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Preparation and characterization of poly(p-phenylene vinylene) Langmuir-Blodgett films formed via precursor method
Synthetic
ELSEVIER
Metals
71 (1995) 2023-2024
Preparation and characterization of poly(p-phenylene vinylene) Langmuir-Blodgett
films formed
via precursor method Jae Hwan Kim“, Young
Kwan Kim’, Byoung Chung Sohn’, Dou-Yol and Chong-Hong Pyund
Kangb, Jung-II Jin’, Chang-Hong
Kimd,
‘Dept. of Chemical Engineering, Hong-Ik University, Seoul 121-791, Korea bDept. of Electrical engineering, Hong-Ik University, Seoul 121-791, Korea “Dep. of Chemistry, Korea University, Seoul 136-701, Korea dKorea Advanced Institute of Science and Technology, Seoul 136-791, Korea Abstract Recent advances on n-conjugated conducting polymers for electroltinescence(EL) have shown a strong possibility for various display applications. In this paper, we will describe the preparation of ultra thin films of oil-soluble poly(p-phenylene vinylene) (PPV) precursor polymer by the Langmuir-Blodgett technique and by conversion into PPV films via a thermal treatment. The characterization of these films was also carried out by FT-IR spectroscopy, UV-VIS absorption spectroscopy, phuoluminescence (PL), and Scanning Tunneling Microscopy (STM). IT-IR, and W-VIS results indicated that multilayer films of PPV precursor polymer were formed well and the elimination of methoxy group in PPV precursor polymer was partially achieved by the thermal treatment. Red shift was observed in PL analysis after thermal treatment. An STM image of a PPV monolayer on graphite showing the polymer chains was observed 1. INTRODUCTION Conjugated polymers have been studied for their electrical conductivity. More recently conjugated polymers, in particular, PPV and its derivatives showed promise in optoelectronic applications, because these polymers can function as the emissive layer in electroluminescent devices using oil-soluble precursor made it possible to process the polymer into technologically useful thin film forms[l]. Recently, our group has been investigating the use of the Iangmuir-Blodgett technique as a means to control the molecular orientation and film thickness in ordered monolayers, so as to understand the basic mechanism of electroluminescence and other electrical properties. 2. EXPERIMENTAL The oil-soluble methoxy leaving group precursor polymer(l) shown below was prepared as reported elsewhere[2].
mrnol/L and filtered just before deposition. The LB films were deposited at a surface pressure of 20 mN/m, compression speed of 1 mN/m/min and dipping speed of 5 mm/min after spreading precursor polymer solution on deionized water at 2O’c. T&e LB films were deposited m Z-type, where the transfer ratio was 0.8. The obtained LB tihns of precursor polymer were heated up to 3ooC in a vacuum( 1Torr) for 12hrs. To follow the elimination of methoxy group, IR spectra were recorded on BIO-RAD Fl’S-10 spectrometer and UV-Visible spectra were recorded on HP 8452A diode army type spectrometer, respectively. The surface morphology of LB films after elimination was investigated by STM, model AutoProbe CP from PSI, operated in air at room temperature. For STM analysis, precursor polymer films were heated as mentioned above. 3. RESULTS
AND DISCUSSION
W-Visible spectra of precursor polymer with various thicknesses of monolayers are shown in Fig. 1. The absorbance at 230 and 325nm in Fig. 1, gives a good linear relationship between the number of layers and the absorlxmces. This suggests that the layer by layer deposition of precursor polymer films at molecular level was successfully controlled. PPV
.
LB films were deposited by using a computer-controlled Kuhn type trough (KSV model 3000). Distilled water as a subphase was purified by a h+iilli-Q Reagent Water System. For W and IR absorption spectroscopic analysis, polished quartz plates and ptype silicon w-afers(lOO) on one side were used as subsaates, and cleaned prior to polished deposition For STM analysis, Highly Oriented Pyrolytic Gtaphite (HOFG) was used as a substrate because its cleaved surface is flat on the atomic scale. Precursor polymer was dissolved in a chloform solution to a concentration of 1
0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03147-X
Science
S.A. All rights
reserved
wavelength
(nm)
Fig. 1. UV-Visible absorption spectra of precursor films with 3, 6, and 9 LB layers on quartz.
polymer
J.H. Kim et al. / Synthetic Metals 71 (1995) 2023-2024
PW
I-
PW Precusor
.
500
15ocl
2500
3500
wavenumber
(cni’)
Pig. 2. FT-IR spectra of (a) precursor polymer film, (b) PPV film prepared via heat treatment of precursor polymer with 4-cm-’ resolution. Fig. 2 shows IR spectra of precursor polymer film and PPV film. The IR stretch at 1097cm.’ in precursor polymer film is due to the methoxy leaving group (C-O-C stretching) and its disappearance was used to monitor the conversion of precursor polymer to PPV[3]. After an appropriate thermal the peak at 1097cm.* in the PPV film treatment, disappeared, which indicated that precursor polymer was converted into PPV.
