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Effects of Cs-doping on morphological, optical and electrical properties of Bi2Te3 nanostructures
Materials Letters 136 (2014) 337–340
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of Cs-doping on morphological, optical and electrical properties of Bi2Te3 nanostructures Punita Srivastava, Kedar Singh n Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi –221005, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 July 2014 Accepted 13 August 2014 Available online 21 August 2014
A new approach for doping of cesium atoms in Bi2Te3 nanostructures using the inexpensive solvothermal route at a temperature of 100 1C is presented. X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), UV–vis spectral studies and electrical studies (I–V measurements) were carried out for both pure Bi2Te3 and Cs-doped Bi2Te3 nano-samples. The experimental outcome shows that the 2 at.% doping of cesium results in the decrement of band gap and significant enhancement of the current by 3 orders of magnitude. The experimental observations also show that the doping effect of Cesium ions in Bi2Te3 nanostructures changes the linear behavior of I–V curve. & 2014 Elsevier B.V. All rights reserved.
Keywords: Doping effect Band gap I–V behavior
1. Introduction In the last two decades, conducting nanomaterials have emerged as a new class of materials for their applications in highly developed technology and electronic devices. Bismuth-telluride (Bi2Te3) is a significant narrow band gap material with a band gap of 0.15 eV. Bismuth-telluride is known as one of the best thermoelectric materials because they acquire the highest thermoelectric figure of merit known at room temperature [1]. In recent times, research on Bi2Te3 has concerned a lot of interest because it is also predicted to be a threedimensional (3D) topological insulator (TI), a latest set of quantum matter with conductive massless Dirac fermions on its surface [2]. The vigorous and nontrivial metallic surface states, influenced by strong spin–orbit coupling of 3D TIs, which are topologically protected and aligned with back scattering from time-reversal invariant defects and impurities, talented realization of dissipationless electron transport in the lack of high magnetic fields [3,4]. Devices based on conducting nanomaterials have established the potential for applications such as sensors [5,6], light weight batteries [7], Schottky diodes [8], field effect transistors [9] and light emitting diodes [10]. The electrical conductivity of conducting nanomaterials can alter their metallic state to the semiconducting state or vice-versa by physical/chemical or electrochemical doping, which is related to the redox state (doping level). In addition to this, other different properties of conducting nanomaterials could be modified by their interaction with other substances. The ability to correctly control the doping of metallic/semiconductor nanocrystals can generate an opportunity for producing functional
n
Corresponding author. Tel.: þ 91 542 2307308; fax: þ 91 542 2368390. E-mail addresses: [email protected], [email protected] (K. Singh).
http://dx.doi.org/10.1016/j.matlet.2014.08.068 0167-577X/& 2014 Elsevier B.V. All rights reserved.
materials with innovative properties, which are of significance for various applications. The synthesis and investigation in to the basic chemistry of uniform nucleation and crystal growth in the presence of impurities in these materials is a field of deep research. Cesium based compounds have been previously investigated in organic photovoltaics (OPVs) and results have indicated that they can serve as an efficient electron transporting layer in superstrate (normal) [11] and inverted OPV structures [12–14]. To the best of the authors' information, synthesis of cesiumdoped Bi2Te3 nanostructures has been rarely reported by the solvothermal route at temperature 100 1C. The recent lack of studies in this region reflects the doping of cesium atoms in Bi2Te3 nanostructures. The focal purpose of the present work is to report our latest results of Cs-doping on Bi2Te3 nanostructures and to investigate their optical and electrical (I–V measurements) properties. Our motivation for the present work on improving the electrical performance of Bi2Te3 by adding cesium is based on the following considerations: (i) an enhancement in the density of states near the Fermi level is possible; and (ii) both inclusions in cesium and the local defects at the interface between Cs and Bi2Te3 can result in additional lattice scattering. An attempt has been made to obtain the doping of Cs and higher values of current in mA range for the doped sample.
2. Experimental details Preparation of Bi2Te3 nanostructures: In the typical synthesis of Bi2Te3 nanostructures, pure tellurium (99.999%) and BiCl3 (analytical grade) purchased from Alfa Aesar were used without further purification. Ethylene glycol and hydrazine hydrate (analytical
338
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
grade) purchased from Merck, Germany, were used. In brief, synthesis process (10 g) BiCl3 and (5 g) tellurium (in powder form) were taken with deionized water; ethylene glycol and
hydrazine hydrate in a volume ratio of 9:7:2 respectively in a 200 mL capacity of conical flask. Then, the solution was refluxed under vigorous stirring at 100 1C for 6 h. Then, the solution was
Fig. 1. (a) Powder X-ray diffraction patterns for Bi2Te3 and Cs-doped Bi2Te3 nanostructures, (b)SEM images for Bi2Te3and Cs-doped Bi2Te3nanostructures and (c) Representative EDAX spectrum of Cs (2 at%) doped Bi2Te3 nanostructures.
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
339
Fig. 2. (a) Absorption spectra for Bi2Te3 and Cs-doped Bi2Te3 nanostructures at room temperature, (b) variation of band gap of Bi2Te3 nanostructures and (c) variation of band gap of Cs-doped Bi2Te3 nanostructures.
heated to 100 1C. Finally, the black precipitates were collected and washed with anhydrous ethanol and hot distilled water several times, and then dried in a vacuum at 50 1C for 5 h. Preparation of cesium (Cs) -doped Bi2Te3 nanostructures: The successful doping process is described here. In this preparation, cesium-doped Bi2Te3 powders containing 2 at.% of dopant were prepared. During the growth process of Bi2Te3 nanostructures at 100 1C, 2 at% of cesium chloride (CsCl) was added and stirred for 4 h. The reaction was maintained at the same temperature (100 1C) for 4 h for diffusing cesium ions into the synthesized bismuthtelluride material. It was observed that this procedure yields cesium-doped Bi2Te3 nanostructures. A similar experimental procedure was adopted to obtain cesium doped Bi2Te3 powder as in the preparation of pure Bi2Te3 nanopowders. At the end, the obtained black powders of Cs-doped Bi2Te3 were used for studies.
3. Results and discussion The XRD patterns of Bi2Te3 and Cs-doped Bi2Te3 are shown in Fig. 1(a). All samples show sharp diffraction peaks, which could be indexed to the rhombohedral crystal structure of Bi2Te3 (JCPDS card no. 15-0863) with lattice constants of a ¼4.385 Å and c ¼30.48 Å. In addition, some other peaks of the Cs2Te phase were observed (JCPDS card no. 36-1400) in the case of the doped sample with lattice constants a ¼9.109 Å, b ¼11.48 Å and c ¼5.867 Å. In the case of the Cs-doped Bi2Te3 sample, a right shift of X-ray peaks was observed. The research proves the that cesium dopant has been incorporated into Bi2Te3 nano-samples. Fig. 1(b) shows SEM images for pure and Cs-doped Bi2Te3 nanostructures. Morphological changes were observed with Cs-doping into the Bi2Te3 nanostructures. Purity and composition of the synthesized samples were studied by the EDX technique and the result for Cs-doped Bi2Te3 nanostructures displayed in Fig. 1(c). As can be seen, the peaks are clearly related to Bi, Te and
Cs elements in the synthesized sample. From the spectrum of the Cs-doped Bi2Te3 sample, it is clear that 30.55 at.% of bismuth, and 67.53 at.% of tellurium and 1.92 at.% of cesium are observed in the spectra. Fig. 2(a) exhibits absorption spectra of the nanostructures. Absorption peak for Bi2Te3 and Cs-doped Bi2Te3 nanostructures are observed at 260 and 270 nm, respectively. The absorption peak shifts from 260 to 270 nm; this confirms the doping effect. Absorption spectroscopy is very useful to calculate the optical band gap (Eg).The plot of (αhv)2 versus hv is shown in Fig. 2(b) and (c). Extrapolating the straight line of this plot, for zero absorption coefficients (α), it gives the direct band gap energy of nanostructures. The direct band gap energy (Eg) of the Bi2Te3 and Bi2Te3:Cs nanostructures was found to be 2.62 and 2.30 eV respectively, and showed large “blueshift” from standard bulk band gap (Eg ¼0.15 eV) of Bi2Te3, which confirms that the conductivity is higher for Cs-doped Bi2Te3 than that for pure Bi2Te3 nanostructures. A similar decrement of band gap by