COMMUNICATION DOI: 10.1002/asia.201402981
Conjugated Donor-Acceptor-Acceptor (D-A-A) Molecule for Organic Nonvolatile Resistor Memory Lei Dong,[a] Guangwu Li,[b] An-Dih Yu,[a] Zhishan Bo,*[b] Cheng-Liang Liu,[c] and Wen-Chang Chen*[a]
Abstract: A new donor-acceptor-acceptor (D-A-A) type of conjugated molecule, N-(4-(N’,N’-diphenyl)phenylamine)-4(4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic monoimide (TPA-NI-DCN), consisting of triphenylamine (TPA) donors and naphthalimide (NI)/dicyanovinylene (DCN) acceptors was synthesized and characterized. In conjunction with previously reported D-A based materials, the additional DCN moiety attached as end group in the D-A-A configuration can result in a stable charge transfer (CT) and charge-separated state to maintain the ON state current. The vacuum-deposited TPA-NI-DCN device fabricated as an active memory layer was demonstrated to exhibit writeonce-read-many (WORM) switching characteristics of organic nonvolatile memory due to the strong polarity of the TPA-NI-DCN moiety.
producibility when compared to the polymeric counterparts.[2–4] However, a rational molecular design for establishing structure-memory property relationships and modulating/optimizing functions has not been fully explored yet. Organic materials with a donor-acceptor (D-A) structure were developed to be switched between at least two different resistance states.[1e, 2–4] Tailoring of molecular planarity,[2a–c] conjugated backbone length,[2d] alkyl chain length,[2e–g] spacer linkage,[2h] and number/strength of donors and acceptors[2i–m, 3, 4] were explored to manipulate the memory performance. In particular, the different arrangement regarding to structural design on D-A small molecules used in memory devices, such as the asymmetric polar push-pull system of D-A-A type molecule, is relatively rare reported. By following these design strategies, herein we report a N(4-(N’,N’-diphenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)phenyl)naphthalene-1,8-dicarboxylic monoimide (see Figure 1) with a specific D-A-A configuration to construct nonvolatile memory coded as TPA-NI-DCN, in which an electron-donating triphenylamine (TPA) moiety is connected to an electron-withdrawing dicyanovinylene (DCN) through another electron-withdrawing naphthalimide (NI) block. The additional aromatic phenyl ring inserted between DCN and NI units can increase the length of p-conjugated
Recently, organic-based resistor memory for resistance switching effect has emerged as one of promising candidates as next-generation data storage devices mainly because of the structural flexibility and electrical property tunability.[1] The simple device cell configuration is built in a form of metal-insulator-metal (MIM) where the insulator acts as the active memory material. To date, studies regarding the formation of electrical properties of nonvolatile/volatile memory using organic small molecules have been carried out based on their attractive advantages of well-defined molecular structures, easy purification, and batch-to-batch re[a] L. Dong,+ A.-D. Yu,+ Prof. W.-C. Chen Department of Chemical Engineering National Taiwan University Taipei 10617 (Taiwan) E-mail: [email protected] [b] G. Li,+ Prof. Z. Bo Beijing Key Laboratory of Energy Conversion and Storage Materials College of Chemistry Beijing Normal University Beijing 100875 (China) E-mail: [email protected] [c] Prof. C.-L. Liu Department of Chemical and Materials Engineering National Central University Taoyuan 32001 (Taiwan) [+] These authors contributed equally to this work.
Figure 1. Scheme of cross-point memory device and optimized geometry, molecular orbitals and ESP surface of D-A-A type TPA-NI-DCN, and structure of D-A type TPA-NI as comparison.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402981.
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spacers. Consequently, the new D-A-A configuration of TPA-NI-DCN for specifically enhancing the device storage lifespan exhibits a write-once-read-many (WORM) memory characteristic through the physical origin of resistance switching. Furthermore, it would be of interest to compare the electrical characteristics of TPA-NI-DCN with that of TPA-NI reported in the literature[2a] to clarify the role of an additional acceptor of DCN on the memory characteristics. The molecular structure of targeted D-A-A type conjugated TPA-NI-DCN materials is outlined in Figure 1. The detailed synthesis procedures are described in Scheme 1. Start-
Figure 2. (a) UV/Vis absorption spectra and (b) cyclic voltammogram of TPA-NI-DCN.
long-wavelength region due to the CT from D to A. Besides, the absorption peak maximum on the evaporated thin film (on quartz substrate) of TPA-NI-DCN in the visible region features a bathochromic shift of about 10 nm and broadening as compared to the peaks in solution (in diluted chloroform), which might be related to the formation of molecular interaction and increased polarizability in the solid state. As a result, the optical band gap estimated from the absorption onset is 2.88 eV. The electrochemical properties of TPA-NIDCN were investigated by cyclic voltammetry (Figure 2 (b)). TPA-NI-DCN exhibits a quasi-reversible oxidation wave, which implies that the HOMO level of TPA-NI-DCN can be estimated to 5.27 eV based on the onset potential of the forward oxidation peak. As expected, the two strong electron-withdrawing cyano arms attached to the backbone significantly lower the LUMO energy level of 2.39 eV, which was determined from the difference between the optical band gap and the HOMO level. The standard two-terminal sandwich configuration of silicon wafer/Al/TPA-NI-DCN/Al cross-point resistor-type memories, where all the active and electrode layers were thermally evaporated, was fabricated to characterize the electrical switching behavior, as shown in Figure 1. Our device is integrated by the logically organized cells on the square with a bit line (as lower electrode), active memory layer, and some word lines (as the upper electrode), thus effectively avoiding the sneak current to the unselected cell. Each memory cell can store one binary bit discussed as follows. During the electrical characterization, a two-terminal scheme was used and the bottom electrode was grounded while quasi-DC sweeps with steps of 0.05 V were applied to the top electrode. All the electrical measurements were performed at room temperature under a N2-filled atmosphere using a Keithley 4200 semiconductor analyzer. Figure 3 (a) shows the current–voltage (I-V) characteristics of a vertical resistor memory device for an optimized TPA-NI-DCN thickness of 40 nm and a cell active joint area of 0.6 0.6 mm2. At the beginning, the device is in its high-resistance state (HRS; OFF state) and the current increases slowly
Scheme 1. Synthesis of TPA-NI-DCN. Reagents and conditions: i) N2H4 H2O, Pd-C, EtOH, 80 8C; ii) 4-bromo-1,8-naphthalic anhydride, EtOH, 80 8C; iii) [Pd2ACHTUNGRE(dba)3], PACHTUNGRE(o-tol)3, 4-formylphenylboronic acid, NaHCO3, THF/H2O, reflux, 90 8C; iv) malononitrile, NEt3, CHCl3, rt.
