Effects of gold based dimers on structural and electronic properties of MoS2

Accepted Manuscript Title: Effects of gold based dimers on structural and electronic properties of MoS2 ¨ Author: Yelda Kadioglu G¨okhan G¨oko˘glu Olcay Uzengi Akturk ¨ PII: DOI: Reference:

S0169-4332(16)32312-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.10.175 APSUSC 34269

To appear in:

APSUSC

Received date: Revised date: Accepted date:

2-8-2016 6-10-2016 26-10-2016

Please cite this article as: Yelda Kadioglu, Gddotokhan Gddotoko˘glu, ¨ Olcay Uzengi Aktddoturk, Effects of gold based dimers on structural and electronic properties of MoS2 , (2016), http://dx.doi.org/10.1016/j.apsusc.2016.10.175 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

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1. Semiconductor MoS2 shows metallic character by AuPt and AuPd adsorption. 2. MoS2 maintains its semiconductor characteristics with a decrease in the band gap values after AuAg, AuCu, and AuAl adsorption. 3. AuPt adsorbed system is the most stable structure energetically. 4. AuAl exhibits the weakest adsorption to MoS2 among the considered dimers.

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Graphical Abstract (for review)

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2.52A Energy (eV)

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EHLG= 1.29 eV

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Energy (eV)

2 1 0 Pd s Pd d

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Effects of gold based dimers on structural and electronic properties of MoS2 Yelda Kadioglu

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G¨okhan G¨oko˘glu

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Department of Physics, Adnan Menderes University, 09100 Aydın, Turkey

Department of Physics, Karab¨uk University, 78050 Karab¨uk, Turkey

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¨ Olcay Uzengi Akt¨urk ∗

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Department of Electrical & Electronics Engineering, Adnan Menderes University, 09100 Aydın, Turkey

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Nanotechnology Application and Research Center, Adnan Menderes University, 09100 Aydın, Turkey

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In view of first principles calculations, we investigate the electronic structure redecoration of monolayer MoS2 upon adsorptions of AuAg, AuPt, AuPd, AuCu, and AuAl bimetallic dimers. Geometrical structure, band structures, electronic density of states, charge density differences of dimer adsorbed MoS2 systems are presented and discussed. All the systems studied have non-magnetic ground states. Charge transfers occur from dimer to surface except for AuPt adsorption. Our results indicate that the semiconductor MoS2 maintains its semiconductor character with decreased band gaps upon AuAg, AuCu, and AuAl adsorptions. However, MoS2 shows metallic behavior by AuPt and AuPd adsorptions, so Pt-d and Pd-d states cross Fermi level yielding metallic character. AuPt adsorbed system has the highest Eads value of 3.15 eV indicating the most stable structure energetically among the dimer adsorbed MoS2 systems considered. Key words: Molybdenum disulfide (MoS2 ), AuAg, AuPt, AuPd, AuCu, AuAl dimers, Electronic structure.

∗ Corresponding Author; [email protected] ¨ Email address: [email protected] (Olcay Uzengi Akt¨urk ).

Preprint submitted to Applied Surface Science

4 November 2016

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1 Introduction

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The most widely studied two dimensional (2D) material graphene lacks an electronic bandgap with zero density of states (DOS) at Fermi level, so it has limited applications in nanoelectronic technology. However, unlike semimetallic graphene, molybdenum disulfide MoS2 is a semiconductor with a large direct band gap ( 1.85eV ) in monolayer form [1,2] opening various possibilities in electronics and photonics applications. MoS2 was reported as an ideal material for electronic devices especially for low-power electronics [3,4]. It has high carrier mobility, at least 200cm2 /V s, at room temperature [3] similar to graphene nanoribbons and has high mechanical strength [5]. MoS2 transistors have current on/off ratios of 108 at room temperature[3]. MoS2 is also used in catalytic applications for hydrodesulfurization and hydrodenitrogenation to remove sulfur from petroleum products [6]. The addition of promoters such as potassium, nickel, or cobalt on MoS2 surface can cause a hydrogenation reaction as a catalyst from syngas to alcohols or higher hydrocarbons [7]. It is also used in high efficiency photoelectrochemical cells because of the optical band gaps consistent with the solar spectrum [8]. Gas sensors are important especially for the detection of toxic gases in the context of environmental pollution, recently MoS2 was suggested as a potential material for gas sensing applications [9,10].

