Semiconductor Quantum Rods Research Articles

Cytoplasmic Dynein Ring Tilting Detected by Combined polTIRF and Sub-Pixel Particle Tracking of Semiconductor Quantum Rods

The mechanism of cytoplasmic dynein, a microtubule-based motor responsible for the majority of minus-end-directed transport in eukaryotes, remains poorly understood compared to myosin and kinesin. While recent crystal and EM structures have given insight into the conformational changes that occur in the AAA+ ring of dynein during ATP hydrolysis, dynamic structural information is lacking. In order to better understand the dynein translocation mechanism we combine polarized total internal reflection fluorescence (polTIRF) microscopy and high precision localization to simultaneously track position and orientation of single dynein molecules in real time. CdSe/CdS rod-in-rod nanoparticles with polarized emission are coated with mercaptoundecanoic acid (MUA) and functionalized with NeutrAvidin. The polarized nanorods are bifunctionally conjugated via biotin-avidin linkages to biotinylation sites inserted in two positions in dynein's AAA+ ring. Both homo- and hetero-dimeric dynein constructs are analyzed. Fluorescence emission of dynein-conjugated rods is split into four channels based on polarization and imaged with an EMCCD camera. The relative intensities of the four channels are used to determine the three-dimensional orientation of the rod, and therefore the dynein AAA+ ring, during stepping. Using this combined polTIRF/tracking method we detect tilting of the ring domain during stepping. Ring rotations are ATP-dependent, highly irregular and are mainly in the plane of the microtubule when correlated with translocation events. Supported by NIH grant P01GM087253.

A general strategy for synthesizing colloidal semiconductor zinc chalcogenide quantum rods.

Quasi-one-dimensional (1D) semiconductor nanocrystals manifest linearly polarized emission, reduced lasing threshold, and improved charge transport compared with their counterparts such as spherical quantum dots. Present investigations of colloidal semiconductor quantum rods are mainly based on cadmium chalcogenide systems because of their facile synthetic accessibility. However, it is still a big challenge to fabricate quasi-1D zinc chalcogenide nanocrystals with controlled aspect ratios. Here we report a general strategy for synthesizing zinc chalcogenide quantum rods via a colloidal chemical synthetic approach. Unlike the most common growth mechanisms of quasi-1D colloidal nanocrystals such as monomer attachment and particle coalescence, the synthesis of zinc chalcogenide quantum rods is performed by a ripening process starting from their respective ultrathin nanowires through thermodynamically driven material diffusion. We anticipate that this strategy is general and could be applied to other systems to construct quasi-1D nanostructures. Moreover, the presence of cadmium-free (or "green") zinc chalcogenide quantum rods synthesized through this strategy provides a desirable platform for eco-friendly photocatalysis, optoelectronic devices, biolabeling, and other applications.

Anisotropic absorption of CdSe/ZnS quantum rods embedded in polymer film

An approach to achieving of spatially homogeneous, ordered ensemble of semiconductor quantum rods in polymer film of polyvinyl butyral is reported. The CdSe/ZnS quantum rods are embedded to the polymer film. Obtained film is stretched up to four times to its initial length. A concentration of quantum rods in the samples is around 2×10 -5 M. The absorption spectra, obtained in the light with orthogonal polarization, confirm the occurrence of spatial ordering in a quantum rod ensemble. Anisotropy of the optical properties in the ordered quantum rod ensemble is examined. The presented method can be used as a low-cost solution for preparing the nanostructured materials with anisotropic properties and high concentration of nanocrystals.

CdSe/CdS Semiconductor Quantum Rods as Robust Fluorescent Probes for Paraffin-Embedded Tissue Imaging

Immunofluorescence techniques on formalin fixed paraffin-embedded sections allow for the evaluation of the expression and spatial distribution of specific markers in patient tissue specimens or for monitoring the fate of labeled cells after in vivo injection. This technique suffers however from the auto-fluorescence background signal of the embedded tissue that eventually confounds the analysis. Here we show that rod-like semiconductor nanocrystals (QRs), intramuscularly injected in living mice, could be clearly detected by confocal microscopy in formalin fixed paraffin-embedded tissue sections. Despite the low amount of QRs amount injected (25 picomoles), these were clearly visible after 24 h in the muscle sections and their fluorescence signal was stronger than that of CdSe/ZnS quantum dots (QDs) similarly functionalized and in the case of QRs only, the signal lasted even after 21 days after the injection.

