Surface quality and cleanness were checked by means of XPS and LEED. The TIPS-Pc molecules were sublimated from a Ta crucible resistively heated at 425 K onto the substrate kept at RT. The overall pressure during deposition was in the low 10−9 mbar range. The evaporation rate was kept constant during the whole experiment by allowing the evaporator to reach thermal equilibrium before exposing the sample to the molecular flux. The evaporation time required to deposit 1 ML of TIPS-Pc molecules, estimated through the analysis of the C 1s spectra, was ∼20 min. The resulting deposition rate of ∼0.05 ML/min was used to evaluate the equivalent coverage for all TIPS-Pc layers. The XPS spectra were acquired with the photon beam impinging on the sample at grazing incidence (70°) while photoelectrons were collected at normal emission angle. The binding energies were calibrated with respect to the Fermi edge measured on the Au crystal. Survey XPS spectra were taken at a photon energy of 650 eV, whereas high-resolution Au 4f, Si 2s, and C 1s spectra were measured at photon energies of 135, 200, and 400 eV, respectively, with an energy resolution below 60 meV. Note that we measured Si 2s rather than the more intense Si 2p spectra because of the close vicinity of the latter to the Au 4f5/2 substrate peak. The C K-edge NEXAFS spectra were measured in the Auger yield mode by revealing the photoelectrons at a kinetic energy of 260 eV corresponding to the C-KLL transition. Under this modality, being the photoelectron escape depth of the order of 8−10 Å,26 the NEXAFS spectra reveal the contribution of the top molecular layers.27 Angular dependent spectra were taken as a function of the angle θ between the electric field E of the photon beam (which was horizontally polarized) and the normal n to the substrate plane. The angle θ was varied between 20° (grazing incidence) and 90° (normal incidence) by rotating the sample. Note that due to the fixed position of the electron analyzer, the geometry with the beam impinging at normal incidence means to reveal the Auger electrons with an emission angle of 70°, whereas the grazing incidence (θ = 70°) configuration means to acquire the spectra at normal emission, enhancing the sensitivity to the bulk of the sample. The gas phase measurements were performed at the 4.2 Circular Polarization beamline, equipped with a normal incidence monochromator and a grazing incidence spherical grating monochromator, sharing the same entrance and exit slits.28 The photoelectron spectra were recorded using a VG, 6channel, 150 mm hemispherical electron energy analyzer, placed at 54.7° with respect to the synchrotron radiation polarization vector. The analyzer was set at 10 eV pass energy, and the overall resolution was about 200 meV at both 120 and 330 eV photon energies, used for the measurement of valence band and C 1s photoelectron spectra, respectively. In the experimental vacuum chamber (base pressure = 7 × 10−8 mbar) the TIPS-Pc was evaporated from a noncommercial, noninductively wound furnace.29 DFT calculations were conducted in a plane-wave pseudopotential framework, as implemented in the Quantum Espresso distribution.30 The Perdew−Becke−Ernzerhof31 exchange-correlation functional and ultrasoft pseudopotentials were used in all calculations.32 The open-source program

face, govern at the microscopic level the crystalline orientation and uniformity. For unmodified Pc the adsorption on insulating and metallic substrates and the transition between different thicknessdependent growth modalities have been deeply investigated. Several studies have demonstrated that on metal surfaces the relatively strong molecule−substrate interaction usually locks Pc into a flat geometry,11−15 the standing-up orientation being recovered only in the multilayer,16−18 whereas on inert substrates, such as SiO2,19 TiO2,20 BN,15 or SAM-covered metals,14,21 the substrate−molecule interaction vanishes and the backbone stands up even in the monolayer (ML). For substituted Pc molecules these aspects have been only rarely explored. For both vacuum evaporated1 and thin solution casting9 TIPS-Pc films, it has been demonstrated that the molecules align with the acene unit “edge-on” oriented touching down with the TIPS groups. However, a recent molecular dynamics study found that the TIPS-Pc molecules lie flat on the SiO2 surface up to 1 ML and, at this coverage, only occasionally stand up.22 In order to understand and control the assembling of TIPS and other modified Pc at the interface with inorganics, fundamental aspects such as the role of the functionalizing groups in screening the acene−surface interaction or the capability of the branched chains to hinder the relaxation toward the equilibrium packing in molecules close to the substrate are of primary importance.23 In particular, the interactions between the molecules and metallic substrates play a major role in driving the emergence of long-range ordering at the submonolayer level24,25 as well as in determining the property of the electric contact in organic electronic devices. In this respect it is believed that the molecular orientation in the first 2 or 3 MLs near the surface of the dielectric gate might significantly affect the charge transport. A better understanding of such issues, which can be investigated in detail for vacuum evaporated films, will also lead to the advancements of solution coating routes. In this study we have followed the adsorption of TIPS-Pc on the Au(111) surface by high-resolution X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption spectroscopy (NEXAS) from the submonolayer to the multilayer regime. The choice of the Au substrate has been dictated by its wide use in contacting electronic devices. We have also investigated isolated TIPS-Pc molecules in the gas phase. By calculating the C 1s core level spectra using density functional theory (DFT) modeling, we have derived the influence of the TIPS groups on the acene binding energies (BE) for the freestanding molecule and the effect of substrate−molecule coupling on the BE shifts of the backbone and ligand atoms for the adsorbed TIPS-Pc. Our results show that the interaction with the Au(111) surface is sufficient for orienting the acene plane in the first monolayer to lie with a modest tilt angle with respect to the substrate, whereas this configuration disappears in thicker films. We also demonstrate that the TIPS-Pc molecules in contact with the Au(111) surface are less thermally stable than in solution casting films due to the fact that around 470 K the TIPS ligands detach from the acene, which gets closer to the substrate mimicking the geometry of the unmodified Pc molecules on Au(111).

