Room-temperature ferromagnetism in Co doped MoS2 sheets.

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Physical Chemistry Chemical Physics View Article Online

DOI: 10.1039/C5CP01509J

ZhongCheng Xianga, Zhong Zhanga,*, XiJin Xua, Qin Zhangb, QingBao Wanga

Published on 13 May 2015. Downloaded by University of Connecticut on 19/05/2015 15:23:22.

Chengwu Yuanc,**

a

School of Physics and Technology, University of Jinan, Jinan 250022, China

b

**

School of Science, Shandong Jiaotong University, Jinan 250357, China

Now with BP America

*

Corresponding author: Tel: (0531)82765976, E-mail: [email protected]

Abstract

Via the hydrothermal method, we synthesized MoS2 nanosheets with varying dopant concentrations of 0%, 3%, 7%, using cobaltous acetate as a promoter, and marked as A, B, and C, respectively. We found that the thickness and flatness of the nanosheets increased with the increase of the co dopant concentrations. Meanwhile, the BET Surface Area of samples (A, B, and C) decreased with the increase of the Co dopant concentrations.

Optical absorption spectroscopy showed that, compared to

sample A, the A1 and B1 excitons of samples B and C were 10 and 23 meV redshifted, respectively. Then, we performed magnetization measurement to investigate the effect of Co-doping; the unique result implied that the values of the magnetic moment decreased with the increase of the Co dopant concentrations. We performed DFT 1

Physical Chemistry Chemical Physics Accepted Manuscript

Room-temperature ferromagnetism in Co doped MoS2 sheets

Physical Chemistry Chemical Physics

Page 2 of 25 View Article Online

DOI: 10.1039/C5CP01509J

that the value of the magnetic moment decreased with the increase of the Co dopant


concentrations, which is in agreement with results of the experiments described above. Keywords: Hydrothermal method, MoS2 nanosheets, dopant, ferromagnetism, and density functional theory

Introduction

MoS2 is a prototypical transition metal chalcogenide material. It is composed of covalently bonded S-Mo-S sheets that are bound by weak van der Waals forces, similar to grapheme, and individual layers can be isolated using traditional mechanical cleavage techniques [1], chemical vapor deposition [2], liquid exfoliation [3], or electrochemical exfoliation [4-5].

In its bulk form, MoS2 is a semiconductor

with an indirect bandgap of about 1eV, and the monolayer material exhibits a direct gap of about 1.8eV due to interlayer interaction, quantum confinement, and long-range Coulomb effects [6]. This makes MoS2 material interesting both for fundamental research and for technological applications [7]. Up to now, MoS2 materials with different morphologies have been synthesized using many different methods in order to obtain various excellent properties for many applications and have been explored in diverse fields; they have been integrated into transistors [8],

2


computations to address the above magnetic result. The computational result indicated

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hydrodesulfurization and hydrogen evolution [11-12].


Although the photoelectric properties of MoS2 have been studied both theoretically and experimentally for many years and the research results have been fruitful, recent studies on the magnetic response of MoS2 are limited to theoretical calculations.

According to these calculations, MoS2 flakes are diamagnetic by

nature but display ferromagnetism as well, which could be due to the magnetic moments from zigzag edges, as seen in MoS2 nanoribbons [13-14], and from sulfur vacancies [15].

Recently, the study of the magnetism of few-layer MoS2 obtained

by doped impurity atoms has attracted great attention. Ramasubramaniam et al. [16] reported the magnetism in layered TMDs via substitutional doping of magnetic transition-metal atoms, and Chen et al. [17] first considered single Co atom adsorption in MoS2 for the magnetism. In addition, magnetism can be artificially induced in few-layer MoS2 through proton irradiation [18] and doping with nonmetal elements [19]. Nevertheless, studies on the magnetism of MoS2 obtained by doped impurity atoms are extremely rare in experimental work. Francis et al. reported that Co doping can be achieved at the MoS2 edge by sulfidation of a mixture of ammonium heptamolybdate and cobalt nitrate in order to obtain the best catalysts [20], which paves the way to transition metal doping by substitution of Mo in MoS2.

