Article Cite This: Chem. Mater. 2018, 30, 781−788
pubs.acs.org/cm
Harnessing Recombinant DnaJ Protein as Reversible Metal Chelator for a High-Performance Resistive Switching Device Sung Kyu Jang,† Sookyung Kim,† Muhammad Saad Salman,‡ Ji-ryang Jang,§ Yu Mi Um,∥ Lihan Tan,⊥ Jin-Hong Park,# Woo-Seok Choe,*,†,‡ and Sungjoo Lee*,†,# †
SKKU Advanced Institute of Nano-Technology (SAINT), ‡School of Chemical Engineering, and #College of Information and Communication Engineering, Sungkyunkwan University (SKKU), Suwon, Gyeonggi-do 16419, Republic of Korea § Research Institute of Biotechnology, CJ CheilJedang Corp., Suwon, Gyeonggi-do 16945, Republic of Korea ∥ Samsung BioLogics, 300 Songdo Bio Way (Daero), Yeonsu-gu, Incheon 21987, Republic of Korea ⊥ Downstream Processing Group, A*STAR, Bioprocessing Technology Institute, 20 Biopolis Way, Centros #06-01, Singapore 138668, Singapore S Supporting Information *
ABSTRACT: A high-performance biomaterial-based resistive switching (RS) device is fabricated by harnessing a thermally denatured protein (hexa-His-tagged recombinant molecular chaperone DnaJ (rDnaJ)) as a switching layer in a Cu/rDnaJ/ Pt configuration on SiO2/Si substrate. The conductivity of the heat-denatured rDnaJ protein layer between the metal electrodes can be reversibly controlled to enable the formation/rupture of conductive Cu filaments by tailoring the metal chelating properties of the amino acid residues in the insulating protein matrix in a pH- and/or redox potential-dependent manner, giving rise to high-performance nonvolatile RS behavior. The rDnaJ-based RS device exhibits extremely low set voltage (∼0.12 V) and reset voltage (∼−0.08 V) with excellent uniformity, along with large memory window (RHRS/RLRS > 106) and long retention time (>106 s). In addition, the rDnaJ RS device, which is fabricated on a flexible poly(ethylene terephthalate) substrate, exhibits an uncompromised switching performance. The present study is the first attempt to explore the use of a recombinant protein as a functional switching layer in RS devices. This approach opens up a new method of harnessing recombinant proteins with engineered properties as powerful building blocks to suit the requirements of next-generation biocompatible, flexible, high-performance, and low power consumption electronics.
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modulated by an electrical stimulus. The RS device, also called a conductive bridging random access memory device or an atomic switching device, is an essential building block for information storage and logic components owing to its high density, large scalability, low power consumption, and high reliability. Recently, several groups have demonstrated the fabrication of RS devices using a variety of biomaterials.3,6−12 Among these, proteins, essential components of organisms, have been researched for use in switching layers.13−23 Such studies have demonstrated that natural biomaterials can effectively simplify the fabrication process and reduce the manufacturing costs associated with organic memory devices. Several studies have shown that the mechanism underlying the resistance switching behavior involves modulating the electrochemical properties of the biomaterials. The switching mechanisms were attributed to filamentary conduction,24 redox reactions,13 and/or charge hopping.14 Biomaterials such
here is a growing demand for biomaterials that can be used as the functional components of future devices. This demand is motivated by the need to fabricate building blocks suitable for next-generation biocompatible electronic/photonic devices for a variety of applications, such as flexible devices and low power consumption green electronics, via a simple fabrication process at a reduced cost. The features of biomaterials that are important for electronic and optoelectronic device applications include their flexibility, scalability, low production cost, biocompatibility, and sustainability. These properties can overcome the limitations of conventional siliconbased electronics by creating passive and active biomaterial components, especially in the areas of wearable information processing electronics, smart skin, and biomedical applications. Several studies have examined the use of biomaterials as active layers in devices, such as transistors,1 diodes,2 memory devices,3 sensors,4 and photonic devices.5 One of the most promising applications of biomaterials in future electronic devices is in resistive switching (RS) devices, used in next-generation information storage/processing devices, wherein the biomaterial acts as the main switching layer with a resistance that is © 2018 American Chemical Society
Received: October 10, 2017 Revised: December 19, 2017 Published: January 11, 2018 781
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Chemistry of Materials
Figure 1. (a) Schematic diagrams of the fabricated rDnaJ RS device arrays (left), the enlarged single device with a conductive filament (middle), and the degree of deprotonation of the imidazole nitrogen in histidine, as a function of pH (right). (b) Surface charge diagram illustrating the degree of protonation of imidazole in histidine.
