Proceedings Volume 3142

Fullerenes and Photonics IV

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Proceedings Volume 3142

Fullerenes and Photonics IV

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Volume Details

Date Published: 1 November 1997
Contents: 7 Sessions, 25 Papers, 0 Presentations
Conference: Optical Science, Engineering and Instrumentation '97 1997
Volume Number: 3142

Table of Contents

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Table of Contents

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  • Optical Probes of Fullerenes
  • Optical and Electrical Properties of Fullerenes
  • Optical Probes of Fullerenes
  • Optical and Electrical Properties of Fullerenes
  • Nonlinear Optical Studies of Fullerenes in Different Media
  • Photoinduced Electron-Transfer Processes in Organic Systems with C60
  • Nonlinear Optical Properties of Charge-Transfer Organic Systems with C60
  • Photoinduced Charge-Transfer Processes with C60/Polymer Blends
  • Poster Session
  • Photoinduced Charge-Transfer Processes with C60/Polymer Blends
Optical Probes of Fullerenes
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Raman studies on single-walled carbon nanotubes
Serge Lefrant, M. Lamy de la Chapelle, C. Journet, et al.
Carbon single walled nanotubes produced in high yields by the electric arc technique have been studied by several techniques including HRTEM. It is shown in particular that they have a narrow diameter distribution around an average value of 1.3 nm. In this paper, we mainly report characterization results obtained by high resolution Raman spectroscopy. Raman spectra exhibit a very rich structure especially in the low frequency range where several components are observed. Using previous calculations, we attribute the main features to armchair tubes with (6,6) to (12,12) geometry, in agreement with the narrow diameter distribution observed by HRTEM measurements.
Magnetic properties and valence states of fullerene complexes
Anton Bartl, Lothar Dunsch, Juergen Froehner, et al.
As a consequence of the bucky-ball structure of the fullerenes two kinds of fullerene complexes are formed: endohedral and exohedral complexes. The endohedral fullerenes Sc at C82, Y at C82 and La at C82 show ESR spectra with hyperfine splittings resulting from the nuclear spin of the metal ion. Scandium, yttrium and lanthanum are in the 3+ state in the endohedral fullerenes. The situation is more difficult in C82 endohedral metallofullerenes with lanthanides. The situation concerning the valence state of the nedohedral complexes is not yet clear. Magnetic measurements should help to solve this problem Magnetization measurements by means of a SQUID magnetometer did not reveal permanent individual magnetic moments in the case of samarium and ytterbium endohedral fullerene complexes, suggesting the presence of Sm++ and Yb++ ions, respectively. Exohedral fullerene complexes were prepared based on C60 with tetrakisdimethylaminoethylene (TDAE) and with lanthanides. ESR measurements give strong single line spectra without any hint of hyperfine structure interaction of the unpaired electron. SQUID measurements indicate permanent individual magnetic moments in all investigated exohedral complexes, quite unexpectedly also in LaC60. The ferromagnetic ordering of TDAE-C60 at 16 K was studied in detail. Moreover, typical ferromagnetic properties such as magnetic hysteresis, coercivity, remanence and viscosity were found for this exohedral structure at low temperatures.
Optical and Electrical Properties of Fullerenes
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Fullerene plasma deposition: its potential for optical carbon coatings
Ulf G. Brauneck, S. Dieckhoff, Alfons Zoeller, et al.
