Fluorescence resonance energy transfer between quantum dot donors and dye-labeled protein acceptors.

We used luminescent CdSe-ZnS core-shell quantum dots (QDs) as energy donors in fluorescent resonance energy transfer (FRET) assays. Engineered maltose binding protein (MBP) appended with an oligohistidine tail and labeled with an acceptor dye (Cy3) was immobilized on the nanocrystals via a noncovalent self-assembly scheme. This configuration allowed accurate control of the donor-acceptor separation distance to a range smaller than 100 A and provided a good model system to explore FRET phenomena in QD-protein-dye conjugates.

Near-infrared fluorescent type II quantum dots for sentinel lymph node mapping.

The use of near-infrared or infrared photons is a promising approach for biomedical imaging in living tissue. This technology often requires exogenous contrast agents with combinations of hydrodynamic diameter, absorption, quantum yield and stability that are not possible with conventional organic fluorophores. Here we show that the fluorescence emission of type II quantum dots can be tuned into the near infrared while preserving absorption cross-section, and that a polydentate phosphine coating renders them soluble, disperse and stable in serum.

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.

Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures.

Type-II band engineered quantum dots (CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures) are described. The optical properties of these type-II quantum dots are studied in parallel with their type-I counterparts. We demonstrate that the spatial distribution of carriers can be controlled within the type-II quantum dots, which makes their properties strongly governed by the band offset of the comprising materials.

Selection of quantum dot wavelengths for biomedical assays and imaging.

Fluorescent semiconductor nanocrystals (quantum dots [QDs]) are hypothesized to be excellent contrast agents for biomedical assays and imaging. A unique property of QDs is that their absorbance increases with increasing separation between excitation and emission wavelengths. Much of the enthusiasm for using QDs in vivo stems from this property, since photon yield should be proportional to the integral of the broadband absorption.

Room temperature measurements of the 3D orientation of single CdSe quantum dots using polarization microscopy.

Simple far-field emission polarization microscopy reveals that the emission transition dipole of CdSe colloidal quantum dots (QDs) is twofold degenerate at room temperature. We measure, model, and compare polarization anisotropy statistics of CdSe QDs and DiI (a one-dimensional emitter). We find excellent agreement between experiment and theory if the transition dipole of CdSe QDs is assumed to be twofold degenerate.

Electroluminescence from single monolayers of nanocrystals in molecular organic devices.

The integration of organic and inorganic materials at the nanometre scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of inorganic nanocrystals. The optimization of such hybrid devices ultimately depends upon the precise positioning of the functionally distinct materials. Previous studies have already emphasized that this is a challenge, owing to the lack of well-developed nanometre-scale fabrication techniques.

Surface-enhanced emission from single semiconductor nanocrystals.

The fluorescence behavior of single CdSe(ZnS) core-shell nanocrystal (NC) quantum dots is dramatically affected by electromagnetic interactions with a rough metal film. Observed changes include a fivefold increase in the observed fluorescence intensity of single NCs, a striking reduction in their fluorescence blinking behavior, complete conversion of the emission polarization to linear, and single NC exciton lifetimes that are >10(3) times faster. The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.

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.

Correlation between fluorescence intermittency and spectral diffusion in single semiconductor quantum dots.

We find a correlation between the dynamics of fluorescence intermittency and spectral diffusion in the spectroscopy of single CdSe nanocrystal quantum dots (QD). A statistical analysis of the data suggests two populations of blinking events: blinking followed by large spectral diffusion shifts and blinking with small or no spectral shifts. Although unexpected from earlier studies, the correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.

Quantization of multiparticle auger rates in semiconductor quantum dots.

We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.

A Solution-Phase Chemical Approach to a New Crystal Structure of Cobalt.

Kinetic control of crystal growth in the presence of a coordinating ligand is critical for the formation of a new structure of elemental cobalt (ε-cobalt, the unit cell with the two different types of cobalt atoms is shown), which was discovered upon analyzing the metallic powder produced by the thermal decomposition of [Co (CO) ] in solution in the presence of trioctylphosphane oxide [TOPO, Eq. (1)].

Quantum-confined stark effect in single CdSe nanocrystallite quantum dots.

The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state ( approximately 10(5) cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening.