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Swaroop Kumar Palai, Mateusz Dyksik, Nikodem Sokolowski, Mariusz Ciorga, Estrella Sánchez Viso, Yong Xie, Alina Schubert, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Duncan K. Maude, Alessandro Surrente, Michał Baranowski, Andres Castellanos-Gomez, Carmen Munuera, Paulina Plochocka

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[Approaching the Intrinsic Properties of Moiré Structures Using Atomic Force Microscopy Ironing](https://mdr.nims.go.jp/datasets/95913dc0-43af-45d6-9433-f734e672b940)

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Approaching the Intrinsic Properties of Moiré Structures Using Atomic Force Microscopy IroningApproaching the Intrinsic Properties of Moire ́ Structures UsingAtomic Force Microscopy IroningSwaroop Kumar Palai, Mateusz Dyksik, Nikodem Sokolowski, Mariusz Ciorga, Estrella Sánchez Viso,Yong Xie, Alina Schubert, Takashi Taniguchi, Kenji Watanabe, Duncan K. Maude, Alessandro Surrente,Michał Baranowski, Andres Castellanos-Gomez, Carmen Munuera,* and Paulina Plochocka*Cite This: Nano Lett. 2023, 23, 4749−4755 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Stacking monolayers of transition metal dichalcogenides(TMDs) has led to the discovery of a plethora of new exotic phenomena,resulting from moire ́ pattern formation. Due to the atomic thickness andhigh surface-to-volume ratio of heterostructures, the interfaces play acrucial role. Fluctuations in the interlayer distance affect interlayercoupling and moire ́ effects. Therefore, to access the intrinsic propertiesof the TMD stack, it is essential to obtain a clean and uniform interfacebetween the layers. Here, we show that this is achieved by ironing withthe tip of an atomic force microscope. This post-stacking proceduredramatically improves the homogeneity of the interfaces, which isreflected in the optical response of the interlayer exciton. Wedemonstrate that ironing improves the layer coupling, enhancingmoire ́ effects and reducing disorder. This is crucial for the investigation of TMD heterostructure physics, which currently suffersfrom low reproducibility.KEYWORDS: transition metal dichalcogenides, atomic force microscopy, heterostructures, interlayer exciton, photoluminescenceVan der Waals (vdW) crystals are considered key enablersof the next-generation electronic and optoelectronicdevices with improved performance.1−6 The absence of latticematching constraints enables new paradigms of materialengineering, in which semiconductors, metals, superconduc-tors, insulators, or topological insulators can be seamlesslycombined.7Fabricating vdW stacks has revolutionized the investigationof graphene8−10 and transition metal dichalcogenides(TMDs).11 Encapsulating these stacks with hexagonal boronnitride (hBN)8,11−13 has provided access to their intrinsicproperties, which are otherwise hidden by charged impuritiesand dielectric disorder.11,13 Simultaneously, the precise controlrelative twist angle in bilayer graphene or TMDs led to thediscovery of new phenomena such as superconductivity14 orlong-lived interlayer optical transitions with controllableselection rules.15,16In this perspective, the quality of the interfaces between thelayers in the stack is of utmost importance to unveil theirintrinsic properties, which are highly sensitive to theirsurroundings.17 Moreover, the mechanical exfoliation of bulkcrystals18 unavoidably entails some form of interface disorder,which include charged impurities, traps,19 wrinkles, orbubbles.20−23 These localized strain gradients impact pro-foundly the electronic properties of layered materials24−26 andare considered to be one of the main sources of disorder inhigh-quality samples.27 Providing long-range homogeneity ofthe interface represents an ongoing challenge, and the lackthereof explains the considerable variations of the optoelec-tronic properties of nominally identical heterostructures.15,16,28Reports on the interlayer exciton in TMD heterobilayers showsignificant variation in the number of peaks and their energy,with a range of up to 110 meV.28−31 Even in high-qualitymonolayers, the energy of the exciton transition can vary asmuch as 10 meV.11Various preparation techniques such as transfer in inertatmosphere32 and annealing33 have been effective inminimizing defects in layered materials. Lately, AFM iron-ing21,23,34,35 has proven to further reduce these defects andenhance optical properties by enabling the observation ofinterlayer exciton at room temperature,21 bilayer Ramanpeaks,28 and moire ́ pattern reconstructions.36 However,systematic studies of the impact of AFM ironing on theoptical properties of the interlayer excitons are needed.Received: December 6, 2022Revised: May 10, 2023Published: June 5, 2023Letterpubs.acs.org/NanoLett© 2023 The Authors. Published byAmerican Chemical Society4749https://doi.org/10.1021/acs.nanolett.2c04765Nano Lett. 2023, 23, 4749−4755Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on June 17, 2023 at 08:47:44 (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="Swaroop+Kumar+Palai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mateusz+Dyksik"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Nikodem+Sokolowski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mariusz+Ciorga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Estrella+Sa%CC%81nchez+Viso"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yong+Xie"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yong+Xie"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alina+Schubert"&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="Duncan+K.