Interface Engineering Modulated Valley Polarization in MoS2/hBN Heterostructure

Layered transition metal dichalcogenides (TMDs) provide a favorable research platform for the advancement of spintronics and valleytronics because of their unique spin-valley coupling effect, which is attributed to the absence of inversion symmetry coupled with the presence of time-reversal symmetry. To maneuver the valley pseudospin efficiently is of great importance for the fabrication of conceptual devices in microelectronics. Here, we propose a straightforward way to modulate valley pseudospin with interface engineering. An underlying negative correlation between the quantum yield of photoluminescence and the degree of valley polarization was discovered. Enhanced luminous intensities were observed in the MoS2/hBN heterostructure but with a low value of valley polarization, which was in stark contrast to those observed in the MoS2/SiO2 heterostructure. Based on the steady-state and time-resolved optical measurements, we reveal the correlation between exciton lifetime, luminous efficiency, and valley polarization. Our results emphasize the significance of interface engineering for tailoring valley pseudospin in two-dimensional systems and probably advance the progression of the conceptual devices based on TMDs in spintronics and valleytronics.


Introduction
Two-dimensional (2D) polarized materials, including ferromagnets [1], ferroelectrics [2], and ferrovalley materials [3,4], demonstrate peculiar behaviors at the quantum realm. Valley pseudospin, which represents the energy band extremes in momentum space, normally exists in periodic solid materials [5,6]. The addressability of valley pseudospin enables the utilization of the momentum states of carriers as a brand-new paradigm in data coding and information handling. Using the research strategies of spintronics [7,8] for reference, a similar concept, valleytronics, arose naturally and vigorously [9]. The novel scientific connotation associated with the manipulation of the valley degree of freedom may result in a transformative impact. Referring to theoretical predictions about the intrinsic properties closely related to the valley pseudospin, rapid experimental advances [10][11][12][13][14] have been performed to observe and manipulate the valley polarization in a way similar to real spin.
Recently, the successful isolation and further experimental characterizations of 2D materials, including but not limited to graphene, hexagonal boron nitride (hBN), and TMDs, enriched our cognition of valley physics [15]. The spatial symmetry breaking along with the time-reversal symmetry enable two sets of the individually addressable valleys, K and K' points in the first Brillouin zone, for TMDs [16]. Especially when the layered TMDs, such as MoS 2 , WS 2 , and MoSe 2 , are mechanically exfoliated from bulk-phase crystals and thinned down to monolayers, a marvelous transition in their electronic structure occurs, viz the evolution from an indirect bandgap to a direct one. The direct bandgaps of monolayer TMDs normally lie in the near-infrared and visible spectral ranges of approximately 1~2 eV, suitable for the investigation of valley pseudospin-related optoelectronic applications. Van der Waals (vdW) heterostructures composed of 2D materials offer a fascinating research platform for tailoring artificial composite constructions with unique properties, novel phenomena [17,18], and widespread potential applications [19][20][21]. The concept of engineering material properties via fabricating mixed-dimensional vdW heterostructures can also be employed to manipulate valley polarization with great convenience and low cost for spintronics and valleytronics [22,23].
Here, we performed circularly polarization-dependent photoluminescence (PL) measurements upon MoS 2 monolayers transferred atop hBN nanoflakes and SiO 2 /Si wafers at room temperature, prepared with the chemical vapor deposition (CVD) method. Substantial variations were observed in these two vertically stacked systems, MoS 2 /hBN and MoS 2 /SiO 2 , especially in their luminous performance. An underlying opposite relationship between the intensity of luminescence and the degree of valley polarization was observed. Specifically, a stronger exciton luminescence was observed in the MoS 2 /hBN heterostructure but with a lower degree of valley-polarized emission, while a weaker luminous performance and a higher degree of valley polarization were obtained in the MoS 2 /SiO 2 heterostructure. We infer that there exists a connection to be examined between the luminous efficiency and the valley polarization. According to the time-resolved circularly polarization-dependent PL measurements, a lower degree of valley polarization is observed in samples that exhibit longer exciton lifetimes. Our work reveals an inverse relationship between luminescence efficiency and the valley polarization value, and emphasizes the significance of interface engineering for modulating the intrinsic new degrees of freedom in ultrathin 2D polarized materials. It may potentially deepen our understanding of 2D quantum systems and advance the realization of the emerging practical applications for next-generation information storage and processing.

