Deterministic particle assembly on nanophotonic chips.

HYPOTHESIS: Controlled particle assembly from a dilute suspension droplet is challenging yet important for many lab-on-a-chip and biosensing applications. The formation of hot spots on the localized surface plasmonic resonance (LSPR) substrates induced by laser excitation can generate microbubbles. These microbubbles, upon the laser removal, shrink and collapse due to electron energy dissipation, leading to guided particle assembly on the LSPR substrate. EXPERIMENTS: After depositing dilute silica particles dispersions on both nanoisland (AuNI) and planar gold (Au) plasmonic substrates (referred to as LSPR and SPR substrates respectively), microbubbles were formed when a laser beam was applied. Particle dispersion concentration, laser power, and the radius of circular laser sequence were varied to produce different sizes of particle clusters on the LSPR substrate after bubble shrinkage upon the laser removal. To stabilize the assembled structures over time, sodium chloride (NaCl) was ad ded to the dispersions. FINDINGS: Even though thermo-plasmonic flow and microbubbles can be produced with SPR substrates, particle assembly is only possible on LSPR substrates because of electron energy dissipation via nanoscale air gaps trapped in the LSPR substrate. By tuning the laser power, the radius of the circular laser sequence, and the particle dispersion concentration, the number of particles in the assembled structure can be controlled. The addition of NaCl to the dispersion can screen the electrostatic charges among the particles and between the particles and substrate, favoring hydrogen bonding and stabilizing the assembled structures for hours. These findings establish a new framework for utilizing nanophotonic chips where particle assembly can be achieved by a single source of light.

Interplay between phase separation and dewetting in PS/PVME thin films: effect of temperature.

We studied the effects of temperature on the interplay between dewetting and phase separation at shallow and deep depths at two-phase temperatures in PS/PVME polymer blend thin films. Optical microscopy, AFM measurements, and ellipsometry analysis were performed to investigate the dewetting behavior of the films. At the deep quench depth (phase separation temperature of 115 °C), a two-layer film formed, consisting of a thin PVME layer directly on the surface of a silicon wafer (as the wetting layer) and a bulk layer which was the upper layer. In the bulk layer, the phase separation mechanism was controlled by an apparent nucleation and growth mechanism, which was driven by entropic and anisotropic limitations rather than thermodynamic preferences. After about 106 min of annealing, liquid-liquid dewetting occurred in the interface of the formed layers, triggered by Laplace pressure differences. However, at the shallow quench depth (phase separation temperature of 95 °C), a tri-layered structure formed in the thin films and concentration fluctuations at the interfaces of the formed layers triggered surface fluctuations and instabilities (dewetting phenomenon).