Surface Structure and Chemistry of Alkanethiols on Au(100)-(1 Ã 1

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Surface Structure and Chemistry of Alkanethiols on Au(100)-(1x1)Substrate Doris Grumelli, Flavia Lobo Maza, Klaus Kern, Roberto C. Salvarezza, and Pilar Carro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09459 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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The Journal of Physical Chemistry

Surface Structure and Chemistry of Alkanethiols on Au(100)-(1x1)Substrate Doris Grumelli+, Flavia Lobo Maza+, Klaus Kern&, Roberto C. Salvarezza+and Pilar Carro*^ +

Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad

de Ciencias Exactas, Universidad Nacional de La Plata - CONICET- Sucursal 4 Casilla de Correo 16, (1900) La Plata, Argentina. &

Max Planck Institute FKF, Stuttgart, Germany. Institute de Physique de la Matière

Condensée, EPFL, Lausanne, Switzerland

*Área de Química Física, Departamento de Química, Facultad de Ciencias, Universidad de La Laguna, Instituto de Materiales y Nanotecnología, Avda. Francisco Sánchez, s/n 38071-La Laguna, Tenerife, Spain

^

Corresponding author

P. Carro email: [email protected]

1 ACS Paragon Plus Environment


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Abstract The surface structure of self-assembled monolayers of hexanethiolate (HT) on the Au(100)-(1×1) surface has been studied by density functional theory (DFT) calculations and their results compared to scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) data. We have found two novel surface structure models consisting of adsorbed thiolates on the Au(100)-(1×1) surface that fairly account for experimental STM and XPS observations. Surprisingly, these models exhibit better thermodynamic stability than those consisting of thiolate-Auad-thiolate moieties on a reconstructed Au(100)-(1×1) surface. Also these proposed models are more stable than the thiolate-Auad-thiolate containing c(4×2) lattice on the reconstructed Au(111) suggesting a surface dependent chemistry for thiolates on Au. However, the possible existence of other models with similar or better stability than those proposed in this work, and also the possible coexistence of different surface structures and chemistry on the substrate surface cannot be excluded.


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Introduction Elucidation of thiolate surface structure and chemistry on metal and semiconductor substrates is a challenging topic in the broad field of nanotechnology since these molecules are widely used to change the physical and chemical properties of large quantity of surfaces. In fact, they have been proposed to improve lubrication, friction and corrosion resistance, to build three dimensional structures of molecules or biomolecules in sensing platforms, and also as functional and/or structural elements in molecular electronic devices.1, 2

In this context the thiolate-Au(111) interface has received particular attention and considerable progress has been made on its knowledge.3 Today, it is accepted that thiolate adsorption on Au(111) induces a surface reconstruction process involving adatom extraction from the surface leading to vacancy islands and the formation of thiolate-Auad-thiolate (Auad = Au adatom) complexes, known as “staples”, that constitute the elemental units of the self-assembled monolayers on this surface.

In contrast to our detailed knowledge on thiolate surface structure and chemistry on Au(111), little is known about the molecular organization and the species present on the thiolate-Au(100) interface. This knowledge can be used to test the existence of an unified model

3

able to describe the surface chemistry of thiols on different single

crystal, polycrystalline and nanoparticle gold surfaces. In particular, small gold nanoparticles consist of {111} and {100} faces, i.e. the complete description of this important system requires the knowledge of thiolates on the (100) surface.4

In this work we propose two novel surface structures based on DFT calculations that fairly account for high resolution STM images and XPS data for hexanethiol (HT) adsorbed on Au(100)-(1×1). HT has been used because there is a large number of 3 ACS Paragon Plus Environment


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experimental and theoretical work concerning the surface structure, mainly formed by c(4x2) domains 5, and surface chemistry consisting of the staples6, thus allowing an easier comparison with results for thiols on the Au(100)-(1×1) surface. Also HT allows to include the hydrocarbon chain effect that is very important to determine the surface structure on Au(111) 7. The experimental and theoretical data suggest that thiolates are the main species on this surface instead the thiolate-Auad-thiolate moieties present on the Au(111) surface suggesting a surface dependent chemistry for adsorbed thiols on Au.

