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At least 127 records · Page 7Linked to original sources
PubMed2004

A new class of far-red and near-infrared biological labels based on alloyed semiconductor quantum dots.

Ternary cadmium selenium telluride quantum dots (CdSe(1-x)Tex, x = mole fraction of tellurium) have been prepared for potential use as constant-size biolabels with tunable fluorescence emission in the far-red and near-infrared (650-850 nm) spectral range. In contrast to particle size tuning reported for binary dots, we show that molar composition can be used to tune the optical and electronic properties of alloyed semiconductor nanocrystals without changing the particle size. A surprising finding is a nonlinear relationship between the composition and the absorption/emission energies, leading to new properties not obtainable from the parent CdSe and CdTe binary systems. Coating the alloy cores with a higher band-gap material such as CdS improves the fluorescence efficiencies to about 40-60% at room temperature and allows the preparation of water-soluble and biocompatible alloyed dots at similar quantum yields. A cadmium-rich surface is found to improve mercapto ligand binding and the long-term stability of water-soluble dots.

Alloys↗
PubMed2005

Quantum dots as cellular probes.

Robust and bright light emitters, semiconductor nanocrystals [quantum dots (QDs)] have been adopted as a new class of fluorescent labels. Six years after the first experiments of their uses in biological applications, there have been dramatic improvements in understanding surface chemistry, biocompatibility, and targeting specificity. Many studies have shown the great potential of using quantum dots as new probes in vitro and in vivo. This review summarizes the recent advances of quantum dot usage at the cellular level, including immunolabeling, cell tracking, in situ hybridization, FRET, in vivo imaging, and other related technologies. Limitations and potential future uses of quantum dot probes are also discussed.

Animals↗
PubMed2005

Optically bright quantum dots in single Nanowires.

We fabricate and demonstrate optically active quantum dots embedded in single nanowires. Observation of photon antibunching proves the zero dimensionality of these heterostructures that can be epitaxially grown on various substrates, including silicon. We show that the nanowire dots are intense single photon sources, typically an order of magnitude brighter than self-assembled quantum dots. Due to control over their composition, size, and position, nanowire dots are ideal building blocks for fully controlled quantum dot molecules.

Electric Wiring↗
PubMed2006

Development of homogeneous binding assays based on fluorescence resonance energy transfer between quantum dots and Alexa Fluor fluorophores.

We studied the fluorescence resonance energy transfer (FRET) between quantum dots emitting at 565, 605, and 655 nm as energy donors and Alexa Fluor fluorophores with absorbance maxima at 594, 633, 647, and 680 nm as energy acceptors. As a first step, we prepared covalent conjugates between all three types of quantum dots and each of the Alexa Fluor fluorophores that could act as an energy acceptor. All of these conjugates displayed efficient resonance energy transfer. Then we prepared covalent conjugates of these quantum dots with biotin, fluorescein, and cortisol and established that the binding of these conjugates to suitable Alexa Fluor-labeled antibodies and streptavidin (in the case of biotin) can be efficiently detected by measuring the resonance energy transfer in homogeneous solutions. Finally, based on these observations, competitive binding assays for these three small analytes were developed. The performance of these assays as a function of the degree of labeling of the quantum dots was evaluated. It was found that decreasing the degree of loading of the quantum dots leads to decreases of the limits of detection. The results show the great potential of this FRET system for the development of new homogeneous binding assays.

Binding, Competitive↗
PubMed2004

Local gate control of a carbon nanotube double quantum dot.

We have measured carbon nanotube quantum dots with multiple electrostatic gates and used the resulting enhanced control to investigate a nanotube double quantum dot. Transport measurements reveal honeycomb charge stability diagrams as a function of two nearly independent gate voltages. The device can be tuned from weak to strong interdot tunnel-coupling regimes, and the transparency of the leads can be controlled independently. We extract values of energy-level spacings, capacitances, and interaction energies for this system. This ability to control electron interactions in the quantum regime in a molecular conductor is important for applications such as quantum computation.

Journal Article↗
PubMed2003

Oligomeric ligands for luminescent and stable nanocrystal quantum dots.

We report a new family of oligomeric alkyl phosphine ligands for nanocrystal quantum dots. These oligomeric phosphines show effective binding affinity to quantum dot surfaces. They form thin and secure organic shells that stabilize quantum dots in diverse environments including serum and polymer matrices. They maintain the initial as-grown photoluminescence quantum yield of the quantum dots and enable homogeneous incorporation into various matrices. They present a chemically flexible structure that can be used for further chemistry, such as cross-linking, copolymerization, and conjugation to biomolecules.

Journal Article↗
PubMed2000

Optical gain and stimulated emission in nanocrystal quantum dots.

The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Journal Article↗
PubMed2005

Spin relaxation and decoherence of holes in quantum dots.

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.

Journal Article↗
PubMed2006

Orbital susceptibilities of PbSe quantum dots.

Different sizes of three-dimensional PbSe quantum dots have been synthesized for the study of orbital magnetic susceptibilities. Two types of orbital susceptibilities have been found, including the Curie susceptibility and finite-size corrections to the Landau susceptibility. The Curie term of a quantum dot manifests itself in the temperature dependence of magnetic susceptibility at low temperatures, while the field dependence of differential susceptibility at high temperatures shows finite-size corrections to the Landau susceptibility. Both of the two kinds of orbital susceptibility, estimated per quantum dot, show linear dependence on the size.

Journal Article↗
PubMed2000

A quantum dot single-photon turnstile device.

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

Journal Article↗
PubMed2001

Near-field spectroscopy of single quantum dots at room temperature.

The observation of photoluminescence spectra of self-assembled single InGaAs quantum dots at room temperature was performed under weak excitation conditions using a near-field scanning optical microscope. Operation in illumination-collection mode with a highly sensitive double-tapered optical fibre probe enabled detection of weak photoluminescence signals at room temperature with high efficiency and high spatial resolution. Each single quantum dot was imaged with a spatial resolution of about 250 nm, which corresponded to a quarter of the wavelength of the photoluminescence from quantum dots. The photoluminescence yields of individual quantum dots were widely distributed and were found to decrease with photoluminescence energy. This result serves as a clue to be pursued for better understanding of the thermal excitation of the carrier from confined states in quantum dots.

Journal Article↗
PubMed2006

Monolayer behavior and Langmuir-Blodgett manipulation of CdS quantum dots.

Cadmium sulfide (CdS) quantum dots (QDs) were prepared and surface modified by dodecanthiol or mercaptosuccinic acid (MSA) to render a surface with alkyl chains (C(12)-CdS) or carboxylic acid groups (MSA-CdS), respectively. Due to the hydrophobic property of C(12)-CdS, the nanoparticles disperse well in chloroform and stay stable at the air/water interface. However, 3-dimensional (3D) aggregative domains and particle-free pores were formed in the monolayer due to poor particle-water interaction. For the MSA-CdS nanoparticles, the surface was hydrophobized through physical adsorption of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The capped MSA on the CdS plays an important role in enhancing the adsorption of CTAB and improving the stability of the QDs at the air/water interface. Due to the reversible adsorption of CTAB on MSA-CdS, a hydrophilic area can be exposed in the water-contacting region of a nanoparticle when it stays at the air/water interface. Thus, the CTAB-MSA-CdS QD behaves as an amphiphilic compound at the air/water interface and has properties superior to those of C(12)-CdS QDs in fabrication of layer-by-layer 2D structure of particulate films. The distinct behaviors of the two QDs at the air/water interface and the related effect on the properties of LB films were studied using a number of methods, including pressure-area (pi-A) isotherm, relaxation and hysteresis experiments, in-situ observation of Brewster angle microscopy (BAM), the postdeposition analysis of atomic force microscopy (AFM), and UV-vis spectroscopy.

Journal Article↗
PubMed2003

Quantum dots as dynamical systems.

Quantum dots show a range of time-dependent behaviours. We show that the polarity of II-VI nanoparticles has important dynamical implications for electronic, vibrational and other phenomena. Polarity-dependent phenomena are found even for nearly spherical stoichiometric clusters of ZnO and ZnS in studies based on interatomic potentials or on a plane-wave density-functional approach. We find a substantial dipole moment for free nanoparticles, whether of the zinc blende or wurtzite structure. This dipole causes a highly non-uniform spin-density distribution on electronic excitation or after a change in the dot's electronic charge state. The spin density of the triplet exciton shows that the dipole aligns so as to reduce the dipole moment in the electronically excited state. The polarity of II-VI dots also affects their vibrational properties. High- and low-frequency tails of the vibration density of states arise from modes strongly localized at surface atoms, near the poles of the dipole. These features, first noted for free clusters, also hold for particles embedded in a wide-gap dielectric, a-SiO(2). We present the results of molecular dynamics of the ZnS particle embedded into the silica glass, and consider the role played by the soft modes in energy-dissipation processes such as dephasing during non-radiative recombination of excitons, and energy transfer from the dot to the matrix.

Journal Article↗
PubMed2004

Cotunneling spectroscopy in few-electron quantum dots.

Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a nonequilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.

Journal Article↗