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%.
We report the composition-dependent optical properties of Bi-doped Cs 2 Ag 1−x Na x InCl 6 nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by a large Stokes shift, and the PL quantum yield increased with the amount of Na up to ∼22% for the Cs 2 Ag 0.4 Na 0.6 InCl 6 stoichiometry. The presence of Bi 3+ dopants was crucial to achieve high PL quantum yields (PLQYs) as nondoped NC systems were not emissive. Density functional theory calculations revealed that the substitution of Ag + with Na + leads to localization of AgCl 6 energy levels above the valence band maximum, whereas doping with Bi 3+ creates BiCl 6 states below the conduction band minimum. As such, the PL emission stems from trapped emission between states localized in the BiCl 6 and AgCl 6 octahedra, respectively. Our findings indicated that both the partial replacement of Ag + with Na + ions and doping with Bi 3+ cations are essential in order to optimize the PL emission of these systems.
Perovskite-related
Cs4PbBr6 nanocrystals
present a “zero-dimensional” crystalline structure where
adjacent [PbBr6]4– octahedra do not share
any corners. We show in this work that these nanocrystals can be converted
into “three-dimensional” CsPbBr3 perovskites
by extraction of CsBr. This conversion drastically changes the optoelectronic
properties of the nanocrystals that become highly photoluminescent.
The extraction of CsBr can be achieved either by thermal annealing
(physical approach) or by chemical reaction with Prussian Blue (chemical
approach). The former approach can be simply carried out on a dried
film without addition of any chemicals but does not yield a full transformation.
Instead, reaction with Prussian Blue in solution achieves a full transformation
into the perovskite phase. This transformation was also verified on
the iodide counterpart (Cs4PbI6).
CsPbBr3 nanocrystals passivated with short molecular
ligands and deposited on a substrate were annealed from room temperature
to 400 °C in inert atmosphere. Chemical, structural, and morphological
transformations were monitored in situ and ex situ by different techniques, while optoelectronic properties
of the film were also assessed. Annealing at 100 °C resulted
in a 1 order of magnitude increase in photocurrent and photoresponse
as a result of partial sintering of the NCs and residual solvent evaporation.
Beyond 150 °C the original orthorhombic NCs were partially transformed
into tetragonal CsPb2Br5 crystals, due to the
desorption of weakly bound propionic acid ligands. The photocurrent
increased moderately until 300 °C although the photoresponse
became slower as a result of the formation of surface trap states.
Eventually, annealing beyond 350 °C removed the strongly bound
butylamine ligands and reversed the transition to the original orthorhombic
phase, with a loss of photocurrent due to the numerous defects induced
by the stripping of the passivating butylamine.
CsPb(Cl 1−x Br x ) 3 perovskite nanocrystals (NCs) doped with Yb 3+ ions have recently attracted large attention for their applications in photovoltaics in view of the high quantum yield, exceeding 100% of Yb 3+ emission at ∼1 μm. In contrast, the particularly relevant Er 3+ emission at 1.5 μm in the third telecommunication window, of high interest in silicon integrated photonics, has been so far largely neglected in view of the weak emission performance displayed by Er 3+ -doped NCs. Comprehensive steady-state and time-resolved spectroscopic measurements provide insights into the underlying mechanisms of Yb 3+ and Er 3+ sensitization to rationalize the anomalous different behavior of these two emitters in singly doped NCs. We propose that single-photon excitation of two Yb 3+ ions possibly occurs through a transient internal redox mechanism in the perovskite host, while this pathway is unviable for Er 3+ . In turn, Yb 3+ -bridged Er 3+ sensitization, boosts the Er 3+ luminescence at ∼1.5 μm by 10 4 -fold compared to Er 3+ singly doped NCs, and a relative high quantum yield of ∼6% and extremely long lifetime (∼3 ms) are obtained. The resulting high Er 3+ excited state densities, combined with the large absorption cross-sections of the semiconducting CsPbCl 3 matrix make Er 3+ -doped perovskite promising innovative materials to realize photonic devices operating at telecommunication wavelengths.
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