Off-axis electron holography (EH) is a powerful method for mapping projected electric potentials, such as built-in potentials in semiconductor devices, in two dimensions (2D) at nanometer resolution. However, not well-defined thickness profiles, surface effects, and composition changes of the sample under investigation complicate the interpretation of the projected potentials. Here, we demonstrate how these problems can be overcome by combining EH with tomographic techniques, that is, electron holographic tomography (EHT), reconstructing electric potentials in 3D. We present EHT reconstructions of an n-type MOSFET including its dopant-related built-in potentials inside the device, as well as of a GaAs/AlGaAs core-multishell nanowire containing a 5 nm thick quantum well tube.

Recently, free-standing nanowires (NWs) based on III-V compound semiconductors have attracted much interest due to their unique physical properties and huge potential for future realization of nanodevices with innovative and/or enhanced functionalities: these systems are indeed, being considered as ideal building blocks for the fabrication of novel and efficient nanophotonic and nano-electronic devices, and III-generation photovoltaic cells. High-quality III-V NW-based heterostructures, with sharp interfaces and good control in size, shape and material composition, can nowadays be grown using various techniques, including metalorganic vapor phase epitaxy (MOVPE. Self-assembly of III-V NWs by MOVPE through the Au-catalyst assisted (so-called Vapour-Liquid-Solid, VLS) mechanism is a most promising technology for the synthesis of NW-based devices, but it still requires demonstrating its entire potentials in terms of materials/device performances and industrial scalability. The growth of device-quality NW structures and the study of their physical properties is crucial in order to improve/optimize the performances of NW-based devices. In this lecture, the self-assembly by MOVPE and the properties of GaAs NWs and core-shell GaAs-AlGaAs NWs will be reported as a case study. The controlled growth of vertically well-aligned GaAs, AlGaAs and GaAs-AlGaAs core-shell NWs will be presented, focusing on both NW properties (morphology, size, growth rate, inner composition, defects content, and luminescence) and on their dependence on MOVPE self-assembly conditions. Moreover, the fabrication of novel single-NW photo-detectors based on MOVPE-grown GaAs and GaAs-AlGaAs core-shell nanostructures will be reported, along with their spectral photocurrent properties. Core-shell structures are advantageous in this respect, as they reduce substantially carrier recombination at the NW surface, thus achieving higher quantum efficiency. Moreover, the GaAs/AlGaAs heterostructure offers other benefits such as an almost strain-free NW system due to the close lattice matching between core and shell materials, and possibility of band bending at the hetero-interface which confines the electrons and facilitates their transport.The potentials of self-assembly MOVPE technology for photovoltaic applications through the growth of large and dense arrays of well-aligned GaAs and GaAs-AlGaAs NWs on Si substrates will be finally presented and discussed.

III-V nanowires (NWs) are considered key elements for the fabrication of future nano-photonic devices. GaAs is a prime candidate for such applications. Recently, optically-pumped lasing has been reported for single GaAs-AlGaAs core-shell NWs.Indeed, the overgrowth of a wider bandgap AlGaAs shell around GaAs NWs leads to effective passivation of GaAs surface states and to improved minority carrier diffusion lenghts, and recombination lifetimes with respect to bare GaAs NWs. However, growth conditions (temperature, V:III precursor ratios in the vapor, etc.) and relevant geometrical parameters (namely, the shell-thickness to core-radius ratio) of GaAs-AlGaAs core-shell NWs affects the nanostructure carrier lifetime, as well as built-in lattice strain and core radiative emission.GaAs-AlGaAs core-shell NWs were grown by low pressure MOVPE in an Aixtron reactor, using trimethylgallium, trimethylaluminum and tertiarybuthylarsine as Ga, Al and As precursors. GaAs core NWs were grown at 400°C on either semi-insulating (111)B-GaAs or (111)Si substrates by the Au-catalyzed method, after which an Al0.33Ga0.67As shell was overgrown at 650°C by conventional MOVPE. In some cases a few-nm thin GaAs cap layer was deposited around the shell to protect it from oxidation. During NW core and shell+cap growth the V:III precursors ratio in the vapor was changed between 5:1 and 30:1.We demonstrate that, under conventional MOVPE growth conditions, the initial diameter, height and density of core NWs strongly impact on the actual shell growth rate. We further explain such findings by validating a detailed MOVPE growth model of AlGaAs shell based on the mass-transport of III-group species. Besides ensuring strict control over shell thickness in core-shell and core-multishell NW structures, the model allows to calculate effective (around the NWs) vapor stoichiometry during shell growth.The 7K photoluminescence of GaAs-AlGaAs core-shell NWs is thus reported as function of the NW relevant geometrical parameter, and values of V:III effective ratios during AlGaAs shell growth obtained from the model. After deconvoluting the strain-dependent red-shift of GaAs excitonic emission from the shell-dependent localization effect in present core-shell NWs, the latter is reported as function of as-calculated V:III effective ratios and discussed.

