Inkjet Printed Silver Nanoparticle Humidity Sensor With ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012

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Inkjet Printed Silver Nanoparticle Humidity Sensor With Memory Effect on Paper Henrik Andersson, Anatoliy Manuilskiy, Tomas Unander, Cecilia Lidenmark, Sven Forsberg, and Hans-Erik Nilsson

Abstract— In this paper, the design and the manufacture of an inkjet printed resistive type humidity sensor on paper are reported. After having been exposed to humidity above a given threshold level, the resistance of the sensor decreases substantially and remains at that level even when the humidity is reduced. It is possible to deduce the humidity level by monitoring the resistance. The main benefit of the printed sensor presented in this case is in relation to its very low production costs. It has also been shown that both the ink type and this paper combination used prove to be crucial in order to obtain the desired sensor effect. More research is required in order to fully understand the humidity sintering effect on the nano particle ink and the role of the substrate. However, the observed effect can be put to use in printed humidity sensors which possess a memory function. The sensor can be used in various applications for environmental monitoring, for example, in situations where a large number of inexpensive and disposable humidity sensors are required which are able to detect whether they have been subjected to high humidity. This could be the checking of transportation conditions of goods or monitoring humidity within buildings. Index Terms— Humidity nanoparticles, sintering.

transducers,

inkjet

printing,

I. I NTRODUCTION

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N THIS work, an inkjet printed resistive humidity sensor manufactured on paper substrate is presented. The resistance in the sensors decreases with increasing humidity and will remain at the lowest value reached after drying. This memory effect is useful in applications where it is desirable to detect whether the humidity level at some point has exceeded a given threshold value, even if it is the case that the level has since decreased. This is useful in order to detect water damage during the transportation of goods or in buildings where a system that constantly monitors the humidity would Manuscript received September 26, 2011; revised November 18, 2011; accepted December 16, 2011. Date of publication December 28, 2011; date of current version April 25, 2012. This work was supported in part by the EU FP7 Program in the PriMeBits Project, under Grant Agreement 215132. The Associate Editor coordinating the review of this paper and approving it for publication was Prof. Michiel J. Vellekoop. H. Andersson, A. Manuilskiy, and H.-E. Nilsson are with the Department of Information Technology and Media, Mid Sweden University, Sundsvall 851 70, Sweden (e-mail: [email protected]; [email protected]; [email protected]). T. Unander is with SCA Research and Development Center, Sundsvall 851 21, Sweden (e-mail: [email protected]). C. Lidenmark and S. Forsberg are with the Department of Natural Sciences, Engineering and Mathematics, Mid Sweden University, Sundsvall 851 70, Sweden (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2011.2182044

prove to be too costly, but, where it could be sufficient to detect the damage when it is checked. From a system perspective, there are some likely scenarii in which such sensors could be used. In the first example, a large number of sensors would be mounted at stationary positions and portable readout systems from which the sensors could be checked at appropriate time intervals would determine whether any changes have occurred or not. In another likely scenario, the readout system is stationary and items with mounted sensors are checked as they pass by, such as would be the case at a goods’ receiving station. The most convenient readout method would be to use some type of wireless readout such as to incorporate the sensors into Radio Frequency Identification (RFID) tags. In this case, the resistance change is detected by the RFID tag which is then triggered when the resistance falls below a set threshold value. To date, many articles have described the possibilities and benefits of using RFID technology to monitor goods in the supply chain [1]–[3]. Also various solutions for adding sensor functionality to RFID tags such as temperature and humidity have been shown [4]–[7]. The main benefit of the printed sensor presented in this case is in relation to its very low production costs which mean that a large number of sensors can be used together with a small number of portable readout systems. The memory function of this sensor means that it is very well suited to providing indications in relation to such described events but would not be suitable for continuously measuring humidity variations because it is not possible to reuse the sensor for lower humidity levels once it has been exposed to a higher level. Actually it is disposable and is meant to be used to detect a humidity change above a set threshold. The observed reduction of the resistance is related to sintering of the silver nanoparticles (NPs) in the ink in which the particles will start to fuse together and thus creating conductive paths. Usually, the sintering of NP inks is performed by heating, either in an oven or electrically [8]–[11], although some results in relation to chemical sintering have been reported [12], [13]. Also, VTT in Finland has shown that the nature of the substrate can facilitate room temperature sintering of printed structures using a similar ANP silver nano particle ink where they discuss the influence of the substrate on the sintering process [14]. The sensor is very cheap to produce as it requires only silver NP ink and photo paper. The simple design also makes it easy to manufacture the sensor directly in a production line by inserting an inkjet step or by integration at a later stage into various sensor systems

