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Chia-Chun Lin, Naomi Tabudlong Paylaga, Chun-Chieh Yen, Yu-Hsuan Lin, Kuang-Hsu Wang, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Chi-Te Liang, Shao-Yu Chen, Wei-Hua Wang

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[Interfacial Elemental Analysis of Slanted Edge-Contacted Monolayer MoS<sub>2</sub> Transistors via Directionally Angled Etching](https://mdr.nims.go.jp/datasets/7ce7283c-664c-45c2-977c-8f50aaccd8c4)

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Interfacial Elemental Analysis of Slanted Edge-Contacted Monolayer MoS2 Transistors via Directionally Angled EtchingInterfacial Elemental Analysis of Slanted Edge-Contacted Monolayer MoS2 Transistors viaDirectionally Angled EtchingChia-Chun Lin, Naomi Tabudlong Paylaga, Chun-Chieh Yen, Yu-Hsuan Lin, Kuang-Hsu Wang,Kenji Watanabe, Takashi Taniguchi, Chi-Te Liang, Shao-Yu Chen, and Wei-Hua Wang*Cite This: ACS Nano 2025, 19, 4452−4461 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Edge contacts offer a significant advantage for enhancing theperformance of semiconducting transition metal dichalcogenide (TMDC)devices by interfacing with the metallic contacts on the lateral side, whichallows the encapsulation of all of the channel material. However, despiteintense research, the fabrication of feasible electrical edge contacts to TMDCsto improve device performance remains a great challenge, as interfacialchemical characterization via conventional methods is lacking. A majorbottleneck in explicitly understanding the chemical and electronic propertiesof the edge contact at the metal−two-dimensional (2D) semiconductorinterface is the small cross section when characterizing nominally one-dimensional edge contacts. Here, we demonstrate a directional angled etchingtechnique that enables the characterization of the interfacial chemistry at the metal−MoS2 junction when in an edge-contactconfiguration. The slanted edge structure provides a substantial cross section for elemental analysis of the edge contact byconventional X-ray photoemission spectroscopy, in which a simple chemical environment and sharp interface were revealed.Facilitated by the well-characterized contact interface, we realized slanted edge-contacted monolayer MoS2 transistorsencapsulated by hexagonal boron nitride. The transport characteristics and photoluminescence of these transistors allowed usto attribute the efficient carrier injection to direct and Fowler−Nordheim tunneling, validating the distinct Au−MoS2interface. The established method represents a viable approach to fabricating edge contacts with encapsulated 2D materialdevices, which is crucial for both the fundamental study of 2D materials and high-performance electronic applications.KEYWORDS: directional etching, edge contact, transition metal dichalcogenides, elemental analysis, interfacial chemical propertyINTRODUCTIONElectrical contact is the key factor for assessing the electronicproperties of two-dimensional (2D) materials.1,2 Efficientelectrical contact with 2D semiconductors (2DSs) encapsu-lated by van der Waals (vdW) heterostructures is important forboth fundamental research and practical applications.3−5 Thetop contact approach is straightforward for 2D geometry andhas been widely developed. On the other hand, edge contactwith 2DSs has several advantages over top contact.3,4,6−8 First,despite significant advances in channel length reduction,contact length scaling remains an issue for transistor scalingowing to the large transfer length in the top contact.Conversely, edge contact is unaffected by contact scaling.9Second, 2DSs exhibit a highly anisotropic crystal structure,with the edge sites and basal planes having very differentchemical and structural properties. In particular, methods thathave been extensively developed for conventional semi-conductors can be employed in edge contact to controlchemical bonding at the metal−semiconductor interface.4,10−12Third, for practical purposes, edge contact naturally endowsdevices with desirable configurations, such as encapsulatedvdW heterostructures for protecting chemically sensitive 2Dmaterials,4,13 efficient carrier modulation,14,15 and uniformcarrier transport.6Examining the interfacial chemistry at the 2DS contact isessential for understanding the contact properties, includingthe contact resistance and Fermi level pinning, and furtherimproving the reliability of edge-contact devices.2 Previously,studies on the chemical bonding that occurs at the metal−2DSReceived: September 26, 2024Revised: January 14, 2025Accepted: January 14, 2025Published: January 21, 2025Articlewww.acsnano.org© 2025 The Authors. Published