Quasi One-Dimensional Metal-Semiconductor Heterostructures.

The band offsets occurring at the abrupt heterointerfaces of suitable material combinations offer a powerful design tool for high performance or even new kinds of devices. Because of a large variety of applications for metal-semiconductor heterostructures and the promise of low-dimensional systems to present exceptional device characteristics, nanowire heterostructures gained particular interest over the past decade. However, compared to those achieved by mature two-dimensional processing techniques, quasi one-dimensional (1D) heterostructures often suffer from low interface and crystalline quality. For the GaAs-Au system, we demonstrate exemplarily a new approach to generate epitaxial and single crystalline metal-semiconductor nanowire heterostructures with atomically sharp interfaces using standard semiconductor processing techniques. Spatially resolved Raman measurements exclude any significant strain at the lattice mismatched metal-semiconductor heterojunction. On the basis of experimental results and simulation work, a novel self-assembled mechanism is demonstrated which yields one-step reconfiguration of a semiconductor-metal core-shell nanowire to a quasi 1D axially stacked heterostructure via flash lamp annealing. Transmission electron microscopy imaging and electrical characterization confirm the high interface quality resulting in the lowest Schottky barrier for the GaAs-Au system reported to date. Without limiting the generality, this novel approach will open up new opportunities in the syntheses of other metal-semiconductor nanowire heterostructures and thus facilitate the research of high-quality interfaces in metal-semiconductor nanocontacts.

Diameter dependence of the thermal conductivity of InAs nanowires.

The diameter dependence of the thermal conductivity of InAs nanowires in the range of 40-1500 nm has been measured. We demonstrate a reduction in thermal conductivity of 80% for 40 nm nanowires, opening the way for further design strategies for nanoscaled thermoelectric materials. Furthermore, we investigate the effect of thermal contact in the most common measurement method for nanoscale thermal conductivity. Our study allows for the determination of the thermal contact using existing measurement setups. The thermal contact resistance is found to be comparable to the wire thermal resistance for wires with a diameter of 90 nm and higher.

Direct band gap wurtzite gallium phosphide nanowires.

The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555-690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality.

Local modification of GaAs nanowires induced by laser heating.

GaAs nanowires were heated locally under ambient air conditions by a focused laser beam which led to oxidation and formation of crystalline arsenic on the nanowire surface. Atomic force microscopy, photoluminescence and Raman spectroscopy experiments were performed on the same single GaAs nanowires in order to correlate their structural and optical properties. We show that the local changes of the nanowires act as a barrier for thermal transport which is of interest for thermoelectric applications.

Spatially resolved Raman spectroscopy on indium-catalyzed core-shell germanium nanowires: size effects.

The structure of indium-catalyzed germanium nanowires is investigated by atomic force microscopy, scanning confocal Raman spectroscopy and transmission electron microscopy. The nanowires are formed by a crystalline core and an amorphous shell. We find that the diameter of the crystalline core varies along the nanowire, down to few nanometers. Phonon confinement effects are observed in the regions where the crystalline region is the thinnest. The results are consistent with the thermally insulating behavior of the core-shell nanowires.

Gallium assisted plasma enhanced chemical vapor deposition of silicon nanowires.

Silicon nanowires have been grown with gallium as catalyst by plasma enhanced chemical vapor deposition. The morphology and crystalline structure has been studied by electron microscopy and Raman spectroscopy as a function of growth temperature and catalyst thickness. We observe that the crystalline quality of the wires increases with the temperature at which they have been synthesized. The crystalline growth direction has been found to vary between <111> and <112>, depending on both the growth temperature and catalyst thickness. Gallium has been found at the end of the nanowires, as expected from the vapor-liquid-solid growth mechanism. These results represent good progress towards finding alternative catalysts to gold for the synthesis of nanowires.

Single crystalline and core-shell indium-catalyzed germanium nanowires-a systematic thermal CVD growth study.

Germanium nanowires were synthesized using thermal chemical vapor deposition (CVD) and indium as a catalyst. The process parameter space for successful growth was studied. By optimizing the growth temperature and gas pressure, high aspect ratio germanium nanowires have been obtained. Scanning electron microscopy investigations indicate that the final diameter of the nanowires is strongly influenced by the growth temperature and the germane partial pressure. High resolution transmission electron microscopy reveals that nanowires grow either as high quality single crystalline, or with a high quality single-crystalline core and a concentric amorphous shell. The occurrence of these two morphologies is found to only depend on the wire diameter. Chemical analysis of the nanowire tip indicates the presence of indium, validating its role as a catalyst. Raman spectroscopy measurements reveal a higher incidence of core-shell structures for nanowires synthesized at 30 Torr and indicate the presence of tensile strain. These results are important towards obtaining high quality germanium nanowires without the use of gold as a catalyst, which is known to degrade the wires' electrical and optical properties.

Acute disseminated histoplasmosis in infancy in Brazil: report of a case and review.

Although early age is an important risk factor for the occurrence of acute disseminated histoplasmosis in non immunosuppressed infants, cases of this condition have been rarely reported, even in highly endemic areas. This is a reason to add a new case, the first one diagnosed in the state of Rio Grande do Sul, and, also, to review and comment on the Brazilian literature of histoplasmosis in patients under three years of age.

Cutaneous cryptococcosis due to Cryptococcus neoformans var. gattii.

A case of cutaneous cryptococcosis due to Cryptococcus neoformans var. gattii in an immunocompetent host is presented. In addition a review of the literature on this subject was carried out and a brief comment made on occurrence of the variety gattii in Brazil.