Scalable fabrication of highly sensitive flexible ...

Sensors and Actuators A 268 (2017) 173–182

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Scalable fabrication of highly sensitive flexible temperature sensors based on silver nanoparticles coated reduced graphene oxide nanocomposite thin films Nagarjuna Neella a , Venkateswarlu Gaddam a , M.M. Nayak b , N.S. Dinesh c , K. Rajanna a,∗ a

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, 560012, India Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore, 560012, India c Department of Electronic Systems Engineering, Indian Institute of Science, Bangalore, 560012, India b

a r t i c l e

i n f o

Article history: Received 20 June 2017 Received in revised form 10 October 2017 Accepted 6 November 2017 Available online 7 November 2017 Keywords: RGO nanosheets Ag nanoparticles RGO-Ag nanocomposite Piezoresistivity Temperature sensor

a b s t r a c t We have demonstrated the scalability of RGO-Ag nanocomposite films based temperature sensor on kapton sheet for the flexible technology platform. The RGO nanosheets are known to have the tendency to form an agglomeration, which was removed by insertion of Ag nanoparticles to the RGO sheets. This resulted in the formation of nanocomposite material having an excellent conductivity suitable for temperature sensor applications. The electro thermal performance of the RGO-Ag nanocomposite sensing film was investigated using the Hot-Cold temperature setup. The sensor developed shows exceptional temperature sensing properties, with a promise for wide range of potential application possibilities. Till today the performance of temperature sensors are limited by the flexibility, lower sensitivity, nonlinearity viz. Thermistors and Platinum based sensors viz. RTD & Pt 100 while linear are expensive. In contrast, the sensor developed in our present work shows negative temperature coefficient (NTC) of −1.64 × 10−3 //K, sensitivity of 0.5553 /K, with a non-linearity of about 1% and hysteresis close to 1%. The measured response time (470 m s) of this sensor is much faster than majority of commercially available temperature sensors with good repeatability and stability. The linearity of the developed sensor lends itself for simpler signal conditioning circuits without requiring linearization front end. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the customer demands are mainly on the integration of micro/nano systems based devices, for example temperature sensors which are playing a key role in areas such as biomedical (for body temperature monitoring, infant incubators, drugs quality monitoring from hospitals to end users), food science (for packaged food quality monitoring from industry to the market), automotive (engine block/oil monitoring, intake & outside air monitoring), electronics & communication (for rechargeable battery packs life monitoring, cellular telephone, refrigerator & freezer temperature control, integrated electronic circuits, micro heaters and gas sensors), military & aerospace (space craft temperature monitoring, satellites, physiological monitoring) and weather/environmental applications (for weather forecast) [1–11]. In the view of above, the human life becomes more convenient

∗ Corresponding author. E-mail address: [email protected] (R. K.). https://doi.org/10.1016/j.sna.2017.11.011 0924-4247/© 2017 Elsevier B.V. All rights reserved.

and comfortable based on the above mentioned applications. Also, the use of temperature sensors, such as Thermistors and platinum resistance temperature detectors (Pt RTD) are becoming extremely useful in our day- to- day life. Basically, the temperature sensors are temperature dependent electrical resistors. In this context, platinum based materials exhibit positive temperature coefficient (PTC) of resistance behavior and metal oxides & semiconductors show negative temperature coefficient (NTC) of resistance [12,13]. Among all temperature sensors, ceramic based Thermistors are advantageous in terms of fabrication cost, sensitivity and response time [14]. In these cases, the sensor elements are fabricated typically in the form of discs and pellets on rigid substrates. However they provide limited flexibility in terms of bendability or stretchability. Furthermore, when they are structurally integrated to mount on large curvatured surfaces, they experiences a large mechanical deformation resulting in their malfunctioning and hence these devices are not suitable for use on flexible platforms [15]. Therefore, there is a tremendous need for the development of nano materials based temperature sensors on flexible platforms to overcome

174

N. Neella et al. / Sensors and Actuators A 268 (2017) 173–182

the problems/limitations mentioned above in case of conventional sensors. Now a days, the flexible electronic devices technology is one of the innovative approaches in the area of the stretchable or bendable electronics. In addition, the bendable electronic technologies offer potential application possibilities in the field of wearable electronic devices [16]. The recent development of these bendable devices are becoming more popular in various applications including elastic conductors, light emitting diodes, transistors, energy harvesting devices, personal electronics, biomedical and sensors [12–17]. Apart from these, the fabrication of the temperature sensors with integrated structurally complex system on the stretchable or bendable components are the key features for light weight devices in real time practical applications. Currently, the nanoscale materials are used/applied for the development of temperature sensors because of their large surface to volume ratio and nanoscale dimensionality [18]. Since the discovery of graphene in 2004 as nanoscale material, it is viewed as two dimensional sheet of one atom thick sp2 bonded carbon atoms that are arranged in a hexagonal lattice. The graphene has emerged as highly promising material with outstanding physical and chemical properties suitable for potential sensor applications [19]. In this regard, the properties of graphene and its derivatives have appeared as a rising star and also attracted as emerging nanomaterial with considerable attention in the area of material science, condensed matter physics, biological science, nanoscience, interdisplinary and aimed to develop various types of technological applications [20]. Considering the above unique properties of graphene, it has been predicted that the thermal conductivity and mechanical stiffness of graphene is comparable to the in plane values (both thermal conductivity, 3000 W/m K and mechanical stiffness, 1060 GPa) of graphite. Also, the fracture strength is comparable to the carbon nanotubes (fracture strength, 1 TPa) [21]. In addition to that, the large surface area (theoretical value 2630 m2 /g) of graphene improves the surface interfacial contact connections [22]. In the recent past, the graphene, graphene derivatives and its composite materials are found to be attractive for making emerging flexible electronic devices [23]. Also, in order to improve the performance and sensitivity of the graphene based devices are reported several ways in the literature. In order to the production of better quality graphene, the ␲-stacked graphene layers are usually exfoliated by micromechanical cleavage of high quality graphite and chemical exfoliate from bulk graphite [20,23]. However, one of the possible ways to utilize the properties from the point of applications could be to incorporate graphene with polymers and other nanoscale particles (called nanocomposite materials). Fabrication of such composites requires not only the high quality production of graphene sheets but also their effective incorporation in various desirable matrices of different nanomaterials. In view of the composites manufacturing, carbon nanostructures have drawn the much attention due to their unique electrical and structural properties and ability to improve the sensing properties [21–23]. Hence graphene based composite materials are becoming an excellent semiconductor temperature sensitive materials. The fabrication of temperature sensors by using nanocomposite materials with integrated structure on flexible system are the key capabilities for practical applications. Kong’s group have reported the inkjet printer graphene Thermistors on flexible PET substrate for temperature sensing [12,13]. The commercially available CNT powder was deposited on the elastic polymer tape by Karimov’s group for temperature measurement [8]. Lithographic filtration method based graphene Thermistors, which are embedded inside the PDMS matrix for temperature measurements have been explored by Pooi’s group [24]. Cunjiang group proposed bulk Si nanoribbons with pre-strained assembly

