12149
2007, 111, 12149-12151 Published on Web 07/31/2007
The c(4 × 2) Structure of Short- and Intermediate-Chain Length Alkanethiolate Monolayers on Au(111): A DFT Study Jian-guo Wang* and Annabella Selloni* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08540 ReceiVed: June 13, 2007
We report on first principles density functional theory calculations for a new adatom + vacancy model (ad+va) of the c(4 × 2) structure of short- and intermediate-length alkanethiolate self-assembled monolayers (SAMs) on Au(111). This model includes the same structural motif, two thiolates bound to an Au adatom and a surface vacancy, that has been recently proposed for the low-coverage and (x3 × x3)R30° phases of shortchain alkanethiolate SAMs on Au(111). We show that the ad+va model is more stable than all available (x3 × x3)R30° one-chain models and can also satisfactorily describe the experimental scanning tunneling microscopy images of the c(4 × 2) phase.
1. Introduction The structure of self-assembled monolayers (SAMs) of alkanethiols on the (111) surface of gold has been the subject of intense research and numerous controversies for almost 20 years.1-6 At monolayer coverage, two main structures have been identified: a commensurate (x3×x3)R30° (hereafter denoted x3) structure, usually assumed to involve a single alkanethiolate, CnH2n+1S, chain per unit cell (for simplicity, in the following we shall use the abbreviated notation CnS instead of CnH2n+1S); and, for n > 1, a c(4 × 2) superstructure with four CnS per unit cell, which is generally believed to be slightly more stable than the x3 structure. For the latter, after many reports indicating that the thiolates adsorb with the S headgroup at the bridge, slightly displaced toward the face-centered cubic (fcc) site (denoted as br-fcc in the following) of the Au(111) surface,7-9 very recent studies have provided a completely different pictures of the adsorption geometry. Adsorption with the S headgroup either at on-top sites of the Au(111) surface or atop an Au adatom occupying a threefold hollow site was proposed on the basis of X-ray photoelectron diffraction10 (XPD) and X-ray standing wave (XSW) experiments,11 respectively. By contrast, a combined first principles theoretical and experimental (angle resolved XPD and grazing incidence X-ray diffraction) study12 of methylthiolate SAMs suggested that the x3 periodicity originates from a dynamic equilibrium between bridge site adsorption and a novel structure where two C1S are bound to an Au adatom that has been lifted from the gold substrate. Interestingly, evidence for an adsorbed species formed by two thiolates bound to an Au adatom was recently obtained also in the case of low-coverage SAMs of short-chain alkanethiolates on Au(111), by means of scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations.13 The structure of the c(4 × 2) superlattice has also been under debate for many years. Although the long-standing question of * To whom correspondence should be addressed. E-mail: (J.-g.W.)
[email protected]; (A.S.)
[email protected].
10.1021/jp0745891 CCC: $37.00
whether the thiols are adsorbed onto the surface as monomers (thiolates) or dimers (disulfides)14 appears to be now settled in favor of the thiolate model (see, e.g., refs 3, 7, 15 and references therein), the atomic-scale details of the structure are not well understood yet. Of particular interest for this work, so far no structural model has proven capable to provide a satisfactory description of the observed STM images of the c(4 × 2) phase.3-5,16-18 Also, previous first principles theoretical studies of the c(4 × 2) structure have not been able to determine a four-chain model more stable than the one-chain br-fcc x3 structure.7,15,19 The aim of this work is to present a new model of the c(4 × 2) superstructure for short- and intermediate-length thiolates, which largely overcomes the above difficulties. This model (see Figure 1) incorporates the same structural motif, two thiolates bound to an Au adatom and a surface vacancy, that has been recently proposed for low-coverage13 and x312 SAMs on Au(111). In particular, by means of DFT calculations we show that this c(4 × 2) model is more stable than the x3 structure, and also that it satisfactorily reproduces the qualitative features of the experimental STM images. 