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To apply in wearable electronics, we propose a stretchable photo sensor that detects an inversely changed resistance by varying light intensity, which is stably operated up to 25% axial strain. Especially, the stretchabity of the proposed photo sensor is achived by using a uniform ridged substrate made of polydimethylsiloxane (PDMS). The proposed device is composed of a thin film of perylene/graphene composite, which is sandwiched between bottom and top indium tin oxide (ITO) transparent electrodes fabricated through electro-hydrodynamic (EHD) technique. The electrical conductivity of perylene is improved by blending graphene with it. The resistance of the proposed photo sensor changes from 108 MΩ to 87 MΩ within the light intensity range of 0 to 400 lux, respectively. Furthermore, the flexibility is verified through a bendability test from 16 mm down to 0 mm and a bending endurance test for more than 1000 cycles. Uniform and smooth deposition of the active layer is tested through surface morphology characterization.
© 2015 Optical Society of America
1. Introduction
Stretchable electronics are a new class of electronics that is introduced in the present decade and got tremendous attraction such as a realization of wearable computers, highly flexible displays, artificial electronic skin, biomedical applications for health monitoring, and biological actuation [1–5]. The integration of multiple stretchable electronic devices is essential to make an autonomous stretchable system which can sense from external world through sensors. This kind of system needs integration of components such as sensors, memory, battery, display unit, and actuators onto elastic polymeric substrate [2]. Many researchers have studied various stretchable core electronic devices, such as transistors [6], resistor [7] light-emitting diodes [8], sensors [9], solar cells [10], and batteries [11]. Sensors are the key components to acquire data from the environment for the further process and actions. To utilize stretchable electronic devices in real time applications the fabrication of a stretchable photo sensor on elastic electronic modules is essential. The selection of the substrate is important in the fabrication of a flexible photo sensor [12].
Various research studies has shown that for the realization of a metal polymer stretchable system, polydimethylsiloxane (PDMS) is widely used as a substrate material mainly because of low cost, intrinsic stretchability, biocompatibility, and excellent molding properties [13–15]. However, PDMS is hydrophobic in nature and poses adhesion problems. On the other hand, in order to achieve high stretchability with minimal resistance change during the stretching, the deposited films must have good adhesion with the substrate. For this reason, various surface modification techniques for the PDMS has been carried out by using methods as ultra violet ozone (UV) treatment, chemical treatments, plasma treatment, and topographical patterning [16–20]. Photo sensors need a material having photoconductive property that can change the resistance in result of incident light. Various materials have researched for the fabrication of the photo sensors [21–24]. Among these photoconductive material, perylene has been widely applied in various optical devices due to its excellent photo-physical properties of high absorption coefficient and high fluorescence quantum yield, charge transfer properties, excellent chemical, thermal and photochemical stability [25]. Since its conductance sensitively varies along the incident light intensity [26,27], the perylene is an efficient photoconductive material, which is widely utilized in solar cells and photo transistors. However, its resistivity becomes very high when it is fabricated as a thin film [26]. To overcome this problem a conductive material would be helpful to blend for the improvement of the electrical property. Graphene could be a good candidate to blend with perylene to achieve the required conductivity because of its extreme, electrical, thermal, and mechanical characteristics [28]. Graphene material is intensively studied thoroughout the literature to enhance electrical, mechanical, and optical properties of the attaching materials. To enhance the light absorbance from 3% to 30%, graphene nano disks array is utilized [29]. Graphene quantum dots are doped to enhance the optical property of MoS2 Monolayers [30]. Au nanoparticles deposited on a MoS2 mono layer has induced a transient reversible 2H to 1T phase transition in MoS2 [31].
