Silver nanoparticles in comparison with ionic-liquid and rgo as. gate dopants for paper-pencil based flexible field-effect transistors

Silver nanoparticles in comparison with ionic-liquid and rgo as gate dopants for paper-pencil based flexible field-effect transistors Soumen Mandal,* a,b Ravi Kumar Arun, a,b Nagahanumaiah, a,b Nripen Chanda, a,b Surajit Das, c Pankaj Agarwal, c Jamil Akhtar, c Prabhash Mishra d a Microsystems Technology Laboratory, CSIR-Central Mechanical Engineering Research Institute, Durgapur, West Bengal India. b Academy of scientific and innovative research, New Delhi, India. c Sensors and nanotechnologies group, CSIR-Central Electronics Engineering Research Institute, Pilani, India. d Nanosensor research laboratory, Jamia Millia Islamia, New Delhi, India. Abstract Nanoparticles based flexible field effect transistors (FET) containing carbon nanotubes (CNTs), silicon nano-wires (SiNWs) have attracted tremendous attention, since their interesting device performance can be utilized for integrated nanoscale electronics. However, use of CNTs and SiNWs on polymer substrates poses serious limitations in terms of their fabrication procedure, repeatability and bio-degradability. In this article, we report for the first time, the fabrication of solution processed FET on paper substrate doped with easily prepared silver nanoparticles (AgNPs) and demonstrated the FET characteristics. To compare the FET performances, we fabricated two other FETs on paper containing ionic liquid, (1- butyl-3-methyimidazolium octyl sulphate, IL) and reduced graphene oxide (rgo) as dopants. We observe that AgNPs based dopant generated good FET characteristics in terms of linear transconductance variations and higher carrier concentration values which showed negligible changes to bending and aging. In comparison to AgNP-FET, rgo and IL based dopants yielded high carrier mobilities, however rgo based FET is more prone to aging and bending. Excellent linearity for I ds -V g curve found in AgNP-FET assures its applicability for devices requiring linear transfer characteristics such as linear amplifiers. Keywords: silver nanoparticle; flexible electronics; field effect transistor; rgo; ionic liquid. 2

1. Introduction Electronics on flexible substrates have immense importance referring to their outstanding bending capability, lightness, low-cost and easy fabrication. All these features essentially make them best fit for disposable electronics, biological sensors for tissue diagnostics, and implantable devices [1-5]. Among various flexible electronic components, nano-wires (NW) and nanoparticle based field effect transistors made of CNTs, inorganic nano-material including silicon nano-wires (SiNWs) especially have attracted tremendous attention, since their interesting device performance can be utilized for integrated nano-scale electronics [6-14]. However, their fabrication requires complicated process steps to perform well-ordered nanomaterial integration on large flexible substrates. Recently, a new method of flexible field effect transistor (FET) fabrication has been reported, [15] where pencil markings on paper substrate acted as channel and a drop of PDMS mixed with ionic liquid (1-butyl-3-methyimidazolium octyl sulphate) poured over the channel served as the gate dielectric. Though fabrication of pencil-on-paper devices does not require any sophisticated facilities and high end fabrication equipment, the use of long chain, non bio-degradable polymer as a material for gate or substrate would become a serious limitation in terms of repeatability, operating condition and biodegradability. Further, as number of phase transitions occurs in long chain polymers, the long term electrical response of the fabricated FETs with aging and bending cannot be reliably assured. Additionally, long chain polymers like polystyrene (PS), polyvinylpyrrolidone (PVP) are not biodegradable leading to the generation of electronic waste. In order to alleviate the above existing issues, we propose a simple, solution processed method for the fabrication of silver nanoparticle-doped paper-pencil FETs (AgNP-FET) using pencil markings on paper substrates. The direct use of nanoscale silver particles on paper substrate will avoid the use of polymers and subsequently reduce the number of complicated steps for the development of flexible FETs. Nano-sized metals provide a potential solution to address present and future technological demand because of their novel properties like dielectric strength enhancement, tunneling effects, Coulomb blockade and so on. The size, size distribution and loading level of metal nanoparticles in the nano-composite is found to have significant influences on the dielectric properties of the nano-composite system. Lu et al. studied the dielectric properties of Ag/Carbon-Black/Epoxy nano-composites and found that the composite with nano-sized Ag particles remarkably increased dielectric constant of the nano-composite due to the piling of charges at the extended interface of the composites [6]. This fact inspired the authors to check the feasibility of AgNP stabilized in paper-cellulose as a gate material for the present paper-pencil FET. Utilization of metallic nanoparticles dopants like AgNPs to fabricate gate 3

