- Email: [email protected]
Controllable organic nanofiber network crystal room temperature NO2 sensor
Organic Electronics 14 (2013) 821–826
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Controllable organic nanofiber network crystal room temperature NO2 sensor Shiliang Ji a,b, Xiujin Wang a, Chengfang Liu a,b, Haibo Wang a, Tong Wang a, Donghang Yan a,⇑ a b
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 21 October 2012 Received in revised form 4 January 2013 Accepted 6 January 2013 Available online 22 January 2013 Keywords: NO2 sensor Organic nanofiber network Room temperature detection Quick recovery
a b s t r a c t We reported an organic room temperature (RT) NO2 sensor based on zinc phthalocyanine (ZnPc) nanofiber network. Compared with traditional polycrystalline ZnPc film devices, the sensors with ZnPc nanofiber network as the sensitive layer exhibit much better recovery characteristics, which could almost recover without any treatments at room temperature. In ZnPc nanofiber network, ultra-thin ZnPc single-crystal fibers not only improve charge transport but also make the charge exchanging process between NO2 and sensitive materials easier. It shortens the response and recovery time and stabilizes the baseline of devices. In addition, we also optimized the sensors by varying the scale of the ZnPc nanofiber. The device performance is obviously improved when the scale of the ZnPc nanofiber becomes smaller. The device could naturally recover and the baseline achieves zero drift. It is attributed to the expansion of the ratio of surface area-to-volume (A/V). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Along with the rapid development of the society, air pollution has become an extremely serious problem [1–3]. As a major origin of acid rain and photochemical smog, nitrogen dioxide (NO2) has been one of the major air pollutants. Hence, in recent years, NO2 detection has become a research focus [1,2,4–9]. Specially, RT NO2 sensors draw more attention because they have lower energy consumption and can be used in the complex environment such as the existing environment of gas explosion [8,10–13]. For most RT NO2 sensors, during the detection process, the baseline of devices constantly rises and is difficult to recover in the absence of any external conditions [8,10–12]. Thus, these devices are hardly to be used in practical application. A direct cause for the phenomenon is that the contact between NO2 and sensitive materials is not sufficient which results in a long interaction time [10,14]. To overcome this draw⇑ Corresponding author. E-mail address: [email protected] (D. Yan). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.01.006
back, a lot of work has been carried out in this field [5,7,9,14–19]. The classic method is to make the sensitive material grow into nano-fiber structure. In the 1980s, Sadaoka prepared the lead phthalocyanine nanofiber perpendicular to the substrate by annealing the film in the air over 3 h [13]. A good response characteristic up to 10 ppm NO2 gas was obtained. However, the harsh process operating near the material sublimation temperature may also cause unexpected reaction or irreversible changes to the sensing layer, which makes the poor reproducibility of the devices. Since 2000, many studies about inorganic oxide nanofiber sensors were reported [14–18,20–24]. Apparently, the nanofiber not only possesses good transport property but also can effectively expand the contact area between the sensitive materials and NO2 gas. As a result, the response and recovery time of devices is dramatically shortened [25]. For the nano-crystal fiber sensor, ratio of surface area-to-volume (A/V) of the active layer is one of the main factors in the device performance [14]. Generally, the response and recovery time of devices will be shorter with a larger ratio of A/V. However, for most inorganic
822
S. Ji et al. / Organic Electronics 14 (2013) 821–826
nano-crystal fiber NO2 sensors, high cost due to the processing method limits their broader application in RT detection field. Hence, low power consumption and room temperature operating sensing devices are greatly desired. In 2010, Wang et al. reported a kind of sensors based on ultra-thin phthalocyanine oxygen vanadium (VOPc) thin film and realized NO2 gas detection at room temperature [8]. Unfortunately, VOPc is not a suitable material for NO2 sensors because it is hardly to recover at room temperature. For all that, it still provides a new concept of organic ultra-thin film for the further development of the organic room temperature sensor. Here, we reported a kind of gas sensors using ZnPc nanofiber network as the active layer, which can detect NO2 gas at room temperature condition. More importantly, the devices show a nice recovery characteristic without any additional process. The result is attributed to ultra-thin ZnPc single-crystal fiber in nanoscale and porous network which accelerates the interaction between ZnPc and NO2. By decreasing the temperature of depositing ZnPc, we could obtain smaller fibers and a larger ratio of A/V network which could further improve the recovery characteristics of devices.
2. Experimental methods 2.1. Film and device preparation The molecular structures of p-6P and ZnPc are given in Fig. 1a, respectively. The ZnPc samples were purchased from Aldrich Company (USA), and the p-6P sample was synthesized according to the method described before [26]. All the materials were purified twice by thermal
gradient sublimation before experiments. The device structure is shown in Fig. 2a. 5 nm p-6P film was sublimed onto SiO2/Si (n++) substrate at the substrate temperature of 180 °C. Then, ZnPc was sublimed subsequently onto the p-6P film to form a nanofiber network as reported [27]. The deposition was performed under a pressure of 10 4–10 5 Pa at a rate of about 1 nm/min. The finger electrode was thermally evaporated with a shadow mask on top of the organic film in another vacuum chamber to obtain the ZnPc ultrathin film device. Each device contains seven pairs of fingers with each finger of 3 mm length and 100 lm distance between each other. 10 nm ZnPc film was also prepared to obtain the common ZnPc polycrystalline film device for contrast. ZnPc nanofiber network preparation at 50 °C: After p-6P was deposited on the substrate, the substrate temperature was reduced to 50 °C, and then ZnPc was deposited. Other procedures were the same to the epitaxial growth of ultrathin films. Preparation of transistors: After p-6P was deposited on the substrate, 20 nm ZnPc film was continuous deposition. The Au electrodes were sublimed with a shadow mask on top of the organic film in another vacuum chamber. The current–voltage measurement was performed with a Keithley 236 source-measurement unit under ambient conditions at room temperature. Field-effect mobility (l) was estimated from the saturation regime (VGS = VDS = 50 V) using the following equation: IDS = W/2LlCi (VGS VT)2 in which L (200 lm) is the channel length, W (6000 lm) is the channel width, Ci (10 nF/cm2) is the capacitance (per unit area) of the insulator, VGS is the gate voltage, VT is the threshold voltage, and IDS is the drainsource current.
2.2. Films characterization and sensor test
Fig. 1. (a and b) Molecular structures of ZnPc and p-6P. (c and d) AFM topography images of the ZnPc (10 nm) film (4 4 lm) and p-6P (5 nm)/ ZnPc (2 nm) network (4 4 lm) on the SiO2 substrate respectively.
The morphologies of the organic thin films were characterized by SPI 3800N instrument (Seiko Instruments Inc.) by tapping mode and lift mode, respectively. The wide angle X-ray diffraction (XRD) pattern was taken from a D8 discovery thin-film diffractometer with Cu Ka radiation (k = 1.54056 Å). The selected voltage and current were 40 kV and 35 mA, respectively. All the devices have been stored in air for 24 h before the sensor test. The electrical measurement was taken by Keithley 2636A double channel source measurement at the voltage of 1 V. Two Mass Flow Controllers (MFCs) (BROOKS 5850S) were used to control the flow rate of air stream used as carrier gas and make NO2 stream generate diluted NO2 sample of 5 ppm, 10 ppm, 15 ppm, 20 ppm, 25 ppm, and 30 ppm. The flow rate in the test was fixed at 100 sccm (standard cm3 per min) and the gas inlet tubes were made of PTFE to avoid analyte adsorption. The sensor test was realized by exposing the devices to a constant flow of NO2 gas doses in a homemade PTFE test chamber which was of 50 mL internal volume at the room temperature and relative humidity of 18%. Each dose lasted 10 min and the carrier gas air was used during the interval. Two devices were placed in the chamber for simultaneously mounting at a time.
S. Ji et al. / Organic Electronics 14 (2013) 821–826
823
Fig. 2. (a) The schematic device configuration of SiO2/p-6P/ZnPc/Au sensor device. (b) the response curve of 10 nm polycrystalline ZnPc film to NO2 pulses, (c) I–V curves of 10 nm polycrystalline film, 2 nm ZnPc nanofiber network before the injection of NO2 gas, and (d) the response curve of 2 nm ZnPc nanofiber network to NO2 pulses.
3. Results and discussion The surface morphologies of 10 nm polycrystalline ZnPc thin films and 2 nm ZnPc nanofiber networks are shown in Fig. 1c and d, respectively. It is clearly observed that the polycrystalline ZnPc film directly grown on SiO2 substrate is composed of grainy crystals. When the film thickness is less than 10 nm, the crystals are almost isolated by grain boundaries and the film is uncontinuous. Hence, the conductivity of the film is hardly detected. On the contrary, ZnPc nanofiber network fabricated by the epitaxial growth method shows a good continuity. The thickness of most nanofibers is around 3 nm which is equal to twice of the length of ZnPc. It is worthy to note that some ends of ZnPc nanofiber could well fuse with each other which improves the continuity of the network. Due to the large size of crystal fibers, the continuous film has a larger signal than that of the polycrystalline film, which ensures the signal can easily be detected. The porous network structure is conducive to the contact between the gas and the sensitive material, reducing gas diffusion and scattering processes in the active layer, beneficial to the gas detection at room temperature. Compared with the out-of-plane X-ray diffraction patterns in Fig. 3b, the same diffraction peak, indexed as (2 0 0) of the a form, indicates the crystal structure of ZnPc does not change. For both devices with ZnPc active layers of different morphologies, current–voltage (I–V) character is measured before injecting NO2 gas and the results are shown in Fig. 2c. In the range of 2 V to 2 V, both I–V curves show linearity which indicates that the device can work in ohmic transport region. Thus the semiconductor film might be
the main sensing element responsible for the observed sensor response [28]. Moreover, the ZnPc (2 nm) nanofiber network displays a higher conductivity than that of the polycrystalline ZnPc (10 nm) film. Highly ordered molecular arrangement in ZnPc single crystal fiber and less grain boundaries between different fibers improve the carrier transport and the conductivity of the network increases [27]. After injecting NO2, two devices show quite different response and recovery performance. As shown in Fig. 2b, for the sensor based on the polycrystalline ZnPc film, when the NO2 is injected, the conductivity of the device shows an obvious increase. But at the end of NO2 injection, the conductivity of the device slowly decreases but could not recover to the initial value. When the NO2 goes through the sensitive area, the charge exchange will occur and NO2 can take the electron of phthalocyanine and leave hole, resulting in the increase of the conductivity of the phthalocyanine [2,4,6]. During several continuous pulsing measurements, the response current of devices increases and the recovery is very poor. The baseline could recover to the initial value after the device was placed in air more than 48 hours. This is an enormous difficulty for continuous measurements of the device and iterative use. This is primarily because the ZnPc film could provide a large quantity of electron to NO2; however, the desorption process of NO2 will diffuse and remain in the active layer, which leads to multiple charge exchange with ZnPc, so that the conductivity decreases very slowly. Obviously, it comes from the fact that the gas cannot successfully leave from the active layer [14]. By contrast, the sensor consisting of ZnPc nanofiber network exhibits a short response and recovery time. More
824
S. Ji et al. / Organic Electronics 14 (2013) 821–826
Fig. 3. (a) AFM topography image of the p-6P (5 nm)/50 °C/ZnPc (2 nm) film (4 4 lm) on the SiO2 substrate, (b) X-ray diffraction spectra of the ZnPc film, p-6P/ZnPc network and p-6P/ZnPc (50 °C) network. The inset gives the I–V curves of 2 nm 50 °C ZnPc nanofiber network, and (c) the response curve of 2 nm 50 °C ZnPc nanofiber network to NO2 pulses.
