JOURNAL OF APPLIED PHYSICS 105, 024314 共2009兲

Modification of molecular transitions in fullerene films under ion impacts Amit Kumar,1,a兲,b兲 A. Podhorodecki,2 J. Misiewicz,2 D. K. Avasthi,3,a兲,c兲 and J. C. Pivin4 1

Institut Néel, Centre National de la Recherche Scientifique (CNRS) and Université Joseph Fourier (UJF), 25 rue des Martyrs, BP166, 38042 Grenoble Cedex 9, France 2 Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland 3 Material Science Group, Inter-University Accelerator Centre, Aruna Asaf Ali Marg, PB-10502, New Delhi 110067, India 4 Centre Spectrometrie Nucleaire et de Spectrometrie de Masse (CSNSM), IN2P3-CNRS, Batiment 108, Orsay Campus 91405, France

共Received 23 October 2008; accepted 16 December 2008; published online 30 January 2009兲 The photoluminescence properties of as-deposited and 90 MeV Si ion irradiated fullerene films are reported. The irradiation of fullerene films leads to some enhancement or quenching in molecular transition intensities 共photoluminescence intensities兲 depending on the ion fluence. These effects are explained in terms of lowering in the energy level symmetry resulting from induced chemical disordering. The structural modifications occurring upon irradiation were investigated using Fourier transform infrared and UV-vis spectroscopies. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3074104兴 I. INTRODUCTION

The properties of fullerene 共C60兲 are attractive for many applications in nanotechnological devices such as single electron transistors, quantum computing, etc.1–4 The understanding of the physical properties and of their modification by external perturbations are topics of tremendous interest. Phase changes have been reported under high temperature and pressure, UV light, e-beam or energetic ions irradiation accompanied by drastic changes in the electrical, magnetic, and/or optical properties.5–9 Fullerene balls have a molecular structure with a semiconductorlike character and a gap of about 1.9 eV between the highest occupied molecular orbital and lowest unoccupied molecular orbital 共HOMO–LUMO兲.10 For an isolated molecule, transitions between the HOMO and LUMO states are dipole forbidden11 and no luminescence could be detected at room temperature 共RT兲 for C60 molecules in solution. Although, for neutral C60 in solid form, a few experiments account for a very weak luminescence at RT, with an energy 共around 1.7 eV兲, which always lies within the HOMO–LUMO gap.12–14 A quenching or enhancement of photoluminescence 共PL兲 or electroluminescence is expected with the change in the crystal symmetry resulting from some treatments.9,10 For instance in studies of fullerene mixed with polystyrene, oxide materials 共ZrO2, Al2O3, and SnO2兲,15 or polymerized and irradiated fullerene films16,17 HOMO–LUMO radiative transitions were observed. The authors suggested that the electronic states of C60 molecules are probably perturbed by a chemical reaction between the dispersed C60 molecules and the oxide matrix. As a consequence, the momentum selection rules are broken and radiative recombinations are liable to occur. A similar effect should be promoted by irradiation a兲

Authors to whom correspondence should be addressed. Tel.: ⫹33–476887469. FAX: ⫹33–476887988. Electronic [email protected]. c兲 Electronic mail: [email protected].

with energetic ions. It is known that the ion irradiation of fullerene thin films induces an ordering/polymerization at low fluence 共⬍1 ⫻ 1013 ions/ cm2兲 and a fragmentation of the molecules at higher fluence 共⬎1 ⫻ 1013 ions/ cm2兲.6,18–20 Earlier investigations on irradiated films showed an increase in oxygen content as a function of the ion fluence.21,22 Therefore, the ion irradiation can be used to create discrete defects, a polymerization or a fragmentation, and oxidation of thin C60 films by choosing the appropriate ion fluence, for tuning of the optical properties. These known changes in the structure prompt us to investigate the modifications in electronic transitions caused by energetic ion impacts. The present work reports the observation of formally forbidden optical transitions in fullerene molecules and the influence of the ion irradiation on these transitions. The structural modifications in fullerene films under ion irradiation have been studied using Fourier transform infrared and UV-vis spectroscopies. II. EXPERIMENTAL

Fullerene thin films were deposited on silicon 共100兲 and quartz substrates in a vacuum better than 1 ⫻ 10−6 Torr by resistive heating, using commercially available C60 共Aldrich兲 in a Ta boat. The thickness of the films 共200 nm兲 was monitored by quartz crystal monitor during film deposition and later confirmed by Rutherford backscattering spectroscopy measurements. The absence of any optically active impurity in the films was carefully checked by x-ray fluorescence 共XRF兲 at Punjab University Chandigarh, India and particle induced x-ray emission 共PIXE兲, at Louvre museum, Paris. Fourier transform infrared 共FTIR兲 and UV-vis measurements were performed on the as deposited and Si ion irradiated C60 films. The FTIR spectra were obtained with a Nexus 670 FTIR spectrometer, in the scan range of 400– 4000 cm−1, with a spectral resolution of 2 cm−1. The optical absorption was recorded with the conventional two-

b兲

mail:

