Ag Functionalization-Induced Conductive Paths in

Article pubs.acs.org/cm

Edge-Specific Au/Ag Functionalization-Induced Conductive Paths in Armchair MoS2 Nanoribbons Weifeng Li,† Meng Guo,‡ Gang Zhang,*,† and Yong-Wei Zhang*,† †

Institute of High Performance Computing, A*STAR, Singapore, 138632 National Supercomputing Center in Jinan, Jinan, China, 250101

‡

ABSTRACT: We perform first-principles calculations to investigate the electronic band structures of chemically functionalized armchair MoS2 nanoribbons by Au and Ag. It is found that the absolute values of the binding energies of these atoms on the edge of armchair MoS2 ribbons are much higher than their surface counterparts, greatly favoring the edge functionalization. Importantly, such edge functionalization causes a transition of the ribbons from semiconducting to metallic, and the conducting path is identified to be via Mo-dominated nonlocalized edge states. Surprisingly, the Fermi velocity of the charge carriers in the edge-functionalized ribbons not only is ultrahigh, comparable to that of graphene, but also can be precisely linearly tuned by varying edge functionalization concentration. The intriguing properties of these edge-functionalized MoS2 ribbons, such as high edge functionalization specificity, semiconducting-to-metallic transition, and also high and tunable Fermi velocity, may open up whole new possibilities for fabricating novel MoS2-based electronic devices.

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INTRODUCTION Recently, two-dimensional (2D) materials have attracted great attention due to their intriguing mechanical, thermal, optical, as well as electronic properties. So far, graphene is by far the most studied 2D material.1 It is well-known that pristine graphene is a semimetal with no band gap and its electronic band structure is characteristically in the form of a Dirac cone. For many electronic applications, however, introduction of a band gap in graphene is often required. To this aim, various band structure engineering techniques, such as cutting graphene into nanoribbons and chemical functionalization, etc., have been employed.2−8 Very recently, 2D transition metal dichalcogenides (TMDs) offer an alternative candidate for the development of nanoscale electronic devices.9,10 Unlike graphene, MoS2, a member of the TMD family, is a semiconductor with a sizable band gap. Depending on the number of stacking layers, the band gap varies from 1.29 eV (indirect-gap) in the bulk crystal to about 1.8 eV (direct-gap) in the monolayer form.11 Currently, monolayer MoS2 has been regarded as a highly competitive candidate for field-effect transistors (FETs) with a high on/off ratio. For many novel applications, other forms of MoS2, such as MoS2 nanotubes,12 nanoflakes,13 and nanoribbons,14,15 are also of great interest because their electronic properties can be different from and thus complementary to their monolayer counterpart. Recently, a MoS2 nanoribbon (MoS2NR) was © XXXX American Chemical Society

successfully synthesized by an electrochemical/chemical route.16,17 It was shown that the electronic properties of MoS2NRs are dependent on their structure and dimensionality. For example, the MoS2NRs with a zigzag edge (Z-MoS2NRs) are metallic.14,15 However, because of the presence of different edge terminations (S or Mo terminated) and also edge reconstructions, Z-MoS2NRs exhibit complicated and volatile electron conducting behavior. Since electronic device applications require stable and well-defined edge structures and robust electronic properties, Z-MoS2NRs are generally not suitable for such applications. In contrast, MoS2NRs with an armchair edge (A-MoS2NRs) are semiconducting, and their edge structure is unique and well-defined.14,15,18,19 Hence, A-MoS2NRs hold great promise for their applications in high-performance fieldeffect transistors. However, to develop A-MoS2NR-based electronic devices, stable and robust conducting interconnects are required. An important question is: Can we engineer AMoS2NRs and turn them into a stable and robust conducting interconnect? MoS2 edges have been shown to exhibit strong chemical activities.20−23 Thus, engineering the edges of A-MoS2NRs by chemical functionalization provides a possible route to address Received: June 16, 2014 Revised: September 10, 2014

