The tunability of electronic and optical properties of semiconductor nanocrystal quantum dots (QDs) has been an important subject in nanotechnology. While control of the emission property of QDs in wavelength has been studied extensively, control of the emission lifetime of QDs has not been explored in depth. In this report, ZnO-CdS core–shell QDs were synthesized in a two-step process, in which we initially synthesized ZnO core particles, and then stepwise slow growth of CdS shells followed. The coating of a CdS shell on a ZnO core increased the exciton lifetime more than 100 times that of the core ZnO QD, and the lifetime was further extended as the thickness of shell increased. This long electron–hole recombination lifetime is due to a unique staggered band alignment between the ZnO core and CdS shell, so-called type II band alignment, where the carrier excitation holes and electrons are spatially separated at the core and shell, and the exciton lifetime becomes extremely sensitive to the thickness of the shell. Here, we demonstrated that the emission lifetime becomes controllable with the thickness of the shell in ZnO-CdS core–shell QDs. The longer excitonic lifetime of type II QDs could be beneficial in fluorescence-based sensors, medical imaging, solar cells photovoltaics, and lasers.
Broadband
plasmonic absorbers are essential components for photovoltaic,
photothermal, and light-emitting devices. They are often made by lithographic
processes that impart out-of-plane surface features with subwavelength
dimensions to metallic films. However, lithographic subwavelength
patterning of inexpensive plasmonic ceramics, such as TiN, is challenging
because of high-temperature processing and the chemical robustness
of these materials. In this work, we show that layer-by-layer assembly
(LbL) of TiN plasmonic nanoparticles with polyelectrolytes results
in spontaneous formation of out-of-plane topography with subwavelength
dimensions. The columnar morphology of these corrugated coatings and
their plasmonic functionality results in broadband absorption capabilities
exemplified by 90% of the light from the ultraviolet to infrared parts
of the spectrum being absorbed. The method is applicable to large,
flexible, and conformal surfaces with complex geometry. It is also
fast, scalable, and environmentally friendly. LBL processing of TiN
nanoparticles demonstrates the possibility of replacement of lithographic
patterning with stochastic self-assembly processes in manufacturing
of photonic metasurfaces.
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