Defect and Substitution-Induced Silicene Sensor to Probe Toxic Gases

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Defect and Substitution Induced Silicene Sensor to Probe Toxic Gases Tanveer Hussain, Thanayut Kaewmaraya, Sudip Chakraborty, and Rajeev Ahuja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08973 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

Defect and Substitution Induced Silicene Sensor to Probe Toxic Gases T. Hussain*1, T. Kaewmaraya2, 5, S. Chakraborty*3 and R. Ahuja3, 4 1

Centre for Theoretical and Computational Molecular Science,

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Qld 4072, Australia 2

Institut d’Électronique Fondamentale, UMR 8622, Université Paris-Sud, 91405 Orsay, France

3

Condensed Matter Theory Group, Department of Physics and Astronomy,

Box 516, Uppsala University, S-75120 Uppsala, Sweden 4

Applied Materials Physics, Department of Materials and Engineering,

Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden 5

Department of Physics, Faculty of Science, Khon Kaen University, 40002 Khon

Kaen, Thailand

[email protected] +61 7 33463976

Abstract: Structural, electronic and gas sensing properties of pure, defected and substituted silicene monolayer has been studied using first principles calculations based on density functional theory. The spin-polarized calculations with van der Waal’s effect into consideration have been revealed that the pristine silicene sheet rarely adsorbs the CO2, H2S and SO2 gas molecules, which restricts the gas sensing application of this two-dimensional material. However, inducing vacancy defect in silicene changes drastically the electronic properties and as a consequence it also improves the binding of exposed gas molecules significantly. Our Bader charge analysis reveals that a considerable amount of charge is being transferred from the defected silicene to the gases resulting in binding energy improvement between silicene and the gas molecules. The change in binding energies has further been explained by plotting density of states. In addition to the vacancy defects, we have also considered the

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substitution of Al, B, N and S in silicene. We could find the sensing propensity of silicene is more sensitive to the vacancy defect as compared to the impurities.

1. Introduction: Since the successful exfoliation of graphene in 2004, two-dimensional nanostructures have been extensively investigated and several ultra-thin novel materials are discovered.1, 2 In recent years, these materials have become promising candidates for a wide range of applications due to their unique electronic properties.3-9 Gas sensing is one such paramount application because the constant development of gas sensors with more efficiency is going on for the last few decades in the scientific community. Due to the large surface to volume ratio, different gas molecules can be exposed to a greater sensing area of such two dimensional nano-structures.10-15 Graphene has been investigated both theoretically and experimentally as an efficient gas sensing material16-20, but the constant growth of it over a large surface area creates an urge for exploring other 2D materials to be experimentally synthesized and applied for gas sensing devices. One such alternate of graphene is the silicon counterpart named silicene, which was experimentally synthesized as 2D buckled honeycomb structure over a large surface area.21 One important difference between graphene and silicene is the respective hybridization that is sp2 in case of graphene while for silicene we can see sp2-sp3 mixed hybridization.22 The compatibility of silicene with the silicon-based nanotechnology is another prime reason of using this material for numerous applications. Under an external electric field the band gap tuning and surface reactivity change with different adsorbate has been found more profoundly in case of silicene as compared to grapheme.23,

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These distinctive

features facilitate silicene to be an appropriate candidate for sensing different gases while adsorbing them, the surface reactivity as well as the band gap changes drastically. To date, silicene with different type of defects for sensing gas has not been envisaged. This is important as defected silicene with more surface reactivity will be of much importance in order to sense gas molecules. Here we have investigated systematically from electronic structure calculations with mono, di ad tri-vacancies in silicene to detect H2S, SO2 and CO2 gases. The toxic hydrogen sulphide (H2S) gas is generally found in fuel cell and by detecting it efficiently, one can prevent the catalyst


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poisoning. Sulfur dioxide (SO2) is also a highly toxic gas and can affect drastically respiratory system through inhalation. The greenhouse gas CO2 is the prime reason behind the global warming and the emission of CO2 should be efficiently detectable using sensor devices. In this work, we are going to envisage silicene-based materials modeling for gas sensing that can detect H2S, SO2 and CO2 with higher sensitivity. It would be interesting to see how these gases can adsorb on defected silicene. For comparison purpose, we have also functionalized silicene with B, Al, N and P and try to adsorb these gases as well. This theoretical investigation would be necessary in order to guide the experimentation as a prior knowledge about defected and functionalized silicene for sensing different gases. We have chosen these three gases from the perspective of environmental issues as mentioned earlier. The sensing properties of the surface for the different gases would be evaluated primarily from band gap change and also from the adsorption energy for the respective cases. The outcome of our work reflects that the defected and functionalization of silicene can certainly increase the surface-adsorbate interaction and therefore it would help improve the sensing activity for silicene based sensor devices.