dielectric for FETs has not been explored yet. For the first time, we explore the effect of AgNPs as a gate dopant for flexible FET on paper. Nevertheless, to compare the electrical transport properties of this AgNP-FET, we also develop two other FETs with reduced-graphene oxide and ionic liquid (rgo-fet and IL-FET) on paper substrates. The application of rgo and IL (1-butyl-3-methyimidazolium octyl sulphate) as gate dielectric is reported on various substrates other than paper. Therefore, their use in paper-pencil FETs is also be reported for the first time in this work. In the present FETs, paper was used both as gate and substrate material. In our proposed FET, metallic AgNPs are used as dopant in paper to built substrates as well as gate. AgNP based dopant generated good FET characteristics in terms of linear transconductance variations and higher carrier concentration values which showed negligible changes to bending and aging. IL and rgo based FETs are prepared to compare the effects on FET performance with AgNP-FET. IL and rgo based dopants yielded high carrier mobilities, however rgo based FET was more prone to aging and bending in comparison to AgNP-FET. In all cases, we introduced paper as an electronic matrix because it has a number of advantages like low cost, easy availability and high flexibility facilitating it to be potential material for flexible electronics. 2. Experimental 2.1. Materials Poly-vinyl alcohol, AgNO 3, NaBH 4, hydrazine and IL (1-butyl-3-methyimidazolium octyl sulphate) was obtained from Sigma Aldrich and they were used as received. GO (Graphene oxide) was synthesized using modified Hummers method [16]. rgo was synthesized in-situ paper matrix by reducing the paper soaked with GO using hydrazine. 2.2. FET fabrication and characterization In this study, the fabricated FETs comprise of filter paper (Whatman, 10 micron pore size) containing AgNPs or rgo or IL, acting as both gate and substrate (Figure 1). Similar strategy to use a single material for both substrate and gate dielectric is claimed by Loi et al for their polymeric mylar-film based FET [17]. Initially, pencil markings were made on a plain paper and a FET was fabricated as per the schematic shown in Figure 1 (a) without any material doping on the paper. The pencil marking acted as a normal resistor however no FET characteristics were observed in this case which shows that paper itself is not capable of showing required dielectric properties required for FET operation. 4

Fig 1. (a) Schematic of the FETs with conventions (b) Fabricated FET using paper containing IL, AgNPs and rgo Fig 2. Drain to source current and gate leakage current of FET containing (a) Ionic liquid (b) AgNPs (c) rgo (d) Drain to source current vs. gate to source voltage for fabricated FETs. (under normal laboratory conditions) Poly-vinyl alcohol (PVA) stabilized AgNPs was synthesized in ethanol and soaked on a piece of filter paper. Fourteen pencil strokes of 1 cm length (Natraj HB 621 pencil) were marked on the filter paper doped with AgNPs. The pencil 5

