抄録
Silicene and germanene are novel two-dimensional honeycomb sheets composed of Si and Ge, respectively [1]. Theoretical studies have shown that the electronic states of free-standing silicene and germanene, as well as their ribbons, possess interesting electronic properties similar to graphene, such as Dirac cones [2]. On the other hand, these materials prefer to form sp3-like hybridized orbitals rather than sp2 orbitals, and thus they have a low buckling structure that differs from graphene. Owing to this low buckling structure, the energy gap can be tuned by an external electric field perpendicular to the surface [3,4]. Furthermore, these materials have larger spin–orbit interactions than graphene. Therefore, it is expected that the quantum spin Hall effect can be observed in these materials at experimentally feasible temperatures [5]. As stated above, these two-dimensional honeycomb sheets have very interesting electronic properties. Therefore, these materials are currently attracting a great deal of attention as future nanoelectronic devices of atomic layer thicknesses.
Silicene and germanene are generally fabricated on metal substrates [6-8]. However, it has been reported that the electronic states of silicene on metal substrates can be dramatically altered by strong interactions with surface atoms [9,10]. Accordingly, it is difficult to measure the intrinsic electronic properties. A hydrogenation of CaSi2 [11] or CaGe2 [12] crystal is one of the most promising methods to fabricate a free-standing silicene or germanene. The hydrogenation yields crystals composed of hydrogenated silicene (silicane) or germanene (germanane), and the free-standing silicane/germanane can be easily exfoliated from the crystal because the interlayer interaction is a van der Waals interaction. In this presentation, we report on the hydrogen desorption characteristics of monolayer and multilayer silicane and germanane crystals. The calculations show a way to create free-standing silicene/germanene from monolayer silicane/ germanane [13,14].
Hydrogenated silicene (silicane) and germanene (germanane) have relatively large energy gaps and exhibit the usual semiconducting electronic states. On the other hand, silicene and germanene are equivalent to a single bilayer on (111) plane of the bulk structure without hydrogen termination, and it is well known that they exhibit topological electronic states. Then, what kind of electronic states do the ultra-thin crystals stacked in (111) direction have? In this presentation, I will also present the results on the electronic structures of the ultra-thin film crystals using the first-principles calculations [15].
References
[1] K. Takeda and K. Shiraishi, Phys. Rev. B 50, 14916 (1994). [2] S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009). [3] M. Ezawa, New J. Phys. 14, 033003 (2012). [4] A. Hattori et al., J. Phys.: Condens. Matter 31, 105302 (2019). [5] C.-C. Liu, W. Feng, and Y. Yao, Phys. Rev. Lett. 107, 076802 (2011). [6] P. Vogt et al., Phys. Rev. Lett. 108, 155501 (2012). [7] A. Fleurence et al., Phys. Rev. Lett. 108, 245501 (2012). [8] J. Yuhara et al., 2D Mater. 8, 045039 (2021). [9] Z.-X. Guo et al., J. Phys. Soc. Jpn. 82, 063714 (2013). [10] M. X. Chen and M. Weinert, Nano Lett. 14, 5189 (2014). [11] H. Nakano and T. Ikuno, Appl. Phys. Rev. 3, 040803 (2016). [12] E. Bianco et al., ACS Nano 7, 4414 (2013). [13] M. Araidai, M. Itoh, M. Kurosawa, A. Ohta, and K. Shiraishi, J. Appl. Phys. 128, 125301 (2020). [14] M. Itoh, M. Araidai, A. Ohta, O. Nakatsuka, and M. Kurosawa, Jpn. J. Appl. Phys. 61, SC1048 (2022). [15] D. Ishihara, M. Araidai, A. Yamakage, and K. Shiraishi, in preparation.
