Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots

Self-assembled halide perovskite quantum dots in polymer thin films showing temperature-controlled exciton recombination

Rationally assembled supramolecular structures of organic chromophores or semiconductor nanomaterials show excitonic properties different from individual molecules or nanoparticles. We report polymer-assisted assembly formation and thermal modulation of excitonic recombination in self-assembled formamidinium lead bromide perovskite quantum dots.

Thermodynamically and Kinetically Controlled Nucleation and Growth of Halide Perovskite Single Crystals

Halide perovskites are ideal for next-generation optical devices and photovoltaics. Although perovskite single-crystals show reproducible optoelectronic properties, significant variations in the crystal size, anisotropy, density, defects, photoluminescence (PL), and carrier lifetime affect the sample properties and device performances. Homogenous size and shape FA/MAPbBr3 single microcrystals (MCs) with controlled edge lengths, crystal densities, PL lifetimes, and PL intensities are prepared by thermodynamically controlling and kinetically separating the crystal nucleation-growth processes using optimum N-cyclohexyl-2-pyrrolidone (CHP) concentration. The crystal growth kinetics at different CHP concentrations and temperatures are estimated spectroscopically by measuring the concentration of Pb (II). High-density cubic MCs with a homogenous size distribution, high PL intensities, and long PL lifetimes are obtained within minutes at high temperatures by the controlled addition of the pyrrolidone derivative. Conversely, the crystal size nonlinearly increases with time at low temperatures. The isotropically grown high-density single crystals at controlled nucleation-growth rates at 190 °C with 20% CHP show the highest PL intensity and the longest PL lifetimes. This method offers thermodynamic and kinetic control of perovskite single-crystal growth with shape control.

Slipping-Free Halide Perovskite Supercrystals from Supramolecularly-Assembled Nanocrystals

Supramolecularly assembled high-order supercrystals (SCs) help control the dielectric, electronic, and excitonic properties of semiconductor nanocrystals (NCs) and quantum dots (QDs). Ligand-engineered perovskite NCs (PNCs) assemble into SCs showing shorter excitonic lifetimes than strongly dielectric PNC films showing long photoluminescence (PL) lifetimes and long-range carrier diffusion. Monodentate to bidentate ligand exchange on ≈ 8 nm halide perovskite (APbX3 ; A:Cs/MA, X:Br/I) PNCs generates mechanically stable SCs with close-packed lattices, overlapping electronic wave functions, and higher dielectric constant, providing distinct excitonic properties from single PNCs or PNC films. From Fast Fourier Transform (FFT) images, time-resolved PL, and small-angle X-ray scattering, structurally and excitonically ordered large SCs are identified. An Sc shows a smaller spectral shift (<35 meV) than a PNC film (>100 meV), a microcrystal (>100 meV), or a bulk crystal (>100 meV). Also, the exciton lifetime (<10 ns) of an SC is excitation power-independent in the single exciton regime 〈N〉<1, comparable to an isolated PNC. Therefore, bidentate-ligand-assisted SCs help overcome delayed exciton or carrier recombination in halide perovskite nanocrystal assemblies or films.

Pressure-Induced Band Gap Engineering of Nontoxic Lead-Free Halide Perovskite CsMgI3 for Optoelectronic Applications

Recently, lead halide perovskites have gained considerable attention by dint of their predominant physiochemical features and potential use in various applications with an improved power conversion efficiency. Despite the incredible technological and research breakthroughs in this field, most of those compounds present an obstacle to future commercialization due to their instability and extreme poisonousness. Because of this, it is preferable to replace lead with alternative stable elements to produce eco-friendly perovskites with equivalent optoelectronic qualities similar to lead-based perovskites. However, Pb-free perovskite-based devices have relatively low power conversion efficiency. Pressure might be considered an effective way for modifying the physical characteristics of these materials to enhance their performance and reveal structure-property correlations. The present study has been done to investigate the structural, electronic, optical, elastic, mechanical, and thermodynamic properties of nontoxic perovskite CsMgI₃ under hydrostatic pressure by using density functional theory (DFT). At ambient pressure, the present findings are in excellent agreement with the available experimental data. Pressure causes the Mg-I and Cs-I bonds to shorten and become stronger. The absorption coefficient in the visible and ultraviolet (UV) zones grows up with the increase in pressure. Additionally, we have observed low reflectivity, a high-intensity conductivity peak, and a dielectric constant in the visible region of the electromagnetic spectrum. As pressure rises, the band gap keeps narrowing, facilitating an electron from the valence band to get excited easily at the conduction band. Furthermore, we analyze the mechanical, elastic, and thermodynamic properties under pressure, which suggests that this compound exhibit ductile behavior. The shrunk band gap and improved physical properties of CsMgI₃ under hydrostatic pressure suggest that this material may be used in solar cells (for photovoltaic applications) and optoelectronic devices more frequently than at ambient pressure. In addition, this paper emphasizes the feasibility of hydrostatic pressure in the systematic modification of the optoelectronic and mechanical characteristics of lead-free halide perovskites.