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dx.doi.org/10.1021/cm5021756 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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mirror interactions. Atomic relaxation was performed until the change of total energy was less than 0.01 meV and all the forces on each atom were less than 0.01 eV/Å, which was sufficient to obtain relaxed structures. A k-point sampling of 1 × 1 × 10 was used for the structure relaxation, while a denser mesh of 1 × 1 × 50 was used to calculate energies, density of states (DOS), and band structures. For Au migration on A-MoS2NR, we have performed minimum energy path profiling using the climbing image nudged elastic band method as implemented in the VASP transition state tools.30,31 The structural convergence criteria were similar to that used in the above-mentioned structure optimization.

the above question. It is well-known that thiols can form a robust self-assembled monolayer on Au/Ag metal nanoparticle surfaces and Au/Ag metal clusters can be synthesized by spontaneous self-assembly of thiols on the metal nanoparticles surfaces.24 In these processes, the presence of strong metal− sulfur covalent bonds is not only important but also essential (the gold−sulfur bond energy is 4.34 eV/atom and the silver− sulfur bond energy is 2.25 eV/atom25). Therefore, the sulfurterminated edge of A-MoS2NRs provides potentially energetically favorable sites for Au/Ag functionalization. In this work, we report an effective and practical route to tune the electronic band structures of A-MoS2NRs by chemically functionalizing their edges using noble Au/Ag atoms. We find that the edge functionalization by Au/Ag is significantly energetically more favorable than their surface adsorption as the absolute values of the binding energy of Au/ Ag on the edge are much higher than that on the surface. Importantly, we also find that such edge functionalization causes a transition of A-MoS2NRs from semiconducting to metallic. Surprisingly, we also find that the Fermi velocity of the charge carriers in Au/Ag-functionalized A-MoS2NRs not only is as high as that in graphene but also can be linearly controlled by varying the edge functionalization concentration.

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RESULTS AND DISCUSSION Targeted Au/Ag Chemical Functionalization at AMoS2NR Edges. We first study the atomic structure and stability of Au/Ag atoms adsorbed on the edges of A-MoS2NRs. For A-MoS2NRs, their two edges can be symmetric (S-AMoS2NR, Figure 1a) or asymmetric (U-A-MoS2NR, Figure 1c), depending on the width of the ribbon. In the present study, we consider both scenarios: S-A-MoS2NR with a width of 1.65 nm and U-A-MoS2NR with a width of 1.45 nm. Since the absorption of Li atoms on MoS2NRs was investigated extensively,32 we also study the adsorption of Li atoms and their effects on electronic band structure and compare them with that of Au and Ag atoms. The initial site of the adsorbate (Au/Ag/Li) was positioned to replace a Mo atom at the edges of A-MoS2NRs. Hence, an edge S atom in the ribbons forms six bonds with 3-fold rotational symmetry (from top view). The metal-functionalized A-MoS2NRs after structure optimization are shown in Figure 1b,d. Each metal atom forms two covalent bonds with two S atoms from the top and bottom layers, respectively. The bond lengths are 2.51 Å for Au−S, 2.53 Å for Ag−S, and 2.29 Å for Li−S, which are also summarized in Table 1. Interestingly, it is found that the distance of Au/Ag

COMPUTATIONAL METHODS

All the first-principles calculations were performed using the Vienna Ab initio Simulation Package (VASP).26,27 Projector-augmented-wave (PAW) potentials28 were used to take into account the electron−ion interactions, while the electron exchange-correlation interactions were treated using a generalized gradient approximation (GGA)29 in the scheme of Perdew−Burke−Ernzerhof. A plane-wave cutoff of 400 eV was used for all the calculations. All atomic positions and the lattice vector along the ribbon growth direction (z, as indicated by the arrows in Figure 1a,c,e) were fully optimized using a conjugate gradient algorithm to obtain the unstrained configuration. In the directions perpendicular to z, vacuum spaces of at least 20 Å were kept to avoid

