RESUMEN
The role of the interfaces on the optoelectronical properties of porphyrin-fullerene composites has been studied by means of ultraviolet-visible (UV-Vis) and Raman spectroscopy, atomic force microscopy (AFM) and electric conductivity measurements. A simple method of synthesis of donor- acceptor complexes has been performed by subsequent deposition of C60 fullerene and tetraphenylporphyrin (H2TPP) thin films, using physical vapor deposition (PVD) on a (100) silicon substrate. UV-Vis spectra showed that the interaction of π-orbitals leads to a more ordering for the dipole moments arrangement and the π-orbitals overlapping between C60 and H2TPP molecules. Besides, Raman spectra presented intensity changes at 960 and 1000 cm-1, both related to the vibration of the pyrrole ring and the rocking of the H on the C atoms within the macrocycle. Therefore, it can be expected that the interface C60-H2TPP should have a main role in the electric response of the multilayer films. The measurements of surface conductivity indicated that interface has specific contribution, and the value of surface conductivity is enhanced by charge delocalization mechanisms occur by π-π stacking interactions. It was found that the transversal conductivity of 3-layer films was enhanced by a factor of 4 in comparison to 2-layer film, due to charge transfer mechanisms occur in the junctions that could extend the diffusion length of the charge carriers. Finally, the interface generated between C60 and H2TPP films, without any linking molecule, enhance charge transport mechanism through the films.
RESUMEN
A detailed optical and photoelectric characterization of pristine fullerene C60 films deposited onto n-silicon substrates (C60/Si), C60 films crosslinked by means of the solvent-free chemical functionalization with 1,8-diaminooctane (C60-DA/Si), and the pristine and crosslinked films decorated with silver nanoparticles (C60-Ag/Si and C60-DA-Ag/Si), was carried out. The reflectance spectra obtained allowed to calculate the absorption coefficient (alpha = 4pik/lamda) spectral dependencies and the spectra of light transmittance in layered barrier structures metal(Au)/fullerene/Si. Photoelectric properties of the films were investigated as well. The experimentally measured values of band gap were in a good agreement with mobility gap values (2.3 +/- 0.1 eV). The decoration of fullerene films (both pristine and chemically crosslinked) with silver nanoparticles did not change the photocurrent spectra as compared to those for undecorated fullerene films, but lowered the values of internal quantum efficiency Qint. The photocurrent generated in fullerene/Si heterostructure, showed a maximum value at lamda to appromimately 450 nm (Qint max approximately = 0.25 for decorated and undecorated C60-DA/Si films), and it was higher for the samples based on pristine C60 films, in accordance with their higher absorption coefficient. Diminishing of Qint for C60-DA/Si and C60-DA-Ag/Si films was observed for the spectral range of photocurrent generated in Si layer. The analysis of dark current-voltage characteristics showed that the barrier properties differ insignificantly, though a certain increase of series resistance was observed for the C60-DA/Si samples.
Asunto(s)
Ácidos Carboxílicos/química , Electroquímica/métodos , Fulerenos/química , Nanoestructuras/química , Nanoestructuras/ultraestructura , Nanotecnología/métodos , Fotoquímica/métodos , Plata/química , Cristalización/métodos , Conductividad Eléctrica , Luz , Sustancias Macromoleculares/química , Ensayo de Materiales , Conformación Molecular , Tamaño de la Partícula , Dispersión de Radiación , Semiconductores , Propiedades de SuperficieRESUMEN
Fullerene C60 films deposited by sublimation were irradiated with Kr-F laser in a wide fluence interval from 15 to 40 mJ/cm2. In situ photoacoustic analysis was applied to study the phase transformation during the irradiation. The results obtained were discussed in conjunction with atomic force microscopy (AFM) and Raman spectroscopy data. It was found that for a irradiation fluence interval from 22 to 30 mJ/cm2, 80% of C60 undergoes photopolymerization (presumably through 2 + 2 cycloaddition). For a laser energy higher than 30 mJ/cm2, a new amorphous carbon phase forms, having a large content of diamond-like, tetra-amorphous carbon (ta-C).
Asunto(s)
Fulerenos/química , Luz , Microscopía de Fuerza Atómica/métodos , Nanotecnología/métodos , Fotoquímica/métodos , Espectrometría Raman/métodos , Acústica , Rayos Láser , Polímeros/química , Dispersión de Radiación , Propiedades de Superficie , Termodinámica , Factores de Tiempo , Rayos UltravioletaRESUMEN
We applied the direct solvent-free functionalization of fullerene C60 with aliphatic bifunctional amine, 1,8-diaminooctane, to prepare chemically cross-linked C60 thin films capable of binding silver nanoparticles. The gas-phase diamine treatment of C60 reduced dramatically the fullerene solubility in toluene, indicating the transformation of pristine C60 into a different solid phase with cross-linked fullerene molecules. Compared to the spectra of pristine C60 film and powder samples, Fourier-transform infrared, UV-Visible, Raman, and 13C nucleic magnetic resonance spectra of the functionalization products exhibited new features, which point to a breaking of C60 ideal structure during the formation of new covalent bonds and to the appearance of sp3 hibridization. The covalent functionalization with 1,8-diaminooctane allowed for a stable and homogeneous deposition of silver nanoparticles of ca. 5-nm diameter onto the functionalized films through the coordination bonding between metal atoms and nitrogen donor atoms of the fullerene derivatives. The proposed mechanism of Ag nanoparticle binding was supported by density functional theory calculations using the hybrid BLYP functional in conjunction with the double numerical basis set DND.