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An all-organic composite system for resistive change memory via the self-assembly of plastic-crystalline molecules An-Na Cha, Sang-A Lee, Sukang Bae, Sang Hyun Lee, Dong Su Lee, Gunuk Wang, and Tae-Wook Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13604 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

An all-organic composite system for resistive change memory via the self-assembly of plasticcrystalline molecules An-Na Cha,‡ Sang-A Lee,‡,† Sukang Bae,‡ Sang Hyun Lee,‡ Dong Su Lee‡, Gunuk Wang,§ and Tae-Wook Kim ‡,* ‡

Applied Quantum Composites Research Center, Institute of Advanced Composite Materials,

Korea Institute of Science and Technology, Jeollabuk-do, 55324, Republic of Korea †

Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju,

561-756, Republic of Korea §

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul

136-701, Republic of Korea

KEYWORDS: Organic composites, Resistive change memory, Space charge limited current, Succinonitrile, Organic craters, Organic disk

*Author to whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT An all-organic composite system was introduced as an active component for organic resistive memory applications. The active layer was prepared by mixing a highly polar plastic-crystalline organic molecule (succinonitrile: SN) into an insulating polymer (poly(methyl methacrylate): PMMA). As increasing concentrations of SN from 0 to 3.0 wt % were added to solutions of different concentrations of PMMA, we observed distinguishable microscopic surface structures on blended films of SN and PMMA at certain concentrations after the spin-casting process. The structures were organic dormant volcanos composed of micron-scale PMMA craters and disk type SN lava. Atomic force microscopy (AFM), cross-sectional transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive x-ray spectrometer (EDX) analysis showed that these structures were located in the middle of the film. Self-assembly of the plastic-crystalline molecules resulted in the phase separation of the SN:PMMA mixture during solvent evaporation. The organic craters remained at the surface after the spin-casting process, indicative of the formation of an all-organic composite film. Because one organic crater contains one SN disk, our system has a coplanar monolayer disk composite system, indicative of the simplest composite type of organic memory system. Current-voltage (I-

V) characteristics of the composite films with organic craters revealed that our all-organic composite system showed unipolar type resistive switching behavior. From logarithmic I-V characteristics, we found that the current flow was governed by space-charge-limited current (SCLC). From these results, we believe that a plastic-crystalline molecule-polymer composite system is one of the most reliable ways to develop organic composite systems as potential candidates for the active components of organic resistive memory applications.


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INTRODUCTION Since the development of prototype wearable smart media, organic based electronic devices such as light emitting diodes, transistors, logic circuits, and memory have been spotlighted due to their potential as flexible electronic components.1-8 The flexibility of organic materials allows their use as active components as well as substrates for truly wearable electronic applications.9-11 Among various organic electronic devices, organic memory devices have been considered to be one of the emerging technologies for next-generation flexible memory applications.9,11 In particular, organic based bi-stable memory devices with a simple sandwich structure, compared with the three-terminal based floating gate types of memory devices, are able to be controlled through their resistance states (high or low) by applying an external electrical field.10 The resistive switching of organic based bi-stable memory devices is accomplished by modulating the resistance states of the active layer. This phenomenon originates from changes in the internal charge transport through the active components. Recent reviews on organic resistive memory devices have been described using the following representative switching mechanisms: filamentary conduction,12-17 space charge and traps,18-20 charge transfer,21-24 conformational change,25-27 and ionic conduction.28,29 Although in-depth studies of charge transport mechanisms have contributed to the understanding of switching behaviors by developing advanced electrical measurements and materials analysis,30,31 the origin of the resistive switching is still unclear. Meanwhile, extensive discussions have provided some clues in the material design of active components. It is possible to design specific material systems and expect corresponding resistive switching behavior with certain switching mechanisms. For example, filamentary conduction is known as the current that flows at the highly localized region in junctions, which are associated with metallic bridges or carbon rich filaments.12-17 Conformational changes of molecules or