importantly, the baseline of the device almost does not drift (Fig. 3d). Double layer ZnPc single crystal fibers and porous nanofiber networks shorten the interaction time between ZnPc and NO2 gas which profits absorption and desorption process of NO2 gas. This is due to porous structures which could make NO2 fully contact sensitive materials of the active layer and leave smoothly away from the sensitive area after desorption. It will make the device have a better recovery and make the baseline rarely drift. At the same time, the device shows different conductivity in different concentration gradients. The response intensity increases with the gas concentration. The natural recovery without any external conditions and different concentration could show different response strength, which make this device applicable in room temperature detection. Though the sensitivity of the device is not the best, it has been able to satisfy the requirement of detecting NO2 at room temperature. The specific value of sensitivity of 50 °C ZnPc nanofiber network is given in Table 1. For the purpose of the improvement of performance, the increase of recovery and the stability of baseline, we can increase the ratio of A/V by decreasing the scale of ZnPc nanofibers. It is a common method to enhance recovery in
Table 1 The sensitivity specific value of 50 °C ZnPc nanofiber network. n (ppm) S (%)
5 40
10 46
15 62
20 70
25 76
30 94
the nanometer fiber device. However, smaller ZnPc nanofibers will reduce the conductivity of network. It is necessary to balance the ratio of A/V and conductivity of the network. By varying the substrate temperature of depositing ZnPc, we can control the size of ZnPc nanofiber and obtain the optimized devices at 50 °C. Fig. 3a shows the surface morphology of ZnPc nanofiber network prepared at 50 °C. It is obviously noted that both the width and the length of ZnPc nanofiber decrease and the amount of ZnPc nanofiber increases. From the out-of-plane X-ray diffraction patterns in Fig. 3b, the position of diffraction peaks for ZnPc nanofiber network grown at 150 °C and 50 °C is the same, which means the crystal structure of ZnPc nanofiber in the network does not change. At the same time, the intensity of ZnPc nanofiber prepared at 50 °C is weak and the molecular order is reduced. The I–V measurement without NO2 gas shows a linear behavior which indicates the device works in ohmic region. It ensures that the device test could not be affected by the contact between metal and semiconductor. As we previously expect, the conductivity of the network displays an obvious reduction. By using field-effect transistors, we calculate the mobility of ZnPc nanofiber networks prepared under different conduction. The result shows that the mobility of ZnPc nanofiber networks decreases from 0.3 cm2/Vs to 0.032 cm2/Vs (Fig. 4). Smaller scales of ZnPc single crystal and more grain boundaries depress the mobility and lead to a lower conductivity, which is in the range of 10 10 S. However, the magnitude of the current signal is enough for measuring. In the response curve
S. Ji et al. / Organic Electronics 14 (2013) 821–826
825
References
Fig. 4. The transfer characteristics (IDS–VGS) of ZnPc (20 nm)/p-6P (5 nm) 1=2 and ZnPc (20 nm) OTFTs at VDS = 50 V and the typical IDS vs. VGS plot (I1=2 50 V. DS —V GS ) at VDS =
shown in Fig. 3c, the device displays a better recovery characteristic. In each NO2 pulse cycle, the value of the baseline varies less than 1%. After six times of NO2 pulse cycles, the baseline drifts only 3% which is comparable with commercial products. Although the conductivity is lower, the response curve becomes smoother. It indicates the device performance is more stable. It is worthy to note that the sensor exhibits a better trend to saturation in 5 ppm NO2 gas. It means the sensor achieves balance of absorption and desorption in a shorter time. Therefore, the device is more suitable for detecting low concentration NO2. 4. Conclusions In conclusion, we have successfully fabricated organic room temperature NO2 sensors. By using ZnPc nanofiber network as the sensitive layer, the devices exhibit the property of quick response and recovery. More importantly, the baseline of sensors almost dose not drift under repeated NO2 pulses. It is completely attributed to the porous nanofiber network composed of double monolayer ZnPc single crystals. Highly ordered ZnPc nanofiber and continuous network greatly improve the conductivity of devices. Ultra-thin sensitive layers shorten the interaction time between ZnPc and NO2. By controlling the substrate temperature of depositing ZnPc, the optimized device is obtained from the balance between ration of A/V and conductivity. It shows much better response characteristics and can completely recover without any treatments at room temperature. It verifies the law that A/V influences response and recovery in organic nanometer fiber sensors. As far as we are concerned it is a convenient and practical method to overcome difficulty in recovering in room temperature sensor. Therefore, this work provides a beginning for the new route in low cost room temperature sensor. More high performance sensors will be expected in near future. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51133007) and The National Basic Research Program (2009CB939702).
