Biodegradable and Flexible Resistive Memory for Transient Electronics

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C: Physical Processes in Nanomaterials and Nanostructures

Biodegradable and Flexible Resistive Memory for Transient Electronics Xinglong Ji, Li Song, Shuai Zhong, Yu Jiang, Kian Guan Lim, Chao Wang, and Rong Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03075 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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The Journal of Physical Chemistry

Biodegradable and Flexible Resistive Memory for Transient Electronics Xinglong Jia, Li Songa, Shuai Zhonga, Yu Jianga, Kian Guan Lima, Chao Wanga and Rong Zhao*a a

Department of Engineering Product Design, Singapore University of Technology and Design, 8

Somapah Road, Singapore, 487372, Singapore *Corresponding author: [email protected] Abstract Physically transient electronics have attracted increasing attention recently due to potential as the basis for building “green” electronics and biomedical devices. In the development of transient devices for biomedical applications, however, the dilemma between the strictly required biodegradability and device performance has brought great difficulties to the material selection. In this paper, we introduced silk fibroin as dielectric layer to fabricate biodegradable resistive memory devices. Comprising a W/Silk fibroin/Mg sandwich structure, stable bipolar resistive switching behaviour with good repeatability and device variability was obtained, surpassing most organic resistive memory and comparable to inorganic resistive memory. The carrier transport evolution process was carefully examined to reveal the mechanism behind resistive switching. A switching model regarding the formation of metallic conductive filament was proposed by considering both the nature of silk fibroin dielectric layer and the key role of active metal electrode. Furthermore, the solubility test in phosphate buffered saline indicates the device exhibiting physically transient behaviour and good biodegradability. Good mechanical property and flexibility were also demonstrated through electrical testing under different bending conditions. These results suggest that our device is a promising memory element candidate for constructing transient electronic system, especially for bio-medical applications. Introduction Electronic waste has become an urgent environmental issue emerging from the rapid growth of consumer electronics.1 This motivated the recent research on transient electronics, a new type of

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technology that endows materials, devices, and systems with the capability of degrading into nontoxic products and absorbing by surrounding environment with minimal or nontraceable remains after a period of stable operation.2 Besides being environmental friendly, transient electronics with biodegradability are highly desired for biomedical applications, especially in implantable medical diagnostic and therapeutic devices.3 For such applications, the transient devices are expected to work in a scheduled time with their predefined function and get absorbed to mammalian body to reduce consecutive surgeries. To prevent potential risks to human health, several criteria must be strictly met when developing transient electronics for biomedical applications 4: (1) The materials should not evoke a sustained inflammatory or toxic response upon implantation in vivo; (2) The degradation products should be nontoxic, and easily metabolized and cleared from body; (3) The degradation time should match the healing or regeneration process; and (4) The materials should have appropriate mechanical strength and flexibility. Such stringent requirements place a significant difficulty to material selection. In the past few years, researchers made great efforts to develop biocompatible or biodegradable devices using both organic materials and inorganic materials.3,5 However, a dilemma between device performance and biocompatibility (or biodegradability) has been noticed by researchers. The usage of organic materials makes the device more biocompatible and biodegradable, but experiencing performance degradation, especially poor endurance and poor uniformity. Although using inorganic material will greatly enhance the device performance, the biodegradable and biocompatible requirements significantly limit the suitability of most widely used CMOS compatible materials, such as Pt, Ag, Al, Si.2 Therefore, it is essential to rebuild electronic system using novel materials to achieve a trade-off between biodegradability and performance. Many biocompatible transient electronic elements have been reported.

