E. Stefan Kooij

Shape and size effects in the optical properties of metallic nanorods

The influence of size and geometrical shape on the optical properties of randomly oriented metallic nanorods is investigated using the discrete dipole approximation (DDA). Our calculations provide a benchmark for an accurate characterisation of nanorod suspensions by frequently used optical spectroscopic techniques. Our DDA results confirm the longitudinal plasmon resonance to be primarily affected by the nanorod aspect ratio, and also verify that the quasi-static (dipole) approximation for ellipsoidal particles is only valid for very small sizes. For prolate ellipsoidal and cylindrical nanorods with an identical aspect ratio, the latter exhibit a longitudinal resonance at significantly longer wavelengths. The importance of phase retardation and multipole contributions for larger nanorod dimensions is discussed. Also, we investigate the influence on the optical spectra of electron surface scattering, which arises from the limited size of the nanorods in comparison to the electron mean free path.

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.

Surface Bubble Nucleation Stability

Recent research has revealed several different techniques for nanoscopic gas nucleation on submerged surfaces, with findings seemingly in contradiction with each other. In response to this, we have systematically investigated the occurrence of surface nanobubbles on a hydrophobized silicon substrate for various different liquid temperatures and gas concentrations, which we controlled independently. We found that nanobubbles occupy a distinct region of this parameter space, occurring for gas concentrations of approximately 100%-110%. Below the nanobubble region we did not detect any gaseous formations on the substrate, whereas micropancakes (micron wide, nanometer high gaseous domains) were found at higher temperatures and gas concentrations. We moreover find that supersaturation of dissolved gases is not a requirement for nucleation of bubbles.

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.

Electrolytically Generated Nanobubbles on Highly Orientated Pyrolytic Graphite Surfaces

Electrolysis of water is employed to produce surface nanobubbles on highly orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles are formed when the HOPG surface acts as negative (positive) electrode. Coverage and volume of the nanobubbles enhance with increasing voltage. The yield of hydrogen nanobubbles is much larger than the yield of oxygen nanobubbles. The growth of the individual nanobubbles during the electrolysis process is recorded in time with the help of AFM measurements and correlated with the total current. Both the size of the individual nanobubbles and the total current saturate after typical 1 minute; then the nanobubbles are in a dynamic equilibrium, meaning that they do not further grow, in spite of ongoing gas production and nonzero current. The surface area of nanobubbles shows a good correlation with the nanobubble volume growth rate, suggesting that either the electrolytic gas emerges directly at the nanobubbles’ surface, or it emerges at the electrode’s surface and then diffuses through the nanobubbles’ surface. Moreover, the experiments reveal that the time constants of the current and the aspect ratio of nanobubbles are the same under all conditions. Replacement of pure water by water containing a small amount of sodium chloride (0.01 M) allows for larger currents, but qualitatively gives the same results.

Smart Design of Stripe-Patterned Gradient Surfaces to Control Droplet Motion

The motion of droplets under the influence of lithographically created anisotropic chemically defined patterns is described and discussed. The patterns employed in our experiments consist of stripes of alternating wettability: hydrophobic stripes are created via fluorinated self-assembled monolayers, and for hydrophilic stripes, the SiO(2) substrate is used. The energy gradient required to induce the motion of the droplets is created by varying the relative widths of the stripes in such a way that the fraction of the hydrophilic area increases. The anisotropic patterns create a preferential direction for liquid spreading parallel to the stripes and confine motion to the perpendicular direction, giving rise to markedly higher velocities as compared to nonstructured surface energy gradients. Consequently, the influence of the distinct pattern features on the overall motion as well as suggestions for design improvements from an application point of view are discussed.

