Synthesis, Photo-physical Studies and Applications of Different Doped and Un-doped Semiconductor Nanocrystals

Abstract

High quality semiconductor nanocrystals (quantum dots, QDs) are small crystal consisting of hundreds to a few thousand atoms each with typical dimension ranging from 1-100 nm which are great interest for fundamental studies and different technological applications such as light emitting devices, lasers, solar cells and biomedical labeing. The quantum mechanical coupling of over hundreds to thousands atoms develops the band structure in semiconductors. In this regime, the spatial confinement of the electronic charge carriers in the nanocrystal leads to a phenomenon known as Quantum Confinement Effect (QCE). Due to this effect, the size and shape of these “artificial atoms” can be used to widely tune the energy of discrete electronic energy states and optical transitions. For this the emission from these particles can be tuned throughout the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges, making them useful for both biological imaging and many types of optoelectronic devices. State-of-the-art semiconductor nanocrystals have been designed to have a quantum efficiency of radiative recombination approaching unity at room temperature, far above what has been achieved from bulk materials. The reason of this high efficiency is also govern by the QCE as the strong overlap between the electron and hole wave functions in the confined structure increases the probability of radiative recombination whereas the exciton in bulk semiconductors is not confined in space and can rapidly dissociate, increasing the probability of non-radiative relaxation process associated with crystalline defects and charge carrier traps on crystal surfaces. Among semiconductor NCs CdSe as the work horse, have been widely studied for their fundamental properties and applications. Despite their so many advantages, the intrinsic toxicity of Cadmium has cast a doubtful commercial future for this promising field. Wide band gap semiconductor nanocrystals, such as zinc chalcogenide doped with transition metal ions, has overcome this concern and yet maintained the advantages of the nanocrystal emitters. Mn and Cu doped zinc chalcogenide semiconductor NCs can give bright yellow orange and tunable blue green emission respectively which has been found very stable and having high quantum yield making them useful for different practical application without having highly toxic Cd metal. Besides their low toxicity by replacing cadmium in CdSe quantum dots with zinc, these doped materials do not also reabsorb the photon which avoids self-quenching, a common phenomenon in quantum dots because of small stokes shift. In contrast, the emission color from a dopant, involving d–states of transition metal ions, to a large extent is fixed and independent of the size of the host. The only way to get substantially different dopant colours is to use different dopant ions. Unfortunately, doping such impurity ions into nanoparticle hosts has proven to be unexpectedly difficult, and different synthesis methods have to be followed to get different d–dots.Research was carried out under the supervision of Prof. Narayan Pradhan of Materials Science division under the SMS [School of Materials Science]Research was conducted under IACS fellowship and DST research gran