Modulation of Surface Charge Transfer through Competing Long

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Modulation of Surface Charge Transfer through Competing Long-Range Repulsive versus Short-Range Attractive Interactions J. Fraxedas,*,† S. García-Gil,† S. Monturet,‡ N. Lorente,† I. Fern
andez-Torrente,§ K. J. Franke,§ J. I. Pascual,§ A. Vollmer,|| R.-P. Blum,^ N. Koch,^ and P. Ordej
on† †

Centre d’Investigaci
o en Nanociencia i Nanotecnologia, CIN2 (CSIC-ICN), Edifici CM7, Campus de Bellaterra, E-08193 Barcelona, Spain Nanoscience Group & MANA Satellite CEMES/CNRS, 29 rue J. Marvig, BP 4347, 31055 Toulouse Cedex, France § Institut f€ur Experimentalphysik, Freie Universit€at Berlin, Arnimallee 14, 14195 Berlin, Germany Helmholtz Zentrum Berlin f€ur Materialien und Energie, Albert Einsteinstrasse 15, 12489 Berlin, Germany ^ Institut f€ur Physik, Humboldt-Universit€at zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany

)

‡

ABSTRACT:

We report a combined experimental and theoretical study of the modulation of surface charge transfer on the tetrathiafulvalene (TTF)/Au(111) interface as a function of coverage in the submonolayer regime by combining low-temperature scanning tunneling microscopy, high-resolution photoemission spectroscopy using synchrotron radiation, and density functional theory (DFT) calculations. The modulation is induced by the competition between long-range repulsive Coulombic interactions and short-range attractive hydrogen-bonding interactions. The system shows the characteristic pattern evolution, from monomeric stripes at low coverages to two-dimensional islands, with the formation of labyrinths in the crossover.

’ INTRODUCTION Many unrelated two- (2D) and three-dimensional (3D) systems exhibit identical domain patterns, such as stripes, islands, and circular droplets (2D) or sheets, tubes, and bubbles (3D), at different length scales spanning from the low nanometer range to the macroscopic range. Such universal patterns arise from the modulation of phases, which are stabilized by competing interactions and are characterized by periodic spatial distributions. The modulation periods display a dependence on external parameters such as temperature, magnetic field, coverage, and so forth.1
5 Among 2D systems exhibiting phase modulation, a few selected examples are mentioned here. In the case of Cu(110)-(2  1)O, a grating of periodic nanometric stripes is formed in the coverage (Θ) range Θ < 0.45 monolayers (ML) induced by the competition between attractive intrastripe short-range Cu
O interactions and long-range interstripe repulsive interactions.6 One-dimensional (1D) stripes have been observed in Cu(001)-c(2  2)N, due to lattice deformation and strain, which under certain conditions evolve toward labyrinths,7 as well as in submonolayer films of 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid, a chiral species, r 2011 American Chemical Society

grown on Au(111) and Ag(111) surfaces, where the period can be tuned by controlling the amount of deposited material.8 The formation of labyrinths, also known as spinodal patterning, has been observed in many systems as organic layers confined between parallel plates.9 Stripe and bubble morphologies also arise in Langmuir films, where dipole
dipole interactions compete with perimetral forces (line tension)10 and in thin liquidcrystalline polymer films when entering the smectic phase.11 On clean Pd(110), and induced by the annealing temperature, ordered islands have been observed, due to the balance between step formation and step
step repulsion.12 Micrometer thick films of different materials, such as ferromagnetic garnets,13 ferrofluids,14 and so forth, exhibit the same universal behavior as a function of the relevant order parameter. Periodic 2D patterning has been modeled from the minimization of the free energy for those systems governed by effective Received: May 31, 2011 Revised: July 29, 2011 Published: August 16, 2011 18640

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The Journal of Physical Chemistry C long-range interactions decaying as r
2, where r stands for a distance.15 A particular and relevant case is indeed the Coulomb interaction involving charges confined on surfaces. The precise knowledge of charge distribution on interfaces (ordering, injection, transfer, etc.) is of paramount importance in order to better understand the underlying mechanisms applying in devices.16
19 A first example is charge ordering. In the case of transition metal oxides, i.e., CuO2 planes in cuprate superconductors, holes exhibit Coulomb repulsion and dipolar attraction. On the basis of Monte Carlo calculations, it has been shown that at low hole densities (low doping) clumps organize into an ordered lattice structure and that, for increasing doping, partially disordered filamentary patterns are formed that evolve toward anisotropic Wigner crystal patterns.20 In a recent example of charge transfer of molecular donors [TTF = tetrathiafulvalene] on Au(111) surfaces, it has been shown that 1D Wigner crystals, characterized by intermonomer distances of 2
3 nm, at low coverages are formed.21 TTF, a π-donor molecule, is the building block of a myriad molecular materials exhibiting magnetic ground states, semiconducting, metallic or superconducting properties.22,23 Here, we combine low-temperature scanning tunnelling microscopy (LTSTM), high-resolution photoemission spectroscopy (PES) using synchrotron radiation, and density functional theory (DFT) calculations in order to compare the real space molecular distribution and the electronic structure of the TTF/ Au(111) system as a function of coverage for Θ < 1 ML covering the whole range of the competing short-range attractive and long-range repulsive interactions.

