Liliana Licea-Jiménez

Infrared photorefractive composites based on polyvinylcarbazole and carbon nanotubes

Photorefractive materials based on unplasticized polymers that have a high glass-transition temperature and the frozen random orientation of chromophores were prepared by layer casting. Under these conditions, only the third-order susceptibility has a nonzero value, increasing with an increase in the conjugation-chain length and reaching considerable values in the case of nanosized molecules, such as single-wall carbon nanotubes (SWNTs). In SWNT-containing polyvinylcarbazole, the photoelectric sensitivity and photorefractive characteristics were measured at 1064 nm. Using electric-field induced second-harmonic generation, the third-order susceptibility was estimated. In a composite containing 0.26 wt % SWNT, the diffraction grating is displaced by 5° relative to the interference pattern, the result that is presumably due to the close mobility of unlike charge carriers. Therefore, the beam-coupling gain coefficient Γ and the net gain Γ-α have low values, which are 53 and 42 cm−1, respectively, at 115 V/μm.

Near-IR Photorefractive Composites Based on Oxidized Single-Wall Carbon Nanotubes

The photoelectric, nonlinear optical, and photorefractive characteristics of polymer composites made from unplasticized polyvinylcarbazole (PVK) and single-wall carbon nanotubes containing surface oxygen groups (at C:O ratio of 77.8:22.2%) were analyzed. The dependences of the quantum efficiency of charge-carrier generation on the applied electric field E measured at 1064 and 1550 nm coincide and are approximated by the Onsager equation expanded to the E4 term, at π0 = 0.012 and r0 = 9.8 Å. The third-order optical nonlinearity determined by the EFISH technique at a nanotube content of 0.26 wt % is 2 × 105 pm2/V2 or 2.3 × 10−29 C4 m/J3 in SI units. The pattern of photorefractive kinetic curves indicates that the mean free path of holes is longer than the mean range of electrons. The photorefractive net gain coefficient of the signal beam measured at 1064 nm in a field of E = 170 V/μm is Γ − α = 55 cm−1 (α = 10 cm−1 is the optical absorption coefficient at 1064 nm). At 1550 nm, the net gain coefficient measured in a field of E = 265 V/μm is Γ − α = 55.7 cm−1 (α = 3.3 cm−1 at 1550 nm). In the presence of oxygen groups, there is no transfer of electrons photogenerated in carbon nanotubes to the external acceptor C60. This effect can be associated with an increase in the ionization potential of nanotubes by almost 0.8 eV as a result of oxidation.

Carbon nanotube-containing photorefractive polymer composites operating at telecommunication wavelengths

The field dependences of photocurrent, the two-beam coupling gain coefficient, and the grating formation time constant in polymer composites made from polyvinylcarbazole (PVK) and single-wall carbon nanotubes (SWNTs) were measured under the conditions of one-photon SWNT excitation with continuous laser radiation at a wavelength of 1550 nm. Carbon nanotubes are responsible for optical electronic absorption up to ∼2000 nm in this composite. The dependence of the quantum efficiency of generation of mobile charge carriers on the electric field E0 as determined from the photocurrent coincides with the curves calculated via the Onsager equation expanded to the (E0)4 term, at a quantum yield of thermalized electron-hole pairs of η0 = 0.07 and a charge separation distance in the pair of r0 = 9.8 Å. An analysis of the photorefractive characteristics showed that the admixture of fullerene C60 in an amount of 3 wt % to the PVK composite with 0.26 wt % SWNT leads to a twofold increase in the beam-coupling gain coefficient. In the PVK-matrix composite containing 0.26 wt % SWNT and 3 wt % C60, the beam-coupling gain coefficient Γ of a 1550-nm laser beam and the net gain Γ-α are 32 and ∼27 cm−1, respectively, at a constant field of E0 = 140 V/μm.

Optical, Electrical and Photocatalytic Properties of the Ternary Semiconductors , and

The effects of vacuum annealing at different temperatures on the optical, electrical and photocatalytic properties of polycrystalline and amorphous thin films of the ternary semiconductor alloys , and were investigated in stacks of binary semiconductors obtained by chemical bath deposition. The electrical properties were measured at room temperature using a four-contact probe in the Van der Pauw configuration. The energy band gap of the films varied from 2.30 to 2.85 eV. The photocatalytic activity of the semiconductor thin films was evaluated by the degradation of an aqueous methylene blue solution. The thin film of annealed under vacuum at 300°C exhibited the highest photocatalytic activity.

Functionalization Effect on Polymer Nanocomposite Coatings Based on TiO2–SiO2 Nanoparticles with Superhydrophilic Properties

In this study, a novel binary nanocomposite system based on TiO2-SiO2 was functionalized with trimethylolpropane triacrylate (TMPTA) and characterized by XPS and XRD. Results revealed that TiO2-SiO2 nanoparticles were covalently functionalized. Functionalized nanoparticles at low concentrations (0.1 wt % and 0.5 wt %) were dispersed in acrylic acid acting as a polymer matrix. Nanocomposite coatings analysis demonstrate to achieve superhydrophilic properties as well as very good optical characteristics. Water contact angle characterization showed the functionalization effect by achieving a superhydrophilic behavior with a contact angle less than 5°. UV-Vis measurements demonstrated high optical transmittance above 95% for the coatings. Based on the obtained results a mechanism describing the chemical interactions of the constituents responsible for the synergy in the nanocomposite as well as the morphological play role in the behavior are presented.

Luminescent Devices Based on Silicon-Rich Dielectric Materials

Luminescent silicon‐rich dielectric materials have been under intensive research due to their potential applications in optoelectronic devices. Silicon‐rich nitride (SRN) and sili‐ con‐rich oxide (SRO) films have been mostly studied because of their high luminescence and compatibility with the silicon‐based technology. In this chapter, the luminescent characteristics of SRN and SRO films deposited by low‐pressure chemical vapor deposi‐ tion are reviewed and discussed. SRN and SRO films, which exhibit the strongest pho‐ toluminescence (PL), were chosen to analyze their electrical and electroluminescent (EL) properties, including SRN/SRO bilayers. Light emitting capacitors (LECs) were fabricated with the SRN, SRO, and SRN/SRO films as the dielectric layer. SRN‐LECs emit broad EL spectra where the maximum emission peak blueshifts when the polarity is changed. On the other hand, SRO‐LECs with low silicon content (~39 at.%) exhibit a resistive switch‐ ing (RS) behavior from a high conduction state to a low conduction state, which produce a long spectrum blueshift (~227 nm) between the EL and PL emission. When the silicon content increases, red emission is observed at both EL and PL spectra. The RS behavior is also observed in all SRN/SRO‐LECs enhancing an intense ultraviolet EL. The carrier transport in all LECs is analyzed to understand their EL mechanism.