Here, we synthesized MoS2 nanosheets with different Co dopant concentrations of 0%, 3%, and 7% by the hydrothermal method using cobaltous

3


sensors [9], and batteries [10], and used as dry lubrication and as catalysts for



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thickness and flatness of the nanosheets increased with the increase of the Co dopant


concentrations

and

the

Brunauer-Emmett-Teller

(BET)

surface

areas

and

optical-absorption spectroscopy of three samples confirmed the results above.

Magnetic measurements were performed using the MicroMag 2900 Series Alternating Gradient Magnetometer at room temperature. The results indicates that the MoS2 nanosheets exhibit ferromagnetic-like behavior with magnetic hysteresis and the value of the magnetic moment decreased with the increase of the Co dopant concentrations, which aroused our interest and we sought to discover the reasons for this. By calculation, our results suggest that the change of the magnetic moment comprises two main terms in the nanosheets: (1) the reduced moment under substitution of one Mo on the Mo edge by Co, which can be attributed to the coupling of the Co moment to the edge state; and (2) variational ferromagnetism possibly originating from a decrease of zigzag edges, with associated magnetism at grain boundaries.

A full understanding of the magnetic properties of MoS2 is crucial for

the successful integration of MoS2 into possible devices.

Experimental details

All the chemicals used in the experiment were analytical grade without further purification. The detailed synthesizing process is as follows: 3*1.4 g of Na2MoO4 •

4


acetate as a promoter and marked as A, B, and C, respectively. We found that the

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(CH3CSNH2) was introduced into the aqueous solution while stirring at room


temperature. The obtained solution was evenly divided into three parts, which were marked as A, B, and C, respectively; then moderate cobaltous acetate was added to the three parts, making Co dopant concentrations of 0%, 3%, and 7%. After 30 min of stirring, the mixture was transferred into a Teflon-lined stainless steel autoclave, which was filled with the aqueous solution up to 80% of the total volume, then sealed and maintained at 180℃ for 24 h. The produced black precipitates were centrifuged, washed with distilled water and ethanol for several times, then taken in a quartz boat and introduced into a Muffle furnace and heated at 60℃ in air for 6 h.

The represented MoS2 nanosheets with different Co dopant concentrations of 0%, 3%, and 7% were identified by XRD using a D8 ADVANCE (BRUKER AXS GMBH) with Cu Kα at λ = 0.15406 nm. The 2θ angle of the XRD spectra were recorded from 10° to 90° at a scanning rate of 51/min, with a working voltage of 80 kV and a current of 10 mA. Their morphologies and sizes were analyzed by SEM (Quanta FEG) at an accelerating voltage of 15 kV and high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM-2100. The UV–vis spectrum was obtained on a UV–vis spectrophotometer (TU-1901). The scanning range was from 600 to 700 nm with an interval of 1 nm. The measurements of magnetic properties were made using the MicroMag 2900 Series Alternating Gradient Magnetometer at room temperature.

5


2H2O was put into 3*60 ml of deionized water; then 3*1.3 g of thioacetamide




Results and Discussion

In Figure 1, we show the XRD patterns of the A, B, and C samples with 0%, 3% and 7% Co doping, respectively, prepared by the hydrothermal method. All the observed main diffraction peaks can be indexed to MoS2 with a hexagonal crystal structure (P63/mmc space group and JCPDS. 37-1492). The results indicate that all the samples with different Co doping keep the hexagonal crystal structure, and the Co was atomically incorporated into MoS2 nanosheets, replaceing the sites of Mo on the Mo edge[20]. That is to say, most Co atoms were located on the edge sites rather than at the basal plane [20-21], which can be confirmed by EDS elemental mapping in figure 3b, 3c. All of the MoS2 samples demonstrated that Bragg reflection of the (002), (100), (103), and (110) planes emerges abruptly (see Fig.1), of which the (002) plane parallels the sandwich layer. We found that the peak of the (002) plane is more and more spiculate with the increase of the Co dopant concentrations; the results indicate the achieving of high crystallinity and a preferential growth direction along the (002) plane

with

a

Co

doping.

