Peter J. Klar

Evolution of Li2O2 Growth and Its Effect on Kinetics of Li–O2 Batteries

Lithium peroxide (Li2O2), the solid and intrinsically electronic insulating discharge product of Li-O2 batteries strongly influences the discharge and charge kinetics. In a series of experiments, we investigated the growth of Li2O2 upon discharge and the corresponding reduction and oxidation processes by varying the depth of discharge. The results indicate that insulating Li2O2 particles with a disc-like shape were formed during the initial discharge stage. Afterward, the nucleation and growth of Li2O2 resulted in the formation of conducting Li2O2 shells. When the discharge voltage dropped below 2.65 V, the Li2O2 discs evolved to toroid-shaped particles and defective superoxide-like phase presumably with high conductivity was formed on the rims of Li2O2 toroids. Both Li2O2 and the superoxide-like phase are unstable in ether-based electrolytes resulting in the degradation of the corresponding cells. Nevertheless, by controlling the growth of Li2O2, the chemical reactivity of the discharge product can be suppressed to improve the reversibility of Li-O2 batteries.

Trends in the electronic structure of dilute nitride alloys

The band-anticrossing (BAC) model has been widely applied to analyse the electronic structure of dilute nitride III-V-N alloys such as GaNxAs1−x. The BAC model describes the strong band gap bowing observed at low N composition in GaNxAs1−x in terms of an interaction between the GaAs host matrix conduction band edge and a higher lying band of localized N resonant states. In practice, replacing As by N introduces a range of N-related defect levels, associated with isolated N atoms, N–N pairs and larger clusters of N atoms. We show that the effect of such defect levels on the alloy conduction band structure is strongly dependent on the relative energy of the defect levels and the host conduction band edge. We first consider GaNxAs1−x, where we show that the unexpectedly large electron effective mass and gyromagnetic ratio, and their non-monotonic variation with x, are due to hybridization between the conduction band edge and specific nitrogen states close to the band edge. The N-related defect levels lie below the conduction band edge in GaNxP1−x. We must therefore explicitly treat the interaction between the higher lying GaP host Γ conduction band minimum and defect states associated with a random distribution of N atoms in order to obtain a good description of the lowest conduction states in disordered GaPN alloys. Turning to other alloys, N-related defect levels should generally lie well above the conduction band minimum in InNSb, with the band dispersion of InNSb then well described by a two-level BAC model. Both InP and InAs are intermediate between InSb and GaAs. By contrast, we calculate that N-related defect levels lie close to the conduction band minimum in GaNSb, and will therefore strongly perturb the lowest conduction states in this alloy. Overall, we conclude that the BAC model provides a good qualitative explanation of the electronic properties of dilute nitride alloys, but that it is in many cases necessary to include the details of the distribution of N-related defect levels to obtain a quantitative understanding of the conduction band structure in dilute nitride alloys.

Synthesis and Characterization of Chiral Benzylic Ether-Bridged Periodic Mesoporous Organosilicas

The first synthesis of a chiral periodic mesoporous organosilica (PMO) carrying benzylic ether bridging groups is reported. By hydrolysis and condensation of the new designed chiral organosilica precursor 1,4-bis(triethoxysilyl)-2-(1-methoxyethyl)benzene (BTEMEB) in the presence of the non-ionic oligomeric surfactant Brij 76 as supramolecular structure-directing agent under acidic conditions, an ordered mesoporous chiral benzylic ether-bridged hybrid material with a high specific surface area was obtained. The chiral PMO precursor was synthesized in a four-step reaction from 1,4-dibromobenzene as the starting compound. The evidence for the presence of the chiral units in the organosilica precursor as well as inside the PMO material is provided by optical activity measurements.

A miniaturized sensor consisting of concentric metallic nanorings on the end facet of an optical fiber.

A polarization-independent optical sensor is created by fabricating a concentric gold ring grating with a period of 900 nm on the end facet of an optical fiber. The sensing function of this miniaturized device is realized by sending white light as a probe to the gold rings and collecting the response signal in the back-reflection through the optical fiber. A pronounced peak due to the Rayleigh anomaly of the gold ring grating is observed in the reflection spectrum, the center wavelength of which is sensitive to the change in the environmental refractive index of the fiber end facet. Theoretical analysis not only shows excellent agreement with the experimental results, but also gives insights into the mechanisms of this kind of sensor. Using the center position of the Rayleigh peak as the response signal, a high sensitivity dλ/dn of 900 nm per unity refractive index is realized for this sensor and a resolution of Δn/n ≈ 1% is demonstrated in preliminary experiments. The sensitivity is solely determined by the period of the grating.

