Probing the Van Der Waals Coupling of 2D Materials by Using Terahertz Ultrasonics
Peng-Jui Wang1*, Vitalyi E. Gusev2, Jinn-Kong Sheu3, Chi-Kuang Sun1,4
1Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, Taiwan
2Laboratoire d’Acoustique UMR CNRS 6613, Université du Maine, Le Mans, France
3Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan
4Molecular Imaging Center and Graduate Institute of Biomedical Electronics and Bioinformatics, National Taiwan University, Taipei, Taiwan
* Presenter:Peng-Jui Wang
Applying 2D materials on semiconductors is a common method for the fabrication of novel electronics or photonics devices, called van der Waals (vdW) heterostructures [1] [2]. The interaction force and energy transfer between adjacent 2D materials and between the 2D-material/substrate interface are crucial to determine the behavior of such devices. However, probing and controlling its interface vdW properties are challenging tasks [3].
In order to better describe such an interface, here we first derived a spring/damping model based on the nature of van der Waals force interaction. With proper assumption and boundary condition, we can simulate the acoustic response under an ultrashort acoustic pulse excitation. Considering a regular vdW adhesion on a 2D material/semiconductor interface, our model expects a resonance frequency above hundreds of GHz.
In order to probe such an interface vdW resonance, an acoustic system with a bandwidth over 1 THz will be needed. In this presentation, we demonstrate the nondestructive probing of the van der Waals interface by using THz Ultrasonics for the first time. Experimentally, we transferred a single layer CVD graphene on GaN to form such a structure. Using femtosecond ultrasonics technology [4], we successfully reported a broadband acoustic response of the adhesion vdW force. The fitting results directly revealed three crucial parameters: effective sheet density of adhesive graphene, spring constant, and damping constant. Our fitting result states that graphene/GaN interface vdW force would result to an oscillation echo with frequency above hundreds of GHz and non-zero damping constant.
Overall, our work provided a unique and powerful method to directly probe the nano-mechanical properties of 2D/3D vdW interface with THz spectrum. We believe it is a first step toward the further realization of interaction mechanism of weak bonding interface in vdW heterostructures.

[1] Deep Jariwala, Tobin J. Marks and Mark C. Hersam. Nature Materials 16, 170–181 (2017).
[2] Qiong Ma, Trond I. Andersen, Nityan L. Nair, Nathaniel M. Gabor, Mathieu Massicotte, Chun Hung Lui, Andrea F. Young, Wenjing Fang, KenjiWatanabe, Takashi Taniguchi, Jing Kong, Nuh Gedik, Frank H. L. Koppens and Pablo Jarillo-Herrero. Nature Physics 12, 455–459 (2016).
[3] Ryan Beardsley, AndreyV.Akimov, Jake D.G.Greener, GarryW. Mudd, Sathyan Sandeep, Zakhar R. Kudrynskyi, Zakhar D. Kovalyuk, Amalia Patanè & Anthony J. Kent. Scientific Reports 6, Article number: 26970 (2016).
[4] Gia-Wei Chern, Kung-Hsuan Lin, and Chi-Kuang Sun. J. Appl. Phys., Vol. 95, No. 3, 1 February 2004.

Keywords: THz Ultrasonics, Graphene , 2-D material, Substrate Effect, Nano-mechanical