Synthesis of Large-Area WS2 monolayers with Exceptional Photoluminescence

Abstract: Monolayer WS2 offers great promise for use in optical devices due to its direct bandgap and high photoluminescence intensity. While fundamental investigations can be performed on exfoliated material, large-area and high quality materials are essential for implementation of technological applications. In this work, we synthesize monolayer WS2 under various controlled conditions and characterize the films using photoluminescence, Raman and x-ray photoelectron spectroscopies. We demonstrate that the introduction of hydrogen to the argon carrier gas dramatically improves the optical quality and increases the growth area of WS2, resulting in films exhibiting mm2 coverage. The addition of hydrogen more effectively reduces the WO3 precursor and protects against oxidative etching of the synthesized monolayers. The stoichiometric WS2 monolayers synthesized using Ar+H2 carrier gas exhibit superior optical characteristics, with photoluminescence emission full width half maximum values below 40 meV and emission intensities nearly an order of magnitude higher than films synthesized in a pure Ar environment.

Synthesis of Large-Area WS₂ Monolayers with Exceptional Photoluminescence

The paper, authored by researchers from the Naval Research Laboratory, examines the synthesis of large-area tungsten disulfide (WS₂) monolayers with superior photoluminescence attributes, which are crucial for various optoelectronic applications. The manuscript delineates a procedure involving chemical vapor deposition (CVD) under different controlled conditions to assess the impact of introducing hydrogen into the growth environment.

Monolayer WS₂ is increasingly meriting attention due to its direct bandgap and substantial photoluminescence intensity, making it a pivotal material for optoelectronic applications. Traditional synthesis methods like mechanical exfoliation allow fundamental studies but fail to meet the demands of large-scale technological deployment due to limitations in coverage and uniformity. This research emphasizes the synthesis of large-area monolayers, which is vital for practical applications.

Key Findings

1. Improved Optical Quality with Hydrogen: The inclusion of hydrogen in the argon carrier gas notably enhances the optical qualities of the WS₂ monolayers. Photoluminescence (PL) measurements reveal that WS₂ synthesized with hydrogen shows full width half maximum (FWHM) values below 40 meV and exhibits PL intensities nearly an order of magnitude higher than those grown in pure argon environments.

2. Impact on Monolayer Coverage: The addition of hydrogen increases the areal coverage of the monolayers significantly up to millimeter scales. This improvement is attributed to the more effective reduction of the WO₃ precursor and protection against oxidative etching.

3. Chemical Analysis: X-ray photoelectron spectroscopy (XPS) confirms that hydrogen incorporation during synthesis results in tungsten with a lower valence state, indicating better reduction and a closer composition to stoichiometric WS₂. Samples without hydrogen exhibited significant oxygen content, underscoring the role of hydrogen in improving chemical purity.

Methodological Approach

The research explores the synthesis process in a quartz tube furnace, using various temperature profiles and gas compositions. Through systematic alterations in the synthesis recipes, notably the carrier gas composition, the team elucidates how hydrogen not only ameliorates photoluminescence properties but also the uniformity and continuity of WS₂ films across larger areas.

Theoretical and Practical Implications

The insights furnished by this research have substantial implications in the synthesis of other two-dimensional transition metal dichalcogenides (TMDs) and their use in van der Waals heterostructures. The findings suggest that hydrogen's role could be pivotal in tuning the electronic properties of other TMD layers, potentially influencing their application in spintronics and optoelectronic devices.

Future Directions

Given the significant influence of hydrogen on the growth process and film quality, future investigations may focus on optimizing H₂:Ar ratios and understanding the fine-mechanisms at the atomic scale. Additionally, expanding this methodology to other TMD materials could yield broader applicability. Furthermore, exploring the interactions between synthesized monolayers and other materials in heterostructures could innovate device architectures. The study sets a precedent for enhancing synthesis protocols to yield materials with superior performance metrics essential for future technological applications.