Phase-transfer induced room temperature ferromagnetic behavior in 1T@2H-MoSe2 nanosheets

Manipulating electronic and magnetic properties of two-dimensional transitional-metal dichalcogenides has raised a lot of attention recently. Herein we report the synthesis and ferromagnetic properties of phase-transfer induced room temperature ferromagnetic behavior in 1 T@2H-MoSe2 nanosheets. Experimental results indicate the saturated magnetization of the 1 T@2H-MoSe2 compound increases first and then decreases as the increasing of 1 T-MoSe2 phase, where 65.58% 1 T-MoSe2 phase incorporation in 2H-MoSe2 could enhance the saturated magnetization from 0.32 memu/g to 8.36 memu/g. Besides, obvious magnetoresistance behaviors are observed in these samples, revealing their potential applications in future spintronics.

. On the other hand, owing to the increasing need of micro devices in modern electronic, nano-scaled semiconductors are more and more popular in applying to these fields [6][7][8] . In this case, two-dimensional (2D) transitional metal dichalcogenides (TMDs) became the ideal materials to meet the demand for applications in nano-scaled semiconductor devices [9][10][11] . In recent years, TMDs have been investigated by many researchers owing to their excellent properties in various fields such as nanoscale field-effect transistors, phototransistors, sensors, lithium-ion battery, and photocatalysts [12][13][14][15] . Among these TMDs, MoX 2 (X = S, Se) are the most studied materials, which are popular for their unique graphene-like structures (X-Mo-X), bounded together by van der Waals interactions 16,17 . Compared with graphene, MoX 2 possesses intrinsic large band gaps (1.3-1.8 eV) in their monolayer form and flexibility of MoX 2 atomic layers, which makes it possible for their applications in nanoelectronic and optoelectronic devices on both conventional and flexible substrates [18][19][20][21] . Beyond that, the well defined spin-splitting property of MoX 2 makes them as promising spintronics devices 22,23 . However, with the development of spintronics, semiconductors possessing excellent magnetic properties are in great demand for applications 24 . Therefore, realizing and manipulating ferromagnetism in MoX 2 nanosheets become the critical issue and challenging problems to be solved. Just like traditional DMSs materials, doping magnetic ions into MoS 2 is an efficient way to induce the RTF, which are reported both theoretically and experimentally in recent years 25 . Results indicate that all the dopants induced ferromagnetism are mainly focusing on the defects, however, magnetic clusters, and secondary phases are possible as the main contributors to the observed ferromagnetism [26][27][28] . For this reason, some non-magnetic ions are selected as the dopants in experiments. As reported in our previous work, copper ions as the dopants are induced in MoS 2 nanosheets to make it become ferromagnetic and obtained a high Curie temperature up to 930 K 29 . What's more, many other ways to introduce ferromagnetism into two dimension TMDs materials have been explored both theoretically and experimentally. It is predicted that ferromagnetism appears when MoS 2 nanoribbons are formed with zig-zag edges [30][31][32][33][34] . Besides, the intrinsic structure transformation could be an efficient method to introduce robust RTF 35,36 , where the structure transformation related RTF in MoSe 2 is seldom reported. In this paper, we synthesize 1 T@2H-MoSe 2 nanosheets with different phase rations by two-step solvothermal method. Results indicate that the 1 T@2H-MoSe 2 nanosheets with different phase rations show the variation magnetic properties. where 65.58% 1 T-MoSe 2 phase incorporation in 2H-MoSe 2 could enhance the saturated magnetization from 0.32 memu/g to 8.36 memu/g. Besides, the obvious magnetoresistance behaviors reveal their intrinsic RTF and their potential applications in future spintronics.

