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Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons
Physics Letters A 376 (2012) 559–562
Contents lists available at SciVerse ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons Xiao-Lin Wei ∗,1 , Yuan-Ping Chen, Wen-Liang Liu, Jian-Xin Zhong 1 Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China
a r t i c l e
i n f o
Article history: Received 6 October 2011 Accepted 21 October 2011 Available online 22 November 2011 Communicated by R. Wu Keywords: Nitrogen vacancy Graphene nanoribbons Electron transport Gas sensor
a b s t r a c t We study the electron transport of nitrogen-vacancy zigzag graphene nanoribbons (ZGNRs) absorbing gas molecules. It is found that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules than the pristine ZGNRs. The gas molecules absorbed on the three-nitrogen vacancies lead to sharp resonant peaks on conductance, while those absorbed on the four-nitrogen vacancies lead to anti-resonant dips. Each kind of gas molecule can be detected by its own unique (different energy) resonant peaks (or dips). This indicates that the nitrogen vacancy can enhance the sensitivity to gas molecules, i.e., nitrogen-vacancy ZGNRs can serve as better gas sensors. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The area of gas sensors has gained a lot of attention due to their important application on the environmental monitoring, control of chemical processes, space missions, etc. [1]. Solid-state gas sensors are renowned for their high sensitivity [2], especially the one-dimensional (1D) nanosensors such as carbon nanotubes (CNT) and semiconductor nanowires can remedy many of the drawbacks of traditional semiconductor-based gas sensors [3–6]. It has been experimentally observed that CNTs could be used to detect small concentration of gas with high sensitivity by measuring changes of the CNTs conductance upon exposure to the gases at room temperature [3]. Graphene, a single atomic layer of graphite, is a new carbonbased nanomaterial, which has been successfully produced in experiments [7,8]. By cutting mechanically exfoliated graphene or patterning epitaxially grown graphene, 1D graphene nanoribbons (GNRs) can be obtained [9,10]. Compared with the 1D CNTs, GNRs possess two long and reactive edges which maximize the effect of absorption and dopant [11–14]. Moreover, GNRs have higher conductivity even in the limit of zero carrier density [15]. Thus the new 1D carbon-based nanoribbons are considered as more ideal gas sensors [16–18]. Some recent studies also indicate that GNRs can present significant conductance changes as absorbing several kinds of gas molecules [16]. However, the interaction of many kinds of gas molecules with the pristine GNRs is not very strong [16]. To remedy this situation,
a strategy is to functionalize them by substitutional doping. In that aspect, nitrogen as a substitutional dopant has been investigated [19–23]. Besides single atom doping, some special doping defects, such as pyridinelike substructures, can be also formed in GNRs [23]. There are two kinds of pyridinelike substructures, namely, three-nitrogen vacancy (3NV) (nitrogen atoms are substituted for three carbon atoms neighboring the vacancy) and four-nitrogen divacancy (4ND). The former study has indicated that the pyridinelike substructures can improve the sensitivity of CNT-based gas sensor [24] because of the interaction between the gas molecule and the nitrogen vacancy present in the nanotube. Then, whether the nitrogen vacancies in GNRs can improve the sensitivity to the gas molecules? In another word, whether the nitrogen-vacancy GNRs are better in gas sensors than the pristine GNRs? To the best of our knowledge, there is no study on this question. In this Letter, we study transport properties of nitrogen-vacancy (3NV and 4ND) zigzag GNRs (ZGNRs) absorbing gas molecules NH3 , CO and N2 . It is found that the absorption of even a single gas molecule will drastically change the conductance of nitrogenvacancy ZGNRs. Different gas molecules lead to different energy resonant or anti-resonant transmission, thus each kind of gas molecules can be detected by its own unique resonant peaks or dips. The resonant peaks or dips are induced by the quasi-bound states localized on the vacancies and molecules. The results indicate that the nitrogen-vacancy ZGNRs are very sensitive to the absorption of gas molecules, thus they are better in gas sensors than the pristine nanoribbons. 2. Computational detail and model
* 1
Corresponding author. E-mail addresses: [email protected] (X.-L. Wei), [email protected] (J.-X. Zhong). Tel.: +86 731 58292063; fax: +86 731 58292061.
