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Time dependent conductive behavior of the layer-by-layer self-assembled SnO2 nanoparticle thin film
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Thin Solid Films 516 (2008) 3167 – 3171 www.elsevier.com/locate/tsf
Time dependent conductive behavior of the layer-by-layer self-assembled SnO2 nanoparticle thin film Y.J. Shin a , Q. Zhang b , F. Hua b,⁎ a
School of Information and Communication Engineering Electrical and Computer Engineering Department SungKyunKwan University 300 CheonCheon-Dong, Suwon 440-746, South Korea b Electrical and Computer Engineering Department Clarkson University Potsdam, NY 13699, USA Received 6 June 2007; received in revised form 25 July 2007; accepted 17 August 2007 Available online 24 August 2007
Abstract The understanding of the electrical conductivity properties of polyion/nanoparticle thin film assemblies has long-term impact for exploring the unique electrical properties in applications such as sensor, actuator and conductive coating. This paper reports on the observations of the time dependent conducting behavior of the SnO2 thin film produced by layer-by-layer self-assembly. The thin film consists of 5 layers of polyion matrix and 12 nm SnO2 nanoparticles that are adsorbed in an alternating sequence. The electrical current flowing through such thin film is not stabilized immediately, but instead increases with time until saturation. We further find a threshold current that is required to trigger the current self-enhancing process. It is hypothesized that the self-heating of the semiconductive nanoparticle explains this phenomenon. Based on this hypothesis, a hybrid channel is designed to completely eliminate this effect. This fundamental study may find applications in different gas and biomedical sensors and transparent electronics. © 2007 Elsevier B.V. All rights reserved. Keywords: Layer-by-layer self-assembly; Nanoparticles; Tin dioxide; Polyion/nanoparticle multilayer; Conductivity
1. Introduction The layer-by-layer (LbL) self-assembly technique has become a powerful tool for the synthesis of nanostructures and hybrid organic–inorganic materials according to the application requirements which represent one of the highest achievements of current chemistry. Its advantages that allow a precise control of molecular order and thickness enable the engineering over macroscopic electrical, optical, magnetic, thermal, mechanical, and other properties, important for many engineering devices and applications [1–5]. The development of thin films composed of polyions and functional nanoparticles is an active field of research. Such films can be obtained via the sequential adsorption of oppositely charged macromolecules from solution. The typical procedure involves the alternate adsorption of two components, the flexible polyion and the nanoparticle. While the flexible ⁎ Corresponding author. Tel.: +1 315 2682126; fax: +1 3152687600. E-mail address: [email protected] (F. Hua). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.08.095
polyion chain ensures a uniform construction of the film, the nanoparticle is used to functionalize the films [6,7]. Functionality can be achieved because the adsorbed components are governed by non-covalent interactions, mainly the electrostatic interactions. Such thin films are regarded as the promising components in numerous applications including rectifying p–n junction [8,9], flexible solar cells [10,11], synthetic catalysts [12,13], antimicrobial agents [14], optical coating [15], and luminescent devices [16]. The electrical property of the self-assembled nanoparticle thin film will play an important role to various devices and systems. SnO2 is a well-known gas sensing material because its conductivity is significantly influenced by the type and amount of gas that is adsorbed on its surface. A large amount of research has been done in developing highly sensitive SnO2 conductivity-based gas sensors [17,18]. It is also widely reported that sensitivity can be dramatically improved if the SnO2 thin film is produced from the smallest possible particles because the sensing is a surface phenomenon [19,20]. This paper reports on the observation of the electrical behavior of the polyion/SnO2
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thin film for the reason that it is an important material and may be representative for the self-assembled polyion/nanoparticle architectures. Interestingly, it is found that when the external voltage is applied across the self-assembled polyion/SnO2 thin film, the electrical current is not stable as it is supposed to be. On the contrary, it shows the time dependent behavior, i. e., it increases steadily until it is eventually stabilized. The I–V characterization of the thin film is analyzed and a hypothesis is proposed. A hybrid channel is then designed to overcome the instability of the current. Finally, a prototype of transparent thermal sensor is demonstrated. 2. Experimental details
Fig. 1. Schematic of the template with ITO as the electrodes and LbL selfassembled SnO2 nanoparticle multilayer as the conductive channel. The order of the multilayer is: (PDDA + PSS)2 + (PDDA + SnO2 nanoparticle)5. The subscript denotes the number of the adsorbed layer. Only 3 bilayers of PDDA/SnO2 were demonstrated.
polymer, mainly polyurethane, which was cured by the irradiation of UV light.
