Ultrafast Energy Transfer Between Semiconductor Nanorods And Colloidal Quantum Dots For Efficient White Light Conversion

White color light emitting diodes (LEDs) have been recorgnized as essential devices to upgrade today’s lighting technology, owing to the high energy conversion efficiency, good color quality, and long working lifetime. Most commercially available white LEDs are constructed with color conversion approach based on re-aborption in which the phosphers absorb the blue light emitted from InGaN LEDs and emit light in green and red bands. Nevertheless, limited by fixed emission wavelength of phosphor emission and relatively inefficient light extraction, there is a large development space to improve the conversion efficiency and color quality.In this dessertation, we study a new color conversion method based on ultrafast energy transfer (ET) between InGaN/GaN quantum well nanorods and colloidal nanocrystals. The new approach can significantly improve the conversion efficiency and color quality. ET between nanoraods and nanocrystals originates from Coulomb interaction between the excitons, which is strongly dependent on the separation distance between donors and acceptors. In order to reduce the separation distances and improve the ET efficiency, we suggest a hybrid structure of side-wall coupled nanorods and nanocrystals in this work.To study the physical mechanisms underlying the ET between nanorods and nanocrystals, we carry out steady and transient photoluminescence (PL) measurements on the samples at different temperatures with carrier density in different regimes. We clarify the carrier dynamics in nanorods at the beginning and explore the possible factors governing the ET. The major findings are listed below:(1) Carrier dynamics in InGaN/GaN quantum well nanorods.We observe faster recombination of carriers in nanorods in comparison to that in plannar quantum wells, induced by the increased surface state density, the strain relaxation, the band filling effect and other impacts post nanofabrication. More importantly, the delay rise observed in the time-resolved PL traces from InGaN/GaN quantum wells is absent in that from nanorods. This difference is a signature of reduced carrier diffusion in nanorods. (2) The mechanisms underlying the ET from nanorods to nanocrystals.Under relatively low or high excitation, ET rate shows different carrier-density dependence, resulting from different electron-hole configurations, i.e. bounded excitons and free carriers. In the localized exciton regime, the ET rate decreases when increasing temperature from20K to200K; However, in the free-carrier regime, the ET rate varies insignificantly in the same temperature range. The temperature dependence in this nanorod-nanocrystal coupling system is different from that in the previously-studied planar quantum well-nanocrystal coupling system. We suggest the reduced carrier diffusion in nanorods to be a major factor for these divergences.Overall, in a hybrid structure, ET efficiency above80%can be achieved with proper carrier density. The highly efficient ET is benificial for white light conversion. The conversion efficiency can increase3-5folders at least in comparison to the conventional absorption-based approach. Considering the tunable emisison color of the nanocrystals, highly efficient white LEDs with excellent color quality is realizable with the ET-based strategy.The dessertation is organized as following. In Chapter1, a general review of white LED is present; Chapter2describes the details of sample and experimetal techniques in this work; Chaptor3and4present the major findings on the carrier dynamics in nanorods and the physical mechanisms underlying ET in the hybrid structures, respectively; Chaptor5present a brief summary and outlook.