A colloidal route to fabricate hierarchical sticky and non-sticky substrates.

We present a facile and inexpensive bottom-up colloidal route to prepare sticky superhydrophobic surfaces and non-sticky ones. Either spin coating to assemble silica microspheres into random multilayered arrays or irreversible adsorption of gold nanoparticles is used to manufacture substrates with a single length scale roughness. Hierarchical roughness with multiple length scales is achieved by decorating the silica spheres with gold nanoparticles. The surface chemistry of the silica surfaces is modified by the adsorption of fluoroalkylsilane self-assembled monolayers, while gold nanoparticles are hydrophobized by dodecanethiol. The wetting properties, both static and dynamic, of surfaces in relation to the morphology of the substrates are addressed. We demonstrate the role of hierarchy in the roughness in converting a sticky into a non-sticky superhydrophobic surface and discuss the results in terms of existing models describing wetting characteristics.

Novel, highly selective gold nanoparticle patterning on surfaces using pure water.

We present a simple, novel procedure to selectively deposit gold nanoparticles using pure water. It enables patterning of nanoparticle monolayers with a remarkably high degree of selectivity on flat as well as microstructured oxide surfaces. We demonstrate that water molecules form a thin "capping" layer on exposed thiol molecules within the mercaptan self-assembled layer. This reversible capping of water molecules locally "deactivates" the thiol groups, therewith inhibiting the binding of metallic gold nanoparticles to these specific areas. This amazing role of water molecules can be used as a tool to pattern flat as well as structured surfaces with gold nanoparticles.

Dipole directed ring assembly of Ni-coated Au-nanorods.

We demonstrate the synthesis of novel core-shell nanoring superstructures consisting of non-dipolar Au nanorods coated with magnetic Ni shells. We show that the magnetic dipole induced self-assembly can be tuned by selective reduction of Ni on gold nanorods. Superstructures range from nanorings containing physically separated particles to solid rigid nanorings.

Superhydrophobic surfaces by anomalous fluoroalkylsilane self-assembly on silica nanosphere arrays.

We present the self-assembled formation of nanosized PFDTS (1H,1H,2H,2H-perfluorodecyltrichlorosilane) features on multilayered silica sphere arrays. We reveal the importance of residual water within the microsphere multilayers during PFDTS deposition and discuss a possible mechanism for the formation of the siloxane nanostructures. The multiscaled roughness induced by these superstructures is shown to lead to superhydrophobic behavior. The role of PFDTS is twofold: it (i) lowers the surface energy and (ii) provides the essential roughness to achieve superhydrophobicity. Moreover, the absence of PFDTS nanostructures on monolayers or in the absence of water leads to considerably smaller contact angles thereby indicating the relevance of multiscaled roughness for superhydrophobicity.

Controlling the morphology of multi-branched gold nanoparticles.

We demonstrate a simple and versatile way to achieve high yield synthesis of shape- and size-controlled multi-branched gold nanoparticles (MBNPs). Control over the shape of the MBNPs was achieved by varying the ratio of gold to the mild reducing agent ascorbic acid, using a seed-mediated growth approach. Higher ascorbate concentrations resulted in the smoothing of branches, leading to the yield of relatively more isotropic particles. Furthermore, we found that using much higher silver concentrations in the growth solution resulted in the formation of rod-shaped micro-features together with MBNPs; we postulate them to be cetyltrimethyl ammonium silver bromide crystals. The as-prepared MBNPs show interesting tunable optical properties that are strongly influenced by the particle shape. The results are discussed in terms of plasmon coupling between the core and branches of the MBNPs.

Quantitative analysis of gold nanorod alignment after electric field-assisted deposition.

We have studied the alignment of colloidal gold nanorods, deposited from solution onto well-defined substrates in the presence of an AC electric field generated by micrometer spaced electrodes. The field strengths employed in our experiments are sufficiently large to overcome Brownian motion and induce accumulation and alignment of the nanorods in the region near the electrodes with their long axis parallel to the field. However, despite the large fields, we find that the degree of alignment is considerably smaller than what was previously reported for field-induced nanorod alignment in suspension. We show that hydrodynamic interactions and capillary effects during drying, as well as friction of nanorods on the substrate surface, to not play a major role. The limited alignment of nanorods is ascribed to the different experimental configuration and the correspondingly larger density of nanorods. The mutual interactions of nanorods give rise to a disturbance of the local electric field and therewith their orientation. For sufficiently large field strengths, these interactions lead to the formation of nanorod chains that ultimately bridge the electrode gap. Furthermore, for small electrode spacing, the nanorods accumulate on the electrode surface, and the screening of their mutual interactions results into considerably improved alignment.