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EXPERIMENTAL SECTION The experiment was performed at the Elettra (Trieste, Italy) synchrotron radiation facility. TIPS-pentacene and pentacene 22523



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Figure 1. XPS analysis of the TIPS-Pc/Au(111) interface. (a) Survey spectra measured at 650 eV for the shortest (5 min) and the longest (150 min) deposition times td corresponding to TIPS-Pc equivalent coverages of 0.25 and 7.5 MLs. (b) Au 4f7/2, (c) Si 2s, and (d) C 1s high-resolution core level spectra measured at increasing td. The spectra are vertically shifted for clarity. The Si 2s and C 1s spectra share the curve legend shown in (c). In (d) the left inset shows the C 1s intensity as a function of td, whereas the right inset shows the low-BE shoulder for the spectra measured at td = 20 and 110 min and normalized to the same height.

where E*A and E*ref are the ionized state total energies of the system when the photoelectron is extracted from atom A and the reference atom, respectively, creating a core hole. The carbon atoms with a core hole is described by a pseudopotential generated in the 1s12s22p23d0 configuration. In accordance with the final-state approximation, E*A and E*ref are calculated in an ionic configuration with a charge of +1 per simulation cell, compensated by a uniform background. The structural optimization and core-level shifts of the gas phase TIPS-Pc molecule were calculated in a large simulation box in the form of a parallelepiped with an edge length of approximately 22 Å using the Γ point for the Brillouin zone integration. All other calculation parameters are the same as listed above.

XCrysDen33 was used for visualization and to produce the figures. The weak interaction between the Au(111) surface and TIPS was modeled using the van der Waals correction (VdWDF)34 to the PBE exchange-correlation functional. During the Broyden−Fletcher−Goldfarb−Shanno (BFGS) geometry optimizations, a force threshold per atom of 0.025 eV/Å was used. The plane-wave basis set was determined by a kinetic energy cutoff of 35 Ry, while a charge density cutoff of 350 Ry was used. The Brillioun zone integration was performed at the Γ point using a Marzari−Vanderbilt smearing35 with a width of 0.01 Ry. The Au(111) surface was modeled by a two-layer slab. All Au atoms were held fixed during the geometric optimization to their bulk coordinates. No rotational or translational restraints were imposed on the molecule. This approach is justified by the weak interaction between the surface and adsorbate. A vertical distance of at least 11 Å was left between the highest atom of the adsorbate and the bottom Au layer of its closest periodic image in order to minimize interaction. Two adsorption geometries were studied as reported in Figure 4. Simulation cells of 6 × 5√3 and 7 × 4√3 were used for the short-edge geometry [A] and the long-edge configuration [B] depicted in Figure 4. Following structural optimization, the C 1s binding energy shifts were calculated in the final state approximation,36 which assumes full relaxation of electronic degrees of freedom by the time the photoelectron is detected. The binding energy of a 1s electron bound to a given carbon atom A is calculated relative to a reference BE using

* Δ = EA* − Eref

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RESULTS AND DISCUSSION Core Level Spectroscopy. Figure 1a shows the XPS spectra measured after the shortest (5 min) and the longest (150 min) evaporation times (td). The spectra exhibit the substrate Au 3d doublet with the 3/2 and 5/2 components at BEs of 353.2 and 335.1 eV and at around 284 eV the C 1s peak arising from the TIPS-Pc molecules. The high-resolution Au 4f7/2, Si 2s, and C 1s spectra measured at increasing td are shown in Figures 1b, 1c, and 1d, respectively. With increasing TIPS-Pc coverage, the Au 4f7/2 core level, beside a strong attenuation, exhibits only weak modifications of its line shape, with a slight upshift of the spectral intensity, which accounts for the change in BE of first layer Au atoms upon the adsorption of TIPS-Pc molecules. The analysis of selected Au 4f7/2 spectra measured at increasing td is