Experimental Methods The Au(100) single-crystal substrate was cleaned by sputtering and annealing in ultrahigh vacuum (UHV) conditions. The Au(100)-(1×1) surfaces were electrochemically generated according to Ref 8. SAMs of hexanethiol (Aldrich, 95%) (HT) were prepared by immersion for 24 h in 10–4 M HT containing ethanolic solutions (BASF 99%). After that, the samples were characterized in a home-made UHV STM operating at room temperature. The STM was calibrated in x,y and z directions using the stripes of the well known Au(100)-(hex) surface reconstruction. WsXM software was used for image analysis.9

Computational Methods Density functional calculations have been performed with the periodic plane-wave basis set code VASP 5.2.12.10-12 We have followed the scheme of non-local functional proposed by Dion et al.13, vdW-DF, and the optimized Becke88 exchange functional optB88-vdW

14

to take into account van der Waals (vdW) interactions. The projector

augmented plane wave (PAW) method has been used to represent the atomic cores15 with PBE potential. The computational parameters that control the theoretical results of 4 ACS Paragon Plus Environment

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this work have been carefully tested and are presented in the supporting information (SI). The electronic wave functions were expanded in a plane-wave basis set with a 420 eV cutoff energy. Optimal grids of Monkhorst-Pack16 k-points 7×2×1, have been used for numerical integration in the reciprocal space of the ቀ

1 7

−1 ቁ and (2x7) surface 6

structures, while for the c(2x2) lattice the optimal grid has been 7×7×1. Gold surfaces were represented by a five atomic layer and a vacuum of ~17 Å that separates two successive slabs. Surface relaxation is allowed in the three uppermost Au layers of the slab, as well as the atomic coordinates of the adsorbed species were allowed to relax without further constraints. The atomic positions were relaxed until the force on the unconstrained atoms was lesser than 0.03 eV Å-1. The adsorbates are placed just on one side of the slab and all calculations include a dipole correction. Radical HT species was optimized in an asymmetric box of 20 Å × 20 Å × 40 Å. The calculated Au lattice constant is 4.16 Å, which compares reasonably well with the experimental value (4.078 Å).24

The average binding energy per adsorbed species on Au(100) surfaces, Eb, is defined in Eq.: Eb =

1 [ E thiol / Au − E Au − N thiol E thiol ] N thiol

[1]

where, Ethiol/Au, EAu and Ethiol stand for the total energy of the adsorbate-substrate system, the total energy of Au slab, and the energy of the alkanethiol radical, respectively, whereas Nthiol is the number of alkanethiol radicals in the surface unit cell. A negative number indicates that adsorption is exothermic with respect to the clean surface and alkanethiol radical. The Gibbs free energy of adsorption of each surface structure (γ ) was approximated through the total energy from DFT calculations by using equation [2]:



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ߛ=

ே೟೓೔೚೗ ா್

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[2]

஺

where A is the unit cell area. Considering that we are concerned with free energy differences, it is reasonable to assume that the contributions coming from the configurational entropy, the vibrations and the work term, pV can be neglected17, 18 . On the other hand, for staple containing models the Eb and γ value were estimated from equations [3] and [4], respectively. Eb =

1 N thiol

ߛ=

[ E thiol / Au − E RAu − N thiol E thiol ] ே೟೓೔೚೗ ஺

ா

ቀ‫ܧ‬௕ + ே ೝ೐೎ ቁ ೟೓೔೚೗

[3]

[4]

where ERAu corresponds to the energy of reconstructed Au(100) surface, and Erec is the surface reconstruction energy per unit cell in the staple containing model that has been calculated as, Au Erec = ERAu − E Au − nad Ebulk

[5]

Au In equation [5] Ebulk is the total energy of a bulk Au atom and nad is the number of Au

adatoms in the surface unit cell. The Erec value is the energy involved in Au adatoms formation needed for the RS-Auad-RS moieties. Core level shifts (CLS) of the S2p levels have been calculated within the final state approximation19, 20 following the Slater-Janak approach 21 in which only an half electron is excited from to the core level to the valence region and placed in the lowest unoccupied orbital.22 The CLS are calculated with respect the binding energy (BE) value of S 2p in (√3x√3)R30º S/Au(111). Constant current STM images of the optimized lattices were simulated by using the Tersoff-Hamman method in its most basic form with the STM tip approximated as a point source.23


