Sulfur Atoms Adsorbed on Cu(100) at Low Coverage: Characterization

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Sulfur Atoms Adsorbed on Cu(100) at Low Coverage: Characterization and Stability Against Complexation Holly Walen, Da-Jiang Liu, Junepyo Oh, Hyun Jin Yang, Peter M. Spurgeon, Yousoo Kim, and Patricia A. Thiel J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07046 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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Sulfur Atoms Adsorbed on Cu(100) at Low Coverage: Characterization and Stability Against Complexation Holly Walen,a,b Da-Jiang Liu,c Junepyo Oh,b Hyun Jin Yang,b Peter M. Spurgeon,a Yousoo Kim,b and Patricia A. Thiela,c,d* a

Department of Chemistry, Iowa State University, Ames, Iowa 50011 USA RIKEN Surface and Interface Science Laboratory, Wako, Saitama 351-0198, Japan c Ames Laboratory of the USDOE, Ames, Iowa 50011 USA d Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011 USA b

* [email protected] (515) 294-8985 Abstract Using scanning tunneling microscopy, we characterize the size and biasdependent shape of sulfur atoms on Cu(100) at low coverage (below 0.1 monolayers) and low temperature (quenched from 300 K to 5 K). Sulfur atoms populate the Cu(100) terraces more heavily than steps at low coverage, but as coverage approaches 0.1 monolayers, close-packed step edges become fully populated, with sulfur atoms occupying sites on top of the step. Density functional theory (DFT) corroborates the preferential population of terraces at low coverage as well as the step adsorption site. In experiment, small regions with p(2x2)-like atomic arrangements emerge on the terraces as sulfur coverage approaches 0.1 monolayer. Using DFT, a lattice gas model has been developed, and Monte Carlo simulations based on this model have been compared with the observed terrace configurations. A model containing 8 pairwise interaction energies, all repulsive, gives qualitative agreement. Experiment shows that atomic adsorbed sulfur is the only species on Cu(100) up to a coverage of 0.09 monolayers. There are no Cu-S complexes. In contrast, prior work has shown that a Cu2S3 complex forms on Cu(111) under comparable conditions. Based on DFT, this difference can be attributed mainly to stronger adsorption of sulfur on Cu(100) as compared with Cu(111). 1. Introduction Sulfur adsorption studies on low-index surfaces of the noble metals Cu, Ag, and Au have led to new insights into sulfur-metal and sulfur-sulfur interactions. These studies have shown a propensity of metals to react with sulfur to form small, mobile complexes, or new ordered structures at low sulfur coverage. For example, sulfur adsorbed on Cu(111) at room temperature forms a heart-shaped Cu2S3 complex.1-2 The energetic motivation for the formation of these complexes relies on the stability of the linear S-M-S substructure (M=Cu, Ag, Au), which is observed also in sulfur complexes on Ag(111)3 and Au(100),4 and is predicted for sulfur adsorption on Ag(100).5 Because this journal issue honors Miquel Salmeron, it is noteworthy that the present work falls at the intersection of two broad topics where he has made major contributions: Sulfur chemistry at surfaces and interfaces,6-15 and adsorbate complexation with surface metal atoms, particularly the role of complexation in catalyst restructuring.16-17

