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Synthesis of Molybdenum Disulfide Nanowire Arrays Using a Block Copolymer Template Wei Wei, Leith Samad, Jonathan W. Choi, Yongho Joo, Austin Way, Michael S. Arnold, Song Jin, and Padma Gopalan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01453 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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Chemistry of Materials

Synthesis of Molybdenum Disulfide Nanowire Arrays Using a Block Copolymer Template Wei Wei,† Leith Samad,‡ Jonathan W. Choi,† Yongho Joo,† Austin Way,† Michael S. Arnold,† Song Jin,‡ Padma Gopalan*†‡ †

Department of Materials Science and Engineering, University of Wisconsin—Madison, Madison, Wisconsin

53706, United States ‡

Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin 53706, United States

■ ABSTRACT

A simple route for the synthesis of arrays of sub-20 nm wide molybdenum disulfide (MoS2) nanowires using self-assembled cylinder forming poly(styrene-b-2-vinylpyridine) thin films is demonstrated. The protonated 2vinylpyridine selectively seeds molybdenum precursors in the aqueous solution, and the precursors are converted to molybdenum sulfide during a sulfur annealing process. An ultraviolet crosslinking step is introduced to ensure the successful transfer of the morphologies of the block copolymer templates to the MoS2 nanowires. The nanowires transition from an amorphous to a crystalline MoS2 phase upon thermal annealing in the presence of sulfur, as confirmed by X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction and transmission electron microscopy. This work provides a pathway to large area, dense, spatially localized arrays of transition metal dichalcogenide nanowires for catalytic and sensing applications.

1 ACS Paragon Plus Environment


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■ INTRODUCTION

Transition metal dichalcogenides (TMDCs) have been studied extensively due to their graphene-like twodimensional (2D) layered structure and tunable electrical properties.1-3 TMDCs follow the common formula MX2, where M represents a transition metal (M = W, Mo, Ta, etc.) and X represents a chalcogen (X = S, Se, Te). MoS2 is one of the most heavily investigated TMDCs due to the relative ease of synthesis and a bandgap range of 1.3-1.8 eV with an indirect-direct-transition resulting from a bulk to monolayer transition.1, 4 Such tunable electronic and optoelectronic properties in the 2D layers with suitable edge and surface structures make MoS2 a promising material for a variety of applications such as transistors, photodetectors, biosensors, catalysis and photoelectrochemical solar energy conversion.2, 5-11 Hence, significant effort has been invested in the synthesis of MoS2 through various methods such as solution phase growth, thermal conversion, exfoliation, chemical vapor deposition (CVD) and atomic layer deposition (ALD).9, 12-16 For many applications, there is a pressing need not only to control the placement of MoS2 but to do so in a scalable manner to make practical devices, a requirement that is not met by current synthetic methods. In an attempt to circumvent this issue, recent studies have focused on controlling the placement of MoS2 flakes using methods such as stamping, masking, and lithography.17-20 The stamping method in particular uses top-down patterning of bulk MoS2 with flake sizes in the sub-micron range, and is limited by poor interfacial connectivity (due to the inherent roughness of bulk MoS2); while the masking process selectively grows circular (diameter ≈ 500 μm) MoS2. A common issue faced by all these methods is the inability to achieve high density of MoS2 nanostructures while maintaining the uniformity and scalability. A high density of patterned features is desirable not only in the semiconductor industry but also for sensors and in catalysts that require large surface areas to increase the number of active sites.7, 10-11, 21


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One promising method to overcome these fabrication challenges and synthesize well-defined nanomaterials such as nanowires and quantum dots is by directed self-assembly of block copolymers (BCPs). BCPs consist of two or more polymer blocks linked by a covalent bond. The chemical differences between the blocks, which is reflected in the Flory-Huggins interaction parameter (χ), combined with the chemical connectivity between the two blocks leads to microphase separation. For a diblock copolymer, morphologies such as spheres, cylinders and lamellae can be achieved through self-assembly by controlling χ, molecular weight of the copolymer, and the volume fraction of the blocks. High density features with typical sizes ranging from 3-50 nm can be formed in thin films with controlled orientation and lateral order on various substrates.22-24 By utilizing the inherent etch contrast of the blocks, one of the domains can be selectively removed to create a mask that can be used to transfer the pattern to the underlying substrate. The etch selectivity can be further enhanced by selective incorporation of metal precursors that are then reduced via plasma, thermal decomposition, or a chemical reducing agent.25-27 For example, poly(styrene-block-2vinylpyridine) (PS-b-P2VP) has been used to grow Pt nanowires within P2VP domains by immersing selfassembled PS-b-P2VP thin film into an aqueous solution of Na2[PtCl4] followed by oxygen plasma exposure.25, 27 Selective nucleation and growth of inorganic nanocrystals such as cadmium sulfide, zinc sulfide, and lead sulfide onto the surface of poly(styrene-block-acrylic acid) (PS-b-PAA) via a low-temperature aqueous-solution chemical bath deposition methods has also been demonstrated.28 Other BCPs such as poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) and poly(styrene-block-ethylene oxide) (PS-b-PEO) have also been used to generate metal and metal oxide nanowires and quantum dots.29-30 Although self-assembled BCP thin films have been used as templates to grow a variety of materials, there are limited studies on TMDCs. For example, earlier studies have demonstrated growth of MoS2 and MoO3 nanoparticles



