The phase behavior of water is a topic of perpetual interest due to its remarkable anomalous properties and importance to biology, material science, geoscience, nanoscience, etc. It is predicted confined water at interface can exist in large amounts of crystalline or amorphous states. However, the experimental evidence of coexistence of liquid water phases at interface is still insufficient. Here, a special folding few-layers graphene film was elaborate prepared to form a hydrophobic/hydrophobic interface, which can provide a suited platform to study the structure and properties of confined liquid water. The real-space visualization of intercalated water layers phases at the folding interface is obtained using advanced atomic force microscopy (AFM). The folding graphene interface displays complicated internal interfacial characteristics. The intercalated water molecules present themselves as two phases, lowdensity liquid (LDL, solid-like) and high-density liquid (HDL, liquid-like), according to their specific mechanical properties taken in two multifrequency-AFM (MF-AFM) modes. Furthermore, the water molecules structural evolution is demonstrated in a series of continuous MF-AFM measurements. The work preliminary confirms the existence of two liquid phases of water in real space and will inspire further experimental work to deeply understanding their liquid dynamics behavior.

本研究利用磁力显微术扫描观测了附着在二氧化硅衬底上的铁铂纳米线样品的表面形貌和磁性结构,实现了对铁铂纳米线磁学信息的成像.将磁学信息成像中的明暗分布与纳米线的形貌成像相结合,分析了铁铂纳米线的磁极分布.磁极方向相反的纳米线聚集在一起的现象表明磁化材料总是趋于能量最低的状态.通过改变外加磁场的强度和方向控制铁铂纳米线内部的磁化状态,观测到磁极反转现象.

The fracture behaviors of single-layer molybdenum disulfide (MoS2) grown on SiO2/Si wafer by chemical vapor deposition (CVD) were systematically investigated here via a technique that combining AFM-based nanoscratch tests with friction force microscopy (FFM). To get a complete cognition of fracture behavior and deepen the understanding of different fracture stages in the process, nanoacratch with progressive and constant force was done step by step. The radius of AFM tip we used is similar to 10 nm which is relatively blunt to produce a tensile fracture rather than pierce the film at the initial stage. As a result, there were novel crack modes with specific normal load in the surface of CVD-grown MoS2 : semi-circular and periodical zigzag cracks, and both crack modes exhibit anisotropy during the generation and propagation. The scratch study of single-layer MoS2 film has exhibited the gradual process of fracture behavior on nanoscale layered films and confirmed the mesoscopic anisotropy of the mechanical failure of 2D materials.

Research about two-dimensional (2D) materials is growing exponentially across various scientific and engineering disciplines due to the wealth of unusual physical phenomena that occur when charge transport is confined to a plane. The applications of 2D materials are highly affected by the electrical properties of these materials, including current distribution, surface potential, dielectric response, conductivity, permittivity, and piezoelectric response. Hence, it is very crucial to characterize these properties at the nanoscale. The Atomic Force Microscopy (AFM)-based techniques are powerful tools that can simultaneously characterize morphology and electrical properties of 2D materials with high spatial resolution, thus being more and more extensively used in this research field. Here, the principles of these AFM techniques are reviewed in detail. After that, their representative applications are further demonstrated in the local characterization of various 2D materials’ electrical properties.

Interfacial engineering, such as molecule intercalation, can modify properties and optimize performance of van der Waals heterostructures and their devices. Here, we investigated the pristine and water molecule intercalated heterointerface of niobium disulphide (NbS2) on hexagonal boron nitride (h-BN) (NbS2/BN) using advanced atomic force microscopy (AFM), and observed the metal-insulator transition (MIT) of first layer (1L-) of NbS2 induced by water molecule intercalation. In pristine sample, interfacial charge transfers were confirmed by the direct detection of trapped static charges at the post-exposed h-BN surface, produced by mechanically peeling off the 1L-NbS2 from the substrate. The interfacial charge transfers facilitate the intercalation of water molecules at the heterointerface. The intercalated water layers make a MIT of 1L-NbS2, while the pristine metallic state of the following NbS2 layers remains preserved. This work is of great significance to help understand the interfacial properties of 2D metal/insulator heterostructures and can pave the way for further preparation of an ultrathin transistor.

Hexagonal two-dimensional (2D) atomic crystals commonly reveal intrinsically isotropic elastic properties, but stretching or bending deformation can lead to their mechanical symmetry breaking. So far, little work has been done on strain-induced in-plane anisotropic shear behaviors of such 2D atomic crystals. Here, in theory, we predict the appearance of in-plane shear stiffness anisotropy under uniaxial normal tension strain in monolayer molybdenum disulfide. We verify experimentally such a shear characteristic based on friction-driven stretch deformation during a contact scan by employing transverse shear microscopy, and we demonstrate the visualization of anisotropic shear deformation as a function of crystallographic orientation. The present work provides deep insights into flexibility governed interactions among friction, deformation, and in-plane elastic characteristics in 2D atomic crystals.

Understanding the process of charge generation, transfer, and diffusion between two-dimensional (2D) materials and their supporting substrates is very important for potential applications of 2D materials. Compared with the systematic studies of triboelectric charging in a bulk sample, a fundamental understanding of the triboelectrification of the 2D material/insulator system is rather limited. Here, the charge transfer and diffusion of both the SiO2 surface and MoS2/SiO2 interface through contact electrification and frictional electrification are investigated systematically in situ by scanning Kelvin probe microscopy and dual-harmonic electrostatic force microscopy. Different from the simple static charge transfer between SiO2 and the PtSi alloy atomic force microscope (AFM) tip, the charge transfer between the tip and the MoS2/SiO2 system is complicated. Triboelectric charges, generated by contact or frictional electrification with the AFM tip, are trapped at the MoS2/SiO2 interface and act as floating gates. The local charge discharge processes can be obtained by monitoring the surface potential. The charge decay time (τ) of the MoS2/SiO2 interface is one (or two) orders of magnitude larger than the decay time τ of the SiO2 surface. This work facilitates an understanding of the triboelectric and de-electrification of the interface between 2D materials and substrates. In addition to the charge transfer and diffusion, we demonstrate the nanopatterns of surface and interfacial charges, which have great potential for the application of self-assembly of charged nanostructures.