Inversion symmetry breaking, strong spin–orbit coupling, and time inversion symmetry give rise to two nonequivalent valleys ( and ) with antiparallel spins in monolayer transition metal dichalcogenides (TMDCs).1
Advanced Photonics Nexus, Volume. 2, Issue 2, 026007(2023)
Controllable valley magnetic response in phase-transformed tungsten diselenide Article Video
Achieving valley pseudospin with large polarization is crucial in the implementation of quantum information applications. Transition metal dichalcogenides (TMDC) with different phase structures provide an ideal platform for valley modulation. The valley splitting has been achieved in hybrid phase WSe2, while its valley polarization remains unstudied. Magnetic field controllable valley polarization is explored in WSe2 with coexistence of H and T phases by an all-optical route. A record high valley polarization of 58.3% is acquired with a 19.9% T phase concentration under a 4-T magnetic field and nonresonant excitation. The enhanced valley polarization is dependent on the phase component and shows various increasing slopes, owing to the synergy between the T phase WSe2 and the magnetic field. The magnetic field controlled local magnetic momentums are revealed as the mechanism for the large valley polarization in H / T-WSe2. This speculation is also verified by theoretical simulations of the nonequilibrium spin density. These results display a considerable valley magnetic response in phase-engineered TMDC and provide a large-scale scheme for valley polarization applications.
1 Introduction
Inversion symmetry breaking, strong spin–orbit coupling, and time inversion symmetry give rise to two nonequivalent valleys ( and ) with antiparallel spins in monolayer transition metal dichalcogenides (TMDCs).1
The strategies for enhancing the valley contrast mainly focus on suppressing the intervalley scattering by plasmonic metasurface,6,7 defects and strain engineering,8,9 or breaking the inversion symmetry by electrical modulation, twist staking, and heterojunction construction.10,11 Alternatively, applying a magnetic field to induce nonequilibrium spin states through the Zeeman field is considered an essential method for regulating the valley dynamics. However, due to the semiconductor property of H phase TMDC materials, a high magnetic field is generally necessary to achieve a larger valley polarization.12 Further, combining with the spin injection through a magnetic heterojunction or ferromagnetic electrode, the magnitude of a required magnetic field can be reduced by taking advantage of the injected nonequilibrium spin carriers.13
In this work, large-scale monolayers with controllable components are synthesized. The structure with the higher T phase concentration is demonstrated to possess larger valley polarization and higher sensitivity to the external magnetic field. A record high degree of valley polarization is achieved in monolayer with 19.9% T phase concentrations. The density functional theory (DFT) simulations indicate the higher electron concentration and nonequilibrium spin density as incorporating the T phase. The local magnetic momentums arranged by external magnetic field are considered as the mechanism for the enhanced valley magnetic response and thus the large valley polarization in hybrid .
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2 Results and Discussion
2.1 Synthesis of Hybrid Phase Monolayer H/T-WSe2
Monolayer with an H/T hybrid phase is prepared through a CVD synthesis followed by a moderate plasma treatment (detailed in Sec. 4). As the optical morphology shows in Fig. 1(a), film after plasma treatment exhibits a uniform thickness contrast and a large scale of more than in lateral size. The atomic force microscopy (AFM) characterization is performed to confirm its monolayer thickness, as provided in Fig. S1 in the Supplementary Material. Figure 1(b) shows the high-resolution transmission electron microscopy (HRTEM) image of the film. The lattices clearly display two different configurations of hexagon honeycomb and zigzag chain, corresponding to the H and T phase , respectively.29,30 Selected area electron diffraction (SAED), shown in Fig. 1(c), has two coexisting hexagonal patterns, which also verify the contribution from the H and T phases, respectively.29 Moreover, the phase component can be well controlled by adjusting the plasma power and treatment time. As is analyzed by X-ray photoelectron spectroscopy (XPS) in Fig. 1(d), two monolayers with different T phase components are verified. Among the four characteristic peaks of , the blue set located at 34.5 and 32.4 eV corresponds to the H phase , and the purple set centered at 33.8 and 31.7 eV corresponds to the T phase. The gray area presents the traces of oxidized tungsten. By fitting the area of XPS peaks, the T phase components are estimated to be 12.2% and 19.9% T, respectively.31 So far, the structure of is confirmed by intuitive transmission electron microscopy (TEM) observation and quantitative XPS analysis.
