Shape control of monodisperse CdS nanocrystals: hexagon and pyramid

The wurtzite CdS nanocrystals with hexagonal or pyramidal geometries were selectively synthesized by tuning the molar ratio of Cd and S precursors in the solution system. For hexagonal nanocrystals, a 2-D or 3-D superlattice assembly could be obtained due to the narrow particle size distribution. The pyramidal CdS nanocrystals were divided into two geometries: the hexagon-based pyramid and the triangle-based pyramid. The realization of the pyramidal geometries further extends the shape multiformity of wurtzite CdS nanocrystals, which may bring new opportunities for the development of CdS semiconductors. The room-temperature absorption spectra of CdS nanoparticles with hexagonal and pyramidal morphologies exhibited a discrepancy in peak positions, revealing the existence of a profound shape-property relationship for the CdS nanophase.     

Synthesis and assembly of monodisperse spherical Cu2S nanocrystals

High-quality monodisperse Cu₂S nanocrystals (sizes from 2 nm to 20 nm) have been successfully synthesized by the reaction of copper stearate (CuSt₂) and dodecanethiol (DDT) in 1-octadecene (ODE). The nanocrystals were characterized using X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), and transmission electron microscopy (TEM). These as-prepared Cu₂S nanocrystals with certain sizes have been found with good self-assembly behaviors, and they were easily to assemble into two-dimensional and three-dimensional superlattice structures. DDT served as both sulfur source and capping ligand, and was found a key factor to affect the growth and the self-assembly behaviors of the Cu₂S nanocrystals.     

Shape evolution of single-crystalline iron oxide nanocrystals

Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature.     

Synthesis of monodisperse spherical nanocrystals

Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.     

Solvothermal synthesis of monodisperse nanocrystals

Monodisperse nanocrystals have drawn tremendous interests in terms of their fundamental sciences and practical applications. In this review, some recent developments in the solvothermal synthesis of monodisperse nanocrystals are discussed, and we present an outlook on the field.     

Solvothermal synthesis of monodisperse PbSe nanocrystals

Size-controllable monodisperse PbSe and PbSe/PbS nanocrystals (NCs) have been successfully synthesized with a solvothermal method. Octadecylamine (ODA) molecules were found effective in organizing the nanocrystals to form an ordered monolayer. It is expected that these narrow-band-gap semiconductor NCs with tunable size would have potential applications in near- and mid-IR telecommunication laser sources, electroluminescence, and solar cell materials.     

Solvothermal synthesis of PbTe/SnTe hybrid nanocrystals

PbTe/SnTe hybrid nanocrystals with designed shape, chemical composition and narrow size distribution were synthesized by an efficient solvothermal approach. This approach enables mass and economical synthesis of PbTe-based nanocrystals. The organic ligands were completely removed by pretreatment with a super-hydride solution, making it possible to fabricate fully dense and robust thermoelectric devices with increased electrical conductivity.     

Synthesis of monodisperse nanocrystals of manganese oxides

We report a simple and effective method for the generation of high-quality nanocrystals of manganese oxide, MnO, with an alkyl capping group. MnO is a model system for the theoretical study of electronic and magnetic properties of rock salt oxides. Synthesis of relatively monodisperse nanocrystals of MnO was achieved over a range of sizes between 7 and 20 nm. The nanocrystals are readily dispersed in nonpolar solvents, and their uniformity allows for the formation of superlattices, as observed by TEM. Fitting size evolution/time data with the Lifshitz-Slyozov-Wagner model indicates that the increase in particle size from 7 nm is dominated by diffusion-limited growth. Controlled chemical oxidation can allow for the preparation of nanocrystals of Mn3O4 from MnO. We demonstrate the use of acetate precursors in the preparation of ligand-capped, transition metal oxide nanocrystals (of MnO and FeO), which are safer and more environmentally benign than their metal carbonyl counterparts.     

