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Arka Karmakar, Tomasz Kazimierczuk, Igor Antoniazzi, Mateusz Raczyński, Suji Park, Houk Jang, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Adam Babiński, Abdullah Al-Mahboob, Maciej R. Molas

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[Excitation-Dependent High-Lying Excitonic Exchange <i>via</i> Interlayer Energy Transfer from <i>Lower</i>-<i>to</i>-<i>Higher</i> Bandgap 2D Material](https://mdr.nims.go.jp/datasets/11927edc-8c67-4182-a3ea-ec50eda13ca0)

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Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D MaterialExcitation-Dependent High-Lying Excitonic Exchange via InterlayerEnergy Transfer from Lower-to-Higher Bandgap 2D MaterialArka Karmakar,* Tomasz Kazimierczuk, Igor Antoniazzi, Mateusz Raczynśki, Suji Park, Houk Jang,Takashi Taniguchi, Kenji Watanabe, Adam Babinśki, Abdullah Al-Mahboob,* and Maciej R. Molas*Cite This: Nano Lett. 2023, 23, 5617−5624 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: High light absorption (∼15%) and strong photoluminescence (PL)emission in monolayer (1L) transition metal dichalcogenides (TMDs) make themideal candidates for optoelectronic device applications. Competing interlayercharge transfer (CT) and energy transfer (ET) processes control the photocarrierrelaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ETcan survive up to several tens of nm, unlike the CT process. Our experiment showsthat an efficient ET occurs from the 1Ls WSe2-to-MoS2 with an interlayerhexagonal boron nitride (hBN), due to the resonant overlapping of the high-lyingexcitonic states between the two TMDs, resulting in enhanced HS MoS2 PLemission. This type of unconventional ET from the lower-to-higher optical bandgapmaterial is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increasedelectron−phonon scattering, destroying the enhanced MoS2 emission. Our work provides new insight into the long-distance ETprocess and its effect on the photocarrier relaxation pathways.KEYWORDS: 2D material, MoS2, WSe2, heterostructure, energy transfer, band-nestingGroup-VI semiconducting transition metal dichalcoge-nides (TMDs) are formed by stacking of stronglybonded two-dimensional (2D) X-M-X layers (M = transitionmetals such as Mo, W, etc. and X = chalcogens such as S, Se,Te, etc.), which are separated by weak bond interlayer van derWaals forces.1 The first mechanical exfoliation of themonolayer (1L) molybdenum disulfide (MoS2) film from abulk crystal in 2010 led us to observe a strong photo-luminescence (PL) emission2,3 due to the indirect-to-directbandgap transition from the bulk-to-1L regime.4,5 Since then,researchers have been exploring exciting excitonic proper-ties6−11 in these 1L TMD materials. In particular, their stronglight−matter interactions and high light absorption of up to∼15% in the solar spectrum12 enabled researchers to realizethe future prospects of 1L TMD-based optoelectronic deviceapplications.13 2D heterostructures (HSs) made by the verticalstacking of different layered materials have shown promisingresults for future ultrathin14−16 and flexible17 optoelectronicdevice applications. Recent advances in direct and patternedgrowth of 2D HSs18,19 to obtain a clean large-area interfacehave also pushed the effort to make commercially availableTMD-based device applications. However, one of the majorchallenges in commercializing the promised optoelectronicdevice applications is the lack of a comprehensive under-standing of the competing interlayer processes, such as theinterlayer charge transfer (CT) and energy transfer (ET)processes, and their roles in the photocarrier recombinationmechanism.CT and ET are the two main carrier relaxation pathways inthe semiconductor HSs. The interlayer CT occurs due to anenergy band offset in the HS,20 and the interlayer ET processhappens when nonradiative energy from the excited donormaterial gets transferred to the acceptor material accompaniedby a fluorescence emission from the acceptor material.21,22 ETis observed as a reduction of the donor fluorescence intensityand carrier lifetime followed by an enhancement of theacceptor fluorescence intensity.22 The interlayer CT can bestopped by placing a thin layer of dielectric material in betweenthe two semiconductors. Britnell et al.23 showed that only fouratomic-layer thick hexagonal boron nitride (hBN) is sufficientas a dielectric medium to block the electron tunneling betweenthe two graphene layers. Unlike the CT process, in TMD HSsthe long-distance interlayer ET process can survive up toseveral tens of nm.24,25 Separating the materials far apart fromeach other to stop the ET process is not practical for ultrathindevice