Showing papers by "Moungi G. Bawendi published in 1999"

Electron and hole relaxation pathways in semiconductor quantum dots

Abstract: Femtosecond (fs) broad-band transient absorption (TA) is used to study the intraband relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC's) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a ``phonon bottleneck,'' we observe a fast subpicosecond $1P$-to-$1S$ electron relaxation, with the rate exceeding that due to phonon emission in bulk semiconductors. The energy relaxation is enhanced with reducing the NC's radius, and does not show any dependence on the NC surface properties (quality of the surface passivation). These data indicate that electron energy relaxation occurs by neither multiphonon emission nor by coupling to surface defects, but is likely meditated by Auger-type electron-hole energy transfer. We use fs infrared TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole relaxation pathways leading to the depopulation of NC quantized states. In contrast to the electron relaxation, which is controlled by NC surface passivation, the depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types of samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

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

Abstract: Kinetic control of crystal growth in the presence of a coordinating ligand is critical for the formation of a new structure of elemental cobalt (e-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 [Co2 (CO)8 ] in solution in the presence of trioctylphosphane oxide [TOPO, Eq. (1)].

Quantum dot white and colored light emitting diodes

Abstract: A light-emitting device comprising a population of quantum dots (QDs) embedded in a host matrix and a primary light source which causes the QDs to emit secondary light and a method of making such a device. The size distribution of the QDs is chosen to allow light of a particular color to be emitted therefrom. The light emitted from the device may be of either a pure (monochromatic) color, or a mixed (polychromatic) color, and may consist solely of light emitted from the QDs themselves, or of a mixture of light emitted from the QDs and light emitted from the primary source. The QDs desirably are composed of an undoped semiconductor such as CdSe, and may optionally be overcoated to increase photoluminescence.

Highly luminescent color-selective materials and method of making thereof

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%.

Photoluminescence from Single Semiconductor Nanostructures

Abstract: We review some recent results in the spectroscopy of single CdSe nanocrystal quantum dots. By eliminating the effects of inhomogeneous broadening and ensemble averaging, single nanocrystal spectroscopy has revealed many new and previously unexpected physical phenomena. Among those discussed in this review are ultra-narrow emission lineshapes (∼600× narrower than ensemble spectra), a highly polarizable emitting state in the presence of strong local electric fields, line broadening as a result of environmental fluctuations, and shifting of the emission spectra over a wide range of energies (from less than 300 μeV to 80 meV). In addition, polarization spectroscopy of single nanocrystals has revealed the presence of a theoretically predicted two-dimensional transition dipole moment oriented in the xy plane of the nanocrystals. As a result, it is, in principle, possible to use polarization spectroscopy to determine the three-dimensional orientation of individual nanocrystals. These and other studies of single quantum dots have provided us with significant insight into the detailed physics and dynamics of this unique and fascinating physical system.

Three-dimensional orientation measurements of symmetric single chromophores using polarization microscopy

Abstract: A complete understanding of any complex molecular system generally requires a knowledge of the three-dimensional (3D) orientation of its components relative both to each other, and to directional perturbations such as interfaces and electromagnetic fields. Far-field polarization microscopy is a convenient and widespread technique for detecting and measuring the orientation of single chromophores. But because the polarized electromagnetic field that is used to probe the system lacks a significant longitudinal component, it was thought that, in general, only 2D orientation information could be obtained1,2,3. Here we demonstrate that far-field polarization microscopy can yield the 3D orientation of certain highly symmetric single chromophores (CdSe nanocrystal quantum dots in the present case). The key requirement is that the chromophores must have a degenerate transition dipole oriented isotropically in two dimensions, which gives rise to a perpendicular ‘dark axis’ that does not couple to the light field. By measuring the fluorescence intensity from the dipole as a function of polarization angle, it is possible to calculate both the tilt angle between the dark axis and the sample plane, as well as the in-plane orientation, and hence obtain the 3D orientation of the chromophore

Block copolymers as photonic bandgap materials

Abstract: Block copolymers self-assemble into one-, two-, and three-dimensional periodic equilibrium structures, which can exhibit photonic bandgaps. This paper outlines a methodology for producing photonic crystals at optical length scales from block copolymers. Techniques for enhancing the intrinsic dielectric contrast between the block copolymer domains, as well as increasing the characteristic microdomain distances, and controlling defects are presented. To demonstrate the applicability of this methodology, a self-assembled one-dimensional periodic structure has been fabricated that reflects visible light. The wealth of structures into which block copolymers can assemble and the multiple degrees of freedom that can be built into these materials on the molecular level offer a large parameter space for tailoring new types of photonic crystals at optical length scales.

Water-soluble fluorescent semiconductor nanocrystals

Abstract: A water-soluble semiconductor nanocrystal capable of light emission is provided. The nanocrystal including a semiconductor nanocrystal core having a selected band gap energy, a shell layer overcoating the core comprised of a semiconductor material having a band gap energy greater than that of the semiconductor nanocrystal, and an outer layer comprised of a molecule having at least one linking group for attachment of the molecule to the overcoating shell layer and at least one hydrophilic group optionally spaced apart from the linking group by a hydrophobic region sufficient to prevent electron charge transfer across the hydrophobic region.

Influence of Spectral Diffusion on the Line Shapes of Single CdSe Nanocrystallite Quantum Dots

Abstract: We study the emission line shapes of single CdSe nanocrystallite quantum dots. Single dot line shapes are found to result from rapid spectral shifting of the emission spectrum rather than the intrinsic physics of the quantum dot. A strong dependence of single dot line widths on excitation intensity, wavelength, temperature, and integration time is found and is correlated with the number of times that the quantum dot is excited during the acquisition of a single spectrum. The observed results are consistent with thermally assisted spectral diffusion, activated by the release of excess excitation energy.

