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Spacing is important: Nanoparticles in dense assemblies can fuse and lose their identity. Changing particle distances in material, on the other hand, lets us tune color, electrical conductivity, or mechanical strength. In this talk, I will discuss how apolar organic ligands set the distance between nanoparticle cores.

Gold and cadmium selenide nanocrystals strongly attract each other. I will show that gold cores with diameters of 10 nm attract each other so strongly that they dominate the agglomeration of nanoparticles with hexadecanethiol shells [1]. In this “core-dominated” regime, ligand shells are compressed. Silica cores of the same diameter attract each other only weakly, and their spacing is set by the geometry of the ligand shells.

Particle geometry affects spacing, too. Ultrathin gold nanowires with oleylamine shells in alkanes spontaneously form bundles with large spacings – their ligand shells do not even touch. I will discuss how the solvent sets their spacing via an entropic mechanism [2]. Changes of the shell disrupt the mechanism and can induce nanowire gelation, as I will show.

Apparently spherical gold nanoparticles with alkylthiol ligands form superlattices with structures that depend on the polarity of the substrate. I will show that such particles can sit on different facets, which changes core-surface and thus, interaction energies [3].

As an outlook I will show that nanoparticle fusion at small spacings can be beneficial: we exploit the fusion of inorganic cores for the printing of transparent flexible electrodes [4] and for chemical sensors.

[1] Kister, T.; Monego, D.; Mulvaney, P.; Widmer-Cooper, A.; Kraus, T. Colloidal Stability of Apolar Nanoparticles: The Role of Particle Size and Ligand Shell Structure. ACS Nano 2018, 12 (6), 5969-5977. DOI: 10.1021/acsnano.8b02202.

[2] Gao, H. Y.; Bettscheider, S.; Kraus, T.; Muser, M. H. Entropy Can Bundle Nanowires in Good Solvents. Nano Letters 2019, 19 (10), 6993-6999. DOI: 10.1021/acs.nanolett.9b02379.

[3] Bo, A.; Liu, Y. W.; Kuttich, B.; Kraus, T.; Widmer-Cooper, A.; de Jonge, N. Nanoscale Faceting and Ligand Shell Structure Dominate the Self-Assembly of Nonpolar Nanoparticles into Superlattices. Advanced Materials 2022, 34 (20). DOI: 10.1002/adma.202109093.

[4] Engel, L. F.; Gonzalez-Garcia, L.; Kraus, T. Flexible and transparent electrodes imprinted from metal nanostructures: morphology and opto-electronic performance. Nanoscale Advances 2022, 4 (16), 3370-3380. DOI: 10.1039/d2na00259k.

Prof. Dr. Tobias Kraus is a chemical engineer and materials scientist trained at TU Munich, MIT, and the University of Neu-châtel. He obtained his PhD at ETH Zürich and the IBM Research Laboratory, where he worked on the assembly of particles at interfaces, particle transfer through controlled adhesion, and the creation of functional interfaces and structures with particles. Today, Tobias works at the INM – Leibniz-Institute for New Materials in Saarbrücken, Germany. He has been Head of the Program Division “Structure Formation” since 2014. In 2016, he became a Full Professor of colloid and interface chemistry at Saarland University, where he teaches and supports the strong collaboration between INM and the University. In his research, Tobias curtails the interactions between particles, polymers, and small molecules. This leads to predictable, hierarchical assemblies for structured interfaces and functional materials. His group investigates hybrid materials for flexible and transparent electronics, reversible interfaces for soft and recyclable electronic devices, optical sensors, and their formation during 2D and 3D printing. Fundamental problems of nanoparticle structure, network formation, and self-assembly are investigated with a combination of small-angle X-ray scattering, light scattering, electron microscopy, and optical spectrometry. The group establishes analytical and digital models of structure formation and applies them to creating new materials with rationally designed morphologies.

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