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Measurement of absolute quantum efficiency and its correlation with spontaneous emission rate in light emitting materials

Authors Tam, Alan Man Chun
Issue Date 2009
Summary Correlation between absolute quantum efficiency (QE) and recombination rate in Au-coated Alq3, ZnO tetrapod and DCG photonic crystal was experimentally studied. Absolute QEs of Au-capped Alq3 films were measured using an integrating sphere. The QEs of uncoated and Au-coated regions of Alq3 are 4-9%and 20- 45% respectively. Photoluminescence (PL) enhancement of 2–6 times from bare Alq3 to metal-coated films clearly indicates surface plasmon (SP) enhanced emission. Comparison of integrating sphere and direct PL measurements in Au-capped Alq3 film suggests that up to 40% of the enhancement in direct measurement comes from mirror and/or enhanced directional scattering effects rather than from SP-mediated emission. In time-resolved measurement, the decrease of PL decay lifetime in Au-coated Alq3 films indicates enhanced radiative recombination rate. The enhanced QE with simultaneous reduction in PL lifetime gives further confirmation of SP coupling effect in Au-capped Alq3 films. Absolute QEs and PL decay lifetimes of ZnO tetrapods grown at different temperatures were measured. All the tetrapods had an UV peak at about 390nm and a very weak defect emission. Measurements showed that the tetrapods have QEs of 2-4%. The sample, grown at optimal temperature, exhibited the largest absolute QE of 4.29% and longest PL decay lifetimes among all the samples. These results showed that precise control of grown temperature plays an important role in making high quality ZnO tetrapods. In time-resolved measurement, the 1/ e PL decay time constant ( τ ) versus temperature is well fitted by the theoretical prediction τ = aT 3/2. This increase in PL lifetime with increasing temperature shows that the excited state relaxation is dominated by radiative recombination. PL spectra and lifetime of some DCG photonic crystals were studied. For substrate side emission, sudden decrease of PL lifetime at the photonic band edge located about 580-590nm was observed and was attributed to the large density of mode at the photonic band edge
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2009
Language English
Format Thesis
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