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Laser ablation of conjugated polymers
ELSEVIER
Synthetic Metals 101 (1999) 234-235
Laser ablation of conjugated polymers M.A. Stevens, l3.A. Weir, G.J. Denton, R.H. Friend Cavendish Laboratory,
UniversiQ of Cambridge, Madingley Road, Cambridge CB3 OHE, UK
Abstract The debris from laser ablation of conjugated polymers can be collected on a substrate to form thin films of the target material. We have deposited thin films of the conjugated polymer poly@-phenylenevinylene), PPV, by laser ablation of a PPV target film. The photoluminescence (PL) spectra of films deposited in this way are very similar to those of pristine, spin-cast material. The UV-visible and infrared absorption spectra of the films produced by ablation also show features characteristic of PPV. We also observe gratings etched onto the surfaces of target films. These grating are-due to interference effects and have the potential to be used in photonic device structures based on PPV or other conjugated polymers. Keywords: Laser ablation, Poly(phenylene
vinylene), Photoluminescence,
1. Introduction Laser ablation of polymers is a potentially useful technique inseveral areas of polymer processing. Laser etching of polymer surfaces with high-intensity laser beams has been the subject of much work [l-4] because this technique has advantages over traditional methods of lithography. It has also been found that the material ejected from the polymer surface during ablation can form a film when collected on a substrate [5-S]. By using a laser wavelength which is strongly absorbed by the polymer and an intensity which is not many times greater than the ablation threshold, the deposited films can be of good optical quality and have compositions similar to those of the target polymer. Recent work has shown that highly oriented polymer films can be deposited using laser ablation [9]. In this paper we describe how laser ablation can be used to deposit thin films of PPV. This method of film deposition is potentially useful for intractable polymers such as PPV, which otherwise require solution processing of a precursor followed by thermal conversion. We have also observed gratings etched onto the surface of the abated target and attribute these to interference effects.
Gratings
Diffraction
deposited films are of good optical quality, while those formed by ablation at poorly absorbed wavelengths are powdery and do not adhere to the substrate well. Thus we consider 35.5 nm a good wavelength for ablation of PPV. The target films were prepared by drop-casting PPV precursor polymer onto spectrosil substrates. The precursor was either ablated directly or thermally converted to produce PPV films which were then ablated. The products of ablation were collected on spectrosil substrates for use in the absorption and PL measurements. The surface of an ablated PPV target was examined using scanning electron microscopy @EM). The target was coated with -30 nm of gold prior to exammation. 3. Results and discussion The absorption and luminescence spectra of the films deposited by ablation are shown in Figures 1 and 2. In both the absorption 2.0
’
matter 0 1999 Elsevier
Science S.A.
1
I
’
. -Ablated ....-.
I
I
Precursor
Ablated
-Spun
1.5 -\ 8
0379-67791991s - see front PII: so379-6779(98)01293-4
’
:
Converted
Film
Film
f\
2. Methods The frequency tripled output from a Q-switched Nd:YAG laser (355 nm, 10 Hz repetition rate, 15 ns pulse duration) was directed onto a target film held in vacuum (
’
0.0
LB
200
’
’
300
1
I
,
400 Wavelength
1
500
I
I
600
(nm)
Fig. 1. LJV-visible absorption spectra from films of PPV formed by: spin-coating; ablation of PPV; ablation of PPV precursor.
All rights reserved.
M.A.
