Inducing High Coercivity in MoS2 Nanosheets by Transition Element

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Inducing high coercivity in MoS2 nanosheets by transition element doping Sohail Ahmed, Xiang Ding, Nina Bao, Pengju Bian, Rongkun Zheng, Yiren Wang, Peter Paul Murmu, John Vedamuthu Kennedy, Rong Liu, Haiming Fan, Kiyonori Suzuki, Jun Ding, and Jia Bao Yi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02593 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Chemistry of Materials

Inducing high coercivity in MoS2 nanosheets by transition element doping Sohail Ahmed1, Xiang Ding1, Nina Bao2, Pengju Bian3, Rongkun Zheng3, Yiren Wang1, Peter Paul Murmu4, John Vedamuthu Kennedy4, Rong Liu5, Haiming Fan6, Kiyonori Suzuki7, Jun Ding2, Jiabao Yi1 *,

1.

School of Materials Science and Engineering, UNSW, Sydney, NSW 2052, Australia

2.

Department of Materials Science and Engineering, National University of Singapore, 119260, Singapore.

3.

School of Physics and the Australian Institute for Nanoscale and Technology, the University of Sydney, 2006, Sydney, Australia.

4.

National Isotope Centre, GNS Science, P.O. Box 31312, Lower Hutt, 5010, New Zealand.

5.

SIMS Facility, Office of the Deputy-Vice Chancellor (Research and Development), Western Sydney University, Locked Bag 1797, Penrith, New South Wales, 2751, Australia.

6.

College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710069, China.

7.

Department of Materials Science and Engineering, Monash University, 3800, Victoria, Australia

*Corresponding Author: [email protected] ABSTRACT:

MoS2 nanosheets were doped with vanadium (V) with a variety of concentrations using a

hydrothermal method. Raman, X-ray photoelectron spectroscopy and electron paramagnetic resonance results indicate the effective substitutional doping in MoS2. Without V doping, oxides such as MoO2 and MoO3 have been observed, whereas, with 5 at% V doping, the oxide disappears. Magnetic measurements show that room temperature ferromagnetism has been induced by V doping. Magnetization tends to increase with the increased V doping concentration. A very large coercivity up to 1.87 kOe has been observed in 5 at% vanadium doped MoS2, which may attribute to a combination effect of localized charge transfer between V and S ions, pinning effect due to the inbetween defects, stress induced by doping and shape anisotropy due to two-dimensional nature of MoS2 ribbons. der Waals forces is the center of attention in past few

1. INTRODUCTION

years. High

carrier mobility and flexibility, large

The discovery of graphene, a two-dimensional (2D)

spin-orbit coupling, high on/off current ratio

material, is considered to be the pivotal point to

easy tuning of bandgap (bulk bandgap 1.2 eV and

revolutionize the future of nano devices 1; as it has

monolayer bandgap 1.8 eV) with various number of

attracted the worldwide focus in investigating the

layers, due to quantum confinement and interlayer

other 2D materials like silicene, boron nitride and

interaction 9, have made MoS2 promising for multiple

transition

metal

dichalcogenides

(TMDCs)

2-4

.

spintronic devices

reported including MoS2, TiSe2, WSe2, NbS2, ZrS2, VSe2 and WS2 etc

and

applications such as transistors 7, solid lubricants

Currently, over 40 different TMDCs have been 5-6

7-8

hydrogen

. Among all the reported TMDCs,

10, 13

nano-devices

11

, solar cells

, sensors

15

12, 14

1

ACS Paragon Plus Environment

,

, catalyst for

, photodetectors

. Recently, electrical

covalently bonded MoS2 stacked with the weak van

12

10

16-17

11

and

, mechanical


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18

and optical 19-20 properties of MoS2 were extensively

Page 2 of 16

2. EXPERIMENTAL DETAILS

investigated experimentally and theoretically with exceptional outcomes. Due to appropriate bandgap

A hydrothermal synthesis method was used to

(1.8 eV) and strong spin-orbit coupling, MoS2 is one

produce V-doped MoS2 nanosheets. All analytical

of

grade chemicals were used in aforesaid experiments

the

promising

candidates

for

spintronics

without any purification.

applications. Spin based data manipulation and storage is an emerging research area owing to

(NH4VO3),

numerous benefits over charge based devices.

