Showing papers by "Moungi G. Bawendi published in 2007"

Renal clearance of quantum dots.

Abstract: The field of nanotechnology holds great promise for the diagnosis and treatment of human disease. However, the size and charge of most nanoparticles preclude their efficient clearance from the body as intact nanoparticles. Without such clearance or their biodegradation into biologically benign components, toxicity is potentially amplified and radiological imaging is hindered. Using intravenously administered quantum dots in rodents as a model system, we have precisely defined the requirements for renal filtration and urinary excretion of inorganic, metal-containing nanoparticles. Zwitterionic or neutral organic coatings prevented adsorption of serum proteins, which otherwise increased hydrodynamic diameter by >15 nm and prevented renal excretion. A final hydrodynamic diameter <5.5 nm resulted in rapid and efficient urinary excretion and elimination of quantum dots from the body. This study provides a foundation for the design and development of biologically targeted nanoparticles for biomedical applications.

CdSe nanocrystal based chem-/bio- sensors

Abstract: Semiconductor nanocrystals (NCs) have found application in biology mostly as optical imaging agents where the photophysical properties of the NCs are insensitive to species in their environment. This tutorial review examines the application of CdSe NCs as optical sensing agents where the NC's photophysical properties are sensitive to species in their environment. For this case, the NC is modified at the surface with a conjugate, which interacts with an external agent by physical (i.e. recognition) or chemical means. Signal transduction in these chem-bio (CB) sensitive NCs is derived primarily from energy transfer between the NC and the external agent, which functions as the energy transfer acceptor or donor. Signaling may be obtained by directly detecting luminescence from the NC and/or the conjugate. New developments for the use of NCs as gain materials in micro-lasing cavities (distributed feedback gratings and spherical resonators) opens the way to designing CB-sensitive NCs for high-gain sensing applications.

Compact Cysteine-Coated CdSe(ZnCdS) Quantum Dots for in Vivo Applications

Abstract: We have developed a versatile nanoparticle construct using a compact cysteine coating on a CdSe(ZnCdS) core(shell) nanocrystal (QD-Cys) that is biologically compatible, exceptionally compact, highly fluorescent, and easily functionalized. The small hydrodynamic diameter of QD-Cys ( approximately 6 nm) allows for renal clearance of these nanoparticles in rat models. Moreover, the ability to directly conjugate to QD-Cys opens up the possibility of functionalized nanocrystals for in vivo targeted imaging, in which small targeting molecules can be appended to QD-Cys, and unbound QDs can be rapidly cleared to achieve high signal/noise ratios and to reduce background toxicity.

Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer.

Abstract: We demonstrate light emitting devices (LEDs) with a broad spectral emission generated by electroluminescence from a mixed-monolayer of red, green, and blue emitting colloidal quantum dots (QDs) in a hybrid organic/inorganic structure. The colloidal QDs are reproducibly synthesized and yield high luminescence efficiency materials suitable for LED applications. Independent processing of the organic charge transport layers and the QD luminescent layer allows for precise tuning of the emission spectrum without changing the device structure, simply by changing the ratio of different color QDs in the active layer. Spectral tuning is demonstrated through fabrication of white QD-LEDs that exhibit external quantum efficiencies of 0.36% (Commission Internationale de l'Eclairage) coordinates of (0.35, 0.41) at video brightness, and color rendering index of 86 as compared to a 5500 K blackbody reference.

Carrier multiplication yields of CdSe and CdTe nanocrystals by transient photoluminescence spectroscopy

Abstract: Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favorable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and $\mathrm{CdSe}$ NCs. We report here an assessment of CM yields in $\mathrm{CdSe}$ and $\mathrm{CdTe}$ NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. Although the technique is particularly sensitive due to enhanced biexciton radiative rates relative to the exciton, ${k}_{BX}^{\mathrm{rad}}g2{k}_{X}^{\mathrm{rad}}$, we find no evidence for CM in $\mathrm{CdSe}$ and $\mathrm{CdTe}$ NCs up to photon energies $\ensuremath{\hbar}\ensuremath{\omega}g3{E}_{g}$, well above previously reported relative energy thresholds.

Carrier multiplication yields in CdSe and CdTe nanocrystals by transient photoluminescence

Abstract: Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. Although the technique is particularly sensitive due to enhanced biexciton radiative rates relative to the exciton, kradBX > 2 kradX, we find no evidence for CM in CdSe and CdTe NCs up to photon energies E > 3 Eg, well above previously reported relative energy thresholds.