’ EXPERIMENTAL AND COMPUTATIONAL METHODS Experimental Details. LTSTM and PES experiments were performed in two independent ultrahigh vacuum (UHV) systems at the Freie Universit€at Berlin and at the synchrotron light source BESSY II (Helmholtz Zentrum f€ur Materialien and Energie, Berlin, Germany), respectively. In both systems, atomically clean and ordered Au(111) surfaces were obtained by repeated cycles of Ar-ion sputtering and annealing up to 700 K, √ until the well-known 23  3 reconstruction was obtained. TTF was deposited by sublimating as-received commercial TTFTCNQ powder (TCNQ = tetracyanoquinodimethane; Aldrich, 99% purity), once properly degassed, with homemade Knudsen cells in UHV while keeping the substrate at room temperature. In the LTSTM system, the deposition was controlled by direct inspection of the images, while in the photoemission experiments it was monitored with a quartz crystal microbalance. LTSTM images were obtained at 5 K with a homemade microscope. Ultraviolet (UPS) and X-ray photoelectron spectroscopy (XPS) experiments were performed at the SurICat endstation (beamline PM4) at BESSY II. Spectra were collected with an hemispherical electron energy analyzer (Scienta SES 100) using excitation photon energies (pω) of 35, 269, and 620 eV. The angle between incident synchrotron radiation and the analyzer entrance was fixed at 60. At 35 eV, the energy resolution of the analyzer was set to 100 meV under normal emission. The presence of sulfur (S2p line) and the absence of nitrogen (N1s line) were checked prior to cooling down to ∼100 K. To prevent beam damage, measurements were taken at different points on the sample, and the radiation was stopped when spectra were not acquired (monochromator setting, change of sample position, etc.). Beam damage was checked by taking short S2p spectra and determining the work function at the same spot at room

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temperature. No significant change could be observed after 2 h. The longest experiments at 100 K lasted 30 min. Computational Details. DFT calculations have been carried out using the Siesta code,24,25 with the exchange correlation functional in the Perdew
Burke
Ernzerhof form26 of the generalized gradient approximation (GGA). As basis sets, we use pseudoatomic orbitals27,28 of double-ζ plus polarization (DZP) size. Given the sensitivity of surface properties on the quality of the basis set size and confinement,29 for the gold atoms we use a basis set specially developed for surfaces in our previous work,30 consisting of a DZP basis optimized in the bulk with a very small fictitious pressure31 of 0.02 GPa (which yields orbitals of relatively long radii, larger than 6.0 bohr), plus a shell of diffuse orbitals with a radius of 9.0 bohr in the surface atoms. With this basis set we obtain results for the surface energy and work function, which differ by only 0.05 and 0.08 eV (respectively) from the plane wave results. The calculations were done using slabs of six gold atomic layers to simulate the surfaces. The molecules were adsorbed only on one side of the slab, and we include the dipole correction32 to compensate the electric field in the vacuum region resulting from the asymmetry of the slabs. The structures were optimized until forces on individual atoms were less than 0.04 eV/Å. For the Brillouin zone integration we have used a Monkhorst-Pack set of special points, ranging from 50 points for the 1.14 ML coverage to 15 points for the 0.15 ML coverage. A converged cutoff of 650 Ry was used for the representation of the density in the real space grid. Core level shifts are calculated using two approximations: initial state and final state. In the first case, the core level shifts are computed from the core level energies, without considering electronic relaxation effects. The calculations are done with the pseudopotential approximation, using first-order perturbation theory,33,34 but without considering the effect of spin
orbit interactions. Final state results were obtained by computing total energy differences as proposed by Pehlke and Scheffler,35 including the effect of the relaxation of the valence electrons in response to the creation of the core hole. In computing core level shifts, we have taken the same reference as in the experiment (i.e., the Fermi level of the metallic substrate). Typical errors in the computed core level shifts between the experimental values and the theory (using the approximations described above) are on the order of 0.1 eV, although they can be significantly larger36 (reaching more than 0.5 eV in some cases). Usually, the final state approximation provides results that are in closer agreement with experiments in systems such as the ones considered here involving adsorption of molecules on metallic surfaces, where there is consensus that final state effects are very important.37

