Optically Detected Magnetic Resonance (ODMR) Study of Isolated C70 Molecules and C70 Films

Abstract

The X-band ODMR of C₇₀ molecules isolated in a toluene/polystyrene (C₇₀:T/PS) glass and C₇₀ films is compared to that of C₆₀. As C₆₀:T/PS, the C₇₀ glasses exhibit only a triplet pattern at g ? 2, clearly due to an exciton delocalized over the molecule; however, it is strikingly asymmetric and narrower, due to the larger size of C₇₀. The ODMR of both C₆₀ and C₇₀ films includes a narrow photoluminescence-enhancing polaron resonance and full- and half- field narrow and broad triplet exciton patterns. As in the glasses, the narrow pattern is due to the exciton delocalized over the molecule and is narrower in C₇₀ The broad pattern is tentatively assigned to a triplet localized on a face adjacent to a neighboring molecule. In contrast to C₆₀, the ODMR of C₇₀ films is detectable up to 295 K, apparently due to the slower rotation.     

Intercalation of O2 and its Effect on the Properties of C60 and C70 Solids

Abstract

The intercalation of O₂ in C₆₀ and C₇₀ solids have been studied. In both gas effusion spectra of C₆₀ and C₇₀ powder exposed to O₂ at room temperature, two evolution peaks of O₂ are found near 80 and 150°C. In both C₆₀ and C₇₀ powder, the peak near 80°C does not depend on a heating ratio and the peak near 150°C shifts to higher temperature with the heating ratio. The activation energy on diffusion of O₂ in C₆₀ and C₇₀ solids are ～0.27eV and ～0.46eV, respectively. The number of O₂ intercalated in C₇₀ powder is larger than that in C₆₀ powder. Electron spin density of 02-intercalated C₆₀ film is larger than that of C₇₀ film. The effect on the properties of C₆₀ and C₇₀ solids by the intercalation of O₂ are discussed.     

Infrared Spectra and Photoinduced Absorption of C60 Doped Polyhexylthiophene

Abstract

We report the IR spectra and the photoinduced absorption of polyhexylthiophene(PHT) doped with C₆₀. The presence of C₆₀ ^?n anions is observed in the IR spectra by a mode at 1383 cm^?1 and a broad electronic transition at 1.2 eV, while most of the C₆₀ molecules stay neutral. This is indicative that for a small fraction of C₆₀ molecules, charge transfer occurs in the ground state.

The photoinduced absorption spectra display a sharp transition at 1.15 eV peculiar to C₆₀ monoanion, the spectral features change with fullerene concentration. The intensity of the polymer photoinduced bands increases 5-10 times due to photoinduced charge transfer to C₆₀.     

Occupied and Unoccupied Orbitals of C60 and C70

Abstract

The full spectrum of occupied and unoccupied σ and π orbitals is presented for solid C₆₀, C₇₀, and graphite, using Cls emission and absorption spectroscopy. There are significant diffcrences between C₆₀ and C₇₀, and even larger changes relative to their infinite analog graphite (C∝). A comparison is made with photocmission and inverse photoemission results, along with first principles quasiparticle calculations.     

Optical Properties of Pyrochlore Lead Scandium Tantalate Crystals

Optically homogeneous Pb₂Sc_0.5Ta_1.5O_6.5 crystals with pyrochlore structure are prepared by the method of high-temperature solution growth (HTSG). The refractive index in the visible region of the spectrum is measured. From the transmission spectra in the visible and infrared region the optical transmission window is estimated. From the fundamental absorption edge in the visible region of the spectrum the optical bandgap is determined corresponding to the intrinsic absorption. From the behaviour of the absorption coefficient α(hν) and α^1/2(hν) and the Raman spectrum permit to suggest that the absorption edge below 2.75 eV is due to indirect transitions.     

