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Air-stable gelatin composite memory devices on a paper substrate
Accepted Manuscript Air-Stable Gelatin Composite Memory Devices on a Paper Substrate
Yu-Chi Chang, Cheng-Jung Lee, Li-Wen Wang, Yeong-Her Wang PII:
S1566-1199(18)30582-2
DOI:
10.1016/j.orgel.2018.11.012
Reference:
ORGELE 4975
To appear in:
Organic Electronics
Received Date:
20 April 2018
Accepted Date:
06 November 2018
Please cite this article as: Yu-Chi Chang, Cheng-Jung Lee, Li-Wen Wang, Yeong-Her Wang, AirStable Gelatin Composite Memory Devices on a Paper Substrate, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.11.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Air-Stable Gelatin Composite Memory Devices on a Paper Substrate Yu-Chi Chang a, Cheng-Jung Lee a, Li-Wen Wang a, and Yeong-Her Wang a a
Institute of Microelectronics, Department of Electrical Engineering,
National Cheng-Kung University, Tainan, 701 Taiwan. Abstract “Green’’ electronics represents an emerging area of research aimed at identifying natural materials that are applicable for environmentally safe and biodegradable devices. Iron (Fe) ions are used in gelatin matrixes (gelatin composites) prepared on commercially available flexible paper substrates through the solution method, thus, the “green” electronics were achieved. Although atomic force microscopy images show that fiber-based paper substrates result in rough Al/paper structure surfaces, the uniform interface between top Al electrodes and gelatin composites can be clearly observed in transmission electron microscopy images. This might be due to the gelatin composite acts as not only the resistive layer, but also the smoothing layer. Moreover, Fe ions play an important role in assisting redox reaction or filament formation. An Al/gelatin/Al/paper device has no resistive switching behavior. Meanwhile, devices with Fe ions on a paper substrate show reproducible resistive switching for 18 cycles, an ON/OFF ratio of over 105, and an excellent current distribution (coefficient of
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variation<60%). The ON/OFF ratio of the device after bending 180 cycles can be maintained at 104. The biodegradable gelatin materials have good ductility and potential in recyclable and green electronic applications.
Keywords: gelatin, green, memory, paper, resistive switching, solution *E-mail: [email protected]
I.
Introduction
Flexible memory devices are worth investigating because they can satisfy the requirements of wearable electronics and bendable displays [1, 2]. Conventionally, plastic substrates, such as polyethylene terephthalate, polyethylene naphthalate, polycarbonate, and polyimide, have been widely used as a technology for flexible electronic products. However, decomposing these plastics decreases the availability of transformed energy for future use along with the production–consumption chain and poses a growing ecological problem [3]. In order to reduce electronic wastes and achieve sustainability in the electronics industry, the natural materials from nature is an important property to successfully enter the future market. Paper, which can be easily recycled [4], is commonly-used in daily life and a
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potential role in the future of “green” electronics. “Green” electronics are applicable in biodegradable and environment-friendly devices. Recently, the paper substrates have been used to develop the degradable material-based memory devices [5]. However, the green memory device still remain limited, due to the biodegradable material that simultaneously shows good memory property and enough flexibility is still a challenge. Gelatin has a high degree of plasticity and is easily absorbed by some organisms. Also, gelatin is suitable for usage in wearable electronic devices due to its skin-friendly. In addition, gelatin solution can be used without any further purification as well as the gelatin thin film can be stored under atmosphere for a period of time [6]. The stable gelatin film facilitates the air-stable memory devices [7]. Metal ions in a gelatin matrix play an important role in improving the ON/OFF ratio of memory devices [8]. In addition, with an appropriate concentration of metal salts, metal ions can assist the formation of an interfacial AlOx layer and improve the memory properties [9]. A previous study shows that when top and bottom electrodes use the same metal, Al ions in gelatin matrix can assist the redox reaction or filament formation [10]. In this work, we presented an air-stable, biodegradable, eco-friendly electronics with an Al/gelatin composite/Al/paper structure. The gelatin composite fabricated by
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dissolving metal salts (ferric nitrate nonahydrate) in gelatin was presented. The ON/OFF ratios of the devices with Fe ions were found to display better performance than those of devices without Fe ions. The protein-based gelatin containing FeIII/FeII redox couples can help resistance switching properties as a consequence of the charge trap/release of FeIII/FeII pairs induced by externally applied voltage. The devices with Fe ions on a paper substrate show reproducible resistive switching for 18 cycles, an ON/OFF ratio of approximately 105, and a good current distribution (coefficient of variation<60%). The ON/OFF ratio of the device after bending 150 cycles maintained at 104. Because our devices are entirely made of biodegradable materials, they can break down into natural materials which is environmentally friendly. These results offer opportunities for achieving green electronics on a single chip of paper.
