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Micro- and nanoscale patterning of luminescent conjugated polymer films
ELSEVIER
Synthetic
Metals
102 (1999)
1042-1045
Micro- and Nanoscale Patterning of Luminescent Conjugated Polymer Films Magnus Granstrom Cavendish
Laboratory,
Dept.
of Physics,
University
of Cambridge,
Madingley
Road,
Cambridge
CB3
OHE,
U.K.
Abstract A luminescent conjugated polymer, poly(phenylene vinylene) (PPV), has been patterned on a sub-micron scale using a modified scanning force microscope (SFM). By using this technique, it is possible to make non-luminescent patterns with linewidths down to -20 nm, and this report shows that the patterning process does not affect the photoluminescence efficiency of the nonpatterned parts of the polymer film. In addition to this, the method used in this work allows for precise control of the patterning, making it possible to create intricate patterns in a neat polymer film. Keywords:
Poly(phenylene microscopy
vinylene) and derivatives, Photoluminescence,
1. Introduction The use of conjugated polymers as emitting materials in light emitting diodes (LEDs) has a number of different advantages, such as ease of fabrication and low cost. Polymer LEDs also exhibit efficiencies on a level unobtainable from large area depositable inorganic materials. In order to make high resolution displays from conjugated polymers, it is necessary to be able to define the size and shape of each pixel. It is shown here that it is possible to obtain micron and submicron sized luminescent areas in a neat polymer film, and that these areas can be given any desired shape. Nano- and micrometer sized LEDs have been demonstrated previously by using small contacts [l] or blends [2, 31, but these have all been randomly distributed. Using photolithographic methods, it is possible to control the position and size of the pixels down to a few microns [4, 51. In order to avoid the photopatterning process that often results in a chemical modification of the polymer and a subsequent loss in efficiency, the approach here has been to modify the polymer film in a reasonably direct and nondestructive way. To be able to do this, scanning probe microscopy techniques have been utilised, since these allow for extremely well controlled lateral positioning and movements. In addition to this, the use of a combined scanning force (SFM) and scanning tunnelling microscope (STM) makes it possible to modify a polymer precursor film locally at the position of the tip by applying a voltage between a metallised tip and the underlying substrate. This allows for a local influence of the 0379.6779/99/$ see front matter PII: SO379-6779(98)00228-S
0 1999 Elsevier
Science
S.A.
All rights
Scanning tunnelling
microscopy, Atomic force
polymer properties, resulting in non-luminescent areas in the converted film. In a similar fashion, luminescent polymer films have been patterned by Wei et al. using another of the scanning probe microscopy techniques, scanning near-field optical microscopy, SNOM [6], reaching a resolution of 0.1 pm. Various other methods have also been used to pattern polymer films, such as laser processing of electrically insulating precursor polymers or a UV/laser sensitive precursor composite into conducting patterns [7]. Electrically conducting patterns can also be created by an electrochemical deposition of a conjugated polymer on a pre-patterned substrate [8]. A problem with the latter methods with respect to luminescent devices is that the polymers are formed in their conducting state, and thus need to be undoped in order to be used as luminescent materials. This often results in lower quality and less pure polymer films, rendering them unsuitable for luminescent devices.
2. Experimental In these experiments, the well known luminescent polymer poly(phenylene vinylene), PPV, has been used, see Figure 1. This conjugated polymer is formed from a solution processible precursor polymer, which can be spin-coated from a methanol solution. This was done on IT0 (indium tin oxide) covered glass or on evaporated gold contacts on silicon or glass. The precursor polymer samples were then mounted in a NanoScope reserved.
M. Granstrtim
I Synthetic
II (Digital Instruments Inc., Santa Barbara), which has been slightly modified in order to use metallised tips in a scanning force head, allowing for the application of a voltage at desired tip positions.