2.1. Materials and equipment The polyelectrolytes used for the self-assembly process were poly(dimethyldiallyl ammonium chloride) (PDDA) (aqueous solution, MW 200 to 300 K, 3 mg/mL, 0.5 M NaCl) and poly (styrenesulfonate) (PSS) (aqueous solution, MW 70 K, 3 mg/ mL, and 0.5 M NaCl), both of which were obtained from Aldrich-Sigma. The dispersions of clear SnO2 nanocrystals, ∼ 12 nm in diameter, were obtained from Nyacol. It was diluted to 10 mg/mL for use. The glass slide coated with indium tin oxide (ITO), with an ITO thickness of about 120 nm and sheet resistivity of 8–12 Ω/square, was obtained from Delta Technologies. The photoresist was NR9-1500P from Futurrex, Inc. The polyurethane used for encapsulating the film was from Norland Products. A quartz crystal microbalance (QCM) which was used to analyze the growth of the thin film was obtained from Stanford Research Systems, Inc. The conductive behavior was characterized by an HP 4156 A semiconductor parameter analyzer. The high-resolution temperature source consists of a Peltier solid-state thermoelectric cooler, a platinum probe and a temperature controller which were all from Melcor with an overall resolution of 0.1 °C. 2.2. Fabrication of the self-assembled SnO2 multilayer The fabrication of the template started from the ITO-coated glass slide. The ITO layer was patterned to form two electrodes on the glass using photolithography. The solution to etch the ITO was the mixture of hydrochloric acid and nitric acid with a volume ratio of 3:1. The width and thickness of the ITO electrode was 1 cm and 120 nm, respectively. The gap between the electrodes was 40 μm. The next lithography step patterned a window on the photoresist over the gap, making the gap exposed to the atmosphere. Two bilayers of polycation and polyanion, PDDA/PSS, were alternately adsorbed into the gap using LbL assembly, in order to construct a firm foundation for the subsequent film growth [21]. Then, 5 bilayers of PDDA/ SnO2 nanoparticle (∼ 12 nm in diameter) were adsorbed on the PDDA/PSS multilayer. Finally, the photoresist was dissolved, leaving the nanoparticle thin film bridging the two ITO electrodes, as shown in Fig. 1. Later, the conductive channel was encapsulated in order to be isolated from the atmosphere by a transparent photo curable
2.3. Electrical characterization of the self-assembled SnO2 multilayer The electrodes were probed on a microprobe station. The voltage and current signals were collected by a HP 4156 A semiconductor parameter analyzer with a current resolution of 1 pA. When measuring the current at a very low level, the “long” measuring mode with the averaging function was selected in an attempt to average out the noise. In order to measure the temperature response of the SnO2 nanoparticle multilayer, the template was placed on the melcor thermoelectric cooler which was controlled by a high-resolution temperature controller with a resolution of 0.1 °C. 3. Results and discussions The LbL assembly makes use of the alternate adsorption of oppositely charged supramolecular species [6,7]. Because the nanoparticles were negatively charged, they were alternately adsorbed onto the substrate with polycation, PDDA. As a wellestablished practice, the substrate should be treated in order to carry the uniform surface charge for the subsequent adsorption. In this experiment, after the ITO substrate was etched in the acidic solution, its surface was oxidized, generating sufficient density of surface charge. Moreover, the growth of the nanoparticle multilayer was monitored by a QCM to control the step growth. The frequency shift corresponding to the adsorption showed that the step growth of PDDA and SnO2 nanocrystals were about 2 and 12 nm, respectively, the later one indicating the formation of a complete nanoparticle monolayer for each dipping. The total thickness of the multilayer was about 76 nm. When the external voltage of 7 V was applied across the electrodes, the current did not stabilize at once. It increased from 2 nA to 730 nA which was the saturated value within 60 min. The time dependence of the current is shown in Fig. 2. The decreasing of the current accelerating rate as a function of the time was observed. This rate gradually decreases as the magnitude of the current reached a higher level. For example, at a current magnitude of ∼2 nA, the rate was ∼ 45 nA/min while at 730 nA, it decreased to zero. It was found that there was a threshold current density, ∼ 1.67 × 103 nA/mm2, required to
Y.J. Shin et al. / Thin Solid Films 516 (2008) 3167–3171
Fig. 2. The time-dependence of the current flowing through the PDDA/SnO2 multilayer. The current basically takes 1 h to stabilize. The multilayer which was isolated from the atmosphere showed a more serious time dependent property.
