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Nanowires of Metal-Organic Complex by Photocrystallization: A System To Achieve Addressable Electrically Bistable Devices and Memory Elements Arup K. Rath,† Koushik Dhara,‡ Pradyot Banerjee,‡ and Amlan J. Pal*,† Department of Solid State Physics and Centre for AdVanced Materials, and Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed December 12, 2007. ReVised Manuscript ReceiVed March 3, 2008 A new method has been achieved to form a Cu:benzoquinone derivative (DDQ) charge-transfer complex by the photoexcitation of [Cu(DDQ)2(CH3COO)2] (1) that has been synthesized by the reaction of DDQ and hydrated cupric acetate in acetonitrile. Photoexcitation of coordinated complex 1 leads to the formation of charge-transfer complex Cu2+(DDQ•-)2 (2). The charge transfer complex 2, when spun on solid substrates, forms nanowires. Sandwich structures of 2 exhibit electrical bistability associated with memory phenomenon. Read-only and random-access memory phenomena are evidenced in nanowires of 2 providing a route to attend the issues pertaining to the addressibility of organic memory devices.
Introduction Downscaling of actual memory elements, such as dynamic random access memory (DRAM), flash or ferroelectric RAM, or magnetoresistive RAM has become more and more challenging due to physical limitations and increasing processing complexity.1 To fulfill the demands of memory technology, organic molecules may play an important role in the near future due to their exciting electronic, optical, and optoelectronic properties.2–7 Memory elements are achieved in electrically bistable devices, where at least two current values are observed at an applied voltage. A suitable voltage pulse generally induces a high-conducting state. Some of the materials, where the high state is stable even after withdrawal of the bias, can be used for memory applications. Electrical bistability in devices based on organic materials has been explained in terms of conformational change,8,9 electroreduction,7,9,10 intermolecular charge transfer,11,12 formation of metal filaments through redox reaction,13 etc. In recent years, read-only and random-access memory applications (ROM and RAM, respectively) have been demonstrated in systems based on different organic materials.7,9 * To whom correspondence should be addressed. Tel.: +91-33-24734971. E-mail:
[email protected]. † Department of Solid State Physics and Centre for Advanced Materials. ‡ Department of Inorganic Chemistry.
(1) Bez, R.; Pirovano, A. Mater. Sci. Semicond. Process. 2004, 7, 349. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (3) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R. W.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521. (4) Arias, A. C.; de Lima, J. R.; Hummelgen, I. A. AdV. Mater. 1998, 10, 392. (5) Ma, D. G.; Wang, G.; Hu, Y. F.; Zhang, Y. G.; Wang, L. X.; Jing, X. B.; Wang, F. S.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2003, 82, 1296. (6) Iimori, T.; Naito, T.; Ohta, N. J. Am. Chem. Soc. 2007, 129, 3486. (7) Bandhopadhyay, A.; Pal, A. J. J. Phys. Chem. B 2003, 107, 2531. (8) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (9) Bandyopadhyay, A.; Pal, A. J. Appl. Phys. Lett. 2004, 84, 999. (10) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid State Lett. 2002, 5, E43. (11) Jiang, G. Y.; Michinobu, T.; Yuan, W. F.; Teng, M.; Wen, Y. Q.; Du, S. X.; Gao, H. J.; Jiang, L.; Song, Y. L.; Diederich, F.; Zhu, D. B. AdV. Mater. 2005, 17, 2170. (12) Potember, R. S.; Poehler, T. O.; Cowan, D. O. Appl. Phys. Lett. 1979, 34, 405. (13) Ssenyange, S.; Yan, H. J.; McCreery, R. L. Langmuir 2006, 22, 10689.
Charge-transfer (CT) complexes between a metal and an organic compound often provide novel systems for memory elements12,14–19 due to a couple of reasons. First of all, the parameters of such complexes can be chosen to achieve suitable functionalities. Second, as memory elements, the CT complexes in general yield high On/Off ratio, the ratio between the two current values at a particular voltage. A high ratio is indeed a key to the use of electrically bistable devices as memory elements. Addessibility is another issue that has to be taken into account. That is, considering the difficulty in addressing a single molecule, it may be prudent to use a cluster of molecules as a memory element. Nanowires or nanorods of suitable complexes17–19 may hence offer a higher On/Off ratio along with their ability to become read or addressed so that a bit can be stored in a single nanowire. In this work, we choose a quinone derivative (DDQ), which is known to exhibit electrical bistability and also form CT complexes with suitable metals.20 We provide a route to form nanowires via photocrystallization21 for use as electrically bistable devices and memory elements. We develop a technique to fabricate a Cu2+(DDQ•-)2 charge transfer salt by photochemical reaction of DDQ dissolved in acetonitrile with copper acetate. In presence of visible light, photocrystallization21 of Cu2+(DDQ•-)2 charge transfer complex shows nanowires in solid substrates.
Experimental Section Materials. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and cupric acetate were purchased from Aldrich Chemicals and (14) Sato, C.; Wakamatsu, S.; Tadokoro, K.; Ishii, K. J. Appl. Phys. 1990, 68, 6535. (15) Muller, R.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2006, 88, 242105. (16) Muller, R.; De Jonge, S.; Myny, K.; Wouters, D. J.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2006, 89, 223501. (17) Muller, R.; Jonge, S.; Myny, K.; Wouters, D. J.; Genoe, J.; Heremans, P. Solid-State Electron. 2006, 50, 601. (18) Xiao, K.; Ivanov, I. N.; Puretzky, A. A.; Liu, Z. Q.; Geohegan, D. B. AdV. Mater. 2006, 18, 2184. (19) Liu, Y. L.; Li, H. X.; Tu, D. Y.; Ji, Z. Y.; Wang, C. S.; Tang, Q. X.; Liu, M.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. J. Am. Chem. Soc. 2006, 128, 12917. (20) Weitz, R. T.; Walter, A.; Engl, R.; Sezi, R.; Dehm, C. Nano Lett. 2006, 6, 2810. (21) O’Mullane, A. P.; Fay, N.; Nafady, A.; Bond, A. M. J. Am. Chem. Soc. 2007, 129, 2066.
10.1021/la703871g CCC: $40.75 2008 American Chemical Society Published on Web 05/02/2008
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Loba Chemicals, respectively. Indium tin oxide (ITO) coated glass substrates, purchased from Optical Filters Inc., had a surface resistance of 20 Ω/0. Complex Formation. Acetonitrile solution containing DDQ and cupric acetate in 2:1 molar ratio was initially stirred for 3 h. The mixture gradually turned from green to dark red indicating formation of [Cu(DDQ)2(CH3COO)2] (1). The solution in a quartz cuvette was then kept in front of a 330 nm UV light source for 2 h to form the Cu2+(DDQ•-)2 (2). A 150 W Newport solar simulator along with a Jobin Yvon monochromator were used to obtain a spot size of diameter 8 mm. The irradiated compound precipitated that was isolated and dried overnight in dark condition. Yield of 2 was above 78% as obtained by the weight ratio after isolation. Characterization. Electronic absorption spectra of the compounds in acetonitrile were recorded with a Shimadzu UV-vis spectrophotometer UV-2550. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet MAGNA-IR 750 spectrometer with samples prepared as KBr pellets. Elemental analysis of the compounds was carried out in a 2400 Series-II CHN analyzer (Perkin-Elmer, USA). The high-resolution mass spectrometry (HRMS) (electrospray ionization (ESI)) spectra were recorded on Qtof Micro YA263 mass spectrometer. Voltammetric experiments were performed with a potentiostat/galvanostat (model 273A, Princeton Applied Research) in a nitrogen environment. Scanning electron microscopy (SEM) images of 1 and 2 were taken with a JEOL JSM 6700F field emission scanning electron microscope. For SEM measurements of 2, a drop of 1, after spreading on a glass substrate to form a liquid thin film, was irradiated from above for 30 min. Device Fabrication and Electrical Characterization. Complex 2 in acetonitrile solution (4 mg/mL) was spun at 1000 rpm on stripped ITO. The films were dried in vacuum for 6 h. Thickness of the film was around 100 nm. Aluminum (Al) was thermally evaporated on the film as orthogonal strips to complete device fabrication. To record current-voltage (I-V) characteristics, the devices were kept in a shielded vacuum chamber. Bias was applied with respect to the Al electrode. Sweep speed of applied voltage was varied between 25 and 100 mV/s. A Keithley 6517A Electrometer was used to record I-V characteristics of the devices.
