Yuanqing He

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.

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 ...

Preparation of luminescent silicon nanoparticles by photothermal aerosol synthesis followed by acid etching

CO2 laser-induced pyrolysis of silane (photothermal aerosol synthesis) was used to produce Si nanoparticles. Particles with an average diameter as small as 5 nm were prepared directly from silane in the gas phase. Etching these particles with mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) is shown to be an effective method to reduce the size of the particles produced by silane pyrolysis. After etching, silicon particles with controlled visible luminescence at room temperature were produced. The wavelength of maximum photoluminescence (PL) intensity can be controlled from over 780–500 nm by controlling the etching time and conditions.

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.

High-rate synthesis and characterization of brightly luminescent silicon nanoparticles with applications in hybrid materials for photonics and biophotonics

This presentation focuses on the synthesis and characterization of luminescent silicon nanoparticles that have potential as components of hybrid inorganic/organic materials for photonic and biophotonic applications. In our lab, silicon nanoparticles with bright visible photoluminescence are being prepared by a new combined vapor-phase and solution-phase process, using only inexpensive commodity chemicals. CO2 laser-induced pyrolysis of silane is used to produce Si nanoparticles at high rates (20 to 200 mg/hour). Particles with an average diameter as small as 5 nm can be prepared directly by this method. Etching these particles with mixtures of hydrofluoric acid (HF) and nitric acid (HNO3) reduces the size and passivates the surface of these particles such that they exhibit bright visible luminescence at room temperature. The wavelength of maximum photoluminescence (PL) intensity can be controlled from above 800 nm to below 500 nm by controlling the etching time and conditions. Particles with blue and green emission are prepared by rapid thermal oxidation of orange-emitting particles. These particles have exciting potential applications in optoelectronics, display technology, chemical sensing, biological imaging, and other areas. The availability of relatively large quantities of these particles is allowing us to begin to functionalize particles for these applications, as well as to study the optical, electronic, and surface chemical properties of them. All of these potential applications require inorganic/organic hybrid materials, in the sense that the nanoparticles must have their surfaces coated with organic molecules that mediate the interaction of the particles with the polymeric or biological host matrix. The particle synthesis methods, photoluminescence measurements on the particles, the stability of the photoluminescence properties with time, chemical quenching of photoluminescence, and functionalization of the particles for incorporation into different organic matrices or for specific interaction with small molecules or biomolecules are discussed in the context of applications to photonics and biophotonics.

Ultrafast dynamics in nanostructured materials

Optical properties of silicon and indium phosphide nanoparticles with emission throughout the visible wavelength range are presented. The peak emission wavelength of these nanoparticles is controlled by the reaction time and by post-growth etching treatments. Ultrafast spectroscopy is used to determine the photoluminescence lifetime in order to correlate the spectral response with the structural and chemical characterization of these nanoparticles. The measured lifetimes are used to identify surfactant, surface, and core nanoparticle emission. The nanoparticles exhibit efficient emission that is quenched when embedded within particular polymeric matrices.

Computational Fluid Dynamics (CFD) Modeling of a Laser-Driven Aerosol Reactor

A detailed 3-dimensional computational fluid dynamics (3D CFD) model of a laser-driven (photothermal) reactor system used in our laboratory to produce nanoparticles of silicon and other materials is presented. This model includes detailed descriptions of the fluid flow, heat and mass transfer, and chemical reactions leading to silane decomposition in the gas phase. Eight chemical reactions and eight chemical species were included in the reacting flow simulation. The overall characteristics of the photothermal reactor system were captured. Temperature and velocity profiles along the axis of the reactor were extracted from the simulations and used with a simple 1D aerosol dynamics model to predict particle size, concentration and size distribution. Background Computational Fluid Dynamics (CFD) refers broadly to the numerical solution, by computational methods, of the governing equations that describe continuum fluid flow: the Navier-Stokes equations (or generalizations thereof), and additional conservation equations, such as energy and species concentration equations (1-3). There are many commercial CFD software packages available, including FLUENT, FIDAP, CFD-ACE, STAR-CD, etc. Jensen et al. (4) examined some of the key issues affecting modeling of Chemical Vapor Deposition (CVD) reactors. While many conventional CVD processes are dominated by surface reactions coupled to gas-phase transport processes, gas to particle conversion processes like that considered here inherently involve gas-phase chemistry that leads to particle formation. Ng (5) and Talukdar (6, 7) carried out preliminary simulations of our laser-driven aerosol reactor using FIDAP. The results were not fully satisfactory due to limitations of the mesh generation and solution algorithms available in the version of FIDAP used at that time. These difficulties worsened when chemical reactions were included in the model. MPSalsa, an unstructured finite element (FE) code developed at Sandia National Laboratories for solving chemically reacting flow problems on massively parallel computers, was employed here (8). It is designed to solve laminar, low Mach number, two- or three-dimensional incompressible and variable density reacting fluid flows using a Petrov-Galerkin finite element formulation. The code can treat coupled fluid flow, heat transfer, multicomponent species transport, and finite-rate chemical reactions. It employs the CHEMKIN libraries to provide a rigorous treatment of multicomponent ideal gas chemical kinetics and transport properties. Chemical reactions occurring in the gas phase and on surfaces are treated by calls to CHEMKIN (9, 10) and SURFACE CHEMKIN