as DNA,6 dronpa,15 ferritin,16 fibroin,17,18 lysozyme,19 gelatin,20 and biocomposites7,21 can provide conduction pathways for redox reactions, and metal ions dissolved from the active electrode8,9 or introduced into the biomaterial10 can form metallic conductive bridges. RS devices based on charge trapping and detrapping in biomaterials,11,14 the composition of the biomaterial,12 and the formation of biomaterial−metal nanoparticle complexes3,22 have also been reported. Although these studies have demonstrated the feasibility and potential advantages of protein-based RS devices, the device performances (e.g., in terms of mechanical stability and power consumption) must be further improved to enable wide usage in new functional memory and electronic circuits. The operation of RS devices critically depends on the formation of conductive bridges between the two electrodes. To satisfy the requirements of a sustainable nonvolatile memory and switching device, comprehensive and precise control over the formation and rupture of conductive nanofilaments embedded in the biomaterials using novel pathways is critically required.
tional inorganic insulating materials. Designer proteins will further enlarge the library of potential protein candidates to be used as the switching layer in a RS device. This will greatly increase the probability of finding a material with high functionality to suit the specific applications. In addition, properly designed recombinant protein can be produced in high yields and at a high purity, facilitating the fabrication of next-generation electronic devices. Second, DnaJ protein is a well-known ubiquitous molecular chaperone found in all prokaryotes and eukaryotes, including humans, which makes it less likely to be immunogenic during implantable bioelectronics applications. Third, DnaJ protein is highly efficient in recognizing the hydrophobic patches (i.e., enriched with aliphatic and/or aromatic amino acid residues) of nascent and/or unfolded polypeptides.25 This property is hypothesized to enable seamless interfacing with the hydrophobic surface of a Pt electrode, ensuring the facile and uniform assembly of an insulating protein layer with a controlled thickness atop the electrode. Fourthly, DnaJ has a unique zinc finger domain known as a CR (cysteine rich) region26 that is likely to be effective in chelating soft metal ions, such as zinc and copper. Considering that our MIM configuration employs copper as an active electrode from which abundant Cu ions may be sourced upon application of a positive potential, conductive filament formation was expected due to the intrinsic copper ion chelation properties of the DnaJ protein layer intervening between the top and bottom electrodes. Finally, the DnaJ protein was engineered to accommodate hexa-histidine residues at its N-terminal permissive site with negligible effect on its hydrophobic interactions with cognate substrates.27 The use of His-tagged recombinant DnaJ (rDnaJ) may empower and/or modulate the chelation of Cu ions by the insulating protein layer in a pH-dependent manner, which is critical to controlling the availability and location of Cu ions immobilized in the insulating protein layer. Unless otherwise stated, a thermally denatured protein, chaperone rDnaJ, was used as a switching layer, based on its protonation-dependent metal ion interaction properties, and was applied in a uniform layer between a Pt bottom electrode and a Cu top electrode. By tailoring the metal chelating properties of the hexa-histidine residues in the protein layer, which displayed a different affinity for metal ions
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RESULTS AND DISCUSSION Protein Resistive Switching Device Integrated with rDnaJ. In this work, we employed a simple two-terminal metal/insulator/metal (MIM) structure comprising a recombinant molecular chaperone DnaJ, Cu, and Pt as the insulating matrix and the top (active) and the bottom (inert) electrode, respectively. The rationale behind the use of recombinant DnaJ is as follows: First, it should be noted that all previously reported RS device studies that have used proteins as the insulating layer have tested natural proteins, such as fibroin,17,18 sericin,14 gelatin,20 albumen,12 and ferritin,16,23 with or without the presence of metal ions. No study has yet addressed the feasibility of using a recombinant protein as the functional switching layer in a RS device component. It would be helpful to understand whether recombinant proteins with genetically engineered properties (e.g., with the inclusion of hexa-His-tag) could result in enhanced performance as the switching layer in a RS device as compared to native proteins. If this is the case, this work provides a new potential approach to harnessing designer proteins in place of natural proteins and/or conven782
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Chemistry of Materials
Figure 2. (a) I−V characteristics of the rDnaJ (prepared at pH 6) RS devices. (b) A double-logarithmic scale plot for the set and reset processes in I−V curves. Linear fits are shown in the figure. (c) The endurance characteristics for the cyclic set/reset process over approximately 100 cycles at Vread = 0.03 V. The inset shows the distribution of the cumulative probabilities of the resistance values at LRS (RLRS) and HRS (RHRS). (d) The data retention performance up to 106 s.