2. Computational details The calculations have been performed using periodic DFT, with the gradient-corrected PW91 functional.20 The neglect of van der Waals (vdW) interactions, implicit in the use of this “standard” DFT approach, is a reasonable approximation for short- and intermediate-length CnS chains (n e 8): for instance, recent QM/MM calculations have found that the vdW contribution to the stabilization of the thiolate against the disulfide model is significant but not decisive for C10S monolayers and decreases steadily thus becoming negligible for n ) 4 chains.15 Ultrasoft pseudopotentials21 were used to describe electron-ion interactions. Plane-wave basis set cutoffs for the smooth part of the wavefunction and the augmented density were 25 and 200 Ry, respectively. The surface was modeled using periodically repeated slabs of four layers, with the CnS (n ) 1-8) species adsorbed on © 2007 American Chemical Society
12150 J. Phys. Chem. C, Vol. 111, No. 33, 2007
Letters TABLE 1: Adsorption Energy Per Molecule, Eads, for Different c(4 × 2) Models of a C8S Monolayer on Au(111)a model
Eads (eV)
model
Eads (eV)
4bri 1ad+1va 1ad+2va
2.11 2.23 2.18
4 va 1ad+3va 2ad+2va
2.11 2.18 2.03
a All values of Eads are referred to the undefected surface. 4 bri and 4 va denote models with 4 identical thiolates at br-fcc sites in a (3 × 2x 3) unit cell of the surface without and with vacancies, respectively. The models actually correspond to x3 one-chain structures, which have been recalculated using a (3 × 2x 3) supercell to allow for a more accurate comparison with the other four-chain models.
Figure 1. Optimized structure of C8S monolayers on Au(111). A (3 × 2x 3) unit cell with four thiolates is shown. Left: 4bri model, corresponding to a x3 structure. The chain tilt angle is ∼23°. Right: 1ad+1va model. The tilt angle is ∼20°(∼25°) for the thiolates at bridge (atop) sites. Sulfur atoms are blue, gold are yellow (large spheres for surface atoms, small sphere for adatom), and carbon are gray. The positions of the gold adatom and vacancy are indicated.
one side of the slab only, and a vacuum width of 15 Å. A (3 × 2x3) unit cell with 12 Au atoms per layer and 4 adsorbed thiolates was used to describe the c(4 × 2) structure. The corresponding Brillouin zone has been sampled with eight special k-points. For the (x3×x3)R30° structure, a unit cell with three Au atoms per layer and one thiolate was used, and the corresponding Brillouin zone was sampled with 48 special k-points. In the geometry optimizations, all coordinates of the adsorbed thiolates and the topmost two layers of the Au(111) slab were relaxed until each component of the residual force on each atom was smaller than 0.03 eV/Å. The adatom formation energy, EfA, was calculated as
EfA ) Etot [Au(111)A] - Etot[Au1(bulk)] - Etot[Au(111)] where Etot [Au(111)A] is the total energy of the Au(111) surface with one adatom per unit cell, Etot[Au1(bulk)] is the total energy of one gold atom in the bulk phase, and Etot[Au(111)] is total energy of the defect-free clean Au(111) surface. Similarly, the average vacancy formation energy, EfV, on a surface, Au(111)V, with mV vacancies per unit cell was determined as
EfV ) (1/mV)(Etot [Au(111)V] + mVEtot[Au1(bulk)] Etot[Au(111)]) The adsorption energy per molecule on defect-free Au(111), and for the 1ad+1va and 2ad+2va models, was calculated from the total energy difference
Eads ) (1/M) (Etot[MCnS/Au(111)] - Etot[Au(111)] MEtot[CnS]) where Etot[MCnS/Au(111)] is the total energy of the most stable structure with M adsorbed CnS per unit cell, and Etot[CnS] is the total energy of an isolated CnS radical. Similarly, the adsorption energy per molecule for the 1ad+2va, 1ad+3va, and 4va models was calculated using
Eads ) (1/M)(Etot[MCnS/Au(111)] - Etot[Au (111)V] MEtot[CnS] + mVEfV) where Au (111)V is a surface with mV ) 1, 2, and 4 vacancies
