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Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates
TSF-35780; No of Pages 4 Thin Solid Films xxx (2017) xxx–xxx
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Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates Seunggen Yang, Kyoungah Cho ⁎, Junggwon Yun, Jinyong Choi, Sangsig Kim ⁎ Department of Electrical Engineering, Korea University, Anam-ro 145, Sungbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 August 2016 Received in revised form 26 January 2017 Accepted 31 January 2017 Available online xxxx Keywords: Silver telluride Nanoparticles Thin film Flexible thermoelectrics Seebeck coefficient
a b s t r a c t In this study, we synthesized Ag2Te nanoparticles (NPs) in an aqueous solution and investigated the thermoelectric characteristics of Ag2Te NP thin films on bendable substrates. The Ag2Te NPs have an average size of 6 nm and a body-centered cubic γ-phase (or, γ-Ag2Te), and they exhibit typical p-type thermoelectric behaviors. The Seebeck coefficient and electrical conductivity of a γ-Ag2Te NP thin film are 1330 μV/K and 0.037 S/m, respectively, and the power factor is calculated to be 0.66 μW/mK2. Furthermore, our bending study shows the stability of the thermoelectric characteristics of γ-Ag2Te NP thin films after the bending cycles. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the progress in low-power electronics, nanomaterial-based thermoelectric devices have been considered promising power generators because the capability of nanomaterials to reduce thermal conductivity has exhibited potential in improving thermoelectric efficiency [1–3]. Because bendable electronics have emerged as nextgeneration devices, the bendability of nanomaterial-based devices becomes important to developing flexible thermoelectric devices for real-life applications. Currently, it is of crucial essence to make nanomaterials with high Seebeck coefficients for the development of thermoelectric devices operating at temperatures lower than the glass transition temperatures of plastics. In recent years, silver telluride (Ag2Te) nanoparticles (NPs) have been investigated as prospective thermoelectric nanomaterials for potential thermoelectric devices, owing to their low thermal conductivity and high mobility [4–6]. In addition, one motivation for using Ag2Te as a thermoelectric material is that the type of charge carrier can be changed by the synthesis parameters [7,8]. In previous studies, the thermoelectric characteristics of Ag2Te NPs were examined under special conditions such as a helium atmosphere and high temperature, far from real-life circumstances [9,10]. In addition, up to now, there has been no attempt to fabricate thermoelectric devices using Ag2 Te NP thin films on bendable plastic substrates. Hence, in this study, we prepared thermoelectric Ag2Te NP thin films on bendable plastic substrates and investigated their ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Cho), [email protected] (S. Kim).
thermoelectric characteristics at room temperature in air. Furthermore, we examined the variation in the Seebeck voltage after bending cycles. 2. Experimental details Ag2 Te NPs were synthesized in an aqueous solution as stated elsewhere [11]. Briefly, 0.4 g of AgNO3 and 1 ml of 1-thioglycerol were dissolved in 250 ml of deionized water, and the pH of the solution was adjusted to 11.4 with NaOH solution of 1 M. Then, Ag2Te NP solution was made from the reaction of the solution and H2Te gas generated by the reaction of 0.3 g of Al2Te3 and HCl solution of 4 M. In order to examine the thermoelectric characteristics of Ag2Te NP thin films, a thermoelectric platform was prepared on a polyether sulfone (PES) substrate with an area of 1 cm 2 as shown in Fig. 1. The PES substrate is suitable for the flexible thermoelectric platform. It has the low thermal conductivity (0.18 W/m·K) and chemical resistance that is an essential condition for the manufacturing process of the flexible thermoelectric platform [18,19]. The thermoelectric platform consisted of a meander line patterned heater and two electrodes (E1 and E2) at a distance of 40 μm, with a Ag2Te NP thin film between E1 and E2. A heater and two electrodes were made of Pt by a sputtering method, and a Ag 2 Te NP thin film was formed by spin-coating re-distributed Ag2Te NP solution. The size of the spincoated Ag2Te NP thin films was 1600 μm2 (40 μm × 40 μm). Prior to the fabrication of the Ag2 Te NP thin film, the patterned PES was treated with O 2 plasma in order to make hydrophilic surface. The re-distributed Ag2Te NP solution was prepared by dissolving 1 mg of the Ag2 Te NP powder obtained by centrifugation in 10 μl of
http://dx.doi.org/10.1016/j.tsf.2017.01.068 0040-6090/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
2
S. Yang et al. / Thin Solid Films xxx (2017) xxx–xxx
Fig. 1. Optical image of a thermoelectric platform consisting of a meander line patterned heater, two Pt electrodes (E1, E2), and a Ag2Te NP thin film on a plastic substrate.
