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Adsorption of COCl2 gas molecule on armchair boron nitride nanoribbons for nano sensor applications
Microelectronic Engineering 146 (2015) 62–67
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Adsorption of COCl2 gas molecule on armchair boron nitride nanoribbons for nano sensor applications Pankaj Srivastava a,⇑, Varun Sharma a, Neeraj K. Jaiswal b a b
Nanomaterials Research Group, ABV-Indian Institute of Information Technology and Management (IIITM), Gwalior 474015, India Discipline of Physics, PDPM-Indian Institute of Information Technology Design and Manufacturing (IIITDM), Jabalpur 482005, India
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 2 March 2015 Accepted 19 March 2015 Available online 26 March 2015 Keywords: Boron nitride Nanoribbons Electronic band structure Transmission spectra Sensor
a b s t r a c t First-principle calculations under the framework of density functional theory have been performed to study the adsorption of COCl2 (Phosgene) gas molecule on armchair boron nitride nanoribbon (ABNNR). Depending upon the geometry of the ribbon, we have considered two possible cases for adsorption of guest molecule i.e. adsorption at closed edge and adsorption at open edge. Adsorption energy (Ead) calculations implies that adsorption at open edge is energetically more favorable compared to closed edge case. The formation of closed ring structure at the open edge in this case also accounts for its stability. Electronic properties reveal that with the adsorption of COCl2 molecule on ABNNR the semiconducting behavior remains intact but a significant modulation in the band gap is observed as compared to bare ribbon. The current voltage (I–V) characteristics have also been investigated using non-equilibrium green’s function (NEGF) approach. The results indicate towards the potential applicability of ABNNR for sensing the toxic COCl2 gas. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The existence of graphene, a pure 2-dimensional crystal [1], has opened a wide area for research community to explore the related inorganic 2-D geometries. Due to its exotic physical properties [2,3], synthesizing some other inorganic 2-D crystals [4–9] is always an unquenched thrust. Such an analogically similar geometry that has recently gained a significant research interest is the hexagonal boron nitride sheet [10,11]. Both theoretical and experimental demonstrations have been done to explore some of its major properties [11–16]. These are somewhere in contrast to graphene [17–19] but can be exploited for various suitable applications. Similar to graphene nanoribbons (GNR), boron nitride nanoribbons (BNNR) can also be classified into two configurations viz. zigzag boron nitride nanoribbons (ZBNNR) and armchair boron nitride nanoribbons (ABNNR). Investigations highlighting the width dependency of BNNRs for both the configurations have already been discussed. Previous efforts described that the band gap of ZBNNR decreases with increasing the ribbon width whereas an oscillatory behavior was observed for the armchair configuration [15]. On account of large intrinsic band gap, its applicability in electronic devices is still limited. The larger band gap is due to the ionic nature of the bond between boron and nitrogen. ⇑ Corresponding author. E-mail address: [email protected] (P. Srivastava). http://dx.doi.org/10.1016/j.mee.2015.03.040 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
However, functionalization and substitutional doping have modulated the band gap to an extent but still the electronic properties for BNNR are not yet explored on a major scale like that of GNR. Earlier findings also suggest that the change in the electronic properties of nanoribbons are mainly attributed to the ribbons edges as they are more reactive as compared to the central part [20,21]. Due to the presence of the dangling bond at the ribbon edge, they are more susceptible to the molecule adsorption or the passivation. Apart from the electronic properties, the exploration of transport property is also an area that is yet to be explored for its suitable applications in future devices [22]. The existing literature related to the studies of transport properties are primarily concerned with the zero bias calculation [21]. Thus, in order to have the complete insight to the physical properties of BNNR a comprehensive study is required. Modulation in the electronic properties by its functionalization through fluorine is reported by Lu et al. [23]. It was revealed that fluorination causes small band gap (direct) semiconducting behavior for both ZBNNR and ABNNR cases. On the other hand, chlorine adsorption reported turns ZBNNR semiconducting [24]. Such an observed behavior is highly suited for its applicability as sensors as suggested for GNR [25–30]. Inspiring from this, the present work focuses on the sensing ability of the BNNR for COCl2 (Phosgene) gas. The sensing of COCl2 is also important as it is a toxic gas. Based on the first-principles calculations, we have explored the most stable configuration for the adsorption of COCl2 molecule on the bare BNNR edges. It is shown here that