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Results and Discussion STM results Adsorption of hexanethiol (HT) on the reconstructed Au(100)-hex surface results in the formation of elongated Au island following the typical stripes of the reconstruction with ordered molecular arrays forming hexagonally distorted patterns with nearest neighbors distances d = 0.48/0.5 nm and surface coverage θ ≈ 0.33 (Figure 1a).8 The hexagonally distorted lattice, also known as the β phase24-26 have been described as a c(2×8) lattice with a (1×4) missing row.10, 27, 28 On the other hand, the unreconstructed Au(100)-(1×1) surface after HT adsorption shows terraces, covered by two different molecular patterns (Figure 1b-d), and well defined square-like islands (Figure 2a). The first molecular structure (Figure 1b, green arrow) is a ribbon-like pattern formed for nearly-square molecular arrangement, known as the α phase, with d ranging from 0.4 to 0.5 nm and θ ≈ 0.44.8, 10, 24-2819 Each ribbon, formed by 4-5 HT molecules, is displaced one site in relation to the adjacent ribbon as can be clearly seen in Figure 1c and inset. The second one is a striped pattern (Figure 1b, blue arrow), which also exhibits square-like molecular structures (Figure 1d) with the same d values. In both cases the rows of molecules intercept the substrate steps forming ≈ 45° angles (Figure 1b, inset).



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Figure 1a-d. STM images of the Au(100)-hex (a) and Au(100)-(1×1) (b-d) after HT adsorption. (b) STM image showing α (ribbon)(green arrow) and stripe (blue arrow) domains. The inset shows the 45º angle formed between the molecules and a surface step. (c) High-resolution STM image of the α structure. The inset shows a detail of the displacement between the rows of molecules (green arrows). (d) High-resolution STM image of the striped structure. Typical tunneling current and bias voltage are 0.5–1 nA and 1 V, respectively.

Other characteristic feature of the HT-covered Au(100)-(1×1) surface is the presence of square-like islands on the terraces (Figure 2a). The cross-section analysis of these structures reveals that they are ≈ 0.2 nm in high (Figure 2b), a figure expected for monoatomic Au islands. 28Also, we observed a few dark spots (Figure 2a, c) on the HTcovered terraces that exhibit ≈ 0.2 nm in depth (Figure 2d) that can be assigned to Au vacancy islands. STM images of the clean Au(100)-(1×1) surface (data not shown) reveal the presence of a similar number of vacancy islands, and, therefore, they are not induced by the HT adsorption. This result contrasts to that observed for the Au(111) surface where HT adsorption produces a remarkable increase in the number of vacancy islands reaching a total coverage close θvac ≈ 0.12. 3


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Figure 2a-d. STM images of the Au(100)-(1×1) after HT adsorption showing the square Au islands (Figure 2a) and vacancy islands (Figure 2c) indicating by arrows. The corresponding cross section analysis is shown in Figures 2 b, d, respectively.

DFT results In Figure 3 we show the optimized ቀ

1 7

−1 ቁ (θ = 0.46) and (2×7) lattices (θ = 0.44) 6

proposed to account for the ribbon and striped surface structures depicted in Figure 1b-d, respectively. The positions of the S-heads are indicated in Figures 3a-3a’ while the HT molecules are shown in more detail in Figures 3c-3c’. Also the side view of the adsorbed HT molecules on the Au(100) substrate are indicated in Figures 3d-3d’. We have also included in Figures 3a’’, 3c’’ and 3d’’ similar schemes for the c(2×2) lattice (d = 0.4, θ = 0.50) as it has also been proposed for thiol

29

and S

30

adsorbed on the

Au(100)-(1×1) surface. The optimized ቀ

1 7

−1 ቁ 6

lattice (Figure 3a) exhibits not only the nearly square

structures 0.42 nm×0.44 nm in size but also the slight displacements in the thiolates species that originate the ribbon-like patterns. On the other hand, the optimized (2×7) lattice (Figure 3a’) has the molecular structure and the characteristic stripe pattern observed in the images, and squared structures 0.42 nm×0.43 nm in size. Note that in 9 ACS Paragon Plus Environment


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both models HT molecules adsorb on bridge sites of the Au(100)-(1×1) surface (Figures 3c,3c’, 3d, 3d’). The calculated STM images of these models (Figures 3b, 3b’, 3b’’) compare very well with the experimental STM images shown in Figure 1b-d. The tilt angle (αtilt ) for the ൬1 7

-1൰ and (2×7) models (Table 1) are close and slightly higher, respectively, than 6

the value αtilt ≈ 14° experimentally measured for alkanethiols on this Au surface.29 Interestingly, the ൬1 7

