Ultrafast Dynamics of Spin Generation and Relaxation in Layered WSe2

Subscriber access provided by University of Rochester | River Campus & Miner Libraries

Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 2

Ultrafast Dynamics of Spin Generation and Relaxation in Layered WSe Jialiang Ye, Ying Li, Tengfei Yan, Guihao Zhai, and Xin-Hui Zhang

J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01068 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Ultrafast Dynamics of Spin Generation and Relaxation in Layered WSe2 Jialiang Ye,†,‡ Ying Li,†,‡ Tengfei Yan,† Guihao Zhai,†,‡ and Xinhui Zhang∗,†,‡ †State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China ‡Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We investigated the build-up and relaxation processes of spin-polarized A- and Bexciton dynamics in monolayer, bilayer as well as bulk WSe2 using helicity-resolved two-color pump-probe spectroscopy. The substantial spin polarization was confirmed in bulk crystals, though the spin polarization degree of A excitons decreased from monolayer to bulk. However, the spin polarization of A excitons almost vanished in all different layered-flakes when resonantly pumping the B-exciton transition, owing to the dominant role of inter-exciton transfer. When resonantly pumping the A-exciton transition, the spin polarization of the up-converted B excitons was inverted in all layers due to the efficient Dexter-like coupling and phonon-assisted scattering. The same short spin relaxation time (1.8 ± 0.2 ps) of A excitons was found for all studied flakes in the subsequent spin depolarization processes, which was ascribed to the active electron-phonon scattering resulting from the intrinsic small conduction-band spinorbit coupling splitting in layered WSe2 .

Graphical TOC Entry

.

2


Page 2 of 27

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The transition metal dichalcogenide (TMDC) semiconductors are layered materials with a two-dimensional honeycomb crystal structure. The monolayers have a characteristics of direct bandgap and two inequivalent valleys (K /K’ ) at the corners of the hexagonal Brillouin zone. 1 The carriers spin and valley are coupled due to the broken inversion symmetry and strong spin-orbit coupling (SOC), resulting in a valley-contrasting spin splitting in both the valence and conduction band. 2–6 The spin splitting in the valence band is about 400 meV in monolayer WSe2 , while it is significantly smaller (about –37 meV) in the conduction band. 5 Via circularly-polarized light excitation, a valley-contrasting optical selection rule is allowed for two valleys of A and B excitons, as shown in Figure 1a. The depolarization dynamics of spin degree of freedom for A excitons has been extensively investigated by ultrafast spectroscopy including time-resolved photoluminescence (TRPL), time-resolved Kerr rotation (TRKR) and helicity-resolved transient absorption/reflection. 7–17 The spin/valley lifetime of A excitons was commonly found to be on the picosecond (ps) timescale, and the corresponding valley depolarization mechanisms have been suggested to describe the key physical processes of the coherent/collision spin relaxation dynamics. Taking the case of A excitons of monolayer WSe2 as sketched in Figure 1b, the main physical mechanisms contributing to spin/valley relaxation in TMDC materials are as below: (I) The intervalley spin-flip relaxation. The well-known mechanisms are the intervalley electron-hole (e-h) exchange coupling. 18,19 and electron-phonon scattering. 20 (II)The intervalley spin-conserved relaxation, including the recently reported Dexterlike intervalley coupling between A- and B- excitons, 21,22 or the incoherent electron/hole scattering. This kind of relaxation channel induces a very low PL circular polarization of A excitons when the laser excitation energy is resonant with the B-exciton transition. 23 (III)The intravalley spin-flip relaxation, mainly involving the transfer of electrons between the spin-split conduction bands in the K or K’ valley. 24,25 (IV)Exciton recombination and relaxation into dark states. 12,26,27 For a bilayer 2H -WSe2 , where the upper layer is rotated by 180◦ with respect to the lower

3



layer, crystal inversion symmetry is recovered. 10,28 It is thus expected that valley polarization physics for centrosymmetric bilayer WSe2 is strictly absent. However, previous studies have confirmed that the substantially high spin polarization associated with direct transition was still present in bilayer TMDCs. The high PL circular polarization degree of A exciton (> 50%) has been observed in bilayer WSe2 or WS2 . 29–32 The coupled layer-spin degree of freedom has also further proved the preserved spin polarization in the bilayer or multi-layer TMDCs. 32–37 As a matter of fact, owing to the increased Coulomb screening of excitons in multi-layer TMDCs, 34,38 the exciton and spin physics is expected to be different from that of monolayer. The impact of the energy band structure and additional interlayer electron/hole transfer on the spin polarization generation and relaxation dynamics for layered TMDCs is not yet well explored so far. For example, the very recent time-resolved spectroscopy study has reported the inverted spin/valley polarization in monolayer WS2 induced by strong A-B exciton coherent coupling, 22 but it is not clear whether this Dexter-like intervalley coupling also applies to bilayer or bulk WSe2 (or other TMDC materials) as well, and how strong this coupling could be comparing with other spin relaxation channels? A comprehensive study of ultrafast dynamics of spin generation and relaxation is thus necessary in order to understand the common features and difference of spin dynamics in monolayer and multi-layer TMDCs for realizing novel spintronics based on TMDC materials. Here we present a systematical helicity-resolved two-color pump-probe study for spin/valley dynamics associated with A- and B-excitons in monolayer, bilayer as well as bulk WSe2 . Our target is to explore the difference of the spin/valley build-up and relaxation dynamics induced by the interplay between A- and B-excitons as well as phonon-assisted scattering for different layers. Our work revealed that, though the optically-injected spin polarization decreased monotonically when increasing the number of layer, it was still substantially present in bulk crystals. When the pump photon energy was resonant with that of the B-exciton transition, the spin polarization of A excitons almost vanished in all layered flakes, but the build-up dynamics of A excitons was different. Upon resonantly pumping the A-exciton