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Gold nanoparticles take attention for potential applications in catalysis [11,12,13,14]. Silver (Ag) catalysts are used in important industrial chemicals for oxidation reactions [15,16], photonics [17], and information storage [18]. Palladium and platinum are the fundamental catalysts in industrial technology [19,20,21,22,23]. However, it is also known that bimetallic catalysts have different properties from its pure metals [24,25,26]. For example, the catalytic performance of Au-Ag alloys are largely improved compared to pure Au and Ag [16]. These Au-Ag bimetallic nanoparticles exhibit high activity in CO oxidation [16]. Recently, Au-Pt catalyst has received much attention as fuel cell electrocatalysts [27]. It was found to have high activity for the CO oxidation [28,29] and higher selectivity in hydrogenation [30]. It is also used for production of hydrocarbons[31,32]. Bimetallic palladium-gold (Pd-Au) alloys show high catalytic property in hydrogenation [33], acetoxylation of ethylene [34], CO oxidation [35,36], and the synthesis of hydrogen peroxide (H2 O2 ) which is an environmentally friendly oxidizing agent compared to pure Pd and Au metals [37,38,39]. Copper-gold alloys have drawn attention because of exhibiting order-disorder phase transitions and the copper mixed oxides show strong enhancement of activity on support for gold nanoparticles [40]. Aluminium-gold alloys are important for wire bonding in microelectronic industry [41]. Due to the various intriguing properties of bimetallic dimers explained above, we are concerned about the adsorption of gold based bimetallic dimers on MoS2 monolayer. The possible applications of the dimer adsorbed MoS2 systems in electronics can be discussed due to modifications in the direct gap semiconducting behavior of 2

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bare MoS2 monolayer. Moreover, it is important to explore the basic relation between the structural and electronic properties, because the activity and selectivity of catalysts are dependent to surface properties significantly. Depending on the modification on physical structure or doping with different atoms, the dramatic changes can occur in electronic properties. In this context, the main purpose of this work is to show how the electronic properties of MoS2 monolayer change with adsorption of gold based dimers. Therefore we investigated the effects of AuAg, AuPt, AuPd, AuCu, and AuAl dimers on geometric and electronic structure of MoS2 monolayer. Self consistent field calculations were performed in a spin-polarized manner, however a net magnetic moment was not observed for all the systems considered in this work.

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2 Computational Details

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All calculations were performed by using Quantum Espresso program [42] which is based on first principles plane wave method within density functional theory (DFT). Van der Waals interaction was also included in calculations as implemented in Quantum Espresso [43]. The Perdew-Burke-Ernzerhof functionals was used for the exchange-correlation potential in generalized gradient approximation (GGA) [44]. The valence states of atoms are as follows; Mo:4s2 4p6 5s1 4d 5 , S:3s2 3p4 , Au:5d 10 6s1 , Pd:4d 9 5s1 , Pt:5d 9 6s1 , Cu:3d 10 4s1 , Ag: 4d 10 5s1 , Al: 3s2 3p1 . Plane wave basis set with kinetic energy cut off value of 952 eV was used. The k-point meshes 9×9×1 was used for Brillouin zone integrations according to MonkhorstPack method [45]. The energy convergence criterion of the electronic self-consistency was chosen as 10−5 eV between two successive iterations. The optimized geometries were obtained with Hellman-Feynman forces on each atom smaller than 0.03 ˚ The vertical distance between the adjacent surfaces was taken as 25 A ˚ which eV/A. provides to avoid interactions. We tried four possible adsorption sites in order to determine the most stable structures. These sites were above the center of hexagonal ring of MoS2 , on top of Mo and S atoms, and above the midpoint of two neighbouring atoms. The dimers were placed to these sites both vertically and horizontally. We also tried orientation of dimer on these sites over both atoms of dimers in vertical positions. The adsorption energy of system was defined as : Ea = Emonolayer + Edimer − Emonolayer+dimer

(1)

where Emonolayer , Edimer , and Emonolayer+dimer are the total energies of the optimized bare monolayer, dimer, and monolayer+dimer system, respectively. Monolayer+dimer system refers to total energy of the system in which the dimers are attached to MoS2 . 3

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The charge differences were calculated as follows: ∆ρ = ρmonolayer+dimer − ρmonolayer − ρdimer

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The amount of charge transfer was calculated by Bader analysis [46].