AH
+
Metals
102 (1999)
1042-1045
a)
>
HCI
+
Fig. 1. Conversion process from the sulfonium polyelectrolyte precursor to the conjugated poly(phenylene vinylene) When the SFM patterning of the films was done, they were transferred to a conversion oven (250°C in vucuo, 10 h) for the conversion to the conjugated PPV. Photoluminescence (PL) efficiencies of the films were measured in an integrating sphere, coupled via a liquid light guide to an Oriel InstaSpec IV spectrophotometer, and imaging of the emissive patterned films was done with a fluorescence microscope (Vickers Photoplan M41). Raman spectroscopy was performed using a Renishaw Ramascope Imaging Raman Microscope, connected to an Olympus BH-2 optical microscope. Excitation of the samples for Raman and PL measurements was made with a Spectra-Physics Model 127 HeNe (632.8 nm) laser and an Omnichrome 543 Argon ion (488 nm) laser.
C)
3. Results and Discussion In order to study the behaviour of the non-converted PPV precursor films under the influence of applied electric fields, several preliminary experiments were performed, where areas of a precursor films were exposed to different field strengths. It was found that a field of the order of 10’ V/m caused enough damage to the film to quench the luminescence. When applying stronger fields, the damaged area enlarges drastically, resulting in uncontrollable shapes. Figure 2 shows SFM images, before and after a voltage has been applied in a square frame pattern, surrounding an undestroyed middle square. To pattern the precursor film, the tip was positioned using the XN movement screws on the microscope head. As can been seen in Figure 2b, there is a topographic difference between the areas to which the voltage has and has not been applied, respectively. Figure 2c shows the same area after conversion. The height differences have become less apparent, showing the well-known shrinkage during conversion of the PPV precursor into PPV, usually resulting in a decrease of the film thickness by 40 to 50%.
Fig. 2. a) Scanning force microscopy image of precursor film before patterning. b) Film after patterning. c) Film topography after conversion.
The behaviour observed during patterning and subsequent heat treatment is regarded as an indication that exposure to an electric field results in conversion of the precursor. Figure 3 shows the same area of the sample in an optical microscope, under UV light, clearly showing that the areas exposed to the
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1044
M. Granstriim
I Synthetic
electric field have been converted in such a way that the photoluminescence is quenched. To show the possibilities and versatility of this method, much smaller and more intricate patterns have been made. An example of this is shown in Figure 4, where the tip positioning has been controlled by software, using the piezo tubes in the scanning force microscope head. In order to resolve a pattern in an optical microscope, the features have to be larger than the diffraction limit (about 270 nm); this means that the patterns in Figure 4 cannot be detected in an ordinary optical microscope.
Metals
102 (1999)
1042-1045
no change in the photoluminescence response between the reference sample and the non-patterned areas of the patterned, converted film. Figure 5 shows Raman spectra, taken with a Raman microscope with a lateral resolution of 1 pm, of the converted reference sample and also patterned and nonpatterned areas of a converted sample like that shown in Figures 2 and 3. The reference and non-patterned spectra are very similar, both resembling other Raman spectra of PPV [912]. The spectrum measured in the patterned area shows some distinct differences. The main differences are found above 1700 cm-’ and at 1550 cm-‘.
Fig. 3. Optical image of UV excited film.
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Raman Shift (cm-‘) Fig. 5. Raman spectra of the reference sample (top), nonpatterned (middle) and patterned (bottom) areas of the treated sample. Both samples have been converted.
nm
Fig. 4. SFM image of a film patterned on a smaller scale than in figure 2.
To make sure that the patterning process does not affect the non-exposed PPV areas, a reference sample was prepared by spin-coating the precursor and converting it to PPV without any patterning process. In this way, photoluminescence spectra could be compared, and these measurements show that there is
In order to interpret these differences, similar measurements were performed on the precursor film, before and after the patterning process, but prior to conversion under vacuum. The results of these measurements are shown in Figure 6, where the dominating peaks of the precursor spectrum (top curve) are found at 1184, 1608, and 1640 cm-‘. These are all assigned to the phenyl ring, the first one to the C-H stretching, and the others the C=C. The Raman spectrum for the E-field exposed area (shown in the bottom half of Figure 6) bears a striking resemblance to the spectrum shown at the bottom of Figure 5. From this, two conclusions can be drawn, first that the patterning process results in conversion of the precursor to a more conjugated film, and second that the resonance at 1552 cm-’ is due to a vinylene stretching, since this peak is virtually absent in the precursor spectrum.
