ARTICLE pubs.acs.org/JPCC
Sulfur-Induced Reconstruction of Ag(111) Surfaces Studied by DFT L. Alvarez Soria, G. Zampieri, and M. L. Martiarena* Centro Atomico Bariloche, Comision Nacional de Energía Atomica, and Consejo Nacional de Investigaciones Científicas y Tecnicas, Av. Bustillos 9500, 8400 S. C. de Bariloche, Argentina ABSTRACT: Density functional theory has√been√ used to investigate the adsorption of sulfur atoms in the ( 7 7)R19.1° unit cell of Ag(111). For the coverages θ = 1/7 and 2/7 the S atoms adsorb at fcc and hcp hollow sites with negligible reconstruction of the surface. For θ = 3/7 a large surface reconstruction occurs. The three Ag atoms around the hcp site that hosts a S atom move vertically and in-plane away from the hollow center provoking two effects: (i) the S atom adsorbed at this site penetrates under the surface and (ii) a top site hosting the third S atom is transformed into a new 3-fold hollow site. A detailed analysis of the different contributions to the adsorption energy shows that the energy cost of deforming the lattice is paid off by the stronger and more favorable bonding of these two S atoms. This picture is confirmed by the new charge distribution after the reconstruction.
W
ith the advent of international legislation on reduction of hazardous substances (July of 2006), the material and process changes required to eliminate lead from electronics was likely to result in new quality and reliability issues. Immersion silver (ImAg) was selected as one of the most popular Pb-free surface finishes by many electronic suppliers because it was readily available, provided good solderability, and was easy to probe (for high in-circuit test yields).13 Therefore, it was a surprise to the industry when electronics in high sulfur industrial environments would fail rather quickly.3 Even though the reaction of silver platings to atmospheric sulfur has been documented for several decades, this fact showed that the fundamentals as to its initiation are still poorly understood.2 There seemed to be a threshold of sulfur and humidity above which creep corrosion occurs producing the unexpected failures. Recently, many studies have described the development of self-assembled monolayers (SAMs) to passivate metal surfaces such as gold, copper, and silver.411 These SAMs are composed of thiol molecules that attach to the surface through the sulfur atom and stand “on-end” with the hydrocarbon tail oriented to the air or solution. These films are a practical method of protection because they provide resistance to atmospheric environments without affecting the appearance of the surface.12 On both topics, the failures in the devices due to sulfur contamination when ImAg are used and the development of possible solutions to this problem via the chemisorption of organosulfur compound on metal surfaces, it is necessary to understand the bonding of sulfur to the surface of Ag. Specially to know the effect of the sulfur atom on the selvage of the metal surface. Therefore, as a representative example we focus here on the interaction of sulfur atoms with Ag(111). Sulfur and thiol monolayers can be easily formed on Ag5,6,10,1323 by simple immersion in sulfide-containing solutions (S2, SH, or thiol species) or by sample exposure to gaseous S2 or SO2. A common finding of all methods of sulfur deposition on Ag(111) at coverages above √ 1/7 √ ML is the formation of a commensurate ordered ( 7 7)R19.1° S phase. This has r 2011 American Chemical Society
been observed either by qualitative low-energy electron diffraction (LEED) or by scanning tunneling microscopy (STM). This phase has been associated with the growth of multilayers of the fcc phase of Ag2S,10,13,22,24,25 but also with the formation of a single S adlayer.1417 In any case, however, there is not yet a conclusive description of the sulfur/metal interface. Therefore, the actual structure of the S/Ag-substrate interface remains in general controversial. Considering the envisioned use of silver surface as a gateway to surfaces with a chemical functionality tailored to the atomic scale, this situation is more than unsatisfactory. The present work aims to supply answers to the open questions related to the structure of the selvage when sulfur is adsorbed on Ag(111) by means of accurate density functional (DF) calculations. Starting from a characterization √ √ of the clean silver surfaces, sulfur adsorption in a ( 7 7)R19.1° unit cell for varying coverage is studied. We show that when the coverage goes to 3/7 it induces a strong reconstruction of the Ag(111) surface. To identify the causes of this reconstruction we analyze the different contributions to the adsorption energy and the changes of the electronic distribution provoked by the reconstruction.
