Dynamic Random Access Memory Devices Based on Functionalized

ARTICLE pubs.acs.org/JPCC

Dynamic Random Access Memory Devices Based on Functionalized Copolymers with Pendant Hydrazine Naphthalimide Group Hua Li, Najun Li, Ru Sun, Hongwei Gu, Jianfeng Ge, Jianmei Lu,* Qingfeng Xu,* Xuewei Xia, and Lihua Wang Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren’ai Road, Suzhou 215123, China Key Laboratory of Optics of Jiangsu Province, Soochow University (Central Campus), 1 Shizi Road, 215006, China ABSTRACT: In this paper, one methacrylate monomer containing naphthalimide as electron acceptor and aromatic hydrazone as electron donor was designed and synthesized. Its copolymer with styrene has been incorporated into sandwiched memory devices which show dynamic random access memory characteristics with highest ON/OFF current ratio up to 106 and a long retention time. Moreover, it was observed that switch threshold voltage of the device varied almost linearly with functional moiety content in the copolymer. The photoluminescence spectra and X-ray diffraction of the copolymer’s film were investigated and the results showed that the functional moieties in the pendant chains occurred as π
π stacking and the distance between each other became closer as the functional moieties content in the copolymer increased The mechanisms associated with dram characteristics were elucidated from molecular simulation results that the slight electron density transition from the HOMO to LUMO surfaces would easily revert to original state once the external electric field was removed.

1. INTRODUCTION Developing high-density data storage (HDDS) devices to meet the information explosion era is one of today’s leading technologies.1 Currently, with most data storage systems that are based on optical or magnetic materials, HDDS is hard to achieve because of the difficulty with continued scaling down of domain size to nanometer scale.2
7 Polymeric and organic bistable memory devices are promising candidates because of their attractive features including simplicity in device structure, good scalability, low-cost potential, low-power operation, and three-dimensional (3D) stacking capability.8
11 Some successful cases have been reported to fabricate excellent memory devices that are based on the π-conjugated polymers containing push
pull electron groups or conductive polymers doped with gold nanoparticles.12
25 Recently, the polymers containing pendant donor
acceptor structures have received considerable attention,26
31 arising from their unique advantages that the carrier injection potentials and mobility of pendant groups in side-chain polymers will be easily varied and controlled. Also, the use of polymeric materials allows rapid, easy, and cost-effective processing of the thin layers required in devices.32 In our previous work, we proved that the devices based on polymethacrylate containing pendent azobenzothiazole show different memory performances as the thickness of polymeric film changes. The device exhibited WORM behavior when the film thickness was 40 nm, while the FLASH behavior was observed when the film thickness was 80 nm.33 However, there are still other important effects of the medium layer such as the functional moiety content and the stacking effect of the side chains remaining unexplored. In this paper, we keep the film r 2011 American Chemical Society

thickness invariable while the content of the functional moiety in the copolymer will be changed to investigate the effect on the memory device’s performance. Hydrazones that contain the —CHdN—Nd functionality possess good hole transport properties for technical applications; the simple synthesis and low cost are the advantages of hydrazones versus other classes of charge transport materials.34 1,8Naphthalimide compounds are an attractive class of electrondeficient organic materials and have been widely utilized in both small molecular and polymer-based organic light-emitting diodes.35 Considering conjugated organic bridges in such D-π-A compounds have been shown to act as molecular “wires”,36 we prepared one naphthalimide derivative Naphthalimide Aromatic-hydrazone Methacrylate (NAM) as shown in Scheme 1, where the naphthalimide acts as electron acceptor and the aromatic hydrazone acts as electron donor and bridge, which is important for charge transfer under an electric field.37 The simple and mature free radical polymerization is used in this paper to prepare pSt-co-NAM (pSNAM) in which the monomer styrene is introduced to adjust the material’s solubility and scalability. The memory devices based on the copolymers with different functional moiety content shows excellent dynamic random access memory (DRAM) performances with ON/OFF current ratio of about 106 and the switch threshold voltage of the device varies approximately linearly with functional moiety content in the copolymer. Received: November 23, 2010 Revised: March 17, 2011 Published: April 05, 2011 8288

