Center for Functional Nanomaterials Seminar

In the post silicon era, type-II transition metal dichalcogenides (TMDCs) heterostructures (HSs) are predicted to be the building blocks for the next generation (opto)electronic device applications. Competing interlayer charge (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in the semiconducting TMDC HSs. In the type-II TMDC HSs, CT process dominates due to its fast timescale (< 100 fs) as compared to the ET process (≤ 1 ps). In this talk, I will present our recent results from an ongoing project to create an understanding on different complex aspects of the ET processes in the TMDC HSs; while main focus on the dominating ET effect over the fast CT process. In the first part of my talk, I will begin with our ET exploration with the HS made by the monolayers (1Ls) of MoSe2 and ReS2 [1]. In this work, we showed that despite forming the type-II HSs an ET process from the ReS2 to MoSe2 layer dominates over the traditional fast interlayer CT process, due to the resonant overlap between the optical bandgaps between the two materials. This ET process resulted ~ 400% MoSe2 photoluminescence (PL) enhancement from the HS area without any charge-blocking interlayer. Upon completely blocking the CT process by placing a charge-blocking interlayer we observed more than one order magnitude higher MoSe2 PL emission from the HS area. This motivated us to explore the effect of resonant overlap between the high-lying absorption states between different materials. Traditionally ET is observed from a higher-to-lower energy state (thermodynamically favorable). In a type-II HS formed between 1Ls of MoS2 and WSe2, high-lying absorption states (band-nesting regions) have resonant overlaps at cryogenic temperature [2]. This enables to observed a unique ET process from the lower-to-higher bandgap (WSe2-to-MoS2) material with a thin charge-blocking interlayer. In the second part, I will talk about the effect of large twist angle in the ET process [3]. Interlayer CT process happens due to the excitonic wavefunctions overlaps. Whereas, the long-distance ET happens mainly via dipole-dipole interaction. By creating large twist angles (~ 30°-60°) in MoSe2 homobilayers fabricated with stacking exfoliated layers on top of the flakes grown by the chemical vapor deposition (CVD), we effectively reduce the CT process. The ~ 3% strain induced lattice mismatch also changes CVD bandgap from direct-to-indirect, making the CT even weaker and allowing the ET process to take over the carrier relaxation channels. References: [1] A. Karmakar et al., Dominating Interlayer Resonant Energy Transfer in Type-II 2D Heterostructure, ACS Nano 16, 3861 (2022). [2] A. Karmakar et al., Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D Material, Nano Lett. 23, 5617 (2023). [3] A. Karmakar et al., Twisted MoSe2 Homobilayer Behaving as a Heterobilayer, Nano Lett. 24, 9459 (2024). Biography: Dr. Arka Karmakar obtained his bachelor degree (B.Tech.) in the Electronics and Communication Engineering from the West Bengal University of Technology, India. Then he joined at the Department of Electrical Engineering in the University at Buffalo (SUNY Buffalo), USA for his PhD program. His thesis entitled 'Terahertz Hybrid Graphene-Metal Reflectarrays' was awarded PhD degree in 2019 under the supervision of Prof. Erik Einarsson. He joined the Okinawa Institute of Science and Technology, Japan as a postdoctoral fellow, before continuing his academic career as an assistant professor at the Faculty of Physics in the University of Warsaw, Poland in 2022. His present research interest is optical spectroscopic studies of the newly emerging layered (2D) materials for (opto)electronic applications.