Memory Effect of Photoswitching Controlled by Voltage Pulse Width

Feb 20, 2009 - Electrical conductivity switching is induced by photoirradiation in ... characteristic curve, and the feature of the hysteresis loop de...
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J. Phys. Chem. C 2009, 113, 4654–4661

Unprecedented Optoelectronic Function in Organic Conductor: Memory Effect of Photoswitching Controlled by Voltage Pulse Width Toshifumi Iimori,† Toshio Naito,‡ and Nobuhiro Ohta*,† Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 001-0020, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: January 16, 2009

Electrical conductivity switching is induced by photoirradiation in single crystals of R-(BEDT-TTF)2I3 below 135 K. Photoirradiated crystals show differential negative resistance (DNR), and bistability is observed in the electrical conductivity as a function of applied voltage. The threshold in voltage, which induces the DNR, increases as the temperature decreases. A hysteresis loop appears in the current versus light intensity characteristic curve, and the feature of the hysteresis loop depends on the pulse width of the applied voltage. The switching to a high conductivity state is initially triggered by the laser light irradiation. The conductivity switching can be repeatedly recovered only by applying the pulsed voltages without further photoirradiation even after the current was reduced to zero, indicating a memory effect in the photoinduced conductivity switching. When pulsed voltages having smaller width and/or height than the corresponding threshold values are applied to the crystal, the field-induced recovery to the high-conductivity state due to the memory effect fades out. The thresholds in width and height of the pulsed voltage for the memory effect can be controlled by the photoirradiation light intensity. These results show that the electrical conductivity can be controlled by the width and height of the pulsed voltage as well as the light intensity without a change in temperature or pressure. 1. Introduction Organic conductors have aroused a great deal of interest because of potential use in advanced functional materials since thediscoveryoforganicconductorsandorganicsuperconductors.1-4 These systems exhibit a number of exotic phenomena including formation of charge and spin density waves, unconventional superconductivity, nonlinear conductivity, magnetoresistance effects, and Mott transition. Recent theoretical and experimental studies have revealed that the charge-ordered (CO) state can exist in organic conductors as one of the possible ground insulating states.5 In the CO state, charges are inhomogeneously localized to charge-rich and charge-poor sites, and Coulomb interaction between conduction electrons plays an important role in the stabilization of the CO state. Organic charge-transfer complexes based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts form a family of organic conductors that have a variety of donor molecule arrangements and extensive polymorphism.6 Because of narrow bandwidth and strong electron correlation, these organic conductors exhibit a variety of electronic states including the CO state.3 Forexample,R-(BEDT-TTF)2I3 undergoesaninsulator-metal (I-M) phase transition at TI-M ) 135 K,7 and the insulating phase has been assigned to the CO state by theoretical and experimental works.5,8-11 The response of the CO state of R-(BEDT-TTF)2I3 to the irradiation of light has been investigated with laser spectroscopic techniques.12-14 The transient photocurrent induced by a visible laser pulse irradiation has been studied at T ) 4 K and 80-115 K.12,13 These studies have shown that the I-M phase transition can be induced by photoirradiation. At T ) 115 K, we have * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected]. † Research Institute for Electronic Science. ‡ Graduate School of Science.

observed a marked persistence of the metallic state generated by photoirradiation. A nonlinear dependence of the amplitude of the photocurrent on the photoirradiation intensity (Iirr) was also characterized.12 By using femtosecond optical pump-probe reflection spectroscopy, on the other hand, Iwai et al. observed (1) a marked increase of the decay time of the metallic state with an increase of the light intensity, and (2) the ultrafast melting of the CO state in a subpicosecond time scale.14 In the presence of external electric fields, photoinduced switching effect in electrical conductivity has also been observed with the pulsed voltage applied to the sample in synchronization with the pulsed laser irradiation.15 Further, we have recently discovered a memory effect controlled by pulsed voltages (hereafter, denoted by MECP) in the photoinduced conductivity switching effect.16 This MECP is explained as follows: (1) The switching to a high conductivity (HC) state is initially triggered by the laser light irradiation having the intensity of the order of 10-5 J/pulse; (2) the conductivity switching can be repeatedly recovered by application of the pulsed voltage without further photoirradiation even after the current was reduced to zero; (3) the recovery to the HC state can be controlled by adjusting the width and/or height of the pulsed voltage. The applied pulsed voltages have a temporal width on the order of 10-3 s and a height of approximately 10 V. The pulse repetition rate was 8.3 Hz (the pulse interval of 120 ms). The MECP is an unprecedented phenomenon found only in the organic conductor. In other materials including semiconductors and thin films, a similar effect has never been reported so far, as far as we know. In this paper, we extend our previous study and comprehensively discuss the MECP in the CO state of the organic conductor R-(BEDT-TTF)2I3. We investigate the conductivity switching in the single crystals by measuring the temporal profile of current in response to the pulsed voltage. We find a bistability in the plots of the current versus irradiation light intensity, and

