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 Cu2-xS 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. This causes the nanorods to fuse via their basal tip facets into chains and then cojoin through diametrically opposed side facets, resulting in atomically coupled, 2D raftlike structures. The stepwise OA of 1D nanocrystals into 2D nanostructures illustrated here expands the range of nanoarchitectures that can be produced via solution-processed methods.

Fluorescent Semiconductor Nanorods for the Solid-Phase Polymerase Chain Reaction-Based, Multiplexed Gene Detection of Mycobacterium tuberculosis.

The spread of infectious diseases with significantly high mortality rates can wreak devastating damage on global health systems and economies, underscoring the need for better disease diagnostic platforms. Solid-phase polymerase chain reaction (SP-PCR) potentially combines the advantages of conventional PCR-based diagnostics with the capability of multiplexed detection, given that the spatial separation between primers circumvents unwanted primer-primer interactions. However, the generally low efficiency of solid-phase amplification results in poor sensitivity and limits its use in detection schemes. We present an SP-PCR-based, multiplexed pulldown fluorescence assay for the detection of Mycobacterium tuberculosis (MTB), utilizing highly fluorescent oligonucleotide-functionalized CdSe/CdS and CdSe1-xSx/CdS nanorods (NRs) as multicolor hybridization probes. The large surface area of the NRs allows for their easy capture and pulldown, but without contributing significantly to the interparticle photon reabsorption when clustered at the pulldown sites. The NR nanoprobes were specifically designed to target the hotspot regions of the rpoB gene of MTB, which have been implicated in resistance to standard rifampicin treatment. The implementation of the semiconductor NRs as photostable multicolor fluorophores in a multiplexed SP-PCR-based detection scheme allowed for the identification of multiple hotspot regions with sub-picomolar levels of sensitivity and high specificity in artificial sputum. While this work demonstrates the utility of semiconductor NRs as highly fluorescent chromophores that can enable SP-PCR as a sensitive and accurate technique for multipathogen diagnostics, the flexible surface chemistry of the NRs should allow them to be applicable to a wide variety of detection motifs.

Branched Heterostructured Semiconductor Nanocrystals with Various Branch Orders via 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 via heterogeneous nucleation and growth processes alone. This work describes a process to facet-link matchstick-like Ag2S-tipped ZnS nanorods via their Ag2S tips, producing branched Ag2S-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. This is attributed to the exposed facets of the monoclinic Ag2S tips, their relative chemical reactivities, and their atomic composition. The formation of particles with an increasing number of branches takes place in a stepwise manner, thus making the facet-linking approach a facile synthesis route to systematically obtaining a diverse set of branched heterostructured semiconductor nanoparticles with a well-defined number of branches.

Thermochromism from Ultrathin Colloidal Sb2 Se3 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 Sb2 Se3 nanowires in an amine-thiol mixture is introduced. The nanowires undergo reversible growth and dissolution with repeated cycles of heating and cooling between 20 and 160 °C, exhibiting intense and contrasting color changes during these processes. Furthermore, the transition temperature in which a change in color first appears can be continuously tuned over a range larger than 100 °C by introducing controlled amounts of Sn2+ . The colloidal nanowire dispersion in the amine-thiol mixture retains its thermochromic properties over hundreds of temperature cycles, continuous heating at 80 °C over months, and shelf life of up to 2 years in an open container under ambient conditions. To illustrate its utility as a robust liquid thermochromic, the nanowire solution is coated onto standard filter paper and its uses as a rewritable surface by thermal scribing, as well as an inexpensive means of visualizing the temperature distribution of an anisotropically heated block are demonstrated.

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. The ability to control color switching online within the same droplet cavity enables sequential detection of multiple target molecules within or around the cavity. As a demonstration of this concept, we show how the presence of FITC-Dextran and methylene blue (MB) in the medium surrounding the lasing droplets can be sequentially detected by the blue and orange laser respectively. The method is simple and can be extended to a range of water-soluble dyes, thus enabling a wide spectral range for the lasing with the use of a single pump laser source.

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.

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. We showcase a simple application of our method, which highlights the advantages of having embedded, spatially segregated laser sources within a droplet containing a model analyte solution. With this method, each microfluidic droplet now functions not only as an isolated experiment flask, but is also capable of on drop sensing that exploits WGM-based lasing, thus expanding the possibilities for online monitoring of biophysical/biochemical processes and sensitive detection of biomolecules in droplet-based microfluidics.

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. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

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 10.7 cm-1, whereas the frequency of the out-of-plane mode remains at ∼20 cm-1. Analysis of the oscillation phases suggests that the coherent acoustic phonon generation mechanism transitions from displacive excitation to subpicosecond Auger hole trapping with increasing pump fluence. The measurements yield Huang-Rhys parameters of ∼10-2 for both acoustic modes. The weak electron-phonon coupling strengths favor the application of NPLs in optoelectronics.

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.

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 Ag2S (Cu2S) domains. Selective removal of the ligands from these inorganic domains results in spontaneous, site-specific bridging of the nanoparticles. Using this method, we demonstrate the fabrication of polymer-like linear and branched nanoparticles with enhanced electrical properties, as well as the stoichiometric formation of nanoparticle homo- and heterodimers and tetramers. We show that linked structures can then be completely cation exchanged with Pb2+ to generate PbSe/PbS-based nanostructured photodetector media with enhanced properties.

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. These nanoplatelets possess very large gain cross-sections of 7.3 × 10(-14) cm(2) and ultralow lasing thresholds of 1.2 and 4.3 mJ cm(-2) under two-photon (λexc=800 nm) and three-photon (λexc=1.3 µm) excitation, respectively. The highly polarized emission from the nanoplatelet laser shows no significant photodegradation over 10(7) laser shots. These findings constitute a more comprehensive understanding of the utility of colloidal semiconductor nanoparticles as the gain medium in high-performance frequency-upconversion liquid lasers.

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.

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. When anisotropic semiconductor nanorods (NRs) were used as the fluorescent probes, the detection limit was found to be 0.4 ng of total RNA, which was much lower than that obtained using spherical quantum dots (QDs) or organic dyes. This was attributed to the large action cross-section of NRs and their high probability of target capture in a pull-down detection scheme. The combination of large scale integrated microfluidics with highly fluorescent semiconductor NRs may find widespread utility in point-of-care devices and multitarget diagnostics.

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. By employing time-resolved spectroscopy measurements, we were able to attribute the wide emission color-tunability to the large valence band offset between InP and CdS. This work highlights the importance of band alignment in the synthetic design of semiconductor nanoheterostructures, which can exhibit color-tunable multiwavelength emission with high efficiency and photostability.

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. Time-resolved spectroscopy measurements on tetrapods revealed the presence of hole traps, which facilitated the localization and subsequent radiative recombination of excitons in the CdS arm regions, whereas excitonic recombination in nanorods took place predominantly within the vicinity of the CdSe core. These observations collectively highlight the role of morphology in the achievement of light emission from the different material components in heterostructured semiconductor nanoparticles, thus showing a way in developing a class of materials which are capable of exhibiting multiwavelength electroluminescence.

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. As the field of semiconductor nanocrystals evolves towards structurally complex multifunctional architectures, the physicochemical attributes of composites of nanocrystals in sol-gel matrices are also dramatically improved. A number of examples from the most recent reports on such structures will be highlighted, along with a brief discussion on the future outlook of such materials.