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P. Srivastava et al. / Microelectronic Engineering 146 (2015) 62–67
Fig. 1. The schematic model of optimized supercell of ABNNR for (a) bare (b) COCl2 molecule adsorbed at closed edge (c) COCl2 molecule adsorbed at open edge with guest molecule approaching perpendicular to the ribbon (ABNNR–COCl\ 2 ) (d) COCl2 molecule adsorbed at open edge with guest molecule approaching parallel to the ribbon (ABNNR–COClk2). Here pink, blue, red, gray, green spheres represents B, N, O, C, Cl atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated bond lengths and adsorption energy for the respective configurations. Particulars
B–N
B–O
O–C
O–N
C–Cl1
C–Cl2
Ead
ABNNR–COCl2 adsorption at closed edge ABNNR–COCl2 adsorption at open edge (ABNNR–COCl\ 2) ABNNR–COCl2 adsorption at open edge (ABNNR–COClk2)
1.35 Å 1.42 Å 1.42 Å
1.58 Å 1.58 Å 1.36 Å
1.24 Å 1.23 Å 1.36 Å
– – 1.44 Å
1.71 Å 1.52 Å 1.77 Å
1.70 Å 1.67 Å 1.78 Å
1.058 eV 3.085 eV 6.328 eV
how the electronic and transport properties changes in accordance with adsorbed molecule. These changes can be further deployed for the application of BNNRs as the nanosensors application.
2. Computational details In the present work we have used first principles approach under the framework of density functional theory (DFT) to perform all the calculations using Atomistic ToolKit Virtual NanoLab (ATKVNL) [31]. The calculations for electronic and transport properties of the COCl2 adsorbed ribbon were performed on the optimized structures with norm-conserving pseudo potentials for describing the ions and linear combinations of atomic orbitals as basis sets. The ribbons under investigation were considered periodic along Z-axis but are confined in the other two transverse directions (X and Y). To account for exchange–correlation effects, we have used generalized gradient approximation (GGA) [32]. Furthermore, as a suitable tradeoff between accuracy and computational overhead we have selected a higher energy mesh cutoff of 100 Ryd with 1 1 100 k-point sampling for the Brillouin zone integration. All the atoms in considered geometries are fully relaxed to change their individual position until the force convergence criteria of 0.05 eV/Å is achieved. In order to avoid any Columbic interactions between the periodic images of ribbon in the transverse directions, a large vacuum space of 10 Å is considered. Two-probe configurations of the optimized geometries were used for the calculation of the transport properties. The
configuration consists of three regions viz. scattering (central) region, left electrode and right electrode. In the current manuscript, we have used 6 repetition of the ABNNR supercell in the central region which seamlessly couples with the left and right electrode. Higher mesh cutoff is set to 150 Ryd for the transport calculations. Earlier study also shows the successful implementation of this model for studying the electronic and transport properties at nano regime [33–35]. ATK-VNL uses DFT coupled with non equilibrium green’s function (NEGF) approach for transport property calculations. As a part of transport properties calculations, first the transmission spectra of the relaxed geometries are calculated. The transmission spectra are further utilized for the transmission coefficient using the relation [31].
TðE; VÞ ¼ T r ½sR ðE; VÞGC ðE; VÞsL ðE; VÞGþC ðE; VÞ
ð1Þ
Here, sL=R ðE; VÞ are the coupling coefficients for the left/right contact respectively, GC ðE; VÞ denotes the green’s function for the scattering region. To calculated the current through the scattering region the Büttiker–Landauer formula [36] is used.
IðVÞ ¼
Z lR lL
TðE; VÞ f ðE lL Þ f ðE lR Þ dE
ð2Þ
Here, lL=R are the chemical potentials of the left/right electrodes respectively.
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P. Srivastava et al. / Microelectronic Engineering 146 (2015) 62–67
(a)
(c)
(b)
(d)
Fig. 2. The calculated band structures of ABNNR for (a) bare, (b) COCl2 molecule adsorbed at B-atom (closed edge), (c) COCl2 molecule adsorbed at B-atom (open edge k (ABNNR–COCl\ 2 )), (d) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2)).
