Fullerene precursors have been shown to result in the growth of diamond films from argon microwave plasmas. In contradistinction to most diamond films grown using conventional methane-hydrogen mixtures, the fullerene-generated films are nanocrystalline and smooth on the nanometer scale. They have recently been shown to have friction coefficients approaching the values of natural diamond. It is clearly important to understand the development of surface morphology during film growth from fullerene precursors and to elucidate the factors leading to surface roughness when hydrogen is present in the chemical vapor deposition (CVD) gas mixtures. To achieve these goals, we are measuring surface reflectivity of diamond films growing on silicon substrates over a wide range of plasma processing conditions. A model for the interpretation of the laser interferometric data has been developed, which allows one to determine film growth rate, root mean square (rms) surface roughness, and bulk losses due to scattering and absorption. The rms roughness values determined by reflectivity are in good agreement with atomic force microscope (AFM) measurements. A number of techniques, including high-resolution transmission electron microscopy (HRTEM) and near-edge x-ray absorption fine structure (NEXAFS) measurements, have been used to characterize the films. A mechanism for diamond-film growth involving the C2 molecule as a growth species is presented. The mechanism is based on (1 the observation that the optical emission spectra of the fullerene-containing plasmas are dominated by the Swan bands of C2 and (2) the ability of C2 to insert directly into C-H and C-C bonds with low activation energy barriers, as shown by recent theoretical calculations of reactions of C2 with carbon clusters.
Recent work on the electronic and vibrational spectra of fullerenes and fullerene assembled materials is discussed. To aid in identifying the potentially useful polymeric phase of fullerene-based materials we compare various aspects of the spectra of the isolated and dimerized fullerene molecules. We discuss the core-level shifts of the spectra that are induced by polymerization and the changes in the electronic density of states near the Fermi level. We discuss three qualitative changes that are expected to occur in the vibrational spectrum upon polymerization and present the calculated infrared (IR) and Raman vibrational spectra for both the isolated and dimerized fullerenes. Some of the energetics associated with dimerization are also briefly discussed.
The nature of the optical excitations at the dipole forbidden optical gap of crystalline C60 is investigated by linear and non-linear spectroscopic techniques. The resonances observed both in the absorption and emission spectra are identified with Herzberg-Teller vibronic coupling of the electronic states to intramolecular vibrational modes. The highly structured fluorescence spectra of both C60 single crystals and crystalline C60 films are interpreted in terms of Frenkel exciton emission from X-traps.
We report the photoluminescence spectrum of well characterized, epitaxially grown single crystal C60 thin films. The highly regular and reproducible spectral features observed can be explained by a simple molecular model which takes into account enhanced coupling of an excited C60 molecule with its nearest neighbor. This model provides an identification of all the observed spectral features at low temperature including a qualitative understanding of the relative peak height ratios, as well as the observed temperature dependence of the PL spectrum.
Electric field induced luminescence quenching in C60 polycrystalline films has been investigated. Its efficiency increases with excitation energy above 2.3 eV, whereas no quenching is observed in the range 1.77 - 2.3 eV, within the experimental resolution. Furthermore, the luminescence efficiency decreases towards higher energy with a drop above 2.3 eV. The photocarrier generation efficiency also shows a sharp increase above 2.3 eV in addition to weak structures in the range 1.5 - 2.3 eV. The increase of the electric field induced luminescence quenching efficiency, the drop of the luminescence yield as well as the sharp increase of the photocarrier generation efficiency above 2.3 eV, are interpreted in terms of branching either in the excitation or relaxation processes involving intermolecular charge transfer excited states. In the range 1.77 - 2.3 eV, the photophysical properties are governed by intramolecular interaction (Frenkel excitons). The solid state specific absorption in the range 2.3 - 3.0 eV has also been correlated to the intermolecular charge transfer states.
