VALLEYTRONICS

Valley-polarized exciton dynamics ina 2D semiconductor heterostructurePasqual Rivera,1* Kyle L. Seyler,1* Hongyi Yu,2 John R. Schaibley,1 Jiaqiang Yan,3,4

David G. Mandrus,3,4,5 Wang Yao,2 Xiaodong Xu1,6†

Heterostructures comprising different monolayer semiconductors provide an attractivesetting for fundamental science and device technologies, such as in the emerging field ofvalleytronics.We realized valley-specific interlayer excitons in monolayer WSe2-MoSe2vertical heterostructures.We created interlayer exciton spin-valley polarization by means ofcircularly polarized optical pumping and determined a valley lifetime of 40 nanoseconds.This long-lived polarization enables the visualization of the expansion of a valley-polarizedexciton cloud over several micrometers.The spatial pattern of the polarization evolvesinto a ring with increasing exciton density, a manifestation of valley exciton exchangeinteractions. Our work introduces van der Waals heterostructures as a promising platformfrom which to study valley exciton physics.

Van der Waals heterostructures of two-dimensional (2D) materials provide anexciting platform for engineering artificialmaterial systems with distinct properties(1). A beautiful example is the demonstra-

tion of the Hofstadter butterfly physics in moirésuperlattice structures composed of grapheneand hexagonal boron nitride (2–4). As the libraryof 2D crystals is explored further, the range ofpossible new phenomena in condensed matterphysics becomes ever more diverse. For example,heterostructures of 2D semiconductors [namely,transitionmetal dichalcogenide monolayers (MX2)]have been assembled with type-II band alignment(5–8), in which electrons and holes energeticallyfavor different layers (Fig. 1A). These heterostruc-tures form atomically thin p-n junctions that canbe used for photon-energy harvesting (9–15) andhost interlayer excitons (XI), with the Coulomb-bound electron andhole located indifferentmono-layers (14–17). This species of exciton has alifetime far exceeding those in monolayer MX2,and the vertical separation of holes and electronsentails a permanent out-of-plane electric dipolemoment, providing an optical means to pumpinterlayer electric polarization and facilitating elec-trical control of interlayer excitons (17).The interlayer excitons in 2D heterostructures

are similar to spatially indirect excitons in III-Vquantum wells (18–22). In both systems, theelectron-hole wave function overlap is reducedin the out-of-plane direction, suppressing the

magnitude of exciton oscillator strength and theelectron-hole exchange interaction. This leads togreatly enhanced population and spin lifetimesfor spatially indirect excitons as compared withtheir direct exciton counterparts.MX2 heterostructures possess several features

distinct from quantum well systems. First, themonolayers’ band edges are at doubly degen-erate corners of the hexagonal Brillouin zone, soXI has an internal degree of freedom specified bythe combination of electron and hole valley in-dices (23). Second, the twist angle between thecrystal axes of constituent monolayers leads to adisplacement of the conduction and valence bandedges in momentum space, making the XI dark—momentum indirect—at its minimum energy andbright only at finite center-of-mass velocities. Thelocation of the XI light cones depends on the twistangle between the monolayers, allowing for con-trol over the optoelectronic properties (24–26),such as the dipole strength and interlayer ex-citon lifetime (27). Third, the constituent MX2

monolayers exhibit valley-contrasting physicalproperties such as spin-valley locking, opticalselection rules (28–30), and Berry curvature(23). The inheritance of valley physics in twistedMX2 heterostructures is predicted to give rise topreviously unknown optical and transport prop-erties of XI (27), allowing the possibility of exci-tonicoptoelectronic circuitswithvalley functionalitiesand providing a platform for investigating exci-tonic superfluidity and condensation (31).We observed long valley lifetime and valley drift-

diffusion of XI in MoSe2-WSe2 heterostructureswith small twist angles. Our devices consist of apair of exfoliatedmonolayers of WSe2 andMoSe2,which are stacked by means of dry transfer (32)on a 285-nm insulating layer of SiO2 on a siliconsubstrate. We used standard electron beam lithog-raphy techniques to fabricate metallic contacts(V/Au) on the heterostructure, and the siliconsubstrate functions as a global backgate, as shownin Fig. 1A (33). The optical brightness of the XI

depends sensitively on the relative alignment of

the two constituent monolayers. Theory showsthat for twist angles near zero or 60°, there existlight cones at small kinematic momenta in whichthe XI can directly interconvert with photons (27).In such heterostructures, the XI can radiativelyrecombine after scattering into these light conesthrough, for example, exciton-phonon or exciton-exciton interactions (27). In our study, we focusedon this type of heterostructure. To fabricate suchsamples, we identified the armchair axes of in-dividual monolayers by means of polarization-resolved second-harmonic generation and thenaligned these axes (fig. S1). This yields heterostruc-tures with twist angles near zero or 60° (33). Assuch, theXI can be observed in photoluminescence(PL), even at room temperature (fig. S2). All data inthe main text are taken at a temperature of 30 Kfrom the device shown in the optical microscopeimage in Fig. 1B,withWSe2 stacked onMoSe2, andthe excitation laser energy in resonancewith the Aexciton of WSe2 (1.72 eV).We first performed polarization-resolved PL at

zero gate voltage (Vg = 0V).We applied circularlypolarized continuous-wave laser excitation andseparately detected the right circular (s+) andleft circular (s−) PL. The s+ (black) and s− (red)components of the XI PL under circularly po-larized excitation are shown in Fig. 1C. Theseresults show that XI emission is strongly co-polarized with the incident light. Denoting thedegree of polarization by r ¼ Iþ−I−

IþþI−, where I± is

the intensity of the s± PL components, we ob-served jrmaxj > 0:3. Similar results were ob-tained from several other samples (fig. S3). Wealso performed measurements in the linearbasis, which do not show appreciable polariza-tion (fig. S4).The observation of circularly polarized PL

demonstrates that the XI can retain memory ofthe excitation light helicity, which is a conse-quence of the valley optical selection rules in 2Dheterostructures (27). In the following, we dis-cuss the generation of valley polarization inheterostructures near AA-like stacking (twistangle near 0°) (Fig. 1D), but similar conclusionscan be drawn for heterostructures near AB-likestacking (twist angle near 60°) (fig. S5) (33). Thevalley configuration of XI is specified by the valleyindices of its electron and hole. With the spin-valley locking in monolayer MX2, a universal as-signment of the valley index is applicable in thetwisted heterostructures, and here we denote thevalley with electron spin up as +K and spin downas −K in both layers. First, s+ excitation createsvalley-polarized intralayer excitons in the +KW

valley in WSe2 and +KM valley in MoSe2. Next,charge carriers relax to the heterostructure bandedges through interlayer charge transfer on sub-picosecond time scales (11, 15) to form XI. Be-cause of the largemomentumdifference, interlayerhopping between+KWand−KMvalleys is stronglysuppressed. Conversely, the +KW and +KM valleyshave small momentum mismatch, and the spin-conserving interlayer hopping between thesevalleys becomes the dominant relaxation channel.Therefore, thes+ excitation leads to valley-polarizedXI (Fig. 1E). The situation for s− excitation can be

688 12 FEBRUARY 2016 • VOL 351 ISSUE 6274 sciencemag.org SCIENCE

1Department of Physics, University of Washington, Seattle, WA98195, USA. 2Department of Physics and Center of Theoreticaland Computational Physics, University of Hong Kong, HongKong, China. 3Materials Science and Technology Division, OakRidge National Laboratory, Oak Ridge, TN 37831, USA.4Department of Materials Science and Engineering, University ofTennessee, Knoxville, TN 37996, USA. 5Department of Physicsand Astronomy, University of Tennessee, Knoxville, TN 37996,USA. 6Department of Materials Science and Engineering,University of Washington, Seattle, WA 98195, USA.*These authors contributed equally to the work. †Correspondingauthor. E-mail: xuxd@uw.edu

RESEARCH | REPORTSon M

ay 29, 2018

http://science.sciencemag.org/

Dow

nloaded from

obtained through time reversal. The radiative re-combination of the valley-polarized XI is facili-tated by the interlayer coupling, which allowsemission of photons that are co-polarized withthe excitation source (27).We found that the degree of XI valley polar-

ization can be electrically controlled by the gate.Shown in Fig. 2A are polarization-resolved PLspectra at selected Vg under s+ excitation with~50-ps laser pulses. There is a strong gate de-pendence of the valley polarization, which isgreatest at +60 V and highly suppressed at −60 V(the full data set is provided in fig. S6). In Fig. 2B,we show the decay of co-polarized (Fig. 2B, black)and cross-polarized (Fig. 2B, red) interlayer ex-citon PL, as well as the degree of polarization(Fig. 2B, blue), at the same Vg values as in Fig. 2A(the full data set is provided in fig. S7). The valleypolarization lifetime increases with Vg, reaching39 ± 2 ns at +60 V, as determined by fitting asingle exponential decay. We also measured longvalley lifetimes in heterostructures with the op-posite stacking order (MoSe2 on WSe2) (fig. S8).These measurements imply a strong suppres-

sion of intervalley scattering for the XI and avalley lifetime several orders of magnitude lon-ger than that of intralayer excitons inmonolayers,in which valley depolarization occurs on pico-second time scales (34–36). Our measurementalso shows that the initial PL polarization of XI is~40% at +60 V. The imperfect initial valleypolarization of XI is likely due to valley depo-larization of intralayer excitons in the constituent

SCIENCE sciencemag.org 12 FEBRUARY 2016 • VOL 351 ISSUE 6274 689

Si

SiO2

Au Au

E

Fig. 1. Interlayer exciton spin-valley polarization in MoSe2-WSe2 hetero-structures. (A) Side view of MoSe2-WSe2 heterostructure device. Boxed regiondepicts the interlayer exciton, with holes (h+) and electrons (e−) located inWSe2 and MoSe2, respectively. (B) Optical image of device, with WSe2 on topof MoSe2. Scale bar, 2 mm. (C) Circular polarization–resolved PL spectra of theinterlayer exciton showing the generation of strong valley polarization. (D) Il-lustration of the Dirac points in the hexagonal Brillouin zone of a MoSe2-WSe2

heterobilayer, with small twisting angle. The +K (red) and −K (blue) valleys atthe conduction band minimum (in MoSe2) and valence band maximum (inWSe2) are nearly aligned in momentum space. (E) Schematic of the interlayerexciton in the +K valley. First, s+ circularly polarized light (black wavy lines)excites intralayer excitons in the +KM and +KW valleys. Fast interlayer chargehopping (blue dotted lines) forms the interlayer exciton in the +K valley. Theoptical selection rules in the +KW and +KM valleys produce co-polarized PL.

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0

0 V

+60 VN

orm

aliz

ed in

tens

ity

-60 V

Time (ns)

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

Degree of polarization

0.0

0.2

0.4

0.6

0.8

5

10

15

10

20

30

1.26 1.28 1.30 1.32 1.34 1.360

20

40

60

80

0 V

-60 V

+60 V

Inte

nsity

(co

unts

µW

-1 s

-1)

Energy (eV)

Fig. 2. Gate-tunable interlayer exciton valley polarization and lifetime. All plots are for s+ pulsedlaser excitation with co-polarized and cross-polarized PL shown in black and red, respectively. (A) Polarization-resolved interlayer exciton PL at selected gate voltages. (B) Time-resolved interlayer exciton PL at selectedgate voltages. The blue curve (right axis) shows the decay of valley polarization. Solid lines are singleexponential fits to valley polarization decay, with lifetimes of 39 ± 2, 10 ± 1, and 5 ± 2 ns for gate voltagesof +60, 0, and −60 V, respectively.

RESEARCH | REPORTSon M

ay 29, 2018

http://science.sciencemag.org/

Dow

nloaded from

monolayers, which mediate the XI formation.Because the XI is dark at the minimum of theenergy dispersion (27), caused by the finite twist-ing angle and slight lattice mismatch betweenthe two layers, it effectively provides a reservoirfrom which the XI are scattered into the lightcones and luminesce. The momentum-indirectnatureofXI is supportedby temperature-dependentmeasurements, which show enhanced lifetimeat low temperature (fig. S9). This complicatedexciton-light coupling is likely responsible forthe subtle but intriguing features in Fig. 2B, suchas the increase of PL lifetime accompanying thedecrease of valley polarization lifetime. However,future studies are required to gain a clear un-derstanding of the microscopic mechanism forthe observed gate-dependent PL dynamics ofthe XI.The long valley lifetime of the XI allows vi-

sualization of their lateral drift and diffusion. Asequence of spatial maps of the XI PL polariza-tion under pulsed excitation (40 MHz repetitionrate) at Vg = 60 V is displayed in Fig. 3A forselected average excitation powers. The spatialpattern of r shows the evolution of a ring with adiameter that increases with excitation intensity(the full data set is available in fig. S10). The pat-tern of polarization stands in contrast to the spatialdistribution of the emission. The polarization-resolved PL intensity spatial maps are shownin Fig. 3B at 20 mW, where both s+ and s− PLcomponents display an approximately Gaussianprofile centered at the excitation spot. For directcomparison of the different spatial profiles,Fig. 3C gives the average PL intensity and r as afunction of the distance from the beam centerfor the 40 mW case. The data show the drift-diffusion of s+ (Fig. 3C, black) and s− (Fig. 3C,red) polarized excitons away from the laser

spot (0.7 mm full-width at half maximum) (Fig.3C, dashed line) as well as the ring of larger r(Fig. 3C, blue), demonstrating the striking differ-ence between the spatial distribution of polar-ization and the total density of XI.One possible explanation for the observed

polarization ring is density-dependent interval-ley scattering. However, consideration of the valleypolarization as a function of excitation intensitysuggests otherwise (fig. S11). Rather, the observedspatial patterns in the valley polarization can beunderstood as manifestations of valley-dependentmany-body interactions in the dense interlayerexcitongas (33). The spin-valleypolarizedXI,whichpossess out-of-plane dipoles, interact throughdipole-dipole and exchange interactions, both ofwhich are repulsive. Because of the small inter-layer separation of ~7 Å, we estimate that theexchange interaction is stronger than the dipole-dipole repulsion (33). Because the exchange inter-actions are appreciable only between excitons ofthe same valley species (33), in a cloud of valley-polarized interlayer excitons the majority valleyexcitons experience stronger mutually repulsiveforce (fig. S13A), leading to more rapid expansionthan that of the minority valley excitons (fig.S13B). On the other hand, the density gradient ofexcitons will also give rise to diffusion, which isvalley-independent and does not produce a ringpattern. Therefore, the relative strength of thediffusion and valley-dependent drift controls thepattern of the spatial polarization. If the inter-layer exciton density is large enough that the valley-dependent repulsive interaction dominates theexpansion of the exciton gas, higher valley po-larization can appear away from the excitationcenter (fig. S15). Indeed, a pronounced ring inthe polarization is generated at sufficiently highexcitation intensity, as seen in Fig. 3A.

A temperature difference between the major-ity and minority XI could, in principle, causethem to expand at different speeds. However,under excitation by polarized laser pulse, theminority excitons are created at the excitationspot through intervalley relaxation. Because therelaxation of this internal degree of freedomdoesnot change the kinetic energy of the exciton, themajority and minority XI are expected to havethe same initial temperature before the expan-sion of the exciton cloud. This precludes valley-dependent temperature as a driving force for thering formation.

REFERENCES AND NOTES

1. A. K. Geim, I. V. Grigorieva, Nature 499, 419–425 (2013).2. C. R. Dean et al., Nature 497, 598–602 (2013).3. B. Hunt et al., Science 340, 1427–1430 (2013).4. L. A. Ponomarenko et al., Nature 497, 594–597 (2013).5. J. Kang, S. Tongay, J. Zhou, J. Li, J. Wu, Appl. Phys. Lett. 102,

012111 (2013).6. K. Kośmider, J. Fernández-Rossier, Phys. Rev. B 87, 075451

(2013).7. H. Terrones, F. López-Urías, M. Terrones, Sci. Rep. 3, 1549

(2013).8. M.-H. Chiu et al., Nat. Commun. 6, 7666 (2015).9. R. Cheng et al., Nano Lett. 14, 5590–5597 (2014).10. M. M. Furchi, A. Pospischil, F. Libisch, J. Burgdörfer, T. Mueller,

Nano Lett. 14, 4785–4791 (2014).11. X. Hong et al., Nat. Nanotechnol. 9, 682–686 (2014).12. C. H. Lee et al., Nat. Nanotechnol. 9, 676–681 (2014).13. Y.-C. Lin et al., Nat. Commun. 6, 7311 (2015).14. Y. Gong et al., Nat. Mater. 13, 1135–1142 (2014).15. F. Ceballos, M. Z. Bellus, H.-Y. Chiu, H. Zhao, ACS Nano 8,

12717–12724 (2014).16. H. Fang et al., Proc. Natl. Acad. Sci. U.S.A. 111, 6198–6202

(2014).17. P. Rivera et al., Nat. Commun. 6, 6242 (2015).18. L. V. Butov, A. C. Gossard, D. S. Chemla, Nature 418, 751–754

(2002).19. D. Snoke, S. Denev, Y. Liu, L. Pfeiffer, K. West, Nature 418,

754–757 (2002).20. J. R. Leonard et al., Nano Lett. 9, 4204–4208 (2009).21. E. Finkman, M. D. Sturge, M. C. Tamargo, Appl. Phys. Lett. 49,

1299–1301 (1986).

690 12 FEBRUARY 2016 • VOL 351 ISSUE 6274 sciencemag.org SCIENCE

-2 -1 0 1 20.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

0

0.2

0.4

0.6

0

120 µW

+

10 µW1 µW 60 µW40 µW20 µW

20 µW-

No

rmal

ized

inte

nsi

ty

Radius (µm)

Fig. 3. Drift-diffusion ofvalley-polarized interlayer excitongas. All plots are for s+ pulsed laserexcitation. (A) Spatial map of valleypolarization under 1 to 60 mWexcitation. Sample outline is shown inwhite overlay. Scale bar, 2 mm.(B) Spatial maps of s+ (left) and s−

(right) interlayer exciton PLnormalized intensity under 20 mWexcitation. (C) Polarization-resolvedspatial profiles of s+ (black) and s−

(red) components of interlayer exciton PL under 40 mWexcitation.The spatial distribution of valley polarization is shown in blue, and the laser excitation profileis shown in gray. Line cuts are radially averaged through the excitation center, and curves are added as guides to the eye.

RESEARCH | REPORTSon M

ay 29, 2018

http://science.sciencemag.org/

Dow

nloaded from

22. G. D. Gilliland et al., Phys. Rev. Lett. 71, 3717–3720(1993).

23. D. Xiao, G. B. Liu, W. Feng, X. Xu, W. Yao, Phys. Rev. Lett. 108,196802 (2012).

24. W.-T. Hsu et al., ACS Nano 8, 2951–2958(2014).

25. A. M. van der Zande et al., Nano Lett. 14, 3869–3875(2014).

26. K. Liu et al., Nat. Commun. 5, 4966 (2014).27. H. Yu, Y. Wang, Q. Tong, X. Xu, W. Yao, Phys. Rev. Lett. 115,

187002 (2015).28. T. Cao et al., Nat. Commun. 3, 887 (2012).29. K. F. Mak, K. He, J. Shan, T. F. Heinz, Nat. Nanotechnol. 7,

494–498 (2012).30. H. Zeng, J. Dai, W. Yao, D. Xiao, X. Cui, Nat. Nanotechnol. 7,

490–493 (2012).31. M. M. Fogler, L. V. Butov, K. S. Novoselov, Nat. Commun. 5,

4555 (2014).32. P. J. Zomer, M. H. D. Guimarães, J. C. Brant, N. Tombros,

B. J. van Wees, Appl. Phys. Lett. 105, 013101 (2014).

33. Materials and methods are available as supplementarymaterials on Science Online.

34. C. Mai et al., Nano Lett. 14, 202–206 (2014).35. Q. Wang et al., ACS Nano 7, 11087–11093

(2013).36. C. R. Zhu et al., Phys. Rev. B 90, 161302

(2014).

ACKNOWLEDGMENTS

We thank D. Cobden and F. Wang for helpful discussion. This workis mainly supported by the U.S. Department of Energy (DOE), BasicEnergy Sciences (BES), Materials Sciences and EngineeringDivision (DE-SC0008145 and SC0012509). The spectroscopy workis partially supported by NSF-EFRI-1433496. H.Y. and W.Y. weresupported by the Croucher Foundation (Croucher InnovationAward), and the Research Grants Counciland University GrantsCommittee of Hong Kong (HKU17305914P, HKU9/CRF/13G, AoE/P-04/08). J.Y. and D.G.M. were supported by the DOE, BES,Materials Sciences and Engineering Division. X.X. acknowledges aCottrell Scholar Award and support from the State of Washington–

funded Clean Energy Institute. Device fabrication was performedat the University of Washington Microfabrication Facility andNSF-funded Nanotech User Facility. Data described in this paperare presented in the supplementary materials and are availableupon request. X.X. and W.Y. conceived and supervised the project.P.R. and K.L.S. fabricated the samples and performed theexperiments, assisted by J.R.S. P.R., K.L.S., and X.X. analyzed data.H.Y. and W.Y. provided theoretical support and performed thesimulation. J.Y. and D.G.M. synthesized and characterized the bulkcrystal. P.R., K.L.S., X.X., W.Y., and H.Y. cowrote the paper. Allauthors discussed the results.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/351/6274/688/suppl/DC1Materials and MethodsFigs. S1 to S15References (37–45)

11 June 2015; accepted 8 January 201610.1126/science.aac7820

MATERIALS SCIENCE

On-chip and freestanding elasticcarbon films for micro-supercapacitorsP. Huang,1,2 C. Lethien,2,3 S. Pinaud,4 K. Brousse,1,2 R. Laloo,1,2 V. Turq,1,2 M. Respaud,4,5

A. Demortière,2,6 B. Daffos,1,2 P. L. Taberna,1,2 B. Chaudret,4 Y. Gogotsi,7 P. Simon1,2*

Integration of electrochemical capacitors with silicon-based electronics is a majorchallenge, limiting energy storage on a chip. We describe a wafer-scale process formanufacturing strongly adhering carbide-derived carbon films and interdigitatedmicro-supercapacitors with embedded titanium carbide current collectors, fullycompatible with current microfabrication and silicon-based device technology.Capacitance of those films reaches 410 farads per cubic centimeter/200 millifaradsper square centimeter in aqueous electrolyte and 170 farads per cubic centimeter/85millifarads per square centimeter in organic electrolyte. We also demonstrate preparationof self-supported, mechanically stable, micrometer-thick porous carbon films with aYoung’s modulus of 14.5 gigapascals, with the possibility of further transfer ontoflexible substrates. These materials are interesting for applications in structural energystorage, tribology, and gas separation.

The plethora of portable electronic devicesand the continued expansion of electronicsinto new mobile applications highlightthe need for high-performance miniatur-ized electrochemical storage devices able

to deliver energy from their environment. Radiofrequency identification (RFID) tags for the devel-opment of smart environments are another crit-ical application that requires compact energystorage. Accordingly, designing efficient mini-aturized energy storage devices for energy de-livery or harvesting with high-power capabilitiesremains a challenge (1).Electrochemical double-layer capacitors (EDLCs),

also known as supercapacitors, store the chargethrough reversible ion adsorption at the surfaceof high-surface-area carbons. Aside from an out-standing cycle life, this electrostatic charge stor-age results in devices with medium energydensity (~6 W·h kg−1) and high power densities(>10 kW kg−1). As a result, supercapacitorscomplement—or can even sometimes replace—batteries in applications from electronics topublic transportation and renewable energy stor-

age, in which high-power delivery and uptakeand very long cycle life are required (2).A large variety ofmaterials such as graphene (3–5),

nanotubes (6, 7), carbide-derived carbons (CDC)(8, 9), and pseudocapacitive materials (1, 10, 11)have been explored in micro-supercapacitors inthe past 5 years.Much of the recentwork focusedon the use of wet processing routes (using col-loidal solutions or suspensions of particles forthe electrode preparation) for the developmentof flexible micro-devices for printable electronics(1, 4, 5, 12). However, the wet processing meth-ods are not fully compatible with semiconduc-tor device manufacturing used in the electronicsindustry, thus hampering supercapacitor manufac-turing on silicon chips. Vapor phase methods,such as atomic layer deposition, are limited tothe preparation of thin layers of active materi-als, limiting the areal energy and power perform-ance of the electrodes (13, 14). Direct growth ofgraphene or carbon nanotubes on a Si waferresults in low-energy-density devices (15). Thepioneering concept of laser scribing of graphenedeveloped by Kaner’s group for preparing flex-

iblemicro-supercapacitors has shown outstandingpower performance but fails to achieve highareal capacitance (< 5 mF cm-2) (3, 5). Anotherstrategy is to use pseudocapacitive materials,which have a larger capacitance, but movingfrompure double-layer to pseudocapacitive chargestorage comes with disadvantages, such as a de-crease in power capabilities and cycle life be-cause of the kinetic limitations of the redoxreactions (10, 11, 16–20). Bulk CDC films (8, 9, 21)have shown high capacitance and high arealand volumetric energy density but poor integrityof the film from cracking and delamination (9).To produce carbon films, TiC coatings of sev-

eral micrometers in thickness were depositedby using a direct current magnetron sputtering(DC-MS) technique (supplementarymaterials, ma-terials and methods). Shown in fig. S1A is a crosssection of a 6.3-mm-thick TiC film deposited ontoa SiO2-coated Si wafer, by using the DC-MS tech-nique. The resistivity, thickness, and mechani-cal stress of the TiC film can be fine-tuned bychanging the deposition parameters (fig. S1, Band C). The film thickness changes linearly withthe deposition time (fig. S1D), and films up to20 mm thick could be prepared, with controlledroughness (fig. S1E). Samples were placed in afurnace and chlorinated at 450°C, thus transforming

SCIENCE sciencemag.org 12 FEBRUARY 2016 • VOL 351 ISSUE 6274 691

1Université Paul Sabatier-Toulouse III, Laboratoire CentreInter-universitaire de Recherche et d’Ingénierie desMatériaux (CIRIMAT), UMR CNRS 5085, 118 route deNarbonne, 31062 Toulouse, France. 2Réseau sur le StockageElectrochimique de l’Energie, FR CNRS n°3459, France.3Université Lille 1 Sciences et Technologies, LaboratoireInstitut d’Electronique de Microélectronique et deNanotechnologie (IEMN), UMR CNRS 8520, Cité scientifique,Avenue Henri Poincaré, CS 60069, 59652 Villeneuve d’Ascqcedex, France. 4Laboratoire de Physique et Chimie desNano-Objets (LPCNO), UMR 5215 Institut National desSciences Appliquées (INSA)–Université Paul Sabatier (UPS)–CNRS, Université de Toulouse, INSA, 135 Avenue deRangueil, 31077 Toulouse, France. 5Atelier Interuniversitairede Micro-nano Électronique (AIME), Université de Toulouse,INSA, UPS, INP, 135 avenue de Rangueil, 31077 ToulouseCedex 4, France. 6Laboratoire de Chimie et de Réactivité desSolides, UMR CNRS 7314, Université de Picardie Jules Verne,80039 Amiens, France. 7Department of Materials Scienceand Engineering, and A. J. Drexel Nanomaterials Institute,Drexel University, 3141 Chestnut Street, Philadelphia, PA19104, USA.*Corresponding author. E-mail: simon@chimie.ups-tlse.fr

RESEARCH | REPORTSon M

ay 29, 2018

http://science.sciencemag.org/

Dow

nloaded from

Valley-polarized exciton dynamics in a 2D semiconductor heterostructurePasqual Rivera, Kyle L. Seyler, Hongyi Yu, John R. Schaibley, Jiaqiang Yan, David G. Mandrus, Wang Yao and Xiaodong Xu

DOI: 10.1126/science.aac7820 (6274), 688-691.351Science 

, this issue p. 688Scienceof either material.

layerdifferent layers. The valley-specific character of such excitons persisted far longer than would be possible in a single to form so that the hole and electron came from the same valley but−−pairs of electrons and holes−−caused excitons

on top of each other and shone circularly polarized light on the structure. The light 2 and WSe2single layers of MoSe placedet al.information carriers. However, electrons easily lose this information by scattering into the other valley. Rivera

structure has two ''valleys.'' Electrons can be distinguished by the valley they reside in, making them act as potential , which, like graphene, has a two-dimensional honeycomb crystal lattice, the electronic2In the material MoSe

Stacking to prolong valley lifetime

ARTICLE TOOLS http://science.sciencemag.org/content/351/6274/688

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2016/02/10/351.6274.688.DC1

REFERENCES

http://science.sciencemag.org/content/351/6274/688#BIBLThis article cites 44 articles, 2 of which you can access for free

PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions

Terms of ServiceUse of this article is subject to the

is a registered trademark of AAAS.Sciencelicensee American Association for the Advancement of Science. No claim to original U.S. Government Works. The title Science, 1200 New York Avenue NW, Washington, DC 20005. 2017 © The Authors, some rights reserved; exclusive

(print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement ofScience

on May 29, 2018

http://science.sciencem

ag.org/D

ownloaded from