5 research outputs found
Optical and Electronic Properties of Nonconcentric PbSe/CdSe Colloidal Quantum Dots
Lead
chalcogenide colloidal quantum dots are attractive candidates
for applications operating in the near infrared spectral range. However,
their function is forestalled by limited stability under ambient conditions.
Prolonged temperature-activated cation-exchange of Cd<sup>2+</sup> for Pb<sup>2+</sup> forms PbSe/CdSe core/shell heterostructures,
unveiling a promising surface passivation route and a method to modify
the dots’ electronic properties. Here, we follow early stages
of an-exchange process, using spectroscopic and structural characterization
tools, as well as numerical calculations. We illustrate that preliminary-exchange
stages involve the formation of nonconcentric heterostructures, presumably
due to a facet selective reaction, showing a pronounced change in
the optical properties upon the increase of the degree of nonconcentricity
or/and plausible creation of core/shell interfacial alloying. However,
progressive-exchange stages lead to rearrangement of the shell segment
into uniform coverage, providing tolerance to oxygen exposure with
a spectral steadiness already on the formation of a monolayer shell
Hydrogen-like Wannier–Mott Excitons in Single Crystal of Methylammonium Lead Bromide Perovskite
A thorough
investigation of exciton properties in bulk CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> perovskite single crystals was carried
out by recording the reflectance, steady-state and transient photoluminescence
spectra of submicron volumes across the crystal. The study included
an examination of the spectra profiles at various temperatures and
laser excitation fluencies. The results resolved the first and second
hydrogen-like Wannier–Mott exciton transitions at low temperatures,
from which the ground-state exciton’s binding energy of 15.33
meV and Bohr radius of ∼4.38 nm were derived. Furthermore,
the photoluminescence temperature dependence suggested dominance of
delayed exciton emission at elevated temperatures, originating from
detrapping of carriers from shallow traps or/and from retrapping of
electron–hole pairs into exciton states. The study revealed
knowledge about several currently controversial issues that have an
impact on functionality of perovskite materials in optoelectronic
devices
Tuning Optical Activity of IV–VI Colloidal Quantum Dots in the Short-Wave Infrared (SWIR) Spectral Regime
The
achievement of tunable optical properties across a wide spectral
range, along with an efficient surface passivation of lead chalcogenide
(PbSe) colloidal quantum dots (CQDs), has significant importance for
scientific research and for technological applications. This paper
describes two comprehensive pathways to tune optical activities of
PbSe CQDs in the near-infrared (NIR, 0.75–1.4 μm) and
the short-wave infrared (SWIR, 1.4–3 μm) ranges. A one-pot
procedure enabled the growth of relatively large PbSe CQDs (with average
sizes up to 14 nm) exploiting programmable temperature control during
the growth process. These CQDs showed optical activity up to 3.2 μm.
In addition, PbSe/PbS core/shell CQDs prepared by an orderly injection
rate led to an energy red-shift of the absorption edge with the increase
of the shell thickness, whereas a postannealing treatment further
extended the band-edge energy toward the SWIR regime. A better chemical
stability of the CQDs with respect to that of PbSe core CQDs was attained
by shelling of PbSe by epitaxial layers of PbS, but limited to a short
duration (<1 day). However, air stability of the relatively large
PbSe as well as the PbSe/PbS CQDs over a prolonged period of time
(weeks) was achieved after a postsynthesis chlorination treatment
Influence of Alloying on the Optical Properties of IV–VI Nanorods
The synthesis and structural and optical characterization
of PbSe<sub><i>x</i></sub>S<sub>1–<i>x</i></sub> and
PbSe/PbSe<sub><i>x</i></sub>S<sub>1–<i>x</i></sub> nanorods with a diameter between 2 and 4.5 nm and a length
of 10 to 38 nm is reported. The energy band gap of the nanorods exhibits
a pronounced variation upon the change in diameter and composition,
with a minor influence on lengths beyond 10 nm. The photoluminescence
spectrum of the nanorods is composed of a dominant band, accompanied
by a satellite band at elevated temperatures. The dominant band shows
an exceptionally small band gap temperature coefficient and negligible
extension of the radiative lifetime at cryogenic temperatures compared
with the photoluminescence processes in PbSe nanorods and in PbSe<sub><i>x</i></sub>S<sub>1–<i>x</i></sub> quantum
dots with similar band gap energy. A theoretical model suggests the
occurrence of independent transitions from a pair of band-edge valleys,
located at the L points of Brillouin zone, related to the dominant
and satellite emission processes. Each valley is four-fold degenerate
and possesses a relatively small electron–hole exchange interaction
Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals
Controlling the synthesis
of narrow band gap semiconductor nanocrystals
(NCs) with a high-quality surface is of prime importance for scientific
and technological interests. This Letter presents facile solution-phase
syntheses of SnTe NCs and their corresponding core/shell heterostructures.
Here, we synthesized monodisperse and highly crystalline SnTe NCs
by employing an inexpensive, nontoxic precursor, SnCl<sub>2</sub>,
the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol.
Moreover, we developed a synthesis procedure for the formation of
SnTe-based core/shell NCs by combining the cation exchange and the
Kirkendall effect. The cation exchange of Sn<sup>2+</sup> by Cd<sup>2+</sup> at the surface allowed primarily the formation of SnTe/CdTe
core/shell NCs. Further continuation of the reaction promoted an intensive
diffusion of the Cd<sup>2+</sup> ions, which via the Kirkendall effect
led to the formation of the inverted CdTe/SnTe core/shell NCs