In this paper, we propose a stretchable photo sensor fabricated on a PDMS substrate having uniform micro ridges. The device construction is that, a perylene/graphene composite thin film is sandwiched between the ITO bottom and top electrodes through electro-hydrodynamic (EHD) technique which is cost effective, environmentally friendly, and non-vacuum e-printing technology [32,33]. The proposed stretchable photo sensor is profound to blue and ultra violet (UV) light. The resistance of the device is inversely related with light intensity in the range of 87 to 108 MΩ. The device showed negligible change in behavior at strain range from 0 to 25%, and maximum stretchability of 50% with reasonable change in resistance but no physical breakdown. Moreover, it is fully flexible and stretchable, tested up to 1000 endurance cycles. Experimental details are given in section 2, results and discussions are given in section 3, and conclusion is summarized in section 4.
2. The proposed photo sensor
2.1 Materials
To fabricate the proposed photo sensor, graphene platelets from Cheap Tubes (less than 4 layers and surface area greater than 750 m2/g), dichloromethane solvent and perylene powder assay 99.5% from Sigma Aldrich are used. The graphene platelets were dispersed in dichloromethane (4 ml) solvent by bath sonication and centrifugation. The dispersion was then bath-sonicated for 30 min at room temperature. The viscosity of the graphene dispersion was measured to be 15.8 mPa by using Viscometer VM-10A system. The surface tension of the dispersion was measured to be 54 to 57 mN/m by using surface-electro-optics (SEO)’s contact angle analyzer. The electrical conductivity of graphene dispersion was found to be 12.2 µS/cm measured by conductivity meter (Cond6 + meter). Perylene powder of 0.15 g was dispersed in 10 ml dichloromethane and stirred for 24 hours by using magnetic stirrer. The prepared ink was filtered with 5 µm filter to remove undispersed particles and then bath sonicated for 30 min prior to use. These two materials were prepared separately and then mixed with various ratios to achieve optimum results experimentally. We found that 1:1/4 is optimal mixing ratio as shown Fig. 1(a). As the mixing ratio increases beyond 1:1/4 electrical conductance also increases, but photo conductance decreases. Ink for silver electrodes was prepared as: Ag nano particle paste sigma Aldrich 50%wt was diluted in 10 ml ethylene glycol solvent and mixed for 1 hour on magnetic stirrer at 1000 RPM and then 20 min bath sonication. The relative humidity during all experiments was 35%.
Fig. 1 (a) Electrical and photo conductance by mixing ratios of perylene/graphene composite. (b) PDMS substrate with the uniform ridges.
2.2 Ridged PDMS substrate
Using Dow Corning's Sylgard 184 elastromer kit containing the PDMS base and the curing agent, we made PDMS composite of the base and curing agent at 10:1 by weight and whisked thoroughly for 10 min to make sure that mold is uniformly cross-linked. After proper mixing, the PDMS solution contained air bubbles in it, which has removed by degassing for 30 min through a vacuum pump. The PDMS substrates with the uniform ridges were prepared by casting the PDMS solution against the ridged plastic homemade die. Die was prepared by producing grooves on plastic substrate though filing. The PDMS castings were annealed inside a furnace at a temperature of 150 °C for 60 min. The PDMS substrates were cut into dimensions of 2.5 cm by 5 mm and thickness of 1 mm as shown in Fig. 1(b). In general, the flat PDMS substrate has a good bendability, but the stretchability is not mechanically stable for thin films. To increase the stretchability, a zigzag patterned substrate made by the PDMS material works like a spring, that is, the stretched length is increased by 50% as compare to the original length and it restores to its original form on release.
