Georgios Palasantzas

Light on the moth-eye corneal nipple array of butterflies.

The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids.

Experimental Observations of Self-Affine Scaling and Kinetic Roughening at Sub-Micron Lengthscales

Experimental observations of self-affine scaling and kinetic roughening at sub-micron length scales are reviewed for thin solid films and ion-beam eroded surfaces.

Effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity

We study the effect of surface roughness on magnetic domain wall thickness, domain size, and coercivity of thin magnetic films. We show that the roughness increases (decreases) the domain wall thickness and domain size for Bloch walls (Neel walls). The surface roughness affects the domain wall movement and causes the increase of coercivity for Neel walls. The coercivity due to domain rotation for Bloch walls decreases with the increase of roughness. The domain wall thickness, domain size, and coercivity are each related to the demagnetizing factor, which depends on the roughness and type of wall (Bloch wall or Neel wall). The calculated coercivity versus thickness is compared with experimental data of ultrathin Co films, where the thickness dependent roughness parameters are available.

WETTING ON ROUGH SURFACES

This paper concentrates on effects of roughness on the wettability. Surface roughness is described by an rms amplitude Δ, a correlation length ξ, and a roughness exponent H (0 θtr. The transition angle θtr appears to be smaller than 90°, and decreases with increasing roughness exponent H.

Gas-phase synthesis of magnesium nanoparticles: A high-resolution transmission electron microscopy study

Magnesium nanoparticles with size above 10nm, prepared by gas-phase syntheses, were investigated by high-resolution transmission electron microscopy. The dominant particle shape is a hexagonal prism terminated by Mg(0002) and Mg{101¯0} facets. Oxidation of Mg yields a MgO shell (∼3nm thick), which has an orientation relation with the Mg. Inhomogeneous facet oxidation influences their growth kinetics resulting in a relatively broad size and shape distribution. Faceted voids between Mg and MgO shells indicate a fast outward diffusion of Mg and vacancy rearrangement into voids. The faceting of polar {220} planes is assisted by electron irradiation.

Influence of roughness on capillary forces between hydrophilic surfaces

Capillary forces have been measured by atomic force microscopy in the plate-sphere setup between gold, borosilicate glass, GeSbTe, titanium, and UV-irradiated amorphous titanium-dioxide surfaces. The force measurements were performed as a function contact time and surface roughness in the range 0.2-15 nm rms and relative humidity ranging between 2% and 40%. It is found that even for the lowest attainable relative humidity ( approximately 2%+/-1%) very large capillary forces are still present. The latter suggests the persistence of a nanometers-thick adsorbed water layer that acts as a capillary bridge between contacting surfaces. Moreover, we found a significantly different scaling behavior of the force with rms roughness for materials with different hydrophilicity as compared to gold-gold surfaces.

Optical properties of gold films and the Casimir force

Precise optical properties of metals are very important for accurate prediction of the Casimir force acting between two metallic plates. Therefore we measured ellipsometrically the optical responses of Au films in a wide range of wavelengths from 0.14 to 33 mu m. The films at various thicknesses were deposited at different conditions on silicon or mica substrates. Considerable variation of the frequency dependent dielectric function from sample to sample was found. Detailed analysis of the dielectric functions was performed to check the Kramers-Kronig consistency, and extract the Drude parameters of the films. It was found that the plasma frequency varies in the range from 6.8 to 8.4 eV. It is suggested that this variation is related with the film density. X-ray reflectivity measurements support qualitatively this conclusion. The Casimir force is evaluated for the dielectric functions corresponding to our samples, and for that typically used in the precise prediction of the force. The force for our films was found to be 5%-14% smaller at a distance of 100 nm between the plates. Noise in the optical data is responsible for the force variation within 1%. It is concluded that prediction of the Casimir force between metals with a precision better than 10% must be based on the material optical response measured from visible to mid-infrared range.

Nanosized iron clusters investigated with in situ transmission electron microscopy

Transmission electron microscopy is employed for investigating the structural stability of nanosized iron clusters as deposited and after in situ annealing treatments under high vacuum conditions. The thin iron oxide shell that is formed around the iron clusters (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of body-centered-cubic (bcc) iron. The oxide shell breaks down upon annealing at relatively low temperatures (∼500 °C) leading to pure iron particles having a bcc crystal structure. Annealing of clusters, which are in contact, leads to their fusion and formation of larger clusters preserving their crystallographic structure and being free of any oxide shell. On the other hand, isolated clusters appear rather immobile (upon annealing). The truncated rhombic dodecahedron was found as the most probable shape of the clusters which differs from former theoretical predictions based on calculations of stable structural forms.

Magnetic versus structural properties of Co nanocluster thin films: A magnetic force microscopy study

Magnetic force microscopy (MFM) has been employed to study thin films consisting of low-energy-deposited cobalt nanoclusters. On continuous cluster layers a clear magnetic stray field pattern can be observed, although measurements on individual clusters are complicated by interference from topography. The magnetic correlation length determined from MFM images is substantially larger than the size of a single cluster. This indicates that the clusters are magnetically coupled to form stable domains associated with the formation of a correlated super-spin-glass state.

Fabrication of Co/Si nanowires by ultrahigh-vacuum scanning tunneling microscopy on hydrogen-passivated Si(100) surfaces

We have fabricated nanometer width Co/Si metal lines on Si(100) surfaces by ultrahigh-vacuum scanning tunneling microscopy (UHVSTM) based nanolithography on the hydrogen-passivated surface, combined with vapor deposition of Co at room temperature and subsequent annealing. The STM tip was used to define depassivated lines (<10 nm in width) by electron stimulated hydrogen desorption, and subsequently Co was deposited at a submonolayer coverage. Annealing of the substrate at 410 °C (just below hydrogen desorption) improves the structure of the wire due to silicidation, whereas the as-deposited wire is very granular (comparable to other materials in previous studies).