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where ρmonolayer+dimer , ρmonolayer , and ρdimer are the total charge on monolayer+dimer, monolayer, and dimer systems, respectively.

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3 Results and Discussion

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We first present a brief summary on the structural and electronic properties of bare single layer MoS2 system to understand the effects of chemisorbed dimers on the physical properties of MoS2 . MoS2 was studied in its stable H-structure form with trigonal prismatic coordination. A 4x4 supercell of monolayer 1H-MoS2 was chosen for the investigation of dimer adsorption as shown in Figure 1. The equilibrium ˚ which is also suffilattice parameter a for this supercell was calculated as 12.77 A cient to avoid interactions between the dimers in adjacent supercells. The selected dimers and their structural and energetic parameters in isolated form are shown in Figure 2. While AuAg, AuCu, and AuAl are nonmagnetic dimers, AuPt and AuPd show magnetic property with 1.0 µB net magnetic moment. The energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are also given in Figure 2. Red colored energy levels refer to spin up states and green colored levels refer to spin down states. Higher HOMO-LUMO gap (HLG) values indicate higher chemical and kinetic stability. Therefore AuAl is the most stable dimer among the systems considered with the highest HLG value of 2.38 eV. AuPt has indirect HLG because of LUMO of spin up states and HOMO of spin down states. It has the lowest HLG value of 1.12 eV. AuPd has a direct HLG between HOMO and LUMO of spin down states. The lowest energy structures of dimer adsorbed MoS2 monolayers are shown in Figure 3 and the structural and energetic parameters are summarized in Table 1. It is seen that dimers prefer primarily to bind to sulfur (S) atoms of MoS2 monolayer as expected because of the weak bond formation tendency between the noble metal atoms and Mo [47]. AuAg dimer binds over Au atom to S atom of MoS2 with 1.33 eV adsorption energy value. This is the lowest energy structure for AuAg adsorption on MoS2 among the possible adsorption sites. However, if we initially place the dimer in parallel direction to MoS2 plane, dimer binds over Ag atom to S atom of MoS2 with an inclination. But there is ≈0.1 eV adsorption energy (Ea ) difference between these two configurations. In other words, AuAg has to overcome an energy barrier of 0.1 eV depending on the initial orientation of dimer to reach the most stable configuration which binds over Au atom to S atom of MoS2 . This small energy barrier is due to the similar features of Au and Ag on MoS2 layer 4

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which were also expressed in literature [47,48,49]. AuAg adsorbed MoS2 system is ˚ is in good agreement with literature nonmagnetic and Au-S bond length of 2.31 A ˚ and do not change after [50]. Before the optimization Au-Ag bond length is 2.56 A the adsorption.

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AuPt, AuPd, and AuCu dimers display similar equilibrium orientations upon adsorption. They prefer to adsorb over Pt, Pd, or Cu atoms. The inclination of dimers is an indicator of effective polarization between dimers and MoS2 layer. However, AuAg and AuAl dimers show distinct orientations. Whereas AuAg dimer has an axis perpendicular to the MoS2 plane, AuAl lies nearly parallel to the MoS2 surface. The highest Eads value of 3.15 eV is found for AuPt dimer adsorption which indicates energetically most stable binding state among the dimers considered. In this structure, Pt atom is bonded to three S atoms and Pt-S bond lengths are calculated ˚ 2.31 A, ˚ and 2.31 A ˚ which are close to experimental values previously as 2.34 A, determined [51,52]. The minimum adsorbate-MoS2 bond length is found at AuPt ˚ which is consistent with the highest Eads value inadsorption by the value of 2.31 A ˚ does not change dicating a strongly covalent type bonding. Au-Pt distance (2.50 A) after the optimization. In the case of AuPd dimer adsorption, it can be mentioned that it is the second most stable system with Eads value of 2.23 eV. Pd is bonded to ˚ 2.36 A, ˚ and 2.35 A ˚ in agreement three S atoms with the bond lengths of 2.39 A, with previous studies in literature [53]. Au-Pd distance remains constant after the optimization. By AuCu dimer adsorption, Eads value of 1.73 eV is calculated. Cu atom forms three bonds with the nearest S atoms and the bond lengths of Cu-S ˚ 2.34 A, ˚ and 2.34 A ˚ in agreement with literature [54]. Au-Cu distance are 2.27 A, ˚ ˚ after the optimization. AuAl dimer adsorpslightly increases from 2.37 A to 2.42 A tion is the weakest adsorption among the considered dimers with the Eads value of ˚ and 2.66 A. ˚ A slight increase 0.41 eV. The distance between Al and S are 2.65 A ˚ to 2.43 A ˚ after the optimization. It can also in Au-Al distance is seen from 2.37 A be concluded that the amount of charge transfers between dimers and substrate are not decisive on the adsorption strength of dimers. It is also remarkable that the magnetic AuPt and AuPd dimers are adsorbed with higher adsorption energies compared to others. The net magnetizations of isolated AuPt and AuPd dimers vanish by the adsorption yielding non-magnetic ground states. These dimers have also the lowest HLG values indicating chemical activeness. We note that there is a clear correlation between adsorption energies and HLG values, so the lower HLG values lead to larger adsorption energies. As a different situation from the other dimers, these dimers result in metallic structure upon adsorption on MoS2 . We can conclude that MoS2 does not provide an active surface for Au atom, accordingly non-Au dimer atom binds to surface in most cases. In addition, the adsorption energies of all the dimers in perpendicular orientation to the surface were calculated as given in Supporting Information. In this case, the adsorption energies display a different trend. This is mainly due to the forcing of dimer orientation, although it prefers to curve during optimization. Moreover, AuAl dimer is not practically adsorbed in this orientation with a negative Eads value. 5

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The electronic band structures are shown in Figure 4 and the band gap values are given in Table 1. MoS2 is a direct gap semiconductor in its single layer form. We calculate the band gap value of bare MoS2 as 1.66 eV which is consistent with the experimental value of 1.8 eV [1,2]. This band gap is restricted by the Mo-d and S-p states. Our results show a significant reduction in the band gap of dimer adsorbed MoS2 systems as compared to that of free-standing monolayer MoS2 . Monolayer MoS2 , which is a well known semiconductor, exhibits metallic behavior after AuPt and AuPd adsorptions as seen from band profiles. In this case, d-states of Pt and Pd atoms are responsible for metallization with crossing Fermi level. These bands are alsmost flat without hybridizations with other states. It should be stated that these flat bands around Fermi level indicate very high effective mass and low electron mobility. Therefore AuPt and AuPd adsorbed metallic systems are expected to have low electrical conductivity. However, MoS2 maintains its semiconductor characteristics even though the band gap values decrease by AuAg, AuCu, and AuAl adsorptions. The electronic band gaps are restricted by the states of adsorbate atoms and Mo-d states. The electronic states around 1-2 eV above the Fermi level are dominated by Mo-d and S-p states for all the systems studied. These states do not show strong hybridizations with the adsorbate states. The electronic states near the Fermi Level are found to be contributed by adsorbate states which form impurity states below the Fermi level. As a result, the calculated energy gaps are reduced compared to bare MoS2 structure. It is known that if the energy of the impurity states is near the middle of the band gap, these are called deep states, if it lies near the band extrema (VBM or CBM) they are called shallow states [55]. There is a shallow state at 1.16 eV below the CBM and 0.49 eV above the VBM in AuAg adsorbed MoS2 . It can be seen from DOS graphs that the impurity states arise from s states of Ag and p states of Au atoms. The impurity state in AuCu system is located 0.5 eV below the Fermi level which is mainly due to Au s states. In the AuAl adsorbed system, one of the two impurity states arises from p states of Al and S atoms ≈0.77 eV below the CBM. Other state, which comprise s states of Au and s and p states of Al, lies just above the VBM. There are two shallow states near the VBM and one deep state at the Fermi level of AuPt adsorbed to MoS2 which makes the system metal. The deep state at the Fermi level is dominated by d states of Mo and p states of S. The degeneracy on the valence band is because of the d states of Au. In the case of AuPd adsorption, there are three shallow states above the VBM and one deep state at the Fermi level as similar with AuPt adsorption. This state, which is responsible for the metallic character of the system, consists of s electrons of Au, d states of Mo, and p states of S atom predominantly. The charge density differences are shown in Figure 3. Red and green shaded areas show charge accumulation and depletion regions, respectively. It can be seen sequential the red and green areas between adsorbate atoms (Ag, Au, Pt, Pd, Cu, Al) and S atoms of MoS2 and the red areas especially around the midpoint of bonds indicating covalent bonding. The covalent bonding of these atoms with S atom has also been reported in literature [47,56,57,58,59,60,61]. The bonding of these atoms with S atom maintains its covalent nature after adsorption. However, the bonding 6