was employed for temperature monitoring [15]. The organic filed effect transistors mounted on the stretchable mesh for temperature detection has been reported by Someya group [25]. Rogers’s group have reported the components in epidermal electronics using serpentine trace of gold metal or Si diodes as temperature sensors [26]. However, limited work is reported on the integration of graphene based temperature sensors with flexible platforms as well as the works related to the structural engineering that limits the device performance in terms of flexibility. It is important to note that the conventional rigid structure devices can easily be adopted onto the flexible substrates. But, the fabrication of flexible/bendable devices are much more challenging, in order to integrate them on flexible platforms. Especially, the reduced graphene oxide and its derivatives have lot of issues mainly, agglomeration to form stack of layers, effective adhesion on various substrates, which are the facts vital for the fabrication of sensor devices. To overcome these limitations, RGO sheets are separated by Ag nanoparticles to form nanocomposite material having an excellent conductivity of bare RGO sheets suitable for temperature sensor applications. In the present paper, we report on the fabrication of flexible RGO-Ag nanocomposite thin sensing film as Thermistor (temperature sensors) with a good bending ability, arising from the embedded base structure. Also, we have studied the nanocomposite sensor performance in detail. Interestingly, it was observed that the semiconducting NTC behavior is linear and better than the conventional temperature sensors. These devices can be made bendable more than 30% without losing/change of the relevant electronic functional properties. Hence, the thermal sensitivity and nature of handling of these devices are useful to future diverse and adaptable real time applications in chemical, biological, moisture and mechanical sensing applications. 2. Experimental section/Materials and methods In this section the details of the procedure followed for the synthesis of Graphite oxide (GO), Reduced graphene oxide (RGO), nanocomposite preparation and sensing film formation. The characterization of the materials and realization of temperature sensors on kapton substrate are also described. Information on the performance study of the sensors is also included. 2.1. Preparation of GO The following Chemicals were used for the synthesis of Graphite Oxide (GO). Graphite powder (flake size 80–120 ␮m, 99%) was procured from M/S. Superior graphite, USA. All the chemicals including concentrated sulfuric acid (AR, H2 SO4 , 98%), potassium permanganate (AR, KMnO4 , 99.5%), hydrogen peroxide (AR, H2 O2 , 30%), hydrochloric acid (AR, HCL, 35%) were obtained from M/S. Merck specialties private limited, India and hydrazine hydrate (AR, N2 H4 , 80%) was procured from M/S. s d fiNE CHEM LimiTEd (SDFCL), India. These chemicals were used without further purification process. Silver nanoparticles were procured from M/S. Siltech corporation conductive inks private limited, India. Flexible kapton sheets/membrane with thickness of 175 ␮m was procured from M/S. DuPont Teijin Films, USA. Graphite oxide (GO) was synthesized by Modified Hummer’s method, in which the fine graphite powder was exfoliated through chemical route [27]. Typically, the graphite powder (2 g) was oxidized using strong acidic environment (concentrated H2 SO4 , 54 ml) through constant magnetic stirring at room temperature. The KMnO4 (6 g) was slowly added to the solution upto 20 min and it was found to be sufficient for the oxidation of graphite. Subsequently, the entire solution mixture concentration was rigor-


175

ously stirred up for 40 min. Also, the deionized water (DI, 100 ml) was added slowly to the solution drop by drop, which results as the oxidation process and continued the stirring process for 90 min. Furthermore, one more time the DI water was added double (200 ml) than the first time to complete the oxidation reaction. Moreover, the entire oxidation reaction process was completed by addition of hydrogen peroxide (H2 O2 , 30 ml) to this solution. After few minutes, it was noticed that the colour of the reaction mixture was changed from black to reddish brown at the end of the reaction process. Further, the mixture solution was washed two or three times with hydrochloric acid (HCL, 100 ml) solution and added DI water in order to remove metal ions as well as un-oxidized graphite from the reaction mixture. The complete reaction process, the entire mixture solution was filtered through Whatman filter paper by using vacuum filtration method. Finally, the filtration cake residue was annealed at 80◦ C for 12 h for further processing.

ment having equipped with an Oxford INCA X-sight X-ray detector. The EDXS data was recorded at an accelerating voltage of 15 kV. The functional defects and number of layers of graphene was analyzed by Raman spectroscopy. Raman spectrum was recorded using Horiba Jobin Yvon (model: Lab RAM HR) spectrometer, which is integrated with a laser diode source of wavelength (532 nm). The spectra were attained with approximate power (2 mW). Transmission electron microscopy (TEM) and high resolution TEM (HR-TEM) data was recorded using a TEM, F30, Tecnai 30 D264 S-Twin microscope operated at 300 kV with a 0.14 nm point resolution. The TEM sample was prepared by using RGO and Ag nanoparticles dispersed in the acetone solution through ultrasonication for 10 min. Finally the colloidal solution was drop casted over the mesh of copper grid and dried at room temperature.

2.2. Preparation of RGO

The Hot-Cold temperature measurement system (Vostch Industries technik laboratory equipment, Model no: VCL 7003 series) was used for the temperature testing applications. The operated temperature range is from −70◦ to 180◦ C with rate of change of heating and cooling in the range of 4 K/min and 3 K/min. Power rating to handle the equipment is maximum about 2 kW.