Result and Discussion Synthesis and Characterization of [Cu(DDQ)2(CH3COO)2] (1). A 20 mL acetonitrile solution of hydrated cupric acetate (0.10 g, 50 mmol) was added slowly to 15 mL of acetonitrile solution of DDQ (0.23 g, 100 mmol). The mixture was stirred in air for 2 h whereby a dark brown solution formed. It was then filtered and kept in air. A dark solid mass was obtained on slow evaporation of the filtrate at ambient temperature after a few days (80% yield). Unfortunately we were unable to get single crystals of 1. The HRMS (ESI) spectrum of the titled complex was obtained in acetonitrile solution. In this solvent, the spectrum shows peaks at 653.5, 593.4, and 463.4 that can be assigned to {[Cu(DDQ)2(CH3COO)2] · H2O + H+}+ (3), [Cu(DDQ)2(CH3COO)]+ · H2O (4), and {[Cu(DDQ)(CH3COO)2] · 3H2O + H+}+ (5). The existence of species 3 in the mass spectrum confirms the possible structure of complex 1 (Figure 1). Structures of 4 and 5 were generated from the mass fragmentation processes from the mother species 3. Further study of HRMS spectra indicates the possible fragmented species that were generated from 3, 4, and 5. Mass fragmentation procedures of species 3 in acetonitrile are shown below:
{[Cu(DDQ)2(CH3COO)2] · H2O + H+}+ (3) (m⁄z ) 653.5,simulated
-CO2
98 m ⁄ z )
(m⁄z ) 463.4,simulated
-CO2
98 m ⁄ z )
m⁄z ) 463.9)
419.5, simulated m ⁄ z ) 419.9
Anal. Calcd for CuC20H6N4O8Cl4: C, 37.80; H, 0.95; N, 8.81. Found: C, 37.51; H, 0.90; N, 8.65. This strongly suggests the possible molecular formula of complex 1. The structure of 1 from the HRMS study is further corroborated by elemental analysis of 1 in the solid state. Complex 1 displayed important FT-IR spectral bands of the coordinated DDQ22,23 and acetate ion. The vibration frequencies of CtN, CdO, CdC, C-C-CN, and C-Cl observed at 2233, 1674, 1554, 897, and 802 cm-1 for DDQ are shifted to 2228, 1627, 1533, 1412, 870, and 608 cm-1, respectively, in 1 (Figure 2a). The UV-visible spectrum of 1 shows peaks at 345, 290, and 254 nm (Figure 3). The spectrum has all the spectral features for DDQ clearly indicating the quantitative presence of the molecule in the complex.24 A Job’s plot analysis reveals that the maximum absorbance is obtained at a Cu2+/DDQ ratio of 1:2. This eventually means that 1 should form a 1:2 complex [Cu2+:(DDQ•-)2] in solution. It further justifies the possible structure of 1 where one copper ion is coordinated with two DDQ molecules (inset of Figure 3). To examine the extent of binding interaction of Cu2+ with DDQ•- in acetonitrile solution, the binding constant was determined based on UV-visible spectral data using modified Benesi-Hildebrand equation.25
1 ⁄ ∆I ) 1 ⁄ ∆IMax + 1 ⁄ K[C]∆IMax where ∆I ) Ix - I0 and ∆IMax ) I∞ - I0. Here I0, Ix, and I∞ are the absorption intensity of DDQ in the absence of Cu2+, at an intermediate Cu2+ concentration, and at a concentration of complete interaction, respectively. K is the binding constant and [C] is the Cu2+ concentration. From the plot of (I∞ I0)/(Ix - I0) against [C]-1 for DDQ (Figure 4), the value of K ((15%) extracted from the slope is 1.6 × 103 M-1. Formation of Cu2+(DDQ•-)2 