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Computational modeling of carbon nanotubes for photoresistor applications
Solid State Communications 309 (2020) 113831
Contents lists available at ScienceDirect
Solid State Communications journal homepage: http://www.elsevier.com/locate/ssc
Computational modeling of carbon nanotubes for photoresistor applications M. Shunaid Parvaiz a, b, Khurshed A. Shah a, *, G.N. Dar b, Prabhakar Misra c a
Department of Physics, S. P. College Campus, Cluster University, Srinagar, Jammu & Kashmir, 190001, India Department of Physics, University of Kashmir, Srinagar, Jammu & Kashmir, 190006, India c Department of Physics & Astronomy, Howard University, Washington, DC, 20059, USA b
A R T I C L E I N F O
A B S T R A C T
Communicated by J Shi
In this paper, photoresistive properties in pristine and homogeneously boron and nitrogen doped semiconducting single-walled carbon nanotubes is studied. The calculations are based on density functional theory in combi nation with Non-Equilibrium Greens Function formalism. The resistance in the SWCNT models is found to decrease with the increasing flux levels. At low electrode voltages, nitrogen doped model shows more photo resistive effect while at high electrode voltages, the most significant photoresistive effect is found in boron doped model. The study reveals that the resistance of the proposed SWCNT systems is dependent on the light intensity, and the conventional boron and nitrogen doping increases the photoresistance by manifold. The models are promising for wide range of applications in the future electronic industry.
Keywords: Carbon nanotubes Photoresistor Doping Optoelectronics
1. Introduction Carbon nanotubes (CNTs) are considered as one of the excellent nanostructures, because of their one dimensional nature and extraor dinary electronic transport properties [1]. A lot of work has been carried out on CNTs for their photonic and electrical applications [2–5], how ever, very little is known about their photoresistive capabilities. Photocurrent of single walled carbon nanotube (SWCNT) films and the optical studies on freestanding transparent films made from nanotubes has been reported earlier [6–8]. The unique density of states present in CNTs allows them to produce electron-hole pairs when exposed to electromagnetic radiations of different wavelengths. Also the tuning of the band gap by controlling diameter and carrier mobility along the tube axis has attracted a lot of attention for their photoresponsive applica tions [9,10]. Furthermore, the CNTs show polarization selectivity, ul traviolet absorption and infrared detection due to their characteristic electronic structure [11–15]. Besides, the anisotropic response to elec tromagnetic radiation is also observed in CNTs due to the constrained electron motion and electron-phonon coupling [16,17]. Therefore, CNTs
are being used in different electronic applications, such as in field effect transistors (FETs) [5], transparent conductors and ring oscillators [18], supercapacitors [19], sensors [20], etc. The CNT based devices are also promising high capabilities for advanced future devices like wearable gadgets and foldable photoresistors [21]. In photoresistive devices, the resistance of the device decreases with increasing light intensity, thus exhibiting photoconductivity [22]. Pho toresistor is a type of resistor in which resistance of the material de creases as the intensity of light increases. The flow of electric current through the photoresistor increases with the increase in the light in tensity. The photoresistor is also referred as LDR (light dependent resistor), semiconductor photoresistor, photoconductor or photocell. Photoresistor changes its resistance only on the exposure to light, and most of these are made up of compound semiconductors like CdS [23], CdSe [24], PbS [25], etc. When using CNT as a photoresistor, in order to produce the net photocurrent along the channel, a potential should be present across the tube. This is done by either keeping the electrodes at different voltages or by doping the CNT channel by atoms of suitable materials [26]. The photovoltaic effect [27–29], and the photo-thermal
* Corresponding author. (Dr. Khurshed A. Shah). E-mail address: [email protected] (K.A. Shah). https://doi.org/10.1016/j.ssc.2020.113831 Received 20 May 2019; Received in revised form 2 December 2019; Accepted 15 January 2020 Available online 24 January 2020 0038-1098/© 2020 Elsevier Ltd. All rights reserved.
