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Photoexcitation spectroscopy of iodine doped polyacetylene
Synthetic Metals, 17 (1987) 3 7 3 - 3 7 6
37 3
PHOTOEXCITATION SPECTROSCOPY OF IODINE DOPED POLYACETYLENE
O. BRAFMAN, E. EHRENFREUND and Z. VARDENY* Physics Department and Solid State Institute, Technion, Haifa (Israel) R. WEAGLEY Xerox Webster Research Center, Webster, NY 14644 (U.S.A.) A. J. EPSTEIN Department of Physics and of Chemistry, Ohio State University, Columbus, OH 43210 (U.S.A.)
ABSTRACT We have measure~l the photomodulation and Raman spectra of iodine doped ci_ss and trans polyacetylene [CH(I3)rl x. The solitonic photoinduced absorption (PA) band gradually disappears /
/
from the spectra consistent with the reduction of neutral solitons density upon doping. The highenergy PA band and its associated interband bleaching shift towards higher energies with doping indicating a substantial increase of the system optical energy gap.
INTRODUCTION The photoexcited gap states in (CH)x have been extensively studied using photomodulation spactroscopy. 1-8 The photoinduced spectrum contains two photoinduced absorption ~ A ) bands: 1'7 a lowenergy (LE) band at 0.45 eV which is due to photoinduced charged solitons (s±) ~ and a high-energy (HE) band 1'7 which is due to photogenerated neutral states, s± are extrinsically photogenerated6-s via an interchain process involving native neutral solitons (sO), while the HE state is an intrinsic intrachain photoexcitafion7"9 and therefore correlates with bleaching of the interband transitions.7 Photomodulation spectra of cis-rich samples were shown7 to arise only from the trans chains in the material. In this work we extend the photomodulation spectroscopy to include iodine doped polyacetylene, [CH(I~)yJx, and study the evolution of the PA bands with increasing y both for initially trans and ci___ssrich samples. We show that the HE PA band in the ei_s-rich materials (which originates from the trans ¢ahains) intensifies with y indicating doping induced isomerization process. The LE band, on the other hand, weakens with y and derives oscillator strength from dopant induced S ± transitions. We ~lso find that doping significantlyr shifts "lthe HE band. For y < 0.02 the HE band of ci__ssLCH(l~)yJx shifts towards that of trans ]CH(l~-)y/x showing again that isomerization occurs during doping, while for y > 0.02 the HE band and the energy gap Eg of both initially trans and cis-rich samples shift to higher energies with increasing y. * Present address: Engineering, Box D, Brown University, Providence, RI 10912, U.S.A. 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
374
Jo
i
I .C
~a..
b
O
% --
-.~.
~
F-I
(
0
Fig. 1.
0.4
o.s I.z ts 2.0 PHOTON ENERGY ( eV )
z.4
Photomo~ulation ~t~ctra of doped and undoped (CH) x at 20K. (a), (b) and (c) are c i s - LCH(I3--)yJx with y=0, 0.01 and 0.017 respectively; (d) is an undoped trans-(CH)x for which different PA bands are identified.
EXPERIMENTAL The experimental set-up for steady state photomodulation measurements consists4'7 of a chopped (~140Hz) Ar + laser beam at 2.7 ev for the pump beam and as a probe beam we have used incandescent light sources dispersed by a monochromator. The induced changes Act in the absorption coefficient ct is studied by taking the ratio (-AT)/T where AT is the induced change in the transmission T. The polyacetyl~e thin films were polymerized onto KBr or sapphire substrates and the dopant concentration was determined 1° by the sample conductivity.
