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...

/pdf/progress-challenges-and-opportunities-in-two-dimensional-1sweg4hljr.pdf

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

Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.

Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers

To obtain graphene-based fluorescent materials, one of the effective approaches is to convert one-dimensional (1D) graphene to 0D graphene quantum dots (GQDs), yielding an emerging nanolight with extraordinary properties due to their remarkable quantum confinement and edge effects. In this review, the state-of-the-art knowledge of GQDs is presented. The synthetic methods were summarized, with emphasis on the top-down routes which possess the advantages of abundant raw materials, large scale production and simple operation. Optical properties of GQDs are also systematically discussed ranging from the mechanism, the influencing factors to the optical tunability. The current applications are also reviewed, followed by an outlook on their future and potential development, involving the effective synthetic methods, systematic photoluminescent mechanism, bandgap engineering, in addition to the potential applications in bioimaging, sensors, etc.

Focusing on luminescent graphene quantum dots: current status and future perspectives

Since its debut in 2004, graphene has attracted enormous interest because of its unique properties. Chemical vapor deposition (CVD) has emerged as an important method for the preparation and production of graphene for various applications since the method was first reported in 2008/2009. In this Account, we review graphene CVD on various metal substrates with an emphasis on Ni and Cu. In addition, we discuss important and representative applications of graphene formed by CVD, including as flexible transparent conductors for organic photovoltaic cells and in field effect transistors.Growth on polycrystalline Ni films leads to both monolayer and few-layer graphene with multiple layers because of the grain boundaries on Ni films. We can greatly increase the percentage of monolayer graphene by using single-crystalline Ni(111) substrates, which have smooth surface and no grain boundaries. Due to the extremely low solubility of carbon in Cu, Cu has emerged as an even better catalyst for the growth of monolayer ...

Review of chemical vapor deposition of graphene and related applications.

The growth of high-quality single crystals of graphene by chemical vapor deposition on copper (Cu) has not always achieved control over domain size and morphology, and the results vary from lab to lab under presumably similar growth conditions. We discovered that oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. Oxygen also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment–limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. The electrical quality of the graphene films was equivalent to that of mechanically exfoliated graphene, in spite of being grown in the presence of O.

http://utw10193.utweb.utexas.edu/Archive/RuoffsPDFs/362.pdf

The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper

Submillimeter single-crystal monolayer and multilayer graphene domains were prepared by an atmospheric pressure chemical vapor deposition method with suppressing nucleation on copper foils through an annealing procedure. A facile oxidation visualization method was applied to study the nucleation density and morphology of graphene domains on copper foils. Scanning electron microscopy, transmission electron microscopy, atomic force microscopy, polarized optical microscopy, and Raman spectra showed that the submillimeter graphene domains were monolayer single crystals.

Controllable Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation

Zinc blende-structured CdTe nanoribbons (NRs) were synthesized for the first time via a two-step process. The electronic, transport, and photoconductive properties of the CdTe NRs were studied systematically. It was revealed that the CdTe NRs showed p-type conductivity, and presented significant photoresponses to visible–NIR (400–800 nm) irradiation with high responsivity and gain. The contribution of the factors such as surface states of NRs, channel length, light intensity, and working bias voltage to the photoresponse characteristics of CdTe NR photodetectors were discussed. Moreover, single CdTe NR-based visible–NIR photodetectors were also demonstrated to have high stability and reliability.

/pdf/visible-nir-photodetectors-based-on-cdte-nanoribbons-4n8an7n0x4.pdf

Visible–NIR photodetectors based on CdTe nanoribbons

A facile chemical solution method was reported for the first time to synthesize CdS/CdSe double-sensitized ZnO nanocable arrays. In this method, CdS was prepared by chemical bath deposition method, and CdSe was formed on the surface of CdS via ion exchange reaction thereafter. Comprehensive characterization techniques were carried out to study the composition, microstructure, and crystallinity of the sensitization layers. The CdS/CdSe ratio was revealed to be controllable by varying the temperature of ion exchange reaction. It was further demonstrated that sensitization of ZnO nanorods with CdS and CdSe exhibited enhanced light-harvesting ability. The photovoltaic performance of the double-sensitized ZnO nanocables in photoelectrochemical solar cells was evaluated. An increase in short-circuit current and an improvement of power conversion efficiency to 3.23% were achieved. In addition, the formation mechanism of the double-sensitizing layer is discussed in the Article.

CdS/CdSe Double-Sensitized ZnO Nanocable Arrays Synthesized by Chemical Solution Method and Their Photovoltaic Applications

In this paper, we report a one-step, high-yield synthesis of one-dimensional Ag heteronanostructures in aqueous solution. Besides the usual fcc phase of Ag, the heteronanostructures also contain a rare hcp (4H polytypic) phase, which favors asymmetrical growth. The rod−needle heteronanostructures (RNHSs) and plate−belt heteronanostructures (PBHSs) are formed through two different mechanisms. For RNHSs, the 4H and fcc phases coexist but have different densities in different segments, while for PBHSs, the fcc and 4H crystal structures exist in plate and belt segments, respectively. Condition-sensitive coexisting phase preferences can be applied as a new way of controlling the shape and thus the properties of Ag nanostructures.

Anisotropic growth of one-dimensional silver rod-needle and plate-belt heteronanostructures induced by twins and hcp phase.

InGaO3(ZnO)m (m = 3, 5) nanowires with perfect superlattice structures were synthesized via a catalyst-assisted vapor−liquid−solid (VLS) growth mechanism. Different from In2O3(ZnO)m nanowires maintaining wurtzite structure, the InGaO3(ZnO)m (m = 3, 5) superlattice nanowires have a monoclinic crystal structure. High resolution transmission electron microscopy (HRTEM) indicates that the nanowires have an axial superlattice structure, which consists of an In−O layer and In, Ga/Zn−O layers stacking alternately along the nanowire length. The period of a InGaO3(ZnO)3 nanowire consists of four atom layers, which is the shortest period in quasi-one-dimensional superlattice structures.

Xing Xie

Papers

Controllable Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation

Visible–NIR photodetectors based on CdTe nanoribbons

CdS/CdSe Double-Sensitized ZnO Nanocable Arrays Synthesized by Chemical Solution Method and Their Photovoltaic Applications

Anisotropic growth of one-dimensional silver rod-needle and plate-belt heteronanostructures induced by twins and hcp phase.

Synthesis and Photoluminescence of InGaO3(ZnO)m Nanowires with Perfect Superlattice Structure