ARTICLE pubs.acs.org/JPCC
Electronic Structure and Bonding of an Ionic Molecular Adsorbate: c-C5H5 on Cu{111} M. Sacchi* and S. J. Jenkins Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom
H. Hedgeland and A. P. Jardine Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, United Kingdom
B. J. Hinch Department of Chemistry & Biological Chemistry, Rutgers University, Piscataway, New Jersey 08854, United States ABSTRACT: Self-assembled monolayers containing conjugated π systems find application in organic electronics to functionalize and modify the electronic properties of metals and metal oxides. Isolated cyclopentadienyl is an aromatic molecular anion similar in size to benzene that, unlike benzene, adsorbs quite strongly even on coinage metal surfaces. In this study, the electronic structure, bonding, and minimum energy configuration of cyclopentadienyl (c-C5H5 or Cp) adsorbed on Cu{111} are calculated via first-principles density theory (DFT). The Cu{111} surface has been √ functional √ modeled within a (2 3 2 3)R30 cell, and the adsorbed Cp has been found to reside preferentially on the hollow sites, with a binding energy of 1.73 eV. Electronic population analysis reveals a net charge transfer of ∼1.1 electrons from the metal to the Cp, indicating that the adsorption is dominated by ionic bonding. The surface diffusion barrier between two adjacent hollow sites was calculated to be 55 meV, in good agreement with previously reported measurements by helium spin echo (HeSE) spectroscopy. It was found that lateral interactions do not significantly influence the binding energy and mobility of the adsorbate. The physicalchemical properties of this strongly bound but weakly mutually interacting molecular adsorbate suggest that Cp could become a model system for ionically adsorbed molecular adsorbates.
1. INTRODUCTION Interest in the bonding properties of aromatic molecules on metals is partially motivated by the fact that organic selfassembled supramolecular networks, often functionalized by aromatic rings, are applied in photonics and electronics to build a continuously increasing number of devices from organic transistors to solar cells, light-emitting diodes, and recently even spintronic devices.15 Aromatic adsorbates on metal surfaces are also of catalytic relevance in industrial chemistry, for example, in the synthesis of styrene (a key monomer in polymer synthesis), which can be produced by catalytic dehydrogenation of ethylbenzene.6 The adsorption of aromatic molecules on metal surfaces represents a challenge for electronic structure first-principle calculations and, in particular, for density functional theory (DFT). Most of the simple and important aromatic molecules like benzene and its derivates adsorb relatively weakly (for molecules of this size) on transition and coinage metals.79 It is well understood that the bonding between benzene and metal surfaces is often dominated by van der Waals (VdW) forces, and DFT, in general, cannot deal very well with this kind of long-range correlation effect. For example, the adsorption energy of benzene on Cu{111}, r 2011 American Chemical Society
Ag{111}, and Au{111} is severely underestimated by DFT calculations.9 On the contrary, cluster-type calculations in which the solid is modeled by means of a small-to-medium size ensemble of atoms10 can apply perturbation theory to account for VdW interactions but completely neglects the effects of any lateral interactions present on real surfaces. The poor convergence of the adsorption energy and substrate properties with the cluster size is also a source of debate because, as shown by Domínguez-Ariza et al.,11 the charge density and the surface potential are often not accurately described by atomic clusters. The performance of DFT-based calculations for modeling the adsorption of aromatic molecules on metal surfaces has recently been the object of an exhaustive review by Jenkins.7 Despite the lack of correctly accounting for VdW forces, DFT calculations can almost always predict quite successfully the favored adsorption site and the adsorbate geometry. Furthermore, when covalent and polar bonding contributes strongly to the adsorption, for example, when benzene adsorbs on Ni{111},12,13 Received: May 27, 2011 Revised: July 12, 2011 Published: July 17, 2011 16134
