Crystalline nanoflowers with different chemical compositions and physical properties grown by limited ligand protection.
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Biomedical subjects
Narayan Pradhan
Publications and source records attributed to Narayan Pradhan.
Formation of nearly monodisperse In2O3 nanodots and oriented-attached nanoflowers: hydrolysis and alcoholysis vs pyrolysis.
Single crystalline and nearly monodisperse In2O3 nanocrystals with both dot and flower shapes were synthesized in a simple reaction system. This system used indium carboxylates as the precursors with or without alcohol as the activating reagents in a hydrocarbon solvent under elevated temperatures. Limited ligand protection (LLP) led to three-dimensional (3D) oriented attachment of nanodots, resulting in 3D nanoflowers. When the system had sufficient ligand protection for the nanocrystals, nanodots were found to be the stable products. The diameters of nearly monodisperse nanodots and nanoflowers were varied in a range from approximately 5 to approximately 15 nm and approximately 15 to approximately 60 nm, respectively. The simple reaction system made it possible to have a systematic study of the reaction mechanisms along with the growth kinetics of nanocrystals. Hydrolysis and alcoholysis were identified as the major paths for this system, as opposed to pyrolysis. Both nearly monodispersed nanodots and nanoflowers can be made through either of the reaction pathways. Hydrolysis was found as a reversible pathway, and alcoholysis was confirmed to be irreversible. Consequently, a sufficient amount of alcohol was able to force the yield of nanocrystals, both dots and flowers, to unity.
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Colloidal CdSe quantum wires by oriented attachment.
We report here a relatively low temperature (100-180 degrees C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as "pearl-necklace" or "string-of-pearls" in the literature, which are loosely associated and chemically fused sections.
Anisotropy↗
An alternative of CdSe nanocrystal emitters: pure and tunable impurity emissions in ZnSe nanocrystals.
The concept, decoupling doping from nucleation and/or growth, allows us to dope nearly all nanocrystals in a given sample which is indicated by complete quenching of the host emission and bright emission from the dopants at characteristic wavelengths tunable in most parts of the visible window using a ZnSe host. In an extreme case, ZnSe coated MnSe nanocrystals (MnSe:ZnSe) emit similarly as commonly known doped nanocrystals. In comparison with CdSe nanocrystals, these alternative emitters not only are intrinsically less toxic but also show some unexpected and expected advantages: stable against thermal and environmental changes, zero reabsorption, and no Forster energy transfer. In addition to their applications to replace CdSe based nanocrystal emitters, the unique structure and properties of the doped nanocrystals are of interest for studying fundamental issues in the field.
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Single-precursor, one-pot versatile synthesis under near ambient conditions of tunable, single and dual band fluorescing metal sulfide nanoparticles.
We present a simple and versatile method for the synthesis of high-quality size-controlled metal sulfide nanoparticles. A single compound (metal xanthate) is the precursor. A Lewis-base solvent is used to achieve a low reaction temperature of 50-150 degrees C, usually in air. Demonstrated with CdS, the precise control over the particle size (by regulating the temperature or the concentration) enables tuning the absorption and emission spectra of the particles. We also can control the relative intensity of the narrow (30-35 nm wide) excitonic emission (tunable in the range 430-480 nm with approximately 2% fluorescence quantum efficiency) and the broad emission associated with deep surface traps (in the range 550-700 nm). Using the same precursor CdS/ZnS core/shell particles are produced with a high PL yield ( approximately 14%).
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