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Valley polarization in MoS2 monolayers byoptical pumpingHualing Zeng1, Junfeng Dai2,1, Wang Yao1,3, Di Xiao4 and Xiaodong Cui1*
Most electronic devices exploit the electric charge of electrons,but it is also possible to build devices that rely on other prop-erties of electrons. Spintronic devices, for example, make useof the spin of electrons1,2. Valleytronics is a more recent devel-opment that relies on the fact that the conduction bands ofsome materials have two or more minima at equal energiesbut at different positions in momentum space35. To make avalleytronic device it is necessary to control the number ofelectrons in these valleys, thereby producing a valley polariz-ation611. Single-layer MoS2 is a promising material for valley-tronics because both the conduction and valence band edgeshave two energy-degenerate valleys at the corners of the firstBrillouin zone12. Here, we demonstrate that optical pumpingwith circularly polarized light can achieve a valley polarizationof 30% in pristine monolayer MoS2. Our results, and similarresults by Mak et al.13, demonstrate the viability of opticalvalley control and valley-based electronic and optoelectronicapplications in MoS2 monolayers.
In many crystalline materials, it often happens that the conduc-tion-band minima and valence-band maxima are located at degener-ate and inequivalent valleys in momentum space. The valley indexcan be regarded as a discrete degree of freedom for low-energycarriers, which is robust against smooth deformation and low-energy phonons because of the large valley separation in momentumspace. To make use of the valley index as an information carrier, thecrucial step is to identify a process in which the valley carriersrespond differently to external stimuli. Recently, a simple schemebased on inversion symmetry breaking has been proposed torealize the manipulation of the valley index through electric, mag-netic and optical means5,11. In particular, it was shown that inversionsymmetry breaking can lead to contrasting circular dichroism indifferent k-space regions, which takes the extreme form of contrast-ing optical selection rules at the high symmetry points of theBrillouin zone11. This enables a valley-dependent interplay ofelectrons with light of different circular polarizations, in analogyto the spin-dependent optical activities in semiconductors suchas GaAs.
Here, we report our observation of selective photoexcitation ofthe degenerate valleys by circularly polarized optical pumping inMoS2 monolayers, an emerging multi-valley two-dimensional semi-conductor with remarkable optical and transport properties14,15.Bulk MoS2 has a hexagonal-crystal layered structure with a covalentlybonded SMoS hexagonal quasi-two-dimensional network packedby weak van der Waals forces. Previous studies have shown that, witha decrease in thickness, MoS2 crosses over from being an indirect-bandgap semiconductor (with multiple layers) to a direct-bandgapsemiconductor (as a monolayer)15,16. The direct bandgap is in thevisible frequency range (1.9 eV) and is therefore ideal for optical
applications, with both conduction and valence band edges locatedat the K (K) points of the two-dimensional hexagonal Brillouinzone. In addition to the changes to its electronic structure, MoS2thin film also undergoes a structural change, as outlined in Fig. 1a.The inversion symmetry present in bulk and in thin films with aneven number of layers is explicitly broken in thin films with an oddnumber of layers, giving rise to a valley-contrasting optical selectionrule11,12,17, where the inter-band transitions in the vicinity of theK (K) point couple exclusively to right (left)-handed circularlypolarized light s (s) (Fig. 1). The direct-bandgap transition atthe two degenerate valleys, together with this valley-contrastingselection rule, suggest that one can optically generate and detectvalley polarizations in a MoS2 monolayer.
Our main result is summarized in the following. We find that inMoS2 monolayers, the photoluminescence has the same helicity inits circularly polarized component as the excitation laser, a signatureof optically pumped valley polarization. Below 90 K, a highphotoluminescence circular polarization is observed, which decayswith temperature. The photoluminescence polarization shows nodependence on the in-plane magnetic field. The absence of theHanle effect is a strong indicator that the polarized photolumines-cence arises from the polarization of the valley rather than spin,as the former cannot be rotated by the magnetic field. Moreover,in MoS2 bilayers, we find that photoluminescence from MoS2bilayers is unpolarized under the same excitation conditions,consistent with the presence of inversion symmetry.
