In Situ WetSTEM Observation of Gold Nanorod Self-Assembly Dynamics in a Drying Colloidal Droplet

Abstract

Direct in situ visualization of nanoparticles in a liquid is an important challenge of modern electron microscopy. The increasing significance of bottom-up methods in nanotechnology requires a direct method to observe nanoparticle interactions in a liquid as the counterpart to the ex situ electron microscopy and indirect scattering and spectroscopy methods. Especially, the self-assembly of anisometric nanoparticles represents a difficult task, and the requirement to trace the route and orientation of an individual nanoparticle is of highest importance. In our approach we utilize scanning transmission electron microscopy under environmental conditions to visualize the mobility and self-assembly of cetyltrimethylammonium bromide (CTAB)-capped gold nanorods (AuNRs) in an aqueous colloidal solution. We directly observed the drying-mediated AuNR self-assembly in situ during rapid evaporation of a colloidal droplet at 4°C and pressure of about 900 Pa. Several types of final AuNR packing were documented including side-by-side oriented chains, tip-to-tip loosely arranged nanorods, and domains of vertically aligned AuNR arrays. The effect of local heating by electron beam is used to qualitatively asses the visco-elastic properties of the formed AuNR/CTAB/water membrane. Local heating induces the dehydration and contraction of a formed membrane indicated either by its rupture and/or by movement of the embedded AuNRs.

In Situ Tracking of Colloidally Stable and Ordered Assemblies of Gold Nanorods

“… 10 , 12 Although early in situ EM studies enabled the observation of large, vertical assemblies of AuNRs, the time scale of the experiments did not allow the observation of single particle events or any qualitative and/or quantitative analysis of the behavior in solution. 28 …”

“…In this work, we demonstrate the use of environmental scanning transmission electron microscopy (WetSTEM 28 ) to characterize in situ the dynamics of relatively large, 2D, vertical assemblies of AuNRs in the wet state. We were able to monitor the dynamics of single nanoparticles and track the self-assembly and disassembly pathways, for orientationally ordered AuNR aggregates.…”

Superlattice growth and rearrangement during evaporation-induced nanoparticle self-assembly

“…the evaporation rate can be controlled 25 . Light microscopy 16, 26 and liquid-cell electron microscopy 27–29 have provided important insight into evaporation-driven phenomena on large and small lengths scales, respectively. Small Angle X-ray Scattering (SAXS) and Grazing Incidence SAXS (GISAXS) have recently evolved to be the methods of choice to obtain time-dependent information on the assembly process 17, 18, 20, 22, 30–37 .…”

Paradoxical Long-Timespan Opening of the Hole in Self-Supported Water Films of Nanometer Thickness

“…The citrate coating confers a net negative charge upon the particles, which allows them to repel one another and stay in solution. Both particle and citrate concentrations are assumed to increase in the film relative to in the bulk drop, which is specific to the film site according to the evaporation rate. A typical film thickness without gold particles was measured to be on the order of 10 nm according to ref , on the basis of the attenuated transmitted beam intensity through the film relative to the hole.…”

“…The initiation of hole formation under an e-beam was attributed to radiology − effects. e-beam heating was mentioned for the self-assembly process of surfactant-treated NPs with the observation of delayed film rupture on the time scale of seconds.…”