ing from diphenylamine, reaction for small molecules proceeded smoothly with a high yield. The newly synthesized TPA-NI-DCN was characterized and verified by 1H NMR and 13C NMR spectroscopies as well as elemental analysis (see Figures S1 and S2 in the Supporting Information). The thermal properties were also investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermogram of TPA-NI-DCN shows about 5 % weight loss at a temperature as high as 412 8C (Figure S3 (a) in the Supporting Information), which can probably be attributed to the rigid rings. No obvious transition peak was detected in the DSC curve at the temperature range of 40 to 340 8C, indicating that the as-prepared TPA-NI-DCN solid is in the amorphous state due to its twisted geometry (Figure S3 (b) in the Supporting Information). The good thermal stability of TPA-NI-DCN suggests it as a potential candidate for achieving thermally durable organic electronics devices. The absorption and charge transfer (CT) characteristics were studied by UV/Vis absorption spectroscopy (Figure 2 (a)). TPA-NI-DCN exhibits a two-band absorption, with one band at the short-wavelength region assigned to aromatic TPA and NI moieties and the other band at the
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and NI groups. Therefore, the switching mechanism of TPANI-DCN can be explained by the electric field-induced CT effect between the donors and acceptors. Under excitations with sufficient energy, electrons are possibly transferred from the electron-donating TPA to the electron-accepting DCN/NI moieties. Therefore, the CT interaction gives rise to the charge separation state that creates the effective charge transport channel and reduces the resistance throughout the TPA-NI-DCN layer. Charge hopping over the thermal barrier between the neighboring local sites is a principal charge conduction mechanism in thin films of organic conjugated materials employed in devices. Colorcoded computer graphics stimulation shows a representation of the ESP surface of TPA-NI-DCN. Note that ESP illustrates the charge distribution of molecules three dimensionally and enables to visualize variably negatively and positively charged regions. Accordingly, a channel flow inside the film for migration of charge carriers between the bottom and top electrodes is formed in the continuous positive ESP through the conjugated backbone, leading to a resistance switching from HRS to LRS. The negative ESP suggests that this electrophilic region acts as a trap site to localize the charge carrier that possibly induces the related memory effects and charge retention.[5] However, the WORM memory behavior of TPA-NI-DCN is supported by the following factors. First, the large calculated dipole moment (4.48 Debye) of the molecule, contributed by the strong electron-withdrawing DCN groups, suggests a high polarity that implies a strong intermolecular interaction in the D-AA architecture. Second, the transferred electrons can be more significantly delocalized in the DCN moiety with a strong electron affinity that provides a deep trapping barrier and stable CT state. Third, the introduction of aromatic TPA donors linked with acceptors suppresses strongly the intramolecular coupling induced by the twisted conformation after turning off the power. Consequently, all these features lead to a more stable/promoted CT state, and an appropriate charge delocalization in the separated moieties leads to the storage of the former transferred charge. It also prevents the segregated cationic and anionic charges from recombination even after power-off or the application of a reverse polarity bias. As a result, the quasi-permanent conductance is observed in the LRS of the fabricated nonvolatile device. Moreover, the current at the LRS increases as the electrode size increases in a nearly linear dependence, which can exclude the switching phenomena from random formation and rupture of conducting metal filaments.[2b] Besides, the obtained vapor phase-deposited TPA-NI-DCN thin film exhibits an extremely smooth surface, with a root-meansquare (RMS) roughness of about 0.38 nm as obtained from the AFM image (Figure S4 in the Supporting Information). This also suggests that the uniform and crack-free surface minimizes the possibility of filamentary resistive switching since no effective pathway can be situated for filaments embedded in the TPA-NI-DCN film.[3] To elucidate the conduction mechanism of resistive switching, the plots of the I-
Figure 3. (a) I–V characteristics and (b) retention time test of the TPANI-DCN memory device.
with the applied voltage. However, the current jumps abruptly from 5 10 8 A to 1 10 5 A at a threshold voltage of about 3.5 V, indicating that the device is switched from the HRS to the low-resistance state (LRS; ON state). The operation for the electrical transition is defined as the “writing” process, with an ON/OFF (HRS to LRS) ratio of ~ 103. The switching voltage of the TPA-NI-DCN device can be reduced if the work function of the deposited electrode is close to the HOMO level of the TPA-NI-DCN. In the LRS, the current then increases smoothly with the applied voltage. The device remains in its LRS once the applied voltage is turned off and then subsequently reapplied to the device either at negative (2nd sweep) or positive voltage (3rd sweep). This device cannot return to the HRS even after switching off the power for 1 day in both polarities (4th or 5th sweep). This indicates that the TPA-NI-DCN device belongs to the nonvolatile WORM memory. To investigate the stability of the small molecule-based device, the retention test was conducted with a reading bias of 1 V, as shown in Figure 3 (b). It is seen that the device exhibits good retention abilities: both the ON and OFF currents do not degrade and thus the ON/OFF ratio does not show any serious deterioration. The test can last for a period of over 104 s and it also proves the nonvolatile nature and non-destructive reading. The optimized molecular geometries and electronic properties such as HOMO and LUMO energy levels and electrostatic potentials (ESP), calculated by density functional theory, are shown in Figure 1. It is seen that HOMO orbitals are localized at the TPA electron donors while LUMO orbitals are mainly localized at the electron acceptors, the DCN
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TPA-NI-DCN device can be proven to exhibit a WORMtype switching behavior due to the attached terminal strong electron-withdrawing strength of DCN in terms of D-A-A configuration and formation of a stable CT complex. Here, the role of the D-A-A motif was designed for the following characteristics: (1) Relative to the single D-A system for memory switching, the formation of stable CT and charge-separated state in a D-A-A framework are adopted and can favor the nonvolatile WORM memory characteristics accordingly. In previous studies, numerous single D-A molecules exhibited weak retention ability of volatile characteristics or poor stability of the charge-separated state of erasable flash characteristics.[2a, c, i, l] (2) Both DA-A and A-D-A molecules with the same D/A ratio of 1/2 have been used as active materials and exhibit WORM behavior. However, the elaborate combination of two connected acceptors in D-A-A perhaps enables the overlapping of trapping potential caused by the neighboring spatial location[2m] and possess binary memory states[2h] as compared to multinary states of the A-D-A counterpart.[2m] (3) Modification in D and A structures play an important role in the data storage properties (volatile vs. nonvolatile), especially in the D/A strength and relative ratio of D/A moieties in the molecular backbone. (4) D-A-A small molecules where the electron donor DCN and electron acceptor TPA are linked by NI units have advantages of reproducible memory switching over the phase-separated blends system. In conclusion, our Al/TPA-NI-DCN/Al device shows a bistable WORM memory characteristic. The theoretical quantum calculation and physical models can well explain resistance switching in terms of the electric induced CT and trapping. The additional acceptors of central NI and terminal DCN units adjacent to the TPA donor in a p-conjugation can strengthen the stability in the LRS. Specifically, the DA-A configuration is effectual for molecular engineering in the design of nonvolatile memory materials.