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mechanism between dimers and MoS2 is a more complex interaction of polarization, covalent, and ionic effects. The orientation of dimers on MoS2 also confirm the polarization between them. According to the Bader analysis, a very small charge transfer is observed from adsorbate atoms to S atom of MoS2 . This is also consistent with covalent type bonding. AuCu and AuPd adsorptions lead about 0.15 electrons charge transfer, while AuAl adsorption results in about 0.8 electrons transfer from dimer to MoS2 . So that, AuAl dimer displays the largest charge transfer to MoS2 surface. There is almost no charge transfer from AuAg and AuPt dimers to MoS2 , but ≈0.15 electrons are transferred between Au-Ag and Au-Pt atoms. However, it is seen that the charge transfers between dimer atoms result in a remarkable electrical polarization in all dimers. This feature can present an active site to interact with adatoms and molecules.

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4 Conclusion

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In summary, we studied the effects of bimetallic dimer adsorption on monolayer MoS2 . We found that whereas monolayer MoS2 maintains its semiconductor characteristics with decreased band gap values after AuAg, AuCu, and AuAl adsorptions, it becomes metallic by AuPt and AuPd adsorptions. All systems were found to be nonmagnetic. AuPt adsorbed to MoS2 system is the most stable structure whereas AuAl adsorption is the weakest adsorption among the considered dimer adsorbed systems. AuAg has to overcome an energy barrier of 0.1 eV for the most energetic configuration which binds over Au atom to S atom of MoS2 compared with binding over Ag atom to S atom. There is almost no charge transfer from AuAg and AuPt dimers to MoS2 . The bonds between S atoms and adsorbate atoms have covalent character in nature. The engineered band gap of MoS2 can lead to design novel electronic devices with specific band gaps such as LEDs, semiconductor lasers; photodetectors, solar cells, gas sensors etc. AuAg + MoS2 system with a band gap of 1.16 eV can be useful for solar cells (to achieve high photocurrent) which matches the minimum of the solar energy spectrum. AuCu + MoS2 can be suitable for photodetectors which desire a band gap especially lower than ≈1 eV. Because of metallic property of MoS2 after AuPt and AuPd adsorptions, it can be said that its electrical conductivity increased compared with bare MoS2 . Our results can provide instrumental knowledge especially for the future design of MoS2 based electronics.

5 Acknowledgements

We would like thank to TUBITAK ULAKBIM, High Performance and Grid Computing Center for numerical calculations. Y.K and O.U.A acknowledges support 7

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from the scientific research projects of Adnan Menderes University (BAP-ADU) under the project number (MF-15003).

