Nature Mater. 10, 669–675 (2011)
Although the radiative rates of semiconductors and molecular systems can be modified by plasmonic structures, the associated enhancement factors have so far been limited to around 10–50. Chang-Hee Cho and co-workers from the University of Pennsylvania in the USA have now used plasmonic nanocavities based on CdS nanowires to achieve radiative rate enhancements of >103 at subpicosecond lifetimes. Each cavity consisted of a CdS nanowire (50–160 nm in diameter) surrounded by a 5-nm-thick SiO2 interlayer and a 15-nm-thick Ag shell. The SiO2 interlayer plays two important roles: electronic passivation and inhibiting the rapid quenching of CdS excitons. The researchers performed microphotoluminescence measurements at 77 K using a continuous-wave Ar+ ion laser at a wavelength of 457.9 nm. The spectra showed hot-exciton emission lines below the laser excitation energy with a separation energy of 38 meV, which corresponds to the CdS longitudinal optical phonon. Time-resolved photoluminescence measurements revealed that the lifetime of the hot exciton was 7 ps, whereas that of excitons observed in a reference nanowire sample without an Ag shell was 1,600 ps. This radiative lifetime shortening is due to the Purcell effect resulting from plasmon whispering-gallery modes.
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