Showing papers by "Moungi G. Bawendi published in 2005"

Optical gain and stimulated emission in nanocrystal quantum dots

Abstract: The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

Fluorescence intermittency in single cadmium selenide nanocrystals

Abstract: SEMICONDUCTOR nanocrystals offer the opportunity to study the evolution of bulk materials properties as the size of a system increases from the molecular scale 1,2 . In addition, their strongly size-dependent optical properties render them attractive candidates as tunable light absorbers and emitters in optoelectronic devices such as light-emitting diodes 3,4 and quantum-dot lasers 5, 6 and as optical probes of biological systems'. Here we show that light emission from single fluorescing nanocrystals of cadmium selenide under continuous excitation turns on and off intermittently with a characteristic timescale of about 0.5 seconds. This intermittency is not apparent from ensemble measurements on many nanocrystals. The dependence on excitation intensity and the change in on/off times when a passivating, high-bandgap shell of zinc sulphide encapsulates the nanocrystal 8,9 suggests that the abrupt turning off of luminescence is caused by photoionization of the nanocrystal. Thus spectroscopic measurements on single nanocrystals can reveal hitherto unknown aspects of their photophysics.

Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites

Abstract: A simple route to the production of high-quality CdE (E = S, Se, Te) semiconductor nanocrystallites is presented. Crystallites from ∼ 12 A to ∼ 115 A in diameter with consistent crystal structure, surface derivatization, and a bigh degree of monodispersity are prepared in a single reaction. The synthesis is based on the pyrolysis of organometallic reagents by injection into a hot coordinating solvent. This provides temporally discrete nucleation and permits controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of crystallites from portions of the growth solution isolates samples with narrow size distributions (<5% rms in diameter). High sample quality results in sharp absorption features and strong "band-edge" emission which is tunable with particle size and choice of material. Transmission electron microscopy and X-ray powder diffraction in combination with computer simulations indicate the presence of bulk structural properties in crystallites as small as 20 A in diameter.

Quantum dots spectrally distinguish multiple species within the tumor milieu in vivo

Abstract: A solid tumor is an organ composed of cancer and host cells embedded in an extracellular matrix and nourished by blood vessels. A prerequisite to understanding tumor pathophysiology is the ability to distinguish and monitor each component in dynamic studies. Standard fluorophores hamper simultaneous intravital imaging of these components. Here, we used multiphoton microscopy techniques and transgenic mice that expressed green fluorescent protein, and combined them with the use of quantum dot preparations. We show that these fluorescent semiconductor nanocrystals can be customized to concurrently image and differentiate tumor vessels from both the perivascular cells and the matrix. Moreover, we used them to measure the ability of particles of different sizes to access the tumor. Finally, we successfully monitored the recruitment of quantum dot–labeled bone marrow–derived precursor cells to the tumor vasculature. These examples show the versatility of quantum dots for studying tumor pathophysiology and creating avenues for treatment.

Large-Area Ordered Quantum-Dot Monolayers via Phase Separation During Spin-Casting†

Abstract: We investigate a new method for forming large-area (> cm2) ordered monolayers of colloidal nanocrystal quantum dots (QDs). The QD thin films are formed in a single step by spin-casting a mixed solution of aromatic organic materials and aliphatically capped QDs. The two different materials phase separate during solvent drying, and for a predefined set of conditions the QDs can assemble into hexagonally close-packed crystalline domains. We demonstrate the robustness and flexibility of this phase-separation process, as well as how the properties of the resulting films can be controlled in a precise and repeatable manner. Solution concentration, solvent ratio, QD size distribution, and QD aspect ratio affect the morphology of the cast thin-film structure. Controlling all of these factors allows the creation of colloidal-crystal domains that are square micrometers in size, containing tens of thousands of individual nanocrystals per grain. Such fabrication of large-area, engineered layers of nanoscale materials brings the beneficial properties of inorganic QDs into the realm of nanotechnology. For example, this technique has already enabled significant improvements in the performance of QD light-emitting devices.

Photodetectors based on treated CdSe quantum-dot films

Abstract: We demonstrate photodetectors of sandwich geometry active in the visible spectrum in which the active layer is a 200 nm thick film of CdSe quantum dots (QDs). The solution-phase treatment of the QD film with n-butylamine after casting greatly increases the exciton dissociation efficiency and charge-transport properties of the film. Under 110mW∕cm2 illumination with light at λ=514nm, the photocurrent to dark current ratio, Iphoto∕Idark, is 103 at V=0V, and the 3 dB frequency is ∼50kHz. At room temperature, we observe zero-bias external quantum efficiencies (EQE) from 0.08% to 0.23% in the wavelength range λ=350nm to λ=575nm, corresponding to an internal quantum efficiency (IQE) of 0.6±0.1% across the tested spectrum. At V=−6V, EQE ranges from 15% to 24%, corresponding to an IQE of 70±10%.

Engineering InAs(x)P(1-x)/InP/ZnSe III-V alloyed core/shell quantum dots for the near-infrared.