Results and Discussion
We synthesize 1 T@2H-MoSe 2 nanosheets by two-step solvothermal method (the schematic diagram is shown in Fig. 1(a)) 35 where phase-transferred MoSe 2 nanosheets with 0 h, 4 h, 8 h, and 20 h are labeled as S0, S4, S8 and S20 respectively in the following passage. X-ray diffraction (XRD) patterns of the samples are shown in Fig. 1(b). According to the standard PDF card of MoSe 2 (JCPDS No. 29-0914), the diffraction peaks are located at 31.4°, 37.9°, and 55.9°, which are related to certain MoSe 2 crystal planes of (100), (103), and (110), respectively. The 2H or 1 T structure can not only be identified by these three peaks owing to the further atomic structure should be investigated, the results of which will be discussed in what follows in the passage. Figure 1(c) shows the SEM image of synthesized MoSe 2 nanosheets (S8), and it can be seen that the obtained MoSe 2 nanosheets are condensed and assembled thin layers. SEM images of other samples are provided in Supplementary Information. Besides, EDS analysis results of S8 are presented in Fig. 1(d-f). Within the uniform distribution of light and shade contrast of elemental images captured by a detector, the existence of molybdenum and selenium can be easily observed, demonstrating the element consistence of MoSe 2 nanosheets. Figure 2(a) is the 1 T@2H TEM image of S8, two different regions are obviously compatible. We can see typical molybdenum atoms in either 2H or 1 T structure possess six selenium atoms, which are triangular prism and octahedral configuration, respectively (white circles). Additionally, the panels can be seen in high resolution transmission electron microscope (HRTEM) image and the interplanar spacing is calculated to be 0.27 nm, indicating the (110) panels of 2H MoSe 2 (JCPDS No. 29-0914). Figure 2(b) shows the high resolution X-ray photoelectron spectra (XPS) of the four samples. In terms of Mo 3d regions, all the spectra can be well fitted by two sets of peaks. The peaks around 229.3 eV and 232.5 eV correspond to 3d 5/2 and 3d 3/2 components of 2H structure MoSe 2 . Yet once the 1 T structure is induced, these two peaks will shift to lower binding energies of 228.4 eV and 231.6 eV. As shown in Fig. 2(c), the 1 T concentrations of S4, S8 and S20 are calculated from Mo 3d spectra as 30.19%, 65.58% and 83.95%, respectively. Figure 2(d) shows the Raman spectra of these four samples, from which the peaks at 150.7 cm −1 and 289.4 cm −1 in S4, S8 and S20 can be observed. These two peaks, marked as J 2 and E 2g 1 , are the exclusive peaks of MoSe 2 1 T structure, consisting with the work reported by Uttam et al. 37 . Herein, the   intensity of these two peaks growing with the amount of 1 T phase, with respect to A 1g peak, which also demonstrates the phase transformation from 2H to 1 T.
Besides, we investigate the magnetic properties of 1 T@2H-MoSe 2 nanosheets. The room temperature magnetic hysteresis loops of samples are shown in Fig. 3(a), where the linear background signals have been subtracted 38 . Compared with S0, the saturate magnetization (M s ) for S4 and S8 increases from 0.32 memu/g to 1.6 memu/g, and then to 8.36 memu/g, respectively, suggesting that introduction of 1 T phase could lead to the ferromagnetic ordering in MoSe 2 nanosheetes. From the inset of Fig. 3(a), it can be seen that the M s increases until the 1 T concentration raises to 65.58%, dramatically, the M s decreases to 2.6 memu/g in S20 (83.95% 1 T phase). Figure 3(b) gives the isothermal hysteresis loops of S4 from 10 K to 300 K, the inset of which shows the zero-field-cooled (ZFC) and field-cooled (FC) curves. Typical ferromagnetism property has been characterized by these curves, thus the M s decreases with the increasing of the measured temperature. The ZFC-FC curves suggest that the Curie temperature of the sample is above the room temperature. Besides, no blocking temperature can be found during the cooling process, indicating that there is no ferromagnetic cluster occurs in S4 39 . Besides, the electron spin resonance spectra (ESR) of four 1 T@2H-MoSe 2 samples are shown in Fig. 3(d). As we can see, the resonance occurs in 325 mT (g = 1.98), corresponding to the paramagnetic resonance of the four samples. Besides, the distinct resonance signal raises up in S4, S8 and S20, nearly at 250 mT (g = 2.57), corresponding to the ferromagnetic resonance of these MoSe 2 nanosheets, in accord with the M-H results in Fig. 3(a). In addition, obvious magetoresistance (MR) behaviors are observed in sample S4, S8 and S20. As described in Fig. 3(d), MR values are negative with the magnetic field range of [− 0.1 T, 0.1 T] and evolves from 0% to − 0.22% with the magnetic field increased to 0.1 T. While for the S4 and S20, the lowest MR value are only − 0.07% and − 0.09%, respectively. The MR values vary with the saturate magnetization of three samples, also confirm the observed ferromagnetism is intrinsic in 1 T phase incorporated MoSe 2 nanosheets 40 .