3
350
450
550
650
wavelength (nm)
Fig. 3. W-Visible absorption spectra of (a) precursor polymer fihn(----), (b) PPV film prepared via heat treatment of precursor polymer film(-). The UV-Visible spectra of the precursor polymer and PPV films with a thermal treatment are shown in Fig. 3. The precursor polymer tihn showed a small peak at around 32Onm, while the PPV film exhibited large peaks in the longer wavelength mgion[4]. According to Fig. 2 and 3, it seemed that the precursor polymer was fully converted into PPV film. However, the photoluminescence spectrum in Fig. 4 indicates that the precursor polymer was not completely converted into PPV, because the main peak was not shifted to the wave length of 55Onm which is typical for a fully converted PPV[5]. Fig. 5 shows the STM image of HOPG and also a partially converted PPV LB film monolayer deposited on HOPG at a surface pressure of 2OmN/m. The STM image of HOPG shows a hexagonal lattice structure with a lattice constant of 2.5A. The STM image of a partially converted PPV LB film monolayer on HOPG does not show any hexagonal structure, but exhibits individual polymer chains which are parallel to each other. The distance between the centers of adjacent polymer chains is about 5A. Further details on the STM results will be published elsewhere.
wavelength (nm)
Fig. 4. Photoluminescence spectra of (a) precursor polymer tihn(----), (b) PPV fihu prepared via heat treatment of precursor polymer film. Measured at 295K with excitation at 35onm(--i
Fig. 5. Unfiltered STM images(2OA X2oA) of (a) HOPG substrate, (b) a partially converted PPV monolayer deposited at a surface pressure of 2OmN/m on HOPG substrate. 4. CONCLUSIONS It was shown that the formation of precursor polymer ultra thin films was well controlled by the LB technique. But, the precursor polymer films were not completely converted into PPV films. The microscopic characterization of a partially converted PPV monolayer on HOPG was studied by STM. An STM image shows that individual polymer chains are parallel to each other, which indicates that the using of LB technique can control the molecular orientation. REFERENCES 1. D.D.C.Bradley, Synthetic Metals, 54(1993) 401. 2. S.Tokito, T.Momii, H.Murata, T.Tsutsui and SSaito, Polymer, 31(1990) 1137. 3. D.D.C.Bradley, R.H.Friend, H.Lindenberger, and SRoth, Polymer, 27(1986) 1709. 4. T.Momii, S.Tokito, T.Tsutsui, and SSaito, Chemistry Letters, (1988) 1201. 5. I.D.W.Samuel, B.Crystall, G.Rumbles, P.L.Bum, A.B. Holmes and R.H.Friend, Synthetic Metals, 54(1993) 281.
ELSEVIER
Metals
71 (1995) 2023-2024
Preparation and characterization of poly(p-phenylene vinylene) Langmuir-Blodgett
films formed
via precursor method Jae Hwan Kim“, Young
Kwan Kim’, Byoung Chung Sohn’, Dou-Yol and Chong-Hong Pyund
Kangb, Jung-II Jin’, Chang-Hong
Kimd,
‘Dept. of Chemical Engineering, Hong-Ik University, Seoul 121-791, Korea bDept. of Electrical engineering, Hong-Ik University, Seoul 121-791, Korea “Dep. of Chemistry, Korea University, Seoul 136-701, Korea dKorea Advanced Institute of Science and Technology, Seoul 136-791, Korea Abstract Recent advances on n-conjugated conducting polymers for electroltinescence(EL) have shown a strong possibility for various display applications. In this paper, we will describe the preparation of ultra thin films of oil-soluble poly(p-phenylene vinylene) (PPV) precursor polymer by the Langmuir-Blodgett technique and by conversion into PPV films via a thermal treatment. The characterization of these films was also carried out by FT-IR spectroscopy, UV-VIS absorption spectroscopy, phuoluminescence (PL), and Scanning Tunneling Microscopy (STM). IT-IR, and W-VIS results indicated that multilayer films of PPV precursor polymer were formed well and the elimination of methoxy group in PPV precursor polymer was partially achieved by the thermal treatment. Red shift was observed in PL analysis after thermal treatment. An STM image of a PPV monolayer on graphite showing the polymer chains was observed 1. INTRODUCTION Conjugated polymers have been studied for their electrical conductivity. More recently conjugated polymers, in particular, PPV and its derivatives showed promise in optoelectronic applications, because these polymers can function as the emissive layer in electroluminescent devices using oil-soluble precursor made it possible to process the polymer into technologically useful thin film forms[l]. Recently, our group has been investigating the use of the Iangmuir-Blodgett technique as a means to control the molecular orientation and film thickness in ordered monolayers, so as to understand the basic mechanism of electroluminescence and other electrical properties. 2. EXPERIMENTAL The oil-soluble methoxy leaving group precursor polymer(l) shown below was prepared as reported elsewhere[2].
