Epitaxial Growth of TMD Monolayers and In-Plane Nanodot Heterostructures for 2D Nanophotonics

Epitaxial growth of transition metal dichalcogenides on sapphire has emerged as a promising approach for synthesis of wafer-scale single crystal monolayer films. Our work has focused on the use of metalorganic chemical vapor deposition (MOCVD) for TMD epitaxy on sapphire as it enables tight control over gas phase precursor concentration and the ability to form heterostructures via modulation of the transition metal and/or chalcogen precursor during growth. We previously developed a three-step process for TMD epitaxy on sapphire involving nucleation, ripening and lateral growth steps for synthesis of W-containing TMDs including WS₂ and WSe₂. Nucleation was observed to occur preferentially on the step edges on the sapphire substrate enabling unidirectional orientation of domains and a significant reduction of inversion domains in coalesced monolayer films. This process was modified for the growth of in-plane MoSe₂-WSe₂ heterostructures by introducing a Mo precursor (Mo(CO)₆) during nucleation to form nanoscale MoSe₂ domains on step edges on sapphire and then switching the transition metal precursor to W(CO)₆ during the lateral growth step to embed the MoSe₂ domains in a continuous WSe₂ matrix. The MoSe₂ nanodots exhibit coherent interfaces with the surrounding WSe₂ and their size can be varied from 10-50 nm through control of the time for the nucleation step. Variable temperature photoluminescence measurements using a confocal microscope which samples a large ensemble of MoSe₂ nanodots within the WSe₂ layer reveals size-dependent confinement of excitons in samples where the dot size is intentionally varied as evidenced by a blue shift in the MoSe₂ exciton emission peak at low temperatures. Samples with the smallest average nanodot size (<10 nm) exhibit characteristics of single-photon emission at 1.6 K. Prospects for the development of large area films of 2D TMD quantum dots using this approach will be discussed.