Controllable Magnetic Proximity Effect and Charge Transfer in 2D Semiconductor and Double-Layered Perovskite Manganese Oxide van der Waals Heterostructure

Yan Zhang, Keisuke Shinokita, Kenji Watanabe, Takashi Taniguchi, Masato Goto, Daisuke Kan, Yuichi Shimakawa, Yutaka Moritomo, Taishi Nishihara, Yuhei Miyauchi, and Kazunari Matsuda*

Recently, monolayer transition metal dichalcogenides (MX2: M = Mo, W, X = S, Se, Te) have gained increased attention, owing to their novel physical properties and potential appli- cations.[1–6] The strong confinement and reduced dielectric screening of optically generated electrons and holes in semi- conducting monolayer MX2 enhance Coulomb interaction. Consequently, this imparts an extraordinarily large binding energy to a neutral exciton as a bound electron–hole pair,and to negatively (positively) charged excitons or trions composed of two elec- trons and a hole (or an electron and two holes).[7–11] The excitons and trions in monolayer MX2 exhibit valley degrees of freedom because monolayer MX2 has a direct bandgap with a band-edge located at the energy degenerate valleys (K, -K), which are situated at the corners of the hexagonal Brillouin zone. The valley degrees of freedom coupled with the spin degrees of freedom (spin-valley locking), which originate from strong spin–orbit interactions and the breaking of inversion symmetry, enable the selective photogen- eration of excitons and trions at the K orOptically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic proper- ties of the materials underneath.

Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe2 on double-layered perovskite Mn oxide ((La0.8Nd0.2)1.2Sr1.8Mn2O7) is demon- strated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photolumines- cence intensity via dielectric screening is observed. Further, a relatively high
trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polari- zation of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic prox- imity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins-K valleys by shining circularly polarizedlight; this leads to the formation of valley- polarized excitons and trions (valley polar- ization) in monolayer MX2.[12–16]

The optically generated excitons and trions are highly sensitive to the phys-
ical properties of substrate materials, owing to the proximity effect.[17–19] The monolayer MX2 on the insulating ferromagnetic materials provides a platform for investigating the exchange interactions between the valley spin-polarized excitons (trions) and ferromagnetic spins via proximity magnetic-exchange effects, which leads to tunable valley spin polarization and large valley splitting. The large valley splitting of excitons and trions in monolayer WSe2 on ferromagnetic insulating EuS is induced

www.advmat.
Y. Zhang, Prof. K. Shinokita, Prof. T. Nishihara, Prof. Y. Miyauchi, Prof. K. Matsuda

Institute of Advanced Energy Kyoto University
Uji, Kyoto 611-0011, Japan

E-mail: [email protected] Dr. K. Watanabe
Research Center for Functional Materials
National Institute for Materials Science

1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
ImageThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202003501.
DOI: 10.1002/adma.202003501 Dr. T. Taniguchi International Center for Materials Nanoarchitectonics National Institute for Materials Science
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Prof. M. Goto, Prof. D. Kan, Prof. Y. Shimakawa Institute for Chemical Research
Kyoto University Uji, Kyoto 611-0011, Japan Prof. Y. Moritomo
Graduate School of Pure & Applied Science and Faculty of Pure & Applied Science University of TsukubaTsukuba, Ibaraki 305-7571, Japan

