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maleimiade copolymers with pendant azobenzene chromophores
European Polymer Journal 47 (2011) 1160–1167
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Flash memory effects based on styrene/maleimiade copolymers with pendant azobenzene chromophores Yuanhua Liu, Najun Li, Xuewei Xia, Jianfeng Ge, Qingfeng Xu ⇑, Jianmei Lu ⇑ Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou, Jiangsu 215123, China
a r t i c l e
i n f o
Article history: Received 11 November 2010 Received in revised form 19 January 2011 Accepted 26 January 2011 Available online 4 February 2011 Keywords: Polymer Electronic memory Flash Device
a b s t r a c t Two styrene/maleimiade copolymers with pendant azobenzene chromophores, poly(styrene-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PS-DP) and poly((4-vinyl-benzyl)-9Hcarbazole-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP), were synthesized. The polymeric memory devices based on each of the two polymer films (ITO/Polymer/Al) show similar rewritable flash memory behaviors but different transition voltages. By introduction of carbazole groups in the polymer side chains, the voltage difference from OFF to ON state of ITO/PVCz-DP/Al is reduced obviously in comparison with that of ITO/PS-DP/ Al, which is beneficial to the protection of devices. Both ITO/PS-DP/Al and ITO/PVCz-DP/ Al show high stability under a constant stress or continuous read pulses voltage of 1.0 V. The memory mechanism is governed by space-charge limited conduction (SCLC) on the basis of the I–V curves of these fabricated memory devices. With excellent flash memory characteristics and simple processability, the memory devices fabricated with these two styrene/maleimiade copolymers have potential applications for the future electronic memory devices. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In recent ten years, polymeric electronic memory devices have attracted lots of attention for their low fabrication cost and super-high data-storage density [1–3]. A wide range of polymer materials, including poly(N-vinylcarbazole) (PVK) [4] and its derivatives [5–7], p conjugated polymers [8–12], dendritic polymers [13] and polymers in doped systems [14–17] have been reported for application in memory devices. Among three major types of memory behaviors, writeonce-read-many times (WORM) memory behavior can be written only once and not allowed to modify the stored data [18]; Dynamic random access memory (DRAM) needs power supply to retain the stored data [19], which disadvantages limit them for wide applications. Flash-type ⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Xu), [email protected] (J. Lu). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.01.012
memories are widely used in portable systems, such as mobile personal computer (PC), personal digital assistant (PDA), and digital camera [20], because it do not need power to keep the stored information and could be repeated written. Previously, many polymeric flash-type memory devices based on doped systems were reported [21]. Although they possess excellent flash memory effects, the phase separation is a hidden danger and may further disturb the stability in devices. Latter, some pure polyamides with complex structure were reported for flash memory applications [9]. Therefore the design and synthesis of a processable and single polymer with good flash memory characteristics are still desirable for fabrication of flash-type memory devices. Azobenzene (Azo) groups, owing to their special chemical structure, have been reported for applications in optical data-storage, switchable supramolecular cavities, nonlinear optics, sensors and molecular motors in recent years [22]. Besides, some azobenzene-containing organic compounds were also reported for electronic memory devices [23,24].
Y. Liu et al. / European Polymer Journal 47 (2011) 1160–1167
In this paper, we introduced azo groups into styrene/maleimiade copolymer systems for their flexible molecular structure and simple synthesis process. With the presence of azo groups, the memory devices based on poly(styrene1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PS-DP) and poly((4-vinyl-benzyl)-9H-carbazole-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP) all showed good flash memory behaviors. In contrast to PS-DP, the introduction of carbazole groups in PVCz-DP was expected to improve the memory effects because carbazole was a well-known electron-donor and hole-transporting group. Furthermore, the I–V characteristics were investigated in details to study the conduction mechanisms. 2. Experiment
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2.2.1. Synthesis of (4-vinylbenzyl)-9H-carbazole (VCz) Potassium hydroxide (4.5 g, 80.0 mmol) was added into a solution of carbazole (7.5 g, 45.0 mmol) in 120 mL N,Ndimethylacetamide (DMF). After stirring for 15 min, 1(chloromethyl)-4-vinylbenzene (6.8 g, 45.0 mmol) was added slowly to the mixture and continued reaction for 5 h at room temperature, the resulting mixture was poured into a large excess of water and the precipitate was collected by filtration. The final white product was washed with hot alcohol several times to remove the residual carbazole. Yield was 83% (10.5 g). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.14–8.12 (d, 2H), 7.44–7.40 (m, 2H), 7.36–7.34 (d, 2H), 7.30–7.27 (m, 2H), 7.25–7.23 (d, 2H), 7.10–7.08 (d, 2H), 6.68–6.60 (m, 1H), 5.70–5.65 (d, 1H), 5.50 (s, 2H), 5.21–5.18 (d, 1H). Anal. calcd. for C21H17N: C, 89.01; H, 6.05; N, 4.94. Found: C, 88.21; H, 5.85; N, 4.91.
2.1. Materials p-Aminoazobenzene (90%) was purchased from Beijing Chemical Reagent Co. Ltd. Maleicanhydride (99%), carbazole (99%), azobisisobutyronitrile (AIBN) (99%), cyclohexanone (99%), potassium hydroxide (KOH) (99%) and other solvents were all purchased from Shanghai Chemical Reagent Co. Ltd. Cyclohexanone was purified by reduced pressure distillation. AIBN was recrystallized from methanol before use. Other reagents were used as received without any further purification. 2.2. Preparation of PS-DP and PVCz-DP The polymer materials for memory devices are prepared according to the route shown in Fig. 1.
2.2.2. Synthesis of 1-(4-phenylazo-phenyl)-pyrrole-2,5-dione (DP) p-Aminoazobenzene (3.0 g, 15.0 mmol), maleicanhydride (1.5 g, 15.0 mmol) and 100 mL ethyl acetate were added to a single-necked flask. After stirring for 8 h at 60 °C, N-(4-phenylazo-phenyl)-maleamic acid was precipitated and collected by filtration. Then N-(4-phenylazo-phenyl)-maleamic acid (4 g, 13.5 mmol), triethylamine (1.5 g, 15.0 mmol) and 150 mL acetone were stirring for 5 min at 60 °C. After the dissolution of reactants, acetic anhydride (8.0 g, 78.0 mmol) and manganese acetate (0.05 g, as catalyst) were added and the mixture was continued to reaction for 4 h at 60 °C. The reaction solution was precipitated in a large excess of water and isolated by filtration, the final yellow product was recrystallized from alcohol. Yield was 65%
Fig. 1. Synthesis scheme and molecular structures of PS-DP and PVCz-DP.
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(2.7 g). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.04–8.02 (d, 2H), 7.94–7.92 (d, 2H), 7.58–7.50 (m, 5H), 6.90 (s, 2H). Anal. calcd. for C16H11N3O2: C, 69.31; H, 4.00; N, 15.15. Found: C, 68.79; H, 4.05; N, 15.18. 2.2.3. Synthesis of poly(styrene-1-(4-phenylazo-phenyl)pyrrole-2,5-dione) (PS-DP) For a typical radical polymerization, styrene (0.35 g), 1diphenyldiazenyl-1H-pyrrole-2,5-dione (DP, 0.55 g), azoisobutyronitrile (AIBN, 3.28 mg) and 5 mL cyclohexanone were stirring at 70 °C for 8 h under nitrogen atmosphere. The reaction mixture was precipitated in a large excess of ethanol and isolated by filtration. The final product was dried at 80 °C under vacuum for 24 h, and the polymer yield was 84% (0.76 g). 2.2.4. Synthesis of poly((4-vinylbenzyl)-9H-carbazole-1-(4phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP) (4-Vinylbenzyl)-9H-carbazole (VCz, 0.28 g), 1-diphenyldiazenyl-1H-pyrrole-2,5-dione (DP, 0.28 g), azoisobutyronitrile (AIBN, 1.64 mg) and 4 mL cyclohexanone were stirring at 70 °C for 8 h under nitrogen atmosphere. After the same process of preparation for PS-DP, the final polymer PVCz-DP was collected with yield of 71% (0.40 g).
microscope. Molecular weights (Mn) and polydispersity (Mw/Mn) were measured by gel permeation chromatography (GPC) utilizing a Waters 1515 pump and a differential refractometer, THF was used as a mobile phase at a flow rate of 1.0 mL min1. 2.4. Fabrication of memory device The memory devices were fabricated as the sandwiched configuration of ITO/Polymer/Al. The indium-tin-oxide (ITO) glass was pre-cleaned by ultrasonic with water, acetone, and alcohol by turns for 30 min. Afterward, PS-DP (or PVCz-DP) solution in 1,2-dichloroethane (10 mg mL1) was filtered through microfilters with a pinhole size of 0.22 lm. The polymer film was prepared by spin-coating the filtered solution onto the pre-cleaned ITO glass substrate at a speed rate of 2000 rpm for 40 s and baked at 80 °C for 12 h under vacuum. The top metal electrode was obtained by a 100 nm-thick film of Al, which was thermally evaporated at about 106 torr through a shadow mask. The active area of the fabricated device was 0.0314 mm2 (a nummular point with a radius of 0.1 mm). All electrical measurements of the device were characterized under ambient conditions, without any encapsulation, using a HP 4145B semiconductor parameter analyzer.
2.3. Instruments 3. Results and discussion 1
H NMR spectra were obtained on an Inova 400 MHz FT-NMR spectrometer. The elemental analysis was performed by Italian 1106 FT analyzer. UV–Vis absorption spectra were carried out at room temperature from 250 to 550 nm with a Perkin-Elmer Lambda spectrophotometer. Thermo gravimetric analysis (TGA) was conducted on a TA instrument Dynamic TGA 2950 at a heating rate of 10 °C min1 under a N2 flow rate of 50 mL min1. SEM images were taken on a Hitachi S-4700 scanning electron
Fig. 2 shows the 1H NMR spectra of PS-DP and PVCz-DP. The number-average molecular weight and the polydispersity of PS-DP (Mn = 29,000, PDI = 1.8) and PVCz-DP (Mn = 13,000, PDI = 1.3) are determined by GPC. In order to obtain the content of DP segments in each copolymer chain, the UV–Vis spectra of DP with different concentrations were investigated with the maximum absorption peak at 333 nm (Fig. 3). And the absorption intensity at 333 nm
Fig. 2. 1H NMR spectra of PS-DP and PVCz-DP in CDCl3.
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Fig. 3. The UV–Vis spectra of DP in THF solution with different concentrations. The inset is the linear equation between the absorption intensity (at 333 nm) and concentration of DP.
shows a linear relationship with the concentration of DP, which is conformed to the following equation: Y = 80.14X + 0.04 Y is the absorption intensity of DP at 333 nm and X is the concentration of DP in THF solution. The absorption intensity at 333 nm (Y) of each copolymer in THF (0.05 g L1) was obtained. Then Y was substituted into the linear equation (Y = 80.14X + 0.04) to acquire X (the concentration of DP) which was 0.32 g L1 for PS-DP and 0.30 g L1 for PVCzDP, respectively. So the content of DP segments in the copolymer PS-DP and PVCz-DP were calculated as 64 wt.% and 60 wt.%, respectively. TGA curves in Fig. 4 reveal that each of the two copolymers has a high decomposition temperature up to 340 °C. Fig. 5(a) gives the schematic diagram of the indium-tin oxide/polymer/aluminum (ITO/Polymer/ Al) device. For each device, the thickness of the intermediate polymer layer was about 75 nm, which was obtained by the scanning electronic microscopic (SEM) image of the cross section of the fabricated storage cell, as shown in Fig. 5(b) and (c).
