Prof. Dr. Tobias Kraus

We report a novel fabrication method for ordered arrays of metal nanoparticles that exploits the uniform arrangement of polymer beads deposited as close-packed monolayers. In contrast to colloidal lithography that applies particles as masks, we used thermal decomposition of the metal-covered particles to precisely define metal structures. Large arrays of noble metal (Au, Ag, Pt) nanoparticles were produced in a three-step process on silicon, fused silica and sapphire substrates, demonstrating the generality of this approach. Polystyrene spheres with diameters ranging between 110 nm and 1 µm were convectively assembled into crystalline monolayers, coated with metal and annealed in a resistive furnace or using an ethanol flame. The thermal decomposition of the polymer microspheres converted the metal layer into particles arranged in hexagonal arrays that preserved the order of the original monolayer. Both the particle size and the interparticle distance were adjusted via the thickness of the metal coating and the sphere diameter, respectively.

The growth of nanoscale gold dendrites was studied in situ in a thin liquid film with transmission electron microscopy (TEM) using a liquid cell with silicon nitride (SiN) windows. Gold nanoparticle seeds were covered by a thin liquid layer containing precursor solution. Dendrite nucleation was induced by the electron beam leading to an initial burst of growth. The growth then settled at tip velocities between 0.1 and 2.0 nm/s for different dendrites. Tip velocities fluctuated as different dendrite geometries grew from the tips. Those dendrites showing granularities in their structure experienced the largest growth speed. Comparison of the observed velocities with diffusion-limited growth rates suggests that dendrite growth in thin films at this scale is limited by diffusion. The described method may find application in research on the mechanisms behind dendrite growth and also to study other types of anisotropic growth of nanomaterials driven by crystal and twin geometries.

The stability of nanoparticle suspensions and the details of their agglomeration depend on the interactions between particles. We study this relationship in gold nanoparticles stabilized with different alkyl thiols in heptane. Temperature-dependent interactions were inferred from small-angle x-ray scattering, agglomeration kinetics from dynamic light scattering, and agglomerate morphologies from transmission electron microscopy. We find that the particles precipitate at temperatures below the melting temperatures of the dry ligands. Agglomerates grow with rates that depend on the temperature: Around precipitation temperature, globular agglomerates form slowly, while at lower temperatures, fibrilar agglomerates form rapidly. All agglomerates contain random dense packings rather than crystalline superlattices. We conclude that ligand-ligand and ligand-solvent interactions of the individual particles dominate suspension stability and agglomeration kinetics. The microscopic packing is dominated by interactions between the ligands of different nanoparticles.

We describe the self-assembly of silver nanocubes (AgNC) into dense bowl-shaped arrays using a template made from polystyrene nanospheres (PSNS). Interestingly, we found that most AgNCs were arranged facet-to-facet. When used as substrates for surface-enhanced Raman scattering (SERS), we observed that the SERS hot spot positions were located at the corners of the cubes. This was confirmed using the formation of a self-assembled monolayer (SAM) of 1-dodecanethiol (DDT) covering the cubes' facet surface, whilst the pinholes in the DDT SAM at the corners were subsequently filled with 2-mercaptopyridine (MPy). Due to the high enhancement from the densely arranged AgNCs, single molecule detection was achieved from this SERS substrate and evidenced using the bi-analyte Raman technique.

This Feature Article discusses two biomimetic aspects of functional particle surface assembly: the fabrication of biologically inspired structures from particles and the arrangement of particles on biomimetic templates. The first part discusses the creation of primary patterns by convective assembly and adsorption of particles that can be modified by a combination of etching and growth steps. Resulting structures mimic moth eyes, Lotus leaves, and the Gecko's adhesive structures, for example. The second part focusses on template assisted self-assembly (TASA) of particles. Herein, biological examples are inspiring in terms of structure formation related processes, rather than in terms of functionality. Template formation is a major bottleneck TASA. It is illustrated how bio-inspired wrinkling processes help overcoming this problem and can be employed for forming highly ordered functional nanoparticle assemblies.