Schematic diagram and optical image of vdW heterostructure. a) Schematic diagram of the vdW heterostructure consisting of the bottom layer of double-layered perovskite Mn oxide ((La0.8Nd0.2)1.2Sr1.8Mn2O7), the interlayer of h-BN with different thicknesses d ranging from 0 to 8.0 nm, and the top layer of 1L-MoSe2. The external magnetic field (B) is applied in the out-of-plane direction. The bottom is the diagram of double-exchange interac- tion, which gives rise to the phase transition of Mn oxide from a ferromagnetic metal to a paramagnetic insulator. b) Optical microscopy image of the vdW heterostructure (1L-MoSe2/h-BN/Mn oxide, d = 1.4 nm). The scale bar is 10 mm. The boundaries of MoSe2, h-BN, and Mn oxide are shown by red, yellow, and black broken curves, respectively. c) Surface roughness of Mn oxide measured by atomic force microscopy (AFM). The scale bar is 200 nm by a strong effective magnetic field from the ferromagnetic spins.[20] Moreover, the switching of the sign on the valley spin polarization of excitons in monolayer WSe2 on few-layer ferro- magnetic semiconducting CrI3 has been demonstrated via the flipping of ferromagnetic magnetization.[21] However, only fer- romagnetic insulators have been employed in monolayer MX2 to lift the valley degeneracy via the magnetic proximity effect, which limits the interest on a large variety of other magnetic materials, such as ferromagnetic metals, and the potential applications of heterostructures.
The strongly correlated electron system of perovskite man- ganese oxide (La1-xSrxMnO3) has been extensively studied as a model system for the double-exchange interactions of d-orbital electrons in Mn sites through O 2p-orbitals.[22,23] Numerous novel physical phenomena such as colossal magnetoresist- ance and charge ordering have emerged in perovskite manga- nese oxides. The eg electrons in perovskite Mn oxide trigger the electronic and magnetic phase transition via double-exchange interactions from the paramagnetic insulator (PI) phase to the ferromagnetic metal (FM) phase at Curie temperature (TC).[22,24] Even though electronic and magnetic phase transition mate- rials are ideal platforms for creating van der Waals (vdW) het- erostructures together with semiconducting 1L-MX2, studies regarding the optical physics of these types of vdW heterostruc- tures have not been reported thus far.

In this work, we studied the charge transfer and magnetic proximity effect of vdW heterostructures, which are composed of monolayer MoSe2 (1L-MoSe2), few-layer h-BN, and double-lay- ered perovskite (Ruddlesden–Popper structure)[23,25,26] Mn oxide ((La0.8Nd0.2)1.2Sr1.8Mn2O7). This vdW heterostructure enabled us to systematically study the interface interactions between the excitonic states (excitons and trions) and magnetic spins as well as charge carriers. We found that the photoluminescence

(PL) properties of 1L-MoSe2 strongly depend on the electronic and magnetic properties of the substrate; the charges are trans- ferred from metallic Mn oxide to 1L-MoSe2, and the valley split- ting and polarization are significantly enhanced by introducing a ferromagnetic substrate (Mn oxide). Moreover, these interac- tions between 1L-MoSe2 and Mn oxide can be modulated and controlled by varying the thickness of the few-layer h-BN.
A schematic of the vdW heterostructure (1L-MoSe2/h-BN/ Mn oxide) under applied magnetic field B in the out-of- plane direction is shown in Figure 1a. The strongly corre- lated electron system of double-layered perovskite Mn oxide ((La0.8Nd0.2)1.2Sr1.8Mn2O7) exhibits a phase transition from a paramagnetic insulator (PI) above TC (80 K) to a ferromag- netic metal (FM) below TC (Figure S1, Supporting Informa- tion), where the simultaneous transport and magnetic phase transition occur via the double-exchange interactions of the d-electrons of the Mn sites, as shown in Figure 1a.[22,24,27] The transfer integral of itinerant electrons in the eg orbitals of Mn sites depends on the alignment of localized t2g electrons with magnetic moments. Moreover, the ferromagnetic alignment of localized t2g electrons allows the itinerant eg electrons to hop through the crystal, inducing the FM phase in Mn oxide.[28] a typical optical microscopy image of the 1L-MoSe2/h-BN/Mn oxide vdW heterostructure with an h-BN thickness (d) of 1.4 nm, which has been fabricated on a SiO2/ Si substrate and is denoted as 1L-MoSe2/h-BN/Mn oxide (d = 1.4 nm). The optical images of the vdW heterostructures used in this study are shown in Figure S2 in the Supporting Information. The regions indicated by red, yellow, and black broken curves correspond to 1L-MoSe2, h-BN, and Mn oxide, respectively. The cleaved surface of Mn oxide was exhibited by atomic force microscopy (AFM) (Figure 1c), indicating that the double-layered Mn oxide has an atomically flat surface The temperature-dependent PL of vdW heterostructure and the reference. a) Contour plots of the temperature-dependent PL spectra of the reference (1L-MoSe2/h-BN) (left) and vdW heterostructure (1L-MoSe2/Mn oxide) (right) from 10 to 300 K. The waterfall plots of the temperature- dependent PL spectra are also shown in Figure S4 in the Supporting Information. b) PL spectra of reference (left) and vdW heterostructure (right) at 10 K. c) Exciton PL intensity (IX) of the reference and vdW heterostructure from 10 to 300 K. Both curves are normalized by the intensity at 10 K.