’ COMPUTATIONAL DETAILS We have carried out DFT calculations within the slab-supercell approach by using the ab initio total energy and molecular dynamics program VASP (Vienna ab initio simulation program).26,27 The one-electron KohnSham orbitals are developed by using a planewave basis set and the interaction with the atomic cores is described through the projector augmented-wave pseudopotential (PAW).28 Exchange and correlation (XC) is described within the generalized gradient approximation (GGA) introduced by Perdew and Wang (PW91),29 which performs well for the global Received: December 22, 2010 Revised: March 11, 2011 Published: April 22, 2011 9587
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√ √ Figure 1. Geometrical configuration after full system optimization of the S/Ag(111) in a ( 7 7)R19.1° unit cell for a sulfur coverage of θ = 1/7, 2/ 7, and 3/7. The yellow and gray balls are the S and Ag atoms, respectively. The color gradient represents the percentage of displacement of the Ag atoms with respect to the positions without relaxation.
energetics of several reactions involving sulfur-containing molecules on Ag(111). The sampling of the Brillouin zone is carried out according to the MonkhorstPack method.30 The cutoff energy chosen is 450 eV, electron smearing is introduced following the MethfesselPaxton technique31 with σ = 0.2 eV and all the energies are extrapolated to 0 K. The convergence of the energy is kept always on the order of 104 eV and the minimization of the forces is assured up to the order of 102 eV/Å. With these parameters and using a 15 15 15 k-point mesh, we obtained for bulk Ag a lattice parameter of 4.160 Å, in excellent agreement with the experimental value (4.078 Å). We√consider √ a six-layer slab to represent the Ag(111) surface, in a ( 7 7)R19.1° unit cell, using a 7 7 1 k-point mesh. A vacuum equivalent in thickness to 10 empty layers was placed to ensure negligible interactions between periodic images normal to the surface. No significant relaxation of the interlayer distance with respect to Ag bulk value is observed. The adsorption energy of n sulfur atoms on Ag(111) is defined as: EAds ðnS=Agð111ÞÞ ¼ EðnS=Agð111ÞÞ nEðSÞ EðAgclean Þ ð1Þ Here, E(nS/Ag(111)) and E(Agclean) are the total energies of the N sulfur atom on the Ag slab, and of the clean unreconstructed surface, respectively. E(S) is the total energy of the S atom in gas phase. The optimization of the geometrical configuration of the system is calculated allowing the relaxation of all coordinates of the S atoms and the Ag atoms of the four topmost layers. Spin polarization is only used for calculating S in the gas phase. To study the effect on the electronic charge of the interaction among the components of the system, we subtract from the total electronic charge of each atom, calculated using the Bader-charge method,32 the number of valence electron of each atom (ne). This difference allows us to characterize the regions of space where an accumulation or deficiency of charge has been produced by the adsorption process. STM images were calculated within the TersoffHamann approach33 by using the charge density obtained with VASP.
’ RESULTS AND DISCUSSION We present first the results of the DFT study of the adsorption process when all the surface atoms are kept frozen in the crystal positions.