Table 1. Structural Parameters of Metal Atom Binding on the Edge of A-MoS2NRa dA−S (Å) dA−Mo (Å)

Au

Ag

Li

2.51 2.68

2.53 2.71

2.29 2.95

a

dA−S stands for the distance between the adsorbate and its nearest S, and dA−Mo stands for the distance between the adsorbate and its nearest Mo, as indicated in Figure 1.

with its nearest Mo as labeled in Figure 1e is almost comparable to that with S, indicating that a weak bond formed between the noble metal atoms with Mo. On the contrary, the distance between a Li atom with its nearest Mo atom is large, indicating that there is no bond formed between them. In order to evaluate the binding strength of these metal adsorbates on the edges of A-MoS2NRs, we define the binding energy, Eb, as E b = (Ecomplex − nmetal × Emetal − EA‐MoS2NR )/nmetal

(1)

where Ecomplex is the total energy of the A-MoS2NR-metal complex, and Emetal and EA‑MoS2NR are the total energies of the metal atom and bare A-MoS2NR, respectively. The denominator nmetal is the number of metal atoms (2 for ribbon edge binding and 1 for ribbon surface binding in the following discussion). As a consequence, Eb has a unit of eV/atom, which is the energy released during the atomic binding. Hence, a larger absolute value (negative value) of the Eb indicates an energetically more stable binding.

Figure 1. Structures of armchair MoS2NRs with bare symmetric edges (a), metal-functionalized symmetric edges (b), bare asymmetric edges (c), and metal-functionalized asymmetric edges (d). The topologies of the metal−S bonds are shown in (e). The dashed lines in (b) and (d) indicate the primitive cell. The blue arrows in (a), (c), and (e) demonstrate the ribbon growth direction. dA−S indicates the distance between the adsorbate and its nearest S atom, and dA−Mo indicates the distance between the adsorbate and its nearest Mo atom. B



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Figure 2. (a) The binding energies of Au/Ag/Li metal atoms on the edges of S-A-MoS2NR (S-R) and U-A-MoS2NR (U-R). (b) The binding energies of Au/Ag/Li on the central hexagonal ring of the surface of S-A-MoS2NR in Figure 1b. (c) The bonding topologies of four representative binding sites. TS: the top site directly above a S atom. H: the hollow site directly above the center of the hexagon. B: the bridging site between two S atoms. TMo: the top site directly above a Mo atom.

Figure 3. (a) Schematic representation of two diffusion paths: P1: T1 → T2 → T3 → T6 → E; and P2: T4 → T5 → T6 → E. The values in the brackets are the binding energies relative to that of the site E. (b) Energy profile for the diffusion path P1. (c) Energy profile for the diffusion path P2. The values of the migration energy barrier are labeled at the top of each corresponding peak.

As shown in Figure 2a, for the Au atom, the value of Eb is −3.01 and −2.97 eV/atom for S-A-MoS2NR and U-AMoS2NR, respectively. For the Li atom, the binding is also strong, with Eb being −3.09 and −3.01 eV/atom for S-AMoS2NR and U-A-MoS2NR, respectively, in good agreement with the previous work.32 Compared with Au atoms, Ag atoms form relatively weaker chemical bonds with S, with Eb being −2.42 and −2.35 eV/atom for S-A-MoS2NR and U-AMoS2NR, respectively. It is noted that, for all the edge functionalization, the binding strength on S-A-MoS2NR is slightly lower than that on U-A-MoS2NR, which may be attributed to the intrinsically higher structural symmetry of the former, which may, in turn, cause less structure distortion during chemical functionalization. Overall, the large absolute values of Eb, which are comparable with that reported previously for S−Au/Ag covalent bonds,25 indicate that the Au/Ag atoms indeed form covalent bonds with S atoms at the edges of A-MoS2NRs. Because of the large surface area of nanoribbon, there are more surface binding sites for Au/Ag which may compete with the edge binding.32,33 Next, we explore the adsorption of Au/ Ag/Li atoms on the surface region of A-MoS2NR. We have placed the adsorbate above the central hexagonal ring in the primitive cell, as shown in Figure 1b, and found four representative binding sites, as indicated in Figure 2c: (1) the top site directly above a S atom (abbreviated as TS site); (2) the

