Engineering electronic structures and optical properties of a MoSi₂N₄ monolayer via modulating surface hydrogen chemisorption

Abstract

Recently, a MoSi₂N₄ monolayer has been successfully synthesized by a delicately designed chemical vapor deposition (CVD) method. It exhibits promising (opto)electronic properties due to a relatively narrow bandgap (∼1.94 eV), high electron/hole mobility, and excellent thermal/chemical stability. Currently, much effort is being devoted to further improving its properties through engineering defects or constructing nanocomposites (e.g., van der Waals heterostructures). Herein, we report a theoretical investigation on hydrogenation as an alternative surface functionalization approach to effectively manipulate its electronic structures and optical properties. The calculation results suggested that chemisorption of H atoms on the top of N atoms on MoSi₂N₄ was energetically most favored. Upon H chemisorption, the band gap values gradually decreased from 1.89 eV (for intrinsic MoSi₂N₄) to 0 eV (for MoSi₂N₄-16H) and 0.25 eV (for MoSi₂N₄-32H), respectively. The results of optical properties studies revealed that a noticeable enhancement in light absorption intensity could be realized in the visible light range after the surface hydrogenation process. Specifically, full-hydrogenated MoSi₂N₄ (MoSi₂N₄-32H) manifested a higher absorption coefficient than that of semi-hydrogenated MoSi₂N₄ (MoSi₂N₄-16H) in the visible light range. This work can provide theoretical guidance for rational engineering of optical and optoelectronic properties of MoSi₂N₄ monolayer materials via surface hydrogenation towards emerging applications in electronics, optoelectronics, photocatalysis, etc.

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1. Introduction

In the past two decades, two-dimensional (2D) nanomaterials^1–3 have attracted enormous attention due to their unique physical and chemical properties, such as high surface area, fast migration of photo-induced electrons and holes to the reaction interface/front, high charge carrier separation efficiency, superior electronic and optical properties and rich reaction sites. Particularly, identification of two-dimensional layered materials in the monolayer limit can lead to the discovering of some new phenomena and unusual properties. Thus, researchers have continuously explored new 2D (monolayer) nanomaterials through experimental and theoretical investigations for various emerging applications in photoelectric and energy-intensive fields.

Recently, Ren's group successfully synthesized a new monolayered MoSi₂N₄ material via a modified chemical vapor deposition (CVD) method.⁴ The MoSi₂N₄ monolayer was constructed by septuple atomic layers of Ni–Si–N–Mo–N–Si–N, as viewed as a MoN₂ layer sandwiched between two Si–N bilayers.⁵ It displayed a bandgap of ∼1.94 eV with a high mechanical strength and excellent chemical stability in atmosphere. These properties endowed MoSi₂N₄ with promising applications in electronic/optoelectronic devices, optical sensors, and photocatalysis.

The great application potential of MoSi₂N₄ monolayer material has spurred increasing interest in further improving its properties by constructing various (van der Waals) heterostructures. For example, Ang et al. reported tunable electronic properties and band alignments in MoSi₂N₄/GaN and MoSi₂N₄/ZnO van der Waals heterostructures.⁶ Nguyen et al. studied the effects of interlayer coupling and under electric fields on the electronic structures of type-II C₃N₄/MoSi₂N₄ heterostructure.⁷ In addition, two-dimensional metal/semiconductor contact in a Janus MoSH/MoSi₂N₄ van der Waals heterostructure⁸ and MoSi₂N₄/MoS₂ van der Waals heterostructure with good optoelectronic performance and tunable electronic properties⁹ were investigated. Apart from heterostructure engineering, Cui et al. found that metal atoms-adsorbed MoSi₂N₄ (ref. 10) systems could enable the manufacture of spintronic and vacuum field emission nanodevices. Luo et al. used first principles calculations to study the structural and electronic properties of organic molecules-doped MoSi₂N₄ monolayers,¹¹ demonstrating the importance of molecular doping in tuning the electronic properties of MoSi₂N₄ with extended applications.