byAmerican Chemical Society4452https://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−4461This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on February 7, 2025 at 03:53:56 (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="Chia-Chun+Lin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Naomi+Tabudlong+Paylaga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Chun-Chieh+Yen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yu-Hsuan+Lin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kuang-Hsu+Wang"&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="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="Chi-Te+Liang"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shao-Yu+Chen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Wei-Hua+Wang"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsnano.4c13581&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/ancac3/19/4?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/4?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/4?ref=pdfhttps://pubs.acs.org/toc/ancac3/19/4?ref=pdfwww.acsnano.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsnano.org?ref=pdfhttps://www.acsnano.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/interface have focused on top contacts because of the directcharacterization of the surface chemistry at the 2D inter-face.16−18 For edge contact via plasma etching, several issuescan occur, including the generation of defects or danglingbonds, chemical termination by oxygen,19 and the formation ofchemical bonds at the metal−2DS edge interface, whichcritically affect the electrical performance of edge-contacteddevices and hinder optimization of the edge contact proper-ties.6 However, investigating the metal−2DS interface in edge-contact geometry is very challenging because of the very smallcross section when characterizing nominally one-dimensionalmetal contacts, as depicted in the upper left panel of Figure 1a,which hampers the advancement of the edge contact approach.Here, we present a directional angled etching technique tofabricate a slanted edge (SE) structure on MoS2 to explicitlyreveal the chemical information on the metal−MoS2 edgeinterface in the edge-contact configuration. Specifically, wedesigned a homemade Faraday cage that encloses the samplesin a reactive ion etching (RIE) system for directional etchingwith an SF6/Ar plasma. As depicted in the lower panel ofFigure 1a, the resulting SE structure yields a substantial crosssection for elemental analysis of the SE contact by conven-tional X-ray photoemission spectroscopy (XPS),20,21 whichrevealed a sharp metal−MoS2 interface. Moreover, aftercharacterizing the interface between the metal and MoS2edge, we successfully demonstrated a high-quality SE contactin the edge-contacted monolayer (ML) MoS2 transistorsencapsulated by hexagonal boron nitride (h-BN).RESULTS AND DISCUSSIONTransition metal dichalcogenides (TMDCs) are an importantclass of 2DSs that show great promise for electronics andphotonics applications.22−24 To yield a controllable SEstructure for surface characterization and subsequent electricalcontact with TMDCs, we developed a MoS2 directionaletching method by employing a Faraday cage,25,26 as depictedin Figure 1b (Supporting Information S1). The trajectory ofthe incident plasma ions is preferentially collimated by thegeometric design of the Faraday cage, yielding various materialsystems with slanted angle profiles under plasma processingconditions.27−30 The advantages of using this technique are 2-fold. First, the etching angle is highly controllable and can bearbitrarily adjusted.27,31 For example, for surface character-ization by XPS, we choose an angle of approximately 45° tooptimize the cross section of the XPS signal. Second, the use ofa Faraday cage is beneficial as the collimated ions can create auniform edge during the etching process. A schematic of theperiodic SE of MoS2 for XPS analysis is depicted in Figure 1c.We employed e-beam lithography to fabricate a large array ofclosely spaced SEs to increase the size of the cross section forsubsequent XPS surface characterization, as shown in thescanning electron microscopy (SEM) image (left inset ofFigure 1c). When used as an etching mask during the etchingprocess, the periodic array of poly(methyl methacrylate)(PMMA) strips fully suppresses the planar MoS2 XPS signal,indicating that all of the observed XPS signals are due to theSEs. The right inset of Figure 1c shows the transmissionFigure 1. (a) Schematic of the comparison of the top, edge, and SE contacts. (b) Schematic of directionally angled etching to fabricate aMoS2 SE sample. The trajectory of the incident plasma ions is preferentially collimated by the Faraday cage. (c) Schematic of the periodic SEof MoS2 for XPS analysis. Left inset: TEM image of the cross section of the SE, which shows an extended, uniform edge for chemical analysisvia XPS. The scale bar is 5 μm. Right inset: SEM image of the periodic SE structure of the bulk MoS2 sample. The scale bar is 10 nm. (d)Typical XPS spectrum of a MoS2 sample fabricated with a periodic SE structure covered by a Au layer.