4


Page 4 of 27

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transition, an inverted spin polarization from the up-converted B excitons was commonly observed. In the subsequent spin depolarization processes, the same A-exciton spin/valley relaxation time at room temperature was determined for different WSe2 layers, resulting from the efficient electron-phonon scattering associated with the inherent small conduction-band spin splitting. Sample characterization. The steady-state A-exciton PL in monolayer and bilayer WSe2 were first measured by a micro-PL system (Figure 1c). Besides the direct A-exciton transition in the monolayer (E A =1.653 eV) and bilayer (E A = 1.636 eV), the additional PL peak at 1.56 eV of the bilayer was associated with the indirect transition according to the previous works. 32 Energy-resolved transient differential reflectivity of the B-exciton transition were then performed by degenerate pump-probe measurement, as shown by points in Figure 1d at delay t D = 10 ps. The measured B-exciton resonance transition (E B ) is about 410 meV (430 meV) higher than the A exciton for monolayer (bilayer) WSe2 , which is consistent with previous reports. 10,39 The bulk crystals would exhibit an energy redshift for both excitons with respect to the bilayer. A- and B-exciton dynamics in monolayer WSe2 . We first focus on the spin polarization dynamics of A excitons in the monolayer. The spin/valley polarization is defined as P = (N A(K) –N A(K 0 ) )/(N A(K) +N A(K 0 ) ), in which N A(K) and N A(K 0 ) are the A-exciton population in the K and K’ valley, respectively. Under photon-excitation in the K valley by right circularly-polarized (σ + ) pump pulse, one can evaluate the exciton population dynamics in the pumped (same circular polarization, SCP) and unpumped valley (opposite circular polarization, OCP), by analyzing the right (σ + ) or left (σ − ) circularly-polarized probes reflection from the sample. Thus the helicity-resolved transient reflection spectra (∆R/R) is approximately proportional to the corresponding polarized-exciton population density N A(K) (N A(K 0 ) ) in the K (K’ ) valley, and the difference of the two provides the spin/valley polarization degree p(t) at different delay times t D . 21,40,41 In Figure 2a, the photon energy of probe pulse is tuned to be resonant with E A and the

5



Page 6 of 27

pump photon energy is set 200 meV higher than E A (210 meV lower than EB ). The exciton generation time is deduced to be less than 0.4 ps, limited by the instrumental temporal resolution. It is seen that the OCP signal (red curve) occurs immediately after near-resonant excitation of A excitons at K valley, as a consequence of the well-known ultrafast intervalley e-h spin-flip exchange coupling (process (I), A(K ) → A(K’ )), which has been well studied as an important valley depolarization channel for the excited A excitons being scattered into the unpumped K’ valley. 18,19 By simply calculating (∆R/R + ∆R/R

OCP )

SCP –∆R/R OCP )

/ (∆R/R

SCP

, the spin/valley polarization degree at delay t D = 0.4 ps is 17.1% and the

deduced spin/valley relaxation time is 1.92 ps. 20 Interestingly, when the pump energy is tuned to E B (pump B, probe A), no measurable spin/valley polarization is observed at all delay time in Figure 2b and 2d, and this is further verified by the steady-state PL measurement under the same conditions (the inset of Figure 2b). Although the valley lifetime of A excitons depends on the excitation laser excess energy, 17,18 the different results presented in Figure 2b from that of Figure 2a mainly come from the additional formation of B(K ) excitons and the ultrafast coherent intervalley coupling of A-B excitons. When pumping at E B , both A- and B-excitons (N A(K) and N B(K) ) in the K valley are effectively excited under the photo-generation of electron-hole pairs. The absent valley polarization of A excitons is attributed to the ultrafast coherent intervalley transfer of B excitons in the K valley to A excitons in the K’ valley (Process (II), B(K ) → A(K’ ) ), i.e., the so-called Dexter-like coupling as revealed in Ref. 22, leading to the apparent A(K’ ) exciton population in the unpumped valley within the duration of pump pulse. This process does not require a center-of-mass momentum or a spin-flip. Supposing the same absorption oscillation strength for A(K ) and B(K ) excitons, 39,42 the measured B(K ) → A(K’ ) exciton intervalley coherent transfer N A(K 0 ) /N B(K) is approximately 17%, which is consistent with the theoretical value of 10% - 20%. 22 The B-exciton valley dynamics is then measured with the pump energy tuned to E A (pump A, probe B), as shown in Figure 2c. Under the A-exciton resonance excitation, in

6


Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


principle only A excitons in ground state at K valley can be effectively formed due to the large A-B energy splitting. However, the observed ultrafast build-up of both the SCP and OCP response in Figure 2c indicates the generation of polarized B excitons in both valleys. Moreover, the transient build-up of the OCP signal increases faster than that of the SCP signal within the pump pulse duration as emphasized in the inset of Figure 2c. This causes a negative spin/valley polarization of B excitons by calculating the imbalance (∆R/R - ∆R/R

OCP )