with thicknesses less than 10 nm, suitable for RS device fabrication, were formed. In particular, it is noted that the use of 2 μM denatured rDnaJ led to the formation of a very thin (4.09 nm) and uniform (root-mean-square roughness of 0.43 nm) insulating protein layer (Figure S2a-1), which is selected as a basis for fabrication of high performance RS devices demonstrated in the present study. Through this optimization process of the thickness, the 4 nm thick rDnaJ switching layer is sufficiently thin for strong electric fields to be applied even at low voltages and thus suppresses unwanted migration of Cu ions due to thermal vibrations. As shown in Figure 1b, the rDnaJ layer sandwiched between the Cu and Pt electrodes worked as a functional switching layer, depending on the degree of chelation of cupric ions oxidized from the Cu electrode under a positive potential bias. The chelation was modulated by the protonation and/or deprotonation status of the imidazole group in histidine residue dependent on the pH. The rDnaJ protein contains 16 histidine residues in total, including a hexa-His-tag fused to its N-terminus. In contrast, 50% of the 10 histidine residues in the wild-type DnaJ are localized to its CR domain where they coordinate cysteine residues to chelate cupric ions. In the present study, a pH range of 5−8 was employed. Because the sulfhydryl group (pKa = 8.2) in cysteine remained mostly protonated in this pH range, it was likely that most of the cupric ion chelation by rDnaJ was mediated by histidine residues. Within this range, histidine residues acted as strong chelators of cupric ions when the imidazole nitrogen was deprotonated, thereby providing a lone pair of electrons to the electron-deficient metal ions. The
depending on the protonation/deprotonation state at a given pH, and thus controlling the metal ion interactions in the protein switching layer, we obtained biomaterial-based highperformance nonvolatile RS devices. The extremely low set and reset voltages of 0.12 and −0.08 V, respectively, were highly uniform, with standard deviations of 0.015 and 0.029 V, respectively. The large memory window (RHRS/RLRS > 106) and a long retention time of >10 6 s were achieved. We demonstrated the implementation of LATCH circuits by integrating chaperone rDnaJ RS devices. Our results show great promise for employing bioengineered materials in the area of future biocompatible nonvolatile information storage devices, information processing switching devices, and their integrated logic circuit applications. Protein RS devices that use chaperone rDnaJ as the RS layer between the Pt bottom electrode and the Cu top electrode were fabricated (Figure 1a). The technique most commonly used to form an organic layer between two electrodes is spincoating, which suffers from the disadvantages of producing a nonuniform coating. We used a thermal denaturation method to form a protein layer on the metal surface, which overcame the aforementioned disadvantage and provided an atomically thin (2−4 nm) and uniform layer that readily enabled electrochemical modulation of the protein switching layer in a protonation-dependent and structure-dependent manner. The thermal denaturation processes and features of the resulting protein layer are described in detail in Figures S1, S2, and S3. In the denatured rDnaJ concentration range from 2 to 8 μM (in Figure S2a−d), highly uniform thin insulating protein layers 783