II. Experimental The inset image of Fig. 1(a) shows the photographic images of the paper substrate used in this study. A4 size inkjet papers having a thickness of 0.28 mm were obtained from Printerfun. To fabricate the devices, a large paper sheet was cut into 2 × 1.5 cm2 pieces to be used as substrates. The paper substrate was cleaned by dipping into
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isopropyl alcohol for 1 min. Followed by a drying process in oven at 100 °C to remove the moisture in the substrate. Al, the bottom electrode, was deposited on the paper substrate by using DC magnetron sputtering. Figure 1(a) shows the preparation of resistive switching layer. Gelatin powder and ferric nitrate nonahydrate powder were purchased from Kodak. Ferric nitrate nonahydrate powder was added to deionized (DI) water upon stirring until the ferric nitrate nonahydrate powder was completely dissolved. The preparation of gelatin solution is used the gelatin powder dissolved in DI water. The 5 mL of gelatin composite solution is prepared by mixing 2.5 mL of ferric nitrate nonahydrate solution with 2.5 mL of gelatin solution. The gelatin composite solution was then spun on the cleaned Al/paper substrate and baked at 60 °C under vacuum conditions. Finally, the Al top electrodes were deposited with 1 mm2 patterned shadow mask by using DC magnetron sputtering. The electrical properties were determined by using an Agilent B1500 semiconductor parameter analyzer. The surface morphology was evaluated by using atomic force microscopy (AFM; SPA 400, SII Nanotechnology Inc., Sunto-gun, Shizuoka, Japan).
III. Results and Discussion A cross-sectional high-resolution transmission electron microscopy (TEM) image
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of the Al/gelatin composite/Al/paper structure is shown in Fig. 1(b). The thicknesses of the Al, gelatin composite, and Al layers were approximately 200, 260, and 280 nm, respectively. The X-ray photoelectron spectroscopy (XPS) analyses were performed on a Perkin-Elmer PHI 5000 Versaprobe system. The FeG/Al/paper sample was used in XPS measurement. The XPS survey spectra of the FeG thin film obtained after 90 s of Ar+ ion sputtering, thereby representing the bulk layer of the FeG thin film. As shown in Fig. 2, the atomic percentages of Fe, N, C, and O were 10%, 7%, 43%, and 40%, respectively. The XPS signals corresponded to Fe2p3, O1s, C1s, and N1s. The C1s spectra were divided into three component peaks, namely, C–C at 285 eV, C–O or C-N at 286.2 eV, and C=O at 288 eV (includes a small C=N bond contribution at 288.0 eV) [11-15]. The O1s peaks at 532.3, 531.1 eV are associated with nitrate groups, C=O groups characteristic of gelatin and to metal-oxygen (529.5 eV) from the ferric nitrate nonahydrate support [16-18]. Due to these abundant oxygen functional groups, gelatin is hydrophilic in nature and easily dispersed in deionized (DI) water. The N1s peak was resolved into two peaks at 400 and 407 eV. The peak at around 400 eV is relevant to N-Fe compounds [19, 20]. The large amount of N associated with Fe, which suggests that N atoms are coordinated with Fe [21, 22]. The N 1s spectrum
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centered at around 407 eV can be ascribed to the nitrate group [23, 24]. The presence of nitrates is mainly due to a NO3− species [25]. Fe2p peak fitting was performed in accordance with the constraints of the Fe0 and Fe3+ components. The two oxidation states were characterized by the Fe0 and Fe3+ components at 711 and 726 eV, respectively. The TEM and EDS mapping images of the Al/gelatin composite/Al/paper structure are shown in Fig. 3(a). Although the fiber-based paper substrate resulted in a rough Al/paper structure surface, the uniform resistive layer with Fe elements were well dispersed into the gelatin matrix. The 3D AFM images in Figs. 3(b–d) reveal the surface morphologies of the paper substrate, Al/paper structure, and gelatin composite/Al/paper structure, respectively. The paper substrate and bottom Al electrode layer were highly rough, with root mean square roughness (Rrms) values of approximately 197 and 28 nm, respectively. When the bottom Al/paper structure was covered with the gelatin composite layer, the surface roughness was significantly reduced. As shown in Fig. 3(c), the Rrms of the gelatin composite thin film was approximately 14 nm. The gelatin composite layer acts not only as the resistive layer but also as the smoothing layer for the Al/paper structure. Figure 4 illustrates the switching I–V characteristics of the Al/gelatin composite/Al/paper device. The devices were reversibly switchable and showed typical