hollow site directly above the center of the hexagon (H site); (3) the bridging site between two S atoms (B site); and (4) the top site directly above a Mo atom (TMo site). After structural optimization, the adsorbate atom is found to be able to form one to three bonds with S, depending on its binding site. In contrast, the TS and B sites for Li are unstable as the Li atom would like to migrate to the nearby H or TMo site during optimization. The binding energies of Au/Ag/Li atoms at different binding sites are summarized in Figure 2b, which clearly show the different binding strengths of the three candidates under examination. It is seen evidently that Au/Ag on the MoS2NR surface has much weak binding energy compared to the strong edge binding. Over all, for Au and Ag, the binding energy differences between edge and surface adsorption can be as large as 2.0 eV/atom, suggesting a significantly high edge binding specificity for Au and Ag. On the contrary, the binding energies for Li atoms on MoS2NR, −2.27 eV/Li (TMo site) and −2.24 eV/Li (H site), are comparable to the edge binding energy. Thus, there is no such specificity for Li functionalization. To examine the edge effect on the Au/Ag adsorption, we systematically explored the Au’s binding at different sites of SA-MoS2NR, as illustrated in Figure 3a. Considering the symmetry of the primitive cell, there are seven representative binding sites: six TS sites (T1−T6 as labeled in Figure 3a) for the basal plane and one edge binding site (E). Taking the E site C



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as the reference state, the relative values of Eb for T1−T6 are summarized in Figure 3a. It is clear that the edge T6 has a significantly lower binding energy than those from T1 to T5. Generally, a trend of stronger binding toward the ribbon edge is found. Hence, the MoS2NR edge acts like an “attractor” for Au atoms if they landed on the ribbon surface initially. For wide ribbons, as there are many more surface sites than edge sites, most dopant atoms may be randomly adsorbed on the surface. Therefore, for Au/Ag atoms that initially landed on the surface, a small diffusion barrier is required in order for them to diffuse to the edge. In the following, we selected two transverse diffusion paths for the Au atom, that is, P1: T1 → T2 → T3 → T6 → E; and P2: T4 → T5 → T6 → E, to migrate toward the MoS2NR edge from the center region, and calculated the migration energy profiles. The results for the two paths are shown in Figure 3b,c, respectively. It is seen that the highest (T1 → T2) and lowest (T3 → T6) energy barriers are found to be 0.11 and 0.06 eV, respectively. Considering the small binding energies, it is reasonable to expect that the diffusion of the Au atom is highly likely. Similar to the case for binding energies, the diffusive activation barriers are also sensitive to the binding position, that is, whether they are close to the edge or not. More specifically, within T1−T6, the Au’s diffusive barrier decreases gradually from the ribbon center toward the edge. Hence, the Au atom prefers to diffuse from the center to the edge. At the ribbon extreme edge, a slightly higher energy barrier of 0.35 eV is found from the T6 site to the E site. However, the energy that was being released from a previous migration step T3 → T6 (or T5 → T6) reaches to a value of 0.67 eV (or 0.72 eV); thus, this energy is expected to help Au to conquer this slightly higher activation barrier. Recently, Shi et al. reported the formation of Au nanoparticles on the edge of MoS2 flakes.34 Hence, our work here provides a possible underlying physical origin for the strong edge selectivity of the observed Au nanoparticle flakes.34 Moreover, the edge-targeted diffusion was also found for Li binding on graphene nanoribbons35 and MoS2NRs.32 It is worth mentioning that, for the case of narrow nanoribbons, surface adsorption may be rare due to the considerably small diffusion barrier toward the edge. However, for ribbons with a large width, the situation is complicated because the large number of surface dopants may induce bending (folding) instability of nanoribbons due to adsorptioninduced surface stress.36,37 Hence, for wide ribbons, postprocessing may be needed, for instance, by annealing at high temperature, as demonstrated in ref 36, which may help the surface adsorbates to desorb or to diffuse toward the ribbon edges. Although chemical functionalization is a simple and effective method to change the physical properties of nanoscale materials, the resulting property change is often sensitively dependent on functionalization site. Traditionally, in the experiment, how to precisely control functionalization sites so as to achieve the desired properties is a critical issue. Our results here show that, with respect to surface adsorption, Au/ Ag atoms predominantly prefer to bind to S atoms at the edges of A-MoS2NRs. This high edge binding specificity makes it possible to realize pure edge functionalization of A-MoS2NRs. Electronic Structure of Edge-Functionalized AMoS2NRs by Au/Ag. As edge functionalization by Au/Ag shows considerably more stability than their surface functionalization, next, we only focus on the effect of Au/Ag edge functionalization on the electronic properties of A-MoS2NRs.