Tetrahedral carbon thin films in amorphous modifications (ta-C) are ofinterest for applications due to their excellent hardness, chemical inertness, low friction, and low wear rates. They are obtained by mass-selected and monoenergetic carbon ion beam deposition and by laser ablation ofgraphite '' . The need ofan ion beam with a narrow distributed kinetic energy per C-atom is currently a fundamental obstacle for the use of such thin films in large area and high volume applications. More recently, the deposition of ions abstracted from a radio frequency excited acetylene plasma has yield a hydrogenated form oftetrahedral carbon with properties comparable to ta-C '.An important technological progress can be expected if an ion source can be made available that delivers carbon ions with a narrow-distributed kinetic energy per Catom, that can easily be scaled up, and that can be integrated into existing deposition systems. Due to their high vapour pressure, fullerenes can easily be sublimed and introduced into ion beam and plasma systems and thus have a potential for the deposition oftetrahedral carbon films by relatively simple technological means. The first approach to the use of fullerenes for the deposition oftetrahedral carbon was a cluster ion beam experiment ,in which fullerene ions were accelerated and disintegrated at their impact on a substrate. In another way, fullerenes can be used to generate a plasma ofhigh C2-content due to the effective dissociation ofstrongly heated fullerenes by C2-emission 6 The first use of such a plasma was a chemical vapour deposition experiment, in which fullerenes were dissociated in an argon microwave plasma and nanocrystalline diamond was grown6. The films obtained hereby show superior surface smoothness, low friction and low wear rates .Forthe deposition of amorphous carbon, a Kaufmann-source supplied with fu8 and a radio-frequency (RF) plasma source supplied with fullerenes and argon 10have been applied. Here we report on the ion beam deposition of amorphous carbon by using a fullerene argon plasma without the admixture ofhydrogen. The films are characterized regarding their Raman-emission, their plasmon loss structure and valence band spectra acquired by high-resolution X-Ray photoelectron spectroscopy (XPS), their optical absorption in the visible wavelength regime, and their wetting angles.
Optical Probes of Fullerenes
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Two-photon spectra of C60 and C70 films between 0.74 and 1.7 um
We present new femtosecond degenerate four-wave (DFWM) measurements on a C70 film in the wavelength range 0.9- 1.6 micrometers in conjunction with previously measured data in C60 and C70 in the ranges 0.74-1.7 (mu) and 0.74-0.88 micrometers , respectively. The new data reveal tow closely spaced peaks in the DFWM spectrum which we interpret as two-photon states with excitation energies of 2.41 +/- 0.05 eV and 2.64 +/- 0.03 eV. We relate these results to nonlinear optical spectra obtained by others in C60 and C70 films. In particular, we compare third-harmonic generation, electro-absorption and DFWM and emphasize the relative advantages of DFWM for two-photon spectroscopy.
Kinetics and spectra of fullerene triplet states
R. Bruce Weisman, Kevin D. Ausman, Angelo F. Benedetto, et al.
Optical kinetic spectroscopy is used to investigate the triplet state properties of solution phase fullerenes. These triplet states, which are the molecules' lowest-energy and longest-lived electronic excitations, show lifetimes and absorption spectra that vary from fullerene to fullerene. In room temperature toluene solution, the intrinsic triplet lifetime of C70 exceeds that of C60 by a factor at of least 80. The 'self-quenching' deactivation of triplet states through encounters with ground state molecules occurs with rate constants of 1.6 X 1070 M-1s-1 for C60 and 3 X 107 M-1s-1 for C70. In solutions containing mixtures of fullerenes, transfer of triplet excitation between species is efficient and reversible, with rate constants up to 2 X 109 M-1s-1. When C60 is aliphatically derivatized at two adjacent sites to form (6,6)-closed dihydrofullerene structure, intrinsic triplet lifetimes are shortened by more than a factor of 3 to approximately 45 microsecond(s) . The triplet spectra of these compounds, in which the derivitizing groups are not sterically strained, are very similar to each other but significantly different from the C60 spectrum. The (6,6)-closed epoxide, C60O, displays unusual triplet state behavior. Its initial triplet spectrum is distinct from that of the other derivatives studied and decays in approximately 6 microsecond(s) through nonexponential kinetics. Evolution of the induced spectrum suggest the possibility of epoxide ring opening form the triplet state of C60O.
Optical and Electrical Properties of Fullerenes
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Anomolous white photoluminescence from C60-based composite films: emission characteristics and structural properties
Ching-ju Wen, Satomi Ohnishi, Nobutsugu Minami
To investigate the possibility of fullerenes as new photofunctional materials making use of their unique molecular and electronic structures, we fabricated composite films consisting of vacuum deposited C60 and sputtered Si, where each layer was alternately and repeatedly deposited. We found that, under green laser irradiation, the composite films emit intense white light that is visible by naked eyes. The emission spectra as revealed by FT-IR spectrophotometry were very broad with no characteristic structures, extending from the visible to mid-IR. The peak position was in the range 1.8-2.3 micrometers , and the spectral shape gave almost perfect fit to the Plank's formula for black-body radiation in the almost entire wavelength range measured. Estimation based on the curve fitting gave surprisingly high effective black-body temperatures in the range 1100-1600 K, depending on the excitation intensity used. In spite of their far exceeding the sublimation temperature of C60 under vacuum, the emission is stable at least for hours. To obtain more insight into this extraordinary emission behavior, we carried out morphological and analytical studies on the emissive and non-emissive spots in the films by optical micrography, AFM, and microscope FT-IR transmittance spectroscopy. The results suggest the involvement of some peculiar microscopic morphology of the composite films in the occurrence of the intense white photoluminescence.