According

to

the

Scherrer

equation

[22],

D = 0.94λ / ( B cos θ ) , where λ is the wavelength of the X-ray source and B is the full width at half maxima of the individual peak at 2θ (where θ is the diffraction angle). The calculated average crystallite size of the (100) plane (c-axis) of the samples A, B, and C was 5.7 nm, 25 nm, and 35 nm, respectively. This means that the thickness of

6


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Fig. 1. XRD pattern of the A, B, and C samples with 0%, 3% and 7% Co doping in MoS2, prepared by the hydrothermal method

Fig. 2 (a-c) shows SEM images of the as-synthesized MoS2 samples A, B, and C, respectively, which revealed the sheet morphology of the samples. The SEM image of sample A demonstrates that the MoS2 nanosheets are bended and the size is a few microns in the in-plane direction. However, the MoS2 nanosheets become more thick and flat due to the presence of the stress in-plane direction as seen in the SEM image

7



the crystallite (c-axis) increased with the increase of the Co dopant concentration.



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hydrothermal method.

The results indicated that the thickness and flatness of the


nanosheets increased with the increase of the Co dopant concentrations. The sizes of the samples also became larger and larger from A to C. We suggest that the Co doping plays the role of a structure-directing agent to control the growth rate of the nanosheets in different directions. Fig. 2 (d-f) shows HRTEM images of the as-synthesized MoS2 samples A, B, and C, respectively. Figure 2d shows that pristine MoS2 (4.5nm thickness) tends to stack with an interlayer distance of 0.62 nm, corresponding to the (002) plane of MoS2. In contrast, the HRTEM image of the sample B and C show that the thickness up to about 15nm and 37nm, respectively. Fig. 3 shows the EDS elemental mapping images. Fig. 3 b shows that most Co atoms were located at the edge sites marked with red dots, rather than at the basal planes. For fig. 3 (c), the increase of the thickness and overlaps of the edge sites make it difficult to observe the edged Co atoms, and only the positions marked with red dots, which was a relatively thin layer, clearly indicate that Co atoms were located at the edge sites. These results are in agreement with previous reports [20-21]. Meanwhile, the BET Surface Area of samples A, B, and C were 22.9498 m2/g, 11.9672 m2/g, and 7.2763 m²/g, respectively, which is in good agreement with the results of the SEM images of the as-synthesized MoS2 samples A, B, and C. This is not hard to understand: with the thickness and flatness of the nanosheets increasing with the increase of the Co dopant concentrations, it is only natural that the BET Surface Area of the samples should decrease.

8


of samples B, and C with 3% and 7% Co doping, respectively, prepared by the

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Fig. 2. SEM and HRTEM images of the as-synthesized MoS2 samples A, B, and C. (a and d) SEM and HRTEM images of the samples A. (b and e) SEM and HRTEM images of the samples B. (c and f) SEM and HRTEM images of the samples C.

9



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Fig. 3. EDS elemental mapping images of (a) samples A ( pure ). (b) samples B ( 3% Co doping ). (c) samples C ( 7% Co doping ). The edge sites of thin layer marked with red dots.

The absorption spectrum was recorded by UV–vis spectrophotometer in the wavelength range of 600–700 nm at room temperature, as shown in Fig. 2. The observed absorption peaks of sample A at about 1.82 eV (683 nm) and about1.96 eV (632 nm) correspond to the A1 and B1 direct excitonic transitions. The A1 and B1 excitons are assigned to transitions at the K point of the Brillouin zone. The A1 and B1 splitting from valence band is due to interlayer interaction and spin-orbit splitting. The splitting values are about 0.15 eV for 2H-MoS2 [23-25]. As shown in Fig. 2, the position of the A1 and B1 excitons of samples B and C are altered in comparison to 10



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A systematic study of the effects of MoS2 material size and number of

atomic layers reveals that the position of the excitons is dependent on the number of


MoS2 layers [26]. The A1 and B1 excitons of samples B and C are 10 and 23 meV redshifted, respectively, compared to sample A. The results also show that the thickness of MoS2 nanosheets (c-axis) increases with the increase of the Co dopant concentrations [27], which is in good agreement with the results of the SEM images and XRD patterns of the as-synthesized MoS2 samples A, B, and C.

