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Adsorption of gas molecules on the defective stanene nanosheets with single vacancy: A DFT study
Applied Surface Science 512 (2020) 145727
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Adsorption of gas molecules on the defective stanene nanosheets with single vacancy: A DFT study Yu Yanga, Hao Zhangb, Lihong Songc, Zhenling Liud,
T
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a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Australian School of Petroleum, the University of Adelaide, Adelaide, SA 5005, Australia c College of Energy, Chengdu University of Technology, Chengdu 610059, China d School of Materials Science and Engineering, Henan University of Technology, China b
ARTICLE INFO
ABSTRACT
Keywords: Density functional theory Charge density difference Defective stanene Gas sensor
Using the first-principles calculations, we have systematically examined the adsorption of gas molecules on the pristine and vacancy defective stanene monolayers. The adsorption of gas molecules on the defective monolayers is much stronger than that on the perfect ones, indicating the suitability of defective stanene nanosheets for adsorption processes. The large adsorption energies for SO2 and SO3 adsorbed complexes indicate the strong reactivity of stanene systems with adsorbed gas molecules, leading to the formation of multiple contacting points at the interface. The formation energies for the defective stanene systems were calculated. Our results indicated that the single vacancy defective stanene monolayers are thermodynamically stable, and can be used as effective gas sensors. The charge density difference calculations indicate that the electronic densities were largely accumulated between the atoms interacting with each other. This strong chemical interaction was also evidenced by the large overlaps of the density of states between the interacting atoms. The band structure calculations reveal the semiconductor features for defective stanene monolayers with adsorbed gas molecules. Our obtained results would be useful to search for promising gas sensor devices based on vacancy defective stanene monolayers.
1. Introduction In recent years, marvelous interests have been devoted to exploring the peculiar properties of two-dimensional (2D) materials owing to their astonishing mechanical [1], optical and electronic properties [2]. These particular properties demonstrate the potential application of 2D layered materials as promising materials for future nanoelectronic devices. Several researchers have done significant efforts to explore the properties of the other honeycomb-like structures constructed from group IV elements such as silicene, graphene and transition-metal dichalcogenides [3–8]. It has been experimentally proved that 2D nanomaterials have many outstanding applications towards optoelectronics and nanoelectronics such as sensing elements, field-effect transistors (FETs), and photovoltaic devices [9,10]. Because of the increasing importance of 2D layered materials in the fabrication of future nanoscale electronic devices, the study on the properties of these materials have triggered tremendous attentions in condensed matter physics and materials science. The 2D layered materials are mainly constructed from the groups IV and V elements of periodic table such as C, Si, Ge and Sn. The 2D arrangements of these elements lead to the formation of
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graphene, silicene, germanium and tin films with honeycomb structures [11,12]. In the past decades, the buckled 2D layered materials such as stanene, germanene, and silicene have attracted enormous interests [13–16]. For example, stanene, a 2D layered material composed of tin atoms has been experimentally synthesized using the epitaxial growth (MBE method) [17]. Recent advances on 2D materials have been focused on layered stanene sheets with buckled geometry because of the weak π–π bonding between the tin atoms [18]. It exhibits a semiconductor characteristics with a Dirac cone located at k point of the band structure plot [19]. Tuning the structural and electronic properties of 2D materials has become an important issue, due to the necessity of extensive applications in electronic and spintronic devices. Recently, Chen et al. [20] suggested that the structural and electronic properties of stanene can be tuned by adsorption of small gas molecules. We systematically examined the adsorption of gas molecules on the pure and defective stanene monolayers to further exploit the gas sensing capability of these systems. It is well known that creating defects is very important for tuning the electronic and magnetic properties of nanomaterials [21,22]. Xiong et al. [23] reported that stanene monolayers with single and double vacancy are energetically stable based on
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.apsusc.2020.145727 Received 19 December 2019; Received in revised form 18 January 2020; Accepted 9 February 2020 Available online 11 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Top and side views of the optimized structures of the pristine (a, c) and single vacancy defective (b, d) stanene monolayers. The buckling parameter (Δz = 0.88 Å) for pristine stanene is shown.
