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Valley polarization in MoS2 monolayers by optical pumping


Most electronic devices exploit the electric charge of electrons, but it is also possible to build devices that rely on other properties of electrons. Spintronic devices, for example, make use of the spin of electrons1,2. Valleytronics is a more recent development that relies on the fact that the conduction bands of some materials have two or more minima at equal energies but at different positions in momentum space3,4,5. To make a valleytronic device it is necessary to control the number of electrons in these valleys, thereby producing a valley polarization6,7,8,9,10,11. Single-layer MoS2 is a promising material for valleytronics because both the conduction and valence band edges have two energy-degenerate valleys at the corners of the first Brillouin zone12. Here, we demonstrate that optical pumping with circularly polarized light can achieve a valley polarization of 30% in pristine monolayer MoS2. Our results, and similar results by Mak et al.13, demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS2 monolayers.

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Figure 1: Schematic of MoS2 monolayer and optical selection rules at K(K') point.
Figure 2: Polarization-sensitive photoluminescence spectra from MoS2 monolayers.
Figure 3: Photoluminescence under an in-plane magnetic field and from bilayers.
Figure 4: Degree of circular polarization P as a function of temperature.

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The authors thank Bairen Zhu, Lu Xie, Dongmei Deng, J.Q. Ning, C.C. Zheng and S.J. Xu for technical assistance. H.Z., J.D., X.C. and W.Y. were supported by the Research Grant Council (HKU10/CRF/08, HKU701810P, HKU706412P) and the University Grant Council (AoE/P-04/08 and SEG_CUHK06) of the government of HKSAR. D.X. was supported by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

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X.C. and W.Y. conceived the project. X.C., H.Z. and J.D. designed the experiments. H.Z. and J.D. performed the experiments. W.Y. and D.X. provided theoretical support. All authors discussed the results and co-wrote the paper.

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Correspondence to Xiaodong Cui.

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The authors declare no competing financial interests.

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Zeng, H., Dai, J., Yao, W. et al. Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech 7, 490–493 (2012).

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