Features of fluorescence of CdSe/ZnS semiconductor quantum rods in multicomponent solutions with pentylcyanobiphenyl

The fluorescence of CdSe/ZnS semiconductor quantum rods (QRs) in the CdSe/ZnS-hexane pentylcyanobiphenyl and CdSe/ZnS-methyl methacrylate pentylcyanobiphenyl multicomponent systems has been investigated. It is shown that the presence of pentylcyanobiphenyl affects differently the QR fluorescence in each composite. The QR fluorescence in hexane solutions is efficiently quenched in the presence of pentylcyanobiphenyl, whereas in the composites based on methyl-methacrylate the QR fluorescence is enhanced as a result of the introduction of pentylcyanobiphenyl in the mesomorphic state. This difference is explained by the different forms of pentylcyanobiphenyl in these systems. The composites based on structured porous polyethylene films containing QRs in a mesomorphic medium have been obtained for the first time. The measured degree of polarization of the QR fluorescence characterizes the composite as strongly ordered.

Lasing in self-assembled microcavities of CdSe/CdS core/shell colloidal quantum rods

Colloidal semiconductor quantum rods have demonstrated many advantageous properties as light emitters such as high quantum yield, tunable emission wavelength, and polarized emission. This makes them an interesting optical gain material for laser applications. We report room-temperature gain lifetimes in core/shell CdSe/CdS quantum rods exceeding 300 ps, and show that the long gain lifetimes result from the significant reduction of Auger recombination in our quantum rods, even though the electrons are delocalized over the rod volume. We also fabricate devices by deposition of small droplets of quantum rod solution onto flat substrates. The evaporation dynamics of the droplets are governed by the coffee stain effect which leads to the formation of well defined micron-size stripes. These stripes consist of densely packed, laterally aligned quantum rods and provide optical feedback originating from the abrupt changes of refractive index at the stripe borders. We exploit the optical gain and the coffee stain mediated self-assembly and show that we can fabricate novel microlasers solely by deposition of droplets of quantum rod solutions on flat substrates.

Tumor Targeting and Imaging in Live Animals with Functionalized Semiconductor Quantum Rods

In this contribution, we demonstrate that highly luminescent CdSe/CdS/ZnS quantum rods (QRs) coated with PEGylated phospholipids and conjugated with cyclic RGD peptide can be successfully used for tumor targeting and imaging in live animals. The design of these targeted luminescent probes involves encapsulation of hydrophobic CdSe/CdS/ZnS QRs with PEGylated phospholipids, followed by conjugation of these PEGylated phospholipids to ligands that specifically target the tumor vasculature. In vivo optical imaging studies in nude mice bearing pancreatic cancer xenografts, both subcutaneous and orthotopic, indicate that the QR probes accumulate at tumor sites via the cyclic RGD peptides on the QR surface binding to the alpha(V)beta(3) integrins overexpressed in the tumor vasculature, following systemic injection. In vivo tumor detection studies showed no adverse effects even at a dose roughly 6.5 times higher than has been reported for in vivo imaging studies using quantum dots. Cytotoxicity studies indicated the absence of any toxic effect in the cellular and tissue levels arising from functionalized QRs. These results demonstrate the vast potential of QRs as bright, photostable, and biocompatible luminescent probes for the early diagnosis of cancer.

Stimulated Rayleigh–Bragg Scattering From a Two-Photon Absorbing CdSe–CdS–ZnS Quantum-Rods System: Optical Power Limiting and Phase-Conjugation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This work reports the properties of stimulated Rayleigh–Bragg scattering (SRBS) from a two-photon absorbing CdSe–Cds–ZnS quantum-rods (QRs) solution in chloroform, excited by 1064-nm and <formula formulatype="inline"> <tex>$\sim$</tex></formula>13-ns laser pulses. The two-photon absorbing capability of the scattering medium, as well as the pump threshold, spectral structure, and pulse waveforms of the backward stimulated scattering were measured. Comparing to a pure solvent or an organic dye-solution, the semiconductor QR system has many advantages such as the lower pump threshold, higher energy transfer efficiency, and better photo-physical and photo-chemical stability. The measured output/input characteristic curve shows that the backward SRBS can enhance the optical power limiting performance that is based on two-photon absorption, backward stimulated scattering, and other nonlinear absorption mechanisms. In addition, the backward SRBS beam from our sample medium exhibits a fairly good optical phase-conjugation capability, so that the distortion influence from an inserted aberrator can be automatically removed. </para>