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Solid State Communications 309 (2020) 113831
Software (Version 2018.06-SP1-1) and its virtual nanolab graphical user interface [41]. The electrodes are considered within the CNT models so that the current quantization effects due to contact regions can be eliminated. During the analysis, the electrodes were kept at the applied voltage range of 0.0 V–1.6 V and photoresistance is calculated at the flux levels of 100–1000 s 1 Å 2 for all the three models, keeping the values of relative permittivity and relative permeability constant at 1 and tem perature at 300 K. The geometry optimization is done for all the models in order to relax the geometry. The central scattering region of all the models consists of 96 carbon atoms of bond length 1.42 Å, while the electrodes are made up of 16 carbon atoms each considering ten percent of their length as scattering region in order to take care of the scattering loses [42]. Generalized Gradient Approximation-1/2 (GGA-1/2) is used as the ex change correlation function for the LCAO calculator. GGA-1/2 has been used in order to reduce the computing time because here we don’t need “Hubbard U” and “van der Waals correction” parameters to be included in the simulation calculations. For proper sampling, k-points are set at the values of 1, 1, 125 with the density mesh cut-off at 55 Hartee. The photons are kept at energies of 0–5 eV and the photoresistance calcu lations are done at 30 energy points. The photons are kept at the linearly Z-polarization along the carbon nanotube axis. The other simulation parameters of the model are given in Table 1.
Fig. 1. Diagram showing the basic plot of the proposed model.
effect [30,31] are reported to be responsible for photocurrent generation in CNTs. The photovoltaic effect occurs in semiconducting CNTs, where electron-hole separation takes place due to the built-in electric field, and the photo-thermal effect in metallic CNTs, in which the current arises because of photo-excited hot carriers [26]. In these devices the electrical resistance changes upon illumination, which makes them feasible for implementation as efficient and cost effective components for use in sensor technologies [32,33]. Some of the important applications of LDRs are inclusion as resistance in random access memories [34], radiation detection devices [35], motion sensing devices [36], and fire sensing instruments [37]. It is because of the intrinsic gain, greater responsivity dynamic range, fast response and ease of use in circuit element design, that the LDRs are generally preferred over capacitive components [38, 39]. Here in this study, we investigated for the first time, the photo resistive properties of (4, 0) zig-zag SWCNT two probe systems, using Atomistic Tool Kit and its graphical interface Virtual Nanolab. We chose (4, 0) zig-zag SWCNT because this one is a semiconducting nanotube with diameter of 3.13 Å. Due to the semiconducting nature, the doping in this type is more efficient because of small tube diameter and the alterations in the bandgap in semiconducting regime making photo exciton generation favourable. The results show that the flux level plays a significant role in photocurrent generation in CNTs, and the CNT photoresistance purely depends upon the amount of flux. This work is important in order to fully understand the potential of CNTs and CNT nanocomposites in future photo-detection devices like photoresistors and photodiodes.
