Reflection Microspectroscopic Study of Laser Trapping
Assembling of Polystyrene Nanoparticles at Air/Solution Interface
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Abstract
We present the formation of a single
nanoparticle assembly with
periodic array structure induced by laser trapping of 200 nm polystyrene
nanoparticles at air/solution interface of the colloidal heavy water
solution. Their trapping and assembling behavior is observed by monitoring
transmission and backscattering images and measuring reflection spectra
under a microscope. Upon the laser irradiation into the solution surface
layer, nanoparticles are gathered at and around the focal spot, and
eventually a nanoparticle assembly with the size much larger than
the focal volume is formed. The assembly gives structural color in
visible range under halogen lamp illumination, indicating that constituent
nanoparticles are periodically arrayed. Reflection spectra of the
assembly show a reflection band, and its peak position is gradually
shifted to short wavelength and the bandwidth becomes narrow with
time, depending on the distance from the focal spot. After the laser
is switched off, red-shift is observed in the reflection band. These
results indicate that nanoparticles are rearranged into a densely
packed periodic array during laser irradiation and diffused out to
the surrounding solution after turning off the laser. These dynamics
are discussed from the viewpoints of the attractive optical trapping
force and the electrostatic repulsive force among nanoparticles