Showing papers by "Moungi G. Bawendi published in 1997"

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

Abstract: We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

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.

The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state

Abstract: We study the luminescence of surface modified CdSe nanocrystallites There has been much speculation as to the origin of the band edge emission in these quantum confined structures Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments Our results are compared with theoretical predictions for the nonresonant and resonant luminescence We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-eth

Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites

Abstract: We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 A in diameter, embedded in a ZnS matrix. The thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470–650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied.

Fabrication of Quantum Dot-Polymer Composites: Semiconductor Nanoclusters in Dual-Function Polymer Matrices with Electron-Transporting and Cluster-Passivating Properties

Abstract: Hybrid inorganic−organic polymer composites have been prepared by a convergent approach in which nearly monodisperse CdSe or (CdSe)ZnS nanoclusters are sequestered within phosphine-containing domains in a charge-transporting matrix. The motivation for these studies is the potential utility of such composites as combined electron-transport and emitter layers in light-emitting devices. Diblock copolymers with electronically passivating and charge-transport capabilities were prepared via ring-opening metathesis polymerization of octylphosphine- and oxadiazole-functionalized norbornenes. Independently prepared CdSe and ZnS-overcoated CdSe nanoclusters, surface-passivated by trioctylphosphine and trioctylphosphine oxide groups, are tethered by polymer-bound phosphine donors, resulting in immediate, sustained increases in fluorescence. Thin films of the CdSe-block copolymer composites, static-cast from dilute solution, exhibit microphase separation, with segregation of nanoclusters within phosphine-rich microdo...