design. Thus, developing a comprehensive under-standing of the long-distance interlayer ET process is anabsolute necessity to create practical device applications.Received: March 24, 2023Revised: May 8, 2023Published: June 8, 2023Letterpubs.acs.org/NanoLett© 2023 The Authors. Published byAmerican Chemical Society5617https://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−5624Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 1, 2023 at 00:34:13 (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="Arka+Karmakar"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomasz+Kazimierczuk"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Igor+Antoniazzi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mateusz+Raczyn%CC%81ski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Suji+Park"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Houk+Jang"&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="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Adam+Babin%CC%81ski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Abdullah+Al-Mahboob"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Maciej+R.+Molas"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.3c01127&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/23/12?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/12?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/12?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/12?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://acsopenscience.org/open-access/licensing-options/In this work, we study the effect of resonant overlapsbetween the high-lying excitonic states of 1Ls tungstendiselenide (WSe2) and MoS2 on the interlayer ET processwith a ∼9 nm thick hBN charge-blocking layer. Both theseTMD materials have overlapping higher energy B and C(MoS2)/D (WSe2) absorption features.26,27 We show thatresonant excitations at the WSe2 B and D absorption regionsresult in MoS2 PL enhancement in the HS area. We report thatthis PL enhancement is due to the interlayer ET process fromthe WSe2-to-MoS2 layer. This type of ET process from thelower-to-higher optical bandgap material is not typical in theTMD HSs, since ET conventionally happens from the higher-to-lower bandgap 2D materials.28−33 In this work, we employmultiple optical spectroscopic techniques at cryogenic temper-ature (8 K); such as μ-PL, μ-photoluminescence excitation(PLE), and differential reflectance contrast (RC), comple-Figure 1. (a) Optical micrograph of the HS. Inset is the schematic illustration of the sample cross-section. The entire MoS2 layer is placed on thesame hBN thickness. (b) Differential reflectance contrast (RC) spectra from the three areas on the sample taken at 8 K. Shaded areas indicate thehigher energy excitonic resonances between MoS2 and WSe2. HS shows the characteristic lower energy absorptions from both the WSe2 (AW) andMoS2 (AM) layers. (c) Single-particle band structure of MoS2 and WSe2 along the Γ-K direction indicating the different optical transitions. Opticalbandgaps were matched with the PL energies. C and D absorption peaks are the results of the “band-nesting” in the Brillouin zone.Figure 2. (a−b) PLE maps of the HS and MoS2 area with the same intensity range taken at 8 K. The WSe2 emission intensity in the HS map is keptsaturated to visualize the MoS2 emission. MoS2 shows a pronounced emission in the HS area. (c−d) (MoS2 in) HS and MoS2 PL emissionintensities at 2.85 and 2.12 eV excitation energies, respectively (along the horizontal dotted lines in parts (a−b). Under both the excited energies,MoS2 emissions in the HS are significantly enhanced as compared to the 1L area. (e) Comparison of the HS and MoS2 excitation profiles at 1.92 eVemission energy (along the vertical solid lines in part (a−b)). Overall MoS2 shows enhanced PLE intensity in the HS area.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−56245618https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asmented by the density functional theory (DFT) calculation ofspin-resolved band structures to study the ET process. Ourwork reveals an unconventional interlayer ET process in theTMD HSs. This will significantly contribute to creating acomprehensive understanding of the long-range interlayer ETprocess and its role to influence the photocarrier radiativerecombination processes in these semiconducting HSs.Figure 1a shows the optical micrograph of the fabricatedMoS2-hBN-WSe2 HS (see the Methods for fabrication details).The inset of Figure 1a shows the schematic illustration of thecross-section of the sample. We introduce a ∼9 nm thickinterlayer hBN (see Supplementary Figure S1) to eliminateany effect related to the interlayer CT in our system.23 Theoptical