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ordering. With increasing TIPS-Pc coverage, molecular overlapping and geometrical rearrangement within the layer will progressively bring all Au surface atoms to interact with the adsorbate, thus canceling completely the S component in the Au 4f7/2 spectrum. The reduction of the Au 4f7/2 intensity to ∼10% at td = 110 min (see the Supporting Information), which corresponds (see below) to a TIPS-Pc coverage slightly above 5 ML, indicates that the layer grows quite homogeneously, without extensive formation of 3D islands. The Si 2s core level spectra (see Figure 1c) show always a single component rising with td and located, for monolayer molecules, at BE of 151.1 eV (for further details see the Supporting Information). The high BE shift (+0.5 eV) measured for the Si 2s spectrum of the thickest with respect to the thinnest layer can be attributed to the absence of metal screening in the multilayer38 and is commonly observed for molecular multilayers on metals.39 The C 1s spectra (see Figure 1d) taken at low td clearly show two main features at 284.1 (P1) and 283.7 eV (P2). This spectral line shape remains constant up to td = 23 min and then evolves converting progressively into a broad unstructured peak that, for the thickest film, is peaked at 284.45 eV. All C 1s spectra show a weak low-BE shoulder (P3) which at low coverage appears at 288.4 eV (see right inset in Figure 1d). A shift of +0.5 eV, similar to that observed in the Si 2s spectra, is estimated between the centers-of-gravity of the C 1s peaks measured at the lowest and the highest TIPS-Pc coverages. The left inset in Figure 1d shows that the C 1s intensity rises linearly up to td ∼ 20 min and then increases with a lower rate. Above this deposition time the spectral intensity starts to upshift and the line shape to modify, thereby indicating the onset of multilayer adsorption.40 The structured profile observed at submonolayer coverage is regulated by the substrate that orients the molecules in certain configurations and interacts preferentially with specific acene and TIPS atoms. Such an effect is lost in the upper TIPS layers, and hence multilayer films exhibit almost structureless line shapes.38,39 Therefore, we take the line shape measured at td = 20 min as representative of 1 ML of TIPS-Pc molecules adsorbed on the Au(111) surface. We assumed constant evaporation rate and sticking coefficients on the growing layer to evaluate the equivalent coverage obtained at different td. With this assumption the highest coverage corresponding to td ∼ 150 min amounts to ∼7.5 ML. C 1s Line Shape Analysis. In order to elucidate the origin of the C 1s spectral features P1, P2, and P3, we have calculated the C 1s BE shifts for the TIPS-Pc molecules adsorbed on the Au(111) surface. However, due to the large number of nonequivalent C atoms, in order to better establish the effect of the interaction with the metal substrate on the C 1s spectrum, we have carried out a parallel investigation on the isolated TIPS-Pc in the gas phase. In this case the BE shifts, as referred to the energy position of the backbone atom C2 (see Figure 3), are listed in Table 3 of the Supporting Information. With respect to unmodified pentacene41 the presence of the TIPS substituents shifts the BE of the C1 atoms by ∼+0.6 eV while leaving almost unaltered the BEs of the other acene atoms. In the TIPS the contribution of C8, the ethynyl C atom bonded to Si, is strongly downshifted with respect to the backbone atoms. Conversely, the C9−C17 atoms, belonging to the isopropylic groups, manifest BE shifts between −0.32 and +0.34 eV. The calculated C 1s spectrum is compared in Figure 3 with the experimental TIPS-Pc gas phase spectrum which exhibits a single unstructured peak centered at 289.8 eV and a

reported in Figure 2a (for further details see the Supporting Information). Component separation was obtained by best-

Figure 2. High-resolution Au 4f7/2 spectra measured on the TIPS-Pc/ Au(111) interface as a function of the deposition time td. (a) Decomposition of selected Au 4f7/2 spectra (black dots) into spectral components: metal bulk (B) and first layer metal atoms in the free surface (S) and interacting with TIPS-Pc molecules (S′); the best-fit curves are represented by the red lines. (b) Relative intensities of the Au4 f 7/2 components vs td.

fitting the spectra with Doniach−S̆unjić functions convoluted with Gaussians and a linear background. In the spectrum of the clean sample the two components B (83.98 eV) and S (83.68 eV) represent bulk and first layer atoms, respectively.37 The intensity of S decreases with increasing td while a new component S′ arises at 83.74 eV due to surface Au atoms interacting with TIPS-Pc molecules. The exiguous difference of +60 meV between the S and S′ BEs indicates that molecular adsorption affects only marginally the electronic structure of the first layer Au atoms. The intensities of the B, S, and S′ components normalized to the total Au 4f7/2 area are plotted in Figure 2b as a function of td. As it will be made clear below, td = 20 min corresponds to a coverage of about 1 ML. The curves show that at this coverage about half of the first layer Au atoms remain unperturbed and that S disappears completely only in the thickest layer. The presence of a consistent S intensity in correspondence to multilayer adsorption might indicate the weakness of the molecule− metal interaction, with a large fraction of the Au surface left unmodified, as well as the presence of uncovered surface regions which would be compatible with the TIPS-Pc conformation combined with the lack of rotational layer 22525



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Figure 3. (left) Ball-and-stick representation of the TIPS-Pc molecule; (right) experimental (hν = 330 eV) and calculated C 1s spectra of gas phase TIPS-Pc. The calculated spectrum was obtained by summing 17 Voigt components, each one with its multiplicity, whose fwhm (0.5 eV), common to all components, was derived by best-fitting the experimental spectrum. The best agreement between the experimental and the calculated spectra was obtained by fixing the BE of the C2 component at 289.92 eV. The bars at the bottom indicate the BEs of the different C atoms used for the calculated C 1s spectrum: acene (purple), ethynyl groups (blue), central (yellow), and terminal (green) C atoms in the isopropylic groups. The numbers in the right panel are relative to the TIPS-Pc scheme on the left. The black curve shows the line shape of each component in the calculated C 1s spectrum.

