Abstract: We report on the Au-catalysed synthesis of GaAs nanowires on hetero-structured GaAs/(111)Si substrates by metalorganic vapour phase epitaxy. It is demonstrated that the deposition of a 40-50 nm thin GaAs epilayer onto Si guarantees a high percentage of straight and vertically-aligned GaAs nanowires. GaAs epilayers were grown at 400°C and subsequently annealed at 700°C. Growth experiments performed on 4°-miscut and exactly-oriented (111)Si substrates show that a higher yield (close to 90%) of vertical nanowires is obtained using miscut substrates, an effect ascribed to the smoother surface morphology of GaAs epilayers on these substrates. Comparison between the cross-sectional shape of nanowires grown on GaAs/(111)Si hetero-substrates and those on (111)A-GaAs and (111)B-GaAs substrates demonstrates that both GaAs epilayers and over-grown nanowires are (111)B-oriented.
Abstract: Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell
nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the
effects of size, geometry, height, packing density, and polarization. Energy dependence of
the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also
the air bands. Simulation of polarization dependence verifies higher absorption with the electric
field along the wire axis. Higher absorption at much lower volume compared to thin film,
combined with excellent charge transport, make core-shell nanowire arrays excellent candidates
for optoelectronics applications.
Abstract: We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy Ï (varying from about 0.1 to about 0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency
values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.
Abstract: Electron holographic tomography (EHT), the combination of off-axis electron
holography with electron tomography, is a technique, which can be applied to the quantitative 3-dimensional (3D) mapping of electrostatic potential at the nanoscale. Here we show the results obtained in the EHT investigation of GaAs and GaAs-AlGaAs core-shell nanowires grown by Au-catalysed metalorganic vapor phase epitaxy. The unique ability of EHT of disentangling the materials mean inner potential (MIP) from the specimen projected thickness allows reconstruction of the nanowire 3D morphology and inner compositional structure, as well as the measurement of the MIP.
Abstract: We report the hot photoexcited electron transfer across the coaxial interface of a cylindrical core-shell nanowire. Modulation of the transfer rates, manifested as a large tunability of the voltage onset of negative differential resistance and of voltage-current phase, is achieved using three different modes. The coupling of electrostatic gating, incident photon energy, and the incident photon intensity to transfer rates is facilitated by the combined influences of geometric confinement and heterojunction shape on hot-electron transfer, and by electron-electron scattering rates that can be altered by varying the incident photon flux, with evidence of weak electron-phonon scattering. Dynamic manipulation of this transfer rate permits the introduction and control of a continuously adjustable phase delay of up to â¼130° within a single nanometer-scale device element.
Abstract: High-speed metal-semiconductor-metal (MSM) photodetectors based on Schottky-contacted core/shell GaAs/AlGaAs and bare GaAs nanowires were fabricated and characterized. The measured core/shell temporal response has a about 10 ps full-width at half-maximum and an estimated corrected
value less than 5 ps. The bare GaAs devices exhibit a slower response (ca. 35 ps) along with a slow decaying persistent photocurrent (ca. 80 s). The core/shell devices exhibit significantly improved dc and high-speed performance over bare nanowires and comparable performance to planar MSM photodetectors. The picosecond temporal response, coupled with picoampere dark current, demonstrate the potential for core/shell nanowires in high-speed imaging arrays and on-chip optical interconnects.
Abstract: The electronic transport and gating characteristics in GaAs and Ge nanowires (NWs) are altered
significantly following either indirect or direct exposure to a focused Ga+ ion beam (FIB), such as
that used to produce Pt electrical contacts to NWs. While these results challenge the assumptions
made in some previously reported work relating to the electronic properties of semiconductor NWs
using FIB-assisted production of contacts and/or their leads, local electron beam induced deposition
is shown to be a reliable and facile route for producing robust electrical contacts to individual vapor
phase-grown NWs in a manner that enables study of their actual carrier transport properties.