’ RESULTS AND DISCUSSION Experimental Results. Figure 1 shows STM images taken at 5 K corresponding to different TTF coverages below 1 ML. Below 0.2 ML (Figure 1a
d) quasiperiodic molecular distributions with TTF pair distances above =2 nm are formed induced by long-range repulsive (Coulomb) interactions upon charge transfer from the TTF monomers to the metallic substrate, which has been interpreted in terms of a 1D molecular Wigner crystal.21 The TTF pair distances decrease monotonously with coverage. At higher coverages, TTF builds self-assembled structures. Upon deposition of ∼0.5 ML (Figure 1e), TTF forms zigzag chainlike structures uniformly distributed over the surface. These chains exhibit a labyrinthine pattern that varies on the hexagonal close-packed (hcp) 18641

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Figure 1. STM images taken at 5 K of TTF/Au(111) for different coverages:38 (a) 0.03 ML (V = 0.9 V, I = 0.3 nA), (b) 0.04 ML (1.2 V, 0.2 nA), (c) 0.08 ML (0.8 V, 0.1 nA), (d) 0.16 ML (V = 1.2 V, I = 0.3 nA), (e) 0.5 ML (
0.05 V, 0.2 nA), and (f) 0.8 ML (0.7 V, 0.13 nA). The molecular coverage is determined from STM images of large surface areas, assuming that 1 ML corresponds to 2 molecules/nm2. Insets in e and f show the arrangement of the TTF molecules on the surface.

and face-centered cubic (fcc) sites of the reconstructed Au(111) surface. While in the fcc domains the chains tend to have a short length and several orientations, in the hcp regions the tendency is reversed with a preferential formation of long chains along the soliton lines. TTF molecules adsorb slightly shifted within the chains, being the structure dominated by S 3 3 3 HC hydrogen bonds (see inset in Figure 1e).39 For higher coverages, ∼ 0.8 ML, 2D TTF islands are formed (Figure 1f) coexisting with zigzag chains. Here, molecules pack densely in a parquetlike structure (see inset). The areas covered by 1D TTF chains exhibit the same zigzag structure previously observed in the 0.5 ML coverage. The evolution of the STM images for increasing surface molecular density is characteristic of a 2D system in which there is a competition between long-range repulsion and short-range attraction.2,3,20,40 When long-range repulsion of Coulombic nature largely dominates, as in the case of very low coverages (∼0.03 ML), the system tends to form a Wigner crystal, as previously reported.21 When short-range attractive interactions participate, molecules order in characteristic labyrinthine patterns in the intermediate coverage regime, ∼0.5 ML. Above ∼0.8 ML, island formation is predominant, as a consequence of the dominant lateral short-range attractive interaction. Let us now introduce the photoemission results. Figure 2 shows the work function (ϕ) of the TTF/Au(111) system as a function of coverage (see discussion below on the definition of coverage). The work function was obtained using the expression ϕ = pω
min
Emin corresponds to the (Emax K K ), where pω = 35 eV, EK kinetic energy from the onset of photoemission (secondary represents the kinetic energy of electron cutoff) and Emax K electrons excited from the Fermi level (EF). Emin K was obtained by applying a
10 V bias to the sample in order to clear the analyzer work function. Note that ϕ decreases with increasing coverage down to a minimum value of 4.8 eV (=
0.7 eV difference with respect to the clean surface) at 0.75 ML and increases for Θ > 0.75 ML to the asymptotic value of 4.9 eV, which corresponds to the measured work function of a thick TTF film grown on the Au(111) substrate. Figure 3a shows the photoemission spectra of the S2p region taken with 269 eV photons as a function of coverage. With such

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Figure 2. Work function ϕ (eV) measured experimentally, as a function of coverage Θ (ML).

Figure 3. (a) Photoemission spectra of the S2p lines as a function of coverage. The spectra have been normalized to their maxima. The binding energies are referred to EF. The measured binding energies of the bulk and surface Au4f7/2 components are 84.0 and 83.6 eV, respectively. (b) Detail of the photoemission spectra of the S2p lines for Θ = 0.05 ML (below) and Θ = 0.96 ML (above) showing a leastsquares fit using a combination of Gaussians and Lorentzians after a Shirley-type background subtraction.

photon energy, the kinetic energy is about 100 eV, leading to a maximum in surface sensitivity due to the small electron mean free path (about 0.5 nm). Each spectrum can be decomposed into two components (see Figure 3b), each consisting of a spin
orbit doublet. At the lowest measured coverage (∼0.05 ML), the Sp3/2 components exhibit binding energies of 163.6 (less intense) and 18642