After studying the structural and optical behaviors of the samples, we performed magnetization measurement to investigate the effect of Co doping. The results of room temperature magnetization versus magnetic field hysteresis loop are shown in Fig. 3, which indicates that all of the Co doped nanosheets are ferromagnetic at room temperature.

The saturated moment of sample A with 0% Co doping (pure sample)

was found to be 2.0103848×10-3emu/g, which is in good agreement with the results of Sefaattin et al. [28]. However, the saturated moments of samples B and C with 3% and 7% Co doping were 0.36×10-3emu/g and 1.19 ×10-3emu/g decreased compared to sample A and were 1.65×10-3emu/g and 0.82×10-3emu/g, respectively. The unique result implies that the values of the magnetic moment decrease with the increase of the Co dopant concentrations. In principle, single crystals of semiconducting MoS2 are expected to be diamagnetic but to display ferromagnetism, as well, which could be due to magnetic moments from the zigzag edges at grain boundaries and from sulfur vacancies. The net magnetic moment arising from these zigzag edges is expected to decrease with increasing grain size (or ribbon width) [28-29], and decreasing bends 11


sample A.



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showed that the thickness and flatness of the nanosheets increased with the increase of


the Co dopant concentrations. From sample A to C, the grain sizes of the samples in the in-plane direction also became larger and larger. The BET Surface Area of the samples increased with the increase of the Co dopant concentrations. It is likely that edge effects would be greater in samples with a smaller number of layers, as well as small areas [31]. Therefore, the values of the magnetic moment decrease with the increase of the Co dopant concentrations.

Fig. 4. Magnetic hysteresis loop ( left ) and UV–vis spectra (right) of the as-synthesized MoS2 samples A, B, and C prepared by hydrothermal method

12


(arising from defects) in the layers [30]. For our samples, the SEM and XRD images

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MoS2 nanosheets? In the following, we performed DFT computations to address the


above issue. Previous DFT calculations and experiments indicate that the Co-promoter atoms incorporate into the MoS2 structure by substituting Mo edge atoms in an alternating configuration [20, 32]. As known, the moment of MoS2 mainly comes from these zigzag edges of MoS2 samples, which contain zigzag edges and armchair edges concurrently [28]. Therefore, the models of MoS2 nanoribbons with zigzag edges with associated magnetism are constructed by cutting a single-layered MoS2 with the desired edges and widths. ZNxy stands for edge type, where x is defined as the number of zigzag lines across the ribbon width and y is defined as the number of amchair lines along the ribbon in the growth direction of the nanoribbons in the supercell. Co-ZNxy stands for the MoS2 nanoribbons with ZNxy edge type in which one Mo on the Mo edge is substituted by Co, as shown in Fig.5(a), is ZN54 (right) and Co-ZN54 (left).

All of the calculations are performed by means of first principles calculations as implemented in the Vienna Ab initio Simulation Package (VASP). Generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) exchange correlation functional was adopted to describe the exchange-correlation interaction and a 500 eV cutoff for the planewave basis set was adopted in all the computations. Our supercells were large enough to ensure that the vacuum space was at least 10 Å, so that the interaction between nanoribbons and their periodic images could be safely avoided, and the convergence threshold was set as 10-6eV in energy and 10-3eV/Å in 13


What is the function of the dopant Co in terms of the magnetic properties of



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geometry optimizations. During the structure optimization, we carried out both


spin-unpolarized and spin-polarized computations to determine the ground state of the MoS2 nanoribbons. The results showed that all of MoS2 nanoribbons with different edge types had a ferromagnetic ground state, since we obtained an energy difference between their spin-unpolarized and spin-polarized total energies. Table1 shows the computed energy difference (∆E) between spin-polarized and spin unpolarized states and total magnetic moments (M) per unit cell for a series of edge type structures. Notably, the considerable ∆E indicates that the ferromagnetic state of MoS2 nanoribbons is rather stable.

