Aloe vera gel as natural organic dielectric in electronic ... - Springer Link

J Mater Sci: Mater Electron (2013) 24:2646–2652 DOI 10.1007/s10854-013-1151-0

Aloe vera gel as natural organic dielectric in electronic application Li Qian Khor • Kuan Yew Cheong

Received: 4 January 2013 / Accepted: 20 February 2013 / Published online: 27 February 2013 Ó Springer Science+Business Media New York 2013

Abstract Aloe vera gel as an environmentally safe and natural material as an organic dielectric layer used in electronic application has been systematically investigated in this work. The commercially purchased gel was deposited on glass substrate by screen printing technique. Effect of drying temperature and duration on the quality of the Aloe vera layer had been examined. The lowest leakage current density was obtained in sample dried at 40 °C for 30 min but electrical breakdown voltage of the sample had reduced as the drying duration was extended more than 40 min. In addition, effect of successive applying the Aloe vera layer and distance between two electrodes on the leakage current of the dielectric had been reported. It was found that single layered Aloe vera had the lowest leakage current density but there was no significant effect of the distance between two electrodes on the leakage current of the dielectric. The produced natural Aloe vera gel after being dried was having a dielectric constant of 3.39 and therefore it had been demonstrated that this material is a potential candidate to be used as a dielectric material in an organic-based electronic device.

1 Introduction Organic materials have been used to fabricate active and passive region of organic electronic devices and are growing exponentially since 1974 [1]. Organic materials

L. Q. Khor K. Y. Cheong (&) Electronic Materials Research Group, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia e-mail: [email protected]

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have been widely used because of their excellent properties, namely lightweight [2], low temperature processing condition [3, 4], flexibility of substrate [5, 6], low production cost for large scale fabrication [7], easy processing technique [8]. With these advantages, organic materials have demonstrated their capability to be used in organic thin film transistor (OFET) [9], organic light-emitting device [10], organic solar cell [11], photovoltaic cell [12], sensor, and memory device [13]. Not only semiconductor layer, as an active region, can be replaced by organic material but organic dielectric, as a passive region, have been demonstrated as a good insulating layer in organic device, such as OFET [14]. Several organic materials have been investigated as the dielectric layer, namely poly(4-vinyl phenol) (PVP) [15], cyanoethylated pullulan (CP) [6, 16], polystyrene (PS) [17], polyimide (PI) [18], benzocyclobutene (BCB), poly(vinly alcohol) (PVA) [19], parylene, poly(methylmethacrylate) (PMMA) [4], Cytop [20, 21]. The device performance may be influenced by using different type of organic dielectric. Therefore, selection of the dielectric has become one of the important criteria when an organic electronic device is being designed [22]. In an OFET, the organic dielectric should provide a low leakage current (\10-3 A/m2) [23] with a high electric breakdown field [24, 25]. Most of the organic dielectrics are based on synthetic polymeric materials but not much study has conducted on natural organic material incorporated in the fabrication of a green electronic device. Chang et al. [26] used chicken albumen as a dielectric layer in OFET and it revealed a higher dielectric constant than most of the synthetic polymeric dielectrics, making it a potential organic dielectric layer for this application. In addition, tetratetracontane presents in some medicinal plants have also been investigated to be used as a natural organic dielectric material

J Mater Sci: Mater Electron (2013) 24:2646–2652

[27]. Besides, dielectric properties of fresh leaves of ficus benghalensis, ficus elastica, ficus religiosa, morus nigra, hibiscus rosa-sinensis, gossypium hirsutum [28], maiza leaf [29], apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato tissues [30] were suggested and reported briefly. Dielectric properties of poplar and monster delicious leaves have also been reported by Helhel et al. [31]. This shows the potential of utilizing natural organic material as the dielectric material in organic electronic application. As an alternative, this work reports on the usage of Aloe vera as a natural organic dielectric layer as the gate insulator in an OFET. Effects of drying temperature and time on the dielectric properties of this material have been investigated. In order to compare the dielectric properties of Aloe vera, five different types of synthetic polymeric dielectrics have also been used; namely poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) (PVA), poly(4-vinylphenol) (PVP), cyanoethyl pullulan (CP) and cyanoethyl polyvinylalcohol (CPVA).

2 Experimental (materials and methods) In this work, commercial purchased Aloe vera gel (CAV) (Fruit of the Earth), poly(methyl methacrylate) (PMMA) (Sigma Aldrich), poly(vinyl alcohol) (PVA) (Merck), poly (4-vinylphenol) (PVP) (Fluka), cyanoethyl pullulan (CP) and cyanoethyl polyvinylalcohol (CPVA) (Shin-Etsu Chemical) had been used without any purification. First, a 10 mm 9 10 mm glass slide was washed by ethanol and de-ionized water in ultrasonic bath and dried in an oven at 100 °C for an hour. Then, aluminum layer was evaporated onto the glass substrate by a thermal evaporator (AUTO 306). Next, organic dielectrics were independently deposited on the glass substrate. Before the deposition, the organic dielectrics were treated as follow. PMMA (10 wt%) was dissolved in toluene and stirred at room temperature. PVP (10 wt%) was dissolved in ethanol, PVA (10 wt%) was dissolved in de-ionized water, CPVA (10 wt%) and CP (10 wt%) were dissolved in acetone. After the entire organic dielectrics had undergone a stirring process, all of the solutions were filter by a filter paper. Screen printing technique was used to deposit the dielectric layer on the aluminum layer. Thin film mask was used so that the thickness of the printed organic dielectric layer can be uniform. During screen printing, suitable amount of organic dielectric material was dropped on one side of the mask, follow by using a glass slide swept the material over on the surface of the mask. The organic dielectric layer on the Al layer was dried at room temperature for 30 min and the mask was removed. The organic dielectric samples were dried at room temperature for another 24 h. For screen printed Aloe vera (with thickness 245 ± 8 nm) on aluminum, effects of drying temperature and time on the dielectric properties have