Materials and Methods
Large-area, continuous MoS 2 monolayers with a lateral size of several millimeters were routinely prepared with a standardized CVD process in a commercial dual-temperaturezone furnace [24,25] using high-purity S and MoO 3 powders as the solid-state powder sources. Mechanically exfoliated hBN flakes were transferred atop SiO 2 /Si wafers with a 285 nm oxidation layer before CVD growth. Highly purified argon was employed as the transport gas. The growth temperature and the required time parameters we used for the CVD process are shown in the inset of Figure 1a. The programming temperatures for the two respective zones, in which the S and MoO 3 powders were placed, were elevated from room temperature to 160 and 650 • C in 40 min and remained unchanged for 5 min. Once the reaction and deposition processes finished, it was cooled naturally. Generally speaking, the typical growth of MoS 2 crystals atop SiO 2 /Si normally results in equilateral triangle shapes under thermodynamically stable conditions. As shown in the optical microscopy image, the MoS 2 samples were successfully deposited onto the 285 nm SiO 2 /Si wafer with some mechanically exfoliated hBN flakes randomly distributed atop its surface (Figure 1a). It can be observed that numerous discrete MoS 2 monolayers connected to each other and merged into a continuous single-layer film. The continuous single-layer film typically exhibits a lateral dimension up to several hundreds of micrometers, beneficial for probing many discrete locations within a sample via an optical means. Yu et al. previously reported that, unlike the growth of MoS 2 on SiO 2 substrates, the growth of MoS 2 on hBN normally follows a lattice alignment epitaxial growth mode [26], where two structurally equivalent 0 • and 60 • stacked MoS 2 /hBN are presented. There is a typical hBN flake obtained with mechanical exfoliation on the commonly used SiO2 (285 nm)/Si wafer before CVD growth. Inset: Temperature and time parameters for the CVD growth of the MoS2 monolayers. (b) Typical Raman spectra obtained from the MoS2 monolayer prepared atop hBN (H1, H2, and H3) and SiO2 (S1, S2, and S3). (c) The schematic diagram of the experimental setup for collecting the circularly polarized photoluminescence signals from the MoS2 sample. Figure 1b presents the Raman scattering spectra acquired from six distinct locations, corresponding to the MoS2 samples prepared atop the hBN and SiO2 substrates, respectively. The characteristic feature located at approximately 520.7 cm −1 is attributed to the underneath Si substrate used for wavenumber calibration. Due to the reliability and repeatability of our synthetic strategy, we can observe two characteristic peaks of MoS2, E 1 2g and A1g, as well as the signal from hBN (≈1366.4 cm −1 ), as shown in the Raman spectra labeled as H1-H3, confirming that MoS2 is successfully grown on the hBN flake. The peak interval between the two characteristic features of MoS2, E 1 2g and A1g, can help us determine the number of layers of the MoS2 samples quantitatively and rapidly (Appendix A). One accessible mode for hBN is the in-plane E2g mode located at approximately 1366.4 cm −1 (Appendix B).