marking acts as a channel. The channel width was ~1.8 mm. The radius of pencil tip was ~210µm before the start of stroking process. The fabrications of rgo-fet and IL-FETs were followed through similar procedure. The detailed fabrication process is illustrated in ESI (Figure S1). The channel material thus contains graphitic layers (in form of pencil trace) doped with materials such as AgNP, IL and rgo which modulates the channel conductance and FET characteristics. Figure 2 displays FET characteristics obtained using Keithley 4200 semiconductor characterization system. The fabricated FETs showed standard ambipolar nature for I ds vs V g and the gate leakage current linearly increased with gate voltage (Figure 2). The ambipolar nature is due to presence of both electrons and holes at the channel (pencil marking)-substrate (dopants) interface. The gate leakage current was high for paper containing AgNPs, while minimum for FET with paper containing rgo as gate material. The shift in Dirac point in case of AgNPs based FET was nominal possibly due to nearly uniform distribution of nanoparticles in cellulose matrix. However, the Dirac point shifted from 0 V due to defects present in the paper material as well as due to non uniform distribution of IL and rgo in paper [18]. FESEM (field emission scanning electron microscopy) for the paper substrates were carried out in order to understand the phenomenon and explained in results and discussion. 3. Results and discussions The FET characteristics as shown in Figure 2 were used to calculate the FET parameters. The channel transconductance (g m ) was found using equation 1. g = di / dv m ds g (1) Drain to source current (I ds ) for negative gate voltage represents FET action due to holes and for positive gate voltage represents action due to electrons. The channel transconductance for electrons and holes in the linear regime of the curves were found to be 0.1 µa/v and 0.108 µa/v for AgNP-FET. The transconductance for electrons and holes were not equal signifying that the mobilities of both are different. In comparison to AgNP-FET, the transconductance values were 0.166 µa/v and 0.385 µa/v for IL-FET, and 0.75 µa/v and 1.33 µa/v for rgo-fet. The electron and hole mobilities were found using equation 2 [19]. g = ( W/ L) µ CV m 0 DS (2) where (W/L) is the width to length ratio, µ is the carrier mobility, C 0 is the gate capacitance per unit area and V DS is the drain to source voltage. The (W/L) ratio was found to be ~0.18. The C 0 value for AgNP-FET was 0.019 µf/cm while for IL-FET and rgo-fet, C 0 were found to be 0.029 µf/cm, and 0.033 µf/cm respectively. The electron and hole mobilities (µ e and µ h ) were found to be 29.24 cm 2 /V-s and 50 cm 2 /V-s for AgNP-FET. In contrast, the electron and hole mobilities 6

were 31.8 cm 2 /V-s and 73.75 cm 2 /V-s for IL-FET, and 126.26 cm 2 /V-s and 223.91 cm 2 /V-s for rgo-fet. These carrier mobilities are in the order of flexible paper based FETs with polymeric gate (PDMS gate) reported in literature and are summarized in Table 1, though the values differ by some extent. It is obvious that the mobility values of these simple devices using only paper substrate without any polymeric materials will not be exactly same as existing polymer based FETs. The extrinsic carrier concentration (n e ) for electrons were estimated using the Drude model from the formula of resistivity (equation 3 and 4) [20, 21] ρ = 1/ ne( µ + µ ) e e h n = ( L/ W){1 / e( µ + µ )}(1 / R) e e h (3) (4) where e is the electron charge, R is the resistance of pencil trace, µ e is the mobility of electrons and ρ is the resistivity. The hole concentration was found using equation 5. n n h e µ K e = µ h (5) Here n h is the concentration of holes and K is the conductance ratio of the holes and electrons found from the transfer characteristics. Value of K was found to be 0.45 for AgNP-FET, 1.257 for IL-FET and 0.51 for rgo-fet. The carrier concentrations for electrons and holes were found as 0.43 10 /cm 2 and 0.11 10 /cm 2 for AgNP-FET, 0.326 10 /cm 2 and 0.176 10 /cm 2 for IL-FET and 0.098 10 /cm 2 and 0.028 10 /cm 2 for rgo-fet. Table 1 Comparitive assessment of carrier mobility and carrier concentration for polymer based paper-pencil FET in literature and FETs fabricated in this work S. No FET Carrier mobility (cm 2 /V-s) Carrier concentration (/cm 2 ) Ion/Ioff Electrons Holes Electrons Holes V dd = 1V Vdd=3 V 1 Ionic liquid-pdms based paper pencil59 106 1.7 10 3 10 ~2.5 ~1.6 FET 15 2 AgNP based FET in this work 29.24 50 0.43 10 0.11 10 0.21 0.16 3 IL based FET in this work 31.8 73.75 0.326 10 0.176 10 0.17 0.33 4 rgo based FET in this work 126.26 223.91 0.098 10 0.028 10 1.1 2.33 3.1. Bending and aging tests 7

