Nonvolatile memory device based on Ag nanoparticle

APPLIED PHYSICS LETTERS 94, 173510 共2009兲

Nonvolatile memory device based on Ag nanoparticle: Characteristics improvement Biswanath Mukherjee1,a兲 and Moumita Mukherjee2 1

Department of Physics, Kalna College, Kalna, Burdwan, West Bengal 713409, India Government College of Engineering and Leather Technology, Salt Lake, Kolkata 700098, India

2

共Received 1 February 2009; accepted 10 April 2009; published online 1 May 2009兲 A single layer memory device based on silver nanoparticles has been fabricated. The device exhibits electrical bistability and nonvolatile memory phenomenon. The performance of the device improved 共in terms of On/Off ratio, switching cycles, and retention time兲 when an additional polymer 共PMMA兲 layer was deposited prior to nanoparticles deposition. The retention time and switching cycles of the device improved a lot and on/off current ratio of the device increased by more than three orders of magnitude. The ability to write, erase, read, and refresh the electrical states of the polymer-nanoparticle composite fulfills the functionality of a dynamic random access memory. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3127233兴 Due to their interesting optoelectronic properties, nanomaterials have attracted widespread interest which boosted the development of highly exciting field of research. Nanometer-sized electronic materials are especially interesting due to quantum confinement of electrons in nanometer dimensions.1–3 Charge transfer through the nanoparticles or nanoparticle/polymer composite material has gained much attention4,5 as an attempt to utilize nanomaterials in different device applications which includes photovoltaic cells,6,7 light emitting diode,8 data storage and memory element,9,10 field effect transistor,11,12 catalytic activity, and biocompound detection.13 To achieve high-density memory elements, different organic materials, composites,14–17 and nanomaterials have been explored which have demonstrated some remarkable improvement in bistable memory device.9,10,18,19 Present-day research aims to achieve finite-sized memory elements having a large On/Off ratio and long retention time. In this communication, we have shown that silver nanoparticles 共Ag NPs兲 can be utilized for the fabrication of switching and memory device. We, further have demonstrated that the performance of the device can be dramatically improved by simply incorporating a layer of poly共methyl methacrylate兲共PMMA兲 over the substrate prior to Ag NP deposition. To synthesize Ag NP, 0.1 g glucose and 0.6 g poly共vinyl pyrrolidone兲共PVP兲 were added to 35 ml distilled water with continuous stirring to get a clear solution. Then 0.5 ml of 0.1M aqueous solution of silver nitrate 共AgNO3兲 was added to the above solution. The mixed solution was transferred to a cylindrical Teflon lined stainless steel chamber and inserted into a preheated oven at 200 ° C for 12 h. After cooling, the precipitate was washed thoroughly in water and ethanol. The x-ray powder diffraction 共XRD, Seifert 3000P, Cu K␣ radiation兲 revealed that the diffraction peaks of the sample could be indexed to face centered cubic phase of silver.20 To fabricate devices, the as prepared samples were centrifuged, dried in room temperature, and then dissolved in ethanol 共1 mg/ml兲. The solution was spun cast 共1500

rpm/30 s兲 on patterned indium tin oxide 共ITO兲-coated glass substrates. For the composite device, PMMA solution in chloroform 共1 mg/ml兲 was spun cast 共3000 rpm兲 over the ITO substrates followed by Ag NP deposition 共3000 rpm兲. The devices were dried at 70 ° C in vacuum 共10−3 Torr兲 for 6 h. Aluminum 共Al兲, thermally evaporated in vacuum 共below 10−5 Torr兲 through a mechanical mask, was the top electrode 共device area⫽6 mm2兲. Electrical characterization of the devices were performed in vacuum 共10−3 Torr兲 by Yokogawa 7651 programmable dc source and Keithley 486 picoammeter. The bias was applied with respect to the top Al electrode at a scan speed of 10–50 mV/s. Impedance spectroscopy of the devices was recorded 共Solartron 1260 Impedance Analyzer兲 with ac test voltage of 100 mV/rms 共frequency 1 Hz to 12 MHz兲. The UV-visible spectrum of the Ag NPs showed the appearance of plasmon resonance weak intense broad peak around 470 nm which can be assigned to the redshift in the peak corresponding to out-of-plane dipole resonance of the Ag NPs.21,22 This peak also indicates the formation of Ag NPs. The morphology and particle sizes of the Ag NPs were studied through scanning electron microscope 共SEM兲共JEOL JSM 6700兲 and transmission electron microscope 共TEM兲共JEOL 2010兲 images. Figure 1 共inset兲 shows the low magnification TEM image of Ag NPs with diameters varying within 40–70 nm.

