A Study of Photophysics and Photochemistry of I-III-VI2 Nanocrystals

Abstract

This thesis, entitled “A Study of Photophysics and Photochemistry of I-III-VI2 Nanocrystals” primarily deals with the properties of I-III-VI2 semiconductor nanocrystals composed of earth abundant, environmentally benign and relatively non-hazardous elements. In initial two chapters, the synthesis and photophysics of CuFeS2 and CuAlS2 QDs have been described. Both materials are potential candidates for various optoelectronic applications, and this makes the study of their physical properties interesting and relevant. Chapter 5 shows the light harvesting potential of I-III-VI2 QDs by using these to perform efficient artificial photosynthesis. Chapter two describes the stable synthesis and interesting optical properties of CuFeS2 and its core shell structures. These materials exhibit a tunable band gap that spans the range of 0.5 – 2 eV (600 nm – 2500 nm). Although the as-prepared material is non-emissive, CuFeS2/CdS core/shell structures are shown to exhibit quantum yields that exceed 80%. Like other members of the I-III-VI2 family QDs, CuFeS2 based nanoparticles exhibit a long- lived emission that is significantly red shifted compared to the band gap. Chapter three shows the various optical properties of CuAlS2 based QDs through calculation and ultrafast studies. CuAlS2/CdS QDs are shown to be associated with cross sections lower than 10-17 cm2 under the emission band. Investigation of this anomaly using spectroscopic techniques are described, and further, it is ascribed to the existence of a strong type-II offset between CuAlS2 and CdS layers. Besides their strong Stokes’ shift, CuAlS2/CdS QDs also exhibit high quantum yields (63%) as well as long emission lifetimes (~1500 ns). Finally the construction of a wide area transparent lighting device with a clear aperture of 7.5 cm2 is discussed. In Chapter four, the physical reason behind the stability of these I-III-VI2 QDs has been investigated. The optical properties of copper containing II-VI alloy quantum dots (CuxZn¬yCd1-x-ySe) were studied. Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases are observed over this composition range. The optical absorption and emission spectra of these materials are observed to be a strong function of copper mole fractions, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply with mole fraction of copper around 12%, suggesting a composition induced change in local electronic structure. In chapter five as photo reductive solar energy harvesters, it is shown that newly synthesized CuAlS2/ZnS QDs offer unprecedented advantages: these are composed of completely biocompatible, earth abundant, inexpensive elements; these exhibit very high solar to chemical energy conversion efficiencies and finally, light harvesting via these materials may be set up to reduce the carbon dioxide already present within the earth’s atmosphere. CuAlS2/ZnS structures can reduce aqueous bicarbonate ions to formate under visible light. The high turnover numbers (>7x104 molecules of sodium formate produced per QD), solar to chemical energy conversion efficiencies (20.2 +/- 0.2) are rationalized through our spectroscopic studies that show a short 550 fs electron dwell times in these structures. The high energy efficiency and the environmentally friendly composition of these materials suggest a future role in solar light harvesting.