+Maude"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alessandro+Surrente"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Micha%C5%82+Baranowski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Micha%C5%82+Baranowski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Andres+Castellanos-Gomez"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carmen+Munuera"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Paulina+Plochocka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.2c04765&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/23/11?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/11?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/11?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/11?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.2c04765?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://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://acsopenscience.org/open-access/licensing-options/Here, we demonstrate that AFM ironing improves theinterface quality and, as a consequence, the optical propertiesof hBN-encapsulated MoS2/MoSe2 heterostructure.37−39We probe the quality of the interface by optical methods,focusing on the interlayer exciton transitions,15,16,29,40−42which are highly sensitive to the interlayer distance. Wedemonstrate that ironing improves the layer coupling,enhancing moire ́ effects and reducing disorder. We begin ourinvestigation by ironing a MoS2 monolayer deposited on anhBN substrate and partially capped with another hBN flake(see Table S1 in the Supporting Information (SI)). With anAFM, we perform an ironing procedure by scanning selectedareas with the tip in contact mode using different pressingforces. We study the evolution of the surface morphology as afunction of the applied force. We then investigate, using opticalspectroscopy, three hBN-encapsulated MoS2/MoSe2 hetero-bilayers: samples A and B with a nominal twist angle of 60°and sample C with 0° alignment, which facilitates stronginterlayer exciton emission16,43 (see Table S1 in the SI forsample details). Within the ironed areas, the surfacehomogeneity can be greatly enhanced, with a reduction inthe root-mean-square (RMS) roughness compared to the non-ironed regions. The increased interface quality of theheterostructure gives rise to a more uniform PL spectrumfrom the ironed parts, whose properties correlate with theapplied ironing force. The high reproducibility of theproperties of van der Waals stacks we demonstrate herequalifies AFM ironing as a strategy for the post-fabricationtreatment of van der Waals heterostructures, providing accessto their intrinsic properties.We investigate the effects of the AFM ironing by examininga MoS2 monolayer exfoliated on an hBN substrate and onlypartially capped by another hBN flake. In Figure 1(a), we showthe AFM image of the as-fabricated sample measured indynamic mode, prior to any AFM treatment. The area weselected straddles the edge of the top hBN flake, highlighted bythe blue dashed line visible in Figure 1(a−c). A large numberof bubbles and wrinkles can be noted, resulting from thestamping process of the exfoliated flakes.20−23 We then scan anarea of 10 μm × 10 μm in contact mode, by applyingsubsequently increasing forces ranging from 5 nN to 120 nN.After each scan, the initial area (12 μm × 12 μm) was imagedin dynamic mode, to verify the effects of the scans in contactmode. Based on the effects on both the surface morphologyand the concentration of bubbles, we identify two ranges offorces. In the low force limit (applied force smaller than 20nN), the debris and adsorbates present on the surfaces areinitially moved and clustered, forming ripple-like features onthe surface of the sample in the direction parallel to the fastscan, before being completely swept away from the scannedarea by the AFM tip. An example of adsorbate clusters is visiblein Figure 1(b), where oblique bright lines can be seen in thescanned area, and is highlighted in Figure S1 of the SI. This, inturn, induced an increase of the roughness in the low forcelimit, which we report in Figure 1(d). Despite the observedchanges, it is important to note that the larger defects,including bubbles and wrinkles, remain unaffected, and noother discernible alterations to the surface morphology areapparent. Their number, shape, and size do not change as longas the applied force is small enough. However, when theapplied force is larger than 30 nN, significantly large areas arecleared of bubbles and wrinkles. This is visible both on thecapped and uncapped MoS2 of Figure 1(c). Notably, it is easierto achieve bubble sweeping in the non-covered MoS2 region.Previous studies have reported that the presence of a top hBNlayer increases the critical force required to sweep bubblesaway with an AFM tip.21,23,34 In particular, bubbles larger than1 μm in size tend to remain in their position, even for forces inthe micronewton regime.34 Reaching these high forces morelikely rips apart the bubbles damaging the heterostructure.34This can be appreciated in the movie available in the SI for thesame sample as in Figure 1, showing the topographic evolutionduring the ironing process for forces up to 200 nN.While the cleaning effect is apparent at low forces whencomparing Figure 1(a) and (b), and is primarily attributed tothe removal of residuals from the top surface, it is notFigure 1. Topographic atomic force microscopy (AFM) images in the dynamic mode of the (a) as-prepared samples and after ironing with (b) 5and (c) 70 nN, demonstrating the impact of incremental ironing forces on the sample. (d) Root-mean-square (RMS) values as a function of theironing forces exerted on the region of interest (1 μm2) indicated by the square in (a). (e) Height profile of the bubble of interest (dotted circle in(a)) taken along the black dash line cross-section, scanned in dynamic mode, before and after ironing with 120 nN. (f) Volume and (g) height ofthe bubble of interest as a function of the applied force during ironing.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.2c04765Nano Lett. 2023, 23, 4749−47554750https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_001.avihttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.2c04765?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asanticipated to significantly affect the optical properties of theheterostructure, as has been shown to be the case for transportcharacteristics in similar systems.34 To obtain more insight onthe cleaning capabilities in the higher force regime, weconducted a quantitative analysis of the evolution of roughnessand bubble morphology in the capped region of Figure 1, forgradually increasing applied forces. Despite the fact that largebubbles were not entirely removed, their shape underwent anoticeable transformation, as depicted in Figure 1(e), whereprofiles taken prior to and after the AFM treatment arecompared for the selected bubble (dotted circle in Figure1(a)) Moreover, both the height and volume evolution as afunction of the applied force can be plotted as illustrated inFigure 1(f) and (g). We note that upon ironing both the heightand volume of analyzed bubbles increase (see also Figure S2 inthe SI). This can be explained by the fact that at these forces,small bubbles can be displaced and adhere to the larger ones,flattening small corrugations at the heterointerface in areasbetween the blisters. This impacts the quality of the interfacesbetween the layers composing the heterostructures in the areascleansed of the smallest bubbles, promoting a more intimatecontact between the layers. This is expected to enhance theoptical quality of the sample, similar to the improved transportcharacteristics demonstrated in prior work.34To prove that, we investigate now a series of three MoS2/MoSe2 heterostructures. In Figure 2(a), we present the opticalimage of sample A (for sample B and sample C see SI). Itconsists of MoS2 and MoSe2 monolayers sandwiched betweenhBN layers (for structural data see Table S1 in the SI). Thetwist angle between monolayers for all structures is determinedby second harmonic generation measurements (SI Figure S5).The results for both tilt angles (0° and 60°) are qualitativelythe same unless otherwise stated. In the main text, we focusmainly on samples A and B (for sample C see the SI).The black dashed triangle in Figure 2(a) denotes theheterostructure region, where the two monolayers overlap. Atypical low-temperature (T = 4.5 K) photoluminescence (PL)spectrum from the heterostructure region is presented inFigure 2(b). We can distinguish features related to MoSe2 andMoS2 intralayer excitons at ∼1.62 eV and ∼1.9 eV,11respectively. These transitions are also visible in the reflectancespectrum of the heterostructure, where we also note anadditional feature, corresponding to the MoSe2 B-excitontransition.44 On the low-energy side, a strong PL peak, visibleonly in the heterostructure region, corresponds to theinterlayer exciton37,38,45,46 (see also SI Figures S3 and S4).As shown in Figure 2(c), we ironed three square-shapedareas, using an AFM tip in contact mode with 30, 60, and 80nN pressing force (for the other samples see SI Table S1,Figure S3, and Figure S4), resulting in an applied pressure ofapproximately 4−6 GPa (Sec. II of the SI). The ironingprocedure considerably improves the flatness of the hetero-structure as evident in the inset of Figure 2(c) which shows aclose-up view of the corner of the ironed area of theheterostructure. The surface of the ironed part shows theremoval of small bubbles and surface adsorbates, and localflattening in between the large bubbles. The presence ofbubbles in the non-ironed part of the sample affected theoptical response as their in-plane distance was lower than thetypical spatial resolution of a μPL setup.Figure 3(a) presents the spatial dependence of the interlayerexciton PL integrated intensity in the heterostructure area. Weobserve that, within the ironed regions indicated by the whitesquares, the PL intensity is enhanced compared with the non-ironed part of the heterostructure. This