[1] B. Bott, T.A. Jones, A highly sensitive NO2 sensor based on electricalconductivity changes in phthalocyanine films, Sensors and Actuators 5 (1984) 43–53. [2] M. Passard, C. Maleysson, A. Pauly, S. Dogo, J.P. Germain, J.P. Blanc, Gas sensitivity of phthalocyanine thin-films, Sensors and Actuators B-Chemical 19 (1994) 489–492. [3] M. Bouvet, G. Guillaud, A. Leroy, A. Maillard, S. Spirkovitch, F.G. Tournilhac, Phthalocyanine-based field-effect transistor as ozone sensor, Sensors and Actuators B-Chemical 73 (2001) 63–70. [4] J.D. Wright, Gas-adsorption on phthalocyanines and its effects on electrical-properties, Progress in Surface Science 31 (1989) 1–60. [5] D. Xie, W. Pan, Y.D. Jiang, Y.R. Li, Erbium bis[phthalocyaninato] complex LB film gas sensor, Materials Letters 57 (2003) 2395–2398. [6] F.I. Bohrer, A. Sharoni, C. Colesniuc, J. Park, I.K. Schuller, A.C. Kummel, W.C. Trogler, Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films, Journal of the American Chemical Society 129 (2007) 5640–5646. [7] X.J. Wang, S.L. Ji, H.B. Wang, D.H. Yan, Highly sensitive gas sensor enhanced by tuning the surface potential, Organic Electronics 12 (2011) 2230–2235. [8] X.J. Wang, S.L. Ji, H.B. Wang, D.H. Yan, Room temperature nitrogen dioxide chemresistor using ultrathin vanadyl-phthalocyanine film as active layer, Sensors and Actuators B-Chemical 160 (2011) 115–120. [9] J.F. Fernandez-Sanchez, I. Fernandez, R. Steiger, R. Beer, R. Cannas, U.E. Spichiger-Keller, Second-generation nanostructured metal oxide matrices to increase the thermal stability of CO and NO2 sensing layers based on iron(II) phthalocyanine, Advanced Functional Materials 17 (2007) 1188–1198. [10] N. Padma, A. Joshi, A. Singh, S.K. Deshpande, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, NO(2) sensors with room temperature operation and long term stability using copper phthalocyanine thin films, Sensors and Actuators B-Chemical 143 (2009) 246–252. [11] J. Kaur, S.C. Roy, M.C. Bhatnagar, Highly sensitive SnO2 thin film NO2 gas sensor operating at low temperature, Sensors and Actuators BChemical 123 (2007) 1090–1095. [12] A. Karthigeyan, R.P. Gupta, K. Scharnagl, M. Burgmair, M. Zimmer, S.K. Sharma, I. Eisele, Low temperature NO2 sensitivity of nanoparticulate SnO2 film for work function sensors, Sensors and Actuators B-Chemical 78 (2001) 69–72. [13] Y. Sadaoka, T.A. Jones, G.S. Revell, W. Gopel, Effects of morphology on NO2 detection in air at room-temperature with phthalocyanine thinfilms, Journal of Materials Science 25 (1990) 5257–5268. [14] S.W. Fan, A.K. Srivastava, V.P. Dravid, Nanopatterned polycrystalline ZnO for room temperature gas sensing, Sensors and Actuators BChemical 144 (2010) 159–163. [15] J. Li, Y.J. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Carbon nanotube sensors for gas and organic vapor detection, Nano Letters 3 (2003) 929–933. [16] V.V. Sysoev, B.K. Button, K. Wepsiec, S. Dmitriev, A. Kolmakov, Toward the nanoscopic ‘‘electronic nose’’: hydrogen vs carbon monoxide discrimination with an array of individual metal oxide nano- and mesowire sensors, Nano Letters 6 (2006) 1584–1588. [17] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, H.J. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [18] Y. Gonullu, G.C.M. Rodriguez, B. Saruhan, M. Urgen, Improvement of gas sensing performance of TiO2 towards NO2 by nano-tubular structuring, Sensors and Actuators B-Chemical 169 (2012) 151–160. [19] E. Comini, G. Faglia, G. Sberveglieri, UV light activation of tin oxide thin films for NO2 sensing at low temperatures, Sensors and Actuators B-Chemical 78 (2001) 73–77. [20] E. Brunet, T. Maier, G.C. Mutinati, S. Steinhauer, A. Kock, C. Gspan, W. Grogger, Comparison of the gas sensing performance of SnO2 thin film and SnO2 nanowire sensors, Sensors and Actuators B-Chemical 165 (2012) 110–118. [21] N.M. Shaalan, T. Yamazaki, T. Kikuta, Synthesis of metal and metal oxide nanostructures and their application for gas sensing, Materials Chemistry and Physics 127 (2011) 143–150. [22] H.Y. Jeong, D.S. Lee, H.K. Choi, D.H. Lee, J.E. Kim, J.Y. Lee, W.J. Lee, S.O. Kim, S.Y. Choi, Flexible room-temperature NO2 gas sensors based on carbon nanotubes/reduced graphene hybrid films, Applied Physics Letters 96 (2010). [23] T. Kida, A. Nishiyama, M. Yuasa, K. Shimanoe, N. Yamazoe, Highly sensitive NO2 sensors using lamellar-structured WO3 particles prepared by an acidification method, Sensors and Actuators BChemical 135 (2009) 568–574.
826
S. Ji et al. / Organic Electronics 14 (2013) 821–826
[24] J. Huang, Q. Wan, Gas sensors based on semiconducting metal oxide one-dimensional nanostructures, Sensors 9 (2009) 9903–9924. [25] G. Kiriakidis, M. Suchea, S. Christoulakis, N. Katsarakis, High performance gas sensing materials based on nanostructed metal oxide films, Reviews on Advanced Materials Science 10 (2005) 215– 223. [26] F. Garnier, G. Horowitz, X.Z. Peng, D. Fichou, Structural basis for high carrier mobility in conjugated oligomers, Synthetic Metals 45 (1991) 163–171.