3. Results and discussions In order to study the photoresistive property of the proposed SWCNTs models, the photocurrent is analyzed by varying the flux over the range 0–1000 s 1Å 2 and the device resistance is calculated at different applied voltages of 0.2 V, 0.6, 1.0 and 1.6 V. Fig. 3 shows the variation of resistance with flux for (a) pristine (b) boron doped, and (c) nitrogen doped SWCNT devices. The results shown in Fig. 3, clearly depicts that, as the flux is increased from 100 to 1000 s 1Å 2, the resistance decreases, which means that the resistance of the CNT devices, inversely varies with the applied flux of light. The photoconductivity occurs due to the photo voltaic effect giving rise to photocurrent due to the optically generated excitons, which under the influence of applied field along the CNT channel separates into free electrons and holes [26]. The reason for the observed results is that the valence electrons absorb energy and become free electrons and when the light energy increases, a large number of valence electrons gain enough energy and jump into the conduction band. A vacancy is created at a particular location in an atom as the electron leaves, called a hole, giving rise to free electrons and holes as pairs. These free charge carriers carry the current. The amount of current flowing through the photoresistor, depends on the number of charge carriers generated, as the light energy increases, the number of charge carriers generated also increases. As a result, the net electric current flowing through the photoresistor increases. This increase in electric current means decrease in resistance. Thus, the resistance of the CNT photoresistor decreases with the increase in the number of photons. Furthermore, from Fig. 3, it is shown that the increase in electrode voltage increases the effect of photoresistivity in all the three models. The increasing field across the tube makes more and more excitons to separate into free electrons and holes, thus increasing the photocurrent and lowering the resistance along the scattering channel. The applied field across the tube adds to the exciton separation process, which in turn increases the generation of photocurrent, and further decrease in the resistance of the channel. Thus it is clear from the results that the photoresistivity in SWCNTs strongly depends upon the magnitude of the applied field across the tube. Table 2 shows the comparison between the various parameters of the three proposed models. It is clear from this table, that in all devices the resistance shows a decreasing trend with increase in flux level from 100 to 1000 s 1Å 2, and boron doped model shows a higher value of resis tance than the pristine and nitrogen doped SWCNT systems at constant
2. Models and methods Fig. 1 shows a zig-zag (4, 0) SWCNT system modeled as a two probe device. All the proposed models consist of three regions, viz., the left electrode, central scattering region lying on a dielectric substrate with dielectric constant K ¼ 1 and the right electrode. Dielectric substrate is used because for all practical purposes the CNT is deposited on a sub strate as shown in previous research works [40]. In our work, the applied electric field along the tube is created by keeping the electrodes at different voltages so that the photogenerated electron-hole pairs are separated, but we can also do the same by using the substrate as a gate in other CNT based devices to produce the required field along the channel. Furthermore, the dielectric constant K is kept at 1 which is the value of dielectric constant for vacuum so that the calculations are simplified and its effect on the channel is negligible. Fig. 2(a) shows a modeled pristine (4, 0) SWCNT, Fig. 2(b) consists of (4, 0) SWCNT as scattering region doped with 16 boron atoms, whereas Fig. 2(c) has (4, 0) SWCNT scattering region doped with 16 nitrogen atoms. The models are simulated using Atomistic Tool Kit (ATK) 2
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Solid State Communications 309 (2020) 113831
Fig. 2. Schematic diagram of (a) pristine, (b) boron doped, (c) nitrogen doped SWCNT systems.
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Solid State Communications 309 (2020) 113831
Table 1 Simulation parameters. Parameter
Value
Calculator Exchange Correlation Function Pseudopotential Density mesh cut-off Occupation method Broadening k-point sampling Maximum Steps Step Size Poisson Solver Electrode Temperature Device Algorithm Formalism
LCAO Generalized Gradient Approximation-1/2. Perdew Burke Ernzerhof functional (GGA-1/2.PBE) PseudoDojo [Z ¼ 4] 55 Hartree Fermi-Dirac 1000 K 1 x 1 x 125 100 0.01 nm Dirichlet 300 K Non-equilibrium Green’s Function
(a)
(b)
(c) Fig. 3. Variation of resistance with flux for (a) pristine, 4 (b) boron doped, (c) nitrogen doped SWCNT systems.
