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The Role of Charge-Charge Correlations and Covalent Bonding in the Electronic Structure of Adsorbed C60: C60/Al Joachim Schiessling,† A. Grigoriev,*,† Mauro Stener,‡ Lisbeth Kjeldgaard,† Thiagarajan Balasubramanian,¶ Piero Decleva,‡ R. Ahuja,†,§ Joseph Nordgren,† and Paul A. Bru¨hwiler†,| Department of Physics and Materials Science, Uppsala UniVersity, Box 530, SE-751 21 Uppsala, Sweden, Dipartimento di Scienze Chimiche, UniVersita´ degli Studi di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy, MAX-lab, Lund UniVersity, Box 118, SE-221 00 Lund, Sweden, Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), 10044, Stockholm, Sweden, and Empa, Swiss Federal Laboratories for Materials Testing and Research, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland ReceiVed: May 5, 2010; ReVised Manuscript ReceiVed: September 3, 2010
Aromatic molecules are central components of model systems for molecular electronics, with C60 one of the most studied. Upon adsorption on (metallic) substrates a splitting of the frontier orbitals is commonly observed, with a strong dependence on substrate material, but little dependence on substrate structure. We report the detailed photoelectron angle dependence of C60/Al(110) over a wide range of energy, finding a strong remnant molecular character. In particular, certain HOMO-derived suborbitals couple strongly, and others weakly, with the metal, which results in final state charging for those weakly coupled. C 1s data correlate well with the assignments made on this basis, as does the comparison of ground state partial densities-of-states (PDOS) to photoelectron spectra. Detailed analysis of the PDOS supports a rough division into surface-near and surfacefar components, in agreement with the molecular picture. The component spectral widths are attributed to intramolecular vibrational coupling, which is suggested to aid in the electronic decoupling of certain suborbitals from the substrate, facilitating the observed final state charging. I. Introduction The electronic structure and bonding of C60 molecules on surfaces has been a topic of high interest for over a decade, with new aspects continually being exposed.1-7 Fullerene-based thin films also serve as prototype systems for molecular devices. The substrate-induced modification of the electronic and structural properties of organic molecules is a major issue when considering the construction of single-molecule devices.8-12 Covering Al electrodes with C60 monolayers improves solar cell charge transfer efficiency.13-15 Such developments rationalize the large effort invested over the years in understanding how the C60-substrate interaction alters the molecular electronic structure.16-33 A fundamental question in these studies which has hardly been addressed so far is the mechanism behind the splitting and broadening of the molecular levels. The significantly increased width of the frontier valence levels in PES has been speculated to be due to the chemical bond,17,30,31,33-37 or nonequivalent carbon sites.38,39 The amount of broadening varies from substrate to substrate, with the largest broadening in valence PES being observed for C60 on Al.34 The relevance of such work to molecular electronics is shown by the similarity of the valence electronic structure observed in scanning tunnelling spectroscopy (STS) of monolayers29 and isolated molecules31 on Ag(001), * To whom correspondence should be addressed. E-mail: anton.grigoriev@ fysik.uu.se. † Uppsala University. ‡ Universita´ degli Studi di Trieste. ¶ Lund University. § Royal Institute of Technology (KTH). | Empa, Swiss Federal Laboratories for Materials Testing and Research.
which is also reflected in the spectrum of a device containing a molecule between (polycrystalline) Ag electrodes.40 This type of robust bonding characteristic is also reflected in the virtual independence of the gross electronic structure of C60 on the surface index for low-index Al surfaces.41 Recent first-principles computational work has focused on improving the ground state description30,31 of the adsorbed molecular system by including such correlation effects semiempirically.32 Final state effects in spectroscopic probes of adsorbed C60 have otherwise not been addressed computationally. We have experimentally studied this question for C60 on Al surfaces using angle-dependent valence photoemission data in which two spectroscopic components are apparent for the first two occupied frontier orbitals.21,41 For C60 on three low-index Al surfaces we compared PES at one angle of emission to ab initio partial density-of-states (PDOS) calculations;42 this is a standard approach to identify final state effects. For all three surfaces, an extra peak at higher binding energy was nonexistent in the PDOS, and is therefore to be associated with a final state charging of the molecule due to lack of charge transfer screening. The position of the charge-transfer screened peak agreed well with the PDOS, and substantiated the earlier analysis, in which molecular suborbitals (e.g., of the 5-fold HOMO hu manifold) were deduced to exhibit strongly differing overlaps with the substrate.21 We concluded that intramolecular correlations, combined with variable covalent interactions between a molecular orbital and the substrate, caused the observed molecular band splitting. A direct comparison of PDOS to PES data41 thus yielded clear proof of the existence of final state effects, whereas the more empirical approach21 yielded a quantitative estimate of the final-state splitting.
