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Strain engineering of selective chemical adsorption on monolayer black phosphorous
Applied Surface Science 503 (2020) 144033
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Strain engineering of selective chemical adsorption on monolayer black phosphorous
T
⁎
Hong-ping Zhanga, , Liangzhi Koub, Yan Jiaoc, Aijun Dub, Youhong Tangd, Yuxiang Nie a
State Key Laboratory of Environmental Friendly Energy Materials, Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Sichuan 621010, China b School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Queensland 4001, Australia c School of Chemical Engineering, The University of Adelaide, South Australia 5005, Australia d Centre for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia 5042, Australia e School of Physical Science and Technology, Southwest Jiaotong University, 610031 Chengdu, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NO2 selective sensing DFT calculation Phosphorene Strain engineering
Controllable sensitivity of the surface chemical adsorption to toxic gases is important for the next generation gas sensors. Two dimensional (2D) layered nanomaterials are the excellent candidates to sensitively detect the chemical species by tuning the properties of the materials. Recently, the single and multi-layer black phosphorous (phosphorene) were discovered to exhibit better performances than graphene in the applications of field effect transistors, PN junctions, and photodetectors, and they are also ultrasensitive to the chemical gases. In this study, by using first principle calculations, we report that the adsorption of NO2 on strained monolayer phosphorene can be noticeably enhanced, while the electronic properties of monolayer phosphorene can be apparently adjusted. The monolayer phosphorene exhibits the superior selective adsorption for NO2 over NO, NH3, CO and CO2. The compressive strains were demonstrated to be able to effectively adjust the adsorptions of NO2. Our findings provide critical information for the novel design of phosphorene-based highly sensitive nanoscale sensors and electromechanical devices.
1. Introduction The investigation of two dimensional (2D) materials has come into a rapid development period since the discovery of graphene in 2004 [1,2]. The significant difference between 2D materials and their bulk counterparts due to the quantum confinement effects attracts lots of research attentions which have motivated the communities to explore the novel materials with excellent physical or chemical properties. During the past few years, graphene has been extensively studied in several areas including composites, energy materials, biomaterials and molecular devices [3–5]. However, owing to the electronic limitation (such as zero band gap) of graphene, several kinds of other 2D materials (such as transitional metal dichalcogenides (TMDs), silicene, germanane, h-BN, mono-chalcogenides (MX, M = Ge, Sn; X = S, Se) and MXenes) were proposed and studied to solve the intrinsic problems in graphene and seek better performance in microelectronic devices. For developing novel generation of nano-devices, the flexible band gaps and high free-carrier mobility are two key issues [1–5]. Recently, monolayer black phosphorous was found to exhibit lots of promising
⁎
properties which can clearly distinguish themselves from graphene based materials [6]. Structurally, in phosphorene, each phosphorus atom connects to three adjacent phosphorus atoms in a puckered honeycomb structure. Electronically, it has a significant advantage over graphene because it exhibits a direct band gap (around 2 eV) and higher measured free-carrier mobility (~1000 cm2/v·s) than MoS2. Phosphorene also exhibits interesting direction-dependent optical and electronic properties due to its anisotropic structure. These outstanding properties have already been studied and found important applications in thin-film solar cells chemical gas sensors and field effect transistor (FET) [7–10]. 2D materials were extensively explored in gas sensing and capture areas due to their vast specific surface area and the related charge transfer between gas molecules and materials substrates. The excellent gas sensing ability of 2D materials, including graphene, MoS2 and phosphorene, have been verified by both theoretical and experimental investigations. They exhibit high sensitivity to the specific gas molecules, and even single gas molecule can be captured and detected [11–18]. Besides the promising applications in chemical sensing and
Corresponding author. E-mail address: [email protected] (H.-p. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144033 Received 25 July 2019; Received in revised form 6 September 2019; Accepted 14 September 2019 Available online 01 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 503 (2020) 144033
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Fig. 1. Ball and stick model of gas-phosphorene interaction systems.
molecules, NO, NO2, CO, CO2 and NH3 on differently strained phosphorene from first-principles calculations. The preferential adsorption sites and the corresponding adsorption energy were determined. Our results show that the interaction strength is highly dependent on the amount of charge transfer between the gas molecules and the phosphorene substrate. The studies provide an important perspective for the controllable gas sensing and the design of flexibility in 2D materials.
