Fabrication of silver nanorods embedded in PDMS ...

Home

Search

Collections

Journals

About

Contact us

My IOPscience

Fabrication of silver nanorods embedded in PDMS film and its application for strain sensing

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys. D: Appl. Phys. 48 445303 (http://iopscience.iop.org/0022-3727/48/44/445303) View the table of contents for this issue, or go to the journal homepage for more

Download details: IP Address: 103.27.8.43 This content was downloaded on 26/10/2015 at 11:26

Please note that terms and conditions apply.

Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 48 (2015) 445303 (4pp)

doi:10.1088/0022-3727/48/44/445303

Fabrication of silver nanorods embedded in PDMS film and its application for strain sensing Pratibha Goel and J P Singh Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India E-mail: [email protected] Received 29 May 2015, revised 3 September 2015 Accepted for publication 14 September 2015 Published 8 October 2015 Abstract

Highly reflective and surface conductive strain gauges have been prepared by embedding the silver nanorods (AgNRs) into polydimethylsiloxane (PDMS). Thermal curing of PDMS on AgNRs grown Si wafer leads to a flexible, reflective and conductive silver surface. The reflectance of the as prepared films were observed to be 60% with a low value of sheet resistance. The reflectance of the film was able to be tuned from 60% to 15% in the visible region. The fabrication of a parallel plate capacitor strain sensor from AgNRs embedded PDMS, and tuning of the capacitance with respect to the applied strain, leads to a gauge factor of ~1. These mechanically tunable AgNRs/PDMS films demonstrate potential application as a strain sensor. Keywords: silver nanorods, polydimethylsiloxane, oblique angle deposition, strain (Some figures may appear in colour only in the online journal)

1. Introduction

their high flexibility, light resistant and anti-bacterial effect of silver. Established approaches such as physical vapour deposition, chemical vapour deposition and electrodeposition have been developed to deposit such metal films on polymer surfaces. Since silver, as a passive metal, does not interact strongly with organic functionalities, the adhesion on a polymer surface is poor [15]. Generating sufficient adhesion between the metal and the base polymer is recognized to be a challenging problem that has not been solved for traditional deposition techniques. Much progress have been made to advance the performance of Ag nanowires based films in terms of sheet resistance and flexibility [18, 19]. Xu et al have demonstrated capacitive strain sensor using Ag nanowires based stretchable conductors [16]. Scardaci et al have achieved a very low value of sheet resistance Rs value with transmittance value of 90% using the spray deposited Ag nanowires [17]. It is important to notice that much of the research focus was on making transparent, stretchable and conducting film. But, there are very few reports which target to achieve the stretchable, highly reflecting and conducting films [18–20].

Materials with conductive and flexible features have been introduced recently, by exploring novel material and through structural consideration [1, 2]. A variety of such materials have been developed such as carbon nanotube (CNT) based composites [3–9], graphene film [10] and wavy thin metal films [11, 12]. There are several limitations for these materials including poor stretchability [9, 10], low conductivity [8, 9] and their resistance value increase under the applied strain [7, 9, 12, 13]. Surface metallization of the polymer films has been extensively investigated due to the combination of excellent mechanical, thermal and chemical properties of the polymer and the superb optical and electrical properties of the metals employed. Silver is a metal of significant interest because of its high reflection coefficient (~0.93) and electrical conductivity (6.3  ×  107 Ω m−1) [14]. Such silvered polymeric films with high surface reflectivity and conductivity have found applications as concentrators in space environments for solar dynamic propulsion and as thin film reflectors in microelectronics [14–17]. Polymer substrates with silver films may also find potential application in food packaging because of 0022-3727/15/445303+4$33.00

