In layered hybrid perovskites, such as (BA)2PbI4 (BA=C4H9NH3), electrons and holes are considered to be confined in atomically thin two dimensional (2D) Pb–I inorganic layers. These inorganic layers are electronically isolated from each other in the third dimension by the insulating organic layers. Herein we report our experimental findings that suggest the presence of electronic interaction between the inorganic layers in some parts of the single crystals. The extent of this interaction is reversibly tuned by intercalation of organic and inorganic molecules in the layered perovskite single crystals. Consequently, optical absorption and emission properties switch reversibly with intercalation. Furthermore, increasing the distance between inorganic layers by increasing the length of the organic spacer cations systematically decreases these electronic interactions. This finding that the parts of the layered hybrid perovskites are not strictly electronically 2D is critical for understanding the electronic, optical, and optoelectronic properties of these technologically important materials.
Hybrid
perovskites have attracted much attention as a promising
photovoltaic material in the past few years. Typically, these hybrid
perovskites such as methyl ammonium lead halides (MAPbX3) undergo dimensionality reduction from three-dimensional (3D) to
zero-dimensional (0D), and finally to PbX2, upon continuous
moisture exposure. Our current study shows that 0D-perovskite-related
structures exhibit a reversible transformation from a transparent
state to a colored 3D state upon exposure to humidity. Fluorescence
imaging of individual microcrystals reveals that the structural phase
transition could be visualized in the solid state, wherein the crystals
transform into cubic crystals. The plausible reason for this transformation
is proposed to be a dynamic dissolution and recrystallization of the
excess methyl ammonium halide with varying humidity. The thermal and
moisture stability are found to be greatly enhanced in the transformed
3D perovskite. Excellent device stability is also demonstrated when
the devices are kept under moist (∼70% RH) conditions.
This paper reports a new method to synthesize Cu-doped ZnSe quantum dots (QDs). Emission properties are tuned from the blue to the green region simply by increasing the size of the QDs. A red shift in optical absorption of Cu:ZnSe QDs compared with undoped ZnSe QDs is observed. The increase in size of QDs is explained by a change in reaction kinematics. PL measurements revealed both a band edge as well as a copper-related emission. Delocalization of electronic wave functions leads to a shift in the copper-related emission with in size. PL excitation spectra recorded at Cu emission shows ZnSe energy levels along with a feature between 350-370 nm. This feature is assigned to excited energy levels of Cu ions. Variation in electron energy levels as a function of size and on Cu incorporation is mapped.
Photo-luminescence (P-L) intermittency (or blinking) in semiconductor nanocrystals (NCs), a phenomenon ubiquitous to single-emitters, is generally considered to be temporally random intensity fluctuations between "bright" ("On") and "dark" ("Off") states. However, individual quantum-dots (QDs) rarely exhibit such telegraphic signal, and yet, the vast majority of single-NC blinking data are analyzed using a single fixed threshold (FT) which generates binary trajectories. Further, blinking dynamics can vary dramatically over NCs in the ensemble, and it is unclear whether the exponents ( ) of single-particle On-/Off-time distributions ( ), which are used to validate mechanistic models of blinking, are narrowly distributed. Here, we sub-classify an ensemble based on the emissivity of QDs, and subsequently compare the (sub)ensembles" behaviors. To achieve this, we analyzed a large number (>1000) of blinking trajectories for a model system, Mn +2 doped ZnCdS QDs, which exhibits diverse blinking dynamics. An intensity histogram dependent thresholding method allowed us to construct distributions of relevant blinking parameters (such as, ). Interestingly, we find that single QD s follow either truncated power law or power law, and their relative proportion vary over sub-populations. Our results reveal a remarkable variation in amongst as well as within sub-ensembles, which implies multiple blinking mechanisms being operational amongst various QDs. We further show that the obtained via cumulative single-particle is distinct from the weighted mean value of all single-particle , an evidence for the lack of ergodicity. Thus, investigation and analyses of a large number of QDs, albeit for a limited time-span of few decades, is crucial to characterize possible blinking mechanisms or heterogeneity therein.
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