- Nano Express
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Ferromagnetism in Transitional Metal-Doped MoS2 Monolayer
© Fan et al. 2016
Received: 21 February 2016
Accepted: 15 March 2016
Published: 22 March 2016
Manipulating electronic and magnetic properties of two-dimensional (2D) transitional-metal dichalcogenides (TMDs) MX2 by doping has raised a lot of attention recently. By performing the first-principles calculations, we have investigated the structural, electronic, and magnetic properties of transitional metal (TM)-doped MoS2 at low and high impurity concentrations. Our calculation result indicates that the five elements of V-, Mn-, Fe-, Co-, and Cu-doped monolayer MoS2 at low impurity concentration all give rise to the good diluted magnetic semiconductors. By studying various configurations with different TM-TM separations, we found that the impurity atoms prefer to stay together in the nearest neighboring (NN) configuration, in which the doped TM atoms are FM coupling except for Fe doping at 12 % concentration. For V, Mn, and Fe doping, the total magnetic moment is smaller than the local magnetic moment of the dopants because the induced spins on the nearby host atoms are antiparallel to that of the doped atoms. In contrast, Co and Cu doping both give the higher total magnetic moment. Especially, Cu doping induces strong ferromagnetism relative to the local spins. However, the atomic structures of Co- and Cu-doped MoS2 deviate from the original prismatic configuration, and the magnetic moments of the doped systems decrease at 12 % impurity concentration although both elements give higher magnetic moments at 8 % impurity concentration. Our calculations indicate that V and Mn are promising candidates for engineering and manipulating the magnetism of the 2D TMDs.
Research on two-dimensional (2D) transitional metal dichalcogenides (TMDs) has attracted considerable attention due to their distinct electronic, optical, and catalytic properties [1–5]. Group 6 TMDs (MX2, M = Mo, W, and X = S, Se, Te) hold promise for flexible and transparent electronics applications owing to their sizeable band gaps ranging from 1 to 2 eV. Current results have indeed revealed that MoS2 and WS2 form an exciting family of transistors [6–11]. On the other side, MoS2 and WS2 are nonmagnetic semiconductors. Accordingly, extensive studies have been performed to investigate the feasible ways to introduce magnetism to MoS2, such as morphology fabricating [12–14], external strain, [15–17] and impurity doping [18–28].
Developing approaches to effectively induce and manipulate magnetism are critical to the use of the magnetic nanostructures in quantum information devices. Among kinds of magnetic property engineering methods, doping attracts more attentions [18–28]. On the basis of previous studies, transitional metal (TM) atom doping can effectively induce magnetism into MoS2. For example, magnetism is observed for Mn [18, 20, 22, 24, 26, 28], Fe [18, 22, 24, 26, 28], Co [18, 22, 24, 26, 28, 29], Cr [18, 24], Zn [22, 24], Cd , and Hg  doping. And the magnetic moment of the 3d TM-doped MoS2 increases with the d-band filling of the TM dopants . Additional, spin polarization was found in MoS2 with S atoms replaced by incomplete d-band atoms, such as Fe and V , and Group VA and III elements, such as N, P, As, B, Al, and Ga . Moreover, adsorption of various atoms, such as H, B, C, N, and F, is also effective to turn MoS2 from nonmagnetic to magnetism . It is worth noting that no magnetism is observed in V-doped MoS2 based on Ref , but according to Ref  and , V doping induces more than 1-μB magnetic moments into monolayer MoS2. And based on Lee’s study , the nonmagnetic element Cu doping brings strong magnetism into the doped MoS2.
More recently, substitutional doping MoS2 monolayer with magnetic atom and the interactions between the doped atoms has draw intensive attentions. Ramasubramaniam  have studied the Mn-doped monolayer MoS2 at concentration of 10–15 % by performing the density functional theory calculations and Monte Carlo simulations, which shows that the doped Mn atoms couple ferromagnetically. Schwingenschlögl et al.  predict that the doped TM atoms are ferromagnetic (FM) ordering for Mn, Zn, Cd, and Hg doping at 6.25 % impurity concentration and antiferromagnetic (AFM) ordering for Fe and Co doping. Similarly, Mishra et al.  predict the FM ordering in fairly diluted Mn doping MoS2, MoSe2, MoTe2, and WS2 and AFM coupling for Fe and Co doping at large separations. In contrast, a later study  found the ground states of Mn-, Fe-, and Co-doped MoS2 are all FM.
