Dopant-controlled selenization in Pd nanocrystals: the triggered Kirkendall effect.

Doping foreign impurities in host nanomaterials can induce new materials properties. In addition, doping can also influence the crystallization process and change the shape and/or phase of the host material. While dopant-induced changes in the properties of materials have been well studied, the concept of doping and its chemistry in the design of different nanostructures has rarely been investigated. In order to further understand the doping chemistry, this study investigated the dopant-controlled enhancement of the rate of the chemical reaction during the transformation from one doped material to another and the consequent effect on the shape evolution of the nanostructures. These are performed during the selenization of metal Pd(0), using Ag dopant. While the controlled process produced cuboidal Pd17Se15 from the quasi-spherical nanocrystals of Pd(0), on doping, the shape of Pd17Se15 transformed into hollow cubes. The rate was also enhanced by more than 30 times for the doped case in comparison to undoped Pd(0). Importantly, while for the undoped nanocrystals, the selenization approached in one direction, where for the doped particles, it occurred all around the nanocrystals and triggered the Kirkendall effect. Detailed investigations were conducted to elucidate the influence of the dopant on both the rate and directional approach of selenization in Pd(0), initiation of the fast diffusion of Pd, change in shape, and formation of the hollow structures. To our understanding, the role of dopants in controlling chemical processes is of fundamental importance, and this will undoubtedly broaden the scope of research on the chemistry of doping and crystal growth in solution.

Chemical sealing of nanotubes: a case study on Sb2S3.

Implementing the solution chemistry, herein, we report the sealing of both ends of Sb2 S3 semiconductor nanotubes following the diffusion-controlled deposition of the sealing material, AgSbS2 . As a consequence, unique dumbbell-shaped hollow nanocapsules having a binary-ternary epitaxial heterojunction were formed in solution. Whereas these capsule-shaped nanostructures were obtained by the introduction of Ag(0) nanocrystals just after the formation of Sb2 S3 nanotubes, the addition of Ag(0) at the beginning of the process, prior to the formation of nanotubes, changed the growth pattern, and solid nanorods of Sb2 S3 were formed. The details of the chemistry involved in the formation of these nanostructures were investigated and are discussed herein.

Intermittency of CsPbBr3 Perovskite Quantum Dots Analyzed by an Unbiased Statistical Analysis.

We analyze intermittency in intensity and fluorescence lifetime of CsPbBr3 perovskite quantum dots by applying unbiased Bayesian inference analysis methods. We apply change-point analysis (CPA) and a Bayesian state clustering algorithm to determine the timing of switching events and the number of states between which switching occurs in a statistically unbiased manner, which we have benchmarked particularly to apply to highly multistate emitters. We conclude that perovskite quantum dots display a plethora of gray states in which brightness, broadly speaking, correlates inversely with decay rate, confirming the multiple recombination centers model. We leverage the CPA partitioning analysis to examine aging and memory effects. We find that dots tend to return to the bright state before jumping to a dim state and that when choosing a dim state, they tend to explore the entire set of states available.

Fano Lineshapes and Rabi Splittings: Can They Be Artificially Generated or Obscured by the Numerical Aperture?

Fano resonances and Rabi splittings are routinely reported in the scientific literature. Asymmetric resonance lineshapes are usually associated with Fano resonances, and two split peaks in the spectrum are often attributed to a Rabi splitting. True Fano resonances and Rabi splittings are unequivocal signatures of coherent coupling between subsystems. However, can the same spectral lineshapes characterizing Fano resonances and Rabi splittings arise from a purely incoherent sum of intensities? Here we answer this question through experiments with a tunable Fabry-Pérot cavity containing a CsPbBr3 perovskite crystal. By measuring the transmission and photoluminescence of this system using microscope objectives with different numerical aperture (NA), we find that even a modest NA = 0.4 can artificially generate Fano resonances and Rabi splittings. We furthermore show that this modest NA can obscure the anticrossing of a bona fide strongly coupled light-matter system. Through transfer matrix calculations we confirm that these spectral artifacts are due to the incoherent sum of transmitted intensities at different angles captured by the NA. Our results are relevant to the wide nanophotonics community, characterizing dispersive optical systems with high numerical aperture microscope objectives. We conclude with general guidelines to avoid pitfalls in the characterization of such optical systems.

