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Brewster angle optical reflection observation of self-limiting nanoparticle monolayer self-assembly at a liquid/liquid interface

Citation

Hu, Jiayang et al. (2021), Brewster angle optical reflection observation of self-limiting nanoparticle monolayer self-assembly at a liquid/liquid interface, Dryad, Dataset, https://doi.org/10.5061/dryad.v9s4mw6tm

Abstract

Real-time optical reflection of incident p-polarized light near Brewster’s angle shows that after drop-casting iron oxide nanoparticles (NPs) in heptane on top of a diethylene glycol (DEG) liquid substrate, an iron oxide NP layer forms at the DEG/heptane interface, and it self-limits to a monolayer even when there are excess NPs dispersed in the upper heptane phase. Most modes of NP self-assembly do not self-limit growth after the formation of a single monolayer. Observations are compared to a reflection model incorporating the reflectances expected at each interface. An effective medium model of the dielectric constant is used to model the reflectance of the NP layer at the DEG/heptane interface.

Methods

To form these NP assemblies, 2 mL of DEG were first deposited into a glass Petri dish (diameter = 3.45 cm) to form a ∼2.14 mm-thick liquid substrate (if the surface were flat). A p-polarized He-Ne laser (632.8 nm, plasma filter) was expanded from below to form a 2.21 cm × 1.32 cm elliptical beam at the center of the DEG/air interface, to average over this region, and all reflected beams other than those from the glass bottom of the Petri dish were imaged on a photodiode, on which they overlap. The laser was adjusted to hit the DEG/air interface at Brewster’s angle (34.65°) to minimize reflectance from this interface. Then, 1400 μL of heptane were deposited on the DEG substrate to form a ∼1.50 mm-thick upper layer reservoir and, finally, 120 μL of a heptane colloid of NPs were drop-cast on top. This quickly mixed with the heptane reservoir to form a more dilute, ∼1.63 mm-thick heptane colloid layer. The Petri dish was then capped to slow the heptane evaporation. The NPs were spherical iron oxide NPs (Fe2O3, 11.8 nm core diameter, capped by oleate) synthesized by standard methods. The number of NPs in the heptane colloid drop was characterized by the number of close-packed NP MLs that would be expected to form on the DEG substrate after heptane evaporation, averaged over the surface; this is termed the number of ML-equivalents being drop-cast.

The light beams reflected from the DEG/heptane colloid and heptane colloid/air interfaces were monitored by a photodiode for 240 min, way before the heptane upper layer evaporated. Before the heptane was added, the reflected fraction from the DEG/air interface was very small, but not zero, because of the surface and laser beam curvatures. After it was added and before the NPs were drop-cast, the reflectances from the DEG/heptane and heptane/air interfaces were still very small because the refractive indices of pure DEG and heptane are nearly the same (below). After the NPs were added and a NP layer formed at the DEG/heptane colloid interface, the reflectance from that interface was expected to initially increase monotonically and linearly with layer thickness at the interface (for up to ∼6 MLs, corresponding to ∼ a quarter wavelength in this medium).

Funding

National Science Foundation, Award: CBET-1603043

National Science Foundation, Award: DGE-1069240