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Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy' 1-3 . Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient 4 or by an osmotic potential resulting from a salt concentration gradient 5 . For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness 5-7 . Here we demonstrate the use of single-layer molybdenum disulfide (MoS 2 ) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10 6 watts per square metre-a current that can be attributed mainly to the atomically thin membrane of MoS 2 . Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment 8-11 -for example, a piezoelectric zinc oxide nanowire array 8 or single-layer MoS 2 (ref. 12). We use our MoS 2 nanopore generator to power a MoS 2 transistor, thus demonstrating a self-powered nanosystem.MoS 2 nanopores have already demonstrated better water-transport behaviour than graphene 13,14 owing to the enriched hydrophilic surface sites (provided by the molybdenum) that are produced following either irradiation with transmission electron microscopy (TEM) 15 or electrochemical oxidation 16 . The osmotic power is generated by separating two reservoirs containing potassium chloride (KCl) solutions of different concentrations with a freestanding MoS 2 membrane, into which a single nanopore has been introduced 13 . A chemical potential gradient arises at the interface of these two liquids at a nanopore in a 0.65-nm-thick, single-layer MoS 2 membrane, and drives ions spontaneously across the nanopore, forming an osmotic ion flux towards the equilibrium state (Fig. 1a). The presence of surface charges on the pore screens the passing ions according to their charge polarity, and thus results in a net measurable osmotic current, known as reverse electrodialysis 1 . This cation selectivity can be better understood by analysing the concentration of each ion type (potassium and chloride) as a function of the radial distance from the centre of the pore, as we show here through molecular-dynamics simulations (Fig. 1b).We fabricated MoS 2 nanopores either by TEM 13 (Fig. 1c) Distance from the centre of the pore (Å)C max /C min = 1,000C max /C min = 500C max /C min = 100 LETTER RESEARCHosmotic current can be expected, owing to the long time required for the system to reach equilibrium. We measured the osmotic current and voltage across the pore by using a pair of Ag/AgCl electrodes to characterize the current-voltage (I-V) response of the nanopore.To gain a better insight into the performance of the MoS 2 nanopore power generator, ...

Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.

Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of physical dimensionality and explore device physics in highly crystalline samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions. Here, we introduce Fe 5−x GeTe 2 as a cleavable material with ferromagnetic behavior at room temperature. We established intrinsic magnetic order at room temperature in bulk crystals (T C = 310 K) through magnetization measurements and in exfoliated, thin flakes (T C ≈ 280 K) using the anomalous Hall effect. Our work reveals Fe 5 GeTe 2 as a prime candidate for incorporating intrinsic magnetism into functional van der Waals heterostructures and devices near room temperature.

We report on the fabrication of field-effect transistors based on single layers and bilayers of the semiconductor WS2 and the investigation of their electronic transport properties. We find that the doping level strongly depends on the device environment and that long in situ annealing drastically improves the contact transparency, allowing four-terminal measurements to be performed and the pristine properties of the material to be recovered. Our devices show n-type behavior with a high room-temperature on/off current ratio of ∼10(6). They show clear metallic behavior at high charge carrier densities and mobilities as high as ∼140 cm(2)/(V s) at low temperatures (above 300 cm(2)/(V s) in the case of bilayers). In the insulating regime, the devices exhibit variable-range hopping, with a localization length of about 2 nm that starts to increase as the Fermi level enters the conduction band. The promising electronic properties of WS2, comparable to those of single-layer MoS2 and WSe2, together with its strong spin-orbit coupling, make it interesting for future applications in electronic, optical, and valleytronic devices.