Semiconducting TiO2SiO2 glasses (II): Optical properties

The refractive index, optical absorption coefficient α and the thermomodulated absorption dα/dT have been measured on 70% TiO₂?30% SiO₂ glasses with up to 8% Ti³⁺. The direct absorption data show intense ligand field absorption at photon energies hν = 1.9 and 3.0 eV, arising from Ti³⁺ in a distorted octahedral environment. In the bandgap region at 3.5 eV α obey αhν ～ (hν ? E_g)²; it is qualitatively different from the bandgap absorption in crystalline TiO₂. The da/dT spectra show peaks in the bandgap region and at 1.1 eV in the near IR. This last peak is attributed to absorption by small polarons, and its line shape is compared with theoretical predictions.     

Structural study of GeS2 glasses permanently densified under high pressures up to 9 GPa

The structures of GeS₂ glasses permanently densified under 1.5, 3.0, 4.5, 6.0 and 9.0 GPa have been investigated by means of Ge–K EXAFS, Ge–K and S–K XANES, X-ray radial distribution, Raman scattering and optical absorption. The experimental results have been analyzed based on the structures of α (high-temperature form)-, β (low-temperature form)- and II (high-pressure form)-GeS₂ crystals. The densities of permanently densified glasses increased monotonously with increasing applied-pressure until 6.0 GPa and then reached a constant value in a pressure range from 6.0 to 9.0 GPa. With increasing densification the structure of GeS₂ glass, which is an intermediate between the structures of α-GeS₂ and β-GeS₂ at atmospheric pressure, was progressively converted into a II-GeS₂-like structure with no large hollows. The red shift of optical absorption edge in the visible region that results from densification exhibited the same pressure dependence as that observed for density.     

Optical absorption spectra of magnesium-implanted GaP

Optical absorption spectra of magnesium-implanted GaP single crystals are measured in the photon energy range below the fundamental edge. It is found that implantation results in the formation of exponential absorption tails with tailing energies up to about 0.8 eV and absorption coefficients up to about 10⁵ cm⁻¹ at 2.0 eV. The results are compared with optical measurements on GaP single crystals implanted with other ions and with optical absorption data of amorphous GaP.     

Optical absorption of glassy semiconductors of the GeSbSe system

The optical transmittance of chalcogenide glasses Ge₂SbSe₇ (I), Ge₃SbSe₆ (II), GeSb₂Se₇ (III) and GeSbSe₃ (IV) was studied in the near infrared spectral region, 0.7–25 μm. The longwavelength tail of the absorption edge can be described by Urbach's rule. At higher absorption levels the absorption coefficient K depends quadratically on the energy of incident radiation. The temperature dependence of the absorption edge is discussed and the optical gaps 1.77 eV (I), 1.67 eV (II), 1.66 eV (III), 1.57 eV (IV) together with the corresponding temperature coefficients are also determined. The studied glasses are quite transparent in the 600–5000 cm^?1 wavenumber range.     

Spectral Characteristics of C60-Conducting Polymer Junctions: Various Molecular D-A Type Photocells

Abstract

We have found recently that thin film layered heterojunction between C₆₀ and conducting polymer, like poly(3-alkylthiophene) (PAT) shows a photovoltaic effect due to photoinduced charge transfer at the interface. Here we describe two other examples of such photocells in RO-PPV/C₆₀ and PPP/C₆₀ heterojunctions and study their spectral characteristics. Contrary to conventional inorganic semiconductor p-n junction photocells, the C₆₀-PAT junction can be rather viewed as molecular donor-acceptor (D-A) type photocell in which the processes of photogeneration and separation of charge carriers are quite distinct. In a p-n junction free electron-hole pairs are known to be primarily photogenerated at interband transition and then separated in the internal electric field of the barrier, while in D-A molecular photocell mainly neutral excitons are first created by light, with charges being primarily separated at the narrow interface region due to D-A type intermolecular charge transfer interactions. We analyze how this processes depend on the wavelength of the pumping light and on the polarity and magnitude of the applied voltage.     

Crystal Structure of Europium C60 Compounds

Abstract

We have studied crystal structures of europium C₆₀ compounds by high resolution x-ray diffraction measurements. In the systematic synthesis of Eu _x C₆₀ for x (0 ≤ x ≤ 6), two stable phases appear at x = 3 and 6. Eu₆C₆₀ has a bcc structure, which is an isostructure to M ₆C₆₀ (M represents an alkali or alkaline earth metal). Eu₃C₆₀ has a superstructure derived by doubling the fcc pristine C₆₀ crystal along three principal axes. This superstructure comes from an ordering of cation vacancies, which is analogous to Yb_2.75C₆₀.     

