Inkjet Printing of Graphene Thin Films for Wireless Sensing Applications

Taoran Le(1), Vasileios Lakafosis(1), Trang Thai(1), Ziyin Lin(2) Manos Tentzeris(1) (1) School of ECE, Georgia Institute of Technology, Atlanta, 30332, USA (2) School of MSE, Georgia Institute of Technology, Atlanta, 30332, USA

Abstract This paper will discuss recent advances in state-of-the-art inkjet printing of novel materials for wireless sensing applications. The work includes development of a prototype sensor based on high performance inkjet printed graphene thin films. The thin films were fabricated from direct write of environmentally friendly, water soluble graphene oxide (GO) inks, which were then reduced to obtain the final film. The resulting sensor demonstrates excellent sensitivity compared to existing thin films, along with remarkable recovery time at room temperature without the use of high temperature or UV treatments as reported in literature. In a response to ammonia gas (NH3), we observed a 6% change in normalized resistance after an exposure to 500 ppm of NH3. In addition, over 30% of material restoration is observed within five minutes in ambient conditions.

1 INTRODUCTION

The expedient and accurate detection of hazardous substances is an essential capability for both humans and the environment. It is therefore extremely useful to both public interest and industry.

Recent work in sensor technology has employed the use of novel nano-materials, such as carbon nanotubes (CNT), in chemical sensing applications. These substances experience material property changes in the presence of a certain substances, due to their ability to adsorb certain compounds. Chemical adsorption produces changes in material properties such as real and imaginary impedance, DC resistance, conductance, and effective dielectric constant [1]. These changes can be exploited and monitored to determine the presence of various chemical compounds by translating the material effects into measurable electrical quantities.

Here, we present the use of reduced graphene oxide (rGO) thin films, produced from environmentally friendly aqueous graphene oxide (GO) inks which can be dispersed well in water. Graphene is being utilized in many applications due to its numerous mechanical, thermal, and electrical advantages [2-3]. The graphene creation process involves chemical oxidation of graphite to form graphene oxide powder, which is then used in the ink development, as discussed in Section 2. The ink is then deposited via direct write techniques, and subsequently reduced to obtain the final thin film sensor material. Thermal reduction processes convert the GO back to graphene and allows recovery of its desirable properties. By integrating the novel graphene-based thin film

sensors to WISP UHF RFID EPC Gen2Tags, we form a new generation of low-cost, passive wireless sensing platforms. As a result, another major contribution of this paper is a reporting of the novel gas sensing capabilities of the RFID tags relying on wireless purely digital transmission of the sensed information. The prototype development will be discussed in detail in Section 3. The experimental setup will be given in Section 4, along with results and discussion in section 5.

2 PREPARATION OF INK & SUBSTRATE

2.1 Graphene Oxide Powder Formation

The graphene oxide (GO) powder was prepared by exfoliation of graphite into individual sheets using chemical oxidation methods first introduced by Hummer and Offeman in 1957 [4]. The introduction of oxygen containing functional groups stretches the lattice, promoting shearing of the graphite into GO sheets. The process used is as follows:

a) Graphite flake was placed into a NaNO3 /concentrated H2SO4 solution in an ice bath. KMnO4 was slowly added to the solution while maintaining the temperature below 20 °C.

b) The mixture was stirred in the ice bath for 2 hours and for another 0.5 hours in 35 °C water bath.

c) Next, 70 °C water was added drop-wise into the flask. The heat generated via exothermic reaction raised the solution temperature up to 98 °C.

d) Next, additional 70 °C water was then added, followed by hydrogen peroxide solution to terminate the reaction. e) The mixture was filtrated and washed with water to remove excess acid and inorganic salts. The resulting GO was dried overnight at 55 °C to produce the GO powder.

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2.2 Ink Formulation and Optimization

Once the graphene oxide powder was obtained, inks were created from a mixture of GO powder, water, and glycerol (to enhance the viscosity). The mixture was sonicated for 24 hours to ensure proper dispersion of the GO platelets and subsequently degassed. The resulting inks demonstrated long term stability at room temperature, printing well after several months and showing little signs of particle precipitation or aggregation. The ink was deposited using a Dimatix DMP-2800 materials deposition system, producing resolution down to 50μm. Optimization of the viscosity was necessary to ensure proper jetting performance. The viscosity required for this particular system ranges between 10-12 centipoise at typical cartridge temperature.

3 PROTPTYPE BUILD

3.1 Inkjet printing of gas sensing thin films

Using the Dimatix system, prototype sensors were

developed via inkjet printing of GO inks on top of inkjet printed conductive silver traces, and then reducing the GO to recover the graphene. Although initial experiments were performed using paper substrate, Kapton® was used in subsequent trials due to its ruggedness and inertness. Both chemical and thermal techniques were explored. Paper substrate was found to partially decompose during the reduction process. After several iterations, the final process utilized thermal reduction on Kapton. The process was as follows:

a) Cabot ® silver ink was deposited using a 10pL cartridge, and the resulting pattern was subsequently annealed at 120 °C for 8 hours.

b) GO ink was deposited using a 1pL cartridge in 5 layer increments. After each printing pass, the GO was annealed at 80°C for 12 hours to remove remaining solvent.

c) The resulting GO thin film samples were then reduced by placing the samples in elevated temperature in a hydrogen and argon atmosphere. The samples were reduced at 200°C for 30 min and 300°C for another 30 min. The heating rate was 5°C/min. After reduction, the samples were left to cool down to room temperature naturally.