initiate the process of current self-enhancement. If the initial current density was less than the threshold which may result from lower external voltage, the current self-enhancement did take place. The effect could happen only when the current density was higher than the threshold. When the SnO2 multilayer was encapsulated by a thin layer of polyurethane which was cured by the UV light illumination, the increasing of the current became more rapid and serious, i. e., the current increases at a much higher speed and stabilizes at a higher current magnitude. The time-dependence of the current for the encapsulated template is also shown in Fig. 2. The SnO2 nanoparticle multilayer is sensitive to the ambient temperature. The current flowing through the SnO2 multilayer which could be the gas sensing material is influenced by the interaction of the SnO2 nanoparticle with the ambient atmospheric molecules, which is a complex function of temperature [22,23]. Overall speaking, for the template
Fig. 3. The current flowing through the PDDA/SnO2 multilayer as a function of temperature which was measured without delay. However, the current would increase with the time so that the temperature related property was covered up. After a discontinuous copper film was added below the PDDA/SnO2 multilayer, this curve became stable with the time.
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produced in this experiment, it was found that the current decreased with the increasing of the temperature. As demonstrated in Fig. 3, when the temperature of the thermoelectric increased, the current of the template immediately decreased in response to the temperature change. However, due to the selfenhancement of the current, the temperature characterization of the nanoparticle multilayer was covered up. As the time went on, the magnitude of the current corresponding to different temperatures automatically increased to the same high level. The time dependent conducting behavior of the selfassembled SnO2 multilayer could be explained by a selfheating mechanism. The nanoparticles are functionalized by coupling chemical groups onto the surface, which separate the adjacent nanoparticles by the chain length. Moreover, they are embedded in the polyion matrix as a result of the LbL assembly. Nanoparticles in different layers are separated by the polyion molecules. The conductivity of such polyion/nanoparticle architecture can be described by the activated tunneling model [24]: EA rðd; T Þ ¼ ro expðbdÞ exp ð1Þ RT
EA ¼
e2 1 1 8keeo r r þ d
ro ¼ nel:
ð2Þ
ð3Þ
In Eq. (1), σ is the conductivity, and σo is the intrinsic conductivity of the nanoparticles as defined by Eq. (3). β represents the electron coupling term and δ is the interparticle distance. EA is the activation energy in kJ/mol, T is the temperature in Kelvin, and R is the universal gas constant. In Eq. (2), the activation energy EA is related to r for the radius of nanoparticles, εo for the permittivity constant, and ε for the relative permittivity. In Eq. (3), the intrinsic value of conductivity, σo, is the product of n for the number of charge
Fig. 4. The optical micrograph of a transparent thermal sensor on the glass slide. The amplified part is the image of part of the channel between electrodes.
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carriers, m for the mobility of the charge carriers m, and e for the electronic charge. The entire conducting cycle is comprised of the ejection of the electron from a nanoparticle and hopping from one particle to the next under the electrical field. When a certain amount of current travels through the nanoparticle, it is expected to excite the nanoparticle through the continuous energy transfer between electrons and nanoparticle, thus generating more “hot” electrons which are more likely to eject from the nanoparticle surface. This self-heating effect in turn provides more mobile charge carriers for the electrical conduction. It is obviously a cumulative process so that the number of mobile electrons increases with the time until saturation is reached when the heating or excitation of the nanoparticle is equal to the dissipation. The much higher increasing rate and saturation magnitude of the current after the template was encapsulated by polyurethane may support this hypothesis because the heat dissipation was made difficult after the conductive channel was isolated from the atmosphere. Based on the above hypothesis, one structure was designed to eliminate the self-heating effect. An extra metallic channel was constructed below the self-assembled nanoparticle film, providing a conductive channel which was immune to the selfheating effect. In particular, a thin copper film of ∼ 15 nm thick was evaporated between the two ITO electrodes. At this thickness, the copper film is just a discontinuous layer with a series of isolated islands. Therefore, the current resulted from the copper film under the same bias of 7 V was measured to be only at the level of 20 pA, largely noise. Then, the same number of self-assembled PDDA/SnO2 nanoparticle multilayer is adsorbed upon