The first step was to calculate the minimum energy with respect to the normal coordinate of a single S atom placed above the surface at several high- and low-symmetry sites. The lowest energy was obtained, as in all the previous calculations,20,34 for the S atom adsorbed at an fcc site; the optimal height was 1.70 Å (dSAg = 2.45 Å) and the adsorption energy was 3.68 eV, which are also in good agreement with the previous results. Next, we added a second S atom and repeated the procedure keeping frozen the position of the first S atom. The lowest energy in this case was for the second atom at 1.69 Å above an hcp site, at 4.49 Å from the fcc site of the first S atom. Subsequently, we repeated the procedure with a third S atom, keeping fixed the positions of the first two S atoms; the lowest energy was in this case for the third S atom adsorbed at a top site, at 2.44 Å from the Ag atom below it, and at 4.49 Å from the hcp site of the second S atom. In the next stage we allowed for vertical and lateral relaxations of all the S atoms and all the Ag atoms in the first four layers. The resulting configurations are shown in Figure 1. The figure shows from left to right the structures for sulfur coverages θ = 1/7, 2/7, and 3/7. The Ag atoms which were allowed to relax have been blackened, and the gray scale represents the percentage of rearrangement. There are several things to be noted: (i) the lateral relaxations of the S atoms are very small and so one can continue referring to them as the atoms adsorbed at fcc, hcp, and top positions (hereafter F, H, and T positions); (ii) the cases θ = 1/7 and 2/7 are very similar, with the S atoms adsorbed at hollow sites and little rearrangement of the Ag atoms; and (iii) at θ = 3/7 there are large vertical relaxations of the three S atoms and, more important, a large reaccommodation of the Ag atoms in the first layer. Figure 2 shows the initial and relaxed positions of the atoms at θ = 3/7 in more detail. Figure 2a is a top view of the S atoms and the Ag atoms of the topmost layer, and Figure 2b shows the vertical displacements of all these atoms and of the atom of the second layer right below the H site. It is seen that the three Ag atoms around the H site (24) are the ones that undergo the largest displacements. The top view shows that these atoms have in-plane displacements toward the unit cell corners, while Figure 2b shows that they also move upward, thereby splitting the initially topmost layer in a bilayer with densities 3/7 and 4/7 of the Ag layers in the bulk. These displacements provoke an important change of the S atom at the H site, which penetrates under the surface and occupies a new position at nearly the same distance from the atoms numbered 2, 3, 4, and 8. This means that after the rearrangement this S atom 9588
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Figure 3. Charge differences around the atoms of Figure 2a: b, before relaxation; O, after relaxation.
Figure 2. (a) Top view of the surface at θ = 3/7; the white and gray circles represent the Ag atoms before and after relaxing the coordinates; T, H, and F indicate the adsorption sites of the S atoms.( b) Vertical displacements of all the atoms in a unit cell plus the Ag atom in the second layer below the H site.
Table 1. Vertical Positions of the Atoms in Figure 2b with Respect to the Topmost Layer of the Clean Ag Surface site
F
H
T
1
24
57
8
h (Å)
1.58
0.47
2.27
0.28
1.40
0.11
2.21
ends in an almost tetrahedral site. Additionally, with the displacement of the Ag atoms toward the corners, accompanied by a slight lowering of the Ag atom number 1, the T site is transformed into something similar to an hcp site. Only the local environment of the F site remains approximately the same after the relaxation. The vertical positions of all the atoms in the surface region (relative to the last layer in the clean surface) are listed in Table 1. Therefore, it can be said that the force that drives the reconstruction is to change an unfavorable top adsorption site into a hollow site and a hollow site into a more favorable tetrahedral site. This last change can be interpreted as a first step toward the formation of a Ag2S phase, which is known to form when the S content is increased.25 We have found, however, that in this calculation at zero temperature the addition of a fourth S atom does not lead to more substrate rearrangement and growth of the sulfide phase. Instead, the fourth atom prefers to adsorb near the outermost S atoms (that at the T site) at a distance of 2.08 A, quite close to that of the S2 molecule. Therefore, to describe the growth of the sulfide phase new calculations are needed including the effect of the temperature and eventually the effect of the substrate vacancies. Further insight into the causes of the reconstruction that occurs at θ = 3/7 can be gained by comparing the charges around the atoms before and after the relaxation. Figure 3 shows the charge differences (Bader charge minus charge of the valence electrons) around the S atoms and the Ag
