Showing papers by "Moungi G. Bawendi published in 2009"

Quantum Dot Light-Emitting Devices with Electroluminescence Tunable over the Entire Visible Spectrum

Abstract: Improvements in quantum dot light-emitting device (QD-LED) performance are achieved by the choice of organic charge transporting layers, by use of different colloidal QDs for the different parts of the visible spectrum, and by utilizing a recently demonstrated robust QD deposition method. Spectrally narrow electroluminescence of our QD-LEDs is tuned over the entire visible wavelength range from λ = 460 nm (blue) to λ = 650 nm (deep red). By printing close-packed monolayers of different QD types inside an identical QD-LED structure, we demonstrate that different color QD-LEDs with QDs of different chemistry can be fabricated on the same substrate. We discuss mechanisms responsible for efficiency increase for green (4-fold) and orange (30%) QD-LEDs as compared to previous reports and outline challenges associated with achieving high-efficiency blue QD-LEDs.

Inkjet-Printed Quantum Dot–Polymer Composites for Full-Color AC-Driven Displays

Abstract: We demonstrate print-deposition of high resolution, patterned,multicolored thin films of luminescent colloidal quantum dot(QD)-polymer composites and use the printed patterns infabricating robust, bright, full-color AC-driven displays. Thebenefits of AC electroluminescent (EL) displays include simple,low-costfabricationandhighreliability;however,findingefficientand stable phosphors for full-colored displays remains a majorchallenge.

Tissue- and organ-selective biodistribution of NIR fluorescent quantum dots

Abstract: A significant portion of the field of nanomedicine is predicated on being able to target nanoparticles to sites of disease. However, in vivo biodistribution and clearance of nanoparticles are poorly understood. In this study, a novel formulation of near-infrared fluorescent InAs(ZnS) quantum dots was synthesized and coated with a systematically increasing chain length of PEG. We found that varying PEG chain length resulted in major changes in organ/tissue-selective biodistribution and clearance from the body.

Selection of metal oxide charge transport layers for colloidal quantum dot LEDs.

Abstract: We investigate the effect of the electronic energy level positioning, conductivity, and morphology of metal oxide charge transport layers on the performance of light emitting devices (LEDs) that consist of a colloidally synthesized quantum dot (QD) luminescent film embedded between electron and hole injecting ceramic layers. We demonstrate that understanding of these material properties and their effect on charging processes in QDs enables the systematic design of higher efficiency QD-LEDs and excitation of QDs with different emission colors using the same device structure.

Insights into the Kinetics of Semiconductor Nanocrystal Nucleation and Growth

Abstract: A model is presented for the colloidal synthesis of semiconductor nanocrystals capturing the reactions underlying nucleation and growth processes. The model combines an activation mechanism for precursor conversion to monomers, discrete rate equations for formation of small-sized clusters, and continuous Fokker-Planck equation for growth of large-sized clusters. The model allows us to track the temporal evolution of the entire cluster size distribution and compute several experimental observables including mean size and size distribution. The model predicts five distinct regions: generation of monomers, small cluster formation, size distribution focusing due to precursor depletion, pseudo steady state region, and size distribution broadening, with the latter three explicitly reproducing available experimental data at larger cluster sizes. Furthermore, we identify two nondimensional parameter combinations and discuss how these can be used to guide experiments to yield a more rational approach to synthesis modification. Contrary to the common hypothesis that diffusion is essential for size distribution focusing, the model shows that focusing can be achieved under pure reaction control. In addition, the model yields new insights into the synthesis of small nanocrystals with narrow size distributions either by modulation of temperature over the duration of nanocrystal synthesis or by introduction of small quantities of additives that enhance the rate of precursor conversion to monomers. We show that for a given set of reaction parameters, there is an optimum in the duration of high temperature and additive concentration minimizing polydispersity.

Alternating current driven electroluminescence from ZnSe/ZnS:Mn/ZnS nanocrystals.

