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.

We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 Å in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 ( 0.1 eV. On the basis of density functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices via direct deposition onto strongly interacting substrates.Graphene 1,2 sa two-dimensional crystalline sheet of carbon atoms arranged in a honeycomb latticesgenerated enormous interest in the research community owing to its ultrathin geometry and properties such as high carrier mobility, 3 excellent thermal conductivity, 4 and high mechanical strength. 5 One of the attractive features of free-standing graphene, a semimetal, is its semiconducting behavior 6,7 at length scales below 500 Å with a size-dependent band gap. [8][9][10] Previous reports [11][12][13] have shown that a band gap can also be opened in graphene grown on insulating SiC(0001) and BN(0001) via interactions with the substrate. Here, we report the formation of semiconducting graphene layers with a band gap of 0.3 ( 0.1 eV on Pd(111), a metallic substrate. Using in situ scanning tunneling microscopy (STM) and spectroscopy (STS), we determine the electronic structure of graphene islands grown in situ via chemical vapor deposition on Pd(111). In contrast to recent reports on nanoribbons, where the band gap originates from size/ edge effects, 8-10 the band gap in epitaxial graphene on palladium is caused by a strong interaction with the Pd substrate and the ensuing breaking of translational symmetry between the two hexagonal close-packed sublattices of graphene. Our experiments illustrate the control over the electronic properties of graphene through interactions with substrates in well-defined epitaxial configurations. This approach opens up the possibility of preparing metal-semiconducting graphene structures and metal-doped graphenebased devices with potentially new applications.Using STM, we followed the formation and growth of graphene on Pd(111) during ethylene deposition over a range of pressures, substrate temperatures, and times. Panels A and B of Figure 1 are representative STM images of graphene islands acquired from a Pd(111) surface in situ during ethylene deposition at 968 K. In our experiments, island sizes vary between 200 and 2000 Å and are commonly observed at or near the Pd step edges as in Figure 1A, or spanning across multiple terraces as in Figure 1B. During ethylene deposition, we observe graphene islands on the...