As shown in Figure 4, bare A-MoS2NR is a semiconductor with a band gap of 0.55 and 0.47 eV for S-A-MoS2NR (Figure 4a)

Figure 4. Electronic band structures of (a) bare S-A-MoS2NR, (b) SA-MoS2NR + Au, (c) S-A-MoS2NR + Ag, (d) bare U-A-MoS2NR, (e) U-A-MoS2NR + Au, and (f) U-A-MoS2NR + Ag. The blue dashed lines at zero indicate the Fermi level, EF.

and U-A-MoS2NR (Figure 4d). In addition, the valence band (VB) and the conduction band (CB) are localized states as the four branches associated with these two orbitals are relatively flat. Just above the CB, there are two delocalized empty bands with a large slope, as indicated with blue arrows in Figure 4a. After Au/Ag edge functionalization, these two empty bands become half-filled, which effectively lowers their energy to cross the Fermi level EF (Figure 4b,c,e,f), thus exhibiting metallic behavior. Comparing the cases of the same adsorbate atom on S-AMoS2NR or U-A-MoS2NR, it appears that the edge symmetry (symmetric or asymmetric) does not show any apparent effect, which is expected since there is little electronic coupling between the two edges (more discussion will be given later). Moreover, there is no significant difference in the electronic band structure between Ag and Au edge functionalization, indicating that the conducting mechanism of the two noble metals is similar. In the following discussion, we only focus on S-A-MoS2NRs. As illustrated by the charge density difference in Figure 5a, it is clear that there is considerable electron transfer from Au atoms to MoS2NR edge atoms, which should account for the metallic bands filling at EF. Structures (b) and (c) in Figure 5 illustrate the spatial distributions of the two metallic bands at EF, respectively. It is seen clearly that the electron conductive channels are composed of edge states, indicating the importance of the edge Au/Ag dopant. In addition, the two edge states are completely separated, indicating the complete decoupling of the two isolated conductive channels, and thus explaining why the effect of edge symmetry (symmetric or asymmetric) is insignificant. In order to quantify the importance of different edge atoms, we have selected four D



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Figure 5. (a) Charge density difference between Au and A-MoS2NR. (b, c) Charge density of the two half-filled bands at the EF of A-MoS2NR with Au. The isosurfaces are 0.02 e/bohr3. The four edge atoms labeled as Mo-1, Mo-2, S, and Au are used for the electron orbital analysis.

