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单层过渡金属硫化物由于其特有的激子效应以及强自旋-谷耦合性质，在光电子学及谷电子学等方面有着很广阔的应用前景.利用超快时间分辨光谱，本文系统地比较了两类钨基单层硫化物（WS2和WSe2）的A-激子动力学和谷自旋弛豫特性.实验结果表明，WS2单层膜的A-激子弛豫表现为双指数过程，而对于WSe2，其A-激子衰减表现为三指数过程，且激子的寿命远长于前者.WS2谷自旋极化弛豫表现为单指数衰减，其寿命约0.35 ps，主要由电子-空穴交换作用所主导.而对于WSe2，谷自旋弛豫表现出双指数弛豫特性：一个寿命为0.5 ps的快过程和一个寿命为28 ps的慢过程.快过程的弛豫来源于电子-空穴交换作用，而慢过程则由于自旋晶格散射形成暗激子的过程.通过调谐抽运光波长，进一步证实WSe2较WS2更容易形成暗激子.Two-dimensional transitional metal dichalcogenide (2D TMD) emerges as a good candidate material in optoelectronics and valleytronics due to its particular exciton effect and strong spin-valley locking. Owing to the enhancement of quantum confinement effect and the decline of dielectric shielding effect, the optical excitation of electron-hole pair is enhanced substantially, which makes large TMD exciton binding energy and makes excitons observed easily at room temperature or even higher temperature. Optical response of 2D TMD is dominated by excitons at room temperature, which provides an ideal medium for studying the generation, relaxation and interaction of excitons or trions. By employing ultrafast time resolved spectroscopy, we investigate experimentally the dynamic behaviors of A-exciton and spin relaxations for two types of TMDs, i.e. WS2 and WSe2 monolayers, respectively. By tuning the excitation wavelength of the degenerate pump and probe laser beam, the WS2 monolayer and WSe2 monolayer are excited at their A-exciton resonance transition position or near their A-exciton resonance transition position in order to compare the dynamical evolutions of band structure and exciton polarization of the two similar WS2 and WSe2 monolayer structures. Our experimental results reveal that the relaxation of A exciton in WS2 shows biexponential decay, while that of WSe2 shows triexponential decay, and the A-exciton life time in WSe2 is much longer than that of WS2 counterpart. The spin relaxation of A exciton in WS2 shows a monoexponential feature with a lifetime of 0.35 ps, which is dominated by the electron-hole exchange interaction. For the case of WSe2, the spin relaxation can be well fitted with biexponential function, the fast component has a lifetime of 0.5 ps and the slow one has a lifetime of 28 ps. The fast relaxation is dominated by the electron-hole exchange interaction, and the slow one comes from the formation of dark exciton via spin-lattice coupling. By tuning the excitation wavelength around A-exciton transition, the formation of dark exciton in WSe2 is demonstrated to be much more effective than that in WS2 monolayer. Our experimental results provide qualitative physical images for an in-depth understanding of the relationship between exciton and TMD structure, and also provide reference for further designing and regulating the TMDs based optoelectronic devices.
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Keywords:
- transitional metal dichalcogenides /
- exciton /
- valley polarization
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[29] Zhao W, Ghorannevis Z, Chu L, Toh M, Kloc C, Tan P H, Eda G 2013 ACS Nano 1 791
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[33] Plechinger G, Nagler P, Arora A, Schmidt R, Chernikov A, Lupton J, Bratschitsch R, Schüller C, Korn T 2017 Solar RRL 11 1700131
[34] Maialle M Z, de Andrada e Silva E A, Sham L J 1993 Phys. Rev. B 47 15776
[35] Vinattieri A, Jagdeep S, Damen T C, Kim D S, Pfeier L N, Maialle M Z, Sham L J 1994 Phys. Rev. B 50 10868