3. Results and discussion We have investigated the sensing abilities of ABNNR for the COCl2 gas molecule. First, we investigated the stability of the ABNNR with COCl2 molecule adsorbed on it. As the COCl2 molecule approaches to the vicinity of bare ABNNR, it prefers to be adsorbed at the edges rather the central region of the ribbon. Furthermore, due to more electronegative character of O-atom the COCl2 molecule prefers the adsorption via O-atom. Based on the ribbon width (Wa = 6 selected for present studies) we have presented the following possible cases viz. – adsorption at the closed edge and adsorption at the open edge. Further, to investigate the effect of guest molecule orientation we have considered two possible cases for open edge adsorption. When guest molecule approaches the ribbon perpendicular to the ribbon (ABNNR–COCl\ 2 ) and, when guest molecule approaches parallel to the ribbon (ABNNR–COClk2). Fig. 1 highlights the optimized geometries in all the cases. In order to find the relative stability amongst the cases under investigation, we have calculated the adsorption energy (Ead) of the guest molecule (COCl2) adsorbed ABNNR. The Ead is calculated using the total energies of bare and COCl2 adsorbed ABNNR using the relation Ead = E(NR + COCl2) E(NR) E(COCl2) [37], where, E(NR + COCl2), E(NR) and E(COCl2) are the total energies of nanoribbons with adsorbed molecule, free standing bare ribbon and a single isolated COCl2 molecule respectively. The calculated adsorption energy and bond lengths for considered configurations are given in Table 1. It is clear from Table 1 that open edge adsorption is energetically more stable in comparison to closed edge adsorption. Further, open edge (ABNNR–COClk2) emerges out as the most stable adsorption
with Ead = 6.33 eV followed by adsorption at open edge (ABNNR–COCl\ 2 ) with Ead = 3.085 eV and lastly adsorption at closed edge with Ead = 1.058 eV. The formation of closed ring in the case of open edge (ABNNR–COClk2) (shown in Fig. 1(d)) also depicts the stability in this case. This type of behavior is in cognizance with the previous literatures [38]. It is also evident from the Fig. 1 that there is no structural deformation in the ribbon due to the adsorption of guest molecule. Further, in all the cases the favorable site of adsorption is B-atom rather than N-atom, the energy calculations confirms the same. The well known semiconducting properties of wide band gap ABNNR still prevail even on the adsorption of COCl2, however, the band gap is modulated by the introduction of new states in the valence as well as conduction band. In order to have further insights about the electronic properties, we have investigated the band structures for all the cases and compared the results with the bare ABNNR as shown in the Fig. 2. The comparison of band structure reveals that the band gap is significantly reduced for the adsorption of COCl2 at the closed edge. Our calculated band gap for bare ABNNR (4.16 eV) is in close agreement with previous results [20,39]. It is observed that adsorption of COCl2 gas molecule at the close edge of the ribbon causes a band gap reduction of about 75.2% and the band gap is now equal to 1.03 eV. In order to analyze this behavior in more detail, the density of states (DOS) have been depicted in Fig. 3(b). The introduction of peaks in the vicinity of Fermi level in DOS accounts for the states in the valence and conduction band around the Fermi level in band structure Fig. 2(b). These new states in the band structure are responsible for decay in the band gap. Thus, for the adsorption of guest
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P. Srivastava et al. / Microelectronic Engineering 146 (2015) 62–67
(a)
(c)
(b)
(d)
Fig. 3. The calculated density of states of ABNNR (a) bare, (b) COCl2 molecule adsorbed at B-atom (closed edge), (c) COCl2 molecule adsorbed at B-atom (open edge (ABNNR– k COCl\ 2 )), (d) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2)).
(a)
(b)
Fig. 4. The calculated transmission spectra of ABNNR for (a) COCl2 molecule is adsorbed at the B-atom (closed edge) (b) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2\)).
