In the past few years, nanopores have emerged as a new powerful tool to interrogate single molecules. They have been successfully used to rapidly characterize biopolymers like DNA 3,4 , RNA 5 , as well as DNA-ligand complexes 6 and local protein structures along DNA 7 at the single-molecule level. A key driving force for nanopore research in the past decade has been the prospect of DNA sequencing. However, a major roadblock for achieving high-resolution DNA sequencing with pores is the finite length of the channel constituting the pore (Fig. 1A). In a long nanopore, the current blockade resulting from DNA translocation is due to a large number of bases (for typical devices ~10-100 bases) present in the pore. Here, we demonstrate that this limitation can be overcome by realizing an ultimately thin nanopore in a graphene monolayer. We obtain single-layer graphene (Fig. 1B) by mechanical exfoliation from graphite on SiO 2 13 . Monolayer graphene is identified by its particular optical contrast 14 in the optical microscope and by Raman measurements (Fig. 1C). At ~ 1590 cm -1 , we measure the socalled G resonance peak and at ~2690 cm -1 the 2D resonance peak. In the case of multilayer graphene, the 2D resonance peak splits off in multiple peaks in contrast to monolayer graphene which has a very sharp single resonance peak. In this way, we are well able to distinguish single-layer graphene from multilayer graphene 15 .Next we select a monolayer of graphene and transfer it onto a SiN support membrane with a 5 micron sized hole 16 by use of our recently developed 'wedging transfer' technique 17 . This transfer procedure is straightforward: flakes can be overlaid to support membranes in less than an hour. Briefly, a hydrophobic polymer is spun onto a hydrophilic substrate (here plasma-oxidized SiO 2 ) with graphene flakes, and wedged off the substrate by sliding it at an angle in water. Graphene flakes are peeled off the SiO 2 along with the -4 -polymer. The polymer is then floating on the water surface, located near a target SiN substrate, the water level is lowered, and the flakes are positioned onto the SiN membrane with micrometer lateral precision. In the final step the polymer is dissolved.We then drill a nanopore into the graphene monolayer using the highly focused electron beam of a transmission electron microscope (TEM). The acceleration voltage is 300 kV, well above the 80-140 kV knock-out voltage for carbon atoms in graphene 18 (see Methods).Drilling the holes by TEM is a robust well-reproducible procedure (we drilled 31 holes with diameters ranging from 2 to 40 nm, in monolayer as well as in multilayer graphene; some examples of pores are shown in Fig. 2). Because of the high acceleration voltage of the electron beam, drilling could potentially induce damage to the graphene around the pore. However, electron beam diffraction measurements across the hole ( Fig. 2B and C) confirm the crystallinity of the monolayer surrounding the hole, as evidenced by the well-defined hexagonal diffraction patterns (Fig. 2C).Su...

Single-layer MoS(2) is an attractive semiconducting analogue of graphene that combines high mechanical flexibility with a large direct bandgap of 1.8 eV. On the other hand, bulk MoS(2) is an indirect bandgap semiconductor similar to silicon, with a gap of 1.2 eV, and therefore deterministic preparation of single MoS(2) layers is a crucial step toward exploiting the large direct bandgap of monolayer MoS(2) in electronic, optoelectronic, and photovoltaic applications. Although mechanical and chemical exfoliation methods can be used to obtain high quality MoS(2) single layers, the lack of control in the thickness, shape, size, and position of the flakes limits their usefulness. Here we present a technique for controllably thinning multilayered MoS(2) down to a single-layer two-dimensional crystal using a laser. We generate single layers in arbitrary shapes and patterns with feature sizes down to 200 nm and show that the resulting two-dimensional crystals have optical and electronic properties comparable to that of pristine exfoliated MoS(2) single layers.