2.3 Fabrication of the proposed sensor
The EHD printing setup is shown in Fig. 2(a), it includes a metallic capillary, a high-voltage power supply, an ink supply section, an X-Y stage control, a Z-axis control for the nozzle, a high-speed camera, a light source and a digital control unit to control and monitor all the operations. The device fabrication process was started with the preprocessing of the PDMS substrate with UV treatment for 5 min to make the PDMS hydrophilic substrate, which can avoid adhesion problem. To deposit each of the three layers of the stretchable photo sensor device, we put the nanoparticles in the ink supply unit (Hamilton, Model 1001 GASTIGHT syringe). The ink was pumped through a Teflon tube to a metallic nozzle (Harvard 33 G - internal diameter of 110 µm and outer diameter of 210 µm) by using a Harvard PHD 2000 infusion pump. The control parameters of the EHD system for the stable cone jet mode in Fig. 2(a) are ink flow rate of 40 µl/h, voltage of 4 kV, and stand-off distance of 3 mm for both top and bottom ITO. The control parameters for the middle layer (or active layer) are as: ink flow rate of 60 µl/h, voltage of 6 kV, and stand-off distance of 12 mm. High voltage was applied on the metallic nozzle by using a Nano NC power supply and was grounded attached with substrate. A substrate holder was used for the X-Y movement of the sample, which is controlled by a computer. A camera and a light source were used to capture and monitor all the operations during the deposition processes. A stable cone jet is needed to deposit the material on the substrate through EHD. To achieve stable cone jet mode for ITO and graphene/perylene composite inks, various applied voltage and ink flow rate combinations were observed which resulted in different modes as shown in Figs. 2(b)-2(e). The fabrication procedure of the device was completed in three steps. In the step one, bottom electrode of ITO was deposited on the PDMS substrate as thin film. In the second step, graphene/perylene active layer was deposited, and in the third step, ITO top electrode was deposited, as the layout diagram is shown in Fig. 3(a). Due to the uniform ridges on the substrate the depositing material got the same shape as of the PDMS substrate. Each layer (ITO/composite/ITO) is very thin and could not fill the ridges on the substrate, hence they adopted the same ridged pattern of the substrate as shown in Fig. 3(b). The sample was cured at 280 °C for 90 min after the deposition of each layer. After curing the device, silver (Ag) pads were placed at both ends of the sensor to connect the external circuits and electronic instruments as shown in Fig. 3(c).
Fig. 2 (a) Schematic diagram of the EHD system. (b) Dripping mode. (c) Unstable jet mode. (d) Stable cone jet mode. (e) Multi jets mode.
Fig. 3 (a) Layout diagram of the proposed stretchable photo sensor. (b) Fabrication steps of the device, 1. ITO is deposited on the PDMS substrate to make bottom electrode, 2. perylene/graphene composite is deposited for active layer, 3. top electrode of ITO. (c) Fabricated device, Ag paste is applied on both sides to make contacts.
3. Characterizations
The working principle of the device is based on photoconductive property of the active layer. In the proposed device, the conductance of a perylene changes proportionally to the incident light intensity, whereas an electrical conductivity of the photoconductive material (perylene) is further enhanced by graphene dispersions. The resistance of the active layer inversely varies with incident light when a ohmmeter is connected across the device. The fabricated stretchable photo sensor was characterized by homemade stretching apparatus having resolution of 10 mm as the schematic diagram is shown in Fig. 4(a). The photo sensor was loaded on the stretching apparatus and placed inside the probe station (Agilent B1500A Semiconductor Analyzer) along a variable light source. Probes were connected to the terminals to measure the resistance as a function of strain while the light source was fixed at 100 lux light intensity. The resistance of the photo sensor at 100 lux was measured by changing the axial strain from 0 to 50% as shown in Fig. 4(b). In the measurement, axial strain percentage (%) is considered as a ratio of actual length and stretched length of the device, for example, if the length of the device is 2 cm at rest, after stretching the length becomes 3cm which is 50% of the original length. The resistance against stretching until 25% is almost stable and after 25% strain the resistance of the device is increased rapidly until 50% strain. The resistance values at 0~25% strain was 95~95.5 MΩ, whereas at the range of 25~50% strain the resistance value changed from 95.5~99.2 MΩ. This result shows the device has a good performance for stretchability between 0~25% axial strain. The homemade stretching apparatus is shown in the inset of Fig. 4(b). While stretching the device until 50% axial strain the proposed photo sensor did not show any physical breakdown as the images shown in Fig. 4(c). The length of the device without stretching is 2 cm, the device length became 2.5 cm at 25% axial strain, and length is 3 cm at 50% strain. The stretchable photo sensor was characterized for its electrical properties by using Agilent B1500A Semiconductor Device Analyzer. The device was placed on a sample holder inside the probes station and probes were connected with its terminal. An LED light source with variable light intensity was used with the lux meter near the photo sensor to detect the light intensity as the diagram shown in the inset of Fig. 4(d). The resistance of the photo sensor was observed from 108 MΩ down to 87 MΩ for the light intensity variation from dark to 400 lux as shown in Fig. 4(d).