Free-standing nanowires (NWs) based on III-V semiconductors are being considered ideal building blocks offering fascinating potential for future technological applications such as the realization of novel and efficient nanophotonic devices and photovoltaic cells. Self-assembly of III-V NWs by metalorganic vapor phase epitaxy (MOVPE) through the Au-catalyzed mechanism, is a most promising technology for the synthesis of NW-based devices, but it still requires demonstrating its entire potentials in terms of materials/device performances and industrial scalability. Here, we report on the self-assembly by MOVPE and properties of core-shell GaAs-AlGaAs NWs, as a case study. In the first part of the talk, the morphology, size, inner composition, structure (defect content, lattice strain) and luminescence of as-grown GaAs and GaAs-AlGaAs NWs and their dependence of MOVPE conditions will be reported. The fabrication of photodetectors based on Schottky-contacted single core-shell GaAs-AlGaAs NWs will be then described. Noteworthy, as-fabricated detectors exhibit relatively strong polarization anisotropy of their spectral photocurrent, and record high external quantum efficiencies (about 10% at 600 nm). Also, core-shell devices exhibit significantly improved dc and high-speed performances over bare GaAs NWs, and comparable to planar MSM photodetectors. Picosecond temporal response coupled with picoampere dark currents, demonstrates the device potential for high-speed imaging arrays and on-chip optical interconnects. The dynamic control of hot electron transfer rates in nanoscale heterojunctions is relevant for novel photovoltaic devices: the hot photoexcited electron transfer across the coaxial interface of a single GaAs/AlGaAs core-shell nanowire device will be reported. The exploitation of GaAs-based NWs for photovoltaics requires the growth of uniform NWs arrays over large areas of relatively low cost substrates. In the second part of the talk we will report on a new approach for the controlled VLS growth of uniform dense arrays of well-aligned GaAs NWs over large substrate areas (over 2" dia. wafers and beyond). The growth of GaAs and GaAs-AlGaAs NWs on GaAs and Si using this new technology will be then demonstrated. The complex interplay between the NW size and height, and their array density in determining the radial (vapor-solid) growth rate of a shell material during the MOVPE process will be finally reported and assessed by validation of a shell vapour growth model.

The core photoluminescence emission of MOVPE-grown GaAs-Al0.33Ga0.67As core-shell nanowires is studied as function of the relevant geometrical parameter of these nanostructures, namely the shell-thickness to core-radius ratio h(s)/R-c. The energy of the dominant emission peak was compared with values of the GaAs heavy- and light-hole excitons redshifted by a uniaxial tensile strain, the latter calculated assuming perfect coherence at the core/shell interface and elastic energy equilibrium within the nanowires. Good agreement is obtained for h(s)/R-c < 1, the intrinsic strain-free excitonic emission being identified at 1.510 eV, and further ascribed to bound heavy-hole excitons. For h(s)/R-c > 1 increasingly larger redshifts (up to similar to 9 meV in excess of values calculated based on our elastic strain model) are observed, and tentatively ascribed to shell-dependent exciton localization effects.

High (>350 degrees C) temperature DC-sputtering deposition of ZnO:Al thin films onto single-crystal (00.1) oriented Al2O3 (sapphire) substrates is reported, using a ultrahigh-density, low-resistivity and low-cost composite ceramic target produced by slip-casting (pressureless) sintering of ZnO-Al2O3 (AZO) powders. The original combination of high-angle theta-2 theta(Bragg-Brentano geometry) X-ray diffraction with low angle theta-2 theta X-ray reflectivity (XRR) techniques allows us to define the AZO target composition and investigate the structural properties and surface/interface roughness of as-sputtered ZnO:Al films; besides, the growth dynamics of ZnO:Al is unambiguously determined. The target turned out composed of the sole wurtzite ZnO and spinel ZnAl2O4 phases. X-ray diffraction analyses revealed highly (00.1)-oriented (epitaxial) ZnO:Al films, the material mean crystallite size being in the 13-20 nm range and increasing with temperature between 350 degrees C and 450 degrees C, while the film growth rate (determined via XRR measurements) decreases appreciably. XRR spectra also allowed to determine rms surface roughness <1 nm for present films and showed ZnO:Al density changes by only a few percent between 350 degrees C and 450 degrees C. The latter result disproves the often-adopted Thornton model for the description of the sputter-grown ZnO films and instead points out toward a reduction of the sticking coefficients of impinging species, as the main origin of film growth rate and grain size dependence with temperature. ZnO:Al films appear highly transparent, with visible-NIR integrated transmittance >97%. (C) 2014 Elsevier B.V. All rights reserved.