Polymer based FETs may possess a huge shift in their characteristics due to phase transitions occurring in long chain glassy polymers. In our fabricated paper based FETs, the aging and bending were mapped to track the change in FET performance in order to ensure their reliability. The fabricated FETs showed negligible variations in their characteristics after 60 days of aging (stored in isolated environment) barring the case of rgo where variations were significantly high. The bending characteristics of the FETs with ~90 angle along the central axis showed no significant variations except for rgo-fet. The maximum variation in I DS vs V g characteristics was within 2% for AgNP-FET. The characteristics are shown in Figure 3. Thus compared to rgo-fet and IL-FET, the AgNP-FET is more robust to aging and bending variations. Fig.3 (a) Bending and aging characteristics for fabricated FETs containing (a) Ionic liquid (b) AgNP (c) rgo 3.2. FESEM based material characterisation In order to understand the phenomenon behind working of the fabricated FETs, FESEM for the gate/ substrate material (paper containing AgNP, IL and rgo) and the channel material (pencil markings on paper) were carried out using Nova 8

NanoSEM instrument. Figure 4 shows FESEM images of (a) paper substrate, (b) paper substrate containing IL, (c) paper containing AgNP and (d) paper containing rgo. The SEM images of the pencil markings on paper (ESI Figure S2(a)) reveals exfoliation of turbostatic graphite layers on the paper. It acts as a layered graphitic system [22]. On application of electric field the stacked layers are mis-oriented along the c-axis causing electronic decoupling which leads to change in the flow of current. The FESEM image of the paper containing AgNPs (Figure 4(c)) reveals that, silver nanoparticles in the range of 70 to 85 nm were trapped in cellulose fibers without agglomeration (ESI Figure S2(b)). Since nano-size silver particles contain pool of electrons in their conduction band, the paper containing AgNP shows good field effect behaviour. The FESEM image of paper containing ionic liquid (Figure 4(b)) reveals that even after drying the sample, ionic liquid remains embedded in the paper substrate in gel form. This leads to the formation of EDL (Electric double layer) due to soaked ionic liquid in the microfibers of the paper. The EDL contributes to the gate field effect leading to FET characteristics [23,24]. It should be noted that whereas dried samples of IL-FET and rgo-fet shows FET characteristics, however drying the AgNP-FET fully does not show any FET characteristics. Thus AgNP based dielectric is effective only in normal environmental conditions where environmental humidity mobilizes the profuse number of charge carriers of AgNP. This won t be a limiting factor for use of AgNP-FET as previously in few research [25], paper based diodes have been fabricated which relies on environmental humidity and proper packaging would help in its reliable operation. Fig.4 FESEM images for (a) Paper substrate (b) Paper containing IL (c) Paper containing AgNP (d) Paper containing rgo 9

