Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

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

Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

Metal oxide nanostructures are promising electrode materials for lithium-ion batteries and supercapacitors because of their high specific capacity/capacitance, typically 2-3 times higher than that of the carbon/graphite-based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications. It is therefore urgent to improve their overall device performance, which depends on not only the development of advanced electrode materials but also in a large part "how to design superior electrode architectures". In the article, we will review recent advances in strategies for advanced metal oxide-based hybrid nanostructure design, with the focus on the binder-free film/array electrodes. These binder-free electrodes, with the integration of unique merits of each component, can provide larger electrochemically active surface area, faster electron transport and superior ion diffusion, thus leading to substantially improved cycling and rate performance. Several recently emerged concepts of using ordered nanostructure arrays, synergetic core-shell structures, nanostructured current collectors, and flexible paper/textile electrodes will be highlighted, pointing out advantages and challenges where appropriate. Some future electrode design trends and directions are also discussed.

Recent Advances in Metal Oxide-based Electrode Architecture Design for Electrochemical Energy Storage

Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.

Synthesis of monodisperse spherical nanocrystals.

The synthesis of nc-Si by reactive evaporation of SiO and subsequent thermal induced phase separation is reported. The size control of nc-Si is realized by evaporation of SiO/SiO2 superlattices. By this method an independent control of crystal size and density is possible. The phase separation of SiO into SiO2 and nc-Si in the limit of ultrathin layers is investigated. Different steps of this phase separation are characterized by photoluminescence, infrared absorption and transmission electron microscopy measurements. The strong room temperature luminescence of nc-Si shows a strong blueshift of the photoluminescence signal from 850 to 750 nm with decreasing crystal size. Several size dependent properties of this luminescence signal, like decreasing radiative lifetime and increasing no-phonon transition properties with decreasing crystal size are in good agreement with the quantum confinement model. Er doping of the nc-Si shows an enhancement of the Er luminescence at 1.54 µm by a factor of 5000 compared to doped SiO2 layers. The decreasing transfer time for the nc-Si to Er transition with decreasing crystal size can be understood as additional proof of increasing recombination probability within the nc-Si for decreasing crystal size.

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Synthesis and size control of Si nanocrystals by SiO/SiO2 superlattices and Er doping

In the last few years nanostructure ZnO is being intensively investigated from the growth control, structural and physical characterization, to device performance. There are now increasing interests in a wide range of ZnO-based ternary compound nanostructures offering even larger possibilities. These compounds can be useful as multi-color light-emitting device, sensors, transparent electrodes, or catalysts. This Review summarizes typical ZnO-based ternary compound nanotubes and nanowires, which are fabricated based on either solid-state reactions with ZnO and/or porous templates, or on the vapour-liquid-solid process. Our focus is put on spinel-type compounds formed with oxides like Al2O3, Ga2O3, Fe2O3, Sb2O3, TiO2, and SnO2. The discussion includes the growth strategies, nanosize effects, and the associated reaction mechanisms. ZnMgO alloy nanowires, another technologically important ternary compound, are touched at the end, too.

ZnO-Based Ternary Compound Nanotubes and Nanowires

We offer a complete structural and optical study of samples containing silicon nanocrystals (Si-NCs) embedded in SiO2/SiON multilayers, varying the oxynitride layer thickness from 2.5 to 7 nm. Using energy-filtered transmission electron microscopy we have determined the size distribution of the precipitated Si-nanoaggregates. Raman scattering measurements were used to investigate the Si-NC size and crystalline quality. By combining both techniques, the nanoaggregate crystalline degree has been evaluated, with values around 50% for all the samples. Photoluminescence spectroscopy has shown a blueshift of the emission at smaller NC sizes, presenting the sample with Si-NCs of 3.9 nm the best emission properties. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the EMRS 2011 symposium organizers: G. Conibeer; Yongxiang Li; J. Poortmans; M. Kondo; A. Slaoui; M. Tao; M. Topic

European Materials Research Society Conference Symp. Advanced Inorganic Materials and Concepts for Photovoltaics Structural and optical characterization of size controlled silicon nanocrystals in SiO 2 /SiO x N y multilayers

Summary form only given. Nanoscaled structures play a major role in optoelectronic and semiconductor research with many applications already in a commercial state. Today's Si nanocrystals research is focused on the preparation of Si nanocrystals embedded in an oxide host. Methods applied for preparation are Si ion implantation into high quality oxides, sputtering of Si rich oxides or reactive evaporation of Si rich oxides. Using these methods the Si crystal size is controlled by the Si content in the SiO/sub 2/ matrix. Crystal size and number cannot be controlled independently. in addition, there is only a limited control of the size distribution. We present a new process that allows the size controlled preparation of silicon nanocrystals on 4 inch wafers.

Si based highly luminescent photonic structures

Beschrieben ist ein Verfahren zur Herstellung einer Halbleiterstruktur mit Siliziumclustern und/oder -nanokristallen, die in verteilter Form in einer Matrix aus einer Siliziumverbindung vorliegen. Das Verfahren umfast die Schritte Abscheiden einer Schicht aus einer thermisch nicht stabilen Siliziumverbindung mit einer Schichtdicke im Bereich zwischen 0,5 nm und 20 nm, speziell zwischen 1 nm und 10 nm und insbesondere zwischen 1 nm und 7 nm auf einem Trager und die thermische Behandlung bei einer Temperatur, die ausreichend ist, um eine Phasenseparation durchzufuhren, um die Siliziumcluster oder -nanokristalle in einer Matrix einer thermisch stabilen Siliziumverbindung zu erhalten. Die Anspruche decken auserdem Halbleiterstrukturen mit solchen verteilten Siliziumclustern oder -nanokristallen ab. Das beschriebene Verfahren ermoglicht eine kostengunstige Herstellung von hochdichten Feldern von Siliziumclustern oder Nanokristallen mit einer engen Grosenverteilung.

Margit Zacharias

Papers

Synthesis and size control of Si nanocrystals by SiO/SiO2 superlattices and Er doping

ZnO-Based Ternary Compound Nanotubes and Nanowires

European Materials Research Society Conference Symp. Advanced Inorganic Materials and Concepts for Photovoltaics Structural and optical characterization of size controlled silicon nanocrystals in SiO 2 /SiO x N y multilayers

Si based highly luminescent photonic structures

Verfahren zur Herstellung einer Halbleiterstruktur mit Siliziumclustern und/oder -nanokristallen und eine Halbleiterstruktur dieser Art