3, 6-8

However, the development of fully

biodegradable transient memory element is still on the weak side, which may hinder the implementation of practical transient electronic system. In this paper, we reported an organic resistive memory device that are biodegradable and disposable while still exhibiting stable bipolar switching performance. Here, we adopted silk fibroin film as the switching layer and water resolvable metal W and Mg as electrode to form a metal-insulator-metal


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structure. By combining the excellent biocompatibility of silk fibroin metallization property of Mg element

10

9

and good electrochemical

, we demonstrated a fully biodegradable organic resistive

memory with good bipolar electrical performance, which outperforms most organic memory devices and is compatible to inorganic memory devices. The flexibility and biodegradability test results indicate that the device exhibits good mechanical property and physically transient behaviour. Furthermore, based on the material properties of silk fibroin and carrier transport analysis, a resistive switching model was put forward to explain the mechanism. Experimental details Silk Fibroin Films Preparation and Characterization. The preparation of silk fibroin solution was based on the standard procedure in Reference [11]. Firstly, the Bombyx mori cocoons boiled in 0.02 M sodium carbonate until sericin was fully removed. Secondly, the silk fibers after drying were dissolved in 9.3 M lithium bromide solution at 60oC for 4 hours. Thirdly, to remove the lithium bromide, the solution was dialyzed against deionized (DI) water for 48 hours. Finally, after twice centrifugation treatment (9000 rpm at 4oC for 20 min), the pure silk fibroin solution can be achieved. Subsequently, we used a spin coater to form the silk albumen film at a spinning rate of 4000 rpm for 60 s. Material characterization. The purity and structure of the silk fibroin film was confirmed by fourier transform infrared spectroscopy (FTIR) with ATR mode and Raman spectrum, respectively. A field emission scanning electron microscope (FESEM) was employed for Silk fibroin film thickness measurement with an acceleration voltage of 3 keV. Atomic force microscopy (AFM) analysis was conducted to investigate the roughness of the film surfaces under a tapping mode at 400 kHz. The transient behaviour was triggered by phosphate buffer solution (PBS, 1M, pH 7.4) at a body temperature (37oC) and were optically inspected periodically in 24 hours. Device fabrication and characterization. We adopted a vertical metal-insulator-metal (MIM) structure to demonstrate our transient device. 50 nm W was deposited over the Si/SiO2 and Polyethylene terephthalate (PET) substrates by a sputter machine with direct current power supply, respectively,



severing as bottom inert electrode. Subsequently, we spin coating silk fibroin film over W layer. Finally, we cover the silk fibroin with a shadow mask and do the Mg deposition to form the top electrode using a sputter machine with a radio frequency power supply. The feature size of the top electrode is ~ 100 µm. And the thickness of the Mg top electrode is 80 nm controlled by deposition time. The electrical characteristics of the W/Silk fibroin/Mg device were measured by a Keithley 4200-SCS semiconductor parameter analyser with high-resolution source and measurement units. Result and Discussion Using silk cocoon as raw material, we prepared silk fibroin film by 3 steps, including boil, dissolve, spin, as shown in Figure 1(a). Figure 1(b) shows the FTIR-ATR absorbance spectra of the silk albumen film. The observed peaks locating at 1243 cm-1 (amide III), 1516 cm-1 (amide II) and 1634 cm-1 (amide I) are the three characteristic peaks of silk fibroin.12 The high relative intensity of the three characteristic peaks indicates good purity of the silk albumen we acquired. In Figure 1(c), the Raman spectrum of silk fibroin is presented. In the Raman spectrum, 13 peaks can be identified in the range of 750 – 4250 cm-1. The detailed peak assignments are provided in Table S1 (Supporting information). The FTIR-ATR and Raman spectra results indicate that the silk fibroin film used in this work has the 18 α-amino acids structure in amorphous state (Figure S1, supporting information).13 SEM was employed to observe the cross-sectional view of the silk fibroin, based on which we can measure the thickness of the silk fibroin film (120 nm). The roughness measurement of the spin coated silk albumen film was also conducted by a AFM system as shown in Figure 1(e). With a 4000 rpm spinning speed, the roughness of the silk albumen film can be controlled within 3 nm. Comparing with the 80 nm thickness of top and bottom electrodes, the 3 nm roughness can be neglected and provides good interface uniformity. Here we employed a metal-insulator-metal structure to demonstrate a resistive memory device, in which W and Mg serve as inert and active electrode, and silk fibroin film serves as the switching layer, respectively (see Figure 2a). Before direct current (DC) sweep, an electroforming step was required. The electroforming voltage is around 5.3 V, as indicated in Figure S2 (supporting information). After the electroforming step the devices can be operated reversibly at lower voltages. Figure 2b shows the 4 ACS Paragon Plus Environment

