RESUMO
Nonlinear conducting states in a strongly correlated organic electronic system α-(BEDT-TTF)2I3 [BEDT-TTF=bis(ethylenedithio)-tetrathiafulvalene] are studied by Raman spectroscopy. Wide-range Raman spectra of nonlinear conducing states provide direct information about conducting properties through the electronic Raman process. A comparison between the behaviors of the electronic modes of BEDT-TTF layers and the vibrational mode of I3 molecules reveals the formation of nonequilibrium states in which only the electronic parts show the change of states. We obtained a spatial map of the conducting regions of the nonlinear conducting states by utilizing the electronic Raman intensity as a measure of the highly conducting states. A spatially inhomogeneous formation of nonlinear conducting states was observed.
RESUMO
We investigate the dynamical properties of photoexcited carriers in a single monolayer of graphene at room temperature in air using femtosecond time-resolved luminescence spectroscopy. The luminescence kinetics are observed in the near-infrared region of 0.7-1.4 eV and analyzed based on the two-temperature model describing the cooling of thermalized carriers via the carrier-optical phonon interaction. The observed luminescence in the range 0.7-0.9 eV is well reproduced by the model. In the range 1.0-1.4 eV, however, the luminescence, which decays in â¼300 fs, cannot be reproduced by this model. These results indicate that the carrier system is not completely thermalized in â¼300 fs. We also show the importance of the carrier-doping effect induced by the substrate and surrounding environment in the carrier cooling dynamics and the predominance of optical phonons over acoustic phonons in the carrier-phonon interactions even at a temperature of â¼400 K.
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Excitation energy transfer has long been an intriguing subject in the fields of photoscience and materials science. Along with the recent progress of photovoltaics, photocatalysis, and photosensors using nanoscale materials, excitation energy transfer between a donor and an acceptor at a short distance (≤1-10 nm) is of growing importance in both fundamental research and technological applications. This Perspective highlights our recent studies on exciton energy transfer between carbon nanotubes with interwall (surface-to-surface) distances of less than â¼1 nm, which are equivalent to or shorter than the size of one-dimensional excitons in carbon nanotubes. We show exciton energy transfer in bundles of single-walled carbon nanotubes with the interwall distances of â¼0.34 and 0.9 nm (center-to-center distances â¼1.3-1.4 and 1.9 nm). For the interwall distance of â¼0.34 nm (center-to-center distance â¼1.3-1.4 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is (1.8-1.9) × 10(12) s(-1), and that to adjacent metallic tubes is 1.1 × 10(12) s(-1). For the interwall distance of â¼0.9 nm (center-to-center distance â¼1.9 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is 2.7 × 10(11) s(-1). These transfer rates are much lower than those predicted by the Förster model calculation based on a point dipole approximation, indicating the failure of the conventional Förster model calculations. In double-walled carbon nanotubes, which are equivalent to ideal nanoscale coaxial cylinders, we show exciton energy transfer from the inner to the outer tubes. The transfer rate between the inner and the outer tubes with an interwall distance of â¼0.38 nm is 6.6 × 10(12) s(-1). Our findings provide an insight into the energy transfer mechanisms of one-dimensional excitons.
Assuntos
Nanotubos de Carbono/química , Transferência de Energia , Medições Luminescentes , Micelas , Semicondutores , Fatores de TempoRESUMO
We study exciton energy transfer in double-walled carbon nanotubes using femtosecond time-resolved luminescence measurements. From direct correspondence between decay of the innertube luminescence and the rise behavior in outertube luminescence, it is found that the time constant of exciton energy transfer from the inner to the outer semiconducting tubes is â¼150 fs. This ultrafast transfer indicates that the relative intensity of steady-state luminescence from the innertubes is â¼700 times weaker than that from single-walled carbon nanotubes.
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Changes of composition profiles in GaInAs layers sandwiched by InP, due to the layer thicknesses, were measured by the x-ray CTR scattering and cross-sectional STM techniques. Both techniques showed quite similar results, which indicates that the x-ray CTR scattering measurements and analyses give us correct composition profiles both for group-III and group-V atoms in the buried heterostructures non-destructively. Limits of the CTR analysis are discussed, especially on the spatial resolution and composition grading below the bottom interface.