The GO ink was deposited and cured in small increments in order to ensure optimum morphology and to reduce the variance in observed intrinsic

properties. After thermal reduction, the resistance of the thin films became as low as 150Ω, approximately 1000 times better than the performance mentioned in the current literature [5]. Figure 1 below demonstrates the completed thin films.

Figure 1: Printed rGO thin films on silver traces.

Note that the rGO thin films were created in different sizes. This was to determine the effect of sensor surface area on gas detection. The resulting thin films on silver traces were integrated into a WISP (microcontroller enabled RFID) using conductive silver epoxy. The complete wireless sensing platform is provided in Figure 2 below.

ADC Input

Graphene-based Inkjet-printed Gas Sensor

Regulated 1.8 V

Figure 2: Sensor integrating rGO thin film with WISP.

4 EXPERIMENTAL SETUP AND RESULTS

In order to test the system, the experimental setup shown in Figure 3 was used. An Environics® S4000 gas mixing system delivered the gas mix. The setup was capable of producing reliable gas mixtures up to 500 ppm of ammonia gas diluted in air and delivered at a rate of 50 ccm. To calibrate and measure the sensor, the samples were placed into a semi-closed glass chamber to direct the flow of gas across the sensor. For this effort, we designed a custom chamber out of Plexiglas, as shown in Figure 4. The custom designed gas flowing chamber has internal dimensions of 4cm x 1.5cm x 1.5cm.

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Figure 3: Gas test setup.

The testing procedure was as follows: a) Air was flowed through the system for 15

minutes to establish a system baseline. b) Next, the gas mixture (500 ppm of

ammonia/air) was introduced into the system and measurements of the resistance were taken at one-minute internals for 15 minutes until near steady state condition in the material was achieved.

c) Finally, the gas source was removed from the device while measurements continued to be taken for another 15-minute interval in order to measure the recovery time.

Figure 4: Flow chamber with DUT inside.

5 RESULTS

The results of 500ppm NH3 gas testing are shown in Figure 5. The inkjet-printed RGO thin films of different dimensions show similar responses to 500 ppm NH3. The electrical resistance rapidly increases in the first few minutes after the introduction of NH3, indicating a fast detection rate. With the continued supply of NH3, the resistance change begins to diminish after 10 minutes, which occurs as the material enters the saturation region.

The maximum sensitivity is observed in the case of pattern 3, which shows greater than 6 % increase in normalized resistance. Although here the largest sensor pad has the highest sensitivity, the effect of

the RGO pattern dimension on the sensor performance is still being investigated.

After the introduction of NH3, pure air was introduced into the system and it was found that the resistance quickly recovered a large portion of its original value (~30%) within 5 minutes.

The increase in resistance upon exposure to NH3 is likely due to the electron donation of NH3 to the p-type RGO film, and is consistent with the literature. Compared to other works on graphene-based gas sensors, our inkjet printed RGO demonstrates superior performance, including a high sensitivity, fast response and quick recovery. Particularly, the short recovery time in natural environmental conditions is hugely advantageous over other reports which used UV and heat treatment to assist the recovery, and is of great importance for practical applications [6]. The sensor exhibits an observable (~10Ω) change in resistance within one minute after introducing NH3, a measure which is certainly in the detectable range of the backend circuitry of the WISP platform.

Figure 5: Thin film performance using 500ppm NH3.

5 CONCLUSIONS

We have demonstrated here the usefulness of graphene based thin films as a component in wireless sensing technologies, highlighting their unique properties and ease of integration with existing wireless packaging technologies such as inkjet printing. We also provide detailed results of our efforts in the development of environmentally friendly, stable, low cost, inkjet-printable GO inks.

The prototype device exceeded our expectations for initial tests, producing a 6% change in resistance at 500 ppm concentration of ammonia gas. Moreover, the sensor demonstrated fast recovery time in comparison to the current state of technology without the use of heat or UV treatments to assist in the material recovery. These results can be improved upon by optimization of the deposition and curing techniques, and with enhancements to the output

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circuitry of the final sensor design. One can imagine the potential that graphene based inks can provide as the foundation for advancements in high-performance inkjet-printed electronics, such as inkjet-printed graphene based diodes, super capacitive devices, and transistors.

Acknowledgments

The authors would like to thank NSF-ECS and IFC-SRC for their support for the research. Author Vasileios Lakafosis would like to acknowledge the support of the Lilian Voudouri Foundation.

References

[1] V. Lakafosis et al.,“Wireless Sensing with Smart Skins”, IEEE Sensors 2011, pp. 623-626.

[2] D. Li et al., “Graphene-Based Materials”,

Science, 320, pp.1170-1171, 2008. [3] A. A. Balandin et al., “Superior Thermal

Conductivity of Single-layer Graphene”, Nano Letters, 2008, vol. 8 ISSU 3, pp. 902-907.

[4] W. Hummers and R. E. Offeman, "Preparation

of Graphitic Oxide," Journal of the American Chemical Society, vol. 80,(1958) pp. 1339-1339.

[5] L. Le et al., “Gaphene supercapacitor eletrodes

fabricated by inkjet printing and thermal reduction of graphene oxide,” Electrochemistry Communication 13, pp. 355 -358, 2011.

[6] F.Schedin et al., “Dection of individual gas

molecules adsorbed on graphene” Nature Materials Vol.6, 2007.

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