Electrochemical Polishing of Two-Dimensional Materials - ACS Nano

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Electrochemical Polishing of Two-Dimensional Materials Amritanand Sebastian, Fu Zhang, Akhil Dodda, Dan May-Rawding, He Liu, Tianyi Zhang, Mauricio Terrones, and Saptarshi Das ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08216 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Electrochemical Polishing of Two-Dimensional Materials Amritanand Sebastian1, Fu Zhang2, Akhil Dodda1, Dan May-Rawding3, He Liu 5, Tianyi Zhang2, Mauricio Terrones2,4,5,6, and Saptarshi Das1,2,* 1

Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802

2

Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802

3

Energy Engineering, Pennsylvania State University, University Park, PA 16802

4

Material Research Institute, Pennsylvania State University, University Park, PA 16802

5

Department of Chemistry, Pennsylvania State University, University Park, PA 16802

6

Department of Physics, Pennsylvania State University, University Park, PA 16802

ABSTRACT: Two-dimensional (2D) layered materials demonstrate their exquisite properties such as high temperature superconductivity, superlubricity, charge density wave, piezotronics, flextronics, straintronics, spintronics, valleytronics, and optoelectronics, mostly, at the monolayer limit. Following initial breakthroughs based on micromechanically exfoliated 2D monolayers, significant progress has been made in recent years towards the bottom-up synthesis of large-area monolayer 2D materials such as MoS2 and WS2 using physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques in order to facilitate their transition into commercial technologies. However, the nucleation and subsequent growth of the secondary, tertiary, and greater numbers of vertical layers poses a significant challenge not only towards the realization of uniform monolayers, but also to maintain their consistent electronic and optoelectronic properties which change abruptly when transitioning from the monolayer to multilayer form. Chemical or physical techniques which can remove the unwarranted top layers without compromising the

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material quality will have tremendous consequence towards the development of atomically flat, large-area, uniform monolayers of 2D materials. Here, we report a simple, elegant, and selflimiting electrochemical polishing technique which can thin down any arbitrary thickness of 2D material, irrespective of whether these are obtained using powder vapor transport (PVT) or mechanical exfoliation, into their corresponding monolayer form at room temperature within a few seconds without compromising their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D transition metal dichalcogenides (TMDCs) owing to the stability of their basal planes, enhanced edge reactivity, and stronger-than-van der Waals (vdW) interaction with the substrate. Our study also reveals that 2D monolayers are chemically more robust and corrosion resistant compared to their bulk counterparts in similar oxidative environments which enables electrochemical polishing of such materials down to a monolayer.

KEYWORDS: two-dimensional (2D) materials, monolayer, physical vapor transport, electrochemical polishing, electro-ablation, corrosion, MoS2, WS2.


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Chemical mechanical polishing (CMP) has played a critical role in the evolution of scaled Si technology as it enabled the development of ultra-flat surfaces with minimal asperities that eventually allowed for higher integration densities and lesser device-to-device variability.1-3 A similar planarizing technology would benefit the two-dimensional (2D) layered materials, especially the semiconducting transition metal dichalcogenides (TMDCs) such as MoS2, WS2, etc., which aspire to replace or at least coexist with the Si industry in the near future.4 In fact, a technology that can scale these materials down to their atomistic thickness without compromising the quality will have far-reaching consequences since most of their extraordinary properties appear at the monolayer limit. For example, monolayer TMDCs are only ≈0.7nm thick which allows aggressive length scaling beyond the state-of-the-art 7nm technology node when used as the semiconducting channel material in a field effect transistor (FET) geometry.5-8 In monolayer form, the TMDCs are direct bandgap semiconductors fostering enhanced light-matter interaction which enables their use in highly efficient photonic and photovoltaic devices, unlike their indirect bandgap multilayer counterparts which are devoid of such interactions.9 Monolayer TMDCs also exhibit spin hall effects (SHE), valley hall effects (VHE),10 enhanced quantum mechanical tunneling phenomena,11, 12 and high temperature excitonic condensate formation which lay the foundation for future ultra-low power electronics.13, 14 Further, monolayer TMDCs can be flexed, stretched, and even squeezed to enable flextronic, piezotronic, and straintronic devices.15-19 These fascinating characteristics, which are prevalent in monolayer TMDCs, are either absent or insignificant in their multilayer counterparts. Moreover, abrupt transitions in the electrical, optical, mechanical, and chemical properties20-23 take place between monolayer and multilayer TMDCs, necessitating techniques similar to CMP which can ensure atomistic uniformity and integrity at the monolayer limit over large areas.