The vdW heterostructures comprised of 2D materials serve as an inspiring platform for tailoring physicochemical properties, exploring novel quantum effects, and ultimately fabricating conceptually new devices [17,18]. A classical paradigm is the demonstration of the fascinating Hofstadter butterfly observed in artificial vertically stacked moiré superlattices comprised of graphene and hBN [27,28]. The strategy to tailor material properties with interface engineering is also available for the vertically stacked structure comprised of TMDs and hBN, which enables the exploration of valley polarization. The circular polarization-sensitive PL measurement was performed in a home-made micro-zone setup as displayed in Figure 1c. The linearly polarized excitation light is converted to the circularly polarized one by a broadband quarter-wave plate. A 50× objective (N.A. 0.55) is used to collect the emission signals from the MoS2 monolayers. The left-handed and righthanded circularly polarized light signals (σ+ and σ−) are converted to horizontal and ver-  Figure 1b presents the Raman scattering spectra acquired from six distinct locations, corresponding to the MoS 2 samples prepared atop the hBN and SiO 2 substrates, respectively. The characteristic feature located at approximately 520.7 cm −1 is attributed to the underneath Si substrate used for wavenumber calibration. Due to the reliability and repeatability of our synthetic strategy, we can observe two characteristic peaks of MoS 2 , E 1 2g and A 1g , as well as the signal from hBN (≈1366.4 cm −1 ), as shown in the Raman spectra labeled as H1-H3, confirming that MoS 2 is successfully grown on the hBN flake. The peak interval between the two characteristic features of MoS 2 , E 1 2g and A 1g , can help us determine the number of layers of the MoS 2 samples quantitatively and rapidly (Appendix A). One accessible mode for hBN is the in-plane E 2g mode located at approximately 1366.4 cm −1 (Appendix B).
The vdW heterostructures comprised of 2D materials serve as an inspiring platform for tailoring physicochemical properties, exploring novel quantum effects, and ultimately fabricating conceptually new devices [17,18]. A classical paradigm is the demonstration of the fascinating Hofstadter butterfly observed in artificial vertically stacked moiré superlattices comprised of graphene and hBN [27,28]. The strategy to tailor material properties with interface engineering is also available for the vertically stacked structure comprised of TMDs and hBN, which enables the exploration of valley polarization. The circular polarization-sensitive PL measurement was performed in a home-made micro-zone setup as displayed in Figure 1c. The linearly polarized excitation light is converted to the circularly polarized one by a broadband quarter-wave plate. A 50× objective (N.A. 0.55) is used to collect the emission signals from the MoS 2 monolayers. The left-handed and right-handed circularly polarized light signals (σ+ and σ−) are converted to horizontal and vertical linearly polarized light signals (I σ+→σ+ and I σ+→σ− ), respectively, via a broadband Nanomaterials 2023, 13, 861 4 of 13 quarter-wave (1/4λ) plate. The two linearly polarized light beams can be separated in real space with a Wollaston prism and then focused to two spots positioned at the entrance slit of the spectrometer equipped with the charged coupled device (CCD) cooled at −75 • C. The intensities of these two orthogonally linearly polarized beams, corresponding to the σ+ and σ− components of the PL signal, are recorded simultaneously with the CCD. By doing so, we greatly reduce the error caused by laser power fluctuation. The steady-state circularly polarized PL spectra were captured under resonant excitation with a continuous-wave 633 nm laser. For the time-resolved PL measurements, a pulsed linearly polarized Ti:sapphire laser with the wavelength of 405 nm was employed.