[(NH4)6Mo7 O24.4H2O]

Therefore, in past years, different materials have been

powder

investigated for suitability such as oxide thin films

molybdenum

and nanowires

21-22

,

Ammonium metavendate

ammonium

were

picked (Mo)

and as and

molybedate

elemental the sulfur

sulfur

vanadium (S)

(S) (V),

sources,

respectively. Initially, 3 x 1 g ammonium molybedate

and 2D materials including due to their unique spin

and 0.35 x 3 g sulfur were added into a 100 ml beaker

dependent properties. Making MoS2 magnetic is one

and then 60 ml of distilled (DI) water and 3 x 8 ml of

of the most important steps for achieving qualified

hydrazine monohydrate (N2H4.H2O), as a reducing

spintronics materials. However, the research of

agent, were further added into a beaker containing

magnetic behaviors of MoS2 mostly depended on

chemicals. After stirring, for 20 mins at 800 RPM, an

theoretical calculations. These calculations showed

aqueous solution was divided into 3 equal parts in

that substitutional doping of transition metals could

different 100 ml beakers. 0.0496 g and 0.102 g of

graphene and TMDCs

23

. Pure MoS2 being a

ammonium metavendate were dropped into the

diamagnetic in pristine form was also reported to

beakers to make V concentration 0, 2.5 and 5 at%,

demonstrate ferromagnetism, which attributed to the

respectively. After stirring, for 30 mins, final mixtures

induce magnetism in MoS2

24-30

. Experimentally,

were transferred to three new / un-used 50 ml Teflon-

ferromagnetism was observed in Fe, Co, Ni, and Mn

lined stainless steel autoclaves to avoid any possibility

doped MoS2 single crystals by ion implantation 33.

of contamination. The autoclaves were further filled

defects and zigzag edges

11, 18, 31-32

with DI water to make final total volume 80 %, then

In this work, we use a hydrothermal synthesis

sealed and maintained at 180 oC in an oven for 48 h.

method to produce MoS2 nanosheets chemically doped with vanadium (V) at different atomic

The produced black precipitates were centrifuged,

concentrations of 0, 2.5 and 5 at%.

Without V

washed with ethanol and DI water successively for

doping, oxide phases have been discovered. However,

numerous times. Finally, the black precipitates were

when the doping concentration reaches 5 at%, the

dried in an air oven at 60 oC for 3 h.

oxide disappears, which provides an effective way to

The phases of nanoribbons were characterized by

prevent the formation of oxide, a notorious issue, in

X-ray diffractometry (XRD, PANalytical Xpert

2D materials during hydrothermal process. In

Multipurpose X-ray Diffraction System) with Cu Kα

addition, for the doped samples, both paramagnetic

radiation and Raman spectroscopy (Renishaw inVia

and

room

Raman Microscope, fitted with a diffraction grating of

Furthermore, an exceptionally large

1800 lines / mm, excited with a radiation of 514 nm

coercivity (1.87 kOe) for 5 at% V-doped MoS2 was

argon ion laser and calibrated with Si Single crystal).

observed at 10 K, which may pave a way for

Microstructure and morphology were investigated

achieving high coercivity in 2D material based DMS.

using scanning electron microscopy (SEM, FEI Nova

ferromagnetic

temperature.

phases

coexist

at

NanoSEM 450) and transmission electron microscopy (TEM, Philips CM200), respectively. TEM samples were prepared by dropping the samples dissolved in ethanol on TEM copper grids. The dropped samples 2


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were dried naturally. Valance state and composition