Figure 1.(a) Optical topography of the CVD-grown large-scale
Considering that higher electron concentration will be beneficial to the valley polarization, the current–voltage () property is further studied to estimate the conductivity of the samples. Figure 1(e) exhibits a larger slope of the curve for 19.9% of the T-phase sample, indicating that the electron concentration is increased with the increasing T-phase component. This can be attributed to the metallicity of the introduced T phase. Local interaction between the T- and H-phase W atoms ( and ) is investigated from the calculated layer-dependent 2D charge distribution, as shown in Fig. 1(f). It can be found that the top three Se atoms of the T phase possess fewer electrons by about 0.03 e than other Se atoms, and the six nearest-neighbor atoms around the atoms lose fewer electrons by about 0.07 e than other atoms. The charge–transfer interactions predict the ability to control the properties of through the introduced T phase.
2.2 Characterization of Valley Polarization
Valley polarization enhancement in with different T-phase concentrations (12.2% and 19.9%) is verified through the circularly polarized PL spectra at 10 K, as shown in Fig. 2. Each spectrum exhibits a single peak around 1.72 eV, consistent with the energy of the neutral exciton (). No additional defect peak or the change of peak position is observed before and after the phase transition, which indicates that the defect number produced by the plasma treatment should be small and its influence on the valley polarization should be negligible. The degree of valley polarization is quantitatively evaluated according to the expression for left-handed circularly polarized excitation () or for right-handed circularly polarized excitation (), where () and () denote the intensities of circularly polarized emissions with a co- and cross-polarized configurations, respectively. The intrinsic valley polarization is calculated as 9.0% [Figs. S2(a) and S2(b) in the Supplementary Material] for and further promotes to 12% [Figs. S2(c) and S2(d) in the Supplementary Material] when the T-phase component increases to 19.9%. It is also noteworthy that the valley polarization is controllable under the external magnetic field. Compared with the co-polarized signal, the suppression of the cross-polarized peak under the magnetic field is a signature of modulation on the optical valley polarization in .3,32
Figure 2.Circularly polarized PL spectra of
The magnetic field-dependent valley polarizations with excitation are compared and depicted in Figs. 3(a) and 3(b) for different T-phase concentrations. The results at 0 T are 11.3% [Fig. 3(a)] and 12.0% for and [Fig. 3(b)], respectively, and the evolution curves exhibit as a “V” pattern with different slopes from −4 to for both the samples. Specifically, in , the valley polarization increases sharply from 0 to 1 T () with a slope of 0.30 (0.28). As the magnetic field further increases, the increasing trend of valley polarization becomes smooth, corresponding to the slopes of 0.08 from to , and 0.02 from to . Compared with the former case, is more sensitive to the magnetic field, exhibiting a larger slope of 0.46 from 0 to , and gradually decreased slopes of 0.06 from to and 0.02 from to . A similar phenomenon is also observed for the excitation, as shown in Fig. S3 in the Supplementary Material.
Figure 3.Magnetic-dependent polarization of (a)
To gain a comprehensive understanding of the role of the magnetic field, the first-principle simulations are performed to compare the spatial distribution of the spin density for monolayer and . As shown in Figs. 3(c) and 3(d), the net spin in is zero, whereas pronounced spin densities are induced by the atom in and distributed around the six nearest atoms. Figures 3(e) and 3(f) show the total spin density-of-states (DOSs) for monolayer before and after the phase transition, respectively. For the , the DOS exhibits a symmetric distribution for the spin-up and spin-down states, indicating a nonmagnetic ground state [Fig. 3(e)]. For , the DOSs split into asymmetric spin-up and spin-down channels, echoing a nonequilibrium distribution in spin density [Fig. 3(f)]. The main contribution of the asymmetric DOSs in is from the 4d state of rather than , as shown in Fig. S4 in the Supplementary Material. In the framework of crystal field theory, has a trigonal prismatic coordination, and the W-4d orbital splits into three energy levels, as shown in Fig. S5 in the Supplementary Material. is an octahedral coordination and the W-4d orbital splits into two energy levels, i.e., double-degenerate and triple-degenerate . The is partly occupied in parallel by the two 4d electrons to reduce the coulomb energy between the 4d electrons, producing a net magnetic moment for the W atom.18 Therefore, the nonequilibrium spin distribution and the induced magnetic moment in the system may be aligned by the external magnetic field, which will facilitate the generation and modulation of the valley polarization.