Heterostructure engineering,^12,13 adsorption and substitutional doping^14–19 have been proven to be effective ways to modify various properties of semiconductor materials. In addition, surface functionalization,^20–24 such as fluoridation and hydrogenation, can also be used to manipulate the properties of 2D materials. Recently, Binh et al. applied the hydrogenation method to change the properties of GeC,²⁵ and InN²⁶ monolayers, largely extending the applications of these materials. Chen et al. demonstrated that the fluorinated MoSi₂N₄ monolayer²⁷ may exhibit a variety of properties and have the potential to fabricate novel optoelectronic devices by simply adjusting the degree of fluorination. Given the structure similarity between GeC/InN and MoSi₂N₄, hydrogenation could be a simple, clean, yet effective approach to engineer the electronic and optical properties of MoSi₂N₄ monolayers and expand its applications.

In this work, we performed a comprehensive first-principles density functional theory study on the hydrogenation of MoSi₂N₄ monolayer. The calculations reveal that the hydrogenated MoSi₂N₄ monolayers are thermodynamically stable. Meantime, electronic structures and optical properties of hydrogenated MoSi₂N₄ can be well regulated as a function of hydrogenation degree on the MoSi₂N₄ surface. The simulation results suggest that the hydrogenated MoSi₂N₄ monolayers can have promising applications in photocatalysis²⁸ and other clean energy-related fields.

2. Computational details

First-principles calculations based on quantum mechanics²⁹ can provide a reliable means to investigate the behavior of electrons and nuclei under specific circumstances. In current work, the electronic structure³⁰ and optical properties of both the pristine and hydrogenated MoSi₂N₄ monolayers were studied. Based on density functional theory (DFT)^31,32 and plane-wave pseudo-potential basis, a collection of analytical tools within Cambridge Sequential Total Energy Package (CASTEP),³³ was used for first principles^34,35 calculations. The exchange correlation functional was represented by generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) functional.³⁶ The semi-empirical DFT-D₂ method of Grimmes correctly considers the weak van der Waals interaction in the material. Because the local density approximation³⁷ (LDA) generally underestimate the equilibrium atomic distances while overestimate the adsorption energy of the model system, and PBE/GGA can provide more precise estimations for geometric optimization. Therefore, the PBE/GGA were applied for geometry optimization and electronic property calculations. The cutoff energy is set as 500 eV. For both electronic and optical analyses, the convergence criterion for the self-consistent electronic loop was set to 2 × 10⁻⁶ eV. Simultaneously, in the Brillouin zone, the 3 × 3 × 1 k-point grid is implemented for calculations. The adsorption energy³⁸ for one H atom adsorbed onto the MoSi₂N₄ monolayer of Mo, Si, or N atom can be confirmed as

E_ads = (E_suf + E_H) − E_H-suf (1)

where E_suf, E_H, and E_H-suf denote the total energy of intrinsic MoSi₂N₄, a free H atom, and total energy of H-atom-adsorbed MoSi₂N₄, separately. Negative (positive) values indicates that the adsorption is an exothermic (endothermic) reaction.

The investigation focused on the thermodynamic stability of hydrogenated MoSi₂N₄ through the computation of the formation energy (E_f) using the following equation:

E_f = (E_MoSi2N4-H − mE_Mo − nE_Si − lE_N − kE_H)/(m + n + l + k) (2)

where E_MoSi2N4-H, E_Mo, E_Si, E_H and E_N denote the total energies of hydrogenated MoSi₂N₄, the chemical potentials of Mo in the metallic bulk Mo, the chemical potentials of Si in the diamond Si, the chemical potential of H in gaseous H₂ molecule, and the chemical potential of N in gaseous N₂ molecule, respectively. The value of m, n, l, and k are the number of Mo, Si, N and H atoms in the system, separately.

