Showing papers by "Moungi G. Bawendi published in 2002"

Electroluminescence from single monolayers of nanocrystals in molecular organic devices

Abstract: The integration of organic and inorganic materials at the nanometre scale into hybrid optoelectronic structures enables active devices that combine the diversity of organic materials with the high-performance electronic and optical properties of inorganic nanocrystals. The optimization of such hybrid devices ultimately depends upon the precise positioning of the functionally distinct materials. Previous studies have already emphasized that this is a challenge, owing to the lack of well-developed nanometre-scale fabrication techniques. Here we demonstrate a hybrid light-emitting diode (LED) that combines the ease of processability of organic materials with the narrow-band, efficient luminescence of colloidal quantum dots (QDs). To isolate the luminescence processes from charge conduction, we fabricate a quantum-dot LED (QD-LED) that contains only a single monolayer of QDs, sandwiched between two organic thin films. This is achieved by a method that uses material phase segregation between the QD aliphatic capping groups and the aromatic organic materials. In our devices, where QDs function exclusively as lumophores, we observe a 25-fold improvement in luminescence efficiency (1.6 cd A(-1) at 2,000 cd m(-2)) over the best previous QD-LED results. The reproducibility and precision of our phase-segregation approach suggests that this technique could be widely applicable to the fabrication of other hybrid organic/inorganic devices.

On the Absorption Cross Section of CdSe Nanocrystal Quantum Dots

Abstract: The linear absorption cross section is a crucial parameter to the design of nanocrystal quantum dot devices and to the interpretation of spectroscopic data. We measure and report the size-dependent absorption cross section of CdSe nanocrystal quantum dots. We compare the results for absorption far above the band edge, where the quantum dot density of states may be approximated as a continuum, to simple theoretical models of light scattering from light-absorbing small particles. Excellent agreement with theory is found for dilute dispersions in hexane. We find that for absorption at 350 nm the per particle absorption cross section Cabs (in cm2) for CdSe is Cabs = (5.501 × 105)a3 cm-1, where a is the particle radius in cm. The absorption cross section is observed to be largely insensitive to the solvent refractive index. Detailed modeling of the effect of the ligand shell may be necessary to understand the lack of sensitivity of the absorption properties of nanocrystal quantum dots to the refractive index o...

Surface-Enhanced Emission from Single Semiconductor Nanocrystals

Abstract: The fluorescence behavior of single CdSe(ZnS) core-shell nanocrystal (NC) quantum dots is dramatically affected by electromagnetic interactions with a rough metal film. Observed changes include a fivefold increase in the observed fluorescence intensity of single NCs, a striking reduction in their fluorescence blinking behavior, complete conversion of the emission polarization to linear, and single NC exciton lifetimes that are >10(3) times faster. The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.

Color-selective semiconductor nanocrystal laser

Abstract: Theoretical predictions of the benefits of three-dimensional quantum confinement have provided motivation for the development of quantum-dot lasers. Such lasers, developed in the case of self-assembled quantum dots, have not been successfully demonstrated with quantum-confined colloidal nanocrystals (NCs). Here, using recently developed NC-titania chemistry, we report the successful development of an optically pumped, NC-based distributed feedback laser, in which the narrow gain profiles of these nanoparticles have been matched with the feedback of a second-order distributed feedback laser. This laser, whose output color can be selected by choosing appropriately sized nanocrystals, operates at 80 K and at room temperature.

From amplified spontaneous emission to microring lasing using nanocrystal quantum dot solids

Abstract: We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.

Transport properties of annealed CdSe colloidal nanocrystal solids

Abstract: Transport properties of artificial solids composed of colloidal CdSe nanocrystals (NCs) are studied from 6 to 250 K, before and after annealing. After the solids are annealed, three changes are observed. First, transmission electron micrographs show that the separation between NCs decreases with annealing. Second, the optical absorption spectrum changes: the excitonic peaks of the NC solids shift to lower energies and broaden with annealing. These redshifts can result from the change of the dielectric environment around the NCs. Last, annealing results in greatly enhanced dark current and photocurrent. This increased current can be attributed to the enhancement of interdot tunneling caused by the decreased separation between NCs and by chemical changes in their organic cap. In addition, the dark current is an exponential function of the applied electric field and it is only weakly temperature dependent. Our measurements also suggest that Coulomb interactions between charges on neighboring NCs play an impo...

Transport properties of annealed CdSe nanocrystal solids

Abstract: Transport properties of artificial solids composed of colloidal CdSe nanocrystals (NCs) are studied from 6 K to 250 K, before and after annealing. Annealing results in greatly enhanced dark and photocurrent in NC solids, while transmission electron microscopy (TEM) micrographs show that the inter-dot separation decreases. The increased current can be attributed to the enhancement of inter-dot tunneling caused by the decreased separation between NCs and by chemical changes in their organic cap. In addition, the absorption spectra of annealed solids are slightly red-shifted and broadened. These red-shifts may result from the change of the dielectric environment around the NCs. Our measurements also indicate that Coulomb interactions between charges on neighboring NCs play an important role in the tunneling current.

Electronic transport in films of colloidal CdSe nanocrystals

Abstract: We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to ${10}^{6} \mathrm{V}/\mathrm{c}\mathrm{m}$ and times as long as $2\ifmmode\times\else\texttimes\fi{}{10}^{4} \mathrm{s}.$ Although the power-law form of the decay is quite general, there are quantitative variations with temperature, applied field, sample history, and the material parameters of the array. Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field scaling of the current at all times. The observation of extremely long-lived current transients suggests the importance of long-range Coulomb interactions between charges on different nanocrystals.

Multiparticle interactions and stimulated emission in chemically synthesized quantum dots

Abstract: We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.

Semiconductor nanocrystal composite

Abstract: A composite including a plurality of semiconductor nanocrystals distributed in a metal oxide matrix can be used as an optical amplifier, a waveguide or a laser.

Temperature-sensing composition

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.

Semiconductor nanocrystal composite material

Abstract: PROBLEM TO BE SOLVED: To provide a composite material containing a semiconductor nanocrystal. SOLUTION: The composite material includes a plurality of semiconductor nanocrystals taken in inorganic matrix. The inorganic matrix may be metal oxide matrix prepared by, for example, a sol-gel process or other low-temperature matrix formation method. The metal oxide matrix may be either crystallinity or noncrystallinity. The metal oxide matrix may be one, for example, that has no light scattering defect such as a crack. COPYRIGHT: (C)2009,JPO&INPIT