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molecular bundles cause resistive switching in organic resistive memory devices.25-27 The formation of space charge in the organic matrix or charge trapping in the nanoparticle-organic matrix system relates to negative differential resistance (NDR) in unipolar switching behavior with space charge limited current (SCLC) conduction.18-20 Among these studies, space charge and trap-related switching mechanisms are mostly cited to explain resistive switching in the nanoscale filler-organic systems.18,20 Nanoscale fillers are known as trapping sites in the organic matrix.18 As an electrical field is applied, space charges accumulate at the trap levels or the interface of the nanoscale filler-organic matrix system. Finally, these electrical interactions prevent more charge injection through the organic matrix, resulting in resistive switching. Because the memory device exhibits different current-voltage (IV) characteristics which are classified into write-once-read-many (WORM), unipolar and bipolar switching behaviors, by adjusting the configuration or concentrations of nanoscale fillers in organic matrixes,18,20 this composite system has been widely used as one of the potential active components for organic resistive memory devices. In particular, both non-organic (metallic or oxide) nanoparticles and semiconducting organic molecules (PCBM) have been introduced as nanoscale fillers to demonstrate resistive switching characteristics exhibiting the unipolar type IV curve with NDR.18,32-35 Based on these experimental results, filler-organic matrix systems have been considered as promising candidates for reliable organic resistive memory systems. However, it is difficult to achieve an ideal and coplanar monolayer disk composite system because most composite systems have been prepared by mixing in the solution state. Although those solutions appeared to be well-dispersed macroscopically, randomly located and aggregated fillers can still be observed in the matrix by micro- or nanoscopic analysis. The location of the fillers in the vertical direction is the most important parameter in studying switching mechanisms


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and suggests material design rules for designing composite type organic memory systems. Therefore, it is very important to develop a coplanar monolayer disk composite system, which is sandwiched between the bottom and top electrodes. Here, we report an all-organic filler-matrix composite system for organic resistive memory applications. The active layer was prepared by mixing a highly polar plastic-crystalline organic molecule (succinonitrile: SN) into an insulating polymer (poly(methyl methacrylate): PMMA). As increasing amounts of succinonitrile, from 0 to 3 wt%, were added into different concentrations of PMMA solutions, we observed distinguishable organic structures by blending films of SN and PMMA at certain concentrations after spin-casting. The organic structures look like droplets of organic material after inkjet printing. The resultant structural feature was an organic dormant volcano composed of a micron-scale PMMA crater and SN lava in the middle of the film, which was confirmed by atomic force microscopy, cross-sectional TEM, SEM and EDX. This all-organic filler-matrix composite was expected to be organized by the selfaggregation of the plastic-crystalline molecules during solvent evaporation from the film. We found that the blended film containing a self-aggregated SN:PMMA composite has reversible resistive switching characteristics.


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EXPERIMENTAL DETAILS To prepare the mixture solutions for the active layer of the organic memory device, poly(methyl methacrylate) (PMMA) (Aldrich, Mw=120,000) was first dissolved in chlorobenzene (CB) at a concentration of 30, 45 and 65 mg/ml; these solutions are denoted as PMMA30, PMMA45 and PMMA65, respectively. The mixture solutions were prepared by adding 1 or 3 wt % of succinonitrile (SN) (Aldrich) into the three different PMMA solutions. All mixture solutions were filtered with a 0.2-µm PTFE filter. A heavily-doped p-type silicon substrate was used as a bottom electrode. The substrates were cleaned using acetone and isopropyl alcohol (IPA) with ultrasonication. The mixture solutions were spin casted on an asprepared, heavily-doped, p-type Si substrate at 2000 rpm for 40 sec. The substrates were baked at 40 °C for 20 min on a hotplate in a nitrogen-filled glove box system. Then, 50-nm thick, 200 µm × 200 µm Au electrodes were deposited using a thermal evaporator at a pressure of ~ 10-7 Torr. The evaporation rate was kept 1.0 Å/s during entire deposition process. The final device structure was Au/PMMA:SN/heavily-doped p-type Si (p++ Si). Its electrical characteristics were determined using semiconductor parameter analyzers (Agilent Technology 4145B or Keithley 4200-SCS) in a nitrogen-filled glove box system. The thicknesses and surface morphologies of the active layers of all samples were estimated using a surface profiler (Kosaka Laboratory, ET 200), AFM (NX-10, Park System) and SEM (FEI, Nova NanoSEM 450). X-ray photoelectron spectroscopy (XPS) was performed using K-Alpha (Thermo Scientific), and transmission electron microscopy (TEM) (FEI, Tecnai F30) was carried out at a 300-kV acceleration voltage. The sample analyzed using cross-sectional TEM imaging was prepared by a focused ion beam (FIB) micro-sampling technique.