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Scheme 1. Synthesis Route and Molecular Structure of the Copolymers

2. EXPERIMENTAL SECTION 2.1. Materials. Hexyl bromide (99%, Wuxi Orient Detergents Technological Co., Ltd., China), 4-bromo-1,8-naphthalic anhydride (97%, Liaoning Liangang Dyes Chemical Co. Ltd., China), and 1-chloro-6-hydroxyhexane (99%, Shandong Zouping Mingxing Chemical Co. Ltd., China) were used as received without purification. Methacryloyl chloride (95%, Shanghai Zhenxing Chemical Reagent Factory) and styrene were purified by reduced pressure distillation. 4-Hydroxy-3,5-dimethoxy-benzaldehyde (99%), 4-nitrophenol (99%), iron powder (99%), and all solvents were purchased from Sinopharm Chemical Reagent Co. Ltd., China, and used as received. 2.2. Synthesis of Compound 1. 4-Hydroxy-3,5-dimethoxybenzaldehyde (18.2 g, 0.1 mol), 1-chloro-6-hydroxyhexane (16.3 g, 0.12 mol), and sodium hydroxide (4.0 g, 0.1 mol) were mixed in 100 mL of water and refluxed overnight. Then 100 mL of ethyl acetate was added to the mixture. The organic layer was extracted and washed with sodium hydroxide solution, dried, and distilled in vacuum to give brown solid compound 1. Yield: 76.5%. 1H NMR (CDCl3, δ, ppm): 9.87 (s, 1H), 7.13 (s, 2H), 4.07 (t, 2H), 3.92 (s, 6H), 3.68 (t, 2H), 1.84 (m, 2H), 1.60 (m, 2H), 1.48 (m, 4H). 2.3. Synthesis of Compound 2. Compound 1 (14.1 g, 0.05 mol), triethylamine (6.0 g, 0.06 mol), and tetrahydrofuran (THF) (100 mL) were added into a flask and stirred at 0 °C. The methacryloyl chloride (6.24 g, 0.06 mol) solution in THF (10 mL) was added dropwise to the mixture. After 1 h, the ice
water bath was removed and the reaction was allowed to continue for 6 h at room temperature. The mixture was filtered and the filtrate was diluted with ethyl acetate and then washed with 5% hydrochloric acid (20 mL  3) and saturated sodium

bicarbonate aqueous solution (20 mL  3). The organic solution was dried over anhydrous magnesium sulfate and concentrated in a rotary evaporator. The resulting product was chromatographed on a silica gel column (ethyl acetate: petroleum ether = 1:3) to give the straw yellow liquid compound 2. Yield: 72.1%. 1H NMR (CDCl3, δ, ppm): 9.87 (s, 1H), 7.13 (s, 2H), 6.11 (s, 1H), 5.56 (s, 1H), 4.17 (t, 2H), 4.04 (t, 2H), 3.94 (t, 6H), 1.96 (s, 3H), 1.87 (m, 2H), 1.74 (m, 2H), 1.51 (m, 4H). 2.4. Synthesis of Compound 3. 4-Nitrophenol (13.9 g, 0.1 mol), hexyl bromide (24.6 g, 0.15 mol), and K2CO3 (27.6 g, 0.2 mol) were stirred in DMF at room temperature for 12 h. Then the solution was diluted with ethyl acetate and washed with 5% sodium hydroxide solution. The organic layer was dried over anhydrous magnesium sulfate and concentrated in a rotary evaporator to give yellow-green liquid (compound 3). Yield: 17.8 g, 79.8%. 1H NMR (CDCl3, δ, ppm): 8.20 (d, 2H), 6.95 (d, 2H), 4.05 (t, 3H), 1.83 (m, 2H), 1.46 (t, 2H), 1.26 (t, 4H), 0.89 (t, 3H). 2.5. Synthesis of Compound 4. Iron powder (3.5 g, 62.5 mmol), ethanol (80 mL), water (10 mL), and several drops of hydrochloric acid were stirred in a three-neck flask and refluxed for 1 h; 3.8 g of compound 3 was added to the mixture and continued refluxing for 8 h. The hot solution was filtered, the filtrate was poured into water and extracted with ethyl acetate, and the organic layer was dried and distilled in vacuum to give a brown liquid (compound 4). Yield: 2.5 g, 65.8%. 1H NMR (CDCl3, δ, ppm): 6.75 (d, 2H), 6.63 (d, 2H), 3.85 (t, 2H), 3.4 (s, 2H), 1.75 (m, 2H), 1.43 (t, 2H), 1.33 (t, 4H), 0.90 (t, 3H). 2.6. Synthesis of Compound 5. 4-Bromide-1,8-naphthalic anhydride (2.5 g, 9 mmol) and compound 4 (2.1 g, 11 mmol) were added to 100 mL of ethanol and stirred vigorously; after 8289