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(a)

(b)

Fig. 1. Photographs showing (a) single line sensor and (b) multiple line sensor. The length of each is 125 μm.

if it is produced separately. Other types of humidity sensors previously presented include capacitive [15] and also flexible resistive sensors [16]. II. E XPERIMENTAL P ROCEDURE The geometry of the sensor is a line with a width of approximately 40 μm and a length of 125 μm between the contact pads, an example is shown to the left in Fig. 1. For particular readout systems the initial and final resistances might possibly be required to be lowered. In this case, multiple lines can be used in which the resistance of each line will be in parallel with the others, and thus reducing the total resistance to the desired value, see Fig. 1 to the right for a multiple line sensor. In this article all data shown are for the single line sensor type. The ink used to fabricate the sensors is the Silverjet DGP-40LT-15C manufactured by Advanced Nano Products (ANP) (S. Korea), with a solid content of about 40–45 wt% silver, a viscosity of about 16 cP and a curing temperature of 100–150 °C according to the manufacturer [17]. The silver NPs have a diameter of approximately 30 nm according to SEM and AFM analysis and they are dispersed in Triethylene Glycol Monoethyl Ether. NP inks are, in general, dispersions of particles in a solvent in which the particles are stabilized by one or more dispersants or by capping agents. The capping agents create an electrostatic and/or steric barrier which prevents the aggregation of the NPs. For the NPs used in this case Polyvinylpyrrolidone (PVP) is used as the capping agent, which is well known [18] and has been used previously in humidity sensors [19]. The presence of PVP in the ink which has been utilized in this work was established by pyrolysis GC/MS. These flocculation experiments of diluted ink showed that the ink is electrostatically stabilized. After printing, the NPs are still imbedded in an insulating polymer matrix of the order of 2-4 nm which impair electrical conductance [10, 20–21]. Sintering is normally performed at elevated temperatures which will soften the polymer and thus allowing the particles to move. At sufficiently high temperatures the organic capping agent is combusted which thus allows for particle coalescence. The printer used was a Dimatix 2831 piezoelectric materials printer, with a 10 pL Dimatix 11610 cartridge. The humidity experiments were performed in a controlled climate chamber with variable humidity levels at 23 °C and the resistance was logged by a Keithley 2400 source meter which has an accuracy of ±0.08 % or better in the measured resistance ranges. The substrates used were the HP Advanced Photo Paper [22] and

Normalized final resistance (a.u.)

100 10−1 10−2 10−3 10−4 10−5 10−6 100

102

104 Initial resistance ()

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Fig. 2. Measured relative resistance change of sensors printed on HP paper with different initial resistance subjected to 90% RH for 24 h.

the Canon PT-101 photo paper [23]. As a reference, the same type of structures were printed using a Cabot (USA) type CCI-300 silver NP ink. III. R ESULTS The combination of the ink and paper substrate is very important for the function of the sensor. For example, identical devices manufactured using Cabot silver ink do not show the humidity sintering effect, regardless of the substrate used. Devices manufactured using the ANP ink on the HP and Canon paper react in a very different manner when subjected to humidity. The initial results obtained from devices printed on the HP substrate and which displayed the useful sensor effect will be shown and following on from this will be the results obtained when printing on the Canon substrate. It has been observed that the initial resistance value affects how large the relative resistance decrease is when the sensors are subjected to humidity. The observed behavior appear to be that, in general, two sensors of the same geometrical dimensions subjected to the same amount of humidity will end up at the same resistance regardless of the initial resistance within quite a broad, 1k to 10M, initial resistance range. This means that a sensor with a higher initial resistance value will show a larger resistance change. As an experiment, sensors with different initial values were subjected to 90 % Relative Humidity (RH) in 23 °C for a period of 24 h. The resistance was measured before the sensors were placed in the climate chamber and after they had been removed and dried in 30% RH. The resulting relative resistance decrease for a number of sensors with initial resistance values from 5 to 20M is displayed in Fig. 2. In Fig. 3 is shown the absolute values of the initial and final resistances of the same sensors as shown in Fig. 2. It can be observed that the final resistance is very consistent and varies only between 40–105 although the initial resistance is within the range 1k-10 M. For values below 1 k the final resistance slowly decreases until it reaches a value of 2 for an initial value

ANDERSSON et al.: INKJET PRINTED SILVER NP HUMIDITY SENSOR WITH MEMORY EFFECT ON PAPER

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Normalized resistance (a.u.)

Final resistance ()

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100 0 10

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104 Initial resistance ()

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90% RH 80% RH 70% RH 60% RH

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Fig. 3. Final resistance versus initial resistance values for the same sensors as shown in Fig. 2 plotted on a log-log scale.