the formation energy results. They found that the formation energy of stanene with single vacancy is −7.4 eV, which indicates that it can be easily formed due to its higher stability. Moreover, Özçelik et al. [24] and Zhang et al. [25] suggested that single vacancy is energetically stable in graphene with negative formation energies. Abbasi et al. [26,27] also studied the effects of gas adsorption and elemental doping on the electronic properties of perfect stanene monolayers. In the recent years, sensors based on metal carbides and other 2D layered materials have been suggested for the capable removal of hazardous materials [28–31]. Sensing air pollutants have aroused tremendous interests because of the negative influences of toxic materials on the air quality, health standards of human beings and environmental safety. Therefore, it is of significant importance to suggest some suitable candidate materials, which detect and eliminate the harmful molecules from the environment. Gas sensors constructed from 2D materials have been attracting incredible attention over the past decades, which can be attributed to their wide surface/volume ratio. Thus, these materials with great surface/volume ratio are beneficial for manufacturing gas sensor devices [32]. Among the 2D layered materials, stanene based systems reveal favorable electronic properties due to their outstanding characteristics and effectiveness in environmental safety subjects both in residential areas and workplaces. Thus, (2D) nanomaterials are promising platforms for constructing nanoscale sensors because of their unique and excellent properties [33–38]. In this paper, we investigated the interaction of some small gas molecules with pure and defective stanene sheets to examine the
possible applicability of defective stanene sheets in sensor devices. Our aim is to provide promising layered stanene based material for future electronic devices like gas sensors. 2. Calculation details The presented electronic structure and relaxation calculations in this research were carried out with the Quantum Espresso (QE) package [39] based on DFT simulations [40,41]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchangecorrelation was used to describe the electronic exchange and correlation effects [42]. The energy cut-off in our calculations was set at 150 Ry. The Brillouin-zone was taken to be at the Γ-centered arrangement with 8 × 8 × 1 Monkhorst–Pack sampling of K points for integration in the structural optimization. We have considered a 5 × 5 × 1 supercell for our simulations of stanene monolayer. Since the higher value of energy cutoff slightly influences the obtained results, the plane wave cutoff energy was set to 150 Ry. The adsorption configurations were relaxed until the energy was converged to 1 × 0−5 eV and the force on each atom was less than 0.02 eV/ Å. We have chosen the K-path of G-KM-K-G for the band structure calculations. Our considered 5 × 5 × 1 supercell of perfect and single vacancy defective (imperfect) stanene monolayers contain 50 and 49 tin atoms, respectively. Moreover, a vacuum space of at least 20 Å is taken into account in the simulation cell to avoid the interactions between periodic images of the monolayers. To evaluate the stability of the adsorption ability of gas
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A1
A2
A3
Fig. 2. Optimized configurations for the adsorption of CO and NO gas molecules on the pristine stanene monolayers. (A1): CO adsorption by its O atom; (A2): CO adsorption by its C atom; (A3): NO adsorption by its O atom; (A4): NO adsorption by its N atom. The gray, brown, blue and red balls represent Sn, C, N and O atoms, respectively.
molecules over the studied defective stanene systems, the adsorption energy (Ead) was computed through the following relation:
Eads = EStanene - Gas -- EStanene - EGas
3. Results and discussion 3.1. Structural stability of single vacancy defective stanene
(1)
It is of highly significance to realize the most stable adsorption configuration for the interaction of gas molecules with pristine and defective stanene monolayers. Following this we discussed the binding of some gas molecules (SO3, SO2, O3, NO, and CO) to the pristine and
In this formula, E Stanene-Gas, E Stanene, and E Gas are the total energies of the gas molecule adsorbed over the defective stanene system, bare stanene based system, and isolated gas molecule, respectively.
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A5
A6
A7
Fig. 3. Optimized configurations for the adsorption of SO2 and SO3 gas molecules on the pristine stanene monolayers. (A5) SO2 adsorption by its two oxygen atoms; (A6) SO3 adsorption by its three oxygen atoms; (A7) SO3 adsorption by its two oxygen atoms; (A8) SO3 adsorption by its one oxygen atom. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
single vacancy defective stanene nanosheets. Before we simulate the adsorption process, it is necessary to analyze the stability of the single vacancy defective stanene monolayer. We have examined the stability of defective stanene systems based on the formation energy analysis. Our calculations show that the formation energy of single vacancy defective stanene is about −7.35 eV, which is very close to the result reported by Xiong et al. [23]. These negative values of the formation energies reveal that defective stanene monolayer with single vacancy can be easily built in future experiments. Thus, we have considered the adsorption of gas molecules on defective
stanene systems with single vacancy, which was created by removing one Sn atom from the structure of perfect stanene. Top and side views of the optimized structures of the perfect and defective stanene monolayers were shown in Fig. 1. The buckling parameter for perfect stanene monolayer was also displayed in Fig. 1c. 3.2. Gas molecules adsorption on the pristine stanene In order to comment on the chemical reactivity of perfect and single vacancy defective stanene monolayers, we have inspected the interaction
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Fig. 4. Projected density of states for the adsorption of SO3 molecule on the pristine stanene monolayers, (a–c) PDOSs of the Sn and O atoms for configuration A6; (d, e) PDOSs of the Sn and O atoms for configuration A7; (f) PDOSs of the Sn and O atoms for configuration A8; (g, h) PDOSs of the S and O atoms of the SO3 molecule for configuration A8.
of gas molecules with these structurally modified systems. Figs. 2 and 3 represent the possible configurations for the adsorption of CO, NO, SO2, and SO3 gas molecules on the pristine stanene sheets. As shown in Fig. 2 for both CO and NO molecules, we can see two stable configurations depending on the orientations of the atoms of gas molecules with respect to the stanene system. For example, the adsorption energy of CO through its carbon site on the pristine stanene (configuration A2) is −0.35 eV, which is much higher than that for CO adsorption through the oxygen site (configuration A1). The adsorption energy for this configuration is −0.26 eV. This indicates that CO molecule is strongly adsorbed on the pristine stanene monolayer from the carbon side. Thus, the carbon atom of the CO molecule strongly interacts with the stanene surface, leading to the most stable configuration. The lower distance between the carbon atom and the stanene sheet would also confirm this. As represented in Fig. 2 for the adsorption of NO, the molecule is similarly placed through both nitrogen and oxygen sites on the stanene monolayer. Configuration A3 represents the adsorption of NO molecule by its oxygen atom, while configuration A4 refers to NO adsorption through the nitrogen atom on the stanene monolayer. The most favorable adsorption conformation with the lowest total energy corresponds to that representing the interaction of nitrogen
atom of NO molecule with stanene system (−0.82 eV). The adsorption of SO2 and SO3 gas molecules based on different adsorption geometries were shown in Fig. 3. A close survey in Fig. 3 represents that SO2 molecule strongly interacts with perfect stanene monolayer from the oxygen sides, which gives rise to a stable bridge configuration. The interaction of SO2 molecule by its oxygen atoms with the stanene monolayer was depicted in configuration A5. The adsorption energy of SO2 on the stanene monolayer is calculated to be −2.54 eV. In the case of SO3 adsorption, we have considered two stable configuration as shown in Fig. 3. Configuration A6 displays the parallel SO3 adsorption by its three oxygen atoms on the stanene monolayer, and configuration A7 refers to the perpendicular adsorption of SO3 by its two oxygen atoms. The perpendicular adsorption of SO3 molecule through one oxygen atom was also depicted in configuration A8. Obviously, the adsorption of SO3 by two oxygen atoms (−2.86 eV) is more actively favorable than that by one oxygen atom on the surface (−2.32 eV). This can be attributed to the stronger interaction between two oxygen and Sn atoms. Finally, the highest adsorption energy of the SO2 and SO3 adsorbed stanene complexes can be ascribed to the strong interaction between them and consequently formation of multiple contacting points at the interface. The discussion regarding the density
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Fig. 5. Optimized configurations for the adsorption of CO and NO gas molecules on the defective stanene monolayers. (B1): CO adsorption by its O atom; (B2): CO adsorption by its C atom; (B3): NO adsorption by its O atom; (B4): NO adsorption by its N atom. The gray, brown, blue and red balls represent Sn, C, N and O atoms, respectively.