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Chemistry of Materials resulting chelate sites act as oxygen vacancies in traditional oxide-based CBRAM to assist redox reaction and transportation of Cu ions,28 contributing to anchoring of oxidized Cu ions at largely uniform spacing in the protein matrix between Cu (top) and Pt (bottom) electrodes. Subsequently, the reduction of chelated Cu ions by electrons sourced from the bottom electrode results in the formation of metallic copper filament. It facilitates the formation of Cu filaments through improved redox reaction and transportation of Cu metal/ions in the resistive switching layer.29 The acid dissociation constant (pKa) of the imidazole side chain in histidine is approximately 6.30 At a pH above this pKa value, the imidazole ring is mostly deprotonated, and metal−protein coordination can occur. At a pH lower than this pKa value, the imidazole ring is mostly protonated, as described by the Henderson−Hasselbalch equation.31 While protonated, the imidazole ring bears two NH bonds and has a positive charge that is evenly distributed across the two nitrogen atoms, producing a net charge of zero on the imidazole group. As a result, the metal chelating capacity of the rDnaJ layer could be controlled by changing the solution pH. This process is summarized in Figure 1b. This property facilitated simple yet effective control of the amount of electrode-sourced Cu ions available in the switching layer, thereby allowing optimization of the device characteristics. Characteristics of the Protonation-Controlled rDnaJ Resistive Switching Devices. The protein RS devices were electrically characterized in the DC voltage sweeping mode over repeated operation cycles (Figure 2a). The DC bias was applied to the top Cu electrode while the bottom Pt electrode was grounded. Initially, the as-prepared device was in a high resistance state (HRS). As the voltage was swept from zero to positive values with current compliance, the current abruptly increased once the bias reached a threshold voltage of 0.12 V. This is called the SET process, and the corresponding voltage is defined as the set voltage (Vset). After the SET process, the device remained in a low resistance state (LRS). As the voltage was swept from zero to negative values, the resistance state changed from the LRS to the HRS at a voltage of −0.08 V, defined as the reset voltage (Vreset). This is called the RESET process. As shown in Figure 2a, the fabricated rDnaJ device displayed stable bipolar RS properties over multiple switching cycles while maintaining extremely low Vset and Vreset values, which is a key factor for satisfying the requirements of low power operation in future electronic devices. In Table 1, the Vset and Vreset values obtained from the rDnaJ switching device are compared with those of other recently reported RS devices using various biomaterials including proteins and carbohydrates. Our rDnaJ RS device exhibited significantly lower Vset and Vreset values with a high on/off ratio above 106, along with excellent uniformity. In Figure 2b, the I−V curve obtained from the rDnaJ protein device was plotted on a double-log scale to further investigate the conduction mechanism. The slopes of I−V curves were ∼1 in both the HRS and LRS over the entire voltage range. These results suggested that the current flow in the rDnaJ protein device was dominated by Ohmic conduction rather than by space charge limiting current conduction, which is generally observed in a variety of RS devices.32,33 Cyclic endurance and data retention tests were carried out to further investigate the stability of the rDnaJ protein RS devices (Figure 2c,d). A cyclic voltage was applied from 0 to 0.5 V and then from 0 to −0.3 V, and a read voltage of 0.03 V was employed to avoid a resistance state change. The rDnaJ RS
Table 1. Performance Comparison of Biomaterial-Based RS Devices switching material tobacco mosaic virus Ag-doped chitosan dry-cured albumen sericin silk fibroin lysozyme/ PSS (PAH/ ferritin)15 NiO/ferritin rDnaJ-6
Vset (V) mean/std. dev.
Vreset (V) mean/ std. dev.