the copper film. It was found that the selfheating effect was effectively eliminated by using this hybrid channel structure. The current was stabilized after the voltage was applied without delay. For example, the current was measured to be 6 nA immediately after 7 V was applied and could stand firm at this value. It looks that, on one hand, the SnO2 nanoparticle film generates more mobile electrons as a result of self-heating from time to time. The generated electrons are likely to hop down into the copper channel which may have a lower resistivity and then travel from one copper island to the next through the void. On the other hand, the copper conductive channel looks immune to the self-heating effect probably because of the less resistivity and scattering to the electrons that travel inside the copper island. For example, when only a thicker copper film, ∼30 nm, was evaporated and a current of ∼ 100 nA was detected, the self-heating did not take place in the copper film. In short, it is hypothesized that the energetic electrons generated in the SnO2 nanoparticle due to the selfheating may prefer to migrate into the conductive path of the discontinuous copper film, thus circumventing the self-heating effect. Because the current component associated with only the thin copper film about ∼ 20 nm thick is negligibly small, the copper conductive channel itself does not generate the resistive current. Its function is to provide an extra channel for those electrons generated inside the SnO2 nanoparticle. Based on this hybrid structure, a prototype transparent thermal sensor is proposed. The transparent sensor can collect not only the optical signal but also other physical and chemical
signals because the light can pass through the sensor's body. Consequently, it provides more insights to the objects under study. The transparent sensor can also find special application for military and homeland security. The visual effect of the transparent thermal sensor on a glass slide is shown in Fig. 4. The optical micrograph above the sensor shows the channel between the two electrodes. It was fabricated as above mentioned on a glass slide with ITO as the electrodes. 20 nm copper film and 5 bilayers of PDDA/SnO2 multilayer were coated across the two electrodes. The optical transmittance of the conductive channel region which is the thickest in the sensor was ∼ 75%. Since the self-heating effect was removed, the stable current could be measured reliably and showed the same trend of current versus the variation of the temperature as shown in Fig. 3. 4. Conclusions In summary, the time dependent conductive behavior of the LbL assembled SnO2 nanoparticle multilayer is studied. It is found that the current of the multilayer does not reach the stabilized value until a process of current self-increasing is finished. A possible explanation of self-heating is provided to address the phenomenon. It suggests that the electrical current passing through the SnO2 nanoparticle may increase the number of the energetic electrons via the energy transfer. The selfheating effect can be effectively overcome by using a hybrid channel structure in which the SnO2 nanoparticle provides the active electrons while the discontinuous copper film provides the conductive channel that does not lead to self-heating. It can find potential applications in sensors and different conductive coatings. References [1] G. Decher, Science 277 (1997) 1232. [2] S. Qin, D. Qin, W.T. Ford, Y. Zhang, N.A. Kotov, Chem. Mater. 17 (2005) 2131. [3] K.M. Chen, X.P. Jiang, L.C. Kimerling, P.T. Hammond, Langmuir 16 (2000) 7825. [4] J. Shen, J. Sun, X. Zhang, Pure Appl. Chem. 72 (2000) 147. [5] T. Shutava, M. Prouty, D. Kommireddy, Y. Lvov, Macromolecules 38 (2005) 2850. [6] R.K. Iler, J. Colloid Interface Sci. 21 (1966) 569. [7] G. Decher, J.D. Hong, Makromol. Chem., Macromol. Symp. 46 (1991) 321. [8] A.P. Alivisatos, MRS Bull. 23 (1998) 18. [9] T. Cassagneau, T.E. Mallouk, J.H. Fendler, J. Am. Chem. Soc. 120 (1998) 7848. [10] J.A. He, R. Mosurkal, L.A. Samuelson, L. Li, J. Kumar, Langmuir 19 (2003) 2169. [11] S. Takenaka, Y. Maeharaa, H. Imaia, M. Yoshikawab, S. Shiratori, Thin Solid Films 438 (2003) 346. [12] H. Qiu, W.Y. Lee, Appl. Catal., A Gen. 314 (2006) 200. [13] B.C. Ku, S.H. Lee, W. Liu, J.A. He, J. Kumar, F.F. Bruno, L.A. Samuelson, J. Macromol. Sci., Pure Appl. Chem. 40 (2004) 1335. [14] J.C. Grunlan, J.K. Choi, A. Lin, Biomacromolecules 6 (2005) 1149. [15] W. Zhizhong, J. Walish, A. Nolte, L. Zhai, R.E. Cohen, M.F. Rubner, Adv. Mater. 18 (2006) 2699. [16] W. Chen, D. Grouquist, J. Roark, J. Nanosci. Nanotechnol. 2 (2002) 47. [17] Y. Hiranaka, T. Abe, H. Murata, Sens Actuators, B 9 (1992) 177.
Y.J. Shin et al. / Thin Solid Films 516 (2008) 3167–3171 [18] A. Heilig, N. Barsan, U. Weimar, M.S. Berberich, W. Gopel, Sens Actuators, B 43 (1997) 45. [19] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Airyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271. [20] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators, B 3 (1991) 147. [21] Y. Lvov, G. Decher, Crystallogr. Rep. 39 (1994) 628.