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Sulfur adsorption on Cu(100) has been previously studied with Auger electron spectroscopy (AES),18-19 low energy electron diffraction (LEED),18-25 scanning tunneling microscopy (STM),25 radioactive tracer analysis,20 x-ray diffraction,26 angle-resolved fine structure, 27-30 high-resolution electron energy loss spectroscopy,31-32 surface-extended xray absorption fine structure,33-35 and x-ray photoemission.36-37 These studies have focused on the p(2 x 2) structure that forms at and near S coverage of 0.25 monolayers, and at room temperature. In this structure the S atoms occupy the four-fold hollow sites of the surface.18, 26-28 A (√17 x √17)R14◦ reconstruction forms at ~0.47 ML (with annealing > 870 K).25, 37 This study is presented in 6 sections. Section 2 contains the experimental and computational details. Section 3 is a presentation and discussion of the experimental evidence for chemisorbed sulfur. Section 4 provides insights from DFT analyses and lattice gas modeling. Section 5 is an overall discussion, including an analysis of why chemisorbed S is stable against complexation on Cu(100) but not Cu(111). Section 6 concludes the paper. 2. Methods. 2.1. Experimental Details. Single-crystal Cu(100) was cleaned under ultrahigh vacuum via Ar+ sputtering (12-14 μA, 2.0 kV, 10 min) and annealing (810 K, 10 min) cycles. The final sputtering was followed by flashing the sample to 770 K. This minimized the number of impurities and defects visible in STM. A typical clean surface produced with such treatment is shown in Fig. 1(a). Some impurities remain visible in STM, such as the bright spot which pins the step edge in Fig. 1(a), but their density is low. Exposure to S2(gas) was performed with the sample held at room temperature. The sulfur source was an in situ electrochemical evaporator following the design by Wagner,38 which has been characterized in detail by Detry et al.39 and Heegemann et al.40 Sulfur coverage (θS) is here defined as the ratio of adsorbed S atoms to the number of Cu atoms in the surface plane, and was determined by counting individual S atoms in a given area. Low temperature STM was the primary experimental technique, and the imaging temperature was 5K. The piezoelectric calibration was checked using atomically-resolved images of the clean Cu(100) surface [Fig. 1(b)]. The Cu(100) lattice constant determined from such images was 0.25 ± 0.01 nm, and the step heights were 0.17 ± 0.01 nm. These values were, within their standard deviations, identical to the accepted values of 0.255 nm and 0.181 nm, respectively.41 Typical imaging conditions were in the range 0.93 – 1.22 nA tunneling current (I) and -1.5 V to +0.2 V sample bias (VS), unless noted otherwise.


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Figure 1. STM images of the clean Cu(100) surface. (a) Large-scale image of step edges with one bright contaminant, 50 x 50 nm2. VS = -0.495 V, I = 1.00 nA. (b) Atomically resolved image, 2 x 2 nm2. VS = -0.020 V, I = 1.24 nA. 2.2. Computational Details. Density functional theory (DFT) calculations were performed using the planewave based VASP code42 with standard PAW potentials43 optimized for the PBE method44-45 that are distributed with versions 5.2 and higher. The energy cutoff was 280 eV. Surface energy calculations were performed using periodic slabs with various thicknesses up to 12 layers for (100) surfaces. All atoms are allowed to relax except the bottom layer. The theoretical lattice constant for Cu using the PBE method is 3.641 Å. For a supercell with side lengths l1 and l 2 (measured in units of a, the surface lattice constant), a gamma-centered (||24/ l 1|| x ||24/ l 2|| x 1) k-point grid is used, where ||x|| denotes the integer nearest to x. All energies reported were obtained by averaging over results from L = 7 to 12, where L is the number of metal layers in the slab, unless noted otherwise. This averaging serves to reduce errors from quantum size effects.46 The estimated uncertainties in energies due to different slab thicknesses are shown via error bars in graphs, and via parentheses in numerical values. Any assessment of the relative stability of species that (potentially) incorporate both S and Cu, such as complexes and reconstructions, must take into account the energetic cost of providing both. The chemical potential per S atom, µS, serves this purpose, where µS is defined as: µS = [E(CumSn + slab) - E(slab) - m µCu]/n - E(S2,g)/2

(1)

Here E is energy, and µCu is the chemical potential of Cu in the bulk metal (at 0 K), which equals the bulk cohesive energy. In Equation (1), µCu serves as the energy reference point for the metal. If bulk and surface are equilibrated, µCu also equals the binding energy of a Cu atom at a step/kink site.47 The energy of the triplet state of the gas-phase dimer, E(S2,g), serves as the energy reference point for S. Choosing gas-phase S2 reduces the significant ambiguity and error that would arise in the calculation of the self energy of an atom using DFT. Since a positive µS thus defined means the system is unstable towards associative desorption of S2, it is also more physically relevant. The integers m and n are the number of Cu and S atoms in the complex, respectively. When m=0, µS reduces to the adsorption energy of a S adatom.