is especially visiblewhen taking a cross-section through all three ironed regionsalong the white dotted line, shown in Figure 3(b). Theintegrated PL intensity exhibits local maxima on the ironedparts (shaded areas). In between the ironed regions, the peakintensity drops. Apart from the enhanced intensity, withincreasing ironing force the interlayer transition redshifts andbroadens and its lineshape changes. PL spectra from non-ironed parts exhibit a low-energy tail while in the ironed partslineshape is more symmetric with increasing ironing force, andfor the highest forces employed, a new feature develops on thehigh-energy side, as illustrated in Figure 3(c), whererepresentative spectra taken along the cross-section arepresented.To corroborate these observations and to provide a deeperinsight into the evolution of interlayer exciton characteristics,we have performed a statistical analysis of the PL spectrummeasured at different spots on the heterostructure. Thehistograms in Figure 4(a−f) show the distribution of the PLpeak energy, line width, and intensity for the different forcesemployed to iron the heterostructure. The ironing results in aredshift of the interlayer exciton transition, as visible in Figure4(a,b,g). This redshift is roughly proportional to the ironingforce, with a coefficient of −286 ± 225 μeV/nN. The averagebroadening of the emission line increases with a coefficient ofFigure 2. (a) Optical micrograph of sample A, highlighting MoS2 and MoSe2 flakes (red and green outlines, respectively) and the heterostructureregion (black outline). (b) Photoluminescence and reflectance spectra obtained at 4.5 K, featuring inter- and intralayer exciton resonances(indicated). (c) Topographic AFM scan in dynamic mode revealing regions ironed with varying forces, with a close-up view of the ironed area (80nN) in the inset.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.2c04765Nano Lett. 2023, 23, 4749−47554751https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.2c04765?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as0.26 ± 0.07 μeV/nN, as we discuss more in detail below.Finally, on average the PL intensity in the ironed part increasesby about 15% compared to non-ironed areas. These results areexplained if we consider the more intimate contact betweenthe layers in the heterostructures, due to the reduced density ofbubbles we obtain after the AFM treatment. Quantitativeestimation of this effect is however complicated by the absenceof more detailed structural information, available withtransmission microscopy techniques. Moreover, in a MoS2/MoSe2 heterostructure, the interlayer exciton transition ispredominantly a strongly hybridized, momentum-indirectexciton,47,48 which makes the quantitative analysis of theimpact of the interlayer distance very challenging.Most importantly, the distributions of all three parameters ofinterest are narrower after ironing, as evident from the directcomparison between Figure 4(a,c,e) and Figure 4(b,d,f), whichrepresent the non-ironed and ironed distributions, respectively.The standard deviation of all the parameters decreases afterironing as shown in Figure 4(j), where we show thedependence of the standard deviation σ of a quantity measuredin the ironed area normalized by the standard deviation of thesame quantity σ0 estimated in non-ironed areas as a function ofthe ironing force. Our analysis demonstrates that the decreasedRMS roughness of the ironed areas and the reduced density ofbubbles and wrinkles are accompanied by a considerablenarrowing (50% decrease of σ/σ0) of the statistical distributionof the interlayer exciton energy, line width, and intensity alongwith a general increase of the PL intensity when forces of 60−80 nN are used to iron the heterostructure. This demonstratesthat reducing the interface roughness and locally improving thequality of the interface of the heterostructure are key toachieving spatially uniform optical spectra.The observed changes in the interlayer exciton emissionproperties after ironing are consistent with the enhancement ofthe moire ́ effects,15,49,50 due to the improved proximity of thelayers and flattening. The redshift of the interlayer excitontransition upon ironing can be understood as an effect of theincreased interlayer exciton binding energy and/or bandhybridization,47 due to a reduction of the interlayer distance.Simultaneously, the improved proximity of the layer togetherwith local flattening deepens the moire ́ trapping poten-tial15,49,50 which also enhances redshift of the interlayerexciton. The ironing procedure has implications beyondproximity enhancement. Specifically, it is also possible thatthis procedure serves to mitigate local strain fluctuations andthus increase the moire ́ effect (see inset of Figure 5).Figure 3. (a) PL intensity map for the interlayer exciton, showinghigher intensity hotspots for the ironed regions. (b) The evolution ofthe PL intensity along the cross-section is indicated by the whitedashed line. A clear enhancement of the PL intensity in ironed part(indicated by the shaded area) is visible. (c) Representative PLspectra along the cross-section show the change of the PL line shapein ironed part. Gray spectra are from the