The direct chemical vapor deposition of WS2 by W(CO)(6) and elemental sulfur as precursors onto epitaxial-graphene on SiC and CVD-graphene transferred on SiO2/Si substrate is presented. This methodology allows the epitaxial growth of continuous WS2 films with a homogeneous and narrow photoluminescence peak without inducing stress or structural defects in the graphene substrates. The control of the WS2 growth dynamics for providing the localized sulfide deposition by tuning the surface energy of the graphene substrates is also demonstrated. This growth methodology opens the way towards the direct bottom up fabrication of devices based on TMDCs/graphene van der Waals heterostructures.

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

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.

We report on the effects of changing the surface densities of MOVPE-grown free-standing GaAs-AlGaAs core-shellnanowires on the resulting nanostructure size and their photoluminescence (PL) properties. It is demonstrated thatdecreasing the local density of GaAs nanowires within the array leads to an increase of the overgrown AlGaAs shellthickness and to a substantial redshift of the nanostructure excitonic emission. Application of a vapor mass-transportlimited growth model of the AlGaAs shell allows explaining the dependence of shell growth rate on nanowire density.The observed redshift of the nanowire PL emission is then experimentally correlated with these density-induced changesof the nanostructure size, namely with the nanowire shell-thickness to core-radius ratio hs/Rc. To account for a possiblecontribution of the nanostructure built-in elastic strain to the energy shift of the peak excitonic emission, the strain fieldin present core-shell nanowires was calculated as function of the nanostructure relevant geometrical parameters, based ona uniaxial elastic energy equilibrium model, and its effect on valence and conduction band shifts of the GaAs coreevaluated by means of the Pikus-Bir Hamiltonian. Good agreement is obtained for hs/Rc<1, the strain-free excitonicemission being identified at 1.510 eV and ascribed to bound heavy-hole excitons. For hs/Rc>1 increasingly largerredshifts (up to ~9 meV in excess of values calculated based on the elastic strain model) are observed, and tentativelyascribed to shell-dependent exciton localization effects.

We report on a detailed MOVPE-growth study of GaAs-AlGaAs core-shell nanowire arrays, and on their related photoluminescence (PL) properties. Validation of a mass transport vapour growth model of AlGaAs shell for nanowire arrays demonstrates the dependence of shell growth rate on nanowire size, height, and density; besides ensuring strict control over shell geometry, the model allows to calculate effective (around the nanowires) vapour conditions during shell growth. PL properties of core-shell nanowires were thus investigated as function of their relevant geometrical parameter, i.e. the shell-thickness to core-radius ratio and effective shell growth conditions. To account for built-in elastic strain effects on PL emission, strain fields in core-shell nanowires were determined by high-resolution XRD, and compared to values based on a uniaxial elastic energy equilibrium model. In addition to the expected strain-dependent red-shift on GaAs excitonic emission, the occurrence of a shell-dependent localization effect in present nanowires is reported and discussed.

