Spatially resolved investigation of solution cast nanoparticle films by X-ray scattering and multidimensional data set classification

Advances in the Development of Sol-Gel Materials Combining Small-Angle X-ray Scattering (SAXS) and Machine Learning (ML)

The requirements for new materials are increasing with each new application, which, in most cases, means an enhancement in the complexity of the development process. Nanoporous sol-gel-based materials, especially aerogels, are promising candidates for thermal superinsulation, electrodes for energy conversion and storage or high-end adsorbers. Their synthesis and processing route is complex, and the relationship between the material/processing parameters and the resulting structural and physical properties is not straightforward. Using small-angle X-ray scattering (SAXS) allows for fast structural characterization of both the gel and the resulting aerogel; combining these results with the respective physical properties of the aerogels and using these data as inputs for machine learning (ML) algorithms provide an approach to predict physical properties on the basis of a structural dataset. This data-driven strategy may be a feasible approach to speed up the development process. Thus, the study aimed to provide a proof of concept of ML-based model derivation from material, process and SAXS data to predict physical properties such as the solid-phase thermal conductivity (λs) of silica aerogels from a structural dataset. Here, we used different data subsets as predictors according to different states of synthesis (wet and dry) to evaluate the model performance.

Understanding Film-To-Stripe Transition of Conjugated Polymers Driven by Meniscus Instability

Meniscus instability during meniscus-guided solution coating and printing of conjugated polymers has a significant impact on the deposit morphology and the charge-transport characteristics. The lack of quantitative investigation on meniscus-instability-induced morphology transition for conjugated polymers hindered the ability to precisely control conjugated polymer deposition for desired applications. Herein, we report a film-to-stripe morphology transition caused by stick-and-slip meniscus instability during solution coating seen in multiple donor-acceptor polymer systems. We observe the coexistence of film and stripe morphologies at the critical coating speed. Surprisingly, higher charge-carrier mobility is measured in transistors fabricated from stripes despite their same deposition condition as the films at the critical speed. To understand the origin of the morphology transition, we further construct a generalizable surface free energy model to validate the hypothesis that the morphology transition occurs to minimize the system surface free energy. As the system surface free energy varies during a stick-and-slip cycle, we focus on evaluating the maximum surface free energy at a given condition, which corresponds to the sticking state right before slipping. Indeed, we observe the increase of the maximum system surface free energy with the increase in coating speed prior to film-to-stripe morphology transition and an abrupt drop in the maximum system surface free energy post-transition when the coating speed is further increased, which is associated with the reduced meniscus length during stripe deposition. Such an energetic change originates from the competition between pinning and depinning forces on a partial wetting substrate which underpins the film-to-stripe transition. This work establishes a quantitative approach for understanding meniscus-instability-induced morphology transition during solution coating. The mechanistic understanding may further facilitate the use of meniscus instability for lithography-free patterning or to suppress instability for highly homogeneous thin film deposition.

Investigating Polymer-Metal Interfaces by Grazing Incidence Small-Angle X-Ray Scattering from Gradients to Real-Time Studies

Tailoring the polymer-metal interface is crucial for advanced material design. Vacuum deposition methods for metal layer coating are widely used in industry and research. They allow for installing a variety of nanostructures, often making use of the selective interaction of the metal atoms with the underlying polymer thin film. The polymer thin film may eventually be nanostructured, too, in order to create a hierarchy in length scales. Grazing incidence X-ray scattering is an advanced method to characterize and investigate polymer-metal interfaces. Being non-destructive and yielding statistically relevant results, it allows for deducing the detailed polymer-metal interaction. We review the use of grazing incidence X-ray scattering to elucidate the polymer-metal interface, making use of the modern synchrotron radiation facilities, allowing for very local studies via in situ (so-called "stop-sputter") experiments as well as studies observing the nanostructured metal nanoparticle layer growth in real time.