M. Granstriim
I Synthetic
Metals
102 (1999)
1042-1045
ment of light and the formation gap structures.
1045 of well defined photonic band-
4. Conclusions To sumrnarise, these results show that it is possible to pattern a luminescent conjugated polymer, PPV, on a submicron scale, and that this patterning does not affect the photoluminescence efficiency of the non-patterned part of the film. Using this technique, it is possible to make nonluminescent patterns in an otherwise luminescent neat polymer film with a spatial linewidth down to -20 nm.
5. Acknowledgements
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The PPV precursor polymer was supplied by Cambridge Display Technology Ltd, Cambridge, U.K., and financial support in the form of a European Commission TMR Marie Curie Fellowship is gratefully acknowledged. The author would like to thank Prof. R.H. Friend and Dr. M.J. Watson for fruitful discussions, and the Microstructural Physics group at the Cavendish Laboratory for the use of the modified scanning force microscope
Raman Shift (cm“) Fig. 6. Raman spectra of the non-converted precursor film (top) prior to patterning, and (bottom) the patterned area.
A possible explanation for the increase of Raman intensity above 1700 cm-’ and the decrease (as seen in Figure 5) of the vinylene signal could be a high carbonyl content in the patterned areas, since it is known that the vinylene bond is susceptible to oxygen attacks. This also corresponds very well with the quenched photoluminescence intensities, as was shown by Papadimitrakopoulos er al. [13]. The results therefore indicate that the application of an electric field induces enough heat into the polymer film to start a localised thermal conversion. Since the patterning is done in the presence of oxygen, the PPV created in this process is likely to contain a large amount of carbonyl defects, hence the low photoluminescence efficiency. The areas where no voltage is applied will then be converted under vacuum and contain fewer defects. Similar results, but on a much longer length-scale, have been achieved by exposing areas of a PPV oligomer film to UV light [14]. The quenching of the luminescence in that case is also attributed to attacks on the vinyl bonds, although not necessarily in the form of oxygen. Some of the main advantages of the method outlined here, are that it is possible to reach dimensions smaller than the wavelength of visible light and to reach an extremely high lateral resolution, thereby opening new prospects for confine-
6. References [l]
M. Granstrom, M. Berggren, 0. Inganls, Science 267 (1995) 1479. [2] M. Granstrom, 0. Inganh, Adv. Mater. 7 (1995) 1012. [3] C. Adachi, S. Hibino, T. Koyama, Y. Taniguchi, Jap. J. Appl. Phys. Part 2 - Lett. 36 (1997) L827. [4] D. G. Lidzey, M. A. Pate, M. S. Weaver, T. A. Fisher, D. D. C. Bradley, Synth. Met. 82 (1996) 141. [5] M. L. Rem&, G. C. Bazan, D. Roitman, Adv. Mater. 9 (1997) 392. [6] P. K. Wei, J. H. Hsu, B. R. Hsieh, W. S. Fann, Adv. Mater. 8 (1996) 573. [7] R. Baumann, J. Bargon, Appl. Surf. Sci. 106 (1996) 287. [8] L. M. Dai, A. W. H. Mau, X. Y. Gong, H. J. Griesser, Synth. Met. 85 (1997) 1379. [9] S. Lefrant, E. Perrin, J. P. Buisson, H. Eckhardt, C. C. Han, Synth. Met. 29 (1989) E 91. [lo] B. Tian, G. Zerbi, K. Mullen, J. Chem. Phys. 95 (1991) 3198. [ 1 l] J. P. Buisson, J. Y. Mevellec, S. Zeraoui, S. Lefrant, Synth. Met. 41-43 (1991) 287. [12] S. Webster, D. N. Batchelder, Polymer 37 (1996) 4961. [ 131 F. Papadimitrakopoulos, K. Konstadinidis, T. M. Miller, R. Opila, E. A. Chandross, M. E. Galvin, Chem. Mater. 6 (1994) 1563. [14] V. E. Choong, Y. Park, B. R. Hsieh, Y. Gao, Phys. Rev. B - Cond. Matt. 55 (1997) 15460.