deionized water. The sputtered Pt has a high electrical conductivity of 2.14 × 106 S/cm as well as the chemical resistance to solvents used in photolithography process for the fabrication of the thermoelectric platform. The thickness of Ag2Te NP thin film was measured to be 70.2 nm with alpha-step stylus profiler (KLA-Tencor, D-100 stylus profiler). The structural properties of the Ag2Te NPs were examined by xray powder diffraction (XRD) measurement (Rigaku, SmartLab) with Cu Kα radiation (λ = 0.154 nm) and 2θ scanning method. And the size and adjacent lattice distance of the Ag2 Te NPs were measured with a transmission electron microscope (TEM) (Fei, Tecnai F20 with an accelerating voltage of 200 kV) and a high resolution TEM (HRTEM) (Jeol, JEM-3010 with an accelerating voltage of 300 kV). In addition, the chemical composition of the Ag 2 Te NPs was analyzed with energy dispersive x-ray spectroscopy (EDS). The thermoelectric properties were measured with a Keithley 4200, and infrared (IR) images were taken by an IR camera (FLIR A645SC) with an uncertainty of 1% and a sensitivity of 30 mK in air at room temperature. The bending test was performed using a home-made machine.
Fig. 2. (a) XRD pattern, (b) TEM and EDS images, and (c) HR-TEM images of the γ-Ag2Te NPs.
3. Results and discussion Fig. 2(a) shows the XRD pattern of the synthesized Ag2 Te NPs. Their XRD pattern is matched with the body-centered cubic γAg2Te phase (JCPDS 15-0081), although their XRD peaks are broad because of the small size of the Ag2Te particles. Compared with γAg 2Te phase of JCPDS 15-0081, the Ag2 Te NPs synthesized in this study have the preferred orientation of (211). Our TEM and EDS analysis reveals that the synthesized NPs have compositional stoichiometry of 2.1:1 (Ag:Te) and that their average size is approximately 6 nm, as shown in Fig. 2(b). From the HR-TEM images of Fig. 2(c), the distance between adjacent lattice fringes is measured to be 0.23 nm corresponding to the interplanar spacing for (211) plane of cubic phase γ-Ag2Te. For bulk Ag2Te, three different polymorphs exist in a temperature range between room temperature and the melting point: monoclinic α-phase, face-centered cubic βphase (at 145 °C), and body-centered cubic γ-phase (at 802 °C) [12–14]. Interestingly, although our Ag2 Te NPs were obtained in aqueous solution at room temperature without any thermal processes, their XRD pattern is identical to that of high temperature γ-Ag2Te. A similar result to ours was reported by Murray and colleagues for Ag 2Te NPs synthesized by rapid injection of trioctylphosphinetelluride into a mixture of silver dodecanethiol and 4-tertbutyltoluene [14], but there are some differences from ours. Considering that our Ag2 Te NPs were synthesized in aqueous solution unlike the synthesis conditions reported in other studies, we suggest