(supporting information, Figure S1) for all samples

were examined by X-ray photoelectron spectroscopy

did not display any peak, indicating the amorphous

(XPS) with Thermo Scientific ESCALAB 250i X-ray

structure of the as-prepared samples. Figure 1(a)

photoelectron Spectrometer (calibrated by C1s=284.8

shows the typical micrograph of the un-doped MoS2

eV). Secondary ion mass spectroscopy (SIMS)

examined by SEM, indicating that the synthesized

analysis was performed to determine the depth profile

MoS2 nanostructures are nanosheets, crosslinked and

and dopant (V) distribution in MoS2 by pressing the

overlapped each other. 2.5 at% vanadium-MoS2 and 5

nanosheets into pellets. SIMS was conducted using

at%

Cameca IMS 5fE7 SIMS instrument operated with

morphologies to that of undoped sample. Further

+

vanadium-MoS2

samples

show

similar

O2 ion gun (8 nA ion current), an impact energy of

analysis was done by high-resolution TEM. Figure

7.5 keV and rastered with 180 x 180 um region of the

1(b-d) shows the high-resolution images of undoped,

surface. Electron paramagnetic resonance (EPR)

2.5 at% vanadium-MoS2 and 5 at% vanadium-MoS2

spectrum

samples

(Bruker

Spectrometer)

EMX-plus

and

X-Band

superconducting

EPR

respectively.

All

the

images

exhibit

quantum

amorphous structures, consistent with SEM analysis.

interference device (SQUID, Quantum design-XL-5)

The insets are corresponding selected area electron

were used for

diffraction

the measurement

of magnetic

properties.

patterns,

indicating

non-

crystalline structures, which supports XRD and highresolution

3. RESULTS AND DISCUSSION

results.

Energy

dispersive

of vanadium inside the MoS2 (supporting information, Figure S2 A & B). In addition, nanoclusters have not

by MoS2, 2.5 at% vanadium-MoS2 and 5 at% respectively.

TEM

spectroscopy (EDS) shows quite uniform distribution

0, 2.5 and 5 at% vanadium-doped MoS2 are denoted vanadium-MoS2,

(SAED)

XRD

been seen in high-resolution TEM images, suggesting

patterns

that

vanadium

3


is

in

substitutional

site.


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(a)

(b)

(c)

(d)

Page 4 of 16

Figure 1. (a) SEM micro image of un-doped MoS2. TEM micro images of (b) Un-doped MoS2. (c) 2.5 at% vanadium-MoS2. (d) 5 at% vanadium-MoS2. The insets are SAED patterns of the corresponding samples. Raman spectroscopy was employed to identify and

377.7 and 401.8 cm-1, whereas, it remains insensitive

examine the structural defects and phases of the

to further V doping (5 at% vanadium-MoS2). This

undoped MoS2, 2.5 at% vanadium-MoS2 and 5 at%

blue shift of Raman peaks, after transition metal

vanadium-MoS2 samples. Figure 2(a) shows the

doping, has also been reported earlier 37-38. Significant

Raman spectra in a large scale. The strong peak at 520

reduction in peak intensity, broadening of peaks and

cm-1 is from the silicon substrate. MoS2 characteristics

peak shift may be related to the doping induced

peaks have also been observed in Figure 2(a). In order

defects 39-41. Terrace and edge terminated structures of

to see more clearly, we enlarge the peak of the MoS2

MoS2 excite the E12g and A1g vibration modes

-1

part at around 400 cm , as shown in Figure 2(b). 1

Dominant peaks, E

2g

the intensity ratio of A1g/ E12g decreases. Hence V

and A1g (out of plane Mo-S mode) are displayed at 379.4 and 403.5 cm

doping in MoS2 reduces the edge-terminated structure 35-36, 38

. Pure MoS2 demonstrates the highest A1g/ E12g

respectively, which are

associated with the MoS2 in all samples 1

at% vanadium-MoS2, the E

2g

34-36

.