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Table 1. Characterization Data of pSNAM GPC result

a

polymer

[St]0/[NA]0

Mn

Mw/Mn

wt %a

onset decomposition temperature (°C)

pSNAM

150/1

14900

1.83

3.6

380.5

100/1

16600

2.10

5.2

389.9

40/1

17100

2.23

16.4

393.4

20/1

16400

1.90

28.6

397.3

10/1

17700

2.39

30.2

400.2

5/1

17400

2.15

29.7

398.5

The weight percentage of naphthalimide moieties in the polymers were measured by UV
vis absorption standard curve.

having refluxed for 6 h, the mixture was cooled and the precipitated solids were filtered and recrystallized from ethanol to give a pale gray product (compound 5). Yield: 3.5 g, 86.2%. 1H NMR (CDCl3, δ, ppm): 8.70 (d, 1H), 8.64 (d, 1H), 8.45 (d, 1H), 8.07 (d, 1H), 7.89 (t, 1H), 7.20 (d, 2H), 7.05 (d, 2H), 4.01 (t, 2H), 1.82 (m, 2H), 1.48 (m, 2H), 1.36 (m, 4H), 0.92 (t, 3H). 2.7. Synthesis of Compound 6. Compound 5 (4.5 g, 10 mmol) and 1.5 mL of 85% hydrazine hydrate were added into 30 mL of methoxyl ethanol and refluxed for 3 h. After the mixture cooled to room temperature, the precipitated solids were filtered, washed with ethanol and chloroform, then recrystallized from methoxyl ethanol, and dried in vacuum to give a pink powdery product (compound 6). Yield: 3.2 g, 79.4%. 1H NMR (d6-DMSO, δ, ppm): 9.11 (s, 1H), 8.68 (d, 1H), 8.63 (d, 1H), 8.46 (d, 1H), 8.17 (d, 1H), 7.63 (t, 1H), 7.21 (d, 2H), 7.01 (d, 2H), 4.69 (s, 2H), 4.00 (t, 2H), 1.86 (m, 2H), 1.48 (m, 2H), 1.34 (m, 4H), 0.93 (t, 3H). 2.8. Synthesis of Compound 7. Compound 2 (0.88 g, 2.5 mmol) and compound 6 (1.0 g, 2.5 mmol) were added into 70 mL of ethanol and refluxed for 4 h. After the mixture cooled to room temperature, the precipitated solids were filtered, dried, and chromatographed on a silica gel column (chloroform:ethyl acetate = 1:3) to give the orange powder compound 7. Yield: 45.2%. Elemental analysis calculated for C43H49N3O8: C, 70.18; H, 6.71; N, 5.71. Found: C, 70.50; H, 6.47; N, 5.65. 1H NMR (CDCl3, 400 MHz) δ (ppm): 9.13 (s, 1H), 8.61 (d, 2H), 8.27 (d, 1H), 7.94 (s, 1H), 7.84 (d, 1H), 7.60 (t, 1H), 7.18 (d, 2H), 6.94 (d, 4H), 6.11 (s, 1H), 5.56 (s, 1H), 4.16 (t, 2H), 4.03 (t, 2H), 3.92 (m, 8H), 1.95 (s, 3H), 1.83
1.68 (m, 6H), 1.57
1.40 (m, 6H), 1.34 (m, 4H), 0.91 (t, 3H). 2.9. Copolymerization of NAM with Styrene. The free radical copolymerization of NAM with styrene was carried out in cyclohexanone solution. The typical procedure is as follows: styrene (1.04 g, 10 mmol), NAM (0.37 g, 0.5 mmol), 2,20 -azobis(isobutyronitrile) (AIBN, 8.2 mg, 0.05 mol), and 1 mL of cyclohexanone were added to a dry glass tube under a N2 atmosphere and the reaction temperature was kept at 85 °C. After 12 h, the mixture was cooled, diluted with a little tetrahydrofuran, and precipitated in a large amount of methanol. The precipitated product was filtrated and further purified by extraction with methanol in a Soxhlet apparatus to remove small molecular residue away to give the orange product. 2.10. Instruments. 1H NMR spectra were obtained on an Inova 400 MHz FT-NMR spectrometer. The elemental analysis was performed by an Italian 1106 FT analyzer. Thermogravimetric analysis (TGA) was conducted on a TA Instruments Dynamic TGA 2950 at a heating rate of 10 °C/min and under an N2 flow rate of 50 mL/min. Molecular weights (Mn) and polydispersity (Mw/Mn) were measured by gel permeation chromatography (GPC) utilizing a Waters 515 pump and a differential refractometer. Tetrahydrofuran