Synthetic
Metals
102 (1999)
1042-1045
Micro- and Nanoscale Patterning of Luminescent Conjugated Polymer Films Magnus Granstrom Cavendish
Laboratory,
Dept.
of Physics,
University
of Cambridge,
Madingley
Road,
Cambridge
CB3
OHE,
U.K.
Abstract A luminescent conjugated polymer, poly(phenylene vinylene) (PPV), has been patterned on a sub-micron scale using a modified scanning force microscope (SFM). By using this technique, it is possible to make non-luminescent patterns with linewidths down to -20 nm, and this report shows that the patterning process does not affect the photoluminescence efficiency of the nonpatterned parts of the polymer film. In addition to this, the method used in this work allows for precise control of the patterning, making it possible to create intricate patterns in a neat polymer film. Keywords:
Poly(phenylene microscopy
vinylene) and derivatives, Photoluminescence,
1. Introduction The use of conjugated polymers as emitting materials in light emitting diodes (LEDs) has a number of different advantages, such as ease of fabrication and low cost. Polymer LEDs also exhibit efficiencies on a level unobtainable from large area depositable inorganic materials. In order to make high resolution displays from conjugated polymers, it is necessary to be able to define the size and shape of each pixel. It is shown here that it is possible to obtain micron and submicron sized luminescent areas in a neat polymer film, and that these areas can be given any desired shape. Nano- and micrometer sized LEDs have been demonstrated previously by using small contacts [l] or blends [2, 31, but these have all been randomly distributed. Using photolithographic methods, it is possible to control the position and size of the pixels down to a few microns [4, 51. In order to avoid the photopatterning process that often results in a chemical modification of the polymer and a subsequent loss in efficiency, the approach here has been to modify the polymer film in a reasonably direct and nondestructive way. To be able to do this, scanning probe microscopy techniques have been utilised, since these allow for extremely well controlled lateral positioning and movements. In addition to this, the use of a combined scanning force (SFM) and scanning tunnelling microscope (STM) makes it possible to modify a polymer precursor film locally at the position of the tip by applying a voltage between a metallised tip and the underlying substrate. This allows for a local influence of the 0379.6779/99/$ see front matter PII: SO379-6779(98)00228-S
0 1999 Elsevier
Science
S.A.
All rights
Scanning tunnelling
microscopy, Atomic force
polymer properties, resulting in non-luminescent areas in the converted film. In a similar fashion, luminescent polymer films have been patterned by Wei et al. using another of the scanning probe microscopy techniques, scanning near-field optical microscopy, SNOM [6], reaching a resolution of 0.1 pm. Various other methods have also been used to pattern polymer films, such as laser processing of electrically insulating precursor polymers or a UV/laser sensitive precursor composite into conducting patterns [7]. Electrically conducting patterns can also be created by an electrochemical deposition of a conjugated polymer on a pre-patterned substrate [8]. A problem with the latter methods with respect to luminescent devices is that the polymers are formed in their conducting state, and thus need to be undoped in order to be used as luminescent materials. This often results in lower quality and less pure polymer films, rendering them unsuitable for luminescent devices.
2. Experimental In these experiments, the well known luminescent polymer poly(phenylene vinylene), PPV, has been used, see Figure 1. This conjugated polymer is formed from a solution processible precursor polymer, which can be spin-coated from a methanol solution. This was done on IT0 (indium tin oxide) covered glass or on evaporated gold contacts on silicon or glass. The precursor polymer samples were then mounted in a NanoScope reserved.
M. Granstrtim
I Synthetic
II (Digital Instruments Inc., Santa Barbara), which has been slightly modified in order to use metallised tips in a scanning force head, allowing for the application of a voltage at desired tip positions.