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The Journal of Physical Chemistry C Ni{100},13 Ni{110},13 and Al{111},14 the energies calculated with DFT are broadly consistent with the experimental results, although in general slightly underestimated because of the missing VdW contributions. DFT calculations of adsorbed fivemembered aromatic rings like pyrrole (C4H4NH), thiophene (C4H4S), and cyclopentandienyl (C5H5) are relatively few. In the case of thiophene, it was suggested that VdW forces are almost totally responsible for the weak binding (0.5 eV) of this molecule on Cu{110} surface,15 whereas pyrrole binds quite strongly (1.30 eV), with a planar geometry, on the flat Mo{110} surface.16 Calculations based on extended H€uckel theory have previously been performed for Cp adsorption on the Pt{111},1719 Ni{111},2022 and Ni{100}22 surfaces. German et al.20have reported a gradient-corrected DFT investigation of Cp adsorption on Ni{111}, and a further DFT investigation of Cp adsorption has recently been reported by Atodiresei et al.23 for Cp adsorption on Fe/W{110}. The high substrate density of states (DOS) at the Fermi level in this latter case makes the binding of Cp particularly strong (2.5 eV). In a previous study,24 we have investigated the potential energy surface and diffusion of Cp on Cu{111}. Our DFT results, combined with the He spin echo (HeSE) measurements, have shown that Cp is a strongly bound but highly mobile molecular species, with an extremely high friction coefficient (η = 2.5 ps1). In the present work, we have performed fully unconstrained DFT geometry optimizations of Cp adsorbed on Cu{111} for several adsorption sites and different molecular orientations. Furthermore, the periodic boundary conditions approach is better suited to the investigation of the properties of polar or ionic adsorbates at moderate-to-high coverages because dipoledipole and Coulomb interactions between coadsorbed molecules are fully taken into account.
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increasing the density of the k-points mesh from 4 4 1 to 6 6 1 decreases the total energy by just 0.1 eV. Vanderbilt Ultrasoft Potentials29 were used in all of the present calculations for Cu, H, and C atoms. The Cp adsorption was modeled by placing the molecule on the top side of the Cu{111} slab, with a vacuum region in the surface calculations equivalent to 19 copper layers, whereas the energy of the ideally isolated gas-phase molecule was estimated by placing a single Cp in a 10 Å side cubic cell, sufficiently large to avoid spurious interactions with the supercell images.
3. RESULTS AND DISCUSSION 3.1. Adsorption Energies and Structural Results. The binding or “adsorption” energy, Eads, of Cp is determined by the difference in energy between the initial state, given by the isolated molecule in gas phase plus a clean relaxed Cu{111} surface, and the final state, where Cp is adsorbed in the same surface unit cell
Eads ¼ ECp=Cuf111g ECuf111g ECp
ð1Þ
where ECp/Cu{111} is the total energy of the adsorbate plus surface system, ECu{111} is the energy of the clean surface, and ECp is the energy of the isolated neutral molecule. To estimate the binding energy of an ionic adsorbate, one should consider the possibility that the molecule may preferentially desorb either as a neutral species, as above, or as an ion. In the latter case, the total adsorption energy is calculated by constructing a thermodynamic BornHaber cycle, as proposed by Giordano and Pacchioni.30 Practically, the energy required to desorb the anionic Cp is given by adding two extra terms to the right-hand side of eq 1 Eads ¼ ECp=Cuf111g ECuf111g ECp