One-pot synthesis of monodisperse CeO2 nanocrystals and superlattices

Monodisperse CeO(2) nanocrystals and superlattice-like colloidal particles have been successfully synthesized in ethanol-water mixed solvent by adopting a one-pot approach using icosahedral (NH(4))(2)Ce(NO(3))(6) as precursor.     

Micro-emulsion synthesis of monodisperse surface stabilized silicon nanocrystals

Silicon nanocrystals with a uniform size distribution were synthesized in inverse micelles using powerful hydride reducing agents. The silicon nanocrystals surfaces were then stabilized with 1-heptene to produce particles with strong blue photoluminescence.     

Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals

A new and simple method has been developed to synthesize large quantities of highly monodisperse tetragonal zirconia nanocrystals. In this synthesis, a nonhydrolytic sol-gel reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride at 340 degrees C generated 4 nm sized zirconia nanoparticles. A high-resolution transmission electron microscopic (HRTEM) image showed that the particles have a uniform particle size distribution and that they are highly crystalline. These monodisperse nanoparticles were synthesized without any size selection process. X-ray diffraction studies combined with Rietveld refinement revealed that the ZrO(2) nanocrystals are the high-temperature tetragonal phase, and very close to a cubic phase. When zirconium(IV) bromide is used as a precursor instead of zirconium chloride, zirconia nanoparticles with an average size of 2.9 nm were obtained. The UV-visible absorption spectrum of 4 nm sized zirconia nanoparticles exhibited a strong absorption starting at around 270 nm. A fluorescence spectrum with excitation at 300 nm showed a broad fluorescence band centered around 370 nm. FTIR spectra showed indication of TOPO binding on the ZrO(2) nanoparticle surface. These optical studies also suggest that the nanoparticles are of high quality in terms of narrow particle size distribution and relatively low density of surface trap states.     

Preparation of monodisperse silicon nanocrystals using density gradient ultracentrifugation

We report the preparation of monodisperse silicon nanocrystals (ncSi) by size-separation of polydisperse alkyl-capped ncSi using organic density gradient ultracentrifugation. The ncSi were synthesized by thermal processing of trichlorosilane-derived sol-gel glasses followed by HF etching and surface passivation with alkyl chains and were subsequently fractionated by size using a self-generating density gradient of 40 wt % 2,4,6-tribromotoluene in chlorobenzene. The isolated monodisperse fractions were characterized by photoluminescence spectroscopy and high-angle annular dark-field scanning transmission electron microscopy and determined to have polydispersity index values between 1.04 and 1.06. The ability to isolate monodisperse ncSi will allow for the quantification of the size-dependent structural, optical, electrical, and biological properties of silicon, which will undoubtedly prove useful for tailoring property-specific optoelectronic and biomedical devices.     

Remarkable amine-TFA self assembly

Supramolecular assemblies that are formed between amines and trifluoroacetic acid were studied using electrospray ionization mass spectrometry. Distinctive association behavior of primary, secondary, and tertiary amines with trifluoroacetic acid upon identical experimental conditions is observed and indicates that steric effects dominate in the formation of these protonated clusters. Extraordinary complexation behavior is observed in the case of R-(+)-alpha-methylbenzylamine and 4-tert-butyl-cyclohexylamine that form high-order clusters. The strong relation between stereochemistry and assembly results in the specific association characteristics of trans 4-tert-butyl-1-phenylcyclohexylamine when compared with the cis isomer. The cis isomer gives rise to a highly abundant [M(4)TFA(3) + H](+) ion (M = amine molecule, TFA = trifluoroacetic acid), as observed for other primary amines. However, the trans isomer generates higher [M(n)TFA(m) + H](+) cluster ions, the largest and most abundant being an [M(7)TFA(6) + H](+) ion. Collision induced dissociation spectra that were recorded for several [M(n)TFA(m) + H](+) cluster ions typically show the consecutive losses of M.TFA moieties. Density functional theory calculations indicate that the highly abundant [M(4)TFA(3) + H](+) clusters are macrocycles and support the formation of these structures with TFA and not with acetic acid.     

