Article Cite This: Chem. Mater. 2017, 29, 9066-9074
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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 Jiabao Yi*,† †
School of Materials Science and Engineering, UNSW, Sydney, New South Wales 2052, Australia Department of Materials Science and Engineering, National University of Singapore, Singapore 119260, Singapore § School of Physics and the Australian Institute for Nanoscale and Technology, The University of Sydney, Sydney 2006, Australia ⊥ National Isotope Centre, GNS Science, P.O. Box 31312, Lower Hutt 5010, New Zealand ¶ SIMS Facility, Office of the Deputy-Vice Chancellor (Research and Development), Western Sydney University, Locked Bag 1797, Penrith, New South Wales 2751, Australia # College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710069, China ∥ Department of Materials Science and Engineering, Monash University, Victoria 3800, Australia ‡
S Supporting Information *
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 disappeared. 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 in-between defects, stress induced by doping, and shape anisotropy due to two-dimensional nature of MoS2 ribbons.
1. INTRODUCTION
with various number of layers, due to quantum confinement and interlayer interaction,9 have made MoS2 promising for multiple applications such as transistors,7 solid lubricants,10 spintronic devices,11 solar cells,12 catalyst for hydrogen,10,13 sensors,12,14 photodetectors,11 and nanodevices.15 Recently, electrical,16,17 mechanical,18 and optical19,20 properties of MoS2 were extensively investigated experimentally and theoretically with exceptional outcomes. Because of appropriate bandgap (1.8 eV) and strong spin−orbit coupling, MoS2 is one of the
The discovery of graphene, a two-dimensional (2D) material, is considered to be the pivotal point to revolutionize the future of nano devices,1 as it has attracted the worldwide focus in investigating the other 2D materials like silicene, boron nitride, and transition metal dichalcogenides (TMDCs).2−4 Currently, over 40 different TMDCs have been reported including MoS2, TiSe2, WSe2, NbS2, ZrS2, VSe2, and WS2, etc.5,6 Among all the reported TMDCs, covalently bonded MoS2 stacked with the weak van der Waals forces is the center of attention in past few years. High carrier mobility and flexibility, large spin−orbit coupling, high on/off current ratio,7,8 and easy tuning of bandgap (bulk bandgap 1.2 eV and monolayer bandgap 1.8 eV) © 2017 American Chemical Society
Received: June 22, 2017 Revised: October 16, 2017 Published: October 16, 2017 9066
DOI: 10.1021/acs.chemmater.7b02593 Chem. Mater. 2017, 29, 9066−9074
Article
Chemistry of Materials
Figure 1. (a) SEM micro image of undoped MoS2. TEM micro images of (b) Undoped MoS2, (c) 2.5 at% vanadium-MoS2, and (d) 5 at% vanadiumMoS2. Insets are SAED patterns of the corresponding samples.
which provides an effective way to prevent the formation of oxide, a notorious issue, in 2D materials during hydrothermal process. In addition, for the doped samples, both paramagnetic and ferromagnetic phases coexist at room temperature. Furthermore, an exceptionally large coercivity (1.87 kOe) for 5 at% V-doped MoS2 was observed at 10 K, which may pave a way for achieving high coercivity in 2D material based DMS.