Abstract: The stoichiometry of single ternary IIIâV semiconductor
nanowires was analyzed by Raman spectroscopy. Free-standing
AlxGa1-xAs nanowires were obtained through metal organic
vapor phase epitaxy (MOVPE) by the vapor liquid solid (VLS)
mechanism on GaAs substrates. The as-grown nanowires were
studied with a scanning confocal Raman spectrometer (spatial
resolution around 300 nm). They were located by Rayleigh imaging
and individual nanowires selected for Raman spectroscopy. The
acquired spectra exhibit 2-mode behavior. The stoichiometry of
single nanowires was determined based on the frequencies of
the GaAs- and AlAs-like transversal optical (TO) and
longitudinal optical (LO) peaks with an accuracy of below
10%. Measurements at different positions along the axis
revealed variations of the composition within single nanowires.
This non-uniformity evidences that the nanowires possess an
internal structure.
Abstract: Coherent gold nanoislands were prepared directly on (100)-oriented Si substrates by a physical methodology, consisting in the thermal evaporation of a very thin Au film (t~2nm) and its successive annealing in the temperature range 350°C<T<814°C. We found that at annealing
temperature of 814°C and in presence of residual oxygen during the annealing process epitaxial monocrystalline gold nanoislands embedded in the Si lattice are formed. The crystallographic orientation and epitaxial relationship between the Au nanoislands and the Si lattice are well defined.
In contrast, at lower annealing temperatures, namely at 350°C and 626°C, the nanoislands are randomly oriented without epitaxial relationships. The morphology, orientation, and crystalline structure of Au nanoislands were investigated by scanning and high-resolution transmission electron microscopy and grazing-incidence X-ray diffraction. A model of the epitaxial Au nanoisland formation on (100)Si is presented in which the Si-atom out-diffusion and the formation of a liquid Au-Si droplet during the annealing process (increasing temperature) and the Si redeposition and oxidation (i.e., SiOx complex formation and removing of the excess Si in the gold islands) during the cooling process (decreasing temperature) play a fundamental role.
Abstract: The luminescence properties of GaAs/AlGaAs coreâshell nanowires grown by metalorganic vapor phase epitaxy (MOVPE) on (1¯ 1¯ 1¯)B GaAs using tertiarybutylarsine, trimethylgallium and trimethylaluminium are reported. Untapered kink-free GaAs nanowires with average diameters around 50â70 nm were grown at 400 °C by the vapourâliquidâsolid method; to this purpose, colloidal Au nanoparticles were used as metal catalyst. Al0.33Ga0.67As shells were grown at 650 °C around GaAs nanowires by conventional MOVPE, with thickness ranging in the 70â160 nm interval. Low-temperature photoluminescence (PL) and high spatial resolution cathodoluminescence (CL) measurements were performed, respectively, on dense ensembles of coreâshell nanowires (still on their original substrates) and single nanowires; comparison between secondary electron and monochromatic CL images of single nanowires led to spatially resolve the major CL emissions. The low-temperature luminescence of nanowires above the GaAs band-gap energy consists of three contributions: (i) the 1.997 eV band-edge (excitonic) emission of the Al0.33Ga0.67As shell, followed (in CL spectra) by a more intense GaAs-like LO-phonon replica; (ii) a broad weaker band at not, vert, similar1.90 eV, ascribed to a donorâacceptor pair recombination associated to residual Si donors in the AlGaAs; and (iii) a dominant and very broad band at not, vert, similar1.67 eV, due to the spatially indirect recombination between electrons in the core and holes in the shell. Comparison between CL and PL spectra suggests that each nanowire has a slightly different GaAs core emission, its peak energy varying in the 1.46â1.49 eV interval.
Abstract: The morphology, structure and phonon properties of well-aligned and kink-free GaAs nanowires grown on (1¯ 1¯ 1¯)B GaAs are reported. The nanowires were grown at temperatures down to 400 1C by Au-catalysed MOVPE using tBuAsH2 and Me3Ga in H2 ambient. Colloidal Au nanoparticles (NPs) with average size of 60â70nm were used as catalyst. Below 425 1C the nanowires consist of almost cylindrical segments, their average diameter closely matching that of the original Au NPs. At higher temperatures, the nanowires show a large tapering. The structural analysis of nanowire samples grown at 450 1C evidences that residual catalyst droplets at the nanowire tips consist of crystalline fcc Au, with a preferred (1 1 1) crystallographic orientation, and orthorhombic GaAu2 alloy (g phase).
Non-resonant Raman scattering experiments carried out on GaAs nanowires showed no evidence of phonon conï¬nement effects.
However, new strong LO phonon contributions arise in the Raman spectra with respect to bulk GaAs, likely due to the nanowire large surface-to-volume ratio.
Abstract: We report the structural and morphological properties of well-aligned ZnO nanowires grown at 750 °C on Au-deposited and annealed (100)Si substrates using carbo-thermal evaporation. As-grown nanowires are made of wurtzite ZnO, have cylindrical shape and carry droplet-like nanoparticles (NPs) at their tips, as expected for vapourâliquidâsolid (VLS) growth. Grazing incidence X-ray diffraction measurements demonstrate that the NPs are made of pure fcc Au. No secondary Au/Zn alloy phases were detected. Bragg diffraction patterns confirmed that the nanowires were grown with their crystal c-axes parallel to the [100] direction of Si (i.e. normal to the substrate surface), while Au NPs are mostly (111)-oriented. The diameter distribution of ZnO nanowires mimics that of the Au NPs at their tips. A quantitative study of the nanostructure size distribution after sequential annealing and growth steps evidences the occurrence of three nanoscale processes: (i) Ostwald ripening and/or coalescence of Au NPs before nanowire nucleation, (ii) Au-catalysed VLS nucleation and axial growth of ZnO nanowires and (iii) radial growth of nanowires by a vapourâsolid process. These processes originate the NP and nanowire size evolution during the experiments. The present findings are interpreted in terms of Zn vapour pressure changes during carbo-thermal evaporation.
Abstract: Au-catalyzed self-assembly of GaAs nanowires on ( )B GaAs by metalorganic vapor phase epitaxy is reported between 375°C and 500°C, using tertiarybutylarsine and trimethylgallium in H2. The nanowires are [ ]B-aligned and kink-free. Below 425°C the nanowires have narrow base diameter distributions, closely matching the size (~60 nm) of the Au nanoparticles at their tip (no tapering). Above 425°C the nanowires show a hexagonal-based pyramidal shape with base edges normal to the ï¡ ï± in-plane substrate directions and base diameters which increase exponentially with temperature, indicating a kinetics limited growth along the nanowire sidewalls. Activation energies ranging ~20-23 kcal/mol were estimated for growth along both the sidewalls and the [ ]B direction.
Abstract: Gold nanoparticles heavily functionalized with oligonucleotides have been used in a variety of DNA detection methods. The optical properties of three-dimensional aggregates of Au nanoparticles in solution or deposited
onto suitable surfaces have been analyzed to detect hybridization processes of specific DNA sequences as possible alternatives to fluorescent labeling methods. This paper reports on the preparation of gold nanoparticles directly deposited onto the surface of silicon (Si) and sapphire (Al2O3) substrates by a physical methodology, consisting in the thermal evaporation of a thin Au film and its successive annealing. The method guarantees the preparation of monodispersed single-crystal Au nanoparticles with a strong surface plasmon resonance (SPR) peak centered at about 540 nm. We show that the changes of SPR excitation before and after DNA functionalization and subsequent hybridization of Au nanoparticles immobilized onto Si and Al2O3 substrates can be exploited to fabricate specific biosensors devices in solid phase.