Lymphatic drainage of the peritoneal space: a pattern dependent on bowel lymphatics.

Abstract: Understanding lymph drainage patterns of the peritoneum could assist in staging and treatment of gastrointestinal and ovarian malignancies. Sentinel lymph nodes (SLNs) have been identified for solid organs and the pleural space. Our purpose was to determine whether the peritoneal space has a predictable lymph node drainage pattern. Rats received intraperitoneal injections of near-infrared (NIR) fluorescent tracers: namely, quantum dots (designed for retention in SLNs) or human serum albumin conjugated with IRDye800 (HSA800; designed for lymphatic flow beyond the SLN). A custom imaging system detected NIR fluorescence at 10 and 20 minutes and 1, 4, and 24 hours after injection. To determine the contribution of viscera to peritoneal lymphatic flow, additional cohorts received bowel resection before NIR tracer injection. Associations with appropriate controls were assessed with the χ2 test. Quantum dots drained to the celiac, superior mesenteric, and periportal lymph node groups. HSA800 drained to these same groups at early time points but continued flowing to the mediastinal lymph nodes via the thoracic duct. After bowel resection, both tracers were found in the thoracic, not abdominal, lymph node groups. Additionally, HSA800 was no longer found in the thoracic duct but in the anterior chest wall and diaphragmatic lymphatics. The peritoneal space drains to the celiac, superior mesenteric, and periportal lymph node groups first. Lymph continues via the thoracic duct to the mediastinal lymph nodes. Bowel lymphatics are a key determinant of peritoneal lymph flow, because bowel resection shifts lymph flow directly to the intrathoracic lymph nodes via chest wall lymphatics.

Anomalous Stokes shift in CdSe nanocrystals

Abstract: We investigate the temperature dependence of exciton fluorescence, absorption, and Stokes shift for five sizes of high quality $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ nanocrystals from the perspective of the fine structure model and exciton-acoustic phonon coupling. The Stokes shift is found to have a weak temperature dependence for all sizes. Within the fine structure model, we use the temperature dependence of the Stokes shift to infer the upper level energy and oscillator strength. We also interpret our ensemble measurements using an exciton-acoustic phonon scattering model. We find that neither the fine structure nor exciton-acoustic phonon scattering is able to adequately explain our data in isolation and conclude that a comprehensive theory which includes the physics of both models on an equal footing is required.

Sentinel lymph node mapping with type-II quantum dots.

Abstract: Sentinel lymph node (SLN) mapping is an important cancer surgery during which the first lymph node draining the site of a tumor is identified, resected, and analyzed for the presence or absence of malignant cells. Fluorescent semiconductor nanocrystals (quantum dots) of the appropriate size, charge, and emission wavelength permit this surgery to be performed rapidly, with high sensitivity, and under complete image guidance. We describe the materials and methods necessary for the production and characterization of type II near-infrared (NIR) fluorescent quantum dots, which have been optimized for SLN mapping. They contain a CdTe core, CdSe shell, and highly anionic, oligomeric phosphine organic coating. We also describe how to utilize such quantum dots in animal model systems of SLN mapping.

Bias-Induced Photoluminescence Quenching of Single Colloidal Quantum Dots Embedded in Organic Semiconductors

Abstract: We demonstrate reversible quenching of the photoluminescence from single CdSe/ZnS colloidal quantum dots embedded in thin films of the molecular organic semiconductor N,N‘-diphenyl-N,N‘-bis(3-methylphenyl)-(1,1‘-biphenyl)-4,4‘-diamine (TPD) in a layered device structure. Our analysis, based on current and charge carrier density, points toward field ionization as the dominant photoluminescence quenching mechanism. Blinking traces from individual quantum dots reveal that the photoluminescence amplitude decreases continuously as a function of increasing forward bias even at the single quantum dot level. In addition, we show that quantum dot photoluminescence is quenched by aluminum tris(8-hydroxyquinoline) (Alq3) in chloroform solutions as well as in thin solid films of Alq3 whereas TPD has little effect. This highlights the importance of chemical compatibility between semiconductor nanocrystals and surrounding organic semiconductors. Our study helps elucidate elementary interactions between quantum dots and...

