Open Access Article

This Open Access Article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence

Yan-Mei Dou,
Chang-Wen Zhang,
Ping Li and
Pei-Ji Wang*

School of Physics and Technology, University of Jinan, Jinan, Shandong 250022, People's Republic of China. E-mail: ss_wangpj@ujn.edu.cn

Received
18th January 2019
, Accepted 11th March 2019

First published on 27th March 2019

Using ab initio calculations, we present a two-dimensional (2D) α-2D-germanene dioxide material with an ideal sp^{3} bonding network which possesses a large band gap up to 2.50 eV. The phonon dispersion curves and molecular dynamics (MD) simulation under the chosen parameters suggest that the novel 2D structure is stable. The dielectric function and absorption spectrum also show the consistent band gap within the electronic structure diagram, suggesting possible application as an ultraviolet light optical detector. The calculated carrier mobility of 4.09 × 10^{3} cm^{2} V^{−1} s^{−1} can be observed along the x direction, which is much higher than that of MoS_{2} (∼3.0 cm^{2} V^{−1} s^{−1}). Finally, we found that α-2D-germanene dioxide could potentially act as an ideal monolayer insulator in so-called van der Waals (vdW) heterostructure devices. These findings expand the potential applications of the emerging field of 2D α-2D-germanene dioxide materials in nanoelectronics.

Recently, Löffler et al. have experimentally grown and determined the structure of crystalline silica sheets on Ru (0001); namely a silica bilayer has recently been synthesized.^{10–15} Furthermore, this hexagonal quasi-2D silica can even be supported by graphene.^{16} In other 2D materials, including MoS_{2} and WS_{2}, h-BN, silicene, germanene, MXene and phosphorene, electrons have freedom only in the 2D plane because of the quantum confinement effect, which could give rise to new phenomena in physics.^{17,18} Germanene dioxide has the same electronic form as silicon dioxide; therefore, as the study matured, we focused on germanene from the same main group.

Other 2D materials also have a high carrier mobility of up to 10^{5} cm^{2} V^{−1} s^{−1} (ref. 19 and 20) and novel in-plane negative Poisson's ratio,^{21} while the excellent bandgap is also beneficial to applications in optoelectronics.^{22} The germanene monolayer has been found to be an insulator with a large bandgap, as well as high carrier mobility (of the order of 10^{5} cm^{2} V^{−1} s^{−1}) due to its linear band dispersion near the Fermi level (E_{F}) at the K point. It has a low-buckled (0.84 Å) structure compared with silicene due to the weak p–p interactions and distinct coupling of s and p bonds between Ge atoms,^{23–26} giving rise to new characteristics beyond silica, such as detectable quantum spin Hall (QSH) and valley-polarized quantum anomalous Hall (QAH) states, for example.^{27,28} In addition, Gao et al.^{29} showed that it can be used for infrared materials, precision instruments and catalysts. Several remarkable features of germanene have been reported.^{26,30,31}

Despite the extensive efforts of 2D germanene dioxide monolayers,^{32} we investigate the electronic structure, dynamic mechanical stabilities and carrier mobilities of germanene dioxide bilayer as a noninteracting dielectric in van der Waals (vdW) electronics employing density functional theory (DFT) in this work. A number of low-energy structures had been investigated based on the of DFT optimization combined with the particle-swarm optimization (PSO) algorithm, and only the bilayer structure with space groups of P6/mmm of the most stable several 2D structures were obtained in our computations.^{33–36} The supercell in the α-2D-germanene dioxide structure is shown in Fig. 1. The optimized lattice constant a is 4.00 Å for α-2D-germanene dioxide in a hexagonal unit cell, as enclosed in the gray dashed lines in Fig. 1a. α-2D-germanene dioxide has an ideal sp^{3} bonding network, which means that all O atoms are connected to Ge atoms with same solid angles 109°28′; i.e., Ge is in a perfect O tetrahedron. The new α-2D-germanene dioxide structure is dynamic and mechanical stabilities. Meanwhile, α-2D-germanene dioxide has good stability, and can be stable up to 300 K. These fantastic properties make α-2D-germanene dioxide more attractive for electric and optical applications.

E_{form} = (E_{total} − N_{Ge}E_{Ge} − N_{O}E_{O})/(N_{Ge} + N_{O}) |

Next, the thermal stabilities of the germanene dioxide configurations are calculated by ab initio molecular dynamics (AIMD) simulations in Fig. 2a. The MD simulations selected the supercell of 4 × 4 × 1 based on the Nośe thermostat at 500 K.^{45} The phonon dispersion (based on Vibra of SIESTA utility^{46}) curves show the dynamic stability of α-2D-germanene dioxide (Fig. 2c). The deformation charge density shown in Fig. 2d suggests the charge transfer from Ge to O yielding the charge of Si to be 3.52e by Bader analysis. The same structure of the silica sheet in Russia (0001) (i.e. double silicon layer) was recently synthesized.^{14} This indicates that our material is likely to be prepared.

where e is the electron charge, ℏ is reduced Planck, k

Direction | Carrier type | |m*| (m_{e}) |
|E_{1}| (eV) |
C_{2D} (N m^{−1}) |
μ (cm^{2} (V^{−1} s^{−1})) |
τ (fs) |
---|---|---|---|---|---|---|

k_{a} |
Holes | 0.41 | 11.08 | 96.73 | 4.09 × 10^{3} |
83.55 |

Electrons | 0.05 | 16.11 | 2.12 × 10^{3} |
43.31 | ||

k_{b} |
Holes | 0.05 | 11.56 | 99.21 | 3.76 × 10^{3} |
76.81 |

Electrons | 0.07 | 14.53 | 1.36 × 10^{3} |
27.78 |

ε(ω) = ε_{1}(ω) + iε_{2}(ω) |

In addition, the absorption coefficient I(ω) was obtained by:

The properties of materials are also directly reflected in the optical properties. The wide bandgap of α-2D-germanene dioxide means that it can more easily be used in short-wavelength light-emitting devices and ultraviolet detection compared with MoS_{2}. Attracted by the proper band gap and excellent electronic properties of α-2D-germanene dioxide, we further explored the light absorption of α-2D-germanene dioxide. As shown in Fig. 4b, α-2D-germanene dioxide shows absorption starting at ∼2.5 eV, with two main absorption peaks from ∼5.5 to ∼7.5 eV, corresponding to significant light absorption at the ultraviolet region of the solar spectrum. The incident light can be effectively absorbed in three directions, with the adsorption coefficients increasing up to the order of 4.5 a.u., as shown in Fig. 4a. As shown in Fig. 4b, the plot of dielectric functions versus energy shows the same trend. Therefore, α-2D-germanene dioxide seems rather attractive for efficient light harvesting and may have promising applications in optoelectronics.

Fig. 4 (a) Computed imaginary absorption coefficient for α-2D-germanene dioxide along different incident light directions. (b) Computed imaginary dielectric functions versus energy. |

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