Sameer Sapra

Evolution of the electronic structure with size in II-VI semiconductor nanocrystals

In order to provide a quantitatively accurate description of the band-gap variation with sizes in various II-VI semiconductor nanocrystals, we make use of the recently reported tight-binding parametrization of the corresponding bulk systems. Using the same tight-binding scheme and parameters, we calculate the electronic structure of II-VI nanocrystals in real space with sizes ranging between 5 and 80 A in diameter. A comparison with available experimental results from the literature shows an excellent agreement over the entire range of sizes.

Synthesis and characterization of mn-doped zno nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

Understanding the quantum size effects in ZnO nanocrystals

In the present work, we report the synthesis of high quality ZnO nanocrystals with sharp absorption edges in four different sizes, namely 3.0, 3.5, 4.7 and 5.4 nm, characterized by X-ray and electron diffraction, as well as transmission electron microscopy. The bandgaps of these samples, in conjunction with further data from the published literature, exhibit a systematic dependence on the nanocrystal size. In absence of any prior reliable theoretical results in the literature to understand this dependence quantitatively, we have analyzed for the first time, the electronic structure of bulk ZnO obtained from the full potential linearized augmented plane wave method using fatbands, density of states and partial density of states. The crystal orbital Hamiltonian population is obtained from linearized Muffin-Tin orbital band structure calculations to understand the range of hopping interactions relevant for an accurate description of the electronic structure. Using these analyses, a realistic tight binding model is proposed. Based on this model, we calculate the variation of the bandgap with the size of ZnO nanocrystals. These theoretical results agree well with all available data over the entire range of sizes, establishing the effectiveness of this approach.

Real-time magnetic resonance imaging and quantification of lipoprotein metabolism in vivo using nanocrystals

Semiconductor quantum dots and superparamagnetic iron oxide nanocrystals have physical properties that are well suited for biomedical imaging. Previously, we have shown that iron oxide nanocrystals embedded within the lipid core of micelles show optimized characteristics for quantitative imaging. Here, we embed quantum dots and superparamagnetic iron oxide nanocrystals in the core of lipoproteins--micelles that transport lipids and other hydrophobic substances in the blood--and show that it is possible to image and quantify the kinetics of lipoprotein metabolism in vivo using fluorescence and dynamic magnetic resonance imaging. The lipoproteins were taken up by liver cells in wild-type mice and displayed defective clearance in knock-out mice lacking a lipoprotein receptor or its ligand, indicating that the nanocrystals did not influence the specificity of the metabolic process. Using this strategy it is possible to study the clearance of lipoproteins in metabolic disorders and to improve the contrast in clinical imaging.

PHOSPHINE-FREE SYNTHESIS OF MONODISPERSE CDSE NANOCRYSTALS IN OLIVE OIL

A new method that uses inexpensive and harmless olive oil as the coordinating solvent and eliminates the need for air-sensitive and toxic trioctylphosphine (TOP), which is commonly used for CdSe nanocrystal synthesis, is presented for the synthesis of highly monodispersed CdSe nanocrystals ranging in diameter between 2.3–6.0 nm which emit over the broad spectral range from 485 to 640 nm with quantum efficiencies of purely band-edge luminescence (FWHM 24–31 nm) as high as 15%.

Dual-color emitting quantum-dot-quantum-well CdSe-ZnS heteronanocrystals hybridized on InGaN/GaN light emitting diodes for high-quality white light generation

We report white light generation by hybridizing green-red emitting (CdSe)ZnS∕CdSe (core)shell/shell quantum-dot-quantum-well heteronanocrystals on blue InGaN∕GaN light emitting diodes with the photometric properties of tristimulus coordinates (x,y)=(0.36,0.30), luminous efficacy of optical radiation LE=278lm∕W, correlated color temperature CCT=3929K, and color-rendering index CRI=75.1. We present the photometric analysis and the quantum mechanical design of these dual-color emitting heteronanocrystals synthesized to achieve high-quality white light when hybridized on light emitting diodes. Using such multicolor emitting heteronanocrystals facilitates simple device implementation while providing good photometric properties.

Realistic tight-binding model for the electronic structure of II-VI semiconductors

We analyze the electronic structure of group II-VI semiconductors obtained within linearized muffin-tin-orbital approach in order to arrive at a realistic and minimal tight-binding model, parametrized to provide an accurate description of both valence and conduction bands. It is shown that a nearest-neighbor

Electronic structure of and quantum size effect in III-V and II-VI semiconducting nanocrystals using a realistic tight binding approach

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Simultaneous control of nanocrystal size and nanocrystal-nanocrystal separation in CdS nanocrystal assembly

model is fairly sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any fictitious

White emitting CdS quantum dot nanoluminophores hybridized on near-ultraviolet LEDs for high-quality white light generation and tuning

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