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While foundational discoveries were reported based on micromechanically exfoliated (ME) single crystalline monolayer TMDCs, recent years have witnessed outstanding progress towards largearea synthesis of monolayers using several bottom-up techniques such as PVD,24 CVD,25 and metal organic chemical vapor deposition (MOCVD).26 Extensive research has gone into understanding and controlling the impact of the carrier gas, reactant concentrations, process temperature, precursors, substrates, and seeding promoters on the nucleation and growth mechanisms of 2D TMDCs.27, 28 Two types of growth mechanisms exist: planar nucleation (Frank-van der Merve growth mechanism) which facilitates the growth of monolayer and bilayer TMDCs, and selfseeding nucleation (Volmer–Weber growth mechanism) which promotes the growth of few-layer and multilayer TMDCs.29 In either case, it is extremely challenging to maintain large-scale uniformity which is critical for reducing device-to-device variability and maintaining performance consistency. Here, we report a room temperature, ultrafast electrochemical polishing mechanism which uses a self-limiting oxidative corrosion process in order to eliminate any unwanted vertical layers from the large-area grown 2D TMDCs and flattens them down to a monolayer without affecting their atomistic integrity. The effectiveness of this electrochemical polishing technique is inherent to 2D TMDCs owing to the stability of their basal planes, enhanced edge reactivity, and stronger-than-van der Waals (vdW) interaction with the substrate.

Results Figure 1a shows the schematic of the PVT system used for the synthesis of large area monolayer TMDCs such as MoS2 and WS2. For MoS2, elemental sulfur and MoO3 were used as precursors. Perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) was drop-casted on the Si/SiO2


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substrate preceding the growth to act as a nucleation agent, promoting a seed-assisted growth process. The hot-zone temperature was ramped to 650 ℃, and the growth time was set to 10 min. During the entire growth process, Ar flux was held at approximately 5 sccm. WS2 monolayers were synthesized following a previous report.30 First, 0.1 g of ammonium metatungstate hydrate (AMT) and 0.2 g of sodium cholate powders were dissolved in 10 mL deionized water. The solution was then spin-coated onto the SiO2/Si substrate to form a solid precursor layer. Subsequently, the precursor-coated substrate was placed inside a tube furnace and sulfurized by vaporized sulfur at 850 ℃ to form WS2 monolayers. The growth time was 15 min. During the synthesis process, 100 sccm of Ar was used as the carrier gas. As shown in the representative optical image in Figure 1a, the PVT growth predominantly yielded monolayer TMDCs. However, multilayer regions and islands were frequently found to coexist with the desired monolayer regions throughout the respective growth substrates for both MoS2 and WS2. In order to perform the electrochemical polishing, MoS2 and WS2 flakes were subsequently transferred from their respective growth substrates onto conductive TiN substrates using a conventional PMMA-assisted wet transfer process as shown in Figure 1b.31 In this process, polymethyl-methacrylate (PMMA) films are spin-casted on the growth substrates encapsulating the MoS2 and WS2 flakes followed by immersion in a 1 M NaOH solution at 90 °C. Capillary action draws the NaOH solution to the PMMA/substrate interface, separating the hydrophobic PMMA/MoS2 and PMMA/WS2 from their respective hydrophilic substrates. The detached films float on the surface which are then rinsed multiple times in deionized water and are finally transferred on to the TiN substrates. Finally, the samples are placed in a three-terminal electrochemical cell with the conductive TiN substrates acting as the working electrode (WE), Platinum (Pt) or graphoil as the auxiliary electrode (AE), and a 1 M LiCl solution as the electrolyte for executing the electrochemical polishing as shown


