MOVPE growth of GaP/GaPN core-shell nanowires: N incorporation, morphology and crystal structure

Dilute nitride III-V nanowires (NWs) possess great potential as building blocks in future optoelectronical and electrochemical devices. Here, we provide evidence for the growth of GaP/GaPN core-shell NWs via metalorganic vapor phase epitaxy, both on GaP(111)B and on GaP/Si (111) hetero-substrates. The NW morphology meets the common needs for use in applications, i.e. they are straight and vertically oriented to the substrate as well as homogeneous in length. Moreover, no parasitical island growth is observed. Nitrogen was found to be incorporated on group V sites as determined from transmission electron microscopy (TEM) and Raman spectroscopy. Together with the incorporation of N, the NWs exhibit strong photoluminescence in the visible range, which we attribute to radiative recombination at N-related deep states. Independently of the N incorporation, a peculiar facet formation was found, with {110} facets at the top and {112} at the bottom of the NWs. TEM reveals that this phenomenon is related to different stacking fault densities within the zinc blende structure, which lead to different effective surface energies for the bottom and the top of the NWs.This work was supported by the Deutsche Forschungsgemeinschaft (DFG, proj. no. HA 3096/4-2 & DA 396/6-2). We thank D Roßberg and D Flock for preparation of the TEM lamellae via FIB, as well as A Müller for technical support of the MOVPE system and W Dziony for AES measurements. We appreciate fruitful discussions with A Paszuk and A Nägelein

Traveling interface modulations and anisotropic front propagation in ammonia oxidation over Rh(110)

The bistable NH3 + O2 reaction over a Rh(110) surface was explored in the pressure range 10−6 -10−3 mbar and in the temperature range 300-900 K using photoemission electron microscopy and low energy electron microscopy as spatially resolving methods. We observed a history dependent anisotropy in front propagation, traveling interface modulations, transitions with secondary reaction fronts, and stationary island structures.Fil: Rafti, Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina. Leibniz-Universität Hannover; AlemaniaFil: Borkenhagen, Benjamin. Technische Universität Clausthal; AlemaniaFil: Lilenkamp, Gerhard. Technische Universität Clausthal; AlemaniaFil: Lovis, Florian. Leibniz-Universität Hannover; AlemaniaFil: Smolinsky, Tim. Leibniz-Universität Hannover; AlemaniaFil: Imbihl, Roland. Leibniz-Universität Hannover; Alemani

A highly selective and self-powered gas sensor via organic surface functionalization of p-Si/n-ZnO diodes

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection

Charge Transfer Characteristics of n-type In0.1Ga0.9N Photoanode across Semiconductor-Liquid Interface

Understanding the mechanisms of charge transfer across the semiconductor/liquid interface is crucial to realize efficient photoelectrochemical devices. Here, the interfacial charge transfer characteristics of n-type In0.1Ga0.9N photoanodes are investigated and correlated to their photo-activity properties measured in phosphate buffered saline solution (pH 7) under illumination conditions. Cyclic voltammetry measurements show evident photoactivity changes as the number of cycles increases. In particular, the photocurrent density reaches its maximum value after 49 voltammetric cycles; meanwhile, the photocurrent onset potential shifts toward more negative cathodic potentials. Electrochemical impedance measurements reveal that, first, the hole transfer process occurs mainly via localized states at the surface and the photocurrent onset potential is dependent on the energetic position of those states. Therefore, the observed initial photocurrent increase and cathodic shift of the photocurrent onset potential can be attributed to a decrease of the transfer resistance and partial passivation of the states at the surface. On the other hand, a gradual oxidation and corrosion of the InGaN surface arises, causing a consequential decrease of the photocurrent. At this point, the charge transfer process occurs predominantly from the valence band. This work provides a basic understanding of the charge transfer mechanisms across the InGaN/liquid interface which can be used to improve the overall photoanode efficiency

Novel approaches towards highly selective self-powered gas sensors

The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented

Effect of antimony on graphite growth in ductile iron

Antimony is used in ductile cast iron melts to avoid graphite degenerations like exploded graphite and chunky graphite. However, the effect of Sb in the melt on the crystallization of ductile cast iron is still unknown, therefore the aim of our investigations. The current study presents thermodynamic calculations showing that at the beginning of the eutectic solidification, Mg3Sb2 is formed. Furthermore, the amount of Sb increases in the austenite and decreases in the remaining melt during solidification. An experiment was conducted, in which a higher amount of Sb than usual was added to a ductile iron melt. In the microstructure of the Sb-containing sample, a higher graphite precipitate count was measured than in the reference sample without Sb. The graphite spheroids of the Sb-containing sample show an almost perfect roundness and a very smooth surface. However, lamellar outgrowths from the graphite spheroids were also observed. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it could be shown that Sb-containing phases have been formed, which are located in the pearlitic matrix and also in graphite spheroids. As a result of spectroscopic investigations, it can be assumed that Sb-containing phases act as nucleant for graphite nucleation. Further investigations by Auger electron spectroscopy showed that a thin layer of Sb at the interface between graphite and matrix has been formed. It seems that this layer constricts the diffusion of carbon to the spheroidal graphite and acts as a surfactant favoring a layer-by-layer growth of the graphite spheroids

Plasmon–Exciton Coupling at Individual Porphyrin-Covered Silver Clusters

The feasibility to couple surface plasmons with molecular excitations is an intriguing feature of plasmonic–organic hybrid systems. To date, investigations of plasmonic–excitonic coupling have largely focused on ensembles of nanoparticles and on purely optical methods. Here we present a single-particle approach based on laser-spectroscopic photoemission electron microscopy. Localized surface plasmons give rise to photoemission resonances in laser excitation spectra of individual silver nanoclusters. As a most striking manifestation of plasmon–exciton coupling, upon deposition of a thin zinc tetraphenylporphyrin (ZnTPP) film, a second resonance near the ZnTPP Soret band appears in our spectra. In accordance with simulations, spectral repulsion of both resonances as well as intensity redistribution between both modes indicates their plasmonic–excitonic hybrid character

Spiky Nickel Electrodes for Electrochemical Oxygen Evolution Catalysis by Femtosecond Laser Structuring

Micro- and nanostructured Ni/NiO surfaces were generated by femtosecond laser structuring for oxygen evolution reaction in alkaline water electrolysis cells. For two different angles between the laser beam and the nickel surface, two different types of laser-structured electrodes were prepared, characterized, and compared with a plane tempered nickel electrode. Their electrochemical activities for the oxygen evolution reaction were tested by using cyclic and linear sweep voltammetry. The chemical surface composition was investigated by X-ray photoelectron spectroscopy. Laser structuring increased the overall electrochemical performance by more than one order of magnitude. The overpotential of the laser-structured electrodes for the oxygen evolution reaction was decreased by more than 100 mV due to high defect densities of the structures created by the laser ablation process