Tunable donor and acceptor impurity states in a WSe₂ monolayer by adsorption of common gas molecules

Abstract

A gas sensor of common gas molecules, such as CO, H₂O, NH₃, O₂, NO and NO₂ on a WSe₂ monolayer is investigated systematically by using first-principle calculations. The results show that the effects of NH₃, H₂O, CO adsorption on the electronic structures of the WSe₂ monolayer are not obvious; while the adsorption of O₂, NO and NO₂ molecules has an obvious influences on electronic structures of the WSe₂ monolayer. Moreover, our calculations show that NH₃ acts as a charge donor, whereas O₂, CO, H₂O, NO and NO₂ gas molecules act as an acceptor impurity. In particular, our results also show that NO₂ molecule adsorptions can result in an effective p-type doping in the WSe₂ monolayer. These results suggest that the carrier type can be tuned effectively in the monolayer WSe₂ by adsorption of common gas molecules.

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1. Introduction

Recently, the two-dimensional (2D) tungsten diselenide (WSe₂) monolayer has attracted intensive attention due to its suitable gap value, typical electronic and optoelectronic properties in the 2D transition-metal dichalcogenides (TMDs).^1–3 The 2D WSe₂ nanosheets were fabricated by micromechanical cleavage exfoliation and applied to ultra-thin UV-light emitting diodes (LEDs).⁴ Campbell et al. studied direct selenization of e-beam evaporated W thin films and used them for manufacturing highly uniform few-layer p-type WSe₂.⁵ Moreover, studies show that 2D WSe₂ photoelectrode is stable under acidic and alkaline environments, and can be used in the photoelectrochemical cell.⁶ For the optoelectronic device applications, carrier type is also a key role in the related p–n junction. Moreover, high performance p-type field-effect transistor (FET) has been successfully fabricated from WSe₂ monolayer with chemically doped contacts.⁷ Liu et al. also studied the role of metal contacts in designing high-performance n-type WSe₂ FET.⁸ Recent experimental studies also show that NO₂ molecules adsorbed on WSe₂ surface can be identified as the dominant configuration leading to p-doping.⁹ Furthermore, studies also show that p-type WSe_2(1−x)S_2x with a tunable band gap is the ideal complement to n-type tunable monolayers in the application of p–n junction-related flexible nanodevices.¹⁰ In addition, the effective mass and negative compressibility have also been calculated in WSe₂ monolayer and bilayer systems.¹¹ These studies show that 2D WSe₂ nanosheets can be a promising optoelectronic material in future studies.

As is well known that surface adsorption is a practicable method to manipulate and control the electronic properties of materials, typical for 2D TMDs material due to their large surface-to-volume ratio and high conductivities, which is extremely appropriate for highly sensitive sensors and other devices. For example, previous studies of MoS₂,^12,13 WS₂,¹⁴ phosphorene¹⁵ and graphene^16,17 indicate that the adsorbates can cause the variation of carrier concentration. Moreover, studies also showed that the molecular adsorption was identified to be a superior method to tune p- and n-doping of graphene.¹⁸ Furthermore, a band gap can be opened in low-buckled silicene by the surface adsorption without degrading its electronic properties.¹⁹ Adsorbates which are close to the carbon nanotube can greatly impact the charge transport properties of nanotube.²⁰ In addition, the adsorbed NH₃ on 2D Ti₂CO₂ lead to the high sensitivity of sensor and could be used as the NH₃ capture.²¹ These results indicate surface adsorption can be an effective method to regulate electronic structure properties of 2D materials.

To our knowledge, there are few theoretical studies involved on gas sensor effects on electronic structures of 2D WSe₂ nanosheets, and its underlying adsorption mechanism is still not clear. Therefore, in order to understand the effects of gas adsorption on electronic structures in the 2D WSe₂ nanosheets, we systematically performed the theoretical studies on the adsorption of small molecules CO, O₂, NH₃, H₂O, NO and NO₂ on WSe₂ monolayer by using a first-principle calculations in this work. Our results show that NO and NO₂ adsorption can accept electrons from the WSe₂ monolayer, leading to a p-type doping in WSe₂ monolayer, while other gas molecules have a slight influences on electronic structures WSe₂ monolayer. Thus, the electronic properties of WSe₂ monolayer can be regulated by selective gas adsorption.

