Exotic 2D Lateral Heterostructures for Optoelectronics

Atomically thin layered materials such as graphene and transition metal dichalcogenides (TMDs) have opened a new and rich field with exotic physical properties and exciting potential applications in the “flatland”. There are enormous possibilities in combining diverse 2D materials for the unique design of ultra-smart and flexible optoelectronic devices, including transistors, light-emitting diodes, photovoltaics, photodetectors, and quantum emitters. Considerable efforts have been devoted to the van der Waals hetero-integration of different 2D layered materials to form vertical superlattices via transfer of their exfoliated or as-grown flakes. On the other hand, lateral heterostructure is possible only via direct growth, which can offer exciting opportunities for engineering the formation, confinement, and transport of electrons, hole, exciton, phonon, and polariton. Unlike vertical heterostructures, lateral heterostructures can be fabricated only via direct growth. Furthermore, the performance of most 2D heterostructure-based devices falls far below the predicted values owing to several intrinsic and extrinsic factors. These significant issues will be discussed. 

We reported the direct fabrication of seamless, high-quality TMDs lateral heterostructures and superlattices in the chemical-vapor-deposition process, only changing the reactive gas environment in the presence of water vapor. Our novel approach offers greater flexibility for the continuous growth of multi-junction TMDs lateral heterostructures, controlled 1D interfaces, alloying, and layer numbers. The extent of the spatial modulation of individual TMD domains and their optical and electronic transition characteristics across the heterojunctions are studied in detail. Electrical transport measurements revealed diode-like responses across the 2D lateral junctions, which were found promising for electroluminescence at room temperature. Using photon energy-resolved photoconductivity mapping, long-term carrier accumulation in MoS2-WS2 lateral heterostructures was observed. At the onset of photoexcitation, local carrier density was increased by two orders of magnitude and persisted for up to several days. Temperature-dependent photoluminescence from neutral exciton, trion, and defect-bound exciton provides a better understanding of the optical properties of these as-grown 2D lateral heterostructures. These studies will further supplement the quantitative evaluation of optical properties of various 2D heterostructures to develop more complex and atomically thin superlattices and exotic devices.