Prof. Dr. Tobias Kraus

Understanding how nanoparticles form stable colloids is fundamental to their practical applications. Nonlinear ligands are known to increase the stability of nanoparticles in apolar solvents compared to shells of linear alkyl chains. Here, we reveal the molecular origin of this colloidal stability. We observe that even a single methyl side chain can suppress disorder–order transitions in the ligand shell, with double bonds or branches leading to drastic decreases in agglomeration temperature in such dispersions. Through a combination of temperature-dependent X-ray scattering and molecular dynamics simulations, we show that these simple structural modifications prevent ligand molecules from forming ordered bundles, maintaining shell disorder even at temperatures approaching solvent freezing. The absence of ligand order enhances colloidal stability by weakening attraction between the ligand shells via a combination of energetic and entropic factors. This mechanism extends dispersion stability by more than 100 K compared to linear ligands of equivalent length. Our findings provide a molecular-level explanation for the enhanced stability previously observed with branched and unsaturated ligands, offering an effective strategy for engineering nanoparticle dispersions that remain stable across broad temperature ranges.

Silver microflakes and -spheres are common fillers for electrically conductive screen-printing pastes. Here, we report on the effects of filler shapes and sizes on conductivity, sintering, and recyclability. We printed pastes based on flakes and spheres, treated them at 110 °C to 300 °C, and evaluated the electrical conductivity of the resulting layers. The electrical conductivity of the layers treated at 110 °C was dominated by particle–particle contact resistances; flakes yielded layers that were five times more conductive than sphere-based layers due to differences in the particle–particle contact area. Increasing temperature led to a reduction of the resistivity of all layers through sintering. At 300 °C, prints based on spheres were 4 times more conductive than those from flakes. Tomography of the sintered structures showed that the difference was caused by a lower tortuosity factor for spheres. In a final study, we showed that silver flakes and spheres could be recycled after sintering and reused for a new generation of prints without losing electrical performance. The more porous structure of sintered flakes allowed for higher recycling yields compared to spheres. At 140 °C, 91.6% of the flakes and 69.7% of the spheres were recovered as reusable dispersions.

The synthesis of an amphiphilic three-arm block copolymer (AB)3-BCP, which consists of poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA) in the hydrophobic inner block, is reported. The hydrophilic block segment is based on poly(2-hydroxyethyl methacrylate) (PHEMA) originating from 2-(trimethylsiloxyl)ethyl methacrylate (HEMA-TMS). The preparation is carried out in two steps using a core-first approach. Using atom transfer radical polymerization (ATRP) as a controlled polymerization technique, three (AB)3-BPCs with HEMA contents of 15 to 38 mol−1 % are prepared, applying different reaction conditions. Porous structures are generated from these BCPs by applying a self-assembly and nonsolvent-induced phase separation (SNIPS) protocol. Complex surface structures are observed using scanning electron microscopy (SEM). Bulk morphologies are investigated for a better understanding of the underlying self-assembly. For PHEMA-rich (AB)3-BCPs, non-regular lamellar microphases are observed in transmission electron microscopy (TEM) and confirmed by small-angle X-ray scattering (SAXS). The porous structures and their expected swelling characteristics are analyzed using atomic force microscopy (AFM) in air and water. Time-resolved measurements in water indicate a rapid swelling after immersion into the water bath. The present study paves the way for exciting porous materials based on the herein synthesized amphiphilic three-arm block copolymers useful for applications as absorber materials and coatings.

Lanthanide-doped upconversion microparticles (UCMP) enable composites for luminescence thermometry with long luminescence lifetime and narrowband absorption and emission spectra. Being non-toxic, easily synthesizable, and having a bright, stable emission makes them an attractive candidate for in-vivo monitoring of key environmental parameters such as temperature. We use them to create soft, biodegradable, miniaturized seed-like robots endowed with fluorescence tags for the sustainable environmental monitoring of topsoil and air above soil environments. Our aim is an airborne platform with a sufficient signal-to-noise ratio to identify the concentration of targeted soil parameters. Here, we study the photoluminescence of Er, Yb: NaYF4 UCMPs embedded in polylactic acid (PLA) polymeric matrix to assess their suitability for remote read-out. We assessed the signal-to-noise ratio in terms of excitation intensity, UCMP concentration, working distance, and sample orientation. We evaluated the signal stability over long exposure time as well as for amplitude-modulated excitation. Finally, we carried out ratiometric and lifetime measurements of luminescence emission in order to demonstrate the feasibility of such sensors in measuring the variation of temperature. Overall, the rare-earth doped UCMPs embedded in biodegradable polymer can be used for remote thermometry, displaying a significant signal-to-noise ratio for luminescence emission detection and subsequent derivation of temperature.

Silver-coated copper microparticles combine the oxidation resistance of silver with the low cost of copper. They are interesting components for printed conductive structures. We studied whether printed films of such particles can be printed and sintered at low temperatures in air to create highly conductive films and whether it is possible to recover the particles from them for recycling. Pastes containing 1.5 μm to 5 μm spheres and 3 μm flakes with L-ascorbic acid were prepared, screen-printed, and treated at temperatures of 110 °C to 300 °C in air. The bulk resistance of films treated below 160 °C were two orders of magnitude higher than that of bulk copper, ρCu, and limited by particle-particle contact resistances. They were reduced by treating the prints at 160 °C to 250 °C, leading to bulk film resistances down to 41ρCu. We demonstrate that the high mobility of silver enables the formation of necks that bridge the copper cores and reduce resistivity in this temperature window. The sintered prints retained their conductivity for at least 6 months. Treatments at higher temperatures in air were detrimental: resistances increased above 250 °C. These temperatures led to dewetting of the silver coating and fast copper oxidation, resulting in a continuously increasing resistance. In a final study, we demonstrated that films treated below 200 °C can be recycled by recovering the metal powder from the printed conductors and that the powder can be printed again.

Hybrid core–shell nanoparticles with metal cores and conductive polymer shells yield materials that are sinter-free and highly conductive but mechanically weak. Conventional composites of such nanoparticles are electrically insulating. Here, we introduce microscale phase-separated nanocomposites of hybrid gold-PEDOT:PPS particles in insulating poly(vinyl alcohol) (PVA). They combine electrical conductivities of up to 2.1 × 105 S/m at 10 vol % PVA with increased mechanical adhesion on polyethylene terephthalate and glass substrates. We studied the effects of the PVA molecular weight, hydrolyzation degree, and volume fraction. Composites with 10 vol % highly hydrolyzed PVA at a MW of 89–98 kDa had the highest conductivities and stabilities; highly hydrolyzed PVA even increased the conductivity of the hybrid particle layers. We propose the formation of hydrogen bonds between PVA and PEDOT:PSS that lead to demixing and the formation of stable, structured composites. Finally, we demonstrated the inkjet-printability of inks containing PVA in water with viscosities of 1.6–2.0 Pa s at 50.1 s–1 and prepared bending-resistant electrical leads.

The structure of supraparticles (SPs) is a key parameter for achieving advanced functionalities arising from the combination of different nanoparticle (NP) types in one hierarchical entity. However, whenever a droplet-assisted forced assembly approach is used, e.g., spray-drying, the achievable structure is limited by the inherent drying phenomena of the method. In particular, mixed NP dispersions of differently sized colloids are heavily affected by segregation during the assembly. Herein, the influence of the colloidal arrangement of Pt and SiO2 NPs within a single supraparticulate entity is investigated. A salt-based electrostatic manipulation approach of the utilized NPs is proposed to customize the structure of spray-dried Pt/SiO2 SPs. By this, size-dependent separation phenomena of NPs during solvent evaporation, that limit the catalytic performance in the reduction of 4-nitrophenol, are overcome by achieving even Pt NP distribution. Additionally, the textural properties (pore size and distribution) of the SiO2 pore framework are altered to improve the mass transfer within the material leading to increased catalytic activity. The suggested strategy demonstrates a powerful, material-independent, and universally applicable approach to deliberately customize the structure and functionality of multi-component SP systems. This opens up new ways of colloidal material combinations and structural designs in droplet-assisted forced assembly approaches like spray-drying.

Soft lithography, in particular microcontact printing (µCP), represents a well-established and widespread class of lithographic patterning techniques. It is based on a directed deposition of molecules or colloidal particles by a transfer process with a micro-structured stamp. A critical aspect of µCP is the adhesion cascade that enables the directed transfer of the objects. Here, the interfacial properties of a µCP-stamp are tuned electrochemically to modify the adhesion cascade. During the printing process, the µCP-stamp is submerged in an electrolyte solution and acted as a working electrode whose surface properties depended on the externally applied potential, thus enabling in situ rapid switching of its adhesion properties. As a proof of principle, defined particle patterns are selectively removed from a monolayer of colloidal particles. The adhesion at the particle/solid interface and the transfer mechanisms are determined by using the colloidal probe technique based on atomic force microscopy (AFM). In this case, a single particle is brought into contact with an electrode with the same surface chemistry as the µCP-stamp. Hence, it became possible to determine the electrochemical potential ranges suitable to establish an adhesion cascade.

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.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.