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been investigated. The samples were dried at 30, 40, 50, 60 and 80 °C for 30 min in an oven. While another set of investigation was performed by fixing the drying temperature at 40 °C but changing the drying time for 20, 30, 40, 50, 60 min in the same oven. With the optimum condition, effect of distance between two electrodes (0.74–6.97 mm) separating the Aloe vera layer and number of Aloe vera layer (1–5 layers) being applied had been investigated. Ultraviolet–visible spectroscopy (UV–Vis) (Perkin Elmer) was used to characterize the transparency of the samples. Field-emission scanning electron microscope (FESEM) (ZEISS SUPRA 35VP) was utilized to measure the thickness of the samples and to analyze the surface morphology of the samples. The surface topography as well as the root mean square (RMS) surface roughness of the organic samples was examined by a non-contact mode atomic force microscope (AFM) (Nano Navi SPI3800 N). Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer) was utilized to obtain the chemical functional group of the samples. The burn off temperature of Aloe Vera gel was measured with Thermogravimetric analysis (TGA) (Mettler Toledo) at a ramping rate of 10 °C/min and temperatures from 25 to 200 °C. In order to investigate the dielectric properties of the samples, capacitor-type of test structure with another Al layer being thermally evaporated on the organic dielectrics was fabricated. The test structure is shown in Fig. 1. A semiconductor parameter analyzer (Agilent 4156C) was used to obtain the current–voltage (I–V) characteristics of the samples. Dielectric test fixture (Agilent HP 16451B) was used to obtain the dielectric constant of the samples.

3 Results and discussion Figure 2 reveals the FTIR spectra for blank glass, synthetic polymers and Aloe vera gel screen printed on glass substrate.

Fig. 1 Schematic of the cross-sectional view of the sample

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Aloe vera gel (labeled as CAV) on glass substrate shows obvious peaks related to OH group (3,130–3,500 cm-1) and C–H bond (2,930 cm-1), which is originated from polysaccharide structure [32]. If compared with the synthetic polymers made of polysaccharide, such as CPVA and CP, [33] similar peaks can be detected. Polysaccharide is a form of polymeric structure that has captured high interest from researchers due to it promising dielectric properties [34–40]. Polysaccharide in Aloe vera gel is rich in mannose (62.9 %) [41]. Mannose is having sigma-bonding electrons with low mobility that confine in single chain bonds and shows insulating property [42]. Figure 3 presents the thermal gravimetry analysis result of Aloe vera gel. It is shown that a significant burn-off happens at 110 °C. Therefore, the drying process of the Aloe vera gel is limited to 80 °C in this study. Typical UV–Vis transmission spectra of Aloe vera gel and the investigated synthetic polymers are shown in Fig. 4. Monitoring the entire visible-light wavelength range (350–700 nm), transmission is approximately 80 % for sample with Aloe vera gel on glass substrate and it is slightly lower than others except for PVA. Figure 5 shows typical FESEM micrographs of surface of the investigated samples. The surface morphology of Aloe vera sample with and without drying process are similar to other synthetic polymer samples, in which no obvious defects being detected. Insets of Fig. 5a, b show the cross-sectional view of as deposited Aloe vera gel on glass substrate and dried Aloe vera on glass substrate, respectively. Thickness of the Aloe vera layer has decreased from 240.1 to 201.3 nm after being dried at 40 °C for 30 min. The root mean square (RMS) roughness of Aloe vera has been decreased from 0.5222 to 0.5182 nm after dried at 40 °C for 30 min (Fig. 6); smooth surface can enhance its dielectric properties. Previous studies showed that the decreasing of RMS roughness enhances the dielectric properties by decreasing the leakage current density, i.e. as the roughened dielectric can generate interface traps, influence the carrier mobility and degrade the dielectric properties [43, 44]. The detail of the leakage current density on samples with and without dry at 40 °C for 30 min will be discussed at below. Current–voltage (I–V) measurement at room temperature by sweeping gate voltage from -10 to 10 V on the test structures has been conducted. After the measurement, I is transformed to current density (J) by dividing the current with area of gate electrode (A = 2 9 10-7 m2). Typical current density–voltage (J–V) characteristics of test structure with Aloe vera as gate insulator that had dried for 30–80 °C for 30 min are presented in Fig. 7. From the J–V measurement, sample with Aloe vera gel dried at 40 °C shows the lower leakage current density (2 9 10-4 A/m2) if compared with samples dried at other temperatures. It is believed that at 30 °C, polysaccharide in the Aloe vera has not been fully cross-linked; whereas at temperature

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Fig. 2 A comparison of FTIR spectra for Aloe vera gel on glass and polymers

Fig. 3 TGA thermogram for Aloe vera gel

Fig. 4 UV–Vis of Aloe vera gel sample

higher than 40 °C, polysaccharide structure may be gradually destroy [45] and consequently reduces its insulating efficiency.