I
et al. I Synthetic
Precursor Converted
550 600 650 Wavelength (nm)
500
(1999)
234-235
235
700
and the PL spectra, the films deposited by ablation of either converted PPV or PPV precursor are very similar, indicating that the ablation process converts the precursor. The blue-shifted absorption edge points towards a shortening of the average conjugation length in deposited films. However, the peaks in the photoluminescence spectra of the deposited films are in almost the same position as in the spun film. This indicates that some long conjugation length segments survive the ablation process intact and are deposited on the substrate. IR absorption spectra of the deposited films (not shown) indicate that their composition is similar to the pristine material. The mechanism for laser ablation of polymers depends on the absorption of the polymer at the laser wavelength. There are two proposed ablation mechanisms [lo]: photochemical and photothermal. Photochemical ablation is prevalent with UV irradiation, which tends to be strongly absorbed by polymers. Direct bond breaking occurs and the fragments are ejected from the surface. Clean etching of the target surface and deposition of films which have the same chemical composition as the target are features of photochemical ablation. In the photothermal model, ener,T is transferred into the vibrational modes of the molecule, which leads to heating and subsequent evaporation. Photothermal ablation prevails at longer wavelengths and results in charring and melting of the polymer target. Figure 3 shows an SEM image of the surface of an ablated target film of converted PPV. The grooves of the grating are formed perpendicular to the direction of the incident beam. From this and other images, the grating period was measured to be 1.3 ?- 0.1 urn. There have previously been reports of gratings on the surfaces of ablated polymers [ 11,121. Akhmanov et al. [ 131 have shown that the grating separation can be predicted using a model where the incident beam interferes with an in-plane surface wave. For an s-polarized beam (as was used in the present experiments) they show that the grating period (A,) is given by: a l-sin9
101
Film
Fig. 2. PL spectra from films of PPV formed spin-coating and by laser ablation.
IL=-
Metals
I
* -Ablated .....-.. Ablated
450
Stevens
(1)
where h is the laser wavelength and 0 is the angle of incidence. For the setup used in our experiments, this expression predicts that A, = 1.21 pm, in good agreement with the measured value,
Fig. 3. SEM image of the surface of an ablated target PPV film. It can be seen from Equation 1 that the grating separation can be varied easily to suit the requirements of a particular application. This factor, along with the simplicity of fabrication, could make this method of direct writing of diffraction gratings onto conducting polymers very useful in device applications. 4. Conclusions Laser ablation of a PPV target has been used to deposit thin films of this conjugated polymer. The luminescence and absorption spectra show features which are characteristic of PPV. We observe grating structures on the surface of target films which result from interference effects. 5. Acknowledgements We would like to thank Cambridge Display Technology Ltd. (Cambridge, UK) for the precursor polymer used in this work. 6. References [l] J.H. Brannon, J.R. Lankard, AI. Baise, F. Burns, J. Kaufman, J. Appl. Phys. 58 (1985) 2036. [2] Y.T.C. Yeh, J. Vat. Sci. Technol. A 4 (1986) 653. [3] L.S. Van Dyke, C.J. Brumlik, CR. Martin, Z. Yu, G.J. Collins, Synth. Met. 52 (1992) 299. [4] T. Lippert, A. Yabe, A. Wokaun, Adv. Mater. 9 (1997) 105. [5] G.B. Blanchet, Appl. Phys. Lett. 62 (1993) 479. [6] G.B. Blanchet, CR. Fincher, CL. Jackson, S.I. Shah, K.H. Gardner, Science 262 (1993) 719. [7] N. Matsumoto, H. Shima, T. Fujii, F. Kannari, Appl. Phys. Lett. 71 (1997) 2469. [8] S.G. Hansen, T.E. Robitaille, Appl. Phys. Lett. 52 (1988) 81. [9] Q. Luo, X. Chen, Z. Liu, Z. Sun, N. Ming, Appl. Surf. Sci. 108 (1997) 89. [lo] B.J. Garrison, R. Srinivasan, J. Appl. Phys. 57 (1985) 2909. [ll] P.E. Dyer, R.J. Farley, J. Appl. Phys. 74 (1993) 1442. [ 121 P.E. Dyer, R.J. Farley, R. Giedl, D.M. Kamakis, Appl. Surf. Sci. 96-98 (1996) 537. 1131 S.A. Akhmanov, VLEmel’yanov, N.I. Korote~ev, V.N. Seminogov, Sov. Phys. Usp. 28 (1985) 1084.
