Sensing Characteristics of Phosphorene Monolayers toward PH

Article pubs.acs.org/JPCC

Sensing Characteristics of Phosphorene Monolayers toward PH3 and AsH3 Gases upon the Introduction of Vacancy Defects Manasi S. Mahabal,† Mrinalini D. Deshpande,*,† Tanveer Hussain,*,‡ and Rajeev Ahuja§,∥ †

Department of Physics, H.P.T. Arts and R.Y.K. Science College, Nasik, Maharashtra 422 005, India Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, Australia § Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden ∥ Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden

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‡

ABSTRACT: Motivated by the exceptional interest of researchers in two-dimensional nanostructures, the current study deals with the structural, electronic, optical, and gassensing properties of recently synthesized monolayer phosphorene. Van der Waals induced first-principles calculations were performed to study the binding mechanism of pristine and defected phosphorene towards the toxic gases PH3 and AsH3. The preferential sites and orientations of these molecules on the phosphorene sheet were determined, and a detailed analysis of the adsorption energetics was performed. Both of the gas molecules interact weakly with the phosphorene sheet, with AsH3 the binding was slightly stronger than PH3. The creation of defects such as monovacancies and divacancies in the phosphorene sheet was found to significantly enhance the adsorption mechanism. The adsorption energies of both PH3 and AsH3 improved by factors of four and three, respectively, as compared to their values on pristine phosphorene. The adsorption mechanism was further investigated by plotting the band structure and density of states. We also studied the optical properties and the static dielectric matrices of these nanostructures using density functional perturbation theory. Our findings showed that defected phosphorene with vacancies can be considered as an efficient sensor for toxic gases. conductors, such as MoS2 (about 200 cm2/V·s). It is expected that the electrical resistivity of phosphorene will also be influenced by the adsorption of gas molecules in a similar way. Given the distinctive electronic properties of phosphorene, exploration of the sensing properties of phosphorene with various gas molecules is highly desirable. Recent first-principles calculations14,15 showed that, upon the adsorption of gas molecules CO, CO2, NH3, NO, and NO2 on phosphorene, the binding of nitrogen-based gas molecules is the strongest among the gas molecules considered. As mentioned earlier, plenty of investigations on the sensing properties of graphene or graphene nanoribbons (GNRs) have been performed, however, little attention has been paid to similar work on phosphorene. The search for novel materials for hydrogen storage, adsorption of natural gases, and sensing of hazardous gases is an issue of great importance for the scientific community. Motivated by this situation, we performed first-principles investigations to reveal the sensing properties of phosphorene for toxic gases. Most of the properties and applications of materials are affected by the

1. INTRODUCTION Recent years have seen rapid progress in the synthesis, characterization, and applications of atomically thin twodimensional (2D) materials, such as graphene, transitionmetal dichalcogenides (TMDs), silicene, germanene, and phosphorene. Two-dimensional materials are also usually good candidates for gas sensors because of their large surfaceto-volume ratios and the associated charge transfer between gas molecules and the substrates. Good sensor properties have already been demonstrated for two-dimensional graphenebased materials in both theoretical1−6 and experimental1,7,8 investigations. These studies showed that the adsorption of gas molecules changes the resistivity of the nanosheet by donating or accepting charge; this property can be used to make highly sensitive sensors. The lack of a band gap in graphene9 and the relatively low carrier mobility in MoS210 have motivated continuing work in a search for more 2D materials that exhibit properties that can lead to specific improved performance. A recent article reported the successful fabrication of few-layer black phosphorous or phosphorene.11 Phosphorene, however, has significant advantages over semimetallic graphene because it exhibits a finite and direct band gap within an appealing energy range12,13 and its measured free-carrier mobility (about 1000 cm2/V·s) is better than those of other typical 2D semi© 2016 American Chemical Society

Received: July 6, 2016 Revised: August 17, 2016 Published: August 17, 2016 20428

DOI: 10.1021/acs.jpcc.6b06791 J. Phys. Chem. C 2016, 120, 20428−20436

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Figure 1. (Left) Top and side views of an optimized phosphorene sheet. (Right) Band structure, total density of states (TDOS), and projected density of states (PDOS) of a phosphorene sheet. The Fermi level is aligned to zero and is indicated by a horizontal dotted line.

