Background:
The emergence of graphene has aroused strong academic interest in two-dimensional materials in the academic community. Graphene has attracted widespread attention due to its unique tunable band structure, quantum electron transport, ultra-high mobility, and elasticity. As graphene-like two-dimensional group IV element compounds, silicene and germanene can be considered as zero-bandgap semiconductors, with a direct bandgap of 1., respectively55 mev and 239 mev。In addition, they have better tunability than graphene. Therefore, the band engineering of silicene and germanene has been widely explored in electronic devices, photodetectors, catalytic materials, batteries, chemical sensors, and topological insulators. Hydrogenation and alloying are the two main methods for adjusting the band gap of materials. The hydrogenation reaction of silicene and germanene can be achieved by the reaction of CASI2 and CCAGE2 with HCl, and the products of the hydrogenation reaction are silane Si2H2 and germanane Ge2H2, both of which have a stable single-layer honeycomb structure. Sixge1 XH alloy is a series of bandgaps between 109 ev~2.Stable structure between 29 EV. However, hydride production in silicon-germanium alloys and carbon-silicon alloys is rarely reported.
Models and calculation methods
All first-principles calculations in this article are based on:Vienna Ab Novo Simulation Package (VASP).Using the Cohen-Schamm theory and the projection augmentation of plane waves, dispersion-corrected D3-Grimme was used to describe the van der Waals interaction. The generalized gradient approximation of the Hyde-Burke-Enzerhoff (GGA-PBE) functional is used to study the electronic, optical and carrier mobility, which are always lower than the band gap value. Plane wave truncation employs a cut-off energy of 500 EV and a k-point grid of 6 6 1. The convergence criterion for geometric optimization is that the Hermann-Feynman force acting on each atom is less than 001 ev with an energy of less than 10 5 ev. A vacuum layer with a thickness of 20 is set along a plane perpendicular to the MNH2 monolayer to avoid mirror interactions between periodic structures.
Under the condition of 500 K, a series of 4 4 1 supercells were constructed for each MnH2 monolayer, and ab initio molecular dynamics (AIMD) simulations were carried out. With a total time duration of 5 ps and a time step of 2 fs, all atoms and molecules can move freely to achieve a full simulation. The structure of the fully optimized MNH2 monolayer is shown in Figure 1, and each monolayer has a folded hexagonal honeycomb structure. For the Si2H2 and Ge2H2 monolayers, the space group is p-3m1, and the optimized lattice constants a and b are 3., respectively89 and 408 å。Compared with the above-mentioned monolayers, the CSIH2 and SigeH2 monolayer films with P3M1 space group lack rotational symmetry. The lattice constants A and B of the optimized CSIH2 and SiGEH2 monolayers are 3., respectively12 and 398 å。The electron localization function (ELF) on the surface of the four monolayer structures (001) is shown in Figure 2, which clearly shows the covalent bonds in the four structures, as well as the aggregation of electrons between M and N atoms. The phonon dispersion curves can be used to evaluate the stability of the lattice dynamics, as shown in Figure 3, no frequencies below zero are found in the phonon dispersion curve in the first Brillouin zone, which confirms the kinetic stability of the four monolayers. The thermodynamic stability of the MNH2 monolayer was confirmed by AIMD simulations at 500 K, as shown in Figure 4. The results show that the energy remains in dynamic equilibrium at the time of 5 PS simulation, and there is no obvious structural change in the monolayer, which proves that the MNH2 monolayer is thermodynamically stable. In addition, the chemical stability can be evaluated by the work function, which is determined by the vacuum energy level and Fermi energy, based on the calculation of the HSE06 functional, the vacuum energy level and Fermi energy of the MNH2 monolayer are obtained, as shown in Figure 5, the work functions of the MNH2 monolayer are 6., respectively20 ev、5.50 ev、5.35 ev and 563 EV, which is even higher than graphene (4.).25 EV), indicating that the MNH2 monolayer has better chemical stability.
Figure 1 Top and side views of the crystal structures of (a) CSIH2, (B) SI2H2, (C) SIGEH2 and (D) GE2H2 monolayers
Fig. 2 Electron localization function (ELF) on the surface of (A) CSIH2, (B) SI2H2, (C) SiGEH2 and (D) GE2H2 monolayer crystal structures (001).
Fig. 3 Phonon dispersion spectra of (a) CSIH2, (B) SI2H2, (C) SIGEH2 and (D) GE2H2 monolayers
Fig. 4 AIMD simulations of (A) CSIH2, (B) SI2H2, (C) SIGEH2 and (D) GE2H2 monolayers
Fig. 5 Electrostatic potential curves for the monolayers of (a) CSIH2, (B) SI2H2, (C) SiGEH2 and (D) GE2H2 (the horizontal part represents the vacuum energy level).