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For lattice gas (LG) modeling, which supplements the analysis of energetics, we constructed a LG model by fitting ~ 50 different DFT energetics (training set), with different supercells and slab thicknesses. The configurations were first chosen systematically, with a single S in different supercells. We then performed Monte Carlo simulations to identify some low energy configurations, and subsequently incorporated DFT calculations of these configurations into the training set, thus obtaining a new set of LG parameters. Finally, we used some additional DFT results for validation. Monte Carlo simulations were performed in a grand canonical ensemble using the Metropolis algorithm. In addition, diffusion of S was included to avoid frustration due to the largely repulsive interactions between S atoms.48 3. Experimental Identification and Characteristics of Chemisorbed Atomic Sulfur. Sulfur deposition on the clean terraces produces small, round, shallow features in STM images, as shown in Fig. 2. These features are sulfur adatoms. The height and width are sensitive to sample bias during imaging. Notably, the features are imaged as protrusions for VS < +0.2 V, and as depressions for VS > +0.4 V, using values of I ~ 1 nA. This change in appearance is reliably reproduced from one experiment to the next—the images in Fig. 2(a), 2(b), and 2(c) were recorded on different days, with rigorous tip cleaning on the clean Cu(100) surface between experiments. The protrusions are surrounded by a dark ring, shown clearly in Fig. 2(c-d). This results in a so-called sombrero shape, observed also in some other systems such as CO/Pt(111).49 We characterize both the height and full-width at half-maximum (FWHM) of these features. When the feature is a protrusion, its height is measured from the minimum of the surrounding ring to the maximum in the center (Fig. 2(c-d)), and when it is a depression, height is measured from the average surrounding terrace to the minimum in the center. Heights of protrusions are small, always < 0.04 nm. (If the baseline of the feature were chosen as the surrounding terrace, rather than the bottom of the sombrero, the height would be even smaller.) For comparison, the vertical height of a sulfur atom above the top plane of Cu(100) atoms is much larger—0.132 nm from DFT (using a (3x3) supercell). The protrusions have constant FWHM = 0.33 ± 0.04 nm, based on a number of measurements N = 2180. As will be discussed in Sec. 5, these two characteristics—anomalously small height, and FWHM ~ 0.3—are fully consistent with identification of the protrusions on Cu(100) as single S atoms.


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Figure 2. Effect of sample bias on S atom imaging in separate experiments. At VS < 0.2 V, the S atoms appear as low, round protrusions with a dark outer ring. At VS > 0.4 V, the S atoms appear primarily as round depressions. All images are 4 x 4 nm2 in size. (a) Sequential images of S atoms from VS = -3 V to +3 V, with I = 1.06 nA, and θS = 0.006 ML. (b) Sequential images of S atoms from VS = -3 V to +3 V in a second experiment, with I = 1.14 nA and θS = 0.002 ML. (c) VS = -0.084 V, θS = 0.061 ML, I = 1.14 nA. (d) Line profile over one S atom.



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On Cu(100), sulfur adatoms are the only species observed over the entire coverage range studied, 0.002 < θS < 0.09 ML. A summary of STM images in this range is shown in Fig. 3. There is no sign of complexes, which would have larger footprints and more intricate shapes,1, 3-4 nor of reconstructions or bulk-like sulfides. The dark ring around individual S atoms becomes less obvious as θS increases (for example, compare Fig. 3(b) with 3(f)). At highest coverage, 0.09 ML, there is some propensity for the appearance of chain motifs of atoms aligned along close-packed directions and separated by 2a, as well as small squares with local p(2 x 2) arrangement, as in Fig. 3(f). This is reasonable, since the p(2x2) is a known phase of S/Cu(100), exhibiting long-range order near its ideal coverage of 0.25 ML (Sec. 1). The small 2a chains and squares evident already at 0.09 ML do not signal longrange order, since there is no correlation between individual domains. They represent regions where θS is locally high as a result of random variations in spatial distribution, and the sulfur atoms are subject to the same interactions that eventually foster the long-range p(2x2). This will be discussed further in Sec. 4. The observation of local p(2x2)-like regions thus supports the assignment of the circular features as sulfur adatoms.