RESULTS AND DISCUSSION In Fig. l we show the photomodulation spectrum of four (CH)x samples at 20K: (a) is an undoped cis-rich (CH)x , Co) and (c) are initially cis-rich doped (CH)x and (d) is an undoped trans-(CH)x. Fig. l(d) defines the main features of the spectrum. The LE PA band at 0.45 eV, its associated2 oscillations at 1.4 to 1.8 eV and the IR active vibrations (IRAV), all o f them 2-a are due to photogenerated S +. The HE band at 1.35 eV and the bleaching part o f the specU'um are due to a photoinduced neutral defect, identified by several groups 1 '7 9' as bound -s0 s0 pairs. In the 80% cis-rich sample (Fig. l(a)) the LE and the IRAV bands are about an order of magnitude weaker than the corresponding bands in trans-(CH)x; similar reducton in spin susceptibility and in the density of s o was observed, tt This correlation is in agreement with the suggestion6-8 that s:~ are photogenerated at the expense o f s °. Note that the LE band appears as a broad feature centered at -0.6eV. The HE at 1.47 eV, the oscillations up to 1.8 eV and the photo-bleaching were all shown 7 to result from the trans chains in the ci._Asmatrix. However, the oscillations above 2 eV belong to the ci__sschains. 2 The gradual isomerization o f the ci..._sschains with y is evident from Figs. l(b) and l(c). For y=0.01 and y=0.017 the cis oscillations in the PA spectrum are considerably weaker showing reduction of the cis content.
375 Also, the HE band intensifies with y; it increases from 10-3 for y=0 to 3xl(P for y-0.017. Since the HE band originates from the trans chains 7 this also indicates that the amount of the trans isomer increases with y. The effect of doping on the isomerization process can be seen also in the resonant Raman scattering spectra shown in Fig. 2. In ci_.ssdoped samples both the ciss and trans Raman scattering lines are observed for all y. However, the relative strength of the ci_sslines weakens with y indicating higher degree of isomerization. The broadening of the Raman lines TM for y=0.05 shown in Fig. 2 and the broadening of the HE band as y increases (Fig. l) indicate increasing disorder upon doping. When the PA spectra for y=0.01 and y=0.017 (Figs. lb and lc) are compared with that of undoped trans-(CH)x (Fig. ld) we see that the LE PA band is common to the doped and the undoped samples. However a new photo-bleaching (PB) band centered at - 0.75 eV appears only in the doped samples. Similar PB band appear 13 also for doped trans-(CH)x (y<0.04). This PB band scales with the LE band, has similar temperature dependence and it increases (like the LE band 3A) sublinearly with the laser power. The origin of this PB band is consistent with the following mechanism. In addidon of new doping induced charged solitons, doping also charges native s o (so - e - ~ (s+)dop) which already exists in the material. The transition associated with (s+)aop appears 1°'t2 at 0.7-0.8 eV, i.e. much closer to midgap than the transition of photogenerated s + (0.45 eV) probably because e-e correlation on (s+)dop is partially screened. The bleaching at 0.75 eV is interpreted6 therefore as transfer of positive charge from (s+)aop to native s o transforming it into s + with transitions at the LE band. The net reaction for this process may be written as s°+(s+)aop--~ s++(s°)d~. Further support for this mechanism was obtained6'13 from the photoinduced IRAV spectrum which shows new photo-induced IRAV which belongs to s + a t the expense of bleaching IRAV which belongs to (s+)aop.
I 70
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Fig. 2. RRS spectra o f iodine doped samples. t and c are Raman lines o f trans and ci_.ssisomer, respectively.
I
1,35Jiw 2
I
4
y (°/o1
Fig. 3. The HE PA band position at 20K, as a function o f doping level (y) for trans and ci__ss doped samples.
All of the photomodulation features associated with the photogenerated s~ (the IRAV, the LE band, the PB band at 0.75 eV and the oscillations) disappear from the spectrum for y - 0.045. This indicates 15 that at this doping level all o f the s o defects are positively charged by the dopant 13 so that s + cannot be photogenerated. For y _ 0.05 we have observed 13 a new PA spectrum which contains two correlated PA bands associated with charged defects which appear at 0.1 eV and at 1.9 eV. This shows for the first time that the insulator-metal phase transition at y--O.0515 can be optically observed.