For the ZN54 edge type structures, the total magnetic moment was 3.201 µB per cell; however, the magnetic moment of the Co-ZN54 edge type structures was 0.4303 µB per cell decreased compared to the ZN54 and was 2.7709 µB, as shown in Table 1. As with the ZN54 and Co-ZN54 edge type structures, the magnetic moment difference between ZN64 and Co- ZN64 was 0.401 µB per cell. The results indicate that the moment reduces to about 0.4 µB per cell under substitution of one Mo on the Mo edge by Co, which can be attributed to the coupling of the Co moment to the edge state. Furthermore, the magnetic moments of the Co-ZN54 and Co- ZN64 edge type structures were almost unchanged. The results indicate that the variation of the nanoribbons width couldn’t cause a change of the magnetic moment for the unit cell, which is in agreement with the results of Li et al. [014].

14


force. The positions of all the atoms in the supercell were fully relaxed during the

Page 15 of 25


Table 1. Energy Difference (∆E) between Spin-Polarized and Spin-Unpolarized States


and Total Magnetic Moments (M) per Unit Cell for a Series of MoS2 edge type structure. The values in parentheses are the single amchair lines moment of the corresponding edge type. Edge type

∆E(eV)

Mtotal(µB)

ZN54

-0.082

3.201(0.800)

Co-ZN56

-0.210

4.569(0.762)

Co-ZN54

-0.132

2.771(0.692)

ZN64

-0.086

3.201(0.800)

Co-ZN64

-0.097

2.800(0.700)

To get further insight into and clarity about the magnetism of the different MoS2 edge type structures, we computed the spatial spin distribution of the different MoS2 edge type structures. As shown in Fig. 5, the unpaired spin mainly concentrates on the Mo edge and S edge of the ZN54 edge type structures, and the inner Mo atoms also contribute a small amount of unpaired spin on the Mo edge. A Bader analysis of the spin-polarized the ZN54 edge type structure shows that the localization of the magnetic states is mainly attributed to the Mo edge; the magnetic moment of the Mo edge (2.049 µB) accounted for a large proportion of the total magnetic moment

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Co atom, the magnetic moment of the Mo edge in the Co-ZN54 edge type was


reduced to 1.623 µB.

The moment of Co on the Mo edge of Co-ZN54 edge type and Mo on the Mo edge of the ZN54 edge type corresponding to the place of Co of Co-ZN54 were 0.310 µB and 0.959 µB, respectively.

Fig. 6a and 6b show the projected density of states

(PDOS) of Co on the Mo edge of Co-ZN54 edge type and Mo on the Mo edge of the ZN54 edge type corresponding to the place of Co of Co-ZN54.

The Co 3d and Mo

4d states undergo an intratomic (Hund’s) exchange splitting, which means the up-spin and down-spin states with the same symmetry have different energies. As shown in Fig. 6a, for dxy, dyz, and dx2-y2 orbitals of Mo, the up-spin states are at lower energies than the down-spin states, which results in the spin-up states being partially occupied near the Fermi energy. As Fig. 6b shows, only the dxy and dx2–y2 orbitals of Co are spin-polarized; this implies that Co substitution leads to a decrease of total magnetic moment.

These results are in agreement with results of the spatial spin distribution.

To further study the magnetic effect of the Co doping concentration in nanoribbons, a Co-ZN56 edge type structure was constructed as show in fig. 5 (right). The Co doping concentration of Co-ZN56 is smaller than Co-ZN54.

As shown in

Table 1, the amchair lines moment of Co-ZN54, Co-ZN56 and ZN54 (undoped case) are 0.692 µB, 0.762 µB and 0.800 µB, respectively. These results indicate that the value of the magnetic moment decreases with the increase of the Co dopant

16


(3.201µB), others were from the S edge ( 1.152 µB ). After replacing Mo atom with

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above.


For the S edge case, the magnetic moments of S edge (S atom on the S edge) in Co-ZN54, Co-ZN56 and ZN54 were 1.148 µB (0.286 µB), 1.710 µB (0.285µB) and 1.152 µB (0.288µB), respectively, with the decrease of the Co dopant concentrations. The results indicate the magnetic moment of S atom of each type almost remain constant with the concentration increase of the Co dopant. Fig. 6(c-e) shows PDOS of S atom on the S edge in Co-ZN54, Co-ZN56 and ZN54 edge style, respectively. It can be observed the shapes are similar for the Py and Pz orbitals of S each type. These results are in agreement with results of the Bader analysis.