From a quantum mechanical point of standpoint, the interaction between photons and electrons in the system is illustrated by time-dependent perturbations in the ground state, where the absorption or emission of photons causes transition between unoccupied and occupied states. The spectrum produced by excitation can be regarded as a joint density of state (DOS) between the conduction bands (CB) and the valence bands (VB). The imaginary part of the dielectric function ε₂(ω) is a function of the photon frequency and can be described as follows:

where k is the reciprocal lattice vector, ω is the frequency of incident photon, u is the vector defining the polarization of the incident electric field, and the superscripts c and v denote the CB and VB, separately. The real part ε₁(ω) of the dielectric function, since the dielectric function exhibits a causal response, can be obtained through the imaginary part of the Kramers–Kroning relations. Then the other optical spectra, such as absorption coefficient (α(ω)), reflectivity R(ω), and refractivity index (η(ω)) can be gained by ε₁(ω) and ε₂(ω):^39–41

3. Results and discussion

3.1. Geometric structure of MoSi₂N₄-nH monolayers

Fig. 1a and b show top- and side-views of the optimized geometry structure of pristine MoSi₂N₄ monolayer with the 4 × 4 × 1 supercell. The MoSi₂N₄ monolayer has a hexagonal crystal symmetry with optimized lattice parameters of a = 2.94 Å and c = 20.31 Å. It has a sandwiched structure consisted of two N–Si–N layers with a Mo layer in the middle.

Fig. 1 (a) and (b) top- and side-views of pristine MoSi₂N₄ monolayer; (c) top view showing the different sites for chemisorption of an H atom on a MoSi₂N₄ monolayer (T_Mo, T_Si, and T_N represent the site atop Mo, Si, and N atoms, respectively); (d) top- and (e) side-view of a MoSi₂N₄ monolayer after chemisorption of an H (MoSi₂N₄-1H).

Next, the surface sites for H chemisorption were considered. Three different sites was computed for comparison to determine the most stable site for H chemisorption on MoSi₂N₄ monolayer. Fig. 1c illustrates the chemisorption of an H on three different sites of MoSi₂N₄ monolayer, referring to atop Mo, Si, and N atoms as defined as T_Mo, T_Si, and T_N, respectively. Fig. 1d and e depicts top- and side-views of an H adsorbed on atop of N in MoSi₂N₄ monolayer (MoSi₂N₄-1H), with the H atom adsorbed on top of an N atom.

The calculated adsorption energies of an H atop of T_Mo, T_Si and T_N are −0.275, −0.948 and −1.601 eV, respectively. The negative adsorption energies suggest that the chemisorption of H on the three sites of the MoSi₂N₄ monolayer is exothermic and thermodynamically stable. Meanwhile, the lowest adsorption energy is −1.601 eV on T_N site indicates it is the most stable adsorption position. Therefore, the following discussion focuses mainly on the chemisorption of different H atoms on T_N sites. The maximum number of H atoms adsorbed at T_N sites on one side of MoSi₂N₄ monolayer can be estimated as 16 based on supercell size.

Then, the calculated formation energy (E_f) for hydrogenated MoSi₂N₄ was investigated to confirm the stability of hydrogenated MoSi₂N₄ further. As shown in Table 1 and Fig. S1 (ESI†), with the number of H increase, the formation energy are slightly increased because of the repulse interaction increase between the H atoms. However, the formation energy of different number of H chemisorption on the MoSi₂N₄ systems are all negative, indicating the stability of the systems. This result implies the occurrence of a favorable exothermic reaction between the MoSi₂N₄ monolayer and the H atom, highlighting the overall chemical stability of the structure.

Table 1 The calculated formation energy of the hydrogenated MoSi₂N₄ monolayer system

	Ef (eV)
MoSi2N4-1H	−1.890
MoSi2N4-2H	−1.864
MoSi2N4-4H	−1.823
MoSi2N4-6H	−1.766
MoSi2N4-8H	−1.742
MoSi2N4-10H	−1.712
MoSi2N4-12H	−1.683
MoSi2N4-14H	−1.635
MoSi2N4-16H	−1.608
MoSi2N4-32H	−1.247

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Fig. 2 shows top and side view of MoSi₂N₄-nH, n is the number of H atoms adsorbed on the surface. As the number of H atoms increases, the resulting MoSi₂N₄-nH retains the sandwich structure. As shown in Fig. 2d, when four H atoms are adsorbed, an adsorption energy is −1.69 eV can be derived, which comparable to that of MoSi₂N₄-1H (−1.60 eV). As the number of adsorbed H atoms increases, the corresponding adsorption energy gradually becomes more negative (e.g., −1.99 eV for MoSi₂N₄-16H). This situation illustrate that the monolayer structures remain stable with increased H atoms.

