Photophysics of resonantly and non-resonantly excited erbium doped Ge nanowires.

We have fabricated Er doped germanium nanowires of different diameters by pulsed laser deposition and chemical methods. Er induced photoluminescence emission due to the intra-4f (4)I(13/2)→(4)I(15/2) transition of Er energy levels at 1.53 µm has been achieved at room temperature using both resonant (980 nm) and non-resonant (325 nm) excitation of Er ions. The observed 1.53 µm photoluminescence signal upon non-resonant 325 nm excitation is attributed to the Ge related oxygen deficiency centers surrounding the Ge core. For direct excitation, the infrared photoluminescence characteristics have been studied as a function of Er concentration, photon flux, and diameter of the nanowires. The Er related emission signal is found to be enhanced with increase in Er concentration, pump flux of 980 nm, and the nanowire diameter. The time resolved characteristics of the Er induced emission peak have been studied as a function of the pump flux as well as the diameter of the Ge nanowires.

Bound electronic and free carrier nonlinearities in Silicon nanocrystals at 1550nm.

We present a detailed investigation of the different processes responsible for the optical nonlinearities of silicon nanocrystals at 1550 nm. Through z-scan measurements, the bound-electronic and excited carrier contributions to the nonlinear refraction were measured in presence of two-photon absorption. A study of the nonlinear response at different excitation powers has permitted to determine the change in the refractive index per unit of photo-excited carrier density sigma(r) and the value of the real bound-electronic nonlinear refraction n(2be) as a function of the nanocrystals size. Moreover at high excitation power, a saturation of the nonlinear absorption was observed due to band-filling effects.

Optical properties and pulse shape discrimination in siloxane-based scintillation detectors.

The possibility to detect fast neutrons as a distinct signal from that one of γ-rays background is surely of great importance for several topics, spanning from homeland security to radiation monitoring in nuclear physics research plants. Nowadays, Helium-3 based detectors are extremely expensive, while the use of large volume liquid scintillators presents serious concerns related to spillage risks and waste disposal. A very attractive alternative is the use of commercially available solid scintillators, which exploits an aromatic polymer matrix entrapping very high loadings of primary dye, thereby enabling the use of pulse shape analysis (PSA) to discriminate between fast neutrons and γ-rays. In this work, we analyse in detail the optical features of a solid scintillator composed by polymethylphenylsiloxane (PMPS) as base polymer loaded with moderate amounts of 2,5-diphenyloxazole (PPO). Furthermore, fluorescence decay kinetics have been correlated to the observed pulse shape discrimination capabilities of this radiation and thermally resistant scintillator, whose performances have been discussed in terms of conformational features and excimers formation revealed by the optical analyses.

TiO2-coated luminescent porous silicon micro-particles as a promising system for nanomedicine.

Porous silicon (pSi) is a sponge-like material obtained by electrochemical etching of a crystalline silicon wafer. Due to quantum confinement effects, this material is photoluminescent and this is a fundamental property from the perspective of bioimaging applications. Limitations in nanomedicine to the use of photoluminescent pSi structures are mainly due to optical quenching in an aqueous environment and to the adverse effects of reactive groups introduced by etching procedures. In this work, we exploited an inorganic TiO2 coating of pSi microparticles by Atomic Layer Deposition (ALD) that resulted in optical stability of pSi particles in a biological buffer (e.g. PBS). The use of a rotary reactor allows deposition of a uniform coating on the particles and enables a fine tuning of its thickness. The ALD parameters were optimized and the photoluminescence (PL) of pSi-TiO2 microparticles was stabilized for more than three months without any significant effect on their morphology. The biocompatibility of the coated microparticles was evaluated by analyzing the release of cytokines and superoxide anion (O2 -) by human dendritic cells, which play an essential role in the regulation of inflammatory and immune responses. We demonstrated that the microparticles per se are unable to significantly damage or stimulate human dendritic cells and therefore are suitable candidates for nanomedicine applications. However, a synergistic effect of the microparticles with bacterial products, which are known to stimulate immune-response, was observed, indicating that a condition unfavorable to the use of inorganic nanomaterials in biological systems is the presence of infection diseases. These results, combined with the proved PL stability in biological buffers, open the way for the use of pSi-TiO2 microparticles as promising materials in nanomedicine, but their ability to increase immune cell activation by other agonists should be considered and even exploited.

