Article pubs.acs.org/Langmuir
Temperature-Dependent Electrical Properties of Graphene InkjetPrinted on Flexible Materials De Kong,† Linh T. Le,† Yue Li,† James L. Zunino,‡ and Woo Lee*,† †
Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States ‡ U.S. Army ARDEC, Picatinny Arsenal, New Jersey 07806, United States ABSTRACT: Graphene electrode was fabricated by inkjet printing, as a new means of directly writing and micropatterning the electrode onto flexible polymeric materials. Graphene oxide sheets were dispersed in water and subsequently reduced using an infrared heat lamp at a temperature of ∼200 °C in 10 min. Spacing between adjacent ink droplets and the number of printing layers were used to tailor the electrode’s electrical sheet resistance as low as 0.3 MΩ/□ and optical transparency as high as 86%. The graphene electrode was found to be stable under mechanical flexing and behave as a negative temperature coefficient (NTC) material, exhibiting rapid electrical resistance decrease with temperature increase. Temperature sensitivity of the graphene electrode was similar to that of conventional NTC materials, but with faster response time by an order of magnitude. This finding suggests the potential use of the inkjet-printed graphene electrode as a writable, very thin, mechanically flexible, and transparent temperature sensor. example of temperature-sensitive and mechanically flexible substrates. In this investigation, inkjet-printed GO sheets were reduced in room environment using an IR heat lamp from a local hardware store with the distance between the substrates and the lamp controlled to be 3 cm while monitoring (1) substrate temperature and (2) electrical resistance (R). As shown in the Figure 1c, the substrate temperature rose to ∼220 °C during the 12 min exposure duration. R became measurable at ∼5 min into the exposure, and continuously decreased until it reached a steady-state value at ∼10 min. Figure 2a shows that GO sheets had various sheet dimensions and shapes. As summarized in Figure 2b, the average lateral dimension was ∼530 nm with ∼35% GO sheets smaller than 300 nm and ∼30% larger than 1000 nm. The formation of a coffee ring was observed from the dried-out structure of a single 10 pL GO ink droplet containing the nominal GO concentration of 2 mg/mL in water (Figure 3a). The coffee-ring structure was similar to what has been observed in various inkjet-printed materials.7,8 As a result of pinning at the edge of the low contact angle area of the droplet, most GO sheets appeared to stack and form aggregated structures 5−10 nm high and 100−200 nm wide at the perimeter (Figure 3b). Interestingly, we consistently observed a “star”-shaped assembly of nanoscale features at the center region of the droplet (Figure 3c), while leaving a significantly lower number of sheets scattered between the center and the perimeter regions. The height and width of these nanoscale features were in the ranges
G
raphene has received significant attention because of its potential as highly flexible electrically conductive electrodes for various applications ranging from optoelectronic to energy storage to biomedical devices.1−3 We recently reported that graphene oxide (GO) sheets dispersed in water can be inkjet-printed and thermally reduced at 200 °C in nitrogen (N2) to produce relatively thick graphene electrodes with promising electrochemical properties for energy storage.4 The broader implication of our previous finding is that hydrophilic GO nanosheets could be dispersed up to 0.2 wt % in pure water, as a scalable ink. In contrast, hydrophobic graphene sheets are difficult to inkjet-print because of the difficulty producing a stable dispersion, even if organic solvents are used.5 Our inkjet printing approach is expected to offer a new, economically viable avenue of producing micropatternable graphene because of (1) active developments in producing large quantities of potentially low-cost GO sheets derived from graphite powder;6 (2) direct phase transformation from simple, environmental friendly water-based inks to graphene micropatterns in an additive, net-shaped manner with minimum material use, handling, and waste generation; and (3) rapid translation of new discoveries for integration with flexible electronics using commercially available inkjet printers ranging from desktop to roll-to-roll. As schematically illustrated in Figure 1a, the goal of this investigation was to evaluate the electrical and optical properties of inkjet-printed and infrared (IR) lamp-reduced graphene electrodes upon optimizing the spacing between adjacent ink droplets (D) and the number of printed layers (N) as two major process parameters. Figure 1b shows an electrically conductive graphene micropattern inkjet-printed on polyethylene terephthalate (PET), which was used as an © 2012 American Chemical Society
Received: May 1, 2012 Revised: August 21, 2012 Published: August 27, 2012 13467
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Figure 1. Flexible graphene micropatterns produced by inkjet-printing of GO sheets and photothermal reduction using an IR heat lamp in ambient environment: (a) illustration of the overall processing concept with the spacing between adjacent ink droplets (D) and the number of printed layers (N) as major printing variables; (b) micropatterns printed on a transparent PET substrate; and (c) electrical resistance and temperature changes measured in real-time during the photothermal reduction step of the inkjet-printed graphene produced at D = 30 μm and N = 3.