Fig. 4. TGA curves of PS-DP and PVCz-DP measured in nitrogen atmosphere at a heating rate of 10 °C min1.
Fig. 5. (a) Schematic diagram of the indium-tin oxide/polymer/aluminum device, (b) SEM image of the cross section of TIO/PS-DP/Al device, (c) SEM image of the cross section of TIO/PVCz-DP/Al device.
The electrical properties of the devices were measured at room temperature under ambient conditions without any encapsulation. For the ITO/PS-DP/Al sandwiched device (as shown in Fig. 6(a)), the current was sharply increased from 105 A (OFF-state) to 103 A (ON-state) at a switch-on voltage of 2.0 V when a voltage sweep from 0 to 4.0 V was applied (sweep 1). This electrical transition can serve as the ‘‘writing’’ process in this memory device. The device remained in its ON-state as the voltage was swept from 4.0 to 0 V (sweep 2). But in a voltage sweep of opposite bias from 0 to 6.0 V (sweep 3), the device could be erased to OFF-state at a switch-off voltage of 5.5 V, and the OFF-state was retained when the voltage sweep was applied again (sweep 4). The OFF-state can be further turned to the ON-state and turned back to the OFF-state by reapplying the voltage sweep (sweep 5, 6, 7, 8). Therefore, PS-DP can be used in a flash-type memory device. Similarly, the memory device based on PVCz-DP also shows flash memory effects (Fig. 7(a)). But the switch-on and switch-off voltages of memory devices based on PVCz-DP were obviously lower than those of PS-DP, as shown in Table 1. Both of the devices show ON/OFF current ratio around 103. For each device, no significant degradation in current of the OFF and ON states was observed after 103 s of continuous stress test at 1.0 V, and also no significant degradation was observed after 105 read cycles at 1.0 V (Figs. 6(b) and 7(b)). The results indicate the two devices have high stabilities in both OFF and ON states. In order to understand the memory mechanisms and explain the difference in transition voltages of these two devices, the energy levels of VCz and DP were carried out at the B3LYP/6-31G(d) level. Styrene groups were not included since they do not significantly affect the electronic properties. For PS-DP, the energy barrier between the work function of ITO bottom electrode and the high-occupiedmolecular-orbital (HOMO) level of DP unit is 1.28 eV, which is smaller than that between the lowest-unoccupied-molecular-orbital (LUMO) level of DP unit and the
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Fig. 6. (a) Current–voltage (I–V) characteristics of the memory device based on PS-DP, (b) effect of retention time on the ON and OFF states of the memory device under a constant stress of 1.0 V, and the inset is effect of read cycles on the ON and OFF state under a read pulse voltage of 1.0 V.
Fig. 7. (a) Current–voltage (I–V) characteristics of the memory device based on PVCz-DP, (b) effect of retention time on the ON and OFF states of the memory device under a constant stress of 1.0 V, and the inset is effect of read cycles on the ON and OFF state under a read pulse voltage of 1.0 V.
Table 1 Switch-on and switch-off voltages of TIO/PS-DP/Al and TIO/PVCz-DP/Al devices. Device
Switch-on voltage (V)
Switch-off voltage (V)
ITO/PS-DP/Al ITO/PVCz-DP/Al
2.0 1.2
5.5 3.8
Table 2 HOMO and LUMO energy levels of each polymer from theoretic calculation. Polymer
HOMO (eV)
LUMO (eV)
Energy barrier (Al ? LUMO) (eV)
Energy barrier (ITO ? HOMO) (eV)
PS-DP PVCzDP
6.08 5.38
2.85 2.85
1.35 1.35
1.28 0.58
work function of Al top electrode (1.35 eV), as shown in Table 2. Thus the hole injection from ITO into the HOMO level of DP is more favorable than electron injection from the Al top electrode into the LUMO level of DP (Fig. 8(a)). And hole injection dominates the conduction process in the devices. Similarly, hole injection also dominates the conduc-
Fig. 8. Energy band diagram of molecular components of PS-DP (a) and PVCz-DP (b), (c) molecular structure and electrostatic potential (ESP) surfaces of DP.
