Investigation On The Optical Characteristics Of Magnetite And Its Nanocomposites With Gold And Silver

In the resent years, magnetite and its hybrid nanocomposites combined with gold and silver have been one of the subjects of scientific and technological interest due to their unique optical properties, including optical limiting, surface enhanced magneto-rotation and two-photon fluorescence, etc. So far, there is lack of systematic research on the absorption spectra and local electric fields of magnetite and its nanocomposites with gold and silver. In this paper, we investigated theoretically the variation laws of absorption efficiency and local electric field distribution of magnetite nanoparticles with different morphology and geometry, and provided a preliminary experimental verification. On the basis of the research on magnetite nanoparticles, we investigated the surface plasmon resonance-enhanced effects of gold-magnetite(Au-Fe3O4) and silver-magnetite(Ag-Fe3O4) dimers, and the dynamic properties of silver-magnetite-gold(Ag-Fe3O4-Au) trimer illuminated by a monochromatic light beam. The research work has been summarized in details as follows:The physical mechanisms and variation laws of absorption efficiency and local electric field of magnetite nanoparticles were investigated. The absorption efficiencies and local electric field distributions of nanosized magnetite particles with four different shapes including a nanosphere, nanocube, nanocylinder and nanodisk were investigated using discrete dipole approximation(DDA). Electrostatic approximation was employed to validate and elucidate the DDA-based simulation. The absorption efficiency and its peak position are closely dependent upon incident wavelength, particle geometry, particle orientation and refractive index of ambient medium. The calculation results indicate that its absorption efficiency increases with an increase in particle size, and increasing the refractive index of ambient medium results in a red shift for two absorption efficiency peaks. Furthermore, different orientation of nanoparticle with respect to incident light corresponds to different depolarization factor, and the resultant absorption efficiency and absorption peak. The maximum red shift of absorption peak may attain when increasing the aspect ratio of particle and refractive index of ambient medium. In addition, for magnetite nanoparticles, because the real part of maximum local electric field is larger than its imaginary part, and the former varies linearly with the real part of polarization, the maximum local electric field shows a similar behavior with the real part of the polarization of magnetite.Oxidation-precipitation and modified co-precipitation methods were first proposed and performed successfully for synthesis of magnetite nanoparticles. Particle sizes of the synthesized spherical magnetite particles via these two methods are 10.7 ± 2.0 nm and 9.6 ± 1.9 nm respectively. The modified co-precipitation method used the organic mixture of isomeric-branched primary alcohol and kerosene as a stabilizing agent to control particle size and improve its dispersion. The system of ferrous chloride, ethanolamine and hydrogen peroxide was engaged to synthesize magnetite particles in an oxidation-precipitation method. This synthetic approach is facile and effective. The average crystallite size and saturation magnetization of the magnetite nanoparticles both increase with the final PH value or reaction temperature of the solution. In addition, the experimental extinction spectrum of spherical magnetite particles with particle radius about 10 nm is in agreement with our theoretical calculated results.Electric dipole vibrating model was firstly employed to elucidate the enhancement effect of metal nanosphere/non-metal dimer system. As the intermediate product of monomer to core/shell structure, gold-magnetite and silver-magnetite dimers exhibit a pronounced plasmonic electric field enhancement effect. The influences of incident wavelength, nanoparticle orientation and metal nanoparticle size on the enhancement effect were systematically investigated with DDA. It is found that the electric field enhancement effect is positive at or above the surface plasmon resonance(SPR) wavelength in longitudinal mode(LM), and negative in nonresonant region in transverse mode(TM). The maximum enhancement effect derived from SPR may be achieved in LM. It is notable that when we estimate the enhancement factors of the dimers, the presence of magnetite nanoparticle can not be neglected because the magnetite nanoparticle significantly changes both the amplitude of the enhanced electric field of metal nanosphere and its distribution. In addition, due to the vanished permittivity for silver, the real part of polarizability is negative, which is responsible for the inverse electric field distribution of silver nanosphere.Silver-magnetite-gold heterotrimer was constructed and its dynamic properties under a monochromatic light beam were investigated. Optical torque efficiency factor, orientation-average optical torque efficiency factor and steady-state angular velocity of the asymmetrical silver-magnetite-gold heterotrimer were calculated in detail with discrete dipole approximation. The results indicate that the optical torque derived from radiation force due to surface plasmon resonance of silver can keep the heterotrimer rotating clockwise or counterclockwise steadily. As the power of optical radiation I=108-109 W/m2, the maximum steady-state angular velocity could reach to about 103-104 rad/s. The peak widths at half height of angular velocity curves for two kinds of heterotrimer are about 35 and 68 nm respectively. When light radiation pressure offers a regular driving force, an array of such grains can serve as micro-mixers or potential nanoscale plasmonic engines. In the meantime, the influences of Brownian rotation and photophoresis torque on the dynamic properties of the heterotrimer were discussed.