Crystalline ZnO with an enhanced surface area obtained by nanocasting

The authors report the synthesis of nanoporous ZnO, which exhibits a periodically ordered, uniform pore system with crystalline pore walls. The crystalline structure is investigated by x-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The large specific surface area and the uniformity of the pore system are confirmed by nitrogen physisorption. Raman spectroscopy along with low-temperature photoluminescence measurements confirms the high degree of crystallinity and gives insight into defects participating in the radiative recombination processes.

Towards ordered arrays of magnetic semiconductor quantum wires

The diluted magnetic semiconductor (Cd, Mn)S has been incorporated into ordered wire-like pores of hexagonal mesoporous silica. X-ray and Raman spectra reveal the wurtzite structure of the incorporated material. Photoluminescence and photoluminescence excitation spectra of the (Cd, Mn)S-wire samples show clearly the optical transitions within the half-filled Mn 3d shell, typical for Mn incorporated in a II–VI host material. The blueshift of the absorption edge of (Cd, Mn)S-wire samples compared to reference crystalline and powder samples of the same Mn content is due to quantum confinement in the nanowires.

Germanium doping of self-assembled GaN nanowires grown by plasma-assisted molecular beam epitaxy

Germanium doping of GaN nanowires grown by plasma-assisted molecular beam epitaxy on Si(111) substrates is studied. Time of flight secondary ion mass spectrometry measurements reveal a constant Ge-concentration along the growth axis. A linear relationship between the applied Ge-flux and the resulting ensemble Ge-concentration with a maximum content of 3.3×1020 cm−3 is extracted from energy dispersive X-ray spectroscopy measurements and confirmed by a systematic increase of the conductivity with Ge-concentration in single nanowire measurements. Photoluminescence analysis of nanowire ensembles and single nanowires reveals an exciton localization energy of 9.5 meV at the neutral Ge-donor. A Ge-related emission band at energies above 3.475 eV is found that is assigned to a Burstein-Moss shift of the excitonic emission.

Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2

Thin films of doped VO2 were deposited, analyzed, and optimized with regard to their solar energy transmittance (Tsol) and visible/luminous light transmittance (Tlum) which are important parameters in the context of smart window applications in buildings. The doping with alkaline earth metals (AEM) like Mg, Ca, Sr, or Ba increased both Tsol and Tlum due to a bandgap widening and an associated absorption edge blue-shift. Thereby, the brown-yellowish color impression of pure VO2 thin films, which is one major hindrance limiting the usage of VO2 as thermochromic window coating, was overcome. Transparent thin films with excellent switching behavior were prepared by sputtering. Highly doped V1−xMexO2 (Me = Ca, Sr, Ba) kept its excellent thermochromic switching behavior up to x(Me) = Me/(Me + V) = 10 at. % doping level, while the optical bandgap energy was increased from 1.64 eV for undoped VO2 to 2.38 eV for x(Mg) = 7.7 at. %, 1.85 eV for x(Ca) = 7.4 at. %, 1.84 eV for x(Sr) = 6.4 at. % and 1.70 eV for x(Ba) =...

Space micropropulsion systems for Cubesats and small satellites: From proximate targets to furthermost frontiers

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

Tailoring the magnetoresistance of MnAs/GaAs:Mn granular hybrid nanostructures

The magnetoresistance properties of GaAs:Mn∕MnAs granular hybrid structures consisting of ferromagnetic MnAs clusters within a paramagnetic GaAs:Mn host differ considerably from those of paramagnetic and ferromagnetic (Ga,Mn)As alloys. We analyze the magnetoresistance effects on the basis of a resistor network model. Typical experimental findings are reproduced and their dependence on cluster density and random spatial arrangement of the clusters are revealed. Controlled spatial positioning of the MnAs clusters within the GaAs:Mn host offers interesting opportunities for optimizing the magnetoresistance properties for applications and for overcoming problems of miniaturization arising from cluster statistics.