Studies on the synthesis and applications of I-III-VI2 semiconductor nanocrystals

Abstract

Nanomaterials have been a topic of extensive research for the past several decades. This is because their properties act as a bridge between their bulk and atomic counterparts. Broadly, nanoparticles can be characterized as one dimensional, 2 dimensional or 3 dimensional depending on the number of directions in which particle size is limited. A semiconductor nanoparticle which is dimensionally limited in all the three directions is known as a Quantum Dot. In Quantum dots (QDs) the photoexcited charge carriers are constrained in a small volume and are not free to move in any direction. This causes an increased overlap between the electron and hole wavefunctions. As a result quantum dots possess many fascinating properties which render them useful in the field of optoelectronics, photovoltaics, and so on. However the use of QDs as efficient photocatalysts is not known. Recently, CuAlS2/ZnS QDs were reported which could reduce aqueous Sodium bicarbonate ions to formate ions using visible radiation. The average energy conversion efficiency obtained was 17% with a maximum of 20% with a turnover number of 7.8 x 104 that is significantly greater than any values reported previously. However, the major reported reduction product is sodium formate. From an energy perspective, it would be much more beneficial to have combustible organics as the reduction product as these could be directly used as fuels. Even though the CuAlS2/ZnS QDs could eventually reduce bicarbonate into organics like butanol, it takes weeks for the reaction to complete. It would hence be highly desirable to have a catalyst that could do this over a much shorter duration. In the second chapter of my thesis I have synthesized CuGaS2/ZnS nanoparticles. These quantum dots are capable of photo-reducing aqueous sodium bicarbonate into a mixture of alcohols using visible light. This is enormously advantageous. The material is made completely of biocompatible elements which makes processing and use of this material entirely safe for environment. The materials used are earth abundant and this reduces the manufacturing cost of the catalysts. The photo-reduced products (mainly butanol) can be used as a fuel and reducing bicarbonate can help reduce the global warming by decreasing CO2 levels in the atmosphere. Third chapter of my thesis elucidates the synthesis and optical properties of Copper Iron Aluminum Sulphide (CuFexAl1-xS2) QDs and its core shell structure with CdS. The band gap of CuAlS2 is 3.45 eV while the band gap of CuFeS2 system is 0.5 eV. The alloyed CuFexAl1-xS2 thus can have a tunable band gap from 3.45 to 0.5 eV. We demonstrate two compositions i.e CuFe0.1Al0.9S2 and CuFe0.35Al0.65S2 which exhibit band gaps of 1.6 eV and 0.7 eV respectively. These hybrid materials are not luminescent as such but coating a CdS layer on top of these materials makes them luminescent by eliminating surface traps. The CdS coated CuFexAl1-xS2 material QDs also exhibit tunable photoluminescence and tunable life time. The CuFexAl1-xS2/CdS system manifests the properties of both CuFeS2/CdS and CuAlS2/CdS i.e high Stokes shift and reasonably high Quantum yields. The potential of these materials for transparent display devices was verified.