2. Computational methods

Our first-principle calculations were performed using Vienna ab initio Simulation Package (VASP)^22–25 which implements a supercell approach to density functional theory (DFT).²⁶ In order to simulate the influences of adsorption molecules on electronic structures of WSe₂ monolayer, a 4 × 4 × 1 WSe₂ supercell is used, which is large enough to neglect the interactions of the adsorbate.^27,28 In this study, we adopted the Perdew–Burke–Ernzerhof (PBE) functional within generalized gradient approximation (GGA)²⁹ to treat the exchange and correlation effects. The energy cutoff is set to 400 eV in all calculations. 5 × 5 × 1 Monkhorst–Pack³⁰ k-points in Brillouin zone are sampled for geometry optimization and 9 × 9 × 1 k-points for static total energy and band structure calculation. Moreover, to ensure good boundary conditions, more than 15 Å vacuum layer is used to avoid periodic interactions between imaging layers. Moreover, four adsorption sites are considered to seek out the most stable one on WSe₂ monolayer: H site (the hollow on the center of a hexagon), B site (the bridge site above the center of W–Se bond), T_w site (the top site above a W atom of sublattice WSe₂ monolayer) and the T_Se site (the top of a Se atom of sublattice WSe₂ monolayer). The corresponding sites are also labeled in Fig. 1.

Fig. 1 Top view (a) and size view (b) of the buckled hexagonal structure of WSe₂ monolayer. The four adsorption sites are marked by the symbol H, T_w, T_Se and B, respectively.

To survey the adsorption properties of these several molecules on WSe₂ monolayer; firstly, we calculate the lattice constant of pristine WSe₂ monolayer, and then explore the configurations of WSe₂ monolayer with different adsorbates including NH₃, H₂O, O₂, CO, NO and NO₂. These molecules in initial orientations are also fully optimized. The convergence criterion for energy is chosen to be less than 10⁻⁵ eV between two consecutive steps, and the atomic positions are relaxed until the residual forces on each atom are smaller than 0.02 eV Å⁻¹. In addition, charge transfer is obtained based on Bader analysis.³¹

3. Results and discussion

To confirm the stability of an adsorption configuration after total energy optimization, the adsorption energy (E_a) of gas molecules on WSe₂ monolayer is defined as:

E_a = E_gas+WSe2 − E_gas − E_WSe2 (1)

where E_gas+WSe2 is the total energy of the compound system, while E_gas and E_WSe2 are the total energies of pristine WSe₂ monolayer and isolated gas molecule, respectively. Moreover, we also define the equilibrium height as the vertical distance between the top of Se-layer of WSe₂ monolayer and the nearest atom of gas molecule in the z-coordinate direction. The calculated results of the adsorption energies, charge transfer and equilibrium height are presented in Table 1.

Table 1 Calculated parameters for WSe₂ monolayer with adsorbed O₂, CO, NH₃, H₂O, NO, NO₂: total magnetic moment before and after gas molecule M/M′ (μ_B), adsorption energy E_a (meV), charge transfer from WSe₂ to gas molecules ΔQ (e), the equilibrium height between the adsorbed molecule and the top-surface WSe₂ h (Å)

Molecule	M/M′ (μB)	Site	Ea (meV)	ΔQ (e)	h (Å)
O2	2/2	H	−8.6	0.0154	3.45
		B	−6.9	0.0166	3.49
		Tw	−8.7	0.0182	3.21
		TSe	−6.6	0.0136	3.69
CO	0/0	H	−7.2	0.0092	3.68
		B	−9.2	0.0089	3.76
		Tw	−8.8	0.0073	3.78
		TSe	−7.1	0.0085	3.68
NH3	0/0	H	−40	−0.0142	3.10
		B	−19	−0.0187	3.24
		Tw	−42	−0.0172	3.11
		TSe	−24	−0.0037	3.60
H2O	0/0	H	−44	0.0179	2.75
		B	−39	0.0177	2.87
		Tw	−45	0.0186	2.78
		TSe	−25	0.0130	3.13
NO	1/1	H	−19	0.0162	3.30
		B	−25	0.0346	2.95
		Tw	−22	0.0244	3.21
		TSe	−21	0.0421	3.24
NO2	1/1	H	−60	0.1110	3.10
		B	−67	0.1165	3.04
		Tw	−57	0.0960	3.21
		TSe	−63	0.1213	3.08