Hybrid graphene-superconductor devices have attracted much attention since the early days of graphene research. So far, these studies have been limited to the case of diffusive transport through graphene with poorly defined and modest-quality graphene/superconductor interfaces, usually combined with small critical magnetic fields of the superconducting electrodes. Here, we report graphene-based Josephson junctions with one-dimensional edge contacts of molybdenum rhenium. The contacts exhibit a well-defined, transparent interface to the graphene, have a critical magnetic field of 8 T at 4 K, and the graphene has a high quality due to its encapsulation in hexagonal boron nitride. This allows us to study and exploit graphene Josephson junctions in a new regime, characterized by ballistic transport. We find that the critical current oscillates with the carrier density due to phase-coherent interference of the electrons and holes that carry the supercurrent caused by the formation of a Fabry-Pérot cavity. Furthermore, relatively large supercurrents are observed over unprecedented long distances of up to 1.5 μm. Finally, in the quantum Hall regime we observe broken symmetry states while the contacts remain superconducting. These achievements open up new avenues to exploit the Dirac nature of graphene in interaction with the superconducting state.

A mechanically exfoliated graphene flake ͑ϳ150ϫ 380 m 2 ͒ on a silicon wafer with 98 nm silicon dioxide on top was scanned with a spectroscopic ellipsometer with a focused spot ͑ϳ100 ϫ 55 m 2 ͒ at an angle of 55°. The spectroscopic ellipsometric data were analyzed with an optical model in which the optical constants were parameterized by B-splines. This parameterization is the key for the simultaneous accurate determination of the optical constants in the wavelength range 210-1000 nm and the thickness of graphene, which was found to be 3.4 Å. © 2010 American Institute of Physics. ͓doi:10.1063/1.3475393͔In 2004, it was discovered that a free-standing single atomic layer can be isolated from its environment by means of micromechanical cleavage.1 Of the different reported twodimensional crystals, the single atomic layer of graphite, graphene, has gained most interesting due to its remarkable electronic properties.2 The vast majority of the studies focuses therefore on its electronic properties. Its optical properties, however, were less explored.Gray et al. 3 studied the optical properties of graphene by near-normal incidence reflectance measurements in the range 190-1000 nm. They acquired reflectance data of graphite flakes of different thicknesses, down to graphene, deposited on a silicon wafer with 300 nm silicon dioxide ͑SiO 2 ͒ on top. They assumed the optical constants to be independent of thickness and that they could be parameterized with five Forouhi-Bloomer oscillators. The parameters of these oscillators and each thickness were fitted simultaneously to all the reflectance data. The thickness was fitted as 3.8 Å. This work was extended by adding spectroscopic ellipsometry and s-polarized reflectance ͑both at 70°, 380-1000 nm͒ to the near-normal incidence reflectance in their data analysis. 4 This time, however, the optical constants were not assumed to be independent of thickness and were parameterized by a proprietary dispersion model. The dispersion parameters, however, were not reported. They found the thickness of graphene was 3.7 Å. Very recently Kravets et al.5 also used spectroscopic ellipsometry on graphene on an oxidized silicon wafer ͑300 nm SiO 2 ͒, and on amorphous quartz. They report optical constants extracted from the variable angle ͑45°-70°͒ ellipsometry data by numerical inversion in the range 240-750 nm for the amorphous quartz wafer ͑240-1000 nm for the oxidized wafer͒, assuming a thickness of 3.35 Å.In this paper we show the optical constants and report dispersion parameters of graphene as found from spectroscopic ellipsometry in the range 210-1000 nm. We show that, without assuming any physical oscillator parameterization beforehand, B-splines allow an uncorrelated, accurate, and simultaneous determination of the optical constants and thickness of graphene. The thickness is in perfect agreement with the thickness as expected from the interlayer spacing in graphite: 3.4 Å. Based on the found optical constants we have simulated transmittance for graphene. We show that this simulation is...

We report a versatile water-based method for transferring nanostructures onto surfaces of various shapes and compositions. The transfer occurs through the intercalation of a layer of water between a hydrophilic substrate and a hydrophobic nanostructure (for example, graphene flakes, carbon nanotubes, metallic nanostructures, quantum dots, etc.) locked within a hydrophobic polymer thin film. As a result, the film entrapping the nanostructure is lifted off and floats at the air-water interface. The nanostructure can subsequently be deposited onto a target substrate by the removal of the water and the dissolution of the polymeric film. We show examples where graphene flakes and patterned metallic nanostructures are precisely transferred onto a specific location on a variety of patterned substrates, even on top of curved objects such as microspheres. The method is simple to use, fast, and does not require advanced equipment.