10.1021/jp810351c CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009

Optoelectronic Function in Organic Conductor

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4655

Figure 3. Current versus voltage (I-V) characteristic curves obtained by scanning the height (∆H) of the applied pulsed voltage in the positive direction. The curves were recorded at 115 K with and without photoirradiation. Figure 1. (a) The circuit diagram. RL represents a load resistance inserted in series with the circuit to avoid damage to the sample. (b) Pulse scheme used in experiments.

Figure 2. Temporal profiles of the current measured at 115 K with RL ) 510 Ω. (a) Current profiles measured with and without photoirradiation. The height (∆H) of the pulsed voltage was 8.5 V. (b) A current profile measured with photoirradiation. The height of the pulsed voltage was 4 V.

the feature of the hysteresis loop in the characteristic curve varies with different widths of the pulsed voltage. The threshold in width and height of the pulsed voltage required to obtain the MECP is determined. It is shown that the threshold in width in the MECP can be controlled by photoirradiation. The mechanism of the MECP is discussed in terms of a high current filament formation in the HC state. 2. Experimental Methods The crystals of R-(BEDT-TTF)2I3 were prepared by galvanostatic anodic oxidation of BEDT-TTF in solution containing (C4H9)4NI3 as a supporting electrolyte.7 The dimensions of a typical sample of R-(BEDT-TTF)2I3 used in the present study were 2 × 1 × 0.03 mm3. The sample was placed in a cryostat using temperature-controlled helium buffer gas. Gold wire (25 µm in diameter) and gold paint were used as the electrodes. The distance between the electrodes was ∼0.4 mm. Figure 1 shows the circuit diagram and pulse scheme used in the experiments. The sample crystal and a square-wave voltage source were connected in series in the circuit. In the measurements of the switching effect, a load resistance inserted in series with the circuit (RL) was necessary to avoid damage to the sample by excessive current loading. The time profile of current was monitored by measuring the voltage drop across the input impedance (50 Ω) of a digital oscilloscope (Tektronix, 2440).

As an excitation laser pulse, we employed a second harmonic of an output from a Nd:YAG laser (QuantaRay, DCR-11) with a pulse width of ∼10 ns and wavelength of 532 nm. An optical fiber was used to deliver the laser pulse to the middle of the crystal surface between the two electrodes. The diameter of the illuminated area on the sample was ∼0.4 mm. The Iirr was adjusted with a variable neutral-density filter. Iirr is given here in units of energy per pulse on the sample. The angle of incidence of the laser light on the a-b plane of the sample was 0°. The polarization direction of the laser light was accordingly parallel to the a-b plane, although it was not intentionally specified to any of these crystal axes. In the conductivity measurements, a square-wave voltage pulse train with specified pulse height (∆H) and pulse width (∆W) was applied to the sample in synchronization with the laser pulse train. The pulse interval was 120 ms, and thus the pulse repetition rate was 8.3 Hz. The irradiation of the laser pulse was delayed against the onset of the pulsed voltage by 50 µs. 3. Photoinduced Switching Effect in Electrical Conductivity 3.1. Time Profile of Current in the Photoinduced Conductivity Switching. The time profile of the transient current signal from the circuit provides direct information concerning the time evolution of the electrical conductivity during the application of electric fields. A typical example of the observed current profiles is shown in Figure 2. Figure 2a shows the current profile observed at T ) 115 K, where the sample was in the insulating CO state, with the pulsed voltage of ∆H ) 8.5 V and ∆W ) 5 ms. Without photoirradiation, it showed a flat current profile stemming from the dark resistance of 1 × 105 Ω. In the current profile observed with photoirradiation at the intensity of Iirr ) 3.4 × 10-5 J/pulse, a rise of the current following the sharp peak was observed, and the current was retained as far as the pulsed voltage was being applied. This observation can be ascribed to the conductivity switching to the HC state by photoirradiation. With a small applied voltage of ∆H ) 4 V, on the other hand, the current decayed with a lifetime shorter than a few microseconds (Figure 2b). Thus, the current profiles measured in the presence of photoirradiation show a marked dependence on ∆H, namely, on the height of the pulsed voltage. At the wavelength of the laser light (532 nm), there are two absorption bands that are ascribed to the intramolecular transitions of both BEDT-TTF and I3-.17 The observed photoresponse accordingly arises from the intramolecular excitation in the crystal. 3.2. Temperature Dependence of the Photoinduced Conductivity Switching Effect. Figure 3 shows the current versus voltage (I-V) characteristic curve observed at 115 K with and