Figure 4. Experimental and fitted electrical characteristics of the TPANI-DCN memory device in the (a) HRS and (b) LRS.
V curves for both LRS and HRS in negative voltage regions were theoretically fitted (Figure 4). Evidently, the I–V characteristic of the LRS shows a linear plot for log (I/V) versus V1/2, which suggests that Poole–Frenkel (PF) emission is obeyed. On the other hand, the I–V characteristic of the HRS is more complicated and can be divided into two regions. At the low-voltage region of HRS (< 1 V), the linear proportion for current to voltage corresponds to an Ohmic conduction behavior. After that it gradually changes to the Childs law region, with a linear relation between I and V3.4 in the higher voltage region of HRS. This behavior is qualitatively interpreted to match the trap-associated spacecharge-limited-conduction (SCLC) theory with a simple equation of I(V) = aV+bVn.[6] The trap-controlled SCLC describes the charge conduction via charge trapping/detrapping where the negative ESP from the NI and DCN strong acceptors can serve as trap sites to result in the HRS followed by driving the resistive switching through the PF emission. The occurrence of PF emission is probably attributed to charge transport of organic materials filled with charge traps.[2b, 2h] The enhanced molecular polarity through the strong donor–acceptor interaction in D-A-A structure arrangement can greatly stabilize the charge-separated state. As a result, the irreversible WORM-type memory behavior is observed in our newly synthesized TPA-NI-DCN. According to the previously reported TPA-NI D-A type molecule (without the DCN end group) by Lu and co-workers (the structure is shown in Figure 1),[2a] this device exhibits a nonvolatile flash memory that indicates CT complex decomposition after applying an opposite voltage. Once again our
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Experimental Section Chemicals and Instrumentation All the reagents were used as received from the commercial suppliers without further purification. Dehydrated tetrahydrofuran (THF) and diethyl ether (Et2O) were distilled from sodium with benzophenone as an indicator under nitrogen atmosphere. Chloroform (CHCl3) was distilled before use. The final products were confirmed by using NMR spectroscopy (1D NMR spectra were recorded on a Bruker AV 400 or DM 300 spectrometer at the resonant frequencies of 400 MHz for 1H and 100 MHz for 13C nuclei, whereas 2D NMR spectra including HSQC and NOESY were recorded on a Bruker AVIII 500 MHz spectrometer at the resonant frequencies of 500 MHz for 1H and 125 MHz for 13C nuclei) and elemental analysis (Flash EA1112 analyzer). Thermal properties were estimated from a Seiko TG/DTA 6300 thermogravimetric analysis system (TGA) and a TA Instruments DSC-Q100 differential scanning calorimeter (DSC) under a nitrogen atmosphere at a heating rate of 10 and 5 8C min 1, respectively. Optical absorption spectra were obtained on a Shimadzu UV-1601 PC UV/Visible spectrometer. The electrochemical behavior of TPA-NI-DCN thin film was investigated by using CV (CHI 630A electrochemical analyzer) with a standard three-electrode electrochemical cell in a 0.1 m tetrabutylammonium tetrafluoroborate solution in
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CH3CN at room temperature at atmosphere with a scanning rate of 0.1 V s 1. A glassy carbon working electrode, a Pt wire counter electrode, and an Ag/AgNO3 (0.01 m in CH3CN) reference electrode were used. The experiments were calibrated with the standard ferrocene/ferrocenium (Fc) redox system and assuming that the energy level of Fc is 4.8 eV below vacuum. UV/Vis absorption spectra were obtained on a SHIMADZU UV/Vis spectrometer model UV-1601PC. The thickness of the organic film as measured with a microfigure measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.).
