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Pulse height defect in pCVD and scCVD diamond based detectors
Diamond & Related Materials 15 (2006) 1986 – 1989 www.elsevier.com/locate/diamond
Pulse height defect in pCVD and scCVD diamond based detectors C. Tuve' a,b,⁎, R. Potenza a,b , M. Chiorboli a,b , M.G. Grimaldi a,f , F. La Rosa a , F. Raimondo a , M. Marinelli c , E. Milani c , A. Tucciarone c , G. Verona Rinati c , M. Donato d , G. Faggio d , G. Messina d , S. Santangelo d , G. Pucella e a
c
Department of Physics and Astronomy, University of Catania, Italy b INFN, Sezione di Catania, Italy Department of Mechanical Engineering, University of Rome “Tor Vergata”, Rome, Italy d Department of Mechanics and Materials, University of Reggio Calabria, Italy e CNR, Rome, Italy f CNR-Matis, Italy Available online 26 September 2006
Abstract Polycrystalline (pCVD) and single crystal (scCVD) diamond films grown from Chemical Vapour Deposition (CVD), if sufficiently pure at Raman analysis, are very good materials for beam or flux monitors inside accelerators or nuclear reactors. This is because they are very hard to damage in high radiation fields and very resistant to high temperatures. Films of pCVD diamond are, however, not so good as spectroscopy detectors due to inhomogeneities induced by their growth in grains with the consequent presence of grain boundaries which worsen their energy resolution. The latter can be significantly improved by growing scCVD diamond films onto HPHT synthetic diamond substrates. We have shown that it is possible to measure the density of defects inside diamond specimens using as probes suitable penetrating nuclear radiations. With the preliminary results reported here we'll show that, bombarding CVD diamond films grown at Roma “Tor Vergata” with energetic protons and 4He, 6 Li and 12C ions produced in the accelerators of Catania laboratories, the pulse height defects are higher than those in silicon detectors and likewise well described by a power law in the deposited energy. Furthermore, we'll show that pulse heights for the same particles seem to depend on the duration of the measurement, thus exhibiting a sort of depolarization of the insulator when exposed to the electric voltage which makes it a particle detector. © 2006 Elsevier B.V. All rights reserved. Keywords: Single crystal CVD diamond detectors; Charge collection efficiency = 100%
1. Introduction The special mechanical and thermal properties of diamond are well known [1–3]. It is the hardest known material and, different from any other material, it is an insulator with a high thermal conductivity. It has a very high melting point (3815 °C). It also possesses the highest refractive index among dielectrics (n = 2.42), which allows its crystals to decompose and reflect light so as to exhibit the characteristic beauty of the well known gem. The relatively high band gap (5.5 eV), which gives diamond its insulating properties, implies that when diamond is traversed ⁎ Corresponding author. Department of Physics and Astronomy, University of Catania, Italy. E-mail address: [email protected] (C. Tuve'). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.07.018
by ionizing particles, an electrical pulse can be collected in an external circuit for either sign of the applied voltage. Moreover, due to this high gap, the dark current is negligible at room temperature, but, on the other hand, the number of carrier pairs produced by the crossing particle is about one fifth of that produced in silicon, with a reduction of the signal amplitude. The above-mentioned properties of diamond seem rather stable against radiation bombardment, which makes diamond a promising substitute of silicon in building nuclear radiation detectors to be employed in regions with strong radiation fields [4]. Natural diamond costs however too much for this aim. Among synthetic diamonds, those obtained through CVD are probably the most suitable ones for the realization of detectors. We have built detectors of diamond obtained from CVD onto silicon backing (pCVD) at the Laboratories of the Department of Mechanical Engineering of the University of Rome “Tor
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
1987
Vergata” and have used them as in-beam monitors for beams from the Tandem accelerator of the Laboratories of Southern Italy (LNS) of the National Institute of Nuclear Physics (INFN) in Catania, Italy, with very good results [5]. In this application, diamond films proved to be very resistant to radiation damage up to fluences of more than 1014 part/cm2. The same or similar pCVD diamond films were also used to detect single particles and measure their energy spectra. Results on these measurements are presented in this paper. 