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[1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V, Firsov A A 2004 Science 306 666
[2] Chen S, Shi G 2017 Adv. Mater. 29 1605448
[3] Tan C L, Cao X H, Wu X J, He Q Y, Yang J, Zhang X, Chen J Z, Zhao W, Han S K, Nam G H, Sindoro M, Zhang H 2017 Chem. Rev. 117 6225
[4] Zeng H L, Dai J F, Yao W, Xiao D, Cui X D 2012 Nat. Nanotech. 7 490
[5] Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A 2011 Nat. Nanotech. 6 147
[6] Bertolazzi S, Brivio J, Kis A 2011 ACS Nano 5 9703
[7] Kang K, Xie S, Huang L J, Han Y, Huang P Y, Mak K F, Kim C J, Muller D, Park J 2015 Nature 520 656
[8] Lu J M, Zheliuk O, Leermakers I, Yuan N F Q, Zeitler U, Law K T, Ye J T 2015 Science 350 1353
[9] Yin X B, Ye Z L, Chenet D A, Ye Y, O'Brien K, Hone J C, Zhang X 2014 Science 344 488
[10] Mak K F, He K L, Shan J, Heinz T F 2012 Nat. Nanotech. 7 494
[11] Yan R H, Ourmazed A, Lee K F 1992 IEEE Trans. Electron Dev. 39 1704
[12] Schwierz F 2010 Nat. Nanotechnol. 5 487
[13] Ross J S, Wu S F, Yu H Y, Ghimire N J, Jones A M, Aivazian G, Yan J, Mandrus D G, Di X, Yao W, Xu X D 2013 Nat. Com. 4 1474
[14] Stébé B, Ainane A 1989 Superlattices Microstruct. 5 545
[15] Ramasubramaniam A 2012 Phys. Rev. B 86 115409
[16] Mak K F, Lee C, Hone J, Shan J, Heinz Tony F 2010 Phys. Rev. Lett. 105 136805
[17] Splendiani A, Sun L, Zhang Y B, Li T S, Kim J, Chim C Y, Galli G, Wang F 2010 Nano Lett. 10 1271
[18] Butler S Z, Hollen S M, Cao L, Cui Y, Gupta J A, Gutiérrez H R, Heinz T F, Hong S S, Huang J X, Ismach A F, Johnston-Halperin E, Kuno M, Plashnitsa V V, Robinson R D, Ruoff R S, Salahuddin S, Shan J, Shi L, Spencer M G, Terrones M, Windl W, Goldberger J E 2013 ACS Nano 7 2898
[19] Yao W, Xiao D, Niu Q 2008 Phys. Rev. B 77 235406
[20] Xiao D, Liu G B, Feng W X, Xu X D, Yao W 2012 Phys. Rev. Lett. 108 196802
[21] Cao T, Wang G, Han W P, Ye H Q, Zhu C R, Shi J R, Niu Q, Tan P H, Wang E, Liu B L, Feng J 2012 Nat. Com. 3 887
[22] Yan T F, Qiao X F, Tan P H, Zhang X H 2015 Sci. Rep. 5 15625
[23] Zhu C R, Zhang K, Glazov M, Urbaszek B, Amand T, Ji Z W, Liu B L, Marie X 2014 Phys. Rev. B 90 161302
[24] Yang L Y, Sinstsyn N A, Chen W B, Yuan J T, Zhang J, Lou J, Crooker S A 2015 Nat. Phys. 11 830
[25] Wang Q S, Ge S F, Xiao L, Qiu J, Ji Y X, Feng J, Sun D 2013 ACS Nano 12 11087
[26] Kioseoglou G, Hanbicki A T, Currie M, Friedman A L, Gunlycke D, Jonker B T 2012 Appl. Phys. Lett. 101 221907
[27] Yan P G, Chen H, Yin J D, Xu Z H, Li J R, Jiang Z K, Zhang W F, Wang J Z, Li I L, Sun Z P, Ruan S 2017 Nanoscale 9 1871
[28] Li Y L, Chernikov A, Zhang X, Rigosi A, Hill H M, van der Zande A M, Chenet D A, Shih E M, Hone J, Heinz T F 2014 Phys. Rev. B 90 205422
[29] Zhao W, Ghorannevis Z, Chu L, Toh M, Kloc C, Tan P H, Eda G 2013 ACS Nano 1 791
[30] Sahin H, Tongay S, Horzum S, Fan W, Zhou J, Li J, Wu J, Peeters F M 2013 Phys. Rev. B 87 165409
[31] Shi H Y, Yan R, Bertolazzi S, Brivio J, Gao B, Kis A, Jena D, Xing H G, Huang L B 2013 ACS Nano 7 1072
[32] Korn T, Heydrich S, Hirmer M, Schmutzler J, Schüller C 2011 Appl. Phys. Lett. 99 102109
[33] Plechinger G, Nagler P, Arora A, Schmidt R, Chernikov A, Lupton J, Bratschitsch R, Schüller C, Korn T 2017 Solar RRL 11 1700131
[34] Maialle M Z, de Andrada e Silva E A, Sham L J 1993 Phys. Rev. B 47 15776
[35] Vinattieri A, Jagdeep S, Damen T C, Kim D S, Pfeier L N, Maialle M Z, Sham L J 1994 Phys. Rev. B 50 10868
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