Magneto‐optical properties of quantum rods and superlattices

AbstractWe show how to compute the optical functions (the complex magneto‐susceptibility, dielectric function, magneto‐reflection spectra) for semiconductor quantum rods (QRDs) and superlattices (SLs) exposed to a uniform magnetic field in the growth direction, including the excitonic effects. The method uses the microscopic calculation of nanostructure excitonic wave functions and energy levels, and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. The electron‐hole screened Coulomb potential and the valence band structure are taken into account. The novelty of our approach is that the solution is obtained in terms of known one‐particle electron and hole eigenfunctions, those omitting the complicated problem of the center‐of‐mass and relative motion separation in the magnetic field. Our approach applies, first of all, for low dimensional systems (QDS, QRDs, nanorings etc.) where, due to confinement effects accompanied by the e‐h Coulomb interaction, the separation of the relative‐ and center‐of‐mass motion is not possible. The convergence of the proposed method is examined. Numerical calculations have been performed for a Ga1x Alx As/GaAs SL and a CdSe QRD. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synchronous Photoluminescence Intermittency (Blinking) along Whole Semiconductor Quantum Wires

Photoluminescence microscopy studies have detected synchronous-photoluminescence-intensity fluctuations along entire cadmium selenide quantum wires under continuous illumination. While similar photoluminescence blinking has been reported previously for semiconductor quantum dots and rods, the observation of synchronous blinking spanning the entire length of quantum wires, with diameters approximately 9 nm and lengths >5 microm, is remarkable. We propose a mechanism to account for the synchronous blinking that is based on a dynamic, photolytic filling of surface-trap sites.

Semiconductor quantum rods as single molecule fluorescent biological labels.

In this paper, we report the development of rod-shaped semiconductor nanocrystals (quantum rods) as fluorescent biological labels. Water-soluble biocompatible quantum rods have been prepared by surface silanization and applied for nonspecific cell tracking as well as specific cellular targeting. Quantum rods are brighter single molecule probes as compared to quantum dots. They have many potential applications as biological labels in situations where their properties offer advantages over quantum dots.

The effect of Auger heating on intraband carrier relaxation in semiconductor quantum rods

The rate at which excited charge carriers relax to their equilibrium state affects many aspects of the performance of nanoscale devices, including switching speed, carrier mobility and luminescence efficiency. A better understanding of the processes that govern carrier relaxation therefore has important technological implications. A significant increase in carrier–carrier interactions caused by strong spatial confinement of electronic excitations in semiconductor nanostructures leads to a considerable enhancement of Auger effects, which can further result in unusual, Auger-process-controlled recombination and energy relaxation regimes. Here, we report the first experimental observation of efficient Auger heating in CdSe quantum rods at high pump intensities, leading to a strong reduction of carrier cooling rates. In this regime, the carrier temperature is determined by the balance between energy outflow through phonon emission and energy inflow because of Auger heating. This equilibrium results in peculiar carrier cooling dynamics that closely correlate with recombination dynamics, an effect never seen before in bulk or nanoscale semiconductors.

Seeded growth of InP and InAs quantum rods using indium acetate and myristic acid

A synthesis of soluble III–V semiconductor quantum rods using gold nanoparticles to direct and catalyze one-dimensional growth is developed. The growth takes place via the solution–liquid–solid (SLS) mechanism where proper precursors are injected into a coordinating solvent. We report the synthesis of InP nanorods using indium acetate and myristic acid with gold nanoparticles as the catalysts in the SLS growth mode. A similar route was successfully developed for the growth of InAs nanorods. We find that the amount of Au catalyst in the reaction is an important parameter to achieve shape control. Transmission electron microscope (TEM) images of InP and InAs nanocrystals revealed that the crystals are mostly rod-shaped, and provide strong evidence for Au presence in one edge. The rods were characterized structurally using X-ray diffraction and high-resolution TEM and optically by absorption and photoluminescence.

Electric Field Induced Switching of the Fluorescence of Single Semiconductor Quantum Rods

The exceptional fluorescence properties of single CdSe quantum rods (QRs) arising from internal and external electric fields are studied. Reversible external field induced switching of the emission in single QRs is reported for the first time. This effect was correlated with local field induced emission intensity reduction and newly observed darkening mechanism. Bimodal spectral jumps under a zero field were also observed and assigned to charged exciton emission, a phenomenon that was likewise directly controlled through an external field. These phenomena point to the use of single QRs as spectrally tunable charge sensitive fluorophores with polarized emission in fluorescence tagging and optical switching applications.

