Showing papers by "Moungi G. Bawendi published in 1998"

Quantum Confined Stark Effect in Single CdSe Nanocrystallite Quantum Dots

Abstract: The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state (∼10 5 cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.

Electroluminescence from heterostructures of poly(phenylene vinylene) and inorganic CdSe nanocrystals

Abstract: Electroluminescence (EL) and photoluminescence (PL) from heterostructure thin films made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals are investigated. In these devices, the organic PPV structure is built next to an indium tin oxide anode, using the technique of molecular layer-by-layer sequential adsorption, and serves primarily as the hole transport layer. The inorganic layer, adjacent to an Al electrode, is made of spin cast CdSe nanocrystals, passivated with either organic groups or with a wider band gap semiconductor, e.g., ZnS in the present case. We find that the electroluminescence signal is almost exclusively generated within the inorganic layer, with a very weak contribution from the PPV layer at higher applied voltage. The performance of these heterostructure devices is influenced by the thickness of the dot layer. Lifetime tests reveal promising stability, with devices operating continuously over 50–100 h. Values of the external quantum efficiency, η...

Raman modes of metallic carbon nanotubes

Abstract: The anomalous resonant behavior of the tangential Raman modes of carbon nanotubes has been studied in the critical region of laser energies 1.7‐2.2 eV. The special enhancement of the Raman modes is explained by a model that takes into account the transition between the singularities in the one-dimensional density of electronic states for the metallic nanotubes and the distribution of diameters in the sample. The results agree with direct measurements of the electronic density of states for the metallic nanotubes and establish their association with the specially enhanced high frequency, first-order Raman modes. @S0163-1829~98!50848-X# Resonant Raman spectroscopy is a very useful tool for the characterization of the one-dimensional ~1D! properties of carbon nanotubes. It has been used to study multiwall nanotubes ~MWNT!, 1 single-wall nanotubes ~SWNT!, 2‐5 and was recently examined theoretically. 6 We show here evidence that special tangential phonon modes of metallic carbon nanotubes are enhanced in a narrow range of laser energies between 1.7 and 2.2 eV by electronic transitions between the first singularities in the 1D electronic density of states ~DOS! in the valence and conduction bands v 1!c 1 . This result establishes the association of the specially enhanced highfrequency, tangential modes with the metallic carbon nanotubes.

Water-soluble thiol-capped 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 the formula, SH(CH2)nX, where X is carboxylate or sulfonate. The particle size of the nanocrystal core is in the range of about 12 Å to about 150 Å, with a deviation of less than 10% in the core. The coated nanocrystal exhibits photoluminescence having quantum yields of greater than 10% in wate.

Magnetic circular dichroism study of CdSe quantum dots

Abstract: We study the magnetic circular dichroism (MCD) of exciton states near the band edge of CdSe nanocrystallites (quantum dots) The experiment probes the difference between left and right circularly polarized transitions in the presence of an external magnetic field Analysis of the MCD signal determines the sign and magnitude of the exciton g-factor which is shown to be highly sensitive to the energy band parameters used in the effective mass approximation The observation of theoretically predicted changes in the sign of the exciton g-factor between the first two transitions is in agreement with recent theory describing the presence of fine structure underlying the optical transitions of CdSe nanocrystallites

Highly luminescent color-selective materials

Abstract: A coated nanocrystal capable of light emission includes a substantially monodisperse nanoparticle selected from the group consisting of CdX, where X = S, Se, Te, and an overcoating of ZnY, where Y = S, Se, uniformally deposited thereon, said coated nanoparticle charaterized in that when irradiated the particles exhibit photoluminescence in a narrow spectral range of no greater than about 60 nm, and most preferably 40 nm at full width half max (FWHM). The particle size of the nanocrystallite core is in the range of about 2 nm (20A) to about 12.5 nm (125A), with a deviation of less than 10% in the core. The coated nanocrystal exhibits photoluminescence having quantum yields of greater than 30%.

Electroluminescence and cathodoluminescence from inorganic CdSe nanocrystals embedded in thin films

Abstract: Electroluminescence (EL) from heterostructure devices made of organic poly (phenylene vinylene), PPV, and inorganic semiconductor CdSe nanocrystals have been investigated, along with cathodoluminescence (CL) from thin films of ZnS doped with CdSe-ZnS core-shell nanocrystals. In the EL devices, the organic PPV structure, built next to the anode using the technique of molecular layer-by-layer sequential adsorption, serves as the hole transport layer. The inorganic layer, adjacent to the electrode and made of spin cast CdSe nanocrystals passivated with either organic groups or with a thin layer of ZnS, is the emitting layer. The ZnS host film in the CL devices, built using chemical vapor deposition, serves as the support medium for the dispersed nanocrystals, but also provides additional passivation to the surface of those nanocrystals. We find that the EL and CL signals almost exclusively originate from the inorganic nanocrystal in both cases, i.e., EL comes from the nanocrystal layer in the heterostructure device while CL is generated from the dispersed particles in the composite film. The external EL quantum efficiency, (eta) EL, is not enhanced by the presence of ZnS overcoating, opposed to the observed increase in the photoluminescence (PL) quantum yield. However, we find that the CL emission and its stability are substantially improved by the presence of ZnS around the emitting nanocrystal cores. These observations reflect a difference in the effects of overcoating ont he various luminescence processes. On the one hand, a ZnS overlayer is associated with an additional energetic barrier that reduces the efficiency of charge injection into the nanocrystals for EL. On the other hand, PL and CL processes only benefit from the surface passivation with ZnS.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.