Fig. 2 (a) Top view of MoSi₂N₄-16H, (b) and (c) side view of MoSi₂N₄-16H and MoSi₂N₄-32H, (d) trend in the adsorption energies of MoSi₂N₄-nH (n = 1, 4, 8, 12, and 16).

3.2. Electronic properties of MoSi₂N₄-nH monolayers

The effects of adsorbed H on the electronics structures of MoSi₂N₄-nH were next investigated by band structure (BS) and density of state (DOS) calculations. Fig. 3 shows the calculation results of BS and DOS of pristine MoSi₂N₄ monolayer. The labels G, F, and H represent high-symmetry points in the Brillouin zone with fractional coordinates of G (0, 0, 0), F (0, 0.5, 0), and H (−0.333, 0.667, 0.5), respectively. The valence band maximum⁴² (VBM) and conduction band minimum⁴³ (CBM) are located at the different positions in the G and H points of Brillouin zone, illustrating that pristine MoSi₂N₄ is an indirect band gap semiconductor. Fig. 3a exhibits the BS of pristine MoSi₂N₄. The calculated band gap is 1.89 eV, near to the experimental value of 1.94 eV.⁴⁴ Fig. 3b shows the computed DOS of pristine MoSi₂N₄ monolayer. The valence band edge⁴⁵ (VBE) in the range of −3 to 0 eV energy level was primarily contributed by the Mo 4d and N 2p state. The conduction band edge (CBE) from 2 to 3 eV is mainly contributed by the Si 3p, Mo 4d and N 2p states.

Fig. 3 (a) Energy band structure and (b) density of states (DOS) plots of a pristine MoSi₂N₄ monolayer. The Fermi level is set at 0 eV.

Fig. 4 shows the calculated BS and DOS profiles of MoSi₂N₄-8H monolayer. Its band gap was 0.98 eV, less than that of pristine MoSi₂N₄. From DOS plot (Fig. 4b), its VBE in the energy range from −4 to 0 eV was primarily composed of Mo 4d and Si 3p state and CBE in the energy range from 1 to 4 eV was mainly comprised of Si 3p, Mo 4d, and H 1s state.

Fig. 4 (a) Energy band structure and (b) DOS of MoSi₂N₄-8H monolayer. The Fermi level is set at 0 eV.

The band structures of the H-adsorbed MoSi₂N₄ monolayer are shown in Fig. 5. From Fig. 5a and b, the MoSi₂N₄-1H and MoSi₂N₄-16H monolayers are both indirect semiconductors with calculated band gap values of 1.88 and 0 eV, respectively. Also, the phenomenon exhibits an indirect band gap for H-adsorbed MoSi₂N₄. Fig. 5c shows the band gap of the MoSi₂N₄-nH as a function of the number of H atoms (“n”) adsorbed on MoSi₂N₄ monolayer. It is evident that the band gap of MoSi₂N₄-nH continuously decreases as the surface-adsorbed H atom increases possibly with an increased impurity level. The decreased band gap of the MoSi₂N₄-nH monolayer suggests its optical absorption can be extended to the visible and infrared regions properties. This enables its promising application as a potential photocatalytic material clean energy conversion and utilization.

Fig. 5 Energy band structures of (a) MoSi₂N₄-1H and (b) MoSi₂N₄-16H; and (c) band gap of MoSi₂N₄-nH monolayer (n = 0, 2, 4, 6, 8, 10, 12, 14 and 16). MoSi₂N₄-nH denotes a MoSi₂N₄ monolayer after chemisorption of “n” H atoms.