0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.10.055
We perform first-principles total-energy calculations to obtain electronic structure and equilibrium geometries of nitrogen-
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Fig. 1. Schematic view of (a) 3NV-ZGNR absorbing an NH3 molecule, (b) 3NV-ZGNR absorbing a CO molecule, (c) 3NV-ZGNR absorbing an N2 molecule; (d) 4ND-ZGNR absorbing an NH3 molecule, (e) 4ND-ZGNR absorbing a CO molecule, (f) 4ND-ZGNR absorbing a CO molecule.
vacancy ZGNRs with absorption of molecules based on the pseudopotential density-functional theory [25]. The exchange-correlation potential is approximated within the generalized gradient approximation using PBE function (GGA.PBE) [26]. The software package atomistix toolkit (ATK) [27], which employs local numerical basis orbitals and nonequilibrium Green’s function formalism to calculate quantum conductance in electrode–device–electrode geometry, has been used in all total-energy and transport calculations. The geometry optimizations of the nitrogen-vacancy ZGNRs with absorption of gas molecules have been relaxed with sufficient vacuum spaces (minimum of 10 Å) to prevent the interactions with periodic images of the structure. The criteria of convergence for total energy and Hellman–Feynman forces were 10−4 eV and 0.05 eV/Å, respectively. The electrostatic potentials were determined on a real space grid with a mesh cutoff energy of 150 Ry. Double-zeta polarized basis sets of local numerical orbitals were employed in our systems. The Brillouin zone has been sampled with (1, 1, 100) points within the Monkhorst Pack k-point sampling scheme. The density of states (DOS), electron density and transport properties of the nitrogen-vacancy ZGNRs with absorption of gas molecules were calculated based on nonequilibrium Green’s function formalism as implemented in ATK [28]. Two types of nitrogen-vacancy ZGNRs adsorbing three types of molecules are considered, as shown in Fig. 1. Figs. 1(a), 1(b) and 1(c) are the 3NV-ZGNRs absorbing an NH3 , CO and N2 respectively, while Figs. 1(d), 1(e) and 1(f) are 4ND-ZGNRs absorbing an NH3 , CO and N2 respectively. The width of ZGNR is N = 5 (zigzag chains). The absorption systems include three parts: two (left and right) semi-infinite electrodes and a central scattering region. All edge carbon atoms have been saturated with hydrogen atoms (white) for a better structural and electronic stability of the ribbons. For all systems, the relaxed position of the adsorbed molecule is at the center of the nitrogen vacancy, and the distance of molecules NH3 , CO and N2 from the ZGNR plane are about 1.73 Å, 2.32 Å and 2.59 Å, respectively. In the case of 3NV-ZGNRs, there is an angle about 6.5◦ between the symmetry axis of NH3 and the ZGNR plane; the dihedral angle is about 87.6◦ between the C–O bond of CO and the ZGNR plane; and the dihedral angle is about 14.5◦ between the N–N bond of N2 and the ZGNR plane. In the case of 4NV-ZGNRs, the angle between the symmetry axis of NH3 and the ZGNR plane is about 1.5◦ ; the dihedral angle between the C–O bond of CO and the ZGNR plane is about 2.3◦ ; and the dihedral angle between the N–N bond of N2 and the ZGNR plane is about 10.4◦ . 3. Results and discussions In Fig. 2(a), we show the effect of nitrogen vacancy on transport properties of pristine ZGNRs. Solid and dotted lines present
Fig. 2. The conductance (a) and DOS (b) as a function of electron energy for 3NVZGNR (solid line), 4ND-ZGNR (dashed line) and perfect ZGNR (dotted line), respectively.
