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1.
Proc Natl Acad Sci U S A ; 119(1)2022 Jan 04.
Artículo en Inglés | MEDLINE | ID: mdl-34969845

RESUMEN

The Late Bronze Age Thera eruption was one of the largest natural disasters witnessed in human history. Its impact, consequences, and timing have dominated the discourse of ancient Mediterranean studies for nearly a century. Despite the eruption's high intensity (Volcanic Explosivity Index 7; Dense Rock Equivalent of 78 to 86 km) [T. H. Druitt, F. W. McCoy, G. E. Vougioukalakis, Elements 15, 185-190 (2019)] and tsunami-generating capabilities [K. Minoura et al., Geology 28, 59-62 (2000)], few tsunami deposits are reported. In contrast, descriptions of pumice, ash, and tephra deposits are widely published. This mismatch may be an artifact of interpretive capabilities, given how rapidly tsunami sedimentology has advanced in recent years. A well-preserved volcanic ash layer and chaotic destruction horizon were identified in stratified deposits at Çesme-Baglararasi, a western Anatolian/Aegean coastal archaeological site. To interpret these deposits, archaeological and sedimentological analysis (X-ray fluorescence spectroscopy instrumental neutron activation analysis, granulometry, micropaleontology, and radiocarbon dating) were performed. According to the results, the archaeological site was hit by a series of strong tsunamis that caused damage and erosion, leaving behind a thick layer of debris, distinguishable by its physical, biological, and chemical signature. An articulated human and dog skeleton discovered within the tsunami debris are in situ victims related to the Late Bronze Age Thera eruption event. Calibrated radiocarbon ages from well-constrained, short-lived organics from within the tsunami deposit constrain the event to no earlier than 1612 BCE. The deposit provides a time capsule that demonstrates the nature, enormity, and expansive geographic extent of this catastrophic event.

3.
Nano Lett ; 15(5): 3533-40, 2015 May 13.
Artículo en Inglés | MEDLINE | ID: mdl-25922974

RESUMEN

Core-shell nanowires (NW) have become very prominent systems for band engineered NW heterostructures that effectively suppress detrimental surface states and improve performance of related devices. This concept is particularly attractive for material systems with high intrinsic surface state densities, such as the low-bandgap In-containing group-III arsenides, however selection of inappropriate, lattice-mismatched shell materials have frequently caused undesired strain accumulation, defect formation, and modifications of the electronic band structure. Here, we demonstrate the realization of closely lattice-matched radial InGaAs-InAlAs core-shell NWs tunable over large compositional ranges [x(Ga)∼y(Al) = 0.2-0.65] via completely catalyst-free selective-area molecular beam epitaxy. On the basis of high-resolution X-ray reciprocal space maps the strain in the NW core is found to be insignificant (ε < 0.1%), which is further reflected by the absence of strain-induced spectral shifts in luminescence spectra and nearly unmodified band structure. Remarkably, the lattice-matched InAlAs shell strongly enhances the optical efficiency by up to 2 orders of magnitude, where the efficiency enhancement scales directly with increasing band offset as both Ga- and Al-contents increase. Ultimately, we fabricated vertical InGaAs-InAlAs NW/Si photovoltaic cells and show that the enhanced internal quantum efficiency is directly translated to an energy conversion efficiency that is ∼3-4 times larger as compared to an unpassivated cell. These results highlight the promising performance of lattice-matched III-V core-shell NW heterostructures with significant impact on future development of related nanophotonic and electronic devices.

4.
Nano Lett ; 14(5): 2256-64, 2014 May 14.
Artículo en Inglés | MEDLINE | ID: mdl-24678960

RESUMEN

We probe and control the optical properties of emission centers forming in radial heterostructure GaAs-Al0.3Ga0.7As nanowires and show that these emitters, located in Al0.3Ga0.7As layers, can exhibit quantum-dot like characteristics. We employ a radio frequency surface acoustic wave to dynamically control their emission energy, and occupancy state on a nanosecond time scale. In the spectral oscillations, we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, different emission lines of a single emission center exhibit pronounced anticorrelated intensity oscillations during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the emission center to a continuum in the radial heterostructure. Using finite element modeling and Wentzel-Kramers-Brillouin theory we identify quantum tunneling as the underlying mechanism. These simulation results quantitatively reproduce the observed switching and show that in our systems these emission centers are spatially separated from the continuum by >10.5 nm.