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614453https://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig1&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aselectron microscopy (TEM) image of the SE, which isextended and uniform for an accurate chemical analysis.Figure 1d shows a typical XPS spectrum of a MoS2 sample witha periodic SE structure. Notably, a pronounced XPS signal wasattained by employing conventional XPS, which facilitates thecharacterization of the interfacial chemistry and optimizationof the SE contact. In addition to XPS, our method can beutilized to identify edge characteristics via other surfacecharacterization tools, such as scanning tunneling microscopy,Raman spectroscopy, and Fourier transform infrared spectros-copy.To exemplify our method, we show that by employingconventional XPS, we can uncover the interfacial chemistry atthe SE contact of MoS2, which is deterministically affected bythe etching of the edge contact. We compare the Mo 3d and S2p core levels for three MoS2 samples, designated A, B, and C,respectively: pristine bulk MoS2, MoS2 SE etched by SF6, andMoS2 SE etched by SF6 with prolonged oxidation underambient conditions for 20 min. Au (3 nm) was deposited on allthe MoS2 SE edge samples to encapsulate the surface andprotect against natural oxidation16 while still allowing thecollection of XPS signals. The area of the periodic SE structureis larger than the spot size of the X-ray incident beam(approximately 100 μm in diameter) to maximize the XPSsignal. The Shirley inelastic background was subtracted fromall XPS data32 for clear presentation even though the XPSsignals are dominant. As a reference, we show the XPS spectraof a controlled MoS2 sample with a pristine surface (sample A)in Figure 2a,b. The XPS spectra of pristine MoS2 exhibitdoublets due to the Mo 3d and S 2p peaks at 230.0 (Mo 3d5/2)and 162.8 eV (S 2p3/2), respectively. These binding energies(BEs) are consistent with pristine MoS2 peaks reportedpreviously.33 All of the Mo 3d and S 2p doublets wereanalyzed with compliance to the constraints of area, full widthat half-maximum (fwhm), and position considering spin−orbitcoupling.34Next, we discuss the XPS spectra of the MoS2 SE samplesetched by SF6, as shown in Figure 2c,d. The Mo 3d5/2 doubletat 229.6 eV (red) can be attributed predominantly to bulkMoS2 with minor doping.35 Remarkably, a well-resolveddoublet (green) with a BE of ∼0.7 eV lower than that of thedoublet corresponding to the underlying bulk MoS2 is clearlyobserved. These XPS spectra are invariant across the SEsamples, indicating uniform chemical conditions at theinterface after SE etching. The decrease in BE is in line withp-type doping in the substoichiometric MoSx layer35,36 andsuggests the presence of undercoordinated Mo upon removalof negatively charged S2− ions during the etching process. Forthe MoS2 basal plane, S vacancies originate from preexistingstructural defects17 and can be caused by various treat-ments,33,37 as evidenced by the decrease in the BE determinedby XPS. Moreover, MoS2 with S vacancies created by ionFigure 2. XPS Mo 3d and S 2p spectra of (a, b) pristine MoS2, (c, d) MoS2 SE sample prepared via the optimized etching process, and (e, f)MoS2 SE sample exposed to ambient conditions for 20 min after etching. (g) Stoichiometry of the MoS2 SE surface as a function of thebinding energy of the Mo 3d5/2 peak. (h) Estimation of the stoichiometry of the MoS2 SE surface calculated from the shifts in the bindingenergies of the Mo 3d5/2 and S 2p3/2 peaks. The error bar was determined from spectra acquired from samples processed under the sameconditions. (i) Comparison of the ratios of the peak areas of the core levels corresponding to S 2s to Mo 3d between samples fabricated viaminimized oxidation (SB−Cond) and samples prepared via prolonged oxidation (SC−Cond).ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614454https://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig2&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asbombardment shows p-type doping, as exemplified by scanningtunneling spectroscopy.38 For the S peak, a well-resolved S2p3/2 doublet at 161.8 eV is observed, which corresponds to aBE of ∼1.0 eV lower than that of bulk MoS2, as shown inFigure 2d. This additional S 2p doublet can be ascribed to themonosulfide,39 supporting the presence of S vacancies at theMoS2 SE edge interface. One may consider that the decrease inthe BE is associated with the formation of metallic Mo.However, we did not observe a larger decrease in the BE of∼1.1 eV corresponding to the reduction of Mo4+ to Mo0,16,18excluding the possibility that the decrease in the BE isassociated with the formation of metallic Mo.The surface structure of the MoS2 SE can be determined byfurther analysis of the XPS spectra. The probing depth of XPSmeasurements is typically 6−9 nm,40 and the capping Au layeris 3 nm thick. Considering the ratio of the intensity of the bulkMoS2 layer to that of the surface layer, it can be estimated thatthe substoichiometric MoSx layer is very thin, at approximately1 nm. Notably, the bulk MoS2 crystal structure extends well tothe edge and is unaffected during etching. The basal plane ofMoS2 requires well-controlled ion sputtering at a grazing angleto etch the top ML.37,41 Importantly, we show that bycontrolling the ion sputtering alignment in the SE geometryplasma etching at the SE naturally occurs at a grazing angle,leading to a substoichiometric MoSx layer that is restrained atthe interface. Furthermore, the well-resolved split Mo 3d and S2p doublets allow us to estimate the stoichiometry of theinterfacial MoSx layer. We first analyze the Mo BE by using theunderlying bulk MoS2 as a reference. As S atoms are removedfrom MoS2, the Mo BE decreases, which reflects the changingelectrostatic environment of the Mo atoms. By adapting therelationship of the MoSx stoichiometry as a result of the Svacancies and Mo BE,35 we plot the distribution of our MoS2SE sample by referencing the underlying bulk MoS2 (Figure2g). Here, it is assumed that the shift in the S 2p3/2 peak BEexhibits the same energy dependence as the Mo 3d5/2 peakbecause the shift in BE is caused by Fermi level alteration.Figure 2h shows the stoichiometry of the MoSx layer in all ofthe SE samples, which were estimated by the shifts in BEs ofboth the Mo 3d5/2 peak and the S 2p3/2 peak, yieldingcomparable S vacancies and a MoSx stoichiometry ofapproximately 1.5.Notably, the fwhm of the Mo 3d5/2 doublet is 0.7 eV, whichis comparable to the value of 0.6 eV for bulk MoS2. Similarly,the fwhm of the additional S 3p3/2 doublet is 0.6 eV, which iscomparable to that of the S 3p3/2 doublet corresponding to theinner MoS2. Because the XPS signal reflects the chemicalconditions, the peak corresponding to the surface of the MoS2edge can broaden after ion bombardment due to the creationof disorders.33,35 The absence of peak broadening in this workFigure 3. (a) Cross-sectional STEM image of a MoS2 SE sample covered with 100 nm of Au. The crystalline MoS2 layers are preserved up tothe Au−MoS2 interface. The scale bar is 2 nm. (b) Energy-dispersive X-ray spectroscopy image of the spatial distribution of Mo and Au inthe MoS2 SE sample. The scale bar is 5 nm. (c) Au 4f XPS spectrum indicating that no bonding occurred between MoS2 and Au. (d) Typicaloptical micrograph of a slanted edge-contacted ML MoS2 field-effect transistor. The red line indicates the SE, and the white dashed linemarks the area of ML MoS2. The scale bar is 2 μm. (e) Raman spectrum of the ML MoS2 sample. Transfer curves for the SE-contacted MLMoS2 transistor (sample D) with ID on the (f) logarithmic and (g) linear scales at T = 290 K.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614455https://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig3&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-assuggests that the substoichiometric MoSx layer has a uniformchemical environment that is comparable to that of crystallineMoS2. This insignificant broadening may be attributed tonegligible disorders or metallic Mo nanoparticles42 in theinterfacial MoSx layer. Generally, the MoS2 edge produced byplasma etching can have many dangling bonds and extensivedisorder, which subsequently lead to the formation of Mooxides in the ambient environment. To assess the extent ofMoS2 SE oxidation, we compared the XPS spectra of sample C,which was kept under ambient conditions (20 °C and 50%relative humidity) for 20 min after SE etching and before beingtransferred to the evaporation chamber, with those of sampleB, where the exposure time was limited to approximately 1min. Figure 2e,f shows the Mo 3d and S 2p spectra of sampleC, respectively, revealing dominant Mo 3d and S 2p doublets.The Mo 3d5/2 doublet at 229.4 eV indicates the presence ofpristine MoS2, which is similar to the SE surface of sample B.Moreover, a small Mo 3d5/2 doublet at 233.0 eV arises,indicating the presence of Mo6+, which is ascribed to theoxidation of Mo4+ (Supporting Information S2).43,44 The Mo3d5/2 doublet corresponding to the substoichiometric MoSxlayer is absent in sample C, suggesting its oxidation uponprolonged exposure. For S, the XPS peaks at higher energiescorresponding to S−O bonds and polysulfide species39,45 werenot observed in either samples B or C. Importantly, thiscomparison indicates that while the samples are exposed toambient air, the oxidation of the etched MoS2 SE can benegligible if the sample transfer time is short (SupportingInformation S3).To examine the effect of oxidation under ambient conditionson the chemical composition of the MoS2 SE, we comparedthe ratio of the core level peak areas corresponding to S 2s tothat of Mo 3d, A(S 2s)/A(Mo 3d), between SB−Cond andSC−Cond, which are plotted in Figure 2i. SB−Cond and SC−Condcorrespond to the samples fabricated via the same oxidationconditions as those used for samples B and C, respectively. Forthe area of the Mo 3d peak, all of the doublets originating frombulk MoS2, the substoichiometric MoSx layer, and the MoO3interfacial layers were included. The ratio A(S 2s)/A(Mo 3d)in the SC−Cond samples is significantly lower than that in theSB−Cond samples, suggesting that the S atoms in the MoSx SElayer are replaced by the O atoms. Notably, sample B does notexhibit any features other than the XPS peaks corresponding tobulk MoS2 and the substoichiometric MoSx layer, indicatingthe absence of detectable interfacial chemical reactions(Supporting Information S2). Generally, for sample B andeven for sample C, the extent of oxidation at the SE of theMoS2 layer is low. Moreover, the XPS spectra of the MoS2 SEsample provide both a quantitative benchmark to assess thequality of the MoS2 SE contact discussed later and directfeedback to optimize the SE contact.The interfacial chemistry of the metal−MoS2 SE interfacecan be further examined by considering the chemical state ofthe Au layer at the interface. Figure 3a shows a TEM image ofthe cross section of the SE, revealing a clear interface betweenFigure 4. Current−voltage scaling relationships of (a) I Vln( / )D DS2 versus ln(1/VDS) and (b) I Vln( / )D DS2 versus −1/VDS for sample D atdifferent temperatures and VGS = 60 V. The clear linear current−voltage scaling relationship indicates that, at low VDS, direct tunnelinggoverns charge transport across the Au−ML MoS2 SE contact. At high VDS, the charge injection mechanism can be attributed to Fowler−Nordheim emission. (c) Output curve for sample D with VGS = 60 V at T = 290 K. Schematics of the band diagram of the charge injectionmechanism (d) for the flat band condition (VGS = VFB), (e) at low VDS, and (f) at high VDS. At low VDS, direct tunneling with negligiblethermionic emission indicates a very small energy barrier. At high VDS and when VGS > VFB, FN tunneling dominates charge injection owingto thinning of the barrier.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614456https://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig4&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe crystalline MoS2 SE and the Au layer. Remarkably, thecrystalline structure of MoS2 is well preserved up to the Au−MoS2 SE interface, which substantiates the inference in theXPS analysis in Figure 2. The darker crystalline structure (tothe left of the interface) is ascribed to the SE not beingperpendicular to the imaging plane. The energy-dispersive X-ray spectroscopy image confirms the clear separation of theMoS2 SE and the Au layer in the interface region, as shown inFigure 3b. The disorder and metal penetration commonlyobserved after deposition are absent, corroborating that thehighly crystalline state extends into the Au−MoS2 SE interfaceat an atomic resolution. To probe the interfacial chemistry ofthe Au−MoS2 SE interface, we present the XPS spectrum ofthe Au 4f core level of sample B, as shown in Figure 3c.Pronounced Au 4f7/2 doublet at 84.0 eV is observed, indicatingthe dominance of metallic gold. XPS peaks corresponding toother chemical states of Au, e.g., Au−S bonding46 and Aunanoparticles,39 are absent, indicating that the Au layerinterfaced with the MoS2 SE without the formation ofchemical bonds.To understand more than simply the chemical informationabout the Au−MoS2 SE interface, we acquire correlatedelectrical characteristics by fabricating a SE-contacted MLMoS2 transistor. Details regarding the fabrication of the SE-contacted ML MoS2 transistors are given in SupportingInformation S4. Figure 3d shows an optical micrograph of atypical SE-contacted ML MoS2 transistor. The SE is fabricatedunder the same etching conditions as those used for sample Bfor the XPS measurements. In contrast to the bulk MoS2 fromthe XPS analysis, the SE in the device is dominated by h-BNencapsulating ML MoS2, as depicted in the inset of Figure 3e(Supporting Information S5). Figure 3e shows the Ramanspectrum of the MoS2 device, revealing peaks at 380.4 and403.4 cm−1, which correspond to the E2g1 and A1g vibrationalmodes, respectively. The two Raman peaks are separated by 20cm−1, indicating that the MoS2 sample is ML,47 which is alsovalidated by the photoluminescence (PL) spectra of the MoS2device shown in Figure 5. We now discuss the transportcharacteristics of the SE-contacted ML MoS2 transistor. Figure3f shows the logarithmic transfer curve, log(ID), as a functionof VGS for the SE-contacted ML MoS2 transistor (sample D) atT = 290 K. Current annealing is conducted on the MoS2 SEsamples to improve the electrical characteristics (SupportingInformation Figure S6). Sample D manifests an ideal turn-oncharacteristic with a large on/off ratio of 1 × 105 and asubthreshold swing of 1 V/dec Figure 3g shows ID as afunction of VGS for sample D on a linear scale at T = 290 K.The threshold is approximately −35 V, indicating n-typetransport behavior, which is in line with the n-type MoS2commonly observed.3,48 Notably, the hysteresis of the transfercurve is negligible, indicating the absence of trap states near theFermi level in both the Au−MoS2 SE contact and the MLMoS2 channel.49−51 The absence of trap states substantiatesthe presence of a clean Au−MoS2 SE interface, which is alsoevidenced by XPS and TEM analysis. Moreover, these findingsconfirm the effectiveness of h-BN encapsulation to protectagainst contamination from ambient molecules. Markedly, theSE-contacted ML MoS2 transistor exhibits a high currentdensity of 5 μA/μm at VGS = 10 V, indicating an effectiveelectrical contact.We now discuss the metal−ML MoS2 SE contact byexamining the electrical conduction mechanism. To under-stand the charge injection and transport mechanism across theAu−ML MoS2 SE contact, we consider direct tunneling52 at alow VDS and Fowler−Nordheim (FN) tunneling53 at a highVDS, where the current−voltage scaling relationships ared e s c r i b e d b y I V Vln( / ) ln(1/ )D DS DS2 a n dI V Vln( / ) 1/D DS DS2 , respectively. Figure 4a shows theplot of I Vln( / )D DS2 versus ln(1/VDS) for sample D at differenttemperatures with VGS = 60 V. At T = 300 and 200 K, thecurrent−voltage relationship is clearly linear, which indicatesthat direct tunneling governs charge transport across the Au−ML MoS2 SE contact. The band alignment at the Au−MLMoS2 SE contact is depicted in Figure 4d. The band gap of MLMoS2 is estimated to be Eg = 2.2 eV.54,55 The ML MoS2channel is strongly n-type, as evidenced by the transfercharacteristics and PL measurements (Figure 5). The directtunneling is observed over the entire VDS range at T = 300 K,reaching a value as small as 0.1 V. The direct tunnelingobserved at T ≥ 200 K indicates a very small energy barriercorresponding to thermionic emission (Supporting Informa-tion S7), as depicted in Figure 4e, facilitating a tunnelingcurrent at the contact interface at low VDS. In addition, thetransport behavior at T = 100 K can no longer be described bythe positive slope, because the direct tunneling is completelysuppressed when thermionic emission decreases.Figure 4b presents the plot of I Vln( / )D DS2 versus −1/VDS forsample D with VGS = 60 V at different temperatures. At highVDS, the charge injection mechanism can be attributed to FNFigure 5. Position-dependent PL spectra of the ML MoS2 SE device measured (a) at the SE contact (d = 0 μm) and (b) away from the SEcontact (d = 3 μm). (c) Ratio of the integrated intensities of the trion to the A exciton as a function of position from the SE contact. Theratio remains at 0.9 away from the SE contact and starts to increase when it approaches the SE contact, reaching 3.2 near the SE contact.ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614457https://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?fig=fig5&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asemission, as evident by the characteristic negative slopes in thecurrent−voltage relationship at different temperatures.56 Thedominance of FN emission over the thermionic emissioncontribution can be ascribed to large band bending at theinterface with a large VDS, as depicted in Figure 4f.Furthermore, the VDS range of the FN emission increases asthe temperature decreases. This is reasonable because as thethermionic emission current decreases, the contribution of theFN emission to the total charge injection increases. Figure 4cshows the output curve, ID as a function