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P. Srivastava et al. / Microelectronic Engineering 146 (2015) 62–67
(a)
(b)
Fig. 5. The calculated I–V characteristics of ABNNR for (a) COCl2 molecule adsorbed at the B-atom (closed edge) (b) COCl2 molecule adsorbed at B-atom (open edge (ABNNR– COCl\ 2 )).
molecule at closed edge, ribbon shows the semiconducting behavior but with a reduced band gap. In case of adsorption at open edge, based on the guest molecule orientation we observe following behavioral changes in the band structures. Firstly, in case of open edge (ABNNR–COCl\ 2 ) we observe from the band structures (shown in Fig. 2(c)) that the band gap gets reduced by 78.04% as compared to bare ribbon (from 4.16 eV to 0.91 eV). The peak in the DOS profile shown in Fig. 3(c) confirms the characteristic 1-D behavior. But for open edge (ABNNR– COClk2), the band structure shows no significant change in the band gap, which is in contrast to previous two cases as shown in Fig. 2(d). Further, the analysis of DOS profile for this case [shown in Fig. 3(d)] shows the absence of any peak around the Fermi level in the range 1.05 eV to 3.0 eV. This implies the non availability of any state in the band structure corresponding to these energy ranges thus ensuring the wide band gap in this case. Examination of the electronic properties of ABNNR with the adsorption of COCl2 molecule reveals some significant changes specifically the band gap modulation for the different adsorption cases. These changes motivate its applicability for sensing the presence of COCl2 gas. In order to observe the implication of these changes on the physically measurable quantities, we have studied the transport properties and calculated the current voltage (I–V) characteristics of the ribbon. Two-probe model under the framework of DFT is employed for the present calculations of transmission spectra as shown in Fig. 4(a) and (b). To be concise, here we have shown only two cases viz. adsorption at closed edge and adsorption at open edge (ABNNR–COCl\ 2 ) since they have lower band gaps (than other considered structures) and hence, are expected to exhibit enhanced transport properties. In case of adsorption at closed edge although we have transmission coefficient (TC) = 0 at the Fermi level for zero bias but just in the vicinity of the Fermi level at around 0.5 eV and 0.5 eV we have TC almost equal to 1. For other structure (ABNNR–COCl\ 2 ) we notice that transmission gap across the Fermi level is slightly reduced and the conduction band peaks are further enhanced [Fig. 4(b)] than the previous configuration. To have further insights of transport properties, the I–V characteristics, calculated for guest molecule adsorbed ribbon has been shown in Fig. 5. The deviation of the current behavior prevails for both the adsorbed configurations as compared to that of bare ABNNR [40]. For adsorption at closed edge, the current increases continuously with increasing the bias voltage [Fig. 5(a)]. On the other hand, the increment in current
for adsorption at open edge (ABNNR–COCl\ 2 ) is much steeper than the former. Moreover, we notice that current is saturating near 3 V biasing beyond which it starts decreasing slowly [Fig. 5(b)]. Thus, the study reveals that there is significant effect of adsorption of COCl2 gas molecule on the current passing through the ribbon for different adsorption cases. This ensures the sensitivity of ABNNR towards sensing the COCl2 gas. 4. Conclusion Systematical investigations have been carried out to evaluate the electronic and transport properties of COCl2 adsorbed ABNNR under different configurations. From the obtained result, it is evident that the preferable site of adsorption in all the cases is the B-atom rather than N-edge. Further analysis shows that open edge adsorption of COCl2 molecule is more energetically favorable as compared to closed edge adsorption. The study of band structure reveals that there is significant modulation in the band gap of the ABNNR upon the adsorption of the COCl2 molecule. The reported I–V characteristics are also in cognizance with the alternations in the electronic properties. On account of these remarkable changes in the electronic and transport properties, the ABNNR stems out as a promising contender for sensing the toxic COCl2 gas. Acknowledgements The work was funded by the Defence Research and Development Organization (DRDO), New Delhi through the project Grant number ERIP/ER/1100401/M/01/1364. Authors are thankful to the Computational Nanoscience & Technology Laboratory (CNTL), ABV-Indian Institute of Information Technology & Management (IIITM), Gwalior, India for providing the computational resources. N.K.J. is also thankful to PDPM-Indian Institute of Information Technology Design and Manufacturing (IIITDM), Jabalpur, India. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [2] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Science 320 (2008) 1308. [3] Y. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201–204.
