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Sihan Chen, Jangyup Son, Siyuan Huang, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Rashid Bashir, Arend M. van der Zande, William P. King

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[Tip-Based Cleaning and Smoothing Improves Performance in Monolayer MoS<sub>2</sub> Devices](https://mdr.nims.go.jp/datasets/7ba40004-628a-4c8f-9660-f0ef5eceb7da)

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Tip-Based Cleaning and Smoothing Improves Performance in Monolayer MoS2 DevicesTip-Based Cleaning and Smoothing Improves Performance inMonolayer MoS2 DevicesSihan Chen, Jangyup Son, Siyuan Huang, Kenji Watanabe, Takashi Taniguchi, Rashid Bashir,Arend M. van der Zande, and William P. King*Cite This: ACS Omega 2021, 6, 4013−4021 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Two-dimensional (2D) materials and heterostructures arepromising candidates for nanoelectronics. However, the quality of materialinterfaces often limits the performance of electronic devices made fromatomically thick 2D materials and heterostructures. Atomic force microscopy(AFM) tip-based cleaning is a reliable technique to remove interfacecontaminants and flatten heterostructures. Here, we demonstrate AFM tip-based cleaning applied to hBN-encapsulated monolayer MoS2 transistors,which results in electrical performance improvements of the devices. Toinvestigate the impact of cleaning on device performance, we compared the characteristics of as-transferred heterostructures andtransistors before and after tip-based cleaning using photoluminescence (PL) and electronic measurements. The PL linewidth ofmonolayer MoS2 decreased from 84 meV before cleaning to 71 meV after cleaning. The extrinsic mobility of monolayer MoS2 field-effect transistors increased from 21 cm2/Vs before cleaning to 38 cm2/Vs after cleaning. Using the results from AFM topography,photoluminescence, and back-gated field-effect measurements, we infer that tip-based cleaning enhances the mobility of hBN-encapsulated monolayer MoS2 by reducing interface disorder. Finally, we fabricate a MoS2 field-effect transistor (FET) from a tip-cleaned heterostructure and achieved a device mobility of 73 cm2/Vs. The results of this work could be used to improve theelectrical performance of heterostructure devices and other types of mechanically assembled van der Waals heterostructures.■ INTRODUCTIONThe quality of material interfaces is critical to the performanceof electronic devices and is particularly important for electronicdevices made from two-dimensional (2D) materials. Commoninterface disorders that degrade device performance includeinterface Coulomb impurities, charge traps,1−4 and localfluctuations in strain5,6 and dielectric screening.7 Carrierscattering due to interface impurities is significant, asatomically thick 2D materials do not have any bulk to screenimpurities.8 Carriers may also scatter at defects that arise fromfolding or wrinkling of the 2D material.9,10Layers of 2D materials can be stacked together via van derWaals forces to create a wide variety of heterostructures thathave novel or improved properties.11 Clean and smoothinterfaces are essential for the performance of electronicdevices made from heterostructures.12−14 Mechanical assemblyis the most common way to fabricate van der Waalsheterostructures,11,15 but it often traps contaminants at theinterfaces, which limits the carrier mobility, device perform-ance, reproducibility, and reliability. Contaminants are trappedat the interfaces as a result of the competition between theelastic energy of the deformed 2D crystal and the adhesionenergy between the 2D crystal and its substrate.16−18 Thesecontaminants come from the ambient environment or areresidual materials from the assembly process.12,19−21 Someresearch studies have been published on the nature of thesecontaminants,12,17,22 which include organic residue at theinterfaces of stacked 2D layers.12,21,23 To minimize interfacecontamination, it is necessary to either prevent thecontamination from forming during fabrication or to removeit after fabrication. Strategies to prevent interface contami-nation include transferring in inert-gas filled glovebox or invacuum,24,25 as well as minimizing exposure of 2D materials topolymers and solvents used in transfer, such as dry pick-up.19,26,27 Dry pick-up technique uses one layer of the 2Dmaterial to pick up another layer by van der Waals forces,which has enabled the fabrication of electronic devices withrecord-high performance and novel properties.14,26,28−30However, there