Electric-current-induced light emission from C60
Thomas T. M. Palstra, Robert C. Haddon, K. B. Lyons
We report the luminescence of C60 crystals and films due to the passage of an electrical current. The current-voltage behavior is highly non-linear with light-emission beyond a threshold voltage. The emission spectrum is featureless and resembles black body radiation with an effective temperature on the order of 1700K. We report experiments aimed at distinguishing between electro- and thermo-luminescence.
Nonlinear Optical Studies of Fullerenes in Different Media
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Ultrafast and nonlinear optical characterization of optical limiting processes in fullerenes
Randolph S. Kohlman, Victor I. Klimov, M. Grigorova, et al.
We present recent results of broadband femtosecond (fs) transient absorption (TA) and broadband nanosecond (ns) optical limiting (OL) studies of C60 and derivatized C60. Improvements in measurement techniques for fs TA spectra provide sensitivity to 10-5 in differential transmission, allowing detailed comparison of excited-state spectra with established energy level diagrams, as well as comparison of the ratio of triplet to singlet excited-state absorption cross sections from TA spectra with those obtained by modeling time transients at different wavelengths. For derivatized fullerenes, which provide enhanced solubility and a ground-state absorption extended into the IR compared with C60 there is no spectral region where the triplet absorption cross section dominates the singlet as strongly as in C60. Wavelength-dependent studies show that the OL response improves monotonically at longer wavelengths, demonstrating broadband limiting in all 6,6 mono-adducts and neat C60. We report new approaches to processing sol-gel encapsulated fullerenes to improve the OL performance of solid-state materials to approach the response of solution limiters.
Nonlinear absorption and optical limiting properties of fullerene materials
Ya-Ping Sun, Jason E. Riggs
Optical limiting results of methanol fullerene derivatives, pendant fullerene polymers, and fullerene-styrene copolymers with different fullerene contents are reported. The optical limiting properties of the t-butyl methanol fullerene carboxylate derivative in polymethylmethacrylate films are also reported and compared with the result in solution.
Photoinduced Electron-Transfer Processes in Organic Systems with C60
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Photoinduced electron transfer in fullerenes containing donor-bridge-acceptor dyads
Dirk M. Guldi, Michele Maggini, Simonetta Mondini, et al.
Excited states properties of supramolecular fullerene donor- bridge-acceptor dyads were studied under various conditions. The steady-state emission yields of C60-Fc and Ru-C60 dyads are noticeably quenched relative to ruthenium and fullerene model compounds, respectively. This indicates that intramolecular electron-energy transfer plays a predominant contribution in the deactivation of the photoexcited chromophores, e.g. the singlet excited state of the fullerene core and ruthenium excited MLCT state. Picosecond and nanosecond resolved photolysis show that the photoexcited C60-Fc and Ru-C60 dyads transform rapidly into charge separated states. Characteristic NIR absorption of the fullerene (pi) -radical anions were complimented by a detailed UV/vis analysis and confirm the existence of long-lived radical pairs. The structure, length, and nature of the bridging spacer separating the two reactive centers had a significant impact on the lifetime nd stabilization of the charge separated states.
Structure and photoelectrochemical properties of porphyrin-linked fullerenes on a gold surface using a self-assembled monolayer technique
Hiroshi Imahori, Takayuki Azuma, Kiminori Ushida, et al.
Self-assembled monolayers (SAMs) have recently attracted much attention as a new methodology among a variety of thin film preparation. There have been several reports of fullerene SAM. These fullerene thin films seem to be the most promising candidates for realizing photovoltaic and optoelectronic devices at the nano-scale level. We have applied this technique to donor-linked fullerene systems and have developed supramolecular porphyrin-C60 molecules that are designed to be self-assembled on gold surface. Such systems are expected to mimic supramolecular photoinduced charge-separation events in photosynthesis. Porphyrin-linked C60 bearing a methylthio group was prepared by 1,3- dipolar cycloaddition of the azomethine ylide porphyrin with C60. SAM was formed by soaking Au electrode into a CH2Cl2 solution of the porphyrin-C60 dyad. Structure of the SAM was investigated by spectroscopic methods including X-ray photoelectron spectroscopy and ellipsometry and electrochemical studies. The large photocurrent with a level of sub-(mu) A/cm2 was observed for the photochemical cell in the presence of methyl viologen under illumination with approximately 10 mW/cm2. Possible mechanism for the photocurrent generation is discussed in terms of the photodynamics of porphyrin-linked fullerenes.
EPR studies of electron transfer processes in composites of substituted sexithiophenes with fullerene derivatives
Anna Lisa Maniero, Luigi Pasimeni, Giovanna Barbarella, et al.
We have examined by EPR spectroscopy a series of films made by composites of hexamethyl and tetramethylsulfanyl substitutes sexithiophenes (T6) with C60 and C60 derivatives. We have found that photoinduced electron transfer (PET) from donor to acceptor molecules occurs in mixed films of C60 with hexamethyl-T6 and in composites of methylsulfanyl substituted T6 with fullerene derivatives. Persistent electron transfer in the ground state was also observed. Spin polarization of the emissive EPR signal attributed to T6+ catio in hexamethyl-T6/C60 composite is consistent with the formation of the radicals from a non-polarized triplet exciplex which gives rise to a loose radical pair after the PET process. In the mixed film of tetramethylsulfanyul-T6 and C60 polymerized with methyl methacrylate, time resolved EPR spectra reveal that a tightly bound radical pair is formed in which the partners are coupled by dipolar and exchange interactions. Such pair relaxes into a loose pair at later times.
Unoccupied density of states of commercially used organic photoreceptors doped with C60
C. Schlebusch, J. Morenzin, Barbara M. Kessler, et al.
TiO-phthalocyanine and H2-phthalocyanine dispersions with polymeric binders are presently used as charge generation layers in xerography. The question arises whether an admixture of the electron acceptor C60 to the dispersion is able to inhibit the recombination of excited states and therefore enhance the charge-generation efficiency. We study the effect of an admixture of 5 percent C60 on the unoccupied density of states by x-ray absorption near edge spectroscopy using tuneable light from the Berlin electron storage ring for synchrotron radiation. Information on the occupied states is obtained by photoelectron spectroscopy with UV-radiation. The spectral features are identified by comparison with spectra from the single components and discussed. The influence of structural differences of H2-Pc on the optical properties is considered using optical transmission data.
Nonlinear Optical Properties of Charge-Transfer Organic Systems with C60
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Nonlinear optical properties of charge-transfer molecular systems with C60
Francois Kajzar, Yoshiko Okada-Shudo, Charles D. Merritt, et al.
Second order nonlinear optical properties of C60 based multilayered charge transfer structures with TPP, TPD and TPN molecules, acting as electron donors, are studied by the transverse optical second harmonic generation. The studied structures are of ABABAB...and ABCABC...type, where layer A is made from electron accepting molecules, layer B from electron donating and layer C from neutral molecules. The typical thickness of individual layers is about 2 nm. A significant increase of SHG is observed in ABCABC...type structures, where an effective charge transfer is expected with creation of a noncentrosymmetry in the perpendicular direction tot eh layers. The largest (chi) (2) susceptibility is obtained with multilayers containing rubrene. The SHG generation experiments performed on thin films with different thicknesses show that the nonlinear optical response comes from the bulk material and not form the interfaces. A quadratic dependence of SHG intensity on input power is also observed, showing that the charge transfer takes place at the ground state.
Photoinduced electron transfer in conjugated polymer/fullerene heterostructures
Andreas Haugeneder, C. Kallinger, Wolfgang Spirkl, et al.