In order to investigate the magnetic couplings between the magnetic moments localized at the edges, we examined two different spin configurations for the MoS2 zigzag nanoribbon: ferromagnetically ordered spins at both edges (FM), and ferromagnetically ordered spins at each edge, but with the opposite spin directions between the edges (AFM). An energy difference between these two magnetic states of Co-ZN54, Co-ZN56 and ZN54 edge style corresponded to E AFM − EFM = 0.10 eV , 0.11eV and 0.054 eV, respectively. This implies that both configurations of the FM could be present at room temperature. These results are in agreement with experimental results of ferromagnetism samples.

17


concentrations, which is in agreement with results of the experiments described



Fig. 5. (a) Molecular models of the optimized geometries of Co-ZN54 and ZN54 edge type structures, Co, Mo and S atoms are represented by blue, gray and yellow balls, respectively. (b) Spatial spin distribution of Co-ZN54 and ZN54 edge type structures with side and top views (left). Spatial spin distribution of ZN64, Co-ZN64 and Co-ZN56 edge type structures (right).

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Fig. 6. (a) Projected density of states (PDOS) of Co on the Mo edge of Co-ZN54 edge type structures and (b) Mo on the Mo edge of the ZN54 edge type structure corresponding to the place of Co in Co-ZN54 (left). (c-e) PDOS of S atom on the S edge in ZN54, Co-ZN56 and Co-ZN54 edge stye, respectively (right).

Conclusion 19



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nanosheets with different Codopant concentrations of 0%, 3%, and 7% using


Cobaltous acetate as a promoter and marked them as samples A, B, and C, respectively. We found that the thickness and flatness of the nanosheets increased with the increase of the Co dopant concentrations. Meanwhile, the BET Surface Area of the samples A, B, and C was 22.9498 m2/g, 11.9672 m2/g, 7.2763 m²/g, respectively. Optical-absorption spectroscopy showed that the A1 and B1 excitons of samples B and C were 10 and 23 meV redshifted, respectively, compared to sample A. We performed magnetization measurement to investigate the effect of Co doping. The results implied that the values of the magnetic moment decreased with the increase of the Co dopant concentrations. We then performed DFT computations to address the above magnetic result. This computational result indicated that the value of the magnetic moment decreased with the increase of the Co dopant concentrations, which was in agreement with results of the experiments described above.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 11304182 and No. 11304120).

References

[1]

Frindt, R. F. Single Crystals of MoS2 Several Molecular Layers Thick, J. Appl. Phys. 1966, 37, 1928–1929.

[2]

Lee, Y. H.; Zhang, X. Q.; Zhang, W. J.; Chang, M. T.; Lin, C. T.; Chang, K. D.; 20


In summary, via the hydrothermal method, we synthesized the MoS2

Page 21 of 25


DOI: 10.1039/C5CP01509J

Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater.


2012, 24, 2320–2325. [3]

Jonathan, N.C.; Mustafa, L. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568.

[4]

Na, L.; Paul, K.; Ji, H. K.; Jun, H. Y.; Sunkook, K.; Cheol, J. L. Large-Area Atomically Thin MoS2 Nanosheets Prepared Using Electrochemical Exfoliation. ACS Nano 2014, 8, 6902–6910.

[5]

You, X. Q.; Liu, N.; Lee, C. J.; Pak, J. J. An Electrochemical Route to MoS2 Nanosheets for Device Applications. Mater Lett 2014, 121, 31-35.

[6]

Rudren, G.; Q, Z. Few-Layer MoS2: A Promising Layered Semiconductor. ACS Nano 2014, 8, 4074–4099.

[7]

Gunnar, B.; Ermin, M. Analytical Approach to Excitonic Properties of MoS2. Phys. Rev. B 2014, 89, 125309.

[8]

Radisavljevic, B.; Radenovic, A.; Brivio1, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nature Nanotech. 2011, 6, 147-150.