376 Unlike the LE band, we have observed t3 that the HE band exists in the PA spectrum for all doping levels. It increases linearly with IL and does not saturate even for the highest IL applied. It is therefore an intrinsic photoexcitafion for the doped materials as well as for the undoped (CH)x. 6 For increasing y the HE band broadens and shifts to higher energies, as shown in Fig. 3. The decrease of the HE peak position of the cis doped (CH) x shows again doping induced isomerization, l° It also shows that the HE peak position directly correlates with the system Eg and therefore the effect of doping on the band structure may be inferred from the changes of the HE position. 13 As seen in Fig. 3, the HE band position for tram-[CH(I~)y|x monotonically increases with y for y> 0.01. Similar upward shifts were observedl'3 " i ntram doped (CH)x for the isosbesfic point and l~or the interband PB. We therefore conclude that Egl6increases upon doping, consistent with the doping process m' conjugation polymers; states are taken from the n-orbitals to create new localized states in the gap. When doping increases these levels overlap 16 forming delocalized bands within the gap. The combined effect of the defect bands within the gap, the reduced ~ bands and the consequent dimerization pattern adjustments have been predicted 13 to increase Eg with y in a pretty close functional dependence as in Fig. 3.
ACKNOWLEDGEMENTS This work was supported in part by the USA-Israel Binational Science Foundation (BSF), by the Israel Academy for Basic Research Jerusalem and by the Fund for the Promotion of Research at the Technion.
REFERENCES 1
J. Orenstein and G. L. Baker, Phys. Rev. Lett., 49 (1982) 1043.
2
J. Orenstein, G. L. Baker and Z. Vardeny, J. Phys. (Paris), 44 (1983) C3-407.
3
G.B. Blanchet, C. R. Fincher, T.-C. C"hung and A. J. Heeger, Phys. Rev. Lett., 50 (1983) 1038.
4
Z. Vardeny, J. Orenstein and G. L. Baker, Phys. Rev. Lett. 50 (1983) 2032.
5
Z. Vardeny, J. Tanaka, H. Fujimoto and M. Tanaka, Solid State Commun., 50 (1984) 937.
6
J. Orenstein, Z. Vardeny, G. L. Baker, G. Eagle and S. Etemad, Phys. Rev.~ 13,30 (1984) 786.
7
Z. Vardeny, E. Ehrenfreund and O. Brafman, Mol. Cryst. Liq. Cryst., 117 (1985) 245.
8
S. Etemad, G. L. Baker, C. B. Roxlo, B. R. Weinberger and J. Orenstein, Mol. Cryst. Liq. Cryst., 117 (1985) 275.
9
S. Kivelson and W.-K. Wu, Bull. Am. Phys. Soe., 31 (1986) 330; and to be published.
10 A. J. Epstein, H. Rommelmann, R. Bigelow, H. W. Gibson, D. M. Hoffman and D. B. Tanner, Phys. Rev. Lett., 50 (1983) 1866. 11 J. C. W. Chien, F. E. Karasz, G. Wnek, A. G. MacDiarmid and A. J. Heeger, J. Polym. Sci. Polym. Lett. Ed.~ 17 (1979) 195. 12 S. Etemad, A. Feldblum, A. G. MacDiarmid and A. J. Heeger, J. Phys. (Paris), 44 (1983) C3-413. 13 E. Ehrenfreund, Z. Vardeny, O. Brafman, A. J. Epstein and R. Weagley, to be published. 14 Z. Vardeny, E. Ehrenfreund, O. Brafman and B. Horovitz, Phys. Rev. Lett. 5zt (1985) 75. 15 S. Ikehata, J. Kaufer, T. Woerner, A. Pron, M. A. Druy, A. Sivak, A. J. Heeger and A. G. MacDiarmid, Phys. Rev. Lett.~ 45 (1980) 1123. 16 J. L. Br6das, B. Themans, J. G. Firpiat, J. M. Andr~ and R. R. Chance, Phys. Rev., B29 (1984) 6761.