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Figure 5. Threshold electric field (Eth) versus temperature (T). Error bar is put on the data at 115 K.

Figure 4. (a) I-V characteristic curves obtained by scanning the height (∆H) of the pulsed voltage in the positive direction at different temperatures. The curves were measured with photoirradiation. (b) Current versus voltage drop across the sample (VS).

without photoirradiation. In the I-V characteristic curves shown here and hereafter, the current observed at the end of the duration of applied pulsed voltage was used as I (see Figure 2). The plots shown in Figure 3 were measured by scanning ∆H in the positive direction, i.e., in the direction toward the increase of ∆H, while Iirr and RL were fixed to be 3.4 × 10-5 J/pulse and 1 kΩ, respectively. Without photoirradiation, the current increased with an increase of ∆H although the magnitude was small. This behavior stems from the CO state, which is hereafter referred to as the low-conductivity (LC) state. With photoirradiation, on the other hand, the observed current showed a leap at ∼7 V, indicating the conductivity switching to the HC state. Figure 4a shows the I-V curves measured at various temperatures below the TI-M. For all the measurements, the same photoirradiation intensity (3.4 × 10-5 J/pulse) was used. As shown in Figure 4a, the threshold in voltage at which the photoinduced conductivity switching took place increased as the temperature decreased. The voltage applied to the sample (VS) is calculated from the relation VS ) ∆H - (R0 + RL) × I, where R0 represents circuit resistance other than that of the sample and RL.18 The current versus VS characteristic curves are shown in Figure 4b. When VS exceeds a certain threshold (Vth), the electrical conductivity changes from the LC state to the HC state along the load line of the circuit, indicating that the differential negative resistance (DNR) is observed in the photoirradiated crystal. Plots of the threshold in electric field, Eth, as a function of the sample temperature are shown in Figure 5. Note that Eth, which was calculated from the Vth divided by the distance between the electrodes, varied from 1 × 102 to 6 × 102 V/cm in the temperature range used in the present experiments. The Eth showed the nearly linear dependence on temperature with a negative slope. 4. Bistability and Memory Effect in Electrical Conductivity 4.1. Hysteresis Loop in the Plots of Current Versus Irradiation Light Intensity. Bistability and hysteresis are expected in a molecular system which shows two different states in a certain range of external physical parameters including temperature, electromagnetic field, or irradiation light intensity,

Figure 6. I-V characteristic curves obtained by scanning the height (∆H) of the pulsed voltage in the positive and negative directions at 115 K with photoirradiation.