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in BCP micelles or composites of BCP and MoS2.31-33 However, selective seeding and spatial localization of MoS2 with BCP templates to achieve high density nanowire arrays is yet to be demonstrated. Likewise direct synthesis of MoS2 nanotubes and fullerenes, synthesis of core-shell MoO3-MoS2 nanowires, and the electrochemical/chemical synthesis of MoS2 nanowires on highly oriented pyrolytic graphite (HOPG) has been shown with little to no control over placement.15, 34-37 A combination of dip coating and self-assembly from precursor solution has also been used to create micron-sized low density (~ 1 nanowire/µm) MoS2/WS2 wire arrays.38 In this paper, we demonstrate the selective seeding of ammonium tetrathiomolybdate (ATTM) or ammonium heptamolybdate (AHM) precursors into the vinyl pyridine domains of a self-assembled PS-b-P2VP thin film followed by annealing process to create molybdenum sulfide nanowire arrays on SiO2/Si substrates. The growth conditions were tuned to produce either amorphous or polycrystalline molybdenum sulfide nanowires, which are promising materials for sensor and catalytic applications due to the large surface area.7, 9-11, 21, 39-40 This work represents a scalable method to synthesize arrays of TMDC nanowires with < 20 nm diameters localized by a BCP template with high density (~ 39 nanowires/µm) and uniformity. During the course of review of this manuscript a similar report by Chaudhari et.al appeared, which uses a solvent annealed lamella forming BCP of PS-b-P4VP, and a MoCl5 precursor deposited on the BCP film to generate MoOx followed by sulfurization, to obtain MoS2 nanowires.41 The solid state reaction used by Chaudhari et.al has advantages from a processing perspective, but the solution based method used in our work gives more complete conversion resulting in high fidelity sub 20 nm polycrystalline nanowires.

■ EXPERIMENTAL SECTION

Materials and Reagents.

Poly(styrene-block-2-vinyl pyridine) (PS-b-P2VP, 23.6 k-10.4 k, Mw/Mn = 1.04) was

purchased from Polymer Source Inc. (Dorval, Quebec, Canada) and used without further purification. (NH4)2MoS4


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(Ammonium Tetrathiomolybdate, 99.97% trace metals basis) was purchased from Aldrich. (NH 4)6Mo7O24·4H2O (Ammonium Heptamolybdate Tetrahydrate, 99.98% trace metals basis), Sulfur (99.5% - 100.5%) and Toluene (ACS reagent, ≥ 99.5%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37%) was purchased from SigmaAldrich and diluted with DI water. DI water was obtained from a Millipore system (resistivity > 18MΩ). Polymer Template Preparation.

Si (100) substrates were cleaned with piranha acid (sulfuric acid : H2O2 = 7 : 3,

highly explosive when meets organics). Solution of PS-b-P2VP in toluene (~0.9% w/w) was spin-coated at 4000 rpm onto Si (100) substrates and self-assembled by thermal annealing at 190℃ under vacuum for 24 h, resulting in one layer of 2VP cylinders oriented parallel to the substrate in the PS matrix. Substrates with self-assembled films were immersed in a pH≈3.2 HCl aqueous solution (10 μL 1 wt% HCl in 4 mL DI water) at room temperature for 3 h and dried by N2 gas, followed by exposure to 2-3 J/cm2 of 254 nm ultraviolet (UV) light in a Spectrolinker XL1500UV Crosslinker (Spectronics Corporation). Precursor Incorporation.

Molybdenum precursors were incorporated by immersing the crosslinked BCP films

in a 0.01 M (NH4)2MoS4 (ATTM) or 0.0014 M (NH4)6Mo7O24·4H2O (AHM) aqueous solution at 90℃ for 4 h followed by a DI water rinse and drying with N2 to remove any residue on the film. Sulfurization Reactions.

