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Transparent and flexible resistive switching memory devices with a very high ON/OFF ratio using gold nanoparticles embedded in a silk protein matrix
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Nanotechnology 24 345202 (http://iopscience.iop.org/0957-4484/24/34/345202) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 345202 (7pp)
doi:10.1088/0957-4484/24/34/345202
Transparent and flexible resistive switching memory devices with a very high ON/OFF ratio using gold nanoparticles embedded in a silk protein matrix Narendar Gogurla1 , Suvra P Mondal1,2 , Arun K Sinha1 , Ajit K Katiyar1 , Writam Banerjee1 , Subhas C Kundu3 and Samit K Ray1 1 2 3
Department of Physics and Meteorology, Indian Institute of Technology Kharagpur, 721302, India Department of Physics, National Institute of Technology Agartala, 799055, India Department of Biotechnology, Indian Institute of Technology Kharagpur, 721302, India
E-mail:
[email protected]
Received 30 March 2013, in final form 2 July 2013 Published 2 August 2013 Online at stacks.iop.org/Nano/24/345202 Abstract The growing demand for biomaterials for electrical and optical devices is motivated by the need to make building blocks for the next generation of printable bio-electronic devices. In this study, transparent and flexible resistive memory devices with a very high ON/OFF ratio incorporating gold nanoparticles into the Bombyx mori silk protein fibroin biopolymer are demonstrated. The novel electronic memory effect is based on filamentary switching, which leads to the occurrence of bistable states with an ON/OFF ratio larger than six orders of magnitude. The mechanism of this process is attributed to the formation of conductive filaments through silk fibroin and gold nanoparticles in the nanocomposite. The proposed hybrid bio-inorganic devices show promise for use in future flexible and transparent nanoelectronic systems. (Some figures may appear in colour only in the online journal)
1. Introduction
the use of silk protein as a flexible substrate, on which they implemented silicon nanomaterial based electronic devices for biomedical applications. Relatively less effort has been made to exploit the use of silk protein and its composite for the fabrication of flexible and transparent electronic devices using wet processing. On the other hand, a number of research studies are being undertaken on the resistive random access memory (RRAM), with the modulation of resistance by an electrical stimulus using a variety of inorganic, organic and biomaterials. The device, also known as a memristor, is a fourth fundamental passive circuit element and is attractive for the realization of new generation nonvolatile memory useful beyond the 10 nm technology node [11], because of high operating speed, high
Natural silk protein is one of the most widely used bio-compatible materials for tissue engineering, regenerative medicine and other biotechnological applications [1–4]. Recently, it has also emerged as a promising candidate for electronic and photonic devices owing to its excellent physical, chemical and biological properties [5–7]. Silk protein has also been explored as an appealing biopolymer for the formation of films because of its mechanical robustness [8], flexibility in thin film form, optical transparency and compatibility with aqueous processing, which are essential for the realization of flexible and transparent bio-electronic devices [9]. Kim et al [10] reported 0957-4484/13/345202+07$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 345202
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For the investigation of the resistive switching behavior of the fabricated devices, the current–voltage (I–V) characteristics were measured using a Keithley 4200-SCS semiconductor characterization system at room temperature and in a normal environment.
stability and scalability [12–19]. The operation of RRAM devices critically depends on the formation of conductive bridges between two electrodes. Therefore, the controlled formation of conductive nano-filaments in the silk protein matrix using novel pathways may lead to the development of large area flexible RRAM devices at reduced cost. In this study we report, for the first time, a novel transparent and flexible nonvolatile resistive switching memory device using silk protein fibroin and gold nanoparticle composites with a low operating voltage (±2 V) and a high switching ratio (106 ). The ON/OFF ratio of the device is comparable to that demonstrated in solid-electrolyte based resistive memory devices using state of the art Si integrated circuit technology. The device fabrication is accomplished by a spin coating process on ITO coated flexible polyethylene terephthalate (PET) substrates, making it compatible with flexible electronics. The simple and soft processing technology is attractive to fabricate protein based nanoparticle incorporated RRAM devices, which may pave the way for the development of bio-compatible transparent and flexible nonvolatile memory devices in the near future.
