Pencil-on-paper: electronic devices

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Lab on a Chip View Article Online

FOCUS

View Journal | View Issue

Pencil-on-paper: electronic devices Cite this: Lab Chip, 2013, 13, 2866

Narendra Kurra and Giridhar U. Kulkarni* Paper based electronics have been rapidly growing in recent years. Drawing with a pencil on paper is perhaps the simplest and easiest way of establishing graphitic circuitry in a solvent-free manner, which in the post-graphene years, has attracted an unusual interest. Here in this focus article, we highlight the

Received 30th March 2013, Accepted 9th May 2013

recent efforts in the literature employing pencil drawings in various ways including sensors, microfluidics, energy storage and microanalytical devices. Even active devices such as piezo and chemiresistive devices as well as field effect transistors have been realised by utilizing pencil-traces. Pencil-on-paper may offer a

DOI: 10.1039/c3lc50406a

viable route for developing lab-on-paper applications through suitable integration of the passive and

www.rsc.org/loc

active roles of the pencil-trace.

1. Introduction Graphite, one of the allotropes of carbon, is naturally found on earth in the form of minerals in metamorphic rocks such as marble, schist and gneiss. Being layered, it can be deposited on a rough surface by exfoliation using a gentle force. Such properties combined with its black color must have impressed early literates to use a piece of graphite as a tool to make impressions on surfaces. The modern version of this tool is the pencil whose history dates back to the 16th century.1 This dayto-day tool is essentially a nanocomposite of graphite with intercalated clay particles and sometimes containing a tiny amount of wax, the latter acting as a binder providing macroscopic continuity.2–4 Depending on the graphite to clay ratio, pencils have been classified into two main grades (H and B). H grade denotes the hardness arising from higher clay content while B grade is for blackness, which is due to higher graphitic content. Typically, HB grade pencils contain 60–70% graphite; the rest is nearly all clay binder. The composition of different grades of commercially available pencils has been investigated by inductively coupled plasma mass spectrometry and time-of-flight secondary ion mass spectrometry.2–4 Pencil rods exhibit somewhat different properties when compared with pure graphite due to the presence of intercalated clay particles.5,6 It has also been shown to exhibit room temperature ferromagnetism due to the disorder induced by the clay particles.6 Stable and intense high-order harmonics with wavelengths ranging from 47 to 70 nm have been generated from pencil lead in an inexpensive manner.7 Pencil rods have also been employed as a source for the extraction of graphene in both electrochemical and mechanical methods.8,9

Chemistry & Physics of Materials Unit and DST Unit on Nanoscience Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India. E-mail: [email protected]

2866 | Lab Chip, 2013, 13, 2866–2873

Paper, another day-to-day material, forms an ideal surface for the exfoliation of graphite particles from pencils through mechanical abrasion leading to a black deposit, what we call the pencil trace. Interestingly, this humble deposit contains interconnected chunks of graphite (numerous graphene layers), which makes it reasonably conducting, in spite of the presence of insulating clay particles.10 Pencil leaves a black trace on a paper surface due to mechanical abrasion which means that the surface has to be sufficiently rough enough to obtain graphitic deposits. Papers have been graded based on their end use, manufacturing process and raw material used, which has led to 10000 different types of paper being available to us.11 However, commonly used papers such as Xerox paper, filter paper, weighing paper and glossy paper are associated with varied roughness, which is pre-designed during the manufacturing process. When the surface roughness is greater (>10 mm), large chunks of graphite get deposited with minimal interconnectivity among the graphitic deposits of the penciltrace. Glossy paper cannot support the pencil-trace as there is no abrasion effect from the smooth surface. Thus, paper surfaces with optimal roughness (1–5 mm) are required to make a pencil-trace, in which the graphitic deposits are well connected in order to obtain conducting graphitic tracks. Certainly, the device performance would vary according to the type of paper employed. Different grades of papers would be useful for different kinds of applications. The specific device application would need a particular type of paper substrate. Though one may end up getting similar device characteristics for the devices fabricated on various types of paper surfaces, it is always important to look for reproducible and better performance of a particular device. In order to accomplish this goal, one has to choose the correct paper surface according to the specific application. One can optimize the device performance on various types of paper surfaces with varied surface roughness and composition to achieve better