Results & Discussion
Fig.6 Band structure of (a) CSIH2, (b) SI2H2, (C) SiGEH2 and (D) GE2H2 monolayers and (e) Split-wave density of MNH2 monolayers
The GGA-PBE functional and HSE06 functional were used to optimize the electronic properties of the MnH2 monolayer, respectively, and the results are shown in Fig. 6(A D). It can be found that all monolayers are direct bandgaps, and the valence band maximum (VBM) and conduction band minimum (CBM) are located at the point. The direct band gap widths of the CSIH2, SI2H2, SIGEH2 and GE2H2 monolayers calculated by the HSE06 functional are 4., respectively85 ev、2.92 ev、2.56 ev and 170 EV, while the bandgap of the mnH2 monolayer calculated by the PBE functional is 395 ev、2.32 ev、1.56 ev and 103 ev。Compared with the HSE06 functional, the band gap obtained by the PBE functional is smaller, and the shape of the rest of the band structures is generally similar. Figure 6(e) shows the projected density of states (PDOS) of the MNH2 monolayer, and it can be found that CBM and VBM are jointly contributed by M and N atoms, respectively, while the charge distribution near CBM and nearby VBM is obviously different, which is conducive to blocking the binding of electrons and holes during photoexcitation and improving the efficiency of photoexcitation. As a catalyst for photocatalytic water splitting and hydrogen production, the band edge of the MNH2 monolayer is related to the oxidation and reduction potential of water decomposition. For O2 H2O, the VBM of the MnH2 monolayer needs to be -5The oxidation potential of 67 EV is more negative, while for H+ H2, the CBM of the MnH2 monolayer needs to be more negative than 444 EV of reduction potential correction. Since the density of states (Dos) at the band-edge of the mnH2 monolayer is mainly contributed by M and N atoms, it can be seen from Figure 6(E) that the lattice structure and atomic position can be significantly changed by applying strain, resulting in a change in the relative positions of M and N atoms. Then, the change in geometry leads to a change in the properties of the electrons, which in turn leads to a change in the energy level, which leads to VBM and CBM. Knowing that strain can effectively modulate the electronic properties of the monolayer, we investigated the band structure of the MNH2 monolayer under strain, and the results are shown in Fig. 7(af).
Fig.7 Effect of the strain calculated by the HSE06 functional on the band edges of the (a) CSIH2, (B) SI2H2, (C) SIGEH2 and (D) GE2H2 monolayers, and the effect of the strains obtained from the (E) PBE functional and (F) HSE06 functional on the band gap
For the CSIH2 monolayer, all tensile strains in the biaxial direction increase the band gap, while all compressive strains result in a decrease in the band gap. Conversely, for the GE2H2 monolayer, all the tensile strains decrease the band gap and the compressive strain increases the band gap. For Si2H2 and SigeH2, the band gap is almost the same when the compressive strain is not greater than -2%, while all tensile strains reduce the band gap, and the strain between -1% and 1% has almost no effect on Si2H2. In fact, the different manifestations under strain are a reflection of atomic number, and the VBM changes of these monolayers exhibit similar characteristics under strain, while the CBM changes are significantly different and correspond to the changes in the band gap. As the atomic number increases, DOS becomes more localized, and this difference may lead to changes in the band gap and strain in these monolayers. In addition, the strain cannot change the CBM and VBM positions of all MNH2 monolayers, and they are still direct bandgaps. Under the action of strain, the band edge of GE2H2 still cannot meet the requirements of water decomposition. CSIH2 has a wide bandgap and is not sensitive to visible light, but is suitable for deep ultraviolet semiconductor devices. The banded edges of the SiGEH2 and Si2H2 monolayers can amplify the oxidation and reduction potentials of water decomposition under biaxial tensile strains, as shown in Figure 8(a b). Therefore, the SigeH2 and Si2H2 monolayers can maintain the activity of photocatalytic water splitting for hydrogen production. The optical absorption of SigeH2 and Si2H2 monolayers under strain was calculated using the HSE06 functional, and the results are shown in Figure 8(Cd). In the visible light range, the peak absorption coefficients of the SigeH2 and Si2H2 monolayers can reach 2., respectively0 105 cm-1 and 40 105 cm-1 and will change with the change of strain, which corresponds to the change in the band structure. In the case of biaxial tensile strain of 2% and 4%, respectively, the absorption edge is redshifted. At the same time, an enhancement of the first absorption peak can also be observed under appropriate tensile strains, which means that these strains can promote the absorption of visible light. Therefore, the smaller tensile strain not only causes the band edges of the SigeH2 and Si2H2 monolayers to cross the redox potential of water, but also improves the photocatalytic performance of the monolayer. In addition, the peak value of the absorption coefficient in Figure 8(cd) corresponds to the band edge in Figure 8(ab), and the optical properties change with strain and correspond to the change in the band gap. Due to the correspondence between optical absorption and electron transition, the decrease of the band gap with strain corresponds to the red shift of the light absorption peak, and the increase of the band gap corresponds to the blue shift of the light absorption peak. Therefore, the SigeH2 and Si2H2 monolayers can effectively capture solar energy, which is the key for applications in photovoltaic devices.