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[1] a) Y. Yang, J. Ouyang, L. Ma, R. J. H. Tseng, C. W. Chu, Adv. Funct. Mater. 2006, 16, 1001 – 1014; b) Q.-D. Ling, D.-J. Liaw, C. Zhu, D. S.H. Chan, E.-T. Kang, K.-G. Neoh, Prog. Polym. Sci. 2008, 33, 917 – 978; c) B. Cho, S. Song, Y. Ji, T.-W. Kim, T. Lee, Adv. Funct. Mater. 2011, 21, 2806 – 2829; d) P. Heremans, G. H. Gelinck, R. M. Muller, K.-J. Baeg, D.-Y. Kim, Y.-Y. Noh, Chem. Mater. 2011, 23, 341 – 358; e) C.-L. Liu, W.-C. Chen, Polym. Chem. 2011, 2, 2169 – 2174; f) Y. Chen, B. Zhang, G. Liu, X. Zhuang, E.-T. Kang, Chem. Soc. Rev. 2012, 41, 4688 – 4772; g) T. Kurosawa, T. Higashihara, M. Ueda, Polym. Chem. 2013, 4, 16; h) W.-P. Lin, S.-J. Liu, T. Gong, Q. Zhao, W. Huang, Adv. Mater. 2014, 26, 570 – 606; i) Q.-D. Ling, Y. Song, S.L. Lim, E. Y.-H. Teo, Y.-P. Tan, C. Zhu, D. S. H. Chan, D.-L. Kwong, E.-T. Kang, K.-G. Neoh, Angew. Chem. Int. Ed. 2006, 45, 2947 – 2951; Angew. Chem. 2006, 118, 3013 – 3017; j) X.-D. Zhuang, Y. Chen, B.-X. Li, D.-G. Ma, B. Zhang, Y. Li, Chem. Mater. 2010, 22, 4455 – 4461; k) C. Ye, Q. Peng, M. Li, J. Luo, Z. Tang, J. Pei, J. Chen, Z. Shuai, L. Jiang, Y. Song, J. Am. Chem. Soc. 2012, 134, 20053 – 20059. [2] a) W. Ren, H. Zhuang, Q. Bao, S. Miao, H. Li, J. Lu, L. Wang, Dyes Pigm. 2014, 100, 127 – 134; b) S. Miao, H. Li, Q. Xu, Y. Li, S. Ji, N. Li, L. Wang, J. Zheng, J. Lu, Adv. Mater. 2012, 24, 6210 – 6215; c) H. Liu, R. Bo, H. Liu, N. Li, Q. Xu, H. Li, J. Lu, L. Wang, J. Mater. Chem. C 2014, 2, 5709 – 5716; d) S. Miao, H. Li, Q. Xu, N. Li, J. Zheng, R. Sun, J. Lu, C. M. Li, J. Mater. Chem. 2012, 22, 16582 – 16589; e) Y. Zhang, H. Zhuang, Y. Yang, X. Xu, Q. Bao, N. Li, H. Li, Q. Xu, J. Lu, L. Wang, J. Phys. Chem. C 2012, 116, 22832 – 22839; f) W. Chen, H. Li, N. Li, Q. Xu, J.-M. Lu, L. Wang, Dyes Pigm. 2012, 95, 365 – 372; g) W. Ren, Y. Zhu, J. Ge, X. Xu, R. Sun, N. Li, H. Li, Q. Xu, J. Zheng, J. Lu, Phys. Chem. Chem. Phys. 2013, 15, 9212 – 9218; h) G. Wang, S. Miao, Q. Zhang, H. Liu, H. Li, N. Li, Q. Xu, J. Lu, L. Wang, Chem. Commun. 2013, 49, 9470 – 9472; i) S. Miao, Y. Zhu, H. Zhuang, X. Xu, H. Li, R. Sun, N. Li, S. Ji, J. Lu, J. Mater. Chem. C 2013, 1, 2320 – 2327; j) H. Zhuang, Q. Zhang, Y. Zhu, X. Xu, H. Liu, N. Li, Q. Xu, H. Li, J. Lu, L. Wang, J. Mater. Chem. C 2013, 1, 3816 – 3824; k) H. Zhuang, Q. Zhou, Y. Li, Q. Zhang, H. Li, Q. Xu, N. Li, J. Lu, L. Wang, ACS Appl. Mater. Interfaces 2014, 6, 94 – 100; l) H. Liu, H. Zhuang, H. Li, J. Lu, L. Wang, Phys. Chem. Chem. Phys. 2014, 16, 17125 – 17132; m) Z. Su, H. Zhuang, H. Liu, H. Li, Q. Xu, J. Lu, L. Wang, J. Mater. Chem. C 2014, 2, 5673 – 5680; n) H. Li, Z. Jin, N. Li, Q. Xu, H. Gu, J. Lu, X. Xia, L. Wang, J. Mater. Chem. 2011, 21, 5860 – 5862; o) H. Li, Q. Xu, N. Li, R. Sun, J. Ge, J. Lu, H. Gu, F. Yan, J. Am. Chem. Soc. 2010, 132, 5542 – 5543. [3] a) W.-Y. Lee, T. Kurosawa, S.-T. Lin, T. Higashihara, M. Ueda, W.-C. Chen, Chem. Mater. 2011, 23, 4487 – 4497; b) Y.-C. Lai, T. Kurosawa, T. Higashihara, M. Ueda, W.-C. Chen, Chem. Asian J. 2013, 8, 1514 – 1522. [4] a) Y. Shang, Y. Wen, S. Li, S. Du, X. He, L. Cai, Y. Li, L. Yang, H. Gao, Y. Song, J. Am. Chem. Soc. 2007, 129, 11674 – 11675; b) Y. Ma, X. Cao, G. Li, Y. Wen, Y. Yang, J. Wang, S. Du, L. Yang, H. Gao, Y. Song, Adv. Funct. Mater. 2010, 20, 803 – 810. [5] a) X.-D. Zhuang, Y. Chen, G. Liu, B. Zhang, K.-G. Neoh, E.-T. Kang, C.-X. Zju, Y.-X. Li, L.-J. Niu, Adv. Funct. Mater. 2010, 20, 2916 – 2922; b) G. Liu, B. Zhang, Y. Chen, C.-X. Zhu, L. Zeng, D. S.-H. Chan, K.-G. Neoh, J. Che, E.-T. Kang, J. Mater. Chem. 2011, 21, 6027 – 6033. [6] M. A. Lampert, Phys. Rev. 1956, 103, 1648.
Synthesis The details of synthetic routes for 4-nitrophenyldiphenylamine (1), 4-diphenylaminophenylamine (2), N-(4-(diphenylamino)phenyl)-4-bromonaphthalene-1,8-dicarboxylic imide (3), and N-(4-(diphenylamino)phenyl)-4-(4-formylphenyl)naphthalene-1,8-dicarboxylic imide (4) are described in the Supporting Information. The final product, N-(4-(N’,N’-diphenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic monoimide (TPA-NI-DCN), was synthesized as described below. A solution of compound 4 (0.50 g, 0.9 mmol) in dry chloroform (50 mL), a few drops of Et3N, and malononitrile (0.067 g, 1.0 mmol) were stirred under N2 at room temperature for 30 min. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel to afford TPA-NI-DCN as a yellow solid Yield: 0.47 g (86 %); 1H NMR (400 MHz, CDCl3): d = 8.63–8.69 (m, 2 H), 8.16 (d, J = 8.4 Hz, 1 H), 8.05 (d, J = 8.2 Hz, 2 H), 7.84 (s, 1 H), 7.64–7.76 (m, 4 H), 7.18–7.28 (m, 4 H), 7.07–7.17 (m, 8 H), 7.00 ppm (t J = 7.3 Hz, 2 H); 13C NMR (100 MHz, CDCl3): d = 164.34, 164.13, 158.80, 148.07, 147.44, 145.03, 144.63, 131.94, 131.88, 131.11, 131.02, 130.95, 129.70, 129.36, 129.14, 129.03, 128.55, 128.00, 127.60, 125.01, 123.41, 123.37, 124.14, 123.07, 113.48, 112.46, 83.91 ppm; elemental anal. calcd (%) for C40H24N4O2 : C 81.07, H 4.08, N 9.45; found: C 81.19, H 4.16, N 9.38. Device Fabrication and Measurements The memory device was fabricated in a cross-point sandwiched architecture with the configuration of Al/TPA-NI-DCN/Al. Before deposition of the Al bottom electrode, the silicon wafer was pre-cleaned by an ultrasonic cleaning process with toluene, acetone, and isopropanol, each for 15 min. Al bottom electrode lines with a thickness of 30 nm were deposited via thermal evaporation at a depositing rate of 1 s 1 under a pressure of around 1 10 6 torr. An active layer of the investigated organic small molecule was successively deposited on top of the bottom electrode by thermal evaporation with a targeted thickness of 40 nm, as determined in situ by a calibrated quartz crystal microbalance (QCM). Top electrode lines were thermally deposited through the shadow mask at the same condition, aligned perpendicular to the bottom lines, leading to crosspoint arrays of memory cells. All the electrical characteristics of the fabricated flexible memory devices were measured by using a Keithley 4200SCS semiconductor parameter analyzer. All electrical measurements were performed in a N2-filled glove box.
Acknowledgements W.-C.C. thanks the financial support from Ministry of Science and Technology (MOST) of Taiwan. Z.B. acknowledges the funding assistant from National Natural Science Foundation of China (No. 21161160443).
Received: August 22, 2014 Revised: September 3, 2014 Published online: && &&, 0000
Keywords: charge transfer · donor-acceptor systems · imides · memory · resistance switching · triphenylamine
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COMMUNICATION Donor-Acceptor Systems Lei Dong, Guangwu Li, An-Dih Yu, Zhishan Bo,* Cheng-Liang Liu, &&&&—&&&& Wen-Chang Chen* Conjugated Donor-Acceptor-Acceptor (D-A-A) Molecule for Organic Nonvolatile Resistor Memory
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The early bird catches the WORM: A donor-acceptor-donor (D-A-A) type conjugated molecule based on linear p-bridging triphenylamine (TPA) donors and naphthalimide (NI)/dicya-
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novinylene (DCN) acceptors was prepared and shown to possess nonvolatile WORM (write-once-read-many) memory behavior.
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Conjugated Donor-Acceptor-Acceptor (D-A-A) Molecule for Organic Nonvolatile Resistor Memory Lei Dong,[a] Guangwu Li,[b] An-Dih Yu,[a] Zhishan Bo,*[b] Cheng-Liang Liu,[c] and Wen-Chang Chen*[a]
Abstract: A new donor-acceptor-acceptor (D-A-A) type of conjugated molecule, N-(4-(N’,N’-diphenyl)phenylamine)-4(4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic monoimide (TPA-NI-DCN), consisting of triphenylamine (TPA) donors and naphthalimide (NI)/dicyanovinylene (DCN) acceptors was synthesized and characterized. In conjunction with previously reported D-A based materials, the additional DCN moiety attached as end group in the D-A-A configuration can result in a stable charge transfer (CT) and charge-separated state to maintain the ON state current. The vacuum-deposited TPA-NI-DCN device fabricated as an active memory layer was demonstrated to exhibit writeonce-read-many (WORM) switching characteristics of organic nonvolatile memory due to the strong polarity of the TPA-NI-DCN moiety.
producibility when compared to the polymeric counterparts.[2–4] However, a rational molecular design for establishing structure-memory property relationships and modulating/optimizing functions has not been fully explored yet. Organic materials with a donor-acceptor (D-A) structure were developed to be switched between at least two different resistance states.[1e, 2–4] Tailoring of molecular planarity,[2a–c] conjugated backbone length,[2d] alkyl chain length,[2e–g] spacer linkage,[2h] and number/strength of donors and acceptors[2i–m, 3, 4] were explored to manipulate the memory performance. In particular, the different arrangement regarding to structural design on D-A small molecules used in memory devices, such as the asymmetric polar push-pull system of D-A-A type molecule, is relatively rare reported. By following these design strategies, herein we report a N(4-(N’,N’-diphenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)phenyl)naphthalene-1,8-dicarboxylic monoimide (see Figure 1) with a specific D-A-A configuration to construct nonvolatile memory coded as TPA-NI-DCN, in which an electron-donating triphenylamine (TPA) moiety is connected to an electron-withdrawing dicyanovinylene (DCN) through another electron-withdrawing naphthalimide (NI) block. The additional aromatic phenyl ring inserted between DCN and NI units can increase the length of p-conjugated
Recently, organic-based resistor memory for resistance switching effect has emerged as one of promising candidates as next-generation data storage devices mainly because of the structural flexibility and electrical property tunability.[1] The simple device cell configuration is built in a form of metal-insulator-metal (MIM) where the insulator acts as the active memory material. To date, studies regarding the formation of electrical properties of nonvolatile/volatile memory using organic small molecules have been carried out based on their attractive advantages of well-defined molecular structures, easy purification, and batch-to-batch re[a] L. Dong,+ A.-D. Yu,+ Prof. W.-C. Chen Department of Chemical Engineering National Taiwan University Taipei 10617 (Taiwan) E-mail: [email protected] [b] G. Li,+ Prof. Z. Bo Beijing Key Laboratory of Energy Conversion and Storage Materials College of Chemistry Beijing Normal University Beijing 100875 (China) E-mail: [email protected] [c] Prof. C.-L. Liu Department of Chemical and Materials Engineering National Central University Taoyuan 32001 (Taiwan) [+] These authors contributed equally to this work.