atoms of the topmost layer before and after relaxing the positions of surface atoms. Before relaxation, the two S atoms adsorbed at hollow sites are negatively charged and all the nearest-neighbor Ag atoms (24 and 57) are positively charged. The S atom at the T site is positively charged and the Ag atom right below (1) is negatively charged. After the relaxation, all the S atoms are negative and have increased the charge around them, and all the Ag atoms are positive. The main changes are again associated with the atoms around the H and T sites. The S atom at the T site is now almost as negative as the other two, and its charge is drawn mainly from the Ag atoms (24) around the H site; these Ag atoms are now bound to two S atoms and its positive charge has increased proportionally. Panels a and b in Figure 4 compare electronic isodensity surfaces for θ = 3/7 before and after relaxing the atoms. The zoomed regions of the isodensity surface before the relaxation show that the S atoms at the F and H sites are above the surface with bonds to the three nearest Ag atoms, while the S atom at the T site has a single bond to the Ag atom underneath. In the reconstructed surface, Figure 4b shows that while the bonding of the S atom at the F site remains practically the same, there are important changes in the other two cases. The atom at the H site, which has penetrated under the surface, has bonds to the three Ag atoms above it and a new bond to a second-layer Ag atom (not visible); it is, therefore, 4-fold coordinated. The S atom at the T site has changed from single to triple coordination. To analyze the interplay of the different interactions at work during the adsorption process we split the adsorption energy into three contributions defined below. Since the adsorption produces both a change of the electronic distribution and a change of the surface atoms configuration, it would be desirable to disentangle the contributions to the adsorption energy of these two effects. To this purpose we sum and subtract in eq 1 the energy of the substrate with the atoms in the configuration that results after the adsorption, E(Agrec). In this way eq 1 becomes: rec rec clean Þ EAds ðnS=AgÞ ¼ Efrozen Ads ðnS=Ag Þ þ ½EðAg Þ EðAg rec rec rec where Efrozen Ads (nS/Ag ) = E(nS/Ag ) nE(S) E(Ag ) is a calculated adsorption energy when the surface atoms are kept in the same positions before and after the adsorption; it is, therefore, an energy change caused solely by the electronic redistribution that occurs during the adsorption. The other term on the
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Table 2. Energies per S Atom (in eV) for the Different Coverages without relaxation θ
EAds/n
ESAg/n
with relaxation
ESS/n
EAds/n
ESAg/n
1/7
3.68
3.68
3.81
3.88
2/7
3.60
3.66
0.059
3.68
3.79
3/7
1.96
3.05
0.083
3.60
4.09
ESS/n
Eelastic/n 0.073
0.053 0.43
0.062 0.92
Table 3. Individual S Adsorption Energies (in eV) Calculated with the Ag Atoms Kept Frozen in Their Positions in the Clean and Reconstructed Surfaces (i) rec Efrozen Ads (S /Ag ) (i) clean ) Efrozen Ads (S /Ag
θ = 1/7 3.88
F
3.68
H
3.64
T
1.82
θ = 2/7
θ = 3/7
3.83
3.78
3.76
4.57 3.92
Eelastic. Thence, with these definitions the adsorption energy can be finally written as: EAds ðnS=AgÞ ¼ ESAg þ ESS þ Eelastic
Figure 4. Top view and 3D zooms around the T, H, and F sulfur atoms of the electronic isodensity surface calculated without (panel a) and with (panel b) geometrical optimization. The red lines define the unit cell.
right-hand side is clearly the energy associated with the surface modification. The electronic part can be further decomposed by summing and subtracting the energies of the S atoms adsorbed (F,H,T) /Agrec). individually on the reconstructed surface, Efrozen Ads (S Equation 1 then becomes: EAds ðnS=AgÞ ¼
∑
i ¼ T, F, H
rec ðiÞ Efrozen Ads ðS =Ag Þ
rec þ ½Efrozen Ads ðnS=Ag Þ
∑
i ¼ T, F, H
Efrozen ðSðiÞ =Agrec Þ
þ ½EðAgrec Þ EðAgclean Þ In this last expression the first term on the right-hand side contains grossly the energy gained by the electrons upon adsorption of the S atoms on the reconstructed surface; we will call it ESAg. The second term is the energy difference between adsorbing these atoms together or individually; it is, therefore, a measure of the interaction among the S atoms in the unit cell and it will be called ESS. Finally, the third term is the energy cost involved in the surface reconstruction and it will be called
ð2Þ
