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Joseph Wragg, Luca Bolzonello, Ludovica Donati, Karuppasamy Pandian Soundarapandian, Riccardo Bertini, Seth Ariel Tongay, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Frank H. L. Koppens, Niek F. van Hulst

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[Dual Action Spectroscopy Exposes the Bright and Dark Excitons of Room-Temperature WSe<sub>2</sub>](https://mdr.nims.go.jp/datasets/864693db-bcd0-4d24-a2e4-941c70bd5c6d)

## Fulltext

Dual Action Spectroscopy Exposes the Bright and Dark Excitons of Room-Temperature WSe2Dual Action Spectroscopy Exposes the Bright and Dark Excitons ofRoom-Temperature WSe2Joseph Wragg,* Luca Bolzonello, Ludovica Donati, Karuppasamy Pandian Soundarapandian,Riccardo Bertini, Seth Ariel Tongay, Kenji Watanabe, Takashi Taniguchi, Frank H. L. Koppens,and Niek F. van Hulst*Cite This: Nano Lett. 2025, 25, 7658−7664 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: While it has long been accepted that the role ofmomentum dark excitons in the photoresponse of transition metaldichalcogenides (TMDs) is critical, their weak optical signatureinhibits their study through conventional means. Here we expose theroom-temperature contributions of both bright and dark excitons tothe behavior of a TMD, WSe2, from monolayer to multilayer to bulk.To do so, we present dual action spectroscopy, a photocurrent- andluminescence-detected Fourier-transform excitation spectroscopyscheme, to microscopically map the energy landscape of WSe2.While bright excitons naturally dominate the luminescence responseof the material, dark excitons dominate the current response.Notably, the dark KK′ exciton is more accessible than the groundstate KΛ, while current maps reveal a disparity in the diffusivity ofthe two states. This work provides the basis for a new, current-detected approach to study the dynamics of dark exciton states acrossdifferent materials.KEYWORDS: transition metal dichalcogenides, 2D materials, action spectroscopy, energy transport, dark excitonsThe physics of bound electron−hole pairs, or excitons, andtheir role in energy transfer carries importance acrossseveral disciplines of science.1−3 Their influence on materialproperties is particularly pronounced in transition metaldichalcogenides (TMDs), a class of two-dimensional materi-als.4 TMDs owe their remarkable electronic properties tostable excitons which exhibit binding energies in the region of0.5 eV.5−7 The planar nature of these excitons, that are largelyconfined to individual TMD layers,8−10 makes them not onlyideal candidates for novel optoelectronic devices but alsouseful platforms for the study of exciton behavior.11−13 Oncean exciton is created, what it does with its energy determinesthe photoresponse of the material. Short lifetimes14 followedby radiative recombination of excitons is prevalent inmonolayer (ML) TMDs.5,15 In the multilayer material,specifically for WSe2 which we study here, this photo-luminescence (PL) is diminished.16−18The loss of luminescence efficiency raises the question: whathappens to the excitons that go “dark”? The decrease inradiative recombination points to the material’s multivalleyband structure that induces myriad allowed and forbiddenexciton states.9,19 Such states unlock a new regime of excitonphysics in TMDs, where lifetimes move from the pico- to thenanosecond20,21 and diffusion lengths move from the nano- tothe micrometer.22 Indeed, great effort has gone into under-standing these forbidden, or dark, excitons as the significanceof their role in the photoresponse of TMDs has becomeincreasingly clear. These efforts include cryogenically emptyingthe population of higher energy bright states to observe darkexciton PL,19−26 application of in-27−30 and out of-plane31fields to induce spin state mixing, and nanopatterning of theTMD substrate to induce strain in the material, encouragingrecombination of the dark exciton.32,33 Understanding darkstates can inform the realization of valleytronic and spintronicdevices with these materials.34,35Here we present dual action spectroscopy, experimentallyseparating the contributions of dark and bright excitons to thephotoresponse of WSe2 at room temperature. We diagnose thevalley of origin of each dark exciton and infer their role in thetransport of energy through the material. This is achieved bysimultaneously measuring the PL and photocurrent (PC)response of a WSe2 flake of multiple thicknesses, from theReceived: December 11, 2024Revised: March 28, 2025Accepted: March 28, 2025Published: April 4, 2025Letterpubs.acs.org/NanoLett© 2025 The Authors. Published byAmerican Chemical Society7658https://doi.org/10.1021/acs.nanolett.4c06349Nano Lett. 2025, 25, 7658−7664This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on May 21, 2025 at 11:21:38 (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="Joseph+Wragg"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Luca+Bolzonello"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ludovica+Donati"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Karuppasamy+Pandian+Soundarapandian"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Riccardo+Bertini"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Riccardo+Bertini"&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="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="Frank+H.