Meera Ramrakhiani

Zinc doped polycrystalline CdSe films for solar energy conversion

Abstract The physical properties of semiconductors define to a great extent the performance of photoelectrochemical (PEC) cells. With a band gap of 1.7 eV, cadmium selenide is an attractive candidate as photoelectrode in PEC cells for the conversion of light into electricity. Polycrystalline films are used for low-cost cells. CdSe films are prepared on a titanium substrate by the electrocodeposition technique from a solution of SeO 2 , CdSO 4 and H 2 SO 4 in galvanostatic conditions, then annealed at 400 °C. Chemical modification of semiconductor surfaces by metal ions has been found to improve the performance of photoelectrodes. Zn ion incorporation on the CdSe film surface was achieved by dipping the film in aqueous solution of ZnSO 4 followed by heating in air. The preparation conditions were optimized to give the best performance. The photovaltaic activity of the films was studied with a polysulphide electrolyte using graphite as a counter electrode and a 200 Watt tungsten lamp as the light source. Addition of Zn ions at the surface produces favourable states in band gap, enhancing charge transfer kinetics at the interface. On heating, Zn diffuses through grain boundaries, reducing recombination centres for majority carriers, thus increasing diffusion length and so photocurrent. In the CdSe lattice, Zn replaces Cd ions which causes an increase in band gap since ZnSe has a band gap higher than CdSe. Hence a small amount of Zn reduces I sc due to an increase in band gap, but as the Zn concentration is increased, greater band bending is obtained, increasing V oc and improving overall performance.

A search for mechanoluminophors capable of pressure-induced thermal population of excited states

During the process of deforming a crystal, a high pressure is developed near the tip of mobile cracks, which may in turn produce a new ground state by thermal electron transfer. Upon sudden release of pressure, the electron can either relax to one atmosphere ground state or remain in the excited state potential well long enough to relax to one atmosphere and radiatively transfer back to the ground state. For analysing the pressure induced thermal population of the excited state, the mechanoluminescence(ML) and high pressure photoluminescence(PL) of several organic and inorganic crystals were measured. The study indicated that usual pressure coefficient of energy shift of the order of 50–100 cm−1/kbar and the stress at the crack-tip of the order of 5–10 kbar, are not sufficient to cause the thermal population of the excited state. If by any means the product of pressure coefficient and stress at the mobile crack-tip can be increased by 50 to 100 times, then the thermal population of the excited states may take place. Using the pressure coefficient of energy shift and the difference in ML and PL spectra, and using independently the change in relative intensities of the vibronic peaks, the pressure at the emitting mechanoluminescent crystal sites is evaluated and it is found to be of the order of several kbar which varies from crystal to crystal.

Effect of heating on the hardness of human bone

Specimens of dry human bone wee subjected to gradual or sudden heating up to different temperatures. After heating they were cooled suddenly or gradually to room temperature. Their hardness was studied by the Vickers and ball indentation tests. The deformation produced was studied by interferometric and phase-contrast techniques. Heating caused the bone to become brittle without any considerable changes in the surface features. Gradual cooling induced surface hardening. Heating was not found to affect indentation-induced surface distortion.

Photo- and Electro- Luminescence of Polyimide Films

Polyimide films have been prepared by chemical process. Their absorption spectrum and photoluminescence have been studied. Absorption spectrum shows stepwise increase in absorption revealing narrow energy bands. Photoluminescence is of exciplex origin and gives dissociation energy of diamine and dianhydride in polyimide as 2.2 eV. Electroluminescence of polyimide film has been observed at 10-40 volts.

Effects of dynamic impacts on human bones

Flat bones of human skeleton were subjected to dynamic indentation with ball indenters. The impacted surface was studied under high magnification and also by using the technique of multiple beam interferometry. The impulse caused the pile up of material at a little distance from the edge of the indent. The diameter of indent is found to increase as fourth root of the energy of impact. Bone structure also has the tendency to minimize the damage caused by external forces. There was about 90% recovery in deformation in the depth of indents due to internal stresses created inside the bone by the impact.