of states of the atoms can be found in Fig. 4, which presents different plots for the PDOSs of the considered atoms. Panels (a–c) in this figure represents the PDOS plots of the Sn and O atoms for configuration A6 providing three chemical Sn-O bonds, while panels (d, e) denotes the PDOS plots of the Sn and O atoms for configuration A7. In this configuration, two chemical bonds were formed between the two Sn atoms and two O atoms of the SO3 molecule. In panel (f) the formation of bond between one Sn atom and one O atom of SO3 was illustrated. In configuration A8, we can see that no bond dissociation occurred between
the S and O atoms of the SO3 molecule because the PDOS diagrams show full overlaps with each other (Panels g, h). Based on the discussion on the PDOS diagrams, we have determined that the sizeable overlaps were responsible for the formation of covalent bonds especially between tin and oxygen atoms. 3.3. Gas molecules adsorption on the defective stanene Next, we turned to study the interaction between single vacancy
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defective stanene monolayers and gas molecules (CO, NO, O3, SO2 and SO3). Fig. 5 shows the adsorption of NO and CO molecules on these defective stanene sheets. Like the adsorption on the perfect stanene monolayers, we can consider two possible configurations for CO and NO adsorptions on the surface. The adsorption energies and corresponding distances for gas adsorption on the defective stanene monolayers were collected as Table 1. The adsorption of CO on the defective stanene monolayer can be classified as a weak physisorption process with a little adsorption energy of −0.37 eV. In the case of NO adsorption, there is a strong chemisorption onto the defective location with the computed binding energy of −0.92 eV, indicating the dominant adsorption of NO by nitrogen atom on the defective stanene. However, the oxygen site of NO molecule presents a very weak interaction with the surface (−0.56 eV) as compared to the adsorption through the nitrogen site. Figs. 6 and 7 show the adsorption of SO3, SO2, and O3 gas molecules on the defective stanene sheets. As can be realized, there are several configurations for the adsorption of these gases on the defective stanene. By closer inspection of all configurations, we found that the oxygen atoms of the SO2, O3 and SO3 molecules were strongly chemisorbed on the defective site of stanene. Configuration B5 represents the SO2 adsorption by its oxygen atoms on top of the Sn atoms, while B6 shows the adsorption by oxygen atoms on the middle of Sn atoms. In configuration B7, SO2 was adsorbed by its sulfur atom on the middle of Sn atoms. In the case of SO3 adsorption,
Table 1 Adsorption energies and distances between the adsorbed gas molecules and defective stanene monolayers. Configuration type
Eads (eV)
d (Å)
Pristine stanene A1 A2 A3 A4 A5 A6 A7 A8
−0.26 −0.35 −0.76 −0.82 −2.54 −2.56 −2.86 −2.32
2.77 2.71 2.68 2.65 1.92 1.93 1.90 1.91
Defective stanene B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11
−0.29 −0.37 −0.56 −0.92 −2.68 −2.70 −2.71 −2.92 −2.88 −2.72 −2.73
2.75 2.68 2.64 2.62 1.91 1.91 1.95 1.90 1.91 1.88 1.89
Fig. 6. Optimized configurations for the adsorption of SO2 gas molecules on the defective stanene monolayers. (B5) SO2 adsorption by its oxygen atoms on top of the Sn atoms; (B6) SO2 adsorption by its oxygen atoms on the middle of Sn atoms; (B7) SO2 adsorption by its sulfur atom on the middle of Sn atoms. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
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Fig. 7. Optimized structures for the adsorption of SO3 and O3 gas molecules on the defective stanene monolayers. (B8) SO3 adsorption by its three oxygen atoms on top of the Sn atoms; (B9) SO3 adsorption by its two oxygen atoms on top of the Sn atoms; (B10) O3 adsorption by its side oxygen atoms on top of the Sn atoms; (B11) O3 adsorption by its side oxygen atoms on the middle of Sn atoms. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
configuration B8 displays the adsorption of SO3 by its three oxygen atoms on top of the Sn atoms and B9 depicts the SO3 binding by its two oxygen atoms to the top of Sn atoms. For O3 adsorption, we can see also the same adsorption configurations as illustrated in Fig. 7. The adsorption energies for different configurations were summarized in Table 1. It is worth noting that the adsorption energies for these bridge configurations were the highest among all the studied configurations.