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RESULTS AND DISCUSSION Succinonitrile, one of the plastic-crystalline molecules with a bcc structure, is known as a solid state, single plastic phase with very high dielectric constant (ε) of 55 at room temperature. Because succinonitrile has a small anisotropic surface tension between the liquid-solid interface (~ 0.5%) and a relatively low melting point (approximately 52 to 62 °C), it has been used as a basic material to study Ostwald ripening in a two-dimensional system.36,37 Impurities in succinonitrile are able to function as nucleation sites for the solidification of the thin succinonitrile film, leading to coarsening of droplet-like crystals in the homogeneous binary mixture.36,37 Due to these properties of succinonitrile, we expected the nucleation and growth of spherical succinonitrile droplets in the organic mixture system and prepared a simple mixture of succinonitrile with the PMMA polymer. We observed distinguished dot like features on the spin-cast organic composite film (3.0 wt % of SN into 65 mg/ml of PMMA solution) using optical microscopy as shown in Figure 1(b). The dots were uniformly formed and distributed on the whole surface of the SN:PMMA film , not showing any serious aggregation of the dots. To identify the exact shape and size of the dots, we performed further microscopic analysis with SEM and AFM. From tilted SEM images, we found that the dots were very similar in shape to the craters on Earth’s moon, as shown in Figure 1(c). The average diameter of the organic craters was ~ 1.8 µm, and most craters were 1.5 to 2.0 µm. (Figure 1(d) and (e)). We determined that the concentration of SN in PMMA is a very important factor for forming organic craters on the mixture film. The systematic study of the morphology changes of the mixture film by varying SN contents reveals that succinonitrile is a necessary component in the formation of the organic craters. The size of the organic craters is


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strongly dependent on the PMMA and succinonitrile concentrations. The size of organic craters were inversely proportional to the concentration of PMMA. (See Figures S1, S2 and S3 in the Supporting Information) For example, by increasing the concentration of PMMA from 30 to 65mg/ml, the crater decreased in size from ~ 2.5 µm to 1.5 µm at constant concentration of 3wt% of SN. Furthermore, by increasing the SN concentration from 1 to 3 wt %, the organic craters increased in size from ~ 1 µm to 2 µm. (See Figures S4 and S5 in the Supporting Information.) Considering the relationship between the formation of the organic craters and SN content, our hypothesis on the origin of organic craters is through the self-assembly of succinonitrile in the PMMA matrix during solvent evaporation. To clarify this mechanism, we performed crosssectional TEM imaging of the organic composite film with 1 wt % of SN into 30 mg/ml of PMMA, as shown in Figure 2. The center of the crater is thin and flat. On the other and, the edge is relative thick due to accumulation of SN. We found that the SN stayed below the organic craters, a very similar feature to magma chambers under volcano craters in nature. SN magma was well separated from the PMMA matrix, implying simultaneous formation of an all-organic composite system (disk type small molecule filler-polymer matrix), as shown in Figure 2. These distinct features was clearly identified by AFM analysis, exhibiting unique dot like features as shown in Figure S2. The line profile from AFM image of the organic crater is well matched with the cross-sectional TEM image of organic composite film with 1 wt % of SN into 30 mg/ml of PMMA as shown in Figure S2 and S3. From XPS of the composite film, as shown in inset of Figure 3(a), we found both strong nitrogen (N 1s) and carbon (C 1s) spectra, indicating the presence of SN in the PMMA matrix. In particular, the nitrogen spectrum, which is an element found in SN molecules, was detected by EDX at the edge region of the scratched organic craters, as shown in Figure 3(c). On the basis of these results, we expect that the formation of organic