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I-V curves of a W/Silk fibroin/Mg sandwich structure under the voltage bias 0 V → 4 V → 0 V → -4 V → 0 V applied to Mg electrode while W electrode was grounded. The as-fabricated devices were in high resistance state. For positive bias sweep, the current maintained low under low electric field range. When the voltage reached a certain value (Set voltage), the total current passing through the device increased abruptly, indicating high-to-low resistive switching (Set process). A compliance current of 1 mA was set as the maximum current during Set operation to prevent the device from breakdown. The larger than 5 orders ON/OFF ratio forms large sensing margin. For the negative bias sweep, the initial state was low resistance state, when a certain negative voltage (Reset voltage) was reached, the resistance switched to high resistance state abruptly, indicating Reset process. The I-V characteristic after electroforming exhibits as typical bipolar resistive switching behaviour. To exclude the unipolar switching, we increased the compliance current to 10 mA and performed positive sweep again from 0 V to 5 V. As seen in Figure S3, the Reset cannot be triggered under positive bias. The data retention was tested at human body temperature, which demonstrated that the metastable state (low resistance state) maintains well in 24 hours (See Figure S4, supplementary information). In Figure 2c, we conducted a statistical analysis on the Set voltage and Reset voltage. The statistical mean value of Set and Reset voltage locate at 2.0 V and -2.2 V, respectively. The narrow distributions of Set voltage and Reset voltage indicate good variability of our device. Reversible switching without degradation up to 100 cycles was demonstrated, as shown in Figure 2d. During the endurance testing, the large ON/OFF ratio maintains sufficient large, which means that the proposed memory device is robust and reliable for transient memory application. The resistive switching of Silk fibroin has been discussed in several papers.14-15 Because silk fibroin is easily damaged by the high energy electron beam, it is a big challenge to conduct a direct observation for the microcosmic mechanism of the resistive switching in silk fibroin layer. However, by combining the nature of silk fibroin protein and the I-V relation extracted from double-logarithmic IV curve, it can provide us clues of the carrier transport process during resistive switching. Here, we carefully examine the switching process under different voltage range and proposed a possible switching model based on our device structure. 16-17



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We replotted the I-V curves in double logarithm scale and did the power function fitting to examine the evolution of carrier transport characteristic during resistive switching, as shown in Figure 3a and b. In the initial stage of Set process, silk fibroin film with the structure of 18 α-amino acids was in high resistance state, and the I-V characterization exhibits linear behaviour (I∝V), dominated by Ohmic law. Although the energy band gap of silk fibroin is very large, there are still a small number of carriers that are generated by thermal excitation as shown in the Figure 3c-1. At very low voltage (0 ~ 0.2 V), the number of injected carrier is much smaller than that of thermal excited carriers, therefore, the conduction current due to mobile carriers is linearly dependent on the electric field. After the Ohmic conduction, space-charge limited conduction (SCLC) begins to take effect. For the second stage (0.2 ~ 0.5 V), with medium injection, the space charges appear, and parts of the traps in dielectric layer will be filled up by injected free carriers, resulting in trap-filled limit SCLC 18:

9

= 8

(1)

where µ is the carrier mobility, ε is the static dielectric constant, θ is the ratio of the free carrier density to total carrier (free and trapped) density, A is the effective area, and d is the thickness of the dielectric layer. An I∝V2 relation can be identified in the I-V curve, as can be seen in Figure 3a. Figure 3c-2 shows the schematic diagram of this stage. With larger injection of carrier, the carrier transport will transform from the trapped characteristic to the trap-free characteristic. When all traps are filled up, the subsequently injected carriers will be free to move in the dielectric layer. The current will increase from the low trap limited value to high trap-free SCLC current. The I-V relation of SCLC in this stage is given by19:

=

2 + 1 +1 + 1

(2)

where q is the elementary charge, Nv is the density of the states in the valence band, Nt is the trap density, and l = Tc/T with Tc is a characteristic temperature related to the trap distribution. Here, l+1 as the exponent of V can be used as a criterion for comparison of the stretching of the exponential 6 ACS Paragon Plus Environment

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distribution. Low slope implies gradual distribution, while higher slope implies abrupt distribution. In our device, a higher exponent than 2 can be observed (I∝V3) for the third stage, indicating the conduction transforming from trap-limited SCLC to trap-free SCLC in the case of strong injection, as shown in Figure 3c-3. The transition of Ohmic conduction - trap-filled limited SCLC - trap-free SCLC can also be found in Reference [18] and [19]. After the SCLC stage, the resistance was switched to low resistance state abruptly at Set voltage, and the conductive filament was formed in this stage, as shown in Figure 3c-4. Due to the low resistance of the conductive filament, the space charge and thermal excitation charge will not contribute to the total current any more. The mechanism of conductive switching of silk fibroin film is still unclear. Two possible mechanisms have been proposed, including the conductive filament formation based on oxidized silk fibroin (Valence change mechanism) 15, and the growth and rupture of metallic filament (A combination of Elemctrochemical mechanism and Thermalchemical mechanism).14 However, we found that neither of the two proposed mechanisms can fully explain our experimental finding because of different device structure. As can be seen from the I-V curve in Figure 2b, our device using Mg as electrode exhibits an abrupt binary switching behaviour, which is quite different from the analog switching behaviour exhibited by those using low activity metals (such as Al) as electrode.15 Therefore, the activity of electrode materials cannot be neglected. On the other hand, by conducting bidirectional sweep for the reset process, we demonstrated that, for our device, the rupture of the filament is driven by electric field rather than Joule heating as proposed in Ref. 14 (see Figure S3, supporting information).20 On this basis, we propose a mechanism considering both the nature of silk fibroin dielectric layer and the key role of active metal electrode in resistive switching. More detailly, when positive bias is applied to Mg electrode, the outer-shell electron of Mg will be pumped to cathode (Mg → Mg2+ + 2e-), forming mobile Mg2+ ions at the Silk fibroin/Mg interface. Under the action of electric field, Mg2+ ions will go into the dielectric layer. Because of the low electronegativity (Pauling scale 1.31) and high activity, Mg usually occurs in the form of compounds. Meanwhile, silk fibroin contains plenty of hydroxy and amino, which may form a coulombic trap sites for Mg2+ ions. It is reasonable to infer that silk fibroin may be oxidized by Mg2+ ions into oxidation states (SF-Mg+ or SF+).21 It should be noted



that, different from typical electrochemical electrodes (Ag and Cu), Mg2+ ions have higher diffusive energy barrier dielectric layer and are hard to be reduced into Mg atoms due to the much higher activity. Therefore, we tend to believe that the role of Mg and Mg2+ may enhance the oxidation process of silk fibroin, which present high conductivity state in oxidation state, rather than form pure metal filaments. The formation of the conductive filament (SF-Mg+ or SF+) finally results in the sudden increase in total current. After resistive switching, the I-V curve of low resistance state shows a linear characteristic, suggesting the conduction in this stage is dominated by metallic conductive filament. When a Reset voltage is applied (as shown in Figure 3b), the injection electrons from cathode will reduce the oxidized filament and electric field will also drive the Mg2+ ions to cathode side, therefore caused the rupture of the conductive filament. Then the space charges and thermal excitation carriers dominate the conduction again. For transient electronics, flexibility is important for the applications on human bodies. It is desired that the devices will not be affected by the mechanical deformation induced by body movement.22 Here we attached our devices to the curved stages with different curvature radiuses to test the impact of bending on electrical performance. As shown in Figure 4a, by bending the device to the curvature radiuses of infinite (no bending), 3 cm, 2 cm, 1 cm, we can acquire the tensile stresses of 0 Pa, 39440 Pa, 59160 Pa, 118320 Pa on the surface, respectively. The tensile modulus of silk fibroin was acquired from Reference [23]. Increasing the tensile stress by 5 orders, the Set and Reset resistance didn’t show obvious change. With different bending loads, we also performed the I-V characterization, as shown in Figure 4b. The near constant I-V curves reveals that the proposed structure exhibits reliable resistive memory properties under bending condition. Therefore, the proposed device structure can be integrated with either Si-semiconductor process or flexible format. As reported, silk fibroin exhibits excellent biodegradability and bioresorbability, which enable the formation of noninflammatory degradation products.24-25 Unlike fibres, silk films are isotropic in nature, with an amorphous phase more prone to swelling, which will accelerate the decomposition rate.26 Therefore, the degradation rate of silk fibroin films is highly programmable and can be configured from minutes to years. In our experiment, the 120 nm silk fibroin film can be hydrolysed 8 ACS Paragon Plus Environment