Crystal Structure of MoS 2 and the Coupled Valley-Spin Excitonic Transition Rules
Transmission electron microscopy (TEM) provides a powerful tool for examining the morphology and lattice structure of low-dimensional materials. As shown in Figure 2a tical linearly polarized light signals (I σ+→σ+ and I σ+→σ− ), respectively, via a broadband q ter-wave (1/4λ) plate. The two linearly polarized light beams can be separated in real sp with a Wollaston prism and then focused to two spots positioned at the entrance sl the spectrometer equipped with the charged coupled device (CCD) cooled at −75 °C. intensities of these two orthogonally linearly polarized beams, corresponding to th and σ− components of the PL signal, are recorded simultaneously with the CCD. By do so, we greatly reduce the error caused by laser power fluctuation. The steady-state ci larly polarized PL spectra were captured under resonant excitation with a continu wave 633 nm laser. For the time-resolved PL measurements, a pulsed linearly polar Ti:sapphire laser with the wavelength of 405 nm was employed.

Crystal Structure of MoS2 and the Coupled Valley-Spin Excitonic Transition Rules
Transmission electron microscopy (TEM) provides a powerful tool for examining morphology and lattice structure of low-dimensional materials. As shown in Figure 2 MoS2 monolayer triangle was transferred atop a carbon-film-coated TEM copper crogrid in the presence of a relatively complete geometric morphology. The recorded lected area electron diffraction (SAED) spots (inset, Figure 2a  From the top view of the crystal structure in MoS2, a hexagonal honeycomb la structure can be observed that generates two sets of degenerate-but-not-equivalent leys, K and K', at the edges of the first Brillouin zone (left panel, Figure 3a). These val that are degenerate in energy exhibit a huge splitting in the valence band (Δv=~148 m induced by spin-orbit coupling and a much smaller one in the conduction band (Δ meV), which is also depicted for completeness (right panel, Figure 3a), for the MoS2 m olayer [29][30][31][32]. The K and K' valleys are differentiated by the opposite spin orientat corresponding to the valence band maximum (VBM) and conduction band minim From the top view of the crystal structure in MoS 2 , a hexagonal honeycomb lattice structure can be observed that generates two sets of degenerate-but-not-equivalent valleys, K and K', at the edges of the first Brillouin zone (left panel, Figure 3a). These valleys that are degenerate in energy exhibit a huge splitting in the valence band (∆ v =~148 meV) induced by spin-orbit coupling and a much smaller one in the conduction band (∆ c =~3 meV), which is also depicted for completeness (right panel, Figure 3a), for the MoS 2 monolayer [29][30][31][32]. The K and K' valleys are differentiated by the opposite spin orientations corresponding to the valence band maximum (VBM) and conduction band minimum (CBM). As schematically displayed in the right panel of Figure 3a, combined with the time reversal symmetry, the spin orientations of the K and K' valleys are anti-symmetric, enabling a locking of the spin and the valley degree of freedom. The remarkable difference guarantees the valley-dependent optical selection rules that the direct excitonic transitions should obey, including A and B, in the MoS 2 monolayer. To be specific, the circularly polarized lights with left-handed helicity (σ+) excite the excitonic transitions in the K valleys exclusively, whereas those lights with right-handed helicity (σ−) only couple to the K' valleys. In our previous report [24], we reported that the A excitonic emission from the MoS 2 monolayer on the hBN flake exhibited a ubiquitous enhancement compared with that on SiO 2 /Si. As plotted in Figure 3b,c, the intensities of the PL signals from MoS 2 on hBN are much stronger than those obtained from MoS 2 on SiO 2 , both under 633 nm and 488 nm excitation. Nanomaterials 2023, 13, 861 5 of 13 (CBM). As schematically displayed in the right panel of Figure 3a, combined with the time reversal symmetry, the spin orientations of the K and K' valleys are anti-symmetric, enabling a locking of the spin and the valley degree of freedom. The remarkable difference guarantees the valley-dependent optical selection rules that the direct excitonic transitions should obey, including A and B, in the MoS2 monolayer. To be specific, the circularly polarized lights with left-handed helicity (σ+) excite the excitonic transitions in the K valleys exclusively, whereas those lights with right-handed helicity (σ−) only couple to the K' valleys. In our previous report [24], we reported that the A excitonic emission from the MoS2 monolayer on the hBN flake exhibited a ubiquitous enhancement compared with that on SiO2/Si. As plotted in Figure 3b and c, the intensities of the PL signals from MoS2 on hBN are much stronger than those obtained from MoS2 on SiO2, both under 633 nm and 488 nm excitation.

Steady-State Circularly Polarized PL Spectra
The valley polarization-resolved luminous property appears a typical representative among the abundant unique physical properties for the MoS2 monolayer. The vertical stacking of MoS2 and hBN enables us to explore the valley polarization utilizing our homemade circular polarization PL measurement system. We further performed the PL measurements upon the MoS2 monolayer on hBN and SiO2 under resonant excitation with a 633 nm laser, widely employed in valley pseudospin-related luminous property investigation. The value for the degree of valley polarization obtained from the MoS2 monolayers with different substrates underneath was calculated based on the measured polarizationresolved PL spectra involving σ− and σ+ components. As shown in Figure 4a and b, we obtained the PL spectra by using the 1.96 eV (633 nm) laser radiation as excitation, that is