Inset shows the normalized trion PL intensity (IT) from 10 to 80 K. d) PL intensity ratio of the trion and exciton (IT/IX) as a function of temperature from 10 to 80 K. Inset shows the exciton and trion dynamics within a three-level model consisting of the exciton (X), trion (T), and ground state (G).
morphology with a root-mean-square (RMS) roughness of only0.2 nm, which enables good contact with the atomically thin- layered MoSe2 and h-BN.Figure 2a shows contour maps of the temperature-dependent PL spectra of 1L-MoSe2/h-BN as a reference (left) and vdW heter- ostructure of 1L-MoSe2/Mn oxide (right), respectively, from 10 to 300 K. The two prominent PL peaks of the neutral excitons (X) and trions (T) are observed at a low temperature below 80 K[29–32] in both maps, while the PL intensities of trions drop signifi- cantly above 60 K with increasing temperature.[29] The PL peaks of the excitons in both maps gradually shift to the lower energy side because of the shrinkage of the bandgap of 1L-MoSe2, which occurs due to thermal lattice expansions.[33,34] In contrast, the temperature dependence of exciton PL intensity (IX) above TC (80 K) shows a clear difference between the reference and the vdW heterostructure, where IX of the reference decreases gradually, while that of the vdW heterostructure increases with increasing temperature. This clear difference in the PL behavior suggests that the excitonic properties are strongly affected by the metal–insulator phase transition of the Mn oxide substrate.Figure 2b presents the PL spectra of the reference (left) and 1L-MoSe2/Mn oxide (right) at 10 K. The PL spectra are decom- posed by multiple Voigt functions from excitons (X, blue), trions (T, orange), and localized excitons from defect states (L, gray) to clarify their peak energies, spectral integrated intensities, and homogeneous linewidths. The most significant difference between the vdW heterostructure and reference is the larger spectral weight of trions in the vdW heterostructure than that in the reference at 10 K, which will be discussed later in detail.Figure 2c and its inset show the temperature dependence of the integrated PL intensity of excitons (IX) and trions (IT) of the vdW heterostructure (1L-MoSe2/Mn oxide) and reference (1L-MoSe2/h-BN), respectively. The normalized IT of both the vdW heterostructure and reference decreases with increasing temperature. The temperature-dependent normalized IX in the reference monotonically decreases, which is consistent with the previously reported result.[30]

