Enhancing nanoparticle electrodynamics with gold nanoplate mirrors.

Mirrors and optical cavities can modify and enhance matter-radiation interactions. Here we report that chemically synthesized Au nanoplates can serve as micrometer-size mirrors that enhance electrodynamic interactions. Because of their plasmonic properties, the Au nanoplates enhance the brightness of scattered light from Ag nanoparticles near the nanoplate surface in dark-field microscopy. More importantly, enhanced optical trapping and optical binding of Ag nanoparticles are demonstrated in interferometric optical traps created from a single laser beam and its reflection from individual Au nanoplates. The enhancement of the interparticle force constant is ≈20-fold more than expected from the increased intensity due to standing wave interference. We show that the additional stability for optical binding arises from the restricted axial thermal motion of the nanoparticles that couples to and reduces the fluctuations in the lateral plane. This new mechanism greatly advances the photonic synthesis of ultrastable nanoparticle arrays and investigation of their properties.

Guiding spatial arrangements of silver nanoparticles by optical binding interactions in shaped light fields.

We demonstrate assembly of spheroidal Ag nanoparticle clusters, chains and arrays induced by optical binding. Particles with diameters of 40 nm formed ordered clusters and chains in aqueous solution when illuminated by shaped optical fields with a wavelength of 800 nm; specifically, close-packed clusters were formed in cylindrically symmetric optical traps, and linear chains were formed in line traps. We developed a coupled-dipole model to calculate the optical forces between an arbitrary number of particles and successfully predicted the experimentally observed particle separations and arrangements as well as their dependence on the polarization of the incident light. This demonstrates that the interaction between these small Ag particles and light is well described by approximating the particles as point dipoles, showing that these experiments extend optical binding into the Rayleigh regime. For larger Ag nanoparticles, with diameters of approximately 100 nm, the optical-binding forces become comparable to the largest gradient forces in the optical trap, and the particles can arrange themselves into regular arrays or synthetic photonic lattices. Finally, we discuss the differences between our experimental observations and the point dipole theory and suggest factors that prevent the Ag nanoparticles from aggregating as expected from the theory.

CdSeS/ZnS alloyed nanocrystal lifetime and blinking studies under electrochemical control.

Alloyed CdSeS nanocrystals allow tuning between the CdSe and CdS band edges while remaining relatively small. The CdSeS cores also lead to a reduced electron confinement energy and a slower biexciton decay rate compared to CdSe cores of similar sizes. A ZnS shell synthesis procedure allows stable CdSeS/ZnS colloidal quantum dots (QDs) suitable for single dot imaging. These are compared to CdSe/ZnS of similar core size. The blinking off-exponents of the CdSeS/ZnS dots in air and on a glass substrate were slightly larger. Using electrochemistry with ensemble and single dot measurements, the trion lifetime of CdSeS/ZnS dot is resolved to be ~0.75 ns, while it is about 0.15 ns for CdSe/ZnS. In addition, the blinking behavior of single CdSeS/ZnS QDs is largely suppressed when in the trion state.