Steady-State Circularly Polarized PL Spectra
The valley polarization-resolved luminous property appears a typical representative among the abundant unique physical properties for the MoS 2 monolayer. The vertical stacking of MoS 2 and hBN enables us to explore the valley polarization utilizing our home-made circular polarization PL measurement system. We further performed the PL measurements upon the MoS 2 monolayer on hBN and SiO 2 under resonant excitation with a 633 nm laser, widely employed in valley pseudospin-related luminous property investigation. The value for the degree of valley polarization obtained from the MoS 2 monolayers with different substrates underneath was calculated based on the measured polarization-resolved PL spectra involving σ− and σ+ components. As shown in Figure 4a and b, we obtained the PL spectra by using the 1.96 eV (633 nm) laser radiation as excitation, Nanomaterials 2023, 13, 861 6 of 13 that is with the left-handed helicity (σ+) on resonance with the A excitonic transition. We determined the degree of valley polarization quantitatively [10,11] as where I σ+→σ+ and I σ+→σ− denote the intensities of the left-handed and right-handed circularly polarized PL signals, respectively, which are excited with a left-handed circular excitation laser. As plotted in Figure 4c, where I σ+→σ+ and I σ+→σ− denote the intensities of the left-handed and right-handed circularly polarized PL signals, respectively, which are excited with a left-handed circular excitation laser. As plotted in Figure 4c, the average valley polarization values, ranging from 656 nm to 676 nm, were calculated to be 0.207 for MoS2 on SiO2 and 0.080 for MoS2 on hBN. It can be observed, among the wavelength range of the direct A excitonic emission for the MoS2 monolayer, the valley polarization for MoS2 on hBN is always lower than that observed from the MoS2 monolayers on SiO2. Based on the numerical analyses of 25 MoS2 monolayer samples on hBN, the statistical average for the degree of valley polarization is P = 0.130 ± 0.046 (Figure 4d), which demonstrates a relatively low degree of valley polarization in the MoS2/hBN heterostructure at room temperature (300 K). In stark contrast to that, the degree of valley polarization for the MoS2 monolayer deposited atop the SiO2 wafer under the identical measurement conditions is relatively high. The statistical average value equals approximately 0.198 ± 0.020 from which an important message can be delivered that the interaction widely existent in 2D materials/supporting substrates may play a critical role in the modulation of valley pseudospin in 2D polarized materials. Combined with the measurement results Based on the numerical analyses of 25 MoS 2 monolayer samples on hBN, the statistical average for the degree of valley polarization is P = 0.130 ± 0.046 (Figure 4d), which demonstrates a relatively low degree of valley polarization in the MoS 2 /hBN heterostructure at room temperature (300 K). In stark contrast to that, the degree of valley polarization for the MoS 2 monolayer deposited atop the SiO 2 wafer under the identical measurement conditions is relatively high. The statistical average value equals approximately 0.198 ± 0.020 from which an important message can be delivered that the interaction widely existent in 2D materials/supporting substrates may play a critical role in the modulation of valley pseudospin in 2D polarized materials. Combined with the measurement results men-tioned above, an enhanced PL intensity and a reduced valley polarization are observed in the MoS 2 /hBN heterostructure. Why does there exist a noticeably opposite relationship between luminous intensity and valley polarization? To answer this question and further clarify the underlying mechanism, we further performed the time-resolved circularly polarization-dependent PL measurements at room temperature.
For the steady-state conditions excited with a continuous wave (CW) laser, the degree of valley polarization, P, can be determined with under a rate model, where P 0 denotes the initial polarization, and τ e and τ v represent the exciton and valley relaxation times, respectively. The derivative process is provided in Appendix C. The value of P increases with either an increase in valley lifetime τ v or a decrease in exciton lifetime τ e , as clearly observed from Equation (2). Previous reports did not identify the influence of substrates on the valley relaxation time τ v for MoS 2 , among which the measured values are close numerically and even the supporting substrates are different. All the as-synthesized MoS 2 monolayers shown in Figure 4 were excited with exactly the same excitation wavelength, pumping power, and exposure time, and exposed to nearly the identical environment. Thus, it is assumed that the valley relaxation time τ v and the initial polarization P 0 are the same. It can be inferred that the degree of valley polarization P will decrease if the exciton relaxation time increases.