RHRS/ RLRS
retention time (s)
ref
3.1/NA
−2.4/NA
103
103
3
0.5/NA
−0.5/NA
105
104
8
0.6/NA
−2.2/NA
103
104
12
2.5/NA 10.4/NA 1.0/NA
−0.8/NA −11.5/NA −1.3/NA
106 10 103
103 103 104
14 17 19
1.8/NA
−1.5/NA
102
104
21
1.8/NA 0.12/0.015
−0.8/NA −0.08/0.029
104 >106
N/A >106
23 this work
device revealed that the LRS and HRS states remained stable over approximately 100 repeated cycling tests (Figure 2c) and a test period of 106 s (Figure 2d). The current ratio between the LRS and HRS state is above 106. The reliability of the device was assessed by measuring the current−voltage characteristics repeatedly in several devices. The inset of Figure 2c shows the distribution of the cumulative probabilities of the resistance at the LRS (RLRS) and HRS (RHRS). Each data point was obtained from a total of 120 switching cycles applied to 12 randomly selected devices. The RLRS and RHRS displayed average resistance values of about 5 kΩ and 2 GΩ, respectively, and these values fluctuated by about 1.1 orders of magnitude, maintaining a memory window of at least 104. The endurance and retention characteristics of the rDnaJ RS devices were superior to those obtained from previously reported switching devices fabricated using other biomaterials (Table 1). Figure 2d plots the RHRS and RLRS values obtained from the rDnaJ RS devices as a function of the retention time. In previous studies, organic RS devices suffered from low on−off ratios and short retention times due to a lack of electrochemical controllability.3,14,18,23 Our results revealed that the use of a controlled rDnaJ switching layer provided biomaterial-based high-performance nonvolatile memory and switching devices. Figure 3a plots the Vset and Vreset values of the rDnaJ protein RS devices, in which the protonation states of the rDnaJ proteins were controlled by varying the pH. For simplicity, RS devices with an rDnaJ switching layer prepared at pH 5, 6, 7, and 8 will be designated hereafter as rDnaJ-5, rDnaJ-6, rDnaJ-7, and rDnaJ-8, respectively. The SET and RESET voltages were found to be the largest in rDnaJ-5 and the smallest in rDnaJ-6, although they further increased as the pH increased. This trend indicated that, to achieve stable low-power switching operation, the rDnaJ RS layer prepared at pH 6 provided the optimal performance. In addition, the pH-dependent controllability of Vset and Vreset of rDnaJ RS devices can be tailored to the requirements of the application circuit, such as noise margin and operating voltage. Nonvolatile switching behavior was obtained from the rDnaJ RS device, attributed to the formation and rupture of conductive filaments. The positive bias induced dissolution of Cu atoms, in the form of Cu ions, into the protein switching layer, with the help of amino acids with a metal binding affinity. The Cu ions then diffused toward the bottom electrode and 784
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Figure 3. (a) The distributions of Vset and Vreset as a function of pH of the buffer used to form the switching layer. (b) Schematic illustration of the Cu atom distribution throughout the rDnaJ RS devices in the LRS and HRS. Red sites represent the deprotonated imidazole groups in histidine. (c) Schematic diagram of the CAFM used to observe the conductive filaments. (d) CAFM current mapping image of a conductive filament in the rDnaJ6 RS device after removing the top Cu electrode. The inset shows an enlarged CAFM image of the indicated area. (e) Cumulative histogram of current distribution profiles measured from the conductive filaments formed within each rDnaJ layer.
affinity of the protein (Figures 1 and S5) and (ii) a low-power operation biomaterial switching device with excellent uniformity could be designed by optimizing the protonation state of the protein (i.e., rDnaJ-6 which displayed extremely low Vset ∼ 0.12 V and Vreset ∼ −0.08 V with limited variability as shown in Figure 3a). As a result, the rDnaJ-6 devices displayed excellent nonvolatile RS characteristics (Figure 2) that outperformed the previously reported experimental results obtained from other similar devices (Table 1). The metal chelation affinity also can be controlled by changing the metal ion or the protein with different amino acid composition. Other metal ions (such as Cu, Ni, Zn, and Co) or other proteins can be used to adjust device operating parameters or implement new functionality. Further investigations are required to fully develop a new method of harnessing recombinant proteins with engineered properties. A variety of RS devices have displayed a quadratic dependence of the current on voltage prior to the SET process, due to current fluctuations caused by space charges and traps in the dielectric film.34−36 The differences observed in our rDnaJ RS device, including observation of the dominant overall Ohmic conduction (Figure 2b), most likely are attributable to the Cu ion-friendly amino acids present in the rDnaJ protein, which lowers the barrier for injection of Cu ions but not electrons. The enhanced Cu ion injection protects the resistance state of the device from charge distributions that produce an unintended transitional state, and the devices can be switched between LRS and HRS states without experiencing charge/trap related conduction. Another widely reported observation on RS devices in general is that a “forming process” at a higher voltage stress is needed when the asprepared device first switches from the HRS to the LRS.37 In contrast, our rDnaJ RS device did not display such a forming process (Figure 2a), which suggests that metal ion transport in the thin rDnaJ switching layer was facilitated by the chelating sites on the proteins. Direct Observation of Conductive Filaments. The conductive filaments formed within the rDnaJ switching layer
were reduced to Cu atoms. This process could be repeated so that a Cu filament formed between the two electrodes, and the resistance state of the device was changed from a HRS to a LRS. This phenomenon was not observed in a device prepared with Pt electrodes at both electrodes (Figure S4). Application of a reverse voltage resulted in filament rupture via thermally assisted electrochemical dissolution. Figure 3b shows a schematic illustration of Cu conductive filament formation for each rDnaJ device under a positive bias applied to the Cu top electrode. The oxidation and reduction of Cu ions at the top Cu electrode was responsible for the RS device operation, and the amount of Cu ions chelated in the switching layer was controlled by the degree of protonation in the rDnaJ layers. The correlation between the amount of chelated Cu ions and the degree of protonation was confirmed by electrochemical measurements (see Figure S5 for details). In rDnaJ-5, most of the lone pair electrons in the imidazole nitrogen in the histidine residues were protonated and hence unavailable to interact with the metal ions. According to the Henderson−Hasselbalch equation,31 the fractions of deprotonated imidazole nitrogen groups in the histidine moieties were 0.09, 0.50, 0.91, and 0.99 at pH 5, 6, 7, and 8, respectively. Therefore, in rDnaJ-5, only a small number of the Cu ions were chelated, and a large voltage was required to SET the device. This led to the formation of thick filaments that increased the voltage required for RESET, resulting in a large operating voltage. These results suggested that the performance could be improved by increasing the chelation of Cu ions into the switching layer. On the other hand, in rDnaJ-8, the chelation of Cu ions was excessive, and thick filaments were formed because it was difficult to precisely control the filament formation. Significant amounts of energy were required to remove any excess Cu ions from the rDnaJ layer. Consequently, a high operating voltage and large SET/ RESET voltage distributions were observed. In this case, it was possible to improve the performance by reducing the rate of Cu ion chelation. The SET and RESET voltages measured from the rDnaJ RS devices (Figure 3a) indicated that (i) the device performances depended significantly on the metal chelation 785
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Figure 4. (a) Schematic diagram of the SR latch and (b) its operating result. (c) Current compliance-mediated multilevel RLRS state characteristics of the rDnaJ RS device. The inset shows the I−V curves obtained using different current compliance values. (d) Resistance (RLRS and RHRS) as a function of bending radius for the rDnaJ RS device fabricated on the flexible PET substrate.
device operation (Figure 3a) for low-power data storage and information processing device applications. Broader current profile distributions were observed for the rDnaJ-5, rDnaJ-7, and rDnaJ-8 devices. These results revealed that the filament size could be effectively controlled by the degree of protonation within the switching layer, as described in Figure 3b. SR Latch Circuit and Flexible Devices. The nonvolatile properties of the rDnaJ RS device could be further utilized to construct a sequential logic circuit (SR latch), a crucial building block in digital information processing systems. The SR latch included two output signals (Q and Q̅ ), the states of which could be controlled and stored with respect to two input signals: S (set) and R (reset). Figure 4a,b plots the schematic diagram and operation of an SR latch circuit composed of two rDnaJ-6 RS devices. When both S and R were low, no voltage was applied to each device, and the rDnaJ-6 latch output held its previous value. If either S or R was high while the other was low, the output Q was forced to assume a high or low state, and Q remained fixed until the signals applied to S or R changed. These results successfully demonstrated the construction of sequential logic circuits using integrated rDnaJ-6 RS devices, revealing that the rDnaJ-6 RS device can form a fundamental building block for future digital electronic systems. In the LRS state, the current of the rDnaJ RS device was found to be inversely proportional to the current compliance. These results suggested that multilevel switching operation could be achieved if the current compliance was controlled during the SET process. Figure 4c plots the current compliance-mediated multilevel state characteristics of the rDnaJ RS device under repeated SET/RESET operation. The SET and RESET voltages remained almost constant whereas the LRS resistance increased as the current compliance conditions changed from 1