Precursor incorporated BCP films were converted to MoS2 using elemental sulfur

loaded into a custom-built CVD system using a silica reaction tube in a Lindberg Blue tube furnace. The substrates were placed along the first half of the furnace hot zone and the sulfur powder (3 g) was loaded into an alumina boat situated at the entrance of the furnace hot zone. The furnace was then heated to 700 oC at a rate of 20 oC/min and held for 1 hour at 760 Torr with an argon flow of 50 sccm. Substrate temperatures were independently verified using a secondary thermocouple inserted into the tube furnace.



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Characterization.

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Electron microscopy images were acquired using LEO-1530 scanning electron microscope (1

kV) and Tecnai 12 transmission electron microscope (120 kV). Raman spectroscopy was performed using an Aramis Horiba Jobin Yvon Confocal Microscope with a 442 nm laser (~15 mW) and ~1 μm2 spot size and collected using a 100× objective in a backscattering geometry. The silicon Raman band at 520.7 cm-1 was used as an internal reference. X-ray photoelectron spectroscopy (XPS) was measured using a Thermo K-alpha XPS with a 400 um2 probing area and monochromatic Al Kα radiation (1486.7 eV). Survey and high-resolution spectra were acquired using analyzer pass energies of 200 and 50 eV, respectively. The XPS spectra was calibrated using the adventitious carbon peak at 284.6 eV. Peak fitting was performed using Gaussian/Lorentzian peak shapes and a Shirley/Smart type background. The crystal structures were characterized using a Bruker D8 Discovery high resolution X-ray diffractometer with Cu Kα source.

■ RESULTS AND DISCUSSION


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Scheme 1. Overview of the synthetic process for block copolymer templated molybdenum disulfide nanowire arrays.

Scheme 1 outlines the synthetic routes explored in this work for MoS2 nanowires using self-assembled cylinder forming PS-b-P2VP (Figure 1a). PS-b-P2VP was used as a template to create nanowire arrays as the electron rich nitrogen in the pyridine group can be protonated and complexed with molybdenum-containing anions, leading to selective incorporation of Mo-precursors into P2VP domains. (NH4)2MoS4 (ATTM) was chosen as molybdenum source initially, due to its common usage in the solution synthesis of molybdenum sulfide.9, 38 After seeding in the ATTM solution, the sample was annealed in H2 atmosphere (H2/Ar 5% H2, 50 sccm) at 350oC to remove the BCP template. However, the resulting nanowires were randomly oriented and ill-defined with respect to the BCP template (Figure 1b). During the H2 annealing process, as the temperature increases above the glass transition temperature of the BCP, the increased chain mobility deforms the morphology in the self-assembled BCP film, resulting in the random orientation of the as-grown nanowires. To solve this problem, an ultraviolet (UV) crosslinking step was



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introduced to stabilize the BCP film before annealing. Exposure to 2~3 J/cm2 254 nm UV light resulted in successful crosslinking for both the PS and P2VP domains (Table S1). The subsequent H2 annealing confirmed that the morphology was preserved, leading to well defined nanowire arrays (Figure 1c). X-ray photoelectron spectroscopy (XPS) surface chemical analysis showed peaks for Mo 3d3/2 at 235.5 eV and Mo 3d5/2 at 232.3 eV, which correspond to the Mo6+ oxidation state,42 and a minor S 2p peak at ~168 eV indicating the presence of a small amount of oxidized sulfur or sulfate (Figure 1e). Elemental analysis (Figure S1) showed a much higher percentage of oxygen relative to sulfur indicating that the formation of MoO3 nanowires, likely formed during the crosslinking and H2 annealing process. To prevent this oxidation, an additional annealing step under saturated sulfur atmosphere was conducted (Figure 1d). This step effectively reduces Mo6+ to Mo4+, with Mo 3d3/2 and Mo 3d5/2 appearing at 232.1 eV and 229.0 eV respectively (Figure 1f).43 However, this process can be further simplified by using an alternate route involving direct seeding of AHM [(NH4)6Mo7O24] followed by sulfur annealing. AHM offers other advantages as it is more economical than ATTM and does not release H2S during the solution growth, and the H2 annealing process can be eliminated all-together. In an AHM aqueous solution, molybdenum exists either in the form of MoO42- or Mo7O246-,44 both of which can be incorporated into the protonated P2VP domains. The oxide precursors were then converted to MoS2 via an annealing process in the saturated sulfur atmosphere to form molybdenum sulfide with simultaneous decomposition of the BCP template.