exists substantial variation in electricalperformance among the devices fabricated by pick-uptechnique,19,31 which suggests a need to further improve thequality of the interfaces after assembly.A few cleaning techniques are available to improve theinterfaces of van der Waals heterostructures after assembly. As2D materials are impermeable to all gases and liquids,32−34Received: December 5, 2020Accepted: January 20, 2021Published: February 1, 2021Articlehttp://pubs.acs.org/journal/acsodf© 2021 The Authors. Published byAmerican Chemical Society4013https://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−4021This is an open access article published under a Creative Commons Non-Commercial NoDerivative Works (CC-BY-NC-ND) Attribution License, which permits copying andredistribution of the article, and creation of adaptations, all for non-commercial purposes.Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 1, 2021 at 05:50:32 (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="Sihan+Chen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jangyup+Son"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Siyuan+Huang"&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="Rashid+Bashir"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arend+M.+van+der+Zande"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arend+M.+van+der+Zande"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="William+P.+King"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsomega.0c05934&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/acsodf/6/5?ref=pdfhttps://pubs.acs.org/toc/acsodf/6/5?ref=pdfhttps://pubs.acs.org/toc/acsodf/6/5?ref=pdfhttps://pubs.acs.org/toc/acsodf/6/5?ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdfhttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttps://http://pubs.acs.org/journal/acsodf?ref=pdfhttp://pubs.acs.org/page/policy/authorchoice/index.htmlhttp://pubs.acs.org/page/policy/authorchoice_ccbyncnd_termsofuse.htmlchemical and plasma-based techniques for cleaning the surfacesof 2D materials are not applicable for cleaning theinterfaces.35−37 Instead, thermal annealing is often used toreduce interface bubbles and increase the bubble-freearea.8,12,31,38 At the annealing temperature (typically 200−500 °C), small bubbles become mobile and migrate oraggregate into large bubbles.38 Annealing relies on the randommotion of interface bubbles and cannot reliably removetrapped contaminants from specific interface regions. Decom-position of contaminants during annealing may also produceradicals that damage 2D materials.39 Alternatively, mechanicalcleaning techniques, such as atomic force microscopy (AFM)tip-based cleaning, remove interface contaminants in acontrolled fashion without damaging 2D materials.21,31,40,41In tip-based cleaning, an AFM tip squeezes trappedcontaminants out from the interface of the targeted cleaningarea, leaving the interface of the scanned area clean and flat.40Tip-based cleaning improved the mobility of bilayer grapheneon hBN by a factor of 60−250%.42 hBN-encapsulatedgraphene and few-layer MoS2 devices also showed improve-ment in their magneto transport properties after tip-basedcleaning.31 Monolayers of 2D semiconductors, such as MoS2,WSe2 and BP, are promising channel materials for nano-electronics, whose intrinsic carrier mobilities are howevertypically limited by extrinsic carrier scattering sources.43−46hBN encapsulation improves device performance of 2Dsemiconductors by reducing extrinsic disorders due to surfaceroughness, charged impurities, and interface charge traps, ashBN has fewer Coulomb impurities than SiO2 and high-κdielectric substrates and is atomically flat.38,46,47 There hashowever been no published research that investigates potentialimprovements for hBN-encapsulated 2D semiconductors usingtip-based cleaning and smoothing.This article reports significant improvement of interfacequalities and electrical performance of hBN-encapsulatedmonolayer MoS2 by tip-based cleaning. The cleaning processreduced nanometer-scale height fluctuations by an order ofmagnitude and reduced the photoluminescence linewidth ofhBN-encapsulated monolayer MoS2 from 84 ± 3 to 71 ± 3meV, both of which indicate a reduction of interface disorder.The mobility of four monolayer MoS2 FETs fabricated on theas-transferred heterostructure increased from an average of 21± 2 to 38 ± 6 cm2/Vs after cleaning, demonstrating that tip-based cleaning is effective in reducing interface disorder andenhancing the mobility of hBN-encapsulated monolayer MoS2.Finally, we demonstrate the utility of this approach byfabricating and testing a MoS2 field-effect transistor (FET)fabricated on a tip-cleaned heterostructure.