AH
+
Metals
102 (1999)
1042-1045
a)
>
HCI
+
Fig. 1. Conversion process from the sulfonium polyelectrolyte precursor to the conjugated poly(phenylene vinylene) When the SFM patterning of the films was done, they were transferred to a conversion oven (250°C in vucuo, 10 h) for the conversion to the conjugated PPV. Photoluminescence (PL) efficiencies of the films were measured in an integrating sphere, coupled via a liquid light guide to an Oriel InstaSpec IV spectrophotometer, and imaging of the emissive patterned films was done with a fluorescence microscope (Vickers Photoplan M41). Raman spectroscopy was performed using a Renishaw Ramascope Imaging Raman Microscope, connected to an Olympus BH-2 optical microscope. Excitation of the samples for Raman and PL measurements was made with a Spectra-Physics Model 127 HeNe (632.8 nm) laser and an Omnichrome 543 Argon ion (488 nm) laser.
C)
3. Results and Discussion In order to study the behaviour of the non-converted PPV precursor films under the influence of applied electric fields, several preliminary experiments were performed, where areas of a precursor films were exposed to different field strengths. It was found that a field of the order of 10’ V/m caused enough damage to the film to quench the luminescence. When applying stronger fields, the damaged area enlarges drastically, resulting in uncontrollable shapes. Figure 2 shows SFM images, before and after a voltage has been applied in a square frame pattern, surrounding an undestroyed middle square. To pattern the precursor film, the tip was positioned using the XN movement screws on the microscope head. As can been seen in Figure 2b, there is a topographic difference between the areas to which the voltage has and has not been applied, respectively. Figure 2c shows the same area after conversion. The height differences have become less apparent, showing the well-known shrinkage during conversion of the PPV precursor into PPV, usually resulting in a decrease of the film thickness by 40 to 50%.
Fig. 2. a) Scanning force microscopy image of precursor film before patterning. b) Film after patterning. c) Film topography after conversion.
The behaviour observed during patterning and subsequent heat treatment is regarded as an indication that exposure to an electric field results in conversion of the precursor. Figure 3 shows the same area of the sample in an optical microscope, under UV light, clearly showing that the areas exposed to the
1043
1044
M. Granstriim
I Synthetic
electric field have been converted in such a way that the photoluminescence is quenched. To show the possibilities and versatility of this method, much smaller and more intricate patterns have been made. An example of this is shown in Figure 4, where the tip positioning has been controlled by software, using the piezo tubes in the scanning force microscope head. In order to resolve a pattern in an optical microscope, the features have to be larger than the diffraction limit (about 270 nm); this means that the patterns in Figure 4 cannot be detected in an ordinary optical microscope.
Metals
102 (1999)
1042-1045
no change in the photoluminescence response between the reference sample and the non-patterned areas of the patterned, converted film. Figure 5 shows Raman spectra, taken with a Raman microscope with a lateral resolution of 1 pm, of the converted reference sample and also patterned and nonpatterned areas of a converted sample like that shown in Figures 2 and 3. The reference and non-patterned spectra are very similar, both resembling other Raman spectra of PPV [912]. The spectrum measured in the patterned area shows some distinct differences. The main differences are found above 1700 cm-’ and at 1550 cm-‘.
Fig. 3. Optical image of UV excited film.
400
-I 1100
1200
1300
1400
1500
1600
1700
1600
Raman Shift (cm-‘) Fig. 5. Raman spectra of the reference sample (top), nonpatterned (middle) and patterned (bottom) areas of the treated sample. Both samples have been converted.
nm
Fig. 4. SFM image of a film patterned on a smaller scale than in figure 2.
To make sure that the patterning process does not affect the non-exposed PPV areas, a reference sample was prepared by spin-coating the precursor and converting it to PPV without any patterning process. In this way, photoluminescence spectra could be compared, and these measurements show that there is
In order to interpret these differences, similar measurements were performed on the precursor film, before and after the patterning process, but prior to conversion under vacuum. The results of these measurements are shown in Figure 6, where the dominating peaks of the precursor spectrum (top curve) are found at 1184, 1608, and 1640 cm-‘. These are all assigned to the phenyl ring, the first one to the C-H stretching, and the others the C=C. The Raman spectrum for the E-field exposed area (shown in the bottom half of Figure 6) bears a striking resemblance to the spectrum shown at the bottom of Figure 5. From this, two conclusions can be drawn, first that the patterning process results in conversion of the precursor to a more conjugated film, and second that the resonance at 1552 cm-’ is due to a vinylene stretching, since this peak is virtually absent in the precursor spectrum.