Table 1 Calculated adsorption distance d between gas molecules (including NO2, NO, NH3, CO2 and CO) and the black phosphorene layer, adsorption energy (Eabs), and electron transfer (e) when the black phosphorene layer is in equilibrium condition.
d (Å) Eabs (eV) etrans
NO2
NO
NH3
CO2
CO
2.40 −0.41 0.22
2.61 −0.18 0.073
2.73 −0.43 0.076
3.39 −0.18 0.002
3.23 −0.18 0.016
2. Theoretical and computational details All the calculations were performed using the density functional theory (DFT) approach based on pseudopotentials with localized atomic-orbital basis sets as implemented in the program Dmol3 in Materials Studio (Accelrys, San Diego, CA). We used the Perdew - Burke -Ernzerhof (PBE) generalized gradient approximation (GGA) for the exchange correlation energy and DFT semi-core pseudopotentials for the core–valence interactions [34–37]. These functional always underestimates the band gap value, but it is a good choice considering the
gas detections, phosphorene is endowed with the significant response to strain in different directions due to its anisotropic structural properties [19–33]. Despite extensive investigation on the gas sensing behavior and the strain engineering of phosphorene, the effects of the strains of phosphorene in distinct directions on the gas molecules adsorption behavior has not been reported. In this study, we report the adsorption of several typical gas
Table 2 Calculated adsorption distance d between gas molecules (including NO2, NO, NH3, CO2 and CO) and the black phosphorene layer, adsorption energy (Eabs), and electron transfer (e) when the black phosphorene layer is under different strains (a or b stands for the different strain direction). Strain direction
Strain rates
NO2
NO
CO
CO2
NH3
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
a
0% 2% 4% 6% 8% 10% −2% −4% −6% −8% −10%
−0.404 −0.319 −0.293 −0.269 −0.256 −0.245 −0.483 −0.629 −0.815 −1.011 −1.186
3.01 2.47 2.53 2.60 2.62 2.64 2.26 2.12 1.94 1.72 1.50
−0.215 −0.186 −0.182 −0.177 −0.175 −0.173 −0.229 −0.253 −0.273 −0.288 −0.291
−0.178 −0.058 −0.076 −0.074 −0.078 −0.090 −0.190 −0.237 −0.297 −0.333 −0.107
2.10 2.88 2.47 2.54 2.44 2.16 2.04 1.96 1.78 1.70 2.22
0.073 0.086 0.087 0.085 0.086 0.087 0.077 0.074 0.062 0.050 0.074
−0.428 −0.178 −0.174 −0.169 −0.165 −0.162 −0.186 −0.192 −0.217 −0.199 −0.207
3.23 3.20 3.20 3.26 3.28 3.27 3.20 3.15 3.04 3.15 2.97
−0.004 −0.004 −0.004 −0.005 −0.005 −0.005 −0.003 −0.003 −0.002 −0.004 −0.004
−0.203 −0.200 −0.198 −0.196 −0.195 −0.193 −0.205 −0.210 −0.454 −0.222 −0.227
3.38 3.31 3.28 3.26 3.21 3.16 3.34 3.27 3.16 3.14 3.21
−0.002 −0.004 −0.004 −0.004 −0.005 −0.004 −0.002 −0.002 −0.001 −0.001 −0.003
−0.182 −0.386 −0.411 −0.402 −0.376 −0.370 −0.396 −0.441 −0.201 −0.451 −0.441
2.83 2.30 2.61 2.64 2.19 2.19 2.32 2.64 2.50 2.52 2.16
0.076 0.040 0.069 0.066 0.048 0.048 0.043 0.079 0.084 0.08 0.063
b
2% 4% 6% 8% 10% −2% −4% −6% −8% −10%
−0.349 −0.331 −0.318 −0.298 −0.285 −0.436 −0.462 −0.580 −0.793 −1.021
2.46 2.51 2.54 2.50 2.45 2.31 2.24 2.07 1.91 1.80
−0.196 −0.194 −0.191 −0.181 −0.167 −0.201 −0.204 −0.207 −0.216 −0.247
−0.142 −0.124 −0.046 −0.184 −0.263 −0.223 −0.140 −0.364 −0.497 −0.872
2.15 2.22 2.97 2.07 1.96 1.96 2.35 2.11 1.82 1.91
0.075 0.071 0.077 0.084 0.092 0.078 0.109 0.094 0.080 −0.035
−0.193 −0.177 −0.176 −0.175 −0.173 −0.184 −0.186 −0.189 −0.199 −0.263
3.05 3.20 3.20 3.21 3.22 3.21 3.21 3.18 2.98 2.88
−0.005 −0.005 −0.005 −0.006 −0.006 −0.004 −0.003 −0.002 −0.001 0.006
−0.200 −0.195 −0.195 −0.192 −0.189 −0.205 −0.208 −0.212 −0.223 −0.307
3.33 3.34 3.34 3.34 3.34 3.35 3.33 3.31 2.84 2.82
−0.002 −0.003 −0.004 −0.003 −0.004 −0.002 −0.002 −0.002 0.000 0.000
−0.428 −0.426 −0.397 −0.399 −0.434 −0.428 −0.393 −0.401 −0.434 −0.507
2.64 2.62 2.26 2.24 2.40 2.68 2.32 2.28 2.65 2.56
0.074 0.073 0.042 0.044 0.070 0.076 0.046 0.051 0.082 0.089
2
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Fig. 2. Electron transfer images of NO2-phosphorene systems with different applied strain (under 0% and 6% strain conditions) along different directions. (The isosurface is 0.002 e/bohr3, and yellow (blue-green) indicates electron accumulation (depletion).
Fig. 3. Calculated adsorption energy as a function of strain for NO2 adsorbed monolayer phosphorene.
3. Results and discussion
time-consuming. Spin polarization was included when calculating the adsorption of NO2 and NO on phosphorene since these molecules are paramagnetic, but not considered in the calculations for other gas molecules. The double numerical basis set with polarisation function (DNP), that is comparable to the 6-31G** basis set, was utilized during the calculations. Special point sampling integration over the Brillouin zone was employed using Monkhorst-Pack schemes with a 5 × 5 × 1 kpoint mesh [38]. A Fermi smearing of 0.005 Ha and a global orbital cutoff of 7 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (1) self-consistent field tolerance of 1.0 × 10−6 Ha/atom, (2) energy tolerance of 1.0 × 10−5 Ha/atom, (3) maximum force tolerance of 0.002 Ha/Å, and (4) maximum displacement tolerance of 0.005 Å. To isolate the phosphorene nanosheets, a vacuum layer of at least 20 Å along the vertical direction was used to avoid the interaction between the neighboring phosphorene layers. The lattice constant of the strain-free phosphorene nanosheet is a0 × b0 (18.5 Å × 16.5 Å), and the strained nanosheet takes the lattice constant of a = (1 + ε) × a0. The strain rate is calcua−a lated by r = a 0 × 100% along a direction. Where a0 stands for the 0 lattice constant of the strain-free phosphorenea long a direction, a stands for the lattice constant of the strained phosphorene, and ε stands for the strain rates.
3.1. Selective adsorption of gases on the phosphorene The formation energy was calculated by the following equation:
Ef =
Et − NEs N
(1)
where Ef refers to the formation energy, Et is the total energy of the 2D phosphorene sheet, Es is the energy per atom of the 2D sheet and N is the number of atoms. The formation energy of single black phosphorene is about 40 meV/atom, which is very similar with the other’s report [39]. The fully relaxed structural configurations of gas adsorption on phosphorene are presented in Fig. 1, where one can see that the gas molecules prefer to locate around the zigzag phosphorous atoms (Fig. 1a). For a comprehensive study of adsorption behavior (NO, NO2, CO, CO2 and NH3) on the phosphorene monolayer, we summarize the obtained results in Table 1, including the adsorption distance (see definition in Fig. 1a), adsorption strength (defined as Eads = ETotal − Ephosphorene − Egas , where Etot, Ephosphorene, and Egas are total energy of gas adsorbed phosphorene and energies of 3
Applied Surface Science 503 (2020) 144033
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Fig. 4. Calculated spin polarized (DOS) of NO2 + phosphorene under a strain of (a) 0%, (b) 2%, (c) 4%, (d) 6%, (e) 8%, and (f) 10% along b direction and (g) 0%, (h) −2%, (i) −4%, (j) −6%, (k) −8%, and (l) −10% along b direction. The vertical dashed line referes to the Fermi level, which is set to zero.
both along a and b directions. In order to further investigate the phenomenon, we take NO2 adsorption on strained phosphorene as a typical example for analyses.
phosphorene and gas molecules, respectively) and electrons transfer. It is interesting to notice that for all N-based molecules including NO, NO2 and NH3, the nitrogen atom points toward to the top zigzag phosphorous atoms of phosphorene (See Fig. 1b). More interestingly, Phosphorene exhibits the apparent selectivity for the different gas molecules. Table 1 shows that NO2 and NH3 own the lower adsorption energies (i.e., more negative, thus stronger binding) compared with NO, CO and CO2. The results are consistent with previous publications [11]. The equilibrate adsorption distance between N atom of NO2 (or NH3) and top P atoms of phosphorene is about 2.4 Å or 2.7 Å while it is 2.6 Å for NO. However, the adsorption energy between NO2 (or NH3) and phosphorene is significantly larger than that of NO-phosphorene. When checking the electron transfer, it can be found that NO2 exhibits the largest amounts of electrons transfer compared with other five gas molecules. According to these analyses, it can be concluded that phosphorene owns the apparent selectivity for the NO2.
3.3. Strain-dependent NO2 adsorption on phosphorene Our calculation indicates that the adsorption of NO2 on phosphorene is significantly sensitive to the strain conditions (including strain magnitudes and directions). In Fig. 2, the isosurface of electron density difference for the NO2 adsorption on phosphorene with and without strain along a or b direction were presented. It can be found that NO2 molecule can be stably adsorbed on the phosphorene surface, the electrons are transferred from phosphorene to the NO2 molecule. Fig. S1 shows the isosurface of electron density difference for the different gas-phosphorene systems. It indicates that NO and NH3 can be stably adsorbed on the phosphorene surface. CO and CO2 show relatively weak interactions with the phosphorene. When the 6% (tensile) strain was applied for phosphorene along a or b direction, the electrons transfer from phosphorene to NO2 tend to be gentle comparing to the pristine layer. The electrons transfer decreased for NO2 - phosphorene with 6% strain, and it is more obvious when the strain is along a direction than that along b direction. On the other hand, when the −6% (compression) strain is applied for phosphorene along a or b direction, it can be found that the electrons transfer from phosphorene to NO2 is be more significant comparing to the pristine layer. The electrons transfer is more obvious along a direction than along b direction (See Fig. 2). The above results could be further demonstrated by the adsorption energy as a function of strain as shown in Fig. 3. It can be found that the compressive strain on phosphorene can enhance the interactions between NO2 and phosphorene more significant than the tensile strain. Furthermore, as a result of anisotropy feature of phosphorene, the compression or tensile strain along different directions exhibit different effect on the NO2 adsorption ability of phosphorene.
3.2. Tunable sensitivity of adsorption by strain To control the adsorption behavior, the effect of the phosphorene’s strains on the adsorption of gas molecules were also investigated, separately along armchair and zigzag directions. The stretching and compressive strains were applied on the phosphorene separately to investigate the adsorption response of the gases. Table 2 shows the typical results of gases adsorbed onto the strained phosphorene with different strain magnitudes and direction. It can be seen that the strain direction and strain manner affect the adsorptions of gases to a great extent. The five different gases all show more obvious response to the compressive strain than the tensile strain. As the materials is anisotropic, b direction can be very effective, which is shown in Figs. 3 and S2. Although all five gas molecules are sensitive to the compressive strain along b direction, NO2 exhibits quite different behavior from other four gases. For instance, NO2 is sensitive to compressive strain 4
Applied Surface Science 503 (2020) 144033
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Fig. 5. Total DOS for the monolayer phosphorene with different strain rates and direction: (a) tensile along a direction; (b) compression along a direction; (c) tensile along b direction; (d) compression along b direction.
after the NO2 adsorption as shown in the DOS plot (Fig. 4), which indicates a p-typing doping effect where NO2 acts as an electrons acceptor. The apparent variation can be found for the spin polarized DOS for the NO2-phosphorenes with compressive strains along both a and b directions. The compressive strains lead the NO2-phosphorenes to the metallic states (See Fig. 5). The effect of the strain on the band gaps of phosphorene was also investigated, as shown in Fig. 6. It is interesting to notice that the strain can not only well control the adsorption behavior by changing the adsorption distance, electron transfer and adsorption strength, but also effectively modulate the electronic properties. It can be seen that the band gaps of phosphorene are significantly decreased with increasing compressive strains. The anisotropic behavior of electronic modulation is also revealed, since the band gaps are more sensitive to the strain along the b direction than that along a direction (See Figs. S3 and S4).
4. Conclusion
Fig. 6. Variation of band gap for phosphorene with different strain along different directions.
The highly sensitive strain dependence of adsorption of NO2 on phosphorene monolayer has been discovered by first principle calculations in this study. The interaction strength as measured by the adsorption energy of NO2 on the mono-layered phosphorene can be controlled and enhanced by applying strains. The electronic properties can also be significantly changed. As a result, the strained phosphorene can be used as the controllable and ultrasensitive markers to detect NO2
To further show the effect of gas adsorption with strain on phosphorene, we plotted the spin polarized density of states (DOS) of NO2 adsorbed on phosphorene with five typical tensile or compressive strained states (2%, 4%, 6%, 8% and 10%) along a and b directions in Figs. 4 and S2. It is found that the Femi level shifts to the valence band 5
Applied Surface Science 503 (2020) 144033
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molecule. Our results reveal that these novel findings are driven by the response of the electrons transfer and band gap’s variation to the applied strains. Strain-induced metallization of the mono-layered phosphorene can be intuitively found for the compression strain along b direction. Since the adsorption of NO2 on the strain-free black phosphorous sensors has been recently demonstrated by related experiments, we expect that our predicted strain engineering of selective chemical adsorption of NO2 on phosphorene would inspire immediate interest and further investigation for fundamental understanding and practical application.
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Full Length Article
Strain engineering of selective chemical adsorption on monolayer black phosphorous
T
⁎
Hong-ping Zhanga, , Liangzhi Koub, Yan Jiaoc, Aijun Dub, Youhong Tangd, Yuxiang Nie a
State Key Laboratory of Environmental Friendly Energy Materials, Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Sichuan 621010, China b School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Queensland 4001, Australia c School of Chemical Engineering, The University of Adelaide, South Australia 5005, Australia d Centre for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia 5042, Australia e School of Physical Science and Technology, Southwest Jiaotong University, 610031 Chengdu, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NO2 selective sensing DFT calculation Phosphorene Strain engineering
Controllable sensitivity of the surface chemical adsorption to toxic gases is important for the next generation gas sensors. Two dimensional (2D) layered nanomaterials are the excellent candidates to sensitively detect the chemical species by tuning the properties of the materials. Recently, the single and multi-layer black phosphorous (phosphorene) were discovered to exhibit better performances than graphene in the applications of field effect transistors, PN junctions, and photodetectors, and they are also ultrasensitive to the chemical gases. In this study, by using first principle calculations, we report that the adsorption of NO2 on strained monolayer phosphorene can be noticeably enhanced, while the electronic properties of monolayer phosphorene can be apparently adjusted. The monolayer phosphorene exhibits the superior selective adsorption for NO2 over NO, NH3, CO and CO2. The compressive strains were demonstrated to be able to effectively adjust the adsorptions of NO2. Our findings provide critical information for the novel design of phosphorene-based highly sensitive nanoscale sensors and electromechanical devices.
1. Introduction The investigation of two dimensional (2D) materials has come into a rapid development period since the discovery of graphene in 2004 [1,2]. The significant difference between 2D materials and their bulk counterparts due to the quantum confinement effects attracts lots of research attentions which have motivated the communities to explore the novel materials with excellent physical or chemical properties. During the past few years, graphene has been extensively studied in several areas including composites, energy materials, biomaterials and molecular devices [3–5]. However, owing to the electronic limitation (such as zero band gap) of graphene, several kinds of other 2D materials (such as transitional metal dichalcogenides (TMDs), silicene, germanane, h-BN, mono-chalcogenides (MX, M = Ge, Sn; X = S, Se) and MXenes) were proposed and studied to solve the intrinsic problems in graphene and seek better performance in microelectronic devices. For developing novel generation of nano-devices, the flexible band gaps and high free-carrier mobility are two key issues [1–5]. Recently, monolayer black phosphorous was found to exhibit lots of promising
⁎
properties which can clearly distinguish themselves from graphene based materials [6]. Structurally, in phosphorene, each phosphorus atom connects to three adjacent phosphorus atoms in a puckered honeycomb structure. Electronically, it has a significant advantage over graphene because it exhibits a direct band gap (around 2 eV) and higher measured free-carrier mobility (~1000 cm2/v·s) than MoS2. Phosphorene also exhibits interesting direction-dependent optical and electronic properties due to its anisotropic structure. These outstanding properties have already been studied and found important applications in thin-film solar cells chemical gas sensors and field effect transistor (FET) [7–10]. 2D materials were extensively explored in gas sensing and capture areas due to their vast specific surface area and the related charge transfer between gas molecules and materials substrates. The excellent gas sensing ability of 2D materials, including graphene, MoS2 and phosphorene, have been verified by both theoretical and experimental investigations. They exhibit high sensitivity to the specific gas molecules, and even single gas molecule can be captured and detected [11–18]. Besides the promising applications in chemical sensing and
Corresponding author. E-mail address: [email protected] (H.-p. Zhang).
https://doi.org/10.1016/j.apsusc.2019.144033 Received 25 July 2019; Received in revised form 6 September 2019; Accepted 14 September 2019 Available online 01 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Ball and stick model of gas-phosphorene interaction systems.
molecules, NO, NO2, CO, CO2 and NH3 on differently strained phosphorene from first-principles calculations. The preferential adsorption sites and the corresponding adsorption energy were determined. Our results show that the interaction strength is highly dependent on the amount of charge transfer between the gas molecules and the phosphorene substrate. The studies provide an important perspective for the controllable gas sensing and the design of flexibility in 2D materials.
Table 1 Calculated adsorption distance d between gas molecules (including NO2, NO, NH3, CO2 and CO) and the black phosphorene layer, adsorption energy (Eabs), and electron transfer (e) when the black phosphorene layer is in equilibrium condition.
d (Å) Eabs (eV) etrans
NO2
NO
NH3
CO2
CO
2.40 −0.41 0.22
2.61 −0.18 0.073
2.73 −0.43 0.076
3.39 −0.18 0.002
3.23 −0.18 0.016
2. Theoretical and computational details All the calculations were performed using the density functional theory (DFT) approach based on pseudopotentials with localized atomic-orbital basis sets as implemented in the program Dmol3 in Materials Studio (Accelrys, San Diego, CA). We used the Perdew - Burke -Ernzerhof (PBE) generalized gradient approximation (GGA) for the exchange correlation energy and DFT semi-core pseudopotentials for the core–valence interactions [34–37]. These functional always underestimates the band gap value, but it is a good choice considering the
gas detections, phosphorene is endowed with the significant response to strain in different directions due to its anisotropic structural properties [19–33]. Despite extensive investigation on the gas sensing behavior and the strain engineering of phosphorene, the effects of the strains of phosphorene in distinct directions on the gas molecules adsorption behavior has not been reported. In this study, we report the adsorption of several typical gas
Table 2 Calculated adsorption distance d between gas molecules (including NO2, NO, NH3, CO2 and CO) and the black phosphorene layer, adsorption energy (Eabs), and electron transfer (e) when the black phosphorene layer is under different strains (a or b stands for the different strain direction). Strain direction
Strain rates
NO2
NO
CO
CO2
NH3
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
Ead (eV)
d (Å)
etrans
a
0% 2% 4% 6% 8% 10% −2% −4% −6% −8% −10%
−0.404 −0.319 −0.293 −0.269 −0.256 −0.245 −0.483 −0.629 −0.815 −1.011 −1.186
3.01 2.47 2.53 2.60 2.62 2.64 2.26 2.12 1.94 1.72 1.50
−0.215 −0.186 −0.182 −0.177 −0.175 −0.173 −0.229 −0.253 −0.273 −0.288 −0.291
−0.178 −0.058 −0.076 −0.074 −0.078 −0.090 −0.190 −0.237 −0.297 −0.333 −0.107
2.10 2.88 2.47 2.54 2.44 2.16 2.04 1.96 1.78 1.70 2.22
0.073 0.086 0.087 0.085 0.086 0.087 0.077 0.074 0.062 0.050 0.074
−0.428 −0.178 −0.174 −0.169 −0.165 −0.162 −0.186 −0.192 −0.217 −0.199 −0.207
3.23 3.20 3.20 3.26 3.28 3.27 3.20 3.15 3.04 3.15 2.97
−0.004 −0.004 −0.004 −0.005 −0.005 −0.005 −0.003 −0.003 −0.002 −0.004 −0.004
−0.203 −0.200 −0.198 −0.196 −0.195 −0.193 −0.205 −0.210 −0.454 −0.222 −0.227
3.38 3.31 3.28 3.26 3.21 3.16 3.34 3.27 3.16 3.14 3.21
−0.002 −0.004 −0.004 −0.004 −0.005 −0.004 −0.002 −0.002 −0.001 −0.001 −0.003
−0.182 −0.386 −0.411 −0.402 −0.376 −0.370 −0.396 −0.441 −0.201 −0.451 −0.441
2.83 2.30 2.61 2.64 2.19 2.19 2.32 2.64 2.50 2.52 2.16
0.076 0.040 0.069 0.066 0.048 0.048 0.043 0.079 0.084 0.08 0.063
b
2% 4% 6% 8% 10% −2% −4% −6% −8% −10%
−0.349 −0.331 −0.318 −0.298 −0.285 −0.436 −0.462 −0.580 −0.793 −1.021
2.46 2.51 2.54 2.50 2.45 2.31 2.24 2.07 1.91 1.80
−0.196 −0.194 −0.191 −0.181 −0.167 −0.201 −0.204 −0.207 −0.216 −0.247
−0.142 −0.124 −0.046 −0.184 −0.263 −0.223 −0.140 −0.364 −0.497 −0.872
2.15 2.22 2.97 2.07 1.96 1.96 2.35 2.11 1.82 1.91
0.075 0.071 0.077 0.084 0.092 0.078 0.109 0.094 0.080 −0.035
−0.193 −0.177 −0.176 −0.175 −0.173 −0.184 −0.186 −0.189 −0.199 −0.263
3.05 3.20 3.20 3.21 3.22 3.21 3.21 3.18 2.98 2.88
−0.005 −0.005 −0.005 −0.006 −0.006 −0.004 −0.003 −0.002 −0.001 0.006
−0.200 −0.195 −0.195 −0.192 −0.189 −0.205 −0.208 −0.212 −0.223 −0.307
3.33 3.34 3.34 3.34 3.34 3.35 3.33 3.31 2.84 2.82
−0.002 −0.003 −0.004 −0.003 −0.004 −0.002 −0.002 −0.002 0.000 0.000
−0.428 −0.426 −0.397 −0.399 −0.434 −0.428 −0.393 −0.401 −0.434 −0.507
2.64 2.62 2.26 2.24 2.40 2.68 2.32 2.28 2.65 2.56
0.074 0.073 0.042 0.044 0.070 0.076 0.046 0.051 0.082 0.089
2
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Fig. 2. Electron transfer images of NO2-phosphorene systems with different applied strain (under 0% and 6% strain conditions) along different directions. (The isosurface is 0.002 e/bohr3, and yellow (blue-green) indicates electron accumulation (depletion).
Fig. 3. Calculated adsorption energy as a function of strain for NO2 adsorbed monolayer phosphorene.
3. Results and discussion
time-consuming. Spin polarization was included when calculating the adsorption of NO2 and NO on phosphorene since these molecules are paramagnetic, but not considered in the calculations for other gas molecules. The double numerical basis set with polarisation function (DNP), that is comparable to the 6-31G** basis set, was utilized during the calculations. Special point sampling integration over the Brillouin zone was employed using Monkhorst-Pack schemes with a 5 × 5 × 1 kpoint mesh [38]. A Fermi smearing of 0.005 Ha and a global orbital cutoff of 7 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (1) self-consistent field tolerance of 1.0 × 10−6 Ha/atom, (2) energy tolerance of 1.0 × 10−5 Ha/atom, (3) maximum force tolerance of 0.002 Ha/Å, and (4) maximum displacement tolerance of 0.005 Å. To isolate the phosphorene nanosheets, a vacuum layer of at least 20 Å along the vertical direction was used to avoid the interaction between the neighboring phosphorene layers. The lattice constant of the strain-free phosphorene nanosheet is a0 × b0 (18.5 Å × 16.5 Å), and the strained nanosheet takes the lattice constant of a = (1 + ε) × a0. The strain rate is calcua−a lated by r = a 0 × 100% along a direction. Where a0 stands for the 0 lattice constant of the strain-free phosphorenea long a direction, a stands for the lattice constant of the strained phosphorene, and ε stands for the strain rates.
3.1. Selective adsorption of gases on the phosphorene The formation energy was calculated by the following equation:
Ef =
Et − NEs N
(1)
where Ef refers to the formation energy, Et is the total energy of the 2D phosphorene sheet, Es is the energy per atom of the 2D sheet and N is the number of atoms. The formation energy of single black phosphorene is about 40 meV/atom, which is very similar with the other’s report [39]. The fully relaxed structural configurations of gas adsorption on phosphorene are presented in Fig. 1, where one can see that the gas molecules prefer to locate around the zigzag phosphorous atoms (Fig. 1a). For a comprehensive study of adsorption behavior (NO, NO2, CO, CO2 and NH3) on the phosphorene monolayer, we summarize the obtained results in Table 1, including the adsorption distance (see definition in Fig. 1a), adsorption strength (defined as Eads = ETotal − Ephosphorene − Egas , where Etot, Ephosphorene, and Egas are total energy of gas adsorbed phosphorene and energies of 3
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Fig. 4. Calculated spin polarized (DOS) of NO2 + phosphorene under a strain of (a) 0%, (b) 2%, (c) 4%, (d) 6%, (e) 8%, and (f) 10% along b direction and (g) 0%, (h) −2%, (i) −4%, (j) −6%, (k) −8%, and (l) −10% along b direction. The vertical dashed line referes to the Fermi level, which is set to zero.
both along a and b directions. In order to further investigate the phenomenon, we take NO2 adsorption on strained phosphorene as a typical example for analyses.
phosphorene and gas molecules, respectively) and electrons transfer. It is interesting to notice that for all N-based molecules including NO, NO2 and NH3, the nitrogen atom points toward to the top zigzag phosphorous atoms of phosphorene (See Fig. 1b). More interestingly, Phosphorene exhibits the apparent selectivity for the different gas molecules. Table 1 shows that NO2 and NH3 own the lower adsorption energies (i.e., more negative, thus stronger binding) compared with NO, CO and CO2. The results are consistent with previous publications [11]. The equilibrate adsorption distance between N atom of NO2 (or NH3) and top P atoms of phosphorene is about 2.4 Å or 2.7 Å while it is 2.6 Å for NO. However, the adsorption energy between NO2 (or NH3) and phosphorene is significantly larger than that of NO-phosphorene. When checking the electron transfer, it can be found that NO2 exhibits the largest amounts of electrons transfer compared with other five gas molecules. According to these analyses, it can be concluded that phosphorene owns the apparent selectivity for the NO2.
3.3. Strain-dependent NO2 adsorption on phosphorene Our calculation indicates that the adsorption of NO2 on phosphorene is significantly sensitive to the strain conditions (including strain magnitudes and directions). In Fig. 2, the isosurface of electron density difference for the NO2 adsorption on phosphorene with and without strain along a or b direction were presented. It can be found that NO2 molecule can be stably adsorbed on the phosphorene surface, the electrons are transferred from phosphorene to the NO2 molecule. Fig. S1 shows the isosurface of electron density difference for the different gas-phosphorene systems. It indicates that NO and NH3 can be stably adsorbed on the phosphorene surface. CO and CO2 show relatively weak interactions with the phosphorene. When the 6% (tensile) strain was applied for phosphorene along a or b direction, the electrons transfer from phosphorene to NO2 tend to be gentle comparing to the pristine layer. The electrons transfer decreased for NO2 - phosphorene with 6% strain, and it is more obvious when the strain is along a direction than that along b direction. On the other hand, when the −6% (compression) strain is applied for phosphorene along a or b direction, it can be found that the electrons transfer from phosphorene to NO2 is be more significant comparing to the pristine layer. The electrons transfer is more obvious along a direction than along b direction (See Fig. 2). The above results could be further demonstrated by the adsorption energy as a function of strain as shown in Fig. 3. It can be found that the compressive strain on phosphorene can enhance the interactions between NO2 and phosphorene more significant than the tensile strain. Furthermore, as a result of anisotropy feature of phosphorene, the compression or tensile strain along different directions exhibit different effect on the NO2 adsorption ability of phosphorene.
3.2. Tunable sensitivity of adsorption by strain To control the adsorption behavior, the effect of the phosphorene’s strains on the adsorption of gas molecules were also investigated, separately along armchair and zigzag directions. The stretching and compressive strains were applied on the phosphorene separately to investigate the adsorption response of the gases. Table 2 shows the typical results of gases adsorbed onto the strained phosphorene with different strain magnitudes and direction. It can be seen that the strain direction and strain manner affect the adsorptions of gases to a great extent. The five different gases all show more obvious response to the compressive strain than the tensile strain. As the materials is anisotropic, b direction can be very effective, which is shown in Figs. 3 and S2. Although all five gas molecules are sensitive to the compressive strain along b direction, NO2 exhibits quite different behavior from other four gases. For instance, NO2 is sensitive to compressive strain 4
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Fig. 5. Total DOS for the monolayer phosphorene with different strain rates and direction: (a) tensile along a direction; (b) compression along a direction; (c) tensile along b direction; (d) compression along b direction.
after the NO2 adsorption as shown in the DOS plot (Fig. 4), which indicates a p-typing doping effect where NO2 acts as an electrons acceptor. The apparent variation can be found for the spin polarized DOS for the NO2-phosphorenes with compressive strains along both a and b directions. The compressive strains lead the NO2-phosphorenes to the metallic states (See Fig. 5). The effect of the strain on the band gaps of phosphorene was also investigated, as shown in Fig. 6. It is interesting to notice that the strain can not only well control the adsorption behavior by changing the adsorption distance, electron transfer and adsorption strength, but also effectively modulate the electronic properties. It can be seen that the band gaps of phosphorene are significantly decreased with increasing compressive strains. The anisotropic behavior of electronic modulation is also revealed, since the band gaps are more sensitive to the strain along the b direction than that along a direction (See Figs. S3 and S4).
4. Conclusion
Fig. 6. Variation of band gap for phosphorene with different strain along different directions.
The highly sensitive strain dependence of adsorption of NO2 on phosphorene monolayer has been discovered by first principle calculations in this study. The interaction strength as measured by the adsorption energy of NO2 on the mono-layered phosphorene can be controlled and enhanced by applying strains. The electronic properties can also be significantly changed. As a result, the strained phosphorene can be used as the controllable and ultrasensitive markers to detect NO2
To further show the effect of gas adsorption with strain on phosphorene, we plotted the spin polarized density of states (DOS) of NO2 adsorbed on phosphorene with five typical tensile or compressive strained states (2%, 4%, 6%, 8% and 10%) along a and b directions in Figs. 4 and S2. It is found that the Femi level shifts to the valence band 5
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molecule. Our results reveal that these novel findings are driven by the response of the electrons transfer and band gap’s variation to the applied strains. Strain-induced metallization of the mono-layered phosphorene can be intuitively found for the compression strain along b direction. Since the adsorption of NO2 on the strain-free black phosphorous sensors has been recently demonstrated by related experiments, we expect that our predicted strain engineering of selective chemical adsorption of NO2 on phosphorene would inspire immediate interest and further investigation for fundamental understanding and practical application.
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