Clearly, current studies on the magnetic interactions in Mn-, Fe-, and Co-doped MoS2 disagree with each other. However, the magnetic ordering of the dopants as well as the orientations of the induced spins on the host atoms are critical factors to determine the magnetic property of the doped system. In this context, we examined the cases of different impurity concentrations and separations of the doped atoms to study the electronic and magnetic properties of TM-doped monolayer MoS2 and to find out the magnetic feature of the TM-doped 2D TMDs. Five 3d TM elements including V, Mn, Fe, Co, and Cu doping were studied in the present work by accurate calculations. Our calculations result indicates that the doped TM atoms prefer to stay in the nearest neighboring configurations and ferromagnetic coupling with each other. Additionally, we found that at high impurity concentrations, the local structures around the dopants were deformed from the original prismatic configurations. More importantly, it was found that V and Mn doping are the good candidate to induce and manipulate the magnetism into 2D TMDs, but Cu is not although it can induce strong magnetism.
The first-principles calculations were carried out by using the Vienna ab initio simulation package (VASP) based on the density functional theory (DFT) . The electron-ion interactions were described by the projector-augmented wave (PAW) method [33, 34]. The generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE-GGA)  formula was used for the electronic exchange-correlation potential. In addition, Hubbard-U parameterization method with a common U value of 3.0 eV was assigned to all the 3d impurities. The U parameterization was not used for the host materials since there little impact on the magnetic ordering [18, 26, 36]. The substitutional TM doping was calculated with a 5 × 5 × 1 supercell. A vacuum region of 15 Å was added to avoid interactions between adjacent images. The Brillouin zone was sampled by the Monkhorst-Pack method  with a 2 × 2 × 1 k-point grid. The wave functions were expanded in a plane wave basis with an energy cutoff of 600 eV. The convergence criterion for the self-consistency process was set to 10−5 eV between two ionic steps, and the convergence criteria of 0.02 eV/Å were adopted for total energy calculations.
Results and Discussions
More importantly, Fig. 3 clearly shows the induced spin polarization for all the five doped systems. The corresponding magnetic moments are 1, 1, 2, 3, and 4.9 μB for V, Mn, Fe, Co, and Cu doping, respectively. An isolated V atom has a 3d 44s 1 electronic configuration with one valence electron less than Mo (4d 55s 1), which reflects the magnetic moment of the V-doped MoS2. The electronic configurations of isolated Mn, Fe, Co, and Cu atom are 3d 54s 2, 3d 64s 2, 3d 74s 2, and 3d 104s 1, respectively; they have one, two, three, and five additional valence electrons compared to Mo atom, which consist with the magnetic moment of Mn-, Fe-, Co-, and Cu-doped system. As shown in Fig. 3, the spin splitting appears near to the Fermi level, which is contributed by the defect states associated with the doped TM atom, p states of the adjacent S atoms, and d states of the nearby Mo atoms. We further calculated the spin-resolved charge density to investigate the distribution of these magnetisms.
In the last part, we have studied TM doping at 4 % impurity concentration by calculating one TM atom replacing one Mo atom in a 5 × 5 × 1 supercell, in which the distance between the dopants is around 16 Å. We further calculated two TM atoms replacing two Mo atoms in a 5 × 5 × 1 surpercell to investigate the TM doping at 8 % impurity concentration. There configurations with different TM-TM separations were considered: NN configurations in which the two TM atoms are in the nearest neighboring position with TM-TM distance of 3.2 Å, the second NN configurations in which the two TM atoms are in the next nearest-neighboring position with TM-TM distance of 5.5 Å, and the third NN configuration in which the distance between the two doped TM atoms are 6.5 Å.
The magnetic moments (Σμi/μtotal) for V-, Mn-, Fe-, Co-, and Cu-doped MoS2 with impurity concentration at 4, 8, and 12 %
As shown in Fig. 6, the spins of the two nearest neighbored dopants are parallel to each other for all the five doped systems. For V, Mn, and Fe doping, the induced spins on the nearby S and Mo atoms are antiparallel to that of the dopants. Thus, the total magnetic moments of the V-, Mn-, and Fe-doped system are smaller than the local magnetic moments on the dopants. As for Co and Cu doping, the total magnetic moments of the doped system are much larger than the local magnetic moments of the impurities because the spin polarizations on the nearby S and Mo atoms are all parallel to that of the dopants. Particularly, the total magnetic moment of the Cu-doped MoS2 in NN configuration at impurity concentration of 8 % is 3.6 μB although the local magnetic moments on the two Cu atoms are only 0.5 μB.