Study of an efficient longitudinal multimode pumping scheme for Si-nc sensitized EDWAs.

We present an efficient multimode longitudinal pumping scheme which overcomes the main limitations of single-mode longitudinal pumping as well as top pumping in Si-nanoclusters sensitized Erbium-doped waveguide amplifiers. The proposed configuration is based on evanescent pump light coupling from a multimode waveguide to a Si-nanoclusters sensitized Er(3+)-doped active core. Theoretical predictions, based on propagation and population-rate equations for the coupled Er(3+)/Sinanoclusters system, point out that the proposed pumping scheme can provide high pump intensity within the active core, also ensuring good uniformity of the population inversion along the waveguide amplifier. Although longitudinal multimode pumping by high power LEDs in the visible can potentially lead to low cost integrated amplifiers, further material optimization is required. In particular, we show that when dealing with high pump intensities, confined carrier absorption seriously affects the amplifier performance, and an optimization of both Si-nc and Er(3+) concentrations is necessary.

A systematic study on the use of ultrasound energy for the synthesis of nickel-metal organic framework compounds.

A nickel metal-organic framework (Ni-MOF) was successfully synthesized using ultrasound irradiation. Further to this, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR), Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and nitrogen adsorption [i.e. Brunauer-Emmett-Teller (BET) Surface Area Analysis] techniques were used to characterize the synthesized Ni-MOF. In addition, the effect of sonication on the surface area, pore diameter and pore volume of the final product was systematically studied using Taguchi technique. The experiments ascertained that manufacturing of the Ni-MOF by means of the ultrasonic-assisted technique is feasible at a relatively shorter time compare to the conventional methods. The final product showed more uniform shape distribution and improved BET properties. The obtained results offered that the synthesized Ni-MOF samples could be used in several applications.

Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition.

Silicon nanocrystals (Si-nc) embedded in SiO2 matrix have been prepared by high temperature thermal annealing (1000-1250 degrees C) of substoichiometric SiOx films deposited by plasma-enhanced chemical vapor deposition (PECVD). Different techniques have been used to examine the optical and structural properties of Si-nc. Transmission electron microscopy analysis shows the formation of nanocrystals whose sizes are dependent on annealing conditions and deposition parameters. The spectral positions of room temperature photoluminescence are systematically blue shifted with reduction in the size of Si-nc obtained by decreasing the annealing temperature or the Si content during the PECVD deposition. A similar trend has been found in optical absorption measurements. X-ray absorption fine structure measurements indicate the presence of an intermediate region between the Si-nc and the SiO2 matrix that participates in the light emission process. Theoretical observations reported here support these findings. All these efforts allow us to study the link between dimensionality, optical properties, and the local environment of Si-nc and the surrounding SiO2 matrix.

Orange and blue luminescence emission to track functionalized porous silicon microparticles inside the cells of the human immune system.

Porous silicon micro-particles (micro-pSi) with size in the range of 1-10 µm are obtained by etching of silicon wafers followed by sonication. The derivatization of the micro-pSi surface by wet chemistry (silylation and coupling with a diamine) yields an interface, which exposes negative (carboxylic) or positive (amine) groups at pH 7.4. The surface modification, beyond the introduction of groups for the drug loading by covalent or electrostatic interactions, stabilizes the intense orange luminescence characteristic of the silicon nano-crystallites. Derivatization by amines introduces also a second emission in the blue region, which follows a different excitation pathway and can be attributed to the interface defects. The micro-pSi are efficiently internalized by human dendritic cells and do not show any toxic effect even at a concentration of 1 mg mL-1. The intrinsic luminescence of the differently functionalized micro-pSi is preserved inside the cells and permits the selective and efficient tracking of the microparticles without using molecular tags and thus leaving the organic coating available for the interaction with the drug. The results obtained suggest that the functionalized micro-pSi are an efficient platform for simultaneous imaging and delivery of therapeutic agents to the disease site.