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Adsorption of COCl2 gas molecule on armchair boron nitride nanoribbons for nano sensor applications Pankaj Srivastava a,⇑, Varun Sharma a, Neeraj K. Jaiswal b a b
Nanomaterials Research Group, ABV-Indian Institute of Information Technology and Management (IIITM), Gwalior 474015, India Discipline of Physics, PDPM-Indian Institute of Information Technology Design and Manufacturing (IIITDM), Jabalpur 482005, India
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 2 March 2015 Accepted 19 March 2015 Available online 26 March 2015 Keywords: Boron nitride Nanoribbons Electronic band structure Transmission spectra Sensor
a b s t r a c t First-principle calculations under the framework of density functional theory have been performed to study the adsorption of COCl2 (Phosgene) gas molecule on armchair boron nitride nanoribbon (ABNNR). Depending upon the geometry of the ribbon, we have considered two possible cases for adsorption of guest molecule i.e. adsorption at closed edge and adsorption at open edge. Adsorption energy (Ead) calculations implies that adsorption at open edge is energetically more favorable compared to closed edge case. The formation of closed ring structure at the open edge in this case also accounts for its stability. Electronic properties reveal that with the adsorption of COCl2 molecule on ABNNR the semiconducting behavior remains intact but a significant modulation in the band gap is observed as compared to bare ribbon. The current voltage (I–V) characteristics have also been investigated using non-equilibrium green’s function (NEGF) approach. The results indicate towards the potential applicability of ABNNR for sensing the toxic COCl2 gas. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The existence of graphene, a pure 2-dimensional crystal [1], has opened a wide area for research community to explore the related inorganic 2-D geometries. Due to its exotic physical properties [2,3], synthesizing some other inorganic 2-D crystals [4–9] is always an unquenched thrust. Such an analogically similar geometry that has recently gained a significant research interest is the hexagonal boron nitride sheet [10,11]. Both theoretical and experimental demonstrations have been done to explore some of its major properties [11–16]. These are somewhere in contrast to graphene [17–19] but can be exploited for various suitable applications. Similar to graphene nanoribbons (GNR), boron nitride nanoribbons (BNNR) can also be classified into two configurations viz. zigzag boron nitride nanoribbons (ZBNNR) and armchair boron nitride nanoribbons (ABNNR). Investigations highlighting the width dependency of BNNRs for both the configurations have already been discussed. Previous efforts described that the band gap of ZBNNR decreases with increasing the ribbon width whereas an oscillatory behavior was observed for the armchair configuration [15]. On account of large intrinsic band gap, its applicability in electronic devices is still limited. The larger band gap is due to the ionic nature of the bond between boron and nitrogen. ⇑ Corresponding author. E-mail address: [email protected] (P. Srivastava). http://dx.doi.org/10.1016/j.mee.2015.03.040 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
However, functionalization and substitutional doping have modulated the band gap to an extent but still the electronic properties for BNNR are not yet explored on a major scale like that of GNR. Earlier findings also suggest that the change in the electronic properties of nanoribbons are mainly attributed to the ribbons edges as they are more reactive as compared to the central part [20,21]. Due to the presence of the dangling bond at the ribbon edge, they are more susceptible to the molecule adsorption or the passivation. Apart from the electronic properties, the exploration of transport property is also an area that is yet to be explored for its suitable applications in future devices [22]. The existing literature related to the studies of transport properties are primarily concerned with the zero bias calculation [21]. Thus, in order to have the complete insight to the physical properties of BNNR a comprehensive study is required. Modulation in the electronic properties by its functionalization through fluorine is reported by Lu et al. [23]. It was revealed that fluorination causes small band gap (direct) semiconducting behavior for both ZBNNR and ABNNR cases. On the other hand, chlorine adsorption reported turns ZBNNR semiconducting [24]. Such an observed behavior is highly suited for its applicability as sensors as suggested for GNR [25–30]. Inspiring from this, the present work focuses on the sensing ability of the BNNR for COCl2 (Phosgene) gas. The sensing of COCl2 is also important as it is a toxic gas. Based on the first-principles calculations, we have explored the most stable configuration for the adsorption of COCl2 molecule on the bare BNNR edges. It is shown here that
