[Paper Review] Two-step flux synthesis of ultrapure transition metal dichalcogenides

Song Liu, Yang Liu+11 more|arXiv (Cornell University)|Mar 28, 2023

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This paper presents a two-step flux synthesis method for producing ultrapure transition metal dichalcogenides (TMDs), significantly reducing defect densities compared to single-step methods. The approach yields monolayer WSe2 with room-temperature hole mobility exceeding 840 cm²/Vs and low-temperature disorder-limited mobility above 44,000 cm²/Vs, enabling superior electrical performance in graphene-WSe2 heterostructures with well-resolved quantum Hall states.

ABSTRACT

Here, we describe synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than in TMDs grown by a single-step flux technique. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/Vs and low-temperature disorder-limited mobility above 44,000 cm2/Vs. Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states.

Motivation & Objective

• To develop a synthesis method that minimizes contamination in transition metal dichalcogenides (TMDs), a critical challenge in 2D materials research.
• To reduce charged and isovalent defect densities in TMD crystals, which degrade electronic performance.
• To enable high-mobility, low-disorder TMDs for use in van der Waals heterostructures and quantum transport studies.
• To achieve ultrapure monolayer WSe2 suitable for advanced electronic and quantum devices.

Proposed method

• Employing a two-step flux growth process: first, a high-temperature flux step dissolves precursors and suppresses impurities; second, a controlled cooling step enables crystal nucleation and growth.
• Using alkali metal fluxes (e.g., Na, K) to lower the melting point and enhance solubility of TMD precursors.
• Optimizing temperature gradients and cooling rates to promote large, single-crystalline TMD flakes with minimal defects.
• Performing in-situ and ex-situ characterization to confirm phase purity and crystal structure.
• Utilizing high-resolution transmission electron microscopy and X-ray diffraction to verify stoichiometry and crystallinity.
• Integrating synthesized WSe2 into graphene-WSe2 heterostructures for electrical transport measurements.

Experimental results

Research questions

• RQ1 Can a two-step flux method reduce defect concentrations in TMDs more effectively than conventional single-step flux growth?
• RQ2 What is the impact of reduced defect density on the intrinsic electronic transport properties of monolayer WSe2?
• RQ3 How does the quality of two-step grown WSe2 affect the performance of graphene-WSe2 heterostructures?
• RQ4 To what extent does the two-step method improve mobility and quantum transport features such as integer quantum Hall states?
• RQ5 Can the two-step flux method be generalized to other transition metal dichalcogenides beyond WSe2?

Key findings

• Monolayer WSe2 synthesized via the two-step method exhibits a room-temperature hole mobility exceeding 840 cm²/Vs.
• Low-temperature disorder-limited mobility reaches above 44,000 cm²/Vs, indicating minimal scattering from defects.
• Charged and isovalent defect densities in the two-step grown TMDs are an order of magnitude lower than in single-step grown counterparts.
• Graphene-WSe2 heterostructures fabricated with two-step grown WSe2 show higher graphene mobility and lower charged impurity density.
• Integer quantum Hall states are well-resolved in the heterostructures, confirming high electronic quality and low disorder.
• The two-step flux method enables the growth of large-area, ultrapure TMD crystals suitable for advanced nanoelectronic applications.