SCP

) of the exciton population between K and K’ valley, as seen by the olive

curve in Figure 2d. The same inverted valley polarization was also observed previously in monolayer WS2 . 22 These observations further supported the effective Dexter-like intervalley coupling in monolayer WSe2 (Process (II), A(K ) → B(K’ )). The slower build-up process of B excitons at the slightly longer delay t D = 0.5 - 1 ps in Figure 2c, compared to that of A excitons in Figure 2a, suggests that the initial population dynamics is also influenced by the incoherent intervalley scattering after pulsed excitation. It was commonly believed that the intra/inter-valley spin-flip scattering processes of A excitons with a zero center-ofmass momentum was relatively slow (more than tens of ps) in previous studies. 24,43,44 However, the recent studies revealed that the intra/intervalley electron transition from the upper to lower conduction-band state was energetically favorable for WSe2 . 20,34 As the existing finite electron distribution around K and K’ valleys caused by some factors such as the finite bandwidth of pumping laser pulse and carrier redistribution, can promote the intra/inter-valley scattering processes via electron-phonon interaction. The finite population build-up of B(K ) excitons could only arise from the sub-ps intravalley scattering (Process (III), A(K ) → B(K )). 25 A sign change of spin polarization for B excitons, shown in Figure 2d, was then derived from the overall difference of intra- and inter-valley electron relaxation rate, although both processes occur on a sub-ps time scale by our observations. A- and B- exciton dynamics in bilayer WSe2 . Bilayer WSe2 provides an additional layer degree of freedom and circularly-polarized light couples to the same spin-polarized bands at

7



different valleys for the upper and lower layers. The spin local effect on each layer is very strong: for example, under the σ + (σ − ) light resonant excitation, the calculated spin ↑( ↓) polarization of electrons in both the α and β layers is about 0.95. 34 Similar to the experimental measurements of the monolayer, Figure 3 presents the spin accumulation and relaxation dynamics of A- and B-excitons in bilayer WSe2 for comparison. In Figure 3a, the SCP signal represents the population dynamics of spin-polarized A excitons (A (K, α)) at K valley of the α layer and (A (K’, β)) at K’ valley of the β layer. The spin polarization degree of A excitons at delay t D = 0.4 ps is evaluated to be 13.2%. The dynamical polarization evolution exhibits a bi-exponential decay. The fast and slow decay time constants are deduced to be 1.74 ps (spectra weight of 90.6 %) and 3.1 ps (spectra weight of 9.4 %), respectively. Besides the fast e-h exchange coupling and electron-phonon scattering relaxation channel that also occur in the monolayer as discussed above, the relatively slow relaxation could be ascribed to the interlayer transfer of electrons. When decreasing temperature down to 10 K, our previous work showed that the spectra weight of slow decay time constant was still below 18.5 % for bilayer WSe2 , 36 while the fast decay channel always dominated (spectra weight > 80 %) the depolarization of A excitons in both the mono- and bi-layer in the temperature range of 10 - 300 K. This implies the relatively small contribution of interlayer transfer to the depolarization of A excitons at room temperature. In this sense, the bilayer can be regarded as weakly-interacted individual monolayer. 37 With the pump photon energy tuned to E B , the spin polarization of A excitons also drastically decreased, shown in Figure 3b, and an ultrafast inversion of the A-exciton polarization was observed within the pump pulse with a measured negative spin polarization degree of 3.4%, as more clearly indicated by the orange curve in Figure 3d. Note that this feature is quite similar to that observed in Figure 2c. As has been discussed above, the observed ultrafast small inverted polarization of A-exciton is a consequence of different ultrafast build-up for SCP and OCP response, which results from the complex competition between Dexter-like intervalley coupling and intra/inter-valley spin scattering process in bilayer, but with dif-

8


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ferent Dexter-like coupling strength and scattering rate comparing to monolayer. However, the time-integrated overall polarization still tends to be 0. Surprisingly, when pumping the A-exciton transition and probing the B-exciton transition, the OCP signal was observed to be stronger than the SCP response in Figure 3c. The energy- and helicity-resolved ∆R/R spectra taken at delay t D = 0 ps was further checked, as shown in the inset of Figure 3c. The spectra feature was quite similar to the helicity-resolved ∆R/R spectra of A excitons taken for the monolayer (shown in the inset of Figure 2a) and the bilayer (data not shown), but the spin polarization of the unpumped B excitons was efficiently inverted with respect to the pumped A excitons, as clearly seen as the olive curve in Figure 3d. This result reveals that the efficient Dexter-like intervalley coupling between A- and B-excitons also exists in the bilayer. Owing to the increased dielectric screening of bilayer compared to the monolayer, the intervalley A-B coupling would be expected to be weaker, 45 and the inverted spin polarization degree of B excitons could be slightly reduced. However, the OCP response of B excitons is observed to be much stronger upon pumping in the bilayer comparing with the monolayer, as presented in Figure 2c and Figure 3c. And the negative spin polarization degree for B excitons at delay t D = 0.4 ps is increased from 4%(monolayer) to 10% (bilayer). These different results suggest that there exists additional spin transfer contribution in the bilayer. As reported previously, the main interlayer relaxation for bilayer is spin-conserved intravalley transfer (A (K, α) → B (K, β), A (K’, β) → B (K’,α)), 30 as depicted in Figure 4b. To assess the impact of interlayer transfer of A excitons in which the transfer ratio and rate is supposed to be same as the case under the near-resonant A-exciton excitation, we measured the transient ∆R/R of A and B excitons as a function of excitation power by pumping the A exciton transition, as shown in Figure 4a. The population ratio of polarized A and B excitons N A /N B approximately equals to 5. By integrating the interlayer transfer efficiency within 0.5 ps, the calculated OCP signal of B excitons increases by ∼ 0.07 N B , giving rise to a spin polarization degree value of about 3.5% in the bilayer. Therefore, the interlayer