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The Journal of Physical Chemistry C 164.0 eV (more intense), respectively, while at 0.96 ML the two components exhibit energies of 163.4 (more intense) and 163.9 eV (less intense), respectively. The ∼163.5 eV value is well above characteristic binding energies corresponding to strong chemisorptive sulfur
gold interactions, as found for thiol-based41,42 and thiophene43 molecules self-assembled on Au(111), with values lying around 162.5 eV. Thus, for TTF/Au(111), TTF exhibits a less strong bonding character. Note the inversion in relative peak intensities for increasing coverages. We thus have two different shifts in binding energy, of 0.2 and 0.4 eV, of different origins. We will discuss the origin of both shifts after the presentation of our DFT calculations. First Principles Calculations Results. Geometries. In order to study the evolution of the bonding and the electronic properties with the molecular coverage of the gold surface, we have considered a number of supercells in which the coverage varies from 0.15 to 1.14 ML. In a previous work,21 it was shown that, since the size of the molecule is not well matched with the interatomic distances between the surface gold atoms, adsorption takes place in a tilted configuration, where the two S atoms in one side of the molecule are tightly adsorbed on two underlying gold atoms forming strong S
Au bonds, whereas the two S atoms in the other side are nearly on top of a hollow site, forming weaker links and being higher above the surface (and appearing brighter in the STM image). Therefore, for all the structures considered in this work, we initially align the long axis of the molecule along the [011] (or equivalent) direction of the surface,21 at around 3 Å above the surface plane. We consider different initial tilt angles, to explore the possibility of multiple adsorption minima. Figure 4 shows the structures corresponding to the lowest and highest coverages: 0.15 and 1 ML, respectively. We note that we have also considered a more densely packed structure with a coverage slightly above 1 ML, which, however, does not seem to be representative of the experimentally observed structures. For the more dilute concentrations (up to Θ e 0.75 ML), we use structures with just one molecule per supercell. However, for coverages larger than 0.5 ML, where dense patterns are experimentally observed, we also build structures following the guidance from the experimental STM images, and consider structures with two molecules per supercell. For instance, for 0.75 ML, we model the structure in which separated rows containing parallel molecules in a zigzag configuration is observed experimentally (also shown in Figure 4). Similarly, for the highest coverages (1 and 1.14 ML), we model the observed highdensity phase where the molecules are tilted on the surface with the long axis along two different directions (forming an angle of around 60 between them). At all coverages considered, the interaction with the surface makes the molecule lose its gas phase boat geometry, acquiring a planar one, but tilted with respect to the surface around the long molecular axis.45,46 After the relaxation, the molecules are adsorbed on the surface at a height that does not significantly change with coverage (around 3 Å). The molecules lay almost flat on the surface, but with a small tilt angle along the long axis of the molecule. This occurs because the two S atoms in one of the sides of TTF lay on top of two gold atoms of the surface layer, whereas the S atoms in the other side lay on top of hollow sites (the only exception being the 1.14 ML coverage, in which the two molecules in the surface unit cell are too compressed, which causes one of them to be displaced slightly from the position on top of gold atoms). The tilt angle obtained after relaxation,

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however, depends on its value in the initial configuration. For the same coverage and structure, we find relaxed structures (within the force tolerance used) that have different values of the tilt angle, and with energies that differ by less than 50 meV/molecule. The tilt angles obtained are always smaller than 30, and fall in two groups in the ranges from 18 to 28 and from 6 to 12 for initial conformations with large and small angles, respectively. We do not observe any correlation between the tilt angle and the coverage. These results indicate that the shape of the potential energy surface with respect to the tilt angle is very flat, so that the molecules will actually explore a large range of values even at small temperatures, around a mean value of about 15. In our simulations, we have done calculations using the relaxed structures with both large and small tilt angles, which will be denoted as “T” and “P”, respectively. Hofmann and co-workers47 have calculated the adsorption geometry of TTF on Au(111) in the low monolayer regime with the long axis of the molecule along both the [011] and [211] directions and concluded that deviations between both orientations are minor. Charge Transfer. We start by analyzing the charge transfer using a Mulliken population analysis. This method, despite wellknown deficiencies such as the dependence of the results on the basis set, gives rather indicative trends on the charge transfer. We have computed it in terms of the number of electrons transferred from the molecule to the substrate. As shown in Figure 5a, for a given coverage, the charge transfer is almost independent of the geometry of the molecule on the surface (tilt angle and molecular arrangement). However, it is clear that it decreases nearly linearly with coverage. At the lowest coverage considered, the charge transfer is of 0.27 electrons from the molecule to the surface, and it extrapolates to around 0.32 electrons for the limit of a single molecule on the surface. For the highest molecular concentration considered, the Mulliken charge transferred drops to nearly zero. To have a better insight on the character of the charge transfer between molecule and surface, we define the induced charge density as 1 Find ðzÞ ¼ ½Fmolþsurface ðzÞ
Fsurface ðzÞ
Fmol ðzÞ A