Synthetic Metals 101 (1999) 234-235
Laser ablation of conjugated polymers M.A. Stevens, l3.A. Weir, G.J. Denton, R.H. Friend Cavendish Laboratory,
UniversiQ of Cambridge, Madingley Road, Cambridge CB3 OHE, UK
Abstract The debris from laser ablation of conjugated polymers can be collected on a substrate to form thin films of the target material. We have deposited thin films of the conjugated polymer poly@-phenylenevinylene), PPV, by laser ablation of a PPV target film. The photoluminescence (PL) spectra of films deposited in this way are very similar to those of pristine, spin-cast material. The UV-visible and infrared absorption spectra of the films produced by ablation also show features characteristic of PPV. We also observe gratings etched onto the surfaces of target films. These grating are-due to interference effects and have the potential to be used in photonic device structures based on PPV or other conjugated polymers. Keywords: Laser ablation, Poly(phenylene
vinylene), Photoluminescence,
1. Introduction Laser ablation of polymers is a potentially useful technique inseveral areas of polymer processing. Laser etching of polymer surfaces with high-intensity laser beams has been the subject of much work [l-4] because this technique has advantages over traditional methods of lithography. It has also been found that the material ejected from the polymer surface during ablation can form a film when collected on a substrate [5-S]. By using a laser wavelength which is strongly absorbed by the polymer and an intensity which is not many times greater than the ablation threshold, the deposited films can be of good optical quality and have compositions similar to those of the target polymer. Recent work has shown that highly oriented polymer films can be deposited using laser ablation [9]. In this paper we describe how laser ablation can be used to deposit thin films of PPV. This method of film deposition is potentially useful for intractable polymers such as PPV, which otherwise require solution processing of a precursor followed by thermal conversion. We have also observed gratings etched onto the surface of the abated target and attribute these to interference effects.
Gratings
Diffraction
deposited films are of good optical quality, while those formed by ablation at poorly absorbed wavelengths are powdery and do not adhere to the substrate well. Thus we consider 35.5 nm a good wavelength for ablation of PPV. The target films were prepared by drop-casting PPV precursor polymer onto spectrosil substrates. The precursor was either ablated directly or thermally converted to produce PPV films which were then ablated. The products of ablation were collected on spectrosil substrates for use in the absorption and PL measurements. The surface of an ablated PPV target was examined using scanning electron microscopy @EM). The target was coated with -30 nm of gold prior to exammation. 3. Results and discussion The absorption and luminescence spectra of the films deposited by ablation are shown in Figures 1 and 2. In both the absorption 2.0
’
matter 0 1999 Elsevier
Science S.A.
1
I
’
. -Ablated ....-.
I
I
Precursor
Ablated
-Spun
1.5 -\ 8
0379-67791991s - see front PII: so379-6779(98)01293-4
’
:
Converted
Film
Film
f\
2. Methods The frequency tripled output from a Q-switched Nd:YAG laser (355 nm, 10 Hz repetition rate, 15 ns pulse duration) was directed onto a target film held in vacuum (
’
0.0
LB
200
’
’
300
1
I
,
400 Wavelength
1
500
I
I
600
(nm)
Fig. 1. LJV-visible absorption spectra from films of PPV formed by: spin-coating; ablation of PPV; ablation of PPV precursor.
All rights reserved.
M.A.