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Solid State Communications 309 (2020) 113831
Table 2 Comparative study of pristine, nitrogen doped, and boron doped SWCNT systems at room temperature. Model
Spectral Peak (nm)
Vmax (Volts)
Operatingtemp. (K)
Resistance at 100 flux (MΩ)
Resistance at 200 flux (MΩ)
Resistance at 300 flux (MΩ)
Resistance at 1000 flux (MΩ)
Pristine Nitrogen doped Boron doped
885.6 310
1.6 1.6
300 300
613 1294
307 650
205 382
63 113
6000
1.6
300
1795
812
586
196
values of temperature and operating voltage. It is also observed from the spectral peaks that the active area of our proposed models lies in the infrared and ultraviolet regions which is discussed in detail in recently reported work [39]. Fig. 4 shows that at low electrode voltage of 0.2 V, photoresistance is more significant over the flux range of 100–1000 s 1Å 2 in nitrogen
doped CNT model and less effective in the pristine CNT model. At low electrode voltage the field is not sufficient to separate the excitonic charges which is further aggravated by the nitrogen doping leading to charge accumulation along the CNT channel, thus increasing the resis tance of the nitrogen doped model. As the electrode voltage is increased, the photoresistive effect becomes more prominent in boron doped model
(a)
(b)
(c)
(d)
Fig. 4. Resistance verses flux curve of pristine, boron doped, and nitrogen doped SWCNT at a constant electrode voltage of (a) 0.2 V (b) 0.6 V (c) 1.0 V (d) 1.6 V. 5
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because of the increase in resistance of CNT channel due to boron doping. At high electrode voltages, charge induced due to nitrogen doping sweeps quickly along the channel thus reducing resistance along the tube [4]. The materials used in LDRs have dark resistance which decreases on exposure to light. But CNTs on the other hand are excellent conductors, thus showing least photoresistivity. While as when we use homogenously doped CNT with boron and nitrogen, it induces p-type and n-type character respectively forming donor-acceptor like states [5]. Thus the majority of the excitonic free carriers recombine with the doping induced free carriers and the field across the tube is not sufficient to separate them [26]. The transitions also gets suppressed because of the Pauli blockade in the low energy CNTs, which in turn increase the resistance [43]. The main feature of Pauli-blockade is the suppression of current for one direction of the source-drain voltage [44,45]. In our model, blockade comes into effect with the boron-nitrogen doping creating a p-n region. This disorder consequently results in valley de generacy causing Pauli-blockade in b-n doped CNT model [46–49]. Resistance in doped CNTs also increases due to the increase in charge screening effect and creation of other doping defects making them favourable material for use in LDRs. The results are comparable with many other previously published reports [27,28,50]. Thus the observed results should be used in designing next generation light dependent electronic devices.
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4. Conclusions In this study simulation based photoresistive property of pristine and doped CNT models is investigated. The results provide information about the photoresistive response of the given CNT devices which arises due to the photogenerated excitonic charge separation by the electric field. The resistance of the modeled devices varies with the incident flux levels. In all the models, resistance is found to decrease with the increase in flux level from 1 to 1000 s 1Å 2 at a constant voltage across the tube. The results reveal that, at low electrode voltages, the nitrogen-doped model shows higher photoresistance when compared with the pristine and the boron-doped SWCNTs. At high voltage, the photoresistance of the boron doped model dominates those of the other two systems. Thus our results show the photoresistive behavior of SWCNTs in the realm of nano-dimensions, therefore providing a new area to design next gener ation nanoscale photon energy dependent electronic devices like photoresistors, photo-detectors, etc. Credit Author Statement First Author: M. Shunaid Parvaiz: Conceptualization, Methodology, Software, Validation, Formal Analysis, Resources, Data Curation, Writing – Original Draft, Visualization. Second Author (Corresponding Author): Khurshed Ahmad Shah: Conceptualization, Methodology, Software, Validation, Formal Analysis, Data Curation, Writing – Review & Editing, Visualization, Supervision, Project Administration, Funding Acquition. Third Author: G. N. Dar: Formal Analysis, Resources, Writing – Review & Editing, Supervision, . Forth Author: Prabhakar Misra: Formal Analysis, Resources, Writing – Review & Editing, Supervision. Acknowledgements This work is supported by the Department of Science and Technol ogy, Science and Engineering Research Board (DST-SERB), New Delhi, India, funded project (Grant No. EMR/002866/2017). References [1] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes Synthesis, Structure, Properties and Applications, Springer-Verlag, Berlin, 2001. [2] P. Avouris, R. Martel, Progress in carbon nanotube electronics and photonics, MRS Bull. 35 (2010) 306–313, https://doi.org/10.1557/mrs2010.553.