Assuntos
Arsênio/química , Gálio/química , Índio/química , Teste de Materiais/métodos , Microscopia de Tunelamento/métodos , Fosfinas/química , Difração de Raios X/métodosRESUMO
The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties of a symmetric acceptor-acceptor-donor-acceptor-acceptor array, C(60)-Co-TTF-Co-C(60), have been described. The precursors, namely, cobalt dicarbonyl complexes Co(C(60)Ar(5))(CO)(2) were synthesized from the penta(organo)[60]fullerenes, C(60)Ar(5)H, as starting materials. In the next step, two cobalt-fullerene complexes were connected to a tetrathiafulvalene (TTF) tetrathiolate bridge to obtain the C(60)-Co-TTF-Co-C(60) array. In addition, the monomeric compounds, Co(C(60)Ar(5))(S(2)C(2)R(2)) (R = CO(2)Me and CN) and Co(C(60)Ar(5))(S(2)C(2)S(2) C = CS(2)C(2)R(2)) were synthesized as references. The C(60)-Co-TTF-Co-C(60) array exhibits very strong transitions in the near-infrared region (lambda(max) = 1,100 nm, epsilon = 30 000 M(-1) x cm(-1)) due to a ligand-to-metal-charge-transfer (LMCT) transition and six reversible electron transfer processes. In the crystal, a fullerene/TTF-layered packing structure is evident. Femtosecond flash photolysis revealed that photoexcitation of the array results in a charge separated state involving the strongly interacting cobaltadithiolene and TTF constituents which electronically relax via a resonance effect that extends all throughout the acceptor parts of the C(60)-Co-TTF-Co-C(60) array. The third-order NLO measurement of the array gave the magnitude of the third-order nonlinear susceptibility, |chi((3))|, values to be 9.28 x 10(-12) esu, suggesting the pi-conjugation of donors and acceptors in the array.
Assuntos
Cobalto/química , Elétrons , Fulerenos/química , Compostos Heterocíclicos/química , Compostos Organometálicos/química , Cristalografia por Raios X , Modelos Moleculares , Conformação Molecular , Espectrofotometria Infravermelho , Espectrofotometria UltravioletaRESUMO
The effect of pressure on the phase behavior of the liquid crystalline complex [Eu(bta)(3)L(2)] (bta is benzoyltrifluoroacetonate, and L is the Schiff base 2-hydroxy-N-octadecyl-4-tetradecyloxybenzaldimine) was studied by X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy. The pressure was varied between ambient pressure and 8.0 GPa. [Eu(bta)(3)L(2)] exhibits a smectic A (SmA) phase at room temperature. The complex undergoes a transition from the SmA phase to a solid lamellar structure around 0.22 GPa and another transition from the solid lamellar phase to an amorphous state from 1.6 to 3.5 GPa. At low pressures, the smectic layer spacing increases, and the intermolecular distance decreases. Above 3.5 GPa, both the interlamellar and the intermolecular spacings hardly change, but the intensity of X-ray reflections exhibits a remarkable decrease and eventually vanishes. An interpretation of the changes in the molecular structure is given. It was found that less interdigitation of the alkyl chains situated in adjacent layers and/or a full extension of the alkyl chains occurred at low pressures and that the second phase transition was accompanied by a transfer of the hydrogen atom from the nitrogen atom of the imine group to the oxygen atom of the Schiff base ligand. The effect of applying pressure equals that of the lanthanide contraction on the phase behavior.
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We have performed electroabsorption spectroscopy on micelle-wrapped single-wall carbon nanotubes. In semiconducting nanotubes, many oscillating structures composed of the increase and decrease of absorption are observed in the spectra in the region of the first and second absorption bands, E11 and E22. The spectral shape is reproduced mainly by the second-derivative curve of the absorption spectrum, which indicates the presence of nearly degenerate bright and dark excitonic states.