[9]

Li, H.; Yin, Z.; He, Q.; Li, H.; Huang, X. ; Lu, G.; Fam, D. W.; Tok, A. I. Y.; Zhang, Q.; Zhang, H. Fabrication of Single- and Multilayer MoS2 Film-Based Field-Effect Transistors for Sensing NO at Room Temperature. Small 2012, 8, 63–67.

[10] Zhao, C. Y.; Kong, J. H.; Yao, X. Y.; Tang, X. S.; Dong, Y. L.; Phua,S. L.; Lu, X. H. Thin MoS2 Nanoflakes Encapsulated in Carbon Nanofibers as High

21


Yu, Y. C.; Wang, T. W.; Chang, C. S.; Li, L. J.; Lin, T. W. Synthesis of



DOI: 10.1039/C5CP01509J

2014, 6, 6392−6398.


[11] Lu, Z. Y.; Zhu, W.; Yu, X. Y. et al Ultrahigh Hydrogen Evolution Performance of Under-Water“Superaerophobic” MoS2 Nanostructured Electrodes. Adv. Mater. 2014, 26, 2683–2687. [12] Zhao, Y. F.; Zhang, Y. X.; Yang, Z. Y.; Yan, Y. M.; Sun, K. N. Synthesis of MoS2 and MoO2 for Their Applications in H2 Generation and Lithium Ion Batteries: a Review. Sci. Technol. Adv. Mater. 2013, 14, 043501. [13] Ouyang, F. P.; Yang, Z. X.; Ni, X.; Wu, N. N.; Chen, Y.; Xiong, X. Hydrogenation-Induced Edge Magnetization in Armchair MoS2 Nanoribbon and Electric Field Effects. Appl. Phys. Lett. 2014, 104, 071901. [14] Li, Y. F.; Zhou, Z.; Zhang, S. B.; Chen, Z. F. MoS2 Nanoribbons: High Stability and Unusual Electronic and Magnetic Properties. J. Am. Chem. Soc. 2008, 130, 16739–16744. [15] Javier, D.; Fuhr; Andres, S. Scanning Tunneling Microscopy Chemical Signature of Point Defects on the MoS2 (0001) Surface. Phys. Rev. L 2004, 92, 026802. [16] Ramasubramaniam, A. Mn-doped Monolayer MoS2: An Atomically Thin Dilute Magnetic Semiconductor. Phys. Rev. B 2013, 87, 195201. [17] Chen, Q.; Ouyang, Y. X.; Yuan, S. J.; Li, R. Z.; Wang, J. L.; Uniformly Wetting Deposition of Co Atoms on MoS2 Monolayer: A Promising Two-Dimensional Robust Half-Metallic Ferromagnet. ACS Appl. Mater. Interfaces 2014, 6, 16835−16840.

22


Performance Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces

Page 23 of 25


DOI: 10.1039/C5CP01509J

MoS2 Induced by Proton Irradiation. Appl. Phys. Lett. 2012, 101, 102103.


[19] He, J. G.; Wu, K. C.; Sa, R. J.; Li, Q. H.; Wei, Y. Q. Magnetic Properties of Nonmetal Atoms Absorbed MoS2 Monolayers. Phys. Rev. L 2010, 96, 082504. [20] Deepak, F. L.; Esparza, R.; Borges, B.; Xochitl, L. L.; Miguel, J. Y. Direct Imaging and Identification of Individual Dopant Atoms in MoS2 and WS2 Catalysts by Aberration Corrected Scanning Transmission Electron Microscopy. ACS Catal. 2011, 1, 537–543. [21] Behal, S. K.; Chianelli, R. R.; Kear, B. H. Edge Plane Segregation in Cobalt-Doped MoS2 Crystals. Mater. Lett. 1985, 3, 381. [22] Cullity, B.D. in: Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1972. [23] Splendiani, A.; Sun, L.; Zhang, Y. B.; Li, T. S.; Kim, J.; Chim, C. Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271–1275. [24] Coehoorn, R.; Hass, C.; Groot, R. A. Electronic Structure of MoSe2, MoS2, and WSe2. II. the Nature of the Optical Band Gaps. Phys. Rev. B 1987, 35, 6203. [25] Mak, K. F.; He,K. L.; Lee, C. G.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nature Mater 2013, 12, 207-211. [26]

Frey, G. L.; Elani, S.; Homyonfer, M.; Feldman, Y.; Tenne, R. Optical-Absorption Spectra of Inorganic Fullerenelike MS2 (M=Mo, W). Phys. Rev. B 1998, 57, 6666-6671.

