Arend van Silfhout

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.

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.

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.

Decomposition of N2O on the Si(100)2×1 and Si(111)7×7 surfaces: Determination of the density of broken bonds

This paper reports on the site-specific decomposition behaviour of N2O onthe clean Si(100)2 × 1 and Si(111)7 × 7 surfaces at room temperature, studied by spectroscopic differential reflectometry (SDR) and Auger electron spectroscopy (AES). It has been established that Si---O bond formation occurs at the toplayer Si atoms, mainly via attachment of the unsaturated broken bonds. In this way it is possible to determine the density of broken (dangling) bonds at the clean Si surface. Our results confirm the high dangling bond density at the 2 × 1 reconstructed Si(100) surface for which we assume a defect density of 15 ± 10%. The density of dangling bonds per 7 × 7 unit cell (49 atoms), 23 ± 4, is in agreement with the established 19 as revealed by current imaging tunneling spectroscopy (CITS).

Chemical titration of clean silicon surfaces with N2O and O2: Atomic nature of "5x1" reconstructed Si(110)

Using spectroscopic differential reflectometry (SDR), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) we have studied the room temperature adsorption behavior of N2O and O2 at the clean low-Miller index Si surfaces

Kinetics of the adsorption of atomic oxygen (N2O) on the Si(001)2x1 surface as revealed by the change in the surface conductance

The adsorption behaviour of N2O on the Si(001)2 × 1 surface at 300 K substrate temperature has been investigated by measuring in situ the surface conductance during the reaction process. For comparison we monitored in the same way the adsorption of O2 on the same surface which ultimately leads to the flat band situation. The adsorption of atomic oxygen as released by decomposition of the N2O molecule, in contrast with molecular oxygen, was found to result in an increase of the band bending. The difference in behaviour of the change of the surface conductance between the two solid-gas reactions can be explained by considering that the adsorption of O2 will also remove deep-lying backbond states in addition to the dangling bond (DB) and dimer bond (DM) related surface states. It is well known that only the DB and DM surface states are affected by N2O. The surface conductance measurements (SCM) presented in this paper complement our previous spectroscopic differential reflectivity measurements and Auger electron spectroscopic results for the system Si(001)2 × 1 + N2O; we have found evidence that the second step of the proposed three-stage adsorption process of atomic oxygen can be divided into two substages. From our SCM data we could derive that the distance between the valence band edge and the Fermi energy of the clean Si(001)2 × 1 surface is 0.32 ± 0.02 eV, which is in agreement with previous photoemission results.

The influence of inter-atomic transitions in Auger valence band spectroscopy: oxygen on Si(001)2 × 1

In this paper we will show that the description of an Auger process in terms of a process confined to one atom is in general not adequate and the Mulliken population is only in very specific cases a good alternative in evaluating the strength of inter-atomic transitions. The ionicity of the chemical bond cannot be used as a direct measure of the contribution of inter-atomic Auger transitions, as will be demonstrated in the case of the oxygen KVV Auger transitions in gaseous molecular oxygen and oxygen chemisorbed on the Si(001) surface. A full evaluation of inter-atomic transition rates shows that their strength depends on the inter-atomic distance as well as on the screening of the initial core hole.

Adsorption of oxygen on the Si(110)5×1 surface via interaction with O2

Auger electron spectroscopy, low-energy electron diffraction, and differential reflectometry in the photon energy range 1.5–4.5 eV have been used to study the room-temperature adsorption of O2 on the Si(110) surface with an initial 5×1 superstructure. The reaction kinetics are complicated. Five adsorption stages can be discerned, the first stage (0–2 L) being remarkably fast with an initial sticking probability near unity. Our results suggest that the surface sites which constitute the higher-order reconstructions and possibly also defects, are highly reactive. In the second stage (2–200 L) the O2 adsorption can be described by a dissociative process on the first-layer Si atoms, the sticking probability being about 2×10–3. Incorporation of oxygen into the subsurface Si lattice and possibly also adsorption of a molecular oxygen species are the main processes of the third stage (200–1000 L). In the fourth stage (103–4×104 L) O2 adsorption completely removes the dangling bonds. This occurs at 0.79±0.15 monolayer oxygen coverage. The number of dangling bonds per 5×1 unit cell (ten atoms) is thus eight or less. The fifth stage (>4×104 L) comprises a slow further oxygen adsorption with a sticking probability of ~10–6: oxygen mainly goes into a bridging position between two first-layer Si atoms in the uppermost chains of the ideal (110) surface. The optical spectrum indicates that the clean Si(110) surface probably has several types of (dangling bond) surface states.

Tuning the dipole-directed assembly of core-shell nickel-coated gold nanorods

We present the dipole-directed assembly of nickel-coated gold nanorods into nanorings and nanowires. We used two different coating methods to synthesise these core-shell superstructures. Surprisingly, the two coating methods lead to very different kinds of dipole directed assembly. We show that the resultant dipole assembly is very sensitive to the reaction conditions and can be tuned to obtain core-shell nanochains, nanorings, and nanowires. In addition to the presented experimental work, cluster moving Monte Carlo simulations of a system of core-shell nanorods were carried out. These simulations are based on a small number of magnetic interaction energy terms and do not explicitly deal with steric interactions or van der Waals forces. The simulation results are in line with the obtained experimental results, confirming that the magnetic self-assembly of core-shell nanorods can be described by means of a relatively simple model.