that the structural property of Ag2Te NPs can be changed with the synthesis conditions. In order to examine the thermoelectric characteristics of a γAg2Te NP thin film, the temperature difference between the ending parts of this thin film touching on two electrodes (E1 and E2) is made by the joule heating of a Pt heater on a thermoelectric platform. The temperature difference is plotted in Fig. 3(a) as a function of heater power. The temperature difference increases linearly with heater power. The Seebeck voltage measured from the Ag2 Te NP thin film is plotted in Fig. 3(b) as a function of temperature difference. The thermoelectric characteristics exhibit the representative p-type with a positive Seebeck coefficient of 1330 μV/K, and the Seebeck coefficient is five or more times larger than those reported in previous studies [8,10,15]. The electrical conductivity is 0.037 S/m. With the Seebeck coefficient and the electrical conductivity, the power factor is calculated to be 0.66 μW/mK2. The Seebeck coefficient reported in this study is the best value for the whole ptype and n-type Ag2Te. The largest Seebeck coefficient may be attributed to a unique structural property of our γ-Ag2Te NPs. Thus far, the thermoelectric characteristics have been investigated only for Ag2Te NPs with a monoclinic α-phase (or, α-Ag2Te NPs) or a face-centered cubic β-phase (or, β-Ag2Te NPs) [10,16]. Tritt et al. have reported that the magnitude of the Seebeck coefficient increases with increasing temperature, and the Seebeck coefficient increases dramatically
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
S. Yang et al. / Thin Solid Films xxx (2017) xxx–xxx
3
this study. In addition, the γ-Ag2Te has a benefit in terms of reducing the thermal conductivity because the γ-Ag2Te has a relatively larger pore space of 32% compared with the β-Ag2Te with a pore space of 26%. Consequently, the thermoelectric characteristics of our thin film made of γ-Ag2Te NPs synthesized in this study are superior to those with the other phases. Fig. 4 represents the Seebeck voltage of the γ-Ag2Te NP thin film at a temperature difference of 2.2 K as a function of bending cycle at a bending radius of 18 mm; the inset shows a photograph of the thin film on a bent plastic substrate inside a homemade bending machine. The average Seebeck voltage is above 4.2 mV under such a small temperature difference. The variation of the Seebeck voltage is less than 6.7% even after 1000 bending cycles, indicating the feasibility of the γ-Ag2Te NP thin film as a bendable thermoelectric material. Our results demonstrate that the γ-Ag2Te NPs are promising thermoelectric materials for the development of flexible thermoelectric devices with high efficiency. 4. Conclusion In this study, we synthesized γ-Ag2Te NPs and investigated the thermoelectric characteristics of a γ-Ag2Te NP thin film constructed on a bendable plastic substrate. The thermoelectric characteristics exhibit typical p-type behaviors. The Seebeck coefficient of the γAg2Te NP thin film is 1330 μV/K, which is significantly larger compared to that of α- or β-Ag2Te NP thin films. The power factor is calculated to be 0.66 μW/mK2 at room temperature in air. Moreover, the stability in the thermoelectric characteristics is maintained even after 1000 bending cycles. Our results demonstrate that the γAg2Te NP thin films are particularly suitable for flexible thermoelectric devices operating in real-life conditions. Acknowledgements
Fig. 3. (a) Temperature of electrodes E1 and E2 as a function of heater power. (b) The Seebeck voltage as a function of temperature difference for the γ-Ag2Te NP thin film.