37 3
PHOTOEXCITATION SPECTROSCOPY OF IODINE DOPED POLYACETYLENE
O. BRAFMAN, E. EHRENFREUND and Z. VARDENY* Physics Department and Solid State Institute, Technion, Haifa (Israel) R. WEAGLEY Xerox Webster Research Center, Webster, NY 14644 (U.S.A.) A. J. EPSTEIN Department of Physics and of Chemistry, Ohio State University, Columbus, OH 43210 (U.S.A.)
ABSTRACT We have measure~l the photomodulation and Raman spectra of iodine doped ci_ss and trans polyacetylene [CH(I3)rl x. The solitonic photoinduced absorption (PA) band gradually disappears /
/
from the spectra consistent with the reduction of neutral solitons density upon doping. The highenergy PA band and its associated interband bleaching shift towards higher energies with doping indicating a substantial increase of the system optical energy gap.
INTRODUCTION The photoexcited gap states in (CH)x have been extensively studied using photomodulation spactroscopy. 1-8 The photoinduced spectrum contains two photoinduced absorption ~ A ) bands: 1'7 a lowenergy (LE) band at 0.45 eV which is due to photoinduced charged solitons (s±) ~ and a high-energy (HE) band 1'7 which is due to photogenerated neutral states, s± are extrinsically photogenerated6-s via an interchain process involving native neutral solitons (sO), while the HE state is an intrinsic intrachain photoexcitafion7"9 and therefore correlates with bleaching of the interband transitions.7 Photomodulation spectra of cis-rich samples were shown7 to arise only from the trans chains in the material. In this work we extend the photomodulation spectroscopy to include iodine doped polyacetylene, [CH(I~)yJx, and study the evolution of the PA bands with increasing y both for initially trans and ci___ssrich samples. We show that the HE PA band in the ei_s-rich materials (which originates from the trans ¢ahains) intensifies with y indicating doping induced isomerization process. The LE band, on the other hand, weakens with y and derives oscillator strength from dopant induced S ± transitions. We ~lso find that doping significantlyr shifts "lthe HE band. For y < 0.02 the HE band of ci__ssLCH(l~)yJx shifts towards that of trans ]CH(l~-)y/x showing again that isomerization occurs during doping, while for y > 0.02 the HE band and the energy gap Eg of both initially trans and cis-rich samples shift to higher energies with increasing y. * Present address: Engineering, Box D, Brown University, Providence, RI 10912, U.S.A. 0379-6779/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
374
Jo
i
I .C
~a..
b
O
% --
-.~.
~
F-I
(
0
Fig. 1.
0.4
o.s I.z ts 2.0 PHOTON ENERGY ( eV )
z.4
Photomo~ulation ~t~ctra of doped and undoped (CH) x at 20K. (a), (b) and (c) are c i s - LCH(I3--)yJx with y=0, 0.01 and 0.017 respectively; (d) is an undoped trans-(CH)x for which different PA bands are identified.
EXPERIMENTAL The experimental set-up for steady state photomodulation measurements consists4'7 of a chopped (~140Hz) Ar + laser beam at 2.7 ev for the pump beam and as a probe beam we have used incandescent light sources dispersed by a monochromator. The induced changes Act in the absorption coefficient ct is studied by taking the ratio (-AT)/T where AT is the induced change in the transmission T. The polyacetyl~e thin films were polymerized onto KBr or sapphire substrates and the dopant concentration was determined 1° by the sample conductivity.