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Contents lists available at ScienceDirect
Solid State Communications journal homepage: http://www.elsevier.com/locate/ssc
Computational modeling of carbon nanotubes for photoresistor applications M. Shunaid Parvaiz a, b, Khurshed A. Shah a, *, G.N. Dar b, Prabhakar Misra c a
Department of Physics, S. P. College Campus, Cluster University, Srinagar, Jammu & Kashmir, 190001, India Department of Physics, University of Kashmir, Srinagar, Jammu & Kashmir, 190006, India c Department of Physics & Astronomy, Howard University, Washington, DC, 20059, USA b
A R T I C L E I N F O
A B S T R A C T
Communicated by J Shi
In this paper, photoresistive properties in pristine and homogeneously boron and nitrogen doped semiconducting single-walled carbon nanotubes is studied. The calculations are based on density functional theory in combi nation with Non-Equilibrium Greens Function formalism. The resistance in the SWCNT models is found to decrease with the increasing flux levels. At low electrode voltages, nitrogen doped model shows more photo resistive effect while at high electrode voltages, the most significant photoresistive effect is found in boron doped model. The study reveals that the resistance of the proposed SWCNT systems is dependent on the light intensity, and the conventional boron and nitrogen doping increases the photoresistance by manifold. The models are promising for wide range of applications in the future electronic industry.
Keywords: Carbon nanotubes Photoresistor Doping Optoelectronics
1. Introduction Carbon nanotubes (CNTs) are considered as one of the excellent nanostructures, because of their one dimensional nature and extraor dinary electronic transport properties [1]. A lot of work has been carried out on CNTs for their photonic and electrical applications [2–5], how ever, very little is known about their photoresistive capabilities. Photocurrent of single walled carbon nanotube (SWCNT) films and the optical studies on freestanding transparent films made from nanotubes has been reported earlier [6–8]. The unique density of states present in CNTs allows them to produce electron-hole pairs when exposed to electromagnetic radiations of different wavelengths. Also the tuning of the band gap by controlling diameter and carrier mobility along the tube axis has attracted a lot of attention for their photoresponsive applica tions [9,10]. Furthermore, the CNTs show polarization selectivity, ul traviolet absorption and infrared detection due to their characteristic electronic structure [11–15]. Besides, the anisotropic response to elec tromagnetic radiation is also observed in CNTs due to the constrained electron motion and electron-phonon coupling [16,17]. Therefore, CNTs
are being used in different electronic applications, such as in field effect transistors (FETs) [5], transparent conductors and ring oscillators [18], supercapacitors [19], sensors [20], etc. The CNT based devices are also promising high capabilities for advanced future devices like wearable gadgets and foldable photoresistors [21]. In photoresistive devices, the resistance of the device decreases with increasing light intensity, thus exhibiting photoconductivity [22]. Pho toresistor is a type of resistor in which resistance of the material de creases as the intensity of light increases. The flow of electric current through the photoresistor increases with the increase in the light in tensity. The photoresistor is also referred as LDR (light dependent resistor), semiconductor photoresistor, photoconductor or photocell. Photoresistor changes its resistance only on the exposure to light, and most of these are made up of compound semiconductors like CdS [23], CdSe [24], PbS [25], etc. When using CNT as a photoresistor, in order to produce the net photocurrent along the channel, a potential should be present across the tube. This is done by either keeping the electrodes at different voltages or by doping the CNT channel by atoms of suitable materials [26]. The photovoltaic effect [27–29], and the photo-thermal
* Corresponding author. (Dr. Khurshed A. Shah). E-mail address: [email protected] (K.A. Shah). https://doi.org/10.1016/j.ssc.2020.113831 Received 20 May 2019; Received in revised form 2 December 2019; Accepted 15 January 2020 Available online 24 January 2020 0038-1098/© 2020 Elsevier Ltd. All rights reserved.
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Software (Version 2018.06-SP1-1) and its virtual nanolab graphical user interface [41]. The electrodes are considered within the CNT models so that the current quantization effects due to contact regions can be eliminated. During the analysis, the electrodes were kept at the applied voltage range of 0.0 V–1.6 V and photoresistance is calculated at the flux levels of 100–1000 s 1 Å 2 for all the three models, keeping the values of relative permittivity and relative permeability constant at 1 and tem perature at 300 K. The geometry optimization is done for all the models in order to relax the geometry. The central scattering region of all the models consists of 96 carbon atoms of bond length 1.42 Å, while the electrodes are made up of 16 carbon atoms each considering ten percent of their length as scattering region in order to take care of the scattering loses [42]. Generalized Gradient Approximation-1/2 (GGA-1/2) is used as the ex change correlation function for the LCAO calculator. GGA-1/2 has been used in order to reduce the computing time because here we don’t need “Hubbard U” and “van der Waals correction” parameters to be included in the simulation calculations. For proper sampling, k-points are set at the values of 1, 1, 125 with the density mesh cut-off at 55 Hartee. The photons are kept at energies of 0–5 eV and the photoresistance calcu lations are done at 30 energy points. The photons are kept at the linearly Z-polarization along the carbon nanotube axis. The other simulation parameters of the model are given in Table 1.
Fig. 1. Diagram showing the basic plot of the proposed model.
effect [30,31] are reported to be responsible for photocurrent generation in CNTs. The photovoltaic effect occurs in semiconducting CNTs, where electron-hole separation takes place due to the built-in electric field, and the photo-thermal effect in metallic CNTs, in which the current arises because of photo-excited hot carriers [26]. In these devices the electrical resistance changes upon illumination, which makes them feasible for implementation as efficient and cost effective components for use in sensor technologies [32,33]. Some of the important applications of LDRs are inclusion as resistance in random access memories [34], radiation detection devices [35], motion sensing devices [36], and fire sensing instruments [37]. It is because of the intrinsic gain, greater responsivity dynamic range, fast response and ease of use in circuit element design, that the LDRs are generally preferred over capacitive components [38, 39]. Here in this study, we investigated for the first time, the photo resistive properties of (4, 0) zig-zag SWCNT two probe systems, using Atomistic Tool Kit and its graphical interface Virtual Nanolab. We chose (4, 0) zig-zag SWCNT because this one is a semiconducting nanotube with diameter of 3.13 Å. Due to the semiconducting nature, the doping in this type is more efficient because of small tube diameter and the alterations in the bandgap in semiconducting regime making photo exciton generation favourable. The results show that the flux level plays a significant role in photocurrent generation in CNTs, and the CNT photoresistance purely depends upon the amount of flux. This work is important in order to fully understand the potential of CNTs and CNT nanocomposites in future photo-detection devices like photoresistors and photodiodes.