atoms, that is, the Au dopant, the edge S, the Mo at the extreme edge (labeled as Mo-1), and the Mo next to the extreme edge (labeled as Mo-2) for the analysis of electron density of states (DOS). In order to understand the origin of the changes in band structure after the edge functionalization, next we analyze the compositions of frontier orbitals. For the cases of S-A-MoS2NR with/without Au binding, the density of states (DOS) and the projections of Mo, S, and Au atoms are summarized in Figure 6. For bare S-A-MoS2NR, as shown in Figure 6a,b, the two delocalized empty bands right above the CB are purely edge states consisting of dxy/dx2−y2 hybrid orbitals from Mo-1. For SA-MoS2NR with Au binding, as shown in Figure 6c, the electron transfer from Au to A-MoS2NR fills these orbitals, which effectively lowers their energy to the EF level. Spatially, the metallic states at the EF level are found to be mainly contributed by the edge Mo atoms, especially Mo-1. Mo-2 and dopant Au atoms also have their contributions to the metallic states. However, the contribution from the edge S atom is found to be negligible. This is consistent with the isosurface analysis where the edge conductive channels are found only at the ribbon edge atoms. Interestingly, it is also found that the composition of the metallic states changes from the dxy orbital dominated (Figure 6b) to the dx2−y2 orbital dominated (Figure 6d). Fermi Velocity Engineering by Varying Au/Ag Edge Functionalization Concentration. From the above analysis, it is found that the metallic states originate from the downward shifting of the higher conduction states in the bare ribbon, more specifically, the bands 1 and 2, as indicated by the blue arrows in Figure 4a. Considering that these bands tend to accept electrons from the Au dopant and their slope changes with the energy level, as demonstrated in Figure 7, an interesting scenario arises: if we can precisely control the electron filling of these bands (for state a, less electron transfer, and for state c, more electron transfer, as illustrated in Figure 7), different locations of the EF level can be realized, leading to the precise control of the Fermi velocity, which corresponds to the slope of bands at EF. This may be technologically important as the Fermi velocity is one of the fundamental physical quantities that bears important information in a variety of materials properties. Two cases of lower edge functionalization concentrations are considered: half-functionalized, that is, one metal atom shared

Figure 6. (a) DOS of edge Mo-1, Mo-2, and S atoms of bare S-AMoS2NR (SR). (b) PDOS of Mo-1 d orbitals of bare S-A-MoS2NR. (c) DOS of edge Mo-1, Mo-2, S, and dopant Au atoms of S-AMoS2NR + Au (SR+Au). (d) PDOS of Mo-1 d orbitals of S-AMoS2NR + Au. The positions of Mo-1, Mo-2, S, and Au can be found in Figure 5b. The states with blue arrows in (b) and (d) are correlated with the bands also highlighted with blue arrows in Figure 4a,b. Fermi level is set to be zero.

by two primitive cells (abbreviated as MoAu1/2, Figure 8a), and one-third-functionalized, that is, one shared by three primitive cells (abbreviated as MoAu1/3, Figure 8b). The electronic band structures of A-MoS2NRs with the two lower functionalization concentrations are shown in Figure 8c,d. It is clear that the half-filled bands are still present in the E



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CONCLUSION The structural and electronic properties of armchair MoS2 nanoribbons functionalized by noble metal atoms (Au and Ag) were investigated by first-principles calculations. For both Au and Ag atoms, the absolute values of the binding energy on the edge of the MoS2 nanoribbon are around 2.0 eV/atom higher than that adsorbed on the MoS2 sheet (nanoribbon) surface, which demonstrates a significantly high edge specificity for Au and Ag binding. Importantly, because of Au/Ag edge functionalization, electron transfers from Au/Ag to MoS2 atoms effectively lower the two nonlocalized hybrid orbitals (dxy/dx2−y2) of Mo atoms to the Fermi level, making the edgefunctionalized A-MoS2NRs to be metallic. Surprisingly, the Fermi velocity of the charge carriers of the Au/Ag edgefunctionalized A-MoS2NRs is comparable with that of graphene and is linearly dependent on the edge functionalization concentration. The intriguing properties of Au/Ag edgefunctionalized A-MoS2NRs revealed here, such as high edge functionalization specificity, semiconducting-to-metallic transition, and also high and tunable Fermi velocity, not only enrich our fundamental physical understandings of 2D materials but also open up whole new possibilities for fabricating novel 2D materials-based electronic devices.

Figure 7. Slope of the delocalized empty bands just above the CB, as indicated in Figure 4a, with respect to the energy level. The insets a to c illustrate the different filling levels of electrons.

two lower concentration cases. However, the slope becomes flattened as the concentration decreases, indicating a change in electron conductivity. Similar effects are also found for Agfunctionalized A-MoS2NRs (not shown). Since the band profile near EF is linearly dependent on the k-point distance, the electron conducting properties can be qualitatively evaluated by Fermi velocity, νF, of the Fermion:

νF = E(k)/ℏk

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Z.). *E-mail: [email protected] (Y.-W.Z.).

(2)

As summarized in Figure 8e, the value of νF for the highest metal concentration is 1.66 × 106 m/s for Au-functionalized AMoS2NR, and 1.75 × 106 m/s for Ag-functionalized AMoS2NR. These values are almost comparable to that of graphene, which is 2.5 × 10 6 m/s from theoretical predictions38,39 and 1 × 106 m/s from experiment.40 When the metal concentration decreases to half-functionalized, the value of νF dramatically decreases to ∼1 × 106 m/s. For the case of one-third-functionalized, the value of νF further drops to 0.46 × 106 m/s for Au-functionalized A-MoS2NR and 0.36 × 106 m/s for Ag-functionalized A-MoS2NR. Within the range of metal concentration investigated, the value of νF is almost linearly dependent on the metal atom concentration for both Au- and Ag-functionalized A-MoS2NR (Figure 8e). This observation suggests a practical way to fabricate A-MoS2NR conductors with the desired electron conducting behavior through controlling the edge functionalization concentration.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the A*STAR Computational Resource Centre through the use of its high performance computing facilities.

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REFERENCES

(1) Neto, A. C.; Guinea, F.; Peres, N.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 2009, 81 (1), 109−162. (2) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 2006, 97 (21), 216803. (3) Long, M.-Q.; Tang, L.; Wang, D.; Wang, L.; Shuai, Z. J. Am. Chem. Soc. 2009, 131 (49), 17728−17729. (4) Huang, B.; Son, Y.-W.; Kim, G.; Duan, W.; Ihm, J. J. Am. Chem. Soc. 2009, 131 (49), 17919−17925.

Figure 8. (a) The atomic structure of Au half-functionalized symmetric ribbon (MoAu1/2). (b) The atomic structure of Au one-third-functionalized symmetric ribbon (MoAu1/3). (c) The electronic band structure of MoAu1/2. (d) The electronic band structure of MoAu1/3. (e) Fermi velocity with respect to edge functionalization concentration. F