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craters occurs in the following three steps, as illustrated in Figure 4: (1) spin casting of the mixture solution, (2) molecule assembly and phase separation during solvent evaporation, and (3) formation of the organic craters. The well-dissolved SN:PMMA mixture in chlorobenzene did not show any phase separation or suspended solids in the solution state. Molecular assembly behavior did not occur in the liquid state (L). The molecules began to assemble in the PMMA matrix during solvent evaporation in the spin-casting process. SN:PMMA was simultaneously in a supersaturated state, implying coexistence of both solid-state SN, PMMA and residual solvent (S+L). It is well known that the crystallinity of organic materials is proportional to the residual solvent residence time in the organic film because more solvent in the organic layer allows more time for molecular ordering.38,39 Finally, self-assembled micron-scale organic-crystalline molecule disks remained below the organic crater after spin-casting. Therefore, the observation of organic craters on the film imply phase separation into SN disks and PMMA matrix. To identify the role of the organic craters on electrical characteristics, we fabricated a metalinsulator-metal (MIM) type memory device, as shown in Figure 5(a). According to the thickness of the composite films and SN content, the devices exhibited different resistive switching characteristics. The existence of SN in the PMMA is necessary for formation of organic craters. The SN to PMMA ratio is the most important factor in the formation of organic craters. For example, the organic craters did not appear in the mixture film with 1 wt % of SN in 65 mg/ml of PMMA (PMMA65). On the other hand, we clearly observed self-assembled organic craters in the mixture film with 3 wt % SN in the same concentration of PMMA after the spin-casting process. Interestingly, we only observed unipolar type resistive switching behavior for the SN:PMMA mixture film with organic craters. In previous studies on organic memory devices, the conventional nanoparticle insulating polymer mixture system exhibited unipolar switching


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characteristics.18,32,35 These composite systems have been considered as promising active component systems for reliable organic resistive memories. Due to the similarity of the active layer structure, which is composed of an organic matrix with a different type of clusters, our allorganic composite system with the craters is also considered to be a potential active component of organic resistive memory devices. Furthermore, our composite system is not only the first allorganic composite system but also coplanar monolayer disk composite system for organic resistive memory applications. Figure 5(b), (c) and (d) show the I-V characteristics of the Au/SN:PMMA/p++ Si organic memory device with different of SN contents (0, 1, and 3 wt %) in PMMA (30, 45 and 65 mg/ml) matrix. The thickness of the active composite film was dependent on the concentration of the PMMA and was measured to be approximately 105, 165 and 310 nm for 30, 45 and 65 mg/ml PMMA, respectively. We could not observe any current flow from the control sample (PMMA without SN), indicating complete electrical insulation between 0 and 15 V. On the other hand, the other films with organic craters exhibited typical unipolar resistive switching behavior between -15 and 15 V, exhibiting more than three orders of the on/off ratio at 0.3 V. The current values at the high and low resistance states exhibited thickness dependent behavior.40,41 From the calculation of the average current values at each resistance states from 16 cells of PMMA65, PMMA45, PMMA30 with 3wt% of SN, the current at low resistance state for PMMA30 was more than 10 times compared with that for PMMA65. Especially, by increasing the thickness from around 100 to 300 nm, the current values at high resistance state was dramatically decreased about two orders of magnitude. (See Figure S7 in the Supporting Information). We could not find any strong relationship between the density or size distribution of organic craters and switching parameters such as operational voltage, on/off ratio and threshold voltage during