Ultrafast dynamics of inter- and intraband transitions in semiconductor nanocrystals : implications for quantum-dot lasers

Abstract: Application of femtosecond transient absorption in the visible and near-IR spectral ranges and time-resolved photoluminescence allows us to separate electron and hole relaxation paths and to map the structure of interband and intraband optical transitions in CdSe and CdS nanocrystals (NC's) with a wide range of surface properties. In contrast to electron relaxation, which is controlled by NC surface passivation, depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results suggest that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

Semiconductor nanocrystals for inventory control

Abstract: A novel encoding system, compositions for use therein and methods for determining the source, location and/or identity of a particular item or component of interest is provided. In particular, the present invention utilizes a collection of one or more sizes of populations of semiconductor nanocrystals having characteristic spectral emissions, to “track” the source or location of an item of interest or to identify a particular item of interest. The semiconductor nanocrystals used in the inventive compositions can be selected to emit a desired wavelength to produce a characteristic spectral emission in narrow spectral widths, and with a symmetric, nearly Gaussian line shape, by changing the composition and size of the semiconductor nanocrystal. Additionally, the intensity of the emission at a particular characteristic wavelength can also be varied, thus enabling the use of binary or higher order encoding schemes.

Composite thin films of CdSe nanocrystals and a surface passivating/electron transporting block copolymer: Correlations between film microstructure by transmission electron microscopy and electroluminescence

Abstract: We present a microscopic characterization of composite thin films made of CdSe semiconductor nanocrystals (quantum dots) dispersed in a polynorbornene-based block copolymer with surface-passivating and electron transport functionalities. We used two types of nanocrystals, CdSe with pure organic capping groups (“bare”) and CdSe–ZnS core-shell nanocrystals, also capped on the outer surface with organic groups. The composite thin films are incorporated in heterostructure light emitting devices where they serve as both the electron transport layer and the emissive layer. A thin layer of self-assembled poly (phenylene vinylene) (PPV) is used as the hole transport layer. We used transmission electron microscopy (TEM) to examine the film structure in plan view and in cross section. The TEM analysis uncovered distinct and complex differences between the microstructures of thin films containing bare and ZnS overcoated nanocrystals. We found a strong correlation between the composite film microstructure and the cor...

Biological applications of semiconductor nanocrystals

Abstract: The present invention provides a composition comprising fluorescent semiconductor nanocrystals associated to a compound, wherein the nanocrystals have a characteristic spectral emission, wherein said spectral emission is tunable to a desired wavelength by controlling the size of the nanocrystal, and wherein said emission provides information about a biological state or event.

Organometallic Synthesis and Spectroscopic Characterization of Manganese-Doped CdSe Nanocrystals

Abstract: The synthesis of II−VI semiconductor nanocrystals doped with transition metals has proved to be particularly difficult. In the case of CdSe quantum dots (QDs) produced via high-temperature pyrolysis in trioctylphosphine oxide (TOPO), specially designed precursors used in this study appear to be necessary to successfully incorporate low levels of Mn. A simple etching experiment and electron paramagnetic resonance (EPR) measurements reveal that most of the dopant atoms reside in the surface layers of the inorganic lattice. The dopant dramatically affects 113Cd magic angle spinning (MAS) nuclear magnetic resonance NMR spectra; the observed paramagnetic shift and decreased longitudinal relaxation time are consistent with Mn incorporated in the QDs. Paramagnetic atoms in QDs generate large effective magnetic fields, which implies that magnetooptical experiments can be performed simply by doping. Results from fluorescence line narrowing (FLN) studies on Mn-doped CdSe QDs mirror previous findings on undoped QDs ...

Polarization label for measuring 3-dimensional orientation

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.

Semiconductor nanocrystal labels

Abstract: A fluorescent semiconductor nanocrystal is used as a tag or label for a biological molecule which is preferably a member of a specific binding pair such as avidin, biotin, antibody, antigen or an oligonucleotide. The nanocrystals may have a core of CdSe, a shell layer of ZnS, a "cap" of trioctylphosphine oxide and a coating of mercaptoundecanoic acid. The latter may be deprotonated to render the nanocrystals water soluble. Biotin-thiol-(as shown) and biotin-amine-nanocrystal complexes are prepared. Nanocrystal-tagged binding members may be used in assays to detect target analytes and particularly in multiplex assays where a plurality of analytes are simultaneously detected by the use of differently tagged binding members, the different nanocrystal labels having emission spectra that are distinct from each other.

Synthesis and Characterization of Strongly Fluorescent CdTe Nanocrystal Colloids

Abstract: We present a synthesis of colloidal CdTe nanocrystals whose absolute room temperature quantum yields are routinely above 60%. The preparation is based on the trioctylphosphine oxide (TOPO) method reported by Murray, with a more stalbe tellurium precursor now used as the chalcogenide source. The photoluminescence is continuously tunable over the range 590-760 nm and is as narrow as 135 meV (45 nm) FWHM. No deep trap luminescence is detected for the diameter range 4-11 nm. CdTe nanocrystals are characterized by UV/vis absorption, photoluminescence emission, transmission electron microscopy, and powder X-ray diffraction.

Femtosecond inter- and intra-band spectroscopy of semiconductor quantum dots

Abstract: Summary form only given. Due to a large energy level spacing and an enhanced surface-to-volume ratio, carrier dynamics in strongly-confined semiconductor nanocrystals (NCs) are significantly different from those in bulk materials. The large level separation has been predicted to significantly inhibit energy relaxation due to a phonon bottleneck which, however, has been argued against by recent experiments. The relaxation of carriers from the lowest quantized states in NCs can be significantly faster than for radiative decay, which is due to enhancement of non-radiative processes associated, e.g., with surface trapping.