5.
Nano Lett ; 13(12): 6189-96, 2013.
Artículo en Inglés | MEDLINE | ID: mdl-24274328

RESUMEN

Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core-shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 10(7) cm(-1) and an electron mobility in the order of 50,000 cm(2)/(V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core-multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2D-electron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.


Asunto(s)
Arsenicales/química , Galio/química , Nanocables/química , Semiconductores , Cristalización , Electrones , Nanotecnología , Puntos Cuánticos/química , Silicio/química
6.
Nano Lett ; 13(12): 6070-7, 2013.
Artículo en Inglés | MEDLINE | ID: mdl-24274597

RESUMEN

Utilizing narrow band gap nanowire (NW) materials to extend nanophotonic applications to the mid-infrared spectral region (>2-3 µm) is highly attractive, however, progress has been seriously hampered due to their poor radiative efficiencies arising from nonradiative surface and Auger recombination. Here, we demonstrate up to ~ 10(2) times enhancements of the emission intensities from InAs NWs by growing an InAsP shell to produce core-shell NWs. By systematically varying the thickness and phosphorus (P)-content of the InAsP shell, we demonstrate the ability to further tune the emission energy via large strain-induced peak shifts that already exceed >100 meV at comparatively low fractional P-contents. Increasing the P-content is found to give rise to additional line width broadening due to asymmetric shell growth generated by a unique transition from {110}- to {112}-sidewall growth as confirmed by cross-sectional scanning transmission electron microscopy. The results also elucidate the detrimental effects of plastic strain relaxation on the emission characteristics, particularly in core-shell structures with very high P-content and shell thickness. Overall, our findings highlight that enhanced mid-infrared emission efficiencies with effective carrier confinement and suppression of nonradiative recombination are highly sensitive to the quality of the InAs-InAsP core-shell interface.


Asunto(s)
Arsenicales/química , Indio/química , Nanocables/química , Silicio/química , Luminiscencia , Nanocáscaras/química , Nanoestructuras/química , Propiedades de Superficie
7.
Nano Lett ; 13(4): 1522-7, 2013 Apr 10.
Artículo en Inglés | MEDLINE | ID: mdl-23517063

RESUMEN

By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 µm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.


Asunto(s)
Aleaciones/química , Arsenicales/química , Galio/química , Nanocables/química , Cristalización , Luminiscencia , Nanoestructuras/química , Tamaño de la Partícula , Silicio/química , Propiedades de Superficie
8.
Theor Med Bioeth ; 33(5): 359-76, 2012 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-23054375

RESUMEN

Peer review is a widely accepted instrument for raising the quality of science. Peer review limits the enormous unstructured influx of information and the sheer amount of dubious data, which in its absence would plunge science into chaos. In particular, peer review offers the benefit of eliminating papers that suffer from poor craftsmanship or methodological shortcomings, especially in the experimental sciences. However, we believe that peer review is not always appropriate for the evaluation of controversial hypothetical science. We argue that the process of peer review can be prone to bias towards ideas that affirm the prior convictions of reviewers and against innovation and radical new ideas. Innovative hypotheses are thus highly vulnerable to being "filtered out" or made to accord with conventional wisdom by the peer review process. Consequently, having introduced peer review, the Elsevier journal Medical Hypotheses may be unable to continue its tradition as a radical journal allowing discussion of improbable or unconventional ideas. Hence we conclude by asking the publisher to consider re-introducing the system of editorial review to Medical Hypotheses.


