Xuegeng Li

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.

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.

Encoding of polycyclic Si-containing molecules for determining species uniqueness in automated mechanism generation

Automated mechanism generation is an attractive way to understand the fundamental kinetics of complex reaction systems such as silicon hydride clustering chemistry. It relies on being able to tell molecules apart as they are generated. The graph theoretic foundation allows molecules to be identified using unique notations created from their connectivity. To apply this technique to silicon hydride clustering chemistry, a molecule canonicalization and encoding algorithm was developed to handle complex polycyclic, nonplanar species. The algorithm combines the concepts of extended connectivity and the idea of breaking ties to encode highly symmetric molecules. The connected components in the molecules are encoded separately and reassembled using a depth-first search method to obtain the correct string codes. A revised cycle-finding algorithm was also developed to properly select the cycles used for ring corrections when thermodynamic properties were calculated using group additivity. In this algorithm, the molecules are expressed explicitly as trees, and all linearly independent cycles of every size in the molecule are found. The cycles are then sorted according to their size and functionality, and the cycles with higher priorities will be used to include ring corrections. Applying this algorithm, more appropriate cycle selection and more accurate estimation of thermochemical properties of the molecules can be obtained.

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.