Anisotropic and passivation-dependent quantum confinement effects in germanium nanowires: a comparison with silicon nanowires

Electronic structures of hydrogen-passivated germanium nanowires (GeNWs) along the [100], [110], [111], and [112] directions are studied by using the density functional theory within the generalized gradient approximation. The band gaps of the fully relaxed GeNWs along the [100], [110], and [111] directions are all direct at the smaller sizes, while those of the wires along the [112] direction remain indirect. The magnitude of the band gaps of the GeNWs for a given size approximately follows the order of E(g)[100] > E(g)[111] > E(g)[112] > E(g)[110]. Compared with silicon nanowires, GeNWs exhibit stronger quantum confinement effects. Replacement of H by the more stable ethine group is found to lead to a weakening of the quantum confinement effects of GeNWs.     

Temperature-dependent Raman scattering of silicon nanowires

Silicon nanowires with narrowly distributed diameters of 20-30 nm have been fabricated by chemical vapor deposition on an anodized aluminum oxide (AAO) substrate. The first-order and second-order Raman scatterings of the silicon nanowires have been studied in a temperature range from 123 to 633 K. Both of the first-order and second-order Raman peaks were found to shift and broaden with increasing temperature. The experimental results were analyzed by combining the phonon confinement effect, anharmonic phonon processes and lattice stress effect. It was found that the intensities of the first-order and second-order Raman bands have different dependences on temperature. The value of relative intensities I(2TA)int/I(2TO)int for silicon nanowires was found to be larger than that of bulk silicon, and increase with rising measurement temperature. We ascribe this phenomenon to the participation of phonons with a large wave vector value of Raman scattering caused by both the phonon confinement effect and the temperature effect.     

Retention measurements of nanofiltration membranes with electrolyte solutions

Retention measurements with single salt solutions of CaCl₂, NaCl and Na₂SO₄ revealed that the rejection mechanism of commercial polymeric nanofiltration membranes investigated in this study may be divided into two categories:

• 1.Membranes for which Donnan exclusion seems to play an important role.
• 2.Membranes for which retention is determined by both Donnan exclusion and size effects.

In category 1 both positively and negatively charged membranes were found.Ceramic γ-Al₂O₃ ultrafiltration membranes with a pore size of 3 nm showed a same type of salt retention behavior as the positively charged polymeric membranes.The extended Nernst–Planck equation in combination with the Donnan equilibrium has been used to model the flux-retention experiments for the salt solutions. The numerical calculations resulted in a good agreement with experimental data and acceptable values for the fixed charge densities have been determined. The effective membrane thicknesses calculated were higher than those observed by scanning electron microscopy.     

Comparison between electrochemical and optoelectrochemical impedance measurements for detection of DNA hybridization

The principles of the electrochemical and optoelectrochemical impedance measurements on bare electrolyte/dielectric/semiconductor structures are described. The analysis of the experimental curves allows access to several indications concerning the electrical behavior of such structures. The application of these techniques to follow the electrical behavior of structures modified with two biological systems was investigated. The antibody/antigen recognition did not change the surface charge and, therefore, did not affect the impedance curves with respect to the applied potential. By contrast, the hybridization of two complementary DNA strands on the surface of the structure induced a variation of flat band potential of the semiconductor leading to a shift of impedance curves along the potential axis. This means that it is possible to detect directly the DNA hybridization without the use of labeled probes. The use of light allows the surface to be probed locally. In the future, the application of this technique for direct detection of hybridization on DNA chips should be possible.     

Particle-based detection of DNA hybridization using electrochemical stripping measurements of an iron tracer

Two particle-based procedures for monitoring DNA hybridization based on electrochemical stripping detection of an iron tracer are described. The first protocol involves probes labeled with gold-coated iron core-shell nanoparticles, while the second route relies on detecting the iron content of magnetic-sphere tags. In both cases, the captured iron-containing particles are dissolved following the hybridization, and the released iron is quantified by cathodic-stripping voltammetry in the presence of the 1-nitroso-2-naphthol ligand and a bromate catalyst. Both protocols offer high sensitivity, a well-defined concentration dependence, and minimal contributions from non-complementary nucleic acids. The iron-containing particle signal amplifiers thus represent a very useful addition to the arsenal of metal tracers employed in electrical bioassays.     