after the structural phase transition from α-Ag2Te to β-Ag2Te [12]. On the basis of the previous result, we claim that the γ-phase of a higher temperature than the β-phase is responsible for the largest Seebeck coefficient. According to the previous research on the polymorphism of Ag2Te [17], the deficiency of silver favors the γ-phase, which accords with the p-type characteristic of Ag2Te observed in
Fig. 4. Seebeck voltage at a temperature difference of 2.2 K as a function of bending cycle. The inset shows a photograph of the thermoelectric platform inside a home-made bending machine.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF2016R1E1A1A02920171, NRF-2015R1A5A7037674), and the Brain Korea 21 Plus Project in 2016. References [1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [2] K.H. Lee, H.S. Kim, S.I. Kim, E.S. Lee, S.M. Lee, J.S. Rhyee, J.Y. Jung, I.H. Kim, Y. Wang, K. Koumoto, Enhancement of thermoelectric figure of merit for Bi0.5Sb1.5Te3 by metal nanoparticle decoration, J. Electron. Mater. 41 (2012) 1165–1169. [3] Y. Wu, S.W. Finefrock, H. Yang, Nanostructured thermoelectric: opportunities and challenges, Nano Energy 1 (2012) 651–653. [4] A. Sahu, L. Qi, M.S. Kang, D. Deng, D.J. Norris, Facile synthesis of silver chalcogenide (Ag2E; E_Se, S, Te) semiconductor nanocrystals, J. Am. Chem. Soc. 133 (2011) 6509–6512. [5] H. Yang, J. Bahk, T. Day, A.M.S. Mohammed, B. Min, G.J. Snyder, A. Shakouri, Y. Wu, Composition modulation of Ag2Te nanowires for tunable electrical and thermal properties, Nano Lett. 14 (2014) 5398–5404. [6] D. Li, H.Z. Xie, J.K. Liu, C.J. Duan, Induced synthesis and characterisation of Ag and Ag2S assembly nanoparticle chains, J. Exp. Nanosci. 6 (2011) 209–216. [7] R. Dalven, R. Gill, Electrical properties of β-Ag2Te and β-Ag2Se from 4.2 to 300 K, J. Appl. Phys. 38 (1967) 753–756. [8] D. Cadavid, M. Ibáñez, A. Shavel, O.J. Durá, M.A. López de la Torre, A. Cabot, Organic ligand displacement by metal salts to enhance nanoparticle functionality: thermoelectric properties of Ag2Te, J. Mater. Chem. A 1 (2013) 4864–4870. [9] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, Electrical properties of α-and βAg2Te, J. Alloys Compd. 387 (2005) 297–299. [10] Y. Sun, H. Fang, L. Pan, M. Han, S. Xu, X. Wang, B. Xu, Y. Wu, Impact of surface-bound small molecules on the thermoelectric property of self-assembled Ag2Te nanocrystal thin films, Nano Lett. 15 (2015) 3748–3756. [11] H. Seong, K. Cho, S. Kim, Photocurrent characteristics of solution-processed HgTe nanoparticle thin films under the illumination of 1.3 μm wavelength light, Semicond. Sci. Technol. 23 (2008) 075011. [12] J. Capps, F. Drymiotis, S. Lindsey, T.M. Tritt, Significant enhancement of the dimensionless thermoelectric figure of merit of the binary Ag2Te, Philos. Mag. Lett. 90 (2010) 677–681. [13] I. Karakaya, W.T. Thompson, The Ag-Te (silver-tellurium) system, J. Phase Equilib. 12 (1991) 56–63.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
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[14] Y.W. Liu, D.K. Ko, S.J. Oh, T.R. Gordon, V. Doan-Nguyen, T. Paik, Y. Kang, X. Ye, L. Jin, C.R. Kagan, C.B. Murray, Near-infrared absorption of monodisperse silver telluride (Ag2Te) nanocrystals and photoconductive response of their self-assembled superlattices, Chem. Mater. 23 (2011) 4657–4659. [15] Y.Y. Wang, K.F. Cai, J.L. Yin, Y. Du, X. Yao, One-pot fabrication and thermoelectric properties of Ag2Te–polyaniline core–shell nanostructures, Mater. Chem. Phys. 133 (2012) 808–812. [16] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, Thermoelectric properties of α-and β-Ag2Te, J. Alloys Compd. 393 (2005) 299–301.