a兲

FIG. 1. 共Color online兲 Typical I-V characteristics of ITO/Ag NP/Al device in three consecutive voltage loops. Arrows show the direction of voltage sweep. Inset shows the low magnification TEM image of the nanoparticles.

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: ⫹91-03454-255032. FAX: ⫹9103454-255861.

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© 2009 American Institute of Physics

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B. Mukherjee and M. Mukherjee

Appl. Phys. Lett. 94, 173510 共2009兲

FIG. 2. 共Color online兲 I-V characteristics of the composite and PMMA based device. Inset shows the voltage dependence of On/Off ratio of both Ag NP based device and that of composite device.

Figure 1 shows I-V characteristics of the ITO/Ag NP/Al device under three consecutive voltage loops which distinctly displays the presence of two conducting states at any voltage during the two sweeps. When the voltage was swept from +VMax to −VMax, the device exhibited its lowconducting state 共off state兲. When the bias exceeded a critical amplitude, the device underwent a transition from the lowconducting to a high-conducting state 共on state兲. The highconducting on state persisted during the sweep from −VMax to +VMax, showing a bistable nature of the device at any voltage. The device finally switched off to its low-conducting state as the voltage approached +VMax. The I-V characteristics are reversible in nature—that is the switching is reproducible over many cycles. When an additional polymer 共PMMA兲 layer was deposited prior to Ag NP deposition, the off-state current of the device decreased and on-state current increased, resulting in a higher on/off ratio between the two conducting states. While the increase in the on-state current is not very large, off-state current decreases by orders of magnitude. The I-V characteristics of the composite device 共ITO/PMMA–Ag NP/ Al兲 have been plotted in Fig. 2 which shows that when the voltage was swept from a negative value, device current was several orders higher in magnitude as compared to the current during the other sweep. The high-conducting state, which is achieved at a negative voltage, is retained when the bias was ramped to positive direction beyond zero. A sufficient positive voltage finally switches the conductivity to its initial low value. Presence of the polymer layer results in very low off-state current and hence high ratio between the currents during the two sweep directions. The on/off ratio in these devices, which showed dependence with VMax, has been more than 104. On the other hand, the ratio reached only ⬃12 in case of ITO/Ag NP/Al device 共inset, Fig. 2兲. As a control experiment, we have fabricated devices with PMMA layer only 共having comparable thickness兲 which shows symmetrical I-V characteristics 共Fig. 2兲 with no signature of switching. Devices with only Ag NP having thicknesses smaller and larger than the composite device have also been tested. The overall current of the devices changed a little but the on/off ratio of the devices was always less than 20. This proves that improvement in the switching parameter 共on/off ratio兲 is due only to the PMMA–Ag NP composite. The role of PMMA in the composite device is thus to decrease the off-state current and effectively to increase the on/off ratio.

FIG. 3. SEM images of the devices based on 共a兲 Ag NP and 共b兲 polymer-Ag NP composite.

The bistability observed in nanoparticles based devices has been explained earlier as due to the charge confinement within the nanoparticles.19,23 However, the higher on-state current and higher on/off ratio in case of PMMA–Ag NP composite device may be attributed to the easy charge transfer through the polymer chain in the on state. The polar groups of PMMA molecule can act as hopping sites to transport the trapped charges in the composite device.24 Figure 3 compares the SEM images of devices based on Ag NP and PMMA–Ag NP composite, indicative of a better percolating channel 共network兲 in case of composite device. Before switching, the charge carriers get trapped within the polymer matrix as well as within the nanoparticles resulting in lower off-state current in composite device as compared to that of Ag NP based device. With increase in voltage more traps are filled up unless at a critical voltage all the traps get filled and the device is switched on. Once the device switched to the on state, the charge carriers move easily through the conjugated network of polymer matrix resulting in higher on-state current. The charges remain trapped even at low bias retaining the high conducting state of the device and became detrapped only at higher positive voltage which brings the device to the initial low conducting off state. For Ag NP based device, because of absence of any such matrix, the trapped charges became detrapped when the bias is gradually decreased. This results in lower on-state current and hence lower on/off ratio. A stress test was carried out by switching the device to either of the states 共⫾4 V, width= 10 s兲 and then probing the states by measuring the current with a small probe voltage 共⫺0.9 V兲. The current under probe voltage pulse for the high conducting state was much higher as compared to that after inducing a low-conducting state 共Fig. 4兲. Also, during the 4 h stress test no significant change in conductivity was noted for the composite device 共Fig. 4兲. In other words, the high and


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Appl. Phys. Lett. 94, 173510 共2009兲

B. Mukherjee and M. Mukherjee

FIG. 4. 共Color online兲 Time response 共stress test兲 of devices based on PMMA–Ag NP composite and Ag NP. Absolute values of current are plotted for comparison. 共Inset兲 WRER voltage sequence 共upper panel兲 and the corresponding current response 共lower panel兲 of both the devices. In both the cases, the high- and low-conducting states were induced by ⫺4 and +4 V 共each of 10 s width兲 respectively, and probed by using ⫺0.9 V.

low states of the composite device are distinguishable for hours. The devices with only Ag NP, on the other hand, exhibited a retention time of less than an hour. Figure 4 共inset兲 presents a representative result of write-read-eraseread 共WRER兲 cycle tests conducted on the devices for hours. In the write-read-erase-read cycle, the “write” 共on兲 is made by applying a negative voltage pulse 共−4 V兲 and the “erase” 共off兲 is achieved through erasing the on state by applying a positive voltage pulse 共+4 V兲. A small probe voltage 共⫺0.9 V兲 pulse is employed to read the states of the device. The probe current in the on state is clearly higher than that in the off state which ensured that the two states of the devices can be flipped-flopped and probed for hours. On the other hand, the WRER performance of ITO/Ag NP/Al device was very poor. The difference between the two states was very small and the degradation of the device took place after few switching cycles 共inset, Fig. 4兲. To get further insight on the mechanism of electrical bistability, we have carried out impedance spectroscopy of the device. Real and imaginary components of complex impedance 共Z⬘ and Z⬙, respectively兲 of the device were measured as a function of frequency at 0 V dc bias. Cole–Cole plots 共Z⬙ versus Z⬘兲 for the high- and low-conducting states for devices based on Ag NP and composites are shown in Fig. 5. The semicircular nature of the curves reveals that the devices can be modeled as parallel combination of a resistor

FIG. 5. 共Color online兲 Cole–Cole plots of the devices based on Ag NP and Polymer-Ag NP composite in their high- and low-conducting states. Measurements were carried out at 0 V dc bias after the on and off states were induced by applying ⫺3.5 and +3.5 V, respectively.

and a capacitor 共C P − R P兲 networks. The diameter of the semicircle, representing the bulk resistance of the device, decreased when switched to a high-conducting state. The change was much higher in the composite systems than in the Ag NP-only device. This decrease in the bulk resistance might be due to charge confinement in the nanoparticles or in the polymer matrix, supporting our results obtained from the I-V characteristics of the two states. The dielectric constant of the active material, however, did not change during conductance switching. In summary, a memory device based on Ag NP showed improvement in device performance with the deposition of an additional polymer layer. The on/off ratio and switching cycles of the composite device become much larger as compared to the device based on Ag NP only. From the currentvoltage characteristics and dielectric spectroscopy, we propose charge storage in the nanoparticles as a possible mechanism for the observed electrical bistability. The presence of conjugation in the polymer chain makes it easier for the transport of charge carriers through the percolating network of polymer-nanoparticle composite, resulting in improved performance in the composite devices. A. P. Alivisatos, Science 271, 933 共1996兲. X. Duan, Y. Huang, Y. Cui, J. Wang, and C. M. Lieber, Nature 共London兲 409, 66 共2001兲. 3 A. M. Michaels, M. Nirmal, and L. E. Brus, J. Am. Chem. Soc. 121, 9932 共1999兲. 4 S. Chen, R. S. Ingram, M. J. Hostetler, J. J. Pietron, R. W. Murray, T. G. Schaaff, J. T. Khoury, M. M. Alvarez, and R. L. Whetten, Science 280, 2098 共1998兲. 5 W. P. Wuelfing, S. J. Green, J. J. Pietron, D. E. Cliffel, and R. W. Murray, J. Am. Chem. Soc. 122, 11465 共2000兲. 6 A. C. Arango, S. A. Carter, and P. J. Brock, Appl. Phys. Lett. 74, 1698 共1999兲. 7 W. U. Huynh, J. J. Dittmer, and A. P. Alivisatos, Science 295, 2425 共2002兲. 8 E. Lai, W. Kim, and P. Yang, Nano Res. 1, 123 共2008兲. 9 S. Paul, C. Pearson, A. Molloy, M. A. Cousins, M. Green, S. Kolliopoulou, P. Dimitrakis, P. Normand, D. Tsoukalas, and M. C. Petty, Nano Lett. 3, 533 共2003兲. 10 J. Ouyang, C.-W. Chu, C. R. Szmanda, L. P. Ma, and Y. Yang, Nature Mater. 3, 918 共2004兲. 11 Z. X. Xu, V. A. L. Roy, and P. Stallinga, Appl. Phys. Lett. 90, 223509 共2007兲. 12 R. T. Weitz, U. Zschieschang, F. Effenberger, H. Klauk, M. Burghard, and K. Kern, Nano Lett. 7, 22 共2007兲. 13 U. Yogeswaran, S. Thiagarajan, and S.-M. Chen, Anal. Biochem. 365, 122 共2007兲. 14 Z. J. Donhauser, B. A. Mantooth, K. F. Kelly, L. A. Bumm, J. D. Monnell, J. J. Stapleton, Jr., D. W. Price, A. M. Rawlett, D. L. Allara, J. M. Tour, and P. S. Weiss, Science 292, 2303 共2001兲. 15 D. Ma, M. Aguiar, J. A. Freire, and I. A. Hümmelgen, Adv. Mater. 共Weinheim, Ger.兲 12, 1063 共2000兲. 16 S. Paul, A. Kanwal, and M. Chhowalla, Nanotechnology 17, 145 共2006兲. 17 B. Mukherjee, S. K. Batabyal, and A. J. Pal, Adv. Mater. 共Weinheim, Ger.兲 19, 717 共2007兲. 18 M. D. Fischbein and M. Drndic, Appl. Phys. Lett. 86, 193106 共2005兲. 19 K. Mohanta, S. K. Majee, S. K. Batabyal, and A. J. Pal, J. Phys. Chem. B 110, 18231 共2006兲. 20 JCPDS Card No. 4–0783. 21 Y. Gao, P. Jiang, L. Song, L. Liu, X. Yan, Z. Zhou, D. Liu, J. Wang, H. Yuan, Z. Zhang, X. Zhao, X. Dou, W. Zhou, G. Wang, and S. Xie, J. Phys. D 38, 1061 共2005兲. 22 G. Zhou, M. Lu, Z. Yang, H. Zhang, Y. Zhou, S. Wang, S. Wang, and A. J. Zhang, J. Cryst. Growth 289, 255 共2006兲. 23 R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner, and Y. Yang, Nano Lett. 5, 1077 共2005兲. 24 K. S. Suh, H. J. Lee, and C. G. Kang, IEEE Trans. Dielectr. Electr. Insul. 2, 460 共1995兲. 1 2