Figure 4. Comparison between experimental and calculated C 1s spectra for adsorbed TIPS-Pc on Au(111). (left) Adsorption geometries considered for the DFT calculations of the C 1s BE shifts; (right top) comparison between the experimental C 1s spectra measured for 1 ML TIPSPc/Au(111) and the curves calculated for the configurations [A] and [B]; (right bottom) BE shifts calculated for the configurations [A] and [B]: acene (purple), ethynyl groups (blue), isopropylic groups in the TIPS T1 (light green), and T2 (green).

weak low-BE shoulder at about 288.4 eV. The bars at the bottom of Figure 3, which depict the calculated BE positions of all C atoms, show that the TIPS and acene contributions mainly overlap. Therefore, the presence of the ligands does not manifest as a distinguished spectral feature except for the weak

low-BE peak at 282.8 eV, absent in the case of pentacene, which arises from the C8 atoms. The effect of the interaction with the substrate was evaluated by calculating the C 1s BE shifts for TIPS-Pc molecules adsorbed on the Au(111) crystal. As shown below, for submonolayer and monolayer coverage, the NEXAFS spectra 22526



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measured for the TIPS-Pc/Au(111) interface indicate that the acene forms an angle of α = 33 ± 2° with the substrate plane. Nevertheless, as it will be better explained in the next paragraph, we cannot determine if the molecular backbone is inclined along its long or short axis. In an attempt to identify the bonding geometry, in our calculations we considered the two configurations sketched in Figure 4: [A] with the molecular backbone inclined along its long axis and leaning on the metal through one of the terminal phenyl rings, and 4 [B] with the backbone inclined along its short axis while only one TIPS is in contact with the substrate. The BE shifts calculated in the two configurations are listed in Table 4 of the Supporting Information and are represented in the bottom right panel of Figure 4 as referred to the most red-shifted energy, which is set at 282.66 eV in correspondence of P3. These correspond in [A] to the two ethynyl atoms (C13 and C35) bonded to Si in the TIPS T1 and T2, respectively, and in [B] to the ethynyl C atom (C13) bonded to Si in the TIPS T1 distant from the Au surface. The C 1s spectra, obtained by summing 44 Doniach−S̆unjić components, are displayed in the upper-right panel of Figure 4 for the configurations [A] and [B] together with the experimental C 1s spectrum of 1 ML TIPSPc/Au(111). The best agreement between the calculated and experimental spectra was obtained by using Doniach−S̆unjić functions with a common Gaussian width ΓG = 0.5 eV, Lorentzian width ΓL = 0.05 eV, and asymmetry α = 0.07 while taking BE(C13) = 282.66 eV. In the calculated spectra, the BEs of the backbone and TIPS atoms interacting with the substrate are only slightly blue-shifted, in agreement with a moderate coupling between the molecule and the substrate. Both configurations satisfactorily replicate the experimental distance of ∼1.4 eV between the P1 and P3 structures. For the configuration [B], the contribution of C35 in T2 is upshifted by ∼1 eV, leaving only its analogue in T1 to contribute to the P3 peak, whose relative intensity is halved with respect to [A]. The bars in the bottom right panel of Figure 4 show that the adsorption through the terminal phenyl ring in [A] partially removes the degeneracy among the atoms that occupy symmetric positions in the acene, whereas the asymmetric adsorption in [B] slightly differentiates the contributions of the two TIPS ligands. Thereby, with respect to the free-standing TIPS-Pc molecule, the adsorption on Au(111) slightly increases the spectral intensity away from the structure P3. None of the simulated curves reproduces the P1 −P2 experimental structures. In particular, the P2 feature seems not to derive from the interaction between the Au(111) substrate and isolated adsorbed TIPS-Pc molecules. The origin of the P2 feature could be then attributed either to a stronger interaction with the surface via defects or to the interaction between TIPS-Pc molecules that were not picked up in our theoretical calculations modeling isolated molecules. However, it must be noted that, at variance with the calculated curves where all C contributions are simply summed, in the experimental spectra the intensities effectively revealed for the different C atoms are damped by the screening effects arising from the complex molecular conformation. Moreover, photoelectron diffraction might also play a role. These effects could enhance only certain spectral components structuring the experimental line shape. Valence Band Spectroscopy. Figure 5 shows the valence band spectra measured at a photon energy of 200 eV for the clean Au(111) surface and for the TIPS-Pc/Au(111) interface at increasing coverage. In all cases the spectra were recorded at

Figure 5. Valence band spectra measured on the Au(111) surface before and after the adsorption of TIPS-Pc molecules at different coverages. The bottom curve shows the valence band measured for gas phase TIPS-Pc.

normal emission angle. The curve measured on the clean metal shows the Au 5d bands at BEs below 10 eV. The adsorbed TIPS-Pc molecules gradually screen the Au spectrum while the molecular bands become visible. The bottom spectrum in Figure 5 shows the valence band measured on gas phase TIPSPc which exhibit the HOMO centered at ionization energy of 6.4 eV, in close agreement with previous results.42,43 The HOMO is not evident in the spectra of the adsorbed molecules due to the residual intensity of the metal substrate and to the spectral broadening which arises from inhomogeneities, intermolecular coupling, and polarization effects within the condensed layers.44,45 NEXAFS Spectroscopy. The C K-edge NEXAFS spectra of 1 ML TIPS-Pc/Au(111) are shown in Figure 6a as a function of the angle θ between the electric field E of the incident photons and the normal n to the sample surface. Similarly to other conjugated molecules,46 in particular pentacene,12,41 the spectra are characterized by sharp resonances between 283 and 287 eV due to excitations of the C 1s electrons to the empty LUMO and LUMO+1 π* states and by broader resonances above 290 eV attributed to excitations into σ* orbitals. In contrast to Pc, the TIPS-Pc spectra show significant intensity also between 287 and 289 eV. This is made evident by the comparison between the 1 ML TIPS-Pc/Au(111) and the ∼1 ML Pc/Au(111) spectra taken at normal (θ = 90°) and grazing (θ = 20°) incidence, shown in Figures 6c and 6d, respectively. The spectral intensity in the 287−289 eV region can be assigned to the C−H and C−C σ* transitions47 in the isopropylic groups: these are unaligned because of the presence of conformers,27 22527



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Figure 6. C K-edge NEXAFS spectra taken on 1 ML TIPS-Pc adsorbed on Au(111): (a) angular dependence of the NEXAFS spectra measured as a function of the angle θ between the direction of the electric field E and the normal n to the sample surface; (b) normalized integral intensity of the π* resonances vs cos2 θ; (c, d) comparison between the NEXAFS spectra taken at normal and grazing incidence, respectively, on 1 ML TIPS-Pc/ Au(111) and ∼1 ML Pc/Au(111).

and hence the corresponding intensity remains high at all θ angles. The behavior of the NEXAFS spectra as a function of θ signals a marked linear dichroism indicative of the presence of a preferential orientation of the conjugated molecular plane on the Au(111) surface. Such orientation can be elucidated by considering the angular dependence of the integral intensity I of the π* resonance. The angle α between the normal n to the substrate surface and the normal N to the acene plane can be derived from the relation 2 obtained from the standard theory applied to the 3-fold symmetry46,48,49 of the Au(111) surface I (θ ) =

A [P cos2 θ(2 − 3 sin 2 α) + sin 2 α] 2

almost vertically with respect to the substrate plane, and the bulky side groups touch down with the stacking direction being parallel to the substrate in a packing nearly identical to that of the bulk crystal. However, the formation of nonequilibrium molecular packing, due to confined molecular motion near the substrate, has been proposed to explain the deviation from the equilibrium crystal lattice in TIPS-Pc films made of less than 10 molecular layers.9 In this respect, recent molecular dynamics simulations find that at low coverage (1 ML) the TIPS-Pc molecules lie nearly flat on the SiO2 surface.22 Accordingly, our results indicate that the interaction with the Au substrate forces the first layer molecular backbones to lie closer to the substrate, whereas, as it will be shown below, a more vertical orientation is adopted in thicker films.50,51 It is worth noting that monolayer Pc molecules lie down on several metal surfaces,11−15 in contrast to a standing-up geometry which is adopted even at monolayer coverage on inert surfaces.15,19−21 An interesting follow-up to this work would be to investigate the first stage of TIPS-Pc adsorption on noninteracting surfaces to find out if also the TIPS-functionalized backbone is prone to align nearly perpendicularly to the substrate or if the presence of the substituent groups favors even on inert surfaces a leaning-down geometry.22 The evolution of the NEXAFS spectra with the TIPS-Pc coverage is shown in Figure 7, which compares the spectra taken at grazing and normal incidence at increasing coverage from 0.5 to 5.5 ML. The corresponding high-resolution spectra of the π* resonances are displayed in the left panels. The

(2)

where A is a constant and P is the polarization factor of the beamline which is ≈1 in our experiments. The intensity I(θ) integrated between 281 and 285.7 eV and normalized to I(θ) = 20° is plotted vs cos2 θ in Figure 6b for TIPS-Pc/Au(111) at coverage of 0.5 and 1 ML. The linear fit of the data gives α = 33 ± 2°, a value that might be slightly overestimated due the π* resonances associated with the px and py orbitals of the ethynyl groups which contribute almost equally at grazing and normal incidence.27 Therefore, our results indicate that the TIPS-Pc molecules lie on the Au surface with the acene moderately inclined with respect to the surface. However, the data do not allow to ascertain whether the molecular backbone is inclined along its short or long axis. It is well-known that in solution-processed50 as well as in vacuum-deposited thin films on SiO251 the TIPS-Pc acenes lie 22528



