Rameshwar N. Bhargava

Homogeneous precipitation of doped zinc sulfide nanocrystals for photonic applications

A process was developed to prepare nanocrystalline and quantum-confined particles of manganese-doped zinc sulfide. By the reaction of diethylzinc with solubilized hydrogen sulfide, particle sizes of 30–36 A were achieved by control of reactant concentration, and size appeared to vary with the thermodynamic considerations indicative of homogeneous precipitation. Managanese doping required the development of an in situ chemical reaction compatible with the homogeneous precipitation reaction. To that end, ethylmagnesium chloride was reacted with manganese chloride to form the metastable diethylmanganese which acted as the dopant source. Quantum confinement of the particles was accomplished by using methacrylic acid and poly(methyl methacrylate) polymer of low molecular weights. These surfactants were transparent to the ultraviolet wavclcngths of light which allowed luminescent excitation of the material and provided surface passivation which enhanced phosphor brightness. The surfactant adsorption and effect of ultraviolet curing of the surfactant on the luminescent efficiency of the doped nanocrystals was investigated by infrared spectroscopy. These results indicate that the chemisorption of the surfactants to the nanoparticle surface and oxidation followed by crosslinking during curing are responsible for the improvement in luminescent efficiency.

Doped Zinc Sulfide Nanocrystals Precipitated within a Poly(Ethylene Oxide) Matrix-Processing and Optical Characteristics

Zinc sulfide nanocrystals doped with a manganese activator element were precipitated within a poly(ethylene oxide) polymer matrix to study the effects of size and quantum confinement on the luminescent properties of ZnS:Mn. Ultraviolet absorption demonstrates the increased bandgap due to quantum confinement. Photoluminescence measurements show the characteristic Mn 2+ emission from within the ZnS host crystal and photoluminescent excitation measurements show a change in excitation which is indicative of the increased bandgap. These characteristics are shown to change with growth of the particles. The growth rate of the ZnS particles is contrasted with the growth rate of CdS by calculating the molecular diffusivity for diffusion controlled growth in each case

Remanent nonlinear magnetic response in superconducting Y‐Ba‐Cu‐O

We report a ‘‘memory’’ effect in the response of sintered Y‐Ba‐Cu‐O to alternating magnetic fields. The application of a steady bias field, above a certain threshold, causes the generation of harmonic components in the response. These harmonics persist after the field is removed, exhibiting a logarithmic decay with time. We explain these phenomena in the framework of the Bean model, taking into account the granularity of the sample. The memory phenomenon offers a principle for a new recording mechanism which may allow extremely high storage capacity.

Mg–O Complexes in GaP—A Yellow Diode

A yellow‐orange luminescence which peaks at 6030 A at 300 °K has been observed in (Mg, O)‐doped GaP and is attributed to a bound excitonic transition at a nearest‐neighbor Mg–O complex. A photoluminescent efficiency at low temperatures of ∼20% has been measured and yellow diodes of external quantum efficiency ≃0.1% at 77 °K have been fabricated.

Harmonic generation in the AC magnetic response of high‐Tc superconductors—applications

Nonlinear magnetic behavior in high‐Tc superconductors as reflected by the appearance of harmonics in ac magnetic response, yields valuable information about transition temperature, critical fields and the dynamics of fluxons. In this paper we discuss the specific applications of the third harmonic measurements in developing (i) a high resolution spectrometer for determining Tc’s and (ii) a microscope which reveals local variation of Tc and the other superconducting properties.

Role of a Single Dopant in Binary and Ternary Nanocrystals

The role of a single dopant in binary and ternary semiconductor nanocrystals has been studied. The changes in the luminescent properties of the dopant brought about by the quantum confinement provided by 2nm to 5nm size host have been studied. Band gap engineering is being sought to develop efficient nanophosphors that are optimized for use for white LEDs and conventional discharge light sources.