2D-Oriented Attachment of 1D Colloidal Semiconductor Nanocrystals via an Etchant.

The oriented attachment (OA) of 0D semiconductor nanocrystals into 1D and 2D nanostructures with unique properties is useful for the fabrication of quantum confined nanomaterials that are otherwise difficult to produce by direct synthesis. Given that the OA of 1D nanocrystals such as nanorods generally produces linear chains, rod-couple structures, or clustered columns, linking them in a facet-specific manner to produce 2D structures is challenging. Here, we report that 1D CuS nanorods undergo etching on exposure to hexylphosphonic acid under mild heating, which results in an increased curvature and a reduction in surface ligands at those sites.

Branched Heterostructured Semiconductor Nanocrystals with Various Branch Orders a Facet-to-Facet Linking Process.

Branched heterostructured semiconductor nanoparticles such as core seeded tetrapods and octapods offer properties not seen in their spherical core-shell counterparts, but are challenging to synthesize with a large diversity of branch numbers heterogeneous nucleation and growth processes alone. This work describes a process to facet-link matchstick-like AgS-tipped ZnS nanorods their AgS tips, producing branched AgS-centered ZnS nanoparticles such as bipods, tripods, and in general multipods with 4 to 16 ZnS arms as a function of reaction time. The angle between nanorods in the bipods and tripods is found to be close to 120°, resulting in unexpected bent and trigonal planar geometry, respectively.

Thermochromism from Ultrathin Colloidal Sb Se Nanowires Undergoing Reversible Growth and Dissolution in an Amine-Thiol Mixture.

Liquid-based thermochromics can be incorporated into an arbitrarily shaped container and provide a visual map of the temperature changes within its volume. However, photochemical degradation, narrow temperature range of operation, and the need for stringent encapsulation processes are challenges that can limit their widespread use. Here, a unique solution-based thermochromic comprising ultrathin colloidal Sb Se nanowires in an amine-thiol mixture is introduced.

Highly fluorescent, monolithic semiconductor nanorod clusters for ultrasensitive biodetection.

We have developed highly fluorescent, monolithic colloidal CdSe seeded CdS nanorod clusters comprising thousands of nanorods. Their use in the sandwich assay detection of a model protein yields a thousand-fold improvement in the detection limit compared to individual nanorods, making them suitable for the detection of low abundance molecular targets.

Multi-color lasing in chemically open droplet cavities.

In this paper, we demonstrate FRET-based multicolor lasing within chemically open droplet cavities that allow online modulation of the gain medium composition. To do this, we generated monodisperse microfluidic droplets loaded with coumarin 102 (donor), where the spherical droplets acted as whispering gallery mode (WGM) optical cavities in which coumarin 102 lasing (~ 470 nm) was observed. The lasing color was switched from blue to orange by the introduction of a second dye (acceptor, rhodamine 6 G) into the flowing droplet cavities; subsequent lasing from rhodamine 6 G (~ 590 nm) was observed together with the complete absence of coumarin 102 emission.

Stable, Ultralow Threshold Amplified Spontaneous Emission from CsPbBr 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 CsPbBr nanoparticles with a mixture of PbBr, 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.

Embedding liquid lasers within or around aqueous microfluidic droplets.

In this paper, we demonstrate the incorporation of dye-based liquid lasers within or around flowing aqueous microfluidic droplets. In particular, we use dye solutions in benzyl alcohol, and either disperse an ensemble of small (∼20 μm) lasing droplets within large (∼500 μm) aqueous droplets flowing in a simple glass capillary-based microfluidic device, or 'wrap' a thin (∼10 μm) lasing benzyl alcohol shell around larger (∼560 μm) microfluidic aqueous droplets. We experimentally and theoretically characterize the lasing behavior in both cases, which is supported by whispering-gallery mode (WGM) optical resonances at the droplet interfaces.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface.

Pump-Power Dependence of Coherent Acoustic Phonon Frequencies in Colloidal CdSe/CdS Core/Shell Nanoplatelets.

Femtosecond optical pump-probe spectroscopy resolves hitherto unobserved coherent acoustic phonons in colloidal CdSe/CdS core/shell nanoplatelets (NPLs). With increasing pump fluence, the frequency of the in-plane acoustic mode increases from 5.2 to 10.

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.

Facet to Facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control.

Nonclassical growth mechanisms such as self-assembly and oriented attachment are effective ways to build complex nanostructures from simpler ones. In the latter case, the nanoparticle components are electronically coupled; however, control over the attachment between nanoparticles is highly challenging and generally requires a delicate balance between dipole-, ligand-, and solvent-based interactions. To this end, we perform incomplete cation exchange with Ag (Cu) on CdSe-seeded CdS nanorods and tetrapods to exclusively convert their tips into small AgS (CuS) domains.

Wet-Chemically Synthesized Colloidal Semiconductor Nanostructures as Optical Gain Media.