Formation of silicon carbide nanotubes and nanowires via reaction of silicon (from disproportionation of silicon monoxide) with carbon nanotubes

One-dimensional silicon-carbon nanotubes and nanowires of various shapes and structures were synthesized via the reaction of silicon (produced by disproportionation reaction of SiO) with multiwalled carbon nanotubes (as templates) at different temperatures. A new type of multiwalled silicon carbide nanotube (SiCNT), with 3.5-4.5 A interlayer spacings, was observed in addition to the previously known beta-SiC (cubic zinc blende structure) nanowires and the biaxial SiC-SiO(x) nanowires. The SiCNT was identified by high-resolution transmission microscopy (HRTEM), elemental mapping, and electron energy loss spectroscopy (EELS). The multiwalled SiCNT was found to transform to a beta-SiC crystalline structure by electron beam annealing under TEM.     

Direct determination of single-to-double stranded DNA ratio in solution using steady-state fluorescence measurements

We report a simple and rapid method for quantitation of single-to-double stranded (ss : ds) DNA ratios in solution, using steady-state measurements of fluorescence from two simultaneously excited intercalated dyes; the ratio of fluorescence intensities from PicoGreen (525 nm) and ethidium bromide (610 nm) is directly proportional to the ss : ds DNA ratio.     

Synthesis of silicon nanowires and nanoparticles by arc-discharge in water

Si nanowires of diameters 5-20 nm and nanoparticles of approximately 4 nm were synthesized by a simple arc-discharge method in water. The TEM analysis reveals that the growth direction of the observed Si nanowires is parallel to the {111} crystal planes.     

Photoelectrochemistry of silicon in HF solution

Photoelectrochemical, photoelectrocatalytic, and electrochemical processes of silicon anodic oxidation and hydrogen evolution in aqueous HF solution are discussed in terms of thermodynamic stability of Si, oxides SiO, SiO₂, and Si surface hydrides. It is shown that photoelectrochemical oxidation of n-type low-resistivity silicon to SiO₂ is catalyzed by Si $^{+}$ photo-hole formation, whereas in the case of p-type Si, the feasibility of this reaction is predetermined by p-type conductivity. It is suggested that anodic oxidation of Si goes through the stage of SiO oxide formation and its subsequent oxidation to SiO₂. Such mechanism accounts for chemical inertness of Si phase in HF solutions as well as for selective, anisotropic, and isotropic etching of Si within E ranges from $-0.5$ to 0.35 V, $0.35-0.8~V,$ and $E > 0.8$ V, respectively. Hydrogen evolution reaction on Si surface proceeds at very large overpotential ( $\geq 0.5$ V) through the stage of surface Si hydride formation: $\mathrm {Si + H_{2}O + e^{-} \rightarrow (SiH)_{surf} + OH^{-}}$ (the rate determining step) and $\mathrm {(SiH)_{surf} + H_{2}O + e^{-} \rightarrow Si + H_{2} + OH^{-}}$ . Illumination-related effects of surface reactions relevant to selective and anisotropic etching and nano/micro-structuring of Si surface are discussed.     

Surface modification of silicon nanowires via copper-free click chemistry

A two-step process based on copper-free click chemistry is described, by which the surface of silicon nanowires can be functionalized with specific organic substituents. A hydrogen-terminated nanowire surface is first primed with a monolayer of an α,ω-diyne and thereby turned into an alkyne-terminated, clickable platform, which is subsequently coupled with an overlayer of an organic azide carrying the desired terminal functionality. The reactive, electron-deficient character of the employed diyne enabled a quantitative coupling reaction at 50 °C without metal catalysis, which opens up a simple and versatile route for surface functionalization under mild conditions without any potentially harmful additives.     

