There are growing concerns over the environmental, climate, and health impacts caused by using non-renewable fossil fuels. The utilization of green energy, including solar and wind power, is believed to be one of the most promising alternatives to support more sustainable economic growth. In this regard, lithium-ion batteries (LIBs) can play a critically important role. To further increase the energy and power densities of LIBs, silicon anodes have been intensively explored due to their high capacity, low operation potential, environmental friendliness, and high abundance. The main challenges for the practical implementation of silicon anodes, however, are the huge volume variation during lithiation and delithiation processes and the unstable solid-electrolyte interphase (SEI) films. Recently, significant breakthroughs have been achieved utilizing advanced nanotechnologies in terms of increasing cycle life and enhancing charging rate performance due partially to the excellent mechanical properties of nanomaterials, high surface area, and fast lithium and electron transportation. Here, the most recent advance in the applications of 0D (nanoparticles), 1D (nanowires and nanotubes), and 2D (thin film) silicon nanomaterials in LIBs are summarized. The synthetic routes and electrochemical performance of these Si nanomaterials, and the underlying reaction mechanisms are systematically described.

Silicon-based Nanomaterials for Lithium-Ion Batteries - A Review

The Kirkendall effect is a consequence of the different diffusivities of atoms in a diffusion couple causing a supersaturation of lattice vacancies. This supersaturation may lead to a condensation of extra vacancies in the form of so-called “Kirkendall voids” close to the interface. On the macroscopic and micrometer scale these Kirkendall voids are generally considered as a nuisance because they deteriorate the properties of the interface. In contrast, in the nanoworld the Kirkendall effect has been positively used as a new fabrication route to designed hollow nano-objects. In this Review we summarize and discuss the demonstrated examples of hollow nanoparticles and nanotubes induced by the Kirkendall effect. Merits of this route are compared with other general methods for nanotube fabrication. Theories of the kinetics and thermodynamics are also reviewed and evaluated in terms of their relevance to experiments. Moreover, nanotube fabrication by solid-state reactions and non-Kirkendall type diffusion processes are covered.

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Formation of nanotubes and hollow nanoparticles based on Kirkendall and diffusion processes: a review.

The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. This Review discusses the various growth processes, with a focus on the vapor-liquid-solid process, which offers an opportunity for the control of spatial positioning of nanowires. Strategies for position-controlled and nanopatterned growth of nanowire arrays are reviewed and demonstrated by selected examples as well as discussed in terms of larger-scale realization and future prospects. Issues on building up nanowire-based electronic and photonic devices are addressed at the end of the Review, accompanied by a brief survey of recent progress demonstrated so far on the laboratory level.

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Semiconductor nanowires: from self-organization to patterned growth

This paper summarizes some of the essential aspects of silicon-nanowire growth and of their electrical properties. In the first part, a brief description of the different growth techniques is given, though the general focus of this work is on chemical vapor deposition of silicon nanowires. The advantages and disadvantages of the different catalyst materials for silicon-wire growth are discussed at length. Thereafter, in the second part, three thermodynamic aspects of silicon-wire growth via the vapor–liquid–solid mechanism are presented and discussed. These are the expansion of the base of epitaxially grown Si wires, a stability criterion regarding the surface tension of the catalyst droplet, and the consequences of the Gibbs–Thomson effect for the silicon wire growth velocity. The third part is dedicated to the electrical properties of silicon nanowires. First, different silicon nanowire doping techniques are discussed. Attention is then focused on the diameter dependence of dopant ionization and the influence of interface trap states on the charge carrier density in silicon nanowires. It is concluded by a section on charge carrier mobility and mobility measurements.

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Silicon Nanowires: A Review on Aspects of their Growth and their Electrical Properties

We present results of ideal epitaxial nucleation and growth of III−V semiconductor nanowires on silicon substrates. This addresses the long-time challenge of integrating high performance III−V semiconductors with mainstream Si technology. Efficient room-temperature generation of light on silicon is demonstrated by the incorporation of double heterostructure segments in such nanowires. We expect that advanced heterostructure devices, such as resonant tunneling diodes, superlattice device structures, and heterostructure photonic devices for on-chip communication, could now become available as complementary device technologies for integration with silicon.

Epitaxial III-V nanowires on silicon

Silicon nanowhiskers in the diameter range of 70 to 200 nm were grown on 〈111〉-oriented silicon substrates by molecular-beam epitaxy. Assuming the so-called “vapor–liquid–solid” (VLS) growth process to operate, we initiated the growth by using small clusters of gold at the silicon interface as seeds. The in situ generation of the Au clusters as well as the growth parameters of the whiskers are discussed. The experimentally observed radius dependence of the growth velocity of the nanowhiskers is opposite to what is known for VLS growth based on chemical vapor deposition and can be explained by an ad-atom diffusion on the surface of the whiskers.

Silicon nanowhiskers grown on 〈111〉Si substrates by molecular-beam epitaxy

Single-mode vertical-cavity surface-emitting lasers based on dense arrays of stacked submonolayer grown InGaAs quantum dots, emitting near 980nm, demonstrate a modulation bandwidth of 10.5GHz. A low threshold current of 170μA, high differential efficiency of 0.53W∕A, and high modulation current efficiency factor of 14GHz∕mA are realized from a 1μm oxide aperture single-mode device with a side mode suppression ratio of >40dB and peak output power of >1mW. The lasers are also suitable for high temperature operation.

Single-mode submonolayer quantum-dot vertical-cavity surface-emitting lasers with high modulation bandwidth

We discuss the formation of a Si/Ge-superlattice (SL) generated by molecular beam epitaxy. Specific growth parameter were chosen to optimize the periodic structure of vertically stacked Ge islands. Optimized SLs show a strong photoluminescence at a wavelength in the region of 1.55 μm up to room temperature. The luminescence is explained by a recombination of electrons in a miniband and holes localized in the Ge islands. The morphology and the crystal structure of the SL, which are influenced by the growth parameters, were analyzed by transmission electron microscopy techniques. It is demonstrated that doping of the SL structure by antimony improves both structural and optical properties.

Room-temperature light emission from a highly strained Si/Ge superlattice

Metalorganic chemical vapor deposition of GaAs-based laser diodes, using self-organized InGaAs quantum dots (QDs), emitting at >1.24 μm is demonstrated. The environment-friendly alternative precursor tertiarybutylarsine is used as a substitute for arsenic hydride. The active region contains ten closely stacked InGaAs QD layers embedded in a GaAs matrix. Lasing emission at such long wavelengths was achieved by overgrowing the In0.65Ga0.35As QDs with a thin In0.2Ga0.8As film. The application of an in situ annealing step leading to the evaporation of plastically relaxed defect clusters is shown to be decisive for the laser performance. A transparency current density of 7.2 A/cm2 per QD layer and an internal quantum efficiency of 75% were achieved at room temperature.

1.24 μm InGaAs/GaAs quantum dot laser grown by metalorganic chemical vapor deposition using tertiarybutylarsine

Metalorganic chemical vapor deposition of laser diodes based on triple stacks of self-organized InxGa1−xAs/GaAs quantum dots (QDs) as active medium using the alternative precursor tertiarybutylarsine (TBAs) is reported. Epitaxy of monodispersed QDs using TBAs is demonstrated. Due to the high cracking efficiency of TBAs at low temperatures, the crucial growth parameters V/III ratio and temperature can be tuned almost independently. Ridge-waveguide QD lasers show a transparency current of 29.7 A/cm2—equivalent to 9.9 A/cm2 per QD layer—an internal quantum efficiency of 91.4%, and an internal optical loss of 2.2 cm−1.

N. D. Zakharov

Papers

Silicon nanowhiskers grown on 〈111〉Si substrates by molecular-beam epitaxy

Single-mode submonolayer quantum-dot vertical-cavity surface-emitting lasers with high modulation bandwidth

Room-temperature light emission from a highly strained Si/Ge superlattice

1.24 μm InGaAs/GaAs quantum dot laser grown by metalorganic chemical vapor deposition using tertiarybutylarsine

Alternative-precursor metalorganic chemical vapor deposition of self-organized InGaAs/GaAs quantum dots and quantum-dot lasers