Charge-Transfer Complex (2). Irradiation at λ ) 330 nm of a liquid thin film of an acetonitrile solution of 1 (1.5 mM) immediately leads to formation of a dark red color which on increasing the irradiation time gradually intensified. While the SEM image of 1 has no nanostructures (Figure 5a), the one of 2 clearly shows presence of submicrometer size nanowire structures over the entire glass substrate (Figure 5b). Average diameter and length of the nanowires were 25 nm and 2.5 µm, respectively. The EDXA (energy dispersive X-ray analysis) of the samples shows the presence of Cu, C, and N, which implies complex formation. Photocrystallization of 1 to form Cu2+(DDQ•-)2 charge-transfer complex at these wavelengths suggests that coordinated DDQ in 1 absorbs a certain wavelength of radiation which is evident from the incomplete structural morphology in SEM.
m⁄z ) 653.8)
609.5, simulated m ⁄ z ) 609.8 -CO2
[Cu(DDQ)2)(CH3COO)]+ ·H2O (4) 98 m ⁄ z ) (m⁄z ) 593.4,simulated
{[Cu(DDQ)(CH3COO)2] · 3H2O + H+}+ (5)
m⁄z ) 593.8)
549.4, simulated m ⁄ z ) 549.8
(22) Miller, J. S.; Krusic, P. J.; Dixon, D. A.; Reiff, W. M.; Zhang, J. H.; Anderson, E. C.; Epstein, A. J. J. Am. Chem. Soc. 1986, 108, 4459. (23) Salman, H. M. A.; Mahmoud, M. R.; Abou-El-Wafa, M. H. M.; Rabie, U. M.; Crabtree, R. H. Inorg. Chem. Commun. 2004, 7, 1209. (24) Durfey, D. A.; Kirss, R. U.; Frommen, C.; Reiff, W. M. Inorg. Chim. Acta 2004, 357, 311. (25) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
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Figure 4. Determination of binding constant K ((15%) of Cu2+ with DDQ from the slope of the plot.
Figure 1. HRMS (ESI) spectrum of complex 1 in acetonitrile solution.
Figure 2. FT-IR spectra of (a) complex 1 and (b) charge-transfer complex 2.
Figure 5. SEM images of (a) thin film of complex 1 and (b) complex 2 formed by irradiation of liquid thin film of complex 1 at λ ) 330 nm.
Figure 3. UV-vis spectra of complex 1 and charge-transfer complex 2 (broken line). Inset shows Job’s plot of analysis from UV-visible spectra recorded at 345 nm showing binding ratio of Cu2+ and DDQ being 1:2 in the formation of complex 1.
The charge-transfer complex Cu2+(DDQ•-)2 shows IR bands at 2220 cm-1, which are due to the formation of DDQ•- species.23
The vibrational frequencies of CtN, CdO, CdC, C-C-CN, and C-Cl are observed at 2220, 1517, 1385, 877, 880, and 605 cm-1, respectively, for the radical anion DDQ•- of the isolated Cu2+(DDQ•-)2 (Figure 2b). It is evident from the experimental data that the C-C-CN stretching band of 1 is slightly blueshifted whereas the other bands are red-shifted. This is probably due to the increased bond ordering of C-C-CN when DDQ is converted to DDQ•-. It is noteworthy that the stretching vibrations
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Figure 6. Cyclic voltammogram obtained at dry nitrogen atmosphere with a 3 mm diameter of Pt electrode in acetonitrile: (a) 0.1 M [NEt4][ClO4] for 1 mM DDQ with scan rate of 50 mV s-1, (b) 0.07 M [NEt4][ClO4] for 0.7 mM 1 with scan rates of 20-200 mV s-1, and (c) 0.12 M [NEt4][ClO4] for 1.2 mM 2 with scan rates of 20-200 mV s-1.