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[22] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, M. Gaitan, P. Chaparala, Sens Actuators, B 33 (1996) 142. [23] A. Ortega, S. Marco, A. Perera, T. Sundic, A. Pardo, J. Samitier, Sens Actuators, B. 78 (2001) 32. [24] G.G. Wang, L. Wang, Q. Rendeng, J. Wang, J. Luo, C. Zhong, J. Mater. Chem. 17 (2007) 457.
Thin Solid Films 516 (2008) 3167 – 3171 www.elsevier.com/locate/tsf
Time dependent conductive behavior of the layer-by-layer self-assembled SnO2 nanoparticle thin film Y.J. Shin a , Q. Zhang b , F. Hua b,⁎ a
School of Information and Communication Engineering Electrical and Computer Engineering Department SungKyunKwan University 300 CheonCheon-Dong, Suwon 440-746, South Korea b Electrical and Computer Engineering Department Clarkson University Potsdam, NY 13699, USA Received 6 June 2007; received in revised form 25 July 2007; accepted 17 August 2007 Available online 24 August 2007
Abstract The understanding of the electrical conductivity properties of polyion/nanoparticle thin film assemblies has long-term impact for exploring the unique electrical properties in applications such as sensor, actuator and conductive coating. This paper reports on the observations of the time dependent conducting behavior of the SnO2 thin film produced by layer-by-layer self-assembly. The thin film consists of 5 layers of polyion matrix and 12 nm SnO2 nanoparticles that are adsorbed in an alternating sequence. The electrical current flowing through such thin film is not stabilized immediately, but instead increases with time until saturation. We further find a threshold current that is required to trigger the current self-enhancing process. It is hypothesized that the self-heating of the semiconductive nanoparticle explains this phenomenon. Based on this hypothesis, a hybrid channel is designed to completely eliminate this effect. This fundamental study may find applications in different gas and biomedical sensors and transparent electronics. © 2007 Elsevier B.V. All rights reserved. Keywords: Layer-by-layer self-assembly; Nanoparticles; Tin dioxide; Polyion/nanoparticle multilayer; Conductivity
1. Introduction The layer-by-layer (LbL) self-assembly technique has become a powerful tool for the synthesis of nanostructures and hybrid organic–inorganic materials according to the application requirements which represent one of the highest achievements of current chemistry. Its advantages that allow a precise control of molecular order and thickness enable the engineering over macroscopic electrical, optical, magnetic, thermal, mechanical, and other properties, important for many engineering devices and applications [1–5]. The development of thin films composed of polyions and functional nanoparticles is an active field of research. Such films can be obtained via the sequential adsorption of oppositely charged macromolecules from solution. The typical procedure involves the alternate adsorption of two components, the flexible polyion and the nanoparticle. While the flexible ⁎ Corresponding author. Tel.: +1 315 2682126; fax: +1 3152687600. E-mail address: [email protected] (F. Hua). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.08.095
polyion chain ensures a uniform construction of the film, the nanoparticle is used to functionalize the films [6,7]. Functionality can be achieved because the adsorbed components are governed by non-covalent interactions, mainly the electrostatic interactions. Such thin films are regarded as the promising components in numerous applications including rectifying p–n junction [8,9], flexible solar cells [10,11], synthetic catalysts [12,13], antimicrobial agents [14], optical coating [15], and luminescent devices [16]. The electrical property of the self-assembled nanoparticle thin film will play an important role to various devices and systems. SnO2 is a well-known gas sensing material because its conductivity is significantly influenced by the type and amount of gas that is adsorbed on its surface. A large amount of research has been done in developing highly sensitive SnO2 conductivity-based gas sensors [17,18]. It is also widely reported that sensitivity can be dramatically improved if the SnO2 thin film is produced from the smallest possible particles because the sensing is a surface phenomenon [19,20]. This paper reports on the observation of the electrical behavior of the polyion/SnO2
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thin film for the reason that it is an important material and may be representative for the self-assembled polyion/nanoparticle architectures. Interestingly, it is found that when the external voltage is applied across the self-assembled polyion/SnO2 thin film, the electrical current is not stable as it is supposed to be. On the contrary, it shows the time dependent behavior, i. e., it increases steadily until it is eventually stabilized. The I–V characterization of the thin film is analyzed and a hypothesis is proposed. A hybrid channel is then designed to overcome the instability of the current. Finally, a prototype of transparent thermal sensor is demonstrated. 2. Experimental details
Fig. 1. Schematic of the template with ITO as the electrodes and LbL selfassembled SnO2 nanoparticle multilayer as the conductive channel. The order of the multilayer is: (PDDA + PSS)2 + (PDDA + SnO2 nanoparticle)5. The subscript denotes the number of the adsorbed layer. Only 3 bilayers of PDDA/SnO2 were demonstrated.
polymer, mainly polyurethane, which was cured by the irradiation of UV light.
2.1. Materials and equipment The polyelectrolytes used for the self-assembly process were poly(dimethyldiallyl ammonium chloride) (PDDA) (aqueous solution, MW 200 to 300 K, 3 mg/mL, 0.5 M NaCl) and poly (styrenesulfonate) (PSS) (aqueous solution, MW 70 K, 3 mg/ mL, and 0.5 M NaCl), both of which were obtained from Aldrich-Sigma. The dispersions of clear SnO2 nanocrystals, ∼ 12 nm in diameter, were obtained from Nyacol. It was diluted to 10 mg/mL for use. The glass slide coated with indium tin oxide (ITO), with an ITO thickness of about 120 nm and sheet resistivity of 8–12 Ω/square, was obtained from Delta Technologies. The photoresist was NR9-1500P from Futurrex, Inc. The polyurethane used for encapsulating the film was from Norland Products. A quartz crystal microbalance (QCM) which was used to analyze the growth of the thin film was obtained from Stanford Research Systems, Inc. The conductive behavior was characterized by an HP 4156 A semiconductor parameter analyzer. The high-resolution temperature source consists of a Peltier solid-state thermoelectric cooler, a platinum probe and a temperature controller which were all from Melcor with an overall resolution of 0.1 °C. 2.2. Fabrication of the self-assembled SnO2 multilayer The fabrication of the template started from the ITO-coated glass slide. The ITO layer was patterned to form two electrodes on the glass using photolithography. The solution to etch the ITO was the mixture of hydrochloric acid and nitric acid with a volume ratio of 3:1. The width and thickness of the ITO electrode was 1 cm and 120 nm, respectively. The gap between the electrodes was 40 μm. The next lithography step patterned a window on the photoresist over the gap, making the gap exposed to the atmosphere. Two bilayers of polycation and polyanion, PDDA/PSS, were alternately adsorbed into the gap using LbL assembly, in order to construct a firm foundation for the subsequent film growth [21]. Then, 5 bilayers of PDDA/ SnO2 nanoparticle (∼ 12 nm in diameter) were adsorbed on the PDDA/PSS multilayer. Finally, the photoresist was dissolved, leaving the nanoparticle thin film bridging the two ITO electrodes, as shown in Fig. 1. Later, the conductive channel was encapsulated in order to be isolated from the atmosphere by a transparent photo curable
2.3. Electrical characterization of the self-assembled SnO2 multilayer The electrodes were probed on a microprobe station. The voltage and current signals were collected by a HP 4156 A semiconductor parameter analyzer with a current resolution of 1 pA. When measuring the current at a very low level, the “long” measuring mode with the averaging function was selected in an attempt to average out the noise. In order to measure the temperature response of the SnO2 nanoparticle multilayer, the template was placed on the melcor thermoelectric cooler which was controlled by a high-resolution temperature controller with a resolution of 0.1 °C. 3. Results and discussions The LbL assembly makes use of the alternate adsorption of oppositely charged supramolecular species [6,7]. Because the nanoparticles were negatively charged, they were alternately adsorbed onto the substrate with polycation, PDDA. As a wellestablished practice, the substrate should be treated in order to carry the uniform surface charge for the subsequent adsorption. In this experiment, after the ITO substrate was etched in the acidic solution, its surface was oxidized, generating sufficient density of surface charge. Moreover, the growth of the nanoparticle multilayer was monitored by a QCM to control the step growth. The frequency shift corresponding to the adsorption showed that the step growth of PDDA and SnO2 nanocrystals were about 2 and 12 nm, respectively, the later one indicating the formation of a complete nanoparticle monolayer for each dipping. The total thickness of the multilayer was about 76 nm. When the external voltage of 7 V was applied across the electrodes, the current did not stabilize at once. It increased from 2 nA to 730 nA which was the saturated value within 60 min. The time dependence of the current is shown in Fig. 2. The decreasing of the current accelerating rate as a function of the time was observed. This rate gradually decreases as the magnitude of the current reached a higher level. For example, at a current magnitude of ∼2 nA, the rate was ∼ 45 nA/min while at 730 nA, it decreased to zero. It was found that there was a threshold current density, ∼ 1.67 × 103 nA/mm2, required to
Y.J. Shin et al. / Thin Solid Films 516 (2008) 3167–3171
Fig. 2. The time-dependence of the current flowing through the PDDA/SnO2 multilayer. The current basically takes 1 h to stabilize. The multilayer which was isolated from the atmosphere showed a more serious time dependent property.