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© 2009 American Institute of Physics

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FIG. 1. 共Color online兲 FTIR spectra of pristine and irradiated fullerene films.

beam method using U-3300 UV-vis spectrophotometer of Hitachi, in order to determine the electronic band gap. PL was recorded using 302 nm Ar laser line with 10 mW laser power and PL spectra were analyzed by HR4000 Ocean Optics spectrometer. The laser beam was not focused to avoid the sample damage. The C60 films were irradiated with 90 MeV Si ions at fluences of 1 ⫻ 1012, 1 ⫻ 1013, and 1 ⫻ 1014 ions/ cm2, using 15 UD tandem Pelletron Accelerator at IUAC, New Delhi, India. Ions transfer energy to the target system in nuclear collisions, which is dominant at low energies 共1–50 keV/ nucleon兲 and by electronic excitations/ionizations, which prevails at high energies 共ions with energies ⬎1 MeV/ nucleon兲. In the case of 90 MeV Si ions, the electronic and nuclear energy losses in C60 films are Se ⬃ 4.6 keV/ nm, Sn ⬃ 4 eV/ nm, respectively, and the range of 90 MeV ion is R p ⬃ 31 ␮m, as simulated by the stopping and range of ions in matter.23 So the electronic stopping is dominant in present case and the 90 MeV Si ions get implanted in the Si substrate in ⬃31 ␮m depth. III. RESULTS AND DISCUSSION

The FTIR spectra of pristine and irradiated fullerene films are shown in Fig. 1共a兲. The fullerene films exhibit four fundamental IR 共T1u兲 active internal modes at 527, 576, 1183, and 1428 cm−1. For better clarity the lower wave number IR modes of C60 are shown in Fig. 1共a兲. The peaks observed are fundamental IR active modes 共T1u兲 consistent with those previously reported in literature and are attributed to the internal modes of the C60 molecule.24 At low fluence 共1 ⫻ 1012 ions/ cm2兲, there is no significant change in the vibration strength, whereas at higher fluence the intensities of C60 IR modes decrease with the increase in the ion fluence. This intensity decrease is attributed to the breaking of fullerene molecules. Figure 1共b兲 shows the IR spectra of selected region to evidence the oxygen attachments to films due to fragmentations of molecules under ion irradiation. The spectra of irradiated films show a peak doublet in the spectral region from 1734 to 1694 cm−1, which is attributed to the C = O vibration. Nuclear reaction analysis of pristine and irradiated films indicated that the pristine films contain

FIG. 2. 共Color online兲 UV-vis spectra of pristine and irradiated fullerene films.

⬃1% oxygen and the oxygen content increases up to ⬃10% in irradiated films.21 Other x-ray photoelectron spectroscopy measurements also give a fair indication of the oxygen attachment in ion irradiated fullerene films.25 The oxidation is most probably due to the formation of dangling bonds, thereafter trapping oxygen or water molecules as the films are exposed to atmospheric environment. Figure 2 shows the UV-visible absorbance spectra of pristine and irradiated films deposited on quartz substrates. The UV-vis absorbance spectra exhibit three broad peaks, centered at ⬃620, ⬃430, and ⬃345 nm. The observed peaks are attributed to three different transitions: the dipole transition from HOMO to LUMO and two other allowed dipole transitions, respectively.25 It is observed that the absorbance spectra of pristine and low fluence 共1 ⫻ 1012 ions/ cm2兲 irradiated film have almost similar features. For better clarity, the absorbance spectrum for the film irradiated at the fluence of 1 ⫻ 1012 ions/ cm2 is slightly shifted upward. The absorbance spectrum of the film irradiated at fluence of 1 ⫻ 1013 ions/ cm2 shows the quenching of the intensity of optical transitions compared to pristine and films irradiated at higher fluence 共1 ⫻ 1014 ions/ cm2兲. The PL spectra of pristine and irradiated films with the excitation at 302 nm are shown in Fig. 3. The spectrum of the pristine film shows two broad emission peaks at around 750 and 500 nm, which are interpreted as the molecular transition Hu − T1u 共HOMO–LUMO兲 and Hu − T1g, respectively.26 Moreover, two more additional molecular transitions at around 440 and 380 nm are also observed, which are attributed to Hg − T1u and Hg − T1g transitions, respectively. The full width at half maximum of the band related to Hg − T1g transition for the pristine and 1 ⫻ 1012 ions/ cm2 fluence irradiated films are about 190⫾ 5 and 220⫾ 5 nm, respec-