Fig. 4 (a) Experimental setup to measure stretchability. (b) Variation of resistance by strain percentage. (c) Stretched photo sensors at 0%, 25%, and 50%. (d) Resistance of the photo sensor along light intensity measured as inset experimental setup.
In our previous work [34], the photo sensor with a comb type electrodes with teeth space of 50 µm was demonstrated, which operates in resistance variation from 78 GΩ to 25 GΩ. Due to 50 µm space between electrodes, it has a low conductance of the film although the thickness was 350 nm. In this paper, the sensing film (66 nm) is sandwiched between two vertical ITO electrodes, the conductivity is improved because here the space between two electrodes is 66 nm with the resistance variation from 108 MΩ to 87 MΩ. Resistance variation was characterized by measuring the resistance of device along the light intensity with 50 lux increment step. A high variation in resistance was observed from 0 lux to 300 lux and at 300~400 lux the resistance variation was reduced whereas, beyond the 400 lux the resistance of the device remain constant. This characterization suggests the device to be used under the 400 lux. Furthermore, the effect of the incident light angle on the behavior of the sensor was analyzed by using an adjustable light source having rotation angle from 0° to 180°. This test was performed as: the sensor was connected to ohmmeter and an adjustable light source of 100 lux was rotated from 0° to 180° as the diagram is shown in Fig. 5(a). It was observed that there is no change in the behavior of the device from 30° to 150° angle as the resistance curve is shown in Fig. 5(b). As the surface of the sensor is zigzag, the shadow effect could change the behavior of the sensor at different incident light angles. However, the proposed sensor is transparent and light passes through it, which ensures its stable behavior at various angles. This result shows that, the ridged shape of the sensor does not affect the sensor performance. Hence, the proposed sensor could be used in applications where the incident light angle is not fixed.
Fig. 5 (a) Setup diagram of the incident light analysis. (b) Resistance vs incident light curve of the stretchable light sensor to different light angles.
Device was characterized against mechanical stresses by bending it on various diameters from 16 down to 0 mm. The device behavior did not show any change against the bending diameters even completely folded as shown in Fig. 6(a). In the insets of the Fig. 6(a), it shows how to take the bendability test of the device manually. To ensure the mechanical reliability of the device it was subjected to over 1000 endurance cycles in an automatic bending machine as shown in the inset of Fig. 6(b). During this test, the resistance changed from 92 MΩ to 95 MΩ as shown in Fig. 6(b). This result suggests that the device can be utilized in foldable and bendable electronic applications.
Fig. 6 (a) Resistance varied by bending diameter. (b) Bending endurance test (resistance vs bending cycles).
Optical characterization was carried out by using UV/Vis Spectroscopy (Shimadzu Corporation - UV-3150). Perylene and its derivatives are maximally absorbs light at longest wavelength of UV and blue light, whereas the minimum absorbance is occurred at small wavelengths of red and infra-red region [33, 35]. The absorbance spectra varies along the wavelengths, which also depends on the solvent. Figures 7(a) and 7(b) shows the absorbance and the transmittance spectrum of the PDMS substrate and ITO electrodes, respectively. The proposed device includes graphene/perylene composite active film in between ITO electrodes on the PDMS substrate was tested in visible spectrum range. The absorbance of the device was observed to be 50% in the range of 465 nm to 535 nm (blue light) and less than 50% in the range from 350 nm to 465 nm (UV and dark blue). The prominent range for the light sensing is from 465 nm to 535 nm as shown in Fig. 7(c). After 535 nm the absorbance goes to zero abruptly, hence the photo sensor cannot detect the light ranging from 550 nm to onward. Figure 7(d) shows the transmittance spectrum of the photo sensor, 1st peak is at 405 nm and 2nd peak at 495 nm where the transmittance is high. The minimum transmittance is at 525 nm, after this peak the transmittance increasing abruptly. The transmittance spectrum shows that the graphene/perylene composite thin film is more suitable for optoelectronic device applications due to the good transparency. From these results, the proposed sensor can be applied to detect a light intensity in various applications including biomedical and electronic circuits. These characteristics show that the proposed device is a promising candidate to be used in stretchable, flexible, transparent, and low cost electronic applications for the sensing of blue and UV light up to 400 lux.
Fig. 7 Optical property of the active layer (the graphene/perylene film) analyzed through UV/Vis Spectroscopy, (a) absorbance of PDMS and ITO, (b) transmittance of PDMS and ITO, (c) Absorbance of the device, and (d) transmittance of the device.
4. Conclusion
A stretchable photo sensor consisted of photoconductive layer of perylene/graphene (66 nm) and ITO electrodes fabricated on uniform ridged PDMS substrate though electro-hydrodynamic (EHD) technique has been presented. The proposed device has shown stable behavior under 25% axial strain. Futhermore, the it has stretchability up to 50% axial strain without electrical and mechanical breakdown. The resistance of the proposed photo detector has varied from 108 MΩ to 87 MΩ against light intensity of 0 ~400 lux, respectively. The maximum detectable light intensity by the sensor is 400 lux and its detectable range is from 465 nm to 535 nm in the visible spectrum. The device has shown bendability down to 0 mm for more than 1000 endurance cycles. In the result of these characterizations, the proposed photo sensor is good candidate for the stretchable, flexible, and transparent electronic applications.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF- 2013R1A1A4A01011554).
References and links
1. J. A. Rogers, T. Someya, and Y. Huang, “Materials and mechanics for stretchable electronics,” Science 327(5973), 1603–1607 (2010). [CrossRef] [PubMed]
2. T. Sekitani and T. Someya, “Stretchable, large-area organic electronics,” Adv. Mater. 22(20), 2228–2246 (2010). [CrossRef] [PubMed]
3. D. H. Kim, N. Lu, R. Ma, Y. S. Kim, R. H. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J. Yu, T. I. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H. J. Chung, H. Keum, M. McCormick, P. Liu, Y. W. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, and J. A. Rogers, “Epidermal electronics,” Science 333(6044), 838–843 (2011). [CrossRef] [PubMed]
4. R. H. Kim, D. H. Kim, J. Xiao, B. H. Kim, S. I. Park, B. Panilaitis, R. Ghaffari, J. Yao, M. Li, Z. Liu, V. Malyarchuk, D. G. Kim, A. P. Le, R. G. Nuzzo, D. L. Kaplan, F. G. Omenetto, Y. Huang, Z. Kang, and J. A. Rogers, “Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics,” Nat. Mater. 9(11), 929–937 (2010). [CrossRef] [PubMed]
5. M. L. Hammock, A. Chortos, B. C. K. Tee, J. B. H. Tok, and Z. Bao, “25th Anniversary Article: The evolution of electronic skin (e-skin): a brief history, design considerations, and recent progress,” Adv. Mater. 25(42), 5997–6038 (2013). [CrossRef] [PubMed]
6. S. H. Chae, W. J. Yu, J. J. Bae, D. L. Duong, D. Perello, H. Y. Jeong, Q. H. Ta, T. H. Ly, Q. A. Vu, M. Yun, X. Duan, and Y. H. Lee, “Transferred wrinkled Al2O3 for highly stretchable and transparent graphene-carbon nanotube transistors,” Nat. Mater. 12(5), 403–409 (2013). [CrossRef] [PubMed]
7. S. Ali, J. Bae, and C. H. Lee, “Design of versatile printed organic resistor based on resistivity (ρ) control,” Appl. Phys., A Mater. Sci. Process. 119(4), 1499–1506 (2015). [CrossRef]
8. M. S. White, M. Kaltenbrunner, E. D. Glowacki, K. Gutnichenko, G. Kettlgruber, I. Graz, S. Aazou, C. Ulbricht, D. A. M. Egbe, M. C. Miron, Z. Major, M. C. Scharber, T. Sekitani, T. Someya, S. Bauer, and N. S. Sariciftci, “Ultrathin, highly flexible and stretchable PLEDs,” Nat. Photonics 7(10), 811–816 (2013). [CrossRef]
9. T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, and T. Sakurai, “A large-area, flexible pressure sensor matrix with organic field-effect transistors for artificial skin applications,” Proc. Natl. Acad. Sci. U.S.A. 101(27), 9966–9970 (2004). [CrossRef] [PubMed]
10. D. J. Lipomi, B. C. K. Tee, M. Vosgueritchian, and Z. Bao, “Stretchable organic solar cells,” Adv. Mater. 23(15), 1771–1775 (2011). [CrossRef] [PubMed]
11. A. M. Gaikwad, A. M. Zamarayeva, J. Rousseau, H. Chu, I. Derin, and D. A. Steingart, “Highly stretchable alkaline batteries based on an embedded conductive fabric,” Adv. Mater. 24(37), 5071–5076 (2012). [CrossRef] [PubMed]
12. R. Guo, Y. Yu, Z. Xie, X. Liu, X. Zhou, Y. Gao, Z. Liu, F. Zhou, Y. Yang, and Z. Zheng, “Matrix-assisted catalytic printing for the fabrication of multiscale, flexible, foldable, and stretchable metal conductors,” Adv. Mater. 25(24), 3343–3350 (2013). [CrossRef] [PubMed]
13. S.-K. Chae, J.-H. Ryoo, and S.-H. Lee, “Thin and large free-standing PDMS membrane by using polystyrene petri dish,” Biochip J. 6(2), 184–190 (2012). [CrossRef]
14. E.-J. Lee, D.-H. Baek, J.-Y. Baek, B.-J. Kim, J. Choi, J. J. Pak, and S.-H. Lee, “PDMS- and silver-ball-based flexible multichannel surface electrode: fabrication and application in nerve conduction study on patients with diabetic polyneuropathy,” IEEE Sens. J. 9(6), 625–632 (2009). [CrossRef]
15. J. Jeong, S. Kim, J. Cho, and Y. Hong, “Stable stretchable silver electrode directly deposited on wavy elastomeric substrate,” IEEE Electron Device Lett. 30(12), 1284–1286 (2009). [CrossRef]
16. Y. Berdichevsky, J. Khandurina, A. Guttman, and Y.-H. Lo, “UV/ozone modification of poly(dimethylsiloxane) microfluidic channel,” Sens. Actuators B Chem. 97(2–3), 402–408 (2004). [CrossRef]
17. J. A. Vickers, M. M. Caulum, and C. S. Henry, “Generation of hydrophilic poly(dimethylsiloxane) for high-performance microchip electrophoresis,” Anal. Chem. 78(21), 7446–7452 (2006). [CrossRef] [PubMed]
18. D. B. H. Chua, H. T. Ng, and S. F. Y. Li, “Spontaneous formation of complex and ordered structures on oxygen-plasma-treated elastomeric polydimethylsiloxane,” Appl. Phys. Lett. 76(6), 721–723 (2000). [CrossRef]
19. P. Mandlik, S. P. Lacour, J. W. Li, S. Y. Chou, and S. Wagner, “Fully elastic interconnects on nanopatterned elastomeric substrates,” IEEE Electron Device Lett. 27(8), 650–652 (2006). [CrossRef]
20. A. P. Robinson, I. Minev, I. M. Graz, and S. P. Lacour, “Microstructured silicone substrate for printable and stretchable metallic films,” Langmuir 27(8), 4279–4284 (2011). [CrossRef] [PubMed]
