Single-layer stanane as potential gas sensor for NO2, SO2, CO2 and NH3 under DFT investigation

Physica E: Low-dimensional Systems and Nanostructures 110 (2019) 100–106

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Single-layer stanane as potential gas sensor for NO2, SO2, CO2 and NH3 under DFT investigation

T

Vipin Kumar, Debesh R. Roy∗ Materials and Biophysics Group, Department of Applied Physics, S. V. National Institute of Technology, Surat, 395007, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Stanane Density functional theory Band structure Density of states Molecular gas sensors Electron density contour

The monolayer stanane in chair form is reported to be a novel sensor for environmentally toxic and non-toxic gas molecules for the first time. The structure, electronic and vibrational properties of all three possible conformations (chair, stirrup and boat) of stanane are studied in detail using density functional theory (DFT) based on an ab-initio technique. The interactions and charge transfer of environmentally toxic (NO2, SO2 and NH3) and non-toxic (CO2) gas molecules on the dynamically most stable hexagonal chair type hydrogenated stanene, viz. stanane has been investigated in detail. The most stable configuration, electronic properties, adsorption energies and charge transfer of these gases on stanane are systematically studied and discussed. The band gap of the pure stanane (0.52 eV) is noticed to be changed after interaction with gases. Moreover, the changes in the energy band gap and charge density is observed upon adsorption of NO2, SO2, NH3 and CO2 gases on p-type stanane based material. The results show that the selectivity of hydrogenated stanene based gas sensors is very important to enhance their sensitivity. It is found that all the gas molecules act as charge donors in which NO2 gas shows maximum adsorption on the stanane surface along with the maximum charge transfer. The nontrivial affectability and selectivity of stanane demonstrate its potential application in the field of gas sensors and superior impetuses.

1. Introduction Rapid and continuous growth of industries and automobiles exerts large amount of toxic and hazardous gases like carbon monoxide, hydrocarbons, sulphur dioxide and nitrogen oxide in the atmosphere. Due to the large emission rate of toxic gas molecules causes acid rain, smog and depletion in ozone layer. As a consequence, it become a challenging and important criterion for sensing toxic gas molecules in the arena of air pollution control, environmental monitoring, device contamination, control of chemical processes, space missions, agricultural and medical purposes [1]. To address such issues, one needs to look for such type of materials which provides low power consumption, higher sensitivity, good selectivity, better stability and quick response. Along this direction many research groups came forward with fascinating results with 2D monolayers due to their large surface area and potential applications in new-generation energy conversion high-speed electronics, optoelectronics and sensing [2–4]. The field is initiated by Novoselov et al. [5,6] who have investigated the 2D monolayer graphene, which exhibits excellent properties like larger surface to volume ratio, less electronic temperature noise, better carrier mobility, higher chemical and thermal stabilities and good response time. Motivated with the

∗

predictions of graphene monolayer, Schedin et al. [7] attempted sensing based theoretical and experimental studies in the development of ultrasensitive sensors with less power consumption, high packing density, higher sensitivity, better selectivity and faster recoverability. Presently, various 2D monolayers of group-IV elements like silicene, germanene etc. have been successfully investigated for the possible applications in terms of electronic devices and nanomaterial sensors [8]. In recent past, experimental work on the successful epitaxial growth of two-dimensional stanene has also been carried out by Zhu et al. [9] It has been reported from the study of molecular sensors that variation in resistivity of monolayer is directly proportional to the amount of toxic gas molecules present in the atmosphere which enables one to predict the quantitative gas concentration. This unique property of monolayer has attracted great attention towards the innovation of novel sensors. On the other hand, buckled monolayer structure of graphene and graphene like materials such as stanene, germanene and silicene with stronger spin orbital coupling facilitates them in band gap engineering, and for being utilized as molecular sensors. In nature, although graphite is available in the form of layered structure, the germanium, stannum and silicon do not exist in such a layered form. Although monolayer structure of silicon (silicene), germanium

Corresponding author. E-mail address: [email protected] (D.R. Roy).

https://doi.org/10.1016/j.physe.2019.02.001 Received 30 November 2018; Received in revised form 13 January 2019; Accepted 3 February 2019 Available online 11 February 2019 1386-9477/ © 2019 Elsevier B.V. All rights reserved.

Physica E: Low-dimensional Systems and Nanostructures 110 (2019) 100–106

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kinetic energy cut-off of 60 Ry, 300 Ry for the wave functions and charge densities, respectively is considered. We adopt the MonkhorstPack scheme for k-point sampling of Brillouin zone integrations with 15 × 15 × 1 Å−3 and 5 × 5 × 1 Å−3 for the unit cell and 3 × 3 × 1 for supercell, respectively. For periodic boundary conditions (PBC), we employed the vacuum which is not smaller than 15 Å in z-direction and is perpendicular to infinite xy-plane of SnH monolayer sheet to avoid the interaction of interface between two adjacent sides of the monolayer. It is well known that GGA doesn't include the van der waals (vdW) interactions in weakly bonded systems like gas adsorption on monolayer as considered in the present study. Therefore, we have incorporated additional functional into standard DFT calculations in order to correctly account for the effect of vdW interactions with the vdW-DF2 functional. During geometry optimization, the cell shape remains unaltered. Between two sequential steps the convergence criterion for energy is set as 10−4 eV. For each configuration, ionic relaxations of atoms are terminated until all maximum HellmannFeynman force applying on each atom is less than 0.03 eV/Å. The geometric structures are drawn and plotted using XCrySDen [27] and p4vasp softwares. The volume of the 3 × 3 × 1 supercell has been kept fixed, but SnH layers and gas molecules allowed to converge until the residual force reach their relaxed position.

(germanene) and stannum (stanene) do not have experimental evidence in past but nowadays, some recent experimental investigations show successful synthesis of two-dimensional monolayer materials like silicene, stanene and germanene [10,11]. It is known that group-IV elements in 2D monolayer form which are semi metallic in nature have negligible band gap and higher carrier mobility [12]. Accordingly, their nearly zero band gap restricts their applications in nano electronic devices such as FET, MOSFET etc. In single molecular gas sensors, band gap usually plays a special role, viz. the variation in energy band gap relevant to adsorption and desorption reaction provides application in chemiresistor for identifying gas molecules for which the resistance of base material changes. The 2D monolayer based studies related to graphene, silicene, germanene and stanene mostly depends on their adsorption properties with respect to toxic/non-toxic gas molecules like SO2, NO2, NH3, CO2, etc. [13,14]. 2D WO3 nanosheets prepared by anodization are also shown to be as the potential sensor for hydrogen gas molecules [15]. Sofo et al. [16] and Li et al. [17] also theoretically investigated the monolayer of graphene, silicene, germanene and stanene. In general, monolayer of stanene possesses buckled honeycomb structure which is more stable compared to its planer counterpart. In the planer architecture of stanene, electrons are localized and show better π-orbital bonding, whereas in case of buckled structure the πorbitals are comparatively weaker. The stanene is reported to be zero band gap material and have approximately 0.1 eV band gap in presence of spin-orbital coupling (SOC), by Shaidu et al. [18] The optical properties of strained stanene and stanane is studied recently by Lu and coworkers where they have reported that strain plays important role in optical absorption [19]. Another study by Nagrajan et al. [20] show the adsorption of alcohols on the monolayer of stanane through charge transfer. The monolayer hydrogenated stanene based study with toxic/ non-toxic molecules, other gas/vapour molecules and volatile organic compounds shows the scope in new-generation electronic gas sensors [21,22]. Very recently, Nagarajan and Chandiramouli [23] studied adsorption of a couple of toxic gases NH3 and NO2 on both the stanene and stanane surfaces and noticed NO2 adsorption is more prominent compared to the other through adsorption energy analysis. Although few of the preliminary work on the adsorption of representative toxic/ non-toxic gas molecule on the stanane surface is carried out, detail investigation on various possible types of stanane and their dynamical stability as well as sensing potential for series of different toxic gases in light of band re-structuring and adsorption energies are still lacking. The purpose of present work is to investigate the stability of stanane (hydrogenated stanene) in its all possible configurations, viz. chair, stirrup and boat. Further, absorption of various toxic gas molecules, viz. NO2, SO2, NH3 and CO2 on the dynamically most stable chair structure of stanane is investigated in detail. The most stable configuration of the absorbed gas molecules on the stanane surface are predicated from the consideration of the possible interaction sites on the stanane surface. The gas sensing efficiency of stanane is predicted through the pre and post adsorption band structure, projected density of states and adsorption energy calculations at different possible sites.

3. Results and discussion Researchers have been performed theoretical investigations on various 2D structure of monolayers like, GeH [28], SiH [29], CH [30] etc. in which for most of the cases chair type structure is found to be more stable compared to the other forms of monolayer. In Fig. 1, we have depicted all three types of possible configurations with their optimized crystal structures, viz. (a) Chair type (b) Stirrup type and (c) Boat type of SnH monolayer sheet. Our optimized bond length for SneSn atom is found to be 2.91 Å, which is comparable to its previously reported value of 2.85 Å [31], and the optimized bond length of SneH atom is achieved as 1.72 Å, which is in close agreement to the past reported value of 1.70 Å [20]. In our calculation, we have chosen crystal symmetry for chair, boat and stirrup conformations as P-3m1 (164), pmna (53) and pmmm (59) respectively. The chair stanane (SnH) monolayer sheet having the buckled parametric value (Δz) of 1.20 Å is little larger than that of a buckled stanene single layer of 0.92 Å [29]. Due to the vertical arrangement of H atom on Sn atoms, the bonding of Sn atoms pull out of the planer configuration and it makes the buckling distance of chair SnH monolayer a bit larger about 0.28 Å, as predicted by Broek et al. [31] Likewise, lattice constant of SnH is also found to be slightly larger than that of stanene, which is caused due to the longer SneSn bond length in the chair structure of SnH. An additional supporting work for the cohesive energy of chair, stirrup and boat type structures shows that those structures are not easily separable into SnH and H2 molecules. The cohesive energy of those configurations are found to be 3.15 eV, 1.58 eV and 1.55 eV respectively. In order to evaluate the stability of the adsorbed gas molecules on monolayer SnH, the adsorption energy (Ead) is calculated as follows [32,33]:

