We report that fully alloyed Ag/Au nanospheres with high compositional homogeneity ensured by annealing at elevated temperatures show large extinction cross sections, extremely narrow bandwidths, and remarkable stability in harsh chemical environments. Nanostructures of Ag are known to have much stronger surface plasmon resonance than Au, but their applications in many areas have been very limited by their poor chemical stability against nonideal chemical environments. Here we address this issue by producing fully alloyed Ag/Au nanospheres through a surface-protected annealing process. A critical temperature has been found to be around 930 °C, below which the resulting alloy nanospheres, although significantly more stable than pure silver nanoparticles, can still gradually decay upon extended exposure to a harsh etchant. Nanospheres annealed above the critical temperature show a homogeneous distribution of Ag and Au, minimal crystallographic defects, and the absence of structural and compositional interfaces, which account for the extremely narrow bandwidths of the surface plasmon resonance and may enable many plasmonic applications with high performance and long lifetime, especially for those involving corrosive species.
Ferrimagnetic inorganic nanorods have been used as building blocks to construct liquid crystals with optical properties that can be instantly and reversibly controlled by manipulating the nanorod orientation using considerably weak external magnetic fields (1 mT). Under an alternating magnetic field, they exhibit an optical switching frequency above 100 Hz, which is comparable to the performance of commercial liquid crystals based on electrical switching. By combining magnetic alignment and lithography processes, it is also possible to create patterns of different polarizations in a thin composite film and control over the transmittance of light in particular areas. Developing such magnetically responsive liquid crystals opens the door toward various applications, which may benefit from the instantaneous and contactless nature of magnetic manipulation.
By using gold nanorods as an example, we report the dynamic and reversible tuning of the plasmonic property of anisotropically shaped colloidal metal nanostructures by controlling their orientation using external magnetic fields. The magnetic orientational control enables instant and selective excitation of the plasmon modes of AuNRs through the manipulation of the field direction relative to the directions of incidence and polarization of light.
One-dimensional photonic nanochains composed of periodically arranged superparamagnetic nanoparticles have been exploited as basic color units for a new type of photonic ink. The photonic response of ink can be rapidly and repetitively switched on and off by tuning the direction of external magnetic fields. Temporary bi-stability is realized in the photonic ink by introducing highly viscous solvents as a dispersant, which suppress the rotational movement of nanochains and allow them to stay temporarily aligned after the removal of external magnetic field and to retain their photonic response for a considerably long period. Increasing the viscosity of the dispersant enhances the bi-stability of photonic inks; for instance, the ink prepared in 95% glycerol solution is able to retain its color up to ten minutes. The length of the chains also plays an important role in their temporary bi-stability. By mixing chains with different photonic properties and lengths, a display has been developed which exhibits two distinct colors in response to the application and removal of external magnetic fields.
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