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this time. In addition, Au deposition on polymer templates might lead to complex growth, and changes in the Au layer morphology.42-45 Because these geometrical or morphological structure may be able to affect the switching behavior, further studies on geometrical/morphological structure of Au electrode on resistive switching behavior is necessary. As an electrical field was applied across the composite type active layer, it was expected that the charges underwent multiple complicated interactions with the pillars in the organic matrix, resulting in the resistive switching behavior of the organic memory devices. Because the pillars (nano or microscale particles) in the conventional composite type organic memory are randomly located in the organic matrix, it is difficult to determine the relationship between the switching parameters (on/off ratio, retention, on or off current, and set or reset voltage, among others) and the geometrical information of the particles (location) in the matrix. Several potential mechanisms have been proposed to explain resistive changes in organic memory systems such as the trap-filled process, the so-called space charge limited current (SCLC),18-20,46 filamentary conduction,12-17 and internal charge transfer between the donor and acceptor.21-24 The switching mechanism of memory systems was generally evaluated by investigating not only electrical analysis, such as the shape of I-V curves, slope of log-log plot of I-V curve, temperature variable I-V measurement and conducting AFM, but also the microscopic observations (cross-sectional TEM) and geometrical information (device structure) of the memory devices. The structure of our organic crater system is quite simple compared to previously reported composite type organic memory systems in which fillers are randomly mixed within matrix because each organic crater contains only one SN disk. Therefore, the charges meet only at one SN disk or not at all while traveling across the composite film when under an electrical field. Based on this geometrical singularity and the high relative permittivity of the SN, which is responsible for the


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distinguished charge separation ability of succinonitrile,47 the switching mechanism of our organic crater system is believed to correspond to trap induced charge transport, which is one reliable origin of resistive switching in well-known filler-organic matrix composite systems. The resistive switching behavior of our organic crater system exhibited typical trap induced charge transport behavior on both current flows at the ON and OFF states from their I-V characteristics, as shown in Figures 6(a) and (b). We clearly observed four distinct regimes in the log-log plot of the I-V curve, as shown in Figure 6(b). Space charge limited current (SCLC) conduction is considered to be a potential resistive switching mechanism of our organic crater system. In low bias regime (1), the current flow was linearly proportional to the applied bias [slope ~ 1.0 at regime (1)], which is due to the thermally activated free carriers in the organic crater system. Our organic crater system contains the SN disk, which has the very high dielectric constant (ε) of 55 at room temperature. It is well known that the high relative permittivity of solid state succinonitrile is responsible for distinguished charge separation ability.47 Therefore, by applying more bias on the top electrode, the charges may become trapped at the interface between the SN disk and PMMA matrix below the set bias, as shown in Schematic (ii) in Figure 6(c). The current flow is governed by the space charge limited current (SCLC), exhibiting that the slope is over ~ 3.2 at the regime (2) from the log-log plot of the I-V characteristics in Figure 6(b). When the bias was further increased, as shown in regime (3), the slope of log-log plot became steeper (over ~ 13) indicating typical trapped charge limited current (TCLC); considerable charge trapping occurs in this regime. After transition of the resistance from high to low, most possible trap sites in the organic crater system are filled, and the charges easily pass through the organic composite layer, as shown in Schematic (iii) in Figure 6(c). At the same time, the current is free from the trap and the I-V characteristics show a V2 dependence [slope ~ 2.0 at regime (4)], as shown in


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Figure 6(b).20 The reset event occurred under higher electric field conditions than those of the set by eliminating trapped charges in the matrix, as shown in Schematic (iv) in Figure 6(c). Finally, the memory device is turned to a high resistance state, showing typical NDR (Negative Differential Resistance) characteristics, as shown in Figure 6(a). For further analysis on our memory devices with organic composite systems, we studied device-to-device uniformity and retention characteristics. The resistance values at both on and off states of the devices with 1 and 3 wt % of SN are shown in Figure 7(a). The resistance states were calculated from the current values at a read voltage of 0.3 V on 32 devices. The calculated resistance values of 90% of the 1 and 3 wt % of SN memory devices were distributed within two orders of magnitude. Moreover, the ratio between the minimum value of HRS and the maximum value of LRS was more than three orders of magnitude. This electrical property implies that our memory device has a sufficient on/off level for digital memory applications. In addition, both the on and off states of each SN memory device maintained their resistance states for more than 104 seconds, showing more than four orders of magnitude of the on/off ratio without any serious degradation, as shown in Figure 7(b). These results imply that our organic composite (SN:PMMA) system is an effective resistive switching memory system for a next generation organic resistive memory platform.


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CONCLUSIONS In this study, we demonstrated an all-organic composite system for organic resistive memory applications via self-assembly of a plastic-crystalline molecule. The active component of the memory device was prepared by mixing a highly polar plastic-crystalline organic molecule, succinonitrile (SN), into an insulating polymer, poly(methyl methacrylate)(PMMA). As increasing concentrations of SN from 0 to 3 wt % were added into different concentrations of PMMA solutions, we observed distinguished microscopic surface structures on blended films of SN and PMMA at a certain concentration after the spin-casting process. Self-assembly of the plastic-crystalline molecules resulted in the phase separation of the SN:PMMA mixture during solvent evaporation. Atomic force microscopy, cross-sectional TEM, SEM and EDX analysis revealed that the structure feature was similar to an organic dormant volcano composed of a micron-scale PMMA crater and disk type SN lava below the organic crater. The organic composite system had a coplanar monolayer disk composite system because one organic crater contained one SN disk. From the current-voltage (I-V) analysis, we found that the organic composite films with the craters had unipolar type resistive switching behavior. The current flow through the composite film was governed by space-charge-limited current (SCLC). The plasticcrystalline molecule-polymer composite system was the origin of charge trapping and responsible for resistive switching behavior, indicating that the all-organic composite is the most reliable organic composite system and can be considered as a potential candidate for the active component of organic resistive memory.


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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Science and Technology (KIST) Young Fellow Program (2V04900) and was partially supported by the National Research Foundation of Korea (NRF-2016R1C1B2007330).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical microscopy, AFM images and line profiles, SEM images, histogram of size distribution of organic craters, retention and cumulative probability of the organic composite memory devices, thickness dependences of average current value at the high and low resistance state.


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FIGURES

Figure 1. (a) Chemical structures of poly(methylmethacrylate) (top) and succinonitrile (bottom). (b) Optical microscopy image of the organic composite film with 3 wt % SN in 65 mg/ml of PMMA. (c) Scanning electron microscopy of the organic craters on the organic composite film. Atomic force microscopy (d) and 3D image (e) of the organic craters.


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Figure 2. (a) Schematics of the organic craters on the organic composite film and cross-sectional view of the organic crater. (b) Cross-sectional TEM image of the organic crater on the organic composite film with 1 wt % of SN into 30 mg/ml of PMMA. TEM image shows the complete phase separation between the disk (succinonitrile) and matrix (PMMA). The self-aggregated succinonitrile disk is located below the crater.


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Figure 3. XPS analysis on the organic composite film with 1 wt % of SN into 30 mg/ml of PMMA (a) Carbon 1s and (b) nitrogen 1s. (c) SEM of the scratched organic crater (left and side) and EDX spectrum at the red point of the organic crater.


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Figure 4. Step-by-step schematics of how organic craters form on the organic composite film. Red bars and black lines indicate SN and PMMA, respectively. (1) Spin-casting of the mixture solution of SN with PMMA; SN and PMMA are completely dissolved in a solvent and both materials are randomly mixed with each other in the solution. (2) Self-assembly of SN molecules begin with solvent evaporation and phase separation between the PMMA matrix and SN happens at the same time. (3) SN molecules form self-assembled disks and the organic crater structure remains on the organic composite film after solvent evaporation.


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Figure 5. (a) Optical microscopy and device schematics of the organic composite resistive memory device with Au/SN:PMMA/p++ Si. I-V characteristics of the memory devices at different concentrations of PMMA matrix, (b) 30, (c) 45 and (d) 65 mg/ml PMMA, with both 1 and 3 wt % SN. The memory devices were programmed by double sweep from 0 to 8, 10, or 12 V (Set) and erased by sweep from 0 to 15 V (Reset). The programming voltage depended on the thicknesses of the organic composite films.


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Figure 6. (a) I-V characteristics of organic composite (45 mg/ml of PMMA with 1 wt % of SN) memory device on positive bias polarity. (b) Log-log plot of the I-V characteristics in the between 0 and 15 V regime. The log-log plot shows a distinct four current regime with different slopes. (c) Schematics of expected charge transport behavior with respect to applied bias.


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Figure 7. (a) Cumulative probability of ON and OFF resistance values of 16 cells and (b) retention characteristics on representative organic composite memory (45 mg/ml of PMMA with 1 and 3 wt % of SN).


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