Fundamental Properties of Hydrogen-Functionalized GaSe Monolayer.

Functionalization reveals potential opportunities for modifying essential properties and designing materials due to the strong interaction between functionalized atoms and the surface. Among them, hydrogenation possesses such a way to control electronic and optical characteristics. In this paper, the stability and transformed electronic, optical properties of H-functionalized GaSe in two cases (single and double sites) were reported that exhibit the effects of hydrogen functionalization via first-principles calculations. Formation energies suggest that H-functionalized GaSe systems are stable for construction. H-GaSe and 2H-GaSe display distinct properties based on the functionalized way (single- or double-site functionalization). Accordingly, H-GaSe is metallic, while 2H-GaSe belongs to a semiconductor. The magnetic configuration with ferro- and anti-ferromagnetic could be found in H- and 2H-functionalized cases through spin distribution, respectively. Especially, the chemical hybridized bonds of Se-H, Ga-Se, and Ga-Ga corresponding to s-sp3 and sp3-sp3 bondings, respectively, are clearly verified in the orbital-projected density of states and charge density. The optical properties of 2H-GaSe could provide the main characteristics of a semiconductor, which is the limited range of transparency by electronic absorption at short and long wavelengths. Moreover, increasing the number of GaSe segments (L) could change the band gap leading to application in the band gap engineering of the 2H-GaSe systems. Thus, hydrogen functionalization could provide the possible manner for adjusting and controlling features of GaSe, promising for the development of electronic devices and applications.

Introduction

Two-dimensional (2D) materials have covered a wide range of specifications[1] and applications[1,2] due to their various features in many fields such as electronics, optoelectronics, conductivity, and energy storage.[3−8] Because of success in graphene and graphene-based investigations,[2,9−15] 2D materials, particularly honeycomb structures, have generally gained more and more attention for designing next-generation devices. In 2D materials, the physical and chemical properties strongly and consistently depend on geometric properties, leading to the necessity and significance of solving structural problems. The surface process including doping, intercalation, or functionalization is one of the main issues of structural problems to mark their signature in applications. Because of the limitations of pristine 2D materials in industrial applicability, this process provides an efficient method for modifying original material properties that have been developed in modern applications.[16] Hence, addressing the surface process becomes a key to determine the potential of the material.

Surface processing of 2D materials has various strategies including doping,[17−19] adsorption,[16,20] substitution,[17] intercalation,[21,22] and functionalization[3,23−26] based on their efficient interacting environment compared to bulk counterparts. Among them, functionalization can create a chemical environment for adjusting properties,[27] which supports fabrication[28] or material growth.[29] Even though the functionalized nature remains unclear, this process has predicted the existence of remarkable effects on a surface reaction for plenty of fields. Functionalization in graphene and related graphene systems indicate high performance in the efficient storage capacity,[3,4,24,30] the hydrogen evolution reaction (HER),[20,26,31−35] growth mechanism,[12] and photoelectrochemical generation.[28] Furthermore, functionalization can tune the electronic and mechanical properties[32,36] of certain 2D materials resulting in tellurene structures[37] and MoS2,[38] respectively. Oxidation and hydrogenation are the two common parameters of functionalization in 2D materials,[3,23−26,29,33−35,37] in which the structure is functionalized by oxygen and hydrogen atoms, respectively. They all also provide useful information concerning the mechanical response[33,38] in graphene, germanene, MoS2, and Ti3C MXene.

On the other hand, with the development of graphene and graphene-based materials,[39−41] the metal chalcogenides group has gained rapidly growing interest.[7,42−47] The metal monochalcogenides labeled MX (M: metal; X: chalcogen atoms)[48−50] are one class of this group that has been synthesized and analyzed in both experimental and theoretical studies.[44,46,50−54] As a typical member of the MX group, GaSe is an emergent candidate due to its excellent performance in nonlinear optics[55] and the generation of electromagnetic waves. To enhance its optical properties, various elements including S, Te, Ag, and Al have been doped on GaSe crystal[56−59] suggesting the further optimum growth technique. Meanwhile, the transformation between a trivial and nontrivial topological insulator of GaSe has been found after functionalizing with oxygen,[60] forming the quantum Hall effect. This motivates one to turn to hydrogen functionalization on GaSe for new opportunities in documented research and applications. Although the natural interaction between hydrogen and a surface through a functionalized structure has been a subject of debate,[4,30] hydrogenation possesses promise and is a potential manner for the design and growth of materials.[3,12,20,24,31,36,38] In addition, hydrogen functionalization has been absent in surface processes of GaSe up to now. Therefore, unique and critical features such as the transformation of electronic properties, band gap engineering, and essential optical properties will be expected to be revealed in the hydrogenation of GaSe. In this work, the stability and electronic and optical properties of H-functionalized GaSe are systematically investigated by implementing first-principles calculations for clarifying the interaction between hydrogen atoms and GaSe surface. As a result, the interacting bondings of hydrogen and the surface as well as the inside interaction of the functionalized system are consistently described through a multiorbital hybridization concept, demonstrating diverse band energy spectra, density of state, and optical properties. Accordingly, frequency dependence of the dielectric function is able to define important optical properties as fundamental for future discussion on exciton effects on the system.