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Fig. 5. Measurements showing resistance change of sensors printed on Canon PT-101 photo paper for different applied humidity levels.

Normalized resistance (a.u.)

100 90% RH 80% RH 70% RH 60% RH 30% RH

10−1

For 90 % RH the resistance increases by up to a factor 1.25 during a 3 h period after which it starts to decrease and after a 23 h period has decreased to a factor of 0.1 of the initial resistance. This shows that it is not only the ink but it is also the paper that has a strong effect on the humidity sintering, and is therefore crucial in order to obtain the desired sensor effect. IV. D ISCUSSION

10−2 0

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Fig. 4. Measurements showing resistance change in different humidity for sensors printed on HP advanced photo paper.

of 5 . Measurements performed in different humidity levels for sensors printed with the ANP ink on HP Advanced photo paper are shown in Fig. 4. The sensors had an initial resistance within the range 3-5k . As presented in Fig. 5, the response is completely different when the device is printed on the Canon PT-101 photo paper. In both Fig. 4 and Fig. 5 the resistance was logged during a 24 h period. It can be seen that the resistance is consistently dropping with increasing humidity levels, by, at most a factor of 100 at 90 % RH, when using the HP paper. The resistance change also occurs at a much faster rate for higher humidity levels. The sensor was not measured for humidity levels below 30 % RH because the main targeted applications are aimed at detecting high humidity events. In addition when considering the small resistance changes measured at 30 % RH, even lower humidity levels would, in practice, not be detectable by the sensor. In the case of the Canon paper, the resistance change observed does not show the same behavior as that for the HP paper. The resistance is observed to decrease or increase by about 10% for humidity levels up to 80 % RH.

Examining the measurements, it is clear that the paper substrate appears to have an important effect on the moisture sensitivity of the sensor. It is possible to discuss and point out some reasonable explanations, but at this stage it is not possible to definitely determine the mechanisms involved. However, the scope of this article is to point out the possibility of using this effect to manufacture a printable sensor rather than to provide a full explanation of the observed effect. The discussion regarding the moisture sintering effect will in this paper be focused on the PVP in the ink, the porosity of the paper coating and the chemical additives in the paper coating. It has recently been presented that it is possible to chemically sinter silver-NPs by neutralizing the charges in the capping polymer to enable particle coalescence [12]. Another investigation has showed that the thickness of the capping agent had a larger impact on the sintering than the melting point of the NPs [24]. The selective removal of the dispersion agents with alcohols showed that silver NPs could be sintered at room temperature [21]. From the data presented in Fig. 4 for sensors printed on the HP paper, it appears likely that a threshold amount of moisture uptake is necessary in order to increase the conductance. The threshold is in the humidity range in which PVP is known to soften, indicating that the moisture uptake of the PVP is important for the sintering. However, this does not explain the different results from the Canon paper because the moisture uptake by the PVP is related to the ink itself, and not to the paper. The second parameter to consider is the porosity of the paper. During printing, the solvent and solutes are drained by

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capillary forces into the coating layer. It is likely that one portion of the PVP is bound to the particles but the excess can be pulled into the paper coating by the capillary forces and therefore, the pore structure will act as a guide to the amount of polymer which is retained in the ink layer. However, porosity measurements conducted on the Canon and HP papers show no significant differences in porosity. The exact chemical compositions of the coating layer in the papers used are not known but, it is well known that it is very common for different kinds of ions and charged complexes to be added in order to achieve the optimum printing quality. When the moisture content in PVP is raised, the ability for ion transport is created within the polymer [25] and ions from the paper coating can be transported into the ink. Together with the moisture-containing PVP, this could reduce the electrostatic barrier. Thus, the chemistry of the paper is of importance for the sintering process. More research is required to fully understand the humidity sintering effect on the NP ink and the role of the substrate, but the observed effect can be put to use in humidity sensors as have been presented. V. C ONCLUSION Low cost printed resistive humidity sensors have been manufactured on paper and characterized. Suitable applications in relation to this are for example, situations in which a large number of inexpensive and disposable humidity sensors are required which are then able to detect whether they have been subjected to high humidity, such as would be the case in the checking of transportation conditions of goods or within buildings. The most efficient readout would be to integrate the sensors in a wireless readout system, such as RFID systems. It has been shown that the ink type and paper combination is crucial for obtaining the desired sensor effect. More research is required to fully understand the humidity sintering effect on the NP ink and the role of the substrate. However, the observed effect can be put to use in printed humidity sensors which have a memory function. R EFERENCES [1] R. Jedermanna, L. Ruiz-Garcia, and W. Langa, “Spatial temperature profiling by semi-passive RFID loggers for perishable food transportation,” Comput. Electron. Agricul., vol. 65, no. 2, pp. 145–154, Mar. 2009. [2] M. Kärkkäinen, “Increasing efficiency in the supply chain for short shelf life goods using RFID tagging,” Int. J. Retail Distr. Manage., vol. 31, no. 10, pp. 529–536, 2003. [3] E. Bottani and A. Rizzi, “Economical assessment of the impact of RFID technology and EPC system on the fast-moving consumer goods supply chain,” Product. Econ., vol. 112, no. 2, pp. 548–569, Apr. 2008. [4] R. Bhattacharyya, C. Floerkemeier, and S. Sarma, “RFID tag antenna based temperature sensing,” in Proc. IEEE Int. Conf. RFID, Apr. 2010, pp. 8–15. [5] K. Chang, Y. H. Kim, Y. J. Kim, and Y. J. Yoon, “Functional antenna integrated with relative humidity sensor using synthesised polyimide for passive RFID sensing,” Electron. Lett., vol. 43, no. 5, pp. 7–8, Mar. 2007. [6] A. Oprea, N. Bârsan, U. Weimar, M.-L. Bauersfeld, D. Ebling, and J. Wöllenstein, “Capacitive humidity sensors on flexible RFID labels,” Sensors Actuat. B, vol. 132, no. 2, pp. 404–410, Jun. 2008. [7] J. Sidén, X. Zeng, T. Unander, and H.-E. Nilsson, “Remote moisture sensing utilizing ordinary RFID tags,” in Proc. IEEE Int. Conf. Sensors, Oct. 2007, pp. 308–311.

[8] A. Alastalo, T. H. Seppä, J. H. Leppäniemi, M. Aronniemi, J. M. L. Allen, and T. Mattila, “Modelling of nanoparticle sintering under electrical boundary conditions,” J. Phys. D: Appl. Phys., vol. 43, no. 48, pp. 485501-1–485501-10, 2010. [9] M. L. Allen, M. Aronniemi, T. Mattila, A. Alastalo, K. Ojanpera, M. Suhonen, and H. Seppä, “Electrical sintering of nanoparticle structures,” Nanotechnology, vol. 19, no. 17, pp. 175201-1–175201-4, 2008. [10] J. R. Greer and R. A. Street, “Thermal cure effects on electrical performance of nanoparticle silver inks,” Acta Mater., vol. 55, no. 18, pp. 6345–6349, Oct. 2007. [11] J. H. Leppäniemi, M. Aronniemi, T. Mattila, A. Alastalo, M. L. Allen, and H. Seppä, “Printed WORM memory on a flexible substrate based on rapid electrical sintering of nanoparticles,” IEEE Trans. Electron Devices, vol. 58, no. 1, pp. 151–159, Jan. 2011. [12] S. Magdassi, M. Grouchko, O. Berezin, and A. Kamyshny, “Triggering the sintering of silver nanoparticles at room temperature,” Acs Nano, vol. 4, no. 4, pp. 1943–1948, Apr. 2010. [13] W. Zapka, W. Voit, C. Loderer, and P. Lang, “Low temperature chemical post-treatment of inkjet printed nano-particle silver inks,” in Proc. 24th Int. Conf. Digital Print. Technol./Digital Fabricat., Pittsburgh, PA, Sep. 2008, pp. 906–911. [14] M. Allen, J. Leppäniemi, M. Vilkman, A. Alastalo, and T. Mattila, “Substrate-facilitated nanoparticle sintering and component interconnection procedure,” Nanotechnology, vol. 21, no. 47, pp. 475204-1–4752046, 2010. [15] P.-G. Su and C.-S. Wang, “Novel flexible resistive-type humidity sensor,” Sensors Actuat. B: Chem., vol. 123, no. 2, pp. 1071–1076, May 2007. [16] H.-J. Chen, Q.-Z. Xue, M. Ma, and X.-Y. Zhou, “Capacitive humidity sensor based on amorphous carbon film/n-Si heterojunctions,” Sensors Actuat. B: Chem., vol. 150, no. 1, pp. 487–489, 2010. [17] Materials for Electronics/Display. (2011, Aug.) [Online]. Available: http://www.anapro.com/english/product/nano_silver.asp [18] H. Wang, X. Qiao, J. Chen, X. Wang, and S. Ding, “Mechanisms of PVP in the preparation of silver nanoparticles,” Mater. Chem. Phys., vol. 94, nos. 2–3, pp. 449–453, Dec. 2005. [19] A. A. A. De Queiroz, D. A. W. Soares, P. Trzesniak, and G. A. Abraham, “Resistive-type humidity sensors based on PVP-Co and PVPI2 complexes,” J. Polym. Sci., Part B: Polym. Phys., vol. 39, no. 4, pp. 459–469, 2001. [20] J. Perelaer, A. W. M. de Laat, C. E. Hendriks, and U. S. Schubert, “Inkjet-printed silver tracks: Low temperature curing and thermal stability investigation,” J. Mater. Chem., vol. 18, no. 27, pp. 3209–3215, 2008. [21] D. Wakuda, K. Chang-Jae, K. Keun-Soo, and K. Suganuma, “Room temperature sintering mechanism of Ag nanoparticle paste,” in Proc. 2nd Electron. Syst. Integr. Technol. Conf., Piscataway, NJ, 2008, pp. 909–914. [22] New HP Advanced Photo Paper. (2011, Aug.) [Online]. Available: http://h10088.www1.hp.com/cda/gap/display/main/index.jsp?zn=gap&cp =20000-20058-20744-20843∧ 23151_4041_100 [23] Photo Paper Pro Platinum [PT-101]. (2011, Aug.) [Online]. Available: http://www.usa.canon.com/app/html/PT101_techguide/PT101_HTML/ index_amr.html [24] B. T. Anto, S. Sivaramakrishnan, L. L. Chua, and P. K. H. Ho, “Hydrophilic sparse ionic monolayer-protected metal nanoparticles: Highly concentrated nano-Au and nano-Ag ‘inks’ that can be sintered to near-bulk conductivity at 150 °C,” Adv. Funct. Mater., vol. 20, no. 2, pp. 296–303, 2010. [25] K. Ogura, A. Fujii, H. Shiigi, M. Nakayama, and T. Tonosaki, “Effect of hygroscopicity of insulating unit of polymer composites on their response to relative humidity,” J. Electrochem. Soc., vol. 147, no. 3, pp. 1105–1109, Mar. 2000.

