Showing papers by "Moungi G. Bawendi published in 2001"

Blinking statistics in single semiconductor nanocrystal quantum dots

Abstract: Statistical studies of fluorescence intermittency in single CdSe nanocrystal quantum dots (QD's) reveal a temperature-independent power-law distribution in the histogram of on and off times---the time periods before the QD turns from emitting to nonemitting (bright to dark) and vice versa. Every QD shows a similar power-law behavior for the off-time distribution regardless of temperature, excitation intensity, surface morphology or size. We propose a dynamic model of tunneling between core and trapped charged states to explain the universal power-law statistics of the blinking events observed. The on-time probability distributions show evidence of both a tunneling mechanism similar to the off-time statistics and a secondary, photoinduced process that leads to a truncation of the power law. The same blinking statistics are also observed for single CdTe nanocrystal QD's.

Biological applications of quantum dots

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.

Water-soluble fluorescent nanocrystals

Abstract: A water soluble semiconductor nanocrystal capable of light emission is provided, including a quantum dot having a selected band gap energy, a layer overcoating the quantum dot, the overcoating layer comprised of a material having a band gap energy greater than that of the quantum dot, and an organic outer layer, the organic layer comprising a compound having a least one linking group for attachment of the compound to the overcoating layer and at least one hydrophilic group space apart from the linking group by a hydrophobic region sufficient to prevent electron charge transfer across the hydrophobic region. The particle size of the nanocrystal core is in the range of about 12Å to about 150Å, with a deviation of less than 10% in the core. The coated nanocrystal exhibits photoluminescende having quantum yield of greater than 10% in water.

Preparation of nanocrystallites

Abstract: A method of manufacturing a nanocrystallite from a M-containing salt forms a nanocrystallite. The nanocrystallite can be a member of a population of nanocrystallites having a narrow size distribution and can include one or more semiconductor materials. Semiconducting nanocrystallites can photoluminesce and can have high emission quantum efficiencies.

Observation of solvatochromism in CdSe colloidal quantum dots

Abstract: We report solvatochromatic shifts in the absorption spectra of colloidal CdSe quantum dots that are consistent with the change in polarization energy of the quantum-confined exciton. Good agreement with theory is found when the screening from the ligand shell is included. The polarization energy also accounts for the spectral shift between dilute dispersions and close-packed quantum dot solids. Experiments with pure and mixed-size quantum dot solids suggest that solvatochromism dominates the redshift observed with quantum dot (2--6-nm diameter) solids.

Bioconjugation of Highly Luminescent Colloidal CdSe–ZnS Quantum Dots with an Engineered Two-Domain Recombinant Protein

Abstract: We present a novel approach, based on molecular self-assembly driven by electrostatic attractions, for conjugating inorganic colloidal semiconductor nanocrystals (quantum dots: QDs) having negatively charged surfaces with a two-domain recombinant protein bearing a positively charged C-terminal leucine zipper domain. Aggregation-free QD/protein conjugate dispersions were prepared. Conjugates retain both properties of the starting materials, i.e., biological activity of the protein and spectroscopic characteristics of the QDs. Such hybrid bio-inorganic conjugates represent a powerful fluorescent tracking tool, because they combine advantages of CdSe–ZnS quantum dots, such as chemical stability and a wide range of size-dependent luminescence emission properties, with a straightforward electrostatic conjugation approach. We describe the design and preparation of a model QD/protein conjugate and present functional characterization of the conjugate using luminescence and bioassays.

Ultrafast carrier dynamics, optical amplification, and lasing in nanocrystal quantum dots

Abstract: Semiconductor materials are widely used in both optically and electrically pumped lasers. The use of semiconductor quantum wells (QWs) as optical-gain media has resulted in important advances in laser technology. QWs have a two-dimensional, step-like density of electronic states that is nonzero at the band edge, enabling a higher concentration of carriers to contribute to the band-edge emission and leading to a reduced lasing threshold, improved temperature stability, and a narrower emission line. A further enhancement in the density of the band-edge states and an associated reduction in the lasing threshold are in principle possible using quantum wires and quantum dots (QDs), in which the confinement is in two and three dimensions, respectively. In very small dots, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), inhibiting thermal depopulation of the lowest electronic states. This effect should result in a lasing threshold that is temperatureinsensitive at an excitation level of only 1 electron-hole (e-h) pair per dot on average. Additionally, QDs in the strongconfinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability over a wide energy range simply by changing the size of the dots.

Inorganic particle conjugates

Abstract: The ionic conjugates include an inorganic particle electrostatically associated with a macromolecule which can interact specifically with predetermined chemical species or biological targets.

Optical gain and lasing in colloidal quantum dots

Abstract: Summary form only given. Semiconductor quantum dots (QDs) promise the lowest lasing threshold for semiconductor media. Additionally, QDs in the strong confinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability simply by changing the dot radius. Lasing has previously been demonstrated for epitaxially grown III-V QDs. Large lateral dimensions and difficulties in size control limit their spectral tunability using quantum confinement effects. An alternative approach to fabricating QDs is through chemical synthesis which can produce semiconductor nanoparticles (colloidal QDs) with radii from 1 to 6 nm and with size dispersions as small as 5%. Such dots show strong quantum confinement and permit size-controlled spectral tunability over an energy range as wide as 1 eV. The combination of tunable electronic energies and chemical flexibility make colloidal QDs ideal building blocks for the bottom-up assembly of optical device structures, including optical amplifiers and lasers. However, despite more than a decade of effort, lasing in small-size colloidal nanoparticles has not been realized. To determine what hinders lasing action, we performed extensive dynamical studies of radiative and nonradiative processes in CdSe colloidal QDs.

Nanocrystal quantum dots: building blocks for tunable optical amplifiers and lasers

Abstract: We study optical processes relevant to optical amplification and lasing in CdSe nanocrystal quantum dots (NQD). NQDs are freestanding nanoparticles prepared using solution-based organometallic reactions originally developed for the Cd chalcogenides, CdS, CdSe and CdTe [J. Am. Chem. Soc. 115, 8706 (1993)]. We investigate NQDs with diameters ranging from 2 to 8 nm. Due to strong quantum confinement, they exhibit size-dependent spectral tunability over an energy range as wide as several hundred meV. We observe a strong effect of the matrix/solvent on optical gain properties of CdSe NQDs. In most of the commonly used solvents (such as hexane and toluene), gain is suppressed due to strong photoinduced absorption associated with carriers trapped at solvent-related interface states. In contrast, matrix-free close packed NQD films (NQD solids) exhibit large optical gain with a magnitude that is sufficiently high for the optical gain to successfully compete with multiparticle Auger recombination [Science 287, 10117 (2000)]. These films exhibit narrowband stimulated emission at both cryogenic and room temperature, and the emission color is tunable with dot size [Science 290, 314 (2000)]. Moreover, the NQD films can be incorporated into microcavities of different geometries (micro-spheres, wires, tubes) that produce lasing in whispering gallery modes. The facile preparation, chemical flexibility and wide-range spectral tunability due to strong quantum confinement are the key advantages that should motivate research into NQD applications in optical amplifiers and lasers.