Novel comparison of microscopy and diffraction techniques on the structure of iron oxide nanoparticle monolayers transferred by Langmuir-Schaefer method

Abstract

Iron oxide nanoparticles undergo self-assembly into well-ordered monolayer films of macroscopic size at the air-water interface. This self-assembly process is the result of the van der Waals forces between the constituent particles. For roughly spherical particles, this monolayer is a 2D hexagonal close packed lattice. With Grazing Incidence X-Ray Diffraction (GID), one can obtain global statistical information about the film's spacing and correlation length. Herein, we demonstrate that comparable structural information can be obtained by a novel Fourier transform analysis method applied to Scanning Electron Microscopy (SEM) images taken of the film after it has been transferred to a silicon substrate. This consists of using numerical methods to isolate the lattice structure of the monolayer in the SEM image to which a 2D discrete Fourier Transform is applied and the result integrated. This results in Bragg peak information akin to that obtained from GID, whose structure shows the same hexagonal close packed lattice with similar spacing and of greater peak contrast. This analysis technique may prove to be a suitable alternative or compliment to GID for many applications.

Stress relaxation in quasi-two-dimensional self-assembled nanoparticle monolayers

“…Second-order intensity autocorrelation functions were calculated using XPCSGUI software designed for sector 8-ID at APS. [22]. Both methods yielded the same center-to-center particle spacing, and correlation length of about 15 particle separations [23].…”

“…Recently, great attention has been paid to the structure, ordering, and variety of elastic responses of thin sheets of nanoparticles at the air-water interface in a Langmuir trough [4,5,17,[19][20][21][22][23][24][25][26][27][28]. Upon drop-casting onto the surface of water, monodisperse nanoparticles (<10% polydispersity) self-assemble into islands of two-dimensional (2D) hexagonally close packed crystalline domains surrounded by large voids.…”

X-ray scattering characterization of iron oxide nanoparticles Langmuir film on water surface and on a solid substrate

“…The difference can be caused by several factors, including imbalance of repulsive and attractive forces [20], the transfer method [23] and physical properties of the particles [24]. Furthermore, an average structural domain size as obtained by GISAXS and SEM can have different values due to the difference in resolution and due to different areas of a sample probed by these techniques [25].…”

Robust Gold Nanoparticle Sheets by Ligand Cross-Linking at the Air–Water Interface

“…Although electron microscopy was not used in this study, Stanley et al 24 reported that the structural information for both the unit cell and domain sizes, determined from in situ X-ray scattering of an NP Langmuir monolayer, is within the experimental precision, equivalent to that determined from the analysis of the SEM images obtained from the same monolayer transferred to a solid substrate. This previous study demonstrated that the packing of the NPs at the air−water interface is reasonably homogeneous over hundreds of mm 2 sampled by X-rays.…”