GaAs nanowires have shown unique potentials for the realization of fast and efficient opto-electronic and photovoltaic nano-devices. The growth of an AlGaAs alloy shell around GaAs nanowires ensures long-term stable passivation of GaAs surface states: enhanced luminescence intensity, improved minority carrier diffusion lengths and recombination lifetimes have been indeed, demonstrated for GaAs-AlGaAs core-shell nanowires. Self-assembly of these core-shell nanostructures has been reported by a variety of growth methods, including MBE and MOVPE. However, the occurrence of defects, elastic strain fields, and growth-dependent inhomogeneity may affect their electronic/optical and functional properties. Understanding the nano-scale properties of these core-shell nanowires is necessary for any future device application.In this talk we present a detailed study of the structural, optical and functional properties of GaAs-AlGaAs core-shell nanowires grown on GaAs and Si substrates by Au-catalyst assisted MOVPE. The morphology (size, shape) and structure of bare GaAs nanowires and of GaAs-AlGaAs core-shell nanowires is investigated, showing that nano-faceting occurs at the shell/core interface as a result of twin defect nucleation. Also, built-in elastic strain fields along the core-shell nanowire axis are experimentally determined by high-resolution XRD and low temperature photoluminescence (PL) measurements, and compared to values calculated based on a uniaxial elastic energy equilibrium model. Application of a vapour mass transport model for AlGaAs shell growth in the case of well-aligned dense nanowire arrays demonstrates the dependence of shell growth rate on nanowire density; this allow us to use a 'combinatorial' approach to explore the PL properties of core-shell nanowires as function of their relevant geometrical parameter, i.e. the shell-thickness to core-radius ratio. Besides the expected strain-dependent effects on the excitonic emission of the GaAs nanowire cores, we demonstrate an additional shell-dependent localization (up to 13 meV), whose possible interface-related origin will be discussed. Spatially indirect electronic transitions in both emission (PL) and absorption (photocurrent) spectra were also recorded for GaAs-AlGaAs core-shell nanowires, further pointing towards a role of the core-shell hetero-interface in determining the nanostructure optical properties. This idea will be discussed also in relation to the previously reported observation of negative differential resistance in core-shell nanowire devices.

We report on the self-assembly by Au-catalyzed metalorganic vapor phase epitaxy (MOVPE) of GaAs-based nanowires(NWs) and their applications to novel and efficient nano-devices. The growth of GaAs and GaAs-AlGaAs core-shellNWs is presented as case study, focusing on the dependence of their structural, optical and electrical properties onMOVPE conditions. MSM diodes fabricated using as-grown core-shell NWs are reported, along with their photoelectricperformances. These devices show potentials for applications as fast photo-detectors and efficient solar cells.

Core-shell nanowires (NWs) of III-V semiconductors possess unique superior characteristics over theirplanar counterparts for the realization of novel logic, photovoltaic, and light emitting devices. As building blocks,they offer fascinating potential for future technological applications, such as the realization of novel and efficientnanophotonic devices and photovoltaic cells. Self-assembly of III-V NWs by metalorganic vapor phase epitaxy(MOVPE) through the Au-catalyzed mechanism, a most promising technology for the synthesis of NW-baseddevices, still requires demonstrating its entire potentials in terms of materials/device performances and industrialscalability. The growth of NW structures and the study of their physical properties are crucial in order toimprove/optimize device performances.In this talk, we report on the optical/electronic and functional properties of GaAs NWs and core-shellGaAs-AlGaAs NWs, as a case study. The micro-structural properties (morphology, size, inner composition andcrystal strain) of these free-standing NW nanostructures will be first presented.The characteristic photoluminescence (PL) core emission of GaAs-AlGaAs core-shell NWs will be thendiscussed as function of the NW relevant geometrical parameters, namely their hs/Rc=(shell thickness)/(core radius)ratio. The GaAs emission appears to redshifts with the hs/Rc ratio. Comparison between the NW excitonicenergy position and the strain-shifted values of heavy- and light-hole excitons calculated upon assuming perfectcoherence at the GaAs-AlGaAs hetero-interface and elastic energy equilibrium within the nanowire, allowidentifying the GaAs core dominant PL emission as due to bound heavy-exciton recombination. Further, a tentativeexplanation in terms of exciton localization of observed spectral redshifts will be given.Understanding of selected electronic and optoelectronic carrier transport properties and devicecharacteristics remains lacking without a direct measurement of band alignment in these NWs. In this respect, theapplication of photocurrent and photoluminescence spectroscopies to core-shell NW systems, allows to build up aband diagram of a single heterostructure nanowire with high spectral resolution, enabling quantification ofconduction band offsets.Finally, the fabrication of photodetectors based on Schottky-contacted single core-shell GaAs-AlGaAs NWswill be presented. Noteworthy, as-fabricated detectors exhibit relatively strong polarization anisotropy of theirphotocurrent, and record high external quantum efficiencies (about 10% at 600 nm). Also, core-shell NW devicesexhibit significantly improved dc and high-speed performances over bare GaAs NWs, and comparable to planarMSM photodetectors. Picosecond temporal response coupled with pA dark currents demonstrate the devicepotential for high-speed imaging arrays and on-chip optical interconnects.

The inner composition, defect content and morphology of AlGaAs nanowires (NWs) grown on (111)B-GaAs by Au-catalyzed MOVPE is reported. The NWs grow tapered with their [111] axis normal to the substrate. The Raman spectra of single AlGaAs NWs were measured in non-resonant conditions with sub-?-meter spatial resolution, allowing determination of the Al content. NWs consist of GaAs for T G<475°C, but show a two-fold compositional structure for T G>475°C, namely an Al xGa 1-xAs core surrounded by an Al yGa 1-yAs (y<x) shell, ascribed to the combination of Au-catalyzed (axial) and conventional (sidewall) growth. The cross-sectional shape of AlGaAs NWs changes from triangular (for T G=500÷525°C) to almost hexagonal (for T G=550°C), due to an exchange between {2 -11} and {110} planes as the slowest to grow. The NWs have free-electron concentrations ~10 18 cm -3, due to Si contamination of the Al source.

Group III-V compound semiconductor nanowires with radial modulation of the materials composition and/or doping in the form of core-shell and core-multishell nanowire heterostructures show promise as novel and high-performance nano-scale light emitting diodes, lasers, photodetectors and solar cells. Strict control over the growth of such radially heterostructured nanowires is, however, necessary. We report the experimental dependence of AlGaAs shell growth by metalorganic vapor phase epitaxy (MOVPE) around free-standing Au-catalysed GaAs nanowires on the relevant sizes and densities of the nanostructures. A model based on (i) the vapor mass transport of group III species, and (ii) perfect conformality between the nanowires and the substrate of AlGaAs deposition is proposed and validated, describing the observed MOVPE growth dynamics of the shell material around dense ensembles of GaAs nanowires. We predict the complex (non-linear) dependence of the shell growth rate on the initial GaAs nanowire diameters (i.e., initial Au catalyst nanoparticle size), heights, local densities on the substrate, and deposition time, which is in very good agreement with experimental data; in particular, a monotonic decrease of AlGaAs shell thickness is expected and observed with increasing nanowire density.

In this work, we report on the microstructural and morphological characterization of III-V semiconductor nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy. As-grown dense (10 8-10 9 cm -2) arrays of few-micron long vertically-aligned (i.e. parallel to the ?111? crystallographic axis) GaAs, Al xGa 1-xAs and core-shell GaAs-Al xGa 1-xAs NWs were investigated, carrying out HRXRD measurements on different (hkl) reflections and by recording reciprocal space maps (RSMs) around the materials (111) reciprocal lattice points (relps). We show that NW diffraction peaks are visible in the RSM by means of characteristic halos. In the case of GaAs NWs, the halo is located at the (111) relp indicating that the NWs are grown along the ?111? direction and parallel to the ?111? axis of the GaAs substrate. On the contrary, for Al xGa 1-xAs NWs or intentional core-shell GaAs-Al xGa 1-xAs NWs the halo is displaced (along the momentum transfer normal to the surface, Q z) with respect to the GaAs (111) relp due to the elastic lattice strain associated with the compositional variation, e.g. the Al molar fraction in the Al xGa 1-xAs alloy, within the nanostructures.

In this work we present new results on the morphological and microstructural properties of GaAs-AlxGa1-xAs (x?0.24) core-shell nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). Optimized growth conditions allowed us to fabricate highly-dense arrays of vertically-aligned (i.e., along the <111> crystallographic orientation) NWs. The NW arrays were investigated by Helium Ion microscopy (HeIM) and X-ray double- and triple-axis measurements and reciprocal space mapping (RSM). We demonstrate that these techniques can be employed in order to correlate some intrinsically local morphological information with statistically relevant (i.e. averaged over millions-to-billions of NWs) data on the NW structural properties.