Facile synthesis and up-conversion properties of monodisperse rare earth fluoride nanocrystals

Monodisperse rare earth (RE) fluoride colloidal nanocrystals (NCs) including REF(3) (RE = La, Pr, Nd), NaREF(4) (RE = Sm-Ho, Y) and Na(5)RE(9)F(32) (RE = Er, Yb, Lu) have been successfully synthesized by a facile one-step method using oleic acid as surfactant and 1-octadecene as solvent. The phase, morphology, size, and photoluminescence properties of as-synthesized NCs were well investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL) spectra. The results reveal that the as-synthesized NCs consist of monodisperse colloidal NCs with narrow size distribution, which can easily disperse in non-polar cyclohexane solvent. The as-prepared NCs exhibit a rich variety of morphologies and different crystal phases (hexagonal or cubic), which may be related to the inherent natures of different rare earth ions. The possible formation mechanism of NCs with diverse architectures has been presented. In addition, representative Yb/Er, Yb/Tm, or Yb/Ho co-doped NaGdF(4) and Na(5)Lu(9)F(32) NCs exhibit intensive multicolor up-conversion (UC) luminescence under a single 980 nm NIR excitation, displaying potential applications in bioimaging and therapy. Moreover, transparent and UC fluorescent NCs-polydimethylsiloxane (PDMS) composites with regular dimensions were also prepared by an in situ polymerization route.     

Shape evolution of CdSe nanocrystals in vegetable oils: A synergistic effect of selenium precursor

In this contribution, common vegetable oils are used as coordination solvents for synthesis of high quality CdSe nanocrystals. Various shaped nanocrystals (quantum dots, quantum rods, multipods, arc structure, etc.) can be produced free of alkylphosphonic acids. Shape evolution can be induced by three types of selenium precursors: ODE-Se, VO-Se and TOP-Se (ODE, 1-octadecene; VO, vegetable oil; TOP, trio-n-octylphosphine). The quantum yields of NCs are 15–40%. The full width at half-maximum (fwhm) of the photoluminescence spectra are 27 ± 1 nm for quantum dots and 23 ± 1 nm for quantum rods/multipods. The obtained quantum dots are zinc blende structure and stable at high temperature. Wurtzite, zinc blende, twinned and dislocation can coexist in the obtained quantum arc structure, which will provide new physical model to study the subtle optoelectronic properties of NCs.     

PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.     

Persistent nucleation and size dependent attachment kinetics produce monodisperse PbS nanocrystals

Modern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed to result from a rapid “burst of nucleation” (La Mer, JACS, 1950, 72(11), 4847–4854) followed by diffusion limited growth and size distribution focusing (Reiss, J. Chem. Phys., 1951, 19, 482). Using a combination of in situ X-ray scattering, optical absorption, and ¹³C nuclear magnetic resonance (NMR) spectroscopy, we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thiourea precursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to be slow and continues during >50% of the precipitation. A population balance model based on a size dependent growth law (1/r) fits the data with a single growth rate constant (k_G) across all three precursors. However, the magnitude of the k_G and the lack of solvent viscosity dependence indicates that the rate limiting step is not diffusion from solution to the nanoparticle surface. Several surface reaction limited mechanisms and a ligand penetration model that fits data our experiments using a single fit parameter are proposed to explain the results.

In situ X-ray scattering kinetics and population balance modeling of crystal nucleation and growth.     

2D assembly of gold-PNIPAM core-shell nanocrystals

2D arrays of Au-PNIPAM core-shell nanocrystals were fabricated using convective deposition and spin-coating. The particle density and ordering were studied by AFM. Annealing at 700 °C removes the polymer shell, while retaining a monolayer of well-separated gold nanoparticles. The surface plasmon modes of the colloid monolayers could be measured by spectroscopic ellipsometry.