Aqueous-Phase Reactions on Hollow Silica-Encapsulated Semiconductor Nanoheterostructures

We introduce a facile and robust methodology for the aggregation-free aqueous-phase synthesis of hierarchically complex metal–semiconductor heterostructures. By encapsulating semiconductor nanostructures within a porous SiO₂ shell with a hollow interior, we can isolate each individual particle while allowing it access to metal precursors for subsequent metal growth. We illustrate this by Pt deposition on CdSe-seeded CdS tetrapods, which we found to be facilitated via the surprising formation of a thin interfacial layer of PtS coated onto the original CdS surface. We took advantage of this unique architecture to perform cation exchange reactions with Ag⁺ and Pd²⁺, thus demonstrating the feasibility of achieving such transformations in complex metal–semiconductor nanoparticle systems

Unusual Selectivity of Metal Deposition on Tapered Semiconductor Nanostructures

We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS nanoheterostructures with conelike, tapered geometries in order to examine the effects of shape on the location-specific deposition of Au under ambient conditions. Although preferential metal deposition at surface defect sites are generally expected, we found suprisingly that Au growth at the side facets of tapered linear and branched structures was significantly suppressed. Further investigation revealed this to be due to a highly efficient electrochemical Ostwald ripening process which was previously thought not to occur in branched nanostructures such as tetrapods. We exploited this phenomenon to fabricate uniform asymmetrically tipped CdSe-seeded CdS tetrapods with conelike arms, where a solitary large Au tip is found on one of the arms while the other three arms bear Ag₂S tips. Importantly, this work presents a synthetic route toward the selective deposition of metals onto branched semiconductor nanostructures whose arms have nearly symmetric reactivity

Synthesis and Characterization of Dually Labeled Pickering-Type Stabilized Polymer Nanoparticles in a Downscaled Miniemulsion System

Dual fluorescently labeled polymer particles were prepared in a downscaled Pickering-type miniemulsion system. Stable dispersions were obtained and the size of the hybrid particles could be varied between ca. 180 and 430 nm. Silica nanoparticles were employed as sole emulsifier, which were labeled by a fluorescein dye (FITC) or (encapsulated) quantum dots, and the polymer core was labeled by a perylene derivative. Downscaling of the Pickering-type miniemulsion system is intriguing by itself as it allows the use of precious nanoparticles as emulsifiers. Here, silica particles with a fluorescent core and an overall diameter between 20 and 40 nm were prepared and employed as stabilizer. The dual excitation and emission of both dyes was tested by fluorescence measurements and confocal laser scanning microscopy (cLSM)

Stable, Ultralow Threshold Amplified Spontaneous Emission from CsPbBr₃ Nanoparticles Exhibiting Trion Gain

Wet-chemically synthesized cesium lead halide nanoparticles have many attractive properties that make them promising as optical gain media, but generally suffer from poor stability under ambient conditions and an optical gain threshold that is widely believed to be dictated by the need for biexcitons. These conditions make it impractical for such particles to be utilized as gain media given the need to undergo repeated stimulated emission processes at above-threshold pump intensities over long periods of time. We demonstrate that the surface treatment of CsPbBr₃ nanoparticles with a mixture of PbBr₂, oleic acid, and oleylamine not only raises their fluorescence quantum yield to nearly unity and prolongs their stability in air from days to months, but it also dramatically increases their trion photoluminescence lifetime from ∼0.9 to ∼1.6 ns. Via a combination of time-resolved photoluminescence and transient absorption spectroscopy, we provide evidence for trion gain at sufficiently low pump intensities in which the likelihood of predominantly biexciton-based gain is small. We then show that, in line with theoretical prediction, the amplified spontaneous emission (ASE) threshold of a thin film of surface-treated CsPbBr₃ nanoparticles reduces to a record low of ∼1.2 μJ/cm² with a corresponding average exciton occupancy per nanoparticle of 0.62. The ultralow pump threshold and increased stability allow for stable ASE over millions of laser shots, paving the way for the deployment of these nanoparticles as viable solution-processed optical gain media

Promoting 2D Growth in Colloidal Transition Metal Sulfide Semiconductor Nanostructures via Halide Ions

Wet-chemically synthesized 2D transition metal sulfides (TMS) are promising materials for catalysis, batteries and optoelectronics, however a firm understanding on the chemical conditions which result in selective lateral growth has been lacking. In this work we demonstrate that Ni₉S₈, which is a less common nonstoichiometric form of nickel sulfide, can exhibit two-dimensional growth when halide ions are present in the reaction. We show that the introduction of halide ions reduced the rate of formation of the nickel thiolate precursor, thereby inhibiting nucleation events and slowing growth kinetics such that plate-like formation was favored. Structural characterization of the Ni₉S₈ nanoplates produced revealed that they were single-crystal with lateral dimensions in the range of ∼100–1000 nm and thicknesses as low as ∼4 nm (about 3 unit cells). Varying the concentration of halide ions present in the reaction allowed for the shape of the nanostructures to be continuously tuned from particle- to plate-like, thus offering a facile route to controlling their morphology. The synthetic methodology introduced was successfully extended to Cu₂S despite its different growth mechanism into ultrathin plates. These findings collectively suggest the importance of halide mediated slow growth kinetics in the formation of nanoplates and may be relevant to a wide variety of TMS

Sub-Picosecond Auger-Mediated Hole-Trapping Dynamics in Colloidal CdSe/CdS Core/Shell Nanoplatelets

Quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a class of semiconductor nanomaterials whose atomically precise monodisperse thicknesses give rise to narrow absorption and emission spectra. However, the sub-picosecond carrier dynamics of NPLs at the band edge remain largely unknown, despite their importance in determining the optoelectronic properties of these materials. Here, we use a combination of femtosecond transient absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate the early time carrier dynamics of CdSe/CdS core/shell NPLs. Band-selective probing reveals sub-picosecond Auger-mediated trapping of holes with an effective second-order rate constant of 3.5 ± 1.0 cm²/s. Concomitant spectral blue shifts that are indicative of Auger hole heating are found to occur on the same time scale as the sub-picosecond trapping dynamics, whereas spectral red shifts that emerge at low excitation densities furnish an electron-cooling time scale of 0.84 ± 0.09 ps. Finally, nonadiabatic molecular dynamics simulations relate the observed sub-picosecond Auger-mediated hole-trapping dynamics to a shallow trap state that originates from the incomplete passivation of dangling bonds on the NPL surface

Continuous Shape Tuning of Nanotetrapods: Toward Shape-Mediated Self-Assembly

We describe a surfactant-driven method to synthesize highly monodisperse CdSe-seeded CdS tetrapods with differing arm lengths and diameters in order to examine their effects on self-assembly. We exploited the phenomena of weak- and strong-binding capping groups to tune the arm length and diameter with uniform shape and achieved >95% yield. Afterward, we utilize these particles to overcome some of the key problems in the assembly of anisotropic shaped particles. Intriguingly, we found that tetrapods with certain arm lengths pack like fishbone chains, which was greatly dependent on particle shape and size. These ordered assembly phenomena were understood with the assistance of computer simulations, which strongly support our experimental observations. Importantly, this work presents a synthetic route toward shape tuning in CdSe-seeded CdS tetrapod structures, which has great influence on their self-assembly behavior at the solution/substrate interface

Hierarchical Multicomponent Nanoheterostructures via Facet-to-Facet Attachment of Anisotropic Semiconductor Nanoparticles

As performance and functionality requirements for solution-processed nanomaterials become more stringent and demanding, there is an ever-growing need for hierarchical nanostructures with sophisticated architecture and complex composition. However, the production of structurally complex nanomaterials is often not possible by direct synthesis. In this work, we describe synthetic methodology to covalently link presynthesized anisotropic semiconductor nanoparticles of different composition in a stoichiometrically controlled manner via specific facet sites at room temperature. We demonstrate that CdSe nanorods can be cojoined with CdTe tetrapods via a competitive cation-exchange process with Ag⁺ that results in linking between the tips of the tetrapod arms with only one end of each nanorod via a Ag₂Se–Ag₂Te interface. This selective linking was engineered by having a large fraction of CdSe nanorods present in the reaction, which sterically hindered homolinking between Ag₂Se-tipped CdSe nanorods and Ag₂Te-tipped CdTe tetrapods with themselves. Cation back-exchange with Cd²⁺ and a size-selective purification to remove unlinked products yields samples enriched in heterolinked CdTe tetrapod–CdSe nanorod structures. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the structure and composition of the nanorod-linked tetrapods, while time-resolved and pump-dependent photoluminescence data were consistent with a type II band offset at the CdTe–CdSe interface. The synthetic approach to colloidal nanoheterostructures described here is highly distinct from traditional methods involving a series of nucleation and growth steps at elevated temperature

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump–probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born–Oppenheimer regime