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the individual C atoms are needed to assign the C 1s → π* transitions and to distinguish between the LUMO and LUMO +1 contributions. Among the various components peaks A and C show the strongest angular dependence, α being 15°−20°, which is consistent with a nearly perpendicular orientation of the corresponding orbitals. In contrast, peak B with α ∼ 50° signals a nearly angularly isotropic orbital orientation. The modest angular dependence of peak B, also in the multilayer spectra, indicates that the orbital distortion with respect to the pure π symmetry is not induced by the Au substrate but it is intrinsic to the TIPS-Pc molecule. Because of that, peak B can be reasonably related to the π orbitals of the ethynyl groups or to those of the central phenyl ring which likely undergo complex rehybridization. In the multilayer, due to orientational disorder and to the complete decoupling from the substrate, the spectral profiles of the π* resonances, taken at grazing and normal incidence, tend to become similar. The behavior observed at intermediate coverage (1.5 ML) can be explained by taking into account that the NEXAFS spectra were measured by revealing the CKK Auger electrons which have a limited escape depth, and because of that, the layer in contact with the substrate and the layer on top are probed with different relative sensitivity at normal and grazing incidence. Therefore, for a coverage of 1.5 ML, at normal incidence (grazing emission) the contribution of the top molecular layer is dominant, and the NEXAFS spectrum closely resembles that of the multilayer, whereas at grazing incidence the similarity vanishes because of the higher spectral weight of the layer in contact with the substrate. Thermal Stability. The thermal stability of the TIPS-Pc molecules adsorbed on Au was studied by following with fastXPS the thermal annealing of the 1.5 ML TIPS-Pc/Au(111) interface up to 660 K. The comparison in Figure 8a between the XPS spectra acquired before and after the thermal annealing demonstrates the occurrence of TIPS-Pc desorption, which reduces the C 1s signal and uncovers the Au substrate. The sequence of C 1s spectra measured while heating the sample and shown as a 2D plot in Figure 8b indicates that the C 1s line shape is almost constant up to 470 K and that above this temperature its intensity and width abruptly decrease, manifesting the occurrence of multilayer desorption as well as of chemical reactions. The high-resolution C 1s spectra shown in Figure 8d indicate the occurrence of slight molecular desorption and, possibly, reorientation already at 430 K, whereas the altered line shape arising after annealing at 540 K changes only marginally upon further heating to 660 K. The analysis of the fast-XPS spectra (see Figure 8c) revealed that, between 480 and 520 K, the C 1s peak intensity is halved and its fwhm reduces by 150 meV, while the center-of-gravity of the peak downshifts by ∼100 meV. The ∼50% reduction of the C 1s intensity allows us to estimate the residual coverage to be less than 0.7 ML, considering that for the pristine coverage of 1.5 ML the intensity of the bottom molecules was partly attenuated by those lying on top. By comparing the C 1s spectra shown in Figure 8d with the calculated spectra of Figure 4, we deduce that the peak narrowing is determined by the intensity loss in correspondence with the spectral contribution of the isopropylic groups. In the Si 2s spectrum (see Figure 8e), the component at 151.1 eV due to adsorbed TIPS molecules loses ∼80% of the initial intensity, which is only partly transferred to a new component (152.1 eV) representing Si atoms in molecular fragments or directly bonded to the Au surface. The molecular dissociation is

Figure 7. (right) C K-edge NEXAFS spectra taken at normal (θ = 90°) and grazing (θ = 20°) incidence on TIPS-Pc/Au(111) interfaces at molecular coverages of 0.5, 1.5, and 5.5 ML and (left) highresolution spectra of the corresponding π* regions. The bottom panels show the survey (right) and high-resolution (left) NEXAFS spectra of gas phase TIPS-Pc.

transition from the submonolayer to multilayer regime corresponds to a change in the acene orientation with respect to the substrate and to a substantial attenuation of the linear dichroism in the multilayer. In this latter case, whereas the angular dependence of the π* resonance gives α = 42 ± 3°, which is only 10° higher than in the monolayer, the spectral line shapes in the σ* region become quite similar when measured at normal and grazing angles, indicating the presence of configurational disorder and/or the coexistence of different phases. The lower panels in Figure 7 show the NEXAFS spectra measured for gas phase TIPS-Pc. The high-resolution spectra (left panel) highlight the presence of evident differences between the π* resonance line shapes taken at grazing and normal incidence on the adsorbed TIPS-Pc. Such differences are marked at low coverage and stem from the interaction with the substrate which reorganizes the pure π symmetry of the molecular orbitals.21 The upper left panels also show a tentative decomposition of the π* resonances spectra into 10 equally broad Gaussian functions, 10 being the minimum number of peaks necessary to adequately fit the data for almost all the θ-dependent sequences of spectra measured at several molecular coverages, while keeping the same energy positions and width of the Gaussian components. Calculations of the empty molecular orbitals for 22529



