Semiconductor nanowires (NWs) are often synthesized by the vapor-liquid-solid (VLS) mechanism, a process in which a liquid droplet-supplied with precursors in the vapor phase-catalyzes the growth of a solid, crystalline NW. By changing the supply of precursors, the NW composition can be altered as it grows to create axial heterostructures, which are applicable to a range of technologies. The abruptness of the heterojunction is mediated by the liquid catalyst, which can act as a reservoir of material and impose a lower limit on the junction width. Here, we demonstrate that this "reservoir effect" is not a fundamental limitation and can be suppressed by selection of specific VLS reaction conditions. For Au-catalyzed Si NWs doped with P, we evaluate dopant profiles under a variety of synthetic conditions using a combination of elemental imaging with energy-dispersive X-ray spectroscopy and dopant-dependent wet-chemical etching. We observe a diameter-dependent reservoir effect under most conditions. However, at sufficiently slow NW growth rates (?250 nm/min) and low reactor pressures (?40 Torr), the dopant profiles are diameter independent and radially uniform with abrupt, sub-10 nm axial transitions. A kinetic model of NW doping, including the microscopic processes of (1) P incorporation into the liquid catalyst, (2) P evaporation from the catalyst, and (3) P crystallization in the Si NW, quantitatively explains the results and shows that suppression of the reservoir effect can be achieved when P evaporation is much faster than P crystallization. We expect similar reaction conditions can be developed for other NW systems and will facilitate the development of NW-based technologies that require uniform and abrupt heterostructures.
Interaction of charge carriers with the surface of semiconductor nanocrystals plays an integral role in determining the ultimate fate of the excited state. The surface contains a dynamic ensemble of trap states that can localize excited charges, preventing radiative recombination and reducing fluorescence quantum yields. Here we report quasi-type-II band alignment in graded alloy CdSxSe1-x nanocrystals revealed by femtosecond fluorescence upconversion spectroscopy. Graded alloy CdSxSe1-x quantum dots are a compositionally inhomogeneous nano-heterostructure designed to decouple the exciton from the nanocrystal surface. The large valence band offset between the CdSe-rich core and CdS-rich shell separates the excited hole from the surface by confining it to the core of the nanocrystal. The small conduction band offset, however, allows the electron to delocalize throughout the entire nanocrystal and maintain overlap with the surface. Indeed, the ultrafast charge carrier dynamics reveal that the fast 1-3 ps hole-trapping process is fully eliminated with increasing sulfur composition and the decay constant for electron trapping (?20-25 ps) shows a slight increase. These findings demonstrate progress toward highly efficient nanocrystal fluorophores that are independent of their surface chemistry to ultimately enable their incorporation into a diverse range of applications without experiencing adverse effects arising from dissimilar environments.
The Gemini Planet Imager is a dedicated facility for directly imaging and spectroscopically characterizing extrasolar planets. It combines a very high-order adaptive optics system, a diffraction-suppressing coronagraph, and an integral field spectrograph with low spectral resolution but high spatial resolution. Every aspect of the Gemini Planet Imager has been tuned for maximum sensitivity to faint planets near bright stars. During first-light observations, we achieved an estimated H band Strehl ratio of 0.89 and a 5-? contrast of 10(6) at 0.75 arcseconds and 10(5) at 0.35 arcseconds. Observations of Beta Pictoris clearly detect the planet, Beta Pictoris b, in a single 60-s exposure with minimal postprocessing. Beta Pictoris b is observed at a separation of 434 ± 6 milliarcseconds (mas) and position angle 211.8 ± 0.5°. Fitting the Keplerian orbit of Beta Pic b using the new position together with previous astrometry gives a factor of 3 improvement in most parameters over previous solutions. The planet orbits at a semimajor axis of [Formula: see text] near the 3:2 resonance with the previously known 6-AU asteroidal belt and is aligned with the inner warped disk. The observations give a 4% probability of a transit of the planet in late 2017.
A synthetic approach has recently been developed which results in Cu(x)In(y)S2 quantum dots (QDs) possessing localized surface plasmon resonance (LSPR) modes in the near-infrared (NIR) frequencies.1 In this study, we investigate the potential benefits of near-field plasmonic effects centered upon light absorbing nanoparticles in a photovoltaic system by developing and verifying nonplasmonic counterparts as an experimental control. Simple QD-sensitized solar cells (QD-SSCs) were assembled which show an 11.5% relative increase in incident photon conversion efficiency (IPCE) achieved in the plasmon-enhanced devices. We attribute this increase in IPCE to augmented charge excitation stemming from near-field "antenna" effects in the plasmonic Cu(x)In(y)S2 QD-SSCs. This study represents the first of its kind; direct interrogation of the influence of plasmon-on-semiconductor architectures with respect to excitonic absorption in photovoltaic systems.
The development of the seeded growth synthesis for gold nanorods provided the first simple, convenient wet chemistry route to these nanomaterials. Over the past decade, the original silver-assisted seeded growth procedure has been the subject of further modifications that have continuously expanded access to anisotropic gold nanoparticles; however, the role of silver in formation of gold nanorods remains poorly understood. We report the first experimental evidence on the position of silver present on gold nanorods using advanced energy dispersive X-ray spectroscopy. Our results indicate the deposition of silver ions on the surface shows no preference for a specific face or axis. Furthermore, we show that the "dog bone" structures developed from gold nanorod solutions show preferential deposition of silver atoms on the ends and in the crevices.
Understanding the precise nature of a surface or interface is a key component toward optimizing the desired properties and function of a material. For semiconductor nanocrystals, the surface has been shown to modulate fluorescence efficiency, lifetime, and intermittency. The theoretical picture of a nanocrystal surface has included the existence of an undefined mixture of trap states that arise from incomplete passivation. However, our recent scanning transmission electron microscope movies and supporting theoretical evidence suggest that, under excitation, the surface is fluctuating, creating a dynamic population of surface and subsurface states. This possibility challenges our fundamental understanding of the surface and could have far-reaching ramifications for nanoparticle-based technologies. In this Perspective, we discuss the current theories behind the optical properties of nanocrystals in the context of fluxionality.
The atomic pair distribution function (PDF) analysis of X-ray powder diffraction data has been used to study the structure of small and ultra-small CdSe nanoparticles. A method is described that uses a wurtzite and zinc-blende mixed phase model to account for stacking faults in CdSe particles. The mixed-phase model successfully describes the structure of nanoparticles larger than 2 nm yielding a stacking fault density of about 30%. However, for ultrasmall nanoparticles smaller than 2 nm, the models cannot fit the experimental PDF showing that the structure is significantly modified from that of larger particles and the bulk. The observation of a significant change in the average structure at ultra-small size is likely to explain the unusual properties of the ultrasmall particles such as their white light emitting ability.
A versatile synthetic method, based on a solvothermal synthesis technique, has been developed for the fabrication of TiO(2) nanocrystals with different shapes, such as nanowires, nanorods and nanospheres. The central characteristic of our approach is the production of a coordination complex at an intermediate stage of the reaction that possesses appropriate symmetry using two distinct ligands, oleic acid and oleylamine. Those ligands have different binding strengths, which specifically dictate the shape of both the coordination complex and the resulting TiO(2) nanostructures. Additionally, this approach yields, for the first time, monodisperse, 4 nm anatase TiO(2) nanocrystals by controlling the symmetry of the intermediate and solvothermal conditions.
Crohns disease (CD) can develop in any region of the gastrointestinal tract, including the stomach. The etiology and pathogenesis of Crohns gastritis are poorly understood, treatment approaches are limited, and there are not many suitable animal models for study. We characterized the features and mechanisms of chronic gastritis in SAMP1/YitFc (SAMP) mice, a spontaneous model of CD-like ileitis, along with possible therapeutic approaches.
Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantum dots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantum dots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantum dot approaches. The article is concluded with a look at the future direction of quantum dots.
Ultrasmall nanocrystals are a growing sub-class of traditional nanocrystals that exhibit new properties at diameters typically below 2 nm. In this review, we define what constitutes an ultrasmall nanoparticle while distinguishing between ultrasmall and magic-size nanoparticles. After a brief overview of ultrasmall nanoparticles, including ultrasmall gold clusters, our recent work is presented covering the optical properties, structure, and application of ultrasmall CdSe nanocrystals. This unique material has potential application in solid state lighting due to its balanced white emission. This section is followed by a discussion on the blurring boundary between what can be considered a nanoparticle and a molecule.
One consistent limitation for high-resolution imaging of small nanoparticles is the high background signal from the amorphous carbon support film. With interest growing for smaller and smaller nanostructures, state of the art electron microscopes are becoming necessary for rudimentary tasks, such as nanoparticle sizing. As a monolayer of carbon, free-standing graphene represents the ultimate support film for nanoparticle imaging. In this work, conventional high-resolution transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM) were used to assess the benefits and feasibility of few-layer graphene support films. Suspensions of few-layer graphene to produce the support films were prepared by simple sonication of exfoliated graphite. The greatest benefit was observed for conventional HRTEM, where lattice resolved imaging of sub 2 nm CdSe nanocrystals was achieved. The few-layer graphene films were also used as a support film for C(s)-corrected STEM and electron energy loss spectroscopy of CuInSe(2) nanocrystals.
White-light emission from ultrasmall CdSe nanocrystals offers an alternative approach to the realization of solid-state lighting as an appealing technology for consumers. Unfortunately, their extremely small size limits the feasibility of traditional methods for nanocrystal characterization. This paper reports the first images of their structure, which were obtained using aberration-corrected atomic number contrast scanning transmission electron microscopy (Z-STEM). With subangstrom resolution, Z-STEM is one of the few available methods that can be used to directly image the nanocrystals structure. The initial images suggest that they are crystalline and approximately four lattice planes in diameter. In addition to the structure, for the first time, the exciton dynamics were measured at different wavelengths of the white-light spectrum using ultrafast fluorescence upconversion spectroscopy. The data suggest that a myriad of trap states are responsible for the broad-spectrum emission. It is hoped that the information presented here will provide a foundation for the future development and improvement of white-light-emitting nanocrystals.
Alternative oxidase (AOX) is a mitochondrial protein that acts as an alternative terminal oxidase to the conventional cytochrome oxidases. It is present in certain prokaryotes, plants, fungi and some protozoa but absent in mammals. AOX activity has previously been described in Acanthamoeba, although no genetic evidence has been reported. Herein, two AOX (AcAOX) genes designated isoforms A and B, were obtained from Acanthamoeba castellanii by a combination of degenerate PCR from cDNA and a series of 5 and 3 rapid amplification of cDNA ends. The corresponding genomic sequences of these AcAOXs were also obtained. Each gene spans six exons over a region of 1607 and 1619bp, respectively. Isoforms A and B have open reading frames of 1113 and 1125bp, respectively. Each encodes a protein with a predicted molecular weight of 42kDa. Each AcAOX protein has a predicted cleavable mitochondrial targeting sequence. The full-length AcAOX is functionally active as it complements hemL-deficient Escherichia coli and inhibited by the inhibitor of AOX, salicylhydroxamic acid (SHAM). SHAM is effective against A. castellanii and Acanthamoeba polyphaga only when used in conjunction with antimycin A, an inhibitor of the conventional cytochrome respiratory pathway. Transcripts for AcAOX are increased during the encystment process, indicating a possible role for alternative respiration during stress.
This research aimed to validate the specificity of the newly developed nanobeacon for imaging the Thomsen-Friedenreich (TF) antigen, a potential biomarker of colorectal cancer. The imaging agent is comprised of a submicron-sized polystyrene nanosphere encapsulated with a Coumarin 6 dye. The surface of the nanosphere was modified with peanut agglutinin (PNA) and poly(N-vinylacetamide (PNVA) moieties. The former binds to Gal-?(1-3)GalNAc with high affinity while the latter enhances the specificity of PNA for the carbohydrates. The specificity of the nanobeacon was evaluated in human colorectal cancer cells and specimens, and the data were compared with immunohistochemical staining and flow cytometric analysis. Additionally, distribution of the nanobeacon in vivo was assessed using an "intestinal loop" mouse model. Quantitative analysis of the data indicated that approximately 2 ?g of PNA were detected for each milligram of the nanobeacon. The nanobeacon specifically reported colorectal tumors by recognizing the tumor-specific antigen through the surface-immobilized PNA. Removal of TF from human colorectal cancer cells and tissues resulted in a loss of fluorescence signal, which suggests the specificity of the probe. Most importantly, the probe was not absorbed systematically in the large intestine upon topical application. As a result, no registered toxicity was associated with the probe. These data demonstrate the potential use of this novel nanobeacon for imaging the TF antigen as a biomarker for the early detection and prediction of the progression of colorectal cancer at the molecular level.
Individual ultrasmall CdSe nanocrystals have recently been found to emit white light, but the ultimate origin of the phenomenon has remained elusive. Here we use a combination of state-of-the-art experiment and theory to show that excitation sets the ultrasmall nanocrystals into a fluxional state. Their energy gaps vary continuously on a femtosecond time scale, so that even an individual nanocrystal can emit across the entire visual range. In addition, we observe the outer layers of the larger monochromatic emitting nanocrystals to be fluxional. Such fluxionality should be considered when optimizing nanocrystals for applications. Thus, small is indeed different, but ultrasmall is different yet again.
A simple treatment method using formic acid has been found to increase the fluorescence quantum yield of ultrasmall white light-emitting CdSe nanocrystals from 8% to 45%. Brighter white-light emission occurs with other carboxylic acids as well, and the magnitude of the quantum yield enhancement is shown to be dependent on the alkyl chain length. Additionally, the nanocrystal luminescence remains enhanced relative to the untreated nanocrystals over several days. This brightened emission opens the possibility for even further quantum yield improvement and potential for use of these white-light nanocrystals in solid-state lighting applications.
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