Eur. Phys. J. AP 14, 3–11 (2001) T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS c  EDP Sciences 2001 Excitonic photoluminescence spectra of C 60 single crystals grown by different techniques V. Capozzi a , M. Santoro, G. Perna, G. Celentano, A. Minafra, and G. Casamassima Dipartimento di Fisica dell’Universit`a di Bari and Istituto Nazionale di Fisica della Materia, Via Amendola 173, 70126 Bari, Italy Received: 7 April 2000 / Revised: 11 January 2001 / Accepted: 24 January 2001 Abstract. We report photoluminescence spectra of C60 single crystals grown by vapor phase transport method using either the sealed ampoule technique or the open tube technique. The spectra for both types of samples show similar features, but different line resolution related to the two different growth techniques. An analysis of temperature and excitation intensity dependencies of the luminescence spectra is reported. The main structures of the spectra have been interpreted according to a model involving intramolecular polaron-exciton recombinations. In particular, emissions due to purely electronic transitions of singlet and triplet or the exciton and related vibronic recombinations have been resolved. At low temperature, emission bands due to X-traps have been observed on the high-energy side of the excitonic singlet purely electronic transition. PACS. 71.35.Aa Frenkel excitons and self-trapped excitons – 78.66.Tr Fullerenes and related materials – 78.55.-m Photoluminescence 1 Introduction Since its discovery, the fullerene C 60 has been studied with great interest because of its peculiar electronic and optical properties due to the high molecular symmetry I h [1–3]. At room temperature (RT) C 60 crystallizes in a molec- ular solid with a fcc lattice in which molecules exhibit a rapid rotational motion (the rotational directions and the rotation molecular axes change continuously and ran- domly) which is quenched by decreasing the temperature (T ). At about 250 K a phase transition to the sc lattice occurs [2,3] and for T < 210 K the crystal exhibits a transition from the disorder to the orientational order [4]: only some rotational molecular axes and some rotational directions are allowed for each single cluster. Moreover at T< 150 K, a transition occurs from the pluriaxial to the uniaxial rotations in which molecular axes jump be- tween two allowed directions [4], corresponding to two dif- ferent energetic configurations for the adjacent faces of two neighboring molecules in the lattice [3,5,6]. At T< 90 K, the molecules are locked in a glassy phase, in which rota- tional axes of C 60 molecules are frozen in the two above different directions [3,7]. Photoluminescence (PL) spectra of C 60 molecules have been explained by Herzberg-Teller (H-T) and Jahn-Teller (J-T) vibronic coupling of the electronic states with ap- propriate molecular vibrational modes; in fact, the low- est energy electronic transition between the h u level (A 1g a e-mail: vito.capozzi@ba.infn.it symmetry) and the t 1u level (T 1g symmetry) is parity for- bidden by the dipole selection rules [8]. Origin of PL emis- sions are less clear for solid C 60 : the presence of crystal imperfections, such as lattice stacking faults or chemical impurities, deeply influences the luminescence pro- cesses [9]. Also in the solid C 60 the electronic transition T 1g −A 1g is forbidden because of the violation of parity for dipole selection rules, and the recombination between the first excited state and the ground state occurs by means of the coupling with the above vibrational modes H-T and J-T [5,8]. The experimental and theoretical evaluation of solid C 60 energy gap (E g ) has been one of fundamental aims of the crystal investigation, together to the determina- tion of the origin and of the energy value of the main optical transitions. In solid C 60 , the intramolecular co- valent bonds are stronger if compared to intermolecular van der Waals bonds; therefore the photoexcited particles are localized on the molecules [10]. This explains the dis- agreement between measurements and calculations of the HOMO-LUMO molecular gap and the theoretical and ex- perimental evaluation of E g for solid C 60 . The forbidden direct gap for solid C 60 with fcc lattice structure ranges between 1.5 and 2.0 eV at the X point of Brillouin zone; this gap in the solid phase corresponds to the forbidden transition between HOMO and LUMO states in the clus- ter. In fact, calculations in local density approximation show that the energetic separation between h u and t 1u in an isolated cluster is about 1.9 eV and E g for the fcc solid is 1.5 eV [11]. The optical absorption onset for solid C 60