2. Computational details: All the calculations presented in this work are carried out by VASP code25-27 within the framework of spin-polarized density functional theory. The exchange-correlation functional is approximated by the gradient-corrected form of Perdew–Burke– Ernzerhof (PBE).28 Projector augmented wave (PAW) method is used to treat electron-ion interactions.29 Van der Waals correction by Grimme approach has been taken into account to describe long-range interactions.30 The freestanding silicene is modelled by 4×4×1 supercell containing 32 Si atoms. The vacuum width of 15 Å along the z-direction is used to negate the possible interactions between the periodically repeated unit cells. Here the Brillouin zone integration is sampled using Monkhorst-Pack grid of 5x5x1 (for structural relaxations) and 15×15×1 (for density of states calculations).31 A cut off energy of 500 eV is used and the structural relaxations are terminated when the residual forces acting on each ion becomes less than 0.01eV/Å. The charge analysis has been performed by using Bader analysis.56 The binding energies of the incident gases on the silicene monolayers has been evaluated using the following relation


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Eb = E (Silicene: X) – E (Silicene) –E (X)

(1)

Where, X=CO2, H2S and SO2 In Eq. (1) 1st, 2nd and 3rd term represents the total energies of silicene sheet with adsorbed gas molecules, pristine silicene sheet and the individual gas molecules, respectively. Additionally, the formations energies of the vacancy defects have been determined by the following formalism 48 Ef = E (defected) – {(x-y)/x} E pristine

(2)

In equation 2, Ef denotes the formation energy. E

defected

and E

pristine

are the total

energies of the defected and pristine systems respectively, x and y are the number of atoms in the silicene monolayer and the number of atoms removed from the monolayer respectively.

3. Results and discussion: The calculated lattice constant (3.86 Å) and Si-Si bond length (2.27 Å) are in good agreement with the previous investigations.32 The optimized structure of pristine silicene sheet is shown in fig.1 (a). Silicene monolayer consists of four sites that can be exposed to bind the gas molecules. We start with the case of CO2 adsorption on silicene monolayer, which is one of the most important greenhouse gases and has precarious effects on the environment. The CO2 molecule can bind on a specific site either horizontally (C-directed), vertically (O-directed) or as a tilted molecule. To find the most preferential binding configuration, we have studied all the possible scenarios and calculated the binding energy in each case. It has been found that CO2 prefers to adsorb horizontally on silicene with relatively weak adsorption energy of -0.09 eV, which is smaller than that of graphene,33 germanene34 and ZnO-sheet 9 with relatively large silicene-CO2 distance of 3.44 Å. For H2S, which is an extremely lethal gas at concentration of higher than 250 ppm,35 the most stable configuration is the one with H atoms of H2S pointing towards silicene monolayer at a distance of 3.19 Å and binding energy of -0.19 eV. Though the binding is smaller than Au doped graphene,36 hydrogenated graphene37 and graphene nanoribbons 38 but it is marginally larger than that of pristine graphene 39, 40 Full structural relaxation shows slight elongation in H-S bond from 1.34 to 1.35 Å and reduction in H-S-H angle from 92.10 to 91.30. Another important sulphur


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containing gas, SO2, which is the main component of industrial waste materials and have toxic effect particularly hazardous for respiratory as well as heart diseases.41, 42 The

minimum energy configuration of SO2 is having the binding energy of -0.80 eV

and the molecule is at a distance of 1.80 Å from the silicene monolayer. This binding energy is sufficiently higher than the corresponding values on SWCNT41, TiO2 surface43, and graphene sheet.44 As compared to CO2 and H2S, the structural properties of SO2 changes considerably upon full geometry relaxation that shows significant increase in S-O bond length from 1.43 Å to 1.60 Å and reduction in S-O-S bond angle from 1190 to 106.30. The lowest energy configurations of CO2, H2S and SO2 on pristine silicene have been shown in fig. 2 (a-c). The binding characteristics of CO2, H2S and SO2 on pristine silicene reveal that apart from SO2, fairly weak physisorption limits the use of this monolayer as an efficient gas sensor. A possible solution to this hindrance is to enhance the binding of the gases to be sensing and the creation of defects and foreign atom substitution are some useful routes of doing so. As mentioned earlier, we consider the vacancy defects in the silicene monolayer to sense the toxic gas molecules. The vacancy defects can be caused due to the exposure of laser or electron beam and they are unavoidable during the synthesis of materials, especially during the exfoliation of monolayers with significant effects on material’s properties.45 One of the major advantages of the vacancy defects is it can enrich the adsorption mechanism of the adsorbate (gas molecules in this case) with the host monolayer

