This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

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Metal-Assisted Chemical Etching of Silicon: A Review

https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

Nanotechnology is providing new solutions and opportunities to ensure sustainable energy and environments for the future. Materials of nanofiberous morphology are attractive to solve numerous energy and environmental issues. Nanofibers can be effectively produced by electrospinning, which is a simple and low cost technique. In addition, electrospinning allows the production of nanofibers from various materials e.g. organics and inorganics in different configurations and assemblies. This is highly beneficial for energy devices, where inorganic materials especially metal oxides can be synthesized and electrospun, improving conducting and ceramic properties. Excitonic solar cells fabricated with aligned nanofiberous metal oxide electrodes provide higher solar–electric energy conversion efficiency, whereas fuel cells made with nanofiberous electrodes enable uniform dispersion of catalysts, and thus increase electrocatalytic activity to obtain higher chemical–electric energy conversion efficiency. The nanofibers used in filtration membranes for environmental remediation, minimize the pressure drop and provide better efficiency than conventional fiber mats. The large surface area-to-volume ratio of nanofiber membranes allows greater surface adsorption of contaminants from air and water, and increases the life-time of the filtration media. This review highlights the potential and application of electrospun nanofiberous materials for solving critical energy and environmental issues.

Electrospun nanofibers in energy and environmental applications

Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO : From basics towards applications

Hydrothermal technology for nanotechnology

Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle.

Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

Using electrochemical etching of a polycrystalline 3$C$-SiC target and subsequent ultrasonic treatment in water solution, we have fabricated suspensions of 3$C$-SiC nanocrystallites that luminesce. Transmission electron microscope observations show that the 3$C$-SiC nanocrystallites, which uniformly disperse in water, have sizes in the range of 1\char21{}6 nm. Photoluminescence and photoluminescence excitation spectral examinations show clear evidence for the quantum confinement of 3$C$-SiC nanocrystallites with the emission band maximum ranging from 440 to 560 nm. Tunable, composite polystyrene/SiC film can be made by adding polystyrene to a toluene suspension of the 3$C$-SiC nanocrystallites and then coating the resulting solution onto a Si wafer.

Experimental evidence for the quantum confinement effect in 3C-SiC nanocrystallites.

From Si nanotubes to nanowires : Synthesis, characterization, and self-assembly

Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.

Current transport studies of ZnO/p-Si heterostructures grown by plasma immersion ion implantation and deposition

We have investigated the role of the solvents in the luminescence from colloidal 3C-SiC suspensions. By dispersing electrochemically etched polycrystalline 3C-SiC wafers in water, ethanol, or toluene, we have fabricated suspensions of 3C-SiC nanocrystals that exhibit intense photoluminescence. By taking into account the quantum confinement effect and observed size distributions of the 3C-SiC crystallites, a simple model is formulated to explain the photoluminescence spectra. Our results show that the colloidal 3C-SiC nanocrystals are robust and intense emitters that have good chemical stability and biocompatibility. They are thus useful in biotechnology and nano-optoelectronics applications.

Gg G. Siu

Papers

Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

Experimental evidence for the quantum confinement effect in 3C-SiC nanocrystallites.

From Si nanotubes to nanowires : Synthesis, characterization, and self-assembly

Current transport studies of ZnO/p-Si heterostructures grown by plasma immersion ion implantation and deposition

Luminescence from colloidal 3C-SiC nanocrystals in different solvents