promising candidates for spintronics applications. Spin based data manipulation and storage are emerging research areas owing to numerous benefits over charge based devices. Therefore, in past years, different materials have been investigated for suitability such as oxide thin films and nanowires21,22 and 2D materials including graphene and TMDCs23 due to their unique spin dependent properties. Making MoS2 magnetic is one of the most important steps for achieving qualified spintronics materials. However, the research of magnetic behaviors of MoS2 mostly depended on theoretical calculations. These calculations showed that substitutional doping of transition metals could induce magnetism in MoS2.24−30 Pure MoS2 being a diamagnetic in pristine form was also reported to demonstrate ferromagnetism, which attributed to the defects and zigzag edges.11,18,31,32 Experimentally, ferromagnetism was observed in Fe, Co, Ni, and Mn doped MoS2 single crystals by ion implantation.33 In this work, we use a hydrothermal synthesis method to produce MoS2 nanosheets chemically doped with vanadium(V) at different atomic concentrations of 0, 2.5, and 5 at%. Without V doping, oxide phases have been discovered. However, when the doping concentration reaches 5 at%, the oxide disappears,
2. EXPERIMENTAL DETAILS A hydrothermal synthesis method was used to produce V-doped MoS2 nanosheets. All analytical grade chemicals were used in aforesaid experiments without any purification. Ammonium metavendate (NH4VO3), ammonium molybedate [(NH4)6Mo7O24.4H2O], and elemental sulfur (S) powder were picked as the vanadium (V), molybdenum (Mo), and sulfur (S) sources, respectively. Initially, 3 × 1 g ammonium molybedate and 0.35 × 3 g sulfur were added into a 100 mL beaker, and then 60 mL of distilled (DI) water and 3 × 8 mL of hydrazine monohydrate (N2H4.H2O), as a reducing agent, were further added into a beaker containing chemicals. After stirring, for 20 min at 800 rpm, an aqueous solution was divided into three equal parts in different 100 mL beakers. Then 0.0496 and 0.102 g of ammonium metavendate were dropped into the beakers to make V concentration 0, 2.5, and 5 at%, respectively. After stirring, for 30 min, final mixtures 9067
DOI: 10.1021/acs.chemmater.7b02593 Chem. Mater. 2017, 29, 9066−9074
Article
Chemistry of Materials
Figure 2. (a) Raman spectra of MoS2 with different V concentrations. (b) Enlarged part of the spectra in panel a. were transferred to three new/unused 50 mL Teflon-lined stainless steel autoclaves to avoid any possibility of contamination. The autoclaves were further filled with DI water to make final total volume 80%, then sealed and maintained at 180 °C in an oven for 48 h. The produced black precipitates were centrifuged and washed with ethanol and DI water successively for numerous times. Finally, the black precipitates were dried in an air oven at 60 °C for 3 h. The phases of nanoribbons were characterized by X-ray diffractometry (XRD, PANalytical Xpert Multipurpose X-ray Diffraction System) with Cu Kα radiation and Raman spectroscopy (Renishaw inVia Raman Microscope, fitted with a diffraction grating of 1800 lines/mm, excited with a radiation of 514 nm argon ion laser and calibrated with Si Single crystal). Microstructure and morphology were investigated using scanning electron microscopy (SEM, FEI Nova 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 were dried naturally. Valence state and composition were examined by X-ray photoelectron spectroscopy (XPS) with Thermo Scientific ESCALAB 250i X-ray photoelectron Spectrometer (calibrated by C 1s = 284.8 eV). Secondary ion mass spectroscopy (SIMS) analysis was performed to determine the depth profile and dopant (V) distribution in MoS2 by pressing the nanosheets into pellets. SIMS was conducted using Cameca IMS 5fE7 SIMS instrument operated with O2+ ion gun (8 nA ion current), an impact energy of 7.5 keV, and rastered with 180 × 180 μm2 region of the surface. Electron paramagnetic resonance (EPR) spectrum (Bruker EMX-plus X-Band EPR Spectrometer) and superconducting quantum interference device (SQUID, Quantum design-XL-5) were used for the measurement of magnetic properties.