Figure 2(b) shows that, with the increase of V doping,

(in-plane Mo-S phonon mode) -1

38

. In 2.5

ratio as compared to other samples. Moreover, Raman

and A1g peaks shift to

spectra of all samples also display some extra peaks

4


Page 5 of 16

associated with MoO2 and MoO3 in the MoS2 without

MoS2. In addition, vanadium oxide can be easily

doping and low doping concentrations of V (i.e. 2.5

transferred to V2S3, having a high decomposition

at% vanadium-MoS2) as shown in Figure 2(a), which

temperature

is a notorious issue unable to avoid during

preventing the formation of molybdenum oxide.

hydrothermal synthesis of 2D materials. However, for

Furthermore, no vanadium related peak could be

5 at% vanadium-MoS2, no trace of oxide could be

discovered in all the spectra, further supporting that V

found, suggesting we may provide an effective way

atoms successfully accommodate as a substitutional

solving the problem by doping. It may be due to that

element at the doping sites of MoS2.

42

, which may be one of the reasons for

V doping promotes the activity for the formation of

(a)

MoS2 (b) 403.5 A1g MoO3 1 E2g 379.4 MoO2 Si-Substrate 5 at%V

Intensity (arb. units)

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5 at%V 2.5 at%V

2.5 at% V MoS2 MoS2 200

300

400

500

600

700

800

900 360

390 420 Raman Shift (cm-1)

-1

Raman Shift (cm )

Figure 2. (a) Raman spectra of MoS2 with different V concentrations. (b) Enlarged part of the spectra in (a). Electronic state and chemical bonding of samples,

peaks at 228.61and 228.84; 232.85 and 232.97 eV are

with and without V doping, were investigated by XPS.

associated with the Mo+4 and Mo+6, respectively

Figure 3(a-b) shows the binding energies of Mo (3d)

47

41, 43-

. Similarly, in Figure 3(b), the un-doped MoS2

and S (2p) core levels, respectively. Figure 3(a)

sample shows three S (2p) doublets with the S (2p3/2)

displays two Mo (3d) doublets of un-doped MoS2 with

peaks at 161.59, 163.37 and 168.32 eV, which are

corresponding binding energies of 228.76 and 232.9

corresponding to S-2, S2-2(poly-sulfide) and S+4,

eV for Mo (3d5/2), which can be related to the Mo+4

respectively

+6

and Mo , respectively. Whereas, extra peak at 225.9 eV is associated with S (2s) of MoS2

41, 43-47

. After V doping, one additional

doublet appears for S (2p) in 2.5 at% vanadium-MoS2

38, 41, 43-44

. In 2.5

and 5 at% vanadium-MoS2, which indicates the

at% vanadium-MoS2 and 5 at% vanadium-MoS2

presence of extra S species. The red and blue color

samples, no new doublet is observed. Hence, valance

curves in Figure 3(b) display the binding energies of S

state remains the same after V doping. The Mo (3d5/2)

(2p3/2). From the overlapping doublets spectra, the 5



corresponding binding energies of peaks at 161.55 and

generates

161.72; 163.47 and 163.68; 168.37 and 168.52 eV can

deficiency which traps the extra electrons is produced

-2

-2

+4

be related to the S , S2 (poly-sulfide) and S , respectively

41, 43-48

50

cationic

vacancies.

Hence,

electron

and can result into higher valance state of the Mo and S 51-52. The V (2p) edge of the spectra is composed of

. Peaks with minor intensities at

166.25 and 166.38 eV are also associated with S+4

49-

one doublet as shown in Figure 3(c). V (2p3/2) peaks

. Moreover, it is evident from the XPS results that

at binding energies of 516.63 and 516.74 eV, with a

+6

tends to rise with

split of 7.6 eV, corresponding to V+4 53-54. The absence

the increase in doping concentration and reaches the

of Vo suggests the successful substitutional V doping

maximum in 5 at% vanadium-MoS2 sample. This

into MoS2. In addition, XPS analysis also indicates

the concentration of Mo

and S

+4

+6

+4

and S may be

that the exact doping concentration of V is 1.42 at%

associated with the disturbance of internal charge

and 3.91 at% for V+4 in 2.5 at% vanadium-MoS2 and 5

balance due to aliovalent dopant (V). In response,

at% vanadium-MoS2, respectively, slightly lower than

compensation defects tend to form in the lattice which

the nominal concentration.

