2 research outputs found
Shape‑, Size‑, and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tunable Band Gaps
Perovskite
nanocrystals have shown themselves to be useful for both absorption-
and emission-based applications, including solar cells, photodetectors,
and LEDs. Here we present a new class of size-, composition-, and
shape-tunable nanocrystals made from Tl<sub>3</sub>PbX<sub>5</sub> (X= Cl, Br, I). These can be synthesized via colloidal methods to
produce faceted spheroidal nanocrystals, and perovskite TlPbI<sub>3</sub> nanowires. Crystal structures for the orthorhombic and tetragonal
phase materials, for both pure and mixed halide species, are compared
to the literature and also calculated from first-principles in VASP.
We show the ability to tune the band gap by halide substitution to
create materials that can absorb strongly between 250 and 450 nm.
In addition, we show evidence of the confinement effect in pure halide
Tl<sub>3</sub>PbBr<sub>5</sub> nanocrystals suggesting size-tuning
is possible as well. By tuning the band gap we can create materials
with specific absorption spectra suitable for photodetection that
display conduction and photoresponse properties similar to previously
observed perovskite nanocrystals. We also observe weak emission consistent
with indirect band-gap materials. Finally, we are able to demonstrate
shape control in these materials, to give some insight into observable
phase changes with varying reaction conditions, and to demonstrate
the utility of the TlPbI<sub>3</sub> perovskite nanowires as wide-band-gap
photoconductors. These novel perovskite nanocrystalline materials
can be expected to find applications in photodetectors, X-ray detectors,
and piezoelectrics
The Evolution of Quantum Confinement in CsPbBr<sub>3</sub> Perovskite Nanocrystals
Colloidal
nanocrystals (NCs) of lead halide perovskites are considered
highly promising materials that combine the exceptional optoelectronic
properties of lead halide perovskites with tunability from quantum
confinement. But can we assume that these materials are in the strong
confinement regime? Here, we report an ultrafast transient absorption
study of cubic CsPbBr<sub>3</sub> NCs as a function of size, compared
with the bulk material. For NCs above ∼7 nm edge length, spectral
signatures are similar to the bulk material–characterized by
state-filling with uncorrelated charges–but discrete new kinetic
components emerge at high fluence due to bimolecular recombination
occurring in a discrete volume. Only for the smallest NCs (∼4
nm edge length) are strong quantum confinement effects manifest in
TA spectral dynamics; focusing toward discrete energy states, enhanced
bandgap renormalization energy, and departure from a Boltzmann statistical
carrier cooling. At high fluence, we find that a hot-phonon bottleneck
effect slows carrier cooling, but this appears to be intrinsic to
the material, rather than size dependent. Overall, we find that the
smallest NCs are understood in the framework of quantum confinement,
however for the widely used NCs with edge lengths >7 nm the photophysics
of bulk lead halide perovskites are a better point of reference