RESULTS AND DISCUSSION In Fig. l we show the photomodulation spectrum of four (CH)x samples at 20K: (a) is an undoped cis-rich (CH)x , Co) and (c) are initially cis-rich doped (CH)x and (d) is an undoped trans-(CH)x. Fig. l(d) defines the main features of the spectrum. The LE PA band at 0.45 eV, its associated2 oscillations at 1.4 to 1.8 eV and the IR active vibrations (IRAV), all o f them 2-a are due to photogenerated S +. The HE band at 1.35 eV and the bleaching part o f the specU'um are due to a photoinduced neutral defect, identified by several groups 1 '7 9' as bound -s0 s0 pairs. In the 80% cis-rich sample (Fig. l(a)) the LE and the IRAV bands are about an order of magnitude weaker than the corresponding bands in trans-(CH)x; similar reducton in spin susceptibility and in the density of s o was observed, tt This correlation is in agreement with the suggestion6-8 that s:~ are photogenerated at the expense o f s °. Note that the LE band appears as a broad feature centered at -0.6eV. The HE at 1.47 eV, the oscillations up to 1.8 eV and the photo-bleaching were all shown 7 to result from the trans chains in the ci._Asmatrix. However, the oscillations above 2 eV belong to the ci__sschains. 2 The gradual isomerization o f the ci..._sschains with y is evident from Figs. l(b) and l(c). For y=0.01 and y=0.017 the cis oscillations in the PA spectrum are considerably weaker showing reduction of the cis content.
375 Also, the HE band intensifies with y; it increases from 10-3 for y=0 to 3xl(P for y-0.017. Since the HE band originates from the trans chains 7 this also indicates that the amount of the trans isomer increases with y. The effect of doping on the isomerization process can be seen also in the resonant Raman scattering spectra shown in Fig. 2. In ci_.ssdoped samples both the ciss and trans Raman scattering lines are observed for all y. However, the relative strength of the ci_sslines weakens with y indicating higher degree of isomerization. The broadening of the Raman lines TM for y=0.05 shown in Fig. 2 and the broadening of the HE band as y increases (Fig. l) indicate increasing disorder upon doping. When the PA spectra for y=0.01 and y=0.017 (Figs. lb and lc) are compared with that of undoped trans-(CH)x (Fig. ld) we see that the LE PA band is common to the doped and the undoped samples. However a new photo-bleaching (PB) band centered at - 0.75 eV appears only in the doped samples. Similar PB band appear 13 also for doped trans-(CH)x (y<0.04). This PB band scales with the LE band, has similar temperature dependence and it increases (like the LE band 3A) sublinearly with the laser power. The origin of this PB band is consistent with the following mechanism. In addidon of new doping induced charged solitons, doping also charges native s o (so - e - ~ (s+)dop) which already exists in the material. The transition associated with (s+)aop appears 1°'t2 at 0.7-0.8 eV, i.e. much closer to midgap than the transition of photogenerated s + (0.45 eV) probably because e-e correlation on (s+)dop is partially screened. The bleaching at 0.75 eV is interpreted6 therefore as transfer of positive charge from (s+)aop to native s o transforming it into s + with transitions at the LE band. The net reaction for this process may be written as s°+(s+)aop--~ s++(s°)d~. Further support for this mechanism was obtained6'13 from the photoinduced IRAV spectrum which shows new photo-induced IRAV which belongs to s + a t the expense of bleaching IRAV which belongs to (s+)aop.
I 70
t
[CH(13)y]
XL. : 600nrr
LeOK
1.65 ¢p v O Z
t.60
• •
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/
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Q
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I
/ /
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/
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•
//
140
qf
/
/ I" i
IO00
i
J
IIO0
A
•
1200
I~SO0
_
, J d
1400
i
L
1500
G) ( c m - I )
Fig. 2. RRS spectra o f iodine doped samples. t and c are Raman lines o f trans and ci_.ssisomer, respectively.
I
1,35Jiw 2
I
4
y (°/o1
Fig. 3. The HE PA band position at 20K, as a function o f doping level (y) for trans and ci__ss doped samples.
All of the photomodulation features associated with the photogenerated s~ (the IRAV, the LE band, the PB band at 0.75 eV and the oscillations) disappear from the spectrum for y - 0.045. This indicates 15 that at this doping level all o f the s o defects are positively charged by the dopant 13 so that s + cannot be photogenerated. For y _ 0.05 we have observed 13 a new PA spectrum which contains two correlated PA bands associated with charged defects which appear at 0.1 eV and at 1.9 eV. This shows for the first time that the insulator-metal phase transition at y--O.0515 can be optically observed.