MoS2 flakes, shown representatively in Fig. 2a, were mechanicallyexfoliated from SiO2/silicon substrates using sticky tape in amanner similar to the technique used to produce graphene sheets.The monolayer, bilayer and multilayer flakes could be identified bytwo characteristic Raman modes: the in-plane vibrational E2g
1 modeand the out-of-plane vibrational A1g mode around 400 cm
(ref. 14). Following ref. 14, we labelled the sample thickness accordingto the frequency difference between the E2g
1 and A1g modes:Dv 19 cm21 indicating monolayers and Dv 21 cm21bilayers (Fig. 2b). The layer assignment was confirmed by atomicforce microscopy (AFM). Photoluminescence spectra around1.9 eV, corresponding to excitons from the direct inter-bandtransition18, were also used as a monolayer and bilayer indicatorfrom the transition from indirect- to direct-gapped semiconductor,as illustrated in refs 15,16.
The polarization-sensitive photoluminescence measurement wascarried out using a confocal-like microscopic set-up. The collimatedbackscattering light was passed through a broadband 1/4l waveplate, a beam-displacing prism to separate the light beam into twoorthogonally polarized beams and a depolarizer, and was thenfocused to two spots at the entrance slit of the monochromatorequipped with a charge-coupled device (CCD) camera. The
1Physics Department, The University of Hong Kong, Pokfulam road, Hong Kong, China, 2Department of Physics, South University of Science and Technologyof China, Shenzhen 518055, China, 3Center of Theoretical and Computational Physics, The University of Hong Kong, Hong Kong, China, 4Materials Scienceand Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, These authors contributed equally to this work.
LETTERSPUBLISHED ONLINE: 17 JUNE 2012 | DOI: 10.1038/NNANO.2012.95
NATURE NANOTECHNOLOGY | VOL 7 | AUGUST 2012 | www.nature.com/naturenanotechnology490
2012 Macmillan Publishers Limited. All rights reserved.
polarization resolved spectrum could then be obtained by analysingthe two branches of dispersion on the CCD. (For details regardingthe set-up, see Supplementary Information.)
Figure 2c presents the circularly polarized luminescence spectraof a pristine MoS2 monolayer, which peak around 1.9 eV with right-and left-handed circularly polarized excitation (HeNe laser,1.96 eV) at a near-resonant condition at T 10 K. The lumines-cence corresponds to a direct inter-band transition at the K (K)valley. The helicity of the luminescence exactly follows that of theexcitation light. In other words, the right-handed circularlypolarized excitation generates right-handed luminescence, and theleft-handed circularly polarized excitation generates left-handedluminescence. To characterize the circular component in the lumi-nescence spectra, we define the degree of circular polarization
P = I(s+) I(s)I(s+) + I(s)
where I(s+) is the intensity of the left (right)-handed circularcomponent. For perfectly circularly polarized light, P 1 (s) or1 (s2) (for details see Supplementary Information). The lumines-cence spectra demonstrate a symmetric polarization for excitationwith opposite helicities: P 0.32 under s excitation andP20.32 under s2 excitation for the most representative MoS2monolayer out of four samples that gave values of P+0.23,+0.25, +0.28 and +0.32 under s+ excitation, respectively, at10 K. This behaviour is fully expected under the mechanism ofthe valley-dependent optical selection rule. As well as the unpolar-ized background, there is also a linearly polarized component, andthe linear polarization shifts by 258 between s and s2 exci-tation. If one switches the excitation light to a higher energy at2.33 eV, no polarization can be observed in the luminescencespectra. We note that the valley selection rule is valid in the vicinityof the K (K) point11,12, whereas the optical transition with 2.33 eVoccurs far away from the K points in band dispersion.
350 400 450 500
1.82 1.84 1.86 1.88 1.90Photon energy (eV)
P = 32 2%
P+ = 32 2%
Left-handed circular excitation
Right-handed circular excitation
Raman shift (cm1)
Figure 2 | Polarization-sensitive photoluminescence spectra from MoS2 monolayers. a, Representative optical image of MoS2 monolayer, bilayer and thin-
film flakes. b, Characteristic Raman spectra from different MoS2 flakes (monolayer, bilayer and thin film), showing an in-plane vibrational E2g1 mode and an
out-of-plane vibrational A1g mode around 400 cm21. c, Polarization resolved luminescence spectra under circularly polarized excitation from a HeNe laser at
1.96 eV and 10 K. Circular polari