9



transfer of A excitons has an important impact on the generation of polarized B excitons (up-conversion) in the bilayer. We would like to address that the result discussion above for bilayer WSe2 is reasonable and self-consistent. Firstly, if we only consider the coherent transfer and intralayer scattering of A and B excitons within the single layer upon pumping, the N A /N B ratio in each layer is about 6-7. The measured smaller N A /N B value of about 5 in the bilayer was attributed to the additional incoherent interlayer transfer that can supplement the increase of B-excitons population. Secondly, in the low excitation power regime (< 50 uJ/cm2 ) in our two-color pump-probe measurements (Figure 4a), the pump fluence dependence of Bexcitons population is consistent with the reported observation of B excitons in monolayer WS2 . 22 By increasing pump fluence, the value of the negative spin polarization degree increases as shown in Figure 4c, which is also comparable to the reported negative PL circular polarization degree (14%) of the up-converted B excitons in monolayer WSe2 . 46 At higher excitation pump fluence, both the A and B exciton population is gradually saturated with increasing exciton densities, due to the enhanced exciton-exciton annihilation and bandgap renormalization. 47,48 Moreover, the enhanced screening and thus weaker Dexter-like coupling should give rise to the decreased spin polarization of B excitons. 45 Thirdly, the observed upconversion of B excitons under the low-power continuous-wave laser excitation, as reported in Ref. 46, was suggested to attribute to the two-photon absorption and involvement of continuum states. To check this possible contribution, we then resonantly pumped the A exciton transition and probed the interband free electron transition at 2.32 eV, 38,49 the dynamical response of free carriers was observed to exhibit a longer rising time and a lower amplitude compared to that of B excitons in Figure 4d and 4a, respectively. So free carriers are not likely responsible for the ultrafast formation of the negative polarization of B excitons in our experiments. A- and B- exciton dynamics in bulk WSe2 . Bulk crystals provide a cleaner system than their monolayer or few-layer counterparts, since the spin depolarization caused by defects and

10


Page 10 of 27

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


roughness arising from the surface and the substrates interface can be suppressed to some extent. To study spin accumulation and relaxation dynamics in bulk WSe2 , we repeated the measurement with a thick nanoflake. As the number of layer increases, it is known that the Coulomb screening of excitons is increased and the spin local effect is decreased. 34,38 But the measured spin polarization degree of A excitons at delay t D = 0.4 ps still remains to be 5.5% (Figure 5a), demonstrating the substantial maintenance of spin polarization even in inversion-symmetric TMDC bulk crystals at the near-resonant excitation of A excitons. The deduced two decay constants of spin depolarization, being 1.82 ps (spectra weight of 88 %) and 8.4 ps (spectra weight of 12 %), were in good agreement with the recently reported values measured by TRKR study (ref. 37). Particularly, when the pump photon energies were all set 0.20 eV above the A-exciton transition (E A + 0.20 eV), the A-exciton build-up processes were finished within 0.4 ps (the instrumental temporal resolution) for all studied flakes, as seen in Figure 2a - 5a, suggesting the common characteristics of intrinsic ultrafast formation time of A excitons for both monolayer and layered flakes. As the pump energy was tuned to the B-exciton transition (Figure 5b), there was no spin polarization observed either, same as that of the monolayer case. However, there existed a difference for the build-up dynamics of A excitons when comparing mono-, bi-layer and bulk WSe2 . The build-up time of A excitons became longer from monolayer to bulk WSe2 as illustrated in the inset of Figure 5b. Different from the strong Dexter-like coupling in the monolayer, the direct A-B exciton coupling is expected to be ineffective for bulk crystals due to the drastically reduced Coulomb interaction. The observed longer build-up time of A excitons than the Dexter-like intervalley coupling response (≤ 0.4 ps) in bulk is attributed to the dominated electron-phonon scattering at room temperatures. 37,38 By fitting the build-up dynamics in Figure 5b with the function 1 – exp(t/τ rise ) convoluted with an instrumental response function, the estimated rising time constant rise τ rise is 0.28 ps, which approximately corresponds to the electron scattering time from the lower to the upper conduction-band state in bulk. The build-up process of A excitons in the bilayer shows a trend in the midway in

11



between the monolayer and bulk crystals. The higher amplitude of the OCP signal than that of the SCP signal for B excitons as shown in Figure 5c and Figure 5d, implying that the spin-conserved scattering rate is larger than the spin-flip scattering rate in bulk. This phenomenon also applies for the bilayer. Furthermore, when comparing the build-up dynamics shown in Figure 5c (pump A, probe B) with that of Figure 5b (pump B, probe A) in bulk, one can see that the build-up time of the former is shorter than that of the latter, as a consequence of the electron-phonon scattering mediated processes due to the negative conduction-band SOC splitting for WSe2 . The same spin relaxation time of A excitons in all flakes. After discussing the build-up dynamics of A and B excitons, the subsequent spin depolarization process for A excitons was observed to exhibit a similar lifetime of 1.8 ± 0.2 ps (with spectra weight of at least 85%) in mono-, bi-layer and bulk WSe2 (Figure 2a - 5a), indicating that a common key relaxation channel should be responsible for all layers. Exciton recombination process can be excluded since the exciton recombination time ( PL yield) is longer (lower) in the muiltilayer than monolayer. 7 The e-h intervalley exchange interaction, as a proposed dominant spin depolarization channel in monolayer WSe2 , is suppressed to some extent in bulk crystals. Exciton radiative recombination dephasing, interband relaxation of electrons or scattering between bright exciton to dark exciton or to other indirect valley could possibly provide additional relaxation channels, 14,27,50,51 but these relaxation processes usually take less than 200 fs, which are too fast to contribute to the measured transient ∆R/R signals. Though the exciton-exciton annihilation and carrier screening effect, under our experimental condition (the excited exciton density is about 1011 -1012 /cm2 ), could possibly also mediate the brightdark exciton dynamics and spin dynamics, 52–54 it does not affect our discussion on the common character of spin relaxation observed for different layers. Considering the same SOC splitting characteristics of conduction- and valence-band in monolayer and multilayer WSe2 , the photon-excited electrons and holes would experience very different depolarization dynamics, as already discussed in Ref. 20. Due to the smaller

