A binary ligand system
composed of aliphatic carboxylic acids and
primary amines of various chain lengths is commonly employed in diverse
synthesis methods for CsPbBr3 nanocrystals (NCs). In this
work, we have carried out a systematic study examining how the concentration
of ligands (oleylamine and oleic acid) and the resulting acidity (or
basicity) affects the hot-injection synthesis of CsPbBr3 NCs. We devise a general synthesis scheme for cesium lead bromide
NCs which allows control over size, size distribution, shape, and
phase (CsPbBr3 or Cs4PbBr6) by combining
key insights on the acid–base interactions that rule this ligand
system. Furthermore, our findings shed light upon the solubility of
PbBr2 in this binary ligand system, and plausible mechanisms
are suggested in order to understand the ligand-mediated phase control
and structural stability of CsPbBr3 NCs.
We
report the nontemplated colloidal synthesis of single crystal
CsPbBr3 perovskite nanosheets with lateral sizes up to
a few micrometers and with thickness of just a few unit cells (i.e.,
below 5 nm), hence in the strong quantum confinement regime, by introducing
short ligands (octanoic acid and octylamine) in the synthesis together
with longer ones (oleic acid and oleylamine). The lateral size is
tunable by varying the ratio of shorter ligands over longer ligands,
while the thickness is mainly unaffected by this parameter and stays
practically constant at 3 nm in all the syntheses conducted at short-to-long
ligands volumetric ratio below 0.67. Beyond this ratio, control over
the thickness is lost and a multimodal thickness distribution is observed.
We show here the first colloidal
synthesis of double perovskite
Cs2AgInCl6 nanocrystals (NCs) with a control
over their size distribution. In our approach, metal carboxylate precursors
and ligands (oleylamine and oleic acid) are dissolved in diphenyl
ether and reacted at 105 °C with benzoyl chloride. The resulting
Cs2AgInCl6 NCs exhibit the expected double perovskite
crystal structure, are stable under air, and show a broad spectrum
white photoluminescence (PL) with quantum yield of ∼1.6 ±
1%. The optical properties of these NCs were improved by synthesizing
Mn-doped Cs2AgInCl6 NCs through the simple addition
of Mn-acetate to the reaction mixture. The NC products were characterized
by the same double perovskite crystal structure, and Mn doping levels
up to 1.5%, as confirmed by elemental analyses. The effective incorporation
of Mn ions inside Cs2AgInCl6 NCs was also proved
by means of electron spin resonance spectroscopy. A bright orange
emission characterized our Mn-doped Cs2AgInCl6 NCs with a PL quantum yield as high as ∼16 ± 4%.
An
increasing number of studies have recently reported the rapid
degradation of hybrid and all-inorganic lead halide perovskite nanocrystals
under electron beam irradiation in the transmission electron microscope,
with the formation of nanometer size, high contrast particles. The
nature of these nanoparticles and the involved transformations in
the perovskite nanocrystals are still a matter of debate. Herein,
we have studied the effects of high energy (80/200 keV) electron irradiation
on colloidal cesium lead bromide (CsPbBr3) nanocrystals
with different shapes and sizes, especially 3 nm thick nanosheets,
a morphology that facilitated the analysis of the various ongoing
processes. Our results show that the CsPbBr3 nanocrystals
undergo a radiolysis process, with electron stimulated desorption
of a fraction of bromine atoms and the reduction of a fraction of
Pb2+ ions to Pb0. Subsequently Pb0 atoms diffuse and aggregate, giving rise to the high contrast particles,
as previously reported by various groups. The diffusion is facilitated
by both high temperature and electron beam irradiation. The early
stage Pb nanoparticles are epitaxially bound to the parent CsPbBr3 lattice, and evolve into nonepitaxially bound Pb crystals
upon further irradiation, leading to local amorphization and consequent
dismantling of the CsPbBr3 lattice. The comparison among
CsPbBr3 nanocrystals with various shapes and sizes evidences
that the damage is particularly pronounced at the corners and edges
of the surface, due to a lower diffusion barrier for Pb0 on the surface than inside the crystal and the presence of a larger
fraction of under-coordinated atoms.
CsPbI3 nanocrystals are still limited in their use because
of their phase instability as they degrade into the yellow nonemitting
δ-CsPbI3 phase within a few days. We show that alloyed
CsPbxMn1–xI3 nanocrystals have essentially the same optical
features and crystal structure as the parent α-CsPbI3 system, but they are stable in films and in solution for periods
over a month. The stabilization stems from a small decrease in the
lattice parameters slightly increasing the Goldsmith tolerance factor,
combined with an increase in the cohesive energy. Finally, hybrid
density functional calculations confirm that the Mn2+ levels
fall within the conduction band, thus not strongly altering the optical
properties.
We report the colloidal synthesis of strongly fluorescent CsPbBr 3 perovskite nanowires (NWs) with rectangular section and with tuneable width, from 20 nm (exhibiting no quantum confinement, hence emitting in the green) down to around 3 nm (in the strong quantumconfinement regime, emitting in the blue), by introducing in the synthesis a short acid (octanoic acid or hexanoic acid) together with alkyl amines (octylamine and oleylamine). Temperatures below 70 °C promoted the formation of monodisperse, few unit cell thick NWs that were free from byproducts. The photoluminescence quantum yield of the NW samples went from 12% for non-confined NWs emitting at 524 nm to a maximum of 77% for the 5 nm diameter NWs emitting at 497 nm, down to 30% for the thinnest NWs (diameter ~ 3nm), in the latter sample most likely due to aggregation occurring in solution.
We devised a colloidal approach for the synthesis of CsPbBr 3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr 3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb 2+ ions. (ii) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (iii) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface.
Halide perovskite
nanocrystals (NCs) are promising solution-processed
emitters for low-cost optoelectronics and photonics. Doping adds a
degree of freedom for their design and enables us to fully decouple
their absorption and emission functions. This is paramount for luminescent
solar concentrators (LSCs) that enable fabrication of electrode-less
solar windows for building-integrated photovoltaic applications. Here,
we demonstrate the suitability of manganese-doped CsPbCl3 NCs as reabsorption-free emitters for large-area LSCs. Light propagation
measurements and Monte Carlo simulations indicate that the dopant
emission is unaffected by reabsorption. Nanocomposite LSCs were fabricated
via mass copolymerization of acrylate monomers, ensuring thermal and
mechanical stability and optimal compatibility of the NCs, with fully
preserved emission efficiency. As a result, perovskite LSCs behave
closely to ideal devices, in which all portions of the illuminated
area contribute equally to the total optical power. These results
demonstrate the potential of doped perovskite NCs for LSCs, as well
as for other photonic technologies relying on low-attenuation long-range
optical wave guiding.
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