The effect of gas adsorption on the electronic structure of defective stanene sheets (charge density difference) were discussed in the last section. Fig. 8 comprises the CDD plots for the combined defective stanene system and O3, NO and CO molecules. A close survey in Fig. 8 represents the appearance of some electronic charges amid the adsorbed gas molecules and defective stanene sheets. As well, the electronic charges were mainly accumulated on the adsorbed O3, NO, and
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Fig. 8. Charge density difference (CDD) plots for the adsorption of CO, NO and O3 gas molecules on the defective stanene monolayers, (a, b) CDD plots for CO adsorption; (c, d) CDD plots for NO adsorption; (e, f) CDD plots for O3 adsorption on the defective stanene.
CO, molecules. The corresponding PDOS diagrams for O3 interaction with defective stanene systems were also given in Fig. S1. Panels (a–b) in this figure represents PDOS spectra of the Sn and O atoms for configuration B10, and (c–f) denotes those PDOS spectra for configuration B11. We also clearly observed that the extensive overlaps between the PDOS spectra are supplied by the oxygen and tin atoms. It was an indication of a covalent bond occurred at the adsorption site. For SO2 adsorption on the defective stanene monolayers, Figs. 9 and S2 show the corresponding CDD and PDOS plots. Fig. 9 shows that there
are electronic charges between the tin and oxygen atoms, indicating the chemisorption process at the interface region. In panels (a–b), we can see the PDOS spectra of the Sn and O atoms (configuration B5). By analyzing the large PDOS overlaps of these atoms (Fig. S2) we can realize that they intensely interacted with each other. Panels (c–f) also depict the PDOS spectra of the Sn and O atoms for configuration B6, suggesting the large overlaps and formation of chemical bonds. Among all the studied gas molecules, SO3 provides the strongest
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Fig. 9. Charge density difference plots for the adsorption of SO2 and SO3 gas molecules on the defective stanene monolayers, (a, b) CDD plots for SO2 adsorption; (c, d) CDD plots for SO3 adsorption on the defective stanene.
interaction with the defective stanene monolayer not only from the energy but also the electronic properties point of view. Fig. 9 also displays the CDD plots for SO3 adsorption on the defective stanene monolayers. For the CDD plots, Figs. 8 and 9 represent two different views for better illustration of the charge densities. We can see that the electronic charges were largely accumulated over the adsorbed gas molecules. The corresponding PDOS profiles were also shown in Fig. S3. Panels (a–c) display the PDOS plots of the Sn and O atoms for configuration B8, providing three covalent Sn-O bonds and panels (d–e) show those PDOS spectra for configuration B9. In this configuration, two chemical bonds were formed between the Sn and O atoms. Similarly, we can see the presence of large PDOS overlaps between the tin and oxygen atoms and consequently chemical characteristics of the interaction between them. To achieve the enviable results, we have performed the band structure calculations for the single vacancy defective stanene monolayers adsorbed with gas molecules. Fig. 10 illustrates the electronic band structure plots for SO3, SO2, and O3 adsorption on the defective stanene sheets. The effects of these adsorptions on the band gap are very strong. For example, molecule adsorption caused a band gap opening nearby Fermi level after the adsorption of gas molecules on the defective systems. For SO2 adsorption on the defective stanene, all the gas adsorbed systems represent semiconductive behavior as shown in Fig. 10a–c. Similarly, SO3 and SO2 adsorbed defective stanene
monolayers exhibit semiconductor characteristics due the band gap region created around the Fermi level (see Fig. 10d–g). 4. Conclusions In this work, DFT calculations were performed to fully inspect the gas molecules adsorption on the perfect and defective stanene sheets to search for the efficient gas sensor devices with improved properties. The formation energy calculations indicate that defective stanene monolayers with single vacancy are energetically stable. So, our study has been mainly focused on effect of single vacancy defects on the properties of stanene. The gas adsorption (especially CO and NO) on the perfect stanene shows a low binding energy, indicating that the process is most likely of physisorption kind. Among all the studied systems, the highest value of adsorption energy corresponds with SO3 adsorption on the defective stanene surface, which indicate the strong interaction between them. These large adsorption energies for SO2 and SO3 adsorbed complexes can be attributed to the robust reactivity of adsorbates above the surface and consequently formation of multiple contacting points at the interface. It was observed that the PDOS spectra of the tin and oxygen atoms show huge overlaps. This result clearly reflects that the gas molecule gets chemisorbed on the defective stanene system with realization of covalent bonds. Our results provide remarkable potential to build sensor which can capture harmful gas molecule.
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Fig. 10. Electronic band structure plots for the adsorption of SO2, SO3 and O3 gas molecules on the defective stanene monolayers, (a–c) Band structures for SO2 adsorption (configurations B5-B7); (d, e) Band structures for SO3 adsorption (configurations B8 and B9); (f, g) Band structures for O3 adsorption (configurations B10 and B11) on the defective stanene system.