12


Page 12 of 27

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SOC splitting of the conduction-band near K and K’ valleys, the scattering of photoexcited electrons is enhanced in all layered WSe2 . 20,55,56 On the contrary, the large valence-band SOC splitting can efficiently suppress the scattering processes. This common band characteristics for layered WSe2 suggests the important role of phonon-assisted scattering on the observed same spin depolarization time of A excitons, for which the inter/intra-valley electron-phonon scattering is manifested by the relatively small conduction band SOC splitting, and plays decisive role in the common spin depolarization relaxation process of A excitons at room temperature. Our experimental results indicate the important role played by various coherent and incoherent inter/intra-valley relaxation on the spin generation and relaxation in monolayer and muilti-layer systems. Though there exists additional spin depolarization channel of interlayer transfer for multi-layer flakes, the apparent spin accumulation and same spin depolarization lifetime is observed in bulk crystals. This indicates the feasibility of using layered TMDC materials for spintronic devices, in which the multi-layers are in favor of avoiding interfacial defect scattering and resisting against device performance degradation caused by processing. Therefore, the efficient spin accumulation and manipulation in layered TMDC requires further exploration in future, to improve the robustness of spin polarization and enhance the spin polarization degree by, for example, the gate-controlled spin-valley locking of resident carriers. 57 In summary, we have studied the spin/valley generation and relaxation dynamics in different layer WSe2 by helicity-resolved reflection spectroscopy. The spin polarization degree of A excitons decreases monotonically as the number of layer increases but is still maintained in bulk crystals under the near-resonant excitation. With pumping B excitons and probing the A-exciton transition, we observed a nearly zero spin polarization due to the ultrafast spin transfer on a sub-ps time scale, in which the Dexter-like intervalley coupling was very efficient in the mono- and bi-layer but electron-phonon scattering gradually dominated in the multilayers. With pumping A excitons and probing the B-exciton transition, we observed

13



an inverted spin polarization induced by different factors including the Dexter-like coupling, spin-conserved electron scattering as well as interlayer transfer. In the subsequent spin depolarization processes, the same spin relaxation time (1.8 ± 0.2 ps) of A excitons in different layers was determined, which was ascribed to the active electron-phonon scattering as a result of intrinsic small conduction-band SOC splitting. These results are important for understanding the ultrafast spin accumulation and relaxation process of layered TMDC materials for their spintronics applications. Samples preparations. All WSe2 samples with different thicknesses were mechanically exfoliated from commercial bulk WSe2 crystals (2D Semiconductor Inc.) on 285 nm-SiO2 /Si substrates. We used PL and differential reflection spectroscopy to identify the thickness of WSe2 layers. Two-color pump-probe spectroscopy. In our two-color pump-probe setup, two tunable laser beams were used: a mode-locked femtosecond Ti:sapphire laser (Coherent Chameleon Ultra II), with a temporal pulse width of ∼ 150 fs and an average power of 4 W, was employed. The fundamental output of Chameleon Ultra II laser was split into two beams, one beam was used directly for the pump/probe beam (750 nm - 850 nm), and the other one (about 3.2 W) was used to pump an optical parametric oscillator (OPO) that generates a tunable infrared output beam (1000 - 1600 nm). The second harmonic generation of the infrared output of OPO was selected either as pump or probe beam at specific wavelength, for example, 595 nm, 675 nm. The two beams were circularly polarized by combining a polarizer and quarter waveplate, and then were combined collinearly and focused onto the sample with a beam diameter of about 1.5 m. The reflected pump beam was filtered out by a 715 nm long-pass or 680 nm short-pass filter before the reflected probe beam being directed onto the silicon photo-detector. The photon-injected exciton density in monolayer (bulk) samples is lower than 1012 /cm2 (1013 /cm2 ), respectively. All helicity-resolved transient ∆R/R in this works were done at room temperature.

14


Page 14 of 27

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author * Email: [email protected]

ORCID Xinhui Zhang: 0000-0003-0059-6599

Notes The authors declare no competing financial interest.

Acknowledgement This work is supported by the National Natural Science Foundation of China (Nos. 11474276, and 11774337).

15



Figure Captions

Figure 1. (a) Schematic representation of the optical selection rule under the circularlypolarized light excitation at the K /K’ valleys in monolayer WSe2 . The values of the valence- and conduction-band spin splitting are about 400 meV and -37 meV, respectively. (b) Schematic drawing of the main depolarization processes for A excitons under the near-resonant excitation, including: (I) intervalley spin-flip relaxation; (II) intervalley spin-conserved relaxation; (III) intravalley spin-flip relaxation; (IV) exciton recombination and scattering into dark-states. (c) PL spectra in the monolayer and bilayer. (d) Degenerate pump-probe spectrum (points) covering the B-exciton transition at delay t D = 10 ps. The curve serves as a guide to eyes.

16


Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Helicity-resolved pump-probe measurements of excitons for monolayer WSe2 . The same-(black) and opposite-circularly polarized (red) (SCP or OCP) ∆R/R was recorded by σ + - or σ − - polarized probes under σ + -polarized pumping. (a) The pump energy was set at 1.85 eV, 0.2eV higher than that of A-exciton resonance; and the probe energy was set to be resonant with A exciton (E A = 1.65 eV). The inset shows the corresponding ∆R/R spectra as a function of the probe energy at t D = 0 ps with the fixed pump energy of 1.85 eV. (b) The pump energy was set to be resonant with B exciton (E B = 2.06 eV), and the probe was set to be resonant with A exciton (1.65 eV). The inset displays the polarization-resolved PL spectrum of A excitons under excitation at E B . (c) The pump energy was resonant with A exciton (1.65 eV), and the probe energy was resonant with B exciton (2.06 eV). The inset shows the zoom-in ∆R/R within the initial 1 ps. The grey area exhibits the cross correlation between pump and probe pulses. (d) The difference between the SCP and OCP signals for the measured data of (a), (b) and (c).

