Raimonds Poplausks

Fabrication of ultra thin anodic aluminium oxide membranes by low anodization voltages

Formation of ultrathin anodised aluminium oxide (AAO) membranes with high aspect ratio by Al anodization in sulphuric and oxalic acids at low potentials was investigated. Low anodization potentials ensure slow electrochemical reaction speeds and formation of AAO membranes with pore diameter and thickness below 20 nm and 70 nm respectively. Minimum time necessary for formation of continuous AAO membranes was determined. AAO membrane pore surface was covered with polymer Paraloid B72TM to transport it to the selected substrate. The fabricated ultra thin AAO membranes could be used to fabricate nanodot arrays on different surfaces.

Optical Scattering by Dense Disordered Metal Nanoparticle Arrays

We address the optical properties of dense disordered yet well-separated metal nanoparticle arrays produced by physical vapor deposition through anodized aluminum oxide membrane masks. Using variations in synthesis parameters, the particle diameters vary from 14 to 50 nm and average center separation from 45 to 112 nm. Ag nanoparticle arrays with no long-range periodicity exhibit apparently random formation of high-intensity depolarized regions relative to orientation of incident electric field. We analyze this behavior numerically using coupled dipole model and explain the contrast formation in recorded scattering images.

Vapor–solid synthesis and enhanced thermoelectric properties of non-planar bismuth selenide nanoplates on graphene substrate

In this work, stoichiometric separate bismuth selenide (Bi2Se3) nanoplates and continuous Bi2Se3 coatings are synthesized on graphene substrate by catalyst-free vapor–solid deposition method. The orientation of synthesized nanoplates relative to the substrate surface varies from heteroepitaxial (planar) to oriented under different angles (non-planar). The non-planar growth of the nanoplates was achieved for the first time by short-term carrier inert gas flow in certain temperature interval of the synthesis process. The crystallographic growth directions of non-planar nanoplates were determined from HRTEM images as well as estimated from the slope angles of non-planar nanoplates. Bi2Se3 coatings consisting of combination of planar and non-planar nanoplates exhibit significantly enhanced in comparison with coating consisting of only planar coalescent Bi2Se3 nanoplates thermoelectric properties. Demonstrated graphene/Bi2Se3/graphene devices may find applications in thermoelectric and photo-detection sensors.

Optical properties of thin metal films with nanohole arrays on porous alumina–aluminum structures

A multilayer system is formed by the deposition of a 10–35 nm thin Au or Ag film with 18–25 nm diameter holes on 75–280 nm thick layers of porous anodized aluminum oxide (AAO) supported by a bulk sheet of aluminum. We present a detailed study of system parameters, which influence the optical response, including the porosity, metal layer thickness and crystallographic orientation of the Al substrate. The spectral properties are mainly governed by the interference of the reflections from the Al substrate and the thin metal film separated by the AAO layer. An enhanced plasmonic attenuation component near 650 nm for the Au films with holes can be observed when the interferometric anti-reflection condition is fulfilled close to this particular wavelength.

Electrochemically etched sharp aluminium probes with nanoporous aluminium oxide coatings: demonstration of addressed DNA delivery

Electrochemical etching of metal wires is widely used to fabricate sharp probes for use in scanning tunnelling microscopy. In this work an electrochemical fabrication method for sharp aluminium probes coated with nanoporous anodised aluminium oxide (AAO) layer is described. The method presented here involves simultaneous anodisation and etching of aluminium wires. The probe apex radius as well as the nanopore length and diameter depend on the etching mode, which could be direct current (DC), alternating current (AC), or pulsed voltage mode (PVM). The probes, coated with a nanoporous AAO layer, were used to demonstrate addressed DNA delivery.

Measurements of SiO2 glass surface parameters by methods of microscopy

In this research we compare chemical and plasma treatment methods for surface of SiO2 glass. For chemical treatment of surface tequila and alcohol were used but for plasma treatment - Ar+As and Ar+Se plasmas. Surface topography was analyzed using atomic force microscope. Comparison of chemical and plasma treatment methods shows that surface treated with plasma is smoother. Because of their various chemical compositions tequila and alcohol show different results.

Variable Thickness Porous Anodic Alumina/Metal Film Bilayers for Optimization of Plasmonic Scattering by Nanoholes on Mirror

Continuously variable thickness porous anodic aluminum oxide (PAAO) films were obtained using electrochemical oxidation of bulk aluminum sheet while both electrodes were simultaneously withdrawn from the electrolyte solution. The thickness gradient was controlled by the withdrawal rate (1–10 mm/min range) and thickness variation demonstrated from below 50 nm to above 1 micrometer. The thickness increased linearly with the sample lateral coordinate, whereas the nanopore structure (diameter and interpore distance) remained unchanged. Effects of the initial pore growth and capillary forces are discussed. The presented method can be used for tuning optimal PAAO thickness for optical and other applications as exemplified by finding maximum plasmonic scattering in structured Al–PAAO–Au multilayers. Enhanced scattering from porous gold film separated by a specific-thickness PAAO layer from aluminum mirror surface is demonstrated.

Polarized interference imaging of dense disordered plasmonic nanoparticle arrays for biosensor applications

We report on light scattering by dense short-range ordered gold and silver nanoparticle arrays with 25 nm diameter and 50 nm center separation produced by masked deposition through anodized aluminum oxide membranes. Local resonant regions are formed, which scatter light with polarization components perpendicular to the incident wave due to electromagnetic coupling between particles at random angles. The observed cross-polarized far-field images have a granular structure that morphs in response to environmental variations in the article near field. We quantify the changes in the recorded images by 2D correlation matrix calculation and demonstrate the application of this approach to biomolecular sensing by using various concentrations of cysteine solution as a model system. The presented method may potentially compete with colorimetric sensor techniques since the detection setup does not require any spectroscopic instruments.