Compositional and Structural Fluctuations in the Medium-Range in Glasses

Nan Jiang (Arizona State University), DMR 0603993

From storage media for nuclear waste, to optical communications, optical filters and refractory coatings, applications of glasses pervade modern society. The characterization of the degree of order in glasses, on which these useful properties depend, is, however, a long-standing problem in materials science. The researchers at ASU have found clear evidence for stoichiometric variations in silicate glasses on a multi-nanometer scale, by studying EELS (electron energy-loss spectroscopy) as a function of electron probe position, and by comparing EELS from glasses and crystals with the same composition - beyond the spatial resolutions of the broad-beam techniques. They also determine that the difference results from structural fluctuations, based on calculations using the SCF full multiple scattering theory (Code FEFF8). It is seen that a small amount of compositional change from stoichiometry CaO-SiO2 (marked as CaSiO3-2M) to the Si rich case of 5CaO-6SiO2 does not induce a recognizable changes in the EELS. However, structural changes from CaSiO3-2M to pseudo-CaSiO3 significantly modifies the shape of the EELS spectra. Therefore, stoichiometry variations do imply the structural fluctuations observed in our study, on the order of the spatial resolution of the instrument used.

0

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0 20 40 60 80 100 120 140 160

area 1 area 2 area 3 area 4 crystal

Energy-loss (eV)

Inte

nsi

ty

X 104

Ca M23

-edge

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110 120 130 140 150

Si L23

-edge

ab

cX 102

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PD

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Si

Energy (eV)

CaSiO3-2M

5CaO-6SiO2

CaSiO3-pseudo

Ca2SiO

4

s+da b

c

Experimental EELS spectra in the CaSiO3 glass (left panel) were acquired from four different regions of the glass using an electron probe 10nm in diameter, and de-convoluted from the zero energy-loss peak. The thickness variations in these four regions are small according to the ratios of the inelastic scattering to the elastic scattering intensities. The stoichiometry variations are obvious from the relative EELS intensities of the Si L23 and Ca M23-edges, by comparing these intensities with those in the compositionally equivalent crystal CaSiO3 (Wollastonite-2M) of known stoichiometry. The calculated spectra in various structure models are given in right panel.

International collaboration on photonic crafts

Nan Jiang (Arizona State University), DMR 0603993

Femtosecond (fs) laser pulses are very powerful in creating artworks inside glasses, such as this beautiful orange butterfly in the top-left image. Drawing pretty pictures in glasses using a fs-laser pen is not just the scientists doing artists’ work. There are full of mysteries to be answered by scientific research. It also has great applications in the fabrication of micro-photonic functional elements such as waveguides, gratings, memories, and possesses the potential applications in controlling light flow, or tuning wavelength. Under an optical microscope, the continuous lines becomes discrete elongated dots, as shown in the bottom-left image, which is a cross-section parallel to line direction. Under transmission electron microscope (TEM), these elongated dots consists of several unique nanometer structures (right column). The state-of-the-art high spatial resolution EELS technique confirms that the tiny dots (~1 – 2nm) surrounding the round-shaped structures are Na nanoparticles or clusters. These TEM images are the first observations of fs-laser induced structures inside glasses. The works are collaborations with scientists in China.

Nanometer structures(nanometer)

Microstructures(micron)

Macrostructures(millimeter)

by J. Qiu, Zhejiang Univ., China

Artworks created using a fs-laser pen