2. THEORETICAL METHOD In this study, all energies were calculated using the CASTEP code.25,26 Electronic exchange and correlation were included through the generalized gradient approximation (GGA) using the PerdewWang ‘91 (PW91) functional.27 We did not find it necessary to include semiempirical corrections via dispersionforce-corrected DFT (DFT+D) methods because the overwhelming contribution of ionic and covalent interactions of Cp with the d orbitals of the Cu{111} surface makes the contributions of long-range dispersion forces negligible. In this regard, our study is comparable to recent work by Atodiresei et al. for Cp on Fe/W{110}.23 √ zones of the (2 3 √Integration over the Brillouin √ √ 2 3)R30, (3 3), and ( 7 7)R19.1 unit cells was achieved by summation over a 4 4 1 Monkhorst-Pack28 kpoints mesh. A plane wave basis set expanded to an energy cutoff of 300 eV was used to describe the electronic wave functions. A seven-layer Cu slab was used to model the Cu{111} surface. The top four substrate layers and the adsorbate molecules were allowed to fully relax, whereas the bottom three layers were kept fixed in the unrelaxed positions optimized in a bulk calculation. Convergence with respect to k-point sampling, kinetic energy cutoff, and slab thickness was tested and found to be satisfactory. In a Cu bulk-calculation, increasing the energy cutoff from 300 to 330 eV, reduces the total energy by 50 meV. Increasing the number of atomic layers in the slab from seven to eight decreases the total energy per copper atom by only 22 meV, whereas
þ ΦCuf111g EACp
ð2Þ
where ΦCu{111} is the electronic work function of the Cu{111} surface, meaning the energy necessary to remove one electron from the metal and taking it to infinite distance, and EACp is the electron affinity of the Cp molecule in the gas phase. To estimate the electron affinity of Cp, we have to calculate the total energy of the isolated Cp anion. Total energy calculations of ionic species in the periodic boundary conditions formalism are not an easy task because electrostatic interaction between charged species requires unusually large supercells for minimizing the Coulomb repulsion between the repeated images. It is also very important to notice that the energy of an infinitely repeating charged system also diverges to infinity, which is why in a (CASTEP) periodic boundary condition calculation of an ionic molecule a uniformly distributed charge is added to the supercell to make the total charge of the cell equal to zero. After the CASTEP calculation for an isolated Cp molecule, the total energy of the system has to be corrected for the artificial contribution because of the compensating uniform positive charge added to the system. This extra energy contribution, known as the Madelung energy (EM), is equal to the energy of a point charge immersed in a neutralizing jellium, and it is given, for a cubic supercell, by31 EM ¼ 16135
q2 R 2L
ð3Þ
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Table 1. Adsorption Energies and Geometrical Parameters √ √ for Cp in a (2 3 2 3)R30 Unit Cell on Cu{111}a
Eads (eV) dCp-Cu (Å) θCH (deg)
fcc
hcp
B
T
1.732
1.732
1.671
1.198
2.25 10.6
2.25 10.7
2.25 10.7
2.42 6.9 0.3
δ(θCH) (deg)
1.9
1.7
1.0
dCC (Å)
1.416
1.415
1.416
1.412
δ(dCC) (Å)
0.0095
0.0084
0.0033
0.0007
dCH (Å)
1.081
1.082
1.081
1.082
δ(dCH) (Å)
0.0008
0.0005
0.0019
0.0013
dCC and dCH are the average CC and CH bond lengths; δ(dCC) and δ(dCH) are the standard deviation in the Cp bond lengths. a
Figure 1. Total energy of isolated Cp as a function of the cubic supercell side L (Å): 9, uncorrected value; b, after correcting for the electrostatic interaction. One can see that the corrected energy values converge more rapidly than the uncorrected energies. It is important to emphasize that for L approaching ∞ the uncorrected energy series will also converge to the correct value, but it is much more efficient to include the Madelung correction analytically than running DFT calculations with an ever increasing L. Because the volume of the supercell increases by L3, it was practically inconvenient to run calculations with L > 50 Å. The solid lines through the data points are guides to the eye.
Figure 2. Sites considered for the adsorption of Cp on Cu{111}. From left to right: hcp hollow site (hcp), bridge site (B), fcc hollow site (fcc), and atop (T) site.
where R is the Madelung constant and L is the supercell side length. We have systematically increased the supercell size of the ionic calculation until it reaches 50 Å along each side, where we have been confident that the energy was converged (Figure 1) and then applied the Madelung energy correction given by eq 3. The energy difference between the Cp and Cp is the EA of the neutral species. Following this procedure, we obtain an EACp of 1.84 eV, in excellent agreement with the experimental32 value of 1.81 eV. The work function for the clean Cu{111} surface was found to be 4.464 eV. It is well known that DFT underestimates metal work functions by typically 0.4 to 0.5 eV, and in fact the calculated ΦCu{111} is ∼0.5 eV lower than the experimental33 value of 4.94 eV. Having calculated ΦCu{111} and EACp, we can estimate that the energy required to desorb the Cp as an anion is 2.62 eV
Figure 3. Main structural parameters of Cp on Cu{111}. The interplanar distance between the top Cu{111} layer and the Cp carbon ring is 2.25 Å. The CH bonds of Cp are tilted 10.6 upward with respect to the plane of the carbon ring.
higher than the desorption energy of the Cp neutral. Therefore, from now on, we will refer to an adsorption energy Eads, as expressed by eq 1, measured relative to the gas-phase neutral. We have calculated the adsorption energy of Cp on √ four different high symmetry adsorption sites in the (2 3 √ 2 3)R30 cell: atop (T), bridge (B), fcc hollow (fcc), and hcp hollow (hcp). (See Figure 2.) The highest binding energies in each site, with respect to the neutral gas-phase Cp radical are reported in Table 1. We found that the most favorable adsorption sites are the fcc and hcp hollow sites with basically the same binding energy of 1.73 eV. The top sites are the least favorable, having more than 0.5 eV lower adsorption energy. The calculated adsorption energies of benzene9 on Cu{111} (neglecting VdW interactions) are negligible (0.05 eV), meaning that the experimentally measured adsorption energy of 0.59 eV34 is almost entirely due to the dispersive VdW interaction between the π system of benzene and the polarizable Cu{111} surface.10 We can observe that the five-membered Cp carbon ring is much more strongly bound than C6H6, although in the gas phase the two molecules are very similar for size, structure, and electronic configuration. Similarly to what is observed for benzene on Cu{111}, we found that for all considered binding sites the molecule resides preferentially with a planar configuration, a little over 2 Å above the surface (interplanar distance), with the hydrogen atoms pointing slightly upward at a ∼10 angle (Figure 3). This geometry is fairly consistent to what was reported for Cp adsorption on the similar Ni{111}surface by German et al.21 In Table 1 are reported the average CC and CH bond lengths (dCC and dCH) and the standard deviation, δ(dCC) and δ(dCH), in the length of these bonds, for the adsorbed molecule. The length of the five CC and CH bonds of Cpads is approximately uniform ((0.0095 Å), meaning that the adsorbed molecule possesses a five-fold rotational symmetry similar to the isolated Cp (Table 2). 16136
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Table 2. Geometrical Parameters of the Isolated Cp and Cpa
Table 4. Charge Distribution (in electrons) in the Adsorbed cp Compared with the Isolated Cp and Cp Molecules
Cp-
Cp
Cp(ads)
dCC (Å)
1.406
1.402
QH
δ(dCC) (Å) dCH (Å)
0.0470 1.081
0.0065 1.084
QC QCp
δ(dCH) (Å)
0.0020
0.0010
Cp
Cp
0.83
1.55
1.15
1.96 1.13
1.55 0.00
2.13 0.98
a
Both the anion and the neutral molecule are planar (θCH = 0). The variation of the CC bond length in the anion, δ(dCC), is almost an order of magnitude lower than that between the CC bonds in the neutral molecule due to the fact that the extra electron fills the bonding π system and the molecule becomes aromatic.35.
Table 3. Adsorption Energy and Average Bond Lengths of Cp on fcc Hollow Cu{111} As a Function of the Unit Cell Sizea √ √ (2 3 2 3)R30
(3 3)
Eads fcc (eV)
1.732
1.664
1.608
dCp-Cu (Å)
2.25
2.26
2.30
θCH (deg)
10.6
10.2
√ √ ( 7 7)R19.1
10.3
dCC (Å)
1.416
1.415
1.414
dCH (Å)
1.081
1.081
1.081
coverage (ML)
0.08
0.11
0.14
a
Surface coverage corresponding to the different unit cells is expressed in monolayers (MLs).
Figure 4. Cp most stable adsorption site is the fcc Cu{111} with one CC bond parallel to the Æ101æ direction. The axis indicates the crystallographic direction on the Cu{111} surface.
The coverage dependence of the Cp adsorption was investigated the size of the unit cell in the calculations√from √by reducing √ a√(2 3 2 3)R30 unit cell to a (3 3) and finally to a ( 7 7)R19.1 cell. The adsorption energy (Table 3) only slightly decreases (7% overall) as the surface coverage increases by up to 70% in the smallest cell, whereas the structure of Cp(ads) is
essentially unchanged with respect to the largest unit cell. These results suggest that from moderate to high coverage the interadsorbate interaction forces are rather weak. In the following, we will show that these observations are reflected in the relatively small dipole moment of the adsorbed molecule. Two rotationally different configurations were investigated for every adsorption site. In the first configuration, the most stable Cp(ads) lies on the surface with a CC bond parallel to the Æ101æ direction, whereas in the second configuration one CC bond direction coincides with one of the Æ121æ directions (Figure 4). The energy difference between these two configurations for Cp on fcc sites is in the 3050 meV range, regardless of coverage. Such orientational insensitivity is likely due to the incompatible symmetries of Cp (five-fold rotational symmetry) and both fcc and hcp hollow sites (three-fold rotational symmetry). Applying a combination of linear synchronous transit and quadratic synchronous transit36 methods (LST/QST), we searched for the position of a transition state for the surface diffusion of Cp. It was found that the transition state is located on the bridge site between adjacent hollow sites, with an energy barrier of 55 meV. Hedgeland et al.24 have recently investigated the surface diffusion of Cp on Cu{111} by helium spinecho spectroscopy. The experimental data are in excellent agreement with our results. They show that Cp diffusion involves jumps between fcc and hcp sites with an estimated energy barrier of 40 ( 3 meV. 3.2. Population Analysis and Surface Dipole Moment. The charge distribution in the adsorbed Cp has been calculated by Mulliken population analysis (MPA).37 It was found that about 1 e is transferred from the Cu{111} surface to the molecule upon adsorption, from which we conclude that the Cp(ads) is ionically bound to the surface. The details of the MPA results are shown in Table 4. QC and QH are, respectively, the total atomic charge on the five carbon and five hydrogen atoms, and QCp is the total charge of the molecule. One can also see that the polarity of the CH bonds is reduced as an effect of the electron transfer, although this does not affect the CC and CH bond lengths of Cp(ads), which are substantially identical to those of the isolated Cp (dCC = 1.414 Å, dCH = 1.081 Å). To estimate the degree of covalent bonding within the interaction between Cp(ads) and the Cu{111} surface, we have also calculated the Mulliken overlap population37 (OP) between the carbon atoms of Cp(ads) and the copper atoms of the threefold coordinated fcc site. The OP is basically a measure of the amount of shared electron density between two atoms, and it provides an objective way to estimate the ionic or covalent character of a chemical bond. A low OP value is characteristic of strongly polar or ionic bonds, whereas high OP values (approaching unity) are associated to pure or prevalently covalent bonds.38 As we can see in Table 5, for all possible bonds between the carbon atoms of Cp(ads) and the nearest surface copper atoms, the OP is close to zero, meaning that the binding of Cp on 16137
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√ Table of Cp on Cu{111} (2 3 √ 5. Overlap Population 2 3)R30 fcc sitesa
a
bond
OP [e]
C1Cu1
0.04
C3Cu3 C4Cu4
0.05 0.05
C2Cu3
0.07
C5Cu4
0.07
C2Cu1
0.07
C5Cu1
0.07
C2Cu2
0.05
C5Cu5
0.05
C4Cu3 C3Cu4
0.06 0.06
Atoms labels are the same as those displayed in Figure 4.