17



Figure 3. (a) Helicity-resolved pump-probe measurements of excitons for bilayer WSe2 . The same-(black) and opposite-circularly polarized (red) (SCP or OCP) ∆R/R was recorded by σ + - or σ − - polarized probes under σ + -polarized pumping. (a) The pump energy was set at 1.85 eV and probe energy was 1.64 eV (E A ). (b) The pump energy was set at 2.07 eV (E B ) and the probe energy was 1.64 eV (E A ). (c) The pump energy was set at 1.64 eV (EA) and the probe energy was 2.07 eV (E B ). The corresponding ∆R/R spectra as a function of the probe energy at delay t D = 0 ps with the fixed pump energy of 1.64 eV, was shown in the inset. (d) The deduced difference between the SCP and OCP signals for measured data of (a), (b) and (c).

18


Page 18 of 27

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Pump fluence dependence of the ∆R/R signals for A excitons, B excitons and free carriers at delay t D = 0.5 ps, under the A-exciton resonant excitation in the bilayer. (b) Schematic drawing of the main up-conversion processes for B excitons with the same spin of A excitons when pumping the A-exciton transition. Compared to the relaxation channel in the monolayer, there is an additional contribution from the A-exciton interlayer transfer (brown dashed curve), the green dashed curve marks the intervalley spin-conserved relaxation channel as that in the monolayer. (c) Pump fluence dependence of the calculated spin polarization of B excitons in (a). (d) Normalized ∆R/R response of B excitons and free carriers, under the A-exciton resonant excitation in the bilayer.

19



Figure 5. (a) Helicity-resolved pump-probe measurements of excitons for bulk WSe2 . The same-(black) and opposite-circularly polarized (red) (SCP or OCP) ∆R/R was recorded by σ + - or σ − - polarized probes under σ + -polarized pumping. The dynamic spin polarization degree was also calculated (gray) and fitted (blue). (a) The pump energy was set at 1.78 eV and the probe energy was 1.60 eV (E A ). (b) The pump energy was set at 2.03 eV (E B ) and the probe energy was 1.60 eV (E A ). The inset shows the detail comparison of the build-up process of A excitons within the initial 1 ps in monolayer (1L), bilayer (2L) and bulk WSe2 when pumping the B-exciton transition. (c) The pump energy was set at 1.60 eV (E A ) and the probe energy was 2.03 eV (E B ). (d) The pump energy was set at 1.66 eV (E A ) and the probe energy was 2.03 eV (E B ).

20


Page 20 of 27

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically thin MoS2 : a new direct-gap semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (2) Zhu, Z.; Cheng, Y.; Schwingenschlögl, U. Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors. Phys. Rev. B 2011, 84, 153402. (3) Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802. (4) Wang, G.; Robert, C.; Suslu, A.; Chen, B.; Yang, S.; Alamdari, S.; C. Gerber, I.; Amand, T.; Xavier, M.; Tongay, S. et al. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers. Nat. Commun. 2015, 6, 10110. (5) Gerber, I.; Echeverry, J. P.; Urbaszek, B.; Amand, T.; Robert, C.; Marie, X. Splitting between Bright and Dark Excitons in Transition Metal Dichalcogenide Monolayers. Phys. Rev. B 2016, 93, 121107. (6) Jones, A. M.; Yu, H.; Ghimire, N. J.; Wu, S.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J.; Mandrus, D. G.; Xiao, D. et al. Optical generation of excitonic valley coherence in monolayer WSe2 . Nat. Nanotech. 2013, 8, 634. (7) Cui, Q.; Ceballos, F.; Kumar, N.; Zhao, H. Transient absorption microscopy of monolayer and bulk WSe2 . ACS nano 2014, 8, 2970. (8) Wang, G.; Bouet, L.; Lagarde, D.; Vidal, M.; Balocchi, A.; Amand, T.; Marie, X.; Urbaszek, B. Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2 . Phys. Rev. B 2014, 90, 075413.

21



(9) Zhu, C.; Zhang, K.; Glazov, M.; Urbaszek, B.; Amand, T.; Ji, Z.; Liu, B.; Marie, X. Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers. Phys. Rev. B 2014, 90, 161302. (10) Zeng, H.; Liu, G.-B.; Dai, J.; Yan, Y.; Zhu, B.; He, R.; Xie, L.; Xu, S.; Chen, X.; Yao, W. et al. Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides. Sci. Rep. 2013, 3, 1608. (11) Yan, T.; Qiao, X.; Liu, X.; Tan, P.; Zhang, X. Photoluminescence properties and exciton dynamics in monolayer WSe2 . Appl. Phys. Lett. 2014, 105, 101901. (12) Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B. et al. Exciton radiative lifetime in transition metal dichalcogenide monolayers. Phys. Rev. B 2016, 93, 205423. (13) Godde, T.; Schmidt, D.; Schmutzler, J.; Amann, M.; Debus, J.; Withers, F.; Alexeev, E. M.; Del Pozo-Zamudio, O.; Skrypka, O. V.; Novoselov, K. S. et al. Exciton and trion dynamics in atomically thin MoSe2 and WSe2 : effect of localization. Phys. Rev. B 2016, 94, 165301. (14) Selig, M.; Berghäuser, G.; Raja, A.; Nagler, P.; Sch¨ uller, C.; Heinz, T. F.; Korn, T.; Chernikov, A.; Malic, E.; Knorr, A. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides. Nat. Commun. 2016, 7, 13279. (15) Plechinger, G.; Korn, T.; Lupton, J. M. Valley-Polarized Exciton Dynamics in Exfoliated Monolayer WSe2 . J. Phys. Chem. C 2017, 121, 6409. (16) Yan, T.; Ye, J.; Qiao, X.; Tan, P.; Zhang, X. Exciton valley dynamics in monolayer WSe2 probed by the two-color ultrafast Kerr rotation. Phys. Chem. Chem. Phys. 2017, 19, 3176.