absorption of the TMD materials reflects their single-electron energy band structure. The low temperature RCspectra (see the Methods for details) measured at 8 K showstrong overlaps between the B peaks of both materials and ofthe WSe2 D peak with the MoS2 C peak (shaded areas inFigure 1b), which agrees well with the previously publishedreports.26,27 In the later sections, we discuss how these strongoverlaps help us to observe the reported ET from the lower-to-higher bandgap (WSe2-to-MoS2) material. The HS spectrum(Figure 1b) shows similar RC resonance positions ascompared to the individual 1Ls, indicating no major strain-induced effect34 in the HS area. A and B excitonic peaks occurdue to the excitonic transitions at the K+/K− valley in the k-space,2,3 and higher energy excitonic transitions, such as C andD, are the results of the “band-nesting”35,36 in the Brillouinzone. “Band-nested” regions occur due to the identicaldispersion in the valence (VB) and conduction (CB) bandsover a region in the Brillouin zone due to the strong van Hovesingularities. “Band-nesting” regions in TMDs are particularlyinteresting as the photogenerated electrons and holespropagate with the same but opposite velocities in CB andVB bands, respectively.35 For 1L MoS2, both the VB maximumand the CB minimum are located at the K+/K− point in theBrillouin zone.2 In the case of WSe2, while the VB maximum islocated at the K+/K− point, the CB minimum is situated at theΛ point.37,38 The “band-nesting” region happens in betweenthe Γ and Λ points.35,36 Figure 1c shows the DFT calculatedelectronic band structures (see Supporting Information fordetails) along the Γ-K+ direction in the Brillouin zone. Forboth the band structures, we match the optical bandgaps withthe corresponding PL energies. All types of optical transitionsare shown with different colors of arrows (Figure 1c).PLE maps (see the Methods for the experimental details)taken at 8 K show the emission landscapes of the threeindividual areas (Figures 2a−2b and S2−S3). In Figure 2a, wesaturate the WSe2 emission intensity in order to visualize theMoS2 emission. After comparing the MoS2 emission intensities,we observe a significantly enhanced MoS2 PL in the HS area ascompared to the 1L region (Figures 2a−2b). The horizontalcuts at the excitation energies of 2.85 and 2.12 eV (blackdotted lines in Figures 2a−2b) reveal that the MoS2 PLemission in the HS is enhanced by a factor of ∼1.9 and ∼1.7,respectively, as compared to the 1L area (Figures 2c−2d). ThePL enhancement factor is defined here as the ratio of PLintensity in the HS area to the 1L area under the sameexcitation and accumulation conditions. Similarly, the PLE(vertical cut along the 1.92 eV emission energy in Figures2a−2b) shows an overall increase of the HS MoS2 PL emissionthroughout the entire excitation range as compared to the 1LMoS2 region (Figure 2e). It is important to mention that thetotal optical absorption in the HS area did not change much ascompared to each 1L area (Figure 1b). However, theenhancement in the HS PLE (Figure 2e) suggests that theinternal PL efficiency of the HS system was increased due tothe ET process. We note that the below-bandgap pronouncedemission from the MoS2 defect states (Figure 2b) is typical forthe exfoliated and nonencapsulated samples.39 We would alsolike to mention that in the HS PLE map we also see anenhancement in the WSe2 excitonic emission (Figures S2−S3)due to the conventional ET from the MoS2-to-WSe2 layer. Wedid not include any discussion related to the enhancement ofthe WSe2 PL in this study, as a similar type of ET has alreadybeen reported in a previous work.28In this paragraph, we take into consideration all otherpossible scenarios in the HS PL enhancement process. We ruleout the possibility of the observed PL enhancement in theMoS2 emission due to the interference of the backscatteringlight, because the entire measured MoS2 area (including theHS) is placed on the same hBN flake (inset of Figure 1a). 1LWSe2 (thickness < 1 nm) in the HS area cannot modulate theinterference pattern considering the ∼9 nm interlayer andthick substrate hBNs. We also rule out the possiblecontribution of ET from the hBN defect states40 to the HSMoS2 PL enhancement process, as the ET from the same hBNthickness cannot result in more HS PL emission as comparedto the 1L MoS2 region. In order to check the datareproducibility, we made another HS with a differentfabrication protocol and nonidentical hBN thickness andobserved an enhanced MoS2 PL emission in the HS area (seeSupporting Information for details and Figure S4). There isanother possibility, that the emitted light from the MoS2 layercould be reflected by the encapsulated WSe2 at lowtemperature,41 increasing the PL enhancement only at theHS area. To verify this, we made a similar HS on thetransparent ultraflat quartz substrate (Figure S5). For thisFigure 3. MoS2 photoluminescence (PL) intensity maps at 8 K under (a) 2.12 and (b) 2.85 eV excitation energy. MoS2 emission in the HS areashows an overall increased PL emission as compared to the 1L region. The scale bars represent 5 μm length.