Abstract: Quantum dots with a core/shell/shell structure consisting of an alloyed core of InAs(x)P(1-x), an intermediate shell of InP, and an outer shell of ZnSe were developed. The InAs(x)P(1-x) alloyed core has a graded internal composition with increasing arsenic content from the center to the edge of the dots. This compositional gradient results from two apparent effects: (1) the faster reaction kinetics of the phosphorus precursor compared to the arsenic precursor, and (2) a post-growth arsenic-phosphorus exchange reaction that increases the arsenic content. The cores have a zinc blend structure for all compositions and show tunable emission in the near-infrared (NIR) region. A first shell of InP leads to a red-shift and an increase in quantum yield. The final shell of ZnSe serves to stabilize the dots for applications in aqueous environments, including NIR biomedical fluorescence imaging. These NIR-emitting core/shell/shell InAs(x)P(1-x)/InP/ZnSe were successfully used in a sentinel lymph node mapping experiment.

Quantum dot-based multiplexed fluorescence resonance energy transfer.

Abstract: We demonstrate the use of luminescent quantum dots (QDs) conjugated to dye-labeled protein acceptors for nonradiative energy transfer in a multiplexed format. Two configurations were explored: (1) a single color QD interacting with multiple distinct acceptors and (2) multiple donor populations interacting with one type of acceptor. In both cases, we showed that simultaneous energy transfer between donors and proximal acceptors can be measured. However, data analysis was simpler for the configuration where multiple QD donors are used in conjunction with one acceptor. Steady-state fluorescence results were corroborated by time-resolved measurements where selective shortening of QD lifetime was measured only for populations that were selectively engaged in nonradiative energy transfer.

Phosphine oxide polymer for water-soluble nanoparticles.

Abstract: A phosphine oxide polymer was developed using bis(dichlorophosphino)ethane and poly(ethylene glycol). This polymer system was used to transfer various nanoparticles from organic solvents to water, retaining their physical properties and reactivities.

Blue semiconductor nanocrystal laser

Abstract: We demonstrate tunable room-temperature amplified spontaneous emission and lasing from blue-emitting core-shell CdS∕ZnS nanocrystals (NCs) stabilized in a sol-gel derived silica matrix. Variable stripe length measurements show that these NC-silica composites have a modal gain of ∼100cm−1 at room temperature. Coating microspheres with a NC-silica composite film via a facile process resulted in uniform resonators that exhibit room-temperature lasing over long periods of continuous excitation. This work opens up a spectral window for emission tunable, microscale NC-based lasers.

Room-Temperature Ordered Photon Emission from Multiexciton States in Single CdSe Core-Shell Nanocrystals

Abstract: We report room-temperature ordered multiphoton emission from multiexciton states of single CdSe(CdZnS) core(-shell) colloidal nanocrystals (NCs) that are synthesized by wet chemical methods. Spectrally and temporally resolved measurements of biexciton and triexciton emission from single NCs are also presented. A simple four level system models the results accurately and provides estimates for biexciton and triexciton radiative lifetimes and quantum yields.

A low-threshold, high-efficiency microfluidic waveguide laser.

Abstract: This communication describes a long (1 cm), laser-pumped, liquid core-liquid cladding (L2) waveguide laser. This device provides a simple, high intensity, tunable light source for microfludic applications. Using a core solution of 2 mM rhodamine 640 perchlorate, optically pumped by a frequency-doubled Nd:YAG laser, we found that the threshold for lasing was as low as 22 muJ (16-ns pulse length) and had a slope efficiency up to 20%. The output wavelength was tunable over a 20-nm range by changing the ratio of solvent components (dimethyl sulfoxide and methanol) in the liquid core.

Quantization of multiparticle auger rates in semiconductor quantum dots

Abstract: We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.

Blue light emitting semiconductor nanocrystal materials

Abstract: A semiconductor nanocrystal includes a core including a first semiconductor material and an overcoating including a second semiconductor material. A monodisperse population of the nanocrystals emits blue light over a narrow range of wavelengths with a high quantum efficiency.

Multiexciton fluorescence from semiconductor nanocrystals

Abstract: We use transient photoluminescence to spectrally resolve the emission from 1, 2, and 3 electron–hole pairs states in CdSe colloidal nanocrystals with radii ranging between 2.3 and 5.2 nm. Temporally and spectrally resolved multiexciton emission from single NCs is also observed. The observation of multiexciton emission enables new experiments and potential applications at both the single NC level and using ensembles of NCs. First we discuss the use of single CdSe(CdZnS) core(shell) colloidal NCs (spheres and rods) to generate triggered photon pair emission at room temperature, with specific ordering of the pair’s constituent photons. Second, we incorporate CdSe/ZnS core-shell nanocrystals into a TiO 2 host matrix and observe simultaneous two-state amplified spontaneous emission and lasing from both multiexcitonic transitions (1S 3/2 –1S e and 1P 3/2 –1P e ) in a surface-emitting distributed feedback CdSe NC laser. From our data we deduce radiative lifetimes, quantum yields, stimulated emission gain, and power dependencies for the multiexciton transitions.

Method and system for transferring a patterned material

Abstract: A method of transferring a material to a substrate includes selectively depositing the material on a surface of an applicator and contacting the surface of the applicator to the substrate. The material can form a pattern on the surface of the applicator. The pattern can be preserved when the material is transferred to the substrate. The material can be deposited on the applicator by ink jet printing.

Properties of the CdSe(0001), (0001̄), and (112̄0) Single Crystal Surfaces: Relaxation, Reconstruction, and Adatom and Admolecule Adsorption

Abstract: Details of the chemical mechanism underlying the growth of colloidal semiconductor nanocrystals remain poorly understood. To provide insight into the subject, we have preformed a comprehensive study of the polar (0001) and (0001) and nonpolar (1120) wurtzite CdSe surfaces that are exposed during crystal growth using first-principles density functional theory (DFT-GGA) calculations. Stabilization of these surfaces by relaxation and reconstruction was considered. Two particular reconstructions of the polar surfaces were examined: vacancy formation on a 2 x 2 unit cell and addition of Se and Cd atoms on the (0001) and (0001) surfaces, respectively. Calculation results indicate that the (1120) is the most stable surface when compared to the two polar surfaces. Furthermore, reconstructions of the (0001) surface are energetically favored when compared to reconstructions of the (0001) facet. Adsorption of Cd and Se atoms and the CdSe molecule on the three relaxed surfaces and two reconstructed (0001) surfaces were also investigated. Several binding sites were considered to determine the most stable binding geometries and energetics. Atomic species preferentially bind in either 2-fold or 3-fold sites, while the CdSe molecule binds parallel to the surface on all of the considered surfaces. Vibrational frequencies of the adspecies were calculated for the most stable binding configurations and were included in the zero point energy correction. Diffusion barriers for the atomic and molecular species were estimated where possible to be between 0.2 and 0.4 eV on the three relaxed surfaces. Thermochemistry of the CdSe molecule binding and dissociation was also investigated. On all considered surfaces, dissociation is preferred to desorption with dissociation only exothermic on the (0001) surface. Comparison of the three relaxed and two reconstructed surfaces indicates that CdSe molecule binding and dissociation is thermodynamically favored on the (0001) surface. This implies that under a reaction-controlled regime, the rate of homoepitaxy would be faster on the (0001) Se terminated surface than on the (0001) and (1120) surfaces, making the (0001) surface of a nanocrystal the primary direction of growth.

Single quantum dot (QD) imaging of fluid flow near surfaces

Abstract: We introduce the use of quantum dot (QD) nanoparticles for near-surface velocimetry and provide preliminary data to demonstrate its feasibility. Evanescent wave illumination is used to image the motion of water-soluble (CdSe)ZnS QDs with a core size of 6 nm within a region of order 100 nm of a surface . Results are presented for the two in-plane components of the velocity field.

Method for fabrication of saturated RGB quantum dot light-emitting devices

Abstract: Creation of patterned, efficient, and saturated color hybrid organic/inorganic quantum dot light emitting devices (QD-LEDs) is dependent on development of integrated fabrication and patterning methods for the QD layer. We show that micro-contact printing can be applied to QD deposition, generating micron-scale pattern definition, needed in pixilated-display applications. We demonstrate saturated color QD-LEDs with external quantum efficiencies in excess of 1%. Combining this technique with the use of wide optical band gap host materials, and a new synthetic route for the creation of blue emitting (CdS)ZnS nanocrystals, it is now possible to fabricate QD-LEDs with saturated color emission in the red, green and blue regions of the spectrum.

Observation of the dark exciton in CdSe quantum dots

Abstract: We use external magnetic fields to identify the band edge emitting statc in CdSe quantum dms. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk. is strongly enhanced by quantum confinement and allows the observation of an optically passive "dark" excitonic state.

Lévy statistics and anomalous transport in quantum-dot arrays

Abstract: A novel model of transport is proposed to explain power law current transients and memory phe- nomena 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 measure- ments that agree well with the predicted non-Poissonian fluctuations in current, and discuss possible mechanisms leading to this behavior.

Self-organization of CdSe nanocrystallites into three-dimensional quantum dot superlattices

Abstract: The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superiattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.

Optical feedback structures and methods of making

Abstract: An optical resonator can include an optical feedback structure (130)disposed on a substrate (110), and a composite (140) including a matrix including a chromophore. (150). The composite is disposed on the substrate and in optical communication with the optical feedback structure. The chromophore can be a semiconductor nanocrystal. The resonator can provide laser emission when excited.

Non-linear transduction strategies for chemo/biosensing on small length scales

Abstract: As the function of optically-based chemo/biosensors (CBsensors) evolves towards micro- and nanoscale dimensions, the sensitivity of the device is compromised because there are simply too few reporting active sites. We see this issue as the fundamental challenge confronting the design of small length scale CBsensors, providing an imperative for the development of new optical transduction strategies. To address this challenge, we are currently fabricating: (i) new materials that will amplify the primary optical signal from the CBsensing active site in micro- and nano-environments and (ii) new sensing elements and devices to achieve high gain from a non-linear optical response. We present here the synthesis and characterization of two platforms poised for sensing by non-linear transduction.