Thermodynamically Stable Synthesis of Large‐Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n–n Heterojunction Devices

Abstract

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full-width-half-maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS /WS heterojunction devices are easily prepared using this synthetic procedure due to the large-sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW ) because of the built-in potential and the majority-carrier transport at the n-n junction. These findings indicate an efficient pathway for the fabrication of high-performance 2D optoelectronic devices.

Vapor‐phase growth of high‐quality wafer‐scale two‐dimensional materials

“…F. Diagram of spin‐coating the MoO 3 solution (left) and the face‐to‐face configuration of source and target substrate (right). Reproduced with permission . Copyright 2017, Wiley‐VCH Verlag GmbH & Co. G. Schematic of MoS 2 growth process in a precursor solution (top) without OH and (down) with OH.…”

“…In the CVD process, transition metal oxides (eg, MoO 3 , WO 3 ) or chlorides (MoCl 5 ) or metal foils/films serve as the metal (molybdenum or tungsten) sources while sulfur or selenium powders act as the source of sulfur or selenium. There are three different precursor supply modes (Figure C): point‐to‐face, horizontal, and face‐to‐face . In the beginning, the point‐to‐face configuration and the horizontal configuration were proposed for growing TMDs with controllable thicknesses using oxides powders.…”

“…It may be a good way for the control of the nucleation density and the thickness in the wafer‐scale growth with high quality by combining the spin‐coating method with the face‐to‐face configuration (Figure F). Lee et al prepared MoO 3 solution in NH 4 OH with very small amounts of MoO 3 (0.01 mg) and then spin‐coated the solution on the SiO 2 source substrate. This method induced a relatively low supersaturation and hence significantly reduced the nucleation density down to 32 nuclei mm −2 over the top target substrate.…”

“…There are three different precursor supply modes ( Figure 2C): point-to-face, 65,66 horizontal, 61,67 and face-to-face. 39,68,69 In the beginning, the point-to-face configuration and the horizontal configuration were proposed for growing TMDs with controllable thicknesses using oxides powders. However, it is difficult to reproducibly produce uniform monolayer films over large size, because of the position-dependent inhomogeneous distribution of the powder source and the limited controllability over the nucleation density.…”

“…Reproduced with permission. 39 Copyright 2017, Wiley-VCH Verlag GmbH & Co. G. Schematic of MoS 2 growth process in a precursor solution (top) without OH and (down) with OH. Reproduced with permission.…”