We report on photoconduction and optical properties of aligned assemblies of core-shell CdSe/CdS nanorods prepared by a seeded growth approach. We fabricate oriented layers of nanorods by drop casting the nanorods from a solution on substrates with prepatterned, micrometer-spaced electrodes and obtain nanorod alignment due to the coffee stain effect. The photoconductivity of the nanorod layers can be improved significantly by an annealing process under vacuum conditions. The spectral response of the photocurrent shows distinct features that can be assigned to the electronic level structure of the core-shell nanorods and that relate well to the spectra obtained by absorption measurements. We study assemblies of nanorods oriented parallel and perpendicular to the applied electric field by the combined use of photocurrent and photoluminescence spectroscopy. We obtain consistent results which show that charge carrier separation and transport are more efficient for nanorods oriented parallel to the electric field. We also investigate the light polarization sensitivity of the photocurrent for the oriented nanorod layers and observe higher conductivity in the case of perpendicular polarization with respect to the long axis of the nanorods.

Tetrapod-shaped CdSe(core)/CdTe(arms) colloidal nanocrystals, capped with alkylphosphonic acids or pyridine, were reacted with various small molecules (acetic acid, hydrazine and chlorosilane) which induced their tip-to-tip assembly into soluble networks. These networks were subsequently processed into films by drop casting and their photoconductive properties were studied. We observed that films prepared from tetrapods coated with phosphonic acids were not photoconductive, but tip-to-tip networks of the same tetrapods exhibited appreciable photocurrents. On the other hand, films prepared from tetrapods coated with pyridine instead of phosphonic acids were already highly photoconductive even if the nanocrystals were not joined tip-to-tip. Based on the current-voltage behavior under light we infer that the tunneling between tetrapods is the dominant charge transport mechanism. In all the samples, chemically-induced assembly into networks tended to reduce the average tunneling barrier. Additionally, pyridine-coated tetrapods and the tip-to-tip networks made out of them were tested as active materials in hybrid photovoltaic devices. Overall, we introduce an approach to chemically-induced tip-to-tip assembly of tetrapods into solution processable networks and demonstrate the enhancement of electronic coupling of tetrapods by various ligand exchange procedures.

We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy ρ (varying from ∼0.1 to ∼0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.

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In this paper, shunt capacitive RF microelectromechanical systems (MEMS) switches are developed in III-V technology using tantalum nitride (TaN) and tantalum pentoxide (Ta 2 O 5 ) for the actuation lines and the dielectric layers, respectively. A compositional, structural, and electrical characterization of the TaN and Ta 2 O 5 films is preliminarily performed, demonstrating that they are valid alternatives to the conventional materials used in III-V technology for RF MEMS switches. Specifically, it is found that the TaN film resistivity can be tuned from 0.01 to 30 Ω · cm by changing the deposition parameters. On the other hand, dielectric Ta 2 O 5 films show a low leakagecurrent density of few nanoamperes per square centimeter for E ∼ 1 MV/cm, a high breakdown field of 4 MV/cm, and a high dielectric constant of 32. The realized switches show good actuation voltages, in the range of 15-20 V, an insertion loss better than −0.8 dB up to 30 GHz, and an isolation of ∼ −40 dB at the resonant frequency, which is, according to bridge length, between 15 and 30 GHz. A comparison between the measured S-parameter values and the results of a circuit simulation is also presented and discussed, providing useful information on the operation of the fabricated switches. [2010-0236] Index Terms-Capacitive switches, RF microelectromechanical systems (MEMS), tantalum nitride (TaN), Ta 2 O 5 .