As-synthesized GO was dispersed in DI water in the weight ratio of 1:1. The diluted GO suspension was ultrasonicated for 90 min [28]. The GO dispersion was reduced by reducing agent hydrazine hydrate (N2 H4 ) using chemical reduction route by maintaining uniform oil heating environment around 95◦ C for 4 h [29,30]. The colour of the reaction mixture is changed from light brown to dark black at the end of the reaction process. Again, the mixture was filtered through Whatman filter paper using vacuum filtration method to remove the unreacted particles and the collected filtration cake residues, which consists of purified RGO nanosheets powder. At the end, RGO nanosheets powder was annealed at 80◦ C for 2 h duration for further preparation of nanocomposites. 2.3. Preparation of RGO and silver nanoparticles based nanocomposite The purified RGO nanosheet powder and Silver (Ag) nanoparticles are dispersed in N-Methyl-2-Pyrrolidone (NMP) organic solvent at a weight ratio of 1:2 through ultrasonication process to form the nanocomposite reaction mixture. Ultrasonication is carried out for 1 h duration in order to achieve the homogeneous dispersion of RGO nanosheets and Ag nanoparticles. Further, the RGO-Ag nanoparticles based nanocomposite dispersion was used for the fabrication of the temperature sensor on a flexible membrane platform. 2.4. Advanced material characterization The synthesized nanostructures of RGO and RGO-Ag nanocomposite were characterized by the following advanced measurement techniques. Brucker D8 Advance diffractometer (model no: A18A100/D76182 Karl sruhe, Germany) was used for the XRD studies. Here, the diffractometer was operated in reactive mode, powered at 40 kV and 30 mA at room temperature. As-synthesized nanomaterial’s crystalline phase was examined using Cu k␣ radiation with wavelength (␭) of 1.5406 Å by using nickel filter at a scan rate of 2◦ /minute. Powders of XRD as well as thin film XRD were used for the preparation of samples by dropping the colloidal solution of dispersed RGO-Ag nanocomposite in acetone solution on the surface of silicon substrate. Field-emission scanning electron microscopy (FE-SEM) images were obtained by Carl Zeiss, Ultra 55 instrument equipped with SE2 and In Lens detectors. The microscopic images were attained with an accelerating voltage in the range of 5–10 kV. Energy dispersive X-ray spectroscopy (EDXS) data was collected with the same instru-

2.5. Device Characterization/Temperature measurements

2.6. The detailed fabrication steps followed for RGO-Ag nanocomposite based temperature Sensor/Device Fabrication steps followed for the realization of temperature sensor on flexible sheet/membrane (kapton) are the following: 1) A flexible kapton membrane thickness (175 ␮m), 2) 50 ␮m thick stainless steel mask designed for deposition of RGO-Ag nanocomposite, 3) Device dimensions being 20 mm × 10 mm × 0.175 mm, 4) Double enameled copper wire (diameter 70 ␮m) was bonded for electrical leads with the help of silver paste on top of the nanocomposite structured film, 5) Parylene coating for the moisture proof. The details of the above mentioned fabrication steps are given below. In our work, flexible kapton (M/S. DuPont Teijin Films, USA) membrane with thickness of 175 ␮m was used as suitable substrate for the development of temperature sensor. The schematic representations (including photographs) of the detailed steps followed for the fabrication are shown in Fig. 1(a–j). Suitable mechanical mask was designed & fabricated using stainless steel (SS 304) sheet of thickness 50 ␮m. As-synthesized/prepared RGO-Ag nanocomposite solution was used to achieve the patterns of the developed micro mold structure onto flexible kapton substrate by drop casting method. The thickness of active area for the nanocomposite film varies with respect to the spacing between the adjacent ink droplets (nanocomposite solution). The number of drops deposited on the kapton sheet/membrane influence on the electrical resistance. Further, the RGO-Ag nanocomposite sensing film was kept for annealing at 80 ◦ C for 1 h. Subjecting the sensing film for annealing process results in removal of the solvent and re-arrangement of the atoms in the nanocomposite film. The thin double enameled copper wires were attached on to patterned films using silver paste. Subsequently, the sensing film was re-annealed at 90 ◦ C for 30 min for the purpose of curing of the electrical contacts and making the device robust. Furthermore, the completely fabricated device was encapsulated with Parylene coating to provide protection from the environmental conditions. The few layers of Parylene were deposited on the device for the purpose of providing complete conformal and uniform thickness as well as making it pinhole free. The formation of Parylene coating on the device enables it more stable and provide long life period of about 10 years at 80 ◦ C.

176


Fig. 1. Schematic view of manufacturing process flow of the RGO-Ag nanocomposite temperature sensor fabrication:(a) Flexible kapton sheet/membrane substarte as a sensor base, (b) Development of structural mechanical mask, (c) Mask arragnement on the flexible kapton substrate, (d) RGO-Ag nanocomposite preparation through ultrasonication process, (e) Development of micro mold structure onto the flexible kapton substrate by drop casting method using RGO-Ag nanocomposite solution, (f) Removal of mask arrangement after transformation of micro mold structure onto the flexible kapton substrate, (g) The electrical leads with thin double enameled copper wires using silver paste on top of the nanocomposite patterned films, (h) Encapsulation of the complete fabricated device with Parylene coating, (i) Photograph of the fabricated piezoresistive Graphene-Ag nanocomposite thin film temperature sensors, (j) Photograph of the fabricated sensor shown in the bending state of more than 30% condition.