An overview on the development of wet-chemically synthesized semiconductor nanostructures as optical gain materials is presented in this Review, beginning with the first demonstration of amplified spontaneous emission in zero-dimensional quantum dots and evolving to more sophisticated heterostructures such as one-dimensional core-seeded nanorods, branched core-seeded tetrapods and two-dimensional nanoplatelets. The advantages and challenges of utilizing strongly quantum-confined colloidal semiconductor materials as gain media are discussed, and a concerted effort is made to elaborate on how the progression towards more structurally complex architectures has allowed for dramatic improvements in performance and stability over the archetypal quantum dot.

Ultralow-threshold multiphoton-pumped lasing from colloidal nanoplatelets in solution.

Although multiphoton-pumped lasing from a solution of chromophores is important in the emerging fields of nonlinear optofluidics and bio-photonics, conventionally used organic dyes are often rendered unsuitable because of relatively small multiphoton absorption cross-sections and low photostability. Here, we demonstrate highly photostable, ultralow-threshold multiphoton-pumped biexcitonic lasing from a solution of colloidal CdSe/CdS nanoplatelets within a cuvette-based Fabry-Pérot optical resonator. We find that colloidal nanoplatelets surprisingly exhibit an optimal lateral size that minimizes lasing threshold.

Gene Detection in Complex Biological Media Using Semiconductor Nanorods within an Integrated Microfluidic Device.

The salient optical properties of highly luminescent semiconductor nanocrystals render them ideal fluorophores for clinical diagnostics, therapeutics, and highly sensitive biochip applications. Microfluidic systems allow miniaturization and integration of multiple biochemical processes in a single device and do not require sophisticated diagnostic tools. Herein, we describe a microfluidic system that integrates RNA extraction, reverse transcription to cDNA, amplification and detection within one integrated device to detect histidine decarboxylase (HDC) gene directly from human white blood cells samples.

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.

Efficient color-tunable multiexcitonic dual wavelength emission from Type II semiconductor tetrapods.

We synthesized colloidal InP/ZnS seeded CdS tetrapods by harnessing the structural stability of the InP/ZnS seed nanocrystals at the high reaction temperatures needed to grow the CdS arms. Because of an unexpected Type II band alignment at the interface of the InP/ZnS core and CdS arms that enhanced the occurrence of radiative excitonic recombination in CdS, these tetrapods were found to be capable of exhibiting highly efficient multiexcitonic dual wavelength emission of equal intensity at spectrally distinct wavelengths of ∼485 and ∼675 nm. Additionally, the Type II InP/ZnS seeded CdS tetrapods displayed a wider range of pump-dependent emission color-tunability (from red to white to blue) within the context of a CIE 1931 chromaticity diagram and possessed higher photostability due to suppressed multiexcitonic Auger recombination when compared to conventional Type I CdSe seeded CdS tetrapods.

Dual wavelength electroluminescence from CdSe/CdS tetrapods.

We fabricated a single active layer quantum dot light-emitting diode device based on colloidal CdSe (core)/CdS (arm) tetrapod nanostructures capable of simultaneously producing room temperature electroluminesence (EL) peaks at two spectrally distinct wavelengths, namely, at ∼500 and ∼660 nm. This remarkable dual EL was found to originate from the CdS arms and CdSe core of the tetrapod architecture, which implies that the radiative recombination of injected charge carriers can independently take place at spatially distinct regions of the tetrapod. In contrast, control experiments employing CdSe-core-seeded CdS nanorods showed near-exclusive EL from the CdSe core.

Formation of hollow iron oxide tetrapods via a shape-preserving nanoscale Kirkendall effect.

Hollow iron oxide tetrapods are synthesized by exposing iron precursors to hollow ultrathin PtS tetrapods. Surprisingly, the heterogeneous nucleation and growth of iron takes place exclusively within the PtS interior. Oxidation of the resulting iron tetrapod produces a hollow iron oxide shell via a shape-preserving Kirkendall effect which preserves the morphology of the original tetrapod with remarkably high precision, offering a robust synthetic route to hollow iron oxide nanostructures of unprecedented geometry.

Semiconductor nanocrystals in sol-gel derived matrices.

Wet-chemically synthesized colloidal semiconductor nanocrystals are unique chromophores which possess properties such as size-dependent absorption and emission, large action cross-sections and flexible surface chemistry. This Perspective summarizes efforts in the field to incorporate these nanocrystals into sol-gel derived matrices, thereby harnessing their salient material properties to enhance or create new avenues for research in applications such as biological imaging, diagnostics and optical amplifiers. A description of the basic chemistry involved in making the semiconductor nanocrystals compatible with the sol-gel process is given, as well as the different strategies developed to localize various types of nanocrystals within the sol-gel network.

Ultralow-threshold two-photon pumped amplified spontaneous emission and lasing from seeded CdSe/CdS nanorod heterostructures.

Ultralow-threshold two-photon pumped amplified spontaneous emission (2ASE) and lasing in seeded CdSe/CdS nanodot/nanorod heterostructures is demonstrated for the first time. Such heterostructures allow the independent tunability of the two-photon absorption (2PA) cross-section (σ(2)) through varying the CdS rod size, and that of the emission wavelength through varying the CdSe dot size. With an enhanced σ(2), 2ASE in these heterostructures is achieved with an ultralow threshold fluence of ~1.

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.

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 SiO(2) 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.