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in 3 hours. Mg top electrode and W bottom electrode also can be fully resolved in DI water or PBS solution undergoing the reactions of Mg + 2H2O → Mg(OH)2 + H2 and 2W + 2H2O + 3O2 → 2H2WO4.26 In the ion forms of Mg2+ and WO42-, the metal element can be readily utilized or metabolized by mammalian physiological system.28 Degradability in body fluid environment is an essential requirement for the applications like implantable electronics and biomedical diagnostics. Here, we used phosphate buffered saline (PBS, pH 7.4) to imitate the body fluid environment to conduct degradability test. Figure 5 shows the dissolution evolution of the W/Silk Fibroin/Mg memory triggered by PBS at 37oC. It should be noted that Mg undergoes a very fast transient behaviour in PBS (see Figure 5(a) – (b)). The Mg electrode disappeared within 2 mins at 37oC after we drop the sample into PBS. And W bottom electrode follow slow but predictable degradation rate (see Figure 5(b) - (h)). As indicated in published work of Yin et al., the dissolution rates of Mg and W are highly dependent on the fabrication method.26 Therefore, we tested the dissolution rates of the sputtered Mg and W using time depending AFM method and summarized the average dissolution rate of Mg and W at room temperature and human body temperature in Table S2 and S3 (Supplementary information). It should be noted that the silk fibroin film dissolved in 3 hours. Though silk fibroin film presents good permeability to oxygen and water, we can still see that W dissolve slower in the covered area than surrounding area, as shown in Figure 5(b) – (g). Eventually, both the metal electrodes and silk fibroin switching layer fully disappear within 24 hours without any visible residue, as shown in Figure 5(h). Conclusion In this paper, we have demonstrated a biodegradable organic resistive memory device based on the W/Silk fibroin/Mg sandwich structure. Compared with existing transient memories, our device shows excellent biodegradability and stable switching performance, which make it a suitable component of transient electronic system for medical use. By combining the nature of silk fibroin protein and the carrier transport information extracted from double-logarithmic I-V curve, we did the conduction analysis and proposed a plausible switching mechanism taking into account the influence of active metal electrode. For flexibility test, different tensile stresses were applied to our device, showing no 9 ACS Paragon Plus Environment


obvious impact on the I-V characteristic. The fully degradation of our device in 24 hours indicates that our device has controllable transient behaviour and great potential for biomedical electronic applications.

Associated content Supporting Information Supporting Information is available on the ACS Publications website. Raman peak assignment of as-fabricated silk fibroin film; Primary structure of the silk fibroin; Electroforming of the W/Silk fibroin/Mg device; Bidirectional I-V sweeping to trigger the Reset; Data retention of the proposed resistive memory at 37oC; Dissolution rate of sputtered Mg and W film.