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bipolar resistive switching behavior. The current achieved the minimum at ∼1 V, which is associated with polarization due to the charge accumulation of ions at the electrode interfaces [26]. Under negative and positive bias, Fe2+/Fe3+ ions accumulated at the Al/gelatin composite and gelatin composite/Al interfaces, respectively. Quantities of multi-charged Fe2+/Fe3+ inside the gelatin composite film, serving as positively charged trap states, can capture electrons and then change into neutral Fe. Also, the captured electrons can escape from the trap states, resulting in a formation of Fe2+/Fe3+. At a set voltage of −3.5–−2.9 V, a reset voltage of 3.8–4.2 V and an ON/OFF ratio of >105 were observed. The Al/gelatin composite/Al/paper structure could maintain the ON/OFF ratio after 18 continuous DC voltage switching cycles. However, no switching behavior was observed for the Al/gelatin/Al/paper device, as shown in the inset in Fig. 4. These results indicate that a supply of Fe3+/Fe2+ ions from the gelatin composite can contribute to the resistance switching properties that were observed as a consequence of the charge trap/release that was induced by the externally applied voltage. This situation was similar to the ITO/ACG/ITO case [10]. To
better
understand
the
switching
mechanism
of
the
Al/gelatin
composite/Al/paper device as shown in Fig. 5(a), we plotted the logarithmic plots of the
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I–V curve for the positive and negative voltage sweep regions. The log I−log V curve in the high resistance state (HRS) at low voltage exhibited a linear region with a slope of approximately 1 and implied the ohmic conduction behavior. In a high-voltage region, space-charge limited conduction can be characterized using the square law [27, 28]. In the current study, when all of the available traps were filled, the current density abruptly increased with an I–V slope of 8. In the positive voltage region, the current states maintained the low resistance state (LRS) and showed an ohmic conduction behavior with a slope of approximately 1, which manifests the formation of many conductive filaments [29]. The Fe ions act as traps to capture/release charges [30]. As the charge carriers trapped by the Fe ions in gelatin are able to build an internal electrical field, similar to BPQD in the PMMA-based memory device [31]. The strong internal electric field built from the trapped charge carriers may facilitate the formation of filaments. Such strong electric field increases the injecting efficiency of the carriers into the traps in the set and reset process, respectively. When the magnitude of applied voltage exceeds Vset, the oxidized Fe ions could be reduced. The device is converted into the LRS. Conversely, after applying a sufficiently positive voltage Vreset, both electrons and oxygen will oxidize Fe elements near the top Al electrode, the device is turned to the HRS. For the
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HRS, injected electrons are captured by traps (or Fe ions) in the gelatin composite film and these behave as space charges. Figure 5(b) plots the scaling trend of the LRS current versus the cell area of the Al/gelatin composite/Al/paper device. The LRS current is mainly a filamentary conduction current and thus only slightly depends on the cell area [32]. Figures 5(c) and (d) present the statistical distribution parameters. Both the LRS and the HRS currents are measured at 0.1 V. The coefficient of variation (CV), which is defined as the ratio of the standard of deviation to the average value, is used for distribution evaluation. The CVs of HRS and LRS currents in the Al/gelatin composite/Al/paper devices were 59% and 46%, respectively. VSet and VReset distributed at the ranges of −4.8–−2 and 3.1–4.6 V, respectively. The wide distributions of set and reset voltages are closely related to the formation and rupture of conductive filaments. The filamentary switching in general exhibits a natural fluctuation, which leads to the non-uniformity of resistive switching parameters [33-35]. The average value of VSet and VReset were −3.2 and 3.9 V, respectively. We fabricated 40 samples via the same preparation process and observed that >80% (32 samples) showed similar resistive switching behaviors. To study the flexibility of the Al/gelatin composite/Al/paper device, we bent the
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device into compression and tension states. The h/d ratio at both states was 1/2. The bipolar resistive switching behaviors of the Al/gelatin composite/Al/paper device at compression and tension states are shown in Fig. 6. When the compressive or tensile strain is applied, the captured electrons can obtain mechanical energy and then escape from traps [36-38]. The ability of capture electrons will be subjected to influence on the mechanical stimuli of compressive/tensile. Because of the compressive and tensile strains can modulate the height of the trap barrier. Compared to compressive strains, the trap barrier height under tensile strains is deeper and more difficult for electrons to emit from one trap and then hop to another [39]. Therefore, tensile strain leads to larger Vset. Both for the compressive and tensile strains, the smaller Vreset than the Vset is due to the rupture of conductive filaments is partial and near the top Al electrode. The ON/OFF ratio was not degraded at both the compression and tension states, implying the flexibility of the Al/gelatin composite/Al/paper device. Figure 7(a) illustrates the flexibility test of Al/gelatin composite/Al/paper device. The ON/OFF ratio was decreased after bending 100 cycles. As shown in the inset of Fig. 7(a), TEM cross-sectional image observation of the fiber paper sheet deformation during bending 100 cycles. Bottom electrode is prone to damage, thus, the device flaked off easily.