In contrast, the exciton PL intensity of the vdW heterostructure initially decreases with increasing temperature from 10 K to the phase transition tem- perature TC (80 K); this result is similar to that observed for the reference below 80 K, while it turns up and gradually increases above 80 K. Similar experimental results have been observed in other vdW heterostructures with different thicknesses of h-BN (d = 0, 1.4, and 3.8 nm) (Figures S3–S6, Supporting Infor- mation). The drastic change in the exciton PL signal across the phase transition temperature suggests that the exciton dynamics of 1L-MoSe2 sensitively depend on the metal–insu- lator transition of Mn oxide underneath.
The typical temperature-dependent exciton PL intensity of monolayer 2D semiconductors can be understood by stronger temperature (T) dependence of the radiative decay rate of 2D excitons (γX µ 1/T), which is in competition with weaker tem- perature dependence of the nonradiative decay rate;[35–38] this result agrees well with that of the reference (1L-MoSe2/h-BN). The temperature dependence of the radiative decay rate of excitons also explains the experimental results of monolayer MoSe2 on metallic Mn oxide below TC. In contrast, the signifi- cant deviation from the temperature dependence of the exciton radiative decay rate above 80 K in the vdW heterostructure is reflected by the change in the dielectric screening of excitons in the range of its binding energy (a few hundreds of meV) across the metal–insulator transition.[39] The dielectric functions of Mn oxide in the energy range of a few hundreds of meV tend to sensitively change depending on the rising temperature, due to the opening of the charge transfer gap (energy gap) from the metal–insulator transition.[23] The reduced screening of exci- tons on Mn oxide would increase its radiative decay rate with increasing temperature, which enhances the exciton PL inten- sity of the vdW heterostructure. This scenario explains the increasing exciton PL intensity toward higher temperatures and suggests that the exciton dynamics of 1L-MoSe2 are strongly affected by the phase transition of Mn oxide.
Figure 2d shows the integrated PL intensity ratio of the trion and exciton components (IT/IX) as a function of temperature below TC (80 K) of Mn oxide. Notably, IT/IX in the vdW hetero- structure is significantly larger than that in the reference, where the Mn oxide is in the metallic state. An enhanced IT/IX value was also obtained in the vdW heterostructure with thin h-BN (Figure S7, Supporting Information). These results suggest that the doped carrier density of MoSe2 on Mn oxide is higher than that of the reference MoSe2 on SiO2/Si.[40]
To gain a deeper understanding of the above experimental results, we derived the relationship between the PL intensity ratio of the exciton and trion (IT/IX) and the doped carrier den- sity of MoSe2 (ne). Based on the rate equation analysis within the framework of the three-level model including the exciton (X), trion (T), and ground state (G) (inset in Figure 2d), the PL intensity ratio between the trions and excitons is described as follows (Supporting Note, Supporting Information) 3a–c show polarization-resolved PL spectra of trion in the reference (1L-MoSe2/h-BN), vdW heterostructure (1L-MoSe2/Mn oxide), and vdW heterostructure (1L-MoSe2/ h-BN/Mn oxide, d = 1.4 nm) under an out-of-plane applied magnetic field of 1 T at 10 K. The PL spectra of 1L-MoSe2 with σ+ and σ- circularly polarized light components excited under σ+ excitation are shown in red and blue, respectively. Enhanced valley degeneracy was not observed in 1L-MoSe2/ Mn oxide (Figure 3b; Figure S9, Supporting Information) compared with the reference. However, large valley split- ting and polarization are obtained when a thin h-BN layer (d = 1.4 nm) is inserted between MoSe2 and Mn oxide, which suggests that a thin insulating layer, such as h-BN, plays an important role in the enhancement of valley splitting and polarization, as will be described in detail later.
Figure 3d shows the polarization-resolved PL spectra of the trion of 1L-MoSe2/h-BN/Mn oxide (d = 1.4 nm) at 10 K under different magnetic fields from 0 to 5 T. The σ+ and σ- com- ponents of the PL spectra of the vdW heterostructure have an identical intensity and spectral shape under a zero-magnetic field,[45,46] because the net magnetization of ferromagnetic Mn oxide with soft magnetic properties is zero at 0 T (Figure S1, Supporting Information).[24] By increasing the applied magnetic field, the PL spectrum of the σ+ component shows a higher intensity than that of the σ- component. In addition, the shift of σ+ and σ- PL peaks to lower and higher energy positions for Eσ+ and Eσ-, respectively, is observed, corresponding to the valley splitting of MoSe2 under a magnetic field.

Valley splitting of trions in the vdW heterostructure is defined as DET = Eσ- – Eσ+, which quantifies the Zeeman effect and magnetic proximity effect.[20,47–49] Figure 3e shows the trion valley splitting of the reference (1L-MoSe2/h-BN) as a function of applied magnetic field. The trion valley splitting shows a linear increase as a function of the applied magnetic field, and results for monolayer MX2 induced by the Zeeman effect under

where DE (31 ± 1 meV) is the energy difference between the excitons and trions reflecting the trion binding energy obtained from the fitting procedures in Figure 2b, γX and γT are the radi- ative decay rates of excitons and trions, respectively; kB is Boltz- mann constant; T is the temperature; GT is the nonradiative decay rate of trions; ktr (k¢tr ) is the formation rate of the trion from the exciton (dissociation rate of trion) through the phonon scattering process, and k0 is the coefficient of the formation rate of trion.[41–44] At low temperature (kBT << DE) and low carrier density region, the ratio of the radiative decay rates of excitons and trions are constant.[35] Under these conditions, Equation (1) IT » æg T ö æ k0 ö µexternal magnetic fields.[47,49] Figure 3f shows the applied mag- netic-field-dependent valley splitting of trions in the vdW heter- ostructure (1L MoSe2/h-BN/Mn oxide, d = 1.4 nm) at 10 K (blue) below TC and 90 K (red) above TC. The trion valley splitting also linearly increases with the applied magnetic field at 90 K, where the Mn oxide of the vdW heterostructure is in the paramagnetic phase. The value of the linear coefficient of trion valley splitting is evaluated as 0.22 meV T-1, and the effective g-factor can be extracted to be 3.6, both of which are consistent with the values of the reference (1L-MoSe2/h-BN, Figure 3e).

Large and nonlinear trion valley splitting of the vdW hetero- structure was experimentally observed at 10 K in the ferromag-intensity ratio of trions and excitons IT/IX is proportional to the doped carrier density ne, which is also depicted in Figure S8 in the Supporting Information.[29] From this relation, the higher intensity ratio of the vdW heterostructure compared with that of the reference (Figure 2d) can be understood by the higher doped carrier density in the vdW heterostructure (Figure S8, Supporting Information).

We also studied the magnetic proximity effect of monolayer increases rapidly and nonlinearly from 0 to 1 T and gradually increases above 1 T, suggesting that 1L-MoSe2 in the vdW het- erostructure experiences both the effective magnetic field (Beff) due to the magnetic proximity effect from ferromagnetic Mn oxide and the applied external magnetic field (Bapp). The valley splitting of trions is described, considering the magnetic prox- imity effect from Mn oxide, as follows MoSe2 and ferromagnetic Mn oxide vdW heterostructures.. Valley splitting and polarization under applied magnetic field. a–c) Circular polarization-resolved PL spectra for the trion of the reference (1L-MoSe2/h-BN) (a), vdW heterostructure (1L-MoSe2/Mn oxide) (b), and vdW heterostructure (1L-MoSe2/h-BN/Mn oxide, d = 1.4 nm) (c) at 10 K under 1 T, where red and blue dots correspond to σ+ and σ- components, respectively. d) Valley splitting of the trion in vdW heterostructure (1L-MoSe2/h-BN/ Mn oxide, d = 1.4 nm) at 10 K under different magnetic field conditions. e) Trion valley splitting of 1L-MoSe2/h-BN as a function of applied magnetic field at 10 K. The line is linearly fitted to the experiment data.

Applied magnetic field dependence of trion valley splitting of vdW heterostructure (1L-MoSe2/h-BN/Mn oxide, d = 1.4 nm). The blue and red dots show data at 10 and 90 K, where the Mn oxide is ferromagnetic and paramagnetic state, respectively. The solid green curve shows the out-of-plane magnetization (MC) of Mn oxide at 10 K under an applied magnetic field of 0.5 T after cooling down to zero field. The gray line is linear fit at 90 K. The black line shows the fitted curve at 10 K, considering the applied magnetic field and the effect of localized spin in Mn oxide, using Equation (2). g) Magnetic-field-dependent valley polarization of the trion in the reference at 10 K.
h) Magnetic-field-dependent valley polarization of the trion in the vdW heterostructure at 10 and 90 K. The error bars in (e,f) come from uncertainties of determination of peak energy due to the CCD pixel size (about 0.1 meV) and fitting procedures. The error bars in (g,h) represent the uncertainties of polarization-resolved PL measurements.

where DEeff and DEapp are the valley splitting caused by Beff and Bapp, respectively, g is the g-factor, and μB is the Bohr mag- neton.[50] The experimental result of valley splitting in Figure 3f can be reproduced using Equation (2), as shown in the black curve with a maximum effective magnetic field of »7.6 T, when the out-of-plane magnetization (MC, green line in Figure 3f) in Mn oxide becomes saturated. In addition, the valley splitting of the exciton is also enhanced at 10 K in the vdW heterostruc- ture with a thin h-BN of d = 1.4 nm (Figure S10, Supporting Information). These results indicate that the large and non- linear enhancement in the valley splitting of monolayer MoSe2 is induced by the magnetic proximity effect from Mn oxide in the vdW heterostructure.

Valley splitting in the magnetic field breaks the valley degen- eracy, enabling the control of valley polarization.[45] Figures 3g,h show the magnetic field dependence of the valley polarization of trions (ρT) in the reference and vdW heterostructure at 10 K, respectively. Here, ρT is defined as ρT = (Iσ+ - Iσ-)/(Iσ+ + Iσ-), where Iσ+ and Iσ- are the integrated trion PL intensities corresponding to σ+ and σ- components, respectively. The ρT of the reference tends to a linear increase with the applied magnetic field.

In contrast, the ρT of the vdW heterostructure exhibits a nonlinear behavior, which is similar to the valley splitting due to the magnetic proximity effect of ferromagnetic Mn oxide under- neath at 10 K. Moreover, a smaller valley polarization of the vdW heterostructure is observed at 90 K above TC, as shown in Figure 3h, which is attributed to the disappearance of the mag- netic proximity effect from the paramagnetic phase of Mn oxide. Further, we investigated the length scales of both the charge transfer and magnetic proximity effect in the vdW heterostruc- ture by changing the h-BN thickness, as schematically shown in Figure 1a. Figure 4a shows the PL spectra of 1L-MoSe2/h- BN/Mn oxide with an h-BN layer of different thicknesses (d = 0, 1.4, 2.9, 3.8, 4.9, and 8.0 nm). The spectral weight of trions com- pared with that of excitons is decreased at 10 K, suggesting that the amount of charge transfer from Mn oxide to 1L-MoSe2 becomes smaller as the h-BN thickness increases from 0 to 8.0 nm. Figure 4b shows the circular polarization-resolved PL Scale lengths of charge transfer and magnetic proximity effect. a) PL spectra of MoSe2/h-BN/Mn oxide with h-BN thicknesses of 0, 1.4, 2.9, 3.8, 4.9, and 8.0 nm at 10 K. b) Circular polarization-resolved PL spectra for the trion of 1L-MoSe2/h-BN/Mn oxide with h-BN thicknesses of 0, 1.4, 2.9, 3.8, 4.9, and 8.0 nm under 1 T at 10 K. c) Trion and exciton intensity ratio (IT/IX) as a function of h-BN thickness d. The right axis indicates the doped carrier density of MoSe2 estimated from IT/IX. The dotted line shows the residual carrier density of »6 ´ 1011 cm-2 in MoSe2. The curve shows the fitting result with a single exponential function. The error bars represent the variation of IT/IX depending on the position of samples (Figure S11, Supporting Information). d) Effective valley splitting of trion as a function of d.

The right axis shows the effective magnetic field arising from the localized spins of Mn oxide to MoSe2. The solid curve shows the fitting result with a single exponential function.spectra of trions in vdW heterostructures (1L-MoSe2/h-BN/Mn oxide) under an out-of-plane magnetic field of 1 T at 10 K. With the h-BN thickness from 1.4 to 8.0 nm, the valley splitting and valley polarization of trions are reduced, indicating a weaker magnetic proximity effect of ferromagnetic Mn oxide. Note that no enhanced valley degeneracy was observed in 1L-MoSe2/Mn oxide (d = 0 nm) (Figure 3b; Figure S9, Supporting Informa- tion). By inserting an extremely thin h-BN, valley splitting and polarization become very obvious due to the magnetic prox- imity effect compared with the reference (1L-MoSe2/h-BN). Importantly, the anomalous behavior of small valley splitting appears in 1L-MoSe2/Mn oxide without h-BN, showing no mag- netic proximity effect of ferromagnetic Mn oxide underneath.

This behavior could be explained by the simultaneous ferro- magnetic and metallic properties of Mn oxide below TC, which is caused by the double-exchange interactions between Mn3+ and Mn4+ ions.[24] The eg electron states of Mn sites are strongly hybridized with O 2p states and change from localized to itin- erant nature in the MnO3 network, as shown in Figure 1a.[22] In the vdW heterostructure (1L-MoSe2/Mn oxide) below TC, metal-induced gap states (MIGS) are formed at the interface between metallic Mn oxide and MoSe.[51–53] The itinerant eg electrons, thus, move to new conductive channels formed by MIGS, instead of hoping between Mn3+ and Mn4+ sites; this prevents the formation of the ferromagnetic phase in the Mn oxide.

Moreover, the orbital hybridization at the interface between the Mn oxide and MoSe2 disturbs the electronic states of the MnO3 network, which would cause the loss of magnet- ization at the surface of the Mn oxide.[54] In contrast, for the
vdW heterostructure (1L-MoSe2/h-BN/Mn oxide) comprising an extremely thin h-BN that possesses a large bandgap, the MIGS and orbital hybridization are not formed at the interfaces between Mn oxide and 1L-MoSe2 by inserting the h-BN, leading to the magnetic proximity effect for MoSe2.[55]
Figure 4c shows the dependence of the PL intensity ratio of trions and excitons (IT/IX) and estimated carrier density (ne) on the h-BN thickness in vdW heterostructures at 10 K. We esti- mated the carrier density using previously reported data of the PL intensities of excitons and trions (Figure S8, Supporting Information).[29] The evaluated doped carrier density rapidly decays with increasing distance between monolayer MoSe2 and Mn oxide (thickness of h-BN), suggesting that the charge transfer from metallic Mn oxide to monolayer MoSe2 via the tunneling barrier of h-BN with a higher bandgap occurs within a few nanometers.[56] The decay profile of the charge transfer is well represented by an exponential decay with a decay length of 2.0 nm from FM Mn oxide and a constant term from residual carriers (»6 ´ 1011 cm-2, dotted line in Figure 4c) in 1L-MoSe2.
Figure 4d shows the decay behavior of the magnetic proximity effect arising from FM Mn oxide on monolayer MoSe2 at 10 K under 1 T. The left axis is the effective valley splitting (DEeff) due to the magnetic proximity effect, and the right axis is the effective magnetic field (Beff) calcu- lated using Equation (2). The magnetic proximity effect in monolayer MoSe2 from FM Mn oxide quickly decays with increasing h-BN thickness. The length scale of a few nano- meters cannot be understood by the leakage magnetic fieldobeying the Biot–Savart law from FM Mn oxide because the typical size of FM Mn oxide in the vdW heterostructure is a large value of several tens of micrometers. A decay of a few nanometers in the experimental result suggests a direct exchange interaction between the ferromagnetic spins of Mn oxide and the excitonic states in K and -K of 1L-MoSe2 via the magnetic proximity effect.

