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FULLERENES, NANOTUBES, AND CARBON NANOSTRUCTURES Vol. 12, Nos. 1 & 2, pp. 297 ­ 300, 2004

Photoluminescence Study of the Two-Dimensional Tetragonal and Rhombohedral Polymers of C60 at High Pressure
K. P. Meletov*
Institute of Solid State Physics, RAS, Chernogolovka, Moscow Region, Russia

ABSTRACT
The pressure behavior of the photoluminescence (PL) spectra of the planar polymeric phases of C60 have been studied at pressure up to 4 GPa. The PL spectra and the pressure-induced shift of the principal bands differ considerably for the pristine C60, two-dimensional tetragonal (2D-T) and rhombohedral (2D-R) polymers of C60. The changes in the PL spectra may be related with the transformation of the electron energy spectrum of polymers. Key Words: Photoluminescence; Tetragonal polymers; Rhombohedral polymers; C60; Pressure.

*Correspondence: K. P. Meletov, Institute of Solid State Physics, RAS, 142432 Chernogolovka, Moscow Region, Russia; E-mail: mele@issp.ac.ru. 297
DOI: 10.1081/FST-120027183 Copyright # 2004 by Marcel Dekker, Inc. 1536-383X (Print); 1536-4046 (Online) www.dekker.com


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The optical transitions to the lowest excited singlet state of C60 (symmetry T1g) are dipole forbidden, whereas the first allowed transition to 1T1u state has noticeably higher energy.[1] The quantum yield of fluorescence is related to vibronically assisted transitions and increases from solution to solid due to the impurities and defects. The polymerization of C60 under high pressure and high temperature treatment changes the crystal structure and phonon spectrum of material and one can expect also changes in the electron energy spectrum of polymers.[2,3] In view of this we have measured carefully the photoluminescence (PL) spectra of the planar C60 polymers, namely crystalline twodimensional rhombohedral (2D-R) and tetragonal (2D-T) polymeric phases, at pressures up to 4.0 GPa and at room temperature. The 2D-R polymer was obtained by subjecting 99.99% pure C60 powder to a pressure of P % 5 GPa at a temperature T % 773 K. The x-ray analysis Ї confirmed the rhombohedral crystal structure (space group: R3m).[2] The 2D-T polymer (space group: P42mmc) was obtained in a similar way at P % 2.2 GPa and T % 820 K. The PL spectra were recorded with spatial resolution 20 mkm by the use of microscope arrangement for excitation and a single monochromator for registration, the spectral width of the system was
1

Figure 1. pressure.

The PL spectra of the pristine C60, 2D-R and 2D-T polymers at ambient


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2D Tetragonal and Rhombohedral Polymers of C60

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0.5 meV. The samples were tested in order to identify the 2D-T and 2D-R phases by means of by micro-Raman probing. Measurements at high pressures were carried out using the diamond anvil cell of Mao-Bell type. The PL spectra of the pristine C60, 2D-R and 2D-T polymers at T % 300 K are shown in Fig. 1. The spectrum of the pristine C60 at room temperature is rather diffuse but becomes well structured at T ј 10 K due to the trapping of exitations on shallow defect levels.[4] The PL spectrum of the 2D-R polymer at T % 300 K is structured while the spectrum of the 2D-T polymer is diffuse. Figure 2 shows the pressure dependence of the principal bands in the PL spectra of the 2D-R, 2D-T polymers and pristine C60. The intensity distributions in the PL spectra and the pressure-induced shifts of the principal bands are quite different for the pristine C60, 2D-T and 2D-R polymers. The pressure coefficients @E/@P are equal to 2 80, 2 25, and 2 11 meV/GPa for main bands of the pristine C60, 2D-R and 2D-T polymers, respectively. The deformation potential, D ј @Eg/@ln(V0/V ) ј 2 B0 б @Eg/@P where Eg is the direct gap and B0 the bulk modulus of a material, drops from 1.15 eV for C60 to

Figure 2. The pressure dependence of the principal bands in the PL spectra of the pristine C60, 2D-R and 2D-T polymers. Circles and squares represent the data for 2D-R and 2D-T polymer, respectively; stars show the pressure dependence of the fluorescence band in the pristine C60; open and solid symbols show increasing and decreasing pressure runs, respectively.


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0.7 eV for 2D-R and to 0.33 eV for 2D-T. The decrease of the pressure coefficients is related with the increase of the bulk moduli B0 of the C60 polymers (14.4, 28.1, and 29.9 GPa for the pristine C60, 2D-R and 2D-T, respectively).[5,6] Despite of proximity of B0 the two planar polymers have rather different pressure coefficients and deformation potentials. In addition, the intensity distribution in the PL spectra of the 2D-R and 2D-T polymers is quite different. Taking into account the changes in the crystal structure and phonon spectra we suppose that polymerization of C60 transforms also the electron energy spectra in a way specified by the number and location of the intermolecular covalent bonds. ACKNOWLEDGMENTS The support by the Russian Foundation for Basic Research, grant 03-0216011, and the support by the Russian State Research Program "Physical properties of the carbon-based nanostructures and development of a new electronic devices" is greatly acknowledged. REFERENCES 1. Negri, F.; Orlandi, G.; Zerbetto, F. Quantum-chemical investigations of Franck-Condon and Jahn-Teller activity in the electronic spectra of Buckminsterfullerene. Chem. Phys. Lett. 1998, 144, 31 ­ 37. 2. Iwasa, Y.; Arima, T.; Fleming, R.M.; Siegrist, T.; Zhou, O.; Haddon, R.C.; Rothberg, L.J.; Lyons, K.B.; Carter, H.L., Jr.; Hebard, A.F.; Tycko, R.; Dabbagh, G.; Krajewski, J.J.; Thomas, G.A.; Yagi, T. New phases of C60 synthesized at high pressure. Science 1994, 264, 1570 ­ 1572. 3. Meletov, K.P.; Arvantidis, J.; Tsilika, I.; Assimopoulos, S.; Kourouklis, G.A.; Ves, S.; Soldatov, A.; Prassides, K. Pressure-induced phase in tetragonal two-dimensional polymeric C60. Phys. Rev. B 2001, 63, 054106-1 ­ 054106-4. Ё Ё 4. Guss, W.; Feldmann, J.; Gobel, E.O.; Taliani, C.; Mohn, H.; Muller, W.; Ё Haussler, P.; ter Meer, H.-U. Fluorescence from X traps in C60 single crystals. Phys. Rev. Lett. 1994, 72, 2644 ­ 2647. 5. Sundqvist, B. Fullerenes under high pressure. Adv. Phys. 1999, 48, 1 ­ 134. 6. Kawasaki, S.; Yao, A.; Matsuoka, Y.; Komiyama, S.; Okino, F.; Touhara, H.; Suito, K. Elastic properties of pressure-polymerized polymers. Solid State Commun. 2003, 125, 637 ­ 640. Submitted July 4, 2003 Accepted August 10, 2003