■ RESULTSFigure 1 shows the tip-based cleaning of 2D heterostructures.The heterostructure system consists of monolayer MoS2encapsulated between monolayer (1L) hBN on top andmultilayer (ML) hBN underneath. Figure 1a illustrates theconcept of tip-based cleaning. As-transferred 2D hetero-structures have surface and interface contaminants and voidsbetween the 2D layers. The interface contaminants and voidsaggregate into isolated pockets with typical lateral sizes from afew nanometers up to micrometers.16,19 In addition to thevoids and contaminants shown in Figure 1, there are also voidsand contaminants between monolayer MoS2 and the multilayerhBN substrate. By scanning the surface of the stacked 2Dlayers with an AFM tip in contact mode, the tip squeezestrapped contaminants out from the interface of stacked 2Dlayers and flattens the stacked 2D layers while pushing surfacecontaminants along the scan direction. The AFM tip scans in araster fashion, as in normal contact mode imaging. Surface andinterface contaminants accumulate at the end of each scan line,leaving the scanned area clean and smooth.We prepared the heterostructure stacks using an establisheddry pick-up technique summarized as follows.19,27 First, weexfoliated hBN and MoS2 flakes onto separate SiO2 on Sisubstrates by Scotch tape. We used monolayers for the tophBN and 8−20 nm thick layers for the bottom hBN. Atomicforce microscopy confirmed the monolayer nature of top hBN.Second, we picked up top hBN with a polycarbonate (PC) filmcoated on a polydimethylsiloxane (PDMS) lens.48 Third, wesequentially picked up monolayer MoS2 and bottom hBN byvan der Waals forces between MoS2 and hBN. Finally, wetransferred monolayer MoS2 encapsulated by top monolayerhBN and bottom multilayer hBN to the final 285 nm SiO2 onthe Si substrate. Figures S1, S2, and S3 show additional detailsof the fabrication. This heterostructure system has two benefitsfor studying the interfaces: First, this system has two interfacesbetween MoS2 and hBN in the top 1−2 nm of theheterostructure, allowing the AFM to reveal details of interfaceinhomogeneity that are largely masked by much thicker tophBN.31 Second, the top monolayer hBN serves as a tunnellayer to help inject charge carriers from contact metals intoMoS2, allowing direct metal deposition to fabricate electronicdevices, without additional processing steps.49,50We performed the tip-based cleaning and measurementexperiments using an Asylum MFP−3D AFM system. For allcleaning experiments, we used a cleaning force of 70−140 nN,and a scan speed of up to 28 μm/s. Mechanical cleaningdepends strongly on the cleaning force but weakly on thespeed. As long as the cleaning force is optimized, the scanspeed should not significantly affect the cleaning. In general,AFM tip-based cleaning is limited by its throughput, so a fasterscan is often better. The optimization of the cleaning force willbe discussed later, while the scan speed was limited by theFigure 1. (a) Schematic of tip-based cleaning of 1L hBN covered 1LMoS2 on a ML hBN substrate. AFM topography images of hBN-encapsulated monolayer MoS2 (b) before and (c) after tip-basedcleaning. (d, e) Line scans along the dashed lines in (b) and (c),respectively. Inset in (d) is the magnified view of the height profileover the position 0−0.5 μm.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−40214014http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig1&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdfcontrol system of MFP-3D, which would be greatly increasedusing a video-rate AFM system.51 The cleaning tips had anominal tip radius of 8 nm. The density of scan lines was 5−7nm/line, smaller than the tip radius to ensure thatcontaminants were pushed out of the cleaned region ratherthan accumulating between scan lines. After cleaning, wereplaced the cleaning tip with an 8 nm radius tapping mode tipfor imaging, which eliminated the potential for recontaminat-ing the scanned area. Devices were imaged in tapping mode tominimize the interaction between the imaging tip and devicesurface. The imaging process did not affect the device surface,as repeated imaging of the same area produced identical AFMimages.Figure 1b−e shows example results of the heterostructurebefore and after tip-based cleaning with a 100 nN cleaningforce. Figure 1b,c shows the topographic maps of theheterostructure before and after tip-based cleaning, respec-tively. Figure 1d,e shows the height profiles along the dashedlines