RSC Advances This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Electron beam lithography for direct patterning of Cite this: RSC Adv., 2021, 11, 19908 MoS2 on PDMS substrates† Open Access Article. Published on 09 June 2021. Downloaded on 1/11/2022 10:21:59 AM. Gil Jumbert,ab Marcel Placidi,cd Francesc Alzina,a Clivia M. Sotomayor Torresae and Marianna Sledzinska *a Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography. In this work we show a fabrication technique based on direct electron beam lithography (EBL) on MoS2 on polydimethylsiloxane (PDMS) substrates. This easy and fast method takes advantage of the interaction of the electron beam with the PDMS, which at high enough doses leads to cross-linking and shrinking of the polymer. At the same time, the adhesion of MoS2 to PDMS is enhanced in the exposed regions. The EBL acceleration voltages and doses are optimized in order to fabricate well-defined microstructures, which can be subsequently transferred to either a flexible or a rigid substrate, to obtain the negative of Received 2nd February 2021 Accepted 24th May 2021 the exposed image. The reported procedure greatly simplifies the fabrication process and reduces the number of steps compared to standard lithography and etching. As no additional polymer, such as DOI: 10.1039/d1ra00885d polymethyl methacrylate (PMMA) or photoresists, are used during the whole process the resulting rsc.li/rsc-advances samples are free of residues. the electron beam or optical lithography8,9 one of the ideas has Introduction been to directly pattern the substrate and achieve the selective 2D materials, such as transition metal dichalcogenides (TMDs), growth in the desired areas.10,11 In the case of MoS2 on SiO2/Si offer unprecedented possibilities in applications such as elec- substrates, photolithography, O2 plasma and a specic chem- tronics, optics or sensing.1,2 However, the precise control of the ical surface treatment was applied before chemical vapour size and shape of 2D materials is critical for the fabrication of deposition (CVD) growth.12 This process resulted in a single functional devices. As for now, their integration in electronic layer MoS2 with desired shapes and a minimal feature lateral circuits requires a large number of fabrication steps, such as size of 2 mm. Similarly chalcogenide crystals of Bi2Se3 and In2Se3 lithography, etching, and transfer.3 In the future the number of were grown by a method which combines solvent nano- these steps will increase with the increasing complexity of the imprinting with van der Waals epitaxy on mica substrates.13 devices. At the same time 2D materials are particularly sensitive Other alternative nanofabrication approaches have been to contamination and residues, which can introduce defects, developed to directly pattern the 2D materials on different doping and seriously degrade their optical and electronic substrates. In order to avoid the use of a photoresist or PMMA it properties.4–7 Moreover, many cleaning techniques commonly has been proposed to pattern graphene lms through a stencil used in silicon industry, such as strong acids or oxygen plasma, mask by magnetic-assisted ultraviolet ozone treatment.14 can no longer be used for 2D materials processing. However, high resolution patterns would be difficult to achieve Therefore, many efforts have been dedicated to the devel- using such a method. Furthermore, laser ablation has been opment of cleaner alternative fabrication methods. Instead of reported as a relatively fast and precise method in order to thin a multilayer MoS2 down to a monolayer,15 bilayer and trilayer16 with both vertical and lateral control and maintaining good a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, BIST, Campus electronic properties. Similarly an optothermoplasmonic UAB, Bellaterra, 08193 Barcelona, Spain. E-mail: marianna.sledzinska@icn2.cat nanolithography is possible for 2D lms placed on top of b Departament de Fisica, Universitat Autònoma de Barcelona, Bellaterra, E-08193 substrates made of gold nanoparticles.17 It takes advantage of Barcelona, Spain c the localized surface plasmon resonances, which enhance Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, Sant Adrià de Besòs, Spain thermal oxidation and sublimation of the lms when the d Photovoltaic Group, Electronic Engineering Department, Polytechnic University of substrate is exposed to the laser irradiation. However, the main Catalonia (UPC), Barcelona, Spain drawback of this method is the need of a metallic substrate. e ICREA, Pg. Lluı́s Companys 23, 08010 Barcelona, Spain In this work, we report an easy patterning method for MoS2 † Electronic supplementary information (ESI) available. See DOI: on PDMS, which is the most common material employed for 2D 10.1039/d1ra00885d 19908 | RSC Adv., 2021, 11, 19908–19913 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances materials exfoliation and transfer. The process is based on exposed pattern (Fig. 1(b)). For the dry transfer process, the direct electron beam lithography (EBL), however, instead of patterned PDMS was brought in contact with the nal substrate using a standard PMMA mask and dry etching pattern transfer, (Si/SiO2 or PET). A nger-pressure was applied for a few we take advantage of the interaction between the electron beam minutes. Then the PDMS was gently peeled off during approx. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. and the MoS2 on the PDMS. The electron beam exposure 5–10 s. In general, we found that the transfer process quality strongly enhances the adhesion of MoS2 to the PDMS and does not strongly depend on the pressure and time. The process aerwards the lm can be dry-transferred either to a rigid was performed in the ambient environment (20  C, 40% relative substrate, such as Si/SiO2, or to another exible substrate, such humidity). as PET, in order to obtain the negative of the exposed image. The main advantage of this method is the signicant reduction Results and discussion of the number of fabrication steps to three (transfer to PDMS, Open Access Article. Published on 09 June 2021. Downloaded on 1/11/2022 10:21:59 AM. EBL and transfer to the nal substrate). The use of EBL allows While PDMS has been extensively used in nanoimprint lithog- for exposition of arbitrary patterns without a physical mask and raphy not many works have been dedicated to its use in EBL with the resolution comparable to the above-discussed and, more specically, to its interaction with an electron beam. processes. Such a study can give rise to a new nanofabrication tool for 2D materials patterning. It has been reported that during e-beam Nanofabrication process exposure methyl groups of the PDMS are removed allowing the backbones of different polymer chains to become cross-linked MoS2 was grown on glass substrates, by direct sulfurization of (Fig. 2(a)). However, at large doses, the electrons also can a Mo thin lm layer. It is a kinetic synthesis method, which has cause chain scission and the formation of other chemical also been used to produce structures such as nanotubes or species.23,24 At the same time, the PDMS hardness increases with fullerene-like nanoparticles.18,19 In this way, both horizontally the increase in the exposure dose, which has been attributed to and vertically aligned layers can be formed, depending on the the increase in cross-link density and decrease in molecular sample thickness and process conditions.20–22 In this work mobility. Higher degree of cross-linking results in a tighter a continuous MoS2 polycrystalline lm with thickness of 2 nm is network, which is responsible for the increase e in hardness.25 used. Detailed sample characterisation can be found in the ESI Moreover, a monotonic increase in the Young's modulus of (ESI, Fig. S1 to S4†). PDMS from 50 to 350 MPa as a function of the e-beam dose Polymer-free, wet transfer was performed in order to transfer was reported.26 the MoS2 from the original glass substrate to PDMS lms In a previous work, we studied the interactions at the inter- (GelPak).21 The direct transfer procedure consisted in bringing face between MoS2 and PDMS induced by an e-beam27 unveiling the PDMS lm into contact with MoS2 surface. A drop of DI that well-dened micro-patterns can be obtained in the MoS2 water was placed with a pipette at the interface of the PDMS and regions. Due to the insulating nature of the PDMS the majority the MoS2/glass. Due to the surface tension difference, the MoS2 of the dose is accumulated on the surface and the MoS2 lm- layer is lied off from the glass substrate. The PDMS is then PDMS interface undergoes strong cross-linking. We have peeled off together with the attached MoS2 during approx. 5– observed stronger adhesion of the MoS2 to the PDMS substrate, 10 s (ESI, Fig. S5†). The process was performed in the ambient enhanced by the short-bonded PDMS chains. environment (20  C, 40% relative humidity). First of all, a set of 200 mm long rectangles with nominal The EBL was performed to directly pattern the MoS2 on width of1 mm have been exposed at three acceleration voltages PDMS samples (Fig. 1(a)). Parameters such as the exposition of 10, 20 and 30 kV and currents of 0.12, 0.14 and 0.15 nA, dose and acceleration voltage are discussed in the section respectively. In Fig. 2(b) we compare the real width of a rect- below. angle designed to be of 1 mm exposed at two doses, 100 and Finally, aer the exposure the MoS2 lm was dry-transferred 700 mC cm 2, for each of the acceleration voltages. Higher to the nal substrate, where we obtained the negative of the acceleration voltages and smaller doses yield thinner patterns. Fig. 1 Schematic representation of the fabrication process. (a) Electron beam lithography on MoS2 on PDMS substrate followed by dry transfer to (b) the final substrate where the negative of the exposed pattern is obtained. © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 19908–19913 | 19909 View Article Online RSC Advances Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 09 June 2021. Downloaded on 1/11/2022 10:21:59 AM. Fig. 2 (a) Schematic of a section of PDMS polymer chain. (b) Real width of the (nominal) 1 mm wide rectangle exposed at 10, 20 and 30 kV and varying doses. Depth of the of the exposed features in MoS2/PDMS as a function of exposure dose for (c) 20 kV and (d) 30 kV acceleration voltage. 3D representations of the AFM images of rectangles exposed at (e) 20 kV and 600 mC cm 2 dose (f) 30 kV and 300 mC cm 2 dose. Clearly, the closest to the original design is the rectangle to deeper penetration of the electrons in the PDMS which in exposed at 30 kV and the dose of 100 mC cm 2 (1.1  0.2 mm). consequence leads to larger depth of exposed features. Exam- Therefore, we can conclude that the easily achievable minimum ples of the AFM proles are shown in Fig. 2(e) and (f). Patterns size of the features exposed on the MoS2/PDMS is between 1 and exposed using 10 kV acceleration voltage yield small contrast 2 mm, comparable to the techniques reported previously. under the optical microscope, therefore it was not possible to Secondly, a set of 1 mm long rectangles with nominal widths locate them for the AFM measurements (Fig. S6†). However, between 1 and 25 mm have been exposed at the same three aer the dry transfer to Si/SiO2 the negatives of the patterns acceleration voltages. At the voltages of 20 and 30 kV the were clearly visible under the optical microscope. This further exposed patterns were clearly visible under the optical micro- conrms that the penetration depth and the resulting PDMS scope because of the strong cross-linking of the PDMS. We contraction are acceleration dependent. In this work we have performed AFM on the set of rectangles exposed with doses opted for exposition using 20 kV acceleration voltage, which from 100 to 2000 mC cm 2 in order to determine the level of combines good visibility under optical microscope and smaller contraction of the PDMS due to the e-beam interaction. The contraction of the PDMS and, accordingly, shallow and narrow results are compared in Fig. 2(c) and (d). For both voltages we patterns. observe a linear increase in depth of the features with the All exposure doses between 100 and 1000 mC cm 2 generate increasing dose. As expected, higher acceleration voltages lead well dened rectangles, as seen in the optical microscope 19910 | RSC Adv., 2021, 11, 19908–19913 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Fig. 3 (a) Optical image of the set of 1 mm long lines with varying widths exposed at 20 kV. Scale bar 100 mm. (b) Detail of the sample. The well- defined lines are only visible where the MoS2 film is present. Scale bar 30 mm. (c) The sample transferred to PET substrate. Scale bar 100 mm. Open Access Article. Published on 09 June 2021. Downloaded on 1/11/2022 10:21:59 AM. Fig. 4 (a) Example of an optical image of MoS2 layer on PDMS after EBL, exposed at 20 kV and 300 mC cm 2 dose. (b) Optical image of MoS2 layer after EBL transferred to Si/SiO2 substrate. Scale bars 20 mm. (c) SEM image of the detail of the transferred pattern. Scale bar 5 mm. (d) Processed image of (c). Red curve indicates the segmentation of the pattern. (e) Distribution of roughness calculated from the segmented pattern. (f) Raman spectra of the MoS2 layer on original growth substrate and transferred to Si/SiO2 after EBL. © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 19908–19913 | 19911 View Article Online RSC Advances Paper (Fig. 3(a) and (b)). The MoS2 can be transferred to the PET lm radiofrequency components with low cross talk and radiative using dry transfer in order to obtain a well-dened negative of losses and thus improved performance. Properly fabricated 2D the exposed patterns (Fig. 3(c) and S7†). The thinnest lines transistor arrays are crucial for scale-up applications.30,31 which can be successfully transferred are of about 2 mm at the Another application which can potentially benet from our This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. exposure dose of 100 mC cm 2. fabrication technique is the hydrogen evolution reaction (HER). However, the patterns are only well exposed in the regions MoS2 has become one of the most promising electrocatalysts for where MoS2 is present on the PDMS. In the regions where the both the HER as well as for hydro-desulfurization (HDS) of MoS2 is not present (Fig. 3(b) and S8†) we observe that PDMS sulfur-rich hydrocarbon fuels.32,33 Specically, the edges of MoS2 colour changed to almost black and the pattern is not well nanostructures are known to be far more active as compared to dened. We believe this is due to the fact that MoS2 is its basal planes. Bulk MoS2 has relatively low electrical conductive and helps eliminate extra charge, in a similar way in conductivity and small number of catalytically active edges, Open Access Article. Published on 09 June 2021. Downloaded on 1/11/2022 10:21:59 AM. which a conductive polymer or thin metal layer is used for EBL which seriously compromises its HER and HDS catalytic on insulating substrates. performance. On the other hand, patterning of MoS2 thin lms To assess the potential of the present method, we patterned increases the number of active edges as well as allows for spatial and transferred a more complex design than the simple rect- tuning of the edge density for local catalytic reactions. The angles, i.e., the ICN2 logo. The design was exposed with EBL possibility for transfer to arbitrary substrates can further using 20 kV and various doses between 100 and 1000 mC cm 2. increase the process efficiency. Optical images of the exposed sample and the transferred MoS2 pattern to Si/SiO2 are shown in Fig. 4(a) and (b), respectively. All of the exposure doses led to a good pattern denition and Conclusions a successful transfer. A SEM image of a detail of the fabricated In this work we have studied the interaction between an elec- sample is shown in Fig. 4(c). tron beam and the PDMS/MoS2 interface. We observed that Using Matlab the SEM image was converted to black and exposure to the e-beam leads to cross-linking and contraction of white (Fig. 4(d)). In the resulting image, the pixels corre- PDMS, where the feature depth depends on the acceleration sponding to MoS2 pattern were black while the pixels of the voltage and is roughly linearly proportional to the exposure background were white. The MoS2 pattern was segmented by an dose. This effect was used to develop a new fabrication tech- arch of a circle tted on the boundary by an iterative process. In nique based on direct EBL on MoS2 on PDMS substrates. This the process the white area inside the arch and black area easy and fast method allows fabricating well-dened micro- outside were maximized. Fig. 4(e) represents the roughness structures, which can be subsequently transferred to either distribution, calculated as the distances between the pattern a rigid or exible substrate, to obtain a negative of the exposed edge and the tted arch. From the latter, we can extract an image. As no additional polymer, such as PMMA, is used during average (root mean square) roughness of the transferred the EBL the resulting samples are free of residues. While this patterns of about 100 nm, which proves that our technique is technique has been proven on MoS2 we believe it can be used suitable for fabricating microstructures. for a variety of conducting and semiconducting 2D materials, as Raman spectra taken on both as-grown and transferred the mechanism behind the technique relies on the interaction layers show no structural changes occurred in the material between the electron beam and the PDMS. during the processing (Fig. 4(f)). Since no carrier polymer is used during the entire process the presence of organic contamination or residues is minimized. However, in some of Experimental methods the samples we have observed wrinkles and fractures of the MoS2 characterisation MoS2 lm aer the transfer from PDMS to the Si/SiO2 (Fig. S9†). This effect is not caused directly by the EBL, but by the fact that Atomic force microscope topographical images were obtained large area transfer of ultrathin 2D materials is difficult to opti- using a Nanoscope IV controller and a Dimension 3100 head mize. The fractures can occur both during the wet transfer to (Veeco). SEM images were taken in the FEI Quanta 650FEG PDMS as well as during the dry transfer to the nal substrate. ESEM and the FEI MAGELLAN 400L. Raman scattering Also polycrystalline MoS2 has a relatively low fracture strength, measurements were performed using the Horiba T64000 in order of 5%, making it prone to breaking.27 However, we Raman spectrometer and a 532 nm laser (cobolt). The Raman believe it should be possible to perform the same process on the spectra at each point was obtained at the same incident laser CVD grown single crystalline MoS2 or exfoliated akes, which power (0.3 mW) and integrated for the same amount of time will have more possibilities for a successful transfer and device (180 s). Elphy Quantum (Inspect F50 FEI SEM based system) was applications. The crystalline MoS2 has much higher Young used for EBL. High-resolution imaging of the structure and modulus and its critical strain increases signicantly when morphology of the samples was obtained using the FEI Tecnai placed on an underlying substrate.28,29 F20 in TEM and scanning TEM (STEM) modes. One possible application of the proposed process could be in the eld of micro- and nanoelectronics. 2D materials offer Conflicts of interest higher mobility than that of silicon and better scalability, which can lead to producing ultra-low noise eld effect transistors and There are no conicts of interest to declare. 19912 | RSC Adv., 2021, 11, 19908–19913 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances 15 A. Castellanos-Gomez, M. Barkelid, A. M. Goossens, Acknowledgements V. E. Calado, H. S. J. van der Zant and G. A. Steele, Nano ICN2 is funded by the CERCA program/Generalitat de Cata- Lett., 2012, 12, 3187. lunya, and is supported by the Severo Ochoa program from 16 L. Hu, X. Shan, Y. Wu, J. Zhao and X. Lu, Sci. Rep., 2017, 7, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Spanish MINECO (Grant No. SEV-2017-0706). 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