-1൰ and (2×7) surface structures show better binding energies 6

(Eb) and thermodynamic stability (γ) (Table 1) than those exhibited by other possible lattices such as the (2×8) surface structure on the Au(100)-(1×1) (θ = 0.44) that according to our DFT calculations exhibit Eb= -2.90 eV and γ = -146.79 meV Å-2. Note also that the surface free energy values, γ, (Table 1) suggest that the (2x7) surface structure (θ = 0.44) should evolve to the slightly denser ൬1 7

-1൰ (θ = 0.46), in 6

agreement with previous observations that the stripes tend to vanish into ribbon domains. 8


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Figure 3. Optimized surface models for HT on Au(100-(1×1). a) a’) and a’’) top view schemes where only the S heads of HT molecules are indicated. b), b’) and b’’) Simulated constant current STM images for HT on the different surface models. c), c’) and c’’) Zoom of the top view surface models showing the HT molecules adsorbed on the substrate. The corresponding unit cells are indicated in a) and c) schemes. d),d’) and d’’) Side view of the HT molecules on the substrate. White: Au atoms, Green: S atoms, Brown: C atoms, Pink: H atoms.

On the other hand, the optimized c(2×2) lattice also exhibits HT preferred adsorption at bridge sites (Figure 3a’’, 3c’’and 3d’’), although the initial configuration was with HT



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molecules placed at the hollow sites. Interestingly the ൬1 7

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-1൰ and (2×7) models 6

exhibit an inward relaxation of the Au atoms of the outmost layer which are not bonded to HT (Figure 3d and 3d’), a fact that is not observed in the c(2×2) lattice (Figure 3d’’) where all these Au atoms are involved in bonding with thiolates. This relaxation is responsible of the ribbons and stripes observed in DFT schemes (Figure 3). The DFT calculations indicate that this lattice has better Eb and γ values than those estimated for the ൬1 7

-1൰ and (2×7) models. However, the c(2×2) lattice has θ = 0.50, a figure 6

exceeding the vale θ ≈ 0.44 experimentally observed for HT SAMs on the Au(100)(1×1) surface.8, 10, 24-28 Also, the calculated STM image of this model (Figure 3b’’) has neither the ribbons nor the stripes observed in the experimental STM images (Figure 1bd). One can speculate that there are steric reasons which hinder thiolate organization in the c(2×2) lattice. In fact, in solid alkanes the lattice constant is 0.445 nm16 a value larger than that corresponding to the c(2×2) lattice (0.407 nm) but close to that found in the ൬1 -1൰ and (2×7) structures. 7 6

Table 1: Energetic and structural data for different HT surface structures on Au(100)-(1×1) and Au(111) 7 Substrate

Au(100) )-(1×1)

Au(111)

Surface Lattice

c(2×2)

(2×7)

(2×7)STAPLE ቀ

θ

0.50

0.43

0.43

1 −1 c(4×2) ቁ STAPLE 7 6 0.46 0.33

Eb/eV

-3.09

-3.05

-3.39

-3.03

-3.49

Er/Nthiol/eV

0.0

0.0

+0.63

0.0

+0.52

γ /meV. Å-2

-178.55 -151.10 -136.45

-161.66

-132.25

z(S-Ausurf)/Å

1.86

1.86

2.38

1.84

2.54

αtilt/º

24.3

21.6

21.2

14.7

32.2 12

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α1/º

50.0

47.6

37.7

49.4

63.5

It is probably that the functional employed in this study, optB88-vdW, underestimate the short-range repulsive forces in the SAM allowing a closer packed arrangement of the hydrocarbon chains in the DFT calculations. Differences in the optimal distances for organic molecules adsorbed on Fe(100) have been also observed by using different functionals with vdW.

31

In any case, when applied to a particular problem, as in our

case, the accuracy of any method used should be tested or verified against experience or other reference data, since any approach, can, in principle, fail for a specific problem. 32 Surface chemistry Now we will discuss the possible existence of a possible unified surface chemistry model for HT on the (100) and (111) surfaces. In fact, there are clear experimental and theoretical evidences that the thiol SAMs on Au(111) are formed by thiolate-Auadthiolate complexes.33, 34 The first result against the unified model picture is the small number of vacancy islands on the HT covered Au(100)-(1×1) terraces (Figure 2). These vacancies should be produced since staple formation requires large amounts of gold adatoms. In fact, the well known c(4×2) for HT on Au(111) has thiolate coverage θ = 0.33, and thus requires θad = 0.165.