3. Results and discussion Fig. 2(a) shows the X-ray diffraction (XRD) pattern of the RGO and as prepared nanocomposite of the RGO–Ag nanoparticles. In the RGO, the diffraction peak at 26.19◦ of 2␪ corresponds to the (002) plane with an interlayer spacing of 0.339 nm along c-axis. It was observed that all other peaks disappear except (002) after the chemical reduction process for 240 min in RGO, indicating that the efficient exfoliation of multilayer’s during the chemical reduction process of Graphene Oxide (GO, see in the supporting information, S1). It appears that, the RGO nanosheets were formed by selfassembly, which was resulted from the reduction. The shoulder peak appearing at 42.82◦ , which is fingerprint peak from graphite. The diffraction peak (100) indicates the reformation of graphite microcrystals on RGO plane because of chemical reduction of the GO [28]. For RGO-Ag nanocomposite system (see in Supporting information, S2 & S3), the Ag nanoparticles were decorated on the RGO nanosheets. The peak at 38.20◦ indicates that the corresponding plane of (111) face centered cubic (FCC) Ag nanoparticles as shown in Fig. 2(a) [31]. The presence of Ag nanoparticles on RGO was also giving three other distinct diffraction peaks at 2. Theta; = 44.57◦ , 64.70◦ , and 77.64◦ respectively, which are indicating the (200), (220), and (311) crystalline planes of Ag, revealing the formation of Ag nanoparticles in this composite system. The RGO was not able to be identified due to the strong Ag signals and other reason probably be involvement of the higher exfoliation degree of RGO after dispersion of Ag nanoparticles. These observations confirm that the Ag nanoparticles were effectively attached to the exfoliated RGO nanosheets. Furthermore, the structural and compositional analysis of RGOAg nanocomposite system was carried out by transmission electron microscope (TEM). The low and high magnification images of RGO nanosheets and RGO-Ag nanocomposite are shown in Fig. 2(b) and (c) respectively. The microscopic images reveal the 2D nanosheet layers with wrinkled morphology (Fig. 2(b)). Because, the RGO nanosheets are basically from mono layers of 3D graphite. The

TEM images of graphene decorated with Ag nanoparticles showed that the diameter in the range of 20–40 nm as shown in Fig. 2(c). In this, the RGO flakes have the tendency to assemble and form few layered structures. The high resolution TEM (HR-TEM) image recorded on the graphene–Ag nanoparticles nanocomposite clearly demonstrate that the graphene nanosheets are covered by the Ag metal nanoparticles on planes and surfaces as shown in Fig. 2(d). The measured d spacing value from HR-TEM image is found to be 0.235 nm. Fig. 2(e) shows the selected area wherein electron diffraction (SAED) pattern recorded on the same nanocomposite region, which reveals the poly crystalline nature. The EDXS measurement recorded on the nanocomposite interface as shown in Fig. 2(f), where in the presence of carbon, oxygen, silver and copper elements are clearly evident. The presence of oxygen reveals that the RGO nanosheets are not completely reduced from oxidation of graphite exfoliation. The presence of silver in the nanocomposite is mainly from silver nanoparticles [32]. The presence of copper element is due to the carbon coated TEM grid used for TEM analysis. The structural characteristics and properties of graphene based materials information was obtained from Raman Spectroscopy. The spectrum of RGO and RGO-Ag nanocomposite are shown in Fig. 3(a), which shows the existence of the D, G and 2D bands. It has been observed that in the RGO, the existence of the D band at 1348.54 cm−1 corresponding to defects of the graphene (RGO) sample. The G (1591.97 cm−1 ) line is usually assigned to the first order scattering of the E2g phonon vibration mode of sp2 bonded C atoms and the D line is the breathing mode of the K-point phonons of A1g Symmetry [28,33]. The 2D band (2696.67 cm−1 ) is originated from second order double resonant Raman Scattering, which gives information about the number of layers (typically more than 10 layers by using I2D /I G ) present in the nanostructures [34,35]. The peak position of 2D band is similar to the monolayer graphene prepared from the mechanical cleavage method [36]. The intensity of 2D-band is sensitive to doping of graphene by either holes or electrons. In the nanocomposite system, both the Raman intensities of the D (1345.32 cm−1 ) and G (1591.97 cm−1 ) bands slightly increases upon the adsorption of Ag nanoparticles with surface


177

Fig. 2. (a) XRD pattern of RGO nanosheets and RGO-Ag nanocomposite. (b) TEM image of single multilayered graphene nanosheets, (c) TEM image of RGO-Ag nanocomposite, (d) HR-TEM image of RGO-Ag nanocomposite interface, (e) SAED pattern of RGO-Ag nanocomposite system and (f) The corresponding EDXS spectrum of RGO-Ag nanocomposite.

Fig. 3. (a) Raman Spectroscopy of RGO nanosheets and RGO-Ag nanocomposite, (b) I–V measurement of RGO-Ag nanocomposite films of R1 resistor, (c) I–V measurement of RGO-Ag nanocomposite films of R2 resistor and, (d) I–V measurement of RGO-Ag nanocomposite films of total of R1 & R2 (combined resistor) resistance.

enhanced Raman scattering activity which is shown in Fig. 3(a). The peak position of 2D (2696.63 cm−1 ) represents graphitic nature in the composite system. In this system, there are two possible mechanisms which contribute to the surface enhancement such as electromagnetic and charge transfer mechanisms [32]. Subrahmanyam group studied the phenomenon of the metal nanoparticle

adsorbate charge transfer interaction based on the first principle calculations [33]. Furthermore, the presence of Ag nanoparticles in RGO-Ag nanocomposite system heightened the relative intensity ratio of D/G, which indicates the degree of disorder in graphene is an ideal tool for the nano electronic devices/applications [36].