This journal is ß The Royal Society of Chemistry 2013

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Lab on a Chip device performance. Thus, control over the surface roughness, porosity and coatings would result in high performance of paper based electronic devices with recyclability and renewability, which is of intense academic interest with the aim of commercial benefits. The role of paper as an inexpensive and widely used medium for writing and printing, and in the display of information in general, as well as in packaging, can not be underestimated. Of late, paper has also become a platform for writing flexible electronic circuitry.12–14 Paper is composed of cellulose fibers with a 3-dimensional hierarchical arrangement giving rise to its unique porous morphology. It is therefore not an easy task to fabricate electronic devices on paper substrates as the associated roughness and porosity may adversely affect device performance.15,16 However, in spite of such severe shortcomings, a great deal of effort has been made in the recent literature towards hosting electronic and optoelectronic devices on paper.12–19 Conductive electrodes have been deposited via inkjet printing,17 sputter and spray coating techniques.18,19 Filling silver ink in a rollerball pen has served to write conducting tracks for the fabrication of light emitting diode (LED) arrays, and three-dimensional (3D) antennae on paper.20 Paper has become, virtually, a platform for cheap, light weight, flexible and disposable electronics.12–20 Compared to the above efforts, pencil drawing on paper is the simplest approach for obtaining conducting tracks. These graphitic tracks are light, robust and stable against radiation, heat and chemical corrosion. Such fascinating characteristics have prompted the development of pencil drawing based paper devices. This focus article highlights recent literature examples wherein various applications have been realized based on pencil drawings on paper. To begin with, we deal with the role of pencil drawings as conducting tracks in fabricating paper based sensors and microfluidic devices. Pencil drawing based fabrication of piezo, chemiresistive and glucose biosensors as well as energy storage devices such as batteries and supercapacitors will be dealt with in subsequent sections. The role of pencil-traces on paper in fabricating RC filters and field effect transistors is also discussed. Finally, we provide an outlook including possible improvements in this emerging field. When pressure is applied to a pencil, graphitic deposits become adhered to the paper matrix (Fig. 1a). The Raman spectrum of a pencil-trace shows characteristic peaks of graphite (see Fig. 1b). The peaks at 1350 and 1575 cm21 correspond to the D and G bands respectively. The G band, also called the graphitic band, comes from the stretching vibrations of the sp2 carbon lattice, while defects give rise to the D band. The stacking periodicity of the graphene layers along the c-axis can be inferred from the shape, position and width of the 2D band.21 The 2D band appears as a single symmetric peak at y2700 cm21 with a width of y90 cm21, which indicates the turbostratic nature of the graphite in the pencil-trace.21 The mis-orientation of graphene layers along the c-axis leads to substantial electronic decoupling making it behave like 2D graphite.21–23

This journal is ß The Royal Society of Chemistry 2013

Focus

Fig. 1 (a) Schematic showing a pencil-trace on paper, and SEM images of cellulose fibers and graphitic deposits on a paper surface. (b) Raman spectrum of the pencil-trace. Images reproduced from ref. 23.

2. Graphite resistors on paper A pencil trace on paper is a graphitic resistor and the resistance varies, as expected, with its physical dimensions – length, width and thickness. As shown in Fig. 2a, graphite resistors on paper with varied resistances can be made by varying the length, width and thickness of the pencil-trace, which, for the sake of demonstration, have been used for controlling the brightness of an LED.

3. Pencil-traces on paper and their applications In the following sections, many literature examples are covered illustrating the use of pencil drawings in various devices as a passive or an active component. Specifically, the use of a pencil trace as a conducting track, sensing element,

Fig. 2 (a) Photograph showing resistors based on pencil drawings on paper and a multimeter reading displaying the resistance value (indicated in kVs, from top to bottom, panel I – 8.6, 14, 19, 25, 30; panel II – 14, 10, 7, 5, 3). (b) Pencil-trace as a variable resistor for controlling the brightness of an LED connected to a 9 V battery.

Lab Chip, 2013, 13, 2866–2873 | 2867

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Focus

Lab on a Chip

Fig. 4 (a) Photograph showing the paper-and-pencil device used for demonstrating the electrokinetic phenomenon. (b) Real-time snap shots of the liquid movement via application of a potential of 50 V on the right electrode. Images reproduced from ref. 26.

