National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Structural, optical and photoluminescence properties of ZnS: Cu nanoparticle thin films as a function of dopant concentration and quantum confinement effect

Abstract: Thin films of ZnS: Cu nanoparticles were deposited in chemical bath by a pH controlled solution synthesis technique. The copper concentration was varied from 0 to 0.1M%. XRD and SEM indicated variations in diffracted intensity and morphology with Cu concentration. The PL spectrum of the undoped ZnS nanoparticles showed emission peaks at 393 and 432nm that could be attributed to the intrinsic defect states of ZnS nanoparticles. For ZnS: Cu samples three peaks in the range of 390nm, 480nm and 525nm were observed. With increase in Cu concentration from 0.001 to 0.1M%, the peak position of 480nm and 525nm did not change, whereas 390nm peak red shifted to longer wavelength region to 422nm. In addition, it was found that the overall photoluminescence intensity reached maximum at 0.01M% and quenched with further increase in Cu concentration. Enhancement of blue and green light emission by seven and twenty fivefold respectively compared to undoped ZnS was observed in ZnS: Cu with optimal dopant concentration. Time resolved decay of photoluminescence showed faster decay for 390 – 420nm purple/ blue emission compared to green (525nm) Cu related emission which is in the microsecond time scale. Optical absorption measurements indicate enhancement of band gap (3.89eV) for undoped ZnS suggesting the quantum confinement effect in the developed nanoparticles of size below the Bohr diameter. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnesium diffusion, surface segregation and oxidation in Al-Mg alloys

Abstract: Auger electron spectroscopy (AES) has been used to follow the surface segregation behaviour of magnesium at the surface of Al-Mg alloys in the temperature range up to 600° C as a function of time The evaporation rate of magnesium from the magnesium-rich surface has also been measured The combination of the competing processes of segregation and evaporation has been treated theoretically and compared with the experimental measurements The measured equilibrium surface enrichment of magnesium fell from a factor of 24 at 100° C to 12 at 200° C At higher temperature the evaporation rate exceeded the segregation rate and the surface layer became magnesium-depleted The data also lead to a low-temperature determination of the diffusivity of magnesium in aluminium The same Al-Mg alloys have been heat-treated, within a similar time-temperature regime, in air The oxide films have been composition-depth profiled using AES with ion sputtering, and measurements of the rate of oxide growth lead to information about the diffusivity of magnesium through the oxide films

A molecular orbital theory of hydrocarbons

Abstract: An independent electron molecular orbital theory is applied to all valence electrons of saturated hydrocarbons, using the 2s and 2p atomic orbitals of carbon and the 1s orbitals of hydrogen as a basis. It is shown that certain results about uniformity of charge distribution, already known for π-electrons, apply to all valence electrons in paraffins under conditions more general than those which lead to localized bonding. An attempt is then made to estimate the extent of electron delocalization in paraffins by calculating long-range molecular orbital bond orders and the associated contributions to energy stabilization. This is done by a perturbation method, the features causing delocalization being introduced as a perturbation of a theory of completely localized bonds. It is concluded that delocalization may arise from (1) the difference between atomic 2s and 2p energies, (2) bonding between non-neighbouring atoms and (3) partial π-bonding in carbon-carbon bonds. The first two causes lead to geminal intera...