We investigate the photophysics in composite systems consisting of fullerene molecules and a conjugated polymer. Photoluminescence (PL) quenching experiments are used to study the photoinduced electron transfer that occurs after photoexcitation of the conjugated polymer In blend systems with various fullerene concentrations we find a strong concentration dependant PL-quenching. By doping a ladder type poly(p-phenylene) (LPPP) with 5 weight percent of a C60-fullerene the polymer PL is quenched by more than one order of magnitude. Time-resolved measurements show that the photoinduced electron transfer can not be described by a single rate. The nonexponential PL-decay is due to complex processes in more detail we have prepared will define heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer. From PL- quenching experiments on these samples we infer a value of 14 nm for the diffusion length of neutral excitations in LPPP.
Photoinduced Charge-Transfer Processes with C60/Polymer Blends
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Electromodulated photoinduced absorption: a new spectroscopy in -conjugated polymer/C60 blends
Martin Liess, Paul A. Lane, Zakya H. Kafafi, et al.
We have applied the electromodulated photoinduced absorption (EPA) technique to a variety of C60-doped and pristine luminescent conducting polymer films. EPA measures the electric field-related change of the absorption in the sample as a result of above-gap illumination. An electric field leads to two major effects. One is a possible Stark shift of photoexcitation energy levels, and the other is a change in the photoexcitation decay kinetics, which leads to reduction or enhancement of PA features. This can be due to an electric field enhanced dissociation of the 1Bu exciton or due to a lifetime reduction of polaron pairs. The EPA technique helps to elucidate the photoinduced absorption bands of the charge transfer complex C60- P+, where P+ is a positively charged polaron on the polymer chain. The following thin films were studied by EPA: MEH-PPV/C60, pristine MEH-PPV, PPP/C60, P3OT/C60, PTSi/C60, and pristine PTSi. We found for the two PA bands of polarons that E(LE) + E(HE) equals Eg. We also found that the polarizability of polaron pairs in three orders of magnitude bigger that the polarizability of the 1Bu exciton.
Ultrafast holography and transient-absorption spectroscopy in charge-transfer polymers
Duncan W. McBranch, Eric S. Maniloff, Dan Vacar, et al.
Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitizes with varying concentrations of C60, holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using non-degenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than previous reports. The charge transfer range at polymer/C60 interfaces was further studied using transient absorption spectroscopy. The fact that charge-transfer occurs in the picosecond-time scale in bilayer structures implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge transfer range to be 80 angstrom and interpret that range as resulting from quantum delocalization of the photoexcitations.
Poster Session
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Relation between intercalated oxygen molecules and localized electronic states in C60 films with/without laser light irradiation
Hitoe Habuchi, Shoji Nitta, Hiroki Maehara, et al.
Localized electronic stats in C60 films related to intercalated oxygen molecules (O2) are studied by means of photoluminescence, electron spin resonance, constant photocurrent method and photothermal deflection spectroscopy. O2 intercalated into C60 films creates localized electronic states in subband gap below the energy of 1.6 eV. These states act as the non-radiative recombination center and the trap level for conduction carriers. Localized electronic stats photo-induced at room temperature are also found in C60 films exposed to air. These states can be related to photo-oxidation of C60 but gradually decrease at room temperature without the exposure to light. On the other hand, localized electronic states in oxygen free C60 films decrease with the laser light irradiation. In this paper, localized electronic states in C60 films with/without the laser light irradiation will be discussed with relating to intercalated O2.
Nanoscale C60 aggregates in solution
Jeung Sun Ahn, Keisuke Suzuki, Yoshihiro Iwasa, et al.
Formation of C60 aggregates has been found in toluene, benzene, and carbon disulfide (CS2) solutions by means of photoluminescence spectroscopy. From the detailed investigation of the temperature dependence of luminescence, it has been confirmed that the C60 aggregates are formed at freezing temperature of these solvents in the cooling process and are weakly bound clusters which are decomposed into C60 molecules at temperatures higher than ca. 220 K in toluene and CS2 solutions. The 0-0 transition energy of their luminescence has been found to be blue-shifted in comparison with that of C60 crystal. It has been also found that the irradiation of ultraviolet light upon the C60 aggregates in benzene at approximately 260 K transforms them to stable substances under atmosphere, which look like round-shaped nanoscale particles in the high-resolution transmission electron microscopy images.
Stability and reversibility of the electrochromic effect in fullerene thin films
The electrochromic effect is observed in fullerene thin films when, under applied electric field, various ionic species are intercalated into the interstitial spaces between fullerene molecules. Within limits, the process is electrically reversible. Residual oxygen, both in the electrolyte system and in the fullerene thin film itself, as well as trace water levels, significantly degrade electrochromic stability. The degree of electrochromic intercalation affects reversibility. However, this is complicated by the mobility of the intercalant ion and the resulting stoichiometric gradients within the fullerene thin film. The details of film preparation and electrolyte preconditioning are discussed, in addition to time-resolved transparency changes during forward and reverse electrochemical intercalation. Potential applications in large area non-volatile flat panel displays and integrated optoelectronic devices are also considered.
Photoluminescence of fullerene thin films prepared at various substrate temperatures
Keiichi Kaneto, Hideki Hasegawa
Photoluminescences (PL) and electrical conductivities of evaporated C70 films prepared at various substrate temperatures are studied. In C70 films prepared at low substrate temperatures, a PL peak at 1.79 eV are strongly observed, besides several peaks at 1.55, 1.67, 1.71 eV observed in the films deposited at high substrate temperatures. Thermal annealing of the films prepared at low substrate temperatures turns into the characteristic of the spectra prepared at high substrate temperatures. A novel memory effect of highly conductive state in fullerene films of C60 and C70, which is attained by a treatment of high electric fields at high substrate temperatures is also mentioned.
Preparation and properties of one-dimensional C60 nanostructures confined in the zeolite FSM-16
Akio Umehara, Shoji Nitta, Takashi Itoh, et al.
Carbon nanotube is an interesting materials as a unique 1D nanosize structure. We have tried to prepare 1D C60 nanostructures confined in the zeolite 'FSM-16' which have 1D holes of the diameter of 2.7 nanometers. 1D C60 embedded in the holes in FSM-16 show new photoluminescence which peak energy is shifted about 0.1 eV higher than that of normal C60 films. The shift of the photoluminescence can be explained by the quantum size effect of the C60 confined to 1D holes. To prepare good samples, it is necessary to effuse out the absorbed gases in FSM-16. The results of gas effusion spectra until 1000 degrees C to clarify the properties of absorbed gases in FSM-16 are also shown and discussed to get good samples without the effect of absorbed gases.
Influence of defects on photoluminescence in C60 crystals
Masaru Tachibana, Kazuki Nishimura, Takeshi Komatsu, et al.
Photoluminescence spectra of C60 crystals with high quality was measured before and after the mechanical deformation . New peak at 685 nm appeared after the deformation. The peak was observed only at temperatures lower than about 90 K, where its intensity decrease with increasing temperature. The temperature dependence of the peak intensity was correlated with that of main other peak at 735 nm, which would originate in intrinsic C60 crystals. From these result, the origin of the peak at 685 nm is discussed in the light of crystal defects, especially dislocations.
Photo-illumination hardening of C60 crystals
Masaru Tachibana, Hiroko Sakuma, Takeshi Komatsu, et al.
The effect of light on the hardness of C60 crystals was investigated. The hardness increased with the development of illumination time, and then reached a constant value. The hardening phenomenon was clearly observed at temperatures lower than about 390 K. In addition, it was shown that the red light with the wavelength of about 700 nm leads to the maximum effect for the hardening. The origin of the hardening is discussed in the light of the photo-induced polymerization of C60 molecules in the crystals.
Photoinduced Charge-Transfer Processes with C60/Polymer Blends
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Photoinduced changes in the complex index of refraction resulting from photoinduced charge transfer in conjugated polymer/fullerene blends
We report steady-state photoinduced absorption (PIA) and photoinduced reflectance (PIR) in films of MEH-PPV and BCHA- PPV blended with fullerene-based acceptors. Absorption from the metastable charge-transferred state is probed by PIA; the modulated absorption spectrum causes changes in the real part of the index of refraction, (Delta) n, which can be measured directly by PIR. The charge transfer gives rise to pronounced features in (Delta) n, including vibrational structure in the mid- and near-IR. Our measurements over a wide spectral range allow quantitative comparison of (Delta) n obtained from PIR with that obtained from Kramers- Kronig transformation of the PIA data. We find good agreement throughout the IR, indicating that our method for measuring (Delta) n is useful as an analytical tool for optical characterization and for prediction of optical spectral ranges for nonlinear optical response.