Fig. 3. (a) The conductance as a function of electron energy for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line), N2 (dotted line) or without absorption (dash-dotted line). Inset: The conductance as a function of electron energy for pristine ZGNRs absorbing NH3 (solid line), CO (dashed line) or N2 (dotted line).
conductance for 3NV-ZGNR and 4ND-ZGNR, respectively. As a comparison, the conductance for pristine ZGNR is also shown (dotted line). One can find that perfect conductance steps appear on the conductance of pristine ZGNR, and the conductance is symmetrical about the Fermi energy. While the conductance steps of 3NV-ZGNR or 4ND-ZGNR are destroyed and the two conductance are obviously lower than that of pristine nanoribbons. In the profile of 3NV-ZGNR there is a zero conductance region at the left side of Fermi energy, while in the profile of 4ND-ZGNR a zero conductance region appears at the right side of Fermi energy. Seen from Fig. 2(b), there are no sharp DOS peaks corresponding to the two zero conductance, thus the zero conductance are not originated from quasi-bound states, just because of strong scattering induced by the vacancies. In Fig. 3, we show the conductance for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line). The dashdotted line presents the conductance for 3NV-ZGNR. One can find that the absorbing molecules significantly change the conductance of 3NV-ZGNR around the energy range [−0.8 eV, −0.1 eV]. Some sharp resonant peaks appear around the zero conductance region
X.-L. Wei et al. / Physics Letters A 376 (2012) 559–562
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Fig. 4. The conductance (a) and DOS (b) as a function of electron energy for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line) in the energy range [−0.8 eV, −0.1 eV]. (c) Top two: The electron densities of quasi-bound states corresponding to the two DOS peaks (solid line) near the Fermi energy; bottom two: The electron densities of quasi-bound states corresponding to the two DOS peaks (dashed line) near the Fermi energy.
of 3NV-ZGNR. Moreover, NH3 , CO and N2 molecules induce different energy conductance peaks, i.e., each kind of gas molecules can be detected by its own unique resonant peaks. As a comparison, in the inset of Fig. 3 we show the conductance for pristine ZGNRs absorbing an NH3 , CO or N2 . It is found that the CO and N2 nearly have no effect on the conductance of pristine ZGNR, while the NH3 also just induces one conductance dip far from the Fermi energy. In addition, we calculate the case of single-vacancy ZGNRs (without nitrogen doping) absorbing NH3 , CO or N2 molecules. The calculated results indicate that the effect of absorbing gas molecules on the conductance is weak (not shown). So, one can conclude that the nitrogen-vacancy ZGNRs become more sensitive to the gas molecules due to the doping nitrogen atoms. To clearly show the effect of absorbing molecules, the conductance and DOS for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line) in the energy range [−0.8 eV, −0.1 eV] are shown in Figs. 4(a) and 4(b) respectively. One can find that the NH3 molecule induces three sharp peaks, while the CO and N2 molecules induce two sharp peaks, respectively. All of the conductance peaks correspond to sharp DOS peaks, indicating that the resonant transmissions are induced by the quasi-bound states in the structures. Fig. 4(c) depicts the electron density profiles of some quasi-bound states. The electrons in these states are mainly localized on the gas molecules and nitrogen atoms. It further shows that the interaction of gas molecule and nitrogen atoms results in the localization of elections and thus significantly changes the conductance profiles. The conductance and DOS for 4ND-ZGNRs absorbing NH3 (solid line), CO (dashed line) or N2 (dotted line) are shown in Fig. 5. Similar to the case of 3NV-ZGNRs, the absorbed molecules only drastically change the conductance for 4ND-ZGNRs in the energy range [−0.8 eV, −0.25 eV], which may be determined by the energy levels of molecules. However, due to the conductance for 4ND-ZGNR in this range is a conductance step rather than a zero conductance region, one can find that the absorbed molecules induce sharp conductance dips, i.e., induce anti-resonant transmission. The absorption of NH3 molecule leads to three conductance dips, while CO and N2 molecules lead to two different energy dips, respectively. All of these dips correspond to sharp DOS peaks, thus these anti-resonant transmissions are also induced by quasibound states localized on the molecules and nitrogen atoms. Because the anti-resonant transmission can be easily observed, each kind of gas molecules can be detected by its own unique resonant dips. Combined with the former results, one can find that the nitrogen-vacancy ZGNRs are better in gas sensor than the pristine GNRs.