in Figure 1b,c, respectively. Figure 1b,d exhibits heightfluctuations in the bubble-free area of the as-transferredheterostructure. As shown in the inset of Figure 1d, the imagedheight fluctuations have a lateral dimension of 50−100 nm andan amplitude of ∼1 nm. Figure 1c,e exhibits a much cleanerand smoother topography of the AFM-cleaned heterostructure,indicating that tip-based cleaning significantly reduces surfaceand interface contaminants and flattens the interfaces. The linescan in Figure 1e exhibits an average step height of 0.7 nmbetween monolayer MoS2 and bottom hBN, indicatingintimate contact between MoS2 and hBN where interfaceimpurities were absent.We next examine the impact of the tip-based cleaning on theoptical and electronic properties of the monolayer MoS2 bycomparing the photoluminescence and field effect transistortransport with and without tip-based cleaning.First, Figure 2 compares the topography and photo-luminescence between cleaned and uncleaned regions of asingle heterostructure. For this measurement, we fabricated aheterostructure consisting of monolayer MoS2 encapsulated bytop monolayer hBN and 8 nm bottom hBN and performed tip-based cleaning on half of the device to create two regions,cleaned and uncleaned. This geometry facilitates side-by-sidecomparison under the same measurement conditions. We thenmeasured the photoluminescence (PL) on both regionssimultaneously.Figure 2a shows the AFM topography of the heterostructure.The uncleaned region had a root-mean-square roughness Rrmsof 2.03 nm, while the cleaned region had a much smallerroughness of 0.41 nm. Figure 2b−d shows the PL peak width(full-width-at-half-maximum) map, characteristic PL spectra,and histogram of PL peak width of monolayer MoS2 in thecleaned and uncleaned regions. Each PL spectrum was fittedwith a Lorentzian peak (red curves in Figure 2c). Both cleanedand uncleaned regions showed a single PL peak at 1.863 ±0.004 eV, corresponding to the A exciton peak in monolayerMoS2.52,53 The cleaned region had an average PL peak widthof 70.5 ± 2.9 meV, while the uncleaned region had a peakwidth of 83.7 ± 2.8 meV.The photoluminescence of 2D materials is sensitive to strain,average doping, and disorder.54−57 The peak width iscorrelated with the disorder within the material,54,55 whilethe peak position is sensitive to doping and strain.56,57 As aresult, the photoluminescence maps reveal the impacts of tip-based cleaning on the electronic properties. First, the cleanedregion had a much smaller PL peak width than the uncleanedregion, indicating that tip-based cleaning reduces disorder inhBN-encapsulated monolayer MoS2.55,58 Second, no A trionpeak was detected in either cleaned or uncleaned region,indicating low electron doping in MoS2 before cleaning andthat tip-based cleaning did not induce electron doping inMoS2.59 Third, there was no measurable difference in peakpositions between the cleaned and uncleaned regions,indicating that tip-based cleaning did not significantly changeaverage doping or strain. With a measurement precision of 4meV, the induced strain and doping should be less than 0.08%and 1012 cm−2, respectively.59−61 Overall, the tip-basedcleaning reduces interface disorder and does not induceaverage doping or strain.Next, in Figures 3 and 4, we explore the optimal tip-basedcleaning force for the heterostructure, as determined bytopography and photoluminescence. In Figure 3, we monitorchanges in topography while scanning the same area of theheterostructure with increasing contact force Fn from 30 to 90nN. Figure 3a shows the AFM images recorded by the cleaningtip as it scanned at each contact force. The surface roughnessRrms values of the heterostructure enclosed by the dashed linesin Figure 3a are listed below each AFM image. Figure 3b showscorresponding topographical changes along the solid line inFigure 3a. The surface roughness Rrms decreased significantlyFigure 2. (a) AFM image of monolayer MoS2 encapsulated by topmonolayer hBN and 8 nm thick bottom hBN. The upper half of theheterostructure was processed by tip-based cleaning, while the lowerhalf was uncleaned. (b) PL peak width map and (c) characteristic PLspectra with a single-peak Lorentzian fit (red curves) of hBN-encapsulated monolayer MoS2 shown in (a). (d) Histogram of PLpeak width of monolayer MoS2 in the cleaned and uncleaned regions.Figure 3. Critical cleaning force. (a) AFM images recorded by thecleaning tip with increasing contact force Fn from 30 to 90 nN.Surface roughness Rrms of the region enclosed by the dashed lines inare listed below each AFM image. (b) Line scans along the solid linein (a) with increasing contact force.