Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.issp.ac.ru/lhpp/PapersMeletov/paper54.pdf Дата изменения: Tue Feb 18 18:10:09 2014 Дата индексирования: Fri Feb 28 03:11:57 2014 Кодировка: Поисковые слова: flare

JETP LETTERS

VOLUME 70, NUMBER 12

25 DEC. 1999

Photoinduced transformation of luminescence centers in C60 crystals at high pressure
V. D. Negri *) and K. P. Meletov
Institute of Solid-State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia

Submitted 22 November 1999 ґ Pis'ma Zh. Eksp. Teor. Fiz. 70, No. 12, 784 ­ 788 25 December 1999 The influence of laser irradiation on the photoluminescence spectra of perfect C60 crystals in the orientationally disordered phase is investigated. It is shown that irradiation of the crystals with low-power light for short durations at T 200 K produces radical changes in the luminescence spectrum. The pressure dependences of the spectral bands of the phototransformed and initial without irradiation spectra differ significantly, indicating a photoinduced structural transformation of the X centers responsible for the luminescence of C60 . The phototransformed C60 crystals are stable against further exposure to light irradiation and pressure. © 1999 American Institute of Physics. S0021-3640 99 00424-7 PACS numbers: 78.55.Hx, 62.50. p, 71.35.Aa, 61.80.Ba

The low-temperature photoluminescence spectra of high-quality C60 crystals are known to have a line structure attributable to the radiative recombination of Frenkel excitons localized at so-called X centers.1,2 A C band associated with the radiative recombination of free Frenkel excitons is also observed in crystals having a relatively low density of X centers and a low luminescence quantum efficiency.3 The properties of X centers and their relationship to the structure of the luminescence spectrum of C60 crystals are of considerable interest and have been investigated in a number of papers. However, despite significant progress in research on the electronic states of C60 , the real nature of X centers remains elusive for the most part. Studies of the luminescence spectra of the purest C60 crystals have shown that the X centers are not associated with impurities, rather they are more likely attributable to defects of the crystal structure. Regardless of the degree of structural perfection of the investigated samples, the presence of orientational defects is a characteristic attribute of fullerite. The occurrence of an orientational ordering phase transition in C60 crystals at T 260 K and the cessation of random molecular rotation below this temperature are well known.4 However, the molecules execute discrete rotations between two energywise close orientational states down to 80 K, below which each molecular motion is frozen, and a phase of the orientational glass type is established.5 It is also known that the dimerization of molecules is observed in the orientationally disordered phase of laser-irradiated C60 , and polymerization of the as-grown material
0021-3640/99/70(12)/6/$15.00 801 © 1999 American Institute of Physics


802

JETP Lett., Vol. 70, No. 12, 25 Dec. 1999

V. D. Negri and K. P. Meletov

takes place when the laser power is increased.6 Detailed studies of the optical spectra and crystal structure of C60 have shown that polymerization also takes place under the combined influence of pressure and high temperatures, resulting in the formation of numerous, structurally different phases.7,8 All these transformations are accompanied by a radical change in the initial photoluminescence spectrum. Modification of the luminescence spectra in laser irradiation has also been observed in the orientationally ordered phase of C60 crystals at standard pressure and at temperatures of 5 K, 77 K and 120 K. This phenomenon was achieved by pre-irradiating local zones of the crystal with a laser beam at power densities ranging from 5 W/cm2 to 50 W/cm2 and then recording the photoluminescence spectra from the irradiated and control zones of the crystal at T 5 K and an excitation power density 1 W/cm2 . It was established that irradiation of the crystals at temperatures in the vicinity of 5 K and 77 K induces a relative variation of the intensities of the emission bands of X2 and X3 centers, and irradiation at 120 K is accompanied by diminution of the X2 - and X3 -center bands and enhancement of the intensity of the C band.9 Similar results on the influence of laser irradiation on the photoluminescence spectra in the low-temperature range are also reported in Ref. 10. Consequently, the existing experimental data indicate that light has a powerful influence on the spectrum of localized electronic states of C60 crystals. Photoinduced processes in the orientationally ordered phase of fullerite, in particular, the low-temperature, photoinduced structural transformation of X centers, are of major interest in this regard. With the latter phenomenon in mind, in the present study we report an investigation of the combined influence of high pressure and laser irradiation on C60 crystals and show that a radical transformation of the photoluminescence spectra takes place in the temperature range 140 ­ 250 K. We confirm that the pressure dependence of the phototransformed and initial photoluminescence spectra differ significantly, and the phototransformed samples are stable against subsequent light irradiation and pressure. We have shown that the photinduced structural transformation of X centers also takes place at high pressure, and when the latter is suddenly dropped to standard pressure, the photoluminescence spectra of the samples transformed at different pressures are identical. The measurements were carried out on a large series of C60 crystals grown from the vapor phase. The photoluminescence spectra were recorded by means of an automated spectrometer incorporating a DFS-12 dual monochromator, a liquid nitrogen-cooled ґ FEU-62 photomultiplier, and a 5S1 photon counting system. All the photoluminescence spectra were normalized to the calibrated spectrum of a tungsten lamp. Luminescence was excited by a helium-neon laser with its power output limited by optical filters. High-pressure measurements in liquid nitrogen or helium vapor were performed using a miniature diamond anvil cell of the Merrill ­ Bassett type enclosed in a helium optical thermostat. Pressure was transmitted by means of a 4:1 methanol-ethanol mixture,11 and its value was determined from the position of the ruby luminescence R 1 line within 0.05-GPa error limits.12 The measurements were performed on crystals having a mirror finish and dimensions of 100 100 40 m, which are close to the dimensions of the working volume of the high-pressure cell. The photinduced structural transformation was investigated over a wide range of temperatures on crystals with a relatively high photoluminescence quantum efficiency, whose spectra were dominated by the emission bands of X3 , X4 , and X5 centers. They


