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Optical properties of fullerence C60 film in the vicinity of orientational phase transition probed by ellipsomerty
Journal of ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 349 (1995) 187-190
optical properties of fullerene c(3-j film in the vicinity of orientational phase transition probed by ellipsomerty. E. G. Bortchagovskya, I. A. Yurchenkoa, W. J. Romanowb and L. Brardc aInstitute of Semiconductor Physics of National Academy of Sciences of Ukraine, Prospect Nauki 45, Kiev-28,252028, Ukraine bDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA CLaboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19 104, USA
In this article we report the effect of dynamic disorder in the vicinity of orientational phase transition (Tc=250-260 K) in solid C6c on reduction of molecular local symmetry. We probe these phenomena by ellipsometry at the wavelength 520 nm, which corresponds to orbitally forbidden molecular absorption of tbllerene. The temperature dependence of optical constants of solid fullerene has been compared with its luminescent properties and depolarization of the reflected light. It has been shown that all data exhibit hysteresis-like behavior nearby Tc.
1. INTRODUCTION Optical methods are efficient tools for investigation of phase transitions. Besides such standard methods like absorption or light scattering, luminescence and ellipsometry also can give helpful information [ 11. Fullerene C6c has the highest molecular symmetry: Ih and, therefore, there is high degeneracy of the electronic levels and vibration modes. For free molecule the lowest (2.0 3.0 eV) optically induced electronic excitation is orbitally forbidden, but can take place vibronically [2]. Our recent results [3] show that luminescence of solid C60 films exhibits pronounced peak near temperature of phase transition Tc: This phenomenon was attributed to lowering of local molecular symmetry due to dynamic disorder. Several papers were focused on the investigation of phase transitions in solid tillerene [see for example the review 41. The first order orientational phase transition of solid Cbc fullerene, which occur in the range Tc=250-260 K, is well defined. We ellipsometrically [5] probed the optical constants of 867A fullerene film on silicon, when temperature stepped in the vicinity of this phase transition. For similar films optical constants of fullerene have been obtained in [6] at the room temperature for a wide spectral region. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V
SSDZ 0022-2860(95)08740-O
All rights reserved
188
2. EXPERIMENTAL All temperature measurements were carried out in helium cryostat. The sample was in thermal contact with the He gas. The temperature of the sample was stepped in 200-270 K and stabilized within 0.05 K. The standard Bitty method was used for ellipsometry. Measurements were made on incident angle 45”. The light source was tilament lamp with stabilized power supply. A parallel light was directed through system to monochromator. Cooled photomultiplier counted the intensity of the reflected light at the wavelength h=520nm: in the spectral region of orbitally forbidden transitions in C,o absorption. We measure angles of rotation of polarizing prisms with the precision of 5’. We took into account the instrumental response: stresses in the windows of the cryostat that had no sudden changes; we also considered the polarization sensitivity of monochromator and dependence of the light intensity over the polarizer from its angle position. The fullerene film on H-terminated silicon used in Preliminary, the our investigation were prepared in the University of Pennsylvania. ellipsometrical parameters of investigated system were obtained by standard null-ellipsometer LEF-3 on the wavelength of He-Ne laser (h=633nm) at the room temperature. The thickness d=867A and optical constants N=l.976-iO.165 were found for fitllerene film from the multiple angle of incidence measurements. For silicon we used optical constants from [7].
3. RESULTS AND DISCUSSION
Fig. 1 Temperature dependencies of the ellipsometrical parameters of firllerene film. Stars: heating; circles: cooling.
Fig. 1 shows temperature dependencies of ellipsometrical parameters of Crju fullerene film when heating and cooling the sample. Smooth approximations are presented there, because error bars do not allow us to discuss the differences in parameters obtained for different temperature points. One can see pronounced peaks in the plot of A and w versus temperature at 250K (cooling) and 257K (heating) which coincides with the first order orientational phase transition. On the base of averaged ellipsometrical parameters on their temperature dependencies and the data obtained by nullellipsometry (thickness), we calculated dielectric constants for C6u. film. Outside the critical temperature range we obtained N=2.107-iO.189, but at the temperature of phase transition N=2.005-i0.082. It should be mentioned that absorption coefficients obtained by ellipsometry are in a good agreement with the data obtained from transmission measurements of the same film. On the other hand they are larger than absorption reported in [6], while the refraction coefficient almost coincides to [6] for both 520 and 633 nm.
189
It has been shown in [3] that luminescent properties of fullerene films also reflect the occurring of the phase transition. As an example we adduce the temperature dependence of the intensity of luminescence of the similar Mlerene film in the region of 730 nm along with the data obtained in this work (Fig.2). Excitation of the luminescence was made by moderate intensity (0.1 W/cm2) of Ar-ion laser with wavelength h =514.5nm. It pays attention, that the peculiar behavior of luminescence begins at Tc and extends to the lower temperatures. The most crucial effect of the phase transition was observed in the temperature behavior of depolarization of reflected light. In this case we measured the intensity of s-polarized light when One one turns incident light to p-polarization. can see on Fig.3 that there were no peculiarities in this parameter when a sample was heated up, but when cooling down we obtained the
7 I\ I\ 27;
9
26:
‘1 ‘b
J
L
Fig.2 Temperature dependencies of w and the intensity of luminescence of similar film when cooling.
Fig.3 Temperature dependence of depolarization of the reflected light. Pointers show the directions of the temperature treatment.
remarkable signal at the temperature of orientational phase transition in Cbo and higher level of it than during heating till lower temperature. Such result might be assigned to a glassy behavior due to orientational freezing of solid Mlerene phase [8] when cooling below the phase transition. Probably this region is characterized in larger extent of disorder in the solid fullerene phase. On our mind, it is such frozen disorder that creates the increasing in the intensity of the luminescence and depolarized reflected light.
4. CONCLUSION The observation of peaks in the temperature dependence of w and A can in fact be viewed as a confirmation of the dependence of the solid C60 optical constants on the local environment of the Cbu molecules. The hysteresis behavior was also observed in depolarization of reflected light. We show pronounced increase of depolarization near Tc and its dependence from the direction of temperature changing (heating or cooling). These results are in a good agreement
190
with previously reported peculiar behavior of orbitally and spin forbidden luminescence of solid Cbo near the orientational phase transition [3].
REFERENCES 1. E. G. Bortchagovsky, V. Z. Lozovski, S. A. Shilo and I. A. Yurchenko, Physica C, 165 (1990) 308. 2. S. Leach, M. Vervloet, A. Despres, E. Breheret, J. P. Hare, T. J. Dennis, H. W. Kroto, R. Taylor, D. R. M. Walton, Chemical Physics 160 (1992) 45 1. 3. I. A. Yurchenko, S. A. Shilo, E. Burstein, J. E. Fischer, W. J. Romanow, L. Brard, Abstract submitted for the 1994 March Meeting of the APS, 21-25 March 1994. 4. P. A. Heiney, J. Phys. Chem. Solids, 53 (1992) 1333; or J. E. Fischer and P. A. Heiney, ibid, (1993) N12. 5. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, North-Holland, Amsterdam, 1977. 6. S. L. Ren, Y. Wang, A. M. Rao, E. McRae, J. M. Holden, T. Hager, Kai An Wang, WenTse Lee, H. F. Ni, J. Selegue and P. C. Eklund, Appl. Phys. Lett., 59 (1991) 2678. 7. D. E. Aspnes, A. A. Studna, Phys. Rev. B, 27 (1983) 985. 8. W. I. F. David, R. M. Ibberson, T. J. S. Dennis, J. P. Hare and K. Prassides, Europhys. Lett., 18 (1992) 219; 735.
MOLECULAR STRUCTURE Journal of Molecular Structure 349 (1995) 187-190
optical properties of fullerene c(3-j film in the vicinity of orientational phase transition probed by ellipsomerty. E. G. Bortchagovskya, I. A. Yurchenkoa, W. J. Romanowb and L. Brardc aInstitute of Semiconductor Physics of National Academy of Sciences of Ukraine, Prospect Nauki 45, Kiev-28,252028, Ukraine bDepartment of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA CLaboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19 104, USA
In this article we report the effect of dynamic disorder in the vicinity of orientational phase transition (Tc=250-260 K) in solid C6c on reduction of molecular local symmetry. We probe these phenomena by ellipsometry at the wavelength 520 nm, which corresponds to orbitally forbidden molecular absorption of tbllerene. The temperature dependence of optical constants of solid fullerene has been compared with its luminescent properties and depolarization of the reflected light. It has been shown that all data exhibit hysteresis-like behavior nearby Tc.
1. INTRODUCTION Optical methods are efficient tools for investigation of phase transitions. Besides such standard methods like absorption or light scattering, luminescence and ellipsometry also can give helpful information [ 11. Fullerene C6c has the highest molecular symmetry: Ih and, therefore, there is high degeneracy of the electronic levels and vibration modes. For free molecule the lowest (2.0 3.0 eV) optically induced electronic excitation is orbitally forbidden, but can take place vibronically [2]. Our recent results [3] show that luminescence of solid C60 films exhibits pronounced peak near temperature of phase transition Tc: This phenomenon was attributed to lowering of local molecular symmetry due to dynamic disorder. Several papers were focused on the investigation of phase transitions in solid tillerene [see for example the review 41. The first order orientational phase transition of solid Cbc fullerene, which occur in the range Tc=250-260 K, is well defined. We ellipsometrically [5] probed the optical constants of 867A fullerene film on silicon, when temperature stepped in the vicinity of this phase transition. For similar films optical constants of fullerene have been obtained in [6] at the room temperature for a wide spectral region. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V
SSDZ 0022-2860(95)08740-O
All rights reserved
188
2. EXPERIMENTAL All temperature measurements were carried out in helium cryostat. The sample was in thermal contact with the He gas. The temperature of the sample was stepped in 200-270 K and stabilized within 0.05 K. The standard Bitty method was used for ellipsometry. Measurements were made on incident angle 45”. The light source was tilament lamp with stabilized power supply. A parallel light was directed through system to monochromator. Cooled photomultiplier counted the intensity of the reflected light at the wavelength h=520nm: in the spectral region of orbitally forbidden transitions in C,o absorption. We measure angles of rotation of polarizing prisms with the precision of 5’. We took into account the instrumental response: stresses in the windows of the cryostat that had no sudden changes; we also considered the polarization sensitivity of monochromator and dependence of the light intensity over the polarizer from its angle position. The fullerene film on H-terminated silicon used in Preliminary, the our investigation were prepared in the University of Pennsylvania. ellipsometrical parameters of investigated system were obtained by standard null-ellipsometer LEF-3 on the wavelength of He-Ne laser (h=633nm) at the room temperature. The thickness d=867A and optical constants N=l.976-iO.165 were found for fitllerene film from the multiple angle of incidence measurements. For silicon we used optical constants from [7].
3. RESULTS AND DISCUSSION
Fig. 1 Temperature dependencies of the ellipsometrical parameters of firllerene film. Stars: heating; circles: cooling.
Fig. 1 shows temperature dependencies of ellipsometrical parameters of Crju fullerene film when heating and cooling the sample. Smooth approximations are presented there, because error bars do not allow us to discuss the differences in parameters obtained for different temperature points. One can see pronounced peaks in the plot of A and w versus temperature at 250K (cooling) and 257K (heating) which coincides with the first order orientational phase transition. On the base of averaged ellipsometrical parameters on their temperature dependencies and the data obtained by nullellipsometry (thickness), we calculated dielectric constants for C6u. film. Outside the critical temperature range we obtained N=2.107-iO.189, but at the temperature of phase transition N=2.005-i0.082. It should be mentioned that absorption coefficients obtained by ellipsometry are in a good agreement with the data obtained from transmission measurements of the same film. On the other hand they are larger than absorption reported in [6], while the refraction coefficient almost coincides to [6] for both 520 and 633 nm.
189
It has been shown in [3] that luminescent properties of fullerene films also reflect the occurring of the phase transition. As an example we adduce the temperature dependence of the intensity of luminescence of the similar Mlerene film in the region of 730 nm along with the data obtained in this work (Fig.2). Excitation of the luminescence was made by moderate intensity (0.1 W/cm2) of Ar-ion laser with wavelength h =514.5nm. It pays attention, that the peculiar behavior of luminescence begins at Tc and extends to the lower temperatures. The most crucial effect of the phase transition was observed in the temperature behavior of depolarization of reflected light. In this case we measured the intensity of s-polarized light when One one turns incident light to p-polarization. can see on Fig.3 that there were no peculiarities in this parameter when a sample was heated up, but when cooling down we obtained the
7 I\ I\ 27;
9
26:
‘1 ‘b
J
L
Fig.2 Temperature dependencies of w and the intensity of luminescence of similar film when cooling.
Fig.3 Temperature dependence of depolarization of the reflected light. Pointers show the directions of the temperature treatment.
remarkable signal at the temperature of orientational phase transition in Cbo and higher level of it than during heating till lower temperature. Such result might be assigned to a glassy behavior due to orientational freezing of solid Mlerene phase [8] when cooling below the phase transition. Probably this region is characterized in larger extent of disorder in the solid fullerene phase. On our mind, it is such frozen disorder that creates the increasing in the intensity of the luminescence and depolarized reflected light.
4. CONCLUSION The observation of peaks in the temperature dependence of w and A can in fact be viewed as a confirmation of the dependence of the solid C60 optical constants on the local environment of the Cbu molecules. The hysteresis behavior was also observed in depolarization of reflected light. We show pronounced increase of depolarization near Tc and its dependence from the direction of temperature changing (heating or cooling). These results are in a good agreement
190
with previously reported peculiar behavior of orbitally and spin forbidden luminescence of solid Cbo near the orientational phase transition [3].
REFERENCES 1. E. G. Bortchagovsky, V. Z. Lozovski, S. A. Shilo and I. A. Yurchenko, Physica C, 165 (1990) 308. 2. S. Leach, M. Vervloet, A. Despres, E. Breheret, J. P. Hare, T. J. Dennis, H. W. Kroto, R. Taylor, D. R. M. Walton, Chemical Physics 160 (1992) 45 1. 3. I. A. Yurchenko, S. A. Shilo, E. Burstein, J. E. Fischer, W. J. Romanow, L. Brard, Abstract submitted for the 1994 March Meeting of the APS, 21-25 March 1994. 4. P. A. Heiney, J. Phys. Chem. Solids, 53 (1992) 1333; or J. E. Fischer and P. A. Heiney, ibid, (1993) N12. 5. R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, North-Holland, Amsterdam, 1977. 6. S. L. Ren, Y. Wang, A. M. Rao, E. McRae, J. M. Holden, T. Hager, Kai An Wang, WenTse Lee, H. F. Ni, J. Selegue and P. C. Eklund, Appl. Phys. Lett., 59 (1991) 2678. 7. D. E. Aspnes, A. A. Studna, Phys. Rev. B, 27 (1983) 985. 8. W. I. F. David, R. M. Ibberson, T. J. S. Dennis, J. P. Hare and K. Prassides, Europhys. Lett., 18 (1992) 219; 735.