Dynamic Structural Evolution and Dual Emission Behavior in Hybrid Organic Lead Bromide Perovskites.

The optoelectronic properties of organic lead halide perovskites (OLHPs) strongly depend on their underlying crystal symmetry and dynamics. Here, we exploit temperature-dependent synchrotron powder X-ray diffraction and temperature-dependent photoluminescence to investigate how the subtle structural changes happening in the pure and mixed A-site cation MA1-xFAxPbBr3 (x = 0, 0.5, and 1) systems influences their optoelectronic properties. Diffraction investigations reveal a cubic structure at high temperatures and tetragonal and orthorhombic structures with octahedral distortion at low temperatures. Steady state photoluminescence and time correlated single photon counting study reveals that the dual emission behavior of these OLHPs is due to the direct-indirect band formation. In the orthorhombic phase of MAPbBr3, the indirect band is dominated by self-trapped exciton (STE) emission due to the higher-order lattice distortions of PbBr6 octahedra. Our findings provide a comprehensive explanation of the dual emission behavior of OLHPs while also providing a rationale for previous experimental observations.

Partial Chemicalization of Nanoscale Metals: An Intra-Material Transformative Approach for the Synthesis of Functional Colloidal Metal-Semiconductor Nanoheterostructures.

Heterostructuring colloidal nanocrystals into multicomponent modular constructs, where domains of distinct metal and semiconductor phases are interconnected through bonding interfaces, is a consolidated approach to advanced breeds of solution-processable hybrid nanomaterials capable of expressing richly tunable and even entirely novel physical-chemical properties and functionalities. To meet the challenges posed by the wet-chemical synthesis of metal-semiconductor nanoheterostructures and to overcome some intrinsic limitations of available protocols, innovative transformative routes, based on the paradigm of partial chemicalization, have recently been devised within the framework of the standard seeded-growth scheme. These techniques involve regiospecific replacement reactions on preformed nanocrystal substrates, thus holding great synthetic potential for programmable configurational diversification. This review article illustrates achievements so far made in the elaboration of metal-semiconductor nanoheterostructures with tailored arrangements of their component modules by means of conversion pathways that leverage on spatially controlled partial chemicalization of mono- and bi-metallic seeds. The advantages and limitations of these approaches are discussed within the context of the most plausible mechanisms underlying the evolution of the nanoheterostructures in liquid media. Representative physical-chemical properties and applications of chemicalization-derived metal-semiconductor nanoheterostructures are emphasized. Finally, prospects for developments in the field are outlined.

In Situ Pressure Controlled Growth of ZnO Nanoparticles: Tailoring Sizes, Defects, and Optical Properties.

ZnO, being an inexpensive, wide band gap semiconductor that possesses high mechanical, thermal, and chemical stabilities and suitable for a wide range of optical and electronic applications, is the preferred semiconductor of this era. In an effort to fully utilize its potential features, ZnO research is receiving increasing attention. This study investigates the influence of pressure on the crystallinity, defect density, size, and morphology of ZnO nanoparticles, synthesized using nonaqueous sol-gel method, and their respective impact on the optical properties. High-crystalline ZnO nanocrystals with a hexagonal wurtzite structure were synthesized at various pressures, including ambient pressure, 25, 37.5, 50, and 100 bars inside a high-pressure reactor. With the increase in pressure, a reduction in particle size was observed, reaching a minimum size (∼10 nm) at 50 bar pressure (ZnO-50). Further increase in pressure causes an enhancement in the particle size. This trend of size variation with pressure is attributed to a tradeoff between esterification and nucleation processes. Contrary to the expectation, smaller ZnO nanocrystals synthesized by the present method possess lesser number of defects, suggesting that high-pressure synthesis is a unique way that offers smaller ZnO nanocrystals of sub-10 nm sizes having high crystallinity and lesser defects in a shorter time span. Also, the optical transmittance of the systems could be greatly enhanced by carefully tuning the particle sizes, with ZnO-50 (∼10 nm particle size) having the highest transmittance (∼95% at 600 nm) among all samples. High crystallinity, uniform morphology, excellent visible transparency, wide band gap, and low defect density make these smaller ZnO nanocrystals a preferred choice for ultraviolet sensors and other optoelectronic devices.

Epitaxial Orientation Angle Tuned Disk-on-Rod Nanoheterostructures for Boosting Charge Transfer.

Controlling the compositions of Se(VI) and Te(VI) ions in a 2D disk on 1D structures of Sb(V) chalcogenides, disk-on-rod heterostructures having three different epitaxial angles with different surface facets are reported. Te injection temperature determined the composition, ensuring heterostructure formation with trigonal Sb2SexTe3-x disks on orthorhombic Sb2Se3 rods having orientation angles 180°, 135°, and 90°. The growth kinetics of disks connected at one/two heads of parent rods is manipulated using an Se precursor as a limiting reagent. Theoretical calculations established the energy minimization of different orientations, their possible formation, and suitability in energy transfer applications. Electrochemical measurements were also in agreement with theoretical calculations. Hence, this is a case study of advanced modular synthesis of disk-on-rod nanostructures, leading a step further in nanocrystal engineering for more desirable complex structures and their charge transfer property.

Au-Cu2-xTe Plasmonic Heteronanostructure Photoelectrocatalysts.