Formation of gold-silver nanowires in thin surfactant solution films

Thin, long gold/silver nanowires were grown on substrates in thin surfactant solution films. This growth process occurred exclusively in thinning aqueous films as the water evaporated, and elongated surfactant template structures were formed. The nanowire growth depended on the presence of a relatively high concentration of silver ions (typical Ag:Au mole ratio of 1:1). Tuning the pH value to about 5 in the growth solution was crucial for the nanowire growth. Further development of this process may lead to a simple wet chemical technique for the fabrication of relatively uniform arrays of metal nanowires on surfaces.     

Kinetic study of DNA/DNA hybridization with electrochemical impedance spectroscopy

The hybridization of immobilized oligonucleotides probe strands with solution phase targets is the underlying principle of microarray-based techniques for the analysis of DNA variation. To study the kinetics of DNA/DNA hybridization, target DNA is often prior labeled with markers. A label-free method of electrochemical impedance spectra (EIS) for study the hybridization in process was reported. The Langmuir model was used to determine the association rate constant (K_on), the dissociation rate constant (K_off) and the affinity rate constant (K_A), for perfect matched DNA hybridization. The results show that, EIS is a successful technique possessing high effectivity and sensitivity to study DNA/DNA hybridization kinetics. This work can provide another view on EIS for the studying of DNA/DNA hybridization.     

Karman vortex street assisted patterning in the growth of silicon nanowires

A Karman vortex street was employed to pattern catalysts and grow nanomaterial arrays, which were made of a disk-like superstructure built of silicon nanowires; there also existed nanowires connected with the disks.     

Simple, one-step synthesis of gold nanowires in aqueous solution

A simple procedure to synthesize gold nanowires based on the reduction of hydrogen tetrachloroaurate by 2-mercaptosuccinic acid in aqueous solution is presented. This procedure requires no additional capping or reduction agent and produces wires with an apparent curly morphology several micrometers in length with diameters as thin as 15 nm. Some of the wires produced end in a ribbonlike structure, finally terminated by a flat triangular prism. Investigations by scanning electron microscopy, transmission electron microscopy (bright and dark field), scanning transmission electron microscopy, and atomic force microscopy as well as conductivity measurements indicate fully connected, polycrystalline gold objects.     

A universal gauge for thermal conductivity of silicon nanowires with different cross sectional geometries

By using molecular dynamics simulations, we study thermal conductivity of silicon nanowires (SiNWs) with different cross sectional geometries. It is found that thermal conductivity decreases monotonically with the increase of surface-to-volume ratio (SVR). More interestingly, a simple universal linear dependence of thermal conductivity on SVR is observed for SiNWs with modest cross sectional area (larger than 20 nm(2)), regardless of the cross sectional geometry. As a result, among different shaped SiNWs with the same cross sectional area, the one with triangular cross section has the lowest thermal conductivity. Our study provides not only a universal gauge for thermal conductivity among different cross sectional geometries, but also a designing guidance to tune thermal conductivity by geometry.     

DNA hybridization detection with water-soluble conjugated polymers and chromophore-labeled single-stranded DNA

A sensor is provided that detects single-stranded deoxyribonucleic acid (ssDNA) with a specific base sequence. The ssDNA sequence sensor comprises an aqueous solution containing a cationic water-soluble conjugated polymer [in this case, poly(9,9-bis(6'-N,N,N-trimethylammonium)-hexyl)-fluorene phenylene), 1] with a ssDNA labeled with a dye (in this case, fluorescein). The emission of light from the sensor solution with the wavelength characteristic of the probe oligonucleotide indicates the presence of ssDNA with a specific base sequence complementary to that of the probe ssDNA-fluorescein. Maximum energy transfer from 1 to the signaling chromophore occurs when the ratio of polymer chains to DNA strands is approximately 1:1. Energy transfer from 1 results in a fluorescein emission that is more intense than that observed by direct excitation of the chromophore. Furthermore, the decrease in energy transfer upon addition of electrolyte indicates that electrostatic forces dominate the interactions between 1 and DNA.