was used as a mobile phase at a flow rate of 1.0 mL/min. Room temperature emission spectra were taken on an Edinburgh-920. Thin layer chromatography was performed on silica gel plates using chloroform as eluent to confirm the bonding of NAM moieties to the polymer chains. The weight percentages of NAM moieties were determined by the UV
vis absorption standard curve. 2.11. Fabrication and Measurement of the Memory Devices. The indium tin oxide (ITO) glass was precleaned with water, acetone, and alcohol, in that order, in an ultrasonic bath for 20 min, respectively. The copolymer solution was prepared in N, N-dimethylformamide (10 mg/mL) and filtered through microfilters with a pinhole size of 0.22 μm, after which the solutions were spin-coated onto ITO controlled at 1500 rpm and the solvent was removed in a vacuum chamber at 10
1 Pa and 60 °C for 12 h. A layer of Al, about 110 nm in thickness and 0.5 mm in diameter, was thermally evaporated and deposited onto the polymer film surface at about 10
6 Torr through a shadow mask to form the top electrode. All electrical measurements of the device were characterized under ambient conditions, without any encapsulation, using a HP 4145B semiconductor parameter analyzer.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Copolymer. The synthesis route and molecular structures of monomers and copolymers are shown in Scheme 1. The weight percentages of NAM moieties in the copolymer were determined by the UV
vis absorption standard curve. The Mn, Polydispersity index (PDI), naphthalimide moieties content, and the onset decomposition temperatures of the copolymers for different monomer ratios are summarized in Table 1. The naphthalimide moieties content in the copolymers increase with the proportion of NAM monomers from 1/150 to 1/20. All the polymers exhibit good thermal stabilities, with an onset decomposition temperature of about 390 °C, which promises the material can endure heat deterioration in the memory devices. The UV
visible absorption spectra of pSNAM with different NAM content in THF solutions are shown in Figure 1a. All of the maxima peaks are at about 450 nm, which is assigned to a π
π* transition in the naphthalimide moieties. The luminescence spectra of pSNAM with the excitation wavelength λex = 388 nm were also investigated in Figure 1b. It is the same with the absorption spectra that the maxima emission peaks are almost the same for all polymers with different NAM content. This result indicates that the polymer backbones are soft and the NAM moieties in the side chains have an irregular arrangement in the solution so that the steric influence of the side chain is negligible. 3.2. Memory Effects. Memory devices of an ITO/polymer/ Al sandwich structure were fabricated using thin films of the 8290