ð1Þ

where the first term Fmol+surface(z) is the self-consistent charge density of the whole system, and the last two terms are those of the surface and molecule isolated, but in the same geometry as for the bonded system. Therefore, this quantity reflects the distortion in the charge density due to the formation of the bond between molecules and surface (without considering structural distortions). All the quantities are defined as dependent only on the z coordinate (in the direction perpendicular to the surface), after averaging in the xy plane: Z 1 Fðx, y, zÞ dx dy ð2Þ FðzÞ ¼ M A (with M and A being the number of molecules and the area of the surface cell, respectively). The results are shown in Figure 6, where only the calculations for the T group are shown (the results for the P group are very similar). Well within the metal, we see the oscillating behavior related with the Friedel oscillations induced by the presence of the adsorbate. At approximately the third
second layer of the surface, more marked variations with different signs start to appear. Most of the charge accumulates in the region just above the metal surface layer, and the corresponding depletion occurs at the molecular site, in agreement with the Mulliken analysis. Precisely 18643

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Figure 4. Structures at the lowest (0.15 ML), intermediate (0.75 ML), and highest (1 ML) coverage.44

at the molecular plane position, the induced charge is close to zero, being an indication of the fact that the molecular levels involved in the charge transfer have a π symmetry. A 3D representation of the induced charge density, shown in Figure 7 for three different coverages, confirms that the molecule is losing charge from π molecular states. In both figures, it is apparent that the shape of the charge distortion due to the molecule
surface interaction is essentially the same regardless of the molecular coverage, while the effect of the coverage is to determine the magnitude of the distortion. The molecule becomes less depleted of charge as the coverage is increased, again in agreement with the linear decrease of the Mulliken charge transfer with coverage. The main contribution for the charge donation from molecule to surface is the HOMO (highest occupied molecular orbital) of TTF. To show this, it is useful to consider the density of states projected on the orbitals of the molecule:48   PDOSm ðεÞ ¼ j Ψn, k jΨmolecule ð3Þ jδðε
εn, k Þ m

∑ n, k

where n, k and εn,k are the band index, wave vector, and band energies of the full system (molecule plus surface), respectively, and m is the band index of the free molecule states. In Figure 8, the projected density of states (PDOS) for some selected coverages are represented, together with the HOMO of the free

molecule. The molecular levels show as resonances in the PDOS curves, due to their interaction and hybridization with the states of the metallic substrate. In all the graphs it can be seen how the charge donation from the molecule to the surface comes from the HOMO, which becomes partially empty. The position of the HOMO changes slightly as we move to higher coverages, indicating the change of occupation of this molecular orbital. We can quantify this change in occupation by integrating the area under the HOMO curves, as shown in Figure 8. The occupation of the HOMO increases with coverage, consistently with the decrease in the charge transfer discussed above. Hoffman and co-workers give a value of 79% for low coverages, in agreement with our results.47 Surface Dipoles and Work Function. The transfer of charge from the adsorbed molecules to the metallic surface leads to changes of the surface dipoles, which in turn produces measurable changes in the work function. In order to understand the change of the work function with molecular coverage, it is convenient to relate it with the dipole moments induced at the surface due to the presence of the adsorbed molecules. From the induced charge density, we define the associated dipole moment (per adsorbed molecule) as Z zvac z0 Find ðz0 Þ dz0 ð4Þ μind ¼ z0

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Figure 5. (a) Charge transfer (computed using the Mulliken scheme) from molecule to surface, as a function of coverage. (b) Induced surface dipole per adsorbed molecule as a function of coverage. (c) Change in the work function caused by the induced dipole μind as a function of coverage with respect to the clean Au(111) surface. Green circles correspond to the experimental values from Figure 2. (d) Adsorption energy as a function of coverage. Black and red symbols indicate molecules in the T and P conformations, respectively, for the structures with one molecule per unit cell. The blue symbols show the results for the structures with two molecules per unit cell. Black diamonds in b, c, and d are the values computed by Hofmann et al.47

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this difference will not influence the general trends for the evolution versus coverage, as we will see below. The behavior of the work function follows quite closely a parabolic dependence with coverage, reaching a minimum at around 0.75
0.8 ML. It is quite independent of the tilt angle of the molecule with respect to the surface, as seen in the small differences between the tested geometries, represented in red, blue, and black in Figure 5c. The coverage corresponding to the photoemission data has been normalized to the calculated values, by assigning the minimum experimental work function value to 0.75 ML (see Figures 2 and 5c). The main contribution to the change in the work function with coverage (Figure 5c) is Δϕind, which comes from the rearrangement of charge due to the formation of the bonds between the adsorbed molecules and surface. This is due to the fact that the intrinsic dipole moment of each molecule (in the planar geometry of the adsorbed configuration) is very small, so that changes of the work function due to these intrinsic dipoles are negligible. The origin of the change in the work function is, therefore, the formation of the dipole layer at the surface, due to the transfer of charge from molecules to metal. Although the dipole per molecule decreases with coverage, the work function change increases as a result of the fact that the number of molecules per unit area increases too. In particular, if the dipole moment per molecule decreases linearly with coverage Θ (as we found in our calculations), μ ¼ μ0
αΘ

ð5Þ

then, through the Helmholtz equation, the change in the work function is quadratic with coverage (since the area is inversely proportional to the coverage): Δϕ 
μ0 Θ þ αΘ2

Figure 6. Induced charge density as a function of z for different molecular coverages, for some of the geometries studied. Positive means a gain of electron density, and negative means a depletion.

where z0 is taken in the middle of the slab (where the perturbation due to the presence of the molecule has been screened by the metal, and Find(z) tends to zero), and zvac is in the middle of the vacuum. The results of the calculation for μind as a function of coverage are shown in Figure 5b. The computed dipoles decrease linearly with coverage, in line with the linear decrease of the charge transfer discussed above. We can correlate the induced dipole with the change in the work function, using the Helmholtz equation: Δϕind =
μind/(ε0A), with ε0 being the vacuum dielectric constant, and A being the surface area per molecule. Figure 5c shows the change in the work function upon adsorption of the TTF molecules, as a function of coverage, obtained by analyzing the electrostatic potential profile curves from the DFT calculations. The work function obtained for the clean surface (4.77 eV) is too low compared with the experimental value of 5.48 eV. This can be attributed to the GGA functional.49 However,

ð6Þ

We note that, if the dipole moment per molecule does not change with coverage (i.e., if α = 0), the work function would decrease linearly. Therefore, a nonlinearity in the work function variation with coverage indicates a change in dipole moment. In our results, the charge transfer and the dipole moment decrease linearly with coverage, leading to a quadratic dependence of the work function. These results, therefore, explain the experimental observations. The system closely follows the depolarization model from Langmuir, in which the strength of the generated dipoles due to charge transfer decreases with the number of dipoles.50 Note that the largest deviation between experimental and calculated values is less than 0.4 eV, corresponding to 0.75 ML, which can be considered acceptable. Finally, we can make a comment about a mechanism that is often invoked to explain the change of the work function of metallic surfaces upon adsorption of closed-shell molecules or atoms, which is the so-called Pauli “push-back” or “pillow” effect.51,52 At the free surface of the metal, the electronic charge spills out of the surface layer, creating a dipole that contributes to the work function of the metal.53 When a closed-shell molecule adsorbs on the surface, even if there is no chemical bond between them, the Pauli repulsion forces the electrons of the metal back into the bulk, reducing the spill-out and also reducing the work function of the system. This effect occurs without any charge transfer between molecule and substrate. In our TTF/Au(111) surface, our results indicate that the Pauli push-back effect is not the main cause of the change of the work function with coverage. In Figure 7 we see that there is a clear charge transfer between molecule and substrate, rather than a redistribution of the charge 18645

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Figure 7. Induced charge density for three of the coverages considered: (a) 0.15 ML, (b) 0.75 ML, and (c) 1 ML. Upper and lower panels show two different views for each configuration. Sharp colors show a 3D view of the isosurface with 5  10
4 e/bohr3, while the lines in the lower panels show a contour plot for a plane that passes through one of the lower S atoms. Blue and red mean a defect and an excess of electrons, respectively.44 For all cases, the adsorption only significantly affects the outermost layers of the substrate.

energy of the bonded system (surface plus molecule) and the sum of the total energies of the free relaxed molecule and the surface as 1 Auð111Þ
ETTF ð7Þ Eads ¼
½ET
ET T  M

Figure 8. Left: PDOS for some of the molecular orbitals around the HOMO and the LUMO (lowest unoccupied molecular orbital), for three different coverages. The HOMO of the free molecule is shown in the top right panel.44 Lower right: Occupation of the HOMO as a function of coverage.

density of the metallic electrons of the surface. The electron density just above the surface increases due to the donation from the molecule, instead of decreasing due to the Pauli push-back. Although, of course, we can not rule out a certain degree of pushback (which is masked out by the opposite effect due to the electron transfer from the molecule), it is certainly not the mechanism that leads to the main effects on the surface dipole and the change of the work function. Adsorption Energy. We have calculated the chemisorption or adsorption energy per molecule as the difference between the total

where M is the number of molecules per surface unit cell. The adsorption or chemisorption energy is a crucial parameter to understand the interactions between the molecule and the substrate. We see in Figure 5d the decrease of the adsorption energy with the coverage, going from 0.86 eV for the lower coverage to 0.33 eV for the largest coverage considered. The adsorption energy follows closely a quadratic dependence with decreasing coverage. At the smallest coverages, the molecules essentially do not interact with each other, reaching the limit of chemisorption energy of the isolated adsorbed molecule. In all cases, the effects of the tilt angle on the chemisorption energy are almost negligible. Again the obtained properties are not very dependent on geometric considerations, for a given coverage. The decrease of adsorption energy upon increase in coverage can be understood from the repulsion between neighboring dipoles, which become larger as these become nearer to each other. This, in turn, also causes the depolarization effect discussed earlier, so the dipole moment between the surface and each molecule decreases with coverage. Core Level Shifts. The core level shifts were computed using both the initial and the final state approximations. Final state effects are expected to be important in this system, due to the metallic screening of the gold surface. We first consider the question whether the two inequivalent S atoms in the tilted adsorbed configuration of TTF (upper and lower S atoms) can explain the existence of two components, shifted by 0.4 eV, in the 18646

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Figure 9. S2p core level shifts between the upper and lower S atoms calculated using the initial state (a) and final state (b) approximations, as a function of coverage. Black and red symbols indicate molecules in the T and P conformations, respectively, for the structures with one molecule per unit cell. The blue symbols show the results for the structures with two molecules per unit cell.

experimental core level spectra. To do that, for each coverage, we calculate the difference in the core level energy between the upper and lower S atoms. As there are four sulfur atoms in each molecule (two up and two down), and in some cases two molecules per unit cell, we calculate an average of all up
down pairs, and the corresponding standard deviation as a measure of the dispersion in the values. The results for the initial and final state approximations are shown in Figure 9. The initial state approximation yields sizable shifts between the upper and lower sulfur atoms. However, these shifts vary significantly with coverage (becoming quite small for the most dense coverages considered), unlike in the experiments, where the splitting between the two components is quite constant as a function of coverage. Additionally, there is another fact that indicates that the existence of S atoms in two inequivalent positions is not the origin of the two components: in the experiment, the height of these components changes dramatically with coverage. Since the relative number of upper and lower S atoms remains constant with composition, one would expect the relative intensity of the two components to be coverage-independent, if that was their origin. Finally, it is clear in Figure 9b that the inclusion of final state effects brings the shifts between upper and lower S atoms to very small values, which allows us to discard this as the origin of the experimental peak splitting. We attribute the presence of two contributions for a given coverage of the S2p lines to the coexistence of TTF molecules in different charge states, i.e., neutral and charged. An analogous effect has been observed in TTF-TCNQ54,55 and in single crystals of TTF derivatives such as (TMTTF)2PF623 [TMTTF = tetramethyl-tetrathiafulvalene], where the S2p spectra evidence two components separated by =1 eV: the lower (higher) binding energy line corresponding to the neutral (charged) state. In both examples, where charge transfer lies between 0.5 and 0.6 electrons per molecule, the coexistence responds to a dynamical configuration, TTF0 (163.5 eV binding energy) and TTF+1 (164.5 eV binding energy), because photoemission is an intrinsically rapid process (10
15 s). A similar comparison has been recently performed on graphene doped with TTF.56 Urban et al. have reported identical binding energy values on exTTF layers grown on Au(111),57 where exTTF stands for 2-[9-(1,3-dithiol2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole, a derivative of TTF with a butterfly shaped nonplanar structure. The authors assign the lower (higher) binding energy feature to the sulfur atoms interacting (not interacting) with the substrate, which is in

Figure 10. S2p core level shifts as a function of coverage, referred to the binding energy for a coverage of 1 ML. Top panels (a,b) show results of the initial state approximation, and bottom panels (c,d) are the final state approximation results. Left panels (a,c) are the shifts for the upper S atom, whereas right panels (b,d) are for the lower S atoms in the TTF molecule. Black and red symbols indicate molecules in the T and P conformations, respectively, for the structures with one molecule per unit cell. The blue symbols show the results for the structures with two molecules per unit cell.

