Raman scattering measurements of acoustic superlocalization in silica aerogels

Large-scale nanoporous metal-coated silica aerogels for high SERS effect improvement

We investigate the optical properties and surface-enhanced Raman scattering (SERS) characteristics of metal-coated silica aerogels. Silica aerogels were fabricated by easily scalable sol-gel and supercritical drying processes. Metallic nanogaps were formed on the top surface of the nanoporous silica network by controlling the thickness of the metal layer. The optimized metallic nanogap structure enabled strong confinement of light inside the gaps, which is a suitable property for SERS effect. We experimentally evaluated the SERS enhancement factor with the use of benzenethiol as a probe molecule. The enhancement factor reached 7.9 × 107 when molecules were adsorbed on the surface of the 30 nm silver-coated aerogel. We also theoretically investigated the electric field distribution dependence on the structural geometry and substrate indices. On the basis of FDTD simulations, we concluded that the electric field was highly amplified in the vicinity of the target analyte owing to a combination of the aerogel's ultralow refractive index and the high-density metallic nanogaps. The aerogel substrate with metallic nanogaps shows great potential for use as an inexpensive, highly sensitive SERS platform to detect environmental and biological target molecules.

Relation between Fractal Inhomogeneity and In/Nb-Arrangement in Pb(In1/2Nb1/2)O3

Relaxor ferroelectrics show substantial responses to electric fields. The key difference from normal ferroelectrics is a temperature-dependent inhomogeneous structure and its dynamics. The lead-based complex perovskite Pb(In_1/2Nb_1/2)O₃ is an intriguing system in which the inhomogeneous structure can be controlled by thermal treatment. Herein, we report investigations of the phase transitions in single crystals of Pb(In_1/2Nb_1/2)O₃ via changing the degree of randomness in which In and Nb occupy the B site of the ABO₃ perovskite structure. We studied the dynamic properties of the structure using inelastic light scattering and the static properties using diffuse X-ray scattering. These properties depend on the degree of randomness with which the B site is occupied. When the distribution of occupied In/Nb sites is regular, the antiferroelectric phase is stabilised by a change in the collective transverse-acoustic wave, which suppresses long-range ferroelectric order and the growth of the inhomogeneous structure. However, when the B site is occupied randomly, a fractal structure grows as the temperature decreases below T^*~475 K, and nanosized ferroelectric domains are produced by the percolation of self-similar and static polar nanoregions.

Experiments on the structure and vibrations of fractal solids

Measurements performed on carefully prepared series of silica aerogels are reported. These materials were studied with small-angle neutron scattering, revealing a fractal structure that can extend up to at least two orders of magnitude in length. The corresponding collective vibrations were investigated by Brillouin, Raman, and inelastic neutron scattering. All results are consistently explained in terms of fractons.