Figure 3. Representative STM images at different coverages. All images are 15 x 15 nm2 and topographic. (a) 0.002 ML; (b) 0.005 ML; (c) 0.02 ML; (d) 0.02 ML; (e) 0.06 ML; (f) 0.09 ML. In (f), arrows show squares of 4 S atoms with local p(2 x 2) arrangement, and a 2a chain is encircled. Two close-packed directions are indicated by arrows to the right of (f). The sample is biased negatively for all images. Values of I and VS for each image are: (a) 1.03 nA, -0.211 V; (b) 1.06 nA, -1.000 V; (c) 1.06 nA, -0.020 V; (d) 0.93 nA, -0.286 V; (e) 1.00 nA, -1.000 V; (f) 0.97 nA, -0.537 V. Focusing next on S interaction with close-packed step edges, experiments show that very little S adsorbs at steps if θS ≤ 0.02 ML [Fig. 4(a)]. At 0.06 ML there is more S visible at steps, but they remain mostly bare [Fig. 4(b)]. At θS = 0.09 ML, there is nearly complete decoration along the close-packed edges, as shown in Fig. 4(c). The S atoms reside at the tops of the steps, separated by 0.50 ± 0.01 nm, i.e. 2a, where a is the surface lattice constant of Cu(100). In short, S populates step edges only as the coverage on terraces accumulates.


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This is in contrast to the (111) surface of Cu, where heavy step decoration occurs even at lowest θS.2

Figure 4. STM images of close-packed Cu(100) step edges with S. All are 10 x 10 nm2. (a) 0.02 ML. (b) 0.06 ML. (c) 0.09 ML. The arrows show two of the close-packed directions.

4. DFT Results. Adsorption and interaction of atomic S. From DFT, the most favorable adsorption site of atomic S is the four-fold hollow site, in agreement with prior work.18, 26-28 The coverage dependence of the adsorption energy of atomic S is calculated using various square supercells, with side length ranging from a to 2√5a. (To clarify this presentation, pairwise interaction distances and nomenclature are defined in Fig. 5.) Following Abufager et al.,50 we plot in Fig. 6 the adsorption energy (the chemical potential μS of a single adsorbed S atom) as a function of 1/θS to demonstrate the stability of various ordered phases. With this choice of axes, the stability of the system is represented by the convex hull (solid line).50 There is strong repulsion between S atoms separated by d = √2a, since μS = -1.879(3) eV for the (√2 x √2)R45o structure at θS =1/2 ML. This is much less favorable (more positive) than μS at lower coverages, where it ranges from -2.42 to -2.48 eV. Interactions between S at larger distances are generally repulsive, but non-monotonic. The unfavorable energy of the (√5 x √5)R27o phase is evidence of substantial repulsion at d = √5a, which can favor formation of p(2x2) domains. The inset in Fig. 6 shows μS calculated for the unrelaxed Cu(100) substrate, which demonstrates much different and weaker coverage dependence for 0.05 < θS < 0.2 ML. This comparison shows that the source of repulsions between S at distances between 2a and 4a is mostly elastic. p(2x2). Using the DFT results in Fig. 6, we can check whether it is physically reasonable to observe small regions of p(2x2)-like structure in the experiment, as in Fig. 3(f). To do this, we derive lattice-gas (LG) models with pair-wise interactions wi, as defined in Fig. 5. The approach is similar to that used in lattice-gas modeling of S on Au(111).51 Figure 7 shows snapshots of Monte Carlo (MC) simulations for a model that includes pairwise interactions up to w8, with values given in the figure caption. The temperature (T) of the MC simulation should be the experimental quench temperature, which is unknown but reasonably lies between 100 and 200 K. The uncertainty in T is unimportant for the present discussion, since quantitative analysis of the p(2x2) motifs shows that their abundance depends much more strongly on θS than on T in the range 100 K < T < 200 K, for this LG model. MC results are shown for T = 200 K and for θS = 0.05 and 0.1 ML, which approximate the middle and upper end of the experimental coverage range.



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Figure 5. Pairwise interactions considered for S/Cu(100). Open circles represent Cu atoms, black dots represent S atoms. In each label, the first entry is the S-S separation d, and the second entry is the corresponding pairwise interaction energy wi. The parameter a is the surface lattice constant.

Figure 6. Adsorption energy as a function of 1/θS for S/Cu(100). The horizontal dashed line shows the adsorption energy at close-packed steps from Fig. 8 and related calculations.


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Figure 7. Snapshots (16 nm x 16 nm) of equilibrium configurations for a LG model with eight pair-wise interactions derived from DFT: w1=2.683 eV; w2=0.275 eV; w3=0.008 eV; w4=0.019 eV; w5=0.003 eV; w6=0.009 eV; w7=0.011 eV; w8=0.001 eV. Interactions are defined in Fig. 5. Results are shown for θS = 0.05 ML and θS = 0.1 ML, at 200 K. The arrows between panels denote close-packed directions. (Note the ~45o rotation relative to STM images.) In the bottom panel, red squares denote 4 S atoms in a p(2x2)-like arrangement, and a 2a chain is encircled. In Figure 7, at the higher coverage, two sets of 4 atoms in a square configuration can be seen, and 2a chains are common. This is consistent with experiment, where similar regions become visible at θS = 0.09 ML. Thus, the LG modeling serves to show that observation of small p(2x2)-like regions of chemisorbed S is plausible under experimental conditions. It should be noted that the LG modeling shows that the local p(2x2) configurations result from long-range repulsive interactions. In an alternative LG model with truncation of interactions to 2a, an attractive w3 is needed to explain p(2x2) ordering near 0.25 ML.52-53 For the present system, such a short-ranged LG model predicts much more prevalent p(2x2) patches at low coverage, in contradiction to the experimental findings.



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Step edges. DFT also corroborates the experimental observation that S populates step edges only as S accumulates on terraces. Figure 8 shows the adsorption energies of S at step edges on Cu surfaces vicinal to the (100). The (511), (711), and (911) surfaces have closepacked steps with different terrace widths, and the (510) surface has open steps. We have examined configurations where these steps are decorated with S atoms on the upper terrace, on the lower terrace, and on both terraces (double row of sulfur).

Figure 8. Configurations of S at steps on 4 different vicinal Cu(100) surfaces, and corresponding μS (in eV) from DFT. In each figure, the red square shows the supercell. Large circles are Cu atoms, white being closest to viewer and darker shades of gray indicating progressively lower layers. Small yellow-orange circles are sulfur atoms, with darker colors again indicating lower levels. Numbers above figures are the adsorption energies in eV, obtained by averaging results for slabs with thickness of approximately 4h to 7h, where h is the interlayer spacing of the (100) surface. (a,d,g) Close-packed step edges with single rows of S atoms attached at the upper terrace. (b,c) Close-packed step edges with single rows of S atoms attached from the lower terrace. (e, f) Close-packed step edges with double rows of S atoms, one row on upper terrace and one on lower terrace. (h) Open step edges with single rows of S atoms, lower terrace. (i) Open step edges with single rows of S atoms, upper


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terrace. (j) Open step edges with double rows of S atoms, one row on upper terrace and one on lower terrace.

The fact that the experiment spans a coverage range of 0.02 to 0.09 ML, means that it is relevant to compare μS for steps with μS for terraces, up to θS ~ 0.1 ML. On terraces, Fig. 6 shows μS = -2.45 eV at 0.1 ML, and μS is more negative at lower coverage. All of the configurations in Fig. 8 are significantly less favorable than terrace adsorption at coverages up to 0.1 ML because all have μS > -2.45 eV, except for three—panels (a), (d), and (g). In each of these, a single row of S adsorbs at the upper terrace along a close-packed step edge. The terrace width increases going from the first to third panel, i.e. from the (511) to (711) to (911) surface. The real surface has terraces that are much wider than the terraces on these vicinal models. However, the fact that there is no significant difference between the values of μS = -2.45 eV for the (711) and (911) indicates convergence with respect to terrace width. The DFT result shows that the preferred adsorption site is at the top of a step, which is in complete agreement with the STM images of Fig. 4. Furthermore, DFT predicts that adsorption sites at steps become energetically equivalent to those on terraces around 0.1 ML. This is also in excellent agreement with the experimental observation that S preferentially populates terraces at low coverages, but step sites become almost fully populated at θS ~ 0.09 ML. The adsorption configurations shown in Fig. 8 all have S atoms separated by 2a along steps. Additional DFT calculations for the (911) and (711) surfaces, with wider S-S separations up to 4a, show that variations in adsorption energies are small and within the uncertainties. In other words, the adsorption energy does not depend on local coverage along the step, for step coverages lower than those shown in Fig. 8 (wider S-S separations). Because of this coverage-independence, the adsorption energy at steps can be shown by the horizontal line in Fig. 6. It may be surprising that the terrace four-fold hollow site is more favorable than the top step four-fold hollow site, in the limit of zero sulfur coverage. Simple localized bonding arguments, i.e. lower coordination of step atoms implying higher reactivity, would favor the step site over the terrace site. Indeed, a DFT analysis of S adsorption with metal atoms frozen at their positions for a clean vicinal surface shows that the step site is more favorable than the terrace site. However, the situation is more subtle. The under-coordination of step edge metal atoms means that under relaxation, their bond lengths decrease to maximize bonding interaction with fewer neighboring metal atoms. DFT calculations on the clean Cu(511) surface show that some of the bond lengths can be reduced by 3.6% from the bulk value. On the other hand, bonding between a step edge atom and the bulk is weakened by S adsorption near the step edge. DFT calculations show that contraction of the bond length is only 2.4% at most. In other words, the ability of a step edge to lower the energy through relaxation can be reduced by the presence of a S atom, thus penalizing adsorption of S near step edges. Relaxation is not the only way the S adsorption energy on different four-fold hollow sites can be affected by step edges. Previously, we conducted theoretical studies of S adsorption on large Cu clusters as models of extended surfaces.54-55 These showed that the Cu cluster must be very large in order to produce convergent adsorption energies and reproduce stronger bonding at (100) surfaces than at (111) surfaces. We concluded that a significant component of the adsorption bond is highly delocalized. Thus four-fold hollow



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sites on Cu(100) are not necessarily equivalent, but rather they present a potential energy surface that depends on the extended structure surrounding each site. The cross-over in adsorption energies between steps and terraces, with increasing S coverage, is probably due to the different abilities of terraces and steps to accommodate elastic S-S interactions. As S coverage increases on terraces, adsorption energy becomes less negative (i.e. weaker) because of these repulsive through-metal interactions. However, the steps can distort more easily and reduce the repulsions, leading to a coverage-independent adsorption energy at steps (at least, up to the 2a limit). This different response leads to a cross-over in the adsorption energies at about 0.1 monolayer, as shown in Fig. 6. Complexes. Finally, DFT can be used to examine the stability of various structures that combine Cu and S, as shown in Fig. 9. The candidate structures at the top of the figure are chosen because their analogs exist on other surfaces.1, 3-4, 56-57 All the localized complexes (CuS2 through Cu4S5) are energetically unfavorable, since their values of μS are higher than the baseline for chemisorbed S. In contrast, the extended √17 reconstruction is more stable than the chemisorbed phase near 0.5 ML. This is consistent with the fact that the reconstruction is observed experimentally.25 DFT analyses of this reconstruction, and its analog on Ag(100), have been discussed in detail elsewhere.56-57

Figure 9. DFT results for Cu-S complexes. Top row: Illustrations of five potential Cu-S complexes and the √17 reconstruction. Gray circles are Cu atoms in the surface plane, brown circles are Cu atoms above the surface plane. Small yellow-orange circles denote S atoms, with darker color indicating closer proximity to the Cu(100) surface. Graph: µS for various Cu-S complexes. For each type of complex, different supercells are used corresponding to different coverages. The solid line represents the stability of chemisorbed atomic S. For all structures covered in this figure, including atomic S, energetics are averaged over L=4 to 7.


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5.Discussion. The present experimental observations for atomic sulfur on Cu(100) can be compared with other systems in which atomic sulfur has been isolated under similar conditions: Au(100),4 Au(110),58 and Au(111).51 From the earlier work, two trends are notable. First, the apparent height of a protrusion is always much smaller than that expected from DFT-based internuclear separations. For example, S atoms on Au(111)51 appear as protrusions at -2 V < VS < +2 V, with height of 0.029 ± 0.013 nm. This is smaller by a factor of 5 than 0.150 nm, the atomic-core-based height derived from DFT (using a (3x3) supercell). Second, the FWHM of protrusions in the other systems ranges from 0.29 to 0.39 nm, which brackets the current value of 0.33 nm. These similarities support the assignment of the protrusions on Cu(100) as single S atoms. Another feature of the STM images is the crossover from sombrero-shaped protrusions to depressions with increasing VS (cf. Fig. 2). Early calculations by Lang59-60 predicted a protrusion-to-depression crossover at approximately +1.3 V, for a sulfur atom adsorbed on jellium, with constant tunneling current. To our knowledge, the present study is the first experimental report of such an effect for S, though it has been reported earlier for O atoms on Ag(100), with a crossover in the range +0.5 V < VS < +0.7 V.61 There, the image features were attributed to a balance between two effects: (1) the influence of the O 2pz orbital in tunneling and its low localized density of states at and around the Fermi level, giving rise to low tunneling current; and (2) a strong resonance between the O 2pz orbital and the s orbital of the metal atom directly beneath the four-fold hollow adsorption site, enhancing tunneling current when the tip is directly above the adsorbed atom.61 Presumably the former factor dominates for electron tunneling into empty states in the sample (VS > 0), producing depressions, and the latter appears for tunneling from filled states in the sample (VS < 0), producing protrusions. In related work, theoretical and experimental investigations have shown that S atoms at different adsorption sites on Mo(100)7 and Re(0001),6 as well as O atoms at different sites on Ag(100),61 can appear as protrusions or depressions at fixed VS and I, depending sensitively upon arrangement of metal atoms at the site. The identification of adsorbed S atoms is supported by very good agreement between experiment and DFT. The agreement encompasses four areas: (1) the emergence of small p(2x2)-like regions at θS ~ 0.1 ML, based on DFT in combination with LG modeling; (2) the preferential population of terrace sites at lowest coverage, with step sites becoming fully populated at θS ~ 0.1 ML; (3) the preferred step sites being four-fold hollow sites on the top terrace of a close-packed step; and (4) the absence of complexes. The absence of complexes in this system is particularly striking, given that a Cu2S3 complex is observed on Cu(111) terraces under very similar conditions.1-2 This naturally raises the question, why are Cu(111) and Cu(100) different in this regard? To address this, we have analyzed the energetics of two relevant CumSn complexes on both surfaces. The complexes are Cu2S3 and CuS2. The latter is chosen because, as noted in Sec. 1, the MS2 unit is a ubiquitous motif in complexes on real surfaces, though it has never been observed directly. The approach is illustrated in Fig. 10. The reaction of interest is that in which S and Cu adatoms form an adsorbed complex CumSn, represented by the arrow at the bottom of Fig. 10. The corresponding energy change is the formation energy of the complex, Eform:



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Eform = E(CumSn,ads + slab) - E(slab) - m µCu - n µS(Sads) = n[µS(CumSn,ads) - µS(Sads)]

(2)

Thus Eform is proportional to the difference in µS between the complex and adsorbed S, cf. Sec. 2.2 and Eq. 1. (This definition of Eform was used by Feibelman in his pioneering work assessing the role of possible S-Cu complexes in coarsening on Cu(111).62) The reaction of interest is then broken down into sequential steps, represented by the upper 3 arrows in Fig. 10. In step 1, surface S desorbs to form S2,gas. The associated energy E1 is simply the desorption energy (negative adsorption energy) of S. In step 2, S2,gas reacts with metallic Cu to form the gas-phase complex, and in step 3 the complex adsorbs on the surface. According to Hess’s Law, Eform = n(E1 + E2 + E3)

(3)

Values of these energies are given in Table 1, from DFT calculations. Focusing first on Cu2S3, Eform is positive for the (100) surface and negative for the (111), consistent with Cu2S3’s existence only on the (111) surface. Closer inspection shows that the adsorption energy of S, E1, contributes the majority (73%) of the difference in Eform. The stability of the adsorbed complex, represented by E3, trends in the same direction, i.e. both S and Cu2S3 adsorb more strongly on the (100) surface than the (111), but the difference is smaller for the complex. This picture is true for CuS2 as well, except that here Eform is positive for both surfaces, consistent with the fact that this complex is not observed on either surface. Also the adsorption energy of S, E1, contributes slightly less (63%) to the difference in Eform. Hence it is primarily the stability of adsorbed S which determines whether these complexes form, the stability being lower (reactivity higher) on the (111) surface than the (100). Why is this? It has long been recognized that S is more stable at four-fold hollow adsorption sites, which are present on (100) terraces, than at three-fold hollow sites, which are present on (111) terraces.63-64 Recently, we have analyzed binding-site preferences of S adsorbed on (100) and (111) facets of Cu nanoclusters. From site-projected density of states analysis, we showed that S adsorbed at a four-fold hollow site on a (100) facet has similar bonding interactions with the metal as at a three-fold hollow site on a (111) facet. However, the antibonding interactions are much weaker on the (100), leading to stronger adsorption overall.55 Hence, at a very basic level, this is the reason why complexes form on Cu(111) but not on Cu(100).


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Figure 10. Decomposition of the surface reaction into elementary steps. The surface reaction to form a CumSn complex is represented by the bottom arrow. The three elementary steps are represented by upper arrows. Table 1. Values of energies, in eV, as defined in Fig. 10 and Eq. (3). Cu2S3 (n=3) CuS2 (n=2) (100) (111) (100) (111) E1 2.44 1.89 2.44 1.89 E2 0.96 1.35 E3 -3.05 -2.85 -3.52 -3.19 Eform 1.04 -0.01 0.55 0.10

Finally, similarities can be drawn between this work and a recent theoretical analysis of AuX2 complexes (X = ligand = S, Cl, CH3S, SiH3S) adsorbed on Au(111).65 There, too, it was concluded that stabilities of adsorbed complexes are controlled by the bond strength of the ligand to the surface. This served to explain, for instance, the existence of Au(CH3S)266-67 but the absence of AuS2,51 at low ligand coverage under similar experimental conditions. The present work thus contributes to a developing conceptual framework which may ultimately predict which systems are most likely to support adsorbed complexes, and which are more likely to support simple adsorbates, as in the case of S/Cu(100). 6. Conclusions. The two main results of this paper are: (1) the identification and characterization of atomic chemisorbed sulfur as the exclusive adsorbed species on Cu(100) at low coverage, θS < 0.1 ML; and (2) determination of why this species forms preferentially on Cu(100) at low coverage, instead of complexes such as those observed on Cu(111) under comparable conditions.



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Regarding (1), the characteristics of chemisorbed S include: image inversion (from sombrero-shaped protrusions to depressions) as a function of tunneling bias in STM; measured height that is significantly less than projected from nuclear coordinates; and a width of 0.33 nm. The latter two features are consistent with known characteristics of chemisorbed S on the three low-index surfaces of Au. The characteristics of chemisorbed S on Cu(100) also include several that are corroborated by DFT: preferential adsorption on terraces (rather than steps) below 0.1 ML; adsorption at top terrace sites on close-packed steps; and emergence of local p(2x2)-like motifs on terraces as coverage approaches 0.1 ML. Using DFT, a lattice gas model has been developed, and Monte Carlo simulations based on this model have been compared with the observed terrace configurations. A model containing 8 pairwise interaction energies, all repulsive, gives good qualitative agreement. Regarding (2), DFT calculations show that chemisorbed atomic S is stable against complexation on Cu(100), mainly because it is strongly bonded to the surface at the four-fold hollow site. By contrast, S is unstable against complexation on Cu(111), mainly because it is more weakly bonded to the surface at the three-fold hollow site. This contributes to a general understanding of why complexes are observed in some systems but not others. Acknowledgements The experimental component of this work was conducted or supervised by HW, PS, JO, HJY, YK, and PAT. The experimental component was supported by three sources. From the U.S., it was NSF Grant CHE-1507223. From Japan, support was provided by a Grant-inAid for Scientific Research on Priority Areas “Electron Transport Through a Linked Molecule in Nano-scale”; and a Grant-in-Aid for Scientific Research(S) “Single Molecule Spectroscopy using Probe Microscope” from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT). The theoretical component of this work was conducted by DJL, with support from the Division of Chemical Sciences, Basic Energy Sciences, U.S. Department of Energy (DOE). The theoretical component of the research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract No. DE-AC02-07CH11358. This part also utilized resources of the National Energy Research Scientific Computing Center, which is a User Facility supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231. We thank J. W. Evans for helpful discussions.


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Sulfur Atoms Adsorbed on Cu(100) at Low Coverage: Characterization

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