Yudhisthira Sahoo

Multi-photon excitation properties of CdSe quantum dots solutions and optical limiting behavior in infrared range.

Multi-photon absorption and excitation properties of CdSe quantum dots in hexane with different dot-sizes have been investigated. The two- and three-photon absorption (2PA and 3PA) coefficients were measured by using ~160-fs laser pulses at wavelengths of ~775-nm and ~1300-nm, respectively. The dependence of one-, two- and three-photon induced fluorescence spectra as well as their double-exponential decay on the dot-sizes was studied. Based on the fluorescence emission spectra and temporal decay constants for a given sample solution excited by one-, two-and three-photon absorption, it can be concluded that the transition pathways for fluorescence emission and decay under one-, two- and three-photon excitation are nearly identical. The optical power limiting capabilities based on 2PA and 3PA mechanisms are demonstrated separately. In addition, a saturation behavior of 3PA at ~1300 nm was observed.

Efficient photoconductive devices at infrared wavelengths using quantum dot-polymer nanocomposites

We report high photoconductivity and narrow luminescence in nanocomposites of PbSe quantum dots in a polymeric matrix. The nanocomposites are photoactive at infrared wavelengths with narrow emission bands, tunable with the quantum dot sizes. The quantum dots sensitize the polymer at distinct wavelengths across the infrared range between 800 nm and 2μm, by virtue of quantum-size effects. Photoconductivity measured in the nanocomposites results in a quantum efficiency of ∼3%, which is among the highest reported so far in this spectral range.

Multiple exciton generation and electrical extraction from a PbSe quantum dot photoconductor

Multiple exciton generation and subsequent electrical extraction from a thin film photoconductive device constructed from PbSe nanocrystal quantum dots are demonstrated. The hydrazine treatment of the PbSe film drastically improves the conductivity of the film while maintaining excellent optical and structural film quality. The effects of multiple exciton generation and electrical extraction (electrons collected per photon absorbed) were quantified as a function of incident photon energy from 1.55to3.1eV. The multiple carrier extraction (>100%) was observed at photon energies greater than 2.8 times of the quantum dot bandgap with ∼210% measured at 4.4 times the bandgap.

Aqueous Ferrofluid of Citric Acid Coated Magnetite Particles

Magnetic nanoparticles have found application in medical diagnostics such as magnetic resonance imaging and therapies such as cancer treatment. In these applications, it is imperative to have a biocompatible solvent such as water at optimum pH for possible bio-ingestion. In the present work, a synthetic methodology has been developed to get a well-dispersed and homogeneous aqueous suspension of Fe 3 O 4 nanoparticles in the size range of 8–10 nm. The surface functionalization of the particles is provided by citric acid. The particles have been characterized using transmission electron microscopy, magnetization measurements with a superconducting quantum interference device, FTIR spectroscopy (for surfactant binding sites), thermogravimetric studies (for strength of surfactant binding), and x-ray photoelectron spectroscopy and x-ray diffraction (for composition and phase information). The carboxylate functionality on the surface provides an avenue for further surface modification with fluorescent dyes, hormone linkers etc for possible cell-binding, bioimaging, tracking, and targeting.

Ferromagnetic ordering in nanostructured Mn-doped InP

We report the observation of ferromagnetic ordering at 25K in a diluted magnetic semiconductor (DMS) nanoparticle system: In0.9Mn0.1P, sized 3nm. These particles were synthesized using a novel nanochemical technique without using any external surfactant. Structural and elemental characterizations established the occurrence of the zinc-blende phase of the DMS without any separate or induced impurity phase. A robust onset of ferromagnetic order is observed in magnetization measurements at around 25K with blocked state behavior below 15K characteristic of magnetic nanoparticles. The system shows strong frequency dependence of the susceptibility, similar to the behavior observed for spin glasses. Reversible transverse susceptibility experiments done using a resonant radio-frequency (rf) method reveal a strong temperature-dependent effective anisotropy.

