Millimeter-Scale Continuous Film of MoS 2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor Maddumage Don Sandeepa Lakshad Wimalananda, Jae-Kwan Kim, Sung Woon Cho,* and Ji-Myon Lee* Cite This: https://doi.org/10.1021/acsomega.1c05052 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS 2 have limited controllability due to their lack of self- controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS 2 film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N 2 atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS 2 grows next to the coating. At 800 °C, the synthesized MoS 2 showed a uniform single-layer MoS 2 film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS 2 lateral growth. The synthesized MoS 2 with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS 2 showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS 2 -based electronic and optoelectronic devices such as transistors and photodetectors. ■ INTRODUCTION The invention of graphene synthesis has driven nano-micro devices to an atomically thin scale. Graphene is a semimetal with a zero band gap at the direct point; thus, bilayer graphene shows band gap tenability up to a certain level. 1,2 Recently, transition metal dichalcogenides (TMDs) like MoS 2 and WSe 2 have shown excellent semiconductor characteristics as an atomically thin-layered material. 3-5 Therefore, TMDs as ultrathin materials have shown a significant potential for use in modern layered devices like transistors, photodetectors, and light-emitting devices. Like graphene, TMDs can also be synthesized using the chemical vapor deposition (CVD) technique with similar merits, notably a broader growth area and larger 2D crystal size. 6-9 However, compared to graphene synthesis, TMDs show a deficiency of self-limiting behavior along with difficulty in 2D crystal formation. 10-13 In general, the nucleation of TMDs on surfaces like SiO 2 /Si requires proper wettability. 10 Nevertheless, the seeding agents and improved surface wettability (for TMDs) on such surfaces showed a nucleation tendency. 10 Previous studies suggest that seeding promoters like PTAS (perylene 3,4,9,10-tetracarboxylic potassium salt) work well on hydrophilic surfaces and that F 16 CuPc (copper(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro- 29H,31H-phthalocyanine) works well even on hydrophobic surfaces. 14 Seeding promoters increase the surface adhesive force of TMDs, which results in the formation of heterogeneous nucleation sites for TMDs and 2D crystal formation. 14 According to Ling et al., the use of seeding promoters lowers the nucleation barrier of TMDs on the surface. 14 Therefore, with the presence of a seeding promoter, TMDs can be synthesized at lower temperatures. 14 Ideally, larger TMD crystals can be synthesized by a lower number of nuclei with rapid growth under any mass flux rate. 10 Further, a rapid growth rate eases larger crystal formation and reduces the multilayer TMD traces. 10 There is currently substantial interest in supporting metallic ions such as Na + in MoS 2 synthesis because Na + lowers the edge-attachment barrier of MoS 2 for rapid synthesis; the effect of Na + has previously been studied for liquid substrates and the vapor- liquid-solid phase synthesis method. 13,15 Received: September 13, 2021 Accepted: November 8, 2021 Article http://pubs.acs.org/journal/acsodf © XXXX The Authors. Published by American Chemical Society A https://doi.org/10.1021/acsomega.1c05052 ACS Omega XXXX, XXX, XXX-XXX Downloaded via 3.236.149.205 on November 17, 2021 at 12:52:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.