21. G. Pace, A. Grimoldi, D. Natali, M. Sampietro, J. E. Coughlin, G. C. Bazan, and M. Caironi, “All-organic and fully-printed semitransparent photodetectors based on narrow bandgap conjugated molecules,” Adv. Mater. 26(39), 6773–6777 (2014). [CrossRef] [PubMed]
22. K. Liu, M. Sakurai, and M. Aono, “ZnO-based ultraviolet photodetectors,” Sensors (Basel) 10(9), 8604–8634 (2010). [CrossRef] [PubMed]
23. C. K. Renshaw, X. Xu, and S. R. Forrest, “A monolithically integrated organic photodetector and thin film transistor,” Org. Electron. 11(1), 175–178 (2010). [CrossRef]
24. A. A. Hussain, A. R. Pal, and D. S. Patil, “High photosensitivity with enhanced photoelectrical contribution in hybrid nanocomposite flexible UV photodetector,” Org. Electron. 15(9), 2107–2115 (2014). [CrossRef]
25. L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, and J. D. MacKenzie, “Self-organized discotic liquid crystals for high-efficiency organic photovoltaics,” Science 293(5532), 1119–1122 (2001). [CrossRef] [PubMed]
26. E. R. Draper, J. J. Walsh, T. O. McDonald, M. A. Zwijnenburg, P. J. Cameron, A. J. Cowan, and D. J. Adams, “Air-stable photoconductive films formed from perylene bisimide gelators,” J. Mater. Chem. C Mater. Opt. Electron. Devices 2(28), 5570–5575 (2014). [CrossRef]
27. H. Niu, C. Wang, X. D. Bai, and Y. Hhuang, “High photoconductivity properties of perylene polyimide containing triarylamine unit,” J. Mater. Sci. Lett. 39(12), 4053–4056 (2004). [CrossRef]
28. K. Kim, J.-Y. Choi, T. Kim, S. H. Cho, and H. J. Chung, “A role for graphene in silicon-based semiconductor devices,” Nature 479(7373), 338–344 (2011). [CrossRef] [PubMed]
29. Z. Fang, Y. Wang, A. E. Schlather, Z. Liu, P. M. Ajayan, F. J. G. de Abajo, P. Nordlander, X. Zhu, and N. J. Halas, “Active tunable absorption enhancement with graphene nanodisk arrays,” Nano Lett. 14(1), 299–304 (2014). [CrossRef] [PubMed]
30. Z. Li, R. Ye, R. Feng, Y. Kang, X. Zhu, J. M. Tour, and Z. Fang, “Graphene quantum dots doping of MoS2 monolayers,” Adv. Mater. 27(35), 5235–5240 (2015). [CrossRef] [PubMed]
31. Y. Kang, S. Najmaei, Z. Liu, Y. Bao, Y. Wang, X. Zhu, N. J. Halas, P. Nordlander, P. M. Ajayan, J. Lou, and Z. Fang, “Plasmonic hot electron induced structural phase transition in a MoS2 monolayer,” Adv. Mater. 26(37), 6467–6471 (2014). [CrossRef] [PubMed]
32. I. Hayati, A. I. Bailey, and T. F. Tadros, “Mechanism of stable jet formation in electrohydro-dynamic atomization,” Nature 319(6048), 41–43 (1986). [CrossRef]
33. S. Ali, J. Bae, K. H. Choi, C. H. Lee, Y. H. Doh, S. Shin, and N. P. Kobayashi, “Organic non-volatile memory cell based on resistive elements through electro-hydrodynamic technique,” Org. Electron. 17(1), 121–128 (2015). [CrossRef]
34. S. Ali, J. Bae, K. H. Choi, C. H. Lee, and Y. H. Doh, “Flexible and passive photo sensor based on perylene/graphene composite,” Sens. Actuators B Chem. 220(1), 634–640 (2015). [CrossRef]
35. S. H. Oh, B. G. Kim, S. J. Yun, M. Maheswara, K. Kim, and J. Y. Do, “The synthesis of symmetric and asymmetric perylene derivatives and their optical properties,” Dyes Pigments 85(1–2), 37–42 (2010). [CrossRef]
