DFT Investigation of Oligothiophenes on a Si(001) Surface - The

Nov 9, 2010 - SMALL MOLECULES ADSORBED ON SILICON SURFACES: TWO CASES THROUGH TIME. MARILENA CARBONE. Surface Review and ...
0 downloads 0 Views 870KB Size
Download PDF
20068

J. Phys. Chem. C 2010, 114, 20068–20075

DFT Investigation of Oligothiophenes on a Si(001) Surface Francesca Costanzo,* Elisabetta Venuti, Raffaele Guido Della Valle, and Aldo Brillante Dipartimento di Chimica Fisica ed Inorganica and INSTM-UdR Bologna, UniVersità di Bologna, Viale Risorgimento 4, I-40137 Bologna, Italy

Pier Luigi Silvestrelli Dipartimento di Fisica, UniVersitá di PadoVa, Via Marzolo 8, I-35131 PadoVa, Italy, and DEMOCRITOS National Simulation Center, Trieste, Italy ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: October 5, 2010

The nature of the interaction of a series of R-oligothiophenes (1T up to 6T) with a Si(001) surface was studied by means of density functional theory (DFT) structural optimizations. For 1T, we found that the products of the [4 + 2] cycloaddition are more thermodynamically stable than those of the [2 + 2] cycloaddition, in agreement with previous cluster approach calculations. Since there are many possible ways of placing the all-trans nT molecule on a Si(001) surface, we have tested a number of possible conformations; in particular, starting from simple geometric considerations, we built the 6T molecule from 1T as a result of [2 + 2] cycloadditions. The binding energy of oligomers with n from 2 to 6 displays a steady drop, attributable to an increased deformation energy. Our study sheds light on the chemisorption of oligothiophenes on Si(001) showing, in particular, that molecular chemisorption is possible for oligothiophenes with up to four thiophene units. For 6T, we estimated the van der Waals contribution to the binding energy, being of the order of 1 eV for molecule-surface distances ≈4 Å, in agreement with the experimental evidence of the presence of 6T physisorbed states on the Si(111) surface. Finally, the calculated work function for 6T on Si(001) is in very good agreement with the experimental estimate. 1. Introduction Organic systems with extended π conjugation have proven to be very interesting materials when used as thin films deposited on inorganic substrates, in the construction of electro-optical devices,1-3 such as organic field effect transistors (OFETs), lightemitting diodes (OLED), and solar cells.4-6 In particular, oligomers of aromatic and heteroaromatic compounds are been widely employed,7-11 as they tend to form self-assembled, highly ordered films with reproducible chemical properties. The nature of the interface and the microscopic molecular arrangement of the material on the substrate play an important role in shaping the charge transport properties of the organic layer in the device.12 Thus, a key area of interest is the description of the molecule-substrate interactions in the monolayer and submonolayer coverage regimes. Depending on the specific type of chemical and physical forces involved in the system under study, different charge-redistribution processes can simultaneously operate at the interfaces.13-17 Theoretical approaches able to describe the molecular structure in its adsorption states and the electric modification of the surface can contribute to clarify these mechanisms at a microscopic level and are therefore extremely useful.18 Among the oligomers with large π conjugation, oligothiophenes have proven to have properties which make them promising compounds for organic electronics,19-21 and among the possible substrates, the group IV semiconductors surfaces are of fundamental interest. The aim of this paper is to provide theoretical insights on the interactions of R-oligothiophenes with the Si(001) surface, by means of density functional theory (DFT) * Author for correspondence, [email protected].

calculations. Indeed, DFT represents an efficient tool to elucidate complex surface processes such as adsorption. The Si(001) surface was chosen for being one for which experimental data were available.23,22 The theoretical treatment started necessarily from the single thiophene unit (1T), which constitutes the repeating unit of ordered films of oligomers and has to be taken as the model system in the understanding of the specific chemical and physical interactions with the surface. Drawings of 1T on Si(001) surfaces appear in Figures 1 and 3 later on, and it may be helpful to give them a glance before reading the rest of the paper. Thiophene, due to its inhomogeneous electron distribution, is expected to show a chemisorption mechanism different from common dienes and benzene,24 but the interpretation of the experimental data has been the subject of some controversy. In particular, early LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and UPS (ultraviolet photoelectron spectroscopy)25 experiments, supported by PM3 calculations, suggested that the thiophene molecule is most likely chemisorbed on the Si(001) surface by forming two C-Si σ-bonds with adjacent carbon atoms. The proposed mechanism corresponds to a [2 + 2] cycloaddition scheme and yields a 2,3-dihydrothiophene-like species. More recently, HREELS (high-resolution electron energy loss spectroscopy), STM (scanning tunneling spectroscopy),24,26 and valence band photoemission experiments27 were carried on both Si(001) and Si(111) surfaces. These experiments suggest that a [4 + 2] cycloaddition scheme is instead a more feasible way of chemisorption, leading to the formation of 2,5-dihydrothiophene-like species. Together with chemisorbed states, also physisorbed thiophene was detected.24

