Electrochromic properties of Phthalocyanine derivative Langmuir Blodgett films

Transcription

1 Electrochromic properties of Phthalocyanine derivative Langmuir Blodgett films Takayuki Kato, obuyuki Iwata and Hiroshi Yamamoto Presented at the 8th International onference on Electronic Materials (IUMRS-IEM 2002, Xi an, hina, June 2002) 545

2 Electrochromic properties of Phthalocyanine derivative Langmuir -Blodgett films Takayuki Kato*, obuyuki Iwata and Hiroshi Yamamoto ollege of Science and Technology, ihon University, arashinodai, Funabashi-shi, hiba , Japan Fax: hyama@ecs.cst.nihon-u.ac.jp Phtalocyanine derivative thin films were accumulated on IT glasses by a Langmuir-Blodgett method. The layered structure and chromogenic properties of the films were investigated. We observed color changes which took place probably according to modulations of electronic distributions in the molecule. The possibility of the electric field induced chromogenic mechanism proposed was confirmed experimentally for the first time. Key words: chromogenic materials, phthalocyanine derivative, LB films, absorption spectrum 1.ITRDUTI Many transition metal oxides 1) and also organic materials 2) have been studied about applications to an electrochromic (E) flat panel display (FPD). The conventional E phenomena had, however, several drawbacks, namely slow responses and/or short lives of color changes according to chemical redox reactions. So applications of the electrochromic materials for a FPD have not been developed. Bucher and Kuhn 3) have reported E phenomenon produced by the electric field application to the single molecule film of coloring matter. According to the report, the response speed was very quick although the amount of color change was so small. Then we have proposed a new mechanism of color changes induced by electric fields without redox reactions. In the model the distribution of electrons is changed intra molecules by applying electric fields. Then we can expect the realization of E FPD s with a high response and a long life. As the substance model aligned organic dye molecules have been adopted. A Langmuir-Blodgett (LB) technique 4) has been widely used to prepare functional organic molecular ultrathin films with characteristic accumulation structures. As the candidate of highly aligned dye molecules we have focused at phthalocyanine derivative and prepared its LB 546

3 film. The obtained films were investigated with respect to the layered structure and chromogenic properties. 2. EXPERIMETAL PREDURES We fabricated LB films by a Kuhn type of apparatus. 5) The phthalocyanine derivative used have the structure of aluminum 1,4,8,11,15,18,22,25-octabutoxy-29H,31H- phthalocyanine triethyl siloxide (PcTr). nce the PcTr is not an amphiphilic molecule, an arachidic acid (AA, H 3 (H 2 ) 18 H) was used in order to stabilize the Langmuir film on the subphase. We prepared a chloroform solution of PcTr at a concentration of 0.64 mmol / l and AA at 3.2 mmol / l. The PcTr was a commercial product of drich-hemical o., Ltd., and the AA was obtained from Kanto-hemical o., Ltd. These molecular structures were schematically shown in Fig. 1. After spreading the solution, a L-film of PcTr / AA was kept on a water subphase at ph = 10.3 containing 2 mmol / l al 2 and 0.1 mmol / l ah. The subphase temperature was maintained at 290K. The film was transferred onto the substrates at a constant surface pressure (24 m / m) through vertical dipping / lifting strokes. The stroke speed was 12 mm / min. The substrates used were IT glasses (Kinoene-Kogaku-Kougyou o., Ltd) cut to the size of 1.5 cm 2. The sheet resistance of the IT glass was 10Ω/ cm 2. The accumulation of the LB films was done by the two kinds of processes; strokes for only PcTr/AA, and alternated dipping/lifting strokes for PcTr/AA and AA. The latter case was named as the hetero-layered LB film. The accumulated structures of the obtained films were investigated by a reflected X-ray diffraction (XRD) (Rigaku o., Ltd). The solid cell to measure the absorption spectra of the reflection has a configuration of Glass/ IT/ Insulator/ ounter Electrode. The insulator was 2 3 or AA films. The counter electrode used was or Au films. Surface Area of counter electrode was about 100mm 2. The absorption spectra of the reflection were measured by a double beam spectrometer (Shimadzu o., Ltd). The voltage-current (V-I) characteristic were measured by a potentiostat / galvanostat (Hokuto-Denko o., Ltd.). The sweep voltage was scanned at the rate of 2 V / min. The structure of the films was observed by a Scanning Electron Microscope (SEM) (Hitachi, Ltd.). 547