M. Granstriim
I Synthetic
Metals
102 (1999)
1042-1045
ment of light and the formation gap structures.
1045 of well defined photonic band-
4. Conclusions To sumrnarise, these results show that it is possible to pattern a luminescent conjugated polymer, PPV, on a submicron scale, and that this patterning does not affect the photoluminescence efficiency of the non-patterned part of the film. Using this technique, it is possible to make nonluminescent patterns in an otherwise luminescent neat polymer film with a spatial linewidth down to -20 nm.
5. Acknowledgements
1100
1200
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1400
1500
1600
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1600
The PPV precursor polymer was supplied by Cambridge Display Technology Ltd, Cambridge, U.K., and financial support in the form of a European Commission TMR Marie Curie Fellowship is gratefully acknowledged. The author would like to thank Prof. R.H. Friend and Dr. M.J. Watson for fruitful discussions, and the Microstructural Physics group at the Cavendish Laboratory for the use of the modified scanning force microscope
Raman Shift (cm“) Fig. 6. Raman spectra of the non-converted precursor film (top) prior to patterning, and (bottom) the patterned area.
A possible explanation for the increase of Raman intensity above 1700 cm-’ and the decrease (as seen in Figure 5) of the vinylene signal could be a high carbonyl content in the patterned areas, since it is known that the vinylene bond is susceptible to oxygen attacks. This also corresponds very well with the quenched photoluminescence intensities, as was shown by Papadimitrakopoulos er al. [13]. The results therefore indicate that the application of an electric field induces enough heat into the polymer film to start a localised thermal conversion. Since the patterning is done in the presence of oxygen, the PPV created in this process is likely to contain a large amount of carbonyl defects, hence the low photoluminescence efficiency. The areas where no voltage is applied will then be converted under vacuum and contain fewer defects. Similar results, but on a much longer length-scale, have been achieved by exposing areas of a PPV oligomer film to UV light [14]. The quenching of the luminescence in that case is also attributed to attacks on the vinyl bonds, although not necessarily in the form of oxygen. Some of the main advantages of the method outlined here, are that it is possible to reach dimensions smaller than the wavelength of visible light and to reach an extremely high lateral resolution, thereby opening new prospects for confine-
6. References [l]
M. Granstrom, M. Berggren, 0. Inganls, Science 267 (1995) 1479. [2] M. Granstrom, 0. Inganh, Adv. Mater. 7 (1995) 1012. [3] C. Adachi, S. Hibino, T. Koyama, Y. Taniguchi, Jap. J. Appl. Phys. Part 2 - Lett. 36 (1997) L827. [4] D. G. Lidzey, M. A. Pate, M. S. Weaver, T. A. Fisher, D. D. C. Bradley, Synth. Met. 82 (1996) 141. [5] M. L. Rem&, G. C. Bazan, D. Roitman, Adv. Mater. 9 (1997) 392. [6] P. K. Wei, J. H. Hsu, B. R. Hsieh, W. S. Fann, Adv. Mater. 8 (1996) 573. [7] R. Baumann, J. Bargon, Appl. Surf. Sci. 106 (1996) 287. [8] L. M. Dai, A. W. H. Mau, X. Y. Gong, H. J. Griesser, Synth. Met. 85 (1997) 1379. [9] S. Lefrant, E. Perrin, J. P. Buisson, H. Eckhardt, C. C. Han, Synth. Met. 29 (1989) E 91. [lo] B. Tian, G. Zerbi, K. Mullen, J. Chem. Phys. 95 (1991) 3198. [ 1 l] J. P. Buisson, J. Y. Mevellec, S. Zeraoui, S. Lefrant, Synth. Met. 41-43 (1991) 287. [12] S. Webster, D. N. Batchelder, Polymer 37 (1996) 4961. [ 131 F. Papadimitrakopoulos, K. Konstadinidis, T. M. Miller, R. Opila, E. A. Chandross, M. E. Galvin, Chem. Mater. 6 (1994) 1563. [14] V. E. Choong, Y. Park, B. R. Hsieh, Y. Gao, Phys. Rev. B - Cond. Matt. 55 (1997) 15460.