Composite material including nanocrystals and methods of making

Abstract: Temperature-sensing compositions can include an inorganic material, such as a semiconductor nanocrystal. The nanocrystal can be a dependable and accurate indicator of temperature. The intensity of emission of the nanocrystal varies with temperature and can be highly sensitive to surface temperature. The nanocrystals can be processed with a binder to form a matrix, which can be varied by altering the chemical nature of the surface of the nanocrystal. A nanocrystal with a compatibilizing outer layer can be incorporated into a coating formulation and retain its temperature sensitive emissive properties.

White light emitting devices

Abstract: A white light emitting semiconductor nanocrystal includes a plurality of semiconductor nanocrystals.

Electrically driven light emission from single colloidal quantum dots at room temperature

Abstract: Light emission from single colloidal CdSe∕ZnS (core/shell) nanocrystals embedded in electrically driven organic light emitting devices is demonstrated at room temperature. Spectral diffusion and blinking from individual quantum dots were observed both in electro- and photoluminescence. The authors propose a model in which the nanocrystals act as seeds for the formation of current channels that lead to enhanced exciton recombination in the vicinity of the quantum dots. This work demonstrates that individual semiconductor nanocrystals can serve as emissive probes in organic light emitting devices and that they can be used to manipulate device structure and properties at the nanometer scale.

Optical structures including nanocrystals

Abstract: An optical structure can include a nanocrystal on a surface of an optical waveguide in a manner to couple the nanocrystal to the optical field of light propagating through the optical waveguide to generate an emission from the nanocrystal.

Electro-optical device

Abstract: An electro-optical device can include a plurality of nanocrystals positioned between a first electrode and a second electrode.

Microspheres including nanoparticles in the peripheral region

Abstract: A microparticle can include a central region and a peripheral region. The peripheral region can include a nanoparticle, such as a metal nanoparticle, a metal oxide nanoparticle, or a semiconductor nanocrystal. The microparticle can be a member of a monodisperse population of particles.

Blue light emitting semiconductor nanocrystals and devices

Abstract: A blue light emitting semiconductor nanocrystal having an quantum yield of greater than 20% can be incorporated in a light emitting device.

Extracting the number of quantum dots in a microenvironment from ensemble fluorescence intensity fluctuations

Abstract: We estimate the number of CdSe quantum dots (QDs) in a microenvironment by analyzing the intensity fluctuation of the fluorescence time trace from a collection of QDs in the fluorescence intensity steady-state regime. The steady state is chosen because the aged blinking dynamics of single QDs leads to a higher probability for observing large on and off waiting times relative to the typical time bins chosen to generate fluorescence time traces. In the steady state, the relative standard deviation of the fluorescence intensity from a collection of QDs is bin-size independent, which enables a simple estimation of the absolute number of QDs under observation. This is not possible for most conventional chromophores, whose triplet blinking dynamics are much faster than a typical bin size, or for QDs undergoing blinking dynamics that are fast compared to the bin size, as occurs prior to the steady state. The estimated number of QDs incorporated into a single silica microsphere as determined by our method agrees well with the number obtained using conventional absorption measurements on an ensemble of microspheres.

Photovoltaic device including semiconductor nanocrystals

Abstract: A photovoltaic 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

Efficient All-Inorganic Colloidal Quantum Dot LEDs

Abstract: We present the first all-inorganic QD-LEDs consisting of radio-frequency sputtered metal-oxide charge transport layers and a colloidal quantum dot electroluminescent region. These devices manifest a 100-fold increase in external quantum efficiency over previously reported structures.

Electro-optical device including nanocrystals

Abstract: An electro-optical device can include a plurality of nanocrystals positioned between a first electrode and a second electrode. The nanocrystal and at least one electrode can have a band gap offset sufficient to inject a charge carrier from the first electrode or second electrode into the nanocrystal. The device can be a secondary photoconductor.

Charge transport in arrays of PbSe nanocrystals

Abstract: We report electrical transport measurements of arrays of PbSe nanocrystals forming the channels of field effect transistors We measure the current in these devices as a function of source-drain voltage, gate voltage and temperature Annealing is necessary to observe measurable current after which a simple model of hopping between intrinsic localized states describes the transport properties of the nanocrystal solid We find that the majority carriers are holes, which are thermally released from acceptor states At low source-drain voltages, the activation energy for the conductivity is given by the energy required to generate holes plus the activation over barriers resulting from site disorder At high source-drain voltages the activation energy is given by the former only The thermal activation energy of the zero-bias conductance indicates that the Fermi energy is close to the highest-occupied valence level, the 1Sh state, and this is confirmed by field-effect measurements, which give a density of states of approximately eight per nanocrystal as expected from the degeneracy of the 1Sh state