Figure 1. Scheme of cross-point memory device and optimized geometry, molecular orbitals and ESP surface of D-A-A type TPA-NI-DCN, and structure of D-A type TPA-NI as comparison.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402981.
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spacers. Consequently, the new D-A-A configuration of TPA-NI-DCN for specifically enhancing the device storage lifespan exhibits a write-once-read-many (WORM) memory characteristic through the physical origin of resistance switching. Furthermore, it would be of interest to compare the electrical characteristics of TPA-NI-DCN with that of TPA-NI reported in the literature[2a] to clarify the role of an additional acceptor of DCN on the memory characteristics. The molecular structure of targeted D-A-A type conjugated TPA-NI-DCN materials is outlined in Figure 1. The detailed synthesis procedures are described in Scheme 1. Start-
Figure 2. (a) UV/Vis absorption spectra and (b) cyclic voltammogram of TPA-NI-DCN.
long-wavelength region due to the CT from D to A. Besides, the absorption peak maximum on the evaporated thin film (on quartz substrate) of TPA-NI-DCN in the visible region features a bathochromic shift of about 10 nm and broadening as compared to the peaks in solution (in diluted chloroform), which might be related to the formation of molecular interaction and increased polarizability in the solid state. As a result, the optical band gap estimated from the absorption onset is 2.88 eV. The electrochemical properties of TPA-NIDCN were investigated by cyclic voltammetry (Figure 2 (b)). TPA-NI-DCN exhibits a quasi-reversible oxidation wave, which implies that the HOMO level of TPA-NI-DCN can be estimated to 5.27 eV based on the onset potential of the forward oxidation peak. As expected, the two strong electron-withdrawing cyano arms attached to the backbone significantly lower the LUMO energy level of 2.39 eV, which was determined from the difference between the optical band gap and the HOMO level. The standard two-terminal sandwich configuration of silicon wafer/Al/TPA-NI-DCN/Al cross-point resistor-type memories, where all the active and electrode layers were thermally evaporated, was fabricated to characterize the electrical switching behavior, as shown in Figure 1. Our device is integrated by the logically organized cells on the square with a bit line (as lower electrode), active memory layer, and some word lines (as the upper electrode), thus effectively avoiding the sneak current to the unselected cell. Each memory cell can store one binary bit discussed as follows. During the electrical characterization, a two-terminal scheme was used and the bottom electrode was grounded while quasi-DC sweeps with steps of 0.05 V were applied to the top electrode. All the electrical measurements were performed at room temperature under a N2-filled atmosphere using a Keithley 4200 semiconductor analyzer. Figure 3 (a) shows the current–voltage (I-V) characteristics of a vertical resistor memory device for an optimized TPA-NI-DCN thickness of 40 nm and a cell active joint area of 0.6 0.6 mm2. At the beginning, the device is in its high-resistance state (HRS; OFF state) and the current increases slowly
Scheme 1. Synthesis of TPA-NI-DCN. Reagents and conditions: i) N2H4 H2O, Pd-C, EtOH, 80 8C; ii) 4-bromo-1,8-naphthalic anhydride, EtOH, 80 8C; iii) [Pd2ACHTUNGRE(dba)3], PACHTUNGRE(o-tol)3, 4-formylphenylboronic acid, NaHCO3, THF/H2O, reflux, 90 8C; iv) malononitrile, NEt3, CHCl3, rt.
ing from diphenylamine, reaction for small molecules proceeded smoothly with a high yield. The newly synthesized TPA-NI-DCN was characterized and verified by 1H NMR and 13C NMR spectroscopies as well as elemental analysis (see Figures S1 and S2 in the Supporting Information). The thermal properties were also investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermogram of TPA-NI-DCN shows about 5 % weight loss at a temperature as high as 412 8C (Figure S3 (a) in the Supporting Information), which can probably be attributed to the rigid rings. No obvious transition peak was detected in the DSC curve at the temperature range of 40 to 340 8C, indicating that the as-prepared TPA-NI-DCN solid is in the amorphous state due to its twisted geometry (Figure S3 (b) in the Supporting Information). The good thermal stability of TPA-NI-DCN suggests it as a potential candidate for achieving thermally durable organic electronics devices. The absorption and charge transfer (CT) characteristics were studied by UV/Vis absorption spectroscopy (Figure 2 (a)). TPA-NI-DCN exhibits a two-band absorption, with one band at the short-wavelength region assigned to aromatic TPA and NI moieties and the other band at the
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and NI groups. Therefore, the switching mechanism of TPANI-DCN can be explained by the electric field-induced CT effect between the donors and acceptors. Under excitations with sufficient energy, electrons are possibly transferred from the electron-donating TPA to the electron-accepting DCN/NI moieties. Therefore, the CT interaction gives rise to the charge separation state that creates the effective charge transport channel and reduces the resistance throughout the TPA-NI-DCN layer. Charge hopping over the thermal barrier between the neighboring local sites is a principal charge conduction mechanism in thin films of organic conjugated materials employed in devices. Colorcoded computer graphics stimulation shows a representation of the ESP surface of TPA-NI-DCN. Note that ESP illustrates the charge distribution of molecules three dimensionally and enables to visualize variably negatively and positively charged regions. Accordingly, a channel flow inside the film for migration of charge carriers between the bottom and top electrodes is formed in the continuous positive ESP through the conjugated backbone, leading to a resistance switching from HRS to LRS. The negative ESP suggests that this electrophilic region acts as a trap site to localize the charge carrier that possibly induces the related memory effects and charge retention.[5] However, the WORM memory behavior of TPA-NI-DCN is supported by the following factors. First, the large calculated dipole moment (4.48 Debye) of the molecule, contributed by the strong electron-withdrawing DCN groups, suggests a high polarity that implies a strong intermolecular interaction in the D-AA architecture. Second, the transferred electrons can be more significantly delocalized in the DCN moiety with a strong electron affinity that provides a deep trapping barrier and stable CT state. Third, the introduction of aromatic TPA donors linked with acceptors suppresses strongly the intramolecular coupling induced by the twisted conformation after turning off the power. Consequently, all these features lead to a more stable/promoted CT state, and an appropriate charge delocalization in the separated moieties leads to the storage of the former transferred charge. It also prevents the segregated cationic and anionic charges from recombination even after power-off or the application of a reverse polarity bias. As a result, the quasi-permanent conductance is observed in the LRS of the fabricated nonvolatile device. Moreover, the current at the LRS increases as the electrode size increases in a nearly linear dependence, which can exclude the switching phenomena from random formation and rupture of conducting metal filaments.[2b] Besides, the obtained vapor phase-deposited TPA-NI-DCN thin film exhibits an extremely smooth surface, with a root-meansquare (RMS) roughness of about 0.38 nm as obtained from the AFM image (Figure S4 in the Supporting Information). This also suggests that the uniform and crack-free surface minimizes the possibility of filamentary resistive switching since no effective pathway can be situated for filaments embedded in the TPA-NI-DCN film.[3] To elucidate the conduction mechanism of resistive switching, the plots of the I-
Figure 3. (a) I–V characteristics and (b) retention time test of the TPANI-DCN memory device.
with the applied voltage. However, the current jumps abruptly from 5 10 8 A to 1 10 5 A at a threshold voltage of about 3.5 V, indicating that the device is switched from the HRS to the low-resistance state (LRS; ON state). The operation for the electrical transition is defined as the “writing” process, with an ON/OFF (HRS to LRS) ratio of ~ 103. The switching voltage of the TPA-NI-DCN device can be reduced if the work function of the deposited electrode is close to the HOMO level of the TPA-NI-DCN. In the LRS, the current then increases smoothly with the applied voltage. The device remains in its LRS once the applied voltage is turned off and then subsequently reapplied to the device either at negative (2nd sweep) or positive voltage (3rd sweep). This device cannot return to the HRS even after switching off the power for 1 day in both polarities (4th or 5th sweep). This indicates that the TPA-NI-DCN device belongs to the nonvolatile WORM memory. To investigate the stability of the small molecule-based device, the retention test was conducted with a reading bias of 1 V, as shown in Figure 3 (b). It is seen that the device exhibits good retention abilities: both the ON and OFF currents do not degrade and thus the ON/OFF ratio does not show any serious deterioration. The test can last for a period of over 104 s and it also proves the nonvolatile nature and non-destructive reading. The optimized molecular geometries and electronic properties such as HOMO and LUMO energy levels and electrostatic potentials (ESP), calculated by density functional theory, are shown in Figure 1. It is seen that HOMO orbitals are localized at the TPA electron donors while LUMO orbitals are mainly localized at the electron acceptors, the DCN
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TPA-NI-DCN device can be proven to exhibit a WORMtype switching behavior due to the attached terminal strong electron-withdrawing strength of DCN in terms of D-A-A configuration and formation of a stable CT complex. Here, the role of the D-A-A motif was designed for the following characteristics: (1) Relative to the single D-A system for memory switching, the formation of stable CT and charge-separated state in a D-A-A framework are adopted and can favor the nonvolatile WORM memory characteristics accordingly. In previous studies, numerous single D-A molecules exhibited weak retention ability of volatile characteristics or poor stability of the charge-separated state of erasable flash characteristics.[2a, c, i, l] (2) Both DA-A and A-D-A molecules with the same D/A ratio of 1/2 have been used as active materials and exhibit WORM behavior. However, the elaborate combination of two connected acceptors in D-A-A perhaps enables the overlapping of trapping potential caused by the neighboring spatial location[2m] and possess binary memory states[2h] as compared to multinary states of the A-D-A counterpart.[2m] (3) Modification in D and A structures play an important role in the data storage properties (volatile vs. nonvolatile), especially in the D/A strength and relative ratio of D/A moieties in the molecular backbone. (4) D-A-A small molecules where the electron donor DCN and electron acceptor TPA are linked by NI units have advantages of reproducible memory switching over the phase-separated blends system. In conclusion, our Al/TPA-NI-DCN/Al device shows a bistable WORM memory characteristic. The theoretical quantum calculation and physical models can well explain resistance switching in terms of the electric induced CT and trapping. The additional acceptors of central NI and terminal DCN units adjacent to the TPA donor in a p-conjugation can strengthen the stability in the LRS. Specifically, the DA-A configuration is effectual for molecular engineering in the design of nonvolatile memory materials.
Figure 4. Experimental and fitted electrical characteristics of the TPANI-DCN memory device in the (a) HRS and (b) LRS.