Quasi-reversible photoluminescence quenching of stable dispersions of silicon nanoparticles

Optically clear and stable dispersions of brightly photoluminescent Si nanoparticles were obtained by covalent attachment of alkenoic compounds to the particles. Quenching of photoluminescence by ethylamine, diethylamine, triethylamine, pyrazine, and piperazine was investigated. The photoluminescence was quenched by the action of these nitrogenous species, but in some cases could be partially restored by the addition of trifluoroacetic acid. The extent of restoration of photoluminescence, after equilibrium is reached, was independent of the sequence of addition of the amine and the acid. The photoluminescence quenching and recovery are influenced by a combination of basicity, polarity, and steric factors of the quencher molecules. The quenching and subsequent restoration occurs gradually at room temperature and it takes several minutes to reach equilibrium.

Solution-processed pentacene quantum-dot polymeric nanocomposite for infrared photodetection

An organic/inorganic polymeric nanocomposite thin film device, consisting of poly-N-vinyl carbazole as host matrix, lead selenide quantum dots as photosensitizer, and the organic semiconductor pentacene as a conductivity booster, is fabricated. Because of the inherent insolubility of pentacene, it is incorporated in the form of a soluble precursor which is made to undergo thermal conversion into pentacene. The device exhibits dramatic enhancement of infrared photocurrent due to pentacene. Efficient photogeneration of carriers coupled with enhanced conductance results in high photoconductive quantum efficiency.

Polymeric nanocomposite infrared photovoltaics enhanced by pentacene

An infrared active thin film polymeric photovoltaic device is fabricated from regioregular poly(3-hexylthiophene), PbSe quantum dots, and the organic semiconductor pentacene. The PbSe quantum dots are infrared photosensitizers. Pentacene is incorporated into the formulation in a soluble precursor form. The current-voltage measurements of the device show that the photovoltaic performance is significantly increased by the introduction of pentacene, with both short-circuit current density and open-circuit voltage increased by a factor of 2. The improved performance of the device is attributed to the high mobility of charge carriers in pentacene probably due to conducting domains provided by it.

An aerosol-mediated magnetic colloid : Study of nickel nanoparticles

A method is presented for the synthesis of high-quality nickel nanoparticles. Laser-driven decomposition of nickel carbonyl vapors is used to produce particles in the form of an aerosol, followed by exposure to a solvent containing an appropriate surfactant to yield a stable dispersion of particles. This method is scalable and yields a substantially monodisperse distribution of particles at a relatively high rate of production. The particles produced by this method are subjected to a detailed characterization using transmission electron microscopy, atomic force microscopy, energy dispersive spectroscopy, and dc magnetization. They have an average diameter of 5 nm, and the observed magnetization curves show no hysteresis above 200 K. The normalized magnetization curves follow a scaling law proportional to the quotient of the applied field over temperature. This data indicates the presence of randomly oriented superparamagnetic particles. The measured magnetization is significantly smaller than that of the ...

Optical Properties of Polymer-Embedded Silicon Nanoparticles

We seek to use electrically conducting polymers, such as those commonly utilized in polymeric LEDs, as hosts for silicon nanoparticles. The proper design of multilayered devices based on these materials will yield efficient light-emitters in which charge carriers localize and recombine within the nanoparticles. Furthermore, these may combine the flexibility and processability of polymeric LEDs with the reliability of inorganic materials. We have synthesized luminescent silicon nanoparticles and have characterized their photoluminescence (PL) using continuous-wave and time-resolved spectroscopy. These particles have been incorporated into a variety of transparent solid hosts. The photoluminescence obtained from particle-containing poly(methyl methacrylate) (PMMA) matrices is very similar to that of the particles in solution, both in spectral content and PL decay characteristics. However, when incorporated into a variety of conducting polymers, such as poly(N-vinylcarbazole) (PVK), the nanoparticles do not retain their photoluminescence properties. A variety of chemical species have been reported as effective PL quenchers for porous silicon. We believe that these polymers quench the luminescence through similar mechanisms. Protective passivation of the nanoparticle surface is suggested as a strategy for overcoming this quenching.