3. Results and discussion Gold nanoparticles (Au NPs) were prepared following the Frens method [20], using an aqueous solution of HAuCl4 (2.5 × 10−3 M) and trisodium citrate solution (1% by wt). Silk protein fibroin solution was prepared by the established protocol [21, 22]. The procedure used for the extraction of silk protein fibroin from the silk cocoons and encapsulation of Au NPs with silk protein is schematically depicted in figure 1. The prepared silk:Au NP solution was used for the device fabrication. The device fabrication process is schematically shown in figure 2(a). Figure 2(b) shows the photograph of a highly transparent and flexible memory device structure, consisting of dispersed Au NPs in a silk protein matrix sandwiched between ITO and Al electrodes. Figure 3(a) shows a typical high resolution transmission electron microscopy (HRTEM) image of gold nanoparticles. Figures 3(b) and (c) show the lattice fringes and the selected area electron diffraction (SAED) patterns of Au NPs, respectively. Highly uniform and crystalline Au NPs are formed. The developed Au NPs are approximately spherical in shape with an average size of 10 nm. The interplanar spacing in the lattice fringes of figure 3(b) indicates the formation of Au nanoclusters oriented in the (111) plane. The SAED pattern in figure 3(c) reveals the growth of nanocrystals with (200) and (220) orientations. The surface plasmon absorption spectrum of Au NPs with increasing trisodium citrate solution is shown in figure 3(d). As seen from the figure, there is a red shift and broadening of the absorption peak with increase in the size of Au NPs. The inset figure shows the variation of color with increasing trisodium citrate solution. Thus the size of gold nanoparticles increases with the concentration of trisodium citrate solution. Figure 4 shows the core level x-ray photoelectron spectroscopy (XPS) of silk:Au nanocomposites using a microfocused (100 µm, 25 W, 15 kV) monochromatic Al Kα radiation of energy 1486.6 eV. The chemical state of the Au cluster has been determined from the XPS analysis. There are four possible electronic states of Au such as Au cations (Au+ and Au3+ ), neutral Au states (Au0 ), partly positively charged Au species (Auδ+ ) and partly negatively charged Au species (Auδ− ) [23, 24]. In figure 4(a), we observe Au 4f5/2 and 4f7/2 peaks at binding energies 86.1 and 82.5 eV, respectively. The binding energy of 4f electrons in Au is found to be shifted to the lower energy side from the pure/bulk metal. The same result is obtained for as-prepared Au NPs and silk protein–Au NP composite, as shown in figure 4(a). This clearly indicates that the surface charge of Au NPs is partially negative, which does not change upon bonding with silk protein. The negative effective charge on Au is formed due to electron transfer from the citrate ion to the Au particle surface with a strong interaction between them. The core level electron binding energies of carbon, nitrogen and oxygen
2. Experimental details 2.1. Preparation of gold nanoparticles Gold nanoparticles (Au NPs) were prepared by boiling 50 ml aqueous solution of HAuCl4 (2.5 × 10−3 M). Then 0.875 ml of trisodium citrate solution (1% by wt) was added to it with continuous stirring. Under these boiling with stirring conditions, the blue color solution suddenly changed to red, indicating the formation of Au NPs. The reaction mixture was boiled for another 30 min for complete reduction of the Au(III) ions. 2.2. Extraction of silk protein from natural cocoons The live and fresh cocoons of the mulberry silkworm, Bombyx mori, were cut into pieces, degummed (removal of the glue protein sericin) by boiling for an hour in sodium bicarbonate solution of 0.2 M and washed thoroughly, followed by drying under a laminar hood. The fibers were dissolved in a 9.3 M solution of lithium bromide. The obtained silk fibroin solution was dialyzed with deionized water several times to remove the traces of lithium bromide. An approximately 2% silk fibroin solution was kept in water for further experimentation. 2.3. Device fabrication For the fabrication of the silk:Au NP nanocomposite device, the aqueous solution was prepared with a concentration ratio of 10:1 of silk and Au NPs. A 200 nm thick film of ITO coated flexible polyethylene terephthalate (PET) substrate (Sigma Aldrich, USA) was used as the bottom electrode (BE). The silk–Au aqueous solution was spin coated onto the ITO BE with a thickness of 150 nm. Finally, a 100 nm thick film of Al metal was deposited as the top electrode (TE) with an area of 0.028 cm2 using thermal evaporation. 2
Nanotechnology 24 (2013) 345202
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Figure 1. Schematic extraction procedure for obtaining silk protein fibroins in aqueous solution from the cocoons of the mulberry silkworm, Bombyx mori, and encapsulation of gold nanoparticles with the silk fibroin solution.
Figure 2. (a) Schematic diagram of the device fabrication process steps. (b) Photograph showing highly transparent flexible memory devices on PET substrate.
in silk protein matrix are shown in figures 4(b), (c) and (d), respectively. The C 1s peak has three main components as shown in figure 4(b). The spectral features for C=O, C=O/N–C=O, and C–C/C–H bonding are located at 283.5, 284.5 and 286.4 eV, respectively. There are two components in the N 1s spectrum located at 398.5 and 399 eV accounting for the C=NH and C–NH2 bonds, respectively, as shown in figure 4(c). The O 1s spectrum shown in figure 4(d) has two distinct peaks at 530.1 and 531.4 eV. The peak at 530.1 eV represents the incorporation of the carbonate component. On the other hand, the peak at 531.4 eV is related to the vacancy of oxygen atoms in the silk protein. Figure 5(a) shows typical bistable current–voltage (I–V) switching characteristics of an ITO/silk:Au-NP/Al RRAM device. The external electrical stimulation is supplied to the device in the form of an applied voltage in a sequence of 0 → +2 → 0 → −2 → 0 V. In order to prevent total
dielectric breakdown during the I–V measurements, a current compliance (ICC ) level is maintained at 10 mA. Initially the device is in the low conducting state (193 pA), i.e. high resistance state (HRS), at a read voltage (VREAD ) of +0.2 V. During the first sweep, the current increases gradually along the sweeping direction and at a voltage of +1.43 V, a pronounced change in the current by about six orders of magnitude is observed. At the same VREAD of +0.2 V, the current is ∼3.6 mA for the high conducting state, i.e. low resistance state (LRS). The process to switch the RRAM device from an HRS to an LRS state is called the SET process. The transition voltage from an HRS to an LRS level, i.e. the SET voltage (VSET ) of ∼+1.43 V, is obtained for the fabricated silk:Au NP RRAM devices, which is comparable to the reported value for the Ferritin protein based RRAM devices [25]. After reaching the high conducting state, the SET state is maintained from +2.0 to −1.31 V, while reverse 3
Nanotechnology 24 (2013) 345202
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Figure 3. (a) Typical HRTEM image of gold nanoparticles with an average diameter of 10 nm. (b) HRTEM image showing the gold lattice fringes. (c) Selected area electron diffraction pattern of gold nanoparticles. (d) UV–vis absorption spectra of gold nanoparticles showing the plasmon band for different diameter 10, 20, and 55 nm nanoparticles. The inset shows the variation of color of gold nanoparticle solution with different sizes.
(VSET ∼ 10.4 V and VRESET ∼ −11.5 V) due to the absence of Au NPs. Thus the proposed silk–Au device is attractive for low power applications, as compared to pure silk, due to lower programming and erasing voltages. In addition, the high switching ratio of the present device is useful to realize multilevel memory operation to achieve higher memory density. This clearly demonstrates the performance enhancement of silk protein based RRAM devices by incorporating Au NPs in the silk switching stack. Due to the charge trapping capability of the negatively charged Au NPs, a superior memory performance is observed as compared to pure silk based devices. Therefore, it is obvious that a hybrid methodology composed of silk protein and Au NPs is useful to provide high performance bipolar switching behavior for RRAM device applications. Several hypothetical models such as ‘trapping and detrapping’, and ‘formation and rupture of conductive filament’ have been proposed to account for the resistive switching phenomena. The formation mechanism of filaments varies from one system to another. Several possible mechanisms for formation of filaments are oxygen vacancies [26, 27] and growth of metallic nano-bridges [28]. The possibility of charge storage within the trap sites in organic [29] and biomolecules [25] resulting in resistive switching has also been reported. It is obvious that both silk protein and Au NPs play an important role in the resistive switching behavior in our device structure. The conductive filament formation during the ‘SET’ process is due to the Coulombic interaction of positively charged silk fibrin chain and negatively charged
sweeping. The device then switches to a low conducting state, which is known as the RESET process. The RESET voltage (VRESET ) is −1.31 V with a current of 4.2 mA. The current overshoot effect is absent during the I–V measurements and a stable output characteristic after multiple switching cycles (10 cycles) is shown in figure 5(a) for the silk:Au NP RRAM devices. In both cycles the SET process occurs at the same voltage, but there is a variation in the RESET voltage for the device measured at the 10th cycle. There is a possibility of forming multiple conduction filaments in the film after repeated cycles during the SET process. During the RESET process, only a few conduction filaments among them may be ruptured at a particular bias. This may result in multiple RESET voltages in the device. For comparison, pure silk protein based RRAM devices using ITO/Silk/Al have been fabricated and measured. The electrical performance is inferior as compared to the silk:Au NP devices shown in the inset of figure 5(a). The performance of the silk:Au NP composite device is dramatically improved over the pure silk protein based RRAM devices, requiring a higher operating voltage (VSET of ∼10 V, VRESET of ∼−10.7 V) with a poor switching ratio of