Time-Resolved Circularly Polarized PL Spectra
According to the theory in semiconductor physics, the exciton relaxation time τ e is closely related to both radiative and non-radiative recombination times through the following relational expression: 1 The time parameters, τ r and τ nr , represent the radiative and non-radiative lifetimes, respectively. For transition metal dichalcogenides, the non-radiative lifetimes can be orders of magnitude shorter than the radiative recombination times. In other words, the magnitude relationship τ nr << τ r exists in MoS 2 monolayers. Hence, there exists an approximation relation τ e ≈ τ nr . For a certain material, the radiative recombination time τ r can be regarded as a constant. Since the luminous intensity is in direct proportion to quantum yield (QY), If the radiative recombination time remains unchanged, a longer exciton relaxation time will lead to a higher QY and, thus, an enhanced PL intensity.
We further performed time-resolved photoluminescence (TRPL) measurements to examine the exciton dynamics and capture the corresponding fluorescence lifetime with which our hypothesis mentioned above may be verified. The measured TRPL spectra are plotted in Figure 5. Two systems, including the MoS 2 monolayers deposited atop hBN and SiO 2 , were measured, which behave remarkably different from each other in the intensity of luminescence and the degree of valley polarization. A pulsed, linearly polarized Ti:sapphire laser with the wavelength of 405 nm was employed to pump the direct excitons into the two degenerate-but-not-equivalent valleys, K and K', in the MoS 2 monolayer simultaneously via an optical means. The pulsed optical pumps were realized with an optical parametric amplification. The subsequent luminescent signals created from the K and K' valleys could then be collected and directed to a time-resolved CCD detector. The dynamic experimental results were quite similar to those observed in the steady-state PL measurement results. parametric amplification. The subsequent luminescent signals created from the K and K' valleys could then be collected and directed to a time-resolved CCD detector. The dynamic experimental results were quite similar to those observed in the steady-state PL measurement results. Figure 5. The circularly polarized time-resolved PL spectra obtained from the MoS2 monolayers on hBN and SiO2, all of which were measured at room temperature. By the deconvolution fitting, the exciton lifetimes could be obtained.
As mentioned above, the MoS2 monolayers on hBN and SiO2 display noticeably different PL intensity as well as distinguishing valley polarization behaviors. The time-resolved emission spectra measured from these samples, as presented here, also exhibit significantly different exciton attenuation kinetics. MoS2 on hBN possesses a longer exciton lifetime than MoS2 on SiO2. Compared with SiO2, hBN is chemically inert and has no defect states or hanging bonds on its surface. This may result in less disorder to the MoS2 monolayer [33], which is mainly stemmed from extrinsic effects, for example, defects and trap states. The difference in the exciton relaxation time explains the enhanced PL intensity and lower valley polarization of MoS2 on hBN.

Conclusions
In conclusion, we successfully synthesized a MoS2 monolayer on hBN nanoflakes via the CVD method and performed room-temperature circularly polarized PL measurements upon the MoS2 samples with different substrates underneath. We observed that there exists an apparent inverse correlation between the intensity of PL signals and the value of valley polarization, which originates from the variations in exciton relaxation time. Compared to MoS2 monolayers atop SiO2, the MoS2 monolayers atop hBN exhibit a relatively low degree of valley polarization. An enhanced PL intensity and a longer exciton lifetime were also experimentally consolidated in the MoS2/hBN system. Our discovery suggests a pathway to tailor carrier dynamics via crystal modification, which can be realized by introducing an appropriate amount of defects or non-radiative recombination sites. By doing so, the room-temperature valley polarization can be effectively modulated, and the coupling between degenerate valleys can be attenuated, which provides new strategies for the realization of state-of-the-art devices in spintronics and valleytronics.
Author Contributions: F.L., H.Z., Y.Z., and M.L. performed the CVD synthesis and sample characterization measurements (Micro-Raman and PL); analyzed and interpreted the corresponding data; Figure 5. The circularly polarized time-resolved PL spectra obtained from the MoS 2 monolayers on hBN and SiO 2 , all of which were measured at room temperature. By the deconvolution fitting, the exciton lifetimes could be obtained.
As mentioned above, the MoS 2 monolayers on hBN and SiO 2 display noticeably different PL intensity as well as distinguishing valley polarization behaviors. The timeresolved emission spectra measured from these samples, as presented here, also exhibit significantly different exciton attenuation kinetics. MoS 2 on hBN possesses a longer exciton lifetime than MoS 2 on SiO 2 . Compared with SiO 2 , hBN is chemically inert and has no defect states or hanging bonds on its surface. This may result in less disorder to the MoS 2 monolayer [33], which is mainly stemmed from extrinsic effects, for example, defects and trap states. The difference in the exciton relaxation time explains the enhanced PL intensity and lower valley polarization of MoS 2 on hBN.

