Search Results - (Author, Cooperation:M. L. Steigerwald)

2013-06-08

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

X-ray powder patterns obtained from three different types of 35–40 A(ring) CdSe nanoclusters are analyzed. We simulate the effects of thermal fluctuations, stacking faults, surface reconstructions, and bond compressions on powder patterns of CdSe clusters using the Debye formula. We find that the spectra of capped and annealed CdSe particles grown in inverse micelles are best fit by a mixture of crystalline structures intermediate between zinc-blende and wurtzite. We describe a new preparation for CdSe clusters, the structure of which appears to be well-defined wurtzite with, on average, less than one stacking fault per cluster. Thermal effects are found to be important and to mask the subtle effects of likely surface and core reconstructions.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The resonance Raman spectrum of 45(+−3) A(ring) diameter CdSe clusters was measured. The incident photons were resonant with the HOMO–LUMO transition in the clusters. At low temperature, one mode at 205 cm−1 is observed, as well as two overtones, with the integrated areas under these peaks in the ratio of 9:3:1. This mode is assigned as the longest wavelength longitudinal optical vibration of the cluster. The strength of the coupling between the lowest electronic excited state and the LO vibration is found to be 20 times weaker in these clusters than in the bulk solid. The CdSe cluster resonance Raman spectrum is shown to be consistent with the recently measured homogeneous cluster absorption spectrum.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A(ring) diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Ta2O5 is a candidate for use in metal–oxide–metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta2O5 MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films 〈300 Å thick, is dominated by a large number of defects, ∼1019–1020 cm−3, located near the metal–oxide interfaces. The interior of thick Ta2O5 films has a substantially reduced number of defects. Modest heating (300–400 °C) of Ta2O5 in contact with a reactive metal electrode such as Al, Ti, or Ta results in interfacial reactions and the diffusion of defects across the thickness of the film. These experiments show that successful integration of Ta2O5 into semiconductor processing requires a better understanding of the impact of defects on the electrical characteristics and a better control of the metal–Ta2O5 interface. © 2000 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Ab initio methods (Hartree–Fock, generalized valence bond, and configuration interaction) have been used to characterize the electronic states of molecular analogs to trimethylaluminum. An understanding of the mechanism of photoexcitation and subsequent dissociation emerges as the calculations reveal that optical excitation results in a metastable bound state rather than a directly dissociative state. The substantial excess energy associated with the transition is stored in electronic and vibrational degrees of freedom other than those associated with the dissociation coordinate. This interpretation allows a large body of observations to be simply understood and has implications for the implementation of the technology of laser photolysis for thin-film deposition.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, we have studied structures used in metal–oxide–metal capacitors including Ta2O5/TiN/Ti, Ta2O5/Ti, Ta2O5/TaN/Ti, Ta2O5/WN/Ti, and Ta2O5/M, where M=Ta, Pt, W, Al, and Si. We find that Ti and Al are able to reduce the Ta2O5 to Ta, forming oxides of Ti and Al, respectively. The diffusion barriers TiN, TaN, and WN hamper the diffusion of oxygen and therefore postpone the reduction of Ta2O5 to higher temperatures. As judged by the temperatures at which the reduction of Ta2O5 occurs, TaN and WN are more effective oxygen-diffusion barriers than TiN. We observe no oxygen remaining in the diffusion barrier when a Ti layer is present underneath. We observe no reduction of Ta2O5 when M=Pt, W, or Si. © 1999 American Institute of Physics.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] C. glabrata and S. pombe respond to Cd salts by synthet y-glutamyl peptides7'8. Cytoplasmic cadmium ions are sequestered within complexes containing these y-Glu pep-tides7'9. Related metal- y-peptide complexes are found in metal-exposed plants and Euglena gracilis10'12. The metal-peptide complexes ...

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