the doping effect was observed in the case of Cs-doped SnO2 [15]. The current–voltage (I–V) characteristics of pellets of assynthesized pure and cesium-doped Bi2Te3 materials were determined by applying the silver paste contact on both sides of pellets at room temperature. We apply the voltage across the pellets of the synthesized sample to measure the current through the sample. Fig. 3(a) shows the I–V characteristic of pure Bi2Te3 nanostructures (pellet) at room temperature, which shows the linear behavior, indicating that the electrical contacts between Bi2Te3 nanostructures and electrodes are ohmic and thermally stable. Fig. 3(b) shows the current–voltage (I–V) characteristics of pellets of Cs-doped Bi2Te3 nanostructures. The I–V characteristic of doped sample shows non-linear behavior and enhancing the values of current by 3 orders of magnitude (from μA to mA range) at room temperature (300 K) due to the metallic nature of Cs atoms which is used as a dopant in Bi2Te3 nanostructures. Csdoped Bi2Te3 nanostructures synthesized by the solvothermal
340
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
Fig. 3. I–V curves of (a) Bi2Te3 (b) Cs-doped Bi2Te3 at room temperature.
route and have a higher value of current, indicating a potential for the application in high efficiency electronic devices.
providing electrical properties measurements and fruitful suggestions in various ways.
4. Conclusion
References
In summary, Bi2Te3 and Cs-doped Bi2Te3 nanostructures were successfully synthesized by the solvothermal route at 100 1C. The EDAX study confirms the presence of cesium in the nanocrystalline Bi2Te3 sample. The UV–vis absorption spectra of the samples showed a very low absorption in the UV–region. Cs doping results in a significant improvement of the current from μA to mA and reduces the band gap from 2.62 to 2.30 eV. Acknowledgments P. Srivastava is highly grateful for support from CSIR, New Delhi for providing financial assistance through Senior Research Fellowship. We are also thankful to Dr. Pawan Kumar Kulriya and Dr. K. Asokan (Materials Science Section, IUAC, New Delhi) for
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Wright DA. Nature 1958;181:834. Zhang H, Liu CX, Qi XL, Dai X, Fang Z, Zhang SC. Nat Phys 2009;5:438. Qi XL, Zhang SC. Phys Today 2010;63:33. Moore JE. Nature 2010;464:194. Wolszczak M, Kroh J, Abdel-Hamid MM. Radiat Phys Chem 1995;45:71. laranjeira JMG, Khoury HJ, de Azevedo WM, da Silva Jr EF, da Vasconcelos EA. Braz J Phys 2002;32:1. Kalayclooglu E, Akbulut U, Toppare L. J Appl Polym Sci 1996;61:1067. Koezuka H, Tsumura A. Synth Met 1989;28:C753–60. Gustafsson G, Treacy GM, Cao Y, Heeger AJ. Synth Met 1993;55–57:4124. Chiang JC, Macdiarmid AG. Synth Met 1986;13:193. Park M-H, Li J-H, Kumar A, Li G, Yang Y. Adv Funct Mater 2009;19:1241. Liao H-H, Chen L-M, Xu Z, Li G, Yang Y. Appl Phys Lett 2008;92:173303. Cheng G, Tong W-Y, Low K-H, Che C-M. Sol Energy Mater Sol Cells 2012;103:164. Xiao T, Cui W, Cai M, Leung W, Anderegg JW, Shinar J, et al. Org Electron 2013;14:267. Kaviyarasu K, Devarajan PA, Xavier SSJ, Thomasand SA, Selvakumar S. J Mater Sci Technol 2012;15–20:28.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of Cs-doping on morphological, optical and electrical properties of Bi2Te3 nanostructures Punita Srivastava, Kedar Singh n Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi –221005, India
art ic l e i nf o
a b s t r a c t
Article history: Received 2 July 2014 Accepted 13 August 2014 Available online 21 August 2014
A new approach for doping of cesium atoms in Bi2Te3 nanostructures using the inexpensive solvothermal route at a temperature of 100 1C is presented. X-ray diffraction (XRD) pattern, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), UV–vis spectral studies and electrical studies (I–V measurements) were carried out for both pure Bi2Te3 and Cs-doped Bi2Te3 nano-samples. The experimental outcome shows that the 2 at.% doping of cesium results in the decrement of band gap and significant enhancement of the current by 3 orders of magnitude. The experimental observations also show that the doping effect of Cesium ions in Bi2Te3 nanostructures changes the linear behavior of I–V curve. & 2014 Elsevier B.V. All rights reserved.
