Wire Growth Direction Research Articles

Electron Tomography Reveals the Droplet Covered Surface Structure of Nanowires Grown by Aerotaxy.

For the purpose of functionalizing III-V semiconductor nanowires using n-doping, Sn-doped GaAs zincblende nanowires are produced, using the growth method of Aerotaxy. The growth conditions used are such that Ga droplets, formed on the nanowire surface, increase in number and concentrations when the Sn-precursor concentration is increased. Droplet-covered wires grown with varying Sn concentrations are analyzed by transmission electron microscopy and electron tomography, which together establish the positioning of the droplets to be preferentially on {-111}B facets. These facets have the same polarity as the main wire growth direction, [-1-1-1]B. This means that the generated Ga particles can form nucleation sites for possible nanowire branch growth. The concept of azimuthal mapping is introduced as a useful tool for nanowire surface visualization and evaluation. It is demonstrated here that electron tomography is useful in revealing both the surface and internal morphologies of the nanowires, opening up for applications in the analysis of more structurally complicated systems like radially asymmetrical nanowires. The analysis also gives a further understanding of the limits of the dopants which can be used for Aerotaxy nanowires.

Magnetoconductance correction in zinc-blende semiconductor nanowires with spin-orbit coupling

We study the effects of spin-orbit coupling on the magnetoconductivity in diffusive cylindrical semiconductor nanowires. Following up on our former study on tubular semiconductor nanowires, we focus in this paper on nanowire systems where no surface accumulation layer is formed but instead the electron wave function extends over the entire cross section. We take into account the Dresselhaus spin-orbit coupling resulting from a zinc-blende lattice and the Rashba spin-orbit coupling, which is controlled by a lateral gate electrode. The spin relaxation rate due to Dresselhaus spin-orbit coupling is found to depend neither on the spin density component nor on the wire growth direction and is unaffected by the radial boundary. In contrast, the Rashba spin relaxation rate is strongly reduced for a wire radius that is smaller than the spin precession length. The derived model is fitted to the data of magnetoconductance measurements of a heavily doped back-gated InAs nanowire and transport parameters are extracted. At last, we compare our results to previous theoretical and experimental studies and discuss the occurring discrepancies.

The Ag+ induced solution–liquid–solid growth, photoluminescence and photocatalytic activity of twinned ZnSe nanowires

Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction are synthesized in the ZnCl2–Na2SeO3–AgNO3–ethylenediamine (EN)-ethylene glycol (EG)-polyvinyl–pyrrolidone (PVP) solvothermal system at 180°C for 12 h. The twinned ZnSe nanowires have diameters of 75±10 nm and lengths of >10 micrometers, and grow along 〈111〉 direction. The role of AgNO3 in the formation of ZnSe nanowires was investigated, and an Ag+ induced solution–liquid–solid growth mechanism is also proposed to account for the conversion of microspheres assembled from ZnSe nanocrystallites into ZnSe nanowires. Compared with ZnSe microspheres, the as-prepared twinned ZnSe nanowires exhibit stronger band edge emissions of the wurtzite- and zinc-blende-structured ZnSe and lower deep defect related emission, and their photocatalytic ability is weaker than that of ZnSe microspheres. The results suggest that this simple, mild, one-step solution approach to fabricate ZnSe nanowires may be employed for the synthesis of other selenium compounds with one dimensional nanostructures, and provides opportunities for both fundamental research and technological applications.

Atomistic Design of Thermoelectric Properties of Silicon Nanowires

We present predictions of the thermoelectric figure of merit ( ZT) of Si nanowires with diameter up to 3 nm, based upon the Boltzman transport equation and ab initio electronic structure calculations. We find that ZT depends significantly on the wire growth direction and surface reconstruction, and we discuss how these properties can be tuned to select silicon based nanostructures with combined n-type and p-type optimal ZT. Our calculations show that only by reducing the ionic thermal conductivity by about 2 or 3 orders of magnitudes with respect to bulk values, one may attain ZT larger than 1, for 1 or 3 nm wires, respectively. We also find that ZT of p-doped wires is considerably smaller than that of their n-doped counterparts with the same size and geometry.

Si Nanowires as Sensors: Choosing the Right Surface

We show, using ab initio calculations based on density functional theory, that for hydrogen-passivated Si nanowires (SiNWs), the relative contribution of surface atoms to the band-edge states varies according to the way these surface atoms are bonded to the core ones. The largest influence occurs when these bonds are oriented along the wire's growth direction, which occurs either on the symmetric (001) 1 x 1 or the monohydrated (111) 1 x 1 surfaces. These results are obtained for wires grown along the [110] direction, with hexagonal cross-sections and facets corresponding to (111) and (001) planes, as observed experimentally. Their diameters range from about 10 to 35 A. On the basis of our results, we propose that particular facets should be more appropriate to be functionalized in order to build SiNW-based sensors. As an example, we have investigated the effect of NH2 adsorbed on some of these surfaces on the electronic structure of these wires.