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Figure 1. The scanning electron microscope (SEM) images of (a) iodine stained PS-b-P2VP (24.6 k-10.3 k, annealed at 190℃/24 hrs) on the SiO2/Si substrate; ATTM selectively seeded PS-b-P2VP self-assembled film after annealing at 350℃ under H2 atmosphere (b) without and (c) with UV crosslinking; (d) nanowires further annealed at 350℃ under a saturated sulfur atmosphere; the corresponding XPS (e) before and (f) after sulfur annealing. Scale bars are 200 nm.

SEM images (Figure 2) of the MoS2 nanowires templated from AHM incorporated BCP films show that the long range ordering of the cylindrical domains is preserved post sulfur annealing at temperatures ranging from 340℃ to 700℃. The average center-to-center distance of the parent self-assembled PS-b-P2VP was 25.9 nm with a P2VP line width of 14.6 nm (Figure S2). In comparison the as-converted molybdenum disulfide nanowires maintained the average center-to-center distance at all annealing temperatures, while the average line width of the nanowires was dilated up to ~18 nm (Figure 2i). This observed dilation is attributed to the protonation of the 2VP



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domain which is known to increase the hydrodynamic diameter,45 and the incorporation of the precursors themselves. The extent of the domain dilation can be controlled by the pH of the solution with optimum conditions at pH≈3.2 (Figure S3). In general, lowering the pH of the solution resulted in overlapping of the neighboring P2VP domains due to increased stretching of the positively charged P2VP chains. However, the averaged line width of the nanowires decreases as the annealing temperature increases above 480℃ (Figure 2i) but still remain larger than the line width of the P2VP template.

Figure 2. The SEM images of AHM incorporated BCP films annealed under saturated sulfur atmosphere at (a) 340℃, (b) 375℃, (c) 410℃, (d) 445℃, (e) 480℃, (f) 515℃, (g) 550℃, and (h) 700℃. Scale bars are 100 nm. (i) The average


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nanowire width at each annealing temperature. Error bars represent one standard deviation from the mean calculated from 100 positions on each sample.

Chemical composition of the nanowires resulting from different annealing temperatures were evaluated by XPS. The appearance of Mo 3d3/2 at 232.2 eV and Mo 3d5/2 at 229.0 eV confirmed the Mo4+ state,43 as shown in Figure 3a. A small shoulder for Mo6+ at ~235.5 eV was also detected,42 possibly due to the formation of a small amount of surface oxide resulting from air exposure. A broad S 2s feature near the Mo 3d5/2 peak indicates multiple chemical states for sulfur. To gain a better understanding of the Mo and S states, detailed peak fitting for 700℃ annealed sample (Figure 3b) revealed two S 2p peaks at 163.1 eV and 161.9 eV corresponding to the divalent sulfide ions (S2-).46 A second set of S 2p at 164.7 eV and 163.5 eV is mainly attributed to elemental sulfur residue deposited during the post-anneal cool down and was readily detected due to the surface sensitivity of XPS. The shoulder appearing at ~168 eV is attributed to a low density of surface sulfates resulting from oxidation of the excess elemental sulfur.47 Though the oxidation states for Mo and S were unaffected by the annealing temperature, the surface sulfur was eliminated at higher annealing temperatures due to increased volatility of the elemental sulfur. The crystallinity of the MoS2 was also improved at higher temperatures with concurrent decrease in the average line width of the nanowires.



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Figure 3. (a) XPS of MoS2 nanowires annealed under sulfur atmosphere at temperatures ranging from 340℃ to 700℃; (b) Peak fitting of 700℃ annealed sample.

Raman spectroscopy was used to examine the crystallinity of MoS2 as a function of annealing temperature (Figure 4a). It is known that the 2H-MoS2 phase has two distinct Raman modes, in-plane E12g at ~383 cm-1 and outof-plane A1g at ~408 cm-1, which are not present in amorphous molybdenum sulfide.9, 48 Samples annealed from 340℃ to 445℃ were predominantly amorphous molybdenum sulfide since no corresponding E12g or A1g peaks were detected. However, for samples annealed above 515℃, the emergence of the E12g and A1g peaks confirmed the formation of 2H-MoS2. As the annealing temperature increases from 515℃ to 700℃, the separation between the two peaks increased from 23.99 cm-1 to 26.29 cm-1 due to the increase in the number of layers in the 2H-MoS2 phase.49 The observed peak intensity for the E12g mode was larger than the A1g mode for samples annealed at higher temperature (700℃). Since the A1g mode is an out-of-plane vibration, more edge planes are likely to be exposed at


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a lower annealing temperature; with the formation of larger crystals at higher temperatures less edge planes are exposed (Figure 4b).21 This is consistent with the previous reports that show higher temperature growth and conversion reactions favor the formation of closed-shell “inorganic fullerene” like nanotubes with fewer exposed edges.15

Figure 4. (a) Raman spectra of molybdenum sulfide nanowires annealed from 340℃ to 700℃ annealed samples; (b) Illustration of the proposed temperature dependent structure variation, showing increase in the size of the crystallites to form a polycrystalline nanowire.

