Fudong Wang

The aggregative growth and oriented attachment of nanocrystals and nanoparticles are reviewed, and they are contrasted to classical LaMer nucleation and growth, and to Ostwald ripening. Kinetic and mechanistic models are presented, and experiments directly observing aggregative growth and oriented attachment are summarized. Aggregative growth is described as a nonclassical nucleation and growth process. The concept of a nucleation function is introduced, and approximated with a Gaussian form. The height (Γmax) and width (Δtn) of the nucleation function are systematically varied by conditions that influence the colloidal stability of the small, primary nanocrystals participating in aggregative growth. The nucleation parameters Γmax and Δtn correlate with the final nanocrystal mean size and size distribution, affording a potential means of achieving nucleation control in nanocrystal synthesis.

The serendipitously discovered solution-liquid-solid (SLS) mechanism has been refined into a nearly general synthetic method for semiconductor nanowires. Purposeful control of diameters and diameter distributions is achieved. The synthesis proceeds by a solution-based catalyzed-growth mechanism in which nanometer-scale metallic droplets catalyze the decomposition of metallo-organic precursors and crystalline nanowire growth. Related growth methods proceeding by the analogous vapor-liquid-solid (VLS) and supercritical fluid-liquid-solid (SFLS) mechanisms are known, and the relative attributes of the methods are compared. In short, the VLS method is most general and appears to afford nanowires of the best crystalline quality. The SLS method appears to be advantageous for producing the smallest nanowire diameters and for variation and control of surface ligation. The SFLS method may represent an ideal compromise. Recent results for SLS growth are summarized.

Here, we elucidate a double-lamellar-template pathway for the formation of CdSe quantum belts. The lamellar templates form initially by dissolution of the CdX(2) precursors in the n-octylamine solvent. Exposure of the precursor templates to selenourea at room temperature ultimately affords (CdSe)(13) nanoclusters entrained within the double-lamellar templates. Upon heating, the nanoclusters are transformed to CdSe quantum belts having widths, lengths, and thicknesses that are predetermined by the dimensions within the templates. This template synthesis is responsible for the excellent optical properties exhibited by the quantum belts. We propose that the templated-growth pathway is responsible for the formation of the various flat, colloidal nanocrystals recently discovered, including nanoribbons, nanoplatelets, nanosheets, and nanodisks.

The solution-liquid-solid (SLS) and related solution-based methods for the synthesis of semiconductor nanowires and nanorods are reviewed. Since its discovery in 1995, the SLS mechanism and its close variants have provided a nearly general strategy for the growth of pseudo-one-dimensional nanocrystals. The various metallic-catalyst nanoparticles employed are summarized, as are the syntheses of III-V, II-VI, IV-VI, group IV, ternary, and other nanorods and nanowires. The formation of axial heterojunctions, core/shell nanowires, and doping are also described. The related supercritical-fluid-liquid-solid (SFLS), electrically controlled SLS, flow-based SLS, and solution-solid-solid (SSS) methods are discussed, and the crystallographic characteristics of the wires and rods grown by these methods are summarized. The presentation of optical and electronic properties emphasizes electronic structures, absorption cross sections, polarization anisotropies, and charge-carrier dynamics, including photoluminescence intermittency (blinking) and photoluminescence modulation by charges and electric fields. Finally, developing applications for the pseudo-one-dimensional nanostructures in field-effect transistors, lithium-ion batteries, photocathodes, photovoltaics, and photodetection are discussed.

Tri-n-octylphosphine oxide (TOPO) is a commonly used solvent for nanocrystal synthesis. Commercial TOPO samples contain varying amounts of phosphorus-containing impurities, some of which significantly influence nanocrystal growth. Consequently, nanocrystal syntheses often give irreproducible results with different batches of TOPO solvent. In this study, we identify TOPO impurities by (31)P NMR, and correlate their presence with the outcomes of CdSe nanocrystal syntheses. We subsequently add the active impurity species, one by one, to purified TOPO to confirm their influence on nanocrystal syntheses. In this manner, di-n-octylphosphine oxide (DOPO) is shown to assist CdSe quantum-dot growth; di-n-octylphosphinic acid (DOPA) and mono-n-octylphosphinic acid (MOPA) are shown to assist CdSe quantum-rod growth, and DOPA is shown to assist CdSe quantum-wire growth. (The TOPO impurity n-octylphosphonic acid, OPA, has been previously shown to assist quantum-rod growth.) The beneficial impurities are prepared on multigram scales and can be added to recrystallized TOPO to provide reproducible synthetic results.