Quantum Confinement and Electronic Properties of Rutile TiO2 Nanowires

The quantum confinement effect, electronic properties, and optical properties of TiO2 nanowires in rutile structure are investigated via first-principles calculations. We calculate the size- and shape-dependent band gap of the nanowires and fit the results with the function E-g = E-g(bulk) + beta/d(alpha). We find that the quantum confinement effect becomes significant for d < 25 angstrom, and a notable anisotropy exists that arises from the anisotropy of the effective masses. We also evaluate the imaginary part of the frequency-dependent dielectric function [epsilon(2)(omega)] within the electric-dipole approximation, for both the polarization parallel [epsilon(parallel to)(2)(omega)] and the perpendicular [epsilon 1/2(omega)] to the axial (c) direction. The band structure of the nanowires is calculated, with which the fine structure of epsilon(parallel to)(2)(omega) has been analyzed.

Optical properties of boron-doped Si nanowires

Raman scattering and photoluminescence (PL) of boron-doped silicon nanowires have been investigated. Raman spectra showed a band at 480 cm(-1), indicating that the crystallinity of the nanowires was suppressed by boron doping. PL taken from B-doped SiNWS at room temperature exhibited three distinct emission peaks at 1.34, 1.42. and 1.47 eV and the PL intensity was much stronger than that of undoped SiNWS. The increased PL intensity should be very profitable for nano-optoelectronics. (C) 2004 Elsevier B.V. All rights reserved.

Hexagonal selenium nanowires synthesized via vapor-phase growth

Hexagonal Se nanowires were synthesized using a simple vapor-phase growth with the assistance of the silicon powder as a source material, which turned out to be very important in the growth of the Se nanowires. The morphology, microstructure, and chemical compositions of the nanowires were characterized using various means (XRD, SEM, TEM, XPS, and Raman spectroscopy). The possible growth mechanism of the Se nanowires was explained. The as-grown Se nanowires may find wide applications in biology and optoelectronics.

Luminescence emission originating from nitrogen doping of beta-Ga2O3 nanowires

Nitrogen-doped beta-Ga2O3 nanowires (GaO NWs) were prepared by annealing the as-grown nanowires in an ammonia atmosphere. The optical properties of the nitrogen-doped GaO NWs were studied by measurements of the photoluminescence and phosphorescence decay at the temperature range between 10 and 300 K. The experimental results revealed that nitrogen doping in GaO NWs induced a novel intensive red-light emission around 1.67 eV, with a characteristic decay time around 136 mus at 77 K, much shorter than that of the blue emission (a decay time of 457 mus). The time decay and temperature-dependent luminescence spectra were calculated theoretically based on a donor-acceptor pair model, which is in excellent agreement with the experimental data. This result suggests that the observed novel red-light emission originates from the recombination of an electron trapped on a donor due to oxygen vacancies and a hole trapped on an acceptor due to nitrogen doping.

Synthesis and optical properties of europium-doped ZnS: Long-lasting phosphorescence from aligned nanowires

Quasi-aligned Eu2+-doped wurtzite ZnS nanowires on Au-coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [0110] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu2+ ions form an isoelectronic acceptor level and yield a high density of bound excitions, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co-dopant Cl- ions can serve not only as donors, producing a donor-acceptor pair transition with the Eu2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu2+, and yield long-lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal-field strength around Eu2+. As a result, not only have an enhanced Eu2+ -4f(6)5d(1)-4f(7) intra-ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu2+ related emissions are shifted to shorter wavelengths.

Synthesis of GaN nanowires and nano-pyramids in a two-hot-boat chemical vapor deposition system via an in-doping technique

A two-hot-boat chemical vapor deposition system was modified from a thermal evaporation equipment. This system has the advantage of high vacuum, rapid heating rate and temperature separately controlled boats for the source and samples. These are in favor of synthesizing compound semiconducting nano-materials. By the system, we have synthesized high-quality wurtzite single crystal GaN nanowires and nanotip triangle pyramids via an in-situ doping indium surfactant technique on Si and 3C-SiC epilayer/Si substrates. The products were analyzed by x-ray diffraction, field emission scanning electron microscopy, highresolution transmission electron microscopy, energy- dispersive x-ray spectroscopy, and photoluminescence measurements. The GaN nanotip triangle pyramids, synthesized with this novel method, have potential application in electronic/ photonic devices for field-emission and laser.

Artificial nanograting woven by self-assembled nanowires

We report on a new simple route to realize a high resolution nanograting. By adopting an InAlGaAs matrix and strain-compensated technique, we have proved that a uniform self-assembled InAs nanowire array can be fabricated by molecular beam epitaxy (MBE). A nanograting woven by self-assembled semiconductor nanowires shows a conspicuous diffraction feature. The good agreement between the theoretical and experimental values of diffraction peak positions indicates that a uniform nanowire array is a promising nanograting. This simple one-step MBE growth method will open exciting opportunities for the field of clever optics design.