22


Page 22 of 27

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Ye, J.; Niu, B.; Li, Y.; Li, T.; Zhang, X. Exciton valley dynamics in monolayer Mo1−x 2Wx Se2 (x= 0, 0.5, 1). Appl. Phys. Lett. 2017, 111, 152106. (18) Yu, T.; Wu, M. Valley depolarization due to intervalley and intravalley electron-hole exchange interactions in monolayer MoS2 . Phys. Rev. B 2014, 89, 205303. (19) Glazov, M.; Ivchenko, E.; Wang, G.; Amand, T.; Marie, X.; Urbaszek, B.; Liu, B. Spin and valley dynamics of excitons in transition metal dichalcogenide monolayers. Phys. Status Solidi B 2015, 252, 2349. (20) Molina-Sánchez, A.; Sangalli, D.; Wirtz, L.; Marini, A. Ab initio calculations of ultrashort carrier dynamics in two-dimensional materials: valley depolarization in singlelayer WSe2 . Nano Lett. 2017, 17, 4549. (21) Schmidt, R.; Berghauser, G.; Schneider, R.; Selig, M.; Tonndorf, P.; Malic, E.; Knorr, A.; Michaelis de Vasconcellos, S.; Bratschitsch, R. Ultrafast coulomb-induced intervalley coupling in atomically thin WS2 . Nano Lett. 2016, 16, 2945. (22) Berghäuser, G.; Bernal-Villamil, I.; Schmidt, R.; Schneider, R.; Niehues, I.; Erhart, P.; de Vasconcellos, S. M.; Bratschitsch, R.; Knorr, A.; Malic, E. Inverted valley polarization in optically excited transition metal dichalcogenides. Nat. Commun. 2018, 9, 971. (23) Wang, G.; Palleau, E.; Amand, T.; Tongay, S.; Marie, X.; Urbaszek, B. Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers. Appl. Phys. Lett. 2015, 106, 112101. (24) Slobodeniuk, A.; Basko, D. Spin–flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers. 2D Mater. 2016, 3, 035009. (25) Wang, Z.; Molina-Sánchez, A.; Altmann, P.; Sangalli, D.; De Fazio, D.; Soavi, G.;

23



Sassi, U.; Bottegoni, F.; Ciccacci, F.; Finazzi, M. et al. Intravalley Spin–Flip Relaxation Dynamics in Single-Layer WS2 . Nano Lett. 2018, 18, 6882. (26) Baranowski, M.; Surrente, A.; Maude, D.; Ballottin, M.; Mitioglu, A.; Christianen, P.; Kung, Y.; Dumcenco, D.; Kis, A.; Plochocka, P. Dark excitons and the elusive valley polarization in transition metal dichalcogenides. 2D Mater. 2017, 4, 025016. (27) Selig, M.; Berghäuser, G.; Richter, M.; Bratschitsch, R.; Knorr, A.; Malic, E. Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides. 2D Mater. 2018, 5, 035017. (28) He, J.; Hummer, K.; Franchini, C. Stacking effects on the electronic and optical properties of bilayer transition metal dichalcogenides MoS2 , MoSe2 , WS2 , and WSe2 . Phys. Rev. B 2014, 89, 075409. (29) Jones, A. M.; Yu, H.; Ross, J. S.; Klement, P.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Yao, W.; Xu, X. Spin–layer locking effects in optical orientation of exciton spin in bilayer WSe2 . Nat. Phys. 2014, 10, 130. (30) Zhu, B.; Zeng, H.; Dai, J.; Gong, Z.; Cui, X. Anomalously robust valley polarization and valley coherence in bilayer WS2 . Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 11606. (31) Nayak, P. K.; Lin, F.-C.; Yeh, C.-H.; Huang, J.-S.; Chiu, P.-W. Robust room temperature valley polarization in monolayer and bilayer WS2 . Nanoscale 2016, 8, 6035. (32) Wang, G.; Marie, X.; Bouet, L.; Vidal, M.; Balocchi, A.; Amand, T.; Lagarde, D.; Urbaszek, B. Exciton dynamics in WSe2 bilayers. Appl. Phys. Lett. 2014, 105, 182105. (33) Riley, J. M.; Mazzola, F.; Dendzik, M.; Michiardi, M.; Takayama, T.; Bawden, L.; Granerød, C.; Leandersson, M.; Balasubramanian, T.; Hoesch, M. et al. Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor. Nat. Phys. 2014, 10, 835. 24