presence of defects, especially point defects.16 It is known that defects in 2D materials severely affect their structural and electronic properties17,18 and, thus, alter their applications; however, little attention has been focused on the sensing properties of phosphorene with defects. In short, in this work, we considered the interaction of pristine and defected phosphorene monolayer with the toxic gas molecules PH3 and AsH3. In particular, we explored the most stable adsorption configurations and the corresponding changes in the electronic properties of phosphorene. Such calculations will be useful for developing efficient nanosensors based on phosphorene in the near future.

the adsorbed atom and phosphorene was estimated from a gridbased Bader charge analysis.25 The cohesive energy per atom (Eb) was calculated as E b(phosphorene sheet) = [−E(phosphorene sheet) + nE(P) + nE(gas molecule)]/[n(P) + n(gas molecule)]

(1)

where E is the total energy of the system and n is the total number of gas molecules, either 0 or 1. The optimized configurations were used to calculate the optical properties. The frequency-dependent dielectric matrix was calculated using VASP 5.2 optical programs. For the electronic structure calculations, the Brillouin zone for the 3 × 3 supercell was sampled by 11 × 11 × 1 k-points grids. The frequency-dependent dielectric matrix calculations were carried out by increasing the number of states by a factor of 3. The imaginary and real dielectric values of the materials were plotted for all photon energies.

2. COMPUTATIONAL DETAILS First-principles calculations were performed within the density functional theory (DFT) framework, as implemented in the Vienna ab Initio Simulation Package (VASP).19 The wave functions were expressed in a plane-wave basis set with an energy cutoff of 500 eV, and Brillouin zone integration was performed using a 3 × 3 × 1 k-point mesh within the Monkhorst−Pack scheme. The ionic potentials were represented by projector augmented wave (PAW) potentials.20−22 The exchange correlation energy was represented by a generalized gradient approximation (GGA) functional proposed by Perdew, Burke, and Ernzerhof (PBE).23 The optimized structures were obtained by relaxing all atomic positions using the quasi-Newton algorithm until all forces were smaller than 0.01 eV/Å. For the gas-molecule−phosphorene structure, we used an orthorhombic supercell (α = β = γ = 90°) with periodic boundary conditions. A large (3 × 3) supercell of phosphorene was used (36 P atoms), giving a nearest adatom distance of ∼12 Å. During the periodic calculations, a vacuum space of 15 Å was inserted along the z direction to minimize the interaction between the periodic images of the phosphorene sheet. In these calculations, we considered van der Waals (vdW) corrections for calculating the energies of the system. The vdW semiempirical corrections were introduced based on Grimme’s method24 as implemented in the VASP code to calculate weak, long-range bonding energies between the gas molecules and the phosphorene sheets. The amount of charge transfer between

3. RESULTS AND DISCUSSION Phosphorene distinguishes itself from other 2D layered materials by its unique structural characteristics: It has a puckered structure along the armchair direction (Figure 1), but it appears as a bilayer configuration along the zigzag direction. The calculated band structure and density of states (DOS), along with the optimized geometry of pristine phosphorene (top and side views), are shown in Figure 1. We found vdWcorrected lattice parameters of a = 3.46 Å, b = 4.94 Å, and c = 10.58 Å. The puckering height was calculated as 2.07 Å. The bond angle along the zigzag direction, known as the hinge angle, was found to be 95.95°, and the adjacent P−P bond length was found to be 2.22 Å; these values are smaller than the corresponding dihedral angle along the zigzag direction (104.12°) and the connecting bond length (2.26 Å). The stability was measured in terms of the cohesive energy, which was calculated to be 3.63 eV/atom for the phosphorene sheet. The relaxed bond length and cohesive energy are consistent with earlier theoretical calculations.26−31 The electronic properties of the system were investigated through the band structure, 20429

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calculated from the total energies of bare and gas-moleculeadsorbed phosphorene using the relation

total density of states (TDOS), and projected density of states (PDOS). As shown in Figure 1, pristine phosphorene is a direct-band gap semiconductor, with its valence-band maximum (VBM) and conduction-band minimum (CBM) both located at the Γ point, which is very beneficial for electron transport. The band gap of the pristine sheet was found to be about 0.86 eV, which agrees well with the previously reported values.14,26 The calculated band gap value is close to the experimentally measured band gap of 1.0−1.5 eV for single-layer phosphorene. From the TDOS and PDOS, it can be seen that the low-lying region of the valence-band maximum is dominated by pz states, whereas the conduction-band region has significant contributions from the px and py states. The contribution of the s orbitals of the P atoms is relatively small. Every semiconductor manufacturer must develop a strategy for the safe handling of gases, because the use of toxic, hazardous, and corrosive gases entails the potential for sudden release into the work environment that could lead to serious injury or property damage. The first step in safe gas handling is to identify every gas used and then identify their effects. It is also necessary to have continuous control of these highly poisonous gases. Phosphine (PH3) and arsine (AsH3) are widely used in the electronics industry. PH3 is a severe pulmonary irritant at high concentrations. At 250 ppm, AsH3 is variably said to be lethal to humans instantly or upon exposure for 30 min. Hence, owing to their toxicity, it is important to sense even traces of these toxic gases. Given the distinctive electronic properties of phosphorene, it is highly desirable to explore the sensing properties of phosphorene with respect to these toxic gas molecules.32,33 To understand the responses of PH3 and AsH3, each gas molecule was allowed to approach freely toward the sheet. To explore the most stable adsorbed configuration, we considered different sites on the phosphorene monolayer, namely, on the P−P bond, in the hollow site, and on the P atom, as well as different orientations of the gas molecule. Among all possible considered configurations, the most stable configurations for PH3- and AsH3-adsorbed phosphorene sheets along with the optimized distances between the gas molecules and sheet are represented in Figure 2. The adsorption energy (Eads) was

Eads = E(phosphorene sheet) + E(gas molecule) − E(phosphorene sheet + gas molecule)

(2)

The calculated Eads values, band gaps from band structure calculations, optimized distances of the gas molecules from the sheet, and work functions for the most stable configurations are presented in Table 1. It is important to note that vdW Table 1. Adsorption Energies (Eads, eV), Distances between Gas and Sheets (d, Å), Band Gaps (Eg, eV), and Work Functions (ϕ, eV) of the Lowest-Energy Configurations of PH3 and AsH3 Gas Molecules on Pristine, MV, and DV Phosphorene Sheets Eads (eV) system

without vdW

with vdW

d (Å)

Eg (eV)

ϕ (eV)

phosphorene MV-phosphorene DV-phosphorene PH3−phosphorene AsH3−phosphorene PH3−MV-phosphorene PH3−DV-phosphorene AsH3−MV-phosphorene AsH3−DV-phosphorene

− − − −0.032 −0.033 −0.62 −0.27 −0.65 −0.29

− − − −0.15 −0.18 −0.70 −0.30 −0.73 −0.34

− − − 3.48 3.65 2.91 2.98 2.81 3.52

0.86 1.0 1.34 0.92 0.90 1.14 1.33 1.14 1.34

4.30 4.59 4.54 4.06 4.11 4.28 4.40 4.20 4.29

interactions dominate the binding between the gases and the phosphorene sheet and significantly enhance the stability of the system.34 The change in conductivity is directly related to the work function of the sheet. Thus, calculating the work function is of great importance when studying the sensing characteristics of phosphorene sheets. To understand the nature of the bonding in these systems, we also performed Bader charge and difference charge density analysis. To understand the electronic properties in detail, the total density of states and the siteprojected density of states were calculated for P and As atom. It was found that the PH3 molecule prefers to interact with the sheet by selecting the hollow site on a pristine phosphorene sheet (Figure 2). PH3 is a trigonal pyramidal molecule with C3v molecular symmetry. The length of the P−H bond is 1.42 Å, and the H−P−H bond angle is 93.5°. The adsorption energy of a PH3 molecule on a phosphorene sheet is −0.15 eV. The optimized distance of the P atom of PH3 from the sheet is ∼3.5 Å. The calculated P−H bond lengths of PH3 and P−P distance in the pristine sheet remain ∼1.43 and 2.22 (2.26) Å, respectively. Between PH3 and AsH3, the sheet shows more affinity toward AsH3, with an adsorption energy of −0.18 eV. In contrast to PH3, AsH3 was found to be sited between the P−P bond positions of the monolayer. The negative adsorption energy indicates that the adsorption is energetically favorable. The optimized distance from monolayer P to As of the AsH3 molecule was found to be 3.65 Å. The large distance between the sheet and the gas molecule reflects the weak interaction between the gas molecule and the sheet. It is evident from Figure 2 that the adsorption of PH3 and AsH3 molecules do not cause any structural distortion in the pristine phosphorene sheet. From the Bader analysis, it was found that the average charge on the P atom in the phosphorene sheet is in the range from −0.08 to 0.1 e. With the adsorption of a gas molecule, a