tion process in ITO/PVCz-DP/Al device (Fig. 8(b)). However, as the sweep voltage increased to the switch-on voltage, electron injection form Al top electrode to LUMO level of
Y. Liu et al. / European Polymer Journal 47 (2011) 1160–1167
DP unit occurs along with hole injection since the low energy barrier (1.35 eV). Double injection from the two electrodes makes the device reach to the high conductivity state. For PS-DP, the memory effects were dominated by DP units. The molecular electrostatic potential (ESP) surfaces of DP are carried out at the B3LYP/6-31G(d) level, which are shown in Fig. 8(C). There are some negative regions (blue parts) in ESP of DP, which come from the electronacceptors of imide-ring and azo groups. These negative regions can serve as ‘‘traps’’ to block the mobility of charge carriers and further hinder the formation of electronic transmission channels. Under a low applied voltage, the ITO/PS-DP/Al device is in a low conductivity state (OFFstate) for the unfilled traps. As the applied voltage increases, the traps are filled by charge carriers and the device can meet the requirement of ON-state when the transition voltage (2.0 V) is reached. However, the traps in PS-DP are shallow and the filled traps can easily be detrapped due to the weak electron-withdrawing abilities of imide-ring and azo groups. So a reverse voltage sweep up to 5.5 V can make the device turn back to OFF-state. The shallow traps lead to the devices exhibit volatile memory effects [25]. And in our devices, the shallow traps of imide-ring and azo groups have significant influence on the observed flash memories. Carbazole is a good electron-donor and hole-transporting group, and can endow some polymers with excellent memory effects, such as the reported poly[3,3-bis(N-ethylenyloxycarbazole)-4,4-biphenylene hexafluoro-isopropylidene diphthalimide] (6F-HAB-CBZ PI) [10]. On the basis of PS-DP, the introduction of carbazole groups in PVCz-DP is expected to improve the memory effects. From the measured I–V curves of both the devices, it is found that the memory device based on PVCz-DP has relatively low transition voltages in contrast to PS-DP as shown in Table 1. These two polymers are both hole-transporting materials and the hole injection dominates the conduction process. So the energy barrier of hole injection between the ITO and polymer interface is an important factor affecting the memory characteristics. The high HOMO level of carbazole makes PVCz-DP have a lower energy barrier (0.58 eV) of hole injection than that of PS-DP (1.28 eV), which lead to
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the ITO/PVCz-DP/Al device have low transition voltages as many literatures reported [9,25]. Besides, in contrast to PSDP, charge carriers transporting in PVCz-DP film is more easily to occur under applied electric fields due to the good hole-transporting ability of pendant carbazole groups in PVCz-DP. And this effect leads the traps in PVCz-DP to be filled by charge carriers more easily, which also results in low transition voltages. In conclusion, the high HOMO level and good hole-transporting ability of carbazole make the devices based on PVCz-DP exhibit lower transition voltages in contrast to PS-DP. And this reasonable decline of transition voltages has beneficial for protection and life extension of the devices. Both PS-DP and PVCz-DP devices have the same conduction mechanisms on OFF and ON states. Take PS-DP for example, at low voltage sweep, holes are injected into polymer layer from the ITO electrode, and charge carriers accumulate near the electrode due to the energy barrier. Mutual repulsion between individual charges limits the charge injection, and the resulting current is space charge-limited current (SCLC). The I–V curve can be fitted by SCLC model [26] as shown in Fig. 9(a):
J/
9ei lV 2 3
8d
where l is the mobility of charge carriers, ei is the dynamic permittivity of the insulator and d is the film thickness. As the sweep voltage reaches to 2.0 V, the energy barrier is overcome and double injection is occurred, which make the current increase rapidly and the current of ON-state can be fitted by ohmic model as shown in Fig. 9(b). In the same way, the OFF-state current of devices based on PVCz-DP is also fitted the SCLC model, whereas the ONstate current is fitted the ohmic model, as shown in Fig. 10. Both of the two devices could switch to ON-state under a negative voltage sweep. But the traps in these polymers are not deep enough to maintain the ON-state, so a reverse voltage sweep could turn the devices back to OFF-state. It is reported that Al top electrode would penetrate into polymer film and coordinate with heteroatom (N or S) to form mental filament. Under different electronic field, the
Fig. 9. Experimental and fitted data of I-V curves for ITO/PS-DP/Al device in the OFF and ON states. (a) OFF-state with SCLC model, and (b) ON-state with ohmic model.
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Fig. 10. Experimental and fitted data of I–V curves for ITO/PVCz-DP/Al device in the OFF and ON states. (a) OFF-state with SCLC model, and (b) ON-state with ohmic model.
Fig. 11. Current–voltage (I–V) characteristics of the memory device based on PS-DP (a) and PVCz-DP (b) with Hg top electrode.
formation and rupture process of mental filament may lead the polymer film to have flash memory effects [2]. In order to eliminate this phenomenon, Hg is used instead of Al as the top electrode to measure the I–V curves on the same polymer film. As shown in Fig. 11, each device based on Hg top electrode showed flash memory behaviors with no significant change in contrast to Al. These phenomena actually indicate that the flash memory type is caused by the intrinsic characteristics of the two polymers themselves and not decided by the device structures. In addition, the switch-on (switch-off) voltage of ITO/PS-DP/Hg and ITO/PVCz-DP/Hg were 2.9 V (6.6 V) and 1.4 V (5.8 V), respectively. Both of the devices based on Hg top electrode show much higher transition voltages than Al because of the high energy barrier between the work function of Hg top electrode and the LUMO energy level of the polymer.
in transition voltages was resulted from the high HOMO level and good hole-transporting ability of carbazole groups. Both PS-DP and PVCz-DP devices have the same conduction mechanism. The current of OFF-state is fitted with the SCLC model, whereas the ON-state current is fitted with the ohmic model. Overall, the memory devices fabricated with these two styrene/maleimiade copolymers have potential applications for the future electronic memory devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (20876101, 20902065, 21076134), Supporting Project of Jiangsu Province (BK2010208) and Project Supported by the Major Fundamental Research Program of Natural Science Foundation of Jiangsu Higher Education Institutions (08KJA430004).