Fig. 5. The conductance (a) and DOS (b) as a function of electron energy for 4NDZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line).
4. Conclusions In conclusion, we study the transport properties of nitrogenvacancy ZGNRs absorbing gas molecules (NH3 , CO and N2 ), by using the density-functional theory in combination with the nonequilibrium Green’s function method. It is found that the absorption of gas molecule induces resonant or anti-resonant transmissions. The size of nitrogen vacancy determines whether the transmission is resonant or anti-resonant. Absorption of different gas molecules induces different energy resonant peaks or antiresonant dips. These resonant or anti-resonant transmissions are induced by quasi-bound states localized on the gas molecules and nitrogen atoms. Utilizing the resonant transmission, each kind of gas molecules can be detected from other gas molecules. The results indicate that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules and thus are better in gas sensor than the pristine ZGNRs. Acknowledgements This work was supported by the Hunan Science and Technology Bureau planned project (Grant No. 2010FJ4130) and the National Natural Science Foundation of China (Grant Nos. 11074213 and 51006086).
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References [1] N. Yamazoe, Sens. Actuators B Chem. 108 (2005) 2. [2] P.T. Moseley, Meas. Sci. Technol. 8 (1997) 223. [3] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622. [4] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [5] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) 1801. [6] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santuccia, Appl. Phys. Lett. 82 (2003) 961. [7] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [8] C. Berger, Z.M. Song, X.B. Li, X.S. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A.N. Marchenkov, et al., Science 312 (2006) 1191. [9] Y. Zhang, Y.-W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201. [10] M.Y. Han, B. Ozyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett. 98 (2007) 206805. [11] Q.M. Yan, B. Huang, J. Yu, F.W. Zheng, J. Zang, J. Wu, B.L. Gu, F. Liu, W.H. Duan, Nano Lett. 7 (2007) 1459. [12] B. Huang, Q.M. Yan, G. Zhou, J. Wu, B.L. Gu, W.H. Duan, F. Liu, Appl. Phys. Lett. 91 (2007) 253122. [13] F. Cervantes-Sodi, G. Csányi, S. Piscanec, A.C. Ferrari, Phys. Rev. B 77 (2008) 165427.
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Contents lists available at SciVerse ScienceDirect
Physics Letters A www.elsevier.com/locate/pla
Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons Xiao-Lin Wei ∗,1 , Yuan-Ping Chen, Wen-Liang Liu, Jian-Xin Zhong 1 Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China
a r t i c l e
i n f o
Article history: Received 6 October 2011 Accepted 21 October 2011 Available online 22 November 2011 Communicated by R. Wu Keywords: Nitrogen vacancy Graphene nanoribbons Electron transport Gas sensor
a b s t r a c t We study the electron transport of nitrogen-vacancy zigzag graphene nanoribbons (ZGNRs) absorbing gas molecules. It is found that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules than the pristine ZGNRs. The gas molecules absorbed on the three-nitrogen vacancies lead to sharp resonant peaks on conductance, while those absorbed on the four-nitrogen vacancies lead to anti-resonant dips. Each kind of gas molecule can be detected by its own unique (different energy) resonant peaks (or dips). This indicates that the nitrogen vacancy can enhance the sensitivity to gas molecules, i.e., nitrogen-vacancy ZGNRs can serve as better gas sensors. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The area of gas sensors has gained a lot of attention due to their important application on the environmental monitoring, control of chemical processes, space missions, etc. [1]. Solid-state gas sensors are renowned for their high sensitivity [2], especially the one-dimensional (1D) nanosensors such as carbon nanotubes (CNT) and semiconductor nanowires can remedy many of the drawbacks of traditional semiconductor-based gas sensors [3–6]. It has been experimentally observed that CNTs could be used to detect small concentration of gas with high sensitivity by measuring changes of the CNTs conductance upon exposure to the gases at room temperature [3]. Graphene, a single atomic layer of graphite, is a new carbonbased nanomaterial, which has been successfully produced in experiments [7,8]. By cutting mechanically exfoliated graphene or patterning epitaxially grown graphene, 1D graphene nanoribbons (GNRs) can be obtained [9,10]. Compared with the 1D CNTs, GNRs possess two long and reactive edges which maximize the effect of absorption and dopant [11–14]. Moreover, GNRs have higher conductivity even in the limit of zero carrier density [15]. Thus the new 1D carbon-based nanoribbons are considered as more ideal gas sensors [16–18]. Some recent studies also indicate that GNRs can present significant conductance changes as absorbing several kinds of gas molecules [16]. However, the interaction of many kinds of gas molecules with the pristine GNRs is not very strong [16]. To remedy this situation,
a strategy is to functionalize them by substitutional doping. In that aspect, nitrogen as a substitutional dopant has been investigated [19–23]. Besides single atom doping, some special doping defects, such as pyridinelike substructures, can be also formed in GNRs [23]. There are two kinds of pyridinelike substructures, namely, three-nitrogen vacancy (3NV) (nitrogen atoms are substituted for three carbon atoms neighboring the vacancy) and four-nitrogen divacancy (4ND). The former study has indicated that the pyridinelike substructures can improve the sensitivity of CNT-based gas sensor [24] because of the interaction between the gas molecule and the nitrogen vacancy present in the nanotube. Then, whether the nitrogen vacancies in GNRs can improve the sensitivity to the gas molecules? In another word, whether the nitrogen-vacancy GNRs are better in gas sensors than the pristine GNRs? To the best of our knowledge, there is no study on this question. In this Letter, we study transport properties of nitrogen-vacancy (3NV and 4ND) zigzag GNRs (ZGNRs) absorbing gas molecules NH3 , CO and N2 . It is found that the absorption of even a single gas molecule will drastically change the conductance of nitrogenvacancy ZGNRs. Different gas molecules lead to different energy resonant or anti-resonant transmission, thus each kind of gas molecules can be detected by its own unique resonant peaks or dips. The resonant peaks or dips are induced by the quasi-bound states localized on the vacancies and molecules. The results indicate that the nitrogen-vacancy ZGNRs are very sensitive to the absorption of gas molecules, thus they are better in gas sensors than the pristine nanoribbons. 2. Computational detail and model
* 1
Corresponding author. E-mail addresses: [email protected] (X.-L. Wei), [email protected] (J.-X. Zhong). Tel.: +86 731 58292063; fax: +86 731 58292061.
0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.10.055
We perform first-principles total-energy calculations to obtain electronic structure and equilibrium geometries of nitrogen-
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X.-L. Wei et al. / Physics Letters A 376 (2012) 559–562
Fig. 1. Schematic view of (a) 3NV-ZGNR absorbing an NH3 molecule, (b) 3NV-ZGNR absorbing a CO molecule, (c) 3NV-ZGNR absorbing an N2 molecule; (d) 4ND-ZGNR absorbing an NH3 molecule, (e) 4ND-ZGNR absorbing a CO molecule, (f) 4ND-ZGNR absorbing a CO molecule.
vacancy ZGNRs with absorption of molecules based on the pseudopotential density-functional theory [25]. The exchange-correlation potential is approximated within the generalized gradient approximation using PBE function (GGA.PBE) [26]. The software package atomistix toolkit (ATK) [27], which employs local numerical basis orbitals and nonequilibrium Green’s function formalism to calculate quantum conductance in electrode–device–electrode geometry, has been used in all total-energy and transport calculations. The geometry optimizations of the nitrogen-vacancy ZGNRs with absorption of gas molecules have been relaxed with sufficient vacuum spaces (minimum of 10 Å) to prevent the interactions with periodic images of the structure. The criteria of convergence for total energy and Hellman–Feynman forces were 10−4 eV and 0.05 eV/Å, respectively. The electrostatic potentials were determined on a real space grid with a mesh cutoff energy of 150 Ry. Double-zeta polarized basis sets of local numerical orbitals were employed in our systems. The Brillouin zone has been sampled with (1, 1, 100) points within the Monkhorst Pack k-point sampling scheme. The density of states (DOS), electron density and transport properties of the nitrogen-vacancy ZGNRs with absorption of gas molecules were calculated based on nonequilibrium Green’s function formalism as implemented in ATK [28]. Two types of nitrogen-vacancy ZGNRs adsorbing three types of molecules are considered, as shown in Fig. 1. Figs. 1(a), 1(b) and 1(c) are the 3NV-ZGNRs absorbing an NH3 , CO and N2 respectively, while Figs. 1(d), 1(e) and 1(f) are 4ND-ZGNRs absorbing an NH3 , CO and N2 respectively. The width of ZGNR is N = 5 (zigzag chains). The absorption systems include three parts: two (left and right) semi-infinite electrodes and a central scattering region. All edge carbon atoms have been saturated with hydrogen atoms (white) for a better structural and electronic stability of the ribbons. For all systems, the relaxed position of the adsorbed molecule is at the center of the nitrogen vacancy, and the distance of molecules NH3 , CO and N2 from the ZGNR plane are about 1.73 Å, 2.32 Å and 2.59 Å, respectively. In the case of 3NV-ZGNRs, there is an angle about 6.5◦ between the symmetry axis of NH3 and the ZGNR plane; the dihedral angle is about 87.6◦ between the C–O bond of CO and the ZGNR plane; and the dihedral angle is about 14.5◦ between the N–N bond of N2 and the ZGNR plane. In the case of 4NV-ZGNRs, the angle between the symmetry axis of NH3 and the ZGNR plane is about 1.5◦ ; the dihedral angle between the C–O bond of CO and the ZGNR plane is about 2.3◦ ; and the dihedral angle between the N–N bond of N2 and the ZGNR plane is about 10.4◦ . 3. Results and discussions In Fig. 2(a), we show the effect of nitrogen vacancy on transport properties of pristine ZGNRs. Solid and dotted lines present
Fig. 2. The conductance (a) and DOS (b) as a function of electron energy for 3NVZGNR (solid line), 4ND-ZGNR (dashed line) and perfect ZGNR (dotted line), respectively.
Fig. 3. (a) The conductance as a function of electron energy for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line), N2 (dotted line) or without absorption (dash-dotted line). Inset: The conductance as a function of electron energy for pristine ZGNRs absorbing NH3 (solid line), CO (dashed line) or N2 (dotted line).
conductance for 3NV-ZGNR and 4ND-ZGNR, respectively. As a comparison, the conductance for pristine ZGNR is also shown (dotted line). One can find that perfect conductance steps appear on the conductance of pristine ZGNR, and the conductance is symmetrical about the Fermi energy. While the conductance steps of 3NV-ZGNR or 4ND-ZGNR are destroyed and the two conductance are obviously lower than that of pristine nanoribbons. In the profile of 3NV-ZGNR there is a zero conductance region at the left side of Fermi energy, while in the profile of 4ND-ZGNR a zero conductance region appears at the right side of Fermi energy. Seen from Fig. 2(b), there are no sharp DOS peaks corresponding to the two zero conductance, thus the zero conductance are not originated from quasi-bound states, just because of strong scattering induced by the vacancies. In Fig. 3, we show the conductance for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line). The dashdotted line presents the conductance for 3NV-ZGNR. One can find that the absorbing molecules significantly change the conductance of 3NV-ZGNR around the energy range [−0.8 eV, −0.1 eV]. Some sharp resonant peaks appear around the zero conductance region
X.-L. Wei et al. / Physics Letters A 376 (2012) 559–562
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Fig. 4. The conductance (a) and DOS (b) as a function of electron energy for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line) in the energy range [−0.8 eV, −0.1 eV]. (c) Top two: The electron densities of quasi-bound states corresponding to the two DOS peaks (solid line) near the Fermi energy; bottom two: The electron densities of quasi-bound states corresponding to the two DOS peaks (dashed line) near the Fermi energy.
of 3NV-ZGNR. Moreover, NH3 , CO and N2 molecules induce different energy conductance peaks, i.e., each kind of gas molecules can be detected by its own unique resonant peaks. As a comparison, in the inset of Fig. 3 we show the conductance for pristine ZGNRs absorbing an NH3 , CO or N2 . It is found that the CO and N2 nearly have no effect on the conductance of pristine ZGNR, while the NH3 also just induces one conductance dip far from the Fermi energy. In addition, we calculate the case of single-vacancy ZGNRs (without nitrogen doping) absorbing NH3 , CO or N2 molecules. The calculated results indicate that the effect of absorbing gas molecules on the conductance is weak (not shown). So, one can conclude that the nitrogen-vacancy ZGNRs become more sensitive to the gas molecules due to the doping nitrogen atoms. To clearly show the effect of absorbing molecules, the conductance and DOS for 3NV-ZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line) in the energy range [−0.8 eV, −0.1 eV] are shown in Figs. 4(a) and 4(b) respectively. One can find that the NH3 molecule induces three sharp peaks, while the CO and N2 molecules induce two sharp peaks, respectively. All of the conductance peaks correspond to sharp DOS peaks, indicating that the resonant transmissions are induced by the quasi-bound states in the structures. Fig. 4(c) depicts the electron density profiles of some quasi-bound states. The electrons in these states are mainly localized on the gas molecules and nitrogen atoms. It further shows that the interaction of gas molecule and nitrogen atoms results in the localization of elections and thus significantly changes the conductance profiles. The conductance and DOS for 4ND-ZGNRs absorbing NH3 (solid line), CO (dashed line) or N2 (dotted line) are shown in Fig. 5. Similar to the case of 3NV-ZGNRs, the absorbed molecules only drastically change the conductance for 4ND-ZGNRs in the energy range [−0.8 eV, −0.25 eV], which may be determined by the energy levels of molecules. However, due to the conductance for 4ND-ZGNR in this range is a conductance step rather than a zero conductance region, one can find that the absorbed molecules induce sharp conductance dips, i.e., induce anti-resonant transmission. The absorption of NH3 molecule leads to three conductance dips, while CO and N2 molecules lead to two different energy dips, respectively. All of these dips correspond to sharp DOS peaks, thus these anti-resonant transmissions are also induced by quasibound states localized on the molecules and nitrogen atoms. Because the anti-resonant transmission can be easily observed, each kind of gas molecules can be detected by its own unique resonant dips. Combined with the former results, one can find that the nitrogen-vacancy ZGNRs are better in gas sensor than the pristine GNRs.
Fig. 5. The conductance (a) and DOS (b) as a function of electron energy for 4NDZGNRs absorbing an NH3 (solid line), CO (dashed line) or N2 (dotted line).
4. Conclusions In conclusion, we study the transport properties of nitrogenvacancy ZGNRs absorbing gas molecules (NH3 , CO and N2 ), by using the density-functional theory in combination with the nonequilibrium Green’s function method. It is found that the absorption of gas molecule induces resonant or anti-resonant transmissions. The size of nitrogen vacancy determines whether the transmission is resonant or anti-resonant. Absorption of different gas molecules induces different energy resonant peaks or antiresonant dips. These resonant or anti-resonant transmissions are induced by quasi-bound states localized on the gas molecules and nitrogen atoms. Utilizing the resonant transmission, each kind of gas molecules can be detected from other gas molecules. The results indicate that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules and thus are better in gas sensor than the pristine ZGNRs. Acknowledgements This work was supported by the Hunan Science and Technology Bureau planned project (Grant No. 2010FJ4130) and the National Natural Science Foundation of China (Grant Nos. 11074213 and 51006086).
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