Table 6. Change in the Cu{111} Work Function (ΔΦ) and Surface Dipole Moment (Δμ) As a Function of Cp Coverage √ √ Cp (2 3 2 3)R30 Cp (3 3) √ √ Cp ( 7 7)R19.1
ΔΦ (eV)
Δμ (Debye)
coverage (ML)
0.799 0.982
1.45 1.33
0.08 0.11
1.033
1.09
0.14
Cu{111} is not localized on specific covalent CCu bonds but is instead prevalently ionic. We have also investigated the influence of Cp adsorption on the Cu{111} work function (ϕ). Because the nature of the Cp binding is mainly ionic, with approximately one electron transferred from the surface to the Cp ring, we would anticipate a significant change in the surface dipole moment and therefore a substantial work function modification. The calculated variation in the Cu{111} work function as a function of Cp coverage is reported in Table 6. The change in work function is on the order of 1 eV, which has the same sign but a smaller value than what is observed experimentally after adsorption of Na on Cu{111} with similar coverage (ΔΦ ≈ 1.5 eV for 0.1 ML coverage).39 We want here to stress that the alkali atoms like Li, Na, and K are essentially purely ionically bound on copper and that they can reduce the work function of a metal by creating an additional surface dipole directed from the ion toward the surface by transfer of electronic charge from the alkali atom to the substrate. Cp also reduces the metal work function despite the electron transfer taking place in the opposite sense from the surface to the molecule so that the adsorbate becomes anionic. This counterintuitive phenomenon is explained, as suggested by Witte et al.10 in the case of benzene and cyclohexane, by a so-called “cushion effect”, which can produce a zone of electron depletion just above the plane of the topmost surface atoms and is generated by the Pauli repulsion between the π system of Cp and the metallic surface states. The net result includes a dipole moment directed toward the bulk of the metal, thereby reducing the work function. An additional component to the dipole moment arises from the polarized CH bonds. These are slightly polarized and simultaneously tilted upward, therefore contributing to the dipole. 3.3. Density of States and Charge Density Analysis. The DOS and projected density of states (PDOS) of Cp on Cu{111} are shown in Figure 5, together with the DOS of clean Cu{111}
Figure 5. (a) DOS of the clean Cu{111} surface. (b) DOS of Cp/ Cu{111} and, shaded, the explicit difference between the DOS of Cp/ Cu{111} and the DOS of the clean Cu{111} surface. (c) Projected density of states (pDOS) for Cp(ads) (at 0.14 ML coverage). (d,e) For comparison the DOS of the isolated Cp anion and the neutral Cp radical. The LUMO levels of the isolated molecules are not reported because they are well above the ionization energy of Cp and therefore in our calculations are strongly mixed with several vacuum states.
and of the isolated Cp molecule in both the gas-phase neutral radical and gas-phase anionic forms. Orbitals for the gas-phase species have been aligned relative to those of the adsorbate on the surface such that the energy of the 2A1 orbital matches. The justification for this is that a state lying so far below the valence band of copper must suffer virtually no change in its energy as the molecule approaches the surface, other than a purely electrostatic effect due to the surface potential; covalent or ionic interactions may be safely ignored for this state. Rigidly shifting the gas-phase orbitals to reflect this electrostatic effect thus accounts approximately for the equivalent shift that would be experienced by the other orbitals in the hypothetical absence of any further adsorbatesubstrate interactions. Any additional modification in the DOS/PDOS (energy shift, broadening, etc.) must therefore be symptomatic of covalent or ionic bonding effects. In the present case, we can see immediately that all adsorbate states lying >5 eV below the Fermi level are virtually unaltered with respect to the gas phase once the rigid electrostatic shift has been applied and so do not take part in bonding to the surface to any significant degree. Only the frontier orbitals (HOMO, LUMO, and/or SOMO) are responsible for chemisorption. In this case, our Mulliken population analysis already makes it clear that the overall direction of electron transfer is from the surface to the molecule, resulting in an a strong ionic bond, but careful consideration of changes in the calculated frontier orbitals will allow us to refine this basic observation somewhat further. 16138
