768 research outputs found
AntiâStokes Photoluminescence of Monolayer WS2
AntiâStokes photoluminescence excitation of a WS2 monolayer flake between 10 and 300âK is reported herein. Even with continuousâwave lasers at low power, the emission of the exciton at excitation 100âmeV below its emission energy at room temperature is observed. A mechanism which involves the trions as the intermediate state is proposed, leading to an efficient upâconversion process. In addition, it is demonstrated that phonons are the source of the additional energy needed by the system. Overall, the results provide evidence that antiâStokes luminescence in transition metal dichalcogenides is very efficient.EC/FP7/259286/EU/Characterizing and Controlling Carbon Nanomaterials/CCCANDFG, 53244630, EXC 315: Neue Materialien und Prozesse - Hierarchische Strukturbildung fĂźr funktionale BauteileDFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeTU Berlin, Open-Access-Mittel - 201
New catalysts with unsymmetrical N-heterocyclic carbene ligands
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG gefĂśrderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The importance of unsymmetrical N-heterocyclic carbenes (uNHCs) as ligands in metal-catalyzed reactions is undeniable. While uNHCs show similar properties as compared with symmetrical NHCs, dissymmetrization allows for further fine-tuning. The introduction of chelatization, hemilability, bifunctionality, shielding effects, and chirality-transfer influences the catalystâs stability, reactivity, and selectivity, thus offering access to tailor-made systems including mono- and multidentate uNHC ligands. Based on selected examples, the structureâreactivity relationship of uNHCs employed in metal catalysts is presented. The focus is on catalytically active complexes, which either offer access to new applications or lead to significantly improved results in metal-catalyzed reactions
Maine\u27s Science and Engineering Brain Drain: How Much and Why?
More than the traditional economic ingredients, the new global economy is being built around talented people with special knowledge and skills, those with the capacity to innovate and the entrepreneurial wherewithal to turn ideas into commercial products. Hence many states are shifting economic development strategies away from efforts that market commodities such as low tax rates and cheap labor, and toward efforts that amass and equip talented people with the tools of innovation. In short, states are beginning to think about strategies for recruiting and/or retaining talented workers. In this regard, the Southern Technology Council conducted a national study to compare states on their performance in retaining their own recent science and engineering graduates and/or attracting similar graduates from states elsewhere in the country. How is Maine doing? they ask. In a word, poorly. The authors compare Maine to other states on a number of performance indicators and predictor variables to assess why this is so. They suggest Maine take bold steps now to prevent the continued loss of its most important commodity of the future
In-situ study and modeling of the reaction kinetics during molecular beam epitaxy of GeO2 and its etching by Ge
Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor
suitable for future power electronics devices while quartz-like GeO2 shows
piezoelectric properties. To explore these crystalline phases for application
and fundamental materials investigations, molecular beam epitaxy (MBE) is a
well-suited thin film growth technique. In this study, we investigate the
reaction kinetics of GeO2 during plasma-assisted MBE using elemental Ge and
plasma-activated oxygen fluxes. The growth rate as a function of oxygen flux is
measured in-situ by laser reflectometry at different growth temperatures. A
flux of the suboxide GeO desorbing off the growth surface is identified and
quantified in-situ by the line-of-sight quadrupole mass spectrometry. Our
measurements reveal that the suboxide formation and desorption limits the
growth rate under metal-rich or high temperature growth conditions, and leads
to etching of the grown GeO2 layer under Ge flux in the absence of oxygen. The
quantitative results fit the sub-compound mediated reaction model, indicating
the intermediate formation of the suboxide at the growth front. This model is
further utilized to delineate the GeO2-growth window in terms of oxygen-flux
and substrate temperature. Our study can serve as a guidance for the thin film
synthesis of GeO2 and defect-free mesa etching in future GeO2-device
processing
Isotopic study of Raman active phonon modes in β-Ga2O3
Holding promising applications in power electronics, the ultra-wide band gap material gallium oxide has emerged as a vital alternative to materials like GaN and SiC. The detailed study of phonon modes in β-Ga2O3 provides insights into fundamental material properties such as crystal structure and orientation and can contribute to the identification of dopants and point defects. We investigate the Raman active phonon modes of β-Ga2O3 in two different oxygen isotope compositions (16O,18O) by experiment and theory: By carrying out polarized micro-Raman spectroscopy measurements on the (010) and (-201) planes, we determine the frequencies of all 15 Raman active phonons for both isotopologues. The measured frequencies are compared with the results of density functional perturbation theory (DFPT) calculations. In both cases, we observe a shift of Raman frequencies towards lower energies upon substitution of 16O with 18O. By quantifying the relative frequency shifts of the individual Raman modes, we identify the atomistic origin of all modes (Ga-Ga, Ga-O or O-O) and present the first experimental confirmation of the theoretically calculated energy contributions of O lattice sites to Raman modes. The DFPT results enable the identification of Raman modes that are dominated by the different, inequivalent O- or Ga-atoms of the unit cell. We find that oxygen substitution on the OII site leads to an elevated relative mode frequency shift compared to OI and OIII sites. This study presents a blueprint for the future identification of different point defects in Ga2O3 by Raman spectroscopy
Factors affecting the adoption of electronic data interchange
Implementation of electronic data interchange (EDI) is thus more desirable and will be one of the major determinants of business success of a company. Despite the current pressure of the public sphere and all the benefits that the adoption of EDI provides, the expansion of this technology is still a minority in the Czech Republic. The aim of this paper is to identify the specifics of EDI adoption, quantify their significance, mutual conditionality and propose a new general model of EDI adoption in businesses. The conclusions of this document are based on the primary data collected through a questionnaire survey in 2015. There were the key factors influencing the likelihood of EDI adoption and their interconnectedness identified. This model reflects the main determinants of the adoption of exchange structured messages for businesses as perceived benefits, external pressure, readiness, attitude of CEO, type of product, participation of trading partners, character of company etc. This study provides a comprehensive survey of motives and barriers of EDI adoption for enterprises, which are aware of the necessary interoperability within the single European market and its highly competitive environment.O
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Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions
Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material propertiesâ associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition
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