Methodology

First-principles calculations implemented in the VASP package[61] were performed for investigating geometric, electronic, and optical properties of hydrogenation GaSe. A vacuum of 15 Å was set to avoid the interaction between neighboring slabs in the structure. For electron exchange-correlation interactions, the generalized gradient approximation (GGA)[62] of the Perdew–Burke–Ernzerhof functional (PBE)[63] was applied with the cutoff energy of 500 eV. Moreover, the hybrid functional Heyd-Scuseria-Ernzerhof (HSE06)[64,65] and Becke, three-parameter, Lee–Yang–Parr (B3LYP)[66] were implemented for more accurate band gap calculations and making a comparison. The values of 10–5 eV and 0.001 eV/ Å were set for the criteria of energy and force, respectively. The k-points of 15 × 15 × 1 in gamma symmetry and high-symmetric Γ-centered grid Brillouin zone were sampled for structural optimization and band energy calculations, respectively.

Results and Discussion

Monolayer GaSe (Figure a) presents a stacked Se–Ga–Ga–Se slab as a member of metal monochalcogenides MX,[48,49] which are formed in a hexagonal structure with the space group of P6m2.[55,67] The hydrogen-functionalized GaSe was constructed in single and double sites in which the hydrogen atoms were directly doped on the top and both sites of selenium atoms as shown in Figure b,c, respectively. In this figure, the optimized structures of pristine and hydrogen-functionalized GaSe were illustrated in single and double sites, namely, H-GaSe and 2H-GaSe, respectively, from the side view. The formation energy (Ef) was considered to confirm the functionalized favorability of GaSe monolayer according to eq where Esys and EGaSe are the total energy of the functionalized and pristine systems, respectively; μSe and μH are, respectively, the chemical potential of selenium in their bulk phases. Number 2 is used to calculate the double-site functionalization. The formation energies indicate that 2H-GaSe is more stable than H-GaSe based on the smaller value (−3.92 eV) as mentioned in Table . Furthermore, this value is close to the formation energies of some stable 2D materials, for example, graphene[68,69] and MoS2.[70] This predicts the possibility of constructing double-site H-functionalized GaSe for practical applications.

Figure 1

Structural optimization of (a) pristine monolayer, (b) single, and (c) double hydrogen functionalization GaSe from the side view with (yz) plane.

Table 1

Structural Parameters of Pristine and H-Functionalized GaSe in the Single and Double Side

				chemical bond (Å)			buckling Δ (Å)
system	Ef (eV)	magnetic moment	lattice constant (Å)	Ga–Ga	Ga–Se	Se–H	(1)	(2)
pristine		0	3.85	2.49	2.48		1.115
H-GaSe	–1.38	–0.01	4.14	2.65	2.7	1.52	1.24	0.96
2H-GaSe	–3.92	0	4.24	3.84	2.8	1.49	1.37	1.36

The structural parameters shown in Table describe the main structural features of H-functionalized GaSe. Functionalization could enlarge the lattice constant from 3.85 to 4.12 and 4.24 Å in single and double types, respectively, leading to increases in the Ga–Ga and Ga–Se bond lengths compared to those of the pristine case as listed in this table. This change might come from the interaction between H and a surface as well as sharing of Se and H charges. A similar feature has been also found in O-functionalized GaSe.[60] Moreover, H-functionalized configurations form two different bucklings, labeled Δ1 and Δ2, while identical ones appear in the pristine case as described in Figure a and Table . 2H-GaSe exhibits a fixed configuration in H-functionalized sites with selenium atoms indicating the larger bond length of the Ga–Ga bond and two buckling values compared to that of the H-GaSe case. Contrarily, the without a H-functionalized site, H-GaSe is more flexible because of the remarkable larger buckling Δ1 that might cause the gapless behavior in the electronic properties. These two cases demonstrate the essential effect of hydrogen functionalization in the 2D structure of GaSe.

GaSe monolayer exhibits a semiconducting configuration with a middle gap of 2.665 eV calculated with the HSE06 hybrid functional as listed in Table . The hydrogen atoms in single-site functionalization reveal the effect of the GaSe monolayer resulting in a gapless structure. However, the double-site H-functionalized case remains the semiconducting configuration with the band gap of 2.08 eV (Table ). Thus, a functionalized method in a single or double site suggests a way to manipulate the band gap, which could become one of the factors for band gap engineering. Moreover, band gap dependence could be realized in various manners related to structural modifications. In fact, the band gap influence of the double-functionalized system could be considered due to varying the number of GaSe segments (L) in a unit cell as described in Figure a and Table . In this table, band gaps with changing L were calculated by implementing PBE and hybrid functional HSE06, B3LYP to get accurate band gap values and make a comparison among them. Although there exists a difference of band gap between these two hybrid functional calculations, the trend in the band gap change is the same as that of increasing L. Accordingly, the dramatic change of band gap is found when going from the L = 1 to L = 2 system for all applied functionals; for example, the band gap increases from 1.5 to 2.04 eV by HSE06 functional calculations. From the L = 2 case, the slight enhancement is shown following the increase of the L parameter. Thus, a double-functionalized structure considering the change of value L of segment GaSe could control the band gap suggesting a potential method for the design and construction of this material.

Table 2

Band Gap Engineering (Eg, eV) Varying the Number of Segment L per Unit Cell in the 2H-GaSe System with PBE, HSE06, and B3LYP Functional for Band Gap Calculations

	2H-GaSe
functional	L = 1	L = 2	L = 3	L = 4	pristine
PBE	0.96	1.68	1.25	1.2	1.8
HSE06	1.5	2.04	2.08	2.1	2.665
B3LYP	2.0	2.573	2.59	2.6	2.96

Even if the band gap depends on the implemented functional as mentioned above, the band energy spectra with main features related to dispersions remain for all cases of functionals. Thus, band energy structures from PBE calculations are presented in Figure . In this figure, the distinct properties between pristine and functionalized cases along the high-symmetry path of M-Γ-K-M in the first Brillouin zone (BZ) of hexagonal lattice are accordingly analyzed. The effects of functionalization are indicated in the energy band with many sub-bands due to the increasing number of atoms and orbitals after functionalization. The main features of monolayer GaSe in Figure a are further explained in Figure S1 of the Supporting Information. In addition, the type of H-functionalized system exhibits different characteristics of atomic doping as a comparison of single- and double-site functionalization (Figure b,c). In Figure b, H-GaSe shows the overlap between conduction and valence bands corresponding to a gapless structure. The sombrero hat dispersion is formed at the Γ point and might cause the arising of a van Hove singularity (vHs) in the band-edge state.[71] Moreover, a spacing appears in the range of (−2, −0.5) eV depending on the two asymmetric structural sites. 2H-GaSe (Figure c) belongs to a narrow-gap semiconductor that remains the indirect band gap feature from the pristine case indicated in the asymmetry of occupied valence and unoccupied conduction bands about the Fermi level. However, the direction of the VBM-CBM (valence band maximum-conduction band minimum) is transformed in which the band gap shifts from the M to the point between the M and Γ points while the CBM locates at the Γ point in GaSe monolayer (Figure a). Furthermore, strong dispersion is demonstrated in various inverted bands. The parabolic dispersions in the valence band could illustrate that the occupation probability for all states below and above the Fermi level are approximately 1 and free, respectively. Especially, partial flat bands found in the energy range from −3.0 to −1.0 around the Γ point display the orbital interactions in which the electron effective mass should be large.[72] Crossing also appears in the whole range in valence and conduction bands due to the compatibility relations between geometry and eigenstates at the symmetry k-points in the BZ. With regard to atom dominance, hydrogen functionalization demonstrates a strong influence in atomic contribution (green circles) to the system. Based on the main contribution of H atoms near the Fermi level, the band gap of H-GaSe is affected leading to the overlap between conduction and valence bands. In contrast, H atoms reveal strong dominance in the conduction band of 2H-GaSe leading to the open band gap in the band energy spectrum. Hence, H-functionalized GaSe could create remarkable conversion in the band energy structures, which would be promising for material design and applications.

Figure 2

Band energy structures of (a) pristine GaSe and (b) single and (c) double side H-functionalized GaSe with L = 3 and considering hydrogen atom dominance. The green circles represent hydrogen atoms for illustrating its contributions to the band structure.

The charge density could provide more information about the type and strength of chemical bonding shown in Figure considering both isolated atoms and compound charge. Isolated atomic charges of all Ga, Se, and H atoms display a spherical-like shape (Figure a–c), while those of H-functionalized cases reveal overlap regions of active atoms (Figure d,e), indicating chemical bonding of Ga–Ga, Ga–Se, and Se–H. Going from H-GaSe to 2H-GaSe, the overlap between two Ga atoms in a Ga–Ga bond approximately becomes misty corresponding to the increase of Ga–Ga bond length. A similar feature is found in the bond of Ga–Se. These changes might come from the strong effect of two sites of hydrogen functionalization and the contribution of s, p, p, and p orbitals to Ga–Se bonds, respectively. In Se–H bonds, red regions between the H and Se atoms indicate the contribution of s orbitals, while yellow-green regions relate to the p, p, and p orbitals of these two atoms. In the system, H and Ga, Se atoms, respectively, provide s and 4s, 4p, 4p, 4p active orbitals for H–Se and Ga–Ga, Ga–Se bondings that could determine their chemical bonds for a thorough understanding of the essential properties of hydrogenation structures. To further clarify the essential electronic properties of H-functionalized GaSe in two cases, the orbital projected density of states (PDOS) is considered in Figure . The peaks around the Fermi level illustrate the asymmetry of occupied valence and unoccupied conduction bands. In Figure a, the strong sp3 bonding is indicated in the coexisting dominance of s and p orbitals corresponding to (4s, 4p, 4p, 4p)-(4s, 4p, 4p, 4p) of the Ga–Se bond. The sp3 bonding is also revealed in 2H-GaSe due to the strong dominance of (4s, 4p, 4p, 4p) of Se and Ga atoms, respectively, as shown in Figure b. The small but significant contribution of H-s orbitals to form s-sp3 bondings of H–Se bonds is shown in both cases. Even though both H-GaSe and 2H-GaSe exhibit sp3 bondings, the orbital contribution is remarkably different, which is caused by the effect of hydrogen atoms based on the functionalized way. These findings additionally indicate the influence of hydrogen atoms on a GaSe monolayer depending on the type of functionalization. Besides, hydrogen-functionalized GaSe displays distinct magnetic behavior corresponding to H-doped single and double sites, resulting in spin density distribution (Figure ). In Figure a, the negative regions with light blue are extended due to Ga–Se bondings, and the interaction between hydrogen and the surface induce the spin-up configurations colored in yellow. This causes the negative value of magnetic moment as shown in Table , leading to a ferromagnetic configuration in this system, while an anti-ferromagnetic spin distribution occurs across two parallel edges as shown in Figure b with zero magnetic moment (see Table ). The spin-up and spin-down environments lead to the spin splitting indicated in edge and center-initiated spin distributions. Thus, hydrogen-functionalized GaSe could manipulate the magnetic configuration depending on the functionalized manner.

Figure 3

Charge density of isolated atoms (a) Ga, (b) Se, (c) H and H-functionalized GaSe in (d) single and (e) double sites, respectively.

Figure 4

Orbital PDOS of (a) single and (b) double H-functionalized GaSe.

Figure 5

Spin distribution of (a) H-GaSe and (b) 2H-GaSe. Positive and negative values corresponding to spin up and spin down, respectively, are reported in yellow and light blue produced using VESTA.