Colloidal particles are continuously assembled into crystalline particle coatings using convective fluid flows. Assembly takes place inside a meniscus on a wetting reservoir. The shape of the meniscus defines the profile of the convective flow and the motion of the particles. We use optical interference microscopy, particle image velocimetry and particle tracking to analyze the particles? trajectory from the liquid reservoir to the film growth front and inside the deposited film as a function of temperature. Our results indicate a transition from assembly at a static film growth front at high deposition temperatures to assembly in a precursor film with high particle mobility at low deposition temperatures. A simple model that compares the convective drag on the particles to the thermal agitation explains this behavior. Convective assembly mechanisms exhibit a pronounced temperature dependency and require a temperature that provides sufficient evaporation. Capillary mechanisms are nearly temperature independent and govern assembly at lower temperatures. The model fits the experimental data with temperature and particle size as variable parameters and allows prediction of the transition temperatures. While the two mechanisms are markedly different, dried particle films from both assembly regimes exhibit hexagonal particle packings. We show that films assembled by convective mechanisms exhibit greater regularity than those assembled by capillary mechanisms.

We exploit the strong optical anisotropy of metal nanorods to measure their mobility in a complex fluid. Gold rods in hot agarose solutions cause dynamic, depolarized scattering with an exponentially decaying autocorrelation. As the solution cools down, its decay constant increases. At a certain temperature, the autocorrelation drastically changes its shape and the dynamic contrast drops. We show that, at this temperature, the gelling liquid confines the rods and dampens their motion almost entirely. Depolarized scattering proves to be extraordinarily sensitive to the transition from Brownian to confined motion. We calculate true mobilities for the particles using Pusey and van Megen's correction of the Siegert relation for nonergodic systems. Multipoint measurements show that the rods are immobilized throughout the gel.

Crystalline and amorphous materials composed of the same atoms exhibit strikingly different properties. Likewise, the behavior of materials composed of mesoscale particles depends on the arrangement of their constituent particles. Here, we demonstrate control over particle arrangement during agglomeration. We obtain disordered and ordered agglomerates of the same alkyl thiol-coated gold nanoparticles depending on temperature and solvent. We find that ordered agglomeration occurs exclusively above the melting temperature of the ligand shells. Many-particle simulations show that the contact mechanics of the ligand shells dominate the order-disorder transition: Purely spherical particle-particle interactions yield order, whereas localized "stiction" between the ligand shells leads to disorder. This indicates that the "stickiness" and the packing of the agglomerates can be switched by the state of the ligand shells. It suggests that contact mechanics govern ordering in a wide range of nanoparticles.

Noble gas and metal atoms form minimum-energy clusters. Here, we present analogous agglomerates of gold nanoparticles formed in oil-in-water emulsions. We exclude interfacial templating and nucleation-and-growth as formation mechanisms of these supraparticles. Similar to atomic clusters, the supraparticles form when a mobile precursor state can reconfigure until the nanoparticles' interactions with each other and with the liquid-liquid interface are maximized. This formation mechanism is in striking contrast to that previously reported for microparticle clusters.

Dense and uniform particle films are deposited using a robust version of the convective particle assembly process. We analyze how the shape of the gas-liquid interface and the three-phase contact line govern the stability of convective deposition and, thus, the achievable quality of films. Interference microscopy indicates that a highly curved meniscus cannot compensate for the ubiquitous perturbation during deposition. A moderately curved meniscus provides flexibility to compensate and localize perturbation and enables reliable homogeneous deposition. We analyze which setup geometry and meniscus velocity yield appropriate meniscus shapes. The quality of the resulting films is analyzed and compared to the deposition conditions. Uniform films over areas beyond the centimeter range are accessible using the optimized process, which is suitable for functional particle coatings and templates for microstructured materials.