39, 44, 46, 47

This essential fact motivates us to create mono, di- and tri-

vacancies in silicene sheet by removing one, two and three Si atoms respectively. For convenience we would refer mono, di- and tri- vacancy silicene by M-silicene, Dsilicene and T-silicene respectively in this work. The optimized structures of Msilicene, D-silicene and T-silicene sheets are shown in fig.1 (b-d). The stabilities of such vacancy defects up to six missing atoms have already been studied and reported earlier based on first principles calculations

45

The formation energies of vacancy

defects determined using eq. 2 are 3.62 eV, 4.32 eV and 5.79 eV for mono, di -and trivacancies respectively. These values are in good agreement with previous study 48 We now proceed towards the adsorption of selected gases with the M-silicene sheet. The procedure of finding the most stable configuration has been repeated again by introducing the gas molecules at various binding sites. The CO2 chemisorbs with binding energy of -1.64 eV, which is remarkably 17 times greater than its value on


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pristine sheet. The surface-adsorbate distance is considerably reduced to 1.70 Å, which is around 51 % less than that of the defect-free pristine sheet. A structural deformation also occurs in CO2 molecules with one C-O bond length of 1.21 Å and another of 1.38 Å and O-C-O angle of 119.40. A noteworthy improvement of CO2 binding energy can be attributed to the charge transfer mechanism between the Msilicene and the corresponding molecule. Being relatively more electronegative, C atom of CO2 attracts charge from Si atoms of M-silicene. According to Bader charge analysis, quantitatively around 2.34 electronic charge has been transferred from the Si atoms those have dangling bonds due to mono vacancy and are in close vicinity to the CO2 molecule. A charge distribution also occurs within the CO2 molecules, where one of the O atoms gathers more charge than the other resulting into the difference in C-O bond lengths as mentioned earlier. The minimum energy configuration is shown in fig. 2 (d) The lowest-energy configuration for the case of H2S gas adsorbed on M-silicene has been shown in fig. 2 (e) with binding energy of -0.98 eV and adsorption distance of 2.3 Å. The binding energy of H2S is comparatively five times higher and the adsorption distance is 36% less on M-silicene than on pristine one. Although less pronounced than CO2 but still H2S molecule undergoes structural change with the enlargement of H-S bond length from 1.34 Å to 1.37 Å and H-S-H angle of 92.10 to 92.50. Here both H and S atoms are more electronegative than Si causing a small amount of charge around 0.52 e- to be migrated from M-silicene to the molecule. Clearly much lesser amount of charge is transferred from M-silicene to H2S than to CO2. This is one of the reasons for smaller increment in binding energy of the former as compared to the later case. In the case of SO2, a series of comprehensive structural optimization sampled from numerous initial geometries, similar to the other two molecules, leads to the lowestenergy configuration depicted in fig. 2 (f). This configuration possesses a binding energy of -2.87 eV, which is the highest value among all the gases considered in this work with a corresponding surface-adsorbate distance of 1.74 Å. As seen from fig. 2 (f), SO2 molecule rearranges its geometry on M-silicene with the elongation of one of the S-O bond angles to 1.62 Å and another to 1.46 Å from the initial value of 1.43 Å. The O-S-O angle also changes and reduces considerably from 1190 to 106.70. Similar to the case of CO2 and H2S, here the charge is also transferred from M-silicene


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towards SO2 molecule due to the higher electronegativity of the later. A careful examination shows that 0.86 e- of the charge migrates from M-silicene towards SO2 molecule. Though the introduction of mono vacancy improved the adsorption mechanism of the studied gases, however the scenario of di- vacancy has been found less promising. Adsorption energies of -0.47 eV, -0.48 eV and -0.34 eV have been obtained on Dsilicene in case of CO2, H2S and SO2 and their respective lowest energy configurations have been shown in fig 2 (g-i) respectively. Here the adsorption energies of CO2 and H2S increase a little as compared to the pristine silicene but in the case of SO2 the adsorption energy is 57% reduction lower. The corresponding adsorption distances of CO2, H2S and SO2 with D-silicene are 1.93 Å, 2.39 Å and 2.66 Å respectively, which are higher than those of mono vacancy silicene for the obvious reason of binding energy difference. The third type of defect case is the tri- vacancy silicene (T-silicene) and the most stable configurations with the corresponding adsorbed gases have been shown in fig 2 (j-l). The trend in binding energies show a significant improvement as compared to pristine sheet with the values of -1.31 eV, -2.13 eV and -1.90 eV for CO2, H2S and SO2 respectively. The binding distances of CO2, H2S and SO2 with the nearest silicene atoms in T-silicene are 1.71 Å, 1.50 Å and 1.70 Å respectively. Similar to the case of M-silicene, here also a strong adsorption of the gas molecules causes the structural changes in the adsorbed gases. For CO2 one of the bond lengths, having O atom of CO2 close to the Si in T-silicene, elongates more with a value of 1.40 Å than the other with 1.22-Å-bond length. The O-C-O bond angle also increase a little to 117.90. A substantial amount (2.3 e-) of charge is transferred from T-silicene to CO2 molecules. The H2S molecule undergoes a dissociated adsorption with one of the H atoms dislodges itself and the other remain in contact with H-S bond length of 1.36 Å, that is slightly larger than it its value (1.34 Å) as an isolated molecule. Significant amount of charge (2.2 é) transferred from the T-silicene to the molecule could have caused the dissociation of H2S, which also resulted into a drastic improvement in adsorption energy of -2.13 eV in T-silicene. However the SO2 molecule preserves its molecular geometry similar to (like CO2) upon its adsorption on the T-silicene with S-O bond lengths of 1.71 Å and 1.49 Å. Similar to CO2, one of the O atoms of SO2 attracted more towards the Si of the T-silicene and ended up having larger bond length than the


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other. There is significant reduction in O-S-O bond length from free state bond angle (1190) to 106.60. In addition to the vacancy defects, we have also investigated the substitution of foreign atoms into silicene monolayer to study the impurity effects on the adsorption behavior of gas molecules. The dopants considered here includes Al, B, N and S atoms, which have already been substituted in silicene sheet for different purposes 50

49,

Though all of these dopants with the doping concentration of 3.12% make stable

structures in silicene, however their existence do not make any appreciable difference in the adsorption mechanism. There is hardly an increment in the binding energies of the gas molecules on Al, B, N and S-doped silicene monolayer with the exception of SO2, which shows reasonable adsorption energy (-2.26 eV) in case of N-doped silicene. To have more profound understanding of the gas molecules adsorbed by the different form of silicene monolayer, the density of states (DOS) have been determined and analyzed. Fig. 3 depicts the DOS of the defect-free silicene adsorbing the gas molecules. One can see that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CO2 and H2S are located away from the Dirac point (Fermi level). Consequently, the Dirac-cone feature of silicene remains unchanged when CO2 and H2S molecules are adsorbed. Hence, they are weakly physisorbed on silicene, resembling the case of CO2 adsorbed on germanene. 34

On the other hand, the electronic property of silicene is greatly altered by the

adsorption of SO2 molecule. This adsorption induces the structural deformation that yields symmetry breaking via the strong covalent Si-S bond. Indeed, the Dirac cone character is completely vanished, revealing that the SO2 molecule is strongly chemisorbed. In addition, there are states near Fermi level, which signifies the p-type doping of silicene by the adsorption of SO2, as supported by the considerable charge being transferred from silicene to SO2. Interestingly, the reactivity of SO2 on silicene is remarkably more pronounced than that on graphene.44 This manifests that Psilicene is more appropriate for sensing SO2 than P-graphene. From the practical point of view, the adsorption energy of SO2 molecule on silicene is moderate and one can desorb it by means of heating. Therefore, silicene is a potential candidate as SO2 gas sensor.


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It is useful to analyse the DOS (as shown in fig. 4) in order to gain fundamental insight regarding these higher adsorption energies induced by the defect formation in silicene monolayer. Unlike P-silicene, the adsorption of CO2 or H2S on M-silicene leads to the substantial modifications in the DOS. This is the consequence of the strong chemisorption of C-Si and S-Si bonds. For adsorption of SO2, there is intensive hybridization of Si-2p and S-2p near Fermi level, which is responsible for the extremely high adsorption energy for this case. Note that M-silicene has become semiconducting after adsorption. This is because of the creation of a single point defect in silicene that results into the localized states in the vacancy and the linearly crossing bands at K-point have opened the band gap due to the symmetry breaking induced by the presence of vacancy 51 Meanwhile, adsorption of gas molecule on D-silicene exhibits decrement in the adsorption energies. This behavior can be due to the inactiveness of D-silicene.

51

The crystal geometry of silicene with double vacancies contains a ring of eight Si atoms, which is surrounded by two pentagons and six hexagons. This character remains visible, shown in fig. 1 (c), although there is a slight elongation of a Si-Si bond in the hexagon where the gas molecule is bonded. There is less hybridization of D-silicene with CO2 or H2S near Fermi level, which implies the smaller adsorption energies. Interestingly, SO2 gets dissociated on D-silicene, where the S and O atoms are located at the positions of missing Si atoms. Therefore, there is a reasonable hybridization of Si-O and Si-S near the Fermi level. The strong adsorption energies are recovered for the gas molecules adsorbed on T-silicene because of the regained chemical activeness of the triple vacancies.49 The presence of trivacancies yields an odd number of dangling bonds. They are subsequently rearranged to form other bonds where each Si atom is surrounded by three atoms. There is one sp2 dangling bond left from which the Si atom possesses the coordination number of two. This dangling bond binds to C or S atom of the adsorbates. As a result, there is hybridization of these molecules with the defected substrate that yields the bands crossing the Fermi level. It is worth noting that P-silicene is strongly sensitive to SO2 molecules with the moderate adsorption energy that is probably applicable for reversible sensing devices due to rapid recovery time upon desorption. However, the defected silicene broadly provides the extremely strong adsorption energies, which are practically


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elusive because of the long recovery time.52 Therefore, defected silicene, could be exploited as irreversible sensing devices. The results presented in the present study complement the sensing characteristics of various pollutants on 2D monolayers. 53-55 Conclusion: In conclusion, we have found the point defects in silicene change the corresponding electronic properties and the binding characteristics improve dramatically. In case of M-silicene, the binding energies of CO2 and H2S improve 17 and 5 times respectively as compared to their values on pristine sheet. Although the higher vacancies improve the corresponding binding energies, however their effects are less pronounced than that of M-silicene with D-silicene is being the least effective case. Because of the less electronegativity of silicon compared to the other elements in the considered systems, Si atoms in defected silicene donate a significant amount of their electronic charges to the gas molecules that resulted into the enhancement of the binding between silicone and gas molecules. The electronic properties depicted by the density of states of the monolayer system with different defects and substitutions are changed as a consequence of the charge transfer mechanism. The functionalization of silicene with Al, B and S dopants do not seem to change the binding energies for the three molecules much except for N-doped silicene that enhances the adsorption of SO2. Acknowledgement: We thank the University of Queensland for support of this project through the UQ Postdoctoral Fellowship Scheme. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme.

References:


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Fig.1 Top view of optimized structures of (a) pure silicene (b) mono vacancy (c) di vacancy and (d) tri vacancy.


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Fig. 2 Top and side views of optimized structures of pure silicene with (a) CO2 (b) H2S (c) SO2, mono vacancy silicene with (d) CO2 (e) H2S (f) SO2, di vacancy silicene with (g) CO2 (h) H2S (i) SO2 and tri vacancy silicene with (j) CO2 (k) H2S (l) SO2 respectively.


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Blue, brown, red, yellow, and pink balls represent Si, C, O, S and H atoms respectively.

Fig. 3 Density of states (DOS) of the defect-free silicene adsorbed by a) CO2, b) H2S and SO2 molecule. The black shaded area represents the total DOS whereas the blue, red and yellow colours account for the projection of DOS onto CO2, H2S and SO2 molecule, respectively. Here, the Fermi level is shifted to zero.


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Fig. 4 Density of states (DOS) of the defected silicene adsorbed by a) CO2, b) H2S and SO2 molecule. The black shaded area represents the total DOS whereas the blue, red and yellow colours account for the projection of DOS onto the molecule. Here, the Fermi level is shifted to zero.


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Fig.5 Van der Waal’s induced adsorption energies of incident gases on pure, defected and substituted silicene


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TOC: Improvement in sensing characteristics of silicene by the introduction of vacancy defects


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Defect and Substitution-Induced Silicene Sensor to Probe Toxic Gases

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