Conclusions
In conclusion, we successfully synthesized a MoS 2 monolayer on hBN nanoflakes via the CVD method and performed room-temperature circularly polarized PL measurements upon the MoS 2 samples with different substrates underneath. We observed that there exists an apparent inverse correlation between the intensity of PL signals and the value of valley polarization, which originates from the variations in exciton relaxation time. Compared to MoS 2 monolayers atop SiO 2 , the MoS 2 monolayers atop hBN exhibit a relatively low degree of valley polarization. An enhanced PL intensity and a longer exciton lifetime were also experimentally consolidated in the MoS 2 /hBN system. Our discovery suggests a pathway to tailor carrier dynamics via crystal modification, which can be realized by introducing an appropriate amount of defects or non-radiative recombination sites. By doing so, the room-temperature valley polarization can be effectively modulated, and the coupling between degenerate valleys can be attenuated, which provides new strategies for the realization of state-of-the-art devices in spintronics and valleytronics.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: All data concerning this study are contained in the present manuscript and in previous articles, whose references have been provided.

Acknowledgments:
The authors are grateful to Yu Ye and Lun Dai form Peking University for helpful discussion.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
Li et al. proposed a general thickness identification technique for atomic-scale, fewlayer MoS 2 on dielectric substrates [34], such as SiO 2 . The peak interval between the two characteristic features of MoS 2 , E 1 2g and A 1g , can determine the number of layers of the MoS 2 samples quantitatively and rapidly.
The zoom-in Raman spectrum of the MoS 2 monolayer on SiO 2 is provided in Figure A1a. We can clearly observe the characteristic modes E 1 2g and A 1g of MoS 2 , the peak interval (~19.8 cm −1 ) between which is consistent with the criterion for CVD-synthesized MoS 2 monolayers on SiO 2 . Figure A1b plots the Raman spectra obtained from 1L, 2L, and 3L MoS 2 on SiO 2 , the thickness of which was determined with the atomic force microscope technique. The three characteristic peaks located among 360~540 cm −1 correspond to two characteristic peaks of MoS 2 , E 1 2g and A 1g , as well as the signal from the underneath Si substrate (520.7 cm −1 ). In the series of Raman spectra, corresponding to 1L, 2L, and 3L MoS 2 , there exists a monotonous variation trend in the peak interval between E 1 2g and A 1g .
and drafted the article. Y.L., Y.Y., J.Y., and S.M. performed the TEM characterizations. E.K. and Y. conceived the project and revised the manuscript. All authors have read and agreed to the publish version of the manuscript. Data Availability Statement: All data concerning this study are contained in the present manuscr and in previous articles, whose references have been provided.

Acknowledgments:
The authors are grateful to Yu Ye and Lun Dai form Peking University for he ful discussion.

Conflicts of Interest:
The authors declare no conflicts of interest.

