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Recently, we suggested an unconventional approach [the so-called Internal-Field-Guarded-Active-Region Design (IFGARD)] for the elimination of the crystal polarization field induced quantum confined Stark effect (QCSE) in polar semiconductor heterostructures. And in this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the QCSE in strongly polar [000-1] wurtzite GaN/AlN nanodiscs while reducing the exciton life times by more than two orders of magnitude. The IFGARD based elimination of the QCSE is independent of any specific crystal growth procedures. Furthermore, the cone-shaped geometry of the utilized nanowires (which embeds the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement- and QCSE-induced emission energy shifts. Due to the IFGARD, both effects become independently adaptable.
One of the most significant limitations for the quantum efficiency of group III-nitride based light emitters is the spatial electron-hole separation due to the quantum-confined Stark effect (QCSE). To overcome this problem, Hönig et al. [1] proposed a novel concept, the Internal-Field-Guarded-Active-Region Design (IFGARD), which suppresses the QCSE for wurtzite crystals in the [0001] direction. Here, we show how encapsulating the active region by additional guard layers results in a strong reduction of the built-in electric field in c-plane wurtzite nanostructures. Even more importantly, we demonstrate the first experimental evidence for the successful realization of an IFGARD structure based on GaN/AlN heterostructures embedded in GaN nanowires. By means of power-dependent and time-resolved µ-photoluminescence (µ-PL) we experimentally proof the validity of the unconventional IFGARD structure. We managed to tune the emission of 4-nm-thick GaN nano-discs up to 3.32 eV at low excitation powers, which is just 150 meV below the bulk GaN bandgap. Our results demonstrate an almost complete elimination of the QCSE in comparison to conventional structures which show approximately 1 eV red-shifted emission. The suppression of the QCSE results in a significant increase of the radiative exciton decay rates by orders of magnitude and demonstrates the potential of IFGARD structures for future light sources based on polar heterostructures.
[1] Hönig et al., Phys. Rev. Applied 7, 024004 (2017)
Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.
Hintergrund und Ziel. Kühlschmierstoffe auf der Basis von Fettsäureestern bieten technologische Vorteile gegenüber mineralölbasierten Produkten. Außerdem sind sie umweltgerechter und leisten einen Beitrag zum nachhaltigen Wirtschaften. Dennoch werden sie in der Praxis kaum eingesetzt, da sie bisher aufgrund von Rohstoffpreisen und Synthesekosten teurer sind, als konventionelle Konkurrenzprodukte. Alternativen hinsichtlich Rohstoffen und Synthesewegen werden betrachtet und die Eigenschaftsbilder resultierender Esterprodukte miteinander verglichen.
Methoden. In die Untersuchungen wurden drei auf enzymatischkatalytischem Weg hergestellte Fettsäure-2-Ethylhexylester auf der Basis von Erdnussfett, Rindertalg und Altspeisefett sowie vier konventionell synthetisierte Ester aus Tier- bzw. Pflanzenfetten und 2-Ethyl-1-Hexanol einbezogen. Bestimmt wurden physikalische Eigenschaftswerte, Fettsäurespektren, Gehalte an freien Fettsäuren sowie Oxidations- und Hydrolysestabilitäten. Ferner wurden Thermolyseexperimente durchgeführt.
Ergebnisse. Die physikalischen Eigenschaften der sieben Ester, d.h. Dichte, Viskosität, Pourpoint und Flammpunkt, waren vergleichbar und ließen die Anwendbarkeit der Produkte als Basisöle für Kühlschmierstoffe erwarten. Hydrolysebeständigkeiten waren hoch und thermische Belastungen der Ester gaben keine Hinweise auf besondere Schadstoffbildungspotentiale. Defizite wiesen insbesondere die enzymatisch hergestellten Ester hinsichtlich Oxidationsstabilitäten und den Gehalten an freien Fettsäuren auf.
Ausblick. Für die Praxis wird es notwendig sein, Mindestqualitäten für Rohstoffe und resultierende Esteröle zu definieren, die als Grundöle im Kühlschmierstoffbereich eingesetzt werden sollen. Außerdem müssen Verfahren ausgearbeitet werden, die es ermöglichen, zu qualitativ hochwertigen Esterprodukten zu gelangen, ohne den Preisvorteil von Rohstoffen wie Altspeise- und Tierfetten durch hohe Produktions- und Veredelungskosten zu verlieren. Ein Beitrag hierzu ist sicherlich die Weiterentwicklung der wenig energieaufwändigen enzymatischen Alkoholyse zur Esterproduktion.
The environmental impact assessment of materials is usually based on laboratory tests, mostly in combination with models describing the longterm fate of the substances of interest in the targeted environmental compartment. Thus, laboratory tests are the fundamental link to achieve appropriate assessment conclusions which makes it essential to generate consistent results. This just as applies to the leaching of cementitious materials. In Europe, the leaching behavior of monolithic building materials is tested in the Dynamic Surface Leaching Test following the specification CEN/TS 16637–2. An interlaboratory comparison on European level regarding this technical specification showed relatively high intra- and interlaboratory variations for the tested materials (monolithic copper slag and cement stabilized coal fly ash). Therefore the German Committee for Structural Concrete (DAfStb) framed a guideline to specify additional testing conditions for cementitious materials. To assess the possible improvement by this guidelines measures, a round robin test with 11 participants from Germany and the Netherlands was conducted. This work aims to provide insight into the factors to be considered in the testing of alkaline materials, including sample preparation, and highlights crucial procedures and their manifestation in the results. All evaluated parameters showed improved results compared to the earlier round robin test. The relative standard deviations for repeatability (RSDr) and reproducibility (RSDR) of the elements calcium, barium, antimony, chromium, molybdenum and vanadium, which are the parameters evaluated in both round robin tests, were RSDr = 4%, 4%, 2%, 5%, 5%, and 5% respectively (4% in average) for this work, in comparison to the European round robin test with an average RSDr of 29% (17%, 17%, 20%, 40%, 36%, and 42%). The RSDR improved from 41% (30%, 36%, 29%, 57%, 40%, and 56%) to 14% (12%, 8%, 6%, 28%, 15%, and 12%). CO2 ingress during testing and the inaccuracy of eluate analytics for concentrations close to the determination limits were identified as the main sources of error.