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schematically in Figure 1c. A relatively small anodic potential is applied against a Ag/AgCl reference electrode (RE) for a short period of time which corrodes the multilayer regions of MoS2 and WS2 flakes regardless of their initial size, shape, and thickness and converts them into their corresponding monolayers over the entire substrate. Figure 2a shows the optical images of PVT MoS2 and, for comparison, Figure 2b shows the optical images of ME MoS2 before and after the electrochemical polishing which was executed at an anodic potential of 1.4V versus the Ag/AgCl reference electrode using a 1 M LiCl solution of pH = 3 at room temperature for 10s and 90s, respectively. Clearly multilayer regions are planarized into monolayers while monolayer regions remain unscathed. Similar observations were made for PVT WS2 in Figure 2c, and ME WS2 in Figure 2d, respectively. For electrochemical polishing of PVT WS2 and ME WS2, an anodic potential of 1.4V versus the Ag/AgCl reference electrode was used in a 1 M LiCl solution of pH = 6 at room temperature for 22.5s and 90s, respectively. Pioneering work on the electrochemical polishing, also referred to as electroablation, of exfoliated 2D materials was reported by Das, S. et al32 and followed-up by Schulman, D. et al.33, 34 It was demonstrated that the electrochemical polishing of ME MoS2 and ME WS2 is minimally impacted by the pH, molarity, and the cationic and anionic components of the electrolyte solution. However, the anodic potential was found to have significant impact on the electrochemical polishing of MoS2 and WS2.33 Figures 2e and 2f, respectively, show the potentio-dynamic (PD) measurements for MoS2/TiN and WS2/TiN obtained by sweeping the anodic potential versus the Ag/AgCl reference electrode. Strong peaks appear at 1.2 V for MoS2 at pH = 3 and 1 V for WS2 at pH = 6 during the first PD scans and are attributed to the irreversible electrochemical polishing and conversion of multilayer MoS2 and WS2 into their corresponding monolayers. Electrochemical polishing executed below these peak potentials were ineffective in the dissolution of multilayer regions.


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Hence, an over-potential of 1.4 V was provided to ensure uniform polishing. For exfoliated TMDCs shown in Figures 2b and 2d, the polishing can sometimes be limited by the passivation of the underlying TiN substrate. At the polishing potential (1.4 V), oxidative species that are responsible for the dissolution of the multilayer MoS2/WS2, also passivate the TiN substrate through the formation of TiO2,35 which is given by the following equation: TiN(s) + 2H2O (l) → TiO2 (s) + 1/2N2 (g) + 4H+(aq) + 4e−

(1)

In general, the passivation of TiN is slow compared to the rate of polishing for the TMDCs. However, it should be noted that the absolute amount of time required to dissolve thicker multilayers may be sufficient to passivate the underlying TiN, leading to incomplete polishing. Since exfoliated flakes are mostly thick (≈20-100nm), they require a longer polishing time (90s for both MoS2 and WS2), passivating TiN and leaving multilayer residues. Whereas PVT grown multilayer islands are relatively thin (≈10-30nm) and, therefore, can be polished faster (10s and 22.5s, for MoS2 and WS2, respectively), yielding uniform monolayers. To obtain cleaner monolayers from exfoliated materials, multiple transfers to fresh TiN substrates yielded improved results. We also explored other conducting substrates such as Pt with limited success.36 A more thorough study on different substrates can provide further insight into the process.

To elucidate the morphology and quality of the polished monolayers, atomic force microscope (AFM) images along with Raman and photoluminescence (PL) spectra are shown in Figure 3. Clearly, uniform monolayers are obtained with no to minimal residue as seen from the AFM images in Figures 3a and 3d for PVT-MoS2 and PVT-WS2, respectively. The reduction in the separation between the A1g and E2g peaks from 25.5 to 20 cm-1 in the Raman spectra of MoS2 in Figure 3b is a clear indication of the successful conversion of multilayer islands to monolayer


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using the electropolishing. Further, the emergence of a PL peak at 1.9eV in Figure 3c, corresponding to the exciton peak for monolayer MoS2, reaffirms the success of the electrochemical polishing. Note that the PL response emerges if there is an indirect to direct bandgap transition, which occurs only at the monolayer limit for 2D TMDCs. Similarly, for WS2, the increase in the ratio of the 2LA(M) peak intensity to A1g peak intensity from 1.6 to 2.38, and the shift in peak position of the 2LA(M) peak from 350.1 to 351.1 cm-1 in the Raman spectra shown in Figure 3e confirms multilayer to monolayer conversion. In addition, the emergence of a PL peak at 1.97eV in Figure 3f corresponding to the exciton peak for monolayer WS2 reasserts electropolishing down to monolayer.

Figures 4a and 4c demonstrate the transfer characteristics (i.e. drain current, IDS versus back-gate voltage, VGS), and Figures 4b and 4d demonstrate the output characteristics (i.e. drain current, IDS versus source to drain voltage, VDS) of as grown and electropolished monolayer MoS2, respectively. The devices were fabricated on a 50 nm Al2O3 back-gate dielectric grown via atomic layer deposition on a Pt coated highly doped Si substrate. Note that the choice of high-k (relative dielectric constant ≈ 10)37 and 50 nm thin Al2O3 over conventionally used 300nm SiO2 is motivated by better gate electrostatics. Clearly, no to minimal difference is observed in the electrical performance of the FETs based on as-grown and electropolished monolayer MoS2.38 The ON current, OFF current, ON-OFF ratio, subthreshold slope (SS), and mobility values extracted from the peak transconductance, shown in the inset of the corresponding figures, convincingly prove that the electrochemical polishing preserves the integrity of the monolayer MoS2. This is a truly intriguing finding given that the material has undergone two transfers processes from SiO2 to TiN for polishing and from TiN to Al2O3 for device fabrication and has


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been in an extremely corrosive environment during the polishing. Yet, the device characteristics do not show any observable changes, illustrating the robustness of monolayer MoS2 to the electrochemical polishing.

Figures 5a, 5b, 5c, and 5d show the optical images and corresponding photoluminescence (PL) maps for PVT MoS2 and PVT WS2 obtained before and after the electrochemical polishing, respectively. As shown in Figure 3c, strong PL emerges at ≈1.89eV from the monolayer edges of the as-grown MoS2 corresponding to its direct bandgap,9 whereas no PL is observed from the multilayer islands owing to their indirect bandgap. Interestingly, as shown in Figure 5b, intense PL is observed across the entire region of the same flake after the electrochemical polishing, which clearly demonstrates the successful removal of the unwanted multilayer regions or islands without affecting the monolayer regions. Similarly, Figure 5c shows no observable PL from the as grown multilayer WS2. However, strong PL emerges from the edges of the same flake at ≈1.97eV corresponding to its direct bandgap at the monolayer limit after the electrochemical polishing as shown in Figure 5d.39 This clearly demonstrates the usefulness of the electrochemical polishing technique and at the same time reveals the extraordinary chemical stability of the monolayers in an oxidative environment that rapidly corrodes their multilayer counterparts.

Figure 6 shows the atomic resolution scanning/transmission electron microscope (S/TEM) imaging of monolayer TMDCs obtained after the electrochemical polishing. Figures 6a, 6c, 6e, and 6g show low magnification STEM images of PVT MoS2, PVT WS2, ME MoS2, and ME WS2 obtained after the electrochemical polishing, respectively. Single versus few layer regions can be distinguished via a linear relationship between the layer number and image intensity in STEM


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mode due to the Z-contrast imaging mechanism in STEM. All the TMDCs were found to preserve their atomistic integrity (hexagonal patterns) after the electrochemical polishing as obvious from the high-resolution STEM images shown in Figures 6b, 6d, 6f, and 6h for PVT MoS2, PVT WS2, ME MoS2, and ME WS2, respectively, articulating the elegance of the self-limiting electrochemical polishing technique. However, it is noted that the monolayer ME-TMDCs were found to be chemically more robust and resilient to corrosion than the PVT-TMDCs. Nanopores with dimensions ≈ 5-10nm can be found in monolayer PVT-TMDCs after the electrochemical polishing. This is expected due to a relatively large number of intrinsic defects such as vacancies and grain boundaries in the monolayer TMDCs introduced by the thermodynamic PVT process. These defect sites are chemically active and facilitate corrosion of the monolayers which are otherwise chemically inert.40

Figure 7 shows the spatiotemporal evolution of the electrochemical polishing of PVT MoS2 and PVT WS2. Note that instead of top-down removal of multilayer regions, the electrochemical polishing begins at the edges of the multilayer flakes and gradually proceeds towards the center regions. This is because the basal plane of multilayer TMDCs is chemically inert, whereas, edge sites are energetically active. However, the edge reactivity is greatly reduced for the monolayer regions owing to their stronger-than-vdW interaction with the substrate, unlike the weak vdW interaction among the individual layers. Density functional theory (DFT) calculations by Das, S et al. found a binding energy value of -1.25 eV for MoS2/TiN interface which is significantly higher than the MoS2/MoS2 vdW interaction of -0.16 eV resulting in the self-limiting nature of the electrochemical polishing.32 However, the dissolution of the multilayer regions and islands during the electrochemical polishing of MoS2 and WS2 involves corrosion of their edge planes via


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oxidizing agents such as O2, H2O2, etc. which are generated at the interface of the TiN and the electrolyte as a result of the applied anodic potentials. We hypothesize that the oxygen is initially chemisorbed at the edge planes of the TMDCs (MX2, where M stands for Mo and W, and X stands for S) to form O-X bonds. This chemisorption is followed by desorption of oxygen in form of O– X species since the DFT calculations suggest that the formation energy of oxygen adsorption from O-X bonds has a relative lower stability compared to the energy of oxygen replacement M-O bonds. Oxygen desorption leaves behind unsaturated bonds for the transition metal, which is then readily available to form M–O bonds in presence of oxygen. Moreover, S vacancies present at the edge planes further facilitate M-O bond formation. M-O-X Pourbaix diagrams36, 41 suggest that, for the pH and anodic potentials used in the electrochemical polishing, the most likely reaction products are MO3 (s), HSO4- (aq), or SO42- (aq) as given by equation 2. MO3

(s)

is known to be

thermodynamically unstable and readily hydrolyzed into molybdate and tungstate following equation 3 which results in the dissolution of the oxide, and hence the completion of the corrosion process for the bulk layers. Other studies also show that elemental species such as MO42- (aq) and SO42- (aq) could be generated at anodic potentials above 1.0 V. Similarly, S vacancies in the basal plane of MoS2, especially for PVT MoS2 act as sites of electrochemical activity, resulting in MO3 formation and its subsequent dissolution. MX2 (s) + H2O (l)/O2 (g) → MO3 (s) + HSO4- or SO42-(aq) MO3 (s) + 2OH- (aq) ↔ MO42- (aq) + H2O (l)

(2)

(3)

Unlike MoS2, the electropolished monolayer WS2 occasionally shows non-uniformities as observed in Figure 7b. This can be explained on the basis of the electro-oxidation dynamics of the S vacancies created during the electropolishing of monolayer MoS2 and WS2. While, S vacancies have a higher energy of formation in WS2 (2.8eV) compared to MoS2 (2.6eV), the corresponding


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adsorption energy of an O2 molecule on these S vacancies are, respectively, 2.1eV and 1.8eV, indicating a weaker oxygen-vacancy interaction for MoS2 compared to WS2.42 Further, MoS2 has a larger energy barrier, 0.93eV for O dissociation versus 0.86eV for WS2, and is thus less susceptible to oxidation at the vacancy sites. Hence, MoS2 is more robust to electropolishing, yielding uniform and high quality monolayers, whereas the same is not always true in the case of WS2. While substantial portions of monolayer WS2 were still intact after the electrochemical polishing, some of the regions were partially etched away. Moreover, the relatively lower quality of the as-grown PVT WS2 compared to the as-grown PVT MoS2 also contributes to the faster etching of monolayer WS2 due to the presence of a larger number of defects acting as nucleation sites for the monolayer etching. With further improvements in the quality of WS2 obtained from PVT growth, more consistent polishing can be achieved.

Conclusion In summary, we have demonstrated an elegant and ultra-fast electrochemical polishing technique for planarizing 2D TMDCs down to their corresponding monolayers, irrespective of their size, shape and thickness at room temperature. Further, we have established the compatibility of the electrochemical polishing for any TMDC source including mechanical exfoliation and powder vapor transport. The electrochemical polishing also revealed the superior stability of the monolayer TMDCs in oxidative environments that rapidly corrode their bulk counterparts.

Methods Electropolishing: Si wafer sputtered with 100nm of TiN from West Coast Silicon is used as the substrate for the electropolishing. TiN is annealed in N2 for 10 min at 500 ˚C to ensure the


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chemical stability of the substrate using an Allwin21 Heatpulse 610 anneal tool. The TMDCs grown using PVT are transferred to the annealed TiN substrates. The methods for the growth and transfer are discussed in detail in the Results section. Single crystal multilayer TMDCs are obtained directly on TiN through micromechanical exfoliation using a scotch tape. The electropolishing is performed using a Solartron Analytical 1287 potentiostat with TiN as the working electrode (WE).

Optical Characterization: The Raman and PL spectra of the multilayers and monolayers before and after electropolishing, respectively are obtained using a Horiba LabRAM HR spectrometer with a 532 nm laser excitation wavelength. AFM images are obtained using Burker Icon using a SCANASYST-AIR tip. For the TEM images, the electropolished monolayers are transferred to a Quantifoil TEM grid. Z-contrast imaging of ADF-STE is obtained from FEI Talos and Titan TEM using an 80kV beam.

Electrical Characterization: For electrical characterization of post-polished and post-transferred PVT monolayers, we fabricated back gated field effect transistor (FET) geometries. The back gate consisted of atomic layer deposition (ALD) grown 50nm alumina (Al2O3) as the gate oxide and a stack of Pt/TiN/p++Si as the back gate electrode. This Al2O3 substrate is sthen pin-coated with methyl methacrylate (MMA) followed by PMMA. Electron-beam lithography is used to expose the desired regions using an EPBG 5200 Vistec E-beam tool, which is then developed using a 1:1 mixture of 4-methyl-2-pentanone (MIBK) and 2-propanol (IPA) for 60 sec. Following this, 40 nm of Ni, and 30 nm of Au are evaporated using a Lab18 E-beam evaporation tool. Lift-off of the evaporated metals is done by immersion in Acetone for 30 min. The FET measurements are done


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in a Lake Shore CRX‐VF probe station in high vacuum (≈10-6 Torr) using a Keysight B1500A parameter analyzer.


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AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Acknowledgement The work of A. S. and S.D. were partially supported through Grant Number FA9550-17-1-0018 from Air Force Office of Scientific Research (AFOSR) through the Young Investigator Program. Authors also acknowledge the National Science Foundation through the I/UCRC Center for Atomically Thin Multifunctional Coatings (ATOMIC), grant No. IIP-1540018. Additional Information: The author declare no competing financial interest.


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Figure Captions

Figure 1. Electrochemical polishing. (a) Schematic of the PVT growth set-up and a representative optical image showing that large-area monolayer TMDC growth leads to undesired nucleation and growth of second and subsidiary layers as islands. (b) Schematics of the wet transfer process for transferring TMDCs from the growth substrate (SiO2) onto the polishing substrate (TiN) using PMMA capping and a NaOH solution. (c) Schematics of the electropolishing set-up and a representative optical image of a polished TMDC.

