Graphene, silicene and germanene are two-dimensional nanomaterials with various applications in novel electronic and spintronic devices. Although having multiple superior properties, they are still plagued with two major disadvantages namely zero-bandgap and needing a suitable substrate to grow. One possible solution to overcome this issue is by forming a heterobilayer where two monolayers are stacked on each other. In our work, silicene and germanene are stacked on top of graphene which works as a substrate. Stabilities and electronic properties of graphene/silicene (Si₂C₆) and graphene/germanene (Ge₂C₆) are investigated in three different stacking configurations which are top, hollow and bridge configurations (Figure 1) using density functional theory. 

Figure 1: Stacking of (√3 ×√3 ) R30° graphene with silicene or germanene in a) top b) bridge and c) hollow configuration.

From structural optimization, both Si₂C₆ and Ge₂C₆ are the most stable when they are in top stacking followed by hollow and bridge configuration. The stability of the structure is considered based on the largest binding energy of the stacking configuration. Due to the broken inversion symmetry from the stacking, the top configuration for Si₂C₆ and Ge₂C₆ produce a bandgap of 32 meV and 60 meV respectively (Figure 2a). Si₂C₆ and Ge₂C₆ are held together by van der Waals (vdW) forces because of the weak charge transfer from graphene to silicene or germanene monolayer (Figure 2c) and the lack of new electronic states formed from the hybridization of C, Si and Ge atomic orbitals. As perpendicular strain increases, the interlayer distance between the monolayers decreases due to a stronger vdW interaction which eventually increases the bandgap of the heterobilayers (Figure 2b). 

The results have shown that the bandgap of the heterobilayers can be altered and modulated by controlling the stacking configuration and interlayer distance. This work provides useful information and options to experimentalists in the development of better materials for nanoelectronics, solar cells, optoelectronics and spintronics.

Figure 2: a) Band structure of top configuration heterobilayer b) interlayer distance and bandgap when strain is applied and c) charge transfer.

References

Hamid, M.A.B., et al., Structural stability and electronic properties of graphene/germanene heterobilayer. Results in Physics, 2021. 28: p. 104545.

Hamid, M.A.B., et al., Structural, electronic and transport properties of silicene on graphene substrate. Materials Research Express, 2019. 6(5): p. 055803.

About Author

Chan Kar Tim (Dr.) Department of Physics, Faculty of Science, Universiti Putra Malaysia Expertise: Mathematics Physics Email: chankt@upm.edu.my

Date of Input: 04/07/2024 | Updated: 04/07/2024 | harithdaniel