Ellipsometric study of percolation in electroless deposited silver films

Using spectroscopic ellipsometry in the visible and near-infrared spectral range we investigate the optical properties of a growing silver film starting from predeposited gold nanoparticles. The effective pseudodielectric functions, obtained by direct inversion of the ellipsometry spectra, reveal a surface plasmon resonance for the nanoparticulate films. Upon prolonged electroless silver deposition, the resonance shifts to lower energies. The redshift is due the longer electron mean free path in larger silver structures and is analyzed by describing the optical response of the developing silver film in terms of a Lorentz line shape. The position of the oscillator, i.e., its resonance energy, is discussed in relation to the transition from isolated nanoparticles to an interconnected, eventually continuous metal film. This transition is also observed in the optical conductivity which exhibits an abrupt, stepwise increase at the same energy where the aforementioned resonance energy becomes zero. For longer deposition times, the optical spectra can be described in terms of a Drude-like free-electron metal. The development of the Drude–Lorentz parameters, i.e., the relaxation time and electron density, are compared to values for bulk silver; the latter were obtained from an optical measurement on a thick bulk silver sample. The saturation values for the relaxation time and thus the conductivity amount to approximately 40% of the bulk value, in agreement with direct current conductivity measurements on these films.

Structural and optical characterization of porous anodic aluminium oxide

Spectroscopic ellipsometry and scanning electron microscopy (SEM) experiments are employed to characterize porous aluminum oxide obtained by anodization of thin aluminum films. Rutherford backscattering spectra and x-ray diffraction experiments provide information on the composition and the structure of the samples. Results on our thin film samples with a well-defined geometry show that anodization of aluminum is reproducible and results in a porous aluminum oxide network with randomly distributed, but perfectly aligned cylindrical pores perpendicular to the substrate. The ellipsometry spectra are analyzed using an anisotropic optical model, partly based on the original work by Bruggeman. The model adequately describes the optical response of the anodized film in terms of three physically relevant parameters: the film thickness, the cylinder fraction, and the nanoporosity of the aluminum oxide matrix. Values of the first two quantities, obtained from fitting the spectra, are in perfect agreement with SEM results, when the nanoporosity of the aluminum oxide matrix is taken into account. The validity of our optical model was verified over a large range of cylinder fractions, by widening of the pores through chemical etching in phosphoric acid. While the cylinder fraction increases significantly with etch time and etchant concentration, the nanoporosity remains almost unchanged. Additionally, based on a simple model considering a linear etch rate, the concentration dependence of the etch rate was determined.

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.

Supramolecular assembly of water-soluble poly(ferrocenylsilanes): multilayer structures on flat interfaces and permeability of microcapsules

We report on the layer-by-layer (LBL) supramolecular assembly of redox responsive, organometallic polyion films on planar and curved (spherical) substrates. Organometallic poly(ferrocenylsilane) (PFS) polyanions and polycations were first used to assemble multilayers on planar quartz, silicon and quartz-crystal microbalance (QCM) electrodes. UV/Vis spectroscopy, spectroscopic ellipsometry and quartz-crystal microgravimetry showed a linear increase of UV absorbance, film thickness and frequency shift with increasing the number of deposited bilayers. Additional ellipsometric studies showed a square-root dependence of the film thickness on solution salt (NaCl) concentration. For the preparation of multilayer films on colloidal particles (manganese carbonate, MnCO), relatively high salt concentrations (0.5 M) were employed. PFS microcapsules were subsequently obtained by colloidal template removal using ethylenediaminetetraacetic acid (EDTA). Following the removal of the spherical template, hollow microcapsules were obtained, whose wall structure-permeability characteristics received particular attention. Atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM) were used to study the wall thickness, integrity and permeability of the capsules. Capsule-wall thickness obtained from AFM indicated the existence of a linear film growth regime when the number of adsorbed bilayers was larger than four. Capsules made of PFS polyanions and rhodamine-labelled PFS polycations were directly visualized by CLSM. Using tetramethylrhodamine isothiocyanate (TRITC)-labelled dextran (∼ 4 400 g mol) as probe, CLSM showed that capsules containing more than four PFS polycation-polyanion bilayers displayed good stability and integrity. These stable capsules are excellent candidates for the investigation of polyelectrolyte microcapsule permeability control triggered by redox stimuli.