contradiction with the mentioned photoemission results. For TTF/Au(111) and at the lowest coverages, charged molecules dominate over neutral ones, indicating a more efficient charge transfer from the molecules into the surface. However, when the coverage increases, the tendency is reversed, meaning that the charge transfer is less efficient due to the increasing importance of short-range lateral interactions. Now, we consider the shift in the position of the S2p core level peaks with coverage observed in the experiment. As observed in Figure 3b, the binding energy decreases by 0.2 eV for increasing coverage, a small but systematic shift having in mind that the estimated error in the determination of the binding energy is (0.1 eV. The sign of the shift is the expected for donor molecules.58,59 Figure 10 shows the results of the calculation. The results are shown for the core level shift of the binding energy relative to that of the 1 ML: for coverage Θ, the core level shift computed is: CLS(Θ) = BE(Θ)
BE(1 ML). The binding energies are obtained taking as a reference the Fermi level of the gold substrate, as done in the experiments. The figure shows the results for both upper and lower S atoms in the tilted adsorption configuration (left and right panels, respectively). The obtained shifts are always rather small in line with the experimental results.

’ CONCLUDING REMAKS The charge transfer of TTF on Au(111) as a function of coverage has been investigated both experimentally, by means of STM and high-resolution photoemission at cryogenic temperatures, and theoretically with the help of DFT calculations. The charge transfer is modulated by two competing interactions: long-range repulsive of Coulombic nature and short-range attractive, leading to characteristic 1D, 2D, and labyrinth-like patterns. The quadratic dependence of the work function with coverage is mainly due to the decrease of the surface dipole moment, following Langmuir’s depolarization model, although a 18647

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The Journal of Physical Chemistry C minor contribution due to the Pauli’s push-back effect cannot be excluded. The dual XPS S2p line composition is attributed to the coexistence of TTF molecules in both neutral and charged states.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Ministerio de Ciencia y Tecnología (Spain), through Projects FIS2009-08355, FIS200912721-C04-01, and CSD2007-00050, by the Generalitat de Catalunya (SGR 00909 and 2009 SGR 186) and by the Deutsche Forschungsgemeinschaft (Germany) through Project FR2726/1. Access to the supercomputing facilities of CESCA and CESGA is fully acknowledged. We also acknowledge support through the BESSY IA-SFS programme (BESSY-ID.08.2.80003, RII 3CT2004-506008). ’ REFERENCES (1) Seul, M.; Andelman, D. Science 1995, 267, 476. (2) Sagui, C.; Desai, R. C. Phys. Rev. Lett. 1993, 71, 3995. (3) Glotzer, S. C.; Stauffer, D.; Jan, N. Phys. Rev. Lett. 1994, 72, 4109. (4) Mu, Y.; Ma, Y. Phys. Rev. B 2003, 67, 014110. (5) Heinig, P.; Helseth, L. E.; Fischer, Th. M. New J. Phys. 2004, 6, 189. (6) Kern, K.; Niehus, H.; Schatz, A.; Zeppenfeld, P.; George, J.; Comsa, G. Phys. Rev. Lett. 1991, 67, 855. (7) Komori, F.; Ohno, S.; Nakatsuji, K. Prog. Surf. Sci. 2004, 77, 1. (8) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. Rev. Lett. 2001, 87, 096101. (9) M€uller-Wiegand, M.; Georgiev, G.; Oesterschulze, E.; Fuhrmann, T.; Salbeck, J. Appl. Phys. Lett. 2002, 81, 4940. (10) Weis, R. M.; McConnell, H. M. J. Phys. Chem. 1985, 89, 4453. (11) Zhang, S.; Terentjeva, E. M.; Donald, A. M. Eur. Phys. J. E 2003, 11, 367. (12) H€ornis, H.; West, J. R.; Conrad, E. H.; Ellialtioglu, R. Phys. Rev. B 1993, 47, 13055. (13) Seul, M.; Wolfe, R. Phys. Rev. Lett. 1992, 68, 2460. (14) Liu, J.; Lawrence, E. M.; Wu, A.; Ivey, M. L.; Flores, G. A.; Javier, K.; Bibette, J.; Richard, J. Phys. Rev. Lett. 1995, 74, 2828. (15) Zeppenfeld, P.; Krzyzowski, M.; Romainczyk, C.; Comsa, G.; Lagally, M. Phys. Rev. Lett. 1994, 72, 2737. (16) Fraxedas, J. Adv. Mater. 2002, 14, 1603. (17) Gershenson, M. E.; Podzorov, V.; Morpurgo, A. F. Rev. Mod. Phys. 2006, 78, 973. (18) Koch, N. Chem. Phys. Chem. 2007, 8, 1438. (19) Beljonne, D.; Cornil, J.; Muccioli, L.; Zannoni, C; Br
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