Nanotechnology and nanomaterials

Structure and nonlinear optical properties of nanocomposites based on metal alkanoates with bimetallic (Ag + Au) nanoparticles
A. Dmytruk1, V. Rudenko1, A. Tolochko1, D. Zhulai1, G. Klimusheva1,
G. Yaremchuk2, T. Mirnaya2

1Institute of Physics of the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03039, Ukraine. 
E-mail: klimush@iop.kiev.ua
2V. I. Vernadsky Institute of General and Inorganic Chemistry, NAS of Ukraine, Palladin prosp., 32/34, Kyiv-03142 Ukraine.

Binary nanoparticles (NPs) of gold and silver demonstrate significant scientific interest due to the manifestation of multifunctional optical and nonlinear optical properties compared with the properties of individual noble metal NPs [1].
The synthesis of the considered bimetallic NPs was carried out directly in the thermotropic liquid crystals phase of cadmium octanoate (CdC8) at 130 -150oC by the simultaneous chemical reduction of gold and silver cations [2]. The total concentration of NPs was 4% mol. or varied according to the formula xAg + (4-x) Au % mol. The variety of shapes and sizes of the NPs was observed by means of transmission electron microscopy. Structural characterization of NPs was also done by the method of small-angle X-ray scattering. 
For spectral and nonlinear optical measurements of nanocomposite samples the following method was used. The powder of nanocomposite was sandwiched between two glass substrates and heated to reach the liquid crystal state.  After this, the cells rapidly cooled down to room temperature. As a result, anisotropic vitrified nanocomposite samples were obtained. Studies of the absorption spectra of bimetallic NPs showed that the ratio of silver and gold in nanocomposite changes the position of the maximum of the surface plasmon resonance band. 
Interesting patterns were found in the study of the nonlinear optical properties of nanocomposites:  CdC8+Ag/Au (3:1); CdC8+Ag/Au (1:1); CdC8+Ag/Au (1:3). Nonlinear optical properties nanocomposites were studied by z-scan method. The measurements were performed by using Nd:YAG pulsed laser, generating 9 ns pulses at 1064 nm with a repetition rate of 0.5 Hz. Laser beam is focused with a F=13 cm focal length lens. Effective (or intensity dependent) nonlinear absorption coefficients and nonlinear refractive indices of the studied materials indicate coexisting of multiple physical mechanisms leading to this complex behavior. Mechanisms related to the nonlinear absorption include the saturation of the absorption, and effective two-photon absorption. 
Composites with bimetallic NPs of the alloy type as well as the core-shell type demonstrate the high value of the third-order nonlinearity X(3) = 5,4 -8.22·10-8 esu. The core-shell NPs lead to a change in the sign of nonlinear refraction from negative to positive and enhancement of the optical nonlinearity of the nanocomposites unlike fused binary metal NPs. So, our multifunctional nanocomposites with different structure NPs may be considered promising candidates for use in optoelectronic and photonic applications.

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