1

© 2015 IOP Publishing Ltd  Printed in the UK

P Goel and J P Singh

J. Phys. D: Appl. Phys. 48 (2015) 445303

AgNRs/PDMS film with AgNRs facing up was placed on it as shown in the schematic in figure 1(e). The thermal curing of the whole sample and peeling off PDMS from Si wafer resulted in the fabrication of a parallel plate capacitor where the top and bottom surfaces of the PDMS were symmetrically covered with AgNRs. The morphology and structural analysis of the as-prepared samples were characterized by scanning electron microscope (SEM) (Zeiss, EVO 50). For the tensile test, a custom-designed tensile tester was used which consists of two posts (stationary and movable) mounted on the optical rails. During tensile experiment, the sample was held in between the two posts and tensile tests were performed by moving the mount post with respect to the stationary post and measuring the increment in the film length. The optical reflection spectra measurements were performed using SHIMADZU UV-2501C UV/vis spectrophotometer. The sheet resistance measurement was conducted in van der Pauw configuration at room temperature using Keithley 2611A series source meter. The capacitance was measured using Agilent E4980A precision LCR meter.

3.  Results and discussion Figure 1.  A schematic illustration of transferring the AgNRs into PDMS resulting in fabrication of parallel plate capacitor configuration.

The SEM micrographs of as prepared AgNRs arrays on Si wafer and AgNRs embedded in PDMS film are shown in figures  2(a) and (b), respectively. The SEM images clearly show infiltration of PDMS into the voids present in the porous columnar silver film. Since, one end of the AgNRs is buried inside PDMS, this makes the adhesion between the silver and the polymeric PDMS film outstanding. Even, the adhesive tapes were found not to remove any silver from this composite AgNRs/PDMS surface. The sheet resistance measurement confirms the high conductivity of the sample. Figure  3(a) shows the reflectance spectra of AgNRs deposited on top of the PDMS film. The reflectance spectra shows dips at 470 nm corresponding to the transverse mode (TM), whereas reflectance dips at 577 and 661 nm correspond to the longitudinal mode (LM) [27]. However, after embedding AgNRs into PDMS the reflection dip at 470 nm which corresponds to TM surface plasmon resonance (SPR) was broadened due to the greater refractive index of PDMS over air [28]. Figure  3(b) shows the variation in reflectivity of the AgNRs embedded PDMS films measured as a function of applied mechanical strain ε (ratio of increase in film length (Δl) to its original film length (l)) varying from 0% to 40%. The specular reflectivity of the as prepared sample at 531 nm wavelength is about 60%. The reflectance dip positions were found to be almost unchanged under application of mechanical strain. However, the depth of the dips at 577 nm and 661 nm decreases with increase in the strain value up to 40%. This is due to an increase in the interrods separation under the application of tensile strain which reduces the surface plasmon coupling between the AgNRs and results in the damping of the longitudinal mode. With increase in strain value the film stretches and the overall coverage of the Ag film decreases resulting into a decrease in the reflectivity value from 60% to 15% in the visible region as shown in figure 3(b).

In this paper, we demonstrate the fabrication of stretchable Ag nanorods (AgNRs) embedded (polydimethylsiloxane) PDMS composite with high surface reflectivity and surface conductivity and show its application as capacitive strain gauge. Polymeric supports offer substantial advantages in weight, flexibility, and packaging options relative to traditional substrates such as glass, ceramics, and metals. 2.  Experimental section Figure 1 shows the major steps for embedding AgNRs in PDMS to achieve flexible parallel plate capacitor configuration. AgNRs arrays were grown over Si (1 0 0) substrates by thermal evaporation of silver powder (99.9%) using oblique angle deposition (OAD) method [21–26]. Before deposition, Si substrates were ultrasonically cleaned using acetone. For the growth of AgNRs film, the substrates were mounted on sample holder such that the angle between the substrate normal and the incident vapour flux was 85°. The chamber pressure during deposition was better than 2  ×  10−6 Torr. The PDMS was prepared using Sylgard 184 silicon elastomer (Dow Corning Inc.) by mixing base with a curing agent in a 10 : 1 ratio at room temperature. The solution was kept in a desiccator attached with rotary pump to remove the trapped air bubbles. After the air bubbles were removed, the mixture was poured on to the AgNRs deposited Si substrate and cured for about 20 min at 80 °C temperature. The PDMS film was then peeled off from the Si wafer and used as AgNRs supported free standing and flexible AgNRs/PDMS films. For the fabrication of capacitive strain sensors liquid PDMS was casted over AgNRs grown Si wafer and before curing, another 2

