Enabling Transient Electronics with Degradation on Demand via Light

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Functional Inorganic Materials and Devices

Enabling Transient Electronics with Degradation on Demand via Light-Responsive Encapsulation of Hydrogel/Oxide Bi-layer Shuai Zhong, Xinglong Ji, Li Song, Yishu Zhang, and Rong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14161 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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ACS Applied Materials & Interfaces

Enabling Transient Electronics with Degradation on Demand via Light-Responsive Encapsulation of Hydrogel/Oxide Bilayer Shuai Zhong, Xinglong Ji, Li Song, Yishu Zhang, Rong Zhao* Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore *[email protected]

Keywords: oxide, hydrogel, encapsulation, transient electronics, active controllability

ABSTRACT Physically transient electronics, which can disappear under certain condition in aqueous solution or biofluids, has attracted increasing attention because of its potential applications as ‘green’ electronics and biomedical devices. Till now the excitation of transient process is achieved by passive dissolution of encapsulation layer, which has a very limited control over the process. Here, we report a novel light triggered encapsulation strategy via a bi-layer of light-responsive hydrogel and oxide to control the degradation on demand in aqueous environment. The hydrogel serving as a barrier between the environment and oxide limited the water’s movement and penetration, leading to improved stable operation time. More importantly, the light responsive hydrogel underwent a gel-to-solution transition upon applying UV light. The drastic change of the water movement enabled a transient process triggered on demand. Via this encapsulation scheme, we demonstrated fully soluble resistors and resistive random access memory devices with UV light triggered transient process. This

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work provides a new pathway to design transient devices with controllable degradation to meet various requirements of green electronics and biomedical devices.

1. INTRODUCTION Transient electronics have become increasingly popular for ‘green’, environmental friendly applications. With the ability of being physically disappeared in a programmable period, they are also greatly favored for sensitive digital data and implantable medical diagnostics.1 Various types of transient devices have been reported so far, including transistors, diodes, sensors, memory, and energy harvesters.2–7 The degradation of these devices is mostly achieved by the dissolution of materials via hydrolysis.8 For transient devices, stable operation is defined as the performance for a specified period of time when the device is first exposed to a stimulus.9 To extend the stable operation time of transient devices, encapsulation incorporating a protective thin outer-layer, such as oxide (MgO, SiO2), nitride (SiNx) and silk, was proposed to prevent them from premature failure.10–12 However, such predefined protective films provide a very limited control over the degradation process, which is unlikely to meet the diverse requirements of different applications. On the other hand, stimuli-responsive materials have been widely studied, with temperature, magnetic field, ultrasound, light, and electric pulses as the exogenous triggers.13–15 Among various physical stimuli, light is a very convenient choice, offering high spatial/temporal precision. By tuning the wavelength, intensity, irradiation time and spot location of light, it can easily signal and control the excitation at the right time and location.16,17

In this work, we propose a new encapsulation strategy for transient devices by introducing a bi-layer of light-responsive hydrogel and oxide thin film, and achieved not only prolonged


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stable operation, but also controllable transient process triggered by UV light. The hydrogel, crosslinked 3D networks of hydrophilic polymer chain, acts as a permeation barrier between the environmental solution and the protective oxide, reducing the amount of solution penetrating through the oxide. More importantly, the hydrogel undergoes a gel-to-sol phase transition in response to ultraviolent (UV) light due to the photo-induced cleavage reaction of the azo bonds, allowing more water to penetrate through the oxide layer and trigger the degradation process. This new strategy enables improved stable operation and controllable degradation of transient electronics on demand, offering the ability to meet various requirements for green electronic and biomedical devices.

2. EXPERIMENTAL SECTION 2.1. Hydrogel synthesis and characterization. Hydrogel has attracted wide investigation in the fields such as tissue engineering, biosensor, and drug delivery because of biocompatibility, biodegradability and stimuli-responsibility.18,19 Here, the hydrogel synthesis followed the process similar to that described in the report.20 Briefly, (E)-4,4′-(diazene-1,2diyl) bis (4-cyanopentanoic acid) and 4-hydroxybenzaldehyde in dichloromethane reacted in an ice bath, then N,N'-Dicyclohexylcarbodiimide (DCC) and 4-Dimethylaminopyridine (DMAP) were added into solution under argon atmosphere. After several hours, the solution was filtered and concentrated. The residual was purified with column chromatography (dichloromethane/methanol = 40:1) and the product was obtained as cross-linker. Then the cross-linker was dissolved in dimethyl sulfoxide and mixed with 4-arm-PEG- NH2 in distilled water to form the hydrogels, as shown in Figure 1b. The structure was identified by nuclear magnetic resonance (NMR) at the frequency of 400 MHz. To test the network of hydrogel, the sample was freeze-dried and then imaged by scanning electron microscope (SEM). For the weight loss testing, the hydrogel was formed in a bottle and heated or shined by UV light