Fig. 3 (a) UV sensor based on ZnO deposited across the interdigitized graphite electrodes on a paper substrate. (b) I–V characteristics of ZnO films before (red) and after UV illumination (blue) in comparison with the paper substrate without ZnO (black). (c) I–V curves of the flexible pencil drawn UV sensor with and without UV illumination. Inset shows the photograph of the screen printed UV sensor based on ZnO nanocrystals with pencil drawings as contact pads. (d) Temporal photoresponse of the ZnO UV sensor at a bias voltage of 1 V (wavelength, 365 nm). Images reproduced from ref. 24, 25.

electrode material and as an active channel material is dealt with. 3.1 As a conducting track Conventional conducting pads/wires are made out of metallic elements such as Cu, Ag, Al and Au. Drawing with a pencil on paper makes graphitic paths, which are essentially conducting tracks (see Fig. 2), and can be utilized in fabricating paper based functional devices. 3.1a UV sensors. ZnO based UV sensors with traditional metallic contacts obtained via lithographic techniques involve multiple steps and are expensive. Alternatively, UV sensors have been made in a cost effective manner with the aid of graphite contacts on a paper substrate. The graphitic tracks were made through pencil drawings to be employed as electrode pads on paper. Initially, graphite lines were drawn using a 4B pencil in the form of interdigitated contact electrodes connected to a probe station (see Fig. 3a). A water suspension of ZnO nanocrystals was then drop coated across the interdigitated graphite electrodes followed by drying on a hot plate at 150 uC to obtain a homogeneous film of ZnO nanocrystals (see Fig. 3a). Due to the porous nature of the paper surface, it could hold the nanocrystals without the need for any binder. As the typical resistivity of the graphite electrodes is at least three orders of magnitude lower compared to that of ZnO, there was no requirement for any low resistivity metal electrodes.24 The characteristic photoresponse of the sensor is shown in Fig. 3b. The photocurrent (blue curve, Fig. 3b) is at least 5–10 times higher when compared with the dark current (red curve, Fig. 3b). Recently, Hasan et al. have demonstrated the fabrication of a UV sensor

2868 | Lab Chip, 2013, 13, 2866–2873

based on screen printed ZnO nanocrystals on paper with pencil drawn circuitry (see inset of Fig. 3c).25 After illuminating with the UV lamp (wavelength, 365 nm), a photoconductive response was observed (see Fig. 3c). The transient UV photoresponse was examined by turning the UV lamp on and off (see Fig. 3d). Thus, the graphitic circuitry on paper offered the fabrication of UV sensors in a simple, easy and economical manner, yet the performance is comparable to those made with complex, expensive procedures. 3.1b Microfluidic devices. Paper based microfluidic devices are being developed towards designing low cost, point-of-care diagnostic kits. The liquid transport in paper based devices is guided by the natural capillary action which is hardly controllable due to the random arrangement of the cellulose fibers. For reliable applications, the liquid flow in paper should be controllable. Mandal et al. have developed a novel and elegantly simple strategy for the active control of liquid transport in an electrical paper-and-pencil microfluidic device.26 The microfluidic channel (width of 1 mm and length of 4 cm) was fabricated on a paper substrate by photolithography, connected to two 1 6 1 cm square pads at the ends (see Fig. 4a). The electrodes were made by sketching with a pencil (grade, 2B) over the two square pads via repeated rubbing, leading to the coverage of a thin layer of graphite. Thin copper wires, attached to the graphite electrode pads by conductive silver paste, provided external control. The liquid transport through this channel was monitored by flowing 1 mM KCl solution, stained with a fluorescent dye. It was found that the flow rate was enhanced upon application of a bias voltage of 50 V (see Fig. 4b). The real time monitoring of the movement of the liquid front revealed enhanced kinetics of the liquid flow with bias, compared to the normal capillary driven flow. This method not only improved the transport of the liquid through the paper medium but also introduced significant controllability. This type of paper-and-pencil device may offer affordable microfluidic devices for diagnostic applications.26

This journal is ß The Royal Society of Chemistry 2013

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Lab on a Chip

Focus

Fig. 5 (a) Normalized resistance change with respect to the applied force of a graphite based piezoresistive sensor. Inset shows the photograph of an array of six graphite-traces on paper based piezoresistive devices. (b) The change in conductance (DG/G0) in response to NH3 (exposed for 200 s) at low ppm concentrations. Vertical error bars represent standard deviations from the mean, based on three exposures of the eight devices on a single paper chip (shown in inset). Images reproduced from ref. 27, 28.