shows the typical micrograph of the undoped MoS2 examined by SEM, indicating that the synthesized MoS2 nanostructures are nanosheets, cross-linked, and overlapped each other. The 2.5 at% vanadium-MoS2 and 5 at% vanadium-MoS2 samples show similar morphologies to that of undoped sample. Further analysis was done by high-resolution TEM. Figure 1b−d shows the high-resolution images of undoped, 2.5 at% vanadiumMoS2 and 5 at% vanadium-MoS2 samples, respectively. All the images exhibit amorphous structures, consistent with SEM analysis. The insets are corresponding selected area electron diffraction (SAED) patterns, indicating noncrystalline structures, which supports XRD and high-resolution TEM results. Energy dispersive spectroscopy (EDS) shows quite uniform distribution of vanadium inside the MoS2 (Supporting Information, Figure S2A,B). In addition, nanoclusters have not been seen in high-resolution TEM images, suggesting that vanadium is in substitutional site. Raman spectroscopy was employed to identify and examine the structural defects and phases of the undoped MoS2, 2.5 at% vanadium-MoS2, and 5 at% vanadium-MoS2 samples. Figure 2a shows the Raman spectra in a large scale. The strong peak at 520 cm−1 is from the silicon substrate. MoS2 characteristics peaks have also been observed in Figure 2a. To see more clearly, we enlarge the peak of the MoS2 part at around 400 cm−1, as shown in Figure 2b. Dominant peaks, E12g (in-plane Mo−S phonon mode) and A1g (out of plane Mo−S mode) are displayed at 379.4 and 403.5 cm−1, respectively, which are associated with the MoS2 in all samples.34−36 In 2.5 at% vanadium-MoS2, the E12g and A1g peaks shift to 377.7 and 401.8 cm−1, whereas they remained insensitive to further V doping (5 at% vanadium-MoS2). This blue shift of Raman peaks, after transition metal doping, has also been reported earlier.37,38 Significant reduction in peak intensity, broadening of peaks, and peak shift may be related to the doping induced
3. RESULTS AND DISCUSSION Vanadium-doped MoS2 at 0, 2.5, and 5 at% is denoted by MoS2, 2.5 at% vanadium-MoS2, and 5 at% vanadium-MoS2, respectively. XRD patterns (Supporting Information, Figure S1) for all samples did not display any peak, indicating the amorphous structure of the as-prepared samples. Figure 1a 9068
DOI: 10.1021/acs.chemmater.7b02593 Chem. Mater. 2017, 29, 9066−9074
Article
Chemistry of Materials
Figure 3. XPS spectra of undoped and V-doped MoS2 associated with (a) Mo(3d), (b) S(2p), and (c) V(2p) core levels.
defects.39−41 Terrace and edge terminated structures of MoS2 excite the E12g and A1g vibration modes.38 Figure 2b shows that, with the increase of V doping, the intensity ratio of A1g/E12g decreases. Hence, V doping in MoS2 reduces the edgeterminated structure.35,36,38 Pure MoS2 demonstrates the highest A1g/E12g ratio as compared to other samples. Moreover, Raman spectra of all samples also display some extra peaks associated with MoO2 and MoO3 in the MoS2 without doping and low doping concentrations of V (i.e., 2.5 at% vanadiumMoS2) as shown in Figure 2a, which is a notorious issue unable to avoid during hydrothermal synthesis of 2D materials. However, for 5 at% vanadium-MoS2, no trace of oxide could be found, suggesting we may provide an effective way solving the problem by doping. It may be due to that V doping promotes the activity for the formation of MoS2. In addition, vanadium oxide can be easily transferred to V2S3, having a high decomposition temperature,42 which may be one of the reasons for preventing the formation of molybdenum oxide. Furthermore, no vanadium related peak could be discovered in all
the spectra, further supporting that V atoms successfully accommodate as a substitutional element at the doping sites of MoS2. Electronic state and chemical bonding of samples, with and without V doping, were investigated by XPS. Figure 3a and b show the binding energies of Mo (3d) and S (2p) core levels, respectively. Figure 3a displays two Mo (3d) doublets of undoped MoS2 with corresponding binding energies of 228.76 and 232.9 eV for Mo (3d5/2), which can be related to the Mo4+ and Mo6+, respectively, whereas the extra peak at 225.9 eV is associated with S (2s) of MoS2.38,41,43,44 In 2.5 at% vanadiumMoS2 and 5 at% vanadium-MoS2 samples, no new doublet is observed. Hence, valence state remains the same after V doping. The Mo (3d5/2) peaks at 228.61 and 228.84; 232.85 and 232.97 eV are associated with the Mo4+ and Mo6+, respectively.41,43−47 Similarly, in Figure 3b, the undoped MoS2 sample shows three S (2p) doublets with the S (2p3/2) peaks at 161.59, 163.37, and 168.32 eV, which correspond to S2−, S2−2 (polysulfide), and S4+, respectively.41,43−47 After V doping, one 9069
DOI: 10.1021/acs.chemmater.7b02593 Chem. Mater. 2017, 29, 9066−9074
Article
Chemistry of Materials
Figure 4. EPR results of (a) undoped 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.