To determine the optical properties of H-functionalized GaSe, related factors including dielectric function, reflectance, refractivity, adsorption, and energy loss spectrum are examined, as shown in Figure and Figure S2. In Figure , the optical properties of 2H-GaSe are presented with these main features. The similar characteristics of H-GaSe could be found in Figure S2 of the Supporting Information. The two parts of the dielectric function are described in Figure a with a remarkably strong peak (the indicated black arrow) and some weak peaks also mentioned in Table in both real (ε1) and imaginary (ε2) parts, colored in violet and red curves. The corresponding relation between these two parts is illustrated via the Kramers–Kronig relationships,[73] which is explained in related formulas (see Supporting Information) and interband transitions in Table and Figure . The ultraviolet absorption of 2H-GaSe is caused by these transitions related to its band structure. The absorption spectrum indicates that the system starts to absorb photons in the ultraviolet regions at 2 eV as shown in Figure b. The zz direction exhibits stronger adsorption in the energy range (4, 10) eV compared to the xx/yy direction. Besides, the valence excitation region extends up to 7.5 eV, which determines the lowest limit of the conduction band and the upper valence band. The real part behaves mainly as a classical oscillator in which it vanishes in an energy range around 4.5–5.5 eV corresponding to maxima of absorption at these frequencies. The absorption band expands beyond 6 up to 10 eV in the zz direction associating with the transition from the valence to conduction bands. Furthermore, peaks of the energy loss function as shown in Figure c are consequently found in the vanished region of ε1 for the xx direction. The loss function corresponds to a broadened peak at approximately 6 and 11 eV in the xx and zz directions, respectively, relating to plasmons and forms of collective excitations. The plasmon is determined by the vanishing real part of the dielectric function as mentioned above and a minimum of the imaginary part. Thus, the plasmon modes associate with the imaginary part that could propagate along the surface. Simultaneously, a small vibration is presented in increasing energy indicated in weak peaks in two parts. It should be noted that the region above 10 eV cannot be interpreted in terms of classical oscillators, which is consistent with interband transitions shown in Figure . In this region, ε1 and ε2 are dominated by linear features in which ε1 increases while there is decreasing of ε2. Additionally, the reflectance and refractive properties are calculated based on the dielectric function shown in Figure d,e. In Figure d, the strong reflectance minimum is determined by the imaginary part of the dielectric constant in the energy range of (0, 2) eV and (5, 6) eV for all directions and zz-direction, respectively, indicating a collective plasma resonance. Simultaneously, the broad maxima in (5, 6) eV and (4, 5) eV of the xx and zz directions, respectively, relate to the strong peak in the dielectric constant. Moreover, the maximum refractive index is found in low-lying energies of approximately 2.5 and 3.5 eV for two directions as described in Figure e. This index accordingly decreases in increasing energy with the increase of wavelength. Especially, the change in the refractive index is produced from the change of absorption through the Kramers–Kronig relationships. These two factors are consistent with characteristics of dielectric function indicating the close relation between dielectric function and linear optical properties, indicated in this relation.

Figure 6

Optical properties of 2H-GaSe through (a) dielectric constant, (b) reflectance, (c) refraction, (d) adsorption coefficient, and (e) energy loss spectrum analysis.

Table 3

Peak Position (eV) of Imaginary Parts and Interband Transition for 2H-GaSe in the Range of (0, 8) eV

peak order	imaginary part	interband transition
1	2.5	M → Γ
2	4.17	M → Γ
3	4.86	Γ → K
4	5.84	Γ → K
5	6.32	Γ → K
6	7.2	M → Γ

Figure 7

Interband transition of 2H-GaSe corresponding to peaks in dielectric function.

Conclusions

In summary, the stability and electronic and optical properties of H-functionalized GaSe monolayer were investigated by implementing first-principles calculations via the VASP package. Double-site functionalization exhibits the stable structure due to formation energy and phonon dispersion. Through the band energy structure and orbital-projected DOS, a transformation of the electronic configuration was realized between two cases of functionalization corresponding to single and double sites, respectively. H-GaSe reveals a gapless structure, while semiconducting behavior was found in 2H-GaSe with a total change of band dispersion. The sp3 bonding was exhibited based on the coexistence of s and p, p, p orbitals in Ga–Ga and Ga–Se, while H–Se bonds were indicated in DOS and charge density. Moreover, the band gap engineering is proposed when increasing the number L of GaSe segments for the double-site functionalization. The functionalized manner could form a different magnetic configuration as mentioned in spin density distribution. In addition, 2H-GaSe exhibits sensitive optical properties indicating the consistency of the main features through dielectric function, absorption, refractivity, reflectivity, and energy loss spectrum. This work provides useful information for the functionalization of GaSe, suggesting a validity to design this material in electronic devices.