3.2 Sensing element Pencil-traces have also been employed as sensing elements for mechanical force, chemicals and even glucose, leading to the fabrication of piezoresistive-, chemiresistive- and bio-sensors, respectively. 3.2a Piezoresistive sensors. In the given example27 (Fig. 5), paper was cut in the form of a cantilever beam followed by drawing with the pencil at its root (see inset of Fig. 5a). This trace could act as a sensing material with the resistance changing according to applied mechanical strain/stress on the cantilever beam. It was found that the change of resistance was linearly proportional to the applied force (see Fig. 5a). This graphite based piezoresistive sensor was sensitive up to a force of 50 mN with a sensitivity of 0.9 mV mN21 and a resolution of 500 mN.27 As the change in resistance could reflect the magnitude of the applied force, this principle was utilized in designing a paper based weighing balance using an array of graphite marks. Such simple paper based piezoresistive devices may find applications in sensing force and acceleration in other day-to-day settings. 3.2b Chemiresistive sensors. Chemiresistive gas sensors have also been fabricated on paper via mechanical abrasion of compressed powders of carbon nanomaterials across the contact pads.28 A plot of conductance response of the devices made out of single walled carbon nanotube films with varying concentrations of NH3 (0.5–80 ppm), is shown in Fig. 5b. The conductance response of eight devices (R, 20–35 kV) on a single paper chip was similar (see inset of Fig. 5b). This technique can be generalized for the fabrication of tunable chemiresistive sensors through tailoring the functionality of the carbon materials, as their electrical conductivity is sensitive to changes in the local chemical environment. 3.2c Glucose biosensor. Demand for developing simple and inexpensive analytical devices is on the rise for glucose biosensing in diabetics. Santhiago and Kubota have developed a simple approach for glucose biosensing by employing paper based analytical devices using graphitic electrodes.29 In the given example, a pencil drawing as the working electrode and silver ink (back face) as the reference and auxiliary electrode

This journal is ß The Royal Society of Chemistry 2013

Fig. 6 (a) A schematic of the paper based biosensing device for the electrochemical detection of glucose: (i) filtration step, (ii) reaction spot and (iii) electrochemical detection. The white circular microzones are hydrophilic (d = 4 mm), while the black parts are hydrophobic. (b) Folded device for storage. (c) Cyclic voltammograms obtained using the device in the absence (dashed line) and the presence (full line) of 1 mM glucose in 0.1 M PBS at pH 7.4. Inset shows a photograph of the device. Images reproduced from ref. 29.

were used for the electrochemical detection of glucose in a two-electrode configuration. Three different regions were made on paper, namely a filtration region, reaction spot and electrochemical detection region (Fig. 6a). The redox mediator, 4-aminophenylboronic acid and glucose oxidase were spotted within the reaction zone. The folding of the device when not in use might help in preventing the contamination of the enzyme present in the microzone region (see Fig. 6b). The electrochemical experiments are performed in the third region (see the inset of Fig. 6c). Here, the oxidation of glucose to gluconic acid is catalyzed by glucose oxidase in the presence of oxygen. Hydrogen peroxide is released as a product of the enzymatic reaction which oxidizes p-aminophenylboronic acid, forming 4-aminophenol (4-AP) and tetrahydroxyborate ions. Thus, the electroactive compound, 4-aminophenol, could be electrochemically detected on the pencil electrode surface at a relatively low overpotential.30 As can be seen in Fig. 6c, in the presence of glucose, two peaks were observed in the studied potential range. The peaks have been attributed to the oxidation of 4-AP in the forward scan and to the reduction of 4-benzoquinone imine (oxidized form of 4-AP) in the backward scan. As the principle of operation is the same for all oxidase enzymes, the developed device is essentially a low cost lab-on-paper or labon-foil technology for bio diagnosis. Due to such excellent electrochemical stability, pencil electrodes have been employed for the determination of target molecules in conventional electrochemical cells as well.31,32 Graphite coating using pencil on conductive surfaces, such as ITO, is being employed as a counter electrode in dye-sensitized solar cells.33

Lab Chip, 2013, 13, 2866–2873 | 2869

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Focus

Fig. 7 (a) A schematic representation of the pencil-drawing process, and the structure of the fabricated lithium–air battery. (b) A charge–discharge curve tested with a current density of 0.25 A g21. (c) Areal and gravimetric capacitances of the pencil drawing on a paper based supercapacitor at different current densities. Inset shows a schematic diagram of the paper supercapacitor device made by pencil drawing. Images reproduced from ref. 34, 35.