Figure 2. (a) SEM image and (b) lateral size distribution of GO sheets deposited on Si from the dried-out structure of one ink droplet containing 0.1 mg/mL GO.
of 10−20 nm and 50−200 nm, respectively. As the evaporation front receded toward the center of the shrinking droplet, it appeared that the droplet became depinned and the GO sheets become entrained, accumulated, and eventually deposited to form the star-shaped assembly at the center region of the droplet. Figure 3d−f shows that the dried-out structure of GO sheets printed on silicon substrates became continuous with decreasing D from 50 to 20 μm at N = 1. Even with increasing N to 5, the structure obtained with D = 50 μm remained largely discontinuous (not shown). On the other hand, the structure produced at D = 40 μm became more interconnected at N = 5. Despite oxygen plasma substrate treatment prior to the printing step, the effect of D on the formation of discontinuous morphology was more pronounced on hydrophobic PET and Kapton substrates than on hydrophilic silicon and glass substrates. Nevertheless, 20 μm was determined to be an
adequate spacing to produce completely continuous morphology even on Kapton and glass substrates used for optical transparency and electrical sheet resistance (Rs) measurements. Note that the printer used for this study was capable of operating with 5 μm resolutions in the x- and y-directions. Prior to the IR lamp treatment, characteristic GO peaks were present in the Fourier transform infrared (FTIR) spectrum (Figure 4a) including the following: (1) CO stretching vibration at 1735 cm−1, (2) OH stretching at 3428 cm−1, (3) OH deformation vibration at 1411 cm−1, (4) aromatic CC stretching vibration at 1610 cm−1, and (5) alkoxy CO stretching vibration at 1041 cm−1.9 After the exposure, the 1411 cm−1 and 1041 cm−1 peaks disappeared with the 3428 cm−1 peak significantly decreased, and the small 1735 cm−1 peak still remained. These changes suggested the significant removal of OH functional groups from the exposed GO sheets. However, the 1735 cm−1 peak did not disappear, suggesting 13468
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Figure 3. Morphology of dried-out structures produced by a single GO ink droplet on Si: (a) SEM and (b,c) AFM images. (d,e,f) SEM images showing the effects of decreasing D on the development of continuous film morphology on Si.
Figure 4. (a) FTIR and (b) Raman spectra of GO sheets before and after IR heat lamp reduction.
Figure 5. Effects of D and N on (a) electrical sheet resistance and (b) optical transparency.
(ID/IG) increased from 0.79 to 0.94 upon reduction. This ratio change suggested that (1) most of the oxygenated functional groups were removed from GO sheets by the reduction step and (2) sp2 network was established. Upon reduction, the G band was slightly shifted to 1602 cm−1 from 1607 cm−1. However, the G and D bands of the reduced GO sheets present at 1602 cm−1 and 1354 cm−1 were considerably higher than those of chemically vapor deposited (CVD) graphene typically
that the CO stretching vibration of six-ring lactones was still present.10 The 1610 cm−1 CC peak was present, indicating that the sp2 structure of carbon atoms was retained.11 Two prominent Raman peaks were observed before and after the IR lamp reduction step (Figure 4b): (1) G band corresponding to the first-order scattering of photons by sp2 carbon atoms and (2) D band arising from small domain-sized graphitic regions.12,13 The intensity ratio of the D to G bands 13469
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Figure 6. (a) Relative electrical resistance changes upon mechanical bending. (b) Experimental configuration. Error bars represent 3 measurements made for each bending angle.
Figure 7. (a) Temperature-dependence on electrical resistance. (b) Linear fit (red) between ln (R) versus T−1. (c) Relative electrical resistance responses upon repeated fingertip tapping. (d) Experimental configuration.
