Structure of Au ultrafine particles in silica glass by x-ray absorption fine structure spectroscopy
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(Received 16 December 1994; accepted 19 June 1995)
The structure of ultrafine gold particles embedded in silica glass by ion implantation has been studied by x-ray absorption fine structure spectroscopy. It is found that the Au-Au interatomic distance in the particles is very similar to that in bulk gold within 0.01 Mean-square relative displacement in the particles is 1.2- 1.3 times larger than that in bulk gold.
A.
Ultrafine particles in glass matrix show high optical nonlinearity, making them useful in nonlinear optical devices.''2 It is known that the optical nonlinearity depends on the size and concentration of the particles. In addition to the size and concentration, the nonlinear optical properties such as nonlinear susceptibility and the time scale of nonlinear response would be affected by the structural defects and disorder which scatter free electrons and phonons in the particles. Although many studies have been made on the structure of ultrafine particles in solutions and on ~ u b s t r a t e s , ~study -~ of the structure of ultrafine particles in glasses is rarely done.' Therefore, it is profitable to study the structure of the ultrafine particles from which high optical nonlinearity originates. In a previous study, we prepared silica glass with dispersed gold ultrafine particles by ion implantation, and have shown by x-ray diffraction study that gold particles in silica glass are highly defective.8 Broadening of the diffraction peak due to the small crystalline size, however, made it impossible to determine the structural parameters precisely. X-ray absorption fine structure (XAFS) spectroscopy is a useful method for studying atomic structure such as coordination number, interatomic distance, and mean-square relative displacement. In particular, fluorescence XAFS spectroscopy is suitable for the measurement of dilute components such as ultrafine particles in glasses. In the present study, the structure of gold ultrafine particles in silica glass has been studied by fluorescence XAFS spectroscopy. Silica glasses with dispersed ultrafine gold particles were prepared by ion implantation. 1.5 MeV Au' ions were implanted into silica glass plates (25 X 15 X 2.3 mm) at fluence levels of 5 X 10l6 and 1 X 1017 ions/cm2. The glass plates implanted with 5 X 10l6 and 1 X 1017 Au+ions/cm2 were heated at 1100 "C for 8 h and at 1000 "C for 36 h, respectively, and then used as samples for XAFS measurement. Preparation of the samples has been described elsewhere in detail.' The 2418
J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
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depth profile of Au atomic distribution had a Gaussian shape which has a maximum at 250 nm from the surface and a full width at half maximum (FWHM) of 260 nm, according to a Rutherford backscattering spectrum.' The average size of gold particles estimated from optical absorption was about 5.8 nm in both samples. On the other hand, the average size of gold particles estimated from x-ray diffraction was about 8.5 nmI2 in the glass implante
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