2. Computational detail

Ead = ESnH+Gas − [E(SnH ) + EGas] We have performed a systematic theoretical investigation on the hydrogenated stanene (SnH) low buckled monolayer sheet with the adsorption of four different toxic/non-toxic gas molecules (NO2, SO2, NH3 and CO2), with their several initial configurations/positions above the stanane sheet, and also calculated various electronic properties. The electronic structure calculations were performed using self-consistent plane wave pseudo potential method under the frame of density functional theory (DFT) [24], as implemented in the QUANTUM ESPRESSO [25] simulation package. The exchange correlation energy was described based on the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) [26], and 5s25p2 electrons for Sn and 1s electrons for H were treated explicitly. A plane-wave basis set up to a

(1)

where E(SnH+Gas) is the total energy of the optimized complex structure, E(SnH) and E(Gas) are energy of SnH monolayer and isolated toxic gas molecules (NO2, SO2, NH3 and CO2) respectively. It may be understood that more negative Ead indicates stronger adsorption of gas molecules on SnH surface [14,34]. Also, a negative value of Ead indicates the exothermic specificity (i.e. adsorption of gas molecules on the surface of SnH layer is energetically favourable), whereas a positive value of Ead represents the endothermic specificity. The adsorption energies determined in our calculations are static which means that the energies are obtained at absolute zero temperatures. The cohesive energy (ECoh) of the system can be defined as [35]: 101

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structure is found to have direct band gap of 0.22 eV at Г-point. Therefore, although the chair and boat confirmation of stanane are found to be narrow direct band semiconductor, the stirrup structure found to be an indirect gap semiconductor. Table 1 presents the symmetries, lattice parameters (a, b, c, α, β, γ), bond parameters, cohesive energies and band gaps of different conformation (chair, stirrup and boat) of stanane (SnH). The SneSn and SneH bond distance for all the three structures are found to be close to each case. It may be noted that the cohesive energies (3.15 eV) of the chair structure is found almost double compared to stirrup (1.59 eV) and boat (1.55 eV) indicating the exceptional relative stability of chair type. In order to understand the role of orbitals in the band structure, the total and partial density of states (PDOS) of these configurations are calculated and shown in the Fig. 3 with energy range from −4 eV to 4 eV. According to the Fig. 3a, p-orbital of Sn atom is dominated near to the Fermi level in VBM where the d-orbital of Sn atom mainly responsible for the contribution at Fermi level for CBM. Similar to the chair structure, it is also noticed for stirrup and boat that the p-orbital of Sn and d-orbital of Sn have major contribution for VBM and CBM, respectively near the Fermi level. The lower band gap (0.22 eV) of boat type structure may be understood due to more dominating behaviour of the p and d orbitals of Sn in VBM and CBM, compared to stirrup structure (0.88 eV). In order to understand the structural rigidly and lattice dynamical stability, we also calculated phonon dispersion frequency spectra for all the considered structure along their high symmetry k-point in the Brillion zones as shown in Fig. 4. The dynamical matrix of the phonon dispersion is explicitly calculated on 4 × 4 × 1 qpoints meshes. For detail study of the phonon frequency spectra, we have considered atoms in the unit cell of all the three structures as 4, 8 and 8 respectively, which gives 12 phonon branches for chair structure and 24 phonon branches for other remaining two structures. The three phonon branches will be considered in acoustic and other remaining branches considered in optical modes. The results show that the dynamically most stable SnH is found in its chair type form, with all positive frequencies in acoustical modes. On the other hand, stirrup structure shows minor negative frequency (up to 5 cm−1, Fig. 4b), whereas boat structure shows frequency up to 50 cm−1 in negative region (Fig. 4c). Overall, the dynamical stability of SnH structure follow the order as Chair > Stirrup > Boat. Therefore, we have considered the most stable crystal structure of stanane, viz. chair form monolayer structure for the further execution of our theoretical investigation. Due to the maximum stability and symmetry of the monolayer chair type SnH configuration, the system is modelled as a 3 × 3 × 1 supercell of SnH with 36 atoms as shown in Fig. 5, and a single gas molecule (from NO2, SO2, NH3 and CO2) is placed above the monolayer for studying their adsorption process. All the four gas molecules are placed on the top of the monolayer with optimized distance between SnH layer and gas molecules in several possible orientations with A, B, C, D and E sites as shown in Fig. 5. Several orientations of these molecules are considered to find the respective configuration of maximum adsorption of gas molecule through the complete optimization of SnH-molecule complex. The A, B, C, D and E positions of molecules indicates the absorption top on the Sn atom, top on the H atom, middle top on the H atom, middle top on the Sn atom and top on hollow site between Sn and H atoms, respectively. The adsorption energies (Ead), charge transfers (ΔQ), geometrical parameters and band gaps (Eg) of gas molecules on stanane chair monolayer are reported in Table 2. The electronic properties of hydrogenated stanene (or stanane) based materials with adsorbed gas molecule are described in terms of energy band structure, partial density of states and charge transfer. The density of states (DOS) spectrum gives the perception on localization of charges in different energy intervals. The density of state spectrum provides understanding on electron transfer between gas molecules and stanane layer, which can be utilized for understanding efficacy of stanane layer as potential chemical sensor. For a better understanding on the changes in the electronic properties of stanane affected by gas adsorption, we have