Figure 2. Optical images and potentio-dynamic (PD) measurements of the electropolishing process. (a) PVT-grown MoS2, (b) ME MoS2, (c) PVT-grown WS2, and (d) ME WS2 before and after electropolishing. Clearly, multilayer islands are removed leaving behind uniform monolayers. Potentio-dynamic (PD) scans for (e) MoS2 and (f) WS2, respectively, obtained by sweeping the anodic potential versus the Ag/AgCl reference electrode at a scan rate of 10mV/s. Strong peaks appear at 1.2 V for MoS2 at pH = 3 and 1 V for WS2 at pH = 6 during the first PD scans and are attributed to the irreversible electrochemical polishing and conversion of multilayer MoS2 and WS2 into their corresponding monolayers. Electrochemical polishing executed below these peak potentials were ineffective in the dissolution of multilayer regions.

Figure 3. Characterization of electropolished monolayers. (a) AFM image of PVT-grown and electropolished monolayer MoS2. (b) Raman spectra of PVT MoS2 before and after electropolishing. The reduction in the separation between A1g and E2g peaks from 25.5 to 20 cm-1 is a clear indication of successful conversion of multilayer islands to monolayer. (c) Emergence of PL at 1.89 eV in electropolished monolayer MoS2. (d) AFM image of PVT-grown and


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electropolished monolayer WS2. (e) Raman spectra of PVT WS2 before and after electropolishing. Increase in the ratio of the 2LA(M) peak intensity to A1g peak (from 1.6 to 2.38), along with the shift in peak position of 2LA(M) from 350.1 to 351.1 cm-1 confirms multilayer to monolayer conversion. (f) Emergence of PL at 1.97 eV in electropolished monolayer WS2.

Figure 4. Performance of field effect transistors (FETs) based on electropolished monolayers. Transfer characteristics of field effect transistors (FETs) based on (a) as grown PVT monolayer MoS2 demonstrating field effect mobility of 3 cm2/Vs, ON-OFF ratio ≈106 and SS= 625 mV/decade, and (c) electropolished monolayer MoS2 with field effect mobility of 2.5 cm2/Vs, ONOFF ratio ≈106 and SS= 335 mV/decade. These FETs were fabricated on 50nm ALD grown Al2O3, used as the back-gate dielectric, on a Pt coated highly doped Si substrate. The field effect mobility values were extracted from peak transconductance. Corresponding output characteristics of (b) as grown PVT monolayer MoS2 and (d) electropolished monolayer MoS2. Clearly, the electrochemical polishing does not impact the electronic properties of monolayer MoS2.

Figure 5. Uniformity of electropolished monolayers using PL. Optical images and corresponding PL maps for PVT-grown MoS2 (a) before and (b) after electrochemical polishing. Strong PL emerges at ≈1.89eV from the monolayer edges of as grown MoS2 corresponding to its direct bandgap. After electrochemical polishing intense PL is observed across the entire region of the same flake. Optical images and corresponding PL maps for PVT-grown WS2 (c) before and (d) after electrochemical polishing. No observable PL is found in as grown WS2. However, strong PL emerges from the edges of the same flake at ≈1.97eV corresponding to its direct bandgap at the monolayer limit after the electrochemical polishing.


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Figure 6. TEM characterization. TEM analyses of electropolished 2D transition metal disulfide materials. Low-mag STEM image of electrochemical polished monolayer (a) PVT-grown MoS2, (c) PVT-grown WS2, (e) ME-MoS2 and (g) ME-WS2, respectively, nanoscale holes were introduced for PVT grown materials during EA process due to extensive intrinsic defects in synthesized materials; Atomic resolution HAADF-STEM image of electropolished monolayer (b) PVT-grown MoS2, (d) PVT-grown WS2, (f) ME-MoS2 and (h) ME-WS2, respectively.

Figure 7. Spatiotemporal evolution of electrochemical polishing. The temporal evolution of electrochemical polishing for (a) PVT-grown MoS2, (b) PVT-grown WS2. Instead of top-down removal of multilayer regions, the electrochemical polishing begins at the edges of the multilayer flakes and gradually proceeds towards the center regions.


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Figure 1


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Figure 2


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Figure 3


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Electrochemical Polishing of Two-Dimensional Materials - ACS Nano

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