Appendix A
Li et al. proposed a general thickness identification technique for atomic-scale, few layer MoS2 on dielectric substrates [34], such as SiO2. The peak interval between the tw characteristic features of MoS2, E 1 2g and A1g, can determine the number of layers of t MoS2 samples quantitatively and rapidly.
The zoom-in Raman spectrum of the MoS2 monolayer on SiO2 is provided in Figu  A1a. We can clearly observe the characteristic modes E 1 2g and A1g of MoS2, the peak inte val (~19.8 cm −1 ) between which is consistent with the criterion for CVD-synthesized Mo monolayers on SiO2. Figure A1b plots the Raman spectra obtained from 1L, 2L, and MoS2 on SiO2, the thickness of which was determined with the atomic force microsco technique. The three characteristic peaks located among 360~540 cm −1 correspond to tw characteristic peaks of MoS2, E 1 2g and A1g, as well as the signal from the underneath substrate (520.7 cm −1 ). In the series of Raman spectra, corresponding to 1L, 2L, and MoS2, there exists a monotonous variation trend in the peak interval between E 1 2g and A Figure A1. (a) Zoom-in Raman spectrum of 1L MoS2. (b) Typical Raman spectra obtained from 1 2L, and 3L MoS2 prepared atop SiO2. The Raman peak located at approximately 520.7 cm −1 cor sponding to Si is used for wavenumber calibration and intensity normalization.
The detailed values of peak positions, as shown in Figure A1b, are summarized Table A1. With the increasing number of layers, there exists a decrease in the peak positi of E 1 2g mode, an increase in the peak position of A1g mode, and an increase in the pe interval Δ, consistent with the experimental results previously reported by Li et al. The detailed values of peak positions, as shown in Figure A1b, are summarized in Table A1. With the increasing number of layers, there exists a decrease in the peak position of E 1 2g mode, an increase in the peak position of A 1g mode, and an increase in the peak interval ∆, consistent with the experimental results previously reported by Li et al. [34]. As mentioned above, it is a feasible calibration method to determine the number of layers of the few-layer MoS 2 samples based on the amplitude of the peak interval ∆.

Appendix B
Raman scattering spectroscopy provides a non-destructive technique for studying hBN. One accessible mode is the in-plane E 2g mode at approximately 1366.4 cm −1 [35]. As displayed in Figure A2, a narrow feature is a representative of highly ordered, defect-free hBN. As we can observe from Figure 1b, the two characteristic peaks of MoS 2 , E 1 2g and A 1g , as well as the signal from hBN (≈1366.4 cm −1 ) obtained from the samples labeled as H1 to H3 in the main text, indicate that MoS 2 is successfully synthesized atop the mechanically exfoliated hBN flakes. Nanomaterials 2023, 13, 861 10 of 13 As mentioned above, it is a feasible calibration method to determine the number of layers of the few-layer MoS2 samples based on the amplitude of the peak interval Δ.

Appendix B
Raman scattering spectroscopy provides a non-destructive technique for studying hBN. One accessible mode is the in-plane E2g mode at approximately 1366.4 cm −1 [35]. As displayed in Figure A2, a narrow feature is a representative of highly ordered, defect-free hBN. As we can observe from Figure 1b, the two characteristic peaks of MoS2, E 1 2g and A1g, as well as the signal from hBN (≈1366.4 cm −1 ) obtained from the samples labeled as H1 to H3 in the main text, indicate that MoS2 is successfully synthesized atop the mechanically exfoliated hBN flakes. Under the steady-state excitation for the neutral exciton (A) generated in the K valley, as employed in our experiment, the right-handed helicity excitation is zero, viz g K' = 0, and the material system is in a state of dynamic equilibrium, which indicates that We determine the degree of valley polarization quantitatively as based on the polarization-dependent PL intensities. The valley polarization can also be written as P = n K − n K n K + n K (A5) based on the polarization-dependent PL intensities. Thus, for the steady-state conditions excited with a CW laser, the degree of valley polarization, P, can be determined with According to the theory in semiconductor physics, the exciton relaxation time τ e is closely related to both radiative and non-radiative recombination times through the following relational expression, τ e −1 = τ r −1 + τ nr −1 , in which the time parameters, τ r and τ nr , represent the radiative and non-radiative lifetimes, respectively. For transition metal dichalcogenides, the non-radiative lifetimes can be orders of magnitude shorter than radiative recombination times, viz τ nr << τ r . Hence, there exists an approximation relation τ e ≈ τ nr . Considering the derivation in initial polarization that is closely related to some parameters, including but not limited to excitation laser wavelength, exposure time, and possible intervalley generation, we multiply the numerator with a coefficient P 0 additively: Consequently, the less-than-perfect (much lower than the ideal 100%) degree of valley polarization observed in the circularly optical measurement results is attributed to inevitable intervalley processes and the competition between the valley lifetime τ v and the recombination relaxation time τ e .