Fig. 1. Schematic diagram of different stanane (hydrogenated stanene) monolayer structures with periodic boundary conditions (PBC): (a) Chair type (b) Stirrup type and (c) Boat type.

ECoh =

(nESn + mEH ) − ESnH n+m

(2)

where ESnH is the total energy of stanane, EH and ESn are the energies of hydrogen and Sn atoms, respectively. Fig. 2 shows band structure of (a) Chair (b) Stirrup and (c) Boat type stanane. The conduction band minimum (CBM) and valence band maximum (VBM) are easily be distinguishable in all cases. It may be noted that for the chair type structure, a direct band gap of 0.58 eV is found through the PBE calculation. Stirrup type structure is noticed to have both direct and indirect band gap, viz. 1.41 eV direct gap at Г-point and indirect band gap of 0.81 eV between Г-point (VBM) and Y-point (CBM). The boat type

Fig. 2. Calculated energy band structure of different stanane monolayer structures: (a) Chair type (b) Stirrup type and (c) Boat type. 102

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Table 1 The symmetries, lattice parameters (a, b, c, α, β, γ), bond parameters, cohesive energies and band gaps of chair, stirrup and boat conformations of stanane (SnH). No.

Type

Symmetry

Lattice Parameters (Å)

Bond Length (Å)

a

b

c

α

β

γ

1

Chair

P-3m1 (164)

4.09

4.09

18.86

90°

90°

120°

2

Stirrup

Pmna (63)

4.08

5.83

19.52

90°

90°

90°

3

Boat

Pmmn (59)

4.06

6.79

19.04

90°

90°

90°

SneSn SneH SneSn SneH SneSn SneH

2.85 1.72 2.88 1.73 2.88 1.73

ECoh(eV)

Eg (eV)

3.15

0.58

1.59

1.41(D), 0.81 (I)

1.55

0.22

Table 2 Binding distance (d), gas molecule angle difference (Δθ), adsorption energy (Ead), charge transfer (ΔQ) and band gap (Eg) for the most stable configurations of various adsorbents on 3 × 3 hydrogenated stanene (or stanane) supercells. The role of stanane layer (style) in each interaction with gas molecules is also reported.

Fig. 3. Calculated projected and total density of states (PDOS) spectrum of different stanane monolayer structures: (a) Chair type (b) Stirrup type and (c) Boat type.

Gas Molecules

Position

d (Å)

Δθ

Ead (eV)

ΔQ (e)

Eg (eV)

Style

NO2

A B C D E A B C D E A B C D E A B C D E

2.31 2.32 2.33 2.02 2.15 2.40 2.72 2.72 2.40 2.34 2.40 2.43 2.42 2.41 2.41 2.40 2.49 2.46 2.40 2.38

128.17 128.31 128.24 128.26 127.71 119.02 119.13 118.79 119.02 118.79 105.24 105.62 105.93 105.21 105.3 179.58 179.71 179.71 179.86 179.86

−2.633 −2.635 −2.635 −2.661 −2.651 −2.504 −2.500 −2.499 −2.504 −2.505 −2.480 −2.461 −2.460 −2.480 −2.480 −2.484 −2.475 −2.472 −2.484 −2.481

0.154 0.158 0.150 0.171 0.175 0.042 0.033 0.026 0.042 0.039 0.017 0.005 0.003 0.017 0.017 0.021 0.017 0.019 0.021 0.020

0.78

Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter Accepter

SO2

NH3

CO2

Fig. 4. Calculated phonon dispersion frequency spectra of different stanane monolayer structures: (a) Chair type (b) Stirrup type and (c) Boat type.

0.38

0.75

0.73

near the Fermi level in CBM, whereas p and d-orbital are responsible near the Fermi level in VBM. For NO2 adsorption on the SnH layer, the band gap decreases up to 0.38 eV (decrement of 0.20 eV). In such case, p and d-orbitals are shifted near the Fermi level in conduction and valence region. Furthermore, in case of the adsorption of CO2 and SO2 molecules on SnH layer, the band gap remains almost same as 0.75 and 0.73 eV, respectively with increase of about 0.20 eV from the pure chair stanane as an effect of their adsorption (Fig. 7c and d). For CO2 adsorption, high peak of orbitals hybridization can be seen for −1.5 to −2.5 eV energy region, but for SO2 (Fig. 7d) maximum peak of hybridization is ranges between −2.50 and −3.50 eV. The p-orbital is observed to be close to the Fermi level in CBM in case of CO2 adsorption, but both the p and d-orbitals are responsible to shift band lines near the Fermi level in CBM for SO2. It may be noted from Fig. 2a and Fig. 6 that except for SO2, for all other cases, the band gaps are found to be increases with respect to the pure stanane whereas the hybridization of orbital plays a critical role. The SO2 molecule having sp2 hybridization and its valence band primarily consist of 5p states of Sn atom, 2p states of O atom and 2p states of S atom near the Fermi level, and in conduction band the 2p states of S atom are having strong hybridization with 5p states of Sn atom resulting the decrease of energy band gap for SO2. On the other hand, hybridization is not much stronger for NO2, NH3 and CO2 and also it doesn't take place near to the Fermi level which provides the increase in energy band gap in those cases. According to these factual changes and hybridization of the orbitals, we looked into the past interaction of SnH layer with molecule for bond lengths between NeO, SeO, NeH and CeO of the gas molecules (NH3, NO2, CO2

Fig. 5. Top and side views of the 3 × 3 × 1 optimized structure of the most stable chair type configuration of stanane monolayer with adsorption of gas molecules (NO2, SO2, NH3 and CO2) with their various possible positions and orientations as A, B, C, D and E on the SnH monolayer. A: top of Sn-atom, B: top of H-atom, C: middle (H atom) of hexagonal structure, D: middle (Sn atom) of hexagonal structure, E: middle of SneH bond.

calculated the band structure, total density of states (DOS), local DOS projected onto adsorbents and total electronic density of the adsorbed 3 × 3 × 1 stanane by NO2, SO2, NH3 and CO2 gas molecules which are shown in Fig. 6a-(d) and Fig. 7a-(d). It may be noted that variation in the band gap of hydrogenated stanene (stanane) monolayer is observed due to gas molecules adsorption on the layer. A similar effect is also noticed for stanene as reported by Chen et al. [20] For NH3 adsorption on the layer, band lines are crossing the Fermi level in conduction region and direct energy band gap is achieved alongside on the gamma point with a slight increment of 0.2 eV with respect to the pure stanene (0.78 eV). Also, from Fig. 7a it may be noted that in PDOS the most dominating orbitals are d-orbital of Sn, p-orbital of O, Sn and N atoms 103

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Fig. 8. Adsorption energies (Ead) and charge transfer of NO2, SO2, NH3 and CO2 gas molecules on the chair type stanane monolayer. The adsorption energy and charge transfer of gas molecules on the surface of the stanane monolayer is shown with their different orientations and positions.

and SO2), angles (Δθ) OeNeO, OeSeO, HeNeH and OeCeO in the gas molecules, adsorption energy (Ead), the charge transfer between the gas molecules and monolayer of SnH (ΔQ), the band gaps (Eg) of total system (SnH layer and gas molecules) with respect to the gas molecules and those values are provided in Table 2 and shown in Fig. 8. The bond lengths of NeO, SeO, NeH and CeO are found after adsorption as 1.27 Å, 1.47 Å, 1.04 Å and 1.20 Å, respectively which are with close agreement with the work by Chen et al. [21] The negative values of adsorption energy (Ead) confirm the adsorption process of gas molecules take place on the stanane layer and are thermodynamically favourable. The negative value of Ead is observed for all the positions A, B, C, D and E (Table 2 and Fig. 8). Overall, it is observed that, the energy band gap of stanane is tuning due to the adsorption of gas molecules on hydrogenated stanene based material, and accordingly the conductivity of hydrogenated stanene monolayer should increase/decrease due to such band gap variations. The working principle of the proposed sensor is based on the working field effect transistors (FET), where the singlelayer stanane is utilized in a proposed FET for detection of the toxic/ non-toxic gas molecules. When such a molecule comes into the contact of stanane surface, the density of charge carriers (electrons and holes) gets modified and accordingly the conductivity (or resistivity) of the FET changes. Table 2 provide the charge transfer values between gas molecules and the hydrogenated stanene monolayer which is a very important factor for molecular gas sensors to understand their effectiveness. The Bader charge transfer is one of the important parameters, which confirms the transfer of charge between the gas molecules to the stanane material. The positive value of the Bader charge in the present case represents charge transfer from gas molecule to the SnH layer, whereas negative value of Bader charge refers as electron transfer from SnH to gas molecule. In the present investigation, Bader charge analysis shows that charge transfer values are positive which refers that charge transfer is executing from gas molecules to the SnH layers, which identifies SnH layer as an acceptor. It can be noticed from the Table 2 that the minimum charge transfer of 0.15e can be seen in NO2 at C-position and is maximum 0.175e at E-site. In case of SO2, charge transfer between 0.026e-0.042e is found where the maximum is observed at A position. Furthermore, charge transfer can be seen for NH3 between 0.003e0.017e and average charge transfer in CO2 is found to be 0.0196e. Overall, it is observed that when NH3 molecule placed at C position on the stanane layer, a minimum charge transfer is noticed (0.003e) compared to others, whereas maximum charge transfer (0.175e) can be found at E-position of NO2 molecule. Similar observation for NO2 is also found recently in terms of adsorption energy by Nagarajan and Chandiramouli [23]. The identified sites for more and less adsorption of energy certainly provide useful information for developing stanane based sensors to detect hazardous gases in the environment. Further, we have also computed a very important property useful for molecular gas sensor, viz. adsorption energy. For that purpose, we optimized the adsorption energy of monolayer with respect to various toxic gas molecules and depicted in Table 2. Our optimized value of adsorption energy