Acknowledgement This work was supported by A*STAR, Science and Engineering Research Council Public Sector Research Funding (Grant number: 1521200085), Singapore. Reference (1) Zoeteman, B. C.; Krikke, H. R.; Venselaar, J.; Handling WEEE waste flows: on the effectiveness of producer responsibility in a globalizing world, Int. J. Adv. Manuf. Techn. 2010, 47, 415-436. (2) Fu, K. K.; Wang, Z.; Dai, J.; Carter, M.; Hu, L.; Transient electronics: materials and devices, Chem. Mater. 2016, 28, 3527-3539. (3) Hwang, S.; Tao, H.; Kim, D.; Cheng, H.; Song, J.; Rill, E.; Brenckle, M. A.; Panilaitis, B.; Won, S. M.; Kim, Y., et al. A physically transient form of silicon electronics. Science, 2012, 337 (6102), 16401644. (4) Cao, Y.; Wang, B.; Biodegradation of silk biomaterials. Int. J. Mol. Sci. 2009, 10, 1514-1524. (5) Hosseini, N. R.; Lee, J. S.; Biocompatible and flexible chitosan-based resistive switching memory with magnesium electrodes. Adv. Funct. Mater. 2015, 25, 5586-5592.


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(6) Hwang, S.-W.; Park, G.; Cheng, H.; Song, J.-K.; Kang, S.-K.; Yin, L.; Kim, J.-H.; Omenetto, F. G.; Huang, Y.; Lee, K.-M., et al. Materials for high‐ performance biodegradable semiconductor devices. Adv. Mater. 2014, 26, 1992-2000. (7) Zhu, H.; Xiao, Z.; Liu, D.; Li, Y.; Weadock, N. J.; Fang, Z.; Huang, J.; Hu, L.; Biodegradable transparent substrates for flexible organic-light-emitting diodes. Energy Environ Sci. 2013, 6, 21052111. (8) Kang, S.-K.; Murphy, R. K. J.; Hwang, S.-W.; Lee, S. M.; Harburg, D. V.; Krueger, N. A.; Shin, J.; Gamble, P.; Cheng, H.; Yu, S., et al. Bioresorbable silicon electronic sensors for the brain. Nature, 2016, 530, 71-76. (9) Zhu, B.; Wang, H.; Leow, W. R.; Cai, Y.; Loh, X. J.; Han, M.; Chen, X.; Silk fibroin for flexible electronic devices. Adv. Mater. 2016, 28, 4250-4265. (10) Wu, S.; Wang, H.; Sun, J.; Song, F.; Wang, Z.; Yang, M.; Xi, H.; Xie, Y.; Gao, H.; Ma, J., et al. Dissolvable and biodegradable resistive switching memory based on magnesium oxide. IEEE. Electron Device Letters, 2016, 37(8), 990-993. (11) Rockwood, D. N.; Preda, R. C.; Yucel, T.; Wang, X.; Lovett, M. L.; Kaplan, D. L.; Materials fabrication from Bombyx mori silk fibroin. Nat. Protocols. 2011, 6, 1612-1631. (12) Chen, X.; Shao, Z.; Marinkovic, N. S.; Miller, L. M.; Zhou, P.; Chance, M. R.; Conformation transition kinetics of regenerated Bombyx mori silk fibroin membrane monitored by time resolved FTIR spectroscopy. Biophys. Chem. 2001, 89, 25-34. (13) Um, I. C.; Kweon, H. Y.; Park, Y. H.; Hudson, S.; Structural characteristics and properties of the regenerated silk fibroin prepared from formic acid, International Journal of Biological Macromolecules, 2001, 29(2), 91-97. (14) Wang, H.; Zhu, B.; Ma, X.; Hao, Y.; Chen, X.; Physically Transient resistive switching memory based on silk protein. Small, 2016, 12(20), 2715-2719.