All these energies are listed in Table 2 for the cases with and without relaxation of the Ag atomic positions. To ease the comparison between the different coverages, the energies are given divided by the number of S atoms. In the calculation without relaxation the elastic term is zero by definition. It is seen in the table that in this case the leading contribution to the adsorption energy comes from ESAg, with the term ESS being always a minor positive (repulsive) contribution. The adsorption energy per atom is practically the same for θ = 1/7 and 2/7, but there is an important decrease when the third S atom is added at the T site, which is energetically much less favorable than the hollow sites. When the Ag atoms are allowed to relax there are not significant changes of the energy terms for θ = 1/7 and 2/7, but important changes occur for θ = 3/7. First, it is seen that the decrease of the adsorption energy per atom that occurs in the unrelaxed calculation is now largely compensated. This is caused by the increment of ESAg, which is around 1 eV larger than in the unrelaxed calculation. Interestingly, the new value of ESAg at θ = 3/7 exceeds the average values obtained at the smaller coverages, when only the hollow sites are occupied. Although ESAg is again the leading term, ESS, which has now turned negative (attractive), is also an important contribution. Finally, Eelastic has also increased considerably as is expected in view of the large reconstruction of the substrate that occurs at this coverage. More information is provided by the analysis of the individual contributions to ESAg for each coverage, which are listed in Table 3 and plotted in Figure 5. For comparison, the energies when the Ag atoms are kept frozen in the clean surface positions are also included. At θ = 1/7 and 2/7 there is not much difference between the adsorptions in the two hollow sites (F and H), similar to what occurs in the clean surface. In passing from θ = 2/7 to 3/7 there is a big change in the energies of two of the three S atoms. The adsorption energy of the atom in the H site, which the reconstruction transforms into a tetrahedral site, increases in almost 1 eV, and the energy of the atom occupying the T site, 9590
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Figure 5. Individual adsorption energies at different coverages: b, adsorption on the clean surface; O, adsorption on the reconstructed surface.
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adsorption energy. There is, however, a very efficient surface reconstruction that ends with two of the three S atoms in much more favorable environments. The most important displacements are those of the three Ag atoms that form the hcp site, which move upward and in-plane away from the hollow center; these displacements provoke two effects: (i) they allow the S atom at the hcp site, which was in 3-fold coordination, to penetrate under the surface and reach a new site with 4-fold coordination, and (ii) they transform the top site in a hcp-like site changing thereby the single coordination of the S atom into triple coordination. We have also found that these calculations at zero temperature are unable to describe the growth of the sulfide phase, as the addition of a fourth S atom produces only the formation of a S2-like molecule above the surface. Further calculations including the effect of the temperature and/or Ag vacancies are in progress.
’ AUTHOR INFORMATION Corresponding Author
*Address correspondence to this author.
’ REFERENCES Figure 6. Simulated STM image in constant-current mode with the optimized configuration for θ = 3/7. Vbias = 0.77 V. Left panel: Zoom of the unit cell (red rhombus); the letters indicate the S adsorption sites.
which the reconstruction transforms into a hcp site, increases in almost 2 eV relative to the energy in the clean surface. The adsorption energy of the atom at the F site remains practically unchanged. Therefore, the above analysis confirms that the driving force for the reconstruction is to build more favorable local environments for two of the three S atoms. The energy cost of deforming the lattice is paid off by the stronger bonding of the S atoms at the H and T sites. Finally, Figure 6 presents an STM image calculated with the final configuration for θ = 3/7 and Vbias = 0.77 V. The zoomed region shows that there are three zones in the unit cell with different brightness. The brightest zones at the four cell corners correspond to the S atoms adsorbed at the T sites, the darkest zone to the pit at the H site where the S atom has penetrated under the surface, and the zone of intermediate brightness to the S atom adsorbed at the F site. It is interesting to note that this and other images calculated with different bias are in good qualitative agreement with experimental images showing a (7/3)1/2 subperiodicity.14,17,22 The comparison, however, must be made with caution since it is not completely clear if the experimental images correspond to the first S layer (as is considered here) or to Ag2S multilayers.
’ CONCLUSION We have used DFT to analyze the first stages of the adsorption of S on Ag(111). The most important finding of this study is the large reconstruction of the substrate atoms that occurs in passing from Θ = 2/7 to 3/7. For Θ =1/7 and 2/7 the S atoms adsorb at hollow sites with little rearrangement of the surface Ag atoms. If one adds a third S atom in the unit cell it occupies a top site that has a high
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