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Figure 1. (a) UV
visible spectra of pSNAM with different NAM content in THF solution; (b) fluorescence intensity spectra of pSNAM with different NAM moiety content in THF solution.

copolymers mentioned above. The device structure is shown schematically in Figure 2a. Figure 2b is the scanning electron microscopic (SEM) image of one storage cell from the direction of a cross-section view; from the top to bottom is the aluminum electrode, pSNAM film, and ITO glass, respectively. The thickness of the pSNAM film can be judged as 90 nm from this image. Electrical transitions in the ITO/polymer/Al devices were investigated in the measurements of the current response to an external applied voltage. ITO was maintained as the ground electrode in all electrical measurements. The current
voltage (I
V) characteristics of the ITO/pSNAM/Al device in Figure 2c exhibited two distinct conductivity states (herein, NAM content in the copolymer is 28.6%). In the first voltage scan from 0 to
3 V, a sharp current jump happened at a threshold voltage of around
1.75 V, indicating the transition from a low-conductivity state (OFF state) to a high-conductivity state (ON state). The device remained in this ON state during the subsequent scan from 0 to
3 V (sweep 2). The distinct bielectrical states in the voltage range of about 0 to
1.75 V, where the ON/OFF current ratio is up to 106, allowed a voltage (e.g.,
1.0 V) to read the “0” or “OFF” signal (before writing) and “1” or “ON” signal (after writing) of the memory device. Sweep 3 was carried out after turning off the power for about 30 s; it was found that the ON state had relaxed to the steady OFF states and the devices could be switched to the ON states again when the threshold voltage of

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about
1.75 V was reapplied. This behavior is characteristic of a dynamic random access memory (DRAM) device. The ON and OFF states of the devices are stable for up to 108 continuous read pulses of
1 V without any resistance degradation. The high ON/OFF current ratios of the devices promise a low misreading rate by precise control over the OFF and ON states. The retention ability of the OFF and ON states of a device was investigated under a constant voltage stress of
1 V and recording of the current passing through the device at regular intervals. Figure 2d shows that the device had both stable OFF and ON state currents within the 200 min time frame, with the high ON/OFF current ratio sustained. Thus, both states are stable under voltage stress and are insensitive to read pulses. To investigate the effect of film thickness of pSNAM on the memory device, the 50 nm thick film was fabricated into a sandwiched device and the I
V characteristics of the device was investigated; it was found that the thinner film did not lead to different memory performance, the device still showed the DRAM behavior, and only the threshold voltage was advanced to
1.52 V. The relationship between the memory behavior and the NAM moieties content in the copolymer was investigated (Figure 3). When the NAM content was below 5%, the devices were always in the OFF state and could not switch to the ON state, even with a large electric field. This may be arising from quite a number of the polymer molecules not containing the NAM moiety, which can be deduced from the Mn of the copolymer and molecular weight of the NAM monomer. When the NAM content in the copolymer was increased to 5.2%, 16.4%, and 28.6%, all the devices based on the copolymers exhibited DRAM characteristics and the switching threshold voltages were corresponding to
3.4,
2.65, and
1.75 V, respectively. It was unexpectedly found that the switching threshold voltage varied approximately linearly with the NAM moiety content in the intermedium copolymer. 3.3. Memory Mechanism Discussion. The fluorescence spectra and X-ray diffraction spectra of the pSNAM film were carried out to understand the memory behavior that the switching threshold voltage varied almost linearly with the NAM moiety content. Figure 4 shows the fluorescence spectra of pSNAM with different NAM moiety content in thin solid films, respectively. The λmax of photoluminescence (PL) in the film is larger than that in the THF solution (Figure 1b), which is due to a high degree of π
π interaction between the side NAM chains in the condensed matter; the λmax of PL red-shifted from 586 to 609 nm as the NAM moiety content in the copolymer side chains increased from 5.8% to 28.6%, which indicated the π
π interaction became more obvious,38 resulting in a stronger overall electronic interaction.39 X-ray diffraction of the pSNAM films was carried out to further understand the regioregular arrangement of the NAM in the side chains. Figure 5 shows that the diffraction peaks centered at 2θ = 19° (4.7 Å), 22.4° (4.0 Å), and 24° (3.7 Å) for pSNAM with different NAM moiety content (number in parentheses represents the corresponding d value for the 2θ peak), which demonstrated that the distance between the NAM groups became closer with the NAM moieties content increasing.40 Thus, the PL spectra and XRD data illustrated well why the switching threshold voltage varied almost linearly with the NAM moiety content. Additional information about the charge transport mechanism can be obtained from the I
V curves in OFF and ON states according to various theoretical models. As shown in Figure 6a, the OFF states for the ITO/pSNAM/Al device can be elucidated as Poole-Frenkel (PF)33 emission in terms of the plots of Log(I/V) versus V1/2 found to be linear in the voltage range of
0.33 to
2.5 V. To investigate the reason that the log(I/V) and V1/2 did not have a strictly linear 8291