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P. Srivastava et al. / Microelectronic Engineering 146 (2015) 62–67
Fig. 1. The schematic model of optimized supercell of ABNNR for (a) bare (b) COCl2 molecule adsorbed at closed edge (c) COCl2 molecule adsorbed at open edge with guest molecule approaching perpendicular to the ribbon (ABNNR–COCl\ 2 ) (d) COCl2 molecule adsorbed at open edge with guest molecule approaching parallel to the ribbon (ABNNR–COClk2). Here pink, blue, red, gray, green spheres represents B, N, O, C, Cl atoms respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The calculated bond lengths and adsorption energy for the respective configurations. Particulars
B–N
B–O
O–C
O–N
C–Cl1
C–Cl2
Ead
ABNNR–COCl2 adsorption at closed edge ABNNR–COCl2 adsorption at open edge (ABNNR–COCl\ 2) ABNNR–COCl2 adsorption at open edge (ABNNR–COClk2)
1.35 Å 1.42 Å 1.42 Å
1.58 Å 1.58 Å 1.36 Å
1.24 Å 1.23 Å 1.36 Å
– – 1.44 Å
1.71 Å 1.52 Å 1.77 Å
1.70 Å 1.67 Å 1.78 Å
1.058 eV 3.085 eV 6.328 eV
how the electronic and transport properties changes in accordance with adsorbed molecule. These changes can be further deployed for the application of BNNRs as the nanosensors application.
2. Computational details In the present work we have used first principles approach under the framework of density functional theory (DFT) to perform all the calculations using Atomistic ToolKit Virtual NanoLab (ATKVNL) [31]. The calculations for electronic and transport properties of the COCl2 adsorbed ribbon were performed on the optimized structures with norm-conserving pseudo potentials for describing the ions and linear combinations of atomic orbitals as basis sets. The ribbons under investigation were considered periodic along Z-axis but are confined in the other two transverse directions (X and Y). To account for exchange–correlation effects, we have used generalized gradient approximation (GGA) [32]. Furthermore, as a suitable tradeoff between accuracy and computational overhead we have selected a higher energy mesh cutoff of 100 Ryd with 1 1 100 k-point sampling for the Brillouin zone integration. All the atoms in considered geometries are fully relaxed to change their individual position until the force convergence criteria of 0.05 eV/Å is achieved. In order to avoid any Columbic interactions between the periodic images of ribbon in the transverse directions, a large vacuum space of 10 Å is considered. Two-probe configurations of the optimized geometries were used for the calculation of the transport properties. The
configuration consists of three regions viz. scattering (central) region, left electrode and right electrode. In the current manuscript, we have used 6 repetition of the ABNNR supercell in the central region which seamlessly couples with the left and right electrode. Higher mesh cutoff is set to 150 Ryd for the transport calculations. Earlier study also shows the successful implementation of this model for studying the electronic and transport properties at nano regime [33–35]. ATK-VNL uses DFT coupled with non equilibrium green’s function (NEGF) approach for transport property calculations. As a part of transport properties calculations, first the transmission spectra of the relaxed geometries are calculated. The transmission spectra are further utilized for the transmission coefficient using the relation [31].
TðE; VÞ ¼ T r ½sR ðE; VÞGC ðE; VÞsL ðE; VÞGþC ðE; VÞ
ð1Þ
Here, sL=R ðE; VÞ are the coupling coefficients for the left/right contact respectively, GC ðE; VÞ denotes the green’s function for the scattering region. To calculated the current through the scattering region the Büttiker–Landauer formula [36] is used.
IðVÞ ¼
Z lR lL
TðE; VÞ f ðE lL Þ f ðE lR Þ dE
ð2Þ
Here, lL=R are the chemical potentials of the left/right electrodes respectively.
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(a)
(c)
(b)
(d)
Fig. 2. The calculated band structures of ABNNR for (a) bare, (b) COCl2 molecule adsorbed at B-atom (closed edge), (c) COCl2 molecule adsorbed at B-atom (open edge k (ABNNR–COCl\ 2 )), (d) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2)).