mrnol/L and filtered just before deposition. The LB films were deposited at a surface pressure of 20 mN/m, compression speed of 1 mN/m/min and dipping speed of 5 mm/min after spreading precursor polymer solution on deionized water at 2O’c. T&e LB films were deposited m Z-type, where the transfer ratio was 0.8. The obtained LB tihns of precursor polymer were heated up to 3ooC in a vacuum( 1Torr) for 12hrs. To follow the elimination of methoxy group, IR spectra were recorded on BIO-RAD Fl’S-10 spectrometer and UV-Visible spectra were recorded on HP 8452A diode army type spectrometer, respectively. The surface morphology of LB films after elimination was investigated by STM, model AutoProbe CP from PSI, operated in air at room temperature. For STM analysis, precursor polymer films were heated as mentioned above. 3. RESULTS
AND DISCUSSION
W-Visible spectra of precursor polymer with various thicknesses of monolayers are shown in Fig. 1. The absorbance at 230 and 325nm in Fig. 1, gives a good linear relationship between the number of layers and the absorlxmces. This suggests that the layer by layer deposition of precursor polymer films at molecular level was successfully controlled. PPV
.
LB films were deposited by using a computer-controlled Kuhn type trough (KSV model 3000). Distilled water as a subphase was purified by a h+iilli-Q Reagent Water System. For W and IR absorption spectroscopic analysis, polished quartz plates and ptype silicon w-afers(lOO) on one side were used as subsaates, and cleaned prior to polished deposition For STM analysis, Highly Oriented Pyrolytic Gtaphite (HOFG) was used as a substrate because its cleaved surface is flat on the atomic scale. Precursor polymer was dissolved in a chloform solution to a concentration of 1
0379-6779/95/$09.50
SSDI
0
1995 Elsevier
0379-6779(94)03147-X
Science
S.A. All rights
reserved
wavelength
(nm)
Fig. 1. UV-Visible absorption spectra of precursor films with 3, 6, and 9 LB layers on quartz.
polymer
J.H. Kim et al. / Synthetic Metals 71 (1995) 2023-2024
PW
I-
PW Precusor
.
500
15ocl
2500
3500
wavenumber
(cni’)
Pig. 2. FT-IR spectra of (a) precursor polymer film, (b) PPV film prepared via heat treatment of precursor polymer with 4-cm-’ resolution. Fig. 2 shows IR spectra of precursor polymer film and PPV film. The IR stretch at 1097cm.’ in precursor polymer film is due to the methoxy leaving group (C-O-C stretching) and its disappearance was used to monitor the conversion of precursor polymer to PPV[3]. After an appropriate thermal the peak at 1097cm.* in the PPV film treatment, disappeared, which indicated that precursor polymer was converted into PPV.
3
350
450
550
650
wavelength (nm)
Fig. 3. W-Visible absorption spectra of (a) precursor polymer fihn(----), (b) PPV film prepared via heat treatment of precursor polymer film(-). The UV-Visible spectra of the precursor polymer and PPV films with a thermal treatment are shown in Fig. 3. The precursor polymer tihn showed a small peak at around 32Onm, while the PPV film exhibited large peaks in the longer wavelength mgion[4]. According to Fig. 2 and 3, it seemed that the precursor polymer was fully converted into PPV film. However, the photoluminescence spectrum in Fig. 4 indicates that the precursor polymer was not completely converted into PPV, because the main peak was not shifted to the wave length of 55Onm which is typical for a fully converted PPV[5]. Fig. 5 shows the STM image of HOPG and also a partially converted PPV LB film monolayer deposited on HOPG at a surface pressure of 2OmN/m. The STM image of HOPG shows a hexagonal lattice structure with a lattice constant of 2.5A. The STM image of a partially converted PPV LB film monolayer on HOPG does not show any hexagonal structure, but exhibits individual polymer chains which are parallel to each other. The distance between the centers of adjacent polymer chains is about 5A. Further details on the STM results will be published elsewhere.
wavelength (nm)
Fig. 4. Photoluminescence spectra of (a) precursor polymer tihn(----), (b) PPV fihu prepared via heat treatment of precursor polymer film. Measured at 295K with excitation at 35onm(--i
Fig. 5. Unfiltered STM images(2OA X2oA) of (a) HOPG substrate, (b) a partially converted PPV monolayer deposited at a surface pressure of 2OmN/m on HOPG substrate. 4. CONCLUSIONS It was shown that the formation of precursor polymer ultra thin films was well controlled by the LB technique. But, the precursor polymer films were not completely converted into PPV films. The microscopic characterization of a partially converted PPV monolayer on HOPG was studied by STM. An STM image shows that individual polymer chains are parallel to each other, which indicates that the using of LB technique can control the molecular orientation. REFERENCES 1. D.D.C.Bradley, Synthetic Metals, 54(1993) 401. 2. S.Tokito, T.Momii, H.Murata, T.Tsutsui and SSaito, Polymer, 31(1990) 1137. 3. D.D.C.Bradley, R.H.Friend, H.Lindenberger, and SRoth, Polymer, 27(1986) 1709. 4. T.Momii, S.Tokito, T.Tsutsui, and SSaito, Chemistry Letters, (1988) 1201. 5. I.D.W.Samuel, B.Crystall, G.Rumbles, P.L.Bum, A.B. Holmes and R.H.Friend, Synthetic Metals, 54(1993) 281.