Chemically Synthesized ZnO Nanostructures : Realization of White Optoelectronic Devices with High CRI Values

Abstract: Recently in a couple of decades, nanotechnology and nanoscience are becoming wide spread fields of research due to the revolutionary advances in the manufacturing processes which enable the realization of infinitesimally modest nanodevices holding a huge variety of fascinating properties and applications. Besides various functional materials, ZnO has captivated interests for a variety of applications in electronics and optoelectronics owing to its unique characteristics; such as, direct wide band gap, large exciton binding energy, semiconducting, photonic, and piezoelectric properties. A distinguished capability of the ZnO material is the effortless synthesis of nanoscale structures with enormous assortments in their morphological and dimensional aspects. Regardless the significant developments in the fabrication of ZnO based homojunction optoelectronic nanodevices, the stable and reproducible p-type conductivity of ZnO material is still a challenge which is one of the paramount factors of the increasing interest for fabrication of heterojunction of ZnO nanostructures with other mainstream ptype semiconductors, such as Si, GaN, and organic materials.Herein, ZnO nanorods, nanotubes and nanoflowers have been synthesized by solution-based methodology at low temperature (<100 ˚C) and a thorough study on the applications of ZnO nanostructures as white light emitting diodes (LEDs) has been perceived. At the outset, ZnO nanotubes have been synthesized by the trimming of aqueous chemically grown ZnO nanorods with 100% yield and their comparative optical properties have been explored through photoluminescence study, and a profound enhancement in ultraviolet and visible emission is observed (paper I). ZnO nanotubes are further exploited for its promising application as an optoelectronic device. Pure white light emission is observed from the ZnO nanotubes/p-GaN based LED. To analyze the location of the recombination of electron–hole and current transport mechanisms, the EL characteristics of n-ZnO nanotubes/p-GaN heterostructure LED have been investigated under forward and reverse bias. The origin of distinctly different EL peaks under both configurations has been suggested and the influence of increasing values of temperature on the device characteristics is also studied under fixed applied current, in order to check its performance under harsh conditions and for practical  applications (paper II-III). Moreover, it is observed that ZnO-nanotubes/GaN heterostructure LED has an ability to produce an environmentally benign alternative of traditional lighting sources with high color rendering index (CRI) of 96 (paper IV). On the basis of EL, cathodoluminescence and transmission electron microscopy investigations; a robust correspondence has been established between the formation of radiative surface defect states in the nanotubes and the pure cool white light with appropriate color temperature. In paper V, a miniaturized white LED has been developed using Au/n-ZnO nanorods integrated on a glass pipette (having a sharp cylindrical tip with the diameter of 700 nm) which exhibits a broad EL band emission covering the whole visible spectrum range and a CRI value of 73. Besides one-dimensional ZnO nanostructures (nanorods and nanotubes), three-dimensional ZnO dahlia-flower nanoarchitectures have also been fabricated at room temperature relying on natural oxidation based aqueous chemically synthetic approach (paper VI).  Glycineassisted multi-oriented ZnO nanoflowers with highly large surface area to volume ratio have been synthesized on Zn foil substrate through the self-assembly of thin nano-petals as building blocks and polar surfaces of ZnO have been anticipated to be  stabilized through the adsorption of reactive hydroxyl and amide functions of glycine.