2. pCVD diamond detectors Energy spectra were obtained of heavy ions from the cited Tandem accelerator as well as of protons and α-particles from the 3.5 MV Singletron Cockcroft–Walton accelerator of the Department of Physics and Astronomy of the University of Catania. As Fig. 1 shows, the resolution was very poor. This is due to the structural defects concentrated near the transition region between silicon and diamond, due to the columnar nature of the pCVD diamond growth. Using the energy spectra of different penetrating particles for both positive and negative signs of the applied voltage, we have found that the electrons and holes have very different mean free paths inside the pCVD diamond, so that only a suitable two-fluid modified Hecht model for charge transport could account for the data [6–8]. In this model it was possible to include: i) the effect of nonuniform ionization density along the particle path (Bragg curve) and ii) the effects of both types of charge absorbing defects, those uniformly distributed in the volume of the specimen, and partly saturated through the priming (pumping) procedure [6], and those more concentrated near the transition region. The model allows to calculate the charge collection efficiency η = Q / Q0 ≡ Qcollected / Qinjected for a detector (η ∼ 1 for silicon). Typical curves, such as those shown in Fig. 2 for light and heavy ions of different energies incident onto a pCVD diamond detector, confirm the peaking of the defect distribution near the transition region (because of the agreement with the model curve, which contains contributions of both uniform and nonuniform defects) and reveal that the mean concentration of
Fig. 1. Energy spectrum of a pCVD diamond detector (50 μm thick; Vbias = ±50 V) for 12C of 16.2 MeV.
Fig. 2. Charge collection efficiency vs. penetration depth for different ions in pCVD diamond in positive polarity, corrected for pulse height defect and compared with calculations from the modified Hecht model with a defect distribution exponentially concentrated near the Si–diamond transition region.
defects is still so large that the electron mean free path is very short (this reduces the efficiency for higher penetrating particles to about 50% of the value for small penetration), as it is also confirmed by the small values of the pulse height in negative polarity [7]. 3. Pulse height defect in pCVD diamond detectors The charge collection efficiency of pCVD detectors depends, at a given penetration, on the incident particle, as shown by Fig. 3. This is the known phenomenon of “pulse height defect”. It is common to all radiation detectors and depends on the ratio between the rate of charge recombination in the plasma created along the track of the incident particle (which in turn depends on the local density of carriers) and their velocity in the applied electric field. As for other detector materials (silicon, scintillators, etc.) a law of the type D = a · E b (1) well describes the pulse height defect D, where E is the
Fig. 3. The same data as Fig. 2, in positive polarity, not corrected for pulse height defect.
1988
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
incident energy, a depends on the particle and b is a common constant. Taking D = 0 for protons and using Eq. (1) fitted to the data, we corrected the data for this pulse height defect and obtained the results reported in Fig. 2, where the agreement between different data appears very good. This effect is common also to the scCVD detectors, though with smaller values of the parameter a in Eq. (1). 4. scCVD diamond detectors and their performance While there is hope to reduce the uniformly distributed defects present in pCVD films, or at least those due to impurities (though the specimens used here appear very pure at Raman examination [7]), the structural non-uniformly distributed defects cannot probably be reduced, so that our group decided to shift the production to scCVD, growing new diamond films onto backings of synthetic diamond too, but obtained from the HPHT technique, with plenty of metal impurities in order to insure a high electrical conductivity. After having tested through Raman spectra that the scCVD films were very pure, we bombarded them with fast protons and α-particles produced at the mentioned 3.5 MV Singletron accelerator and with 12C ions accelerated at the mentioned Tandem of the LNS. A typical 12C particle spectrum obtained with these early built scCVD diamond detectors is reported in Fig. 4 for comparison with that of a pCVD detector (Fig. 1). The energy resolution is now much better and is comparable to that of a silicon detector. Some data on charge collection efficiency are reported in Fig. 5 after correction for the pulse height defect. It is to be strongly remarked that for the first time we succeeded in reaching η = 100%, that is the same as silicon, except for the smaller number of charge carriers produced in diamond. Data are compared with theoretical curves now calculated introducing only a uniform distribution of defects. There was no need to introduce concentrations of defects near the transition region since the mismatch of reticular constants had disappeared, as what the agreement between theory and experiment demonstrates. How-
Fig. 4. Energy spectrum of a scCVD diamond detector (110 μm thick; Vbias = 80 V) for 12C of 16.2 MeV.