initiate the process of current self-enhancement. If the initial current density was less than the threshold which may result from lower external voltage, the current self-enhancement did take place. The effect could happen only when the current density was higher than the threshold. When the SnO2 multilayer was encapsulated by a thin layer of polyurethane which was cured by the UV light illumination, the increasing of the current became more rapid and serious, i. e., the current increases at a much higher speed and stabilizes at a higher current magnitude. The time-dependence of the current for the encapsulated template is also shown in Fig. 2. The SnO2 nanoparticle multilayer is sensitive to the ambient temperature. The current flowing through the SnO2 multilayer which could be the gas sensing material is influenced by the interaction of the SnO2 nanoparticle with the ambient atmospheric molecules, which is a complex function of temperature [22,23]. Overall speaking, for the template
Fig. 3. The current flowing through the PDDA/SnO2 multilayer as a function of temperature which was measured without delay. However, the current would increase with the time so that the temperature related property was covered up. After a discontinuous copper film was added below the PDDA/SnO2 multilayer, this curve became stable with the time.
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produced in this experiment, it was found that the current decreased with the increasing of the temperature. As demonstrated in Fig. 3, when the temperature of the thermoelectric increased, the current of the template immediately decreased in response to the temperature change. However, due to the selfenhancement of the current, the temperature characterization of the nanoparticle multilayer was covered up. As the time went on, the magnitude of the current corresponding to different temperatures automatically increased to the same high level. The time dependent conducting behavior of the selfassembled SnO2 multilayer could be explained by a selfheating mechanism. The nanoparticles are functionalized by coupling chemical groups onto the surface, which separate the adjacent nanoparticles by the chain length. Moreover, they are embedded in the polyion matrix as a result of the LbL assembly. Nanoparticles in different layers are separated by the polyion molecules. The conductivity of such polyion/nanoparticle architecture can be described by the activated tunneling model [24]: EA rðd; T Þ ¼ ro expðbdÞ exp ð1Þ RT
EA ¼
e2 1 1 8keeo r r þ d
ro ¼ nel:
ð2Þ
ð3Þ
In Eq. (1), σ is the conductivity, and σo is the intrinsic conductivity of the nanoparticles as defined by Eq. (3). β represents the electron coupling term and δ is the interparticle distance. EA is the activation energy in kJ/mol, T is the temperature in Kelvin, and R is the universal gas constant. In Eq. (2), the activation energy EA is related to r for the radius of nanoparticles, εo for the permittivity constant, and ε for the relative permittivity. In Eq. (3), the intrinsic value of conductivity, σo, is the product of n for the number of charge
Fig. 4. The optical micrograph of a transparent thermal sensor on the glass slide. The amplified part is the image of part of the channel between electrodes.
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carriers, m for the mobility of the charge carriers m, and e for the electronic charge. The entire conducting cycle is comprised of the ejection of the electron from a nanoparticle and hopping from one particle to the next under the electrical field. When a certain amount of current travels through the nanoparticle, it is expected to excite the nanoparticle through the continuous energy transfer between electrons and nanoparticle, thus generating more “hot” electrons which are more likely to eject from the nanoparticle surface. This self-heating effect in turn provides more mobile charge carriers for the electrical conduction. It is obviously a cumulative process so that the number of mobile electrons increases with the time until saturation is reached when the heating or excitation of the nanoparticle is equal to the dissipation. The much higher increasing rate and saturation magnitude of the current after the template was encapsulated by polyurethane may support this hypothesis because the heat dissipation was made difficult after the conductive channel was isolated from the atmosphere. Based on the above hypothesis, one structure was designed to eliminate the self-heating effect. An extra metallic channel was constructed below the self-assembled nanoparticle film, providing a conductive channel which was immune to the selfheating effect. In particular, a thin copper film of ∼ 15 nm thick was evaporated between the two ITO electrodes. At this thickness, the copper film is just a discontinuous layer with a series of isolated islands. Therefore, the current resulted from the copper film