In the fabrication of diluted magnetic semiconductors, effective substitutional doping of a dopant is vital. Electron paramagnetic resonance (EPR) is one of the effective ways for identifying the effective doping.55 Figure 4 shows EPR spectra of pure MoS2, 2.5 at% vanadium-MoS2, and 5 at% vanadiumMoS2, respectively. EPR spectrum of undoped MoS2 shows a weak symmetrical signal at (g = 2.003), which is attributed to the unpaired electrons in the sample (inset of Figure 4a).56 However, in V-doped MoS2 samples, EPR spectra of both samples demonstrate similar hyperfine lines, which correspond to the interaction of 3d1 electron spin with a nuclear spin of 51 V(I = 7/2) paramagnetic center (Figure 4a).57,58 EPR results bolster the earlier claim of substitutional doping of V at Mo sites. At higher concentrations of V, the dipole−dipole interaction, between the electronic moment of the paramagnetic V+4 ions and the magnetic moment of the 51V nucleus, tends to increase.59 It may be the reason for a higher intensity in 5 at% vanadium-MoS2 samples. According to a n upper = e−(ΔE / kT ), where Maxwell−Boltzmann distribution, n
additional doublet appears for S (2p) in 2.5 at% vanadiumMoS2 and 5 at% vanadium-MoS2, which indicates the presence of extra S species. The red and blue color curves in Figure 3b display the binding energies of S (2p3/2). From the overlapping doublets spectra, the corresponding binding energies of peaks at 161.55 and 161.72; 163.47 and 163.68; 168.37 and 168.52 eV can be related to the S2−, S2−2 (polysulfide), and S4+, respectively.41,43−48 Peaks with minor intensities at 166.25 and 166.38 eV are also associated with S4+.49,50 Moreover, it is evident from the XPS results that the concentration of Mo6+ and S4+ tends to rise with the increase in doping concentration and reaches the maximum in 5 at% vanadium-MoS2 sample. This increase in the concentration of Mo6+ and S4+ may be associated with the disturbance of internal charge balance due to aliovalent dopant (V). In response, compensation defects tend to form in the lattice, which generates cationic vacancies. Hence, electron deficiency, which traps the extra electrons, is produced and can result into higher valence state of the Mo and S.51,52 The V (2p) edge of the spectra is composed of one doublet as shown in Figure 3c. V (2p3/2) peaks at binding energies of 516.63 and 516.74 eV, with a split of 7.6 eV, corresponding to V+4.53,54 The absence of Vo suggests the successful substitutional V doping into MoS2. In addition, XPS analysis also indicates that the exact doping concentration of V is 1.42 at% and 3.91 at% for V+4 in 2.5 at% vanadium-MoS2 and 5 at% vanadium-MoS2, respectively, slightly lower than the nominal concentration. To confirm the doping concentration in MoS2, we used SIMS to identify the dopants concentration and distribution. The 5 at% vanadium-MoS 2 sample shows higher V concentration compared to 2.5 at% vanadium-MoS2, in consistence with XPS analysis (Supporting Information, Figure S3).
lower
nupper and nlower are the population of the magnetic centers at 1 1 S = 2 and S = − 2 energy levels. ΔE is the difference between 1
1
the S = 2 and S = − 2 energy levels, k is a Boltzmann constant, and T is the EPR measuring temperature. At room temperature, n upper ≈ 0.998 results in a very low EPR signal. However, at n lower
lower temperature (