(15) Hota, M. K.; Bera, M. K.; Kundu, B.; Kndu, S. C.; Maiti, C. K.; A natural silk fibroin proteinbased transparent bio-memristor. Adv. Funct. Mater. 2012, 22, 4493-4499. (16) Shang, D. S.; Wang, Q.; Chen, L. D.; Dong, R.; Li, X. M.; Zhang, W. Q.; Effect of carrier trapping on the hysteretic current-voltage characteristics in Ag/La0.7Ca0.3MnO3/Pt hererostructures. Phys. Rev. B, 2006, 73, 245427. (17) Rubi, D.; Tesler, F.; Alposta, I.; Kalstein, A.; Ghenzi, N.; Gomez-Marlasca, F.; Rozenberg, M.; Levy, P.; Two resistive switching regimes in thin film maganite memory devices on silicon. Appl. Phys. Lett. 2013, 103, 163506. (18) Chiu, F. C.; A review on conduction mechanisms in dielectric films. Advances in Materials Science and Engineering, 2014. (19) Chandra, W.; Ang, L. K.; Pey, K. L.; Ng, C. M.; Two-dimensional analytical Mott-Gurney law for a trap-filled solid. Appl. Phys. Lett. 2007, 90, 153505. (20) Ielmini, D.; Bruchhaus, R.; and Waser, R.; Thermaochemical resistive switching: materials, mechanisms and scaling projections. Phase Transitions, 2011, 84 (7), 570-602. (21) Ault, B.; Evans, R. H.; Francis, A. A.; Oakes, D. J.; Watkins, J. C.; Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations. J. Physiol. 1980, 307, 413-428. (22) Hwang, S. W.; Lee, C. H.; Cheng, H.; Jeong, J. W.; Kang, S. K.; Kim, J. H.; Shin, J.; Yang, J.; Liu, Z.; Ameer, G. A., et al. Biodegradable elastomers and silicon nanomembranes/nanoribbons for stretchable, transient electronics, and biosensors. Nano Lett. 2015, 15, 2801-2808. (23) Robson, R. M.; In fiber chemistry handbook of science and technology, Edited by Lewin, M.; and Pearce. E.; Marcel Dekker, New York, 1970, vol. IV, pp. 647-700. (24) Zhu, B.; Wang, H.; Leow, W. R.; Cai, Y.; Loh, X. J.; Han, M.; Chen, X.; Silk fibroin for flexible electronic devices. Adv. Mater. 2016, 28, 4250-4265.


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(25) Wang, Y.; Rudym, D. D.; Walsh, A.; Abrahamsen, L.; Kim, H.; Kim, H. S.; Kirker-Head, C.; Kaplan, D. L.; In vivo degradation of three-dimensional silk fibroin scaffolds. Biomaterials, 2008, 29(24-25), 3415-3428. (26) Arai, T.; Freddi, G.; Innocenti, R.; Tsukada, M.; Biodegradation of bombyx mori silk fibroin fibers and films. J. Appl. Polym. Sci. 2004, 91, 2383-2390. (27) Yin, L.; Cheng, H.; Mao, S.; Haasch, R.; Liu, Y.; Xie, X.; Huang, S.; Jain, H.; Kang, S. K.; Su, Y., et al. Dissolvable metals for transient electronics. Adv. Funct. Mater. 2014, 24, 645-658. (28) Slutsky, I.; Abumaria, N.; Wu, L.; Huang, C.; Zhang, L.; Li, B.; Zhao, X.; Govindarajan, A.; Zhao, M.; Tonegawa, S., et al. Enhancement of learning and memory by elevating brain magnesium. Neuron, 2010, 65(2), 165-177.



Figure 1. Silk fibroin film fabrication and characterization. (a) the schematic diagram of silk fibroin film fabrication process: silk cocoon – silk fibroin fiber – silk fibroin solution - silk film. Inset: the scanning electron microscope (SEM) image of silk fiber. (b) FTIR (ATR) spectra of silk fibroin film. (c) Raman spectra of the silk fibroin film. (d) the cross-sectional SEM image of the Silk fibroin film on SiO2/Si substrate. The thickness of the spin coated film is ~ 120 nm. (e) the AFM image of the silk fibroin film with a 5 × 5 µm2 scanning area, showing the roughness of the film is less than 3 nm.


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Figure 2. Electrical performance of the W/Silk fibroin/Mg device. (a) the device structure of W/Silk fibroin/Mg device and the optical images of the corresponding device on SiO2 and PET substrate. (b) Typical DC I-V characteristic of the W/Silk fibroin/Mg devices. (c) Set voltage and Reset voltage distributions of the W/Silk fibroin/Mg devices. (d) Endurance cycling performance of W/Silk fibroin/Mg based RRAM device. During endurance test, the resistances were read at a 0.5 V voltage bias.