of VDS, for sample Dwith VGS = 60 V and T = 290 K. The output curve is nonlinear,which is consistent with tunneling behavior.52,57 Generally,electrical transport is determined by carrier conduction in thechannel and carrier injection across the SE contact. Becauseelectrical transport has been well characterized by thetunneling mechanism, the SE-contacted ML MoS2 transistoris governed by injection-limited conduction, suggesting a lowchannel resistance in the ML MoS2 channel encapsulated by h-BN layers. Notably, the electron injection at the edge contactof the ML MoS2 transistor is dominated by thermionic/tunneling effects, indicating a thin tunneling barrier for efficientcarrier injection (Supporting Information S8).Finally, we performed PL measurements on the ML MoS2channel region to confirm the doping effect at the contactinterface and determine its effect on the electrical properties ofthe SE contact devices. Figure 5a shows the PL spectrum ofML MoS2 adjacent to the SE contact in the ML MoS2transistor (sample D). PL is measured at d = 0 μm, which isdefined as the distance from the SE contact, as depicted in theinset of Figure 5c. In the PL spectrum, the trion (A−), A0, andB excitons are identified with PL energies of 1.90, 1.93, and2.09 eV, respectively.58 The strong PL signal and absence of anadditional indirect transition clearly indicate ML MoS2.59 Thetrion BE is estimated by determining the energy differencebetween the A− and A0 peaks, yielding a value of approximately35 meV. This trion BE is consistent with that reported for MLMoS2,60 validating our assignment of sample D as ML MoS2.The pronounced PL peak of the trion indicates the presence ofan excess of electrons in MoS2, which is typical of MoS2.61Figure 5b shows the PL spectrum of ML MoS2 measured atapproximately d = 3 μm away from the SE contact. Comparedwith the PL spectra measured at d = 0 μm, the ratio of the PLintensity of the A− and A0 excitons apparently decreases(Supporting Information S9). To estimate the doping level, weplot the position dependence of the PL intensity ratio of theA− and A0 excitons, which is sensitive to the density ofnonequilibrium electrons associated with the doping level,62 inFigure 5c. The ratio remains at 0.9 °C from the SE contact andstarts to increase when it approaches the SE contact, reaching3.2 °C near the SE contact. This strong n-type doping in thechannel of the ML MoS2 transistor corroborates the n-typeconduction determined from the transport measurement.Moreover, n-type doping near the SE contact may beattributed to efficient charge injection at the SE contact dueto the tunneling effect.CONCLUSIONSIn summary, by fabrication of a periodic SE structure,conventional XPS can be utilized to investigate the metal−MoS2 interface at the edge contact. The SE structure is realizedby the developed directional etching technique that enablesfeasible elemental analysis of the edge contact, yielding explicitchemical bonding conditions at the Au−MoS2 SE interface.We present distinct XPS spectra from the MoS2 SE, whichreveals pristine bulk MoS2 and an atomically thin substoichio-metric MoSx surface layer at the Au−MoS2 SE interface.Facilitated by the well-characterized Au−MoS2 interface, weachieve high-quality contact in the SE-contacted ML MoS2transistors encapsulated by h-BN. By analyzing the temper-ature-dependent transport characteristics and position-depend-ent PL of the SE-contacted ML MoS2 transistors, we discussthe efficient carrier injection governed by direct and FNtunneling. This edge contact method offers a viable approachto fabricating 2D material devices encapsulated in vdWheterostructures to advance the study of 2D materials forhigh-performance electronic and multifunctional applications.METHODSFabrication of the Periodic SE Structure of MoS2 for XPSAnalysis. We employed e-beam lithography to fabricate a large arrayof closely spaced SEs to increase the size of the cross section forsubsequent XPS surface characterization. The plasma power of thereactive ion etching (RIE) system used for etching is 10 W. The TEMimages of the MoS2 sample with a periodic SE confirm the extendedand uniform SE structure for accurate chemical analysis. When usedas an etching mask during the etching process, the periodic array ofpoly(methyl methacrylate) (PMMA) strips fully suppresses the planarMoS2 XPS signal, indicating that all the observed XPS signals are dueto the SEs.Faraday Cage for Directional Etching of MoS2 SE. Thedimensions of the Faraday cage and relative sample position aredepicted in the Supporting Information. The distance between thesample and the mesh is approximately 2 mm. The Faraday cageincluding the grid is made of stainless steel. The side length of thesquare hole is 0.56 mm and the diameter of the grid is 0.2 mm. Thegeneral rule to shield the radiation is that the diameter of the grid holeshould be less than one-tenth of the wavelength of the radiation. Forour RIE system, the wavelength is approximately 22 m correspondingto a radio frequency of 13.56 MHz. The diameter of the grid hole of0.56 mm is significantly smaller than the wavelength, therefore leadingto complete shielding of the RF electric field by the Faraday cage.ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsnano.4c13581.Faraday cage for directional etching of MoS2 slantededge (SE); S 2p spectra of MoS2 SE samples afternatural oxidation for different durations; XPS analysis ofthe controlled MoS2 SE sample with Cr/Au cappinglayer; fabrication of the SE-contacted MoS2 transistors;slanted edge of h-BN characterized by SEM; Hybridcurrent/thermal annealing of the SE-contacted MoS2transistors; electrical transport mechanism; usingSimmon’s model for tunneling current; power depend-ence of the PL on the ML MoS2 adjacent to the SEcontact of the ML MoS2 transistor (PDF)AUTHOR INFORMATIONCorresponding AuthorWei-Hua Wang − Institute of Atomic and Molecular Sciences,Academia Sinica, Taipei 106319, Taiwan; orcid.org/0000-0003-1737-6648; Phone: +886 2 2366 8208;Email: wwang@sinica.edu.twAuthorsChia-Chun Lin − Molecular Science Technology Program,Taiwan International Graduate Program, Academia Sinica,ACS Nano www.acsnano.org Articlehttps://doi.org/10.1021/acsnano.4c13581ACS Nano 2025, 19, 4452−44614458https://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsnano.4c13581?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsnano.4c13581/suppl_file/nn4c13581_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Wei-Hua+Wang"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-1737-6648https://orcid.org/0000-0003-1737-6648mailto:wwang@sinica.edu.twhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Chia-Chun+Lin"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfwww.acsnano.org?ref=pdfhttps://doi.org/10.1021/acsnano.4c13581?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asTaipei 115201, Taiwan; National Central University,Taoyuan 320317, Taiwan; Institute of Atomic andMolecular Sciences, Academia Sinica, Taipei 106319, TaiwanNaomi Tabudlong Paylaga − Molecular Science TechnologyProgram, Taiwan International Graduate Program,Academia Sinica, Taipei 115201, Taiwan; National CentralUniversity, Taoyuan 320317, Taiwan; Institute of Atomicand Molecular Sciences, Academia Sinica, Taipei 106319,TaiwanChun-Chieh Yen − Institute of Atomic and MolecularSciences, Academia Sinica, Taipei 106319, Taiwan; PresentAddress: Max Planck Institute for Solid State Research,Heisenbergstraße 1, 70569 Stuttgart, Germany (C.-C.Y.)Yu-Hsuan Lin − Department of Physics, National TaiwanUniversity, Taipei 106319, TaiwanKuang-Hsu Wang − Department of Physics, National TaiwanUniversity, Taipei 106319, TaiwanKenji Watanabe − Research Center for Electronic and OpticalMaterials, National Institute for Materials Science, Tsukuba305-0044, Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − Research Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Chi-Te Liang − Department of Physics, National TaiwanUniversity, Taipei 106319, Taiwan; Taiwan SemiconductorResearch Institute (TSRI), Hsinchu 300091, TaiwanShao-Yu Chen − Center of Atomic Initiative for NewMaterials and Center for Condensed Matter Sciences,National Taiwan University, Taipei 106319, Taiwan;orcid.org/0000-0003-3423-9768Complete contact information is available at:https://pubs.acs.org/10.1021/acsnano.4c13581Author ContributionsC.-C.L., N.T.P., C.-C.Y., and W.-H.W. conceived and designedthis project. C.-C.L. fabricated and characterized the MoS2 SEsamples and devices with assistance from N.T.P., Y.-H. L., K.-H. W.C.-C.L. performed the electrical measurements withassistance from C.-C.Y. and Y.-H.L. C.-C.L. performed theoptical measurements with assistance from S.-Y.C. C.-C.L.,N.T.P., C.-T.L., S.-Y.C., and W.-H.W. performed data analysis.K.W. and T.T. provided the h-BN crystals. The manuscript waswritten through the contributions of all authors. All authorshave approved the final version of the manuscript.NotesThe authors declare no competing financial interest.ACKNOWLEDGMENTSW.-H.W. thanks Prof. Mei-Yin Chou for insightful discussions.K.W. and T.T. acknowledge support from the JSPS KAKENHI(grant numbers 20H00354, 21H05233, and 23H02052) andWorld Premier International Research Center Initiative (WPI),MEXT, Japan. W.-H.W. acknowledges NSTC for researchsupport (NSTC 112-2740-M-002-006). 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