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−56245619https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-astransparent sample we observe shifts in the absorption peaksdue to the change in the dielectric environment, whichdestroys the resonant overlap of the B-excitonic levels betweenthe two materials. As a result, we see a quenching in the HSMoS2 PL emission and an increase in the HS WSe2 emission,proving that a one-way ET occurred from the MoS2-to-WSe2Figure 4. (a−b) HS and MoS2 PLE maps at 25 K. MoS2 PL emission in the HS area shows an enhancement as compared to the 1L area. (c) HSand MoS2 PLE comparison along the vertical lines in parts (a−b). HS shows a slightly reduced MoS2 PLE enhancement as compared to the 8 Kmap. (d−e) HS and MoS2 PLE maps taken at 100 K. (f) Similar HS and MoS2 PLE comparison at 100 K. MoS2 in the HS area does not show anyintensity enhancement at 100 K as compared to the 1L area. In all the HS maps, WSe2 emission intensities are kept saturated to visualize the MoS2emission.Figure 5. (a) Schematic illustration of the valence band (VB) and conduction band (CB) splitting at the K valley in MoS2 and WSe2, respectively.(b−c) Calculated MoS2 optical transitions along the K−-Γ-K+ direction from VB2 to CB1 and VB1 to CB2 (as shown in part (a)), respectively. (d−e) Similar calculated WSe2 momentum-space energy landscape along the K−-Γ-K+ direction from VB2 to CB2 and VB1 to CB1 (as shown in part(a)), respectively. (f−g) Schematic illustration of the photocarrier relaxation pathways from the higher energy levels to the ground state (GS) inMoS2 due to the energy transfer (ET) from WSe2 after resonant excitation at the (WSe2) B and D excitonic levels, respectively. Different types oftransition are shown in the MoS2 layer; such as intravalley scattering (kiv), intervalley transition (kv), and radiative recombination (kr).Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−56245620https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?fig=fig5&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aslayer.28 This result proves that the reflection of the MoS2 PLfrom the encapsulated WSe2 layer has no effect here in thereported HS PL enhancement process. We conclude that theMoS2 PL enhancement in the HS area is a result of aninterlayer ET process from the WSe2 layer.Strong overlaps between the higher energy absorptions inboth the investigated materials (Figure 1b) help us to study theeffect of the interlayer ET process under those “resonant”excitation conditions. The PL intensity map taken at 8 K underthe excitation of 2.12 eV (B resonances overlap region) showsan overall enhanced MoS2 emission in the HS area (Figure 3a).Similarly, an excitation at 2.85 eV energy (WSe2 D and MoS2C peaks overlap region) shows an increased MoS2 PL emissionthroughout the HS area (Figure 3b), thus proving that at boththe excitation energies an efficient ET happened from theWSe2-to-MoS2 layer as discussed in the later section. The PLintensity maps (Figures 3a−3b) also show that the observedenhancement of the MoS2 PL emission in the HS area is not alocalized phenomenon. We note that although there is somenonuniformity in the HS PL intensity due to the typicalinhomogeneous nature of the exfoliated samples, but the HSPL emission is always higher than the 1L MoS2 area.In order to study the effect of increasing temperature in ourexperiments, we performed PLE maps at 25, 100, and 200 K(Figure 4 and Figure S6). At 25 K, MoS2 emissions in the HSarea under both the excitation energies at ∼2.83 and 2.2 eVshow a similar enhancement factor of ∼1.6 (Figure S7). Thesevalues are a slight reduction from the 8 K data. The PLE alsoshows a similar overall enhancement in the MoS2 HS emissionat 25 K (Figure 4c). Upon further increasing the temperatureat 100 and 200 K, we observe a complete vanishing of theMoS2 PL enhancement in the HS (Figures 4d−4f). A slightquenching of the HS MoS2 PLE at 100 K (Figure 4f) could bedue to the conventional type-II HS ET28 from the higher-to-lower bandgap material (MoS2-to-WSe2).For MoS2 and WSe2, the schematics of the A and Btransitions based on the VB and CB splitting are shown inFigure 5a. In these TMD monolayers, VB (VB1 and VB2) andCB (CB1 and CB2) spin splitting occurs due to the spin−orbitcoupling and lack of inversion