This Perspective
outlines basic structural and optical properties
of lead halide perovskite colloidal nanocrystals, highlighting differences
and similarities between them and conventional II–VI and III–V
semiconductor quantum dots. A detailed insight into two important
issues inherent to lead halide perovskite nanocrystals then follows,
namely, the advantages of defect tolerance and the necessity to improve
their stability in environmental conditions. The defect tolerance
of lead halide perovskites offers an impetus to search for similar
attributes in other related heavy metal-free compounds. We discuss
the origins of the significantly blue-shifted emission from CsPbBr3 nanocrystals and the synthetic strategies toward fabrication
of stable perovskite nanocrystal materials with emission in the red
and infrared parts of the optical spectrum, which are related to fabrication
of mixed cation compounds guided by Goldschmidt tolerance factor considerations.
We conclude with the view on perspectives of use of the colloidal
perovskite nanocrystals for applications in backlighting of liquid-crystal
TV displays.
Emission color controlled, high quantum yield CH3NH3PbBr3 perovskite quantum dots are obtained by changing the temperature of a bad solvent during synthesis. The products for temperatures between 0 and 60 °C have good spectral purity with narrow emission line widths of 28–36 nm, high absolute emission quantum yields of 74% to 93%, and short radiative lifetimes of 13–27 ns.
We alloyed Zn2+ into CsPbI3 perovskite nanocrystals
by partial substitution of Pb2+ with Zn2+, which
does not change their crystalline phase. The resulting alloyed CsPb0.64Zn0.36I3 nanocrystals exhibited an
improved, close-to-unity photoluminescence quantum yield of 98.5%
due to the increased radiative decay rate and the decreased non-radiative
decay rate. They also showed an enhanced stability, which correlated
with improved effective Goldschmidt tolerance factors, by the incorporation
of Zn2+ ions with a smaller radius than the Pb2+ ions. Simultaneously, the nanocrystals switched from n-type (for CsPbI3) to nearly ambipolar for the alloyed
nanoparticles. The hole injection barrier of electroluminescent LEDs
was effectively eliminated by using alloyed CsPb0.64Zn0.36I3 nanocrystals, and a high peak external quantum
efficiency of 15.1% has been achieved.
Semiconductor nanocrystals produced by means of colloidal chemistry in a solvent medium are an attractive class of nanometer-sized building blocks from which to create complex materials with unique properties for a variety of applications. Their optical and electronic properties can be tailored easily, both by their chemical composition and particle size. While colloidal nanocrystals emitting in the infrared region have seen a burst of attention during the last decade there is clearly a paucity of review articles covering their synthesis, assembly, spectroscopic characterization, and applications. This Review comprehensively addresses these topics for II-VI, III-V, and IV-VI nanocrystals, examples being HgTe and Cd(x)Hg(1-) (x)Te, InP and InAs, and PbS, PbSe, and PbTe, respectively. Among the applications discussed here are optical amplifier media for telecommunications systems, electroluminescence devices, and noninvasive optical imaging in biology.
Carbon-dot based light-emitting diodes (LEDs) with driving current controlled color change are reported. These devices consist of a carbon-dot emissive layer sandwiched between an organic hole transport layer and an organic or inorganic electron transport layer fabricated by a solution-based process. By tuning the device structure and the injecting current density (by changing the applied voltage), we can obtain multicolor emission of blue, cyan, magenta, and white from the same carbon dots. Such a switchable EL behavior with white emission has not been observed thus far in single emitting layer structured nanomaterial LEDs. This interesting current density-dependent emission is useful for the development of colorful LEDs. The pure blue and white emissions are obtained by tuning the electron transport layer materials and the thickness of electrode.
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