2.3 Qualitative and Quantitative Analysis of Valley Polarization in H/T-WSe2
Based on the above experimental and simulation results, the enhanced valley polarizations in are qualitatively analyzed in terms of nonequilibrium spin distribution in and quantitatively calculated from the increased electron concentration in , as depicted in Figs. 4(a)–4(e). The Hamiltonian of the valley exciton can be expressed as38
Figure 4.Schematic diagram of magnetic-field-modulated valley dynamic process of monolayer
As is well known, the spin–orbital coupling (SOC) interaction will induce the splitting in the valence band, producing the spin-valley locking effect [Fig. 4(a)] and circular polarization in the PL helicities. with higher electron concentration will involve more electrons in the excitons process, transferring oscillator strength from the exciton to the attractive Fermi polaron, resulting in the polarization enhancement.39 Therefore, the valley polarization can be quantitatively increased with the increasing T-phase concentration under the and excitations, as observed in the above two different components of monolayer . Without the magnetic field, the nonequilibrium spin in generates local magnetic moments with random directions. The overall statistical average should not show a long-range magnetism that can be equivalent to a magnetic field. Under a perpendicular magnetic field, the induced Zeeman energy results in the two-branch exciton dispersion by opening up a finite gap at , as shown in both the conduction and valence bands in Fig. 4(b). Simultaneously, the T-phase component with certain magnetic momentums in acts as local magnetic fields. If these local magnetic momentums can be arranged in a unified direction, a valley splitting of will be acquired [Fig. 4(c)]. Taking the 19.9% T-phase concentration as an example, the maximum is calculated about 38.5 meV, as shown in Fig. S6 in the Supplementary Material. The induced intrinsic nonequilibrium spin distribution is much larger than the latest reported spin injection from or any other intermedium manipulating the valley polarization.13 Therefore, although the absolute value of valley-conserving rate and valley-flipping rate will switch with the change of magnetic field direction, resulting in larger valley polarization for () excitation than for () excitation under positive (negative) magnetic field [Figs. 4(d) and 4(e)], the effect of magnetic field on the valley excitons dispersion is limited. Consequently, the valley polarization increases with both the increased positive or negative magnetic fields, as shown in Figs. 3(a) and 3(b).
According to above analysis, it can be concluded that there are three possible physical processes dominating the polarization: (i) the increased electron concentration. Without the magnetic field, the valley polarization is mainly determined by the optical valley polarization and enhanced by the increased electron concentration, i.e., the T-phase component. (ii) The synergy between the T phase and the external magnetic field. With the magnetic field, the T-phase-induced nonequilibrium spins are well aligned, resulting in an obvious () response with the spin-up (spin-down) electrons. This explains the sharply increased slopes of valley polarization from 0 to , as the dark blue lines shown in Fig. 3(a). (iii) The magnetic field induced Zeeman effect. The initial magnetic field within could already align most of the magnetic momentums of the T phase. As the magnetic field increases further, it will align the rest small part of nonequilibrium spins of the T phase and increases the Zeeman field simultaneously, resulting in a moderate increased slope of valley polarizations from to [light blue lines in Fig. 3(b)]. Although in the magnetic field range from to , only the magnetic-field-induced Zeeman effect dominates the enhancement of valley polarizations, corresponding to even smaller slopes, as the pink lines depict in Figs. 3(a) and 3(b). Thus the enhanced valley polarization is achieved in the lateral . Besides the lateral structure, the valley polarization may also realize a vertical H/T heterostructure. Compared with the strong bonding interaction, the van der Waals interaction in the vertical heterostructure is much weaker, and the induced local magnetic moment may be smaller. However, the contact area of H and T phases in the vertical heterostructure can be larger than that of the lateral one, and there may be new properties and interactions. Therefore, the exploration of valley polarization in vertical H/T phase heterostructure is ongoing.