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The Journal of Physical Chemistry C Incidentally, it ought to be noted in passing that the anionic nature of the adsorbed molecules is strikingly at odds with previous analysis based on extended H€uckel theory for Cp on Pt{111} and Ni{111}, which actually suggested cationic adsorption.1721 An obvious question to ask at this juncture is whether this difference represents a genuine physical characteristic of coinage versus transition metals or rather a discrepancy between the methods employed. In this regard, we note that our own DFT calculations for Cp on Ni{111} show anionic adsorption, as opposed to the cationic adsorption predicted by extended H€uckel theory. This seems, therefore, to be a disagreement between first-principles DFT and the previous semiempirical approach. In fact, our results not only show that the molecule is anionic when bound to the surface but also indicate that a little more than a single electron is transferred, so that the charge on the adsorbed species becomes 1.13 e. For an open-shell species, such as the neutral gas-phase Cp radical, the position of the SOMO is critical in determining the overall direction of electron transfer. Here it is clear that including the downward shift in orbital energy as the molecule enters into the electrostatic influence of the surface leaves the SOMO well below the Fermi level of the metal. (See Figure 5.) Thus, electron transfer to the adsorbate is favorable and at least sufficient to occupy fully the SOMO. Granted that this degree of electron transfer occurs, the SOMO becomes part of the doubly degenerate HOMO of Cp, and one may interpret any further bonding interactions with the surface in terms of normal expectations for a closed-shell species. That is, the formation of covalent bonding and antibonding combinations between the HOMO and states near the Fermi level of the metal provide one contribution to the overall binding, together with a partial transfer of electrons from adsorbate to surface, whereas the formation of bonding and antibonding combinations between the LUMO and states near the Fermi level of the metal provide another contribution to the overall binding, together with a partial transfer of electrons from surface to adsorbate. In effect, two polar covalent bonds are formed, which we consider to be distinct from the ionic bond associated with filling of the radical SOMO. Assuming that both the HOMO and LUMO can overlap equally well with states deriving from the surface, the dominant contribution, both to the binding and to the net flow of electrons, will derive from the molecular state lying closest to the surface Fermi level after accounting for the electrostatic shift in orbital energies due to the surface potential. In the present case, the (doubly degenerate) HOMO of Cp lies ∼2 eV below the Fermi level, after alignment of the 2A1 orbital between gas-phase Cp and the adsorbed species. The extreme broadening of this orbital upon adsorption, to the extent that it is smeared almost flat across a range of several electronvolts, indicates that its contribution to covalent bonding is quite marked. (See Figure 5.) In contrast with the case of the HOMO, the gas-phase anion does not have a well-defined LUMO lying below the vacuum energy; in our calculations, we see only poorly defined molecular resonances among the vacuum states, which begin around 2 eV above the Cp HOMO. Nevertheless, the general downward shift of orbital energies due to the surface potential should allow some of these resonances to emerge as genuine bound orbitals, of which the lowest would be the LUMO, and these in turn would be available to form bonding and antibonding combinations with the surface. The absence of a strong LUMO-related peak in the DOS/PDOS of the adsorbate (Figure 5) is evidence that any such states must also contribute markedly to covalent bonding between the adsorbate and the
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Figure 6. Charge density difference plot showing charge transfer upon adsorption of Cp on the Cu{111} fcc site. Red contours indicate electron density increase by 0.02 electrons/Å3; blue contours indicate electron density decrease by 0.02 electrons/Å3.
surface. We thus should expect not only electron transfer from the adsorbate to the surface (due to polar covalent bonding involving the HOMO) but also electron transfer from the surface to the adsorbate (due to polar covalent bonding involving the LUMO). The fact that the adsorbed species acquires a more negative net charge than would be expected simply from the formation of a singly charged anion suggests that it is the LUMO that dominates the net polar covalent interaction. In context, however, the net polar covalent component of the charge transfer here (i.e., 0.13 e) is rather small when compared both with the ionic component (i.e., 1.00 e) and with the polar covalent component calculated for other aromatic species on transition metals (e.g., 0.45 to 0.50 e for benzene on Ni{111}40). Therefore, we might reasonably conclude that the covalent contribution to the adsorption energy in the present case is probably also rather small. Certainly, we know that the covalent contribution to the bonding of benzene on Cu{111} is essentially negligible,9 and the same is likely to be true here. We furthermore speculate that if the individual strengths of the HOMO and LUMO contributions to the overall polar covalent bonding vary in proportion to one another, then the net charge transfer associated with the polar covalent bonding should itself be approximately proportional to the total polar covalent bond strength and vice versa. This approximate proportionality may hold true when comparing similar adsorbates on similar substrates but almost certainly is not more generally applicable. Noting that the polar covalent charge transfer in the present system (0.13 e) is about one-quarter of that observed for benzene on Ni{111} (0.45 to 0.50 e), we might therefore imagine that the polar covalent contribution to the bonding of Cp on Cu{111} is also around one-quarter of that found for benzene on Ni{111} (previously calculated to be 0.91 eV40). We thus estimate that polar covalent bonding is responsible for around 0.91/4 = 0.23 eV of the overall binding energy of 1.73 eV for Cp on Cu{111}, leaving the remaining 1.50 eV attributable to ionic bonding. Finally, we note that the so-called charge density difference distribution can provide striking visual confirmation of the 16139
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of Cp and other ionically bound molecular species on ferromagnetic surfaces, where injection of single spin electrons from a substrate into organic overlayers is a promising way of building organic spintronics devices.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
Figure 7. Calculated 1E00 1 π orbital of gas-phase Cp anion.
various kinds of electron transfer that occur upon adsorption. Defining this quantity as the difference between the charge density of the adsorbate-covered surface and the summed charge densities of the clean surface and the neutral isolated molecule (each frozen in the geometry adopted upon adsorption), we thus have δF ¼ FCp=Cuf111g ðFCuf111g þ FCp Þ
ð4Þ
Clearly, when compared with the gas-phase neutral radical, the electron redistribution will be dominated by transfer from the surface to the molecule, and indeed, we find that the electron accumulation isosurface matches very closely the SOMO/ HOMO orbital of the isolated radical/anion (Figure 7); electron depletion regions (mostly flat and slightly below the molecular plane) on the molecule are of minor significance in comparison (Figure 6). Conversely, the substrate is dominated by electron depletion, with relatively much less electron accumulation.
4. CONCLUSIONS In conclusion, in this work, we have shown that Cp is in all respects a unique example of an aromatic molecule that becomes an ionic molecular adsorbate with very interesting properties. First, the adsorption energy of Cp on Cu{111} is one order of magnitude higher than other aromatic molecules like benzene on the same surface, allowing the molecule to be stable under UHV conditions up to 400 K.24 Second, Cp diffuses from one hollow site to the next (fcc and hcp are basically equivalent for the adsorption) through a small energy barrier. Both the thermal stability and the high mobility of Cp on Cu{111} are completely consistent with helium spinecho measurements. This combination of stability and high mobility suggests that Cp, or a Cp derivative, could be potentially employed as functional groups to build organic self-assembled monolayers. Our results show that about one electron per molecule is transferred from the surface to Cp, which thus adsorbs as an anion, and that the polarized CH bonds are bent upward. Further charge transfer due to covalent interaction is observed. The combination of charge transfer and internal dipole moment of the adsorbed molecule reduces the metal work function by as much as 1 eV at high coverage (0.14 ML). In contrast with what is observed for alkali metal adsorption, the lateral interactions between coadsorbed molecules do not reduce significantly the Cp adsorption energy. This is an interesting property of the system that we would like to investigate further in future work. In fact, tuning the dipole moment of metalorganic interfaces by polar organic layers is a technique often applied in organic electronics and requires stable organic precursors capable of adsorbing on metal surfaces up to high coverage. Finally, we are currently exploring the properties
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