Aerosol Technologies for Fabrication, Collection, and Deposition of Engineered Nanoparticles

aerosol;ambipolar charging;CVD;design;DMA;Femlab;Fluent;heterogeneous;homogeneous;mobility analysis;Multiphysics;nanocrystalline;nanoparticle;nanotechnology;neutralizer;nucleation;photoluminescence;RDMA;reactor;silane;silicon;thermophoresis

Holunga, Dean Marcu ; Flagan, Richard C. (advisor)

University:California Institute of Technology

Department:Chemistry and Chemical Engineering

关键词: aerosol;    ambipolar charging;    CVD;    design;    DMA;    Femlab;    Fluent;    heterogeneous;    homogeneous;    mobility analysis;    Multiphysics;    nanocrystalline;    nanoparticle;    nanotechnology;    neutralizer;    nucleation;    photoluminescence;    RDMA;    reactor;    silane;    silicon;    thermophoresis;   

Others  :  https://thesis.library.caltech.edu/3768/1/Holunga_dm_2006.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

We demonstrate a turbulent mixing reactor capable of producing highly monodisperse, σG ≈ 1.1, heterogeneous oxide-coated silicon nanoparticles from pyrolytic decomposition of silane. Particle concentrations approach 10⁹ cm⁻³ as measured with a radial differential mobility analyzer and fA resolution electrometer. Turbulent mixing power, induced by locally high-momentum jets that actually remain below turbulent Reynolds numbers, induce mechanical mixing within a pathlength comparable to the diameter of the major flow channel. Timescales for transport are enhanced orders of magnitude above laminar processes, enabling nanoparticle evolutionary processes such as densification and crystallization to complete in the absence of significant agglomeration. Use of multiple jets in series may well enable the homogeneous introduction of additional reagents to facilitate additional heterogenous particle development.

Particles formed in the Inconel reactor were further studied using both transmission electron microscopy and photoluminescence measurements. Spherical particle morphology with faceted and unfaceted crystalline cores were observed, and thermal oxides appeared uniform. Particle purity and a high quality passivation of the particles were demonstrated by photoluminescence, although particles occasionally required additional processing to complete O₂ passivation. Photoluminescence measurements are in good agreement with models of quantum-confined exciton recombination, both in emitted wavelength and photoluminescence decay. Particle contamination studies using Electron Energy Loss Spectroscopy and Energy Dispersive X-Ray Spectroscopy found no evidence of metal contamination within particles studied for both native oxide and thermal oxide-coated particles. A phenomenological comparison of size information from the radial differential mobility analyzer and photoluminescence spectra demonstrated that thermally grown oxide shells and native oxide shell have initially opposite trends in the variation of thickness with particle size, although over time, native oxide shells thicken considerably.

A thermophoretic deposition chamber was designed for uniform deposition on wafers ranging in size from 100 mm–300 mm and over a range of flowrates from 500 sccm to 15000 sccm. A power-law hyperbolic inlet nozzle was shown theoretically to minimize separation. A uniform axial temperature gradient is developed using programmable temperature controlled heaters along with active cooling. Characterization by atomic force microscopy studies on 150 mm wafers demonstrated uniform coverage both radially and in the azimuth, in good agreement with model results. Deposition uniformity is predicted on larger wafers, up to 300 mm.

Pyrolysis reactions in small diameter tubular reactors foul the reactors’ walls continuously, with deposition morphology ranging from thin-films to dendritic, filter-like structures. The particle number concentration decays linearly with time. Hybridization of the turbulent mixing reactor with high energy seed reactors, such as a microplasma discharge, shows promise that may significantly reduce fouling, maintain or increase particle number concentration, maintain or increase particle monodispersity, expand chemistries available, and retain the ability to produce heterogeneous particles.

Laminar flow reactors are well suited to the production of monodisperse, σG ≈ 1.1, aerosols. The rate of pyrolytic decomposition of silane precursor is kept relatively slow during a gentle thermal ramp wherein the low temperature favors vapor deposition growth over additional nucleation. The resulting reduction in silane inhibits further nucleation as the temperature is increased. Slow flowrates, wherein diffusional losses of precursor assists the inhibition of additional nucleation, also contributed to maintaining lower nucleation rates, but are not necessary to achieve monodispersity or higher yield.