178


Fig. 3(b–d) show the current (I) verses voltage (V) behavior of the fabricated RGO-Ag nanocomposite based temperature sensor at room temperature for two different resistors (R1 , R2 ) of the device on the same base substrate material (kapton). It can be seen that the current increases linearly with the increase of voltages from −10 V to +10 V. It was observed that this behavior remains unaltered for multiple cycles. Both of the resistors (R1 and R2 ) of Fig. 1 (i) showed a linear variation of I–V characteristics behavior (Fig. 3(b) & (c)), indicating the ohmic contact between the RGO-Ag nanocomposite and electrodes. Similar experiments were conducted for other different device samples and the results obtained were found to be same as above. Hence, it is clear that the silver nanoparticles can provide the conductive pathway to improve the electrical conductivity of graphene in the fabricated device for faster electron transfer, which occurs between RGO-Ag nanocomposite sensing element and the electrodes. Similar experiments were also conducted for the combined resistors (R1 + R2 = Total R of Fig. 1(i)) and the results obtained (Fig. 3(d)) are similar in nature. These results clearly demonstrate the good electrical conductivity of RGO-Ag nanocomposite films and hence their suitability for sensing applications. The schematic view of the arrangements as well as the photograph of the fabricated piezoresistive Graphene-Ag nanocomposite thin film temperature sensors are shown in Fig. 1(h) & (i) (details are provided in Supporting information, S4) respectively. The active structural sensing area of the device is around 4 mm × 1 mm and integrated on the flexible kapton sheet/membrane (20 mm × 10 mm × 0.175 mm). In order to study the performance of the device such as resistive element behavior, it was placed inside the Hot-Cold measurement system (shown in Supporting information, S5). As can be observed, the resistive element was subjected to thermal heating environment by using temperature calibration system. The electrical leads of the temperature sensor were connected to a 6 ½ digital multimeter (Gwinstek GDM-8261). The sensor was subjected to both negative and positive temperature using temperature controlled oven. As the nanocomposite sensing film is exposed to heating environment, the conductance of nanocomposite sensing film varies resulting in corresponding change in resistance (highly sensitive to local electrical and chemical changes). When temperature increases, only surface absorbed molecules and intercalated RGO nanosheets lattice expansion are sensitive to the charge carrier’s movement. Hence, the changes in electrical resistance of the nanocomposite system resulted as resistance decrease in significantly observed. Also, when temperature decreases, the charge carrier’s movements are not in function and hence changes of electrical resistance of nanocomposite system resulted as resistance increase in significantly. The temperature variation was adjusted by digitally controlled meter and the corresponding resistance variation of the temperature sensor changes were recorded using the 6 ½ digital multimeter. It is observed that, the fabricated nanocomposite film behaves as negative temperature coefficient (NTC) sensing element, similar to the semiconducting device [30]. It is important to note that, it was observed that the electrical resistance decreases linearly with the increase of temperature. The resistance versus temperature measurement data recorded in the year 2014 and 2016 are shown in Fig. 4(a) & (b). Interestingly, the NTC value of the RGO-Ag nanocomposite sensing films was found to be same (−1.64 × 10−3 //K) as shown in Fig. 4(e) & (f). It was noticed that the obtained value is ten times lower than the reported value (−0.0148 //K) of other graphene based films [12,13]. Fig. 4(a) & (b) show the sensitivity (0.5553 /K) of our presently developed sensor in the temperature range of 248 K–353 K. It is evident that, the measured sensitivity value is better than the value reported for other semiconductor NTC materials.

The NTC resistance of the temperature sensor was calculated using the following equation R(T)=R(T0 )(1 + ␣ T)

(1)

The sensitivity of the sensor was calculated by using the following equation Sensitivity =

R(T) − R(T0 ) R = (T − T0 ) T

(2)

Where ␣ is the negative temperature coefficient of resistance of the temperature sensor, T is the change in temperature, R is the change in resistance and R is the initial resistance of the sensor. The temperature dependent electrical properties of graphene attract significant attention due to its outstanding electrical properties of the pristine graphene derived from mechanical cleavage method. In this context, it gives the diverse results both in increasing and decreasing resistivity trends of device at increasing temperature were reported by [37,38]. The resistivity dependence on temperature is the combined effect of semiconducting behavior as dominated by the thermal charge carries (resistance decreases at higher temperature) and metallic behavior as dominated by the charge carriers scattering (resistance increases at higher temperature). These results are different from the device, measurement conditions. Also, the device results depend on the graphene crystallinity, numbers of layers, carrier density, interference of scattering wave etc. [38]. For example, the semiconducting nature is observed from the samples with Fermi level near to the Dirac point (charge neutrality) and metallic nature is observed from the graphene with carrier density. However, the studies on chemically reduced graphene oxide suggested that the quite like semiconducting behavior [39,40]. In essence, this nature is because of the difficulty to reduce the graphene oxide completely, in the presence of remnant oxygen and its hydroxyl groups as well as functional groups on the surface to convert the zero gap graphene in to finite gap semiconductor or even insulator. Our observations are consistent with the above, where in the measured NTC characteristics of our flexible RGO-Ag nanocomposite Thermistor are clear indicators of extrinsic semiconducting behaviors with proper doping of Ag nanoparticles on their surface and plane edges. The temperature performance of the sensors is affected by sensitivity, non-linearity, hysteresis and repeatability. Surprisingly, to the best of our knowledge although there are reports on graphene related materials for sensor applications, the information on the use of graphene derivatives with the Ag metal nanoparticles based nanocomposite for the temperature sensing is not available. In our work, particularly we chose to integrate RGO-Ag nanocomposite on flexible kapton sheet/membrane because of its attractive sensing properties due to their defects and functionalization of Ag nanoparticles. The operation of the sensor at room temperature stabilizes quickly with insignificant value of non-linearity. Also we notice that, the same sensor (resistor R1 ) performance results recorded after a time gap of two years (2014 and 2016) was almost invariant. Hence it clearly indicates the fact that the presently developed sensors exhibit good temporal stability & repeatability with negligible hysteresis and non-linearity. The observed average static parameter values of same sensors for both the years (2014 and 2016) were calculated. The maximum non-linearity & hysteresis was found to be around 1.02% & 0.98% respectively at full scale range (FSO) at 248 K–353 K. The sensors exhibited a good repeatable value of about 98.87% at FSO. The root mean square (RMS) error and root sum square (RSS) error were also calculated and found to be 56.78 and 98.31 respectively. These characteristics are very important in case of indirect detection of surface target analytes such as gas, humidity, mechanical stresses or stains and biological species.


179

Fig. 4. R-T variations exhibits the behavior of linear relationship of same sensor (resistor R1 ) in both the years (a) The non-linearity curve of 2014, (b) The non-linearity curve of 2016, (c) Repeartability of sensor in between the period of 2 years and, (d) Hysteresis behavior of sensor in the superimpose position in the period of 2 years, (e) The relative change in resistance as change in temperature resulting as TCR in the 2014 and its best linear fit, (f) The relative change in resistance as change in temperature resulting as TCR in the 2016 and its best linear fit.

We have demonstrated the temperature sensing property of the developed nanocomposite based RGO-Ag temperature sensors. The devices have been tested experimentally and compared their data with the commercially available standard resistance based temperature sensors namely Pt 100 and Thermistor 471. Also, the RGO-Ag sensor is compared with LM 35 and Thermocouple (Ktype). Fig. 5(a) show the photograph of mounting arrangement of commercial devices with our present RGO-Ag nanocomposite based temperature sensor encapsulated with Al foil on an isothermal base. The experiments were performed by keeping all the devices/sensors together in the Hot-Cold encloser set up. Care was taken to maintain the temperature and stabilization (1 h) time uniformly for all the devices during the measurement process. It has been observed that the resistance of Pt 100 decreases with

the decrease of temperature indicating the positive temperature coefficient (PTC) behavior. Whereas, Thermistor 471 and RGO-Ag nanocomposite show the increase of resistance with the decrease of temperature indicating the negative temperature coefficient (NTC) behavior. The present study addresses the NTC behavior, wherein the resistance changes linearly with respect to the applied temperature in the range of 213 K–353 K. The sensitivity (0.555 /◦ C) of the device is found to be better than the standard Pt 100 temperature sensor (0.38 /◦ C), which is shown in Fig. 5(b). Although, the Thermistor (Fig. 5(c)) shows a better sensitivity (14.85 /◦ C) compared to Pt 100 and RGO- Ag nanocomposite sensor, it exhibits noticeable non-linear behavior. However, LM 35 and Thermocouple K-type cannot be compared with the present study, due to the different type of the sensing mechanism. It is important to note

180


Fig. 5. (a) Photograph of the commercial devices with the present work (RGO-Ag nanocomposite sensor encapsulated with Al foil) based temperature sensors mounted on isothermal base for comparative performance study, (b) Comparative performance results of RGO-Ag with Pt 100, (c) Comparative performance results of RGO-Ag with Thermistor 471 and, (d) Comparative performance results of RGO-Ag with Commercial Temperature sensors (Pt 100 and Thermistor 471).

that compared to the above mentioned commercial temperature sensing devices, the presently developed RGO-Ag nanocomposite based sensor has the advantage of simpler electronic circuitry, light in weight and the possibility for mass production with low cost. In order to study the response time of the temperature sensors namely Pt 100, Thermistor and RGO-Ag, we have conducted the experiments by dipping the sensors in ice bath and hot water bath for cold and hot responses. Care was taken to ensure that the device was fully in physical contact with proper thermal equilibrium between ices bath/hot bath and the device environment. The observed response times for the above sensors (Pt 100, Thermistor and RGO-Ag) in cold bath are 22 s, 49.3 s and 470 m s respectively. Similarly, the observed response times in hot water bath for the same sensors are 17.5 s, 2.7 s and 3.45 s respectively. It is evident that the fabricated RGO-Ag sensor has ultrafast response time (470 m s, can be seen in Supporting information, S6) in comparison with the commercial Pt 100 (22.0 s) and Thermistor 471 (49.3 s) sensor when it was kept in cold bath. Also, the fabricated RGO-Ag sensor has better response time (3.45 s, can be seen in Supporting information, S7) in comparison with commercial Pt 100 (17.5 s) when it was kept in hot bath. The comparative performance study results for RGO-Ag and commercial temperature sensors Pt 100, Thermistor 471 are shown in Fig. 5(d). It can be clearly observed that Pt 100 and RGO-Ag sensors exhibit linear response compared to Thermistor type sensor. Moreover, the TCR value (−1.64 × 10−3 //K) for the presently developed RGO-Ag sensor is better than the Pt 100 (3.85 × 10−3 //K) and Thermistor 471 (−3.9 × 10−3 //K). Hence the presently developed sensor (RGO-Ag) can easily replace the Thermistors type sensors in the market. The detailed comparative performance study data of both the commercial and RGO-Ag nanocomposite temperature sensors are given in the following Table 1. In essence, since the response of the RGO-Ag sensor is linear and repeatable with respect to the temperature variation; the electronic

readout circuitry becomes simpler. In our present work, we have demonstrated the effectiveness of nanocomposite on the flexible platform enabling the possibility of using it as a temperature sensor. The fabricated sensor is compact in size, low cost and flexible. The same sensor can be applied for the indirect measurement of strain, pressure, force, acoustic, speed, humidity, gas and biological sensing applications.

4. Conclusions We have successfully developed the RGO-Ag nanocomposite film integrating on the flexible kapton sheets/membrane platform enabling as a temperature sensor for generic temperature monitoring applications. It was found that the obtained NTC value is ten times lower than the reported value of other graphene based films. The measured response time is better than the commercially available temperature sensors. The device developed can be adapted to any substrates with an insulating layer in case of conducting substrate. The sensor fabrication and characterization do not require expensive clean room facility for mass production. The major notable advantages of the present work are: i) Easy to fabricate, ii) Easy to productionize for scaling up, iii) Can be made either flexible or as rigid, iv) The temperature coefficient of resistance (TCR) and sensitivity of presently developed sensor are −1.64 × 10−3 //K and 0.5553 /K respectively, v) Repeatable and stable at specific temperature values, vi) Clean room and sophisticated laboratory equipment’s are not required for fabrication, vii) Low cost, viii) Skilled person is not necessarily required for fabrication, ix) Packaging can be made simpler using readily available materials, x) Resistance of the sensor can be tailored, xi) Configuration of the sensor can easily be made in accordance with the application, xii) Response is linear and controllable based on the geometry & substrate materials. Moreover, the mechanism of electrical resistance change of nanocomposite films can also be affected by moisture, chemical, biological and mechanical param-


181

Table 1 Comparative study data of commercially available sensors with the presently developed RGO-Ag nanocomposite based Temperature sensor. S.No

Parameters

PT 100 (SS Encapsulated)

Thermistor (Bare)

RGO-Ag (Al foil Encapsulated)

LM35 (Bare)

Thermocouple K-Type (SS Encapsulated)

1. 2. 3. 4. 5. 6. 7. 8.

Sensing type Sensitivity TCR Linearity Temperature Range Weight Type of device Resistance/Voltage at 0 ◦ C Response Time Cost

PTC 0.38 /◦ C 3.85 × 10−3 //K Linear −50 ◦ C to 230 ◦ C 36.87 gm Metal 100

NTC 14.85 /◦ C −3.90 × 10−3 //K Non linear −40 ◦ C to 125 ◦ C 0.18 gm Ceramics 1532

NTC 0.555 /◦ C −1.64 × 10−3 //K Linear −60 ◦ C to 170 ◦ C 0.4 gm Nanocomposite 313

Semiconductor 10 mV/◦ C – Non-linear −55 ◦ C to 150 ◦ C 0.17 gm Semiconductor 1.733 mV

Seebeck 41 ␮V/◦ C – Linear 9300 ◦ C to 1350 ◦ C 56.29 gm Alloy 0.0105 mV

Cold – 22.0 s Hot –17.5 s High (x)

Cold – 49.3 s Hot – 2.7 s Low(x/10)

Cold – 0.470 s Hot – 3.45 s Very Low (×/15)

–

–

Moderate (x/12)

High(x/2)

9. 10.

eters and hence this property can be utilized to measure various other physical parameters as well. Acknowledgements The authors are thankful to the Advanced Facility for Microscopy and Microanalysis (AFMM) for providing the microscopy facility, Indian Institute of Science (IISc), Bangalore. The authors would like to thank the Centre for Nano Science and Engineering (CeNSE), IISc for providing the FE-SEM facility, Raman spectroscopy facility and Hot-Cold temperature calibration facility Bangalore. Thanks to the IPC department, IISc for providing XRD facility. We also sincerely thank Dr. K. S. Vasu, Dept. of Physics, IISc for various technical discussions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.sna.2017.11.011. References [1] J. Jeon, H.B.R. Lee, Z. Bao, Flexible wireless temperature sensors based on Ni microparticle-filled binary polymer composites, Adv. Mater. 25 (2013) 850–855. [2] G. Yang, L. Xie, M. Mäntysalo, X. Zhou, Z. Pang, L. Da Xu, et al., A health-iot platform based on the integration of intelligent packaging, unobtrusive bio-sensor, and intelligent medicine box, IEEE Trans. Ind. Inf. 10 (2014) 2180–2191. [3] M. Sibinski, M. Jakubowska, M. Sloma, Flexible temperature sensors on fibers, Sensors 10 (2010) 7934–7946. [4] J. Li, Z. Lyv, Y. Li, H. Liu, J. Wang, W. Zhan, et al., A theranostic prodrug delivery system based on Pt (IV) conjugated nano-graphene oxide with synergistic effect to enhance the therapeutic efficacy of Pt drug, Biomaterials 51 (2015) 12–21. [5] I. Kang, M.J. Schulz, J.H. Kim, V. Shanov, D. Shi, A carbon nanotube strain sensor for structural health monitoring, Smart Mater. Struct. 15 (2006) 737. [6] C. Wang, L. Zhang, Z. Guo, J. Xu, H. Wang, K. Zhai, et al., A novel hydrazine electrochemical sensor based on the high specific surface area graphene, Microchim. Acta 169 (2010) 1–6. [7] S.S. Varghese, S. Lonkar, K. Singh, S. Swaminathan, A. Abdala, Recent advances in graphene based gas sensors, Sens. Actuators B: Chem. 218 (2015) 160–183. [8] K.S. Karimov, F. Khalid, M.T.S. Chani, A. Mateen, M.A. Hussain, A. Maqbool, et al., Carbon nanotubes based flexible temperature sensors, Optoelectron. Adv. Mater.-Rapid Commun. 6 (2012) 194–196. [9] H. Zhang, S. Jia, M. Lv, J. Shi, X. Zuo, S. Su, et al., Size-dependent programming of the dynamic range of graphene oxide-DNA interaction-based ion sensors, Anal. Chem. 86 (2014) 4047–4051. [10] B.C. Marin, S.E. Root, A.D. Urbina, E. Aklile, R. Miller, A.V. Zaretski, et al., Graphene–metal composite sensors with near-zero temperature coefficient of resistance, ACS Omega 2 (2017) 626. [11] O. Gohardani, M.C. Elola, C. Elizetxea, Potential and prospective implementation of carbon nanotubes on next generation aircraft and space vehicles: a review of current and expected applications in aerospace sciences, Prog. Aerosp. Sci. 70 (2014) 42–68. [12] W.Y. Lee, L.T. Le, D. Kong, Graphene-based films in sensor applications, Google Patents (2015).

[13] D. Kong, L.T. Le, Y. Li, J.L. Zunino, W. Lee, Temperature-dependent electrical properties of graphene inkjet-printed on flexible materials, Langmuir 28 (2012) 13467–13472. [14] H. Wu, K. Cai, J. Zhou, B. Li, L. Li, Unipolar memristive switching in bulk negative temperature coefficient thermosensitive ceramics, PLoS One 8 (2013) e79832. [15] C. Yu, Z. Wang, H. Yu, H. Jiang, A stretchable temperature sensor based on elastically buckled thin film devices on elastomeric substrates, Appl. Phys. Lett. 95 (2009) 141912. [16] D. Vilela, A. Romeo, S. Sánchez, Flexible sensors for biomedical technology, Lab Chip 16 (2016) 402–408. [17] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (2010) 1603–1607. [18] W. Chen, L. Yan, In situ self-assembly of mild chemical reduction graphene for three-dimensional architectures, Nanoscale 3 (2011) 3132–3137. [19] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [20] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M. Katsnelson, I. Grigorieva, et al., Two-dimensional gas of massless Dirac fermions in graphene, Nature 438 (2005) 197–200. [21] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282–286. [22] A. Peigney, C. Laurent, E. Flahaut, R. Bacsa, A. Rousset, Specific surface area of carbon nanotubes and bundles of carbon nanotubes, Carbon 39 (2001) 507–514. [23] D. Wang, D. Choi, J. Li, Z. Yang, Z. Nie, R. Kou, et al., Self-assembled TiO2-graphene hybrid nanostructures for enhanced Li-ion insertion, ACS Nano 3 (2009) 907–914. [24] C. Yan, J. Wang, P.S. Lee, Stretchable graphene thermistor with tunable thermal index, ACS Nano 9 (2015) 2130–2137. [25] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, et al., Conformable flexible, large-area networks of pressure and thermal sensors with organic transistor active matrixes, Proc. Natl. Acad. Sci. U. S. A. 102 (2005) 12321–12325. [26] R.C. Webb, A.P. Bonifas, A. Behnaz, Y. Zhang, K.J. Yu, H. Cheng, et al., Ultrathin conformal devices for precise and continuous thermal characterization of human skin, Nat. Mater. 12 (2013) 938–944. [27] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958), 1339-1339. [28] K. Vasu, B. Chakraborty, S. Sampath, A. Sood, Probing top-gated field effect transistor of reduced graphene oxide monolayer made by dielectrophoresis, Solid State Commun. 150 (2010) 1295–1298. [29] N. Neella, V. Gaddam, K. Rajanna, M. Nayak, T. Srinivas, Highly flexible and sensitive graphene-silver nanocomposite strain sensor, Sens. IEEE (2015) 1–4. [30] N. Neella, V. Gaddam, K. Rajanna, M. Nayak, Negative temperature coefficient behavior of graphene-silver nanocomposite films for temperature sensor applications, Nano/Micro Engineered and Molecular Systems (NEMS), IEEE 11th Annual International Conference On, IEEE (2016) 329–332. [31] W.-P. Xu, L.-C. Zhang, J.-P. Li, Y. Lu, H.-H. Li, Y.-N. Ma, et al., Facile synthesis of silver@ graphene oxide nanocomposites and their enhanced antibacterial properties, J. Mater. Chem. 21 (2011) 4593–4597. [32] J. Qi, W. Lv, G. Zhang, Y. Li, G. Zhang, F. Zhang, et al., A graphene-based smart catalytic system with superior catalytic performances and temperature responsive catalytic behaviors, Nanoscale 5 (2013) 6275–6279. [33] K. Subrahmanyam, A.K. Manna, S.K. Pati, C. Rao, A study of graphene decorated with metal nanoparticles, Chem. Phys. Lett. 497 (2010) 70–75. [34] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P. Eklund, Raman scattering from high-frequency phonons in supported n-graphene layer films, Nano Lett. 6 (2006) 2667–2673. [35] A. Kesarwani, O. Panwar, S. Dhakate, V. Singh, R. Rakshit, A. Bisht, et al., Determining the number of layers in graphene films synthesized by filtered cathodic vacuum arc technique, Fullerenes Nanotubes Carbon Nanostruct. 24 (2016) 725–731.