Asunto(s)
Políticas Editoriales , Revisión de la Investigación por Pares , Publicaciones Periódicas como Asunto , Informe de Investigación , Ciencia , Sesgo de Selección , Creatividad , Humanos , Variaciones Dependientes del Observador , Publicaciones Periódicas como Asunto/ética , Publicaciones Periódicas como Asunto/normas , Publicaciones Periódicas como Asunto/tendencias , Ciencia/ética , Ciencia/normas , Ciencia/tendencias
9.
Nano Lett ; 11(9): 3848-54, 2011 Sep 14.
Artículo en Inglés | MEDLINE | ID: mdl-21823601

RESUMEN

We identify a new noncatalytic growth regime for molecular beam epitaxially grown GaAs nanowires (NWs) that may provide a route toward axial heterostructures with discrete material boundaries and atomically sharp doping profiles. Upon increase of the As/Ga flux ratio, the growth mode of self-induced GaAs NWs on SiO(2)-masked Si(111) is found to exhibit a surprising discontinuous transition in morphology and aspect ratio. For effective As/Ga ratios <1, in situ reflection high-energy electron diffraction measurements reveal clear NW growth delay due to formation of liquid Ga droplets since the growth proceeds via the vapor-liquid-solid mechanism. In contrast, for effective As/Ga ratios >1 an immediate onset of NW growth is observed indicating a transition to droplet-free, facet-driven selective area growth with low vertical growth rates. Distinctly different microstructures, facet formation and either the presence or absence of Ga droplets at the apex of NWs, are further elucidated by transmission electron microscopy. The results show that the growth mode transition is caused by an abrupt change from As- to Ga-limited conditions at the (111)-oriented NW growth front, allowing precise tuning of the dominant growth mode.

10.
Appl Radiat Isot ; 69(11): 1605-12, 2011 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-21530282

RESUMEN

The Kyra sequence is a volcanic eruption sequence originating from the eastern flank of Nisyros volcano, Greece. Its eruptions products can be found not only on Nisyros itself but also on the nearby non-volcanic island of Tilos. In an extensive sampling campaign, outcrops of the Kyra eruption products on Nisyros were sampled and corresponding samples on Tilos were taken. The clear stratigraphical relationship between the different units within in the individual outcrops, combined with the chemical information gained by the application of instrumental neutron activation analysis (INAA) to the samples, made a detailed chemo-stratigraphy of the complete eruption sequence possible. It can be shown that the sequence is separated into eight distinguishable eruptions. Furthermore, no eruption products of the caldera-forming eruptions from Nisyros (Lower- and Upper Caldera Pumice) or from Santorin were found on Tilos.

11.
Appl Radiat Isot ; 69(7): 929-35, 2011 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-21367609

RESUMEN

We discuss geochemical and sedimentological characteristics of 12 tephra layers, intercalated within the finely laminated sediments of Lake Van. Within the about 15kyr long sediment record studied, volcanic activity concentrated in the periods 2.6-7.2 and 11.9-12.9kyr B.P. Concentrations of 25 elements provide the geochemical fingerprint of each tephra layer and allow comparison to literature values of potential source volcanoes such as Mts. Nemrut and Süphan. The youngest two tephra layers (and probably also the other three ashes from the 2.6-7.2kyr B.P. eruptions) originate from the Nemrut volcano. The source of the older tephra (11.9-12.9kyr B.P.), however, remains unidentified.

12.
Nano Lett ; 11(4): 1512-7, 2011 Apr 13.
Artículo en Inglés | MEDLINE | ID: mdl-21355606

RESUMEN

We report on optical experiments performed on individual GaAs nanowires and the manipulation of their temporal emission characteristics using a surface acoustic wave. We find a pronounced, characteristic suppression of the emission intensity for the surface acoustic wave propagation aligned with the axis of the nanowire. Furthermore, we demonstrate that this quenching is dynamical as it shows a pronounced modulation as the local phase of the surface acoustic wave is tuned. These effects are strongly reduced for a surface acoustic wave applied in the direction perpendicular to the axis of the nanowire due to their inherent one-dimensional geometry. We resolve a fully dynamic modulation of the nanowire emission up to 678 MHz not limited by the physical properties of the nanowires.


Asunto(s)
Arsenicales/química , Galio/química , Nanoestructuras/química , Nanoestructuras/ultraestructura , Acústica , Luz , Ensayo de Materiales , Tamaño de la Partícula , Dispersión de Radiación , Vibración
13.
Nanotechnology ; 22(6): 065301, 2011 Feb 11.
Artículo en Inglés | MEDLINE | ID: mdl-21212484

RESUMEN

We present a method to fabricate multiple metal nanogap electrodes of tailored width and distance in parallel, on the cleaved plane of a GaAs/AlGaAs heterostructure. The three-dimensional patterned structures are obtained by a combination of molecular-beam-epitaxial regrowth on a crystal facet, using the cleaved-edge-overgrowth (CEO) method, and subsequent wet selective etching and metallization steps. SEM and AFM studies reveal smooth and co-planar electrodes of width and distance of the order of 10 nm. Preliminary electrical characterization indicates electrical gap insulation in the 100 MΩ range with kΩ lead resistance. We propose our methodology to realize multiple electrode geometries that would allow investigation of the electrical conductivity of complex nanoscale objects such as branched organic molecules.

14.
Naturwissenschaften ; 97(4): 403-10, 2010 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-20135300

RESUMEN

Seven pumice samples from excavations in North Sinai have been investigated with respect to their geochemical composition. This type of volcanic rock has been used as an abrasive and thus has been an object of trade since antiquity. With the help of Instrumental Neutron Activation Analysis, six of these Bronze Age samples could be correlated to their volcanic sources on the islands of Santorini, Nisyros and Giali (Greece) using the typical element concentrations ("chemical fingerprint"). The source of one pumice sample remains unidentified excluding, however, the Santorini eruption as a possible source. The concluding section of this article discusses the possible contribution, however indirect, of the pumice from Sinai and elsewhere in the Eastern Mediterranean to the controversial issue of the accurate date of the "Minoan" eruption of Santorini.


Asunto(s)
Silicatos , Erupciones Volcánicas , Egipto , Elementos Químicos , Grecia , Historia Antigua , Historia Medieval , Islas del Mediterráneo , Sicilia , Silicatos/química
15.
Appl Radiat Isot ; 67(12): 2069, 2009 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-19560932
16.
Appl Radiat Isot ; 67(12): 2117-22, 2009 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-19428263

RESUMEN

Carnivorous plants use animals as fertiliser substitutes which allow them to survive on nutrient deficient soils. Most research concentrated on the uptake of the prey's nitrogen and phosphorus; only little is known on the utilisation of other elements. We studied the uptake of three essential nutrients, potassium, iron and manganese, in three species of carnivorous pitcher plants (Cephalotus follicularis LaBilladiere, Sarracenia purpureaL., Heliamphora nutans Bentham). Using relatively short-lived and gamma-emitting radiotracers, we significantly improved the sensitivity compared to conventional protocols and gained the following results. We demonstrated the uptake of trace elements like iron and manganese. In addition, we found direct evidence for the uptake of potassium into the pitcher tissue. Potassium and manganese were absorbed to virtually 100% if offered in physiological concentrations or below in Cephalotus. Analysis of pitcher fluid collected in the natural habitat showed that uptake was performed here as efficiently as in the laboratory. The absorption of nutrients is an active process depending on living glandular cells in the pitcher epidermis and can be inhibited by azide. Unphysiologically high amounts of nutrients were taken up for a short time, but after a few hours the absorbing cells were damaged, and uptake stopped. Absorption rates of pitcher leaves from plants under controlled conditions varied highly, indicating that each trap is functionally independent. The comparison of minerals in typical prey with the plants' tissues showed that a complete coverage of the plants' needs by prey capture is improbable.


Asunto(s)
Hierro/metabolismo , Manganeso/metabolismo , Plantas/metabolismo , Potasio/metabolismo , Análisis de Activación de Neutrones
17.
Phys Rev Lett ; 100(2): 026801, 2008 Jan 18.
Artículo en Inglés | MEDLINE | ID: mdl-18232902

RESUMEN

We study the formation of shock waves in a nanomechanical resonator with an embedded two-dimensional electron gas using surface acoustic waves. The mechanical displacement of the nanoresonator is read out via the induced acoustoelectric current. Applying acoustical standing waves, we are able to determine the so-called anomalous acoustocurrent. This current is found only in the regime of shock wave formation.