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Figure 2. (a) Schematic diagram of the device consisting of a thin film of the copolymers sandwiched between an ITO bottom electrode and an Al top electrode; (b) SEM image of one storage cell (cross-section view); (c) current
voltage (I
V) characteristics and effect of read pulse of 1 V on the device; (d) effect of operation time (at
1 V) on OFF and ON states of the ITO/pSNAM/Al device under a constant stress of
1 V (herein, NAM moiety content in the active layer is 28.6%).

Figure 3. Current
voltage curves of several devices based on pSNAM with different NAM moieties content.

relationship in the entire OFF state, calculation of electrostatic potentials (ESP) for functional group NAM was carried out at B3LYP/6-31G(d) level with the Gaussian03 program package to further understand the charge carriers migration through the polymer film. The monomer molecular surfaces with the continuous positive ESP (Figure 7) in the red area along the NAM side chain indicated that charge carriers can migrate through this open channel. However, there are some negative ESP regions (blue) which arise from the amide and phenoxy groups. These negative regions can serve as “traps” to block the mobility of charge carriers. Thus, the low current level in the OFF state was due to the PF emission models and the “traps” lying in the conjugated backbone of the side chains. However, the currents in the ON states are almost linear dependent on applied voltage just as metallic conduction (Figure 6b); this indicates that in the ON states the charge transport is dominated by ohmic model. So the change of the current conduction from the OFF state to the ON

Figure 4. Fluorescence spectra of the thin film of pSNAM with different NAM moiety content.

state for both types of devices is attributed to the PF emission model and “traps limited” model to the Ohmic model. In previous work, it has been demonstrated that the memory characteristic of the device depends on whether the molecular structure with donor
acceptor structures has a potential to form a intramolecule charge-separated state under electric field.24,41 If the electron acceptor has a strong electron trapping ability, upon undergoing electric field, the electron located on the electron donor side would totally transit to the electron acceptor side to form a permanent charge-separated state and show write once read many times memory (WORM) characteristics, while the electron acceptor is a moderate one that the electron located on donor side would only partly or slightly transit to the acceptor side and this transition could be easily detrapped through reverse voltage or even automatically revert to the pristine state and thus lead to the device’s flash or DRAM memory performance. 8292

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Figure 7. HOMO, LUMO, and ESP surfaces of the monomer units of pSNAM obtained by molecule simulation. Figure 5. X-ray diffraction of pSNAM film with different NAM moiety content.

Figure 6. Experimental and fitted data of I
V curves for ITO/ pSNAM/Al device in the OFF and ON states. (a) is the OFF state with the Poole-Frenkel model (
0.25 to
1.96 V); (b) is the ON state with the Ohmic current model.

Because the HOMO surface represents the localization of electron density on the electron donor in the ground state and the LUMO surface represents the localization of electron density

on the electron acceptor in the excited state, through analysis of the electron density change from the HOMO to LUMO surfaces, we can judge how the charge-separated state will exist after the electric field is applied. The HOMO and LUMO surfaces of NAM monomer were simulated and summarized in Figure 7. The HOMO surface shows the electron density not only locating on the electron donor methoxylbenzene side but also on the electron acceptor naphthalimide side, which indicates the electron pull-push effect in the NAM is not so obvious. Upon undergoing the HOMO to LUMO transition, the electron density underwent only a slight transition from the methoxylbenzene side to the naphthalimide side, so a charge-separated state could not occur and once the external electric field was removed, the slight electron density transition would revert to the pristine state and the device would exhibit the DRAM memory effects.

4. CONCLUSION One naphthalimide derivative NAM was synthesized and the devices based on the copolymers pSNAM exhibited DRAM behaviors with high ON/OFF ratios up to 106. It is found that the switching threshold voltage varies almost linearly with the functional moiety content, which is attributed to the distance between the NAM groups in the side chains becoming closer. The conduction processes of the devices was found to be dominated by hole injection and governed by PF emission model and “traps limited” model to the Ohmic model during the OFF to ON state transition. The mechanism of the DRAM behavior was further elucidated from the molecular simulations of electron density in the HOMO and LUMO surfaces; the slight electron density shift from donor to acceptor aside indicates the steady charge-separated state could not be maintained permanently and would automatically revert to its original state after the voltage scan was completed. We envision that our proposed mechanism is promising for providing a molecular design direction in studying high-density data storage devices. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-512-6588-0368. Fax: þ86-512-6588-0367. E-mail: [email protected]; [email protected]. 8293