+L.+Koppens"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Niek+F.+van+Hulst"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Niek+F.+van+Hulst"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.4c06349&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/25/19?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/19?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/19?ref=pdfhttps://pubs.acs.org/toc/nalefd/25/19?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c06349?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-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/monolayer to bulk, to retrieve the spatially resolved excitationspectrum of the TMD through both observation channels. Thecombination of the two detection mechanisms enables theseparation of bright and dark excitons due to their respectivedominance in the PL and PC response. Naturally, radiativerecombination in the material is representative of the brightexciton response, which is shown in the PL excitationspectrum. Conversely, the more stable, but less abundant,dark excitons are more likely to contribute to the PC generatedin the material, that requires exciton diffusion over micro-meters. This natural filtering of the bright exciton contributionenables the observation of these dark states that, due to theirlow yield (<1 × 10−6), evade detection in typical experimentalapproaches such as conventional absorption and differentialreflectance. Probing the excitation spectrum in both domainsalso offers insight into the states that are directly accessiblewith optical stimulation instead of those to which the systemrelaxes as in conventional PL spectroscopy. We hope that theinsight we provide can be of use to the community in thedesign of devices using these materials going forward.Although many of the plethora of charge and energy carryingstates available in WSe2 at cryogenic temperatures aredestroyed by phonons at room temperature, the multivalleyform of the TMD’s conductance band means that many tightlybound exciton states can still exist. Spin−orbit coupling, as wellas weak intralayer dielectric screening, lead to a complex bandstructure. A heavily simplified version of the diagram, showingthe excitonic states that will be relevant for this study, iscompiled from the theoretical calculations of refs 9, 10, and 36in Figure 1.Across both conductance and valence bands, spin−orbitcoupling induces splitting between the two electronic spinstates throughout the first Brillouin zone.37 In the valenceband, the splitting is so pronounced (100s of meV10) that forthe band edge transitions that we are able to resolve here, weonly need to consider one spin configuration. For the sake ofFigure 1, we show this configuration as spin ↑. There are anumber of transitions available to an electron sitting at thevalence band maximum (VBM) of the monolayer, located atthe ↑K high symmetry point. Of these transitions, only onenear the band edge is allowed. This is the bright exciton,29shown in orange in Figure 1. It is responsible for the large PLresponse of monolayer WSe2. The other three transitionsshown all represent dark excitons. The first, shown as a hollowred ellipse, is the spin forbidden transition to the ground stateof the K point. The final two, shown as purple ellipses, are themomentum-forbidden transitions of the electron across the k-space, with valleys lying at the Λ and K′ points.Adding extra layers to the material changes the bandstructure. Although excitons remain largely confined to a singlelayer, overlap of the out-of-plane orbital components withneighboring layers relaxes the energy of the conductance bandedge across the Brillouin zone.10 The introduction of newmagnetic fields from surrounding layers specifically relaxes thehigher energy spin states at the K and K′ points, bringing thetwo spins close to degeneracy. This shift is indicated by dottedlines in Figure 1. The largest relaxation occurs at the Λsymmetry point, which incurs a red shift of ∼200 meV relativeto the VBM from its monolayer value. This makes the dark KΛtransition the ground state exciton for bulk WSe2. The shiftrepresents the origin of the PL suppression in multilayerTMDs, as electrons excited into the conductance band fromthe VBM are far more likely to relax into the energeticallylower valleys away from the K point, where the correspondinghole remains.Although defining the shift in the ground state exciton is auseful way of defining the behavior of the semiconductor, all ofthe