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Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 °C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

Adaptive nanocomposite coating materials that automatically and reversibly adjust their surface composition and morphology via multiple mechanisms are a promising development for the reduction of friction and wear over broad ranges of ambient conditions encountered in aerospace applications, such as cycling of temperature and atmospheric composition. Materials selection for these composites is based on extensive study of interactions occurring between solid lubricants and their surroundings, especially with novel in situ surface characterization techniques used to identify adaptive behavior on size scales ranging from 10 −10 to 10 −4 m. Recent insights on operative solid-lubricant mechanisms and their dependency upon the ambient environment are reviewed as a basis for a discussion of the state of the art in solid-lubricant materials. 297 Click here for quick links to Annual Reviews content online, including: • Other articles in this volume • Top cited articles • Top downloaded articles • Our comprehensive search Further ANNUAL REVIEWS Tribology: the study of surfaces in relative motion, from the Greek word tribos, meaning rubbing Friction coefficient: the ratio of tangential force to applied normal force, or the ratio of shear strength to pressure if both forces are divided by contact area Abrasive wear: the plowing away of softer material by harder particles at contact interfaces Adhesive wear: asperities from both surfaces in contact adhere, and material from the softer surface is sheared away as the counterface moves Fatigue wear: the removal of material due to crack growth and propagation over repeated load cycles Wear rate: the rate of material loss from a surface due to sliding contact, often reported in units of volume lost per unit sliding distance per unit of applied normal force (e.g., mm 3 m −1 N −1) FUNDAMENTALS OF SOLID LUBRICATION IN DIVERSE ENVIRONMENTS A spray of liquid lubricant from a can available at the hardware store solves most familiar friction and wear problems on Earth under ordinary conditions of ambient pressure, humidity, and temperature. When any one of those operating parameters reaches the limits of advanced engineering materials, however, the availability of suitable lubricants is diminished significantly. Moreover, if operation throughout a broad range of any one of these environmental factors is desired, the number of choices becomes exactly zero; tribologists have found that no single surface can lubricate effectively outside a narrow span of ambient humidity or temperature (1-3). In fact, one of the primary technological barriers for the development of new jet engines for sustained highmach-number (>5) flights and other key aerospace advances is the ability to lubricate mechanical assemblies from ambient temperature to >1000 • C. Only recently have discoveries of self-adaptive mechanisms and subsequent developments in tribological coatings, made possible with modern instrumentation and pioneering efforts to observe phenomena at atomic through microscopic size scales at contact ...

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