increase in the concentration of Mo

(a) MoS2-Mo(3d)

Mo+4 (3d5/2)

Mo+4 (3d3/2)


(b) MoS2-S(2p)

MoS2 Mo3d5-A Mo3d3-A Mo3d5-B Mo3d3-B S2s-A

Mo+6 (3d5/2)

S-2 (2p3/2)

S-2 2(2p3/2)

+6 (2p3/2)

S

MoS2 S2p3-A S2p3-A S2p3-B S2p3-B S2p-C

Mo+6 (3d3/2)

S(2S)


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Page 6 of 16

2.5 at% V-Mo(3d)

5 at% V-Mo(3d)

2.5 at% V Mo3d5-A Mo3d3-A Mo3d5-B Mo3d3-B S2s-A

2.5 at% V-S(2p)

5 at% V Mo3d5-A Mo3d3-A Mo3d5-B Mo3d3-B S2s-A

5 at% V-S(2p)

S+4 (2p3/2)

2.5 at% V S2p3-A S2p3-A S2p3-B S2p3-B S2p3-C S2p3-C S2p3-D S2p3-D

5 at% V S2p3-A S2p3-A S2p3-B S2p3-B S2p3-C S2p3-C S2p3-D S2p3-D

224 226 228 230 232 234 236 238 240158 160 162 164 166 168 170 172 174 (c) V(2p)

V+4 (2p3/2)

Binding Energy (eV)

V+4 (2p1/2) 5 at% V 2.5 at% V

508 510 512 514 516 518 520 522 524 526

Binding Energy (eV)

6


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Figure 3. XPS spectra of un-doped and V-doped MoS2 associated with (a) Mo(3d), (b) S(2p) and (c) V(2p) core levels.

In order to confirm the doping concentration in

intensity

in

5

at%

vanadium-MoS2

samples.

MoS2, we used SIMS to identify the dopants

According to a Maxwell-Boltzmann distribution,

concentration and distribution. 5 at% vanadium-MoS2

sample shows higher V concentration compared to 2.5

population of the magnetic centers at =

at% vanadium-MoS2, in consistence with XPS

= −

analysis (supporting information, Figure S3). In

the

fabrication

of

diluted

magnetic

and

energy levels, respectively. ∆ is the

levels,

dopant is vital. Electron paramagnetic resonance

is a Boltsmann constant and ! is the EPR

measuring

(EPR) is one of the effective ways for identifying the 55

difference between the = and = − energy

semiconductors, effective substitutional doping of a

effective doping

∆

= () , where and are the

. Figure (4) shows EPR spectra of

temperature.

At

room

temperature,

~0.998 results in a very low EPR signal.

pure MoS2, 2.5 at% vanadium-MoS2 and 5 at%

However, at lower temperature (< 300 K),

vanadium-MoS2 respectively. EPR spectrum of un-

tends to decrease which means the lower number of

doped MoS2 shows a weak symmetrical signal at (g =

the spin at higher energy level ( = ), the larger

2.003), which is attributed to the unpaired electrons in the sample (Inset of Figure. 4(a))

56

. However, in V-

signals, which may be very weak or undetectable at

to the interaction of 3d1 electron spin with a nuclear

room temperature (300 K), can be visualized at

V(I=7/2) paramagnetic center (Figure. 4(a))

relatively lower temperature than 300 K

57-58

. EPR results bolster the earlier claim of

substitutional doping of V at Mo sites.