3.3 Energy storage devices Electrochemical energy storage devices such as batteries and supercapacitors store energy by Faradaic and non-Faradaic processes, respectively. Pencil drawing has also made an entry as an electrode material in the fabrication of these devices. 3.3a Batteries. Wang and Zhou34 employed pencil drawing as an air electrode after drawing on the surface of a ceramicstate electrolyte (not on paper in this case!). They used a lithium superionic conductor (LISICON) film as a solid surface for the pencil-drawing (see the photograph in Fig. 7a). The pencil-trace acts as a catalytic electrode in a lithium–air battery. The schematic layout of the lithium–air battery has the following structure – ‘‘Li | organic electrolyte | LISICON | pencil-trace thin-film electrode’’. Here, the pencil-trace serves as the air catalytic electrode (cathode), while the LISICON film acts as a solid state electrolyte film. During the discharge process, the migration of the Li+ ions from the Li anode to the top LISICON film takes place through the organic electrolyte. The migrated Li+ ions combine with O2 from air to form LixOy within the pencil-trace. The reverse mechanism happens during the charging process. The galvanostatic charge and discharge characteristics of the lithium–air battery were investigated at a current density of 0.25 A g21 within a voltage window between 5.0 and 2.0 V (see Fig. 7b). The charging potential was found to be higher during the charging process compared to that of discharging. Importantly, the capacitance value remained similar for both charging and discharging processes. This result also demonstrates that Li+ ions from the electrolyte and electrons from the external circuit combine reversibly with O2 from the air. Hence, the pencil-trace on the Li+ ion conducting ceramic surface can serve as a catalytic electrode in a lithium–air battery, where a capacity of 500–700 mA h g21 has been achieved.34

2870 | Lab Chip, 2013, 13, 2866–2873

Lab on a Chip

Fig. 8 (a) Photograph showing the paper based resistor–capacitor (RC) low pass filter using pencil-trace and ion gel dielectric and its circuit diagram. (b) The input and output voltage waveforms of the RC filter at different frequencies of 0.1 kHz, 10 kHz and 100 kHz with a voltage amplitude of 1 V (input and output wave forms are shown with sky blue and yellow, respectively). Images reproduced from ref. 23.

3.3b Supercapacitors. Solid state supercapacitors have been fabricated by employing pencil drawings on paper as porous electrodes (see inset of Fig. 7c). Here, there is a favorable synergy among the components involved. Graphite is an excellent electrode material allowing facile formation of an electrochemical double layer. The paper substrate is an efficient dielectric separator and, being porous, allows rapid diffusion of ionic species. In the given example of a supercapacitor,35 a gel electrolyte was sandwiched between the two electrodes of the pencil drawings on paper. Areal and gravimetric capacitances of the supercapacitor at different current densities are presented in Fig. 7c. Electrochemical testing of the supercapacitors showed high areal capacitance (2.3 mF cm22) and excellent long term cycling performance. The authors found that the specific energy and power densities of the paper supercapacitors are reasonable compared to commercial and other carbon based supercapacitors. Thus, pencil drawings on paper offer a viable alternative approach for low cost and environmentally friendly energy storage devices. 3.4 Passive and active electronic devices Pencil-traces on paper have also been employed as a resistor element in fabricating passive devices such as RC filters and also as active channel material in field effect transistors (FETs). 3.4a RC filter. To be compatible with paper, an ion gel, prepared by mixing PDMS with ionic liquid (1-butyl-3-methylimidazolium octylsulfate) in the ratio 5 : 1 (similar to other formulations known in the literature),36 has been used as the dielectric (Fig. 8a). Two electrode pads were defined using conductive Ag ink. The DC specific capacitance of the ion gel