Above results indicate that the M s decreases when the concentration of 1 T phase increases up to 83.95%. Therefore, we assume that the observed ferromagnetism is related to the relative ratio of both 1 T and 2H phase in MoSe 2 nanosheets. To verify this, it is necessary to conduct further experiment. Zhao et al. report that transformation from 1 T to 2H can be conducted by annealing the 1 T samples under Ar ambitions 41 , so we perform the re-transformation with annealing the S20 in high purity Ar for 1 h and 2 h under 250 °C, respectively. The obtained samples are subsequently studied by using M-H hysteresis loops and Raman spectra. It can be seen from Fig. 4(a), the M s increases by four times after annealing for 1 h, but decreases as the annealing time prolonged to 2 h. In Fig. 4(b), the peak of A 1g mode appears together with the peaks of J 2 and E 2g 1 modes decaying after annealed for one and two hours. Both the two results indicate that the coexistence of 1 T and 2H phase is the ultimate condition for observed ferromagnetism in MoSe 2 nanosheets. To explore the origin of the observed ferromagnetism, it is necessary to point out how the magnet moment induced with the 1 T phase incorporated in 2H matrix.  35,37 . For the robust ferromagnetism observed in 1 T@2H MoSe 2 nanosheets, bounded magnetic polaron (BMP) model is suitable to explain the magnetic origin. Figure 4(d) gives the schematic diagram of BMPs in 1 T incorporated MoSe 2 nanosheets. During the solvothermal synthesis process, many selenium vacancies formed in MoSe 2 nanosheets, as a result, BMPs could be developed with localized holes and a large number of Mo 4+ spins are bounded around the Se vacancies. The Mo 4+ spins near a Se vacancies could align their spins parallel to the vacancy spin, leading to the formation of a BMP. It has been reported by Cai et al. that the 2H-1 T transformation always occurs near the defects 35 . Based on this phenomenon, we proposed that in low 1 T concentration case, the Mo 4+ spins emerged near the selenium vacancies and form BMPs. These BMPs began to overlap and ferromagnetic coupled, giving the origin of ferromagnetism and risen of magnetization. However, when the amount of 1 T concentration added up to 83.95%, the 1 T regions expand and produce more Mo 4+ spins in the regions where the Se vacancy density is much lower, as shown in Fig. 4(d). In this case, majority of these Mo atoms are around Se atoms compared with Se vacancies, and they are either anti-ferromagnetic coupled by Se atoms or existed as isolated Mo 4+ spins, resulting the decreasing of magnetization macroscopically. This could cause the weaken of ferromagnetism in MoSe 2 nanosheets and this is why we observe the decreased M s in S20.

Conclusions
In summary, we synthesize the 1 T phase incorporated 2H-MoSe 2 nanosheets by solvothermal method, the crystallinity of all samples have been confirmed by structural characterization methods. After the phase transformation, RTF of MoSe 2 can be improved, with the M s from 0.32 memu/g up to 8.36 memu/g. At the same time, the obtained MoSe 2 nanosheets exhibit obvious magnetoresistance behavior with MR value up to − 0.22% when the external magnetic field applied to ± 0.1 T. The induced Se vacancies may affect the formation of the BMPs and their interactions, in turn controlling the magnetic moments of the 1 T phase incorporated MoSe 2 nanosheets. The obtained results enlighten on the development of ferromagnetic MoSe 2 nanosheets and provide them a paradigm of application of spintronics devices.