3. Results and discussion We have investigated the sensing abilities of ABNNR for the COCl2 gas molecule. First, we investigated the stability of the ABNNR with COCl2 molecule adsorbed on it. As the COCl2 molecule approaches to the vicinity of bare ABNNR, it prefers to be adsorbed at the edges rather the central region of the ribbon. Furthermore, due to more electronegative character of O-atom the COCl2 molecule prefers the adsorption via O-atom. Based on the ribbon width (Wa = 6 selected for present studies) we have presented the following possible cases viz. – adsorption at the closed edge and adsorption at the open edge. Further, to investigate the effect of guest molecule orientation we have considered two possible cases for open edge adsorption. When guest molecule approaches the ribbon perpendicular to the ribbon (ABNNR–COCl\ 2 ) and, when guest molecule approaches parallel to the ribbon (ABNNR–COClk2). Fig. 1 highlights the optimized geometries in all the cases. In order to find the relative stability amongst the cases under investigation, we have calculated the adsorption energy (Ead) of the guest molecule (COCl2) adsorbed ABNNR. The Ead is calculated using the total energies of bare and COCl2 adsorbed ABNNR using the relation Ead = E(NR + COCl2) E(NR) E(COCl2) [37], where, E(NR + COCl2), E(NR) and E(COCl2) are the total energies of nanoribbons with adsorbed molecule, free standing bare ribbon and a single isolated COCl2 molecule respectively. The calculated adsorption energy and bond lengths for considered configurations are given in Table 1. It is clear from Table 1 that open edge adsorption is energetically more stable in comparison to closed edge adsorption. Further, open edge (ABNNR–COClk2) emerges out as the most stable adsorption
with Ead = 6.33 eV followed by adsorption at open edge (ABNNR–COCl\ 2 ) with Ead = 3.085 eV and lastly adsorption at closed edge with Ead = 1.058 eV. The formation of closed ring in the case of open edge (ABNNR–COClk2) (shown in Fig. 1(d)) also depicts the stability in this case. This type of behavior is in cognizance with the previous literatures [38]. It is also evident from the Fig. 1 that there is no structural deformation in the ribbon due to the adsorption of guest molecule. Further, in all the cases the favorable site of adsorption is B-atom rather than N-atom, the energy calculations confirms the same. The well known semiconducting properties of wide band gap ABNNR still prevail even on the adsorption of COCl2, however, the band gap is modulated by the introduction of new states in the valence as well as conduction band. In order to have further insights about the electronic properties, we have investigated the band structures for all the cases and compared the results with the bare ABNNR as shown in the Fig. 2. The comparison of band structure reveals that the band gap is significantly reduced for the adsorption of COCl2 at the closed edge. Our calculated band gap for bare ABNNR (4.16 eV) is in close agreement with previous results [20,39]. It is observed that adsorption of COCl2 gas molecule at the close edge of the ribbon causes a band gap reduction of about 75.2% and the band gap is now equal to 1.03 eV. In order to analyze this behavior in more detail, the density of states (DOS) have been depicted in Fig. 3(b). The introduction of peaks in the vicinity of Fermi level in DOS accounts for the states in the valence and conduction band around the Fermi level in band structure Fig. 2(b). These new states in the band structure are responsible for decay in the band gap. Thus, for the adsorption of guest
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(a)
(c)
(b)
(d)
Fig. 3. The calculated density of states of ABNNR (a) bare, (b) COCl2 molecule adsorbed at B-atom (closed edge), (c) COCl2 molecule adsorbed at B-atom (open edge (ABNNR– k COCl\ 2 )), (d) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2)).
(a)
(b)
Fig. 4. The calculated transmission spectra of ABNNR for (a) COCl2 molecule is adsorbed at the B-atom (closed edge) (b) COCl2 molecule adsorbed at B-atom (open edge (ABNNR–COCl2\)).