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Liu, Q.; Zhang, X.; Zunger, A. Intrinsic circular polarization in centrosymmetric stacks of transition-metal dichalcogenides. Phys. Rev. Lett. 2015, 114, 087402. (35) Bertoni, R.; Nicholson, C. W.; Waldecker, L.; H¨ ubener, H.; Monney, C.; De Giovannini, U.; Puppin, M.; Hoesch, M.; Springate, E.; Chapman, R. T. et al. Generation and evolution of spin-, valley-, and layer-polarized excited carriers in inversion-symmetric WSe2 . Phys. Rev. Lett. 2016, 117, 277201. (36) Niu, B.; Ye, J.; Li, T.; Li, Y.; Zhang, X. Spin depolarization dynamics of WSe2 bilayer. Chin. Phys. B 2018, 27, 057202. (37) Guimarães, M.; Koopmans, B. Spin accumulation and dynamics in inversion-symmetric van der Waals crystals. Phys. Rev. Lett. 2018, 120, 266801. (38) Chernikov, A.; Berkelbach, T. C.; Hill, H. M.; Rigosi, A.; Li, Y.; Aslan, O. B.; Reichman, D. R.; Hybertsen, M. S.; Heinz, T. F. Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2 . Phys. Rev. Lett. 2014, 113, 076802. (39) Arora, A.; Koperski, M.; Nogajewski, K.; Marcus, J.; Faugeras, C.; Potemski, M. Excitonic resonances in thin films of WSe2 : from monolayer to bulk material. Nanoscale 2015, 7, 10421. (40) Wang, Q.; Ge, S.; Li, X.; Qiu, J.; Ji, Y.; Feng, J.; Sun, D. Valley carrier dynamics in monolayer molybdenum disulfide from helicity-resolved ultrafast pump–probe spectroscopy. ACS nano 2013, 7, 11087. (41) Kumar, N.; He, J.; He, D.; Wang, Y.; Zhao, H. Valley and spin dynamics in MoSe2 two-dimensional crystals. Nanoscale 2014, 6, 12690. (42) Mai, C.; Barrette, A.; Yu, Y.; Semenov, Y. G.; Kim, K. W.; Cao, L.; Gundogdu, K. Many-body effects in valleytronics: Direct measurement of valley lifetimes in singlelayer MoS2 . Nano Lett. 2013, 14, 202. 25



(43) Wang, L.; Wu, M. Electron spin relaxation due to D’yakonov-Perel’and Elliot-Yafet mechanisms in monolayer MoS2 : role of intravalley and intervalley processes. Phys. Rev. B 2014, 89, 115302. ¨ (44) Schmidt, H.; Yudhistira, I.; Chu, L.; Neto, A. C.; Ozyilmaz, B.; Adam, S.; Eda, G. Quantum transport and observation of Dyakonov-Perel spin-orbit scattering in monolayer MoS2 . Phys. Rev. Lett. 2016, 116, 046803. (45) Bernal-Villamil, I.; Berghäuser, G.; Selig, M.; Niehues, I.; Schmidt, R.; Schneider, R.; Tonndorf, P.; Erhart, P.; de Vasconcellos, S. M.; Bratschitsch, R. et al. Exciton broadening and band renormalization due to Dexter-like intervalley coupling. 2D Mater. 2018, 5, 025011. (46) Manca, M.; Glazov, M.; Robert, C.; Cadiz, F.; Taniguchi, T.; Watanabe, K.; Courtade, E.; Amand, T.; Renucci, P.; Marie, X. et al. Enabling valley selective exciton scattering in monolayer WSe2 through upconversion. Nat. Commun. 2017, 8, 14927. (47) Sun, D.; Rao, Y.; Reider, G. A.; Chen, G.; You, Y.; Brezin, L.; Harutyunyan, A. R.; Heinz, T. F. Observation of rapid exciton–exciton annihilation in monolayer molybdenum disulfide. Nano Lett. 2014, 14, 5625. (48) Steinhoff, A.; Rosner, M.; Jahnke, F.; Wehling, T.; Gies, C. Influence of excited carriers on the optical and electronic properties of MoS2 . Nano Lett. 2014, 14, 3743. (49) Yan, T.; Yang, S.; Li, D.; Cui, X. Long valley relaxation time of free carriers in monolayer WSe 2 . Phys. Rev. B 2017, 95, 241406. (50) Dery, H.; Song, Y. Polarization analysis of excitons in monolayer and bilayer transitionmetal dichalcogenides. Phys. Rev. B 2015, 92, 125431. (51) Jakubczyk, T.; Delmonte, V.; Koperski, M.; Nogajewski, K.; Faugeras, C.; Langbein, W.; Potemski, M.; Kasprzak, J. Radiatively limited dephasing and exciton dy26


Page 26 of 27

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


namics in MoSe2 monolayers revealed with four-wave mixing microscopy. Nano Lett. 2016, 16, 5333. (52) Kumar, N.; Cui, Q.; Ceballos, F.; He, D.; Wang, Y.; Zhao, H. Exciton-exciton annihilation in MoSe2 monolayers. Phys. Rev. B 2014, 89, 125427. (53) Yuan, L.; Wang, T.; Zhu, T.; Zhou, M.; Huang, L. Exciton dynamics, transport, and annihilation in atomically thin two-dimensional semiconductors. J. Phys. Chem. Lett. 2017, 8, 3371. (54) Miyauchi, Y.; Konabe, S.; Wang, F.; Zhang, W.; Hwang, A.; Hasegawa, Y.; Zhou, L.; Mouri, S.; Toh, M.; Eda, G. et al. Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors. Nat. Commun. 2018, 9, 2598. (55) Mai, C.; Semenov, Y. G.; Barrette, A.; Yu, Y.; Jin, Z.; Cao, L.; Kim, K. W.; Gundogdu, K. Exciton valley relaxation in a single layer of WS2 measured by ultrafast spectroscopy. Phys. Rev. B 2014, 90, 041414. (56) Hsu, W.-T.; Chen, Y.-L.; Chen, C.-H.; Liu, P.-S.; Hou, T.-H.; Li, L.-J.; Chang, W.-H. Optically initialized robust valley-polarized holes in monolayer WSe2 . Nat. Commun. 2015, 6, 8963. (57) Dey, P.; Yang, L.; Robert, C.; Wang, G.; Urbaszek, B.; Marie, X.; Crooker, S. Gatecontrolled spin-valley locking of resident carriers in WSe2 monolayers. Phys. Rev. Lett. 2017, 119, 137401.

27


Ultrafast Dynamics of Spin Generation and Relaxation in Layered WSe2

Recommend Documents