non-ironed region along thecross-section direction.Figure 4. (a−f) Histograms showing the statistical distribution of PL energy, line width, and intensity for non-ironed (top panels) and ironed(bottom panels) areas of the heterostructure. (g−i) Average values of interlayer exciton transition energy, line width, and PL intensity as a functionof ironing force. Dashed lines are linear fits. (j) Interlayer exciton transition energy, line width, and PL intensity standard deviation σ normalized tostandard deviation of the corresponding quantity measured on the non-ironed areas σ0 as a function of the ironing force. Dashed line stands forglobal linear fit of the relative standard deviation of all the parameters. The fitting procedure in panels (g) and (h) exclude the 0 nN data point,which corresponds to the as-prepared heterostructure, over which we have no control.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.2c04765Nano Lett. 2023, 23, 4749−47554752https://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.2c04765?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asThe enhancement of the moire ́ effect can also explain thesurprising, increased broadening of the interlayer exciton PLspectrum with the increasing ironing force. However, at thesame time, the line width distribution is narrower. Thissuggests that the increasing broadening is not related to thedisorder, but results from the evolution of the intrinsicproperties of the interlayer exciton. This is demonstrated in theevolution of the interlayer exciton PL lineshape upon ironingas shown in Figure 3(c). On the non-ironed part, the PLspectrum exhibits a low-energy tail, which is a typicalindication of disorder in semiconductors.41,51−54 In the ironedareas, the PL spectrum is more symmetric or, in some cases, ahigh-energy shoulder appears (see Figure 3(c)). To describethe lineshape evolution quantitatively, we analyze how thedifference δE between the maximum of the PL peak (Emax) andthe center-of-mass energy (i.e., δE = Emax − ·E I EI E( )( )) changes asa function of the ironing force. δE describes the distribution ofthe PL intensity with respect to the peak energy. A positive δEindicates a spectral tail at the low-energy side, whereas negativeδE is related to the presence of a high-energy shoulder. Fornon-ironed regions the distribution for δE is centered around∼15 meV (Figure 5). With ironing δE decreases andapproaches 0 meV, and, for higher forces, it becomes negative,confirming the change of PL lineshape. Again, we expect thatthis stems from the enhanced coupling between monolayersbuilding the heterostructure after ironing. This is schematicallypresented in Figure 5. After ironing, the potential landscapebecomes more uniform (as attested by reduced variation of fullwidth half-maximum) and the increased layers couplingdeepens the moire ́ potential. This facilitates the bounding ofextra states, which contribute to the emission on the high-energy side of the PL peak.15,49 This explains the apparentcontradiction of an improved spatial uniformity of the linewidth, observed in conjunction with a constant (or slightlyincreasing) average value of line width which does not decreaseafter ironing.In conclusion, we have investigated the effect of AFMironing on the optical quality of TMD heterostructures. Weidentify two classes of effects. At low forces, the surfaceroughness of the heterostructure initially increases and thendecreases due to a “sweeping-out” of surface adsorbates. Atforces above 30 nN, ironing moves small, more mobile bubblesand merges them into larger bubbles. We show that AFMironing can significantly improve the optical quality ofheterostructures. It improves the homogeneity of the interface(enhancing local flattening and reducing corrugations andstrain fluctuations), leading to more uniform optical spectraand enhanced interlayer coupling and moire ́ pattern impact.Our findings demonstrate that AFM ironing is a promisingmethod to unveil the intrinsic moire ́ physics in TMDheterostructures, which suffer from low reproducibility andinconsistent results related to low interface and surface quality.Therefore, AFM ironing, in addition to hBN encapsulation,could be a crucial post-stacking step to improve the quality ofTMD stacks.■ METHODSA description of sample preparation and AFM and opticalspectroscopy setups can be found in the SI.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765.Nail-polish fabrication technique used for the fabricationof samples, second harmonic generation measurementsand photoluminescence experiments used to character-ize the sample, and details of the AFM ironing procedureincorporated (PDF)Video showcasing the movement and coalescence ofbubbles under different ironing forces (AVI)■ AUTHOR INFORMATIONCorresponding AuthorsCarmen Munuera − Materials Science Factory, Instituto deCiencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain; Email: cmunuera@icmm.csic.esPaulina Plochocka − Laboratoire National des ChampsMagnétiques Intenses, EMFL, CNRS UPR 3228, UniversitéToulouse, 31400 Toulouse, France; orcid.org/0000-0002-4019-6138; Email: paulina.plochocka@lncmi.cnrs.frAuthorsSwaroop Kumar Palai − Laboratoire National des ChampsMagnétiques Intenses, EMFL, CNRS