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

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Background Since at least 400 C.E., when the Mayans first used layered clays to make dyes, people have been harnessing the properties of layered materials. This gradually developed into scientific research, leading to the elucidation of the laminar structure of layered materials, detailed understanding of their properties, and eventually experiments to exfoliate or delaminate them into individual, atomically thin nanosheets. This culminated in the discovery of graphene, resulting in a new explosion of interest in two-dimensional materials. Layered materials consist of two-dimensional platelets weakly stacked to form three-dimensional structures. The archetypal example is graphite, which consists of stacked graphene monolayers. However, there are many others: from MoS 2 and layered clays to more exotic examples such as MoO 3 , GaTe, and Bi 2 Se 3 . These materials display a wide range of electronic, optical, mechanical, and electrochemical properties. Over the past decade, a number of methods have been developed to exfoliate layered materials in order to produce monolayer nanosheets. Such exfoliation creates extremely high-aspect-ratio nanosheets with enormous surface area, which are ideal for applications that require surface activity. More importantly, however, the two-dimensional confinement of electrons upon exfoliation leads to unprecedented optical and electrical properties. Liquid exfoliation of layered crystals allows the production of suspensions of two-dimensional nanosheets, which can be formed into a range of structures. (A) MoS 2 powder. (B) WS 2 dispersed in surfactant solution. (C) An exfoliated MoS 2 nanosheet. (D) A hybrid material consisting of WS 2 nanosheets embedded in a network of carbon nanotubes. Advances An important advance has been the discovery that layered crystals can be exfoliated in liquids. There are a number of methods to do this that involve oxidation, ion intercalation/exchange, or surface passivation by solvents. However, all result in liquid dispersions containing large quantities of nanosheets. This brings considerable advantages: Liquid exfoliation allows the formation of thin films and composites, is potentially scaleable, and may facilitate processing by using standard technologies such as reel-to-reel manufacturing. Although much work has focused on liquid exfoliation of graphene, such processes have also been demonstrated for a host of other materials, including MoS 2 and related structures, layered oxides, and clays. The resultant liquid dispersions have been formed into films, hybrids, and composites for a range of applications. Outlook There is little doubt that the main advances are in the future. Multifunctional composites based on metal and polymer matrices will be developed that will result in enhanced mechanical, electrical, and barrier properties. Applications in energy generation and storage will abound, with layered materials appearing as electrodes or active elements in devices such as displays, solar cells, and batteries. Particularly important will be the use of MoS 2 for water splitting and metal oxides as hydrogen evolution catalysts. In addition, two-dimensional materials will find important roles in printed electronics as dielectrics, optoelectronic devices, and transistors. To achieve this, much needs to be done. Production rates need to be increased dramatically, the degree of exfoliation improved, and methods to control nanosheet properties developed. The range of layered materials that can be exfoliated must be expanded, even as methods for chemical modification must be developed. Success in these areas will lead to a family of materials that will dominate nanomaterials science in the 21st century.

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Liquid Exfoliation of Layered Materials

Disclosed is a method of producing an LnCuOX single-crystal thin film (wherein Ln is at least one selected from the group consisting of lanthanide elements and yttrium, and X is at least one selected from the group consisting of S, Se and Te), which comprises the steps of growing a base thin film on a single-crystal substrate, depositing an amorphous or polycrystalline LnCuOX thin film on the base thin film to form a laminated film, and then annealing the laminated film at a high temperature of 500° C. or more. While a conventional LnCuOX film produced by growing an amorphous film through a sputtering process under appropriate conditions and then annealing the film at a high temperature was unexceptionally a polycrystalline substance incapable of achieving high emission efficiency and electron mobility required for a material of light-emitting devices or electronic devices, the method of the present invention can grow a thin film with excellent crystallinity suitable as a single crystal to an building black of light-emitting diodes, semiconductor leasers, filed-effect transistors, or a hetero-bipolar transistors.

LnCuO(S,Se,Te)monocrystalline thin film, its manufacturing method, and optical device or electronic device using the monocrystalline thin film

BiCuSeO oxyselenides have recently acquired ever-increasing attention and have been extensively studied as very promising thermoelectric materials. The ZT of the BiCuSeO system was significantly increased from 0.5 to 1.4 in the past three years, which indicates that BiCuSeO oxyselenides are robust candidates for energy conversion applications. In this review, we first discuss and summarize the crystal structures, microstructures, electronic structures and physical/chemical properties of BiCuSeO oxyselenides. Then, the approaches that successfully enhanced the thermoelectric performances in the BiCuSeO system are outlined, which include increasing carrier concentration, optimizing Cu vacancies, a simple and facile ball milling method, multifunctional Pb doping, band gap tuning, and increasing carrier mobility through texturing. Theoretical calculations to predict a maximum ZT in the BiCuSeO system are also described. Finally, a discussion of future possible strategies is proposed to aim at further enhancing the thermoelectric figure of merit of these materials.

BiCuSeO oxyselenides: new promising thermoelectric materials

Rare earth intermetallic compounds in Gd(Ag, Cd, In) and Gd(Cu, Ag, Au) systems have been studied. Most of these compounds have the CsCl type of crystal structure. Magnetization measurements have been made on these compounds to determine the type of the magnetic order and its relationship to the number of conduction electrons. GdAg, GdAg 0.9 In 0.1 , GdAg 0.2 In 0.8 , GdAg 0.1 In 0.9 , and GdIn in the Gd(Ag, In) system and GdCu, GdAg, GdAg 0.7 Au 0.3 , and GdAg 0.5 Au 0.5 in the Gd(Cu, Ag, Au) system are antiferromagnetic at low temperatures. On the contrary, GdCd, GdAg 0.8 In 0.2 , GdAg 0.7 In 0.3 , GdAg 0.5 In 0.5 , and GdAg 0.3 In 0.7 are ferromagnetic at low temperatures. GdCd has a very high Curie temperature of 262°K. In the Gd(Cu, Ag, Au) system, the Neel temperature increases with the increase in the lattice constant. The remarkable difference in magnetic behavior between the Gd(Ag, Cd, In) system and the Gd(Cu, Ag, Au) system is considered to be associated with the fact that the number of conduct...

Magnetic Properties of Rare Earth Intermetallic Compounds in Gd(Ag, Cd, In) and Gd(Cu, Ag, Au) Systems

Electrical resistivity and photoluminescence spectrum of layered oxysulfide (LaO)CuS

酸化硫化物(LaO)CuS及び(La 1-x Ca x O)Cu 1-x Ni x S(x≦0.10)の電子構造の光電子放出及び逆光電子放出分光法による研究

A superconducting transition with an onset temperature of about 40 K has been observed in the La-Cu-O system which is not doped with alkaline earth ions. The compounds with nominal compositions, La2CuO4-δ, La3Cu2Oy and La4Cu3Oz were prepared. In this system, the occurrence of superconductivity is attributed to the orthorhombic (distorted K2NiF4 type) phase. The superconducting transition occurs following a semiconductor-like increase in the resistivity with decreasing temperature, which is characteristic of the low carrier density superconductors. The effects of an annealing in oxygen at atmospheric pressure are also reported.

Superconductivity in the La–Cu–O System

Magnetic properties of disilicides of Gd, Tb, Dy, Ho, and Er have been investigated at temperatures ranging from 4.2°K to room temperature. All the disilicides except ErSi 2 have the orthorhombic ThSi 2 type of crystal structure and are shown to be antiferromagnetic. The Neel temperatures of GdSi 2 , TbSi 2 , DySi 2 and HoSi 2 are 27°K, 17°K, 17°K, and 18°K, respectively. The compound ErSi 2 has the hexagonal AlB 2 type of crystal structure and is paramagnetic at above 4.2°K. The paramagnetic Curie temperature of these compounds decreases with the number of f electrons of the rare earth ion.

Kazuko Sekizawa

Papers

Magnetic Properties of Rare Earth Intermetallic Compounds in Gd(Ag, Cd, In) and Gd(Cu, Ag, Au) Systems

Electrical resistivity and photoluminescence spectrum of layered oxysulfide (LaO)CuS

酸化硫化物(LaO)CuS及び(La 1-x Ca x O)Cu 1-x Ni x S(x≦0.10)の電子構造の光電子放出及び逆光電子放出分光法による研究

Superconductivity in the La–Cu–O System

Antiferromagnetism of Disilicides of Heavy Rare Earth Metals