I
et al. I Synthetic
Precursor Converted
550 600 650 Wavelength (nm)
500
(1999)
234-235
235
700
and the PL spectra, the films deposited by ablation of either converted PPV or PPV precursor are very similar, indicating that the ablation process converts the precursor. The blue-shifted absorption edge points towards a shortening of the average conjugation length in deposited films. However, the peaks in the photoluminescence spectra of the deposited films are in almost the same position as in the spun film. This indicates that some long conjugation length segments survive the ablation process intact and are deposited on the substrate. IR absorption spectra of the deposited films (not shown) indicate that their composition is similar to the pristine material. The mechanism for laser ablation of polymers depends on the absorption of the polymer at the laser wavelength. There are two proposed ablation mechanisms [lo]: photochemical and photothermal. Photochemical ablation is prevalent with UV irradiation, which tends to be strongly absorbed by polymers. Direct bond breaking occurs and the fragments are ejected from the surface. Clean etching of the target surface and deposition of films which have the same chemical composition as the target are features of photochemical ablation. In the photothermal model, ener,T is transferred into the vibrational modes of the molecule, which leads to heating and subsequent evaporation. Photothermal ablation prevails at longer wavelengths and results in charring and melting of the polymer target. Figure 3 shows an SEM image of the surface of an ablated target film of converted PPV. The grooves of the grating are formed perpendicular to the direction of the incident beam. From this and other images, the grating period was measured to be 1.3 ?- 0.1 urn. There have previously been reports of gratings on the surfaces of ablated polymers [ 11,121. Akhmanov et al. [ 131 have shown that the grating separation can be predicted using a model where the incident beam interferes with an in-plane surface wave. For an s-polarized beam (as was used in the present experiments) they show that the grating period (A,) is given by: a l-sin9
101
Film
Fig. 2. PL spectra from films of PPV formed spin-coating and by laser ablation.
IL=-
Metals
I
* -Ablated .....-.. Ablated
450
Stevens
(1)
where h is the laser wavelength and 0 is the angle of incidence. For the setup used in our experiments, this expression predicts that A, = 1.21 pm, in good agreement with the measured value,
Fig. 3. SEM image of the surface of an ablated target PPV film. It can be seen from Equation 1 that the grating separation can be varied easily to suit the requirements of a particular application. This factor, along with the simplicity of fabrication, could make this method of direct writing of diffraction gratings onto conducting polymers very useful in device applications. 4. Conclusions Laser ablation of a PPV target has been used to deposit thin films of this conjugated polymer. The luminescence and absorption spectra show features which are characteristic of PPV. We observe grating structures on the surface of target films which result from interference effects. 5. Acknowledgements We would like to thank Cambridge Display Technology Ltd. (Cambridge, UK) for the precursor polymer used in this work. 6. References [l] J.H. Brannon, J.R. Lankard, AI. Baise, F. Burns, J. Kaufman, J. Appl. Phys. 58 (1985) 2036. [2] Y.T.C. Yeh, J. Vat. Sci. Technol. A 4 (1986) 653. [3] L.S. Van Dyke, C.J. Brumlik, CR. Martin, Z. Yu, G.J. Collins, Synth. Met. 52 (1992) 299. [4] T. Lippert, A. Yabe, A. Wokaun, Adv. Mater. 9 (1997) 105. [5] G.B. Blanchet, Appl. Phys. Lett. 62 (1993) 479. [6] G.B. Blanchet, CR. Fincher, CL. Jackson, S.I. Shah, K.H. Gardner, Science 262 (1993) 719. [7] N. Matsumoto, H. Shima, T. Fujii, F. Kannari, Appl. Phys. Lett. 71 (1997) 2469. [8] S.G. Hansen, T.E. Robitaille, Appl. Phys. Lett. 52 (1988) 81. [9] Q. Luo, X. Chen, Z. Liu, Z. Sun, N. Ming, Appl. Surf. Sci. 108 (1997) 89. [lo] B.J. Garrison, R. Srinivasan, J. Appl. Phys. 57 (1985) 2909. [ll] P.E. Dyer, R.J. Farley, J. Appl. Phys. 74 (1993) 1442. [ 121 P.E. Dyer, R.J. Farley, R. Giedl, D.M. Kamakis, Appl. Surf. Sci. 96-98 (1996) 537. 1131 S.A. Akhmanov, VLEmel’yanov, N.I. Korote~ev, V.N. Seminogov, Sov. Phys. Usp. 28 (1985) 1084.