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(a)
(b)
Fig. 5. The calculated I–V characteristics of ABNNR for (a) COCl2 molecule adsorbed at the B-atom (closed edge) (b) COCl2 molecule adsorbed at B-atom (open edge (ABNNR– COCl\ 2 )).
molecule at closed edge, ribbon shows the semiconducting behavior but with a reduced band gap. In case of adsorption at open edge, based on the guest molecule orientation we observe following behavioral changes in the band structures. Firstly, in case of open edge (ABNNR–COCl\ 2 ) we observe from the band structures (shown in Fig. 2(c)) that the band gap gets reduced by 78.04% as compared to bare ribbon (from 4.16 eV to 0.91 eV). The peak in the DOS profile shown in Fig. 3(c) confirms the characteristic 1-D behavior. But for open edge (ABNNR– COClk2), the band structure shows no significant change in the band gap, which is in contrast to previous two cases as shown in Fig. 2(d). Further, the analysis of DOS profile for this case [shown in Fig. 3(d)] shows the absence of any peak around the Fermi level in the range 1.05 eV to 3.0 eV. This implies the non availability of any state in the band structure corresponding to these energy ranges thus ensuring the wide band gap in this case. Examination of the electronic properties of ABNNR with the adsorption of COCl2 molecule reveals some significant changes specifically the band gap modulation for the different adsorption cases. These changes motivate its applicability for sensing the presence of COCl2 gas. In order to observe the implication of these changes on the physically measurable quantities, we have studied the transport properties and calculated the current voltage (I–V) characteristics of the ribbon. Two-probe model under the framework of DFT is employed for the present calculations of transmission spectra as shown in Fig. 4(a) and (b). To be concise, here we have shown only two cases viz. adsorption at closed edge and adsorption at open edge (ABNNR–COCl\ 2 ) since they have lower band gaps (than other considered structures) and hence, are expected to exhibit enhanced transport properties. In case of adsorption at closed edge although we have transmission coefficient (TC) = 0 at the Fermi level for zero bias but just in the vicinity of the Fermi level at around 0.5 eV and 0.5 eV we have TC almost equal to 1. For other structure (ABNNR–COCl\ 2 ) we notice that transmission gap across the Fermi level is slightly reduced and the conduction band peaks are further enhanced [Fig. 4(b)] than the previous configuration. To have further insights of transport properties, the I–V characteristics, calculated for guest molecule adsorbed ribbon has been shown in Fig. 5. The deviation of the current behavior prevails for both the adsorbed configurations as compared to that of bare ABNNR [40]. For adsorption at closed edge, the current increases continuously with increasing the bias voltage [Fig. 5(a)]. On the other hand, the increment in current
for adsorption at open edge (ABNNR–COCl\ 2 ) is much steeper than the former. Moreover, we notice that current is saturating near 3 V biasing beyond which it starts decreasing slowly [Fig. 5(b)]. Thus, the study reveals that there is significant effect of adsorption of COCl2 gas molecule on the current passing through the ribbon for different adsorption cases. This ensures the sensitivity of ABNNR towards sensing the COCl2 gas. 4. Conclusion Systematical investigations have been carried out to evaluate the electronic and transport properties of COCl2 adsorbed ABNNR under different configurations. From the obtained result, it is evident that the preferable site of adsorption in all the cases is the B-atom rather than N-edge. Further analysis shows that open edge adsorption of COCl2 molecule is more energetically favorable as compared to closed edge adsorption. The study of band structure reveals that there is significant modulation in the band gap of the ABNNR upon the adsorption of the COCl2 molecule. The reported I–V characteristics are also in cognizance with the alternations in the electronic properties. On account of these remarkable changes in the electronic and transport properties, the ABNNR stems out as a promising contender for sensing the toxic COCl2 gas. Acknowledgements The work was funded by the Defence Research and Development Organization (DRDO), New Delhi through the project Grant number ERIP/ER/1100401/M/01/1364. Authors are thankful to the Computational Nanoscience & Technology Laboratory (CNTL), ABV-Indian Institute of Information Technology & Management (IIITM), Gwalior, India for providing the computational resources. N.K.J. is also thankful to PDPM-Indian Institute of Information Technology Design and Manufacturing (IIITDM), Jabalpur, India. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [2] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Science 320 (2008) 1308. [3] Y. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201–204.
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