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We describe the synthesis and three-dimensional structure of a new single-crystalline "hexameric" nanocrystal composed of six near-spherical PbSe nanocrystals arranged at the vertices of an octahedron. We examine the detailed three-dimensional structure of these nanocrystals using electron tomography and demonstrate single-crystal to single-crystal cation exchange to CdSe. We reveal that the growth of these nanocrystals, which form under conditions similar to other anisotropic PbSe nanocrystals, depends on the initial presence of lead oleate particles with approximate diameters of 1.7–3.1 nm that form upon heating lead(II) acetate hydrate in the presence of oleic acid. These lead oleate particles, which are visible by transmission electron microscopy, constitute the beginning of nearly every synthesis of anisotropic PbSe nanocrystals. We show that the lead oleate particles play a definitive role in determining the morphology of the resultant PbSe nanocrystals. We note that the acetate anion, which was previously identified as the key factor in achieving anisotropic PbSe growth, greatly accelerates the formation of the lead oleate particles, and thus appears to be responsible for the subsequent PbSe morphology. However, we demonstrate that acetate is not required for lead oleate particle formation, nor indeed for anisotropic PbSe growth. The potential role of these new particles in other PbSe synthetic preparations from lead(II) oleate is of high interest for future study.

First principles electronic structure simulations are used to study the atomistic detail of the interaction between organic surfactant molecules and the surfaces of CdSe semiconductor nanoparticles. These calculations provide insights into the relaxed atomic geometry of organics bound to semiconductor surfaces at the nanoscale as well as the electronic charge transfer between surface atoms and the organics. We calculate the binding energy of phosphine oxide, phosphonic and carboxylic acids, and amine ligands to a range of CdSe nanoparticle facets. The dominant binding interaction is between oxygen atoms in the ligands and cadmium atoms on the nanoparticle surfaces. The most strongly bound ligands are phosphonic acid molecules, which bind preferentially to the facets forming the sides of CdSe nanorods. The calculated relative binding strengths of ligands to different facets support the hypothesis that these binding energies control the relative growth rates of different facets, and therefore the resulting geometry of the nanoparticles.

Localization of carrier wave functions to the quantum-well portion of the CdS/HgS quantum-dot quantum well (QDQW) is investigated. Nanosecond hole-burning (HB) spectra measure the photoinduced exciton coupling to a ${250\ensuremath{-}\mathrm{c}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}$ HgS phonon mode indicative of localization. Femtosecond pump-probe spectroscopy of these QDQW, however, show the photoinduced exciton couples to coherent ${300\ensuremath{-}\mathrm{c}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}$ CdS longitudinal optical-phonon modes, which is indicative of delocalization throughout the QDQW. Femtosecond HB and three pulse pump-dump experiments reveal these results are dependent on the time scale of the experiment. These experiments indicate that the initially photoexcited electron and hole wave functions are weakly confined to the HgS monolayer. Only after long times (\ensuremath{\sim}400 fs) will the exciton localize to the HgS well. These results indicate that the primary optical interaction excites electrons from a delocalized QDQW ground state and not from a localized HgS well state.

Branched mesoporous silica SBA-15 materials have been prepared in a simple process using non-ionic surfactant in acidic conditions in the presence of metal ions.

The mechanism of formation of recently fabricated CdS-Ag(2)S nanorod superlattices is considered and their elastic properties are predicted theoretically based on experimental structural data. We consider different possible mechanisms for the spontaneous ordering observed in these 1D nanostructures, such as diffusion-limited growth and ordering due to epitaxial strain. A simplified model suggests that diffusion-limited growth partially contributes to the observed ordering, but cannot account for the full extent of the ordering alone. The elastic properties of bulk Ag(2)S are predicted using a first principles method and are fed into a classical valence force field (VFF) model of the nanostructure. The VFF results show significant repulsion between Ag(2)S segments, strongly suggesting that the interplay between the chemical interface energy and strain due to the lattice mismatch between the two materials drives the spontaneous pattern formation.

Colloidal metal phosphide nanocrystals have been extensively explored for energy conversion, optoelectronic devices, and bioimaging applications. However, colloidal synthesis to achieve precise control over size, morphology, and composition remains extremely challenging for these phosphide nanocrystals. This is partly due to the unbalanced reactivity of the metal and phosphide precursors and a poor understanding of their formation mechanisms. Herein, we have utilized defined prenucleation metal-aminophosphine-alkylamine complexes as precursors for the highly tunable synthesis of colloidal metal phosphide nanomaterials with accurate control over morphology and composition. We show that precise control over morphology and composition can be achieved using a highly customizable prenucleation-complex-based method for metal phosphide nanocrystals. Using 31P-nuclear magnetic resonance spectroscopy and mass spectrometry, we reveal the prenucleation complex formation mechanisms and provide a structural analysis of the M-P (M = Cu, Co, Cd, and Ni) complexes. Moreover, we investigated the morphology and structure of the phosphide nanocrystals using electron microscopy and X-ray diffraction experiments. We have further used first-principles calculations and boundary element method-based simulations to gain insights into the structural evolution of these nanocrystals as well as into the impact of the morphology and composition on their plasmonic response. We anticipate that the generalized prenucleation-based colloidal metal phosphide synthesis with precise control of size, shape, and composition could pave the way to yield high-quality metal phosphide nanocrystals with tailored chemical and optoelectronic properties.

For over a decade, much effort has been focused on passivation of the high density of localized electronic trap states in colloidal semiconductor quantum dots (QDs), which lead to reduced performance in solar cell, light-emitting diode, laser, and photoconductor applications. However, here we take advantage of the naturally occurring high density of trap states to demonstrate solution-processed high-speed PbSe quantum dot near-infrared photodetectors. Carrier transport dynamics studies reveal multiple trapping and release transport dynamics in band tail states. A sandwich microstrip transmission line photodetector utilizing these QD films was fabricated to achieve high performance by allowing carriers to be swept to the electrodes before they fall into the band tail states. As a result, this device demonstrates external quantum efficiency, responsivity, and response time (full width at half-maximum) of 54%, 0.36 A/W, and 74 ps, respectively.

Lead halide perovskite nanocrystals have attracted intense interest due to their desirable optical properties, diverse structural features, and size-tunable excitonic structure. Here we show that, under ambient conditions, a non-monotonic trend in radiative lifetime emerges from the interplay of size, lattice symmetry and excitonic structure. Small nanocrystals exhibit long radiative lifetimes due to weakly emissive excitons, but the oscillator strength increases and shortens the lifetime for nanocrystals approaching intermediate confinement. For larger nanocrystals with higher exciton density of states (DOS), the radiative lifetime is lengthened due to depopulation of the bright exciton manifold into thermally accessible dim states. A size-dependent structural symmetry lowering transition from cubic to orthorhombic is observed by XRD and MD simulations, and the non-monotonic radiative lifetime trend emerges only in lower symmetry structures with an increased dim exciton DOS. These findings shed light on the impact of nanocrystal size and structure on radiative lifetime and pave the way for tailored optical materials in various optical applications.

We describe the synthesis of water-soluble semiconductor nanoparticles and discuss and characterize their properties. Hydrophobic CdSe/ZnS core/shell nanocrystals with a core size between 2 and 5 nm are embedded in a siloxane shell and functionalized with thiol and/or amine groups. Structural characterization by AFM indicates that the siloxane shell is 1−5 nm thick, yielding final particle sizes of 6−17 nm, depending on the initial CdSe core size. The silica coating does not significantly modify the optical properties of the nanocrystals. Their fluorescence emission is about 32−35 nm fwhm and can be tuned from blue to red with quantum yields up to 18%, mainly determined by the quantum yield of the underlying CdSe/ZnS nanocrystals. Silanized nanocrystals exhibit enhanced photochemical stability over organic fluorophores. They also display high stability in buffers at physiological conditions (>150 mM NaCl). The introduction of functionalized groups onto the siloxane surface would permit the conjugation of the nanocrystals to biological entities.

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionSymmetry of Annealed Wurtzite CdSe Nanocrystals: Assignment to the C3v Point GroupJ. J. Shiang, A. V. Kadavanich, R. K. Grubbs, and A. P. AlivisatosCite this: J. Phys. Chem. 1996, 100, 32, 13886Publication Date (Web):August 8, 1996Publication History Published online8 August 1996Published inissue 1 January 1996https://pubs.acs.org/doi/10.1021/jp963478rhttps://doi.org/10.1021/jp963478rcorrectionACS PublicationsCopyright © 1996 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views482Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (131 KB) Get e-Alerts

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.