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Our results show that all gas molecules are physically adsorbed on WSe₂ monolayer. It can be seen that for H₂O, O₂ and NH₃, the most stable position is T_w site, while CO, NO and NO₂ prefer to be adsorbed on the B site. Moreover, we can see that the most stable adsorption configurations of H₂O and NH₃ are parallel to the WSe₂ surface, while O₂ is perpendicular to the WSe₂ monolayer. The H atoms of H₂O point towards the WSe₂ monolayer and the H atoms of NH₃ point away from the WSe₂ surface as shown in Fig. 2(b), (c) and (e). In Fig. 2(a), (d) and (f) the homologous structures of NO and CO are sloping to the surface with N and C atoms close to the substrate. O atoms of NO₂ are parallel to the WSe₂ surface. Furthermore, the optimized bond lengths of the isolate molecules CO, O₂, H₂O, NH₃, NO and NO₂ are found to be 1.14 Å, 1.23 Å, 0.97 Å, 1.02 Å, 1.16 Å and 1.21 Å, respectively. These isolate molecules' bond lengths are in good accordance with previous theoretical and experimental results.^32,33 In addition, the equilibrium height for O₂, CO, NH₃, H₂O, NO and NO₂ is 3.21 Å, 3.76 Å, 3.11 Å, 2.78 Å, 2.95 Å, 3.04 Å with corresponding adsorption energies −8.7 meV, −9.2 meV, −42 meV, −45 meV, −25 meV, −67 meV, respectively. Combined with Table 1, we find that NH₃ and H₂O are not easy to be adsorbed on WSe₂ monolayer. The adsorption energy of NO and NO₂ systems are much higher than that of O₂ system. We also see stronger charge transfer among the NO and NO₂ systems compared with the O₂ system as shown in Table 1. In view of these results, it is anticipated that WSe₂ will be rather sensitive when used as NO and NO₂ sensor device.

Fig. 2 The most stable adsorption configurations of WSe₂ monolayer adsorbed with (a) CO, (b) NH₃, (c) O₂, (d) NO₂, (e) H₂O, (f) NO from the top view and side view. The blue and orange balls represent W and Se atoms. H, C, O and N atoms are represented by white, gray, red and iceblue balls, respectively. The equilibrium height is labeled around gas molecules.

To understand the electronic properties of various adsorbates on WSe₂ monolayer, we calculated the total density of states (TDOS) of adsorbate systems and the relevant local DOS (LDOS) projected on the adsorbed gas molecules, partial density of states (PDOS) of W(Se) atom and band structures of the WSe₂ monolayer under different configurations as shown in Fig. 3 and 4. In Fig. 4(a) and (e), the band structures of pristine WSe₂ has a direct band gap of 1.51 eV at the high symmetry point K, which is in consistent with the previous result.³⁴ Furthermore, when CO, NH₃, and H₂O are adsorbed, the band structures of WSe₂ monolayer are not distinctly changed, the band gap value remain around 1.51 eV as shown in Fig. 4(b)–(d). The spin-up and spin-down TDOS are symmetric, which indicates that these systems exhibit the non-magnetic ground states. It's also in good agreement with the calculated total magnetic moment of 0 μB in the CO, NH₃ and H₂O adsorbed WSe₂ systems (see Table 1). In addition, for NO, O₂ and NO₂, they lead to spin polarized ground states in the WSe₂ monolayer. From the calculation, for NO and NO₂ adsorbates, the total magnetic moment are 1.00 μ_B, while adsorption of O₂ leads to a total magnetic moment of 2.00 μ_B as shown in Table 1. More interestingly, these adsorbates induced impurity states in the gap from the TDOS and band structure as shown in Fig. 3(d)–(f) and 4(f)–(h). It shows that three impurity states appeared after the adsorption of NO: one occupied down-spin state 0.006 eV below the Fermi level, one unoccupied down-spin state 0.26 eV above the Fermi level, and one unoccupied up-spin state close to the bottom of the conduction band with an energy separation of 0.57 eV. NO₂ introduces an unoccupied down-spin state 0.10 eV above the Fermi level as shown in Fig. 4(g). In addition, O₂ introduces a down-spin state 0.32 eV above the Fermi level in the band as shown in Fig. 4(f). Besides, from the LDOS of the adsorbed gas molecules in Fig. 3, we know that the contribution of impurity states is mainly from the adsorbed gas molecules. The phenomenon of Fermi-level pinning in the case of gas molecules CO, H₂O, NH₃, O₂, NO and NO₂ adsorbed on 2D TMDs has not been systematically evaluated before. In particular, though adsorption of O₂, NO, and NO₂ induced impurity states in the bandgap of WSe₂ monolayer, there have almost no influence on the bandgap energy of pristine WSe₂ monolayer due to the weak physisorption interaction. Moreover, these impurity states are all localized around the adsorbed gas molecules because there is no obvious hybridization between these impurity states and WSe₂ substrate.

Fig. 3 The spin-polarized TDOS of WSe₂ monolayer after adsorption of (a) CO, (b) NH₃, (c) H₂O, (d) O₂, (e) NO₂ and (f) NO, respectively. The corresponding LDOS projected on the adsorbed gas molecules as well as the partial DOS of their nearest W and Se atoms are also displayed. The Fermi level is indicated by the green dotted line.

Fig. 4 Band structures of (a) pristine, (b) CO, (c) NH₃ and (d) H₂O on WSe₂ monolayer, respectively. The spin-polarized band structures of (e) pristine, (f) O₂, (g) NO₂, and (h) NO on WSe₂ with up-spin bands and down-spin bands are indicated in black and red, respectively. The dashed green line donates the Fermi level. And the Fermi energy is set as reference zero energy.

In order to explore the adsorption-induced charge transfer on the system, Bader charge analysis method is used to analyze the charge transfer.³⁵ The calculated results are listed in Table 1. Numerical results show that NH₃ behaves as a charge donor, providing 0.0172 e to the WSe₂ substrate, while O₂, CO, NO, H₂O, NO₂ behave as charge acceptors, which obtain 0.0182 e, 0.0089 e, 0.0346 e, 0.0186 e, 0.1165 e from the substrate, respectively. Our charge analysis reveals that the adsorbed NO₂ accepts 0.1165 e from WSe₂ monolayer, indicating that NO₂ works as an acceptor with obvious charge transfer happening. It is also reflected by its high adsorption energy as described in Table 1. In view of this conclusion, the charge transfer is expected to induce changes on the conductivity of the system.

To further gain insight into the charge transfer between gas adsorbates and WSe₂ monolayer, we calculated the electron charge density difference for these considered adsorption configurations. The charge density difference is calculated using the following equation:

Δρ = ρ_gas+WSe2 − (ρ_WSe2 + ρ_gas) (2)

where ρ_gas+WSe2, ρ_WSe2, ρ_gas are the charge density of the molecule-adsorbed WSe₂ monolayer, pristine WSe₂ monolayer, and isolated gas molecule, respectively. The results are shown in Fig. 5, where the blue distribution represents the charge depletion and the yellow one represents the charge accumulation. It is indicated that there is a charge depletion on WSe₂ surface for CO, H₂O, O₂, NO and NO₂ adsorbates, whereas NH₃ presents a charge accumulation area near the WSe₂ monolayer. Meanwhile it is demonstrated that there is large charge transfer occurs between NO₂ and WSe₂ monolayer, which also gives larger adsorption energy as mentioned above in Table 1. The charge depletion in WSe₂ monolayer reduces the number of electrons in the monolayer, which results in the lower charge carriers and more intensive resistance. These results show that the interactions between WSe₂ and adsorption gas molecules can lead to remarkable conductivity change and provide a charming basis for application in gas sensing.

Fig. 5 The isosurface plots of electronic charge density defference for (a) CO, (b) H₂O, (c) NH₃, (d) NO, (e) O₂, and (f) NO₂ on WSe₂ monolayer. The isosurface value for all of the cases is 0.002 e Å⁻³ (top and side view are provided for each adsorbed molecules). Charge accumulation and depletion are represented in yellow and blue plots, respectively.

4. Conclusions

In conclusion, on the basis of first-principle calculations, we systematically investigated common gas molecules, such as CO, H₂O, NH₃, O₂, NO and NO₂ on WSe₂ monolayer. Various adsorption sites and molecule orientations are considered to judge the most favorable configurations. We also analyzed the electronic properties of WSe₂ monolayer upon adsorption of gas molecules. Numerical results show that for H₂O, O₂ and NH₃, the most stable position is on the top site above the W atom of sublattice WSe₂ monolayer, while CO, NO and NO₂ prefer to be adsorbed on the bridge site above the center of W–Se bond. In addition, Bader analysis indicates that all adsorbed gas molecules behave as either charge donors or charge acceptors. Moreover, when NH₃, H₂O or CO is adsorbed, the electronic structures of WSe₂ monolayer do not change significantly. However, O₂, NO and NO₂ molecules can introduce the lowest unoccupied molecular orbitals around the Fermi level. Furthermore, the marked charge transfers from 2D WSe₂ nanosheets to NO, NO₂ and O₂ molecules suggest that the physical adsorption is an effective approach for modulating the carries density on WSe₂ monolayer. All these results show that 2D WSe₂ is a promising candidate for molecular sensor application.

Acknowledgements

This research was supported by the National Natural Science Foundation of China under Grant No. U1304518, 61674053 and 11204121. The calculations are also supported by The High Performance Computing Center of Henan Normal University.

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