symmetry,10,42 allowing possibleabsorptions based on the optical selection rule.43,44 Thecorresponding PL emission, which comes from the directradiative recombination at the optical bandgap, stronglydepends on the spin-state of the CB (CB1 or CB2) electronand the VB (VB1 or VB2) hole at the K+/K− point. Based onthe allowed electron recombination from the CB1 or CB2 tothe hole situated at the top of the VB (VB2), the materials aredivided into two categories: “bright” or “dark”, respectively.10The calculated momentum-space energy landscape for theallowed optical transitions from VB2-to-CB1 and VB1-to-CB2in the MoS2 layer shows a smaller separation of ∼150 meV atthe K+/K− point due to the spin splitting (Figures 5b−5c andS8a), which matches well with the previous results.45,46 WSe2shows a comparatively larger separation of ∼500 meV at theK+/K− point47,48 for the VB2-to-CB2 and VB1-to-CB1transitions (Figures 5d−5e and S8b). The spin- andmomentum-resolved energy landscapes (Figures 5b−5e) helpus to visualize the optical transitions corresponding to the PLEenergies and eventually deduce the clear picture of the possiblephotocarriers’ relaxation pathways.Optical excitation at the “band-nested” region (MoS2 C andWSe2 D peaks) excites electrons in the valley in between theΓ−Λ point in the MoS2 CB and around the Λ valley in theWSe2 CB. These excited photocarriers (electron and hole)instantly relax to their immediate band extreme points: the Λvalley for the electrons and the Γ hill for the holes.27 Thesecarriers then further transfer to the band extrema via theextremely fast (<500 fs) intravalley scattering (kiv).49−51 In ourHS, to describe the PL intensity map under the 2.12 eVexcitation (Figure 3a), the only possible mechanism is shownas a schematic illustration in Figure 5f. Upon excitation withthe 2.12 eV photons, photoexcited carriers are generated at theWSe2 B excitonic level. Due to the resonant overlap with theMoS2 B level (Figure 1b), the WSe2 B excitonic energyimmediately transfers to the MoS2 B and A bands, resulting inmore carriers in the MoS2 layer. The extra carriers at the MoS2B level transfer to the subsequent band extremum via anintervalley transition (kv, i.e., BK+/K‑ to AK‑/K+ transitions),followed by a radiative recombination process (kr) to theground state (GS). Thus, we obtain an enhanced MoS2 PLemissions in the HS area with an excitation of 2.12 eV (Figure3a). However, at an excitation energy of 2.85 eV (MoS2 C andWSe2 D peaks overlap region, Figure 1b), two possible ETchannels can play a crucial role. First, ET from the WSe2 Dlevel can directly generate more carriers at the MoS2 C leveldue to the resonant overlapping. These extra carriersradiatively recombine at the band extremum via intravalleytransition (kiv) and give rise to more MoS2 PL emissions in theHS area, as shown in the schematic of Figure 5g (gray coloredET process). Another possibility is that upon the 2.85 eVexcitation carriers generated at the WSe2 D level scatter to theWSe2 B level via the analogous kiv process and then transfer tothe MoS2 B and A levels via the ET process. Finally, itincreases the MoS2 PL emission similar to the 2.12 eVexcitation process (black colored ET process in Figure 5g).Interestingly, an excitation at the WSe2 C absorption peak(2.56 eV) does not result in any MoS2 PL emission (FigureS9), indicating that interlayer coupling between the suitablelevels was not possible at this excitation due to the immediatephotoexcited carrier transfer to the WSe2 A level. Hence, noenhancement in the MoS2 HS PL emission due to the ETprocess is also apparent.Our model to describe the enhanced MoS2 PL emissionfrom the HS area also supports the temperature-dependentdata. Photocarriers go through a series of phonon scatteringbefore relaxing to the ground state. At low temperature,electron−phonon scattering dominates.52 With the increasingtemperature, other types of scattering processes, such asanharmonic phonon−phonon scattering and phonon structurescattering,53 start to dominate. Thus, with the increasingtemperature, the intravalley transition becomes weaker due tothe multiple-phonon scattering and eventually a minor fractionof the photocarriers generated at the “band-nested” region canbe transferred to the K+/K− point for radiative recombination.Furthermore, the thermal activation should make the “hot”carrier transfer to the band extremum extremely faster (<100fs),54 preventing the coupling between the materials’corresponding energy levels. These eventually result in acomplete disappearance of the MoS2 PL enhancement in theHS area at higher temperatures (100 and 200 K).Considering the temperature-dependent data, we canconclude that at higher excitation energy (∼2.85 eV) the ETprocess via the WSe2 B level to the MoS2 B and A levelsdominates (black colored ET process in Figure 5g) in ourexperiment. Otherwise, with increasing the temperature weshould observe an enhanced MoS2 HS PL emission. AtNano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−56245621https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascryogenic temperature, the fast intravalley scattering (kiv) inTMDs occurs at the ∼100−500 fs time scale,49−51,54 whereasintervalley transitions (kv) occur at a longer time scale of a fewps range.55,56 Our study suggests that the reported EThappened at a time scale faster than the intervalley transitionand slower than the intravalley transition. Otherwise, the ETfrom the lower optical bandgap WSe2 cannot excite morecarriers in the higher bandgap MoS2, resulting in an enhancedHS MoS2 PL emission. Finding the “true” ET time scale in ourexperiment will require an ultrafast study, which is beyond thescope of this work. We would like to point out that the effect oftrions participating in the ET process cannot be excluded, asthe binding energies between the trions and excitons at lowtemperature are only in the order of a few tens of meV inWSe257 and MoS2.58 Resolving the MoS2 excitons−trionscontribution is beyond the resolution limit of our instrumentalsetup. However, this does not change the overall picture of ourwork. It is also important to mention that with the increasingtemperature the effect of band renormalization in the ETprocess to alter the radiative recombination pathway of thephotocarriers cannot be ignored. A thorough investigation ofthe band renormalization effect in the ET process is required inthe future work.In conclusion, our study shows that strong light−matterinteraction in the 1L MoS2 and WSe2 “band-nested” regionallows us to observe an unusual ET process from the lower-to-higher bandgap (WSe2-to-MoS2) material. The excitation-dependent PL intensity maps prove that the reported HS MoS2PL enhancement is not a localized phenomenon due to thematerial’s local property change; the entire HS area shows thisenhanced PL emission. Finally, the temperature-dependentstudy proves that with the increasing temperature due to thegrowing electron−phonon scattering, the carriers’ transfer tothe band extremum becomes faster, preventing ET from theWSe2 (smaller gap) to the MoS2 (larger gap) layer. Ourfindings provide an important insight into the interlayer ETprocess in these layered materials and will help to build acomprehensive understanding about the competing interlayerprocesses for developing future TMD-based optoelectronicdevice applications.■ METHODSHS Fabrication. We fabricated three HSs using twofabrication protocols. HSs in Figures 1a and S5, werefabricated using the PDMS-based dry transfer technique atthe University of Warsaw. The bottom hBN layer was directlycleaved on the SiO2/Si substrate. MoS2-hBN-WSe2 layers wereexfoliated onto the Gel-Pak (PDMS) films and were stackedlayer-by-layer (in reverse order) onto each other using a home-built semiautomatic transfer stage. The other HS in Figure S4was partially fabricated using a robotic fabrication system(QPress) at the Brookhaven National Laboratory (the detailsin Supporting Information). MoS2, WSe2, and hBN bulkcrystals for exfoliation were obtained from the GrapheneSupermarket, HQ Graphene, and National Institute forMaterials Science, respectively.Characterization. We used a Bruker Dimension Icon withNanoScope 6 controller in ‘ScanAsyst’ (peak force tapping)mode to obtain the high-resolution AFM images.The differential RC measurements were performed using asupercontinuum light source (without a monochromator)focused by a Nikon L Plan 100× (N.A. 0.7) objective anddirected into a spectrometer. Samples were loaded in a cryostatand cooled with a continuous flow of liquid helium (LHe).The differential reflectance is defined by (Rs − Rsub)/(Rs +Rsub), where Rs is the reflected light intensity from the TMDsample areas and Rsub is that from the hBN/Si substrate.We performed the μ-PL/PLE experiments by using asupercontinuum light source coupled with a monochromatoras an excitation source. The incident light was focused using aMitutoyo M Plan 50× (N.A. 0.75) objective. The excitationpower was constant throughout the measurements, and theaverage power on the sample was kept at ∼50 μW (spotdiameter ∼ 1 μm) to avoid any high-power-induced nonlineareffects from the sample. For the PLE experiment sample wasloaded in a LHe cryostat to reach the minimum temperature of∼5 K during the experiments.■ ASSOCIATED CONTENTData Availability StatementAll the data necessary to conclude the results are presented inthe manuscript and Supporting Information. The technicaldetails of the theoretical calculations are available from thecorresponding authors upon reasonable request.