[27] H.B. Wang, F. Zhu, J.L. Yang, Y.H. Geng, D.H. Yan, Weak epitaxy growth affording high-mobility thin films of disk-like organic semiconductors, Advanced Materials 19 (2007) 2168–2171. [28] K.A. Miller, R.D. Yang, M.J. Hale, J. Park, B. Fruhberger, C.N. Colesniuc, I.K. Schuller, A.C. Kummel, W.C. Trogler, Electrode independent chemoresistive response for cobalt phthalocyanine in the space charge limited conductivity regime, Journal of Physical Chemistry B 110 (2006) 361–366.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Controllable organic nanofiber network crystal room temperature NO2 sensor Shiliang Ji a,b, Xiujin Wang a, Chengfang Liu a,b, Haibo Wang a, Tong Wang a, Donghang Yan a,⇑ a b
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 21 October 2012 Received in revised form 4 January 2013 Accepted 6 January 2013 Available online 22 January 2013 Keywords: NO2 sensor Organic nanofiber network Room temperature detection Quick recovery
a b s t r a c t We reported an organic room temperature (RT) NO2 sensor based on zinc phthalocyanine (ZnPc) nanofiber network. Compared with traditional polycrystalline ZnPc film devices, the sensors with ZnPc nanofiber network as the sensitive layer exhibit much better recovery characteristics, which could almost recover without any treatments at room temperature. In ZnPc nanofiber network, ultra-thin ZnPc single-crystal fibers not only improve charge transport but also make the charge exchanging process between NO2 and sensitive materials easier. It shortens the response and recovery time and stabilizes the baseline of devices. In addition, we also optimized the sensors by varying the scale of the ZnPc nanofiber. The device performance is obviously improved when the scale of the ZnPc nanofiber becomes smaller. The device could naturally recover and the baseline achieves zero drift. It is attributed to the expansion of the ratio of surface area-to-volume (A/V). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Along with the rapid development of the society, air pollution has become an extremely serious problem [1–3]. As a major origin of acid rain and photochemical smog, nitrogen dioxide (NO2) has been one of the major air pollutants. Hence, in recent years, NO2 detection has become a research focus [1,2,4–9]. Specially, RT NO2 sensors draw more attention because they have lower energy consumption and can be used in the complex environment such as the existing environment of gas explosion [8,10–13]. For most RT NO2 sensors, during the detection process, the baseline of devices constantly rises and is difficult to recover in the absence of any external conditions [8,10–12]. Thus, these devices are hardly to be used in practical application. A direct cause for the phenomenon is that the contact between NO2 and sensitive materials is not sufficient which results in a long interaction time [10,14]. To overcome this draw⇑ Corresponding author. E-mail address: [email protected] (D. Yan). 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.01.006
back, a lot of work has been carried out in this field [5,7,9,14–19]. The classic method is to make the sensitive material grow into nano-fiber structure. In the 1980s, Sadaoka prepared the lead phthalocyanine nanofiber perpendicular to the substrate by annealing the film in the air over 3 h [13]. A good response characteristic up to 10 ppm NO2 gas was obtained. However, the harsh process operating near the material sublimation temperature may also cause unexpected reaction or irreversible changes to the sensing layer, which makes the poor reproducibility of the devices. Since 2000, many studies about inorganic oxide nanofiber sensors were reported [14–18,20–24]. Apparently, the nanofiber not only possesses good transport property but also can effectively expand the contact area between the sensitive materials and NO2 gas. As a result, the response and recovery time of devices is dramatically shortened [25]. For the nano-crystal fiber sensor, ratio of surface area-to-volume (A/V) of the active layer is one of the main factors in the device performance [14]. Generally, the response and recovery time of devices will be shorter with a larger ratio of A/V. However, for most inorganic
822
S. Ji et al. / Organic Electronics 14 (2013) 821–826
nano-crystal fiber NO2 sensors, high cost due to the processing method limits their broader application in RT detection field. Hence, low power consumption and room temperature operating sensing devices are greatly desired. In 2010, Wang et al. reported a kind of sensors based on ultra-thin phthalocyanine oxygen vanadium (VOPc) thin film and realized NO2 gas detection at room temperature [8]. Unfortunately, VOPc is not a suitable material for NO2 sensors because it is hardly to recover at room temperature. For all that, it still provides a new concept of organic ultra-thin film for the further development of the organic room temperature sensor. Here, we reported a kind of gas sensors using ZnPc nanofiber network as the active layer, which can detect NO2 gas at room temperature condition. More importantly, the devices show a nice recovery characteristic without any additional process. The result is attributed to ultra-thin ZnPc single-crystal fiber in nanoscale and porous network which accelerates the interaction between ZnPc and NO2. By decreasing the temperature of depositing ZnPc, we could obtain smaller fibers and a larger ratio of A/V network which could further improve the recovery characteristics of devices.
2. Experimental methods 2.1. Film and device preparation The molecular structures of p-6P and ZnPc are given in Fig. 1a, respectively. The ZnPc samples were purchased from Aldrich Company (USA), and the p-6P sample was synthesized according to the method described before [26]. All the materials were purified twice by thermal
gradient sublimation before experiments. The device structure is shown in Fig. 2a. 5 nm p-6P film was sublimed onto SiO2/Si (n++) substrate at the substrate temperature of 180 °C. Then, ZnPc was sublimed subsequently onto the p-6P film to form a nanofiber network as reported [27]. The deposition was performed under a pressure of 10 4–10 5 Pa at a rate of about 1 nm/min. The finger electrode was thermally evaporated with a shadow mask on top of the organic film in another vacuum chamber to obtain the ZnPc ultrathin film device. Each device contains seven pairs of fingers with each finger of 3 mm length and 100 lm distance between each other. 10 nm ZnPc film was also prepared to obtain the common ZnPc polycrystalline film device for contrast. ZnPc nanofiber network preparation at 50 °C: After p-6P was deposited on the substrate, the substrate temperature was reduced to 50 °C, and then ZnPc was deposited. Other procedures were the same to the epitaxial growth of ultrathin films. Preparation of transistors: After p-6P was deposited on the substrate, 20 nm ZnPc film was continuous deposition. The Au electrodes were sublimed with a shadow mask on top of the organic film in another vacuum chamber. The current–voltage measurement was performed with a Keithley 236 source-measurement unit under ambient conditions at room temperature. Field-effect mobility (l) was estimated from the saturation regime (VGS = VDS = 50 V) using the following equation: IDS = W/2LlCi (VGS VT)2 in which L (200 lm) is the channel length, W (6000 lm) is the channel width, Ci (10 nF/cm2) is the capacitance (per unit area) of the insulator, VGS is the gate voltage, VT is the threshold voltage, and IDS is the drainsource current.