3. Results and discussions In order to study the photoresistive property of the proposed SWCNTs models, the photocurrent is analyzed by varying the flux over the range 0–1000 s 1Å 2 and the device resistance is calculated at different applied voltages of 0.2 V, 0.6, 1.0 and 1.6 V. Fig. 3 shows the variation of resistance with flux for (a) pristine (b) boron doped, and (c) nitrogen doped SWCNT devices. The results shown in Fig. 3, clearly depicts that, as the flux is increased from 100 to 1000 s 1Å 2, the resistance decreases, which means that the resistance of the CNT devices, inversely varies with the applied flux of light. The photoconductivity occurs due to the photo voltaic effect giving rise to photocurrent due to the optically generated excitons, which under the influence of applied field along the CNT channel separates into free electrons and holes [26]. The reason for the observed results is that the valence electrons absorb energy and become free electrons and when the light energy increases, a large number of valence electrons gain enough energy and jump into the conduction band. A vacancy is created at a particular location in an atom as the electron leaves, called a hole, giving rise to free electrons and holes as pairs. These free charge carriers carry the current. The amount of current flowing through the photoresistor, depends on the number of charge carriers generated, as the light energy increases, the number of charge carriers generated also increases. As a result, the net electric current flowing through the photoresistor increases. This increase in electric current means decrease in resistance. Thus, the resistance of the CNT photoresistor decreases with the increase in the number of photons. Furthermore, from Fig. 3, it is shown that the increase in electrode voltage increases the effect of photoresistivity in all the three models. The increasing field across the tube makes more and more excitons to separate into free electrons and holes, thus increasing the photocurrent and lowering the resistance along the scattering channel. The applied field across the tube adds to the exciton separation process, which in turn increases the generation of photocurrent, and further decrease in the resistance of the channel. Thus it is clear from the results that the photoresistivity in SWCNTs strongly depends upon the magnitude of the applied field across the tube. Table 2 shows the comparison between the various parameters of the three proposed models. It is clear from this table, that in all devices the resistance shows a decreasing trend with increase in flux level from 100 to 1000 s 1Å 2, and boron doped model shows a higher value of resis tance than the pristine and nitrogen doped SWCNT systems at constant
2. Models and methods Fig. 1 shows a zig-zag (4, 0) SWCNT system modeled as a two probe device. All the proposed models consist of three regions, viz., the left electrode, central scattering region lying on a dielectric substrate with dielectric constant K ¼ 1 and the right electrode. Dielectric substrate is used because for all practical purposes the CNT is deposited on a sub strate as shown in previous research works [40]. In our work, the applied electric field along the tube is created by keeping the electrodes at different voltages so that the photogenerated electron-hole pairs are separated, but we can also do the same by using the substrate as a gate in other CNT based devices to produce the required field along the channel. Furthermore, the dielectric constant K is kept at 1 which is the value of dielectric constant for vacuum so that the calculations are simplified and its effect on the channel is negligible. Fig. 2(a) shows a modeled pristine (4, 0) SWCNT, Fig. 2(b) consists of (4, 0) SWCNT as scattering region doped with 16 boron atoms, whereas Fig. 2(c) has (4, 0) SWCNT scattering region doped with 16 nitrogen atoms. The models are simulated using Atomistic Tool Kit (ATK) 2
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Fig. 2. Schematic diagram of (a) pristine, (b) boron doped, (c) nitrogen doped SWCNT systems.
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Table 1 Simulation parameters. Parameter
Value
Calculator Exchange Correlation Function Pseudopotential Density mesh cut-off Occupation method Broadening k-point sampling Maximum Steps Step Size Poisson Solver Electrode Temperature Device Algorithm Formalism
LCAO Generalized Gradient Approximation-1/2. Perdew Burke Ernzerhof functional (GGA-1/2.PBE) PseudoDojo [Z ¼ 4] 55 Hartree Fermi-Dirac 1000 K 1 x 1 x 125 100 0.01 nm Dirichlet 300 K Non-equilibrium Green’s Function
(a)
(b)
(c) Fig. 3. Variation of resistance with flux for (a) pristine, 4 (b) boron doped, (c) nitrogen doped SWCNT systems.