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Figure 8. Thermal annealing of the 1.5 ML TIPS-Pc/Au(111) interface. (a) XPS spectra taken on the sample at RT and after annealing at 660 K. (b) 2D plot of the fast-XPS C 1s spectra measured during sample heating; the bottom and the top spectra are the high-resolution C 1s spectra measured before and after thermal annealing. (c) Intensity, full width at half-maximum (fwhm), and BE of the center-of-gravity of the C 1s spectra vs annealing temperature. (d) C 1s and (e) Si 2s spectra measured at RT and after heating the sample. For the Si 2s spectra the components and the best-fit curves are also shown. (f) Valence band spectra measured before and after thermal annealing. The inset shows a magnification of the spectra between 30 and 10 eV. (g) Comparison between the NEXAFS spectra taken at grazing and normal incidence on the TIPS-Pc layer annealed at 660 K and on ∼1 ML Pc adsorbed at RT on Au(111).

massive detachment of the TIPS substituents from the acene backbone.

confirmed by the valence band spectrum (see Figure 8f) that for the annealed film reveals strong changes in the molecular bands between 10 and 30 eV. All the spectral evidence indicates that the molecules start to decompose and the TIPS ligands to desorb slightly above 470 K, leaving the backbone on the Au(111) surface. This conclusion is supported by the comparison between the NEXAFS spectra measured for the 1.5 ML TIPS-Pc/Au(111) interface before (Figure 7) and after annealing (Figure 8g). For the annealed TIPS-Pc, the profile in the π* region appears strongly smoothed, and the spectral intensity due to the ligand σ* bonds in the 287−289 eV range significantly decreased at both grazing and normal incidence. The spectra of the annealed layer appear similar to those measured for ∼1 ML Pc/Au(111), shown for comparison at the bottom of Figure 8g, suggesting that the backbone of the decomposed TIPS-Pc molecules tends to orient on the Au(111) surface like the unmodified Pc molecules. For drop-cast TIPS-Pc films, differential scanning calorimetry measurements showed a reversible solid-state phase transition at 397 K and an endothermic melting at 534 K followed by an exothermic irreversible degradation around 539 K.50 The lowtemperature transition was associated with a conformational reorganization of the TIPS groups, accompanied by a slight decrease in the acene-to-acene spacing and a shift in the overlap among neighboring acenes.50 Such structural modifications, responsible for mechanical cracking in the TIPS-Pc films,50 remain quite undetectable by our spectroscopic techniques, besides the slight modification observed in the C 1s spectrum taken after annealing at 430 K (Figure 8d). The hightemperature degradation was related to a complete decomposition of TIPS-Pc, most likely by Diels−Alder polymerization between the alkyne substituent and the Pc backbone.50 Our results show that for monolayer TIPS-Pc adsorbed on Au(111) the thermal degradation occurs 60 K below and consists in the

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CONCLUSIONS The RT adsorption of TIPS-Pc on Au(111) surface and the thermal stability of the interface were investigated by combining electronic spectroscopies and DFT modeling. By using the calculated C 1s BE shifts for the different C atoms, we could fully reproduce the core level line shape of isolated molecules and simulate the main spectral features measured for the adsorbed molecules, demonstrating that the TIPS ligands broaden the spectrum of the acene backbone toward high BEs and add a distinguished low-BE feature arising from the ethynyl C atoms bonded to Si. The slight C 1s BE shifts, measured and calculated for the atoms in contact with Au, indicate a moderate electronic coupling with the substrate, in agreement with the modest modifications revealed in the metal core level spectra. TIPS-Pc molecules were found to adsorb with the acene plane forming a moderate tilt angle with the substrate plane, in agreement with recent modeling and different from the configuration with the acene plane vertical with respect to the substrate observed in thicker solution processed or evaporated film. For multilayer film the NEXAFS spectra indicate the onset of configurational disorder, suggesting the presence of a transition layer where the acene planes rearrange from a nearly flat to a vertical configuration, which might regulate the electronic coupling between the organic layer and the Au substrate. We found that TIPS-Pc molecules in contact with Au are less thermally stable than in thick films. Therefore, the interaction with the substrate, although quite weak, causes the early detachment of the TIPS substitutents from the acene backbone, which occurs 60 K below the temperature of the thermal decomposition in thick drop-cast films. The detailed understanding of the TIPS-Pc-metal interface formation studied 22530



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here for vacuum-evaporated films will also contribute to the advancements of solution coating routes extensively used for TIPS-Pc film deposition.