Abstract: We present a novel technique for room temperature, solution-based fabrication of alternating current thin-film electroluminescent (AC-TFEL) devices using phosphor-doped nanocrystals. Synthesis for stable ZnSe/ZnS:Mn/ZnS nanocrystals that exhibit a quantum yield of 65 ± 5% is outlined, and their electroluminescence is demonstrated in structures consisting of only wide band gap ceramic layers. Both the nanocrystal and the ceramic films have minimal absorption across the visible light spectrum, enabling us to demonstrate transparent AC-TFEL devices.

InAs(ZnCdS) Quantum Dots Optimized for Biological Imaging in the Near-Infrared

Abstract: We present the synthesis of InAs quantum dots (QDs) with a ZnCdS shell with bright and stable emission in the near-infrared (NIR, 700-900 nm) region for biological imaging applications. We demonstrate how NIR QDs can image tumor vasculature in vivo at significantly deeper penetration depths and with higher contrast than visible emitting CdSe(CdS) QDs. Targeted cellular labeling is also presented and may enable multiplexed and low autofluorescence cellular imaging.

Two-Photon Absorbing Nanocrystal Sensors for Ratiometric Detection of Oxygen

Abstract: Two nanocrystal-osmium(II) polypyridyl (NC-Os(II)PP) conjugates have been designed to detect oxygen in biological environments. Polypyridines appended with a single free amine were linked with facility to a carboxylic acid functionality of a semiconductor NC overlayer to afford a biologically stable amide bond. The Os(II)PP complexes possess broad absorptions that extend into the red spectral region; this absorption feature makes them desirable acceptors of energy from NC donors. Fluorescence resonance energy transfer (FRET) from the NC to the Os(II)PP causes an enhanced Os(II)PP emission with a concomitant quenching of the NC emission. Owing to the large two-photon absorption cross-section of the NCs, FRET from the NC to the Os(II)PP can be established under two-photon excitation conditions. In this way, two-photon processes of metal polypyridyl complexes can be exploited for sensing. The emission of the NC is insensitive to oxygen, even at 1 atm, whereas excited states of both osmium complexes are quenc...

Heterojunction photovoltaics using printed colloidal quantum dots as a photosensitive layer.

Abstract: We demonstrate a bilayer photovoltaic device consisting of a heterojunction between colloidal cadmium selenide (CdSe) quantum dots (QDs) and a wide band gap organic hole-transporting thin film of N,N'-diphenyl-N,N'-bis(3-methylphenyl)[1,1'-biphenyl]-4,4'-diamine (TPD) molecules. The active light-absorbing film of QDs is nondestructively printed onto TPD using microcontact stamping. Indium-tin-oxide (ITO) provides the top contact. The resulting device structure can accommodate different size QDs, produces an exceptionally large open circuit voltage (0.8 V) for an architecture with symmetric electrodes, and yields an internal quantum efficiency of 10% at the first QD absorption peak.

Electrostatic Formation of Quantum Dot/J-aggregate FRET Pairs in Solution

Abstract: We report the formation and photonic properties of CdSe/ZnS quantum dot (QD)/J-aggregate donor−acceptor Forster resonant energy transfer (FRET) pairs that are electrostatically bound and dispersed in water. Assembly occurs when the positively charged dye, 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (TTBC), binds electrostatically to QDs that are coated with a negatively charged amphiphilic polymer. QD/J-aggregate FRET pairs display the broadband absorption in the visible and the ultraviolet (UV) part of the spectrum typical of quantum dots, along with the narrow emission linewidths characteristic of J-band emitters (∼12 nm full width at half-maximum). We use dynamic light scattering, transmission electron microscopy, photoluminescence spectroscopy, and photoluminescence lifetimes to conclude that the size of the aggregates formed in the presence of QDs is between 1−6 nm. We find the FRET radius of the QD/J-aggregate to be 5.1 nm. We further demonstrate electrostatic binding as a ge...

Narrow-band absorption-enhanced quantum dot/J-aggregate conjugates.