Figure 3. Mechanism explanation of the W/Silk fibroin/Mg resistive switching. (a) Doublelogarithmic plot with the space-charge-limited model fitting for Set process. (b) Double-logarithmic 1 to ○ 5 indicate plot with the space-charge-limited model fitting for Reset process. The labels from ○

the sweeping direction. (c) Schematic of carrier distribution in silk fibroin film under different voltage bias, including ohmic conduction under weak carrier injection, trap-filled-limited SCLC under medium carrier injection, trap-free SCLC under strong carrier injection, and metallic filament conduction.


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Figure 4. Flexibility test for the W/Silk fibroin/Mg memory device. (a) Set state resistance and reset state resistance evolution with different curvature radiuses. Inset: optical images of the flexible device attached to the bending stages with different curvature radiuses: no bending; curvature radius equals to 3 cm; curvature radius equals to 2 cm; curvature radius equals to 1 cm, respectively. (b) I-V characteristics of the memory devices with different bending radiuses.



Figure 5. Microscopy images recording the dissolving of the W/Silk Fibroin/Mg structure device in PBS. (a) Initial state. (b) < 1 minute. (c) 4 hours. (d) 8 hours. (e) 12 hours. (f) 16 hours. (g) 20 hours. (h) 24 hours.


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ToC Graphic



Silk fibroin film fabrication and characterization. (a) the schematic diagram of silk fibroin film fabrication process: silk cocoon – silk fibroin fiber – silk fibroin solution - silk film. Inset: the scanning electron microscope (SEM) image of silk fiber. (b) FTIR (ATR) spectra of silk fibroin film. (c) Raman spectra of the silk fibroin film. (d) the cross-sectional SEM image of the Silk fibroin film on SiO2/Si substrate. The thickness of the spin coated film is ~ 120 nm. (e) the AFM image of the silk fibroin film with a 5 × 5 µm2 scanning area, showing the roughness of the film is less than 3 nm. 290x115mm (200 x 200 DPI)

ACS Paragon Plus Environment

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Electrical performance of the W/Silk fibroin/Mg device. (a) the device structure of W/Silk fibroin/Mg device and the optical images of the corresponding device on SiO2 and PET substrate. (b) Typical DC I-V characteristic of the W/Silk fibroin/Mg devices. (c) Set voltage and Reset voltage distributions of the W/Silk fibroin/Mg devices. (d) Endurance cycling performance of W/Silk fibroin/Mg based RRAM device. During endurance test, the resistances were read at a 0.5 V voltage bias. 78x61mm (300 x 300 DPI)



Mechanism explanation of the W/Silk fibroin/Mg resistive switching. (a) Double-logarithmic plot with the space-charge-limited model fitting for Set process. (b) Double-logarithmic plot with the space-charge-limited model fitting for Reset process. The labels from ○1 to ○5 indicate the sweeping direction. (c) Schematic of carrier distribution in silk fibroin film under different voltage bias, including ohmic conduction under weak carrier injection, trap-filled-limited SCLC under medium carrier injection, trap-free SCLC under strong carrier injection, and metallic filament conduction. 80x41mm (300 x 300 DPI)


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Flexibility test for the W/Silk fibroin/Mg memory device. (a) Set state resistance and reset state resistance evolution with different curvature radiuses. Inset: optical images of the flexible device attached to the bending stages with different curvature radiuses: no bending; curvature radius equals to 3 cm; curvature radius equals to 2 cm; curvature radius equals to 1 cm, respectively. (b) I-V characteristics of the memory devices with different bending radiuses. 49x19mm (300 x 300 DPI)



Microscopy images recording the dissolving of the W/Silk Fibroin/Mg structure device in PBS. (a) Initial state. (b) < 1 minute. (c) 4 hours. (d) 8 hours. (e) 12 hours. (f) 16 hours. (g) 20 hours. (h) 24 hours. 55x20mm (300 x 300 DPI)


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Biodegradable and Flexible Resistive Memory for Transient Electronics

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