[17] A. Frueh, The use of Zone theory in problems of sulfide mineralogy. 3., polymorphism of Ag2Te and Ag2S, Am. Mineral. 46 (1961) 654. [18] J. Brandrup, E. Immergut, E. Grulke, Polymer Handbook, fourth ed. Wiley, New York, 1999. [19] E. Arkhangelsky, D. Kuzmenko, V. Gitis, Impact of chemical cleaning on properties and functioning of polyethersulfone membranes, J. Membr. Sci. 305 (2007) 176–184.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates Seunggen Yang, Kyoungah Cho ⁎, Junggwon Yun, Jinyong Choi, Sangsig Kim ⁎ Department of Electrical Engineering, Korea University, Anam-ro 145, Sungbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 27 August 2016 Received in revised form 26 January 2017 Accepted 31 January 2017 Available online xxxx Keywords: Silver telluride Nanoparticles Thin film Flexible thermoelectrics Seebeck coefficient
a b s t r a c t In this study, we synthesized Ag2Te nanoparticles (NPs) in an aqueous solution and investigated the thermoelectric characteristics of Ag2Te NP thin films on bendable substrates. The Ag2Te NPs have an average size of 6 nm and a body-centered cubic γ-phase (or, γ-Ag2Te), and they exhibit typical p-type thermoelectric behaviors. The Seebeck coefficient and electrical conductivity of a γ-Ag2Te NP thin film are 1330 μV/K and 0.037 S/m, respectively, and the power factor is calculated to be 0.66 μW/mK2. Furthermore, our bending study shows the stability of the thermoelectric characteristics of γ-Ag2Te NP thin films after the bending cycles. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the progress in low-power electronics, nanomaterial-based thermoelectric devices have been considered promising power generators because the capability of nanomaterials to reduce thermal conductivity has exhibited potential in improving thermoelectric efficiency [1–3]. Because bendable electronics have emerged as nextgeneration devices, the bendability of nanomaterial-based devices becomes important to developing flexible thermoelectric devices for real-life applications. Currently, it is of crucial essence to make nanomaterials with high Seebeck coefficients for the development of thermoelectric devices operating at temperatures lower than the glass transition temperatures of plastics. In recent years, silver telluride (Ag2Te) nanoparticles (NPs) have been investigated as prospective thermoelectric nanomaterials for potential thermoelectric devices, owing to their low thermal conductivity and high mobility [4–6]. In addition, one motivation for using Ag2Te as a thermoelectric material is that the type of charge carrier can be changed by the synthesis parameters [7,8]. In previous studies, the thermoelectric characteristics of Ag2Te NPs were examined under special conditions such as a helium atmosphere and high temperature, far from real-life circumstances [9,10]. In addition, up to now, there has been no attempt to fabricate thermoelectric devices using Ag2 Te NP thin films on bendable plastic substrates. Hence, in this study, we prepared thermoelectric Ag2Te NP thin films on bendable plastic substrates and investigated their ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Cho), [email protected] (S. Kim).
thermoelectric characteristics at room temperature in air. Furthermore, we examined the variation in the Seebeck voltage after bending cycles. 2. Experimental details Ag2 Te NPs were synthesized in an aqueous solution as stated elsewhere [11]. Briefly, 0.4 g of AgNO3 and 1 ml of 1-thioglycerol were dissolved in 250 ml of deionized water, and the pH of the solution was adjusted to 11.4 with NaOH solution of 1 M. Then, Ag2Te NP solution was made from the reaction of the solution and H2Te gas generated by the reaction of 0.3 g of Al2Te3 and HCl solution of 4 M. In order to examine the thermoelectric characteristics of Ag2Te NP thin films, a thermoelectric platform was prepared on a polyether sulfone (PES) substrate with an area of 1 cm 2 as shown in Fig. 1. The PES substrate is suitable for the flexible thermoelectric platform. It has the low thermal conductivity (0.18 W/m·K) and chemical resistance that is an essential condition for the manufacturing process of the flexible thermoelectric platform [18,19]. The thermoelectric platform consisted of a meander line patterned heater and two electrodes (E1 and E2) at a distance of 40 μm, with a Ag2Te NP thin film between E1 and E2. A heater and two electrodes were made of Pt by a sputtering method, and a Ag 2 Te NP thin film was formed by spin-coating re-distributed Ag2Te NP solution. The size of the spincoated Ag2Te NP thin films was 1600 μm2 (40 μm × 40 μm). Prior to the fabrication of the Ag2 Te NP thin film, the patterned PES was treated with O 2 plasma in order to make hydrophilic surface. The re-distributed Ag2Te NP solution was prepared by dissolving 1 mg of the Ag2 Te NP powder obtained by centrifugation in 10 μl of
http://dx.doi.org/10.1016/j.tsf.2017.01.068 0040-6090/© 2017 Elsevier B.V. All rights reserved.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
2
S. Yang et al. / Thin Solid Films xxx (2017) xxx–xxx
Fig. 1. Optical image of a thermoelectric platform consisting of a meander line patterned heater, two Pt electrodes (E1, E2), and a Ag2Te NP thin film on a plastic substrate.
