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Berlin Brandenburg

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  • 1
    Language: English
    In: Analytical chemistry, 01 April 2011, Vol.83(7), pp.2469-75
    Description: The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of (235)U/(238)U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of uranium oxide targets with essentially no sample preparation.
    Keywords: Uranium -- Chemical Properties ; Mass Spectrometry -- Management ; Accuracy And Precision -- Research ; Lasers -- Properties ; Bandwidth -- Research ; Isotopes -- Measurement;
    ISSN: 00032700
    E-ISSN: 1520-6882
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  • 2
    Language: English
    In: Science, 328(5975):224-228, 09 April 2010, Vol.328(5975)
    Description: Production of the industrial chemical propylene oxide is energy-intensive and environmentally unfriendly. Catalysts based on bulk silver surfaces with direct propylene epoxidation by molecular oxygen have not resolved these problems because of substantial formation of carbon dioxide. We found that unpromoted, size-selected Ag3 clusters and ~3.5-nanometer Ag nanoparticles on alumina supports can catalyze this reaction with only a negligible amount of carbon dioxide formation and with high activity at low temperatures. Density functional calculations show that, relative to extended silver surfaces, oxidized silver trimers are more active and selective for epoxidation because of the open-shell nature of their electronic structure. The results suggest that new architectures based on ultrasmall silver particles may provide highly efficient catalysts for propylene epoxidation.
    Keywords: Inorganic, Organic, Physical And Analytical Chemistry ; Nanoscience And Nanotechnology ; Catalysts ; Electronic Structure ; Propylene ; Silver ; Catalytic Effects ; Nanostructures ; Oxidation ; Epoxides ; Environmental Molecular Sciences Laboratory ; Sciences (General) ; Biology
    ISSN: 0036-8075
    E-ISSN: 1095-9203
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  • 3
    Language: English
    In: Journal of Physical Chemistry C, 01 January 2011, Vol.115(50)
    Description: Niobium carbide thin films were synthesized by atomic layer deposition (ALD) using trimethylaluminum (TMA), NbF{sub 5}, and NbCl{sub 5} precursors. In situ quartz crystal microbalance (QCM) measurements performed at 200 and 290 C revealed controlled, linear deposition with a high growth rate of 5.7 and 4.5 {angstrom}/cycle, respectively. The chemical composition, growth rate, structure, and electronic properties of the films were studied over the deposition temperature range 125-350 C. Varying amounts of impurities, including amorphous carbon (a-C), AlF{sub 3}, NbF{sub x}, and NbCl{sub x}, were found in all samples. A strong growth temperature dependence of film composition, growth rate, and room temperature DC resistivity was observed. Increasing film density, decreasing total impurity concentration, and decreasing resistivity were observed as a function of increasing deposition temperature for films grown with either NbF{sub 5} or NbCl{sub 5}. Superconducting quantum interference device (SQUID) magnetometry measurements down to 1.2 K revealed a superconducting transition at T{sub c} = 1.8 K in a 75 nm thick film grown at 350 C with TMA and NbF{sub 5}. The superconducting critical temperature could be increased up to 3.8 K with additional use of NH{sub 3} during ALD film growth.
    Keywords: Engineering ; Carbon ; Chemical Composition ; Critical Temperature ; Deposition ; Impurities ; Microbalances ; Niobium ; Niobium Carbides ; Quartz ; Squid Devices ; Superconductors ; Temperature Dependence ; Thin Films ; Engineering ; Chemistry
    ISSN: 1932-7447
    E-ISSN: 1932-7455
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  • 4
    Language: English
    In: J. Phys. Chem. C, 22 February 2011, Vol.115(Feb. 22, 2011)
    Description: Growing interest in Fe{sub 2}O{sub 3} as a light harvesting layer in solar energy conversion devices stems from its unique combination of stability, nontoxicity, and exceptionally low material cost. Unfortunately, the known methods for conformally coating high aspect ratio structures with Fe{sub 2}O{sub 3} leave a glaring gap in the technologically relevant temperature range of 170-350 C. Here, we elucidate a self-limiting atomic layer deposition (ALD) process for the growth of hematite, {alpha}-Fe{sub 2}O{sub 3}, over a moderate temperature window using ferrocene and ozone. At 200 C, the self-limiting growth of Fe{sub 2}O{sub 3} is observed at rates up to 1.4 {angstrom}/cycle. Dense and robust thin films grown on both fused quartz and silicon exhibit the expected optical bandgap (2.1 eV). In situ mass spectrometric analysis reveals the evolution of two distinct cyclic reaction products during the layer-by-layer growth. The readily available and relatively high vapor pressure iron precursor is utilized to uniformly coat a high surface area template with aspect ratio 150.
