O. Teschke

Interfacial aqueous solutions dielectric constant measurements using atomic force microscopy

Abstract The exchange of the volume of a region of the electric double layer of a mica surface immersed in aqueous solutions, with a dielectric constant ϵ DL , by a nanosized radius tip, with a dielectric constant ϵ Tip , is responsible for the repulsion at large distances from the surface (starting at ∼100 nm, diffuse layer) and followed by an attraction when the tip is immersed in the inner layer (∼10 nm). The calculated dielectric constant as a function of the distance to the charged interface obtained by fitting the force versus distance curves, allows the mapping of the inner layer dielectric constant profiles with a nanometer resolution.

Dielectric exchange: The key repulsive or attractive transient forces between atomic force microscope tips and charged surfaces

Attractive as well as repulsive forces between electrically neutral tips and charged surfaces are measured directly with an atomic force microscope. The exchange of the volume of a region of the electric double layer of a mica surface immersed in water with a dielectric constant eDL by the tip with a dielectric constant eTip is responsible for the repulsion at large distances from the surface (starting at ∼100 nm, diffuse layer) and followed by an attraction when the tip is immersed in the Stern layer (∼2 nm). The force versus distance measured curves for high approaching velocities (⩾30 μm/s) were fitted to the expression of the dielectric exchange force derived by using a continuum theory for a sharpened pyramidal tip immersed in a spatially variable dielectric constant double-layer electric field. The dielectric exchange effect gives a consistent description of the force acting on the tip by assuming a double-layer region of disorganized water with eDL∼80 at distances far away from the surface followed...

Time resolved photoluminescence of porous silicon: Evidence for tunneling limited recombination in a band of localized states

Time resolved photoluminescence of porous silicon at room temperature was measured for several emission energies under 2 ns nitrogen laser excitation. For each emission energy studied there is a broad distribution of lifetimes extending over a few decades. The mean value of the distribution varies with the emission energy, from 3 (2.77 eV) to 50 μs (1.96 eV). The results can be explained by assuming a tunneling limited recombination mechanism between bands of localized states. We associate this behavior with a superficial disordered Si:O:H compound rather than with quantum confinement effects.

Viscous drag effect on imaging of linearized plasmid deoxyribonucleic acid in liquid medium with the atomic force microscope

In many attempts to image biomolecules like deoxyribonucleic acid with the atomic force microscope, the apparent width of the molecules exceeds the expected width as obtained by x-ray diffraction. This increase in size was explained by a geometrical tip convolution, but the increased width seems to persist despite improvements to the tip. Experimental evidence is shown that part of this increase is due to the liquid drag force when molecules are imaged under liquid. The liquid drag force is calculated using standard fluid dynamics where the tip motion in the liquid is modeled by the relative motion of a cylinder through a constant velocity fluid. The Reynold’s number for the experimental configuration is smaller than 1, characterizing a laminar flow and the calculated drag force is 80 pN, which is in agreement with the experimentally measured force for ethanol and relative tip velocity of 100 μm/s. Both the viscous drag force and the apparent width increase may be modeled by a vk dependence, where v is th...

Structure imaging by atomic force microscopy and transmission electron microscopy of different light emitting species of porous silicon

The complex pattern of the nanowire skeletons of different light emitting porous silicon structures is investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Diffraction lines and dark field images are used to identify and determine the crystallite specimen long range order. TEM images give the size and particle orientation, and AFM images show a three‐dimensional pattern formed by an interconnecting skeleton of particles. Near infrared photoluminescent porous silicon (0.006 Ω cm) structures show a skeleton of nanosized silicon aggregates which form domains of spatially oriented crystallites. For red photoluminescent samples (4.9 Ω cm) the electron diffraction spots are discontinuously split into tiny intensity maxima. The diameter of the wire structure forming porous silicon as measured by TEM allows us to estimate the distortion of the AFM images due to the finite size of the tip radius. A critical angle α0=2 arctan[K/(1−K)]1/2, where K is the ratio of the height of th...

