Simon Blum

Salts induce the agglomeration and assembly of gold nanocrystals that are stabilized by charged ligand shells. This destabilization is known to partially deviate from the predictions of classical DLVO theory for larger colloids, but existing studies focus on limited concentration ranges or ion types. Here, we use a high-throughput approach to test the agglomeration efficacy of 17 different salts at concentrations ranging from 0.16 mM to 2 M on negatively charged nanocrystals with shells of 11-mercaptoundecanoic acid and/or triethylene glycol mono-11-mercaptoundecyl ether. Automated pipetting is used to create a large dataset of close to 10000 UV–vis absorbance spectra. We analyze the spectral shifts to find the onset of agglomeration, identify critical salt concentrations, and characterize the nature of the agglomeration transition. The results are compared to classical DLVO theory using conventional analysis, and the effects of ion concentration and anion valency are shown to be consistent with DLVO predictions. Cation valencies only partially follow the predictions, suggesting local ion-specific interactions that dominate when screening lengths reach molecular length scales. A Random Forest Regression model is used as additional “black box” analysis of the results to correlate the ionic strength, ligand shell composition, and intrinsic ion properties, and to rank their relative importance to the colloidal stability of gold nanocrystals. The ranking combines classical DLVO effects and specific ion interactions with subtle effects on the agglomerate structure that affect plasmon resonance shifts, providing a complementary interpretation of the data.

Conductive polymer composites (CPCs) combine the stretchability of an elastomeric matrix with the electrical conductivity of a metallic filler. The 3D structure of this filler particle network (FPN) and the contact resistances between particles above percolation, key factors in the conductivity, are not well understood. Here, we introduce 3D reconstructions of FPNs of micron-sized spherical silver particles in polydimethylsiloxane from focused ion beam scanning electron microscopy tomography. Analysis of the tomographic images provides the length and number of parallel conductive paths. The results show that the average contact resistance drops five orders of magnitude when increasing the silver loading from 36 vol% to 53 vol%, highlighting its dominating role for macroscopic conductivity rather than network structure. This links to 33% larger average area-equivalent diameters of the contact spots. Diffusional tortuosity, a metric that quantifies flow restriction through narrow contact spots, proves that higher contact forces decrease current flow restrictions and thus, increase overall electrical conductivity. These conclusions are verified using a segregated CPC, and it is found that the addition of 20 vol%
of insulating fillers at a constant silver loading of 30 vol% increases the conductivity 37-fold and decreases the average contact resistance by two orders of magnitude.

Recycling of Waste from Electrical and Electronic Equipment (WEEE) is crucial in preventing resource depletion and promoting a circular economy. The increasing fraction of printed and in-mold electronics is particularly challenging. The combinations of polymers and printed metals are difficult to disassemble due to the strong interfaces that are formed to create reliable in-mold devices. The relatively low metal content makes recycling uneconomical and those valuable materials are then lost to landfill or incineration. Separation layers enable design-for-recycling with minimal modifications during the fabrication process, while preserving product performance and reliability. We present a scalable method for preparing polymer separation layers for printed and in-mold electronics. Slot-die coating is used to prepare water-soluble polymer films with a dry thickness of less than 10 μm on commodity polymer substrates. This separation layer improves the bending stability of inkjet- and screen-printed circuits. Furthermore, it is compatible with typical polymer processing methods, such as thermoforming and injection molding. Various methods, including plasma treatment, are presented to ensure adhesion of the modified interfaces. Finally, we investigate the material recovery and demonstrate the release of the integrated metal within a few minutes by dissolving the separation layer in water. This material recovery process can be readily integrated into current WEEE recycling processes.