Fig.8 Optical absorption curves of (A) SiGEH2 and (B) GE2H2 monolayers for (A) SiGEH2 and (B) GE2H2 monolayers under different strains calculated by HSE06 functional
Since the photocatalytic water cracking process includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the free energies of all intermediates in HER and OER were calculated to test the catalytic activity of the SiGEH2 and Si2H2 monolayers. As can be seen in Figure 9(a), the HER Gibbs free energy of the SigeH2 and Si2H2 monolayers is 0., respectively98 ev and 023 EV (close to zero). For OER, there are four electron transfer processes, as shown in Figure 6(b), the Gibbs free energy changes in the SiGEH2 and Si2H2 monolayers are very similar, and in the third, the formation of OOH*, is the one where the free energy changes the most. The OER overpotentials of the SigeH2 and Si2H2 monolayers were calculated to be 0., respectively54 V and 018 V, even lower than with a single layer of graphene or metal oxide (0.49-1.70 V) covers many transition metals.
Fig.9 Gibbs free energy curves of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of SiGEH2 and Si2H2 monolayers
By measuring the mobility of the carriers, their photocatalytic activity can be quantitatively evaluated. Changes under uniaxial strain, as shown in Table 1. The results show that these MNH2 monolayers have sufficiently high mobility to reach about 104 cm2V 1S1. For each monolayer, the electron mobility is higher than the hole mobility in both the direction along the plane where the bond or atom is located and in the zigzag direction in the direction of the plane where the bond or atom is located. In addition, anisotropy is also observed in the MnH2 monolayer, especially in the Si2H2 monolayer, and the results show that the mobility of electrons is more than 5 times higher than the hole mobility in the direction of the vertical bond or the plane where the atom is located due to the relatively small effective mass and deformation potential constant. The significant differences between these holes and electrons suggest that there is a promising effective charge separation in the MNH2 monolayer. The maximum electron mobility is 8 of the Si2H2 monolayer15 104 cm2v 1s 1, which is even higher than graphene. Therefore, due to the high carrier mobility, the MNH2 monolayer may be a potential material for electron, spintron, and photovoltaic applications. Photocatalytic water splitting is a method to improve the energy conversion efficiency of solar energy utilization. For photocatalytic splitting of water to produce hydrogen, the efficiency of solar hydrogen production (STH) is required to be at least 10%. Therefore, it is necessary to calculate the light absorption efficiency (ABS), carrier utilization rate (Cu) and STH (STH) of the SigeH2 and Si2H2 monolayers, and the results are shown in Table 2, and it can be found that the ABS of the Si2H2 monolayer can reach 5694%, while the Cu of the SigeH2 monolayer is higher than 45%. Both the SigeH2 and Si2H2 monolayers had an STH of 10% and 19., respectively27% and 1322%, which is higher than the current GA2S3 (6.).4%), Al2Se3 (8%), and IN2S3 (14.)4%), confirming that the photocatalytic energy conversion efficiency of both monolayers is high enough to obtain hydrogen from the splitting of water.
Table 1 Carrier effective mass, elastic modulus, deformation potential constant, and carrier mobility of the MNH2 monolayer
Table 2 Si2H2 and SigeH2 monolayer energy conversion efficiencies (light absorption efficiency, carrier utilization, and STH).
Conclusions and prospects
In this paper, the geometry, thermodynamic and chemical stability, electronic and optical properties, carrier mobility, and band structure engineering of the MNH2 monolayer are studied. The results show that all MnH2 monolayers are stable. The band edges of the Si2H2 and SiGeH2 monolayers can amplify the oxidation and reduction potentials of water splitting under specific strains, which means that these monolayers meet the prerequisites for photocatalytic water splitting, while the VBMs of the Ge2H2 and CSIH2 monolayers cannot meet the water splitting requirements under all the strains considered. The mobility of the CSIH2, Si2H2, SiGEH2 and GE2H2 monolayers can reach 2., respectively52×104 cm2v−1s.15×104 cm2v−1s.08 104 cm2v 1s 1 and 138 104 cm2v 1s 1, which is significantly higher than the 1 of graphene50 104 cm2v 1s 1 and other reported values for graphene-like materials. The results also show that there is efficient absorption of the MnH2 monolayer in the visible region, and the absorption of the Si2H2 monolayer can be further enhanced by tensile strain. Further, both the Si2H2 and SigeH2 monolayers have high energy conversion efficiency enough to obtain hydrogen from water splitting. Therefore, these two monolayers can be identified as candidate materials for photocatalytic water splitting, while the GE2H2 and CSIH2 monolayers are not satisfied. These findings are expected to promote the practical application of MNH2 monolayer in optoelectronic devices and provide new and efficient catalysts for photocatalytic hydrogen production.
Bibliographic information
q. li, h. liu, h. yang, y. zheng. high carrier mobilities and tunable band structures in two-dimensional mnh2 (m, n= c, si, ge) monolayers. appl sur sci
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