Raman scattering from GaP nanowires

A complete Raman study of GaP nanowires is presented. By comparison with the Raman spectra of GaP bulk material, microcrystals and nanoparticles, we give evidence that the Raman spectrum is affected by the one-dimensional shape of the nanowires. The Raman spectrum is sensitive to the polarization of the laser light. A specific shape of the overtones located between 600 and 800 cm(-1) is actually a signature of the nanowires. Some phonon confinement and thermal behavior is also observed for nanowires.

Raman scattering and photoluminescence studies of Zn1-xMnxO nanowires via vapor phase growth

In this paper, we investigated the Raman scattering and photoluminescence of Zn1-xMnxO nanowires synthesized by the vapor phase growth. The changes of E-2(High) and A(1(LO)) phonon frequency in Raman spectra indicate that the tensile stress increases while the free carrier concentration decreases with the increase of manganese. The Raman spectra exited by the different lasers exhibit the quantum confinement effect of Zn1-xMnxO nanowires. The photoluminescence spectra reveal that the near band emission is affected by the content of manganese obviously. The values Of I-UV/G decrease distinctly with the manganese increase also demonstrate that more stress introduced with the more substitution of Mn for Zn.

Surface morphology control of strained InAs/GaAs(331)A films: From nanowires to island-pit pairs

We have studied the effect of molecular beam epitaxy growth conditions on the surface morphology of strained InAs/GaAs(331)A films. Our results reveal that InAs nanowires aligned along the [1 (1) over bar0] direction are formed under As-rich conditions, which is explained by the effect of anisotropic buffer layer surface roughing. Under In-rich conditions, however, the surface morphology of the InAs layers is characterized by a feature of island-pit pairs. In this case, cooperative nucleation of islands and pits can lower the activation barrier for domain growth. These results suggest that the surface morphology of strained InAs layers is highly controllable. (C) 2005 American Institute of Physics.

Boron-doped silicon nanowires grown by plasma-enhanced chemical vapor deposition

Boron-doped ( B-doped) silicon nanowires have been successfully synthesized by plasma-enhanced chemical vapor deposition (PECVD) at 440degreesC using silane as the Si source, diborane( B2H6) as the dopant gas and An as the catalyst. It is desirable to extend this technique to the growth of silicon nanowire pn junctions because PECVD enables immense chemical reactivity.

Electronic structure of paramagnetic In1-xMnx as nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In1-xMnxAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k(z) (k(z) is the wave vector in the wire direction), and the spin-splitting bands cross at k(z) = 0, whose k(z)-positive part and negative part are symmetrical. A proper magnetic field makes the k(z)-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k(z). In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.

High and electric field tunable Curie temperature in diluted magnetic semiconductor nanowires and nanoslabs

The Curie temperature of diluted magnetic semiconductor (DMS) nanowires and nanoslabs is investigated using the mean-field model. The Curie temperature in DMS nanowires can be much larger than that in corresponding bulk material due to the density of states of one-dimensional quantum wires, and when only one conduction subband is filled, the Curie temperature is inversely proportional to the carrier density. The T-C in DMS nanoslabs is dependent on the carrier density through the number of the occupied subbands. A transverse electric field can change the DMS nanowires from the paramagnet to ferromagnet, or vice versae. (c) 2007 American Institute of Physics.

Influence of N doping on the Rashba coefficient, semiconductor-metal transition, and electron effective mass in InSb1-xNx nanowires: Ten-band k center dot p model

The electronic structures of InSb1-xNx nanowires are investigated using the ten-band k center dot p method. It is found that nitrogen increases the Rashba coefficient of the nanowires dramatically. For thick nanowires, the Rashba coefficient may increase by more than 20 times. The semiconductor-metal transition occurs more easily in InSb1-xNx nanowires than in InSb nanowires. The electronic structure of InSb1-xNx nanowires is very different from that of the bulk material. For fixed x the bulk material is a semimetal, while the nanowires are metal-like. In InSb1-xNx bulk material and thick nanowires, an interesting decrease of electron effective mass is observed near k=0 which is induced by the nitrogen, but this phenomenon disappears in thin nanowires.

Stark and rashba effects in GaN nanowires

The effects of an external electric field on the electronic structure of GaN nanowires, as well as GaAs nanowires for comparison, are investigated theoretically. It is found that there is an anti-crossing effect in GaN nanowires caused by a small electric field, the hole energy levels, hole wave functions, and optical oscillator strengths change dramatically when the radius (R) is around a critical radius (R-c), while this effect is absent in GaAs nanowires. When R is slightly smaller than R-c, the highest hole states are optically dark in the absence of the electric field, and a small electric field can change them to be optically bright, due to the coupling of hole states brought by the field. The Rashba spin-orbit effect is also studied. The electron Rashba coefficient alpha increases linearly with the electric field. While the hole Rashba coefficients beta do not increase linearly, but have complicated relationships with the electric field.