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−40214015https://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig3&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdfwhen Fn was increased from 30 to 50 nN and from 50 to 70nN but decreased only slightly from 70 to 90 nN. The heightprofiles in Figure 3b also show no further topographicalchanges as Fn was increased from 70 to 90 nN.Figure 4 shows the AFM topographies and correspondingPL peak width mappings of the same heterostructure as-transferred, after tip-based cleaning at 70 nN, and afteradditional cleaning at 140 nN. On average, the PL peak widthof monolayer MoS2 was 60.6 ± 3.0 meV as-transferred, 56.6 ±1.7 meV after cleaning at 70 nN, and 57.0 ± 1.1 meV aftercleaning at 140 nN. Since additional cleaning at 140 nN didnot further reduce the PL peak width, 70 nN was sufficient tooptimize the PL of the heterostructure. In summary, the criticalcleaning force for the heterostructure beyond which noimprovement in topography or PL could be detected wasaround 70 nN.While 70 nN is shown in this work to be the critical cleaningforce for monolayer MoS2 covered by monolayer hBN, theforce needed to flatten heterostructures with multilayer tophBN remains debatable. One article reports that the requiredcleaning force increases with the thickness of top hBN, and a2.1 μN force was used to flatten heterostructures with 18 nmthick top hBN.40 Another work used a 50−150 nN force toflatten heterostructures with up to 50 nm thick top hBN.31 Theconflicting results may be explained by their differences inheterostructure fabrication: the former work used a water/solvent mixture in fabrication, which resulted in a few-nanometer-thick liquid contamination layer at the interfaces,while the latter used dry pick-up technique with minimalinterface contamination. Future research should explorewhether the optimal cleaning force is independent of thethickness of top hBN when the heterostructure is fabricatedusing dry pick-up.To quantify the effects of tip-based cleaning on the electricalperformance of hBN-encapsulated monolayer MoS2, wefabricated four separate field-effect transistors on an as-transferred heterostructure and compared their performanceas fabricated and after tip-based cleaning. Figure 5 shows thesedevices. Figure 5a shows the schematic of the device structureand electrical measurement. The device consists of amonolayer MoS2 channel, electrical contacts for the sourceand drain consisting of 30 nm gold on 5 nm nickel, a 13 nmhBN on 285 nm SiO2 gate dielectric, and a degenerately p-doped silicon back gate. We take advantage of a recentlyreported technique, where the top monolayer hBN serves as atunnel layer to help inject charge carriers from contact metalsinto MoS2 and thus reduces contact resistance.49,50 Figure 5b,cshows the AFM topographies of the FETs (Devices A−D)before and after tip-based cleaning, respectively. Surfaceroughness Rrms of the MoS2 channel region decreased from1.36 to 0.34 nm after cleaning, excluding trapped bubbles thatpersisted. Figure 5d shows the transfer curves of the FETsbefore (black) and after (red) tip-based cleaning.We extracted four performance metrics of the FETs from thetransfer curves: extrinsic field-effect mobility μ = (L/WCgVds)(dIds/dVbg), threshold voltage Vth, subthreshold swing SS, andhysteresis H. We assumed a gate dielectric capacitance per unitarea of 285 nm thick SiO2 in series with 13 nm thick hBN (CgFigure 4. Determining the optimal cleaning force of theheterostructure by photoluminescence. AFM images and correspond-ing PL peak width maps of the same heterostructure as-transferred,after tip-based cleaning at 70 nN, and after additional cleaning at 140nN. Average PL peak widths are listed below each PL map. Variationsof outlines in PL maps resulted from misalignment and/or stage shiftduring measurement.Figure 5. (a) Cross-sectional view of the FETs, along with the electrical connections to characterize the devices. AFM topography of the FETs (b)before and (c) after tip-based cleaning. (d) Transfer curves of the FETs before (black) and after (red) tip-based cleaning in both linear and semi-log scale. Vds = 0.1 V. (e) Extrinsic mobilities of the FETs before and after tip-based cleaning.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−40214016https://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig5&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdf= 11.6 nF/cm2).62 The tip-based cleaning affects mobility,threshold voltage, subthreshold swing, and hysteresis of theFETs measured in air. As shown in Figure 5e, the extrinsicmobility of every FET consistently increased after tip-basedcleaning, by 60−93%. To exclude the effect of contactresistance, we further extracted the intrinsic mobilities usingthe Y-function method,75 as shown in Figure S7. On average,before tip-based cleaning, the four FETs had an extrinsicmobility of 21 ± 2 cm2/Vs, an intrinsic mobility of 24 ± 3cm2/Vs, a threshold voltage of −54.2 ± 4.5 V in forward sweepand −52.6 ± 4.8 V in reverse sweep, a subthreshold swing of3.6 ± 1.3 V/dec, and a hysteresis of 1.6 ± 0.9 V. After tip-based cleaning, the FETs had an extrinsic mobility of 38 ± 6cm2/Vs, an intrinsic mobility of 46 ± 5 cm2/Vs, a thresholdvoltage of −35.7 ± 1.0 V in forward sweep and −10.7 ± 5.5 Vin reverse sweep, a subthreshold swing of 7.0 ± 1.5 V/dec, anda hysteresis of 25.1 ± 5.3 V.We ascribe the improvement in mobility to reducedinterface disorder by tip-based cleaning. Cleaning andflattening of the interfaces reduced local strain fluctuations,spatial inhomogeneities in dielectric screening, and interfaceCoulomb impurities.4,6,7 As shown in a recent study,63 theremoval of surface contaminants does not improve the electronmobility of MoS2 likely because ambient adsorbates couldeasily readsorb onto the surfaces after tip-based cleaning andstill scatter charge carriers.64−66 Unexpectedly, thresholdvoltage, subthreshold swing, and hysteresis are all increasedafter tip-based cleaning. The threshold voltage increased by anaverage of 18.4 V in forward sweep and 41.9 V in reversesweep, the subthreshold swing nearly doubled, and thehysteresis increased by an average of 23.5 V. Previous reportsshow that annealing of MoS2 or graphene on hBN reduces p-type doping, inducing a negative shift in thresholdvoltage.8,13,38 It is peculiar that the initial hysteresis beforecleaning is small since these devices had only a monolayer ofhBN on top, compared with the much thicker layers of hBN>10 nm typically used in encapsulation.38,67 We hypothesizethat the positive shift in threshold voltage and the increase insubthreshold swing and hysteresis are all resulted fromincreased ambient effects after tip-based cleaning. The initialsurface adsorbates that were cleaned away degraded themobility but were relatively immobile. After tip-based cleaning,the effects of ambient adsorbates increased. Ambientadsorbates served as p-type dopants and charge traps,64−66leading to positive shift in threshold voltage and increasedsubthreshold swing and hysteresis after tip-based cleaning. Ingeneral, the increased role of ambient adsorbates is a tradeoffof using the monolayer top hBN, which had the advantage ofreduced contact resistance49,50 and enabled easy visualizationof the interfaces. For applications, tip-based cleaning should becombined with thicker top hBN encapsulation.For the device in Figure 5, we deposited the electrodesbefore cleaning to enable before and after comparison ofdevice behavior. However, any residue or interfacial disorderunder the electrodes will still affect contact resistance and thuslimit device performance.68 To examine the role of precleaningthe interface, we fabricated a FET in the tip-cleaned region ofthe heterostructure shown in Figure 2. Figure 6 shows thetransfer curve of the FET characterized in air in a two-probeconfiguration in which a drain-source bias was applied acrossthe outer two leads, leaving the inner two leads floating. Theinset shows the SEM image of the FET, exhibiting a channelwidth W of 2.42 μm and a channel length L of 0.99 μm. Thedevice achieved an extrinsic electron mobility of 73 cm2/Vs,which is among the highest reported room-temperatureextrinsic two-probe mobility values for monolayer MoS2 (seeTable S1).14,26,38,50,69−74 We extracted the intrinsic mobilitywithout the effect of contact resistance using the Y-functionmethod75 (see Figure S6) and obtained a high intrinsicelectron mobility of 102 cm2/Vs. If we account for thedifferences in device geometry and Vds (0.1 V in Figure 5 and 1V in Figure 6), the transfer curve in Figure 6 is not verydifferent from the ones after tip-based cleaning in Figure 5.The device in Figure 6 showed higher mobility than thedevices in Figure 5, which likely resulted from lower contactresistance. However, we cannot rule out the possibility ofdevice-to-device variations, which is ubiquitous in bothexfoliated and synthetic monolayer MoS2.71,72,76 Since topmonolayer hBN is not sufficiently thick to screen chargedimpurities from ambient air,77 which significantly limit theelectrical performance of monolayer MoS2,64,65 we expectfurther improvement in electron mobility after passivation withthick hBN or high-k dielectrics.4,69,74,78■ DISCUSSIONThe major advantage for AFM tip-based cleaning andsmoothing of the van der Waals heterostructure is that it canbe applied to a wide variety of van der Waals heterostructuresassembled by various techniques. Tip-based learning is purelymechanical and insensitive to the chemistry of the contami-nants and 2D layers. The cleaning procedure could be used notonly for lateral FETs like hBN-encapsulated MoS2 but also forvertical heterojunction transistors,79−81 which are increasinglyimportant for high-frequency applications. Other types of vander Waals heterostructures can also benefit from reducedinterface impurities by tip-based cleaning.11,41,82 AFM tip-based cleaning also has some limitations. First, it is challengingto remove bubbles much larger than the AFM tip radius. Thus,this cleaning technique is more suitable with van der Waalsheterostructures without micron-scale interface impurities.Other techniques such as thermal annealing31,38 and micro-dome cleaning83 can be used to remove microbubbles trappedin between van der Waals heterostructures. Second, if the top2D layer is fragile, the cleaning tip could damage the 2Dmaterial before interface contaminants are removed. Third, thistechnique is limited by low throughput (∼6 μm2/min in thiswork), which is typical of all scanning probe-basedtechniques.84■ CONCLUSIONSWe demonstrated reliable cleaning and smoothing of theinterfaces of hBN-encapsulated monolayer MoS2, by scanningthe heterostructure with an AFM tip in contact mode. TheFigure 6. Transfer curve of a FET fabricated in the tip-cleaned regionof the heterostructure shown in Figure 2. Vds = 1 V. Inset: SEM imageof the FET.ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−40214017http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?fig=fig6&ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdfAFM tip-based cleaning reduced interface disorder asevidenced by reduced height fluctuations of the hetero-structure and reduced photoluminescence linewidth ofmonolayer MoS2. The mobility of hBN-encapsulated mono-layer MoS2 improved substantially after tip-based cleaning.Combining the results from AFM topography, photolumines-cence, and back-gated field-effect measurements, we infer thattip-based cleaning enhances the mobility of hBN-encapsulatedmonolayer MoS2 by reducing interface disorder. Finally, wesurmise that tip-based cleaning will also significantly improvethe electrical properties of other mechanically assembled vander Waals heterostructures by cleaning and flattening theirinterfaces.■ METHODSFabrication of the Heterostructures. We assembled andtransferred the heterostructures onto SiO2 on Si substratesusing established van der Waals pick-up techniques.19,27 First,we exfoliated 2D flakes onto separate SiO2 on Si substrates byScotch tape. We used 90 nm thick oxide substrates with hBNexfoliation85 and 285 nm thick oxide substrates with MoS2exfoliation for sufficient optical contrast to identify number oflayers. We used monolayers for top hBN and 8−20 nm thicklayers for bottom hBN. Atomic force microscopy confirmedthe monolayer nature of top hBN (see Figure S3). Second, weprepared a PDMS lens on a glass slide and coated a layer of thePC film onto the PDMS lens.48 Then, we fixed the glass slidewith PC on PDMS stamp onto a micromanipulator. Third, wesequentially picked up 1L hBN, 1L MoS2, and ML hBN withthe PC film on a PDMS lens at 90 °C. Fourth, we contacted 1LMoS2 encapsulated by top 1L hBN and bottom ML hBN withthe final 285 nm SiO2 on the Si substrate at 90 °C and meltedthe PC film at 170 °C to complete transfer. Last, we removedthe PC film on the heterostructure in a chloroform bath atroom temperature for 24 h.Tip-Based Cleaning and Measurements. All tip-basedcleaning and measurement experiments were performed usingan Asylum MFP−3D AFM system. For all cleaning experi-ments, we used a cleaning force of 70−140 nN and a scanspeed of up to 28 μm/s. The cleaning tips (RFESP-75, Bruker)had a nominal tip radius of 8 nm and a spring constant of 3 N/m. The density of scan lines was 5−7 nm/line, smaller than thetip radius to ensure that contaminants were pushed out of thecleaned region rather than accumulating between scan lines.After cleaning, we replaced the cleaning tip with an 8 nmradius tapping mode tip (HQ:NSC15/AL_BS, MikroMasch)for imaging, which eliminated the potential for recontaminat-ing the scanned area.Photoluminescence Measurements. We performed PLmeasurements on a