2.2. Films characterization and sensor test
Fig. 1. (a and b) Molecular structures of ZnPc and p-6P. (c and d) AFM topography images of the ZnPc (10 nm) film (4 4 lm) and p-6P (5 nm)/ ZnPc (2 nm) network (4 4 lm) on the SiO2 substrate respectively.
The morphologies of the organic thin films were characterized by SPI 3800N instrument (Seiko Instruments Inc.) by tapping mode and lift mode, respectively. The wide angle X-ray diffraction (XRD) pattern was taken from a D8 discovery thin-film diffractometer with Cu Ka radiation (k = 1.54056 Å). The selected voltage and current were 40 kV and 35 mA, respectively. All the devices have been stored in air for 24 h before the sensor test. The electrical measurement was taken by Keithley 2636A double channel source measurement at the voltage of 1 V. Two Mass Flow Controllers (MFCs) (BROOKS 5850S) were used to control the flow rate of air stream used as carrier gas and make NO2 stream generate diluted NO2 sample of 5 ppm, 10 ppm, 15 ppm, 20 ppm, 25 ppm, and 30 ppm. The flow rate in the test was fixed at 100 sccm (standard cm3 per min) and the gas inlet tubes were made of PTFE to avoid analyte adsorption. The sensor test was realized by exposing the devices to a constant flow of NO2 gas doses in a homemade PTFE test chamber which was of 50 mL internal volume at the room temperature and relative humidity of 18%. Each dose lasted 10 min and the carrier gas air was used during the interval. Two devices were placed in the chamber for simultaneously mounting at a time.
S. Ji et al. / Organic Electronics 14 (2013) 821–826
823
Fig. 2. (a) The schematic device configuration of SiO2/p-6P/ZnPc/Au sensor device. (b) the response curve of 10 nm polycrystalline ZnPc film to NO2 pulses, (c) I–V curves of 10 nm polycrystalline film, 2 nm ZnPc nanofiber network before the injection of NO2 gas, and (d) the response curve of 2 nm ZnPc nanofiber network to NO2 pulses.
3. Results and discussion The surface morphologies of 10 nm polycrystalline ZnPc thin films and 2 nm ZnPc nanofiber networks are shown in Fig. 1c and d, respectively. It is clearly observed that the polycrystalline ZnPc film directly grown on SiO2 substrate is composed of grainy crystals. When the film thickness is less than 10 nm, the crystals are almost isolated by grain boundaries and the film is uncontinuous. Hence, the conductivity of the film is hardly detected. On the contrary, ZnPc nanofiber network fabricated by the epitaxial growth method shows a good continuity. The thickness of most nanofibers is around 3 nm which is equal to twice of the length of ZnPc. It is worthy to note that some ends of ZnPc nanofiber could well fuse with each other which improves the continuity of the network. Due to the large size of crystal fibers, the continuous film has a larger signal than that of the polycrystalline film, which ensures the signal can easily be detected. The porous network structure is conducive to the contact between the gas and the sensitive material, reducing gas diffusion and scattering processes in the active layer, beneficial to the gas detection at room temperature. Compared with the out-of-plane X-ray diffraction patterns in Fig. 3b, the same diffraction peak, indexed as (2 0 0) of the a form, indicates the crystal structure of ZnPc does not change. For both devices with ZnPc active layers of different morphologies, current–voltage (I–V) character is measured before injecting NO2 gas and the results are shown in Fig. 2c. In the range of 2 V to 2 V, both I–V curves show linearity which indicates that the device can work in ohmic transport region. Thus the semiconductor film might be
the main sensing element responsible for the observed sensor response [28]. Moreover, the ZnPc (2 nm) nanofiber network displays a higher conductivity than that of the polycrystalline ZnPc (10 nm) film. Highly ordered molecular arrangement in ZnPc single crystal fiber and less grain boundaries between different fibers improve the carrier transport and the conductivity of the network increases [27]. After injecting NO2, two devices show quite different response and recovery performance. As shown in Fig. 2b, for the sensor based on the polycrystalline ZnPc film, when the NO2 is injected, the conductivity of the device shows an obvious increase. But at the end of NO2 injection, the conductivity of the device slowly decreases but could not recover to the initial value. When the NO2 goes through the sensitive area, the charge exchange will occur and NO2 can take the electron of phthalocyanine and leave hole, resulting in the increase of the conductivity of the phthalocyanine [2,4,6]. During several continuous pulsing measurements, the response current of devices increases and the recovery is very poor. The baseline could recover to the initial value after the device was placed in air more than 48 hours. This is an enormous difficulty for continuous measurements of the device and iterative use. This is primarily because the ZnPc film could provide a large quantity of electron to NO2; however, the desorption process of NO2 will diffuse and remain in the active layer, which leads to multiple charge exchange with ZnPc, so that the conductivity decreases very slowly. Obviously, it comes from the fact that the gas cannot successfully leave from the active layer [14]. By contrast, the sensor consisting of ZnPc nanofiber network exhibits a short response and recovery time. More
824
S. Ji et al. / Organic Electronics 14 (2013) 821–826
Fig. 3. (a) AFM topography image of the p-6P (5 nm)/50 °C/ZnPc (2 nm) film (4 4 lm) on the SiO2 substrate, (b) X-ray diffraction spectra of the ZnPc film, p-6P/ZnPc network and p-6P/ZnPc (50 °C) network. The inset gives the I–V curves of 2 nm 50 °C ZnPc nanofiber network, and (c) the response curve of 2 nm 50 °C ZnPc nanofiber network to NO2 pulses.