dx.doi.org/10.1021/jp1111668 |J. Phys. Chem. C 2011, 115, 8288–8294

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’ ACKNOWLEDGMENT This work was financially supported by the Chinese Natural Science Foundation (NSFC 20876101, 21076134), NSF of Jiangsu Province (08KJA430004, BK2008158, BK2010208), and project supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070285003). ’ REFERENCES (1) Scott, J. C.; Bozano, L. D. Adv. Mater. 2007, 19, 1452. (2) Moller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. Nature 2003, 426, 166. (3) Rozenberg, M. J.; Inoue, I. H.; Sanchez, M. J. Phys. Rev. Lett. 2004, 92, 178302-1. (4) Hagen, R.; Bieringer, T. Adv. Mater. 2001, 12, 1805. (5) Kawata, S.; Kawata, Y. Chem. Rev. 2000, 100, 1777. (6) Wang, J. P. Nat. Mater. 2005, 4, 191. (7) Osaka, T.; Takai, M.; Hayashi, K.; Ohashi, K.; Saito, M.; Yamada, K. Nature 1998, 329, 796. (8) Kapetanakis, E.; Douvas, A. M.; Velessiotis, D.; Makarona, E.; Argitis, P.; Glezos, N.; Normand, P. Adv. Mater. 2008, 20, 4568. (9) Yang, Y.; Ouyang, J. Y.; Ma, L. P.; Tseng, R. J.; Chu, C. W. Adv. Funct. Mater. 2006, 16, 1001. (10) Ling, Q. D.; Liaw, D. J.; Zhu, C. X.; Chan, D. S. H.; Kang, E. T.; Neoh, K. G. Prog. Polym. Sci. 2008, 33, 917. (11) Li, H.; Xu, Q. F.; Li, N. J.; Sun, Ru.; Ge, J. F.; Lu, J. M.; Gu, H. W.; Yan, F. J. Am. Chem. Soc. 2010, 132, 5542. (12) Park, S.; Lee, T. J.; Kim, D. M.; Kim, J. C.; Kim, K.; Kwon, W.; Ko, Y. G.; Choi, H.; Chang, T.; Ree, M. J. Phys. Chem. B 2010, 114, 10294. (13) Lee, T. J.; Park, S.; Hahm, S. G.; Kim, D. M.; Kim, K.; Kim, J. C.; Kwon, W. S.; Kim, Y.; Chang, T.; Ree, M. J. Phys. Chem. C 2009, 113, 3855. (14) Lee, D.; Baek, S.; Ree, M.; Kim, O. IEEE Electron Device Lett. 2008, 29, 694. (15) Choi, S.; Hong, S. H.; Cho, S. H.; Park, S. D.; Park, S. M.; Kim, O.; Ree, M. Adv. Mater. 2008, 20, 1766. (16) Baek, S.; Lee, D.; Kim, J.; Hong, S.; Kim, O.; Ree, M. Adv. Funct. Mater. 2007, 17, 2637. (17) Kim, J.; Cho, S.; Choi, S; Baek, S.; Lee, D.; Kim, O.; Park, S. M.; Ree, M. Langmuir 2007, 23, 9024. (18) Hong, S. H.; Kim, O.; Choi, S.; Ree, M. Appl. Phys. Lett. 2007, 91, 093517. (19) Kim, D. M.; Park, S.; Lee, T. J.; Hahm, S. G.; Kim, K.; Kim, J. C.; Kwon, W.; Ree, M. Langmuir 2009, 25 (19), 11713. (20) Ling, Q. D.; Chang, F. C.; Song, Y.; Zhu, C. X.; Liaw, D. J.; Chan, D. S. H.; Kang, E. T.; Neoh, K. G. J. Am. Chem. Soc. 2006, 128, 8732. (21) Wang, K. L.; Tseng, T. Y.; Tsai, H. L.; Wu, S. C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6861. (22) Tseng, R. J.; Huang, J.; Ouyang, J. Y.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077. (23) Chen, Q.; Zhao, L.; Li, C.; Shi, G. Q. J. Phys. Chem. C 2007, 111, 18392. (24) You, N. H.; Chueh, C. C.; Liu, C. L.; Ueda, M.; Chen, W. C. Macromolecules 2009, 42, 4456. (25) Kuorosawa, T.; Chueh, C. C.; Liu, C. L.; Higashihara, T.; Ueda, M.; Chen, W. C. Macromolecules 2010, 43, 1236. (26) Lim, S. L.; Li, N. J.; Lu, J. M.; Ling, Q. D.; Zhu, C. X.; Kang, E. T.; Neoh, K. G. Appl. Mater. Interfaces 2009, 1, 60. (27) Ling, Q. D.; Wang, W.; Song, Y.; Zhu, C. X.; Chan, D. S.; Kang, E. T.; Neoh, K. G. J. Phys. Chem. B 2006, 110, 22995. (28) Hahm, S. G.; Choi, S.; Hong, S.-H.; Lee, T. J.; Park, S..; Kim, D. M.; Kwon, W.-S.; Kim, K.; Kim, O.; Ree, M. Adv. Funct. Mater. 2008, 18, 3276–3282. (29) Hahm, S. G.; Choi, S.; Hong, S.-H.; Lee, T. J.; Park, S.; Kim, D. M.; Kim, J. C.; Kwon, W.-S.; Kim, K.; Kim, M.-J.; Kim, O.; Ree, M. J. Mater. Chem. 2009, 19, 2207–2214.

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