CRediT authorship contribution statement
Declaration of Competing Interest
Yu Yang: Conceptualization, Project administration, Methodology, Software, Validation. Hao Zhang: Software, Data curation, Writing original draft. Lihong Song: Visualization, Investigation, Writing original draft, Writing - review & editing. Zhenling Liu: Software, Supervision, Writing - review & editing.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Full Length Article
Adsorption of gas molecules on the defective stanene nanosheets with single vacancy: A DFT study Yu Yanga, Hao Zhangb, Lihong Songc, Zhenling Liud,
T
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a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Australian School of Petroleum, the University of Adelaide, Adelaide, SA 5005, Australia c College of Energy, Chengdu University of Technology, Chengdu 610059, China d School of Materials Science and Engineering, Henan University of Technology, China b
ARTICLE INFO
ABSTRACT
Keywords: Density functional theory Charge density difference Defective stanene Gas sensor
Using the first-principles calculations, we have systematically examined the adsorption of gas molecules on the pristine and vacancy defective stanene monolayers. The adsorption of gas molecules on the defective monolayers is much stronger than that on the perfect ones, indicating the suitability of defective stanene nanosheets for adsorption processes. The large adsorption energies for SO2 and SO3 adsorbed complexes indicate the strong reactivity of stanene systems with adsorbed gas molecules, leading to the formation of multiple contacting points at the interface. The formation energies for the defective stanene systems were calculated. Our results indicated that the single vacancy defective stanene monolayers are thermodynamically stable, and can be used as effective gas sensors. The charge density difference calculations indicate that the electronic densities were largely accumulated between the atoms interacting with each other. This strong chemical interaction was also evidenced by the large overlaps of the density of states between the interacting atoms. The band structure calculations reveal the semiconductor features for defective stanene monolayers with adsorbed gas molecules. Our obtained results would be useful to search for promising gas sensor devices based on vacancy defective stanene monolayers.
1. Introduction In recent years, marvelous interests have been devoted to exploring the peculiar properties of two-dimensional (2D) materials owing to their astonishing mechanical [1], optical and electronic properties [2]. These particular properties demonstrate the potential application of 2D layered materials as promising materials for future nanoelectronic devices. Several researchers have done significant efforts to explore the properties of the other honeycomb-like structures constructed from group IV elements such as silicene, graphene and transition-metal dichalcogenides [3–8]. It has been experimentally proved that 2D nanomaterials have many outstanding applications towards optoelectronics and nanoelectronics such as sensing elements, field-effect transistors (FETs), and photovoltaic devices [9,10]. Because of the increasing importance of 2D layered materials in the fabrication of future nanoscale electronic devices, the study on the properties of these materials have triggered tremendous attentions in condensed matter physics and materials science. The 2D layered materials are mainly constructed from the groups IV and V elements of periodic table such as C, Si, Ge and Sn. The 2D arrangements of these elements lead to the formation of
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graphene, silicene, germanium and tin films with honeycomb structures [11,12]. In the past decades, the buckled 2D layered materials such as stanene, germanene, and silicene have attracted enormous interests [13–16]. For example, stanene, a 2D layered material composed of tin atoms has been experimentally synthesized using the epitaxial growth (MBE method) [17]. Recent advances on 2D materials have been focused on layered stanene sheets with buckled geometry because of the weak π–π bonding between the tin atoms [18]. It exhibits a semiconductor characteristics with a Dirac cone located at k point of the band structure plot [19]. Tuning the structural and electronic properties of 2D materials has become an important issue, due to the necessity of extensive applications in electronic and spintronic devices. Recently, Chen et al. [20] suggested that the structural and electronic properties of stanene can be tuned by adsorption of small gas molecules. We systematically examined the adsorption of gas molecules on the pure and defective stanene monolayers to further exploit the gas sensing capability of these systems. It is well known that creating defects is very important for tuning the electronic and magnetic properties of nanomaterials [21,22]. Xiong et al. [23] reported that stanene monolayers with single and double vacancy are energetically stable based on
Corresponding author. E-mail address: [email protected] (Z. Liu).
https://doi.org/10.1016/j.apsusc.2020.145727 Received 19 December 2019; Received in revised form 18 January 2020; Accepted 9 February 2020 Available online 11 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Top and side views of the optimized structures of the pristine (a, c) and single vacancy defective (b, d) stanene monolayers. The buckling parameter (Δz = 0.88 Å) for pristine stanene is shown.
the formation energy results. They found that the formation energy of stanene with single vacancy is −7.4 eV, which indicates that it can be easily formed due to its higher stability. Moreover, Özçelik et al. [24] and Zhang et al. [25] suggested that single vacancy is energetically stable in graphene with negative formation energies. Abbasi et al. [26,27] also studied the effects of gas adsorption and elemental doping on the electronic properties of perfect stanene monolayers. In the recent years, sensors based on metal carbides and other 2D layered materials have been suggested for the capable removal of hazardous materials [28–31]. Sensing air pollutants have aroused tremendous interests because of the negative influences of toxic materials on the air quality, health standards of human beings and environmental safety. Therefore, it is of significant importance to suggest some suitable candidate materials, which detect and eliminate the harmful molecules from the environment. Gas sensors constructed from 2D materials have been attracting incredible attention over the past decades, which can be attributed to their wide surface/volume ratio. Thus, these materials with great surface/volume ratio are beneficial for manufacturing gas sensor devices [32]. Among the 2D layered materials, stanene based systems reveal favorable electronic properties due to their outstanding characteristics and effectiveness in environmental safety subjects both in residential areas and workplaces. Thus, (2D) nanomaterials are promising platforms for constructing nanoscale sensors because of their unique and excellent properties [33–38]. In this paper, we investigated the interaction of some small gas molecules with pure and defective stanene sheets to examine the
possible applicability of defective stanene sheets in sensor devices. Our aim is to provide promising layered stanene based material for future electronic devices like gas sensors. 2. Calculation details The presented electronic structure and relaxation calculations in this research were carried out with the Quantum Espresso (QE) package [39] based on DFT simulations [40,41]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchangecorrelation was used to describe the electronic exchange and correlation effects [42]. The energy cut-off in our calculations was set at 150 Ry. The Brillouin-zone was taken to be at the Γ-centered arrangement with 8 × 8 × 1 Monkhorst–Pack sampling of K points for integration in the structural optimization. We have considered a 5 × 5 × 1 supercell for our simulations of stanene monolayer. Since the higher value of energy cutoff slightly influences the obtained results, the plane wave cutoff energy was set to 150 Ry. The adsorption configurations were relaxed until the energy was converged to 1 × 0−5 eV and the force on each atom was less than 0.02 eV/ Å. We have chosen the K-path of G-KM-K-G for the band structure calculations. Our considered 5 × 5 × 1 supercell of perfect and single vacancy defective (imperfect) stanene monolayers contain 50 and 49 tin atoms, respectively. Moreover, a vacuum space of at least 20 Å is taken into account in the simulation cell to avoid the interactions between periodic images of the monolayers. To evaluate the stability of the adsorption ability of gas
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A1
A2
A3
Fig. 2. Optimized configurations for the adsorption of CO and NO gas molecules on the pristine stanene monolayers. (A1): CO adsorption by its O atom; (A2): CO adsorption by its C atom; (A3): NO adsorption by its O atom; (A4): NO adsorption by its N atom. The gray, brown, blue and red balls represent Sn, C, N and O atoms, respectively.
molecules over the studied defective stanene systems, the adsorption energy (Ead) was computed through the following relation:
Eads = EStanene - Gas -- EStanene - EGas
3. Results and discussion 3.1. Structural stability of single vacancy defective stanene
(1)
It is of highly significance to realize the most stable adsorption configuration for the interaction of gas molecules with pristine and defective stanene monolayers. Following this we discussed the binding of some gas molecules (SO3, SO2, O3, NO, and CO) to the pristine and
In this formula, E Stanene-Gas, E Stanene, and E Gas are the total energies of the gas molecule adsorbed over the defective stanene system, bare stanene based system, and isolated gas molecule, respectively.
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A5
A6
A7
Fig. 3. Optimized configurations for the adsorption of SO2 and SO3 gas molecules on the pristine stanene monolayers. (A5) SO2 adsorption by its two oxygen atoms; (A6) SO3 adsorption by its three oxygen atoms; (A7) SO3 adsorption by its two oxygen atoms; (A8) SO3 adsorption by its one oxygen atom. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
single vacancy defective stanene nanosheets. Before we simulate the adsorption process, it is necessary to analyze the stability of the single vacancy defective stanene monolayer. We have examined the stability of defective stanene systems based on the formation energy analysis. Our calculations show that the formation energy of single vacancy defective stanene is about −7.35 eV, which is very close to the result reported by Xiong et al. [23]. These negative values of the formation energies reveal that defective stanene monolayer with single vacancy can be easily built in future experiments. Thus, we have considered the adsorption of gas molecules on defective
stanene systems with single vacancy, which was created by removing one Sn atom from the structure of perfect stanene. Top and side views of the optimized structures of the perfect and defective stanene monolayers were shown in Fig. 1. The buckling parameter for perfect stanene monolayer was also displayed in Fig. 1c. 3.2. Gas molecules adsorption on the pristine stanene In order to comment on the chemical reactivity of perfect and single vacancy defective stanene monolayers, we have inspected the interaction
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Fig. 4. Projected density of states for the adsorption of SO3 molecule on the pristine stanene monolayers, (a–c) PDOSs of the Sn and O atoms for configuration A6; (d, e) PDOSs of the Sn and O atoms for configuration A7; (f) PDOSs of the Sn and O atoms for configuration A8; (g, h) PDOSs of the S and O atoms of the SO3 molecule for configuration A8.
of gas molecules with these structurally modified systems. Figs. 2 and 3 represent the possible configurations for the adsorption of CO, NO, SO2, and SO3 gas molecules on the pristine stanene sheets. As shown in Fig. 2 for both CO and NO molecules, we can see two stable configurations depending on the orientations of the atoms of gas molecules with respect to the stanene system. For example, the adsorption energy of CO through its carbon site on the pristine stanene (configuration A2) is −0.35 eV, which is much higher than that for CO adsorption through the oxygen site (configuration A1). The adsorption energy for this configuration is −0.26 eV. This indicates that CO molecule is strongly adsorbed on the pristine stanene monolayer from the carbon side. Thus, the carbon atom of the CO molecule strongly interacts with the stanene surface, leading to the most stable configuration. The lower distance between the carbon atom and the stanene sheet would also confirm this. As represented in Fig. 2 for the adsorption of NO, the molecule is similarly placed through both nitrogen and oxygen sites on the stanene monolayer. Configuration A3 represents the adsorption of NO molecule by its oxygen atom, while configuration A4 refers to NO adsorption through the nitrogen atom on the stanene monolayer. The most favorable adsorption conformation with the lowest total energy corresponds to that representing the interaction of nitrogen
atom of NO molecule with stanene system (−0.82 eV). The adsorption of SO2 and SO3 gas molecules based on different adsorption geometries were shown in Fig. 3. A close survey in Fig. 3 represents that SO2 molecule strongly interacts with perfect stanene monolayer from the oxygen sides, which gives rise to a stable bridge configuration. The interaction of SO2 molecule by its oxygen atoms with the stanene monolayer was depicted in configuration A5. The adsorption energy of SO2 on the stanene monolayer is calculated to be −2.54 eV. In the case of SO3 adsorption, we have considered two stable configuration as shown in Fig. 3. Configuration A6 displays the parallel SO3 adsorption by its three oxygen atoms on the stanene monolayer, and configuration A7 refers to the perpendicular adsorption of SO3 by its two oxygen atoms. The perpendicular adsorption of SO3 molecule through one oxygen atom was also depicted in configuration A8. Obviously, the adsorption of SO3 by two oxygen atoms (−2.86 eV) is more actively favorable than that by one oxygen atom on the surface (−2.32 eV). This can be attributed to the stronger interaction between two oxygen and Sn atoms. Finally, the highest adsorption energy of the SO2 and SO3 adsorbed stanene complexes can be ascribed to the strong interaction between them and consequently formation of multiple contacting points at the interface. The discussion regarding the density
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Fig. 5. Optimized configurations for the adsorption of CO and NO gas molecules on the defective stanene monolayers. (B1): CO adsorption by its O atom; (B2): CO adsorption by its C atom; (B3): NO adsorption by its O atom; (B4): NO adsorption by its N atom. The gray, brown, blue and red balls represent Sn, C, N and O atoms, respectively.