Free-standing nanowires (NWs) based on III-V semiconductors are considered ideal building blocks offering fascinating potential for future technological applications such as the realization of novel and efficient nanophotonic devices and photovoltaic cells. Self-assembly of III-V NWs by metalorganic vapor phase epitaxy (MOVPE) through the Au-catalyzed mechanism, is a most promising technology for the synthesis of NW-based devices, but it still requires demonstrating its entire potentials in terms of materials/device performances and industrial scalability. The growth of device-quality NW structures and the study of their physical properties is crucial in order to improve/optimize the performances of NW-based devices. In this lecture, we report on the self-assembly by MOVPE and the properties of GaAs NWs and core-shell GaAs-AlGaAs NWs, as a case study. The controlled growth of vertically well-aligned GaAs, AlGaAs and GaAs-AlGaAs core-shell NWs will be presented, focusing on both NW properties (morphology, size, growth rate, inner composition, defects content, and luminescence) and on their dependence on MOVPE self-assembly conditions. The potentials of self-assembly MOVPE technology for photovoltaic applications through the growth of large and dense arrays of well-aligned GaAs and GaAs-AlGaAs NWs on GaAs/Si hetero-substrates will be then discussed. The complex interplay between the NW size and height, and their array density in determining the radial (vapor-solid) growth rate of a shell material during the MOVPE process will be reported and assessed by validation of a shell vapour growth model.The fabrication of photodetectors based on Schottky-contacted single core-shell GaAs-AlGaAs NWs will be then presented. Noteworthy, as-fabricated detectors exhibit relatively strong polarization anisotropy of their spectral photocurrent, and record high external quantum efficiencies (about 10% at 600 nm). Also, core-shell devices exhibit significantly improved dc and high-speed performances over bare GaAs NWs, and comparable to planar MSM photodetectors. Picosecond temporal response coupled with picoampere dark currents, demonstrates the device potential for high-speed imaging arrays and on-chip optical interconnects. The dynamic control of hot electron transfer rates in nanoscale heterojunctions is relevant for novel photovoltaic devices: the hot photoexcited electron transfer across the coaxial interface of a single GaAs/AlGaAs core-shell nanowire device will be reported.

We report on the development of electron holographic tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as x-ray tomography.

Electron holography at medium resolution simultaneously probes projected electrostatic and magnetostatic potentials as well as elastic and inelastic attenuation coefficients with a spatial resolution of a few nanometers. In this work, we derive how the elastic and inelastic attenuation can be disentangled. Using that result, we perform the first three dimensional tomographic reconstruction of potential and (in) elastic attenuation in parallel. The technique can be applied to distinguish between functional potentials and composition changes in nanostructures, as demonstrated using the example of a GaAs-Al0.33Ga0.67As core-shell nanowire. (C) 2014 AIP Publishing LLC.

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.

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.

We report on the photoluminescence (PL) of GaAs-Al0.32Ga0.68As core-shell nanowires grown by MOVPE, and their dependence on the precursors V:III molar ratio utilized in the vapor during growth. It is shown that the PL emission of the GaAs nanowire core red-shifts with decreasing the V:III ratio from 30:1 to 4:1, an effect tentatively ascribed to the build-up of a space-charge region at the core-shell hetero-interface, the latter associated to the unintentional incorporation of impurities, namely C in GaAs and Si in AlGaAs.

We report on the growth of GaAs-AlGaAs core-multishell nanowire quantum heterostructures by metalorganic vapor phase epitaxy, and their photoluminescence (PL) properties. Dense arrays of vertically-aligned GaAs nanowires were fabricated onto (111)B-GaAs wafers by Au-catalyzed self-assembly, and radially overgrown by two AlGaAs shells between which a few-nm thin GaAs shell was introduced to form a quantum well tube (QWT). Besides the GaAs nanowire core emission band peaked at around 1.503 eV, 7K PL spectra showed an additional broad peak in the 1.556-1.583 eV energy interval, ascribed to the transition between electron and hole confined states within the QWT. The emission blue-shifts with the shrinkage of as-grown GaAs well tubes, as the nanowire local (on the substrate) density and height change.

The photoconduction properties of individual GaAs/AlGaAs core-shell nanowires under uniform and local optical excitation are investigated, allowing the external quantum efficiency, the polarization anisotropy, and the role of the nanocontacts to be valuated. © OSA 2013.

Conductivity and photoconductivity properties of individual GaAs/AlGaAs core-shellnanowires (NWs) are reported. The NWs were grown by Au-assisted metalorganic vaporphase epitaxy, and then dispersed on a substrate where electrical contacts were defined on theindividual NWs by electron beam induced deposition. Under dark conditions, the carriertransport along the NW is found to be limited by Schottky contacts, and influenced by thepresence of an oxide layer. Nonetheless, under illumination, the GaAs/AlGaAs core-shell NWshows a significant photocurrent, much higher than the bare GaAs NW. The spatialdependence of the photocurrent within the single core-shell NW, evaluated by a mappingtechnique, confirms the blocking behavior of the contacts. Moreover, local spectralmeasurements were performed which allow one to discriminate the contribution of carriersphotogenerated in the core and in the shell.

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 similar to 10 ps full-width at half-maximum and an estimated corrected value less than 5 ps. The bare GaAs devices exhibit a slower response (similar to 35 ps) along with a slow decaying persistent photocurrent (similar to 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.

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 rho (varying from similar to 0.1 to similar to 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.

The stoichiometry of single ternary III-V 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 approximate to 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.

We demonstrate the possibility of fast and efficient solvent-free functionalization of buckypaper (BP) mats prefabricated from oxidized multiwalled carbon nanotubes (MWCNTs-ox), by using three representative amines of different structure: one monofunctional aliphatic amine, octadecylamine (ODA), one monofunctional aromatic amine, 1-aminopyrene (AP), and one aromatic diamine, 1,5-diaminonaphthalene (DAN). The functionalization procedure, which relies on the formation of amide bonds with carboxylic groups of MWCNTs-ox, is performed at 150-180 degrees C under reduced pressure and takes about 4 h including auxiliary degassing. The amine-treated BP samples (BP-ODA, BP-AP and BP-DAN, respectively) were characterized by means of a variety of analytical techniques such as Fourier-transform infrared and Raman spectroscopy, thermogravimetric and differential thermal analysis, scanning and transmission electron microscopy, scanning helium ion microscopy, and atomic force microscopy. The highest amine content was found for BP-ODA, and the lowest one was observed for BP-DAN, with a possible contribution of non-covalently bonded amine molecules in all three cases. Despite of some differences in spectral and morphological characteristics for amine-functionalized BP samples, they have in common a dramatically increased stability in water as compared to pristine BP and, on the other hand, a relatively invariable electrical conductivity. (C) 2015 Elsevier B.V. All rights reserved.

We demonstrate spatial probing of carrier transport within GaAs/AlGaAs core-shell nanowires with nanometer lateral resolution and subsurface sensitivity by energy-variable electron beam induced current imaging. Carrier drift that evolves with applied electric field is distinguished from a coupled drift-diffusion length. Along with simulation of injected electron trajectories, combining beam energy tuning with precise positioning for selective probing of core and shell reveals axial position- and bias-dependent differences in carrier type and transport along parallel conduction channels. These results indicate how analysis of transport within heterostructured nanomaterials is no longer limited to nonlocal or surface measurements.

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 degrees C and subsequently annealed at 700 degrees C. Growth experiments performed on 4 degrees-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.

Atomic force microscopy (AFM) and scanning probe microscopy (SPM) were used to study the surface topography of electrodeposited pure nickel with different grain sizes from ultra-fine to nanoscale level. Such technique, coupled with nanoindentation and nanoscratch experiments allowed to map the nanotribomechanism acting on the same pure metal with different mechanical and topographical properties.

We report the hot photoexcited electron transfer across the coaxial interface of a cylindrical core-shellnanowire. Modulation of the transfer rates, manifested as a large tunability of the voltage onset of negativedifferential resistance and of voltage-current phase, is achieved using three different modes. The couplingof electrostatic gating, incident photon energy, and the incident photon intensity to transfer rates isfacilitated by the combined influences of geometric confinement and heterojunction shape on hot-electrontransfer, 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 theintroduction and control of a continuously adjustable phase delay of up to
about 130 within a singlenanometer-scale device element.

With ongoing instrumental improvements Raman spectroscopy (RS) is advancing into the study of surface vibrational modes of semiconductors. On compound semiconductors, like the IIIV's, two distinct types of surface modes occur, microscopic modes being vibrations confined to the surface and macroscopic modes penetrating much deeper into the bulk, dependent on the electromagnetic boundary conditions. Both mode types depend on surface properties, the microscopic ones on the atomic scale, and the macroscopic ones on the scale of the surface polariton wavelength. While the former one delivers surface atomic structure information, the latter one may be useful for instance to study the morphology of surfaces, such as the shape of semiconductor nanowires. We discuss Raman spectra obtained on the atomically well-defined III-V(110) model surfaces and recent results obtained on isolated III-V nanowires. The comparison of both gives insight in the capabilities of Raman scattering from surface phonons: The contributions of surface phonons, surface resonances, and macroscopic modes (Fuchs-Kliewer modes, surface phonon polaritons) to the Raman spectra become evident. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim