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  • 1995-1999  (8)
  • 1990-1994  (11)
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  • 1
    Electronic Resource
    Electronic Resource
    Magnetic circular dichroism study of CdSe quantum dots (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4242-4247 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475823
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  • 2
    Electronic Resource
    Electronic Resource
    The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9869-9882 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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-ethylhexyl)phosphate. The lack of dependence of our data on surface modification is consistent with a dark exciton description of the band edge luminescence. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473875
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  • 3
    Electronic Resource
    Electronic Resource
    Characterization of CdSe nanocrystallite dispersions by small angle x-ray scattering (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 9890-9896 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use x-ray scattering at small angles (SAXS) to study interparticle interactions in dilute dispersions of CdSe nanocrystallite particles. In particular, we probe the dependence of these interactions on a few key parameters, including the nature of the cap molecules attached to the surface of the particles and the solvent. Alkyl capped nanocrystallites with relatively long hydrocarbon chains are stable in a range of solvents. Nanocrystallites capped with smaller molecules show attractive interparticle interactions. Existence of association (e.g., dimers and trimers) in dispersions characterized by attractive interactions is observed in the scattering data for pyridine capped particles. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472854
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  • 4
    Electronic Resource
    Electronic Resource
    Laboratory observation of hot bands of H+3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6200-6209 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The (2ν2,l=2←ν2), (2ν2,l=0←ν2), and (ν1+ν2←ν1) hot bands of H+3 were observed. The vibrationally hot ions were produced in a liquid nitrogen cooled 6 kHz ac discharge using gas mixtures of H2 and He. The spectra were detected in direct absorption using a newly extended tunable difference frequency spectrometer using both LiNbO3 and LiIO3 crystals as nonlinear optical elements. The range of this spectrometer is now ∼5300–∼1900 cm−1. The positions of the rovibrational transitions compare extremely well with the theoretical predictions of Miller and Tennyson. A vibrational temperature study of the discharge indicates a significant population inversion between the ν1 and ν2 levels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458989
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  • 5
    Electronic Resource
    Electronic Resource
    Luminescence properties of CdSe quantum crystallites: Resonance between interior and surface localized states (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 946-954 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use time-, wavelength-, temperature-, polarization-resolved luminescence to elucidate the nature of the absorbing and "band edge'' luminescing states in 32 A(ring) diameter wurtzite CdSe quantum crystallites. Time-resolved emission following picosecond size-selective resonant excitation of the lowest excited state shows two components—a temperature insensitive 100 ps component and a microsecond, temperature sensitive component. The emission spectrum, showing optic phonon vibrational structure, develops a ∼70 wave number red shift as the fast component decays. Photoselection shows the slow component to be reverse polarized at 10 K, indicating this component to be the result of a hole radiationless transition. The 100 ps emitting state is repopulated thermally as temperature increases from 10 to 50 K. All available data are interpreted by postulating strong resonant mixing between a standing wave molecular orbital delocalized inside the crystallite and intrinsic surface Se lone pair states. The apparent exciton transition is assigned to a ∼130 wave number wide band of eigenstates with the hole localized principally on the surface. The band contains strongly emitting "doorway'' states and weakly emitting "background'' states. The hole becomes mobile among these states as T increases to 50 K. It is suggested that such resonant mixing may be general in II–VI and III–V crystallites.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462114
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  • 6
    Electronic Resource
    Electronic Resource
    Electroluminescence from CdSe quantum-dot/polymer composites (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 1316-1318 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electroluminescence is obtained from nearly monodisperse CdSe nanocrystallites (quantum dots) incorporated into thin films (1000 A(ring)) of polyvinylcarbazole (PVK) and an oxadiazole derivative (t-Bu-PBD) and sandwiched between ITO and Al electrodes. The electroluminescence and photoluminescence spectra (bandwidths ≤40 nm) are nearly identical at room temperature and are tunable from ∼530 to ∼650 nm by varying the size of the dots. Voltage studies at 77 K 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. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.113227
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  • 7
    Electronic Resource
    Electronic Resource
    Electrospray organometallic chemical vapor deposition—A novel technique for preparation of II–VI quantum dot composites (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2795-2797 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.112568
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  • 8
    Electronic Resource
    Electronic Resource
    Investigation of the surface morphology of capped CdSe nanocrystallites by 31P nuclear magnetic resonance (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3297-3300 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466420
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  • 9
    Electronic Resource
    Electronic Resource
    Stark spectroscopy of CdSe nanocrystallites: The significance of transition linewidths (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5236-5245 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use Stark spectroscopy to examine the nature of the excited states of CdSe nanocrystallites. The Stark spectra we obtain are in the small coupling limit in which the changes induced by the electric field to the absorption spectrum are small compared to the transition linewidths. Within this limit, we theoretically examine the dependence of the line shape of Stark difference spectra on the linewidth of the transitions involved. For systems such as CdSe nanocrystallites, which have overlapping transitions coupled by the electric field, we find that the usual association of derivatives of absorption features with dipole moments and polarizabilities is problematic. We show that the Stark absorption spectrum of the CdSe nanocrystallites can be explained by polarizable and delocalized nonpolar excited states. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470559
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  • 10
    Electronic Resource
    Electronic Resource
    Structure in the lowest absorption feature of CdSe quantum dots (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5260-5268 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We use transient differential absorption experiments to investigate the "single dot'' absorption line shape of CdSe quantum dots. We observe both a narrow (full width half maximum ∼5 meV) and a broad (∼50 meV) bleach component within the inhomogeneously broadened first absorption line of our samples. We deduce the single dot absorption line shape which is most consistent with the experimental results. This line shape, which contains structure in the lowest quantum dot absorption feature, explains the large "Stokes'' shift seen in the full band edge luminescence of CdSe quantum dots. We discuss the structure within the context of several competing models. The data appear inconsistent with models which use surface states to explain the anomalous emission behavior of II–VI quantum dots. Instead they imply that exciton fine structure is observed in our samples. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470561
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