Keywords: Doping effect Band gap I–V behavior
1. Introduction In the last two decades, conducting nanomaterials have emerged as a new class of materials for their applications in highly developed technology and electronic devices. Bismuth-telluride (Bi2Te3) is a significant narrow band gap material with a band gap of 0.15 eV. Bismuth-telluride is known as one of the best thermoelectric materials because they acquire the highest thermoelectric figure of merit known at room temperature [1]. In recent times, research on Bi2Te3 has concerned a lot of interest because it is also predicted to be a threedimensional (3D) topological insulator (TI), a latest set of quantum matter with conductive massless Dirac fermions on its surface [2]. The vigorous and nontrivial metallic surface states, influenced by strong spin–orbit coupling of 3D TIs, which are topologically protected and aligned with back scattering from time-reversal invariant defects and impurities, talented realization of dissipationless electron transport in the lack of high magnetic fields [3,4]. Devices based on conducting nanomaterials have established the potential for applications such as sensors [5,6], light weight batteries [7], Schottky diodes [8], field effect transistors [9] and light emitting diodes [10]. The electrical conductivity of conducting nanomaterials can alter their metallic state to the semiconducting state or vice-versa by physical/chemical or electrochemical doping, which is related to the redox state (doping level). In addition to this, other different properties of conducting nanomaterials could be modified by their interaction with other substances. The ability to correctly control the doping of metallic/semiconductor nanocrystals can generate an opportunity for producing functional
n
Corresponding author. Tel.: þ 91 542 2307308; fax: þ 91 542 2368390. E-mail addresses: [email protected], [email protected] (K. Singh).
http://dx.doi.org/10.1016/j.matlet.2014.08.068 0167-577X/& 2014 Elsevier B.V. All rights reserved.
materials with innovative properties, which are of significance for various applications. The synthesis and investigation in to the basic chemistry of uniform nucleation and crystal growth in the presence of impurities in these materials is a field of deep research. Cesium based compounds have been previously investigated in organic photovoltaics (OPVs) and results have indicated that they can serve as an efficient electron transporting layer in superstrate (normal) [11] and inverted OPV structures [12–14]. To the best of the authors' information, synthesis of cesiumdoped Bi2Te3 nanostructures has been rarely reported by the solvothermal route at temperature 100 1C. The recent lack of studies in this region reflects the doping of cesium atoms in Bi2Te3 nanostructures. The focal purpose of the present work is to report our latest results of Cs-doping on Bi2Te3 nanostructures and to investigate their optical and electrical (I–V measurements) properties. Our motivation for the present work on improving the electrical performance of Bi2Te3 by adding cesium is based on the following considerations: (i) an enhancement in the density of states near the Fermi level is possible; and (ii) both inclusions in cesium and the local defects at the interface between Cs and Bi2Te3 can result in additional lattice scattering. An attempt has been made to obtain the doping of Cs and higher values of current in mA range for the doped sample.