Selective growth of metal tips onto semiconductor quantum rods and tetrapods.

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.

Polarization surface-charge density of single semiconductor quantum rods.

Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.

Optical phonons in semiconductor quantum rods

Surface polar-optical phonons are analyzed for semiconductor quantum rods (QR). The cylindrical symmetry is modelled in the form of a prolate spheroid where the aspect-ratio is identified with the corresponding ratio of the ellipsoid semiaxes. Using a theory based on the dielectric continuum approach, we consider a single CdSe QR inserted in a host material, assumed as an infinite dielectric, and viewed as an inactive rigid medium to the oscillations. Our results for CdSe are discussed, together with the differences from spherical and quasi-spherical quantum dots. We believe these are important steps to help the understanding of future experiments in this new class of materials.

Synthesis of InP and InAs quantum rods using Indium Acetate and Myristic acid

ABSTRACTThe development of solution based synthesis approaches for preparing nanocrystals of III-V semiconductor presents a significant &amp; important challenge especially with relation to shape control to achieve rod growth. To this end, a novel approach for synthesis of soluble semiconductor quantum rods using metal nanoparticles to direct and catalyze one-dimensional growth is developed. The synthesis method is useful in particular for III-V semiconductor with cubic lattice, where the utilization of surfactant-controlled rod-growth is not easily realized. The growth takes place via the solution–liquid–solid (SLS) mechanism where proper precursors are injected into a coordinating solvent as we reported in earlier work for InAs nanorods. Herein, we report the synthesis of high quality InP nanorods using Indium Acetate and myristic acid with gold nanoparticles as the catalysts in the SLS growth mode. A similar route was successfully developed for the growth of InAs nanorods. We find that the amount of Au catalyst in the reaction is an important parameter to achieving shape control. Transmission electron microscope (TEM) images of InP and InAs nanocrystals revealed that the crystals are mostly rod-shaped. XRD measurements, absorption spectra were preformed for the nanorods characterization.

Direct Observation of Highly Polarized Non-Linear Absorption Dipole of Single Semiconductor Quantum Rods

AbstractPolarization fluorescence microscopy was used to study the nature of the emission and nonlinear absorption dipole of single CdSe/ZnS quantum rods. Rods, with aspect ratios ranging from 2.75 to 15, showed strongly polarized emission consistent with previous one-photon studies. Non- linear excitation showed a sharp angular dependence fully consistent with the predicted two- photon absorption process. Two-photon absorption probes different transitions than linear absorption due to modified parity and angular momentum selection rules. The two-photon absorption dipole was found to be parallel to the emission polarization, and allows achieving highly orientation selective excitation of quantum rods. This is yet a further demonstration of single molecule measurements in unraveling basic principles of light-matter interaction that are otherwise masked by ensemble averaging.

Shape control of III-V semiconductor nanocrystals: synthesis and properties of InAs quantum rods.

A novel approach for synthesis of soluble semiconductor quantum rods using metal nanoparticles to direct and catalyze one-dimensional growth is developed. The method is useful in particular for III-V semiconductors with cubic lattice, where the utilization of surfactant-controlled rod-growth is not easily realized. The growth takes place via the solution-liquid-solid (SLS) mechanism where proper precursors are injected into a coordinating solvent. Centrifugation is used for separation of rod-fractions with different lengths. The reaction is demonstrated for InAs, InP and GaAs. Focusing on InAs rods as a model system, we examined the effects of the type of metal catalyst, and the tuning of reaction conditions with respect to temperature, concentration, catalyst content and reaction time. Within the three types of metal catalysts used--Au, Ag and In, Au was found to provide the best control for achieving rod-growth even though the melting point of bulk gold is significantly higher then the reaction temperature. The structural properties of the rods were characterized by transmission electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Rods have a cubic lattice and grow mainly along the [111] direction. The relative gold content decreases in shorter rods suggesting Au depletion as a cause for limiting the growth. Room and low temperature absorption and photoluminescence measurements show that the band-gap shifts to the red upon increasing rod length revealing strong quantum confinement along the long axis in InAs rods, providing spectral coverage of the near-IR range relevant for telecommunication applications. Emission intensity also decreases with increased rod-length. These length dependent properties manifest the transition from 0D to 1D quantum confined systems.