P Goel and J P Singh

J. Phys. D: Appl. Phys. 48 (2015) 445303

Figure 2.  SEM image of the (a) obliquely inclined AgNRs arrays, (b) bottom surface of PDMS embedded AgNRs.

Figure 3.  (a) Reflectance spectra of the AgNRs grown over PDMS, (b) change in reflectance of AgNRs embedded PDMS film with applied

strain.

Figure 4.  Variation in capacitance with (a) frequency, (b) log frequency, (c) relative change in capacitance as a function of strain, (d) capacitance modulation of the AgNRs embedded PDMS strain sensor under cyclic applied strain. 3

P Goel and J P Singh

J. Phys. D: Appl. Phys. 48 (2015) 445303

1. These highly flexible, reflective and conductive properties have potential applications in strain sensors.

A capacitive strain gauge prepared from AgNRs/PDMS was subjected to the measurements with a strain profile, where the mechanical strain was applied progressively from 0% to 40%. The capacitance of parallel plate capacitor made from AgNRs embedded PDMS film was measured with varying frequency 0.02 kHz and 2000 kHz, under constant level of 1.0 V. A typical exponential decay in capacitance with increase in frequency was observed as shown in figure 4(a). This is further supported by almost a linear nature of the capacitance when plotted on frequency with log scale (figure 4(b)). Since, higher excitation frequencies are beneficial for improving the signal to noise performance and capacitive characteristics, so we have used an excitation frequency of 100 kHz for all the capacitance versus strain measurements. As a result of uniaxial stretching the sample size changes, in particular its length (l) increases while its width (w) and thickness (t) decrease due to the Poisson effect as l = (1 + ε)l 0, w = (1 − υ  ε)w0 and t = (1 − υ  ε)t0, where l 0, w0 and t0 denote the length, width and thickness of the sensor in its unstretched state and ε is the applied strain with υ as the Poisson ratio of the PDMS. Thus, the capacitance (C) with the