Quantifying Strain and Dislocation Density at Nanocube Interfaces after Assembly and Epitaxy.

Nanoparticle self-assembly and epitaxy are utilized extensively to make 1D and 2D structures with complex shapes. High-resolution transmission electron microscopy (HRTEM) has shown that single-crystalline interfaces can form, but little is known about the strain and dislocations at these interfaces. Such information is critically important for applications: drastically reducing dislocation density was the key breakthrough enabling widespread implementation of light-emitting diodes, while strain engineering has been fundamental to modern high-performance transistors, solar cells, and thermoelectrics. In this work, the interfacial defect and strain formation after self-assembly and room temperature epitaxy of 7 nm Pd nanocubes capped with polyvinylpyrrolidone (PVP) is examined. It is observed that, during ligand removal, the cubes move over large distances on the substrate, leading to both spontaneous self-assembly and epitaxy to form single crystals. Subsequently, atomically resolved images are used to quantify the strain and dislocation density at the epitaxial interfaces between cubes with different lateral and angular misorientations. It is shown that dislocation- and strain-free interfaces form when the nanocubes align parallel to each other. Angular misalignment between adjacent cubes does not necessarily lead to grain boundaries but does cause dislocations, with higher densities associated with larger rotations.

Close-Packed Ultrasmooth Self-assembled Monolayer of CsPbBr3 Perovskite Nanocubes.

The use of colloidal self-assembly to form the complex multiscale patterns in many optoelectronic devices has been a long-standing dream of the nanoscience community. While great progress has been made using charged colloids in polar solvents, controlled assembly from nonpolar solvents is much more challenging. The major challenge is colloidal clustering caused by strong van der Waals (vdW) attraction between long-chain surface capping ligands passivating the surface of nanocrystals. Such clustering degrades ordering in packing during the self-assembly process. While ligand exchange to provide colloidal stability in polar phases is often an option, this is not the case for the exciting new class of halide perovskites due to the material's solubility in essentially all polar solvents. Here, we report surface-functionalized self-assembly of luminescent CsPbBr3 perovskite nanocubes by partially replacing long-chain oleyl groups (18 carbon chain) with short-chain thiocyanate (SCN-). This enables the fabrication of ultrasmooth monolayer thin films of nanocubes with a root-mean-square (RMS) roughness of around 4 Å. This ultrasmooth large area self-assembled layer could act as high-efficiency optoelectronic devices like solar cells, light-emitting diodes (LEDs), transistors, etc. We correlate our experimental results with simulations, providing detailed predictions for lattice constants with chain conformations showing reduced free energy for cubes grafted with short-chain thiocyanate compared to long-chain oleyl groups, thus facilitating better self-assembly.

Soral crypsis: protective mimicry of a coccid on an Indian fern.

Herbivory with crypsis is not well documented in ferns. The present record of cryptic coloration of coccid Saissetia filicum Boisduval (Homoptera: Coccidae) to the sori of a fern species Asplenium nidus L. (Aspleniaceae) is unique. Predatory beetles (Jauravia sp., Coleoptera: Coccinellidae) that feed on the coccids, are suggested to be selective pressure for the development of the present homopteran soral crypsis. A higher rate of effective predation is noticed in the vegetative leaves than the fertile leaves. Aggressive ants were found harvesting honeydew secretions from the coccids and defending the trophobionts as well as the host fern from their natural enemies. In addition, a possible three-way mutualistic relationship among the coccids, its host fern and the tending ant is suggested. Differential numbers of coccids on vegetative and fertile leaves is correlated with their phenol content and degree of predation by beetles. Such coloration mimicry by the coccids may enable them to obtain the necessary blend of sorus of the host fern needed to evade beetle detection and attack.

Monodisperse AuCuSn trimetallic nanocube catalysts.

Monodisperse trimetallic AuCuSn nanocubes are reported where Sn and Cu are inserted into the crystal lattice following co-precipitation protocols along with Au. These trimetallics are explored further towards catalytic hydrazine electrooxidation and their enhanced catalytic activity compared with their bimetallic counterpart AuCu is reported.

Diffusion-Induced Shape Evolution in Multinary Semiconductor Nanostructures.