confocal Raman microscope (NanophotonRaman 11) using a 532 nm laser with a 100× objective at anexcitation power of 0.5 mW with a grating of 600 L/mm. Thelateral resolution of the equipment was 350 nm. The PL mapin Figure 2 had a pixel size of 0.2 μm and an acquisition time of0.1 s per pixel. The PL maps in Figure 3 had a pixel size of 0.2μm and an acquisition time of 3 s per pixel. Longer acquisitiontime increased signal but induced obvious stage drift. Weperformed all the measurements at room temperature inambient laboratory conditions.Fabrication of MoS2 Transistors and ElectricalMeasurement. First, we patterned large contact pads andleads consisting of 30 nm Au on 5 nm Ni onto a 285 nm SiO2on the degenerately p-doped silicon substrate using opticallithography. Second, we transferred the hBN-encapsulatedmonolayer MoS2 onto prepatterned SiO2 on the Si substrate.Third, we defined the contact electrodes to MoS2 consisting of30 nm Au on 5 nm Ni by e-beam lithography (eLINE, Raith)using a polymethyl methacrylate (PMMA) resist (A4 950k,Microchem) at an accelerating voltage of 20 kV, a beamcurrent of 30 pA, and a dose of 240 μC/cm2. We performed allthe electrical measurements in air at room temperature using asemiconductor parameter analyzer (Agilent, 4155C).■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsomega.0c05934.Schematic of heterostructure preparation, optical images,additional AFM images, Raman spectroscopy, outputcurves, extractions of intrinsic mobility, additionalsurface roughness data, and Tables S1 and S2 (PDF)■ AUTHOR INFORMATIONCorresponding AuthorWilliam P. King − Department of Mechanical Science andEngineering, University of Illinois at Urbana-Champaign,Urbana, Illinois 61801, United States; orcid.org/0000-0001-8606-1290; Email: wpk@illinois.eduAuthorsSihan Chen − Department of Mechanical Science andEngineering, University of Illinois at Urbana-Champaign,Urbana, Illinois 61801, United StatesJangyup Son − Department of Mechanical Science andEngineering, University of Illinois at Urbana-Champaign,Urbana, Illinois 61801, United StatesSiyuan Huang − Department of Mechanical Science andEngineering, University of Illinois at Urbana-Champaign,Urbana, Illinois 61801, United StatesKenji Watanabe − National Institute for Materials Science,Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − National Institute for Materials Science,Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-1467-3105Rashid Bashir − Department of Mechanical Science andEngineering and Department of Bioengineering, University ofIllinois at Urbana-Champaign, Urbana, Illinois 61801,United StatesArend M. van der Zande − Department of Mechanical Scienceand Engineering, University of Illinois at Urbana-Champaign,Urbana, Illinois 61801, United States; orcid.org/0000-0001-5104-9646Complete contact information is available at:https://pubs.acs.org/10.1021/acsomega.0c05934NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported in part by Taiwan SemiconductorManufacturing Company (TSMC) under grant no. 089401.J.S. acknowledges support from the Korea Institute of Scienceand Technology Institution Program (2K02420, 2Z06030).ACS Omega http://pubs.acs.org/journal/acsodf Articlehttps://dx.doi.org/10.1021/acsomega.0c05934ACS Omega 2021, 6, 4013−40214018http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsomega.0c05934?goto=supporting-infohttp://pubs.acs.org/doi/suppl/10.1021/acsomega.0c05934/suppl_file/ao0c05934_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="William+P.+King"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttp://orcid.org/0000-0001-8606-1290http://orcid.org/0000-0001-8606-1290mailto:wpk@illinois.eduhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sihan+Chen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jangyup+Son"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Siyuan+Huang"&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=pdfhttp://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttp://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Rashid+Bashir"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Arend+M.+van+der+Zande"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttp://orcid.org/0000-0001-5104-9646http://orcid.org/0000-0001-5104-9646https://pubs.acs.org/doi/10.1021/acsomega.0c05934?ref=pdfhttp://pubs.acs.org/journal/acsodf?ref=pdfhttps://dx.doi.org/10.1021/acsomega.0c05934?ref=pdfThis work was carried out in part in the Materials ResearchLaboratory Central Facilities at the University of Illinois.■ REFERENCES(1) Terman, L. 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