The X-band photoluminescence (PL) and PL-detected magnetic resonance (PLDMR) of C60- and C70-doped pristine 2,5-dihexoxy poly(p-phenylene vinylene) (DHOPPV), poly(3-dodecyl thiophene) (P3DT), and 2,5-dibutoxy poly(p-phenylene ethynylene) (DBOPPE) are described and discussed. While light (approximately 0.1 mol.%) doping of pristine DHOPPV sharply weakens the PL, it strongly enhances the PL-enhancing polaron and triplet exciton resonances, which are very weak in the undoped film. The polaron PLDMR is attributed to magnetic resonance enhancement of nonradiative trapped polaron pair recombination, which reduces the polaron population and consequently the rate at which they nonradiatively quench singlet excitons. Its behavior in 0.1 mol.% C60:DHOPPV is discussed in relation to the potential effects of C60-counterions, including the photogeneration of trapped polaron pairs at the expense of singlet excitons, and the fission of singlet excitons to such pairs. The emergence of the triplet resonance is discussed in relation to intersystem crossing from the singlet to the triplet manifold induced by the C60 dopant. At higher doping levels, the polaron and triplet exciton resonances weaken with increasing C60 content. The reduced polaron resonance is discussed in relation to the smaller relative change in the polaron population at the field for resonance, and the potential effects of the dissociation of the polaron pairs. The behavior of C70-doped pristine DHOPPV at up to 1 mol.% C70 is qualitatively similar to that of C60: DHOPPV, but the polaron resonance in P3DT and DBOPPE is less sensitive to C60. The triplet resonance is almost completely suppressed even in lightly doped P3DT and DBOPPE, which is attributed to quenching by the injected polarons.
The general features of charge transfer processes fullerene/conducting polymer (CP) systems, such as energetics of photoinduced charge transfer (PCT) between C60 and CP (pi) - electronic states, geometry of (pi) -(pi) overlapping and the role of self-trapping effects to polaronic states on C60 and CP chains on the PCT dynamics are analyzed. Persistent photoconductivity and electroluminescence quenching recently found in C60/CP composites additionally to photoconductivity enhancement and photoluminescence quenching observed earlier, indicate that photogenerated C60 radicals may be extremely long living in CP matrices, due to multicharging of C60 as suggested by us accompanied with deep self-trapping to polaron/bipolaron states. The anisotropy of PCT is proposed to arise due to orientational modulation of overlapping between polaronic rings on C60 and CP which strongly suppresses back recombination. The strategy to increase the efficiency of C60CP donor-acceptor (DA) photocells by improving PCT is analyzed, particularly considering multilayered structures with polarization barriers at interfaces, and increased intralayer mobilities of carriers. To increase the efficiency of photons collection in photocells we suggest three layered D-M-A structures, with molecular 'photon pump' layers strongly absorbing photons. The prospects for novel photonic applications of various C60CP systems, such as NLO devices and photomodulated field effect transistors (FETs) are discussed and illustrated by the newest results. New results on superconductivity of C60/CP upon alkali metal doping are presented, and exciting possibilities for novel superconducting phases in this system are discussed.
In this review we discus the photophysical properties of the supramolecular composites of two (pi) -electron semiconductors; e.g. conjugated polymers as electron donors and Buckminsterfullerene as electron acceptor. Conjugated, polymeric semiconductors have been found to be effective donors upon photoexcitation of the valence band electrons across the bandgap into the conduction band. The Buckminsterfullerene, C60 is a powerful acceptor moeity upon photoexcitation. Thus, the supramolecular composite of these two conjugated materials exhibit an ultrafast, reversible, metastable photoinduced electron transfer and charge separation. This process, similar to the primary steps of photosynthesis, has been utilized in conjugated polymer/C60 based heterojunction as well as Schottky type devices for effective conversion of the solar photon energy into electricity. Other related applications of the above mentioned photophysics include photolithographic and xerographic processes. Furthermore, quantum well like heterostructures based on organic donor-acceptor layers are proposed to exhibit interesting photoinduced phenomena. Non-linear optical (NLO) properties of these composites are reported in comparative studies with the components alone. The results show a strong enhancement of the NLO coefficients in the composite materials compared to the conjugated polymer as well as C60 alone.
Experimental studies of photoinduced absorption, PA, have been carried out by several groups on composites of C60 with conducting polymers such as poly[2-methoxy, 5-(ethyl hexyloxy) -p- phenylene vinylene], MEH-PPV, to understand the quenching of photoluminescence and increase of photoconductivity caused by the addition of small amounts of C60. To account for the observed PA it is first necessary to know where the C60 molecules sit relative to the MEH-PPV chains. We found with the use of a Monte Carlo cooling algorithm that the most probable position of the C60 is in a channel formed by the alkoxy side chains, its center approximately 10 angstrom from that of the MEH-PPV backbone. We identify the PA peak seen in MEH-PPV as the transition between the lower and upper levels of a polaron coupled to one of opposite sign on a neighboring chain. Calculations are carried out for the energy levels of neutral C60 and C60- using a tight binding Hamiltonian with the parameters obtained by Harigaya. To determine the effect on the levels of the Coulomb interaction between C60- and a positively charged polaron, P+, on MEH-PPV we use coupled equations. Finally, we suggest that the peak of the PA in the composite is due to transitions between the two polaron levels of a free P+ on MEH-PPV. Additional PA features in the composite can be identified as due to C60- and neutral C60.
We have studied photoexcitations in various fullerene thin films using transient photomodulation and photoluminescence from 100 fs to 50 ms and absorption-detected magnetic resonance (ADMR). We show that singlet Frenkel type excitons are the primary photoexcitations; their recombination kinetics in the picosecond time domain are dispersive as a result of inhomogeneity. The long-lived photoexcitations, however, are shown to be triplet excitons and charged polarons, identified by the correlation found between their associated optical transitions and ADMR signals with spin 1 and 1/2, respectively.
We discuss theoretically the role of intermolecular interaction in the optical properties of C60 solids. Two kinds of intermolecular interaction, namely, electron transfer and Coulomb interaction, are taken into account in terms of an extended Pariser-Parr-Pople model. Configuration-interaction calculations have been carried out for systems of up to four molecules. It is demonstrated that the two kinds of interactions are both important in understanding the optical spectra of C60 solids. The Coulomb interaction is relevant for the overall absorption spectrum in the sense that those states which contribute to the main peaks are essentially Frenkel exciton states. The intermolecular hopping of a singlet exciton is governed by ordinary dipole-dipole coupling. Intermolecular electron transfer among different molecular orbitals leads to the mixing of Frenkel and charge transfer states. These mixed states are responsible for the shoulders on the low and high energy sides of the molecular absorption peak.
The complete knowledge of the intrinsic electronic properties of new materials like fullerenes is essential for their technical application in optoelectronic and photonic devices, as well as in terms of the fundamental physical processes. The value of the intrinsic energy gap, the shape of the bandedge, and the nature and origin of the radiative recombination channels after photoexcitation of C60 are still unsettled. We report on photoluminescence emission and excitation spectroscopy on high-quality C60 single crystals and high-quality C60 thin films grown by molecular beam epitaxy (MBE) on mica substrates.
Novel multiphasic nanostructured inorganic : polymer composites of C60 have been prepared by sol-gel processing. These composites offer the prospect of introducing multifunctionality by optimizing each phase for a particular response. Since the phase separation is at a nanometer size scale, the composites are optically transparent. Results are presented form the studies of spectroscopic characterization, optical power limiter behavior, and third-order nonlinear optical response.
In this presentation, we shall describe the mechanisms and dynamics of two newly observed phenomena in fullerene C60 doped liquid crystal, namely, holographic grating formation in thin film structures and nonlinear pulsed propagation and optical limiting in isotropic liquid crystal cored fibers and fiber arrays. The unusually large photorefractive like nonlinearity of C60 doped nematic film allows us to write permanent holographic gratings which can be electronically modulated. In fullerene-doped liquid crystalline cored fibers, we have observed greatly enhanced nonlinear optical transmission and propagation effects caused by nonlinear scattering, defocusing, increasing absorption and possibly other nonlinear processes. The resulting lower threshold and reduced interaction-length requirement enable one to design practical liquid crystal fiber arrays that could function as imaging faceplate as well as sensor protection devices.
We present here our first-principles density functional theory (DFT) calculations of the potential energy and dipole moment function for Li+C60. We also present the results for the equilibrium geometry, dipole moment and polarizability dispersion obtained from ab initio Hartree-Fock (HF) calculations. The calculated equilibrium position of Li inside the C60 cage agrees with other theoretical calculations. The dipole moments calculated by the DFT and HF calculations, while differing from each other by almost a factor of two, are considerably smaller than a recent DFT result. The polarizability tensor calculated at lambda equals infinity (static) and at lambda equals 1064 nm are very nearly isotropic, indicating little deformation of the spherically symmetric charge distribution of C60. Quantitatively, the alpha values of the Li+C60 complex are very close to the corresponding value for the C60 molecule.
Transient absorption and pump-probe experiments have been performed on degassed toluene solutions of C60 and eta2-C60Pd(PPh3)2. Previous reports of a 1.2 ns singlet state lifetime for C60 have been confirmed. For the eta2-C60Pd(PPh3)2 compound an extremely fast decay of 12 ps has been observed which is believed to be derived from the rapid depopulation of a charge transfer band which has formed due to electron transfer from metal to ligand.
Calculations of the dispersion of the second order hyperpolarizability of C60, C70 and C76 have been performed with a sum over states -- complete neglect of differential overlap/spectroscopic parametrization, plus configuration interaction method. The third harmonic generation spectra of C60 and C70 thin films have been measured in a wide spectral range, and they are in excellent agreement with calculations. The validity of the essential states model description for fullerenes is discussed in terms of symmetry properties of the molecular system.
We use time resolved degenerate four-wave-mixing with femtosecond pulses to measure magnitude, phase, and dispersion of all nonzero components of the third order nonlinear optical susceptibility tensor (chi) (3)(-(omega) ; (omega) , (omega) , -(omega) ) of a polycrystalline C70 film. Rise and fall times of the nonlinearities measured are short compared to the (112 plus or minus 5 fs) pulses employed. Accordingly, the cw symmetry relation (chi 1111) equals 2 (chi 1212) plus (chi 1221) is experimentally found to be satisfied. The magnitude of (chi 1221) is measured to be (5.04 plus or minus 0.19) 10-13 esu relative to fused silica independent of wavelength. The ratio (chi 1212)/(chi 1221) is wavelength dependent and varies between 1.87 plus or minus 0.12 and 1.44 plus or minus 0.09. The magnitudes of phase angles for (chi 1111) and (chi 1212) are (120 plus or minus 22) degree(s) and (105 plus or minus 21) degree(s), respectively. The intensity dependence of the observed signals is cubic for intensities up to 20 GW/cm2 at all wavelengths. Good agreement between data derived from degenerate four-wave-mixing and third-harmonic generation in C70 as well as in C60 films is found.
The wavelength-dependence of optical limiting in the reverse saturable absorber C60 has been studied in detail over a large spectral range and compared to that of two phthalocyanines. The reverse saturable absorption of C60 is found to increase at wavelengths longer than the standardly measured 532 nm. The optical limiting data have been analyzed in terms of the dynamical population rate equations to obtain the excited state absorption cross-sections. The computational effort of the analysis is greatly reduced by a semi-analytic solution to the rate equations that we have developed. We have also demonstrated that the spectral window for optical limiting of fullerenes can be increased to include longer wavelengths at which C60 is transparent. Both a C60 charge-transfer complex and C76 have been shown to exhibit strong reverse saturable absorption at long wavelengths.
Complex formations in C60 solutions and the reverse saturable absorption (RSA) properties have previously been observed. In a wide survey of solvents conducted in our laboratory, several solvents exhibited complexing behavior with C60 when analyzed with UV-Vis and fluorescence spectroscopy. We have studied the formation of complexes in C60/toluene solutions when benzonitrile, N-methyl-pyrrolidinone, and triphenylamine, and diethylaniline were added. Formation of the complexes was followed with UV-Vis spectroscopy. Several plots of the complexes were done to determine the equilibrium constant and the change in molar extinction coefficient in each system. We report the behavior of the reverse saturable absorption (RSA) properties as a function of complexing agent and concentration.
We report the ground-state and excited-state optical absorption spectra in the visible and near infrared for several substituted fullerenes and higher fullerenes in toluene solutions. Based on these measurements, broadband predictions of the optical limiting performance of these molecules can be deduced. These predictions are then tested in the wavelength range from 532 nm to 700 nm in intensity-dependent transmission measurements. We observe optical limiting in all fullerenes measured; higher fullerenes show the greatest potential for limiting in the near infrared (650 - 1000 nm), while substituted C60 shows optimal limiting in the visible (450 - 700 nm). We observe dramatically reduced limiting for solid forms of C60 (thin films and C60-doped porous glasses), indicating that efficient optical limiting in fullerenes requires true molecular solutions.
Nonlinear optical absorption in solid films of poly(3-octyl thiophene) (P3OT) sensitized with methanofullerene was investigated for wavelengths from 620 to 960 nm. The nonlinear absorption is enhanced over that in either of the component materials by more than two orders of magnitude at 760 nm. The large nonlinearity results from efficient photoinduced intermolecular charge transfer from P3OT to methanofullerene, followed by absorption in the charge separated excited state. P3OT/fullerene films are promising as optical limiters; the transmission clamps at an average fluence of approximately 0.1 J/cm2.