182


[36] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U. Waghmare, et al., Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (2008) 210–215. [37] S. Morozov, K. Novoselov, M. Katsnelson, F. Schedin, D. Elias, J.A. Jaszczak, et al., Giant intrinsic carrier mobilities in graphene and its bilayer, Phys. Rev. Lett. 100 (2008) 016602. [38] K. Bolotin, K. Sikes, J. Hone, H. Stormer, P. Kim, Temperature-dependent transport in suspended graphene, Phys. Rev. Lett. 101 (2008) 096802. [39] V. Skákalová, A.B. Kaiser, J.S. Yoo, D. Obergfell, S. Roth, Correlation between resistance fluctuations and temperature dependence of conductivity in graphene, Phys. Rev. B 80 (2009) 153404. [40] J. Ito, J. Nakamura, A. Natori, Semiconducting nature of the oxygen-adsorbed graphene sheet, J. Appl. Phys. 103 (2008) 113712.

Biographies

Nagarjuna Neella was born in 1988 in India. He received the M. Sc. degree in Physics from the Dept. of Physics, Sri Venkateswara University, India. He is currently pursuing the Ph.D.degree in the Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, where he is involved in the synthesis of Graphene as well as design and development of nanocomposite based flexible sensors and other devices.

Venkateswarlu Gaddam presently working as Research associate at Dept. of Instrumentation & Applied Physics, Indian Institute of Science, Bangalore. He did his engineering graduation in Electronics and Instrumentation Engineering from Andhra University (2006), India. His post graduation in Instrumentation Engineering from NIT Kurukshetra (2010), India. He received the PhD degree (2016) from at Dept. of Instrumentation & Applied Physics, Indian Institute of Science, Bangalore. His field of interests are on synthesis of nanostructures (1D nanorods/2D nanosheets), thin films, Graphene devices and piezoelectric sensors. Manjunath Nayak presently working as visiting Professor, Center for Nano Science and Engineering (CeNSE), Indian Institute of Science, Bangalore (August 2011–Present). He worked as Deputy Director Semiconductor Laboratory, Department Of Space, Chandigarh, India. He also holds the additional charge as Group Director, Transducer Development and Production group of LPSC, Indian Space Research Organisation, Bangalore, India. His post graduation in electronic design is from the Department of Center For Electronic Design Technology (CEDT), Indian Institute Of Science (1982), India. PhD in thin film Instrumentation from Indian Institute of Science (1992). He has eight patents on sensors. He has published more than 45 papers in international Journals. He has been awarded NRDC award, Kirloskar Electric Company Award, SEP award for the Export of Transducers. His interests are development of MEMS Sensors, space grade transducers and nano sensors.

Dinesh N.S. presently working as Professor at Dept. of Electronics Systems Engineering (DESE), Indian Institute of Science, Bangalore, India. He also worked as Principal Research Scientist at Centre for Electronics Design and Technology (CEDT), IISc, Bangalore, India Received Bachelor of Engineering degree from Mysore University (1976), India, masters in electrical engineering from Indian Institute of Technology, Madras (1979), India. PhD from Indian Institute of Science (1992), India. He worked for Indian telephone Industries for few years before taking up teaching and research. He teaches courses in Motion Control System, Mechatronics and Electronics Packaging. He is involved in the development of products related to mechatronics, communication, robotics and sensors and actuators. He is working on the concept of automated stringed puppetry, which uses a robot to manipulate the puppet. He has undertaken industrial consultancy projects in the area of weather monitoring, cryogenic sensors, aerospace applications, and equipment for biotechnology and system automation. K. Rajanna was born in Karnataka, India. He received the M. Sc. degree in physics from the University of Mysore, India, in 1976 and the M. Sc. (Engg.) and Ph.D. degrees from the Indian Institute of Science (IISc), Bangalore, in 1988 and 1993, respectively. At present, he is a Professor at the Department of Instrumentation and Applied Physics, IISc. He was a Visiting Fellow of the Japan Society for the Promotion of Science (JSPS) twice from 1997 to 1998 at the Toyohashi University of Technology, Japan. He was also a Visiting Professor at Tohoku University, Japan, during 2000 and 2002. He was invited to serve on the program committee of the Asia-Pacific Conference of Transducers and Micro-Nano Technology 2004 (APCOT’04) to be held in Sapporo, Japan. His current areas of interest include thin-film sensors and micro/nano sensors (including MEMS/NEMS), vacuum and thin-film technology, energy harvesting devices and novel materials (Graphene) engineering for sensor applications. Dr. Rajanna served as a member of the Editorial Review Committee of the IEEE Transactions on Instrumentation and Measurement.