The classical mechanism of crystal growth for architecting different nanomaterials in solution, although widely studied, is mainly restricted to binary semiconductor systems. However, this method is not applicable to multinary nanomaterials, which have multivalent cations possessing different reactivity under identical reaction conditions. Hence, the shape architectures of these nanostructures, which require a more sophisticated approach, remain relatively unexplored compared to those of binary semiconductors. Owing to the importance of the multinary materials, which are emerging as excellent green materials for both light harvesting and light emission, we investigated the diffusion-rate-controlled formation of ternary AgGaSe2 nanostructures and studied their heterostructures with noble metals. Controlling the changes in the rate of diffusion of the Ag ions resulted in the formation of tadpole-shaped AgGaSe2 ternary nanostructures. In situ study by collecting a sequential collection of samples has been carried out, and the conversion of amorphous Ga-selenide to crystalline AgGaSe2 has been monitored. In addition, heterostructures of tadpole AgGaSe2 with noble metals, Au and Pt, were designed, and their photocatalytic behaviors were studied.

Efficient Superionic Conductor Catalyst for Solid in Solution-Solid-Solid Growth of Heteronanowires.

How efficient could a superionic conductor catalyst be? Beyond the traditionally used molecular precursors when the solution dispersed solid nanomaterials of variable size, shape and phase are introduced under certain reaction condition; the catalyst is found to digest all these structures in minutes irrespective of their phase and morphology, resulting unique heteronanowires. This has been inspected here by employing different ZnSe nanostructures as precursor for Ag2Se nanocrystal catalyst in its superionic conductor phase to obtain the Ag2Se-ZnSe heteronanowires. This dissolution and formation process of these nanostructures is correlated with the change in the reaction temperature profile, the phase of the catalyst, the shape/phase and surface ligands of the source nanostructures, and the possible mechanism of the unique heteronanowires growth has been investigated.

Synthesis and photo-darkening/photo-brightening of blue emitting doped semiconductor nanocrystals.

By programming the synthetic reaction chemistry, stable blue emitting Cu(i) or Ag(i) doped Al(iii) co-doped ZnS (Al,Cu:ZnS or Al,Ag:ZnS) semiconductor nanocrystals are designed. Further, the photostability of the obtained intense blue-violet emission is studied, and the effects of doping/co-doping are correlated. Finally, it is revealed that the strong binding surface ligand 1-dodecanethiol and Al(iii) co-doping play pivotal roles in achieving such stable blue emitting doped nanocrystals.

Tuning the growth pattern in 2D confinement regime of Sm2O3 and the emerging room temperature unusual superparamagnetism.

Programming the reaction chemistry for superseding the formation of Sm2O3 in a competitive process of formation and dissolution, the crystal growth patterns are varied and two different nanostructures of Sm2O3 in 2D confinement regime are designed. Among these, the regular and self-assembled square platelets nanostructures exhibit paramagnetic behavior analogous to the bulk Sm2O3. But, the other one, 2D flower like shaped nanostructure, formed by irregular crystal growth, shows superparamagnetism at room temperature which is unusual for bulk paramagnet. It has been noted that the variation in the crystal growth pattern is due to the difference in the binding ability of two organic ligands, oleylamine and oleic acid, used for the synthesis and the magnetic behavior of the nanostructures is related to the defects incorporated during the crystal growth. Herein, we inspect the formation chemistry and plausible origin of contrasting magnetism of these nanostructures of Sm2O3.

The Redox Chemistry at the Interface for Retrieving and Brightening the Emission of Doped Semiconductor Nanocrystals.

Photo-oxidation of semiconductor quantum dots is the prime concern during their processability, as it often induces nonradiative states and quenches the band edge excitonic emission. Nevertheless, similar effects have been observed for light emitting doped semiconductor nanocrystals, and the dopant emissions are also quenched due to the surface oxidation. This is more pronounced for selenide-based host semiconductors. To overcome this, we study the interface chemistry of Cu-doped and Mn-doped ZnSe nanocrystals and report here the retrieving and brightening of the emission from completely quenched months old doped nanocrystals. This has been obtained by treating the doped nanocrystals with appropriate organic thiol ligands which remove the surface oxidative states as well as resist further oxidation of the nanocrystals. Here, we investigate details of the redox chemistry at the interface and study related photophysics in retrieving the dopant emission.