and a hysteresis loop in the I-V characteristic curves may arise from the DNR of the materials.18 The hysteresis loop observed in the I-V characteristic curve of R-(BEDT-TTF)2I3 is shown in Figure 6, which was obtained at T ) 115 K by scanning ∆H in the positive and negative directions with photoirradiation at a constant intensity (Iirr ) 3.4 × 10-5 J/pulse). It is noted that different load resistances as well as different samples were used from each other in Figure 6 and in Figures 2 and 3. Accordingly, the magnitude of the current in the HC state in Figure 6 is different from the others. As shown in Figure 6, the photoinduced LC-to-HC switching occurs at ∆H ≈ 7 V, while the reverse switching to the LC state occurs at a lower voltage in the negative voltage scan. Thus, two states with the different electrical conductivities from each other exist in a certain voltage range. A bistability was also observed in the plots of current versus irradiation light intensity, namely, in I-Iirr characteristic curves, obtained with a constant ∆H. Figure 7a-c shows the results at T ) 115 K, where ∆W of 2, 3, and 4 ms were used, respectively. In each case, photoirradiation was done regularly at a repetition rate of 8.3 Hz in synchronization with the pulsed voltage with a scan of Iirr. Irrespective of ∆W, the LC-to-HC switching was observed at essentially the same value of Iirr (∼3.2 × 10-5 J/pulse), indicating that ∆W does not affect the threshold of Iirr necessary for the photoinduced LC-to-HC switching. In the negative scan of Iirr, however, the I-Iirr characteristic curves showed the ∆W dependence. For example, the crystal restored the LC state at Iirr ≈ 2 × 10-5 J/pulse with ∆W ) 2 ms and at Iirr ≈ 5 × 10-6 J/pulse with ∆W ) 3 ms, respectively. As ∆W increased, the width of the hysteresis loop in the I-Iirr curve expanded, and with ∆W ) 4 ms, the LC state was not observed even after the photoirradiation was ceased. The HC state can be thus sustained with an appropriate width of pulsed voltage even without photoirradiation, once the switching to the HC state is induced by a combination of pulsed voltage and photoirradiation. This fact amounts to the MECP phenomenon. With small ∆W, the crystal returns to the LC state from the HC state without photoirradiation, though a bistability appears in the I-Iirr characteristic curve. Thus, the width of the hysteresis

Optoelectronic Function in Organic Conductor

Figure 7. Current versus photoirradiation intensity (Iirr) curves. The pulsed voltages having 2, 3, and 4 ms widths (∆W) were used in a-c, respectively. RL ) 510 Ω and ∆H ) 10 V.

Figure 8. Sequences of current profiles obtained for 10 successively applied voltage pulses with ∆H ) 11 V. The order in the sequence is indicated by the numbers. (a) ∆W ) 7 ms. (b) ∆W ) 6 ms.

loop in the I-Iirr characteristic curve can be controlled by ∆W of the pulsed voltage. The current profiles observed in the presence and absence of the MECP are shown in Figure 8. Two sequences of the current profiles were measured for 10 consecutive pulsed voltages for the cases of ∆W ) 7 and 6 ms. In both cases, photoirradiation was done only at the first pulsed voltage, by which the LC-toHC switching was induced. At the second and following pulses, photoirradiation was turned off. With ∆W ) 7 ms (Figure 8a), the HC state was obtained repeatedly even without further photoirradiation; the MECP of the photoinduced switching was observed, and the HC state was sustained continuously without