Fig. 6. Electronic band structures for (a) NO2, (b) SO2,(c) NH3 and (d) CO2 gas molecules interacting with the top surface of the chair type stanane structure (3 × 3 × 1 SnH monolayer) for all the gas molecules with A-type configuration.

Fig. 7. Projected and total density of states (PDOS) spectrum for (a) NO2, (b) SO2,(c) NH3 and (d) CO2 gas molecules interacting with the top surface of the hydrogenated stanene of chair structure (3 × 3 × 1 SnH monolayer) for all the gas molecules with A-type configuration.

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The most stable chair type monolayer stanane is therefore considered as the substrate to investigate adsorption process and charge transfer with NO2, SO2, NH3 and CO2 gases. It is also noticed from the calculated band structures that band structure/gap of pure stanane (0.58 eV) does changes significantly (ranging between 0.38 and 0.78 eV) with an effect of adsorption of gas molecules on its surface, confirming sensing potential of chair type monolayer stanane, and such effect can be reflected with colour changes in relevant experiments like photoluminescence and UV–Vis spectroscopy. For all the molecular-stanane interactions porbital of Sn atom is found to dominate near to the Fermi level in VBM, whereas the d-orbital of Sn atom noticed to be responsible for the major contribution at Fermi level for CBM. To understand maximum adsorption energy and charge transfer, all possible orientations (different positions A to E) of the gas molecules on the stanane (chair) surface are considered. Among all the twenty orientations (five for each gas molecules), maximum charge transfer (0.175e) between gas-stanane is found for the E-position of NO2 molecule, whereas maximum adsorption energy is found as 2.66 eV at D position of NO2 and such information will certainly be of help for their possible synthesis. The present work certainly predicts chair type monolayer stanane as a novel potential sensor for environmental toxic/non-toxic gas molecules. Fig. 9. The charge density difference diagrams for NO2, SO2, NH3 and CO2 gas molecules interacting with the top surface of the chair type stanane monolayer with E-type, D-type, A-type and A-type configurations, respectively.

Conflicts of interest The authors declare that they have no conflict of interest.

is in good agreement with previously reported values by Chen et al. [21] on the stanene monolayer with respect to toxic gas molecules as reported between −0.345 and −1.165 eV, and in an another study by Feng et al. [36] of silicene monolayer with respect to the different gas molecules they found the adsorption energy value varies from −0.17 to −2.69 eV. The study by Xia et al. [37] on germanene monolayer with respect to toxic gas molecules, the adsorption energy as reported varies from −0.13 to −1.08 eV. Furthermore, adsorption energy of gas molecules on graphene nano ribbon show between −0.18 and −2.70 eV as reported by Huang, Bing et al. [14] It is hearting to be note that the maximum adsorption energy for all the respective gas molecule viz. NO2, SO2, NH3 and CO2 in their most active sites range between 2.46 and 2.66 eV, identifying stanane (SnH) as a promising future toxic/nontoxic molecular gas sensor. The changes in band gap (structure) as well as in PDOS due to the adsorption of all the gas molecules also confirm the efficiency of the stanane monolayer as a useful molecule sensor, and such effect can be reflected with colour change in relevant experiments like photo luminescence and UV–Vis spectroscopy. To understand the nature of interaction and bonding between gas molecules and SnH sheet, we also have calculated the charge density contour for the four molecules adsorbed at the most active sites of the monolayer SnH and presented in Fig. 9. It may be noticed that for all the cases a week interaction is found between the hydrogen atoms of stanane and most electronegative centre of the gas molecules as follows: N … H (NO2), S … H (SO2), N … H (NH3) and O … H (CO2), in which N … H (NO2) is found to be strongest as supported with its maximum charge transfer (0.175e at E position) and maximum adsorption energy (2.66 eV at D position).

Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements DRR is thankful to the SERB, New Delhi, Govt. of India for financial support (Grant No. EMR/2016/005830). VK is thankful to the SERB, New Delhi for his fellowship. DRR and VK are also thankful for the High-Performance Computing facility at CDAC, Pune and IUAC, New Delhi. References [1] Kong, null Franklin, null Zhou, null Chapline, null Peng, null Cho, null Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [2] J. Klinovaja, D. Loss, Spintronics in MoS2 monolayer quantum wires, Phys. Rev. B 88 (2013) 075404, , https://doi.org/10.1103/PhysRevB.88.075404. [3] K.S. Novoselov, V.I. Fal′ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192, https://doi.org/10.1038/ nature11458. [4] S.Z. Butler, S.M. Hollen, L. Cao, Y. Cui, J.A. Gupta, H.R. Gutiérrez, T.F. Heinz, S.S. Hong, J. Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V.V. Plashnitsa, R.D. Robinson, R.S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M.G. Spencer, M. Terrones, W. Windl, J.E. Goldberger, Progress, challenges, and opportunities in two-dimensional materials beyond graphene, ACS Nano 7 (2013) 2898–2926, https://doi.org/ 10.1021/nn400280c. [5] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669, https://doi.org/10.1126/science.1102896. [6] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197, https://doi.org/10.1038/nature04233. [7] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652, https://doi.org/10.1038/nmat1967. [8] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asensio, A. Resta, B. Ealet, G. Le Lay, Silicene: compelling experimental evidence for graphenelike two-dimensional silicon, Phys. Rev. Lett. 108 (2012) 155501, , https:// doi.org/10.1103/PhysRevLett.108.155501. [9] F. Zhu, W. Chen, Y. Xu, C. Gao, D. Guan, C. Liu, D. Qian, S.-C. Zhang, J. Jia, Epitaxial growth of two-dimensional stanene, Nat. Mater. 14 (2015) 1020, https:// doi.org/10.1038/nmat4384. [10] L. Li, S. Lu, J. Pan, Z. Qin, Y. Wang, Y. Wang, G. Cao, S. Du, H.-J. Gao, Buckled germanene formation on Pt(111), Adv. Mater. 26 (2014) 4820–4824, https://doi. org/10.1002/adma.201400909.

4. Conclusions A detail investigation for understanding the potential of stanane as a novel sensor for detection of environmental toxic/non-toxic gas molecules, viz. NO2, SO2, NH3 and CO2 is performed under density functional framework. It is found that among all the possible configurations of monolayer stanane, viz. chair, stirrup and boat, the chair form is noticed to be dynamically most stable through phonon dispersion analysis. The dynamical stability of chair type structure is also supported with its cohesive energy (3.15 eV) which is found to be almost double compared to stirrup (1.59 eV) and boat (1.55 eV) conformations. 105

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[11] B. Feng, Z. Ding, S. Meng, Y. Yao, X. He, P. Cheng, L. Chen, K. Wu, Evidence of silicene in honeycomb structures of silicon on Ag(111), Nano Lett. 12 (2012) 3507–3511, https://doi.org/10.1021/nl301047g. [12] S. Cahangirov, M. Topsakal, E. Aktürk, H. Şahin, S. Ciraci, Two- and one-dimensional honeycomb structures of silicon and germanium, Phys. Rev. Lett. 102 (2009) 236804, , https://doi.org/10.1103/PhysRevLett.102.236804. [13] R. Chandiramouli, A. Srivastava, V. Nagarajan, NO adsorption studies on silicene nanosheet: DFT investigation, Appl. Surf. Sci. 351 (2015) 662–672, https://doi.org/ 10.1016/j.apsusc.2015.05.166. [14] S.M. Aghaei, M.M. Monshi, I. Calizo, A theoretical study of gas adsorption on silicene nanoribbons and its application in a highly sensitive molecule sensor, RSC Adv. 6 (2016) 94417–94428, https://doi.org/10.1039/C6RA21293J. [15] M.B. Rahmani, M.H. Yaacob, Y.M. Sabri, Hydrogen sensors based on 2D WO3 nanosheets prepared by anodization, Sensor. Actuator. B Chem. 251 (2017) 57–64, https://doi.org/10.1016/j.snb.2017.05.029. [16] J.O. Sofo, A.S. Chaudhari, G.D. Barber, Graphane: a two-dimensional hydrocarbon, Phys. Rev. B 75 (2007), https://doi.org/10.1103/PhysRevB.75.153401. [17] Y. Li, Z. Chen, XH/π (X = C, Si) interactions in graphene and silicene: weak in strength, strong in tuning band structures, J. Phys. Chem. Lett. 4 (2013) 269–275, https://doi.org/10.1021/jz301821n. [18] Y. Shaidu, O. Akin-Ojo, First principles predictions of superconductivity in doped stanene, Comput. Mater. Sci. 118 (2016) 11–15, https://doi.org/10.1016/j. commatsci.2016.02.029. [19] P. Lu, L. Wu, C. Yang, D. Liang, R. Quhe, P. Guan, S. Wang, Quasiparticle and optical properties of strained stanene and stanane, Sci. Rep. 7 (2017) 3912, https:// doi.org/10.1038/s41598-017-04210-w. [20] V. Nagarajan, R. Chandiramouli, Interaction of alcohols on monolayer stanane nanosheet: a first-principles investigation, Appl. Surf. Sci. 419 (2017) 9–15, https:// doi.org/10.1016/j.apsusc.2017.05.017. [21] X. Chen, C. Tan, Q. Yang, R. Meng, Q. Liang, M. Cai, S. Zhang, J. Jiang, Ab initio study of the adsorption of small molecules on stanene, J. Phys. Chem. C 120 (2016) 13987–13994, https://doi.org/10.1021/acs.jpcc.6b04481. [22] X. Li, S. Zhang, F.Q. Wang, Y. Guo, J. Liu, Q. Wang, Tuning the electronic and mechanical properties of penta-graphene via hydrogenation and fluorination, Phys. Chem. Chem. Phys. 18 (2016) 14191–14197, https://doi.org/10.1039/ C6CP01092J. [23] V. Nagarajan, R. Chandiramouli, Adsorption behavior of NH3 and NO2 molecules on stanene and stanane nanosheets – a density functional theory study, Chem. Phys. Lett. 695 (2018) 162–169, https://doi.org/10.1016/j.cplett.2018.02.019. [24] R.G. Parr, Density functional theory of atoms and molecules, Horizons of Quantum Chemistry, Springer, Dordrecht, 1980, pp. 5–15, , https://doi.org/10.1007/978-94009-9027-2_2. [25] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli,

[26]

[27]

[28]

[29] [30] [31]

[32]

[33]

[34] [35]

[36]

[37]

106

G.L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A.P. Seitsonen, A. Smogunov, P. Umari, R.M. Wentzcovitch, QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials, J. Phys. Condens. Matter 21 (2009) 395502, , https://doi.org/10.1088/0953-8984/21/39/395502. C.-L. Lin, R. Arafune, K. Kawahara, M. Kanno, N. Tsukahara, E. Minamitani, Y. Kim, M. Kawai, N. Takagi, Substrate-induced symmetry breaking in silicene, Phys. Rev. Lett. 110 (2013) 076801, , https://doi.org/10.1103/PhysRevLett.110.076801. A. Kokalj, XCrySDen—a new program for displaying crystalline structures and electron densities, J. Mol. Graph. Model. 17 (1999) 176–179, https://doi.org/10. 1016/S1093-3263(99)00028-5. X. Luo, E. Zurek, Crystal structures and electronic properties of single-layer, fewlayer, and multilayer GeH, J. Phys. Chem. C 120 (2016) 793–800, https://doi.org/ 10.1021/acs.jpcc.5b11770. Y. Ding, Y. Wang, Electronic structures of silicene fluoride and hydride, Appl. Phys. Lett. 100 (2012) 083102, , https://doi.org/10.1063/1.3688035. D.K. Samarakoon, X.-Q. Wang, Chair and twist-boat membranes in hydrogenated graphene, ACS Nano 3 (2009) 4017–4022, https://doi.org/10.1021/nn901317d. B. van den Broek, M. Houssa, E. Scalise, G. Pourtois, V.V. Afanas‘ev, A. Stesmans, Two-dimensional hexagonal tin: ab initio geometry, stability, electronic structure and functionalization, 2D Mater. 1 (2014) 021004, , https://doi.org/10.1088/ 2053-1583/1/2/021004. V. Nagarajan, R. Chandiramouli, TeO2 nanostructures as a NO2 sensor: DFT investigation, Comput. Theor. Chem. 1049 (2014) 20–27, https://doi.org/10.1016/j. comptc.2014.09.009. L. Liu, Q. Yang, H. Ye, X. Chen, G. Zhang, Adsorption of gases on monolayer GeSe: a first principle study, 2017 18th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE), 2017, pp. 1–5, , https://doi.org/10.1109/EuroSimE. 2017.7926280. Gas adsorption on MoS2 monolayer from first-principles calculations, Chem. Phys. Lett. 595–596 (2014) 35–42, https://doi.org/10.1016/j.cplett.2014.01.043. T.P. Kaloni, R.P. Joshi, N.P. Adhikari, U. Schwingenschlögl, Band gap tunning in BN-doped graphene systems with high carrier mobility, Appl. Phys. Lett. 104 (2014) 073116, , https://doi.org/10.1063/1.4866383. J. Feng, Y. Liu, H. Wang, J. Zhao, Q. Cai, X. Wang, Gas adsorption on silicene: a theoretical study, Comput. Mater. Sci. 87 (2014) 218–226, https://doi.org/10. 1016/j.commatsci.2014.02.025. W. Xia, W. Hu, Z. Li, J. Yang, A first-principles study of gas adsorption on germanene, Phys. Chem. Chem. Phys. 16 (2014) 22495–22498, https://doi.org/10.1039/ C4CP03292F.