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.

Ultrathin amorphous Bi films, patterned with a nano-honeycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper Pairs (CP) of electrons between localized states. Here we show that the magnetoresistance (MR) of this Cooper Pair Insulator (CPI) phase is positive and grows exponentially with decreasing temperature, T , for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.Below its transition temperature, T c0 , a conventional superconductor, like Pb, can be driven into its non-superconducting, normal state by applying a magnetic field, H. The temperature, T , dependent sheet resistance, R (T ), of this state joins smoothly to the normal state resistance just above T c0 , R N , and assumes the dependence expected for a simple metal of unpaired electrons [1]. The behavior of this low T normal state changes substantially when these superconductors are made as thin films with R N ∼ R Q = h/4e2 . For example, applying a H to superconducting (SC) films of either Indium oxide For granular Pb films, the formation of a CPI phase has strong intuitive appeal and experimental support. STM experiments show that they consist of islands of grains that can naturally localize CPs [9]. Indeed, tunneling experiments on insulating films confirmed the existence of these localized pairs by showing the energy gap in the density of states that accompanies CP formation [6]. Also, these insulators exhibit giant negative Magneto-Resistance (MR) that can be attributed to the enhancement of inter-island quasi-particle tunneling [10]. By contrast, InO x and TiN films lack any obvious structure that could localize CPs. Moreover, these films exhibit a giant positive MR [3,4,5,11,12], which can peak orders of magnitude above R N at sufficiently low T . The mechanism behind this spectacular giant positive MR [3,4,5,11,12] and whether it is a property of a CPI phase remains unresolved despite significant attention [13,14,15,16].Theories of the positive MR peak presume that CPs spontaneously localize into islands or puddles [13,14,15,16]. On each island, the complex SC order parameter has a well defined amplitude, but electrostatic interactions between islands prevents the development of the long range phase coherence necessary for CP delocalization. A magnetic field induces more phase disorder and localization through the direct coub.

Black phosphorus (BP) is receiving significant attention because of its direct 0.4–1.5 eV layer-dependent bandgap and high mobility. Because BP devices rely on exfoliation from bulk crystals, there is a need to understand the native impurities and defects in the source material. In particular, samples are typically p-doped, but the source of the doping is not well understood. Here, we use scanning tunneling microscopy and spectroscopy to compare the atomic defects of BP samples from two commercial sources. Even though the sources produced crystals with an order of magnitude difference in impurity atoms, we observed a similar defect density and level of p-doping. We attribute these defects to phosphorus vacancies and provide evidence that they are the source of p-doping. We also compare these native defects to those induced by air exposure and show that they are distinct and likely more important for the control of electronic structure. These results indicate that impurities in BP play a minor role compared to vacancies, which are prevalent in commercially available materials, and call for better control of vacancy defects.

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