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4657 further photoirradiation over more than 10 000 pulses without noticeable change. In contrast, with a smaller value of ∆W ) 6 ms (Figure 8b), the recovery to the HC state weakened as time passed, and the current due to the HC state completely disappeared after eight pulses. It should be noted that the threshold in width to obtain the MECP (∆WT) depends on the current in the HC state. In the measurements of the I-Iirr characteristic curve shown in Figure 7, the RL was 510 Ω, and the current in the HC state was in the range from 12 to 14 mA. Then, ∆WT was in the range from 3 to 4 ms. On the other hand, RL ) 1 kΩ in Figure 8, and the observed current in the HC state was ∼8 mA. The result in Figure 8 also indicates that ∆WT is a certain value between 6 and 7 ms. Thus, the ∆WT is related to the magnitude of current in the HC state. 4.2. Threshold in Pulse Width of the Applied Voltage for the Memory Effect. The MECP can be controlled by ∆W, as described above. The experimental results shown above were obtained with ∆W fixed to a certain value in each experiment. In Figure 8a, for example, the pulsed voltage having ∆W ) 7 ms was always applied to the sample to obtain each current profile; the voltage pulse width was not changed during the measurements. In this section, the ∆WT, with which the memory effect can appear, is determined under different conditions. In the evaluation of ∆WT, although the current profiles were similarly acquired at each 120 ms interval, we shortened ∆W of the pulsed voltage that was successively applied to the crystal for the measurement of the current profile. The HC state was initially induced by application of both laser light and pulsed voltage (∆W ) 7 ms). After the photoinduced switching, we changed the Iirr to certain values and then shortened the ∆W. When ∆W became smaller than ∆WT, the crystal returned to the LC state with a transient response similar to the one shown in Figure 8b; the memory effect disappeared by shortening ∆W without changing temperature or Iirr. The experimental results for the evaluation of ∆WT at T ) 115 K, RL ) 510 Ω, and Iirr ) 0 (that is, without photoirradiation) are shown in Figure 9a. Note that the initial LC-to-HC switching was induced by using Iirr that was strong enough to induce the switching and that Iirr which is necessary to induce the LC-to-HC switching is independent of ∆W (see Figure 7). The applied pulsed voltage was fixed to ∆H ) 10 V, and the initial value of ∆W was set to 7 ms. MECP could be obtained at ∆W ) 7 ms without photoirradiation following the photoinduced LC-to-HC switching; the HC state appeared by application of the pulsed voltage even without photoirradiation. The result with ∆W ) 7 ms is shown in the most outer curve in Figure 9a. Note that the current profile was measured by taking the average of 8 shots of the waveform output from the oscilloscope. Next, ∆W for the current measurements was changed to shorter values; current profiles were similarly measured with different values of ∆W without photoirradiation. Figure 9a shows the current profiles obtained in such a manner at ∆W of 7 ms, 5.8 ms, 4.9 ms, 4 ms, and 3.7 ms, respectively. When ∆W was set shorter than 3.8 ms, the response to the applied voltage pulses reverted to the LC state, indicating that ∆WT was 3.8 ms in this case. We have examined the photoirradiation effect on ∆WT by using a similar technique. Following the photoinduced LC-toHC switching, the photoirradiation was not stopped completely, but weak laser light whose intensity was given by Iirraft was irradiated to the sample during the measurements of the current profile. Note that Iirraft was not strong enough to induce the LCto-HC switching. As described in section 4.1, the critical value of Iirr to induce the initial LC-to-HC switching was ∼3 × 10-5

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Figure 9. ∆W dependence of the memory effect. (a) Current profiles were measured without photoirradiation, following the photoinduced LC-to-HC switching. We initially used the pulsed voltage with ∆W ) 7 ms. Following the photoinduced switching, the laser light was stopped. The current profile observed at ∆W ) 7 ms is shown as the outermost line in this panel. The magnitude of ∆W was shortened to 5.8 ms, 4.9 ms, 4 ms, and 3.7 ms, respectively, following the observation with ∆W ) 7 ms. The vertical broken lines indicate the position of ∆WT. The innermost line, measured with ∆W ) 3.7 ms and showing I ≈ 2 × 10-4 A, represents the LC state, while the others represent the HC state. (b) Same as (a) except for continued photoirradiation with Iirr of 1.8 × 10-5 J/pulse even after the photoinduced LC-to-HC switching was induced with ∆W ) 7 ms. Five profiles measured with ∆W ) 5 ms, 4.1 ms, 3.4 ms, 2.6 ms, and 2.3 ms are shown. (c) Reversible photoswitching behavior observed with the pulsed voltage of ∆W ) 3.4 ms. The current profile having the greater current was recorded with photoirradiation, and the other was recorded without photoirradiation.

Figure 10. Threshold width (∆WT) as a function of Iirraft at 115 K. See the text about the definition of Iirraft. A broken line is a guide for the eyes.

J/pulse. Figure 9b shows the result for the evaluation of ∆WT with Iirraft ) 1.8 × 10-5 J/pulse, which is approximately 60% of the critical intensity necessary for the initial LC-to-HC switching. When the pulsed voltage having ∆W shorter than 2.4 ms was used, the MECP disappeared, indicating that ∆WT was shortened from 3.8 to 2.4 ms with the photoirradiation of Iirraft ) 1.8 × 10-5 J/pulse. Thus, ∆WT has the Iirraft dependence, as shown in Figure 10; ∆WT decreases as Iirraft increases. As a result, we can control ∆WT in the MECP by adjusting Iirraft.