This journal is ß The Royal Society of Chemistry 2013

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Lab on a Chip was found to be 0.3 mF cm22. The higher gate capacitance of the ion gel is due to the formation of electrochemical double layers upon application of a potential.36 An input square wave form (amplitude of 1 V) was applied, using a function generator, to the series combination (both the resistor and capacitor together) and the output signal (Vout) was measured using an oscilloscope across the capacitor (see Fig. 8b). The resistance of the pencil-trace was found to be 425 kV. Fig. 8b shows the voltage response of the filter at different frequencies of the square wave-shaped signal, 100 Hz, 10 kHz, and 100 kHz respectively (blue curves in Fig. 8b). The response of the integrator changed dramatically with increasing frequency. At low frequencies (100 Hz), the input and output waveforms were of similar shapes except that the amplitude of the output was lower compared to the input due to charging of the capacitor (see Fig. 8b). At higher frequencies (y10 kHz), the output voltage response was converted to a triangular shaped waveform (yellow curve in Fig. 8b) with respect to the input square wave signal. This shaping of the waveforms in the low pass RC filters can be attributed to the frequency dependent reactance of the capacitor (see Fig. 8b). The triangular waveform consisted of alternate but equal positive and negative ramps. At higher frequencies (y100 kHz), the output became a well shaped triangular wave (see Fig. 8b). The performance of the circuit is as expected of a RC filter; the higher the input frequency, the lower the output amplitude due to decreasing capacitive reactance (Xc = 1/2pfC where f is the frequency and C is the capacitance).23 3.4b Field effect transistor. A field effect transistor has also been made by employing a pencil-trace as an active channel material. This device was hand-made by keeping Ag contacts as the source and drain electrodes followed by placing a drop of the ion gel (the same as in the example given above) on top of the pencil trace, which formed the gate dielectric. The gate electrode was made using Ag paint during the process of baking (see Fig. 9a). The chosen pencil-trace has a base resistance of y11 kV at VG = 0. Transfer characteristics (Fig. 9b) reflected the ambipolar nature of the pencil-trace with a broad Dirac point minimum. The asymmetric nature of the transfer curve indicates the unequal mobilities of holes and electrons. The gate leakage current (see blue curve in Fig. 9b) is at least 3 orders of magnitude lower compared to the channel current. It is clear that the ion gel, due to its high gate capacitance, is effective in deriving the electric field effect from the pencil trace at such low operating voltages. The hole and electron mobilities were estimated to be mh y 109 cm2 V21 s21 and me y 59 cm2 V21 s21.23 Typical charge carrier concentrations of the order of y3 6 1013 cm22 and y1.7 6 1013 cm22 were found for the holes and electrons, respectively.

4. Summary and perspective Graphitic circuitry on paper defined by pencil-drawings is a rather simple, inexpensive and solvent-free technique. Due to its light weight, chemical stability and resistance to heat and radiation, it has become quite attractive in fabricating various kinds of active and passive electronic circuits. However, as

This journal is ß The Royal Society of Chemistry 2013

Focus

Fig. 9 (a) Photograph showing the pencil-trace as an active channel material with ion gel as the gate dielectric. Alongside is the geometry of the pencil-trace FET. (b) Transfer characteristics of the pencil-trace transistor. Images reproduced from ref. 23.

paper is hygroscopic, a certain amount of moisture may assist in giving rise to ionic currents, affecting the electrical properties of the pencil-trace on paper. This issue can be resolved by encapsulation or lamination of the pencil-trace on paper between plastic films in order to obtain stable device performance. Conducting properties of the pencil-traces have been utilized for the fabrication of paper based UV sensors and microfluidic devices in a cost effective manner. Besides its role as a conducting track, pencil drawings have also been employed as the active material in the fabrication of piezoresistive and chemiresistive sensors. Additionally, energy storage devices such as batteries and supercapacitors were made by employing the pencil drawings as electrodes. Basic electronic devices such as RC filters have also been fabricated by employing pencil-traces as resistors and an ion gel as a dielectric. Amazingly, the electric field effect was also derived from the graphite crystallites of a pencil-trace using an ion gel dielectric. Due to their good electrical conductivity and excellent electrochemical stability, pencil drawings were also employed as electrode systems in the fabrication of paper based micro analytical devices. Graphite coatings using a pencil are also being employed as a counter or negative electrode in dye-sensitized solar cells.33 Indeed, this is a directwrite method which consumes less time for device fabrication. Thus, pencil-drawing on paper is a simple approach to build devices all the way from basic circuit components such as resistors, through to advanced energy storage devices and biosensors in a cost-effective manner. It is particularly useful in use-and-throw applications not involving critical performance criteria yet reliable enough for certain kind of applications. The fabrication of pencil-on-paper devices does not require any sophisticated facilities (clean room) and high end fabrication equipment. Thus, pencil-drawing on paper can

Lab Chip, 2013, 13, 2866–2873 | 2871

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Focus also be made as demo experiments for children to illustrate its electrical properties towards making paper based variable resistors, capacitive touch sensors, etc. This solvent free method of defining electrodes and active elements may be generalized by casting the material of interest in the form of pencil rods followed by drawing on paper. Some of these ideas have been implemented recently for the fabrication of gas sensors through mechanical abrasion of compressed pellets made from various carbon nanomaterials such as multi, single walled carbon nanotubes; these may offer tunable properties of the sensor elements on paper.28 Nanopencils are one such approach, in which a graphite microcrystal attached to a cantilever arm of an atomic-force microscope scratches the substrate surface with controlled force to deposit thin graphene flakes.37 The path of development is never smooth—among the many hurdles in developing paper based electronics, the major issue is deterioration due to the large surface roughness, porosity and chemical impurities present in the paper matrix. A few electronic devices such as LEDs, solar cells and FETs may require a smooth interface to function effectively; this requirement may be addressed by employing polymer coatings such as PDMS. On the other hand, in some other applications, the porosity (and roughness) of paper is actually advantageous in obtaining good adhesion of the printed material, such as in energy storage and microfluidic devices. The robustness of the ion gel based transistor fabrication may allow large-scale manufacturing directly onto recyclable paper substrates. However, further progress is definitely required to improve the stability and performance of paper based electronic devices. As controlling pencil exfoliation on paper is difficult, a uniform deposition may be realized by drawing with a plotter with adjustable pneumatic control. The electronic properties of the pencil trace can be modified by decorating with metal nanoparticles, semiconductor quantum dots and other functional nanomaterials such as ZnO. Given the diverse possibilities, all carbon based paper electronics may not be a distant dream. Extending the concept to inorganic analogues, MoS2, WS2 etc., and integrating them with graphitic traces may open up a brand new arena for highly scalable, low cost, paper based electronics. This approach of pencil-on-paper has many implications towards making biosensors, energy storage devices and also suitable integration of passive and active roles of pencil-traces will eventually lead to developing lab-on-paper technology. Recently invented nanopaper may also help further exploration in this direction.38

Acknowledgements The authors thank Professor C. N. R. Rao, FRS for his encouragement. Support from the Department of Science and Technology, Government of India is gratefully acknowledged. NK acknowledges the CSIR for funding. GUK acknowledges the Sheikh Saqr Senior Fellowship.

