Spatially Resolved Investigation of Solution Cast Nanoparticle Films by X-ray Scattering and Multidimensional Data Set Classification

Abstract

A combinatorial high-throughput approach is used to investigate a solution cast gradient consisting of colloidal gold nanoparticles on top of a silicon substrate by means of a X-ray nanobeam. Classification algorithms are used to reveal and visualize structural transitions from a frozen colloidal solution to a well-defined nanostructure. Prominent length scales on the order of 100 nm are observed. A periodic change in the nanostructure along the gradient is explained by a simplified stick-slip model.The fundamental understanding of wetting and flow behavior of nanoparticle and (bio)polymeric solutions and blends on solid substrates is crucial for their application in many technological fields, for example, proteomics, optical coatings, or data storage applications.1-6 Typical deposition methods are spin-coating, Langmuir-Blodgett/Langmuir-Schaefer 8 techniques, or solution casting.9,10 The latter is widely applied in producing designed colloidal, polymeric, and biopolymeric thin films. 4,11,12 It allows for realizing dedicated morphologies and morphological gradients in order to perform combinatorial investigations of thin films 4,9 for different applications, for example, to produce two-and threedimensional photonic crystals. 13,14 Combinatorial methods 15,16 are routinely used for material testing and optimization. By testing a large physical parameter space in short time, one is able to determine the structure-function relationship desired from the application. Solution casting of individual droplets allows one to study structures and morphologies present after deposition on solid wetting substrates. This offers a deeper understanding of the structures induced by the solvent evaporation and its implications for coating, which is of great interest for inkjet printing. 17 In an individual droplet, a multitude of structures is formed, depending on the wetting behavior of the substrate, for example, the rim and a wetting region. The wetting region might extend far from the droplet rim and could be used for installing large scale arrays of ordered colloidal and nanoparticles with respect to high-throughput applications such as surface enhanced Raman scattering (SERS), 18 colorimetric detection of biopolymers, 19 or sensing. 20,21Nanostructuring during colloidal solution casting is a complex, nonequilibrium process. The self-assembly involves the interplay of several flow and time-dependent mechanisms. It involves rheology, phase changes, and nonequilibrium thermodynamics. 22,23 In detail, the nanostructuring takes places at the triple phase contact line air-colloidal solution-substrate.11 The process can be described as follows: During solvent evaporation, the droplet cools at the interface and the concentration of the colloidal particles increases. The evaporation rate is nonuniform and increased near the contact line. This leads to a convective flow and material transport toward the contact line 24 or in the inverse direction, depending on the ratio of the surface tensions of solute and solvent.

Possible origin of the crack pattern in deposition films formed from a drying colloidal suspension

“…We consider that the clusters continuingly grow and separate each other with a distance ζ in the solvent. This physical configuration is confirmed by the x-ray scattering experiment for drying nanoparticle suspension [15,17]. After k coalescences (k doublet reactions), the number and radius of clusters in the suspension are…”

“…[7,15]), which indicates that no flow replenishes solvent loss owing to the evaporation. The particle diffusion from inner suspension to the reaction area (drying front) can be neglected (Appendix subsection B).…”

Advanced grazing-incidence techniques for modern soft-matter materials analysis

“…So far, the minimum demonstrated X-ray beam size in GISAXS experiments is 300 nm and the corresponding experiments are named nanobeam GISAXS (nGISAXS). With a beam diameter of 300 nm, GISAXS measurements have been performed on ordered nanoparticles at the airwater-substrate boundary in colloidal solutions (Roth, Autenrieth et al, 2007) and solution-cast gradients consisting of colloidal gold nanoparticles on top of a silicon substrate (Roth et al, 2010). The use of a high-brilliance 300 nm X-ray beam allows for acquisition times of 120 s per two-dimensional GISAXS pattern and this has enabled in situ investigations such as the nanoscale structuring that occurs during evaporation of water (Roth, Autenrieth et al, 2007).…”

Time-Resolved Ultrathin Cobalt Film Growth on a Colloidal Polymer Template

“…7 Therefore, much effort has been put into better understanding of the growth process and structural transition of nanocomposites. 8,9 It has been shown that the optical and electrical properties of a metal-polymer film are very sensitive to small changes in the composition and/or morphology of the film and that this sensitivity is drastically increased near the percolation threshold, 10 when a conducting path is formed for the first time.…”