Dielectric exchange force: a convenient technique for measuring the interfacial water relative permittivity profile

Water relative permittivity profiles perpendicular to mica surfaces have been measured by atomic force microscopy using the force acting on uncharged tips when immersed in the mica double-layer. This force is modelled by the gradient of the electrostatic energy variation (dielectric exchange force) involved in the immersion of the tip with a relative permittivity eTip in the double layer region with eDL. The measured variable permittivity profile starting at e≈4 at the interface and increasing to e≈80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of mica interfacial charges. Changes in water polarization are therefore responsible for the hydration or structural forces acting on the tips immersed in the inner double layer. Corroboration for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal-coated tips (eTip≈∞) are immersed in the mica double layer. Support for the change in the water relative permittivity at the interface is given by measurements of only a repulsive force component when silicon nitride and silicon tips are immersed in solvent where there is no interaction between the mica surface and the solvent and, consequently, no solvent structuring at the interface.

Amorphous alloys as anodic and cathodic materials for alkaline water electrolysis

Abstract The general tendency of operating modern industrial electrolyzers at high current densities has stimulated research on materials with improved catalytic properties for the hydrogen and oxygen evolution reaction in alkaline water electrolyzers. The performances of electrocatalytic materials have usually been improved by increasing the ratio between the real and the apparent surface area. A less common way of improving the electrocatalytic activity is by modifying the microstructure of the alloys. In this paper we describe the electrocatalytic properties of various amorphous alloys. Among those investigated, amorphous FeNiSiB exhibit oxygen potential vs Hg HgO of around 600 mV at 300 mA/cm2 in 30 wt.% KOH at 60 °C. Polycrystalline nickel was less electroactive, indicating that the amorphous state is a better precursor for the formation of an active surface layer. In the alloys tested, metal-metalloid glasses are chemically unstable for high concentrations of iron and the electrode overvoltage decreases with an increase of iron. Consequently, corrosion resistance and electrode performance were found to have opposite dependence on iron concentration.

Visualization of nanostructured porous silicon by a combination of transmission electron microscopy and atomic force microscopy

The observation of red‐, near‐infrared, and nonphotoluminescent porous silicon structures both by transmission electron microscopy as well as by atomic force microscopy provides a consistent structural picture of the species responsible for the visible luminescence observed in these samples. The topography of the tops are seen in detail by atomic force microscopy, while the valleys are better observed by transmission electron microscopy; the observation techniques are therefore complementary. By measuring the apex radius of curvature of the atomic force microscope tip, it is possible to determine the porous silicon particle size by simple geometrical arguments. For red‐photoluminescent porous silicon, the measured particle diameter using atomic force microscopy was 55 A and the particle height 4 A. The corrected particle diameter of 17 A is in close agreement with the 15 A value measured by transmission electron microscopy.

Nanosize structures connectivity in porous silicon and its relation to photoluminescence efficiency

Transmission electron microscopy is used to reveal the existence of an interconnected nanosize structure in porous silicon films. The interconnections of this nanostructure determine the photoexcited electron‐hole pair separation and consequently the luminescence efficiency of the material. Efficient photoluminescence is obtained from structures which shows no connectivity.

Electrostatic response of hydrophobic surface measured by atomic force microscopy

The arrangement of water molecules at aqueous interfaces is an important question in material and biological sciences. We have measured the force acting on neutral tips as a function of the distance to hydrophobic silicon surfaces and cetyltrimethylammonium bromide monolayers covering mica surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer) which is generated by the reorientation of water molecular dipoles. The exchange of a volume of this region with a dielectric permittivity (eint) by the tip with a dielectric permittivity (etip) is responsible for the tip attraction when it is immersed in the polarization (hydration) layer. Variable permittivity profiles starting at e≈11 at the interface and increasing to e=80 about 10 nm from hydrophobic silicon surfaces and about 50 nm from cetyltrimethylammonium bromide monolayer covering mica surfaces were measured.