Fig. 5. Charge collection efficiency vs. penetration for different ions in scCVD diamond (45 μm thick; Vbias = 130 V) in positive polarity, corrected for pulse height defect and compared with calculations from the modified Hecht model with a uniform defect distribution (no concentration near HPHT–CVD diamond transition region).
ever, the inequality between the mean free paths of the charge carriers is still present, as the dependence of the efficiency curve on the particle penetration indicates, even though it was impossible to collect data in negative polarity (the specimens underwent discharge for an unforeseen injection of positive carriers from the backing). 5. Time dependence of pulse heights A second unexpected phenomenon appeared during our measurements on pCVD diamond: the pulse heights produced by the used diamond detector varied during each measurement. We saw that taking data for different short time intervals during the same run, the mean value of the efficiency decreased progressively. At the end of each run, we reversed the polarity, thus alternating polarity from one run to the next. We saw that every
Fig. 6. Time dependence of pulse heights produced during a measurement of charge collection efficiency with protons of Einc = 1.5 MeV. a) for a pCVD; b) for a scCVD.
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
second run, when the previous polarity was restored, the data took again the same initial values, beginning again to decrease, so showing a complete recovery after the application of a reverse bias for a few minutes. Fig. 6a) reports the trend of η during a typical run with incident protons. Using this trend we were able to correct the data taken in long runs without interruptions. We saw this deterioration data also in scCVD diamond detectors. In Fig. 6 there is a comparison between the two types of diamond detectors with protons at Einc = 1.5 MeV. The figure shows a similar trend of charge collection efficiency vs. measurement time. Since this effect is present in both pCVD and scCVD a possible explanation is some sort of polarization due e.g. to carrier pile-up at contacts or space charge formation in the bulk of the crystal. Up to now no measurement has been dedicated specifically to studying this effect. We will perform these measurements in the near future, varying independently beam current and run duration and varying also the temperature of the sample, in order to confirm (or exclude) the slow depolarization of the samples. 6. Conclusions The two-fluid modified Hecht model for charge transport inside the diamond with the addition of the correction for pulse height defect works very well in describing the gross structure of defect distribution in diamond films. Application of this model to pCVD and scCVD diamond shows the big step forward achieved in obtaining good radiation hard nuclear detectors: uniform homoepitaxial growth of diamond crystals without grain formation and consequently without grain
1989
boundary defects concentrated near the transition region between the crystal and substrate. The improvement of the energy resolution well confirms this. A charge collection efficiency η = 1, obtained for the first time in diamond is a further confirmation. More work seems however necessary to realize truly ohmic contacts and to avoid the effects of dielectric delayed polarization, if any, during measurements. Acknowledgement The authors wish to thank Ms. S. Tatì for her help in measurements at the Physics Department of University of Catania. References [1] S. Pan, et al., Journal of Applied Physics 74 (1993) 1086. [2] A. Paoletti, A. Tuccuarone (Eds.). The Physics of Diamond, Proceedings of the 135th course of the International School of Physics "Enrico Fermi" 1996, Vatenna, Italy, IOS Press, Amsterdam. [3] M. Friedl, Diamond Detectors for Ionizing Radiation, Diploma Thesis, University of Technology, Wien 1999. [4] W. Adam, et al., The RD42 Collaboration, Development of Diamond Tracking Detectors for High Luminosity Experiments at the LHC-CERNLHCC-97-003 Report — Geneva, 1996. [5] M. Marinelli, E. Milani, A. Paoletti, A. Tucciarone, G. Verona Rinati, S. Albergo, V. Bellini, V. Campagna, C. Marchetta, A. Pennisi, G. Poli, R. Potenza, F. Simone, L. Sperduto, C. Sutera, Diamond and Related Materials 10 (2001) 706. [6] Marco Marinelli, et al., Diamond and Related Materials 10 (2001) 645. [7] C. Tuve', et al., Diamond and Related Materials 12 (2003) 499. [8] R. Potenza, C. Tuve', in: G. Messina, S. Santangelo (Eds.), Carbon: the Future Material for Advanced Technology Applications, Springer Series Topics in Applied Physics, 2005, p. 267.