per unit cell, respectively, and EfV is the corresponding formation energy per vacancy. 3. Results and Discussion As a first step, we have verified that with our computational setup the structural and energetic properties of one-chain x3 models of CnS monolayers agree well with those reported in previous first principles theoretical studies.7-9,19,21 Indeed, we find that CnS adsorption at the br-fcc site with a binding energy Eads ∼2.1 eV is preferred for n ) 1-6 on the defect-free surface. However, bridge adsorption in presence of a vacancy (for which the calculated formation energy is EfV ) 0.69 eV) is more favorable by about 0.1 eV, whereas adsorption in the presence of an Au adatom (computed formation energy EfA ) 0.67 eV) with no vacancy is about 0.3 eV less stable. To check how this result is affected by the choice of the DFT exchange and correlation functional, we have compared bridge adsorption of C1S on the undefected surface and in the presence of a vacancy using the local density approximation (LDA). Although the LDA is known to overestimate binding energies, it yields somewhat better values of surface and vacancy formation energies than the PW91 and other gradient-corrected functionals.22 Within the LDA, we obtain Eads ) 2.79 and 2.74 eV on the defect-free and defected surfaces, respectively, with EfV) 0.95 eV. While adsorption on the defect-free surface is slightly favored in the LDA, these calculations confirm that the formation of Au surface vacancies is extremely facile in the presence of adsorbed thiolates. Turning next to the four-chain c(4 × 2) structure, we examined a very large number of configurations of the CnS (n ) 2-8) species both on the defect-free and on different defected surfaces. The latter included structures with mA adatoms and mV vacancies per (3 × 2x3) unit cell, where mA ) 0, 1, and 2 and mV ) 0, 1, 2, 3, and 4. The calculated energetics, summarized in Table 1 for the case of C8S, indicates that the most stable configuration is obtained on a defected surface involving an Au adatom and a surface vacancy (see Figure 1). We also tested configurations involving different relative positions of the adatom and vacancy in the (3 × 2x3) unit cell and found that it is energetically more favorable for the surface vacancy to be not underneath the adatom, as in the model of ref 12, but a few sites away from it. In this c(4 × 2) model, hereafter denoted 1ad+1va, two of the four thiolates are bound to the Au adatom (S-Au bond distance: dS-Au ) 2.32 Å) and occupy atop sites on the Au(111) surface, while the other two are at br sites (dS-Au ) 2.47 Å). The S-S distance, dS-S ∼4.6 Å, between the two atop thiolates is ∼0.5 Å shorter than the typical intermolecular distance for thiolate monolayers on Au(111), consistent with recent STM observations.3 In addition, the S headgroups of the atop (bridge) thiolates are at height of ∼2.6 (2.0) Å above the surface with a height difference, ∼0.6 Å, in agreement with XSW measurements.23 The adsorption
Letters
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12151
Figure 2. Calculated STM images for C8S monolayers on Au(111). From left to right: (x3×x3)R30° structure; 1ad+1va; 1ad+2va; and 1ad+3va. The thin red line indicates the unit cell of the c(4 × 2) structure. For all images, a bias voltage of 0.8 V (tunneling into SAM’s empty states) has been used.
eV below EF for bridge and Au adatom-bound species, respectively. The computed STM images for 1ad+2va and 1ad+3va, also shown in Figure 2, exhibit somewhat different contrasts with respect to that of 1ad+1va. For instance, for 1ad+3va both the brightest and the dimmest spots are due to bridge thiolates, the former arising however from a C8S at a regular bridge site and the other from a C8S near a vacancy. All these computed images show a clear similarity to the experimental ones, especially if one takes into account the broadening effect caused by the finite tip size. In conclusion, DFT calculations show that adatom + vacancy models of c(4 × 2) alkanethiolate SAMs are energetically more stable than all known x3 one-chain structures and agree well with a variety of experimental data. In particular, our results suggest that these structural models could provide the basis for a detailed explanation of all observed STM contrasts.4,5,16
Figure 3. Partial density of states for the different sulfur headgroups in the (1ad+1va) model of C8S monolayers on Au(111).