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The device had a maintained ON/OFF ratio of >103 after repeated bending for 180 cycles and an extreme bending radius of 8 mm. This finding suggests that the excellent mechanical flexibility of the Al/gelatin composite/Al/paper device is attributed to the gelatin composite resistive layer ductility. Figure 7(b) shows the ON/OFF ratio as a function of ageing time for the Al/gelatin composite/Al/paper devices under the atmosphere. The ON/OFF ratio did not significantly change when the device was placed under atmospheric environment for 7 weeks but was approximately 104 when the device was placed under atmospheric environment for >8 weeks. The oxidation of Al electrode or Fe, which was induced through oxygen diffusion within the gelatin matrix, is hypothesized to play a role. Notably, the devices were measured and stored in ambient conditions, that is, at room temperature of ~23 °C and a relative humidity of ~80%, for the whole investigation period, thus lending weight to the performance of the memory in air.
IV. Conclusions Eco-friendly electronics were demonstrated with Al/gelatin composite/Al/paper structure. The gelatin composite acts as the smoothing layer for the Al/paper structure, thereby, forming uniform interface between the top Al electrode and gelatin composite.
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The gelatin composite memory device showed reproducible bipolar resistive switching with an ON/OFF ratio of >105, and uniform current distribution. This can be attributed to the protein-based gelatin containing FeIII/FeII redox couples, which can help resistance switching properties. The Al/gelatin composite/Al/paper device also has good flexibility and air stability. The ON/OFF ratio was not degraded after repeated bending for 180 cycles and extreme bending radius of 6 mm at the compression state. The ON/OFF ratio of ~105 did not significantly change when the device was placed under atmospheric environment for 7 weeks. These demonstrations of high-performance bioelectronics on paper substrate indicated that truly “green” electronics is potential. This work aims to create paths for the production of human- and environment-friendly electronics.
Acknowledgements This work was supported in part by the National Science Council of Taiwan under Contracts NSC102-2221-E-006-182-MY3 and NSC 105-2221-E-006-193-MY3.
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Figure
Figure 1 (a) Preparation of gelatin composite thin film. The inset image shows the photographic images of the paper substrate. (b) TEM image of the Al/gelatin composite/Al/paper structure.
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Figure 2 C1s, O1s, N1s, and Fe2p3 peaks from the XPS spectra.
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Figure 3 AFM image of the (a) paper, (b) Al/paper, and (c) gelatin composite/Al/paper structure. (d) TEM image of the Al/gelatin composite/Al/paper structure. EDS mapping analysis result for the Al/gelatin composite/Al/paper structure.
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Figure 4 Switching cycles of the Al/gelatin composite/Al/paper device. The inset shows the I-V curve of the Al/gelatin/Al/paper device.
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Figure 5 (a) Log I–log V curve for the positive and negative voltage regions. (b) LRS current versus cell area of the Al/gelatin composite/Al/paper device. (c) Voltage and (d) current statistical distributions of the Al/gelatin composite/Al/paper device.
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Figure 6 Typical bipolar resistive switching characteristics of the Al/gelatin composite/Al/paper device at compression and tension state. The inset images show the photograph of the device at compression and tension states.
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Figure 7 (a) Continuous bending effect of the Al/gelatin composite/Al/paper device. The inset shows the TEM cross-sectional image of the Al/gelatin composite/Al/paper structure after bending 100 cycles. (b) ON/OFF ratio as a function of ageing time for the Al/gelatin composite/Al/paper devices under the atmosphere.
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Highlight 1. An
air-stable
and
green
electronic
with
an
Al/gelatin
composite/Al/paper structure has been demonstrated. 2. The Al/gelatin composite/Al/paper structure showed forming-free, an ON/OFF ratio of over 105, and uniform current distribution. Moreover, the ON/OFF ratio of the device after bending 150 cycles can be maintained at 104. 3. A supply of Fe3+/Fe2+ ions from the gelatin composite can contribute to the resistance switching properties.
