Charge Separation Outcompetes Interlayer Exciton Formation for Defect-Engineered Liquid Phase Exfoliated 2D Lateral Hetero-Networks

Van der Waals heterostructures formed by stacking monolayers of two-dimensional (2D) semiconductors offer innovative opportunities for creating hybrid materials and (opto)electronic devices with extraordinary properties not found in the individual components. To achieve high-quality, low-defect 2D semiconductors for optoelectronic devices, either mechanical exfoliation or chemical vapor deposition techniques are typically employed. However, these methods yield either microscopic samples or require energy-intensive processes. Liquid-phase exfoliation (LPE), in contrast, is a high-yield, energy-efficient, and scalable method for preparing 2D semiconductors suitable for large-area printed optoelectronic devices. Unfortunately, LPE often produces highly defective flakes with limited control over component selectivity. Heterostructures prepared using LPE tend to form thick films of overlapping nanosheets, with high defect densities and poor charge transport, leading to exciton trapping and the formation of strongly bound interlayer excitons that hinder (opto)electronic performance.

In this work, we engineer a uniform, large-area transition metal dichalcogenide (TMD) lateral hetero-network by covalently linking neighboring MoS₂ and WS₂ nanosheets using dithiolated molecules [1]. The π-conjugated, dithiolated bidentate systems based on benzodithiophene (BDT) heal sulfur vacancies in the TMDs, enabling more efficient charge transfer and improved inter-flake electronic connectivity. Femtosecond transient absorption (TA) spectroscopy reveals that unlinked heterostructures suffer from interlayer charge-transfer excitons, limiting their electrical performance. In contrast, we demonstrate that BDT-linked lateral hetero-networks enable efficient charge separation, resulting in field-effect transistors with lower turn-on currents and reduced subthreshold swings. Additionally, the BDT molecules reduce the density of trapped excitons within the lateral hetero-network compared to unlinked heterostructures.

[1] A. G. Ricciardulli, C. E. Petoukhoff, et al., “Defect-engineering of liquid-phase exfoliated 2D semiconductors: stepwise covalent growth of electronic lateral hetero-networks,” Mater. Horiz., Advance Article (2024).