10.1021/jp106627z  2010 American Chemical Society Published on Web 11/09/2010

Oligothiophenes on a Si(001) Surface The computational results presented in this paper confirm indeed that for 1T the [4 + 2] cycloaddition mechanism yields the most stable species, with the formation of two σ-bonds between the C atoms in positions 2 and 5 of the thiophene ring and the Si atoms of the surface. In extending the treatment to the molecules 2T to 6T, a common adsorption mechanism for all oligomers was searched. Especially interesting was of course the case of the R-sexithiophene (6T) molecule, since this has been considered particularly interesting and has been used as a prototype system for several devices.30,31,28,29 However, works on the growth of 6T films on clean semiconductor surfaces such as Si are relatively few. Scanning tunneling microscopy (STM) images22 showed that 6T molecules are prone to dissociation in a submonolayer regime on Si(001), yielding adsorbed SC4 H3 and SC4 H2 monomers. The analysis of the images, combined with the results of DFT calculations22 suggested that the monomeric fragments were bound to the surface by two C-Si bonds on adjacent C atoms. On the Si(111)-7 × 7 surface,32 UPS data suggested instead that, at a monolayer coverage, 6T molecular films grow in a layer-by-layer fashion, with molecules oriented parallel to the surface. The goal of the present paper was to understand whether nT molecular chemisorption is energetically possible or if dissociation is to be expected with an increasing number of thiophene units. The occurrence of physisorption was also investigated by applying a novel method33 to estimate the van der Waals (vdW) contributions to the molecule-surface interactions, which are not appropriately described by standard DFT methods. We eventually calculated the electric work function for the 6T molecule on Si(001) and compared it to the experimental results.32 The paper is organized as following. In section 2 the employed theoretical methods are described. In section 3, subsection 3.1, results of the calculations for 1T are reported, with a detailed discussions of the energetics of a number of possible adsorption geometries. Subsections 3.2 and 3.3 are dedicated to the presentation of the results for the oligomers 2T-6T, and to the discussion of the physisorption mechanism for the systems studied. Finally, we draw the conclusions in section 4. 2. Computational Methods 2.1. DFT Calculations. We performed DFT calculations by using the Quantum-ESPRESSO ab initio package.34 Ultrasoft pseudopotentials were adopted and the generalized gradient approximation (GGA), in the PW91 flavor,35 was used for the exchange-correlation energy. Wave functions were expanded in a plane-wave basis set with an energy cutoff of 25 Ry, while a cutoff of 150 Ry was applied to the charge density. We explicitly checked that structural and binding properties of our systems were well converged at these cutoff values. The calculations were carried out by including only the Γ point of the Brillouin zone. We have used the PW91 functional because it often reproduces reasonably well both chemisorbed and physisorbed configurations,36 such as those investigated in the present study (although weak interactions in physisorbed structures are probably reproduced as a result of an error cancellation rather than a genuine description of weak longrange interactions).37 To our knowledge, with the exception of a work on thiophene radicals,22 no previous work has been reported which describes adsorption processes of oligothiophenes on Si(001) using an ab initio approach with plane waves. Adopting localized wave

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20069

Figure 1. Models of nT on Si(001) surface slabs. Top left: view on the yz plane of 1T in 2,3| configuration on the smallest p(4 × 4) slab (or, in fact, on any p(4 × 4) slab). Top right and bottom: view on the xz plane of 1T in 2,3| configuration on the smallest p(4 × 4) slab and of 6T on the largest slab, respectively. The rings of 6T are labeled R1 through R6.

functions would usually permit a more efficient use of hybrid DFT functionals (often more accurate than the standard GGA in a number of selected applications, such as bond description). However, to study a periodic system with DFT method, we made the natural choice of representing the electron density in a plane wave basis set. 2.2. Modeling the Surface. To model the Si(001) surface in the 2 × 2 reconstruction a slab model was employed. All together, four different slabs were used in the calculations, to adapt the size of the surface to the increasing length of the oligomer. Moreover, to check whether different surface coverages or finite size effects had any influence on the energies, calculations were performed for at least two distinct slabs of different size for each molecular system, with the exception of 1T, whose adsorption was tested on all surfaces, and of 5T and 6T, for which only the largest surface was instead suitably sized. This results in a saturation coverage close to 40%. The smallest Si slab was formed by five layers of Si atoms with a p(8 × 8)R45° periodicity,38 i.e., with four Si surface dimers. A slab characterized by a p(4 × 4) periodicity was then built, containing six Si layers with 16 atoms on each layer, i.e., eight Si surface dimers, and with dimensions a ≈ 29 b (b ) 0.52918 Å), b ) a, and c ≈ 26 b. From this, two larger supercells were obtained, by increasing only the a dimension to ≈43 and to ≈58 b, while keeping b and c constant, to comprise 12 and 16 dimers, respectively. In all cases, a monolayer of hydrogen atoms was used to saturate the dangling bonds on the lower surface of the slab. A vacuum region, 6 Å wide, separating the repeated images of the slabs, completed the supercells. Each slab underwent a preliminary structural optimization, in which all the atoms, except for the bottommost Si and H atoms, were fully relaxed. We verified that, by starting with the unreconstructed, clean Si(001) surface, the optimization procedure correctly reproduced asymmetric surface dimers, with a dimer bond length and buckling angle in good agreement with previous, highly converged, ab initio calculations.39 Pictures of some of the slabs used in the calculations appear in Figure 1. Further views appear in Figure S1 in the Supporting Information.

20070

J. Phys. Chem. C, Vol. 114, No. 47, 2010

2.3. Treatment of the van der Waals Interactions. A known drawback of standard DFT methods concerns their failure to describe van der Waals (vdW) interactions, and in particular the leading term -C6/R6, which result from electron correlation effects. An adequate treatment of these effects is nonetheless essential to the study of physisorption states, weakly bonded configurations where vdW dispersion interactions play a key role. We overcame the DFT limitations by applying the method reported in full detail in ref 33. The method is based on the generation of the maximally localized Wannier functions (MLWFs) and uses as only input the ground state Kohn-Sham orbitals computed in the conventional DFT approach. MLWFs, a generalization of the localized Boys’ orbitals40 for systems characterized by periodic boundary conditions, allow the total electronic density to be partitioned into individual fragment contributions, in a chemically transparent fashion. Once the MLWFs are obtained, the leading -C6/R6 vdW correction terms are evaluated by suitable expressions for long-range interactions between separated fragments of matter41 thanks to the fact that the C6 coefficients can be calculated directly from the basic information (center positions and spreads) given by the MLFWs.33 The method has been successfully applied to small molecules, bulk, and surface systems.42,43 It is true that, adding vdW corrections to the binding energies obtained at the PW91 level, an overestimate of the bonding in physisorbed configurations is expected. However our aim was mainly to determine the effect of the vdW contribution only, and this is expected to depend only midly on the precise choice of the DFT functional. 2.4. Work Functions. A key phenomenon associated with the formation of semiconductor/organic interfaces is the occurrence of interfacial dipoles.16 One of the measurable physical quantities linked to this process is the variation ∆W of the work function W of the material upon adsorption of the organic system. Even weakly interacting adsorbates are known to change the metal work function by as much as 1 eV. For chemisorbed systems, the picture is even more complex, due to the possible interface charge rearrangement determined by bond formation.44 The mechanism affecting the work function W and the electronic level alignment in molecular adsorbates on semiconductor surfaces is an open issue with important implications for the design of electronic devices.15 The possibility of estimating work functions and local dipoles by first principles methods is very appealing, but the problem is not trivial.16,17,44,45 By definition, the work function W is the minimum energy required to extract one electron to an infinite distance from the surface

W ) Vel(+∞) + EN-1 - EN where Vel(+∞) is the electrostatic potential energy of the electron far from the surface and EN and EN-1 are the total energies of the system with N and N - 1 electrons, respectively. The quantity W can be expressed45 as

W ) ∆Vel - EF ) Vel(+∞) - Vel(-∞) - EF where ∆Vel is the variation in the mean electrostatic potential energy across the surface and EF is the Fermi energy. If the energies are instead referred to the potential in the vacuum far from the surface, the work function can be expressed as W ) -EF. In the present study the DFT calculations are carried out by expanding the Kohn-Sham orbitals in a set of plane waves and

Costanzo et al.

Figure 2. View on the yz plane of 6T on the symmetric Si(001) surface slab used to calculate the work function. The darker Si atoms drawn in the central part of the slab are kept fixed during the relaxation. Coordinates along the z axis (in Å) correspond to those used in Figure 5. A less congested view of 6T appears in Figure 1.

allow one to obtain the electron density F(x,y,z) on a grid in real space. The electrostatic potential Vel(x,y,z) is then computed by solving the Poisson equation and, assuming that the surface normal is along the z axis, the value of Vel(+∞) can be determined by computing, as a function of z, the plane averaged potential

j el(z) ) 1 V A

A Vel(x, y, z) dx dy

j el(z) where A is the area of the unit cell surface. Typically, V reaches its asymptotic value already within a distance of about 5 Å from the surface, and DFT calculations give work functions that are within 0.1-0.2 eV of the experimental values.44 The calculation of the work function through the average j el is equivalent to a calculation through electrostatic potential V the dipole17 but may be easier and more accurate.44 As long as one is interested in relatiVe changes ∆W in the work function W, which is typically the case in adsorption processes (see, for j el(z) at the two sides of instance, ref 44), only the values of V the slab are necessary. If one is instead interested in the absolute value of the work function, the position of the bulk Fermi level needs to be determined additionally. The 6T-Si(001) system was treated by building a symmetric surface with a and b dimensions taken from the largest slab but modified in the c direction, i.e., along the normal to the surface. The idea, in fact, is to create a slab with a dipole moment inherently equal to zero and with a central part mimicking the silicon bulk. This is obtained by replacing the bottom silicon and hydrogen atoms with exact replicas of the upper layers, from the fifth to the second. Finally, the bottom layer is obtained by reflection of the first layer of buckled dimers. The slab now contains 10 silicon layers as shown in Figure 2. The first and the bottommost three layers are relaxed in the calculations while the central silicon layers, shown as darker atoms in the figure, are kept fixed at the bulk reference distances. The work function change ∆W can then be calculated j el(z) as the difference between the electrostatic potentials V between the two sides of the slab, the side with the adsorbed

Oligothiophenes on a Si(001) Surface

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20071

Figure 3. Local minima for the 1T unit on the Si(001) surface. The silicon atoms on the surface are labeled A, B, C, and D. The other silicon atoms are not labeled.