Abstract: We report narrow-band absorption enhancement of semiconductor nanocrystals via Forster resonance energy transfer from cyanine J-aggregates. These J-aggregated dyes associate electrostatically with short quantum-dot (QD) surface ligands in solution. Energy transfer efficiencies approach unity for this light sensitization and result in a 5-fold enhancement in the QD excitation near the J-aggregate absorption maximum. Because a thin layer of J-aggregates attenuates the same amount of light (at peak absorbance) as a far thicker film of monomer dye, these absorption-enhanced materials may have applications in light-sensitizing applications such as photodetection and optical down-conversion.

Lateral heterojunction photodetector consisting of molecular organic and colloidal quantum dot thin films

Abstract: We demonstrate a heterojunction photodetector of lateral geometry that utilizes an evaporated film of the hole-transporting molecular material N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD) as a charge transport layer and that is sensitized across visible wavelengths by a thin film of colloidal CdSe nanocrystal quantum dots (QDs). High photon-to-electron quantum conversion efficiencies are obtained at room temperature as a result of photoconductive gain. With an electric field of 3.0×105 V/cm applied across the electrodes, we measure the external quantum efficiency at the first QD absorption peak (at wavelength λ=590 nm) to be 13%, corresponding to an internal quantum efficiency of approximately 80%. The operating mechanism of these devices is discussed, noting that the optical response is dominated by the QD absorption spectrum while the charge transport nearly exclusively takes place in the spiro-TPD.

High-Pressure/High-Temperature Microreactors for Nanostructure Synthesis

Abstract: Chemically synthesized nanomaterials, such as nanocrystalline quantum dots, are being considered as the active element in many applications, including photovoltaics, displays, and biochem sensing. To realize the promise of these devices, it will be critical to have an efficient, reproducible synthesis technique of the nanostructures. Currently, nanoparticles are synthesized in a batch mode in small volumes, which is appropriate for studying the fundamental properties of nanosized structures and for developing proof of principle device structures. However, batch synthesis suffers from control of size, size distribution, and quality of the nanomaterial from batch to batch. Moreover, there is an inherent difficulty in scaling up to quantities more reasonable for device development and optimization. Continuous-flow reactors based on microfluidics (microreactors) integrated with heaters and fluid control elements offer a solution to these problems and additional advantages. We describe continuous synthesis of ...

Quantitative phase contrast imaging of THz electric fields in a dielectric waveguide.

Abstract: We apply phase contrast imaging to enable, sharply focused visualization of terahertz waves in electro-optic media. The approach allows quantitative characterization of THz waves in the 60 GHz – 4.5 THz frequency range in a thin dielectric slab and in-focus observation of THz waves in polaritonic structures.

Electrically Controlling and Monitoring InP Nanowire Growth from Solution.

Abstract: Indium phosphide nanowires are of significant technological interest for applications ranging from single junction solar cells to high speed electronics. However, the efficient placement and integration of nanowires into devices remains a significant challenge. Here we extend the technique of electrically controlled solution-liquid-solid (EC-SLS) catalytic nanowire growth to indium phosphide. We are able to control the amount of nanowire growth by varying the bias voltage between the electrodes in solution, and to monitor nanowire bridging across the electrodes by recording the conductivity as a function of growth time. The as-grown indium phosphide nanowires exhibit n-type conductivity as was determined by the in situ integration of nanowires into a field effect transistor geometry. The ability to monitor nanowire growth and electrically control nanowire placement are valuable tools for fabricating nanowire devices. The EC-SLS process has the potential to aid in the fabrication of nanowire devices that could find applications in nanoelectronics, and as electrodes in solar cells and batteries.

Mechanistic Insights into the Formation of InP Quantum Dots

Abstract: MIT-Harvard Center for Cancer Nanotechnology Excellence (National Institutes of Health (U.S.) 1U54-CA119349)

Revealing single emitter spectral dynamics from intensity correlations in an ensemble fluorescence spectrum.

Abstract: We show that the single emitter linewidth underlying a broadened ensemble emission spectrum can be extracted from correlations among the stochastic intensity fluctuations in the ensemble spectrum. Spectral correlations can be observed at high temporal and spectral resolutions with a cross-correlated pair of avalanche photodiodes placed at the outputs of a scanning Michelson interferometer. As illustrated with simulations in conjunction with Fluorescence Correlation Spectroscopy, our approach overcomes ensemble and temporal inhomogeneous broadening to provide single emitter linewidths, even for emitters under weak, continuous, broadband excitation.

Compact biocompatible quantum dots via RAFT-mediated synthesis of imidazole-based random copolymer ligand

Abstract: We present a new class of polymeric ligands for quantum dot (QD) water solubilization to yield biocompatible and derivatizable QDs with compact size (∼10−12 nm diameter), high quantum yields (>50%), excellent stability across a large pH range (pH 5−10.5), and low nonspecific binding. To address the fundamental problem of thiol instability in traditional ligand exchange systems, the polymers here employ a stable multidentate imidazole binding motif to the QD surface. The polymers are synthesized via reversible addition−fragmentation chain transfer-mediated polymerization to produce molecular weight controlled monodisperse random copolymers from three types of monomers that feature imidazole groups for QD binding, polyethylene glycol (PEG) groups for water solubilization, and either primary amines or biotin groups for derivatization. The polymer architecture can be tuned by the monomer ratios to yield aqueous QDs with targeted surface functionalities. By incorporating amino-PEG monomers, we demonstrate cova...

System and method for large field of view, single cell analysis

Abstract: A method and system for medical imaging employs an excitation source configured to cause an object having a plurality of cells to at least one of emit, reflect, and fluoresce light An optical receptor is employed that is configured to receive the light from the object A filter assembly receives the light from the optical receptor and filters the light An image processor having a field of view (FOV) substantially greater than a diameter of a cell of the object and an analysis resolution substantially matched to the diameter of a cell of the object that receives the filtered light from the filter and analyzes the filtered light corresponding to each cell in the FOV A feedback system is provided that is configured to provide an indication of a state of each cell in the FOV and a location of a cell in the FOV meeting a predetermined condition

Design Considerations for Tumor-Targeted Nanoparticles

Abstract: DIAGNOSTIC AGENTS—Nanoparticles functionalized with ligands that target tumors can be cleared from the body through the kidneys if they have a hydrodynamic diameter of less than 5.5 nm.

Tissue- and Organ-Selective Biodistribution of NIR Fluorescent Quantum Dots

Abstract: National Science Foundation (U.S.) (Massachusetts Institute of Technology. Biophysical Instrumentation Facility 0070319)

Electrically controlled catalytic nanowire growth

Abstract: A population of nanowires can be prepared by a method involving electric field catalyzed growth and alteration based on surface charge density.

Materials and methods for biological imaging

Abstract: Water soluble InAs(ZnCdS) semiconductor nanocrystals with bright and stable emission in the near infrared (NIR) wavelength range have been prepared. The NIR semiconductor nanocrystals can be functionalized to enable imaging of specific cellular proteins. In addition, the utility of the NIR region for in vivo biological imaging is clearly demonstrated by the superior ability of InAs(ZnCdS) semiconductor nanocrystals to image tumor vasculature.

Quantitative phase contrast imaging of THz electric fields in a dielectric waveguide

Abstract: We apply phase contrast imaging to enable, sharply focused visualization of terahertz waves in electro-optic media. The approach allows quantitative characterization of THz waves in the 60 GHz – 4.5 THz frequency range in a thin dielectric slab and in-focus observation of THz waves in polaritonic structures.

Revealing single emitter spectral dynamics from intensity correlations in an ensemble fluorescence spectrum

Abstract: We show that the single emitter linewidth underlying a broadened ensemble emission spectrum can be extracted from correlations among the stochastic intensity fluctuations in the ensemble spectrum. Spectral correlations can be observed at high temporal and spectral resolutions with a cross-correlated pair of avalanche photodiodes placed at the outputs of a scanning Michelson interferometer. As illustrated with simulations in conjunction with Fluorescence Correlation Spectroscopy, our approach overcomes ensemble and temporal inhomogeneous broadening to provide single emitter linewidths, even for emitters under weak, continuous, broadband excitation.