4. Conclusion References Two styrene/maleimiade copolymers with pendant azobenzene chromophores, PS-DP, PVCz-DP were synthesized for fabrication of memory devices. Both of the two devices showed rewritable flash memory characteristics with the ON/OFF current ratio around 103. But the switch-on and switch-off voltages of memory devices based on PVCz-DP were obviously lower than those of PS-DP. This difference
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Flash memory effects based on styrene/maleimiade copolymers with pendant azobenzene chromophores Yuanhua Liu, Najun Li, Xuewei Xia, Jianfeng Ge, Qingfeng Xu ⇑, Jianmei Lu ⇑ Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou, Jiangsu 215123, China
a r t i c l e
i n f o
Article history: Received 11 November 2010 Received in revised form 19 January 2011 Accepted 26 January 2011 Available online 4 February 2011 Keywords: Polymer Electronic memory Flash Device
a b s t r a c t Two styrene/maleimiade copolymers with pendant azobenzene chromophores, poly(styrene-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PS-DP) and poly((4-vinyl-benzyl)-9Hcarbazole-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP), were synthesized. The polymeric memory devices based on each of the two polymer films (ITO/Polymer/Al) show similar rewritable flash memory behaviors but different transition voltages. By introduction of carbazole groups in the polymer side chains, the voltage difference from OFF to ON state of ITO/PVCz-DP/Al is reduced obviously in comparison with that of ITO/PS-DP/ Al, which is beneficial to the protection of devices. Both ITO/PS-DP/Al and ITO/PVCz-DP/ Al show high stability under a constant stress or continuous read pulses voltage of 1.0 V. The memory mechanism is governed by space-charge limited conduction (SCLC) on the basis of the I–V curves of these fabricated memory devices. With excellent flash memory characteristics and simple processability, the memory devices fabricated with these two styrene/maleimiade copolymers have potential applications for the future electronic memory devices. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In recent ten years, polymeric electronic memory devices have attracted lots of attention for their low fabrication cost and super-high data-storage density [1–3]. A wide range of polymer materials, including poly(N-vinylcarbazole) (PVK) [4] and its derivatives [5–7], p conjugated polymers [8–12], dendritic polymers [13] and polymers in doped systems [14–17] have been reported for application in memory devices. Among three major types of memory behaviors, writeonce-read-many times (WORM) memory behavior can be written only once and not allowed to modify the stored data [18]; Dynamic random access memory (DRAM) needs power supply to retain the stored data [19], which disadvantages limit them for wide applications. Flash-type ⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Xu), [email protected] (J. Lu). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2011.01.012
memories are widely used in portable systems, such as mobile personal computer (PC), personal digital assistant (PDA), and digital camera [20], because it do not need power to keep the stored information and could be repeated written. Previously, many polymeric flash-type memory devices based on doped systems were reported [21]. Although they possess excellent flash memory effects, the phase separation is a hidden danger and may further disturb the stability in devices. Latter, some pure polyamides with complex structure were reported for flash memory applications [9]. Therefore the design and synthesis of a processable and single polymer with good flash memory characteristics are still desirable for fabrication of flash-type memory devices. Azobenzene (Azo) groups, owing to their special chemical structure, have been reported for applications in optical data-storage, switchable supramolecular cavities, nonlinear optics, sensors and molecular motors in recent years [22]. Besides, some azobenzene-containing organic compounds were also reported for electronic memory devices [23,24].
Y. Liu et al. / European Polymer Journal 47 (2011) 1160–1167
In this paper, we introduced azo groups into styrene/maleimiade copolymer systems for their flexible molecular structure and simple synthesis process. With the presence of azo groups, the memory devices based on poly(styrene1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PS-DP) and poly((4-vinyl-benzyl)-9H-carbazole-1-(4-phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP) all showed good flash memory behaviors. In contrast to PS-DP, the introduction of carbazole groups in PVCz-DP was expected to improve the memory effects because carbazole was a well-known electron-donor and hole-transporting group. Furthermore, the I–V characteristics were investigated in details to study the conduction mechanisms. 2. Experiment
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2.2.1. Synthesis of (4-vinylbenzyl)-9H-carbazole (VCz) Potassium hydroxide (4.5 g, 80.0 mmol) was added into a solution of carbazole (7.5 g, 45.0 mmol) in 120 mL N,Ndimethylacetamide (DMF). After stirring for 15 min, 1(chloromethyl)-4-vinylbenzene (6.8 g, 45.0 mmol) was added slowly to the mixture and continued reaction for 5 h at room temperature, the resulting mixture was poured into a large excess of water and the precipitate was collected by filtration. The final white product was washed with hot alcohol several times to remove the residual carbazole. Yield was 83% (10.5 g). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.14–8.12 (d, 2H), 7.44–7.40 (m, 2H), 7.36–7.34 (d, 2H), 7.30–7.27 (m, 2H), 7.25–7.23 (d, 2H), 7.10–7.08 (d, 2H), 6.68–6.60 (m, 1H), 5.70–5.65 (d, 1H), 5.50 (s, 2H), 5.21–5.18 (d, 1H). Anal. calcd. for C21H17N: C, 89.01; H, 6.05; N, 4.94. Found: C, 88.21; H, 5.85; N, 4.91.
2.1. Materials p-Aminoazobenzene (90%) was purchased from Beijing Chemical Reagent Co. Ltd. Maleicanhydride (99%), carbazole (99%), azobisisobutyronitrile (AIBN) (99%), cyclohexanone (99%), potassium hydroxide (KOH) (99%) and other solvents were all purchased from Shanghai Chemical Reagent Co. Ltd. Cyclohexanone was purified by reduced pressure distillation. AIBN was recrystallized from methanol before use. Other reagents were used as received without any further purification. 2.2. Preparation of PS-DP and PVCz-DP The polymer materials for memory devices are prepared according to the route shown in Fig. 1.