A deep look into the spray coating process in real-time-the crucial role of x-rays

Tailoring functional thin films and coating by rapid solvent-based processes is the basis for the fabrication of large scale high-end applications in nanotechnology. Due to solvent loss of the solution or dispersion inherent in the installation of functional thin films and multilayers the spraying and drying processes are strongly governed by non-equilibrium kinetics, often passing through transient states, until the final structure is installed. Therefore, the challenge is to observe the structural build-up during these coating processes in a spatially and time-resolved manner on multiple time and length scales, from the nanostructure to macroscopic length scales. During installation, the interaction of solid-fluid interfaces and between the different layers, the flow and evaporation themselves determine the structure of the coating. Advanced x-ray scattering methods open a powerful pathway for observing the involved processes in situ, from the spray to the coating, and allow for gaining deep insight in the nanostructuring processes. This review first provides an overview over these rapidly evolving methods, with main focus on functional coatings, organic photovoltaics and organic electronics. Secondly the role and decisive advantage of x-rays is outlined. Thirdly, focusing on spray deposition as a rapidly emerging method, recent advances in investigations of spray deposition of functional materials and devices via advanced x-ray scattering methods are presented.

Thermal transformations of self-assembled gold glyconanoparticles probed by combined nanocalorimetry and X-ray nanobeam scattering

Noble metal nanoparticles with ligand shells are of interest for applications in catalysis, thermo-plasmonics, and others, involving heating processes. To gain insight into the structure-formation processes in such systems, self-assembly of carbohydrate-functionalized gold nanoparticles during precipitation from solution and during further heating to ca. 340 °C was explored by in situ combination of nanobeam SAXS/WAXS and nanocalorimetry. Upon precipitation from solution, X-ray scattering reveals the appearance of small 2D domains of close-packed nanoparticles. During heating, increasing interpenetration of the nanoparticle soft shells in the domains is observed up to ca. 81 °C, followed by cluster formation at ca. 125 °C, which transform into crystalline gold nuclei at around 160 °C. Above ca. 200 °C, one observes the onset of coalescence and grain growth resulting in gold crystallites of average size of about 100 nm. The observed microstructural changes are in agreement with the in situ heat capacity measurements with nanocalorimetry.

Probing droplets on superhydrophobic surfaces by synchrotron radiation scattering techniques

Droplets on artificially structured superhydrophobic surfaces represent quasi contact-free sample environments which can be probed by X-ray microbeams and nanobeams in the absence of obstructing walls. This review will discuss basic surface wettability concepts and introduce the technology of structuring surfaces. Quasi contact-free droplets are compared with contact-free droplets; processes related to deposition and evaporation on solid surfaces are discussed. Droplet coalescence based on the electrowetting effect allows the probing of short-time mixing and reaction processes. The review will show for several materials of biological interest that structural processes related to conformational changes, nucleation and assembly during droplet evaporation can be spatially and temporally resolved by raster-scan diffraction techniques. Orientational ordering of anisotropic materials deposited during solidification at pinning sites facilitates the interpretation of structural data.

Nanomaterial processing using self-assembly-bottom-up chemical and biological approaches

Nanotechnology is touted as the next logical sequence in technological evolution. This has led to a substantial surge in research activities pertaining to the development and fundamental understanding of processes and assembly at the nanoscale. Both top-down and bottom-up fabrication approaches may be used to realize a range of well-defined nanostructured materials with desirable physical and chemical attributes. Among these, the bottom-up self-assembly process offers the most realistic solution toward the fabrication of next-generation functional materials and devices. Here, we present a comprehensive review on the physical basis behind self-assembly and the processes reported in recent years to direct the assembly of nanoscale functional blocks into hierarchically ordered structures. This paper emphasizes assembly in the synthetic domain as well in the biological domain, underscoring the importance of biomimetic approaches toward novel materials. In particular, two important classes of directed self-assembly, namely, (i) self-assembly among nanoparticle-polymer systems and (ii) external field-guided assembly are highlighted. The spontaneous self-assembling behavior observed in nature that leads to complex, multifunctional, hierarchical structures within biological systems is also discussed in this review. Recent research undertaken to synthesize hierarchically assembled functional materials have underscored the need as well as the benefits harvested in synergistically combining top-down fabrication methods with bottom-up self-assembly.

An X-ray chamber for in situ structural studies of solvent-mediated nanoparticle self-assembly

Spontaneous ordering of nanoparticles (NPs) occurring as a consequence of solvent evaporation can yield highly ordered and extended NP superlattices bearing both fundamental scientific interest and potential for technological application. A versatile experimental chamber has been developed allowing (i) controlled in situ deposition of NP solutions on solid substrates, (ii) rate-controlled evaporation of the bulk solvent, and (iii) adsorption/desorption of nano-thick solvent films onto preformed NP assemblies. Within this hermetically sealed chamber all the stages of self-assembly, including macroscopic solution evaporation, NP thin-film formation and its subsequent structural transformation induced by nano-thick solvent films, can be characterized in situ by X-ray scattering techniques. Here, technical design and calibration details are provided, as well as three experimental examples highlighting the chamber's performances and potential. Examples include the controlled adsorption of thin toluene films on flat silicon wafers, the observation of transient accumulation of gold NPs near the toluene-vapour interface, and preliminary data on the structural effects of fast macroscopic solvent evaporation followed by nanoscale solvent adsorption/desorption from a vapour phase. By combining bulk evaporation rate control, fine tuning of the thickness of adsorbed solvent films and in situ X-ray characterization capabilities, this cell enables explorations of both near-to-equilibrium and far-from-equilibrium routes to NP self-assembly.

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

The fracture mechanics was usually employed to explain the crack propagation in the deposition produced by drying colloidal suspension. However, more complex than conventional fracture, those cracks periodically distribute and make up a unique pattern. Inspired by the concept of spinodal decomposition, here we develop the theory to illustrate the possible mechanism of the spatial arrangement of the cracks. It indicates that before the cracks develop and propagate in the deposition under the law of fracture mechanics, the periodically distributed flaws are generated by the phase separation of colloidal clusters and solvent. Then the cracks originate at the sites of those flaws in terms of fracture mechanics. It concludes that the crack spacing results from the wavelength of the concentration fluctuation during the phase separation, linearly growing with the increase of the deposition thickness and initial particle concentration, which is consistent with experimental results.

P03, the microfocus and nanofocus X-ray scattering (MiNaXS) beamline of the PETRA III storage ring: the microfocus endstation

The P03 beamline, also called the microfocus and nanofocus X-ray scattering (MiNaXS) beamline, exploits the excellent photon beam properties of the low-emittance source PETRA III to provide a microfocused/nanofocused beam with ultra-high intensity for time-resolved X-ray scattering experiments. The beamline has been designed to perform X-ray scattering in both transmission and reflection geometries. The microfocus endstation started user operation in May 2011. An overview of the beamline status and of some representative results highlighting the performance of the microfocus endstation at MiNaXS are given.

Influence of nanoparticle surface functionalization on the thermal stability of colloidal polystyrene films

The installation of large scale colloidal nanoparticle thin films is of great interest in sensor technology or data storage. Often, such devices are operated at elevated temperatures. In the present study, we investigate the effect of heat treatment on the structure of colloidal thin films of polystyrene (PS) nanoparticles in situ by using the combination of grazing incidence small-angle X-ray scattering (GISAXS) and optical ellipsometry. In addition, the samples are investigated with optical microscopy, atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). To install large scale coatings on silicon wafers, spin-coating of colloidal pure PS nanoparticles and carboxylated PS nanoparticles is used. Our results indicate that thermal annealing in the vicinity of the glass transition temperature T(g) of pure PS leads to a rapid loss in the ordering of the nanoparticles in spin-coated films. For carboxylated particles, this loss of order is shifted to a higher temperature, which can be useful for applications at elevated temperatures. Our model assumes a softening of the boundaries between the individual colloidal spheres, leading to strong changes in the nanostructure morphology. While the nanostructure changes drastically, the macroscopic morphology remains unaffected by annealing near T(g).

In situ observation of cluster formation during nanoparticle solution casting on a colloidal film

We present a real-time study of the nanostructuring and cluster formation of gold nanoparticles deposited in aqueous solution on top of a pre-structured polystyrene colloidal thin film. Cluster formation takes place at different length scales, from the agglomerations of the gold nanoparticles to domains of polystyrene colloids. By combining in situ imaging ellipsometry and microbeam grazing incidence small-angle x-ray scattering, we are able to identify different stages of nanocomposite formation, namely diffusion, roughness increase, layer build-up and compaction. The findings can serve as a guideline for nanocomposite tailoring by solution casting.

Structure and flow of droplets on solid surfaces

The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro- and nanometer-sized beams, GISAXS and in situ imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed in situ during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed.