Hybrid organic-inorganic perovskites possess promising signal transduction properties, which can be exploited in a variety of sensing applications. Interestingly, the highly polar nature of these materials, while being a bane in terms of stability, can be a boon for sensitivity when they are exposed to polar gases in a controlled atmosphere. However, signal transduction during sensing induces irreversible changes in the chemical and physical structure, which is one of the major lacuna preventing its utility in commercial applications. In the context of developing alkylammonium lead(II) iodide perovskite materials for sensing, here we address major issues such as reversibility of structure and properties, correlation between instability and properties of alkylamines, and relation between packing of alkyl chains inside the crystal lattice and the response time toward NH gas. The current investigation highlights that the vapor pressure of alkylamine formed in the presence of NH determines the reversibility and stability of the original perovskite lattice. In addition, close packing of alkyl chains inside the perovskite crystal lattice reduces the response toward NH gas. The mechanistic study addresses three important factors such as quick response, reversibility, and stability of perovskite materials in the presence of NH gas, which could lead to the design of stable and sensitive two-dimensional hybrid perovskite materials for developing sensors.
Incorporation of suitable lanthanide (Ln 3+ ) ions into semiconducting WO 3 can be useful to produce host-sensitized luminescence for solid-state lighting applications. Codoping of another Ln 3+ ion can assist in transferring energy from the host to the activator Ln 3+ ion to produce bright luminescence depending upon the electronic structure of the doped system. As a case study, Eu 3+ and Tb 3+ ion−doped WO 3 phosphors (WO 3 :Tb 3+x Eu 3+ y , x = 0−0.05 and y = 0−0.20) have been prepared over a wide range of doping concentrations to investigate the role of the Tb 3+ ion as a sensitizer and realize host-sensitized emission from Eu 3+ ions. The steady-state and time-resolved photoluminescence (TRPL) data for WO 3 :Tb 3+x Eu 3+ y (x = 0−0.05 and y = 0−0.10) samples confirm that Tb 3+ ions assist in excitation of Eu 3+ ions via sequential energy transfers from the host to Tb 3+ ions followed by Tb 3+ to Eu 3+ ions. The energy transfer process is controlled by optimizing their doping concentrations, and a single-phase white-light-emitting phosphor with a composition WO 3 :Tb 3+ 0.05 Eu 3+ 0.0005 has been developed. The electronic band structures and projected density of state plots for the WO 3 :Tb 3+ 0.03125 Eu 3+ 00.03125 system obtained using density functional theory (DFT)-based simulations confirm the formation of impurity states due to Eu 3+ and Tb 3+ ions within the forbidden gap of WO 3 . Based on the TRPL and DFT data, it is confirmed that the Tb 3+ ions act as a bridge between the conduction band edge of WO 3 and excited states of Eu 3+ ions to transfer energy and facilitate characteristic emission from europium species.
Gas-induced growth of organic–inorganic hybrid perovskites, especially methylammonium lead iodide (MAPbI3), has shown interesting properties and applications in the area of optoelectronics. In this report, we introduce a method of gas-induced band gap engineering of thin films of MAPbI3 due to systematic dimensional confinement–deconfinement along the crystallographic c axis of growing MAPbI3. Interestingly, such a restricted growth phenomenon was observed when the hexylammonium lead iodide (two-dimensional hybrid perovskite) film was exposed to methylamine gas instead of the conventional PbI2 film–methylamine gas precursor pair. Hexylamine, formed due to the cation exchange reaction, interacts selectively with the Pb centers of growing MAPbI3 crystals, and this induces an enormous restriction in the growth of MAPbI3 along the crystallographic c direction, leading to a unique sheet-type MAPbI3 film having a much higher band gap (2.18 eV) compared to conventional bulk MAPbI3. However, careful control of exposure timing gradually evaporates the hexylamine, leading to systematic dimensional deconfinement, enabling modulation of the band gap from 2.18 to 1.69 eV. An interplay of adsorption and desorption of hexylamine is also utilized for generating patterns of two different fluorescent hybrid perovskite materials in a single pixel. This new mechanistic investigation highlighting gas-induced interplay of dimensional confinement–deconfinement associated with band gap tuning provides smooth thin films, which can be used to develop optoelectronic devices.
Crystal growth involves an increase in the average size of crystals even during Ostwald ripening, wherein larger crystals grow at the expense of smaller crystals. In contrast, we observed particle size reduction due to rupture during the growth of polycrystalline LnF 3 (Ln = Lu, Y, La) nanoparticles. We rationalize this by the feature of the growing crystal to evolve toward the most stable native crystal form via several metastable intermediate nonnative amorphous or crystalline structures with a significant change in density, resulting in high misfit strain and generation of stress within the single nanoparticle. The stress promotes the propagation of already existing flaws in the crystal, leading to the rupture of nanocrystals and a subsequent decrease in crystallite size. Results from finite element elastic simulations also support the propagation of flaws leading to nanocrystal rupture if the fracture toughness of the materials is low. The ligand plays an essential role in the observation of this phase of crystal growth by retarding the growth kinetics.
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