[27] Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M. W.; Chhowalla, M.

23


[18] Mathew, S.; Gopinadhan, K.; Chan, T. K.; Yu, X. J.; Zhan, D. Magnetism in



DOI: 10.1039/C5CP01509J

5111–5116.


[28] Sefaattin, T.; Varnoosfaderani, S. S.; Appleton, B. R.; Wu, J. Q.; Hebard, A. F. Magnetic Properties of MoS2: Existence of Ferromagnetism. Appl. Phys. Lett. 2012, 101, 123105. [29] Zhang, J.; Soon, J. M.; Loh, K. P.; Yin, J. H.; Ding, J.; Sullivian, M. B.; Wu, P. Magnetic Molybdenum Disulfide Nanosheet Films. Nano Lett. 2007, 7, 2370-2376. [30] Rao, C. N. R.; Maitra, U.; Waghmare, U. V. Extraordinary Attributes of 2-Dimensional MoS2 Nanosheets. Chem. Phys. Lett. 2014, 609, 172–183. [31] Ramakrishna Matte, H. S. S.; Subrahmanyam, K. S.; Rao, C. N. R. Novel Magnetic Properties of Graphene: Presence of Both Ferromagnetic and Antiferromagnetic Features and Other Aspects. J. Phys. Chem. C 2009, 113, 9982-9985. [32] Krebs, E.; Silvi, B.; Raybaud, P. Mixed Sites and Promoter Segregation: A DFT Study of the Manifestation of Le Chatelier’s Principle for the Co(Ni)MoS Active Phase in Reaction Conditions. Catal.Today 2008, 130, 160–169.

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Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11,



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Electronic and magnetic properties of Co doped MoS2 monolayer.

Ferromagnetism in ultrathin MoS2 nanosheets: from amorphous to crystalline.

Antibacterial activity of two-dimensional MoS2 sheets.

Encapsulation of Fe3O4 Nanoparticles into N, S co-Doped Graphene Sheets with Greatly Enhanced Electrochemical Performance.

The electronic structure and optical properties of Mn and B, C, N co-doped MoS2 monolayers.

Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3.

Spin-dependent thermoelectric effects in Fe-C6 doped monolayer MoS2.

Strain tuning of magnetism in Mn doped MoS2 monolayer.

S-rich single-layered MoS2 nanoplates embedded in N-doped carbon nanofibers: efficient co-electrocatalysts for the hydrogen evolution reaction.

Elasticity of MoS2 Sheets by Mechanical Deformation Observed by in Situ Electron Microscopy.

Nature of charge transport and p-electron ferromagnetism in nitrogen-doped ZrO2: an ab initio perspective.

Co spheres.

Realization of high Curie temperature ferromagnetism in atomically thin MoS2 and WS2 nanosheets with uniform and flower-like morphology.

MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes.

Atomically Thin B doped g-C3N4 Nanosheets: High-Temperature Ferromagnetism and calculated Half-Metallicity.

Confocal absorption spectral imaging of MoS2: optical transitions depending on the atomic thickness of intrinsic and chemically doped MoS2.

Ferromagnetism and topological surface states of manganese doped Bi2Te3: insights from density-functional calculations.

Formation of ferromagnetic Co-H-Co complex and spin-polarized conduction band in Co-doped ZnO.

A general approach for fabrication of nitrogen-doped graphene sheets and its application in supercapacitors.

Yellow laser performance of Dy³⁺ in co-doped Dy,Tb:LiLuF₄.

Unconventional charge order in a co-doped high-Tc superconductor.

Bi co-doped silica glass.

Co3O4 porous nanocomposites derived from MOFs: room-temperature ferromagnetism and high catalytic oxidation of CO.

Bottom-up synthesis of nitrogen-doped graphene sheets for ultrafast lithium storage.