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(12) Söhnchen, S.; Lukas, S.; Witte, G. Epitaxial Growth of Pentacene Films on Cu110. J. Chem. Phys. 2004, 121, 525−534. (13) McDonald, O.; Cafolla, A.; Carty, D.; Sheerin, G.; Hughes, G. Photoemission, NEXAFS and STM Studies of Pentacene Thin Films on Au(100). Surf. Sci. 2006, 600, 3217−3225. (14) Käfer, D.; Ruppel, L.; Witte, G. Growth of Pentacene on Clean and Modified Gold Surfaces. Phys. Rev. B 2007, 75, 085309. (15) Ng, M. L.; Preobrajenski, A. B.; Zakharov, A. A.; Vinogradov, A. S.; Krasnikov, S. A.; Cafolla, A. A.; Mårtensson, N. Effect of Substrate Nanopatterning on the Growth and Structure of Pentacene Films. Phys. Rev. B 2010, 81, 115449. (16) Satta, M.; Iacobucci, S.; Larciprete, R. Molecular Adsorption and Multilayer Growth of Pentacene on Cu(100):Layer Structure and Energetics. Phys. Rev. B 2007, 75, 155401. (17) Käfer, D.; Witte, G. Evolution of Pentacene Films on Ag(111): Growth Beyond the First Monolayer. Chem. Phys. Lett. 2007, 442, 376−383. (18) zu Heringdorf, F. M.; Reuter, M.; Tromp, R. Growth Dynamics of Pentacene Thin Films. Nature 2001, 412, 517−520. (19) Schwieger, T.; X. Liu, D. O.; Knupfer, M.; Schmidt, T. Orientation and Electronic Properties of Pentacene Molecules on SiO2 and GeS(0001) Studied Using X-Ray Absorption Spectroscopy. J. Appl. Phys. 2004, 96, 5596−5600. (20) Lanzilotto, V.; Sanchez-Sanchez, C.; Bavdek, G.; Cvetko, D.; Lopez, M.; Martin-Gago, J.; Floreano, L. Planar Growth of Pentacene on the Dielectric TiO2(110) Surface. J. Phys. Chem. C 2011, 115, 4664−4672. (21) Chiodi, M.; Gavioli, L.; Beccari, M.; Castro, V. D.; Cossaro, A.; Floreano, L.; Morgante, A.; Kanjilal, A.; Mariani, C.; Betti, M. Interaction Strength and Molecular Orientation of a Single Layer of Pentacene in Organic-Metal Interface and Organic-Organic Heterostructure. Phys. Rev. B 2008, 77, 115321. (22) Chen, S.; Ma, J. Substituent Effects on Packing Entropy and Film Morphologies in the Nucleation of Functionalized Pentacenes on SiO2 Substrate: Molecular Dynamics Simulations. J. Chem. Phys. 2012, 137, 074708. (23) Gundlach, D.; Royer, J.; et al. S. P. Contact-induced Crystallinity for High-Performance Soluble Acene-based Transistors and Circuits. Nat. Mater. 2008, 7, 216−221. (24) Tomba, G.; Stengel, M.; Schneider, W.-D.; Baldereschi, A.; Vita, A. D. Supramolecular Self-Assembly Driven by Electrostatic Repulsion: The 1D Aggregation of Rubrene Pentagons on Au(111). ACS Nano 2010, 4, 7545−7551. (25) Blüm, M.-C.; Cávar, E.; Pivetta, M.; Patthey, F.; Schneider, W.D. Supramolecular Self-Assembly Driven by Electrostatic Repulsion: The 1D Aggregation of Rubrene Pentagons on Au(111). Angew. Chem. 2005, 117, 5468−5471. (26) Lesiak, B.; Kosinski, A.; Jablonski, A.; Köver, L.; Tȯth, J.; Varga, D.; Cserny, I.; Zagorska, M.; Kulszewicz-Bajer, I.; Gergely, G. Determination of the Inelastic Mean Free Path of Electrons in Polythiophenes Using Elastic Peak Electron Spectroscopy Method. Appl. Surf. Sci. 2001, 174, 70−85. (27) Burke, K.; Shu, Y.; Kemppinen, P.; Singh, B.; Bown, M.; Liaw, I.; Williamson, R.; Thomsen, L.; Dastoor, P.; Belcher, W.; et al. Single Crystal X-ray, AFM, NEXAFS, and OFET Studies on Angular Polycyclic Aromatic Silyl-Capped 7,14-Bis(ethynyl)dibenzo[b,def ]chrysenes. Cryst. Growth Des. 2012, 12, 725−731. (28) Derossi, A.; Lama, F.; Piacentini, M.; Prosperi, T.; Zema, N. High Flux and High-Resolution Beamline for Elliptically Polarized Radiation in the Vacuum-Ultraviolet and Soft-X-Ray Regions. Rev. Sci. Instrum. 1995, 66, 1718−1720. (29) Catone, D.; Stener, M.; Decleva, P.; Contini, G.; Zema, N.; Prosperi, T.; Feyer, V.; Prince, K. C.; Turchini, S. Resonant Circular Dichroism of Chiral Metal-Organic Complex. Phys. Rev. Lett. 2012, 108, 083001. (30) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; et al. I. D. QUANTUM ESPRESSO: a Modular and Open-Source Software

ASSOCIATED CONTENT

S Supporting Information *

C 1s and Au 4f7/2 intensities vs deposition time; analysis of the Au 4f7/2 and Si 2s spectra: best fit curves, residuals, fit parameters; C 1s BE shifts calculated for the TIPS-Pc molecules in the gas phase and adsorbed on Au(111). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors dedicate this paper to the memory of Gianluca Latini, who had initiated this project with passion and enthusiasm. The support of the SuperESCA and CiPo beamline staffs of Elettra is deeply acknowledged. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/20072013) under grant agreement no. 226716.

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REFERENCES

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Au(111) Interface: Coupling

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