2.2.2. Synthesis of 1-(4-phenylazo-phenyl)-pyrrole-2,5-dione (DP) p-Aminoazobenzene (3.0 g, 15.0 mmol), maleicanhydride (1.5 g, 15.0 mmol) and 100 mL ethyl acetate were added to a single-necked flask. After stirring for 8 h at 60 °C, N-(4-phenylazo-phenyl)-maleamic acid was precipitated and collected by filtration. Then N-(4-phenylazo-phenyl)-maleamic acid (4 g, 13.5 mmol), triethylamine (1.5 g, 15.0 mmol) and 150 mL acetone were stirring for 5 min at 60 °C. After the dissolution of reactants, acetic anhydride (8.0 g, 78.0 mmol) and manganese acetate (0.05 g, as catalyst) were added and the mixture was continued to reaction for 4 h at 60 °C. The reaction solution was precipitated in a large excess of water and isolated by filtration, the final yellow product was recrystallized from alcohol. Yield was 65%
Fig. 1. Synthesis scheme and molecular structures of PS-DP and PVCz-DP.
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(2.7 g). 1H NMR (CDCl3, 400 MHz), d (ppm): 8.04–8.02 (d, 2H), 7.94–7.92 (d, 2H), 7.58–7.50 (m, 5H), 6.90 (s, 2H). Anal. calcd. for C16H11N3O2: C, 69.31; H, 4.00; N, 15.15. Found: C, 68.79; H, 4.05; N, 15.18. 2.2.3. Synthesis of poly(styrene-1-(4-phenylazo-phenyl)pyrrole-2,5-dione) (PS-DP) For a typical radical polymerization, styrene (0.35 g), 1diphenyldiazenyl-1H-pyrrole-2,5-dione (DP, 0.55 g), azoisobutyronitrile (AIBN, 3.28 mg) and 5 mL cyclohexanone were stirring at 70 °C for 8 h under nitrogen atmosphere. The reaction mixture was precipitated in a large excess of ethanol and isolated by filtration. The final product was dried at 80 °C under vacuum for 24 h, and the polymer yield was 84% (0.76 g). 2.2.4. Synthesis of poly((4-vinylbenzyl)-9H-carbazole-1-(4phenylazo-phenyl)-pyrrole-2,5-dione) (PVCz-DP) (4-Vinylbenzyl)-9H-carbazole (VCz, 0.28 g), 1-diphenyldiazenyl-1H-pyrrole-2,5-dione (DP, 0.28 g), azoisobutyronitrile (AIBN, 1.64 mg) and 4 mL cyclohexanone were stirring at 70 °C for 8 h under nitrogen atmosphere. After the same process of preparation for PS-DP, the final polymer PVCz-DP was collected with yield of 71% (0.40 g).
microscope. Molecular weights (Mn) and polydispersity (Mw/Mn) were measured by gel permeation chromatography (GPC) utilizing a Waters 1515 pump and a differential refractometer, THF was used as a mobile phase at a flow rate of 1.0 mL min1. 2.4. Fabrication of memory device The memory devices were fabricated as the sandwiched configuration of ITO/Polymer/Al. The indium-tin-oxide (ITO) glass was pre-cleaned by ultrasonic with water, acetone, and alcohol by turns for 30 min. Afterward, PS-DP (or PVCz-DP) solution in 1,2-dichloroethane (10 mg mL1) was filtered through microfilters with a pinhole size of 0.22 lm. The polymer film was prepared by spin-coating the filtered solution onto the pre-cleaned ITO glass substrate at a speed rate of 2000 rpm for 40 s and baked at 80 °C for 12 h under vacuum. The top metal electrode was obtained by a 100 nm-thick film of Al, which was thermally evaporated at about 106 torr through a shadow mask. The active area of the fabricated device was 0.0314 mm2 (a nummular point with a radius of 0.1 mm). All electrical measurements of the device were characterized under ambient conditions, without any encapsulation, using a HP 4145B semiconductor parameter analyzer.
2.3. Instruments 3. Results and discussion 1
H NMR spectra were obtained on an Inova 400 MHz FT-NMR spectrometer. The elemental analysis was performed by Italian 1106 FT analyzer. UV–Vis absorption spectra were carried out at room temperature from 250 to 550 nm with a Perkin-Elmer Lambda spectrophotometer. Thermo gravimetric analysis (TGA) was conducted on a TA instrument Dynamic TGA 2950 at a heating rate of 10 °C min1 under a N2 flow rate of 50 mL min1. SEM images were taken on a Hitachi S-4700 scanning electron
Fig. 2 shows the 1H NMR spectra of PS-DP and PVCz-DP. The number-average molecular weight and the polydispersity of PS-DP (Mn = 29,000, PDI = 1.8) and PVCz-DP (Mn = 13,000, PDI = 1.3) are determined by GPC. In order to obtain the content of DP segments in each copolymer chain, the UV–Vis spectra of DP with different concentrations were investigated with the maximum absorption peak at 333 nm (Fig. 3). And the absorption intensity at 333 nm
Fig. 2. 1H NMR spectra of PS-DP and PVCz-DP in CDCl3.