Iimori et al. The height of the pulsed voltage, that is, ∆H, is also the parameter which can be used to control the appearance of MECP. When ∆H was set to lower levels than a threshold (∆HT), the HC state returned to the LC state even without the change in ∆W. With the same sample as used in Figure 9, for example, the MECP obtained with ∆W ) 6 ms disappeared without photoirradiation, unless ∆H was higher than 8 V. As ∆W of the applied voltage became shorter, ∆HT became higher. Thus, the photoinduced HC state cannot be sustained without photoirradiation unless both ∆W and ∆H exceed the threshold values. Iirraft also affects ∆WT, as mentioned above. Consequently, the conductivity of the R-(BEDT-TTF)2I3 crystal can be controlled by Iirr, Iirraft, ∆W, and ∆H. The presence of MECP means that the photoinduced conductivity switching is regarded as irreversible; the original LC state does not reappear even when the irradiation light is turned off. As already described, the MECP is realized when ∆W > ∆WT. By using the pulsed voltage with ∆W < ∆WT, on the contrary, a reversible photoswitching becomes feasible, as shown in Figure 9c. The HC state was induced by photoirradiation having Iirr ≈ 3 × 10-5 J/pulse and the pulsed voltage of ∆W ) 3.4 ms, and the LC state was recovered when the photoirradiation was stopped. Thus, the reversibility and irreversibility of the photoswitching behavior can be controlled by changing ∆W of the pulsed voltage applied to the crystal. The ∆W dependence of the photoswitching behavior is related to the ∆W dependence of the shape of the hysteresis loop appearing in the I-Iirr characteristic curve (see Figure 7). 5. Initial Process Following Photoirradiation When ∆H and Iirr smaller than the threshold that could induce the LC-to-HC switching were employed, photoirradiation to the R-(BEDT-TTF)2I3 crystal in the LC state at temperatures close to the TI-M generated a transient photocurrent, which showed a decay with a lifetime shorter than a few microseconds (see Figure 2b and ref 12). When strong irradiation light intensity was used, persistent decay profiles and a nonlinear dependence of the photocurrent amplitude on Iirr were observed, consistent with the photoinduced I-M phase conversion. In the Iirr dependence of the photocurrent, the magnitude of the current measured at t ) 0.6 µs showed a threshold at Iirr ≈ 3 × 10-5 J/pulse, above which the current abruptly increased with an increase of Iirr.12 Similarly, using ultrafast time-resolved reflection spectroscopy, Iwai et al. have observed that the slow component becomes dominant for strong Iirr.14 It is pertinent to note that the threshold observed in the photocurrent measurement is in good agreement with the critical value of Iirr to induce the LC-to-HC switching (see Figure 7). This result implies that the slow and persistent component in the photoconductivity is closely related to the induction of the conductivity switching. The ultrafast time-resolved experiment has shown that a photoinduced I-M phase transition takes place immediately after the photoexcitation ( ∆HT were employed, the bistability of the conductivity in the absence of photoirradiation was observed. One could accordingly observe the memory effect in the photoinduced conductivity switching in the CO state of R-(BEDTTTF)2I3. The hysteresis loop in the I-Iirr characteristic curve was confirmed to depend on ∆W, suggesting that the appearance of the memory effect can be controlled by varying ∆W without changing either the temperature or Iirr. ∆WT depended both on Iirr and on ∆H. ∆WT and ∆HT were characterized at different temperatures below TI-M. The electrical power dissipation calculated from the current profiles was inconsistent with a Joule heating effect, and the mechanism of the MECP phenomenon was discussed in terms of the formation of high current filament. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (A) (Grant No. 20043005) from the Ministry of Education, Culture, Sports, Science and Technology in Japan. References and Notes (1) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.; Geiser, U.; Wang, H. H., Kini, A. M.; Whangbo, M. -H. In Organic Superconductors (including fullerenes); Prentice Hall: Englewood, NJ, 1996.