UPR 3228, UniversitéToulouse, 31400 Toulouse, FranceMateusz Dyksik − Department of Experimental Physics,Faculty of Fundamental Problems of Technology, WrocławUniversity of Science and Technology, 50-370 Wrocław,Poland; orcid.org/0000-0003-4945-8795Nikodem Sokolowski − Laboratoire National des ChampsMagnétiques Intenses, EMFL, CNRS UPR 3228, UniversitéToulouse, 31400 Toulouse, FranceMariusz Ciorga − Department of Experimental Physics,Faculty of Fundamental Problems of Technology, WrocławUniversity of Science and Technology, 50-370 Wrocław,PolandEstrella Sánchez Viso − Materials Science Factory, Instituto deCiencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain; orcid.org/0000-0003-0202-4340Figure 5. Evolution of the difference δE between the maximum of thePL peak and the PL center of mass as a function of ironing force. Apositive δE indicates a spectral tail on the low-energy side, whilenegative δE indicates a high-energy tail. The inset is a schematicillustration of the evolution of the moire ́ potential and PL spectraupon ironing. The enhanced proximity of the layers in the flattenedstructure increases the amplitude of the moire ́ potential, which alsobecomes more uniform, and on average deeper, therefore it canaccommodate more states.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.2c04765Nano Lett. 2023, 23, 4749−47554753https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_001.avihttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_002.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.2c04765/suppl_file/nl2c04765_si_001.avihttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Carmen+Munuera"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfmailto:cmunuera@icmm.csic.eshttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Paulina+Plochocka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-4019-6138https://orcid.org/0000-0002-4019-6138mailto:paulina.plochocka@lncmi.cnrs.frhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Swaroop+Kumar+Palai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mateusz+Dyksik"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-4945-8795https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Nikodem+Sokolowski"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mariusz+Ciorga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Estrella+Sa%CC%81nchez+Viso"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-0202-4340https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yong+Xie"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.2c04765?fig=fig5&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.2c04765?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asYong Xie − Materials Science Factory, Instituto de Ciencia deMateriales de Madrid (ICMM-CSIC), Madrid E-28049,Spain; orcid.org/0000-0001-7904-664XAlina Schubert − Materials Science Factory, Instituto deCiencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, SpainTakashi Taniguchi − International Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba, Ibaraki 305-004, Japan; orcid.org/0000-0002-1467-3105Kenji Watanabe − Research Center for Functional Materials,National Institute for Materials Science, Tsukuba, Ibaraki305-004, Japan; orcid.org/0000-0003-3701-8119Duncan K. Maude − Laboratoire National des ChampsMagnétiques Intenses, EMFL, CNRS UPR 3228, UniversitéToulouse, 31400 Toulouse, FranceAlessandro Surrente − Department of Experimental Physics,Faculty of Fundamental Problems of Technology, WrocławUniversity of Science and Technology, 50-370 Wrocław,Poland; orcid.org/0000-0003-4078-4965Michał Baranowski − Department of Experimental Physics,Faculty of Fundamental Problems of Technology, WrocławUniversity of Science and Technology, 50-370 Wrocław,Poland; orcid.org/0000-0002-5974-0850Andres Castellanos-Gomez − Materials Science Factory,Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC),Madrid E-28049, Spain; orcid.org/0000-0002-3384-3405Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.2c04765NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSWe would like to thank Thomas Pucher and Alvaro RodriguezRodriguez whose timely help with the exfoliation of TMDflakes helped us a lot in the final stages of the work. This workreceived funding from the European Union’s Horizon 2020research and innovation program under grant agreements956813 (2Exciting) and 755655 (ERC-St G 2017 project 2D-TOPSENSE). M.B. acknowledges National Science CentrePoland within the SONATA BIS program (Grant No. 2020/38/E/ST3/00194). Funding was also received from theMinistry of Science and Innovation (Spain) through theproject PID2020-115566RB-I00 and the EU FLAG-ERAproject “To2Dox” under the program PCI2019-111893-2.This study has been partially supported through the EUR grantNanoX no. ANR-17-EURE-0009 in the framework of the“Programme des Investissements d’Avenir”. M.D. acknowl-edges the support from the Polish National Agency forAcademic Exchange (grant no. BPN/BKK/2021/1/00002/U/00001). K.W. and T.T. acknowledge support from JSPSKAKENH I (Grant Numbers 19H05790, 20H00354, and21H05233)■ REFERENCES(1) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.Single-layer MoS2 transistors. 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