Ants Lõhmus

Force interactions and adhesion of gold contacts using a combined atomic force microscope and transmission electron microscope

Force interactions and adhesion of gold contacts using a combined atomic force microscope and transmission electron microscope

Preparation of smooth siloxane surfaces for AFM visualization of immobilized biomolecules

Development of scanning probe microscopy (SPM) techniques during past decade has made possible to study bioprocesses at a molecular level. Common protocols for immobilization of biomolecules for SPM studies are based on their adsorption to alkylsiloxane monolayers on hydroxylated surfaces. The same procedure is also widely used for DNA chip and biosensor fabrication. For SPM studies of immobilized biomolecules smooth carrier surfaces are mandatory. Several studies refer to the lack of reproducibility in formation of smooth silanized surfaces. In this work a new method based on carrier treatment with linearly polymerized 3-aminopropyltrimethoxysilane for preparation of reproducibly smooth silanized surfaces is proposed.

Characterization of glucose oxidase immobilization onto mica carrier by atomic force microscopy and kinetic studies

Glucose oxidase (E.C 1.1.3.4) immobilized onto activated surface of mica was analyzed by enzymatic kinetics and visualization with atomic force microscopy (AFM). The activity of the immobilized enzyme decreased with the decrease of concentration of gamma-aminopropyltrimethoxysilane used for the first step of activation of mica, while AFM analysis showed similar homogeneous filling of the surface with the enzyme. The comparison of enzyme activity with its surface filling revealed that there has to be additional vertical structures, which cannot be visualized by the methods of AFM. The simultaneous decrease of the silanizing agent and the concentration of the enzyme led to molecular resolution for the enzyme on the surface of mica. This allows to propose the described method also for analyzing other surfaces of solid materials with coupled biomolecules.

Shape Restoration Effect in Ag–SiO2 Core–Shell Nanowires

The combination of two different materials in a single composite core-shell heterostructure can lead to improved or even completely novel properties. In this work we demonstrate the enhancement of the mechanical properties of silver (Ag) nanowires (NW) achieved by coating them with a silica (SiO2) shell. In situ scanning electron microscope (SEM) nanomechanical tests of Ag-SiO2 core-shell nanowires reveal an improved fracture resistance and an electron-beam induced shape restoration effect. In addition, control experiments are conducted separately on uncoated Ag NWs and on empty SiO2 shells in order to gain deeper insight into the peculiar properties of Ag-SiO2. Test conditions are simulated using finite-element methods; possible mechanisms responsible for the shape restoration and the enhanced fracture resistance are discussed.

Pinching of alkoxide jets—a route for preparing nanometre level sharp oxide fibres

The pinching phenomenon is used to shape jets of Sn(OBu)4 based viscous oligomeric melts into nanometre range sharp oxide needles. The influence of viscosity of the liquid, humidity of surrounding environment and pulling speed on formation of the needles is investigated. Optimal conditions enable reproducible preparation of needles that have tip radii down to 15–25 nm, i.e. in the range that is of considerable interest for many nanotechnological applications.

Alkoxide-based precursors for direct drawing of metal oxide micro- and nanofibres

Abstract The invention of electrospinning has solved the problem of producing micro- and nanoscaled metal oxide fibres in bulk quantities. However, until now no methods have been available for preparing a single nanofibre of a metal oxide. In this work, the direct drawing method was successfully applied to produce metal oxide (SnO2, TiO2, ZrO2, HfO2 and CeO2) fibres with a high aspect ratio (up to 10 000) and a diameter as small as 200 nm. The sol–gel processing includes consumption of precursors obtained from alkoxides by aqueous or non-aqueous polymerization. Shear thinning of the precursors enables pulling a material into a fibre. This rheological behaviour can be explained by sliding of particles owing to external forces. Transmission (propagation) of light along microscaled fibres and their excellent surface morphology suggest that metal oxide nanofibres can be directly drawn from sol precursors for use in integrated photonic systems.

A variable temperature scanning SQUID microscope

We present a design of a scanning SQUID microscope (SSM) operating in a temperature range between about 5 K (2 K with pumping) and 100 K.

Micro- and nanoscale structures by sol-gel processing

An overview of the practical output of sol-gel chemistry is demonstrated from the viewpoint of its contribution to the design of 1D and 2D materials. Different structures like thin films, fibres, needles, microtubes, -patterns and -rolls are discussed. In most cases, the advantages of sol-gel processing feasibility and cost-effectiveness are clearly evident. Accompanied by flexibility of production and good quality of the formed structures, the method offers interesting possibilities for practical applications such as sensors, opto-electronic devices, scintillators, etc.

Synthesis of ZrC-TiC Blend by Novel Combination of Sol-Gel Method and Carbothermal Reduction

Titanium Carbide and zirconium carbide binary mixtures (ZrC – TiC) were prepared by sol-gel processing followed by carbothermal reduction. Solution-based synthesis was applied to achieve a molecular dispersion of the reactants. Titanium – (IV) – and zirconium – (IV)butoxides were used as metal sources and hydroquinol was used as carbon source for precursor preparation. Mixture of titanium and zirconium carbides was synthesized by carbothermal reduction at temperatures up to 1500 o C in argon and vacuum environment. The structural transformation of the polymeric materials into the carbides was characterized by SEM, X – ray diffraction analysis and Raman spectroscopy. Characterizations of heat treated samples at 800 o C to 1500 o C in argon and vacuum has showed that the carbothermal reduction of the binary solid carbide mixture (ZrC – TiC) polymeric precursor began in vacuum at lower temperature (1100 o C) than in argon environment.

High Tc SQUID systems for magnetophysiology

Abstract Magnetophysiology is the use of a superconducting quantum interference device (SQUID) based instrument to detect neuromagnetic fields evoked by electrical stimulation of brain tissue slices. In this paper we show that a SQUID based on high temperature superconductors (HTSs) would have considerable advantages over a low T c device in this application. We construct a model of electrical activity in a hippocampal brain slice, which enables the neuromagnetic field to be determined as a function of position and distance from the tissue. We then describe the design of HTS SQUID systems for magnetophysiology and the two styles of system we are developing. Finally we use our model to show that an existing HTS SQUID magnetometer would give a superior signal to noise ratio compared to a low T c system for the hippocampal brain slice preparation at least.