Showing papers by "Moungi G. Bawendi published in 2003"

Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures

Abstract: 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. This allows access to optical transition energies that are not restricted to band gap energies. The type-II quantum dots reported here can emit at lower energies than the band gaps of comprising materials. The type-II emission can be tailored by the shell thickness as well as the core size. The enhanced control over carrier distribution afforded by these type-II materials may prove useful for many applications, such as photovoltaics and photoconduction devices.

Selection of Quantum Dot Wavelengths for Biomedical Assays and Imaging

Abstract: 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. In this study, we demonstrate that tissue scatter and absorbance can sometimes offset increasing QD absorption at bluer wavelengths, and counteract this potential advantage. By using a previously validated mathematical model, we explored the effects of tissue absorbance, tissue scatter, wavelength dependence of the scatter, water-to-hemoglobin ratio, and tissue thickness on QD performance. We conclude that when embedded in biological fluids and tissues, QD excitation wavelengths will often be quite constrained, and that excitation and emission wavelengths should be selected carefully based on the particular application. Based on our results, we produced near-infrared QDs optimized for imaging surface vasculature with white light excitation and a silicon CCD camera, and used them to image the coronary vasculature in vivo. Taken together, our data should prove useful in designing fluorescent QD contrast agents optimized for specific biomedical applications.

Oligomeric Ligands for Luminescent and Stable Nanocrystal Quantum Dots

Abstract: 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.

Quantum-dot optical temperature probes

Abstract: The steady-state photoluminescence (PL) properties of cadmium selenide quantum dots (QDs) with a zinc sulfide overlayer [(CdSe)ZnS] can be strongly dependent on temperature in the range from 100 to 315 K. The PL intensity from 50 to 55 A (CdSe)ZnS QDs in poly(lauryl methacrylate) matrices increases by a factor of ∼5 when the temperature is decreased from 315 to 100 K, and the peak of the emission band is blueshifted by 20 nm over the same range. The change in PL intensity is appreciable, linear, and reversible (−1.3% per °C) for temperatures close to ambient conditions. These properties of (CdSe)ZnS dots are retained in a variety of matrices including polymer and sol–gel films, and they are independent of excitation wavelength above the band gap. The significant temperature dependence of the luminescence combined with its insensitivity to oxygen quenching establishes (CdSe)ZnS dots as optical temperature indicators for temperature-sensitive coatings.

Light emitting device including semiconductor nanocrystals

Abstract: A light emitting device includes a semiconductor nanocrystal and a charge transporting layer that includes an inorganic material. The charge transporting layer can be a hole or electron transporting layer. The inorganic material can be an inorganic semiconductor.

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

Abstract: 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. This implies that the three-dimensional orientation of the unique crystal axis in QDs can be determined at room temperature with polarization microscopy. We describe an optical setup to measure four polarization angles of multiple single QDs simultaneously and in real time (≈16 Hz). We use this setup in a proof-of-concept experiment to demonstrate that the rotational motion of QDs can be monitored in various host matrices.

Semiconductor nanocrystal heterostructures

Abstract: A semiconductor nanocrystal heterostructure has a core of a first semiconductor material surrounded by an overcoating of a second semiconductor material. Upon excitation, one carrier can be substantially confined to the core and 5 the other carrier can be substantially confined to the overcoating.

Stabilized semiconductor nanocrystals

Abstract: A semiconductor nanocrystal associated with a polydentate ligand. The polydentate ligand stabilizes the nanocyrstal.

Observation of Bimolecular Carrier Recombination Dynamics in Close-Packed Films of Colloidal CdSe Nanocrystals

Abstract: We study the intensity dependence of the photoconductivity in close-packed films of CdSe quantum dots in which the photoconductivity is characterized by a low density of trapped charges. We find that the intensity dependence of the photocurrent is nonlinear, which is indicative of bimolecular charge carrier recombination dynamics. We present a physical model for the intensity dependence of the photocurrent that is used to fit the data and extract a ratio of the density of carriers to the density of trapped charges. As our model predicts, we show that the current vs intensity plot becomes more linear when the carrier density is decreased.

Semiconductor nanocrystal heterostructures having specific charge carrier confinement

Abstract: A semiconductor nanocrystal heterostructure has a core of a first semiconductor material surrounded by an overcoating of a second semiconductor material. Upon excitation, one carrier can be substantially confined to the core and the other carrier can be substantially confined to the overcoating.

Creating photon atoms

Abstract: A narrow linewidth fluorescent emitter (100) can incorporate a chromophore into a microcavity (130) that can support low-order optical modes.

Imaging the charge transport in arrays of CdSe nanocrystals

Abstract: A method to image charge is used to measure the diffusion coefficient of electrons in films of CdSe nanocrystals at room temperature. This method makes possible the study of charge transport in films exhibiting extremely high resistances or very small diffusion coefficients.

Highly luminescnt color-selective nanocrystalline materials

Abstract: A coated nanocrystal capable of light emission includes a substantially monodisperse nanoparticle selected from the group consisting of CdX, where x=S, Se, Te and an overcoating of ZnY, where Y=S, Se, uniformly deposited thereon, said coated nanoparticle characterized in that when irradiated the particles exhibit photoluminescence in a narrow spectral range of no greater than about 60 nm, and most preferably 40 nm, at full width half max (FWHM). The particle size of the nanocrystallite core is in the range of about 20 Å to about 125 Å, with a deviation of less than 10% in the core. The coated nanocrystal exhibits photoluminescence having quantum yields of greater than 30%.

Method for measuring 3-dimensional orientation of a photoactive moiety

Abstract: A photoactive moiety exhibiting an anisotropic transition dipole. The moiety exhibits emission of polarized light in response to energy absorption. In a preferred embodiment, the moiety comprises a particle from the group consisting of a crystalline arrangement of photoactive molecules and a photoactive nanocrystal. The moiety may include a matrix in which photoactive objects exhibiting an anisotropic emission dipole are embedded. The moiety may be photobleached to product the anisotropy and the photoactive objects may have a one dimensional transition dipole in their natural state.

Anomalous Transport in Quantum Dot Arrays

Abstract: A novel model of transport is proposed to explain power law current transients and memory phenomena observed in partially ordered arrays of semiconducting nanocrystals. The model describes electron transport by a stationary Levy process of transmission events and thereby requires no time dependence of system properties. The waiting time distribution with a characteristic long tail gives rise to a nonstationary response in the presence of a voltage pulse. We report on noise measurements that agree well with the predicted non-Poissonian fluctuations in current, and discuss possible mechanisms leading to this behavior.

Method of preparing a coated nanocrystal

Abstract: A semiconductor nanocrystal heterostructure has a core of a first semiconductor material surrounded by an overcoating of a second semiconductor material. Upon excitation, one carrier can be substantially confined to the core and the other carrier can be substantially confined to the overcoating. The present invention concerns a method of preparing such a coated nanocrystal.