# Fileset [alexeev-et-al-2024-nature-of-long-lived-moiré-interlayer-excitons-in-electrically-tunable-mos2-mose2-heterobilayers.pdf](https://mdr.nims.go.jp/filesets/fbc0118b-c7ad-4e68-b540-75ab5bae0540/download) ## Creator Evgeny M. Alexeev, Carola M. Purser, Carmem M. Gilardoni, James Kerfoot, Hao Chen, Alisson R. Cadore, Bárbara L.T. Rosa, Matthew S. G. Feuer, Evans Javary, Patrick Hays, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Seth Ariel Tongay, Dhiren M. Kara, Mete Atatüre, Andrea C. Ferrari ## Rights [Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/) ## Other metadata [Nature of Long-Lived Moiré Interlayer Excitons in Electrically Tunable MoS₂/MoSe₂ Heterobilayers](https://mdr.nims.go.jp/datasets/f5595e8c-6ca1-45f8-be5d-d4577474f9ce) ## Fulltext Nature of Long-Lived Moiré Interlayer Excitons in Electrically Tunable MoS2/MoSe2 HeterobilayersNature of Long-Lived Moire ́ Interlayer Excitons in ElectricallyTunable MoS2/MoSe2 HeterobilayersEvgeny M. Alexeev,* Carola M. Purser, Carmem M. Gilardoni, James Kerfoot, Hao Chen,Alisson R. Cadore, Bárbara L.T. Rosa, Matthew S. G. Feuer, Evans Javary, Patrick Hays, Kenji Watanabe,Takashi Taniguchi, Seth Ariel Tongay, Dhiren M. Kara, Mete Atatüre,* and Andrea C. Ferrari*Cite This: Nano Lett. 2024, 24, 11232−11238 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Interlayer excitons in transition-metal dichalcogenide hetero-bilayers combine high binding energy and valley-contrasting physics with a longoptical lifetime and strong dipolar character. Their permanent electric dipoleenables electric-field control of the emission energy, lifetime, and location.Device material and geometry impact the nature of the interlayer excitons viatheir real- and momentum-space configurations. Here, we show that interlayerexcitons in MoS2/MoSe2 heterobilayers are formed by charge carriers residingat the Brillouin zone edges, with negligible interlayer hybridization. We find thatthe moire ́ superlattice leads to the reversal of the valley-dependent opticalselection rules, yielding a positively valued g-factor and cross-polarized photoluminescence. Time-resolved photoluminescencemeasurements reveal that the interlayer exciton population retains the optically induced valley polarization throughout itsmicrosecond-long lifetime. The combination of a long optical lifetime and valley polarization retention makes MoS2/MoSe2heterobilayers a promising platform for studying fundamental bosonic interactions and developing excitonic circuits for opticalinformation processing.KEYWORDS: layered materials heterostructures, transition-metal dichalcogenides, interlayer excitons, moire ́ superlattice,valley polarization, Stark shift, photoluminescenceLayered materials heterostructures (LMHs) comprisingmonolayer transition-metal dichalcogenides (1L-TMDs)are promising platforms for optoelectronics1−4 and quantumtechnology5 as they combine optically addressable spin andvalley degrees of freedom6−8 with unique tunability throughthe choice of material combination9−11 and rotationalalignment.12−14 TMD heterobilayers have drawn particularinterest due to their ability to host interlayer excitons (iXs)15,16which offer lifetime approaching 200 μs,17 strong repulsivedipolar interaction,18,19 and high sensitivity to rotationalalignment,20−22 strain,17,23 and electric24 and magnetic25 fields.Different TMD combinations give rise to iX with drasticallydifferent properties, including oscillator strength,26,27 center-of-mass momentum,20,21 and degree of interlayer hybridiza-tion.19,22 Of the plethora of possible TMD combinations, themajority of research effort focused on 1L-MoSe2/1L-WSe227−30 and 1L-WS2/1L-WSe2.31−33 For other materialcombinations, key aspects of the iX nature, such as real- andmomentum-space configuration, remain elusive due to thecomplexity of the underlying physics.In this work, we investigate iX in 1L-MoS2/1L-MoSe2 usingpolarization-resolved magneto-photoluminescence spectrosco-py. We find that iX photoluminescence (PL) is visible only indevices with relative twist angle less than 5°. This indicates thatthe constituent iX charge carriers reside at the edges of theBrillouin zone. We study the iX PL response to out-of-planeelectric and magnetic fields and show