were investigated by performing conductive atomic force microscopy (CAFM) measurements. Figure 3c presents a schematic diagram of the CAFM measurement configuration. The sample was prepared by etching the Cu top electrode of each rDnaJ device after applying the electrical SET process. The local filamentary conducting paths within the rDnaJ layer were scanned using the CAFM tip within a 5 × 5 μm2 area. AFM measurements were simultaneously collected to investigate the surface morphology of thermally denatured rDnaJ on the platinum layer and to monitor damage caused by Cu overetching. The surface of the rDnaJ layer, onto which the Cu electrode was etched, displayed a uniform root-mean square roughness of less than 1 nm, irrespective of the degree of protonation. Figure 3d presents a surface current mapping image of the rDnaJ-6 device obtained from the CAFM measurements. These results revealed that the filamentary conducting paths formed in the LRS but not in the HRS state (Figure S6). The high-magnification CAFM image displayed in the inset of Figure 3d reveals a well-defined local filament with an area of ∼90 nm2. Figure 3e presents the current distribution profiles measured from the conductive filaments formed within each rDnaJ layer prepared with different degrees of protonation (rDnaJ-5, rDnaJ-6, rDnaJ-7, and rDnaJ-8). The CAFM results under each condition used in the graph may be found in Figure S7. The maximum filament currents measured from the rDnaJ5, rDnaJ-6, rDnaJ-7, and rDnaJ-8 layers were 2.2, 1.8, 2.1, and 2.4 nA, respectively. Importantly, each sample displayed a distinct current distribution profile, indicating the size of the conductive filaments in each switching layer. A narrower distribution in the filament current was measured in the case of rDnaJ-6, indicating thinner filament formation, which is critical to lowering the Vset/Vreset and suppressing variability in the RS 786
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
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Chemistry of Materials to 60 μA. The LRS states were completely restored to the HRS state through the RESET process. A LRS multilevel state with a resistance ratio of ∼100 was deterministically implemented by adjusting the current compliance. The SET and RESET voltages remained approximately constant while the LRS resistance and current levels changed over orders of magnitude in response to variations in the compliance current. The excellent switching characteristics and nonvolatile capabilities of the rDnaJ RS device were also employed to demonstrate the sequential logic circuits (Figure 4a). In addition, flexible devices have attracted increasing attention due to their potential applications in wearable electronic devices and biomedical devices. A flexible switching device based on a biocompatible material would be highly desirable. The rDnaJ RS devices were fabricated on polyethylene terephthalate (PET) transparent and flexible substrates. We measured the resistance of LRS and HRS as a function of the bending radius to investigate the mechanical bending stability of the rDnaJ RS device on a PET substrate. As shown in Figure 4d, no significant variations in the resistance or ON/OFF ratio were observed above an ON/OFF ratio of 106, demonstrating the excellent flexibility of our rDnaJ RS device.
ORCID
Jin-Hong Park: 0000-0001-8401-6920 Sungjoo Lee: 0000-0003-1284-3593 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) of the National Research Foundation of Korea (NRF) and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2014 M3C1A3053024), and the Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government (MSIP) (grant nos.: 2015R1D1A1A09057297, 2015M3A7B7045496, 2017R1A4A1015400, and 2017R1A2B2011341).
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CONCLUSION In summary, we successfully prepared high-performance biocompatible nonvolatile data storage and RS devices by integrating an rDnaJ switching layer, with controllable metal ion (cupric ions) chelating properties, into an rDnaJ RS device. The low-power operation (Vset ∼ 0.12 V, Vreset ∼−0.08 V) and nonvolatile RS devices were fabricated by integrating pHcontrollable metal chelating sites in the form of the rDnaJ protein. The rDnaJ thin switching layer was deposited by thermal denaturation, and optimal conductive bridges within the switching layer could be controllably formed by adjusting the protonation state of the protein. CAFM and cyclic voltammetry measurements revealed the structures of the filaments and the dependency of electrical switching characteristics on protonation states of the rDnaJ switching layer. The excellent switching properties and nonvolatile characteristics of the rDnaJ RS devices were deployed in SR latch sequential logic circuits. rDnaJ RS devices were shown to successfully enable multilevel switching operation and the reliable operation of flexible devices. These results show great promise of the rDnaJ RS device as a fundamental building block for various device applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04261. Cloning, expression, and purification of rDnaJ; preparation of the rDnaJ-coated substrates; device fabrication and electrical characterization; electrochemical measurements (cyclic voltammetry); conductive atomic force microscopy images (PDF)
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[email protected] (S.L.). 787
DOI: 10.1021/acs.chemmater.7b04261 Chem. Mater. 2018, 30, 781−788
Article
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