*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127.Details of the theoretical calculations, AFM heightprofile, extended PLE maps and spectra of different HSs,RC spectra of HS sample on transparent substrate, PLintensity map, and spin-resolved DFT calculated energylandscape (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsArka Karmakar − Institute of Experimental Physics, Faculty ofPhysics, University of Warsaw, 02-093 Warsaw, Poland;orcid.org/0000-0002-8351-2268;Email: arka.karmakar@fuw.edu.pl, karmakararka@gmail.comAbdullah Al-Mahboob − Center for FunctionalNanomaterials, Brookhaven National Laboratory, Upton,New York 11973, USA; Email: aalmahboo@bnl.govMaciej R. Molas − Institute of Experimental Physics, Facultyof Physics, University of Warsaw, 02-093 Warsaw, Poland;orcid.org/0000-0002-5516-9415; Email: maciej.molas@fuw.edu.plAuthorsTomasz Kazimierczuk − Institute of Experimental Physics,Faculty of Physics, University of Warsaw, 02-093 Warsaw,Poland; orcid.org/0000-0001-6545-4167Igor Antoniazzi − Institute of Experimental Physics, Faculty ofPhysics, University of Warsaw, 02-093 Warsaw, PolandMateusz Raczyński − Institute of Experimental Physics,Faculty of Physics, University of Warsaw, 02-093 Warsaw,Poland; orcid.org/0000-0003-0443-1943Suji Park − Center for Functional Nanomaterials, BrookhavenNational Laboratory, Upton, New York 11973, USA;orcid.org/0000-0002-2269-7705Houk Jang − Center for Functional Nanomaterials,Brookhaven National Laboratory, Upton, New York 11973,USATakashi Taniguchi − International Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01127Nano Lett. 2023, 23, 5617−56245622https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01127?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01127/suppl_file/nl3c01127_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arka+Karmakar"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-8351-2268https://orcid.org/0000-0002-8351-2268mailto:arka.karmakar@fuw.edu.plmailto:karmakararka@gmail.commailto:karmakararka@gmail.comhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Abdullah+Al-Mahboob"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfmailto:aalmahboo@bnl.govhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Maciej+R.+Molas"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-5516-9415https://orcid.org/0000-0002-5516-9415mailto:maciej.molas@fuw.edu.plmailto:maciej.molas@fuw.edu.plhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomasz+Kazimierczuk"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-6545-4167https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Igor+Antoniazzi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mateusz+Raczyn%CC%81ski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-0443-1943https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Suji+Park"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-2269-7705https://orcid.org/0000-0002-2269-7705https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Houk+Jang"&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=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01127?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asTsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-1467-3105Kenji Watanabe − Research Center for Functional Materials,National Institute for Materials Science, Tsukuba, Ibaraki305-0044, Japan; orcid.org/0000-0003-3701-8119Adam Babiński − Institute of Experimental Physics, Faculty ofPhysics, University of Warsaw, 02-093 Warsaw, PolandComplete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.3c01127Author ContributionsA.K. and A.A.M. conceived the project. A.K., A.A.M., andM.R.M. designed the experiments. A.K., S.P., and H.J.fabricated the samples. T.K., A.K., I.A., M.R., and M.R.M.performed the experiments. A.K. and A.A.M. analyzed the data.A.A.M. performed the theoretical calculations. A.K., A.A.M.,M.R.M., and A.B. interpreted the results. T.T. and K.W.provided the bulk hBN for exfoliation. A.K. wrote themanuscript with feedback taken from all the coauthors.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe work has been supported by the National Science Centre,Poland (Grant No. 2017/27/B/ST3/00205 and 2018/31/B/ST3/02111). K.W. and T.T. acknowledge support from theJSPS KAKENHI (Grant No. 19H05790, 20H00354, and21H05233). This research used the quantum material press(QPress) of the Center for Functional Nanomaterials (CFN),which is a U.S. Department of Energy Office of Science UserFacility, at Brookhaven National Laboratory under ContractNo. DE-SC0012704. 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