Department of Physics

Indian Institute Of Technology Madras , Chennai

Two Dimensional Materials and Their Heterostructures

Speaker : Dr. Pramod Kumar Nayak, Ulsan National Institute of Science and Technology (UNIST), Republic of Korea

2016-11-15

Abstract :

Two-dimensional (2D) materials such as graphene, hexagonal boron nitrides (h-BN), transition metal dichalcogenides (TMDs), and so on have attracted considerable attention in the past few years because of their novel properties and versatile potential applications [1]. These 2D layers can be integrated into a monolayer (lateral 2D heterostructure) or a multilayer stack (van der Waals 2D heterostructure) [2], resulting artificial 2D structures with intriguing physical properties completely different from their individual components. While stacking different 2D crystals together, charge redistribution might occur between the neighboring crystals that facilitate novel electro-optical phenomena in these hybrid systems [3]. In this talk, I will present a new optical phenomenon i.e. interlayer excitons (i.e., bound electron-hole pairs assigned to different layers) observed at the heterojunctions in van der Waals heterostructures consisting of atomic thin MoSe2 and WSe2 layers. Interestingly, we found that interlayer excitons are highly susceptible to the strength of interlayer coupling between the two layers i.e. the photoluminescence (PL) intensity of the interlayer excitons is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles due to weak coupling [4]. Moreover, by the incorporation of a hexagonal boron nitride monolayer between MoSe2 and WSe2, it is observed that the interlayer coupling becomes insignificant and PL of interlayer excitons disappears completely irrespective of the twist angle. This finding explores new optical phenomena concealed in these heterostructures, which paves a step further towards application of vdW HS in optoelectronic devices. Finally, several aspects of this growing research area including synthetic strategy of heterostructure fabrication, new technological opportunities towards optoelectronics and nanoelectronics will be discussed.

References:
[1]M. Chhowalla et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nature Chemistry 5, 263-275 (2013).
[2] A. K. Geim, I. V. Grigorieva, Van der Waals heterostructures. Nature 499, 419-425 (2013).
[3] P. Rivera, et al. Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures Nat. Commun. 6, 6242 (2015).
[4] P. K. Nayak, et al. Twist-angle-dependent interlayer excitons in MoSe2/WSe2 van der Waals heterostructures Nat. Commun. (Submitted 2016)

About the Speaker:

Pramoda Kumar Nayak has performed his undergraduate and post graduate studies in Physics from Utkal University, Orissa. Then he perused his doctoral programme in Physics at Indian Institute of Technology Guwahati under supervision of Prof. S. Ravi. After completion of his Ph.D in 2007, he worked as a post-doctoral researcher and visiting scientist at Institute for Plasma Research, Gandhinagar, India. In 2009, he moved to Taiwan to carry out post- doctoral research work in abroad, where he exposed to field of nanoscience and nanotechnology. From 2009 to 2012, he worked at National Cheng Kung University, Taiwan under Chair Prof. Jow Lay Huang and shifted to National Tsing Hua University, Taiwan in 2013, where he started work on two dimensional (2D) materials including Graphene and transition metal dichaclogenides (TMD) in Prof. Po-Wen Chiu's group. Currently, he is a senior researcher in Prof. Hyeon Suk Shin's lab at Centre for Low Dimensional Carbon Materials, Ulsan National Institute of Science and Technology, Republic of Korea from 2015 onwards. His current research interest is focused on van der Waals heterostructures made out of 2D materials. He has published more than 40 international journals, 5 book chapters, 02 edited books and one book so far in his research carrier.

Key Speaker Dr. Pramod Kumar Nayak, Ulsan National Institute of Science and Technology (UNIST), Republic of Korea
Guests None
Place Seminar Room
Start Time 3:00 PM
Finish Time 4:00 PM
External Link None