Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

https://science.sciencemag.org/content/sci/291/5510/1947.full.pdf

Nanobelts of Semiconducting Oxides

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

A method combining laser ablation cluster formation and vapor-liquid-solid (VLS) growth was developed for the synthesis of semiconductor nanowires. In this process, laser ablation was used to prepare nanometer-diameter catalyst clusters that define the size of wires produced by VLS growth. This approach was used to prepare bulk quantities of uniform single-crystal silicon and germanium nanowires with diameters of 6 to 20 and 3 to 9 nanometers, respectively, and lengths ranging from 1 to 30 micrometers. Studies carried out with different conditions and catalyst materials confirmed the central details of the growth mechanism and suggest that well-established phase diagrams can be used to predict rationally catalyst materials and growth conditions for the preparation of nanowires.

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A laser ablation method for the synthesis of crystalline semiconductor nanowires

Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties1,2 that depend sensitively on both size and shape3,4, and are of both fundamental and technological interest In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate5,6,7,8,9,10,11,12 And, in the case of optically active II–VI and III–V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects6,8,9,10 Thus, except for some metal nanocrystals13, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions For semiconductors, a benchmark preparation is the growth of nearly spherical II–VI and III–V nanocrystals by injection of precursor molecules into a hot surfactant14,15 Here we demonstrate that control of the growth kinetics of the II–VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments16,17 and as chromophores in light-emitting diodes18,19

Shape control of CdSe nanocrystals

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Until now, micrometer-scale or larger crystals of the III-V semiconductors have not been grown at low temperatures for lack of suitable crystallization mechanisms for highly covalent nonmolecular solids. A solution-liquid-solid mechanism for the growth of InP, InAs, and GaAs is described that uses simple, low-temperature (≤203°C), solution-phase reactions. The materials are produced as polycrystalline fibers or near-single-crystal whiskers having widths of 10 to 150 nanometers and lengths of up to several micrometers. This mechanism shows that processes analogous to vapor-liquid-solid growth can operate at low temperatures; similar synthesis routes for other covalent solids may be possible.

https://science.sciencemag.org/content/sci/270/5243/1791.full.pdf

Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-Liquid-Solid Growth

Methanolysis of {t-Bu2In[μ-P(SiMe3)2]}2 (1) in aromatic solvents gives polycrystalline InP fibers (dimensions 10−100 nm × 50−1000 nm) at 111−203 °C. The chemical pathway consists of a molecular component, in which precursor substituents are eliminated, and a nonmolecular component, in which the InP crystal lattices are assembled. The two components working in concert comprise the solution−liquid−solid (SLS) mechanism. The molecular component proceeds through a sequence of isolated and fully characterized intermediates:  1 → [t-Bu2In(μ-OMe)]2 (2) → [t-Bu2In(μ-PHSiMe3)]2 (3) → 2 → [t-Bu2In(μ-PH2)]3 (4). Complex 4, which is alternatively prepared from t-Bu3In and PH3, undergoes alkane elimination, the last steps of which are catalyzed by the protic reagent MeOH, PhSH, Et2NH, or PhCO2H. In the subsequent nonmolecular component of the pathway, the resulting (InP)n fragments dissolve into a dispersion of molten In droplets, and recrystallize as the InP fibers. Important criteria are identified for crystal growt...

Solution−Liquid−Solid Growth of Indium Phosphide Fibers from Organometallic Precursors: Elucidation of Molecular and Nonmolecular Components of the Pathway

Turning down the heat on semiconductor growth: Solution‐chemical syntheses and the solution‐liquid‐solid mechanism

Reaction of MoCl3 and Si at 500 °C produces phase-pure MoSi2 powders in near-quantitative yields. Use of an inert diluent (LiCl or ZnCl2) enables the production of nanocrystalline powders. The diluent can be subsequently removed by washing with methanol or by sublimation. Consolidation of diluent-free powders by hot pressing at 1400 °C and 375 MPa for 0.5−4 h affords dense compacts (>95% theoretical density) possessing nanocrystalline grains (<50 nm) and exhibiting hardnesses of 15.0−16.3 GPa. This constitutes a 60−75% enhancement in hardness over conventional, microcrystalline MoSi2.

Preparation of nanocrystalline molybdenum disilicide (MoSi2) by a chlorine-transfer reaction

Sonochemical co-reduction of MoCl5 and SiCl4 with NaK alloy in a hexane dispersion using 600 W, 20 kHz irradiation, followed by annealing at 900 °C produces nanocrystalline MoSi2 powders with ≈90% yield. Consolidation by hot pressing at 1170 °C, 140 MPa for 4 h gives a 78%-dense compact with 31 nm average grain sizes and a Vickers microhardness of 1484 (10% kg/mm2. Consolidation by hot pressing at 1200 °C, 100 MPa for 10 min. followed by hot isostatic pressing at 1100 °C, 200 MPa for 4 h gives a 83%-dense compact with 41 nm average grain sizes and a Vickers microhardness of 1587 ± 10% kg mm−2. The second compact exhibits a compressive yield strength of 2875 MPa, and fractures without significant plastic deformation. The microhardness values and compression strengths are 50–70% higher in nanocrystalline MoSi2 than in conventional, coarse-grained MoSi2.

Timothy J. Trentler

Papers

Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-Liquid-Solid Growth

Solution−Liquid−Solid Growth of Indium Phosphide Fibers from Organometallic Precursors: Elucidation of Molecular and Nonmolecular Components of the Pathway

Turning down the heat on semiconductor growth: Solution‐chemical syntheses and the solution‐liquid‐solid mechanism

Preparation of nanocrystalline molybdenum disilicide (MoSi2) by a chlorine-transfer reaction

Sonochemical synthesis of nanocrystalline molybdenum disilicide (MoSi2)