2D Materials and Heterostructures for Electronic, Optoelectronic, and Thermoelectric Device Applications

In this talk, I will review our recent progress in the area of 2D materials and heterostructures. We have demonstrated several approaches for converting many-layer transition metal dichalcogenides (TMDs) from indirect to direct band gap semiconductors using radiation exposure and plasma treatment techniques through an inter-layer decoupling mechanism.^1-3 In addition to converting the band structure of these materials, these material processing techniques result in substantial reduction in unintentional doping and improved charge neutrality,⁴ as well as strong circularly polarized photoluminescence.⁵ We have also investigated cross-plane transport in vertically-stacked 2D materials, including graphene/BN and graphene/MoS₂ heterostructures, for applications is thermoelectric and thermionic energy conversion,^{6, 7} as well as photodiodes.⁸

1. Dhall, et al., Direct Bandgap Transition in Many-Layer MoS₂ by Plasma-Induced Layer Decoupling. Advanced Materials, 27, 1573 (2015).
2. Wang, et al., Radiation-induced direct bandgap transition in few-layer MoS₂. Applied Physics Letters, DOI:10.1063/1.5005121 (2017).
3. Li, et al., Layer Control of WSe₂ via Selective Surface Layer Oxidation. ACS Nano, 10, 6836-6842 (2016).
4. Dhall, et al., Charge neutral MoS₂ field effect transistors through oxygen plasma treatment. Journal of Applied Physics, 120, 195702 (2016).
5. Dhall, et al., Strong Circularly Polarized Photoluminescence from Multilayer MoS2 Through Plasma Driven Direct-Gap Transition. Acs Photonics, 3, 310-314 (2016).
6. Li, et al., Cross-Plane Seebeck Coefficient Measurement of Misfit Layered Compounds (SnSe)(n)(TiSe2)(n), (n=1,3,4,5). Nano Letters, 17, 1978-1986 (2017).
7. Poudel, et al., Cross-plane Thermoelectric and Thermionic Transport across Au/h-BN/Graphene Heterostructures. Scientific Reports, DOI:10.1038/s41598-017-12704-w (2017).
8. Li, et al., Highly efficient, high speed vertical photodiodes based on fewlayer MoS₂. 2D Materials, 4, 015003 (2016).