that iX is formed bycharge carriers at the ±K valleys with negligible degree ofinterlayer hybridization. Our time- and polarization-resolvedPL measurements reveal microsecond-scale retention ofoptically induced valley polarization, demonstrating thepotential of 1L-MoS2/1L-MoSe2 for opto-valleytronic applica-tions.Figure 1a shows an optical microscope image of one of ourelectric-field-tunable 1L-MoS2/1L-MoSe2. The hexagonalboron nitride (hBN) layers provide a flat and clean dielectricenvironment for 1L-MoS2/1L-MoSe2, and the transparent few-layer graphene (FLG) top and bottom gates allow opticalmeasurements under an out-of-plane electric field. Each of theeight devices is fabricated using deterministic mechanicaltransfer,34,35 with constituent monolayers obtained throughmicromechanical exfoliation of bulk TMD crystals prepared byReceived: June 4, 2024Revised: August 20, 2024Accepted: August 22, 2024Published: August 30, 2024Letterpubs.acs.org/NanoLett© 2024 The Authors. Published byAmerican Chemical Society11232https://doi.org/10.1021/acs.nanolett.4c02635Nano Lett. 2024, 24, 11232−11238This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 8, 2025 at 00:25:53 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Evgeny+M.+Alexeev"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carola+M.+Purser"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carmem+M.+Gilardoni"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="James+Kerfoot"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hao+Chen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alisson+R.+Cadore"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alisson+R.+Cadore"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ba%CC%81rbara+L.T.+Rosa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Matthew+S.+G.+Feuer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Evans+Javary"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Patrick+Hays"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Seth+Ariel+Tongay"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Dhiren+M.+Kara"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mete+Atatu%CC%88re"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Andrea+C.+Ferrari"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.4c02635&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/24/36?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/36?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/36?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/36?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c02635?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/flux zone growth. Thickness and quality of constituent layersare characterized using Raman36 and PL spectroscopy (seeMethods and Supporting Information Figures S1, S2). Figure1b presents a schematic of the type-II alignment of electronicbands within 1L-MoS2/1L-MoSe2, with conduction-bandminimum (valence-band maximum) occurring in 1L-MoS2(1L-MoSe2).37,38 The type-II band alignment leads tointerlayer charge separation and the formation of iX, with PLlower in energy compared to the intralayer PL of theconstituent monolayers. The devices offer a range of twistangles between the 1L-TMD θ, enabling the investigation of iXmomentum-space configuration. Figure 1c compares room-temperature (RT) PL spectra of two devices with θ = 1° (top)and θ = 28° (bottom). We identify θ using polarization-resolved second-harmonic generation (SHG) (Figure 1d) andnote that our measurements do not allow us to distinguishparallel from antiparallel alignment between monolayers(Supporting Information Figure S3). Both devices show PLpeaks corresponding to the A exciton in 1L-MoSe2 (1L-MoSe2XA) at 1.55 eV and the A and B excitons in 1L-MoS2 (1L-MoS2Figure 1. iX in 1L-MoS2/1L-MoSe2. (a) Optical microscope image of an electrically tunable 1L-MoS2/1L-MoSe2 device. 