3 Conclusion
In summary, large-scale monolayer with controllable phase concentrations are synthesized through the CVD method followed by an plasma treatment. The polarized PL spectra suggest a component-dependent valley polarization, which can be further modulated by the external magnetic field. Owing to the enhanced valley magnetic response, a record-high valley polarization of 58.3% is achieved successfully in monolayer under a magnetic field of 4 T and nonresonant excitation. DFT calculations indicate the high electron concentration and nonequilibrium spin density distribution in . Accordingly, three possible physical mechanisms, including the increased electron concentration, the synergy between the T phase and the external magnetic field, and the magnetic-field-induced Zeeman effect are analyzed qualitatively and quantitatively to understand the enhanced valley magnetic response and thus the large valley polarization. All these results fully explore the role of T phase in valley polarization under the effect of external magnetic field, and provide a promising perspective for a large-scale, all-optics-controlled valley dynamic manipulation.
4 Methods
4.1 Preparation of Monolayer H-WSe2 and H/T-WSe2
CVD technology compatible with large-scale growth is used to prepare monolayer on sapphire substrates. Deposited (Alfa Aesar, 99.9%) film on an chip and the Se powder (Aladdin, 99.99%) serve as the precursors of W and Se sources, respectively.23 is obtained based on monolayer through an plasma treatment. The radio-frequency power and flow rate of high pure Ar gas are set as 1 W and 0.5 sccm, respectively, under a 100-Pa background pressure at room temperature. A cold, moderate plasma atmosphere is formed on the surface of , enabling partial H to T phase transition.18,40 Since the formation energies of W and Se vacancy defects are higher than the energy required for the H-to-T phase transition,40
4.2 First-Principles Calculations
The differential charge densities of and are calculated using the DFT. The supercells of and monolayers are constructed by considering the measured phase components. The T phase in is introduced by wrenching the Se-W bonds locally by 60 deg around the W atom.44 To avoid the artificial interaction between the periodic slabs, an optimized vacuum layer of 20 Å is set. A Monkhorst–Pack grid of the points is sampled with a mesh in the Brillouin zone, and the plane-wave cutoff energy is set at 500 eV. The residual forces converge to and the energy for structural optimization.
4.3 Characterizations
The morphology and structural characterizations are performed by XPS (Quantum 2000), AFM (SPA400-Nanonavi), and TEM (JEM-2100, 200 kV) techniques. The magnetic-field-dependent circularly polarized PL spectra are obtained at 10 K with a 633-nm laser, and the magnetic fields are applied perpendicularly to the plane of the samples. The optical path diagram is shown in Fig. S7 in the Supplementary Material. For the polarization resolved PL system, the left- and right-handed circularly polarized excitation lights are produced through a linear polarizer and a quarter-wave plate () in the excitation path. The polarization of emission lights is analyzed by the quarter-wave plate and another linear polarizer in the detection path.
Yaping Wu is a professor in the Department of Physics at Xiamen University. She is committed to the research of new semiconductor structural materials and new functional devices. Her SCI-indexed papers have been cited more than 3000 times.
Zhiming Wu is a professor in the Department of Physics at Xiamen University and a deputy director of the Department of Physics. He mainly studies the growth and characteristics of semiconductor nanomaterials, surface structure, spin transport, and solar cells.
Biographies of the authors are not available.
[17] S. Cho et al. Phase patterning for ohmic homojunction contact in
[40] B. Ouyang. Phase engineering of low dimensional transition metal dichalcogenides(2017).
[41] H. G. Nam. Phase-controlled synthesis for 1T’ phase
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Haiyang Liu, Zongnan Zhang, Yingqiu Li, Yaping Wu, Zhiming Wu, Xu Li, Chunmiao Zhang, Feiya Xu, Junyong Kang. Controllable valley magnetic response in phase-transformed tungsten diselenide[J]. Advanced Photonics Nexus, 2023, 2(2): 026007
Category: Research Articles
Received: Jan. 25, 2023
Accepted: Feb. 6, 2023
Published Online: Mar. 3, 2023
The Author Email: Wu Yaping (ypwu@xmu.edu.cn), Wu Zhiming (zmwu@xmu.edu.cn)