observed at 1575 cm−1 and 1350 cm−1. These peak shifts indicated the relative lack of sp2 character and the remaining presence of some oxygenated functional groups, consistent with the FTIR results. The FTIR and Raman results suggested that the IR heat lamp treatment was effective in reducing printed GO films to graphene films to a significant extent, but not completely. The IR lamp reduction method is expected to be particularly useful for printing onto thermally and chemically sensitive materials and devices. Also, this method is advantageous for easy integration with roll-to-roll, additive manufacturing since it only takes minutes as opposed to hours required for the thermal and chemical methods without the need for controlled reduction environments and equipment. As shown in Figure 5a, Rs of the graphene electrodes fabricated on Kapton decreased with (1) decreasing D and (2) increasing N. At D = 40 μm, the films were not conductive at N = 2, but became conductive with N = 3 at ∼26 MΩ/□ and with N = 5 at 14 MΩ/□. The high Rs values of these samples
could be explained by (1) the development of noncontinuous morphology at large D and small N and (2) consequently blocking of electron transport paths. At D = 20 μm, Rs decreased from ∼12 MΩ/□ to ∼0.3MΩ/□ with increasing N from 2 to 5. As shown in Figure 5b, graphene electrodes printed on glass substrates became less transparent with (1) reducing D and (2) increasing N. At D = 20 μm, transparency rapidly decreased from ∼76% to 45% upon increasing N from 2 to 5. It is wellknown that an increase in the stacking of CVD graphene layers decreases light transparency of 2.3% per graphene sheet.14 Assuming this number for our sample obtained at N = 2, we roughly estimated that ∼10 graphene sheets may be stacked on average to result in 76% transparency. This estimation was consistent with the average thickness of the dried out structure of each ink droplet being on the order of ∼10 nm as suggested by the AFM data in Figures 3b and 3c. Based on the above results, D = 20 μm and N = 2 were determined to be optimum printing parameters for producing 13470
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continuous electrode morphology with Rs = 12 MΩ/□ at 76% transparency. This optoelectrical performance is similar to that reported by Torrisi et al.5 with Rs = 102 MΩ/□ at 74% transparency for graphene sheets exfoliated by ultrasonicating graphite powder, dispersed in an organic solvent, and inkjetprinted. However, in comparison to CVD graphene,15,16 Rs of our sample was about 7 orders of magnitude higher at a given transparency of 86%. The lower Rs of the CVD graphene was expected since it contains relatively defect-free graphene structure. Nevertheless, the comparison highlights a significant challenge associated with the use of inkjet-printed graphene for optoelectronic applications. Figure 6 shows that R of the electrode printed on Kapton at D = 20 μm and N = 2 decreased with increasing the degree of bending (2θ). The overall decrease in R was 5.6% at 2θ = 27.4°. Apparently, local bending stresses increased the effective mobility of electrons, although the mechanism behind this behavior is not clear. Some hysteresis was observed during recovery, but the resistance ultimately returned to the initial value prior to bending. This recovery behavior implies that the mechanical structure of the graphene electrode remained to be relatively stable during the mechanical bending test. Figure 7a shows that R of the graphene electrode decreased significantly with temperature. The effect of the temperature on the electrode resistance is similar to what has been recently observed by (1) Sahoo et al.17 for filter-deposited and chemically reduced GO sheets using hydrazine vapor and (2) Zhuge et al.18 with filter-deposited and metal-defused GO sheets. As shown in Figure 7b, the following equation was used to model the observed temperature dependence as a negative temperature coefficient (NTC) behavior
The response time to the touching was about 0.5 s, and the recovery time to its initial resistance value upon removing the finger tip was about 10 s. In comparison, typical response time for conventional NTC metal oxide materials is more than 10 s,21 suggesting an order-of-magnitude faster temperaturesensing function of the inkjet-printed graphene electrode. The observed NTC behavior suggests the inkjet-printed graphene functions as an intrinsic semiconductor with perhaps thermally activated transfer of electrons between the reduced domains of the GO sheets as well as between the sheets. It appears that a major reason for the fast time response of the graphene electrode is a very small volume of the inkjet-printed electrode and therefore a significantly lower thermal mass involved with transient heat transfer. In conclusion, our results suggest that micropatternable graphene electrodes can be easily fabricated by inkjet printing of GO sheets and subsequent photothermal reduction using the IR heat lamp in ambient environment in about 10 min. D and N were optimized as the major printing parameters to produce the continuous morphology of the graphene electrode for optimum Rs and transparency. R of the electrode decreased during mechanical bending, but returned to its initial value upon recovery, suggesting the electrode’s structural stability with mechanical flexing. Also, the electrode’s NTC behavior with high temperature sensitivity and fast response time suggests new potential as a writable, very thin, flexible, and transparent temperature sensor.