Figure 6 also shows that for V, Mn, Fe, and Co doping in the second NN and third NN configurations, the two dopants are FM coupling or even weakly AFM coupling (the energy difference between the FM and AFM states is 6 meV for V doping in the second configuration). This is similar with the NN configuration. Additionally, the magnetic orderings among the dopants and the nearby host atoms in second and third NN configuration for the four elements are similar with the situation in the NN configuration. In detail, the induced spins on the nearby S and Mo atoms are antiparallel to the impurities for V, Mn, and Fe doping, which leads to the smaller total magnetic moment relative to the local magnetic moments on the dopants, while the FM coupling between the doped Co atoms and the nearby S and Mo atoms makes the total magnetic moment larger than the local ones on the dopants. Moreover, the local magnetic moments of the Fe and Co dopants in second and third NN configurations are larger than those in the NN configuration; thus, the total magnetic moments of second and third NN configurations are larger than those in the NN configuration. For Cu doping in the second and third NN configurations, the AFM states are energetically more stable than the FM states; this is differing from the NN configuration. Figure 6 shows the AFM coupling between the two doped Cu atoms with large separations and the FM exchange with the nearby host S and Mo atoms like the NN configuration. Hence, the total magnetic moments for Cu doping in the second and third NN configurations are very close to 0.
According to our calculations, for the five elements except for Cu doping, the magnetic ordering between the doped atoms and host atoms in the second and third configurations is similar with those in the NN configurations. In contrast, Mishra et al.  predicted AFM coupling for the dopants with large separations and FM coupling for the dopants in NN configurations for Fe and Co doping. Additionally, according to Schwigenschlogal et al.’s study , Fe and Co doping also lead to AFM ground state in large separations. In this situation, we recalculated the NN, second and third configurations without U parameterizations. The energy differences between the FM and AFM states are summarized in Additional file 1: Figure S2. It shows that for Fe and Co doping in large separations, the AFM states are more favorable energetically, which agrees with previous results [37, 39]. More importantly, we found that the NN configurations are more favorable than the other two configurations with large separations. The total energy of the NN configuration is less than the second and third configurations by 0.2, 0.5, 0.8, 1.0, and 1.3 eV for V, Mn, Fe, Co, and Cu doping, respectively. This is consistent with Liu’s study which shows that the V atoms prefer to stay together in MoS2 monolayer.
Figure 7 shows that the magnetic ordering among the dopants and the nearby host atoms at 12 % impurity concentration is similar with the situation at 8 % impurity concentration; for V and Mn (Co and Cu) doping, the induced spins on the nearby host atoms are antiparallel (parallel) to those of the dopants. Thus, the total magnetic moment of V and Mn (Co and Cu) doping is less (larger) than the local magnetic moments of the three dopants. Additionally, our calculations result indicates that for V and Mn doping, the magnetic moments of the doped MoS2 increase as the increasing impurity concentration, whereas the magnetisms of Co- and Cu-doped system decrease when impurity concentration increases from 8 to 12 %.
Our study on MoS2 with TM doping at 4 % concentration tells us all the five 3d elements of V, Mn, Fe, Co, and Cu doping which give rise to the good diluted magnetic semiconductors. Additionally, we have found that the doped TM atoms prefer to stay in the nearest neighboring positions at high concentrations and couple with each other ferromagnetically. For V, Mn, and Fe doping, the induced spins on the nearby host atoms are antiparallel to that of the impurities, whereas for Co and Cu doping, they are parallel to that of the dopants. It indicates that the local structures around the impurities are deformed from the original prismatic configurations for Co and Cu doping at high impurity concentration although both doping induce strong ferromagnetism into the doped system. Our calculations show that, besides Mn, V is also good candidate to induce and manipulate the magnetism in 2D TMDs.
This work was supported by the National Natural Science Foundation of China (NNSFC) (21273172), the program for New Century Excellent Talents in University (NCET-13-0471). This work was also supported by the 111 Project (B08040) and the Fundamental Research Funds for the Central Universities (3102015BJ (II) JGZ005, 3102015BJ023) in China. The supports from the Beijing Computational Science Research Center and the CAEP Chengdu Science and Technology Development Center and the Chengdu Green Energy and Green Manufacturing Technology R&D Center are also appreciated.
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