1L-MoS2 and 1L-MoSe2regions are outlined in red and green, respectively. Solid (dashed) black lines show the position of top (bottom) FLG gates. (b) Schematic bandalignment of 1L-MoS2 and 1L-MoSe2. (c) RT PL spectra recorded in two devices with different θ. The closely rotationally aligned device (toppanel, θ = 1°) shows intralayer 1L-MoS2 B and A excitons and 1L-MoSe2 A exciton peaks, as well as an iX peak appearing in a lower-energy range(highlighted in copper), not visible in the PL spectrum of the strongly misaligned device (bottom panel, θ = 28°). (d) Polarization-resolved SHGintensity recorded in isolated (red) 1L-MoS2 and (green) 1L-MoSe2 regions of the two devices, confirming θ = 1° (top) and θ = 28° (bottom).Figure 2. Electric field tuning of iX. (a) Normalized iX PL under out-of-plane electric field. The iX PL energy shows a linear shift with slope∼0.31 eV nm V−1, corresponding to a dipole size of d = 0.56(3) nm, closely matching the expected interlayer distance. (b) Variation of iX PL decaytime as a function of electric field. Gray and black circles correspond to fast (τ1) and slow (τ2) time constants, respectively. (c) PL decay acquired at+0.24, 0, and −0.24 V nm−1. Red curves are biexponential fits to the data.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c02635Nano Lett. 2024, 24, 11232−1123811233https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c02635?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asXA and XB) at 1.85 and 2.0 eV, respectively.39,40 Crucially, theiX PL peak at ∼1.3 eV is visible only in the device with θ = 1°.Of the eight devices with θ ranging from 1° to 28°, only thosewith θ ≤ 5° reveal the iX PL peak at RT (SupportingInformation Figure S4), consistent with ref 41. Thus, closerotational alignment is critical for the observation of iX in 1L-MoS2/1L-MoSe2.The ∼3.7% mismatch in lattice constants of 1L-MoS2 and1L-MoSe242 eliminates θ dependence of the interlayer distanceas an underlying source of this behavior.43 Instead, the highsensitivity of the iX PL intensity to θ indicates that iX isformed by the charge carriers residing in valleys at the edges ofthe Brillouin zone (BZ). Homo- and heterobilayers where atleast one of the charge carriers resides at the Γ valley at the BZcenter of display iX PL throughout the entire θ range, as themomentum-space separation between electron and holeremains unchanged.21,44,45 In contrast, in heterobilayerswhere both constituent charges reside at the BZ edges, largemomentum-space separation of electron and hole suppressesradiative recombination of iX in devices with θ away from 0 or60°,13,20 consistent with our observations.Our device structure enables control of doping and theelectric field independently. We use this to identify the real-space configuration of iX by studying its response to an out-of-plane electric field in the neutral regime. We note that thedoping dependence of iX emission is reminiscent of what isobserved in 1L-WS2/1L-WSe219,46,47 (Supporting InformationFigure S5). Figure 2a presents the normalized iX PL spectrumrecorded as a function of electric field at 4K. The iX PL energyshifts linearly with a rate ∼0.31 eV nm V−1 and can be tunedover a 144-meV range within the gate tuning limits of ourdevice. We find an average tuning response across threedevices of ∼0.30 eV nm V−1, yielding an average dipole size∼0.55(3) nm19,24 (Supporting Information Figure S6), in goodagreement with the ∼0.6 nm separation between the layers.48A similar dipole size was observed in 1L-MoSe2/1L-WSe219,where iX is formed by nonhybridized electrons and holes,while MoSe2 homobilayers show a reduced dipole size of 0.26nm due to charge-carrier hybridization.49 Comparatively, ourresults suggest negligible interlayer hybridization for ourdevices.Figure 2b shows the iX PL decay time constants as afunction of electric field. We extract these constants from abiexponential fit to the time-resolved PL. Figure 2c presentsexamples of a PL decay trace recorded at three applied fieldvalues along with their corresponding fit curves. We observe amicrosecond-long iX lifetime, with a fast time constant τ1 =1.0(1) μs and a slow time constant τ2 = 4.4(4) μs at zeroelectric field�an order of magnitude longer than typicallifetimes of 10−100 ns reported for 1L-MoSe2/1L-WSe2.13The slow time constant in other devices ranges from 0.1 to 3.0μs (Supporting Information Figure S7). The variability of PLlifetime measured across different devices supports theassignment of this time scale to iX PL, rather than defect-bound PL. The fast time constant is mostly field-independent,except for ∼0.06 V nm−1, where it increases to 1.3 μs.Shortening of τ1 for the electric field away from this value islikely caused by inadvertent electrostatic doping induced by aslight asymmetry in the thicknesses of the bottom and topdielectric layers. The slow time constant τ2 shows a gradualdecrease with increasing electric field, consistent with a changein radiative lifetime due to a field-induced variation ofelectron−hole separation. For electric field antiparallel to theiX electric dipole moment, the separation between the twocharge carriers is reduced, leading to an increased probabilityof radiative recombination. The opposite process takes placefor parallel field alignment. That said, the PL decay timeremains slow (τ1 ≥ 0.04 μs, τ2 ≥ 1.9 μs) throughout the entirefield-tuning range.We use polarization-resolved magneto-PL spectroscopy toidentify the valley configuration of iX. Figure 3a shows iX PLspectra recorded using right circularly polarized (σ+), 1.94 eVexcitation as a function of applied out-of-plane magnetic field Branging from −6 to 6 T; blue (red) curves correspond to PLFigure 3. Magneto-PL spectroscopy of iX. (a) Helicity-resolved iX PL spectra recorded under out-of-plane magnetic field ranging from −6 to 6 Tusing σ+ polarized 1.94 eV optical excitation; blue (red) curves correspond to PL with σ+ (σ−) polarization. (b) Energy splitting between σ+ and σ−polarized PL as a function of out-of-plane magnetic field for (top) iX in heterobilayer and (bottom) neutral excitons (X0) in 1L-MoSe2 regions.Lande ́ g-factors extracted using linear fits are listed next to each plot. (c) Optically induced valley polarization calculated as ρopt = (I++ + I−− − I+− −I−+)/(I++ + I−− + I+− + I−+) as a function of magnetic field for iX (top) and 1L-MoSe2 X0 (bottom), where IXY is the intensity of PL with σY-polarization collected under σX-polarized excitation. Unfilled circles in the panel and the inset are extracted from a fine scan around 0 T, showingthe small-field dependence of ρopt for iX.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c02635Nano Lett. 2024, 24, 11232−1123811234https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c02635?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aswith σ+ (σ−) polarization. The iX PL remains cross-polarizedwith respect to the excitation laser throughout the entiremagnetic field range. Two distinct mechanisms are known togive rise to this behavior in TMD heterobilayers: 1) directionalintervalley scattering50; 2) moire-́induced reversal of the valley-dependent optical selection rules.51We identify the underlying mechanism in 1LMoS2/1L-MoSe2 based on the sign of the energy splitting between σ+and σ− polarized PL under magnetic field. Figure 3b is a plot ofthe energy splitting (ΔE) as a function of B for the iX in theheterobilayer region (top panel) and the neutral intralayerexcitons (X0) in an isolated 1L-MoSe2 region (bottom panel).We define ΔE as Eσ+ − Eσ− = gμBB, where Eσ+(Eσ−) is theenergy of the σ+ (σ−) polarized PL, g is the effective Lande ́ g-factor, and μB is the Bohr magneton. For X0 in 1L-MoSe2, weextract g = −3.90(2), consistent with previous reports,52 wherethe minus sign stems from valley-Zeeman interaction andvalley-dependent optical selection rules for 1L-TMDs:53 σ+-polarized light couples to optical transitions in the +K valley,which has lower energy at positive B. In contrast, we obtain g =+2.50(7) for iX�the positive sign of g shows that iX PL fromthe +K valley appears with σ− polarization, confirming thereversal of optical selection rules with respect to the monolayercase. All devices show positive iX g-factors with an averagevalue of +4.5. In TMD heterobilayers, the reversal of theselection rules arises from local changes in crystal symmetryinduced by the moire ́ superlattice.51 We observe iX g-factorsthat are always positive, but range from +1.0 to +8.0 acrossdevices, with variability between different positions withinindividual devices (see Supporting Information Figure S8).The variability is likely a consequence of the difference in themoire ́ superlattice parameters arising from different θ and localstrain in different devices.Two mechanisms can give rise to the observed PLpolarization under finite magnetic field: 1) optically inducedvalley polarization54; 2) Zeeman-shift-induced valley thermal-ization.39 The former is limited by nondirectional intervalleyscattering, while the latter arises from exciton relaxation intothe lower-energy valley. We calculate the degree of opticallyinduced valley polarization independently as =opt++ + +++ + +++ + +I I I II I I I, where IXY represents the intensity of PLwith σY-polarization collected under σX-polarized excitation.Figure 3c displays the dependence of ρopt on magnetic field foriX and 1L-MoSe2 X0. 1L-MoSe2 X0 shows a constant PLpolarization degree ∼4%, consistent with earlier reports.55,56 Incontrast, |ρopt| for iX shows a distinct increase with increasing |B|, saturating at ∼6% above ±20 mT (see inset in Figure 3c).This dependence is consistent across all devices, with |ρopt|ranging from 6% to 14%. In the absence of a magnetic field,|ρopt| ranges from 0% to 7%. Similar sharp changes in valleypolarization degree with magnetic field have been observed foriX in 1L-MoSe2/1L-WSe257 and 1L-MoS2/1L-WSe258, as wellas intralayer excitons in 1L-WS2 and 1L-WSe2.59 This effectwas attributed to the suppression of intervalley scattering ofintralayer excitons within the monolayer with dark excitonicground state.6 However, we observe device-specific saturationfield for ρopt (Bsat) ranging from 0.02 to 3 T (SupportingInformation Figure S9), indicating that it is not defined by theproperties of individual monolayers, but the collective propertyof the assembled LMH.Figure 4a presents the polarization-resolved decay of iX PLrecorded at 0 and 40 mT in a device with Bsat ∼ 200 mT. Thedifference in intensity for cross-co polarization allows us tomonitor |ρopt| as a function of time. Figure 4b plots the time-resolved ρopt extracted from the iX PL decay measured at 0 and40 mT; the solid curves are guides to the eye. Withoutmagnetic field (black filled circles), iX has a low polarizationdegree (|ρopt| < 2%) throughout the measurement range. Incontrast, at B = 40 mT (gray filled circles) |ρopt| starts at 6%and shows a gradual decay toward zero, with a characteristic 1/e time ∼2 μs. These results indicate that the loss of valleypolarization for iX is governed by at least two processesoccurring at different time scales: 1) fast intervalley relaxationwith a characteristic time shorter than 10 ns (i.e., timingresolution of our measurement) dominates at zero magneticfield. 2) This process is suppressed at 40 mT, revealing aslower microsecond-scale relaxation. We note that this timescale directly reflects the loss of valley polarization.In conclusion, we showed that iX in 1L-MoS2/1L-MoSe2 isformed by electrons and holes residing at the edges of theBrillouin zone, with a negligible degree of interlayer hybrid-ization. We find that iX retains its optically induced valleypolarization, with the cross-polarized iX PL stemming from themoire-́induced reversal of selection rules. Magnetic fieldenhances valley-polarization retention by suppressing fastintervalley scattering. The typical magnetic field required forthis (≤200 mT) is within reach of a variety of readily accessibletechniques, including assembling heterostructures on magneticsubstrates60 or using rare-earth magnets.61 In some devices, weobserved |ρopt| up to 7% at zero field, allowing for magnet-freeoperation. The combination of microsecond-long iX PLlifetime and the retention of valley polarization offers theprospect of combining excitonic and valleytronic function-alities in a single optoelectronic device.■ METHODSSample Fabrication. All flakes used for the fabrication ofthe electrically tunable 1L-MoS2/1L-MoSe2 are produced bymicromechanical cleavage of bulk crystals. Bulk TMD crystalsare prepared by a flux zone growth method,62 and bulk hBNcrystals are grown by the temperature-gradient method.63Figure 4. Temporal evolution of iX valley polarization. (a)Polarization-resolved iX PL decay acquired at 0 and 40 mT usingσ+ polarized excitation; blue (red) curve corresponds to PL intensityco- (cross-) polarized with the excitation laser; the data sets are offsetfor clarity and normalized to the intensity of copolarized componentat zero delay. (b) Time-resolved changes of ρopt for B = 0 mT (black)and B = 40 mT (gray). The solid curves are guides to the eye.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c02635Nano Lett. 2024, 24, 11232−1123811235https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c02635?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asGraphite crystals are sourced from NGS. The thickness ofexfoliated crystals is estimated using optical contrast64 andconfirmed using PL and Raman spectroscopy for TMDs under532 nm (2.33 eV) laser illumination (LabRAM HR Evolution,Horiba) and atomic force microscopy for hBN (DimensionIcon, Bruker). Electrically tunable heterobilayer devices areassembled by deterministic dry mechanical transfer usingpolymer stamps.34,35 θ is identified using polarization-resolvedSHG65 measured at RT using a custom-built optical setup. TheSHG laser is a Chameleon Compact Optical ParametricOscillator providing ∼200 fs pulses with a repetition rate of 80MHz centered at 1320 nm. To minimize chromaticaberrations, a linearly polarized laser beam with ∼5 mWpower is focused onto the sample using a 40x reflectiveobjective (numerical aperture of 0.5, LMM40X-P01, Thor-labs). Polarization orientation is controlled using a super-achromatic half-wave plate (SAHWP05M-1700, Thorlabs)mounted in a motorized rotational mount. Electrical contactsto TMD layers and transparent FLG gates are created by directlaser lithography (LW-405B+, Microtech) with a positive resist(AZ5214E, MicroChemicals) followed by electron beamevaporation (PVD200Pro, Kurt J. Lesker) of 5 nm of Crfollowed by 45 nm of Au. The resist excess metal layer is thenlifted off by immersion in acetone and isopropanol for 30 min.Photoluminescence Measurements. RT PL measure-ments are performed using a LabRAM HR Evolution Ramanmicroscope under 532 nm (2.33 eV) laser illumination.Helicity-resolved magneto-optical measurements are done ina close-cycle bath cryostat (Attodry 1000, Attocube) equippedwith a superconducting magnet at a nominal sampletemperature of 4 K. Excitation and collection light passthrough a home-built confocal microscope in reflectiongeometry, with a 0.81 numerical aperture apochromaticobjective (LT-307 APO/NIR/0.81, Attocube). The PLmeasurements are taken using 638 nm (1.94 eV) continu-ous-wave excitation (MCLS1-638, Thorlabs), with incidentpower below 5 μW. The PL signal collected in epi-direction isisolated using a long-pass filter (FELH0700, Thorlabs) anddetected by a 0.75-m spectrometer (SpectraPro 2750,Princeton Instruments) with 150 l mm−1 grating and anitrogen-cooled CCD camera (Spec-10, Princeton Instru-ments). Time-resolved measurements are performed using asingle-photon avalanche photodiode (SPCM-AQRH-16-FC,Excelitas Technologies) and a time-to-digital converter(quTAU, qutools GmbH) with a 81 ps timing resolution.For these measurements, the intensity of the CW laser ismodulated using an acousto-optic modulator (MT350-A0.12-VIS, AA Opto Electronic), producing 200 ns pulses with the100-kHz repetition rate. A dual-channel source meter (2612B,Keithley) is used for electric field tuning.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635.Additional data and analysis, including table with thesummary of device parameters, Raman spectra, identi-fication of stacking configuration, room-temperaturephotoluminescence spectra, second harmonic generationplots, atomic force microscopy characterization, Starktuning for additional devices, summary of photo-luminescence lifetimes, g-factors, and magnetic-fielddependence of optically induced valley polarization forthe six closely aligned devices used in the study (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsEvgeny M. Alexeev − Cambridge Graphene Centre, Universityof Cambridge, CB3 0FA Cambridge, U.K.; CavendishLaboratory, University of Cambridge, Cambridge CB3 0HE,U.K.; orcid.org/0000-0002-8149-6364; Email: ea529@cam.ac.ukMete Atatüre − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.; orcid.org/0000-0003-3852-0944; Email: ma424@cam.ac.ukAndrea C. Ferrari − Cambridge Graphene Centre, Universityof Cambridge, CB3 0FA Cambridge, U.K.; orcid.org/0000-0003-0907-9993; Email: acf26@cam.ac.ukAuthorsCarola M. Purser − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.; CambridgeGraphene Centre, University of Cambridge, CB3 0FACambridge, U.K.Carmem M. Gilardoni − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.; orcid.org/0000-0001-5318-3363James Kerfoot − Cambridge Graphene Centre, University ofCambridge, CB3 0FA Cambridge, U.K.; orcid.org/0000-0002-6041-4833Hao Chen − Cambridge Graphene Centre, University ofCambridge, CB3 0FA Cambridge, U.K.Alisson R. Cadore − Cambridge Graphene Centre, Universityof Cambridge, CB3 0FA Cambridge, U.K.; BrazilianNanotechnology National Laboratory (LNNano), BrazilianCenter for Research in Energy and Materials (CNPEM),13083-849 Sao Paulo, Brazil; orcid.org/0000-0003-1081-0915Bárbara L.T. Rosa − Cambridge Graphene Centre, Universityof Cambridge, CB3 0FA Cambridge, U.K.Matthew S. G. Feuer − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.Evans Javary − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.; École NormaleSupérieure, PSL, Paris 75005, FrancePatrick Hays − Materials Science and Engineering, School forEngineering of Matter,Transport and Energy, Arizona StateUniversity, Tempe, Arizona 85287, United StatesKenji Watanabe − Research Center for Electronic and OpticalMaterials, National Institute for Materials Science, Tsukuba305-0044, JapanTakashi Taniguchi − Research Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Seth Ariel Tongay − Materials Science and Engineering,School for Engineering of Matter,Transport and Energy,Arizona State University, Tempe, Arizona 85287, UnitedStates; orcid.org/0000-0001-8294-984XDhiren M. Kara − Cavendish Laboratory, University ofCambridge, Cambridge CB3 0HE, U.K.Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.4c02635Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c02635Nano Lett. 2024, 24, 11232−1123811236https://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c02635/suppl_file/nl4c02635_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Evgeny+M.+Alexeev"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-8149-6364mailto:ea529@cam.ac.ukmailto:ea529@cam.ac.ukhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mete+Atatu%CC%88re"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3852-0944https://orcid.org/0000-0003-3852-0944mailto:ma424@cam.ac.ukhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Andrea+C.+Ferrari"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-0907-9993https://orcid.org/0000-0003-0907-9993mailto:acf26@cam.ac.ukhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carola+M.+Purser"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carmem+M.+Gilardoni"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-5318-3363https://orcid.org/0000-0001-5318-3363https://pubs.acs.org/action/doSearch?field1=Contrib&text1="James+Kerfoot"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-6041-4833https://orcid.org/0000-0002-6041-4833https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hao+Chen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alisson+R.+Cadore"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-1081-0915https://orcid.org/0000-0003-1081-0915https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ba%CC%81rbara+L.T.+Rosa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Matthew+S.+G.+Feuer"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Evans+Javary"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Patrick+Hays"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-1467-3105https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Seth+Ariel+Tongay"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-8294-984Xhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Dhiren+M.+Kara"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c02635?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c02635?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asNotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSWe acknowledge funding from the EU Graphene andQuantum Flagships, ERC Grants Hetero2D, GSYNCOR,GIPT, EIC Grant CHARM, and EPSRC Grants EP/K01711X/1, EP/K017144/1, EP/N010345/1, EP/L016087/1, EP/X015742/1, EP/V000055/1, The Netherlands Organ-isation for Scientific Research (NWO 019.221EN.004,Rubicon 2022-1 Science), DOE-SC0020653 (materials syn-thesis), JSPS KAKENHI (Grant Nos. 21H05233 and23H02052) and World Premier International Research CenterInitiative (WPI), MEXT, Japan. 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