signal. This discussion also explains that some

demonstrate similar hyperfine lines, which correspond 51

levels. This in return generates the enhanced EPR

doped MoS2 samples, EPR spectra of both samples

spin of

energy difference between = and = − energy

60

. Figure

4(b) shows the temperature dependence of the spectra

At higher

of 5 at% vanadium-MoS2. Absence of peak other than

concentrations of V, the dipole-dipole interaction,

V+4 and significant enhancement in EPR signal

between the electronic moment of the paramagnetic

intensities further endorse the above claim of the

V+4 ions and the magnetic moment of the 51V nucleus,

effective doping of V+4 ions into the as-prepared

tends to increase 59. It may be the reason for a higher

samples.

7


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Page 8 of 16

Temp-300K (b) 5 at% V-MoS2

(a)

250K 225K

5 at%V MoS2

175K 100200 300400 500 600 Magnetic Field (mT)

2.5 at%V MoS2

125K

200 300 400 500 300 350 400 Magnetic Feild (mT) Magnetic Field (mT) Figure 4. EPR results of (a) Un-doped and V-doped MoS2 at 300 K. The inset is the enlarged part of the spectrum of the undoped MoS2 and (b) 5 at% vanadium-MoS2 at various temperatures lower than 300 K. MoS2, which shows diamagnetism in its pure

magnetization of 0.067 emu/g is achieved at 10 K,

form, can display ferromagnetism in the presence of

suggesting that the enhanced magnetization may be

zigzag edges or sulfur vacancy 11, 18, 31-32. In this work,

from V doping, not from defects. Reduction in the

the magnetic hysteresis loops of the three samples

edge terminated structure, which promotes the

were measured at different temperatures, as shown in

formation of ferromagnetism

Figure

weak

increase of V concentration, as evident from Raman

ferromagnetic signal (0.0019 emu/g at 15 kOe) after

results (Figure 2(b)), further supports the claim of

removing linear signal (to clearly visualize the

enhanced magnetization due to the higher V

ferromagnetic signals, as shown in supporting

concentration instead of edge terminated structure of

information, Figure S4). Edge terminated structure,

MoS2. Similar variation in magnetization due to the

defects or sulfur/oxygen vacancy may be attributed to

dopant concentration in MoS2

5(a-c).

Un-doped

the ferromagnetism

11, 18, 31-32

MoS2

exhibits

11, 18, 31-32

, with an

was also reported

61

. The hysteresis loops of

earlier , in which magnetization was enhanced from

2.5 at% vanadium-MoS2 at different temperatures

0.003 to 0.023 emu/g at 300 K with the increase of

show co-existence of paramagnetic and ferromagnetic

dopant (Cu) concentration from 2.5 at% to 5 at% in

signals. The magnetization at the applied magnetic

MoS2. Furthermore, a very high coercivity of 1.87

field of 15 kOe at 10 K is enhanced significantly

kOe is observed in the sample of 5 at% vanadium-

(0.0125 emu/g) compared to that at room temperature.

MoS2 (Figure. 5(c)), which is very high for a

For 5 at% vanadium-MoS2, similar to that of 2.5 at%

transition

vanadium-MoS2, very low magnetization is observed

semiconductor. The highest coercivity reported so far

at room temperature. However, the further enhanced

in the diluted magnetic semiconductor is around 1.3

metal

8


doped

diluted

magnetic

Page 9 of 16

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kOe 62. In Figure 5(d), 5 at% vanadium-MoS2 shows a