2872 | Lab Chip, 2013, 13, 2866–2873

Lab on a Chip

References 1 D. Harper, (June 27, 2012). ‘‘Pencil’’. Online Etymology Dictionary. Retrieved June 27, 2012. 2 S. Cain, A. A. Cantu, R. Brunnelle and A. Lyter, J. Forensic Sci., 1978, 23, 643–661. 3 J. A. Zoro and R. N. Totty, J. Forensic Sci., 1980, 25, 675–678. 4 J. A. Denman, I. M. Kempson, W. M. Skinner and K. P. Kirkbride, Forensic Sci. Int., 2008, 175, 123–129. 5 M. S. Dresselhaus and G. Dresselhaus, Adv. Phys., 2002, 51, 1–186. 6 R. N. Bhowmik, Composites, Part B, 2012, 43, 503–509. 7 Y. Pertot, L. B. Elouga Bom, V. R. Bhardwaj and T. Ozaki, Appl. Phys. Lett., 2011, 98, 101104. 8 V. V. Singh, G. Gupta, A. Batra, A. K. Nigam, M. Boopathi, P. K. Gutch, B. K. Tripathi, A. Srivastava, M. Samuel, G. S. Agarwal, B. Singh and R. Vijayaraghavan, Adv. Funct. Mater., 2012, 22, 2352. ´gin, O. Ersen, P. Bernhardt, 9 I. Janowska, F. Vigneron, D. Be T. Romero, M. J. Ledoux and C. Pham-Huu, Carbon, 2012, 50, 3106. 10 L. D. Woolf and H. H. Streckert, Phys. Teach., 1996, 34, 440–441. 11 http://www.paperonweb.com/grade.htm#a. 12 D.-H. Kim, Y.-S. Kim, J. Wu, Z. Liu, J. Song, H.-S. Kim, Y. Y. Huang, K.-C. Hwang and J. A. Rogers, Adv. Mater., 2009, 21, 3703–3707. 13 A. C. Siegel, S. T. Phillips, M. D. Dickey, N. Lu, Z. Suo and G. M. Whitesides, Adv. Funct. Mater., 2010, 20, 28–35. 14 M. Berggren, D. Nilsson and N. D. Robinson, Nat. Mater., 2007, 6, 3–5. ¨ sterbacka, Adv. Mater., 2011, 23, ¨rk and R. O 15 D. Tobjo 1935–1961. ¨m, A. Ma ¨¨ ¨nen, D. Tobjo ¨rk, P. Ihalainen, 16 R. Bollstro atta ¨ sterbacka, J. Peltonen and M. Toivakka, N. Kaihovirta, R. O Org. Electron., 2009, 10, 1020–1023. 17 B.-J. de Gans, P. C. Duineveld and U. S. Schubert, Adv. Mater., 2004, 16, 203. 18 A. C. Siegel, S. T. Phillips, B. J. Wiley and G. M. Whitesides, Lab Chip, 2009, 9, 2775. 19 S. S. Kim, S. I. Na, J. Jo, G. Tae and D. Y. Kim, Adv. Mater., 2007, 19, 4410–4415. 20 A. Russo, B. Y. Ahn, J. J. Adams, E. B. Duoss, J. T. Bernhard and J. A. Lewis, Adv. Mater., 2011, 23, 3426–3430. 21 M. A. Pimenta, G. Dresselhaus, M. S. Dresselhaus, L. G. Cançado, A. Jorioa and R. Saito, Phys. Chem. Chem. Phys., 2007, 9, 1276–1290. 22 L. M. Malard, M. A. Pimenta, G. Dresselhaus and M. S. Dresselhaus, Phys. Rep., 2009, 473, 51–87. 23 N. Kurra, D. Dutta and G. U. Kulkarni, Phys. Chem. Chem. Phys., 2013, 15, 8367–8372. ´n ˜ ez-Limn and J. M. Seminario, J. 24 A. J. Gimenez, J. M. Ya Phys. Chem. C, 2011, 115, 282–287. 25 K. u. Hasan, O. Nur and M. Willander, Appl. Phys. Lett., 2012, 100, 211104. 26 P. Mandal, R. Dey and S. Chakraborty, Lab Chip, 2012, 12, 4026–4028. 27 T.-L. Ren, H. Tian, D. Xie and Y. Yang, Sensors, 2012, 12, 6685–6694. 28 K. A. Mirica, J. G. Weis, J. M. Schnorr, B. Esser and T. M. Swager, Angew. Chem., 2012, 124, 10898.

This journal is ß The Royal Society of Chemistry 2013

View Article Online

Published on 10 May 2013. Downloaded by Jawaharlal Nehru Centre for Advanced Scientific Research on 25/01/2014 10:33:15.

Lab on a Chip 29 M. Santhiago and L. T. Kubota, Sens. Actuators, B, 2013, 177, 224–230. 30 L. Hu, S. Han, Z. Liu, S. Parveen, Y. Yuan and G. Xu, Electrochem. Commun., 2011, 13, 1536–1538. 31 B. Dogan-Topal and S. A. Ozkan, Electrochim. Acta, 2011, 56, 4433–4438. 32 B. B. Prasad, D. Kumar, R. Madhuri and M. P. Tiwari, Sens. Actuators, B, 2011, 160, 418–427. 33 I. V. Lightcap and P. V. Kamath, Acc. Chem. Res., 2012, DOI: 10.1021/ar300248f.

This journal is ß The Royal Society of Chemistry 2013

Focus 34 Y. Wang and H. Zhou, Energy Environ. Sci., 2011, 4, 1704. 35 G. Zheng, L. Hu, H. Wu, X. Xiec and Y. Cui, Energy Environ. Sci., 2011, 4, 3368–3373. 36 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge and C. D. Frisbie, Nat. Mater., 2008, 7, 900–906. 37 A. K. Geim and P. Kim, Carbon Wonderland, Sci. Am., 2008, 298, 90. 38 J. Huang, H. Zhu, Y. Chen, C. Preston, K. Rohrbach, J. Cumings and L. Hu, ACS Nano, 2013, 7, 2106–2113.

Lab Chip, 2013, 13, 2866–2873 | 2873