Self-assembled quantum dots in a nanowire system for quantum photonics.

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Chemical to electrical transduction mechanisms from single metal oxide nanowire measurements: response time constant analysis.

Insight into chemical to electrical transduction mechanisms taking place at the surface of a single metal oxide nanowire is reported due to its outstanding importance for determining the characteristics of resistive solid state gas sensors. The surface chemical reaction kinetics is discussed considering competitiveness phenomena among different active sites and gas species on the nanowire taken as a metal oxide monocrystal at the nanoscale level. Experimental results for different representative gas molecules are shown to determine and understand sensor selectivity. The reported gas species are carbon monoxide and water vapour as general reference molecules, and ethanol and ammonia species as special references for gas-solid interactions, respectively, on acid and basic sites. Kinetic properties are proposed as particular signatures for each of the possible surface chemical reactions, allowing their identification and distinction. Likewise, features such as thermal inertia limitation and effects of the molecular and monoatomic absorbed oxygen are also estimated considering operation working modes based on nanowire self-heating. Furthermore, the applicability of a surface electrical field on a one-dimensional metal oxide nanostructure to enhance the surface ionization of the absorbed molecules is also reviewed as a new type of metal oxide based nanosensor for achieving improved selectivity.

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.

Bidimensional versus tridimensional oxygen vacancy diffusion in SnO(2-x) under different gas environments.

Metal oxides present oxygen defects that induce different chemical and physical properties. Experiments performed in SnO(2-x) sensors show that the dynamics of these vacancies are strongly affected by the presence of different gases in the environment. Experimentally, the electrical resistance of individual metal oxide SnO(2-x) nanowires shows modulation: when the environment is oxygen rich long term drifts (hours) are observed indicating extended vacancy dynamics. Instead, if CO is present, drifts disappear in minutes. Density functional theory indicates that changes in resistance follow the extension of reoxidation. For oxygen-poor environments, oxygen vacancy excorporation and healing are confined to the near-surface layer of SnO(2-x) (bidimensional or near-surface diffusion), and completed in short times. Under oxygen-rich conditions, tridimensional diffusion of oxygen vacancies towards the surface takes place at room temperature. In this case, a push-pull mechanism allows bulk-to-surface diffusion and as a consequence resistance drifts are longer and the vacancy quenching is more extensive.

Direct imaging of the visible emission bands from individual ZnO nanowires by near-field optical spectroscopy.

Room-temperature photoluminescence (PL) measurements have been performed on single-crystal ZnO nanowires grown on SiO2/Si and quartz substrates by the vapor transport method using Au as a catalyst. Two emission bands are apparent, one in the UV spectral region around 380 nm (3.26 eV) associated with exciton recombination processes and a much broader structure in the visible range from 420 to 700 nm, which exhibits two distinct peak-like features around 520 and 590 nm (2.38 and 2.10 eV). Spectrally resolved scanning near-field optical microscopy (SNOM) of single ZnO nanowires have been performed for a direct imaging of the PL emission with spatial resolution below 100 nm. SNOM results provide evidence that the yellow emission band observed at 590 nm is a unique property of the ZnO nanowires, being most likely related to radiative recombination processes associated with Au impurities introduced during the catalytic growth.

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.

A practical non-enzymatic urea sensor based on NiCo2O4 nanoneedles.

We propose a new facile electrochemical sensing platform for determination of urea, based on a glassy carbon electrode (GCE) modified with nickel cobalt oxide (NiCo2O4) nanoneedles. These nanoneedles are used for the first time for highly sensitive determination of urea with the lowest detection limit (1 µM) ever reported for the non-enzymatic approach. The nanoneedles were grown through a simple and low-temperature aqueous chemical method. We characterized the structural and morphological properties of the NiCo2O4 nanoneedles by TEM, SEM, XPS and XRD. The bimetallic nickel cobalt oxide exhibits nanoneedle morphology, which results from the self-assembly of nanoparticles. The NiCo2O4 nanoneedles are exclusively composed of Ni, Co, and O and exhibit a cubic crystalline phase. Cyclic voltammetry was used to study the enhanced electrochemical properties of a NiCo2O4 nanoneedle-modified GCE by overcoming the typical poor conductivity of bare NiO and Co3O4. The GCE-modified electrode is highly sensitive towards urea, with a linear response (R 2 = 0.99) over the concentration range 0.01-5 mM and with a detection limit of 1.0 µM. The proposed non-enzymatic urea sensor is highly selective even in the presence of common interferents such as glucose, uric acid, and ascorbic acid. This new urea sensor has good viability for urea analysis in urine samples and can represent a significant advancement in the field, owing to the simple and cost-effective fabrication of electrodes, which can be used as a promising analytical tool for urea estimation.

Portable microsensors based on individual SnO2 nanowires.

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Improved selectivity for partial oxidation of methane to methanol in the presence of nitrite ions and BiVO4 photocatalyst.

Nitrite ions are shown to have significant influence on the selectivity of the photocatalytic oxidation of methane to methanol. An almost complete inhibition of undesired CO2 has been achieved with BiVO4 in the presence of a low concentration of nitrite, which might act both as a UV filter and as a hydroxyl radical scavenger.

Solid electrolyte interphase in semi-solid flow batteries: a wolf in sheep's clothing.

The formation of the alkyl carbonate-derived solid electrolyte interphase (SEI) enables the use of active materials operating at very cathodic potentials in Li-ion batteries. However, the SEI in semi-solid flow batteries results in a hindered electron transfer between a fluid electrode and the current collector restricting the operating potentials to ca. 0.8 V vs. Li/Li(+) for EC-based electrolytes.

Mesoporous silica: a suitable adsorbent for amines.

Mesoporous silica with KIT-6 structure was investigated as a preconcentrating material in chromatographic systems for ammonia and trimethylamine. Its adsorption capacity was compared to that of existing commercial materials, showing its increased adsorption power. In addition, KIT-6 mesoporous silica efficiently adsorbs both gases, while none of the employed commercial adsorbents did. This means that KIT-6 Mesoporous silica may be a good choice for integrated chromatography/gas sensing micro-devices.

The effects of electron-hole separation on the photoconductivity of individual metal oxide nanowires.

The responses of individual ZnO nanowires to UV light demonstrate that the persistent photoconductivity (PPC) state is directly related to the electron-hole separation near the surface. Our results demonstrate that the electrical transport in these nanomaterials is influenced by the surface in two different ways. On the one hand, the effective mobility and the density of free carriers are determined by recombination mechanisms assisted by the oxidizing molecules in air. This phenomenon can also be blocked by surface passivation. On the other hand, the surface built-in potential separates the photogenerated electron-hole pairs and accumulates holes at the surface. After illumination, the charge separation makes the electron-hole recombination difficult and originates PPC. This effect is quickly reverted after increasing either the probing current (self-heating by Joule dissipation) or the oxygen content in air (favouring the surface recombination mechanisms). The model for PPC in individual nanowires presented here illustrates the intrinsic potential of metal oxide nanowires to develop optoelectronic devices or optochemical sensors with better and new performances.

Water vapor detection with individual tin oxide nanowires.

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32 000 ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on ten individual devices (tested >6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.