importantly, the baseline of the device almost does not drift (Fig. 3d). Double layer ZnPc single crystal fibers and porous nanofiber networks shorten the interaction time between ZnPc and NO2 gas which profits absorption and desorption process of NO2 gas. This is due to porous structures which could make NO2 fully contact sensitive materials of the active layer and leave smoothly away from the sensitive area after desorption. It will make the device have a better recovery and make the baseline rarely drift. At the same time, the device shows different conductivity in different concentration gradients. The response intensity increases with the gas concentration. The natural recovery without any external conditions and different concentration could show different response strength, which make this device applicable in room temperature detection. Though the sensitivity of the device is not the best, it has been able to satisfy the requirement of detecting NO2 at room temperature. The specific value of sensitivity of 50 °C ZnPc nanofiber network is given in Table 1. For the purpose of the improvement of performance, the increase of recovery and the stability of baseline, we can increase the ratio of A/V by decreasing the scale of ZnPc nanofibers. It is a common method to enhance recovery in
Table 1 The sensitivity specific value of 50 °C ZnPc nanofiber network. n (ppm) S (%)
5 40
10 46
15 62
20 70
25 76
30 94
the nanometer fiber device. However, smaller ZnPc nanofibers will reduce the conductivity of network. It is necessary to balance the ratio of A/V and conductivity of the network. By varying the substrate temperature of depositing ZnPc, we can control the size of ZnPc nanofiber and obtain the optimized devices at 50 °C. Fig. 3a shows the surface morphology of ZnPc nanofiber network prepared at 50 °C. It is obviously noted that both the width and the length of ZnPc nanofiber decrease and the amount of ZnPc nanofiber increases. From the out-of-plane X-ray diffraction patterns in Fig. 3b, the position of diffraction peaks for ZnPc nanofiber network grown at 150 °C and 50 °C is the same, which means the crystal structure of ZnPc nanofiber in the network does not change. At the same time, the intensity of ZnPc nanofiber prepared at 50 °C is weak and the molecular order is reduced. The I–V measurement without NO2 gas shows a linear behavior which indicates the device works in ohmic region. It ensures that the device test could not be affected by the contact between metal and semiconductor. As we previously expect, the conductivity of the network displays an obvious reduction. By using field-effect transistors, we calculate the mobility of ZnPc nanofiber networks prepared under different conduction. The result shows that the mobility of ZnPc nanofiber networks decreases from 0.3 cm2/Vs to 0.032 cm2/Vs (Fig. 4). Smaller scales of ZnPc single crystal and more grain boundaries depress the mobility and lead to a lower conductivity, which is in the range of 10 10 S. However, the magnitude of the current signal is enough for measuring. In the response curve
S. Ji et al. / Organic Electronics 14 (2013) 821–826
825
References
Fig. 4. The transfer characteristics (IDS–VGS) of ZnPc (20 nm)/p-6P (5 nm) 1=2 and ZnPc (20 nm) OTFTs at VDS = 50 V and the typical IDS vs. VGS plot (I1=2 50 V. DS —V GS ) at VDS =
shown in Fig. 3c, the device displays a better recovery characteristic. In each NO2 pulse cycle, the value of the baseline varies less than 1%. After six times of NO2 pulse cycles, the baseline drifts only 3% which is comparable with commercial products. Although the conductivity is lower, the response curve becomes smoother. It indicates the device performance is more stable. It is worthy to note that the sensor exhibits a better trend to saturation in 5 ppm NO2 gas. It means the sensor achieves balance of absorption and desorption in a shorter time. Therefore, the device is more suitable for detecting low concentration NO2. 4. Conclusions In conclusion, we have successfully fabricated organic room temperature NO2 sensors. By using ZnPc nanofiber network as the sensitive layer, the devices exhibit the property of quick response and recovery. More importantly, the baseline of sensors almost dose not drift under repeated NO2 pulses. It is completely attributed to the porous nanofiber network composed of double monolayer ZnPc single crystals. Highly ordered ZnPc nanofiber and continuous network greatly improve the conductivity of devices. Ultra-thin sensitive layers shorten the interaction time between ZnPc and NO2. By controlling the substrate temperature of depositing ZnPc, the optimized device is obtained from the balance between ration of A/V and conductivity. It shows much better response characteristics and can completely recover without any treatments at room temperature. It verifies the law that A/V influences response and recovery in organic nanometer fiber sensors. As far as we are concerned it is a convenient and practical method to overcome difficulty in recovering in room temperature sensor. Therefore, this work provides a beginning for the new route in low cost room temperature sensor. More high performance sensors will be expected in near future. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51133007) and The National Basic Research Program (2009CB939702).
