Since the discovery of extremely high and non-saturated magnetoresistance in WTe₂ in 2014,¹ WTe₂ has attracted a lot of attention. Detailed studies on WTe₂ band structure indicates that well balanced electron and hole concentration in WTe₂ man plays an importance role in its large magnetoresistance.² Moreover, WTe₂ was predicted to be a Type-II Weyl semimetal, where two topological nontrivial Weyl nodes exist near its Fermi level.³ And the prediction that the single layer WTe₂ is a 2D topological insulator⁴ makes the study of single layer WTe₂ transport properties essentially important. Till now, some work has been done on the transport properties of bulk WTe₂ and show some hints of its exotic properties arouse from its nontrivial band structure through quantum transport measurement.⁵ But the study of thin films is still limited. The surface or even the whole thin film of WTe₂ degrades quickly in atmosphere,⁶ which will ruin the transport properties of thin film WTe₂ devices a lot.

To solve this problem, we introduced hexagonal BN (h-BN) to encapsulate WTe₂ thin film so that it is absolutely isolated from oxygen and moisture in atmosphere. Thin film WTe₂ encapsulated by h-BN was achieved by a series of dry transfer processes. The process was illustrated in Figure 1. Here, graphene with special shape was selected be used as source/drain electrodes in WTe₂ devices. Due to the strong van der Waals interaction among 2D materials, WTe₂ was perfectly sealed by BN and graphene and is isolated from ambient environment. Raman spectroscopy on encapsulated WTe₂ indicates that the encapsulation interrupts the degradation of WTe₂ effectively. Then, e-beam lithography, inductively couple plasma etching, and e-beam evaporation of Ti and Au were used to pattern metal electrodes. As the surface of WTe₂ was protected by BN, the transport performance of WTe₂ devices was largely enhanced. This dry transfer and encapsulation method also provide a good platform for 2D flexible devices. By transfering the whole encapsulated structure to a flexible substrate, a 2D flexible device with high performance could be envisioned.

 References

¹ Mazhar N. Ali, Jun Xiong, Steven Flynn, Jing Tao, Quinn D. Gibson, Leslie M. Schoop, Tian Liang, Neel Haldolaarachchige, Max Hirschberger, N. P. Ong, and R. J. Cava, Nature 514 (7521), 205 (2014).

² I. Pletikosić, Mazhar N. Ali, A. V Fedorov, R. J Cava, and T. Valla, Phys. Rev. Lett. 113 (21), 216601 (2014).

³ Alexey A. Soluyanov, Dominik Gresch, Zhijun Wang, QuanSheng Wu, Matthias Troyer, Xi Dai, and B. Andrei Bernevig, Nature 527 (7579), 495 (2015).

⁴ Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li, Science 346 (6215), 1344 (2014).

⁵ Yongkang Luo, H. Li, Y. M. Dai, H. Miao, Y. G. Shi, H. Ding, A. J. Taylor, D. A. Yarotski, R. P. Prasankumar, and J. D. Thompson, Appl. Phys. Lett. 107 (18), 182411 (2015); Zengwei Zhu, Xiao Lin, Juan Liu, Benoît Fauqué, Qian Tao, Chongli Yang, Youguo Shi, and Kamran Behnia, Phys. Rev. Lett. 114 (17), 176601 (2015).

⁶ Fan Ye, Jaesung Lee, Jin Hu, Zhiqiang Mao, Jiang Wei Wei, and Philip X.-L. Feng, Small 12 (42), 5802 (2016).