Electric-field (EF) effects on two-dimensional (2D) materials play an essential role in developing functional devices such as transistors, sensors, and photonic-devices. Silicene has an analogous electronic structure to graphene, namely, Dirac band structure and it is increasingly expected to be utilized in highly-functional 2D materials. Although the silicene is unstable in air and therefore its applications to functional devices are demanding, very recently Tao et al. reported silicene field effect transistors [1].

To intelligently design devices operating by the EF effects on silicene, it is required to reveal the electronic interactions at the interface region between silicene and heterogeneous materials. Ni et al. studied EF effects on boron-nitride sandwiched silicene by carrying out the first-principles calculations, and showed that the variation in the band gap by an applied EF was enhanced by electronic interactions between silicene and boron-nitride [2]. On the other hand, chemical interactions such as charge transfer and covalent bond at an interface have crucial influence on the electronic structures [3, 4]. Kaloni et al. showed that some organic molecules were adsorbed on silicene more strongly than on graphene, and the adsorption caused a large band gap [3]. However, the relationship between the chemical interactions and the EF effects still remains to be less studied.

In this study, we performed first-principles study of the EF effects on silicene-amine 2D heterogeneous systems focusing on the interactions at the silicene-amine interface. We analyzed the Si-N chemical bond and the internal EF, and thereby revealed the mechanism of electronic structure change induced by the applied EF.

Figure 1 shows the variation in the band gap of silicene-amine (NH₂CH₃ and NH₃) systems with respect to the gap without the EF, when applying the EFs of 4 V/nm and −4 V/nm. The variation for a silicene monolayer at the field of 4 V/nm is the same as that at −4 V/nm because of the geometrical symmetry. However, the band gaps of silicene-amine increase by the positive EF, whereas decrease by the negative EF. It is also found that when applying the positive EF, the band-gap variation in silicene-amine becomes smaller than that in the silicene monolayer.

The band-gap variation in silicene-amine depending on the applied EF is due to the change in the bond strength between silicene and amine. It is known that NH₃ strongly adsorbs on silicene, and thereby the band gap increases [3]. The binding energy for NH₃ adsorption without an applied EF is 0.30 eV, and the energy increases by 0.14 eV at the positive EF whereas decreases by 0.09 eV at the negative EF. The binding energy for NH₂CH₃ adsorption also shows an analogous dependence on the applied EF to that for NH₃ adsorption.

Since the amine­ adsorption follows electron transfer from amine­ to silicene, the internal EFs in the silicene-amine systems are significantly different from those in the silicene monolayer. Figure 2 shows the change in the internal EF by NH₃ adsorption (a), and the change by the applied EF of −4 V/nm (b). The blue and yellow distributions indicate the positive and negative internal EFs, respectively. A strong internal EF is found in Fig. 2(a) around Si binding to N. However, the −4 V/nm EF reduces the internal EF as seen from Fig. 2(b), and thus, the gap decreases. In other words, the changes in the bond strength and internal EF determine the band-gap variation induced by the applied EF.

[1] L. Tao, E. Cinquanta, D. Chiappe, et al., Nature Nanotech., 10, 227 (2015).

[2] Z. Ni, Q. Liu, K. Tang, et al., Nano lett., 12, 113 (2012).

[3] T. P. Kaloni, G. Schreckenbach, M. S. Freund, J. Phys. Chem. C, 118, 23361 (2014).

[4] W. Hu, N. Xia, X. Wu, et al., Phys. Chem. Chem. Phys., 16, 6957 (2014).