35

The gold adatoms need to be extracted from

terraces, resulting in vacancy island formation, or step edges as the lifting of the herringbone reconstruction induced by the thiol adsorption only provides (θad = 0.05). 3 In principle, the amount of metal adatoms produced by the lifting of the Au(100)-(hex) reconstruction after thiol adsorption to form the (1×1) surface (θad = 0.25) should be enough to form all the thiolate-Auad-thiolate complexes for thiolate surface structures with θ ≈ 0.44 (θad = 0.22). However, in our experimental system these adatoms are not available to form the staples as thiol adsorption takes place directly on an



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unreconstructed Au(100)-(1×1) surface. In fact, we induce the electrochemical lifting of the Au(100)-(hex) surface reconstruction before thiol adsorption, so that the spelled Auad nucleate and growth on terraces forming well defined square and rectangular Au islands on the clean Au substrate surface.5 These islands still remain visible after thiol adsorption as shown in Figure 2a. Therefore, Auad should be taken from terraces or step edges, a process that is also energetically favored on the Au(100)36. Gold adatom removal from terraces by the thiol molecules to form the staples should result in a large density of vacancy islands a fact not observed in the STM images of the Au(100)-(1×1) surface (Figure 2). Although, Au adatoms uptake from step edges could be consistent with the serrated island edges observed in Figure 2a the absence of a high density of vacancy islands, a fingerprint for the formation of thiolate-Auad-thiolate complexes on the Au(111) surface, suggests that no extensive staple formation takes place on the Au(100)-(1×1) terraces. The question is why should the HT molecules behave differently on the Au(111) and Au(100) )-(1×1) surfaces leading to different species?. A possible answer emerges from the analysis of the energetic and thermodynamics data in Table 1. Interestingly, the ൬1 7

-1൰ and (2×7) structures formed by adsorbed HT radicals on the Au(100)-(1×1) 6

exhibit better thermodynamic stability than the HT staple containing c(4×2) lattice on Au(111).

7

Also we have calculated the thermodynamic stability for a hypothetical

(2×7) lattice on Au(100)-(1×1) formed by the thiolate-Auad-thiolate complexes with the same result: it is thermodynamically unstable in relation to the adsorbed HT radicals (Table 1). In fact, in contrast to that found for HT on Au(111)3, the large Eb value of the staple containing (2×7) lattice on the Au(100)-(1×1) surface (Table 1) cannot compensate the extra energy cost, (Erec), to reconstruct the Au(100) by adding the Au adatoms needed

to form

the RS-Auad-RS moieties, thus turning the SAM 14


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thermodynamically less stable (Table 1). Note that Erec (equation 5) is independent of the origin of the Au adatoms, i.e. the staple containing (2×7) lattice on the Au(100)(1×1) surface is unstable irrespective that Auad are removed from terraces or from the energetically favored step edges. On the other hand, it has been impossible to form the ribbon-like pattern with staple units by steric reasons. Therefore, these results are then consistent with the low number of vacancy islands for HT on the Au(100)-(1×1) surface. We have also used the experimental difference between the S 2p CLS for HT adsorbed in Au(100)-(1×1) with that obtained for HT on the Au(111) surface (∆CLS = -0.15 eV)8 to validate our models and the absence of staple species on the Au(100)-(1×1) substrate. The DFT calculations show that the ∆CLS between the S 2p CLS for ൬1 7

-1൰ and 6

(2×7) HT models on Au(100)-(1×1) with respect to the S 2p CLS for c(4×2) HT staple containing model on Au(111) is ∆CLS ≈ -0.2eV, in fair agreement with the experimental difference (Table 2). Considering that the Bader charge is practically the same for all the studied surface structures (Table 2), we assign the observed difference in S 2p CLS to the work function difference (∆Φ Φ) between the HT covered Au(100)(1×1) and Au(111) surfaces, which according to the DFT calculations is ∆Φ Φ≈ -0.20/-25 eV (Table 2). Interestingly, the staple containing (2×7) model on Au(100)-(1×1) exhibits only ∆CLS = 0.02 eV with respect to the staple c(4×2) lattice on Au(111), far from the -0.15 eV experimental difference, providing further evidence for the absence of thiolate-Auad-thiolate complexes on the Au(100)-(1×1) surface.