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Fig. 5 Typical FESEM micrographs of a Aloe vera without heat treatment, b Aloe vera heated at 40 °C for 30 min (the inserts show the thickness of the Aloe vera layer), c PMMA, d PVP, e PVA, f CP, and g CPVA

Figure 8 demonstrates the J–V characteristics of test structures with Aloe vera dried at different duration with a fixed dry temperature (40 °C). It is notices that decreasing

of leakage current density is revealed for samples dried for 0 to 40 min. But, samples dried for 50 and 60 min have breakdown voltage at 1.9 and 4.8 V with leakage current

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Fig. 6 Typical AFM topography of Aloe vera/Al/Glass structure. a Sample without heat treatment. b Sample heated at 40 °C for 30 min Fig. 8 Current density–voltage characteristics of Al/Aloe vera/Al structure heated at 40 °C for different duration

Fig. 7 Current density–voltage characteristics of Al/Aloe vera/Al structure heated at different temperatures for 30 min 1

-3

2

density 2.48 9 10 and 2.69 9 10 A/m , respectively. As the drying duration extended beyond 40 min, aloe’s polysaccharides might be destroy [45]. The lowest leakage current density is obtained for samples dried for 30 min. If compared with other test structures with synthetic polymeric gate insulators, dried Aloe vera at 40 °C for 30 min has demonstrated the lowest leakage current density (Fig. 9), which fulfill the requirement for stable operation, leakage current density \10-3 A/m2 [23]. In addition, dielectric constant (k) of the Aloe vera sample dried at 40 °C for 30 min and other synthetic polymeric samples were calculated by measuring the capacitance (C) of the samples using Eq. (1), C ¼ k e0 A=t

ð1Þ

Where e0 is permittivity of free space, A is area of gate and t is thickness of dielectric [46]. The calculated k values are showed in Table 1, in which k value of the Aloe vera is 3.39. With low leakage current density and reasonably good k value,

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Fig. 9 Current density–voltage characteristics of Al/Aloe vera/Al structure heated at 40 °C, Al/PMMA/Al structure, Al/PVA/Al structure, Al/PVP/Al structure, Al/CP/Al structure and Al/CPVA/Al structure

Aloe vera gel can be considered as a potential candidate for dielectric layer applied in organic electronic devices. Figure 10 shows the effect of distance between two metal electrodes on the leakage current density of the dried Aloe vera (40 °C for 30 min). It is clearly shown that no significant differences of those samples. In addition, effect of thickness of dried Aloe vera with respect to the number of layer successively screen printed on the leakage current density is presented in Fig. 11. It is revealed that sample with only one layer of dried Aloe vera has demonstrated the lowest leakage current density if compared with those test structures with 2, 3 and 5 layers. It is believed that the polysaccharides in between Aloe vera layers have not been fully cross-linked; lead to porosity appears inside the layer. Porosity is the major factor affecting the leakage current density [47, 48].

J Mater Sci: Mater Electron (2013) 24:2646–2652 Table 1 Dielectric constant for polymer samples and Aloe vera sample Sample

Dielectric constant (k)

Aloe vera heated at 40 °C for 30 min

3.39

PMMA

3.45

PVA

1.99

PVP

2.18

CRM

4.85

CRS

4.70

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synthetic polymer usually being used as dielectric layer. Commercially purchased Aloe vera gel had been screenprinted on glass substrate for structural and electrical studied. The leakage current density could be affected by the drying temperature, duration and thickness of Aloe vera layer. Overall, the sample heated at 40 °C for 30 min with one screen printed layer had displayed the best dielectric properties with dielectric constant of 3.39. Acknowledgments One of the authors (L.Q.K.) would like to acknowledge the financial support given by the Ministry of Higher Education Malaysia under the MyMaster scholarship program and Shin-Etsu Chemical, Japan, for providing the cyanoethyl pullulan (CP) and cyanoethyl polyvinylalcohol (CPVA) for this research. The authors would also like to thank the Research Grant provided by Universiti Sains Malaysia through RU-PRGS (Grant No. 8035003) and USM Short Term Grant (Grant No. 60311034).

References

Fig. 10 Current density–voltage characteristics of Al/Aloe vera/Al structure on glass substrate with difference distance between two electrodes

Fig. 11 Current density–voltage characteristics of Al/Aloe vera/Al structure on glass substrate with difference layer of Aloe vera been screen printed

4 Conclusions This work had utilized Aloe vera gel as an organic dielectric material used in electronic application. The characterization showed the Aloe vera gel contained polysaccharide that alike

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