(365 nm). After a certain time, the solution was carefully removed by a needle, and the remaining hydrogel was weighed. 2.2. Device fabrication and characterization. Two types of transient devices were fabricated to verify the effect of hydrogel-oxide bi-layer encapsulation including a dissolvable U-shape resistor and a 3×3 transient resistive random access memory (RRAM) crossbar array. The U-shape Mg (100 nm) resistor was fabricated by using e-beam lithography with its two ends covered by TiW to facilitate electrical testing during dissolution test. An area of 60 µm2 was patterned on top of the Mg resistor and subsequently deposited with MgO film or hydrogel-oxide bi-layer as the encapsulation. The 3×3 transient RRAM crossbar array was fabricated by standard semiconductor process. It was composed of W (70 nm)/MgO (10 nm)/Mg (100 nm), where W, MgO, and Mg served as the bottom electrode, switching layer, and top electrode, respectively. Nine transient RRAM devices were formed in a 3×3 crossbar array with TiW extended electrodes to facilitate electrical testing during dissolution test. Similarly, an area of 60 µm2 was patterned on top of the RRAM array and subsequently deposited with MgO film or hydrogel-oxide bi-layer as the encapsulation. In addition, thin films of W, MgO, Mg with 50 × 50 µm2 patterns were prepared individually for material dissolution study. The film thickness was measured by atomic force microscope. All the thin films were sputtered at room temperature. All the electrical characterizations were conducted by using Keithley 4200 parameter analyzer. The optical images during transient process were captured using microscope in real-time.

3. RESULTS AND DISCUSSION The structure of crosslinker and the morphology of hydrogel was identified by NMR and SEM (Figure S1). Under the stimulus of UV light, the hydrogel experienced a gel-to-sol transition, as shown in Figure 1a. In this transition, the azo bond of hydrogel absorbed UV


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light and cleaved, resulting in the degradation of 3D network and the subsequent irreversible phase change.21–23 The Fourier-transform infrared spectroscopy (FTIR) of gel and sol state also supports this claim, as plotted in Figure S2. To understand this phenomenon thoroughly, light-responsive performance of hydrogel was investigated in detail. Figure 1d presents the weight loss of hydrogel under UV light with 365 nm wavelength and different intensities from 100 mW/cm2 to 300 mW/cm2. The tiny changes of weight loss for hydrogel with density of 100 mW/cm2 indicated that it remained at gel state during the experimental period. This is because the energy of the low intensity light was insufficient to break the azo bonds and failed to induce a phase transition. As light intensity increased, the hydrogel gained more energy causing a phase transition from gel to sol state. These results further exhibited that a 100% increase in light intensity led to an about 150% reduction in phase transition time, indicating an accelerated phase transition with increased energy.

High thermal stability is important for biomedical applications of transient electronics. To assess the potential risk posed on human body, we studied the thermal performance of hydrogel at different temperatures. The data plotted in Figure 1e shows that the hydrogel was very stable at 37 °C, with no observed weight loss. The gel-to-sol transition was only activated when the temperature raised up to ~70 °C, which was further proved by the images of hydrogel taken at various temperatures as shown in Figure S3. This considerable good thermal stability suggests that hydrogel is suitable for applications in human body and other scenarios at below ~70 °C.



Figure 1. (a) Schematic illustration of the hydrogel-oxide bi-layer encapsulation design for controllable degradation of transient electronics. (b) Chemical structure of the 4-arms-PEG-NH2 and crosslinker and the reaction to form hydrogel. (c) Optical images showing the gel-to-sol transition of hydrogel under UV/heat condition. (d) Weight loss of hydrogel when it is triggered by 365 nm UV light with different energy intensities. (e) Thermal stability of hydrogel at different temperatures.


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Utilizing the light-responsivity of hydrogel, we designed a new encapsulation composed with a bi-layer of hydrogel and oxide as illustrated in Figure 1a. To evaluate its encapsulation functions, we fabricated U-shape Mg resistors and Mg/MgO/W RRAM devices as described in the experimental section. Here, Mg was selected to form a resistor because of ultrahigh sensitivity to biofluids. Tiny amount of biofluids reacting with Mg will cause a large resistance change, enabling an easy detection. RRAM was chosen as a representative transient device because memory is an important component of most electronic systems. W, MgO, Mg were used as bottom electrode, switching layer, and top electrode of memory device, because they are not only soluble, but also biocompatible owning to the nontoxic hydrolysis products without immune response. The crossbar array structure was employed to minimize the memory size for potential compact and light biomedical product design.

We first investigated the transient behaviors of the Mg resistors and Mg/MgO/W RRAM devices. The solubility of Mg, MgO and W thin film was tested individually in deionized (DI) water and phosphate buffered saline (PBS) at room temperature. Undergoing hydrolysis reactions

of

Mg+2H2O→Mg(OH)2+H2,

MgO+H2O→Mg(OH)2,

and

2W+2H2O+3O2→2H2WO4, the materials were fully dissolved in the DI and PBS solutions.24 The average room temperature dissolution rates of Mg, MgO and W were 0.66 nm/s, 0.34 nm/min, 1.13 nm/h in DI water, respectively, and 2.56 nm/s, 11 nm/min, 8 nm/hour in PBS, respectively, as shown in Figure 2a. Generally, the dissolution rate in PBS is much faster than that in DI water due to the ion-rich environment (such as Na+, Cl-, HPO42-) in PBS that speeds up the reaction.24 Figure 2b tracks the dissolution process of a U-shape Mg resistor in DI water. It can be noticed that the Mg resistor was quickly dissolved within 80 seconds with little residue because of high reaction rate with water. Figure 2c describes the time-resolved dissolution of the 3×3 RRAM array in DI water. After 1 minute of soaking, the Mg top



electrode was disappeared, and after two hours, the MgO layer was fully dissolved. Eventually, the whole RRAM structure vanished from Si wafer after about 24 hours’ soaking in DI water and only the insoluble TiW extended electrodes were remained. These results proved that the Mg resistor and Mg/MgO/W RRAM device are fully dissolvable, which are suitable for the functionality test of the proposed hydrogel-oxide encapsulation.

Figure 2. (a) Dissolution behavior of Mg, MgO, W in DI water and PBS solution at room temperature. (b) Optical images describing the dissolution of transient Mg resistor. (c) Optical images showing the timesequential dissolution of 3×3 transient RRAM crossbar array. The inset is a zoom-in image of the 3×3 RRAM crossbar array before soaking into the solution.

Degradation mechanism is very important for transient electronics, which lays a foundation for designing suitable encapsulation strategies. In order to understand the underneath mechanism, the U-shape Mg resistor was protected by two encapsulation designs individually,


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including a single MgO thin film and a hydrogel-MgO bilayer. The MgO film served as an oxide protection due to its high transparency, resistivity, and degradability (Figure S4). We first investigated the degradation mechanism of Mg/MgO structure, as shown in Figure 3a. Figure 3a plots the change in resistance of Mg resistors with different MgO thicknesses. The Mg resistors had initial low resistance value owning to high conductivity of Mg. When the Mg resistor was soaked in the DI water, it showed an abrupt increase of resistance within one minute because of the fast dissolution rate of bare Mg in water, as previously discussed in Figure 2a. This indicated that the stable operation time of Mg resistor was prolonged by covering a MgO layer. For Mg resistor covered with 50 nm, 100 nm, and 200 nm, the stable operation time was extended to 4 mins, 7 mins and 14 mins, respectively. However, it is worth noting that the measured stable operation times were much shorter than the calculated ones based on the dissolution rate of MgO thin film in Figure 2a. For example, considering the 0.34 nm/min dissolution rate of MgO film at room temperature in water, it took ~588 minutes to fully dissolve a 200 nm thick MgO protective layer, which was much longer than the measured 16 minutes in Figure 3a. The discrepancy was attributed to the pinhole defects in the MgO film. 25,26 The defects provided penetration paths for water molecules to permeate through the oxide film and reach the metal-oxide interface, resulting in the dissolution of Mg resistor. On the basis of the above discussion, we can deduce that the dissolution of Mg/MgO has two main stages: Stage 1 is the permeation of water molecules through the oxide protective layer, in which the morphology and electrical properties of Mg resistors were not affected; Stage 2 is the degradation of Mg resistor. When enough water permeates through oxide layer, rapid dissolution of Mg occurs resulting in significant increase in resistance. It can be expected that the stable operation time of transient electronics is mainly governed by Stage 1 as the oxide encapsulation usually has much slower degradation rate than other



components of the transient devices. By varying the penetration distance (oxide layer thickness), the stable operation time of transient electronics can be tuned precisely.

The Mg resistors protected with the hydrogel/MgO bilayer encapsulation were then investigated. The resistance testing without UV light irradiation was firstly performed to identify the degradation mechanism. Figure 3b plots the change in resistance against the thickness of MgO protective layer while maintaining the same hydrogel layer. The stable operation times were 10 mins, 20 mins and 45 mins, for the resistor with 50 nm, 100 nm, 200 nm thick MgO, respectively, showing significant improvement compared with pure MgO protection. Such improvement is due to hydrogel serving as a permeation barrier between the environmental solution and the protective oxide barrier, owning to two possible factors. Firstly, the solution environment of hydrogel is different from that of DI water. There are three states of water in hydrogel: unfrozen trapped water, frozen trapped water and free water.27–29 Unfrozen trapped water is closely associated with a polymer matrix. Frozen trapped water is the fraction bound to the matrix less closely and shows different behaviors from free water. Only free water shows similar properties as pure water and can cause the degradation of MgO and Mg. In our hydrogel layer, most water was trapped in the crosslink structure. There was a very small amount of free water existing in the polymer matrix. Secondly, the permeation of water molecules through the hydrogel to reach oxide is highly dependent on diffusion, which is notably prohibited by the friction between the hydrogel network.30,31 In order to understand this point clearly, the stable operation time of transient resistor covered by hydrogel and pure water was tested, where the hydrogel and pure water has the same amount of water (Figure S5). The results show that the transient resistor with hydrogel has much longer lifetime than that with water, which means that only a small portion of water in hydrogel is free that can affect the stable operation time of transient


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device. Due to the presence of hydrogel, less free water reaches the surface of MgO layer within the same time comparing without hydrogel, resulting in increased stable operation time. The stable operation time of transient resistor with different thickness of hydrogel was also studied, as shown in Figure S6. It can be found that the increased thickness of hydrogel has little effect on the stable operation time of the transient resistor. The high water permeation rate inside the hydrogel maybe responsible for this phenomenon (Figure S7). It takes only several seconds for water to permeate through hydrogel with different thickness.

We next evaluated the light responsive behavior of Mg resistor with the bi-layer encapsulation of hydrogel/MgO (200 nm) by applying UV light. As the device has a stable operation time of about 45 mins, we irradiated 365 nm UV light with 300 mW/cm2 intensity at 10 mins and 20 mins after soaking in DI water, respectively. As shown in Fig 3c, if UV light was triggered at 10 mins, the degradation of Mg resistor occurred at ~20 mins; while if it was triggered at 20 mins, the stable operation was extended to ~30 mins. The results showed that Mg resistor was degraded at ~10 min after UV irradiation was applied. The high energy of UV light destroyed the 3D network of hydrogel by breaking the azo bond and resulted in a gel-to-sol transition. The collapse of network, either complete or partially complete, allowed more water to pass through the hydrogel and penetrate through the MgO protective layer, leading to a failure of the Mg resistor. Under this circumstance, the stable operation of transient devices contains two main parts: normal stable operation and the delay after triggering. The normal stable operation is the operation time of Mg resistor covered by encapsulation without UV treatment before failure; while the trigger delay time refers to the stable operation time after UV irradiation. From the above discussion, we notice that the normal stable operation is highly dependent on the thickness of oxide layer, while the trigger delay time is almost fixed at ~10 mins. Only when the normal stable operation time is larger



than the trigger delay time, the UV trigger is effective that could enable a controlled degradation on demand. Hence, the oxide protective layer must meet a minimum thickness requirement. To find out the thickness design requirement and the suitable working window that the triggering action can be imposed, we plotted the stable operation times of the hydrogel-oxide encapsulated device (red line) without UV irradiation and of the oxide encapsulation (black line), respectively in Fig. 3d. The latest effective time (blue line)of applying a UV light can be calculated by minus 10 mins (the trigger delay time) from the stable operation time of the hydrogel-oxide encapsulated device without UV irradiation. The corresponding thickness of the intersection point A represents the required minimum thickness of oxide. The shaded area indicates the working window, in which the triggering via UV light can be effectively activated for a controllable degradation.