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Table 2 : S 2p core level shift, S Bader charge and Φ work function of HT species on Au(100) and Au(111) for the different surface models. Substrate Surface lattice Au(111) c(4x2)STAPLE c(2x2) (2x7) Au(100) (2x7)STAPLE

ቀ

1 7

−1 ቁ 6

S 2p CLS/eV +1.15 +0.95 +0.89 +1.17

S Bader Charge/ u.a. -0.18 -0.19 -0.18 -0.18

Φ/ eV 4.10 3.84 3.97 3.62

+0.90

-0.18

3.88

However, these results should be taken with caution due to the plastic surface of Au, and the large differences between the experimental conditions (298 K, adsorption from solution) and the DFT calculations (0 K, vacuum). In fact, experimental data and molecular dynamic calculations for methanethiol33 and HT6 on the Au(111) surface have shown that thiolates adsorbed at bridge sites can coexist with thiolate-Auad-thiolate moieties. Therefore, a possible scenario at T = 298 K for the Au(100)-(1×1) substrate could also involve the presence of unreconstructed (1×1) domains with the adsorbed thiolates at bridge sites and reconstructed regions with thiolate-Auad-thiolate moieties, in particular near step edges where one could expect that adatom removal is easier due to its low energy of desorption36.

Conclusions We have proposed two novel surface models for HT SAMs on the Au(100)-(1×1) surface, ቀ

1 −1 ቁ and (2x7), 7 6

that account for the ribbon-like and striped patterns

observed in the experimental STM images and XPS data after HT adsorption on this surface. Surprisingly, in these models adsorbed thiolates on the plain Au(100)-(1x1) substrate exhibit better thermodynamic stability than the same models on a reconstructed surface formed by thiolate-Auad-thiolate moieties. These models are also


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more stable than the thiolate-Auad-thiolate containing c(4×2) on the reconstructed Au(111). These results suggest that staple formation is not favored on Au(100)-(1×1) surface, a fact consistent with the absence of a high density of vacancy islands on the terraces, and also with the S 2p CLS difference measured between HT-covered Au(100)-(1×1) and HT-covered Au(111) surfaces. Therefore, our results suggest that the chemistry of the thiolate-Au interface is strongly dependent on the crystal face. It should be noted, however, that the possible existence of other models with similar or better stability can not be excluded, as well as the coexistence of domains with different surface structure and chemistry on the Au(100)-(1×1). In particular, there are significant differences between the experimental conditions (298 K, adsorption from solution) and DFT calculations (0 K, vacuum). Clearly, more experimental and theoretical work for thiol adsorption on this surface is needed to shed light on this elusive system. Acknowledgment The authors acknowledge financial support from ANPCyT (PICT 2554) from Argentina, and CTQ2011-24784, Spain. P. C. thankfully acknowledges the computer resources provided by Atlante, Canary Islands Supercomputing Infrastructure- Red Española de Supercomputación and by the Computer Support Service for Research (SAII) at La Laguna University. Supporting Information Available: Convergence test used for DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.



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Figure 1a-d. STM images of the Au(100)-hex (a) and Au(100)-(1×1) (b-d) after HT adsorption. (b) STM image showing α (ribbon)(green arrow) and stripe (blue arrow) domains. The inset shows the 45º angle formed between the molecules and a surface step. (c) High-resolution STM image of the α structure. The inset shows a detail of the displacement between the rows of molecules (green arrows). (d) High-resolution STM image of the striped structure. The arrows indicate the stripes. Typical tunneling current and bias voltage were 0.5–1 nA and 1 V, respectively. 84x84mm (300 x 300 DPI)


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Figure 2a-d. STM images of the Au(100)-(1×1) after HT adsorption showing the square Au islands (Figure 2a) and vacancy islands (Figure 2c) indicating by arrows. The corresponding cross section analysis is shown in Figures 2 b,d, respectively. 84x75mm (300 x 300 DPI)



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Figure 3. Optimized surface models for HT on Au(100-(1×1). a) a’) and a’’) top view schemes where only the S heads of HT molecules are indicated. b), b’) and b’’) Simulated constant current STM images for HT on the different surface models. c), c’) and c’’) Zoom of the top view surface models showing the HT molecules adsorbed on the substrate. The corresponding unit cells are indicated in a) and c) schemes. d),d’) and d’’) Side view of the HT molecules on the substrate. White: Au atoms, Green: S atoms, Brown: C atoms, Pink: H atoms.

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Surface Structure and Chemistry of Alkanethiols on Au(100)-(1 Ã 1

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Surface Structure and Chemistry of Alkanethiols on Au(100)-(1 Ã 1