Pressure and temperature dependence of the electrical resistivity of bulk Al23 Te77 glass

Electrical resistivity of bulk amorphous Al₂₃T₇₇ samples has been studied as a function of pressure (up to 80 kbar) and temperature (down to 77 K). At atmospheric pressure the temperature dependence of resistivity obeys the relation ? = π₀ exp(δE/RT) with two activation energies. In the temperature range 300 K ? T > 234 K the activation energy is 0.58 eV and for 234 >T ? 185 K the value is δE = 0.30 ev. The activation energy has been measured as a function of pressure. The electrical resistivity decreases exponentially with the increase of pressure and at 70 kbar pressure the electrical behaviour of the sample shows a metallic nature with a positive temperature coefficient. The high pressure phase of the sample is found to be a crystalline hexagonal phase.     

Wide band-gap organic molecules containing benzodithiophene and difluoroquinoxaline derivatives for solar cell applications

Abstract

Two new semiconducting organic small molecules, namely BDTQ-BDT(EH) and BDTQ-BDT(OC), were prepared by attaching electron accepting 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (DTQ) unit on 2,6-position of electron donating 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(EH)) and 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(OC)) units. Molecule BDTQ-BDT(EH) showed higher thermal stability (5% weight loss temperature, T_d “349 ^оC), slightly lower band-gap (E_g “2.10?eV) and deeper highest occupied molecular orbital energy level (HOMO “–5.36?eV) level compared to those (T_d “336 ^оC, E_g “2.11?eV, and HOMO “–5.30?eV, respectively.) of the molecule BDTQ-BDT(OC). The organic solar cells (OSCs) made with the synthesized molecules as an electron donor and [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₀BM) as an electron acceptor gave a maximum power conversion efficiency (PCE) of 1.20% and 0.83%, respectively, for BDTQ-BDT(EH) and BDTQ-BDT(OC). This study confirmed that the substituents attached on the 4,8-position of BDT unit greatly alter the properties of the resulting molecules.     

A density functional theory study on multiple exciton generation in lead chalcogenides

Abstract

Quantum confined structure-based solar cell is promising two folds increment of the maximum theoretical photovoltaic conversion efficiency i.e., > 60% in comparison with that of the bulk analogs e.g., silicon-based and dye sensitized solar cell (ca. 32% of maximum theoretical efficiency). The key to the significant increment is the ability of the fluorophore to exhibit multiple exciton generation upon absorption photon with sufficient energy. Small size of lead chalcogenides (PbS, PbSe, PbTe) crystals have been reported and proven experimentally could exhibit this unique property. We have investigated few clusters of narrow bandgap lead chalcogenides nanocrystals i.e., (PbS)n, (PbSe)n and (PbTe)n; which n?=?4 - 80. The cluster models were optimized using quantum chemical calculations to the lowest energy geometry at B3LYP/lanl2dz level of theory. The predicted realistic (PbS)₈₀, (PbSe)₅₀, and (PbTe)₇₄ clusters with the size, and bandgap of 4.58?nm (2.00?eV), 4.03?nm (1.51?eV), and 4.84?nm (1.55?eV) are smaller than that of their exciton Bohr radius i.e., 5.01, 13.1, and 24.8?nm respectively. Therefore, the occurrence of multi exciton generation in the clusters is hypothesized upon absorption of photon with E_photon = 2E_g.     

Thin layers deposited from V2O5 gels: II. An optical absorption study

Absorption spectra of V₂O₅ layers deposited from gels of various V⁴⁺ contents C were studied from near UV down to near IR. The high absorption region due to the charge transfer transition extends above ≈ 2 eV. Whatever C the optical gap is ≈ 2.25 eV. On the low energy side an Urbach tail is observed whose slope increases with C. In the near infrared absorption is related to the V⁴⁺ ions and increases linearly with C. The absorption band due to the optically induced polaron hopping is detected. The corresponding maximum E_max suffers a red shift when C increases. The experimental E_max data are very close to the values predicted from the polaron and disorder energies as deduced from the conductivity study in an earlier paper. The slight difference between the two sets of data allowed us to estimate the transfer integral J ≈ 0.015 eV.     

Characteristic Interaction of Na-H-C60 and C60 with Hydrogen and Rare Gases

Abstract

Rare gases such as He, Ne and Ar are trapped in the lattice of solid C₆₀ and (NaH)₄C₆₀ under conditions of ambient temperature and pressure. The mass analyzed thermal desorption reveals that rare gases are desorbed above 400K and their desorption amounts are in nonstoichiometric level. Ar2p and Ar3s X-ray photoelectron spectra show large chemical shifts. These results suggest that He, Ne and Ar have charge and are in valence state.     

Optical studies of aromatic polyazomethine thin films

Optical transmission and fundamental reflectivity spectra of aromatic polyazomethine thin films, obtained by vacuum evaporation via the polycondensation process have been investigated in the wide spectral range 0.49–6.2 eV. As initial monomers, terephaldehyde (TPA) and one of four different amines, i.e. paraphenylene-diamine (PPDA), 7,2-diamino-fluorene, 1,1′-biphenyl-3,3′,4,4′-tetramine and 3-amino-4-(1-naphthyldiazenyl)phenylamine (fat brown RR) have been used, respectively. Amorphous character of these films was confirmed by the results of X-ray diffraction and AFM investigations. Absorption coefficient of the films has been obtained and the edge of absorption seems to be similar to the absorption edge typical for amorphous semiconductors, what allowed to obtain the Urbach energy and E_T parameter. The energy gaps of the films, following the Tauc relation, are found to change from 2.05 to 2.4 eV, depending on the length of conjugated part of the polymer chain. Absorption bands above the absorption edge, observed for different polyazomethine films are connected with their chemical structure and possible electronic transitions.     

Optical Investigation of Defects in p-type CuInSe2 Single Crystals

Optical absorption spectra in the photon energy range from 0.03 to 1.1 eV and photoreflectance spectra in the range of the fundamental edge are measured on p-type CuInSe₂ single crystals. Besides a dominant contribution to the absorption coefficient due to intervalence band transitions below about 0.75 eV the spectra revealed five additional structures that can be ascribed to defect induced optical transitions with characteristic energies between 0.48 and 0.72 eV. Based on a comparison of the near-edge optical absorption and photoreflectance spectra a shallow defect (donor or acceptor) with an ionisation energy of about 46 meV was identified.     

Optical properties of fluorine-substituted zinc bismuth phosphate glasses

The effects of the substitution of fluoride ions for oxide ions on the thermal and optical properties of ternary ZnO-Bi₂O₃-P₂O₅ glass with low-P₂O₅ content (20-25 mol%) were investigated. Fluoride ions were introduced into the glass up to about 12 mol% as ZnF₂. Raman spectra indicated that fluoride ions were substituted for oxide ions connected with bismuth ions. Deformation and glass transition temperatures decreased monotonically with fluorine concentration. The absorption edge shifted toward higher energies with increasing fluorine concentration by about 0.3 eV for 12 mol% ZnF₂ substitution. The blue shift of the absorption edge is attributable to two effects. One was a blue shift of an absorption band which was observed as a peak at 4.7 eV in the reflection spectra and was attributed to the spin forbidden 6s-6p interband transition in Bi³⁺ ions. The blue shift originates from a change in electron-donating ability through anions as expected from electronegativity or optical basicity. Another is a disappearance of a shoulder at around 4.3 eV in the reflection spectra. The latter was the major reason for the large blue shift of the absorption edge energy, because the band relating to the 4.3 eV shoulder is close to the absorption edge.     

Optical absorption behaviour of magnesium-implanted GaAs

Optical absorption spectra of magnesium implanted GaAs single crystals are measured in the photon energy range below the fundamental absorption edge. It is found that the absorption coefficient increases with increasing fluence up to a saturation value of about 5 · 104 ⁴ cm⁻¹ at 1.36 eV. The results are compared with optical measurements on GaAs single crystals implanted with other ions.