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Fig. 3. The UV–Vis spectra of DP in THF solution with different concentrations. The inset is the linear equation between the absorption intensity (at 333 nm) and concentration of DP.
shows a linear relationship with the concentration of DP, which is conformed to the following equation: Y = 80.14X + 0.04 Y is the absorption intensity of DP at 333 nm and X is the concentration of DP in THF solution. The absorption intensity at 333 nm (Y) of each copolymer in THF (0.05 g L1) was obtained. Then Y was substituted into the linear equation (Y = 80.14X + 0.04) to acquire X (the concentration of DP) which was 0.32 g L1 for PS-DP and 0.30 g L1 for PVCzDP, respectively. So the content of DP segments in the copolymer PS-DP and PVCz-DP were calculated as 64 wt.% and 60 wt.%, respectively. TGA curves in Fig. 4 reveal that each of the two copolymers has a high decomposition temperature up to 340 °C. Fig. 5(a) gives the schematic diagram of the indium-tin oxide/polymer/aluminum (ITO/Polymer/ Al) device. For each device, the thickness of the intermediate polymer layer was about 75 nm, which was obtained by the scanning electronic microscopic (SEM) image of the cross section of the fabricated storage cell, as shown in Fig. 5(b) and (c).
Fig. 4. TGA curves of PS-DP and PVCz-DP measured in nitrogen atmosphere at a heating rate of 10 °C min1.
Fig. 5. (a) Schematic diagram of the indium-tin oxide/polymer/aluminum device, (b) SEM image of the cross section of TIO/PS-DP/Al device, (c) SEM image of the cross section of TIO/PVCz-DP/Al device.
The electrical properties of the devices were measured at room temperature under ambient conditions without any encapsulation. For the ITO/PS-DP/Al sandwiched device (as shown in Fig. 6(a)), the current was sharply increased from 105 A (OFF-state) to 103 A (ON-state) at a switch-on voltage of 2.0 V when a voltage sweep from 0 to 4.0 V was applied (sweep 1). This electrical transition can serve as the ‘‘writing’’ process in this memory device. The device remained in its ON-state as the voltage was swept from 4.0 to 0 V (sweep 2). But in a voltage sweep of opposite bias from 0 to 6.0 V (sweep 3), the device could be erased to OFF-state at a switch-off voltage of 5.5 V, and the OFF-state was retained when the voltage sweep was applied again (sweep 4). The OFF-state can be further turned to the ON-state and turned back to the OFF-state by reapplying the voltage sweep (sweep 5, 6, 7, 8). Therefore, PS-DP can be used in a flash-type memory device. Similarly, the memory device based on PVCz-DP also shows flash memory effects (Fig. 7(a)). But the switch-on and switch-off voltages of memory devices based on PVCz-DP were obviously lower than those of PS-DP, as shown in Table 1. Both of the devices show ON/OFF current ratio around 103. For each device, no significant degradation in current of the OFF and ON states was observed after 103 s of continuous stress test at 1.0 V, and also no significant degradation was observed after 105 read cycles at 1.0 V (Figs. 6(b) and 7(b)). The results indicate the two devices have high stabilities in both OFF and ON states. In order to understand the memory mechanisms and explain the difference in transition voltages of these two devices, the energy levels of VCz and DP were carried out at the B3LYP/6-31G(d) level. Styrene groups were not included since they do not significantly affect the electronic properties. For PS-DP, the energy barrier between the work function of ITO bottom electrode and the high-occupiedmolecular-orbital (HOMO) level of DP unit is 1.28 eV, which is smaller than that between the lowest-unoccupied-molecular-orbital (LUMO) level of DP unit and the
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Fig. 6. (a) Current–voltage (I–V) characteristics of the memory device based on PS-DP, (b) effect of retention time on the ON and OFF states of the memory device under a constant stress of 1.0 V, and the inset is effect of read cycles on the ON and OFF state under a read pulse voltage of 1.0 V.
Fig. 7. (a) Current–voltage (I–V) characteristics of the memory device based on PVCz-DP, (b) effect of retention time on the ON and OFF states of the memory device under a constant stress of 1.0 V, and the inset is effect of read cycles on the ON and OFF state under a read pulse voltage of 1.0 V.
Table 1 Switch-on and switch-off voltages of TIO/PS-DP/Al and TIO/PVCz-DP/Al devices. Device
Switch-on voltage (V)
Switch-off voltage (V)
ITO/PS-DP/Al ITO/PVCz-DP/Al
2.0 1.2
5.5 3.8
Table 2 HOMO and LUMO energy levels of each polymer from theoretic calculation. Polymer
HOMO (eV)
LUMO (eV)
Energy barrier (Al ? LUMO) (eV)
Energy barrier (ITO ? HOMO) (eV)
PS-DP PVCzDP
6.08 5.38
2.85 2.85
1.35 1.35
1.28 0.58
work function of Al top electrode (1.35 eV), as shown in Table 2. Thus the hole injection from ITO into the HOMO level of DP is more favorable than electron injection from the Al top electrode into the LUMO level of DP (Fig. 8(a)). And hole injection dominates the conduction process in the devices. Similarly, hole injection also dominates the conduc-
Fig. 8. Energy band diagram of molecular components of PS-DP (a) and PVCz-DP (b), (c) molecular structure and electrostatic potential (ESP) surfaces of DP.