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Figure 3. (a) Degradation time of Mg resistor with various thickness of MgO encapsulation layer. (b) Transient time of Mg resistor with oxide-hydrogel encapsulation design. (c) Control the degradation time of transient electronic device by UV light using hydrogel-oxide encapsulation. (d) Trigger time area and minimum thickness of oxide investigation by relationship between stable operation time and thickness of MgO oxide.

Transient RRAM has drawn a lot of attention recently due to its simple structure, high scalability, low power consumption and great potential in bio-application and data security.5,32–35 It has been reported to store information during the therapy.33 However, controllable stable operation time is not realized yet on transient RRAM. Here, we applied the light responsive hydrogel-MgO (200 nm) bi-layer encapsulation on the transient W/MgO/Mg RRAM array, which has a device structure as shown in Figure S8. The setup of the light response test is schematically described in Figure 4a. We first tested the electrical performance of the transient RRAM device. It required a forming process to display a bipolar



switching behavior (Figure S9). The inset of Fig. 4a shows a typical current-voltage behavior. Applying a voltage sweeping, the device switched from low resistance state (LRS) to high resistance state (HRS) at 0.6 V, and then returned back to HRS at -0.6 V. It also exhibited a good retention and cycling performance, as shown in Figure S10. The switching mechanism can be explained by the well reported filament model that is associated to the formation and rupture of metallic Mg filament in MgO switching layer (Figure S11).36

The stable operation time of W/MgO/Mg device without any protection in DI water was then studied. The device displayed a very fast degradation within 1min due to rapid dissolution of Mg, indicated by an electrical failure in Figure 4b. The broken Mg electrode caused the RRAM device to remain at the HRS state and impossible to switch back to the LRS. For the W/MgO/Mg device with the hydrogel/MgO bi-layer encapsulation, it displayed a significant improvement on stable operation time, as shown in Figure 4c. A stable LRS-HRS-LRS switching was achieved for 40 mins before a device fail. After that, the whole device was gradually dissolved in the solution (Figure S12). We then performed the light triggered degradation test as presented in Figure 4d. UV light was applied at two different times before its self-degradation occurred: at the point that the device was soaked into DI water (refers as 0 min) and 10 mins after soaking (refers as 10 mins). For the device triggered at 0 min, an electrical fault was occurred at ~ 12 mins while for the device triggered at 10 mins, a device failure occurred at ~18 mins. These results demonstrate that the stable operation time of transient memory device can be manipulated on demand via the hydrogel-oxide encapsulation strategy. By exploring other types of oxide materials and hydrogel, it can be expected that the stable operation with various duration, from mins to days or years, is achievable.


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Figure 4. (a) Light trigger testing scheme and typical I-V curve of transient W/MgO/Mg resistive memory device. (b) Electrical performance of RRAM device before and after degradation. (c) Degradation investigation of memory with 200 nm MgO and hydrogel layer without light trigger. (d) Controllable degradation of transient RRAM device by UV light.

4. CONCLUSION In summary, we demonstrate that through a novel light responsive hydrogel-oxide bi-layer encapsulation, the degradation on-demand of transient RRAMs and resistors with tunable stable operation time was achieved. The light-responsive hydrogel functions as an obstacle to separate the oxide and the aqueous solution or biofluids, which limits water permeation through the oxide layer, leading to prolonged stable operation time of transient electronics. More important, the gel-to-sol transition of hydrogel under UV light provides the



controllability of triggering the degradation. The required minimum thickness of the oxide layer to effectively implement this new encapsulation strategy was analyzed, which can be used as a design guideline for further research. The demonstration in this work provides a new pathway to develop transient electronics with degradation on-demand.

ASSOCIATED CONTENT Supporting Information The supporting information includes the NMR of crosslinker and SEM image of hydrogel, FTIR

spectrum of hydrogel with gel state and sol state, hydrogel state under different temperatures, UV-VIS transmission spectrum and leakage current of MgO film, degradation time of transient resistor under different condition: hydrogel and water, resistance of transient resistor with various hydrogel thicknesses, schematic of experimental design to test the permeation rate of hydrogel, schematic diagram and optical image of transient memory device, forming process of RRAM device, data retention and cycling performance of RRAM device, resistive switching mechanism of W/MgO/Mg device, captured image of dissolution of the whole structure.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by A*STAR, Public Sector Research Funding (Grant number: 1521200085), Singapore.


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Enabling Transient Electronics with Degradation on Demand via Light

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