6T molecule and that of the clean surface (top and bottom sides in Figure 2, respectively). All calculations are for a thickness of z ) 25 Å, thus leaving 13 Å of empty space between periodic replica. In fact, special care must be taken to ensure that the vacuum region separating the slab from its replicas is thick enough46 to avoid spurious interactions in the z direction. In order to further reduce the effects of such interactions, the Neugebauer-Scheffler dipole correction has been applied.44,47 3. Results and Discussion 3.1. 1T on Si(001). An earlier experimental and theoretical study25 suggested that thiophene chemisorption on the Si(001) 2 × 2 surface takes place via a [2 + 2] cycloaddition mechanism, involving the carbon atoms C2 and C3 of the thiophene ring. More recent experiments and calculations,26,27,23 however, were in favor of a [4 + 2] cycloaddition scheme, involving the carbon atoms C2 and C5, in R position to the

sulfur (see Figure 3). In light of the experimental findings,24-27 and of previous DFT calculations with a cluster model approach,23,48 we investigated the local minima obtained by assuming that the single thiophene unit on Si(001) can link the Si dimers either by a [2 + 2] or a [4 + 2] cycloaddition mechanism. Altogether, four local minima involving the formation of two C-Si σ bonds were investigated, and they are drawn in Figure 3. The figure also shows two stable structures involving the formation of four C-Si σ bonds, with two Si dimers linked to the thiophene unit. As reported elsewhere,49 the reaction of a CdC double bond with the surface Si dimers can occur through different mechanisms. If one assumes that the surface dimers are symmetric and have a genuine double bond character, then a true [2 + 2] “cycloaddition” reaction appears to be feasible, but the whole process is forbidden by symmetry. The symmetry restriction is removed by the buckling of the surface dimers, which are asymmetric and therefore have a “zwitterionic” character. The reaction thus becomes allowed, with the reactive electrons lying in nonbonding orbitals on each Si atoms. In a possible [2 + 2] cycloaddition for 1T, the C2dC3 double bond reacts with a single SidSi dimer, leading to a four-atom ring with an arrangement in which the C2-C3 single bond is aligned above a dimer row and parallel to the underlying Si-Si dimer bond, in what we call a “parallel” (|) configuration (2,3| in Figure 3, also shown in Figure 1 with different perspectives). Alternatively, the reaction of the C2dC3 bond may take place with Si atoms belonging to two distinct, adjacent SidSi dimers. In this case the C2-C3 bond of the chemisorbed thiophene unit is aligned in a “perpendicular” (⊥) way with respect to the Si dimer bonds (configuration 2,3⊥). A [4 + 2] cycloaddition for 1T involves atoms C2 and C5, in R position to the sulfur, and a single SidSi dimer, leading to a six-atom ring (configuration 2,5|). The analogous reaction involving two adjacent SidSi dimers yields the configuration 2,5⊥. Binding and deformation energies (Ebind and Edeform) of all 1T structures shown in Figure 3, computed for the case of the smallest Si(001) surface with p(8 × 8)R45° periodicity, are given in Table 1, together with structural data. Binding energies are obtained as differences between the energy of the complex

TABLE 1: Binding and Deformation Energies (eV), Distances (Å), and Bending and Dihedral Angles (deg) for the 1T Configurations on Si(001)a

a

config

2,3|

2,3⊥

2,5|

2,5⊥

2,3,4,5|

2,3,4,5⊥

Ebind Edeform A-B C-D A-D C2-Si C3-Si C4-Si C5-Si C2-C5 C2-C3 C3-C4 C4-C5 C2-S C5-S Si-C2-C3 Si-C3-C2-Si C3-C2-S-C5

1.04 2.14 2.367 2.362 4.014 1.972 1.996

0.91 2.59 2.371 2.349 3.468 1.994 2.039

1.27 2.21 2.404 2.312 3.946 1.962

1.38 2.03 2.393 2.422 3.766 1.957

2.596 1.570 1.486 1.348 1.843 1.748 101.8 0.8 -1.8

2.627 1.582 1.487 1.347 1.852 1.762 119.7 6.3 -9.2

1.955 2.558 1.502 1.346 1.500 1.843 1.837 108.9

1.959 2.693 1.497 1.346 1.494 1.846 1.852 109.8

1.61 6.34 2.382 2.352 3.410 1.975 1.999 2.015 1.961 2.551 1.564 1.584 1.565 1.843 1.844 113.3

35.3

19.4

48.3

1.56 5.30 2.358 2.358 3.457 1.967 2.014 2.015 1.968 2.628 1.570 1.586 1.570 1.823 1.823 101.3 21.0 31.7

free

2.345 2.280 4.023

2.477 1.376 1.424 1.376 1.719 1.718 0.0

Molecule-surface distances (C2-Si through C5-Si) are as indicated in Figure 3. The column labeled “free” reports the distances in the clean slab or in the isolated molecule.

20072

J. Phys. Chem. C, Vol. 114, No. 47, 2010

(nT + surface) and of the fully dissociated system (isolated nT molecule plus free surface). Deformation energies are calculated as differences between the energy of the isolated molecule and the energy of the molecule deformed after the chemisorption on the surface. As anticipated in section 2, we checked for convergence, finite surface size and coverage effects, by calculating the binding energy also for 1T adsorbed on the other three larger p(4 × 4) slabs used in this work. The values found for each adsorption configuration are all within the 10% of the energies reported in Table 1, which we consider within the reproducibility of the method. Analogous energy differences were obtained for 2T, 3T, and 4T on their test surfaces. In general, we can expect the energetic ordering of different adsorption configurations to result from two competing effects:50 (i) the deformation of both molecule and surface required to allow the formation of adsorbate-substrate bonds; (ii) the bond formation energy, which increases with the number of adsorbate-substrate bonds. So, when comparing the 2,3| and 2,3⊥ configurations, we have an example where the first factor should determine the energy order. As can be seen from Table 1, the difference in the binding energy between these two cases is small (≈0.13 eV) but still in favor of the 2,3| configuration, and this can probably be ascribed to the larger strain which both the molecule and surface undergo in the 2,3⊥ configuration. In fact, the molecule deformation energy for this configuration is higher than that in the 2,3| minimum by 0.45 eV, in agreement with the C2-C3 distance exceeding the length of a C-C single bond and with a slight loss of planarity of the ring (as indicated by the nonzero dihedral angle C3-C2-S-C5). Furthermore, both C-Si bonds in the 2,3⊥ geometry are longer than in the 2,3| one, while the large Si-C2-C3 angle (≈120°) is a further indicator of strain in the structure of the complex. The presence of two dangling bonds on the Si atoms certainly adds to the surface instability. Quite interestingly, in the case of 2,3|, an increase of only 0.022 Å is computed for the Si-Si bond length of the A-B dimer reacting with the thiophene molecule. This is in agreement with previous findings for other hydrocarbons49 and was interpreted as the lack of a genuine [2 + 2] cycloaddition mechanism. According to Lu et al.23,48 thiophene most probably behaves as a cisoid conjugated diene for [4 + 2] cycloadditions with dienophiles, thus giving the 2,5| as the most stable chemisorbed configuration among the di-σ-bonded structures. The [4 + 2] cycloaddition reaction of thiophene with the Si dimers would correspond to a Diels-Alder reaction in cyclic dienes, with the C atoms in the R positions highly susceptible to the electrophilic attack. In fact, the product of the [4 + 2] cycloaddition, namely, the 2,5| configuration, is found to be more thermodynamically stable than the 2,3| configuration, with a binding energy of 1.27 eV. These configurations correspond to those labeled as LM1 and LM2, respectively, in ref 23. While the energy ordering is the same as that computed in the cluster model approach,23 the energy difference (≈0.23 eV) is slightly smaller and indeed we find that our 2,5| minimum is less deep than that reported in the literature.23 As can be seen from Table 1, the deformation energy paid by linking the atoms in positions 2 and 5 is only slightly higher than that computed for the 2,3| configuration and is accompanied by a marked loss of planarity. The 2,5| chemisorbed species is characterized by relatively shorter C-Si distances compared to the 2,3| one, by the C3-C4 distance

Costanzo et al. matching that of a double C-C bond and finally by C2-C3 and C4-C5 distances within the upper limit of a single C-C bond. In our calculations, we find that the 2,5⊥ configuration, not included among the minima of ref 23, is more stable than the 2,5| one, being characterized by the highest binding energy (1.38 eV) and the smallest deformation energy among the di-σ-bonded configurations. Although this is not the product of a genuine [4 + 2] cycloaddition, it still involves two distinct SidSi dimers of the surface, leaving basically unaltered the Si-Si distances for all the atoms involved in the process. Starting from a [4 + 2] cycloaddition process, a [2 + 2] cycloaddition can follow.23 This reaction would involve the C3 and C4 atoms and two more Si atoms, yielding the stable tetraσ-bonded 2,3,4,5| configuration of Figure 3, which has been described23 as the possible final product of the adsorption process of 1T. The analogous 2,3,4,5⊥ configuration is also conceivable. As can be seen from Table 1, the tetra-σ-bonded species have higher binding energy than the di-σ-bonded species accompanied by a higher deformation energy, in agreement with the literature.23 3.2. Chemisorption of nT Oligomers on the Si(001) Surface. To the best of our knowledge, there are only two papers22,32 dealing with the adsorption of R-oligothiophenes on clean silicon surfaces, and both concern the case of 6T. In ref 22 no molecular 6T was observed on a clean Si(001) surface. In fact, at very low coverage regimes, only fragments corresponding to the dissociation of 6T into monomeric units were detected. Longer nT fragments, arising from a partial breaking of the CR-CR′ bonds, were also observed in a submonolayer regime, presumably when most of the very reactive dangling bonds of the silicon surface had already been saturated. A thin film of 6T on Si(111) was studied in ref 32; the analysis of the photoemission spectra for nominal thickness 4 Å and the behavior of the work function suggested that the film grows in a layer-by-layer fashion, with molecules physisorbed parallel to the surface. As mentioned in the “Introduction” our theoretical approach included all the systems from 2T to 6T, in the attempt to study the occurrence of chemisorbed states common to all the oligomers and therefore applicable also to polymers. It is also interesting to check whether the experimentally reported chemical dissociation could be preceded by molecular chemisorption, despite the fact that the experiments22 are in a low coverage regime not approached in our calculations. Starting from the results obtained for 1T, many possible ways of placing the all-trans nT molecule on the Si(001) surface, and a number of possible adsorption configurations, have been tested, involving the formation of a number of bonds e2n. In fact, most of them failed to converge to bonded states. Since no free energy landscape sampling techniques have been used, nothing can actually be said about activation barriers for the chemisorptions or physisorption of nT oligomers. By assuming that the chemisorption of any nT oligomer takes place by forming two C-Si σ bonds for each ring, simple geometric considerations show that this can only happen when the carbon atoms in positions 2 and 3 of the ring react with a single Si surface dimer. That is, multiple bonds between the nT oligomer and the Si(001) surface are geometrically compatible only with multiple 2,3| cycloaddition processes, but not with the 2,5 cycloadditions, which instead prove to be the more energetically stable for the single thiophene unit. On the other hand, from the agreement between experimental and simulated STM profiles for the thiophene fragment SC4 H2,22 detected as the product of

Oligothiophenes on a Si(001) Surface

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20073

Figure 4. Binding and deformation energies (upper and lower panels, respectively) of the nT molecules chemisorbed on the largest Si(001) slab and forming 2n σ bonds with a row of n adjacent Si dimers.

TABLE 2: Distances (Å) and Dihedral Angles (deg) for 6T on Si(001)a C2-A C3-B C3-C2-S-C5 a

R1

R2

R3

R4

R5

R6

1.989 1.975 -27.9

2.090 1.941 26.7

2.106 1.950 -28.6

2.104 1.950 28.7

2.086 1.942 -27.3

2.071 1.952 7.7

The rings of 6T are labeled R1 through R6, as shown in Figure 1.

the dissociation of 6T on Si(001), it is reasonable to infer that the 2,3| adsorption mechanism is the preferred one for nT oligomers. Views of 1T and 6T adsorbed in 2,3| configuration on Si(001) appear in Figure 1. Views of 2T through 6T appear in Figure S2 in the Supporting Information. Figure 4 shows the binding and deformation energies of the nT molecules chemisorbed on the largest Si(001) surface (a ≈ 58 b, 16 Si dimers), by forming 2n σ bonds with a row of n adjacent Si dimers. The same energies are also listed in Table S3 in the Supporting Information. As shown by the figure, only 2T has a binding energy comparable to 1T, whereas the following nT molecules display a steady drop of the binding energy which is likely largely due to the increased deformation energy, on average 2.8 eV per each added ring. In fact, already for R-5T, the binding energy is almost null. Clearly, although the reactivity of Si dangling bonds with the C-C double bonds of the thiophene rings is very high, the deformation undergone by both the molecule and the surface bonds is also very large, due to the mismatch between molecular and surface geometries. The 2n C-Si σ bond lengths and dihedral angles for 6T are given in Table 2. The C-Si bond lengths for 6T are comparable to those for the strongly bonded 1T (Table 1), indicating significant interactions. The geometric mismatch between surface and 6T molecule is well evidenced by the dihedral angles reported in Table 2, mostly around (28°, which clearly signal a strong deformation of the thiophene rings R1 through R5 in comparison with those of 1T on Si(001) (see Table 1) or of the

isolated 6T molecule, where the dihedral angles are nearly zero. As shown in Figure 1, the last ring of 6T, R6, is adsorbed in an arrangement closer to that of 1T. Since it is less constrained, R6 exhibits a more relaxed dihedral angle. These findings make very unlikely that a molecular chemisorption of the whole nT molecule is the actual process that precedes the fragmentation into monomeric units observed in ref 22. Therefore, more likely, the whole process proceeds through the chemisorption involving a limited number of the double bonds of a single oligomer, and it is followed by dissociation of the molecule into progressively shorter reactive fragments able to interact with the still available dangling bonds. 3.3. Physisorbed States for 6T. The experimental evidence32 of the presence of 6T physisorbed states on the Si(111) surface suggested to us to check for the energy of such states on Si(001). The results can be extended to any Si surface, as physisorption mainly depends on vdW interactions, which are not directional and not particularly sensitive to the specific position of the molecule on the surface and to the specifically chosen surface face. In fact, physisorption and dissociative chemisorption have been observed to occur simultaneously in the interaction of reactive Si surfaces with organic molecules.51 In the presence of strong vdW interactions with the surface, molecules tend to lie on the surface itself. Keeping this in mind, and in agreement with the data of ref 32, 6T was placed parallel to the surface, and the vdW contribution was computed over a range of distances from the slab for which neither chemical bonding nor charge overlap with Si atoms occur. The vdW energy was computed with the procedure proposed by Silvestrelli33 and described in section 2.3. Since the reliability of the method is based on the assumption of nonoverlapping Wannier functions, we analyzed the spreads relative to the Wannier functions of each fragments, at different molecule-surface distances d, and we concluded that the estimated vdW correction is valid for d > 4 Å, being of the order of 1 eV at d = 4 Å. Such a value is within the range of observed desorption energies for physisorbed organics,52-54 and supports the occurrence of very stable physisorbed states. 3.4. Work Function of 6T on Si(001). Figure 5 shows the j el(z) of the system shown in average electrostatic potential V Figure 2 (the symmetric silicon slab with 6T adsorbed on one side only). As mentioned in section 2.4, to avoid the artificial macroscopic electrostatic field due to the periodic boundary conditions, we followed the Neugebauer-Scheffler procedure,47 introducing a planar dipole layer in the middle of the vacuum j el(z) on the two sides (inset of region. The difference between V Figure 5) is the variation ∆W of the work function W of the silicon surface due to physisorption of 6T. Our calculations indicate that physisorption of 6T causes a lowering of the work function by about 0.5 eV, which indicates the formation of a strong dipole at the interface. This result is in good agreement with the experimental value for sexithiophene on Si(111) in the low thickness regime,32 ∆W ) 0.5 eV. Clearly, since the Si(001) and Si(111) surfaces are inequivalent, for this particular quantity, the comparison with this reference value can be, at most, semiquantitative. 4. Summary and Conclusions The chemical adsorption of the thiophene molecule on the Si(001) surface has been studied by an ab initio approach, using a plane-wave basis set and periodic boundary conditions. In agreement with previous theoretical studies23,48 and experimental evidence,24,26,27 the [4 + 2] cycloaddition mechanism yields the most stable chemisorbed configurations, with the formation 2,5-

20074

J. Phys. Chem. C, Vol. 114, No. 47, 2010

Costanzo et al. Framework Programme (FP7/2007-2013) under Grant Agreement NO 212311 of the ONE-P project. We thank Jérôme Cornil for helpful discussions. Supporting Information Available: Figures of Si(001) surface models, figures of 2T through 6T on the Si(001) surface, table of binding and deformation energies of nT chemisorbed on Si(001). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

j el(z) for 6T adsorbed Figure 5. Plane averaged electrostatic potential V on the symmetric Si(001) slab (structure shown in Figure 2). Inset: j el(z) g -0.25 eV. enlargement of the region with V

dihydrothiophene-like species. The chemisorption of nT thiophene oligomers, however, very unlikely may take place with the same mechanism. This consideration is supported by the findings of STM measurements for 6T on Si(001),22 in which fragments SC4 H3 and SC4 H2 appear to be linked to the Si dimers with bonds involving the C2 and C3 atoms of the thiophene monomers. With the idea of seeking chemisorbed states with multiple bonds with the surface for oligomers, we studied many possible configurations and found that stable bound states are obtainable with the nT molecules forming 2n C-Si bonds involving adjacent dimers. The steady drop of the binding energy with n, together with the increase of the deformation energy, suggests that multiple bond formation involves an increasing strain of the complex and it is probably followed/accompanied by an efficient bond breaking. Our study sheds light on the chemisorption of oligothiophenes on Si(001) showing, in particular, that molecular chemisorption is possible for oligothiophenes with up to four thiophene units. Chemisorption can be followed by dissociation and fragmentation on the surface, in agreement with experimental work. The coexistence of chemi- and physisorbed states has been experimentally observed in a number of organics, even on very reactive surfaces such as Si.24,51 We are here able to quantify the vdW physisorption binding energy for the 6T molecule lying flat of the Si(001) surface. Our calculated decrease for the work function, ∆W ≈ 0.5 eV for a single 6T molecule on the surface, is consistent with the experiments by photoemission spectra32 of 6T on Si(111), giving us confidence in the method. As a complement to our study and as a further investigation to our problem system, in the future it could be interesting to study by ab initio techniques possible reaction paths (see our previous studies)49,55,56 for chemisorption of 6T on Si(001), in order to describe also the kinetics of the adsorption processes. Acknowledgment. The research leading to these results has received funding from the European Community’s Seventh

(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Edman, L.; Pauchard, M.; Liu, B.; Bazan, G.; Moses, D.; Heeger, A. Appl. Phys. Lett. 2003, 82, 3961. (3) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (4) Handbook of Oligo-and Poly-thiophenes; Fichou, D., Ed.; WileyVCH: Wienheim, 1999. (5) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (6) Gigli, G.; Ingana¨s, O.; Anni, M.; De Vittorio, M.; Cingolani, R.; Barbarella, G.; Favaretto, L. Appl. Phys. Lett. 2001, 78, 1493. (7) Lu, X.; Lin, M. C.; Xu, X.; Wang, N.; Zhang, Q. PhysChemComm 2001, 13, 1. (8) Carbone, M.; Piancastelli, M. N.; Casaletto, M. P.; Zanoni, R.; Comtet, G.; Dujardin, G.; Hellner, L. Phys. ReV. B 2000, 61, 8531. (9) Wolkow, R. A.; Moffatt, D. J. J. Chem. Phys. 1995, 103, 10696. (10) MacPherson, C. D.; Leung, K. T. Surf. Sci. 1995, 324, 202. (11) Cao, Y.; Wang, Z.; Deng, J. F.; Xu, G. Q. Angew. Chem., Int. Ed. 2000, 39, 2740. (12) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (13) Michaelides, A.; Hu, P.; Lee, M.-H.; Alavi, A.; King, D. A. Phys. ReV. Lett. 2003, 90, 246103. (14) Leung, T. C.; Kao, C. L.; Su, W. S.; Feng, Y. J.; Chan, C. T. Phys. ReV B 2003, 68, 195408. (15) De Renzi, V.; Rousseau, R.; Marchetto, D.; Biagi, R.; Scandolo, S.; del Pennino, U. Phys. ReV. Lett. 2005, 95, 046804. (16) Natan, A.; Zidon, Y.; Shapira, Y.; Kronik, L. Phys. ReV. B 2006, 73, 193310. (17) Natan, A.; Kronik, L.; Shapira, Y. Appl. Surf. Sci. 2006, 252, 7608. (18) Jeong, H.; Jung, D. Y.; Yeom, H. W. Phys. ReV. B 2008, 78, 073305. (19) Mas-Torrent, M.; Rovira, C. Chem. Soc. ReV. 2008, 37, 827. (20) Horowitz, G.; Fichou, D.; Peng, X. Z.; Xu, Z. G.; Garnier, F. Solid State Commun. 1989, 72, 381. (21) de Bettignies, R.; Nicolas, Y.; Blanchard, P.; Levillain, E.; Nunzi, J.-M.; Roncali, J. AdV. Mater. 2003, 15, 1939. (22) Lin, R.; Galili, M.; Quaade, U. J.; Brandbyge, M.; Bjørnholm, T.; Degli Esposti, A.; Biscarini, F.; Stokbro, K. J. Chem. Phys. 2002, 117, 321. (23) Lu, X.; Xu, X.; Wang, N.; Zhang, Q.; Lin, M. C. J. Phys. Chem. B 2001, 105, 10069. (24) Qiao, M. H.; Cao, Y.; Tao, F.; Liu, Q.; Deng, J. F.; Xu, G. Q. J. Phys. Chem. B 2000, 104, 11211. (25) Jeong, H. D.; Lee, Y. S.; Kim, S. J. Chem. Phys. 1996, 105, 5200. (26) Cao, Y.; Yong, K. S.; Wang, Z. H.; Deng, J. F.; Lai, Y. H.; Xu, G. Q. J. Chem. Phys. 2001, 115, 3287. (27) Rousseau, G. B. D.; Dhanak, V.; Kadodwala, M. Surf. Sci. 2001, 494, 251. (28) Horowitz, G.; Hajlaoui, M. E.; Hajlaoui, R. J. Appl. Phys. 2000, 87, 4456. (29) Garnier, F. Acc. Chem. Res. 1999, 32, 209. (30) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (31) Horowitz, G. J. Mater. Chem. 1999, 9, 2021. (32) Ivanco, J.; Krenn, J. R.; Ramsey, M. G.; Netzer, F. P.; Haber, T.; Resel, R.; Haase, A.; Stadlober, B.; Jakopic, G. J. Appl. Phys. 2004, 96, 2716. (33) Silvestrelli, P. L. Phys. ReV. Lett. 2008, 100, 053002. (34) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; Fabris, S.; Fratesi, G.; de Gironcoli, S.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter 2009, 21, 395502. (35) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (36) Bergès, J.; Caillet, J.; Langlet, J.; Kozelka, J. Chem. Phys. Lett. 2001, 344, 573.

Oligothiophenes on a Si(001) Surface (37) Tsuzuki, S.; Lu¨thi, H. P. J. Chem. Phys. 2001, 114, 3949. (38) Wood, E. A. J. Appl. Phys. 1964, 35, 1306. (39) Shkrebtii, A. I.; Di Felice, R.; Bertoni, C. M.; Del Sole, R. Phys. ReV. B 1995, 51, 11201. (40) Boys, S. F. ReV. Mod. Phys. 1960, 32, 296. (41) Andersson, Y.; Langreth, D. C.; Lundqvist, B. I. Phys. ReV. Lett. 1996, 76, 102. (42) Silvestrelli, P. L.; Benyahia, K.; Grubisicˆ, S.; Ancilotto, F.; Toigo, F. J. Chem. Phys. 2009, 130, 074702. (43) Silvestrelli, P. L. J. Phys. Chem. A 2009, 113, 5224. (44) Rusu, P. C.; Brocks, G. J. Phys. Chem. B 2006, 110, 22628. (45) Fall, C. J.; Binggeli, N.; Baldereschi, A. J. Phys.: Condens. Matter 1999, 11, 2689. (46) Kajita, S.; Nakayama, T.; Yamauchi, J. J. Phys.: Conf. Ser. 2006, 29, 120. (47) Neugebauer, J.; Scheffler, M. Phys. ReV. B 1992, 46, 16067. (48) Lu, X.; Wang, X.; Yuan, Q.; Zhang, Q. J. Am. Chem. Soc. 2003, 125, 7923.

J. Phys. Chem. C, Vol. 114, No. 47, 2010 20075 (49) Costanzo, F.; Silvestrelli, P. L.; Ancilotto, F. J. Phys. Chem. B 2005, 109, 819. (50) Hafner, J. Monatsh. Chem. 2008, 139, 373. (51) Yanagi, H.; Schlettwein, D.; Nakayama, H.; Nishino, T. Phys. ReV. B 2000, 61, 1959. (52) Mete, E.; Demirogˇlu, I.; Danis¸man, M. F.; Ellialtiogˇlu, S¸. J. Phys. Chem. C 2010, 114, 2724. ¨ stro¨m, H.; Ogasawara, H.; Na¨slund, L.-Å.; Pettersson, L. G. M.; (53) O Nilsson, A. Phys. ReV. Lett. 2006, 96, 146104. (54) Cafe, P. F.; Larsen, A. G.; Yang, W.; Bilic, A.; Blake, I. M.; Crossley, M. J.; Zhang, J.; Wackerbarth, H.; Ulstrup, J.; Reimers, J. R. J. Phys. Chem. C 2007, 111, 17285. (55) Costanzo, F.; Sbraccia, C.; Silvestrelli, P. L.; Ancilotto, F. J. Phys. Chem. B 2003, 107, 10209. (56) Costanzo, F.; Sbraccia, C.; Silvestrelli, P. L.; Ancilotto, F. Surf. Sci. 2004, 566-568, 971.

JP106627Z