The charge piling in gate dielectric of AgNP-FET results in carrier concentration values (0.43 10 /cm 2 and 0.11 10 /cm 2 for electrons and holes respectively) which is about 1.3 times higher than IL- FET (0.326 10 /cm 2 and 0.176 10 /cm 2 for electrons and holes respectively) and about 4 times higher than rgo-fet (0.098 10 /cm 2 and 0.028 10 /cm 2 for electrons and holes respectively). The pool of charge carriers gets oriented when gate potential is applied resulting in gate field which also reveals that formation of EDL is not a mandatory condition for gate dielectric of FETs. For rgo based FET, rgo layers were trapped in cellulose fibers as seen in the FESEM image. The electrically conductive graphitic layers lead to polarization effect due to orientation of layers on application of gate voltage and hence contribute to the gate field. For the same gate voltage (Vg), the gate leakage current (Ig) for AgNPs and IL based FET were comparable, but was higher than rgo based FET. This may be attributed to higher 2-D continuum percolation effect [26, 27] leading to enhanced conductance of AgNPs and IL as a result of embedded conducting filler in insulated matrix. For rgo based FET the 2-D percolation effect is lower, that may be due to 3-D layered structure of reduced graphene oxide which lies out-of plane of the paper substrate. This low gate leakage current also produce a direct impact on I DS, as change in I DS with respect to V G becomes high for rgo based FET. Nevertheless, the fillers present in paper as gate (rgo) and in channel (exfoliated graphite) are graphene based compositions which lead to lesser material defects and better adhesion between gate and channel material [28]. This similarity in material characteristic renders higher delocalization of electrons from the graphitic domain leading to higher current variations with gate field. However, rgo-fet showed high variations in drain-to-source current (I DS ) with respect to gate voltage (V G ) when bending and aging took place. This may be due to alteration in layered like structure of rgo embedded in paper matrix which is absent for AgNPs or ionic liquid. As the carrier concentration is higher for AgNPs based FET it results in a uniform field distribution all over the gate material. It could also be justified from this point that the Ids vs V g curve (Figure 2 (b)) for AgNP-FET follows almost a linear pattern with minimal distortion in the characteristics. This also adds to the fact that AgNP based FETs could be used for devices requiring linear transconductance characteristics such as linear amplifiers [29]. The Ion/Ioff ratio for rgo-fet was of the order of the polymeric gate based FET in reference paper [15] which indicates that rgo based FET only using paper substrates have a potential to replace the polymer containing FETs. The Ion/Ioff ratios for the IL-FET and AgNP-FET are much lower in compared to the rgo-fet. However, it should be 10

noted that, simple paper-pencil based polymer free FETs merely using paper substrate fabricated in this work are not to be compared with polymeric gate FETs in terms of their Ion/Ioff ratios. 4. Conclusions In conclusion, we demonstrate paper based FETs containing metallic silver nanoparticles and compare the parameters with the fabricated FETs containing IL and graphene oxides (rgo) as gate dopants. Unlike other reported FETs, all three FETs presented here are fabricated in one-step by pencil markings on materials doped paper substrate without any use of polymer support. The carrier concentration of AgNP-FET is higher while its electron and hole mobilities are in the order of IL-FET and rgo-fet. Further, AgNP-FET has been established as a stable FET with good reliability towards bending and aging. However, there remains much scope towards improvement of the AgNP-FET to achieve higher Ion/Ioff ratios for their complete utilization in electronic devices. The successful application of AgNPs thus opens up new vistas towards development of field effect transistor which does not require EDL formation and produce comparative FET performance on flexible substrates. The process is cost effective, repeatable and uses biodegradable materials. Acknowledgements This research was supported by Department of Biotechnology, Govt. of India, Grant No: GAP101612 and Council of scientific and industrial research, Govt. of India, Grant No: ESC0112. The authors acknowledge Madan Reddy, Mohd Afroz Akhtar, Saurav Haldar, Kalyan Chatterjee, Peuli Nath and Preeti Singh from CSIR-CMERI, Durgapur, India for their assistance in making the CAD models and preparing the chemicals. Abbreviations PS, polystyrene; PDMS, polydimethylsiloxane; AgNP, silver nanoparticle; rgo, reduced graphene oxide; FET, field effect transistor, IL; ionic liquid; EDL, electric double layer; PVP, polyvinylpyrrolidone; FESEM, field emission scanning electron microscope; G, gate contact; D, drain contact; S, source contact. Electronic Supplementary Information (ESI) available: Method for synthesis of AgNP with particle size in the range of 70-85 nm. Process to fabricate the IL-FET, AgNP-FET and rgo-fet. FESEM image demonstrating the size of AgNPs trapped in paper substrate. Exfoliation of the graphitic layers in pencil markings on paper substrate revealed through SEM image. 11

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