Figure 2. Stable configurations of a gas molecule of (a) PH3 and (b) AsH3 on a pristine phosphorene sheet. The dark blue spheres represent phosphorus atoms. The pink sphere represents the As atom, and the light blue spheres represent hydrogen atoms. 20430

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Figure 3. Difference charge density isosurfaces of pristine phosphorene sheet with (a) PH3 and (b) AsH3 gas molecule at the one-sixth value of the maximum level.

very small amount of charge transfer is observed from the gas molecule to the sheet. In the presence of PH3, the charges on the P atoms of the monolayer range from −0.04 to 0.41 e. In contrast, upon the adsorption of AsH3, the charges on the P atoms of the monolayer range from −0.03 to 0.01 e. In the difference charge density plots, one can see the charge density on the P atoms in the vicinity of the gas molecule (Figure 3). This small charge transfer does not change the electronic properties of the pristine sheet significantly. From the band structure calculations (not shown), a small increase in the band gap is observed as compared to that of the pristine sheet. In the presence of PH3 and AsH3, the band gap of the sheet increases from 0.86 eV for the pristine sheet to 0.92 and 0.90 eV, respectively. Although a larger size of phosphate sheets has been fabricated, in reality, the presence of defects in layered materials is almost inevitable during the manufacturing process. As such, it is of fundamental interest to study the interactions of gas molecules with defective phosphorene sheets.35−37 We now shift the focus to the gas molecules adsorbed on the defective phosphorene sheet. Specifically, we considered a monovacancy (MV) and a divacancy (DV) in the pristine sheet. Our calculations showed that, by introducing defects, even a singlepoint vacancy, in the phosphorene sheet, one can significantly enhance the sensitivity of the gas sensor. Here, we demonstrate that defective phospherene should be a highly sensitive sensor for monitoring PH3 and AsH3. We created vacancy defects by removing P atoms from the pristine sheet. In particular, we considered a monovacancy (MV) and a divacancy (DV). To understand the stability of defects in phosphorene, we define the formation energy as Ef = E Phospho − NpEp

(3)

where EPhospho represents the total energy of defective phosphorene, Ep is the energy per phosphorus atom in a perfect phosphorene sheet, and Np corresponds to the number of phosphorus atoms in the phosphorene sheet. We found that the formation energy of a monovacancy is 2.18 eV. The lowestenergy configuration is spin-polarized with a magnetic moment of 1 μB. The formation energy for a divacancy sheet is 1.77 eV. The lower formation energy is correlated with the large structural deformation. Such deformations bond every P atom to three nearby P atoms, forming new sp3 hybridization by structural deformation, and this should also be one reason that a divacancy is more energetically favorable than a single vacancy. For the pristine sheet, the P−P bond lengths are 2.22 and 2.26 Å. Near a monovacancy, the P−P bond lengths are 2.19 and 2.26 Å, respectively, whereas near a divacancy, the P− P bond distances are 2.21 and 2.26 Å, respectively. The spinpolarized total DOS are shown in Figure 4. It can be seen that the levels near the Fermi level are mainly contributed from the P atoms near the vacancy. The introduction of a vacancy leads

Figure 4. Spin-up (↑) and spin-down (↓) total density of states for pristine, MV-, and DV-phosphorene sheets, as well as PH3 and AsH3 gas molecules adsorbed on pristine, MV, and DV sheets. The labels indicate the contributions of a particular atom state to the total density of states. The Fermi level is aligned to zero.

to a shift of the majority spin below the Fermi level. The major contribution to the magnetic moment is now from the minority 20431

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Figure 5. Stable configurations of a gas molecule of (a,c) PH3 and (b,d) AsH3 on (a,b) MV- and (c,d) DV-phosphorene sheets. The dark blue spheres represent phosphorus atoms. The pink sphere represents the As atom, and the light blue spheres represent hydrogen atoms.

Figure 6. Difference charge density plots of (a,b) MV- and (c,d) DV-phosphorene sheets with (a,c) PH3 and (b,d) AsH3 gas molecule at the onesixth value of the maximum level.

monovacancy was found to be −0.70 eV as compared to the pristine sheet (−0.17 eV). The distance between the sheet and one of the H atoms of the PH3 gas molecule is 2.78 Å. In the presence of a monovacancy, the AsH3 adsorption energy is slightly higher (−0.73 eV) than that of PH3. The distance between the sheet and one of the H atoms of AsH3 is 2.89 Å. In the presence of a divacancy, the adsorption energies for PH3 and AsH3 decrease as compared to the corresponding values on the MV-phosphorene sheet, but they are still higher than the pristine sheet. The adsorption energies for PH3 and AsH3 were found to be −0.30 eV and −0.34 eV, respectively. The distance from a sheet with a DV was found to be 2.98 and 3.52 Å for PH3 and AsH3, respectively. To understand the variation in adsorption energy in the presence of defects, we plotted the isosurfaces of the difference charge densities of the systems. Panels (a) and (b) of Figure 6 show the isosurfaces of the difference charge densities for PH3 and AsH3 gas molecules, respectively, adsorbed on an MVphosphorene sheet. Because of the presence of the gas molecule and the occurrence of structural deformation, a polarized charge density is observed near the vacancy. The red region shows charge accumulation, whereas the green region represents charge depletion. It is seen that the MV-

spin. The shifting of the spectra at slightly lower energy increases the band gap of the MV sheet (1.0 eV) as compared to that of the pristine sheet (0.86 eV). In the case of a divacancy, the structural deformation leads to delocalization of valence-band states. As compared to the case for a monovacancy, further shifting of the spectra to a lower energy increases the band gap from 1.0 to 1.34 eV. From the DOS, it is noted that the spin-up and spin-down states are symmetric in the DV sheet, suggesting that the system is nonmagnetic. Further, the gas molecules such as PH3 and AsH3 were made to adsorb on the defect-induced phosphorene sheet specifically near a vacancy and on a vacancy. The optimized geometries are shown in Figure 5. The adsorption energies, band gaps, and distances between the gas molecule and the sheet for MV- and DV-phosphorene are presented in Table 1. A more negative Eads value of a gas molecule from the gas-molecule-adsorbed MV-phosphorene system indicates increased stability as compared to pristine phosphorene. Close inspection of the results reveals that both gas molecules interact with the phosphorene surface through the hydrogen atoms of the gas molecule. In general, PH3 gas found to be slightly less sensitive to the defect-induced sheet than the AsH3 gas molecule. The adsorption energy for the PH3 gas molecule in the presence of a 20432

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Table 2. Bader Charge Analysis of Lowest-Energy Configurations of a PH3 and AsH3 Gas Molecule on a Phosphorene Sheet without and with Vacancies system phosphorene MV-phosphorene DV-phosphorene PH3 AsH3 PH3−phosphorene PH3−MV-phosphorene PH3−DV-phosphorene AsH3−phosphorene AsH3−MV-phosphorene AsH3−DV-phosphorene

charge on sheet (e) from from from − − from from from from from from

charges on atoms of molecule (e)

−0.08 to 0.11 −0.009 to 0.01 −0.003 to 0.05

−0.04 to 0.41 −0.09 to −0.03 −0.4 to 0.5 −0.03 to 0.01 −0.02 to 0.07 −0.03 to 0.05

P, 1.10; H, −0.37 As, 0.64; H, −0.21 P, 1.17; H, from −0.37 to −0.41 P, 1.30; H, −0.4, −0.5 P, 1.31; H, −0.4 As, 0.66; H, −0.21 As, 0.86; H, −0.9 As, 0.92; H, −0.4

Figure 7. Real (ε1) parts of the dielectric function for pristine, MV-, and DV-phosphorene sheets, as well as the pristine, MV-, and DV-PH3 and pristine, MV-, and DV-AsH3 systems.

phosphorene sheet is considerably polarized upon the adsorption of gas molecules, and electrostatic interaction plays a role in the attractive interaction. The enhanced charge transfers within the sheet and from the gas molecule are also manifested in the Bader analysis in Table 2. The results indicate that the vacancies act as adsorption sites. The strong interaction between MV-phosphorene and the gas molecules substantially modifies the electronic structure of the defective phosphorene. From Figure 4, it can be seen that the total density of states is more delocalized than for bare MV-phosphorene. The states from the P atom of the PH3 molecule appear in the middle part of the spectra. The spin-up states from the P atoms of the phosphorene sheet move slightly away from the Fermi level, which results in an increase in the band gap as compared to that of the bare MV-phosphorene sheet. Upon the adsorption of a

PH3 gas molecule, the band gap of MV-phosphorene increases from 1.0 to 1.14 eV. For the AsH3-adsorbed MV-phosphorene sheet, the band gap also increases from 1.0 to 1.14 eV. We also found that the induced magnetic moment of the MV sheet (1 μB) remains unchanged after the adsorption of a gas molecule. The calculated total density of states (DOS) further provides an explanation for the magnetic behavior of these systems. DV-phosphorene has no dangling bonds, and because of the large hollow area at the DV, the gas molecules are less strongly bonded to the sheet than to MV-phosphorene, which leads to lower adsorption energies than for MV-phosphorene but still higher than for the pristine sheet. The longer bonds between the sheet and the gas molecule reflect the weak bonding of the gas molecule over DV-phosphorene. The DOS of the gas molecule adsorbed on DV-phosphorene is similar in nature to 20433

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Figure 8. Imaginary (ε2) parts of the dielectric function for pristine, MV-, and DV-phosphorene sheets, as well as the pristine, MV-, and DV-PH3 and pristine, MV-, and DV-AsH3 systems.

that of the gas molecule adsorbed on the pristine sheet. The difference charge density plots (Figure 6 - c,d) for DVphosphorene with the gas molecules are almost similar in nature to those of the pristine sheet with the gas molecules. As mentioned earlier, that the band gap of DV-phosphorene is 1.34 eV. In the presence of the PH3 gas molecule, the band gap is 1.33 eV, whereas in the presence of AsH3, it remains 1.34 eV. Because of the small adsorption energy and large separation height, the interaction between the gas molecules and the sheet surface can thus be characterized as physisorption. To understand the effects of adsorption of these gas molecule on phosphorene, we calculated the work function, Φ, of the bare and physisorbed sheets. The work function is the minimum amount of energy required to remove an electron from the Fermi level to infinity. The work function of the gas molecule adsorbed on the phosphorene sheet can be estimated as Φ = V (Φ) − Ef

systems, the PH3 systems have lower work functions than the AsH3 systems. This is similar to the trend observed in the adsorption energy. The adsorption energies are lower for the pristine and defective phosphorene sheets in the presence of PH3 gas molecules than in the presence of AsH3 gas molecules. Because the band gaps and work functions of the pristine and defective phosphorene sheets can effectively be tuned by the physisorption of gas molecules, some effects on the optical absorption of phosphorene are highly expected. Finally, we studied the optical properties of gas molecule−phosphorene systems in comparison with those of isolated pristine and defective phosphorene sheets. The real (ε1) and imaginary (ε2) parts along the three directions for isolated and PH3- and AsH3adsorbed sheets are shown in Figures 7 and 8, respectively, as a function of photon energy. The real part of the dielectric function ε1 is shown in Figure 7. The natures of the curves for the bare and gas-moleculeadsorbed systems are nearly the same. From these spectra, we calculated the static dielectric constant, ε1(0). Because of the anisotropy of phosphorene along the x, y, and z directions, the dielectric constants show large variations. Table 3 presents the dielectric constants for the pristine, MV, DV, and gas-moleculeadsorbed systems. For the pristine sheet, the static dielectric constant is 2.56, whereas the dielectric constant calculated by Srivastava et al.27 is 2.6. Here, it is notable that the dielectric constant was found to decreases from pristine (2.56) to MV (2.54) to DV (2.48) sheets, which is inversely proportional to the band gap. The dielectric constants for the PH3-adsorbed

(4)

where Φ, V(Φ), and Ef are the work function, electrostatic potential at the vacuum level, and Fermi energy of the phosphorene sheet, respectively. The work function of the monolayer is 4.30 eV. It increases in the presence of defects. For MV and DV sheets, it is 4.59 and 4.54 eV, respectively. The calculated work functions of the pristine and defective phosphorene sheets decrease when the gas molecules are brought within their vicinity, which explains the affinities of the sheets toward both gas molecules. Between the PH3 and AsH3 20434

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and work functions of both pristine and defected systems can be effectively tuned by the physisorption of gas molecules, and the effects can be observed in the optical properties. Finally, the optical properties of gas-molecule-adsorbed systems were compared with those of the bare systems. As a result, the pristine and defective phosphorene sheets exhibited wide light absorption in three polarized directions. The adsorption of gas molecules on the surface of the sheets decreases the absorption intensity in each direction. The static dielectric constant [ε1(0)] and band gap were found to be inversely proportional to each other for all of the systems.

Table 3. Dielectric Constants along the x, y, and z Axes from the Real Part dielectric constant system

x

y

z

phosphorene PH3−phosphorene AsH3−phosphorene MV-phosphorene PH3−MV-phosphorene AsH3−MV-phosphorene DV-phosphorene PH3−DV-phosphorene AsH3−DV-phosphorene

5.39 5.36 5.42 9.26 10.92 10.88 4.65 4.70 4.74

4.19 4.24 4.25 12.14 8.00 8.03 4.28 4.33 4.35

2.56 2.64 2.65 2.54 2.60 2.63 2.48 2.53 2.55

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +91-253-2316779. *E-mail: [email protected]. Phone: +61-451635750.

systems are slightly lower than those for the AsH3-adsorbed systems. As noted in Table 1, for the PH3-adsorbed systems, the band gaps are slightly higher than for the AsH3- adsorbed systems. The imaginary parts of the dielectric functions (ε2) along the three directions for the bare and gas-molecule-adsorbed phosphorene systems are shown in Figure 8. As mentioned earlier, because of the anisotropy of phosphorene along the three directions, the imaginary parts of the dielectric function in the x, y, and z directions show different characteristics. Along the y and z directions, the absorption spectra show a localized nature as compared to the spectra along the x direction. In the spectra, the presence of multiple peaks is evidently seen along the x direction in the lower energy range. As noted earlier, the removal of a P atom from the pristine sheet leads to asymmetry in the population of the majority and minority spins near the Fermi level. This spin polarization induces a magnetic moment of 1 μB. For the MV sheet, the presence of more than one dominant peak in the lower energy range reflects the asymmetry in the majority and minority spins. As shown in Figure 8, the imaginary parts (ε2) for the gasadsorbed systems have little influence on the optical spectra of the bare systems. For all of the systems, a wide absorption-band region is observed from 1 to ∼9 eV for all three directions. As compared to the MV and DV sheets as well as the gas-adsorbed MV and DV sheets, the pristine sheet shows a higher absorption intensity. After the introduction of the vacancies and the adsorption of the gas molecule on the phosphorene sheet, a small shift in the absorption edge is observed to a higher energy range compared with that of the bare system, which is consistent with the increase in the band gap of the system. These two gas molecules turn out to be effective in tuning the band gaps and dielectric properties of pristine and defective phosphorene sheets.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.D.D. and M.S.M. gratefully acknowledge financial assistance from the Department of Science and Technology (DST), Government of India, and the University Grants Commission (UGC), New Delhi, India. T.H. thanks the University of Queensland for support of this project through the UQ Postdoctoral Fellowship Scheme. R.A. acknowledges the Carl Tryggers Stiftelse for Vetenskaplig Forskning (CTS), Swedish Research Council (VR), and StandUP for financial support. M.D.D. and M.S.M. acknowledge the Center for Development of Advance Computing (CDAC), Pune and Bangalore, India; SNIC; and UPPMAX for providing computing time.

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4. CONCLUSIONS The present study invoved a comprehensive analysis of the structural, electronic, optical, and gas-sensing properties of pristine and defected phosphorene monolayers upon exposure to the toxic gases PH3 and AsH3. The results of DFT-based calculations were analyzed in terms of energetics, band structure, density of states, and absorption spectra. The sensitivities of the pristine and defective phosphorene surfaces were tested for PH3 and AsH3 gases by calculating the corresponding adsorption energies. The monovacancy and divacancy defects were found to improve the adsorption mechanism. The sheet with a monovacancy showed strong interactions with both types of gas molecules. The band gaps 20435

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Article

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DOI: 10.1021/acs.jpcc.6b06791 J. Phys. Chem. C 2016, 120, 20428−20436