semiconductors in a thin film or bulk structures have

very low coercivity (81 Oe) at 300 K, whereas, with

not shown so high coercivity. Shape anisotropy has

the decrease in temperature, the coercivity tends to

induced high coercivity in pure Fe, Co and Ni

increase. At 100 K, the coercivity rises up to 550 Oe

nanowires65-67. In this research, the anisotropy field is

and then decrease to 150 Oe at 50 K. From ZFC/FC

as high as 15 kOe (Figure 5(c)), suggesting that even

measurement as shown in Figure 7(c), 50 K is close to

higher coercivity may be achieved by optimizing the

the boundary temperature (38 K), at which, the frozen

fabrication parameters. Figure 7(a-c) shows zero field

or aligned spins becomes much less aligned due to the

cooling (ZFC) and field cooling (FC) curves of V-

thermal vibration energy. Hence, the low coercivity at

doped MoS2 samples. All the samples show high

50 K may be associated with the boundary of

magnetization at very low temperature, suggesting the

transition. At 30 K, the coercivity slightly increases,

existence of paramagnetic phase. It is to note that the

but still very low (500 Oe). At 10 K, coercivity shows

hump in the curves of the pure MoS2 sample is from

a significant increase and reaches its maximum value

the signal of oxygen. For 2.5 at% V doped MoS2,

of 1.87 kOe. It is to note that MoS2 ribbons in this

ZFC/FC and reverse susceptibility curves (shown in

work are amorphous. Hence, MoS2 itself without

the inset) both suggest that the sample exhibits a

crystalline anisotropy cannot induce high coercivity

paramagnetic like behavior at the temperature above 5

The coercivity should be due to the combination effect

K. For 5 at% vanadium-MoS2, there is an evident

of the doping of

V and doping induced stress or

transition temperature at around 38 K, as seen from

defects in MoS2. V doping promotes the charge

the reverse susceptibility curve in the inset of Figure

transfer between V and S ions and generates the

7(c). From Figure. 7(c), high coercivity can only be

anisotropic geometry of

V – S bonds (Figure 6).

observed below the transition temperature, indicating

Charge transfer between V and S has been evidenced

the high coercivity is associated with the transformed

by XPS analysis. This anisotropy in localized charge

ferromagnetic phase at low temperature. Figure 7(d)

distribution due to V – S bonding and pinning effect

shows the magnetic moment of V, given that all the

by the defects in-between or pinning effect induced by

moment is from V element. It shows that 2.5 at%

63-64

. In order to

vanadium-MoS2 indicates a higher magnetic moment

further understand the mechanism, we annealed 5 at%

than that of 5 at% vanadium-MoS2. The low magnetic

vanadium-MoS2 to remove the stress and the

moment of dopant with high concentration doping is

coercivity is significantly reduced (378 Oe at 10 K)

very common for diluted magnetic semiconductors,

(supporting

EPR

which is due to the antiferromagnetic coupling of

measurement indicated that some of V left the

dopant moments for higher doping concentration. For

substitutional sites (supporting information, Figure

direct measurement, magnetic force microscopy

stress may induce the high coercivity

information,

Figure

S5).

S6). Most importantly, different from thin films, the

(MFM) of un-doped MoS2, 2.5 at% and 5 at%

2D material has strong shape anisotropy due to its

vanadium doped MoS2 was performed at 300 K. Very

two-dimensional

also

weak magnetic domains have been observed. It may

contribute to the high coercivity since, in diluted

be due to the very weak ferromagnetism at room

magnetic semiconductors, transition metal doped

temperature (supporting information, Figure S7).

confinement,

which

may

9


0.002 (a) MoS2

0.010

10K

0.001 0.000 150K

0.0002

-0.005

0.0000 -0.0002 -0.0004

-0.002 -15

-10

-5

0

-0.010

-0.4 -0.2 0.0 0.2 0.4

5

10

15

0.06 (c) 5 at% V 0.04 0.02 0.00

15x-300K 15x-150K 0.06

-0.02

150K

-0.03 -0.06

-0.06 -15

10K

0.03 300K 0.00

-0.04 -10

-5

0

Page 10 of 16

10K 300K

0.000 150K

0.0004

-0.001

(b) 2.5 at% V

0.005

300K

Coercivity (KOe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Magnetization (emu/g) Magnetization (emu/g)