Acknowledgments The author PG kindly acknowledges Council of Scientific and Industrial Research (CSIR), India for the senior research fellowship. The technical support of IUAC, New Delhi and Nanoscale Research Facility, IIT Delhi are highly acknowledged. We thank B C Vaagensmith, SDSU, Brookings, USA for English proof reading of the manuscript. References [1] Sekitani T and Someya T 2010 Adv. Mater. 22 2228–46 [2] Kim D-H, Xiao J, Song J, Huang Y and Rogers J A 2010 Adv. Mater. 22 2108–24 [3] Cohen D J, Mitra D, Peterson K and Maharbiz M M 2012 Nano Lett. 12 1821–5 [4] Lipomi D J, Vosgueritchian M, Tee B C-K, Hellstrom S L, Lee J A, Fox C H and Bao Z 2011 Nat. Nanotechnol. 6 788–92 [5] Yamada T, Hayamizu Y, Yamamoto Y, Yomogida Y, Izadi-Najafabadi A, Futaba D N and Hata K 2011 Nat. Nanotechnol. 6 296–301 [6] Cai L et al 2013 Sci. Rep. 3 3048 [7] Hu L, Yuan W, Brochu P, Gruner G and Pei Q 2009 Appl. Phys. Lett. 94 161108 [8] Sekitani T, Noguchi Y, Hata K, Fukushima T, Aida T and Someya T 2008 Science 321 1468–72 [9] Liu K, Sun Y, Liu P, Lin X, Fan S and Jiang K 2011 Adv. Funct. Mater. 21 2721–8 [10] Kim K S, Zhao Y, Jang H, Lee S Y, Kim J M, Kim K S, Ahn J-H, Kim P, Choi J-Y and Hong B H 2009 Nature 457 706–10 [11] Bowden N, Brittain S, Evans A G, Hutchinson J W and Whitesides G M 1998 Nature 393 146–9 [12] Gray D S, Tien J and Chen C S 2004 Adv. Mater. 16 477 [13] Lacour S P, Wagner S, Huang Z and Suo Z 2003 Appl. Phys. Lett. 82 2404 [14] Southward R E and Stoakley D M 2001 Prog. Org. Coat. 41 99–119 [15] Green P F and Berger L L 1993 Thin Solid Films 224 209–16 [16] Xu F and Zhu Y 2012 Adv. Mater. 24 5117–22 [17] Scardaci V, Coull R, Lyons P E, Rickard D and Coleman J N 2011 Small 7 2621–8 [18] Southward R E and Thompson D W 2001 Mater. Des. 7 565–76 [19] Yang S, Wu D, Qi S, Cui G, Jin R and Wu Z 2009 J. Phys. Chem. B 113 9694–701 [20] Qi S-L, Wu D-Z, Wu Z-P, Wang W-C and Jin R-G 2006 Polymer 47 3150–6 [21] Robbie K, Brett M J and Lakhtakia A 1996 Nature 384 616 [22] Zhao Y-P, Ye D-X, Wang G-C and Lu T-M 2002 Nano Lett. 2 351–4 [23] Karabacak T, Wang G-C and Lu T-M 2004 J. Vac. Sci. Technol. A 22 1778 [24] Singh D P, Goel P and Singh J P 2012 J. Appl. Phys. 112 104324 [25] Goel P, Singh K and Singh J P 2014 RSC Adv. 4 11130 [26] Mark A G, Gibbs J G, Lee T-C and Fischer P 2013 Nat. Mater. 12 802–7 [27] Zhang Z-Y and Zhao Y-P 2006 Appl. Phys. Lett. 89 023110 [28] Noguez C 2007 J. Phys. Chem. C 111 3806–19

l w

applied strain becomes C = (1 + ε)C0, where C0 = ε0εr 0t  0 0 denotes the initial capacitance, ε0 and εr represents the permittivity of vacuum and the relative permittivity of PDMS, respectively. The effect of temperature can be neglected since the measurements were performed at a steady ambient temperature. Therefore, the parallel plate model predicts a linear variation in the capacitance under applied strain leading to a

(

)

capacitive gauge factor ∆C C0 /ε equal to ~1. Figure 4(c) plots the capacitance change as the function of applied strain (up to 40%). The percentage change in the capacitance value is same as the percentage change in the applied strain value. This is in good agreement with the theoretical prediction from parallel plate model. Hence, our strain sensor shows an excellent linearity under a strain value up to 40%. The gauge factor of the strain sensor comes out to be about 1. The capacitive strain gauge prepared from AgNRs/PDMS was found to be robust and provided reproducible piezocapacitive values after multiple operations of stretching and releasing cycles as shown in figure 4(d). Due to the flexibility and the stable chemical inertness of PDMS, the AgNRs/PDMS strain gauge can be mounted onto a variety of substrates with an excellent adhesion, regardless of environmental impacts. In addition to such sensor applications, its variable capacitance with respect to strain enables it as a force sensitive variable capacitor. This technique can be readily extended to large scale fabrication of several other noble metals nanostructures embedded polymeric film. 4. Conclusions We have described the process for embedding the AgNRs into PDMS. Both conductive and reflective films were obtained with a maximum reflectivity of over 60%. The reflectivity of the surface was tuned under applied mechanical strain. The capacitance value of the parallel plate capacitor configuration of the sample was found to be tuned with respect to the applied strain up to 40% leading to the gauge factor of about 4