Showing papers by "Moungi G. Bawendi published in 1994"

Measurement of the size dependent hole spectrum in CdSe quantum dots

Abstract: We combine a new synthesis with transient optical hole burning to observe and assign the size evolution (19 to 115 \AA{} diameter, \ensuremath{\sigma}5%) of a series of excited states in CdSe quantum dots. We observe an avoided crossing around the spin orbit energy in the hole spectra for \ensuremath{\sim}65 \AA{} dots, indicating the importance of valence band complexities in the description of the excited states. Comparsion with dc Stark data shows that bleach spectra are consistent with localized carrier induced electric fields.

Fluorescence-line narrowing in CdSe quantum dots: Surface localization of the photogenerated exciton

Abstract: The electronic properties of shallow band-edge surface traps in nanometer-size CdSe quantum dots are probed using fluorescence-line-narrowing spectroscopy. We find large changes in electron-hole-pair radiative lifetimes and couplings to LO phonons as the temperature is changed from 1.75 to 10 K. We attribute these changes to the localization of the photogenerated hole at the surface of the dots, accompanied by thermally activated motion between these surface localized states. A simple model based on the observed exciton--LO-phonon couplings is constructed to estimate the extent of hole localization in the luminescing state. A size-dependent study (20--80 \AA{} diameter) indicates that surface effects diminish rapidly with increasing size.

Investigation of the surface morphology of capped CdSe nanocrystallites by 31P nuclear magnetic resonance

Abstract: Room temperature 31P magic angle sample spinning (MAS) nuclear magnetic resonance (NMR) is used to study the phosphine chalconide species coordinating the surface of CdSe nanocrystallites. Two surface species are identified: trioctylphosphine oxide (TOPO) and trioctylphosphine selenide (TOPSe). The TOPO coordinates Cd surface sites while trioctylphosphine (TOP) coordinates Se to produce surface TOPSe species. Chemical removal of TOPSe linkages produces CdSe nanocrystallites passivated exclusively with TOPO. A double resonance experiment (31P and 77Se) characterizes the TOPSe species. Quantitative studies indicate coordination to nearly all surface Cd atoms and allows the development of a model for the morphology of an average crystallite surface.

Electrospray organometallic chemical vapor deposition—A novel technique for preparation of II–VI quantum dot composites

Abstract: A novel technique combining electrospray and organometallic chemical vapor deposition (OMCVD) has been developed for the synthesis of new II–VI quantum dot composites. CdSe nanocrystals (quantum dots) of selected size are dispersed in a pyridine/acetonitrile mixture. The nanocrystals are transferred by electrospray into the growth zone of an OMCVD reactor and codeposited on a ZnSe matrix grown from hydrogen selenide and diethyl zinc. Composites consisting of CdSe nanocrystals and an amorphous or polycrystalline ZnSe matrix have been deposited on glass substrates at temperatures of 150–250 °C. Room‐temperature absorption and photoluminescence spectra show optical transitions characteristic of the initial nanocrystal dispersions. The emission wavelength may be tuned in a broad spectral region by incorporating nanocrystals of varying sizes. The composites have been characterized by Auger electron spectroscopy, high resolution transmission electron microscopy, and scanning transmission electron microscopy.

Size Dependent Electroluminescence from CdSe Nanocrystallites (Quantum Dots)

Abstract: We obtain spectrally narrow (FWHM < 40 nm) electroluminescence from nearly monodisperse CdSe nanocrystallites (quantum dots) incorporated into thin films of polyvinyl carbazole (PVK) and an oxadiazole derivative (PBD) sandwiched between aluminum and ITO electrodes. The electroluminescence and photoluminescence spectra are nearly identical at room temperature and are tunable from ~530 nm to ~650 nm by varying the size of the dots. Voltage studies at 77K indicate that while only the dots electroluminesce at the lower voltages, both the dots and the PVK matrix electroluminesce at higher applied voltages. Variable temperature studies indicate that the electroluminescence efficiency increases substantially as the films are cooled down to cryogenic temperatures.

Synthesis, Structural Characterization, and Optical Spectroscopy of Close Packed CdSE Nanocrystallites

Abstract: We describe a method to synthesize optical quality thin and thick films of close packed CdSe nanocrystallites (quantum dots). The average dot size is tunable from ≈12-150 A in diameter with ≈3.5% rms size distribution. The identity of each particle and the high monodispersity of the sample are maintained in the films as revealed by small-angle x-ray scattering, transmission electron microscopy, and optical spectroscopy. We use small-angle x-ray diffraction patterns to obtain form factors for the individual dots and to generate radial distribution functions for the densely packed films. We observe the random close packing of dots in the solid state using transmission electron microscopy. Comparing optical spectra of particles close packed in a film with those of nanocrystallites dispersed in alkanes reveals similar optical resonances in absorption while the emission lineshape is modified and its position is red shifted. Differences in film emission are consistent with electronic energy transfer between close packed dots within the inhomogeneous distribution of the sample.