of C-H and CdO (antisymmetric) of acetate anion are absent in the IR spectrum of 2 due to photochemical decomposition of acetate anion to methanol (a methyl radical might have reacted with a hydroxyl moiety to form methanol). Formation of methanol after photoirradiation is also evident from the gas chromatography experiment. Anal. Calcd for 2 formulated as CuC16N4O4Cl4: C, 37.13; N, 10.82. Found: C, 36.92; H, 0.15; N, 8.65. The data indicate that the amount of hydrogen atoms in 2 is negligible. This corroborates decomposition of acetate ions from 1 during formation of 2 along with structural formulation of the final species. UV-visible spectral studies show characteristic band of the anion radical of DDQ at 415 nm for the CT complex (Figure 3). For DDQ•- there are two types of transitions that can occur due to the presence of a singly occupied molecular orbital (SOMO).22 The HOMO-LUMO energy gap in DDQ becomes the SOMO-NHOMO (next highest occupied molecular orbital) in DDQ•-. The difference in the SOMO-NHOMO energy in DDQ•- will be less than that of the HOMO-LUMO energy difference in DDQ. Thus the HOMO-LUMO transition in DDQ is now an “internal” transition for DDQ•- and should be significantly shifted to the red as compared to that of DDQ. Figure 6a shows the cyclic voltamogram of DDQ in acetonitrile. The reversibility of DDQ can be observed from the sharp oxidation and reduction peaks in the diagram. For coordination complex 1 (Figure 6b), sharp peaks changed to broader ones, which emphasize the quasi-reversibility of the complex without any shift in the peak position. This again implies that DDQ is in a neutral state in 1. But in the case of 2, the oxidation peak shifts at a low voltage because in 2 DDQ was present in its reduced state, so that oxidation of DDQ becomes comparatively easier. The quasi-reversible nature of cyclic voltamogram however has remained unaltered. This is shown in Figure 6c. Electrical Bistability in Cu(DDQ) Films. I-V characteristics of devices based on Cu:DDQ CT complex were recorded by scanning the applied voltage in loops (from 0 to +VMax, from +VMax to -VMax, and from -VMax to +VMax, in sequence). We have varied the amplitude of VMax to generate a range of I-V characteristics. At a suitable forward bias, the devices switched to a high-conducting state (Figure 7a). The higher level of conductivity is retained until a suitable reverse bias reinstated the low conductivity. The characteristics of the devices in the +VMax to -VMax and -VMax to +VMax scans did not match; that is, the device has exhibited electrical bistability. At any voltage, the device current, which did not respond on the sweep speed of the applied voltage, was much higher during the sweep from +VMax than that during the sweep from -VMax. Bistability in the conductivity or device current is associated with a memory
phenomenon. In other words, no bias is required to sustain the high-conducting state. The basis of the conductance switching in Cu:DDQ CT complex can be explained in terms of
[Cu2+(DDQ·-)2]n T [Cu0]x + [DDQ0]x + [Cu2+(DDQ·-)2]n-x “OFF” state
“ON” state
Here, the nonstoichiometric Cu phase is being produced along with neutral DDQ and Cu0 to yield a mixed valence film.12,26 Complex 1 shows little evidence of electrical bistability (On/Off ratio is an order of magnitude lower as compared to that in device 2), since no such reaction could take place in [Cu(DDQ)2(CH3COO)2]. In the high-conducting state, it is argued that the repulsion of charge carriers is lowered by the availability of empty molecular orbitals of the neutral DDQ for conduction electrons, as opposed to the situation when an electron is moved from one DDQs radical to another radical anion DDQs. When VMax g 3.5 V, during the sweep from +VMax, magnitude of current at -0.8 V increases abruptly followed by a sharp decrease. A similar jump in current is observed at a voltage of +0.8 V when scanned from -3.5 to +3.5 V (Figure 7b). During a sweep from (3.5 V, large numbers of neutral Cu and DDQ molecules are formed. When the voltage is reversed, the neutral copper atoms become ionized to form continuous channels of copper ions