Structural Studies Of Amorphous Si-Ni-H

It is well know that the transition from an insulator to a metallic conductor may be induced in amorphous semiconductor : metal alloys by increasing the metal concentration above a certain critical limit. However, without a detailed understanding of the changes taking place in the atomic scale structure, it is difficult to ascribe a mechanism to the process. We have investigated the microstructure of one such alloy system, a-Si1-yNiyH, using EXAFS as the principal technique. Thin film samples, prepared by rf co-sputtering, were studied over the composition range 0<y<0.3. Both silicon and nickel K-edge EXAFS results are presented, together with complimentary data from Raman scattering, neutron diffraction and scanning calorimetry experiments. The results indicate that the samples contain two separate amorphous phases: a Ni:Si alloy which is embedded in the surviving, modified a-Si host network. The measured electrical conductivity is discussed in the light of this structural model

The Structure Of a-Si(1-X)Sn(X)H Thin Films

The doping of a-Si:H with Sn is known to modify the electrical and optical properties of the material. The optical band gap decreases as the doping level is increased, however, there is no insulator-metal transition of the type observed, for example, when transition metals are used as dopants. In order to increase the understanding of the conductivity processes that occur in a-Si:metal:H alloys we have measured the atomic scale structure of a series of a-Si(1-x)Sn(x):H thin-films using EXAFS. Samples were prepared by RF reactive co-sputtering and both Si and Sn K-edge EXAFS examined. The results indicate that the Sn atoms are substituted randomly into the a-Si tetrahedral random network. Both Si and Sn atoms retain fourfold co-ordination over the composition range studied (0-less-than-or-equal-to-x-less-than-or-equal-to-0.18). In contrast to results obtained using transition metal dopants there is no local modification of the tetrahedral random network

An EXAFS Study Of The Cluster Molecule Au55(PPh3)12Cl6

Gold L3 edge EXAFS has been used to study the coordination environment of gold atoms in the cluster molecule Au55(PPh3)12Cl6. The mean coordination of 7 by other gold atoms is consistent with a 3-shell cuboctahedral structure for the Au55 cluster. The first direct measurement of the Au-Au distance in this cluster shows that the spacing is significantly shorter than that in bulk metallic gold and is consistent with calorimetric work which has shown the Au-Au interactions are stronger in the cluster than in bulk gold. There is no evidence to suggest a significant spread of Au-Au distances in Au55(PPh3)12Cl6, in contrast to lower-nuclearity gold cluster molecules which have peripheral Au-Au distances typically 0.2Å longer than those to interstitial gold atoms

Total Synthesis of Glycosylated Human Interferon-y

Interferon-γ (IFN-γ) is a glycoprotein that is responsible for orchestrating numerous critical immune induction and modulation processes and is used clinically for the treatment of a number of diseases. Herein, we describe the total chemical synthesis of homogeneously glycosylated variants of human IFN-γ using a tandem diselenide-selenoester ligation-deselenization strategy in the C- to N-terminal direction. The synthetic glycoproteins were successfully folded, and the structures and antiviral functions were assessed.Xiaoyi Wang, Anneliese S. Ashhurst, Luke J. Dowman, Emma E. Watson, Henry Y. Li, Antony J. Fairbanks, Mark Larance, Ann Kwan, and Richard J. Payn

Structural Studies Of Amorphous Semiconductor-Metal Alloys

It is well known that a semiconductor to metal transition may be induced in amorphous semiconductor-metal alloys by increasing the metal concentration above a critical limit. However, without a knowledge of the atomic scale structure of the alloy it is difficult to ascribe a mechanism to this process. Three alloy systems (a-Si1?xNix---H, a-Ge1?xAux and a-Si1?xSnx---H) have been prepared as thin films by rf reactive co-sputtering over pertinent composition ranges. The micro-structure of these alloys has been investigated using EXAFS. Both a-Si1?xNix---H and a-Ge1?xAux appear to consist of two separate phases, regions of an amorphous Ni---Si alloy and a crystalline Ge---Au alloy being embedded in an amorphous matrix provided by a-Si and a-Ge, respectively. In contrast, however, Sn atoms are substituted randomly into the a-Si tetrahedral random network