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Table 2 Comparative study of pristine, nitrogen doped, and boron doped SWCNT systems at room temperature. Model
Spectral Peak (nm)
Vmax (Volts)
Operatingtemp. (K)
Resistance at 100 flux (MΩ)
Resistance at 200 flux (MΩ)
Resistance at 300 flux (MΩ)
Resistance at 1000 flux (MΩ)
Pristine Nitrogen doped Boron doped
885.6 310
1.6 1.6
300 300
613 1294
307 650
205 382
63 113
6000
1.6
300
1795
812
586
196
values of temperature and operating voltage. It is also observed from the spectral peaks that the active area of our proposed models lies in the infrared and ultraviolet regions which is discussed in detail in recently reported work [39]. Fig. 4 shows that at low electrode voltage of 0.2 V, photoresistance is more significant over the flux range of 100–1000 s 1Å 2 in nitrogen
doped CNT model and less effective in the pristine CNT model. At low electrode voltage the field is not sufficient to separate the excitonic charges which is further aggravated by the nitrogen doping leading to charge accumulation along the CNT channel, thus increasing the resis tance of the nitrogen doped model. As the electrode voltage is increased, the photoresistive effect becomes more prominent in boron doped model
(a)
(b)
(c)
(d)
Fig. 4. Resistance verses flux curve of pristine, boron doped, and nitrogen doped SWCNT at a constant electrode voltage of (a) 0.2 V (b) 0.6 V (c) 1.0 V (d) 1.6 V. 5
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because of the increase in resistance of CNT channel due to boron doping. At high electrode voltages, charge induced due to nitrogen doping sweeps quickly along the channel thus reducing resistance along the tube [4]. The materials used in LDRs have dark resistance which decreases on exposure to light. But CNTs on the other hand are excellent conductors, thus showing least photoresistivity. While as when we use homogenously doped CNT with boron and nitrogen, it induces p-type and n-type character respectively forming donor-acceptor like states [5]. Thus the majority of the excitonic free carriers recombine with the doping induced free carriers and the field across the tube is not sufficient to separate them [26]. The transitions also gets suppressed because of the Pauli blockade in the low energy CNTs, which in turn increase the resistance [43]. The main feature of Pauli-blockade is the suppression of current for one direction of the source-drain voltage [44,45]. In our model, blockade comes into effect with the boron-nitrogen doping creating a p-n region. This disorder consequently results in valley de generacy causing Pauli-blockade in b-n doped CNT model [46–49]. Resistance in doped CNTs also increases due to the increase in charge screening effect and creation of other doping defects making them favourable material for use in LDRs. The results are comparable with many other previously published reports [27,28,50]. Thus the observed results should be used in designing next generation light dependent electronic devices.
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4. Conclusions In this study simulation based photoresistive property of pristine and doped CNT models is investigated. The results provide information about the photoresistive response of the given CNT devices which arises due to the photogenerated excitonic charge separation by the electric field. The resistance of the modeled devices varies with the incident flux levels. In all the models, resistance is found to decrease with the increase in flux level from 1 to 1000 s 1Å 2 at a constant voltage across the tube. The results reveal that, at low electrode voltages, the nitrogen-doped model shows higher photoresistance when compared with the pristine and the boron-doped SWCNTs. At high voltage, the photoresistance of the boron doped model dominates those of the other two systems. Thus our results show the photoresistive behavior of SWCNTs in the realm of nano-dimensions, therefore providing a new area to design next gener ation nanoscale photon energy dependent electronic devices like photoresistors, photo-detectors, etc. Credit Author Statement First Author: M. Shunaid Parvaiz: Conceptualization, Methodology, Software, Validation, Formal Analysis, Resources, Data Curation, Writing – Original Draft, Visualization. Second Author (Corresponding Author): Khurshed Ahmad Shah: Conceptualization, Methodology, Software, Validation, Formal Analysis, Data Curation, Writing – Review & Editing, Visualization, Supervision, Project Administration, Funding Acquition. Third Author: G. N. Dar: Formal Analysis, Resources, Writing – Review & Editing, Supervision, . Forth Author: Prabhakar Misra: Formal Analysis, Resources, Writing – Review & Editing, Supervision. Acknowledgements This work is supported by the Department of Science and Technol ogy, Science and Engineering Research Board (DST-SERB), New Delhi, India, funded project (Grant No. EMR/002866/2017). References [1] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes Synthesis, Structure, Properties and Applications, Springer-Verlag, Berlin, 2001. [2] P. Avouris, R. Martel, Progress in carbon nanotube electronics and photonics, MRS Bull. 35 (2010) 306–313, https://doi.org/10.1557/mrs2010.553.
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