376 Unlike the LE band, we have observed t3 that the HE band exists in the PA spectrum for all doping levels. It increases linearly with IL and does not saturate even for the highest IL applied. It is therefore an intrinsic photoexcitafion for the doped materials as well as for the undoped (CH)x. 6 For increasing y the HE band broadens and shifts to higher energies, as shown in Fig. 3. The decrease of the HE peak position of the cis doped (CH) x shows again doping induced isomerization, l° It also shows that the HE peak position directly correlates with the system Eg and therefore the effect of doping on the band structure may be inferred from the changes of the HE position. 13 As seen in Fig. 3, the HE band position for tram-[CH(I~)y|x monotonically increases with y for y> 0.01. Similar upward shifts were observedl'3 " i ntram doped (CH)x for the isosbesfic point and l~or the interband PB. We therefore conclude that Egl6increases upon doping, consistent with the doping process m' conjugation polymers; states are taken from the n-orbitals to create new localized states in the gap. When doping increases these levels overlap 16 forming delocalized bands within the gap. The combined effect of the defect bands within the gap, the reduced ~ bands and the consequent dimerization pattern adjustments have been predicted 13 to increase Eg with y in a pretty close functional dependence as in Fig. 3.
ACKNOWLEDGEMENTS This work was supported in part by the USA-Israel Binational Science Foundation (BSF), by the Israel Academy for Basic Research Jerusalem and by the Fund for the Promotion of Research at the Technion.
REFERENCES 1
J. Orenstein and G. L. Baker, Phys. Rev. Lett., 49 (1982) 1043.
2
J. Orenstein, G. L. Baker and Z. Vardeny, J. Phys. (Paris), 44 (1983) C3-407.
3
G.B. Blanchet, C. R. Fincher, T.-C. C"hung and A. J. Heeger, Phys. Rev. Lett., 50 (1983) 1038.
4
Z. Vardeny, J. Orenstein and G. L. Baker, Phys. Rev. Lett. 50 (1983) 2032.
5
Z. Vardeny, J. Tanaka, H. Fujimoto and M. Tanaka, Solid State Commun., 50 (1984) 937.
6
J. Orenstein, Z. Vardeny, G. L. Baker, G. Eagle and S. Etemad, Phys. Rev.~ 13,30 (1984) 786.
7
Z. Vardeny, E. Ehrenfreund and O. Brafman, Mol. Cryst. Liq. Cryst., 117 (1985) 245.
8
S. Etemad, G. L. Baker, C. B. Roxlo, B. R. Weinberger and J. Orenstein, Mol. Cryst. Liq. Cryst., 117 (1985) 275.
9
S. Kivelson and W.-K. Wu, Bull. Am. Phys. Soe., 31 (1986) 330; and to be published.
10 A. J. Epstein, H. Rommelmann, R. Bigelow, H. W. Gibson, D. M. Hoffman and D. B. Tanner, Phys. Rev. Lett., 50 (1983) 1866. 11 J. C. W. Chien, F. E. Karasz, G. Wnek, A. G. MacDiarmid and A. J. Heeger, J. Polym. Sci. Polym. Lett. Ed.~ 17 (1979) 195. 12 S. Etemad, A. Feldblum, A. G. MacDiarmid and A. J. Heeger, J. Phys. (Paris), 44 (1983) C3-413. 13 E. Ehrenfreund, Z. Vardeny, O. Brafman, A. J. Epstein and R. Weagley, to be published. 14 Z. Vardeny, E. Ehrenfreund, O. Brafman and B. Horovitz, Phys. Rev. Lett. 5zt (1985) 75. 15 S. Ikehata, J. Kaufer, T. Woerner, A. Pron, M. A. Druy, A. Sivak, A. J. Heeger and A. G. MacDiarmid, Phys. Rev. Lett.~ 45 (1980) 1123. 16 J. L. Br6das, B. Themans, J. G. Firpiat, J. M. Andr~ and R. R. Chance, Phys. Rev., B29 (1984) 6761.