10.1021/jp104090d 2010 American Chemical Society Published on Web 10/08/2010
Electronic Structure of Adsorbed C60: C60/Al Here we present further details of our investigations, including more data on the angle dependence to expose molecular aspects of the electronic structure available to direct probing. We develop the discussion of the electron self-energy effects41 in the observed orbital splitting in more detail, extending the analysis to the core level spectra. Finally, we present a spatial analysis of the PDOS calculations that suggests possible ground state properties of relevance to the observed excited states. II. Experimental and Computational Details The experiments were carried out at Beamline 33 at MAXlab.42 The end-station is equipped with a standard ultrahigh vacuum preparation chamber connected via a sample transfer system to a photoelectron analysis chamber. The base pressure in the preparation chamber was 4 × 10-10 mbar, and that in the analysis chamber was 6 × 10-11 mbar. The Al(110) singlecrystal substrate was cleaned by cycles of Ar+ sputtering and subsequent annealing. The ordered monolayer was formed by subliming C60 from a home-built Ta crucible at 400 °C onto the cleaned (110) surface while the substrate was held at 400 °C. Since this temperature is well above the sublimation temperature of solid C60, only one monolayer sticks to the surface. Confirmation that no higher layers were present came from low-energy electron diffraction (LEED), which showed the typical c4 × 4 monolayer (ML).21,34 All measurements were carried out at room temperature. The analysis chamber was equipped with a goniometer-mounted electron energy analyzer with variable acceptance angle, chosen to be 2°. The emission could be varied over 2π radians, with tighter constraints for certain sample orientations. Spectra were taken in the plane defined by the light polarization and light incidence by scanning the polar emission angle in 6° steps. All angles are given with respect to the surface normal. The common plane of light incidence and photoelectron emission was always perpendicular to the sample surface. Given the deduction that the molecules adsorb with a 5-6 bond in contact with the surface, the plane of photoelectron emission was perpendicular to the 5-6 bond direction.21 We employed photon energies between 24 and 110 eV, with an overall energy resolution of better than 80 meV. The photon energy was calibrated by measuring the difference in the kinetic energy of the highest occupied molecular orbital (HOMO) excited by first- and second-order light. The kinetic energy scale was calibrated with the monolayer work function of 5.25 eV.34,43 The spectra are normalized to the light flux, which was measured as photocurrent on a gold mesh placed in the beamline in front of the analysis chamber. The calculations of the partial density-of-states (PDOS) were performed within the generalized gradient approximation of DFT, using the SIESTA program.44,45 We used the parametrization of ref 46 for the exchange-correlation functional, and normconserving pseudopotentials47 with nonlinear core correction.48 The employed reference configuration was 3s23p13d04f0 for Al and 2s22p23d04f0 for C. Valence wave functions were expanded in a set of numerical localized pseudoatomic orbitals.45,49 Similar results were obtained for the SIESTA single- (presented) and double-ζ polarized basis sets.44 The finite range of the orbitals was defined by an orbital confinement energy49 of 0.02 Ry. Calculations were performed in a 4 × 5 orthorhombic supercell geometry. Three Al atomic layers were fixed in bulk positions at one side of the slab, and the rest were allowed to fully relax to model the free surface. The molecule was first oriented as found experimentally.21 Then the surface with the molecule was fully optimized, with residual forces less than 0.04 eV/Å. The
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18687 adsorption sites were determined by shifting the molecule in the unit cell to achieve the energy minimum. The minimal energy molecular orientation was always consistent with experiment. The overlayer was not close-packed, which was found to be irrelevant in test calculations. The resolution of the realspace mesh was defined by a 105 Ry cutoff, assuring energy and force convergence. We used the Monkhorst Pack 3 × 3 × 1 scheme, yielding 6 k-points in the BZ. A smooth PDOS was obtained by convoluting the line PDOS with a 0.42 eV-wide Gaussian to account for experimentally derived broadening.21 The photoionization differential cross section was calculated in the dipole-approximation, using an approach that combines density-functional theory (DFT) with a basis consisting of a large One Center Expansion (OCE) of products of radial B-spline functions and angular real spherical harmonics; see refs 50-52 for more details. In the present work we employed a conventional Kohn-Sham Hamiltonian,53 with the LB94 exchange-correlation potential.54 The complete Ih point group symmetry was assumed. Once the continuum functions were obtained, the dipole integrals between bound and unbound states were calculated, imposing incoming wave S-matrix boundary conditions. From such matrix elements, the differential cross section for the given fixed molecular orientation was then calculated by means of standard expressions.55,56 Because a 2-fold symmetry plane of the molecule was coincident with the emission plane, only those components of the HOMO manifold which were allowed by symmetry to photoemit were included. The calculations of the adsorbed molecular partial density-ofstates (PDOS) have been described in detail elsewhere.41 III. Results and Discussion a. Component Line Shapes. A series of PES spectra at three photon energies taken for a range of emission angles is shown in Figure 1. Large intensity variations are seen in the data. In particular the two frontier bands, denoted HOMO and HOMO1, show rich structure, whereas peak C exhibits little structure, with notable photon energy dependence in all cases. Four contributions can be identified by visual inspection in the HOMO- and HOMO-1-derived region from the angle dependence, which is more complex than observed57 for solid C60. We focus on the HOMO-derived components, which we label HOMO-a (closer to EF) and HOMO-b.21 We present details of our analysis of the photon energy and angle dependence of the data in the Supporting Information. Summarizing that analysis, the angular distributions are consistent with molecular orbital splitting due to interaction with the substrate, and agree well with free molecule DFT calculations at photon energies near 32 eV. A more detailed comparison would require including scattering effects induced by the metal surface. To isolate the contributions and quantify the angle dependence of the data, they were fitted21 by using empirical line shapes derived from differences between spectra; these line shapes are collected in Figure 2 for comparison to results given below. Note that the quality of the fit is expected to be best for energies closest to EF.21 The solid C60 data are included there as a visual aid to estimating the widths of the features, which are seen to be slightly larger, but of closely the same order of magnitude. The empirically determined components concretize the picture of strongly split molecular orbitals, with component spectral widths comparable to those of solid, gas phase, and physisorbed58 C60. b. C 1s Level. As generally observed, the core level line shape of C60 monolayer systems is much broader than that
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Figure 2. Example of the empirical line shape analysis of the data of Figure 1. (a) Fit at 32 eV (note that the quality of the fit is expected to deteriorate for the deeper levels). (b) Comparison of the derived line shapes to the corresponding spectra of solid C60. See ref 21 for further details and for a comparison to gas-phase data.
Figure 3. C 1s PES for34 C60/Al(110) and multilayer43 C60, taken at comparable resolutions. The spectrum of the multilayer is shifted in energy for better comparison.
Figure 1. Three sets of angle-dependent photoemission spectra at the indicated excitation energies, with features derived from the frontier and next levels of the free molecule labeled. The light polarization was 25°. All spectra are normalized to the light flux. The emission angles are given with respect to the sample normal. See the text for more details.
observed for multilayers. However, as we show in Figure 3, C60/Al(110) constitutes an exception, which is also the case for the Al(111) surface.34 A visual comparison shows that the width of the C60/Al C 1s line is very similar to that of the multilayer spectrum.59 This suggests that there is a similar internal screening of the core hole in both cases,34,59 which shows in turn that the distance of the excited C atom on the molecule to the surface is of secondary importance for the screening.34,60 For isolated C60, a core hole produces a uniform redistribution of the positive charge via static polarization of the molecule in the final state.59 We therefore ascribe the similar widths to the internal polarization of C60 upon 1s ionization.59,61,62 The comparable widths could be compatible with either complete or negligible charge transfer screening; given that the C 1s binding energy marks the X-ray absorption threshold,34,60 it is
clear63,64 that metallic screening of the (core-excited) charged molecule takes place. Here we consider the screening mechanisms for core excitation of C60/Al(110) in more detail, since they provide further insight into the present question of valence excitation screening. It has already been pointed out21,34 that HOMO-a is the logical choice for a band affected by substrate bonding due to its surface-dependent width, with a charge-transfer screening approaching that of the metallic valence band, and thereby much smaller intramolecular correlation. This implies that the HOMO-a component and the C 1s level are screened similarly. Other studies60,65 have established that the C 1s hole breaks the degeneracy of the LUMO,66-68 inducing one component with large weight on the core hole, and two distributed further away on the molecule. These were demonstrated to have a good overlap with the substrate when in proximity to it,60,65 thereby enabling a fast charge-transfer screening relative to the few femtosecond time scale for core excitations on any carbon atom. We propose that this, coupled with the excellent internal molecular polarization, explains the essentially identical screening of the C 1s and HOMO-a level. This is therefore also the explanation of the identical separation of those levels of 282.6 eV and the C 1s-HOMO separation for charge transfer systems (see Table 1 of ref 69), for which the screening mechanism (polarization) is the same in both cases. Thus, the screening
Electronic Structure of Adsorbed C60: C60/Al
Figure 4. Comparison of valence band electronic structure for the indicated systems. Photoelectron spectra are shown with markers and the broadened DFT PDOS as lines. Major differences between photoemission and calculation are marked by (vertical) arrows for the HOMO-derived band. The solid C60 spectrum and the corresponding calculation have been positioned to simplify comparisons with the monolayer PDOS. Further details are discussed in the text and in ref 41.
for the core level is deduced to be quite close to that of the HOMO-a, and the core hole will be neutralized via the covalently bonded suborbital components of the core-excited (LUMO-derived) state which contribute to the polarization screening of the molecule. Electrodes in many test cases are based on Au, the electronic structure of which has also been studied for monolayers on low-17,70 and high-index71 surfaces. Early results indicated charge transfer bonding (and no apparent covalent bonding), as summarized in Table 1 of ref 34. For the low-index surfaces, the HOMO-C 1s separation is69 282.7 eV, and