energy per molecule for 1ad+1va is 0.12 eV larger than for the x3 defect-free and x3 vacancy structures with thiolates at br-fcc sites. Slightly less stable than 1ad+1va are also structures involving one Au adatom and two (1ad+2va) or three (1ad+3va) surface vacancies, while models involving two Au adatoms and two or more vacancies, for example, 2ad+2va, are considerably less stable and may be thus ruled out. Extensive studies of the c(4 × 2) structure by STM have shown a variety of contrasts.3-5,16,17 This has led to the suggestion that distinct c(4 × 2) phases can exist.3,16 Our theoretical STM images for the adatom + vacancy models of C8S monolayers have been calculated from the local density of states in the vacuum region22 at distance of ∼1.0 Å from the highest molecular apex (see Figure 2). A justification of this approach to describe tunneling through monolayers of short and intermediate length molecules is given in ref 24. For 1ad+1va, the computed image shows two brighter and two darker spots, as frequently observed in experiment. These spots originate from the CH3 endgroups with no clear evidence of the Au adatoms. However, the S-Au bonds do have an influence on the image. In particular, for the 1ad+1va model, the methyl endgroups of thiolates at bridge sites are brighter than those of thiolates at atop sites, even though the latter are geometrically more protruding by ∼0.2 Å. To rationalize this difference, we have calculated the partial density of states (PDOS) for the sulfur headgroups at bridge and atop sites (see Figure 3). It appears that around the Au Fermi energy (EF) the PDOS of sulfur atoms at bridge sites is higher than that of sulfurs at atop sites, and the highest peak of the PDOS (roughly corresponding to the molecular highest occupied molecular orbital24) is at ∼1 and 2
Acknowledgment. This work was supported by NSF Grant DMR-0213706 to the MRSEC-Princeton Center for Complex Materials. We are grateful to R. Rousseau and collaborators for sharing their results on the c(4 × 2) structure prior to publication, and we thank R. Rousseau and G. Scoles for helpful discussions. References and Notes (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (3) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (4) Vericat, C.; Vela, M. E.; Benitez, G. A. et al. J. Phys. C: Condensed Matter 2006, 18, R867. (5) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K. et al. Chem. ReV. 2005, 105, 1103. (7) Vargas, M. C.; Giannozzi, P.; Selloni, A. et al. J. Phys. Chem. B 2001, 105, 9509. (8) Gottschalck, J.; Hammer, B. J. Chem. Phys. 2002, 116, 784. (9) Hayashi, T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (10) Kondoh, H.; Iwasaki, M.; Shimada, T. et al. Phys. ReV. Lett. 2003, 90, 066102. (11) Yu, M.; Bovet, N.; Satterley, C. J. et al. Phys. ReV. Lett. 2006, 97, 166102. (12) Mazzarello, R.; Cossaro, A.; Verdini, A. et al. Phys. ReV. Lett. 2007, 98, 016102. (13) Maksymovych, P.; Sorescu, D. C.; Yates, J. T., Jr. Phys. ReV. Lett. 2006, 97, 146103. (14) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (15) Fischer, D.; Curioni, A.; Andreoni, W. Langmuir 2003, 19, 3567. (16) Riposan, A.; Liu, G. J. Phys. Chem. B 2006, 110, 23926. (17) Zhang, J.; Chi, Q. J.; Ulstrup, J. Langmuir 2006, 22, 6203. (18) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 4038. (19) Morikawa, Y.; Liew, C. C.; Nozoye, H. Surf. Sci. 2002, 514, 389. (20) Perdew, J. P.; Chevary, J. A.; Vosko, S. H. et al. Phys. ReV. B 1992, 46, 6671. (21) Molina, L. M.; Hammer, B. Chem. Phys. Lett. 2002, 360, 264. (22) Mattsson, T. R.; Mattsson, A. E. Phys. ReV. B 2002, 66, 214110. (23) Fenter, P.; Schreiber, F.; Berman, L. et al. Surf. Sci. 1998, 412/ 413, 213. (24) Sun, Q.; Selloni, A. J. Phys. Chem. A 2006, 110, 11396.