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[47] W. Li, M. Guo,G. Zhang,Y.W. Zhang, Edge-Specic Au/Ag Functionalization-Induced Conductive Paths in Armchair MoS2 Nanoribbons , Chem. Mater. 26 (2014) 5625. [48] F.E.Brenker, G. Jordan, Nanoscopic Approaches in earth and planetary sciences, Cambridge (2010).

ip t

[49] M. H. Hakala, O. H. Pakarinen, and A. S. Foster, First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces, Phys. Rev. B 78 (2008) 045418.

cr

[50] M. W. Heaven, A. Dass, P. S. White, K. M. Holt, R. W. Murray, Crystal structure of the gold nanoparticle [N(C8 H17 )4 [Au25 (SCH2CH2 Ph)18 ], J. am. Chem. Soc. 130 (2008) 3754.

an

us

[51] S. G. Almeida, J.L. Hubbard, N. Farrell, Spectroscopic and structural properties of dimethyl-, diphenyl- and methylphenylsulfoxide platinum complexes. The crystal and molecular structures of cis-dichlorobis(methylphenylsulfoxide)-platinum(II) and potassiumtrichloro(phenylsulfoxide)platinum(II)-acetone solvate, Inorg. Chim. Acta 193 (1992) 149.

M

[52] V.W. Yam, R.P.Tang, K.M. Wong, C. C. Ko, K.K Cheung, Synthesis and ion-binding studies of a platinum (II) terpyridine complex with crown ether pendant. X-ray crystal structure of [Pt(trpy)(S-benzo-15-crown-5)]PF6, Inorg. Chem. 40 (2001) 571.

te

d

[53] A. I. Matesanz, J M. Perez, P. Navarro, J. M. Moreno, E. Colacio, P. Souza, Synthesis and characterization of novel palladium(II) complexes of bis(thiosemicarbazone). Structure, cytotoxic activity and DNA binding of Pd(II)benzyl bis(thiosemicarbazonate), J. Inorg. Biochem. 76 (1999) 29.

Ac ce p

[54] M. Ralle, S. Lutsenko, N. J. Blackburn, X-ray Absorption Spectroscopy of the Copper Chaperone HAH1 Reveals a Linear Two-coordinate Cu(I) Center Capable of Adduct Formation with Exogenous Thiols and Phosphines, J. Bio. Chem. 278 (2003) 23163. [55] E. Kaxiras, Atomic and Electronic Structure of Solids, Cambridge University Press Newyork (2003). [56] A. Longo,G. Carotenuto, M. Palomba, S.D. Nicola, Dependence of optical and microstructure properties of thiol-capped silver nanoparticles embedded in polymeric matrix, Polymers 3 (2011) 1794. [57] Z. Yin, Q, He, X. Huang, J. Zhang, S. Wu, P. Chen, G. Lu, P. Chen, Q. Zhang, Q. Yana, H. Zhang, Real-time DNA detection using Pt nanoparticle-decorated reduced graphene oxide eld-effect transistors, Nanoscale 4 (2012) 293. [58] G-Wu Yang, G-Yu Gao, G-Yu Zhao, H-Lin Li, Effective adhesion of Pt nanoparticles on thiolated multi-walled carbon nanotubes and their use for fabricating electrocatalysts, Carbon 45 (2007) 3036. [59] R.J.T Houk, K.J. Wallace, H.S. Hewage, E.V. Anslyn, A colorimetric chemodosimeter for Pd (II): a method for detecting residual palladium in cross-coupling reactions, Tetrahedron 64 (2008) 8271.

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[60] R. Langen, I.J. Chang, J.P. Germanas, J.H. Richards, Electron tunneling in proteins: coupling through a beta strand , Science 268-5218 (1995) 1733.

Ac ce p

te

d

M

an

us

cr

ip t

[61] G. H. Lu, M. Kohyama, R. Yamamoto, First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms, Phil. Mag. Lett. 83 (2003) 159.

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Page 14 of 19

ip t cr us

2

an Eg,d= 1.66 eV

0

M

Energy (eV)

1

ed

-1

-2

Γ

Μ

Mo s Mo p Mo d Ss Sp Total Κ

Γ

p dos

Ac ce

pt

Fig. 1. Adsorption sites, band structure, and projected density of states (PDOS) of bare MoS2 monolayer.