4 3. RESULTS AD DISUSSI Figure 2 shows XRD patterns of the PcTr / AA and the hetero-layered PcTr / AA LB film. A lattice spacing of the obtained PcTr / AA film evaluated from XRD peak was about 5.3 nm which was approximately twice as long as a molecular length of the AA. Accumulated films were confirmed to have a symmetry structure (Y-type) of lattice spacing 54. From the result we expected that the molecular planes of the phthalocyanine were aligned parallel to the film plane. Furthermore PcTr molecules in the hetero-layered films were aligned nonsymmetrically as shown in the inset of Fig. 1. Figure 3 shows a SEM photograph of the cross section of the Au electrode film deposited on the hetero-layered LB film. The thickness of the LB film was about 350 nm of which the value was consistent with that estimated from the number of accumulation. Figure 4 (a) shows the typical color changes in the absorption spectra of the Y-type film. Then the electrode used was. The spectrum of the original green state has a λ max at about 750 nm and a subpeak at about 680 nm. The color of the cell changed slightly, when a voltage above 5 V was applied at the IT electrode. The spectrum has a large peak at about 740 nm and a subpeak at about 670 nm. n the other hand, the color of the cell changed largely by applying a voltage of 5 V. Then λ max shifted to about 770 nm. These shifts did not reveal good reproducibility and were transient in the most cases. The cyclic voltammetry (V) was measured at the cell. Figure 5 (a) shows the typical V characteristics of such the cell. The 1st cycle revealed a large hysteresis in the cell using electrodes. The successive V after 2nd was stable. n the other hand, the hysteresis of V was not observed in the case of Au electrodes. The leak current flowed in the cell because of poor insulation of the film. Figure 4 (b) shows the typical color changes in the absorption spectra of the hetero-layered LB film. Then the electrode was Au. The color of the cell changed slightly when a voltage above 3 V was applied. The spectrum had a large peak at about 760 nm. When a voltage above -3 V was applied, the spectrum has as well as a large peak at about 760 nm. When the voltage was 0 V, the spectrum turned back the nonbiased condition. Then the reversible color changes were confirmed. Figure 5 (b) shows the V characteristics at the hetero-layered LB film. A large hysteresis was not observed. The result suggested that nonreversible chemical redox reactions did not take place near the electrode. It should be noticed the result that the absorption peak shifted in spite of nonhysteresis V characteristics at the hetero-layered LB 548

5 film. These chromogenic responses are thought to be caused by electric fields rather than by redox reactions and demonstrate the possibility of a novel electrochromic mechanism. 4. LUSI PcTr / AA and/or hetero-layered films on the IT glasses were prepared by a LB method. Each obtained films had the lattice spacing of about 5.3 nm and had a Y-type of accumulated structure. We prepared the E cell with metal counter electrodes above on LB films/it glasses. When electrodes were used for the Y-type of LB films, transient color changes by applying voltages were observed accompanying with a hysteresis V characteristic. The color change observed here may be considered to redox reactions. n the other hand, the reversible color changes were observed in the hetero-layered LB film cell with Au electrodes. As a conclusion the novel electrocromic phenomenon induced by electric fields was confirmed for the first and it is expected to be applied to a future FPD. AKWLEDGEMETS The authors wish to thank Advanced Materials Science enter in their ollege for helps by physical analyses. REFEREES 1). G. Granqvist, Handbook of Inorganic Electrochromic Materials (1995). 2) P.. Moskalev and. I. imova, Russ. J. Inorg. hem., 20, 1474 (1975). 3) H. Bucher, and H. Kuhn, Z. aturforsch, 25b (1971) ) K. B. Blodgett, J. Am. hem. Soc., 57, 1007 (1935). 5) H. Kuhn, D. Mobius, and H. Bucher, Techniques of hemistry, edited by A. Weissberger and B. W. Rossiter, Wiley (1973). 549

6 Figure aption Fig. 1. Molecular structures of the arachidic acid (AA) and the phthalocyanine derivative (PcTr). so the schematic hetero-layered structure of the LB film. Fig. 2. XRD patterns of the AA and PcTr / AA LB films. (a) PcTr / AA LB films (b) Hetero by AA and PcTr / AA Fig. 3. ross section SEM image of the E cell with the hetero-layered LB film. The LB film was prepared with the 40 couples of hetero-layered PcTr/AA films and 30 AA layers. The thickness of the couple of PcTr/AA and one AA layer was 5.3nm and 2.7nm, respectively. Fig. 4. olor changes observed by applying voltages of (a) only PcTr/AA and (b) the hetero-layered PcTr /AA LB film. The inset is the schemtic cell structure for the measurement of chromogenic properties. Fig. 5. The V-I characteristics of the E cell. The triangular voltage was scanned at the rate of 2 V / min in the cell of (a) IT / LB films / and (b) IT / LB films / Au. The area of the electrode was about 100 mm

7 H H H H H H Electric Field H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 H2H3 AA PcTr 551

8 (001) Intensity (a.u.) (003) Intensity (a.u.) (004) (005) θ (deg.) (001) (a) 100 (002) (003) 0 (004) (005) θ (deg.) (b) 552

9 Au LB films IT Glass 553

10 1.2 Electrode 2 3 LB film IT Absorbance (a.u.) Light Glass hromogenic ell -5V 0V +5V Wavelength / nm (a) Absorbance (a.u.) IT V Au Electrode AA LB film Light Glass hromogenic ell 0 V +3 V 0 V -3 V 0 V -3 V Wavelength / nm (b) 554

11 0.4 urrent (A) 2nd, 3rd 1st -5 5 Voltage (V) -0.4 (a) 0.6 urrent (A) -3 Voltage (V) (b) 555