-2

5

-1

0

10

1

2

15

0.0008 10K 0.0004 150K 0.0000 300K -0.0004 -0.0008 -0.4 -0.2 0.0 0.2 0.4

-15 -10 -5 0 5 10 15 2.0 (d) 10K 5 at% V 1.8 1.6 1.4 1.2 1.0 0.8 100K 150K 0.6 30K 200K 0.4 250K 300K 0.2 50K 0.0

Magnetic Field (kOe) Figure 5. Hysteresis loop of (a) MoS2. (b) 2.5 at% vanadium-MoS2 and (c) 5 at% vanadium-MoS2. (d) Coercivity of 5 at% vanadium-MoS2 at different temperatures.

Figure 6. Vanadium Doping in MoS2 and charge transfer between V and S ions.

10


M (emu/g)

0.0004 (a) MoS2

0.010

FC500 ZFC500

0.0003 0.0002

0.006

0.0001

0.004

0.0000

0.002

0.05 (c) 5

at% V

FC500 ZFC500

0

50 100 150 200 250 300

Temperature (K)

0.000 50

100

150

200

at% V

250

300

FC500 ZFC500 χ (1/emu)

0.04 0.03 0.02 0.01

0 0.90 (d)

50

100

150

2.5 at% V-MoS2

Temp=5K

0.75

38 K 50 100 150 200 250 300

0.70

Temperature (K)

100

300

0.80

0

50

250

0.85

5 at% V-MoS2

0.00 0

200

uB / V Atom

-0.0002 0

(b) 2.5

0.008

-0.0001

M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


χ (1/emu)

Page 11 of 16

150

200

250

300

0.65

Temperature (K) Figure 7: Zero field cooling (ZFC) and field cooling curves (FC) of (a) un-doped MoS2. (b) 2.5 at% vanadium doped MoS2 with magnetic reverse susceptibility in the inset (c) 5 at% vanadium-MoS2 with magnetic reverse susceptibility in the inset (d) Magnetic moment per V atom for 2.5 at% vanadium MoS2 and 5 at% vanadiumMoS2 samples at 5 K.

observed in 5 at% vanadium-MoS2 sample. This high 4. CONCLUSION

coercivity may attribute to the combination effects of V doping, anisotropy due to V – S bonds, pinning

V-doped MoS2, 2D nanosheets, with different V

effects by defects or stress and shape anisotropy of

concentrations (0, 2.5 and 5 at%), were synthesized

nanosheets due to its 2D nature.

using a hydrothermal method. TEM, Raman, XPS and EPR analysis demonstrate effective substitutional

AUTHOR INFORMATION

doping of V atoms at Mo sites. Impurities such as MoO2 and MoO3 are observed for pure MoS2 or MoS2

Corresponding Author

with a low doping concentration of V. However, for 5

* [email protected]

at% vanadium doped MoS2, the oxides disappear. ACKNOWLEDGMENT

Pure MoS2 show very weak ferromagnetism due to the existence of defects. V doping induces enhanced

This work is funded by Australian Research Council

magnetism. In addition, a very large coercivity of 1.87

Discovery

kOe, below the transition temperature (31 K), is

Project

DP140103041

Fellowship FT160100205. 11


and

Future


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ferromagnetism

5 at% V

0.06 Temp : 10K 0.04 0.02 0.00

2.5 at% V

-0.02

Coercivity (KOe)

Magnetic Properties. J. Phs. Chem. B 2005, 109, 3094-3098. (66) Sellmyer, D. J.; Zheng, M.; Skomski, R., Magnetism of Fe, Co and Ni nanowires in self-assembled arrays. J. Phys.: Condens. Matter 2001, 13, R433.

Magnetization (emu/g)

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Page 16 of 16

-0.04 -0.06 -8 -6 -4 -2

0

MoS2

2 1 0

10K

5 at% V

100K150K 250K 30K 50K 200K 300K

2

4

6

Magnetic Field (kOe)

16


8

Inducing High Coercivity in MoS2 Nanosheets by Transition Element

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