Semiconductor nanocrystals coupled with plasmonic Au nanoparticles have been widely studied as photoelectrocatalysts for solar water splitting. Among these, heterostructures of copper chalcogenides with Au remained a unique category for their dual plasmon character. However, while sulfides and selenides of copper have been extensively reported, heterostructures of copper tellurides with Au have not been explored. Herein, the plasmonic semiconductor Cu2-xTe disks grown on Au nanoparticles (disk-on-dot) were explored as efficient photoelectrocatalysts for hydrogen evolution reactions (HER). This has been successfully designed by growing Cu2-xTe disks on presynthesized Au nanoparticles under optimized reaction conditions. The resulting heterostructured nanocrystals acted as efficient photoelectrocatalysts for the H2 evolution reaction with a low Tafel slope and less cathodic overpotential in the presence of light. Details of their synthesis, characterization, optical properties, and electrocatalytic activities are studied and reported in this letter.

Modulated Triple-Material Nano-Heterostructures: Where Gold Influenced the Chemical Activity of Silver in Nanocrystals.

For efficient charge separations, multimaterial hetero-nanostructures are being extensively studied as photocatalysts. While materials with one heterojunction are widely established, the chemistry of formation of multijunction heterostructures is not explored. This needs a more sophisticated approach and modulations. To achieve these, a generic multistep seed mediated growth following controlled ion diffusion and ion exchange is reported which successfully leads to triple-material hetero-nanostructures with bimetallic-binary alloy-binary/ternary semiconductors arrangements. Ag2 S nanocrystals are used as primary seeds for obtaining AuAg-AuAgS bimetallic-binary alloyed metal-semiconductor heterostructures via partial reduction of Ag(I) using Au(III) ions. These are again explored as secondary seeds for obtaining a series of triple-materials heterostructures, AuAg-AuAgS-CdS (or ZnS or AgInS2 ), with introduction of different divalent and trivalent ions. Chemistry of each step of the gold ion-induced changes in the rate of diffusion and/or ion exchanges are investigated and the formation mechanism for these nearly monodisperse triple material heterostructures are proposed. Reactions without gold are also performed, and the change in the reaction chemistry and growth mechanism in presence of Au is also discussed.

Digestive Ripening of Au Nanoparticles Using Multidentate Ligands.

The efficiency of multidentate ligands as digestive ripening (DR) agents for the preparation of monodisperse Au nanoparticles (NPs) was investigated. This systematic investigation was performed using ligands possessing one, two, or three thiol moieties as ligands/DR agents. Our results clearly establish that among the different ligands, monodentate ligands and the use of temperature in the range of 60-120 °C offer the best conditions for DR. In addition, when DR was carried out at lower temperatures (e.g., 60 °C), the NP size increased as the number of thiol groups per ligand increased. However, in the case of ligands possessing two and three thiol moieties, when they were heated with polydispersed particles at higher temperatures (120 or 180 °C), the etching process dominated, which affected the quality of the NPs in terms of their monodispersity. We conclude that the temperature-dependent strength of the interaction between the ligand headgroup and the NP surface plays a vital role in controlling the final particle sizes.

Time and temperature effects on the digestive ripening of gold nanoparticles: is there a crossover from digestive ripening to Ostwald ripening?

The effects of time and temperature on the gold nanoparticle sizes obtained by digestive ripening have been investigated. In digestive ripening, a polydisperse colloid, upon refluxing with a surface-active ligand in a solvent, gets converted to a nearly monodisperse one. In this study, a polydisperse gold nanoparticle system was heated in 4-tert-butyltoluene with hexadecanethiol at different temperatures, viz., 60, 90, 120, 150, and 180 °C for different time periods, and the trends in particle size variations were recorded. At lower temperatures such as 60 and 90 °C, after the initial narrowing of the size distribution, the particle sizes remain constant even though the refluxing step is continued for 24 h, substantiating the prevalence of the digestive ripening process. However, at elevated temperatures (120, 150, and 180 °C) particle sizes grow continuously, indicating a deviation from the digestive ripening behavior to an Ostwald ripening-type phenomenon.

Synthesis and in situ observation of 3D superlattices of gold nanoparticles using oil-in-water emulsion.

In this work oil-in-water emulsion has been successfully used as a confined environment to grow 3D superlattices of gold nanoparticles. The superlattices were grown from 5 nm uniform gold nanoparticles using slow destabilization method. The confined environment was created by forming a stable emulsion where the gold colloid suspended in toluene was used as oil phase. Superlattices were also formed in bulk solution using the same slow destabilization method. A comparative study reveals that compact superlattices form more readily inside the emulsion drops as compared to bulk precipitation. The unstable colloid (in bulk or as emulsion) was aged for various periods at 5 °C to form more compact superlattices. The best superlattices with sharp corners are observed when the superlattices are formed inside the emulsion and aged for a month. Two key parameters, the incubation temperature and anti-solvent concentration, are optimized to obtain larger superlattices with sharp features. A new method is also demonstrated for in situ observation of superlattice formation using an optical microscope.

Fine control of nanoparticle sizes and size distributions: temperature and ligand effects on the digestive ripening process.

It is demonstrated that a fine control over the size and size distribution of nanoparticles could be achieved using digestive ripening at different temperatures. Such variations in size and size distributions hugely influence the self-assembled processes in nanoparticles, and result in superlattice structures that are controlled by subtle interplay between ligand orientational entropy and their interdigitation and the van der Waals attraction between the metal cores.