V curves for both LRS and HRS in negative voltage regions were theoretically fitted (Figure 4). Evidently, the I–V characteristic of the LRS shows a linear plot for log (I/V) versus V1/2, which suggests that Poole–Frenkel (PF) emission is obeyed. On the other hand, the I–V characteristic of the HRS is more complicated and can be divided into two regions. At the low-voltage region of HRS (< 1 V), the linear proportion for current to voltage corresponds to an Ohmic conduction behavior. After that it gradually changes to the Childs law region, with a linear relation between I and V3.4 in the higher voltage region of HRS. This behavior is qualitatively interpreted to match the trap-associated spacecharge-limited-conduction (SCLC) theory with a simple equation of I(V) = aV+bVn.[6] The trap-controlled SCLC describes the charge conduction via charge trapping/detrapping where the negative ESP from the NI and DCN strong acceptors can serve as trap sites to result in the HRS followed by driving the resistive switching through the PF emission. The occurrence of PF emission is probably attributed to charge transport of organic materials filled with charge traps.[2b, 2h] The enhanced molecular polarity through the strong donor–acceptor interaction in D-A-A structure arrangement can greatly stabilize the charge-separated state. As a result, the irreversible WORM-type memory behavior is observed in our newly synthesized TPA-NI-DCN. According to the previously reported TPA-NI D-A type molecule (without the DCN end group) by Lu and co-workers (the structure is shown in Figure 1),[2a] this device exhibits a nonvolatile flash memory that indicates CT complex decomposition after applying an opposite voltage. Once again our
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Experimental Section Chemicals and Instrumentation All the reagents were used as received from the commercial suppliers without further purification. Dehydrated tetrahydrofuran (THF) and diethyl ether (Et2O) were distilled from sodium with benzophenone as an indicator under nitrogen atmosphere. Chloroform (CHCl3) was distilled before use. The final products were confirmed by using NMR spectroscopy (1D NMR spectra were recorded on a Bruker AV 400 or DM 300 spectrometer at the resonant frequencies of 400 MHz for 1H and 100 MHz for 13C nuclei, whereas 2D NMR spectra including HSQC and NOESY were recorded on a Bruker AVIII 500 MHz spectrometer at the resonant frequencies of 500 MHz for 1H and 125 MHz for 13C nuclei) and elemental analysis (Flash EA1112 analyzer). Thermal properties were estimated from a Seiko TG/DTA 6300 thermogravimetric analysis system (TGA) and a TA Instruments DSC-Q100 differential scanning calorimeter (DSC) under a nitrogen atmosphere at a heating rate of 10 and 5 8C min 1, respectively. Optical absorption spectra were obtained on a Shimadzu UV-1601 PC UV/Visible spectrometer. The electrochemical behavior of TPA-NI-DCN thin film was investigated by using CV (CHI 630A electrochemical analyzer) with a standard three-electrode electrochemical cell in a 0.1 m tetrabutylammonium tetrafluoroborate solution in
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CH3CN at room temperature at atmosphere with a scanning rate of 0.1 V s 1. A glassy carbon working electrode, a Pt wire counter electrode, and an Ag/AgNO3 (0.01 m in CH3CN) reference electrode were used. The experiments were calibrated with the standard ferrocene/ferrocenium (Fc) redox system and assuming that the energy level of Fc is 4.8 eV below vacuum. UV/Vis absorption spectra were obtained on a SHIMADZU UV/Vis spectrometer model UV-1601PC. The thickness of the organic film as measured with a microfigure measuring instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.).
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Synthesis The details of synthetic routes for 4-nitrophenyldiphenylamine (1), 4-diphenylaminophenylamine (2), N-(4-(diphenylamino)phenyl)-4-bromonaphthalene-1,8-dicarboxylic imide (3), and N-(4-(diphenylamino)phenyl)-4-(4-formylphenyl)naphthalene-1,8-dicarboxylic imide (4) are described in the Supporting Information. The final product, N-(4-(N’,N’-diphenyl)phenylamine)-4-(4’-(2,2-dicyanovinyl)phenyl) naphthalene-1,8-dicarboxylic monoimide (TPA-NI-DCN), was synthesized as described below. A solution of compound 4 (0.50 g, 0.9 mmol) in dry chloroform (50 mL), a few drops of Et3N, and malononitrile (0.067 g, 1.0 mmol) were stirred under N2 at room temperature for 30 min. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel to afford TPA-NI-DCN as a yellow solid Yield: 0.47 g (86 %); 1H NMR (400 MHz, CDCl3): d = 8.63–8.69 (m, 2 H), 8.16 (d, J = 8.4 Hz, 1 H), 8.05 (d, J = 8.2 Hz, 2 H), 7.84 (s, 1 H), 7.64–7.76 (m, 4 H), 7.18–7.28 (m, 4 H), 7.07–7.17 (m, 8 H), 7.00 ppm (t J = 7.3 Hz, 2 H); 13C NMR (100 MHz, CDCl3): d = 164.34, 164.13, 158.80, 148.07, 147.44, 145.03, 144.63, 131.94, 131.88, 131.11, 131.02, 130.95, 129.70, 129.36, 129.14, 129.03, 128.55, 128.00, 127.60, 125.01, 123.41, 123.37, 124.14, 123.07, 113.48, 112.46, 83.91 ppm; elemental anal. calcd (%) for C40H24N4O2 : C 81.07, H 4.08, N 9.45; found: C 81.19, H 4.16, N 9.38. Device Fabrication and Measurements The memory device was fabricated in a cross-point sandwiched architecture with the configuration of Al/TPA-NI-DCN/Al. Before deposition of the Al bottom electrode, the silicon wafer was pre-cleaned by an ultrasonic cleaning process with toluene, acetone, and isopropanol, each for 15 min. Al bottom electrode lines with a thickness of 30 nm were deposited via thermal evaporation at a depositing rate of 1 s 1 under a pressure of around 1 10 6 torr. An active layer of the investigated organic small molecule was successively deposited on top of the bottom electrode by thermal evaporation with a targeted thickness of 40 nm, as determined in situ by a calibrated quartz crystal microbalance (QCM). Top electrode lines were thermally deposited through the shadow mask at the same condition, aligned perpendicular to the bottom lines, leading to crosspoint arrays of memory cells. All the electrical characteristics of the fabricated flexible memory devices were measured by using a Keithley 4200SCS semiconductor parameter analyzer. All electrical measurements were performed in a N2-filled glove box.
Acknowledgements W.-C.C. thanks the financial support from Ministry of Science and Technology (MOST) of Taiwan. Z.B. acknowledges the funding assistant from National Natural Science Foundation of China (No. 21161160443).
Received: August 22, 2014 Revised: September 3, 2014 Published online: && &&, 0000
Keywords: charge transfer · donor-acceptor systems · imides · memory · resistance switching · triphenylamine
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COMMUNICATION Donor-Acceptor Systems Lei Dong, Guangwu Li, An-Dih Yu, Zhishan Bo,* Cheng-Liang Liu, &&&&—&&&& Wen-Chang Chen* Conjugated Donor-Acceptor-Acceptor (D-A-A) Molecule for Organic Nonvolatile Resistor Memory
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The early bird catches the WORM: A donor-acceptor-donor (D-A-A) type conjugated molecule based on linear p-bridging triphenylamine (TPA) donors and naphthalimide (NI)/dicya-
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novinylene (DCN) acceptors was prepared and shown to possess nonvolatile WORM (write-once-read-many) memory behavior.
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