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?

Green synthesis of metal nanoparticles using plants

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A ∼10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200−350 °C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod−polymer solar cells.

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General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds

The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.

Metal oxides for solid-state gas sensors: What determines our choice?

Well-aligned ZnO nanowires were synthesized by simple physical vapor deposition using c-oriented ZnO thin films as substrates without catalysts or additives. The synthesized ZnO nanowires have two typical average diameters: 60nm in majority and 120nm in minority. They are about 4μm in length and well aligned along the normal direction of the substrate. Most of the synthesized ZnO nanowires are single crystalline in a hexagonal structure and grow along the [001] direction. The c-oriented ZnO thin films control the growth direction. Photoluminescence spectrum was measured showing a single strong ultraviolet emission (380nm). Such result indicates that the ZnO nanowire arrays can be applied to excellent optoelectronic devices.

Synthesis of well-aligned ZnO nanowires by simple physical vapor deposition on c-oriented ZnO thin films without catalysts or additives

By a simple method, that is, by heating raw materials in a flowing gas at ambient pressure, Si3N4, Ga2O3, and ZnO nanowires, SiC nanocables, and SiO2 amorphous nanowires are synthesized without metal catalysts. The diameters of these one-dimensional nanoscale materials are greatly affected by synthesis temperatures. At suitable synthesis temperatures, their diameters are <100 nm. The growth mechanisms of these nanowires are discussed preliminarily.

A Simple Method To Synthesize Nanowires

Aluminum nitride nanowires have been synthesized in bulk from carbon nanotubes (CNTs) at relatively low temperatures. This method produces AlN nanowires through the reaction of the carbon nanotubes, Al, and Al2O3 in a flowing NH3 atmosphere. The diameters of the products, mainly in the range of 10−50 nm, correspond with the diameters of the carbon nanotubes, which provides a promising way to control the diameters of the AlN nanowires. The AlN nanowires fabricated in this way are single crystals covered by a thin amorphous layer. The small diameter and single crystal form make the AlN nanowires highly flexible. The growth mechanism of the AlN nanowires and the factors that allow a decrease in the synthesis temperature are discussed.

Synthesis of Aluminum Nitride Nanowires from Carbon Nanotubes

A simple method to synthesize Si3N4 and SiO2 nanowires from Si or Si/SiO2 mixture

Some non-magnetic semiconductors have been found to exhibit potentially useful electrical responses to magnetic fields. Such effects, termed inhomogeneity-induced magnetoresistance, occur when the conductivity of the material is not uniform. Wan et al. now show how this phenomenon can be engineered in silicon using the geometry (shape) of the device, leading to enhanced sensitivities that could prove attractive to the magnetic-field-sensing industry. Inhomogeneity-induced magnetoresistance (IMR) reported in some non-magnetic semiconductors1,2,3,4,5,6,7,8, particularly silicon1,6,7,8, has generated considerable interest owing to the large magnitude of the effect and its linear field dependence (albeit at high magnetic fields). Various theories implicate9,10,11,12,13,14,15,16,17,18 spatial variation of the carrier mobility as being responsible for IMR. Here we show that IMR in lightly doped silicon can be significantly enhanced through hole injection, and then tuned by an applied current to arise at low magnetic fields. In our devices, the ‘inhomogeneity’ is provided by the p–n boundary formed between regions where conduction is dominated by the minority and majority charge carriers (holes and electrons) respectively; application of a magnetic field distorts the current in the boundary region, resulting in large magnetoresistance. Because this is an intrinsically spatial effect, the geometry of the device can be used to enhance IMR further: we designed an IMR device whose room-temperature field sensitivity at low fields was greatly improved, with magnetoresistance reaching 10% at 0.07 T and 100% at 0.2 T, approaching the performance of commercial giant-magnetoresistance devices19,20. The combination of high sensitivity to low magnetic fields and large high-field response should make this device concept attractive to the magnetic-field sensing industry. Moreover, because our device is based on a conventional silicon platform, it should be possible to integrate it with existing silicon devices and so aid the development of silicon-based magnetoelectronics.

Xiaozhong Zhang

Papers

Synthesis of well-aligned ZnO nanowires by simple physical vapor deposition on c-oriented ZnO thin films without catalysts or additives

A Simple Method To Synthesize Nanowires

Synthesis of Aluminum Nitride Nanowires from Carbon Nanotubes

A simple method to synthesize Si3N4 and SiO2 nanowires from Si or Si/SiO2 mixture

Geometrical enhancement of low-field magnetoresistance in silicon