18.
Appl Radiat Isot ; 66(1): 1-8, 2008 Jan.
Artículo en Inglés | MEDLINE | ID: mdl-17719793

RESUMEN

Chemical fingerprint techniques are frequently applied to airborne volcanic eruption products, so-called tephra, such as ash and pumice for archeological and geoscientific purposes. However, in some cases, a meaningful interpretation of the results is complicated by superficial contaminations. Therefore, this situation was simulated by the use of powdered rhyolitic pumice to investigate its capability to adsorb several ions from aqueous solutions. Using neutron activation analysis, adsorption could be proven for Cr(3+), Cr(2)O(7)(2-) (dichromate), Fe(3+), Co(2+), HAsO(4)(2-) (hydrogen arsenate), Rb(+), Sr(2+), Cs(+), Ba(2+), La(3+), Ce(3+), Ce(4+), Sm(3+), Th(4+) and UO(2)(2+), which is a clear evidence for the interaction of those ions with the volcanic glass. In our experiments, pumice powder showed the ability to adsorb ions in the range from 1.8 mg kg(-1) (in case of HAsO(4)(2-)) to 5.8 wt% (in case of Fe(3+)). Adsorption is probably due to ion-exchange reactions. It could also be shown that a few ions are not adsorbed in detectable quantities: Na(+), K(+), Fe(2+), Zn(2+) and Nd(3+). The knowledge about adsorption of ions enables us not only to examine the possible influence of contaminations where chemical fingerprinting methods are applied to volcanic material for archaeometry, but it also suggests the technical application of pumiceous materials for technical purposes, like water purification or as an adsorbent in the final storage of nuclear waste. In another series of INAA supported experiments, the influence of chemicals like ascorbic acid, acetic acid, HCl, HF, HNO(3), H(2)O, H(2)O(2), H(3)PO(4), H(2)SO(4), NaOH and NH(3) on the bulk composition of pumice powder was investigated-resulting in no detectable change. We conclude that superficially contaminated tephra can be washed in diluted HF to remove contaminations without influencing the chemical fingerprint.

19.
Nanotechnology ; 19(4): 045303, 2008 Jan 30.
Artículo en Inglés | MEDLINE | ID: mdl-21817500

RESUMEN

We present experimental results on the controlled synthesis of InAs ordered nanostructures with three different grades of complexity: nanowires, quantum dot arrays, and double quantum dot arrays. A model for the diffusion of In adatoms on (110) surfaces explains the observed ordering and establishes general criteria for the optimized fabrication of the three different InAs nanostructure configurations, as a function of the growth conditions. These results are important for the use of ordered InAs nanostructures in future optoelectronic applications.

20.
Sci Total Environ ; 374(1): 36-42, 2007 Mar 01.
Artículo en Inglés | MEDLINE | ID: mdl-17270253

RESUMEN

Gas mantles for camping gas lanterns sometimes contain thorium compounds. During the last years, the use of thorium-free gas mantles has become more and more popular due to the avoidance of a radioactive heavy metal. We investigated a gas mantle type that is declared to be "non-radioactive" and that can be bought in Austria at the moment. Methods used were Instrumental Neutron Activation Analysis (INAA), gamma-spectroscopy, and Liquid Scintillation Counting (LSC). We found massive thorium contents of up to 259 mg per gas mantle. Leaching experiments showed that only 0.4% of the Th but approximately 90% of the decay products of (232)Th can be leached under conditions simulating sucking and chewing with human saliva. In this paper, the investigation of these gas mantles including the consideration of the environmental hazard caused by disposed mantles and the health hazard for unsuspecting consumers is presented and legal consequences are discussed for this fraud.


Asunto(s)
Iluminación/métodos , Torio/análisis , Gases , Iluminación/instrumentación , Iluminación/legislación & jurisprudencia , Radiometría/métodos
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