tion process in ITO/PVCz-DP/Al device (Fig. 8(b)). However, as the sweep voltage increased to the switch-on voltage, electron injection form Al top electrode to LUMO level of
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DP unit occurs along with hole injection since the low energy barrier (1.35 eV). Double injection from the two electrodes makes the device reach to the high conductivity state. For PS-DP, the memory effects were dominated by DP units. The molecular electrostatic potential (ESP) surfaces of DP are carried out at the B3LYP/6-31G(d) level, which are shown in Fig. 8(C). There are some negative regions (blue parts) in ESP of DP, which come from the electronacceptors of imide-ring and azo groups. These negative regions can serve as ‘‘traps’’ to block the mobility of charge carriers and further hinder the formation of electronic transmission channels. Under a low applied voltage, the ITO/PS-DP/Al device is in a low conductivity state (OFFstate) for the unfilled traps. As the applied voltage increases, the traps are filled by charge carriers and the device can meet the requirement of ON-state when the transition voltage (2.0 V) is reached. However, the traps in PS-DP are shallow and the filled traps can easily be detrapped due to the weak electron-withdrawing abilities of imide-ring and azo groups. So a reverse voltage sweep up to 5.5 V can make the device turn back to OFF-state. The shallow traps lead to the devices exhibit volatile memory effects [25]. And in our devices, the shallow traps of imide-ring and azo groups have significant influence on the observed flash memories. Carbazole is a good electron-donor and hole-transporting group, and can endow some polymers with excellent memory effects, such as the reported poly[3,3-bis(N-ethylenyloxycarbazole)-4,4-biphenylene hexafluoro-isopropylidene diphthalimide] (6F-HAB-CBZ PI) [10]. On the basis of PS-DP, the introduction of carbazole groups in PVCz-DP is expected to improve the memory effects. From the measured I–V curves of both the devices, it is found that the memory device based on PVCz-DP has relatively low transition voltages in contrast to PS-DP as shown in Table 1. These two polymers are both hole-transporting materials and the hole injection dominates the conduction process. So the energy barrier of hole injection between the ITO and polymer interface is an important factor affecting the memory characteristics. The high HOMO level of carbazole makes PVCz-DP have a lower energy barrier (0.58 eV) of hole injection than that of PS-DP (1.28 eV), which lead to
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the ITO/PVCz-DP/Al device have low transition voltages as many literatures reported [9,25]. Besides, in contrast to PSDP, charge carriers transporting in PVCz-DP film is more easily to occur under applied electric fields due to the good hole-transporting ability of pendant carbazole groups in PVCz-DP. And this effect leads the traps in PVCz-DP to be filled by charge carriers more easily, which also results in low transition voltages. In conclusion, the high HOMO level and good hole-transporting ability of carbazole make the devices based on PVCz-DP exhibit lower transition voltages in contrast to PS-DP. And this reasonable decline of transition voltages has beneficial for protection and life extension of the devices. Both PS-DP and PVCz-DP devices have the same conduction mechanisms on OFF and ON states. Take PS-DP for example, at low voltage sweep, holes are injected into polymer layer from the ITO electrode, and charge carriers accumulate near the electrode due to the energy barrier. Mutual repulsion between individual charges limits the charge injection, and the resulting current is space charge-limited current (SCLC). The I–V curve can be fitted by SCLC model [26] as shown in Fig. 9(a):
J/
9ei lV 2 3
8d
where l is the mobility of charge carriers, ei is the dynamic permittivity of the insulator and d is the film thickness. As the sweep voltage reaches to 2.0 V, the energy barrier is overcome and double injection is occurred, which make the current increase rapidly and the current of ON-state can be fitted by ohmic model as shown in Fig. 9(b). In the same way, the OFF-state current of devices based on PVCz-DP is also fitted the SCLC model, whereas the ONstate current is fitted the ohmic model, as shown in Fig. 10. Both of the two devices could switch to ON-state under a negative voltage sweep. But the traps in these polymers are not deep enough to maintain the ON-state, so a reverse voltage sweep could turn the devices back to OFF-state. It is reported that Al top electrode would penetrate into polymer film and coordinate with heteroatom (N or S) to form mental filament. Under different electronic field, the
Fig. 9. Experimental and fitted data of I-V curves for ITO/PS-DP/Al device in the OFF and ON states. (a) OFF-state with SCLC model, and (b) ON-state with ohmic model.
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Fig. 10. Experimental and fitted data of I–V curves for ITO/PVCz-DP/Al device in the OFF and ON states. (a) OFF-state with SCLC model, and (b) ON-state with ohmic model.
Fig. 11. Current–voltage (I–V) characteristics of the memory device based on PS-DP (a) and PVCz-DP (b) with Hg top electrode.
formation and rupture process of mental filament may lead the polymer film to have flash memory effects [2]. In order to eliminate this phenomenon, Hg is used instead of Al as the top electrode to measure the I–V curves on the same polymer film. As shown in Fig. 11, each device based on Hg top electrode showed flash memory behaviors with no significant change in contrast to Al. These phenomena actually indicate that the flash memory type is caused by the intrinsic characteristics of the two polymers themselves and not decided by the device structures. In addition, the switch-on (switch-off) voltage of ITO/PS-DP/Hg and ITO/PVCz-DP/Hg were 2.9 V (6.6 V) and 1.4 V (5.8 V), respectively. Both of the devices based on Hg top electrode show much higher transition voltages than Al because of the high energy barrier between the work function of Hg top electrode and the LUMO energy level of the polymer.
in transition voltages was resulted from the high HOMO level and good hole-transporting ability of carbazole groups. Both PS-DP and PVCz-DP devices have the same conduction mechanism. The current of OFF-state is fitted with the SCLC model, whereas the ON-state current is fitted with the ohmic model. Overall, the memory devices fabricated with these two styrene/maleimiade copolymers have potential applications for the future electronic memory devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (20876101, 20902065, 21076134), Supporting Project of Jiangsu Province (BK2010208) and Project Supported by the Major Fundamental Research Program of Natural Science Foundation of Jiangsu Higher Education Institutions (08KJA430004).
4. Conclusion References Two styrene/maleimiade copolymers with pendant azobenzene chromophores, PS-DP, PVCz-DP were synthesized for fabrication of memory devices. Both of the two devices showed rewritable flash memory characteristics with the ON/OFF current ratio around 103. But the switch-on and switch-off voltages of memory devices based on PVCz-DP were obviously lower than those of PS-DP. This difference
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