Henrik Andersson was born in 1975. He received the M.Sc. degree from Umeå University, Umeå, Sweden, and the Ph.D. degree in electronics from Mid Sweden University, Sundsvall, Sweden, in 2003 and 2008, respectively. He is currently a Research Engineer with Electronics Design Division, Mid Sweden University. His current research interests include semiconductor photon detectors, printed electronics, and printed sensor technology.

ANDERSSON et al.: INKJET PRINTED SILVER NP HUMIDITY SENSOR WITH MEMORY EFFECT ON PAPER

Anatoliy Manuilskiy was born in 1943. He received the Ph.D. degree in lasers and optics properties from the Department of Radio Physics, Kiev State University, Kiev, Ukraine, in 1971. He is currently a Senior Researcher with Electronics Design Division, Mid Sweden University, Sundsvall, Sweden. His current research interests include development of laser systems, laser optics, and printed nano structures. Tomas Unander received the M.S. degree in telecommunication and the Lic.Eng. degree from Mid Sweden University, Sundsvall, Sweden, in 2002 and 2009, respectively. He was with AddMarkable Technologies, Sundsvall, as a Development Engineer from 2002 to 2005. Since 2006, he has been with SCA R&D Center, Sundsvall, as a Research Engineer, working on wood fiber based printable sensors with a focus on printable sensors integrated with radio frequency identification. Cecilia Lidenmark was born in 1976. He received the Masters degree from Mid Sweden University, Sundsvall, Sweden, in 2002. Her current research interests include chemical aspects of printing and sintering of nanoparticle-inks.

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Sven Forsberg was born in 1956. He received the Masters degree in chemistry and the Lic.Eng. degree in physical chemistry from the Royal Institute of Technology, Stockholm, Sweden, in 1982 and 1990, respectively. He was with Surface Chemistry Institute, Stockholm, for seven years. He has ten years of experience from industrial research in the paper industry. He is currently with Mid Sweden University, Sundsvall, Sweden, with paper and printed electronics. His current research interests include novel applications of paper.

Hans-Erik Nilsson received the Ph.D. degree in solid state electronics from the Royal Institute of Technology, Stockholm, Sweden, in 1997. He was a Senior Researcher with Mid Sweden University, Sundsvall, Sweden, in 1997, focusing on the modeling of advanced semiconductor devices. In 2002, he became a Full Professor in electronics. His current research interests include quantum transport in electron devices, radiation imaging detectors, radio frequency electron devices, printed radio frequency identification antennas, printed sensor technology, and wireless sensor networks.