Topological insulators are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi2Te3 and Bi2Se3 crystals. Theoretical models, materials properties, and experimental results on two-dimensional and three-dimensional topological insulators are reviewed, and both the topological band theory and the topological field theory are discussed. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.

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Topological insulators and superconductors

Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Topological insulators represent a new quantum state of matter which is characterized by peculiar edge or surface states that show up due to a topological character of the bulk wave functions. This review presents a pedagogical account on topological insulator materials with an emphasis on basic theory and materials properties. After presenting a historical perspective and basic theories of topological insulators, it discusses all the topological insulator materials discovered as of May 2013, with some illustrative descriptions of the developments in materials discoveries in which the author was involved. A summary is given for possible ways to confirm the topological nature in a candidate material. Various synthesis techniques as well as the defect chemistry that are important for realizing bulk-insulating samples are discussed. Characteristic properties of topological insulators are discussed with an emphasis on transport properties. In particular, the Dirac fermion physics and the resulting peculiar qu...

http://inspirehep.net/record/1229385/files/arXiv:1304.5693.pdf

Topological Insulator Materials

Topological insulators represent a new quantum state of matter which is characterized by peculiar edge or surface states that show up due to a topological character of the bulk wave functions. This review presents a pedagogical account on topological insulator materials with an emphasis on basic theory and materials properties. After presenting a historical perspective and basic theories of topological insulators, it discusses all the topological insulator materials discovered as of May 2013, with some illustrative descriptions of the developments in materials discoveries in which the author was involved. A summary is given for possible ways to confirm the topological nature in a candidate material. Various synthesis techniques as well as the defect chemistry that are important for realizing bulk-insulating samples are discussed. Characteristic properties of topological insulators are discussed with an emphasis on transport properties. In particular, the Dirac fermion physics and the resulting peculiar quantum oscillation patterns are discussed in detail. It is emphasized that proper analyses of quantum oscillations make it possible to unambiguously identify surface Dirac fermions through transport measurements. The prospects of topological insulator materials for elucidating novel quantum phenomena that await discovery conclude the review.

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

Metal/Semiconductor Hybrid Nanostructures for Plasmon-Enhanced Applications

Dendrite-like ZnO@Ag heterostructure nanocrystals are designed and fabricated by a facial two-step chemical method in a large scale. The heterostructure nanocrystals are composed of single crystal Ag nanowires as trunks and highly dense (0001) oriented ZnO nanorods as branches. ZnO nanorods with diameters of about 50−400 nm are vertically grown on the six lateral surfaces of the Ag nanowires. Ultrathin ZnO nanowires or nanotubes with a diameter of less than 30 nm are decorated on the ZnO nanorods. The photocatalysis test shows that the ZnO@Ag heterostructures exhibit a higher photocatalytic activity than the pure ZnO nanorods, thereby implying that the Ag/ZnO interfaces promote the separation of photogenerated electron−hole pairs and enhance the photocatalytic activity.

Growth and Photocatalytic Activity of Dendrite-like ZnO@Ag Heterostructure Nanocrystals

The epitaxial growth of thin films of the topological insulator Bi2Se3 on nominally flat and vicinal Si(111) substrates was studied. In order to achieve a planar growth front and better quality epifilms, a two-step growth method was adopted for the van der Waals epitaxy of Bi2Se3 to proceed. By using vicinal Si(111) substrate surfaces, the in-plane growth rate anisotropy of Bi2Se3 was exploited in order to achieve single crystalline Bi2Se3 epifilms, in which threading defects and twins are effectively suppressed. The optimization of the growth parameters has resulted in the vicinal Bi2Se3 films showing a carrier mobility of ~2000 cm2 V−1 s−1 and a background doping of ~3×1018 cm−3 of the as-grown layers. Such samples not only show a relatively high magnetoresistance but also show a linear dependence on the magnetic field.

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The Van der Waals Epitaxy of Bi 2 Se 3 on the Vicinal Si(111) Surface: An Approach for Preparing High-quality Thin Films of a Topological Insulator

Without the aid of surfactants or catalysts, one-dimensional (1D) nanostructures of Te (nanowires with a small quantity of nanotubes and nanoribbons) have been synthesized directly from template-free electrodeposition (TFED) in an aqueous solution at low temperature. As observed by electron microscopy, the as-grown Te nanomaterials are single crystalline trigonal structure and contain few defects. The preferential growth of these 1D nanostructures along the [001] direction is attributed to the intrinsic anisotropic crystal structure of Te and the special growth condition of TFED. It is anticipated that TFED could be used as a versatile approach to synthesize various 1D materials which contain intrinsic highly anisotropic crystal structures.

Template-Free Electrodeposition of One-Dimensional Nanostructures of Tellurium

A self-entanglement mechanism for continuous pulling of carbon nanotube yarns

We present a two-stage fabrication method for Zn2TiO4−ZnO nanowire axial heterostructures using the site-specific deposition of TiO2 on the tips of ZnO nanowires, followed by an annealing process. As revealed by the electron microscopy investigation, the cubic spinel Zn2TiO4 single crystal heads possess two kinds of oriented relationships with ZnO nanowires after high-temperature reaction. The reaction between TiO2 nanoparticles and ZnO nanowires shows a unilateral diffusion characteristic of ZnO into TiO2 which provides a useful method for rational design and fabrication of nanostructures based on nanostructured phase transformation.

Tailun Wong

Papers

Growth and Photocatalytic Activity of Dendrite-like ZnO@Ag Heterostructure Nanocrystals

The Van der Waals Epitaxy of Bi 2 Se 3 on the Vicinal Si(111) Surface: An Approach for Preparing High-quality Thin Films of a Topological Insulator

Template-Free Electrodeposition of One-Dimensional Nanostructures of Tellurium

A self-entanglement mechanism for continuous pulling of carbon nanotube yarns

Zn2TiO4-ZnO Nanowire Axial Heterostructures Formed by Unilateral Diffusion