Pulse height defect in pCVD and scCVD diamond based detectors C. Tuve' a,b,⁎, R. Potenza a,b , M. Chiorboli a,b , M.G. Grimaldi a,f , F. La Rosa a , F. Raimondo a , M. Marinelli c , E. Milani c , A. Tucciarone c , G. Verona Rinati c , M. Donato d , G. Faggio d , G. Messina d , S. Santangelo d , G. Pucella e a
c
Department of Physics and Astronomy, University of Catania, Italy b INFN, Sezione di Catania, Italy Department of Mechanical Engineering, University of Rome “Tor Vergata”, Rome, Italy d Department of Mechanics and Materials, University of Reggio Calabria, Italy e CNR, Rome, Italy f CNR-Matis, Italy Available online 26 September 2006
Abstract Polycrystalline (pCVD) and single crystal (scCVD) diamond films grown from Chemical Vapour Deposition (CVD), if sufficiently pure at Raman analysis, are very good materials for beam or flux monitors inside accelerators or nuclear reactors. This is because they are very hard to damage in high radiation fields and very resistant to high temperatures. Films of pCVD diamond are, however, not so good as spectroscopy detectors due to inhomogeneities induced by their growth in grains with the consequent presence of grain boundaries which worsen their energy resolution. The latter can be significantly improved by growing scCVD diamond films onto HPHT synthetic diamond substrates. We have shown that it is possible to measure the density of defects inside diamond specimens using as probes suitable penetrating nuclear radiations. With the preliminary results reported here we'll show that, bombarding CVD diamond films grown at Roma “Tor Vergata” with energetic protons and 4He, 6 Li and 12C ions produced in the accelerators of Catania laboratories, the pulse height defects are higher than those in silicon detectors and likewise well described by a power law in the deposited energy. Furthermore, we'll show that pulse heights for the same particles seem to depend on the duration of the measurement, thus exhibiting a sort of depolarization of the insulator when exposed to the electric voltage which makes it a particle detector. © 2006 Elsevier B.V. All rights reserved. Keywords: Single crystal CVD diamond detectors; Charge collection efficiency = 100%
1. Introduction The special mechanical and thermal properties of diamond are well known [1–3]. It is the hardest known material and, different from any other material, it is an insulator with a high thermal conductivity. It has a very high melting point (3815 °C). It also possesses the highest refractive index among dielectrics (n = 2.42), which allows its crystals to decompose and reflect light so as to exhibit the characteristic beauty of the well known gem. The relatively high band gap (5.5 eV), which gives diamond its insulating properties, implies that when diamond is traversed ⁎ Corresponding author. Department of Physics and Astronomy, University of Catania, Italy. E-mail address: [email protected] (C. Tuve'). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.07.018
by ionizing particles, an electrical pulse can be collected in an external circuit for either sign of the applied voltage. Moreover, due to this high gap, the dark current is negligible at room temperature, but, on the other hand, the number of carrier pairs produced by the crossing particle is about one fifth of that produced in silicon, with a reduction of the signal amplitude. The above-mentioned properties of diamond seem rather stable against radiation bombardment, which makes diamond a promising substitute of silicon in building nuclear radiation detectors to be employed in regions with strong radiation fields [4]. Natural diamond costs however too much for this aim. Among synthetic diamonds, those obtained through CVD are probably the most suitable ones for the realization of detectors. We have built detectors of diamond obtained from CVD onto silicon backing (pCVD) at the Laboratories of the Department of Mechanical Engineering of the University of Rome “Tor
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
1987
Vergata” and have used them as in-beam monitors for beams from the Tandem accelerator of the Laboratories of Southern Italy (LNS) of the National Institute of Nuclear Physics (INFN) in Catania, Italy, with very good results [5]. In this application, diamond films proved to be very resistant to radiation damage up to fluences of more than 1014 part/cm2. The same or similar pCVD diamond films were also used to detect single particles and measure their energy spectra. Results on these measurements are presented in this paper. 2. pCVD diamond detectors Energy spectra were obtained of heavy ions from the cited Tandem accelerator as well as of protons and α-particles from the 3.5 MV Singletron Cockcroft–Walton accelerator of the Department of Physics and Astronomy of the University of Catania. As Fig. 1 shows, the resolution was very poor. This is due to the structural defects concentrated near the transition region between silicon and diamond, due to the columnar nature of the pCVD diamond growth. Using the energy spectra of different penetrating particles for both positive and negative signs of the applied voltage, we have found that the electrons and holes have very different mean free paths inside the pCVD diamond, so that only a suitable two-fluid modified Hecht model for charge transport could account for the data [6–8]. In this model it was possible to include: i) the effect of nonuniform ionization density along the particle path (Bragg curve) and ii) the effects of both types of charge absorbing defects, those uniformly distributed in the volume of the specimen, and partly saturated through the priming (pumping) procedure [6], and those more concentrated near the transition region. The model allows to calculate the charge collection efficiency η = Q / Q0 ≡ Qcollected / Qinjected for a detector (η ∼ 1 for silicon). Typical curves, such as those shown in Fig. 2 for light and heavy ions of different energies incident onto a pCVD diamond detector, confirm the peaking of the defect distribution near the transition region (because of the agreement with the model curve, which contains contributions of both uniform and nonuniform defects) and reveal that the mean concentration of
Fig. 1. Energy spectrum of a pCVD diamond detector (50 μm thick; Vbias = ±50 V) for 12C of 16.2 MeV.
Fig. 2. Charge collection efficiency vs. penetration depth for different ions in pCVD diamond in positive polarity, corrected for pulse height defect and compared with calculations from the modified Hecht model with a defect distribution exponentially concentrated near the Si–diamond transition region.
defects is still so large that the electron mean free path is very short (this reduces the efficiency for higher penetrating particles to about 50% of the value for small penetration), as it is also confirmed by the small values of the pulse height in negative polarity [7]. 3. Pulse height defect in pCVD diamond detectors The charge collection efficiency of pCVD detectors depends, at a given penetration, on the incident particle, as shown by Fig. 3. This is the known phenomenon of “pulse height defect”. It is common to all radiation detectors and depends on the ratio between the rate of charge recombination in the plasma created along the track of the incident particle (which in turn depends on the local density of carriers) and their velocity in the applied electric field. As for other detector materials (silicon, scintillators, etc.) a law of the type D = a · E b (1) well describes the pulse height defect D, where E is the
Fig. 3. The same data as Fig. 2, in positive polarity, not corrected for pulse height defect.
1988
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
incident energy, a depends on the particle and b is a common constant. Taking D = 0 for protons and using Eq. (1) fitted to the data, we corrected the data for this pulse height defect and obtained the results reported in Fig. 2, where the agreement between different data appears very good. This effect is common also to the scCVD detectors, though with smaller values of the parameter a in Eq. (1). 4. scCVD diamond detectors and their performance While there is hope to reduce the uniformly distributed defects present in pCVD films, or at least those due to impurities (though the specimens used here appear very pure at Raman examination [7]), the structural non-uniformly distributed defects cannot probably be reduced, so that our group decided to shift the production to scCVD, growing new diamond films onto backings of synthetic diamond too, but obtained from the HPHT technique, with plenty of metal impurities in order to insure a high electrical conductivity. After having tested through Raman spectra that the scCVD films were very pure, we bombarded them with fast protons and α-particles produced at the mentioned 3.5 MV Singletron accelerator and with 12C ions accelerated at the mentioned Tandem of the LNS. A typical 12C particle spectrum obtained with these early built scCVD diamond detectors is reported in Fig. 4 for comparison with that of a pCVD detector (Fig. 1). The energy resolution is now much better and is comparable to that of a silicon detector. Some data on charge collection efficiency are reported in Fig. 5 after correction for the pulse height defect. It is to be strongly remarked that for the first time we succeeded in reaching η = 100%, that is the same as silicon, except for the smaller number of charge carriers produced in diamond. Data are compared with theoretical curves now calculated introducing only a uniform distribution of defects. There was no need to introduce concentrations of defects near the transition region since the mismatch of reticular constants had disappeared, as what the agreement between theory and experiment demonstrates. How-
Fig. 4. Energy spectrum of a scCVD diamond detector (110 μm thick; Vbias = 80 V) for 12C of 16.2 MeV.
Fig. 5. Charge collection efficiency vs. penetration for different ions in scCVD diamond (45 μm thick; Vbias = 130 V) in positive polarity, corrected for pulse height defect and compared with calculations from the modified Hecht model with a uniform defect distribution (no concentration near HPHT–CVD diamond transition region).
ever, the inequality between the mean free paths of the charge carriers is still present, as the dependence of the efficiency curve on the particle penetration indicates, even though it was impossible to collect data in negative polarity (the specimens underwent discharge for an unforeseen injection of positive carriers from the backing). 5. Time dependence of pulse heights A second unexpected phenomenon appeared during our measurements on pCVD diamond: the pulse heights produced by the used diamond detector varied during each measurement. We saw that taking data for different short time intervals during the same run, the mean value of the efficiency decreased progressively. At the end of each run, we reversed the polarity, thus alternating polarity from one run to the next. We saw that every
Fig. 6. Time dependence of pulse heights produced during a measurement of charge collection efficiency with protons of Einc = 1.5 MeV. a) for a pCVD; b) for a scCVD.
C. TuveT et al. / Diamond & Related Materials 15 (2006) 1986–1989
second run, when the previous polarity was restored, the data took again the same initial values, beginning again to decrease, so showing a complete recovery after the application of a reverse bias for a few minutes. Fig. 6a) reports the trend of η during a typical run with incident protons. Using this trend we were able to correct the data taken in long runs without interruptions. We saw this deterioration data also in scCVD diamond detectors. In Fig. 6 there is a comparison between the two types of diamond detectors with protons at Einc = 1.5 MeV. The figure shows a similar trend of charge collection efficiency vs. measurement time. Since this effect is present in both pCVD and scCVD a possible explanation is some sort of polarization due e.g. to carrier pile-up at contacts or space charge formation in the bulk of the crystal. Up to now no measurement has been dedicated specifically to studying this effect. We will perform these measurements in the near future, varying independently beam current and run duration and varying also the temperature of the sample, in order to confirm (or exclude) the slow depolarization of the samples. 6. Conclusions The two-fluid modified Hecht model for charge transport inside the diamond with the addition of the correction for pulse height defect works very well in describing the gross structure of defect distribution in diamond films. Application of this model to pCVD and scCVD diamond shows the big step forward achieved in obtaining good radiation hard nuclear detectors: uniform homoepitaxial growth of diamond crystals without grain formation and consequently without grain
1989
boundary defects concentrated near the transition region between the crystal and substrate. The improvement of the energy resolution well confirms this. A charge collection efficiency η = 1, obtained for the first time in diamond is a further confirmation. More work seems however necessary to realize truly ohmic contacts and to avoid the effects of dielectric delayed polarization, if any, during measurements. Acknowledgement The authors wish to thank Ms. S. Tatì for her help in measurements at the Physics Department of University of Catania. References [1] S. Pan, et al., Journal of Applied Physics 74 (1993) 1086. [2] A. Paoletti, A. Tuccuarone (Eds.). The Physics of Diamond, Proceedings of the 135th course of the International School of Physics "Enrico Fermi" 1996, Vatenna, Italy, IOS Press, Amsterdam. [3] M. Friedl, Diamond Detectors for Ionizing Radiation, Diploma Thesis, University of Technology, Wien 1999. [4] W. Adam, et al., The RD42 Collaboration, Development of Diamond Tracking Detectors for High Luminosity Experiments at the LHC-CERNLHCC-97-003 Report — Geneva, 1996. [5] M. Marinelli, E. Milani, A. Paoletti, A. Tucciarone, G. Verona Rinati, S. Albergo, V. Bellini, V. Campagna, C. Marchetta, A. Pennisi, G. Poli, R. Potenza, F. Simone, L. Sperduto, C. Sutera, Diamond and Related Materials 10 (2001) 706. [6] Marco Marinelli, et al., Diamond and Related Materials 10 (2001) 645. [7] C. Tuve', et al., Diamond and Related Materials 12 (2003) 499. [8] R. Potenza, C. Tuve', in: G. Messina, S. Santangelo (Eds.), Carbon: the Future Material for Advanced Technology Applications, Springer Series Topics in Applied Physics, 2005, p. 267.