The junctions formed at the contact between metallic electrodes and semiconductor materials are crucial components of electronic and optoelectronic devices . Metal-semiconductor junctions are characterized by an energy barrier known as the Schottky barrier, whose height can, in the ideal case, be predicted by the Schottky-Mott rule on the basis of the relative alignment of energy levels. Such ideal physics has rarely been experimentally realized, however, because of the inevitable chemical disorder and Fermi-level pinning at typical metal-semiconductor interfaces. Here we report the creation of van der Waals metal-semiconductor junctions in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning. The Schottky barrier height, which approaches the Schottky-Mott limit, is dictated by the work function of the metal and is thus highly tunable. By transferring metal films (silver or platinum) with a work function that matches the conduction band or valence band edges of molybdenum sulfide, we achieve transistors with a two-terminal electron mobility at room temperature of 260 centimetres squared per volt per second and a hole mobility of 175 centimetres squared per volt per second. Furthermore, by using asymmetric contact pairs with different work functions, we demonstrate a silver/molybdenum sulfide/platinum photodiode with an open-circuit voltage of 1.02 volts. Our study not only experimentally validates the fundamental limit of ideal metal-semiconductor junctions but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics.

Bimetallic platinum-nickel (Pt-Ni) nanostructures represent an emerging class of electrocatalysts for oxygen reduction reaction (ORR) in fuel cells, but practical applications have been limited by catalytic activity and durability. We surface-doped Pt3Ni octahedra supported on carbon with transition metals, termed M-Pt3Ni/C, where M is vanadium, chromium, manganese, iron, cobalt, molybdenum (Mo), tungsten, or rhenium. The Mo-Pt3Ni/C showed the best ORR performance, with a specific activity of 10.3 mA/cm(2) and mass activity of 6.98 A/mg(Pt), which are 81- and 73-fold enhancements compared with the commercial Pt/C catalyst (0.127 mA/cm(2) and 0.096 A/mg(Pt)). Theoretical calculations suggest that Mo prefers subsurface positions near the particle edges in vacuum and surface vertex/edge sites in oxidizing conditions, where it enhances both the performance and the stability of the Pt3Ni catalyst.

An activity lift for platinum Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al. ). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies. Science , this issue p. 1410 , p. 1414 ; see also p. 1378

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm(2)/(V s) in MoS2 at low temperature.