    Keywords: Solar Energy ; Aspect Ratio ; Coatings ; Deposition ; Ferrocene ; Harvesting ; Hematite ; Iron ; Ozone ; Precursor ; Quartz ; Silicon ; Solar Energy Conversion ; Stability ; Surface Area ; Thin Films ; Vapor Pressure ; Windows ; Chemistry
    ISSN: 1932-7447
    E-ISSN: 1932-7455
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  • 5
    Language: English
    In: Analytical chemistry, 01 June 2005, Vol.77(11), pp.3531-5
    Description: The quartz crystal microbalance is extremely useful for in situ monitoring of thin-film growth by atomic layer deposition (ALD) in a viscous flow environment. Unfortunately, conventional AT-quartz sensors are limited to growth temperatures below approximately 300 degrees C. Gallium orthophosphate (GaPO4) is an alternative piezoelectric material offering much greater high-temperature frequency stability than AT-quartz (SiO2). Our measurements reveal that the temperature coefficient for Y-11 degrees GaPO4 decreases linearly with temperature reaching 3 Hz/ degrees C at 450 degrees C. In contrast, the temperature coefficient for the SiO2 sensor increases as the cube of the sensor temperature to 650 Hz/ degrees C at 390 degrees C. To examine the effect of temperature fluctuations on the sensor frequency, we exposed the SiO2 and GaPO4 sensors to helium pulses at 400 degrees C. The resulting frequency change measured for the SiO2 sensor was approximately 40 times greater than that of the GaPO4 sensor. Next, we performed Al2O3 ALD using alternating tri-methylaluminum/water exposures at 400 degrees C and monitored the growth using the SiO2 and GaPO4 sensors. The GaPO4 sensor yielded well-defined pulse shapes in agreement with predictions, while the SiO2 pulses were severely distorted. Measurements during TiO2 ALD using alternating titanium tetrachloride/water exposures at 450 degrees C with the GaPO4 sensor also showed well-defined ALD mass steps.
    Keywords: Membranes, Artificial ; Temperature ; Aluminum Oxide -- Analysis ; Gallium -- Chemistry ; Phosphates -- Chemistry ; Titanium -- Analysis
    ISSN: 0003-2700
    E-ISSN: 15206882
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  • 6
    Language: English
    In: Applied Physics Letters, 10 December 2007, Vol.91(24)
    Description: We describe a technique for depositing materials at controlled depths within porous substrates based upon the passivating effect produced by one precursor to prevent the adsorption of a second precursor. For example, a surface exposed to trimethyl aluminum is not reactive toward diethyl zinc. This effect, combined with Knudsen diffusion in which the precursor exposure times dictate the depth of penetration of the deposited layer, enables spatially controlled “stripe coating” within porous supports. We demonstrate ZnO stripes in anodic alumina and model the results using Monte Carlo simulations. Etching is identified as a potential problem for certain precursor combinations.
    Keywords: Nanoscale Science And Design
    ISSN: 0003-6951
    E-ISSN: 1077-3118
    Source: © 2007 American Institute of Physics (AIP)〈img src=http://exlibris-pub.s3.amazonaws.com/AIP_edited.gif style="vertical-align:middle;margin-left:7px"〉
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  • 7
    Language: English
    In: Applied Physics Letters, 17 December 2007, Vol.91(25)
    Description: The mechanism for zirconium oxide atomic layer deposition using bis(methylcyclopentadienyl)methoxymethyl zirconium and H 2 O was examined using ab initio calculations of hydrolysis energies to predict the order of ligand loss. These predictions were tested using in situ mass spectrometric measurements which revealed that the methyl ligand, and 65% of the methylcyclopentadienyl ligands are lost during the zirconium precursor adsorption. The remaining 35% of the methylcyclopentadienyl ligands and the methoxy ligand are lost during the subsequent H 2 O exposure. These measurements agree very well with the predictions, demonstrating that thermodynamic calculations are a simple and accurate predictor for the reactivities of these compounds.
    Keywords: Nanoscale Science And Design
    ISSN: 0003-6951
    E-ISSN: 1077-3118
    Source: © 2007 American Institute of Physics (AIP)〈img src=http://exlibris-pub.s3.amazonaws.com/AIP_edited.gif style="vertical-align:middle;margin-left:7px"〉
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  • 8
    Language: English
    In: Microscopy and Microanalysis, 08/2015, Vol.21(S3), pp.1143-1144
    ISSN: 1431-9276
    E-ISSN: 1435-8115
    Source: CrossRef
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  • 9
    Language: English
    In: Journal Name: Journal of Radioanalytical and Nuclear Chemistry, na, na, August 1, DOI 10.1007/s10967-012-2028-z
    Description: not provided
    Keywords: 42 Engineering ; 36 Materials Science ; 58 Geosciences
    Source: University of North Texas
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  • 10
    Description: We report results of high-resolution TOF SIMS (time of flight secondary ion mass spectrometry) depth profiling experiments on a nanolayered structure, a stack of 16 alternating MgO and ZnO ~5.5 nm layers grown on a Si substrate by atomic layer deposition. The measurements were performed using a newly developed approach implementing a low energy direct current normally incident Ar+ ion beam for sample material removal by sputtering (250 eV and 500 eV energy), in combination with a pulsed 5 keV Ar+ ion beam at 60{\deg} incidence for TOF SIMS analysis. By this optimized arrangement, a noticeably improved version of known dual-beam (DB) approach to TOF SIMS depth profiling is introduced, which can be called gentleDB. We apply the mixing-roughness-information model to detailed analysis of experimental results. It reveals that the gentleDB approach allows ultimate depth resolution by confining the ion beam mixing length to about 2 monolayers. This corresponds to the escape depth of secondary ions, the fundamental depth resolution limitation in SIMS. Other parameters deduced from the measured depth profiles indicate that a single layer thickness equals to 6 nm so that "flat" layer thickness d is of 3 nm and interfacial roughness {\sigma} is of 1.5 nm thus yielding d+2\bullet{\sigma}=6 nm. In essence, we have demonstrated that the gentleDB TOF SIMS depth profiling with noble gas ion beams is capable of revealing structural features of a stack of nanolayers, resolving its original surface and estimating the roughness of interlayer interfaces, which is difficult to obtain by traditional approaches. Comment: 7 pages and 4 figures
    Keywords: Condensed Matter - Materials Science ; Physics - Chemical Physics
    Source: Cornell University
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