Michael T. Kelly

High efficiency chemical etchant for the formation of luminescent porous silicon

Hydrofluoric acid solutions containing high concentrations of the ion NO+ produce an etchant capable of reproducibly generating a porous silicon layer on both single‐crystal and polycrystalline silicon surfaces. Room‐temperature photoluminescence from porous silicon that has been chemically etched in such solutions has been observed. The photoluminescent intensity is superior to that obtained using HNO3/HF based stain etches. Reproducibility with respect to etch induction time, and the quality of the porous silicon layer are also improved when compared to classic stain etchants. Although, prior work has suggested that HNO2 is the active oxidant in silicon stain etching processes, the present work points to NO+ as the active species.

Nickel ferrocyanide modified electrodes as active cation-exchange matrices : real time XRD evaluation of overlayer structure and electrochemical behavior

The dynamic variations in the structure of nickel ferrocyanide modified electrodes was determined using in situ diffractoelectrochemical techniques. X-ray powder diffraction peaks from the nickel ferrocyanide modified electrode were recorded during electrochemical oxidation/reduction of the surface overlayer and reversible changes in the unit cell lattice parameter observed. The electrochemical changes that occur during oxidation/reduction in various alkali cation electrolytes are correlated with changes in the lattice parameter. It is postulated that micro-domains form when two cations having markedly different radii such as cesium and potassium are intercalated into the nickel ferrocyanide structure so that distinct regions of the nickel ferrocyanide layer containing cesium exist separated from regions with the lighter alkali cations.

MECHANISMS OF PHOTOLUMINESCENT QUENCHING OF OXIDIZED POROUS SILICON: APPLICATIONS TO CHEMICAL SENSING

Abstract Silicon can be caused to photoluminesce in the visible by rapid anisotropic etching of bulk samples to form a porous material. Both electrochemical and chemical etching procedures based on an HF reagent have been developed for this purpose. Porous silicon is found to contain nanoscopic silicon particles which have been identified as the luminescent species. The observed photoluminescence is very sensitive to the surface preparation of the porous silicon. Light oxidation of the porous silicon substrate produces a material which is selectively sensitive to Bronsted bases, sulfur dioxide, and halogens. Selective quenching of oxidized porous silicon is associated with the presence of specific types of dangling bond surface states (Pb0 and Pb1) at the silicon/silicon oxide interface. Interfaces of this type can be fashioned into environmental sensors. Using this approach a gas phase sulfur dioxide sensor has been demonstrated.

Perspective on the Storage of Hydrogen: Past and Future

There are clear advantages to using hydrogen as a fuel, but storage of hydrogen in a light-weight and compact form remains a challenge. This review highlights past breakthroughs that led to the current thinking in hydrogen storage methodology. Metal organic frameworks are discussed briefly, with greater attention to the storage of hydrogen in other molecules. Many molecular storage strategies rely on the thermal decomposition of hydrogen hetero atom bonds and formation of hydrogen to hydrogen bonds. An acid-base thought model is presented to explain observed behaviors and to guide future endeavors.