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EXPERIMENTAL SECTION
Commercially available GO sheets (Cheap Tubes, Brattleboro, VT) dispersed in water (2 mg/mL) were used to prepare inks at several GO concentrations by dilution for some initial experiments. For most experiments, 2 mg/mL was used as the nominal concentration of the GO ink. The viscosity, surface tension, and ζ-potential of the nominal GO ink were measured to 1.06 mPa·s, 68 N/m, and −20 mV, respectively.4 Glass slides (1.2 mm thick, Thermo Scientific, Portsmouth, NH), Kapton-HN (DuPont, Wilmington, DE), and PET (3M, St. Paul, MN) films were used as examples of transparent substrates. Also, polished Si (University Wafer, Boston, MA) was used for characterization purposes. Glass and Si substrates were cleaned using a piranha solution and deionized water several times, then dried with nitrogen gas prior to printing. Si, Kapton, and PET were treated with O2 plasma for 30 s prior to printing using Plasma Cleaner (Harrick Plasma, Ithaca, NY). As previously described,4 a Dimatix Material Printer (DMP 2831, Fujifilm Dimatix, Santa Clara, CA) was used to print the GO inks using cartridges that generate 10 pL droplets. The cartridge height and substrate temperature were maintained at 0.5 mm and 25 °C, respectively. GO electrodes were inkjet-printed as 0.8 cm × 0.8 cm square patterns. The GO electrodes were reduced with an infrared (IR) heat lamp (250 W, GE, Cleveland, OH). Raman spectroscopy (Spectra Pro 2300i, Princeton Instrument, Trenton, NJ) was conducted using the excitation line of 632.8 nm. FTIR (TENSOR Series 27 FT-IR Spectrometers, Bruker Optics, Billerica, MA) was performed in a transparency mode using 100 μL droplet-cast samples on silicon before and after reduction. The drop casting method was used for the FTIR measurements, since the signal from the printed samples was not strong enough to be measured. The morphology and pattern formation of the printed GO electrodes were characterized by optical microscopy (SMZ1500, Nikon, Melville, NJ) and scanning electron microscopy (SEM, Carl Zeiss SMT Auriga FIB-SEM workstation, Peabody, MA), and atomic force microscopy (AFM, Nanoink, Skokie, IL). Transparency was recorded at 560 nm using a multimode microplate reader (Synergy HT, BioTek Instruments, Inc., Winooski, VT).
⎛ (T − T ) ⎞ RT = R 0 exp⎜B 0 ⎟ T ·T0 ⎠ ⎝
where RT is the electrical resistance as a function of temperature (T), B is the material constant and a measure of temperature sensitivity, and R0 is the resistance at the reference temperature (T0 = 298 K). From the data fitting, B was determined to be 1860 K in the temperature range of 298 to 358 K with the respective resistance changes from 4.4 × 106 to 2.4 × 106 Ω. This B value is close to that of the conventional metal oxide NTC materials, typically in the range of 2000 to 5000 K.19 The temperature coefficient of resistance (α) was also used as another measure of temperature sensitivity where α = R−1·(dR/ dT). α for our graphene electrodes was determined to be −0.0148 K−1 at 298 K, which is about 1 order of magnitude larger than that of the chemically reduced GO sheets17 as well as that of metal-defused GO sheets.18 Also, the α value of our graphene electrode is about 3 orders of magnitude higher than that of carbon nanotubes.20 As shown in Figure 7c,d, temperature-sensing function of the graphene electrode was evaluated by tapping the electrode with a human finger in the ambient room environment. The repeated taps resulted in the resistance decreases shown in the Figure 7c. In contrast, no change in the resistance was observed when the electrode was tapped with other objects that were in thermal equilibrium with the room environment (not shown). This observation also indicated that the effect of slight substrate flexing during tapping on the resistance changes was much smaller than that of touching with the finger tip. These results suggested that the resistance changes were as a result of heat transfer between the finger tip and the electrode. 13471
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Rs was measured using a digital multimeter (Keithley Instruments Inc., Cleveland, OH) and a custom-made four-point probe configuration shown in Figure 5a. The four-point probe was prepared by inkjet printing silver nanoparticles ink (Cabot Corporation, Boston, MA) onto Kapton followed by annealing at 200 °C using a hot plate (Corning, Lowell, MA) in the air. Electrical resistance changes during the reduction process were measured by the multimeter with a distance of 2 mm between two probes. Similarly, electrical resistance changes during mechanical bending were measured with a distance of 0.8 mm between two probes. Temperature dependence characterization was conducted similarly using a tunable hot plate (Corning, Lowell, MA) in the air and a thermocouple attached to the graphene electrode. The fingertip tapping experiment was performed with the 4point probe device by applying a constant voltage of 10 V across the sample and recording the corresponding current change using the multimeter. The graphene electrode surface was covered with Scotch tape, and a plastic glove was worn, as shown in Figure 7d.