deionized water. The sputtered Pt has a high electrical conductivity of 2.14 × 106 S/cm as well as the chemical resistance to solvents used in photolithography process for the fabrication of the thermoelectric platform. The thickness of Ag2Te NP thin film was measured to be 70.2 nm with alpha-step stylus profiler (KLA-Tencor, D-100 stylus profiler). The structural properties of the Ag2Te NPs were examined by xray powder diffraction (XRD) measurement (Rigaku, SmartLab) with Cu Kα radiation (λ = 0.154 nm) and 2θ scanning method. And the size and adjacent lattice distance of the Ag2 Te NPs were measured with a transmission electron microscope (TEM) (Fei, Tecnai F20 with an accelerating voltage of 200 kV) and a high resolution TEM (HRTEM) (Jeol, JEM-3010 with an accelerating voltage of 300 kV). In addition, the chemical composition of the Ag 2 Te NPs was analyzed with energy dispersive x-ray spectroscopy (EDS). The thermoelectric properties were measured with a Keithley 4200, and infrared (IR) images were taken by an IR camera (FLIR A645SC) with an uncertainty of 1% and a sensitivity of 30 mK in air at room temperature. The bending test was performed using a home-made machine.
Fig. 2. (a) XRD pattern, (b) TEM and EDS images, and (c) HR-TEM images of the γ-Ag2Te NPs.
3. Results and discussion Fig. 2(a) shows the XRD pattern of the synthesized Ag2 Te NPs. Their XRD pattern is matched with the body-centered cubic γAg2Te phase (JCPDS 15-0081), although their XRD peaks are broad because of the small size of the Ag2Te particles. Compared with γAg 2Te phase of JCPDS 15-0081, the Ag2 Te NPs synthesized in this study have the preferred orientation of (211). Our TEM and EDS analysis reveals that the synthesized NPs have compositional stoichiometry of 2.1:1 (Ag:Te) and that their average size is approximately 6 nm, as shown in Fig. 2(b). From the HR-TEM images of Fig. 2(c), the distance between adjacent lattice fringes is measured to be 0.23 nm corresponding to the interplanar spacing for (211) plane of cubic phase γ-Ag2Te. For bulk Ag2Te, three different polymorphs exist in a temperature range between room temperature and the melting point: monoclinic α-phase, face-centered cubic βphase (at 145 °C), and body-centered cubic γ-phase (at 802 °C) [12–14]. Interestingly, although our Ag2 Te NPs were obtained in aqueous solution at room temperature without any thermal processes, their XRD pattern is identical to that of high temperature γ-Ag2Te. A similar result to ours was reported by Murray and colleagues for Ag 2Te NPs synthesized by rapid injection of trioctylphosphinetelluride into a mixture of silver dodecanethiol and 4-tertbutyltoluene [14], but there are some differences from ours. Considering that our Ag2 Te NPs were synthesized in aqueous solution unlike the synthesis conditions reported in other studies, we suggest
that the structural property of Ag2Te NPs can be changed with the synthesis conditions. In order to examine the thermoelectric characteristics of a γAg2Te NP thin film, the temperature difference between the ending parts of this thin film touching on two electrodes (E1 and E2) is made by the joule heating of a Pt heater on a thermoelectric platform. The temperature difference is plotted in Fig. 3(a) as a function of heater power. The temperature difference increases linearly with heater power. The Seebeck voltage measured from the Ag2 Te NP thin film is plotted in Fig. 3(b) as a function of temperature difference. The thermoelectric characteristics exhibit the representative p-type with a positive Seebeck coefficient of 1330 μV/K, and the Seebeck coefficient is five or more times larger than those reported in previous studies [8,10,15]. The electrical conductivity is 0.037 S/m. With the Seebeck coefficient and the electrical conductivity, the power factor is calculated to be 0.66 μW/mK2. The Seebeck coefficient reported in this study is the best value for the whole ptype and n-type Ag2Te. The largest Seebeck coefficient may be attributed to a unique structural property of our γ-Ag2Te NPs. Thus far, the thermoelectric characteristics have been investigated only for Ag2Te NPs with a monoclinic α-phase (or, α-Ag2Te NPs) or a face-centered cubic β-phase (or, β-Ag2Te NPs) [10,16]. Tritt et al. have reported that the magnitude of the Seebeck coefficient increases with increasing temperature, and the Seebeck coefficient increases dramatically
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
S. Yang et al. / Thin Solid Films xxx (2017) xxx–xxx
3
this study. In addition, the γ-Ag2Te has a benefit in terms of reducing the thermal conductivity because the γ-Ag2Te has a relatively larger pore space of 32% compared with the β-Ag2Te with a pore space of 26%. Consequently, the thermoelectric characteristics of our thin film made of γ-Ag2Te NPs synthesized in this study are superior to those with the other phases. Fig. 4 represents the Seebeck voltage of the γ-Ag2Te NP thin film at a temperature difference of 2.2 K as a function of bending cycle at a bending radius of 18 mm; the inset shows a photograph of the thin film on a bent plastic substrate inside a homemade bending machine. The average Seebeck voltage is above 4.2 mV under such a small temperature difference. The variation of the Seebeck voltage is less than 6.7% even after 1000 bending cycles, indicating the feasibility of the γ-Ag2Te NP thin film as a bendable thermoelectric material. Our results demonstrate that the γ-Ag2Te NPs are promising thermoelectric materials for the development of flexible thermoelectric devices with high efficiency. 4. Conclusion In this study, we synthesized γ-Ag2Te NPs and investigated the thermoelectric characteristics of a γ-Ag2Te NP thin film constructed on a bendable plastic substrate. The thermoelectric characteristics exhibit typical p-type behaviors. The Seebeck coefficient of the γAg2Te NP thin film is 1330 μV/K, which is significantly larger compared to that of α- or β-Ag2Te NP thin films. The power factor is calculated to be 0.66 μW/mK2 at room temperature in air. Moreover, the stability in the thermoelectric characteristics is maintained even after 1000 bending cycles. Our results demonstrate that the γAg2Te NP thin films are particularly suitable for flexible thermoelectric devices operating in real-life conditions. Acknowledgements
Fig. 3. (a) Temperature of electrodes E1 and E2 as a function of heater power. (b) The Seebeck voltage as a function of temperature difference for the γ-Ag2Te NP thin film.
after the structural phase transition from α-Ag2Te to β-Ag2Te [12]. On the basis of the previous result, we claim that the γ-phase of a higher temperature than the β-phase is responsible for the largest Seebeck coefficient. According to the previous research on the polymorphism of Ag2Te [17], the deficiency of silver favors the γ-phase, which accords with the p-type characteristic of Ag2Te observed in
Fig. 4. Seebeck voltage at a temperature difference of 2.2 K as a function of bending cycle. The inset shows a photograph of the thermoelectric platform inside a home-made bending machine.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF2016R1E1A1A02920171, NRF-2015R1A5A7037674), and the Brain Korea 21 Plus Project in 2016. References [1] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008) 105–114. [2] K.H. Lee, H.S. Kim, S.I. Kim, E.S. Lee, S.M. Lee, J.S. Rhyee, J.Y. Jung, I.H. Kim, Y. Wang, K. Koumoto, Enhancement of thermoelectric figure of merit for Bi0.5Sb1.5Te3 by metal nanoparticle decoration, J. Electron. Mater. 41 (2012) 1165–1169. [3] Y. Wu, S.W. Finefrock, H. Yang, Nanostructured thermoelectric: opportunities and challenges, Nano Energy 1 (2012) 651–653. [4] A. Sahu, L. Qi, M.S. Kang, D. Deng, D.J. Norris, Facile synthesis of silver chalcogenide (Ag2E; E_Se, S, Te) semiconductor nanocrystals, J. Am. Chem. Soc. 133 (2011) 6509–6512. [5] H. Yang, J. Bahk, T. Day, A.M.S. Mohammed, B. Min, G.J. Snyder, A. Shakouri, Y. Wu, Composition modulation of Ag2Te nanowires for tunable electrical and thermal properties, Nano Lett. 14 (2014) 5398–5404. [6] D. Li, H.Z. Xie, J.K. Liu, C.J. Duan, Induced synthesis and characterisation of Ag and Ag2S assembly nanoparticle chains, J. Exp. Nanosci. 6 (2011) 209–216. [7] R. Dalven, R. Gill, Electrical properties of β-Ag2Te and β-Ag2Se from 4.2 to 300 K, J. Appl. Phys. 38 (1967) 753–756. [8] D. Cadavid, M. Ibáñez, A. Shavel, O.J. Durá, M.A. López de la Torre, A. Cabot, Organic ligand displacement by metal salts to enhance nanoparticle functionality: thermoelectric properties of Ag2Te, J. Mater. Chem. A 1 (2013) 4864–4870. [9] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, Electrical properties of α-and βAg2Te, J. Alloys Compd. 387 (2005) 297–299. [10] Y. Sun, H. Fang, L. Pan, M. Han, S. Xu, X. Wang, B. Xu, Y. Wu, Impact of surface-bound small molecules on the thermoelectric property of self-assembled Ag2Te nanocrystal thin films, Nano Lett. 15 (2015) 3748–3756. [11] H. Seong, K. Cho, S. Kim, Photocurrent characteristics of solution-processed HgTe nanoparticle thin films under the illumination of 1.3 μm wavelength light, Semicond. Sci. Technol. 23 (2008) 075011. [12] J. Capps, F. Drymiotis, S. Lindsey, T.M. Tritt, Significant enhancement of the dimensionless thermoelectric figure of merit of the binary Ag2Te, Philos. Mag. Lett. 90 (2010) 677–681. [13] I. Karakaya, W.T. Thompson, The Ag-Te (silver-tellurium) system, J. Phase Equilib. 12 (1991) 56–63.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068
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[14] Y.W. Liu, D.K. Ko, S.J. Oh, T.R. Gordon, V. Doan-Nguyen, T. Paik, Y. Kang, X. Ye, L. Jin, C.R. Kagan, C.B. Murray, Near-infrared absorption of monodisperse silver telluride (Ag2Te) nanocrystals and photoconductive response of their self-assembled superlattices, Chem. Mater. 23 (2011) 4657–4659. [15] Y.Y. Wang, K.F. Cai, J.L. Yin, Y. Du, X. Yao, One-pot fabrication and thermoelectric properties of Ag2Te–polyaniline core–shell nanostructures, Mater. Chem. Phys. 133 (2012) 808–812. [16] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, Thermoelectric properties of α-and β-Ag2Te, J. Alloys Compd. 393 (2005) 299–301.
[17] A. Frueh, The use of Zone theory in problems of sulfide mineralogy. 3., polymorphism of Ag2Te and Ag2S, Am. Mineral. 46 (1961) 654. [18] J. Brandrup, E. Immergut, E. Grulke, Polymer Handbook, fourth ed. Wiley, New York, 1999. [19] E. Arkhangelsky, D. Kuzmenko, V. Gitis, Impact of chemical cleaning on properties and functioning of polyethersulfone membranes, J. Membr. Sci. 305 (2007) 176–184.
Please cite this article as: S. Yang, et al., Thermoelectric characteristics of γ-Ag2Te nanoparticle thin films on flexible substrates, Thin Solid Films (2017), http://dx.doi.org/10.1016/j.tsf.2017.01.068