Nanometre-sized GaAs wires grown by organo-metallic vapour-phase epitaxy

We grew GaAs wires as thin as 20 nm on a GaAs(111)B substrate using organo-metallicvapour-phase epitaxy (OMVPE), with Au as a growth catalyst. To investigate the growthcharacteristics, we compared two methods of depositing Au. In the first, Au was depositedby vacuum evaporation, and the deposition thickness was varied to form a planar Au layer.We found that an Au layer thickness of 1 nm was best for forming cylindrical shaped wires.Next, a new method of injecting Au onto an area of a few micrometres was testedusing a focused ion beam (FIB), and this method was found to be effective forgrowing wires as thin as 30–80 nm. However, the wire width did not depend on theinjected density of Au. We based our analysis of the results on an ion implantationmodel. GaAs wires with a p–n junction along the direction were formed by changing dopants from silicon to carbon during growth. Weobserved an optical emission with a peak intensity at the wavelength of 910–920 nm duringcontinuous current injection into the wires at 300 K. A spectral blue-shift in the lightemission and a polarization along the wire growth direction were also revealed at 77 K.

Electronic band structure of high‐index silicon nanowires

AbstractWe calculated the electronic properties of high‐index free‐standing silicon nanowires. [11$ \bar 2 $] nanowires are indirect semiconductors for diameters down to 0.8 nm; [110] wires have a direct band gap at the Γ‐point, but the density of states is very small at the conduction band edge. Confinement arguments show that only [001] nanowires are expected to develop a direct gap with a large density of electronic states at the band edges for diameters in the nm range. The magnitude of the gap depends strongly on the wire growth direction, which is due to the different effective confinement length and effective masses for the Γ X‐derived silicon states. Correcting for the extension of the wave functions we find our calculated energies to agree with recent scanning tunneling experiments. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Supercritical Fluid—Liquid—Solid Synthesis of Gallium Phosphide Nanowires.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Supercritical Fluid−Liquid−Solid Synthesis of Gallium Phosphide Nanowires

A new GaP nanowire synthesis method is employed to produce a high yield of single crystalline wires. Reaction at 500 °C in supercritical hexane using gold seed particles results in wires that average 20 nm in diameter and tens of micrometers in length. Some wires generate twinning faults orthogonal to the wire growth direction.

Defect-free InP nanowires grown in [001] direction on InP (001)

We report on [001]InP nanowires grown by metalorganic vapor phase epitaxy directly on (001)InP substrates. Characterization by scanning electron microscopy and transmission electron microscopy reveals wires with nearly square cross sections and a perfect zinc-blende crystalline structure that is free of stacking faults. Photoluminescence measurements of single [001] nanowires exhibit a narrow and intense emission peak at approximately 1.4eV, whereas ⟨111⟩B grown reference wires show additional broad luminescence peaks at lower energy. The origin of this uncommon wire growth direction [001] is discussed as a means of controlled formation of [00l]-oriented nanowires on (001) substrates.

Size‐Dependent Periodically Twinned ZnSe Nanowires

Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction (see Figure) have been synthesized using thermal evaporation with a Au catalyst. The periodicity of the alternating twins has a linear dependence on the diameter of the nanowires. Sharp excitonic peaks dominating the low-temperature photoluminescence spectrum of the nanowires reveal the high quality of their electronic structure despite the large unpassivated surface and interface associated with the twinned nanowire configuration.

GaAs free-standing quantum-size wires

Ultrathin GaAs wires as thin as 15–40 nm and about 2 μm long have been grown on a GaAs substrate by metal-organic vapor-phase epitaxy. The wires, which consist of whiskers, are grown between 380 and 550 °C using trimethylgallium and arsine (AsH3) as source materials. It is found that the wire growth direction is parallel to the [111] arsenic dangling-bond direction and can be perfectly controlled by the crystallographic orientation of the GaAs substrate surface. From transmission electron microscopic analysis it is revealed that the crystal structure of the wire coincides with the zinc-blende type for the growth temperature range of 460–500 °C, but it changes to the wurtzite type at 420 °C and temperatures higher than 500 °C. It is also found that the wires have a twin-type structure around the [111] growth axis for zinc blende and [0001] growth axis for wurtzite. Photoluminescence study of these wires shows that the luminescence peak energy shifts to a higher energy as the wire width decreases from 100 to about 35 nm. In terms of luminescence polarization it is confirmed that the luminescence intensity parallel to the wires is four times greater than that perpendicular to the wires. These results clearly indicate the quantum-size effect of carriers confined in the wire. As a preliminary application to devices, a p-n junction has been formed along the GaAs wire. Light emission by current injection to the p-n junction wires has been observed in continuous operation at room temperature.