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 field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth.We demonstrate a colloidal route to synthesizing CdS-Ag 2 S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization. The nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.

The combined effects of strain and phonon confinement are seen to explain why the Raman peak near 464 cm Ϫ1 in CeO 2Ϫy nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric ͑on the low-energy side͒ as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.

Control of nanocrystal surface defects for efficient charge extraction in polymer-ZnO photovoltaic systems J. Appl. Phys. 112, 066103 (2012) Experimental surface-enhanced Raman scattering response of two-dimensional finite arrays of gold nanopatches Appl. Phys. Lett. 101, 111606 (2012) Nano-hillock formation in diamond-like carbon induced by swift heavy projectiles in the electronic stopping regime: Experiments and atomistic simulations Appl. Phys. Lett. 101, 113115 (2012) Mass transport and thermal stability of TiN/Al2O3/InGaAs nanofilms

We demonstrate dual interface formation in nanocrystals (NCs) through cation exchange, creating epitaxial heterostructures within spherical NCs. The thickness of the inner-disk layer can be tuned to form two-dimensional (2D), single atomic layers (<1 nm). During the cation exchange reaction from copper sulfide to zinc sulfide (ZnS), we observe a solid-solid phase transformation of the copper sulfide phase in heterostructured NCs. As the cation exchange reaction is initiated, Cu ions replaced by Zn ions at the interfaces are accommodated in intrinsic Cu vacancy sites present in the initial roxbyite (Cu1.81S) phase of copper sulfide, inducing a full phase transition to djurleite (Cu1.94S)/low chalcocite (Cu2S), a more thermodynamically stable phase than roxbyite. As the reaction proceeds and reduces the size of the copper sulfide layer, the epitaxial strain at the interfaces between copper sulfide and ZnS increases and is maximized for a copper sulfide disk ∼ 5 nm thick. To minimize this strain energy, a second phase transformation occurs back to the roxbyite phase, which shares a similar sulfur sublattice to wurtzite ZnS. The observation of a solid-solid phase transformation in our unique heterostructured NCs provides a new pathway to control desired phases and an insight into the influence of cation exchange on nanoscale phase transitions in heterostructured materials.

A novel method is reported to create inorganically connected nanocrystal (NC) assemblies for both II-VI and IV-VI semiconductors by removing surfactant ligands using (NH4)2S. This surface modification process differs from ligand exchange methods in that no new surfactant ligands are introduced and the post-treated NC surfaces are nearly bare. The detailed mechanism study shows that the high reactivity between (NH4)2S and metal-surfactant ligand complexes enables the complete removal of surfactant ligands in seconds and converts the NC metal-rich shells into metal sulfides. The post-treated NCs are connected through metal-sulfide bonding and form a larger NCs film assembly, while still maintaining quantum confinement. Such "connected but confined" NC assemblies are promising new materials for electronic and optoelectronic devices.

In this work, we have developed a new fabrication method for nanoparticle (NP) assemblies for Li-ion battery electrodes that require no additional support or conductive materials such as polymeric binders or carbon black. By eliminating these additives, we are able to improve the battery capacity/weight ratio. The NP film is formed by using electrophoretic deposition (EPD) of colloidally synthesized, monodisperse cobalt NPs that are transformed through the nanoscale Kirkendall effect into hollow Co(3)O(4). EPD forms a network of NPs that are mechanically very robust and electrically connected, enabling them to act as the Li-ion battery anode. The morphology change through cycles indicates stable 5-10 nm NPs form after the first lithiation remained throughout the cycling process. This NP-film battery made without binders and conductive additives shows high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm(3)) even after 50 cycles. Because similar films made from drop-casting do not perform well under equal conditions, EPD is seen as the critical step to create good contacts between the particles and electrodes resulting in this significant improvement in battery electrode assembly. This is a promising system for colloidal nanoparticles and a template for investigating the mechanism of lithiation and delithiation of NPs.

A nonhydrolytic sol-gel process closely following that of Hyeon et al. for the synthesis of ZrO 2 nanocrystals (J. Am. Chem. Soc. 2003, 125, 6553-6557) was used to synthesize highly crystalline and monodisperse HfO 2 nanoparticles. Reactions of hafnium isopropoxide with hafnium halides at high temperature in a strongly coordinating solvent yield nanometersized particles of HfO 2 . The size, shape, and crystalline phase of the hafnia particles depend on both the reaction temperature and the halide. The nonhydrolytic cross-condensation method was also extended to the binary metal oxides nanocrystals, i.e., HfO 2 -ZrO 2 , ZrO 2 -TiO 2 , and HfO 2 -TiO 2 . Efforts to prepare nanocrystals of Hf x Zr 1-x O 2 over a wide range of x were successful; however, this method could not be used to prepare either Zr x Ti 1-x O 2 or Hf x Ti 1-x O 2 . In conjunction with X-ray powder diffraction and high-resolution transmission electron microscopy, Raman spectroscopy identifies the structural phase of the nanocrystals and also proves the formation of the Hf-Zr solid solution nanoparticles via the crosscondensation method. In the case of Hf x Zr 1-x O 2 , for x < 0.5, highly monodisperse, roughly spherical particles of tetragonal Hf x Zr 1-x O 2 were formed, while for x > 0.5, small nanorods of the monoclinic phase of the binary oxide were obtained.