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(5) Han, M. Y.; Ozyilmaz, B.; Zhang, Y. B.; Kim, P. Phys. Rev. Lett. 2007, 98 (20), 206805. (6) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Science 2008, 319 (5867), 1229−1232. (7) Jiao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Nature 2009, 458 (7240), 877−880. (8) Xia, F.; Farmer, D. B.; Lin, Y.-m.; Avouris, P. Nano Lett. 2010, 10 (2), 715−718. (9) Grigorieva, A. G. I. Nature 2013, 499 (7459), 419−425. (10) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5 (4), 263−275. (11) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Phys. Rev. Lett. 2010, 105 (13), 136805. (12) Li, W.; Zhang, G.; Guo, M.; Zhang, Y.-W. Nano Res. 2014, 7 (4), 1−10. (13) Wang, X.; Feng, H.; Wu, Y.; Jiao, L. J. Am. Chem. Soc. 2013, 135 (14), 5304−5307. (14) Li, Y.; Zhou, Z.; Zhang, S.; Chen, Z. J. Am. Chem. Soc. 2008, 130 (49), 16739−16744. (15) Pan, H.; Zhang, Y.-W. J. Mater. Chem. 2012, 22 (15), 7280− 7290. (16) Li, Q.; Newberg, J.; Walter, E.; Hemminger, J.; Penner, R. Nano Lett. 2004, 4 (2), 277−281. (17) Wang, Z.; Li, H.; Liu, Z.; Shi, Z.; Lu, J.; Suenaga, K.; Joung, S.K.; Okazaki, T.; Gu, Z.; Zhou, J. J. Am. Chem. Soc. 2010, 132 (39), 13840−13847. (18) Ataca, C.; Sahin, H.; Akturk, E.; Ciraci, S. J. Phys. Chem. C 2011, 115 (10), 3934−3941. (19) Cai, Y.; Zhang, G.; Zhang, Y.-W. J. Am. Chem. Soc. 2014, 136 (17), 6269−6275. (20) Kibsgaard, J.; Lauritsen, J. V.; Lægsgaard, E.; Clausen, B. S.; Topsøe, H.; Besenbacher, F. J. Am. Chem. Soc. 2006, 128 (42), 13950− 13958. (21) Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsøe, H.; Clausen, B. S.; Lægsgaard, E.; Besenbacher, F. Nat. Nanotechnol. 2007, 2 (1), 53−58. (22) Lauritsen, J. V.; Nyberg, M.; Vang, R. T.; Bollinger, M.; Clausen, B.; Topsøe, H.; Jacobsen, K. W.; Lægsgaard, E.; Nørskov, J.; Besenbacher, F. Nanotechnology 2003, 14 (3), 385. (23) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. J. Am. Chem. Soc. 2011, 133 (19), 7296−7299. (24) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801−802. (25) Longo, A.; Carotenuto, G.; Palomba, M.; Nicola, S. D. Polymers 2011, 3 (4), 1794−1804. (26) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54 (16), 11169− 11186. (27) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6 (1), 15− 50. (28) Blochl, P. E. Phys. Rev. B 1994, 50 (24), 17953. (29) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46 (11), 6671−6687. (30) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. J. Chem. Phys. 2000, 113 (22), 9901−9904. (31) Henkelman, G.; Jónsson, H. J. Chem. Phys. 2000, 113 (22), 9978−9985. (32) Li, Y.; Wu, D.; Zhou, Z.; Cabrera, C. R.; Chen, Z. J. Phys. Chem. Lett. 2012, 3 (16), 2221−2227. (33) Yang, S.; Li, D.; Zhang, T.; Tao, Z.; Chen, J. J. Phys. Chem. C 2011, 116 (1), 1307−1312. (34) Shi, Y.; Huang, J.-K.; Jin, L.; Hsu, Y.-T.; Yu, S. F.; Li, L.-J.; Yang, H. Y. Sci. Rep. 2013, 3, 1839. (35) Uthaisar, C.; Barone, V. Nano Lett. 2010, 10 (8), 2838−2842. (36) Yu, D.; Liu, F. Nano Lett. 2007, 7 (10), 3046−3050. (37) Zang, J.; Liu, F. Nanotechnology 2007, 18 (40), 405501. (38) Hwang, C.; Siegel, D. A.; Mo, S.-K.; Regan, W.; Ismach, A.; Zhang, Y.; Zettl, A.; Lanzara, A. Sci. Rep. 2012, 2, 590.

(39) Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotechnol. 2007, 2 (10), 605−615. (40) Mafra, D.; Samsonidze, G.; Malard, L.; Elias, D.; Brant, J.; Plentz, F.; Alves, E.; Pimenta, M. Phys. Rev. B 2007, 76 (23), 233407.

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