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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 34

Electron dynamics in gold and gold–silver alloy nanoparticles: The influence of a nonequilibrium electron distribution and the size dependence of the electron–phonon relaxation

(Georgia Institute of Technology, 1999-07-15) Link, S. ; Burda, C. ; Wang, Z. L. (Zhong Lin) ; El-Sayed, Mostafa A.

Electron dynamics in gold nanoparticles with an average diameter between 9 and 48 nm have been studied by femtosecond transient absorption spectroscopy. Following the plasmon bleach recovery after low power excitation indicates that a non-Fermi electron distribution thermalizes by electron–electron relaxation on a time scale of 500 fs to a Fermi distribution. This effect is only observed at low excitation power and when the electron distribution is perturbed by mixing with the intraband transitions within the conduction band (i.e., when the excitation wavelength is 630 or 800 nm). However, exciting the interband transitions at 400 nm does not allow following the early electron thermalization process. Electron thermalization with the lattice of the nanoparticle by electron–phonon interactions occurs within 1.7 ps under these conditions, independent of the excitation wavelength. In agreement with the experiments, simulations of the optical response arising from thermalized and nonthermalized electron distributions show that a non-Fermi electron distribution leads to a less intense bleach of the plasmon absorption. Furthermore, the difference between the response from the two electron distributions is greater for small temperature changes of the electron gas (low excitation powers). No size dependence of the electron thermalization dynamics is observed for gold nanoparticles with diameters between 9 and 48 nm. High-resolution transmission electron microscopy (HRTEM) reveals that these gold nanoparticles possess defect structures. The effect of this on the electron–phonon relaxation processes is discussed. 18 nm gold–silver alloy nanoparticles with a gold mole fraction of 0.8 are compared to 15 nm gold nanoparticles. While mixing silver leads to a blue-shift of the plasmon absorption in the ground-state absorption spectrum, no difference is observed in the femtosecond dynamics of the system.
Plasmon energy shift in mesoporous and double length-scale ordered nanoporous silica

(Georgia Institute of Technology, 1999-05-03) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)

Electron energy-loss spectroscopy studies are reported on three different types of structures: solid silica spheres, mesoporous silica, and the double length-scale ordered (DLSO) porous silica. The mesoporous silica has porosity at the length scale of nanometers. The DLSO porous silica has an additional ordering on submicron hollows created by the template polystyrene spheres. The plasmon energy of the porous silica shows a significant shift in comparison to that of the bulk, suggesting that the local density of the bound electrons in the porous structures is lower than that in the bulk. This gives the possibility of tuning the electronic structure of silica by varying its porosity, leading to even lower dielectric loss.
Synthesis and properties of Sr₂CeO₄ blue emission powder phosphor for field emission displays

(Georgia Institute of Technology, 1999-03-22) Jiang, Yongdong ; Zhang, Fuli ; Summers, Christopher J. ; Wang, Z. L. (Zhong Lin)

A blue emission powder phosphor Sr₂CeO₄ for field emission displays was prepared using a chemical coprecipitation technique, which is most suitable for large-scale production. The powders were fired at different temperatures to optimize the properties. Firing the powder at 1200 °C for 2 h gave the highest luminescence efficiency of 5.4 lm/W at 4 kV and 29.0 lm/W at 10 kV. The emission peak of this phosphor is at ~ 470 nm and Commission International de l'Eclairage coordinates are x = 0.19, y = 0.26.
Synthesis of cobalt oxide nanocrystal self-assembled materials

(Georgia Institute of Technology, 1999-02) Yin, Jinsong ; Wang, Z. L. (Zhong Lin)

Self-assembling of size-, shape-, and phase-selected nanocrystals into superlattices is a new approach for synthesizing a new generation of advanced materials with functionality. In this paper, high purity and monodispersive tetrahedral nanocrystals of CoO, with edge-lengths of 4.4 ± 0.2 nm, have been synthesized and separated from Co nanocrystals using colloidal chemistry and magnetic separation. The tetrahedral CoO nanocrystals behave like a molecular matter, and their assembling forms superlattices with translational symmetry. The phase transformation of the CoO nanocrystals is examined by ex situ annealing in oxygen, and the results showed the formation of Co₃O₄ with spinel structure.
Conductance quantization in multiwalled carbon nanotubes

(Georgia Institute of Technology, 1999) Poncharal, P. ; Frank, S. ; Wang, Z. L. (Zhong Lin) ; de Heer, Walter A.

We present results of carbon nanotube conductance measurements. The experiments were performed using an scanning probe microscope (SPM) system where a carbon nanotube fiber is connected to the SPM tip and then lowered into a liquid mercury contact. Experiments were also performed using a modified transmission electron microscope (TEM) specimen holder supplied with piezo and micrometer positioning system. Thus the contacting process of the nanotubes with the mercury could be monitored while simultaneously recording the conductance. These measurements and observations confirm previously reported conductance quantization (Frank et al.: Science 280, 1744 (1998)) of the nanotubes while providing additional details concerning the mercury nanotube contacts.We also report conductance versus voltage characteristics of carbon nanotubes
Low k, Porous Methyl Silsesquioxane and Spin-On-Glass

(Georgia Institute of Technology, 1999) Kohl, Abbe T. ; Mimna, Richard ; Shick, Robert ; Rhodes, Larry ; Wang, Z. L. (Zhong Lin) ; Kohl, Paul A.

Low dielectric constant, porous silica was made from commercially available methyl silsesquioxane (MSQ) by the addition of a sacrificial polymer, substituted norbornene polymer containing triethoxysilyl groups (NB), to the MSQ. The silsesquioxane-NB polymer film mixture was thermally cured followed by decomposition of the NB at temperatures above 400°C. The dielectric constant of the MSQ was lowered from 2.7 to 2.3 by creating 70 nm pores in the MSQ. The voids created in the MSQ exhibited a closed-pore structure. The concentration of NB in the MSQ affected the number of pores but not their size. Porous films were also created in a methyl siloxane spin-on-glass and its dielectric constant was lowered from 3.1 to 2.7. Infrared spectroscopy was used to follow the curing of the MSQ and decomposition of the NB.
Synthesis and characterization of Y₂O₃: Eu³⁺ powder phosphor by a hydrolysis technique

(Georgia Institute of Technology, 1998-10) Jiang, Yongdong ; Wang, Z. L. (Zhong Lin) ; Zhang, Fuli ; Paris, Henry G. ; Summers, Christopher J.

A forced hydrolysis technique is used for preparing Y₂O₃: Eu³⁺ powders at low processing temperatures. The technique uses yttrium oxide, europium oxide, nitric acid and urea, and has the potential for large-scale production for industrial applications. Several experimental conditions have been examined to optimize the luminescence efficiency. The best result was found to be at 2 mol% Eu doping and a 2 hour firing of 1400°C. Micro-structural information provided by x-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been applied to interpret the observed luminescent properties
Plasmon excitations in graphitic carbon spheres

(Georgia Institute of Technology, 1998-06-15) Stöckli, Thomas ; Bonard, Jean-Marc ; Châtelain, André ; Wang, Z. L. (Zhong Lin) ; Stadelmann, Pierre

Electron energy loss spectroscopy in a high-resolution transmission electron microscope has recently been used with success to characterize the electronic properties of closed cage nanometer-size graphitic particles. In the plasmon region, the experimental data reveal interesting size-dependent variations, which are not yet fully understood. The difficulties encountered in the interpretation of the spectra are principally due to the lack of a complete theoretical treatment of the anisotropic dielectric response in nanometer-size particles. In order to obtain a better understanding of the experimental data we propose a model based on nonrelativistic local dielectric response theory for electrons penetrating through a nested concentric-shell fullerene or the so-called ‘‘carbon onion.’’ The anisotropy of the electronic properties of the sphere is taken into account via the frequency-dependent dielectric tensor of graphite. The model can be applied to simulate electron energy loss spectra as well as line scans through energy filtered images and allows thus a direct comparison to experimental data.
Plasmon excitations in carbon onions: Model vs. measurements

(Georgia Institute of Technology, 1998-03) Stöckli, Thomas ; Wang, Z. L. (Zhong Lin) ; Bonard, Jean-Marc ; Stadelmann, Pierre ; Châtelain, André

Non-relativistic local dielectric response theory has proven successful in the interpretation of Electron Energy Loss data of nanometer-size isotropic particles of different geometries. In previous work, we have adapted this model to take into account anisotropy as encountered in the case of carbon onions. We have shown that this anisotropy needs to be taken into account since important deviations with respect to an isotropic model can be observed. In this contribution, we report on the first energy filtered images of carbon onions and compare intensity profiles across the spheres to our calculations.
Analysis of Cation Valences and Oxygen Vacancies in Magnetoresistive Oxides by Electron Energy-Loss Spectroscopy

(Georgia Institute of Technology, 1998) Wang, Z. L. (Zhong Lin) ; Yin, Jinsong ; Berta, Yolande ; Zhang, Jiming

Magnetic oxides of (La,A)MnO₃ and (La,A)CoO₃ have two typical structural characteristics: cations with mixed valences and oxygen vacancies, which are required to balance the charge introduced by cation doping. The consequences introduced by each can be different, resulting in different properties. It is important to quantitatively determine the percentage of charges balanced by each, but this analysis is rather difficult particularly for thin films. This paper has demonstrated that electron energy-loss spectroscopy (EELS) can be an effective technique for analyzing Mn and Co magnetic oxides with the use of intensity ratio of white lines, leading to a new technique for quantifying oxygen vacancies in functional and smart materials.