Yu-Chi Chang, Cheng-Jung Lee, Li-Wen Wang, Yeong-Her Wang PII:
S1566-1199(18)30582-2
DOI:
10.1016/j.orgel.2018.11.012
Reference:
ORGELE 4975
To appear in:
Organic Electronics
Received Date:
20 April 2018
Accepted Date:
06 November 2018
Please cite this article as: Yu-Chi Chang, Cheng-Jung Lee, Li-Wen Wang, Yeong-Her Wang, AirStable Gelatin Composite Memory Devices on a Paper Substrate, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.11.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Air-Stable Gelatin Composite Memory Devices on a Paper Substrate Yu-Chi Chang a, Cheng-Jung Lee a, Li-Wen Wang a, and Yeong-Her Wang a a
Institute of Microelectronics, Department of Electrical Engineering,
National Cheng-Kung University, Tainan, 701 Taiwan. Abstract “Green’’ electronics represents an emerging area of research aimed at identifying natural materials that are applicable for environmentally safe and biodegradable devices. Iron (Fe) ions are used in gelatin matrixes (gelatin composites) prepared on commercially available flexible paper substrates through the solution method, thus, the “green” electronics were achieved. Although atomic force microscopy images show that fiber-based paper substrates result in rough Al/paper structure surfaces, the uniform interface between top Al electrodes and gelatin composites can be clearly observed in transmission electron microscopy images. This might be due to the gelatin composite acts as not only the resistive layer, but also the smoothing layer. Moreover, Fe ions play an important role in assisting redox reaction or filament formation. An Al/gelatin/Al/paper device has no resistive switching behavior. Meanwhile, devices with Fe ions on a paper substrate show reproducible resistive switching for 18 cycles, an ON/OFF ratio of over 105, and an excellent current distribution (coefficient of
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variation<60%). The ON/OFF ratio of the device after bending 180 cycles can be maintained at 104. The biodegradable gelatin materials have good ductility and potential in recyclable and green electronic applications.
Keywords: gelatin, green, memory, paper, resistive switching, solution *E-mail: [email protected]
I.
Introduction
Flexible memory devices are worth investigating because they can satisfy the requirements of wearable electronics and bendable displays [1, 2]. Conventionally, plastic substrates, such as polyethylene terephthalate, polyethylene naphthalate, polycarbonate, and polyimide, have been widely used as a technology for flexible electronic products. However, decomposing these plastics decreases the availability of transformed energy for future use along with the production–consumption chain and poses a growing ecological problem [3]. In order to reduce electronic wastes and achieve sustainability in the electronics industry, the natural materials from nature is an important property to successfully enter the future market. Paper, which can be easily recycled [4], is commonly-used in daily life and a
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potential role in the future of “green” electronics. “Green” electronics are applicable in biodegradable and environment-friendly devices. Recently, the paper substrates have been used to develop the degradable material-based memory devices [5]. However, the green memory device still remain limited, due to the biodegradable material that simultaneously shows good memory property and enough flexibility is still a challenge. Gelatin has a high degree of plasticity and is easily absorbed by some organisms. Also, gelatin is suitable for usage in wearable electronic devices due to its skin-friendly. In addition, gelatin solution can be used without any further purification as well as the gelatin thin film can be stored under atmosphere for a period of time [6]. The stable gelatin film facilitates the air-stable memory devices [7]. Metal ions in a gelatin matrix play an important role in improving the ON/OFF ratio of memory devices [8]. In addition, with an appropriate concentration of metal salts, metal ions can assist the formation of an interfacial AlOx layer and improve the memory properties [9]. A previous study shows that when top and bottom electrodes use the same metal, Al ions in gelatin matrix can assist the redox reaction or filament formation [10]. In this work, we presented an air-stable, biodegradable, eco-friendly electronics with an Al/gelatin composite/Al/paper structure. The gelatin composite fabricated by
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dissolving metal salts (ferric nitrate nonahydrate) in gelatin was presented. The ON/OFF ratios of the devices with Fe ions were found to display better performance than those of devices without Fe ions. The protein-based gelatin containing FeIII/FeII redox couples can help resistance switching properties as a consequence of the charge trap/release of FeIII/FeII pairs induced by externally applied voltage. The devices with Fe ions on a paper substrate show reproducible resistive switching for 18 cycles, an ON/OFF ratio of approximately 105, and a good current distribution (coefficient of variation<60%). The ON/OFF ratio of the device after bending 150 cycles maintained at 104. Because our devices are entirely made of biodegradable materials, they can break down into natural materials which is environmentally friendly. These results offer opportunities for achieving green electronics on a single chip of paper.