Experimentally, a large effective magnetic field reaching over 7 T is obtained in the vdW heterostructure with the bilayer h-BN. A larger effec- tive magnetic field of over 15 T is predicted in the 1L-MoSe2/ h-BN/Mn oxide using a thinner h-BN monolayer, according to the extrapolation of experimental results in Figure 4d and the ab initio theoretical calculation.[57,58] These results reveal that the effective magnetic field decays within a few nanometers due to exchange interactions between 1L-MoSe2 and atomically flat ferromagnetic materials via the magnetic proximity effect.
In summary, we studied the charge transfer and magnetic proximity effect for excitons and trions in the 1L-MoSe2/ h-BN/Mn oxide vdW heterostructure. The discontinuous change in the dielectric screening of excitons is attributed to the metal- to-insulator transition of Mn oxide. The higher IT/IX of the vdW heterostructure than that of the reference is caused by the charge transfer to 1L-MoSe2 from metallic Mn oxide below TC. Moreover, large excitonic valley splitting and polarization have been demonstrated by the ferromagnetic spins of the FM states of Mn oxide with a bilayer h-BN between semiconducting MX2 and ferromagnetic metal, which provides a new experimental approach to introduce a ferromagnetic metal substrate, thereby lifting the valley degeneracy of MX2 with a strong magnetic proximity effect. The vdW structures using electronic and mag- netic phase transition materials demonstrated here can pro- vide new opportunities for the modulation and controllability of excitonic states via dielectric screening, charge carriers, and magnetic spins.

Experimental Section
Preparation of vdW Heterostructures: The double-layered perovskite (Ruddlesden–Popper structure) Mn oxide ((La0.8Nd0.2)1.2Sr1.8Mn2O7) was used in this study because it was easy to form a mechanical exfoliation from a single crystal. Moreover, it exhibited a sharp transition from a paramagnetic insulator to a ferromagnetic metal as a function of temperature, which was contrasting to La1-xSrxMnO3.[24] The double- layered perovskite Mn oxide was mechanically exfoliated on a SiO2/ Si substrate with a SiO2 thickness of 270 nm. Further, 1L-MoSe2 and hexagonal boron nitride (h-BN) on poly(dimethylsiloxane) (PDMS) films were mechanically exfoliated from their respective bulk crystals. The number of MoSe2 layers was determined from the optical contrast and the peak position of the PL spectra. Then, the vdW heterostructures of
Polarization-Resolved PL Spectra under a Magnetic Field: The polarization-resolved PL measurement was performed using a He–Ne laser as an excitation light source (photon energy of 1.959 eV) with circularly polarized light. The right- and left-handed circularly polarized PL spectra labeled as σ+ and σ-, respectively, were detected using a CCD camera after the monochromator was used under applied magnetic fields that ranged from 0 to 5 T; here, the magnetic field was applied in the out-of-plane direction of the samples.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements
The discussion and experimental help from Wenjin Zhang, Xiaofan Wang, Takashi Someya, Kenya Tanaka, and Masaru Onga are gratefully acknowledged. This work was supported by JSPS KAKENHI (Grant No. JP16H00911, JP15K13337, JP15H05408, JP16H00910, JP16H06331, JP17H06786, JP17K19055, JP19K14633, JP20H02605, JP19H05823, JP19K22142, and JP20H05664), by JST CREST (Grant No. JPMJCR16F3
and JPMJCR18I5), by the Keihanshin Consortium for Fostering the Next Generation of Global Leaders in Research (K-CONNEX) established by the Human Resource Development Program for Science and Technology, MEXT, and by the Asahi Glass Foundation.

Conflict of Interest
The authors declare no conflict of interest.

Author Contributions
Y.Z., K.S., Y.M., and K.M. designed the study. Y.Z. performed the optical SR1 experiments, analyzed the data, with the support of K.S., T.N., and K.M., K.W., T.T., and Y.M. provided the crystals, and M.G., D.K., and Y.S. supported the measurement. All authors discussed the results and contributed to the manuscript.

RAMANtouch) and a He-flow cryostat with a semiconductor laser (photon energy of 2.33 eV).
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