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FIG. 3. 共Color online兲 PL spectra of pristine and irradiated fullerene films with excitation at 302 nm.

tively. Moreover, PL intensity of this band, for the film irradiated at lower fluence 共1 ⫻ 1012 ions/ cm2兲, is about two times higher as compared to the pristine fullerene film. PL spectrum of film irradiated at fluence of 1 ⫻ 1012 ions/ cm2 shows a decrease in HOMO–LUMO transitions intensity together with a redshift of 56 nm, compared to pristine PL spectra. Additionally, from Fig. 3, the evidence of two other higher in energy bands corresponding to Hg − T1u and Hg − T1g transitions can be observed for pristine as well for the film irradiated at the fluence of 1 ⫻ 1012 ions/ cm2. Further irradiation of the films induces the quenching of these bands. It is worth noticing that these bands were not previously reported in PL studies. The low fluence irradiation leads to partial modification or fragmentation of fullerene molecules. These partially fragmented molecules have dangling bonds, which can react with other species, especially oxygen atoms. The fragmentation and oxygen attachment lead to the lowering of the energy level symmetry and breaks the selection rules. Thus, when the partially distorted fullerene films are excited by the laser beam, the radiative transition become possible. The electronic energy states of distorted fullerene vary widely, since there is a wide variety of dangling bond sites at the surface of irradiated fullerene films. This variety of electronic states results in a broad luminescence band as observed at low fluence 共1 ⫻ 1012 ions/ cm2兲 irradiated fullerene film. At higher fluence the molecules are destroyed, therefore a quenching of PL occurs. Recently, FTIR and x-ray diffraction measurements on low fluence irradiated 共⬍5 ⫻ 1011 ions/ cm2兲 fullerene films showed some ordering or recrystallization, which were explained by a pseudothermal annealing caused by the dense electronic excitations by ions.6 To explain the above mentioned optical transitions, we consider the electronic structure of C60 molecule, proposed

FIG. 4. 共a兲 Schematic of the electronic energy states in fullerene molecule as calculated by Huckel model and 共b兲 experimentally observed optical transitions.

by Huckel,26 shown in Fig. 4共a兲. The energy of T1u orbital 共LUMO兲 with respect to Fermi level is 0.348 eV, while that of Hu 共HOMO兲 is ⫺1.549 eV. These calculations show that the energy band gap Eg = 1.897 eV in solution 共single C60兲, but in the crystal state, Eg is 10%–15% smaller. In most of the studies reported so far, 488 nm excitation was used to study the PL properties of C60 molecules/films,11,15–17,25 which is favorable to study the HOMO–LUMO 共Hu − T1u兲 transitions in molecules. In present work, we experimentally report all the molecular electronic transitions, below 4 eV energy along with HOMO–LUMO 共⬃1.7 eV兲 molecular transition, as evident in PL measurement. Figure 4共b兲 shows the schematics of the experimentally observed optical transitions with 302 nm wavelength excitation. All the observed transitions have also been identified in PL spectra given in Fig. 3. These transitions are not due to any impurity in fullerene films, as XRF and PIXE analysis ruled out these possibilities. These transitions were not reported experimentally so far. These investigations revealed that we can tailor the PL intensities in fullerene films by selecting the ion fluence. IV. CONCLUSION

This work reports the optical molecular transitions in pristine and ion irradiated fullerene films using PL spectroscopy, at 302 nm excitation wavelength. It has been shown

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that irradiation at low fluence 共1 ⫻ 1012 ions/ cm2兲 leads to enhancement of PL emission, while quenching of PL intensity occurs at higher fluence 共⬎1 ⫻ 1013 ions/ cm2兲. The enhancement of PL intensity is explained by lowering of the fullerene molecule symmetry caused by the ion irradiation, which leads to the breaking of the selection rules for optical transition. The quenching of PL intensities at higher fluence is explained due to destruction completely of fullerene molecules. ACKNOWLEDGMENTS

A.K. is thankful to the Council of Scientific and Industrial Research 共CSIR兲, New Delhi, India for providing financial support during this research. We are thankful to the crew of 15UD accelerator at IUAC New Delhi for providing stable beam. We are also thankful to Dr. Devindra Metha, Punjab University Chandigarh, India, and to L. Pichon, J. Salomon, of C2RMF, Louvre museum, Paris, France, for XRF and PIXE measurements, respectively. One of the authors 共A.P.兲 acknowledges financial support from the Foundation for Polish Science. 1

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