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Effect of Thermal Fluctuations on the Radiative Rate in Core/Shell Quantum Dots
The
effect of lattice fluctuations and electronic excitations on the radiative
rate is demonstrated in CdSe/CdS core/shell spherical quantum dots
(QDs). Using a combination of time-resolved photoluminescence spectroscopy
and atomistic simulations, we show that lattice fluctuations can change
the radiative rate over the temperature range from 78 to 300 K. We
posit that the presence of the core/shell interface plays a significant
role in dictating this behavior. We show that the other major factor
that underpins the change in radiative rate with temperature is the
presence of higher energy states corresponding to electron excitation
into the shell. These effects should be present in other core/shell
samples and should also affect other excited state rates, such as
the rate of Auger recombination or the rate of charge transfer
Unraveling the Impurity Location and Binding in Heavily Doped Semiconductor Nanocrystals: The Case of Cu in InAs Nanocrystals
The doping of colloidal semiconductor
nanocrystals (NCs) presents
an additional knob beyond size and shape for controlling the electronic
properties. An important problem for doping with aliovalent elements
is associated with resolving the location of the dopant and its structural
surrounding within small NCs, an issue directly connected with self-purification.
Here we used a postsynthesis diffusion-based doping method for introducing
Cu impurities into InAs quantum dots. X-ray absorption fine structure
(XAFS) spectroscopy experiments along with first-principle density
functional theory (DFT) calculations were used to probe the impurity
sites. The concentration dependence was investigated for a wide range
of doping levels, helping to derive a self-consistent picture where
the Cu impurity occupies an interstitial site within the InAs lattice.
Moreover, at extremely high doping levels, Cu–Cu interactions
are identified in the XAFS data. This structural model is supported
by X-ray diffraction data, along with the DFT calculation. These findings
establish the reproducibility of the diffusion-based doping strategy
giving rise to new opportunities of correlating the structural details
with emerging electronic properties in heavily doped NCs
Semiconductor Seeded Nanorods with Graded Composition Exhibiting High Quantum-Yield, High Polarization, and Minimal Blinking
Seeded
semiconductor nanorods represent a unique family of quantum confined
materials that manifest characteristics of mixed dimensionality. They
show polarized emission with high quantum yield and fluorescence switching
under an electric field, features that are desirable for use in display
technologies and other optical applications. So far, their robust
synthesis has been limited mainly to CdSe/CdS heterostructures, thereby
constraining the spectral tunability to the red region of the visible
spectrum. Herein we present a novel synthesis of CdSe/Cd<sub>1–<i>x</i></sub>Zn<sub><i>x</i></sub>S seeded nanorods
with a radially graded composition that show bright and highly polarized
green emission with minimal intermittency, as confirmed by ensemble
and single nanorods optical measurements. Atomistic pseudopotential
simulations elucidate the importance of the Zn atoms within the nanorod
structure, in particular the effect of the graded composition. Thus,
the controlled addition of Zn influences and improves the nanorods’
optoelectronic performance by providing an additional handle to manipulate
the degree confinement beyond the common size control approach. These
nanorods may be utilized in applications that require the generation
of a full, rich spectrum such as energy-efficient displays and lighting