between the electrodes resulting in an abrupt increase in current. When the Cu ion concentration is high enough, the device is in its high conductive state due to a metallization effect.13,27,28 As voltage increases, the copper ions turn to neutral atoms causing a drop in current in the forward bias. A memory phenomenon in devices based on Cu:DDQ CT complex for ROM applications is demonstrated in Figure 8a. The current under a probe voltage is plotted as a function of time for both the high and low states. The device was switched to either a high or a low state by applying +3.5 and -3.5 V pulses (10 s width), respectively. The states of the devices were “read” by measuring current under a -0.1 V pulse for several hours. The magnitude of current under the probe voltage for the high-conducting state was much higher than that for the low state. While the response of the low state has remained almost steady, the response for the high-conducting state has showed an initial decay. The two states are clearly distinguishable (26) Heintz, R. A.; Zhao, H. H.; Xiang, O. Y.; Grandinetti, G.; Cowen, J.; Dunbar, K. R. Inorg. Chem. 1999, 38, 144. (27) Wang, M. T.; Lin, Y. C.; Chen, M. C. J. Electrochem. Soc. 1998, 145, 2538. (28) Ma, L. P.; Xu, Q. F.; Yang, Y. Appl. Phys. Lett. 2004, 84, 4908.
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Figure 7. I-V characteristics of a device based on complex 2 in three consecutive voltage loops with (a) VMax ) 3.0 V and (b) VMax ) 3.5 V.
even after 6 h of continuous probing demonstrating ROM applications. Reversibility of the memory phenomenon in the bistable devices can best be studied under a “write-read-erase-read” pulse sequence. In such a voltage sequence in cycles, the high and low conducting states are induced (“write” and “erase”, respectively) repeatedly and the states are monitored or “read” in between (Figure 8b). When currents under the “read” voltage pulses are compared, its magnitude is higher in probing the high state than while probing the low state. The results show that the devices can be flip-flopped between the two conducting states for RAM applications. It is worth mentioning that electrical bistability and memory phenomena of 2 are stable also in ambient condition making the devices suitable for applications. The amplitude of current under such a condition was however a little lower as compared to the measurements carried out in vacuum.
Conclusions In conclusion, we have showed that nanowires of Cu:DDQ complex have formed upon photoreaction. The nanowires, as imaged by SEM, have been characterized via electronic absorption, cyclic voltametry, etc. In sandwiched structures, the Cu: DDQ nanowires have exhibited electrical bistability. The basis of the conductance switching in Cu:DDQ CT complex has been explained in terms of appearance of neutral DDQ molecules
Figure 8. (a) ROM and (b) RAM application of a device based on complex 2 based device. In (a), amplitude of current under -0.1 V is plotted after a high state (open symbols) and a low state (filled symbols) were induced by applying +3.5 and -3.5 V pulses, respectively. In (b), voltage pulse sequence and corresponding current have been shown in the upper and the lower panels, respectively.
along with high Cu ion concentration that occurs due to metallization effect. Bistability is associated with memory phenomenon. ROM and RAM applications of the nanowires allowed storage of a bit in a nanowire to achieve addressable of memory elements. Acknowledgment. A.K.R. acknowledges CSIR Junior Research Fellowship No. 09/080(0505)/2006-EMR-I (Roll No. 503974). K.D. acknowledges CSIR, New Delhi, India, for financial support. The Department of Science & Technology, Government of India financially supported the work through Ramanna Fellowship SR/S2/RFCMP-02/2005. Supporting Information Available: SEM images of 2, current-voltage characteristics of a device based on 1, current-voltage characteristics of a device based on 2 at different scan speeds, and measurements carried out in air of a device based on 2. This material is available free of charge via the Internet at http://pubs.acs.org. LA703871G