13

Page 15 of 19

Cu

µ=0.0 µB

-2

µ=1.0 µB

0

us EHLG= 1.12 eV

-2

Pd

µ=0.0 µB

0

EHLG= 2.38 eV

-2 -4

0

EHLG= 1.29 eV

Ac ce

µ=1.0 µB

2

Al

pt

c) Au

Energy (eV)

o

e) Au

2

ed

-4

2.52 A

an

Pt

o

2.38A

M

b) Au

-4

2

Energy (eV)

o

EHLG= 1.86 eV

-2

-4

2.50A

0

ip t

d) Au

EHLG= 2.06 eV

2

cr

0

2.37A

Energy (eV)

µ=0.0 µB

Energy (eV)

Ag

a) Au

o

2

Energy (eV)

o

2.56 A

-2 -4

Fig. 2. Bond length, magnetization, molecular orbital energy scheme with HOMO-LUMO gap values of the dimers considered in this work. Red and green energy levels belong to spin up and down states, respectively.

14

Page 16 of 19

a)

d) AuCu

AuPt

c)

AuPd

e)

AuAl

Ac ce

pt

ed

M

an

b)

us

cr

ip t

AuAg

Fig. 3. Geometrical structures and charge differences of a) AuAg, b) AuPt, c) AuPd, d) AuCu, e) AuAl adsorbed MoS2 monolayer. Red and green colours indicate the area of charge accumulation and depletion regions, respectively.

15

Page 17 of 19

Ef

0

Ag s Ag d

-1 -2 -3 2

-1

-3 2

0

Pt s Pt d

-1

Energy (eV)

1

an

e)

Cu s Cu d

1 0

Al s Al p

-1 -2

-3 2

ed

-3

1 0 -1

pt

Energy (eV)

0

-2

-2

c)

1

M

Energy (eV)

b)

ip t

1

2

cr

d)

us

2

Energy (eV)

Energy (eV)

a)

Ac ce

-2

Pd s Pd d

Κ

Γ

p dos

Mo s

Sp

Au s

Total

Mo p

Ss

Au p

bare MoS2

Γ

Μ

Mo d

Au d

-3

Γ

Μ

Κ

Γ

p dos

Fig. 4. DOS and band diagram of a) AuAg, b) AuPt, c) AuPd, d) AuCu, e) AuAl adsorbed MoS2 monolayer. Fermi levels are set to zero and displayed with red dashed line.

16

Page 18 of 19

Ea (eV)

AuAg

1.33

˚ R (A) dAu−S = 2.32

˚ ddimer (A) dAu−Ag = 2.56

EG (eV)

ρ

us

Dimer

cr

ip t

Table 1 Adsorption Energy (Ea ), adsorbate-monolayer bond length (R), dimer bond length (ddimer ), smallest band gap (EG ) and charge transfer (ρ ) values of the AuAg, AuPt, AuPd, AuAl, AuCu adsorbed MoS2 . “ave” denotes average values of related bonds. Minus sign indicates charge decrease on atoms.

1.16

Au: -0.17

3.15

ave = 2.32 dPt−S

dPt−Mo = 2.78

1.73

dAu−S = 2.47

dAu−Pd = 2.52

ed

AuCu

2.23

metal

Pt: -0.15

metal

Au: -0.13 Pd: -0.05

ave = 2.37 dPd−S

MoS2 : 0.18

ave = 2.32 dCu−S

Ac ce 0.41

Au: 0.13

MoS2 : -0.02

dAu−Cu = 2.42

0.92

dCu−Mo = 2.84

AuAl

MoS2 : 0.02

dPd−Mo = 2.88

pt

AuPd

dAu−Pt = 2.50

M

AuPt

an

Ag: 0.16

ave = 2.65 dAl−S

Au: 0.07 Cu: -0.22 MoS2 : 0.15

dAu−Al = 2.43

0.77

Au: -0.22 Al: -0.57 MoS2 : 0.79

17

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