2. Experimental details Preparation of Bi2Te3 nanostructures: In the typical synthesis of Bi2Te3 nanostructures, pure tellurium (99.999%) and BiCl3 (analytical grade) purchased from Alfa Aesar were used without further purification. Ethylene glycol and hydrazine hydrate (analytical
338
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
grade) purchased from Merck, Germany, were used. In brief, synthesis process (10 g) BiCl3 and (5 g) tellurium (in powder form) were taken with deionized water; ethylene glycol and
hydrazine hydrate in a volume ratio of 9:7:2 respectively in a 200 mL capacity of conical flask. Then, the solution was refluxed under vigorous stirring at 100 1C for 6 h. Then, the solution was
Fig. 1. (a) Powder X-ray diffraction patterns for Bi2Te3 and Cs-doped Bi2Te3 nanostructures, (b)SEM images for Bi2Te3and Cs-doped Bi2Te3nanostructures and (c) Representative EDAX spectrum of Cs (2 at%) doped Bi2Te3 nanostructures.
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
339
Fig. 2. (a) Absorption spectra for Bi2Te3 and Cs-doped Bi2Te3 nanostructures at room temperature, (b) variation of band gap of Bi2Te3 nanostructures and (c) variation of band gap of Cs-doped Bi2Te3 nanostructures.
heated to 100 1C. Finally, the black precipitates were collected and washed with anhydrous ethanol and hot distilled water several times, and then dried in a vacuum at 50 1C for 5 h. Preparation of cesium (Cs) -doped Bi2Te3 nanostructures: The successful doping process is described here. In this preparation, cesium-doped Bi2Te3 powders containing 2 at.% of dopant were prepared. During the growth process of Bi2Te3 nanostructures at 100 1C, 2 at% of cesium chloride (CsCl) was added and stirred for 4 h. The reaction was maintained at the same temperature (100 1C) for 4 h for diffusing cesium ions into the synthesized bismuthtelluride material. It was observed that this procedure yields cesium-doped Bi2Te3 nanostructures. A similar experimental procedure was adopted to obtain cesium doped Bi2Te3 powder as in the preparation of pure Bi2Te3 nanopowders. At the end, the obtained black powders of Cs-doped Bi2Te3 were used for studies.
3. Results and discussion The XRD patterns of Bi2Te3 and Cs-doped Bi2Te3 are shown in Fig. 1(a). All samples show sharp diffraction peaks, which could be indexed to the rhombohedral crystal structure of Bi2Te3 (JCPDS card no. 15-0863) with lattice constants of a ¼4.385 Å and c ¼30.48 Å. In addition, some other peaks of the Cs2Te phase were observed (JCPDS card no. 36-1400) in the case of the doped sample with lattice constants a ¼9.109 Å, b ¼11.48 Å and c ¼5.867 Å. In the case of the Cs-doped Bi2Te3 sample, a right shift of X-ray peaks was observed. The research proves the that cesium dopant has been incorporated into Bi2Te3 nano-samples. Fig. 1(b) shows SEM images for pure and Cs-doped Bi2Te3 nanostructures. Morphological changes were observed with Cs-doping into the Bi2Te3 nanostructures. Purity and composition of the synthesized samples were studied by the EDX technique and the result for Cs-doped Bi2Te3 nanostructures displayed in Fig. 1(c). As can be seen, the peaks are clearly related to Bi, Te and