[1] B. Bott, T.A. Jones, A highly sensitive NO2 sensor based on electricalconductivity changes in phthalocyanine films, Sensors and Actuators 5 (1984) 43–53. [2] M. Passard, C. Maleysson, A. Pauly, S. Dogo, J.P. Germain, J.P. Blanc, Gas sensitivity of phthalocyanine thin-films, Sensors and Actuators B-Chemical 19 (1994) 489–492. [3] M. Bouvet, G. Guillaud, A. Leroy, A. Maillard, S. Spirkovitch, F.G. Tournilhac, Phthalocyanine-based field-effect transistor as ozone sensor, Sensors and Actuators B-Chemical 73 (2001) 63–70. [4] J.D. Wright, Gas-adsorption on phthalocyanines and its effects on electrical-properties, Progress in Surface Science 31 (1989) 1–60. [5] D. Xie, W. Pan, Y.D. Jiang, Y.R. Li, Erbium bis[phthalocyaninato] complex LB film gas sensor, Materials Letters 57 (2003) 2395–2398. [6] F.I. Bohrer, A. Sharoni, C. Colesniuc, J. Park, I.K. Schuller, A.C. Kummel, W.C. Trogler, Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films, Journal of the American Chemical Society 129 (2007) 5640–5646. [7] X.J. Wang, S.L. Ji, H.B. Wang, D.H. Yan, Highly sensitive gas sensor enhanced by tuning the surface potential, Organic Electronics 12 (2011) 2230–2235. [8] X.J. Wang, S.L. Ji, H.B. Wang, D.H. Yan, Room temperature nitrogen dioxide chemresistor using ultrathin vanadyl-phthalocyanine film as active layer, Sensors and Actuators B-Chemical 160 (2011) 115–120. [9] J.F. Fernandez-Sanchez, I. Fernandez, R. Steiger, R. Beer, R. Cannas, U.E. Spichiger-Keller, Second-generation nanostructured metal oxide matrices to increase the thermal stability of CO and NO2 sensing layers based on iron(II) phthalocyanine, Advanced Functional Materials 17 (2007) 1188–1198. [10] N. Padma, A. Joshi, A. Singh, S.K. Deshpande, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, NO(2) sensors with room temperature operation and long term stability using copper phthalocyanine thin films, Sensors and Actuators B-Chemical 143 (2009) 246–252. [11] J. Kaur, S.C. Roy, M.C. Bhatnagar, Highly sensitive SnO2 thin film NO2 gas sensor operating at low temperature, Sensors and Actuators BChemical 123 (2007) 1090–1095. [12] A. Karthigeyan, R.P. Gupta, K. Scharnagl, M. Burgmair, M. Zimmer, S.K. Sharma, I. Eisele, Low temperature NO2 sensitivity of nanoparticulate SnO2 film for work function sensors, Sensors and Actuators B-Chemical 78 (2001) 69–72. [13] Y. Sadaoka, T.A. Jones, G.S. Revell, W. Gopel, Effects of morphology on NO2 detection in air at room-temperature with phthalocyanine thinfilms, Journal of Materials Science 25 (1990) 5257–5268. [14] S.W. Fan, A.K. Srivastava, V.P. Dravid, Nanopatterned polycrystalline ZnO for room temperature gas sensing, Sensors and Actuators BChemical 144 (2010) 159–163. [15] J. Li, Y.J. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Carbon nanotube sensors for gas and organic vapor detection, Nano Letters 3 (2003) 929–933. [16] V.V. Sysoev, B.K. Button, K. Wepsiec, S. Dmitriev, A. Kolmakov, Toward the nanoscopic ‘‘electronic nose’’: hydrogen vs carbon monoxide discrimination with an array of individual metal oxide nano- and mesowire sensors, Nano Letters 6 (2006) 1584–1588. [17] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, H.J. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [18] Y. Gonullu, G.C.M. Rodriguez, B. Saruhan, M. Urgen, Improvement of gas sensing performance of TiO2 towards NO2 by nano-tubular structuring, Sensors and Actuators B-Chemical 169 (2012) 151–160. [19] E. Comini, G. Faglia, G. Sberveglieri, UV light activation of tin oxide thin films for NO2 sensing at low temperatures, Sensors and Actuators B-Chemical 78 (2001) 73–77. [20] E. Brunet, T. Maier, G.C. Mutinati, S. Steinhauer, A. Kock, C. Gspan, W. Grogger, Comparison of the gas sensing performance of SnO2 thin film and SnO2 nanowire sensors, Sensors and Actuators B-Chemical 165 (2012) 110–118. [21] N.M. Shaalan, T. Yamazaki, T. Kikuta, Synthesis of metal and metal oxide nanostructures and their application for gas sensing, Materials Chemistry and Physics 127 (2011) 143–150. [22] H.Y. Jeong, D.S. Lee, H.K. Choi, D.H. Lee, J.E. Kim, J.Y. Lee, W.J. Lee, S.O. Kim, S.Y. Choi, Flexible room-temperature NO2 gas sensors based on carbon nanotubes/reduced graphene hybrid films, Applied Physics Letters 96 (2010). [23] T. Kida, A. Nishiyama, M. Yuasa, K. Shimanoe, N. Yamazoe, Highly sensitive NO2 sensors using lamellar-structured WO3 particles prepared by an acidification method, Sensors and Actuators BChemical 135 (2009) 568–574.
826
S. Ji et al. / Organic Electronics 14 (2013) 821–826
[24] J. Huang, Q. Wan, Gas sensors based on semiconducting metal oxide one-dimensional nanostructures, Sensors 9 (2009) 9903–9924. [25] G. Kiriakidis, M. Suchea, S. Christoulakis, N. Katsarakis, High performance gas sensing materials based on nanostructed metal oxide films, Reviews on Advanced Materials Science 10 (2005) 215– 223. [26] F. Garnier, G. Horowitz, X.Z. Peng, D. Fichou, Structural basis for high carrier mobility in conjugated oligomers, Synthetic Metals 45 (1991) 163–171.
[27] H.B. Wang, F. Zhu, J.L. Yang, Y.H. Geng, D.H. Yan, Weak epitaxy growth affording high-mobility thin films of disk-like organic semiconductors, Advanced Materials 19 (2007) 2168–2171. [28] K.A. Miller, R.D. Yang, M.J. Hale, J. Park, B. Fruhberger, C.N. Colesniuc, I.K. Schuller, A.C. Kummel, W.C. Trogler, Electrode independent chemoresistive response for cobalt phthalocyanine in the space charge limited conductivity regime, Journal of Physical Chemistry B 110 (2006) 361–366.