Photoinduced interfacial electron transfer from perovskite quantum dots to molecular acceptors for solar cells

Bandgap-engineered inorganic and hybrid halide perovskite (HP) films, nanocrystals, and quantum dots (PQDs) are promising for solar cells. Fluctuations of photoinduced electron transfer (PET) rates affect the interfacial charge separation efficiencies of such solar cells. Electron donor- or acceptor-doped perovskite samples help analyze PET and harvest photogenerated charge carriers efficiently. Therefore, PET in perovskite-based donor-acceptor (D-A) systems has received considerable attention. We analyzed the fluctuations of interfacial PET from MAPbBr₃ or CsPbBr₃ PQDs to classical electron acceptors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 1,2,4,5-tetracyanobenzene (TCNB) at single-particle and ensemble levels. The significantly negative Gibbs free energy changes of PET estimated from the donor-acceptor redox potentials, the donor-acceptor sizes, and the solvent dielectric properties help us clarify the PET in the above D-A systems. The dynamic nature of PET is apparent from the decrease in photoluminescence (PL) lifetimes and PL photocounts of PQDs with an increase in the acceptor concentrations. Also, the acceptor radical anion spectrum helps us characterize the charge-separated states. Furthermore, the PL blinking time and PET rate fluctuations (10⁸ to 10⁷ s^-1) provide us with single-molecule level information about interfacial PET in perovskites.

Simultaneous Photoluminescence and Photothermal Investigation of Individual CH3NH3PbBr3 Microcrystals

The photoluminescence (PL) of CH₃NH₃PbBr₃ (MAPbBr₃), from thin films to nanoparticles, has been widely studied, providing information about charge carrier dynamics. However, the other energy dissipative channel, nonradiative relaxation, has not been thoroughly investigated due to a lack of proper technology. In this work, we simultaneously investigated the PL and photothermal (PT) properties of single MAPbBr₃ microcrystals (MCs) by a home-built PL and PT microscope. In addition to the direct observation of the heterogeneity of the PL and PT images and kinetics of different MCs, we demonstrated the variation in the absorption of single MAPbBr₃ MCs, which was believed to be constant. We also proved that more absorbed energy dissipated from the nonradiative channel at higher heating power. These results show that PL and PT microscopy is an effective and convenient method to investigate the charge carrier behaviors of optoelectronic materials at the single particle level for a deep understanding of their photophysical processes.

Metal-Halide Perovskite Nanocrystal Superlattice: Self-Assembly and Optical Fingerprints

Self-assembly of nanocrystals into superlattices is a fascinating process that not only changes geometric morphology, but also creates unique properties that considerably enrich the material toolbox for new applications. Numerous studies have driven the blossoming of superlattices from various aspects. These include precise control of size and morphology, enhancement of properties, exploitation of functions, and integration of the material into miniature devices. The effective synthesis of metal-halide perovskite nanocrystals has advanced research on self-assembly of building blocks into micrometer-sized superlattices. More importantly, these materials exhibit abundant optical features, including highly coherent superfluorescence, amplified spontaneous laser emission, and adjustable spectral redshift, facilitating basic research and state-of-the-art applications. This review summarizes recent advances in the field of metal-halide perovskite superlattices. It begins with basic packing models and introduces various stacking configurations of superlattices. The potential of multiple capping ligands is also discussed and their crucial role in superlattice growth is highlighted, followed by detailed reviews of synthesis and characterization methods. How these optical features can be distinguished and present contemporary applications is then considered. This review concludes with a list of unanswered questions and an outlook on their potential use in quantum computing and quantum communications to stimulate further research in this area.

Nanomaterials for Fluorescence and Multimodal Bioimaging

Bioconjugated nanomaterials replace molecular probes in bioanalysis and bioimaging in vitro and in vivo. Nanoparticles of silica, metals, semiconductors, polymers, and supramolecular systems, conjugated with contrast agents and drugs for image-guided (MRI, fluorescence, PET, Raman, SPECT, photodynamic, photothermal, and photoacoustic) therapy infiltrate into preclinical and clinical settings. Small bioactive molecules like peptides, proteins, or DNA conjugated to the surfaces of drugs or probes help us to interface them with cells and tissues. Nevertheless, the toxicity and pharmacokinetics of nanodrugs, nanoprobes, and their components become the clinical barriers, underscoring the significance of developing biocompatible next-generation drugs and contrast agents. This account provides state-of-the-art advancements in the preparation and biological applications of bioconjugated nanomaterials and their molecular, cell, and in vivo applications. It focuses on the preparation, bioimaging, and bioanalytical applications of monomodal and multimodal nanoprobes composed of quantum dots, quantum clusters, iron oxide nanoparticles, and a few rare earth metal ion complexes.

Halide Perovskite Single Crystals and Nanocrystal Films as Electron Donor-Acceptor Heterojunctions

Halide perovskites are materials for future optical displays and solar cells. Electron donor-acceptor perovskite heterostructures with distinguishing halide compositions are promising for transporting and harvesting photogenerated charge carriers. Combined e-beam lithography and anion exchange are promising to develop such heterostructures but challenging to prepare multiple heterojunctions at desired locations in single crystals. We demonstrate swift laser trapping-assisted band gap engineering at the desired locations in MAPbBr₃ microrods, microplates, or nanocrystal thin films. The built-in donor-acceptor double and multi-heterojunction structures let us transport and trap photogenerated charge carriers from wide-band gap bromide to narrow-band gap iodide domains. We discuss the charge carrier transport and trapping mechanisms from the viewpoints of engineered bands and band continuity. This work offers a convenient method for designing single-, double- and multi-heterojunction donor-acceptor halide perovskites for photovoltaic, photonic, and electronic applications.