Cs elements in the synthesized sample. From the spectrum of the Cs-doped Bi2Te3 sample, it is clear that 30.55 at.% of bismuth, and 67.53 at.% of tellurium and 1.92 at.% of cesium are observed in the spectra. Fig. 2(a) exhibits absorption spectra of the nanostructures. Absorption peak for Bi2Te3 and Cs-doped Bi2Te3 nanostructures are observed at 260 and 270 nm, respectively. The absorption peak shifts from 260 to 270 nm; this confirms the doping effect. Absorption spectroscopy is very useful to calculate the optical band gap (Eg).The plot of (αhv)2 versus hv is shown in Fig. 2(b) and (c). Extrapolating the straight line of this plot, for zero absorption coefficients (α), it gives the direct band gap energy of nanostructures. The direct band gap energy (Eg) of the Bi2Te3 and Bi2Te3:Cs nanostructures was found to be 2.62 and 2.30 eV respectively, and showed large “blueshift” from standard bulk band gap (Eg ¼0.15 eV) of Bi2Te3, which confirms that the conductivity is higher for Cs-doped Bi2Te3 than that for pure Bi2Te3 nanostructures. A similar decrement of band gap by the doping effect was observed in the case of Cs-doped SnO2 [15]. The current–voltage (I–V) characteristics of pellets of assynthesized pure and cesium-doped Bi2Te3 materials were determined by applying the silver paste contact on both sides of pellets at room temperature. We apply the voltage across the pellets of the synthesized sample to measure the current through the sample. Fig. 3(a) shows the I–V characteristic of pure Bi2Te3 nanostructures (pellet) at room temperature, which shows the linear behavior, indicating that the electrical contacts between Bi2Te3 nanostructures and electrodes are ohmic and thermally stable. Fig. 3(b) shows the current–voltage (I–V) characteristics of pellets of Cs-doped Bi2Te3 nanostructures. The I–V characteristic of doped sample shows non-linear behavior and enhancing the values of current by 3 orders of magnitude (from μA to mA range) at room temperature (300 K) due to the metallic nature of Cs atoms which is used as a dopant in Bi2Te3 nanostructures. Csdoped Bi2Te3 nanostructures synthesized by the solvothermal
340
P. Srivastava, K. Singh / Materials Letters 136 (2014) 337–340
Fig. 3. I–V curves of (a) Bi2Te3 (b) Cs-doped Bi2Te3 at room temperature.
route and have a higher value of current, indicating a potential for the application in high efficiency electronic devices.
providing electrical properties measurements and fruitful suggestions in various ways.
4. Conclusion
References
In summary, Bi2Te3 and Cs-doped Bi2Te3 nanostructures were successfully synthesized by the solvothermal route at 100 1C. The EDAX study confirms the presence of cesium in the nanocrystalline Bi2Te3 sample. The UV–vis absorption spectra of the samples showed a very low absorption in the UV–region. Cs doping results in a significant improvement of the current from μA to mA and reduces the band gap from 2.62 to 2.30 eV. Acknowledgments P. Srivastava is highly grateful for support from CSIR, New Delhi for providing financial assistance through Senior Research Fellowship. We are also thankful to Dr. Pawan Kumar Kulriya and Dr. K. Asokan (Materials Science Section, IUAC, New Delhi) for
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Wright DA. Nature 1958;181:834. Zhang H, Liu CX, Qi XL, Dai X, Fang Z, Zhang SC. Nat Phys 2009;5:438. Qi XL, Zhang SC. Phys Today 2010;63:33. Moore JE. Nature 2010;464:194. Wolszczak M, Kroh J, Abdel-Hamid MM. Radiat Phys Chem 1995;45:71. laranjeira JMG, Khoury HJ, de Azevedo WM, da Silva Jr EF, da Vasconcelos EA. Braz J Phys 2002;32:1. Kalayclooglu E, Akbulut U, Toppare L. J Appl Polym Sci 1996;61:1067. Koezuka H, Tsumura A. Synth Met 1989;28:C753–60. Gustafsson G, Treacy GM, Cao Y, Heeger AJ. Synth Met 1993;55–57:4124. Chiang JC, Macdiarmid AG. Synth Met 1986;13:193. Park M-H, Li J-H, Kumar A, Li G, Yang Y. Adv Funct Mater 2009;19:1241. Liao H-H, Chen L-M, Xu Z, Li G, Yang Y. Appl Phys Lett 2008;92:173303. Cheng G, Tong W-Y, Low K-H, Che C-M. Sol Energy Mater Sol Cells 2012;103:164. Xiao T, Cui W, Cai M, Leung W, Anderegg JW, Shinar J, et al. Org Electron 2013;14:267. Kaviyarasu K, Devarajan PA, Xavier SSJ, Thomasand SA, Selvakumar S. J Mater Sci Technol 2012;15–20:28.