valleys across the conduction band can provide a stableFigure 1. Monolayer to bulk WSe2, bright to dark excitons. Band structure and corresponding Brillouin zones of monolayer (solid lines) and bulk(dashed lines) WSe2 using data compiled from refs 9, 10, and 36. The spin states of each band are represented in red and blue, indicating whether atransition between states is spin allowed or forbidden. Notable exciton states are outlined with ellipses. The only allowed transition is bright KKshown in orange, which accounts for the observable fluorescence response of the material. All other excitons shown here are forbidden. The groundstate of monolayer WSe2 is the spin-forbidden K↑K↓ exciton (red hollow ellipses). Moving to the bulk, conduction band relaxation at the Λsymmetry point means the momentum-forbidden (purple hollow ellipses) KΛ exciton becomes the ground state, while throughout all layers theKK′ (purple hollow ellipses) maintains its energy shift relative to the VBM, due to offsetting effects of spin relaxation across inverse points in the k-space.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c06349Nano Lett. 2025, 25, 7658−76647659https://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c06349?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asbasis for exciton formation and propagation. Here, we untanglethe roles of the bright KK exciton, dark KK′ exciton, and darkKΛ exciton toward the photoresponse of WSe2 at roomtemperature. To do so, we use microscopic broad-band Fouriertransform excitation spectroscopy to spatially resolve theexcitation spectrum of the material through two detectionpathways.A simplified schematic of the experimental approach isshown in Figure 2d. A WSe2 staircase flake (a flake starting atmonolayer thickness and increasing in steps up to bulk values)was mounted on a sapphire substrate across two goldelectrodes (Figure 2a) and encapsulated with hBN. To obtainspectra, the frequency of a broad-band pulse was modulated inthe Fourier domain. The PC response to illumination wasmonitored with a lockin amplifier where excitons are onlydetected once they reach the contact between the device andthe active electrode, i.e., the electrode that supports conditionsfor charge tunneling and separation (more information inSupporting Information Figure 3). This means PC is onlydetected for excitons that diffuse from the point of illuminationto the active electrode. PL was measured with a photoncounting camera in transmission using a long-pass filter toblock the illumination beam. The illumination was focused to a750 nm spot and scanned across the flake with galvo mirrors sothe spectrum of the WSe2 could be resolved with microscopicspatial resolution. Maps, like the ones shown in Figure 2g,h,were then constructed for both detection mechanisms. Eachpixel of these maps has a corresponding excitation spectrum forthat specific part of the flake, indicated in Figure 2e,f. For thoseinterested in reproducing the results here or using theexperimental technique, a detailed explanation of theprocedure is given in the Supporting Information. For all ofthe spectra presented here, true data points are shown, as wellas a curve retrieved through zero padding and frequencyfiltering in the Fourier domain. Full descriptions of the datahandling and the spectral acquisition process are also availablein the Supporting Information.The layer number of each step in the staircase is determinedthrough absorption. The enhanced fluorescence in the thinnestsection of the sample (Figure 2g) means we can confidentlyassign this section as a monolayer and use this absorption as areference for the other layers. The absorption coefficients,plotted in Figure 3a, reveal the steps as 3, 6, and 12 layers (L)thick.Spectra recovered in PL and PC show a clear distinctionbetween exciton states more likely to recombine and statesmore likely to produce photocurrent in WSe2. Figure 3b,cdemonstrates this difference in the spectral ranges of 1.6−2.1eV (780−590 nm), specifically at the band edge between1.6−1.7 eV, that contains the transition energies between theVBM and the three conduction band valleys for the monolayer.In PL, a 40 meV red shift is visible with increasing thickness,shown in the inset of Figure 3c. The shift is indicative ofrelaxation of the bright state as splitting between spin statesdecreases at the K points. The red shift in PL is accompaniedby a decrease in amplitude through 3L, 6L, and 12L. This isconsistent with the quenching that occurs at multilayerthicknesses as electrons are funnelled toward the lower energyregions of the k-space, away from the allowed transition.One such forbidden state dominates the exciton response inPC. Shown in Figure 3b, the KK′ exciton peak (marked with adashed line) remains fixed and red-shifted by ∼30 meV fromthe bright state in the monolayer throughout all thicknesses.Although all K points experience a shift in energy fromintroducing layers, the counteraction of spin−orbit coupling ofelectrons in the K′ minima and holes in the K maxima cancelsout any overall change in the KK′ resonance. Access to themomentum forbidden states in excitation is enabled throughcoupling to phonon modes at room temperature. The sameaccess cannot be gained to the spin-forbidden exciton, as thelikelihood of simultaneous photon absorption and spin flip isvanishingly small. We can conclude from these initial spectrathat the photocurrent response of WSe2 is dominated bymomentum dark excitons. The forbidden transition requiredfor recombination ensures longer lifetimes and a significantlyFigure 2. Separating bright and dark excitons through dual action spectroscopy mapping. A staircase flake, with multiple steps of increasingthickness from monolayer to bulk (bright field image in (b)), was mounted across two gold electrodes (a) on a sapphire substrate and encapsulatedin hBN. It was ensured that a part of each thickness region made contact with both electrodes; monolayer contact is confirmed with fluorescenceimaging in (c). The area of the sample that was mapped in this work is marked by dashed lines in (a), (b), and (c). A simplified experimentalapproach is shown in (d). Using a broad-band laser, the flake is illuminated with a 750 nm spot. The photoresponse is then measured by capturingboth the fluorescence and photocurrent generated by the laser as the frequency components of the pulse are modulated. The excitation spectra arerecovered through Fourier transform, and the process is repeated for each 1 × 1 μm portion of the flake to provide spatial resolution. Theamplitude of the spectrum measured at each point is mapped in fluorescence (g) and photocurrent (h). The area of the flake that bridges theelectrodes is cropped from (b). and thickness steps are labeled in (e). The same area of the sample is shown through bright-field microscopy in (f).Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c06349Nano Lett. 2025, 25, 7658−76647660https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c06349/suppl_file/nl4c06349_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c06349/suppl_file/nl4c06349_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.4c06349/suppl_file/nl4c06349_si_002.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c06349?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asgreater chance to diffuse across the device. Lifetime may not bethe only factor, as previous studies have established that theorbital character of each valley can vary dramatically, where in-plane orbitals (dxy, dx2−y2), that are expressed more in themomentum-dark valleys,10 can contribute more effectively totransport. Naturally an increase in transport increases thelikelihood of detection through PC, as the further an excitondiffuses, the higher the chance it has of reaching the potentialbarrier at the active electrode.At this laser fluence (∼1000 μJ cm−2) saturation occurs inthe state from 6 to 12 layers. An increase in signal is expectedwith added layers simply due to the increase in absorption, asobserved in the blue part of each spectra, whereas the KK′transition approaches full saturation faster. We assign thiseffect to the enhanced dispersion efficiency that higher-energyexcitations can access as they vibrationally relax to aconduction band minimum. Faster dispersion out of theinteraction area would lead to a slower approach to saturationconditions. As alluded to in Figure 1, the CBM of multilayerWSe2 lies at the Λ symmetry point, 200 meV below the KK′resonance. In order to inspect the bulk ground state darkexciton KΛ, we shifted the spectral excitation window to 1.4−1.7 eV (885−729 nm). Moving into this spectral windowmeans that PL is no longer measurable, as the signal atwavelengths longer than 885 nm is below a detectable level atroom temperature. The fluence was decreased to ∼50 μJ cm−2in these measurements to avoid saturation of the KK′ stateseen in Figure 3b. The fluence used in the PC-PLmeasurements was necessary to achieve the required signal-to-noise ratio in PL.The excitation spectrum from 1.4 to 1.7 eV for each step ofthe flake is shown in Figure 4a. All multilayer steps showexcitation peaks representing two momentum-forbidden states,the KΛ exciton at ∼1.46 eV and the KK′ exciton at 1.64 eV.The absence of the KΛ resonance in the monolayer spectrumis due to the dark states’ degeneracy in 1L, in agreement withprevious study.8 The small redshift of 10 meV observed in theposition of the ground state from 3L to 6L reflects furtherinterlayer orbital overlap at multilayer thicknesses, where theseeffects become less pronounced as the band structure movescloser to that of the bulk material. There is significantly moreoff-resonance excitation in classical absorption spectra (Figure4c) than in action spectra, obscuring the shape of the excitonpeaks. The positions of all three excitons recorded across theexcitation spectra are marked on the plot. The competitionbetween the KK and KK′ valleys is, for the most part, obscuredby their near degeneracy at multilayer thicknesses. In themonolayer, two transitions are visible, while absorption in 3L,6L, and 12L thicknesses falls off below 1.46 eV, inferring KΛ isthe ground state exciton.Notably, the inequality of amplitudes between the KΛ andKK′ peaks across both spectral detection mechanisms indicatesa greater probability of absorption into the K′ valley than theΛ. The curvature and width of each valley of interest willdefine the range of phonon momenta and photon energy withwhich an electron lying at the K point of the valence band caninteract to access the conduction band valleys. The stability ofeach of these excitons can be gauged by the spatial dependenceof their signal, relative to the active electrode. The amplitude ofthe overall PC signal is highly dependent on which electrode isused as active, an effect that is visible in Figure 4b. Leveragingthis property, the spatial dependence of the spectra at eachthickness is investigated in Figure 5.The distance dependence of the PC excitation spectrumacross the different thickness (Figure 5a,d) steps demonstratesthe inequality in the stability between the two exciton states.The amplitude of the two exciton peaks with respect toFigure 3. Absorption by step (a) and PC (b) and PL (c) excitationspectra for all 4 regions of the WSe2 flake. In PL, the expected shift inboth the spectral position and intensity of the bright exciton isobserved (shown in the inset), as interlayer orbital overlap induces ared shift of the direct transition along with a switch in the groundstate of the conduction band. In PC, we observe that the main excitonpeak (marked with purple dashed line) directly corresponds to theliterature value for the red shift of the dark KK′ exciton.9 No shift isobserved in this exciton with layer thickness, while the amplitude ofthe peak saturates at this laser power (∼20 μW) above the bilayerthickness.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c06349Nano Lett. 2025, 25, 7658−76647661https://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c06349?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asFigure 4.Momentum dark excitons of WSe2. Appearance of the ground state dark exciton, KΛ, near 1.46 eV above monolayer thicknesses is shownin the PC signal of each region of the staircase, shown in (a). Absence of the peak in the monolayer region of the flake confirms the single WSe2layer, while we see a smaller red shift of the KΛ peak with thickness (shown inset) than that seen in PL.18 The spectra in (a) are produced byaveraging the signal from each region of the flake as shown in the top PC map in (b). Evidence of the position dependence of the signal is shown byflipping the location of the active electrode. Absorption measurements for each sample region (c) show that off-resonance absorption into excitonstates is heavily filtered in the action spectra, while the peak of absorption follows the resonance of the bright exciton for each step.Figure 5. Diffusion of dark KΛ and KK′ excitons. Mapping the sample spectrally (a,d) allows analysis of not only the spectra at different samplethicknesses but also the way the peaks change as excitons need to travel further to undergo charge separation. By isolating a cross section in the 12layer part of the flake (marked with a dashed line in (b)) and stacking the plots (a), the spectral transformation is clear when moving the point ofillumination away from the active electrode. Overall the amplitude of both states increases toward the electrode. The normalized slope of thisincrease is shown for each exciton in (c) for 12 L and (e) for all others. The KΛ exciton is lower in amplitude across all spectra than the KK′, butits spatial decay is lower, implying greater stability and more effective diffusion. Moving to fewer layers, the stability of both excitons across thedevice increases, with almost no change in amplitude in KΛ in 3L and KK′ in the monolayer, respectively.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.4c06349Nano Lett. 2025, 25, 7658−76647662https://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349?fig=fig5&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.4c06349?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aselectrode distance is normalized and plotted in Figure 5c for12L and Figure 5e for all other thicknesses. Although theground state has a smaller overall contribution to the PCsignal, it is more stable than the KK′ exciton in space.Extrapolating from the lines drawn in Figure 5c, we canestimate that the photocurrent signal could be drawn fromeach exciton peak over distances of 10 and 20 μm for KK′ andKΛ, respectively. Although enhanced exciton−exciton annihi-lation in the more densely populated valley may contribute tothe inequality, both the linear nature of the signal degradationand increase in stability of both states at fewer layers implyannihilation is not dominant in the decay. The KΛ amplitudeis almost unchanged over the device at a 3L thickness. Theapparent stability but small amplitude of the KΛ exciton pointstoward the inaccessibility of the electronic transition neededfor both recombination and excitation. Theoretical work onmonolayer WSe2 predicts different diffusion constants fordifferent effective valley masses,38 which could also point to thebehavior that we see here. This study predicted intervalleycoupling in the monolayer at room temperature, leading to anaveraging of the diffusion constant, given the near degeneracyof the three valleys in the single-layer structure of the material.Probing deeper into the bulk, we observe that the energeticbarriers introduced in the multilayer band structure inhibit thesame degree of intervalley coupling, which presents anopportunity to use such a difference in diffusivity. Thisproperty opens the door for many promising applications innovel electronics, leveraging the valley polarization available inthese materials, over lengths far beyond what’s needed inmodern devices. Room-temperature measurements of valleybias in TMDs bring the potential of utilizing valleytronics inworking devices away from the confines of cryogenicconditions. Moving to an environment rich in thermal energy,we have shown that while the myriad of states available in thelow temperature regime disappears, the momentum forbiddendark excitons not only remain stable at room temperature butdominate the photoelectric response of the material over manymicrometers.We have presented a novel technique for the detection anddiagnosis of energy transfer in photoactive materials. Wedemonstrate that in the presence of both bright and darkexcitons, the photocurrent response of WSe2 is dominated bythe dark KK′ state, which acts as the main state for thepropagation of energy more generally. We also observed thatalthough the ground state exciton, KΛ, contributes less to theexciton population, its inaccessibility in transition leads tostability, which manifests into diffusion over tens of micro-meters. The relative simplicity of the experimental approachpaves the way for further studies of energy transfer inphotoactive materials, which brings the reality of valleytronicsapplications a step closer.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.4c06349.■ AUTHOR INFORMATIONCorresponding AuthorsJoseph Wragg − ICFO - Institut de Ciencies Fotoniques, TheBarcelona Institute of Science and Technology, Castelldefels,Barcelona 08860, Spain; Email: joseph.wragg@icfo.euNiek F. van Hulst − ICFO - Institut de Ciencies Fotoniques,The Barcelona Institute of Science and Technology,Castelldefels, Barcelona 08860, Spain; ICREA, InstitucióCatalana de Recerca i Estudis Avançats, Barcelona, Spain.08010; orcid.org/0000-0003-4630-1776;Email: niek.vanhulst@icfo.euAuthorsLuca Bolzonello − ICFO - Institut de Ciencies Fotoniques,The Barcelona Institute of Science and Technology,Castelldefels, Barcelona 08860, Spain; orcid.org/0000-0003-0893-5743Ludovica Donati − LENS, European Laboratory for Non-Linear Spectroscopy, 50019 Sesto Fiorentino FI, ItalyKaruppasamy Pandian Soundarapandian − ICFO - Institutde Ciencies Fotoniques, The Barcelona Institute of Scienceand Technology, Castelldefels, Barcelona 08860, Spain;orcid.org/0000-0002-9664-9095Riccardo Bertini − ICFO - Institut de Ciencies Fotoniques,The Barcelona Institute of Science and Technology,Castelldefels, Barcelona 08860, SpainSeth Ariel Tongay − SEMTE, The School for Engineering ofMatter, Transport and Energy, Arizona State University,Tempe, AZ 85287, United States; orcid.org/0000-0001-8294-984XKenji Watanabe − National Institute for Materials Science,Tsukuba 305-0047 Ibaraki, Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − National Institute for Materials Science,Tsukuba 305-0047 Ibaraki, Japan; orcid.org/0000-0002-1467-3105Frank H. L. Koppens − ICFO - Institut de CienciesFotoniques, The Barcelona Institute of Science andTechnology, Castelldefels, Barcelona 08860, Spain; ICREA,Institució Catalana de Recerca i Estudis Avançats, Barcelona,Spain. 08010; orcid.org/0000-0001-9764-6120Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.4c06349NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSJ.W., L.B., and N.F.v.H. acknowledge support through theMCIN/AEI projects PID2021-123814OB-I00, TED2021-129241BI00, the ”Severo Ochoa” program for Centres ofExcellence in R&D CEX2019-000910-S, Fundacio PrivadaCellex, Fundacio Privada Mir-Puig, and the Generalitat deCatalunya through the CERCA program. N.F.v.H. acknowl-edges financial support from the European Commission (ERCAdvanced Grant 101054846-FastTrack). This work is part ofthe ICFO Clean Planet Program supported by Fundació JoanRibas Araquistain (FJRA).■ REFERENCES(1) Baldereschi, A.; Lipari, N. C. 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