Optoelectronics of excitonic van der Waals heterostructures
Goki Eda1, 2, 3 *
1 Department of Physics, National University of Singapore, Singapore 117551
2 Department of Chemistry, National University of Singapore, Singapore 117543
3 Centre for Advanced 2D Materials, National University of Singapore, Singapore 117546
Excitons in two-dimensional (2D) semiconductors possess giant oscillator strength and play a fundamental role in mediating the strong light-matter interaction. Among others, monolayer transition metal dichalcogenides exhibit strong excitonic absorption due to band nesting  and allow exploration of hybrid quasi-particle states such as plexcitons  through strong dipole-dipole coupling . The first part of this talk will focus on our approaches to realizing electrical generation, manipulation, and detection of excitons and their complexes based on various van der Waal heterostructures. Specifically, I will discuss how MIS-type heterostructures allow electrically tunable excitonic electroluminescence and electro-optic upconversion in linear optics regime. We demonstrate that hexagonal boron nitride can serve as a unipolar tunnel barrier that allows hot carrier injection and energy harvesting . The second part will discuss our recent discovery of a novel monolayer MoS2 growth mechanism based on vapor-liquid-solid conversion . We show that alkali metal plays a key role in reducing the melting point of the precursors and triggering the vapor-liquid-solid mode, yielding expitaxial growth of monolayer nanoribbons.
 D. Kozawa et al. “Photocarrier relaxation pathway in two-dimensional semiconducting transition metal dichalcogenides” Nat. Comm. 5, 4543 (2014).
 W. Zhao et al. “Exciton-plasmon coupling and electromagnetically induced transparency in monolayer semiconductors hybridized with Ag nanoparticles” Adv. Mater. 28, 2709 (2016).
 D. Kozawa et al. “Efficient interlayer energy transfer via 2D dipole coupling in MoSe2/WS2 heterostructures” Nano Lett. 16, 4087 (2016).
 S. Wang et al. “Efficient carrier-to-exciton conversion in field emission tunnel diodes based on MIS-type van der Waals heterostack” Nano Lett. 17, 5156 (2017).
 S. Li, et al. “Vapor-liquid-solid growth of monolayer MoS2 nanoribbons” Nat. Mater. 17, 535 (2018).