Optoelectronic Function in Organic Conductor (2) Ishiguro, T.; Yamaji, K.; Saito, G. In Organic Superconductors, 2nd ed., Springer-Verlag: Heidelberg, Germany, 1998. (3) Saito, G.; Yoshida, Y. Bull. Chem. Soc. Jpn. 2007, 80, 1–137. (4) Fukuyama, H J. Phys. Soc. Jpn. 2006, 75, 051001(Special Issue on Physics of Molecular Conductors). (5) Takahashi, T.; Nogami, Y.; Yakushi, K. J. Phys. Soc. Jpn. 2006, 75, 051008. (6) Shibaeva, R. P.; Yagubskii, E. B. Chem. ReV. 2004, 104, 5347– 5378. (7) Bender, K.; Hennig, I.; Schweitzer, D.; Dietz, K.; Endres, H.; Keller, H. J. Mol. Cryst. Liq. Cryst. 1984, 108, 359–371. (8) Seo, H. J. Phys. Soc. Jpn. 2000, 69, 805–820. (9) Kino, H.; Fukuyama, H. J. Phys. Soc. Jpn. 1996, 65, 2158–2169. (10) Wojciechowski, R.; Yamamoto, K.; Yakushi, K.; Inokuchi, M.; Kawamoto, A. Phys. ReV. B 2003, 67, 224105. (11) Kakiuchi, T.; Wakabayashi, Y.; Sawa, H.; Takahashi, T.; Nakamura, T. J. Phys. Soc. Jpn. 2007, 76, 113702. (12) Iimori, T.; Naito, T.; Ohta, N. Chem. Lett. 2007, 36, 536–537. (13) Tajima, N.; Fujisawa, J.; Naka, N.; Ishihara, T.; Kato, R.; Nishio, Y.; Kajita, K. J. Phys. Soc. Jpn. 2005, 74, 511–514. (14) Iwai, S.; Yamamoto, K.; Kashiwazaki, A.; Hiramatsu, F.; Nakaya, H.; Kawakami, Y.; Yakushi, K.; Okamoto, H.; Mori, H.; Nishio, Y. Phys. ReV. Lett. 2007, 98, 097402. (15) Iimori, T.; Naito, T.; Ohta, N. Appl. Phys. Lett. 2007, 90, 262103.

J. Phys. Chem. C, Vol. 113, No. 11, 2009 4661 (16) Iimori, T.; Naito, T.; Ohta, N. J. Am. Chem. Soc. 2007, 129, 3486– 3487. (17) Sugano, T.; Yamada, K.; Saito, G.; Kinoshita, M. Solid State Commun. 1985, 55, 137–141. (18) Sawano, F.; Terasaki, I.; Mori, H.; Mori, T.; Watanabe, M.; Ikeda, N.; Nogami, Y.; Noda, Y. Nature 2005, 437, 522–524. (19) Miyano, K.; Tanaka, T.; Tomioka, Y.; Tokura, Y. Phys. ReV. Lett. 1997, 78, 4257–4260. (20) Sze, S. M. In Physics of Semiconductor DeVices; John Wiley and Sons, Inc.: New York, 1969. (21) Ridley, B. K. Proc. Phys. Soc. 1963, 82, 954–966. (22) Yamamoto, K.; Iwai, S.; Boyko, S.; Kashiwazaki, A.; Hiramatsu, F.; Okabe, C.; Nishi, N.; Yakushi, K. J. Phys. Soc. Jpn. 2008, 77, 074709. (23) Fleming, R. M.; Schneemeyer, L. F. Phys. ReV. B 1986, 33, 2930– 2932. (24) Ido, M.; Okajima, Y.; Wakimoto, H.; Oda, M. Physica 1986, 143B, 54–58. (25) Coppersmith, S. N.; Littlewood, P. B. Phys. ReV. B 1987, 36, 311– 317. (26) Cox, S.; Singleton, J.; McDonald, R. D.; Migliori, A.; Littlewood, P. B. Nat. Mater. 2008, 7, 25–30. (27) Dressel, M.; Gru¨ner, G.; Pouget, J. P.; Breining, A.; Schweitzer, D. J. Phys. I (Paris) 1994, 4, 579–594.

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