of states of the atoms can be found in Fig. 4, which presents different plots for the PDOSs of the considered atoms. Panels (a–c) in this figure represents the PDOS plots of the Sn and O atoms for configuration A6 providing three chemical Sn-O bonds, while panels (d, e) denotes the PDOS plots of the Sn and O atoms for configuration A7. In this configuration, two chemical bonds were formed between the two Sn atoms and two O atoms of the SO3 molecule. In panel (f) the formation of bond between one Sn atom and one O atom of SO3 was illustrated. In configuration A8, we can see that no bond dissociation occurred between
the S and O atoms of the SO3 molecule because the PDOS diagrams show full overlaps with each other (Panels g, h). Based on the discussion on the PDOS diagrams, we have determined that the sizeable overlaps were responsible for the formation of covalent bonds especially between tin and oxygen atoms. 3.3. Gas molecules adsorption on the defective stanene Next, we turned to study the interaction between single vacancy
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defective stanene monolayers and gas molecules (CO, NO, O3, SO2 and SO3). Fig. 5 shows the adsorption of NO and CO molecules on these defective stanene sheets. Like the adsorption on the perfect stanene monolayers, we can consider two possible configurations for CO and NO adsorptions on the surface. The adsorption energies and corresponding distances for gas adsorption on the defective stanene monolayers were collected as Table 1. The adsorption of CO on the defective stanene monolayer can be classified as a weak physisorption process with a little adsorption energy of −0.37 eV. In the case of NO adsorption, there is a strong chemisorption onto the defective location with the computed binding energy of −0.92 eV, indicating the dominant adsorption of NO by nitrogen atom on the defective stanene. However, the oxygen site of NO molecule presents a very weak interaction with the surface (−0.56 eV) as compared to the adsorption through the nitrogen site. Figs. 6 and 7 show the adsorption of SO3, SO2, and O3 gas molecules on the defective stanene sheets. As can be realized, there are several configurations for the adsorption of these gases on the defective stanene. By closer inspection of all configurations, we found that the oxygen atoms of the SO2, O3 and SO3 molecules were strongly chemisorbed on the defective site of stanene. Configuration B5 represents the SO2 adsorption by its oxygen atoms on top of the Sn atoms, while B6 shows the adsorption by oxygen atoms on the middle of Sn atoms. In configuration B7, SO2 was adsorbed by its sulfur atom on the middle of Sn atoms. In the case of SO3 adsorption,
Table 1 Adsorption energies and distances between the adsorbed gas molecules and defective stanene monolayers. Configuration type
Eads (eV)
d (Å)
Pristine stanene A1 A2 A3 A4 A5 A6 A7 A8
−0.26 −0.35 −0.76 −0.82 −2.54 −2.56 −2.86 −2.32
2.77 2.71 2.68 2.65 1.92 1.93 1.90 1.91
Defective stanene B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11
−0.29 −0.37 −0.56 −0.92 −2.68 −2.70 −2.71 −2.92 −2.88 −2.72 −2.73
2.75 2.68 2.64 2.62 1.91 1.91 1.95 1.90 1.91 1.88 1.89
Fig. 6. Optimized configurations for the adsorption of SO2 gas molecules on the defective stanene monolayers. (B5) SO2 adsorption by its oxygen atoms on top of the Sn atoms; (B6) SO2 adsorption by its oxygen atoms on the middle of Sn atoms; (B7) SO2 adsorption by its sulfur atom on the middle of Sn atoms. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
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Fig. 7. Optimized structures for the adsorption of SO3 and O3 gas molecules on the defective stanene monolayers. (B8) SO3 adsorption by its three oxygen atoms on top of the Sn atoms; (B9) SO3 adsorption by its two oxygen atoms on top of the Sn atoms; (B10) O3 adsorption by its side oxygen atoms on top of the Sn atoms; (B11) O3 adsorption by its side oxygen atoms on the middle of Sn atoms. The gray, yellow and red balls represent Sn, S and O atoms, respectively.
configuration B8 displays the adsorption of SO3 by its three oxygen atoms on top of the Sn atoms and B9 depicts the SO3 binding by its two oxygen atoms to the top of Sn atoms. For O3 adsorption, we can see also the same adsorption configurations as illustrated in Fig. 7. The adsorption energies for different configurations were summarized in Table 1. It is worth noting that the adsorption energies for these bridge configurations were the highest among all the studied configurations.
The effect of gas adsorption on the electronic structure of defective stanene sheets (charge density difference) were discussed in the last section. Fig. 8 comprises the CDD plots for the combined defective stanene system and O3, NO and CO molecules. A close survey in Fig. 8 represents the appearance of some electronic charges amid the adsorbed gas molecules and defective stanene sheets. As well, the electronic charges were mainly accumulated on the adsorbed O3, NO, and
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Fig. 8. Charge density difference (CDD) plots for the adsorption of CO, NO and O3 gas molecules on the defective stanene monolayers, (a, b) CDD plots for CO adsorption; (c, d) CDD plots for NO adsorption; (e, f) CDD plots for O3 adsorption on the defective stanene.
CO, molecules. The corresponding PDOS diagrams for O3 interaction with defective stanene systems were also given in Fig. S1. Panels (a–b) in this figure represents PDOS spectra of the Sn and O atoms for configuration B10, and (c–f) denotes those PDOS spectra for configuration B11. We also clearly observed that the extensive overlaps between the PDOS spectra are supplied by the oxygen and tin atoms. It was an indication of a covalent bond occurred at the adsorption site. For SO2 adsorption on the defective stanene monolayers, Figs. 9 and S2 show the corresponding CDD and PDOS plots. Fig. 9 shows that there
are electronic charges between the tin and oxygen atoms, indicating the chemisorption process at the interface region. In panels (a–b), we can see the PDOS spectra of the Sn and O atoms (configuration B5). By analyzing the large PDOS overlaps of these atoms (Fig. S2) we can realize that they intensely interacted with each other. Panels (c–f) also depict the PDOS spectra of the Sn and O atoms for configuration B6, suggesting the large overlaps and formation of chemical bonds. Among all the studied gas molecules, SO3 provides the strongest
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Fig. 9. Charge density difference plots for the adsorption of SO2 and SO3 gas molecules on the defective stanene monolayers, (a, b) CDD plots for SO2 adsorption; (c, d) CDD plots for SO3 adsorption on the defective stanene.
interaction with the defective stanene monolayer not only from the energy but also the electronic properties point of view. Fig. 9 also displays the CDD plots for SO3 adsorption on the defective stanene monolayers. For the CDD plots, Figs. 8 and 9 represent two different views for better illustration of the charge densities. We can see that the electronic charges were largely accumulated over the adsorbed gas molecules. The corresponding PDOS profiles were also shown in Fig. S3. Panels (a–c) display the PDOS plots of the Sn and O atoms for configuration B8, providing three covalent Sn-O bonds and panels (d–e) show those PDOS spectra for configuration B9. In this configuration, two chemical bonds were formed between the Sn and O atoms. Similarly, we can see the presence of large PDOS overlaps between the tin and oxygen atoms and consequently chemical characteristics of the interaction between them. To achieve the enviable results, we have performed the band structure calculations for the single vacancy defective stanene monolayers adsorbed with gas molecules. Fig. 10 illustrates the electronic band structure plots for SO3, SO2, and O3 adsorption on the defective stanene sheets. The effects of these adsorptions on the band gap are very strong. For example, molecule adsorption caused a band gap opening nearby Fermi level after the adsorption of gas molecules on the defective systems. For SO2 adsorption on the defective stanene, all the gas adsorbed systems represent semiconductive behavior as shown in Fig. 10a–c. Similarly, SO3 and SO2 adsorbed defective stanene
monolayers exhibit semiconductor characteristics due the band gap region created around the Fermi level (see Fig. 10d–g). 4. Conclusions In this work, DFT calculations were performed to fully inspect the gas molecules adsorption on the perfect and defective stanene sheets to search for the efficient gas sensor devices with improved properties. The formation energy calculations indicate that defective stanene monolayers with single vacancy are energetically stable. So, our study has been mainly focused on effect of single vacancy defects on the properties of stanene. The gas adsorption (especially CO and NO) on the perfect stanene shows a low binding energy, indicating that the process is most likely of physisorption kind. Among all the studied systems, the highest value of adsorption energy corresponds with SO3 adsorption on the defective stanene surface, which indicate the strong interaction between them. These large adsorption energies for SO2 and SO3 adsorbed complexes can be attributed to the robust reactivity of adsorbates above the surface and consequently formation of multiple contacting points at the interface. It was observed that the PDOS spectra of the tin and oxygen atoms show huge overlaps. This result clearly reflects that the gas molecule gets chemisorbed on the defective stanene system with realization of covalent bonds. Our results provide remarkable potential to build sensor which can capture harmful gas molecule.
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Fig. 10. Electronic band structure plots for the adsorption of SO2, SO3 and O3 gas molecules on the defective stanene monolayers, (a–c) Band structures for SO2 adsorption (configurations B5-B7); (d, e) Band structures for SO3 adsorption (configurations B8 and B9); (f, g) Band structures for O3 adsorption (configurations B10 and B11) on the defective stanene system.
CRediT authorship contribution statement
Declaration of Competing Interest
Yu Yang: Conceptualization, Project administration, Methodology, Software, Validation. Hao Zhang: Software, Data curation, Writing original draft. Lihong Song: Visualization, Investigation, Writing original draft, Writing - review & editing. Zhenling Liu: Software, Supervision, Writing - review & editing.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Appendix A. Supplementary material
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