The structure and composition in the nanowires annealed at 700℃ were characterized further by transmission electron spectroscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). To transfer molybdenum sulfide nanowires to a TEM grid, a floating transfer process was utilized due to the rough surface presented by the TEM grid.50 MoS2 nanowires fabricated from the self-assembled PS-b-P2VP film on a SiO2 substrate were immersed into a 20% HF solution to etch away SiO2, floating the molybdenum sulfide nanowire film on the water surface, which was then transferred to DI water and collected on a 200-mesh copper TEM grid. The TEM image of the nanowires annealed at 700℃ (Figure 5a) and the corresponding selective area electron



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diffraction (SAED) pattern (Figure 5a inset) unequivocally confirmed that the nanowires are indeed polycrystalline MoS2 and the composition of these nanowires measured through EDS shows a Mo:S ratio of 1:2.61. The XRD peaks from the molybdenum sulfide nanowires on the SiO2/Si substrate were overwhelmed by predominantly Si peaks from the substrate, with small peaks seen from the MoS2 (not shown here) due to relatively small amount of material. To increase the amount of material for XRD detection a thick homopolymer P2VP film was subjected to identical steps to seed the AHM precursor (see supporting information for details). The XRD pattern of the resulting MoS2 agreed well with the standard PXRD pattern for MoS2 (Figure 5b), further confirming the formation of crystalline MoS2. The lattice constants calculated from the measured XRD pattern for MoS2, a = 3.15 ± 0.01 Å and c = 12.56 ± 0.06 Å, are in good agreement with those reported for 2H-MoS2.51


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Figure 5. (a) TEM of MoS2 nanowires annealed at 700℃, scale bar is 50 nm, with inset showing the corresponding SAED, and scale bar is 2 nm-1. (b) XRD of the MoS2 sample prepared from the P2VP film and annealed at 700℃ in comparison with the standard pattern for MoS2.

■ CONCLUSION

In conclusion, we have demonstrated the selective seeding and conversion synthesis of arrays of MoS2 nanowires with sub-20 nm width in a PS-b-P2VP block copolymer thin-film template. This method takes advantage of the self-assembly of BCPs over large areas to create uniform high density features with sub-20 nm widths. The 2VP domain provides the required functionality to selectively seed the Mo precursors from the solution. The optimization



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of the protonation of the VP domains to maximize the interaction with a precursor counter ion was found to be equally important. For dichalcogenides the challenges with using BCP templates are to control the crystallinity and the chemical composition, both of which require a high temperature conversion process. By using a UV crosslinking step to stabilize the PS-b-P2VP structure at a high temperature we have demonstrated successful spatial localization of the as-grown MoS2 nanowires. By using a combination of characterization tools such as XPS, Raman spectroscopy, TEM and XRD, the resulting MoS2 wires were characterized. At higher annealing temperatures the amorphous phase transitioned to the polycrystalline 2H-MoS2 phase, and the excess elemental sulfur was reduced. This work demonstrates the potential for using BCP templates to prepare patterned arrays of polycrystalline MoS2 nanowires over a large area with an inexpensive solution process, which is potentially interesting for catalytic and sensing applications.

■ ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. UV crosslink study of homo PS and P2VP thin films, XPS survey of the sample annealed under H2 atmosphere, small angle X-ray (SAXS) data of the PS-b-P2VP used in this work, protonation study with different pH, detailed preparation of XRD sample and atomic force microscope (AFM) images of MoS2 nanowires annealed at 700℃. ■ AUTHOR INFORMATION Corresponding Author *[email protected]


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Notes The authors declare no competing financial interest. ■ TABLE OF CONTENT

■ ACKNOWLEDGMENT

A.W., M.A., and P.G. acknowledge NSF grant CMMI-1462771 for support. W.W. acknowledges support from Wisconsin Energy Institute for partial financial support. L.S. thanks the support from the NSF Graduate Research Fellowship Program. S. J. thanks the support by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-09ER46664. We acknowledge support from the staff and the use of equipment at the Materials Science Center and Wisconsin Center for Microelectronics.

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