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(12) Lin, Z.; Yao, Y.; Li, Z.; Liu, Y.; Li, Z.; Wong, C.-P. SolventAssisted Thermal Reduction of Graphite Oxide. J. Phys. Chem. C 2010, 114, 14819−14825. (13) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 2000, 61, 14095− 14107. (14) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine structure constant defines visual transparency of graphene. Science 2008, 320, 1308. (15) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, 706−710. (16) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-roll production of 30-in. graphene films for transparent electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (17) Sahoo, S.; Barik, S. K.; Sharma, G. L.; Khurana, G.; Scott, J. F.; Katiyar, R. S. Reduced graphene oxide as ultra fast temperature sensor. 2012, arXiv:1204.1928v1 [cond-mat.mes-hall]. (18) Zhuge, F.; Hu, B.; He, C.; Zhou, X.; Liu, Z.; Li, R. Mechanism of nonvolatile resistive switching in graphene oxide thin films. Carbon 2011, 49, 3796−3802. (19) Kang, J.; Ryu, J.; Han, G.; Choi, J.; Yoon, W.; Hahn, B.; Kim, J.; Ahn, C.; Choi, J.; Park, D. LaNiO3 conducting particle dispersed NiMn2O4 nanocomposite NTC thermistor thick films by aerosol deposition. J. Alloys Compd. 2012, 534, 70−73. (20) Bartolomeo, A. D.; Sarno, M.; Giubileo, F.; Altavilla, C.; Iemmo, L.; Piano, S.; Bobba, F.; Longobardi, M.; Scarfato, A.; Sannino, D.; Cucolo, A. M.; Ciambelli, P. Multiwalled carbon nanotube films as small-sized temperature sensors. J. Appl. Phys. 2009, 105, 064518. (21) Kanungo, J.; Saha, H.; Basu, S. Pd sensitized porous silicon hydrogen sensorInfluence of ZnO thin film. Sens. Actuators, B: Chem. 2010, 147, 128−136.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the U.S. Army - ARDEC for funding this project under the contract of W15QKN-05-D-0011. This research effort used microscope resources partially funded by the National Science Foundation through NSF Grant DMR0922522. We also thank Andrew Ihnen at Stevens and Brian Fuchs at ARDEC for various discussions.
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REFERENCES
(1) Eda, G.; Fanchini, G.; Chhowalla, M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nat. Nanotechnol. 2008, 3, 270−274. (2) Cohen-Karni, T.; Qing, Q.; Li, Q.; Fang, Y.; Lieber, C. M. Graphene and nanowire transistors for cellular interfaces and electrical recording. Nano Lett. 2010, 10, 1098−1102. (3) Shan, C.; Yang, H.; Song, J.; Han, D.; Ivaska, A.; Niu, L. Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene. Anal. Chem. 2009, 81, 2378−2382. (4) Le, L. T.; Ervin, M. H.; Qiu, H.; Fuchs, B. E.; Lee, W. Y. Graphene supercapacitor electrodes fabricated by inkjet printing and thermal reduction of graphene oxide. Electrochem. Commun. 2011, 13, 355−358. (5) Torrisi, F.; Wu, T. H., W.; Sun, Z.; Lombardo, A.; Kulmala, T.; Hshieh, G. W.; Jung, S. J.; Bonaccorso, F.; Paul, P. J.; Chu, D. P.; Ferrari, A. C. Ink-Jet Printed Graphene Electronics. ACS Nano 2012, 6 (4), 2992−3006. (6) Jacoby, M. Graphene Moves Toward Applications. Chem. Eng. News November 21, 2011. (7) Kang, J. S.; Kim, H. S.; Ryu, J.; Hahn, H. T.; Jang, S.; Joung, J. W. Inkjet printed electronics using copper nanoparticle ink. J. Mater. Sci. Mater. Electronics 2010, 21, 1213−1220. (8) Yoshitsugu, K.; Ueno, I. Effect of suspended particles on spreading of volatile droplet on solid substrate. J. Vis. 2011, 14, 285− 294. (9) Ren, P. G.; Yan, D. X.; Ji, X.; Chen, T.; Li, Z. M. Temperature dependence of graphene oxide reduced by hydrazine hydrate. Nanotechnology 2011, 22, 055705. (10) Mei, X. G.; Ouyang, J. Y. Ultrasonication-assisted ultrafast reduction of graphene oxide by zinc powder at room temperature. Carbon 2011, 49, 5389−5397. (11) Chen, W. F.; Yan, L. F.; Bangal, P. R. Preparation of graphene by the rapid and mild thermal reduction of graphene oxide induced by microwaves. Carbon 2010, 48, 1146−1152. 13472
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