JETP Lett., Vol. 70, No. 12, 25 Dec. 1999

V. D. Negri and K. P. Meletov

803

FIG. 1. Photoluminescence spectra of nonirradiated a­d and laser-irradiated e­g C ture T 80 K and pressures up to 3.0 GPa.

60

crystals at a tempera-

show that the variations of the photoluminescence spectra takes place in the temperature interval 180 ­ 240 K and are appreciable even at a very low excitation level 5 10 3 W/cm2 . The temperature dependence of the phototransformation efficiency at standard pressure is bell-shaped with a maximum in the vicinity of T 200 K. Consequently, at T 200 K laser irradiation of the crystal for 15 min at a power density 0.2 W/cm2 leads to radical more than 95% restructuring of the initial spectrum. It is important to note that direct measurements of the photoluminescence spectra were performed at T 80 K, i.e., at a temperature where phototransformation is frozen. Figure 1 shows the initial a and phototransformed e photoluminescence spectra determined at a pressure 0.2 GPa under the above-stated irradiation conditions. The measurements show that spectrum e is stable in the presence of further irradiation of the crystal over the entire temperature range up to T 260 K. Temperature cycling of the samples in the interval 5 ­ 300 K without irradiation by any kind of light does not produce appreciable changes in the high-temperature photoluminescence spectra. On the other hand, the irradiation of these crystals at T 300 K for the sample laser excitation parameters alters the low-temperature photoluminescence spectrum in connection with flare-up and pronounced broadening of the bands of the initial spectrum. Similar effects have been observed in all the C60 crystals at our disposal, but irradiation does not have such a pronounced influence on their photoluminescence spectra. Figure 1 also shows the photoluminescence spectra of the as-grown and phototransformed crystals at various pressures. Spectra a ­ d refer to the nonirradiated crystal, and spectra e ­ g to a crystal preirradiated at a temperature T 200 K and pressure of 0.2 GPa. It is evident from Fig. 1 a­d that, apart from an overall shift of the photolu-


804

JETP Lett., Vol. 70, No. 12, 25 Dec. 1999

V. D. Negri and K. P. Meletov

FIG. 2. Pressure dependence of the band positions in the low-temperature photoluminescence spectrum of nonirradiated dark symbols and irradiated light symbols C60 crystals. The dark and light dots represent the pressure dependence of the position of the C band.

minescence spectrum into the long-wavelength region, its shape also changes as the pressure is increased. At P 1.2 GPa a band that is scarcely perceptible in the initial photoluminescence spectrum flares up in the short-wavelength region. On the other hand, fine structure becomes increasing evident in the broad bands of the initial photoluminescence spectrum as the pressure is increased. Such changes have also been observed in the spectra of the preirradiated crystal, but are not nearly as pronounced. Figure 2 shows the pressure dependence of the positions of the spectral bands in the photoluminescence spectra of the as-grown dark symbols and phototransformed light symbols crystals. It is evident from Fig. 2 that the pressure variations of the bands of preirradiated and nonirradiated crystals differ significantly in the low-pressure range, but then they become more alike as the pressure is increased, and at P 1.8 GPa they essentially coincide. The difference in the pressure dependence of the photoluminescence bands of the as-grown and irradiated crystals in the initial pressure range indicates that what happens during irradiation is not merely a redistribution of the photoluminescence intensity among different luminescence centers of the as-grown crystal, but a transformation of the core structure of the crystal. At the same time, the increasing similarity of the photoluminescence spectra of the as-grown and phototransformed samples with increasing pressure suggests that the latter could be unstable under the influence of high pressure and revert to the as-grown state. It is also essential to note that pressureenhanced band in the spectrum of the nonirradiated crystals is in the same position and exhibits the same pressure dependence as the C band, which we have previously identified with the emission of free excitons.3 We have carried out an experiment to answer the question of whether the phototransformed samples are stable and whether phototransformation takes place at high pressure; the results are shown in Fig. 3. Curves a and b in Fig. 3 represent the photoluminescence spectra of the nonirradiated crystal at a temperature T 80 K and


JETP Lett., Vol. 70, No. 12, 25 Dec. 1999

V. D. Negri and K. P. Meletov

805

FIG. 3. Luminescence spectra of C60 crystals phototransformed at various pressures.

pressures of 0.2 GPa and 2.3 GPa, respectively. The crystal was then irradiated with light at the above-indicated power and duration at a pressure of 2.3 GPa and temperature T 200 K. The photoluminescence spectrum of this crystal at T 80 K is shown in Fig. 3 c . On the whole, its shape is quite close to that of the spectrum of the nonirradiated crystal b other than a certain broadening of the bands and diminution of the intensity of the short-wavelength C band. After the pressure is dropped back to the standard level the photoluminescence spectrum of this same crystal at T 80 K acquires the form d . It is evident from the figure that it is not similar to the initial spectrum a , but is more like the characteristic spectrum e for the crystal preirradiated at standard pressure. The observable difference between spectra d and e is probably attributable to the presence of residual stresses in the crystal and cracking of the crystal after the pressure is dropped to the standard level, so that the spectral bands broaden, and the contribution of the initial spectrum increases somewhat. The results of the investigations show that spectrum d is stable against further irradiation of the crystal up to T 300 K. Consequently, the results indicate that when C60 crystals are irradiated by a laser beam in the temperature interval 180 ­ 240 K and at pressures up to 2.3 GPa, the photoluminescence spectra undergo a radical transformation as a result of restructuring of the radiative recombination X centers. At standard pressure this process attains its maximum efficiency at T 200 K and falls off sharply near the point of transition to the orientationally disordered phase. The drop in efficiency of transformation of the centers at T 260 K, when the molecules execute random rotation, is most likely indicative of the definite role played by the orientation of the C60 molecules during photoinduced transformation of the X centers. In closing, the authors are grateful to R. K. Nikolaev for furnishing the C60 crystals, to the Russian Fund for Fundamental Research for partial financial support of the study Project #99-02-17555 , and to the NATO Research Committee for its support under the


806

JETP Lett., Vol. 70, No. 12, 25 Dec. 1999

V. D. Negri and K. P. Meletov

auspices of the program ``Collaboration Research'' HTECH.CRG #972317 .
* Deceased.

1 2

3 4 5 6 7 8 9 10 11 12

Ё W. Guss, J. Feldmann, E. O. Gobel et al., Phys. Rev. Lett. 72, 2644 1994 . ґ ґ V. V. Kveder, V. D. Negri, E. A. Shtenman et al., Zh. Eksp. Teor. Fiz. 113, 734 1998 . K. P. Meletov and V. D. Negri, JETP Lett. 68, 248 1998 . P. A. Heiney, J. E. Fischer, A. R. McGhie et al., Phys. Rev. Lett. 66, 2911 1991 . C. Meingast and F. Gugenberger, Mod. Phys. Lett. B B7, 1703 1993 . A. M. Rao, P. Zhou, K.-A. Wang et al., Science 259, 955 1993 . I. O. Bashkin, A. P. Moravski, E. G. Ponyatovski et al., JETP Lett. 59, 279 1994 Y. Iwasa, T. Arima, R. M. Fleming et al., Science 264, 1570 1994 . V. D. Negrii, V. V. Kveder, Yu. A. Ossipyan et al., Phys. Status Solidi B 199, 587 U. D. Venkateswaran, M. G. Schall, Y. Wang et al., Solid State Commun. 96, 951 A. Jayaraman, Rev. Sci. Instrum. 57, 1013 1986 . D. Barnett, S. Block, and G. J. Piermarini, Rev. Sci. Instrum. 44, 1 1973 .

1998

JETP 86, 405

. 1997 . 1995 .

Translated by James S. Wood