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Condensation and arrangement of ions at water–solid interfaces are of great importance in the formation of electrical double layers (EDL) and the transport of ions under a confined geometry. So far, the microscopic understanding of interfacial ion configurations is still far from complete, especially when the local ion concentration is high and ion–ion interactions become prominent. In this study, we directly visualized alkali metal cations within the hydrogen-bonding network of water on graphite and Cu(111)-supported graphene surfaces, using qPlus-based noncontact atomic force microscopy (NC-AFM). We found that the codeposition of the alkali cations and water molecules on the hydrophobic graphite surface leads to the formation of an ion-doped bilayer hexagonal ice (BHI) structure, where the ions are repelled from each other and scattered in a disordered distribution. In contrast, the hydrated alkali cations aggregate in one dimension on the more hydrophilic graphene/Cu(111) surface, forming a nematic state with a long-range order. Such a nematic state arises from the delicate interplay between water–ion and water–water interactions under surface confinement. These results reveal the high sensitivity of ion–ion interactions and ionic ordering to the surface hydrophobicity and hydrophilicity. [ABSTRACT FROM AUTHOR]

Graphene with unique linear band structure and enhanced electron–electron interaction exhibits peculiar hot‐carrier generation. The thermalized hot carriers in graphene have potential to achieve high energy which is well above energy of the incident photons. Together with its outstanding optical and electrical properties, the exotic thermalization of hot carriers makes graphene a promising material for applications in optical up‐conversion devices and ultrafast photonics. However, the excitation efficiency of hot carriers is limited by its weak absorption and atomically thin light–matter interaction length. Here, the efficient hot‐carrier amplification is demonstrated in graphene by adjacent quantum dots (QD). With pump–probe experiments, this hot carrier amplification is found to stem from the charge collection of the two‐photon excited hot carriers in QD. Taking advantages of strong absorption of QD, the hot‐carrier excitation efficiency of graphene can be efficiently improved under near‐infrared sub‐bandgap excitation of QD. The transferred hot carriers are with energy higher than those originally excited in graphene, leading to raised hot‐carrier temperature and enhanced hot‐carrier up‐conversion. This research demonstrates QD's immense potential to realize high efficiency hot‐carrier injection in graphene‐based devices, which will promote their ability for hot‐carrier‐driven devices, optical up‐conversion devices and high‐frequency optoelectronics. [ABSTRACT FROM AUTHOR]

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS 2 ), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS 2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS 2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Low-dimensional water transport can be drastically enhanced under atomic-scale confinement. However, its microscopic origin is still under debate. In this work, we directly imaged the atomic structure and transport of two-dimensional water islands on graphene and hexagonal boron nitride surfaces using qPlus-based atomic force microscopy. The lattice of the water island was incommensurate with the graphene surface but commensurate with the boron nitride surface owing to different surface electrostatics. The area-normalized static friction on the graphene diminished as the island area was increased by a power of ~-0.58, suggesting superlubricity behavior. By contrast, the friction on the boron nitride appeared insensitive to the area. Molecular dynamic simulations further showed that the friction coefficient of the water islands on the graphene could reduce to <0.01.