under the same bias of 7 V was measured to be only at the level of 20 pA, largely noise. Then, the same number of self-assembled PDDA/SnO2 nanoparticle multilayer is adsorbed upon the copper film. It was found that the selfheating effect was effectively eliminated by using this hybrid channel structure. The current was stabilized after the voltage was applied without delay. For example, the current was measured to be 6 nA immediately after 7 V was applied and could stand firm at this value. It looks that, on one hand, the SnO2 nanoparticle film generates more mobile electrons as a result of self-heating from time to time. The generated electrons are likely to hop down into the copper channel which may have a lower resistivity and then travel from one copper island to the next through the void. On the other hand, the copper conductive channel looks immune to the self-heating effect probably because of the less resistivity and scattering to the electrons that travel inside the copper island. For example, when only a thicker copper film, ∼30 nm, was evaporated and a current of ∼ 100 nA was detected, the self-heating did not take place in the copper film. In short, it is hypothesized that the energetic electrons generated in the SnO2 nanoparticle due to the selfheating may prefer to migrate into the conductive path of the discontinuous copper film, thus circumventing the self-heating effect. Because the current component associated with only the thin copper film about ∼ 20 nm thick is negligibly small, the copper conductive channel itself does not generate the resistive current. Its function is to provide an extra channel for those electrons generated inside the SnO2 nanoparticle. Based on this hybrid structure, a prototype transparent thermal sensor is proposed. The transparent sensor can collect not only the optical signal but also other physical and chemical
signals because the light can pass through the sensor's body. Consequently, it provides more insights to the objects under study. The transparent sensor can also find special application for military and homeland security. The visual effect of the transparent thermal sensor on a glass slide is shown in Fig. 4. The optical micrograph above the sensor shows the channel between the two electrodes. It was fabricated as above mentioned on a glass slide with ITO as the electrodes. 20 nm copper film and 5 bilayers of PDDA/SnO2 multilayer were coated across the two electrodes. The optical transmittance of the conductive channel region which is the thickest in the sensor was ∼ 75%. Since the self-heating effect was removed, the stable current could be measured reliably and showed the same trend of current versus the variation of the temperature as shown in Fig. 3. 4. Conclusions In summary, the time dependent conductive behavior of the LbL assembled SnO2 nanoparticle multilayer is studied. It is found that the current of the multilayer does not reach the stabilized value until a process of current self-increasing is finished. A possible explanation of self-heating is provided to address the phenomenon. It suggests that the electrical current passing through the SnO2 nanoparticle may increase the number of the energetic electrons via the energy transfer. The selfheating effect can be effectively overcome by using a hybrid channel structure in which the SnO2 nanoparticle provides the active electrons while the discontinuous copper film provides the conductive channel that does not lead to self-heating. It can find potential applications in sensors and different conductive coatings. References [1] G. Decher, Science 277 (1997) 1232. [2] S. Qin, D. Qin, W.T. Ford, Y. Zhang, N.A. Kotov, Chem. Mater. 17 (2005) 2131. [3] K.M. Chen, X.P. Jiang, L.C. Kimerling, P.T. Hammond, Langmuir 16 (2000) 7825. [4] J. Shen, J. Sun, X. Zhang, Pure Appl. Chem. 72 (2000) 147. [5] T. Shutava, M. Prouty, D. Kommireddy, Y. Lvov, Macromolecules 38 (2005) 2850. [6] R.K. Iler, J. Colloid Interface Sci. 21 (1966) 569. [7] G. Decher, J.D. Hong, Makromol. Chem., Macromol. Symp. 46 (1991) 321. [8] A.P. Alivisatos, MRS Bull. 23 (1998) 18. [9] T. Cassagneau, T.E. Mallouk, J.H. Fendler, J. Am. Chem. Soc. 120 (1998) 7848. [10] J.A. He, R. Mosurkal, L.A. Samuelson, L. Li, J. Kumar, Langmuir 19 (2003) 2169. [11] S. Takenaka, Y. Maeharaa, H. Imaia, M. Yoshikawab, S. Shiratori, Thin Solid Films 438 (2003) 346. [12] H. Qiu, W.Y. Lee, Appl. Catal., A Gen. 314 (2006) 200. [13] B.C. Ku, S.H. Lee, W. Liu, J.A. He, J. Kumar, F.F. Bruno, L.A. Samuelson, J. Macromol. Sci., Pure Appl. Chem. 40 (2004) 1335. [14] J.C. Grunlan, J.K. Choi, A. Lin, Biomacromolecules 6 (2005) 1149. [15] W. Zhizhong, J. Walish, A. Nolte, L. Zhai, R.E. Cohen, M.F. Rubner, Adv. Mater. 18 (2006) 2699. [16] W. Chen, D. Grouquist, J. Roark, J. Nanosci. Nanotechnol. 2 (2002) 47. [17] Y. Hiranaka, T. Abe, H. Murata, Sens Actuators, B 9 (1992) 177.
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