II. Experimental The inset image of Fig. 1(a) shows the photographic images of the paper substrate used in this study. A4 size inkjet papers having a thickness of 0.28 mm were obtained from Printerfun. To fabricate the devices, a large paper sheet was cut into 2 × 1.5 cm2 pieces to be used as substrates. The paper substrate was cleaned by dipping into
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isopropyl alcohol for 1 min. Followed by a drying process in oven at 100 °C to remove the moisture in the substrate. Al, the bottom electrode, was deposited on the paper substrate by using DC magnetron sputtering. Figure 1(a) shows the preparation of resistive switching layer. Gelatin powder and ferric nitrate nonahydrate powder were purchased from Kodak. Ferric nitrate nonahydrate powder was added to deionized (DI) water upon stirring until the ferric nitrate nonahydrate powder was completely dissolved. The preparation of gelatin solution is used the gelatin powder dissolved in DI water. The 5 mL of gelatin composite solution is prepared by mixing 2.5 mL of ferric nitrate nonahydrate solution with 2.5 mL of gelatin solution. The gelatin composite solution was then spun on the cleaned Al/paper substrate and baked at 60 °C under vacuum conditions. Finally, the Al top electrodes were deposited with 1 mm2 patterned shadow mask by using DC magnetron sputtering. The electrical properties were determined by using an Agilent B1500 semiconductor parameter analyzer. The surface morphology was evaluated by using atomic force microscopy (AFM; SPA 400, SII Nanotechnology Inc., Sunto-gun, Shizuoka, Japan).
III. Results and Discussion A cross-sectional high-resolution transmission electron microscopy (TEM) image
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of the Al/gelatin composite/Al/paper structure is shown in Fig. 1(b). The thicknesses of the Al, gelatin composite, and Al layers were approximately 200, 260, and 280 nm, respectively. The X-ray photoelectron spectroscopy (XPS) analyses were performed on a Perkin-Elmer PHI 5000 Versaprobe system. The FeG/Al/paper sample was used in XPS measurement. The XPS survey spectra of the FeG thin film obtained after 90 s of Ar+ ion sputtering, thereby representing the bulk layer of the FeG thin film. As shown in Fig. 2, the atomic percentages of Fe, N, C, and O were 10%, 7%, 43%, and 40%, respectively. The XPS signals corresponded to Fe2p3, O1s, C1s, and N1s. The C1s spectra were divided into three component peaks, namely, C–C at 285 eV, C–O or C-N at 286.2 eV, and C=O at 288 eV (includes a small C=N bond contribution at 288.0 eV) [11-15]. The O1s peaks at 532.3, 531.1 eV are associated with nitrate groups, C=O groups characteristic of gelatin and to metal-oxygen (529.5 eV) from the ferric nitrate nonahydrate support [16-18]. Due to these abundant oxygen functional groups, gelatin is hydrophilic in nature and easily dispersed in deionized (DI) water. The N1s peak was resolved into two peaks at 400 and 407 eV. The peak at around 400 eV is relevant to N-Fe compounds [19, 20]. The large amount of N associated with Fe, which suggests that N atoms are coordinated with Fe [21, 22]. The N 1s spectrum
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centered at around 407 eV can be ascribed to the nitrate group [23, 24]. The presence of nitrates is mainly due to a NO3− species [25]. Fe2p peak fitting was performed in accordance with the constraints of the Fe0 and Fe3+ components. The two oxidation states were characterized by the Fe0 and Fe3+ components at 711 and 726 eV, respectively. The TEM and EDS mapping images of the Al/gelatin composite/Al/paper structure are shown in Fig. 3(a). Although the fiber-based paper substrate resulted in a rough Al/paper structure surface, the uniform resistive layer with Fe elements were well dispersed into the gelatin matrix. The 3D AFM images in Figs. 3(b–d) reveal the surface morphologies of the paper substrate, Al/paper structure, and gelatin composite/Al/paper structure, respectively. The paper substrate and bottom Al electrode layer were highly rough, with root mean square roughness (Rrms) values of approximately 197 and 28 nm, respectively. When the bottom Al/paper structure was covered with the gelatin composite layer, the surface roughness was significantly reduced. As shown in Fig. 3(c), the Rrms of the gelatin composite thin film was approximately 14 nm. The gelatin composite layer acts not only as the resistive layer but also as the smoothing layer for the Al/paper structure. Figure 4 illustrates the switching I–V characteristics of the Al/gelatin composite/Al/paper device. The devices were reversibly switchable and showed typical
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bipolar resistive switching behavior. The current achieved the minimum at ∼1 V, which is associated with polarization due to the charge accumulation of ions at the electrode interfaces [26]. Under negative and positive bias, Fe2+/Fe3+ ions accumulated at the Al/gelatin composite and gelatin composite/Al interfaces, respectively. Quantities of multi-charged Fe2+/Fe3+ inside the gelatin composite film, serving as positively charged trap states, can capture electrons and then change into neutral Fe. Also, the captured electrons can escape from the trap states, resulting in a formation of Fe2+/Fe3+. At a set voltage of −3.5–−2.9 V, a reset voltage of 3.8–4.2 V and an ON/OFF ratio of >105 were observed. The Al/gelatin composite/Al/paper structure could maintain the ON/OFF ratio after 18 continuous DC voltage switching cycles. However, no switching behavior was observed for the Al/gelatin/Al/paper device, as shown in the inset in Fig. 4. These results indicate that a supply of Fe3+/Fe2+ ions from the gelatin composite can contribute to the resistance switching properties that were observed as a consequence of the charge trap/release that was induced by the externally applied voltage. This situation was similar to the ITO/ACG/ITO case [10]. To
better
understand
the
switching
mechanism
of
the
Al/gelatin
composite/Al/paper device as shown in Fig. 5(a), we plotted the logarithmic plots of the
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I–V curve for the positive and negative voltage sweep regions. The log I−log V curve in the high resistance state (HRS) at low voltage exhibited a linear region with a slope of approximately 1 and implied the ohmic conduction behavior. In a high-voltage region, space-charge limited conduction can be characterized using the square law [27, 28]. In the current study, when all of the available traps were filled, the current density abruptly increased with an I–V slope of 8. In the positive voltage region, the current states maintained the low resistance state (LRS) and showed an ohmic conduction behavior with a slope of approximately 1, which manifests the formation of many conductive filaments [29]. The Fe ions act as traps to capture/release charges [30]. As the charge carriers trapped by the Fe ions in gelatin are able to build an internal electrical field, similar to BPQD in the PMMA-based memory device [31]. The strong internal electric field built from the trapped charge carriers may facilitate the formation of filaments. Such strong electric field increases the injecting efficiency of the carriers into the traps in the set and reset process, respectively. When the magnitude of applied voltage exceeds Vset, the oxidized Fe ions could be reduced. The device is converted into the LRS. Conversely, after applying a sufficiently positive voltage Vreset, both electrons and oxygen will oxidize Fe elements near the top Al electrode, the device is turned to the HRS. For the
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HRS, injected electrons are captured by traps (or Fe ions) in the gelatin composite film and these behave as space charges. Figure 5(b) plots the scaling trend of the LRS current versus the cell area of the Al/gelatin composite/Al/paper device. The LRS current is mainly a filamentary conduction current and thus only slightly depends on the cell area [32]. Figures 5(c) and (d) present the statistical distribution parameters. Both the LRS and the HRS currents are measured at 0.1 V. The coefficient of variation (CV), which is defined as the ratio of the standard of deviation to the average value, is used for distribution evaluation. The CVs of HRS and LRS currents in the Al/gelatin composite/Al/paper devices were 59% and 46%, respectively. VSet and VReset distributed at the ranges of −4.8–−2 and 3.1–4.6 V, respectively. The wide distributions of set and reset voltages are closely related to the formation and rupture of conductive filaments. The filamentary switching in general exhibits a natural fluctuation, which leads to the non-uniformity of resistive switching parameters [33-35]. The average value of VSet and VReset were −3.2 and 3.9 V, respectively. We fabricated 40 samples via the same preparation process and observed that >80% (32 samples) showed similar resistive switching behaviors. To study the flexibility of the Al/gelatin composite/Al/paper device, we bent the
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device into compression and tension states. The h/d ratio at both states was 1/2. The bipolar resistive switching behaviors of the Al/gelatin composite/Al/paper device at compression and tension states are shown in Fig. 6. When the compressive or tensile strain is applied, the captured electrons can obtain mechanical energy and then escape from traps [36-38]. The ability of capture electrons will be subjected to influence on the mechanical stimuli of compressive/tensile. Because of the compressive and tensile strains can modulate the height of the trap barrier. Compared to compressive strains, the trap barrier height under tensile strains is deeper and more difficult for electrons to emit from one trap and then hop to another [39]. Therefore, tensile strain leads to larger Vset. Both for the compressive and tensile strains, the smaller Vreset than the Vset is due to the rupture of conductive filaments is partial and near the top Al electrode. The ON/OFF ratio was not degraded at both the compression and tension states, implying the flexibility of the Al/gelatin composite/Al/paper device. Figure 7(a) illustrates the flexibility test of Al/gelatin composite/Al/paper device. The ON/OFF ratio was decreased after bending 100 cycles. As shown in the inset of Fig. 7(a), TEM cross-sectional image observation of the fiber paper sheet deformation during bending 100 cycles. Bottom electrode is prone to damage, thus, the device flaked off easily.
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The device had a maintained ON/OFF ratio of >103 after repeated bending for 180 cycles and an extreme bending radius of 8 mm. This finding suggests that the excellent mechanical flexibility of the Al/gelatin composite/Al/paper device is attributed to the gelatin composite resistive layer ductility. Figure 7(b) shows the ON/OFF ratio as a function of ageing time for the Al/gelatin composite/Al/paper devices under the atmosphere. The ON/OFF ratio did not significantly change when the device was placed under atmospheric environment for 7 weeks but was approximately 104 when the device was placed under atmospheric environment for >8 weeks. The oxidation of Al electrode or Fe, which was induced through oxygen diffusion within the gelatin matrix, is hypothesized to play a role. Notably, the devices were measured and stored in ambient conditions, that is, at room temperature of ~23 °C and a relative humidity of ~80%, for the whole investigation period, thus lending weight to the performance of the memory in air.
IV. Conclusions Eco-friendly electronics were demonstrated with Al/gelatin composite/Al/paper structure. The gelatin composite acts as the smoothing layer for the Al/paper structure, thereby, forming uniform interface between the top Al electrode and gelatin composite.
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The gelatin composite memory device showed reproducible bipolar resistive switching with an ON/OFF ratio of >105, and uniform current distribution. This can be attributed to the protein-based gelatin containing FeIII/FeII redox couples, which can help resistance switching properties. The Al/gelatin composite/Al/paper device also has good flexibility and air stability. The ON/OFF ratio was not degraded after repeated bending for 180 cycles and extreme bending radius of 6 mm at the compression state. The ON/OFF ratio of ~105 did not significantly change when the device was placed under atmospheric environment for 7 weeks. These demonstrations of high-performance bioelectronics on paper substrate indicated that truly “green” electronics is potential. This work aims to create paths for the production of human- and environment-friendly electronics.
Acknowledgements This work was supported in part by the National Science Council of Taiwan under Contracts NSC102-2221-E-006-182-MY3 and NSC 105-2221-E-006-193-MY3.
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Figure
Figure 1 (a) Preparation of gelatin composite thin film. The inset image shows the photographic images of the paper substrate. (b) TEM image of the Al/gelatin composite/Al/paper structure.
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Figure 2 C1s, O1s, N1s, and Fe2p3 peaks from the XPS spectra.
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Figure 3 AFM image of the (a) paper, (b) Al/paper, and (c) gelatin composite/Al/paper structure. (d) TEM image of the Al/gelatin composite/Al/paper structure. EDS mapping analysis result for the Al/gelatin composite/Al/paper structure.
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Figure 4 Switching cycles of the Al/gelatin composite/Al/paper device. The inset shows the I-V curve of the Al/gelatin/Al/paper device.
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Figure 5 (a) Log I–log V curve for the positive and negative voltage regions. (b) LRS current versus cell area of the Al/gelatin composite/Al/paper device. (c) Voltage and (d) current statistical distributions of the Al/gelatin composite/Al/paper device.
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Figure 6 Typical bipolar resistive switching characteristics of the Al/gelatin composite/Al/paper device at compression and tension state. The inset images show the photograph of the device at compression and tension states.
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Figure 7 (a) Continuous bending effect of the Al/gelatin composite/Al/paper device. The inset shows the TEM cross-sectional image of the Al/gelatin composite/Al/paper structure after bending 100 cycles. (b) ON/OFF ratio as a function of ageing time for the Al/gelatin composite/Al/paper devices under the atmosphere.
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Highlight 1. An
air-stable
and
green
electronic
with
an
Al/gelatin
composite/Al/paper structure has been demonstrated. 2. The Al/gelatin composite/Al/paper structure showed forming-free, an ON/OFF ratio of over 105, and uniform current distribution. Moreover, the ON/OFF ratio of the device after bending 150 cycles can be maintained at 104. 3. A supply of Fe3+/Fe2+ ions from the gelatin composite can contribute to the resistance switching properties.