3.3. Optical property variation of MoSi₂N₄-nH monolayers

The dielectric function of MoSi₂N₄-nH (n = 0, 1, 8 and 16) monolayers is calculated to evaluate the influence of electronic structure change on its optical properties (Fig. 6). Fig. 6a demonstrates the real part ε₁(ω) of pristine and nH-adsorbed MoSi₂N₄ monolayer systems. The pristine MoSi₂N₄ monolayer exhibits a dielectric constant ε₁(0) of 5.1. For H atom-adsorbed MoSi₂N₄ monolayer, the ε₁(0) are 4.8 (1H-chemisorption), 5.5 (4H-chemisorption) and 10.9 (16H-chemisorption), respectively. The result illustrates that ε₁(0) of the nH-adsorbed systems is significantly larger than that of the system without adsorbed hydrogen, except that of MoSi₂N₄-1H monolayer possibly due to the occasional calculation error.

Fig. 6 Dielectric functions of the pristine MoSi₂N₄ and MoSi₂N₄-nH (n = 0, 1, 8 and 16) monolayers: (a) real part; (b) imaginary part.

A higher dielectric constant can lead to a stronger ability to bind electrons and a less polarization. Therefore, with the increase of H atom chemisorption, the polarization intensity decreases and the thermal stability of MoSi₂N₄-nH system increases. Fig. 6b shows the imaginary parts of the dielectric functions. For pristine MoSi₂N₄, it can be seen that the imaginary part has three main peaks at 3.4, 6.1, and 9.1 eV, corresponding to the three intrinsic plasma frequencies. By analyzing the electronic band structure, we speculate that the peak at 3.4 eV may be caused by the electron transition between the N 2p state and the Mo 4d state in the conduction band minimum. The peaks at 6.1 and 9.1 eV may result from electronic transitions between the N 2p, Mo 4d and Si 3p states. It can also be seen that the line shapes of MoSi₂N₄-nH monolayers are almost the same in all energy ranges, and the peaks of MoSi₂N₄-16H monolayer at 3.4, 6.1 and 9.1 eV are consistent with the peaks of MoSi₂N₄-nH monolayer, but a weak peak is noted at 0.5 eV. However, the MoSi₂N₄-16H monolayer exhibits a distinctly different peak shape in the 0–2 eV range. In the high energy range, the line shapes of MoSi₂N₄-nH (n = 0, 1 and 8), these observations indicate that the difference in the number of H atoms adsorbed mainly affect the optical properties in the low energy zone.

The absorption coefficients α(ω) of the intrinsic and H-decorated MoSi₂N₄ monolayers were calculated using the obtained ε₁(ω) and ε₂(ω). Fig. 7a shows the absorption spectra of pristine MoSi₂N₄ and MoSi₂N₄-nH monolayer. As can be seen from the figure, the absorption region becomes significantly wider after chemisorption of H, and the absorption coefficient in the visible region is greatly improved compared with the pristine MoSi₂N₄. It suggests that H chemisorption is favorable for photocatalytic activity, making MoSi₂N₄-nH a potential candidate for photoelectrochemical applications.

Fig. 7 (a) Absorption spectra, (b) energy loss function plots, (c) reflectivity spectra, and (d) refractivity index spectra for MoSi₂N₄ and MoSi₂N₄-nH (n = 0, 1, 8 and 16). The pink areas indicate the visible light region.

Fig. 7b–d shows the energy-loss spectrum, that is, the reflectivity (R(ω)) and the refractive indices (n(ω)) of MoSi₂N₄-nH monolayers in the range of 0–10 eV. Fig. 7b exhibits the energy loss plots, where the spike near 4.2 eV can be attributed to plasma oscillations. It is worth mentioning that the peak in the energy loss spectrum corresponds to the valleys in the reflection spectrum. It can be seen from Fig. 7c that the reflectivity drops suddenly at about 4.3 eV and keeps the same trend as the peak value of the energy loss spectrum. In the light energy range from 1.8–3.5 eV, MoSi₂N₄-nH shows reflectivity peaks at 3.5 eV and refractivity peaks at 2.6 eV. The results show that H-adsorbed MoSi₂N₄ has a higher refractive index and substantially unchanged reflectance in visible region.

3.4. The electronic and optical properties of semi and full-hydrogenated MoSi₂N₄

The work function is defined as the minimum energy for an electron escaping from the surface as can be calculated by the following equation:

Φ = E₀ − E_F (7)

where E_F and E₀ are the Fermi level and the electronic potential at the vacuum level, respectively. In order to study the effect of the chemisorption of H atoms on the work function, the work functions of the pristine and adsorbed MoSi₂N₄ monolayer system were calculated. Fig. 8a indicates the variation of work function after the H chemisorption on MoSi₂N₄. The calculated work functions are 5.44 and 2.09 eV for pristine and full-hydrogenated MoSi₂N₄ monolayers, respectively. The change of Φ is closely related to the change of surface conductivity, especially for monolayer. Accurate calculation of Φ also helps to determine the direction of surface charge flow.

Fig. 8 (a) The change of work function Φ for H adsorbed MoSi₂N₄ systems, (b) the band gap of MoSi₂N₄-nH (n = 0 and 32), (c) density of state of 32H-adsorbed MoSi₂N₄ monolayer, (d) absorption spectra of MoSi₂N₄-nH (n = 0, 16 and 32).

Fig. 8b shows the band gap variations of pristine and full-hydrogenated MoSi₂N₄ systems. With the increase of the hydrogenation degree, the band gap of MoSi₂N₄ monolayer system decreases continuously. The result shows that the band gap of pristine MoSi₂N₄ is 1.89 eV and the full-hydrogenated MoSi₂N₄ is 0.25 eV. In combination with Fig. 8c, there are two impurity energy levels of Si 3p and N 2p states, and the two states are hybridized to form the impurity band. The impurity energy band of H 1s enters the valence band and is connected with the valence band to form a new degenerated energy band, which leads to the decrease of the gap bandwidth. The above phenomena show that hydrogenation is the main reason for the band gap decrease. Therefore, the hydrogenation can effectively modulate the size of band gap for MoSi₂N₄.

Based on the electronic property discussions, the optical properties variations of the MoSi₂N₄ system are discussed. Fig. 8d depicts the absorption spectra of pristine, semi- and full-hydrogenated MoSi₂N₄, respectively. In the visible range, the absorption coefficients of full-hydrogenated MoSi₂N₄ monolayers are much larger than those of the other two cases. These results indicate that full-hydrogenated MoSi₂N₄ monolayer is more suitable for solar cell and photocatalytic applications.

4. Conclusions

In this study, the structural, electronic, and optical properties of H-adsorbed MoSi₂N₄ monolayers were investigated using first principles calculations. The calculated energy of formation (E_f) for MoSi₂N₄ hydrogenation was determined to be negative, indicative of thermodynamic stability. This finding suggests the presence of a beneficial exothermic reaction between the MoSi₂N₄ monolayer and the H atom, emphasizing the overall chemical stability of the structure. Also, the negative adsorption energy indicates the stability of MoSi₂N₄ monolayer structure adsorbed with H. The electronic structure analysis shows that the pristine MoSi₂N₄ monolayer is an indirect band gap semiconductor (with a band gap of 1.89 eV). With the increase of H atoms adsorbed on MoSi₂N₄ monolayer, the band gaps gradually decrease from 1.89 eV to 0 eV (16H-adsorbed) and 0.25 eV (32H-adsorbed), which indicates hydrogenation can effectively change the width of band gap and make MoSi₂N₄ system into adjustable band-gap structure. The results of work function analysis show that H chemisorption on MoSi₂N₄ monolayers reduces the work function value, and full-hydrogenated monolayer has a minimum work function, which is conducive to the electron transition application for photocatalysis. The optical study reveals that the chemisorption of H atoms greatly increases the dielectric constant of MoSi₂N₄. The visible light absorption range and visible light absorption coefficient are also increased. Moreover, compared with semi-hydrogenated chemisorption, the absorption coefficient of full-hydrogenated monolayer in the visible range is much larger. Overall, due to the unique physical and chemical properties, H-adsorbed MoSi₂N₄ monolayers are promising candidates for novel photocatalytic materials and optoelectronic devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 52262042) and Spring City Plan: The High-level Talent Promotion and Training Project of Kunming (2022SCP005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra04428a

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